A multimode optical fiber spatial coherence experimental analysis system and method

Abstract

The present invention relates to the field of optical measurement technology, and discloses a multimode optical fiber spatial coherence experimental analysis system and method, the system comprising: an LED light source is arranged in front of the front end face of a sapphire optical fiber, an aperture diaphragm is arranged behind the rear end face of the sapphire optical fiber, a beam expander, a collimator, a beam splitter, a compensation plate and a second right-angle reflector are arranged in sequence behind the aperture diaphragm, the first right-angle reflector is arranged above the beam splitter, a plane reflector is arranged below the beam splitter, a focusing lens is arranged behind the plane reflector, and a photodetector is arranged behind the focusing lens. The present invention can eliminate the phase imbalance caused by the difference in arm length of the interference arm, improve the clarity of the interference fringes, and enhance the accuracy of the measurement result.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a multimode optical fiber spatial coherence experimental analysis system and method. Background Art

[0002] With the rapid development of aerospace and internal combustion engine industries, the technical requirements for high-temperature monitoring under extreme conditions are increasing. Sapphire optical fiber, with its excellent high-temperature resistance and excellent easy multiplexing characteristics, has shown broad application prospects in the field of temperature and pressure sensor manufacturing in high-temperature environments. However, as a multimode optical fiber, when the light source is injected into it, the spatial distribution of the output light becomes uneven due to the dispersion effect of the multimode optical fiber and the combined effect of various nonlinear effects, thereby reducing the coherence of the light source. This phenomenon will have an adverse effect on the quality of the sensing spectrum signal.

[0003] Complex coherence is an important measure of the coherence of the light field. In the prior art, the spatial coherence of sunlight and vortex fields is measured by using a Sagnac interferometer. However, this interferometer cannot directly obtain the complex coherence when measuring the coherence of the light field, but measures the coherence function. In fact, the modulus of the complex coherence is the accurate measure to characterize the spatial coherence. In the prior art, a transverse shear delay jitter Mach-Zehnder interferometer is also used to measure the complex coherence of a broadband light source output through a multimode optical fiber. Although this method overcomes the limitations of traditional methods in measuring the spatial coherence of light fields, it still has shortcomings. Specifically, the two beams of light formed after the light source passes through the beam splitter have an optical path difference due to the different number of times they pass through the beam splitter, which leads to the blurring of the interference fringes, increasing the complexity and difficulty of the experimental operation.

[0004] Therefore, there is an urgent need for a multimode optical fiber spatial coherence experimental analysis system and method that can eliminate the phase imbalance caused by the difference in arm length of the interference arm, improve the clarity of the interference fringes, and enhance the accuracy of the measurement results. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a multimode optical fiber spatial coherence experimental analysis system and method, which can eliminate the phase imbalance caused by the difference in arm length of the interference arm, improve the clarity of the interference fringes, and enhance the accuracy of the measurement results.

[0006] The present invention provides a multimode optical fiber spatial coherence experimental analysis system, comprising:

[0007] LED light source, sapphire optical fiber, aperture stop, beam expansion and collimation system, beam splitter, compensation plate, first right-angle reflector, second right-angle reflector, plane reflector, focusing lens and photodetector;

[0008] The LED light source is arranged in front of the front end face of the sapphire optical fiber, and is used to emit the original light signal, which is transmitted through the sapphire optical fiber and output from the rear end face of the sapphire optical fiber;

[0009] The aperture diaphragm is arranged behind the rear end face of the sapphire optical fiber to filter out stray light in the optical signal output by the sapphire optical fiber;

[0010] The beam expansion and collimation system consists of a beam expander and a collimator, which are arranged behind the aperture diaphragm in sequence to expand and collimate the light signal passing through the aperture diaphragm;

[0011] The beam splitter is arranged behind the collimator to split the light signal after beam expansion and collimation into a lateral interference arm and a vertical interference arm; wherein the lateral interference arm is the transmitted light and the vertical interference arm is the reflected light;

[0012] The compensation plate is arranged behind the beam splitter to make the number of times the horizontal interference arm and the vertical interference arm pass through the beam splitter consistent;

[0013] The first right-angle reflector is arranged above the beam splitter and is used to reflect the vertical interference arm;

[0014] The second right-angle reflector is arranged behind the compensation plate and is used to reflect the lateral interference arm;

[0015] The plane reflector is arranged below the beam splitter to reflect the light that passes through the beam splitter for the second time;

[0016] The focusing lens is arranged behind the plane reflector and is used for forming a focusing light spot on the plane of the photoelectric detector with the light beam reflected by the plane reflector.

[0017] Furthermore, the beam splitter and the compensation plate are parallel glass plates with exactly the same thickness and refractive index.

[0018] Furthermore, the sapphire optical fiber is axially parallel to the optical axes of the focusing lens, the beam expander and the collimator, the optical axes of the beam expander and the collimator are parallel to the horizontal plane, and the beam splitter and the compensation plate form an angle of 45° with the horizontal plane.

[0019] Furthermore, the first right-angle reflector is mounted on the translation stage and can move laterally, and the first right-angle reflector is perpendicular to the vertical interference arm;

[0020] The second right-angle reflector is mounted on the piezoelectric controller and can move laterally, and a first reflective surface of the second right-angle reflector forms an angle of 45° with the horizontal plane.

[0021] The present invention also provides a multimode optical fiber spatial coherence experimental analysis method, which is implemented based on a multimode optical fiber spatial coherence experimental analysis system described in any one of the above items, and comprises the following steps:

[0022] S1. Build a multimode optical fiber spatial coherence experimental analysis system and record the initial position of the first right-angle reflector;

[0023] S2. The original light signal is emitted by the LED light source, transmitted through the sapphire optical fiber, and output from the rear end face of the sapphire optical fiber. Among the light signals output from the rear end face of the sapphire optical fiber, two modes of light are selected and recorded as P and S2. 1 and P 2 ;

[0024] S3, P 1 and P 2 After the stray light is filtered out by the aperture diaphragm, it enters the beam expansion and collimation system to obtain the P after beam expansion and collimation. 1 and P 2 , P after beam expansion and collimation 1 and P 2 After passing through the beam splitter, P 1 The lateral interference arm and vertical interference arm, P 2 The lateral interference arm and the vertical interference arm;

[0025] S4, P 1 The lateral interference arm and vertical interference arm, P 2 The horizontal interference arm and the vertical interference arm pass through the beam splitter, the compensation plate, the first right-angle reflector, the second right-angle reflector, the plane reflector, and the focusing lens, and finally form two displacement images on the photodetector;

[0026] S5, calculating the modulus of complex coherence according to the interference fringes of the two displacement images, calculating the transverse shear amount according to the current position and initial position of the first right-angle reflector and the core diameter of the sapphire optical fiber, and obtaining a modulus-transverse shear amount curve of complex coherence;

[0027] S6. The spatial coherence of the original optical signal after being transmitted through the multimode optical fiber is measured according to the mode value-transverse shear amount curve of the complex coherence.

[0028] Furthermore, in S4, P 1 The lateral interference arm and vertical interference arm, P 2 The lateral interference arm and the vertical interference arm, after passing through the beam splitter, the compensation plate, the first right-angle reflector, the second right-angle reflector, the plane reflector, and the focusing lens, finally form two displacement images on the photodetector, including:

[0029] S41, P 1 The lateral interference arm is recorded as the transmitted light PT1 , P 2 The lateral interference arm is recorded as the transmitted light P T2 , P 1 The vertical interference arm is recorded as the reflected light P F1 , P 2 The lateral interference arm is recorded as the transmitted light P F2 ;

[0030] S42, P T1 and P T2 After passing through the compensation plate, P T1 and P T2 They are respectively reflected by the two right-angle reflection surfaces of the second right-angle reflector and pass through the compensation plate again; then, P T1 and P T2 It is reflected by the beam splitter, then by the plane mirror, and enters the focusing lens;

[0031] S43, P F1 and P F2 After being reflected by the two right-angle reflection surfaces of the first right-angle reflector, they are transmitted through the beam splitter (G1), then reflected by the plane reflector, and finally enter the focusing lens;

[0032] S44, through the focusing lens, P T1 and P F1 The first displacement image, P, is formed on the photodetector. T2 and P F2 A second shifted image is formed on the photodetector.

[0033] Further, in S5, calculating the modulus of the complex coherence according to the interference fringes of the two displacement images includes:

[0034] S51, by moving the first right-angle reflector laterally, adjust P 1 The vertical interference arm and P 2 The phase difference between the lateral interference arms is used to obtain the maximum and minimum light intensity of the interference fringes of the two displacement images;

[0035] S52, by moving the second right-angle reflector laterally, adjust P 1 , P 2 The time delay between the vertical interferometer arm and the horizontal interferometer arm makes the interference fringes of the two displacement images clearest;

[0036] S53, calculating the modulus of the complex coherence according to the interference fringes of the two adjusted displacement images; the calculation formula is as follows:

[0037] ;

[0038] in, The modulus of the complex coherence, S 1 Indicates P 2 The light intensity of the vertical interferometer arm, S 2 Indicates P 1 The light intensity of the transverse interference arm, max(S) represents the maximum light intensity of the interference fringes, and min(S) represents the minimum light intensity of the interference fringes.

[0039] Further, in S5, calculating the transverse shear amount according to the current position and initial position of the first right-angle reflector and the core diameter of the sapphire optical fiber includes:

[0040] S54, calculating the moving distance of the first right-angle reflector according to the difference between the current position and the initial position of the first right-angle reflector;

[0041] S55. Calculate the transverse shearing amount according to the ratio of the moving distance of the first right-angle reflector to the core diameter of the optical fiber.

[0042] The present invention has the following technical effects:

[0043] The present invention can make the number of times that the lateral interference arm and the vertical interference arm of the light beam pass through the beam splitter consistent by introducing a compensation plate, and by making the number of times that the lateral interference arm and the vertical interference arm pass through the beam splitter equal, it can be ensured that the lateral interference arm and the vertical interference arm have the same phase relationship when they re-converge, effectively eliminating the phase imbalance caused by the difference in arm length of the interference arm, improving the contrast and clarity of the interference fringes, thereby improving the accuracy and reliability of the measurement of the spatial coherence of the light field. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0045] Figure 1 It is a structural schematic diagram of a multimode optical fiber spatial coherence experimental analysis system provided by an embodiment of the present invention;

[0046] Figure 2 It is a flow chart of a multimode optical fiber spatial coherence experimental analysis method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention.

[0048] The present invention proposes a multimode optical fiber spatial coherence experimental analysis system. Figure 1 is a schematic diagram of a multimode optical fiber spatial coherence experimental analysis system provided by an embodiment of the present invention, see Figure 1 , specifically including:

[0049] LED light source (not shown in the figure), sapphire optical fiber, aperture stop, beam expansion and collimation system, beam splitter G1, compensation plate G2, first right-angle reflector M1, second right-angle reflector M2, plane reflector M3, focusing lens L3 and photodetector;

[0050] The LED light source is arranged in front of the front end face of the sapphire optical fiber, and is used to emit the original light signal, which is transmitted through the sapphire optical fiber and output from the rear end face of the sapphire optical fiber;

[0051] The aperture diaphragm is arranged behind the rear end face of the sapphire optical fiber to filter out stray light in the optical signal output by the sapphire optical fiber;

[0052] The beam expansion and collimation system is composed of a beam expander L1 and a collimator L2. The beam expander L1 and the collimator L2 are sequentially arranged behind the aperture stop to expand and collimate the light signal passing through the aperture stop.

[0053] The beam splitter G1 is arranged behind the collimator L2, and is used to split the light signal after beam expansion and collimation into a horizontal interference arm and a vertical interference arm; wherein the horizontal interference arm is the transmitted light, and the vertical interference arm is the reflected light;

[0054] The compensation plate G2 is arranged behind the beam splitter G1 to make the number of times the horizontal interference arm and the vertical interference arm pass through the beam splitter G1 consistent;

[0055] The first right-angle reflector M1 is disposed above the beam splitter G1 and is used to reflect the vertical interference arm;

[0056] The second right-angle reflector M2 is arranged behind the compensation plate G2 to reflect the lateral interference arm;

[0057] The plane reflector M3 is disposed below the beam splitter G1 and is used to reflect the light that passes through the beam splitter G1 for the second time;

[0058] The focusing lens L3 is arranged behind the plane reflector M3, and is used for forming a focusing spot on the plane of the photodetector with the light beam reflected by the plane reflector M3.

[0059] Furthermore, the beam splitter G1 and the compensation plate G2 are parallel glass plates with exactly the same thickness and refractive index; wherein, there is no mandatory requirement for the thickness of the beam splitter G1, as long as it can achieve light splitting.

[0060] Furthermore, the sapphire fiber axis is parallel to the optical axis directions of the focusing lens L3, the beam expander L1 and the collimator L2, the optical axes of the beam expander L1 and the collimator L2 are parallel to the horizontal plane, and the beam splitter G1 and the compensation plate G2 form an angle of 45° with the horizontal plane.

[0061] Furthermore, the first right-angle reflector M1 is mounted on the translation stage and can move laterally, and the first right-angle reflector M1 is perpendicular to the vertical interference arm;

[0062] The second right-angle reflector M2 is mounted on the piezoelectric controller and can move laterally, and a first reflective surface of the second right-angle reflector M2 forms an angle of 45° with the horizontal plane.

[0063] The present invention can make the number of times that the lateral interference arm and the vertical interference arm of the light beam pass through the beam splitter consistent by introducing a compensation plate, and by making the number of times that the lateral interference arm and the vertical interference arm pass through the beam splitter equal, it can be ensured that the lateral interference arm and the vertical interference arm have the same phase relationship when they re-converge, effectively eliminating the phase imbalance caused by the difference in arm length of the interference arm, improving the contrast and clarity of the interference fringes, thereby improving the accuracy and reliability of the measurement of the spatial coherence of the light field.

[0064] Figure 2 is a flow chart of a multimode optical fiber spatial coherence experimental analysis method provided by an embodiment of the present invention, which is implemented based on a multimode optical fiber spatial coherence experimental analysis system described in the above embodiment, such as Figure 2 As shown, the specific steps include:

[0065] S1. Build a multimode optical fiber spatial coherence experimental analysis system and record the initial position of the first right-angle reflector M1.

[0066] S2. The original light signal is emitted by the LED light source, transmitted through the sapphire optical fiber, and output from the rear end face of the sapphire optical fiber. Among the light signals output from the rear end face of the sapphire optical fiber, two modes of light are selected and recorded as P and S2. 1 and P 2 .

[0067] S3, P 1 and P 2 After the stray light is filtered out by the aperture diaphragm, it enters the beam expansion and collimation system to obtain the P after beam expansion and collimation.1 and P 2 , P after beam expansion and collimation 1 and P 2 After passing through the beam splitter G1, P 1 The lateral interference arm and vertical interference arm, P 2 The lateral interference arm and the vertical interference arm.

[0068] S4, P 1 The lateral interference arm and vertical interference arm, P 2 The lateral interference arm and the vertical interference arm pass through the beam splitter G1, the compensation plate G2, the first right-angle reflector M1, the second right-angle reflector M2, the plane reflector M3, and the focusing lens L3, and finally form two displacement images on the photodetector.

[0069] The specific steps are as follows:

[0070] S41, P 1 The lateral interference arm is recorded as the transmitted light P T1 , P 2 The lateral interference arm is recorded as the transmitted light P T2 , P 1 The vertical interference arm is recorded as the reflected light P F1 , P 2 The lateral interference arm is recorded as the transmitted light P F2 .

[0071] S42, P T1 and P T2 After passing through the compensation plate G2, P T1 and P T2 They are respectively reflected by the two right-angle reflection surfaces of the second right-angle reflector M2 and pass through the compensation plate G2 again;

[0072] Then, P T1 and P T2 It is reflected by the beam splitter G1 and then by the plane mirror M3 and enters the focusing lens L3.

[0073] S43, P F1 and P F2 After being reflected by the two right-angle reflection surfaces of the first right-angle reflection mirror M1, it is transmitted through the beam splitter G1, then reflected by the plane reflection mirror M3, and finally enters the focusing lens L3.

[0074] S44, through focusing lens L3, P T1 and P F1 The first displacement image, P, is formed on the photodetector. T2 and P F2 A second shifted image is formed on the photodetector.

[0075] S5. Calculate the modulus of complex coherence according to the interference fringes of the two displacement images, calculate the transverse shear amount according to the current position and initial position of the first right-angle reflector M1 and the core diameter of the sapphire optical fiber, and obtain a modulus-transverse shear amount curve of complex coherence.

[0076] The specific steps are as follows:

[0077] S51, by moving the first right-angle reflector M1 laterally, adjust P 1 The vertical interference arm and P 2 The phase difference between the lateral interference arms is used to obtain the maximum and minimum light intensity of the interference fringes of the two displacement images.

[0078] Specifically, when the positions of the beam splitter G1 and the compensation plate G2 are fixed, the position of the first right-angle reflector M1 is adjusted laterally. The purpose of this operation is to adjust the optical path of the reflected light so that an optical path difference is generated between the reflected light and the transmitted light, thereby promoting P F1 and P T2 The phase difference between them changes, and the relationship between the optical path difference and the phase difference is:

[0079] ;

[0080] in, represents the phase difference (in radians), λ represents the wavelength of the light wave, It represents the optical path difference, that is, the optical path difference between the two beams of light when they reach the interference point.

[0081] Phase difference is one of the conditions for interference to occur. When the phase difference is an integer multiple of the wavelength, the light waves will undergo constructive interference to form bright stripes. When the optical path difference is a half-integer multiple of the wavelength, the light waves will undergo destructive interference to form dark stripes.

[0082] By controlling this phase difference, P can be precisely adjusted. F1 and P T2 The interference of the image formed. F1 and P T2 The light waves are coherently superimposed in space. When the phase difference between the two light waves is 0 or π, the intensity of the interference fringes reaches the maximum or minimum. On the contrary, when the phase difference is π / 2 or 3π / 2, the interference effect between the light waves is weak, and the intensity of the generated interference fringes is relatively small. F1 and P T2 The phase difference can be used to adjust the intensity of the interference fringes, thereby forming the desired interference fringes on the photodetector plane. 1 and P 2 The light intensity of the horizontal interference arm and the light intensity of the vertical interference arm, specifically, when the beam is expanded and collimated, P1 and P 2 After passing through the beam splitter (G1), P 1 The lateral interference arm and vertical interference arm, P 2 The horizontal interference arm and vertical interference arm of P 1 The lateral interference arm and P 2 The lateral interference arm is measured to be P 1 The light intensity of the vertical interferometer arm and P 2 The light intensity S of the vertical interferometer arm 1 , and then cover P 1 The vertical interference arm and P 2 The vertical interference arm of 1 The light intensity S of the lateral interference arm 2 and P 2 The light intensity of the lateral interferometer arm.

[0083] Exemplarily, the interferometer in this embodiment may be a Mach-Zehnder interferometer.

[0084] S52, by moving the second right-angle reflector M2 laterally, adjust P 1 , P 2 The time delay between the vertical interferometer arm and the lateral interferometer arm makes the interference fringes of the two displacement images clearest.

[0085] Specifically, the change in optical path will cause the change in the phase of the optical signal, thereby causing the phase delay jitter. The contrast of the interference fringes changes with the delay τ. When τ=0, the interference fringes are the clearest. As the delay τ gradually increases, the contrast of the interference fringes decreases. The light delay between the vertical interference arm and the lateral interference arm can be quickly adjusted by moving the second right-angle reflector M2 laterally, which can realize interference delay adjustment and rapid scanning between multi-level fringes, and then obtain the time interference pattern generated by rapid delay scanning. The time interference pattern is manifested as fluctuations that change with time. The peak value of the fluctuation represents high light intensity, and the valley value represents low light intensity.

[0086] S53, calculating the modulus of the complex coherence according to the interference fringes of the two adjusted displacement images.

[0087] Specifically, the light intensity signal detected by the photodetector will oscillate with the change of time delay. When the time delay τ is small enough, the detected light intensity signal S will oscillate with a slow envelope and will cover at least one interference fringe near τ=0, where S represents P F1 and P T2 If the time delay τ is not small enough, the piezoelectric controller is adjusted to adjust the position of the second right-angle mirror M2, thereby changing the time delay τ.

[0088] When the time delay τ is small enough, the calculation formula is as follows:

[0089] ;

[0090] in, The modulus of the complex coherence, S 1 Indicates P 2 The light intensity of the vertical interferometer arm, S 2 Indicates P 1 The light intensity of the transverse interference arm, max(S) represents the maximum light intensity of the interference fringes, and min(S) represents the minimum light intensity of the interference fringes.

[0091] S54. Calculate the moving distance of the first right-angle reflecting mirror M1 according to the difference between the current position and the initial position of the first right-angle reflecting mirror M1.

[0092] S55. Calculate the transverse shearing amount according to the ratio of the moving distance of the first right-angle reflector M1 to the core diameter of the sapphire optical fiber.

[0093] For example, assuming that the current position of the first right-angle reflector M1 moves 5 microns compared with the initial position, and the core diameter of the sapphire optical fiber is 105 microns, the transverse shear amount is 5 / 105=0.0476 microns.

[0094] S56. Obtain a complex coherence modulus-lateral shear amount curve graph according to the calculated complex coherence modulus and lateral shear amount.

[0095] S6. The spatial coherence of the original optical signal after being transmitted through the multimode optical fiber is measured according to the mode value-transverse shear amount curve of the complex coherence.

[0096] Specifically, the closer the modulus value of the complex coherence is to 1, the stronger the spatial coherence is, and the closer the modulus value of the complex coherence is to 0, the weaker the spatial coherence is.

[0097] It should be noted that the terms used in the present invention are only for describing specific embodiments, rather than limiting the scope of the present application. As shown in the present specification, unless the context clearly indicates an exception, the words "one", "a", "a kind of" and / or "the" do not specifically refer to the singular, but may also include the plural. The terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method or device. In the absence of more restrictions, the elements defined by the sentence "include one..." do not exclude the presence of other identical elements in the process, method or device including the elements.

[0098] It should also be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. Unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", etc. should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be a connection between the two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the technical solutions of the embodiments of the present invention.

Claims

1. A multimode optical fiber spatial coherence experimental analysis system, characterized in that: include: LED light source, sapphire optical fiber, aperture stop, beam expansion and collimation system, beam splitter (G1), compensation plate (G2), first right-angle reflector (M1), second right-angle reflector (M2), plane reflector (M3), focusing lens (L3) and photodetector; The LED light source is arranged in front of the front end face of the sapphire optical fiber, and is used to emit an original light signal, which is transmitted through the sapphire optical fiber and output from the rear end face of the sapphire optical fiber; The aperture stop is arranged behind the rear end face of the sapphire optical fiber and is used to filter out stray light in the optical signal output by the sapphire optical fiber; The beam expansion and collimation system is composed of a beam expander (L1) and a collimator (L2), wherein the beam expander (L1) and the collimator (L2) are sequentially arranged behind the aperture stop and are used to expand and collimate the optical signal passing through the aperture stop; The beam splitter (G1) is arranged behind the collimator (L2) and is used to split the light signal after beam expansion and collimation into a lateral interference arm and a vertical interference arm; wherein the lateral interference arm is a transmitted light, and the vertical interference arm is a reflected light; The compensation plate (G2) is arranged behind the beam splitter (G1) and is used to make the number of times that the horizontal interference arm and the vertical interference arm pass through the beam splitter (G1) remain consistent; The first right-angle reflector (M1) is arranged above the beam splitter (G1) and is used to reflect the vertical interference arm; The second right-angle reflector (M2) is arranged behind the compensation plate (G2) and is used to reflect the lateral interference arm; The plane reflector (M3) is arranged below the beam splitter (G1) and is used to reflect the light that passes through the beam splitter (G1) for the second time; The focusing lens (L3) is arranged behind the plane reflector (M3) and is used to form a focusing spot on the plane of the photodetector with the light beam reflected by the plane reflector (M3); The sapphire optical fiber is axially parallel to the optical axes of the focusing lens (L3), the beam expander (L1) and the collimator (L2); the optical axes of the beam expander (L1) and the collimator (L2) are parallel to the horizontal plane; and the beam splitter (G1) and the compensation plate (G2) form an angle of 45° with the horizontal plane. The first right-angle reflector (M1) is installed on the translation stage and can move laterally, and the first right-angle reflector (M1) is perpendicular to the vertical interference arm; The second right-angle reflector (M2) is mounted on a piezoelectric controller and can move laterally, and a first reflective surface of the second right-angle reflector (M2) forms an angle of 45° with a horizontal plane.

2. A multimode optical fiber spatial coherence experimental analysis system according to claim 1, characterized in that: The beam splitter (G1) and the compensation plate (G2) are parallel glass plates with exactly the same thickness and refractive index.

3. A method for analyzing the spatial coherence of a multimode optical fiber, implemented based on a system for analyzing the spatial coherence of a multimode optical fiber as described in any one of claims 1 to 2, characterized in that: The steps include: S1. Build a multimode optical fiber spatial coherence experimental analysis system and record the initial position of the first right-angle reflector (M1); S2, an original light signal is emitted by an LED light source, transmitted through a sapphire optical fiber, and outputted from a rear end face of the sapphire optical fiber, and two modes of light are selected from the light signal outputted from the rear end face of the sapphire optical fiber and are respectively recorded as P1 and P2; After passing through the aperture diaphragm to filter out stray light, S3, P1 and P2 enter the beam expansion and collimation system to obtain P1 and P2 after beam expansion and collimation. After passing through the beam splitter (G1), the lateral interference arm and vertical interference arm of P1 and the lateral interference arm and vertical interference arm of P2 are generated respectively. S4, the lateral interference arm and the vertical interference arm of P1, the lateral interference arm and the vertical interference arm of P2, after passing through a beam splitter (G1), a compensation plate (G2), a first right-angle reflector (M1), a second right-angle reflector (M2), a plane reflector (M3), and a focusing lens (L3), finally form two displacement images on a photodetector; S5, calculating the modulus of complex coherence according to the interference fringes of the two displacement images, calculating the transverse shear amount according to the current position and initial position of the first right-angle reflector (M1) and the core diameter of the sapphire optical fiber, and obtaining a modulus-transverse shear amount curve of complex coherence; S6. The spatial coherence of the original optical signal after being transmitted through the multimode optical fiber is measured according to the mode value-transverse shear amount curve of the complex coherence.

4. The method for analyzing the spatial coherence of a multimode optical fiber according to claim 3, characterized in that: In S4, the lateral interference arm and the vertical interference arm of P1, and the lateral interference arm and the vertical interference arm of P2, after passing through a beam splitter (G1), a compensation plate (G2), a first right-angle reflector (M1), a second right-angle reflector (M2), a plane reflector (M3), and a focusing lens (L3), finally form two displacement images on a photodetector, including: S41, record the lateral interference arm of P1 as the transmitted light P T1 , the lateral interference arm of P2 is recorded as the transmitted light P T2 , the vertical interference arm of P1 is recorded as the reflected light P F1 , the lateral interference arm of P2 is recorded as the transmitted light P F2 ; S42, P T1 and P T2 After passing through the compensation plate (G2), P T1 and P T2 are respectively reflected by the two right-angle reflection surfaces of the second right-angle reflection mirror (M2), and pass through the compensation plate (G2) again; then, P T1 and P T2 After being reflected by the beam splitter (G1), it is reflected by the plane mirror (M3) and enters the focusing lens (L3); S43, P F1 and P F2 After being reflected by the two right-angle reflection surfaces of the first right-angle reflection mirror (M1), the light is transmitted through the beam splitter (G1), then reflected by the plane reflection mirror (M3), and finally enters the focusing lens (L3); S44, through the focusing lens (L3), P T1 and P F1 The first displacement image, P, is formed on the photodetector. T2 and P F2 A second shifted image is formed on the photodetector.

5. The method for analyzing the spatial coherence of a multimode optical fiber according to claim 4, characterized in that: In S5, calculating the modulus of the complex coherence according to the interference fringes of the two displacement images includes: S51, by laterally moving the position of the first right-angle reflector (M1), adjusting the phase difference between the vertical interference arm of P1 and the lateral interference arm of P2, and obtaining the maximum and minimum light intensities of the interference fringes of the two displacement images; S52, by moving the position of the second right-angle reflector (M2) laterally, adjusting the time delay between the vertical interference arms and the lateral interference arms of P1 and P2, so that the interference fringes of the two displacement images are clearest; S53, calculating the modulus of the complex coherence according to the interference fringes of the two adjusted displacement images; the calculation formula is as follows: ； in, represents the modulus of the complex coherence, S1 represents the light intensity of the vertical interference arm of P2, S2 represents the light intensity of the lateral interference arm of P1, max(S) represents the maximum light intensity of the interference fringes, and min(S) represents the minimum light intensity of the interference fringes.

6. The method for analyzing the spatial coherence of a multimode optical fiber according to claim 4, characterized in that: In S5, calculating the transverse shear amount according to the current position and initial position of the first right-angle reflector (M1) and the core diameter of the sapphire optical fiber includes: S54, calculating the moving distance of the first right-angle reflecting mirror (M1) according to the difference between the current position and the initial position of the first right-angle reflecting mirror (M1); S55. Calculate the transverse shearing amount according to the ratio of the moving distance of the first right-angle reflector (M1) to the core diameter of the optical fiber.

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