Polarization maintaining method for a galvanometer mirror type coherent laser communication system

Abstract

The application discloses a polarization maintaining device and method for a swing mirror type coherent laser communication system, which is composed of a polarization maintaining fiber laser light source, a collimating mirror, a polarization beam splitter, a half-wave plate, a quarter-wave plate, a rear optical path and a telescope system, and a swing mirror. The light emitted by the polarization maintaining fiber laser light source is collimated by the collimating mirror, and is polarized into horizontal polarized light by the polarization beam splitter. After being modulated by the half-wave plate and the quarter-wave plate, the light is changed into circular polarized light after passing through the rear optical path, the telescope system and the swing mirror. The phase delay of the rear optical path, the telescope system and the swing mirror is obtained by measurement before installation. The azimuth and the pitch angle are obtained in real time by the attitude of the swing mirror. The angles of the half-wave plate and the quarter-wave plate are calculated based on the corresponding parameter matrix of each system and device by a Mueller-Stokes algorithm, so that the state of the system polarization state is finally maintained.

Description

Technical Field

[0001] This invention relates to a polarization maintaining device and method for a mirror-type coherent laser communication system, specifically to real-time compensation for the dynamic degradation of beam polarization state caused by dynamic optical systems in space laser communication. It can be applied to polarization-dependent optical design for space laser communication and optical design for space quantum communication. Background Technology

[0002] Laser communication is a method of transmitting information using laser light as the information carrier. Depending on the transmission medium, it can be divided into space laser communication and polarization-maintaining fiber communication. Space laser communication has advantages such as high bandwidth, high security, high transmission speed, and low cost, making it the main method for high-data-volume communication in future space applications. Polarization is a fundamental property of laser light, and coherent laser communication, which utilizes the polarization state of laser light to transmit information, is currently the main method of laser communication.

[0003] Coherent laser communication typically transmits right-handed circularly polarized light and receives left-handed circularly polarized light. Optical components, such as mirrors, which are part of the optical path, introduce additional phase delays to the arriving light, causing degradation of the circularly polarized light transmitted and received by the system. Mirror-type optical systems, due to their simple optical path and small moment of inertia, are a common design for steering mechanisms in space optical systems. However, during system operation, the mirrors are constantly adjusting their attitude. Different pitch angles of the mirrors cause varying phase delays, and different azimuth angles produce varying basis vector rotations. Therefore, the polarization state of the transmitted and received light dynamically degrades, requiring real-time compensation for this degradation.

[0004] Polarization compensation is typically achieved using methods such as extruded polarization-maintaining fibers and waveplates. Waveplates are the primary means of polarization modulation in space laser communication due to their high compensation accuracy and stability. Currently, in engineering applications, waveplates are usually made of materials with birefringence, such as quartz crystals, mica, or electro-optic crystals. This invention is based on the Mueller-Stokes algorithm in polarization optics. It utilizes measurements of various system parameters of the optical system before loading and the real-time attitude of the tilting mirror to achieve real-time compensation for polarization degradation by rotating a waveplate. This method has broad application prospects. Summary of the Invention

[0005] Specifically, it involves real-time compensation for the dynamic degradation of beam polarization caused by dynamic optical systems in space laser communication, and can be applied to the design of polarization-coded space laser communication optical systems and space quantum communication optical systems.

[0006] The purpose of this invention is to provide a polarization maintaining device and method for a mirror-type coherent laser communication system. Based on the measurement of the parameters of each device in the system before loading and the real-time attitude of the mirror, the polarization state is modulated and the dynamic polarization degradation is compensated in real time by rotating the angle of the waveplate.

[0007] The polarization compensation device of the present invention is as follows: Figure 1 As shown, the device consists of a polarization-maintaining fiber laser source 1, a collimating lens 2, a polarization beam splitter 3, a half-wave plate 4, a quarter-wave plate 5, a rear optical path and telescope system 6, and a tilting mirror 7. The horizontally polarized light emitted from the polarization-maintaining fiber laser source 1 is collimated by the collimating lens 2, and after transmission through the polarization beam splitter 3, it maintains a horizontal polarization state. After being modulated by the half-wave plate 4 and the quarter-wave plate 5, the polarized light is then converted into right-handed or left-handed circularly polarized light and emitted after passing through the rear optical path and telescope system 6 and the tilting mirror 7.

[0008] The collimating mirror 2 uses a wavelength compatible with the polarization-maintaining fiber laser source 1, and the size of the collimated beam spot matches the size of the polarization beam splitter 3. The polarization extinction ratio of the polarization beam splitter 3 is greater than 2000:1, the surface accuracy of the four light-transmitting surfaces is better than λ / 20@632.8nm, and it is coated with an anti-reflection film. Its working wavelength matches the wavelength of the polarization-maintaining fiber laser source 1. The parallelism requirements of the half-wave plate 4 and the quarter-wave plate 5 are better than 5”, and the phase delay error is less than λ / 250. Their working wavelength matches the wavelength of the polarization-maintaining fiber laser source 1. The tilting mirror (7) is used to adjust the pointing of the system and can realize pitch rotation ① and azimuth rotation ②.

[0009] The polarization preservation method of the present invention includes the following steps.

[0010] 1) Phase delay of the optical path and telescope system 6 before loading Then the optical path and telescope system 6 transmission matrix It can be represented as:

[0011]

[0012] 2) Before loading, measure the phase delay of the pendulum mirror 7 at different pitch angles, and obtain the pitch rotation angle ① of the pendulum mirror based on the attitude of the pendulum mirror 7. When, its phase angle is denoted as Then its transmission matrix It can be represented as:

[0013]

[0014] 3) When the mirror-type coherent laser communication system is working, it acquires the azimuth rotation angle ② of the mirror 7. The corresponding rotation matrix is ​​expressed as:

[0015]

[0016] 4) Rotate the half-wave plate at 4 angles and rotating a quarter-wave plate at 5 degrees Taking the outgoing light as right-handed polarized light as an example, the communication light becomes horizontally polarized light after passing through polarization beam splitter 3. After waveplate compensation, the optical path and the transmission system of telescope system 6 and oscillating mirror 7 ensure that the light ultimately emitted from oscillating mirror 7 is right-handed circularly polarized light. The specific calculation process is as follows:

[0017] Azimuth angle is The transmission matrix of half-wave plate 4 is:

[0018]

[0019] Azimuth angle is The transmission matrix of quarter-wave plate 5 is:

[0020]

[0021] , , represented by Stokes parameters, , Therefore, according to the Mueller-Stokes algorithm, the system polarization transfer equation can be expressed as:

[0022]

[0023] This equation can be transformed into:

[0024]

[0025] After expanding the transmission matrix, the waveplate rotation angle can be obtained according to the following formula:

[0026]

[0027] 5) Rotate the half-wave plate 4 and the quarter-wave plate 5 to the angle corresponding to the orientation of the tilting mirror 7, and adjust the angles of the half-wave plate 4 and the quarter-wave plate 5 according to the change in the orientation of the tilting mirror 7. This ensures that the polarization state of the emitted light from the coherent laser communication system remains right-handed circularly polarized.

[0028] This method provides a polarization-maintaining device and method for optical signals in a mirror-type coherent laser communication system. Its advantages are: 1) The compensation device of this invention has a simple structure; 2) The method of this invention can compensate for the dynamic degradation of polarization state caused by rotating devices such as mirrors in real time; 3) Compared with the compensation method of extruded polarization-maintaining fiber, the compensation accuracy of this invention is higher. Attached Figure Description

[0029] Figure 1 Polarization holding device for a mirror-type coherent laser communication system. Detailed Implementation

[0030] The following describes in detail, with reference to the accompanying drawings, examples of implementation of the method of the present invention.

[0031] The main components used in the embodiments of this invention are described as follows:

[0032] It mainly includes a polarization-maintaining fiber laser source 1, a collimating lens 2, a polarization beam splitter 3, a half-wave plate 4, a quarter-wave plate 5, a rear optical path and telescope system 6, and a tilting mirror 7.

[0033] 1) Polarization-maintaining fiber laser source 1: A polarization-maintaining fiber laser source used for specific tasks, such as an 850nm polarization-maintaining fiber-coupled laser, which outputs laser light in a horizontal polarization state after being fixedly installed.

[0034] 2) Collimator 2: The collimator is a Thorlabs product, model F810APC-850, with the following main performance parameters: calibration wavelength 850nm, focal length 36.2mm, numerical aperture 0.25, polarization-maintaining fiber-coupled input via FC / APC port, and exit pupil 24.0mm.

[0035] 3) Polarization beam splitter 3: The polarization beam splitter adopts Ibtek's product, model PBS24-850, with the following main performance parameters: working wavelength 850nm, polarization extinction ratio greater than 3000:1, transmission wavefront difference of λ / 4, and size of 25.4mm×25.4mm×25.4mm.

[0036] 4) Half-wave plate 4: The half-wave plate is an Ibtek product, model HWP10-850B, with a working wavelength of 850nm, a delay accuracy of λ / 300, a light-transmitting aperture of 25.4mm, a transmittance greater than 98%, and a parallelism better than 5”.

[0037] 5) Quarter-wave plate 5: The quarter-wave plate is an Ibtek product, model QWP10-850B, with a working wavelength of 850nm, a delay accuracy of λ / 300, a light-transmitting aperture of 25.4mm, a transmittance greater than 98%, and a parallelism better than 5”.

[0038] 6) Rear optical path and telescope system: The rear optical path and telescope system is the conventional optical path of the laser communication system;

[0039] 7) Swing Mirror 7: The swing mirror is a reflector used in laser communication systems to achieve pointing and tracking.

[0040] The schematic diagram of the main optical path of the method of the present invention is attached. Figure 1 As shown, the specific situation is described below:

[0041] 1) Measurement of the phase retardation of the optical path and telescope system 6 at 850nm before loading The measurement results are 12° then the optical path and telescope system 6 transmission matrix It can be represented as:

[0042]

[0043] 2) At a certain moment, the pendulum mirror 7 is at an azimuth angle of... pitch angle is °, when the pitch angle is measured to be 35° before loading, the phase delay corresponding to the pendulum mirror 7 is . At this time, the transmission matrix of the pendulum mirror 7 It can be represented as:

[0044]

[0045] 3) When the mirror-type coherent laser communication system is working, it acquires the azimuth rotation angle ② of the mirror 7. The corresponding rotation matrix is ​​expressed as:

[0046]

[0047] 4) Rotate the half-wave plate at 4 angles and rotating a quarter-wave plate at 5 degrees Taking the outgoing light as right-handed polarized light as an example, the communication light becomes horizontally polarized light after passing through polarization beam splitter 3. After waveplate compensation, the optical path and the transmission system of telescope system 6 and oscillating mirror 7 ensure that the light ultimately emitted from oscillating mirror 7 is right-handed circularly polarized light. The specific calculation process is as follows:

[0048] Azimuth angle is The transmission matrix of half-wave plate 4 is:

[0049]

[0050] Azimuth angle is The transmission matrix of quarter-wave plate 5 is:

[0051]

[0052] , , represented by Stokes parameters, , Therefore, according to the Mueller-Stokes algorithm, the system polarization transfer equation can be expressed as:

[0053]

[0054] This equation can be transformed into:

[0055]

[0056] According to the following formula:

[0057]

[0058] The rotation angle of the half-wave plate can be obtained. Or 51.33°, or rotation angle Or 38.67°.

[0059] The angles of the half-wave plate 4 and the quarter-wave plate 5 are rotated to the corresponding posture of the pendulum mirror 7, and the angles of the half-wave plate 4 and the quarter-wave plate 5 are adjusted according to the change in the posture of the pendulum mirror 7 to achieve dynamic compensation.

Claims

1. A polarization preserving method for a galvanometer mirror based coherent laser communication system, characterized in that: The swing mirror type coherent laser communication system comprises a polarization maintaining optical fiber laser light source (1), a collimating mirror (2), a polarization beam splitter (3), a half wave plate (4), a quarter wave plate (5), a rear optical path and telescope system (6) and a swing mirror (7). The horizontal polarization light emitted by the polarization maintaining optical fiber laser light source (1) is collimated by the collimating mirror (2), and the horizontal polarization state is maintained after being transmitted through the polarization beam splitter (3); the polarization light is modulated by the half wave plate (4) and the quarter wave plate (5), and then becomes right-handed or left-handed circularly polarized light after passing through the rear optical path and telescope system (6) and the swing mirror (7); the angles of the half wave plate (4) and the quarter wave plate (5) are determined by the phase delay of the rear optical path and telescope system (6) and the azimuth and elevation angles of the swing mirror (7), and the angle values are obtained by calculating the Mueller-Stokes algorithm; The method comprises the following steps: 1) Post-loading measurement of the phase delay of the post-optical path with respect to the telescope system (6) The post-optical path with respect to the telescope system (6) transmission matrix is then represented as: The post-optical path with respect to the telescope system (6) transmission matrix is then represented as: ； 2) Measure the phase delay of the mirror (7) at different pitch angles before loading, and obtain the pitch rotation angle of the mirror (7) according to the attitude of the mirror (7) The phase angle is denoted as The transmission matrix of the mirror is The transmission matrix of the mirror is ； 3) In the working process of the swing mirror type coherent laser communication system, the angle of the azimuth rotation ② of the swing mirror (7) is acquired The corresponding rotation matrix is represented as: ； 4) Rotating half-wave plate (4) angle and rotating quarter-wave plate (5) angle , the outgoing light is right-handed polarized light, and the communication light is horizontal polarized light after passing through the polarized beam splitter (3) , the wave plate group compensates the light path and the telescope system (6), the mirror (7) transmission system, so that the light finally emitted from the mirror (7) is right-handed circularly polarized light The specific calculation process is as follows: The azimuth angle is The transmission matrix of the half-wave plate with the azimuth angle of 0 is ； The azimuth angle is The transmission matrix of the quarter-wave plate with the azimuth angle of 0 is ； , , in terms of Stokes parameters, , , the system polarization transfer equation is expressed according to the Mueller-Stokes algorithm: ； The equation is transformed into: ； After the transmission matrix is expanded, the wave plate rotation angle satisfies the following formula: ； 5) The angles of the rotating half wave plate (4) and the rotating quarter wave plate (5) are adjusted according to the change of the attitude of the swing mirror (7), that is, the polarization state of the outgoing light of the coherent laser communication system is ensured to be right-handed circularly polarized light.

2. The polarization preserving method for a galvanometer mirror based coherent laser communication system according to claim 1, wherein: The collimating mirror (2) has a center wavelength that is adapted to the polarization maintaining optical fiber laser light source (1), and the size of the collimated light spot is matched with the size of the polarization beam splitter (3).

3. The polarization preserving method for a galvanometer mirror based coherent laser communication system according to claim 1, wherein: The polarization beam splitter (3) has a polarization extinction ratio greater than 2000:1, a surface type accuracy of four light transmission surfaces better than λ / 20, λ=632.8nm, and an antireflection film, and the working wavelength is matched with the wavelength of the polarization maintaining optical fiber laser light source (1).

4. The polarization preserving method for a galvanometer mirror based coherent laser communication system according to claim 1, wherein: The half wave plate (4) and the quarter wave plate (5) have a parallelism better than 5", a phase delay error less than λ / 250, and a working wavelength λ matched with the wavelength of the polarization maintaining optical fiber laser light source (1).

5. The polarization preserving method for a galvanometer mirror based coherent laser communication system according to claim 1, wherein: The swing mirror (7) can realize the elevation and azimuth rotation.

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