1. An apparatus for improving signal-to-noise ratio of a laser doppler coherent velocimetry system, the apparatus comprising: the seed laser (1), the first beam splitter (2), the acousto-optic modulator (3), the optical fiber amplifier module (4), the circulator module (5) and the optical antenna module (6) are sequentially connected through optical fibers;

the first beam splitter (2) is sequentially connected with the variable optical attenuator module (7), the coupler module (8) and the balanced photoelectric detector module (9) through optical fibers; the circulator module (5) is connected with the coupler module (8) through optical fibers; the balanced photoelectric detector module (9) is electrically connected with the signal processing module (10);

the signal processing module (10) is electrically connected with the variable optical attenuator module (7); given a feedback voltage signal, the signal processing module (10) loads the given feedback voltage signal to the variable optical attenuator module (7) in real time to change the insertion loss of the variable optical attenuator module (7).

2. The apparatus of claim 1, wherein the given feedback voltage signal is: according to the signal-to-noise ratio obtained by resolving the signal sent by the balanced photoelectric detector module (9) by the control beat signal processing module (10), comparing the signal-to-noise ratio with the signal-to-noise ratios obtained by resolving by the two previous continuous control beat signal processing modules (10) to obtain a comparison value, and selecting positive feedback voltage, negative feedback voltage or 0 to carry out voltage adjustment on the variable optical attenuator module (7); the positive feedback voltage is 0.1-0.5V, and the negative feedback voltage is-0.1-0.5V.

3. The apparatus for improving the signal-to-noise ratio of a laser doppler coherent velocimetry system according to claim 1, wherein the circulator module (5) comprises a circulator, the optical antenna module (6) comprises an optical antenna, the coupler module (8) comprises a coupler, the variable optical attenuator module (7) comprises a variable optical attenuator, and the balanced photodetector module (9) comprises a balanced photodetector.

4. The apparatus for improving the signal-to-noise ratio of the laser doppler coherent velocimetry system of claim 1, wherein the apparatus further comprises a second beam splitter;

the circulator module (5) comprises a plurality of circulators, the optical antenna module (6) comprises a plurality of optical antennas, the coupler module (8) comprises a plurality of couplers, the variable optical attenuator module (7) comprises a plurality of variable optical attenuators, and the balanced photodetector module (9) comprises a plurality of balanced photodetectors; the optical fiber amplifier module (4) comprises a plurality of optical fiber amplifiers;

the local oscillation light of the first beam splitter (2) is divided into a plurality of paths of local oscillation optical signals through the second beam splitter and respectively enters a plurality of variable optical attenuators;

the variable optical attenuators, the couplers and the balance photodetectors are connected in sequence in a one-to-one correspondence manner;

the plurality of optical fiber amplifiers are sequentially connected with the plurality of circulators and the plurality of optical antennas in a one-to-one correspondence manner;

the circulators are connected with the couplers in a one-to-one correspondence manner.

5. The device for improving the signal-to-noise ratio of the laser Doppler coherent velocimetry system according to claim 4, wherein the optical fiber amplifier module (4) comprises an optical fiber amplifier and a third beam splitter;

and the third beam splitter is respectively connected with the plurality of circulators and the optical fiber amplifier.

6. The device for improving the signal-to-noise ratio of the laser Doppler coherent velocity measurement system according to claim 4, wherein the optical fiber amplifier module (4) comprises a fourth beam splitter and a plurality of optical fiber amplifiers;

the fourth beam splitter is respectively connected with the acousto-optic modulator (3) and the plurality of optical fiber amplifiers; the plurality of optical fiber amplifiers are connected with the plurality of circulators in a one-to-one correspondence manner.

7. The device for improving the signal-to-noise ratio of the laser Doppler coherent velocity measurement system according to claim 4, wherein the optical fiber amplifier module (4) is a multistage optical fiber amplifier; and a fifth beam splitter is connected between the two stages of optical fiber amplifiers.

8. The device for improving the signal-to-noise ratio of the laser Doppler coherent velocity measurement system according to claim 4, wherein the optical antenna module (6) comprises an optical antenna capable of simultaneously transmitting and receiving a plurality of laser signals with different field angles; the optical antenna is connected to a plurality of circulators.

9. A method for improving the signal-to-noise ratio of a laser doppler coherent velocimetry system, using the measurement device of any one of claims 1 to 7, the method comprising:

the signal processing module (10) is used for calculating a signal-to-noise ratio in real time according to a signal sent by the balanced photoelectric detector module (9), comparing the signal-to-noise ratio with a set signal-to-noise ratio threshold value Ts, and if the signal-to-noise ratio is smaller than the threshold value, the signal processing module (10) does not provide feedback voltage for the variable optical attenuator module (7);

if the signal-to-noise ratio is larger than the threshold value, comparing and calculating the signal-to-noise ratio obtained by the control shooting and the signal-to-noise ratios obtained by two continuous control shots stored previously, and judging whether the signal-to-noise ratio reaches the maximum or not;

if the signal-to-noise ratio obtained by the second control beat in the continuous 3 control beats is not the maximum, the signal processing module (10) provides positive feedback voltage or negative feedback voltage to the variable optical attenuator module (7) until the signal-to-noise ratio obtained by the second control beat in the continuous 3 control beats is the maximum, and the loading voltage of the variable optical attenuator module (7) corresponding to the second control beat is the best.

10. The method for improving the signal-to-noise ratio of the laser Doppler coherent velocimetry system of claim 9,

setting the initial voltage loaded on the variable optical attenuator to be V0, and setting the signal-to-noise ratios obtained by resolving through the continuous 3 control intra-beat signal processing modules (10) to be S respectively_n-2、S_n-1And S_nN represents the nth control beat;

if S_n>S_n-1Setting the voltage adjustment direction coefficient K to 1 if S_n<S_n-1Setting a voltage adjusting direction coefficient K to be-1, and setting the adjusting voltage corresponding to single feedback to be KxDeltaV;

up to S_n<S_n-1And S_n-1>S_n-2When the voltage is equal to V0+ Σ (K × Δ V), Δ V is the voltage adjustment interval.

Background

The laser Doppler velocity measurement technology is based on Doppler effect, calculates the motion velocity value of an object by utilizing the linear relation between the motion velocity of the measured object and Doppler frequency shift quantity, has the advantages of high measurement precision, high spatial resolution, fast dynamic response, wide velocity measurement range, non-contact measurement and the like, is widely applied to various fields, such as measurement of fluid velocity and solid velocity in industrial fields and scientific researches, monitoring of blood flow velocity of blood vessels in the medical field, measurement and positioning of wind velocity, turbulence, wind shear, automobile and the like in the safety monitoring field, measurement of aircraft velocity, measurement of engine tail flame airflow field, measurement of airspeed and the like in the aerospace field, and has wide application market and prospect.

The laser Doppler velocity measurement system can be divided into a direct detection scheme and a coherent detection scheme. Coherent detection has the advantages of detection capability close to the shot noise limit, high detection sensitivity, high detection precision and signal-to-noise ratio, small noise power, insensitivity to solar background light and the like, so that the method is widely applied to the field of laser measurement. Besides the signal light, the coherent detection system also needs to introduce local oscillator light which is homologous with the signal light, the purpose of the coherent detection system is heterodyne interference with the returned signal light, and the signal-to-noise ratio of the system can be greatly improved by utilizing the amplification effect of the local oscillator light on the signal light (the local oscillator light is several orders of magnitude higher than the signal light).

Because the frequency mixing efficiency of the local oscillator light and the signal light affects the signal-to-noise ratio of the system, and the frequency mixing efficiency depends on the degree of matching of the local oscillator light and the signal light in polarization, phase, amplitude (power) and the like, polarization and phase matching is easy to realize for an all-fiber speed measurement system, and therefore, the main factor affecting the frequency mixing efficiency or the signal-to-noise ratio of the system is amplitude (power) matching. Theoretically, the signal-to-noise ratio of the system increases with the increase of the local oscillator optical power and finally approaches to a constant value, and the signal-to-noise ratio reaches the signal-to-noise ratio under the shot noise limit. In fact, since the responsivity of the photodetector is nonlinear, the detector is saturated with the increase of the input optical power, and the signal-to-noise ratio is reduced with the increase of the local oscillator optical power. This means that there is an optimum local oscillator optical power to maximize the system mixing efficiency and signal-to-noise ratio. In 1992, Rod g.frehlich indicated that there was a trend of decreasing the signal-to-noise ratio of the detection with increasing local oscillator optical power by analyzing the nonlinear characteristics of the detector. In 1995, j.fred Holmes studied the theory and determination method of the optimal local oscillator optical power of a coherent detection system using a photodiode, and presented a method of determining the optimal local oscillator optical power by measuring detector parameters. In 2008, the local oscillator light power optimization of all-fiber coherent laser radar is researched by Mazong and the like, a PIN photodiode with a tail fiber is used as a detector, a single-mode fiber interferometer is used as an optical mixer, the influence of the local oscillator light power on the coherent detection signal-to-noise ratio and the local oscillator light power are given, and corresponding tests are carried out, and the result shows that the optimization method can give a proper working point of a coherent detection system. The researches accurately indicate the reason that coherent detection cannot reach shot noise limit detection, and provide a method for calculating the optimal local oscillator optical power to realize the maximum signal-to-noise ratio by measuring the nonlinear parameters of the detector. However, the method in the above document has large operation difficulty, large measurement error and no real-time property in practice, and has the following reasons: 1. in practice, most photoelectric detectors for coherent detection adopt a transimpedance amplification working mode, a transimpedance amplification circuit is saturated before a photodiode, and nonlinear parameters of the photoelectric detectors are difficult to measure; 2. the nonlinear parameters are calculated by a curve fitting method, and calculation errors are large for some photoelectric detectors, so that the determined optimal local oscillator optical power has deviation from a theoretical value; 3. the actual system test environment changes can cause the local oscillator optical power and the detector parameters to change, and the maximum signal-to-noise ratio of the system is difficult to guarantee in real time at the moment.

Disclosure of Invention

The purpose of the invention is as follows: the invention provides a practical device and method for improving the signal-to-noise ratio of a coherent speed measurement system aiming at the problems of the traditional method for optimizing the signal-to-noise ratio of the coherent speed measurement system, which can meet the requirement that the coherent detection system quickly determines the optimal local oscillator light power parameter in real time and ensure that the maximum signal-to-noise ratio can be realized in the process of testing the environmental change.

The technical scheme of the invention is as follows: in one aspect, an apparatus for improving a signal-to-noise ratio of a laser doppler coherent velocity measurement system is provided, where the apparatus includes: the seed laser 1, the first beam splitter 2, the acousto-optic modulator 3, the optical fiber amplifier module 4, the circulator module 5 and the optical antenna module 6 are sequentially connected through optical fibers;

the first beam splitter 2 is sequentially connected with the variable optical attenuator module 7, the coupler module 8 and the balanced photoelectric detector module 9 through optical fibers; the circulator module 5 is connected with the coupler module 8 through optical fibers; the balanced photoelectric detector module (9) is electrically connected with the signal processing module (10);

the signal processing module 10 is electrically connected with the variable optical attenuator module 7; given the feedback voltage signal, the signal processing module 10 loads the given feedback voltage signal to the variable optical attenuator module 7 in real time to change the insertion loss of the variable optical attenuator module 7.

Optionally, the given feedback voltage signal is: resolving signals sent by the balanced photoelectric detector module 9 according to the control beat signal processing module 10 to obtain a signal-to-noise ratio, comparing the signal-to-noise ratio with the signal-to-noise ratios obtained by resolving the two previous continuous control beat signal processing modules 10 to obtain a comparison value, and selecting positive feedback voltage, negative feedback voltage or 0 to perform voltage adjustment on the variable optical attenuator module 7; the positive feedback voltage is 0.1-0.5V, and the negative feedback voltage is-0.1-0.5V.

Optionally, the circulator module 5 comprises a circulator, the optical antenna module 6 comprises an optical antenna, the coupler module 8 comprises a coupler, the variable optical attenuator module 7 comprises a variable optical attenuator, and the balanced photodetector module 9 comprises a balanced photodetector.

Optionally, the apparatus further comprises a second beam splitter;

the circulator module 5 comprises a plurality of circulators, the optical antenna module 6 comprises a plurality of optical antennas, the coupler module 8 comprises a plurality of couplers, the variable optical attenuator module 7 comprises a plurality of variable optical attenuators, and the balanced photodetector module 9 comprises a plurality of balanced photodetectors; the optical fiber amplifier module 4 includes a plurality of optical fiber amplifiers;

the local oscillator light of the first beam splitter 2 is divided into a plurality of paths of local oscillator optical signals through the second beam splitter and respectively enters a plurality of variable optical attenuators;

the variable optical attenuators, the couplers and the balance photodetectors are connected in sequence in a one-to-one correspondence manner;

the plurality of optical fiber amplifiers are sequentially connected with the plurality of circulators and the plurality of optical antennas in a one-to-one correspondence manner;

the circulators are connected with the couplers in a one-to-one correspondence manner.

Optionally, the fiber amplifier module 4 includes one fiber amplifier and a third splitter;

and the third beam splitter is respectively connected with the plurality of circulators and the optical fiber amplifier.

Optionally, the fiber amplifier module 4 includes a fourth splitter and a plurality of fiber amplifiers;

the fourth beam splitter is respectively connected with the acousto-optic modulator 3 and the plurality of optical fiber amplifiers; the plurality of optical fiber amplifiers are connected with the plurality of circulators in a one-to-one correspondence manner.

Optionally, the fiber amplifier module 4 is a multi-stage fiber amplifier; and a fifth beam splitter is connected between the two stages of optical fiber amplifiers.

Alternatively, the optical antenna module 6 includes an optical antenna that can simultaneously transmit and receive a plurality of laser signals with different field angles; the optical antenna is connected to a plurality of circulators.

In another aspect, a method for improving the signal-to-noise ratio of a laser doppler coherent velocimetry system is provided, which uses the measuring apparatus as described above, and the method includes:

the signal processing module 10 calculates the signal sent by the balanced photoelectric detector module 9 in real time to obtain a signal-to-noise ratio, compares the signal-to-noise ratio with a set signal-to-noise ratio threshold value Ts, and if the signal-to-noise ratio is smaller than the threshold value, the signal processing module 10 does not provide feedback voltage for the variable optical attenuator module 7;

if the signal-to-noise ratio is larger than the threshold value, comparing and calculating the signal-to-noise ratio obtained by the control shooting and the signal-to-noise ratios obtained by two continuous control shots stored previously, and judging whether the signal-to-noise ratio reaches the maximum or not;

if the signal-to-noise ratio obtained by the second control beat in the continuous 3 control beats is not the maximum, the signal processing module 10 provides positive feedback voltage or negative feedback voltage to the variable optical attenuator module 7 until the signal-to-noise ratio obtained by the second control beat in the continuous 3 control beats is the maximum, and the voltage loaded by the variable optical attenuator module 7 corresponding to the second control beat is the best.

Optionally, the initial voltage applied to the variable optical attenuator is set to V0, and the signal-to-noise ratios calculated by the signal processing module 10 in 3 consecutive control beats are respectively set to S_n-2、S_n-1And S_nN represents the nth control beat;

if S_n>S_n-1Setting the voltage adjustment direction coefficient K to 1 if S_n<S_n-1Setting a voltage adjusting direction coefficient K to be-1, and setting the adjusting voltage corresponding to single feedback to be KxDeltaV;

up to S_n<S_n-1And S_n-1>S_n-2When the voltage is equal to V0+ Σ (K × Δ V), Δ V is the voltage adjustment interval.

One control beat is defined as a complete process of loading a given feedback voltage signal to the variable optical attenuator module 7 from the signal processing module 10 to the signal processing module 10 resolving a signal sent by the balanced photodetector module 9 to obtain a signal-to-noise ratio and calculating the feedback voltage signal.

The invention has the advantages that: the invention provides a device and a method for improving the signal-to-noise ratio of a laser Doppler coherent velocity measurement system, and aims to realize the maximum system signal-to-noise ratio by changing the local oscillator light power value in real time and judging by a certain method. Compared with the method for improving the signal-to-noise ratio disclosed by the existing literature, the device and the method have the advantages of convenience in measurement, small error, real-time adjustment and the like, and can be used for improving the signal-to-noise ratio of a laser Doppler coherent velocity measurement system and other laser coherent detection systems.

Description of the drawings:

FIG. 1 is a schematic diagram of a scheme for improving the signal-to-noise ratio of a laser Doppler coherent velocity measurement system;

FIG. 2 is a schematic diagram of a signal power spectrum;

FIG. 3 is a flow chart of local oscillator optical power closed loop control;

FIG. 4 is a schematic diagram of a scheme for simultaneously increasing the signal-to-noise ratio of multi-channel laser Doppler coherent velocity measurement

Description of reference numerals: the device comprises a seed laser 1, a first beam splitter 2, an acousto-optic modulator 3, an optical fiber amplifier 4, a circulator 5, an optical antenna 6, a variable optical attenuator 7, a coupler 8, a balanced photodetector 9, a signal processing 10, a second beam splitter 11 and a third beam splitter 12.

The specific implementation mode is as follows:

example 1

In this embodiment, referring to fig. 1, this embodiment provides a device for improving a signal-to-noise ratio of a laser doppler coherent velocity measurement system, where the device includes: the seed laser 1, the first beam splitter 2, the acousto-optic modulator 3, the optical fiber amplifier module 4, the circulator module 5 and the optical antenna module 6 are sequentially connected through optical fibers;

the first beam splitter 2 is sequentially connected with the variable optical attenuator module 7, the coupler module 8 and the balanced photoelectric detector module 9 through optical fibers; the circulator module 5 is connected with the coupler module 8 through optical fibers; the balanced photoelectric detector module 9 is electrically connected with the signal processing module 10;

the signal processing module 10 is electrically connected with the variable optical attenuator module 7; given the feedback voltage signal, the signal processing module 10 loads the given feedback voltage signal to the variable optical attenuator module 7 in real time to change the insertion loss of the variable optical attenuator module 7.

The signal processing module 10 calculates the signal to noise ratio sent by the balanced photodetector module 9 according to the control beat signal, compares the signal to noise ratio with a set signal to noise ratio threshold value Ts, if the signal to noise ratio is smaller than the threshold value, the adjustment is finished, otherwise, compares the signal to noise ratio with the signal to noise ratios obtained by calculation of the two previous continuous control beat signal processing modules 10 to obtain a comparison value, and selects the positive feedback voltage, the negative feedback voltage or 0 to perform voltage adjustment on the variable optical attenuator module 7; the positive feedback voltage is 0.1-0.5V, and the negative feedback voltage is-0.1-0.5V. The feedback voltage with small value has good control stability and high precision, but the adjusting speed is slow, and the phenomenon that the front and back signal-to-noise ratios are almost unchanged is easy to generate, the feedback coefficient with large value has high adjusting speed, but the stability and the precision are poor, and sometimes the optimal control parameters are difficult to obtain.

As one of the preferred modes of the present embodiment, the circulator module 5 includes a circulator, the optical antenna module 6 includes an optical antenna, the coupler module 8 includes a coupler, the variable optical attenuator module 7 includes a variable optical attenuator, and the balanced photodetector module 9 includes a balanced photodetector.

Specifically, in this embodiment, the apparatus for laser doppler velocity measurement includes a seed laser 1, a beam splitter, an acousto-optic modulator, an optical fiber amplifier, a circulator, an optical antenna, a variable optical attenuator, an 50/50 coupler, a balanced photodetector, and a signal acquisition, processing and driving circuit.

The seed laser, the beam splitter, the acousto-optic modulator, the optical fiber amplifier, the circulator and the optical antenna are sequentially connected, the input end 1 of the 50/50 coupler is connected with three ports of the circulator, the input end of the variable optical attenuator is connected with the other output end of the beam splitter 2, the output end of the variable optical attenuator is connected with the input end 2 of the 50/50 coupler, the two output ends W of the 50/50 coupler are connected with the balanced photoelectric detector, the balanced photoelectric detector is connected with the signal processing module through an electrical interface, and the signal processing module is connected with the seed laser, the acousto-optic modulator, the optical fiber amplifier and the variable optical attenuator through an electrical interface. The optical devices are all connected with each other through polarization maintaining fiber welding or through flanges by using connectors such as FC/APC.

As one of the preferable modes of this embodiment, in conjunction with fig. 4, the apparatus for laser doppler velocity measurement includes a second beam splitter 11; the circulator module 5 comprises a plurality of circulators, the optical antenna module 6 comprises a plurality of optical antennas, the coupler module 8 comprises a plurality of couplers, the variable optical attenuator module 7 comprises a plurality of variable optical attenuators, and the balanced photodetector module 9 comprises a plurality of balanced photodetectors. In this embodiment, the fiber amplifier module 4 includes a plurality of fiber amplifiers.

The local oscillator light of the first beam splitter 2 is divided into multiple local oscillator light signals by the second beam splitter 11 and enters a plurality of variable optical attenuators respectively; the variable optical attenuators, the couplers and the balance photodetectors are connected in sequence in a one-to-one correspondence manner; the plurality of optical fiber amplifiers are connected with the plurality of circulators and the plurality of optical antennas in a one-to-one correspondence mode in sequence. The circulators are connected with the couplers in a one-to-one correspondence manner.

As one of the preferable modes of the present embodiment, referring to fig. 4, in this embodiment, the fiber amplifier module 4 includes a fiber amplifier and a third splitter 12; the third beam splitter is respectively connected with the plurality of circulators and the optical fiber amplifier; the optical fiber amplifier is connected with the acousto-optic modulator 3.

As one of preferable modes of the present embodiment, the optical fiber amplifier module 4 includes a fourth beam splitter and a plurality of optical fiber amplifiers; the fourth beam splitter is respectively connected with the acousto-optic modulator 3 and the plurality of optical fiber amplifiers; the plurality of optical fiber amplifiers are connected with the plurality of circulators in a one-to-one correspondence manner.

As one of the preferable modes of the present embodiment, the optical fiber amplifier module 4 is a multistage optical fiber amplifier; and a fifth beam splitter is connected between the two stages of optical fiber amplifiers.

As one of the preferable modes of the present embodiment, the optical antenna module 6 includes an optical antenna that can simultaneously transmit and receive a plurality of laser signals with different field angles; the optical antenna is connected to a plurality of circulators.

The working principle of the embodiment is as follows: laser light emitted by the seed laser is divided into two beams by a beam splitter, one beam enters an acousto-optic modulator as signal light, the other beam enters a variable optical attenuator as local oscillation light, the signal light is modulated into a pulse signal by the acousto-optic modulator and then enters an optical fiber amplifier, the pulse signal is amplified and then enters a circulator, the signal enters an optical antenna through two ports of the circulator and then is emitted into the air, a signal scattered by air molecules enters the circulator through the optical antenna, enters an 50/50 coupler from three ports of the circulator and enters a 50/50 coupler together with the local oscillation light output by the variable optical attenuator to realize optical frequency mixing, then the signal after frequency mixing is divided into two beams of light with equal power by a 50/50 coupler and respectively enters two input ends of a balanced photoelectric detector to realize heterodyne detection, and heterodyne signals output by the balanced photoelectric detector enter a signal processing module, the signal processing module provides driving current for the seed laser and the optical fiber amplifier, and provides driving voltage and modulation signals for the acousto-optic modulator. In order to realize the optimal signal-to-noise ratio, the signal processing module calculates feedback voltage according to the collected heterodyne signals and outputs the feedback voltage to the variable optical attenuator for changing the local oscillator optical power, so that the maximum signal-to-noise ratio is realized.

Example 2

In this embodiment, as shown in fig. 3, a method for improving a signal-to-noise ratio of a laser doppler coherent velocity measurement system is provided, where the measuring apparatus in embodiment 1 is utilized, and the method includes:

the signal processing module 10 calculates the signal sent by the balanced photoelectric detector module 9 in real time to obtain a signal-to-noise ratio, compares the signal-to-noise ratio with the signal-to-noise ratios obtained by two previous continuous control beats, and judges whether the signal-to-noise ratio reaches the maximum;

if the signal-to-noise ratio of the second control beat in the continuous 3 control beats is not the maximum, the signal processing module 10 provides a positive feedback voltage or a negative feedback voltage to the variable optical attenuator module 7 until the signal-to-noise ratio of the second control beat is the maximum, and the control parameter corresponding to the second control beat is the best.

Referring to fig. 1, a control beat is defined as a complete process from loading a given feedback voltage signal to the variable optical attenuator module 7 by the signal processing module 10 to calculating the signal-to-noise ratio of the signal sent by the balanced photodetector module 9 by the signal processing module 10, and comparing the signal-to-noise ratio with the stored signal-to-noise ratios obtained by two previous consecutive control beats to calculate the feedback voltage signal.

In this embodiment, as shown in fig. 3, firstly, pulse data is collected and extracted, an initial voltage loaded on the variable optical attenuator is set as V0, and a signal-to-noise ratio threshold Ts is set, and when calculating the signal-to-noise ratio S_n<When Ts is reached, stopping adjusting the local oscillator optical power, otherwise calculating and storing the signal-to-noise ratio obtained by the previous two continuous control beats, and calculating the signal-to-noise ratio S of the control beat_n(ii) a If amount S_n<S_n-1And S_n-1>S_n-2Stopping adjusting the local oscillator optical power, or else, if S_n>S_n-1Setting a voltage adjustment direction coefficient K to be 1; if S_n<S_n-1Setting the voltage regulation direction coefficient K to-1, and calculating the voltage regulationThe variable optical attenuator voltage is adjusted by calculating the voltage V applied to the variable optical attenuator, which is V0+ Σ (K × Δ V). The value of delta V is 0.1-0.5. Therefore, local oscillator optical power closed-loop control is formed, the purpose of adjusting the local oscillator optical power in real time is achieved, and the system works in the state of the optimal signal-to-noise ratio.

Specifically, the method for improving the signal-to-noise ratio of the coherent velocity measurement system comprises the following steps: after heterodyne signals output by the balanced photoelectric detector 9 are sampled by the signal processing module 10, the signal-to-noise ratio of the system is calculated in real time by a certain method, and the feedback quantity is calculated according to a certain control algorithm flow; and changing the local oscillator optical power by changing the driving voltage of the variable optical attenuator 7 so as to change the signal-to-noise ratio of the system, and repeating the steps until the signal-to-noise ratio of the system is maximum.

Further, this embodiment provides two methods for calculating the signal-to-noise ratio of the system, namely, a cross-correlation method and a spectrum analysis method, so as to achieve the purpose of accurately calculating the signal-to-noise ratio of the system in real time. The cross-correlation method is to take N pulse acquisition data, perform self-correlation operation on each pulse data and calculate a zero value of each pulse data, wherein the zero value is the maximum value of the self-correlation operation, perform cross-correlation operation on adjacent pulse acquisition data and calculate the maximum value, and calculate the signal-to-noise ratio of the system according to the following formula.

In the formula, Q_i,i+1Computing maximum, Q, for cross-correlation of adjacent pulse data_i,i(0) And calculating a zero value for each pulse data autocorrelation.

The spectrum analysis method comprises collecting M (M is greater than or equal to 1) pulse data, calculating frequency spectrum of each pulse signal, calculating modulus of frequency spectrum, squaring to obtain power spectrum, and adding M power spectra to obtain final power spectrum, as shown in FIG. 2, f_LIs the lowest cut-off frequency of the signal, f_HIs the highest cut-off frequency of the signal, f_CTo measure bandwidth, | X (f) non-conducting²Representing the Power Spectral Density (PSD), will f_L～f_HThe sum of the power spectral density values is regarded as the signal power, and the whole power spectral density is taken asThe signal power is added and subtracted as the noise power, and the signal-to-noise ratio is calculated using the following formula.