1. A demodulation method for detecting gas-solid interface sound waves by a sinusoidal phase modulation laser interferometer is characterized by comprising the following steps:

suppose that the carrier modulation depth of the phase modulation is C and the carrier frequency is omega_cAnd the carrier wave is a sine wave;

obtaining interference signal I according to the principle of Michelson interferometer_s(t)；

Using Bessel identity and trigonometric function formula to convert the interference signal I_s(t) decomposing such that the interference signal is decomposed into a sum of the harmonic terms;

according to an FFT algorithm, the amplitude of each frequency component in the interference signal can be obtained;

according to the frequency N omega in the interference signal_n、(Nω_n+ω_c)、(Nω_n+2ω_c) And (N ω)_n+3ω_c) Defining an attenuation ratio as a function of the carrier modulation depth C;

determining the value of the modulation depth C by a table look-up method;

the interference signal I_sAnd (t) filtering the direct current component by an alternating current coupling sampling method, mixing a frequency doubling carrier signal and a frequency doubling carrier signal with the interference signal, and performing low-pass filtering to obtain an orthogonal interference signal pair:

a frequency-doubled carrier signal and the interference signal I with 90 degree phase shift_s(t) obtaining signals after frequency mixing and low-pass filtering;

the carrier phase delay theta can be obtained from the signal and the quadrature interference signal pair_cA value of (d);

according to the obtained value of the modulation depth C and the carrier phase delay theta_cFor said pair of quadrature interference signalsNormalization processing is carried out to obtain the visibility coefficient I of the interference fringes₁：

The carrier phase delay theta is determined according to the value of the modulation depth C_cAnd the interference fringe visibility coefficient I₁To obtain the phase difference related to the vibration of the measured point

Phase difference related to vibration of measured pointAnd (4) carrying out band-pass filtering to obtain gas-solid interface sound waves.

2. The demodulation method for detecting the acoustic wave at the gas-solid interface by the sinusoidal phase modulation laser interferometer of claim 1, wherein the interference signal I is obtained according to the principle of Michelson interferometer_s(t) is:

wherein, I₀Is equal to the DC component; i is₁Is the amplitude of the alternating current component of the interference signal; i is₁/I₀Is the interference fringe visibility;the phase difference is related to the vibration of the measured point.

3. The demodulation method for detecting the acoustic wave at the gas-solid interface by the sinusoidal phase modulation laser interferometer of claim 2, wherein the interference signal I is obtained by using Bessel's identity and trigonometric function formula_s(t) decomposing such that the sum of the harmonic terms of the interference signal is:

wherein J represents a Bessel function, J_nIs a Bessel function of order n;

using Bessel identity to compareAndand further decomposing, so that the interference signal is decomposed into the sum of each harmonic term, and the amplitude of the component with the frequency of omega in the interference signal is recorded as A (omega).

4. The demodulation method for detecting the gas-solid interface acoustic wave by the sinusoidal phase modulation laser interferometer of claim 3, wherein the frequency in the interference signal is N ω_nAnd its component amplitude through 2 omega_cThe amplitude after frequency shifting is:

wherein, C_nModulation depth, C, for low frequency environmental disturbances in the interference signal_n＝2kA_n；C_aThe phase modulation depth, C, of the gas-solid interface sound wave in the interference signal_a＝2kD_a；

The frequency of the interference signal is N omega_n+ω_cAnd its component amplitude through 2 omega_cThe amplitude after frequency shifting is:

the frequency of the interference signal is N omega_n、(Nω_n+ω_c)、(Nω_n+2ω_c) And (N ω)_n+3ω_c) Effective frequency ofThe number of rate components is N_eThen, then

Attenuation ratio R_a2Comprises the following steps:

5. the demodulation method for detecting the gas-solid interface acoustic wave by the sinusoidal phase modulation laser interferometer of claim 4, wherein the quadrature interference signal pair:

wherein, theta_cIs the carrier phase delay.

6. The demodulation method for detecting the gas-solid interface acoustic wave by the sinusoidal phase modulation laser interferometer of claim 5, wherein the signal:

so that theta is_cComprises the following steps:

7. the demodulation method for detecting the gas-solid interface acoustic wave by the sinusoidal phase modulation laser interferometer of claim 6, wherein the step of normalizing the quadrature interference signal pair comprises:

dividing Q (t) signal by J in the quadrature interference signal pair₁(C)cosθ_cI (t) signal divided by J₂(C)cos2θ_cAnd further obtaining a processed orthogonal interference signal pair:

then

8. The demodulation method for detecting the gas-solid interface acoustic wave by the sinusoidal phase modulation laser interferometer of claim 7, wherein the 2kD gas-solid interface acoustic wave to be detected can be extracted by high-pass filtering_aThe value of (t).

Background

In the current technical development, the laser interference technology is the optimal non-contact gas-solid interface acoustic wave detection means. The amplitude of the sound wave at the gas-solid interface is very weak, often only a few nanometers. To realize the resolution of the measured displacement nanometer, phase demodulation of interference signals is necessary; to achieve phase demodulation of the interference signals, quadrature interference signal pairs must be obtained. There are currently 3 methods of acquiring quadrature interference signal pairs: the output signal of the heterodyne interferometer is obtained by processing through a specific algorithm; the single-frequency laser interferometer is obtained by light splitting through a polarizing device; the signal is obtained by processing the output signal of the sine phase modulation interferometer through a specific algorithm. The first two methods suffer from device polarization leakage, which causes phase demodulation to produce non-linear errors that are difficult to eliminate. More and more researchers are focusing on sinusoidal phase modulation single-frequency laser interferometers, and phase demodulation of interference signals can be achieved by utilizing a PGC-DCM algorithm or a PGC-Arctan algorithm. However, since the solid interface is not an ideal reflecting surface, it causes power fluctuation of the interference signal intensity, which causes amplitude fluctuation of the interference signal pair, so that the non-linear error of the demodulation result caused by the phase modulation depth and the carrier phase delay is large.

Disclosure of Invention

Aiming at the defects existing in the problems, the invention provides a demodulation method for detecting gas-solid interface sound waves by a sinusoidal phase modulation laser interferometer.

In order to achieve the above object, the present invention provides a demodulation method for detecting a gas-solid interface acoustic wave by a sinusoidal phase modulation laser interferometer, comprising:

suppose that the carrier modulation depth of the phase modulation is C and the carrier frequency is omega_cAnd the carrier wave is a sine wave;

obtaining interference signal I according to the principle of Michelson interferometer_s(t)；

Using Bessel identity and trigonometric function formula to convert the interference signal I_s(t) decomposing such that the interference signal is decomposed into a sum of the harmonic terms;

according to an FFT algorithm, the amplitude of each frequency component in the interference signal can be obtained;

according to the frequency N omega in the interference signal_n、(Nω_n+ω_c)、(Nω_n+2ω_c) And (N ω)_n+3ω_c) Defining an attenuation ratio as a function of the carrier modulation depth C;

determining the value of the modulation depth C by a table look-up method;

the interference signal I_sAnd (t) filtering the direct current component by an alternating current coupling sampling method, mixing a frequency doubling carrier signal and a frequency doubling carrier signal with the interference signal, and performing low-pass filtering to obtain an orthogonal interference signal pair:

a frequency-doubled carrier signal and the interference signal I with 90 degree phase shift_s(t) obtaining signals after frequency mixing and low-pass filtering;

the carrier phase delay theta can be obtained from the signal and the quadrature interference signal pair_cA value of (d);

according to the obtained value of the modulation depth C and the carrier phase delay theta_cThe orthogonal interference signal pair is normalized to obtain the visibility coefficient I of the interference fringe₁：

The carrier phase delay theta is determined according to the value of the modulation depth C_cAnd the interference fringe visibility coefficient I₁To obtain the phase difference related to the vibration of the measured point

Phase difference related to vibration of measured pointAnd (4) carrying out band-pass filtering to obtain gas-solid interface sound waves.

Preferably, the interference signal I is obtained according to the principle of the michelson interferometer_s(t) is:

wherein, I₀Is equal to the DC component; i is₁Is the amplitude of the alternating current component of the interference signal; i is₁/I₀Is the interference fringe visibility;the phase difference is related to the vibration of the measured point.

Preferably, the interference signal I is calculated using Bessel's identity and trigonometric function_s(t) decomposing such that the sum of the harmonic terms of the interference signal is:

wherein J represents a Bessel function, J_nIs a Bessel function of order n;

using Bessel identity to compareAndand further decomposing, so that the interference signal is decomposed into the sum of each harmonic term, and the amplitude of the component with the frequency of omega in the interference signal is recorded as A (omega).

Preferably, the frequency of the interference signal is N ω_nAnd its component amplitude through 2 omega_cThe amplitude after frequency shifting is:

wherein, C_nModulation depth, C, for low frequency environmental disturbances in the interference signal_n＝2kA_n；C_aThe phase modulation depth, C, of the gas-solid interface sound wave in the interference signal_a＝2kD_a；

The frequency of the interference signal is N omega_n+ω_cAnd its component amplitude through 2 omega_cThe amplitude after frequency shifting is:

the frequency of the interference signal is N omega_n、(Nω_n+ω_c)、(Nω_n+2ω_c) And (N ω)_n+3ω_c) Has an effective frequency component number of N_eThen, then

Attenuation ratio R_a2Comprises the following steps:

preferably, the pair of quadrature interference signals:

wherein, theta_cIs the carrier phase delay.

Preferably, the signal:

so that theta is_cComprises the following steps:

preferably, the step of normalizing the pair of quadrature interference signals comprises:

to what is neededThe Q (t) signal of the quadrature interference signal pair is divided by J₁(C)cosθ_cI (t) signal divided by J₂(C)cos2θ_cAnd further obtaining a processed orthogonal interference signal pair:

then

Preferably, the 2kD of the detected gas-solid interfacial acoustic wave can be extracted by high-pass filtering_aThe value of (t).

Compared with the prior art, the invention has the beneficial effects that:

the method can accurately calculate the carrier phase modulation degree and the phase delay, and the algorithm obtains theta through a carrier phase delay estimation algorithm_cAnd obtaining a modulation depth C through a phase modulation depth estimation algorithm, and carrying out pre-normalization processing on the orthogonal interference signal pair by utilizing the carrier phase delay and the phase modulation depth, so that the nonlinear error of a demodulation result caused by the phase modulation depth and the carrier phase delay is obviously reduced.

Drawings

FIG. 1 is a flow diagram of a demodulation method of the present invention;

fig. 2 is a graph of modulation depth C versus attenuation ratio in the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.

Referring to fig. 1, the invention provides a demodulation method for detecting gas-solid interface sound waves by a sinusoidal phase modulation laser interferometer, which comprises the following steps:

suppose that the carrier modulation depth of the phase modulation is C and the carrier frequency is omega_cAnd the carrier wave is a sine wave;

obtaining interference signal I according to the principle of Michelson interferometer_s(t) is:

wherein, I₀Is a direct current component; i is₁Is the amplitude of the alternating current component of the interference signal; i is₁/I₀Is the interference fringe visibility;phase difference related to vibration of a measured point;

demodulating interference signals I_s(t) obtaining:

wherein k is the wave number; a. the_nAmplitude of sound wave at gas-solid interface; omega_nThe angular frequency of the gas-solid interface sound wave;is the initial phase of the gas-solid interface sound wave;is the initial optical path difference of the reference arm and the measuring arm; d_a(t) particle displacement caused by gas-solid interface sound waves;

the 2kD of the measured gas-solid interface sound wave can be extracted by high-pass filtering_aThe value of (t).

Phase difference related to vibration of measured pointAfter band-pass filtering, gas-solid can be obtainedInterfacial acoustic waves;

wherein interference signal I is obtained by using Bessel identity equation and trigonometric function formula_s(t) decomposition into:

wherein J represents a Bessel function, J_nIs a Bessel function of order n; direct current component I₀Filtering by an alternating current coupling sampling method;

using Bessel identity to compareAndfurther decomposing to enable the interference signal to be decomposed into the sum of each harmonic term, and recording the amplitude of the component with the frequency of omega in the interference signal as A (omega);

frequency of interference signal is N omega_nAnd its component amplitude through 2 omega_cThe amplitude after frequency shifting is:

wherein, C_nModulation depth, C, for low frequency environmental disturbances in interference signals_n＝2kA_n；C_aFor modulating the phase depth, C, of the gas-solid interface acoustic wave in the interference signal_a＝2kD_a；

Frequency of interference signal is N omega_n+ω_cAnd its component amplitude through 2 omega_cThe amplitude after frequency shifting is:

frequency of interference signal is N omega_n、(Nω_n+ω_c)、(Nω_n+2ω_c) And (N ω)_n+3ω_c) Has an effective frequency component number of N_eThen, then

Attenuation ratio R_a2Comprises the following steps:

attenuation ratio R_a1Comprises the following steps:

attenuation ratio R_a1And R_a2Is a function of C with respect to the modulation depth of the carrier, and the value of the attenuation ratio can be obtained by inverting the modulation depth C from the functional relationship. That is, the amplitude of the low-frequency component in the interference signal is regularly attenuated or increased after the carrier frequency shift, and the proportion of the attenuation or increase is determined by the carrier phase modulation depth. Further analysis shows that even if the phase of the interference signal contains more low-frequency vibration, the low-frequency component of the interference signal still meets the rule after the carrier frequency shift. As shown in FIG. 2, the attenuation ratio R_a1、R_a2With respect to the curve of the modulation depth C, the function R can be seen from FIG. 1_a1(C) And R_a2(C) Not a monotonic function, R_a1(C) In the interval (0, 2.4050)]Intrinsic existence of an inverse function, R_a2(C) In the interval (0, 3.8320)]An inverse function is present. Function R_a1(C) And R_a2(C) The inverse function of (2) is difficult to find an analytic form, so that a table look-up method is generally adopted to determine the value of the modulation depth C after the attenuation ratio is obtained;

in engineering, the amplitude of each frequency component in the interference signal can be obtained by FFT algorithm, and the attenuation ratio R is calculated_a1Then, using a look-up R_a1Estimating the carrier modulation depth C by a function value table; due to the value of the depth C when phase modulating<At 2.405, N ω in the interference signal_n+3ω_cThe amplitude of the frequency component has been very small, when the calculated attenuation ratio R is_a1A large error is involved so that the calculation accuracy of the modulation depth C is lowered. Therefore, if the value of the depth C is modulated<At 2.405, calculate the attenuation ratio R_a2Then refer to R_a2The function value table obtains the phase modulation depth C.

Determining the value of the modulation depth C by a table look-up method;

interference signal I_s(t) medium direct current component I₀Filtering by AC coupling sampling method, and using a frequency-doubled carrier signal cos (omega)_ct) and a frequency-doubled carrier signal cos (2 ω)_ct) and interference signals are mixed, and then orthogonal interference signal pairs can be obtained after low-pass filtering:

wherein, theta_cThe carrier phase delay represents the phase difference between the carrier signal acquired by mixing and the carrier signal generated by the actual phase modulator;

frequency-multiplied carrier signal sin (omega) with 90 DEG phase shift_ct) and interference signal I_s(t) frequency mixing, low pass filtering, and sin (ω)_ct) from a carrier signal cos (ω)_ct) is obtained through Hilbert transformation, and a signal can be obtained:

so that theta_cComprises the following steps:

the algorithm calculates the carrier phase delay and has some noise points which can be eliminated by using median filtering.

According to the obtained value of the modulation depth C and the carrier phase delay theta_cIs adjusted toThe cross interference signal pair is subjected to normalization processing, namely:

dividing the Q (t) signal by J₁(C)cosθ_cDividing the I (t) signal by J₂(C)cos2θ_c. Further, a processed orthogonal signal pair can be obtained;

obtaining the visibility coefficient I of interference fringe₁：

Therefore, the fringe contrast at each time can be calculated, so that the influence of the fringe contrast fluctuation on the phase demodulation can be eliminated.

Carrier phase delay theta according to the value of modulation depth C_cValue of (d) and interference fringe visibility coefficient I₁To obtain the phase difference related to the vibration of the measured point

The method can accurately calculate the carrier phase modulation degree and the phase delay, and the algorithm obtains theta through a carrier phase delay estimation algorithm_cObtaining a modulation depth C through a phase modulation depth estimation algorithm, and performing pre-normalization processing on the orthogonal interference signal pair by utilizing carrier phase delay and the phase modulation depth, so that nonlinear errors of a demodulation result caused by the phase modulation depth and the carrier phase delay are obviously reduced; and through self-mixing the preprocessed orthogonal interference signal pairs, the influence of interference signal fringe visibility on a demodulation result is eliminated, the complete normalization of the orthogonal interference signal pairs of the sine phase modulation interferometer is realized, the stability of PGC demodulation is improved, the intensity information of a phase demodulation result is completely reserved, and the environment adaptability of the detection system is improved.

The above is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes will occur to those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.