A carrier demodulation method based on third harmonic to eliminate the influence of modulation depth

Abstract

The present invention discloses a carrier demodulation method based on eliminating the influence of modulation depth by third harmonics. The method uses a modulation depth elimination module to eliminate the influence of light intensity disturbance and modulation depth drift by performing operations such as subtraction, self-differentiation multiplication, and division on the sine and cosine components after frequency mixing and filtering, followed by single, double, and triple frequency multiplication. After the phase resolution module performs power reduction operations and sign judgment, the phase to be resolved can be accurately calculated, and the modulation depth value of the current system can be accurately calculated by the modulation depth resolution module. The present invention can eliminate the influence of the modulation depth of the external carrier on the demodulated signal, and can also eliminate the influence of light intensity disturbance on the demodulated signal, so that the demodulation result has a high signal-to-noise ratio and low harmonic distortion, thereby improving the accuracy of signal amplitude detection and the stability of the demodulation system. It can be widely used in high-precision optical fiber measurement and sensing systems.

Description

Technical Field

[0001] The present invention belongs to the field of optical fiber interferometer phase demodulation algorithms, relates to a carrier (PGC) demodulation method, and particularly relates to a carrier demodulation method based on third harmonics for eliminating modulation depth influence. Background Art

[0002] Interference fiber optic sensors are composed of light sources, transmission fibers, sensing fibers, modulation units, photoelectric detection units, and demodulation units. The sensing mechanism of interference fiber optic sensors is that the external measured physical quantity (such as pressure, acceleration, temperature, displacement, etc.) acts on the sensing fiber, affecting the phase and intensity of light. The optical signal is modulated, and through photoelectric conversion, the demodulation unit calculates the change in the measured physical quantity. Compared with traditional sensors, it has the advantages of anti-electromagnetic interference, high sensitivity, large dynamic range, diversified structure, and convenience for large-scale arrays and networking. It is widely used in fiber optic seismometers, fiber optic strain gauges, and fiber optic hydrophones. Since this type of sensor is based on the principle of light interference, the change in the external measured physical quantity is converted into the phase change of the interference signal. In order to calculate the measured signal, phase demodulation technology is required. There are mainly the following phase demodulation technologies: active zero-difference method, 3D-based method, There are three methods: passive homodyne method with 3-coupler, heterodyne method, passive homodyne method based on phase generated carrier (PGC), etc. Among them, the PGC passive homodyne method has the advantages of high resolution and strong real-time demodulation capability, and is the most widely used in engineering.

[0003] Traditional PGC demodulation algorithms are susceptible to interference from external factors, such as phase modulation depth C drift and carrier phase delay. The two most classic PGC algorithms are the cross-multiplication-based PGC algorithm (PGC-DCM) and the arctangent-based PGC algorithm. The PGC-DCM demodulation algorithm uses a cross-differential multiplication method that is dependent on light intensity and suffers from poor stability when light intensity changes rapidly. The PGC-Arctan demodulation algorithm demodulates two signals by dividing and performing an arctangent operation. This algorithm requires the modulation depth C to remain at its optimal value of 2.63. When the modulation depth C deviates from this optimal value, the demodulation result will exhibit nonlinear changes, resulting in severe harmonic distortion.Zhang Min and others from Tsinghua University conducted in-depth research on noise suppression for standalone fiber-optic hydrophones (CN102359797B) and multiplexed hydrophone arrays (CN102680072B) based on the PGC principle. The 715th Research Institute of China Shipbuilding Industry Corporation proposed a PGC complex demodulation method for large-scale fiber-optic hydrophone arrays (CN101604957A), which meets the demodulation needs of large-scale hydrophone arrays. In the same year, the 715th Research Institute of China Shipbuilding Industry Corporation proposed a portable multifunctional fiber-optic hydrophone signal demodulation method (CN101615888A). This method is low-power, portable, and multifunctional, providing a reliable and convenient tool for the study of fiber-optic hydrophones and their arrays. David B. Hall of Northrop Grumman Corporation in the United States also published a patent related to array demodulation (US 7038784 B2). In 2017, Anton V. Volkov et al. proposed a PGC demodulation algorithm based on phase modulation depth evaluation and correction. The algorithm calculates the modulation depth C value by introducing the third and fourth harmonics, and then uses the PI control algorithm to correct the C value (Phase Modulation Depth Evaluation and Correction Technique for the PGC Demodulation Scheme in Fiber-Optic Interferometric Sensors); in 2019, Yang Jun et al. from Harbin Engineering University proposed a real-time self-calibration PGC demodulation algorithm. The algorithm uses a combination of elliptical fitting and PID control to correct the modulation depth C value to 2.63, and the actual system signal-to-noise ratio can reach 61.33dB (Real-time self-calibration PGC-Arctan demodulation algorithm in fiber-optic interferometric sensors); in 2021, Yan Liping et al. from Zhejiang University of Technology proposed a PGC demodulation nonlinear error compensation method based on carrier phase delay and phase modulation depth. This algorithm solves the nonlinear error caused by phase delay and modulation depth in the PGC algorithm (Nonlinear Error Compensation of PGC Demodulation With the Calculation of Carrier Phase Delay and Phase Modulation Depth).However, the above algorithms all increase the computational complexity of the system, prolong the time required for signal processing, affect the real-time performance of the system, and easily cause the system to generate large harmonic distortion during large signal demodulation.

[0004] For the demodulation system, eliminating the influence of modulation depth and light intensity disturbance while improving the signal-to-noise ratio, suppressing harmonic distortion, and reducing signal distortion at a low cost is of great significance and practical value. Summary of the Invention

[0005] To overcome the aforementioned shortcomings and deficiencies of the prior art, the present invention provides a carrier demodulation method based on third harmonic modulation depth elimination. By loading a third harmonic carrier signal, the method enables calculation and demodulation of the modulation depth of an external carrier. This method eliminates the effects of the external carrier modulation depth on the demodulated signal, as well as the effects of optical intensity disturbances. This method results in a high signal-to-noise ratio and low harmonic distortion in the demodulated signal, improving the accuracy of signal amplitude detection and the stability of the demodulation system. The method is widely applicable to high-precision fiber-optic measurement and sensing systems.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A carrier demodulation system based on third harmonic elimination of modulation depth influence includes five parts: a signal modulation module, a mixing and filtering module, a modulation depth elimination module, a modulation depth calculation module, and a phase calculation module.

[0008] The signal modulation module includes a synchronous start module, a data acquisition module and a modulation output module. The synchronous start module is used for the synchronous operation of the data acquisition module and the modulation output module. The data acquisition module is used to collect the interference signal output by the interferometer module. After the interference signal undergoes photoelectric conversion, it is converted from an optical signal to an electrical signal. The modulation output module outputs a sine wave to the light source modulator for modulating the light source, and the modulated light is injected into the interferometer module.

[0009] The mixing and filtering module includes a first frequency multiplier, a second frequency multiplier, a first multiplier, a second multiplier, a third multiplier, a first low-pass filter, a second low-pass filter, and a third low-pass filter. The signals collected by the data acquisition module are respectively sent to the first multiplier, the second multiplier, and the third multiplier. The signals of the modulation output module are respectively sent to the first frequency multiplier, the second frequency multiplier, and the first multiplier. The output result obtained by the first frequency multiplier is sent to the second multiplier, and the output result obtained by the second frequency multiplier is sent to the third multiplier. The output results of the first multiplier, the second multiplier, and the third multiplier are respectively sent to the first low-pass filter, the second low-pass filter, and the third low-pass filter.

[0010] The module for eliminating the influence of modulation depth includes a sine component being fed into a subtractor, a first differentiator, a second differentiator, a fourth multiplier, a fifth multiplier, a sixth multiplier, a seventh multiplier, a first divider and a second divider, the sine components of the measured signal extracted by the first low-pass filter and the third low-pass filter are respectively fed into the subtractor, the sine components calculated by the subtractor are respectively fed into the sixth multiplier, the second differentiator and the fifth multiplier, the cosine component of the measured signal extracted by the second low-pass filter is respectively fed into the sixth multiplier, the first differentiator and the fourth multiplier, the output result of the first differentiator is respectively fed into the seventh multiplier and the fourth multiplier, the output result of the second differentiator is respectively fed into the seventh multiplier and the fifth multiplier, the output results of the fourth multiplier and the fifth multiplier are simultaneously fed into the first divider, and the output results of the sixth multiplier and the seventh multiplier are simultaneously fed into the second divider;

[0011] The modulation depth calculation module includes a first power reduction module, and the output result of the first divider is subjected to a power reduction operation by the first power reduction module to obtain the modulation depth value of the current system;

[0012] The phase solution module includes a second power reduction module, a sign judgment module and an integrator. The second power reduction module performs an inversion and power reduction operation on the output signal of the second divider to obtain a phase signal differential value with a positive sign; then, the sign judgment module judges the sign of the phase signal differential value based on the signs of the sine component output by the first low-pass filter and the cosine component output by the second low-pass filter as conditions, and then obtains the true tangent value of the phase signal through the integrator.

[0013] A carrier demodulation method based on third harmonic elimination of modulation depth effects using the above-mentioned carrier demodulation system, wherein the modulation depth effect elimination module eliminates the effects of light intensity disturbance and modulation depth drift by performing operations such as subtraction, self-differentiation multiplication, and division on the sine and cosine components after frequency mixing filtering, doubled, and tripled. After the power reduction operation and sign judgment in the phase solution module, the phase to be solved can be accurately calculated, and the modulation depth value of the current system can be accurately calculated by the modulation depth solution module. Specifically, the method includes the following steps:

[0014] Step 1: Use the data acquisition module to collect the interference signal output by the interferometer module. After the interference signal undergoes photoelectric conversion, it is converted from an optical signal to an electrical signal; use the modulation output module to output a sine wave to the light source modulator for modulating the light source, and inject the modulated light into the interferometer module;

[0015] Step 2: Send the signal of the modulation output module to the first frequency multiplier, the second frequency multiplier, and the first multiplier respectively; send the output result of the first frequency multiplier to the second multiplier; send the output result of the second frequency multiplier to the third multiplier; send the output results of the first multiplier, the second multiplier, and the third multiplier to the first low-pass filter, the second low-pass filter, and the third low-pass filter respectively;

[0016] Step 3: The sine components of the measured signal extracted by the first low-pass filter and the third low-pass filter are respectively input into the subtractor, and the sine components calculated by the subtractor are respectively input into the sixth multiplier, the second differentiator, and the fifth multiplier. The cosine component of the measured signal extracted by the second low-pass filter is respectively input into the sixth multiplier, the first differentiator, and the fourth multiplier. The output results of the first differentiator are respectively input into the seventh multiplier and the fourth multiplier. The output results of the second differentiator are respectively input into the seventh multiplier and the fifth multiplier. The output results of the fourth multiplier and the fifth multiplier are simultaneously input into the first divider, and the output results of the sixth multiplier and the seventh multiplier are simultaneously input into the second divider.

[0017] Step 4: Send the output result of the first divider to the first power reduction module for power reduction operation to obtain the modulation depth value of the current system;

[0018] Step 5. Send the output result of the second divider to the second power reduction module, perform inversion and power reduction operations, and obtain a phase signal differential value with a positive sign; then pass through the sign judgment module, use the signs of the sine component output by the first low-pass filter and the cosine component output by the second low-pass filter as conditions to judge the sign of the phase signal differential value, and then obtain the true tangent value of the phase signal through the integrator.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] (1) The modulation depth C value can be calculated by eliminating the modulation depth influence module, and the modulation depth C value under the current environment of the demodulation system can be monitored in real time;

[0021] (2) By eliminating the modulation depth effect module, the signal containing the AC intensity B value and the modulation depth C value is eliminated, so that the output item only contains the differential value of the phase signal, eliminating the influence of light intensity disturbance on the demodulated signal, avoiding the uncertainty of the demodulation result caused by factors such as light source instability, making the demodulation result have a high signal-to-noise ratio and low harmonic distortion, and improving the accuracy of signal amplitude detection and the stability of the demodulation system;

[0022] (3) Reduce nonlinear errors. When the modulation depth slightly shifts due to environmental changes, no distortion term will be generated. This can effectively suppress harmonics and eliminate the demodulation result's dependence on the modulation depth introduced by the external carrier wave. This makes the demodulation result have a high signal-to-noise ratio and low harmonic distortion, thereby improving the accuracy of signal amplitude detection and the stability of the demodulation system.

[0023] (4) The computational complexity is not high, and the system compatibility is good, so it can be widely used in high-precision fiber optic measurement and sensing systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a flow chart of the carrier demodulation algorithm based on the third harmonic to eliminate the influence of modulation depth;

[0025] Figure 2 This is a diagram of the interferometer modulation, demodulation and detection optical path device;

[0026] Figure 3 The waveform of the demodulated signal is a carrier demodulation algorithm based on the third harmonic to eliminate the influence of modulation depth;

[0027] Figure 4 It is the harmonic distortion of the demodulated signal after the algorithm is improved;

[0028] Figure 5 It is the comparison of the modulation depth C on the signal demodulation amplitude before and after the algorithm improvement;

[0029] Figure 6 It is the comparison of the interference signal AC intensity B and the signal demodulation amplitude before and after the algorithm improvement. DETAILED DESCRIPTION

[0030] The technical solution of the present invention is further described below with reference to the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0031] The present invention provides a carrier demodulation system based on the third harmonic to eliminate the influence of modulation depth, such as Figure 1 As shown, the carrier demodulation system includes five parts: a signal modulation module, a frequency mixing and filtering module, a modulation depth influence elimination module, a modulation depth calculation module, and a phase calculation module. The interference signal collected by the signal modulation module 10 passes through the frequency mixing and filtering module 11, the modulation depth influence elimination module 12, the modulation depth calculation module 13, and the phase calculation module 14 in sequence, and finally outputs the phase demodulation signal and the modulation depth value, where:

[0032] The signal modulation module 10 includes a synchronous start module 101, a data acquisition module 102, and a modulation output module 103. The data acquisition module 102 is used to collect the interference signal output by the interferometer module 22. After the interference signal undergoes photoelectric conversion, it is converted from an optical signal to an electrical signal. The modulation output module 103 outputs a sine wave to the light source modulator 232 for modulating the light source 211. The modulated light is injected into the interferometer module 22. The modulation frequency of the light source modulator 232 is between 2kHz and 50MHz, and the modulation amplitude is set in the range of 1 to 6rad to ensure the stability of the interference fringes.

[0033] The mixing and filtering module 11 includes a first frequency multiplier 111, a second frequency multiplier 112, a first multiplier 113, a second multiplier 114, a third multiplier 115, a first low-pass filter 116, a second low-pass filter 117, and a third low-pass filter 118. The signals collected by the data acquisition module 102 are respectively sent to the first multiplier 113, the second multiplier 114, and the third multiplier 115. The signals of the modulation output module 103 are respectively sent to the first frequency multiplier 111, the second frequency multiplier 112, the first multiplier 113, the second multiplier 114, and the third multiplier 115. The output result obtained by the first frequency multiplier 111 is sent to the second multiplier 114, the output result obtained by the second frequency multiplier 112 is sent to the third multiplier 115, and the output results of the first multiplier 113, the second multiplier 114, and the third multiplier 115 are sent to the first low-pass filter 116, the second low-pass filter 117, and the third low-pass filter 118 respectively. The cutoff frequencies of the first low-pass filter 116, the second low-pass filter 117, and the third low-pass filter 118 are selected to be between 1 kHz and 25 MHz according to the frequency of the carrier signal.

[0034] The modulation depth influence elimination module 12 includes a sine component input to a subtractor 121, a first differentiator 122, a second differentiator 123, a fourth multiplier 124, a fifth multiplier 125, a sixth multiplier 126, a seventh multiplier 127, a first divider 128 and a second divider 129. The sine components of the measured signal extracted by the first low-pass filter 116 and the third low-pass filter 118 are respectively input to the subtractor 121. The sine components calculated by the subtractor 121 are respectively input to the sixth multiplier 126, the second differentiator 123 and the fifth multiplier 125. The measured signal extracted by the second low-pass filter 117 is The cosine components of the phase differential are respectively sent to the sixth multiplier 126, the first differentiator 122 and the fourth multiplier 124. The output results of the first differentiator 122 are respectively sent to the seventh multiplier 127 and the fourth multiplier 124. The output results of the second differentiator 123 are respectively sent to the seventh multiplier 127 and the fifth multiplier 125. The output results of the fourth multiplier 124 and the fifth multiplier 125 are simultaneously sent to the first divider 128. The output results of the sixth multiplier 124 and the seventh multiplier 125 are simultaneously sent to the second divider 129 to obtain a signal containing only the square term of the phase differential value. The above calculation can eliminate the influence of light intensity disturbance and modulation depth drift.

[0035] The modulation depth calculation module 13 includes a first power reduction module 131. The output result of the first divider 128 is subjected to a power reduction operation by the first power reduction module 131 to obtain the modulation depth value of the current system.

[0036] The phase solution module 14 includes a second power reduction module 141, a sign judgment module 142 and an integrator 144. The second power reduction module 141 performs an inversion and power reduction operation on the output signal of the second divider 129 to obtain a phase signal differential value with a positive sign; then, the sign judgment module 142 judges the sign of the phase signal differential value based on the signs of the sine component output by the first low-pass filter 116 and the cosine component output by the second low-pass filter 117 as conditions, and then the true tangent value of the phase signal is obtained through the integrator 144.

[0037] The present invention also provides a carrier demodulation method based on the third harmonic elimination of modulation depth effects using the above-mentioned carrier demodulation system. The modulation depth effect elimination module eliminates the effects of light intensity disturbance and modulation depth drift by performing operations such as subtraction, self-differentiation multiplication, and division on the sine and cosine components after frequency doubling, doubling, and tripling mixing filtering. After the power reduction operation and symbol judgment in the phase resolution module, the phase to be resolved can be accurately calculated, and the modulation depth value of the current system can be accurately calculated through the modulation depth resolution module. This method is an algorithmic improvement to the phase generated carrier (PGC) demodulation algorithm. The improved algorithm principle is as follows: Figure 1 As shown:

[0038] The signal modulation module 10 includes a data acquisition module 102 and a modulation output module 103. The modulation output module 103 outputs a sine wave to the light source modulator 232 for modulating the light source 211 to generate a phase carrier, so that the light phase is added with a change The modulated light is injected into the interferometer module 22; the data acquisition module 102 is used to collect the result of the photoelectric conversion output by the interferometer module 22 to obtain the interference signal in the form of formula (1):

[0039] (1)

[0040] Where: I1 and I2 are the light intensities of the two arms of the interferometer module respectively; θ(t) is the optical path difference; A is the DC component of the light intensity, B is the AC component of the light intensity, C is the phase modulation depth, ω0 is the carrier signal frequency, is the signal to be measured.

[0041] The interference signal obtained by formula (1) is expanded using the Bessel function to obtain the spectral components of the interference signal:

[0042] (2)

[0043] Among them, A is the DC intensity of the output signal, B is the AC intensity, C is the modulation depth, ω0 is the carrier frequency, is the signal to be measured, J k (C) is the Bessel function coefficient, k is the high-order component of the signal, and the amplitude of each sideband component around the zero frequency is proportional to J k (C). The larger the C value, the higher the J k The slower the speed at which (C) approaches zero, and the smaller the C value, the better it is for reducing the system phase noise.

[0044] The three components containing phase-shifted signals are obtained through the mixing and filtering module 11:

[0045] (3)

[0046] (4)

[0047] (5)

[0048] In actual systems, G, H, and K are the amplitudes of the detection wave signal. Generally, G=H=K. Parameters B and C drift due to changes in the external environment or internal instability of the system.

[0049] By performing the operation of the modulation depth elimination module 12 on the above three components, the purpose of eliminating the influence of light intensity disturbance and modulation depth drift is achieved. The specific implementation process is as follows:

[0050] According to the Bessel coefficient relationship:

[0051] (6)

[0052] The following relationship can be obtained:

[0053] (7)

[0054] The intermediate variable relationship can be obtained:

[0055] (8)

[0056] The square value of the modulation depth and the square value of the phase differential to be measured can be calculated through a series of operations including the first differentiator 122, the second differentiator 123, the fourth multiplier 124, the fifth multiplier 125, the sixth multiplier 126, the seventh multiplier 127, and the first divider 128. The expressions thereof are as follows:

[0057] (9)

[0058] (10)

[0059] The square value of the modulation depth is subjected to the operation of the modulation depth calculation module 13 to calculate the modulation depth value 132 in the current system.

[0060] The square value of the phase differential to be measured is subjected to the phase solution module 14 operation. The sign judgment module 142 performs the following operation on the orthogonal components (3) and (4): I1(t) / I2(t) is used as a condition to judge the sign of the tangent value of the phase signal. If I1(t) / I2(t)<0, then The signal sign is negative, otherwise positive, to obtain the true differential value of the phase signal. The result is then sent to integrator 144 to obtain PGC demodulation result 145. This result does not contain the AC intensity B value and the modulation depth C value, and is not affected by light intensity disturbances and modulation depth drift.

[0061] (11)

[0062] Example:

[0063] This embodiment provides a phase generation carrier demodulation method based on Mach-Zehnder interferometer, the interferometer modulation and demodulation device is as follows: Figure 2As shown, it includes a light source module 21, an interferometer module 22, a photoelectric acquisition circuit 20, and a data processing module 23. The light source module 21 includes a light source 211 and an isolator 212. The interferometer module 22 includes a first coupler 221, a second coupler 221, a phase modulator 222, a first fiber ring 223, and a second fiber ring 224. The data processing module 23 includes a light source modulator 232, a computer 234, and a data acquisition module 236. The device selection and parameters of the interferometer measurement device are as follows:

[0064] (1) The input light source 211 is an ASE broadband light source with a central wavelength of 1550 nm, a half-width greater than 45 nm, and a power of 10 mW.

[0065] (2) The operating wavelength of the optical fiber isolator 212 is 1550nm±5nm, the insertion loss is ≤1.0dB (at an operating temperature of 23℃), and the return loss is ≥55dB.

[0066] (3) The operating wavelength of coupler No. 1 221 is 1550 nm, and the splitting ratio is 50% / 50%; the operating wavelength of coupler No. 225 is 1550 nm, and the splitting ratio is 50% / 50%.

[0067] (4) The operating wavelength of the No. 1 optical fiber ring 223 is 1550 nm, the ring crosstalk is less than -18 dB, the ring attenuation is less than 0.3 dB / km, the ring inner diameter is 50 mm, the ring outer diameter is 80 mm, and the optical fiber length is 300 m; the operating wavelength of the No. 2 optical fiber ring 224 is 1550 nm, the ring crosstalk is less than -18 dB, the ring attenuation is less than 0.3 dB / km, the ring inner diameter is 50 mm, the ring outer diameter is 80 mm, and the optical fiber length is 300 m.

[0068] (5) The phase modulator 222 is a cylindrical piezoelectric ceramic ring wrapped with an optical fiber, with a resonant frequency of 2000 Hz, a resonant resistance of less than 200 ohms, a capacitance of 50 nF ± 30%, a ring thickness of 1 mm, a ring height of 10 mm, and an outer diameter of 20 mm. A 1 m long optical fiber is wrapped around the piezoelectric ceramic ring and bonded with strong glue.

[0069] (6) The photodetection module 20 contains two photodetectors for differential detection. The detector type is an InGaAs photodetector, the connection mode is a pigtail FC / PC, the operating wavelength is 1100nm to 1650nm, the light intensity responsivity R = 0.85A / W, and the capacitance is 0.35pF.

[0070] (7) The acquisition module 236 is an NI-7856R acquisition card with a sampling rate of 4Mbps, an input voltage amplitude of ±10V, and a sampling clock that is the internal clock of the acquisition card.

[0071] The specific process of the algorithm is as follows:

[0072] (1) The system runs the signal modulation module 10. First, the computer 234 controls the light source modulator 232 through the second data transmission line 233. Then, the light source 221 is frequency modulated through the first data transmission line 231. The carrier signal with an amplitude of 2.6 rad and a frequency of 5 kHz is set. The carrier signal does not change with factors such as environmental changes. At the same time, a calibration signal with an amplitude of 1 rad and a frequency of 400 Hz is applied to the phase modulator 222. The frequency modulated optical signal is injected into the interferometer module 22 through the optical fiber isolator 212. After passing through coupler No. 1 221, the signal is divided into two paths. One optical signal passes through the optical path with phase modulator 222 and optical fiber ring No. 1 223, and the other optical signal passes through optical fiber ring 224. The two optical signals interfere with each other at coupler No. 2 225, and are differentially detected by photoelectric detection module 20 and converted into differential interference electrical signals. The differential interference electrical signals are transmitted to data acquisition module 236 by electrical conductor No. 2 237, and then transmitted to computer 234 via data transmission line No. 2 235 for carrier phase demodulation. The entire process is carried out synchronously.

[0073] (2) The data acquisition module 102 obtains an interference signal including a DC bias, the peak-to-peak value of the signal is 4V, and the DC bias is about 2V.

[0074] (3) Perform mixing and filtering operations on the interference signal. Set the first filter 114 and the second filter 115 to FIR Blackman windows with parameters of passband cutoff frequency 2 kHz, stopband cutoff frequency 3 kHz, attenuation -80 dB, passband ripple 0.01 dB, and order 265. After the data passes through the filter, three signals are obtained.

[0075] (4) The two signals are passed through the modulation depth elimination module 12. At this time, the influence of the drift of the AC intensity B and modulation depth C caused by changes in the external environment or instability in the system is eliminated.

[0076] (5) The signal is sent to the modulation depth calculation module 13 for corresponding calculations to obtain the tangent value of the phase signal with a positive sign. The sign judgment module 133 uses the signs of the sine component 121 and the cosine component 122 as the judgment conditions to perform sign judgment on the tangent value and obtain the true tangent value of the phase signal.

[0077] (6) The signal is sent to the phase solution module 14 for corresponding operation to obtain the differential value of the phase signal with a positive sign. The sign judgment module 142 uses the signs of the sine component output by the first low-pass filter 116 and the cosine component output by the second low-pass filter 117 as conditions to perform sign judgment on the tangent value to obtain the true differential value of the phase signal; the differential value of the signal is solved by the integrator 144 to obtain the final result, and the phase demodulation result 145 is obtained.

[0078] Figure 3 Shown is the demodulated output waveform of a signal with a frequency of 400Hz.

[0079] Figure 4 The figure shows the harmonic distortion of the demodulated signal after the algorithm is improved. Figure 4 It can be seen that the harmonic distortion value is -88.25dB, and the harmonic suppression effect is good.

[0080] Figure 5 The figure shows the comparison of signal demodulation results of the original PGC-Arctan algorithm and the improved PGC algorithm when the modulation depth C is changed. Figure 5 It can be seen that when the modulation depth C changes from 1 rad to 3.5 rad, the signal amplitude obtained by the original PGC demodulation algorithm changes with the modulation depth. When the C value shifts to 1 rad, the demodulation result of the signal amplitude changes by more than 0.4 rad. When the C value shifts to 3.5 rad, the demodulation result of the signal amplitude changes by more than 0.55 rad. The signal amplitude obtained by the improved PGC demodulation algorithm changes by less than 0.2 rad when the C value changes from 2 rad to 3.5 rad.

[0081] Figure 6 The figure shows the comparison of the interference signal AC intensity B and the signal demodulation amplitude before and after the improvement. Figure 6 It can be seen that when the AC intensity B value changes from 1 rad to 3.5 rad, the signal amplitude obtained by the improved PGC demodulation algorithm changes by less than 0.01 rad.

Claims

1. A carrier demodulation system based on third harmonic elimination of modulation depth influence, characterized in that The carrier demodulation system includes five parts: a signal modulation module, a frequency mixing and filtering module, a modulation depth influence elimination module, a modulation depth calculation module, and a phase calculation module, wherein: The signal modulation module includes a synchronous start module, a data acquisition module and a modulation output module. The synchronous start module is used for the synchronous operation of the data acquisition module and the modulation output module. The data acquisition module is used to collect the interference signal output by the interferometer module. After the interference signal undergoes photoelectric conversion, it is converted from an optical signal to an electrical signal. The modulation output module outputs a sine wave to the light source modulator for modulating the light source, and the modulated light is injected into the interferometer module. The mixing and filtering module includes a first frequency multiplier, a second frequency multiplier, a first multiplier, a second multiplier, a third multiplier, a first low-pass filter, a second low-pass filter, and a third low-pass filter. The signals collected by the data acquisition module are respectively sent to the first multiplier, the second multiplier, and the third multiplier. The signals of the modulation output module are respectively sent to the first frequency multiplier, the second frequency multiplier, and the first multiplier. The output result obtained by the first frequency multiplier is sent to the second multiplier, and the output result obtained by the second frequency multiplier is sent to the third multiplier. The output results of the first multiplier, the second multiplier, and the third multiplier are respectively sent to the first low-pass filter, the second low-pass filter, and the third low-pass filter. The module for eliminating the influence of modulation depth includes a sine component being sent to a subtractor, a first differentiator, a second differentiator, a fourth multiplier, a fifth multiplier, a sixth multiplier, a seventh multiplier, a first divider and a second divider, the sine components of the measured signal extracted by the first low-pass filter and the third low-pass filter are respectively sent to the subtractor, the sine components calculated by the subtractor are respectively sent to the sixth multiplier, the second differentiator and the fifth multiplier, the cosine components of the measured signal extracted by the second low-pass filter are respectively sent to the sixth multiplier, the first differentiator and the fourth multiplier. The output results of the first differentiator are respectively sent to the seventh multiplier and the fourth multiplier, the output results of the second differentiator are respectively sent to the seventh multiplier and the fifth multiplier, the output results of the fourth multiplier and the fifth multiplier are simultaneously sent to the first divider, and the output results of the sixth multiplier and the seventh multiplier are simultaneously sent to the second divider. The sine component and cosine component after the single-frequency, double-frequency, and triple-frequency mixing filtering are eliminated by the modulation depth elimination module through subtraction, self-differentiation multiplication, and division operations to eliminate the influence of light intensity disturbance and modulation depth drift; The modulation depth calculation module includes a first power reduction module, and the output result of the first divider is subjected to a power reduction operation by the first power reduction module to obtain the modulation depth value of the current system; The phase solution module includes a second power reduction module, a sign judgment module and an integrator. The second power reduction module performs an inversion and power reduction operation on the output signal of the second divider to obtain a phase signal differential value with a positive sign; then, the sign judgment module judges the sign of the phase signal differential value based on the signs of the sine component output by the first low-pass filter and the cosine component output by the second low-pass filter as conditions, and then obtains the phase signal through the integrator.

2. The carrier demodulation system based on eliminating the influence of modulation depth by the third harmonic according to claim 1 is characterized in that The modulation frequency of the light source modulator is between 2kHz and 50MHz, and the modulation amplitude is set within the range of 1 to 6rad.

3. The carrier demodulation system based on eliminating the influence of modulation depth by the third harmonic according to claim 1 is characterized in that The cutoff frequencies of the first low-pass filter, the second low-pass filter, and the third low-pass filter are selected between 1 kHz and 25 MHz according to the frequency of the carrier signal.

4. The carrier demodulation system based on eliminating the influence of modulation depth by the third harmonic according to claim 1 is characterized in that The interference signal output by the interferometer module is: Where: I1 and I2 are the light intensities of the two arms of the interferometer module respectively; θ(t) is the optical path difference; A is the DC component of the light intensity, B is the AC component of the light intensity, C is the phase modulation depth, ω0 is the carrier signal frequency, is the signal to be measured.

5. The carrier demodulation system based on eliminating the influence of modulation depth by the third harmonic according to claim 1 is characterized in that The sign determination module determines the sign of the differential value of the phase signal in the following manner: the sign of the tangent value of the phase signal is determined using I1(t) / I2(t) as a condition. If I1(t) / I2(t)<0, the sign is negative, otherwise it is positive, where: , G and H are the amplitudes of the detection wave signal, B is the AC intensity, and C is the modulation depth. is the signal to be measured, J k (C) is the Bessel function coefficient, k is the high-order component of the signal, and I1(t) and I2(t) are the components containing phase-shifted signals obtained by the mixing and filtering module.

6. A carrier demodulation method for eliminating the influence of modulation depth based on the third harmonic using the carrier demodulation system according to any one of claims 1 to 5, characterized in that The method comprises the following steps: Step 1: Using the data acquisition module to collect the interference signal output by the interferometer module, the interference signal is converted from an optical signal to an electrical signal after photoelectric conversion; The modulation output module outputs a sine wave to the light source modulator for modulating the light source, and the modulated light is injected into the interferometer module; Step 2: Send the signal of the modulation output module to the first frequency multiplier, the second frequency multiplier, and the first multiplier respectively; send the output result of the first frequency multiplier to the second multiplier; send the output result of the second frequency multiplier to the third multiplier; send the output results of the first multiplier, the second multiplier, and the third multiplier to the first low-pass filter, the second low-pass filter, and the third low-pass filter respectively; Step 3: The sine components of the measured signal extracted by the first low-pass filter and the third low-pass filter are respectively input into the subtractor, and the sine components calculated by the subtractor are respectively input into the sixth multiplier, the second differentiator, and the fifth multiplier. The cosine component of the measured signal extracted by the second low-pass filter is respectively input into the sixth multiplier, the first differentiator, and the fourth multiplier. The output results of the first differentiator are respectively input into the seventh multiplier and the fourth multiplier. The output results of the second differentiator are respectively input into the seventh multiplier and the fifth multiplier. The output results of the fourth multiplier and the fifth multiplier are simultaneously input into the first divider, and the output results of the sixth multiplier and the seventh multiplier are simultaneously input into the second divider. Step 4: Send the output result of the first divider to the first power reduction module for power reduction operation to obtain the modulation depth value of the current system; Step 5. Send the output result of the second divider to the second power reduction module, perform inversion and power reduction operations, and obtain a phase signal differential value with a positive sign; then pass through the sign judgment module, use the signs of the sine component output by the first low-pass filter and the cosine component output by the second low-pass filter as conditions to judge the sign of the phase signal differential value, and then obtain the phase signal through the integrator.

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