JP2004343360A - Optical transmitter and optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter capable of easily increasing the availability of light frequency, and to provide an optical communication system. <P>SOLUTION: A high pass cut-off differential NRZI signal pair is impressed respectively to two branching paths of a push-pull type intensity modulator 12, the amplitude level of the differential NRZI signal pair and the DC bias level of the light intensity modulator 12 are appropriately set, and thus inverted RZ optical signals for which a phase is inverted for each adjacent mark are generated. Further, the bandpass of the inverted RZ optical signals is limited by an optical filter 13 and the spectrum width is suppressed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmission device used for a wavelength division multiplexing optical communication system and an optical communication system applicable to the wavelength division multiplexing optical communication system.
[0002]
[Prior art]
In a wavelength multiplexing optical communication system, the utilization efficiency of an optical frequency in an optical fiber as a transmission path is an important index. Duobinary light modulation and vestigial sideband (VSB) light modulation are known as light modulation methods that can narrow the band of transmission light and can use the optical frequency band with high efficiency.
[0003]
Patent Literature 1 below discloses details regarding a duobinary light modulation scheme. In this method, each band of a binary differential NRZI (Non Return to Zero Inverted) signal pair is limited by a low-pass filter having a band of, for example, one third or less of a bit rate to generate a ternary signal. . The ternary signal is provided as a drive signal to a light intensity modulator that modulates each of the signal pairs. When one level, which is an intermediate value among the levels (0, 1, 2) of the ternary signal, is applied to the light intensity modulator, the light intensity becomes minimum, and the other levels are input to the light intensity modulator. Then, the driving conditions of the light intensity modulator are adjusted so that the light intensity becomes maximum.
[0004]
In this method, since a band limitation below the bit rate is applied to the modulator driving signal, the band of the optical signal output from the modulator can be made very compact. However, in this method, since the cutoff frequency of the low-pass filter is in the main band of the binary NRZ signal, one level of the ternary signal is different from the center value of the other levels depending on the transmission characteristics and group delay characteristics of the low-pass filter. However, one level may be further binarized. In such a case, the extinction ratio of the light intensity modulated light deteriorates. Further, when handling ultra-high-speed signals of the order of several tens of Gbit / s, severe characteristics are required for a low-pass filter. Therefore, if the required severe transmission and group delay characteristics are to be satisfied, the yield of the low-pass filter is reduced, and there is a concern that the cost of the optical transmission unit may be increased.
[0005]
Non-Patent Document 1 below discloses details regarding the VSB light modulation method. In this method, an RZ (Return to Zero) optical pulse output from a light source is encoded by an NRZ (Non Return to Zero) data signal to generate an RZ optical signal, and the band is limited by a narrow-band optical filter. ing. However, in this method, it is necessary to accurately set the center transmission frequency of the narrow band optical filter to a position shifted from the oscillation frequency of the light source. That is, the center transmission frequency of the narrow-band optical filter is not located at the point where the transmitted light intensity of the narrow-band optical filter becomes maximum. Therefore, there is a problem that it is very difficult to set and control the optimum value of the oscillation frequency of the light source and the transmission frequency of the narrow band optical filter. Related techniques are also disclosed in Non-Patent Documents 2 and 3 below. Non-Patent Document 2 discloses an example in which optical phase modulation is performed using an NRZI signal and an RZ optical signal in which an optical carrier component is suppressed is generated by a Mach-Zehnder interferometer. Non-Patent Document 3 discloses that an RZ (Return to Zero) optical signal generated by combining a phase modulator and a Mach-Zehnder optical interferometer is extremely excellent in nonlinear tolerance.
[0006]
[Patent Document 1]
JP-A-8-139681 (paragraph numbers [0019] to [0025], FIGS. 1A to 1D)
[0007]
[Non-patent document 1]
IEEE Photonics Technology Letters vol. 7 1995 pp. 929-931
[0008]
[Non-patent document 2]
IEEE Photonics Technology Letters, vol. 13 2001 pp. 1298-1300
[0009]
[Non-Patent Document 3]
2002 IEICE Society Conference Program: Lecture Number B-10-63 Evaluation of Basic Characteristics of Carrier-Suppressed RZ Modulation Using Differential Phase
[0010]
[Problems to be solved by the invention]
As described above, in the conventional optical modulation system, severe characteristics are required for devices such as an optical filter and a light source in order to narrow the band of transmission light. In addition, the optical device must be controlled by a special method, and it cannot be said that the efficiency of using the optical frequency can be easily increased.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical transmission device and an optical communication system capable of easily increasing the optical frequency utilization efficiency.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, an optical transmission apparatus according to the present invention provides a signal generation unit (for example, a signal generation unit) that generates an NRZI (Non Return to Zero Inverted) signal according to transmission information and an inverted NRZI signal whose logic is inverted. Source 14, precoder 15, and driver 16) and drive signal generating means (for example, low-pass filters 17, 18) for generating first and second drive signals by blocking high-frequency components of the NRZI signal and the inverted NRZI signal, respectively. ), A laser light source (for example, a semiconductor laser 11) that generates continuous light, the continuous light is branched into two, and each of the branched lights is individually phase-shifted in accordance with the first and second drive signals. A light intensity modulator (for example, a push-pull type light intensity modulator 12) for generating and outputting intensity-modulated light through a wave; Level control means for controlling a DC level of at least one of the first and second drive signals so that the output intensity modulated light is an inverted RZ (Return to Zero) optical signal (for example, a driver 16 and a DC voltage source 19; A band limiting means (for example, an optical filter 13) for limiting the band of the intensity modulated light output from the light intensity modulator.
[0012]
By taking such means, an inverted RZ optical signal is output from the optical intensity modulator. This inverted RZ optical signal has the maximum extinction ratio and the optical phase is inverted for each adjacent mark. As a result, the optical fiber has a strong resistance to the chromatic dispersion characteristics of the optical fiber.
[0013]
The inverted RZ optical signal is an optical signal obtained by inverting the logic of a normal (ie, non-inverted) RZ optical signal, and its tolerance to fiber nonlinear effects is known to be substantially the same as that of a normal RZ optical signal. . In addition, the optical spectrum of the inverted RZ optical signal is usually more compact than that of the RZ optical signal, and the optical carrier component is suppressed in principle.
[0014]
That is, according to the above configuration, it is possible to generate an inverted RZ optical signal having high resistance to the dispersion effect and the non-linear effect of the optical fiber. Therefore, it is possible to easily limit the band by a simple method using an optical filter or the like. Become. As a result, it is possible to increase the band use efficiency of the optical frequency by a simple method.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present embodiment will be described in detail with reference to the drawings.
(1st Embodiment)
FIG. 1 is a functional block diagram illustrating a configuration of the optical transmission device according to the first embodiment of the present invention. In FIG. 1, a signal source 14 generates a binary NRZ (Non Return to Zero) signal according to transmission information and inputs the signal to a precoder 15. The precoder 15 generates an NRZI signal from the NRZ signal and inputs the signal to the driver 16. The driver generates a differential NRZI signal pair amplified to a predetermined level from the NRZI signal. Among them, the NRZI signal is input to the push-pull type light intensity modulator 12 as a drive signal after a high frequency component is cut off via a low pass filter (LPF) 17. After the high frequency component of the inverted NRZI signal is cut off via the low-pass filter 18, the inverted NRZI signal is input to the push-pull type light intensity modulator 12 as a drive signal.
[0016]
A semiconductor laser (LD) 11 generates and outputs a single-mode continuous light, and inputs the light to a light intensity modulator 12. The continuous light is branched into two in the light intensity modulator 12 and introduced into the first and second branch paths, respectively. The phase of the continuous light introduced into the first branch is shifted according to the NRZI signal from which the high frequency component has been cut off. The phase of the continuous light introduced into the second branch is shifted according to the inverted NRZI signal from which the high-frequency component has been cut off. Thereafter, the phase-shifted branched lights are multiplexed, whereby the light intensity modulator 12 generates and outputs intensity-modulated light. The intensity-modulated light is band-limited by the optical filter 13 and then output to an optical fiber (not shown).
[0017]
A DC voltage source 19 that generates a DC bias voltage is connected to the light intensity modulator 12. The DC bias voltage is added to the inverted NRZI signal and applied to the second branch of the light intensity modulator 12. By appropriately setting the signal level and the DC bias voltage of the differential NRZI signal pair in such a configuration, the waveform of the intensity modulated light output from the light intensity modulator 12 becomes an inverted RZ optical signal. Hereinafter, the intensity modulated light output from the light intensity modulator 12 is referred to as an inverted RZ optical signal.
[0018]
FIG. 2 is a diagram illustrating a process of generating an inverted RZ optical signal in the optical transmission device of FIG. For example, assuming that the transmission information has a bit string of (010011101100), the signal source 14 and the precoder 15 in FIG. 1 output the NRZ signal and the NRZI signal, respectively, as illustrated. Since the NRZI signal is applied to the first branch of the optical intensity modulator 12, the optical phase of the first branch takes two states according to the state of the NRZI signal. Conversely, since the inverted NRZI signal is applied to the second branch, the optical phase of the second branch takes two states inverted to the state of the NRZI signal.
[0019]
By appropriately setting the signal level and the DC bias voltage of the NRZI signal and the inverted NRZI signal, the optical phase of the first branch path is set to one of -π / 2 and π / 2, and The optical phase of the two branch paths can be in one of -π / 2 and -3π / 2. Thereby, the operating point of the light intensity modulator 12 can be set so that the phase difference between the branch paths is either 0 or 2π according to the state of the NRZI signal. In the falling part, the light intensity of the optical signal output from the light intensity modulator 12 can be minimized. That is, by appropriately setting the amplitude level and the DC bias level of the differential NRZI signal pair, the differential NRZI signal maximizes the light intensity of the optical signal output from the light intensity modulator 12 at the time of a mark or space. it can.
[0020]
By keeping the light intensity modulator 12 in the above state, the light intensity modulator 12 outputs an inverted RZ optical signal having the maximum extinction ratio. The inverted RZ optical signal thus generated is driven by the differential NRZI signal pair across the minimum output point of the light intensity modulator 12. Therefore, the phase of the inverted RZ optical signal is inverted for each adjacent mark (at the time when the light intensity is maximum). From this, the inverted RZ optical signal output from the optical intensity modulator 12 has a high resistance to the chromatic dispersion characteristics of the optical fiber used as the optical transmission line.
[0021]
In general, since an optical fiber has chromatic dispersion characteristics, as an optical signal propagates through the optical fiber, an optical pulse width is widened, and overlapping between adjacent marks occurs. On the other hand, since the optical phase of the inverted RZ optical signal obtained by the present embodiment is inverted for each adjacent mark, the optical signals cancel each other out at the overlapping portion between the adjacent marks. As a result, the influence of intersymbol interference is extremely small, and therefore, there is a large resistance to chromatic dispersion of the fiber.
[0022]
In addition to the immunity to chromatic dispersion, another important fiber transmission characteristic is a fiber nonlinearity (distortion) effect. This is an effect that when the fiber input light intensity is increased, the refractive index of the fiber changes in accordance with the light intensity, thereby distorting the optical signal, making it difficult to identify the binary signal on the receiving side. . It is known that an inverted RZ optical signal has almost the same tolerance to fiber nonlinear effects as a normal RZ optical signal.
[0023]
FIG. 3 is a diagram showing the optical spectrum of the inverted RZ optical signal in comparison with the optical spectrum of the RZ optical signal. In FIG. 3, the horizontal axis indicates the light wavelength, and the vertical axis indicates the light power. The thick line plotted indicates the inverted RZ optical signal, and the thin line indicates the RZ optical signal. The bit rates are both 10 Gb / s.
[0024]
Since the inverted RZ optical signal is an optical signal obtained by inverting the logic of the normal RZ optical signal, its optical spectrum becomes compact as shown in FIG. Is suppressed.
[0025]
Further, in the present embodiment, as shown in FIG. 1, the optical filter 13 limits the band limitation of the inverted light RZ signal. As a result, unnecessary side lobe components in the optical signal spectrum of the inverted light RZ signal are further removed. Therefore, according to the present embodiment, even in a high-density wavelength-division multiplexed optical communication system in which the bit rate of an optical signal and the channel optical frequency interval are almost the same, it is possible to maintain a high level of wavelength separation between channels. It becomes.
[0026]
FIG. 4 is a diagram showing an optical spectrum when the inverted light RZ signal is subjected to band suppression by an optical narrow band filter and a signal waveform after photoelectric conversion. In FIG. 4, a third-order Gaussian optical narrow band filter is applied as the optical filter 13, and the bit rate of the inverted RZ signal is set to 40 Gb / s.
[0027]
FIG. 4A shows an optical spectrum and an optical signal waveform when the optical filter 13 is not provided. From this state, it can be seen that as the optical filter 13 is provided and its band is narrowed, the signal waveform changes from the inverted RZ optical signal to the NRZ signal as shown in FIGS. 4B and 4C. In FIG. 4, it can be seen that an identifiable signal waveform can be obtained even if the band of the optical filter 13 is reduced to about half the bit rate. In particular, when an optical filter whose transmission band characteristic is trapezoidal is used and the cut-off frequency width of 20 dB is about twice the bit rate, the shape of the inverted RZ optical signal can be maintained. I understand. That is, since the band of the inverted RZ optical signal can be easily limited by using a simple optical filter, the use of the inverted RZ optical signal obtained according to the present embodiment can improve the efficiency of using the optical frequency band. It turns out to be effective.
[0028]
FIGS. 4B and 4C show a case where the band of the inverted RZ optical signal is limited around the optical carrier (center) frequency of the inverted RZ optical signal, and the optical spectrum is suppressed. In addition, as shown in FIG. 4 (d), even if an asymmetric band limitation such that the center transmission frequency of the optical filter 13 and the optical carrier frequency are slightly detuned is performed, the signal waveform with little deterioration of the eye opening degree is obtained. Is obtained.
[0029]
Specifically, for a 40 Gb / s-class inverted RZ optical signal, the deviation between the optical carrier frequency and the transmission center optical frequency of the optical filter is allowed to be about 0 to 8 GHz. By optimizing the shift amount, it is possible to suppress the spectrum width of the optical signal by about 10% as compared with the case where the symmetric band limitation is performed, and it is possible to further increase the optical frequency use efficiency.
[0030]
As described above, in the present embodiment, the differential NRZI signal pair whose high band is cut off is applied to each of the two branch paths of the push-pull type optical intensity modulator 12, and the amplitude level of the differential NRZI signal pair and the optical intensity modulator 12 By appropriately setting the DC bias level, an inverted RZ optical signal whose phase is inverted for each adjacent mark is generated. Further, the band of the inverted RZ optical signal is limited by the optical filter 13 so as to suppress the spectrum width.
[0031]
The inverted RZ optical signal has a high resistance to the chromatic dispersion of the optical fiber. Even if the band is limited using the optical filter 13, an optical signal with less waveform deterioration can be obtained. Further, in the present embodiment, the optical filter 13 is provided immediately after the light intensity modulator 12, so that even if high-density wavelength multiplexing is performed, optical communication with less influence of crosstalk between adjacent wavelength channels can be performed. . From these facts, it is possible to provide an optical transmitter capable of easily increasing the optical frequency utilization efficiency.
[0032]
(Second embodiment)
FIG. 5 is a functional block diagram illustrating a configuration of an optical transmission device according to the second embodiment of the present invention. In FIG. 5, portions common to FIG. 1 are denoted by the same reference numerals, and only different portions will be described here.
[0033]
In FIG. 5, a low-frequency oscillator 58 generates a low-frequency signal in a low-frequency region that does not affect transmission of an ultra-high-speed signal as a signal for monitoring. This low-frequency signal is superimposed on the current injected into the semiconductor laser 11, whereby the continuous light from the semiconductor laser 11 is subjected to weak frequency modulation. At this time, if the center transmission frequency of the optical filter 13 deviates from the center oscillation frequency of the semiconductor laser 11, the optical frequency modulation component is converted into a light intensity modulation component by the optical frequency transmission characteristics of the optical filter 13.
[0034]
A part of the inverted RZ optical signal that has passed through the optical filter 13 is branched by the coupler 55 and input to the photodetector (PD) 56. The photodetector 56 detects a light intensity modulation component included in the inverted RZ optical signal. The output of the photodetector 56 is multiplied by a low-frequency signal by the mixer 59 via the band-pass filter 51, that is, synchronously detected. At the output of the mixer 59, a signal component twice as high as the oscillator frequency and a DC component mainly appear. This is further input to a low-pass filter 510 to extract only the DC component.
[0035]
By detecting the sign of the DC component (that is, the output of the low-pass filter 510), it is possible to monitor whether the oscillation light frequency of the semiconductor laser 11 is shifted to the positive or negative with respect to the center transmission frequency of the optical filter 13. . Further, the deviation of the optical frequency may be obtained by detecting the absolute value of the DC component. In principle, the value of the DC component is minimized when the optical frequency shift is eliminated.
[0036]
With such a configuration, the optical frequency shift between the semiconductor laser 11 and the optical filter 13 can be grasped. Based on this, the value of the current injected from the DC bias source 511 into the semiconductor laser 11 can be controlled. Eleven oscillation frequencies can be set.
[0037]
In the above description, the oscillation light frequency of the semiconductor laser 11 is controlled to correct the deviation of the optical frequency. However, the center transmission frequency of the optical filter 13 is adjusted to the center light frequency of the semiconductor laser 11. May be changed.
[0038]
Instead of the above configuration, the level of the inverted RZ optical signal itself may be detected by the photodetector (PD) 56, and the output optical frequency of the semiconductor laser 11 may be controlled to maximize the level. Also in this case, the oscillation light frequency of the semiconductor laser 11 and the center transmission frequency of the optical filter 13 can be matched.
[0039]
(Third embodiment)
FIG. 6 is a functional block diagram showing the configuration of the optical communication system according to the third embodiment of the present invention. This optical communication system includes a plurality of the optical transmission devices of FIG. 1 or FIG. Each optical transmitter outputs inverted RZ optical signals having different frequencies f1 to fn. The inverted RZ optical signal of each wavelength is wavelength-multiplexed by the optical multiplexer 64 to generate a wavelength-division multiplexed light, which is transmitted to the optical fiber 66. The wavelength multiplexed light reaches the optical demultiplexer 65 via a repeater and the like, and is separated into inverted RZ optical signals of each wavelength. The separated inverted RZ optical signal is input to the optical receiver 62 via the optical filter 61 for each wavelength.
[0040]
As described above, in the present embodiment, the optical filter 61 is disposed immediately before the input stage of the optical receiver 62. As described in the first embodiment, the inverted optical RZ signal has a large dispersion tolerance and a nonlinear tolerance equivalent to that of a normal RZ optical signal. Further, as described in the first and second embodiments, the inverted optical RZ signal is also an optical modulation signal whose optical frequency utilization efficiency can be improved by limiting its band. However, when the band of the inverted optical RZ signal is limited immediately after the transmission stage of the optical transmission device, there is a possibility that the nonlinear tolerance is deteriorated because the spectrum width is narrowed.
[0041]
Therefore, in the present embodiment, as shown in FIG. 6, in addition to the optical filter 13 provided in the optical signal transmitting device, an optical filter 61 is disposed immediately before the input of the optical receiver 62. Then, the amount of band limitation by the optical filter 13 is relaxed to the extent that optical crosstalk between adjacent channels in the optical fiber does not cause a problem in transmission characteristics, and the band is again limited by the optical filter 61 on the receiving side, whereby the inverted RZ Complete NRZ conversion of the optical signal is achieved. In the two optical filters 13, 61, the optical multiplexer 64, and the optical demultiplexer 65, the inverted RZ optical signal is band-limited, but passes through the allowable range of the band-limit after passing through these four optical devices. Estimating the bandwidth (full width at half maximum) is about 70% of the bit rate frequency.
[0042]
With such a configuration, it is possible to prevent deterioration of the nonlinear tolerance of the inverted RZ optical signal. In addition, compared to the bit rate of the ultra-high-speed optical signal, when there is no margin in the band of the photodetector and the electric circuit constituting the optical receiver 62, and the phase characteristics and the group delay characteristics of the entire optical receiver cannot be sufficiently secured. Also, the band limitation by the optical filter 61 is effective. As described above, according to the present embodiment, since the optical filter 61 is inserted on the input side of the optical receiver 62, in addition to the effects obtained by the first and second embodiments, the reception band of the optical receiver 62 Can be relaxed.
[0043]
(Fourth embodiment)
FIG. 7 is a functional block diagram illustrating a configuration of an optical communication system according to the fourth embodiment of the present invention. The optical communication system shown in FIG. 7 includes an ADD-DROP filter 72 that is provided through an optical fiber 66 and multiplexes / demultiplexes an inverted RZ optical signal included in each wavelength multiplexed signal in wavelength units. This system also includes a periodic optical filter 71 on the input side of the ADD-DROP filter 72. Each ADD-DROP filter 72 is connected to a pair of optical receivers and optical transmitters for each of the frequencies f1 to fn. For example, in a system corresponding to the frequency f1, an inverted RZ signal of the frequency f1 is dropped from the wavelength multiplexed light by the ADD-DROP filter 72 and input to the optical receiver. Further, an inverted RZ signal having the same frequency f1 is output from the optical transmitter, added to the wavelength multiplexed light by the ADD-DROP filter 72, and transmitted downstream. The same applies to systems corresponding to other frequencies f2 to fn. The add / drop processing of an optical signal is an operation of loading and unloading an optical signal having the same wavelength but different data to be superimposed.
[0044]
FIG. 8 is a block diagram showing a configuration example of the ADD-DROP filter 72 of FIG. In FIG. 8, the wavelength-division multiplexed light enters a fiber Bragg grating 73 via a circulator 74. The fiber Bragg grating 73 reflects only the signal component of a channel having a specific center optical frequency, and allows the signal light of another channel that is not reflected to pass through as it is. By such a function, for example, only the optical signal fn is returned to the circulator 74 again and separated into a drop path. On the other hand, the wavelength multiplexed light that has passed through the fiber Bragg grating 73 is incident on the circulator 75, is multiplexed with the optical signal fn having information different from that dropped earlier, and is output to the optical transmission line.
[0045]
FIG. 9 is a block diagram showing another configuration example of the ADD-DROP filter 72 of FIG. In FIG. 9, a Mach-Zehnder interferometer having two optical branches is formed by couplers 77 and 78 on a substrate (not shown) interposed in the optical waveguide, and each optical branch has the same optical frequency reflection characteristic. The fiber Bragg gratings 73 and 76 are arranged. For example, if the fiber Bragg gratings 73 and 76 reflect the frequency fn, an optical signal having the frequency fn can be added and dropped.
[0046]
In general, the dispersion characteristic of a Bragg grating filter is relatively large, and if there is a deviation between the center frequency of an optical signal and the transmission center frequency of the Bragg grating filter, the signal waveform after dropping will be disturbed.
[0047]
Therefore, in the present embodiment, the periodic optical filter 71 is inserted before the add / drop processing of the optical signal so as to narrow the bandwidth of each optical signal as much as possible. As a result, it is possible to widen the allowable range of the optical frequency shift between the optical signal and the Bragg grating filter. As the periodic optical filter 71, a Mach-Zehnder optical interferometer, a Fabry-Perot optical resonator, or the like can be used. do it.
[0048]
As described above, in the present embodiment, the periodic optical filter 71 is inserted into the optical fiber 66, and the inverted RZ optical signal that is hardly affected by the dispersion effect (pulse spread) and the non-linear effect (distortion) of the optical fiber 66 is obtained. The signal is transmitted to the position (that is, the insertion position of the periodic optical filter 71). As a result, an optical signal having a narrow band is generated in the periodic optical filter 71, and the add / drop processing of the optical signal can be performed with high accuracy regardless of the wavelength shift between the optical filter and the optical signal.
[0049]
(Fifth embodiment)
FIG. 10 is a functional block diagram illustrating a configuration of an optical communication system according to the fifth embodiment of the present invention. In FIG. 10, an optical add / drop unit is formed by a pair of optical multiplexer / demultiplexers 81 and 87. In FIG. 10, the wavelength division multiplexed light transmitted via the optical fiber 66 is split by the optical splitter 81 into optical signals of each wavelength. The optical signal of each wavelength is input to the optical switch 82, and it is individually determined whether to drop or pass through.
[0050]
The optical signal of the channel dropped by the optical switch 82 is band-limited through an optical narrow-band filter 83 whose central transmission frequency is set corresponding to the central optical frequency, and is input to an optical receiver 84. At that time, an optical signal of the same wavelength is newly generated from the optical transmitter 85 and guided to the optical multiplexer 87 via the optical switch 86.
[0051]
On the other hand, the optical signal of the channel passed through the optical switch 82 passes through the optical switch 86 as it is and enters the optical multiplexer 87. The optical switch 82 and the optical switch 86 are switched in conjunction with each other according to the state of the add or the drop.
[0052]
In FIG. 10, an optical narrow-band filter 83 is provided at each output port of the optical demultiplexer 81. However, a filter having a narrow transmission bandwidth is applied to the optical demultiplexer 81 so as to increase the optical resolution. Then, the optical narrow band filter 83 can be omitted.
[0053]
In general, an array grating waveguide type grating often used as an optical multiplexer / demultiplexer has a good dispersion characteristic, and the transmittance is flat especially in the vicinity of the optical frequency where the transmittance of the optical frequency-transmission characteristic peaks. An optical multiplexer / demultiplexer has excellent dispersion characteristics. Therefore, even if the band of the inverted RZ optical signal is limited by narrowing the transmission bandwidth at each channel port, an optical waveform with less waveform distortion due to dispersion can be obtained.
[0054]
Also, in ultra-high-speed optical communication in which the transmission rate of an optical signal exceeds 40 Gb / s, the band and group delay characteristics of an electric circuit inside the optical receiver are often insufficient. Even if a photoelectric conversion unit such as a PD (photodetector) has a band margin, a signal having a high-frequency component such as an RZ pulse is somewhat distorted, and a difference occurs in the rise and fall response times. May be deteriorated.
[0055]
On the other hand, according to the present embodiment, the inverted RZ optical signal can be converted into NRZ by the optical band limitation in the optical demultiplexer 81, and the band and the group delay characteristics of the electric circuit constituting the optical receiver 84 can be reduced. The restriction can be relaxed, which is advantageous in cost.
[0056]
On the other hand, when the band is limited by the optical demultiplexer 81, the optical signals subjected to THROUGH by the optical switches 82 and 86 are also band-limited, so that the resistance to the optical nonlinear effect is deteriorated.
[0057]
In order to cope with this, in FIG. 10, the transmission bandwidth of each output port of the optical demultiplexer 81 is not narrowed, the optical narrow band filter 83 is provided, and nothing is provided on the add side. With such a configuration, the optical signal is subjected to add / drop processing in the middle of the transmission line while utilizing the dispersion tolerance and nonlinear tolerance of the inverted RZ optical signal, and the bandwidth to the optical receiver at each node performing the add / drop processing Can be reduced. Note that the transmission bandwidth of the optical multiplexer 87 may be appropriately selected according to the modulation method and bandwidth of the optical signal to be added.
[0058]
(Sixth embodiment)
FIG. 11 is a functional block diagram illustrating a configuration of an optical transmission device according to the sixth embodiment of the present invention. In the present embodiment, an inverted NRZ optical signal is taken into the Mach-Zehnder optical interferometer 91, and an inverted NRZ optical signal and an RZ optical signal with a suppressed carrier component (DPCS-RZ (Differential Phase Carrier Suppress-RZ) optical signal). And is generated.
[0059]
FIG. 12 is a diagram showing signal waveforms at ports # 1 and # 2 of the Mach-Zehnder optical interferometer 91 in the configuration of FIG. In FIG. 12, the transmittance of the Mach-Zehnder optical interferometer 91 becomes sinusoidal with respect to the optical frequency, and the relationship between the peak and the valley of the transmittance is reversed between the two output ports # 2.
[0060]
That is, the optical signal is split into two inside the Mach-Zehnder optical interferometer 91, and one of the split lights is delayed and given to be multiplexed so that the two split lights interfere with each other. One of the output ports # 2 is added and the other is subtracted. As a result, a waveform as shown in FIG. 13 is generated.
[0061]
FIG. 13 is a diagram showing signal waveforms output from the output ports # 1 and # 2 of the Mach-Zehnder optical interferometer 91 in the configuration of FIG. FIG. 13 shows a case where the delay amount inside the Mach-Zehnder optical interferometer 91 is equivalent to one bit of the transmission signal. As shown in FIG. 13, the output from one output port # 2 of the Mach-Zehnder interferometer is inverted. An NRZ optical signal is output, and an RZ optical signal is output from the other output port # 2.
[0062]
Specifically, an inverted NRZ signal is obtained at a port that narrows the optical spectrum of the inverted RZ optical signal, and a carrier-suppressed RZ signal is generated at a port that suppresses the carrier component and provides a band twice as high as the bit rate. It turns out that it is done.
[0063]
The inverted NRZ optical signal is generated by suppressing the band of the optical signal centered on the carrier optical frequency. However, in the optical modulator that generates the inverted RZ optical signal, a set of differential NRZI signals for driving the optical intensity modulator 12 are subjected to a differential NRZI signal by an electric low-pass filter having a band corresponding to half the bit rate frequency. It is advisable to relax the rise and fall times of the.
[0064]
As described in the first embodiment, the phase is inverted between adjacent mark pulses of the inverted RZ optical signal. Therefore, the inverted RZ optical signal is not only an intensity modulation signal but also a phase modulation signal. Therefore, an RZ optical signal is generated by using the Mach-Zehnder optical interferometer 91 whose delay amount is equivalent to 1 bit as an optical delay detector. An example of generating an RZ optical signal in which optical phase modulation is performed by an NRZI signal and an optical carrier component is suppressed by a Mach-Zehnder interferometer is described in Non-Patent Document 2, and the operating principle of generating an RZ optical signal in the present invention is as follows. This is equivalent to the method described in the literature.
[0065]
However, when compared with the RZ optical signal generated by combining the phase modulator and the Mach-Zehnder optical interferometer, the inverted RZ optical signal generated according to the present invention is obtained by simultaneously performing intensity modulation in addition to phase modulation. . In addition, since the inverted RZ optical signal in which the carrier component is suppressed is taken into the Mach-Zehnder optical interferometer, the carrier suppression amount of the generated RZ optical signal is further increased, and the rise and fall time of the RZ optical pulse can be reduced. it can.
[0066]
FIG. 14 is a diagram illustrating the amount of dispersion with respect to an optical signal in comparison with the present embodiment and the related art. As shown in the figure, according to the present embodiment, it is possible to obtain a better waveform of the eye opening as compared with the related art, and to increase the resistance to the influence of the RZ optical pulse width spread due to the dispersion of the optical fiber. Can be.
[0067]
FIG. 15 shows a waveform when the amount of phase shift of the phase modulator is halved (90 °), the amount of optical frequency shift generated at the time of phase change in the phase modulator is reduced, and the influence of dispersion of the optical fiber is reduced. FIG. 5 is a diagram showing a comparison between an inverted RZ optical signal and a waveform of an RZ optical pulse generated by a Mach-Zehnder interferometer. As shown in the figure, even in such a case, it can be seen that the method of the present embodiment is superior in dispersion resistance to the conventional method.
[0068]
FIG. 16 shows a waveform when the phase shift amount in the phase modulator is maximized (180 °), the optical frequency shift amount is increased to reduce the effect of the nonlinear effect of the optical fiber, the inverted RZ optical signal and the Mach-Zehnder signal. FIG. 9 is a diagram showing a waveform of an RZ optical pulse generated by an interferometer in comparison with a case where the optical fiber input light intensity is increased. Non-Patent Document 3 describes that an RZ optical signal generated by combining a phase modulator and a Mach-Zehnder optical interferometer is extremely excellent in nonlinear tolerance.
[0069]
On the other hand, from the comparison of FIG. 16, it can be seen from the comparison of the RZ optical signal according to the present embodiment that the effect of the signal distortion due to the nonlinear effect of the optical fiber on the optical signal waveform is almost the same. From these facts, according to the present embodiment, it is possible to generate an optical signal capable of maintaining a high nonlinear resistance while increasing the resistance to chromatic dispersion of the optical fiber. That is, according to the present embodiment, by using the Mach-Zehnder interferometer 91 having a delay amount of 1 bit or less as an optical filter, an inverted NRZ optical signal having a narrow band from an inverted RZ optical signal and an optical carrier component having excellent nonlinear tolerance and excellent non-linear tolerance. A suppressed RZ optical signal can be generated.
[0070]
(Seventh embodiment)
FIG. 17 is a functional block diagram illustrating a configuration of an optical transmission device according to the seventh embodiment of the present invention. FIG. 17 corresponds to a modification of the configuration in FIG. In FIG. 17, a Mach-Zehnder optical filter 141 is provided on the optical transmission device side, and the optical switches 142 and 143 select the code format of an optical signal to be transmitted according to the status of the transmission line of the optical communication system. .
[0071]
When the dispersion value of the optical fiber to be laid has a large fluctuation in the longitudinal direction and it is difficult to strictly compensate for the dispersion, the optical switches 142 and 143 are switched so that the inverted RZ optical signal or the inverted NRZ signal is output. On the other hand, when the transmission path length is long, or when the distance to the optical amplifier used as a repeater is long and it is desired to increase the optical fiber input light intensity at the transmission end as much as possible, the optical signal is output so that the RZ optical signal is output. The switches 142 and 143 are switched.
[0072]
(Eighth embodiment)
FIG. 18 is a functional block diagram illustrating a configuration of an optical transmission device according to the eighth embodiment of the present invention. FIG. 18 corresponds to a modification of the configuration in FIG. When the optical transmitter of FIG. 17 is applied to a wavelength division multiplexing optical communication system, a Mach-Zehnder optical filter 151 is individually inserted into the output of the optical transmitter of each channel as shown in FIG. The point that the output optical signal is switched by the optical switches 152 and 153 is the same as in FIG.
[0073]
(Ninth embodiment)
FIG. 19 is a functional block diagram showing the configuration of the optical transmission device according to the ninth embodiment of the present invention. FIG. 19 corresponds to a modification of the configuration in FIG. When the optical transmitter shown in FIG. 17 is applied to a wavelength division multiplexing optical communication system, optical switches 162 and 163 are provided at the subsequent stage of the optical multiplexer 64 as shown in FIG. You may do it. In this case, a Mach-Zehnder optical interferometer 161 having a free spectrum range (periodic frequency of transmittance) corresponding to a channel frequency interval, and optical switches 162 and 163 are provided on the output side of an optical multiplexer 64 that bundles optical signals of respective channels. It is good to arrange.
[0074]
(Tenth embodiment)
FIG. 20 is a functional block diagram illustrating a configuration of an optical transmission device according to the tenth embodiment of the present invention. FIG. 20 shows a case where an add / drop unit combining a pair of optical demultiplexers 171 and an optical multiplexer 172 is provided in a transmission line, and the configuration of FIG. 17 is applied to this configuration. Further, a Mach-Zehnder interferometer 173 having a free spectrum range equivalent to the channel frequency interval is inserted on the input side of the add / drop unit.
[0075]
Here, when the input / output port of the Mach-Zehnder interferometer is set so that each channel optical signal input to the add-drop unit becomes an inverted NRZ optical signal, the Mach-Zehnder interferometer not connected to the add-drop unit A wavelength-multiplexed RZ optical signal is generated (DROP) from the output port of (1), and fiber transmission excellent in nonlinear tolerance is possible.
[0076]
On the other hand, when the channel frequency interval of the optical multiplexer 172 in the add / drop unit is set to half of the channel frequency interval of the optical demultiplexer 171 or the wavelength-multiplexed inverted RZ optical signal, the output port of the optical demultiplexer 171 is optically coupled. If every other input port is connected to the input port of the optical multiplexer 172, the half input port of the optical multiplexer 172 takes in the inverted NRZ optical signal generated from the Mach-Zehnder optical interferometer 173, and the other half port receives the other channel. Optical signals can be captured. With such a configuration, it is possible to double the wavelength multiplexing density in the add / drop unit.
[0077]
(Eleventh embodiment)
FIG. 21 is a functional block diagram showing the configuration of the optical communication system according to the eleventh embodiment of the present invention. The configuration shown in FIG. 21 eliminates the optical narrow-band filter 83 of FIG. 10 and replaces this with a Mach-Zehnder interferometer 181 on the input side of the optical demultiplexer 182, so that each channel light inputted to the add / drop unit is provided. The signal is configured to be an RZ optical signal. In such a configuration, the signal output from the other output port (the side not connected to the optical demultiplexer 182) of the Mach-Zehnder interferometer 181 is an inverted NRZ optical signal. It may be received separately using a filter or the like.
[0078]
Inside the add / drop unit, an RZ optical signal can be dropped at a node using optical multiplexer / demultiplexer pairs 182 and 183 and optical switches 184 and 185 having the same channel optical frequency interval. Also, the multiplexed wavelength multiplexed signal has excellent nonlinear tolerance.
[0079]
In the above description, an example in which the Mach-Zehnder optical interferometer 181 is disposed immediately before the add / drop unit is described. However, a plurality of Mach-Zehnder optical interferometers 181 are prepared, and each output port of the optical demultiplexer 182 and the optical multiplexer 183 And one of the outputs of each Mach-Zehnder interferometer 181 can be used, for example, for monitoring the optical signal of each channel.
[0080]
(Twelfth embodiment)
FIG. 22 is a functional block diagram showing the configuration of the optical receiver according to the twelfth embodiment of the present invention. FIG. 22 shows an example in which the configuration of FIG. 17 is applied to an optical receiver. In FIG. 22, a Mach-Zehnder optical interferometer 191 is inserted at the receiving end of an optical signal.
[0081]
Of the two output ports of the Mach-Zehnder interferometer 191, the port to which the inverted NRZ optical signal is output is taken into the baseband optical receiver 192 to generate a data signal. On the other hand, the port from which the RZ optical signal is output is connected to the narrow band optical receiving section 193 centered on the bit rate frequency, and the clock signal is reproduced. The original signal can be reproduced from these two signals.
[0082]
With such a configuration, it is possible to prevent a load on the band of the baseband light receiving unit 192. Further, even if the bit rate is high, it is sufficient to prepare an optical receiving unit whose operation is guaranteed only in a band near the bit rate frequency for reproducing the clock signal.
[0083]
As described above, according to each of the above embodiments, even if the band of the data signal for driving the light intensity modulator is limited to about half of the bit rate frequency, the optical signal is used in combination with the optical modulation section and the optical filter. Can be narrowed. Therefore, each of the above embodiments can be particularly suitably used when applied to a wavelength division multiplexing optical communication system in which the interval between optical wavelengths is narrow.
[0084]
According to each of the above embodiments, the transmission optical frequency is stabilized by an easy control method that only needs to match the center transmission frequency of the optical filter with the oscillation frequency of the light source while limiting the band with the optical filter. It becomes possible. From these, it is possible to provide an optical transmission device and an optical communication system that can easily increase the optical frequency use efficiency.
[0085]
The present invention is not limited to the above embodiment. For example, in each of the above embodiments, the band of the inverted RZ optical signal is limited by the optical filter. Instead, for example, the transmission band of the optical multiplexer 64 and the optical demultiplexer 65 in FIG. By doing so, the band of the inverted RZ optical signal may be limited.
[0086]
Furthermore, the present invention can be embodied by modifying the constituent elements in the implementation stage without departing from the scope of the invention. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiments. For example, some components may be deleted from all the components shown in the embodiment. Further, components of different embodiments may be appropriately combined.
[0087]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to provide an optical transmission device and an optical communication system capable of easily increasing the optical frequency utilization efficiency.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing a configuration of an optical transmission device according to a first embodiment of the present invention.
FIG. 2 is a diagram showing a process of generating an inverted RZ optical signal in the optical transmission device of FIG. 1;
FIG. 3 is a diagram showing an optical spectrum of an inverted RZ optical signal in comparison with an optical spectrum of an RZ optical signal.
FIG. 4 is a diagram showing an optical spectrum and a signal waveform after photoelectric conversion when band-suppression is applied to an inverted light RZ signal by an optical narrow-band filter.
FIG. 5 is a functional block diagram showing a configuration of an optical transmission device according to a second embodiment of the present invention.
FIG. 6 is a functional block diagram showing a configuration of an optical communication system according to a third embodiment of the present invention.
FIG. 7 is a functional block diagram showing a configuration of an optical communication system according to a fourth embodiment of the present invention.
FIG. 8 is a block diagram showing a configuration example of an ADD-DROP filter 72 of FIG. 6;
FIG. 9 is a block diagram showing another configuration example of the ADD-DROP filter 72 of FIG. 6;
FIG. 10 is a functional block diagram showing a configuration of an optical communication system according to a fifth embodiment of the present invention.
FIG. 11 is a functional block diagram showing a configuration of an optical transmission device according to a sixth embodiment of the present invention.
12 is a diagram showing signal waveforms at ports # 1 and # 2 of the Mach-Zehnder optical interferometer 91 in the configuration of FIG.
13 is a diagram showing signal waveforms output from output ports # 1 and # 2 of the Mach-Zehnder optical interferometer 91 in the configuration of FIG.
FIG. 14 is a diagram showing the amount of dispersion with respect to an optical signal in comparison with the present embodiment and the related art.
FIG. 15 is a waveform when the phase shift amount of the phase modulator is halved (90 °) and the optical frequency shift amount generated when the phase changes in the phase modulator is reduced to reduce the influence of the optical fiber dispersion. FIG. 5 is a diagram showing a comparison between an inverted RZ optical signal and a waveform of an RZ optical pulse generated by a Mach-Zehnder interferometer.
FIG. 16 shows a waveform when the amount of phase shift in the phase modulator is maximized (180 °), the amount of optical frequency shift is increased to reduce the effect of the nonlinear effect of the optical fiber, the inverted RZ optical signal, and the Mach-Zehnder. The figure which shows the waveform of the RZ light pulse produced | generated by the interferometer in comparison with the case where the optical fiber input light intensity is increased.
FIG. 17 is a functional block diagram showing a configuration of an optical transmission device according to a seventh embodiment of the present invention.
FIG. 18 is a functional block diagram showing a configuration of an optical transmission device according to an eighth embodiment of the present invention.
FIG. 19 is a functional block diagram showing a configuration of an optical transmission device according to a ninth embodiment of the present invention.
FIG. 20 is a functional block diagram showing a configuration of an optical transmission device according to a tenth embodiment of the present invention.
FIG. 21 is a functional block diagram showing a configuration of an optical communication system according to an eleventh embodiment of the present invention.
FIG. 22 is a functional block diagram showing a configuration of an optical receiving device according to a twelfth embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Semiconductor laser, 12 ... Push-pull type light intensity modulator, 13 ... Optical filter, 14 ... Signal source, 15 ... Precoder, 16 ... Driver, 17, 18 ... Low pass filter, 19 ... DC voltage source, 51 ... Band pass 55, coupler, 56, photodetector, 58, low frequency oscillator, 59, mixer, 61, optical filter, 62, optical receiver, 64, optical multiplexer, 65, optical demultiplexer, 66, optical fiber, 71 ... Periodic optical filter, 72 DROP filter, 73, 76 Fiber Bragg grating, 74, 75 circulator, 77, 78 coupler, 81 optical splitter, 82, 86, 142, 143, 152, 153 162, 163, 184, 185 optical switch, 83 optical narrow band filter, 84 optical receiver, 85 optical transmitter, 87 optical multiplexer, 41,151 ... Mach-Zehnder optical filter, 171,182 ... optical demultiplexer, 172,183 ... optical multiplexer, 192 ... baseband optical receiver, 193 ... optical receiver, 510 ... low pass filter, 511 ... DC bias source

Claims (12)

Signal generating means for generating an NRZI (Non Return to Zero Inverted) signal corresponding to the transmission information and an inverted NRZI signal whose logic is inverted; Drive signal generation means for generating the first and second drive signals;
A laser light source for generating continuous light,
An optical intensity modulator that splits the continuous light into two, shifts each split light individually according to the first and second drive signals, and then multiplexes to generate and output intensity modulated light;
Level control means for controlling a DC level of at least one of the first and second drive signals so as to convert an optical signal output from the intensity modulation optical device into an inverted RZ (Return to Zero) optical signal;
An optical transmission device comprising: a band limiting unit configured to limit a band of the intensity-modulated light output from the light intensity modulator. 2. The optical transmission apparatus according to claim 1, further comprising frequency control means for controlling the frequency of the continuous light so as to maximize the level of the intensity-modulated light output from the band limiting means. The frequency control means,
An oscillator for generating a monitor signal;
Modulating means for modulating the frequency of the continuous light by the monitor signal,
Branching means for partially branching the intensity-modulated light output from the band-limiting means,
3. The optical transmitter according to claim 2, wherein the frequency of the continuous light is controlled by using a synchronous detection output of a signal obtained from the split light split by the splitter and the monitor signal. The band limiting means splits the intensity-modulated light into two, delays one of the split lights by a phase amount longer than a half bit length of the intensity-modulated light and equal to or less than 1 bit length, and then combines the split light with the other split light. 4. The optical transmission device according to claim 1, wherein the optical transmission device is a Mach-Zehnder type optical interferometer. A plurality of optical transmitters each outputting intensity-modulated light having a different wavelength,
Multiplexing means for wavelength-multiplexing the intensity-modulated light output from the plurality of optical transmission devices to generate a wavelength-multiplexed light,
A transmission path for transmitting the wavelength multiplexed light,
Separating means for separating the wavelength multiplexed light transmitted via this transmission path into intensity modulated light of each wavelength,
First band limiting means for limiting the band of the intensity modulated light of each wavelength separated by the separating means;
A plurality of optical receiving devices each receiving the intensity-modulated light of each wavelength whose band is limited by the first band limiting unit,
Each of the plurality of optical transmission devices,
Signal generation means for generating an NRZI (Non Return to Zero Inverted) signal corresponding to the transmission information and an inverted NRZI signal whose logic is inverted,
Drive signal generating means for generating first and second drive signals by blocking high-frequency components of the NRZI signal and the inverted NRZI signal, respectively;
A laser light source for generating continuous light,
An optical intensity modulator that splits the continuous light into two, shifts each split light individually according to the first and second drive signals, and then multiplexes to generate and output intensity modulated light;
Level control means for controlling a DC level of at least one of the first and second drive signals so as to convert an optical signal output from the intensity modulation optical device into an inverted RZ (Return to Zero) optical signal;
An optical communication system comprising: a second band limiting unit that limits a band of the intensity modulated light output from the light intensity modulator. The optical communication system according to claim 5, wherein at least one of the first and second band limiting units is a Mach-Zehnder optical interferometer. A plurality of optical transmitters each outputting intensity-modulated light having a different wavelength,
Multiplexing means for wavelength-multiplexing the intensity-modulated light output from the plurality of optical transmission devices to generate a wavelength-multiplexed light,
A transmission path for transmitting the wavelength multiplexed light,
Add / drop means interposed in the transmission line and adding / dropping any one of the intensity modulated lights multiplexed to the wavelength multiplexed light,
Each of the plurality of optical transmission devices,
Signal generation means for generating an NRZI (Non Return to Zero Inverted) signal corresponding to the transmission information and an inverted NRZI signal whose logic is inverted,
Drive signal generating means for generating first and second drive signals by blocking high-frequency components of the NRZI signal and the inverted NRZI signal, respectively;
A laser light source for generating continuous light,
An optical intensity modulator that splits the continuous light into two, shifts each split light individually according to the first and second drive signals, and then multiplexes to generate and output intensity modulated light;
Level control means for controlling a DC level of at least one of the first and second drive signals so as to convert an optical signal output from the intensity modulation optical device into an inverted RZ (Return to Zero) optical signal;
An optical communication system comprising: a band limiting unit that limits a band of the intensity modulated light output from the light intensity modulator. The band limiting means splits the intensity-modulated light into two, delays one of the split lights by a phase amount longer than a half bit length of the intensity-modulated light and equal to or less than 1 bit length, and then combines the split light with the other split light. The optical communication system according to claim 7, wherein the optical communication system is a Mach-Zehnder type optical interferometer. The add / drop means includes:
A Bragg grating filter that selectively reflects only intensity-modulated light to be dropped and transmits other intensity-modulated light,
A first optical circulator that causes the wavelength multiplexed light arriving via the transmission path to enter the Bragg grating filter and guide the intensity modulated light reflected by the Bragg grating filter to a drop path;
A second optical circulator for guiding the intensity-modulated light to be added to the transmission path together with the intensity-modulated light transmitted by the Bragg grating filter,
8. The optical communication system according to claim 7, further comprising: a filter provided interposed in said transmission path to limit a band of each intensity-modulated light included in said wavelength multiplexed light. The optical communication system according to claim 9, wherein the filter unit is a periodic optical filter. The add / drop means includes:
Demultiplexing means for demultiplexing wavelength multiplexed light arriving via the transmission path into intensity modulated light of each wavelength,
Light path control means for selectively guiding the intensity-modulated light to be dropped among the intensity-modulated lights of the respective wavelengths demultiplexed by the demultiplexing means to a drop path, and allowing other intensity-modulated light to pass therethrough;
Filter means for limiting the band of the intensity-modulated light guided to the drop path by the light path control means,
8. The optical communication apparatus according to claim 7, further comprising a multiplexing unit that multiplexes the intensity-modulated light that has passed through the optical path control unit and the intensity-modulated light to be added and sends the multiplexed light to the transmission line. system. The filter means splits the intensity-modulated light into two, delays one of the split lights by a phase amount longer than a half bit length of the intensity-modulated light and equal to or less than 1 bit length, and then combines the split light with the other split light. The optical communication system according to claim 11, wherein the optical communication system is a Mach-Zehnder optical interferometer.

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