A wide field of view ocean laser radar system based on mach-zehnder interference

Abstract

The application discloses a wide field of view ocean laser radar system based on Mach-Zehnder interference, belonging to the field of hyperspectral laser radar, comprising a laser emission module, a receiving detection module, a comprehensive control and data processing module. The core frequency discrimination component of the receiving detection module adopts a Mach-Zehnder interference assembly, comprising two half-reflective half-transmissive cubic prisms, two total reflection prisms and interference arms; the interference arms have two paths: one path is inserted into a glass arm-air gap mixed structure in a short path between the two half-reflective half-transmissive cubic prisms, and the other path is inserted into three glass arms in a long path between the two half-reflective half-transmissive cubic prisms, and through reasonable design of the materials and lengths of the glass arms, the field of view is widened while good thermal stability is achieved. The Mach-Zehnder interference assembly outputs two light fields, and after complementary processing, has high energy utilization rate and signal-to-noise ratio, can expand the action distance of the hyperspectral laser radar, and has significance in the field of ocean laser radar detection.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of hyperspectral lidar, and particularly relates to a wide-field-of-view ocean profile detection lidar system based on Mach-Zehnder interference, which can detect the optical parameter profile of the ocean. BACKGROUND

[0002] After the interaction of laser and water body, elastic scattering and inelastic scattering occur, the elastic scattering includes Mie scattering and Rayleigh scattering, the central wavelength of the scattering does not change relative to the original laser wavelength, the inelastic scattering includes Brillouin scattering and Raman scattering, the central wavelength of the scattering changes relative to the laser wavelength. Since the scattering cross section of Brillouin scattering of the water body is large, the echo energy is strong, and the frequency spectrum information of the Brillouin scattering signal is strongly related to the optical parameters, temperature and salinity of the water body, therefore, the Brillouin ocean lidar can be used to retrieve the water body parameters. Generally, the Brillouin frequency shift of the blue-green laser caused by the water body is 7-8 GHz, and the measurement of the small frequency shift belongs to the high spectral resolution lidar. The core component of the high spectral resolution lidar is a high-precision frequency discriminator, and common frequency discriminators include an atomic molecular vapor absorption cell, a Fabry-Pérot interferometer and a Michelson interferometer. The atomic molecular vapor absorption cell has a high spectral filtering effect on certain specific wavelengths according to the absorption peaks of specific atoms or molecules, and cannot be widely used for any wavelength.

[0003] The conventional Fabry-Pérot interferometer and Michelson interferometer adopt a light beam interference mode to adjust the transmittance of the laser spectrum, and in theory, the Michelson interferometer can modulate any wavelength, but is very sensitive to the field of view, and deviating from the best field of view will reduce the contrast of the interference fringes and affect the application. The field-of-view-widened Michelson interferometer provides a solution of a wide field of view without wavelength limitation, for example, the patent document with the application number CN 103308926B discloses an atmospheric detection hyperspectral lidar based on the field-of-view-widened Michelson interferometer frequency discriminator, but the structure of the Michelson interferometer makes part of the light return to the light source direction after interference, which is not convenient to use.

[0004] A new type of wide-field-of-view high spectral resolution lidar is established, which is not limited by wavelength, has a wide field of view and high energy utilization rate, can expand the effective distance of the hyperspectral lidar, and has significance in the field of ocean lidar detection. SUMMARY

[0005] The technical problem solved by the application is to overcome the shortcomings of the prior art, and to provide a wide-field-of-view ocean lidar system based on Mach-Zehnder interference, which is not limited by wavelength, has a wide field of view and high energy utilization rate, has strong laser echo energy, and can improve the effective distance of the ocean lidar optical parameter profile measurement.

[0006] The technical solution of the present application is:

[0007] A wide field of view ocean laser radar system based on Mach-Zehnder interference, comprising a receiving detection module for receiving a laser radar echo signal scattered back by seawater, the receiving detection module comprising a Mach-Zehnder interference assembly; the Mach-Zehnder interference assembly comprising a first semi-reflective semi-transmissive cube prism, a second semi-reflective semi-transmissive cube prism, a first total reflection prism, a second total reflection prism, and an interference arm.

[0008] The incident light passes through the first semi-reflective semi-transmissive cube prism to obtain reflected light and transmitted light, the reflected light passes through the first total reflection prism and the second total reflection prism in sequence to be reflected, and the transmitted light passes through the second semi-reflective semi-transmissive cube prism to interfere with the reflected light, and the light is divided into two paths.

[0009] The transmission exit surface of the first semi-reflective semi-transmissive cube prism is parallel to and opposite to the entrance surface of the second semi-reflective semi-transmissive cube prism; the reflection exit surface of the first semi-reflective semi-transmissive cube prism is parallel to and opposite to the entrance surface of the first total reflection prism, the exit surface of the first total reflection prism is parallel to and opposite to the entrance surface of the second total reflection prism, the exit surface of the second total reflection prism is parallel to and opposite to the entrance surface of the second semi-reflective semi-transmissive cube prism, the two semi-reflective semi-transmissive cube prisms are depolarization beam splitting prisms, the beam splitting film is placed at an angle of 45° with the entrance surface, and the cross section of the two total reflection prisms is an isosceles right triangle, the entrance surface is a right angle side, and the angle between the oblique side coated with a total reflection film and the right angle side is 45°.

[0010] The interference arm between the reflection exit surface of the first semi-reflective semi-transmissive cube prism and the entrance surface of the first total reflection prism, the interference arm between the exit surface of the first total reflection prism and the entrance surface of the second total reflection prism, and the interference arm between the exit surface of the second total reflection prism and the entrance surface of the second semi-reflective semi-transmissive cube prism are all filled with a cuboid glass; the three interference arms are made of the same glass material, and the three interference arms are collectively referred to as a glass arm glass assembly.

[0011] The interference arm between the transmission exit surface of the first semi-reflective semi-transmissive cube prism and the entrance surface of the second semi-reflective semi-transmissive cube prism is a hybrid arm, the hybrid arm is a glass arm-air gap hybrid structure, and the hybrid arm comprises a hybrid arm glass assembly and a hybrid arm air gap; the hybrid arm glass assembly is composed of a cuboid glass, one end of the hybrid arm glass assembly is in close contact with the transmission exit surface of the first semi-reflective semi-transmissive cube prism, and the other end of the hybrid arm glass assembly is parallel to and opposite to the entrance surface of the second semi-reflective semi-transmissive cube prism; the two ends of the hybrid arm glass assembly are provided with anti-reflection films.

[0012] In summary, the present application at least has the following beneficial technical effects:

[0013] (1) It can be used at any wavelength, and has a wider wavelength working range than an atomic molecular filter;

[0014] (2) By designing the material composition and length of the interference arm of the Machzell interferometer, the field of view of the filter is expanded. Compared with the field of view expansion scheme using a single field compensation plate, the present invention optimizes the parameters by using multiple glass components, which not only expands the field of view but also has better thermal stability.

[0015] (3) Compared with the Michelson interferometer based on field of view widening which has only one interference output end, the present invention uses a Machzed interferometer component with two interference output ends. Through the complementary processing of the two interference output ends, it has higher energy utilization and higher signal-to-noise ratio. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of the present invention.

[0017] Figure 2 This invention relates to the main prisms and their relative positions in the wide field-of-view Mach-Zehnder interferometer assembly.

[0018] Figure 3 This is a schematic diagram of the wide field-of-view Mach-Zehnder interferometer assembly and interferometer arm of the present invention.

[0019] Figure 4 This describes the field-of-view widening of the Mach-Zehnder interferometer component designed in this embodiment of the invention.

[0020] Figure 5 This describes the temperature stability of the wide-field Mach-Zehnder interferometer component designed in this embodiment of the invention.

[0021] Explanation of reference numerals in the attached figures: 1. Single-frequency narrow-linewidth and narrow-pulse-width laser; 2. Beam expander; 3. Telescope; 4. Narrow-band filter; 5. Polarization beam splitter; 6, 8, 10, 11. Photodetectors; 7. Beam splitter; 9. Mach-Zehnder interferometer assembly; 12. High-speed data acquisition unit; 13. Integrated control and data processing module;

[0022] 14. First semi-reflective and semi-transparent cubic prism; 15. Second semi-reflective and semi-transparent cubic prism; 16. First total internal reflection prism; 17. Second total internal reflection prism; 18. Glass arm glass assembly; 19. Hybrid arm glass assembly; 20. Hybrid arm air gap. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings:

[0024] A wide-field-of-view marine lidar system based on Machzell interferometry, such as Figure 1 As shown, it includes a laser emitting module, a receiving and detection module, and a comprehensive control and data processing module.

[0025] The laser emission module comprises a single-frequency narrow line width narrow pulse width laser 1 and a beam expander 2; the receiving and detecting module comprises a telescope 3, a narrow-band filter 4, a polarization beam splitter 5, a photodetector 6, a beam splitter 7, a photodetector 8, a Mach-Zehnder interference component 9, a photodetector 10, a photodetector 11, and a data high-speed acquisition unit 12; the data processing module receives the digital signals of the four photodetectors output by the data high-speed acquisition unit 12, processes them through the marine optical parameter inversion algorithm of the comprehensive control and data processing module 13, and outputs the marine optical parameter information of the water body at different depths.

[0026] The light beam emitted by the single-frequency narrow line width narrow pulse width laser 1 is emitted to the water body through the beam expander 2 and the atmosphere, the laser radar echo signal scattered back by the seawater is collected by the telescope 3, the background scattered light is filtered out by the narrow-band filter 4, the light is split by the polarization beam splitter 5, the P-polarization echo signal is reflected into the photodetector 6, the S-polarization echo signal is split by the beam splitter 7, the reflected part enters the photodetector 8, and the transmitted part is split into two paths by the Mach-Zehnder interference component 9 after frequency discrimination and is received by the photodetectors 10 and 11. The receiving signals of the four photodetectors are sent to the data high-speed acquisition unit 12 for sampling and converted into digital signals, and the digital signals are sent to the comprehensive control and data processing module 13 for processing.

[0027] The single-frequency narrow line width narrow pulse width laser 1 has a blue-green wavelength, a wavelength range of 480-533 nm, a pulse width of 5-10 ns, a line width of less than 100 MHz, and a frequency stability of less than ±20 MHz.

[0028] The photodetectors 6, 8, 10 and 11 are high-sensitivity PMT detectors, and the working mode is linear mode.

[0029] As shown in Figure 2 The wide-field Mach-Zehnder interference component 9 is composed of two half-reflective half-transmissive cubic prisms and two total reflection prisms, the two half-reflective half-transmissive cubic prisms are a first half-reflective half-transmissive cubic prism 14 and a second half-reflective half-transmissive cubic prism 15, and the two total reflection prisms are a first total reflection prism 16 and a second total reflection prism 17.

[0030] The transmission exit surface 141 of the first semi-reflective cubic prism 14 is parallel and opposite to the incident surface 151 of the second semi-reflective cubic prism 15; the reflection exit surface 142 of the first semi-reflective cubic prism 14 is parallel and opposite to the incident surface 161 of the first total internal reflection prism 16; the exit surface 162 of the first total internal reflection prism 16 is parallel and opposite to the incident surface 171 of the second total internal reflection prism 17; and the exit surface 172 of the second total internal reflection prism 17 is parallel and opposite to the incident surface 152 of the second semi-reflective cubic prism 15. The two semi-reflective cubic prisms 14 and 15 are depolarizing beam splitters, with the beam splitting film placed at a 45° angle to the incident surface. The cross-sections of the two total internal reflection prisms 16 and 17 are isosceles right triangles, with the incident surface forming a right-angled side, making a 45° angle with the hypotenuse coated with the total internal reflection film.

[0031] like Figure 3 As shown, the interference arm 181 between the reflecting exit surface 142 of the first semi-reflective cubic prism 14 and the incident surface 161 of the first total internal reflection prism 16, the interference arm 182 between the exit surface 162 of the first total internal reflection prism 16 and the incident surface 171 of the second total internal reflection prism, and the interference arm 183 between the exit surface 172 of the second total internal reflection prism 17 and the incident surface 152 of the second semi-reflective cubic prism 15 are filled with cuboid glass. The two ends of the interference arms 181, 182, and 183 are in close contact with the incident or exit surfaces of the adjacent cubic prisms and total internal reflection prisms and are provided with anti-reflection coatings. The three interference arms 181, 182, and 183 are made of the same glass material, and the three interference arms are referred to as a whole as glass arm glass assembly 18.

[0032] The interference arm between the transmission exit surface 141 of the first semi-reflective cubic prism 14 and the incident surface 151 of the second semi-reflective cubic prism 15 is a hybrid arm, which is a glass arm-air gap hybrid structure. The hybrid arm includes a hybrid arm glass assembly 19 and a hybrid arm air gap 20. The hybrid arm glass assembly 19 is made of cuboid glass, with one end in close contact with the transmission exit surface 141 of the first semi-reflective cubic prism 14, and the other end parallel to and opposite to the incident surface 151 of the second semi-reflective cubic prism 15. Anti-reflective coatings are provided at both ends of the hybrid arm glass assembly.

[0033] Specifically, the incident light passes through the first semi-reflective and semi-transparent cubic prism (14) to obtain reflected light and transmitted light. The reflected light is reflected by the first total internal reflection prism (16) and the second total internal reflection prism (17) in sequence. After interfering with the transmitted light through the second semi-reflective and semi-transparent cubic prism (15), it is split into two paths and received by the photodetector (10) and the photodetector (11) respectively.

[0034] The polarizing beam splitter 5 divides the echo signal into P-polarization and S-polarization, filters out the P-polarization echo signal, and reduces the influence of the sea surface Fresnel reflection. The beam splitter 7 is a depolarizing beam splitter. The output signal entering the photodetector 8 through the beam splitter 7 is the reference signal S1, which contains the scattering signals of water molecules and water body particles, and represents the water body backscattering mixed channel. The transmitted light through the beam splitter 7 enters the Mach-Zehnder interference component 9 for frequency discrimination. The echo signals output by the photodetectors 10 and 11 are complementary to the reference signal output by the photodetector 8 and are processed as the overall frequency discrimination output signal S2, which contains only the scattering signals of water molecules and represents the water body backscattering molecular channel. The energy ratio factor of the water body backscattering mixed channel and the molecular channel is calculated by combining the light splitting ratio M1:M2 (reflection ratio: transmission ratio) of the beam splitter 7 for retrieving the water body backscattering coefficient. The method for retrieving the water body backscattering coefficient is described in detail in the literature YUDI ZHOU, DONG LIU, PEITUO XU, CHONG LIU, JIAN BAI, LIMING YANG, ZHONGTAO CHENG, PEIJUN TANG, YUPENG ZHANG, AND LIN SU 'Retrieving the seawater volume scattering function at the 180°

[0035] scattering angle with a high-spectral-resolution lidar.'Opt Express 25, 11813-11816 (2017).

[0036] Example 2

[0037] The light beam emitted by the single-frequency narrow linewidth laser 1 passes through the beam expander 2 and is emitted to the water body through the atmosphere. The laser radar echo signal scattered back by the seawater body is collected by the telescope 3, the background scattering light is filtered out through the narrow-band optical filter 4, the light is divided by the polarizing beam splitter 5, the P-polarization echo signal is reflected into the photodetector 6, the S-polarization echo signal is divided by the beam splitter 7, the reflected part enters the photodetector 8, and the transmitted part is frequency discriminated by the wide-field Mach-Zehnder interference component 9 and then received by the photodetector 10 and the photodetector 11 in two ways. The received signals of the four photodetectors are sent to the data high-speed acquisition unit 12 for sampling and converted into digital signals, and the digital signals are sent to the comprehensive control and data processing module 13 for processing.

[0038] The comprehensive control and data processing module 13 performs complementary processing on the data obtained by the third photodetector 10 and the fourth photodetector 11, and outputs the overall frequency discrimination of the marine laser radar system. Based on the received signals of the four photodetectors, the comprehensive control and data processing module 13 uses a marine optical parameter inversion algorithm to invert the marine optical parameter information of the water body at different depths from the digital signals.

[0039] The single-frequency narrow linewidth laser 1 has a wavelength of 532 nm, a pulse width of 8 ns, a linewidth of less than 100 MHz, and a frequency stability of ±20 MHz.

[0040] The bandwidth of the narrowband filter 4 is ±1 nm, and the center wavelength is 532 nm.

[0041] The photodetectors 6, 8, 10, and 11 are high-sensitivity PMT detectors, and the working mode is linear mode.

[0042] The reflectivity and transmissivity of the beam splitter 7 correspond to a splitting ratio of 1:20.

[0043] The Mach-Zehnder interference assembly 9 is composed of two half-reflective half-transmissive cube prisms and two total reflection prisms. The two half-reflective half-transmissive cube prisms are a first half-reflective half-transmissive cube prism 14 and a second half-reflective half-transmissive cube prism 15, and the two total reflection prisms are a first total reflection prism 16 and a second total reflection prism 17. The coated surface of the half-reflective half-transmissive cube prism is parallel to the total reflection surface of the total reflection prism and is placed at an angle of 45° with the interference arm. The interference arm 181 between the adjacent two surfaces of the first half-reflective half-transmissive cube prism 14 and the first total reflection prism 16, the interference arm 182 between the adjacent two surfaces of the first total reflection prism 16 and the second total reflection prism 17, and the interference arm 183 between the lower adjacent two surfaces of the second total reflection prism 17 and the second half-reflective half-transmissive cube prism 14 are all pure glass arms, and each glass arm has the same material and is provided with an anti-reflection film at both ends. The three glass assemblies are combined into one whole, which is called a glass arm glass assembly 18. The interference arm between the half-reflective half-transmissive cube prism 14 and the half-reflective half-transmissive cube prism 15 is a mixed arm, which adopts a mixed structure of glass assembly-air assembly, and the mixed arm glass assembly 19 is provided with an anti-reflection film at both ends.

[0044] The materials and sizes of the above-mentioned glass arm glass assembly 18 and mixed arm glass assembly 19 can be obtained by the following steps:

[0045] S1, let n1, n2, n3 be the refractive index of the glass arm glass assembly 18, the mixed arm glass assembly 19, and the mixed arm air gap 20 respectively, θ0 be the angle between the optical axis of the incident light and the normal incidence direction of the Mach-Zehnder interference assembly, θ be the angle between the incident light and the normal incidence direction of the Mach-Zehnder interference assembly, the normal incidence direction of the Mach-Zehnder interference assembly be the incidence perpendicular to the incidence plane of the half-reflective half-transmissive cube prism 14, θ1, θ2, θ3 be the refraction angle of the light in the glass arm glass assembly 18, the mixed arm glass assembly 19, and the mixed arm air gap 20, h1, h2, h3 be the length of the light in the glass arm glass assembly 18, the mixed arm glass assembly 19, and the mixed arm air gap 20. The distance between the two ends of the interference arm parallel to the prism surface is the length. The optical path difference of the two output light paths of the adjustable height inner and outer threaded fitting honeycomb reinforcing bushing of the Mach-Zehnder interference assembly 9 is:

[0046]

[0047] Where, let the half-angle of the incident light be α, then the range of the incident angle θ of the incident light is: θ ∈ [θ0-α, θ0+α]. σ0 is the optical path difference introduced by the reflection and transmission of light inside the cube prism and the total reflection prism, which is related to the prism material and size and is a fixed constant.

[0048] S2: Since θ0 is a small inclination angle, expand σ(θ) near sin 2 θ0 in Taylor series,

[0049] σ(θ) = σ(θ0) + f(θ0) × (sin 2 θ-sin 2 θ0) + o((sin 2 θ-sin 2 θ0)) (2)

[0050] In the above formula, o(·) represents the second and higher infinitesimal of (sin 2 θ-sin 2 θ0),

[0051]

[0052] S3: Let f(θ0) = 0 (4)

[0053] As a condition for field compensation, the coefficient of sin 2 θ-sin 2 θ0 in σ(θ) is 0 at this time, and σ(θ) changes slowly with the change of the incident angle θ;

[0054] S4: Let

[0055] As the condition of temperature stability, the change of σ(θ0) with temperature T is slow.

[0056] And let σ'(θ0) and σ''(θ0) represent the first and second derivatives of the optical path length of the glass arm glass component (18), the mixed arm glass component (19), and the mixed arm air gap (20) with respect to temperature.

[0057] Then, Expand f T (h1, n1)-f T (h2, n2)-f T (h3, n3) = 0 (5)

[0058] Wherein, is the relative refractive index temperature coefficient in air or glass medium, is the thermal expansion coefficient of air gap or glass material. And The specific values of σ'(θ0) and σ''(θ0) can be found in the optical glass parameter table.

[0059]

[0060] The design value is determined according to the frequency discrimination requirement of the marine laser radar. First, the spectral characteristics of the two outputs of the Mach-Zehnder interference component require that the transmittance of the center wavelength of the laser reaches a maximum or minimum value. Second, the free spectral range reaches 15GHz.

[0061] S6, the formula of steps S3-S5 is solved, that is, (4) (5) (6) is solved, combined with the design value of σ(θ0), the relative refractive index temperature coefficient and the thermal expansion coefficient of the glass structure 18 and the mixed arm glass component 19, h1, h2, h3 can be calculated as the design basis of the interference arm of the Mach-Zehnder interference component 9.

[0062] S7, σ(θ0) is the inherent optical path difference of the interferometer, and the change amount of the optical path difference introduced by the incident angle is

[0063] δ(θ) = σ(θ)-σ(θ0) (7)

[0064] Substitute h1, h2, h3 into formula (7) to evaluate the change value of the optical path difference introduced by different incident angles, and calculate the field of view expansion range of the wide field of view interferometer according to the design input constraint on the change of the optical path difference.

[0065] S8, let be the optical path difference of the interferometer under the theoretical working temperature T0 condition, wherein n i (T0) = n i , h i (T0) = h i(i = 1, 2, 3) are the designed refractive index and the length of the interference arm, respectively;

[0066] Let The optical path difference of the interferometer under the actual working temperature T, where The actual refractive index and the actual length of the interference arm, respectively.

[0067] The relative refractive index temperature coefficient The thermal expansion coefficient The actual temperature T is substituted into the formula δ(T) = σ(n′ i (T), h′ i (T) - σ(n i (T0), h i (T0)), the optical path difference change amount δ(T) introduced by the actual environmental temperature T is obtained. According to the constraint of the temperature stability of the design input, the temperature expansion range of the wide-field interferometer thermal stability is calculated.

[0068] Taking the case of the incident laser emission wavelength 532 nm, the angle θ0 between the optical axis of the incident light and the normal direction of the Mach-Zehnder interference assembly 10 mrad, and the theoretical working temperature T0 = 20℃, the material of the glass arm glass assembly 18 is selected as glass N-SF66, the total length is 47.5934mm, and it is placed in three sections. The first section is placed in the interference arm 181 between the first half-reflection half-transmission cube prism 14 and the first total reflection prism 16, with a length of 3.17845mm. The second section is placed in the interference arm 182 between the first total reflection prism 16 and the second total reflection prism 17, with a length of 41.2635mm. The third section is placed in the interference arm 183 between the second total reflection prism 17 and the second half-reflection half-transmission cube prism 15, with a length of 3.17845mm. The interference arm between the first half-reflection half-transmission cube prism 14 and the second half-reflection half-transmission cube prism 15 is a glass-air hybrid arm, with a total length of 41.2365mm. The material of the hybrid arm glass assembly 19 is selected as glass N-SF57, with a length of 36.0712mm. The length of the hybrid arm air gap 20 is 5.1653mm.

[0069] Figure 4 It is shown that when the incident angle deviation Δθ = θ - θ0 changes in [-75mrad, 75mard], the change amount of the optical path difference of the interferometer can be kept within 0.1 wavelength.

[0070] Figure 5 It is shown that when the temperature change amount ΔT = T - T0 changes in [-5℃, 5℃], the change amount of the optical path difference of the interferometer can be kept within 0.1 wavelength.

[0071] Although the present application is disclosed with reference to the preferred embodiments above, it is not intended to limit the present application, and any person skilled in the art can make possible variations and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application should be defined by the scope of the claims.

Claims

1. A wide-field-of-view marine lidar system based on Mach-Zehnder interferometry, characterized in that: It includes a receiving and detection module for receiving the lidar echo signal scattered back by seawater. The receiving and detection module includes a Machzell interferometer (9). The Machzell interferometer (9) includes a first semi-reflective cube prism (14), a second semi-reflective cube prism (15), a first total reflection prism (16), a second total reflection prism (17), and an interferometer arm. The incident light passes through the first semi-reflective and semi-transparent cubic prism (14) to obtain reflected light and transmitted light. The reflected light is reflected by the first total internal reflection prism (16) and the second total internal reflection prism (17) in sequence. After interfering with the transmitted light through the second semi-reflective and semi-transparent cubic prism (15), it is split into two paths. The interference arms (181) between the reflecting exit surface (142) of the first semi-reflective cubic prism (14) and the incident surface (161) of the first total internal reflection prism (16), the interference arms (182) between the exit surface (162) of the first total internal reflection prism (16) and the incident surface (171) of the second total internal reflection prism, and the interference arms (183) between the exit surface (172) of the second total internal reflection prism (17) and the incident surface (152) of the second semi-reflective cubic prism (15) are all filled with cuboid glass. The three interference arms (181, 182, 183) are made of the same glass material, and the three interference arms are referred to as a whole as the glass arm glass assembly (18).

2. The wide-field-of-view marine lidar system based on Mach-Zehnder interferometry according to claim 1, characterized in that: The transmission exit surface (141) of the first semi-reflective cubic prism (14) is parallel and opposite to the incident surface (151) of the second semi-reflective cubic prism (15); the reflection exit surface (142) of the first semi-reflective cubic prism (14) is parallel and opposite to the incident surface (161) of the first total internal reflection prism (16); the exit surface (162) of the first total internal reflection prism (16) is parallel and opposite to the incident surface (171) of the second total internal reflection prism (17). Parallel and opposite, the exit surface (172) of the second total reflection prism (17) is parallel and opposite to the incident surface (152) of the second semi-reflective cube prism (15). The two semi-reflective cube prisms (14) and (15) are depolarizing beam splitters. The beam splitting film is placed at 45° with the incident surface. The cross sections of the two total reflection prisms (16) and (17) are isosceles right triangles. The incident surface is the right-angled side, and the angle between it and the hypotenuse coated with the total reflection film is 45°.

3. A wide-field-of-view marine lidar system based on Mach-Zehnder interferometry according to claim 1 or 2, characterized in that: The interference arm between the transmission exit surface (141) of the first semi-reflective cubic prism (14) and the incident surface (151) of the second semi-reflective cubic prism (15) is a hybrid arm. The hybrid arm is a glass arm-air gap hybrid structure. The hybrid arm includes a hybrid arm glass assembly (19) and a hybrid arm air gap (20). The hybrid arm glass assembly (19) is made of cuboid glass. One end is in close contact with the transmission exit surface (141) of the first semi-reflective cubic prism (14), and the other end is parallel to and opposite to the incident surface (151) of the second semi-reflective cubic prism (15). Anti-reflective coatings are provided at both ends of the hybrid arm glass assembly (19).

4. A wide-field-of-view marine lidar system based on Mach-Zehnder interferometry according to claim 1, characterized in that: The materials and dimensions of the glass arm glass assembly (18) and the hybrid arm glass assembly (19) are determined through the following steps: S1: Let n1, n2, and n3 be the refractive indices of the glass arm assembly (18), the mixed arm assembly (19), and the air gap (20) of the mixed arm, respectively; θ0 is the angle between the incident light axis and the normal incident direction of the Mach-Zehnder interferometer assembly (9); θ is the angle between the incident light and the normal incident direction of the Mach-Zehnder interferometer assembly (9); θ1, θ2, and θ3 are the refraction angles of the light in the glass arm assembly (18), the mixed arm assembly (19), and the air gap (20); h1, h2, and h3 are the lengths of the glass arm assembly (18), the mixed arm assembly (19), and the air gap (20); the distance between the two ends of the interferometer arm parallel to the prism surface is the length of the interferometer arm; σ0 is the optical path difference introduced by the reflected and transmitted light when it propagates inside the cubic prism and the total internal reflection prism; σ0 is related to the prism material and size and is a fixed constant; the optical path difference introduced by the two interferometer arm paths at the Mach-Zehnder interferometer assembly (9) is: Let the half-angle of divergence of the incident light be α, then the range of the incident angle θ of the incident light is: θ∈[θ0-α, θ0+α]; S2: Since θ0 is a small tilt angle, Taylor expand σ(θ) around sin 2 θ0 near θ0, σ(θ)=σ(θ0)+f(θ0)×(sin 2 θ-sin 2 θ0)+o((sin 2 θ-sin 2 θ0)) In the above formula, o(·) represents (sin 2 θ-sin 2 Infinitesimals of order two and above (θ0), S3: Let f(θ0) = 0; S4: Order And order So, Expand to f T (h1,n1)-f T (h2,n2)-f T (h3,n3)=0, in, It is the temperature coefficient of relative refractive index in air or glass media. It is the coefficient of thermal expansion of the air gap or the glass material; and For specific values, please refer to the optical glass parameter table; S5 The design values ​​are determined based on the frequency discrimination requirements of the marine lidar. First, the spectral characteristics of the two outputs of the Machzell interferometer component must be such that the transmittance to the center wavelength of the laser reaches a maximum or minimum value. Second, the free spectral range must reach 15 GHz. S6. Combine the formulas from steps S3-S5 to calculate h1, h2, and h3.

5. A wide-field-of-view marine lidar system based on Mach-Zehnder interferometry according to claim 4, characterized in that: The normal incident direction of the Machzed interferometer component (9) is perpendicular to the incident direction of the first semi-reflective and semi-transparent cubic prism (14).

6. A wide-field-of-view marine lidar system based on Mach-Zehnder interferometry according to claim 1, characterized in that: The receiving and detection module also includes a telescope (3), a narrowband filter (4), a polarization beam splitter (5), a beam splitter (7), four photodetectors, and a high-speed data acquisition unit (12). The laser radar echo signal scattered back by the seawater is collected by the telescope, and the background scattered light is filtered out by the narrowband filter (4). The polarization beam splitter (5) splits the light to obtain the P-polarized echo signal and the S-polarized echo signal. The P-polarized echo signal is reflected into the first photodetector (6), and the S-polarized echo signal is split by the beam splitter (7). The reflected part enters the second photodetector (8), and the transmitted part is divided into two paths after frequency discrimination by the Mach-Zehnder interferometer (9) and received by the third photodetector (10) and the fourth photodetector (11). The received signals of the four photodetectors are sent to the high-speed data acquisition unit (12) for sampling and conversion into digital signals. The digital signals are sent to the integrated control and data processing module (13) for processing.

7. A wide-field-of-view marine lidar system based on Mach-Zehnder interferometry according to claim 6, characterized in that: The integrated control and data processing module (13) performs complementary processing on the data obtained by the third photodetector (10) and the fourth photodetector (11) as the overall frequency discrimination output of the marine lidar system.

8. A wide-field-of-view marine lidar system based on Mach-Zehnder interferometry according to claim 1 or 6, characterized in that: It also includes a laser emitting module for emitting lasers. The laser emitting module includes a single-frequency narrow-linewidth narrow-pulse-width laser (1) and a beam expander (2). The single-frequency narrow-linewidth narrow-pulse-width laser (1) emits lasers, and the lasers are expanded by the beam expander (2) and then taken into seawater. Single-frequency narrow-linewidth and narrow-pulse-width lasers belong to the blue-green laser category. The center wavelength is selected in the range of 480nm to 533nm, the pulse width of the output laser is in the range of 5 to 10ns, the linewidth is less than 100MHz, and the frequency stability is less than ±20MHz.

9. A wide-field-of-view marine lidar system based on Mach-Zehnder interferometry according to claim 4, characterized in that: σ(θ0) is the inherent optical path difference of the interferometer, and the change in optical path difference introduced by the incident angle is δ(θ)=σ(θ)-σ(θ0); set up As the interferometer optical path difference under the theoretical design temperature T0, where n i (T0)=n i h i (T0)=h i (i = 1, 2, 3) represent the designed refractive index and interference arm length, respectively; set up As the interferometer optical path difference under the actual operating temperature T, where These are the actual refractive index and the actual interference arm length, respectively. temperature coefficient of relative refractive index coefficient of thermal expansion Substituting the actual temperature T into the equation δ(T)=σ(n) i ′(T),h i ′(T))-σ(n i (T0),h i (T0)), to obtain the change in optical path difference δ(T) introduced by the actual ambient temperature T.

Images