CN116519028A - Carrier demodulation method based on third harmonic elimination modulation depth influence - Google Patents
Carrier demodulation method based on third harmonic elimination modulation depth influence Download PDFInfo
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Abstract
The invention discloses a carrier demodulation method based on third harmonic elimination modulation depth influence, which is characterized in that a modulation depth influence elimination module eliminates the influence of light intensity disturbance and modulation depth drift by subtracting, multiplying by self-differentiation, dividing by and the like, and then can accurately calculate to-be-calculated phase after power reduction operation and symbol judgment in a phase calculation module, and can accurately calculate to-be-calculated modulation depth value of a current system through a modulation depth calculation module. The invention can eliminate the influence of the modulation depth of the externally added carrier wave on the demodulation signal, and also can eliminate the influence of the light intensity disturbance on the demodulation signal, so that the demodulation result has high signal-to-noise ratio and low harmonic distortion, the accuracy of signal amplitude detection and the stability of a demodulation system are improved, and the invention can be widely applied to high-precision optical fiber measurement and sensing systems.
Description
Technical Field
The invention belongs to the field of optical fiber interferometer phase demodulation algorithms, relates to a carrier wave (PGC) demodulation method, and in particular relates to a carrier wave demodulation method for eliminating modulation depth influence based on third harmonic.
Background
The interference type optical fiber sensor is composed of a light source, a transmission optical fiber, a sensing optical fiber, a modulation unit, a photoelectric detection unit, a demodulation unit and the like. The sensing mechanism of the interference type optical fiber sensor is as follows: external measured physical quantities (such as pressure, acceleration, temperature, displacement and the like) act on the sensing optical fiber, influence the phase, intensity and the like of light, optical signals are modulated, and the change of the measured physical quantities is calculated by the demodulation unit through photoelectric conversion. Compared with the traditional sensor, the sensor has the advantages of electromagnetic interference resistance, high sensitivity, large dynamic range, diversified structure, convenience for large-scale array and networking and the like, and is widely applied to optical fiber seismometers, optical fiber strain gauges and optical fiber hydrophones. Because such sensors are based on the principle of interference of light, the change of the external measured physical quantity is converted into the phase change of the interference signal, and the phase demodulation technology is needed to calculate the measured signal. The phase demodulation technique mainly comprises the following steps: an active homodyne method, a passive homodyne method based on 3*3 coupler, a heterodyne method, a passive homodyne method based on Phase Generated Carrier (PGC), and the like. The PGC passive homodyne method has the advantages of high resolution, strong real-time demodulation capability and the like, and is most widely applied to engineering.
Traditional PGC demodulation algorithms are susceptible to interference from external factors such as drift of the phase modulation depth C value, carrier phase delay, etc. The most classical two PGC algorithms are a PGC algorithm (PGC-DCM) based on cross multiplication and a PGC algorithm based on arctangent, and a PGC-DCM demodulation algorithm adopts a method based on cross differential multiplication, and is related to light intensity, and the stability is poor when the light intensity changes rapidly; the PGC-Arctan demodulation algorithm performs division and arctangent on two paths of signals to achieve demodulation, and the algorithm requires that the modulation depth C is kept at an optimal value of 2.63, and when the modulation depth C is shifted from the optimal value by 2.63, the demodulation result can generate nonlinear change, so that serious harmonic distortion is caused. The university of Qinghua Zhang Min et al conducted intensive studies on noise suppression of a separate fiber optic hydrophone (CN 102359797B) and a multiplexed hydrophone array (CN 102680072B) based on the PGC principle; the seventh and fifth research of China shipping heavy industry group company provides a PGC complex demodulation method (CN 101604957A) for a large-scale optical fiber hydrophone array, which meets the demodulation requirement of the large-scale hydrophone array; the seventh and fifth research institute of middle ship reworking provides a portable multifunctional optical fiber hydrophone signal demodulation method (CN 101615888A), which has the characteristics of low power consumption, portability and multifunction, and provides a reliable and convenient tool for optical fiber hydrophone and array research thereof; northrop Grumman, inc. DavidB.Hall, U.S. Pat. No. 5,087,784 B2, also issued with related patents for array demodulation; in 2017, anton v.volkov et al, university of mechanical and optical research, russian, proposed a PGC demodulation algorithm based on phase modulation depth estimation and correction, which calculated the modulation depth C value by introducing third and fourth harmonics, and then corrected the C value using PI control algorithm (Phase Modulation Depth Evaluation and Correction Technique for the PGC Demodulation Scheme in Fiber-Optic Interferometric Sensors); yang Jun of Harbin engineering university et al in 2019 proposes a Real-time self-calibrating PGC demodulation algorithm, which adopts a method of combining ellipse fitting with PID control, can correct the modulation depth C value to be 2.63, and the number of signal susceptances of an actual system can reach 61.33dB (Real-time self-calibration PGC-Arctan demodulation algorithm in fiber-optic interferometric sensors); in 2021, the university of Zhejiang, ind. Toril et al, proposed a method for compensating nonlinear error of PGC demodulation based on carrier phase delay and phase modulation depth, which solves nonlinear error (Nonlinear Error Compensation of PGC Demodulation With the Calculation of Carrier Phase Delay and Phase Modulation Depth) caused by phase delay and modulation depth in PGC algorithm. However, the algorithm increases the calculation complexity of the system, the time side length required by signal processing is increased, the real-time performance of the system can be influenced, and the system is easy to generate larger harmonic distortion in the large signal demodulation process.
For a demodulation system, under the condition of low cost, the method eliminates the influence of modulation depth and light intensity disturbance, improves the signal to noise ratio, inhibits harmonic distortion and reduces signal distortion, and has very important significance and practical value.
Disclosure of Invention
In order to overcome the defects and shortcomings of the prior art, the invention provides a carrier demodulation method based on eliminating the influence of modulation depth by third harmonic, and the calculation and demodulation of the modulation depth of an additional carrier are realized by loading a third harmonic carrier signal. The invention can eliminate the influence of the modulation depth of the externally added carrier wave on the demodulation signal, and also can eliminate the influence of the light intensity disturbance on the demodulation signal, so that the demodulation result has high signal-to-noise ratio and low harmonic distortion, the accuracy of signal amplitude detection and the stability of a demodulation system are improved, and the invention can be widely applied to high-precision optical fiber measurement and sensing systems.
The invention aims at realizing the following technical scheme:
a carrier demodulation system based on third harmonic elimination modulation depth influence comprises a signal modulation module, a mixing filtering module, a modulation depth influence elimination module, a modulation depth resolving module and a phase resolving module, wherein:
the signal modulation module comprises a synchronous starting module, a data acquisition module and a modulation output module, wherein the synchronous starting module is used for synchronously working the data acquisition module and the modulation output module, the data acquisition module is used for acquiring an interference signal output by the interferometer module, and the interference signal is converted into an electric signal from an optical signal after photoelectric conversion; the modulation output module outputs sine waves to the light source modulator for modulating a light source, and the modulated light is injected into the interferometer module;
the frequency mixing filter module comprises 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, signals acquired by the data acquisition module are respectively sent to the first multiplier, the second multiplier and the third multiplier, signals of the modulation output module are respectively sent to the first frequency multiplier, the second frequency multiplier and the first multiplier, output results obtained by the first frequency multiplier are sent to the second multiplier, output results obtained by the second frequency multiplier are sent to the third multiplier, and 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 modulation depth influence eliminating module comprises a sinusoidal component, a subtracter, 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, wherein the sinusoidal component of a measured signal, which is respectively extracted by the first low-pass filter and the third low-pass filter, is sent to the subtracter, the sinusoidal component calculated by the subtracter is respectively sent to the sixth multiplier, the second differentiator and the fifth multiplier, the cosine component of the measured signal, which is extracted by the second low-pass filter, is respectively sent to the sixth multiplier, the first differentiator and the fourth multiplier, the output result of the first differentiator is respectively sent to the seventh multiplier and the fourth multiplier, the output result of the second differentiator is 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 modulation depth resolving module comprises a first exponentiation module, and the output result of the first divider is subjected to exponentiation operation by the first exponentiation module to obtain a modulation depth value of the current system;
the phase resolving module comprises a second exponentiation module, a symbol judging module and an integrator, wherein the second exponentiation module performs inverse and exponentiation operations on an output signal of the second divider to obtain a phase signal differential value with a positive symbol; and then a symbol judgment module judges the symbol of the differential value of the phase signal by taking the symbols 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 the true tangent value of the phase signal is obtained through an integrator.
The carrier demodulation method based on third harmonic elimination modulation depth influence by utilizing the carrier demodulation system comprises the steps that a modulation depth influence elimination module eliminates the influence of light intensity disturbance and modulation depth drift by subtracting, self-differentiating multiplication, dividing and the like, a power reduction operation and symbol judgment in a phase calculation module can accurately calculate to-be-calculated phase, and a modulation depth value of a current system can be accurately calculated by the modulation depth calculation module, and the method specifically comprises the following steps:
step one, a data acquisition module is used for acquiring interference signals output by an interferometer module, and the interference signals are converted into electric signals from optical signals after photoelectric conversion; outputting a sine wave to a light source modulator by using a modulation output module for modulating a light source, and injecting the modulated light into an interferometer module;
step two, the signals of the modulation output module are respectively sent to a first frequency multiplier, a second frequency multiplier and a first multiplier, the output result obtained by the first frequency multiplier is sent to the second multiplier, the output result obtained by the second frequency multiplier is sent to a third multiplier, and the output results of the first multiplier, the second multiplier and the third multiplier are respectively sent to a first low-pass filter, a second low-pass filter and a third low-pass filter;
step three, the sine components of the measured signals extracted by the first low-pass filter and the third low-pass filter are respectively sent to a subtracter, the sine components calculated by the subtracter are respectively sent to a sixth multiplier, a second differentiator and a fifth multiplier, the cosine components of the measured signals extracted by the second low-pass filter are respectively sent to the sixth multiplier, the first differentiator and the fourth multiplier, the output result of the first differentiator is respectively sent to a seventh multiplier and a fourth multiplier, the output result of the second differentiator is 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 a first divider, and the output results of the sixth multiplier and the seventh multiplier are simultaneously sent to a second divider;
step four, sending the output result of the first divider into a first exponentiation module for exponentiation operation to obtain a modulation depth value of the current system;
step five, sending the output result of the second divider into a second exponentiation module, and performing inverse and exponentiation operations to obtain a differential value of the phase signal with a positive sign; and then a symbol judgment module judges the symbol of the differential value of the phase signal by taking the symbols 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 the true tangent value of the phase signal is obtained through an integrator.
Compared with the prior art, the invention has the following advantages:
(1) The modulation depth C value can be calculated by the modulation depth influence elimination module, so that the modulation depth C value in the current environment of the demodulation system can be monitored in real time;
(2) The signal containing the alternating current intensity B value and the modulation depth C value is eliminated through the modulation depth influence elimination module, so that the output item only contains the phase signal differential value, the influence of light intensity disturbance on the demodulation signal is eliminated, uncertainty of a demodulation result caused by factors such as unstable light source is avoided, the demodulation result has high signal-to-noise ratio and low harmonic distortion, and the accuracy of signal amplitude detection and the stability of a demodulation system are improved;
(3) The nonlinear error is reduced, a distortion item is not formed when the modulation depth generates tiny offset due to environmental change, harmonic waves can be effectively restrained, dependence of a demodulation result on the modulation depth introduced by external loading waves is eliminated, the demodulation result has high signal-to-noise ratio and low harmonic distortion, and the accuracy of signal amplitude detection and the stability of a demodulation system are improved;
(4) The method has low calculation complexity and good compatibility with the system, and can be widely applied to high-precision optical fiber measurement and sensing systems.
Drawings
FIG. 1 is a flow chart of a carrier demodulation algorithm based on third harmonic cancellation modulation depth effects;
FIG. 2 is a diagram of an interferometric modem probe optical path arrangement;
FIG. 3 is a waveform of a demodulated signal based on a carrier demodulation algorithm with third harmonic cancellation modulation depth effects;
FIG. 4 is a plot of the harmonic distortion of the demodulated signal after algorithm modification;
FIG. 5 is a comparison of modulation depth C versus signal demodulation amplitude before and after algorithm improvement;
fig. 6 is a comparison of the alternating current intensity B of the interference signal versus the amplitude of the signal demodulation before and after the algorithm improvement.
Detailed Description
The following description of the present invention is provided with reference to the accompanying drawings, but is not limited to the following description, and any modifications or equivalent substitutions of the present invention should be included in the scope of the present invention without departing from the spirit and scope of the present invention.
The invention provides a carrier demodulation system based on third harmonic elimination modulation depth influence, as shown in fig. 1, the carrier demodulation system comprises a signal modulation module, a mixing filter module, a modulation depth influence elimination module, a modulation depth resolving module and a phase resolving module, wherein interference signals acquired by the signal modulation module 10 sequentially pass through the mixing filter module 11, the modulation depth influence elimination module 12, the modulation depth resolving module 13 and the phase resolving module 14, and finally phase demodulation signals and modulation depth values are output, wherein:
the signal modulation module 10 comprises a synchronous starting module 101, a data acquisition module 102 and a modulation output module 103, wherein the data acquisition module 102 is used for acquiring an interference signal output by the interferometer module 22, and the interference signal is converted into an electric signal from an optical signal after photoelectric conversion; the modulation output module 103 outputs sine waves to the light source modulator 232 for modulating the light source 211, the modulated light is injected into the interferometer module 22, wherein the modulation frequency of the light source modulator 232 is 2 kHz-50 MHz, and the modulation amplitude is set within the range of 1-6 rad to ensure the stability of interference fringes;
the mixing filtering module 11 includes a first multiplier 111, a second 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 collecting module 102 are respectively sent to the first multiplier 113, the second multiplier 114, and the third multiplier 115, the signals of the modulating output module 103 are respectively sent to the first multiplier 111, the second multiplier 112, and the first multiplier 113, the output result obtained by the first multiplier 111 is sent to the second multiplier 114, the output result obtained by the second multiplier 112 is sent to the third multiplier 115, the output result obtained by the first multiplier 113, the second multiplier 114, and the third multiplier 115 are respectively sent to the first low-pass filter 116, the second low-pass filter 117, and the third low-pass filter 118, and the cut-off frequencies of the first low-pass filter 116, the second low-pass filter 117, and the third low-pass filter 118 are selected between 1kHz and 25MHz according to the carrier signal frequency;
the modulation depth influence eliminating module 12 includes a sinusoidal component sending 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 sinusoidal component of the measured signal respectively extracted by the first low-pass filter 116 and the third low-pass filter 118 sending to the subtractor 121, the sinusoidal component calculated by the subtractor 121 sending to the sixth multiplier 126, the second differentiator 123 and the fifth multiplier 125, the cosine component of the measured signal respectively extracted by the second low-pass filter 117 sending to the sixth multiplier 126, the first differentiator 122 and the fourth multiplier 124, the output result of the first differentiator 122 sending to the seventh multiplier 127 and the fourth multiplier 124, the output result of the second differentiator 123 sending to the seventh multiplier 127 and the fifth multiplier 125, respectively, the output result of the fourth multiplier 124 and the fifth multiplier 125 sending to the first divider 128, the output result of the sixth multiplier 124 and the seventh multiplier 125 sending to the second multiplier 129 simultaneously to obtain a signal having a square difference value only, and the disturbance depth influence eliminating the light intensity drift can be calculated by the square difference value;
the modulation depth resolving module 13 comprises a first exponentiation module 131, and the output result of the first divider 128 is subjected to exponentiation operation by the first exponentiation module 131 to obtain a modulation depth value of the current system;
the phase resolving module 14 includes a second exponentiation module 141, a symbol judging module 142 and an integrator 144, where the second exponentiation module 141 performs inverse and exponentiation operations on the output signal of the second divider 129 to obtain a differential value of the phase signal with a positive symbol; then, the sign of the differential value of the phase signal is determined by the sign determining module 142, and the true tangent value of the phase signal is obtained by the integrator 144, under the condition that 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.
The invention also provides a carrier demodulation method based on third harmonic elimination modulation depth influence by utilizing the carrier demodulation system, which comprises the steps of eliminating the influence of light intensity disturbance and modulation depth drift by subtracting, self-differentiating multiplication, dividing and the like through the sine component and the cosine component after frequency multiplication, frequency doubling and frequency tripling mixing filtering by the elimination modulation depth influence module, accurately calculating to obtain a phase to be calculated after power reduction operation and symbol judgment in the phase calculation module, and accurately calculating to obtain the modulation depth value of the current system by the modulation depth calculation module. The method is an algorithm improvement of a Phase Generation Carrier (PGC) demodulation algorithm, and the improved algorithm principle is shown in figure 1:
the signal modulation module 10 includes a data acquisition module 102 and a modulation output module 103, where 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 wave to add a change to the optical phaseModulated light injection into interferometer modules22; the data acquisition module 102 is configured to acquire the result of the interferometer module 22 after the photoelectric conversion is completed, and obtain an interference signal as formula (1):
wherein: i 1 And I 2 The light intensities of the two arms of the interferometer module are respectively; θ (t) is the optical path difference; a is the direct current component of the light intensity, B is the alternating current component of the light intensity, C is the phase modulation depth omega 0 For the carrier signal frequency,is the signal to be measured.
Expanding the interference signal obtained in the formula (1) by using a Bessel function to obtain a frequency spectrum component of the interference signal:
wherein A is the DC intensity of the output signal, B is the AC intensity, C is the modulation depth, omega 0 As a function of the carrier frequency,for the signal to be measured, J k (C) As Bessel function coefficient, k is higher order component of signal, and the amplitude of each side frequency component around zero frequency is proportional to J k (C) A. The invention relates to a method for producing a fibre-reinforced plastic composite The greater the C value, J k (C) The slower the speed towards zero, and the smaller value of C is beneficial to reduce the system phase noise.
Three components containing the phase-shifted signal are obtained by the mixing filter module 11:
in an actual system, G, H, K is the amplitude of the detected wave signal, and g=h=k, and the parameter B, C drift due to an external environment change or instability inside the system.
The three components are operated by the modulation depth influence eliminating module 12, so that the purposes of eliminating the light intensity disturbance and the modulation depth drift influence are achieved, and the specific implementation process is as follows:
according to the Bessel coefficient relation:
the following relationship can be obtained:
an intermediate variable relationship can be obtained:
the square value of the modulation depth square value and the square value of the phase differential to be measured can be calculated through a series of operations of the first differentiator 122, the second differentiator 123, the fourth multiplier 124, the fifth multiplier 125, the sixth multiplier 126, the seventh multiplier 127, the first divider 128 and the like, and the expressions are as follows:
the modulation depth square value is operated by the modulation depth resolving module 13, so that the modulation depth value 132 in the current system can be calculated.
The square value of the above-mentioned phase differential to be measured is subjected to the operation of the phase resolving module 14. The symbol judgment module 142 performs the following operations on the orthogonal components (3) and (4): i is as follows 1 (t)/I 2 (t) determining the sign of the phase signal tangent value as a condition, if I 1 (t)/I 2 (t)<0, thenThe signal sign of the phase signal is negative, otherwise, the signal sign is positive, and then the real differential value of the phase signal is obtained; the result is then fed to an integrator 144 to obtain PGC demodulation result 145. The result does not contain an alternating current intensity B value and a modulation depth C value, and is not influenced by light intensity disturbance and modulation depth drift.
Examples:
the present embodiment provides a carrier demodulation method based on a mach zehnder interferometer, an interferometer modem device is shown in fig. 2, and includes a light source module 21, an interferometer module 22, a photoelectric acquisition circuit 20 and a data processing module 23, where 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 loop 223, a second fiber loop 224, and 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 measuring apparatus are as follows:
(1) The input light source 211 is an ASE broadband light source with a central wavelength of 1550nm, a half spectral width of more than 45nm and a power of 10mW.
(2) The working wavelength of the optical fiber isolator 212 is 1550nm plus or minus 5nm, the insertion loss is less than or equal to 1.0dB (at the working temperature of 23 ℃), and the return loss is more than or equal to 55dB.
(3) The working wavelength of the first coupler 221 is 1550nm, and the spectral ratio is 50%/50%; the second coupler 225 operates at a wavelength of 1550nm and a 50%/50% spectral ratio.
(4) The working wavelength of the first optical fiber ring 223 is 1550nm, the ring crosstalk is < -18dB, the ring attenuation is less than 0.3dB/km, the ring inner diameter is 50mm, the ring outer diameter is 80mm, and the optical fiber length is 300m; the working wavelength of the second optical fiber ring 224 is 1550nm, the ring crosstalk is < -18dB, the ring attenuation is less than 0.3dB/km, the ring inner diameter is 50mm, the ring outer diameter is 80mm, and the optical fiber length is 300m.
(5) The phase modulator 222 is a cylindrical piezoelectric ceramic ring wound with an optical fiber, the resonance frequency is 2000Hz, the resonance resistance is less than 200 ohms, the capacitance is 50 nF+/-30%, the thickness of the ring is 1mm, the height of the ring is 10mm, the outer diameter of the ring is 20mm, and the piezoelectric ceramic ring is wound with an optical fiber with the length of 1m and is bonded by using strong glue.
(6) The photoelectric detection module 20 has two photoelectric detectors for differential detection. The detector is an InGaAs type photoelectric detector, the connection mode belongs to a fiber-pigtail type FC/PC, the working wavelength is 1100 nm-1650 nm, the light intensity responsivity R=0.85A/W, and the capacitance is 0.35pF.
(7) The acquisition module 236 is an NI-7856R acquisition card, the sampling rate is 4Mbps, the input voltage amplitude is +/-10V, and the sampling clock is the internal clock of the acquisition card.
The specific flow of the algorithm is as follows:
(1) The system operates the signal modulation module 10, first, the computer 234 controls the light source modulator 232 through the second data transmission line 233, then the first data transmission line 231 modulates the frequency of the light source 221, and sets a carrier signal with amplitude of 2.6rad and frequency of 5kHz, and the carrier signal does not change along with factors such as environmental transformation; meanwhile, a calibration signal with the amplitude of 1rad and the frequency of 400Hz 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, the optical signal is divided into two paths after passing through the first coupler 221, one path of optical signal passes through an optical path with the phase modulator 222 and the first optical fiber ring 223, the other path of optical signal passes through the optical fiber ring 224, the two paths of optical signals interfere at the second coupler 225, differential detection is carried out through the photoelectric detection module 20, the differential interference electric signal is converted into a differential interference electric signal, the differential interference electric signal is transmitted to the data acquisition module 236 through the second electric wire 237, and then is transmitted to the computer 234 through the second data transmission wire 235, and carrier phase demodulation is carried out.
(2) The data acquisition module 102 obtains an interference signal containing a dc offset, the peak-to-peak value of the signal is 4V, and the dc offset is about 2V.
(3) The interference signals are subjected to mixing filtering operation, the first filter 114 and the second filter 115 are set as FIR Blackman windows, parameters are passband cut-off frequency 2kHz, stopband cut-off frequency 3kHz, attenuation-80 dB, passband ripple is 0.01dB, order is 265 th order, and three paths of signals are obtained after the data pass through the filters.
(4) The two signals are passed through the cancel modulation depth influence module 12, where the influence of the PGC algorithm due to the ac intensity B and drift of the modulation depth C caused by external environmental changes or instability during the system interior is cancelled.
(5) The signal is sent to the modulation depth resolving module 13 to perform corresponding operation to obtain the tangent value of the phase signal with positive sign, and the sign judging module 133 uses the signs of the sine component 121 and the cosine component 122 as judging conditions to judge the tangent value to obtain the true tangent value of the phase signal.
(6) The signals are sent to the phase resolving module 14 to perform corresponding operation to obtain differential values of phase signals with positive signs, and the sign judging module 142 performs sign judgment on the tangent values by taking the signs of the sine components output by the first low-pass filter 116 and the cosine components output by the second low-pass filter 117 as conditions to obtain real differential values of the phase signals; the differential value of the signal is solved for the final result by the integrator 144, resulting in the phase demodulation result 145.
Fig. 3 shows a demodulation output waveform for a signal having a frequency of 400 Hz.
Fig. 4 shows the harmonic distortion of the demodulated signal after the algorithm is improved, and fig. 4 shows that: the harmonic distortion value is-88.25 dB, and the harmonic suppression effect is good.
Fig. 5 shows the comparison of the signal demodulation result of the original PGC-Arctan algorithm and the modified PGC algorithm by changing the modulation depth C. As can be seen from fig. 5: when the modulation depth C is changed from 1rad to 3.5rad, the signal amplitude obtained by the original PGC demodulation algorithm is changed along with the change of the modulation depth, when the C value is shifted to 1rad, the demodulation result of the signal amplitude is changed to be more than 0.4rad, when the C value is shifted to 3.5rad, the demodulation result of the signal amplitude is changed to be more than 0.55rad, and when the C value is changed to be 3.5rad, the signal amplitude obtained by the improved PGC demodulation algorithm is changed to be less than 0.2rad.
Fig. 6 shows the comparison of the alternating current intensity B of the interference signal before and after improvement with the demodulation amplitude of the signal. As can be seen from fig. 6: when the alternating current intensity B value is changed from 1rad to 3.5rad, the amplitude change of the signal obtained by the improved PGC demodulation algorithm is smaller than 0.01rad.
Claims (6)
1. The carrier demodulation system based on the third harmonic elimination modulation depth influence is characterized by comprising a signal modulation module, a mixing filtering module, a modulation depth influence elimination module, a modulation depth resolving module and a phase resolving module, wherein:
the signal modulation module comprises a synchronous starting module, a data acquisition module and a modulation output module, wherein the synchronous starting module is used for synchronously working the data acquisition module and the modulation output module, the data acquisition module is used for acquiring an interference signal output by the interferometer module, and the interference signal is converted into an electric signal from an optical signal after photoelectric conversion; the modulation output module outputs sine waves to the light source modulator for modulating a light source, and the modulated light is injected into the interferometer module;
the frequency mixing filter module comprises 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, signals acquired by the data acquisition module are respectively sent to the first multiplier, the second multiplier and the third multiplier, signals of the modulation output module are respectively sent to the first frequency multiplier, the second frequency multiplier and the first multiplier, output results obtained by the first frequency multiplier are sent to the second multiplier, output results obtained by the second frequency multiplier are sent to the third multiplier, and 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 modulation depth influence eliminating module comprises a sinusoidal component, a subtracter, 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, wherein the sinusoidal component of a measured signal, which is respectively extracted by the first low-pass filter and the third low-pass filter, is sent to the subtracter, the sinusoidal component calculated by the subtracter is respectively sent to the sixth multiplier, the second differentiator and the fifth multiplier, the cosine component of the measured signal, which is extracted by the second low-pass filter, is respectively sent to the sixth multiplier, the first differentiator and the fourth multiplier, the output result of the first differentiator is respectively sent to the seventh multiplier and the fourth multiplier, the output result of the second differentiator is 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 modulation depth resolving module comprises a first exponentiation module, and the output result of the first divider is subjected to exponentiation operation by the first exponentiation module to obtain a modulation depth value of the current system;
the phase resolving module comprises a second exponentiation module, a symbol judging module and an integrator, wherein the second exponentiation module performs inverse and exponentiation operations on an output signal of the second divider to obtain a phase signal differential value with a positive symbol; and then a symbol judgment module judges the symbol of the differential value of the phase signal by taking the symbols 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 the true tangent value of the phase signal is obtained through an integrator.
2. The carrier demodulation system based on the third harmonic cancellation modulation depth influence according to claim 1, wherein the modulation frequency of the light source modulator is 2 kHz-50 MHz, and the modulation amplitude is set in the range of 1-6 rad.
3. The carrier demodulation system based on third harmonic cancellation modulation depth influence according to claim 1, wherein the cut-off frequencies of the first, second and third low pass filters are selected between 1kHz and 25MHz according to the carrier signal frequency.
4. The carrier demodulation system based on third harmonic cancellation modulation depth effect according to claim 1, wherein the interference signal output by the interferometer module is:
wherein: i 1 And I 2 The light intensities of the two arms of the interferometer module are respectively; θ (t) is the optical path difference; a is the direct current component of the light intensity, B is the alternating current component of the light intensity, C is the phase modulation depth omega 0 For the carrier signal frequency,is the signal to be measured.
5. The carrier demodulation system based on third harmonic cancellation modulation depth effect according to claim 1, wherein the symbol judgment module judges the symbol of the phase signal differential value as follows: i is as follows 1 (t)/I 2 (t) determining the sign of the phase signal tangent value as a condition, if I 1 (t)/I 2 (t)<0, the sign takes negative, otherwise positive, wherein:G. h, K the amplitude of the detected wave signal, B is the AC intensity, C is the modulation depth, ++>For the signal to be measured, J k (C) Is a Bessel function coefficient, and k is a higher-order component of the signal.
6. A carrier demodulation method based on third harmonic cancellation modulation depth effects by using the carrier demodulation system according to any one of claims 1-5, characterized in that the method comprises the steps of:
step one, a data acquisition module is used for acquiring interference signals output by an interferometer module, and the interference signals are converted into electric signals from optical signals after photoelectric conversion; outputting a sine wave to a light source modulator by using a modulation output module for modulating a light source, and injecting the modulated light into an interferometer module;
step two, the signals of the modulation output module are respectively sent to a first frequency multiplier, a second frequency multiplier and a first multiplier, the output result obtained by the first frequency multiplier is sent to the second multiplier, the output result obtained by the second frequency multiplier is sent to a third multiplier, and the output results of the first multiplier, the second multiplier and the third multiplier are respectively sent to a first low-pass filter, a second low-pass filter and a third low-pass filter;
step three, the sine components of the measured signals extracted by the first low-pass filter and the third low-pass filter are respectively sent to a subtracter, the sine components calculated by the subtracter are respectively sent to a sixth multiplier, a second differentiator and a fifth multiplier, the cosine components of the measured signals extracted by the second low-pass filter are respectively sent to the sixth multiplier, the first differentiator and the fourth multiplier, the output result of the first differentiator is respectively sent to a seventh multiplier and a fourth multiplier, the output result of the second differentiator is 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 a first divider, and the output results of the sixth multiplier and the seventh multiplier are simultaneously sent to a second divider;
step four, sending the output result of the first divider into a first exponentiation module for exponentiation operation to obtain a modulation depth value of the current system;
step five, sending the output result of the second divider into a second exponentiation module, and performing inverse and exponentiation operations to obtain a differential value of the phase signal with a positive sign; and then a symbol judgment module judges the symbol of the differential value of the phase signal by taking the symbols 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 the true tangent value of the phase signal is obtained through an integrator.
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