Vortex beam coordinate transformation reconstruction method and system

Abstract

The invention discloses a vortex beam coordinate transformation reconstruction method and system. Based on coordinate transformation of the light field, the beam splitting surface light field broadening principle of coordinate transformation when the vortex light beam is shielded is explained, and the corresponding relation between beam splitting surface light field broadening and shielded vortex light beam spectrum broadening is explained. A vortex beam reconstruction method is constructed through the principle, a beam splitting surface broadening light field is subjected to spatial filtering so as to narrow the beam splitting surface light field, then the beam splitting surface light field is reversely input into a coordinate transformation structure, and wavefront reconstruction of a shielded vortex beam can be achieved. A coordinate transformation diffractive optical device is designed and manufactured, a vortex beam reconstruction system is constructed through combination of the coordinate transformation diffractive optical device, a Fourier transformation lens, a polarization control element, a reflective programmable device and the like, wavefront reconstruction of a shielded vortex beam is achieved, and wavefront reconstruction of shielded single-mode and multi-mode vortex beams can be supported. The vortex beam coordinate transformation reconstruction method is complete in theory, good in reconstruction effect, simple in system structure, high in stability and wide in prospect in the field of high-capacity laser communication.

Description

Technical Field

[0001] The present invention relates to the field of optoelectronic technology, and in particular to a vortex light beam coordinate transformation and reconstruction method and system. Background Art

[0002] A vortex beam is a beam with a spiral wavefront. The special beam (l is the topological charge number, It carries orbital angular momentum (OAM), and its eigenvalue l can be any integer. The different eigenvalue OAM components of this light beam are orthogonal to each other, forming an infinite-dimensional Hilbert space, so theoretically it can provide infinite orthogonal dimensions. Through OAM keying technology, large-capacity optical communication can be achieved, bringing a revolutionary increase in information transmission rate and data capacity, and has broad prospects in the fields of intersatellite communication and GPU interconnection.

[0003] The use of vortex beams for optical communication requires receiving complete light field information, and then detecting the OAM components carried by the beam through interference, diffraction and other methods to achieve decoding. However, the transmission size and divergence angle of the vortex beam increase significantly with the increase of the OAM eigenvalue, which makes it difficult to miniaturize the receiver in the free space communication link; in addition, when there are obstructions in the link, bit errors are prone to occur. The above problems limit the development of vortex optical communication. Therefore, how to reconstruct the complete wavefront for decoding when the complete light field cannot be received is one of the key issues to be solved in this field. Summary of the invention

[0004] In view of this, the present invention discloses a vortex beam reconstruction method and system, which can filter out the mode broadening of the light beam caused by the occlusion when the light beam is partially blocked, thereby realizing beam reconstruction.

[0005] The vortex beam reconstruction method of the present invention is based on the coordinate transformation principle, and core components are designed and processed, including a de-ring device, a phase compensation device and a Fourier transform lens, to achieve the broadening mode splitting of the blocked vortex beam.

[0006] The principle of the vortex beam reconstruction method of the present invention is that when the light beam is partially blocked, its OAM spectrum will be broadened. If the broadened side lobes can be filtered out, the reconstruction of the original light beam can be achieved. The vortex light beam is pattern-splitting using a coordinate transformation structure, and the broadening characteristics of the split light field are consistent with the broadening characteristics of the OAM spectrum. The narrowing of the split light field can be achieved by spatially filtering the broadened split light field using a reflective programmable electrically controlled reflector at the pattern splitting position. Since the coordinate transformation structure can work in both directions, the narrowed light field is transmitted in the reverse direction along the original optical path, and the reverse coordinate transformation can be performed into a vortex light beam with a complete wavefront.

[0007] The vortex beam reconstruction system of the present invention is composed of a coordinate transformation structure, a first depolarizing beam splitter prism, a polarization control element group, a reflective programmable device, and a receiving control component, wherein:

[0008] The coordinate transformation structure has a de-circulation device, a phase compensation device, a Fourier transform lens and a depolarization beam splitter prism, among which:

[0009] The de-ring device is used to transform the vortex light field from a polar coordinate system to a rectangular coordinate system, and to provide an additional focusing effect to achieve Fourier transform of the light field;

[0010] The phase compensation device is placed in the laser light path behind the de-ring device, and is used to compensate for the extra phase introduced by the de-ring device so that the light emitted from the device is still collimated;

[0011] The Fourier transform lens is used for Fourier transform of the light field to obtain a mode-splitting strip light field.

[0012] The first depolarizing beam splitter is placed in the laser light path before the coordinate transformation structure, and is used to accommodate the incident blocked vortex beam and the outgoing reconstructed vortex beam.

[0013] The polarization control element group is used to control the polarization state of the light beam so that the de-circulation device and the phase compensation device can realize the designed phase modulation, and has a polarizer, a first quarter wave plate, a second quarter wave plate, and a polarization beam splitter prism, wherein:

[0014] The polarizer is placed in the laser light path in front of the depolarizing beam splitter prism, and is used to convert the incident vortex light beam into horizontal linear polarized light, so as to further convert the vortex light beam into left-handed circularly polarized light;

[0015] The first quarter wave plate is placed in the laser light path between the polarizer and the de-circling device, and is used to convert the horizontal linearly polarized vortex beam into a left-handed circularly polarized vortex beam, so that the de-circling device can achieve correct phase modulation;

[0016] The second quarter wave plate is placed in the laser light path behind the Fourier transform lens, and is used to convert the left-handed circularly polarized vortex beam into a horizontal linearly polarized vortex beam to meet the polarization state requirements of the subsequent reflective programmable device;

[0017] The polarization beam splitter prism is placed in the laser light path behind the reflection end of the depolarization beam splitter prism to filter out stray light.

[0018] The reflective programmable device can be a liquid crystal spatial light modulator or a digital microlens array, etc., which is placed in the laser light path behind the second quarter wave plate and is used for spatial filtering of the broadened split beam light field.

[0019] The receiving control component is used to receive the split beam field, generate a spatial filter mask by analyzing the broadening situation and load it into the reflective programmable device, and is equipped with a second depolarizing beam splitter prism, a planar array detector, and a computer, wherein:

[0020] The second depolarizing beam splitter prism is placed in the laser light path between the second quarter wave plate and the reflective programmable device, and is used to reflect one path of laser light for observation and analysis without affecting the main laser light path;

[0021] The planar array detector is placed in the laser light path behind the reflection path of the depolarizing beam splitter prism and is used to observe and analyze the distribution of the split beam light field;

[0022] The computer is connected to the planar array detector and the reflective programmable device, and is used to generate a spatial filtering mask according to the light intensity information received by the planar array detector and load the mask onto the reflective programmable device.

[0023] The present invention has the following beneficial effects:

[0024] 1. The vortex beam reconstruction method of the present invention is theoretically complete and can realize light field reconstruction of a vortex beam that is greatly obstructed, and has important application prospects in the field of optical communications;

[0025] 2. The vortex beam reconstruction of the present invention has a small volume, a compact structure, and high stability. It can be used as a key unit of an optical communication receiver, greatly reducing the size of the vortex optical communication receiver. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The structure diagram of the vortex beam reconstruction system of the present invention, wherein: 1-polarizer, 2-first depolarizing beam splitter prism, 3-first quarter wave plate, 4-de-ring device, 5-phase compensation device, 6-Fourier transform lens, 7-second quarter wave plate, 8-second depolarizing beam splitter prism, 9-reflective programmable device, 10-array detector, 11-polarizing beam splitter prism, 12-computer;

[0027] Figure 2 (a) is the phase distribution of the de-looping device. Figure 2 (b) is the phase distribution of the phase compensation device;

[0028] Figure 3 (b) is the intensity distribution of the 4th-order vortex beam; Figure 3 (a) Figure 3 (b) OAM spectrum of the corresponding light field; Figure 3 (c) Figure 3 (b) The intensity distribution of the corresponding light beam incident on the phase compensation device in the coordinate transformation structure, Figure 3 (d) Figure 3(b) The intensity distribution of the corresponding light beam incident on the focal plane (beam splitting plane) behind the Fourier transform lens in the coordinate transformation structure; Figure 3 (f) Figure 3 (b) The intensity distribution of the vortex beam after being angularly blocked; Figure 3 (e) Figure 3 (b) OAM spectrum of the obscured vortex beam; Figure 3 (g) Figure 3 (b) The intensity distribution of the blocked light beam incident on the phase compensation device; Figure 3 (h) Figure 3 (b) The intensity distribution of the blocked beam on the beam splitting surface; Figure 3 (l) is the spatial filter mask loaded on the reflective programmable device, black represents filtering and white represents transmission; Figure 3 (k) Figure 3 (h) The back-propagating light field after spatial filtering of the splitting surface light field, Figure 3 (j) is the output reconstructed light intensity distribution after the reverse coordinate transformation structure. Figure 3 (i) Figure 3 (j) OAM spectrum of the reconstructed light field.

[0029] Figure 4 (a) is the incident beam carrying four OAM modes; Figure 4 (c) Figure 4 (a) OAM spectrum of the multimode vortex light field; Figure 4 (b) Figure 4 (a) The intensity distribution of the middle beam at the beam splitting surface after the coordinate transformation structure; Figure 4 (d) Figure 4 (a) Light intensity distribution after the middle beam is blocked; Figure 4 (f) Figure 4 (d) The OAM spectrum of the blocked light field, compared with the unblocked Figure 4 (c), obvious spectrum broadening occurs; Figure 4 (e) Figure 4 (d) The intensity distribution of the blocked beam on the beam splitting surface of the coordinate transformation structure, compared with Figure 4 (b), broadening also occurs; Figure 4 (h) Figure 4 (e) The light intensity distribution at the beam splitting surface after spatial filtering by the reflective programmable device is significantly improved; Figure 4 (g) Figure 4 (d) The reconstructed light intensity distribution of the blocked light beam obtained by the reconstruction system of the present invention, Figure 4 (i) Figure 4 OAM spectrum of the reconstructed light field in (g).

[0030] Figure 5This is a flow chart of the vortex beam reconstruction method of the present invention. DETAILED DESCRIPTION

[0031] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0032] First, the vortex beam coordinate transformation splitting principle of the present invention is introduced. The present invention adopts a design of coordinate transformation pure phase grating: a de-ring grating and a phase compensation grating are combined with a Fourier transform lens to realize the logarithmic-polar coordinate transformation of the OAM light field, such as Figure 1 As shown in the components 4, 5, and 6, the phase compensation grating is placed at the front focal plane of the Fourier transform lens, and the distance between the de-ring grating and the phase compensation grating is f 1 The phase distribution of the coordinate transformation de-looping device and the phase compensation device used in the present invention is as follows: Figure 2 As shown, and

[0033]

[0034] Where (x, y) and (u, v) are the rectangular coordinates at the de-looping device and the phase compensation device respectively; parameter a represents 1 / 2π of the rectangular width at the phase compensation grating after the vortex light field is de-looped from a ring to a rectangle, and parameter b represents the translation of the rectangular light field in the u direction at the phase compensation grating; λ is the wavelength of the input light; f 1 is the distance between the de-ring grating and the compensation grating.

[0035] The phase distribution function of the de-ringed grating consists of two parts. The first part The incident light beam is mapped from the polar coordinate system to the rectangular coordinate system in a logarithmic-polar coordinate system, and then 1 The lens is focused, and the mapped rectangular light field can be obtained on the rear focal plane of the lens; the second part A focusing effect can be achieved, thereby converting the aforementioned focal length f 1 The lens effect is integrated into the de-ringed grating to make the system more compact.

[0036] The phase compensation grating is calculated by the fixed phase approximation method based on the de-ring grating structure, which can compensate the phase distortion introduced by the de-ring grating to obtain a collimated rectangular light beam at the front focal plane of the Fourier transform lens. The rectangular light field can be transformed into a strip light field at its rear focal plane (beam splitting plane) by the Fourier transform lens. Since the phase gradients of the rectangular light field corresponding to the vortex beams of different modes are different, the focusing positions of different modes at the rear focal plane are different.

[0037] The vortex beam reconstruction method of the present invention is theoretically explained as follows: the phase-compensated rectangular light field can be simply expressed as rect is the gate function, whose width is 2πa, l is the eigenvalue of the input vortex beam, and the strip light field after passing through the Fourier transform lens is When the light beam is blocked, the annular light field is defective, which leads to the truncation of the rectangular light field in the coordinate transformation, that is, the gate width of the gate function becomes smaller. According to the scale transformation property of the Fourier transform, the sinc function of the strip light field will be widened. Although the beam splitting surface light field is widened in the case of blocking, the central main maximum position is only related to the input OAM mode eigenvalue. Since the coordinate transformation structure can operate in both forward and reverse input, it is possible to perform a similar toe-cutting operation on the strip light field on the beam splitting surface, filter out the widened part of the sinc function through spatial filtering, and then reversely input the coordinate transformation structure. The output beam can basically reconstruct the wavefront of the blocked beam. Figure 5 This is a flow chart of the vortex beam reconstruction method of the present invention.

[0038] The de-ring device and phase compensation device of the present invention are prepared by processing liquid crystal polymer materials by ultraviolet laser direct writing. Based on geometric phase modulation, each liquid crystal pixel unit of the device can be regarded as a sub-half-wave plate function, and a phase difference of π can be introduced between the o-light and the e-light on the incident unit structure. By controlling the orientation angle α of the liquid crystal molecules, geometric phase modulation can be introduced. This modulation introduces -2α and 2α geometric phases to the left-handed and right-handed circularly polarized components of the incident light field, respectively. Since the designed de-ring device requires left-handed polarized light input, the phase compensation device also requires left-handed circularly polarized light input when the coordinate transformation system is input in reverse; and reflective programmable devices often use digital microlens arrays or liquid crystal spatial light modulators, wherein the liquid crystal spatial light modulator requires the input to be horizontal linear polarized light. In order to be compatible with vortex light beams of any polarization state, and considering the polarization state requirements of the above-mentioned key devices, a polarization control element group is introduced, and the design is as follows: Figure 1 The vortex beam reconstruction system shown in the figure transforms the polarization state of the input vortex beam into left-handed circular polarization through the polarizer 1 and the first quarter-wave plate 3 to meet the requirements of the liquid crystal device (de-circularization device 4); the left-handed circularly polarized light passing through the Fourier transform lens 6 is converted into a horizontal linear polarization state through the second quarter-wave plate 7; after being reflected by the reflective programmable device, it is still horizontally polarized light, and re-passes through the above structure, which meets the polarization state requirements for normal operation of the system; at the output end, the polarization state of the system output beam is horizontally polarized, and the interference light caused by the reflection of the device surface in the system is vertically polarized, so the polarization splitter prism 11 can be used to filter out the interference light.

[0039] Combined with the theoretical explanation of the vortex beam reconstruction method of the present invention, the spatial filter mask generation method loaded by the reflective programmable device is as follows: Figure 5As shown, the beam splitting surface broadening light field intensity signal output by the area array detector 10 is input into the computer 12. The computer calculates the main maximum position of the light field and then generates a spatial filtering mask through the beam splitting surface parameters when there is no obstruction vortex input or identifies the secondary maximum position, and loads it on the reflective programmable device. In order to eliminate the influence caused by the unfilled pixel part of the reflective programmable device, the system tilts the reflective programmable device (no more than 4° from the main optical axis) and loads a reference blazed grating on the device to achieve the overlap of the reflected light beam and the incident light beam.

[0040] The following briefly introduces a vortex beam reconstruction method and system of the present invention in combination with two embodiments.

[0041] Example 1: When the 4th-order single-mode vortex beam is angularly blocked by 30%, the reconstructed vortex beam is output

[0042] In this embodiment, the incident beam is a single-mode 4th-order vortex beam, which is input into the vortex beam reconstruction system of the present invention under the conditions of no obstruction and 30% angular obstruction, respectively. Figure 3 (b) and (f) show the angular occlusion situation. Figure 3 (f) The mask in the upper right corner (white represents unobstructed, black represents obstructed); the corresponding OAM spectra of the two are as follows Figure 3 As shown in (a) and (e), it can be seen that when the beam is blocked, its OAM spectrum is centered on its carrying mode and is broadened on both sides. When the vortex beams in these two cases are input into the coordinate transformation structure, the light field distribution at the phase compensation device is as follows: Figure 3 As shown in (c) and (g), the strip light field is truncated when the light is blocked compared to when it is not blocked. After the two light beams are further focused by the Fourier transform lens, the light intensity on the beam splitting surface is as follows: Figure 3 As shown in (d) and (h), the light field of the beam splitting surface with occlusion is broadened in the vertical direction compared with the unobstructed case. Figure 5 The filter mask is made in the following way: Figure 3 As shown in (l), it is loaded into a reflective programmable device, and the light beam is modulated by the reflective programmable device to obtain Figure 3 The light field shown in (k) has achieved spatial narrowing of the broadened light field; the narrowed light field is input into the coordinate transformation structure in reverse, and the output light field is as follows: Figure 3 As shown in (j), the unobstructed ring structure is basically restored, and the output light field OAM spectrum is as follows: Figure 3 (i) shows that the broadened mode is filtered out by the system. This embodiment verifies the theoretical explanation of the vortex beam reconstruction method proposed in the present invention.

[0043] Example 2: When a multimode vortex beam is blocked by a circular receiving aperture, a reconstructed vortex beam is output

[0044] In this embodiment, the incident light beam includes four OAM eigenmodes, namely -6, -3, +3, and +6, which are input into the vortex light beam reconstruction system of the present invention under the conditions of no shielding and shielding of the receiving aperture, respectively. Figure 4 (a) and (d) show that the circular aperture is Figure 4 (d) The upper left sub-image (white represents passing, black represents blocking); the corresponding OAM spectra of the two are as follows Figure 4 As shown in (c) and (f), it can be seen that when the beam is blocked, its OAM spectrum is significantly broadened, and it is difficult to identify the mode it originally carries. The vortex beams in these two cases are input into the coordinate transformation structure, and the intensity of the splitting surface is respectively as follows: Figure 4 As shown in (b) and (e), the light field of the beam splitting surface in the blocked state is greatly broadened in the vertical direction compared with the unblocked state. Figure 5 The filter mask is made in the following way: Figure 4 (h) As shown in the upper left sub-figure (the black part is filtered out, and the other parts are transmitted), it is loaded on the reflective programmable device, and the light beam is modulated by the reflective programmable device to obtain Figure 4 (h) shows the light field, which realizes the spatial narrowing of the broadened light field; the narrowed light field is input into the coordinate transformation structure in reverse, and the output light field is as follows: Figure 4 As shown in (g), the wavefront recovery of the blocked light field is achieved, and the OAM spectrum of the output light field is shown in Figure 4 As shown in (i), by comparing the OAM spectrum of the directly measured blocked light field, the output light field of the vortex beam reconstruction system of the present invention can be used to determine the original OAM eigenmode through its OAM spectrum. This embodiment verifies the applicability of the vortex beam reconstruction method proposed in the present invention to multimode OAM beams.

[0045] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A vortex beam coordinate transformation and reconstruction method, characterized in that: When a partially blocked vortex light beam is incident, the incident light beam is first pattern-split by the coordinate transformation structure of the present invention, the light intensity on the splitting surface is collected, its broadening condition is analyzed based on theory, the main maximum position of the light field is identified, a spatial filtering mask is generated and loaded into a reflective programmable device, the broadened split light field is compressed, and then the compressed light beam is transmitted in the reverse direction along the optical path of the coordinate transformation to obtain a vortex light beam with a reconstructed wavefront.

2. A vortex beam coordinate transformation and reconstruction system, characterized in that: The vortex beam reconstruction system of the present invention is composed of a coordinate transformation structure, a first depolarizing beam splitter prism, a polarization control element group, a reflective programmable device, and a receiving control component, wherein: The coordinate transformation structure has a de-circulation device, a phase compensation device, a Fourier transform lens and a depolarization beam splitter prism, among which: The de-ring device is used to transform the vortex light field from a polar coordinate system to a rectangular coordinate system, and to provide an additional focusing effect to achieve Fourier transform of the light field; The phase compensation device is placed in the laser light path behind the de-ring device, and is used to compensate for the extra phase introduced by the de-ring device so that the light emitted from the device is still collimated; The Fourier transform lens is used for Fourier transform of the light field to obtain a mode-splitting strip light field; A first depolarizing beam splitter prism is placed in the laser light path before the coordinate transformation structure, and is used to accommodate the incident blocked vortex beam and the outgoing reconstructed vortex beam; The polarization control element group is used to control the polarization state of the light beam so that the de-circulation device and the phase compensation device can realize the designed phase modulation, and has a polarizer, a first quarter wave plate, a second quarter wave plate, and a polarization beam splitter prism, wherein: The polarizer is placed in the laser light path in front of the depolarizing beam splitter prism, and is used to convert the incident vortex light beam into horizontal linear polarized light, so as to further convert the vortex light beam into left-handed circularly polarized light; The first quarter wave plate is placed in the laser light path between the polarizer and the de-circling device, and is used to convert the horizontal linearly polarized vortex beam into a left-handed circularly polarized vortex beam, so that the de-circling device can achieve correct phase modulation; The second quarter wave plate is placed in the laser light path behind the Fourier transform lens, and is used to convert the left-handed circularly polarized vortex beam into a horizontal linearly polarized vortex beam to meet the polarization state requirements of the subsequent reflective programmable device; The polarization beam splitter prism is placed in the laser light path behind the reflective end of the depolarization beam splitter prism to filter out stray light; The reflective programmable device may use a liquid crystal spatial light modulator or a digital microlens array, etc., and is placed in the laser light path behind the second quarter wave plate for spatial filtering of the broadened split beam light field. When the device uses a liquid crystal spatial light modulator, the device is placed at an angle of no more than 4° with the main optical axis; The receiving control component is used to receive the split beam field, generate a spatial filter mask by analyzing the broadening situation and load it into the reflective programmable device, and is equipped with a second depolarizing beam splitter prism, a planar array detector, and a computer, wherein: The second depolarizing beam splitter prism is placed in the laser light path between the second quarter wave plate and the reflective programmable device, and is used to reflect one path of laser light for observation and analysis without affecting the main laser light path; The planar array detector is placed in the laser light path behind the reflection path of the depolarizing beam splitter prism and is used to observe and analyze the distribution of the split light field; The computer is connected to the planar array detector and the reflective programmable device, and is used to generate a spatial filtering mask according to the light intensity information received by the planar array detector and load the mask onto the reflective programmable device.

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