Design Method of Off-Axis Cascaded Hyperstructure Surface Orbital Angular Momentum Demultiplexing Device

Abstract

This invention discloses a design method for an off-axis cascaded metasurface orbital angular momentum demultiplexing device, comprising an infrared laser, a reflective vortex beam generation module, a beam-shrinking module, a demultiplexing module, and a microscopy module arranged sequentially along the optical path. The demultiplexing module includes a first-stage metasurface and a second-stage metasurface, which are an off-axis system. The phase distribution of the second phase correction element is optimized using a method of reverse calculation of the actual diffraction field. The off-axis design eliminates stray light generated by the less than 100% modulation efficiency of the metasurface, greatly reducing the volume of the orbital angular momentum demultiplexing system. The stray light can be guided to a fixed region without affecting the performance of the cascaded device.

Description

Technical Field

[0001] This invention relates to the field of micro-nano optics technology, and in particular to a design method for off-axis cascaded metasurface orbital angular momentum demultiplexing devices. Background Technology

[0002] Orbital angular momentum (OAM) has attracted widespread attention in high-speed optical communication and quantum communication in recent years due to its spatial orthogonality. A beam carrying OAM satisfies the spatial phase distribution exp(ilθ), where the topological charge l can take unbounded integer values. Therefore, theoretically, different OAM modes can provide an infinite number of channels, which can greatly increase communication capacity. Thus, demultiplexing and mode detection of vortex beams are crucial for their application. Early OAM beam mode detection methods employed interferometry and geometrical optical transformation. Interferometry mainly utilizes the interference between the OAM beam and other beams, then determines the OAM mode of the incident light through the characteristic distribution of the interferogram. However, it can only measure the mode of one topological charge of the OAM beam at a time. To design the separation of N modes, N-1 cascaded interferometers and corresponding beam splitters are required, leading to system complexity and increased error. This method has been validated in high-speed optical communication. However, it suffers from a fatal problem in terms of efficiency: efficiency is inversely proportional to the amount of topological charge. To address this issue, geometrical optical transformation is employed to detect OAM modes. This involves using a special phase plate to map the field of the incident OAM beam between different coordinate systems, and another phase plate acting as a phase corrector to correct the tilted wavefront and focus different OAM modes to different lateral positions in space. Theoretically, the efficiency of generating or separating beams of different OAM modes is close to 100%, offering advantages such as high demodulation efficiency and simple setup, making it a promising approach in current demultiplexing schemes. Geometrical optical transformation methods can be categorized into traditional logarithmic polar coordinate transformation and high-resolution spiral transformation based on their mapping methods. Traditional logarithmic polar coordinate transformation separates different OAM modes by mapping the angular concentric circle phase to the lateral gradient phase and focusing them to different lateral positions, but this can lead to partial overlap between adjacent topological charge modes after separation. Spiral transformation, on the other hand, unfolds the angular spiral phase into the lateral gradient phase along a spiral path, achieving mode replication and thus enabling high-resolution demultiplexing, making it more suitable for CCD collection and detection of the output beam spot.

[0003] Due to design and fabrication precision limitations, current metasurfaces cannot achieve the same high efficiency as traditional optical mirrors in the short term. Stray light is unavoidable in practical applications. While the most efficient metasurface designs currently in research can achieve some simple focusing and imaging, they cannot perform complex imaging and therefore cannot be widely applied. Meanwhile, the properties of individual metasurfaces have been extensively studied. Since complex systems cannot be constructed from single devices, cascaded metasurfaces have become an inevitable future trend. In conventional coaxial metasurface cascade systems, the modulation efficiency is limited. A portion of the light passing through the metasurface is modulated into the target beam, while the remaining portion is undemonicated as background noise. This leads to increased stray light in the final output signal and reduced demultiplexing efficiency. The most significant problem with existing technologies is that in miniature demultiplexed systems based on orbital angular momentum, enhanced diffraction effects increase the original phase distribution error. The limited efficiency of coaxial cascaded metasurfaces significantly impacts the cascaded system, and currently, no off-axis cascaded metasurface technology has been found. Summary of the Invention

[0004] To address the aforementioned issues, this invention provides a design method for an off-axis cascaded hyperstructure surface orbital angular momentum demultiplexer, which has the advantages of correcting phase errors, reducing output stray spots, further miniaturizing phase errors in integrated and miniaturized systems, and reducing the size of the OAM demultiplexer.

[0005] The technical solution of this invention is:

[0006] A design method for an off-axis cascaded metasurface orbital angular momentum demultiplexing device includes an infrared laser, a reflective vortex beam generation module, a beam shrinking module, a demultiplexing module, and a microscopy module arranged sequentially along the optical path. The demultiplexing module includes a first-level metasurface and a second-level metasurface, which are an off-axis system.

[0007] The formula for the phase distribution of the first-order metasurface is:

[0008]

[0009]

[0010] Phase represents the off-axis amount introduced by the off-axis cascaded metasurface; α, β, and θ are proportional transformation parameters; r0 is the starting point of the helical transformation spiral; λ is the working wavelength; and f is the focal length of the lens that realizes the Fourier transform, which is also the lateral distance between the two metasurfaces.

[0011] The formula for the phase distribution of the second-order metasurface is:

[0012]

[0013] φ PC (u,v)=2π-arctan(Im{U(u,v)} / Re{U(u,v})-Phase

[0014] Among them FT and FT -1 The mapping relationship between rectangular coordinates (x, y) and logarithmic polar coordinates (u, v) for Fourier transform and inverse Fourier transform is as follows:

[0015] .

[0016] The second-level metasurface is mounted on a three-dimensional platform.

[0017] The off-axis angle of the first-level metasurface and the second-level metasurface is 1 to 4°.

[0018] The reflective vortex beam generating module includes a first plano-convex lens, a spatial light modulator, and a reflector arranged sequentially along the optical path.

[0019] The beam-shrinking module includes a second plano-convex lens and a first objective lens arranged sequentially along the optical path.

[0020] The beam-shrinking module is used to match the waist radius of the incident vortex beam after beam shrinkage with the cross-sectional area of ​​the metasurface.

[0021] The microscope module includes a second objective lens, a third plano-convex lens, and an inductive coupling element arranged sequentially along the direction of the optical path.

[0022] The inductively coupled element is a CCD.

[0023] The beneficial effects of this invention are:

[0024] This paper describes a method for achieving the phase required by a spiral transformation-based orbital angular momentum demultiplexing system using metasurfaces. This method is easily integrated and miniaturized. The phase distribution of the phase corrector in the spiral transformation is obtained by reverse calculation using the diffraction propagation formula. Traditional phase correction methods are based on the propagation of light along straight lines. Therefore, the phase distribution error based on the geometric optics phase formula is corrected by reverse calculation of the actual diffraction field, resulting in fewer stray spots in the final output. This can further reduce the phase error in integrated and miniaturized systems and reduce the size of the OAM demultiplexer. For the first time, an off-axis cascaded metasurface system is combined with an orbital angular momentum demultiplexing system to guide stray light into a fixed region, realizing a highly efficient orbital angular momentum demultiplexing technology using off-axis cascaded metasurfaces. Attached Figure Description

[0025] Figure 1This is a schematic diagram of the overall structure of the design method for the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device described in this embodiment of the invention;

[0026] Figure 2 This is a schematic diagram of the demultiplexing module structure of the design method for the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device according to an embodiment of the present invention;

[0027] Figure 3 This is a schematic diagram illustrating the concept of geometric optical coordinate mapping as described in an embodiment of the present invention;

[0028] Figure 4 This is a schematic diagram of the spiral transformation described in an embodiment of the present invention;

[0029] Figure 5 This is a schematic diagram illustrating the change of the spiral phase unfolding into a transverse gradient phase according to an embodiment of the present invention;

[0030] Figure 6 This is a partial schematic diagram of the metasurface unit structure and metasurface device described in the embodiments of the present invention;

[0031] Figure 7 This refers to the signal-to-noise ratio of adding different off-axis angles under different polarization conversion rates as described in the embodiments of the present invention;

[0032] Figure 8 It is a focal spot pattern of different topological charge modes output under the traditional geometric optics phase correction method;

[0033] Figure 9 This is a diagram showing the stray light ratio of different topological charge modes and their adjacent topological charge modes to the target mode under the traditional geometric optics phase correction method.

[0034] Figure 10 This is the output focal spot diagram of different topological charge modes under the actual diffraction field reverse calculation correction method adopted in the embodiments of the present invention;

[0035] Figure 11 This is a stray light ratio diagram of different topological charge modes and their adjacent topological charge modes relative to the target mode under the actual diffraction field reverse calculation correction method adopted in the embodiments of the present invention.

[0036] Explanation of reference numerals in the attached figures:

[0037] 1 is an infrared laser, 2 is a reflective vortex beam generation module, 3 is a beam shrinking module, 4 is a demultiplexing module, 5 is a microscopy module, 21 is a first plano-convex lens, 22 is a spatial light modulator, 23 is a reflector, 31 is a second plano-convex lens, 32 is a first objective lens, 41 is a first-level metasurface, 42 is a second-level metasurface, 51 is a second objective lens, 52 is a third plano-convex lens, and 53 is an inductively coupled element. Detailed Implementation

[0038] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0039] Example:

[0040] like Figures 1-3 As shown, the design method of the off-axis cascaded metasurface orbital angular momentum demultiplexing device includes an infrared laser 1, a reflective vortex beam generation module 2, a beam shrinking module 3, a demultiplexing module 4 and a microscopy module 5 arranged sequentially along the optical path. The demultiplexing module 4 includes a first-level metasurface 41 and a second-level metasurface 42, and the first-level metasurface 41 and the second-level metasurface 42 are an off-axis system.

[0041] The demultiplexing module 4 consists of two levels of metasurfaces. After the first-level metasurface 41 is aligned with the beam-constricting light source, the second-level metasurface 42 is mounted on a three-dimensional platform for alignment with the designed off-axis angle between the second and first-level metasurfaces. Alignment is achieved through XY movement of the three-dimensional platform, and Z-axis adjustment further eliminates the influence of calculation errors and processing errors on the substrate refractive index.

[0042] Geometric optics transformation maps the angular gradient phase of vortex light to a transverse gradient phase through coordinate transformation. The first-stage unwrapper maps the angular gradient phase to a transverse phase with a tilted wavefront, and the second-stage phase corrector corrects the tilted wavefront into a plane wave. The coordinate mapping concept is as follows: Figure 3 As shown, the phase distribution formula for the first-order metasurface 41 is:

[0043]

[0044]

[0045] Phase represents the off-axis amount introduced by the off-axis cascaded metasurface; α, β, and θ are proportional transformation parameters; r0 is the starting point of the helical transformation spiral; λ is the working wavelength; and f is the focal length of the lens that realizes the Fourier transform, which is also the lateral distance between the two metasurfaces.

[0046] The formula for the 42-phase distribution of the second-level metasurface is:

[0047]

[0048] φ PC (u,v)=2π-arctan(Im{U(u,v)} / Re{U(u,v)})-Phase

[0049] Among them FT and FT -1The mapping relationship between rectangular coordinates (x, y) and logarithmic polar coordinates (u, v) for Fourier transform and inverse Fourier transform is as follows:

[0050] .

[0051] The phase distribution of the second metasurface 42 is calculated based on the Rayleigh-Sommerfeld diffraction formula. The phase distribution of the plane wave is calculated in reverse after it is mediated by the first metasurface 41 and propagates to the Fourier plane.

[0052] The off-axis angle of the first-level metasurface 41 and the second-level metasurface 42 is 3°.

[0053] The reflective vortex beam generation module 2 includes a first plano-convex lens 21, a spatial light modulator 22, and a reflector 23 arranged sequentially along the optical path. The output light is modulated by the spatial light modulator to generate a vortex beam. By changing the phase diagram loaded on the spatial light modulator, vortex beams with different topological kernel numbers can be generated in real time.

[0054] The beam-shrinking module 3 includes a second plano-convex lens 31 and a first objective lens 32 arranged sequentially along the optical path direction. It is used to match the waist radius of the incident vortex beam after beam shrinkage with the cross-sectional area of ​​the metasurface, and to align the first-level metasurface with the incident light through a three-dimensional platform.

[0055] The microscopic module 5 includes a second objective lens 51, a third plano-convex lens 52 and an inductive coupling element 53 arranged sequentially along the optical path. After being aligned with the output beam of the second-stage metasurface 42, the beam is magnified and used by the inductive coupling element 53 to acquire the output image.

[0056] The inductive coupling element 53 is a CCD.

[0057] like Figure 4 , Figure 5 As shown, the geometric optical transformation method described in this embodiment is a spiral transformation to achieve high-resolution orbital angular momentum demultiplexing. Through this transformation, the input vortex light with angular phase is unfolded into a transverse gradient phase along the spiral path, achieving the effect of mode replication and obtaining a narrower intensity peak output spot, thereby achieving high-resolution demultiplexing and reducing crosstalk between adjacent topological charge modes.

[0058] like Figure 6 As shown, the metasurface described in this embodiment employs a rectangular dielectric nanopillar unit structure. This structure can achieve phase abrupt changes in light waves by simply altering the rotation angle of the micro / nano structure, thereby enabling artificial control of the phase distribution and significantly reducing the complexity of designing and fabricating the metasurface. At infrared wavelengths, the metasurface unit structure dimensions are set on the micrometer scale, the structural material is Si, and it is fabricated using laser direct-write lithography.

[0059] like Figure 7 As shown, in this embodiment, after adding different off-axis angles to the metasurface at different polarization conversion rates, it can be observed that the signal-to-noise ratio is significantly improved when the off-axis angle is between 1 and 4°. Through optimization of the metasurface unit structure, the metasurface adopts a rectangular dielectric nanopillar unit structure with a unit period S = 0.9 μm, height H = 1.2 μm, length L = 0.52 μm, and width W = 0.11 μm. At this point, the polarization conversion rate reaches over 90%, and the transmittance T in the cross direction of the unit structure is also high. X =95%, T y =98%.

[0060] like Figure 8 , Figure 9 As shown, in the final output light field diagram of the traditional geometric optics phase correction method, the noise of the adjacent topological charge mode to the target mode reaches 40%, and the light spot focusing efficiency is poor with a lot of stray spots.

[0061] like Figure 10 , Figure 11 As shown, in the final output optical field map of the actual diffraction field reverse calculation correction method used in this embodiment, the noise ratio of adjacent topological charge modes to the target mode is reduced to 10%, and the light spot focusing efficiency is good with only a small number of stray spots.

[0062] The embodiments described above are merely illustrative of specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A design method for an off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device, characterized in that, The system includes an infrared laser, a reflective vortex beam generation module, a beam shrinking module, a demultiplexing module, and a microscopy module, which are arranged sequentially along the optical path. The demultiplexing module includes a first-level metasurface and a second-level metasurface, which are an off-axis system. The formula for the phase distribution of the first-order metasurface is: Phase represents the off-axis amount introduced by the off-axis cascaded metasurface; α, β, and θ are proportional transformation parameters; r0 is the starting point of the helical transformation spiral; λ is the working wavelength; and f is the focal length of the lens that realizes the Fourier transform, which is also the lateral distance between the two metasurfaces. The formula for the phase distribution of the second-order metasurface is: φ PC (u,v)=2π-arctan(Im{U(u,v)} / Re{U(u,v)})-Phase Among them FT and FT -1 The mapping relationship between rectangular coordinates (x, y) and logarithmic polar coordinates (u, v) for Fourier transform and inverse Fourier transform is as follows: 。 2. The design method of the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device according to claim 1, characterized in that, The second-level metasurface is mounted on a three-dimensional platform.

3. The design method of the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device according to claim 1, characterized in that, The off-axis angle of the first-level metasurface and the second-level metasurface is 1 to 4°.

4. The design method of the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device according to claim 1, characterized in that, The reflective vortex beam generating module includes a first plano-convex lens, a spatial light modulator, and a reflector arranged sequentially along the optical path.

5. The design method of the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device according to claim 1, characterized in that, The beam-shrinking module includes a second plano-convex lens and a first objective lens arranged sequentially along the optical path.

6. The design method of the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device according to claim 1, characterized in that, The beam-shrinking module is used to match the waist radius of the incident vortex beam after beam shrinkage with the cross-sectional area of ​​the metasurface.

7. The design method of the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device according to claim 1, characterized in that, The microscope module includes a second objective lens, a third plano-convex lens, and an inductive coupling element arranged sequentially along the direction of the optical path.

8. The design method of the off-axis cascaded hyperstructure surface orbital angular momentum demultiplexing device according to claim 7, characterized in that, The inductive coupling element is a CCD.

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