IQ modulator bias voltage automatic control system and control method based on MPD

Abstract

The invention discloses an MPD-based IQ modulator bias voltage automatic control system and control method, the system comprises a continuous light source, an IQ modulator, an MPD acquisition module, a data processing module and a voltage output module, and three MPDs are arranged in the IQ modulator; after two arms of I, Q and P paths of the IQ modulator are combined, a beam of light is split out and enters three MPDs respectively, and an MPD acquisition module reads light currents of the three MPDs in real time; the voltage output module applies initial bias voltage to the bias electrodes I, Q and P; and the data processing module judges whether the I, Q and P paths are at correct working points under the current initial bias voltage according to the light currents of the three MPDs, and controls the voltage output module to correct the initial bias voltage, so that the IQ modulator is at the correct working point. Automatic locking and long-term control functions of the working point of the IQ modulator are achieved, the structure is simple, and pilot frequency interference is avoided.

Description

Technical Field

[0001] The present invention relates to optical signal modulation technology, and in particular to an automatic control system and a control method for an IQ modulator bias voltage based on MPD. Background Art

[0002] The IQ modulator is a key component for high-speed optical communication. By precisely controlling the phase and amplitude of the optical signal, it can support a variety of complex modulation formats and meet the requirements of modern optical communication systems for high data rates and high signal quality. It can be widely used in digital modulation methods such as BPSK, QPSK, QAM, as well as OFDM modulation, single-sideband optical signal generation, etc.

[0003] The IQ modulator usually adopts Mach-Zehnder modulator (MZM) as the basic structure, which consists of 2 sub-MZMs and 1 parent MZM. During use, these 3 MZMs need to be phase controlled, which is usually achieved by setting appropriate bias voltages for the 3 channels, that is, locking the optimal operating point.

[0004] During the use of the IQ modulator, factors such as temperature and stress can cause the bias point of the IQ modulator to drift over time, causing distortion and deterioration of the output optical signal. Therefore, in order to achieve precise bias point control and obtain better modulation quality, it is necessary to use a peripheral control system to adjust the three bias voltages of the IQ modulator in real time to achieve the optimal state.

[0005] At the output port of a traditional IQ modulator, a small portion of the output light is usually split and sent to an integrated MPD for output optical power detection. With the improvement of process capabilities, IQ modulators with integrated 3-way MPDs have appeared on the market, providing the possibility for a better working point control method.

[0006] In addition, the current mainstream modulator automatic bias control methods are mainly based on the pilot-free control method (power method) of direct optical power detection and the pilot-based method (pilot method). The traditional pilot-free method is based on the analysis of the DC power and AC power of the optical power to determine whether the bias point is offset, and then iteratively optimizes and stabilizes the modulator bias point through continuous adjustment. The pilot method detects the bias point by adding a periodic disturbance signal, monitoring and analyzing the relevant harmonic power components of the pilot, and then completes the regulation.

[0007] However, the traditional power method requires AC signal analysis, so the circuit is relatively complex. The pilot method introduces multiple pilot signals, which will cause signal degradation; the pilot signals will also interfere with each other, which brings difficulties to circuit design and further causes signal degradation. Summary of the invention

[0008] Invention content: One object of the present invention is to provide an IQ modulator bias voltage automatic control system based on MPD, which has the advantages of simple structure and no pilot interference; another object of the present invention is to provide an IQ modulator bias voltage automatic control method based on MPD, which can adjust the three-way bias voltage of the IQ modulator to return it to the optimal state.

[0009] Technical solution: The present invention discloses an automatic control system for bias voltage of an IQ modulator based on MPD, comprising a continuous light source, an IQ modulator, an MPD acquisition module, a data processing module and a voltage output module, wherein the IQ modulator has three built-in MPDs;

[0010] The optical signal output by the continuous light source enters the optical input port of the IQ modulator and is then divided into two paths, one path entering the sub-MZM of the I path of the IQ modulator and the other path entering the sub-MZM of the Q path of the IQ modulator;

[0011] The optical signals passing through the two arms of the child MZM of the I path are further divided into two beams after being combined, one of which is used as the optical signal of one arm of the parent MZM of the P path, and the other is received by the MPD of the I path; the optical signals passing through the two arms of the child MZM of the Q path are further divided into two beams after being combined, one of which is used as the optical signal of the other arm of the parent MZM of the P path, and the other is received by the MPD of the Q path;

[0012] After the optical signals of the two arms of the parent MZM of the P path are combined, they are also divided into two beams again, one of which is used as the output optical signal, and the other is received by the MPD of the P path;

[0013] The voltage output module is used to apply initial bias voltages to the bias electrodes of the I, Q, and P paths in the IQ modulator respectively; the MPD acquisition module is used to read the photocurrents of the three MPDs in real time, thereby monitoring the working status of the I, Q, and P paths; the data processing module is connected to the MPD acquisition module, and reads the photocurrents of the three MPDs according to the MPD acquisition module, and judges whether the I, Q, and P paths are at the correct working points under the current initial bias voltage, and controls the voltage output module to correct the initial bias voltage of one path that is not at the correct working point, so as to finally put the IQ modulator at the correct working point.

[0014] Furthermore, two RF input ports are provided on the IQ modulator, and two groups of sinusoidal RF signals with the same frequency and a phase difference of 90° output by the RF source are applied to an arm where the bias electrodes of the two sub-MZMs in the IQ modulator are located through the two RF input ports.

[0015] Furthermore, the two MZM arms of the I, Q and P paths of the IQ modulator respectively adopt a 2×2 MMI structure when beam-combining, and the two output ports thereof lead to the optical waveguide and MPD of the path respectively.

[0016] Furthermore, the optical power received by the MPD of each of the I, Q, and P paths of the IQ modulator is in a synchronous and complementary relationship with the optical power entering the output optical waveguide of the path.

[0017] Furthermore, the MPD is a low-bandwidth device used to detect average optical power.

[0018] Furthermore, the IQ modulator adopts an IQ thermo-optical bias modulator or an IQ electro-optical bias modulator.

[0019] Furthermore, the voltage output module adopts an adjustable voltage source; the MPD acquisition module is one including but not limited to a single-chip microcomputer ADC or an ADC sampling module; the data processing module is one including but not limited to a single-chip microcomputer, FPGA, and LabView.

[0020] Based on the same inventive concept, an automatic control method for bias voltage of an IQ modulator based on MPD of the present invention is applied to the above-mentioned automatic control system for bias voltage of an IQ modulator based on MPD, and the method comprises:

[0021] Build an automatic control system for the bias voltage of the IQ modulator based on MPD, turn on the optical signal generated by the continuous light source, and inject it into the optical input port of the IQ modulator, and observe the spectrum of the output optical signal through a spectrometer;

[0022] Turn on the RF source to generate two RF signals with a phase difference of 90°, and inject them into two RF input ports of the IQ modulator, wherein the two RF signals have the same frequency and power;

[0023] The MPD acquisition module reads the photocurrents of the three MPDs in real time to monitor the working status of the I, Q, and P paths;

[0024] The data processing module reads the photocurrents of the three MPDs according to the MPD acquisition module, and determines whether the I, Q, and P paths are at the correct working points under the current bias voltage, and controls the voltage output module to correct the bias voltage of the path that deviates from the correct working point;

[0025] The voltage output module provides a corrected bias voltage to a bias electrode of one of the I, Q, and P paths in the IQ modulator that deviates from the correct operating point, thereby ultimately placing the IQ modulator at the correct operating point.

[0026] Furthermore, the data processing module reads the photocurrents of the three MPDs according to the MPD acquisition module, and respectively determines whether the I, Q, and P paths are at the correct working points under the current bias voltage, and controls the voltage output module to correct the bias voltage of one path that deviates from the correct working point, including:

[0027] I and Q path coarse adjustment: The voltage output module applies 0-15V scanning voltage to the I and Q paths of the IQ modulator in turn. The photocurrent output by the MPD of the I and Q paths is collected and read by the MPD acquisition module, and all voltages and their corresponding MPD currents within the scanning range are recorded. During the scanning process, the current values ​​of the two MPDs have two peaks. After the scanning is completed, the bias voltage of the I and Q paths returns to the second peak value I of the MPD current. max-I ,I max-Q The corresponding voltage V 0-I 、V 0-Q ;

[0028] I, Q circuit fine adjustment: V 0-I As the center, perform voltage scan on I channel within the range of ±0.5V and set the scan step size; if a peak of MPD current of I channel is read in this scan, I max-I Replace V 0-I is replaced by the voltage corresponding to the peak value; if a peak of the MPD current of the I path is not read in this scan, the I max-I To operate, directly set V 0-I Replace it with the voltage corresponding to the maximum value of MPD current of I channel in this scan; repeat the above operation 3 times, lock the bias voltage of I channel at the null point, and fine-tune Q channel according to the fine-tuning method of I channel;

[0029] I, Q channel drift correction: read the current MPD current reading of I channel in real time I , and with I max-I Compare and determine whether the I channel has drifted according to the comparison result. If drift has occurred, correct the I channel; read the current MPD current reading I of the Q channel in real time. Q , and I max-Q Compare and determine whether the Q path has drifted according to the comparison result, and correct the Q path if drift has occurred;

[0030] P circuit control: perform voltage scan on P circuit. The scanning method is the same as I and Q coarse adjustment. After the scan is completed, return to the second positive Q point MPD current I quad-P The corresponding voltage; when the P-way MPD current I P >I quad-P When the MPD current exceeds the threshold, the bias voltage is gradually increased until the MPD current returns to the threshold. P quad-P And when the threshold is exceeded, the bias voltage is gradually reduced until the MPD current returns to within the threshold;

[0031] ​Long-term maintenance: After completing the P-channel control, continue to execute the I and Q-channel drift correction steps and the P-channel control steps, and perform long-term drift judgment on the I, Q, and P channels. When one of the channels drifts, correct it to return to the correct working point.

[0032] Furthermore, the current MPD current reading I of channel I is read in real time. I , and I max-I A comparison is made, and whether the I channel has drifted is determined according to the comparison result. If drift has occurred, the I channel is calibrated according to the I and Q channel fine-tuning steps, including:

[0033] Drift determination: real-time reading of the current MPD current reading I of channel I I , and I max-I For comparison; if I I >I max-I , then I max-I Replace update with I I ;if I I max-I and This indicates that the current I path has deviated from the correct working point and needs to be corrected. Indicates the set threshold;

[0034] Correction method: If drift occurs, it will deviate from the correct working point. At this time, according to the I and Q path fine-tuning method, set the scanning voltage range to ±0.15V, and the scanning step is smaller than the scanning step during the I and Q path fine-tuning. At the same time, the number of scans is not fixed at 3 times, and it continues until the correct working point is found to achieve correction of the I path.

[0035] Beneficial effects: Compared with the prior art, the significant technical effects of the present invention are as follows:

[0036] The three MPDs built into the IQ modulator are used to accurately monitor the working status of the I, Q, and P paths, thereby achieving efficient and accurate automatic bias control. Compared with the traditional method, it does not require complex peripheral circuits and has low cost; it does not require the use of pilot signals, thus avoiding interference caused by the introduction of pilot signals; the control circuit and algorithm are simple and the response is fast. Therefore, the present invention uses a simple structure and algorithm to achieve automatic control of the bias voltage of the IQ modulator, which has the advantages of simple structure, low cost, and high precision.

[0037] ​In the present invention, the MPD acquisition module, the data processing module, and the voltage output module cooperate with each other. The data processing module can determine whether the IQ modulator is at the correct working point according to the sizes of the three MPD currents collected by the MPD acquisition module, and control the voltage output module to correct the three bias voltages, so that the deviation from the correct working point returns to the correct working point, and repeat the above process, real-time reading and continuous control, finally realizing the automatic locking and long-term control of the IQ modulator working point. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram of the structure of an automatic control system for bias voltage of an IQ modulator based on MPD disclosed in an embodiment of the present invention;

[0039] Figure 2 The diagram is a curve showing the change of the MZM transfer function with the bias voltage and a schematic diagram of the operating point disclosed in the embodiment of the present invention;

[0040] Figure 3 A schematic diagram of the first type of 0th order Bessel function curve and the first two positive zero points disclosed in an embodiment of the present invention;

[0041] Figure 4 Different RF signals V disclosed in the embodiments of the present invention PP The following is the curve of the average optical power of MZM changing with the bias voltage;

[0042] Figure 5 It is a curve diagram of the average optical output power and MPD current of the P path of the IQ thermo-optical modulator disclosed in the embodiment of the present invention changing with the bias voltage;

[0043] Figure 6 A schematic diagram of the implementation structure of an automatic control system for an IQ modulator bias voltage based on MPD disclosed in an embodiment of the present invention;

[0044] Figure 7 A schematic flow chart of an automatic control method for bias voltage of an IQ modulator based on MPD disclosed in an embodiment of the present invention;

[0045] Figure 8 A flowchart of a data processing module disclosed in an embodiment of the present invention;

[0046] Fig. 9 This is a spectrum diagram of a carrier suppressed single sideband signal generated by controlling an IQ thermo-optical bias modulator disclosed in an embodiment of the present invention. DETAILED DESCRIPTION

[0047] The technical solution of the present invention is described in detail below in conjunction with specific implementation methods and the accompanying drawings of the specification.

[0048] Example 1

[0049] like Figure 1 As shown, the MPD-based IQ modulator bias voltage automatic control system of the present invention includes a continuous light source, an IQ modulator, a radio frequency source, an MPD acquisition module, a data processing module, a voltage output module, and a spectrometer, wherein the IQ modulator has three built-in MPDs (monitoring photodetectors). Among them, Bias-I, Bias-Q, and Bias-P represent the bias electrodes of the I, Q, and P circuits, respectively.

[0050] The optical signal output by the continuous light source enters the optical input port of the IQ modulator, and is then divided into two paths, one path entering the sub-MZM (Mach-Zehnder modulator) of the I path of the IQ modulator, and the other path entering the sub-MZM of the Q path of the IQ modulator; the optical signals passing through the two arms of the sub-MZM of the I path are further divided into two beams after beam combination, one of which is used as the optical signal of one arm of the parent MZM of the P path, and the other is received by the MPD of the I path; the optical signals passing through the two arms of the sub-MZM of the Q path are further divided into two beams after beam combination, one of which is used as the optical signal of the other arm of the parent MZM of the P path, and the other is received by the MPD of the Q path; the optical signals passing through the two arms of the parent MZM of the P path are also divided into two beams again after beam combination, one of which is used as the output optical signal, and the other is received by the MPD of the P path. Therefore, the optical signal power received by the three MPDs is related to the optical signal power transmitted to the optical waveguide of the path, that is, the working state of the I, Q, and P paths can be determined by the photocurrent of the three MPDs.

[0051] The RF source provides the I and Q sub-MZMs with sinusoidal RF signals with the same frequency and a phase difference of 90°. Specifically, two RF input ports are set on the IQ modulator, and two sets of sinusoidal RF signals with the same frequency and a phase difference of 90° output by the RF source are applied to one arm where the bias electrodes of the two sub-MZMs in the IQ modulator are located through the two RF input ports.

[0052] The MPD acquisition module, data processing module and voltage output module should be used after providing optical input to the IQ modulator and injecting RF signals into the I and Q paths. When a continuous light source provides optical input to the IQ modulator and injects RF signals with a phase difference of 90° into the I and Q RF (radio frequency) input ports, the voltage output module applies an initial bias voltage to the 3 bias electrodes of the IQ modulator.

[0053] The voltage output module applies initial bias voltage to the bias electrodes of the I, Q, and P paths in the IQ modulator. The spectrometer is used to observe the spectrum of the output optical signal. The MPD acquisition module is responsible for reading the photocurrents of the three MPD outputs in real time and transmitting them to the data processing module to monitor the working status of the I, Q, and P paths; the data processing module is connected to the MPD acquisition module, and according to the size of the photocurrents of the three MPDs read by the MPD acquisition module, it judges whether the I, Q, and P paths are at the correct working point under the current bias voltage. If one path is not at the correct working point, the control voltage output module corrects the initial bias voltage of the path that is not at the correct working point, and finally makes the IQ modulator at the correct working point. Repeat the above process, read in real time and control continuously, so that the present invention can realize automatic locking and long-term control of the working point of the IQ modulator.

[0054] In this embodiment, the voltage output module is an adjustable voltage source, including but not limited to a source meter, a DAC controlled by a single chip microcomputer or an FPGA, etc.

[0055] In this embodiment, the MPD acquisition module includes but is not limited to a single-chip microcomputer ADC or an ADC sampling module.

[0056] In this embodiment, the data processing module is one including but not limited to a single chip microcomputer, FPGA, and LabView.

[0057] In this embodiment, the IQ modulator adopts an IQ thermo-optical bias modulator or an IQ electro-optical bias modulator. The IQ modulator is analyzed below by taking the IQ electro-optical bias modulator as an example.

[0058] IQ modulators have multiple application scenarios and usage methods, among which generating carrier suppressed single sideband signals is an important capability of IQ modulators. To generate carrier suppressed single sideband signals using IQ modulators, it is necessary to make the I and Q sub-MZMs work at the null point and the P-path parent MZM work at the Q point. The transfer function of the MZ electro-optical bias modulator can be expressed as:

[0059]

[0060] Among them, V B Represents the DC bias voltage, V π represents the half-wave voltage of the MZ modulator, v RF Indicates the RF modulation signal. When no RF signal is added, the curve and operating point of the transfer function are shown as follows Figure 2 shown.

[0061] When the amplitude of V is injected into the IQ modulator I RF , when the RF signal has an angular frequency of ω, the I-channel transfer function can be expressed as:

[0062]

[0063] Formula (2) can be expressed as:

[0064]

[0065] Among them, J n (x) represents the nth-order Bessel function of the first kind.

[0066] In this embodiment, the MPD used is a low-bandwidth device for detecting average optical power, and the output is the average photocurrent, which cannot detect high-frequency modulation signals. Therefore, what is received by I-channel MPD is the average optical power, which does not contain high-frequency components, so formula (3) can be simplified as:

[0067]

[0068] From formula (4), it can be seen that the curve of the transfer function of the I-channel MZM with the change of bias voltage is related to the injected RF signal amplitude. The curve of the first-order 0th-order Bessel function J0(x) is as follows: Figure 3 As shown in the figure, the first two zero points greater than 0 are 2.405 and 5.520 respectively. Therefore, the RF signal amplitude V RF The size of determines the change trend of the transfer function of the I-channel MZM. When 0<πV RF / V π <2.405, that is, 0<V RF <0.7655V π When the average optical power of channel I is V B =V π The null point has the minimum value; when 2.405<πV RF / V π <5.520, i.e. 0.7655V π <V RF <1.757V π When the average optical power of channel I is V B =V π The null point has the maximum value. Figure 4 Shows V PP =2V RF =V π / 2,V π , 3V π / 2,2V π The schematic diagram of the optical power of the I-channel MZM changing with the bias voltage is shown in FIG. The Q-channel MZM has the same situation as the I-channel MZM, which will not be repeated here.

[0069] The difference between the IQ thermo-optic bias modulator and the IQ electro-optic bias modulator is that the IQ electro-optic bias modulator directly changes the refractive index of one arm of the MZM waveguide through the bias voltage to change the phase of the optical signal, while the bias voltage of the IQ thermo-optic bias modulator is applied to the thermal adjustment resistor, and the heat generated by the thermal adjustment resistor changes the refractive index of the waveguide, thereby changing the phase of the optical signal. Therefore, the above theory is also applicable to the IQ thermo-optic bias modulator.

[0070] In this embodiment, the two MZM arms of the I, Q, and P paths of the IQ modulator adopt a 2×2 MMI (multi-mode interferometer) structure when combining, and its two output ports lead to the optical waveguide and MPD of the path respectively. The splitting ratio of the two output ports is related to the phase of the two input ports. When a bias voltage is applied to one of the arms, the phase of the optical signal passing through the arm is changed, and the splitting ratio of the MMI output port changes. Therefore, the optical power received by the MPD of the I, Q, and P paths of the IQ modulator is in a synchronous and complementary relationship with the optical power entering the optical waveguide of the path. When the optical power entering the optical waveguide reaches the maximum, the optical power received by the MPD is the minimum. Conversely, when the optical power entering the optical waveguide is the minimum, the optical power received by the MPD is the maximum. Therefore, by reading the photocurrent size of the 3-path MPD, the current working state of the 3 paths of the IQ modulator can be accurately judged, and then the automatic control of the bias voltage can be realized. The optical signals received by the MPD on the I, Q, and P paths are respectively related to the optical signals transmitted to the I, Q, and P path optical waveguides. Figure 5 The curves of the P-path optical power and the P-path MPD of the IQ thermo-optical bias modulator changing with the bias voltage are shown. Since the optical power of the thermo-optical bias modulator changes with the heat in a sinusoidal curve, and the heat change is proportional to the square of the bias voltage, Figure 5 It can be seen that the optical power presents a gradually compressed sine curve as the bias voltage increases, and the MPD current is synchronously anti-phase with the optical power.

[0071] like Figure 6 As shown, in a specific implementation, the IQ modulator adopts an IQ thermal-optical bias modulator, and the single-chip microcomputer integrates four parts: a DAC module, an MCU, an ADC module, and a logarithmic amplifier. Among them, the DAC module serves as a voltage output module, the MCU serves as a data processing module, the ADC module serves as an MPD acquisition module, and the logarithmic amplifier amplifies the photocurrent of the 3-way MPD output.

[0072] Example 2

[0073] like Figure 7 As shown, an automatic control method for bias voltage of an IQ modulator based on MPD of the present invention is applied to the automatic control system for bias voltage of an IQ modulator based on MPD described in Example 1, and the method comprises the following steps:

[0074] S1. Build an automatic control system for the bias voltage of the IQ modulator based on MPD, turn on the optical signal generated by the continuous light source, and inject it into the optical input port of the IQ modulator, and observe the spectrum of the output optical signal through a spectrometer.

[0075] S2. Turn on the RF source to generate two RF signals with a phase difference of 90°, and inject them into two RF input ports of the IQ modulator, wherein the two RF signals have the same frequency and power.

[0076] S3, MPD acquisition module reads the photocurrents of the three MPDs in real time, thereby monitoring the working status of I, Q, and P paths.

[0077] S4, the data processing module reads the photocurrents of the three MPDs according to the MPD acquisition module, and determines whether the I, Q, and P paths are at the correct working points under the current bias voltage, and controls the voltage output module to correct the bias voltage of the path that deviates from the correct working point. The specific steps are as follows:

[0078] S4.1, I, Q path coarse adjustment: The voltage output module applies 0~15V scanning voltage to the I and Q paths of the IQ modulator in turn. The photocurrent output by the MPD of the I and Q paths is collected and read by the MPD acquisition module, and all voltages and their corresponding MPD currents within the scanning range are recorded; during the scanning process, the current values ​​of the two MPDs have two peaks. After the scanning is completed, the bias voltage of the I and Q paths returns to the second peak value I of the MPD current. max-I ,I max-Q The corresponding voltage V 0-I 、V 0-Q ;

[0079] S4.2, I, Q circuit fine adjustment: V 0-I As the center, perform voltage scan on I channel within the range of ±0.5V and set the scan step size; if a peak of MPD current of I channel is read in this scan, I max-I Replace V 0-I is replaced by the voltage corresponding to the peak value; if a peak of the MPD current of the I path is not read in this scan, the I max-I To operate, directly set V 0-I Replace it with the voltage corresponding to the maximum value of MPD current of I channel in this scan; repeat the above operation 3 times, lock the bias voltage of I channel at the null point, and fine-tune Q channel according to the fine-tuning method of I channel;

[0080] S4.3, I, Q channel drift correction: real-time reading of the current MPD current reading of I channel I , and I max-ICompare and determine whether the I channel has drifted according to the comparison result. If drift has occurred, correct the I channel; read the current MPD current reading I of the Q channel in real time. Q , and I max-Q A comparison is made, and whether the Q path has drifted is determined based on the comparison result. If drift has occurred, the Q path is corrected.

[0081] The specific process of step S4.3 is as follows:

[0082] S4.3.1. Drift determination: Read the current MPD current reading I of channel I in real time I , and I max-I For comparison; if I I >I max-I , then I max-I Replace update with I I ;if I I max-I and This indicates that the current I path has deviated from the correct working point and needs to be corrected. Indicates the set threshold; reads the current MPD current reading I of Q circuit in real time Q , and I max-Q A comparison is made and whether the Q path drifts is determined based on the comparison result. The drift determination condition of the Q path is the same as that of the I path.

[0083] S4.3.2, Correction method: If drift occurs, it will deviate from the correct working point. At this time, according to the I and Q fine-tuning method, set the scanning voltage range to ±0.15V, and the scanning step is smaller than the scanning step during the I and Q fine-tuning. At the same time, the number of scans is not fixed to 3 times, and it continues until the correct working point is found to achieve the correction of the I path. Correct the Q path according to the correction method of the I path.

[0084] S4.4, P circuit control: perform voltage scan on P circuit. The scanning method is the same as I and Q coarse adjustment. After the scanning is completed, return to the second positive Q point MPD current I quad-P The corresponding voltage; when the P-way MPD current I P >I quad-P When the MPD current exceeds the threshold, the bias voltage is gradually increased until the MPD current returns to the threshold. P quad-P And when the threshold is exceeded, the bias voltage is gradually reduced until the MPD current returns to within the threshold;

[0085] S4.5, long-term maintenance: After completing the P-channel control, continue to execute steps S4.3 and S4.4 to perform long-term drift judgment on the I, Q, and P channels. When one of the channels drifts, it is corrected to return to the correct working point. ​​

[0086] S5. The voltage output module provides a corrected bias voltage to the bias electrode of one of the I, Q, and P paths in the IQ modulator that deviates from the correct operating point, thereby ultimately placing the IQ modulator at the correct operating point.

[0087] The present invention can realize the automatic locking and long-term control functions of the IQ modulator working point, has the advantages of simple structure and no pilot interference, and can be widely used in various digital modulation modes such as BPSK, QPSK, QAM, as well as OFDM modulation, single-sideband optical signal generation and other fields.

[0088] The above-mentioned automatic control method of the bias voltage of an IQ modulator based on MPD is verified by a specific embodiment below.

[0089] like Figure 7 As shown, an automatic control method of IQ modulator bias voltage based on MPD of the present invention comprises the following steps:

[0090] S1, press Figure 6 The control system is built as shown in the structural schematic diagram. The continuous light source is turned on to generate an optical signal with a wavelength of 1550nm and a power of 15dBm and injected into the light input port of the IQ thermo-optical bias modulator. The spectrum of the output optical signal is observed through a spectrometer.

[0091] S2. Turn on the RF source to generate two RF signals with a phase difference of 90° and inject them into the I and Q RF input ports of the IQ thermo-optical bias modulator respectively. The two RF signals have the same frequency and power, which are 10 GHz and 18 dBm respectively. The matching resistance of the IQ thermo-optical bias modulator is 50Ω, and the V corresponding to the 18 dBm power is PP About 5V, V RF The half-wave voltage of the I and Q paths of the IQ thermo-optical bias modulator at 10GHz is about 5V, which meets V RF <0.7655V π Therefore, when the I and Q paths work at the null point, the average optical power passing through the I and Q output waveguides is the smallest, and the average optical power received by the two MPDs is the largest. When performing automatic bias control, the I and Q paths need to be locked at the maximum current readings of the two MPDs.

[0092] S3, supply power to the microcontroller, after the logarithmic amplifier amplifies the photocurrent output by the three MPDs, the ADC module reads the photocurrent of the three MPDs in real time, thereby monitoring the working status of the I, Q, and P paths.

[0093] S4, MCU determines whether the I, Q, and P paths are at the correct working point under the current bias voltage according to the photocurrents of the three MPDs read by the ADC module, and controls the DAC module to correct the bias voltage of the path that deviates from the correct working point. Figure 8 As shown, the specific steps include:

[0094] S4.1, I, Q path coarse adjustment: The DAC module in the single-chip microcomputer applies scanning voltage to the I, Q paths of the IQ thermo-optical bias modulator in turn. The two MPD output photocurrents are amplified by the logarithmic amplifier and then collected and read by the ADC module, recording all voltages within the scanning range and their corresponding MPD currents.

[0095] The thermal resistor value of the IQ thermo-optical bias modulator is 4kΩ, and the applied voltage scan range is 0-15V with a step size of 0.1V. During the scan, the MPD current value has two peaks. After the scan, the bias voltage returns to the second peak value of the MPD current I max-I ,I max-Q The corresponding voltage V 0-I 、V 0-Q The second peak is chosen instead of the first peak because the thermal stability of the IQ thermo-optical biased modulator is better at a larger voltage.

[0096] S4.2, I, Q fine adjustment. 0-I The voltage of I channel is scanned in the range of ±0.5V with a scanning step of 0.01V. If a peak of MPD current is read in this scan, I max-I Replace V 0-I If a peak of MPD current is not read in this scan, I max-I To operate, V 0-I Replaced by the voltage corresponding to the maximum value of MPD current in this scan. The judgment condition for MPD current to read a peak is that there are at least 8 values ​​on both sides of the MPD maximum value that are smaller than the maximum value.

[0097] Repeat the above operation 3 times, and the bias voltage of I channel is locked near the null point with a small error. After completing the fine adjustment of I channel, fine adjust Q channel in the same way as I channel.

[0098] S4.3, I, Q path drift correction

[0099] Drift determination: real-time reading of the current MPD current reading I of channel I I , and I max-I For comparison. I >I max-I , then for I max-I Perform a replacement update; if II max-I and This indicates that the current I path has deviated from the correct working point and needs to be corrected. Indicates the set threshold. The judgment conditions of Q channel are the same as those of I channel.

[0100] Correction method: When the I and Q paths drift due to external environmental factors such as temperature, they will deviate from the correct operating point and need to be corrected. The correction operation is basically the same as the I and Q path fine-tuning method, except that the scanning voltage range becomes ±0.15V, the scanning step becomes 0.005V, and the number of scans is not fixed at 3 times, but continues until the correct operating point is found.

[0101] S4.4, P circuit control: perform voltage scan on P circuit, the scan setting is the same as I, Q coarse adjustment. After the scan is completed, return to the second positive Q point MPD current I quad-P Between the peak and valley values ​​on both sides of the positive Q point, the optical power of the P channel increases monotonically with the increase of the bias voltage, while the MPD current decreases monotonically, which is used as the judgment condition of the positive Q point.

[0102] Before the ADC module, a logarithmic amplifier is used to amplify the MPD current. The working principle of the logarithmic amplifier is that when the input current exceeds the threshold and increases exponentially by 10, the output voltage increases linearly. Assuming the MPD output photocurrent is x, the signal collected by the ADC module after being amplified by the logarithmic amplifier can be expressed as:

[0103] y=Alog 10 (x-x0)(5)

[0104] Wherein, A is a constant, and x0 represents the response threshold of the logarithmic amplifier (8). It can be seen from formula (5) that the value of the P-channel MPD current can be obtained through the data collected by the ADC module (6a), which can be expressed as:

[0105] x=10 yA +x0(6)

[0106] The MPD current corresponding to the Q point is the average of the maximum and minimum values ​​of the MPD current. Through the above method, the MPD current corresponding to the P-way Q point and its corresponding bias voltage can be accurately obtained.

[0107] The I and Q path control is to make the MPD current at the maximum value. Therefore, the control can be performed according to the data collected by the ADC module without converting it into the MPD current value.

[0108] ​When the P circuit drifts due to external environmental factors such as temperature, it will deviate from the correct operating point and needs to be corrected. P >I quad-P When the MPD current exceeds the threshold, the bias voltage is gradually increased until the MPD current returns to the threshold. P quad-P When the threshold is exceeded, the bias voltage is gradually reduced until the MPD current returns to within the threshold.

[0109] S4.5, long-term maintenance: After completing the P-channel control, continue to execute the drift judgment of the I, Q, and P channels in steps S4.3 and S4.4. When a channel drifts due to factors such as ambient temperature, it is corrected to return to the correct working point.

[0110] S5. The DAC module provides a corrected bias voltage to the bias electrode of one of the I, Q, and P paths in the IQ modulator that deviates from the correct operating point, thereby ultimately placing the IQ modulator at the correct operating point.

[0111] Through the above steps, the automatic locking and long-term control functions of the IQ thermo-optical modulator working point are realized. Fig. 9 The spectrum of the carrier suppressed single sideband signal generated by this embodiment is shown. It can be seen from the figure that the carrier and sideband suppression ratio exceeds 30dB.​

Claims

1. An IQ modulator bias voltage automatic control system based on MPD, characterized in that: It includes a continuous light source, an IQ modulator, an MPD acquisition module, a data processing module and a voltage output module, wherein the IQ modulator has three built-in MPDs; The optical signal output by the continuous light source enters the optical input port of the IQ modulator and is then divided into two paths, one path entering the sub-MZM of the I path of the IQ modulator and the other path entering the sub-MZM of the Q path of the IQ modulator; The optical signals passing through the two arms of the child MZM of the I path are further divided into two beams after being combined, one of which is used as the optical signal of one arm of the parent MZM of the P path, and the other is received by the MPD of the I path; the optical signals passing through the two arms of the child MZM of the Q path are further divided into two beams after being combined, one of which is used as the optical signal of the other arm of the parent MZM of the P path, and the other is received by the MPD of the Q path; After the optical signals of the two arms of the parent MZM of the P path are combined, they are also divided into two beams again, one of which is used as the output optical signal, and the other is received by the MPD of the P path; The voltage output module is used to apply initial bias voltages to the bias electrodes of the I, Q, and P paths in the IQ modulator respectively; the MPD acquisition module is used to read the photocurrents of the three MPDs in real time, thereby monitoring the working status of the I, Q, and P paths; the data processing module is connected to the MPD acquisition module, and reads the photocurrents of the three MPDs according to the MPD acquisition module, and judges whether the I, Q, and P paths are at the correct working points under the current initial bias voltage, and controls the voltage output module to correct the initial bias voltage of one path that is not at the correct working point, so as to finally put the IQ modulator at the correct working point.

2. The MPD-based IQ modulator bias voltage automatic control system according to claim 1, characterized in that: The IQ modulator is provided with two RF input ports, and two groups of sinusoidal RF signals with the same frequency and a phase difference of 90° output by the RF source are applied to an arm where the bias electrodes of the two sub-MZMs in the IQ modulator are located through the two RF input ports.

3. The MPD-based IQ modulator bias voltage automatic control system according to claim 1, characterized in that: The two MZM arms of the I, Q and P paths of the IQ modulator respectively adopt a 2×2 MMI structure when beam-combining, and the two output ports thereof lead to the optical waveguide and MPD of the path respectively.

4. The MPD-based IQ modulator bias voltage automatic control system according to claim 3, characterized in that: The optical power received by the MPD of each of the I, Q, and P paths of the IQ modulator is in a synchronous and complementary relationship with the optical power entering the output optical waveguide of the path.

5. The MPD-based IQ modulator bias voltage automatic control system according to claim 3, characterized in that: The MPD is a low-bandwidth device used to detect average optical power.

6. The MPD-based IQ modulator bias voltage automatic control system according to claim 1, characterized in that: The IQ modulator adopts an IQ thermo-optical bias modulator or an IQ electro-optical bias modulator.

7. The MPD-based IQ modulator bias voltage automatic control system according to claim 1, characterized in that: The voltage output module adopts an adjustable voltage source; the MPD acquisition module is one including but not limited to a single-chip microcomputer ADC or an ADC sampling module; the data processing module is one including but not limited to a single-chip microcomputer, FPGA, and LabView.

8. An automatic control method for bias voltage of an IQ modulator based on MPD, characterized in that: The method is applied to the MPD-based IQ modulator bias voltage automatic control system according to any one of claims 2 to 7, and the method comprises: Build an automatic control system for the bias voltage of the IQ modulator based on MPD, turn on the optical signal generated by the continuous light source, and inject it into the optical input port of the IQ modulator, and observe the spectrum of the output optical signal through a spectrometer; Turn on the RF source to generate two RF signals with a phase difference of 90°, and inject them into two RF input ports of the IQ modulator, wherein the two RF signals have the same frequency and power; The MPD acquisition module reads the photocurrents of the three MPDs in real time to monitor the working status of the I, Q, and P paths; The data processing module reads the photocurrents of the three MPDs according to the MPD acquisition module, and determines whether the I, Q, and P paths are at the correct working points under the current bias voltage, and controls the voltage output module to correct the bias voltage of the path that deviates from the correct working point; The voltage output module provides a corrected bias voltage to a bias electrode of one of the I, Q, and P paths in the IQ modulator that deviates from the correct operating point, thereby ultimately placing the IQ modulator at the correct operating point.

9. The MPD-based IQ modulator bias voltage automatic control method according to claim 8, characterized in that: The data processing module reads the photocurrents of the three MPDs according to the MPD acquisition module, and determines whether the I, Q, and P paths are at the correct working points under the current bias voltage, and controls the voltage output module to correct the bias voltage of one path that deviates from the correct working point, including: I and Q path coarse adjustment: The voltage output module applies 0-15V scanning voltage to the I and Q paths of the IQ modulator in turn. The photocurrent output by the MPD of the I and Q paths is collected and read by the MPD acquisition module, and all voltages and their corresponding MPD currents within the scanning range are recorded. During the scanning process, the current values ​​of the two MPDs have two peaks. After the scanning is completed, the bias voltage of the I and Q paths returns to the second peak value I of the MPD current. max-I ,I max-Q The corresponding voltage V 0-I 、V 0-Q ; I, Q circuit fine adjustment: V 0-I As the center, perform voltage scan on I channel within the range of ±0.5V and set the scan step size; if a peak of MPD current of I channel is read in this scan, I max-I Replace V 0-I is replaced by the voltage corresponding to the peak value; if a peak of the MPD current of the I path is not read in this scan, the I max-I To operate, directly set V 0-I Replace it with the voltage corresponding to the maximum value of MPD current of I channel in this scan; repeat the above operation 3 times, lock the bias voltage of I channel at the null point, and fine-tune Q channel according to the fine-tuning method of I channel; I, Q channel drift correction: read the current MPD current reading of I channel in real time I , and with I max-I Compare and determine whether the I channel has drifted according to the comparison result. If drift has occurred, correct the I channel; read the current MPD current reading I of the Q channel in real time. Q , and with I max-Q Compare and determine whether the Q path has drifted according to the comparison result, and correct the Q path if drift has occurred; P circuit control: perform voltage scan on P circuit. The scanning method is the same as I and Q coarse adjustment. After the scan is completed, return to the second positive Q point MPD current I quad-P The corresponding voltage; when the P-way MPD current I P >I quad-P When the MPD current exceeds the threshold, the bias voltage is gradually increased until the MPD current returns to the threshold. P quad-P And when the threshold is exceeded, the bias voltage is gradually reduced until the MPD current returns to within the threshold;​ Long-term maintenance: After completing the P-channel control, continue to execute the I and Q-channel drift correction steps and the P-channel control steps, and perform long-term drift judgment on the I, Q, and P channels. When one of the channels drifts, correct it to return to the correct working point.

10. The MPD-based IQ modulator bias voltage automatic control method according to claim 9, characterized in that: Real-time reading of the current MPD current reading I I , and with I max-I A comparison is made, and whether the I channel has drifted is determined according to the comparison result. If drift has occurred, the I channel is calibrated according to the I and Q channel fine-tuning steps, including: Drift determination: real-time reading of the current MPD current reading I of channel I I , and with I max-I For comparison; if I I >I max-I , then I max-I Replace update with I I ;if I I max-I and This indicates that the current I path has deviated from the correct working point and needs to be corrected. Indicates the set threshold;​ Correction method: If drift occurs, it will deviate from the correct working point. At this time, according to the I and Q path fine-tuning method, set the scanning voltage range to ±0.15V, and the scanning step is smaller than the scanning step during the I and Q path fine-tuning. At the same time, the number of scans is not fixed at 3 times, and it continues until the correct working point is found to achieve correction of the I path.

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