CN113204133A - Electro-optical modulator bias voltage automatic control device based on FPGA - Google Patents

Electro-optical modulator bias voltage automatic control device based on FPGA Download PDF

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CN113204133A
CN113204133A CN202110432107.3A CN202110432107A CN113204133A CN 113204133 A CN113204133 A CN 113204133A CN 202110432107 A CN202110432107 A CN 202110432107A CN 113204133 A CN113204133 A CN 113204133A
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bias voltage
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electro
fpga
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CN113204133B (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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Beijing Aerospace Control Instrument Institute
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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
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Abstract

The invention discloses an automatic control device for bias voltage of an electro-optical modulator based on an FPGA (field programmable gate array), which relates to the field of optical control and comprises a photoelectric detector, a digital potentiometer, a trans-impedance amplification circuit, an A/D (analog/digital) acquisition module, a main control FPGA, a D/A (digital/analog) conversion module and a bias voltage conditioning circuit. The main control FPGA generates DC bias voltage V in the electro-optical modulator through an internal DDS digital signal generator, a D/A conversion module and a bias voltage conditioning circuitbiasOn the basis of applying a small sine disturbance signal VsinThe signal is input to the FPGA through the electro-optical modulator, and subjected to band-pass filtering in the FPGA to be subjected to V-sumsinPerforming cross-correlation operation and feedback control, thereby reducing the complexity of peripheral circuits and improving the feedback control speed; meanwhile, the amplification factor of the transimpedance amplification circuit can be changed through the digital potentiometer, the dynamic range of the circuit and the adaptability of a system are improved, and the digital potentiometer has good practical value for stabilizing the optimal working point of the electro-optic modulator.

Description

Electro-optical modulator bias voltage automatic control device based on FPGA
Technical Field
The invention relates to an automatic control device for bias voltage of an electro-optical modulator based on an FPGA (field programmable gate array), belonging 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 the information of the physical quantity to be measured is obtained by analyzing the interference or scattering condition of a light path, wherein the extinction ratio of the pulse light is one of the most important parameters of the system, and the signal-to-noise ratio and the stability of a subsequent system are directly influenced. Niobium lithiate LiNbO3The electro-optical modulator has the advantages of low loss, high responsivity, small frequency modulation chirp, small wavelength dependence and the like, and is widely applied to a pulse light modulation system. However, due to the waveguide material and the structure thereof, the waveguide is susceptible to the influence of external environments such as temperature and vibration, and under the long-time working condition, the accumulation of the heat thereof also easily causes the drift or the jitter of the optimal working point thereof, which causes the extinction ratio of the output pulse light to be reduced, and affects the system performance or even causes the system to be incapable of working normally. Therefore, the bias voltage of the electro-optical modulator is automatically controlled to be stabilized at the optimal working point, and the method has important significance for long-time stable operation of a system.
At present, most of bias voltage feedback control devices of electro-optical modulators use a single chip as a main control chip, and an internal A/D, D/A module is used for completing collection of photovoltage and output of bias voltage. Because the working main frequency of the single chip microcomputer is generally not more than 20MHz, and 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 above hardware schemes are mainly classified into three categories:
(1) direct optical power measurement: the output light 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 of the electro-optic modulator corresponding to different frequency points fluctuates in the frequency sweeping process, the feedback control process is diverged due to the fact that the mode of directly measuring the optical power is easily influenced by the frequency sweeping;
(2) phase-locked mixing integration method: and (3) superposing a sine disturbance signal on the basis of the bias voltage for modulation, integrating the output signal of the electro-optic modulator and the sine disturbance signal after mixing by a hardware circuit, and positioning the optimal working point by detecting an integral zero point. Because the scheme can not distinguish the phase phi of the optimal working point of the output characteristic curve of the electro-optical modulator0And (phi)0+ pi) so that the optimum operating point scan needs to be performed first 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, special algorithm hardware modules such as a sine signal generator, an adder, a mixer and an integrating circuit are needed, and the scanning process takes longer time.
(3) And (3) second harmonic detection: a sine disturbance signal is superposed on the basis of bias voltage for modulation, and the optimal working point is positioned by detecting the ratio of the intensity of the second harmonic component and the first harmonic component of the output signal of the electro-optical modulator. Because the slope k of the transmission characteristic curve of the electro-optical modulator is approximately equal to 0 near the optimal working point, the ratio of the intensity of the corresponding second harmonic component to the intensity of the corresponding first harmonic component cannot form an obvious peak value, the practical application is difficult, a frequency multiplier is required to generate a double-frequency signal, and the hardware cost and the complexity are further improved compared with the scheme (2).
The three schemes can not adjust the amplification factor of the photoelectric amplification circuit through software, the photoelectric voltage can be saturated when the output optical power of the electro-optical modulator is larger, and the signal-to-noise ratio of the photoelectric voltage signal can be reduced when the optical power is smaller. The system may not work properly due to the larger or smaller optical power.
Disclosure of Invention
The technical problem solved by the invention is as follows: the device overcomes the defects of the prior art, provides an automatic control device for the bias voltage of the electro-optical modulator based on the FPGA, aims at the defects of complex hardware circuit, low response speed, insufficient control precision, poor system adaptability and the like of the traditional automatic control device for the bias voltage of the electro-optical modulator based on a single chip microcomputer, realizes a digital signal processing flow and feedback control logic inside a main control FPGA, simplifies peripheral circuits, fully utilizes the advantages of FPGA parallel processing, improves the feedback control speed, improves the dynamic range of a trans-resistance amplifying circuit by flexibly configuring the output of a digital potentiometer, and enhances the adaptability of the device to the difference of peripheral light paths.
The technical scheme of the invention is as follows: an electro-optical modulator bias voltage automatic control device based on FPGA comprises: the device comprises a photoelectric detector (1), a transimpedance amplification 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 amplification circuit (3) is provided with a digital potentiometer (2); the digital potentiometer (2) is used as a transimpedance in the transimpedance amplifying circuit (3);
an external electro-optical modulator outputs an optical signal, 5% of the power of the output optical signal is input to a photoelectric detector (1) through a light splitting sheet, photoelectric conversion is carried out through the photoelectric detector (1), a photocurrent signal is generated and sent to a trans-resistance 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 amplification circuit (3) according to the control instruction of the FPGA (5);
the transimpedance amplification circuit (3) converts the photocurrent signal into an analog photovoltage signal according to the adjusted transimpedance value, and transmits 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 to perform analog-to-digital conversion to form a digital photovoltage signal which is used as the 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 then the bias voltage signal is subjected to amplitude linear amplification by the bias voltage conditioning circuit (6) and is output to the electro-optical modulator, so that the electro-optical modulator works at the optimal working point.
Preferably, the digital potentiometer (2) is used as a transimpedance in the transimpedance amplification circuit (3), the main control FPGA (5) is connected with a control interface of the digital potentiometer (2), and an output value of the digital potentiometer (2) is set so as to adjust the amplification factor of the transimpedance amplification circuit (3), so that the dynamic range of the transimpedance amplification 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 16bits) 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; and having an output; wherein, the input end 1 of the adder (701) is connected to the external-1.25V DC bias voltage, and the input end 2 of the adder (701) is connected to the output 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 an external electro-optical modulator bias voltage pin, the amplification factor A of the bias voltage conditioning circuit (7) is 10, and the output bias voltage range of the bias voltage conditioning circuit (7) is-12.5V- + 12.5V.
Preferably, the main control FPGA (5) controls the digital potentiometer (2) to set a 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.
Preferably, the master FPGA (5) comprises: the DDS digital signal generating circuit comprises a DDS digital signal generator (501), a data gating device (502), a digital adder (503), a D/A control module (504), a bias voltage feedback control module (505), an A/D control module (506), a band-pass filter (507) and a cross-correlation operation module (508).
Preferably, the master FPGA (5) comprises: the DDS digital signal generating circuit comprises a DDS digital signal generator (501), a data gating device (502), a digital adder (503), a D/A control module (504), a bias voltage feedback control module (505), an A/D control module (506), a band-pass filter (507) and a cross-correlation operation module (508);
the DDS digital signal generator (501) generates a DC bias signal VbiasAnd a sinusoidal perturbation signal Vsin；
The data strobe (502) is provided with an input end 1, an input end 2, a strobe end and an output end;
the digital adder (503) has an input terminal 1, an input terminal 2, and an output terminal;
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;
generated DC bias signal VbiasAre two paths which are completely the same, and are respectively a signal Vbias1And Vbias2: signal Vbias1Connected to input 1 of data strobe (502), signal Vbias2To input 1 of a digital adder (503); generating a sinusoidal disturbance signal VsinTwo identical paths are respectively as follows: signal Vsin1And Vsin2(ii) a Signal Vsin1Connected to input 2 of digital adder (503), signal Vsin2The output end of the digital adder (503) is connected to the input end 1 of the cross-correlation operation module (508), the output end of the data gate (502) is connected to the input end 2 of the data gate (502), the output end of the data gate (502) is connected to the input end of the D/A control module (504), and the gate end of the data gate (502) is connected to the output end of the bias voltage feedback control module (505);
the A/D acquisition module (4) acquires analog photovoltaic signals under the drive of the A/D control module (506) to perform analog-to-digital conversion to form digital photovoltaic signals which are used as input of feedback control, the digital photovoltaic signals are two paths of completely identical signals which are respectively Vdc1 and Vdc 2: 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) is subjected to band-pass filtering, 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);
(4) the bias voltage feedback control module (505) controls the data gate (502) to output the bias voltage of the electro-optical modulator by adopting preset feedback control logic 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 bias voltage feedback control module (505) in the main control FPGA (5) comprises three stages: the method comprises a primary optimal working point positioning stage, a transimpedance amplifier circuit adjusting stage and an optimal working point accurately positioning and stabilizing stage, and comprises the following steps:
the initial stage of 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 (508)bias1Setting the DC bias voltage output by the D/A control module (504) as Vbias；
(2) The DDS digital signal generator (501) drives the D/A control module (504) to sequentially output Vbias＝Vbias1-Vstep1、Vbias＝Vbias1、Vbias＝Vbias1+Vstep1And collects the digital light voltage signal output (V) corresponding to the A/D control module (506)dc1) Is provided with Vdc1Respectively is Vad1、Vad2、Vad3In which V isstep1Coarse adjustment step size for the set bias voltage;
(3) if Vad1>Vad2>Vad3Update Vbias1＝Vbias1+Vstep1Repeating the step (2); if Vad1<Vad2<Vad3Update Vbias1＝Vbias1-Vstep1Repeating the step (2); if Vad1≥Vad2≤Vad3Entering the step (4); suppose the optimum operating point of the electro-optic modulator is Vbias0At this time, the bias voltage Vbias1Close to the optimal operating point: vbias0-Vstep1≤Vbias≤Vbias0+Vstep1；
The stage of adjusting the transimpedance amplification circuit is as follows:
(4) when V isad2≤VLWhen the voltage is over, the resistance value of the digital potentiometer (2) is adjusted upwards; when V isad2≥VHWhen the voltage is over, the resistance value of the digital potentiometer (2) is adjusted downwards until V is reachedL≤Vad2≤VH；
The stage of accurately positioning and stabilizing the optimal working point comprises the following steps:
(5) controlling the data Strobe (508) to strobe the adder (503) output Vadd＝Vsin1+Vbias2,VaddAt a DC component Vbias2On the basis of the signal, a tiny sine disturbance signal V is superposedsin1；
(6) Cross correlation operation module calculating VacAnd Vsin1Zero correlation peak of
Figure BDA0003031778480000061
If Corvzero>delta, indicating that the optimum working point of the electro-optical modulator shifts towards the negative direction, and updating Vbias＝Vbias-Vstep2(ii) a If Corvzero<Delta, indicating that the optimum working point of the electro-optical modulator shifts to the positive direction, and updating Vbias＝Vbias+Vstep2(ii) a If CorvzeroDelta is less than or equal to | indicates the current bias voltage VbiasLocating the optimal working point of the electro-optical modulator; wherein Vstep2For bias voltage trimming step size, | Corv when the electro-optic modulator is at the optimum operating pointzeroAnd | is approximately equal to 0, and delta is the optimal working point judgment threshold.
Compared with the prior art, the invention has the advantages that:
(1) the invention relates to an electro-optical modulator bias voltage automatic control device based on FPGA, which takes the FPGA as a main control chip, realizes DDS digital signal generator, band-pass filter and cross-correlation operation and feedback control in the FPGA, 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 is used for replacing a fixed resistor as the transimpedance, the main control FPGA can adjust the amplification factor of the transimpedance amplification circuit by controlling the output of the digital potentiometer, the dynamic range of the circuit is improved, and the adaptability of the device to the difference of peripheral light paths is enhanced;
(3) compared with the prior art, the high-precision low-temperature drift analog converter DAC8560 is adopted for digital-analog conversion, the output voltage is pure, heat accumulation is not easy to generate in the operation process of the device, the error caused by temperature drift is controllable, and the operation stability of the device is improved;
(4) compared with the prior art, the feedback control process comprises three stages: the method comprises the steps of initially positioning an optimal working point, adjusting a transimpedance amplification circuit and accurately positioning and stabilizing the optimal working point, wherein in the whole process, the bias voltage of the electro-optical modulator is coarsely adjusted and then finely adjusted, and the positioning accuracy and the convergence speed of the optimal working point of the electro-optical modulator are considered;
(5) compared with the prior art, the method has the advantages that the optimal working point is automatically positioned by adopting a feedback control process and a recursive control mode, the whole voltage range does not need to be scanned, the convergence speed is high, and the working efficiency is high;
(6) compared with the prior art, the method has the advantages that the zero-point correlation peak of alternating current components of the sine disturbance signal and the output signal of the electro-optical modulator is calculated in the FPGA, the optimal working point drift direction 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 composition of an automatic control device for bias voltage of an electro-optical modulator based on FPGA according to the present invention;
FIG. 2 is a schematic diagram of the internal digital signal processing flow of the main control 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 figures and specific embodiments.
The invention discloses an automatic control device for bias voltage of an electro-optical modulator based on an FPGA (field programmable gate array), which relates to the field of optical control and comprises a photoelectric detector, a digital potentiometer, a trans-impedance amplification circuit, an A/D (analog/digital) acquisition module, a main control FPGA, a D/A (digital/analog) conversion module and a bias voltage conditioning circuit. The main control FPGA generates DC bias voltage V in the electro-optical modulator through an internal DDS digital signal generator, a D/A conversion module and a bias voltage conditioning circuitbiasOn the basis of applying a small sine disturbance signal VsinThe electro-optical modulator is connected with the light splitter5% of output optical power is sent to a photoelectric detector, is input to the FPGA after trans-impedance amplification and A/D conversion, and is subjected to band-pass filtering in the FPGA and then is subjected to VsinAnd performing cross-correlation operation, and judging the drift direction of the optimal working point of the electro-optical modulator through the positive and negative of the zero-point correlation peak so as to perform feedback control. The invention takes the FPGA as a main control chip, realizes the DDS digital signal generator, the band-pass filter and the cross-correlation operation in the FPGA and carries out feedback control, reduces the complexity of a peripheral circuit and improves the feedback control speed; meanwhile, the amplification factor of the transimpedance amplification circuit can be changed through the digital potentiometer, the dynamic range of the circuit and the adaptability of a system are improved, and the digital potentiometer 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 FPGA, which adopts parallel processing FPGA to replace the traditional microcontroller MCU as a control core, realizes bias voltage feedback control logic in the FPGA, simplifies peripheral hardware circuit and improves feedback control speed; the device feedback control process comprises three stages: the method comprises a stage of initially positioning an optimal working point, a stage of adjusting a transimpedance amplification circuit and a stage of accurately positioning and stabilizing the optimal working point. The feedback control process can position the optimal working point without scanning the whole bias voltage working range, thereby shortening the power-on starting time of the device; the amplitude of the photovoltage signal after I-V conversion is adaptively adjusted according to the input optical power of the device in the stage of adjusting the transimpedance amplifier circuit, so that the dynamic range of the transimpedance amplifier circuit is improved, the adaptability of the device to peripheral optical path difference is enhanced, and the practical application value of the device is greatly improved.
As shown in fig. 1, an automatic control device for bias voltage of an electro-optical modulator based on an FPGA is composed of a photoelectric detector (1), a digital potentiometer (2), a transimpedance amplification circuit (3), an a/D acquisition module (4), a main control FPGA (5), a D/a conversion module (6) and a bias voltage conditioning circuit (7).
The preferred scheme is as follows: the electro-optical modulator inputs 5% of output optical power to the photoelectric detector (1) through the light splitting sheet for photoelectric conversion, and a photocurrent signal output by the photoelectric detector (1) is sent to the transimpedance amplifier circuit (3), wherein the transimpedance is the digital potentiometer (2). The main control FPGA (5) adjusts the amplification factor of the transimpedance amplification circuit (3) by adjusting the output resistance of the digital potentiometer (2) according to the input optical power, so that the saturation or the too low amplitude of a photovoltage signal is avoided; the transimpedance amplification circuit outputs an optical voltage signal to the A/D acquisition module (4), the main control FPGA (5) drives the ADC to acquire the optical voltage signal to perform 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) linearly amplifies the amplitude of the analog voltage signal output by the D/A conversion module (6) to finally generate the bias voltage required by the electro-optical modulator;
the preferred 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 preferred scheme is as follows: the digital potentiometer (2) is an AD5251 single channel, 128 bits, an I2C interface and a nonvolatile digital potentiometer, an I2C control interface is connected to a main control FPGA, and a 100K omega gear is selected;
the preferred scheme is as follows: the transimpedance amplification circuit (3) selects the digital potentiometer (2) as a transimpedance, and the amplification factor of the transimpedance amplification circuit (3) is adjusted by changing the output resistance of the digital potentiometer (2);
the preferred 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 master control FPGA controls the A/D acquisition module to sample through the SPI interface;
the preferred scheme is as follows: the main control FPGA (5) selects EP3C40F144C8N to perform digital signal processing and feedback control, and has the advantages of low cost and small volume;
the preferred scheme is as follows: the D/A conversion module (6) adopts a TI analog-to-digital conversion chip DAC8560 to perform analog-to-digital conversion on the bias voltage of the electro-optic modulator, 2.5V reference voltage is integrated in the chip, 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 D/A conversion module has important significance for improving the operation stability of the device;
the preferred scheme is as follows: the offset voltage conditioning circuit (7) is composed of an adder (701) and a proportional amplification circuit (702), the input of the adder (701) is respectively a D/A conversion module (6) to output an analog voltage and a-1.25V direct current offset voltage, the output of the adder (701) is sent to the proportional amplification circuit (702), wherein the output voltage range of the D/A conversion module (6) is 0-2.5V, and after the-1.25V direct current offset is superposed, the output voltage range of the adder is-1.25V; the amplification factor A of the proportional amplification circuit is 10, the output of the proportional amplification circuit is connected to the electro-optic modulator, and the output voltage range is-12.5V;
the preferred scheme is as follows: the main control FPGA (5) and the internal data signal processing flow are shown in fig. 2, and the main control FPGA (5) comprises: the DDS digital signal generating circuit comprises a DDS digital signal generator (501), a data gating device (502), a digital adder (503), a D/A control module (504), a bias voltage feedback control module (505), an A/D control module (506), a band-pass filter (507) and a cross-correlation operation module (508);
the DDS digital signal generator (501) generates a DC bias signal VbiasAnd a sinusoidal perturbation signal Vsin；
The data strobe (502) is provided with an input end 1, an input end 2, a strobe end and an output end;
the digital adder (503) has an input terminal 1, an input terminal 2, and an output terminal;
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;
generated DC bias signal VbiasAre two paths which are completely the same, and are respectively a signal Vbias1And Vbias2: signal Vbias1Connected to input 1 of data strobe (502), signal Vbias2To input 1 of a digital adder (503); generating a sinusoidal disturbance signal VsinTwo identical paths are respectively as follows: signal Vsin1And Vsin2(ii) a LetterNumber Vsin1Connected to input 2 of digital adder (503), signal Vsin2The output end of the digital adder (503) is connected to the input end 1 of the cross-correlation operation module (508), the output end of the data gate (502) is connected to the input end 2 of the data gate (502), the output end of the data gate (502) is connected to the input end of the D/A control module (504), and the gate end of the data gate (502) is connected to the output end of the bias voltage feedback control module (505);
the A/D acquisition module (4) acquires analog photovoltaic signals under the drive of the A/D control module (506) to perform analog-to-digital conversion to form digital photovoltaic signals which are used as input of feedback control, the digital photovoltaic signals are two paths of completely identical signals which are respectively Vdc1 and Vdc 2: 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) is subjected to band-pass filtering, 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);
(4) the bias voltage feedback control module (505) controls the data gate (502) to output the bias voltage of the electro-optical modulator by adopting preset feedback control logic 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 preferred scheme is as follows: as shown in fig. 3, the internal bias voltage feedback control logic of the main control FPGA preferably includes three stages:
the method comprises a primary optimal working point positioning stage, a transimpedance amplifier circuit adjusting stage and an optimal working point accurately positioning and stabilizing stage, and comprises the following steps:
the initial stage of positioning the optimal working point, obtaining the coordinates of three points on the transmission characteristic curve of the electro-optical modulator, and obtaining the recursion direction of the bias voltage by identifying the minimum point, the recursion step length Vstep1＝(1％～10％)Vπ，VπFor the half-wave voltage of the electro-optical modulator, the preferred scheme is as follows:
(1) the electro-optic modulator bias voltage automatic control device is powered on to control the data gate (508) to gate the signalNumber Vbias1Setting the DC bias voltage output by the D/A control module (504) as Vbias；
(2) The DDS digital signal generator (501) drives the D/A control module (504) to sequentially output Vbias＝Vbias1-Vstep1、Vbias＝Vbias1、Vbias＝Vbias1+Vstep1And collects the digital light voltage signal output (V) corresponding to the A/D control module (506)dc1) Is provided with Vdc1Respectively is Vad1、Vad2、Vad3In which V isstep1Coarse adjustment step size for the set bias voltage;
(3) if Vad1>Vad2>Vad3Update Vbias1＝Vbias1+Vstep1Repeating the step (2); if Vad1<Vad2<Vad3Update Vbias1＝Vbias1-Vstep1Repeating the step (2); if Vad1≥Vad2≤Vad3Entering the step (4); suppose the optimum operating point of the electro-optic modulator is Vbias0At this time, the bias voltage Vbias1Close to the optimal operating point: vbias0-Vstep1≤Vbias≤Vbias0+Vstep1；
The stage of adjusting the transimpedance amplification circuit is as follows:
(4) when V isad2≤VLWhen the voltage is over, the resistance value of the digital potentiometer (2) is adjusted upwards; when V isad2≥VHWhen the voltage is over, the resistance value of the digital potentiometer (2) is adjusted downwards until V is reachedL≤Vad2≤VH；
The stage of accurately positioning and stabilizing the optimal working point comprises the following steps:
(5) controlling the data Strobe (508) to strobe the adder (503) output Vadd＝Vsin1+Vbias2,VaddAt a DC component Vbias2On the basis of the signal, a tiny sine disturbance signal V is superposedsin1Asin (ω t), where A is a sinusoidal perturbation signal Vsin1A is less than or equal to 1 ‰ Vπ(ii) a Omega is the angular frequency of the disturbance signal，ω≤10kHz/2π；
Vadd＝Vbias2+Asin(ωt)； (1)
(6) The electro-optic modulator transfer characteristic curve may be expressed as:
Figure BDA0003031778480000111
wherein alpha is a coefficient, PinFor electro-optical modulators input optical power, V is a modulation voltage, VπIs a half-wave voltage, and is,
Figure BDA0003031778480000112
is an intrinsic phase whose value is susceptible to changes in the environment, such as temperature, mechanical stress and aging, P0The leakage optical power of the electro-optical modulator is obtained by substituting the formula (1) into the formula (2) to obtain an output modulation signal of the electro-optical modulator:
Figure BDA0003031778480000121
filtering out the direct current component by a band-pass filter (507):
Figure BDA0003031778480000122
calculating VacAnd a disturbance signal Vsin1Zero-point correlation peak Conv ofzero：
Figure BDA0003031778480000123
Where ρ is a constant, Conv is known from the formula (5)zeroWith transfer characteristic curve of electro-optical modulator at DC bias voltage Vbias2The slope is linear, when the optimum working point is shifted negatively, Convzero>0; conv when the optimum operating point drifts forwardzero<0; when V isbiasConv at optimum operating pointzero＝0；
The preferred scheme is as follows: a cross-correlation operation module (508) for calculating VacAnd Vsin1Zero correlation peak of
Figure BDA0003031778480000124
Wherein N is the sequence VacAnd Vsin1Number of correlation operations, N-10 fadc/(2πω)，fadcThe sampling frequency is in Hz for the A/D control module (506). If Convzero>delta, indicating that the optimum working point of the electro-optical modulator shifts towards the negative direction, updating Vbias2＝Vbias2-Vstep2(ii) a If Convzero<Delta, indicating that the optimum working point of the electro-optical modulator shifts to the positive direction, and updating Vbias2＝Vbias2+Vstep2(ii) a If | Convzero|<delta, indicating the present bias voltage Vbias2At the optimum operating point of the electro-optic modulator. Conv when the electro-optic modulator is at the optimum operating pointzeroAnd | is approximately equal to 0, delta is the optimal working point judgment threshold, and can be adjusted according to the actual condition of hardware by taking the delta to be 255, wherein Vstep2Step size fine tuning for bias voltage2＝2A。
The invention relates to an electro-optical modulator bias voltage automatic control device based on FPGA, which takes the FPGA as a main control chip, realizes DDS digital signal generator, band-pass filter and cross-correlation operation and feedback control in the FPGA, 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 is used for replacing a fixed resistor as a transimpedance, the main control FPGA can adjust the amplification factor of the transimpedance amplification circuit by controlling the output of the digital potentiometer, the dynamic range of the circuit is improved, and the adaptability of the device to peripheral light path difference is enhanced;
the invention adopts the high-precision low-temperature drift analog converter DAC8560 to carry out digital-to-analog conversion, the output voltage is pure, the heat accumulation is not easy to generate in the operation process of the device, the error caused by temperature drift is controllable, and the operation stability of the device is improved; the feedback control process of the invention comprises three stages: the method comprises the steps of initially positioning an optimal working point, adjusting a transimpedance amplification circuit and accurately positioning and stabilizing the optimal working point, wherein in the whole process, the bias voltage of the electro-optical modulator is coarsely adjusted and then finely adjusted, and the positioning accuracy and the convergence speed of the optimal working point of the electro-optical modulator are considered;
compared with the prior art, the method has the advantages that the optimal working point is automatically positioned by adopting a feedback control process and a recursive control mode, the whole voltage range does not need to be scanned, the convergence speed is high, and the working efficiency is high; compared with the prior art, the method has the advantages that the zero-point correlation peak of alternating current components of the sine disturbance signal and the output signal of the electro-optical modulator is calculated in the FPGA, the optimal working point drift direction is judged through the positive and negative of the correlation peak, and the digital signal processing process is flexible and controllable.

Claims (8)

1. An electro-optical modulator bias voltage automatic control device based on FPGA is characterized by comprising: the device comprises a photoelectric detector (1), a transimpedance amplification 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 amplification circuit (3) is provided with a digital potentiometer (2); the digital potentiometer (2) is used as a transimpedance in the transimpedance amplifying circuit (3);
an external electro-optical modulator outputs an optical signal, 5% of the power of the output optical signal is input to a photoelectric detector (1) through a light splitting sheet, photoelectric conversion is carried out through the photoelectric detector (1), a photocurrent signal is generated and sent to a trans-resistance 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 amplification circuit (3) according to the control instruction of the FPGA (5);
the transimpedance amplification circuit (3) converts the photocurrent signal into an analog photovoltage signal according to the adjusted transimpedance value, and transmits 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 to perform analog-to-digital conversion to form a digital photovoltage signal which is used as the 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 then the bias voltage signal is subjected to amplitude linear amplification by the bias voltage conditioning circuit (6) and is output to the electro-optical modulator, so that the electro-optical modulator works at the optimal working point.
2. The device for automatically controlling the bias voltage of the electro-optical modulator based on the FPGA as claimed in claim 1, wherein: the digital potentiometer (2) is used as a transimpedance in the transimpedance amplification circuit (3), the main 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 amplification circuit (3), so that the dynamic range of the transimpedance amplification circuit (3) is improved.
3. The device for automatically controlling the bias voltage of the electro-optical modulator based on the FPGA as claimed in 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 of 0V-2.5V.
4. The device for automatically controlling the bias voltage of the electro-optical modulator based on the FPGA as claimed in claim 1, wherein: the bias voltage conditioning circuit (7) comprises an adder (701) and an in-phase proportional amplification circuit (702); the adder (701) has two inputs, input 1 and input 2; and having an output; wherein, the input end 1 of the adder (701) is connected to the external-1.25V DC bias voltage, and the input end 2 of the adder (701) is connected to the output 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 an external electro-optical modulator bias voltage pin, the amplification factor A of the bias voltage conditioning circuit (7) is 10, and the output bias voltage range of the bias voltage conditioning circuit (7) is-12.5V- + 12.5V.
5. The device for automatically controlling the bias voltage of the electro-optical modulator based on the FPGA as claimed in claim 1, wherein: the main control FPGA (5) controls the digital potentiometer (2) to set a 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.
6. The device for automatically controlling the bias voltage of the electro-optical modulator based on the FPGA as claimed in claim 1, wherein: master FPGA (5), comprising: the DDS digital signal generating circuit comprises a DDS digital signal generator (501), a data gating device (502), a digital adder (503), a D/A control module (504), a bias voltage feedback control module (505), an A/D control module (506), a band-pass filter (507) and a cross-correlation operation module (508).
7. The device for automatically controlling the bias voltage of the electro-optical modulator based on the FPGA as claimed in claim 1, wherein: the main control FPGA (5) comprises: the DDS digital signal generating circuit comprises a DDS digital signal generator (501), a data gating device (502), a digital adder (503), a D/A control module (504), a bias voltage feedback control module (505), an A/D control module (506), a band-pass filter (507) and a cross-correlation operation module (508);
the DDS digital signal generator (501) generates a DC bias signal VbiasAnd a sinusoidal perturbation signal Vsin；
The data strobe (502) is provided with an input end 1, an input end 2, a strobe end and an output end;
the digital adder (503) has an input terminal 1, an input terminal 2, and an output terminal;
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;
generated DC bias signal VbiasAre two paths which are completely the same, and are respectively a signal Vbias1And Vbias2: signal Vbias1Connected to input 1 of data strobe (502), signal Vbias2To input 1 of a digital adder (503); generating a sinusoidal disturbance signal VsinTwo identical paths are respectively as follows: signal Vsin1And Vsin2(ii) a Signal Vsin1Connected to input 2 of digital adder (503), signal Vsin2Is connected to the input end 1 of the cross-correlation operation module (508), and the output end of the digital adder (503) is connectedTo 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 gating 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 analog photovoltaic signals under the drive of the A/D control module (506) to perform analog-to-digital conversion to form digital photovoltaic signals which are used as input of feedback control, the digital photovoltaic signals are two paths of completely identical signals which are respectively Vdc1 and Vdc 2: 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) is subjected to band-pass filtering, 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);
(4) the bias voltage feedback control module (505) controls the data gate (502) to output the bias voltage of the electro-optical modulator by adopting preset feedback control logic according to the signal Vdc1 output by the A/D control module (506) and the signal output by the cross-correlation operation module (508).
8. The device for automatically controlling the bias voltage of the electro-optical modulator based on the FPGA as claimed in claim 7, wherein: the preset feedback control logic adopted by the bias voltage feedback control module (505) in the main control FPGA (5) comprises three stages: the method comprises a primary optimal working point positioning stage, a transimpedance amplifier circuit adjusting stage and an optimal working point accurately positioning and stabilizing stage, and comprises the following steps:
the initial stage of 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 (508)bias1Setting the DC bias voltage output by the D/A control module (504) as Vbias；
(2) The DDS digital signal generator (501) drives the D/A control module (504) to sequentially output Vbias＝Vbias1-Vstep1、Vbias＝Vbias1、Vbias＝Vbias1+Vstep1And collects the digital light voltage signal output (V) corresponding to the A/D control module (506)dc1) Is provided with Vdc1Respectively is Vad1、Vad2、Vad3In which V isstep1Coarse adjustment step size for the set bias voltage;
(3) if Vad1>Vad2>Vad3Update Vbias1＝Vbias1+Vstep1Repeating the step (2); if Vad1<Vad2<Vad3Update Vbias1＝Vbias1-Vstep1Repeating the step (2); if Vad1≥Vad2≤Vad3Entering the step (4); suppose the optimum operating point of the electro-optic modulator is Vbias0At this time, the bias voltage Vbias1Close to the optimal operating point: vbias0-Vstep1≤Vbias≤Vbias0+Vstep1；
The stage of adjusting the transimpedance amplification circuit is as follows:
(4) when V isad2≤VLWhen the voltage is over, the resistance value of the digital potentiometer (2) is adjusted upwards; when V isad2≥VHWhen the voltage is over, the resistance value of the digital potentiometer (2) is adjusted downwards until V is reachedL≤Vad2≤VH；
The stage of accurately positioning and stabilizing the optimal working point comprises the following steps:
(5) controlling the data Strobe (508) to strobe the adder (503) output Vadd＝Vsin1+Vbias2,VaddAt a DC component Vbias2On the basis of the signal, a tiny sine disturbance signal V is superposedsin1；
(6) Cross correlation operation module calculating VacAnd Vsin1Zero correlation peak of
Figure FDA0003031778470000041
If Corvzero>delta, indicating that the optimum working point of the electro-optical modulator shifts towards the negative direction, and updating Vbias＝Vbias-Vstep2(ii) a If Corvzero<Delta, indicating that the optimum working point of the electro-optical modulator shifts to the positive direction, and updating Vbias＝Vbias+Vstep2(ii) a If CorvzeroDelta is less than or equal to | indicates the current bias voltage VbiasLocating the optimal working point of the electro-optical modulator; wherein Vstep2For bias voltage trimming step size, | Corv when the electro-optic modulator is at the optimum operating pointzeroAnd | is approximately equal to 0, and delta is the optimal working point judgment threshold.
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