A two-way atmospheric transmission and adaptive correction device based on coherent aperture array transceiver

Abstract

The present invention relates to a bidirectional atmospheric transmission and adaptive correction device based on a coherent aperture array transceiver, comprising a fiber collimator array, a transmission fiber, an integrated optical module, and a phase control module. The present invention combines integrated optics with fiber optics, uses the principle of adaptive optics to correct wavefront distortion, and can reliably realize the array emission, transmission correction and array reception functions of array vortex light beams under real atmospheric turbulence conditions under the closed-loop control of an optimization algorithm. The device composition structure of the present invention is simple, and each unit is highly modularized. It is easy to increase the number of channel units to achieve equivalent large-aperture optical transceiver, which greatly reduces costs; the array multiplexing transceiver can reduce optical components, and based on the reversibility of the optical path, it has more advantages in double-end alignment, common optical path aberration correction, etc.; the system can still operate normally when a small number of apertures cannot work normally, which can enhance the reliability of actual communication.

Description

Technical Field

[0001] The present invention relates to a bidirectional atmospheric transmission and adaptive correction device based on a coherent aperture array transceiver, belongs to the field of optical engineering and instrument science, and has important application prospects in free space optical communication. Background Art

[0002] In recent years, theoretical and experimental research on OAM vortex multiplexing to improve communication capacity has made breakthrough progress, but there are still many technical difficulties to be overcome in order to realize long-distance communication applications in actual atmospheric turbulence environments. At present, the verification of optical communication systems based on OAM multiplexing is mainly carried out under short-distance transmission conditions. The current research status shows (YFZhao, et al. Experimental demonstration of 260-meter security free-space optical data transmission using 16-QAM carrying orbital angular momentum (OAM) beams multiplexing. Proceedings of 2016Optical Fiber Communications Conference and Exhibition, Anaheim, CA, USA, 2016, 1-3) that the unfavorable factors such as beam drift, phase distribution fluctuation, and light intensity flicker caused by atmospheric turbulence disturbances will destroy the OAM mode, causing crosstalk and degradation between different modes, resulting in a surge in bit error rate and even communication failure.

[0003] Traditional adaptive optics (AO) research is basically aimed at AO compensation of plane wave or spherical wave distorted beams, and there is little research on direct detection and correction of wavefronts of OAM multiplexed beams. The mainstream of current research is to detect wavefront distortion based on Gaussian beams as probes, and then use wavefront correction algorithms to obtain the required phase correction information based on this information to achieve optical compensation of distorted wavefronts. However, it has limited scope of application, complex algorithms, and cannot track the random characteristics of turbulent channels in real time, so its application prospects are limited. In addition, the high technical threshold and economic cost of traditional AO have limited the practical application of OAM mode multiplexing wireless optical communications to a certain extent. Fiber laser array technology can also alleviate the impact of atmospheric turbulence, so that the aberration on the overall aperture is distributed to the tilt aberration and piston aberration on the unit aperture. Among them, the fiber-integrated phase modulation technology makes the available bandwidth of piston phase actuators reach GHz. In 2016, the Vorontsov team at the University of Dayton in the United States conducted a 7 km target in the loop (TIL) experiment with 21 low-power fiber amplifiers (T. Weyrauch, et al. Deep turbulence effects mitigation with coherent combining of 21 laser beams over 7 km. Optics Letters, 2016, 41 (4), 840-843). For strong turbulence conditions with atmospheric turbulence Rytov variables between 1 and 6, TIL can still effectively control, while the control effect of the traditional single-aperture AO system can only work in the range of 0.3 to 0.5. This verifies that the coherent aperture array transmission system has better atmospheric turbulence correction capabilities than the single-aperture transmission system. Coherent multi-aperture arrays can not only be used to transmit laser beams, but also to receive beams. The multi-aperture receiving scheme uses an array of multiple smaller-aperture receiving telescopes to be equivalent to a large-aperture receiving telescope to achieve efficient beam reception (DJ Geisler, et al. Multi-aperture digital coherent combining for free-space optical communication receivers. Optics Express, 2016, 24 (12): 12661-12671). Compared with the traditional single-aperture transmitting AO system, it has the following advantages: 1) The damage of a few local receiving sub-apertures does not affect the operation of the entire communication system, which can effectively reduce the probability of communication interruption and reduce the risk of failure; 2) The system effectively increases the received signal light by increasing the number of sub-apertures, thereby improving communication performance; 3) The single sub-aperture has a small aperture, so the cost is low, which can effectively reduce costs.At the same time, the premise of traditional wavefront detection is that the optical wavefront is continuous, which is not applicable to the situation where there are discontinuous singular points in the wavefront under strong turbulence conditions. Therefore, AO technology represented by coherent aperture array and wavefront aberration correction is the most effective means to alleviate the impact of atmospheric turbulence on OAM mode multiplexing wireless optical communications. Summary of the invention

[0004] Based on the principle of coherent superposition of diffraction of multi-aperture array beams and combined with adaptive optical wavefront aberration correction technology, the present invention proposes a system consisting of a fiber collimator array, a Gaussian probe, a transmission fiber, an integrated waveguide module, and a phase control module. The system takes into account the array emission, array reception, and simultaneous transmission and reception functions of beams under real atmospheric conditions, and can specifically realize the correction and integrated transmission and reception functions of vortex beams of various mode numbers under atmospheric turbulence conditions.

[0005] The technical problems to be solved by the present invention are: solving the problem of the applicability of the existing atmospheric turbulence correction system to the transmission multiplexed OAM vortex beam; and realizing the bidirectional transmission of vortex beams of various mode numbers in the real atmosphere.

[0006] The technical solution adopted by the present invention to solve the technical problem is: a bidirectional atmospheric transmission and adaptive correction device based on a coherent aperture array transceiver, comprising a fiber collimator array, a Gaussian probe, a transmission fiber, an integrated waveguide module, and a phase control module, wherein:

[0007] The fiber collimator array is composed of a multi-aperture array, and the fiber end face is placed on the focal plane of the collimator lens to achieve the coupled reception or reverse collimated emission of the laser beam from free space to the single-mode fiber. The Gaussian probe is emitted from two terminals; the probe wavelengths of the same terminal are the same, and different terminals can use probes of different wavelengths λ1 / λ2; and an independent collimator is set at the center of the annular aperture array to introduce Gaussian probes to eliminate the influence of atmospheric turbulence. The transmission fiber used is a single-mode polarization-maintaining fiber. The integrated waveguide module of each terminal includes two sets of planar waveguide OAM mode classifiers, star couplers and waveguide phase shifters (thermal phase shifters) in a structure; one set is for a wavelength of λ1 light beam, including a preloaded phase controller, a turbulence compensation phase shifter and a planar waveguide OAM mode classifier; the other set is for a wavelength of λ2, including a turbulence compensation phase shifter and a planar waveguide OAM mode classifier; the planar waveguide OAM mode classifier couples multiple groups of linearly arranged light beams to the corresponding OAM mode number output port or reversely divides a single light beam into multiple groups of energy-equalized light beams. The uniform and phase-linearly arranged beams are output from the array end to achieve mode multiplexing of the transmitted OAM beam and mode demultiplexing of the received OAM beam; the waveguide phase shifter is used to compensate for the internal phase difference of the system and the piston phase difference caused by atmospheric turbulence under the closed-loop control of the optimization algorithm, ensuring that the phase distribution of each beam when reaching the planar waveguide OAM mode classifier is consistent with the average phase distribution of the OAM vortex beam on each sub-aperture at the array receiving plane; the phase control module is composed of a return light sampler, a fiber circulator, a probe laser, a photodetector and a controller. The transceiver terminal aperture array receives the opposite Gaussian probe beam (l = 0, λ1 / λ2), and the multi-channel receiving beam is transmitted to the integrated optical module through the optical fiber. After being split by the wavelength division multiplexer (WDM), it enters the star coupler module corresponding to the OAM mode signal transmission. The l = 0 port will output the Gaussian probe beam, and the closed-loop control is performed with the light intensity as the performance indicator, and the phase is modulated to maximize the light energy value. At this time, the full-path aberration from the opposite aperture array plane to the local OAM mode transmitting port is corrected, so the locally emitted signal light (λ1 / λ2) will be transmitted in a pre-compensated manner, and when it reaches the opposite array aperture plane, it is an OAM vortex beam that has eliminated the turbulence effect. In the same way, the OAM mode beam (λ2 / λ1) emitted by the other party will also pass through the atmospheric turbulence in a pre-compensated manner to reach the local array aperture receiving plane. In order to achieve the demultiplexing of the signal light, the return light sampler reflects part of the collimated beam back, and the sampling beam returning along the original path is closed-loop controlled by setting a weaker probe beam (λ2 / λ1) at the l=0 port of the local OAM receiving module to clean the optical path from the local aperture array plane to the OAM receiving module. Here, the return light sampling device adopts coating and other methods to only sample the beam with a wavelength of λ2 / λ1. In addition, since there is no OAM mode in the optical signal emitted from the opposite side, which is an l=0 beam, the closed-loop process will not be affected by the opposite signal light.

[0008] Furthermore, the device is not limited to the transmission and reception of vortex light beams, but is also applicable to general coherently synthesized light beams.

[0009] Furthermore, the aperture array is arranged in a two-dimensional plane, which can be a ring, regular hexagon, square, triangle or other irregular arrangement, and the arrangement of the transmission optical fiber connected to the aperture array needs to be reasonably configured.

[0010] Furthermore, the planar waveguide OAM mode classifier includes but is not limited to a star coupler, which outputs all the array receiving light beams arranged in parallel and with tilted phases from the output port of the OAM mode number corresponding to the tilted phase slope and couples them into their respective optical fibers or transmits them in reverse, and inputs them from the planar waveguide OAM mode classifier ports with different OAM mode numbers, and diffracts them to the array end of the planar waveguide OAM mode classifier to form array light beams arranged in parallel and with tilted phases. The parameters of the planar waveguide OAM mode classifier are reasonably designed according to actual usage requirements, so that the energy of the light beams at both ends of the planar waveguide OAM mode classifier is uniform, and the phase of the light beams at the array end is linearly arranged, to ensure that no crosstalk occurs between different OAM modes or only limited OAM mode crosstalk is generated.

[0011] Furthermore, the waveguide phase shifter includes but is not limited to a thermally adjustable phase shifter, an electrically adjustable phase shifter, etc., which is used to compensate and lock the optical path difference inside the system and correct the phase difference caused by atmospheric turbulence. When the system is working, the output beam will achieve phase locking at the array exit plane.

[0012] Furthermore, the optimization control algorithms used for controller phase locking in the phase control module include but are not limited to random parallel gradient descent algorithm, hill climbing method, multi-dither method, single dither method, genetic algorithm, simulated annealing algorithm, particle swarm algorithm, neural network algorithm, and evolutionary algorithm.

[0013] Furthermore, the return light sampler includes but is not limited to a corner cone, an off-axis parabolic mirror, a planar diffraction element, etc., and uses the reflected probe laser as a closed-loop control signal.

[0014] Furthermore, when transmitting and receiving vortex beams of various mode numbers and correcting atmospheric turbulence, a multi-layer aperture array control scheme should be used to increase the equivalent transmitting and receiving areas; through phase-locked control between different layer units, unified OAM vortex light generation and multiplexing / demultiplexing can be achieved on the coherent aperture array composed of multi-layer apertures.

[0015] Compared with the prior art, the present invention has the following characteristics and beneficial technical effects:

[0016] 1. The present invention can reduce the influence of atmospheric turbulence on the vortex beam and improve the stability of the vortex beam mode.

[0017] 2. The present invention can improve the quality and power of the transmitted or received vortex light beam and enhance the communication effect.

[0018] 3. The transmitter and receiver are integrated, which can transmit and multiplex vortex beams, and also receive and demultiplex vortex beams.

[0019] 4. The system of the present invention has strong scalability, can increase equivalent transmitting and receiving apertures, and has low cost.

[0020] 5. The present invention can continue to work even if part of the collimator is damaged and will not malfunction.

[0021] 6. The present invention is not limited to the transmission and reception of vortex light beams, but is also applicable to general coherently synthesized light beams. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a two-way atmospheric transmission and adaptive correction scheme based on a coherent aperture array transmitter and receiver. DETAILED DESCRIPTION

[0023] The present invention will be further described in detail below with reference to the accompanying drawings and through specific embodiments, but these embodiments are only used to illustrate some specific implementations of the present invention and should not be construed as any limitation on the protection scope of the present invention.

[0024] The fiber collimator array is composed of a multi-aperture array, and the fiber end face is placed on the focal plane of the collimator lens to achieve the coupled reception or reverse collimated emission of the laser beam from free space to the single-mode fiber. The Gaussian probe is emitted from two terminals; the probe wavelengths of the same terminal are the same, and different terminals can use probes of different wavelengths λ1 / λ2; and an independent collimator is set at the center of the annular aperture array to introduce Gaussian probes to eliminate the influence of atmospheric turbulence. The transmission fiber used is a single-mode polarization-maintaining fiber. The integrated waveguide module of each terminal includes two sets of planar waveguide OAM mode classifiers, star couplers and waveguide phase shifters (thermal phase shifters) in a structure; one set is for a wavelength of λ1 light beam, including a preloaded phase controller, a turbulence compensation phase shifter and a planar waveguide OAM mode classifier; the other set is for a wavelength of λ2, including a turbulence compensation phase shifter and a planar waveguide OAM mode classifier; the planar waveguide OAM mode classifier couples multiple groups of linearly arranged light beams to the corresponding OAM mode number output port or reversely divides a single light beam into multiple groups of energy-equalized light beams. The uniform and phase-linearly arranged beams are output from the array end to achieve mode multiplexing of the transmitted OAM beam and mode demultiplexing of the received OAM beam; the waveguide phase shifter is used to compensate for the internal phase difference of the system and the piston phase difference caused by atmospheric turbulence under the closed-loop control of the optimization algorithm, ensuring that the phase distribution of each beam when reaching the planar waveguide OAM mode classifier is consistent with the average phase distribution of the OAM vortex beam on each sub-aperture at the array receiving plane; the phase control module is composed of a return light sampler, a fiber circulator, a probe laser, a photodetector and a controller. The transceiver terminal aperture array receives the opposite Gaussian probe beam (l = 0, λ1 / λ2), and the multi-channel receiving beam is transmitted to the integrated optical module through the optical fiber. After being split by the wavelength division multiplexer (WDM), it enters the star coupler module corresponding to the OAM mode signal transmission. The l = 0 port will output the Gaussian probe beam, and the closed-loop control is performed with the light intensity as the performance indicator, and the phase is modulated to maximize the light energy value. At this time, the full-path aberration from the opposite aperture array plane to the local OAM mode transmitting port is corrected, so the locally emitted signal light (λ1 / λ2) will be transmitted in a pre-compensated manner, and when it reaches the opposite array aperture plane, it is an OAM vortex beam that has eliminated the turbulence effect. In the same way, the OAM mode beam (λ2 / λ1) emitted by the other party will also pass through the atmospheric turbulence in a pre-compensated manner to reach the local array aperture receiving plane. In order to achieve the demultiplexing of the signal light, the return light sampler reflects part of the collimated beam back, and the sampling beam returning along the original path is closed-loop controlled by setting a weaker probe beam (λ2 / λ1) at the l=0 port of the local OAM receiving module to clean the optical path from the local aperture array plane to the OAM receiving module. Here, the return light sampling device adopts coating and other methods to only sample the beam with a wavelength of λ2 / λ1. In addition, since there is no OAM mode in the optical signal emitted from the opposite side, which is an l=0 beam, the closed-loop process will not be affected by the opposite signal light.For how to multiplex and demultiplex OAM vortex beams, please refer to the patent that has been applied for (a device for generating, multiplexing and demultiplexing vortex beams based on coherent aperture arrays, patent application number: 202111322723.X).

[0025] So far, the present invention has completed a detailed description of a bidirectional atmospheric transmission and adaptive correction device based on a coherent aperture array vortex beam. The contents not described in detail in the present invention specification belong to the well-known technology of those skilled in the art.

Claims

1. A two-way atmospheric transmission and adaptive correction device based on a coherent aperture array transceiver, characterized in that: The device comprises two symmetrically arranged terminals, each terminal comprising a fiber collimator array, a Gaussian probe, a transmission fiber, an integrated optical module and a phase control module, wherein: The fiber collimator array is composed of a multi-aperture array, which is arranged in a ring shape. A collimator lens is set on each hole of the array, and the fiber end face is placed on the focal plane of the collimator lens to achieve the coupling reception or reverse collimation emission of the laser beam from free space to the single-mode fiber; The Gaussian probe emits laser light from two terminals respectively; the probes at the same terminal have the same wavelength, while probes at different terminals use probes with different wavelengths. The wavelengths of the two probes are λ 1, λ 2. An independent collimator is set at the center of the annular aperture array to introduce the laser emitted by the Gaussian probe to eliminate the influence of atmospheric turbulence. The transmission optical fiber used is a single-mode polarization-maintaining optical fiber. The integrated optical module of each terminal includes two sets of coupled structures; for one of the terminals, one set of coupled structures in the terminal is for a wavelength of λ 1 beam, including a preload phase controller, a turbulence phase compensation controller, two sets of waveguide phase shifters and planar waveguide OAM mode classifiers; the other set is for wavelength λ 2, comprising a turbulence phase compensation controller, a waveguide phase shifter and a planar waveguide OAM mode classifier; for another terminal, a set of coupled structures in the terminal for a wavelength of λ 2 beams, including a preload phase controller, a turbulence phase compensation controller, two sets of waveguide phase shifters and planar waveguide OAM mode classifiers; the other set is for wavelength λ 1, including a turbulence phase compensation controller, a waveguide phase shifter and a planar waveguide OAM mode classifier; the planar waveguide OAM mode classifier couples multiple groups of linearly arranged light beams to the output ports corresponding to the number of OAM modes or reversely divides a single light beam into multiple groups of light beams with uniform energy and linearly arranged phases and outputs them from the array end, thereby realizing mode multiplexing of the transmitted OAM light beam and mode demultiplexing of the received OAM light beam; the waveguide phase shifter is used to compensate for the internal phase difference of the system and the piston phase difference caused by atmospheric turbulence under the closed-loop control of the optimization algorithm, so as to ensure that the phase distribution of each light beam when reaching the planar waveguide OAM mode classifier is consistent with the average phase distribution of the OAM vortex light beam on each sub-aperture at the array receiving plane; the planar waveguide OAM mode classifier includes a star coupler; The phase control module is composed of a return light sampler, a fiber circulator, a probe laser, a photodetector and a controller; the fiber collimator array of the transceiver terminal receives the Gaussian probe beam from the opposite side, where the OAM mode number of the Gaussian probe beam is l =0, wavelength is λ 1 or λ 2. The multi-channel receiving light beams are transmitted to the integrated optical module through optical fibers, and then separated by the wavelength division multiplexer and enter the corresponding star coupler in the integrated optical module. l =0 port will output the Gaussian probe beam, and the intensity of the beam will be used as the performance index for closed-loop control, and the phase will be modulated to maximize the light energy value; The wavelength is launched from the opposite fiber collimator aperture array plane to the local integrated optical module λ 1 or λ The full-path aberration of the port of the star coupler 2 is corrected, so the wavelength emitted by the local integrated optical module is λ 1 or λ The signal light of 2 will be transmitted in a pre-compensated manner, and when it reaches the aperture plane of the fiber collimator array on the opposite side, it is an OAM vortex beam with turbulence effects eliminated. In the same way, the wavelength emitted by the other integrated optical module is λ 1 or λ The signal light of 2 will also pass through the atmospheric turbulence in a pre-compensated manner and reach the receiving plane of the local aperture array.

2. The device according to claim 1, characterized in that In order to realize the demultiplexing of the signal light, the return light sampler reflects part of the collimated light beam back, and the sampling light beam returning along the original path passes through the star coupler in the local integrated optical module. l =0 port sets a weaker probe beam with a wavelength of λ 2 or λ 1. Closed-loop control is performed to clean the optical path from the local aperture array plane to the star coupler in the integrated optical module. Here, the return light sampler adopts a coating method and only samples the wavelength λ 2 or λ 1; in addition, since the signal light emitted from the opposite side is a fundamental mode Gaussian beam with the same wavelength, the closed-loop process will not be affected by the signal light from the opposite side.

Images