Free space optical communication link demodulation method and system based on helical fractional orbital angular momentum

Abstract

This invention provides a demodulation method and system for free-space optical communication links based on helical fractional orbital angular momentum. N data streams are loaded onto a Gaussian beam, and at least one modulated beam of helical fractional OAM (Optical Aspect-Oriented Motion) is generated through a spatial optical phase modulation module. After coaxial beam combining, the beam is emitted in free space and imaged onto the demodulation phase modulation module plane via an optical relay imaging system. A conjugate helical phase map is loaded to demodulate the target OAM, and the focusing optics module is adjusted to couple the peak of the target OAM beam into a single-mode fiber, while non-target OAM beams are isolated. After verifying the demodulation isolation, the demodulation phase map is switched to sequentially recover each data stream. This invention achieves hybrid multiplexing of helical fractional and integer OAM, and achieves a mode isolation of ≥5dB through a dual isolation mechanism of phase conjugate demodulation and spatial optimal focusing. This improves fiber coupling efficiency and communication link robustness, significantly expanding the channel capacity and spectral efficiency of free-space optical communication.

Description

Technical Field

[0001] This invention relates to the field of optical communication technology, and in particular to a free-space optical communication link demodulation method and system based on helical fractional orbital angular momentum. Background Technology

[0002] With the rapid development of information technologies centered on the Internet, including mobile Internet, Internet of Things, cloud computing, and big data, the amount of information is expanding rapidly, which puts forward higher requirements for the transmission capacity of optical communication systems.

[0003] Currently, traditional channel multiplexing techniques, such as wavelength division multiplexing (WDM), time division multiplexing (TDM), and polarization multiplexing (PDM), have reached their limits in terms of increasing communication capacity. To further significantly improve communication capacity, spatial division multiplexing (SDM) technology has become an important research direction in the field of communications. Due to the phase singularity and theoretically infinite orthogonal orbital angular momentum (OAM) mode space of vortex beams, it has shown broad application prospects in multiple fields such as optical communication, optical micromanipulation, optical imaging, and quantum information. In particular, orbital angular momentum (OAM) spatial division multiplexing technology has become a promising technique for addressing the ever-increasing communication capacity requirements.

[0004] Traditional OAM spatial division multiplexing techniques are mainly based on integer-order OAM modes. In recent years, fractional-order OAM has gradually attracted research interest due to its advantages such as continuous mode tunability. By applying helical coordinate transformation as an efficient optical field manipulation method to the precise operation of orbital angular momentum modes, this technique's potential in high-resolution OAM mode selection was demonstrated, and its high efficiency and flexibility in handling complex modes were verified, laying the foundation for subsequent research. Through this mechanism, when the generated OAM has a non-integer-order topological charge, a special fractional vortex beam with a helical intensity distribution cross-section and radial phase discontinuity can be generated. This contrasts sharply with traditional fractional OAM modes, which exhibit open-loop intensity and abrupt azimuth phase changes. Furthermore, by adjusting different parameters, the focusing position and depth of focus of the helical fractional-order OAM can be precisely controlled, allowing it to self-focus into a bright, sharp ring at a specific distance. In contrast, traditional fractional OAM can only maintain a radially open-loop distribution within a specific propagation distance, evolving into a multi-lobed petal-shaped spot, with the number of petals related to the fractional part of the topological charge. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a demodulation method and system for free-space optical communication links based on helical fractional orbital angular momentum. This invention can be extended to the generation, stable transmission, and efficient demodulation of multi-channel helical fractional OAM beams, so as to realize multi-channel multiplexing and demultiplexing communication between helical fractional OAMs, and multi-channel multiplexing and demultiplexing communication between helical fractional OAMs and integer-order OAMs.

[0006] In a first aspect, the present invention provides a demodulation method for a free-space optical communication link based on helical fractional orbital angular momentum, comprising the following steps:

[0007] S1) Load the N data signals to be transmitted into the N beams respectively, collimate them and output a Gaussian beam;

[0008] S2) By applying a spiral phase with a specific topological charge value to each Gaussian beam through the spatial light phase modulation module, N modulated beams carrying different orbital angular momentum modes are generated.

[0009] S3) Using a beam combiner module, N modulated beams are coaxially combined to generate a multiplexed orbital angular momentum beam and emitted into free space;

[0010] S4) Receive the multiplexed orbital angular momentum beam imaged to the plane of the demodulation spatial optical phase modulation module through the optical relay imaging system, and load a matching demodulation phase map on the demodulation spatial optical phase modulation module according to the current target topological charge value to be demodulated.

[0011] S5) The beam demodulated by the spatial light phase modulation module is incident on the single-mode fiber coupling module, and the beam incident state is adjusted by the focusing optics module so that the beam with the target topological charge value is coupled into the fiber, while other beams with non-target topological charge values ​​are isolated.

[0012] S6) Adjust the focusing optical module and the single-mode fiber coupling module to make the coupled optical power reach the peak value and restore the data signal of the current target channel;

[0013] S7) Measure the single-mode fiber coupling power between the target topological charge beam and the non-target topological charge beam. After confirming that the coupling power of the target channel is more than 5dB higher than that of the non-target channel, switch the matching demodulation phase diagram on the demodulation spatial optical phase modulation module and repeat steps S4) to S6) to demodulate the data signals carried by each modulation beam in sequence.

[0014] Preferably, in step S1), the Gaussian beam distribution... for:

[0015]

[0016] In the formula, Radial coordinates; These are the polar coordinates of the Gaussian beam; Let be the waist radius of the Gaussian beam.

[0017] Preferably, in step S2), the spatial light modulation element applies a spiral phase with a specific topological charge value to each Gaussian beam by loading different order spiral fractional OAM phase maps or different order integer OAM phase maps, thereby generating a spiral fractional OAM beam or an integer OAM modulated beam.

[0018] Preferably, in step S2), among the N modulated beams, except for at least one spiral fractional-order OAM beam, the remaining N-1 beams are either spiral fractional-order OAM beams or integer-order OAM modulated beams.

[0019] Preferably, in step S2), the topological charge spectrum of the spiral fractional-order OAM beam is composed of multiple integer orbital angular momentum eigenstates superimposed, and its angular momentum density is distributed in the outer ring region of the beam cross section.

[0020] Preferably, in step S2), the spiral fractional-order OAM beam and the open-ring fractional-order OAM beam with the same non-integer topological charge value are mutually converted through a spiral phase piecewise function; the spiral phase piecewise function for:

[0021]

[0022] In the formula, It is a scaling factor and is a fraction; Scaling factor The denominator; For topological load values;

[0023] When the helical phase piecewise function is superimposed on the helical coordinate transformation, its effect on the helical fractional-order vortex beam is as follows: the helical phase piecewise function reconstructs its radial phase jump into an azimuth phase jump, thereby converting it into a beam with the same non-integer topological charge value. The open-ended annular fractional-order vortex beam; the function of the open-ended annular fractional-order vortex beam is: through inverse transformation, the spiral phase piecewise function can restore its azimuth phase jump to radial phase jump, thereby converting it into a spiral fractional-order vortex beam.

[0024] Preferably, in step S2), the spiral fractional-order OAM beam is generated by performing a spiral coordinate transformation through a spatial optical phase modulation module. The spiral fractional-order OAM beam has a spiral intensity distribution and a phase jump along the radial direction, and its complex amplitude is expressed as:

[0025]

[0026]

[0027]

[0028] In the formula, For complex amplitude; Radial coordinates; The beam waist radius; For axial coordinates; Indicates the wavelength of light.

[0029] Preferably, in step S4), the matched demodulation phase diagram is a conjugate spiral phase diagram corresponding to the target topological charge value; the conjugate spiral phase diagram is used to convert the spiral wavefront of the orbital angular momentum beam carrying the target topological charge value into a planar or spherical wavefront with an approximately Gaussian distribution.

[0030] Preferably, in step S5), the other non-target topological charge beams, due to maintaining the spiral phase distribution characteristics, exhibit a central dark spot or ring intensity distribution at the fiber core end face of the single-mode fiber coupling module. The energy coupled into the single-mode fiber is intrinsically suppressed, and the coupling power difference with the target beam is greater than 5dB.

[0031] Preferably, in step S6), by fixing the spatial position of the single-mode fiber, a matching demodulation phase map corresponding to the current target topological charge value to be demodulated is loaded on the demodulation spatial light phase modulation module. By continuously adjusting the focusing optical module, the optical power coupled into the single-mode fiber reaches its peak value. At this time, the orbital angular momentum beam corresponding to the target topological charge value is demodulated and coupled into the fiber, while beams carrying other non-target topological charge values ​​are spatially isolated.

[0032] Preferably, in step S6), the matched demodulation phase map loaded on the demodulation spatial light phase modulation module and the focusing optical module are kept unchanged; the single-mode fiber coupling power when only the target channel beam is transmitted and the single-mode fiber coupling power when only the non-target channel beam is transmitted are measured respectively; if the coupling power of the target channel beam is more than 5dB higher than the coupling power of the non-target channel beam, the current demodulation state is confirmed to be valid, the target channel is successfully demodulated, and the non-target channel is effectively isolated.

[0033] Preferably, in step S6), the focusing optical module is a focusing phase distribution, and the focusing phase distribution is a secondary phase distribution.

[0034] Secondly, the present invention provides a free-space optical communication link demodulation system based on helical fractional orbital angular momentum, comprising:

[0035] The Gaussian beam generation module is used to load N data signals to be transmitted into N beams respectively, and output Gaussian beams after collimation;

[0036] The transmitter spatial light phase modulation module is used to apply a spiral phase with a specific topological charge value to each Gaussian beam to generate N modulated beams carrying different orbital angular momentum modes.

[0037] The beam combining module is used to coaxially combine N modulated beams to generate a multiplexed orbital angular momentum beam and emit it;

[0038] An optical relay imaging system is used to receive the multiplexed orbital angular momentum beam imaging onto the plane of the demodulated spatial light phase modulation module;

[0039] The demodulation module uses a spatial optical phase modulation module to load a matched demodulation phase map based on the current target topological charge value to be demodulated, and then demodulates the target topological charge value using the OAM beam.

[0040] A focusing optical module is used to adjust the incident state of the demodulated beam, so that the beam with the target topological charge value is coupled into the fiber, while isolating other beams with non-target topological charge values.

[0041] A single-mode fiber coupling module is used to receive the demodulated beam with the adjusted target topological charge value and couple it into a single-mode fiber.

[0042] The verification module is used to verify the single-mode fiber coupling power when only the target channel beam is transmitted, and the single-mode fiber coupling power when only the non-target channel beam is transmitted. If the coupling power of the target channel beam is more than 5dB higher than the coupling power of the non-target channel beam, the current demodulation state is confirmed to be valid, the target channel is successfully demodulated, and the non-target channel is effectively isolated.

[0043] The beneficial technical effects of this invention are as follows:

[0044] 1. This invention applies helical fractional-order OAM to spatial multiplexing in free-space optical communication. The helical fractional-order OAM beam generated by helical coordinate transformation has a helical intensity distribution and radial phase jump. The self-focusing position and depth of focus can be precisely controlled by parameters, which solves the problem of unstable optical field structure and evolution into petal-shaped light spots during propagation of traditional fractional-order OAM, laying the foundation for efficient fiber coupling.

[0045] 2. This invention converts the target OAM wavefront into a Gaussian distribution only by loading a conjugate spiral phase diagram, while the non-target OAM wavefront is disturbed; at the same time, by utilizing the self-focusing characteristics of spiral fractional-order OAM, the optimal focal point is used as the fiber coupling position, and the non-target OAM is in a defocused state. After energy diffusion, it is further suppressed by the spatial filtering effect of single-mode fiber, achieving a mode isolation of ≥5dB in hybrid multiplexing scenarios and significantly reducing channel crosstalk.

[0046] 3. This invention adjusts the focusing optical element to form the smallest and most concentrated spot of the target OAM beam on the fiber end face, achieving the best match with the fundamental mode of the single-mode fiber, improving coupling efficiency, enhancing the power budget of the communication link, reducing the bit error rate, and improving the system's robustness to mechanical vibration and environmental disturbances.

[0047] 4. This invention realizes the hybrid multiplexing of spiral fractional-order and integer-order OAM, expands the channel dimension of OAM spatial multiplexing, and significantly improves the spectral efficiency and channel capacity of free-space optical communication; by superimposing spiral phase piecewise functions, it realizes the bidirectional conversion between spiral fractional-order OAM and open-ring fractional-order OAM, expanding the flexibility of optical field control. Attached Figure Description

[0048] Figure 1 This is a flowchart illustrating Embodiment 1 of the present invention;

[0049] Figure 2 This is a flowchart of Embodiment 1 of the present invention;

[0050] Figure 3 This is a simulation diagram of the beam profile as the transmission distance increases after demodulation of a 1.5-order spiral fractional OAM beam using a 1.5-order VPP in Embodiment 1 of the present invention.

[0051] Figure 4 This is a beam profile diagram of Embodiment 1 of the present invention after demodulation of a 1.5-order spiral fractional OAM beam at a certain transmission distance;

[0052] Figure 5 This is a flowchart illustrating Embodiment 2 of the present invention;

[0053] Figure 6 This is a comparison chart of SMF coupling power when using 1.5-order VPP demodulation for dual-channel demodulation based on 1.5-order spiral fractional OAM and -0.5-order spiral fractional OAM, -1-order OAM, and -2-order OAM in Embodiment 2 of the present invention.

[0054] Figure 7 This is a simulation diagram of the beam profile as the transmission distance increases after the -0.5 order spiral fractional OAM beam is demodulated using a 1.5 order VPP.

[0055] Figure 8 This is a simulation diagram of the beam profile as the transmission distance increases after the -1st order spiral fractional OAM beam is demodulated using a 1.5th order VPP in Embodiment 2 of the present invention.

[0056] Figure 9 This is a simulation diagram of the beam profile as the transmission distance increases after the -2nd order spiral fractional OAM beam is demodulated using a 1.5th order VPP. Detailed Implementation

[0057] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0058] Example 1

[0059] like Figure 1 and 2 As shown, this embodiment provides a demodulation method for a free-space optical communication link based on the fractional-order orbital angular momentum of a spiral, specifically including the following steps:

[0060] S1) A continuous wave laser with a wavelength of 1550nm is used as the light source. Its output laser beam is Gaussian after passing through a single-mode fiber coupler and collimator.

[0061] S2) The collimated Gaussian beam is perpendicularly incident into the spatial light phase modulation module, which is a spatial light modulator SLM1; a 1.5-order spiral fractional OAM phase diagram is loaded through the spatial light modulator SLM1 to generate a corresponding 1.5-order spiral fractional OAM beam. The complex amplitude distribution of the 1.5-order spiral fractional OAM beam is as follows:

[0062]

[0063]

[0064]

[0065] In the formula, For complex amplitude; Radial coordinates; The beam waist radius; For axial coordinates; Indicates the wavelength of light.

[0066] S3) The 1.5th order spiral fractional OAM beam is received by the optical relay imaging system and accurately imaged onto the plane of the demodulated spatial light phase modulation module SLM2. The optical relay imaging system adopts a 4f optical system, which consists of two lenses Lens1 and Lens2, each with a focal length of 150mm. The spatial light phase modulation module SLM1 and the demodulated spatial light phase modulation module SLM2 are located at the front focal plane and the rear focal plane of the 4f optical system, respectively, to ensure that the light field can be accurately transmitted.

[0067] S4) Based on the current target topological charge value to be demodulated, a -1.5 order spiral fractional OAM phase map is loaded onto the demodulation spatial optical phase modulation module SLM2 to demodulate the incident 1.5 order spiral fractional OAM beam;

[0068] S5) Fix the spatial position of the single-mode fiber, adjust the phase of the lens loaded on the SLM, and monitor the output optical power of the single-mode fiber coupling unit to make the coupling power after demodulation of the 1.5th order spiral fractional OAM beam reach its peak value.

[0069] In this embodiment, the demodulated 1.5th order spiral fractional OAM beam profile simulation diagram as the transmission distance increases is as follows: Figure 3 As shown, the center is a single bright spot, and the surrounding light spots exhibit a spiral distribution, gradually spreading out as the propagation distance increases. The beam profile observed through a CCD camera is as follows... Figure 4 As shown, the center is a single bright spot, and the surrounding light spots exhibit a spiral distribution. Simultaneously, by measuring the coupling power of the SMF to the beam using an optical power meter, it can be determined that the coupling power reaches its maximum value when the main intensity distribution of the beam profile converges into a single spot.

[0070] Example 2

[0071] This embodiment provides a free-space optical communication dual-channel multiplexing and demodulation method based on spiral fractional OAM, such as... Figure 5 As shown, the specific implementation steps are as follows:

[0072] S6) The first 1550nm laser Laser1 is incident on the spatial light modulator SLM1 loaded with a phase diagram of 1.5th order spiral fractional OAM to generate a 1.5th order spiral fractional OAM beam;

[0073] S7) The second 1550nm laser Laser2 is used as crosstalk and incident on another spatial light modulator SLM2. If this SLM2 is loaded with a phase diagram of 0.5th order spiral fractional OAM, a 0.5th order spiral fractional OAM beam is generated; if this SLM2 is loaded with a phase diagram of 1st order OAM, a 1st order OAM beam is generated; if this SLM2 is loaded with a phase diagram of 2nd order OAM, a 2nd order OAM beam is generated.

[0074] S8) The two OAM beams are transmitted for 150mm respectively, passing through Lens1 and Lens2 with a focal length of 150mm respectively, and then transmitted for 300mm. During the 300mm transmission process, the two OAM beams are first combined by the 50:50 beam combining module BS before reaching Lens3 with a focal length of 150mm. The combined OAM beam is then transmitted for 150mm to the demodulated spatial light modulator SLM3.

[0075] S9) The phase diagram of -1.5 order OAM is loaded onto the demodulator SLM3 to demodulate the 1.5 order spiral fractional OAM. The demodulated 1.5 order spiral fractional OAM beam is received and measured by a single-mode fiber coupling unit through an optical power meter. By loading a lens phase onto the demodulator SLM3 that generates the 1.5 order spiral fractional OAM, the coupling power of the demodulated 1.5 order spiral fractional OAM beam reaches its peak value. At this time, the main light intensity distribution can be observed to converge into a light spot in the beam profile of the fiber coupling through a CCD.

[0076] S10) On another spatial light modulator SLM that serves as crosstalk, phase diagrams of 0.5th order spiral fractional OAM, 1st order OAM, and 2nd order OAM are loaded respectively, generating 0.5th order spiral fractional OAM, 1st order OAM, and 2nd order OAM respectively. The VPP and lens phase of the 1.5th order spiral fractional OAM loaded on the spatial light modulator SLM3 are kept unchanged. Then, the coupling power of this crosstalk single-channel input is measured respectively.

[0077] Simulation as Figure 6 As shown, the coupling power of the 1.5th order spiral fractional OAM, which fully utilizes the self-focusing effect, is more than 10dB higher than that of the 0.5th order spiral fractional OAM, 1st order OAM, and 2nd order OAM, which are used as crosstalk. This indicates that the 1.5th order spiral fractional OAM is demodulated, while the 0.5th order spiral fractional OAM, 1st order OAM, and 2nd order OAM are isolated.

[0078] Simulation diagram of the beam profile of the 0.5th order spiral fractional OAM demodulated using the -1.5th order spiral fractional OAM phase diagram as the transmission distance increases is shown below. Figure 7 As shown, the ring at the center of the beam profile gradually expands as the propagation distance increases.

[0079] The simulation diagram of the beam profile of the first-order spiral fractional OAM after demodulation using a 1.5-order VPP as the transmission distance increases is shown below. Figure 8 As shown, after demodulation, the spiral of the beam profile of the first-order OAM gradually expands and diverges as the propagation distance increases.

[0080] The simulation diagram of the beam profile of the second-order spiral fractional OAM after demodulation using a 1.5-order VPP as the transmission distance increases is shown below. Figure 9 As shown, after demodulation, the central spot and surrounding spiral lines of the beam profile of the 2nd order OAM gradually expand and diverge as the propagation distance increases.

[0081] The results show that the free-space optical communication link construction and demodulation method based on spiral fractional order OAM provided in this embodiment can achieve efficient generation and stable transmission of different spiral fractional order OAM modes. The mode isolation when using dual-channel multiplexing between spiral fractional order OAM and dual-channel multiplexing between spiral fractional order OAM and integer order OAM is higher than 5dB, providing an effective technical solution for future high-speed free-space optical communication systems.

[0082] Example 3

[0083] This embodiment provides a free-space optical communication link demodulation system based on helical fractional orbital angular momentum, including:

[0084] The Gaussian beam generation module is used to load N data signals to be transmitted into N beams respectively, and output Gaussian beams after collimation;

[0085] The transmitter spatial light phase modulation module is used to apply a spiral phase with a specific topological charge value to each Gaussian beam to generate N modulated beams carrying different orbital angular momentum modes.

[0086] The beam combining module is used to coaxially combine N modulated beams to generate a multiplexed orbital angular momentum beam and emit it;

[0087] An optical relay imaging system is used to receive the multiplexed orbital angular momentum beam imaging onto the plane of the demodulated spatial light phase modulation module;

[0088] The demodulation module uses a spatial optical phase modulation module to load a matched demodulation phase map based on the current target topological charge value to be demodulated, and then demodulates the target topological charge value using the OAM beam.

[0089] A focusing optical module is used to adjust the incident state of the demodulated beam, so that the beam with the target topological charge value is coupled into the fiber, while isolating other beams with non-target topological charge values.

[0090] A single-mode fiber coupling module is used to receive the demodulated beam with the adjusted target topological charge value and couple it into a single-mode fiber.

[0091] The verification module is used to verify the single-mode fiber coupling power when only the target channel beam is transmitted, and the single-mode fiber coupling power when only the non-target channel beam is transmitted. If the coupling power of the target channel beam is more than 5dB higher than the coupling power of the non-target channel beam, the current demodulation state is confirmed to be valid, the target channel is successfully demodulated, and the non-target channel is effectively isolated.

[0092] The embodiments and descriptions above are merely illustrative of the principles and preferred embodiments of the present invention. Various changes and modifications may be made to the present invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed.

Claims

1. A demodulation method for free-space optical communication links based on fractional-order spiral orbital angular momentum, characterized in that, Includes the following steps: S1) Load the N data signals to be transmitted into the N beams respectively, collimate them and output a Gaussian beam; S2) By applying a spiral phase with a specific topological charge value to each Gaussian beam through the spatial light phase modulation module, N modulated beams carrying different orbital angular momentum modes are generated. S3) Using a beam combiner module, N modulated beams are coaxially combined to generate a multiplexed orbital angular momentum beam and emitted into free space; S4) Receive the multiplexed orbital angular momentum beam imaged to the plane of the demodulation spatial optical phase modulation module through the optical relay imaging system, and load a matching demodulation phase map on the demodulation spatial optical phase modulation module according to the current target topological charge value to be demodulated. S5) The beam demodulated by the spatial light phase modulation module is incident on the single-mode fiber coupling module, and the beam incident state is adjusted by the focusing optics module so that the beam with the target topological charge value is coupled into the fiber, while other beams with non-target topological charge values ​​are isolated. S6) Adjust the focusing optical module and the single-mode fiber coupling module to make the coupled optical power reach the peak value and restore the data signal of the current target channel; S7) Measure the single-mode fiber coupling power between the target topological charge beam and the non-target topological charge beam. After confirming that the coupling power of the target channel is more than 5dB higher than that of the non-target channel, switch the matching demodulation phase diagram on the demodulation spatial optical phase modulation module and repeat steps S4) to S6) to demodulate the data signals carried by each modulation beam in sequence.

2. The demodulation method for free-space optical communication links based on fractional-order spiral orbital angular momentum according to claim 1, characterized in that: In step S2), the spatial light modulation element applies a spiral phase with a specific topological charge value to each Gaussian beam by loading different order spiral fractional OAM phase maps or different order integer OAM phase maps, thereby generating a spiral fractional OAM beam or an integer OAM modulated beam.

3. The demodulation method for free-space optical communication links based on fractional-order spiral orbital angular momentum according to claim 2, characterized in that: In step S2), among the N modulated beams, except for at least one spiral fractional-order OAM beam, the remaining N-1 beams are either spiral fractional-order OAM beams or integer-order OAM modulated beams.

4. The free-space optical communication link demodulation method based on helical fractional orbital angular momentum according to claim 3, characterized in that: In step S2), the topological charge spectrum of the spiral fractional-order OAM beam is composed of multiple integer orbital angular momentum eigenstates superimposed, and its angular momentum density is distributed in the outer ring region of the beam cross section.

5. The free-space optical communication link demodulation method based on helical fractional orbital angular momentum according to claim 4, characterized in that: In step S2), the spiral fractional-order OAM beam and the open-ring fractional-order OAM beam with the same non-integer topological charge value are mutually converted through a spiral phase piecewise function; the spiral phase piecewise function for: In the formula, It is a scaling factor and is a fraction; Scaling factor The denominator; This represents the topological load value.

6. The free-space optical communication link demodulation method based on helical fractional orbital angular momentum according to claim 5, characterized in that: In step S2), the spiral fractional-order OAM beam is generated by performing a spiral coordinate transformation through a spatial light phase modulation module. The spiral fractional-order OAM beam has a spiral intensity distribution and a phase jump along the radial direction, and its complex amplitude is expressed as: In the formula, For complex amplitude; Radial coordinates; The beam waist radius; For axial coordinates; Indicates the wavelength of light.

7. The demodulation method for free-space optical communication links based on fractional-order spiral orbital angular momentum according to claim 1, characterized in that: In step S5), the other non-target topological charge beams, due to maintaining the spiral phase distribution characteristics, exhibit a central dark spot or ring intensity distribution at the fiber core end face of the single-mode fiber coupling module. The energy coupled into the single-mode fiber is intrinsically suppressed, and the coupling power difference with the target beam is greater than 5dB.

8. The demodulation method for free-space optical communication links based on fractional-order spiral orbital angular momentum according to claim 1, characterized in that: In step S6), by fixing the spatial position of the single-mode fiber, a matching demodulation phase map corresponding to the current target topological charge value to be demodulated is loaded on the demodulation spatial light phase modulation module. By continuously adjusting the focusing optical module, the optical power coupled into the single-mode fiber reaches its peak value. At this time, the orbital angular momentum beam corresponding to the target topological charge value is demodulated and coupled into the fiber, while beams carrying other non-target topological charge values ​​are spatially isolated.

9. The demodulation method for free-space optical communication links based on fractional-order spiral orbital angular momentum according to claim 1, characterized in that: In step S7), the matched demodulation phase map loaded on the demodulation spatial light phase modulation module and the focusing optical module are kept unchanged; the single-mode fiber coupling power when only the target channel beam is transmitted and the single-mode fiber coupling power when only the non-target channel beam is transmitted are measured respectively; if the coupling power of the target channel beam is more than 5dB higher than the coupling power of the non-target channel beam, the current demodulation state is confirmed to be valid, the target channel is successfully demodulated, and the non-target channel is effectively isolated.

10. A demodulation system for a free-space optical communication link based on fractional-order spiral orbital angular momentum, characterized in that, include: The Gaussian beam generation module is used to load N data signals to be transmitted into N beams respectively, and output Gaussian beams after collimation; The transmitter spatial light phase modulation module is used to apply a spiral phase with a specific topological charge value to each Gaussian beam to generate N modulated beams carrying different orbital angular momentum modes. The beam combining module is used to coaxially combine N modulated beams to generate a multiplexed orbital angular momentum beam and emit it; An optical relay imaging system is used to receive the multiplexed orbital angular momentum beam imaging onto the plane of the demodulated spatial light phase modulation module; The demodulation module uses a spatial optical phase modulation module to load a matched demodulation phase map based on the current target topological charge value to be demodulated, and then demodulates the target topological charge value using the OAM beam. A focusing optical module is used to adjust the incident state of the demodulated beam, so that the beam with the target topological charge value is coupled into the fiber, while isolating other beams with non-target topological charge values. A single-mode fiber coupling module is used to receive the demodulated beam with the adjusted target topological charge value and couple it into a single-mode fiber. The verification module is used to verify the single-mode fiber coupling power when only the target channel beam is transmitted, and the single-mode fiber coupling power when only the non-target channel beam is transmitted. If the coupling power of the target channel beam is more than 5dB higher than the coupling power of the non-target channel beam, the current demodulation state is confirmed to be valid, the target channel is successfully demodulated, and the non-target channel is effectively isolated.

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