CN112923863A - Secondary frequency conversion fiber grating resonance peak tracking detection system - Google Patents

Secondary frequency conversion fiber grating resonance peak tracking detection system Download PDF

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CN112923863A
CN112923863A CN202110105072.2A CN202110105072A CN112923863A CN 112923863 A CN112923863 A CN 112923863A CN 202110105072 A CN202110105072 A CN 202110105072A CN 112923863 A CN112923863 A CN 112923863A
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signal
frequency
modulation
multiplier
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CN112923863B (en
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杨军
张毅博
金威
邹晨
刘志海
苑勇贵
苑立波
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Harbin Engineering University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
    • G01B11/18—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge using photoelastic elements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D3/00—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups
    • G01D3/028—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups mitigating undesired influences, e.g. temperature, pressure
    • G01D3/036—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups mitigating undesired influences, e.g. temperature, pressure on measuring arrangements themselves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/36—Forming the light into pulses
    • G01D5/38—Forming the light into pulses by diffraction gratings

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Abstract

The invention provides a secondary frequency conversion fiber grating resonance peak tracking detection system, wherein laser is loaded into a fiber grating after being modulated by a high-speed signal (>20MHz) through a Y waveguide, and a return signal of the laser is converted into an electric signal through a photoelectric detector and then is subjected to primary frequency conversion through an analog mixer. And then, a digital acquisition system acquires and realizes secondary frequency conversion, carrier recovery, signal demodulation and feedback control through an algorithm. The demodulation loop uses an improved COSTAS loop, and phase mismatch caused by time delay of a high-speed signal on a line can be eliminated. Compared with the common PDH demodulation method, the front end of the demodulation scheme uses the analog mixer for down-conversion without the limitation of A/D sampling speed, and after down-conversion to intermediate frequency, 1/f noise of a circuit system can be effectively avoided, and the signal-to-noise ratio of signals is improved. The system can work under the modulation signals from several MHz to several hundred MHz and has wider applicability.

Description

Secondary frequency conversion fiber grating resonance peak tracking detection system
Technical Field
The invention relates to a secondary frequency conversion fiber grating resonance peak tracking detection system, and belongs to the technical field of photoelectric detection.
Background
Due to the advantages of small size, high-speed corresponding characteristic, easy multiplexing and the like, the fiber bragg grating sensor (FBG) is effectively applied to the aspects of intelligent materials and structure monitoring. However, for high-precision temperature strain observation and other aspects, a single fiber grating cannot meet the requirement due to the fact that the reflection peak is too wide. Therefore, the fiber grating can be used for forming a resonant cavity to reduce the width of a reflection peak and obtain a more accurate observation signal. Therefore, many improved fiber grating measurement schemes have been proposed, such as using phase-shifted fiber gratings, fiber grating fabry-perot interferometers instead of ordinary fiber gratings, using laser frequency-locking techniques to improve the measurement accuracy of fiber gratings, etc. The fiber grating can obtain extremely high strain measurement precision by combining a PDH laser frequency locking technology.
The PDH laser frequency stabilization technology, also called phase modulation optical heterodyne technology, uses the resonant frequency of an external standard resonant cavity as a reference frequency to lock the frequency of a laser, and belongs to an active frequency stabilization technology. The frequency stabilization technology has the advantages of strong anti-interference capability of a system, high stability, fast servo response, difficult lock losing and the like, and becomes a commonly adopted frequency stabilization method at present.
In the demodulation process of the PDH signal, a single balanced mixer is used to perform mixing process at present. For example, in 1983, R.W.P.Drever et al, a PI controller was constructed using a mixer directly in combination with an analog operational amplifier to directly feedback control the system (Drever R W, Hall J L, Kowalski F V, et al laser phase and frequency stabilization using an optical resonator [ J ]. Applied Physics B,1983,31(2): 97-105.). But the result of direct mixing can be a delay in phase due to the increase in frequency, causing a change in signal amplitude. Therefore, in the later system, an rf delay is added to the front end of the mixer to perform fine phase modulation on the system. In 2011, Liuqing et al, Shanghai university of traffic, corrected the phase mismatch of the system by adding a phase delay (Liu Q, Tokunaga T, He Z, et al, Ultra-high-resolution large-dynamic-range optical fiber static sensor using round-Drever-Hall technique [ J ]. Optics Letters,2011,36(20): 4044) -4046.).
In order to more accurately eliminate the effect of phase mismatch at the front end of the mixer, a lock-in amplifier may also be used to demodulate the signal. In 2016, Shanghai university of traffic, Xin Yi et al, used LIA to demodulate the error signal (Chen J, Liu Q, Fan X, et al, ultra high resolution optical fiber transmitter using dual Pound-Drever-Hall feedback loops [ J ]. Optics Letters,2016,41(5): 1066-. However, the lock-in amplifier is usually expensive, so that the quadrature demodulation with the same principle as the lock-in amplifier can be used for processing, such as the delay matching of the error signal in the patents (CN109883348A, CN109916533A), i.e. using similar techniques.
However, the demodulation method is mostly suitable for low frequency systems, and the requirement of signal bandwidth generates a large demand on an acquisition system under high frequency modulation, so in order to reduce the system cost, analog equipment is generally used for processing, and then digital acquisition is performed for further calculation. But due to the introduction of the amplifier, 1/f noise of the circuit is introduced. In order to improve the signal quality and the anti-interference capability of the system, the influence of 1/f noise can be reduced by processing the system by using a secondary frequency conversion technology in a communication system. As described in the patent (CN204836148U), the double-conversion technique can enhance the anti-interference capability of the system and reduce the influence of 1/f noise.
The invention provides a secondary frequency conversion fiber grating resonance peak tracking detection system by combining a secondary frequency conversion technology and improving on the basis. By utilizing the fine reflection peak structure of the fiber grating resonant cavity and combining a PDH high-precision demodulation scheme, the frequency drift generated by the influence of environmental physical quantity on the fiber grating can be accurately detected. The demodulation system utilizes analog mixing and secondary frequency conversion technology, can work under a higher frequency point (>20MHz), simultaneously reduces the requirements of a rear FPGA acquisition module on sampling speed and computing capacity, reduces the cost of the system, can avoid the influence of 1/f noise of the analog system, and improves the signal-to-noise ratio. Meanwhile, an improved COSTAS loop is realized in the FPGA, and the influence caused by phase mismatch can be eliminated in a self-adaptive manner. The scheme realizes a high-precision resonant peak tracking and measuring system of the fiber grating resonant cavity, and can be used for tracking and detecting resonant peaks.
Disclosure of Invention
The invention aims to provide a secondary frequency conversion fiber bragg grating (PDH) resonance peak tracking detection system based on a PDH technology, which realizes demodulation of a PDH error signal under the condition of high-speed modulation and realizes high-precision resonance peak tracking measurement.
The purpose of the invention is realized as follows: the system comprises a light source modulation and driving module 100, a PDH light path modulation and acquisition module 110, a fiber bragg grating resonant cavity 120, a first-order down-conversion module 130 and an FPGA acquisition and secondary frequency conversion post-processing module 140, wherein the fiber bragg grating resonant cavity 120 is connected with the first-order down-conversion module 130 through a coaxial cable; the intermediate frequency signal generated by the first-order down-conversion module 130 is connected with the FPGA acquisition and secondary frequency conversion post-processing module 140 through a coaxial cable; the feedback correction signal generated by the FPGA acquisition and secondary frequency conversion post-processing module 140 is connected with the light source modulation and driving module 100 through a cable to provide a correction signal; the light source modulation and driving module 100 is connected with the PDH light path modulation and acquisition module 110 through an optical fiber jumper, and provides a narrow-linewidth single-frequency laser for the PDH light path modulation and acquisition module 110; the PDH optical path modulation and acquisition module 110 is connected to the fiber grating resonator 120 through an optical fiber jumper, and is converted into an electrical signal through the internal circulator 114 and the photodetector PD 113.
The invention also includes such structural features:
1. the first-order down-conversion module 130 includes a second rf signal generator 131, an analog double-balanced mixer 132, and an analog anti-aliasing filter 133; a constant frequency difference exists between the second radio frequency signal generator 131 and the modulation frequency generated by the first radio frequency signal generator 112 in the PDH optical path modulation and acquisition module 110, and the modulation signal frequency generated by the second radio frequency signal generator 131 is smaller than the modulation signal frequency generated by the first radio frequency signal generator 112; the analog double balanced mixer 132 operates in an ac coupled mode; the analog anti-aliasing filter 133 is a low-pass filter, and the passband bandwidth of the analog anti-aliasing filter is 2 times of the frequency difference between the second radio frequency signal generator 131 and the modulation frequency generated by the PDH optical path modulation and the first radio frequency signal generator 112 in the acquisition module 110; the second rf signal generator 131, the analog double balanced mixer 132, and the analog anti-aliasing filter 133 are connected by a coaxial cable.
2. The FPGA acquisition and secondary frequency conversion post-processing module 140 comprises an A/D sampling module 141, an improved COSTAS phase-locked demodulation circuit 142, a loop filter feedback circuit 143 and a D/A correction signal generation module 144, wherein the working frequency band of the A/D sampling module 141 is more than 2 times of the passband bandwidth of the analog anti-aliasing filter 133 in the first-order down-conversion module 130, the working frequency band of the D/A correction signal generation module 144 is more than or equal to the passband bandwidth of the analog anti-aliasing filter 133 in the first-order down-conversion module 130, and the improved COSTAS phase-locked demodulation circuit 142 and the loop filter feedback circuit 143 are pure digital modules and are realized by an FPGA internal algorithm.
3. The improved COSTAS phase-locked demodulation circuit 142 comprises an input signal register module 201, an I-path multiplier 202, an I-path low-pass filter module 203, a Q-path multiplier 204, a Q-path low-pass filter module 205, an IQ-path multiplier module 206, an arc tangent calculation module 207, a threshold determination module 208, a loop filter module 209 and a direct digital frequency synthesizer module (210), wherein the acquired input signal is firstly sent to the input signal register module 201 for storage; collected signals read from the input signal register module 201 are multiplied by digital I, Q carrier signals generated by a direct digital frequency synthesizer module (210) through an I-path multiplier 202 and a Q-path multiplier 204 respectively, and are sent to an I-path low-pass filter module 203, and a Q-path low-pass filter module 205 is filtered to generate corresponding I, Q signals; the signals passing through the I path low pass filter module 203 and the Q path low pass filter module 205 are sent to the IQ path multiplier module 206, and the arctangent calculation module 207 performs calculation; the IQ path multiplier module 206 and the arctangent calculation module 207 calculate the obtained signals and send the signals to the threshold value judgment module 208 for processing, and send the processed results to the loop filter module 209 for calculating the phase correction information, and send the obtained calculation results to the direct digital frequency synthesizer module (210) for calculation and phase correction of the generated digital I, Q carrier signals; the final calculation result is the I-path signal 211, which is sent to the post-loop filter feedback loop 143 for further processing.
4. The threshold decision module 208 includes threshold decision logic 501, and when the absolute value of the input product is compared with the threshold and meets the condition of being smaller than the threshold, the output result is 0; and when the absolute value of the input product is compared with the threshold value and the condition that the absolute value is larger than or equal to the threshold value is met, outputting the result of the arc tangent calculation.
5. The loop filter module 209 consists of a multiplier 1301, a multiplier 2302, an adder 303, an adder 2305 and a unit delay module 304, wherein the signal input into the loop filter passes through the multiplier 1301, and the multiplier 2302 is multiplied by a corresponding constant term; the adder 303 adds the calculation result of the multiplier 2302 and the output signal result, and sends the result to the unit delay module 304; the adder 2305 adds the calculation result of the unit delay block 304 and the calculation result of the multiplier 1301, directly outputs the result as a loop output signal, and feeds the result to the adder 303.
6. The loop filter feedback loop 143 includes: the circuit comprises a multiplier 3401, an adder 3402, an adder 4405, a unit delay module 2403 and a unit delay module 3404, wherein a loop input signal is multiplied by a constant through the multiplier 3401; the calculation results of the multiplier 3401 are respectively sent to the adder 3402 and the unit delay module 2403 for processing; the adder 3402 adds the output of the multiplier 3401 and the output of the adder 4405, outputs the sum as a loop signal calculation result, and simultaneously sends the result to the unit delay module 3404; the adder 4405 performs addition calculation on the outputs from the unit delay module 2403 and the unit delay module 3404, and sends the sum to the adder 3402.
7. The light source modulation and driving module 100 comprises a narrow linewidth laser 101 and a light source driving module 102, wherein the output linewidth of the narrow linewidth laser 101 is narrow, external sweep frequency voltage generated by the light source driving module 102 is received to realize sweep frequency, and narrow linewidth single-frequency laser generated by the narrow linewidth laser 101 is connected with a PDH light path modulation and acquisition module 110 through an optical fiber jumper; the light source driving module 102 receives a correction signal generated by the FPGA acquisition and secondary frequency conversion post-processing module 140, processes the correction signal, generates a laser driving sweep frequency voltage, and drives the narrow linewidth laser 101 to perform sweep frequency operation; the light source driving module 102 is connected with the narrow linewidth laser 101 through a cable; the working wavelength of the fiber grating resonator 120 is matched with the wavelength of the laser light source provided by the light source modulation and driving module 100.
8. The PDH optical path modulation and acquisition module 110 includes an optical fiber phase modulator PM111, a first radio frequency signal generator 112, a photodetector PD113, and an optical fiber circulator 114, where the optical fiber phase modulator PM111 receives a high-frequency modulation driving signal generated by the first radio frequency signal generator 112 to perform phase modulation on the single-frequency narrow-linewidth laser generated by the light source modulation and driving module 100; the frequency of the modulation signal generated by the first radio frequency signal generator 112 is >10 MHz; the optical fiber circulator 114 receives the modulated optical signal from the fiber phase modulator PM111 and sends the modulated optical signal into the rear-end fiber grating resonator 120, and receives the echo signal from the fiber grating resonator 120 and sends the echo signal into the photodetector PD 113; the photoelectric detector PD113 is a high-speed photoelectric detector, the detection bandwidth of the photoelectric detector PD113 is more than 20MHz, the optical fiber phase modulator PM111, the optical fiber loop device 114 and the photoelectric detector PD113 are cascaded through optical fibers, and the first radio frequency signal generator 112 and the optical fiber phase modulator PM111 are cascaded through cables.
Compared with the prior art, the invention has the beneficial effects that: (1) the method can be suitable for manufacturing a PDH (digital-to-analog) resonance peak tracking and measuring system under the condition of low-cost high-speed modulation, and compared with the traditional tracking system, the method is low in price and easy to realize. (2) The system introduces a secondary frequency conversion technology, so that carrier jitter and 1/f noise interference of analog equipment can be eliminated, and the anti-interference capability and stability of the system are improved. (3) The system uses the scheme of combining an analog system and a digital system, ensures the precision of system operation and demodulation, has wide application range, and can realize the demodulation of single-board multipath on the basis. (4) The system uses the improved pure digital COSTAS loop demodulation algorithm technology, can eliminate the influence of clock jitter, avoids large errors caused by small signals, and can be realized by an FPGA system. (5) The system has wide application range and flexible design, and can realize double-path and multi-path tracking demodulation on the basis.
Drawings
Fig. 1 is a schematic diagram of an apparatus of a PDH fiber grating resonance peak tracking detection system for high-speed modulation according to the present invention.
Fig. 2 is a block diagram of an internal algorithm of the improved COSTAS phase-locked demodulation loop module.
Fig. 3 is a block diagram of a signal processing flow of the threshold determination module.
Fig. 4 is a block diagram of an algorithm implementation of an internal loop filter module of the improved COSTAS phase-locked demodulation loop module.
Fig. 5 is a block diagram of an algorithm inside a loop filter feedback loop module.
Fig. 6 is a schematic diagram of an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
The invention provides a secondary frequency conversion fiber grating resonance peak tracking detection system, which comprises: the system comprises a light source modulation and driving module 100, a PDH light path modulation and acquisition module 110, a fiber bragg grating resonant cavity 120, a first-order down-conversion module 130, an FPGA acquisition and secondary frequency conversion post-processing module 140, wherein the fiber bragg grating resonant cavity 120 is connected with the first-order down-conversion module 130 through a coaxial cable; the intermediate frequency signal generated by the first-order down-conversion module 130 is connected with the FPGA acquisition and secondary frequency conversion post-processing module 140 through a coaxial cable; the feedback correction signal generated by the FPGA acquisition and secondary frequency conversion post-processing module 140 is connected to the light source modulation and driving module 100 through a cable to provide a correction signal. The light source modulation and driving module 100 is connected with the PDH light path modulation and acquisition module 110 through an optical fiber jumper, and provides a narrow-linewidth single-frequency laser for the PDH light path modulation and acquisition module 110; the PDH optical path modulation and acquisition module 110 is connected to the fiber grating resonator 120 through an optical fiber jumper, and is converted into an electrical signal through the internal circulator 114 and the photodetector PD 113;
the first-order down-conversion module 130 includes a second rf signal generator 131, an analog double-balanced mixer 132, and an analog anti-aliasing filter 133. A constant frequency difference exists between the second rf signal generator 131 and the modulation frequency generated by the first rf signal generator 112 in the PDH optical path modulation and acquisition module 110, and the modulation signal frequency generated by the second rf signal generator 131 is less than the modulation signal frequency generated by the first rf signal generator 112. The analog double balanced mixer 132 operates in an ac coupled mode. The analog anti-aliasing filter 133 is a low-pass filter, and the passband bandwidth of the analog anti-aliasing filter is 2 times the frequency difference between the second rf signal generator 131 and the PDH optical path modulation and modulation frequency generated by the first rf signal generator 112 in the acquisition module 110. The second rf signal generator 131, the analog double balanced mixer 132, and the analog anti-aliasing filter 133 are connected by a coaxial cable.
The FPGA acquisition and secondary frequency conversion post-processing module 140 includes: an A/D sampling module 141, an improved COSTAS phase-locked demodulation circuit 142, a loop filter feedback circuit 143, and a D/A correction signal generation module 144. The operating frequency band of the a/D sampling module 141 is more than 2 times the passband bandwidth of the analog anti-aliasing filter 133 in the first-order down-conversion module 130. The D/a correction signal generation module 144 operates at a frequency band greater than or equal to the passband bandwidth of the analog anti-aliasing filter 133 in the first-order down-conversion module 130. The improved COSTAS phase-locked demodulation circuit 142 and the loop filtering feedback circuit 143 are pure digital modules and are realized by an internal algorithm of the FPGA.
The improved COSTAS phase-locked demodulation circuit 142 comprises: an input signal register module 201, an I-path multiplier 202, an I-path low-pass filter module 203, a Q-path multiplier 204, a Q-path low-pass filter module 205, an IQ-path multiplier module 206, an arctangent calculation module 207, a threshold determination module 208, a loop filter module 209, a direct digital frequency synthesizer module (210), and the like. The collected input signals are firstly sent to the input signal registering module 201 for storage; collected signals read from the input signal register module 201 are multiplied by digital I, Q carrier signals generated by a direct digital frequency synthesizer module (210) through an I-path multiplier 202 and a Q-path multiplier 204 respectively, and are sent to an I-path low-pass filter module 203, and a Q-path low-pass filter module 205 is filtered to generate corresponding I, Q signals; the signals passing through the I path low pass filter module 203 and the Q path low pass filter module 205 are sent to the IQ path multiplier module 206, and the arctangent calculation module 207 performs calculation; the IQ path multiplier module 206 and the arctangent calculation module 207 calculate the obtained signals and send the signals to the threshold value judgment module 208 for processing, and send the processed results to the loop filter module 209 for calculating the phase correction information, and send the obtained calculation results to the direct digital frequency synthesizer module (210) for calculation and phase correction of the generated digital I, Q carrier signals; the final calculation result is the I-path signal 211, which is sent to the post-loop filter feedback loop 143 for further processing.
The threshold determination module 208 is mainly composed of a threshold determination logic 501, and when the absolute value of the input product is compared with a threshold and meets the condition that the absolute value is smaller than the threshold, the output result is 0; and when the absolute value of the input product is compared with the threshold value and the condition that the absolute value is larger than or equal to the threshold value is met, outputting the result of the arc tangent calculation.
The loop filter module 209 is composed of a multiplier 1301, a multiplier 2302, an adder 303, an adder 2305 and a unit delay module 304, wherein a signal input into the loop filter passes through the multiplier 1301, and the multiplier 2302 is multiplied by a corresponding constant term; the adder 303 adds the calculation result of the multiplier 2302 and the output signal result, and sends the result to the unit delay module 304. The adder 2305 adds the calculation result of the unit delay block 304 and the calculation result of the multiplier 1301, directly outputs the result as a loop output signal, and feeds the result to the adder 303.
The loop filter feedback loop 143 includes: the circuit comprises a multiplier 3401, an adder 3402, an adder 4405, a unit delay module 2403 and a unit delay module 3404, wherein a loop input signal is multiplied by a constant through the multiplier 3401. The calculation result of the multiplier 3401 is sent to the adder 3402 and the unit delay module 2403 for processing. The adder 3402 adds the output of the multiplier 3401 and the output of the adder 4405, outputs the result as a loop signal calculation result, and sends the result to the unit delay module 3404. The adder 4405 performs addition calculation on the outputs from the unit delay module 2403 and the unit delay module 3404, and sends the sum to the adder 3402.
The light source modulation and driving module 100 includes a narrow linewidth laser 101 and a light source driving module 102. The narrow linewidth laser 101 has a narrow output linewidth, and can receive an external sweep voltage generated by the light source driving module 102 to realize frequency sweep. A narrow linewidth single-frequency laser generated by the narrow linewidth laser 101 is connected with the PDH light path modulation and acquisition module 110 through an optical fiber jumper; the light source driving module 102 receives a correction signal generated by the FPGA acquisition and secondary frequency conversion post-processing module 140, processes the correction signal, generates a laser driving sweep frequency voltage, and drives the narrow linewidth laser 101 to perform sweep frequency operation; the light source driving module 102 is connected to the narrow linewidth laser 101 through a cable.
The PDH optical path modulation and acquisition module 110 includes an optical fiber phase modulator PM111, a first radio frequency signal generator 112, a photodetector PD113, and an optical fiber circulator 114. The fiber phase modulator PM111 receives the high-frequency modulation driving signal generated by the first radio frequency signal generator 112 to perform phase modulation on the single-frequency narrow linewidth laser generated by the light source modulation and driving module 100; the frequency of the modulation signal generated by the first radio frequency signal generator 112 is more than 10 MHz; the optical fiber circulator 114 receives the modulated optical signal from the fiber phase modulator PM111 and sends the modulated optical signal into the rear-end fiber grating resonator 120, and receives the echo signal from the fiber grating resonator 120 and sends the echo signal into the photodetector PD 113; the photodetector PD113 is a high-speed photodetector, and the detection bandwidth is more than 20 MHz. The optical fiber phase modulator PM111, the optical fiber circulator 114 and the photodetector PD113 are cascaded through an optical fiber, and the first radio frequency signal generator 112 and the optical fiber phase modulator PM111 are cascaded through a cable.
The working wavelength of the fiber grating resonator 120 is matched with the wavelength of the laser light source provided by the light source modulation and driving module 100.
The invention is an improvement based on PDH fiber grating technology. In order to reduce the requirement of a system on a hardware system of a demodulation system in the demodulation process and improve the anti-interference capability of the system under the condition of high-speed modulation, the system combines a secondary frequency conversion technology to solve the demodulation of an error signal under high-frequency modulation, and high-precision resonant peak tracking measurement is realized. The working principle is shown in figure 1. The narrow linewidth laser generated by the narrow linewidth laser 101 is modulated by the fiber phase modulator PM111 in the PDH optical path modulation and acquisition module 110, and then sent to the sensing fiber grating resonant cavity 120 through the fiber circulator 114. The reflected signal from the fiber grating resonator 120 enters the photodetector PD113 through the fiber loop 114 again and is converted into an electrical signal. The electric signal generated by the photodetector PD113 is collected by the back-end FPGA and collected by the secondary frequency conversion post-processing module 140 to generate a correction signal for correcting the wavelength deviation of the laser, and then is modulated by the light source and driven by the driving module 100 to correct the frequency deviation, so as to track the wavelength of the fiber grating resonator 120. The magnitude of the correction voltage can reflect the change characteristic of the external physical quantity.
After the output laser signal passes through the phase modulator, the laser signal in the optical fiber satisfies the relation (1):
ei(ωt+βsin(2πΩt)) (1)
in the formula, omega is the frequency of laser generated by the laser, beta is the modulation depth, and omega is the modulation frequency. After passing through the fiber grating resonator 120, the corresponding reflected signals are:
F(ωeff)ei(ωt+βsin(2πΩt)) (2)
wherein, F (ω)eff) As a function of the reflection characteristics of the fiber grating resonator, ωeffIs the equivalent instantaneous frequency.
At the port of the photodetector, the photodetector PD113 is converted under the influence of the detection range and detection principle of the photodetectorOf (2) an electric signal SinIs a narrow-band signal, signal S being a high-speed modulationinSatisfies the following formula:
Sin＝PDC-P0Im{F(ω)}sin(Ωt) (3)
in the formula, PDCFor the direct-current term of the input signal, P0F (omega) is a function of the reflection characteristics of the fiber grating resonator for gain factor, which is related to the input laser intensity.
Under the high-speed condition, because the modulation frequency Ω is often greater than 10MHz, if a D/a acquisition card is directly used for acquisition, the sampling frequency of the acquisition card needs to be more than twice of the modulation frequency Ω according to the nyquist sampling law, and if an analog mixing device is directly used for mixing, a constant phase difference exists between the acquired signal and the modulated carrier due to the line loss and the mismatch of the carrier line length, thereby causing signal distortion. Meanwhile, the analog mixing equipment has 1/f noise of about DC-1 KHz, and extra interference is also introduced into the system. Therefore, the noise is prevented from being introduced by combining the analog and digital acquisition technologies, and the stability of the system is improved. If the sampling signal after removing the direct current term is assumed to be:
SinAC＝Ferr(ω)sin(Ωt) (4)
then after first order mixing and filtering, the signal becomes:
SinAC＝Ferr(ω)cos(ΔΩt+Δφ) (5)
wherein, the frequency difference between the mixed carrier and the modulated carrier used by the first-order mixing of Δ Ω, and Δ Φ is the phase difference between the mixed carrier and the modulated carrier used by the first-order mixing. After the mixed low-frequency signal is subjected to alternating current coupling, the mixed low-frequency signal is collected by the FPGA and further demodulated.
The collected signal is firstly subjected to carrier extraction and envelope extraction through an internal improved COSTAS loop. The core principle of the method is shown in figure 2. After I, Q paths of input signals are multiplied by carriers and filtered, corresponding I, Q paths of signals can be obtained, and the signals have the following forms:
Figure RE-RE-GDA0002986216480000081
Figure RE-RE-GDA0002986216480000082
wherein, Δ ω is the frequency difference between the internal DDS generated carrier and the first-order mixed frequency difference Δ Ω, and the term Δ Φ is the accumulated phase difference between the DDS generated carrier and the mixed signal, which is the total phase difference of the system. F (ω, t) is the desired error function.
The corresponding phase difference can be directly obtained by directly solving the signals of I, Q, and the phase difference is taken as a reference, so that the frequency of the carrier signal generated by the DDS can be adjusted, the magnitude of the phase difference can be corrected, and the tracking of the carrier can be realized. When the phase difference is 0, SIThe output signal of the channel is the envelope signal to be obtained.
In order to suppress the influence of the envelope on carrier extraction, it is necessary to eliminate interference in the case of small signals. So here the effect is removed using a threshold decision algorithm. As shown in fig. 3, when the absolute value of the signal product of the IQ two-path is smaller than a set threshold, the output of the correction signal is temporarily turned off, and only tracking is performed, and only after the signal strength recovers over time, tracking and locking are performed again. The method can eliminate the interference of calculation errors under the condition of small signals.
The loop filters used in the system tracking process are respectively a PI controller and an I controller, and the structures of the loop filters are shown in fig. 4 and fig. 5, and respectively correspond to a COSTAS loop filter and a fiber grating tracking adjustment loop control filter. The size of the corresponding control constant is determined according to the actual system design result.
The implementation case is as follows: strain sensor based on fiber bragg grating FP
In the embodiment shown in fig. 6, the sensor is composed of a plurality of modules, including a computer 601, an FPGA data processing and collecting module 602, an analog mixer module 603, a photodetector module 604, a DDS module 605, a narrow linewidth laser light source 606, an optical fiber phase modulator 607, an optical fiber circulator 608, and an armored strain sensor 609. The entire sensing system is interconnected according to fig. 6.
The DDS module 605 sets two generated carrier signals, the frequency difference between the two carrier signals is 100kHz, and the data acquisition rate of the FPGA data processing and acquiring module 602 is 1 MHz. The high frequency carrier signal generated by the DDS module 605 is sent to the fiber phase modulator 607, and the low frequency carrier signal is sent to the analog mixer module 603. The DDS module 605 is set and controlled by the FPGA data processing and collecting module 602 as a slave module. In the system, the rear end of the analog mixer module 603 is provided with a passive anti-aliasing filtering module for filtering out high-frequency out-of-band signals. The control signal generated by the FPGA data processing and collecting module 602 is sent to the laser light source to adjust the wavelength of the laser, and the calibration signal is directly sent to the computer 601 to be post-processed. The FPGA data processing and collecting module 602 completes carrier recovery, error signal extraction, feedback signal operation, and the like.
When demodulation starts, the FPGA data processing and collecting module 602 issues an instruction to control the DDS module 605 to perform modulation and demodulation, the FPGA data processing and collecting module 602 starts collection and performs operation processing, and a feedback signal is transmitted to a computer and a laser light source through a communication cable in real time. When the wavelength of the armored strain sensor 609 is changed due to the influence of external strain, the FPGA data processing and collecting module 602 generates a corresponding calibration signal and feeds the calibration signal back to the light source and the computer.
In the scheme, the method comprises the following steps:
(1) the wavelength of the working center of the polarization maintaining fiber grating FP used by the sensor is 1550nm, the peak reflectivity at two ends is 99%, the length of the used fiber grating FP cavity is 20cm, and the fiber grating FP cavity is inscribed on a standard single-mode fiber.
(2) The carrier frequency is set to 40MHz and 39.9MHz at the analog mixer.
(3) The used laser is a 1550nm narrow linewidth laser, and the laser has a frequency sweeping function and can be controlled through an external signal. The linewidth of the laser is <10 KHz.
(4) The used phase modulator is a straight waveguide modulator, the working frequency band is 1550nm, the modulation frequency is more than 500MHz, and the fast axis or the slow axis works.
(5) The modulated carrier signal is a standard sine wave.
According to the technical scheme, the double-frequency-conversion fiber bragg grating resonance peak tracking detection system eliminates the phase difference caused by line time delay under the high-frequency condition by introducing a method of combining a double-frequency-conversion technology and a digital analog system, avoids the interference influence of low-frequency 1/f noise of the analog system, improves the stability of the system, reduces the requirement on the FPGA performance and reduces the cost. The system also keeps the high-precision characteristic of the PDH system, and ensures the real-time response and detection capability of the sensor to external strain signals. Because of the high integration of the modules, the module has higher combination freedom and better expansion capability. The system also has better real-time performance, and can meet the strain signal detection from low frequency to audio frequency range.
In summary, the invention is an improvement of the PDH modulation and demodulation technology of the fiber bragg grating, and provides an error signal demodulation scheme with low noise and high stability by utilizing a secondary frequency conversion method. Laser is loaded into the fiber grating after being modulated by a high-speed signal (>20MHz) through the Y waveguide, and a return signal of the laser is converted into an electric signal through a photoelectric detector and then is subjected to primary frequency conversion through an analog mixer. And then, a digital acquisition system acquires and realizes secondary frequency conversion, carrier recovery, signal demodulation and feedback control through an algorithm. The demodulation loop uses an improved COSTAS loop, and phase mismatch caused by time delay of a high-speed signal on a line can be eliminated. Compared with the common PDH demodulation method, the front end of the demodulation scheme uses the analog mixer for down-conversion without the limitation of A/D sampling speed, and after down-conversion to intermediate frequency, 1/f noise of a circuit system can be effectively avoided, and the signal-to-noise ratio of signals is improved. The system can work under the modulation signals from several MHz to several hundred MHz and has wider applicability.

Claims (9)

1. The utility model provides a secondary frequency conversion fiber grating formant tracking detecting system which characterized in that: the device comprises a light source modulation and driving module (100), a PDH light path modulation and acquisition module (110), a fiber grating resonant cavity (120), a first-order down-conversion module (130) and an FPGA acquisition and secondary frequency conversion post-processing module (140), wherein the fiber grating resonant cavity (120) is connected with the first-order down-conversion module (130) through a coaxial cable; intermediate frequency signals generated by the first-order down-conversion module (130) are connected with the FPGA acquisition and secondary frequency conversion post-processing module (140) through a coaxial cable; the feedback correction signal generated by the FPGA acquisition and secondary frequency conversion post-processing module (140) is connected with the light source modulation and driving module (100) through a cable to provide a correction signal; the light source modulation and driving module (100) is connected with the PDH light path modulation and acquisition module (110) through an optical fiber jumper, and provides narrow-linewidth single-frequency laser for the PDH light path modulation and acquisition module (110); the PDH optical path modulation and acquisition module (110) is connected with the fiber grating resonant cavity (120) through an optical fiber jumper, and is converted into an electric signal through an internal circulator (114) and a photoelectric detector PD (113).
2. The system according to claim 1, wherein the system comprises: the first-order down-conversion module (130) comprises a second radio-frequency signal generator (131), an analog double-balanced mixer (132) and an analog anti-aliasing filter (133); a constant frequency difference exists between the second radio frequency signal generator (131) and the modulation frequency generated by the first radio frequency signal generator (112) in the PDH optical path modulation and acquisition module (110), and the modulation signal frequency generated by the second radio frequency signal generator (131) is less than that generated by the first radio frequency signal generator (112); the analog double balanced mixer (132) operates in an AC coupling mode; the analog anti-aliasing filter (133) is a low-pass filter, and the passband bandwidth of the analog anti-aliasing filter is 2 times of the frequency difference between the second radio frequency signal generator (131) and the modulation frequency generated by the first radio frequency signal generator (112) in the PDH optical path modulation and acquisition module (110); the second radio frequency signal generator (131), the analog double balanced mixer (132) and the analog anti-aliasing filter (133) are connected by a coaxial cable.
3. The system according to claim 2, wherein the system comprises: the FPGA acquisition and secondary frequency conversion post-processing module (140) comprises an A/D sampling module (141), an improved COSTAS phase-locked demodulation loop (142), a loop filtering feedback loop (143) and a D/A correction signal generation module (144), wherein the working frequency band of the A/D sampling module (141) is more than 2 times of the passband bandwidth of an analog anti-aliasing filter (133) in a first-order down-conversion module (130), the working frequency band of the D/A correction signal generation module (144) is more than or equal to the passband bandwidth of the analog anti-aliasing filter (133) in the first-order down-conversion module (130), the improved COSTAS phase-locked demodulation loop (142) and the loop filtering feedback loop (143) are pure digital modules and are realized by an FPGA internal algorithm.
4. The system according to claim 3, wherein the system comprises: the improved COSTAS phase-locked demodulation loop (142) comprises an input signal registering module (201), an I-path multiplier (202), an I-path low-pass filtering module (203), a Q-path multiplier (204), a Q-path low-pass filtering module (205), an IQ-path multiplier module (206), an arc tangent calculating module (207), a threshold value judging module (208), a loop filter module (209) and a direct digital frequency synthesizer module (210), wherein the acquired input signal is firstly sent to the input signal registering module (201) for storage; collected signals read from the input signal register module (201) are multiplied by digital I, Q carrier signals generated by the direct digital frequency synthesizer module (210) through an I-path multiplier (202) and a Q-path multiplier (204) respectively and are sent to an I-path low-pass filter module (203), and a Q-path low-pass filter module (205) carries out filtering processing to generate corresponding I, Q signals; the signals passing through the I path low-pass filtering module (203) and the Q path low-pass filtering module (205) are sent to the IQ path multiplier module (206), and the arctangent calculation module (207) performs calculation; the signal calculated by the IQ path multiplier module (206) and the arc tangent calculation module (207) is sent to the threshold value judgment module (208) for processing, the processed result is sent to the loop filter module (209) for calculating phase correction information, the obtained calculation result is sent to the direct digital frequency synthesizer module (210) for calculation, and the phase correction is carried out on the generated digital I, Q carrier signal; the final calculation result is an I-path signal (211) and is sent to a rear loop filter feedback loop (143) for further processing.
5. The system according to claim 4, wherein the system comprises: the threshold decision module (208) comprises threshold decision logic (501), and when the absolute value of the input product is compared with the threshold and meets the condition of being smaller than the threshold, the output result is 0; and when the absolute value of the input product is compared with the threshold value and the condition that the absolute value is larger than or equal to the threshold value is met, outputting the result of the arc tangent calculation.
6. The system according to claim 5, wherein the system comprises: the loop filter module (209) is composed of a multiplier 1(301), a multiplier 2(302), an adder (303), an adder 2(305) and a unit delay module (304), wherein the signal input into the loop filter passes through the multiplier 1(301), and the multiplier 2(302) is multiplied by a corresponding constant term; the adder (303) adds the calculation result of the multiplier 2(302) and the output signal result, and sends the result to the unit delay module (304); the adder 2(305) adds the calculation result of the unit delay module (304) and the calculation result of the multiplier 1(301), and directly outputs the result as a loop output signal to the adder (303).
7. The double-conversion fiber grating resonance peak tracking detection system according to any one of claims 3 to 6, characterized in that: the loop filter feedback loop (143) comprises: multiplier 3(401), adder 3(402), adder 4(405), unit delay module 2(403), unit delay module 3(404), loop input signal through multiplier 3(401) is multiplied by a constant; the calculation results of the multiplier 3(401) are sent to the adder 3(402) and the unit delay module 2(403) respectively for processing; adder 3(402) adds the output of multiplier 3(401) and the output of adder 4(405), and outputs the result as the loop signal calculation result, and sends the result to unit delay module 3 (404); the adder 4(405) adds the outputs from the unit delay modules 2(403), 3(404) and sends them to the adder 3 (402).
8. The double-conversion fiber grating resonance peak tracking detection system according to any one of claims 1 to 6, characterized in that: the light source modulation and drive module (100) comprises a narrow linewidth laser (101) and a light source drive module (102), wherein the output linewidth of the narrow linewidth laser (101) is narrow, external sweep frequency voltage generated by the light source drive module (102) is received to realize sweep frequency, and narrow linewidth single-frequency laser generated by the narrow linewidth laser (101) is connected with a PDH light path modulation and acquisition module (110) through an optical fiber jumper; the light source driving module (102) receives a correction signal generated by the FPGA acquisition and secondary frequency conversion post-processing module (140), processes the correction signal, generates a laser driving frequency sweep voltage, and drives the narrow linewidth laser (101) to perform frequency sweep operation; the light source driving module (102) is connected with the narrow-linewidth laser (101) through a cable; the working wavelength of the fiber bragg grating resonant cavity (120) is matched with the wavelength of a laser light source provided by the light source modulation and driving module (100).
9. The double-conversion fiber grating resonance peak tracking detection system according to any one of claims 1 to 6, characterized in that: the PDH optical path modulation and acquisition module (110) comprises an optical fiber phase modulator PM (111), a first radio frequency signal generator (112), a photoelectric detector PD (113) and an optical fiber loop device (114), wherein the optical fiber phase modulator PM (111) receives a high-frequency modulation driving signal generated by the first radio frequency signal generator (112) to perform phase modulation on single-frequency narrow-linewidth laser generated by the light source modulation and driving module (100); the frequency of the modulation signal generated by the first radio frequency signal generator (112) is more than 10 MHz; the optical fiber circulator (114) receives a modulated optical signal from the optical fiber phase modulator PM (111) and sends the modulated optical signal into the rear-end fiber grating resonant cavity (120), and receives an echo signal from the fiber grating resonant cavity (120) and sends the echo signal into the photoelectric detector PD (113); the photoelectric detector PD (113) is a high-speed photoelectric detector, the detection bandwidth of the photoelectric detector PD (113) is more than 20MHz, the optical fiber phase modulator PM (111), the optical fiber loop device (114) and the photoelectric detector PD (113) are cascaded through optical fibers, and the first radio frequency signal generator (112) and the optical fiber phase modulator PM (111) are cascaded through cables.
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