JP3447664B2 - Optical transmitter and optical transmitter control method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter and an optical transmitter control method for generating an optical signal to be transmitted to a transmission medium such as an optical fiber having wavelength dispersion that deteriorates transmission quality.

[0002]

2. Description of the Related Art In an optical transmission system, deterioration of transmission quality caused by waveform deterioration due to wavelength dispersion of an optical fiber as a transmission medium becomes a problem. This occurs because the group velocity dispersion of the optical fiber acts on the bandwidth of the optical signal to increase its optical pulse width and cause interference in adjacent time slots.

FIG. 1 shows a conventional example disclosed in Japanese Patent Laid-Open No. 10-79705. In this conventional example, in order to suppress the deterioration due to group velocity dispersion, an optical modulator of the type in which a pre-chirp is added to an optical signal in a transmitter to cancel the group velocity dispersion is proposed. This is to suppress the waveform deterioration at the receiving end in the optical transmitter by adding in advance a frequency chirp of an amount that substantially matches the group velocity dispersion of the optical fiber. In FIG. 1, the light source 101
The continuous light generated by is divided by the clock generation means 102 into two optical clocks. The clock generating means 102 is, for example, a Mach-Zehnder type optical modulator,
It is generated by driving this with a sinusoidal electric signal. At this time, the prechirp means 106 controls the bias voltage and the like to the optical modulator to control the value and sign of the chirp. When an electro-absorption optical modulator is used as the clock generation means 102, the pre-chirp means 106 adjusts the amount of chirp by controlling its bias voltage. Then, the optical clock whose chirp is controlled is encoded by the first and second data modulators 103 and 104 which are the optical modulators of the next stage,
By multiplexing with the optical multiplexer 105, a desired optical signal is obtained.

However, in the optical modulator, since the amount of frequency chirp that cancels the group velocity dispersion is added in advance,
The bandwidth of the optical signal spectrum is increased. The proof stress against group velocity dispersion has an inverse square relationship with the optical spectrum band, and the proof stress decreases as the band increases. Therefore, the method of adding pre-chirp as in the conventional example has a low dispersion resistance and impairs stable operation of the transmission system. That is, the transmission quality is deteriorated by a slight difference in group velocity dispersion.

Further, when an electro-absorption type optical modulator is used as the clock generation means 102, there is a problem that the bias voltage dependency of the extinction ratio is not linear as shown in the conventional example. That is, when the bias voltage is changed to control the frequency chirp, the pulse width of the clock also changes, so the band in the optical spectrum also changes, and the tolerance against the dispersion also changes. Since the chirp and the pulse width cannot be changed independently, it is difficult to stably set the desired chirp amount.

Further, in the clock generation means 102,
The optical power loss for cutting out the clock light from the continuous light by the gate by the modulator is large, and the two encoded clock lights are extinguished with each other due to the interference effect of the light at the time of multiplexing in the optical multiplexer 105. Therefore, the loss further increases. This causes a decrease in the optical power at the output end of the optical modulator and a decrease in the S / N ratio during transmission.

In FIG. 2, the bit rate is 80 Gbit / s.
3 is a graph showing the relationship between the chromatic dispersion and the dispersion tolerance of the optical clock of FIG. 3 as a parameter of the relative optical phase of two encoded optical clocks at the time of multiplexing in the optical multiplexer. As a result, when the bit rate is different, the absolute value of the dispersion value on the horizontal axis (| D | (PS / nm) changes, but the relative relationship between the duty ratio and the penalty amount due to the relative optical phase difference is Is the same. Here, in-phase means out-of-p when the relative optical phase difference is 0 or an integral multiple of 2π.
hase means middle-p when the phase difference is an odd multiple of π
The hase refers to the case where the phase difference is π / 2 or an odd multiple thereof. As shown here, when the duty ratio is excessively increased, the reception sensitivity is significantly deteriorated. In other words, it is necessary to optimally control the duty ratio in order to achieve both high dispersion tolerance and low deterioration in reception sensitivity. However, in the conventional example, the optical clock is generated by placing the driving point in the linear part of the modulator with a sine wave having a frequency equal to the bit rate before multiplexing. Must be changed, and the deterioration of the extinction ratio becomes a problem, and the signal waveform is deteriorated due to the interference effect at the time of multiplexing.

Further, Japanese Patent Laid-Open No. 10-79705 discloses an example of driving with a rectangular wave, but since the optical spectrum is excessively widened by driving with a rectangular wave, the dispersion resistance becomes small. Will end up. For this reason,
It is considered that the influence of the group velocity dispersion of the optical fiber on the transmission distance becomes more severe. In addition, since driving with a sine wave can only realize an excessively wide duty ratio, a device having a division number of 3 or more as disclosed in Japanese Patent Laid-Open No. 10-79705 has a large effect of interference between adjacent optical clocks.
Since the light extinguishes each other, it becomes difficult to generate a practically effective optical signal.

Further, in the conventional example disclosed in Japanese Patent Laid-Open No. 9-261207, when the outputs of the optical modulators 111 and 112 are multiplexed by the optical multiplexer 113 as shown in FIG. The low frequency from the low frequency oscillator 115 is superimposed on the signal by phase modulation, and the optical branching unit 114 and the optical phase detection control unit 116 partially monitor the optical signal after the multiplexing, so that the low frequency signal By controlling the phase by the optical phase controller 110 so that the intensity of the intensity modulation component is minimized, the relative optical phase difference is automatically maintained.
However, since the minimum value control becomes dull in terms of sensitivity, it is inevitably necessary to superimpose a low frequency with a relatively large amplitude, as shown in JP-A-9-261207. However, as described above, since the relative optical phase difference is intentionally shifted from π, the relative optical phase difference in which the dispersion proof is significantly deteriorated approaches the condition of π / 2. That is, the dispersion yield strength deteriorates due to the control itself.

[0010]

As described above, an optical clock is generated by a sine wave or the like, and at this time, the chirp of the optical clock is controlled to suppress the deterioration of transmission quality due to group velocity dispersion. In the conventional optical transmitter, firstly, there is a problem that an excessive optical spectrum band is occupied in order to add the chirp, and the tolerance against the group velocity dispersion becomes small.

Further, it is possible to control the chirp small and arbitrarily by driving the push-pull type Mach-Zehnder type optical modulator in both phases, but there is a problem that the duty ratio of the optical clock cannot be controlled. is there.

Furthermore, when the electroabsorption modulator is used, the chirp amount itself is large, and the chirp parameter and the duty ratio are not independent parameters, which makes it difficult to add a desired chirp, and the chirp parameter is added. In addition, in order to control the duty ratio arbitrarily, there is a problem that the loss fluctuates greatly, the S / N ratio fluctuates during communication, and the transmission quality fluctuates.

The present invention has been made in view of the above,
The purpose is to provide a stable optical transmitter and optical transmitter control method that has high resistance to group velocity dispersion of optical fiber, little deterioration in reception sensitivity, and is not easily affected by group velocity dispersion when the network scale is expanded. To provide.

[0014]

In order to achieve the above object, the present invention can generate an optical clock pulse synchronized with a signal bit rate with a constant duty ratio and variably set the duty ratio of the optical clock pulse. A light source section and a relative optical phase difference between the optical clock pulses of adjacent time slots is set to an odd multiple of π, and the optical clock pulse is encoded by an electrical signal synchronized with the optical clock pulse. And an optical transmitter having a section.

Further, in the present invention, the light source section is of a mode-locked laser type capable of generating an optical clock pulse synchronized with a signal bit rate at a constant duty ratio and variably setting the duty ratio of the optical clock pulse. It is characterized by having a light source.

Further, in the present invention, the encoding section includes a demultiplexing unit that demultiplexes the optical clock pulse, and an electrical signal that synchronizes each of the output signals demultiplexed by the demultiplexing unit with the optical clock pulse. A coding delay means for coding with a signal and delaying another output signal with respect to one of the output signals demultiplexed by the demultiplexing means, and an output signal of this coding delay means are combined. And a delay provided to the other output signal by the encoding delaying means, where n is the number of multiplexing in the multiplexing means and m is an arbitrary integer The time width of the time slot is (k / n) + m (k = 1, 2, ..., (n-1)) times, and the relative optical phase difference is an odd multiple of π.

In the present invention, the encoding delay means encodes each of the output signals demultiplexed by the demultiplexing means with an electric signal synchronized with the optical clock pulse, and the encoding means. It is characterized by comprising delay means for delaying at least a part of the output signal coded by the coding means.

In the present invention, the coding delay means delays at least a part of the output signal demultiplexed by the demultiplexing means, the output signal delayed by the delay means, and If there is any, it is characterized by comprising an encoding means for encoding each of the output signals not delayed by the delay means by an electric signal synchronized with the optical clock pulse.

Further, in the present invention, an optical spectrum extracting means for branching a part of the signals multiplexed by the multiplexing means and extracting a central component of the optical spectrum or a line spectrum adjacent to the central component is provided. And a feedback means for generating a feedback signal based on the center component or the line spectrum extracted by the optical spectrum extracting means.

Further, in the present invention, the coding delay means has a variable delay means capable of varying the delay amount, and the feedback means has the maximum center component or line spectrum extracted by the optical spectrum extraction means. Alternatively, it has a delay control means for generating a feedback signal for controlling the variable delay means so as to be minimized.

Further, the present invention comprises a light source control means for generating a feedback signal for controlling the light source section so that the central component or line spectrum extracted by the optical spectrum extraction means becomes maximum or minimum. And

Further, according to the present invention, the part branched in the optical spectrum extracting means is a phase-inverted component of the signal multiplexed in the multiplexing means.

Further, in the present invention, the light source section comprises a continuous light generating means for generating continuous light and a push-pull type Mach-Zehnder modulator driven at zero point, and the coding section has the push-pull type Mach-Zehnder modulation. And a coder for coding the output of this push-pull type Mach-Zehnder modulator.

Further, in the present invention, the push-pull type Mach-Zehnder modulator is driven by a clock signal having a repetition frequency which is a half of the repetition frequency of the optical clock pulse, and the continuous light generated by the continuous light generation means is used to generate the continuous light. An optical clock pulse is generated, and the relative optical phase difference between the optical clock pulses of adjacent time slots is set to an odd multiple of π.

Further, in the present invention, the push-pull type Mach-Zehnder modulator is characterized in that the duty ratio of the optical clock pulse can be variably set by controlling the drive amplitude.

Further, in the present invention, the push-pull type Mach-Zehnder modulator is characterized in that the waveform of the optical clock pulse is optimized to have a high dispersion tolerance by controlling the drive waveform.

Further, in the present invention, the coding section is composed of an LN modulator having at least one set of push-pull type modulation electrodes, and is provided at a corresponding position on an upper arm and a lower arm of the LN modulator. It is characterized in that the signals inputted to the push-pull type modulation electrodes are inverted to each other and are driven at zero point.

Also, in the present invention, the LN modulator has at least two sets of push-pull type modulation electrodes, and multiplexes signals input to each set of the at least two sets of push-pull type modulation electrodes. It is characterized by

Further, in the present invention, the light source section is provided on the input side of the CW light source for generating continuous light and the push-pull type modulation electrode of the LN modulator, and the duty ratio for electrically controlling the duty ratio is provided. It is characterized by having a controller.

Further, the present invention is characterized by including an optical band limiting means for removing an unnecessary harmonic component contained in the optical signal generated in the optical transmitter.

Further, in the present invention, the optical band limiting means has a transmission band having a characteristic of blocking a component other than a necessary optical signal band so as to remove an unnecessary harmonic component. .

Further, the present invention is an optical transmitter which is provided in parallel and is set so as to output optical signals of different optical wavelengths, each optical transmitter being synchronized with a signal bit rate. An optical clock pulse is generated with a constant duty ratio, and the relative optical phase difference between the optical clock pulses in adjacent time slots and the light source section in which the duty ratio of the optical clock pulse can be variably set is set to an odd multiple of π. And a coding section for coding the optical clock pulse by an electric signal synchronized with the optical clock pulse, and an optical signal of a different optical wavelength output from each of the plurality of optical transmitters. And an optical wavelength multiplexing means for multiplexing and wavelength-multiplexing and outputting.

Further, according to the present invention, the optical wavelength multiplexing means has a periodic transmission band having a characteristic of blocking a component other than a required optical signal band so as to remove an unnecessary harmonic component. And

Further, according to the present invention, the duty ratio of the optical clock pulse is variably set, and the duty ratio of the optical clock pulse is set to a value that reduces interference between the pulses, and the duty ratio is set to Generating the optical clock pulse that is constant and synchronized with the signal bit rate,
An optical transmission characterized in that the relative optical phase difference between the optical clock pulses of adjacent time slots is set to an odd multiple of π, and the optical clock pulse is encoded by an electric signal synchronized with the optical clock pulse. A method for controlling a vessel.

Therefore, the present invention provides an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate, and an optical means having an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. It is a transmitter, and the gist is that the duty ratio of the optical clock pulse is variable.

As a result, since the duty ratio of the optical clock pulse is variable, the duty ratio can be set to an arbitrary value, and both high dispersion tolerance and small deterioration in reception sensitivity can be achieved.

The present invention further includes an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate, and an optical means having an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. The gist of the present invention is to have a means for setting the relative optical phase difference to an odd multiple of π or in the vicinity thereof for each pulse of the optical clock pulse.

As a result, the relative optical phase difference is set to an odd multiple of π or in the vicinity thereof for each optical clock pulse, so that a high dispersion tolerance can be stably maintained.

The present invention further includes an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate, and an optical means having an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. A transmitter is characterized in that the repetition frequency of the optical clock pulse is twice the frequency of an electric signal for driving the optical pulse generating means.

Further, the present invention is an optical clock pulse generating means for generating an optical clock pulse which is synchronized with a signal bit rate and whose duty ratio can be varied, a demultiplexing means for demultiplexing the optical clock pulse, and this demultiplexing means. Encoding means for encoding each of the output signals demultiplexed by the means by an electric signal synchronized with the optical clock pulse, and other than at least one of the output signals encoded by the encoding means A delay means for delaying the output signal; and a multiplexing means for multiplexing the other output signal delayed by the delay means and the at least one output signal not delayed by the delay means, The delay in the delay means is k / n (k: 1, ..., (n-1)) where the number of multiplexing in the multiplexing means is an integer n with respect to the time slot for encoding.
Alternatively, it is summarized as a value obtained by adding an integer multiple of a time slot to this and its vicinity, and the relative optical phase difference is an odd multiple of π or its vicinity.

The present invention also relates to an optical clock pulse generating means for generating an optical clock pulse whose duty ratio is variable in synchronization with the signal bit rate, a demultiplexing means for demultiplexing the optical clock pulse, and this demultiplexing means. Delay means for delaying other output signals except at least one of the output signals demultiplexed by the means, and the other output signal delayed by the delay means and not delayed by the delay means Encoding means for encoding each of the at least one output signal by an electric signal synchronized with the optical clock pulse; and multiplexing means for multiplexing the output signals encoded by the encoding means, The delay in the delay means is k / n (k: 1, ..., (n−), where the number of multiplexing in the multiplexing means is an integer n with respect to the encoding time slot.
1)) or a value obtained by adding an integer multiple of a time slot thereto and its vicinity, and the relative optical phase difference is an odd multiple of π or its vicinity.

Further, in the present invention, the delay means is a variable delay means capable of varying the delay amount, branches a part of the signals multiplexed by the multiplexing means, and adjoins the center of the optical spectrum. The gist of the present invention is to further include optical spectrum extraction means for extracting the line spectrum, and delay control means for controlling the variable delay means so that the line spectrum extracted by the optical spectrum extraction means becomes maximum.

Further, in the present invention, the delay means is a variable delay means capable of varying the delay amount, branches a part of the signals multiplexed by the multiplexing means, and outputs the central component of the optical spectrum thereof. The gist of the present invention is to further include an optical spectrum extracting means for extracting and a delay control means for controlling the variable delay means so that the center component of the optical spectrum extracted by the optical spectrum extracting means is minimized.

Further, in the present invention, the delay means is a variable delay means capable of varying the delay amount, and branches a part of the signals multiplexed by the multiplexing means and adjoins the center of the optical spectrum thereof. It is further characterized by further comprising: an optical spectrum extracting means for extracting the linear spectrum, and a light source control means for controlling the optical clock pulse generating means so that the linear spectrum extracted by the optical spectrum extracting means becomes maximum. .

Further, the gist of the present invention is that the part is a phase-inverted component of a signal multiplexed by the multiplexing means.

The present invention further includes an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate, and an optical means having an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. In the transmitter, the gist is that the encoding means has a signal multiplexing function.

As a result, since the encoding means has a signal multiplexing function, it operates with a signal having a bit rate that is half or less than the optical signal to be generated. Therefore, one multiplexing circuit can be omitted, which is economical. Can be achieved.

Further, the present invention is an optical transmitter having a continuous light generating means for generating continuous light and an encoding means for encoding the continuous light, wherein each pulse of an optical signal generated by the encoding means. The gist is to have means for setting the relative optical phase difference to an odd multiple of π or in the vicinity thereof.

Thus, the relative optical phase difference is controlled to be an odd multiple of π or in the vicinity thereof for each pulse of the optical signal, so that high dispersion resistance can be stably maintained.

Further, the present invention is an optical transmitter having continuous light generating means for generating continuous light and coding means for coding the continuous light, wherein the coding means has a signal multiplexing function. That is the summary.

Since the encoding means has a signal multiplexing function, it operates with a signal having a bit rate that is half or less than the optical signal to be generated. Therefore, one multiplexing circuit can be omitted, which is economical. Can be achieved.

Further, the gist of the present invention is to have optical band limiting means for removing unnecessary harmonic components contained in the optical signal generated in the optical transmitter.

As a result, since unnecessary harmonic components contained in the optical signal generated in the optical transmitter are removed by the optical band limiting means, the generated optical signal occupies an excessive band than the required band. As a result, it is possible to prevent a decrease in band utilization efficiency and improve the band utilization efficiency.

In the present invention, a plurality of the above-mentioned optical transmitters are provided in parallel, and the plurality of optical transmitters are set so as to output optical signals having different optical wavelengths. The characteristics of blocking the components other than the required optical signal band so that unnecessary harmonic components can be removed, as well as multiplexing and wavelength-multiplexing the optical signals of different optical wavelengths output from each The gist of the present invention is to have optical multiplexing means having the periodic transmission band.

As a result, since unnecessary harmonic components are removed, unnecessary harmonic components can be suppressed, and the generated optical signal occupies an excessive band than the required band, resulting in a decrease in band utilization efficiency. It is possible to prevent this, improve band utilization efficiency, or prevent deterioration of transmission characteristics due to crosstalk between optical signals of different wavelengths.

Further, in the present invention, the optical band limiting means is
It is desirable to have an optical multiplexing means having a transmission band having a characteristic of blocking a component other than a required optical signal band so as to remove unnecessary harmonic components, while multiplexing a plurality of optical signals of different optical wavelengths. Use as a summary.

[0057]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

First, the main features of the optical transmitter and the optical transmitter control method of the present invention will be briefly described.

FIG. 4 shows the basic configuration of the optical transmitter of the present invention. The optical transmitter comprises a light source section 1000 for generating an optical clock pulse synchronized with a signal bit rate, and an encoding section 2000 for encoding an optical clock pulse with an electric signal synchronized with the optical clock pulse.
The light source section 1000 has a duty ratio variable setting function 1001 capable of variably setting the duty ratio of an optical clock pulse, and a constant duty ratio optical pulse generation function of generating an optical clock pulse synchronized with a signal bit rate at a constant duty ratio. 1002, and the encoding section 2000 has an alternating phasing function 2001 for setting a relative optical phase difference between optical clock pulses of adjacent time slots to an odd multiple of π, and an alternating phased optical clock. It has an encoding function 2002 for encoding a pulse with an electric signal synchronized with an optical clock pulse.

In the optical transmitter control method of the present invention, the optical transmitter having such a configuration is controlled as follows according to the procedure shown in FIG.

First, by using the duty ratio variable setting function 1001 of the light source section 1000 described above, the duty ratio of the optical clock pulse is set to a value that reduces the interference between the pulses, so that the interference effect is suppressed. (Step S1).

Next, using the constant duty ratio optical pulse generation function 1002 of the light source section 1000 described above,
By generating an optical clock pulse synchronized with the signal bit rate while keeping the duty ratio of the optical clock pulse thus set constant, the pulse width is stabilized (step S2).

Next, the encoding section 2000 described above.
By setting the relative optical phase difference between the optical clock pulses of the adjacent time slots to an odd multiple of π using the alternating phasing function 2001, the dispersion tolerance can be kept stable (step S3).

Then, the encoding section 200 described above is used.
The encoding function 2002 of 0 is used to encode the alternating phased optical clock pulse with an electrical signal synchronized with the optical clock pulse (step S4).

As a result, since the duty ratio of the optical clock pulse is variable, the duty ratio can be set to an appropriate value that reduces the interference between the pulses, so that both high dispersion resistance and small deterioration in reception sensitivity can be achieved. In addition to being settable, the relative optical phase difference between the optical clock pulses of adjacent time slots is set to an odd multiple of π, so that it is also possible to stably maintain a high dispersion tolerance. Further, since the pulse width is constant, the amplitude after equalization is stable, and the eye opening deterioration as seen in the conventional optical transmitter does not occur at all. Further, since it becomes possible to optimize the waveform of the optical clock pulse to a waveform having a higher dispersion tolerance, the dispersion tolerance itself can be increased to about twice that of the conventional one. Therefore,
It is possible to provide a stable optical transmitter and an optical transmitter control method that have high resistance to group velocity dispersion of optical fibers, little deterioration in reception sensitivity, and are not easily affected by group velocity dispersion when the network scale is expanded. Therefore, it is suitable for constructing a larger scale network.

In the description below, for example, the relative optical phase difference between the optical clock pulses of adjacent time slots is set to an odd multiple of π or in the vicinity thereof. Exactly π
This is because it is considered that the present invention is substantially effective not only in the idealized case of setting an odd number of times, but also in the case of allowing a certain amount of error to this. In this case, the dispersion yield strength deteriorates as compared with the idealized case, but if the error of the dispersion yield strength is within 20% of the idealized case, it is considered to be practically within the allowable range. The neighborhood means an error within such an allowable range.

Embodiments relating to a specific configuration example of the optical transmitter of FIG. 4 will be described in detail below with reference to FIGS.

FIG. 6 shows the configuration of the optical transmitter according to the first embodiment of the present invention. In the optical transmitter shown in FIG. 6, the duty ratio can be variably set, and the duty ratio variable optical pulse generation means 1 for generating an optical clock pulse synchronized with the signal bit rate, the duty ratio variable optical pulse generation means 1 can be used. Optical demultiplexing means 3 for demultiplexing an optical clock pulse generated from the optical demultiplexing means, first and second encoding means 5, 7 for encoding each output signal demultiplexed by the optical demultiplexing means 3, respectively. An optical delay device 9 for delaying the output signal encoded by the first encoding means 5 among the first and second encoding means 5, 7.
And an optical multiplexing means 11 for multiplexing the output signal delayed by the optical delay device 9 and the output signal from the second encoding means 7. Here, the duty ratio variable optical pulse generation means 1 constitutes the light source section 1000 described above, and the optical demultiplexing means 3, the first and second encoding means 5 and 7, the optical delay device 9 and the optical multiplexing means 11 are provided. It constitutes the coding section 2000 described above.

In the optical transmitter configured as described above,
The optical pulse generated by the duty ratio variable optical pulse generation means 1 is split into two by the optical demultiplexing means 3. The demultiplexing ratio at this time is 1: 1. The demultiplexed light pulses are coded independently by the first and second coding means 5 and 7. This coding bit rate is equal to the repetition frequency of the optical pulse. Of these demultiplexed and coded optical pulses, one component is delayed by the optical delay device 9 by an odd multiple of half the repetition period of the optical pulse or a time in the vicinity thereof. Further, this delay amount is set so that, when multiplexed by the optical multiplexing means 11, the relative optical phase difference between the two demultiplexed and encoded components is an odd multiple of π or its vicinity.

Here, the variable duty ratio optical pulse generating means 1 is a modulator-integrated mode-locked laser diode (K. Sato et al., Electron. Lett., Vol. 34, No. 20, pp. 194).
4-1946, 1998), fiber ring type mode-locked laser, supercontinuum light source, and other laser light sources (herein, these are collectively referred to as mode-locked laser type light source). Alternatively, the duty ratio may be controlled by limiting the band by combining the mode-locked laser type light source and the optical bandpass filter.
The optical demultiplexing means 3 is a Y branch or a directional coupler.
The coding means 5 and 7 are Mach-Zehnder interferometer type modulators or electroabsorption type modulators using Y branch or directional coupler. The optical delay device 9 is a delay line or the like. The optical multiplexer 11 is a Y branch or a directional coupler. It should be noted that it is convenient for the above optical component to be manufactured on an LN or PLC substrate.

Next, the optical transmitter of FIG. 6 will be described more specifically with reference to FIG. The circuit configuration shown in the upper part of FIG. 7 shows a specific configuration of the optical transmitter shown in FIG.
The lower part of FIG. 7 shows the waveform of each part of the optical transmitter shown in FIG. 7.

In the optical transmitter shown in the upper part of FIG. 7, the duty ratio variable optical pulse generation means 1 is composed of a semiconductor mode-locked laser 21, and the optical demultiplexing means 3 is a Y branch 2.
3, the first and second encoding means 5 and 7 are the first and second Mach-Zehnder interferometer type (MZ) modulators 2.
5, 27, the optical multiplexing means 11 is a directional coupler 3
It is composed of 1.

In the optical transmitters of FIGS. 6 and 7 having such a configuration, the variable duty ratio optical pulse generating means 1 constituted by the semiconductor mode-locked laser 21 causes the optical pulse of a desired duty ratio without frequency chirp. To generate. The generated optical pulse is incident on the optical demultiplexing means 3 constituted by the Y branch 23, and is split into two at a branching ratio of 1: 1. The divided optical pulses are coded by coding means 5 and 7 composed of Mach-Zehnder interferometer type (MZ) modulators 25 and 27 driven by NRZ code, respectively, and become RZ code. The NRZ code used for driving is synchronized with the divided optical pulse.

One component of the encoded optical pulse is delayed by the optical delay unit 9 by an odd multiple of half the repetition period of the optical pulse or in the vicinity thereof as shown by the shaded waveform in FIG. If a heater is built into this optical delay device 9, the amount of delay can be controlled with high precision below the wavelength of light by the thermo-optical effect by controlling the power supplied to that portion. Precise delay control utilizing this thermo-optic effect allows the directional coupler 3 between the two divided optical pulses to be separated.
The relative optical phase difference in the optical multiplexing means 11 constituted by 1 is adjusted to be an odd multiple of π or its vicinity.

By doing so, the optical pulse output from the optical multiplexing means 11 is characterized in that the relative optical phase difference between the optical pulses of the adjacent time slots is always π. This control realizes the out-of-phase condition shown in Fig. 2 and has high dispersion resistance. Also, the deterioration of reception sensitivity is small.

FIG. 8 shows the configuration of an optical transmitter according to the second embodiment of the present invention. The optical transmitter shown in FIG. 8 has a coding section 2000 as compared with the embodiment shown in FIG.
2 uses an optical variable delay device 35 capable of varying the delay time instead of the optical delay device 9 and branches a part of the signal multiplexed by the optical multiplexing device 11 from the monitor port of the optical multiplexing device 11. From the optical spectrum selecting means 37 for selectively extracting only the central component of the optical spectrum, the photoelectric converter 39 for converting the central component of the optical spectrum extracted by the optical spectrum selecting means 37 into an electric signal, and the photoelectric converter 39. The difference lies in that a delay control circuit 41 is provided which receives an electric signal and controls the variable optical delay device 35 so that the center component of the optical spectrum is maximized based on the electric signal.

In the optical transmitter configured as described above,
The optical pulse generated by the duty ratio variable optical pulse generation means 1 is split into two by the optical demultiplexing means 3. The demultiplexing ratio at this time is 1: 1. The demultiplexed optical pulses are encoded independently by the first and second encoding means 5 and 7. This coding bit rate is equal to the optical pulse repetition frequency. Of the demultiplexed and coded optical pulses, one component is delayed by the variable optical delay device 35 by a time that is an odd multiple of half the repetition period of the optical pulse or a time in the vicinity thereof. Further, this delay amount is set so that, when multiplexed by the optical multiplexing means 11, the relative optical phase difference between the two demultiplexed and encoded components is an odd multiple of π or its vicinity. Optical spectrum selection means 3
Only the central component of the optical spectrum is taken out by 7 and converted into an electric signal by the photoelectric converter 39. This is input to the delay control circuit 41, and the optical variable delay device 35 is controlled so that this optical spectrum center component becomes maximum.

Here, the optical spectrum selecting means 37 is a Fabry-Perot interferometer type optical filter, a dielectric multilayer film type optical filter, an arrayed waveguide type optical filter, a Mach-Zehnder interferometer type optical filter, or the like. The photoelectric converter 39 is a photodiode, a photomultiplier tube, a CCD, or the like. The delay control circuit 41 is a combination of an operational amplifier, an A / D converter, a calculator and the like.

Next, the optical transmitter of FIG. 8 will be described more specifically with reference to FIG. The circuit configuration shown in the upper part of FIG. 9 shows a specific configuration of the optical transmitter shown in FIG. 8, and the lower part of FIG. 9 shows the optical signals at the monitor port and the output port of the optical multiplexing means 11 of the optical transmitter shown in FIG. The optical spectrum which took out a part is shown.

In the optical transmitter shown in the upper part of FIG. 9, the duty ratio variable optical pulse generation means 1 of FIG. 8 is composed of a semiconductor mode-locked laser 21, and the optical demultiplexing means 3 is of Y branch, as in FIG. 23, the first and second coding means 5 and 7 are composed of first and second Mach-Zehnder interferometer type (MZ) modulators 25 and 27, and the optical multiplexing means 1
1 is composed of a directional coupler 45. In this embodiment, a Mach-Zehnder interferometer in which the directional coupler 45 is combined is used as the optical multiplexer 11.

FIG. 10 shows an example of an optical spectrum when part of the light at the monitor port and the output port of the optical multiplexer 11 of the optical transmitter shown in FIG. 9 is extracted.
In particular, in FIGS. 10A, 10B, and 10C, the relative optical phase difference between the optical pulses demultiplexed by the optical demultiplexing unit 3 is 0,
The case of π and π / 2 is shown. In order to have a high dispersion tolerance, this phase difference needs to be an odd multiple of π.
The spectrum at that time has a shape in which the central component is suppressed as shown in FIG. In this Mach-Zehnder interferometer, an optical pulse whose optical phase is inverted is output to a port that forms a pair of output ports. For this reason, in the monitor port, the central component becomes the largest in this state. Therefore, in order to maintain the phase difference at the output port at an odd multiple of π or in the vicinity thereof, the central component of the monitor port may be maximized. Therefore, the central component is cut out by the optical spectrum selection means 37, replaced by an electric signal by the photoelectric converter 39, and the optical variable delay device 35 is controlled so as to maximize the signal strength. With this feedback control, even if the phase of the two divided optical pulses fluctuates, a high dispersion tolerance can always be realized.

By the way, in the conventional example disclosed in Japanese Patent Laid-Open No. 9-261207, the fluctuation of the optical power due to the extinction of the overlapping portion of the optical pulses to be multiplexed is detected and controlled to the minimum. However, this variation has a low S / N ratio because the difference is whether the overlapping portion of the optical pulse is extinguished or constructive, and thus the control accuracy is poor. In addition, there is a problem that the sensitivity varies due to the difference in duty ratio.

On the other hand, in the present embodiment, only the central component is cut out by the optical bandpass filter,
Since this is controlled to the maximum, the S / N ratio is high and the control can be performed accurately. In addition, the change in sensitivity due to the difference in duty ratio is very small.

Other basic operations are the same as those in the first embodiment.

In the above operation, the feedback control is performed so that the central component of the monitor port is maximized. However, a part of the optical signal component from the output port is monitored to minimize the central component of the optical spectrum. You may control.
Further, a part of the optical signal component from the output port or the monitor port may be monitored, and the line spectrum adjacent to the central component of the optical spectrum may be controlled to be maximum or minimum.

FIG. 11 shows the configuration of an optical transmitter according to the third embodiment of the present invention. The optical transmitter shown in FIG.
Compared with the embodiment shown in FIG.
00, a light source control circuit 46 for controlling the variable duty ratio optical pulse generation means 1 is provided in place of the delay control circuit 41 for controlling the variable optical delay device 35, and the variable optical delay device 35 has the same optical delay as that of the first embodiment. 8 is the same as that of the embodiment of FIG.

In the optical transmitter shown in FIG. 11, the optical pulse generated by the variable duty ratio optical pulse generating means 1 is demultiplexed into two by the optical demultiplexing means 3. The demultiplexing ratio at this time is 1: 1. The demultiplexed optical pulses are encoded independently by the first and second encoding means 5 and 7. This coding bit rate is equal to the optical pulse repetition frequency. Of these demultiplexed and coded optical pulses, one component is delayed by the optical delay device 9 by an odd multiple of half the repetition period of the optical pulse or a time in the vicinity thereof. Further, this delay amount is set so that, when multiplexed by the optical multiplexing means 11, the relative optical phase difference between the two demultiplexed and encoded components is an odd multiple of π or its vicinity. Only the central component of the optical spectrum is taken out by the optical spectrum selecting means 37 and converted into an electric signal by the photoelectric converter 39. This is the light source control circuit 4
6 and controls the wavelength of the variable duty ratio optical pulse generation means 1 so that the center component of the optical spectrum becomes maximum.

Here, the optical spectrum selection means 37 is a Fabry-Perot interferometer type optical filter, a dielectric multilayer film type optical filter, an arrayed waveguide type optical filter, a Mach-Zehnder interferometer type optical filter, etc., and the second embodiment Use a narrower band than that used in the form. This is because the detection of the wavelength fluctuation requires higher accuracy. The photoelectric converter 39 is a photodiode, photomultiplier tube, CC
D etc. The light source control circuit 46 is a combination of an operational amplifier, an A / D converter, a calculator, and the like.

The operation of the third embodiment shown in FIG. 11 will be described in more detail. In this embodiment, a Mach-Zehnder interferometer in which a directional coupler is combined is used as the optical multiplexing means 11.

In the embodiment shown in FIG. 11, as in the second embodiment, the central component of the optical spectrum of the monitor port or the output port or the line spectrum adjacent to the central component may be maximized or minimized. The reason is shown in the second embodiment. Therefore, the optical spectrum selecting means 37 having a narrower band than that used in the second embodiment.
The central component is cut out by and is converted into an electric signal by the photoelectric converter 39, and the duty ratio variable optical pulse generation means 1 is controlled so as to maximize the signal strength. By this feedback control, it becomes possible to keep the oscillation wavelength of the variable duty ratio optical pulse generation means 1 always constant.

It has been reported that the group velocity dispersion of an optical fiber varies depending on the wavelength at 0.07 ps / nm ² / km (KSKim et al., J. Appl. Phys., Vol. 73, N.
o.5, pp. 2069-2074, 1993). Therefore, when the wavelength of the light source fluctuates, the value of the group velocity dispersion that the optical pulse undergoes changes, which may cause deterioration of the waveform of the optical pulse and deterioration of the transmission quality. Deterioration can be prevented in advance, leading to improved system reliability.

Other basic operations are the same as in the first and second embodiments.

FIG. 12 shows the configuration of an optical transmitter according to the fourth embodiment of the present invention. The optical transmitter shown in FIG.
Coding section 2000 as compared to the first embodiment.
In place of the optical demultiplexing means 3, the optical demultiplexing means 51 for demultiplexing the optical clock pulse from the variable duty ratio optical pulse generation means 1 into four is used, and the first and second encoding means 5 and 7 are used. Instead of the four first to fourth encoding means 53 to
56 is used to encode each of the four output signals of the optical demultiplexing means 51, and the optical delay device 9 is replaced by first to third optical signals.
Of the first to fourth coding means 53 to 56 using the three optical delay devices 57 to 59 of FIG.
.. to 55. Only the output signals from the three encoding means of .about.55 are delayed, and the delayed output signals from the fourth encoding means 56 are input to the optical multiplexing means 61 for multiplexing. Except that the other configurations and operations are the same as those of the embodiment of FIG.

In the optical transmitter of the fourth embodiment having such a configuration, the optical pulse generated by the variable duty ratio optical pulse generating means 1 is converted into 4 by the optical demultiplexing means 51.
It is split into two. The demultiplexing ratio at this time is equal (1: 1: 1:
1). The demultiplexed light pulses are coded by the coding means 53 to 56 independently. This coding bit rate is equal to the optical pulse repetition frequency.

As shown in FIG. 13, of at least three components of these demultiplexed and encoded optical pulses, one of the time widths of one encoding time slot is set.
/ 4, 2/4, 3/4 times, or a time obtained by adding an integer multiple of the time width of one time slot of encoding to the optical delay devices 57 to 59. In addition, more generally, the delay in the optical delay unit is the number of multiplexing in the optical multiplexing means is n,
When an arbitrary integer is m, (k / n) + m (k = 1, 2, ..., (n−) of the time width of one time slot for encoding.
1)) times.

Further, this delay amount is set so that, when multiplexed by the optical multiplexing means 61, the relative optical phase difference between the four demultiplexed and encoded components is an odd multiple of π or its vicinity. To do.

Only the central component of the optical spectrum is taken out by the optical spectrum selecting means 37 and is taken out by the photoelectric converter 39
Is converted into an electric signal. This is input to the light source control circuit 46, and the wavelength of the variable duty ratio optical pulse generation means 1 is controlled so that this optical spectrum component becomes maximum.

Here, the light demultiplexing means 51 uses three two-way branching light demultiplexing means. In this method, the first unit is divided into two, and the remaining two units are each divided into two to be divided into four. The optical demultiplexing means 51 may use a 4-branch star coupler, a PLC circuit, or an LN waveguide. The optical multiplexer 61 may use these circuits correspondingly. The optical spectrum selection means 37 is a Fabry-Perot interferometer type optical filter, a dielectric multilayer film type optical filter, an arrayed waveguide type optical filter, a Mach-Zehnder interferometer type optical filter, or the like, which is different from that used in the second embodiment. Also uses a narrow band. This is because the detection of the wavelength fluctuation requires higher accuracy. Photoelectric converter 3
Reference numeral 9 is a photodiode, a photomultiplier tube, a CCD or the like. The light source control circuit 45 is a combination of an operational amplifier, an A / D converter, a calculator and the like.

Next, the operation of the embodiment shown in FIG. 12 will be described in more detail. In this embodiment, a directional coupler on the PLC is used as the optical multiplexing means 61. Also in the present embodiment, it is shown in the second embodiment that the central component of the optical spectrum of the monitor port or the output port or the line spectrum adjacent to the central component may be maximized or minimized as in the second embodiment. It depends on the reason. Therefore, the second
The central component of the monitor port is cut out by the optical spectrum selection means 37 having a narrower band than that used in the above embodiment, converted into an electric signal by the photoelectric converter 39, and the duty ratio is varied so as to maximize the signal strength. The optical pulse generation means 1 is controlled. By this feedback control,
It is possible to always keep the oscillation wavelength of the variable duty ratio optical pulse generation means 1 constant. The effectiveness of this control is the same as that of the third embodiment described above.

Other basic operations are the same as those in the above-mentioned embodiments.

It should be noted that, by modifying the configuration of FIG. 12, as shown in FIG. 14, four first to fourth encoding means 53 to 56 are provided.
Of the output signals demultiplexed by the optical demultiplexing means 51 by exchanging the positional relationship of the first to third optical delay devices 57 to 59. After delaying each of the three, the first to fourth encoding means 53-56 respectively output the delayed output signals of the optical delay devices 57-59 and the undelayed output signals of the optical demultiplexing means 51. May be encoded.

Further, in the above embodiment, the control of the variable optical delay device 35 and the control of the variable duty ratio optical pulse generating means 1 are described separately, but these may be used in combination or may be selectively used as needed. Therefore, an optical transmitter which is more stable and has a high resistance to group velocity dispersion may be configured.

FIG. 15 shows the configuration of an optical transmitter according to the fifth embodiment of the present invention. The optical transmitter shown in FIG.
Compared with the first embodiment shown in FIG. 2, the optical band limiting means 71 is provided after the optical multiplexing means 11 in the encoding section 2000 of the optical transmitter, and the other configurations and operations are the same. Therefore, the same components are given the same reference numerals.

The optical band limiting means 71 has a transmission band for cutting off components other than the required optical signal band, thereby removing unnecessary harmonic components and thereby improving band utilization efficiency. It is a thing.

Instead of providing the optical band limiting means 71 on the output side of the optical multiplexing means 11 as in the optical transmitter shown in FIG. 15, the optical band limiting means 71 is used as shown in FIG. It is also possible to configure the optical transmitter when it is provided on the output side of the pulse generation means 1. As described above, even if the optical band limiting means 71 is provided on the output side of the duty ratio variable optical pulse generation means 1, unnecessary harmonic components can be suppressed as in the optical transmitter of FIG.

The operation of the optical transmitter shown in FIG. 15 or 16 will be described with reference to FIG. 17 is shown in FIG.
5 or 16 schematically shows an example of an optical spectrum of an optical signal generated in the optical transmitter shown in FIG.

As shown in FIG. 17 (a), unnecessary harmonic components in the duty ratio variable optical pulse generation means 1 or the encoding means 5 constituting the optical transmitter need to be indicated as the optical signal bit rate. Appears on both sides of the signal band. As shown in FIG. 17B, the optical band limiting unit 71 has a transmission band characteristic of blocking the above-mentioned unnecessary harmonic component, that is, a component other than the required optical signal band. Therefore, by providing the optical band limiting means 71 having such a transmission band characteristic, unnecessary harmonic components appearing on both outer sides can be suppressed as shown in FIG. 17 (c). As a result, it is possible to prevent a decrease in band utilization efficiency due to the generated optical signal occupying an excessive band than the required band.

In each of the above embodiments, the pulse width of the variable duty ratio optical pulse generation means is controlled so that the case where the number of demultiplexed waves is 3 or more can be easily dealt with.

Further, in each of the above embodiments, one of the demultiplexed light pulses is not delayed, but all the demultiplexed light pulses are delayed so that the relative light pulses between the respective light pulses are delayed. The delay amount may be set as described above.

FIG. 18 shows the configuration of an optical transmitter according to the sixth embodiment of the present invention. The optical transmitter shown in FIG.
The light source 65, the harmonic wave pulse generation means 67, and the encoding means 69 are included. Here, the CW light source 65 and the harmonic wave pulse generation means 67 configure the light source section 1000 described above, and the harmonic wave pulse generation means 67 and the encoding means 69 configure the coding section 2000 described above.

In the present embodiment, the continuous light generated by the CW light source 65 is modulated by the harmonic pulse generation means 67 into a clock having a repetition frequency twice the drive frequency supplied to the harmonic pulse generation means 67. This harmonic clock is encoded by the encoding means 69. This coding bit rate is equal to the repetition frequency of the optical clock. That is, the drive frequency of the clock supplied to the harmonic pulse generation means 67 is half the bit rate. Here, the CW light source 65 is a laser light source such as DFB-LD, FP-LD. The harmonic pulse generation means 67 is a push-pull type Mach-Zehnder modulator or the like on the LN substrate. The encoding means 69 is a push-pull type Mach-Zehnder modulator or the like on the LN substrate.

Next, the operation of the optical transmitter according to the sixth embodiment shown in FIG. 18 will be described with reference to FIG. In the present embodiment, as shown in FIG. 19, the first and second push-pull type Mach-Zehnder modulators 67a and 69a formed in series on the LN substrate 66 are used as the harmonic pulse generator 67 and the encoder 69. To use.

Continuous light generated by the CW light source 65 is modulated by the first Mach-Zehnder modulator 67a into alternating phase optical clock pulses as shown in FIG. In this alternating-phase optical clock pulse, the relative optical phase difference between the optical pulses of adjacent time slots is an odd multiple of π or its vicinity, as in the case of the above-described embodiments. At this time, the repetition frequency of the electric signal for driving this may be half the frequency of the alternating phase optical clock pulse, as shown in FIG. Here, the first Mach-Zehnder modulator 67a is driven by a zero-point drive in which the driving point is at or near the point where the transmittance becomes minimum during non-modulation, thereby realizing such alternating phase. .

The alternating phase optical clock pulse is chirp-free encoded by the second Mach-Zehnder modulator 69a. The bit rate obtained by this embodiment is 4
An example of an optical waveform of 0 Gbit / s and its optical spectrum is shown in FIG.
It shows in 0. When the relative optical phase difference is 0 or π,
Since it has an optical spectrum equivalent to that in the case of the optical multiplexing shown in each of the above-described embodiments, it can be seen that the out-of-phase condition having high dispersion tolerance is realized also in this embodiment.

Here, the optical waveform having a relative optical phase difference of π is realized at the point where the transmittance of the first Mach-Zehnder modulator 67a becomes minimum when no modulation is performed.

In the configuration shown in FIG. 19, the duty ratio of the alternating phase optical clock pulse is changed from 1/2 to 2 / by controlling the drive amplitude to the first Mach-Zehnder modulator 67a.
It can be controlled to 3. Also, the waveform of the alternating phase clock can be controlled by changing the drive waveform, and the waveform can be optimized to have a higher dispersion tolerance. By these controls, it is possible to minimize the deterioration of the eye opening due to the interference effect between adjacent pulses at the time of zero dispersion.

Although both the Mach-Zehnder modulators are driven in both phases in the configuration of FIG. 19, the second Mach-Zehnder modulator 69 which constitutes the encoding means 69 as shown in FIG.
Similar coding can be performed for a using single-sided driving. However, in this case, a small amount of chirp remains in the encoding, but the influence on the basic operation is small. For easier analogy, the Mach-Zehnder modulator 67a that constitutes the overtone pulse generation means 67 can also use the one-sided drive. At this time, the driving point is set to a point at which the transmittance is the minimum when there is no modulation or the vicinity thereof, as in the above-described embodiment. In this case as well, a slight chirp remains in the generated overtone pulse, but the effect on the basic operation is small.

Further, in the present embodiment, the case where the alternating phase optical clock pulse is first generated and then encoded is described, but the encoding may be performed first and then the alternating phase clock may be used. In that case, the positional relationship between the above-mentioned harmonic pulse generation means 67 and the encoding means 69 is reversed.

Further, in the present embodiment, the harmonic pulse generating means and the encoding means are integrated on the same substrate,
You may comprise these on separate substrates.

22 and 23 show an optical transmitter in the case where the optical band limiting means 71 shown in FIGS. 15 and 16 is provided to the optical transmitter of the sixth embodiment shown in FIG. 18, respectively. 22 shows a configuration in which an optical band limiting means 71 is provided on the output side of the encoding means 69, and FIG. 23 shows an optical band limiting means 71 on the output side of the harmonic pulse generation means 67.
Is provided. By providing the optical band limiting means 71 in this way, as in the case of the fifth embodiment described above, unnecessary harmonic components can be removed, and the optical signal generated by this means has an excessive band than the required band. By occupying, it is possible to prevent a decrease in bandwidth utilization efficiency.

In each of the above embodiments, since the duty ratio of the optical clock pulse is variable, the duty ratio is set to 0.5.
By setting an appropriate value in the vicinity, both high dispersion resistance and small deterioration in reception sensitivity can be achieved. Further, since the relative optical phase difference between the optical clock pulses of adjacent time slots is set to an odd multiple of π or in the vicinity thereof, high dispersion tolerance can be stably maintained.

FIG. 24 shows the structure of an optical transmitter according to the seventh embodiment of the present invention. The optical transmitter shown in FIG. 24 includes a pulse light source 151 forming a light source section 1000 for generating an optical clock pulse synchronized with a signal bit rate,
An LN modulator 153 and four wide band amplifiers 155a that form an encoding section 2000 that encodes an optical clock pulse with an electrical signal synchronized with the optical clock pulse.
.About.155d. LN modulator 153
Are two sets of push-pull type modulation electrodes 153a to 153d.
And the outputs of the broadband amplifiers 155a to 155d are connected to the modulation electrodes 153a to 153d, respectively.

The pulse light source 151 generates an optical pulse having a duty ratio of 0.5 or less with respect to the time width of one time slot in the generated optical signal. Further, the electric signal input to the present optical transmitter is an RZ code having a duty ratio in the optical signal to be generated of 0.5 or in the vicinity thereof. The input RZ code corresponds to the codes CH1, CH2 and these codes. CH1, C with overline
The wideband amplifiers 155a to 155a, as indicated by the inverted code of H2 (hereinafter, the inverted code with this overline is described by adding a dash "'" like CH1' and CH2 ').
Is input to the wide band amplifier 155a.
˜155d, the voltage is amplified to a voltage sufficient to drive the LN modulator 153, that is, near Vπ / 2. Then, the optical pulse from the pulse light source 151 is encoded into an alternating phase RZ code by the electric signals amplified by the broadband amplifiers 155a to 155d.

The pulse light source 151 is an optical pulse generator such as a semiconductor mode-locked laser, a ring laser, a modulator integrated light source, or a combination of a CW light source and an external modulator such as an LN modulator. Collectively referred to as mode-locked laser light source). Further, the LN modulator 1 having two sets of push-pull type modulation electrodes 153a to 153d.
53 is a Mach-Zehnder interferometer type optical intensity modulator,
The Mach-Zehnder interferometer type optical intensity modulator has two modulation electrodes provided in series on each of the upper and lower arms, and controls the phase of light independently. The wideband amplifiers 155a to 155d amplify the amplitude of the electric signal input to the LN modulator 153 to a value necessary to drive the LN modulator 153.

In the optical transmitter configured as described above, the optical pulse output from the pulse light source 151 with a duty ratio of 0.5 or less is LN modulated by having two sets of push-pull type modulation electrodes 153a to 153d. The LN modulator 153 branches the signal to a branch ratio of 1: 1 and the upper and lower arms of the LN modulator 153 independently add phase shifts. Here, FIG.
As shown in FIG. 6, the LN modulator 153 is driven by two-phase driving in which the signals input to the modulation electrodes provided at the corresponding positions on the upper arm and the lower arm of the LN modulator 153 are mutually inverted signals. ing.

The electric signals CH1 and CH2 '(inversion of CH2) applied to the upper arm of the LN modulator 153 have drive amplitudes shown in FIGS. 25 (a) and 25 (b), respectively. These electric signals CH1 and CH2 'are shown in FIG.
As shown in (a) and (b), the duty ratio is 0.5 or in the vicinity thereof, and the amplitude is wide band amplifier 155.
It is set to Vπ / 2 by a and 155c. Furthermore, the temporal relative phases of these two electric signals are set so as to be displaced by 1/2 time slot as shown in FIG. With this setting, a phase shift as shown in FIG. 25C is added as a total phase shift in the upper arm of the Mach-Zehnder interferometer type optical intensity modulator. That is, a maximum π phase shift is added.

The electric signals CH1 'and CH2 applied to the other lower arm of the LN modulator 153 are shown in FIG.
As shown in (a) and (b), it is the reverse logic of the electric signals CH1 and CH2 ′ applied to the upper arm. That is,
With respect to the lower arm of the Mach-Zehnder interferometer type optical intensity modulator, the drive amplitude of the inverted logic of CH1 is applied to the first modulation electrode and the drive amplitude of CH2 is applied to the next modulation electrode. . Therefore, the phase shift applied in the lower arm is the reverse of that in the upper arm as shown in FIG.

Finally, when the phase shifts of the two arms, the upper arm and the lower arm, are summed up, the relative phase difference has a total amplitude of 2π as shown in FIG. 27 (a). The bias voltage is adjusted so that the driving point shown in FIG. 27A coincides with the extinction voltage of the Mach-Zehnder interferometer type optical intensity modulator. That is, as shown in FIG.
Zero point drive is performed so that the direction of drive amplitude is inverted by the electric signals CH1 and CH2. By doing so, the phase of the Mach-Zehnder interferometer type optical intensity modulator is inverted at the extinction point, so that the generated optical signal has alternating phase inversion as shown in FIG. 27 (b). Becomes

The optical transmitter of this embodiment is the LN modulator 15
3 of the two sets of push-pull type modulation electrodes 153a
˜153d has a multiplexing function of being able to multiplex two electric signals by applying the electric signals CH1 and CH2 via the wide band amplifiers 155a to 155d, and at least half or less of the optical signal to be generated. Since it operates with a signal of a bit rate, one multiplexing circuit can be omitted, and an inexpensive amplifier for driving the modulator can be used.

If the multiplexing function is not required, the optical transmitter of this embodiment may be constructed by using an LN modulator having a set of push-pull type modulation electrodes.

FIG. 29 shows the structure of an optical transmitter according to the eighth embodiment of the present invention. The optical transmitter shown in FIG.
In the optical transmitter shown in FIG. 4, a CW light source 161 that generates stable continuous light is used instead of the pulse light source 151, and duty ratio controllers 157a to 157d capable of electrically controlling the duty ratio are connected to wide band amplifiers 155a to 155a.
Only the points provided on the input side of 5d are different, the other configurations and operations are the same, and the same components are denoted by the same reference numerals. In this case, the light source section 100
0 is the CW light source 161 and the duty ratio controller 15
7a to 157d.

The CW light source 161 is, for example, a DFB laser,
It is a light source such as a wavelength tunable laser.

Duty ratio controllers 157a-15
7d is provided to control the duty ratio of the optical pulse to be generated, and the duty ratio can be electrically controlled as shown in FIG. This makes it possible to generate an optical pulse with a stable duty ratio while suppressing interference between the optical pulses without using a pulse light source with a variable duty ratio. Such a duty ratio controller can be realized by a combination of digital ICs and the like. When the duty ratio is controlled in order to suppress the interference between the optical pulses, the above-mentioned FIG. 25, FIG. 26, and FIG. 27 are as shown in FIG. 31, FIG. 32, and FIG. 33, respectively. Basically, except that
6, the same as FIG. 27.

Further, each duty ratio controller 15
By providing a waveform shaping means such as an electric low pass filter on the output side of 7a to 157d, the drive waveform may be smoothed and the generation of higher harmonics may be prevented.

Also in the optical transmitter of the eighth embodiment configured as described above, as in the case of FIG. 24, the continuous light from the CW light source 161 is amplified by the wide band amplifiers 155a to 155d, and the electric signal of the RZ code. The signals are modulated by the LN modulator 153 by CH1 and CH2, encoded into an alternating phase RZ code, multiplexed, and output.

FIG. 34 shows the structure of an optical transmitter according to the ninth embodiment of the present invention. The optical transmitter shown in FIG.
LN modulator 15 having two sets of push-pull type modulation electrodes 153a to 153d, as compared with the optical transmitter shown in FIG.
4 sets of push-pull type modulation electrodes 163a-
The only difference is that the LN modulator 163 having 163h is used, and eight wide band amplifiers 155a to 155h are used by combining the wide band amplifiers 155a to 155d with four sets of push-pull type modulation electrodes 163a to 163h. Yes, other configurations and operations are the same.

The optical transmitter of the ninth embodiment configured as described above has the same basic operation as the optical transmitter of the seventh embodiment shown in FIG. 24, but four sets of push-pull modulations are used. The electric signals applied to the arms of the LN modulator 163 having the electrodes 163a to 163h are arranged to be inverted between the modulation electrodes facing each other. Then, the optical pulse from the pulse light source 151 is converted into the broadband amplifiers 155a to 155.
RZ code electric signals CH1, CH2, C amplified by h
It is modulated by the LN modulator 163 by H3 and CH4, encoded into the alternating phase RZ, multiplexed and output.

FIG. 35 shows the structure of an optical transmitter according to the tenth embodiment of the present invention. Compared with the optical transmitter shown in FIG. 30, the optical transmitter shown in FIG.
CW light source 161 for generating stable continuous light is used instead of the above, and duty ratio controllers 157a to 157h capable of electrically controlling the duty ratio are connected to the wide band amplifier 155.
Only the points provided on the input sides of a to 155h are different, and the other configurations and operations are the same.

In the optical transmitter of the tenth embodiment configured as described above, the CW light source 161 is also used as in the case of FIG.
RZ code electric signals CH1, CH2, CH3, C obtained by amplifying the continuous light from the broadband amplifiers 155a to 155h.
It is modulated by the LN modulator 163 by H4, encoded into an alternating phase RZ code, multiplexed and output.

FIG. 36 shows the structure of the optical transmitter in the case where the optical band limiting means 71 shown in FIGS. 15 and 16 is provided in the optical transmitter of the seventh embodiment shown in FIG. The optical band limiting means 71 is provided on the output side of the LN modulator 153. By providing the optical band limiting means 71 in this way, as in the case of the fifth embodiment described above, unnecessary harmonic components can be removed, and the optical signal generated by this means has an excessive band than the required band. By occupying, it is possible to prevent a decrease in bandwidth utilization efficiency.

Also in the seventh to tenth embodiments described above, the optical pulse output from the LN modulators 153 and 163 always has a relative optical phase difference between optical clock pulses of adjacent time slots of π. , Has high dispersion proof stress. Further, in the ninth and tenth embodiments, the number of multiplexing is 4, but this can be easily expanded when the number of channels is even.

FIG. 37 shows the structure of the optical transmitter according to the eleventh embodiment of the present invention. The optical transmitter shown in FIG. 37 is provided with a plurality of the optical transmitters shown in FIG. 6 in parallel, and the plurality of optical transmitters are set to output optical signals having different optical wavelengths. The optical wavelength multiplexing means 73 which multiplexes the optical signals output from the respective optical transmitters, wavelength-multiplexes them, and outputs them.

Further, the optical wavelength multiplexing means 73 has a periodic transmission band having a characteristic of cutting off components other than the required optical signal band, whereby unnecessary harmonic components are removed. Is.

The operation of the optical transmitter shown in FIG. 37 will be described with reference to FIG. FIG. 38A schematically shows an example of an optical spectrum of optical signals having different wavelengths output from a plurality of optical transmitters arranged in parallel in the optical transmitter shown in FIG. As shown in FIG. 38 (a),
In the duty ratio variable optical pulse generation means 1 or the encoding means 5 constituting each optical transmitter, unnecessary harmonic components appear on both outsides of the required signal band indicated as the optical signal bit rate before wavelength multiplexing. ing.

As shown in FIG. 38B, the optical wavelength multiplexing means 73 has a periodic transmission band having a characteristic of blocking such an unnecessary harmonic component, that is, a component other than the required optical signal band. is doing. Therefore, by providing the optical wavelength multiplexing means 73 having such a periodic transmission band characteristic, unnecessary harmonic components appearing on both outsides are eliminated as shown in FIG.
It can be suppressed as shown in (c). As a result, it is possible to prevent a decrease in the band utilization efficiency due to the generated optical signal occupying an excessive band than the required band, or to prevent deterioration of transmission characteristics due to crosstalk between optical signals of different wavelengths. it can.

In FIG. 37, a plurality of the optical transmitters shown in FIG. 6 are provided in parallel, but instead of these, a plurality of the optical transmitters shown in FIG. 18 and the optical transmitter shown in FIG. 24 are provided in parallel. It may be provided in the.

The present invention is not limited to the above-described embodiments, but can be implemented with various modifications without departing from the spirit of the invention.

[0148]

As described above, according to the present invention,
Since the duty ratio of the optical clock pulse is variable, it is possible to set the duty ratio to an appropriate value and achieve both high dispersion tolerance and small deterioration of reception sensitivity. Further, according to the present invention, since the relative optical phase difference between the optical clock pulses of the adjacent time slots is set to an odd multiple of π or in the vicinity thereof, high dispersion resistance can be stably maintained. That is, a stable optical transmitter can be configured, which is suitable for constructing a larger-scale network.

Further, according to the present invention, since the optical transmitter has a signal multiplexing function and can operate with a signal having a bit rate which is half or less than the optical signal to be generated, the multiplexing is possible. One circuit can be omitted and an inexpensive amplifier can be used to drive the modulator.
Economical can be achieved.

Further, according to the present invention, since unnecessary harmonic components contained in the optical signal generated in the optical transmitter are removed by the optical band limiting means, the generated optical signal is excessive in the required band. It is possible to prevent a decrease in band utilization efficiency due to occupying a different band and improve the band utilization efficiency.

According to the present invention, since unnecessary harmonic components are removed by the optical multiplexing means having the periodic transmission band having the characteristic of blocking the components other than the required optical signal band, the generated optical signal is required. It is possible to prevent a decrease in band utilization efficiency due to occupying an excessive band than the above band, improve the band utilization efficiency, and prevent deterioration of transmission characteristics due to crosstalk between optical signals of different wavelengths. be able to.

[Brief description of drawings]

FIG. 1 is a block diagram showing a conventional example disclosed in Japanese Patent Laid-Open No. 10-79705.

FIG. 2 is a graph showing the relationship between optical clock waveform dispersion and dispersion tolerance at 80 Gbit / s.

FIG. 3 is a block diagram showing another conventional example disclosed in JP-A-9-261207.

FIG. 4 is a block diagram showing a basic configuration of an optical transmitter according to the present invention.

FIG. 5 is a flowchart showing a control procedure of the optical transmitter control method of the present invention.

FIG. 6 is a block diagram showing a configuration of an optical transmitter according to the first embodiment of the present invention.

FIG. 7 is a diagram showing a specific configuration of the optical transmitter shown in FIG. 6 and a waveform of each part.

FIG. 8 is a block diagram showing a configuration of an optical transmitter according to a second embodiment of the present invention.

9 is a diagram showing a specific configuration of the optical transmitter shown in FIG. 8 and an optical spectrum obtained by extracting a part of light at a monitor port and an output port of the optical multiplexing means.

10 is a diagram showing an example of an output optical spectrum under each phase condition of the optical transmitter shown in FIG.

FIG. 11 is a block diagram showing a configuration of an optical transmitter according to a third embodiment of the present invention.

FIG. 12 is a block diagram showing a configuration of an optical transmitter according to a fourth embodiment of the present invention.

13 is a diagram showing how the optical delay device in the optical transmitter shown in FIG. 12 gives a delay.

14 is a diagram showing a modified configuration of the optical transmitter shown in FIG.

FIG. 15 is a block diagram showing a configuration example of an optical transmitter according to a fifth embodiment of the present invention.

FIG. 16 is a block diagram showing another configuration example of the optical transmitter according to the fifth embodiment of the present invention.

FIG. 17 is a diagram for explaining the operation of the optical transmitter shown in FIGS. 15 and 16.

FIG. 18 is a block diagram showing a configuration of an optical transmitter according to a sixth embodiment of the present invention.

FIG. 19 is a diagram for explaining the operation of the optical transmitter shown in FIG.

20 is a diagram showing an optical waveform and an optical spectrum in the optical transmitter shown in FIG.

FIG. 21 is a diagram showing a case where the Mach-Zehnder modulator that constitutes the encoding means in the optical transmitter shown in FIG. 18 is driven on one side.

22 is a block diagram showing a configuration example in which an optical band limiting unit is provided for the optical transmitter shown in FIG.

23 is a block diagram showing another configuration example in which an optical band limiting unit is provided for the optical transmitter shown in FIG.

FIG. 24 is a diagram showing a configuration of an optical transmitter according to a seventh embodiment of the present invention.

25 is a diagram for explaining the operation of the optical transmitter shown in FIG. 24, showing a phase shift in the upper arm of the Mach-Zehnder interferometer type optical intensity modulator.

FIG. 26 is a diagram for explaining the operation of the optical transmitter shown in FIG. 24, showing a phase shift in the lower arm of the Mach-Zehnder interferometer type optical intensity modulator.

FIG. 27 is a diagram for explaining the operation of the optical transmitter shown in FIG. 24, showing the total phase shift and intensity.

FIG. 28 is a diagram for explaining the operation of the embodiment shown in FIG. 24, and a diagram showing driving points.

FIG. 29 is a diagram showing the configuration of an optical transmitter according to an eighth embodiment of the present invention.

30 is a diagram for explaining the operation of the duty ratio controller of the optical transmitter shown in FIG.

31 is a diagram for explaining the operation of the optical transmitter shown in FIG. 29, and a diagram showing a phase shift in the upper arm of the Mach-Zehnder interferometer type optical intensity modulator.

32 is a diagram for explaining the operation of the optical transmitter shown in FIG. 29, and is a diagram showing a phase shift in the lower arm of the Mach-Zehnder interferometer type optical intensity modulator.

FIG. 33 is a diagram for explaining the operation of the optical transmitter shown in FIG. 29, showing a total phase shift and intensity.

FIG. 34 is a diagram showing a configuration of an optical transmitter according to a ninth embodiment of the present invention.

FIG. 35 is a diagram showing the configuration of an optical transmitter according to a tenth embodiment of the present invention.

36 is a block diagram showing a configuration in which an optical band limiting unit is provided for the optical transmitter shown in FIG.

FIG. 37 is a block diagram showing the configuration of an optical transmitter according to an eleventh embodiment of the present invention.

FIG. 38 is a diagram for explaining the operation of the optical transmitter shown in FIG. 33.

[Explanation of symbols]

1 Duty ratio variable optical pulse generation means 3,51 Optical demultiplexing means 5,7,53 to 56,69 Coding means 9 Optical delay device 11,61 Optical multiplexing means 21 Semiconductor mode-locked laser 23 Y branch 25,27 Mach-Zehnder interferometer modulator 31,45 Directional coupler 35, 57-59 Optical variable delay device 37 Optical spectrum selection means 39 photoelectric converter 41 Delay control circuit 46 Light source control circuit 65,161 CW light source 66 LN board 67 Overtone pulse generation means 67a, 69a Push-pull type Mach-Zehnder modulator 71 Optical band limiting means 73 Optical wavelength multiplexing means 151 pulse light source 153 LN modulator 155 Wide band amplifier 157 Duty ratio controller 1000 light source section 2000 coding section

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI H04B 10/142 H04B 9/00 D 10/152 L 10/18 H04J 14/08 H04L 25/02 303 25/03 Patent Law No. 30 Application for application of Article 1 (Electronics Letters 20th January 2000 Vol. 36, No. 2 pp. 153-155, "320 Gbit / s WDM field experiment using 40 Gbit / s ETDM channel els over 176 km dispersion shifted fibrous with non-third vines, 3rd generation," Fukuto district, Tokyo "," Nagoya, Tokyo " No. Nippon Telegraph and Telephone Corp. (72) Inventor Masao Yube 2-3-1, Otemachi, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corp. (56) Reference JP-A-10-79705 (JP, A) Kaihei 9-261207 (JP, A) Japanese Patent Laid-Open No. 10-178397 (JP, A) Akira Hirano et al. High dispersion tolerance 80 Gbit / s optical time division multiplexing system by duty ratio and optical phase control, 1999 Electronic Information Communication Proceedings of IEICE Communication Society Conference 2, Japan, The Institute of Electronics, Information and Communication Engineers, 1999 August 19, B-10-55, p. 232 (58) Fields surveyed (Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02F 1/03 502 G02F 1/035 H04L 25/02 303 H04L 25 / 03 JISST file (JOIS)

Claims (28)

(57) [Claims] 1. A light source section capable of generating an optical clock pulse synchronized with a signal bit rate with a constant duty ratio and variably setting the duty ratio of the optical clock pulse, and between the optical clock pulses of adjacent time slots. And an encoding section for encoding the optical clock pulse by an electric signal synchronized with the optical clock pulse, the relative optical phase difference being set to an odd multiple of π. 2. The light source section has a mode-locked laser type light source capable of generating an optical clock pulse synchronized with a signal bit rate with a constant duty ratio and variably setting the duty ratio of the optical clock pulse. The optical transmitter according to claim 1, wherein: 3. The encoding section encodes the demultiplexing means for demultiplexing the optical clock pulse, and each of the output signals demultiplexed by the demultiplexing means with an electric signal synchronized with the optical clock pulse. At the same time, the encoding delay means for delaying one of the output signals demultiplexed by the demultiplexing means and the other output signal, and the multiplexing means for multiplexing the output signal of the encoding delay means. And a delay given to the other output signal by the encoding delay means, where n is the number of multiplexing in the multiplexing means and m is an arbitrary integer, the time of one time slot of encoding is Width (k / n) + m
2. The optical transmitter according to claim 1, wherein the optical transmitter is (k = 1, 2, ..., (n-1)) times and the relative optical phase difference is an odd multiple of π. 4. The encoding delay means encodes each of the output signals demultiplexed by the demultiplexing means by an electric signal synchronized with the optical clock pulse, and an encoding means by the encoding means. 4. The optical transmitter according to claim 3, further comprising delay means for delaying at least a part of the converted output signal. 5. The encoding delay means delays at least a part of the output signal demultiplexed by the demultiplexing means, the output signal delayed by the delay means, and the delay if any. 4. The optical transmitter according to claim 3, further comprising coding means for coding each of the output signals not delayed by the means by an electric signal synchronized with the optical clock pulse. 6. An optical spectrum extracting means for branching a part of the signals multiplexed by the multiplexing means and extracting a central component of the optical spectrum or a line spectrum adjacent to the central component, and the optical spectrum extracting means. 4. The optical transmitter according to claim 3, further comprising: feedback means for generating a feedback signal based on the center component or line spectrum extracted in [4]. 7. The encoding delay means has a variable delay means capable of varying a delay amount, and the feedback means maximizes or minimizes a center component or a line spectrum extracted by the optical spectrum extracting means. 7. The optical transmitter according to claim 6, further comprising delay control means for generating a feedback signal for controlling the variable delay means. 8. The light source control means for generating a feedback signal for controlling the light source section so that the center component or the line spectrum extracted by the light spectrum extraction means becomes maximum or minimum. 6. The optical transmitter according to item 6. 9. The part branched in the optical spectrum extracting means is a phase-inverted component of a signal combined in the combining means.
The optical transmitter described. 10. The light source section includes a continuous light generating unit that generates continuous light and a push-pull type Mach-Zehnder modulator driven at zero point, and the encoding section includes the push-pull type Mach-Zehnder modulator. The optical transmitter according to claim 1, comprising an encoder that encodes an output of the push-pull type Mach-Zehnder modulator. 11. The push-pull Mach-Zehnder modulator is driven by a clock signal having a repetition frequency that is half the repetition frequency of the optical clock pulse, and outputs the optical clock pulse from the continuous light generated by the continuous light generation means. 11. The optical transmitter according to claim 10, wherein the relative optical phase difference between the optical clock pulses of the adjacent time slots is set to an odd multiple of π while being generated. 12. The optical transmitter according to claim 10, wherein the push-pull Mach-Zehnder modulator is capable of variably setting the duty ratio of the optical clock pulse by controlling the drive amplitude. 13. The optical transmitter according to claim 10, wherein the push-pull Mach-Zehnder modulator optimizes the waveform of the optical clock pulse to have a high dispersion tolerance by controlling a drive waveform. . 14. The coding section comprises an LN modulator having at least one set of push-pull type modulation electrodes, and push-pulls provided at corresponding positions on an upper arm and a lower arm of the LN modulator. 2. The optical transmitter according to claim 1, wherein the optical transmitter is driven at a zero point by using signals that are input to the type modulation electrodes as signals that are inverted from each other. 15. The LN modulator has at least two sets of push-pull type modulation electrodes, and multiplexes signals input to each set of the at least two sets of push-pull type modulation electrodes. 15. The optical transmitter according to claim 14. 16. The light source section has a CW light source for generating continuous light, and a duty ratio controller provided on the input side of a push-pull type modulation electrode of the LN modulator and capable of electrically controlling the duty ratio. 15. The optical transmitter according to claim 14, wherein: 17. The optical transmitter according to claim 1, further comprising an optical band limiting unit that removes an unnecessary harmonic component included in an optical signal generated in the optical transmitter. 18. The optical band limiting means has a transmission band having a characteristic of blocking a component other than a necessary optical signal band so that an unnecessary harmonic component can be removed. Optical transmitter. 19. An optical transmitter, which is provided in parallel and set to output optical signals of different optical wavelengths, each optical transmitter transmitting an optical clock pulse synchronized with a signal bit rate. Generated at a constant duty ratio, the light source section capable of variably setting the duty ratio of the optical clock pulse, and the relative optical phase difference between the optical clock pulses of adjacent time slots is set to an odd multiple of π, And a coding section for coding the optical clock pulse by an electric signal synchronized with the optical clock pulse, and a wavelength obtained by multiplexing optical signals of different optical wavelengths respectively output from the plurality of optical transmitters. An optical transmitter comprising: an optical wavelength multiplexing unit that multiplexes and outputs. 20. The optical wavelength division means has a periodic transmission band having a characteristic of blocking a component other than a necessary optical signal band so as to remove an unnecessary harmonic component. 19. The optical transmitter according to item 19. 21. A duty ratio of an optical clock pulse is variably set, a duty ratio of the optical clock pulse is set to a value that reduces interference between the pulses, and the duty ratio is kept constant. The optical clock pulse synchronized with the bit rate is generated, the relative optical phase difference between the optical clock pulses between adjacent time slots is set to an odd multiple of π, and the electrical signal synchronized with the optical clock pulse is used. An optical transmitter control method comprising encoding the optical clock pulse. 22. An optical transmitter having optical pulse generation means for generating an optical clock pulse synchronized with a signal bit rate, and encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. An optical transmitter, wherein the duty ratio of the optical clock pulse is variable. 23. Optical clock pulse generation means for generating an optical clock pulse whose duty ratio can be varied in synchronization with a signal bit rate, demultiplexing means for demultiplexing the optical clock pulse, and demultiplexing by this demultiplexing means. Encoding means for encoding each of the output signals encoded by an electrical signal synchronized with the optical clock pulse; and delaying other output signals except at least one of the output signals encoded by the encoding means. The delay means, and the combining means for combining the other output signal delayed by the delay means and the at least one output signal not delayed by the delay means. The delay is k / n (k: 1, ..., (n-1)) or an integer of the time slot, where n is the number of multiplexing in the multiplexing means with respect to the encoding time slot. The doubling and its neighborhood,
An optical transmitter characterized in that the relative optical phase difference is an odd multiple of π or in the vicinity thereof. 24. An optical clock pulse generating means for generating an optical clock pulse whose duty ratio is variable in synchronism with a signal bit rate, a demultiplexing means for demultiplexing the optical clock pulse, and a demultiplexing means by the demultiplexing means. Delaying means for delaying the other output signal except at least one of the outputted output signals, the other output signal delayed by the delaying means, and the at least one not delayed by the delaying means. In the delay means, there is provided an encoding means for encoding each of the output signals by an electric signal synchronized with the optical clock pulse, and a multiplexing means for multiplexing the output signals encoded by the encoding means. The delay is an integer n representing the number of multiplexing in the multiplexing means with respect to the encoding time slot.
Then, k / n (k: 1, ..., (n-1)) or a value obtained by adding an integer multiple of a time slot to it and its vicinity, and the relative optical phase difference is an odd multiple of π or its vicinity. An optical transmitter characterized by being. 25. The delay means is a variable delay means capable of varying the delay amount, and branches a part of the signals multiplexed by the multiplexing means to generate a line spectrum adjacent to the center of the optical spectrum. An optical spectrum extracting means for extracting,
25. The optical transmitter according to claim 23, further comprising delay control means for controlling the variable delay means so that the line spectrum extracted by the optical spectrum extraction means becomes maximum. 26. The delay means is a variable delay means capable of varying a delay amount, and an optical spectrum for branching a part of the signals multiplexed by the multiplexing means and extracting a central component of the optical spectrum thereof. 25. The method according to claim 23, further comprising: an extracting means and a delay control means for controlling the variable delay means so that the center component of the optical spectrum extracted by the optical spectrum extracting means is minimized. Optical transmitter. 27. The delay means is a variable delay means capable of varying the delay amount, and branches a part of the signals multiplexed by the multiplexing means to generate a line spectrum adjacent to the center of the optical spectrum. An optical spectrum extracting means for extracting,
25. The optical transmitter according to claim 23, further comprising a light source control means for controlling the optical clock pulse generation means so that the line spectrum extracted by the optical spectrum extraction means becomes maximum. 28. The optical transmitter according to claim 25, wherein the part is a phase-inverted component of a signal multiplexed by the multiplexing means.