CN114838831A - Wave-splitting surface orthogonal polarization interference system and method for measuring atmospheric coherence length - Google Patents

Abstract

The invention discloses a wave-division plane orthogonal polarization interference system for measuring atmospheric coherence length, comprising a laser emitting unit, a target reflection target surface, a front two-way optical receiving system, an orthogonal polarization four-way interference system, as well as signal acquisition and data processing The system; the front two-way optical receiving system is used to realize beam collection and collimation; the polarization state of the two beams is controlled by the orthogonal polarization interference system, and four channels of interference signals with different polarization states are output; the signal acquisition and solution system will Simultaneously record the four interference signals, calculate the phase difference and count the phase structure function, and finally obtain the atmospheric coherence length on the transmission path. Also disclosed is an orthogonal polarization interferometry method for measuring the atmospheric coherence length of the partial wave planes. The invention selects two symmetrically distributed sub-beams in the received echo light field, and combines the polarization element and the interference system to convert the phase difference fluctuation into the change of the intensity of the interference pattern, thereby realizing the measurement of the turbulent intensity signal on the transmission path.

Description

A split-wave plane orthogonal polarization interferometry system for measuring atmospheric coherence length and its method

technical field

The invention relates to the technical field of atmospheric optical detection, in particular to a wave-division plane orthogonal polarization interference system for measuring atmospheric coherence length and a method thereof.

Background technique

When light waves are transmitted in random turbulent media, they are affected by the fluctuation of the refractive index of the atmosphere, resulting in phase fluctuations and light intensity flickering, which limit the application of systems such as free-space optical communication and high-resolution optical imaging in the atmosphere. The atmospheric coherence length on the propagation path is of great significance for evaluating the working efficiency of advanced optoelectronic systems.

Since Fried proposed the concept of atmospheric coherence length r ₀ in 1965, the measurement technology of r ₀ has been continuously developed and perfected. Among them, the differential imaging motion method (DIMM) has gradually become the most widely used r ₀ measurement at this stage due to its anti-vibration interference characteristics. method, the principle of which is to obtain r ₀ by using the fluctuation of the arrival angle of different points on the same wavefront. However, because the differential image motion method belongs to direct detection, it has the characteristics of limited detection accuracy and is not suitable for low-light conditions, while coherent detection can greatly improve the detection efficiency and detection accuracy, and has better low-light detection performance.

与大气相干长度r₀的关系反推出r₀，在VonKarman谱条件下，二者之间的关系如下：Since atmospheric turbulence is a random process, the physical quantity to measure its intensity is the second-order statistic of the light wave, that is, the structure function, and the structure function of the light wave is the sum of the amplitude structure function and the phase structure function. Under the paraxial approximation, the amplitude structure function is about Zero, the phase structure function is approximately equal to the wavefront structure function, so the phase fluctuation of the beam can be measured and its structure function can be counted, and then according to the phase structure function

The relationship between the atmospheric coherence length r ₀ and the atmospheric coherence length r ₀ can be deduced inversely. Under the condition of VonKarman spectrum, the relationship between the two is as follows:

where Δr represents the distance between two points, z _r represents the transmission distance of the beam, κ ₀ =2π/L ₀ , L ₀ is the outer scale of turbulence, Γ(·) is the gamma function, and K _5/6 is the third type of correction Bessel function.

Since coherent detection is a holographic detection method with high detection sensitivity and accuracy, the turbulence intensity measurement scheme based on coherent detection is _worthy of in-depth research. Methods. In addition, the traditional laser interferometer can only provide an interference signal of one polarization state, and the four-step phase shift method is often required to solve the light wave phase difference, and the movement of the optical lens will inevitably reduce the stability of the measurement and cause measurement errors.

Therefore, there is an urgent need to provide a novel split-wave plane orthogonal polarization interferometer to solve the above problems.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a demultiplexing plane orthogonal polarization interference system for measuring atmospheric coherence length and a method thereof, which can be combined with polarization elements to build an interference optical path, and can synchronously acquire sinusoidal term information including the wavefront phase difference of two sub-beams and For the cosine term information, the obtained real-time phase difference sequence is phase unwrapped, and the phase structure function is counted, and then the atmospheric coherence length on the transmission path is obtained.

In order to solve the above-mentioned technical problems, a technical solution adopted in the present invention is to provide a demultiplexing plane orthogonal polarization interference system for measuring atmospheric coherence length, including a laser emitting unit, a target reflecting target surface, a front dual-path optical receiving system, a positive Cross-polarization four-way interference system and signal acquisition and data processing system;

The laser emitting unit includes a laser and a beam expanding system for emitting a laser beam with a specified radius and wavelength band;

The target reflecting target surface is used to reflect the laser beam;

The front dual-path optical receiving system is used for collecting signal light and selecting two symmetrical sub-beams as received light;

The orthogonal polarization four-way interference system includes a filter, an attenuator, a polarizer, a half-wave plate, and two plane mirrors arranged along the transmission paths of the two sub-beams, and then a quarter-wave is arranged on one of the beam paths. The two beams are adjusted to make the transmission paths equal in length; the orthogonal polarization four-way interference system also includes a beam splitting prism, two polarized beam splitting prisms, and the transmitted light and reflected light of the beam splitter It is the result of the interference of two sub-beams, including the interference information of the horizontal polarization state and the vertical polarization state at the same time. The transmitted light and the reflected light respectively pass through the polarization beam splitter to separate the horizontal polarization state interference result and the vertical polarization state interference result to form four-way interference signals. ;

The signal acquisition and calculation system includes four photodetectors for collecting four-way polarization interference signals, and inputs the signals to a computer through a data acquisition card for calculating phase information.

In a preferred embodiment of the present invention, the target reflecting target surface is a plane reflecting mirror or a corner reflecting mirror.

In a preferred embodiment of the present invention, the front dual-path optical receiving system includes a telescope and a diaphragm.

In a preferred embodiment of the present invention, the polarization direction of the polarizer is horizontal, and the emitted light is horizontal linearly polarized light.

In a preferred embodiment of the present invention, the fast axis direction of the half-wave plate forms an angle of 22.5° with the polarization direction of the polarizer, and the polarization direction of the outgoing beam forms an angle of 45° with the horizontal direction.

In a preferred embodiment of the present invention, the direction of the fast axis of the quarter-wave plate is the horizontal direction or the vertical direction, so that the phase delay of π/2 is increased in the horizontal direction or the vertical direction of the light beam.

In a preferred embodiment of the present invention, the transmittance of the beam splitting prism is T=50%, so that the light intensity constant term in the transmitted light and the reflected light is eliminated, and only the interference term of the two sub-beams is included.

In a preferred embodiment of the present invention, the four photodetectors select a CCD camera for imaging detection when the structure is adjusted and installed, and after the optical path adjustment is completed, a photomultiplier tube PMT is used for data acquisition to achieve high frame rate. high-speed, high-sensitivity signal acquisition.

In order to solve the above-mentioned technical problems, another technical solution adopted in the present invention is: to provide a method of dividing wave plane orthogonal polarization interference for measuring atmospheric coherence length, adopting the method of measuring atmospheric coherence length according to any one of the above-mentioned dividing wave plane orthogonal polarization interference method. system, including the following steps:

Step 1: The laser passes through the beam expander system to form a parallel beam with a small divergence angle. The beam passes through the turbulent atmosphere to the target reflection target surface, and is reflected by the target reflection target surface and returns to the front dual optical receiving system at the same end as the laser;

Step 2: After the light beam passes through the front two-way optical receiving system, it forms two incident sub-beams, which are transmitted into the orthogonal polarization four-way interference system;

Step 3: The two light beams first pass through a filter, an attenuator and a polarizer whose polarization direction is horizontal to become horizontal linearly polarized light;

Step 4: After passing through the half-wave plate, the two linearly polarized lights become linearly polarized lights whose polarization direction and the horizontal direction are at an angle of 45°, and then adjust the transmission angle of the beam through two mirrors to maintain the horizontal transmission of the beam;

Step 5: After adjusting the propagation direction of the two linearly polarized lights by the mirror, they enter a beam splitting prism with a transmittance T of 50%, in which the fast axis is inserted in a beam of light (let it be A light) as the horizontal direction (or the vertical direction) The quarter-wave plate of , so that the beam produces a leading phase of π/2 in the horizontal polarization direction (or vertical polarization direction), and the other light (let it be B light) passes through two mirrors that adjust the optical path, Realize the effect of arm length of A light and B light;

The complex amplitude of the A-path light in the horizontal and vertical directions can be expressed as:

The complex amplitudes of the B-path light in the horizontal and vertical directions can be expressed as:

Step 6: The beam splitting prism realizes the interference of the transmitted light of the A light and the reflected light of the B light (let it be the C light), and the reflected light of the A light and the transmitted light of the B light (let it be the D light). The C light and D light emitted from the prism contain the interference information of the horizontal polarization state and the vertical polarization state at the same time;

Step 7: Both the C light and the D light pass through the polarization beam splitter prism to separate the interference light of the two polarization states to form four interference lights, which are the horizontal polarization state of the C light, the vertical polarization state of the C light, and the horizontal polarization state of the D light. state and vertical polarization state of D light, four photodetectors simultaneously collect four channels of interference information containing different polarization states;

Step 8: Use four photodetectors for synchronization signal acquisition. When installing and debugging the orthogonal polarization four-way interference system, the photodetector selects a CCD camera to receive the interference pattern. By adjusting the mirrors in steps 3 and 4 There is no inclination angle between the two incident lights, forming an interference pattern with a spatial frequency of 1;

解算出相位差并进行实时显示、储存操作，统计相位差序列的方差获得相位结构函数，并根据相位结构函数与大气相干长度的函数关系，推得大气相干长度r₀。Step 9: The interference signal received by the photodetector is transmitted into the data acquisition card and transmitted to the computer, and the real-time phase calculation is carried out through the solution program. The solution program subtracts the collected four-way interference signals two by two to obtain respectively Contains the information of the sine and cosine terms of the phase difference on the transmission paths of the two beams, further according to Euler's formula

Calculate the phase difference and perform real-time display and storage operations, count the variance of the phase difference sequence to obtain the phase structure function, and deduce the atmospheric coherence length r ₀ according to the functional relationship between the phase structure function and the atmospheric coherence length.

In a preferred embodiment of the present invention, the specific steps of the solution program include:

C光的垂直偏振态光强记为

D光的水平偏振态光强记为

D光的垂直偏振态光强记为

S901: Simultaneously record four-way polarization interference signals, and the four light intensity signals received by the photodetector are: the horizontal polarization state light intensity of C light is recorded as

The vertical polarization state light intensity of C light is recorded as

The horizontal polarization state light intensity of D light is recorded as

The vertical polarization state light intensity of D light is recorded as

S902: Subtract the interference signals of the same polarization state two by two to obtain the sine term and cosine term containing the phase difference between the two sub-beams respectively. For the horizontal polarization state interference signal:

For vertically polarized interference signals:

S903: According to Euler's formula e ^ix =cos(x)+i sin(x), the phase difference between the A-path light and the B-path light is calculated as:

S904: perform statistics on the calculated phase sequence to obtain the phase structure function

与传输路径上大气相干长度r₀的关系为：S905: Consider the Von Karman spectrum of the atmosphere, and calculate the phase structure function between the two sub-beams

The relationship with the atmospheric coherence length r ₀ on the transmission path is:

where κ ₀ =2π/L ₀ , L ₀ is the outer scale of the turbulent flow, Γ(·) is the gamma function, and K _5/6 is the third type of modified Bessel function;

Further obtain the atmospheric coherence length on the transmission path.

The beneficial effects of the present invention are:

(1) The present invention provides a wave-splitting orthogonal polarization interferometer, which utilizes the wavefront of the laser echo to perform orthogonal polarization coherence detection, thereby obtaining the atmospheric coherence length on the measurement path. This method does not need to move the optical path during measurement. It can solve the wavefront phase difference in real time, and has high detection sensitivity and detection frequency;

(2) The present invention combines polarization elements to synchronously collect four polarization interference components, and can measure the phase fluctuation on the transmission path in real time, and then calculate the phase structure function according to the phase fluctuation, and invert the atmospheric coherence length r ₀ . Compared with the existing direct detection system based on differential image motion, the photomultiplier tube is used as the detector in this system, which can greatly improve the detection frequency and detection sensitivity;

(3) Compared with the traditional interferometer, the present invention obtains four-channel interference signals synchronously by controlling the polarization state of the beam, and calculates the phase difference information between the two beams in real time. During the measurement process, the system as a whole does not move. components to improve the stability of the system;

(4) This interference system adopts the design of equal arm length. The optical path difference between the two beams in the receiving system is fixed and basically zero. Therefore, the turbulence on the transmission path is the main reason for the phase difference between the two beams. The fluctuation of the phase difference reflects the intensity change of the atmospheric turbulence. The invention belongs to the coherent detection technology and greatly improves the detection sensitivity.

Description of drawings

Fig. 1 is the structural block diagram of the demultiplexing plane orthogonal polarization interference system for measuring atmospheric coherence length of the present invention;

Fig. 2 is the optical path structure diagram of the demultiplexing plane orthogonal polarization interference system for measuring atmospheric coherence length;

Fig. 3 is the schematic diagram of polarizing element regulating and controlling the light field;

Fig. 4 is the flow chart of the orthogonal polarization interferometry method of described measuring atmospheric coherence length;

Figure 5 is a flowchart of the phase demodulation algorithm.

The labels of the components in the accompanying drawings are as follows: 1, laser, 2, beam expander system, 3, target reflecting target surface, 4, telescope, 5, diaphragm, 6, filter, 7, attenuator, 8, polarizer , 9, half-wave plate, 10, mirror, 11, quarter-wave plate, 12, beam splitter prism, 13, polarized beam splitter prism, 14, photodetector, 15, data acquisition card, 16, computer, 17, Laser emitting unit, 18. Front two-way optical receiving system, 19. Quadrature polarization four-way interference system, 20. Signal acquisition and data processing system.

Detailed ways

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, and the protection scope of the present invention can be more clearly defined.

Please refer to FIG. 1 and FIG. 2, the embodiments of the present invention include:

A demultiplexing plane orthogonal polarization interference system for measuring atmospheric coherence length, comprising a laser emitting unit 17, a target reflection target surface 3, a front two-way optical receiving system 18, an orthogonal polarization four-way interference system 19, as well as signal acquisition and data processing system 20.

The laser emitting unit 17 includes a laser 1 and a beam expansion system 2 for emitting a laser beam with a specified radius and a wavelength band; the target reflecting target surface 3 is a plane mirror or a corner mirror for reflecting the laser beam; the The front two-way optical receiving system 18 includes two telescopes 4 and two apertures 5, which are used to collect signal light and select two symmetrical sub-beams as the received light; the orthogonal polarization four-way interference system 19 comprises The optical filter 6, the attenuation plate 7, the polarizer 8, the half-wave plate 9, and the two plane mirrors 10 are arranged in the transmission path of the sub-beam, and then a quarter-wave plate 11 and a mirror 10 are arranged on one of the optical paths. The other path of light passes through the two mirrors 20, and the two beams are adjusted to make the transmission path lengths equal; It is the result of the interference of the two sub-beams, and includes the interference information of the horizontal polarization state and the vertical polarization state. The transmitted light and the reflected light pass through the polarization beam splitter prism 13 to separate the horizontal polarization state interference result and the vertical polarization state interference result respectively, forming a four-way interference Signal; the signal acquisition and calculation system 20 includes four photodetectors 14 for collecting four-way polarization interference signals, and inputs the signals to the computer 16 through the data acquisition card 15 for calculating the phase information.

Further, the power of the laser 1 is adjustable, and the emitted laser light belongs to the visible light band, and the focal length of the beam expanding system 2 can be adjusted according to the transmission distance.

Further, in the orthogonal polarization four-way interference system 19, the filter 6 is a narrow-band filter, which can reduce the influence of background light. The two flat reflection mirrors 10 behind the half-wave plate 9 are single-wavelength high reflection mirrors, which are used to adjust the optical path difference between the two light beams and reduce the influence of ambient light. The transmittance T of the beam splitting prism 12 is 50%, and the transmitted light and the reflected light are the result of the interference of two incident sub-beams. The transmittance T of the two polarizing beam splitting prisms 13 is 50%, which separates the horizontal polarization state and the vertical polarization state of the interference light respectively, thereby forming four-way interference signals.

Further, the four photodetectors 14 use a CCD camera or a photomultiplier tube PMT to synchronously record the polarization interference information including the phase difference of the two beams of light.

The optical path principle of the orthogonal polarization interference system of the splitter plane is as follows:

The laser starts from the transmitting unit, passes through the turbulent path to the target reflection target surface 3 at a distance L, and then returns to the front dual optical receiving system 18 through the same transmission path. The beam passes through the front dual optical receiving system 18. It is collimated and incident on the orthogonal polarization four-way interference system 19. First, the two incident beams pass through the filter 6 and the attenuation plate 7 to weaken the ambient background light, and then polarize into the horizontal linearly polarized light through the polarizing plate 8, and then the horizontal linearly polarized light. Through the half-wave plate 9 whose fast axis is at an angle of 22.5° to the horizontal direction, the light beam becomes linearly polarized light whose polarization direction is at an angle of 45° to the horizontal direction.

After passing through the half-wave plate 9, the two beams of 45° linearly polarized light enter the corresponding interference arms through the two mirrors 10 respectively, and a quarter-wave plate 11 is inserted into one of the interference arms, and this interference arm is called A light, The fast axis of the quarter-wave plate is in the horizontal direction (or vertical direction), so that the optical path generates a leading phase of π/2 in the horizontal polarization direction (or vertical polarization direction), and the other interference arm is B light passing through two The adjustment arm length of the mirror 10 is equal to the arm length of the A light. It should be pointed out that, when adjusting the mirror 10 of the interference optical path, the spatial frequency of the interference fringes needs to be adjusted to 1, so that the overall phase of the interference pattern is equal. When the turbulent intensity on the transmission path changes, the difference between the two sub-beams The phase difference will fluctuate, thereby causing the brightness of the interference pattern to change. At this time, the sine and cosine terms of the phase difference of the two sub-beams are included in the four-way polarization interference signal.

The two light beams interfere through the beam splitter prism 12, and the beam splitter prism 12 realizes the interference of the transmitted light of the A light and the reflected light of the B light, so that it is the C light, and the reflected light of the A light and the transmitted light of the B light interfere, so that it is the D light At this time, the C light and D light emitted by the beam splitter prism 12 contain the interference information of the horizontal polarization state and the vertical polarization state at the same time. Further, the interference light passes through the polarization beam splitter prism 13 to separate the horizontal polarization state and the vertical polarization state to form four interference lights, which are the horizontal polarization state of C light, the vertical polarization state of C light, the horizontal polarization state of D light, and the horizontal polarization state of D light. In the vertical polarization state, the four-way interference signal will be further transmitted into the signal acquisition and data processing system 20 .

The signal acquisition and data processing system 20 first collects four paths of interference light with different polarization states simultaneously through the four photodetectors 14. The core part of the interference of the two sub-beams and the details of the polarization states are shown in FIG. The optical signal is converted into an electrical signal through the photodetector 14, and is transmitted to the computer 16 through the data acquisition card 15, and the real-time phase calculation is performed through the calculation program.

Referring to FIG. 4 , an example of the present invention also provides a method for measuring the coherence length of the atmospheric coherence by dividing the wave plane orthogonal polarization interference method, which includes the following steps:

Step 1: The laser passes through the beam expander system 2 to form a parallel beam with a small divergence angle. The beam passes through the turbulent atmosphere to the target reflection target surface 3, and is reflected back to the front dual optical receiver at the same end as the laser 1 through the target reflection target surface 3. system 18;

Step 2: After the light beam passes through the front two-way optical receiving system 18, two incident sub-beams are formed, which are transmitted into the orthogonal polarization four-way interference system 19;

Step 3: The two beams first pass through the filter 6, the attenuation plate 7 and the polarizer 8 whose polarization direction is horizontal to become horizontal linearly polarized light;

Step 4: the two linearly polarized lights become linearly polarized light whose polarization direction and the horizontal direction are at an angle of 45° after passing through the half-wave plate 9, and then adjust the transmission angle of the light beam through two mirrors 10 to keep the light beam horizontally transmitted;

Step 5: After adjusting the propagation direction of the two linearly polarized lights by the reflector 10, they enter the beam splitting prism 12 with a transmittance T of 50%, wherein the fast axis is inserted in a beam path (let it be the A light) in the horizontal direction (or vertical direction). direction), so that the beam produces a leading phase of π/2 in the horizontal polarization direction (or vertical polarization direction), and the other light (let it be B light) is reflected by two adjusted optical paths After the mirror 10, the effect of the arm length of the A light and the B light is realized;

The complex amplitude of the A-path light in the horizontal and vertical directions can be expressed as:

The complex amplitudes of the B-path light in the horizontal and vertical directions can be expressed as:

Step 6: The beam splitter prism 12 realizes the interference of the transmitted light of the A light and the reflected light of the B light (let it be the C light), and the reflected light of the A light and the transmitted light of the B light interfere (let it be the D light). The C light and the D light emitted by the beam splitter prism 12 contain the interference information of the horizontal polarization state and the vertical polarization state at the same time;

Step 7: Both the C light and the D light pass through the polarization beam splitter prism 13, and the interference light of the two polarization states is separated to form four interference lights, which are the horizontal polarization state of the C light, the vertical polarization state of the C light, and the horizontal polarization state of the D light. The polarization state, the vertical polarization state of the D light, and the four photodetectors 14 simultaneously collect four-way interference information including different polarization states;

Step 8: Use four photodetectors 14 to collect synchronization signals. When installing and debugging the orthogonal polarization four-way interference system, the photodetectors 14 select a CCD camera to receive the interference pattern. The reflector 10 makes no inclination angle between the two incident lights, and forms an interference pattern with a spatial frequency of 1, so that the overall phase of the interference pattern is equal. Since the system has no moving structure and no additional phase difference is introduced, the phase caused by the turbulent atmosphere at this time is The difference change is manifested as the light and dark fluctuation of the interference pattern, that is, the light and dark change of the interference pattern is only caused by the turbulent atmosphere. After the debugging is completed, in order to improve the weak light detection capability and detection frequency of the system, it is necessary to replace the CCD with a photomultiplier tube PMT;

Step 9: The interference signal received by the photodetector 14 is transmitted into the data acquisition card 15 and transmitted to the computer 16, and the real-time phase calculation is carried out through a solution program, which subtracts the collected four-way interference signals two by two , obtain the information of the sine and cosine terms containing the phase difference on the transmission paths of the two beams respectively, and further according to Euler's formula e ^ix =cos(x)+i sin(x), solve the phase difference and perform real-time display and storage operations, The phase structure function is obtained by counting the variance of the phase difference sequence, and the atmospheric coherence length r ₀ is deduced according to the functional relationship between the phase structure function and the atmospheric coherence length.

With reference to Figure 5, the specific steps of the solution program include:

C光的垂直偏振态光强记为

D光的水平偏振态光强记为

D光的垂直偏振态光强记为

S901: synchronously record four-way polarization interference signals, the four light intensity signals received by the photodetector 14 are respectively: the horizontal polarization state light intensity of C light is recorded as

The vertical polarization state light intensity of C light is recorded as

The horizontal polarization state light intensity of D light is recorded as

The vertical polarization state light intensity of D light is recorded as

S902: Subtract the interference signals of the same polarization state two by two to obtain the sine term and cosine term containing the phase difference between the two sub-beams respectively. For the horizontal polarization state interference signal:

For vertically polarized interference signals:

S903: According to Euler's formula e ^ix =cos(x)+i sin(x), the phase difference between the A-path light and the B-path light is calculated as:

S904: perform statistics on the calculated phase sequence to obtain the phase structure function

与传输路径上大气相干长度r₀的关系为：S905: Consider the Von Karman spectrum of the atmosphere, and calculate the phase structure function between the two sub-beams

The relationship with the atmospheric coherence length r ₀ on the transmission path is:

where κ ₀ =2π/L ₀ , L ₀ is the outer scale of the turbulent flow, Γ(·) is the gamma function, and K _5/6 is the third type of modified Bessel function;

Further obtain the atmospheric coherence length on the transmission path.

The above descriptions are only the embodiments of the present invention, and are not intended to limit the scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the description and drawings of the present invention, or directly or indirectly applied to other related technologies Fields are similarly included in the scope of patent protection of the present invention.

Claims (10)

1. A wave-splitting surface orthogonal polarization interference system for measuring atmospheric coherence length is characterized by comprising a laser emission unit, a target reflection target surface, a front-mounted two-path optical receiving system, an orthogonal polarization four-path interference system and a signal acquisition and data processing system;

the laser emitting unit comprises a laser and a beam expanding system and is used for emitting laser beams with specified radius and wave bands;

the target reflection target surface is used for reflecting laser beams;

the preposed two-way optical receiving system is used for collecting signal light and selecting two symmetrical sub-beams as received light;

the orthogonal polarization four-path interference system comprises an optical filter, an attenuation plate, a polarizing plate, a half-wave plate and two plane reflectors which are arranged along the transmission paths of the two sub-beams, then a quarter-wave plate and a reflector are arranged on one light path, and the other light path passes through the two reflectors and the two light beams are adjusted to enable the lengths of the transmission paths to be equal; the cross-polarization four-path interference system also comprises a beam splitter prism and two polarization beam splitter prisms, wherein transmitted light and reflected light of the beam splitter prism are interference results of two sub-beams and simultaneously contain interference information of a horizontal polarization state and a vertical polarization state, and the transmitted light and the reflected light are respectively subjected to polarization beam splitter prism to separate the interference results of the horizontal polarization state and the vertical polarization state so as to form four-path interference signals;

the signal acquisition and calculation system comprises four photoelectric detectors for acquiring four paths of polarization interference signals, and inputting the signals to a computer through a data acquisition card for calculating phase information.

2. The system according to claim 1, wherein the target surface is a plane mirror or an angle mirror.

3. The system according to claim 1, wherein the front-mounted two-way optical receiving system comprises a telescope and a diaphragm.

4. The system according to claim 1, wherein the polarization direction of the polarizer is horizontal, and the emergent light is horizontally linearly polarized light.

5. The system according to claim 1, wherein the half-wave plate has a fast axis at an angle of 22.5 ° to the polarization direction of the polarizer, and the emergent beam has a polarization at an angle of 45 ° to the horizontal.

6. The system according to claim 1, wherein the fast axis direction of the quarter-wave plate is horizontal or vertical, so that the phase delay of pi/2 is increased in the horizontal or vertical direction of the light beam.

7. The system according to claim 1, wherein the transmission T of the splitting prism is 50%, so that the light intensity constant term in the transmitted light and the reflected light is eliminated, and only the interference term of the two sub-beams is included.

8. The system according to claim 1, wherein the four photodetectors are configured to perform imaging detection by using a CCD camera during structure adjustment and installation, and perform data acquisition by using a photomultiplier tube PMT after completion of optical path adjustment, so as to achieve high frame rate and high sensitivity signal acquisition.

9. A method for measuring orthogonal polarization interference of a partial wave surface of an atmospheric coherence length, which adopts the orthogonal polarization interference system of the partial wave surface of the atmospheric coherence length according to any one of claims 1 to 8, and comprises the following steps:

step 1: the laser forms parallel beams with smaller divergence angle through the beam expanding system, the beams pass through turbulent atmosphere to reach a target reflection target surface, and are reflected back to a front-mounted two-way optical receiving system at the same end as the laser through the target reflection target surface;

step 2: after passing through a front two-path optical receiving system, the light beam forms two incident sub-light beams which are transmitted into an orthogonal polarization four-path interference system;

and step 3: the two light beams firstly pass through a light filter, an attenuation sheet and a polarizing sheet with horizontal polarization direction to become horizontal linear polarization light;

and 4, step 4: the two linearly polarized lights become linearly polarized lights with the polarization direction forming an angle of 45 degrees with the horizontal direction after passing through the half-wave plate, and then the transmission angle of the light beam is adjusted through the two reflectors to keep the light beam horizontally transmitted;

and 5: two lines of polarized light enter a beam splitting prism with the transmissivity T of 50% after the transmission direction is adjusted by a reflector, wherein a quarter-wave plate with a fast axis in the horizontal direction (or the vertical direction) is inserted into one light path (the light is A light), so that the light beam generates a pi/2 advanced phase in the horizontal polarization direction (or the vertical polarization direction), and the other light (the light is B light) passes through the two reflectors with the optical path adjusted, so that the effect of equal arm length of the light A and the light B is realized;

the complex amplitudes of the a-way light in the horizontal and vertical directions can be expressed as:

the complex amplitudes of the B-path light in the horizontal and vertical directions can be expressed as:

step 6: the beam splitter prism realizes interference of the transmitted light of the light A and the reflected light of the light B (the transmitted light is made to be C light), interference of the reflected light of the light A and the transmitted light of the light B (the transmitted light is made to be D light), and the C light and the D light emitted by the beam splitter prism simultaneously contain interference information of a horizontal polarization state and a vertical polarization state;

and 7: the C light and the D light pass through the polarization beam splitter prism to split the two polarization state interference lights to form four paths of interference lights which are respectively the horizontal polarization state of the C light, the vertical polarization state of the C light, the horizontal polarization state of the D light and the vertical polarization state of the D light, and four photoelectric detectors simultaneously acquire four paths of interference information containing different polarization states;

and 8: using four photoelectric detectors to acquire synchronous signals, selecting a CCD camera by the photoelectric detectors to receive interference patterns when installing and debugging the orthogonal polarization four-path interference system, and adjusting the reflectors in the step 3 and the step 4 to ensure that no inclination angle exists between two incident lights so as to form the interference patterns with the spatial frequency of 1;

and step 9: interference signals received by the photoelectric detector are transmitted into a data acquisition card and transmitted to a computer, real-time phase solution is carried out through a solution program, the solution program subtracts every two of the four paths of interference signals to respectively obtain sine item and cosine item information containing phase difference on two light beam transmission paths, and the information is further processed according to an Euler formula

Resolving phase difference, performing real-time display and storage operation, counting the variance of the phase difference sequence to obtain a phase structure function, and deriving the atmospheric coherence length r according to the functional relationship between the phase structure function and the atmospheric coherence length ₀ 。

10. The method for measuring the orthogonal polarization interference of the wave-splitting surface of the atmospheric coherence length according to claim 9, wherein the specific steps of the calculation program comprise:

s901: four paths of polarization interference signals are synchronously recorded, and four light intensity signals received by the photoelectric detector are respectively as follows: the intensity of the horizontally polarized light of C light is recorded as

The intensity of vertically polarized light of C light is recorded as

Of D lightThe intensity of horizontally polarized light is recorded as

The intensity of the vertically polarized light of D light is recorded as

S902: subtracting interference signals in the same polarization state pairwise to respectively obtain a sine term and a cosine term containing the phase difference between the two sub-beams, and for the interference signals in the horizontal polarization state:

for a vertical polarization state interference signal:

s903: according to Euler's formula e ^ix The phase difference between the a-path light and the B-path light is calculated as:

s904: the calculated phase sequence is counted to obtain a phase structure function

S905: considering the atmosphere as Von Karman spectrum, counting the phase structure function between two sub-beams

Length r of coherence with atmosphere on transmission path ₀ The relationship of (1) is:

in the formula kappa ₀ ＝2π/L ₀ ，L ₀ For the turbulent outer dimension, Γ (·) is the gamma function, K _5/6 A modified Bessel function for the third class;

further obtaining the atmospheric coherence length on the transmission path.

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