JP5542620B2 - Optical transmitter and optical communication system - Google Patents

Description

  The present invention relates to an optical transmitter, an optical receiver, and an optical communication system including these for a long-distance high-speed optical communication system.

  In recent years, in response to an increase in communication demand, there is an increasing demand for large capacity for submarine cable optical communication systems and transcontinental land optical communication systems using repeaters. On the other hand, the capacity of the optical communication system can be increased by increasing the number of wavelength multiplexing, but this is starting to show the limit. Against this background, in recent years, there is an increasing demand for improving the transmission speed from 10 Gb / s to 40 Gb / s.

  On the other hand, the deterioration factor of transmission performance that becomes apparent in long-distance transmission is known. For example, there are spontaneous emission (ASE) noise, non-linear effects and dispersion generated in an erbium-doped fiber amplifier (EDFA). Also, polarization mode dispersion (PMD) can degrade transmission performance. Further, in a wavelength division multiplexing (WDM) optical communication system, crosstalk between channels can also be a problem.

  It is known that the performance deterioration due to the above factors becomes more and more remarkable as the transmission speed is improved. For example, as the speed increases, the influence of ASE noise increases. Therefore, a method of increasing the light intensity of the transmission light and maintaining the resistance to ASE noise is taken. However, if the light intensity of the transmitted light is increased too much, the nonlinear effect becomes obvious, and the transmission characteristics rather deteriorate. Furthermore, the inter-channel crosstalk also increases as the light intensity increases. That is, as the transmission speed is improved, it is difficult to satisfy both ASE noise and non-linear effects. Further, the influence of dispersion and PMD increases in proportion to the square of the transmission rate.

  In particular, in the conventional intensity modulation system, when the transmission speed is 40 Gb / s, the above-described performance degradation is significant, and it is difficult to realize long-distance optical transmission. In order to solve this problem, optical transmission systems of various modulation schemes have been devised. As one of the methods, there is an alternating polarization modulation RZ-DQPSK (Return-to-Zero Differential Quadrature Phase Shift Keying) method (see, for example, Patent Document 1).

  Here, the DQPSK system is a phase modulation system that transmits and receives an optical signal having phase differences of 0, π / 2, π, and 3π / 2 between the preceding and following time slots. Can send and receive information. RZ is a modulation method for shaping an optical signal in each time slot into an RZ pulse. In the alternating polarization modulation, the polarization state of the optical signal is changed for each time slot by A, B, A, B,. . . And the two polarization states are repeated. In particular, as the polarization states A and B, linear polarization states orthogonal to each other are often used.

  Furthermore, in the optical communication system disclosed in Patent Document 1, by using a variable delay device, each modulator (RZ modulator, DQPSK modulator, and polarization modulator) is synchronized to adjust the skew, We are trying to ensure performance.

Special table 2009-529834

As described above, in the optical communication system disclosed in Patent Document 1, each modulator can be synchronized by using a variable delay device. On the other hand, the delay time difference (the difference between the time when the modulator operates and the time when the optical signal is input to the modulator) when synchronization is performed using the variable delay device depends on the temperature. However, in the optical communication system disclosed in Patent Document 1, temperature is not taken into consideration, and it is difficult to operate in a wide temperature range, and it is difficult to synchronize with high accuracy, so that good transmission characteristics can be obtained. There was no problem.
In addition, when PMD is present, jitter occurs in the time waveform of the alternating polarization RZ-DQPSK modulated light transmitted from the transmitter, and the alternating signals are overlapped with each other, so that good transmission characteristics can be obtained. There was no problem.

  The present invention has been made to solve the above-described problems, and operates in a wide temperature range and performs accurate synchronization, and also suppresses signal jitter and signal interference between adjacent bits. An object of the present invention is to provide an optical transmitter, an optical receiver, and an optical communication system capable of stably realizing good transmission performance.

An optical transmitter of an optical communication system according to the present invention includes a first delay function unit that outputs an electric clock signal in which a delay time is adjusted based on a set delay amount, and an electric clock from the first delay function unit An RZ modulator that RZ-modulates the input optical signal according to the signal and a phase modulation of the optical signal that is RZ-modulated by the RZ modulator according to the input transmission data. A data modulator, a second delay function unit that outputs an electrical clock signal whose delay time is adjusted based on the set delay amount, and a data modulator that is provided in a subsequent stage of the data modulator, In accordance with the electrical clock signal, the polarization modulator that alternately polarizes the optical signal phase-modulated by the data modulator, the optical coupler that branches the optical signal from the polarization modulator, and the optical coupler that branches Optical signal strength A monitor unit that converts into an electric signal, a detection unit for measuring a peak value of the electric signal from the monitor unit, based on the peak value of the electrical signal, a controller for adjusting the delay of second delay function unit It is equipped with.

  According to the present invention, since it is configured as described above, it can be operated in a wide temperature range, can be accurately synchronized, and can suppress signal jitter and signal interference between adjacent bits. And good transmission performance can be stably realized.

It is a figure which shows the structure of the optical communication system which concerns on Embodiment 1 of this invention. It is a graph which shows the delay time of the optical fiber which connects the RZ modulator and DQPSK modulator in Embodiment 1 of this invention. It is a graph which shows the delay time difference between the DQPSK modulator and RZ modulator in Embodiment 1 of this invention. It is a graph which shows the delay time difference between the polarization modulator and DQPSK modulator in Embodiment 1 of this invention. It is a figure which shows the structure of the conventional optical communication system. It is a graph which shows the delay time difference between the conventional DQPSK modulator and the RZ modulator. It is a graph which shows the delay time difference between the conventional polarization modulator and DQPSK modulator. It is a graph which shows the calculation error of the delay time difference between the DQPSK modulator and RZ modulator in Embodiment 1 of this invention. It is a graph which shows the calculation error of the delay time difference between the polarization modulator and DQPSK modulator in Embodiment 1 of this invention. It is a graph which shows the calculation error of the delay time difference between the conventional DQPSK modulator and the RZ modulator. It is a graph which shows the calculation error of the delay time difference between the conventional polarization modulator and DQPSK modulator. It is a figure which shows the structure of the optical communication system which concerns on Embodiment 2 of this invention. It is a graph which shows the output of the monitor part in Embodiment 2 of this invention. It is a figure which shows another structure of the optical communication system which concerns on Embodiment 2 of this invention. It is a graph which shows the output of the detection part of the optical communication system shown in FIG. It is a figure which shows another structure of the optical communication system which concerns on Embodiment 2 of this invention. FIG. 17 is a graph showing an output (when the delay time is optimum) of the polarization beam splitter of the optical communication system shown in FIG. 16. It is a graph which shows the output (when delay time has shifted half phase) of the polarization beam splitter of the optical communication system shown in FIG. It is a graph which shows the output of the detection part of the optical communication system shown in FIG. It is a figure which shows the structure of the optical communication system which concerns on Embodiment 3 of this invention. It is a figure which shows the time waveform (when there is no PMD) of the alternating polarization RZ-DQPSK modulated light in Embodiment 3 of this invention. It is a figure which shows the time waveform (when there exists PMD) of the alternating polarization RZ-DQPSK modulated light in Embodiment 3 of this invention. It is a figure which shows the structure of the polarization mode dispersion compensation part in Embodiment 3 of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, a long-distance high-speed optical communication system using the alternating polarization modulation RZ-DQPSK system will be described.
Embodiment 1 FIG.
1 is a diagram showing a configuration of an optical communication system 1 according to Embodiment 1 of the present invention.
As shown in FIG. 1, the optical communication system 1 includes an optical transmitter 2 that transmits an optical signal (alternate polarization RZ-DQPSK modulated optical signal), and a transmission path 3 that transmits the optical signal from the optical transmitter 2. And an optical receiver 4 that receives an optical signal from the optical transmitter 2 via the transmission path 3.

The optical transmitter 2 includes a light source (LD) 201, a MUX function unit 202, a variable delay function unit (first delay function unit) 203, an RZ modulation drive unit 204, an RZ modulator 205, and a precode (Prec) function unit 206. , DQPSK modulator driving unit 207, DQPSK modulator (data modulator) 208, variable delay function unit (second delay function unit) 209, polarization modulation driving unit 210, polarization (PoM) modulator 211, temperature measurement The unit 212 and the controller 213a are included. In the optical transmitter 2, the functional units to which optical signals are input / output are connected by an optical fiber (double line portion), and the functional units to / from which electric signals are input / output are connected by an electric path (single line portion). It is connected.
Here, each modulator is arranged in the order of RZ modulator 205, DQPSK modulator 208, and polarization modulator 211.

  The light source 201 emits CW light. The CW light emitted from the light source 201 is output to the RZ modulator 205.

The MUX function unit 202 generates an electric clock signal or a transmission data signal. The MUX function unit 202 generates an electric clock signal CLK1 in order to cause the RZ modulator 205 to RZ-modulate the CW light. The electric clock signal CLK1 generated by the MUX function unit 202 is output to the variable delay function unit 203.
Further, the MUX function unit 202 bundles the transmission data di (i = 1 to m) from the precoding function unit 206 and generates two transmission data signals TI and TQ for the I channel and the Q channel. The transmission data signals TI and TQ generated by the MUX function unit 202 are output to the DQPSK modulator driving unit 207.
In addition, the MUX function unit 202 generates the electrical clock signal CLK <b> 2 in order to cause the polarization modulator 211 to alternately polarize the RZ-DQPSK modulated light. The electrical clock signal CLK2 has a speed that is half the symbol rate in order to cause the polarization modulator 211 to perform alternating polarization modulation. The electric clock signal CLK2 generated by the MUX function unit 202 is output to the variable delay function unit 209.

  The variable delay function unit 203 adjusts the delay time of the electrical clock signal CLK1 from the MUX function unit 202 based on the variable delay amount set by the controller 213a. The electric clock signal CLK1 whose delay time is adjusted by the variable delay function unit 203 is output to the RZ modulation driving unit 204.

  The RZ modulation driving unit 204 amplifies the electric clock signal CLK1 from the variable delay function unit 203. The electric clock signal CLK1 amplified by the RZ modulation driving unit 204 is output to the RZ modulator 205.

  The RZ modulator 205 performs RZ modulation on the CW light from the light source 201 in accordance with the electric clock signal CLK1 from the RZ modulation driving unit 204 to generate RZ modulated light. The RZ modulated light generated by the RZ modulator 205 is output to the DQPSK modulator 208.

  The precoding function unit 206 performs DQPSK modulation on the RZ modulated light by the DQPSK modulator 208, so that the m transmission data signals di (i = 1 to m) having a speed of 1 / m of the bit rate. 2 bit delayed DQPSK precode processing is performed. The transmission data signal di on which the 2-bit delayed DQPSK precoding processing has been performed by the precoding function unit 206 is output to the MUX function unit 202.

  The DQPSK modulator driving unit 207 amplifies the transmission data signals TI and TQ from the MUX function unit 202, respectively. The transmission data TI and TQ amplified by the DQPSK modulator driving unit 207 are output to the DQPSK modulator 208 to drive the DQPSK modulator 208.

  The DQPSK modulator 208 performs DQPSK modulation on the RZ modulated light from the RZ modulator 205 in accordance with the transmission data TI and TQ from the DQPSK modulator driving unit 207, and generates RZ-DQPSK modulated light having a symbol rate. Is. The RZ-DQPSK modulated light generated by the DQPSK modulator 208 is output to the polarization modulator 211.

  The variable delay function unit 209 adjusts the delay time of the electric clock signal CLK2 from the MUX function unit 202 based on the variable delay amount set by the controller 213a. The electric clock signal CLK 2 whose delay time is adjusted by the variable delay function unit 209 is output to the polarization modulation driving unit 210.

  The polarization modulation driving unit 210 amplifies the electric clock signal CLK2 from the variable delay function unit 209. The electric clock signal CLK2 amplified by the polarization modulation driver 210 is output to the polarization modulator 211.

  The polarization modulator 211 performs alternating polarization modulation on the RZ-DQPSK modulated light from the DQPSK modulator 208 in accordance with the electric clock signal CLK2 from the polarization modulation driving unit 210, thereby alternating polarization modulation RZ-DQPSK modulation. It generates light. The alternating polarization modulated RZ-DQPSK modulated light generated by the polarization modulator 211 is transmitted to the optical receiver 4 via the transmission path 3.

  The temperature measurement unit 212 measures the temperature of the optical fiber 214 that connects the RZ modulator 205 and the DQPSK modulator 208 and the optical fiber 215 that connects the DQPSK modulator 208 and the polarization modulator 211. The temperature measurement unit 212 measures a voltage signal corresponding to the temperature using, for example, a temperature monitor IC or a thermistor. A voltage signal corresponding to the temperature measured by the temperature measuring unit 212 is output to the controller 213a.

  The controller 213a adjusts the variable delay amounts of the variable delay function units 203 and 209 based on the voltage signal from the temperature measurement unit 212. The controller 213a includes an A / D converter that converts a voltage signal into a digital signal, and a microcomputer that calculates variable delay amounts of the variable delay function units 203 and 209 based on the converted digital signal. Yes. Here, the microcomputer calculates the difference between the time during which the DQPSK modulator 208 operates and the time during which the RZ modulated light is input to the DQPSK modulator 208 based on the digital signal, and outputs the difference to the variable delay function unit 203. Calculate the optimal variable delay amount. Similarly, based on the digital signal, the difference between the time during which the polarization modulator 211 operates and the time during which the RZ-DQPSK modulated light is input to the polarization modulator 211 is calculated to Calculate the optimal variable delay amount. Signals indicating the variable delay amount calculated by the controller 213a are output to the variable delay function units 203 and 209, respectively.

  The optical receiver 4 includes a 2-bit delay interference unit (2 bit ODI) 401, a quad light detection unit (QPD) 402, and a DMUX function unit 403. In the optical receiver 4, functional units to which optical signals are input / output are connected by an optical fiber (double line portion), and functional units to which electric signals are input / output are connected by an electric path (single line portion). It is connected.

  The 2-bit delay interference unit 401 separates the optical signal (alternating polarization RZ-DQPSK modulated optical signal) from the optical transmitter 2 into four optical intensity signals Ori. The 2-bit delay interference unit 401 includes a branching unit that splits an optical signal into two parts and an asymmetric Mach-Zehnder interference unit that has two waveguides that cause the two-branched optical signals to interfere with each other. The two waveguides are provided with an optical path length difference for two symbols. Therefore, in the asymmetric Mach-Zehnder interference unit, optical signals whose time slots are separated by two symbols can be caused to interfere with each other, and the optical intensity signal Ori corresponding to the transmission phase information of the optical signal can be output. The four light intensity signals Ori separated by the 2-bit delay interference unit 401 are output to the quad light detection unit 402.

  The quad light detection unit 402 converts the four light intensity signals Ori from the 2-bit delay interference unit 401 into electric signals Er1 and Er2 having a symbol rate. The electrical signals Er1 and Er2 converted by the quad light detection unit 402 are output to the DMUX function unit 403.

  The DMUX function unit 403 shapes the electric signals Er1 and Er2 from the quad light detection unit 402, reproduces the received electric signal r having the same speed as the bit rate, and then has an n having a speed 1 / n of the bit rate. The received electric signal R of the book is obtained.

Next, the delay time of the optical fiber 214 connecting the RZ modulator 205 and the DQPSK modulator 208 in the transmitter 2 configured as described above will be described with reference to FIG. In addition, the left end and right end of the horizontal axis shown in FIG. 2 are the minimum value and the maximum value of the operating temperature range, respectively.
It can be seen that the delay time of the optical fiber 214 is a monotonically increasing function of temperature, as shown in FIG. In addition, a delay time from the output of the two transmission data signals TI and TQ from the MUX function unit 202 to the operation of the DQPSK modulator 208, and the RZ modulation after the electrical clock signal CLK1 is output from the MUX function unit 202 The delay time until the device 205 operates also changes with temperature fluctuations, but the degree is relatively small.

Here, when the variable delay amount of the variable delay function unit 203 is constant, the difference between the time when the DQPSK modulator 208 operates and the time when the RZ modulated light is input to the DQPSK modulator 208 is shown in FIG. It becomes street. That is, the delay time difference is a monotonically increasing function of temperature, and the slope thereof is m ₂ when approximated by a straight line.

Similarly, when the variable delay amount of the variable delay function unit 209 is made constant, the difference between the time during which the polarization modulator 211 operates and the time during which the RZ-DQPSK modulated light is input to the polarization modulator 211 is As shown in FIG. That is, the delay time difference is a monotonically increasing function of temperature, and the slope thereof is m ₄ when approximated by a straight line.

On the other hand, in the conventional optical communication system, as shown in FIG. 5, each modulator is arranged in the order of a DQPSK modulator 608, an RZ modulator 605, and a polarization modulator 611.
In the conventional optical communication system in which each modulator is arranged as shown in FIG. 5, when the variable delay amount of the variable delay function unit 603 is constant, the time when the DQPSK modulator 608 operates and the RZ-modulated light is DQPSK modulated. The difference from the time input to the device 608 is as shown in FIG. That is, the delay time difference is a monotonically decreasing function of temperature, and the slope thereof is −m ₂ when approximated by a straight line.

Similarly, when the variable delay amount of the variable delay function unit 609 is constant, the difference between the time during which the polarization modulator 611 operates and the time during which the RZ-DQPSK modulated light is input to the polarization modulator 611 is As shown in FIG. That is, the delay time difference becomes a monotonically increasing function of temperature, and the slope thereof is m ₂ + m _{4 when} approximated by a straight line. This is because the delay time due to the optical fiber 614 and the optical fiber 615 becomes dominant in the delay time difference. As described above, in the conventional optical communication system, since the inclination becomes high, the amount of change in the delay time difference becomes large. In this case, there is a possibility that the capability of the variable delay function unit 609 may be exceeded.

  Also, these delay time differences are factors that degrade the transmission characteristics, and are ideally zero. Therefore, the temperature measurement unit 212 measures the temperature, the controller 213a calculates the delay time difference as a function of temperature, and adjusts the variable delay amounts of the variable delay function units 203 and 209. Here, in the calculation by the controller 213a, the approximation accuracy tends to be increased by increasing the order of the approximation function. However, when the order of the approximate function is increased, a high calculation amount is required, so that the time required for adjustment becomes longer. Therefore, in general, approximation is often performed with a linear function. In this case, the slope and intercept, which are parameters of the linear approximation function, are stored in the nonvolatile memory, and the delay time difference is calculated based on the temperature that is always measured by the temperature measurement unit 212. Thereafter, based on the calculation result, the variable delay function units 203 and 209 are adjusted to the optimum variable delay amount.

Here, in the optical communication system according to Embodiment 1, the calculation (prediction) error of the difference between the time when the DQPSK modulator 208 operates and the time when the RZ-modulated light is input to the DQPSK modulator 208 is as shown in FIG. As shown by the solid line. The dotted line is the zero point of the calculated value. As shown in FIG. 8, the calculation error is not necessarily zero, deviates from zero, the maximum value of the calculated error is D _2.
Similarly, the calculation error of the difference between the time when the polarization modulator 211 operates and the time when the RZ-DQPSK modulated light is input to the polarization modulator 211 is as shown by the solid line in FIG. As shown in FIG. 9, the maximum value of the calculation error is D _4.

On the other hand, in the conventional optical communication system shown in FIG. 5, the calculation error of the difference between the time when the DQPSK modulator 608 operates and the time when the RZ modulated light is input to the DQPSK modulator 608 is indicated by a solid line in FIG. As shown. As shown in FIG. 10, the maximum value of the calculation error is consistent with the maximum value of the calculated error when the optical communication system according to Embodiment 1 shown in D _2, and the Fig.
However, in the conventional optical communication system, the calculation error of the difference between the time when the polarization modulator 611 operates and the time when the RZ-DQPSK modulated light is input to the polarization modulator 611 is shown by a solid line in FIG. It becomes street. In this case, unlike the optical communication system according to Embodiment 1 shown in FIG. 9, the maximum value of the calculation error is D ₂ + D ₄ , which is a higher value. That is, during actual operation, the delay time becomes high, so that the transmission characteristics are more significantly degraded.

  As described above, according to the first embodiment, in the optical transmitter 2 using the alternating polarization modulation RZ-DQPSK modulation, the modulators are the RZ modulator 205, the DQPSK modulator 208, and the polarization modulator. Since the arrangement is made in the order of 211, the delay time of the polarization modulator 211 can be adjusted over a wide temperature range using the variable delay function unit 209, and the optimum polarization modulation state is always maintained. Can do. In addition, since the temperature fluctuation amount of the delay time can be suppressed, the delay time can be adjusted with high accuracy and signal interference between adjacent bits can be suppressed, so that good transmission characteristics can be maintained.

Embodiment 2. FIG.
In the first embodiment, the delay time difference is calculated based on the constantly measured temperature, and the variable delay amount of the variable delay function units 203 and 209 is adjusted. In the second embodiment, the output of the polarization modulator 211 is always measured, and based on this output, the time during which the polarization modulator 211 operates and the RZ-DQPSK modulated light are input to the polarization modulator 211. The following is an example of measuring the difference with the time to adjust the variable adjustment amount of the variable delay function unit 209.
FIG. 12 is a diagram showing a configuration of an optical communication system 1 according to Embodiment 2 of the present invention.
The optical communication system 1 according to the second embodiment shown in FIG. 12 deletes the temperature measurement unit 212 and the controller 213a from the optical transmitter 2 according to the first embodiment shown in FIG. (PD) 217 and controller 213b are added. Other configurations are the same, and the same reference numerals are given and description thereof is omitted.

  The optical coupler 216 is provided at the subsequent stage of the polarization modulator 211 and branches the optical signal (alternate polarization RZ-DQPSK modulated optical signal) from the polarization modulator 211 into two. The optical coupler 216 has a polarization dependent loss. Among the optical signals branched into two by the optical coupler 216, one optical signal is transmitted to the optical receiver 4 through the transmission path 3, and the other optical signal is output to the monitor unit 217.

  The monitor unit 217 converts the intensity of the optical signal from the optical coupler 216 into a current signal. The monitor unit 217 can measure the average value of the intensity of the optical signal branched into two. Here, the current signal output from the monitor unit 217 is as shown in FIG. The current signal converted by the monitor unit 217 is output to the controller 213b.

The controller 213b adjusts the variable delay amount of the variable delay function unit 209 based on the current signal from the monitor unit 217. The controller 213b includes a low-speed transimpedance circuit that converts a current signal into a voltage signal, an A / D converter that converts the converted voltage signal into a digital signal, and a variable delay function based on the converted digital signal. And a microcomputer for calculating the variable delay amount of the unit 209. Here, the microcomputer calculates the difference between the time during which the polarization modulator 211 operates and the time during which the RZ-DQPSK modulated light is input to the polarization modulator 211 based on the digital signal, and the variable delay function. The optimum variable delay amount for the unit 209 is calculated. A signal indicating the variable delay amount calculated by the controller 213b is output to the variable delay function unit 209.
The variable delay function unit 209 adjusts the delay time of the electric clock signal CLK2 from the MUX function unit 202 based on the variable delay amount set by the controller 213b.

  Here, since the optical coupler 216 has a polarization dependent loss, the output of the optical coupler 216 changes depending on the delay time of the polarization modulator 211. It is assumed that the output of the optical coupler 216 is maximized when the polarization angle is 0 degree. This can be realized when the optical coupler 216 is a polarization preserving type. When the polarization modulator 211 is synchronized with the RZ-DQPSK modulated light, the output of the optical coupler 216 is minimal (the optimum point A shown in FIG. 13). On the other hand, when the polarization modulator 211 is shifted by a half phase to the RZ-DQPSK modulated light, the output of the optical coupler 216 is maximized. As described above, the delay time of the polarization modulator 211 is adjusted by using the polarization dependent loss of the optical coupler 216 and adjusting the variable delay amount of the variable delay function unit 209 so that the output of the optical coupler 216 is minimized. Can be adjusted.

Next, another configuration of the optical communication system according to Embodiment 2 is shown in FIG. The difference from the optical communication system according to the second embodiment shown in FIG. 12 is that a detection unit (DET) 218 is provided in the subsequent stage of the monitor unit 217.
The detection unit 218 measures the peak value of the current signal from the monitor unit 217. The detector 218 is constituted by a detector circuit using, for example, a Schottky diode. Here, the peak value measured by the detection unit 218 is as shown in FIG. 15, and the change in the delay time of the polarization modulator 211 can be measured with higher sensitivity than the average value shown in FIG. Therefore, the delay time of the polarization modulator 211 can be stably adjusted with high accuracy.

FIG. 16 shows still another form of the optical communication system according to the second embodiment. The difference from the optical communication system shown in FIG. 14 is that a polarization beam splitter (PBS) 219 is provided in front of the monitor unit 217.
The polarization beam splitter 219 separates the optical signal from the optical coupler 216 into 45 degree polarization and −45 degree polarization, and outputs the 45 degree polarization to the monitor unit 217. Here, the time waveform of the output of the polarization beam splitter 219 when the delay time of the polarization modulator 211 is optimum is as shown in FIG. In this case, the time pulse that is not output follows the time pulse output from the polarization beam splitter 219, and this is repeated.

On the other hand, the time waveform of the output of the polarization beam splitter 219 when the delay time of the polarization modulator 211 is shifted by a half phase is as shown in FIG. In this case, an optical signal with a peak of 0 degrees and an optical signal with a peak of 90 degrees are alternately input to the polarization beam splitter 219. Therefore, the polarization beam splitter 219 outputs time pulses having the same intensity.
As described above, the peak value measured by the detector 218 using the polarization measured by the polarization beam splitter 219 is as shown in FIG. 19 and is compared with the average value and peak value shown in FIGS. Thus, the change in the delay time of the polarization modulator 211 can be measured with higher sensitivity. Therefore, the delay time of the polarization modulator 211 can be adjusted more stably and with high sensitivity.

  As described above, according to the second embodiment, since a part of the output light of the polarization modulator 211 is converted into a current signal and adjusted so that the current value is minimized, The polarization modulator 211 can be easily synchronized, and the optimum polarization modulation state can always be maintained. Also, by providing the detection unit 218 at the subsequent stage of the monitor unit 217, the delay time of the polarization modulator 211 can be adjusted stably and with high accuracy. Furthermore, by providing the polarization beam splitter 219 in front of the monitor unit 217, the delay time of the polarization modulator 211 can be adjusted more stably and with high sensitivity.

Embodiment 3 FIG.
FIG. 20 is a diagram showing a configuration of an optical communication system 1 according to Embodiment 3 of the present invention.
An optical communication system 1 according to the second embodiment shown in FIG. 20 is obtained by adding a polarization mode dispersion compensation unit (PMDC) 404 to the optical receiver 4 according to the first embodiment shown in FIG. Other configurations are the same, and the same reference numerals are given and description thereof is omitted.

First, the time waveform of the alternating polarization RZ-DQPSK modulated light received by the optical receiver 4 will be described.
FIG. 21 is a time waveform showing ideal alternating polarization RZ-DQPSK modulated light when there is no polarization mode dispersion (PMD). In the case of alternating polarized RZ-DQPSK modulated light, when there is no PMD, as shown in FIG. 21, the time waveform has little jitter, and the overlap between the alternating signals is small.

On the other hand, FIG. 22 is a time waveform of the alternating polarization RZ-DQPSK modulated light when there is PMD. As shown in FIG. 22, when the alternating polarization RZ-DQPSK modulated light has PMD, jitter occurs in the time waveform, and the alternating signals overlap. The jitter occurs because the group delay time of the optical signal depends on the polarization due to the PMD of the transmission path 3. Further, the overlap between the alternating signals occurs because the polarization state of the transmission light is different and the group delay time of the optical signal changes due to PMD of the transmission path 3.
Therefore, in order to compensate for PMD of the transmission path 3, the polarization mode dispersion compensation unit 404 is provided before the 2-bit delay interference unit 401.

FIG. 23 is a diagram showing the configuration of the polarization mode dispersion compensation unit 404 according to Embodiment 3 of the present invention.
The polarization mode dispersion compensation unit 404 includes a polarization control unit 4041, a circulator 4042, a polarization separation unit 4043, optical fiber cables 4044 and 4045, and center wavelength control devices 4046 and 4047.

  The polarization controller 4041 adjusts the rotation angle of the polarization of the input optical signal. As a result, the polarization axis of the optical signal propagating through the input-side optical fiber cable substantially matches the optical axis of the output-side optical fiber cable. The optical signal whose polarization rotation angle is adjusted by the polarization separation unit 4043 is output to the circulator 4042.

  The circulator 4042 outputs the optical signal from the polarization control unit 4041 to the polarization separation unit 4043, and outputs the optical signal from the polarization separation unit 4043 to the 2-bit delay interference unit 401.

  The polarization separation unit 4043 separates the optical signal from the circulator 4042 into a TM (Transverse Magnetic) wave and a TE (Transverse Electric) wave. The optical signal separated by the polarization separation unit 4043 is output to the optical fiber cables 4044 and 4045. In addition, the reflected wave from the optical fiber cables 4044 and 4045 is output to the circulator 4042.

  Each of the optical fiber cables 4044 and 4045 is a fiber having a chirped grating, and compensates for the group delay of the TM wave and the TE wave by reflecting the optical signal from the polarization separation unit 4043. The reflected wave reflected by the optical fiber cables 4044 and 4045 is output to the polarization separation unit 4043.

  The center wavelength control devices 4046 and 4047 are provided in the optical fiber cables 4044 and 4045, respectively, and compensate the polarization mode dispersion of the optical signal by controlling the center wavelength of the wavelength band of the optical signal reflected by the chirped grating. Is.

  As described above, according to the third embodiment, the polarization mode dispersion compensation unit 404 that compensates for polarization mode dispersion is used in combination as a method for suppressing signal interference and jitter (caused by PMD) between adjacent bits. With this configuration, signal jitter and signal interference between adjacent bits can be suppressed, and a high-quality electric signal can be obtained.

  In the optical communication system according to the third embodiment, the case where the transmitter 2 that controls the variable adjustment amount based on the temperature shown in the first embodiment is applied as the transmitter is described. The present invention is similarly applicable to the optical transmitter 2 that controls the variable adjustment amount based on the output of the polarization modulator 211.

In the optical communication system according to Embodiment 1-3, the case where the DQPSK modulator 208 is provided as the data modulation unit and the differential quaternary phase modulation (DQPSK modulation) is performed has been described. The present invention is not limited to this, and can be similarly applied to a case where a DPSK modulator is provided as a data modulation unit and differential binary phase modulation (DPSK modulation) is performed.
In the present invention, within the scope of the invention, any combination of the embodiments, or any modification of any component in each embodiment, or omission of any component in each embodiment is possible. .

  DESCRIPTION OF SYMBOLS 1 Optical communication system, 2 Optical transmitter, 3 Transmission path, 4 Optical receiver, 201 Light source (LD), 202 MUX function part, 203 Variable delay function part (1st delay function part), 204 RZ modulation drive part, 205 RZ modulator, 206 Precode (Prec) function unit, 207 DQPSK modulator drive unit, 208 DQPSK modulator (data modulator), 209 Variable delay function unit (second delay function unit), 210 Polarization modulation drive , 211 Polarization (PoM) modulator, 212 Temperature measurement unit, 213a, 213b Controller, 214, 215 Optical fiber, 216 Optical coupler, 217 Monitor unit (PD), 218 Detector unit (DET), 219 Polarized beam Splitter (PBS), 401 2-bit delay interference unit (2 bit ODI), 402 Quad photodetection unit (QPD), 403 DM X functional unit, 404 a polarization mode dispersion compensator (PMDC), 4041 polarization control unit, 4042 a circulator, 4043 polarization splitter, 4044,4045 optical fiber cable, 4046,4047 center wavelength control device.

Claims (8)

A first delay function unit that outputs an electric clock signal in which a delay time is adjusted based on a set delay amount;
An RZ modulator that performs RZ (Return-to-Zero) modulation on an input optical signal in accordance with an electrical clock signal from the first delay function unit;
A data modulator that is provided at a subsequent stage of the RZ modulator, and phase-modulates an optical signal that has been RZ-modulated by the RZ modulator in accordance with input transmission data;
A second delay function unit that outputs an electrical clock signal in which the delay time is adjusted based on the set delay amount;
A polarization modulator that is provided at a subsequent stage of the data modulator, and that alternately modulates an optical signal phase-modulated by the data modulator in accordance with an electrical clock signal from the second delay function unit ;
An optical coupler for branching the optical signal from the polarization modulator;
A monitor unit for converting the intensity of the optical signal branched by the optical coupler into an electrical signal;
A detector for measuring a peak value of an electric signal from the monitor unit;
A controller that adjusts a delay amount in the second delay function unit based on a peak value of the electrical signal;
An optical transmitter comprising: And a polarization beam splitter for separating the optical signal from the optical coupler into 45-degree polarization and -45-degree polarization and outputting the 45-degree polarization to the monitor unit. The optical transmitter according to claim 1. The data modulator, using the transmission data input, the RZ modulator by the RZ modulated optical signal DPSK (Differential Phase Shift Keying) according to claim 1 or claim 2, wherein the modulating Optical transmitter. The data modulator, using the transmission data input, the RZ modulator RZ modulated optical signal DQPSK (Differential Quadrature Phase Shift Keying) according to claim 1 or claim 2, wherein the modulating the Optical transmitter. In an optical communication system comprising: an optical transmitter that transmits an alternating polarization RZ phase modulation optical signal; and an optical receiver that receives the alternating polarization RZ phase modulation optical signal from the optical transmitter.
The optical transmitter is
A first delay function unit that outputs an electric clock signal in which a delay time is adjusted based on a set delay amount;
An RZ modulator that RZ-modulates an input optical signal in response to an electrical clock signal from the first delay function unit;
A data modulator that is provided at a subsequent stage of the RZ modulator, and phase-modulates an optical signal that has been RZ-modulated by the RZ modulator in accordance with input transmission data;
A second delay function unit that outputs an electrical clock signal in which the delay time is adjusted based on the set delay amount;
A polarization modulator that is provided at a subsequent stage of the data modulator, and that alternately modulates an optical signal phase-modulated by the data modulator in accordance with an electrical clock signal from the second delay function unit ;
An optical coupler for branching the optical signal from the polarization modulator;
A monitor unit for converting the intensity of the optical signal branched by the optical coupler into an electrical signal;
A detector for measuring a peak value of an electric signal from the monitor unit;
An optical communication system , comprising: a controller that adjusts a delay amount in the second delay function unit based on a peak value of the electric signal . The optical transmitter is
  A polarization beam splitter that separates the optical signal from the optical coupler into 45-degree polarization and -45-degree polarization, and outputs the 45-degree polarization to the monitor;
The optical communication system according to claim 5. The data modulator, using the transmission data input, the RZ optical communication system according to claim 5 or claim 6, wherein the optical signal RZ modulated by the modulator, characterized in that DPSK modulation. The data modulator, using the transmission data input, the RZ optical communication system according to claim 5 or claim 6, wherein the optical signal RZ modulated by the modulator, characterized in that DQPSK modulation.

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