阅读说明：本技术 一种光学调制装置及调制方法 (Optical modulation device and modulation method ) 是由 朱石超 曹权 于 2021-09-30 设计创作，主要内容包括：一种光学调制装置及调制方法,涉及通信技术领域,装置包括调制单元和控制单元,调制单元包括依次光路连接的分束器、耦合器、MZ调制器和监测光探测器,分束器分出的一条光路上还设置有改变光信号相位的相移器；所述控制单元分别与监测光探测器、相移器和MZ调制器信号连接,用于监测光探测器的信号进行低通滤波处理得到目标低频信号,还用于根据监测光探测器信号和目标低频信号,交替调整MZ调制器偏置电极的电压和相移器的电压,使目标低频信号达到最小,本发明可以提升消光比。(An optical modulation device and a modulation method relate to the technical field of communication, the device comprises a modulation unit and a control unit, wherein the modulation unit comprises a beam splitter, a coupler, an MZ modulator and a monitoring optical detector which are sequentially connected by an optical path, and one optical path split by the beam splitter is also provided with a phase shifter for changing the phase of an optical signal; the control unit is respectively in signal connection with the monitoring optical detector, the phase shifter and the MZ modulator, is used for performing low-pass filtering on the signal of the monitoring optical detector to obtain a target low-frequency signal, and is also used for alternately adjusting the voltage of the bias electrode of the MZ modulator and the voltage of the phase shifter according to the signal of the monitoring optical detector and the target low-frequency signal to minimize the target low-frequency signal.)

1. An optical modulation device is characterized by comprising a modulation unit and a control unit, wherein the modulation unit comprises a beam splitter, a coupler, an MZ modulator and a monitoring optical detector which are sequentially connected through an optical path, and one optical path split by the beam splitter is also provided with a phase shifter for changing the phase of an optical signal;

the control unit is respectively in signal connection with the monitoring optical detector, the phase shifter and the MZ modulator, is used for performing low-pass filtering processing on signals of the monitoring optical detector to obtain target low-frequency signals, and is further used for alternately adjusting the voltage of the bias electrode of the MZ modulator and the voltage of the phase shifter according to the signals of the monitoring optical detector and the target low-frequency signals to enable the target low-frequency signals to be minimum.

2. The optical modulation device according to claim 1, wherein when the control unit adjusts the voltage of the bias electrode of the MZ modulator, if the voltage is adjusted to the operating point voltage, the control unit adjusts the voltage of the phase shifter instead; and the working point voltage is the voltage of the bias electrode corresponding to the minimum value of the output optical power of the MZ modulator.

3. The optical modulation device according to claim 2,

if the control unit firstly adjusts the voltage of the phase shifter and the target low-frequency signal is smaller than the first preset threshold, or if the control unit adjusts the voltage of the phase shifter and the current target low-frequency signal is lower than the last target low-frequency signal, the control unit changes to adjust the voltage of the bias electrode of the MZ modulator.

4. The optical modulation device according to claim 3,

if the control unit firstly adjusts the voltage of the bias electrode of the MZ modulator, and if the control unit adjusts the voltage of the phase shifter and the target low-frequency signal is smaller than the first preset threshold value, the alternate adjustment is finished.

5. The optical modulation device according to claim 1, wherein the control unit is configured to apply a dc voltage to the phase shifter or a dc voltage signal with low frequency disturbances to adjust the phase shift amount of the phase shifter and reduce the target low frequency signal.

6. The optical modulation apparatus of claim 1, wherein the MZ modulator comprises two modulation arms, at least one modulation arm being the modulation cell.

7. A modulation method based on the optical modulation device according to claim 1, comprising:

the input optical signal is divided into two paths for transmission by the beam splitter, wherein after one path of optical signal is subjected to phase shift by the phase shifter, the two paths of optical signals are firstly coupled together by the coupler, then are divided into two paths of optical signals with phase difference, and then are output after being modulated by the MZ modulator;

the monitoring optical detector detects light of the MZ modulator in real time and converts the light into a current signal, the control unit performs low-pass filtering processing on the current signal to obtain a target low-frequency signal, and alternately adjusts the voltage of a bias electrode of the MZ modulator and the voltage of the phase shifter according to the current signal and the target low-frequency signal to enable the target low-frequency signal to be minimum.

8. The modulation method according to claim 7, wherein when the control unit adjusts the voltage of the bias electrode of the MZ modulator, if the control unit adjusts the voltage to the operating point voltage, the control unit changes to adjust the voltage of the phase shifter; and the working point voltage is the voltage of the bias electrode corresponding to the minimum value of the output optical power of the MZ modulator.

9. The modulation method according to claim 8, wherein the control unit performs low-pass filtering processing on the current signal to obtain a target low-frequency signal;

if the control unit firstly adjusts the voltage of the phase shifter and the target low-frequency signal is smaller than a first preset threshold value, or when the control unit adjusts the voltage of the phase shifter and the current target low-frequency signal is lower than the last target low-frequency signal, the control unit changes to adjust the voltage of the bias electrode of the MZ modulator;

if the control unit firstly adjusts the voltage of the bias electrode of the MZ modulator, and if the control unit adjusts the voltage of the phase shifter and the target low-frequency signal is smaller than the first preset threshold value, the alternate adjustment is finished.

10. The modulation method of claim 7, wherein the control unit adjusting the voltage of the phase shifter comprises:

the control unit performs low-pass filtering on the current signal to obtain a target low-frequency signal y0, adjusts the voltage of the phase shifter according to the adjusting direction, changes the adjusting direction of the phase shifter if the adjusted target low-frequency signal y1 is larger than or equal to y0, assigns y1 to y0, and adjusts the voltage of the phase shifter according to the adjusting direction again until y1 is smaller than y 0;

if y1 is less than y0, y1 is greater than or equal to a second preset threshold, the adjusting direction of the phase shifter is kept unchanged, y1 is assigned to y0, the voltage of the phase shifter is adjusted again according to the adjusting direction, and until y1 is less than y0, y1 is less than the second preset threshold;

wherein, the preset adjusting direction is as follows: increasing the voltage of the phase shifter when the last adjustment direction is positive; the last adjustment direction is negative, and the voltage of the phase shifter is reduced; and when the adjustment direction is unknown at the last time, initializing the phase shifter to adjust the direction to be positive.

Technical Field

The present invention relates to the field of communications technologies, and in particular, to an optical modulation apparatus and a modulation method.

Background

With the development of optical communication technology, communication capacity is gradually increased, and the integration level of optical devices is higher and higher. The integrated optical device has the advantages of low energy consumption, high bandwidth, ultrahigh frequency spectrum utilization rate and the like, so that a plurality of electrical devices are replaced in the fields of optical interconnection, optical sensing, optical communication, quantum communication and the like. A Mach-Zehnder (MZ) modulator is one of core devices for applying an electrical signal to an optical wave to realize intensity modulation and phase modulation of the light. The extinction ratio is one of the important indicators for measuring the performance of MZ modulators. The extinction ratio reflects the balance degree of the light field intensity of the two modulation arms of the MZ modulator, and the high extinction ratio is beneficial to improving the quality of the modulated optical signal.

MZ modulators typically comprise a beam splitter, a waveguide, a phase shifter, a travelling wave electrode and a beam combiner. Due to manufacturing process errors and the like, and the loss of the two modulation arms of the MZ modulator are different, which all result in low extinction ratio. In addition, passive structures in the MZ modulator, such as a beam splitter and a waveguide, are greatly affected by temperature, so that the extinction ratio is degraded with temperature and time, and the communication quality is affected.

In the technical field of optical communication, the communication rate and the encoding complexity are still further improved, and higher requirements are put on the extinction ratio of the MZ modulator, and even the requirements become key factors influencing the performance of an optical communication system gradually.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide an optical modulation device and a modulation method, which can improve the extinction ratio.

In order to achieve the above object, on one hand, an optical modulation device is adopted, which comprises a modulation unit and a control unit, wherein the modulation unit comprises a beam splitter, a coupler, an MZ modulator and a monitoring optical detector which are sequentially connected by an optical path, and a phase shifter for changing the phase of an optical signal is further arranged on one optical path divided by the beam splitter;

the control unit is respectively in signal connection with the monitoring optical detector, the phase shifter and the MZ modulator, is used for performing low-pass filtering processing on signals of the monitoring optical detector to obtain target low-frequency signals, and is further used for alternately adjusting the voltage of the bias electrode of the MZ modulator and the voltage of the phase shifter according to the signals of the monitoring optical detector and the target low-frequency signals to enable the target low-frequency signals to be minimum.

Preferably, when the control unit adjusts the voltage of the bias electrode of the MZ modulator, if the control unit adjusts the voltage to the working point, the control unit changes to adjust the voltage of the phase shifter; and the working point voltage is the voltage of the bias electrode corresponding to the minimum value of the output optical power of the MZ modulator.

Preferably, if the control unit first adjusts the voltage of the phase shifter and the target low-frequency signal is smaller than the first preset threshold, or if the control unit adjusts the voltage of the phase shifter and the current target low-frequency signal is lower than the previous target low-frequency signal, the control unit changes to adjust the voltage of the bias electrode of the MZ modulator.

Preferably, if the control unit first adjusts the voltage of the MZ modulator bias electrode, and if the control unit adjusts the voltage of the phase shifter, the alternate adjustment is ended if the target low-frequency signal is smaller than the first preset threshold.

Preferably, the control unit is configured to add a dc voltage to the phase shifter, or add a dc voltage signal with low-frequency disturbance to adjust a phase shift amount of the phase shifter, so as to reduce the target low-frequency signal.

Preferably, the MZ modulator includes two modulation arms, and at least one modulation arm is the modulation unit.

In another aspect, a modulation method based on the optical modulation apparatus is also provided, including:

the input optical signal is divided into two paths for transmission by the beam splitter, wherein after one path of optical signal is subjected to phase shift by the phase shifter, the two paths of optical signals are firstly coupled together by the coupler, then are divided into two paths of optical signals with phase difference, and then are output after being modulated by the MZ modulator;

the monitoring optical detector detects light of the MZ modulator in real time and converts the light into a current signal, the control unit performs low-pass filtering processing on the current signal to obtain a target low-frequency signal, and alternately adjusts the voltage of a bias electrode of the MZ modulator and the voltage of the phase shifter according to the current signal and the target low-frequency signal to enable the target low-frequency signal to be minimum.

Preferably, when the control unit adjusts the voltage of the bias electrode of the MZ modulator, the control unit adjusts the voltage of the phase shifter if the control unit adjusts the voltage to the operating point voltage; and the working point voltage is the voltage of the bias electrode corresponding to the minimum value of the output optical power of the MZ modulator.

Preferably, the control unit performs low-pass filtering processing on the current signal to obtain a target low-frequency signal;

if the control unit firstly adjusts the voltage of the phase shifter and the target low-frequency signal is smaller than a first preset threshold value, or when the control unit adjusts the voltage of the phase shifter and the current target low-frequency signal is lower than the last target low-frequency signal, the control unit changes to adjust the voltage of the bias electrode of the MZ modulator;

if the control unit firstly adjusts the voltage of the bias electrode of the MZ modulator, and if the control unit adjusts the voltage of the phase shifter and the target low-frequency signal is smaller than the first preset threshold value, the alternate adjustment is finished.

Preferably, the adjusting the voltage of the phase shifter by the control unit includes:

the control unit performs low-pass filtering on the current signal to obtain a target low-frequency signal y0, adjusts the voltage of the phase shifter according to the adjusting direction, changes the adjusting direction of the phase shifter if the adjusted target low-frequency signal y1 is larger than or equal to y0, assigns y1 to y0, and adjusts the voltage of the phase shifter according to the adjusting direction again until y1 is smaller than y 0;

if y1 is less than y0, y1 is greater than or equal to a second preset threshold, the adjusting direction of the phase shifter is kept unchanged, y1 is assigned to y0, the voltage of the phase shifter is adjusted again according to the adjusting direction, and until y1 is less than y0, y1 is less than the second preset threshold;

wherein, the preset adjusting direction is as follows: increasing the voltage of the phase shifter when the last adjustment direction is positive; the last adjustment direction is negative, and the voltage of the phase shifter is reduced; and when the adjustment direction is unknown at the last time, initializing the phase shifter to adjust the direction to be positive.

One of the above technical solutions has the following beneficial effects:

under the condition that the insertion loss of the device is not reduced, partial light of the real-time MZ modulator of the optical detector is monitored, the phase shifter and the MZ modulator are alternately adjusted by the control unit, when a target low-frequency signal is minimum, the extinction ratio is maximum, the extinction ratio of the device can be compensated and improved, and the problem that the extinction ratio of the MZ modulator drifts along with temperature and time is solved. The invention is suitable for intensity modulation and also suitable for phase modulation.

Drawings

FIG. 1 is a schematic diagram of an optical modulation device according to an embodiment of the present invention;

FIG. 2 is a more detailed schematic diagram of the optical modulation device of FIG. 1;

FIG. 3 is a graph showing the variation of extinction ratio with the amount of phase shift of the phase shifter after compensation according to the present invention;

FIG. 4 is a graph of the phase shift of the target low frequency signal with the phase shifter according to the present invention;

FIG. 5 is a graph showing the variation of extinction ratio with the target low frequency signal after compensation according to the present invention;

fig. 6 is a schematic diagram of adjusting the voltage of the phase shifter by the control unit according to the embodiment of the invention.

Reference numerals:

1. a modulation unit; 10. an input waveguide; 20. a beam splitter; 21. a first optical path; 22. a second optical path; 30. a phase shifter; 40. a coupler; 50. an MZ modulator; 51. a first modulation arm; 52. a second modulation arm; 60. monitoring the light detector; 70. an output waveguide; 2. a control unit.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

As shown in fig. 1, an embodiment of an optical modulation device is provided. The optical modulation device comprises a modulation unit 1 and a control unit 2, wherein the control unit 2 controls the modulation unit 1 to perform optical modulation. Specifically, the control unit 2 includes a beam splitter 20, a coupler 40, an MZ modulator 50, and a monitoring photodetector 60, which are connected in sequence by optical paths, the beam splitter 20 splits two optical paths, and the phase shifter 30 is disposed on one of the optical paths; the control unit 2 is in signal connection with the monitoring light detector 60, the phase shifter 30 and the MZ modulator 50, respectively.

The beam splitter 20 is used to split the input optical signal into two optical paths for transmission. In this embodiment, an input optical signal is transmitted through the input waveguide 10, and the two optical paths are divided into a first optical path 21 and a second optical path 22.

A phase shifter 30 is arranged on one of the optical paths, in this embodiment on the first optical path 21, for changing the phase of the optical signal on the first optical path. The proper phase change of the light can improve the extinction ratio and even perfectly compensate (the extinction ratio tends to infinity).

The coupler 40 is configured to couple optical signals of the two optical paths together first, and the coupled optical signals are split into two optical paths for transmission, where the optical signals on the two optical paths are both coupled signals, but the two optical signals have a phase difference. Preferably, the coupler 40 may be a Directional Coupler (DC) or a2 × 2 multimode interferometer (MMI).

And an MZ modulator 50 for modulating the optical signal output by the coupler 40 and outputting the modulated optical signal. As shown in fig. 2, the MZ modulator 50 includes two modulation arms, which are divided into a first modulation arm 51 and a second modulation arm 52 in the present embodiment. And the two modulation arms are provided with RF electrodes (radio frequency electrodes) which modulate optical signals after being loaded with radio frequency signals. The bias electrodes are arranged on both modulation arms, or one of the modulation arms, and different voltages are loaded on the bias electrodes, so that the working point of the MZ modulator 50 can be regulated.

And a monitoring optical detector (MPD)60 for detecting a part of the light split by the MZ modulator and converting the detected light into a current signal.

The control unit 2 is configured to perform low-pass filtering on the current signal detected by the monitoring optical detector 60 to obtain a target low-frequency signal, and is further configured to alternately adjust the voltage of the bias electrode of the MZ modulator and the voltage of the phase shifter according to the current signal and the target low-frequency signal, so that the target low-frequency signal is minimized.

Specifically, the control unit 2 is configured to adjust the voltage of the phase shifter 30 according to the target low-frequency signal, so as to change the phase shift amount of the phase shifter 30, and reduce the target low-frequency signal. Preferably, the control unit 2 sets the signal power obtained by passing the current signal through a low-pass filter (10kHz or less) as the target low-frequency signal. The control unit 2 is used for adjusting the voltage of the bias electrode of the MZ modulator according to the current signal.

In some embodiments, the control unit 2 performs the alternating adjustment of the voltage of the bias electrode of the MZ modulator 50 and the voltage of the phase shifter 30, and may first adjust the voltage of the bias electrode of the MZ modulator 50 and then perform the repeated alternating; the voltage of the phase shifter 30 may be adjusted first and then repeated.

When the control unit 2 adjusts the voltage of the bias electrode of the MZ modulator 50, if the voltage is adjusted to the operating point voltage, the control unit 2 adjusts the voltage of the phase shifter 30 instead. The working point voltage is the voltage of the corresponding bias electrode when the output optical power of the MZ modulator 50 reaches the minimum value, and the condition that the working point voltage needs to be satisfied is determined by the application scenario of the MZ modulator 50.

In an embodiment, when the voltage of the bias electrode of the MZ modulator 50 is adjusted after the voltage of the phase shifter is adjusted, since the voltage of the bias electrode of the MZ modulator 50 is periodic, the voltage of the bias electrode of the MZ modulator 50 in one period is recorded and a curve is drawn, an abscissa of the curve is a voltage value loaded on the bias electrode, an ordinate of the curve is a current value converted by the monitoring photodetector 60, a lowest point of the curve is a minimum value of the output optical power of the MZ modulator 50, and a corresponding abscissa is a working point voltage of this adjustment.

If the control unit 2 first adjusts the voltage of the phase shifter 30 and the target low-frequency signal is smaller than the first preset threshold, or if the control unit 2 adjusts the voltage of the phase shifter 30 and the current target low-frequency signal is lower than the previous target low-frequency signal, the control unit 2 changes to adjust the voltage of the bias electrode of the MZ modulator 50.

If the control unit 2 first adjusts the voltage of the bias electrode of the MZ modulator 50, and if the control unit 2 adjusts the voltage of the phase shifter 30, the entire alternation adjustment is finished if the target low-frequency signal is smaller than the first preset threshold value.

In an alternative embodiment, a dc voltage V is applied to the phase shifter 30, the signal power obtained by passing the current signal detected by the monitoring photodetector 60 through a low-pass filter (below 10 kHz) is used as the target low-frequency signal, and the dc voltage V is controlled to adjust the phase shift amount of the phase shifter 30, so that the target low-frequency signal is reduced. As shown in fig. 4, when the target low frequency signal is minimum, the extinction ratio is maximum.

In another alternative embodiment, the phase shifter 30 is applied with a dc voltage signal V + V0 with low frequency disturbance, where V is a dc voltage, V0 is a low frequency disturbance voltage signal, and V0 may be a sinusoidal signal or a square wave signal with a frequency of f 0. The current signal detected by the monitoring photodetector 60 is passed through a low-pass filter (10kHz or less) to obtain a component having a frequency f0 as a target low-frequency signal, and the phase shift amount of the phase shifter 30 is adjusted by controlling V + V0 so that the target low-frequency signal is reduced. As shown in fig. 4, when the target low frequency signal is minimum, the extinction ratio is maximum.

The principle of the optical modulation device will be described below based on the above-described embodiments.

When the amplitude of the output light field of the first modulation arm 51 and the amplitude of the output light field of the second modulation arm 52 of the conventional MZ modulator 50 are b1 and b2, respectively, the extinction ratio ER of the optical modulation apparatus is:

it can be seen that the closer b1 is to b2, the greater the extinction ratio ER, which tends to infinity when b1 is b 2. Therefore, in order to increase the extinction ratio ER, the amplitude of the optical fields of the two modulation arms of the MZ modulator 50 needs to be equalized.

As shown in fig. 2, the optical modulation apparatus of the above embodiment is added with a control unit 2, a phase shifter 30, a coupler 40, and a monitoring photodetector 60 in addition to a conventional MZ modulator 50. Assuming that the optical field of the first optical path 21 is denoted as a1, the optical field of the second optical path 22 is denoted as a2 × exp (j θ), the amount of phase shift generated by the phase shifter 30 is α, and the transfer function of the coupler 40 can be expressed as:

wherein, delta₁And kappa₁Satisfies δ as a coupling coefficient₁ ²+κ₁ ²≤1。

Considering the imbalance of the two modulation arms of the original MZ modulator and the imbalance of the coupling coefficient of the beam combiner of the MZ modulator, the two imbalances are equivalent to the loss coefficient of the amplitude of the optical field of the MZ modulator 50, and let b be assumed respectively₁′，b₂', the DC bias voltages of the two modulation arms are respectively V₁And V₂Radio frequency voltage of V_rfHalf-wave voltage of radio frequency V_πrfBias DC half-wave voltage of V_πdcThen the transfer function of MZ modulator 50 can be expressed as:

the two modulation arm optical fields output by coupler 40 can be expressed as:

the light field E of the first modulation arm 51_out1And the light field E of the second modulation arm 52_out2Respectively as follows:

the extinction ratio ER of the present embodiment is:

in order to maximize the extinction ratio ER, | E is actually required_out1|＝|E_out2Obtaining:

since the first optical path 21 and the second optical path 22 are passive structures, there is not much loss, it is easy to achieve a1 ≈ a2, and the coupler 40 can also easily achieve substantial splitting balance, i.e., δ₁≈κ₁Therefore:

therefore, the phase shift amount α can always be found to satisfy the formula (9), that is, the phase shift amount α can be found to satisfy | E_out1|＝|E_out2To maximize the extinction ratio ER. The phase shift amount α generated by the phase shifter 30 can be controlled by loading a voltage V on the phase shifter 30, which satisfies:

wherein, V_πIs the half wave voltage of the phase shifter 30. Therefore, the optical modulation apparatus according to the embodiment of the present invention can compensate the extinction ratio of the MZ modulation, so that the total extinction ratio is improved, as shown in fig. 3.

Based on equations (5) and (6), the optical power P of the optical modulator can be calculated as follows:

where Re represents the real part and x represents the complex conjugate. The operating point of the MZ modulator 50 is set to the voltage of the bias electrode corresponding to the minimum output optical power, i.e. the DC bias voltage V of the two modulation arms₁And V₂P reaches a minimum value. Due to V₁、V₂And E_out1And E_out2The amplitude is independent, and therefore, when the output optical power P reaches a minimum,E_out1and E_out2The phases are exactly opposite, i.e. the phase difference is pi or pi +2n pi, n is an integer. At this time, the output optical power may be expressed as follows:

when | E_out1|＝|E_out2I, the output optical power P reaches a minimum value. That is, the minimum value of the output optical power P corresponds to the maximum value of the extinction ratio ER, and the output optical power P can reach the minimum value by adjusting the phase shift amount α by adjusting the voltage V on the phase shifter 30, and at this time, the extinction ratio ER of the optical modulation device also reaches the maximum value. The output optical power P may be the target low frequency signal.

Based on the equation (11), the curve of the target low-frequency signal varying with the phase shift amount of the phase shifter 30 can be simulated and calculated as shown in fig. 4, and the simulation condition is V_rf0, i.e. no radio frequency voltage is applied; in practice, the curve will also be similar to fig. 4 after loading with a radio frequency voltage. The curve of the change of the compensated extinction ratio ER along with the target low-frequency signal is shown in FIG. 5, and the simulation condition is consistent with that in FIG. 4; in fact, after applying the rf voltage, the curve will be similar to that of fig. 5, that is, the target low frequency signal is decreased, which can increase the extinction ratio ER of the optical modulation apparatus in the embodiment of the present invention.

Based on the above embodiments, the present invention also provides another embodiment of an optical modulation device. In the nested optical modulation device, one of the first modulation arm 51 and the second modulation arm 52 of the MZ modulator 50 may be the modulation unit 1, or both of them may be the modulation unit 1, and the remaining components are not changed.

Based on the above-described embodiments, a nested optical modulation device embodiment is provided in which both arms are modulation units 1. The outer MZ modulator 50 is called a mother MZ modulator, and the inner 2 MZ modulators are child MZ modulators. The nested optical modulation apparatus can be used as an IQ modulator for coherent optical communication. In this embodiment, IQ signal intensity can be equalized, and the nested optical modulation apparatus can substantially increase the extinction ratio.

The present invention further provides an embodiment of a modulation method, which is applicable to the optical modulation apparatus, and includes:

the input optical signal is divided into two paths for transmission by the beam splitter 20, wherein after one optical signal is phase-shifted by the phase shifter 30, the two optical signals are coupled together by the coupler 40, then divided into two paths of optical signals with phase difference, and then modulated by the MZ modulator 50 for output.

The monitoring optical detector 60 detects part of light of the MZ modulator 50 in real time and converts the part of light into a current signal, the control unit 2 performs low-pass filtering on the current signal to obtain a target low-frequency signal, and alternately adjusts the voltage of the bias electrode of the MZ modulator 50 and the voltage of the phase shifter 30 according to the current signal and the target low-frequency signal, so that the target low-frequency signal is minimized.

Further, the voltages of the bias electrodes of the MZ modulator 50 and the phase shifter 30 are alternately adjusted, the voltages of the bias electrodes of the MZ modulator 50 can be adjusted first, then the voltages of the phase shifter 30 and the voltages … … of the bias electrodes of the MZ modulator 50 can be adjusted to alternate cyclically; it is also possible to first adjust the voltage of the phase shifter 30, then the MZ modulator 50 biases the voltage of the electrodes, and then adjust the voltage … … of the phase shifter 30 to cyclically alternate.

In the above adjustment process, when the control unit 2 adjusts the voltage of the bias electrode of the MZ modulator 50, the control unit 2 adjusts the voltage of the phase shifter 30 if the operating point voltage is adjusted (the operating point voltage is the voltage of the bias electrode corresponding to the minimum MZ modulator output optical power).

In some embodiments, if the control unit 2 first adjusts the voltage of the phase shifter 30, and the target low-frequency signal is smaller than the first preset threshold; or, when the control unit 2 adjusts the voltage of the phase shifter 30, the current target low-frequency signal is lower than the previous target low-frequency signal; the control unit 2 becomes to adjust the voltage of the bias electrode of the MZ modulator 50.

In some embodiments, if the control unit 2 first adjusts the voltage of the bias electrode of the MZ modulator 50, the control unit 2 ends the entire alternation adjustment when adjusting the voltage of the phase shifter 30 if the target low frequency signal is less than the first preset threshold.

That is, in the adjustment process of the cyclic alternation, except that the control unit 2 firstly adjusts the voltage of the phase shifter 30, and the target low-frequency signal is smaller than the first preset threshold; and taking the target low-frequency signal smaller than a first preset threshold value as an end condition of the whole cycle alternating adjustment.

As shown in fig. 6, an embodiment of the control unit 2 for adjusting the voltage of the phase shifter 30 is provided, which specifically includes the following steps:

s1, the control unit 2 performs low-pass filtering processing on the current signal detected by the monitoring light detector 60 to obtain a target low-frequency signal y 0.

S2, the control unit 2 adjusts the voltage of the phase shifter 30 according to the adjustment direction. Specifically, the adjusting direction is as follows: increasing the voltage of the phase shifter 30 when the last adjustment direction is positive; when the last adjustment direction is negative, the voltage of the phase shifter 30 is reduced; the last time the adjustment direction was unknown, the initialization phase shifter 30 adjusts the direction to positive.

And S3, obtaining the adjusted target low-frequency signal y 1.

S4, judging whether y1 is less than y0, if yes, entering S6; if not, the process proceeds to S4.

S5, the adjustment direction of the phase shifter is changed, and the process proceeds to S8.

S6, judging whether y1 is smaller than a second preset threshold value, and if yes, ending; if not, the process proceeds to S7.

S7, the adjustment direction of the phase shifter is kept unchanged, and the process proceeds to S8.

S8, assigning y1 to y0, and turning to S2.

The present invention is not limited to the above embodiments, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the scope of the claims of the present invention which are filed as the application.