Automatic optical fiber coupling device and method based on double galvanometers

Abstract

The invention discloses an automatic optical fiber coupling device and method based on double galvanometers. The automatic optical fiber coupling device is characterized by comprising a laser light source, a first galvanometer, a second galvanometer, an optical fiber coupling head, a single-mode optical fiber, a photoelectric detector and a control panel which are sequentially mounted on an optical base. The first galvanometer and the second galvanometer are two-dimensional galvanometers, and four adjusting freedom degrees are provided in total. And the photoelectric detector feeds back coupling power to the control panel, and the control panel controls the double galvanometers to perform swing search, so that light emitted by the second galvanometer can be incident to the optical fiber coupling head at different angles and different positions, thereby realizing automatic recovery of coupling efficiency. The device is low in complexity, high in stability, easy and convenient to adjust, capable of achieving automatic recovery of coupling efficiency, capable of saving manual adjustment time, compact in structure and capable of achieving modularization while sufficient adjustment of the freedom degree is guaranteed.

Description

Technical Field

[0001] The present invention relates to automatic fiber optic coupling, and is a device mainly used to automatically restore the coupling efficiency after the optical path is misaligned, and can be used in integrated optical systems or optical path systems without human intervention. Background Art

[0002] The automatic fiber optic coupling system has important application value in laser communication without human intervention or other coupling optical path systems in unmanned environments (such as the optical path system of the space station atomic clock, the automatic alignment and compensation system for free space optical communication). It can solve the problem of the decrease in coupling efficiency caused by mechanical vibration and other reasons in the unmanned intervention environment. The coupling and tracking technology of spatial light to single-mode fiber is a key technology in multiple fields such as lidar, astronomical optical interferometers, and space optical communication.

[0003] The mode field of a single-mode fiber can be approximated as a Gaussian mode field. The spatial Gaussian beam is coupled through a coupling lens. The coupling efficiency is related to the matching of the mode field radius, the lateral offset of the beam, the longitudinal offset of the coupling lens from the fiber end face, and the alignment of the beam direction. When a Gaussian beam is coupled into a single-mode fiber, the theoretical coupling efficiency under the condition of parameter matching is close to 1. In the field of astronomy or space-ground communication, the incident light is a plane wave, which is focused on the fiber end face to form an Airy disk. Since the Airy disk and the single-mode fiber mode field cannot be completely matched, the theoretical maximum coupling efficiency is lower than the case of Gaussian beam incidence. Usually, the fiber optic coupling device mainly includes four parts: a single-mode fiber, a fiber optic interface, a coupling (lens) assembly, and an adjustment mechanism. In terms of the adjustment mechanism, the commonly used manual adjustment is two two-dimensional mirrors, which is time-consuming and laborious. Regarding automatic fiber optic coupling, classified according to the actuator, the following schemes have been reported. (1) The nutation scanning scheme based on a fast steering mirror. For example, in 2019, Zhao Baiqiu et al. proposed a nutation coupling algorithm by combining a fast steering mirror and a fiber optic photodetector, and conducted a coupling experiment on a laser nutation system, achieving a coupling efficiency of 59.63% under static conditions; (2) The automatic coupling system based on a rotating double optical wedge, such as the 2019 scheme of the Shanghai Institute of Optics and Fine Mechanics and the 2020 scheme of Cao et al. In the scheme based on the rotating double optical wedge, the authors realized the automatic coupling of single-mode fiber by combining coarse and fine scanning and a greedy algorithm; (3) The system based on a laser scanning galvanometer, such as the automatic coupling scheme proposed by Zhang Haitao et al. of Tsinghua University in 2022, which combines a galvanometer and a five-dimensional fiber adjustment frame; however, the system complexity is too high and the stability is unknown. (4) The automatic coupling scheme based on an electric high-precision displacement platform. Commercial electric adjustment mechanisms are usually large in size, difficult to integrate into the optical path, expensive, and manual adjustment requires the assistance of special software and the adjustment algorithm is complex.

[0004] There are still many problems with existing automatic optical fiber coupling devices: (1) Insufficient degrees of freedom, such as the fast steering mirror nutation scheme and the rotating double optical wedge scheme. (2) Emphasizing generality, the system has many degrees of freedom and a complex system composition, and the stability in specific scenarios is unknown, such as the scheme of galvanometer combined with five-dimensional optical fiber adjustment frame by Zhang Haitao et al. from Tsinghua University in 2022. (3) The commercial electric adjustment platform is expensive, the adjustment requires the manufacturer's dedicated software, the secondary development of the automatic coupling algorithm is difficult, and the flexibility is low, and it cannot be integrated and miniaturized.

[0005] At present, the domestic laser galvanometer technology is mature and widely used in the fields of laser processing, etc. The single-mirror swing resolution in a two-dimensional galvanometer can reach the micro-radian level. Without the encapsulated shell, the length, width, and height of the galvanometer are all no more than 6 cm, which is easy to integrate, and there are rich supporting software and hardware resources, the control method is simple, and the stability is high. Using two two-dimensional galvanometers for automatic coupling, the degrees of freedom and adjustment principle are completely equivalent to manual coupling of two mirrors, which can ensure the effectiveness of automatic recovery of coupling efficiency, and at the same time take into account the simplicity and stability of the system, and is easy to integrate. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the above-mentioned existing systems, solve the problems of low coupling efficiency, complex adjustment, repeated misalignment, etc. existing in the traditional optical fiber coupling process, and achieve high-precision and automated optical fiber coupling. The present invention provides an automatic optical fiber coupling device mainly for free space light - single-mode optical fiber. The system uses two two-dimensional galvanometers, which can provide 4 degrees of freedom, and is completely equivalent to the method of manually adjusting two mirrors for coupling. While ensuring sufficient degrees of freedom, the system structure complexity is minimized, and the coupling power is continuously monitored. After detecting that the coupling power drops to a certain threshold, automatic coupling is performed again. The device has a simple structure, convenient adjustment, strong stability, can realize automatic recovery of coupling efficiency, is easy to be integrated into the optical path, and can be used for the coupling adjustment of the daily optical path in the laboratory or an unattended experimental environment.

[0007] To achieve the above object, an automatic optical fiber coupling device and method based on a double galvanometer disclosed by the present invention is characterized in that: the automatic optical fiber coupling device mainly consists of a laser light source, a first galvanometer, a second galvanometer, an optical fiber coupling head, a single-mode optical fiber, a photodetector, and a control board, which are sequentially installed on an optical base. Among them, the laser light source emits the free space light beam to be coupled, and the light beam enters the optical fiber coupling head after being reflected by the first galvanometer and the second galvanometer in sequence.

[0008] Preferably, the output spot diameter of the laser light source to be coupled is in the range of 0.5 mm - 4 mm. To achieve a better coupling effect, a beam size transformation device can be installed behind the laser light source, so that after being focused by the coupling lens in the coupling head, it can be mode-matched with the single-mode optical fiber.

[0009] Preferably, both the first galvanometer and the second galvanometer are two-dimensional galvanometers, providing a total of 4 adjustment degrees of freedom, enabling the light emitted by the second galvanometer to enter the fiber optic coupler at different angles and positions.

[0010] Preferably, the surfaces of the two galvanometers are coated with high-reflection films according to the wavelength of the light source to be coupled, and the swing resolution of the galvanometer motors is better than 8urad, ensuring the accuracy and stability of beam reflection.

[0011] Preferably, the installation distance between the laser light source and the first galvanometer is not greater than 20 cm, the installation distance between the first galvanometer and the second galvanometer is not greater than 10 cm, and the installation distance between the second galvanometer and the fiber optic coupler is not greater than 5 cm. The reasonable spacing design helps to ensure the stability of the system and the compactness of the structure.

[0012] The fiber optic coupler has a diameter of 8 mm - 12 mm and is equipped with an aspherical coupling lens with an anti-reflection coating on its surface; the fiber optic coupler has a fiber optic interface for installing the fiber, and the coupling lens and the fiber optic interface have been adjusted and optimized to ensure that after installing the fiber, the fiber end face is located in the focal plane of the coupling lens, and the center line of the fiber end face is aligned with the optical axis of the coupling lens.

[0013] Preferably, the fiber optic coupler is installed on a bracket with manual pitch adjustment, up-down, left-right adjustment functions, and a locking function, facilitating manual adjustment in necessary situations.

[0014] The control board is connected to the first galvanometer and the second galvanometer for controlling the swing of the first galvanometer and the second galvanometer; the control board receives the coupled optical power value output by the photodetector to achieve feedback control of the galvanometer swing. At the same time, the control board has the function of manually adjusting the galvanometer swing and is provided with a locking function button.

[0015] Preferably, in practical applications, the single-mode fiber can adopt a one-to-two fiber, where a part of the light is used for the photodetector to monitor the coupling power, and the other part is used for the subsequent optical path. The splitting ratio can be customized according to the actual scenario.

[0016] The output gain of the photodetector is adjustable and is connected to the control board through a coaxial cable to transmit the output voltage signal. In fact, a converter with a fiber optic interface is also required in front of the photodetector to fix the fiber, ensuring that the laser energy coupled into the fiber is effectively received and detected by the photodetector.

[0017] The present invention also provides a method for automatic fiber coupling, including:

[0018] Step S1, installing each component of the system in place according to the requirements;

[0019] Step S2: Insert the optical fiber at the photodetector end into the fiber laser pen. Using the principle of reversible light path, observe the spot size of the laser to be coupled and the spot size of the beam output from the coupling head. If there is a large difference, a beam size transformation device should be installed behind the laser source to adjust the beam size of the laser to be coupled to be equivalent to the spot size of the beam of the fiber laser pen output in the reverse direction of the coupling head.

[0020] Step S3: According to the method of manual adjustment of the coupling principle, manually adjust the swing of the first galvanometer and the second galvanometer through the control board to make the laser to be coupled and the beam output from the coupling head basically coincide, and quickly achieve preliminary coupling. Adjust the coupling head bracket if necessary.

[0021] Step S4: Remove the fiber laser pen and place the fiber terminal at the input port of the photodetector.

[0022] Step S5: Press the button for automatic coupling on the control board, and the system enters the automatic coupling search program.

[0023] Step S6: The control program sequentially changes the four control signals of the first galvanometer and the second galvanometer, searches for the maximum power value, and sets the control signal to the value corresponding to the maximum power in this step. Repeat this process.

[0024] Step S7: Repeat the process of Step S6. When the change amount of the coupling output power value is less than the set threshold for 5 consecutive times, stop the automatic coupling program and continuously monitor the coupling power until the coupling power drops to a certain threshold and then repeat Step S6.

[0025] Technical effects of the present invention:

[0026] (1) The system uses two two-dimensional galvanometers, providing 4 degrees of freedom, which is completely equivalent to the method of manually adjusting two mirrors for coupling. While ensuring sufficient degrees of freedom and the effectiveness of automatic recovery of coupling, the structural complexity of the system is minimized.

[0027] (2) The control board has the function of manually adjusting the swing of the galvanometer, is compatible with manual methods, and can achieve preliminary and rapid alignment of coupling.

[0028] (3) The control board controls the galvanometer to automatically search for and quickly achieve high-efficiency coupling. At the same time, it continuously monitors the coupling power, can timely correct the optical path deviation and restore the coupling efficiency, greatly shortening the time for establishing and restoring coupling, and improving the system stability.

[0029] (4) The present invention makes the optical system simple in structure, high in stability, and can be integrated. Description of the Drawings

[0030] Figure 1 is a structural block diagram of the automatic fiber optic coupling device based on dual two-dimensional galvanometers of the present invention. In the figure, 1 is the laser light source to be coupled, 2 and 3 are galvanometers, 4 is the fiber optic coupling head, 5 is the single-mode fiber, 6 is the photodetector, and 7 is the control board.

[0031] Figure 2 is a schematic diagram of the main structure of the two-dimensional galvanometer applied in the automatic fiber optic coupling device based on dual two-dimensional galvanometers of the present invention. Detailed implementation mode

[0032] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0033] Figure 1 is a structural block diagram of an embodiment of the automatic fiber optic coupling device based on dual two-dimensional galvanometers of the present invention. As can be seen from the figure, an automatic fiber optic coupling device based on dual two-dimensional galvanometers includes a laser light source 1, a first galvanometer 2, a second galvanometer 3, a fiber optic coupling head 4, a single-mode fiber 5, a photodetector 6, and a control board 7 that are sequentially installed on an optical base. Figure 2 is a schematic diagram of the main structure of the two-dimensional galvanometer applied in the automatic fiber optic coupling device based on dual two-dimensional galvanometers of the present invention, indicating that the two-dimensional galvanometer actually integrates two one-dimensional galvanometers.

[0034] The laser light source 1 to be coupled has a model of HNL050, an output spot diameter of 0.81 mm, a wavelength of 632.8 nm, and a power of 5 mW.

[0035] The surfaces of the first galvanometer 2 and the second galvanometer 3 are both coated with a 632.8 nm high-reflection film, and the single-axis swing accuracy is 4 urad.

[0036] The first galvanometer 2 and the second galvanometer 3 are both two-dimensional galvanometers, and the two together provide 4 adjustment degrees of freedom, enabling the light emitted from the second galvanometer 3 to be incident on the fiber optic coupling head 4 at different angles and positions.

[0037] The first galvanometer 2 and the second galvanometer 3 are connected to the control board 7, and their swings are controlled by the control board 7.

[0038] The focal length of the coupling lens in the fiber optic coupling head 4 is 6.2 mm, and the coupling lens is coated with a 632.8 nm high-transmission film. The fiber optic interface of the fiber optic coupling head 4 is FC / APC (round thread / bevel physical contact). The distance and coaxiality between the coupling lens and the fiber optic interface have been optimized at the factory and do not require adjustment.

[0039] The described optical fiber coupler 4 is installed on a bracket with manual pitch adjustment, up / down / left / right adjustment functions, and a locking function, facilitating manual adjustment.

[0040] The installation distance between the described laser light source 1 and the first galvanometer 2 is 20 cm, the installation distance between the first galvanometer 2 and the second galvanometer 3 is 10 cm, and the installation distance between the second galvanometer 3 and the optical fiber coupler 4 is 5 cm. The reasonable spacing design helps to ensure the stability of the system and the compactness of the structure.

[0041] The described single-mode optical fiber 5 is DH-FSM600-APC, with a mode field diameter of approximately 4 um @ 633 nm. To achieve a better coupling effect, a beam size transformation device can be installed after the laser light source. After calculation, the beam expansion ratio is 1.5 times, and the mode field matching is the best. The beam diameter after expansion is 1.2 mm.

[0042] The described photodetector 6 is PDA100A2, which outputs a voltage signal with adjustable gain and is connected to the control board 7 through a coaxial cable. In fact, a converter with an optical fiber interface is also required in front of the photodetector to fix the optical fiber, ensuring that the laser energy coupled into the optical fiber is effectively received and detected by the photodetector.

[0043] The described control board 7 is connected to the first galvanometer 2 and the second galvanometer 3, used to control the swing of the first galvanometer 2 and the second galvanometer 3, and receive the coupled optical power value output by the photodetector 6 to achieve feedback control of the galvanometer swing. At the same time, the control board 7 has the function of manually adjusting the swing of the galvanometer and is provided with a locking function button.

[0044] The specific steps of the embodiment of the automatic optical fiber coupling method based on a dual two-dimensional galvanometer disclosed in the present invention are as follows:

[0045] Step S1, install each component of the system in place according to requirements;

[0046] Step S2, insert the optical fiber at the photodetector 6 into an optical fiber laser pen. Using the principle of optical path reversibility, observe the spot size of the laser to be coupled and the spot size of the output beam of the coupler. If the difference is large, a beam size transformation device should be installed after the laser light source 1 to adjust the beam size of the laser to be coupled to be equivalent to the spot size of the beam of the optical fiber laser pen output in the reverse direction by the coupler 4;

[0047] Step S3, according to the method of manual adjustment of the coupling principle, manually adjust the swing of the first galvanometer 2 and the second galvanometer 3 through the control board 7 so that the laser to be coupled and the output beam of the coupler 4 are basically coincident, quickly achieving preliminary coupling; adjust the coupler bracket if necessary;

[0048] Step S4, remove the optical fiber laser pen and place the optical fiber terminal at the input port of the photodetector 6;

[0049] Step S5: Press the button on the control panel 7 to start automatic coupling, and the system enters the automatic coupling search program.

[0050] Step S6: The control program sequentially changes the four-way control signals of the first galvanometer 2 and the second galvanometer 3, searches for the maximum power value, and sets the control signals to the corresponding values of the maximum power at this step. This process is repeated in a loop.

[0051] Step S7: Repeat the process of Step S6. When the change in the coupled output power value is less than the set threshold for 5 consecutive times, stop the automatic coupling program and continuously monitor the coupling power until the coupling power drops to a certain threshold and then restart Step S6.

[0052] The embodiment of the present invention uses two high-precision two-dimensional galvanometers, which can provide 4 degrees of freedom, and is completely equivalent to the method of manually adjusting two mirrors for coupling. While ensuring sufficient degrees of freedom, the structural complexity of the system is minimized. The device has a simple structure, is easy to adjust, and has strong stability. It can automatically restore the coupling efficiency, continuously monitor the coupling power, and restart the automatic coupling after detecting that the coupling power drops to a certain threshold. The device has a compact structure and is easy to integrate into the optical path. It can be used for the coupling adjustment of the daily optical path in the laboratory or an unattended experimental environment.

[0053] The above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. An automatic optical fiber coupling device based on a double two-dimensional galvanometer, characterized in that: It includes a laser light source (1), a first galvanometer (2), a second galvanometer (3), an optical fiber coupler (4), a single-mode optical fiber (5), a photodetector (6), and a control board (7) that are sequentially installed on an optical base; the spatial light beam to be coupled emitted by the laser light source (1) reaches the first galvanometer (2) and is reflected, and the primary reflected light reaches the second galvanometer (3) and undergoes a second reflection, and the secondary reflected light enters the coupler (4).

2. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claim 1, characterized in that: Preferably, the spot diameter of the laser light source (1) is 0.5 mm to 4 mm, and a beam size conversion device can be installed behind it, so that after being focused by the coupling lens in the coupler, it can be mode-matched with the single-mode optical fiber mode field.

3. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claim 1, characterized in that: The first galvanometer (2) and the second galvanometer (3) are two-dimensional galvanometers, providing a total of 4 adjustment degrees of freedom, so that the light emitted from the second galvanometer (3) can enter the optical fiber coupler (4) at different angles and different positions.

4. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claims 1 and 3, characterized in that: The surfaces of the first galvanometer (2) and the second galvanometer (3) can be coated with high-reflection films according to the wavelength of the light source to be coupled. The swing resolution of the galvanometer motor is better than 8 urad.

5. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claims 1 and 3, characterized in that: The installation distance between the laser light source (1) and the first galvanometer (2) is not greater than 20 cm; the installation distance between the first galvanometer (2) and the second galvanometer (3) is not greater than 10 cm; the installation distance between the second galvanometer (3) and the optical fiber coupler (4) is not greater than 5 cm.

6. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claim 1, characterized in that: The control board (7) is connected to the first galvanometer (2) and is used to control the swing of the first galvanometer (2); the control board (7) is connected to the second galvanometer (3) and is used to control the swing of the second galvanometer (3); the control board (7) is used to receive the coupled optical power value output by the photodetector (6) and is used to feedback and control the swing of the first galvanometer (2) and the second galvanometer (3).

7. The automatic optical fiber coupling device based on double two-dimensional galvanometers according to claim 1 and claim 3, characterized in that: The diameter of the optical fiber coupler (4) is 8 mm to 12 mm, and it contains an aspherical coupling lens with an antireflection film on its surface; the coupler (4) has an optical fiber interface for installing the optical fiber; the coupling lens and the optical fiber interface have been adjusted and optimized. After installing the optical fiber, the optical fiber end face is located in the focal plane of the coupling lens, and the optical fiber center line is aligned with the optical axis of the coupling lens.

8. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claim 1, wherein: The coupler (4) is installed on a coupler bracket and is fixed to the optical base through the coupler bracket; the coupler bracket has manual pitch adjustment and up / down / left / right adjustment functions and is equipped with a locking function.

9. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claims 1 and 6, characterized in that: The control board (7) has the function of manually adjusting the swing of the first galvanometer (2) and the second galvanometer (3) and is equipped with a locking function button.

10. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claim 1 and claim 7, characterized in that: In actual application of the device, the single-mode optical fiber (5) can be a one-for-two optical fiber. Part of the light is used for the photodetector (6) to monitor the coupling power, and the other part is used for the subsequent optical path. The specific splitting ratio is customized according to the actual scenario.

11. The automatic optical fiber coupling device based on a double two-dimensional galvanometer according to claims 1 and 6, characterized in that: The output gain of the photodetector (6) is adjustable, and the output voltage signal is connected to the control board using a coaxial cable.

12. Automatic optical fiber coupling method based on double two-dimensional galvanometers, characterized in that: The method can be applied to the system described in any one of claims 1 to 11. The method includes: Step S1, install each component of the system in place according to the requirements; Step S2: Insert the optical fiber at the photodetector end into the fiber laser pen. Using the principle of reversible light path, observe the spot size of the laser to be coupled and the spot size of the beam output from the coupling head. If there is a large difference, according to claim 2, a beam size transformation device should be installed behind the laser source to adjust the beam size of the laser to be coupled to be equivalent to the spot size of the beam of the fiber laser pen output in the reverse direction of the coupling head. Step S3: According to the method of manual adjustment of the coupling principle, manually adjust the swing of the first galvanometer (2) and the second galvanometer (3) through the control board to make the laser to be coupled basically coincide with the beam output from the coupling head, and quickly achieve preliminary coupling; adjust the coupling head bracket if necessary. Step S4: Remove the fiber laser pen and place the fiber terminal at the input port of the photodetector. Step S5: Press the button for starting automatic coupling on the control board, and the system enters the automatic coupling search program. Step S6: The control program sequentially changes the four-way control signals of the first galvanometer (2) and the second galvanometer (3), searches for the maximum power value, and sets the control signal to the corresponding value of the maximum power at this step, and loop this process. Step S7: Loop and execute the process of step S6. When the change amount of the coupling output power value is less than the set threshold for 5 consecutive times, stop the automatic coupling program and continuously monitor the coupling power until the coupling rate drops to a certain threshold and then re-perform step S6.

Images