CN109541621B - A vibration compensation method for frequency scanning interferometric absolute ranging system - Google Patents

Abstract

The invention discloses a vibration compensation method of a frequency scanning interference absolute ranging system, which utilizes a single-frequency laser and two acousto-optic modulators to generate beat frequency signals with Doppler frequency shift information, simultaneously, a tunable laser can also generate measurement beat frequency signals containing Doppler information through a measurement interference light path, and the two measurement beat frequency signals are processed to eliminate the influence of vibration on the frequency scanning interference absolute ranging system. Compared with a double-tunable-laser measuring system, the double-tunable-laser measuring system has the advantages that the hardware cost and the device complexity are reduced, meanwhile, compared with a single-frequency laser, the single-frequency laser has smaller volume and mass and is easier to integrate and use on site, the double-tunable-laser also needs the same frequency modulation speed, which is almost impossible at present, because the tunable laser generally has the phenomenon of frequency modulation nonlinearity, and the single-frequency laser is used, so that the problem is well avoided.

Description

A vibration compensation method for frequency scanning interferometric absolute ranging system

technical field

The invention relates to the field of frequency scanning interferometric absolute ranging, in particular to a vibration compensation method of a frequency scanning interferometric absolute ranging system.

Background technique

The absolute distance measurement system can be used for distance measurement up to tens of meters with relative uncertainty less than 3ppm, which is of great significance in the field of measurement. Absolute distance measurement systems can improve the manufacturing efficiency and precision of large assemblies in areas such as aircraft construction, automotive engineering and modern windmill blade production. Current absolute distance measurement systems are generally based on three different optical methods: time-of-flight method, synthetic wavelength method, and frequency-sweeping interferometry. Frequency scanning interferometric absolute ranging technology is a ranging method that can quickly measure the surface information of diffuse reflectors without targets or markers, and has high measurement accuracy, so it has received extensive attention.

But in actual measurement, with the existence of Doppler effect caused by vibration, the distance spectrum will be broadened, which will cause the measurement error to be more than 1000 times larger than the actual change of optical path difference. In order to solve the influence of this vibration, Swinkels et al. used a triangular wave frequency scanning laser, and calculated the distance value to eliminate the influence of vibration through four continuous phase measurements; Jia et al. The sensor is combined to track the instantaneous movement of the target, and the standard deviation of the measurement is only 2.5μm for the target moving at a speed of 1mm/s.

SUMMARY OF THE INVENTION

Aiming at the situation that the existing target to be measured is difficult to ensure that the measurement is in a completely static condition in practical application occasions, and the measurement error of the traditional frequency scanning interferometric absolute ranging system is large, the present invention proposes a frequency scanning interferometric absolute ranging system. The vibration compensation method uses a single-frequency laser plus two acousto-optic modulators and a tunable laser to generate a measurement beat frequency signal containing vibration information, and processes the two beat frequency signals to eliminate vibration and frequency scanning interference absolute ranging The influence of the system, and the auxiliary beat signal is used to eliminate the influence of the tunable laser frequency modulation nonlinearity on the measurement beat signal. The invention does not need two tunable lasers, but replaces one tunable laser in the double tunable laser system with a single-frequency laser, which reduces hardware cost and device complexity, and does not use real-time measurement of vibration displacement for compensation. Instead, the distance value that offsets the vibration displacement is directly obtained through two synchronous measurement beat signals, which simplifies the algorithm and makes the frequency scanning interferometric absolute ranging system more widely used.

The technical scheme adopted in the present invention is: a vibration compensation method for a frequency scanning interference absolute ranging system, which utilizes a single-frequency laser, a tunable laser, two acousto-optic modulators, several couplers, an optical circulator, and a collimating lens. , the piezoelectric displacement stage, the target mirror, and the coarse wavelength division multiplexer obtain two measurement beat signals containing Doppler frequency shift information, and the two measurement beat signals contain the vibration information of the target mirror to be measured; A tunable laser and a time-delay fiber obtain an auxiliary beat frequency signal; after using the auxiliary beat frequency signal to eliminate the influence of the tunable laser frequency modulation nonlinearity on the two measured beat frequency signals, the two measured beat frequency signals are Perform processing to eliminate the influence of vibration on the frequency scanning interference absolute ranging system, and calculate the distance value that eliminates the influence of vibration.

Further, a vibration compensation method of a frequency scanning interference absolute ranging system of the present invention specifically includes the following steps:

Generation of ranging signals:

Step 1-1. A single-frequency laser generates a single-frequency signal, and a tunable laser generates a frequency scanning signal; the single-frequency signal is divided into a path A and a path B through the first beam splitter, and the frequency scanning signal passes through the second beam splitter. The device is divided into C-channel and D-channel, A-channel, B-channel, C-channel enter the measurement interference system, D-channel enters the auxiliary interference system;

Step 1-2, in the measurement interference system: the A-channel signal reaches the first coupler through the second acousto-optic modulator, the C-channel signal is divided into the E-channel and the F-channel through the third beam splitter, and the E-channel signal and the A-channel signal are separated. Coupled at the first coupler and sent into an optical circulator; the optical circulator adopts a first, second, and third port for cyclically transmitting light from the first port to the second port, and from the second port. A 3-port optical circulator whose port is transmitted to a third port, the first port of the optical circulator is connected to the output of the first coupler, the second port is connected to the collimating lens, and the third port is connected to the first coupler. One input end of the three couplers; the B-channel signal reaches the second coupler through the first acousto-optic modulator, and the F-channel signal and the B-channel signal are coupled in the second coupler and sent to the other input end of the third coupler; The A- and E-channel lasers pass through the first coupler, optical circulator, and collimating lens, and after being reflected by the target mirror, return to the optical circulator in the same way, and then enter an input end of the third coupler , in which the A-channel laser and B-channel laser converge at the third coupler and interfere, and the E-channel laser and the F-channel laser meet and interfere at the third coupler, because the A-channel laser and B-channel laser from the single-frequency laser The E-channel laser and the F-channel laser from the tunable laser are laser signals of two frequency bands. The laser signal of the two frequency bands is separated by a coarse wavelength division multiplexer. The A-channel laser and the B-channel laser are detected in the first photoelectric detection. A first measurement beat signal is formed on the detector, and a second measurement beat signal is formed on the second photodetector by the E-channel laser and the F-channel laser;

Steps 1-3, in the auxiliary interference system: the signal of channel D is divided into channel G and channel H through the fourth beam splitter, and the laser beam of channel G enters the fourth coupler after passing through the delay fiber with constant length and known optical path difference, The G-channel laser and the H-channel laser converge at the fourth coupler and interfere, and the output end of the fourth coupler is connected to the third photodetector, and the G-channel laser and the H-channel laser form an auxiliary beat frequency on the third photodetector Signal;

Among them, A path and B path form one measurement interference optical path, E path and F path form another measurement interference optical path, G path and H path form a reference interference optical path;

Synchronized data collection:

The synchronous data acquisition system performs synchronous sampling on the first measurement beat frequency signal and the second measurement beat frequency signal generated by the measurement interference system and the auxiliary beat frequency signal generated by the auxiliary interference system, and the steps are as follows:

2-1. Synchronize the initialization of the data acquisition system, set the sampling time t _s and the sampling frequency f _s ;

2-2. Data acquisition. During the acquisition process, perform error detection and judgment on the first measurement beat signal, the second measurement beat signal and the auxiliary beat signal collected by the synchronous data acquisition system. If there is no error, proceed to the next step, otherwise Repeat steps 2-2;

data processing:

The optical frequency output by the currently used tunable laser is not completely linearly modulated. When the output optical frequency is not completely linearly modulated, the measurement beat frequency signal will be severely broadened, resulting in a great measurement error. Therefore, auxiliary interference is used. The system eliminates the influence of frequency modulation nonlinearity of the measurement interferometric system, which specifically includes the following steps:

Step 3-1: Use the auxiliary beat frequency signal passing through the synchronous data acquisition system as a clock signal, and perform equal optical frequency resampling on the first measurement beat frequency signal and the second measurement beat frequency signal at the same time;

Step 3-2. For a moving object, the A-channel laser and the B-channel laser will interfere. Since both the first measurement beat signal and the second measurement beat signal contain the motion information of the object, the equal-frequency resampled The first measurement beat signal and the second measurement beat signal are multiplied to obtain a new signal, the new signal contains two cosine terms, one of which contains the Doppler frequency shift cosine term is an interference term, and the other frequency does not contain The cosine term of the Doppler frequency shift is the desired term; the frequency of the desired term is obtained by using fast Fourier transform or chirp-z transform, and the true distance value to be measured is obtained according to the frequency of the desired term.

Among them, step 3-1 specifically includes:

The first measurement beat signal is expressed as:

P ₁ (t)=Acos{2π[(10 ⁶ -f _d )t+f _d τ+f ₀ τ+80×10 ⁶ τ]} (1)

In formula (1), P ₁ (t) is the first measurement beat signal, A represents the amplitude of the first measurement beat signal, f _d represents the Doppler frequency shift introduced in the measurement beat signal due to object motion, f ₀ represents the frequency of the single-frequency laser, τ represents the time delay corresponding to the distance to be measured, and t represents the time;

The second measurement beat signal is expressed as:

In formula (2), P ₂ (t) is the second measurement beat signal, B represents the amplitude of the second measurement beat signal, α ₁ represents the modulation speed of the tunable laser, and f ₁ represents the initial laser emission of the tunable laser. frequency, ε ₁ (t) represents the frequency nonlinearity error deviating from the ideal linear sweep;

The expression of the auxiliary beat signal is:

In formula (3), P ₃ (t) is the auxiliary beat signal, M represents the amplitude of the auxiliary beat signal, and τ _r represents the time delay corresponding to the optical path difference of the reference interference optical path.

The first measurement beat signal and the second measurement beat signal are resampled by taking the peak and valley position points of the auxiliary beat signal, and the result is:

In formula (4), P ₁ (k) is the resampled first measurement beat signal; in formula (5), P ₂ (k) is the resampled second measurement beat signal.

Among them, the 3-2 steps specifically include:

Multiply the first measurement beat signal and the second measurement beat signal after equal optical frequency resampling to obtain a new signal:

里包含了多普勒频移f_d，为干扰项；第二个余弦项的频率项

里没有了多普勒频移f_d，为所需项；In formula (6), P ₄ (k) is the new signal obtained by multiplying the first measurement beat signal and the second measurement beat signal after equal optical frequency resampling; it can be known from formula (6) that the first cosine term is the frequency term of

contains the Doppler frequency shift f _d , which is the interference term; the frequency term of the second cosine term

There is no Doppler shift f _d in , which is the required term;

Let the number of points of the resampled first measurement beat signal and the second measurement beat signal be N, perform chirp-z transformation on the new signal to obtain the frequency of the desired item, and the spectral peak position of the frequency of the desired item is p, then The formula for calculating the peak frequency f _beat is:

得到：By the frequency term of the second cosine term of equation (6)

get:

Combining formula (7) and formula (8), we can get:

The formula for calculating the distance to be measured is:

In formula (10), R is the real distance value to be measured, and c is the speed of light.

The beneficial effects of the present invention are:

On the basis of the original frequency scanning interferometric absolute ranging system, a single frequency laser and two acousto-optic modulators are added. The purpose is to generate a beat frequency signal with the vibration information of the target mirror, and the light emitted by the tunable laser is measured. After the interference system, a beat frequency signal containing the vibration information of the target mirror can also be generated. By processing these two beat frequency signals, the distance value that eliminates the influence of vibration can be obtained. Compared with the dual tunable laser method, this method reduces the Hardware cost and device complexity, and there is no need to ensure the same frequency sweep speed of the two tunable lasers (often this requirement is difficult to achieve, because the light output by the current tunable lasers on the market is not completely linearly modulated), and this At the same time, for the practical application environment, the integration of the system is also relatively convenient, tunable lasers tend to be larger in size, while single-frequency lasers are small in size and light in weight, and the system designed by the present invention is more practical.

Description of drawings

Fig. 1 is the flow chart of the vibration compensation method of a kind of frequency scanning interference absolute ranging system of the present invention;

FIG. 2 is a schematic structural diagram of a frequency scanning interferometric absolute ranging system device used in the present invention.

1. Single frequency laser; 2. The first beam splitter; 3. The second acousto-optic modulator; 4. The third beam splitter; 5. The first acousto-optic modulator; 6. The first coupler ; 7, the second coupler; 8, the optical circulator; 9, the third coupler; 10, the collimating lens; 11, the coarse wavelength division multiplexer; 12, the piezoelectric stage and the objective lens; 13, the first photodetector; 14, second photodetector; 15, measurement interference system; 16, auxiliary interference system; 17, third photodetector; 18, fourth coupler; 19, delay fiber; 20, fourth point beam splitter; 21, second beam splitter; 22, tunable laser; 23, synchronous data acquisition system; 24, data processing system;

P1, the first measurement beat signal; P2, the second measurement beat signal; P3, the auxiliary beat signal; P4, the new signal obtained by multiplying the resampled first measurement beat signal and the second measurement beat signal .

Detailed ways

In order to further understand the content of the invention, features and effects of the present invention, the following embodiments are exemplified and described in detail with the accompanying drawings as follows:

The invention proposes a vibration compensation method using a single-frequency laser, an acousto-optic modulator and the original frequency scanning interference absolute ranging system, and its feasibility is verified through theoretical derivation.

The frequency scanning interference absolute ranging system device used in the present invention is shown in FIG. 2 , including a single-frequency laser 1 and a tunable laser 22 , and the single-frequency laser 1 is divided into a path A and a path B through the first beam splitter 2 , the tunable laser 22 is divided into a C path and a D path through the second beam splitter 21 , the A path, the B path, and the C path enter the measurement interference system 15 , and the D path enters the auxiliary interference system 16 .

In the measurement interferometry system 15: the A-channel signal enters the second acousto-optic modulator 3 to reach the first coupler 6, the C-channel signal is divided into the E-channel and the F-channel through the third beam splitter 4, and the E-channel signal and A are separated. The channel signal is coupled at the first coupler 6 and sent to the optical circulator 8; the optical circulator 8 adopts a first, second and third port for cyclically transmitting light from the first port to the second port port, a 3-port optical circulator transmitted from the second port to the third port, the first port of the optical circulator 8 is connected to the output of the first coupler 6, and the second port is connected to the collimating lens 10. connection, the third port is connected to an input end of the third coupler 9; the B-channel signal reaches the second coupler 7 through the first acousto-optic modulator 5, and the F-channel signal and the B-channel signal are coupled in the second coupler 7 and sent to the other input end of the third coupler 9; the A and E lasers pass through the first coupler 6, the optical circulator 8, the collimating lens 10, and after being reflected by the target mirror, return to the original path and enter the The optical circulator 8 enters an input end of the third coupler 9, where the A-channel laser and the B-channel laser converge and interfere in the third coupler 9, and the E-channel laser and the F-channel laser are in the third coupler. 9 Convergence and interference occurs, since the A-channel laser and B-channel laser from the single-frequency laser 1 and the E-channel laser and F-channel laser from the tunable laser 22 are laser signals of two frequency bands (that is, the A-channel laser, B-channel laser The lasers of channel E and channel F are in the same frequency band, and the two frequency bands do not intersect), so the output end of the third coupler 9 is connected to a coarse wavelength division multiplexer 11. The coarse wavelength division multiplexer 11 is used to separate the laser signals of two frequency bands. The A-channel laser and the B-channel laser form the first measurement beat signal P1 on the first photodetector 13, and the E-channel laser and the F-channel laser form the first measurement beat signal P1. The laser forms a second measurement beat signal P2 on the second photodetector 14 .

In the auxiliary interference system 16: the D-channel signal is divided into G-channel and H-channel through the fourth beam splitter 20, and the G-channel laser enters the fourth coupler 18 after passing through the delay fiber 19 with a constant length and a known optical path difference. , the G-channel laser and the H-channel laser converge at the fourth coupler 18 and interfere, the output end of the fourth coupler 18 is connected to the third photodetector 17 , and the G-channel laser and the H-channel laser are connected to the third photodetector 17 The auxiliary beat signal P3 is formed thereon.

The outputs of the first photodetector 13 , the second photodetector 14 and the third photodetector 17 are respectively connected to the three channels of the synchronous data acquisition system 23 , and the output of the synchronous data acquisition system 23 is connected to the data processing system 24 .

A vibration compensation method of a frequency scanning interference absolute ranging system of the present invention utilizes a single frequency laser 1, a tunable laser 22, a first acousto-optic modulator 5 and a second acousto-optic modulator 3, several couplers, an optical circulator 8. The collimating lens 10, the piezoelectric displacement stage, the objective mirror 12, the coarse wavelength division multiplexer 11, etc. obtain the first measurement beat signal P1 and the second measurement beat signal P2 containing Doppler frequency shift information, so The first measurement beat signal P1 and the second measurement beat signal P2 contain the vibration information of the target mirror to be measured; Utilize the tunable laser 22, the delay fiber 19, etc. to obtain the auxiliary beat signal P3; Use the auxiliary beat frequency After the signal P3 eliminates the influence of the frequency modulation nonlinearity of the tunable laser 22 on the first measurement beat signal P1 and the second measurement beat signal P2, the first measurement beat signal P1 and the second measurement beat signal P2 P2 is processed to eliminate the influence of vibration on the frequency scanning interference absolute ranging system, and the distance value to eliminate the influence of vibration is calculated. The specific implementation steps are as follows:

Build the experimental device as shown in Figure 2, and then as shown in Figure 1, the tunable laser 22 and the single-frequency laser 1 are turned on first in the frequency scanning interference absolute ranging system device, and then the parameters of the tunable laser 22 (scanning bandwidth and scanning speed), the parameters of the piezoelectric stage (vibration frequency and vibration amplitude), and the parameters of the synchronous data acquisition system 23 (sampling frequency and sampling time).

After the equipment is warmed up and initialized, the frequency scanning interference absolute ranging begins. The specific steps are as follows:

Generation of ranging signals

Step 1-1, the single-frequency laser 1 generates a single-frequency signal with a single frequency, and the tunable laser 22 generates a frequency sweep signal; the single-frequency signal is divided into A and B through the first beam splitter 2, and the frequency sweep The signal passes through the second beam splitter 21 and is divided into C-path and D-path, A-path, B-path, C-path enter the measurement interference system 15, and D-path enters the auxiliary interference system 16;

Steps 1-2, in the measurement interference system 15: the A-way signal enters the second acousto-optic modulator 3 to reach the first coupler 6, the C-way signal passes through the third beam splitter 4 and is divided into E-way and F-way, E-way The signal and the A-channel signal are coupled at the first coupler 6 and sent to the optical circulator 8; the optical circulator 8 adopts a first, second, and third port to transmit light cyclically from the first port To the second port, from the second port to the 3-port optical circulator of the third port, the first port of the optical circulator 8 is connected to the output of the first coupler 6, and the second port is connected to the collimator The lens 10 is connected, and the third port is connected to an input end of the third coupler 9; the B-channel signal reaches the second coupler 7 through the first acousto-optic modulator 5, and the F-channel signal and the B-channel signal are in the second coupler. 7 is coupled and sent to the other input end of the third coupler 9; the lasers of the A and E paths pass through the first coupler 6, the optical circulator 8, the collimating lens 10, and are reflected by the objective mirror 12, and the original path is Return to the optical circulator 10, and then enter an input end of the third coupler 9, where the A-channel laser and the B-channel laser converge at the third coupler 9 and interfere, and the E-channel laser and the F-channel laser are in the third coupler 9. The third coupler 9 merges and interferes, because the A-channel laser and B-channel laser from the single-frequency laser 1 and the E-channel laser and F-channel laser from the tunable laser 22 are laser signals of two frequency bands (ie, A The laser of channel B and the laser of channel B are in the same frequency band, the laser of channel E and the laser of channel F are in the same frequency band, and these two frequency bands do not intersect), so the output end of the third coupler 9 is connected to a coarse wave The division multiplexer 11, the coarse wavelength division multiplexer 11 is used to separate the laser signals of two frequency bands, the A-channel laser and the B-channel laser form the first measurement beat frequency signal P1 on the first photodetector 13, and the E-channel laser The laser and the F-channel laser form a second measurement beat signal P2 on the second photodetector 14;

Steps 1-3, in the auxiliary interference system 16: the D channel signal is divided into the G channel and the H channel through the fourth beam splitter 20, and the G channel laser enters the fourth channel after passing through the delay fiber 19 with a constant length and a known optical path difference. Coupler 18, the G-channel laser and the H-channel laser converge in the fourth coupler 18 and interfere, and the output end of the fourth coupler 18 is connected to the third photodetector 17, and the G-channel laser and the H-channel laser are connected to the third photoelectric detector 17. An auxiliary beat signal P3 is formed on the detector 17;

Among them, the A path and the B path form one measurement interference optical path, the E path and the F path form another measurement interference optical path, and the G path and the H path form the reference interference optical path.

Synchronized data collection

The synchronous data acquisition system 23 performs synchronous sampling on the first measurement beat signal P1 and the second measurement beat signal P2 generated by the measurement interference system 15 and the auxiliary beat signal P3 generated by the auxiliary interference system 16, and the steps are as follows:

2-1. Synchronize the initialization of the data acquisition system 23, set the sampling time t _s and the sampling frequency f _s ;

2-2. Data collection. During the collection process, the first measurement beat signal P1, the second measurement beat signal P2 and the auxiliary beat signal P3 collected by the synchronous data collection system 23 are subjected to error detection and judgment, and if there is no error, the next step is performed. Step 1, otherwise perform steps 2-2 again.

data processing

The optical frequency output by the currently used tunable laser 22 cannot be completely linearly modulated. When the output optical frequency is not completely linearly modulated, the measurement beat frequency signal will be severely broadened, resulting in a great measurement error. Here, the auxiliary interferometric system 16 is needed to eliminate the influence of the frequency modulation nonlinearity of the measurement interferometric system 15, which specifically includes the following steps:

Step 3-1, using the auxiliary beat signal P3 passing through the synchronous data acquisition system 23 as a clock signal, and performing equal optical frequency resampling on the first measurement beat signal P1 and the second measurement beat signal P2 at the same time;

Step 3-2. For a moving object, the A-channel laser and the B-channel laser will interfere. Since the first measurement beat signal P1 and the second measurement beat signal P2 both contain the motion information of the object, the equal optical frequency will be resampled. After the first measurement beat signal P1 and the second measurement beat signal P2 are multiplied to obtain a new signal P4, the new signal P4 contains two cosine terms, one of which contains the cosine term of the Doppler frequency shift as an interference term, and the other is an interference term. The cosine term that does not include the Doppler frequency shift is the required term; the frequency of the required term is accurately obtained by using fast Fourier transform or chirp-z transform, and the true distance value to be measured is obtained according to the frequency of the required term.

The vibration compensation method of a frequency scanning interference absolute ranging system of the present invention is further described below in conjunction with specific formulas:

Set the frequency of the first acousto-optic modulator 5 to 81MHz and the frequency of the second acousto-optic modulator 3 to be 80MHz, then for a moving target, the first measurement beat signal P1 detected by the first photodetector 13 can be expressed as:

P ₁ (t)=Acos{2π[(10 ⁶ -f _d )t+f _d τ+f ₀ τ+80×10 ⁶ τ]} (1)

In formula (1), P ₁ (t) is the first measurement beat signal P1, A represents the amplitude of the first measurement beat signal P1, and f _d represents the Doppler frequency introduced into the measurement beat signal due to object motion. shift, f ₀ represents the frequency of the single-frequency laser 1, τ represents the time delay corresponding to the distance to be measured, and t represents the time.

The second measurement beat signal P2 detected by the second photodetector 14 can be expressed as:

In formula (2), P ₂ (t) is the second measurement beat signal P2, B represents the amplitude of the second measurement beat signal P2, α ₁ represents the modulation speed of the tunable laser 22, and f ₁ represents the tunable laser 22 The initial frequency of the lasing, ε ₁ (t) represents the frequency nonlinearity error away from the ideal linear sweep.

In order to eliminate the influence of frequency modulation nonlinearity, the auxiliary beat signal P3 of the auxiliary interference system 16 is used to resample the first measurement beat signal P1 and the second measurement beat signal P2 (although the first measurement beat signal P1 is not There is no nonlinear term, but in order to be able to synchronize the first measurement beat signal P1 and the second measurement beat signal P2, the first measurement beat signal P1 is also resampled here), the auxiliary beat signal P3 The expression is:

In formula (3), P ₃ (t) is the auxiliary beat signal P3, M represents the amplitude of the auxiliary beat signal P3, and τ _r represents the time delay corresponding to the optical path difference of the reference interference optical path.

The first measurement beat signal P1 and the second measurement beat signal P2 are resampled by taking the peak and valley position points of the auxiliary beat signal P3, and the result is:

In formula (4), P ₁ (k) is the resampled first measurement beat signal P1, and in formula (5), P ₂ (k) is the resampled second measurement beat signal P2. It can be seen from equation (4) that the nonlinear term in the first measurement beat signal P1 has been eliminated, and the resampled first measurement beat signal P1 and the second measurement beat signal P2 are multiplied to obtain:

里包含了多普勒频移f_d，为干扰项；第二个余弦项的频率项

里没有了多普勒频移f_d，为频率单一的项，即为所需项，且相对于式(2)第二个余弦项大大减少了相位调制。干扰项与所需项的频率相差2MHz左右，故两者分布在频率域的不同区域(频率差由第一声光调制器5和第二声光调制器3的频率差决定，可根据实际需要选择)，因此能够很容易的区分出所需项。In Equation (6), P ₄ (k) is the new signal P4. From Equation (6), it can be known that the frequency term of the first cosine term is

contains the Doppler frequency shift f _d , which is the interference term; the frequency term of the second cosine term

There is no Doppler frequency shift f _d in it, it is a single frequency term, that is, the required term, and compared with the second cosine term of equation (2), the phase modulation is greatly reduced. The frequency difference between the interference item and the desired item is about 2MHz, so the two are distributed in different regions of the frequency domain (the frequency difference is determined by the frequency difference between the first acousto-optic modulator 5 and the second acousto-optic modulator 3, which can be determined according to actual needs. selection), so the desired item can be easily distinguished.

Set the number of points of the resampled first measurement beat signal P1 and the second measurement beat signal P2 to N, and perform chirp-z (or fast Fourier transform or other algorithms that can obtain accurate frequencies) _on the new signal P4 The frequency of the desired item is obtained by transformation, and the peak position of the spectrum of the desired item frequency is p, then the calculation formula of the frequency of the peak point f _beat is:

得到：By the frequency term of the second cosine term of equation (6)

get:

Combining formula (7) and formula (8), we can get:

The formula for calculating the distance to be measured is:

In formula (10), R is the real distance value to be measured, and c is the speed of light. It can be seen from the formula (10) that the influence of vibration on the frequency scanning interference absolute ranging technique has been eliminated by the method of the present invention.

Although the preferred embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments. Under the inspiration of the present invention, without departing from the spirit of the present invention and the protection scope of the claims, personnel can also make many forms, which all fall within the protection scope of the present invention.

Claims (3)

1. a vibration compensation method of frequency scanning interference absolute ranging system, is characterized in that, utilizes single frequency laser, tunable laser, two acousto-optic modulators, some couplers, optical circulator, collimating lens, piezoelectric The displacement stage, the target mirror, and the coarse wavelength division multiplexer obtain two measurement beat signals containing Doppler frequency shift information, and the two measurement beat signals contain the vibration information of the target mirror to be measured; using a tunable laser and delaying the optical fiber to obtain an auxiliary beat frequency signal; after using the auxiliary beat frequency signal to eliminate the influence of the tunable laser frequency modulation nonlinearity on the two measurement beat frequency signals, the two measurement beat frequency signals are processed, Eliminate the impact of vibration on the frequency scanning interference absolute ranging system, and calculate the distance value for eliminating the impact of vibration; the specific steps include the following: Generation of ranging signals: Step 1-1. A single-frequency laser generates a single-frequency signal, and a tunable laser generates a frequency scanning signal; the single-frequency signal is divided into a path A and a path B through the first beam splitter, and the frequency scanning signal passes through the second beam splitter. The device is divided into C-channel and D-channel, A-channel, B-channel, C-channel enter the measurement interference system, D-channel enters the auxiliary interference system; Step 1-2, in the measurement interference system: the A-channel signal reaches the first coupler through the second acousto-optic modulator, the C-channel signal is divided into the E-channel and the F-channel through the third beam splitter, and the E-channel signal and the A-channel signal are separated. Coupled at the first coupler and sent into an optical circulator; the optical circulator adopts a first, second, and third port for cyclically transmitting light from the first port to the second port, and from the second port. A 3-port optical circulator whose port is transmitted to a third port, the first port of the optical circulator is connected to the output of the first coupler, the second port is connected to the collimating lens, and the third port is connected to the first coupler. One input end of the three couplers; the B-channel signal reaches the second coupler through the first acousto-optic modulator, and the F-channel signal and the B-channel signal are coupled in the second coupler and sent to the other input end of the third coupler; The A- and E-channel lasers pass through the first coupler, optical circulator, and collimating lens, and after being reflected by the target mirror, return to the optical circulator in the same way, and then enter an input end of the third coupler , in which the A-channel laser and B-channel laser converge at the third coupler and interfere, and the E-channel laser and the F-channel laser meet and interfere at the third coupler, because the A-channel laser and B-channel laser from the single-frequency laser The E-channel laser and the F-channel laser from the tunable laser are laser signals of two frequency bands. The laser signal of the two frequency bands is separated by a coarse wavelength division multiplexer. The A-channel laser and the B-channel laser are detected in the first photoelectric detection. A first measurement beat signal is formed on the detector, and a second measurement beat signal is formed on the second photodetector by the E-channel laser and the F-channel laser; Steps 1-3, in the auxiliary interference system: the signal of channel D is divided into channel G and channel H through the fourth beam splitter, and the laser beam of channel G enters the fourth coupler after passing through the delay fiber with constant length and known optical path difference, The G-channel laser and the H-channel laser converge at the fourth coupler and interfere, and the output end of the fourth coupler is connected to the third photodetector, and the G-channel laser and the H-channel laser form an auxiliary beat frequency on the third photodetector Signal; Among them, A path and B path form one measurement interference optical path, E path and F path form another measurement interference optical path, G path and H path form a reference interference optical path; Synchronized data collection: The synchronous data acquisition system performs synchronous sampling on the first measurement beat frequency signal and the second measurement beat frequency signal generated by the measurement interference system and the auxiliary beat frequency signal generated by the auxiliary interference system, and the steps are as follows: 2-1. Synchronize the initialization of the data acquisition system, set the sampling time t _s and the sampling frequency f _s ; 2-2. Data acquisition. During the acquisition process, perform error detection and judgment on the first measurement beat signal, the second measurement beat signal and the auxiliary beat signal collected by the synchronous data acquisition system. If there is no error, proceed to the next step, otherwise Repeat steps 2-2; data processing: The optical frequency output by the currently used tunable laser is not completely linearly modulated. When the output optical frequency is not completely linearly modulated, the measurement beat frequency signal will be severely broadened, resulting in a great measurement error. Therefore, auxiliary interference is used. The system eliminates the influence of frequency modulation nonlinearity of the measurement interferometric system, which specifically includes the following steps: Step 3-1: Use the auxiliary beat frequency signal passing through the synchronous data acquisition system as a clock signal, and perform equal optical frequency resampling on the first measurement beat frequency signal and the second measurement beat frequency signal at the same time; Step 3-2. For a moving object, the A-channel laser and the B-channel laser will interfere. Since both the first measurement beat signal and the second measurement beat signal contain the motion information of the object, the equal-frequency resampled The first measurement beat signal and the second measurement beat signal are multiplied to obtain a new signal, the new signal contains two cosine terms, one of which contains the Doppler frequency shift cosine term is an interference term, and the other frequency does not contain The cosine term of the Doppler frequency shift is the desired term; the frequency of the desired term is obtained by using fast Fourier transform or chirp-z transform, and the true distance value to be measured is obtained according to the frequency of the desired term. 2. the vibration compensation method of a kind of frequency scanning interference absolute ranging system according to claim 1, is characterized in that, 3-1 step specifically comprises: The first measurement beat signal is expressed as: P ₁ (t)=Acos{2π[(10 ⁶ -f _d )t+f _d τ+f ₀ τ+80×10 ⁶ τ]} (1) In formula (1), P ₁ (t) is the first measurement beat signal, A represents the amplitude of the first measurement beat signal, f _d represents the Doppler frequency shift introduced in the measurement beat signal due to object motion, f ₀ represents the frequency of the single-frequency laser, τ represents the time delay corresponding to the distance to be measured, and t represents the time; The second measurement beat signal is expressed as:

In formula (2), P ₂ (t) is the second measurement beat signal, B represents the amplitude of the second measurement beat signal, α ₁ represents the modulation speed of the tunable laser, and f ₁ represents the initial laser emission of the tunable laser. frequency, ε ₁ (t) represents the frequency nonlinearity error deviating from the ideal linear sweep; The expression of the auxiliary beat signal is:

In formula (3), P ₃ (t) is the auxiliary beat signal, M represents the amplitude of the auxiliary beat signal, and τ _r represents the time delay corresponding to the optical path difference of the reference interference optical path; The first measurement beat signal and the second measurement beat signal are resampled by taking the peak and valley position points of the auxiliary beat signal, and the result is:

In formula (4), P ₁ (k) is the resampled first measurement beat signal; in formula (5), P ₂ (k) is the resampled second measurement beat signal. 3. the vibration compensation method of a kind of frequency scanning interference absolute ranging system according to claim 2, is characterized in that, 3-2 step specifically comprises: Multiply the first measurement beat signal and the second measurement beat signal after equal optical frequency resampling to obtain a new signal:

In formula (6), P ₄ (k) is the new signal obtained by multiplying the first measurement beat signal and the second measurement beat signal after equal optical frequency resampling; it can be known from formula (6) that the first cosine term is the frequency term of

contains the Doppler frequency shift f _d , which is the interference term; the frequency term of the second cosine term

There is no Doppler shift f _d in , which is the required term; Let the number of points of the resampled first measurement beat signal and the second measurement beat signal be N, perform chirp-z transformation on the new signal to obtain the frequency of the desired item, and the spectral peak position of the frequency of the desired item is p, then The formula for calculating the peak frequency f _beat is:

By the frequency term of the second cosine term of equation (6)

get:

Combining formula (7) and formula (8), we can get:

The formula for calculating the distance to be measured is:

In formula (10), R is the real distance value to be measured, and c is the speed of light.

Images