JP3763803B2 - Optical transmitter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical transmission device for an optical fiber transmission system.
[0002]
[Prior art]
Today, the advancement and popularization of the information communication field is remarkable, and there is an increasing expectation for speeding up the transmission technology that supports the information communication system. In particular, in the field of optical fiber transmission systems, the development of basic and subscriber communication systems is considered essential in the future, and the practical application of high-speed optical transmission technology is highly expected. A modulation method capable of maintaining necessary and sufficient performance in terms of transmission capacity and transmission distance in addition to the realization of an optical modulator capable of performing high-speed modulation on the order of several tens of Gbits on an optical wave when transmitting high-speed optical signals. It is important to establish optical transmission technology that incorporates
[0003]
In general, it is well known that characteristics inherent to an optical fiber, such as chromatic dispersion characteristics and nonlinear optical effects, limit transmission reachable distances of optical signals. When an ultrahigh-speed optical signal of about 40 Gbit / s is transmitted through an optical fiber, it is susceptible to the chromatic dispersion of the optical fiber in order to handle a signal with a narrow bandwidth with a wide band. In addition, stricter conditions are imposed on the circuit configuration of electric systems for generating and receiving broadband ultra-high-speed pulses, the mounting accuracy of these circuits, and the like.
[0004]
Optical signal modulation methods include a non-return-to-zero (NRZ) code method that has been widely used since light wave modulation is possible with a simple configuration, and zero having the characteristic of being less affected by the nonlinear optical effect of an optical fiber. There are a return (RZ) code system and a duobinary system which can improve the efficiency of optical signal band use because the modulation band can be reduced. Among these, the RZ code method is particularly excellent in terms of resistance to non-linear effects, and can increase the transmission optical power, and thus has attracted attention as a method suitable for a long-distance transmission system.
[0005]
As described in Non-Patent Document 1, for example, an optical transmission device that generates an RZ-coded optical signal has a configuration including two optical intensity modulators. That is, a single-mode semiconductor laser, a first optical intensity modulator for generating an RZ code pulse, a clock signal source for supplying a clock signal, a signal source serving as an ultrafast NRZ code optical signal data source, and a signal source The second light intensity modulator for modulating the light wave by the ultrahigh-speed data. The light wave emitted from the semiconductor laser is converted into an optical pulse by the first light intensity modulator, and is modulated by the NRZ code data signal in the second light intensity modulator. At this time, it is necessary that the pulse repetition frequency of the clock signal source corresponds to half of the bit rate of the NRZ code data signal emitted from the signal source, and that the clock signal source and the signal source are always synchronized.
[0006]
However, in the configuration of such an RZ code optical transmission device, it is necessary to prepare two light intensity modulators capable of ultra-high speed modulation, compared with the NRZ code method using only one light intensity modulator. Cost becomes high. Further, since the insertion loss of the optical intensity modulator is large, the optical output of the optical transmission device is further weakened, and the signal-to-noise ratio (S / N ratio) of the signal light is deteriorated.
[0007]
In addition, since the optical waveguide substrate used for the light intensity modulator has a large polarization dependency, it is necessary to use a constant polarization optical fiber for connection between the first and second light intensity modulators. In this case, it is indispensable to connect the polarization optical fiber and the individual polarization axes of the optical waveguide substrate of the light intensity modulator in a well-aligned manner. Increase manufacturing costs.
[0008]
In order to solve this problem, a method of generating an RZ-coded optical signal using a single optical intensity modulator composed of an optical phase modulator and a Mach-Zehnder interferometer is disclosed in P. Jay. Proposed by Windsor et al. (See Non-Patent Document 2).
[0009]
In this system, an NRZ code data signal emitted from a signal source is converted into a non-zero return inversion (NRZI) code by a precoder circuit. The NRZI code signal is applied to the phase modulator through the driver circuit, and the light wave emitted from the semiconductor laser is phase-modulated. The phase-modulated light wave is phase-adjusted by a Mach-Zehnder interferometer to generate an RZ code optical signal. According to this method, the trouble of synchronizing the signals input to the two light intensity modulators is eliminated. Further, since the optical phases of adjacent mark (“1” code) bits are inverted, the resistance to intersymbol interference is large.
[0010]
However, in this configuration, since the light wave emitted from the semiconductor laser passes through the phase modulator and the Mach-Zehnder interferometer, the optical loss increases and the S / N ratio of the output optical signal is deteriorated. In the Mach-Zehnder interferometer, the branched light wave is caused to interfere with an optical path length difference of about 3 to 4 digits of the normal wavelength. Accordingly, since the phase fluctuation of the semiconductor laser is converted into light intensity as noise, the noise of the mark level of the output optical signal increases, and the transmission characteristics are deteriorated. Further, in order to obtain a uniform output light or extinction state of the Mach-Zehnder interferometer, a highly accurate control system for precisely matching the optical frequency of the semiconductor laser and the wavelength characteristic of the Mach-Zehnder interferometer is required.
[0011]
Further, the extinction ratio of the RZ code optical signal is determined by the extinction ratio of the Mach-Zehnder interferometer, as shown in FIG. Considering manufacturing errors and the like, the extinction ratio of the Mach-Zehnder interferometer is expected to be about 15 dB. Therefore, from FIG. 32, the extinction ratio of the RZ code optical signal is suppressed to about 8 dB, and a good eye opening cannot be obtained. It will be. Further, since the phase modulator and the Mach-Zehnder interferometer have polarization dependency, they are connected by a polarization-maintaining optical fiber. However, the extinction ratio deteriorates due to the polarization axis shift at this time.
[0012]
[Non-Patent Document 1]
Y. Miyamoto et.al, OFC2000 Post ndeadline papers, PD26 (2000)
[Non-Patent Document 2]
PJ Winzer et. Al. IEEE Photonics Technology Letters, Vol. 13, No. 12, pp. 1298-1300, 2001
[0013]
[Problems to be solved by the invention]
As described above, in the conventional RZ code optical transmitter, two light intensity modulators are required, and the polarization axis needs to be aligned with a constant polarization optical fiber for connection between the light intensity modulators. There have been problems such as an increase in the number of signals applied to the two light intensity modulators and a deterioration in the S / N ratio due to the insertion loss of the light intensity modulators. In order to solve this problem, a method of generating an RZ encoded optical signal using one optical intensity modulator that modulates an optical wave with an NRZI encoded signal has been proposed. However, since the proposed optical transmitter uses a phase modulator in front of the optical intensity modulator, the insertion loss is large, the S / N ratio of the output optical signal is degraded, and mark level noise is also present. There is a problem that the transmission characteristic is deteriorated and transmission characteristics are deteriorated. Furthermore, there is a problem that the extinction ratio of the RZ code optical signal is suppressed and a good eye opening cannot be obtained.
[0014]
The present invention has been made to solve the above-described problems. The object of the present invention is to output an output signal light of an RZ code with a low noise and a large extinction ratio by using a single light intensity modulator. An object of the present invention is to provide an optical transmission device for generation.
[0015]
Another object of the present invention is to provide an optical transmitter having a high tolerance against intersymbol interference.
[0016]
[Means for Solving the Problems]
In order to achieve the above object, one feature of the present invention is that (a) when the first logic code of the NRZ code signal is input, the level of the NRZ code signal is converted, and the second logic of the NRZ code signal is converted. When the code is input, the level of the NRZ code signal is not converted, and the NRZI code signal generated from the NRZ code signal and the inverted NRZI code signal generated by inverting the code of the NRZI code signal are amplified respectively. , A code conversion module that outputs an NRZI modulation signal and an inverted NRZI modulation signal, (b) a DC bias power source that supplies a DC bias, (C) a semiconductor laser that outputs a light wave, and (D) a light wave at the input unit 2 An optical waveguide that branches and multiplexes at the output section, a modulation section that applies an output pulse code signal and an inverted output pulse code signal to each of the two branched light waves, and a direct current to the optical waveguide Provided with adjustment electrode for applying a bias, by adjusting the DC bias, the NRZI modulated signal and an optical signal pulse code of the inverted NRZI modulation signal is outputted from the optical intensity modulator in a section that inverts Minimum strength The gist of the present invention is that it is an optical transmission device including an optical intensity modulator that modulates the intensity of a light wave branched in two.
[0017]
According to the above feature of the present invention, it is possible to provide an optical transmission device that generates output signal light of an RZ code with a low noise and a high extinction ratio using a single optical intensity modulator.
[0018]
In the above feature of the present invention, since the optical waveguide of the light intensity modulator is made of a dielectric having an electro-optic effect, the pulse code of the output pulse code signal and the inverted output pulse code signal is inverted by adjusting the DC bias. The light wave can be intensity-modulated so as to output an optical signal in a section. Also, a differential amplifier circuit having an output terminal for amplifying and outputting the output pulse code signal and an inverting output terminal for inverting the amplified output pulse code signal to an inverted output pulse code signal is connected to the code conversion circuit and the light intensity. If the configuration is inserted between the modulators, the circuit configuration can be simplified. Further, it is preferable that the light intensity modulator and the differential amplifier circuit are integrated in a hybrid manner and mounted on the light modulator module.
[0019]
Further, if the light intensity modulator is made of a semiconductor having an electro-optic effect or an electro-absorption effect, monolithic integration can be achieved. Furthermore, it is preferable that an optical filter is connected to the output part of the light intensity modulator, and the output signal light is single sideband signal light. Thus, by making the signal light to be transmitted into a single sideband, the occupied band of the optical frequency can be reduced, and frequency multiplex transmission with high band utilization efficiency becomes possible.
[0020]
In addition, the present invention is characterized in that (a) the level of the NRZ code signal is converted when the first logical code of the NRZ code signal is input, and the level of the NRZ code signal is input when the second logical code of the NRZ code signal is input. A code conversion module that generates an NRZI code signal from an NRZ code signal without converting the level of the code signal, a splitter that divides the NRZI code signal into two, a delay unit that is connected to an output on one side of the splitter, (B) a DC bias power supply for supplying a DC bias; (c) a semiconductor laser that outputs a light wave; (d) an optical waveguide that divides the light wave into two at the input part and multiplexes at the output part; , A light intensity provided with a modulation unit for applying an output pulse code signal and an output pulse code signal subjected to time delay in the delay unit, and an adjustment electrode for applying a DC bias to the optical waveguide And summarized in that an optical transmitting apparatus comprising a regulator.
[0021]
According to the above aspect of the present invention, there is provided an optical transmission device that generates an output signal light of an RZ code having a low noise, a large extinction ratio, and a high tolerance against intersymbol interference using a single optical intensity modulator. Can be provided.
[0022]
In the above-mentioned feature of the present invention, the light intensity modulator adjusts the DC bias, and the rising edge of the output pulse code signal and the falling edge of the delayed output pulse code signal, and the falling edge of the output pulse code signal and the delayed output pulse code signal. In order to intensity-modulate the light wave branched into two so as to output the optical signal in the delay section of the delay time at the rising edge of the signal, the phase of the adjacent output signal light is inverted, and the interference between the signal light codes in the transmission optical fiber The resistance to can be increased. Further, it is preferable that the two-branched light wave is substantially phase-modulated by the output pulse code signal and the delayed output pulse code signal at a phase substantially corresponding to a half wavelength of the light wave. When light waves that have undergone half-wave phase modulation are combined, the light intensity becomes maximum, so that output signal light having a high extinction ratio can be obtained with low noise.
[0023]
It is preferable that a waveform shaping filter for shaping the pulse waveforms of the output pulse code signal and the delayed output pulse code signal is provided between the code conversion circuit and the light intensity modulator. Further, the delay time is shorter than the unit pulse code time of the output pulse code signal and the delayed output pulse code signal, and longer than the rise time and the fall time of the pulse waveform of the output pulse code signal and the delayed output pulse code signal. Is preferred.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Next, first to third embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic and shapes and dimensions are different from actual ones. Therefore, specific shapes and dimensions should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.
[0025]
(First embodiment)
The optical transmission apparatus 1 according to the first embodiment of the present invention (a) converts the level of the NRZ code signal A1 when the first logical code of the NRZ code signal A1 is input, When the second logic code is input, the level of the NRZ code signal A1 is not converted, and the NRZI code signal A3 generated from the NRZ code signal A1 and the inversion generated by inverting the sign of the NRZI code signal A3 The code conversion module 80 that amplifies the NRZI code signal A4 and outputs the NRZI modulation signal A5 and the inverted NRZI modulation signal A6, (b) the DC bias power supply 6 that supplies the DC bias A7, and (c) the light wave B1. A semiconductor laser 10 to be output; and (d) first and second optical waveguides 15, 16, 2 that divide the light wave B 1 into two at the light wave input unit 13 and combine at the light combining unit 14. The first and second phase modulators 17 and 18 that apply the NRZI code signal A3 and the inverted NRZI code signal A4 to each of the branched first and second branched light waves B3 and B5, and the DC bias A7 to the optical waveguide Is composed of a light intensity modulator 9 provided with a phase adjusting unit 19 for applying.
[0026]
As shown in FIG. 1, a signal source 3 is connected to the optical transmitter 1, and the NRZ code signal A <b> 1 and the inverted NRZ code signal A <b> 2 of transmission data emitted from the signal source 3 are the first and second signals of the code conversion module 80. Are converted into an NRZI code signal A3 and an inverted NRZI code signal A4 by the precoder circuits 4 and 5, respectively, and the NRZI code signal A3 and the inverted NRZI code signal A4 are amplified by the driver circuits 7 and 8 of the code conversion module 80 to be NRZI modulated signals. A5 and an inverted NRZI modulation signal A6 are output from the code conversion module 80, a light wave B1 is transmitted from the semiconductor laser 10 of a single mode light source, a DC bias A7 is set by the DC bias power supply 6, and the light intensity modulator 9 NRZI modulation signal A5 and inverted NRZI modulation signal A6 and direct current bias A6 Modulated output signal light B2 is output to the transmission path optical fiber.
[0027]
The light wave B1 input from the semiconductor laser 10 to the light intensity modulator 9 is branched into two first and second branch optical waveguides 15 and 16 in the light intensity modulator 9 (see FIGS. 6 and 7). ). The first and second branched optical waveguides 15 and 16 shown in FIGS. 6 and 7 are made of a medium having an electro-optic effect (Pockels effect) in which the refractive index changes according to an electric field, and an optical path length difference between the two optical waveguides. Is not provided.
[0028]
The phase of each of the first and second branch light waves B3 and B5 propagating through the first and second branch optical waveguides 15 and 16 changes according to the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 shown in FIG. When the phase changes, the optical frequency of the branched light wave increases or decreases instantaneously. The relative phase relationship between the first and second branched light waves B3 and B5 can be adjusted by the level of the DC bias A7 to the light intensity modulator 9, and the output levels of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6. Is appropriately set, the light intensity of the output signal light B2 of the light intensity modulator 9 can be minimized when the logic level does not change, and the light intensity can be maximized instantly when the logic level is switched. Ratio can be realized.
[0029]
The NRZI code signal A3 is a bit string whose code is inverted when the NRZ code signal A1 is a mark bit, while the inverted NRZI code signal A4 has a code when the inverted NRZ code signal A2 is a space (“0” code) bit. It is a bit string to be inverted. Therefore, the output signal light B2 whose light intensity is modulated by the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 becomes the signal light of the RZ code.
[0030]
In the first embodiment of the present invention, the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 are generated using the NRZ code signal A1 and the inverted NRZ code signal A2 output from the signal source 3. However, the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 can be generated using only the NRZ code signal A1.
[0031]
For example, the NRZ code signal A1 of the signal source 3 may be branched into two, and one branched NRZ code signal A1 may be input to the second precoder circuit 5 via a negation circuit. Further, after the NRZ code signal A1 is converted into the NRZI code signal A3 by the first precoder circuit 4, the NRZI code signal A3 is branched into two, and one of the branched NRZI code signals A3 is inverted through a negation circuit. The NRZI code signal A4 may be generated and input to the driver circuit 8. In this case, the second precoder circuit 5 can be omitted, and the cost can be reduced. Further, after the NRZI code signal A3 is amplified by the driver circuit 7, the NRZI modulation signal A5 is branched into two, and one of the branched NRZI modulation signals A5 is input to the negative circuit to generate the inverted NRZI modulation signal A6 You can also. In this case, the second precoder circuit 5 and the driver circuit 8 can be omitted, and the cost can be further reduced.
[0032]
According to the optical transmission apparatus 1 according to the first embodiment of the present invention, the output signal light B2 from the NRZ code signal A1 to the RZ code is composed of one signal source 3 and one light intensity modulator 9. Therefore, the insertion loss can be reduced. In addition, since no optical path length difference is provided in the branched optical waveguide of the light intensity modulator 9, noise at the mark level can be reduced, and a good signal-to-noise ratio can be obtained.
[0033]
As shown in FIG. 2A, the first precoder circuit 4 according to the first exemplary embodiment of the present invention includes an exclusive OR (XOR) circuit 11a and a 1-bit delay line 11b. The 1-bit delay line 11b is composed of, for example, a microstrip line or a coplanar line. Here, the initial value of the 1-bit delay line 11b is a “0” code. The exclusive OR circuit 11a calculates the exclusive OR of the NRZ code signal A1 of the data signal and the 1-bit delay signal obtained by delaying the NRZI code signal A3 by 1-bit by the 1-bit delay circuit 11b. The exclusive OR circuit 11a outputs a “0” code when the values of the NRZ code signal A1 and the 1-bit delay signal are the same, and outputs a “1” code when they are different.
[0034]
The exclusive OR circuit 11a calculates the exclusive OR of the initial value “0” code and the NRZ code signal A1 when the first NRZI code signal A3 is not yet output. As shown in FIG. 2B, the second precoder circuit 5 that converts the inverted NRZ code signal A2 into the inverted NRZI code signal A4 is arranged between the exclusive OR circuit 11a and the branch point of the 1-bit delay line 11b. In this configuration, a negative circuit 11c is added. In the second precoder circuit 5, the exclusive OR circuit 11b calculates an exclusive OR between the inverted NRZ code signal A2 and the 1-bit delayed signal obtained by delaying the inverted NRZI code signal A4 by 1 bit, and the negation circuit. 11c inverts the operation result and converts it into an inverted NRZI code signal A4.
[0035]
For example, as shown in FIG. 3A, when the NRZ code signal A1 is a bit string “010011101100”, as shown in FIG. 3B, the first precoder circuit 4 uses the NRZI code of the “011101001000” bit string. Converted to signal A3. The inverted NRZ code signal A2 is a “101100010011” bit string as shown in FIG. 3C, and the second precoder circuit 5 inverts the “100010110111” bit string as shown in FIG. It is converted into the NRZI code signal A4. Here, the time axis of FIGS. 3A to 3D is the time slot T of the reciprocal of the bit rate of the data signal emitted from the signal source 3. _bit Time separated by (t / T _bit ).
[0036]
The light intensity modulator 9 according to the first embodiment of the present invention includes a dielectric having a relatively large electro-optic coefficient, such as lithium niobate (LiNbO). ₃ ) To form an optical waveguide. As shown in FIG. 6, the light wave input unit 13 of the light intensity modulator 9 branches the light wave B <b> 1 input from the semiconductor laser 10 into the first branch optical waveguide 15 and the second branch optical waveguide 16. In the first branch optical waveguide 15, for example, when the NRZI modulation signal A5 of the bit string shown in FIG. 3B is applied to the first phase modulation unit 17, the first branch light wave B3 is caused by the electro-optic effect. As shown in FIG. 4A, the phase-modulated first modulated light wave B4 is obtained.
[0037]
Here, the NRZI modulation signal A5 is 0V when the code is [0], and when the code is “1”, the voltage at which the phase of the light wave is shifted by a half wavelength, that is, a half-wave voltage, V _π The output level of the driver circuit 7 is set so that Also in the second branching waveguide 16, when the inverted NRZI modulation signal A6 of the bit string shown in FIG. 3 (d) is applied to the second phase modulation unit 18, the second branching light wave B5 is shown in FIG. 4 (c). As shown in FIG. 5, the second modulated light wave B6 is subjected to phase modulation. Here, the inverted NRZI modulation signal A6 is 0V when the [1] code is used, and is half-wave voltage −V when the “0” code is used. _π The output level of the driver circuit 8 is set so that
[0038]
The first and second modulated light waves B4 and B6 advance in phase in the rising section of the phase pulse waveform shown in FIGS. 4A and 4C, respectively, and delay in phase in the falling section. Therefore, the optical frequency f ₀ Are changed in pulses by Δf and −Δf, respectively, in the rising and falling intervals of the pulse waveform. As shown in FIGS. 4B and 4D, a frequency-modulated signal of a differential waveform pulse having a pulse width corresponding to the rising width and falling time width of the pulse waveform and having opposite polarities is obtained.
[0039]
The first and second modulated light waves B4 and B6 are combined in the optical combining unit 14 to become combined light B7 having the phase and optical frequency as shown in FIGS. 5 (a) and 5 (b). Here, the second branching waveguide 16 is provided with a phase adjusting unit 19, and the DC bias A7 causes the phase difference between the first modulated light wave B4 and the second modulated light wave B6 to be a half wavelength of the light wave, Adjust to π.
[0040]
When the phase difference is adjusted in this way, as shown in FIG. 5C, the combined light B7 is the optical frequency f of the first and second modulated light waves B4 and B6. ₀ When the light is constant, the light is extinguished and the optical frequency is f ₀ + Δf and f ₀ When the pulse changes to -Δf, the light intensity becomes maximum. In this way, the bit string of the NRZ code signal A1 is restored to the RZ code by optical intensity modulation, and the output signal light B2 having a higher extinction ratio is output from the optical intensity modulator 9.
According to the optical transmission device 1 according to the first embodiment of the present invention, the single optical intensity modulator 9 can be used to generate the output signal light B2 of the RZ code from the NRZ code signal A1, Since the number of modulators is smaller than the method using two conventional modulators, the insertion light loss due to the optical modulator can be reduced. Further, since there is no difference in optical path length between the first and second branch optical waveguides 15 and 16 of the light intensity modulator 9, noise at the mark level can be reduced and a good signal-to-noise ratio can be obtained. Furthermore, output signal light having a high extinction ratio can be output by appropriately setting the output levels of the DC bias A7, the NRZI modulation signal A5, and the inverted NRZI modulation signal A6.
[0041]
In addition, since the branching waveguide in the light intensity modulator 9 is a Y-shaped branching path, the branching ratio of the light wave to the two-branching waveguides 15 and 16 compared to the 3 dB coupler provided in the Mach-Zehnder interferometer. Can be higher than the Mach-Zehnder interferometer. In FIG. 32, an extinction ratio of 20 dB or more can be obtained, and an extinction ratio of 10 dB or more is obtained as an RZ optical signal.
[0042]
Further, since the output signal light B2 of the RZ code is generated in a section where the codes of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 change, the band limitation is performed on the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 by a low pass filter. The rising / falling times of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 may be changed. In this case, since the band limitation by the low-pass filter can be shortened to about half of the bit rate, the band of the modulator driving circuit unit such as a driver circuit can be halved, and the cost of the optical transmission unit can be reduced.
[0043]
Next, the structure and mounting form of the light intensity modulator 9 according to the first embodiment of the present invention will be described with reference to FIGS. The light intensity modulator 9 includes first and second phase modulations on a modulator substrate 21 having an optical waveguide composed of a light wave input unit 13, an optical multiplexing unit 14, and first and second branch optical waveguides 15 and 16. The electrodes 17 and 18 and the electrodes constituting the phase adjusting unit 19 are arranged. In the first embodiment of the present invention, lithium niobate is used as the modulator substrate 21, and titanium (Ti) diffusion layers formed by selective thermal diffusion technology are used as the first and second optical waveguides 15 and 16. Due to titanium diffusion, the refractive index n of the lithium niobate layer is Δn = 2 to 5 × 10 6. ^-3 As a result, an optical waveguide with a low loss can be formed.
[0044]
For example, as shown in FIG. 7, the first and second optical waveguides 15 and 16 extend left and right with respect to the paper surface, and the first and second phase modulation electrodes 22 and 24, the first and second phase modulation electrodes Each of the ground electrodes 23 and 25 and the common ground electrode 26 is disposed substantially parallel to each other, is in contact with the upper end of the modulator substrate 21, and extends in a direction perpendicular to the first and second branch optical waveguides 15 and 16. And a portion bent halfway to the left of the modulator substrate 21 in the middle and extending in parallel with the first and second branch optical waveguides 15 and 16, and further bent downward of the modulator substrate 21. The first and second branch optical waveguides 15 and 16 extend in a direction perpendicular to the first and second branch optical waveguides 15 and 16 and have a crank shape including a portion in contact with the lower end of the modulator substrate 21. Further, the portions extending in parallel with the first and second branch optical waveguides 15 and 16 are first the first ground electrode 23 and the first branch optical waveguide 15 from the upper end to the lower end of the modulator substrate 21. The first phase modulation electrode 22, the common ground electrode 26, the second phase modulation electrode 24, and the second ground electrode 25 are arranged in this order across the second branch optical waveguide 16.
[0045]
Here, the first phase modulation unit 17 includes a portion in which the first phase modulation electrode 22 and the first ground electrode 23 are arranged in parallel with a part of the first branch optical waveguide 15 interposed therebetween. . Similarly, the second phase modulation unit 18 includes a portion in which the second phase modulation electrode 24 and the second ground electrode 25 are arranged in parallel with a part of the second branch optical waveguide 16 interposed therebetween. .
[0046]
FIG. 8 is an AA cross-sectional structure of the first and second phase modulation units 17 and 18 cut out in a step shape shown in FIG. The first and second phase modulators 17 and 18 are opposed first phases provided on the surface of the modulator substrate 21 with the first and second branch optical waveguides 15 and 16 made of a titanium diffusion layer in between. The modulation electrode 22 and the first ground electrode 23, and the second phase modulation electrode 24 and the second ground electrode 25, respectively. The common ground electrode 26 is disposed between the first and second phase modulation units 17 and 18. In the first embodiment of the present invention, the first and second phase modulation electrodes 22, 24, the first and second ground electrodes 23, 25, and the common ground electrode 26 are each configured in a crank shape. However, as long as the first and second phase modulators 17 and 18 are arranged in parallel with a part of the first and second branch optical waveguides 15 and 16 in between, in any shape Of course, it does not matter.
[0047]
In the first embodiment of the present invention, the lengths of the first and second branch optical waveguides 15 and 16 are equal within the range of manufacturing error, and the first and second phase modulation units 17 and 18 are used. Similarly, the lengths are made equal within the range of manufacturing errors. As shown in FIG. 7, the optical path difference between the first and second branched optical waveguides 15 and 16 until the first and second branched light waves B3 and B5 reach the first and second phase modulators 17 and 18. The first and second phase modulation electrodes 22 and 24 until the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 reach the first and second phase modulation units 17 and 18. Line length difference and transmission line electrical length difference L2.
[0048]
If the light wave B1 is introduced from the left end with respect to the paper surface of FIG. 7 and the electric signal is introduced from the upper end, the time for the second branched light wave B5 to arrive at the second phase modulator 18 is the first branched light wave B3. The time when the NRZI modulation signal A5 arrives at the first phase modulation section 17 is earlier than the time when the first NRZI modulation signal A6 arrives at the first phase modulation section 17, and the time when the NRZI modulation signal A6 arrives at the first phase modulation section 17 It becomes earlier by the transmission line electrical length difference L2 with respect to the time arriving at the second phase modulation unit 18. The speed of the light wave propagating through the dielectric having a refractive index of n is expressed as c / n. Here, c is the speed of the light wave in vacuum.
[0049]
Further, the first phase modulation electrode 22 and the first ground electrode 23, and the second phase modulation electrode 24 and the second ground electrode 25 form a coplanar strip transmission line as shown in FIG. In the case of a coplanar strip transmission line, the propagation speed of the microwave signal is c / ((1 + n ² ) / 2) ^1/2 Is approximated by In the case of lithium niobate, n = 2.15 at a wavelength of 1.55 μm, while n˜4.2 for microwaves. Therefore, if the ratio between the optical waveguide length difference L1 and the transmission line electrical length difference L2 is set to ˜2, the effective line length can be set to be the same.
[0050]
The DC bias electrode 27 is arranged in a U shape from the upper end of the modulator substrate 21 so as to straddle the first and second branch optical waveguides 15, 16, and the DC bias ground electrode 28 is first from the upper end of the modulator substrate 21. It is arranged across only the branch optical waveguide 15. The phase adjustment unit 19 includes a DC bias electrode 27 and a DC bias ground electrode 28 arranged in parallel with a part of the second branch optical waveguide 16 interposed therebetween.
[0051]
In the first embodiment of the present invention, the phase adjustment unit 19 provided in a part of the second branch optical waveguide 16 is disposed across the first optical waveguide 15. Of course, the arrangement of can be selected as appropriate. For example, the DC bias electrode 27 is arranged in an inverted U shape from the lower end of the modulator substrate 21 so as not to straddle both the first and second optical waveguides 15 and 16, and straddles the first optical waveguide 15. You may make it oppose with the direct current | flow bias ground electrode 28 arrange | positioned. Alternatively, the DC bias electrode 27 may be disposed in an inverted U shape across the second branch optical waveguide 16 from the lower end of the modulator substrate 21, and the DC bias ground electrode 28 may be disposed at the lower end of the modulator substrate 21. .
[0052]
As shown in FIG. 9, the modulator substrate 21 having the light intensity modulator 9 is mounted in contact with the mounting substrate 31 on which the driver circuits 7 and 8 are arranged. The first phase modulation electrode 22 and the first ground electrode 23 of the light intensity modulator 9 are connected to the driver circuit 7 by bonding wires 38c and 38d at the upper end of the modulator substrate 21 shown in FIG. Similarly, the modulation electrode 24 and the second ground electrode 25 are connected to the driver circuit 8 by bonding wires 38b and 38a. The common ground electrode 26 is connected to the high frequency ground wiring 30 on the mounting substrate 31 by a bonding wire 38e. Then, the mounting substrate 31 is attached to the package 39. The light wave input unit 13 and the optical multiplexing unit 14 of the light intensity modulator 9 are each connected to an optical fiber. The driver circuits 7 and 8 are connected to the high frequency connectors 32 and 33 by wiring, and are connected to the power supply connector 35, respectively.
[0053]
The high frequency connectors 32 and 33 are connected to the first and second precoder circuits 4 and 5 outside the package 39 by a coaxial cable. The power connector 35 is connected to a DC power supply and supplies a power supply voltage to the driver circuits 7 and 8. The DC bias electrode 27 is connected to a DC bias wiring 40 wired to the DC bias power supply connector 36 by a bonding wire 38f. The DC bias power supply connector 36 is connected to the DC bias power supply 6 outside the package 39.
[0054]
The DC bias ground electrode 28 is connected to the DC bias ground wiring 60 on the mounting substrate 31 by a bonding wire 38g. The first and second phase modulation electrodes 22 and 24 of the light intensity modulator 9 are provided outside the package 39 through the high frequency connector 34 at the lower end of the modulator substrate 21 shown in FIG. 9, and the other end is grounded. Connected to the terminator 37.
[0055]
In the first embodiment of the present invention, the outputs of the driver circuits 7 and 8 are connected to the half-wave voltage of the light intensity modulator 9, V _π The extinction ratio of the output signal light B2 of the light intensity modulator 9 becomes the highest when it is increased to _π There is no need to increase to V _π There may be a deviation of about 15%.
[0056]
The pulse width of the output signal light B2 of the RZ code output from the light intensity modulator 9 is the rising and falling intervals of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 that drive the light intensity modulator 9. It depends on the time span. Accordingly, waveform shaping is performed on the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 using a waveform shaping filter such as a low pass filter, and the rising and falling intervals of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 By adjusting the time width, it is possible to generate the output signal light B2 having an RZ code having a desired pulse width. When performing such waveform shaping, the waveform shaping filter may be inserted at the output stage of the first and second precoder circuits 4 and 5 or the output stage of the driver circuits 7 and 8. Good.
[0057]
Further, since the output signal light B2 of the RZ code is generated in the section where the codes of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 change, the band limitation is performed on the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 by a low pass filter. The rising / falling times of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 may be changed.
[0058]
In this case, since the band limitation by the low-pass filter can be shortened to about half of the bit rate, the band of the modulator driving circuit unit such as the driver circuit can be halved, and the rising of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6. -Since the fall time increases, an RZ optical signal can be generated even if a time lag occurs between two phase-modulated optical signals generated by the NRZI modulated signal A5 and the inverted NRZI modulated signal A6 generated in the optical intensity modulator. is there. In this case, the allowable amount of time shift is one time slot, and in terms of the line length, it is about 3 cm for a 10 Gb / s signal.
[0059]
Further, in the situation where the rise / fall time of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 is increased, the time differential value of the modulation signal in the rise / fall interval is small, so that even if there is a variation in line length error. There is a feature that the pulse width of the output signal light B2 of the RZ code can be kept substantially constant.
[0060]
As shown in FIGS. 7 to 9, the electrodes of the first and second phase modulation units 17 and 18 and the phase adjustment unit 19 are directly formed on the surface of the modulator substrate 21. However, since the resistance of the lithium niobate crystal is not sufficiently high, a voltage drop of the applied DC bias A7 occurs, causing long-term drift of the light intensity modulator 9. Therefore, as shown in FIG. 10, a dielectric film 29 such as a silicon oxide film having a high insulating property is provided on the surface of the modulator substrate 21 on which the first and second branch optical waveguides 15 and 16 are arranged. On the dielectric film 29, the first phase modulation electrode 22 and the first ground electrode 23 constituting the first phase modulation unit 17, and the second phase modulation electrode 24 constituting the second phase modulation unit 18 are formed. And the second ground electrode 25 may be disposed. In addition, the electro-optic effect of the lithium niobate crystal is anisotropic, and the arrangement of the electrodes of the phase modulation unit varies depending on the plane orientation of the modulator substrate 21 to be used.
[0061]
In the light intensity modulator 9 shown in FIGS. 7 to 10, the first and second phase modulation electrodes 22 and 24 of the first and second phase modulation units 17 and 18 are the first and second optical waveguides 15 and 16. The modulator substrate 21 selects the optical axis direction with the maximum electro-optic coefficient in the left-right direction with respect to the paper surface. When the optical axis direction of the modulator substrate 21 is selected in the vertical direction with respect to the paper surface, as shown in FIG. 11, the first and second phase modulation electrodes 22 of the first and second phase modulation units 17 and 18, Reference numeral 24 denotes a structure that is disposed directly above the first and second optical waveguides 15 and 16, respectively. In this case, a dielectric film 29 such as a silicon oxide film is required in order to prevent the phenomenon that the light wave is absorbed by the first or second phase modulation electrode 22 or 24 metal.
[0062]
In FIG. 11, the first ground electrode 23, the second ground electrode 25, and the common ground electrode 26 are the dielectric films on the surface of the modulator substrate 21, similarly to the first and second phase modulation electrodes 22 and 24. However, as shown in FIG. 12, a common ground electrode 26 may be arranged on the back surface of the modulator substrate 21 to constitute a microstrip transmission line. In FIG. 12, a dielectric film 29 is provided on the surface of the modulator substrate 21 having the first and second optical waveguides 15 and 16, and the first and second phase modulation electrodes 22 and 24 are formed on the dielectric film 29. Is disposed immediately above the first and second optical waveguides 15 and 16, and the common ground electrode 26 is disposed on the rear surface of the modulator substrate 21. In such a microstrip transmission line, the first and second ground electrodes can be omitted.
[0063]
Further, as shown in FIG. 13, in the modulator substrate 21, after the first and second optical waveguides 15a and 16a are formed by titanium diffusion, both sides of the first and second optical waveguides 15a and 16a are etched. Therefore, a ridge structure may be used. With the ridge structure, the electric field density under the first and second phase modulation electrodes 22 and 24 of the first and second phase modulation units 17 and 18 is increased, and the modulation efficiency is improved. In FIG. 13, the first and second ground electrodes 23 and 25 and the common ground electrode 26 are provided on the dielectric film 29 on the same surface of the modulation substrate 21 as the first and second phase modulation electrodes 22 and 24. However, it goes without saying that a microstrip type transmission line structure in which the common ground electrode 26 as shown in FIG.
[0064]
(First modification)
In the optical transmission apparatus according to the first modification of the first embodiment of the present invention, as shown in FIG. 14, an optical filter 65 is provided between the optical intensity modulator 9 and the transmission line optical fiber, It differs from the first embodiment described above in that single sideband (SSB) output signal light B8 is output. Others are the same as those in the first embodiment, and thus redundant description is omitted.
[0065]
As shown in FIG. 14, the optical transmission device 1a according to the first modification of the first embodiment of the present invention outputs an optical signal to the output of the optical intensity modulator 9 of the optical transmission device 1 shown in FIG. The optical filter 65 is inserted through the fiber. The optical filter 65 is a bandpass filter as shown in FIG.
[0066]
Since the output signal light B2 of the optical intensity modulator 9 is a combination of optical signals whose phases are inverted, the optical frequency spectrum of the output signal light B2 is as shown in FIG. 5B. Optical frequency f of 10 light waves ₀ Is a double sideband (DSB) spectrum that is spread positively and negatively by the signal component Δf. Even if one sideband component is extracted by the optical filter 65 from the output signal light B2 component of both sidebands and is converted into a single sideband (SSB), the signal information is not impaired. Therefore, it is possible to reduce the occupied band of the optical frequency along with the generation of the output signal light B2 of the RZ code. By saving the bandwidth as described above, the number of channels can be increased, and high-density wavelength division multiplexing transmission is possible.
[0067]
The peak of the transmission characteristic of the optical filter 65 is the center optical frequency f of the light wave B1 of the semiconductor laser 10, as shown in FIGS. ₀ It is shifted to the lower frequency side. Therefore, the optical frequency spectrum of the output signal light B2 has an optical frequency f as shown in FIG. ₀ More negative (f ₀ -Δf), the signal spectral component, ie, the lower sideband, is extracted and the positive side (f ₀ The signal spectral component of + Δf), ie the upper sideband, is removed. As a result, as shown in FIG. 15D, the SSB output signal light B8 having the signal light intensity of the RZ code that has been converted into the single sideband is output to the transmission line optical fiber. Since the SSB output signal light B8 can convert phase-modulated light into light-intensity-modulated light by the transmission characteristics of the optical filter 65, if the steepness of the transmission characteristics of the optical filter 65 is increased, the optical filter 65 is not used. Thus, the extinction ratio of the SSB output signal light B8 can be increased.
[0068]
In the first modification, the lower sideband is used. However, the peak of the transmission characteristic of the optical filter 65 is the center optical frequency f of the light wave B1 of the semiconductor laser 10. ₀ Of course, the upper sideband may be shifted to a higher frequency side. In addition, a band pass filter is used as the optical filter 65. For example, a high pass filter can be used when the lower sideband is used, and a low pass filter can be used when the upper sideband is used. Of course. Further, as shown in FIG. 14 (a), an optical filter is inserted into two output units of the phase modulation unit in the optical intensity modulator, and the optical filter is given a bandpass characteristic that cuts out one sideband. SSB signal output can be obtained.
[0069]
According to the optical transmission device 1a according to the first modification of the first embodiment of the present invention, the optical filter 65 is inserted into the output of the optical intensity modulator 9, and the output signal light B2 is transmitted as a single sideband. Thus, frequency multiplex transmission with high band utilization efficiency becomes possible.
[0070]
(Second modification)
As shown in FIG. 16, the second modification of the first embodiment of the present invention is provided with a plurality of optical transmission apparatuses 1 (see FIG. 1) according to the first embodiment of the present invention. The output signal lights B2a to B2c are input to a plurality of channels of input ports P. _i ~ P _{(I + 2)} The wavelength multiplexing is performed by the optical multiplexer 62 having Others are the same as those in the first embodiment, and thus redundant description is omitted. When transmitting the double sideband components of the output signal light B2 of the optical transmitter 1 according to the first embodiment, the transmission ports corresponding to the double sideband components are transmitted to the input ports of the respective channels of the optical multiplexer 62. Bandwidth is required.
[0071]
In order to increase the number of channels by narrowing the channel wavelength interval of the output signal light B2, the single sideband of the output signal B2 of the RZ code may be achieved as in the first modification. The optical multiplexer 62 uses a plurality of thin film filters or optical waveguides having different transmission characteristics, and has the same bandpass filter function as the optical filter 65.
[0072]
As shown in FIG. 16, the plurality of NRZ code signals A1a to A1c and the plurality of inverted NRZ code signals A2a to A2c output from the signal sources 3a to 3c of the i to (i + 2) channels are the first and second precoder circuits. 4 and 5 are converted into NRZI code signals A3a to A3c and inverted NRZI code signals A4a to A4c, the signal levels are amplified by driver circuits 7 and 8, and NRZI modulation signals A5a to A5c and inverted NRZI modulation signals A6a to A6c are obtained. It is output to each light intensity modulator 9a-9c. The semiconductor lasers 10a to 10c emit optical frequencies f1 of the oscillating light waves B1a to B1c. ₀ Is each input port P _i ~ P _{(I + 2)} The semiconductor lasers 10a to 10c are controlled in accordance with the injection current or the operating temperature so as to be different from each other corresponding to the transmission characteristics. The light intensity modulators 9 a to 9 c respectively output the output signal lights B <b> 2 a to B <b> 2 c in both sidebands to the i to (i + 2) channel input ports P of the optical multiplexer 62. _i ~ P _{(I + 2)} Output to. The optical multiplexer 62 converts the output signal lights B2a to B2c in both side bands into single sidebands, multiplexes them, and outputs the multiplexed output signal light B9 to the transmission optical fiber.
[0073]
For example, as shown in FIG. 17A, the input port P of each channel of the optical multiplexer / demultiplexer 62 _i ~ P _{(I + 2)} And the center optical frequency f of the output signal light B2a to B2c. _{0, i} ~ F _{0, (i + 2)} It is out of place. Therefore, as shown in FIG. 15, the output signal lights B2a to B2c of the RZ code are converted into single sidebands. By using the optical multiplexer 62 having a high optical frequency resolution, it is possible to increase the extinction ratio of the output signal lights B2a to B2c from the optical intensity modulators 9a to 9c and perform wavelength multiplexing.
[0074]
According to the second modification of the first embodiment of the present invention, the output signal light B2 of the plurality of optical transmission devices 1 is converted into a single sideband by the optical multiplexer 62, so that the band use efficiency is high. Frequency multiplex transmission is possible.
[0075]
(Third Modification)
In the third modification of the first embodiment of the present invention, the light wave of the semiconductor laser is branched into orthogonal polarization components by a polarization beam splitter, and the RZ code signal light is generated by the light intensity modulator 9 for each branched polarization. Then, the output signal light B2 of the adjacent channel whose polarization directions are orthogonal is multiplexed by the polarization beam splitter. Others are the same as those in the first embodiment, and thus redundant description is omitted.
[0076]
As shown in FIG. 18, light waves B1 of the semiconductor lasers 10d, 10e,. _i , B1 _{(I + 1)} ,... Are branched into orthogonal polarization components by the first polarization beam splitter 63, and the first polarization light wave D1. _i , D1 _{(I + 1)} , ... and the second polarized light wave D2 _i , D2 _{(I + 1)} ,... Are input to the light intensity modulator 9, respectively. First polarized light wave D1 _i Is modulated by the NRZI code modulation signal A5d and the inverted NRZI code modulation signal A6d, and the first modulated polarized light wave M1 _i Is input to the second polarization beam splitter 64, and the second polarization light wave D2 _i Is modulated by the NRZI code modulation signal A5e and the inverted NRZI code modulation signal A6e, and the second modulated polarized light wave M2 _i Is input to the second polarization beam splitter 64.
[0077]
Similarly, the first polarized light wave D1 _{(I + 1)} Is modulated by the NRZI code modulation signal A5f and the inverted NRZI code modulation signal A6f, and the first modulated polarized light wave M1 _{(I + 1)} Is input to the second polarization beam splitter 64, and the second polarization light wave D2 _{(I + 1)} Is modulated by the NRZI code modulation signal A5g and the inverted NRZI code modulation signal A6g, and the second modulated polarized light wave M2 _{(I + 1)} Is input to the second polarization beam splitter 64. Further, the first polarized light wave D1 _{(I + 2)} Is modulated by the NRZI code modulation signal A5h and the inverted NRZI code modulation signal A6h, and the first modulated polarized light wave M1 _{(I + 2)} Is input to the second polarization beam splitter 64.
[0078]
In this way, the second polarization beam splitter 64 has the first modulated polarized light wave M1 of each adjacent channel. _i And the second modulated polarized light wave M2 _(I-1) , First modulated polarized light wave M1 _{(I + 1)} And the second polarized light wave M2 _i , First modulated polarized light wave M1 _{(I + 2)} And the second polarized light wave M2 _{(I + 1)} ,... Are combined and output signal light B2 of the RZ code _i , B2 _{(I + 1)} , B2 _{(I + 2)} , ... are output. The optical multiplexer 62 outputs the output signal light B2 in both sidebands. _i , B2 _{(I + 1)} , B2 _{(I + 2)} ,... Are converted into single sidebands and further multiplexed to output multiplexed output signal light B9a to the transmission optical fiber.
[0079]
In the third modification of the first embodiment of the present invention, as shown in FIG. 17A, the input port P of each channel of the optical multiplexer / demultiplexer 62 is used. _i ~ P _{(I + 2)} Maximum transmitted light frequency and output signal light B2 _i ~ B2 _{(I + 2)} Center optical frequency f _{0, i} ~ F _{0, (i + 2)} It is out of place. For example, input port P _{(I + 1)} Output signal light B2 input to the _{(I + 1)} As shown in FIG. 17B, the first polarized light wave D1 _{(I + 1)} The center optical frequency of the component is f _{0, (i + 1)} And the second polarized light wave D2 _i The center optical frequency of the component is f _{0, i} Are combined. Input port P _{(I + 1)} As shown in FIG. 17 (c), the first polarized light wave D1 is obtained by the transmission characteristics of _{(I + 1)} The component is converted into a lower sideband, and the second polarized light wave D2 _i The component is upper sidebanded.
[0080]
Thus, in the adjacent channel, the center optical frequency f ₀ Are combined and the output signal light having polarization directions orthogonal to each other is multiplexed and single-sidebanded at each input port and frequency-multiplexed. By performing such multiplexing, frequency multiplex transmission with high band utilization efficiency becomes possible.
[0081]
(Fourth modification)
In the configuration of FIG. 1, the output signal light B2 can be converted into an inverted RZ optical signal by adjusting the DC bias A7. In this case, the light intensity of the output signal light B2 is minimized during the period in which the signs of the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 change (FIG. 19). In other words, since the intensity of the output signal light B2 is minimized in the section where the time differential value (optical frequency chirp) of the optical phase in the phase modulators 17 and 18 is the largest, the light by the optical frequency chirp of the output signal light B2 Spectral spread can be suppressed.
[0082]
FIG. 20 shows the optical spectrum (both 10 Gb / s signal) when the RZ signal is generated as the output optical signal B2 by adjusting the DC bias A7 and when the inverted RZ signal is generated. In the optical spectrum of the RZ signal, a peak component corresponding to the bit rate is generated in both sidebands at a frequency separated by ± 10 GHz around the optical carrier component, but such a peak component is not generated in the inverted RZ signal, The spread of the optical spectrum is suppressed. Therefore, when the inverted RZ signal is generated, the occupied optical band can be shortened, so it is suitable for high-density wavelength multiplexing transmission with a narrow wavelength interval, such as an optical multiplexer / demultiplexer or optical filter with a narrow transmission bandwidth. When used together, it is possible to reduce the occupied band as much as the SSB signal as described in the first modification, and to further increase the density of wavelength multiplexing.
[0083]
In order to receive such an inverted RZ signal, it is necessary to further invert the code on the receiving side and decode it into an RZ signal. Such decoding may be performed by providing an inverting unit in the optical receiver, More specifically, an amplifier or a negation circuit whose logic code is inverted and output may be provided in the optical receiver.
[0084]
(Second Embodiment)
As shown in FIG. 21, the optical transmission apparatus 1a according to the second embodiment of the present invention includes a code conversion module 8 that converts an NRZ code signal A1 of transmission data emitted from a signal source 3 into an NRZI code signal A3. The configuration of the code conversion module 82 including the precoder circuit 4 and the differential amplification driver circuit 50 that amplifies the NRZI code signal A3 and outputs the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 is the first embodiment. This is different from the related optical transmitter 1. Since others are the same as those of the first embodiment of the present invention, redundant description is omitted.
[0085]
Since the differential amplifier driver circuit 50 used in the second embodiment of the present invention includes the inverting output terminal 51 that inverts the logic of the sign, only one precoder circuit 4 may be used. The number of parts can be reduced. In the first embodiment of the present invention, as shown in FIG. 1, the signal transmission system is branched into two from the signal source 3 and passes through the precoder circuits 4 and 5 and the driver circuits 7 and 8. Are connected to the first and second phase modulators 17 and 18 of the light intensity modulator 9, respectively.
[0086]
In order to make the light intensity modulator 9 function effectively and obtain the output signal light B2 having a sufficient extinction ratio, the NRZI modulated signal A5 and the time-aligned NRZI modulation signals A5 and It is desirable to derive the inverted NRZI modulation signal A6. For this purpose, it is necessary that the total transmission line length difference of the branched signal transmission system be aligned with an accuracy of about 1/10 with respect to the wavelength of the transmission signal.
[0087]
For example, when a 40 Gb / s NRZ code signal A1 is used, the overall transmission line length difference needs to be 1 mm or less. When the time differential value of the modulation signal at the rise and fall is reduced by adding a band limitation to the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 by inserting a low-pass filter in the modulator driving circuit. When the NRZ code signal A1 of 40 Gb / s is used, the overall transmission line length difference is reduced to 7.5 mm. Although a phase shifter or the like capable of adjusting the time delay due to the transmission line length difference may be inserted into the transmission line, there is a problem in operation stability and reliability.
[0088]
In the second embodiment of the present invention, since the differential amplifier driver circuit 50 having the inverting output terminal 51 is used, the signal source 3 is connected to the differential amplifier driver circuit via the precoder circuit 4. There is one transmission line, and the setting of the transmission line length can be simplified.
The optical transmitter according to the second embodiment of the present invention can generate RZ code output signal light from an NRZ code signal with a simple configuration using a single optical intensity modulator. Insertion loss can be reduced. Further, since no optical path length difference is provided in the branch optical waveguide of the light intensity modulator, noise at the mark level can be reduced, and a good signal-to-noise ratio can be obtained. Furthermore, output signal light having a high extinction ratio can be output by appropriately setting the DC bias and the output levels of the light intensity modulator NRZI modulation signal and the inverted NRZI modulation signal.
[0089]
The light intensity modulator 9 used in the second embodiment of the present invention is the same as that used in the first embodiment, as shown in FIG. The first and second phase modulation electrodes 22, 24, the first and second ground electrodes 23, 25, common so that the effective line lengths to the first and second branch optical waveguides 15, 16 are the same. The structure and length of the ground electrode 26 are determined. The modulator substrate 21 having the light intensity modulator 9 is mounted in contact with the mounting substrate 41 on which the differential amplification driver circuit 50 is disposed.
[0090]
The first phase modulation electrode 22 and the first ground electrode 23 of the light intensity modulator 9 are connected to the differential amplifier driver circuit 50 at the upper end of the modulator substrate 21 shown in FIG. The phase modulation electrode 24 and the second ground electrode 25 are connected to the inverting output terminal 51 of the differential amplifier driver circuit 50, and the common ground electrode 26 is connected to the ground wiring of the differential amplifier driver circuit 50 through bonding wires 48. Then, the mounting substrate 41 is attached to the package 49. The light wave input unit 13 and the optical multiplexing unit 14 of the light intensity modulator 9 are each connected to an optical fiber. The differential amplifier driver circuit 50 is connected to the high frequency connector 42 by wiring and is also connected to the power connector 45. The high frequency connector 42 is connected to the precoder circuit 4 outside the package 49 by a coaxial cable. The power connector 35 is connected to a DC power supply and supplies a power supply voltage to the differential amplification driver circuit 50. Further, a phase shift generated between the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 of the differential amplification output in the differential amplification driver circuit 50, the differential amplification driver circuit 50, and the first and second phase modulation units 17 , 18 may be provided with a phase difference adjustment signal input port connected to the high frequency connector 43 in the differential amplifier driver circuit 50 in order to compensate for a phase difference due to a slight error in the line length between 18 and 18.
[0091]
By adopting such a configuration, the length of the bonding wire 48 from the differential amplification driver circuit 50 and the inverting output terminal 51 of the differential amplification driver circuit 50 to the first and second phase adjustment units 17 and 18 can be adjusted. It becomes unnecessary. The DC bias electrode 27 is connected to the DC bias power supply connector 46 and connected to the DC bias power supply 6. The DC bias ground electrode 28 is connected to a DC bias ground wiring on the mounting substrate 31. The first and second phase modulation electrodes 22 and 24 of the light intensity modulator 9 are provided outside the package 49 through the high-frequency connector 44 at the lower end of the modulator substrate 21 shown in FIG. 9, and the other end is grounded. Connected to the terminator 47.
[0092]
(First modification)
As shown in FIG. 23, the first modification of the second embodiment of the present invention includes first and second phase modulation electrodes 22 and 24 arranged on a modulator substrate 21, and first and second phase modulation electrodes. The wiring structure of the ground electrodes 23 and 25 and the form of mounting on the package 49 are different from those of the second embodiment. Others are the same as those of the second embodiment of the present invention, and redundant description is omitted.
[0093]
The first phase adjustment electrode is U-shaped, and both ends thereof are in contact with the upper end of the modulator substrate 21 and are disposed across the first branch optical waveguide 15. The first ground electrode 23 is in contact with the upper end of the modulator substrate 21 and is disposed in parallel with the first phase adjustment electrode 22 and the first branch optical waveguide 15 interposed therebetween. Similarly, the second phase modulation electrode 24 and the second ground electrode 25 are disposed in contact with the lower end of the modulation substrate 21 with the second branch optical waveguide 16 interposed therebetween. Since the first and second phase modulation electrodes 22 and 24 are formed symmetrically with respect to the central axis on the left and right sides of the modulation substrate 21, the optical waveguide length difference L1 and the transmission line electrical length difference L2 are substantially the same. It becomes zero. In the modified example of the second embodiment of the present invention, a U-shaped mounting board 41 is installed so as to surround the light wave input unit 13 of the phase modulation board 21, and ends of wirings 57 and 58 on the mounting board 41. Are arranged so that the ends of the first and second phase modulation electrodes 22 and 24 on the light wave input unit 13 side are in contact with each other, and are connected by a bonding wire 48. The lengths of the wirings 57 and 58 on the mounting substrate 41 are the same and are led to the differential amplifier driver circuit 50.
[0094]
The differential amplification driver circuit 50 mounted on the mounting substrate 41 is connected to the high frequency connector 43 and the DC power supply connector 45 for inputting the phase difference adjustment signal via the wirings 55 and 56, and the NRZI modulation signal A5 and the inverted NRZI. It is directly connected to the high frequency connector 42 for inputting the modulation signal A6. The differential amplifier driver circuit 50 and the wirings 55 to 58 are connected by bonding wires 48. A terminator 47 provided inside the package 49 is connected to the other ends of the first and second phase modulation electrodes 22 and 24, respectively. Thus, by providing the terminator 47 inside the package 49, the high-frequency connector for the terminator 47 can be reduced. These terminators 47 and the first and second ground electrodes 23 and 25 are connected to a high-frequency ground wiring. The DC bias electrode 27 is connected to the DC bias power supply connector 46 and connected to the DC bias power supply 6. The DC bias ground electrode 28 is connected to a DC bias ground wiring in the package 49.
[0095]
According to the first modification of the second embodiment of the present invention, since the differential amplifier driver circuit 50 having the inverting output terminal 51 is used, the differential signal is transmitted from the signal source 3 via the precoder circuit 4. The transmission line connected to the amplification driver circuit is one system, and the transmission line lengths of the wirings 57 and 58 with respect to the first and second phase modulation electrodes 22 and 24 on the mounting substrate 41 are the same. The output signal light B2 having a high extinction ratio can be easily obtained with noise.
[0096]
(Second modification)
A second modification of the second embodiment of the present invention uses a semiconductor crystal as the modulator substrate. As a substrate material having an electro-optic effect, a compound semiconductor crystal such as GaAs can be used in addition to a dielectric crystal such as lithium niobate. In addition, in semiconductors such as InP and GaP, light intensity modulation by the electroabsorption effect is possible, and it can be used as a modulator substrate. Others are the same as those of the second embodiment of the present invention, and redundant description is omitted.
[0097]
As shown in FIG. 24, each electrode shape of the light intensity modulator 9 is the same as that in FIG. For example, when a high-speed semiconductor crystal such as InP is used as the modulator substrate 61, the differential amplification driver circuit 50a can be integrated on the modulator substrate 61. In this case, not only the mounting board is unnecessary, but also the differential amplification driver circuit 50a and the first and second phase modulation electrodes 22, 24, the first and second ground electrodes 23, 25, or the common ground electrode. No wire bonding is required and the connection is made only on the input connector side, so that further cost reduction and stabilization operation can be expected. Further, since the terminator 47 can be built in the modulator substrate 61, the number of high frequency connectors for the terminator can be reduced.
[0098]
As shown in FIG. 25, when a modulator substrate 61 made of a semiconductor crystal such as InP is used, an optical waveguide structure with strong optical confinement can be realized by surrounding the optical waveguide core with a cladding layer having a small refractive index. Therefore, the optical waveguide can be greatly bent by the optical wave input unit 13 and the optical multiplexing unit 14, and the size of the optical intensity modulator circuit can be reduced. Further, the differential amplifier driver circuit 50b can be disposed in a region surrounded by the first and second branch optical waveguides on the light wave input unit 13 side, and the first and second phase modulation electrodes 22 and 24 are provided. In addition, the wiring path to the first and second ground electrodes 23 and 25 can be shortened, and the time difference between the NRZI modulation signal A5 and the inverted NRZI modulation signal A6 can be reduced. The first and second phase modulation electrodes 22, 24 are connected to a terminator 47 formed on the modulator substrate 61. The differential amplifier driver circuit 50b includes a high-frequency connector 43 that inputs a phase difference adjustment signal, a DC power connector 45, and a high-frequency connector that inputs an NRZI modulation signal A5 and an inverted NRZI modulation signal A6 via wirings 55a, 56a, and 59. 42.
[0099]
(Third Modification)
The third modification of the second embodiment of the present invention relates to an implementation in the case of performing light intensity modulation using a signal with a high bit rate, and the others are the second embodiment of the present invention. Since it is the same as the form of, overlapping description is omitted.
[0100]
When light intensity modulation is performed using a signal with a high bit rate, the size of the light intensity modulator 9 is reduced, and accordingly the modulator substrate 21 is inevitably thinner. When the thin film modulator substrate 21 is used, the structure of the microstrip transmission line shown in FIG. 12 can increase the electric field strength of the applied signal to further increase the phase modulation efficiency of the light wave. As shown in FIG. 26A, a modulator having a common ground electrode 26 on the back surface and first and second phase modulation electrodes 22, 24 disposed on the dielectric film 29 on the front surface on the mounting substrate 41. A substrate 21 and a differential amplification driver circuit 50 are mounted.
[0101]
In such a case, since the thickness of the modulator substrate 21 is thinner than the thickness of the differential amplification driver circuit 50, the bonding pad 52 of the differential amplification driver circuit 50 and the first and second phase modulation electrodes 22, The length of the bonding wire 48 used for 24 wiring becomes long, and there exists a problem that flattening of a high frequency characteristic becomes difficult. Therefore, as shown in FIG. 26B, a spacer 53 made of a dielectric substrate such as a silicon oxide film is inserted between the modulator substrate 21 and the mounting substrate 41, so that the first and second phase modulations are performed. The heights of the electrodes 22 and 24 and the bonding pad 52 of the differential amplifier driver circuit 50 should be substantially the same. With such a height setting, the length of the bonding wire 48 can be shortened, and the high-frequency characteristics of transmission from the differential amplification driver circuit 50 to the first and second phase modulation electrodes 22 and 24 can be flattened. it can. Therefore, light intensity modulation using a signal with a higher bit rate is possible.
[0102]
(Fourth modification)
In the fourth modification of the second embodiment of the present invention, the output signal light shown in the first to third modifications of the first embodiment is converted into a single sideband, and the band use efficiency is high. The present invention relates to an optical transmission apparatus capable of frequency multiplexing transmission. That is, the optical transmission device 1 shown in FIGS. 14, 16 and 18 or a portion corresponding to the optical transmission device 1 is disclosed in the second embodiment of the present invention or the first and second modifications. The various types of optical transmission devices 1b can be replaced and used.
According to the fourth modification of the second embodiment of the present invention, since the differential amplification driver circuit 50 having the inverting output terminal 51 is used for amplification of the signal for performing the light intensity modulation, the transmission line length is increased. The setting can be simplified, and further, frequency multiplex transmission with high band utilization efficiency is possible by making the output signal light B2 into a single sideband.
[0103]
(Third embodiment)
The optical transmission apparatus 1c according to the third embodiment of the present invention (a) converts the level of the NRZ code signal A11 when the first logical code of the NRZ code signal A1 is input, When the second logical code is input, the level of the NRZ code signal A11 is not converted, and the NRZI code signal A12 generated from the NRZ code signal A1 and the NRZI code signal A12 are branched by the splitter and output. A code conversion module 82 for amplifying two NRZI code signals A13 and A14 and an NRZI code delay signal A18 generated by giving a delay time Td to the NRZI code signal A14 by the delay unit 70 and outputting an NRZI modulation signal A15; (B) a DC bias power source 6 that supplies a DC bias A17; and (c) a semiconductor laser 1 that outputs a light wave B1. (D) NRZI modulation signal A15 and delayed NRZI modulation signal A19 are respectively added to the first and second optical waveguides 15 and 16 that split the light wave B1 into two at the input unit and combine at the output unit. A light intensity modulator 9d provided with a first and second phase modulators 17 and 18 for applying a voltage, and a phase adjusting unit 19 for applying a DC bias A17 to the first and second optical waveguides 15 and 16; including.
[0104]
In the third embodiment, the delay unit 70 is provided in the one-side output A14 of the splitter 69, the delayed NRZI code delay signal A18 is generated, and the light intensity modulator 9d performs light intensity modulation. And different from the second embodiment. Since others are the same as those of the first and second embodiments of the present invention, redundant description is omitted.
[0105]
As shown in FIG. 27, the signal source 3 is connected to the optical transmission device 1c, and the NRZ code signal A11 of the transmission data emitted from the signal source 3 is converted into the NRZI code signal A12 by the precoder circuit 4 of the code conversion module 82. The Further, the one-side output A14 of the splitter 69 is delayed by a delay time Td by the delay unit 70 to generate a delayed NRZI code signal A18. The NRZI code signal A13 output from the precoder circuit 4 and the delayed NRZI code delay signal A18 from the delay unit are amplified by the driver circuits 7 and 8 of the code conversion module 82 and output as the NRZI modulation signal A15 and the delayed NRZI modulation signal A9. . On the other hand, the light wave B1 is transmitted from the semiconductor laser 10 of the single mode light source to the light intensity modulator 9d.
[0106]
Further, a DC bias A17 is set by the DC bias power supply 6 in the light intensity modulator 9d. In the light intensity modulator 9d, the light wave B1 is light intensity modulated by the NRZI modulation signal A15, the delayed NRZI modulation signal A19, and the DC bias A17, and the output signal light B2 is output to the transmission line optical fiber.
[0107]
The light intensity modulator 9d of the third embodiment of the present invention has the same configuration as that shown in FIG. In the first phase modulation unit 17 of the optical intensity modulator 9d, the first branched light wave B3 is phase-modulated by the NRZI modulation signal A15, and in the second phase modulation unit 18, the second phase is modulated by the delayed NRZI modulation signal A19. The branched light wave B5 is phase-modulated. Here, the DC bias A17 has a maximum optical output when the phase difference between the first and second modulated light waves B4 and B6 modulated by the NRZI modulation signal A15 and the delayed NRZI modulation signal A9 is 0, and when the phase difference is π. A DC bias A17 is applied to the phase adjuster 19 so that the light output is minimized. Since the delayed NRZI modulation signal A19 is delayed by a delay time Td from the NRZI modulation signal A15, the rise and fall of the pulses of the NRZI modulation signal A15 and the delayed NRZI modulation signal A19 are shifted. The phase difference between the first and second modulated light waves B4 and B6 is 0 in the section, and the phase difference is π in other time zones.
[0108]
In this way, output signal light B2 of RZ code is obtained. Since the phase of the multiplexed light B7 of the optical multiplexing unit 14 is inverted between adjacent optical pulses, the resistance to interference between optical pulses is improved.
[0109]
According to the optical transmission device 1c according to the third embodiment of the present invention, the output signal light B2 of the RZ code can be generated from the NRZ code signal A11 with a simple configuration, and the insertion loss can be reduced. it can. Further, since there is no difference in optical path length between the first and second branch optical waveguides 15 and 16 of the light intensity modulator 9d, noise at the mark level can be reduced and a good signal-to-noise ratio can be obtained. Furthermore, the output signal light B2 having a high extinction ratio can be output by appropriately setting the output levels of the DC bias A17, the NRZI modulation signal A15, and the inverted NRZI modulation signal A19. Furthermore, since the phase between adjacent optical pulses is inverted, the tolerance to intersymbol interference can be increased.
[0110]
In the third embodiment of the present invention, for example, as shown in FIG. 28A, when the NRZ code signal A11 output from the signal source 3 is a bit string of “010011101100”, FIG. As shown in the figure, the precoder circuit 4 converts it into an NRZI code signal A13 having a “011101001000” bit string. The NRZI code signal A14, which is one side output of the splitter 69, is delayed by Td by the delay unit to become a delayed NRZI code delay signal A18. The NRZI code signal A13 and the delayed NRZI code delay signal A18 are respectively sent to a predetermined level, for example, a half-wave voltage V _π And is input to the optical intensity modulator 9d as the NRZI modulation signal A15 and the delayed NRZI modulation signal A19.
[0111]
The first branched light wave B3 is modulated by the NRZI modulation signal A15 applied to the first phase modulation unit 17, and becomes the first modulated light wave B4 that is phase-modulated as shown in FIG. Similarly, the second branched light wave B5 is modulated by the delayed NRZI modulation signal A19 applied to the second phase modulation unit 18, and the second modulated light wave B6 phase-modulated as shown in FIG. 28 (d). It becomes. For example, when the phase of the first modulated light wave B4 rises from −π to 0, the second modulated light wave B6 rises from 0 to π with a delay of Td. Further, when the phase of the first modulated light wave B4 falls from 0 to −π, the second modulated light wave B6 falls from π to 0 with a delay of Td. Here, the delay time Td by the delay unit 70 is equal to the time slot T. _bit About 60%.
[0112]
As shown in FIG. 28 (e), the output signal light B2 output by combining the first and second modulated light waves B4 and B6 is the phase pulse of the first and second modulated light waves B4 and B6. The phase pulse changes by π or −π at the time of deviation of the delay Td in the rising section or the falling section. The phase difference between the first and second modulated light waves B4 and B6 is 0 when the phase of the output signal light B2 is π or −π, as shown in FIGS. 28C and 28D. Therefore, as shown in FIG. 28 (f), the output signal light B2 has a maximum light intensity when the phase is π or −π, and is extinguished when the phase is 0. In this way, the output signal light B2 optically modulated to the RZ code is generated corresponding to the NRZ code signal A11.
In the third embodiment of the present invention, the delay time Td is set to the time slot T. _bit However, the pulse shape of the output signal light B2 changes depending on Td. If the NRZI modulation signal A15 is a bit string of “01011010”, the delay time Td is the time slot T _bit If it is smaller, for example, 50%, as shown in FIG. 29A, output signal light B2 of RZ code is obtained. The smaller the delay time Td, the smaller the duty ratio of the pulse of the RZ code. However, when Td becomes smaller than the pulse rise time or fall time (10% -90%) of the NRZI modulation signal A15 or the delayed NRZI modulation signal A19, RZ. The phase pulse of the sign does not change up to ± π, and the output signal light intensity also decreases.
[0113]
The delay time Td is 1 time slot T _bit If they are equal, the light wave pulse of the output signal light B2 becomes a pulse of the NRZ code as shown in FIG. Further, when the delay time Td is larger than one time slot, for example, 150%, as shown in FIG. 29C, when the bit string is “10”, the space ([0] code When the bit string is “111” and the mark bit continuous pulse, the pattern effect that the pulse width of the mark bit sandwiched therebetween becomes smaller is observed. Therefore, in order to faithfully reproduce the bit string of the NRZ code signal A11 emitted from the signal source 3 with the RZ code, the delay time Td in the delay unit 70 is equal to the time slot T _bit Must be set to:
[0114]
In the third embodiment of the present invention, the level of the NRZI modulation signal A15 or the delayed NRZI modulation signal A19 applied to the first and second phase adjustment units 17 and 18 is the half-wave voltage V. _π A large extinction ratio is obtained with the phase modulation amount of the first and second modulated light waves B4 and B6 being π. The relationship between the phase modulation rate with the phase modulation amount normalized by π and the extinction ratio is as shown in FIG. 30, and it can be seen that an extinction ratio of 15 dB or more is obtained when the phase modulation rate is 0.88 or more. Therefore, the level of the NRZI modulation signal A15 or the inverted NRZI modulation signal A19 is 0.88 × V _π If it is above, sufficient extinction ratio is realizable.
[0115]
According to the optical transmission apparatus according to the third embodiment of the present invention, the output signal light B2 of the RZ code can be generated by the single optical intensity modulator 9d, and strict wavelength control is not required. And cost reduction. Further, since the light wave B1 of the semiconductor laser passes only through the light intensity modulator 9d1, only a light loss is small and a good signal-to-noise ratio can be obtained. Further, the extinction ratio of the output signal light B2 of the output RZ code is set so that the phase modulation amounts in the two phase modulation sections are approximately π, whereby an extinction ratio of 15 dB or more is obtained. Also, since the optical path length difference when the light intensity modulator 9d interferes with the two lights is at most the wavelength level of the light, the phase noise of the semiconductor laser is not converted into intensity noise. Furthermore, since the phase is inverted between adjacent output light pulses, the resistance to inter-pulse interference is improved, which is effective against characteristic deterioration due to nonlinear phenomena in the optical fiber.
(Modification)
As shown in FIG. 31, an optical transmission device 1d according to a modification of the third embodiment of the present invention has a waveform shaping filter 71 between the driver circuits 7 and 8 of the code conversion module 82 and the optical intensity modulator 9d. , 72 is different from the third embodiment described above. Others are the same as those in the third embodiment, and a duplicate description is omitted.
[0116]
The NRZI modulation signal A15 and the delayed NRZI modulation signal A19 are input to the light intensity modulator d after the rise and fall times of the pulse waveform are adjusted by the waveform shaping filters 71 and 72. As shown in FIG. 28, the waveform of the output optical pulse greatly depends not only on the delay time Td of the delay unit 70 but also on the rise and fall times of the pulse waveform of the NRZI modulation signal A15 or the delayed NRZI modulation signal A19. . By adjusting the rising and falling times of the pulse waveform of the NRZI modulation signal A15 or the delayed NRZI modulation signal A19 by the waveform shaping filters 71 and 72 and reducing the delay time Td, an output optical pulse having a narrow pulse width can be obtained. .
[0117]
Of course, as described above, the delay time Td is longer than the rise and fall times of the pulse waveform of the NRZI modulation signal A15 or the delay NRZI modulation signal A19. In this way, it is possible to easily output an optical pulse having a shape suitable for the transmission characteristics of the transmission line optical fiber.
In the modification of the third embodiment of the present invention, the waveform shaping filters 71 and 72 are arranged after the driver circuits 7 and 8 of the code conversion module 82. However, in the code conversion module 82, the driver circuits 7 and 8 and It may be inserted between the precoder circuits 4. According to the optical transmission device 1d according to the modification of the third embodiment of the present invention, the NRZ modulation signal A15 and the delayed NRZI modulation signal A19 are subjected to the rise and fall times of the pulse waveform by the waveform shaping filters 71 and 72, respectively. Since the light intensity is adjusted and input to the light intensity modulator d, an optical pulse having a shape suitable for the transmission characteristics of the transmission line optical fiber can be easily output.
[0118]
(Other embodiments)
As described above, the first to third embodiments of the present invention have been described. However, it should not be understood that the description and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art.
[0119]
For example, as the modulator substrate, the first to third embodiments of the present invention, or the modifications and variations thereof, have been described using lithium niobate, but the dielectric film exhibiting various electro-optic effects For example, lithium tantalate (LiTaO ₃ ), Potassium titanyl phosphate (KTiOPO) ₄ ) Etc. can of course be used. Further, GaAs having an electro-optic effect or semiconductor crystals such as InP or GaP having an electro-absorption effect can be used. Further, a polymer such as a dye-doped PMMA, an acrylic polymer, a urethane-urea polymer or a polyurethane exhibiting an electro-optic effect. Of course, can also be used.
[0120]
In addition, the optical transmitters according to the first to third embodiments of the present invention can be reduced in size by being mounted as one module. In particular, a smaller optical transmitter can be realized by monolithically integrating the semiconductor laser and the light intensity modulator on the same semiconductor substrate. In this case, an optical fiber connecting the semiconductor laser and the light intensity modulator is not necessary, and the cost can be reduced. It is also conceivable to monolithically integrate the precoder circuit and driver circuit or differential amplifier driver circuit. Furthermore, a smaller optical transmitter can be realized by hybrid mounting an integrated precoder circuit and driver circuit or differential amplifier driver circuit, and an integrated semiconductor laser and optical intensity modulator. It becomes.
[0121]
As described above, the present invention naturally includes various embodiments not described herein. Therefore, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.
[0122]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the optical transmitter which produces | generates the output signal light of the RZ code | cord | chord with a low noise and a big extinction ratio using a single optical intensity modulator can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an optical transmission apparatus according to a first embodiment of the present invention.
FIG. 2 is a diagram illustrating a configuration of first and second precoder circuits of the optical transmission apparatus according to the first embodiment of the present invention.
FIG. 3 is a diagram illustrating an example of (a) an NRZ code signal, (b) an NRZI code signal, (c) an inverted NRZ code signal, and (d) an inverted NRZI code signal according to the first embodiment of the present invention. It is.
FIGS. 4A and 4B are diagrams showing examples of (a) and (c) phases and (b) and (d) optical frequencies of modulated light waves according to the first embodiment of the present invention.
FIG. 5 is a diagram illustrating an example of (a) phase, (b) optical frequency, and (c) signal light intensity of combined light according to the first embodiment of the present invention.
FIG. 6 is a diagram illustrating a configuration of an optical intensity modulator of the optical transmission device according to the first embodiment of the present invention.
FIG. 7 is a plan view for explaining an example of the configuration of an optical intensity modulator of the optical transmission apparatus according to the first embodiment of the present invention.
FIG. 8 is a cross-sectional view illustrating an example of a configuration of an optical intensity modulator of the optical transmission device according to the first embodiment of the present invention.
FIG. 9 is a configuration diagram illustrating an example of mounting of an optical intensity modulator of the optical transmission apparatus according to the first embodiment of the present invention.
FIG. 10 is a cross-sectional view illustrating another example of the configuration of the optical intensity modulator of the optical transmission device according to the first embodiment of the invention.
FIG. 11 is a cross-sectional view illustrating another example of the configuration of the optical intensity modulator of the optical transmission device according to the first embodiment of the invention.
FIG. 12 is a cross-sectional view illustrating another example of the configuration of the optical intensity modulator of the optical transmission device according to the first embodiment of the invention.
FIG. 13 is a cross-sectional view illustrating another example of the configuration of the optical intensity modulator of the optical transmission device according to the first embodiment of the invention.
FIG. 14 is a diagram illustrating a configuration of an optical transmission apparatus according to a first modification of the first embodiment of the present invention.
FIG. 15 is a diagram for explaining the operation of the optical transmission apparatus according to the first modification of the first embodiment of the present invention;
FIG. 16 is a diagram illustrating a configuration of an optical transmission apparatus according to a second modification of the first embodiment of the present invention.
FIG. 17 is a diagram explaining the operation of the optical transmission apparatus according to the third modification of the first embodiment of the present invention.
FIG. 18 is a diagram illustrating a configuration of an optical transmission apparatus according to a third modification of the first embodiment of the present invention.
FIG. 19 is a diagram for explaining the operation of the optical transmission apparatus according to the fourth modification of the first embodiment of the present invention.
FIG. 20 is a diagram illustrating the characteristics of the optical transmission device according to the fourth modification of the first embodiment of the present invention.
FIG. 21 is a diagram illustrating a configuration of an optical transmission apparatus according to a second embodiment of the present invention.
FIG. 22 is a configuration diagram illustrating an example of mounting of an optical intensity modulator of an optical transmission device according to a second embodiment of the present invention.
FIG. 23 is a configuration diagram illustrating an example of mounting of an optical intensity modulator of an optical transmission apparatus according to a first modification of the second embodiment of the present invention.
FIG. 24 is a configuration diagram illustrating another example of mounting the optical intensity modulator of the optical transmission device according to the second modification of the second embodiment of the present invention.
FIG. 25 is a configuration diagram illustrating another example of mounting the optical intensity modulator of the optical transmission device according to the second modification of the second embodiment of the present invention;
FIG. 26 is a cross-sectional configuration diagram illustrating an example of mounting of an optical intensity modulator of an optical transmission apparatus according to a third modification of the second embodiment of the present invention.
FIG. 27 is a diagram illustrating a configuration of an optical transmission apparatus according to a third embodiment of the present invention.
FIGS. 28A and 28B are (a) an NRZ code signal, (b) an NRZI code signal, (c) a first modulated light wave phase, (d) a second modulated light wave phase according to a third embodiment of the present invention, It is a figure which shows an example of e) output signal light phase and (f) output signal light intensity.
FIG. 29 is a diagram for explaining the relationship between the output signal light intensity pulse shape and the delay time Td according to the third embodiment of the present invention.
FIG. 30 is a diagram for explaining the relationship between the extinction ratio of the output signal light and the phase modulation rate according to the third embodiment of the present invention.
FIG. 31 is a diagram for explaining the configuration of an optical transmission apparatus according to a modification of the third embodiment of the present invention.
FIG. 32 is a diagram showing the relationship between the extinction ratio of an optical signal and the extinction ratio of a Mach-Zehnder interferometer.
[Explanation of symbols]
1, 1a to 1d ... optical transmitter,
3, 3a-3c ... signal source,
4 ... First precoder circuit,
5 ... Second precoder circuit,
4a, 4b ... Precoder circuit,
6 ... DC bias power supply,
7, 8 ... driver circuit,
9, 9a to 9d ... light intensity modulator,
10, 10a to 10e... Semiconductor laser,
11a: exclusive OR circuit,
11b 1-bit delay line,
11c ... negative circuit,
12: Optical filter,
13: Light wave input unit,
14: Optical multiplexing part,
15, 15a ... first branch optical waveguide,
16, 16a, second branched optical waveguide,
17 ... 1st phase modulation part,
18 ... second phase modulation section,
19: Phase adjustment unit,
21, 61 ... modulator substrate,
22: First phase modulation electrode,
23... First ground electrode,
24 ... second phase modulation electrode,
25 ... second ground electrode,
26: Common ground electrode,
27 ... DC bias electrode,
28: DC bias ground electrode,
29 ... dielectric film,
30 ... high frequency grounding wiring,
31, 41... Mounting board,
32-34, 42-44 ... high frequency connectors,
35, 45 ... power connector,
36, 46 ... DC bias power connector,
37 ... Terminator,
38a-38g, 48 ... bonding wire,
39, 49 ... package,
40: DC bias wiring,
50, 50a, 50b... Differential amplification driver circuit,
51... Inverted output terminal,
52 ... Bonding pad,
53 ... spacer,
55-59, 55a, 56a ... wiring,
60: DC bias ground wiring,
62: Optical multiplexer,
63... First polarization beam splitter,
64 ... second polarization beam splitter,
65: Optical filter,
69 ... splitter,
70 ... delay part,
71, 72 ... waveform shaping filter,
80, 81, 82 ... code conversion module,
A1, A1a to A1c, A11... NRZ code signal,
A2, A2a to A2c ... inverted NRZ code signal,
A3, A3a to A3c, A12, A13, A14... NRZI code signal,
A4, A4a to A4c ... inverted NRZI code signal,
A5, A5a to A5h, A15... NRZI modulated signal,
A6, A6a to A6h ... inverted NRZI modulation signal,
A7, A17 ... DC bias,
A18: Delayed NRZI code signal,
A19: Total NRZI modulation signal,
A20: NRZI waveform shaping modulation signal,
A21: Delayed NRZI waveform shaping modulation signal,
B1, B1a-B1c, B1 _i , B1 _{(I + 1)} ... light waves,
B2, B2a to B2c, B2 _i ~ B2 _{(I + 2)} ... Output signal light,
B3: First branched light wave,
B4 ... first modulated light wave,
B5: Second branched light wave,
B6 ... second modulated light wave,
B7 ... Combined light,
B8: SSB output signal light,
B9, B9a ... multiplexed output signal,
D1 _i ~ D1 _{(I + 2)} ... First polarized light wave,
D2 _i , D2 _{(I + 1)} ... Second polarized light wave,
M1 _i ~ M1 _{(I + 2)} ... First modulated polarized light wave,
M2 _(I-1) ~ M2 _{(I + 1)} ... Second modulated polarized light wave,
L1 ... Optical waveguide length difference,
L2: Transmission line electrical length difference,
P _i ~ P _{(I + 2)} ... Input port.

Claims (7)

The level of the NRZ code signal is converted when the first logical code of the NRZ code signal is input, and the level of the NRZ code signal is not converted when the second logical code of the NRZ code signal is input. The NRZI code signal generated from the NRZ code signal and the inverted NRZI code signal generated by inverting the code of the NRZI code signal are respectively amplified to output the NRZI modulation signal and the inverted NRZI modulation signal. Module,
A DC bias power source for supplying a DC bias, a semiconductor laser for outputting light waves,
An optical waveguide that divides the light wave into two at the input unit and combines at the output unit, two phase modulation units that apply the NRZI modulation signal and the inverted NRZI modulation signal to each of the two branched light waves, and the optical waveguide to the optical waveguide A light intensity modulator provided with a phase adjustment unit for applying a DC bias,
The two-branched light wave is adjusted so that the intensity of the optical signal output from the optical intensity modulator is minimized during the period in which the pulse codes of the NRZI modulation signal and the inverted NRZI modulation signal are inverted by adjusting the DC bias. An optical transmitter characterized in that intensity modulation is performed. The code conversion module generates a first precoder circuit that generates the NRZI code signal from the NRZ code signal, and a second precoder circuit that generates the inverted NRZI code signal from an inverted NRZ code signal obtained by inverting the logical code of the NRZ code signal. The optical transmitter according to claim 1, further comprising: a precoder circuit.   The code conversion module includes a precoder circuit that generates the NRZI code signal from the NRZ code signal, and a differential amplifier circuit that includes an inverting output terminal that inverts the NRZI modulation signal to the inverted NRZI modulation signal. The optical transmission device according to claim 1, wherein:   In two lines that guide the NRZI modulation signal and the inverted NRZI modulation signal emitted from the code conversion module to two phase modulation units of the optical intensity modulator, the time delay amount due to the effective length difference between the lines is the NRZ modulation. The optical transmission apparatus according to claim 1, wherein the optical transmission apparatus is one time slot or less of a signal.   4. The optical transmission device according to claim 3, wherein the optical intensity modulator and the differential amplifier circuit are integrated and mounted on a modulator substrate.   The optical filter is connected to the output unit of the light intensity modulator, and the output signal light output from the output unit is a single sideband signal light. The optical transmission device described.   An optical filter unit that extracts a single side wave component from the light wave phase-modulated by the NRZI modulation signal and the inverted NRZI modulation signal in the light intensity modulator, and the phase modulation unit and the output unit of the light intensity modulator. The optical transmission device according to claim 1, wherein the optical transmission device is provided between the two.

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