JP2001339346A - Optical transmitter and optical transmitter control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stable optical transmitter that is highly immune to dispersion in a group velocity of an optical fiber and that hardly deteriorates in the receiving sensitivity and may not be affected by the dispersion in the group velocity for the extension of the network scale. SOLUTION: The optical transmitter consists of a light source section that generates an optical clock pulse synchronously with a signal bit rate at a prescribed duty ratio and that variably sets the duty ratio of the optical clock pulse and a coding section that sets a relative optical phase difference between optical clock pulses in adjacent time slots to an odd number multiple of πin order to encode the optical clock pulse by an electric signal in synchronism with the optical clock pulse.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical transmitter for generating an optical signal to be transmitted to a transmission medium such as an optical fiber having chromatic dispersion that degrades transmission quality, and an optical transmitter control method.

[0002]

2. Description of the Related Art In an optical transmission system, there is a problem of deterioration of transmission quality caused by waveform deterioration due to chromatic dispersion of an optical fiber as a transmission medium. This is caused by the fact that the group velocity dispersion of the optical fiber acts on the bandwidth of the optical signal, thereby increasing the optical pulse width and causing interference within adjacent time slots.

FIG. 1 shows a conventional example disclosed in JP-A-10-79705. In this conventional example, in order to suppress deterioration due to group velocity dispersion, a type of optical modulator in which a transmitter adds prechirping to an optical signal to cancel the group velocity dispersion has been proposed. This is to suppress the waveform deterioration at the receiving end by adding in advance an amount of frequency chirp substantially balanced with the group velocity dispersion of the optical fiber in the optical transmitter. In FIG. 1, a light source 101
Is divided into two optical clocks by the clock generation means 102. The clock generating means 102 is, for example, a Mach-Zehnder type optical modulator.
This is generated by driving with a sine wave electric signal. At this time, the pre-chirp means 106 controls the value and sign of the chirp by controlling the bias voltage or the like to the optical modulator. When an electro-absorption type optical modulator is used as the clock generation means 102, the pre-chirp means 106 controls the amount of chirp by controlling the bias voltage. Then, the optical clock whose chirp is controlled is encoded by first and second data modulating means 103 and 104 which are optical modulators of the next stage,
A desired optical signal is obtained by multiplexing by the optical multiplexer 105.

However, in the optical modulation device, an amount of frequency chirp that cancels the group velocity dispersion is added in advance.
The bandwidth of the optical signal spectrum increases. The tolerance to group velocity dispersion has an inverse square relationship with the optical spectrum band, and the tolerance decreases as the band increases. Therefore, the method of adding prechirp as in the conventional example has a small dispersion tolerance, and impairs stable operation of the transmission system. That is, the transmission quality deteriorates due to a slight difference in the group velocity dispersion.

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

In the clock generating means 102,
There is a large loss in optical power for cutting out the clock light from the continuous light at the gate by the modulator, and the two encoded clock lights are extinguished due to the optical interference effect 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 causes a decrease in the S / N ratio during transmission.

FIG. 2 shows that the bit rate is 80 Gbit / s.
6 is a graph showing the relationship between the chromatic dispersion and dispersion tolerance of the optical clock of FIG. 1 using the relative optical phase of two encoded optical clocks as a parameter at the time of multiplexing in the optical multiplexer. As a result, when the bit rate is different, the absolute value (| D | (PS / nm)) of the dispersion value on the horizontal axis 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 is the middle-p when the phase difference is an odd multiple of π.
hase indicates a 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. That is, it is necessary to optimally control the duty ratio in order to achieve both high dispersion tolerance and low reception sensitivity deterioration. However, in the conventional example, the optical clock is generated by placing a driving point on a linear portion of the modulator with a sine wave having a frequency equal to the bit rate before multiplexing. Needs to be changed, and the extinction ratio deteriorates, and the signal effect deteriorates due to the interference effect at the time of multiplexing.

Japanese Patent Application Laid-Open No. H10-79705 discloses an example in which driving is performed by using a rectangular wave. However, when the device is driven by using a rectangular wave, the optical spectrum is excessively widened, and the dispersion tolerance is reduced. Would. 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. Moreover, since only an excessively wide duty ratio can be realized by driving with a sine wave, a device having a division number of 3 or more as disclosed in JP-A-10-79705 has a large interference effect between adjacent optical clocks.
Since they are extinguished with each other, it is difficult to generate a practically effective optical signal.

In the conventional example disclosed in Japanese Patent Application Laid-Open No. 9-261207, when the outputs of the optical modulators 111 and 112 are multiplexed by the optical multiplexing unit 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 multiplexed optical 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 deliberately shifted from π, the relative optical phase difference in which the dispersion tolerance is significantly deteriorated approaches the condition of π / 2. That is, the control itself causes the deterioration of the dispersion proof stress.

[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 transmission device, firstly, there is a problem that an excessive optical spectrum band is occupied due to the addition of chirp, and the tolerance to the group velocity dispersion is reduced.

Although the push-pull type Mach-Zehnder type optical modulator can be controlled to be small and arbitrarily controlled by bi-phase driving, the duty ratio of the optical clock cannot be controlled. is there.

Further, when an electro-absorption modulator is used, the amount of chirp itself is large and the chirp parameter and the duty ratio are not independent parameters, so that there is a problem that it is difficult to add a desired chirp. However, in order to arbitrarily control the duty ratio, the loss also fluctuates greatly, causing a change in the S / N ratio at the time of communication and further causing a variation in transmission quality.

The present invention has been made in view of the above,
The objective is to provide an optical transmitter and an optical transmitter control method that have a high tolerance to group velocity dispersion of an optical fiber, have a small reception sensitivity degradation, and are not easily affected by the group velocity dispersion when expanding the network scale. To provide.

[0014]

According to the present invention, an optical clock pulse synchronized with a signal bit rate is generated at a constant duty ratio, and the duty ratio of the optical clock pulse can be variably set. A light source section and a relative optical phase difference between the optical clock pulses in adjacent time slots are set to odd multiples of π, and the optical clock pulse is encoded by an electric signal synchronized with the optical clock pulse. And an optical transmitter.

In the present invention, the light source section may generate an optical clock pulse synchronized with a signal bit rate with a constant duty ratio, and may be a mode-locked laser type that can variably set the duty ratio of the optical clock pulse. It has a light source.

Further, in the present invention, the encoding section comprises a demultiplexing means for demultiplexing the optical clock pulse, and an electric signal which synchronizes each of the output signals demultiplexed by the demultiplexing means with the optical clock pulse. Encoding delay means for encoding the output signal with respect to one of the output signals demultiplexed by the demultiplexing means, and 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 a multiplexing number in the multiplexing means and m is an arbitrary integer, and each of the delays is 1 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.

Further, in the present invention, the encoding delay means comprises: encoding means for encoding each of the output signals demultiplexed by the demultiplexing means with an electric signal synchronized with the optical clock pulse; And delay means for delaying at least a part of the output signal encoded by the converting means.

Further, in the present invention, the encoding delay means includes a delay means for delaying at least a part of the output signal split by the splitting means, and an output signal delayed by the delay means. If there is, there is provided encoding means for encoding each of the output signals which have not been delayed by the delay means with an electric signal synchronized with the optical clock pulse.

Further, in the present invention, an optical spectrum extracting means for splitting a part of the signal multiplexed by the multiplexing means and extracting a center component of the optical spectrum or a line spectrum adjacent to the center component, Feedback means for generating a feedback signal based on the center component or line spectrum extracted by the optical spectrum extraction means.

Further, in the present invention, the encoding delay means has a variable delay means capable of varying a delay amount, and the feedback means has a function of maximizing a center component or a line spectrum extracted by the optical spectrum extraction means. Alternatively, there is provided a delay control means for generating a feedback signal for controlling the variable delay means so as to minimize it.

Further, the present invention is characterized in that there is provided a 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. And

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

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

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

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

Further, in the present invention, the push-pull Mach-Zehnder modulator optimizes a waveform of the optical clock pulse to a waveform having a high dispersion tolerance by controlling a drive waveform.

In the present invention, the encoding section comprises an LN modulator having at least one set of push-pull type modulation electrodes, and is provided at a corresponding position on the upper arm and the lower arm of the LN modulator. The signal inputted to the push-pull type modulation electrode is inverted to each other, and is driven at zero point.

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

In the present invention, the light source section may include a CW light source for generating continuous light, and a duty ratio provided on an input side of a push-pull type modulation electrode of the LN modulator and capable of electrically controlling a duty ratio. It has a controller.

Further, the present invention is characterized in that there is provided an optical band limiting means for removing unnecessary harmonic components 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 components other than a necessary optical signal band so that unnecessary harmonic components can be removed. .

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

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

Further, according to the present invention, the duty ratio of the optical clock pulse is set to a value that reduces interference between the pulses by using a configuration capable of variably setting the duty ratio of the optical clock pulse, and the duty ratio is set to a value that reduces interference between the pulses. Generating the optical clock pulse constant and synchronized with the signal bit rate,
Optical transmission characterized in that a relative optical phase difference between the optical clock pulses in 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 container is provided.

Accordingly, the present invention provides an optical pulse generator having an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate and an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. A transmitter, wherein the duty ratio of the optical clock pulse is variable.

Thus, 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 low reception sensitivity deterioration can be achieved.

According to the present invention, there is provided an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate, and 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 that the transmitter includes means for setting the relative optical phase difference to an odd multiple of π or its vicinity for each pulse of the optical clock pulse.

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

The present invention also provides an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate, and an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. In the transmitter, a repetition frequency of the optical clock pulse is twice as high as a frequency of an electric signal for driving the optical pulse generation unit.

Further, the present invention provides an optical clock pulse generating means for generating an optical clock pulse in which a duty ratio can be varied in synchronization with a signal bit rate, a demultiplexing means for demultiplexing the optical clock pulse, and the demultiplexing means. Encoding means for encoding each of the output signals demultiplexed by the means with an electric signal synchronized with the optical clock pulse, and another encoding means excluding at least one of the output signals encoded by the encoding means. Delay means for delaying an output signal, and multiplexing means for multiplexing the another 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 multiplexes in the multiplexing means is an integer n with respect to a time slot of encoding.
Alternatively, the sum is a sum of an integer multiple of the time slot and its vicinity, and the relative optical phase difference is an odd multiple of π or its vicinity.

Further, the present invention provides an optical clock pulse generating means for generating an optical clock pulse in synchronization with a signal bit rate and having a variable duty ratio, a demultiplexing means for demultiplexing the optical clock pulse, and a demultiplexing means. Delay means for delaying another output signal except at least one of the output signals split 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 signal encoded by the encoding means, The delay in the delay means is k / n (k: 1,..., (N-
1)) or a value obtained by adding an integer multiple of the time slot to the value and its vicinity, and the relative optical phase difference is an odd multiple of π or its vicinity.

Further, according to the present invention, the delay means is a variable delay means capable of varying a delay amount, and a part of the signal multiplexed by the multiplexing means is branched, and the signal is adjacent to the center of the optical spectrum. The present invention further comprises an optical spectrum extracting means for extracting the extracted line spectrum, and a delay control means for controlling the variable delay means so that the line spectrum extracted by the optical spectrum extracting means is maximized.

Also, in the present invention, the delay means is a variable delay means capable of varying a delay amount, and a part of the signal multiplexed by the multiplexing means is branched, and the center component of the optical spectrum is changed. The gist of the present invention is to further include an optical spectrum extracting unit to be extracted, and a delay control unit that controls the variable delay unit so that a central component of the optical spectrum extracted by the optical spectrum extracting unit is minimized.

Further, according to the present invention, the delay means is a variable delay means capable of varying a delay amount, and a part of the signal multiplexed by the multiplexing means is branched, and the signal is adjacent to the center of the optical spectrum. The invention further comprises an optical spectrum extracting means for extracting the extracted line spectrum, and a light source controlling means for controlling the optical clock pulse generating means so that the line spectrum extracted by the optical spectrum extracting means is maximized. .

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

Further, according to the present invention, there is provided an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate, and an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. In a transmitter, 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 equal to or less than half of the optical signal to be generated. Can be achieved.

The present invention also relates to an optical transmitter having a continuous light generating means for generating continuous light and an encoding means for encoding the continuous light, wherein each optical signal generated by the encoding means has a pulse. It is essential to have means for setting the relative optical phase difference to an odd multiple of π or its vicinity.

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

The present invention is also 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 gist.

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

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

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

Further, according to the present invention, a plurality of any one of the above optical transmitters is provided in parallel, and the plurality of optical transmitters are set so as to output optical signals having different optical wavelengths. In addition to multiplexing and wavelength-multiplexing optical signals of different optical wavelengths respectively output from the devices, it has the characteristic of blocking components other than the necessary optical signal band so that unnecessary harmonic components can be removed. The point is to have an optical multiplexing means having a periodic transmission band.

As a result, unnecessary harmonic components are removed, so that unnecessary harmonic components can be suppressed, and a decrease in band use efficiency due to the generated optical signal occupying an excessive band over the required band. Thus, the band use efficiency can be improved, or deterioration of transmission characteristics due to crosstalk between optical signals having different wavelengths can be prevented.

Further, according to the present invention, the optical band limiting means preferably comprises:
An optical multiplexing unit having a transmission band having a characteristic of blocking a component other than a necessary optical signal band so as to remove unnecessary harmonic components and to multiplex optical signals of a plurality of different optical wavelengths. Make a summary.

[0057]

Embodiments of the present invention will be described below in detail with reference to the drawings.

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

FIG. 4 shows a basic configuration of an optical transmitter according to the present invention. The optical transmitter includes a light source section 1000 that generates an optical clock pulse synchronized with a signal bit rate, and an encoding section 2000 that encodes the 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 that can variably set the duty ratio of the optical clock pulse, and a duty ratio constant optical pulse generation function that generates an optical clock pulse synchronized with the signal bit rate at a constant duty ratio. 1002, and the encoding section 2000 includes an alternating phasing function 2001 for setting the relative optical phase difference between optical clock pulses in adjacent time slots to an odd multiple of π, and an alternating phasing 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 to set the duty ratio of the optical clock pulse to a value that reduces the interference between the pulses, the interference effect can be suppressed. (Step S1).

Next, using the above-described light pulse generation function 1002 of the light source section 1000 with a constant duty ratio,
The pulse width is stabilized 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 (step S2).

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

Then, the above-described encoding section 200
Using the encoding function 2002 of 0, the optical clock pulse that has been alternately phased is encoded by an electric 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 for reducing the interference between pulses, so that both high dispersion tolerance and low reception sensitivity deterioration can be achieved. In addition to the setting, the relative optical phase difference between the optical clock pulses in the adjacent time slots is set to an odd multiple of π, so that a high dispersion tolerance can be stably maintained. Further, since the pulse width is constant, the amplitude after the equalization is stabilized, and the eye opening deterioration as seen in the conventional optical transmitter does not occur at all. Further, since the waveform of the optical clock pulse can be optimized to a waveform having a higher dispersion tolerance, the dispersion tolerance itself can be increased about twice as compared with the related art. Therefore,
It is possible to provide an optical transmitter and an optical transmitter control method that have a high tolerance to group velocity dispersion of an optical fiber, have small reception sensitivity degradation, and are not easily affected by group velocity dispersion when the network scale is expanded. Therefore, it is suitable for constructing a larger network.

In the following description, for example, the relative optical phase difference between optical clock pulses in adjacent time slots is described as being set to an odd multiple of π or in the vicinity thereof. Is exactly π
In consideration of the fact that the present invention is practically effective not only in the case of idealization set to an odd multiple of but also in the case of allowing some error to this, such an error is allowed. In this case, the dispersion proof stress deteriorates more than in the idealized case. However, if the error is within 20% of the idealized dispersion proofness deterioration, it is considered that the error is actually within the allowable range. The vicinity means an error within such an allowable range.

Referring to FIGS. 6 to 34, an embodiment relating to a specific configuration example of the optical transmitter in FIG. 4 will be described in detail.

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 set variably, and the duty ratio variable optical pulse generating means 1 generates an optical clock pulse synchronized with the signal bit rate. Optical demultiplexing means 3 for demultiplexing an optical clock pulse generated from the optical demultiplexing means, first and second encoding means 5 and 7 for encoding the output signals demultiplexed by the optical demultiplexing means 3, respectively. 1, an optical delay unit 9 for delaying an output signal encoded by the first encoding unit 5 among the second encoding units 5 and 7;
And an optical multiplexing means 11 for multiplexing the output signal delayed by the optical delay unit 9 and the output signal from the second encoding means 7. Here, the variable duty ratio optical pulse generating means 1 constitutes the above-described light source section 1000, and the optical demultiplexing means 3, the first and second encoding means 5, 7, the optical delay unit 9, and the optical multiplexing means 11 comprise The coding section 2000 described above is configured.

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

Here, the duty ratio variable 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), and laser light sources such as a fiber ring type mode-locked laser and a super continuum light source (here, these are collectively referred to as a mode-locked laser type light source). In addition, a combination of these mode-locked laser light sources and an optical band-pass filter may be used to limit the band and control the duty ratio.
The optical demultiplexing means 3 is a Y-branch or a directional coupler.
The encoding means 5, 7 is a modulator of the Mach-Zehnder interferometer type using a Y-branch or a directional coupler, or an electro-absorption modulator. The optical delay unit 9 is a delay line or the like. The optical multiplexing means 11 is a Y-branch or a directional coupler. It is convenient that the optical component is 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 of FIG.
The lower part of FIG. 7 shows the waveform of each part of the optical transmitter shown in FIG.

In the optical transmitter shown in the upper part of FIG. 7, the variable duty ratio optical pulse generating 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, 7 are first and second Mach-Zehnder interferometer (MZ) modulators 2.
5 and 27, and the optical multiplexing means 11 includes the directional coupler 3
1.

In the optical transmitters of FIGS. 6 and 7 configured as described above, the duty ratio variable optical pulse generation means 1 constituted by the semiconductor mode-locked laser 21 uses the optical pulse having a desired duty ratio without frequency chirp. 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 split ratio of 1: 1. Each of the divided optical pulses is encoded by encoding means 5 and 7 composed of Mach-Zehnder interferometer type (MZ) modulators 25 and 27 driven by NRZ codes to become RZ codes. The NRZ code used for driving is synchronized with the divided optical pulse.

One of the components of the encoded light pulse is delayed by the optical delay unit 9 by an odd multiple of half the repetition period of the light pulse or in the vicinity thereof, as shown by the hatched waveform in FIG. If a heater is built in the optical delay unit 9, the amount of delay can be controlled with a high degree of accuracy equal to or less than the wavelength of light by controlling the energizing power to that part by the thermo-optic effect. Due to the precise delay control using the thermo-optic effect, the directional coupler 3 between the two divided light pulses
The relative optical phase difference in the optical multiplexing means 11 constituted by 1 is adjusted to be an odd multiple of π or in the vicinity thereof.

In this way, the optical pulse output from the optical multiplexing means 11 has such a feature that the relative optical phase difference between the optical pulses in adjacent time slots always becomes π. With this control, the out-of-phase condition shown in FIG. 2 is realized, and a high dispersion tolerance is achieved. Also, the deterioration of the receiving 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 differs from the embodiment shown in FIG.
In the above, an optical variable delay device 35 capable of changing the delay time is used in place of the optical delay device 9, and a part of the signal multiplexed by the optical multiplexing means 11 is branched from the monitor port of the optical multiplexing means 11. An optical spectrum selecting means 37 for selectively extracting only the central component of the optical spectrum, a 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 a signal from the photoelectric converter 39. The difference is that an electric signal is input and a delay control circuit 41 for controlling the optical variable delay device 35 based on the electric signal is provided so that the center component of the optical spectrum is maximized.

In the optical transmitter configured as described above,
The optical pulse generated by the variable duty ratio optical pulse generator 1 is split into two by the optical splitter 3. The splitting ratio at this time is 1: 1. The split optical pulses are independently encoded by the first and second encoding means 5 and 7, respectively. This coding bit rate is equal to the repetition frequency of the light pulse. The optical variable delay unit 35 delays one component of the demultiplexed and encoded optical pulses by an optical multiple of a half of the repetition period of the optical pulse or a time near the odd multiple. The amount of delay is set such that the relative optical phase difference between the two components, which are demultiplexed and coded, when multiplexed by the optical multiplexing means 11 is an odd multiple of π or in the vicinity thereof. Optical spectrum selection means 3
7 extracts only the central component of the optical spectrum, and converts it into an electric signal by the photoelectric converter 39. This is input to the delay control circuit 41, and the variable optical delay unit 35 is controlled so that the center component of the optical spectrum becomes maximum.

Here, the optical spectrum selecting means 37 is a Fabry-Perot interferometer type optical filter, a dielectric multilayer type optical filter, an array waveguide type optical filter, a Mach-Zehnder interferometer type optical filter, or the like. The photoelectric converter 39 is a photodiode, a photomultiplier, 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 shown in 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 light coupling at the monitor port and the output port of the optical multiplexing means 11 of the optical transmitter shown in FIG. The light spectrum which extracted a part is shown.

In the optical transmitter shown in the upper part of FIG. 9, the variable duty ratio optical pulse generating means 1 of FIG. 8 is constituted by a semiconductor mode-locked laser 21 and the optical demultiplexing means 3 is Y-branched similarly to FIG. 23, and the first and second encoding means 5 and 7 are constituted by first and second Mach-Zehnder interferometer (MZ) modulators 25 and 27, respectively.
Reference numeral 1 denotes a directional coupler 45. In this embodiment, a Mach-Zehnder interferometer combined with a directional coupler 45 is used as the optical multiplexing unit 11.

FIG. 10 shows an example of an optical spectrum when a part of the light is extracted from the monitor port and the output port of the optical multiplexing means 11 of the optical transmitter shown in FIG.
In particular, FIGS. 10A, 10B, and 10C respectively show that the relative optical phase difference between the optical pulses demultiplexed by the optical demultiplexing means 3 is 0,
The case of π and π / 2 is shown. In order to have a high dispersion tolerance, the phase difference needs to be an odd multiple of π.
The spectrum at that time has a form in which the central component is suppressed as shown in FIG. In the present Mach-Zehnder interferometer, an optical pulse having an inverted optical phase is output to a pair of output ports. Therefore, 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 with an electric signal by the photoelectric converter 39, and the optical variable delay unit 35 is controlled so as to maximize the signal intensity. By this feedback control, even if the phase of the two divided optical pulses fluctuates, a high dispersion tolerance can always be stably realized.

In the conventional example disclosed in Japanese Patent Application Laid-Open No. 9-261207, a change in optical power due to extinction of an overlapping portion of an optical pulse to be multiplexed is detected and controlled to a minimum. However, this variation has only a difference of whether the overlapping portion of the light pulse is extinguished or strengthened, so that the S / N ratio is low, and the control accuracy is poor. In addition, there is a problem that the sensitivity varies depending on the duty ratio.

On the other hand, in the present embodiment, only the center component is cut out by the optical band-pass filter,
To control this to the maximum, the S / N ratio is high and control can be performed with high accuracy. Further, the variation in sensitivity due to the difference in duty ratio is very small.

The other basic operations are the same as in the first embodiment.

In the above operation, feedback control was performed so as to maximize the central component of the monitor port. However, a part of the optical signal component from the output port was monitored, and the central component of the optical spectrum was minimized. It may be controlled.
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 to the embodiment shown in FIG.
At 00, a light source control circuit 46 for controlling the variable duty ratio optical pulse generating means 1 is provided in place of the delay control circuit 41 for controlling the variable optical delay unit 35, and the variable optical delay unit 35 has the same optical delay as the first embodiment. The only difference is that the container 9 is used, and other configurations and operations are the same as those of the embodiment of FIG.

In the optical transmitter shown in FIG. 11, the optical pulse generated by the variable duty ratio optical pulse generator 1 is split into two by the optical splitter 3. The splitting ratio at this time is 1: 1. The split optical pulses are independently encoded by the first and second encoding means 5 and 7, respectively. This coding bit rate is equal to the repetition frequency of the light pulse. The optical delay unit 9 delays one component of these demultiplexed and encoded optical pulses by an optical multiple of half the repetition period of the optical pulse or a time near the odd multiple. The amount of delay is set such that the relative optical phase difference between the two components, which are demultiplexed and coded, when multiplexed by the optical multiplexing means 11 is an odd multiple of π or in the vicinity thereof. Only the central component of the optical spectrum is extracted 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 duty ratio variable optical pulse generation means 1 so that the center component of the optical spectrum is maximized.

Here, the optical spectrum selecting means 37 is a Fabry-Perot interferometer type optical filter, a dielectric multilayer type optical filter, an array waveguide type optical filter, a Mach-Zehnder interferometer type optical filter, or the like. A narrower band than that used in the embodiment is used. This is because higher accuracy is required for detecting wavelength fluctuation. The photoelectric converter 39 includes a photodiode, a photomultiplier, and a CC.
D and the like. 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 combined with a directional coupler is used as the optical multiplexing means 11.

In the embodiment shown in FIG. 11, similarly to 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. This is based on the reason described 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 the photoelectric converter 39 and replaced with an electric signal by the photoelectric converter 39, and the duty ratio variable optical pulse generating means 1 is controlled so as to maximize the signal intensity. This feedback control makes it possible to always keep the oscillation wavelength of the duty ratio variable optical pulse generation means 1 constant.

It has been reported that the group velocity dispersion of an optical fiber varies at a wavelength of 0.07 ps / nm ² / km with wavelength (KSKim et al., J. Appl. Phys., Vol. 73, N.
o.5, pp.2069-2074, 1993). For this reason, when the wavelength of the light source fluctuates, the value of the group velocity dispersion that the optical pulse suffers changes, which may cause waveform degradation of the optical pulse and degradation of transmission quality. Deterioration can be prevented beforehand, which leads to improvement in system reliability.

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

FIG. 12 shows the configuration of the optical transmitter according to the fourth embodiment of the present invention. The optical transmitter shown in FIG.
Compared to the embodiment of FIG.
, An optical demultiplexing unit 51 for demultiplexing the optical clock pulse from the duty ratio variable optical pulse generating unit 1 into four is used in place of the optical demultiplexing unit 3, and the first and second encoding units 5, 7 are used. Instead of the first to fourth four encoding means 53 to
56, each of the four output signals of the optical demultiplexing means 51 is encoded.
Of the first to fourth encoding units 53 to 56 using the three optical delay units 57 to 59.
-55, only the output signals from the three encoding means are delayed, and the output signals from the fourth encoding means 56 which have not been delayed are input to the optical multiplexing means 61 and multiplexed. , And the other configuration and operation are the same as those of the embodiment of FIG.

In the optical transmitter according to the fourth embodiment configured as described above, the optical pulse generated from the variable duty ratio optical pulse generating means 1 is converted by the optical demultiplexing means 51 into four pulses.
It is split into two. The demultiplexing ratio at this time is equal (1: 1: 1:
1). The demultiplexed optical pulses are encoded independently by the encoding means 53 to 56. This coding bit rate is equal to the repetition frequency of the light pulse.

As shown in FIG. 13, at least three components of these demultiplexed and encoded optical pulses each have a time width of one time slot of encoding.
It is delayed by the optical delay units 57 to 59 by / 4, 2/4, 3/4 times, or a time obtained by adding an integral multiple of the time width of one time slot for encoding. More generally, the delay in the optical delay device is represented by n,
When an arbitrary integer is m, (k / n) + m (k = 1, 2,..., (N−
1)) Double.

This delay amount is set such that the relative optical phase difference between the four components which are demultiplexed and coded, when multiplexed by the optical multiplexing means 61, is an odd multiple of π or in the vicinity thereof. I do.

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

Here, the light branching means 51 uses three branching light branching means. In this method, the first device is divided into two, and the remaining two devices are further divided into two to separate the light into four. Further, the optical demultiplexing means 51 may use a four-branch star coupler, a PLC circuit, or an LN waveguide. The optical multiplexing means 61 may use these circuits correspondingly. 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, and is different from that used in the second embodiment. Also use a narrow band. This is because higher accuracy is required for detecting wavelength fluctuation. Photoelectric converter 3
Reference numeral 9 denotes a photodiode, a photomultiplier, a CCD, and 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 the present embodiment, a directional coupler on the PLC is used as the optical multiplexing means 61. In the present embodiment, 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 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 selecting means 37 having a narrower band than that used in the embodiment, and converted into an electric signal by the photoelectric converter 39, and the duty ratio is changed so as to maximize the signal intensity. The light pulse generator 1 is controlled. With this feedback control,
The oscillation wavelength of the variable duty ratio optical pulse generation means 1 can be kept constant at all times. The effectiveness of this control is the same as in the third embodiment described above.

The other basic operations are the same as in the above embodiments.

By modifying the configuration of FIG. 12, as shown in FIG. 14, first to fourth four encoding means 53 to 56
And the first to third optical delay units 57 to 59 are exchanged with each other, and the first to third optical delay units 57 to 59 are output from the output signals divided by the optical demultiplexing unit 51. After the three delays, the first to fourth encoding means 53-56 respectively output the delayed output signal of the optical delay units 57-59 and the undelayed output signal of the optical demultiplexing means 51. May be encoded.

In the above-described embodiment, the control of the variable optical delay unit 35 and the control of the variable duty ratio optical pulse generating means 1 are described separately. However, these may be used together or used as needed. An optical transmitter which is more stable and has high tolerance against 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.
As compared with the first embodiment shown in FIG. 1, the only difference is that an optical band limiting unit 71 is provided after the optical multiplexing unit 11 in the encoding section 2000 of the optical transmitter, and the other configurations and operations are the same. The same components are denoted by the same reference numerals.

The optical band limiting means 71 has a transmission band for blocking components other than the necessary optical signal band, thereby removing unnecessary harmonic components, thereby improving the band use efficiency. Things.

Note that, 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 changed to a duty ratio variable light as shown in FIG. It is also possible to adopt a configuration of an optical transmitter provided on the output side of the pulse generation means 1. Thus, even if the optical band limiting unit 71 is provided on the output side of the duty ratio variable optical pulse generating unit 1, unnecessary harmonic components can be suppressed as in the optical transmitter of FIG.

Referring to FIG. 17, the operation of the optical transmitter shown in FIG. 15 or 16 will be described. FIG.
17 schematically illustrates an example of an optical spectrum of an optical signal generated in the optical transmitter illustrated in FIG.

As shown in FIG. 17A, it is necessary that the unnecessary duty cycle component is indicated as the optical signal bit rate in the duty ratio variable optical pulse generating means 1 or the encoding means 5 constituting the optical transmitter. It 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 unnecessary harmonic components as described above, that is, components other than the necessary 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. As a result, it is possible to prevent a decrease in band use efficiency due to the generated optical signal occupying an excessive band more than a necessary band.

In each of the above embodiments, it is possible to easily cope with the case where the number of demultiplexing is 3 or more by controlling the pulse width of the duty ratio variable optical pulse generating means.

In each of the above embodiments, one of the demultiplexed optical pulses is not delayed. However, all the demultiplexed optical pulses are delayed so that the relative distance between the respective optical pulses can be reduced. An appropriate 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.
It comprises a light source 65, a harmonic pulse generating means 67, and an encoding means 69. Here, the CW light source 65 and the harmonic pulse generating means 67 constitute the light source section 1000 described above, and the harmonic pulse generating means 67 and the encoding means 69 constitute the encoding section 2000 described above.

In this embodiment, the continuous light generated from the CW light source 65 is modulated by the harmonic pulse generating means 67 into a clock having a repetition frequency twice as high as the driving frequency supplied to the harmonic pulse generating means 67. This harmonic clock is encoded by the encoding means 69. This encoding bit rate is equal to the repetition frequency of the optical clock. That is, the driving frequency of the clock supplied to the harmonic pulse generating means 67 is half the bit rate. Here, the CW light source 65 is a laser light source such as a DFB-LD and an FP-LD. The harmonic pulse generating means 67 is a push-pull Mach-Zehnder modulator or the like on the LN substrate. The encoding means 69 is a push-pull 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, first and second two push-pull Mach-Zehnder modulators 67a and 69a formed in series on an LN substrate 66 as harmonic pulse generating means 67 and encoding means 69. Is used.

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

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
FIG. 2 shows an example of an optical waveform of 0 Gbit / s and its optical spectrum.
0 is shown. When the relative optical phase difference is 0 or π,
Since the optical spectrum is equivalent to that of the optical multiplexing described in each of the above-described embodiments, it can be seen that the out-of-phase condition having high dispersion tolerance is also realized in this embodiment.

Here, the optical waveform having a relative optical phase difference of π is realized by driving the first Mach-Zehnder modulator 67a at a point where the transmittance at the time of non-modulation becomes minimum.

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

In the configuration shown in FIG. 19, both Mach-Zehnder modulators are bi-phase driven. However, as shown in FIG.
For a, the same coding can be performed by using one-sided driving. However, in this case, a small chirp remains during encoding, but the influence on the basic operation is small. As can be easily analogized, the Mach-Zehnder modulator 67a constituting the harmonic pulse generating means 67 can also use one-sided driving. At this time, the driving point is set at or near the point where the transmittance is minimum when no modulation is performed, as in the above-described embodiment. Also in this case, a slight chirp remains in the generated harmonic pulse, but the influence on the basic operation is small.

Further, in this embodiment, the case where the alternating phase optical clock pulse is generated first and then encoded is described. However, the encoding may be performed first and then the alternating phase clock may be used. At that time, the positional relationship between the harmonic pulse generating means 67 and the encoding means 69 is reversed.

In the present embodiment, the harmonic pulse generating means and the encoding means are integrated on the same substrate.
These may be constituted by separate substrates.

FIGS. 22 and 23 show an optical transmitter in which the optical band limiting means 71 shown in FIGS. 15 and 16 is provided for the optical transmitter of the sixth embodiment shown in FIG. 18, respectively. FIG. 22 shows a configuration in which an optical band limiting unit 71 is provided on the output side of the encoding unit 69, and FIG. 23 shows an optical band limiting unit 71 on the output side of the harmonic pulse generating unit 67.
Is provided. By providing the optical band limiting means 71 in this manner, unnecessary harmonic components can be removed as in the case of the above-described fifth embodiment, and the generated optical signal has an excess band over the required band. By occupying the bandwidth, it is possible to prevent a decrease in band use 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, it is possible to achieve both high dispersion tolerance and small deterioration in reception sensitivity. Further, since the relative optical phase difference between the optical clock pulses in the adjacent time slots is set to an odd multiple of π or in the vicinity thereof, a high dispersion tolerance can be stably maintained.

FIG. 24 shows the configuration of the optical transmitter according to the seventh embodiment of the present invention. The optical transmitter shown in FIG. 24 includes a pulse light source 151 constituting a light source section 1000 that generates an optical clock pulse synchronized with a signal bit rate;
An LN modulator 153 and four broadband amplifiers 155a constituting an encoding section 2000 for encoding an optical clock pulse by an electric signal synchronized with the optical clock pulse.
To 155d. LN modulator 153
Are two sets of push-pull type modulation electrodes 153a to 153d.
And the outputs of the wideband amplifiers 155a to 155d are connected to the respective modulation electrodes 153a to 153d.

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. The electrical signal input to the optical transmitter is an RZ code having a duty ratio of 0.5 or near in the generated optical signal, and the input RZ code includes codes CH1 and CH2 and these codes. CH1, C with overline
As shown by the inverted code of H2 (hereinafter, the inverted code with the overline is indicated by adding a dash "'" like CH1' and CH2 '),
155d, thereby providing a wideband amplifier 155a
155d amplifies the voltage to a voltage sufficient to drive the LN modulator 153, that is, around Vπ / 2. Then, the optical pulse from the pulse light source 151 is encoded into an alternating phase RZ code by the electric signal 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. Mode-locked laser type 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 light intensity modulator,
The Mach-Zehnder interferometer type light intensity modulator has two modulation electrodes provided in series on upper and lower arms, respectively, and independently controls the phase of light. The wideband amplifiers 155a to 155d amplify the amplitude of the electric signal input to the LN modulator 153 to a value required to drive the LN modulator 153.

In the optical transmitter configured as described above, an optical pulse output from the pulse light source 151 and having a duty ratio of 0.5 or less is subjected to LN modulation having two sets of push-pull type modulation electrodes 153a to 153d. The LN modulator 153 branches the signal into a branch ratio of 1: 1 and independently adds a phase shift in the upper arm and the lower arm of the LN modulator 153. Here, FIG.
As shown in (1), the LN modulator 153 performs bi-phase driving in which signals input to modulation electrodes provided at corresponding positions on the upper arm and the lower arm of the LN modulator 153 are inverted signals from each other. ing.

The electric signals CH1 and CH2 ′ (inversion of CH2) applied to the upper arm of the LN modulator 153 have driving amplitudes as shown in FIGS. 25 (a) and 25 (b), respectively. These electric signals CH1 and CH2 ′ are
As shown in (a) and (b), the duty ratio is at or near 0.5 and its amplitude is
a, 155c is set to Vπ / 2. Further, the temporal relative phases of these two electric signals are set so as to be shifted 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 phase shift of π is added.

Electric signals CH1 'and CH2 applied to the other lower arm of LN modulator 153 are respectively 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,
For the lower arm of the Mach-Zehnder interferometer type optical intensity modulator, the driving amplitude of the inverted logic of CH1 is applied to the first modulation electrode, and the driving amplitude of CH2 is applied to the next modulation electrode. . Therefore, the phase shift applied to the lower arm is inverted from that of 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, the relative phase difference has an overall amplitude of 2π as shown in FIG. The bias voltage is adjusted so that the driving point shown in FIG. 27A matches the extinction voltage of the Mach-Zehnder interferometer type optical intensity modulator. That is, as shown in FIG.
Zero point driving is performed such that the direction of the driving 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 an alternating phase as shown in FIG. Becomes

The optical transmitter according to the present embodiment comprises an LN modulator 15
3 and two sets of push-pull modulation electrodes 153a.
153d through the wideband amplifiers 155a to 155d to multiplex the two electric signals by applying the electric signals CH1 and CH2, and a half or less of the generated optical signal. Since it operates with a bit rate signal, one multiplexing circuit can be omitted, and an inexpensive amplifier for driving the modulator can be used.

When the multiplexing function is not required, the optical transmitter according to the present embodiment may be constituted by using an LN modulator having a set of push-pull type modulation electrodes.

FIG. 29 shows the configuration of the 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 for generating stable continuous light is used in place of the pulse light source 151, and duty ratio controllers 157a to 157d capable of electrically controlling the duty ratio are replaced with broadband amplifiers 155a to 155.
Only the points provided on the input side of 5d are different, and 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 tunable laser.

Duty ratio controllers 157a to 157
7d is provided for controlling the duty ratio of the generated light pulse, and can electrically control the duty ratio as shown in FIG. Accordingly, it is possible to generate an optical pulse having a stable duty ratio by suppressing interference between optical pulses without using a pulse light source having a variable duty ratio. Such a duty ratio controller can be realized by a combination of a digital IC and the like. When the duty ratio is controlled to suppress the interference between the light pulses, the above-described FIGS. 25, 26, and 27 are as shown in FIGS. 31, 32, and 33, respectively. 25 and 2 except for
6, similar to FIG.

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 to prevent generation of harmonics.

In the optical transmitter according to the eighth embodiment having the above-described configuration, the continuous light from the CW light source 161 is converted into an electric signal of an RZ code amplified by the wideband amplifiers 155a to 155d, as in the case of FIG. The signal is modulated by the LN modulator 153 by CH1 and CH2, encoded into an alternating phase RZ code, multiplexed, and output.

FIG. 34 shows the configuration of the optical transmitter according to the ninth embodiment of the present invention. The optical transmitter shown in FIG.
Compared with the optical transmitter shown in FIG. 4, an LN modulator 15 having two sets of push-pull type modulation electrodes 153a to 153d
4 sets of push-pull modulation electrodes 163a-
163h is used, and eight wideband amplifiers 155a to 155h are used in accordance with four sets of push-pull modulation electrodes 163a to 163h. Yes, and the other configurations and operations are the same.

The optical transmitter of the ninth embodiment configured as described above has the same basic operation as that of the optical transmitter of the seventh embodiment shown in FIG. 24, but has four sets of push-pull type modulation. The electric signals applied to each arm of the LN modulator 163 having the electrodes 163a to 163h are configured to be inverted between the opposing modulation electrodes. Then, the optical pulses from the pulse light source 151 are transmitted to the broadband amplifiers 155a to 155.
h, RZ code electric signals CH1, CH2, C
The signal is modulated by the LN modulator 163 using H3 and CH4, encoded into an alternating phase RZ, multiplexed, and output.

FIG. 35 shows the configuration of the optical transmitter according to the tenth embodiment of the present invention. The optical transmitter shown in FIG. 35 is different from the optical transmitter shown in FIG.
Instead of using a CW light source 161 that generates stable continuous light, the duty ratio controllers 157a to 157h capable of electrically controlling the duty ratio are replaced with a broadband 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, similarly to the case of FIG. 34, the CW light source 161 is used.
Signals CH1, CH2, CH3, C of RZ code amplified by the broadband amplifiers 155a to 155h from the continuous light from
The signal is modulated by the LN modulator 163 by H4, encoded into an alternating phase RZ code, multiplexed, and output.

FIG. 36 shows the configuration of an optical transmitter in the case where the optical band limiting means 71 shown in FIGS. 15 and 16 is provided for 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 manner, unnecessary harmonic components can be removed as in the case of the above-described fifth embodiment, and the generated optical signal has an excess band over the required band. By occupying the bandwidth, it is possible to prevent a decrease in band use efficiency.

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

FIG. 37 shows the configuration 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 optical transmitters shown in FIG. 6 in parallel, and the plurality of optical transmitters are set so as to output optical signals having different optical wavelengths, respectively. The optical wavelength multiplexing means 73 for multiplexing and wavelength-multiplexing the optical signals output from the respective optical transmitters is provided.

The optical wavelength multiplexing means 73 has a periodic transmission band having a characteristic of blocking components other than the required optical signal band, thereby removing unnecessary harmonic components. It is.

Referring to FIG. 38, the operation of the optical transmitter shown in FIG. 37 will be described. FIG. 38A schematically shows an example of an optical spectrum of optical signals having different wavelengths output from a plurality of optical transmitters provided in parallel in the optical transmitter shown in FIG. As shown in FIG.
Unnecessary harmonic components appear on both outer sides of a necessary signal band shown as an optical signal bit rate before wavelength multiplexing in the variable duty ratio optical pulse generating means 1 or the encoding means 5 constituting each optical transmitter. ing.

As shown in FIG. 38B, the optical wavelength multiplexing means 73 has a periodic transmission band having a characteristic of blocking such unnecessary harmonic components, that is, components other than the required optical signal band. are doing. Therefore, by providing the optical wavelength multiplexing means 73 having such a periodic transmission band characteristic, unnecessary harmonic components appearing on both outer sides can be reduced as shown in FIG.
It can be suppressed as shown in FIG. As a result, it is possible to prevent a decrease in band use efficiency due to the generated optical signal occupying an excessive band more than a 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 optical transmitters shown in FIG. 6 are provided in parallel. Instead, a plurality of optical transmitters shown in FIG. 18 and optical transmitters shown in FIG. May be provided.

The present invention is not limited to the above-described embodiment, but can be implemented in various modifications without departing from the gist thereof.

[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 both the duty ratio to an appropriate value and to achieve both high dispersion tolerance and small deterioration in 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, a high dispersion tolerance can be stably maintained. That is, since a stable optical transmitter can be configured, it is suitable for constructing a larger network.

Further, according to the present invention, the optical transmitter has a signal multiplexing function and can operate with a signal having a bit rate equal to or less than half the optical signal to be generated. One circuit can be omitted, and an inexpensive amplifier for driving the modulator can be used.
Economy can be achieved.

Further, according to the present invention, unnecessary harmonic components contained in the optical signal generated by the optical transmitter are removed by the optical band limiting means, so that the generated optical signal is more than the required band. It is possible to prevent a decrease in the band use efficiency due to occupying a new band, and to improve the band use efficiency.

According to the present invention, unnecessary harmonic components are removed by the optical multiplexing means having a periodic transmission band having a characteristic of blocking components other than the required optical signal band. Occupying an excessive band rather than a narrow band can prevent a decrease in band use efficiency, improve band use efficiency, and prevent deterioration of transmission characteristics due to crosstalk between optical signals of different wavelengths. be able to.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a conventional example disclosed in JP-A-10-79705.

FIG. 2 is a graph showing a 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 the 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 waveforms of respective units.

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 an optical multiplexing unit.

FIG. 10 is a view showing an example of an output light spectrum of the optical transmitter shown in FIG. 8 under each phase condition.

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.

FIG. 13 is a diagram showing how to give a delay by an optical delay unit in the optical transmitter shown in FIG. 12;

FIG. 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 view 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 view for explaining the operation of the optical transmitter shown in FIG. 18;

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

FIG. 21 is a diagram illustrating a case where a Mach-Zehnder modulator constituting an encoding unit in the optical transmitter illustrated in FIG. 18 is driven on one side;

FIG. 22 is a block diagram showing a configuration example in a case where an optical band limiting unit is provided for the optical transmitter shown in FIG. 18;

FIG. 23 is a block diagram showing another configuration example when an optical band limiting unit is provided for the optical transmitter shown in FIG. 18;

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, and is a diagram showing a phase shift in the upper arm of the Mach-Zehnder interferometer type optical intensity modulator.

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

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

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

FIG. 29 is a diagram illustrating a 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. 29.

FIG. 31 is a diagram for explaining the operation of the optical transmitter shown in FIG. 29, 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 illustrating a configuration of an optical transmitter according to a ninth embodiment of the present invention.

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

FIG. 36 is a block diagram showing a configuration in a case where an optical band limiting unit is provided for the optical transmitter shown in FIG. 24;

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

FIG. 38 is a view for explaining the operation of the optical transmission device shown in FIG. 33;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Duty ratio variable optical pulse generation means 3,51 Optical demultiplexing means 5,7,53-56,69 Encoding means 9 Optical delay unit 11,61 Optical multiplexing means 21 Semiconductor mode-locked laser 23 Y branch 25,27 Mach-Zehnder interference Measuring modulator 31, 45 Directional coupler 35, 57 to 59 Variable optical 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 Generated harmonic pulse Means 67a, 69a Push-pull Mach-Zehnder modulator 71 Optical band limiting means 73 Optical wavelength multiplexing means 151 Pulse light source 153 LN modulator 155 Broadband amplifier 157 Duty ratio controller 1000 Light source section 2000 Coding section

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/152 H04B 9/00 D 10/142 L 10/04 10/06 H04L 25/02 303 25/03 Applied for the application of Article 30, Paragraph 1 of the Patent Act. Published in "ELECTRONICS LETTERS Vol. 35, No. 25" issued on December 9, 1999. No. 1 Nippon Telegraph and Telephone Corporation (72) Inventor Noriyoshi Sato 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Masao Yube Otemachi, Chiyoda-ku, Tokyo 2-3-1, Nippon Telegraph and Telephone Corporation F-term (reference) 2H079 AA02 AA12 BA01 CA04 DA03 EA05 FA02 GA01 GA03 HA11 KA18 KA20 5K002 AA02 BA02 BA04 BA05 BA13 CA01 CA 03 CA12 CA14 DA02 DA06 DA31 FA01 5K029 AA03 CC04 DD02 GG03 GG05 JJ01

Claims (36)

[Claims] 1. A light source section 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, and a light source section between the optical clock pulses in adjacent time slots. An encoding section that sets the relative optical phase difference to an odd multiple of π, and encodes the optical clock pulse with an electric signal synchronized with the optical clock pulse. 2. The light source section includes a mode-locked laser type light source that generates an optical clock pulse synchronized with a signal bit rate at a constant duty ratio and variably sets the duty ratio of the optical clock pulse. The optical transmitter according to claim 1, wherein: 3. The encoding section encodes each of the output signals divided by the optical clock pulse with an electric signal synchronized with the optical clock pulse. Encoding delay means for delaying one of the output signals demultiplexed by the demultiplexing means with respect to one of the output signals, and multiplexing means for multiplexing the output signal of the encoding delay means And the delay given by the encoding delay means to the other output signal is, assuming that the number of multiplexes in the multiplexing means is n and an arbitrary integer is m, the time of one time slot of encoding (K / n) of width + m
2. The optical transmitter according to claim 1, wherein (k = 1, 2,..., (N-1)) times, and the relative optical phase difference is an odd multiple of π. 4. The encoding delay means comprises: encoding means for encoding each of the output signals demultiplexed by the demultiplexing means by an electric signal synchronized with the optical clock pulse; and encoding 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 includes a delay means for delaying at least a part of the output signal demultiplexed by the demultiplexing means, and an output signal delayed by the delay means and the delay signal, if any. 4. The optical transmitter according to claim 3, further comprising encoding means for encoding each of the output signals not delayed by the means with an electric signal synchronized with the optical clock pulse. 6. A light spectrum extracting means for splitting a part of a signal multiplexed by said multiplexing means and extracting a center component of the light spectrum or a line spectrum adjacent to the center component, and said light 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 the line spectrum extracted in step (b). 7. The encoding delay unit includes a variable delay unit that can vary a delay amount, and the feedback unit minimizes a central component or a line spectrum extracted by the optical spectrum extraction unit. 7. The optical transmitter according to claim 6, further comprising delay control means for generating a feedback signal for controlling said variable delay means. 8. A light source control means for generating a feedback signal for controlling the light source section so that a center component or a line spectrum extracted by the light spectrum extraction means becomes maximum or minimum. 6. The optical transmitter according to 6. 9. The apparatus according to claim 6, wherein said part branched by said optical spectrum extracting means is a phase-inverted component of a signal multiplexed by said multiplexing means.
An optical transmitter as described. 10. The light source section includes continuous light generating means for generating continuous light, and a push-pull Mach-Zehnder modulator driven at zero point. The encoding section includes the push-pull Mach-Zehnder modulator. 2. The optical transmitter according to claim 1, comprising an encoder for encoding the output of the push-pull 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 converts the optical clock pulse from continuous light generated by the continuous light generation means. 11. The optical transmitter according to claim 10, wherein the optical transmitter generates and sets a relative optical phase difference between the optical clock pulses in adjacent time slots to an odd multiple of π. 12. The optical transmitter according to claim 10, wherein the push-pull Mach-Zehnder modulator can variably set a duty ratio of the optical clock pulse by controlling a driving amplitude. 13. The optical transmitter according to claim 10, wherein the push-pull Mach-Zehnder modulator optimizes a waveform of the optical clock pulse to a waveform having a high dispersion tolerance by controlling a driving waveform. . 14. The encoding section comprises an LN modulator having at least one set of push-pull modulation electrodes, the push-pull being provided at corresponding positions on the upper and lower arms of the LN modulator. 2. The optical transmitter according to claim 1, wherein the signals input to the pattern modulation electrode are driven to zero as inverted signals. 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. The optical transmitter according to claim 14, wherein: 16. The light source section includes a CW light source for generating continuous light, and a duty ratio controller provided on an input side of a push-pull type modulation electrode of the LN modulator and capable of electrically controlling a duty ratio. 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 unnecessary harmonic components included in an optical signal generated by the optical transmitter. 18. The apparatus according to claim 17, wherein said optical band limiting means has a transmission band having a characteristic of blocking components other than a necessary optical signal band so as to remove unnecessary harmonic components. Light transmitter. 19. An optical transmitter provided in parallel and configured to output optical signals of different optical wavelengths, wherein each optical transmitter outputs an optical clock pulse synchronized with a signal bit rate. A light source section that is generated at a constant duty ratio and that can variably set the duty ratio of the optical clock pulse, and sets a relative optical phase difference between the optical clock pulses of adjacent time slots to an odd multiple of π, An encoding section for encoding the optical clock pulse by an electric signal synchronized with the optical clock pulse; and optical signals having different optical wavelengths respectively output from the plurality of optical transmitters. And an optical wavelength multiplexing means for multiplexing and outputting. 20. The optical wavelength multiplexing means having a periodic transmission band having a characteristic of blocking components other than a necessary optical signal band so as to remove unnecessary harmonic components. 20. The optical transmission device according to 19. 21. Using a configuration in which the duty ratio of an optical clock pulse can be variably set, the duty ratio of the optical clock pulse is set to a value that reduces interference between pulses, and the signal is output while the duty ratio is kept constant. Generating the optical clock pulse synchronized with the bit rate; setting the relative optical phase difference between the optical clock pulses between adjacent time slots to an odd multiple of π; An optical transmitter control method comprising encoding an optical clock pulse. 22. An optical transmitter comprising: an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate; and an 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. An optical transmitter comprising: an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate; and an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. An optical transmitter having 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. 24. An optical transmitter comprising: an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate; and an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. An optical transmitter, wherein a repetition frequency of the optical clock pulse is twice a frequency of an electric signal for driving the optical pulse generating means. 25. An optical clock pulse generating means for generating an optical clock pulse synchronized with a signal bit rate and having a variable duty ratio, a demultiplexing means for demultiplexing the optical clock pulse, and a demultiplexing means for demultiplexing the optical clock pulse. Encoding means for encoding each of the outputted output signals with an electric 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. And delay 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 the number of multiplexes in the multiplexing means is an integer n with respect to the time slot of encoding. Doubled and its vicinity,
An optical transmitter characterized in that the relative optical phase difference is an odd multiple of π or its vicinity. 26. An optical clock pulse generating means for generating an optical clock pulse capable of changing a duty ratio in synchronization with a signal bit rate, a demultiplexing means for demultiplexing the optical clock pulse, and a demultiplexing means for demultiplexing the optical clock pulse. Delay means for delaying another output signal other than at least one of the output signals, and the other output signal delayed by the delay means and the at least one output signal not delayed by the delay means. Encoding means for encoding each of the output signals with an electric signal synchronized with the optical clock pulse, and multiplexing means for multiplexing the output signals encoded by the encoding means; The delay is obtained by changing the number of multiplexes in the multiplexing means with respect to a time slot of encoding to an integer n.
Then, k / n (k: 1,..., (N-1)) or a value obtained by adding an integer multiple of the time slot to the value and its vicinity, and the relative optical phase difference is an odd multiple of π or its vicinity. An optical transmitter, comprising: 27. The delay means is a variable delay means capable of varying a delay amount, and branches a part of the signal multiplexed by the multiplexing means;
There is further provided an optical spectrum extracting means for extracting a line spectrum adjacent to the center of the optical spectrum, and a delay control means for controlling the variable delay means so that the line spectrum extracted by the optical spectrum extracting means is maximized. The optical transmitter according to claim 25 or 26, wherein: 28. The delay means is a variable delay means capable of varying a delay amount, and branches a part of the signal multiplexed by the multiplexing means;
Further comprising: an optical spectrum extracting means for extracting a central component of the optical spectrum; and a delay control means for controlling the variable delay means such that the central component of the optical spectrum extracted by the optical spectrum extracting means is minimized. The optical transmitter according to claim 25 or 26, wherein: 29. The delay means is a variable delay means capable of varying a delay amount, and branches a part of the signal multiplexed by the multiplexing means;
Light spectrum extraction means for extracting a line spectrum adjacent to the center of the light spectrum; and light source control means for controlling the optical clock pulse generation means so that the line spectrum extracted by the light spectrum extraction means is maximized. The optical transmitter according to claim 25, further comprising: 30. The optical transmitter according to claim 27, wherein the part is a component obtained by inverting the phase of the signal multiplexed by the multiplexing means. 31. An optical transmitter comprising: an optical pulse generating means for generating an optical clock pulse synchronized with a signal bit rate; and an encoding means for encoding the optical clock pulse with an electric signal synchronized with the optical clock pulse. An optical transmitter, wherein the encoding means has a signal multiplexing function. 32. An optical transmitter comprising continuous light generating means for generating continuous light and coding means for coding the continuous light, wherein the relative light level is determined for each pulse of the optical signal generated by the coding means. An optical transmitter comprising means for setting a phase difference to an odd multiple of π or a vicinity thereof. 33. 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. Optical transmitter. 34. The optical transmission according to claim 25, further comprising an optical band limiting unit that removes unnecessary harmonic components contained in an optical signal generated by the optical transmitter. vessel. 35. A plurality of optical transmitters according to claim 25, wherein said plurality of optical transmitters are set so as to output optical signals having different optical wavelengths, respectively. Multiplexes and wavelength-multiplexes optical signals of different optical wavelengths output from each optical transmitter, and blocks components other than the necessary optical signal band so that unnecessary harmonic components can be removed. An optical transmitter, comprising: an optical multiplexing unit having a periodic transmission band having a characteristic of: 36. The optical band limiting means has a characteristic of combining optical signals of a plurality of different optical wavelengths and blocking components other than the necessary optical signal band so as to remove unnecessary harmonic components. The optical transmitter according to claim 34, further comprising an optical multiplexing unit having a transmission band.