JP4838775B2 - Optical transmission circuit - Google Patents

Description

  The present invention relates to an optical transmission circuit of an optical communication system using a phase modulation code.

  In a system that transmits a long distance such as a submarine optical communication system, a highly sensitive transmission code is indispensable. In the conventional optical communication system, an intensity modulation method for detecting information of “1” and “0” based on the presence or absence of light is widely used. In a system that requires high sensitivity, RZ that shortens an optical data signal is shortened. (Return-to-Zero) signals are often used. This is because the RZ signal can be improved in sensitivity by about 3 dB as compared with an NRZ (Non-Return-to-Zero) signal that does not shorten the data. However, for further high speed and large capacity, there is a limit to high sensitivity in the approach using the intensity modulation method.

  In recent years, attention has been focused on a differential phase shift keying method and a differential four-phase phase shift keying method for transmitting information in the phase of light as new transmission methods that replace the intensity modulation method. These phase modulation signals can be improved in sensitivity by 3 dB compared to the conventional NRZ signal by making the receiving circuit a differential balance configuration. By converting the phase-modulated light into RZ, a further improvement in sensitivity of 3 dB can be expected. Therefore, it can be said to be a suitable transmission method for a long-distance optical communication system.

  In order to generate a phase-modulated signal converted to RZ, as shown in FIG. 18, a phase modulator 12 for phase modulation and an intensity modulator 14 for RZ (intensity) modulation are connected in series, and each of them is connected to data. Modulation is performed using the signal D and the clock signal C1 (see, for example, Patent Document 1). At this time, the clock signal C1 for driving the intensity modulator 14 is modulated not at the phase transition point of each symbol of the phase modulated light L1 input to the intensity modulator 14, but at the center (optimum phase) where the phase is determined. It must be input to the intensity modulator 14 at such timing. Therefore, it is necessary to adjust the propagation delay time of the clock signal path R1 or the data signal path R2 to optimize the relative phase between the phase modulated light L1 and the clock signal C1 in the intensity modulator 14. The relative phase optimization needs to satisfy the following requirements.

[Request 1]
In recent optical communication systems, it is necessary to accommodate inter-computer communication data such as Ether in addition to signals obtained by multiplexing telephone lines, and the operation speed varies depending on which signal the system accommodates. In a system employing an error correction code, the operation speed varies depending on the design of the correction gain. It is required that the relative phase can be optimized corresponding to such a plurality of operation speeds (multirate).

[Request 2]
Even if the delay time of the signal path fluctuates due to temperature fluctuation or aging fluctuation, it is required to keep the optimum state continuously. As a means for satisfying the above requirements, it is possible to strictly align the absolute delays of the clock signal path and the data signal path. However, in an actual transmission circuit, different electronic circuits and optical components are inserted in each signal path, and the signal path is also composed of electrical wiring and optical fiber. It cannot be said that it can be done. Even if manual adjustment is performed using a semi-fixed phase shifter or the like at the initial stage of operation, subsequent temperature and aging fluctuations cannot be compensated. For this reason, the relative phase adjustment means is required to have a function of performing phase optimization corresponding to initial individual variation and switching of operation speed and continuing to maintain the optimum state thereafter.

  FIG. 19 shows a conventional example of a means for generating an RZ optical signal (see, for example, Patent Document 2). In this configuration, the continuous light L0 from the light source 71 is used as the optical input, and the continuous light is intensity-modulated by the clock signal C1 that has passed through the phase shifter 13 to generate an optical pulse train L3 having the same frequency as the clock signal C1. And a second modulator 94 that performs data modulation on the optical pulse train L3 by the output of the retiming circuit 96 that uses the optical pulse train L3 as an optical input and rewrites the data signal D in synchronization with the clock signal C1. The control circuit 17 outputs a signal for controlling the phase of the clock signal C1 to the phase shifter 13 from the phase comparison result obtained by synchronously detecting the output of the first modulator 92 and the clock signal C1. .

  The operation principle of this configuration is as follows. The optical pulse train L3 output from the first modulator 92 is branched and subjected to photoelectric conversion, and then clock recovery is performed to obtain a recovered clock signal having the same frequency as the clock signal. The synchronous detection circuit 22 uses the reproduced clock signal and the clock signal C1 as input signals, and outputs the result of phase comparison between them. By setting the control voltage of the phase shifter 13 so that the error of the phase comparison result becomes zero, the optical pulse train L3 can be synchronized with the clock signal C1. Here, by adding an adder to the output of the synchronous detection circuit 22 and adding an offset voltage to the output, the relative phase between the optical pulse train L3 and the clock signal C1 can be set to a desired state.

The phase of the data signal D for intensity-modulating the second modulator 94 is controlled by the retiming circuit 96 using the clock signal C1 as a clock input, but from the optical distributor 15 to the second modulator 94. The delay time until the delay time until the clock signal C1 is input to the retiming circuit 96 and the data is rewritten and input to the second modulator 94 is different, so that the first modulator 92 is driven. The clock signal phase and the data phase for driving the second modulator 94 do not match. Therefore, in this configuration, the offset voltage of the adder is adjusted so that both phases are synchronized.
Japanese Patent No. 3625726 Japanese Patent No. 3681865

  Therefore, in the conventional example, a high-speed timing extraction circuit and a phase comparator are required to adjust the relative phase between the optical pulse train L3 input to the second modulator 94 and the data signal after retiming. Furthermore, in order to synchronize the phase of the two modulators, it is necessary to adjust the offset voltage of the adder. However, this method can be used to observe the waveform using an oscilloscope or to measure the error rate by connecting to a receiving circuit. It is assumed that manual adjustment is performed while Since the object of evaluation is a high-speed signal and the measurement accuracy is maintained, there is a problem that the corresponding measuring instruments are limited to high-precision ones.

  Further, the delay time (time from clock input to data rewriting) of the retiming circuit 96 varies depending on temperature, power supply voltage, and deterioration with time. Even if the offset voltage is adjusted with the initial delay time, the deviation from the phase optimum point cannot be compensated automatically due to the variation of the delay time due to the subsequent variation factor, so that the waveform quality of the output pulse is deteriorated.

  Therefore, in order to solve the problem, an object of the present invention is to provide an optical transmission circuit that eliminates the need for a high-speed timing extraction circuit and a phase comparator and can automatically optimize the relative phase between two modulators. .

  In order to achieve the above object, an optical transmission circuit according to the present invention includes a phase modulator that performs phase modulation with an output from a differential encoding circuit, and an intensity that modulates the intensity of the phase modulated light from the phase modulator with a clock signal. A modulator is provided, and the relative phase between the phase modulator and the intensity modulator is optimized so that the average intensity of the output light from the intensity modulator is maximized.

  Specifically, an optical transmission circuit according to the present invention includes a differential encoding circuit that converts a differential encoding signal of NRZ synchronized with an input clock signal and outputs the differential signal, and continuous light from a light source. By branching, by differential output from the differential encoding circuit, (a) one of the two branched continuous lights based on '0' / '1' of the differential encoding signal is changed to + π phase / 0 phase, The other is phase-modulated to -π phase / 0 phase. (B) One of the two split continuous lights based on '0' / '1' of the differentially encoded signal is set to 0 phase / + π phase, and the other is set to 0. Phase / phase reduced to −π phase and phase-modulated continuous light as in (a) or (b) is combined to reduce the light intensity at the time of phase transition of the phase-modulated continuous light A phase modulator that outputs modulated light, and a phase shifter that controls the phase of the input clock signal and outputs a phase adjusted clock signal. An intensity modulator that modulates the intensity of the phase-modulated light from the phase modulator with a phase adjustment clock signal from the phase shifter and outputs it as output light, and monitors the output light from the intensity modulator, And a control circuit that causes the phase shifter to shift the phase of the clock signal so that the intensity of the signal becomes maximum.

  By using a phase modulator that modulates the optical phase with the electrical data signal after differential encoding, the optical output is attenuated only at the data transition point, and when the clock phase is optimal, the intensity modulator When the optical output is maximized and the phase difference between the data and the clock is increased, the optical output is attenuated. Using this characteristic, the control circuit causes the phase shifter and the intensity modulator to adjust the phase of the input clock signal so that the average intensity of the output light from the intensity modulator is maximized. The relative phase with the vessel can be automatically optimized.

  Therefore, the present invention can provide an optical transmission circuit that eliminates the need for a high-speed timing extraction circuit and a phase comparator and can automatically optimize the relative phase between the two modulators.

  An optical transmission circuit according to the present invention includes: a differential encoding circuit that converts a differentially encoded signal of NRZ synchronized with an input clock signal and outputs the differential signal; a continuous light from a light source into two branches; Depending on the differential output from the dynamic encoding circuit, (a) one of the two branched continuous lights based on '0' / '1' of the differential encoding signal is set to + π phase / 0 phase, and the other is −π phase. Phase modulation to / 0 phase, (b) One of the two branched continuous light based on '0' / '1' of the differential encoded signal is set to 0 phase / + π phase, and the other is set to 0 phase / −π phase And phase-modulated light in which the light intensity at the time of phase transition of the phase-modulated continuous light is reduced is output by combining the continuous light that has been phase-modulated into (a) or (b). A phase modulator, a phase shifter for controlling a phase of the input clock signal and outputting a phase-adjusted clock signal; An intensity modulator that modulates the intensity of the phase-modulated light from the phase modulator with the phase adjustment clock signal from the phase shifter and outputs it as an output light, and a reference signal of the generated reference frequency is input to the phase shifter, An oscillator that varies the phase of the adjustment clock signal at the reference frequency of the reference signal, and a synchronous detection circuit that synchronously detects the reference signal from the oscillator and the light intensity of the output light from the intensity modulator and outputs a detection signal And a control circuit that causes the phase shifter to shift the center phase of the phase of the phase adjustment clock signal so that the frequency of the detection signal from the synchronous detection circuit is twice the reference frequency of the reference signal from the oscillator And comprising.

  By synchronously detecting the reference signal from the oscillator and the light intensity of the output light from the intensity modulator, it is possible to detect whether the phase of the input clock signal is advanced or delayed in the intensity modulator.

  The optical transmission circuit according to the present invention preferably further includes a low-pass filter that adjusts a time during which a code of the differential encoded signal from the differential encoding circuit transitions and inputs the signal to the phase modulator.

  By providing the low-pass filter, in addition to the effects described above, it is possible to narrow a phase region in which it is difficult to detect the optical output power with respect to the optimum phase shift.

  The optical transmission circuit according to the present invention preferably further includes a ½ frequency divider that converts the frequency of the clock signal to ½ times and the amplitude to twice and inputs the clock signal to the phase shifter.

  By providing the ½ frequency divider, in addition to the above-described effects, it is possible to narrow the phase region where it is difficult to detect the optical output power with respect to the optimum phase shift.

  According to the present invention, it is possible to provide an optical transmission circuit that eliminates the need for a high-speed timing extraction circuit and a phase comparator and can automatically optimize the relative phase between the two modulators.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(First embodiment)
FIG. 1 is a block diagram of an optical transmission circuit 101 according to the first embodiment. The optical transmission circuit 101 converts the input data signal D into an NRZ differential encoding signal synchronized with the clock signal C1 from the clock signal generator 72, and outputs the differential encoding circuit 11 and the light source 71. The continuous light L0 is branched into two, and by the differential output from the differential encoding circuit 11, (a) one of the continuous lights branched into two based on '0' / '1' of the differential encoded signal is + π Phase modulated to phase / 0 phase and the other to -π phase / 0 phase, and (b) one of the continuous light branched into two based on '0' / '1' of differential encoded signal is 0 phase / + π phase In addition, the light intensity at the time of phase transition of the phase-modulated continuous light is obtained by merging the continuous light phase-modulated as in (a) or (b) by phase-modulating the other to 0 phase / −π phase. The phase modulator 12 that outputs the lowered phase modulated light L1 and the phase of the clock signal C1 from the clock signal generator 72 are controlled. The phase shifter 13 that outputs the phase adjustment clock signal C2 and the intensity modulation that modulates the intensity of the phase modulated light L1 from the phase modulator 12 with the phase adjustment clock signal C2 from the phase shifter 13 and outputs it as the output light L2. A part of the output light L2 from the intensity modulator 14 extracted by the optical device 14 and the optical distributor 15 is monitored by the light receiving element 16, and the phase of the clock signal C1 is shifted so that the intensity of the output light L2 is maximized. And a control circuit 17 that controls the phase shifter 13 to output the phase adjustment clock signal C2.

  The differential encoding circuit 11 differentially encodes an input data signal D and outputs a differential encoded signal. The differential encoding is an encoding process for inverting the code output for each input of ‘1’ from ‘1’ to ‘0’ or ‘0’ to ‘1’. The sign to be output may be inverted every time “0” is input. Further, the differential encoding circuit 11 differentially drives the differentially encoded signal and outputs a differential encoded signal. Specifically, when the differentially encoded code is “1”, “+1” is output to the + output of the differential encoding circuit 11 and “−1” is output to the − output. When the differentially encoded code is “0”, “+” and −output of the differential encoding circuit 11 are both output “0”.

  FIG. 2 shows the phase modulation operation in the phase modulator 12. The optical input of the phase modulator 12 is continuous light L0 from the light source 71, and modulates the continuous light L0 with a differential encoding signal from a differential encoding circuit 11 (not shown).

  FIG. 3 is a diagram illustrating the operation of the phase modulator 12 of FIGS. 1 and 2. As shown in FIG. 3, the voltage of “1” / “0” of the differentially encoded signal is set to match the bias voltages of two transmission maximum points in the transmission characteristics of the phase modulator 12 that are close to each other. The phase-modulated light L1 output from the phase modulator 12 is always in a transmissive state except that it is extinguished at the moment when the differential encoded data signal transitions from “1” to “0” or from “0” to “1”. The optical phase becomes “0” / “π” (or “π” / “0”) corresponding to “1” / “0” of the differentially encoded signal, and the phase modulation is completed.

Specifically, the phase modulator 12 of FIGS. 1 and 2 branches the continuous light L0 from the light source 71 into two branches. The phase modulator 12 performs phase modulation as shown in (a) or (b) below by the differential output from the differential encoding circuit 11.
(A) Based on “0” / “1” of the differentially encoded signal, one of the two branched continuous lights is phase-modulated to + π phase / 0 phase and the other is phase-modulated to −π phase / 0 phase.
(B) Based on “0” / “1” of the differentially encoded signal, one of the two branched continuous lights is phase-modulated to 0 phase / + π phase and the other is phase-modulated to 0 phase / −π phase.
For example, for one continuous light, the optical phase is advanced to “0” / “π” (or “π” / “0”) by “0” / “1” of the differentially encoded signal as shown in FIG. Make it compatible. For the other continuous light, the optical phase is delayed to “0” / “− π” (or “−π” / “0”) by “0” / “1” of the differentially encoded signal. Thereafter, the phase modulator 12 merges the phase-modulated continuous light.

  The case (b) will be described. When the code input to the differential encoding circuit 11 is “1” and the output code is changed from “0” to “1” as a result of differential encoding, the + output of the differential encoding circuit 11 is “+1”. ',-Output outputs' -1'. One waveguide of the phase modulator 12 corresponds to the + output of the differential encoding circuit 11, and the other waveguide corresponds to the − output of the differential encoding circuit 11. Therefore, continuous light propagating through one waveguide of the phase modulator 12 is phase-modulated from 0 phase to + π phase, and continuous light propagating through the other waveguide is phase-modulated from 0 phase to −π phase.

  Conversely, if the code input to the differential encoding circuit 11 is “1” and the output code is changed from “1” to “0” as a result of differential encoding, the + output of the differential encoding circuit 11 -Outputs “0” for both outputs. Therefore, the continuous light propagating through one waveguide of the phase modulator 12 and the continuous light propagating through the other remain zero phase.

  Since the phase modulator 12 merges the phase-modulated continuous light, the continuous light propagating through one waveguide is in the + π phase, and the continuous light propagating through the other waveguide is in the −π phase. When both are in the 0 phase, the outputs are added.

  However, when the differentially encoded signal transitions from “1” to “0” or from “0” to “1”, the phase of the continuous light propagating through one waveguide becomes “π / 2”. There is. Similarly, there is a moment when the phase of continuous light propagating through the other waveguide becomes ‘−π / 2’. At this time, since the continuous light propagating through one waveguide and the continuous light propagating through the other waveguide cancel each other, the light intensity of the phase-modulated light L1 output from the phase modulator 12 decreases.

  Therefore, every time the code input to the differential encoding circuit 11 is ‘1’, the light intensity of the phase-modulated light L <b> 1 output from the phase modulator 12 decreases during the transition process.

  On the other hand, the case (a) is as follows. When the code input to the differential encoding circuit 11 is “1” and the output code is changed from “1” to “0” as a result of differential encoding, the + output of the differential encoding circuit 11 is “+1”. ',-Output is' -1', continuous light propagating through one waveguide of the phase modulator 12 changes from 0 phase to + π phase, and continuous light propagating through the other waveguide passes from 0 phase to -π Phase modulated to phase.

  On the contrary, when the code input to the differential encoding circuit 11 is “1” and the output code is changed from “0” to “1” as a result of differential encoding, the + output of the differential encoding circuit 11 Both the -output outputs "0", and the continuous light propagating through one waveguide of the phase modulator 12 and the continuous light propagating through the other remain in the zero phase.

  Therefore, also in the case of (a), the code input to the differential encoding circuit 11 is “1”, and the phase transition process is performed with “π / 2” continuous light and “−π / 2”. Since the continuous light cancels out, the light intensity of the phase-modulated light L1 output from the phase modulator 12 decreases.

  4 shows the relative phase and output of the clock signal 42 with respect to the phase modulated light 41 when the phase modulated light 41 is used as the optical input of the intensity modulator 14 described in FIG. It is the result of calculating the relationship with the light 43. FIG. 4A shows the calculation result when the phase of the clock signal 42 is optimal. FIG. 4B shows a calculation result when the phase of the clock signal 42 is shifted from the phase of the phase-modulated light 41. 4A and 4B, the horizontal axis indicates time, and the vertical axis indicates light intensity or amplitude. As described with reference to FIGS. 2 and 3, the phase-modulated light 41 generated using the phase modulator always has light except for the extinction point at which the optical phase transitions. Therefore, when the phase of the clock signal 42 is optimum, the output light 43 of the intensity modulator 14 becomes an optical pulse train having the same frequency as that of the clock signal 42 as shown in FIG. On the other hand, when the phase of the clock signal 42 deviates from the optimum point, the output light 43 of the intensity modulator 14 is transmitted through the portion where the extinction point and the peak of the clock signal 42 overlap as shown in FIG. Therefore, an optical pulse train in which some of the optical pulses are cut off is obtained. In FIG. 4B, the portion where the light pulse has been cut is indicated by a circle. Since the output power is attenuated by removing the optical pulse, the phase shift of the clock signal 42 can be detected as the average power of the output light 43 of the intensity modulator 14.

  FIG. 5 shows the operation of intensity modulation in the intensity modulator 14. The intensity modulator 14 modulates the intensity of the phase-modulated light L1 with the phase adjustment clock signal C2. The output light L 2 output from the intensity modulator 14 is distributed by the light distributor 15, and the light intensity is measured by the light receiving element 16.

  FIG. 6 shows the result of calculating the relationship between the relative phase of the phase adjustment clock signal C2 with respect to the phase modulated light L1 in the intensity modulator 14 described in FIG. 1 and the light intensity of the output light L2 output from the intensity modulator 14. (Hereinafter, “the relative phase of the phase adjustment clock signal C2” is abbreviated as “clock phase”, and “the light intensity of the output light L2 output from the intensity modulator 14” is abbreviated as “output power”). The output power is maximized at the clock phase optimum point, and the output power is minimized at a point where the clock phase is shifted by a half cycle from the phase optimum point. This output power fluctuation is repeated in the cycle of the clock signal. The light receiving element 16 monitors the output power of the output light L2, and the control circuit 17 controls the phase shifter 13 based on the result. Specifically, the phase shifter 13 shifts the phase of the clock signal C1 so as to maximize the output power, and outputs the phase adjustment clock signal C2.

  Therefore, the optical transmission circuit 101 can maintain the clock phase optimum point for the phase-modulated light L1 without providing a high-speed timing extraction circuit or a phase comparator. Further, since the optical transmitter circuit 101 can automatically adjust the relative phase of the two modulators, there is no need to manually adjust the relative phase as in the prior art, and it is automatically optimized even if the relative phase changes with time. be able to.

(Second Embodiment)
FIG. 7 is a block diagram of the optical transmission circuit 102 according to the second embodiment. The optical transmission circuit 102 converts the input data signal D into an NZR differential encoding signal synchronized with the clock signal C 1 from the clock signal generator 72, and outputs a differential output, and a light source 71. The continuous light L0 is branched into two, and by the differential output from the differential encoding circuit 11, (a) one of the continuous lights branched into two based on '0' / '1' of the differential encoded signal is + π Phase modulated to phase / 0 phase and the other to -π phase / 0 phase, and (b) one of the continuous light branched into two based on '0' / '1' of differential encoded signal is 0 phase / + π phase In addition, the light intensity at the time of phase transition of the phase-modulated continuous light is obtained by merging the continuous light phase-modulated as in (a) or (b) by phase-modulating the other to 0 phase / −π phase. The phase modulator 12 that outputs the lowered phase modulated light L1 and the phase of the clock signal C1 from the clock signal generator 72 are controlled. The phase shifter 13 that outputs the phase adjustment clock signal C2 and the intensity modulation that modulates the intensity of the phase modulated light L1 from the phase modulator 12 with the phase adjustment clock signal C2 from the phase shifter 13 and outputs it as the output light L2. 14 and the generated reference frequency reference signal S1 is input to the phase shifter 13 via the adder 24, and the clock phase is changed by the reference frequency of the reference signal S1 and the optical distributor 15 extracts the clock phase. A synchronous detection circuit 22 for synchronously detecting a measurement signal obtained by measuring a part of the light intensity of the output light L2 from the intensity modulator 14 with the light receiving element 16 and the reference signal S1 from the oscillator 23, and outputting a detection signal; The phase shifter 13 is arranged so that the frequency of the detection signal from the synchronous detection circuit 22 that has passed through the filter 21 is twice the reference frequency of the reference signal S1 from the oscillator 23. And a control circuit 27 for shifting the varying phase center phase of the adjusted clock signal C2, the.

  The difference between the optical transmission circuit 102 in FIG. 7 and the optical transmission circuit 101 in FIG. 1 is that the measurement signal of the light receiving element 16 is synchronously detected by the reference signal S 1 from the oscillator 23. FIG. 8 schematically illustrates fluctuations in the output power of the intensity modulator 14 due to the reference signal S1 in order to explain the principle of the synchronous detection circuit 22. In the figure, the clock phase for the phase-modulated light L1 is shown in three states from α to γ. Of these, β is the phase optimum point at which the output power is maximum, and α and γ indicate the advance and delay of the relative phase, respectively. When the phase of the phase shifter 13 is modulated by the reference signal S1 in each state, the output power varies due to the change of the clock phase. At the phase optimum point of β, the clock phase is modulated to the left and right with the output power maximum point as the center, so that the output power fluctuation fluctuates with a period twice the reference signal S1. In the state of α, the clock phase is modulated while the output power changes to the right with respect to the clock phase, so that an output power fluctuation in phase with the reference signal S1 occurs. Similarly, in the γ state, output power fluctuations having a phase opposite to that of the reference signal S1 occur.

  Therefore, it is possible to detect whether the clock phase is advanced or delayed by synchronously detecting the output power with the reference signal S1 and comparing the phases. FIG. 9 shows the detection signal output from the synchronous detection circuit 22 with respect to the clock phase. When the clock phase is advanced, the detection signal output is positive. Conversely, when the clock phase is delayed, the detection signal output is negative. The control circuit 27 detects a deviation from zero of the detection signal output as an error signal, and controls to delay the clock phase if the error signal is positive and to advance the clock phase if the error signal is negative.

  Therefore, the optical transmission circuit 102 can maintain the clock phase optimum point for the phase-modulated light L1 without providing a high-speed timing extraction circuit or a phase comparator. Further, since the optical transmitter circuit 102 can automatically adjust the relative phase of the two modulators, there is no need to manually adjust the relative phase as in the prior art, and it is automatically optimized even if the relative phase changes with time. be able to.

(Third embodiment)
FIG. 10 is a block diagram of the optical transmission circuit 103 according to the third embodiment. The difference between the optical transmission circuit 103 and the optical transmission circuit 101 of FIG. 1 is that a 1/2 frequency divider 31 is inserted in the previous stage of the phase shifter 13. The ½ divider circuit 31 converts the frequency of the clock signal C1 to ½ times and the amplitude to twice and inputs it to the phase shifter 13. As described with reference to FIG. 6, the relationship between the clock phase and the output power in the optical transmission circuit 101 in FIG. 1 becomes flat near the phase optimum point, and it is difficult to measure the clock phase shift. The optical transmission circuit 103 includes the ½ frequency divider 31 so that the clock phase shift can be measured even near the phase optimum point.

  The reason will be described below. FIGS. 11A to 11C show the relationship between the transmission characteristic of the intensity modulator 14 and the drive signal (phase adjustment clock signal C2). FIG. 11A is a diagram showing the relationship between the clock signal and the output power transmitted through the intensity modulator 14. Of the waveform of the clock signal, the 50% RZ clock signal corresponds to the phase adjustment clock signal C2 shown in FIG. 1 and represents a state of driving with an amplitude matching the half-wave voltage of the intensity modulator. At this time, the output light L2 of the intensity modulator 14 becomes a pulse with a duty of 50% as shown in FIG. On the other hand, of the waveform of the clock signal in FIG. 11A, the 67% RZ clock signal corresponds to the phase adjustment clock signal C2 that has passed through the ½ divider circuit 31 in FIG. The midpoint of the 67% RZ clock signal is set as the extinction point of the intensity modulator 14 and driven with an amplitude twice that of the half-wave voltage. The output light L2 of the intensity modulator 14 is turned back, and as shown in FIG. 11B, an optical pulse is generated with a period twice the driving frequency, and its duty is 67%.

  FIG. 12 shows the result of calculating the relationship between the phase of the 67% RZ clock signal and the output power of the intensity modulator 14. FIG. 12 also shows the relationship between the phase of the 50% RZ clock signal described in FIG. 6 and the output power of the intensity modulator 14. In the case of the 67% RZ clock signal, the power fluctuation near the clock phase optimum point is steep compared to the case of the 50% RZ clock signal. FIG. 13 shows the relationship between the bottom of the pulse and the extinction point of the phase modulation signal when the clock phase starts to deviate from the optimum point. FIG. 13A shows a 67% RZ clock signal, and FIG. 13B shows a 50% RZ clock signal. The 67% RZ clock signal has a pulse tail at the extinction point of the phase modulation signal from a state where the deviation of the clock phase from the optimum point is small, and the tail G1 in FIG. As shown in FIG. 12, the power fluctuation near the clock phase optimum point becomes steep.

  Therefore, in addition to the effect described in the optical transmission circuit 101 of FIG. 1, the optical transmission circuit 103 has an effect of being able to measure a clock phase shift even near the phase optimum point.

  The 1/2 divider circuit 31 can also be incorporated in the optical transmission circuit 102 of FIG. In the present embodiment, the ½ divider circuit 31 is configured to divide the clock signal by ½ so that the amplitude is twice that of the half-wave voltage. However, a 1/1 frequency clock signal is used. The same effect can be obtained by setting the amplitude to be slightly larger than the half-wave voltage of the intensity modulator 14 and increasing the duty by slightly shifting the DC bias of the intensity modulator 14 to the transmission side. .

(Fourth embodiment)
FIG. 14 is a block diagram of the optical transmission circuit 104 according to the fourth embodiment. The difference between the optical transmission circuit 104 and the optical transmission circuit 102 in FIG. 7 is that a limiter amplifier 26 is inserted in the subsequent stage of the low-pass filter 21.

  The limiter amplifier 26 amplifies the detection signal of the synchronous detection circuit 22 that has passed through the low-pass filter 21. As described with reference to FIG. 6, the relationship between the clock phase and the output power is flat near the phase optimum point. For this reason, the synchronous detection of the synchronous detection circuit 22 tends to be difficult. The optical transmission circuit 104 band-limits the detection signal from the synchronous detection circuit 22 with an LPF and amplifies the detection signal with a high gain limiter amplifier. Further, by clamping the output amplitude with a certain voltage, it is possible to narrow the phase range in which the detection sensitivity of the clock phase shift is difficult.

  Therefore, in addition to the effect described with reference to the optical transmission circuit 101 in FIG. 1, the optical transmission circuit 104 has an effect that it can measure a clock phase shift even near the phase optimum point.

(Fifth embodiment)
FIG. 15 is a block diagram of the optical transmission circuit 105 of the fifth embodiment. The difference between the optical transmission circuit 105 and the optical transmission circuit 101 in FIG. 1 is that a low-pass filter 51 is inserted between the differential encoding circuit 11 and the phase modulator 12. The low pass filter 51 adjusts the time during which the code of the differential encoded signal from the differential encoding circuit 11 transitions and inputs the adjusted signal to the phase modulator 12. Specifically, the low-pass filter 51 adjusts the rising time Tr for transition from “0” to “1” and the falling time Tf for transition from “1” to “0” of the differentially encoded signal.

  FIG. 16 shows the relationship between the clock phase and the output power of the intensity modulator 14 when the rise time Tr / fall time Tf of the differentially encoded signal is changed from 20% to 40%. By increasing the rise time Tr / fall time Tf, the power fluctuation near the clock phase optimum point becomes steep as shown in FIG. Therefore, it is possible to narrow the phase region where the detection sensitivity of the clock phase shift is difficult.

  Therefore, in addition to the effect described in the optical transmission circuit 101 in FIG. 1, the optical transmission circuit 105 has an effect of being able to measure a clock phase shift even near the phase optimum point.

  The low-pass filter 51 can also be incorporated in the optical transmission circuit 102 of FIG.

(Sixth embodiment)
FIG. 17 is a block diagram of the optical transmission circuit 106 according to the sixth embodiment. The difference between the optical transmission circuit 106 and the optical transmission circuit 102 of FIG. 7 is that an amplifier 61 and a second bandpass filter 64 are inserted between the light receiving element 16 and the synchronous detection circuit 22 and the output of the amplifier 61. Is fed back to the intensity modulator 14 via the first band-pass filter 62 and the automatic bias control circuit 63. The optical transmission circuit 106 branches the output of the light receiving element 16 and uses one for controlling the phase shifter 13 and the other for controlling the intensity modulator 14.

The intensity modulator for optical transmission is widely used LiNbO ₃ modulator. In this modulator, since the operating bias point drifts, the drift is generally compensated by an automatic bias control circuit. The optical transmission circuit 106 includes an automatic bias control circuit 63. Although not shown in FIG. 17, a synchronous detection circuit can also be used for the automatic bias circuit 63.

  The optical transmission circuit 106 is characterized in that the input of the clock phase control circuit and the automatic bias control circuit 63 is obtained from a common light receiving element 16. The clock phase control circuit includes a second bandpass filter 64, a synchronous detection circuit 22, a lowpass filter 21, a control circuit 27, an oscillator 23, and an adder 24. Reference signals having different frequencies are assigned to the respective control circuits, and the output of the light receiving element 16 is branched. Then, the first band pass filter 62 and the second band pass filter 64 extract the respective transmission frequency reference signals and perform synchronous detection. multiply. By adopting such a configuration, it is possible to share main components and suppress interference between reference signals, thereby enabling individual control.

  In the optical transmission circuit 101 to the optical transmission circuit 106, the phase shifter 13 is inserted into the path of the clock signal C1. However, the phase shifter 13 is inserted before or after the differential encoding circuit 11 in the path of the data signal D. The same effect can be obtained. Further, in the optical transmission circuit 101 to the optical transmission circuit 106, the phase modulator 12 is provided on the light source 71 side, but the same effect can be obtained even if the intensity modulator 14 is provided on the light source 71 side.

  The optical transmission circuit according to the present invention can also be applied to a differential four-phase phase shift keying method.

1 is a block diagram of an optical communication circuit according to the present invention. It is the figure which showed the operation | movement of the phase modulation in a phase modulator. It is a figure explaining operation | movement of a phase modulator. It is the figure which showed the relationship between the relative phase of the clock signal with respect to phase modulation light, and output light. (A) is the figure which showed the calculation result of the state with the optimal phase of a clock signal. (B) is the figure which showed the calculation result of the state which has produced the shift | offset | difference with the phase of the phase modulation light in the phase of a clock signal. It is a figure explaining operation | movement of an intensity modulator. FIG. 6 is a diagram showing a relationship between a clock phase with respect to phase-modulated light and an output power of an intensity modulator. 1 is a block diagram of an optical communication circuit according to the present invention. It is a figure explaining the principle of a synchronous detection circuit. It is the figure which showed the detection signal output from the synchronous detection circuit with respect to a clock phase. 1 is a block diagram of an optical communication circuit according to the present invention. The relationship between the transmission characteristic of an intensity modulator and a drive signal is shown. (A) is the figure which showed the relationship between a clock signal and the output power of the intensity | strength modulator 14. FIG. (B) is the figure which showed the output light of the intensity | strength modulator which is a pulse of 67% of a duty. (C) is the figure which showed the output light of the intensity modulator which is a pulse of 50% duty. It is the figure which showed the result of having calculated the relationship between the phase of a clock signal, and the output power of an intensity modulator. It is the figure which showed the relationship between the tail of a pulse when a clock phase begins to shift | deviate from the optimal point, and the quenching point of a phase modulation signal. (A) is a figure explaining the case of the clock signal for 67% RZ. (B) is a diagram illustrating the case of a 50% RZ clock signal. 1 is a block diagram of an optical communication circuit according to the present invention. 1 is a block diagram of an optical communication circuit according to the present invention. It is the figure which showed the relationship between the clock phase and the output power of the intensity | strength modulator 14, when changing the rise time Tr / fall time Tf of a differential encoding signal from 20% to 40%. 1 is a block diagram of an optical communication circuit according to the present invention. It is a block diagram of the conventional optical communication circuit. It is a block diagram of the conventional optical communication circuit.

Explanation of symbols

101-106 Optical transmission circuit 11 Differential encoding circuit 12 Phase modulator 13 Phase shifter 14 Intensity modulator 15 Optical distributor 16 Light receiving element 17, 27 Control circuit 21 Low-pass filter 22 Synchronous detection circuit 23 Oscillator 24 Adder 26 Limiter amplifier 31 1/2 frequency divider circuit 41 phase modulated light 42 clock signal 43 output light 51 low pass filter 61 amplifier 62 first band pass filter 63 automatic bias control circuit 64 second band pass filter 71 light source 72 clock signal generator 92 first 1 modulator 94 second modulator 96 retiming circuit L0 continuous light L1 phase modulated light L2 output light L3 optical pulse train L4 output light D data signal C1 clock signal C2 phase adjustment clock signal S1 reference signal R1 clock signal path R2 data Signal path

Claims (4)

A differential encoding circuit that converts the differentially encoded signal of NRZ in synchronization with the input clock signal and outputs the differential signal;
The continuous light from the light source is branched into two, and by the differential output from the differential encoding circuit,
(A) Phase modulation of one of the bifurcated continuous light based on '0' / '1' of the differential encoded signal to + π phase / 0 phase and the other to -π phase / 0 phase;
(B) Phase modulation of one of the bifurcated continuous light based on '0' / '1' of the differential encoded signal to 0 phase / + π phase and the other to 0 phase / −π phase;
In the state where the differential encoded signal remains “0” or “1” and does not transit by joining the continuous light phase-modulated as in (a) or (b), the optical phase after joining is 0. Phase-modulated light having a constant intensity is output as a phase or π-phase, and the optical phase after joining at the time of phase transition in which the differentially encoded signal transitions from “0” to “1” or “1” to “0” A phase modulator that outputs phase-modulated light that cancels out and reduces the light intensity;
A phase shifter for controlling a phase of the input clock signal and outputting a phase adjustment clock signal;
An intensity modulator that intensity-modulates the phase-modulated light from the phase modulator with a phase adjustment clock signal from the phase shifter and outputs it as output light;
A control circuit that monitors the output light from the intensity modulator and causes the phase shifter to shift the phase of the clock signal so that the intensity of the output light is maximized;
An optical transmission circuit comprising: A differential encoding circuit that converts the differentially encoded signal of NRZ in synchronization with the input clock signal and outputs the differential signal;
The continuous light from the light source is branched into two, and by the differential output from the differential encoding circuit,
(A) Phase modulation of one of the bifurcated continuous light based on '0' / '1' of the differential encoded signal to + π phase / 0 phase and the other to -π phase / 0 phase;
(B) Phase modulation of one of the bifurcated continuous light based on '0' / '1' of the differential encoded signal to 0 phase / + π phase and the other to 0 phase / −π phase;
In the state where the differential encoded signal remains “0” or “1” and does not transit by joining the continuous light phase-modulated as in (a) or (b), the optical phase after joining is 0. Phase-modulated light having a constant intensity is output as a phase or π-phase, and the optical phase after joining at the time of phase transition in which the differentially encoded signal transitions from “0” to “1” or “1” to “0” A phase modulator that outputs phase-modulated light that cancels out and reduces the light intensity;
A phase shifter for controlling a phase of the input clock signal and outputting a phase adjustment clock signal;
An intensity modulator that intensity-modulates the phase-modulated light from the phase modulator with a phase adjustment clock signal from the phase shifter and outputs it as output light;
An oscillator that inputs a reference signal of the generated reference frequency to the phase shifter and varies the phase of the phase adjustment clock signal at the reference frequency of the reference signal;
A synchronous detection circuit that synchronously detects the reference signal from the oscillator and the light intensity of the output light from the intensity modulator, and outputs a detection signal;
A control circuit that causes the phase shifter to shift the center phase of the phase of the phase adjustment clock signal so that the frequency of the detection signal from the synchronous detection circuit is twice the reference frequency of the reference signal from the oscillator;
An optical transmission circuit comprising:   The optical transmission circuit according to claim 2, further comprising a low-pass filter that adjusts a time during which a code of the differentially encoded signal from the differential encoding circuit transitions and inputs the time to the phase modulator.   4. The frequency division circuit according to claim 1, further comprising a ½ divider circuit that converts the frequency of the clock signal to ½ times and the amplitude to twice and inputs the clock signal to the phase shifter. 5. Optical transmission circuit.

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