A precise tracking and detection optical path system for free space laser communication terminals

Abstract

The present invention discloses a precision tracking detection optical path system for a free space laser communication terminal, and belongs to the field of satellite laser communication technology. The present invention includes a receiving optical path convergence lens, an inclined end-face optical fiber, a relay magnifying lens, and a precision tracking detector; the core of the inclined end-face optical fiber is located at the focus of the receiving optical path convergence lens, and the core of the inclined end-face optical fiber and the center point of the precision tracking detector satisfy the object-image conjugate relationship for the relay magnifying lens; the optical axis of the receiving optical path convergence lens, the optical axis of the relay magnifying lens, the optical axis of the inclined end-face optical fiber, and the normal of the inclined end-face optical fiber are in the same plane; the present invention collects the reflected light beam of the inclined end-face optical fiber by using the relay magnifying lens and images it to a high-resolution array detector, and without introducing additional laser energy loss, realizes high-precision detection of the angle of the received laser beam, and the result is not affected by the angle drift of each optical element of the receiving branch.

Description

Technical Field

[0001] The invention belongs to the technical field of satellite laser communication, and in particular relates to a precise tracking and detection optical path system for a free space laser communication terminal. Background Art

[0002] In free-space laser communication systems, precise tracking accuracy plays a vital role in the coupling efficiency of single-mode optical fibers, which is directly related to the stability and reliability of communication quality. Since the core diameter of single-mode optical fibers is very small, usually only a few microns, the laser beam must be aligned and coupled into the optical fiber with extremely high precision. Any slight deviation will cause the coupling efficiency of the beam to decrease, which in turn causes the loss of signal power and the degradation of communication quality. When free-space laser communication is carried out between satellites or ground stations, factors such as atmospheric disturbances, satellite attitude changes, and orbital deviations will cause the laser beam to jitter or offset. Therefore, a high-precision tracking and control system must be used to adjust the beam direction in real time to ensure that the laser always accurately enters the single-mode optical fiber and ensure the stability and efficient transmission of the communication link.

[0003] In the precision tracking system of laser communication terminals, an array detector or a four-quadrant detector is usually used to detect the angle of the received laser beam. By using a beam splitter to sample the received laser beam and using a converging lens with a longer focal length to form a laser spot on the above detector, the change in the spot position can be analyzed by algorithms such as the centroid algorithm, and the offset angle of the incident laser can be accurately measured. The fast mirror in the optical path is further corrected based on the offset angle to guide the laser beam to align with the single-mode optical fiber. However, laser communication terminals often work in relatively harsh mechanical and thermal environments. The optical elements such as the sampling beam splitter and the folding plane mirror used in the optical path are deformed under various mechanical factors (such as changes in the direction of gravity, impact during satellite launch) and thermal factors (such as temperature differences, temperature gradients), thereby changing the beam angle, causing the spot on the spot detector to drift, causing the zero position of the tracking subsystem to produce a systematic error, and reducing the tracking accuracy. As the tracking accuracy decreases, the laser beam cannot be accurately coupled into the single-mode optical fiber, resulting in a decrease in the fiber coupling efficiency, which directly affects the signal transmission power and stability of the communication link, and ultimately deteriorates the performance and quality of the overall communication system. To ensure efficient communication quality, the thermal and mechanical stability of the optomechanical subsystem must be considered in the system design to make the optomechanical system sufficiently robust. It is also necessary to consider using additional calibration modules to calibrate the mechanical and thermal deformations in the optomechanical system to alleviate the problem of fine tracking errors caused by mechanical and thermal factors in the laser communication terminal.

[0004] In recent years, some laser communication terminals have adopted nutation technology to achieve precise tracking in order to solve the problem of precision tracking system errors caused by environmental changes. Nutation technology uses a precision aiming mirror to perform high-frequency circular motion in the angle space. The laser beam performs circular motion with an appropriate radius r0 near the core diameter of the optical fiber. The distance between the laser beam and the center point of the optical fiber core diameter is determined by the periodic change of the light intensity during the circular motion. This precision tracking method does not rely on traditional precision tracking detectors and is not affected by the above-mentioned precision tracking system errors. However, this method also has significant disadvantages: first, there is a basic angle offset in the nutation technology, and the average distance between the laser spot and the center point of the optical fiber core diameter is always greater than its nutation radius r0. This deviation leads to a decrease in laser coupling efficiency, and the typical achievable coupling efficiency is 50% to 60%; secondly, the effective range of nutation tracking is small, and it is difficult to cope with a larger range of angle offsets. For this reason, the system generally requires an additional first-level tracking link with a larger tracking angle range, which brings additional receiving power loss to the laser communication terminal and increases the complexity of the system, which is not conducive to the miniaturization of laser communication terminals.

[0005] In summary, for free-space laser communication terminals, especially satellite laser communication terminals used in space environments, advanced precision tracking technology is of great significance to ensure the stability and efficiency of satellite laser communication links and improve communication capacity and communication quality. High-performance free-space communication systems require precision tracking technology with technical characteristics such as large tracking range, high energy utilization, and high stability. In addition, for satellite laser communication terminals, the miniaturization and lightweight of precision tracking modules are conducive to reducing the size, weight, and power consumption of the terminals, which is of great significance for reducing satellite resource consumption and reducing launch costs. Summary of the invention

[0006] Aiming at the needs of free-space laser communication terminals, the present invention proposes a precision tracking detection optical path system for free-space laser communication terminals. By using a relay magnifying lens to collect the reflected light beam of the tilted end-face optical fiber and image it to a high-resolution array detector, high-precision detection of the angle of the received laser beam is achieved without introducing additional laser energy loss, and the angle detection result is not affected by the angle drift of each optical element of the receiving branch. This method has the technical advantages of a large tracking range, high energy utilization, and high stability. On the basis of miniaturization of the precision tracking module, the system error of the precision tracking module can be eliminated to achieve high-precision and high-stability precision tracking, which provides a strong guarantee for the communication performance and communication quality of the free-space laser communication terminal, and is also of great significance for reducing satellite resource consumption and reducing launch costs.

[0007] The above purpose is achieved through the following technical solutions:

[0008] The present invention first provides a precision tracking detection optical path system for a free space laser communication terminal, comprising a receiving optical path converging lens, an inclined end-face optical fiber, a relay magnifying lens, and a precision tracking detector; the core of the inclined end-face optical fiber is located at the focus of the receiving optical path converging lens, and the core of the inclined end-face optical fiber and the center point of the precision tracking detector satisfy an object-image conjugate relationship for the relay magnifying lens; the optical axis of the receiving optical path converging lens, the optical axis of the relay magnifying lens, the optical axis of the inclined end-face optical fiber, and the normal of the inclined end-face optical fiber are in the same plane;

[0009] Let the end face inclination angle of the inclined end face optical fiber be θ, and the angle between the optical axis of the inclined end face optical fiber and the optical axis of the receiving optical path converging lens be ψ, and the two satisfy:

[0010] ψ=(n-1)θ

[0011] Where n is the core refractive index of the optical fiber with the tilted end face;

[0012] Let the NA of the receiving light path converging lens be α 1 The angle between the optical axis of the receiving optical path converging lens and the normal line of the optical fiber end face of the inclined end face optical fiber is φ 1 , the NA of the relay magnifying lens is α 2 The angle between the optical axis of the relay magnifying lens and the normal line of the fiber end face of the inclined end face fiber is φ 2 , the above parameters satisfy the following relationship:

[0013]

[0014] Among them, NA fb is the fiber NA value of the optical fiber with tilted end face.

[0015] Furthermore, the optical fiber with the inclined end face is realized by polishing the end face of a common optical fiber by optical cold working so that the end face has an inclination angle of θ.

[0016] Furthermore, the end face of the inclined end face optical fiber is coated outside the range of 1.2 to 1.8 times of the core diameter, and the coating film system is a metal reflective film or a dielectric reflective film.

[0017] Furthermore, the end face of the inclined end face optical fiber is plated with a dielectric beam splitting film, and the dielectric beam splitting film can reflect the precise tracking detection wavelength in the precise tracking detection optical path system and transmit the signal light wavelength in the precise tracking detection optical path system.

[0018] Furthermore, a filter is added between the inclined end-face optical fiber and the relay magnifying lens or between the relay magnifying lens and the fine tracking detector.

[0019] Furthermore, the precision tracking detection optical path system also includes a plane reflector, and a module composed of the plane reflector and the inclined end-face optical fiber realizes the function of coupling signal light into the optical fiber and reflecting the laser beam to the relay amplifier lens; the optical surface of the plane reflector is coplanar with the optical fiber end face of the inclined end-face optical fiber; the plane reflector has a center hole, and the inclined end-face optical fiber is located in its center hole.

[0020] The present invention also provides a post-receiving optical path system, which includes the above-mentioned precise tracking detection optical path system and a photoelectric detection module, wherein the photoelectric detection module is connected to the inclined end-face optical fiber.

[0021] The present invention also provides a laser communication terminal, which includes the above-mentioned post-receiving optical path system, post-transmitting optical path, beam splitter, precision aiming mirror and optical antenna; the post-transmitting optical path includes a transmitting light source, a transmitting collimating lens and an advance aiming mirror, the laser beam emitted by the transmitting light source passes through the transmitting collimating lens to form a collimated laser beam, and the beam passes through the advance aiming mirror for angle correction; when the laser communication terminal receives a laser beam from another laser communication terminal, it is successively beam-contracted by the optical antenna, accurately angle-corrected by the precision aiming mirror, and then split by the beam splitter, and then focused on the core of the inclined end-face optical fiber by the receiving optical path converging lens, coupled into the optical fiber, and then enters the photoelectric detection module; the reflected laser of the inclined end-face optical fiber is converged on the precision tracking detector through the relay amplifier lens to form a laser spot; the precision control module of the laser communication terminal controls the deflection angle of the precision aiming mirror based on the position of the laser spot on the precision tracking detector, so as to realize the laser communication terminal to accurately correct the angle of the incident laser, so that it can always be coupled to the inclined end-face optical fiber with high efficiency, so as to realize the laser communication terminal to accurately track another laser communication terminal.

[0022] Furthermore, a nutating mirror is provided between the beam splitter and the receiving light path converging lens for realizing circular motion of the light beam in the angular space.

[0023] Furthermore, a corner cone is provided on the reflective surface of the beam splitter for calibrating the angle of the emission optical axis of the post-emission optical path: the emission optical axis of the post-emission optical path is reflected by the beam splitter, then reflected along the original path through the corner cone, and then transmitted through the beam splitter, enters the post-receiving optical path, and is received by the precision tracking detector; by deviating the sight from a known angle in advance, the two light spots of the emission laser beam of the post-emission optical path and the normal receiving laser beam on the precision tracking detector are separated by a specified distance, and based on the deviation between the actual relative position and the theoretical relative position of the two light spots, the angular error between the receiving optical axis and the transmitting optical axis of the laser communication terminal is calibrated.

[0024] Aiming at the needs of free-space laser communication terminals, the present invention proposes a precise tracking detection optical path system with high angular stability, and its beneficial effects include the following aspects:

[0025] 1. Simple structure, reducing the number of optical components;

[0026] 2. Compared with the traditional fine tracking method, there is no need to split the beam for the fine tracking branch, which improves the utilization rate of the laser energy of the receiving beam;

[0027] 3. Compared with the nutation precision tracking method, it does not need to introduce basic angle deviation, which improves the utilization rate of the laser energy of the receiving beam;

[0028] 4. Compared with the nutation precision tracking method, it effectively expands the precision tracking field of view, and does not require an additional tracking optical path with a larger effective field of view;

[0029] 5. Compared with the traditional precision tracking method, it eliminates the precision tracking system error caused by the zero drift of the tracking subsystem caused by various mechanical influencing factors (such as changes in the direction of gravity, impact during satellite launch) and thermal influencing factors (such as temperature difference changes, temperature gradient);

[0030] 6. Reduce the optical-mechanical stability requirements of the laser communication terminal, thereby indirectly reducing the size and weight of the laser communication terminal and reducing the transmission cost;

[0031] 7. Improve the stability of satellite laser communication links. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the optical path system of the present invention;

[0033] Figure 2 It is a schematic diagram of the angle relationship between the receiving light path converging lens, the relay magnifying lens, and the inclined end-face optical fiber of the present invention;

[0034] Figure 3 is a schematic structural diagram of an inclined end-face optical fiber with a coplanar plane reflector as described in Example 5;

[0035] Figure 4 is a schematic diagram of an optical path system with a nutating mirror as described in Example 8;

[0036] Figure 5 is a schematic diagram of the optical path system with a pyramid described in Example 9;

[0037] Description of each component in the figure: 1. Optical antenna; 2. Precision aiming mirror; 3. Beam splitter; 4. Post-transmission optical path; 401. Transmission light source; 402. Transmission collimating lens; 403. Advance aiming mirror; 5. Post-reception optical path system; 6. Inclined end-face optical fiber; 601. Plane reflector; 7. Photoelectric detection module; 8. Relay magnifying lens; 9. Precision tracking detector; 10. Reception optical path converging lens; 11. Nutating mirror; 12. Corner cone. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 making creative work are within the scope of protection of the present invention.

[0039] Embodiment 1:

[0040] like Figure 1 As shown, the present embodiment is a precision tracking detection optical path system for a free space laser communication terminal, comprising a receiving optical path converging lens 10, an inclined end-face optical fiber 6, a relay magnifying lens 8, and a precision tracking detector 9; the core of the inclined end-face optical fiber 6 is located at the focus of the receiving optical path converging lens 10, and between the core of the inclined end-face optical fiber 6 and the center point of the precision tracking detector 9, the object-image conjugate relationship is satisfied for the relay magnifying lens 8; the optical axis of the receiving optical path converging lens 10, the optical axis of the relay magnifying lens 8, the optical axis of the inclined end-face optical fiber 6, and the normal of the inclined end-face optical fiber 6 are in the same plane;

[0041] like Figure 2 As shown, let the end face inclination angle of the inclined end face optical fiber 6 be θ, and the included angle between the optical axis of the inclined end face optical fiber 6 and the optical axis of the receiving light path converging lens 10 be ψ, and the two satisfy:

[0042] ψ=(n-1)θ

[0043] Where n is the core refractive index of the inclined end-face optical fiber 6;

[0044] Let the NA of the receiving light path converging lens 10 be α 1 The angle between the optical axis of the receiving optical path converging lens 10 and the normal line of the optical fiber end face of the inclined end face optical fiber 6 is φ 1 , the NA of the relay magnifying lens 8 is α 2 The angle between the optical axis of the relay magnifying lens 8 and the normal line of the fiber end face of the inclined end face optical fiber 6 is φ 2 , the above parameters satisfy the following relationship:

[0045]

[0046] Among them, NA fb is the fiber NA value of the inclined end face optical fiber 6.

[0047] Embodiment 2:

[0048] The difference between this embodiment and embodiment 1 is that the inclined end face optical fiber 6 is realized by optically cold processing the end face of the ordinary optical fiber to polish the end face to an end face inclination angle of θ. For a small NA, a commercial product of an APC connector optical fiber can be used, and its end face inclination angle is 8 degrees. 1 and φ 21 The angle is about 12 degrees, and the separation angle between the receiving light path converging lens 10 and the relay magnifying lens 8 is relatively small, which may cause difficulties in the optical machine design.

[0049] Embodiment 3:

[0050] The difference between this embodiment and embodiment 1 is that, in order to further improve the energy utilization rate in the precision tracking optical path, the end face of the inclined end-face optical fiber 6 is slightly larger than the portion outside the core area (outside the range of 1.2 to 1.8 times the core diameter, for example, for a typical SMF28+ optical fiber, the core diameter is about 8.2um, except for the portion outside the core of Φ12um) is coated to improve the overall reflectivity of the end face, and the coating film system adopts a metal reflective film or a dielectric reflective film.

[0051] Embodiment 4:

[0052] The difference between this embodiment and embodiment 1 is that, in order to adapt to the situation where the signal light wavelength is different from the precise tracking detection wavelength, a dielectric beam splitting film is plated on the end face of the inclined end face optical fiber 6 to reflect its precise tracking detection wavelength and transmit its signal light wavelength; the receiving light path converging lens 10 is achromatic for the signal light wavelength and the precise tracking detection wavelength.

[0053] In this embodiment, a filter may be added between the tilted end-face optical fiber 6 and the relay amplifier lens 8 or between the relay amplifier lens 8 and the fine tracking detector 9 as needed to suppress interference of light beams outside the fine tracking detection wavelength on fine tracking.

[0054] Embodiment 5:

[0055] like Figure 3 As shown, in order to expand the effective field of view of precise tracking, the module composed of the plane reflector 601 and the inclined end-face optical fiber 6 realizes the function of coupling signal light into the optical fiber and reflecting the laser beam to the relay amplifier lens 8; the optical surface of the plane reflector 601 is coplanar or approximately coplanar with the fiber end face of the inclined end-face optical fiber 6, and the plane reflector 601 has a central hole, and the inclined end-face optical fiber 6 is located in its central hole and the gap between them is minimized as much as possible. In terms of its process implementation, after punching a hole in the plane reflector 601, the inclined end-face optical fiber 6 can be inserted and fixed with glue, and then the two can be polished as a whole to their working surfaces (the reflection surface of the plane reflector 601 and the fiber end face of the inclined end-face optical fiber 6).

[0056] Embodiment 6:

[0057] like Figure 1 As shown, this embodiment provides a post-receiving optical path system 5 , which includes the above-mentioned precise tracking detection optical path system and a photoelectric detection module 7 , and the photoelectric detection module 7 is connected to the inclined end-face optical fiber 6 .

[0058] Embodiment 7:

[0059] like Figure 1 As shown, the present invention also provides a laser communication terminal, which includes the above-mentioned post-receiving optical path system 5, post-transmitting optical path 4, beam splitter 3, precision aiming mirror 2 and optical antenna 1; the post-transmitting optical path 4 includes a transmitting light source 401, a transmitting collimating lens 402, and an advance aiming mirror 403. The laser beam emitted by the transmitting light source 401 passes through the transmitting collimating lens 402 to form a collimated laser beam, and the beam passes through the advance aiming mirror 403 for angle correction; when the laser communication terminal receives a laser beam from another laser communication terminal, it is sequentially beam-contracted through the optical antenna 1, and the precision aiming mirror 2 performs precise angle correction. After being split by the beam splitter 3, it is focused on the core of the tilted end-face optical fiber 6 by the receiving light path convergence lens 10, coupled into the optical fiber, and then enters the photoelectric detection module 7; the reflected laser of the tilted end-face optical fiber 6 is converged on the precision tracking detector 9 through the relay amplifier lens 8 to form a laser spot; the precision control module of the laser communication terminal controls the deflection angle of the precision aiming mirror 2 based on the position of the laser spot on the precision tracking detector 9, so as to realize the precise correction of the angle of the incident laser by the laser communication terminal, so that it can always be coupled to the tilted end-face optical fiber 6 with high efficiency, so as to realize the precise tracking of the laser communication terminal to another laser communication terminal.

[0060] like Figure 5 As shown, in this embodiment, a corner cone 12 can also be provided on the reflective surface of the beam splitter 3 to calibrate the angle of the emission optical axis of the post-emission optical path 4: the emission optical axis of the post-emission optical path 4 is reflected by the beam splitter 3, then reflected along the original path by the corner cone 12, and then transmitted through the beam splitter 3, enters the post-receiving optical path system 5, and is received by the precision tracking detector 9; by deviating the sighting mirror 403 from a known angle in advance, the two light spots of the emission laser beam of the post-emission optical path 4 and the normal receiving laser beam on the precision tracking detector 9 are separated by a specified distance, and based on the deviation between the actual relative position and the theoretical relative position of the two light spots, the angular error between the receiving optical axis and the transmitting optical axis of the laser communication terminal is calibrated.

[0061] The method for calibrating the angle error between the receiving optical axis and the transmitting optical axis of the laser communication terminal comprises the following steps:

[0062] (1) Continuously calculate the center positions of the two light spots. The light spot positions at a certain moment are recorded as (m(t), n(t)), where m(t) and n(t) are the light spot positions in the x and y directions, respectively, in pixels.

[0063] (2) Continuously calculate the sum of the grayscale values ​​of each pixel of the light spot, recorded as S(t), or measure the optical power P(t) coupled into the optical fiber;

[0064] (3) Based on the statistical results of S(t), (or P(t)) and (m(t), n(t)) within a period of time (e.g., 10s), find the spot position (m) corresponding to the minimum value of S(t) (or the maximum value of P(t)). 0 ,n 0 ), as the reference position, which corresponds to the optical fiber core diameter position, and the value can be continuously updated during the tracking process;

[0065] (4) Beam angle deviation vector (γ x (t),γ y (t)) is:

[0066]

[0067] Where, Γ is the angular magnification of the optical antenna 1, f Rx M is the focal length of the focusing lens 10 for the receiving light path, relay is the magnification of the relay magnifying lens 8.

[0068] Embodiment 8:

[0069] like Figure 4 As shown, the difference between this embodiment and embodiment 7 is that a nutating mirror 11 is provided between the beam splitter 3 and the receiving light path converging lens 10 to realize circular motion of the light beam in the angular space.

[0070] Embodiment 9:

[0071] The precise tracking detection optical path system of the present invention is used in a Φ80mm aperture satellite laser communication terminal of a certain project. It has been verified by ground experiments and obtained good results. While meeting the precise tracking accuracy requirements, the laser energy utilization rate of the received light beam is increased by 10%. Since an optical axis calibration module is not required and the mechanical and thermal stability of the optomechanical structure is reduced, a higher lightweight ratio can be used. After the optimized design, the total weight of the optomechanical part of the laser communication terminal is reduced by about 15%.

[0072] Its main technical details are as follows:

[0073] The optical antenna adopts an off-axis reflective type with a Mersenne structure, an aperture of 80mm, and an angular magnification of 8×; the precision aiming mirror adopts a 20mm-aperture commercial product of the piezoelectric ceramic type;

[0074] The beam splitter uses a flat dichroic mirror, with a reflectivity of better than 97% for the 1563nm receiving signal light and a transmittance of better than 95% for the 1541nm transmitting signal light;

[0075] The focal length of the receiving convergent lens is 47mm and it is composed of three optical lenses. A 1563nm narrowband filter is used between the beam splitter and the receiving convergent lens. The peak wavelength transmittance is about 93%, and the wavelength within the cutoff range meets OD>4.

[0076] The inclined end-face optical fiber uses Corning's SMF28+ and is used in combination with a 15mm-diameter plane reflector. The fiber end face inclination angle is 15 degrees, and the working surface of the optical fiber and the plane reflector except for the core diameter range of Φ12um is coated with a dielectric film, and the reflectivity is better than 98%; the photodetector uses a high-speed avalanche diode, and an EDFA is connected between the optical fiber and the photodetector to amplify the received weak laser signal.

[0077] The relay magnifying lens is composed of five lenses with a magnification of 3×. The precision tracking detector used is a CMOS array detector with a resolution of 2048×2048 and a center-to-center distance of 5.5um between adjacent pixels.

[0078] The receiving converging lens and the relay amplifying lens are located on both sides of the normal line of the optical fiber end face and its plane reflector, and the angle between their optical axes and the normal line is about 22 degrees.

Claims

1. A precision tracking detection optical path system for a free space laser communication terminal, comprising a receiving optical path converging lens (10), an inclined end-face optical fiber (6), a relay magnifying lens (8), and a precision tracking detector (9); characterized in that: The core of the tilted end-face optical fiber (6) is located at the focus of the receiving light path convergence lens (10), and the core of the tilted end-face optical fiber (6) and the center point of the precision tracking detector (9) satisfy the object-image conjugate relationship for the relay magnifying lens (8); the optical axis of the receiving light path convergence lens (10), the optical axis of the relay magnifying lens (8), the optical axis of the tilted end-face optical fiber (6) and the normal of the tilted end-face optical fiber (6) are in the same plane; The end face inclination angle of the inclined end face optical fiber (6) is θ, and the included angle between the optical axis of the inclined end face optical fiber (6) and the optical axis of the receiving light path converging lens (10) is ψ, and the two satisfy the following relationship: ψ=(n-1)θ Wherein n is the core refractive index of the inclined end-face optical fiber (6); The NA of the receiving light path converging lens (10) is α1, the angle between the optical axis of the receiving light path converging lens (10) and the normal line of the optical fiber end face of the inclined end face optical fiber (6) is φ1, the NA of the relay magnifying lens (8) is α2, the angle between the optical axis of the relay magnifying lens (8) and the normal line of the optical fiber end face of the inclined end face optical fiber (6) is φ2, and the above parameters satisfy the following relationship: Among them, NA fb is the optical fiber NA value of the inclined end face optical fiber (6); The inclined end face optical fiber (6) is realized by polishing the end face of a common optical fiber by optical cold processing so that the end face has an inclination angle of θ; The end face of the inclined end face optical fiber (6) is coated outside the range of 1.2 to 1.8 times the core diameter, and the coating film system adopts a metal reflection film or a dielectric reflection film; The end face of the inclined end face optical fiber (6) is plated with a dielectric beam splitting film, and the dielectric beam splitting film can reflect the precise tracking detection wavelength in the precise tracking detection optical path system, and transmit the signal light wavelength in the precise tracking detection optical path system; The precise tracking detection optical path system also includes a plane reflector (601), and a module composed of the plane reflector (601) and the inclined end-face optical fiber (6) realizes the functions of coupling signal light into the optical fiber and reflecting the laser beam to the relay amplifier lens (8); the optical surface of the plane reflector (601) is coplanar with the optical fiber end face of the inclined end-face optical fiber (6); the plane reflector (601) has a central hole, and the inclined end-face optical fiber (6) is located in the central hole.

2. According to claim 1, a precise tracking detection optical path system for a free space laser communication terminal is characterized in that: An optical filter is added between the inclined end-face optical fiber (6) and the relay magnifying lens (8) or between the relay magnifying lens (8) and the precise tracking detector (9).

3. A post-receiving optical path system, characterized in that: The post-receiving optical path system comprises the precise tracking detection optical path system according to any one of claims 1 to 2 and a photoelectric detection module (7), wherein the photoelectric detection module (7) is connected to the inclined end-face optical fiber (6).

4. A laser communication terminal, characterized in that: The laser communication terminal comprises the post-receiving optical path system (5) as claimed in claim 3, the post-transmitting optical path (4), a beam splitter (3), a precision aiming mirror (2) and an optical antenna (1); the post-transmitting optical path (4) comprises a transmitting light source (401), a transmitting collimating lens (402) and an advance aiming mirror (403); the laser beam emitted by the transmitting light source (401) passes through the transmitting collimating lens (402) to form a collimated laser beam, and the beam passes through the advance aiming mirror (403) for angle correction; when the laser communication terminal receives a laser beam from another laser communication terminal, it is sequentially beam-contracted through the optical antenna (1) and accurately angle-corrected by the precision aiming mirror (2). After being split by the beam splitter (3), the beam is focused by the receiving optical path convergence lens (10) on the core of the tilted end-face optical fiber (6), coupled into the optical fiber, and then enters the photoelectric detection module (7); the reflected laser of the tilted end-face optical fiber (6) is converged on the precision tracking detector (9) through the relay amplifier lens (8) to form a laser spot; the precision control module of the laser communication terminal controls the deflection angle of the precision aiming mirror (2) based on the position of the laser spot on the precision tracking detector (9), so as to achieve the precise correction of the angle of the incident laser by the laser communication terminal, so that it can always be coupled to the tilted end-face optical fiber (6) with high efficiency, so as to achieve the precise tracking of another laser communication terminal by the laser communication terminal.

5. The laser communication terminal according to claim 4, characterized in that: A nutating mirror (11) is arranged between the beam splitter (3) and the receiving light path converging lens (10) for realizing circular motion of the light beam in an angular space.

6. The laser communication terminal according to claim 4, characterized in that: A cone (12) is provided on the reflective surface of the beam splitter (3) for calibrating the angle of the emission optical axis of the post-emission optical path (4): the emission optical axis of the post-emission optical path (4) is reflected by the beam splitter (3), then reflected along the original path by the cone (12), then transmitted by the beam splitter (3), enters the post-receiving optical path system (5), and is received by the precision tracking detector (9); by deviating the sighting mirror (403) from a known angle in advance, the two light spots of the emission laser beam of the post-emission optical path (4) and the normal receiving laser beam on the precision tracking detector (9) are separated by a specified distance, and based on the deviation between the actual relative position and the theoretical relative position of the two light spots, the angle error between the receiving optical axis and the emission optical axis of the laser communication terminal is calibrated.

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