CN113204133B - Automatic bias voltage control device of electro-optical modulator based on FPGA - Google Patents

Automatic bias voltage control device of electro-optical modulator based on FPGA Download PDF

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CN113204133B
CN113204133B CN202110432107.3A CN202110432107A CN113204133B CN 113204133 B CN113204133 B CN 113204133B CN 202110432107 A CN202110432107 A CN 202110432107A CN 113204133 B CN113204133 B CN 113204133B
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CN113204133A (en
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马胜利
孙巧英
孔令兵
姚李刚
唐才杰
潘亮
吴海涛
王宇
高腾宇
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Beijign Institute of Aerospace Control Devices
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
    • G02F1/0327—Operation of the cell; Circuit arrangements

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Abstract

The invention relates to an automatic bias voltage control device of an electro-optical modulator based on an FPGA, which relates to the field of optical control and comprises a photoelectric detector, a digital potentiometer, a transimpedance amplifying circuit, an A/D acquisition module, a master control FPGA, a D/A conversion module and a bias voltage regulatorAnd (5) processing the circuit. The main control FPGA is used for controlling the direct-current bias voltage V of the electro-optic modulator through the internal DDS digital signal generator, the D/A conversion module and the bias voltage conditioning circuit bias On the basis of applying a small sinusoidal disturbance signal V sin Input to FPGA through electro-optic modulator, pass band-pass filter and V sin The cross-correlation operation and the feedback control are carried out, so that the complexity of the peripheral circuit is reduced, and the feedback control speed is improved; meanwhile, the amplification factor of the transimpedance amplifying circuit can be changed through the digital potentiometer, the dynamic range of the circuit and the adaptability of the system are improved, and the method has good practical value for stabilizing the optimal working point of the electro-optic modulator.

Description

Automatic bias voltage control device of electro-optical modulator based on FPGA
Technical Field
The invention relates to an automatic bias voltage control device of an electro-optical modulator based on an FPGA, and belongs to the technical field of optical control.
Background
The technical fields of optical fiber communication, distributed sensing and the like mostly adopt pulse light modulation, and information of physical quantity to be measured is obtained by analyzing interference or scattering conditions of an optical path, wherein extinction ratio of pulse light is one of the most important parameters of a system, and signal-to-noise ratio and stability of a subsequent system are directly influenced. Niobium lithium LiNbO 3 The electro-optical modulator is widely applied to pulse optical modulation systems by virtue of the advantages of low loss, high responsivity, small frequency modulation chirp, small wavelength dependence and the like. However due to its waveguide material and its own structureThe reason is that the system is easily influenced by external environment such as temperature and vibration, and under the condition of long-time working, the accumulation of heat of the system is easy to cause the drift or shake of the optimal working point, so that the extinction ratio of the output pulse light is reduced, the system performance is influenced, and even the system cannot work normally. Therefore, the bias voltage of the electro-optical modulator is automatically controlled to be stabilized at an optimal working point, and the method has important significance for long-time stable operation of the system.
At present, a single chip microcomputer is used as a main control chip in most bias voltage feedback control devices of the electro-optical modulator, and an internal A/D, D/A module is used for completing acquisition of the optical voltage and output of the bias voltage. As the main frequency of the singlechip is generally not more than 20MHz and the bit width of the internal integrated AD and DA modules is limited, the defects of low response speed, insufficient control precision and the like exist in the bias voltage control process.
Bias voltage feedback control methods based on the hardware scheme are mainly divided into three types:
(1) Direct optical power measurement: the output optical power of the electro-optical modulator is directly measured, and the optimal working point is positioned through the minimum value point. Because the optical power corresponding to different frequency points of the electro-optical modulator fluctuates in the frequency sweeping process, the mode of directly measuring the optical power is easy to be influenced by the frequency sweeping, so that the feedback control process diverges;
(2) Phase-locked mixing integration method: and superposing a sinusoidal disturbance signal on the basis of the bias voltage for modulation, mixing the output signal of the electro-optical modulator with the sinusoidal disturbance signal by a hardware circuit, integrating, and positioning the optimal working point by detecting an integration zero point. Because the scheme can not distinguish the optimal working point phase phi of the output characteristic curve of the electro-optic modulator 0 Sum (phi) 0 +pi), it is first necessary to scan the optimum operating point over the entire bias voltage control range, and then drift tracking is performed by this method. The circuit design scheme corresponding to the method is complex, an algorithm special hardware module comprising a sinusoidal signal generator, an adder, a mixer, an integrating circuit and the like is needed, and the scanning process takes longer time.
(3) Second harmonic detection: and superposing a sinusoidal disturbance signal on the basis of the bias voltage for modulation, and positioning an optimal working point by detecting the ratio of the intensity of the second harmonic component and the intensity of the first harmonic component of the output signal of the electro-optical modulator. Because the slope k apprxeq 0 of the transmission characteristic curve of the electro-optic modulator near the optimal working point, the corresponding ratio of the second harmonic component to the first harmonic component intensity cannot form an obvious peak value, the practical application is difficult, and a frequency multiplier is required to generate a frequency doubling signal, compared with the scheme (2), the hardware cost and complexity are further improved.
The three schemes can not adjust the amplification factor of the photoelectric amplifying circuit through software, and when the output optical power of the photoelectric modulator is larger, the photoelectric voltage can be saturated, and when the optical power is smaller, the signal-to-noise ratio of the photoelectric voltage signal can be reduced. A larger or smaller optical power may cause the system to fail to operate properly.
Disclosure of Invention
The technical problems solved by the invention are as follows: the utility model provides an electro-optic modulator bias voltage automatic control device based on FPGA has been provided to overcoming prior art shortcoming such as current electro-optic modulator bias voltage automatic control device based on singlechip has that hardware circuit is complicated, response speed is slow, control accuracy is not enough and system adaptability is poor, realize digital signal processing flow and feedback control logic in master control FPGA, simplify peripheral circuit, make full use of FPGA parallel processing's advantage, feedback control speed has been improved, and through flexible configuration digital potentiometer output has improved the dynamic range of transimpedance amplifier circuit, the adaptability of device to peripheral light path difference has been strengthened.
The technical scheme of the invention is as follows: an automatic control device for bias voltage of an electro-optical modulator based on an FPGA, comprising: the device comprises a photoelectric detector (1), a transimpedance amplifying circuit (3), an A/D acquisition module (4), a master control FPGA (5), a D/A conversion module (6) and a bias voltage conditioning circuit (7);
the transimpedance amplifying circuit (3) is provided with a digital potentiometer (2); the digital potentiometer (2) is used as a transimpedance in the transimpedance amplifying circuit (3);
the external electro-optical modulator outputs optical signals, a light splitting sheet inputs 5% of the power of the output optical signals to the photoelectric detector (1), the photoelectric detector (1) performs photoelectric conversion to generate photocurrent signals, and the photocurrent signals are sent to the transimpedance amplifying circuit (3);
the FPGA (5) sends a control instruction to the digital potentiometer (2), and the digital potentiometer (2) adjusts a transimpedance value in the transimpedance amplifying circuit (3) according to the control instruction of the FPGA (5);
the transimpedance amplifying circuit (3) converts the photocurrent signal into an analog photovoltage signal according to the adjusted transimpedance value, and sends the analog photovoltage signal to the A/D acquisition module (4), and the main control FPGA (5) controls the A/D acquisition module (4) to acquire the analog photovoltage signal for analog-to-digital conversion to form a digital photovoltage signal which is used as input of feedback control; the main control FPGA (5) drives the D/A conversion module (5) to generate a bias voltage signal through feedback control, and the bias voltage signal is amplified linearly in amplitude by the bias voltage conditioning circuit (6) and then output to the electro-optical modulator, so that the electro-optical modulator works at an optimal working point.
The master control FPGA (5) comprises: the DDS digital signal generator (501), the data strobe (502), the digital adder (503), the D/A control module (504), the bias voltage feedback control module (505), the A/D control module (506), the band-pass filter (507) and the cross correlation operation module (508);
the DDS digital signal generator (501) generates a DC offset signal V bias And sinusoidal disturbance signal V sin ；
The data strobe (502) has an input 1, an input 2, a strobe, and an output;
the digital adder (503) has an input 1, an input 2, and an output;
the bias voltage feedback control module (505) is provided with an input end 1, an input end 2 and an output end;
the cross-correlation operation module (508) is provided with an input end 1, an input end 2 and an output end;
the generated DC bias signal V bias Is two paths which are identical and are respectively the signal V bias1 And V bias2 : signal V bias1 Connected to input 1 of the data strobe (502), signal V bias2 Is connected to the input 1 of the digital adder (503); generated sinusoidal disturbance signal V sin The two paths are identical, and are respectively: signal V sin1 And V sin2 The method comprises the steps of carrying out a first treatment on the surface of the Signal V sin1 Connected to input 2 of the digital adder (503), signal V sin2 The digital adder (503) is connected to the input end 1 of the cross correlation operation module (508), the output end of the digital adder (503) is connected to the input end 2 of the data strobe (502), the output end of the data strobe (502) is connected to the input of the D/A control module (504), and the strobe end of the data strobe (502) is connected to the output end of the bias voltage feedback control module (505);
under the drive of the A/D control module (506), the A/D acquisition module (4) acquires analog photovoltage signals to perform analog-to-digital conversion to form digital photovoltage signals, wherein the digital photovoltage signals are identical two paths of signals which are respectively signals Vdc1 and Vdc2 and serve as input of feedback control: the signal Vdc1 is connected to the input end 1 of the bias voltage feedback control module (505), the signal Vdc2 is connected to the input end of the band-pass filter (507), after the band-pass filter (507) carries out band-pass filtration, the output of the band-pass filter (507) is connected to the input end 2 of the cross-correlation operation module (508), and the output end of the cross-correlation operation module (508) is connected to the input end 2 of the bias voltage feedback control module (505);
the bias voltage feedback control module (505) adopts preset feedback control logic to control the data strobe (502) to output the bias voltage of the electro-optic modulator according to the signal Vdc1 output by the A/D control module (506) and the signal output by the cross correlation operation module (508).
Preferably, the preset feedback control logic adopted by the feedback control module (505) of the internal bias voltage of the main control FPGA (5) comprises three stages: the method comprises the steps of initially positioning an optimal working point stage, adjusting a transimpedance amplifying circuit stage and accurately positioning and stabilizing the optimal working point stage, and comprises the following steps:
the stage of initially positioning the optimal working point is as follows:
(1) The bias voltage automatic control device of the electro-optical modulator is electrified to control the gating signal V of the data gating device (502) bias1 Let the DC bias voltage outputted by the D/A control module (504) be V bias ；
(2) The DDS digital signal generator (501) drives the D/A control module (504) to sequentially output V bias ＝V bias1 -V step1 、V bias ＝V bias1 、V bias ＝V bias1 +V step1 And collect the digital photovoltage signal output (V) corresponding to the A/D control module (506) dc1 ) Let V be dc1 The values of (2) are respectively V ad1 、V ad2 、V ad3 Wherein V is step1 Coarse tuning the step length of the set bias voltage;
(3) If V ad1 >V ad2 >V ad3 Update V bias1 ＝V bias1 +V step1 Repeating step (2); if V ad1 <V ad2 <V ad3 Update V bias1 ＝V bias1 -V step1 Repeating step (2); if V ad1 ≥V ad2 ≤V ad3 Step (4) is entered; let the optimal operating point of the electro-optic modulator be V bias0 At this time bias voltage V bias1 Near the optimal operating point: v (V) bias0 -V step1 ≤V bias ≤V bias0 +V step1 ；
The stage of adjusting the transimpedance amplifying circuit is as follows:
(4) When V is ad2 ≤V L When the trans-impedance is smaller, the resistance value of the digital potentiometer (2) is adjusted upwards; when V is ad2 ≥V H When the trans-impedance is large, the resistance value of the digital potentiometer (2) is adjusted downwards until V L ≤V ad2 ≤V H ；
The stage of accurately positioning and stabilizing the optimal working point is as follows:
(5) Controlling the data strobe (502) to strobe the adder (503) output V add ＝V sin1 +V bias2 ,V add In the direct current component V bias2 On the basis of which a small sinusoidal disturbance signal V is superimposed sin1 ；
(6) The cross-correlation operation module calculates V ac And V sin1 Zero-point correlation peak of (2)
Figure GDA0004045703210000051
If Corv zero >delta, then shows that the optimal working point of the electro-optic modulator drifts to the negative direction, and V is updated bias ＝V bias -V step2 The method comprises the steps of carrying out a first treatment on the surface of the If Corv zero <Delta, it indicates that the optimal operating point of the electro-optic modulator is shifted in the forward direction, updating V bias ＝V bias +V step2 The method comprises the steps of carrying out a first treatment on the surface of the if|Corv zero Delta is smaller than or equal to the current bias voltage V bias The optical modulator is positioned at the optimal working point of the electro-optical modulator; wherein V is step2 To fine tune the step size of the bias voltage, |Corv when the electro-optic modulator is at the optimum operating point zero And delta is the optimal operating point decision threshold.
Preferably, the digital potentiometer (2) is used as a transimpedance in the transimpedance amplifying circuit (3), the master control FPGA (5) is connected with a control interface of the digital potentiometer (2), and the output value of the digital potentiometer (2) is set so as to adjust the amplification factor of the transimpedance amplifying circuit (3), so that the dynamic range of the transimpedance amplifying circuit (3) is improved.
Preferably, the D/A conversion module (6) adopts a D/A conversion chip DAC8560 with high precision (more than or equal to 16 bits) and low temperature drift (less than 10 ppm/DEG C), and can generate an electro-optic modulator bias voltage signal of 0V-2.5V.
Preferably, the bias voltage conditioning circuit (7) comprises an adder (701) and an in-phase proportional amplifying circuit (702); the adder (701) has two inputs, input 1 and input 2 respectively; and has an output; wherein the input end 1 of the adder (701) is connected to an external-1.25V direct current bias voltage, and the input end 2 of the adder (701) is connected to the output end of the D/A conversion module (6); the input end of the in-phase proportional amplifying circuit (702) is connected to the output end of the adder (701), the output end of the in-phase proportional amplifying circuit (702) is connected to the bias voltage pin of the external electro-optic modulator, the amplification factor A=10 of the bias voltage conditioning circuit (7), and the output bias voltage range of the bias voltage conditioning circuit (7) is-12.5V to +12.5V.
Preferably, the main control FPGA (5) controls the digital potentiometer (2) to set a trans-resistance value, controls the A/D acquisition module (4) to acquire a photovoltage signal and perform analog-digital conversion, performs feedback control, and drives the D/A conversion module (6) to generate bias voltage.
Compared with the prior art, the invention has the advantages that:
(1) The invention relates to an automatic bias voltage control device of an electro-optical modulator based on an FPGA, which takes the FPGA as a main control chip, realizes DDS digital signal generator, band-pass filter and cross correlation operation in the FPGA and performs feedback control, reduces the complexity of peripheral circuits, fully utilizes the advantage of parallel processing of the FPGA and improves the feedback control speed;
(2) Compared with the prior art, the digital potentiometer replaces a fixed resistor as a transimpedance, and the master control FPGA can adjust the amplification times of the transimpedance amplifying circuit by controlling the output of the digital potentiometer, so that the dynamic range of the circuit is improved, and the adaptability of the device to peripheral light path differences is enhanced;
(3) Compared with the prior art, the high-precision low-temperature drift analog converter DAC8560 is adopted for digital-to-analog conversion, the output voltage is pure, heat accumulation is not easy to generate in the running process of the device, the error introduced by temperature drift is controllable, and the running stability of the device is improved;
(4) Compared with the prior art, the feedback control flow comprises three stages: the method comprises the steps of preliminary positioning of an optimal working point stage, adjustment of a transimpedance amplifying circuit stage and accurate positioning and stabilization of the optimal working point stage, wherein in the whole process, firstly, rough adjustment and fine adjustment are carried out on bias voltage of an electro-optic modulator, and positioning accuracy and convergence speed of the optimal working point of the electro-optic modulator are considered;
(5) Compared with the prior art, the invention adopts a feedback control flow to automatically position the optimal working point in a recursion control mode, does not need to scan the whole voltage range, and has high convergence speed and high working efficiency;
(6) Compared with the prior art, the method has the advantages that the zero correlation peak of the sine disturbance signal and the alternating current component of the output signal of the electro-optical modulator is calculated in the FPGA, the drift direction of the optimal working point is judged through the positive and negative of the correlation peak, and the digital signal processing process is flexible and controllable.
Drawings
FIG. 1 is a schematic diagram of the system components of an automatic control device for the bias voltage of an electro-optic modulator based on an FPGA;
FIG. 2 is a schematic diagram of the internal digital signal processing flow of the master FPGA of the present invention;
FIG. 3 is a flow chart of the internal feedback control logic of the master FPGA of the present invention.
Detailed Description
The invention is described in further detail below with reference to the drawings and the specific embodiments.
The invention discloses an automatic bias voltage control device of an electro-optical modulator based on an FPGA, which relates to the field of optical control and comprises a photoelectric detector, a digital potentiometer, a transimpedance amplifying circuit, an A/D acquisition module, a master control FPGA, a D/A conversion module and a bias voltage conditioning circuit. The main control FPGA is used for controlling the direct-current bias voltage V of the electro-optic modulator through the internal DDS digital signal generator, the D/A conversion module and the bias voltage conditioning circuit bias On the basis of applying a small sinusoidal disturbance signal V sin The electro-optical modulator sends 5% of output optical power to the photoelectric detector through the light splitting sheet, and the output optical power is input to the FPGA after transimpedance amplification and A/D conversion, and is subjected to band-pass filtration and V in the FPGA sin And performing cross-correlation operation, judging the drift direction of the optimal working point of the electro-optical modulator through the positive and negative of the zero correlation peak, and performing feedback control. According to the invention, the FPGA is used as a main control chip, the DDS digital signal generator, the band-pass filter and the cross correlation operation are realized in the FPGA, and the feedback control is performed, so that the complexity of a peripheral circuit is reduced, and the feedback control speed is improved; meanwhile, the amplification factor of the transimpedance amplifying circuit can be changed through the digital potentiometer, the dynamic range of the circuit and the adaptability of the system are improved, and the method has good practical value for stabilizing the optimal working point of the electro-optic modulator.
The electro-optical modulator is used as a key element in the technical fields of optical fiber communication, distributed sensing and the like, and whether the electro-optical modulator stably works at an optimal working point directly influences the signal-to-noise ratio and the stability of a subsequent system. The invention discloses an electro-optical modulator bias voltage control device based on an FPGA, which adopts a parallel processing FPGA to replace a traditional micro controller MCU as a control core, realizes bias voltage feedback control logic in the FPGA, simplifies peripheral hardware circuits and improves feedback control speed; the device feedback control flow includes three phases: preliminary positioning of the optimal working point stage, adjustment of the transimpedance amplification circuit stage and accurate positioning and stabilization of the optimal working point stage. The feedback control flow can position the optimal working point without scanning the whole bias voltage working range, so that the power-on starting time of the device is shortened; the step of adjusting the transimpedance amplifying circuit adaptively adjusts the amplitude of the photo-voltage signal after the I-V conversion according to the magnitude of the input optical power of the device, thereby improving the dynamic range of the transimpedance amplifying circuit, enhancing the adaptability of the device to peripheral optical path difference and greatly improving the practical application value of the device.
As shown in fig. 1, the automatic bias voltage control device of the electro-optical modulator based on the FPGA consists of a photoelectric detector (1), a digital potentiometer (2), a transimpedance amplifying circuit (3), an A/D acquisition module (4), a master control FPGA (5), a D/A conversion module (6) and a bias voltage conditioning circuit (7).
The preferable scheme is as follows: the photoelectric modulator inputs 5% of output optical power into the photoelectric detector (1) through the light splitting sheet to perform photoelectric conversion, and a photoelectric current signal output by the photoelectric detector (1) is sent to the transimpedance amplifying circuit (3), wherein the transimpedance is the digital potentiometer (2). The main control FPGA (5) adjusts the amplification factor of the transimpedance amplifying circuit (3) according to the size of the input optical power by adjusting the output resistance of the digital potentiometer (2) so as to avoid saturation of an optical voltage signal or excessively low amplitude; the transimpedance amplifying circuit outputs a photovoltage signal to the A/D acquisition module (4), the main control FPGA (5) drives the ADC to acquire the photovoltage signal for digital signal processing, and the FPGA internal feedback control logic drives the D/A conversion module (6) to output an analog voltage signal according to a digital signal processing result and sends the analog voltage signal to the bias voltage conditioning circuit (7); the bias voltage conditioning circuit (7) carries out amplitude linear amplification on the analog voltage signal output by the D/A conversion module (6) to finally generate bias voltage required by the electro-optic modulator;
the preferable scheme is as follows: the photoelectric detector (1) adopts an InGaAs photoelectric detector, and the working mode is in a reverse bias mode, so that the photoelectric detection sensitivity is improved;
the preferable scheme is as follows: the digital potentiometer (2) is an AD5251 single-channel, 128-bit, I2C interface and nonvolatile digital potentiometer, the I2C control interface is connected to the main control FPGA, and a 100KΩ gear is selected;
the preferable scheme is as follows: the transimpedance amplifying circuit (3) is used for selecting the digital potentiometer (2) as a transimpedance, and the amplification factor of the transimpedance amplifying circuit (3) is adjusted by changing the output resistance of the digital potentiometer (2);
the preferable scheme is as follows: the A/D acquisition module (4) selects an AD7720 single-channel 16-bit, 12.5MSPS and SPI interface analog-to-digital converter, and the main control FPGA controls the A/D acquisition module to sample through the SPI interface;
the preferable scheme is as follows: the main control FPGA (5) is used for carrying out digital signal processing and feedback control by selecting EP3C40F144C8N, and has the advantages of low cost and small volume;
the preferable scheme is as follows: the D/A conversion module (6) adopts a TI analog-to-digital conversion chip DAC8560 to carry out analog-to-digital conversion of the bias voltage of the electro-optic modulator, and 2.5V reference voltage is integrated in the chip, so that the chip has a typical value of 2 ppm/DEG C temperature drift, 0.15nV-s burr pulse energy and power consumption as low as 1.377mW, the output voltage is pure, heat accumulation is not easy to generate in the operation process, errors caused by the temperature drift are controllable, and the chip has important significance for improving the operation stability of the device;
the preferable scheme is as follows: the offset voltage conditioning circuit (7) is composed of an adder (701) and a proportional amplifying circuit (702), wherein the input of the adder (701) is respectively the analog voltage output by the D/A conversion module (6) and the direct current offset voltage of-1.25V, the output of the adder (701) is sent to the proportional amplifying circuit (702), the output voltage range of the D/A conversion module (6) is 0-2.5V, and after the direct current offset of-1.25V is overlapped, the output voltage range of the adder is-1.25V; the amplification factor A=10 of the proportional amplification circuit, the output is connected to the electro-optic modulator, and the output voltage range is-12.5V;
the preferable scheme is as follows: the internal data signal processing flow of the master control FPGA (5) is shown in fig. 2, and the master control FPGA (5) comprises: the DDS digital signal generator (501), the data strobe (502), the digital adder (503), the D/A control module (504), the bias voltage feedback control module (505), the A/D control module (506), the band-pass filter (507) and the cross correlation operation module (508);
the DDS digital signal generator (501) generates a DC offset signal V bias And sinusoidal disturbance signal V sin ；
The data strobe (502) has an input 1, an input 2, a strobe, and an output;
the digital adder (503) has an input 1, an input 2, and an output;
the bias voltage feedback control module (505) is provided with an input end 1, an input end 2 and an output end;
the cross-correlation operation module (508) is provided with an input end 1, an input end 2 and an output end;
the generated DC bias signal V bias Is two paths which are identical and are respectively the signal V bias1 And V bias2 : signal V bias1 Connected to input 1 of the data strobe (502), signal V bias2 Is connected to the input 1 of the digital adder (503); generated sinusoidal disturbance signal V sin The two paths are identical, and are respectively: signal V sin1 And V sin2 The method comprises the steps of carrying out a first treatment on the surface of the Signal V sin1 Connected to input 2 of the digital adder (503), signal V sin2 The digital adder (503) is connected to the input end 1 of the cross correlation operation module (508), the output end of the digital adder (503) is connected to the input end 2 of the data strobe (502), the output end of the data strobe (502) is connected to the input of the D/A control module (504), and the strobe end of the data strobe (502) is connected to the output end of the bias voltage feedback control module (505);
under the drive of the A/D control module (506), the A/D acquisition module (4) acquires analog photovoltage signals to perform analog-to-digital conversion to form digital photovoltage signals, wherein the digital photovoltage signals are identical two paths of signals which are respectively signals Vdc1 and Vdc2 and serve as input of feedback control: the signal Vdc1 is connected to the input end 1 of the bias voltage feedback control module (505), the signal Vdc2 is connected to the input end of the band-pass filter (507), after the band-pass filter (507) carries out band-pass filtration, the output of the band-pass filter (507) is connected to the input end 2 of the cross-correlation operation module (508), and the output end of the cross-correlation operation module (508) is connected to the input end 2 of the bias voltage feedback control module (505);
the bias voltage feedback control module (505) adopts preset feedback control logic to control the data strobe (502) to output the bias voltage of the electro-optic modulator according to the signal Vdc1 output by the A/D control module (506) and the signal output by the cross correlation operation module (508).
The preferable scheme is as follows: the control flow preferably comprises three stages as shown in fig. 3, wherein the feedback control logic of the bias voltage in the main control FPGA is as follows:
the method comprises the steps of initially positioning an optimal working point stage, adjusting a transimpedance amplifying circuit stage and accurately positioning and stabilizing the optimal working point stage, and comprises the following steps:
preliminary positioning of the optimal working point stage, obtaining coordinates of three points on the transmission characteristic curve of the electro-optic modulator, obtaining the direction of bias voltage recursion by identifying the minimum point, and recursion step length V step1 ＝(1％～10％)V π ，V π The optimal scheme is as follows:
(1) The bias voltage automatic control device of the electro-optical modulator is electrified to control the gating signal V of the data gating device (502) bias1 Let the DC bias voltage outputted by the D/A control module (504) be V bias ；
(2) The DDS digital signal generator (501) drives the D/A control module (504) to sequentially output V bias ＝V bias1 -V step1 、V bias ＝V bias1 、V bias ＝V bias1 +V step1 And collect the digital photovoltage signal output (V) corresponding to the A/D control module (506) dc1 ) Let V be dc1 The values of (2) are respectively V ad1 、V ad2 、V ad3 Wherein V is step1 Coarse tuning the step length of the set bias voltage;
(3) If V ad1 >V ad2 >V ad3 Update V bias1 ＝V bias1 +V step1 Repeating step (2); if V ad1 <V ad2 <V ad3 Update V bias1 ＝V bias1 -V step1 Repeating step (2); if V ad1 ≥V ad2 ≤V ad3 Step (4) is entered; let the optimal operating point of the electro-optic modulator be V bias0 At this time bias voltage V bias1 Near the optimal operating point: v (V) bias0 -V step1 ≤V bias ≤V bias0 +V step1 ；
The stage of adjusting the transimpedance amplifying circuit is as follows:
(4) When V is ad2 ≤V L When it is indicated thatThe trans-resistance is small, and the resistance value of the digital potentiometer (2) is adjusted upwards; when V is ad2 ≥V H When the trans-impedance is large, the resistance value of the digital potentiometer (2) is adjusted downwards until V L ≤V ad2 ≤V H ；
The stage of accurately positioning and stabilizing the optimal working point is as follows:
(5) Controlling the data strobe (502) to strobe the adder (503) output V add ＝V sin1 +V bias2 ,V add In the direct current component V bias2 On the basis of which a small sinusoidal disturbance signal V is superimposed sin1 =asin (ωt), wherein a is the sinusoidal disturbance signal V sin1 The amplitude A is less than or equal to 1% π The method comprises the steps of carrying out a first treatment on the surface of the Omega is the angular frequency of a disturbance signal, and omega is less than or equal to 10kHz/2 pi;
V add ＝V bias2 +Asin(ωt)； (1)
(6) The electro-optic modulator transmission characteristic can be expressed as:
Figure GDA0004045703210000111
wherein alpha is a coefficient, P in The optical power is input to the electro-optical modulator, V is the modulation voltage, V π Is a half-wave voltage, and is a voltage of a half-wave,
Figure GDA0004045703210000112
is an intrinsic phase whose value is susceptible to changes by environmental influences such as temperature, mechanical stress and aging, P 0 The electro-optical modulator leaks optical power, and the electro-optical modulator outputs a modulation signal by bringing the formula (1) into the formula (2):
Figure GDA0004045703210000113
filtering the direct current component by a band-pass filter (507):
Figure GDA0004045703210000114
calculate V ac And disturbance signal V sin1 Zero-point correlation peak Conv zero ：
Figure GDA0004045703210000121
Wherein ρ is a constant, conv is known from equation (5) zero With the transmission characteristic curve of the electro-optic modulator at a DC bias voltage V bias2 The slope is in a linear relation, and Conv is when the optimal working point is in negative drift zero > 0; conv when the optimum operating point drifts forward zero < 0; when V is bias Conv when located at the optimal operating point zero ＝0；
The preferable scheme is as follows: cross-correlation operation module (508) for calculating V ac And V sin1 Zero-point correlation peak of (2)
Figure GDA0004045703210000122
Wherein N is the sequence V ac And V sin1 Correlation calculation point, n=10f adc /(2πω)，f adc The sampling frequency is in Hz for the A/D control module (506). If Conv zero The delta is more than delta, which shows that the optimal working point of the electro-optic modulator drifts to the negative direction, and V is updated bias2 ＝V bias2 -V step2 The method comprises the steps of carrying out a first treatment on the surface of the If Conv zero The < -delta indicates that the optimal working point of the electro-optic modulator drifts to the positive direction and updates V bias2 ＝V bias2 +V step2 The method comprises the steps of carrying out a first treatment on the surface of the If |Conv zero |<delta, then indicates the current bias voltage V bias2 At the optimum operating point of the electro-optic modulator. |Conv when the electro-optic modulator is at the optimum operating point zero The delta is the optimal working point judgment threshold value, delta=255 is taken, and the adjustment can be carried out according to the actual condition of hardware, wherein V step2 Step for trimming step size for bias voltage 2 ＝2A。
The invention relates to an automatic bias voltage control device of an electro-optical modulator based on an FPGA, which takes the FPGA as a main control chip, realizes DDS digital signal generator, band-pass filter and cross correlation operation in the FPGA and performs feedback control, reduces the complexity of peripheral circuits, fully utilizes the advantage of parallel processing of the FPGA and improves the feedback control speed; compared with the prior art, the digital potentiometer replaces a fixed resistor as a transimpedance, and the master control FPGA can adjust the amplification times of the transimpedance amplifying circuit by controlling the output of the digital potentiometer, so that the dynamic range of the circuit is improved, and the adaptability of the device to peripheral light path differences is enhanced;
the high-precision low-temperature drift analog-to-digital converter DAC8560 is adopted for digital-to-analog conversion, the output voltage is pure, heat accumulation is not easy to generate in the running process of the device, the error introduced by temperature drift is controllable, and the running stability of the device is improved; and the feedback control flow of the invention comprises three stages: the method comprises the steps of preliminary positioning of an optimal working point stage, adjustment of a transimpedance amplifying circuit stage and accurate positioning and stabilization of the optimal working point stage, wherein in the whole process, firstly, rough adjustment and fine adjustment are carried out on bias voltage of an electro-optic modulator, and positioning accuracy and convergence speed of the optimal working point of the electro-optic modulator are considered;
compared with the prior art, the invention adopts a feedback control flow to automatically position the optimal working point in a recursion control mode, does not need to scan the whole voltage range, and has high convergence speed and high working efficiency; compared with the prior art, the method has the advantages that the zero correlation peak of the sine disturbance signal and the alternating current component of the output signal of the electro-optical modulator is calculated in the FPGA, the drift direction of the optimal working point is judged through the positive and negative of the correlation peak, and the digital signal processing process is flexible and controllable.

Claims (5)

1. An automatic control device for bias voltage of an electro-optical modulator based on an FPGA is characterized by comprising: the device comprises a photoelectric detector (1), a transimpedance amplifying circuit (3), an A/D acquisition module (4), a master control FPGA (5), a D/A conversion module (6) and a bias voltage conditioning circuit (7);
the transimpedance amplifying circuit (3) is provided with a digital potentiometer (2); the digital potentiometer (2) is used as a transimpedance in the transimpedance amplifying circuit (3);
the external electro-optical modulator outputs optical signals, a light splitting sheet inputs 5% of the power of the output optical signals to the photoelectric detector (1), the photoelectric detector (1) performs photoelectric conversion to generate photocurrent signals, and the photocurrent signals are sent to the transimpedance amplifying circuit (3);
the main control FPGA (5) sends a control instruction to the digital potentiometer (2), and the digital potentiometer (2) adjusts a transimpedance value in the transimpedance amplifying circuit (3) according to the control instruction of the main control FPGA (5);
the transimpedance amplifying circuit (3) converts the photocurrent signal into an analog photovoltage signal according to the adjusted transimpedance value, and sends the analog photovoltage signal to the A/D acquisition module (4), and the main control FPGA (5) controls the A/D acquisition module (4) to acquire the analog photovoltage signal for analog-to-digital conversion to form a digital photovoltage signal which is used as input of feedback control; the main control FPGA (5) drives the D/A conversion module (6) to generate a bias voltage signal through feedback control, and the bias voltage signal is amplified linearly in amplitude by the bias voltage conditioning circuit (7) and then output to the electro-optical modulator, so that the electro-optical modulator works at an optimal working point;
the master control FPGA (5) comprises: the DDS digital signal generator (501), the data strobe (502), the digital adder (503), the D/A control module (504), the bias voltage feedback control module (505), the A/D control module (506), the band-pass filter (507) and the cross correlation operation module (508);
the DDS digital signal generator (501) generates a DC offset signal V bias And sinusoidal disturbance signal V sin ；
The data strobe (502) has an input 1, an input 2, a strobe, and an output;
the digital adder (503) has an input 1, an input 2, and an output;
the bias voltage feedback control module (505) is provided with an input end 1, an input end 2 and an output end;
the cross-correlation operation module (508) is provided with an input end 1, an input end 2 and an output end;
the generated DC bias signal V bias Is two paths which are identical and are respectively the signal V bias1 And V bias2 : signal V bias1 Connected to input 1 of the data strobe (502), signal V bias2 Is connected to the input 1 of the digital adder (503); generated sinusoidal disturbance signal V sin The two paths are identical, and are respectively: signal V sin1 And V sin2 The method comprises the steps of carrying out a first treatment on the surface of the Signal V sin1 Connected to input 2 of the digital adder (503), signal V sin2 The digital adder (503) is connected to the input end 1 of the cross correlation operation module (508), the output end of the digital adder (503) is connected to the input end 2 of the data strobe (502), the output end of the data strobe (502) is connected to the input end of the D/A control module (504), and the strobe end of the data strobe (502) is connected to the output end of the bias voltage feedback control module (505);
the A/D acquisition module (4) acquires the analog photovoltage signal to perform analog-to-digital conversion under the drive of the A/D control module (506) to form a digital photovoltage signal, and the digital photovoltage signal is taken as the input of feedback control and is two paths of identical signals which are respectively the signal V dc1 And V dc2 : signal V dc1 Input terminal 1 connected to bias voltage feedback control module (505), signal V dc2 The output end of the band-pass filter (507) is connected to the input end 2 of the cross-correlation operation module (508), and the output end of the cross-correlation operation module (508) is connected to the input end 2 of the bias voltage feedback control module (505) after the band-pass filter (507) carries out band-pass filtration;
the bias voltage feedback control module (505) outputs a signal V according to the A/D control module (506) dc1 And the signal outputted by the cross correlation operation module (508) adopts preset feedback control logic to control the data strobe (502) to output the bias voltage of the electro-optic modulator;
the preset feedback control logic adopted by the feedback control module (505) of the internal bias voltage of the main control FPGA (5) comprises three stages: the method comprises the steps of initially positioning an optimal working point stage, adjusting a transimpedance amplifying circuit stage and accurately positioning and stabilizing the optimal working point stage, and comprises the following steps:
the stage of initially positioning the optimal working point is as follows:
(1) The bias voltage automatic control device of the electro-optical modulator is electrified to control the gating signal V of the data gating device (502) bias1 Let the DC bias voltage outputted by the D/A control module (504) be V bias ；
(2) The DDS digital signal generator (501) drives the D/A control module (504) to sequentially output V bias ＝V bias1 -V step1 、V bias ＝V bias1 、V bias ＝V bias1 +V step1 And collect the digital photovoltage signal output V corresponding to the A/D control module (506) dc1 Let V be dc1 The values of (2) are respectively V ad1 、V ad2 、V ad3 Wherein V is step1 Coarse tuning the step length of the set bias voltage;
(3) If V ad1 >V ad2 >V ad3 Update V bias1 ＝V bias1 +V step1 Repeating step (2); if V ad1 <V ad2 <V ad3 Update V bias1 ＝V bias1 -V step1 Repeating step (2); if V ad1 ≥V ad2 ≤V ad3 Step (4) is entered; let the optimal operating point of the electro-optic modulator be V bias0 At this time bias voltage V bias1 Near the optimal operating point: v (V) bias0 -V step1 ≤V bias ≤V bias0 +V step1 ；
The stage of adjusting the transimpedance amplifying circuit is as follows:
(4) When V is ad2 ≤V L When the trans-impedance is smaller, the resistance value of the digital potentiometer (2) is adjusted upwards; when V is ad2 ≥V H When the trans-impedance is large, the resistance value of the digital potentiometer (2) is adjusted downwards until V L ≤V ad2 ≤V H ；
The stage of accurately positioning and stabilizing the optimal working point is as follows:
(5) Controlling the data strobe (502) to strobe the digital adder (503) output V add ＝V sin1 +V bias2 ,V add In the direct current component V bias2 On the basis of which a small sinusoidal disturbance signal V is superimposed sin1 ；
(6) The cross-correlation operation module calculates a sequence V ac And V sin1 Zero-point correlation peak of (2)
Figure FDA0004196587370000031
If Conv zero >delta, it shows that the optimal working point of the electro-optic modulator drifts to the negative direction, and moreNew V bias ＝V bias -V step2 The method comprises the steps of carrying out a first treatment on the surface of the If Conv zero <Delta, it indicates that the optimal operating point of the electro-optic modulator is shifted in the forward direction, updating V bias ＝V bias +V step2 The method comprises the steps of carrying out a first treatment on the surface of the If |Conv zero Delta is smaller than or equal to the current bias voltage V bias The optical modulator is positioned at the optimal working point of the electro-optical modulator; wherein V is step2 For fine-tuning the step size of the bias voltage, |Conv when the electro-optic modulator is at the optimum operating point zero I is approximately equal to 0, delta is the optimal working point judgment threshold value, V step1 Is a recursive step length; v (V) ac For the sequence of photoelectric signals output by the band-pass filter (507), V L Output voltage amplitude V for A/D acquisition module (4) ad2 Lower limit value of V H Output voltage amplitude V for A/D acquisition module (4) ad2 Upper limit value of (2); n is the sequence V ac And V sin1 The number of relevant operation points, i is the serial number.
2. The automatic control device for bias voltage of electro-optical modulator based on FPGA of claim 1, wherein: the digital potentiometer (2) is used as a transimpedance in the transimpedance amplifying circuit (3), the master control FPGA (5) is connected with a control interface of the digital potentiometer (2), and the output value of the digital potentiometer (2) is set so as to adjust the amplification factor of the transimpedance amplifying circuit (3), so that the dynamic range of the transimpedance amplifying circuit (3) is improved.
3. The automatic control device for bias voltage of electro-optical modulator based on FPGA of claim 1, wherein: the D/A conversion module (6) adopts a D/A conversion chip DAC8560 with high precision and low temperature drift, and can generate an electro-optic modulator bias voltage signal with 0V-2.5V.
4. The automatic control device for bias voltage of electro-optical modulator based on FPGA of claim 1, wherein: the bias voltage conditioning circuit (7) comprises an adder (701) and an in-phase proportional amplifying circuit (702); the adder (701) has two inputs, input 1 and input 2 respectively; and has an output; wherein the input end 1 of the adder (701) is connected to an external-1.25V direct current bias voltage, and the input end 2 of the adder (701) is connected to the output end of the D/A conversion module (6); the input end of the in-phase proportional amplifying circuit (702) is connected to the output end of the adder (701), the output end of the in-phase proportional amplifying circuit (702) is connected to the bias voltage pin of the external electro-optic modulator, the amplification factor A=10 of the bias voltage conditioning circuit (7), and the output bias voltage range of the bias voltage conditioning circuit (7) is-12.5V to +12.5V.
5. The automatic control device for bias voltage of electro-optical modulator based on FPGA of claim 1, wherein: the main control FPGA (5) controls the digital potentiometer (2) to set a trans-resistance value, controls the A/D acquisition module (4) to acquire a photovoltage signal and perform analog-to-digital conversion, performs feedback control, and drives the D/A conversion module (6) to generate bias voltage.
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