A high-order mode heterodyne detection free-space laser communication system and communication method

Abstract

The present invention discloses a high-order mode heterodyne detection free-space laser communication system and a communication method, belonging to the field of communication technology. The system comprises a tunable laser module, a signal generating module, an electro-optical modulation module, a high-order mode signal light excitation module, an atmospheric turbulence channel module, a high-order mode local oscillator light excitation module, a photoelectric detection module and a digital signal processing module. The tunable laser module and the signal generating module are both connected to the electro-optical modulation module, which is sequentially connected to the high-order mode signal light excitation module, the atmospheric turbulence channel module and the photoelectric detection module. The high-order mode local oscillator light excitation module is sequentially connected to the photoelectric detection module and the digital signal processing module. The present invention uses high-order mode signal light for transmission at a transmitting end, adopts light superimposed by multiple high-order modes as local oscillator light for heterodyne detection at a receiving end, and uses a KK algorithm to restore phase in a digital domain. The system has the advantages of strong anti-atmospheric turbulence effect, simple structure and stable system.

Description

Technical Field

[0001] The present invention belongs to the field of communication technology, and in particular relates to a high-order mode heterodyne detection free-space laser communication system and a communication method. Background Art

[0002] Free-space laser communication is a communications technology that uses light as a carrier and free space as a transmission medium. Compared to microwave communications, it offers advantages such as high bandwidth, compact size, rich spectrum resources, and resistance to electromagnetic interference. It has broad application potential in areas such as intersatellite, satellite-to-ground communications, near-Earth communications, and disaster-affected emergency communications. It is also a key component in establishing last-mile all-optical communication links. In recent years, interest in this technology has continued to grow, with both domestic and international research on laser communication technology in atmospheric environments being conducted in depth, with the goal of promoting this technology towards practical engineering applications.

[0003] The transmission medium for free-space laser communication is the atmosphere. Atmospheric turbulence causes optical signals to experience fluctuations in intensity and phase, beam expansion, and beam drift during space transmission, leading to performance degradation in free-space laser communication systems. Therefore, compensating for atmospheric turbulence is a key technology in free-space laser communication.

[0004] In recent years, researchers have utilized the large mode area and multi-mode characteristics of few-mode optical fibers to propose various atmospheric turbulence compensation schemes based on mode diversity reception, which can effectively alleviate the negative effects of atmospheric turbulence. However, it is worth noting that the transmitters of these schemes all use fundamental mode lasers as signal light to transmit information into the atmospheric channel. However, under the influence of atmospheric turbulence, high-order modes have better anti-turbulence performance than low-order modes, which causes the fundamental mode signal light to be more affected by turbulence. Therefore, it is very meaningful to explore the transmission performance of high-order mode signal light in free-space laser communication systems. Summary of the Invention

[0005] In response to the problem that the fundamental mode signal light used in traditional heterodyne detection free-space laser communication systems is greatly affected by turbulence, the present invention proposes a free-space laser communication system and communication method with high-order mode heterodyne detection. At the transmitting end, high-order mode signal light is used instead of the fundamental mode signal light for transmission. At the receiving end, light superimposed by multiple high-order modes is used as the local oscillator light for heterodyne detection. The Kramers-Kronig (KK) algorithm is used in the digital domain to restore the phase and reconstruct the light field information of the signal. The system has the advantages of strong resistance to atmospheric turbulence, simple structure, and system stability.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A free-space laser communication system for high-order mode heterodyne detection includes a tunable laser module 1, a signal generating module 2, an electro-optical modulation module 3, a high-order mode signal light excitation module 4, an atmospheric turbulence channel module 5, a high-order mode local oscillator light excitation module 6, a photoelectric detection module 7, and a digital signal processing module 8; wherein, the output port of the tunable laser module 1 is connected to an input port of the electro-optical modulation module 3, the output port of the signal generating module 2 is connected to another input port of the electro-optical modulation module 3, the output port of the electro-optical modulation module 3 is connected to the input port of the high-order mode signal light excitation module 4, the output port of the high-order mode signal light excitation module 4 is connected to the input port of the atmospheric turbulence channel module 5, the output port of the atmospheric turbulence channel module 5 is connected to an input port of the photoelectric detection module 7, the output port of the high-order mode local oscillator light excitation module 6 is connected to another input port of the photoelectric detection module 7, and the output port of the photoelectric detection module 7 is connected to the input port of the digital signal processing module 8.

[0008] Furthermore, the tunable laser module 1 generates a continuous light wave with a wavelength of 1550 nm and an output power of 10 dBm;

[0009] The radio frequency signal generated by the signal generating module 2 is output to the electro-optical modulation module 3 via the radio frequency line and then modulated;

[0010] The electro-optical modulation module 3 uses an IQ modulator to modulate the radio frequency signal of the signal generation module 2 onto the optical carrier generated by the tunable laser module 1 in the form of amplitude and phase information.

[0011] Furthermore, the high-order mode signal light excitation module 4 adopts a mode-selective photon lantern to convert the signal light modulated by the electro-optical modulation module 3 into the required high-order mode signal light, with an average insertion loss of 2.5dB. The output port of the electro-optical modulation module 3 is connected to the few-mode input port 41 of the photon lantern, and the single-mode output port 42 of the photon lantern is connected to the input port of the atmospheric turbulence channel module 5.

[0012] Furthermore, the atmospheric turbulence channel module 5 includes a transmitting end fiber collimator 51, a spatial light modulator 52, and a receiving end fiber collimator 53; the output port of the high-order signal light excitation module 4 is connected to the input port of the transmitting end fiber collimator 51, and the output port of the transmitting end fiber collimator 51 transmits the modulated signal light into free space, and then the spatial light modulator 52 reflects and distorts the light beam, and the distorted light beam is collimated by the receiving end fiber collimator 53 and connected to the input port of the photoelectric detection module 7 through the few-mode fiber;

[0013] The spatial light modulator 52 is a reflective phase spatial light modulator with 1920×1080 pixels.

[0014] Furthermore, the high-order mode local oscillation light excitation module 6 includes a local oscillator laser 61 and a non-mode selective photon lantern 62. The output port of the local oscillator laser 61 is connected to the single-mode input port of the non-mode selective photon lantern 62. The non-mode selective photon lantern 62 converts the fundamental mode light generated by the local oscillator laser 61 into a local oscillation light superimposed by multiple high-order modes. The input port of the photoelectric detection module 7 is connected to the few-mode output port of the high-order mode local oscillation light excitation module 6.

[0015] The local oscillator laser 61 generates a continuous light wave with a wavelength of 1550 nm and an output power of 0 dBm.

[0016] Furthermore, the photoelectric detection module 7 includes a 3dB coupler 71, a photodetector 72 and a digital oscilloscope 73. The output port of the atmospheric turbulence channel module 5 is connected to the input port 711 of the 3dB coupler 71, the few-mode output port of the non-mode selective photon lantern 62 is connected to the input port 712 of the 3dB coupler 71, and then the output port 713 of the 3dB coupler 71 is connected to the input port of the photodetector 72, the output port of the photodetector 72 is connected to the input port of the digital oscilloscope 73, and the output port of the digital oscilloscope 73 is connected to the input port of the digital processing module 8.

[0017] The photodetector 72 is a multi-mode photodetector with a bandwidth of 20 GHz. It receives the signal light at the output of the 3 dB coupler 71 and converts the signal light into an electrical signal for output.

[0018] The digital oscilloscope 73 adopts a bandwidth of 20 GHz and a sampling frequency of 100 GS / s.

[0019] Furthermore, the digital signal processing module 8 is used to measure the electrical signal output by the photoelectric detection module 7, including resampling, KK algorithm, orthogonal imbalance compensation, clock synchronization, frequency offset compensation, carrier phase recovery and decision output, so as to improve signal quality and compensate for noise and interference.

[0020] On the other hand, the present invention also provides a communication method for a high-order mode heterodyne detection free-space laser communication system, which specifically includes the following steps:

[0021] A tunable laser module generates continuous light to provide an optical carrier. The radio frequency signal is modulated on the optical carrier in the form of amplitude and phase information by the electro-optical modulation module. The modulated signal light is excited into a high-order mode signal light by the high-order mode signal light excitation module and emitted into free space through the atmospheric turbulence channel module. In the atmospheric channel simulation part, the spatial light modulator of the atmospheric turbulence channel module simulates atmospheric turbulence, causing random fluctuations in the refractive index of the channel, reflecting and distorting the light beam. The distorted light beam is collimated and coupled into the few-mode fiber through the collimator at the receiving end and connected to the photoelectric detection module. It is mixed with the local oscillator light of the high-order mode generated by the local oscillator, and then the photoelectric conversion is completed by the photoelectric detection module. Finally, the light field reconstruction, signal compensation and recovery are completed in the digital signal processing module.

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] The present invention provides a high-order mode heterodyne detection free-space laser communication system and communication method. At the transmitting end, high-order mode signal light is used instead of fundamental mode signal light for transmission. At the receiving end, light superimposed by multiple high-order modes is used as local oscillator light for heterodyne detection. Compared with the traditional fundamental mode heterodyne detection free-space laser communication system, the transmitting end's transmission power budget can be improved, and the system has the advantages of strong resistance to atmospheric turbulence, simple structure, and system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly describes the drawings required for the specific embodiments or the description of the prior art. Similar elements or parts are generally identified by similar reference numerals throughout the drawings. Elements or parts in the drawings are not necessarily drawn to scale.

[0025] Figure 1 Schematic diagram of the structure of the high-order mode heterodyne detection free-space laser communication system of the present invention;

[0026] Figure 2 Schematic diagram of the structure of the high-order mode signal light excitation module of the present invention;

[0027] Figure 3 Schematic diagram of the structure of the high-order mode local oscillator light excitation module of the present invention;

[0028] Figure 4 Schematic diagram of the 3dB coupler structure of the present invention;

[0029] Figure 5 This is a structural diagram of a digital signal processing module of the present invention;

[0030] Figure 6The bit error rate curves of the system under three different turbulence conditions: strong, medium, and weak; when different modes are used as signal light;

[0031] (a) shows the bit error rate curve of the system under weak turbulence (D / r0 = 3.04) when different modes are used as signal light;

[0032] (b) Bit error rate curves of the system under moderate turbulence (D / r0 = 9.37) when different modes are used as signal light;

[0033] (c) Bit error rate curves of the system under strong turbulence (D / r0 = 16.6) when different modes are used as signal light;

[0034] Figure 7 When different modes are used as signal light, the system interruption probability curves under three different turbulence conditions: strong, medium, and weak;

[0035] Among them, (a) is the interruption probability curve of the system under weak turbulence (D / r0=3.04) when different modes are used as signal light;

[0036] (b) The interruption probability curve of the system under medium turbulence (D / r0=9.37) when different modes are used as signal light;

[0037] (c) The interruption probability curve of the system under strong turbulence (D / r0 = 16.6) when different modes are used as signal light;

[0038] In the figure: tunable laser module 1, signal generation module 2, electro-optical modulation module 3, high-order mode signal light excitation module 4, atmospheric turbulence channel module 5, high-order mode local oscillator light excitation module 6, photoelectric detection module 7 and digital signal processing module 8, mode-selective photon lantern few-mode input terminal 41, mode-selective photon lantern single-mode output terminal 42, transmitting end fiber collimator 51, spatial light modulator 52, receiving end fiber collimator 53, local oscillator laser 61, non-mode-selective photon lantern 62, non-mode-selective photon lantern single-mode input terminal 621, non-mode-selective photon lantern few-mode output terminal 622, 3dB coupler 71, 3dB coupler input port 711, 3dB coupler input port 712, 3dB coupler output port 713, photoelectric detector 72, digital oscilloscope 73, digital signal processing module 8. DETAILED DESCRIPTION

[0039] The embodiments of the technical solution of the present invention will be described in detail below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention and are therefore only examples and cannot be used to limit the scope of protection of the present invention.

[0040] It should be noted that, unless otherwise specified, the technical or scientific terms used in this application should have the common meanings understood by those skilled in the art to which the present invention belongs.

[0041] Example 1

[0042] This example establishes a high-order mode heterodyne detection free space laser communication system, the structural diagram of which is shown in the figure below. Figure 1 As shown, it consists of a tunable laser module 1, a signal generation module 2, an electro-optical modulation module 3, a high-order mode signal light excitation module 4, an atmospheric turbulence channel module 5, a high-order mode local oscillator light excitation module 6, a photoelectric detection module 7 and a digital signal processing module 8; the connection method is as follows:

[0043] The output port of the tunable laser module 1 is connected to an input port of the electro-optical modulation module 3, the output port of the signal generation module 2 is connected to another input port of the electro-optical modulation module 3, the output port of the electro-optical modulation module 3 is connected to the few-mode input port 41 of the high-order mode signal light excitation module 4, the single-mode output port 42 of the high-order mode signal light excitation module 4 is connected to the emission end fiber collimator 51 of the atmospheric turbulence channel module 5 and is emitted into the atmospheric channel, the spatial light modulator 52 reflects and distorts the light beam, resulting in light wavefront distortion, and the distorted light beam passes through The receiving end optical fiber collimator 53 is collimated and connected to the input port 711 of the 3dB coupler 71; the output port of the local oscillator laser 61 is connected to the single-mode input port 621 of the non-mode-selective photon lantern 62, the single-mode output port 622 of the non-mode-selective photon lantern 62 is connected to the input port 712 of the 3dB coupler 71, the output port 713 of the 3dB coupler 71 is connected to the photodetector 72, the output port of the photodetector 72 is connected to the input port of the digital oscilloscope 73, and the output port of the digital oscilloscope 73 is connected to the digital processing module 8.

[0044] In this example, the tunable laser module 1 uses a narrow-linewidth laser from NKT Photonics, which generates a continuous 1550nm wavelength light wave with an output power of 10dBm. The narrow-linewidth laser has low phase noise and has little impact on the performance of the system in this example.

[0045] The signal generating module 2 is a Fujitsu LEIA-DK DAC development board. In this example, the output frequency is set to a 4 GHz RF signal. The RF signal generated by the DAC development board is connected to the electrical signal input port of the electro-optical modulator module 3 via a cable and loaded onto the optical carrier to achieve conversion from electrical signal to optical signal, generating an 8 Gbps QPSK signal light.

[0046] The electro-optical modulation module 3 uses a lithium niobate modulated I / Q modulator to modulate the radio frequency signal of the signal generation module 2 onto the optical carrier generated by the tunable laser module 1 in the form of amplitude and phase information.

[0047] The principle diagram of the high-order mode signal light excitation module 4 in this example is as follows Figure 2 As shown, a mode-selective photon lantern is used with an average insertion loss of 2.5 dB, which converts the signal light modulated by the electro-optical modulation module 3 into LP 01 LP 11 LP 21 LP 02 Four modes of signal light, LP 11 LP 21 LP 02 The three modes of signal light are used as high-order mode signal light in this example, and are respectively connected to the input port of the atmospheric turbulence channel module 5 through the single-mode output port 42 of the photon lantern and emitted into the atmospheric channel.

[0048] The atmospheric turbulence channel module 5 in this example includes a transmitting end fiber collimator 51, a spatial light modulator 52, and a receiving end fiber collimator 53. The spatial light modulator 52 uses a reflective pure phase spatial light modulator with 1920×1080 pixels, and each pixel is 8um×8um; each pixel has 256 grayscales, and D / r0 is introduced to characterize the atmospheric turbulence intensity, where D is the diameter of the transmitting beam and r0 is the atmospheric coherence length parameter. The turbulence model adopts the modified Von Karman turbulence model, and the power spectrum formula under this model is:

[0049]

[0050] Where κ0=2π / L0 and κ m =5.92 / l0, f is the spatial frequency, f m is the frequency corresponding to the inner scale, f0 is the frequency corresponding to the outer scale, l0 is the inner scale of turbulence, L0 is the outer scale of turbulence, r0 is the atmospheric coherence length, and the atmospheric structure constant Inversely proportional.

[0051] The principle diagram of the high-order mode local oscillator light excitation module 6 in this example is as follows: Figure 3As shown, the system comprises a local oscillator laser 61 and a non-mode-selective photon lantern 62. The local oscillator laser 61 is a narrow-linewidth tunable laser from NKT Photonics with an output power of 0 dBm. To meet the minimum phase condition of the KK relationship, the center frequency f2 of the local oscillator light power is adjusted to 4 GHz, which is the center frequency f1 of the signal light. After mixing, the signal is converted into an electrical signal by a multi-mode photodetector. The non-mode-selective photon lantern 62 has an average insertion loss of 3.5 dB and converts the fundamental mode light generated by the local oscillator laser 61 into a superposition of multiple higher-order modes of local oscillator light for coherent detection.

[0052] The photoelectric detection module 7 in this example has a schematic diagram as shown below: Figure 4 As shown, it includes a 3dB coupler 71, a photodetector 72 and a digital oscilloscope 73. The 3dB coupler 71 is a 180° mixer, which is essentially a heterodyne detection. Traditional heterodyne detection demodulation methods include synchronous demodulation and asynchronous demodulation, but both methods require strict matching of the frequency of the local oscillator light and the signal light and phase locking. They are usually implemented in an optical phase-locked loop structure, which is expensive and relatively difficult to implement. This paper uses heterodyne detection based on the Kramers-Kronig (KK) relationship to reconstruct the light field in the digital domain through the signal amplitude information obtained by the optical front end. The signal light and the local oscillator light must meet the minimum phase condition of the KK relationship, namely: (1) the amplitude of the local oscillator light is at least greater than the amplitude of the signal light (2) the center frequency of the local oscillator light is outside the frequency band of the signal light. At this time, the phase information of the original signal can be restored by measuring the intermediate frequency signal.

[0053] The photoelectric detector 72 is a multi-mode photoelectric detector from Beijing Kangguan Company with a bandwidth of 20 GHz. It receives the signal light at the output end of the 3dB coupler 71 and converts the signal light into an electrical signal for output.

[0054] The digital oscilloscope 73 is a Tektronix DSA72004C with a bandwidth of 20 GHz and a sampling frequency of 100 GS / s.

[0055] The digital signal processing module 8 in this example is used to measure the electrical signal output by the photoelectric detection module 7. The principle diagram thereof is shown in FIG. Figure 5 As shown in the figure, offline processing is adopted, and the KK algorithm is used for light field reconstruction to restore the amplitude and phase information of the original signal. The Gram-Schmidt algorithm is used to compensate for IQ imbalance and orthogonalize it. The Gardner algorithm is used for clock synchronization. The Viterbi-Viterbi algorithm is used to compensate for frequency offset and phase noise. Finally, the signal demodulation and reception are completed through the decision output.

[0056] Example 2

[0057] This embodiment provides a communication method for a high-order mode heterodyne detection free-space laser communication system, specifically comprising the following steps:

[0058] First, the output power of the tunable laser is set to 10dBm and the wavelength is set to 1550.12nm to provide an optical carrier for the system. The DAC development board generates an RF drive signal, which is modulated onto the optical carrier via the IQ modulator to generate an 8Gbps QPSK signal light, thus realizing the conversion of electrical signals to optical signals. The modulated optical signal excites the LP through the mode-selected photon lantern. 01 LP 11 LP 21 LP 02 The four modes of signal light are emitted into free space through the transmitting fiber collimator and incident on the spatial light modulator. The spatial light modulator simulates atmospheric turbulence to reflect and distort the light beam. The distorted light beam is collimated by the receiving collimator and then coupled into the few-mode fiber. It is mixed with the local oscillator light generated by the high-order mode local oscillator light excitation module, which is a superposition of multiple high-order modes. The photoelectric conversion is then completed by the photodetector. Finally, the digital signal processing module first reconstructs the light field, followed by signal compensation and recovery. This includes resampling, the KK algorithm, orthogonal imbalance compensation, clock synchronization, frequency offset compensation, carrier phase recovery, and decision output, completing the system's offline digital signal processing operations.

[0059] Due to the orthogonal nature of the modes in a few-mode system, each mode can be considered an independent channel. For the multiple modes of signal light received by the FMF in an atmospheric laser communication system, if mixed with another local oscillator light source containing the corresponding mode, this can be considered as mixing each mode's optical signal with the corresponding local oscillator light source. Then, photoelectric conversion is performed, enabling coherent detection of each mode of the received optical signal. The detected photocurrent is the sum of the corresponding photocurrents for each mode, essentially a form of equal-gain combining after coherent detection of each mode.

[0060] Figure 6 Given LP 01 The traditional heterodyne detection system and LP 11 LP 21 LP 02 The bit error rate curves of the high-order mode heterodyne detection system with three modes as signal light under strong, medium and weak turbulence conditions. Under low turbulence intensity, BER = 3×10 -4 When the turbulence intensity is high, the transmission power budget of high-order mode signal light can be increased by 1-2dB compared with the fundamental mode signal light. Under medium and high turbulence intensity, the transmission power budget of high-order mode signal light can be increased by 2-4dB compared with the fundamental mode signal light.

[0061] Figure 7 Given LP01 The traditional heterodyne detection system and LP 11 LP 21 LP 02 The outage probability curves for a high-order mode heterodyne detection system using three different modes as signal light under strong, moderate, and weak turbulence conditions are shown. At low turbulence intensity, with an outage probability of 20%, the high-order mode signal light can increase the transmit power budget by 1-2 dB compared to the fundamental mode signal light. At medium-to-high turbulence intensity, the high-order mode signal light can increase the transmit power budget by 2-5 dB compared to the fundamental mode signal light. This demonstrates that a high-order mode heterodyne detection system using high-order mode signal light has better resistance to atmospheric turbulence.

[0062] The embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0063] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0064] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A free-space laser communication system with high-order mode heterodyne detection, characterized in that: The invention comprises a tunable laser module (1), a signal generating module (2), an electro-optical modulation module (3), a high-order mode signal light excitation module (4), an atmospheric turbulence channel module (5), a high-order mode local oscillator light excitation module (6), a photoelectric detection module (7) and a digital signal processing module (8); wherein the output port of the tunable laser module (1) is connected to an input port of the electro-optical modulation module (3), the output port of the signal generating module (2) is connected to another input port of the electro-optical modulation module (3), the output port of the electro-optical modulation module (3) is connected to the input port of the high-order mode signal light excitation module (4), the output port of the high-order mode signal light excitation module (4) is connected to the input port of the atmospheric turbulence channel module (5), the output port of the atmospheric turbulence channel module (5) is connected to an input port of the photoelectric detection module (7), the output port of the high-order mode local oscillator light excitation module (6) is connected to another input port of the photoelectric detection module (7), and the output port of the photoelectric detection module (7) is connected to the input port of the digital signal processing module (8); The high-order mode signal light excitation module (4) uses a mode-selective photon lantern to convert the signal light modulated by the electro-optical modulation module (3) into the required high-order mode signal light, with an average insertion loss of 2.5 dB. The output port of the electro-optical modulation module (3) is connected to the few-mode input port (41) of the photon lantern, and the single-mode output port (42) of the photon lantern is connected to the input port of the atmospheric turbulence channel module (5); The high-order mode local oscillation light excitation module (6) comprises a local oscillation laser (61) and a non-mode selective photon lantern (62), wherein the output port of the local oscillation laser (61) is connected to the single-mode input port of the non-mode selective photon lantern (62), and the non-mode selective photon lantern (62) converts the fundamental mode light generated by the local oscillation laser (61) into local oscillation light with a plurality of high-order modes superimposed. The input port of the photoelectric detection module (7) is connected to the few-mode output port of the high-order mode local oscillation light excitation module (6); The local oscillator laser (61) generates a continuous light wave with a wavelength of 1550 nm and an output power of 0 dBm; The communication method of the free-space laser communication system specifically comprises the following steps: A tunable laser module generates continuous light to provide an optical carrier. The radio frequency signal is modulated on the optical carrier in the form of amplitude and phase information by the electro-optical modulation module. The modulated signal light is excited into a high-order mode signal light by the high-order mode signal light excitation module and emitted into free space through the atmospheric turbulence channel module. In the atmospheric channel simulation part, the spatial light modulator of the atmospheric turbulence channel module simulates atmospheric turbulence, causing random fluctuations in the refractive index of the channel, reflecting and distorting the light beam. The distorted light beam is collimated and coupled into the few-mode fiber through the collimator at the receiving end and connected to the photoelectric detection module. It is mixed with the local oscillator light of the high-order mode generated by the local oscillator, and then the photoelectric conversion is completed by the photoelectric detection module. Finally, the KK algorithm is used to restore the phase in the digital signal processing module to complete the light field reconstruction, signal compensation and recovery.

2. A free-space laser communication system with high-order mode heterodyne detection according to claim 1, characterized in that: The tunable laser module 1 generates a continuous light wave with a wavelength of 1550nm and an output power of 10dBm; The radio frequency signal generated by the signal generating module (2) is output to the electro-optical modulation module 3 via the radio frequency line and then modulated; The electro-optical modulation module (3) uses an IQ modulator to modulate the radio frequency signal of the signal generation module (2) in the form of amplitude and phase information onto the optical carrier generated by the tunable laser module (1).

3. The free-space laser communication system with high-order mode heterodyne detection according to claim 1, wherein: The atmospheric turbulence channel module (5) comprises a transmitting end optical fiber collimator (51), a spatial light modulator (52), and a receiving end optical fiber collimator (53); the output port of the high-order mode signal light excitation module (4) is connected to the input port of the transmitting end optical fiber collimator (51); the output port of the transmitting end optical fiber collimator (51) emits the modulated signal light into free space, and then the spatial light modulator (52) reflects and distorts the light beam; the distorted light beam is collimated by the receiving end optical fiber collimator (53) and connected to the input port of the photoelectric detection module (7) through the few-mode optical fiber; The spatial light modulator (52) is a reflective phase spatial light modulator with 1920×1080 pixels.

4. The free-space laser communication system with high-order mode heterodyne detection according to claim 1, wherein: The photoelectric detection module (7) includes a 3dB coupler (71), a photoelectric detector (72) and a digital oscilloscope (73); the output port of the atmospheric turbulence channel module (5) is connected to the input port (711) of the 3dB coupler (71); the few-mode output port of the non-mode selective photon lantern (62) is connected to the input port (712) of the 3dB coupler (71); then the output port (713) of the 3dB coupler (71) is connected to the input port of the photoelectric detector (72); the output port of the photoelectric detector (72) is connected to the input port of the digital oscilloscope (73); and the output port of the digital oscilloscope (73) is connected to the input port of the digital signal processing module (8); The photoelectric detector (72) is a multi-mode photoelectric detector with a bandwidth of 20 GHz, which receives the signal light from the output end of the 3dB coupler (71) and converts the signal light into an electrical signal for output; The digital oscilloscope (73) adopts a bandwidth of 20 GHz and a sampling frequency of 100 GS / s.

5. The free-space laser communication system with high-order mode heterodyne detection according to claim 1, wherein: The digital signal processing module (8) is used to measure the electrical signal output by the photoelectric detection module (7), including resampling, KK algorithm, orthogonal imbalance compensation, clock synchronization, frequency offset compensation, carrier phase recovery and decision output, so as to achieve signal quality improvement and noise and interference compensation.

Images