1. A non-coherent superposition keying method based on orbital angular momentum states of light beams is characterized by comprising the following steps:

s1: the first electronic pulse control laser, the second electronic pulse control laser, the third electronic pulse control laser and the Nth electronic pulse control laser generate N paths of optical pulses with different sequences, and the generated Gaussian beams are simultaneously and respectively modulated by the first spatial light modulator, the second spatial light modulator, the third spatial light modulator and the Nth spatial light modulator;

s2: the modulation signal output by the first spatial light modulator and the modulation signal output by the second spatial light modulator pass through the first optical beam splitter, the modulation signal output by the third spatial light modulator and the modulation signal output by the fourth spatial light modulator pass through the second optical beam splitter until the modulation signal output by the N-1 th spatial light modulator and the modulation signal output by the N-2 th spatial light modulator pass through the N/2 th optical beam splitter; the first optical beam splitter and the second optical beam splitter pass through an N/2+1 optical beam splitter, the third optical beam splitter and the fourth optical beam splitter pass through an N/2+2 optical beam splitter until the N/2 optical beam splitter and the N/2-1 optical beam splitter pass through an N/2+ N/4 optical beam splitter, the N/2+1 optical beam splitter and the N/2+2 optical beam splitter pass through an N/2+ N/4+1 optical beam splitter, and the like until the N-3 optical beam splitter and the N-2 optical beam splitter pass through an N-1 optical beam splitter to obtain incoherent superposed composite vortex beams;

s3, the composite vortex light beam sequentially passes through an atmospheric turbulence channel and a CCD camera to obtain an intensity distribution graph of the composite vortex light beam changing along with time under the influence of atmospheric turbulence;

s4: the intensity distribution graph is trained, identified and demodulated through a deep learning network on a host computer to obtain a transmitted digital signal, so that transmitted image information is obtained.

2. The method according to claim 1, wherein the encoding of the image information to be transmitted is implemented by a sequence of a first electrical pulse, a second electrical pulse, a third electrical pulse and an Nth electrical pulse, the sequence being determined according to the image information to be transmitted; wherein the first electronic pulse sequence is the most significant bit of the number of bits.

3. The method according to claim 2, wherein the spatial light modulator is controlled by the host, the first spatial light modulator, the second spatial light modulator, the third spatial light modulator, and the nth spatial light modulator correspond to holograms formed by combining orbital angular momentum mode values of different vortex beams and normalized amplitude values of different vortex beams, respectively, and the modulation and coding of the digital signal is to modulate two different dimensions, namely, orbital angular momentum state and amplitude of the beam, respectively, assuming OAM and amplitude dimensions to represent m-ary and N-ary symbols, respectively, and to represent log symbols, respectively₂m and log₂n bits of data information, then when both dimensions are used simultaneously, a total of log can be generated₂(mn) bit information; the modulation coding may be considered as an extension coding in the OAM amplitude dimension, so that higher order bits may be encoded with fewer OAM states.

4. The method according to claim 3, wherein the deep learning network on the host is a classical LeNet-5 architecture.

5. A non-coherent superposition keying system based on light beam orbital angular momentum states is characterized by comprising an electronic pulse control laser, a spatial light modulator, a 1xN multiplexer, a CCD camera, a data transmission line and a host;

the electronic pulse control laser is used as a light source and used for generating a Gaussian beam and outputting the Gaussian beam which changes along with time according to the sequence of the electronic pulse;

the spatial light modulator is loaded with a series of holograms which are specially designed according to signals to be coded to modulate Gaussian beams to realize signal coding and obtain vortex beams generated by respectively modulating two different dimensions of orbital angular momentum states and amplitudes of the beams;

the 1xN multiplexer realizes incoherent superposition of four paths of different vortex light beams;

the CCD camera is arranged in a laser light path of the 1xN multiplexer and used for collecting an intensity distribution diagram of the multiplexed vortex light beam;

the data transmission line is used for connecting the CCD camera and the host to realize real-time transmission of signals;

the host is used for analyzing an intensity distribution diagram acquired by the CCD camera, obtaining coded digital signals by using an image classification method in deep learning, and realizing signal demodulation so as to obtain transmitted images.

Background art:

the Orbital Angular Momentum (OAM) has dimensions different from wavelength/frequency, time, complex amplitude (amplitude, phase) and polarization, and is a new physical dimension and a new degree of freedom, which are independent from other dimensions. The vortex light carrying the OAM has a continuous helical phase front and the intensity profile presents a bright ring centered as a dark spot. Theoretically, because the value of the OAM mode value is infinite and the light beams carrying different integral-order OAM are orthogonal to each other, it is inspired that people can use the space dimension resource of the OAM light beam as the carrier of information and apply it to the Free Space Optical (FSO) communication field to get rid of the dilemma that the communication resource is increasingly tightened.

The generation of the composite vortex light beam is divided into two modes of coherent superposition and incoherent superposition. The conditions required to be met for realizing coherent superposition are harsh, coherent superposition of a plurality of light sources is difficult to realize, a light field after coherent superposition is interference light, and the light intensity of the interference light is easily influenced by the phase difference of coherent light field superposition areas of different vortex light beams. The superposition of incoherent light fields belongs to light intensity superposition, the total light intensity is equal to the sum of the light intensities of all light beams everywhere, and the realization is easy. Both coherent and non-coherent superposition retain the property of "swirl".

In the OAM coding technology, although the coding method based on two different dimensions of the orbital angular momentum state and the amplitude of the light beam can effectively realize the coding of higher bit information amount, its demodulation structure is more complex and has higher requirement on the spatial light modulator, so a new technology needs to be developed at present, so that the demodulation speed and the accuracy rate of the vortex light beam generated by using two different dimensions of the coding method are faster and higher, and the requirement on the modulation rate of the spatial light modulator can be reduced.

Disclosure of Invention

The invention aims to provide a non-coherent superposition keying method and a system based on a light beam orbital angular momentum state, aiming at the defects of low modulation rate and low OAM light beam demodulation speed of the existing spatial light modulator.

In order to achieve the purpose, the invention adopts the following technical scheme:

a non-coherent superposition keying method based on orbital angular momentum states of light beams is characterized by comprising the following steps:

s1: the first electronic pulse control laser, the second electronic pulse control laser, the third electronic pulse control laser and the Nth electronic pulse control laser generate N paths of optical pulses with different sequences, and the generated Gaussian beams are simultaneously and respectively modulated by the first spatial light modulator, the second spatial light modulator, the third spatial light modulator and the Nth spatial light modulator.

S2: the modulation signal output by the first spatial light modulator and the modulation signal output by the second spatial light modulator pass through the first optical beam splitter, the modulation signal output by the third spatial light modulator and the modulation signal output by the fourth spatial light modulator pass through the second optical beam splitter until the modulation signal output by the N-1 th spatial light modulator and the modulation signal output by the N-2 th spatial light modulator pass through the N/2 th optical beam splitter. The first optical beam splitter and the second optical beam splitter pass through the N/2+1 optical beam splitter, the third optical beam splitter and the fourth optical beam splitter pass through the N/2+2 optical beam splitter until the N/2 optical beam splitter and the N/2-1 optical beam splitter pass through the N/2+ N/4 optical beam splitter, the N/2+1 optical beam splitter and the N/2+2 optical beam splitter pass through the N/2+ N/4+1 optical beam splitter, and the like until the N-3 optical beam splitter and the N-2 optical beam splitter pass through the N-1 optical beam splitter, and the incoherent superimposed compound vortex beams are obtained.

And S3, the composite vortex light beam sequentially passes through the atmospheric turbulence channel and the CCD camera to obtain an intensity distribution graph of the composite vortex light beam changing along with time under the influence of atmospheric turbulence.

S4: the intensity distribution graph is trained, identified and demodulated by a deep learning network on the host computer through the host computer to obtain the transmitted digital signals, so that the transmitted image information is obtained.

Furthermore, the encoding of the image information to be transmitted is realized by a sequence consisting of a first electronic pulse, a second electronic pulse, a third electronic pulse and an Nth electronic pulse, and the sequence is determined according to the image information to be transmitted. Wherein the first electronic pulse sequence is the most significant bit of the number of bits.

Furthermore, the spatial light modulator is controlled by the host, the first spatial light modulator, the second spatial light modulator, the third spatial light modulator and the Nth spatial light modulator are respectively corresponding to holograms formed by combining orbital angular momentum mode values of different vortex beams and normalized amplitude values of different vortex beams, modulation coding of digital signals is to modulate two different dimensions of orbital angular momentum states and amplitudes of the beams respectively, and it is assumed that OAM and amplitude dimensions respectively represent m-ary symbols and N-ary symbols and respectively represent log₂m and log₂n bits of data information, then when both dimensions are used simultaneously, a total of log can be generated₂(mn) bits of information. The modulation coding may be considered as an extension coding in the OAM amplitude dimension, so that higher order bits may be encoded with fewer OAM states.

Furthermore, the deep learning network on the host adopts a classic LeNet-5 architecture.

Correspondingly, the incoherent superposition keying system based on the light beam orbital angular momentum state comprises an electronic pulse control laser, a spatial light modulator, a 1xN multiplexer, a CCD camera, a data transmission line and a host.

The electronic pulse control laser is used as a light source and used for generating a Gaussian beam and outputting the Gaussian beam which changes along with time according to the sequence of the electronic pulse;

the spatial light modulator is loaded with a series of holograms which are specially designed according to signals to be coded to modulate Gaussian beams to realize signal coding and obtain vortex beams generated by respectively modulating two different dimensions of orbital angular momentum states and amplitudes of the beams;

the 1xN multiplexer realizes incoherent superposition of four paths of different vortex light beams;

the CCD camera is arranged in a laser light path of the 1xN multiplexer and used for collecting an intensity distribution diagram of the multiplexed vortex light beam;

the data transmission line is used for connecting the CCD camera and the host to realize real-time transmission of signals;

the host is used for analyzing an intensity distribution diagram acquired by the CCD camera, obtaining coded digital signals by using an image classification method in deep learning, and realizing signal demodulation so as to obtain transmitted images.

Compared with the prior art, the method combines vortex light beams generated by respectively modulating two different dimensions of the orbital angular momentum state and the amplitude of the light beams by an incoherent superposition method, and demodulates the generated composite vortex light beams by utilizing a deep learning network, so that the demodulation speed and the accuracy of the system are improved. At present, the OAM-SK research mostly adopts the mode that Gaussian beams generated by one path of laser are modulated, the generation rate of vortex beams is influenced by the modulation rate of a spatial light modulator, the scheme adopts the mode that multiple paths of Gaussian beams are modulated simultaneously, and the generated vortex beams are independent from each other and are not limited by the modulation rate of the spatial light modulator, so that the requirement on the modulation rate of the spatial light modulator is reduced. The demodulation structure is very simple and the system cost is reduced. Compared with the existing demodulation method based on the orbital angular momentum state of the light beam, the demodulation method based on the orbital angular momentum state of the light beam has great improvement and has wide application prospect in a free space optical communication system.

Drawings

FIG. 1 is a schematic diagram of a structure of incoherent superposition keying based on orbital angular momentum states of a light beam;

FIG. 2 is a graph of the intensity distribution of a vortex beam generated by a four-way electronically pulsed laser producing a Gaussian beam while being modulated by a first spatial light modulator, a second spatial light modulator, a third spatial light modulator, and a fourth spatial light modulator, respectively;

FIG. 3 is a mapping relationship between intensity profiles of generated composite vortex beams by three beam splitters and corresponding 4-bit symbol symbols of different intensity profiles;

FIG. 4 is an experimental graph of the recognition rate of intensity distribution graph after passing through an atmospheric turbulence channel through a deep learning network, wherein the x-axis is Epoch, and the atmospheric turbulence intensity is

Wherein, 1, a first electronic pulse controls the laser; 2. a second electronic pulse controlled laser; 3. a third electronic pulse controlled laser; 4. the Nth electronic pulse controls the laser; 5. a first spatial light modulator; 6. a second spatial light modulator; 7. a third spatial light modulator; 8. an Nth spatial light modulator; 9. a first optical beam splitter; 10. a second optical beam splitter; 11. an N-1 optical beam splitter; 12. an atmospheric turbulence channel model; a CCD camera; 14. data transmission line and host computer.

Detailed Description

The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict.

The invention aims to provide a non-coherent superposition keying method and a non-coherent superposition keying system based on orbital angular momentum states of light beams, aiming at the defects of the prior art.

Example one

As shown in FIG. 1, the present invention provides a non-coherent superposition keying system based on orbital angular momentum state of light beam, 1. a first electronic pulse controls a laser; 2. a second electronic pulse controlled laser; 3. a third electronic pulse controlled laser; 4. a fourth electronic pulse controlled laser; 5. a first spatial light modulator; 6. a second spatial light modulator; 7. a third spatial light modulator; 8. a fourth spatial light modulator; 9. a first optical beam splitter; 10. a second optical beam splitter; 11. a third optical beam splitter; 12. an atmospheric turbulence channel model; a CCD camera; 14. data transmission line and host computer.

The embodiment provides a non-coherent superposition keying method based on a light beam orbital angular momentum state, which comprises the following steps:

as shown in fig. 1, the 4-bit information is transmitted as an example.

S1: the first electronic pulse control laser, the second electronic pulse control laser, the third electronic pulse control laser and the fourth electronic pulse control laser generate four paths of light pulses with different sequences, and the generated Gaussian beams are simultaneously and respectively modulated by the first spatial light modulator, the second spatial light modulator, the third spatial light modulator and the fourth spatial light modulator.

S2: the modulation signal output by the first spatial light modulator and the modulation signal output by the second spatial light modulator pass through the first optical beam splitter, the modulation signal output by the third spatial light modulator and the modulation signal output by the fourth spatial light modulator pass through the second optical beam splitter, and the first optical beam splitter and the second optical beam splitter pass through the third optical beam splitter to obtain the incoherent superimposed composite vortex light beam.

And S3, the composite vortex light beam sequentially passes through the atmospheric turbulence channel and the CCD camera to obtain an intensity distribution graph of the composite vortex light beam changing along with time under the influence of atmospheric turbulence.

S4: the intensity distribution graph is trained, identified and demodulated by a deep learning network on the host computer through the host computer to obtain the transmitted digital signals, so that the transmitted image information is obtained.

As shown in fig. 1, a schematic structural diagram of a non-coherent superposition keying method based on orbital angular momentum states of a light beam according to the present invention is shown, 1. a first electronic pulse controls a laser; 2. a second electronic pulse controlled laser; 3. a third electronic pulse controlled laser; 4. a fourth electronic pulse controlled laser; 5. a first spatial light modulator; 6. a second spatial light modulator; 7. a third spatial light modulator; 8. a fourth spatial light modulator; 9. a first optical beam splitter; 10. a second optical beam splitter; 11. a third optical beam splitter; 12. an atmospheric turbulence channel model; a CCD camera; 14. data transmission line and host computer.

In step S2, the encoding for transmitting the image information is implemented by a sequence of a first electrical pulse, a second electrical pulse, a third electrical pulse and an nth electrical pulse, the sequence being determined according to the image information to be transmitted. Wherein the first electronic pulse sequence is the most significant bit of the number of bits.

In step S2, the spatial light modulator is controlled by the host, and the first spatial light modulator, the second spatial light modulator, the third spatial light modulator, and the fourth spatial light modulator correspond to holograms in which OAM mode values (+4, +15) and 2 normalized amplitude values (1,0.707) are combined in pairs, respectively, where the first spatial light modulator corresponds to a hologram in which a mode value is +4 and a normalized amplitude value is 0.707 are combined, the second spatial light modulator corresponds to a hologram in which a mode value is +4 and a normalized amplitude value is 1 are combined, the third spatial light modulator corresponds to a hologram in which a mode value is +15 and a normalized amplitude value is 0.707 are combined, and the fourth spatial light modulator corresponds to a hologram in which a mode value is +15 and a normalized amplitude value is 1 are combined. Fig. 2 shows a distribution diagram of intensity of vortex beams generated by a gaussian beam generated by a laser after the gaussian beam is modulated by a first spatial light modulator, a second spatial light modulator, a third spatial light modulator and a fourth spatial light modulator respectively.

In step S3, the intensity distribution map of the composite vortex beam corresponds to the mapping relationship of the 4-bit symbol, and fig. 3 shows the intensity distribution map of the composite vortex beam and the mapping relationship of the 4-bit symbol symbols corresponding to different intensity distribution maps;

in step S4, the convolutional neural network on the host is adopted as the classic LeNet-5 architecture in deep learning. FIG. 4 shows an experimental graph of the recognition rate of a composite vortex beam intensity profile through a deep learning network.

Compared with the prior system, the incoherent superposition keying system based on the light beam orbital angular momentum state has the advantages that the incoherent superposition method is used for carrying out composite combination on vortex light beams generated by respectively modulating two different dimensions of the light beam orbital angular momentum state and the amplitude, the generated composite vortex light beams are demodulated by utilizing the deep learning network, the demodulation speed and the accuracy of the system are improved, and meanwhile, the requirement on the modulation rate of the spatial light modulator is reduced. The demodulation structure is very simple and the system cost is reduced.

It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in greater detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.