Wave plane orthogonal polarization interference system and method for measuring atmospheric coherence length

Abstract

The present invention discloses a wave-plane orthogonal polarization interference system for measuring atmospheric coherence length, including a laser emitting unit, a target reflection target surface, a front two-way optical receiving system, an orthogonal polarization four-way interference system, and a signal acquisition and data processing system; the front two-way optical receiving system is used to realize beam acquisition and collimation; the orthogonal polarization interference system regulates the polarization state of two light beams, and outputs four interference signals with different polarization states; the signal acquisition and solution system will synchronously record the four-way interference signals, solve the phase difference and statistically calculate the phase structure function, and finally obtain the atmospheric coherence length on the transmission path. A wave-plane orthogonal polarization interference method for measuring atmospheric coherence length is also disclosed. The present invention selects two symmetrically distributed sub-beams in the received echo light field, combines the polarization element and the interference system to convert the phase difference fluctuation into the change of the interference pattern intensity, thereby realizing the measurement of the turbulence intensity signal on the transmission path.

Description

Technical Field

[0001] The invention relates to the technical field of atmospheric optical detection, and in particular to a wave-plane orthogonal polarization interference system for measuring atmospheric coherence length and a method thereof. Background Art

[0002] When light waves are transmitted in random turbulent media, they are affected by the fluctuations in the atmospheric refractive index, resulting in phase fluctuations and light intensity flickering, which limits the application of free-space optical communications, high-resolution optical imaging and other systems in the atmosphere. Measuring the atmospheric coherence length on the light propagation path is of great significance for evaluating the working efficiency of advanced optoelectronic systems.

[0003] Fried proposed the atmospheric coherence length r in 1965. 0 After the concept of 0 The measurement technology is constantly developing and improving. Among them, the differential image motion method (DIMM) has gradually become the most widely used method due to its anti-vibration interference characteristics. 0 The measurement method is based on the fluctuation of the arrival angle at different points on the same wavefront to obtain r 0 However, since the differential image motion method belongs to direct detection, it has the characteristics of limited detection accuracy and is not suitable for weak light conditions. Coherent detection can greatly improve the detection efficiency and detection accuracy, and has better weak light detection performance.

[0004] Since atmospheric turbulence is a random process, the physical quantity that measures its intensity is the second-order statistics of light waves, namely the structure function. The structure function of light waves is the sum of the amplitude structure function and the phase structure function. Under the paraxial approximation, the amplitude structure function is approximately zero, and the phase structure function is approximately equal to the wavefront structure function. Therefore, the phase fluctuation of the light beam can be measured and its structure function can be statistically analyzed, and then the phase structure function can be used to obtain the amplitude structure function. Coherence length with atmosphere r 0 The relationship between 0 , under the VonKarman spectrum condition, the relationship between the two is as follows:

[0005]

[0006] Where Δr represents the distance between two points, z r represents the transmission distance of the light beam, κ 0 =2π / L 0 , L 0 is the outer scale of turbulence, Γ(·) is the gamma function, K 5 / 6 is the modified Bessel function of the third kind.

[0007] Since coherent detection belongs to the holographic detection method with high detection sensitivity and accuracy, the turbulence intensity measurement scheme based on coherent detection deserves further study. However, there is no method to obtain the phase structure function by coherent detection of laser wavefront and further calculate r. 0 In addition, the traditional laser interferometer can only provide an interference signal of a polarization state, and the four-step phase shift method is often required to solve the phase difference of light waves. The movement of the optical lens will inevitably reduce the stability of the measurement and cause measurement errors.

[0008] Therefore, there is an urgent need to provide a new type of waveplane orthogonal polarization interferometer to solve the above problems. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide a waveplane orthogonal polarization interference system and method for measuring the atmospheric coherence length. By combining polarization elements to build an interference optical path, the sine term information and cosine term information containing the wavefront phase difference of two sub-beams can be synchronously obtained. By phase unwrapping the acquired real-time phase difference sequence and statistically analyzing the phase structure function, the atmospheric coherence length on the transmission path can be obtained.

[0010] In order to solve the above technical problems, a technical solution adopted by the present invention is: to provide a wave plane orthogonal polarization interference system for measuring atmospheric coherence length, including a laser emitting unit, a target reflection target surface, a front two-way optical receiving system, an orthogonal polarization four-way interference system and a signal acquisition and data processing system;

[0011] The laser emitting unit includes a laser and a beam expansion system, which is used to emit a laser beam of a specified radius and wavelength band;

[0012] The target reflective surface is used to reflect the laser beam;

[0013] The front dual-path optical receiving system is used to collect signal light and select two symmetrical sub-beams as received light;

[0014] The orthogonal polarization four-way interference system comprises a filter, an attenuation plate, a polarizer, a half-wave plate, and two plane reflectors arranged along the transmission paths of the two sub-beams, and then a quarter-wave plate and a reflector are arranged on the path of one of the light beams, and the other light beam passes through the two reflectors, and the two light beams are adjusted so that the transmission path lengths are equal; the orthogonal polarization four-way interference system also comprises a beam splitter prism and two polarization beam splitter prisms, the transmitted light and the reflected light of the beam splitter prism are the results of the interference of the two sub-beams, and contain the interference information of the horizontal polarization state and the vertical polarization state, and the transmitted light and the reflected light pass through the polarization beam splitter prism respectively to separate the interference result of the horizontal polarization state and the interference result of the vertical polarization state, so as to form a four-way interference signal;

[0015] The signal acquisition and solution system includes four photoelectric detectors for acquiring four-way polarization interference signals, and inputting the signals into a computer through a data acquisition card for solving phase information.

[0016] In a preferred embodiment of the present invention, the target reflection target surface is a plane reflector or a corner reflector.

[0017] In a preferred embodiment of the present invention, the front two-way optical receiving system includes a telescope and an aperture.

[0018] In a preferred embodiment of the present invention, the polarization direction of the polarizer is horizontal, and the output light is horizontal linear polarized light.

[0019] 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 light beam forms an angle of 45° with the horizontal direction.

[0020] In a preferred embodiment of the present invention, the fast axis direction of the quarter wave plate is in the horizontal direction or the vertical direction, so that a phase delay of π / 2 is added in the horizontal direction or the vertical direction of the light beam.

[0021] In a preferred embodiment of the present invention, the transmittance T of the beam splitter 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.

[0022] In a preferred embodiment of the present invention, the four photodetectors select a CCD camera for imaging detection during structural adjustment and installation, and after the optical path adjustment is completed, a photomultiplier tube PMT is used for data acquisition to achieve high frame rate and high sensitivity signal acquisition.

[0023] In order to solve the above technical problems, another technical solution adopted by the present invention is: to provide a wave plane orthogonal polarization interference method for measuring atmospheric coherence length, using the wave plane orthogonal polarization interference system for measuring atmospheric coherence length as described in any of the above items, comprising the following steps:

[0024] Step 1: The laser passes through the beam expansion system to form a parallel beam with a small divergence angle. The beam passes through the turbulent atmosphere to reach the target reflection target surface, and is reflected back to the front dual-path optical receiving system at the same end as the laser.

[0025] Step 2: After the light beam passes through the front two-way optical receiving system, it forms two incident sub-beams and transmits into the orthogonal polarization four-way interference system;

[0026] Step 3: The two light beams first pass through a filter, an attenuator, and a polarizer with a horizontal polarization direction to become horizontally polarized light;

[0027] Step 4: After passing through the half-wave plate, the two linearly polarized lights become linearly polarized lights with a polarization direction at an angle of 45° to the horizontal direction. After that, the transmission angle of the light beam is adjusted through two reflectors to keep the light beam transmitted horizontally.

[0028] Step 5: After the propagation direction of the two linearly polarized lights is adjusted by the reflector, they enter the beam splitter with a transmittance T of 50%. A quarter-wave plate with a fast axis in the horizontal direction (or vertical direction) is inserted into one beam path (let it be light A), so that the light beam produces a π / 2 leading phase in the horizontal polarization direction (or vertical polarization direction). After the other light (let it be light B) passes through two reflectors for adjusting the optical path, the effect of equal arm lengths of light A and light B is achieved;

[0029] The complex amplitude of light path A in the horizontal and vertical directions can be expressed as:

[0030]

[0031] The complex amplitude of light path B in the horizontal and vertical directions can be expressed as:

[0032]

[0033] Step 6: The beam splitter prism realizes the interference of the transmitted light of light A and the reflected light of light B (let it be light C), and the interference of the reflected light of light A and the transmitted light of light B (let it be light D). At this time, the light C and light D emitted by the beam splitter prism contain interference information of both horizontal polarization state and vertical polarization state;

[0034] Step 7: Both light C and light D pass through a polarization beam splitter to separate the two polarization interference lights, forming four interference lights, namely, the horizontal polarization state of light C, the vertical polarization state of light C, the horizontal polarization state of light D, and the vertical polarization state of light D. Four photodetectors simultaneously collect interference information of four paths containing different polarization states;

[0035] Step 8: Use four photodetectors to collect synchronous signals. When installing and debugging the orthogonal polarization four-way interference system, the photodetector selects the CCD camera to receive the interference pattern. By adjusting the reflectors in steps 3 and 4, there is no inclination between the two incident lights, forming an interference pattern with a spatial frequency of 1.

[0036] Step 9: The interference signal received by the photodetector is transmitted to the data acquisition card and then to the computer. The real-time phase solution is performed by the solution program. The solution program subtracts the collected four interference signals in pairs to obtain the sine term and cosine term information containing the phase difference on the transmission path of the two light beams. Further, according to the Euler formula e ix=cos(x)+isin(x), calculate the phase difference and display and store it in real time, calculate the variance of the phase difference sequence to obtain the phase structure function, and deduce the atmospheric coherence length r according to the functional relationship between the phase structure function and the atmospheric coherence length 0 .

[0037] In a preferred embodiment of the present invention, the specific steps of the solution program include:

[0038] S901: Synchronously record the four-way polarization interference signal. The four light intensity signals received by the photodetector are: The horizontal polarization intensity of C light is recorded as The vertical polarization intensity of C light is recorded as The horizontal polarization intensity of light D is recorded as The vertical polarization intensity of D light is recorded as

[0039] S902: Subtract the interference signals of the same polarization state from each other to obtain the sine term and cosine term containing the phase difference between the two sub-beams. For the horizontal polarization interference signal:

[0040]

[0041] For the vertical polarization interference signal:

[0042]

[0043] S903: According to Euler's formula e ix =cos(x)+i sin(x), the phase difference between light path A and light path B is calculated as:

[0044]

[0045] S904: Perform statistics on the calculated phase sequence to obtain the phase structure function

[0046] S905: Considering the atmosphere as a Von Karman spectrum, calculate the phase structure function between the two sub-beams The atmospheric coherence length r on the transmission path 0 The relationship is:

[0047]

[0048] Where κ 0 =2π / L 0 , L 0 is the outer scale of turbulence, Γ(·) is the gamma function, K 5 / 6 is the third kind of modified Bessel function;

[0049] The atmospheric coherence length on the transmission path is further obtained.

[0050]

[0051] The beneficial effects of the present invention are:

[0052] (1) The present invention provides a wavefront orthogonal polarization interferometer, which uses the wavefront of the laser echo to perform orthogonal polarization coherent detection, thereby obtaining the atmospheric coherence length on the measurement path. This method does not require moving components in the optical path during measurement, can solve the wavefront phase difference in real time, and has high detection sensitivity and detection frequency;

[0053] (2) The present invention combines polarization elements to synchronously collect four polarization interference components, which can measure the phase fluctuation on the transmission path in real time, and then calculate the phase structure function based on the phase fluctuation, and invert the atmospheric coherence length r 0 Compared with the existing direct detection system based on differential image motion, the use of photomultiplier tubes as detectors in this system can greatly improve the detection frequency and detection sensitivity;

[0054] (3) Compared with the traditional interferometer, the present invention synchronously obtains four-channel interference signals by regulating the polarization state of the light beam, and calculates the phase difference information between the two light beams in real time. During the measurement process, the system as a whole has no moving parts, which improves the stability of the system;

[0055] (4) This interference system adopts an equal arm length design. The optical path difference between the two light 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 light beams. The fluctuation of the phase difference calculated by the interference system reflects the intensity change of atmospheric turbulence. The present invention belongs to coherent detection technology and greatly improves the detection sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is a structural block diagram of the wave plane orthogonal polarization interference system for measuring atmospheric coherence length of the present invention;

[0057] Figure 2 is a diagram of the optical path structure of the wave-plane orthogonal polarization interference system for measuring the atmospheric coherence length;

[0058] Figure 3 It is a schematic diagram of the regulation of the light field by the polarization element;

[0059] Figure 4 It is a flow chart of the wave plane orthogonal polarization interference method for measuring the atmospheric coherence length;

[0060] Figure 5 is a flow chart of the phase demodulation algorithm.

[0061] The components in the accompanying drawings are marked as follows: 1. Laser, 2. Beam expansion system, 3. Target reflection target surface, 4. Telescope, 5. Aperture, 6. Filter, 7. Attenuation plate, 8. Polarizer, 9. Half-wave plate, 10. Reflector, 11. Quarter-wave plate, 12. Spectral prism, 13. Polarization spectroscopic prism, 14. Photoelectric detector, 15. Data acquisition card, 16. Computer, 17. Laser emitting unit, 18. Front two-way optical receiving system, 19. Orthogonal polarization four-way interference system, 20. Signal acquisition and data processing system. DETAILED DESCRIPTION

[0062] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0063] See also Figure 1 and Figure 2 , the embodiment of the present invention includes:

[0064] A wave plane orthogonal polarization interference system for measuring atmospheric coherence length comprises 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 and a signal acquisition and data processing system 20.

[0065] The laser emitting unit 17 includes a laser 1 and a beam expansion system 2, which are used to emit a laser beam of a specified radius and band; the target reflection target surface 3 is a plane reflector or an angle reflector, which is used to reflect the laser beam; the front dual-path 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 received light; the orthogonal polarization four-path interference system 19 includes a filter 6, an attenuation plate 7, a polarizer 8, a half-wave plate 9, and two plane reflectors 10 arranged along the transmission paths of the two sub-beams, and then a quarter-wave plate 11 and a reflector 10 are arranged on one of the light paths, and the other light passes through two reflectors 20 , adjusting the two light beams so that the transmission path lengths are equal; the orthogonal polarization four-way interference system 19 also includes a beam splitter prism 12 and two polarization beam splitter prisms 13. The transmitted light and reflected light of the beam splitter prism 12 are the results of interference between the two sub-beams, and contain interference information of the horizontal polarization state and the vertical polarization state. The transmitted light and the reflected light are respectively separated by the polarization beam splitter prism 13 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 solution system 20 includes four photodetectors 14, which are used to collect four-way polarization interference signals, and input the signals to the computer 16 through the data acquisition card 15 to solve the phase information.

[0066] Furthermore, the power of the laser 1 is adjustable, and the emitted laser belongs to the visible light band, and the focal length of the beam expansion system 2 can be adjusted according to the transmission distance.

[0067] Furthermore, in the orthogonal polarization four-way interference system 19, the filter 6 is a narrow-band filter that can reduce the influence of background light. The two plane reflectors 10 behind the half-wave plate 9 are single-wavelength high reflectors that 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 splitter prism 12 is 50%, and its transmitted light and reflected light are the result of interference of two incident sub-beams. The transmittance T of the two polarization beam splitters 13 is 50%, which separates the horizontal polarization state and the vertical polarization state of the interference light, thereby forming a four-way interference signal.

[0068] Furthermore, the four photodetectors 14 use CCD cameras or photomultiplier tubes PMTs to synchronously record polarization interference information including the phase difference between the two light beams.

[0069] The optical path principle of the split-wave plane orthogonal polarization interference system is:

[0070] The laser starts from the transmitting unit, passes through the turbulent path to reach the target reflective target surface 3 at a distance of L, and then returns to the front two-way optical receiving system 18 through the same transmission path. The light beam is collimated after passing through the front two-way optical receiving system 18 and is incident on the orthogonal polarization four-way interference system 19. First, the two incident light beams pass through the filter 6 and the attenuation plate 7 to weaken the ambient background light, and then pass through the polarizer 8 to polarize into horizontal linear polarized light. Subsequently, the horizontal linear polarized light passes through the half-wave plate 9 whose fast axis is at an angle of 22.5° to the horizontal direction, so that the light beam becomes linear polarized light whose polarization direction is at an angle of 45° to the horizontal direction.

[0071] After passing through the half-wave plate 9, the two beams of 45° linear polarized light respectively pass through two reflectors 10 to enter the corresponding interference arms. A quarter-wave plate 11 is inserted into one interference arm and this interference arm is called A light, wherein the fast axis of the quarter-wave plate is in the horizontal direction (or vertical direction), so that the optical path produces a leading phase of π / 2 in the horizontal polarization direction (or vertical polarization direction). The other interference arm is B light, and the arm length is adjusted to be equal to the arm length of A light through two reflectors 10. It should be pointed out that when adjusting the reflector 10 of the interference optical path, the spatial frequency of the interference fringes needs to be adjusted to 1 so that the interference pattern is equal in phase as a whole. When the turbulence intensity on the transmission path changes, the phase difference between the two sub-beams will fluctuate, thereby causing the brightness of the interference pattern to change. At this time, the sine and cosine terms of the phase difference between the two sub-beams are included in the four-way polarization interference signal.

[0072] The two light beams interfere with each other after passing through the beam splitter prism 12. The beam splitter prism 12 realizes the interference of the transmitted light of light A and the reflected light of light B, making it light C. The reflected light of light A and the transmitted light of light B interfere with each other, making it light D. At this time, the light C and light D emitted by the beam splitter prism 12 contain interference information of the horizontal polarization state and the vertical polarization state. The interference light is further separated from the horizontal polarization state and the vertical polarization state by the polarization beam splitter prism 13 to form four interference lights, namely the horizontal polarization state of light C, the vertical polarization state of light C, the horizontal polarization state of light D, and the vertical polarization state of light D. The four interference signals will be further transmitted to the signal acquisition and data processing system 20.

[0073] The signal acquisition and data processing system 20 first collects four interference lights of different polarization states through four photodetectors 14 at the same time. The core part of the interference of the two sub-beams and the details of the polarization states are as follows: Figure 3 As shown, the four-way interference light passes through the photoelectric detector 14 to convert the optical signal into an electrical signal, and transmits it to the computer 16 through the data acquisition card 15, and performs real-time phase solution through the solution program.

[0074] See also Figure 4 The present invention also provides a method for measuring atmospheric coherence length by using orthogonal polarization interference of wave planes, comprising the following steps:

[0075] Step 1: The laser passes through the beam expansion system 2 to form a parallel beam with a small divergence angle. The beam passes through the turbulent atmosphere to reach the target reflection target surface 3, and is reflected by the target reflection target surface 3 back to the front dual-path optical receiving system 18 at the same end as the laser 1;

[0076] Step 2: After the light beam passes through the front two-way optical receiving system 18, two incident sub-beams are formed and transmitted into the orthogonal polarization four-way interference system 19;

[0077] Step 3: The two light beams first pass through the filter 6, the attenuation plate 7 and the polarizer 8 with horizontal polarization direction to become horizontal linear polarized light;

[0078] Step 4: After passing through the half-wave plate 9, the two linearly polarized lights become linearly polarized lights with a polarization direction at an angle of 45° to the horizontal direction, and then pass through two reflectors 10 to adjust the transmission angle of the light beam to keep the light beam transmitted horizontally;

[0079] Step 5: After the propagation direction of the two linearly polarized lights is adjusted by the reflector 10, they enter the beam splitter prism 12 with a transmittance T of 50%, wherein a quarter wave plate 11 with a fast axis in the horizontal direction (or vertical direction) is inserted into one beam path (let it be A light), so that the light beam generates an advanced 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 reflectors 10 for adjusting the optical path, so that the effect of equal arm length of A light and B light is achieved;

[0080] The complex amplitude of light path A in the horizontal and vertical directions can be expressed as:

[0081]

[0082] The complex amplitude of light path B in the horizontal and vertical directions can be expressed as:

[0083]

[0084] Step 6: The beam splitter prism 12 realizes interference between the transmitted light of light A and the reflected light of light B (let it be light C), and interference between the reflected light of light A and the transmitted light of light B (let it be light D). At this time, the light C and light D emitted by the beam splitter prism 12 contain interference information of both horizontal polarization state and vertical polarization state.

[0085] Step 7: Both the C light and the D light pass through the polarization beam splitter prism 13, and the two polarization interference lights are separated to form four interference lights, namely, 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. Four photodetectors 14 simultaneously collect interference information of four paths containing different polarization states;

[0086] Step 8: Use four photodetectors 14 to collect synchronous signals. When installing and debugging the orthogonal polarization four-way interference system, the photodetector 14 selects the CCD camera to receive the interference pattern. By adjusting the reflector 10 in steps 3 and 4, there is no inclination between the two incident lights, forming an interference pattern with a spatial frequency of 1, so that the interference pattern is uniformly phased. Since the system has no moving structure and no additional phase difference is introduced, the phase difference change caused by the turbulent atmosphere at this time is manifested as the light and dark fluctuations of the interference pattern, that is, the light and dark changes of the interference pattern are 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, the CCD needs to be replaced with a photomultiplier tube PMT;

[0087] Step 9: The interference signal received by the photodetector 14 is transmitted to the data acquisition card 15 and then to the computer 16. The real-time phase solution is performed by the solution program. The solution program subtracts the collected four interference signals in pairs to obtain the sine term and cosine term information containing the phase difference on the transmission path of the two light beams. Further, according to the Euler formula e ix = cos(x) + i sin(x), calculate the phase difference and display and store it in real time, calculate the variance of the phase difference sequence to obtain the phase structure function, and deduce the atmospheric coherence length r according to the functional relationship between the phase structure function and the atmospheric coherence length 0 .

[0088] Combination Figure 5 , the specific steps of the solution procedure include:

[0089] S901: Synchronously record the four-way polarization interference signals. The four light intensity signals received by the photodetector 14 are: The horizontal polarization state intensity of C light is recorded as The vertical polarization intensity of C light is recorded as The horizontal polarization intensity of light D is recorded as The vertical polarization intensity of D light is recorded as

[0090] S902: Subtract the interference signals of the same polarization state from each other to obtain the sine term and cosine term containing the phase difference between the two sub-beams. For the horizontal polarization interference signal:

[0091]

[0092] For the vertical polarization interference signal:

[0093]

[0094] S903: According to Euler's formula e ix =cos(x)+i sin(x), the phase difference between light path A and light path B is calculated as:

[0095]

[0096] S904: Perform statistics on the calculated phase sequence to obtain the phase structure function

[0097] S905: Considering the atmosphere as a Von Karman spectrum, calculate the phase structure function between the two sub-beams The atmospheric coherence length r on the transmission path 0 The relationship is:

[0098]

[0099] Where κ 0 =2π / L 0 , L 0 is the outer scale of turbulence, Γ(·) is the gamma function, K 5 / 6 is the third kind of modified Bessel function;

[0100] The atmospheric coherence length on the transmission path is further obtained.

[0101]

[0102] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A wave-plane orthogonal polarization interferometer system for measuring atmospheric coherence length, It is characterized in that It includes a laser emitting unit, a target reflection target surface, a front two-way optical receiving system, an orthogonal polarization four-way interference system, and a signal acquisition and data processing system; The laser emitting unit includes a laser and a beam expansion system, which is used to emit a laser beam of a specified radius and wavelength band; The target reflective surface is used to reflect the laser beam; The front dual-path optical receiving system is used to collect signal light and select two symmetrical sub-beams as received light; The orthogonal polarization four-way interference system comprises a filter, an attenuation plate, a polarizer, a half-wave plate, and two plane reflectors arranged along the transmission paths of the two sub-beams, and then a quarter-wave plate and a reflector are arranged on the path of one of the light beams, and the other light beam passes through the two reflectors, and the two light beams are adjusted so that the transmission path lengths are equal; the orthogonal polarization four-way interference system also comprises a beam splitter prism and two polarization beam splitter prisms, the transmitted light and the reflected light of the beam splitter prism are the results of the interference of the two sub-beams, and contain the interference information of the horizontal polarization state and the vertical polarization state, and the transmitted light and the reflected light pass through the polarization beam splitter prism respectively to separate the interference result of the horizontal polarization state and the interference result of the vertical polarization state, so as to form a four-way interference signal; The signal acquisition and data processing system includes four photoelectric detectors for acquiring four-way polarization interference signals, and inputs the signals into a computer through a data acquisition card for calculating phase information.

2. The wave plane orthogonal polarization interference system for measuring atmospheric coherence length according to claim 1, It is characterized in that The target reflection target surface is a plane reflector or an angle reflector.

3. The wave-plane orthogonal polarization interference system for measuring atmospheric coherence length according to claim 1, It is characterized in that The front two-way optical receiving system includes a telescope and an aperture.

4. The wave-plane orthogonal polarization interferometer system for measuring atmospheric coherence length according to claim 1, It is characterized in that The polarization direction of the polarizer is horizontal, and the emitted light is horizontal linear polarized light.

5. The wave-plane orthogonal polarization interference system for measuring atmospheric coherence length according to claim 1, It is characterized in that 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 light beam forms an angle of 45° with the horizontal direction.

6. The wave-plane orthogonal polarization interference system for measuring atmospheric coherence length according to claim 1, It is characterized in that The fast axis direction of the quarter wave plate is in the horizontal direction or the vertical direction, so that a phase delay of π / 2 is added in the horizontal direction or the vertical direction of the light beam.

7. The wave-plane orthogonal polarization interference system for measuring atmospheric coherence length according to claim 1, It is characterized in that The transmittance T of the beam splitter 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 wave plane orthogonal polarization interference system for measuring atmospheric coherence length according to claim 1, It is characterized in that The four photoelectric detectors are installed by adjusting the structure and using a CCD camera for imaging detection. After the optical path is adjusted, a photomultiplier tube (PMT) is used for data acquisition to achieve high frame rate and high sensitivity signal acquisition.

9. A method for measuring atmospheric coherence length by using a wavefront orthogonal polarization interference system for measuring atmospheric coherence length according to any one of claims 1 to 8, It is characterized in that The following steps are involved: Step 1: The laser passes through the beam expansion system to form a parallel beam with a small divergence angle. The beam passes through the turbulent atmosphere to reach the target reflection target surface, and is reflected back to the front dual-path 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 and transmits into the orthogonal polarization four-way interference system; Step 3: The two light beams first pass through a filter, an attenuator, and a polarizer with a horizontal polarization direction to become horizontally polarized light; Step 4: After passing through the half-wave plate, the two linearly polarized lights become linearly polarized lights with a polarization direction at an angle of 45° to the horizontal direction. After that, the transmission angle of the light beam is adjusted through two reflectors to keep the light beam transmitted horizontally. Step 5: After the propagation direction of the two linearly polarized lights is adjusted by the reflector, they enter the beam splitter with a transmittance T of 50%. In one beam path, let it be A light, insert a quarter-wave plate with a fast axis in the horizontal direction or the vertical direction, so that the light beam produces a π / 2 leading phase in the horizontal polarization direction or the vertical polarization direction. The other light is let be B light, and after passing through two reflectors for adjusting the optical path, the effect of equal arm length of A light and B light is achieved; The complex amplitude of light path A in the horizontal and vertical directions is expressed as: The complex amplitude of light path B in the horizontal and vertical directions is expressed as: Step 6: The beam splitter prism realizes the interference of the transmitted light of light A and the reflected light of light B, which is called light C. The reflected light of light A and the transmitted light of light B interfere with each other, which is called light D. At this time, the light C and light D emitted by the beam splitter prism contain interference information of both horizontal polarization state and vertical polarization state. Step 7: Both light C and light D pass through a polarization beam splitter to separate the two polarization interference lights, forming four interference lights, namely, the horizontal polarization state of light C, the vertical polarization state of light C, the horizontal polarization state of light D, and the vertical polarization state of light D. Four photodetectors simultaneously collect interference information of four paths containing different polarization states; Step 8: Use four photodetectors to collect synchronous signals. When installing and debugging the orthogonal polarization four-way interference system, the photodetector selects the CCD camera to receive the interference pattern. By adjusting the reflectors in steps 3 and 4, there is no inclination between the two incident lights, forming an interference pattern with a spatial frequency of 1. Step 9: The interference signal received by the photodetector is transmitted to the data acquisition card and then to the computer. The real-time phase solution is performed by the solution program. The solution program subtracts the collected four interference signals in pairs to obtain the sine term and cosine term information containing the phase difference on the transmission path of the two light beams. Further, according to the Euler formula e ix = cos(x) + i sin(x), calculate the phase difference and display and store it in real time, calculate the variance of the phase difference sequence to obtain the phase structure function, and deduce the atmospheric coherence length r according to the functional relationship between the phase structure function and the atmospheric coherence length 0 .

10. The wave plane orthogonal polarization interference method for measuring atmospheric coherence length according to claim 9, It is characterized in that The specific steps of the solution procedure include: S901: Synchronously record the four-way polarization interference signal. The four light intensity signals received by the photodetector are: The horizontal polarization intensity of C light is recorded as The vertical polarization intensity of C light is recorded as The horizontal polarization intensity of light D is recorded as The vertical polarization intensity of D light is recorded as S902: Subtract the interference signals of the same polarization state from each other to obtain the sine term and cosine term containing the phase difference between the two sub-beams. For the horizontal polarization interference signal: For the vertical polarization interference signal: S903: According to Euler's formula e ix =cos(x)+i sin(x), the phase difference between light path A and light path B is calculated as: S904: Perform statistics on the calculated phase sequence to obtain the phase structure function S905: Considering the atmosphere as a Von Karman spectrum, calculate the phase structure function between the two sub-beams The atmospheric coherence length r on the transmission path 0 The relationship is: Where κ 0 =2π / L 0 , L 0 is the outer scale of turbulence, Γ(·) is the gamma function, K 5 / 6 is the third kind of modified Bessel function; Further obtain the atmospheric coherence length on the transmission path

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