JP3900874B2 - Optical transmitter and optical modulation method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical modulation system, and more particularly, to an optical transmitter and an optical modulation method excellent in nonlinear tolerance in an optical fiber communication system.
[Prior art]
In high-speed optical fiber communication, waveform distortion due to a synergistic effect of chromatic dispersion and nonlinear effects (particularly, self-phase modulation and cross-phase modulation) of an optical fiber that is a transmission path becomes a factor that limits transmission speed and transmission distance. Therefore, in order to facilitate system design, a modulation method and a transmission method having excellent nonlinear tolerance are indispensable.
[0002]
As a modulation method having excellent nonlinear proof strength, for example, “NTT Miyamoto et al., November 1999, Electronics Letters, Vol. 35, No. 23, pages 2041 to 2042 (Y.Miyamoto et al.,“ 320 Gbit / s ( 8 × 40Gbit / s) WDM transmission over 367km with 120km repeater spacing using carrier-suppressed return-to-zero format ", Electronics letters, vol.35, no.23, pp.2041-2042, 1999)" As described above, a carrier-suppressed return-to-zero modulation scheme (CS-RZ modulation scheme) has been proposed.
[0003]
FIG. 9 is a block diagram showing an example of the CS-RZ modulation method. The configuration for realizing the CS-RZ modulation method shown in FIG. 9 includes a light source 1 that generates an optical signal having a carrier frequency, a clock modulation unit 13 that performs clock modulation on the optical signal generated by the light source 1, and an optical amplifier 2. A data modulation unit 10 that performs data modulation on an optical signal and an optical amplifier 3 are included.
[0004]
In the first light intensity modulator 14, CW (Continuous Wave) light emitted from the light source 1 generates a pair of electric clock signals (1/2 clock signals) having a half frequency of the bit rate of the data signal. In addition, clock modulation (intensity modulation) is performed by a push-pull (bipolar drive) operation. The phase between the pair of ½ clock signals differs by π (180 degrees) due to the phase shift 15. By performing the push-pull operation, it is possible to perform clock modulation such that the phase is inverted every bit slot without being given chirp (phase shift according to intensity accompanying intensity modulation) during intensity modulation. Become.
[0005]
FIG. 10A shows an optical waveform (eye pattern) after clock modulation (output T11 of the first intensity modulator 14), and FIG. 10B shows an optical spectrum. In FIG. 10 (and similar figures below), the diagram showing the eye pattern shows time on the horizontal axis and the amplitude on the vertical axis, and in the diagram showing the optical spectrum, the horizontal axis shows the frequency of the carrier wave emitted from the light source. The relative frequency, and the vertical axis indicates the output level. The feature of clock modulation in the CS-RZ modulation system is that the output level of the carrier wave is suppressed as the name suggests. In FIG. 10B, it should be noted that the carrier wave is suppressed in the optical spectrum, as can be seen from the fact that the output level of the carrier wave (the point where the relative frequency is zero) is zero.
[0006]
Next, the clock-modulated optical signal is optically amplified by the first optical amplifier 2 and then incident on the second optical intensity modulator 11. The second light intensity modulator 11 performs data modulation using an electrical data signal (non-return to zero signal). As shown in FIG. 9, the modulation operation may be a push-pull operation using a pair of data signals created by using the phase shift 12, or a single end (single-pole drive) using one data signal. ) Operation is acceptable. The optical CS-RZ signal generated by the above process is optically amplified by the second optical amplifier 3 and emitted to the transmission line.
[0007]
FIGS. 10C and 10D show the optical waveform and optical spectrum of the CS-RZ signal at the output T12 of the second intensity modulator 11. FIG. In order to clarify the difference from a normal RZ modulation signal, FIGS. 11 (a) and 11 (b) show the optical waveform and optical spectrum of the CS-RZ modulation system, and FIGS. 11 (c) and 11 (d) show the RZ modulation. The optical waveform and optical spectrum of the method are shown respectively.
[0008]
Since this CS-RZ signal is subjected to clock modulation by a 1/2 clock signal with the minimum value of the extinction curve (minimum value of transmission characteristics) in the first light intensity modulator 14 as a bias point, FIG. As shown in (b), the main lobe has only a frequency band of about 75% as compared with the normal RZ signal [FIG. 11 (d)]. For this reason, in wavelength division multiplexing, the CS-RZ signal has less optical spectrum overlap between adjacent channels than the RZ signal, is less susceptible to adjacent channel interference in the frequency domain, and uses an optical filter or the like. Thus, it is easy to demultiplex only a desired channel.
[0009]
In addition, according to the above-mentioned document, it is reported that the CS-RZ signal can be transmitted with a transmission power 1.4 dB higher than the RZ signal at the time of 1-channel transmission. It also has a higher proof strength than the RZ signal against waveform distortion due to the synergistic effect of self-phase modulation.
[0010]
[Problems to be solved by the invention]
The problem in the above prior art is that it is necessary to adjust the phase, amplitude, and bias between a pair of 1/2 clock signals used for clock modulation. The problem is described in more detail below.
[0011]
In the conventional technique, it is necessary to use a push-pull (bipolar drive) intensity modulator in order to perform chirpless clock modulation. For this reason, a pair of 1/2 clock signals are required, and further, phase adjustment in which the phase difference between the 1/2 clock signals is mutually π (180 degrees) and amplitude adjustment in which the amplitudes are equal are indispensable. Further, since it is necessary to set the bias voltage to the minimum value of the extinction curve (transmission characteristic) of the modulator, adjustment of the bias voltage is also indispensable.
[0012]
An object of the present invention is to provide a modulation scheme that is more excellent in non-linear tolerance than the CS-RZ modulation scheme. Another object of the present invention is to eliminate the need for phase adjustment, amplitude adjustment, and bias adjustment between a pair of 1/2 clock signals used for clock modulation. Furthermore, another object of the present invention is to enable CS-RZ modulation by appropriately selecting the phase modulation index of the phase modulator.
[0013]
[Means for Solving the Problems]
A first optical transmitter according to the present invention includes a CW (Continuous Wave) light source, a clock modulation unit, and a data modulation unit. In the optical transmitter in which data modulation is performed after clock modulation is performed, 1 A clock modulation unit that extracts only the carrier wave and the first double sideband wave after phase modulation of the CW light emitted from the light source using the / 2 clock signal is provided.
[0014]
It is another feature that CS-RZ modulation can be obtained by adjusting the phase modulation index with simpler adjustment than the conventional configuration.
[0015]
A second optical transmitter according to the present invention includes a clock modulation unit and a data modulation unit, and the CW light emitted from the light source is received in the optical transmitter that is subjected to data modulation after being subjected to clock modulation. After branching into two, intensity modulation is applied to one branched CW light so that the phase is inverted every bit slot using a 1/2 clock signal, and the phase and gain of the other branched CW light are also obtained. And a clock modulation unit for combining the two branched signals.
[0016]
A third optical transmitter according to the present invention includes a CW light, a clock modulation unit, and a data modulation unit. In the optical transmitter in which data modulation is performed after clock modulation is performed, a ½ clock signal And a clock modulation unit for extracting only the carrier wave and the first double sideband wave by performing intensity modulation so that the phase is inverted every 1 bit slot and chirp is applied. .
[0017]
In the present invention, after the phase modulation, only the carrier wave and the first double sideband are extracted, so that an optical spectrum width similar to or narrower than the conventional CS-RZ modulation method can be realized.
[0018]
In the present invention, since the phase modulation index can be varied, the amount of phase chirp between adjacent codes can be varied, and optimization can be performed to reduce interference between adjacent codes in the transmission path. Further, since the carrier wave can be suppressed by appropriately selecting the phase modulation index, it is possible to perform the conventional CS-RZ modulation with the configuration of the present invention.
[0019]
Further, in the present invention, since phase modulation is performed, it is not necessary to add a clock bias, and therefore, adjustment of the bias is unnecessary.
[0020]
Furthermore, in the present invention, since modulation is performed using only one ½ clock signal, phase adjustment and amplitude adjustment between a pair of ½ electric clock signals in the conventional CS-RZ modulation are unnecessary. It becomes.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described in detail with reference to the drawings. In each drawing described below, the same components as those in FIG.
[0022]
A first embodiment of an optical transmitter according to the present invention will be described with reference to FIG. In the optical transmitter shown in FIG. 1, CW (Continuous Wave) light emitted from the light source 1 is transmitted by an optical phase modulator 24 to an electrical clock signal (1/2 electrical frequency) having a frequency that is 1/2 the bit rate of the data signal. Phase modulation by a clock signal). A sine wave is preferable as the 1/2 clock signal. This is because it is easy to produce a signal.
[0023]
In the phase modulator 24, if the voltage required to shift the phase of light by π is Vπ, and the voltage amplitude of the 1/2 clock signal is Vclk, the phase modulation index can be expressed as π × Vclk / Vπ. The range of the modulation index is preferably 0.3π to 0.765π. This is because the non-linear proof stress is better in that range than CS-RZ.
[0024]
More specifically, about 0.5π is optimal. This is because at this time, the level of the carrier wave is equal to the level of the first sideband, and the nonlinear proof stress is the best. Also, when the phase modulation index is about 0.765π (the first zero in the first type Bessel function), the level of the carrier wave is suppressed (theoretically becomes 0), which is equivalent to CS-RZ modulation. A modulation result is obtained.
[0025]
2A and 2B, the light at the optical output T21 of the phase modulator 24 when phase modulation is performed with a phase modulation index of 0.5π using a 20 GHz sine wave as a 1/2 clock signal. Waveform (eye pattern) and optical spectrum are shown. The passband of the phase-modulated light is limited by the optical filter 25. As a pass band, it is preferable to pass only the carrier wave and the first double sideband wave. This is because the optical spectrum after data modulation can be narrowed and the dispersion tolerance can be improved.
[0026]
Further, as a pass band, only the carrier wave and the first and second double sideband waves may be allowed to pass. This is because although the dispersion tolerance is reduced, the nonlinear tolerance is improved and extraction by an optical filter is easy. Further, the third and subsequent double sideband waves may be passed. However, paying attention to the reception sensitivity, the reception sensitivity is the best when only the first double sideband wave is allowed to pass. When the pass band of the filter is expanded, the reception sensitivity converges to a state that does not pass through the filter. I will do it.
[0027]
Here, when data modulation is performed in a state where only the carrier wave and the first and second double sideband waves are extracted by the optical filter 25, and when clock modulation and data modulation are performed without using the optical filter 25 And compare. The main lobe occupancy band of the optical spectrum when data modulation is performed after passing through the optical filter 25 is the data bit rate frequency of the optical spectrum band after being subjected to clock modulation and data modulation without using the optical filter 25. This corresponds to limiting to 4 times or less.
[0028]
FIGS. 2C and 2D show the optical waveform (eye pattern) and optical spectrum at the optical output T22 of the optical filter 25 when only the carrier wave and the first double sideband wave are allowed to pass through. It can be seen that a 40 GHz clock signal is generated after photoelectric conversion by limiting the passband with an optical filter. The optical signal after passing through the optical filter 25 is amplified by the first optical amplifier 2 and then data-modulated by the optical intensity modulator 21.
[0029]
In the configuration shown in FIG. 1, the clock modulation unit 23 includes the optical phase modulator 24 and the optical filter 25. In FIG. 1, the configuration of the data modulation unit 20 is the same as that of the conventional data modulation unit (10 in FIG. 9) of CS-RZ modulation. That is, the light intensity modulator 21 and the phase shift 22 of the data modulation unit 20 have the same configuration as the light intensity modulator 11 and the phase shift 12 of FIG.
[0030]
The optical signal after data modulation is amplified by the second optical amplifier 3 and then transmitted to the transmission line. FIGS. 2E and 2F show the optical waveform (eye pattern) and optical spectrum at the output T23 of the optical intensity modulator 21. FIG.
[0031]
In the first embodiment of the present invention, the optical filter 25 is disposed in front of the first optical amplifier 2, but it may be disposed behind the first optical amplifier 2. You may arrange | position between steps.
[0032]
Further, as shown in FIG. 3, an optical filter 26 in place of the optical filter 25 can be provided between the optical intensity modulator 21 and the optical amplifier 3. However, in that case, it is desirable to adjust the pass band of the optical filter 26 so as to pass only the main lobe of the optical signal spectrum in the transmitter configuration of FIG.
[0033]
In the case of wavelength multiplexing transmission, the effect of suppressing crosstalk with an adjacent channel is also obtained by adjusting the optical filter to pass through the wavelength grid or less. In the case of the transmitter configuration in FIG. 3, the clock modulation unit 23 is arranged in front of the data modulation unit 20. However, the arrangement is not limited thereto, and the arrangement may be interchanged.
[0034]
In the data modulation unit 20, a push-pull operation Mach chain light intensity modulator has been described. However, the present invention is not limited to this, and a single-ended Mach chain light intensity modulator or an electroabsorption optical modulator may be used.
[0035]
Next, effects of the first exemplary embodiment of the present invention will be described. In the present embodiment, the transmission rate of data transmitted by the signal light is 40 Gb / s, and a NRZ (Non-return to zero) signal is used as the data modulation signal. As the 1/2 electric clock signal, a 20 GHz sine wave was used, and the phase modulation degree was 0.5π. The optical filter 25 is assumed to be an ideal rectangular filter having a pass band of 0.4 nm. The transmission path is 80 km × 5 relays, and each relay span is normally composed of 50 km of a single mode fiber (standard single mode fiber) and 30 km of a reverse dispersion fiber (reverse dispersion fiber).
[0036]
The dispersion value, dispersion slope, loss, core diameter, and nonlinear coefficient of the single mode fiber are +16 (ps / nm / km), +0.07 (ps / nm ² / km), 0.2 (dB / km), 10.4 (μm), 2.62 × 10 ⁻²⁰ (m ² / W) was used. Further, the dispersion value, dispersion slope, loss, core diameter, and nonlinear coefficient of the inverse dispersion fiber are −26.66 (ps / nm / km), −0.08 (ps / nm ² / km), 0. 3 (dB / km), 5.64 (μm), 2.6 × 10 ⁻²⁰ (m ² / W) were used.
[0037]
FIG. 4 is a diagram showing the degree of deterioration of the eye pattern after 400 km transmission with respect to the transmission power. When the 1 dB eye pattern deterioration is allowed, it is possible to transmit with a transmission power that is higher by about 2.5 dB than the conventional CS-RZ modulation method. In other words, the nonlinear proof stress is improved by 2.5 dB.
[0038]
Next, a second embodiment of the present invention will be described with reference to FIG. This embodiment is different from the first embodiment only in the clock modulation unit 33. In the clock modulation unit 33, the CW light emitted from the light source 1 is branched into two by the separation circuit 36. One CW light is phase-shifted by a phase shift 39 after the optical power is adjusted by the gain variable unit 38. The other CW light is subjected to clock modulation by a push-pull operation using a phase shift 35 in the light intensity modulator 34 and a 1/2 electric clock signal.
[0039]
Clock modulation is performed using the minimum value of the extinction curve in the light intensity modulator 34 as a bias point, which is the same as the clock modulation unit (13 in FIG. 9) described in the conventional CS-RZ modulation method. When the optical signals are multiplexed by the multiplexing circuit 37, the CW component is added to the clock component of the CS-RZ signal. The clock signal emitted from the clock modulation unit 33 is optically amplified by the first optical amplifier 2 and then subjected to data modulation by the data modulation unit 30. The optical signal after data modulation is amplified by the second optical amplifier 3 and then transmitted to the transmission line. The configuration of the data modulation unit 30 is the same as that of the conventional data modulation unit of CS-RZ modulation (10 in FIG. 9). That is, the light intensity modulator 31 and the phase shift 32 of the data modulation unit 30 have the same configuration as the light intensity modulator 11 and the phase shift 12 of FIG.
[0040]
FIG. 6 shows an output waveform and an optical spectrum of each part in the second embodiment. 6 (a) and 6 (b) show the optical output waveform (eye pattern) and optical spectrum of the light intensity modulator 34, (c) and (d) show the eye pattern and optical spectrum of the multiplexing circuit 37, and ( e) and (f) show the eye pattern and optical spectrum of the light intensity modulator 31.
[0041]
In the configuration of FIG. 5, the gain variable unit 38, the phase shift 39, and the multiplexing circuit 37 are converted into a CS-RZ clock modulation spectrum [optical spectrum of the optical output of the light intensity modulator 34 shown in FIG. 6B]. As shown in FIG. 6D (optical spectrum of the optical output of the multiplexing circuit 37), it is used to add the carrier waves.
[0042]
The gain variable unit 38 is used to vary the strength of the added carrier wave. This variation in strength has the same effect as varying the modulation index of the phase modulator 24 in the first embodiment. Have. Therefore, the amount of phase chirp between adjacent codes can be varied by changing the strength of the added carrier wave using the gain variable unit 38. Adjust the strength of the added carrier wave so that it is at the same level as the first double sideband wave of the CS-RZ clock modulation spectral component [the two bright line spectra in the middle of FIG. 6 (b)]. Is desirable. This is because the non-linear proof stress is excellent.
[0043]
The phase shift 39 is used to adjust the phase relationship between the added carrier wave and the CS-RZ clock modulation spectral component. The phase shift amount is preferably adjusted so that the phase of the added carrier is in phase with one of the first two sidebands of the CS-RZ clock modulation spectral component. Which component has the same phase depends on the dispersion map of the transmission path.
[0044]
The signal light generated in the present embodiment is substantially equivalent to the signal light generated in the first embodiment. For this reason, it has the nonlinear proof stress equivalent to 1st Embodiment.
[0045]
The gain variable unit 38 described above can be realized by, for example, an optical amplifier or an optical attenuator, or a combination of both. The phase shift 39 can be realized by a phase modulator and an optical delay device, for example.
[0046]
A narrow band optical filter may be inserted between the multiplexing circuit 37 and the optical amplifier 2, between the stages of the optical amplifier 2, or between the optical amplifier 2 and the optical intensity modulator 31. As an optical filter to be inserted, it is preferable to pass only the carrier wave and the first double sideband wave. This is because the bandwidth of the optical transmission waveform spectrum can be narrowed without degrading the non-linear tolerance, and is suitable for high-density wavelength division multiplexing transmission. Further, a narrow band optical filter may be inserted between the light intensity modulator 34 and the multiplexing circuit 37. In this case, it is preferable to pass only the first double sideband wave as the optical filter to be inserted.
[0047]
Next, a third embodiment of the present invention will be described with reference to FIG. This embodiment is different from the first embodiment only in the clock modulation unit 43. In the clock modulation unit 43, the CW light emitted from the light source 1 is intensity-modulated using a 1/2 electric clock signal in a light intensity modulator 44 having phase chirp (phase shift corresponding to intensity). Although the intensity modulation is performed using the minimum value of the extinction curve in the light intensity modulator 44 described above as a bias point, since the intensity modulator 44 has a phase chirp, it differs from the clock modulation unit 13 of CS-RZ modulation. The carrier wave is not suppressed.
[0048]
The passband of the intensity-modulated light is limited by the optical filter 45. As a pass band, it is preferable to pass only the carrier wave and the first double sideband wave. The clock signal emitted from the clock modulation unit 43 is optically amplified by the first optical amplifier 2 and then subjected to data modulation by the data modulation unit 40. The optical signal after data modulation is amplified by the second optical amplifier 3 and then transmitted to the transmission line.
[0049]
The configuration of the data modulation unit 40 is the same as the data modulation unit (10 in FIG. 9) of the conventional CS-RZ modulation. That is, the light intensity modulator 41 and the phase shift 42 of the data modulation unit 40 have the same configuration as the light intensity modulator 11 and the phase shift 12 of FIG. FIG. 8 shows the output waveform and optical spectrum of each part in the third embodiment. 8A and 8B show the optical output waveform (eye pattern) and optical spectrum of the light intensity modulator 44, FIGS. 8C and 8D show the eye pattern and optical spectrum of the optical filter 45, and ( e) and (f) show the eye pattern and optical spectrum of the light intensity modulator 41, respectively.
[0050]
The signal light generated in the present embodiment is substantially equivalent to the signal light generated in the first embodiment. For this reason, it has the nonlinear proof stress equivalent to 1st Embodiment.
[0051]
While the embodiments have been described with reference to the light modulation system of the present invention, the present invention can be implemented in various other modes. First, in the optical modulation system and the optical transmitter of the present invention, only the case of 1-channel transmission has been described. However, the system is also effective for multi-channel transmission.
[0052]
Further, it goes without saying that any circuit or component can be applied to the various circuits and components in the present invention as long as their functions are satisfied. Further, although an optical amplifier is used for the compensation of the modulator loss, it is irrelevant to the signal modulation itself and can be omitted.
[0053]
Furthermore, in the phase modulation unit of the above-described embodiment, phase modulation is performed by an electric clock signal (1/2 electric clock signal) having a half frequency of the bit rate of the data signal. The phase modulation may be performed by an electric clock signal having a frequency of 1/2 ⁿ (n is an integer). Further, the processing in the data modulation unit may be performed before the processing in the phase modulation unit.
[0054]
【The invention's effect】
The first effect is that the nonlinear tolerance of the optical transmission signal is remarkably improved. According to the result of the numerical calculation shown in FIG. 4, it has a non-linear proof stress 2.5 dB higher than the conventional configuration. This is because in the present invention, the level of each signal (carrier wave, sideband) in the frequency domain can be made closer.
[0055]
The second effect is that an optical transmission spectrum width comparable to or narrower than that of the conventional CS-RZ modulation method can be realized. This is because only the carrier wave and the first double sideband are extracted after the phase modulation.
[0056]
The third effect is that it is not necessary to adjust the clock bias. The reason is that it is not necessary to add a clock bias because phase modulation is performed.
[0057]
The fourth effect is that the phase adjustment between the 1/2 electric clock signals and the amplitude adjustment are unnecessary. This is because the phase modulation is performed using only one 1/2 electric clock signal to generate the clock signal.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first embodiment of the present invention.
FIG. 2 is a diagram showing a waveform and an optical spectrum in each component of the first embodiment of the present invention.
FIG. 3 is a diagram showing a modification of the configuration shown in FIG.
FIG. 4 is a diagram showing an effect in the first embodiment of the present invention.
FIG. 5 is a diagram showing a second embodiment of the present invention.
FIG. 6 is a diagram showing a waveform and an optical spectrum in each component of the second embodiment of the present invention.
FIG. 7 is a diagram showing a third embodiment of the present invention.
FIG. 8 is a diagram showing a waveform and an optical spectrum in each component of the third embodiment of the present invention.
FIG. 9 is a diagram showing a conventional example.
FIG. 10 is a diagram showing a waveform and an optical spectrum in each component of a conventional example.
FIG. 11 is a diagram illustrating waveforms and optical spectra of the CS-RZ modulation method and the RZ modulation method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Light source 2, 3 Optical amplifier 20, 30, 40 Data modulation part 21, 31 Light intensity modulator 22, 32, 35,
39, 42 Phase shift 23, 33, 43 Clock modulator 24 Optical phase modulator 25, 45 Optical filter 34, 41 Optical intensity modulator 36 Separation circuit 37 Multiplex circuit 38 Gain variable 44 Optical intensity modulator with chirp

Claims (10)

A CW (Continuous Wave) light source that emits an optical signal having a carrier frequency;
A clock modulation unit that performs phase modulation using only one electric clock signal having a frequency that is ½ of a data bit rate with respect to an optical signal having a carrier frequency emitted from the CW light source;
In the optical transmitter having a data modulation unit that performs data modulation using the data bit rate to the phase-modulated optical signal NRZ (Non-return to zero) electric signal by the clock modulation unit,
Light that limits the optical signal spectral bandwidth after phase modulation by the clock modulation unit between the clock modulation unit and the data modulation unit to a frequency that is four times the data bit rate frequency or less. An optical transmitter comprising: a filter, wherein a phase modulation index in the clock modulation unit is approximately in a range of 0.3π to 0.765π . It said optical filter is an optical transmitter according to claim 1, wherein the benzalkonium to extract the carrier and the first sidebands. It said optical filter is an optical transmitter according to claim 1, wherein the benzalkonium to extract the carrier and the first sideband wave and the second sideband. A CW (Continuous Wave) light source that emits an optical signal having a carrier frequency;
A data modulation unit that performs data modulation using an NRZ (Non-return to zero) electrical signal of a data bit rate to an optical signal having a carrier frequency emitted from the CW light source;
An optical transmitter having a clock modulation unit that performs phase modulation using only one electric clock signal having a frequency that is ½ of the data bit rate for the optical signal that is data-modulated by the data modulation unit ;
After the clock modulation unit, phase modulation is performed by the clock modulation unit.
An optical filter for limiting the subsequent optical signal spectral bandwidth to a frequency equal to or lower than four times the data bit rate frequency is disposed, and the phase modulation index in the clock modulation unit is approximately in the range of 0.3π to 0.765π. An optical transmitter characterized by that. A CW (Continuous Wave) light source that emits an optical signal having a carrier frequency;
Separating means for bifurcating the CW light emitted from the CW light source, and an electric clock signal having a frequency that is ½ of the data bit rate for one of the CW light branched by the separating means Means for performing intensity modulation with the minimum value of the extinction curve in the optical modulator as a bias point, means for varying the gain and phase of the other of the CW light branched by the separating means, A clock modulation unit including an optical signal that has undergone intensity modulation and a multiplexing unit that combines the optical signal with variable gain and phase;
An optical transmitter having a data modulation unit for performing data modulation using an NRZ (Non-return to zero) electrical signal of the data bit rate to the optical signal multiplexed by the multiplexing means ;
The means for varying the gain and the phase includes a gain variable range less than or equal to the first sideband of the optical signal spectral component subjected to the intensity modulation, and a first optical signal spectral component subjected to the intensity modulation. The optical transmitter is characterized in that the phase is adjusted so as to be substantially in phase with one of the sideband waves of . A process of performing phase modulation using only one electric clock signal having a frequency that is ½ of the data bit rate with respect to an optical signal having a carrier frequency emitted from a CW (Continuous Wave) light source;
A method of performing data modulation using an NRZ (Non-return to zero) electrical signal of the data bit rate on the optical signal subjected to the phase modulation ;
The optical signal spectral bandwidth after being subjected to the phase modulation by the optical filter is limited to a frequency that is four times the data bit rate frequency, and the phase modulation index of the phase modulation is approximately 0.3π to 0. An optical modulation method having a range of 765π . It said optical filter is an optical modulation method according to claim 6, wherein the benzalkonium to extract the carrier and the first sidebands. The optical filter carrier and the first sideband wave and the second optical modulation method according to claim 6, wherein the benzalkonium to extract and sideband. A process of performing data modulation using an NRZ (Non-return to zero) electrical signal of a data bit rate to an optical signal of a carrier frequency emitted from a CW (Continuous Wave) light source;
A method of performing phase modulation on the data-modulated optical signal using only one electric clock signal having a frequency that is ½ of the data bit rate ;
The optical signal spectral bandwidth after the phase modulation is limited by an optical filter to a frequency that is four times the data bit rate frequency, and the phase modulation index of the phase modulation is approximately 0.3π to 0 A light modulation method characterized by being in the range of .765π . A step of 2 branches C W (Continuous Wave) CW light emitted from the light source, using the electric clock signal having a half to become the frequency of the data bit rate for one of the branched optical CW Te, comprising the steps of subjecting the intensity modulation bias point a minimum value of the extinction curve in a light modulator, a process for varying the gain and phase relative to the other of the front SL min Toki been CW light, the intensity A process of combining the modulated optical signal and the optical signal with variable gain and phase;
An optical modulation method for performing data modulation using a NRZ (Non-return to zero) electric signal of the data bit rate before Kigo wave optical signal,
The process of changing the gain and phase includes a gain variable range that is less than or equal to the first sideband of the optical signal spectral component subjected to the intensity modulation, and the first optical signal spectral component subjected to the intensity modulation. A light modulation method characterized in that the phase is adjusted so as to be substantially in phase with one of the sideband waves .

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