All-fiber achromatic perfect vortex beam generating device

Abstract

The invention provides an all-fiber achromatic perfect vortex beam generating device. The device is characterized by comprising a single-mode fiber (1), a coreless fiber (2) and an achromatic metasurface structure (3). The coreless optical fiber (2) is welded on the single-mode optical fiber (1) and plays a role in beam expansion of a transmission light field in a fiber core of the single-mode optical fiber. The achromatic metasurface structure (3) is arranged on the end face of the coreless optical fiber (2) and is composed of rectangular nano-columns of different sizes, and the orientation angles and the sizes of the rectangular nano-columns at different positions are designed according to phase modulation and chromatic aberration compensation needed by an emergent light field of the end face of the coreless optical fiber (2). When circularly polarized light with different wavelengths is coupled and incident into the device, the circularly polarized light is expanded by the coreless optical fiber (2) and then is irradiated on the achromatic metasurface structure (3), and achromatic perfect vortex light beams with the same ring radius can be generated at the same position away from the end face of the optical fiber. The invention belongs to the technical field of optical fiber microstructure devices.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber microstructure device technology, specifically involving the generation of achromatic perfect vortex beams by designing and fabricating achromatic metasurface structures on the end face of optical fibers and coupling in circularly polarized light. Background Technology

[0002] As a novel means of optical field manipulation, fiber-optic combined metasurface structures have advantages such as high integration, strong anti-interference ability, and efficient optical transmission. Although optical field manipulation based on fiber-optic integrated metasurfaces has received widespread attention, there are still many unresolved issues regarding the effective generation of optical fields in fiber-optic achromatic structures, which urgently need to be addressed.

[0003] Vortex beams, as structured beams carrying orbital angular momentum, possess a phase-dependent Hilbert factor. After long-term development, vortex beams have been extensively studied and applied in various optical fields, such as optical imaging, quantum information communication, wireless optical communication, and optical tweezers. However, the beam radius of traditional vortex beams increases with the increase of topological charge, significantly increasing the difficulty of practical applications and limiting their applicability. To overcome this problem, Ostrovsky et al. proposed the concept of perfect vortex beams (PVBs), whose annular intensity diameter remains unchanged regardless of the topological charge [Optics letters 39.18 (2014): 5305-5308]. Vaity et al. further theoretically proved that perfect vortex beams can be obtained through the Fourier transform of Bessel-Gaussian beams [Optics letters 40.4 (2015): 597-600]. Today, perfect vortex beams are widely used in research fields such as wireless optical communication, light trapping, and optical imaging. However, traditional perfect vortex beams are still limited by the incident light wavelength. Due to the dispersive properties of traditional optical devices, the ring radius of the generated perfect vortex beam will still change when the incident light wavelength changes. This is not conducive to applications that require the use of different light wavelengths, such as wavelength division multiplexing (WDM) communication.

[0004] Building upon the aforementioned background technology, this invention successfully generates a perfect achromatic vortex beam with an achromatic ring radius that does not change with the incident wavelength by designing an achromatic metasurface structure at the fiber end. This overcomes the shortcomings of existing perfect vortex beams in practical applications. The structure designed in this invention not only possesses excellent achromatic capability but also features high integration density, small size, and high flexibility, making it plug-and-play. It provides new ideas for the design of all-fiber metasurface structure devices, light-matter interactions, and achromatic processing. Summary of the Invention

[0005] This invention provides an all-fiber achromatic perfect vortex beam generating device.

[0006] The all-fiber achromatic perfect vortex beam generating device provided by this invention is implemented as follows:

[0007] The aforementioned all-fiber achromatic perfect vortex beam generating device, such as Figure 1 As shown, it consists of a single-mode fiber 1, a coreless fiber 2, and an achromatic metasurface structure 3. The coreless fiber 2 is fused to the single-mode fiber 1, serving to expand the transmitted light field in the core of the single-mode fiber 1. The achromatic metasurface structure 3 is located at the end face of the coreless fiber 2. When circularly polarized light is incident, the transmitted light field expanded by the coreless fiber 2 illuminates the achromatic metasurface structure 3. The achromatic metasurface structure 3 performs phase modulation and chromatic aberration compensation on the incident light field, realizing the output of a perfect achromatic vortex beam at the fiber end.

[0008] The specific design principle of the achromatic metasurface structure 3 is as follows: to generate a perfect vortex beam in the experiment, firstly, a spiral phase plate is used to convert the Gaussian beam into a higher-order Laguerre-Gaussian (LG) beam; then, an axicon mirror is used to convert the LG beam into a corresponding Bessel-Gaussian (BG) beam; finally, a Fourier lens is used to convert the BG beam into a perfect vortex beam. Therefore, to design the achromatic metasurface structure 3 to generate an achromatic perfect vortex beam, the achromatic metasurface structure 3 must have a full-phase profile of the spiral phase plate, the axicon, and the Fourier transform lens, that is:

[0009] (1)

[0010] in, , , in the formula , These are coordinates in a cylindrical coordinate system with the center of the fiber end face as the origin. λ is the wavelength of the incident light. Represents the phase distribution of the axial pyramid, where The angle of deflection of the incident light after passing through the prism. Indicates the spiral phase. This represents the phase distribution of the Fourier lens. is the focal length of the Fourier lens.

[0011] In experiments, the input Gaussian beam is typically assumed to have a planar phase distribution. However, unlike this, in our all-fiber achromatic perfect vortex beam generator, the output light field of single-mode fiber 1, after being expanded by coreless fiber 2, exhibits a non-planar phase. Considering the refractive index of coreless fiber 2... The Gaussian mode in single-mode fiber 1 is ,in It is a constant. Let be the mode field radius of the Gaussian mode in single-mode fiber 1. Then, the light field reaching the output end face of the coreless fiber 2 after beam expansion, i.e., illuminating the achromatic metasurface structure 3, is:

[0012] (2)

[0013] in , , and H represents the length of the fusion spliced ​​coreless fiber. For simplicity, the transmission phase and Gouy phase are omitted here because these two phases have a uniform spatial distribution and do not affect the overall transverse phase distribution of the optical field.

[0014] Therefore, designing an achromatic metasurface structure 3 to generate a perfect vortex beam requires adding an additional phase distribution function to compensate for the phase distribution caused by the fiber beam expansion. Thus, achromatic perfect vortex light is generated at the fiber end, and the phase distribution required for achromatic metasurface structure 3 becomes:

[0015] (3)

[0016] The following section further elaborates on how the achromatic metasurface structure 3 is designed. To design the achromatic metasurface structure 3 capable of generating perfectly achromatic vortex beams, we first select a specific wavelength. Then, according to formula (3), the phase distribution required for this wavelength is designed using geometric phase. The geometric phase is determined solely by the orientation of the artificial atoms on the metasurface and is independent of wavelength; therefore, if the metasurface only encodes the geometric phase distribution... Arbitrary wavelength All beams of light passing through this metasurface will gain phase. The phase distribution in formula (3) cannot be satisfied. The requirement for wavelength variation results in chromatic aberration in the final fiber-tipped perfect vortex beam. Therefore, to achieve achromatic aberration, it is necessary to utilize the dispersive properties of artificial atomic transport phase to construct a wavelength-varying phase distribution. As compensation, it satisfies:

[0017] (4)

[0018] In this invention, such as Figure 2The diagram shows a schematic of the rectangular nanopillar structure that makes up the achromatic metasurface structure 3. Titanium dioxide was chosen to construct the rectangular nanopillars with a height of T = 4.5 μm, a length of L (120 nm ≤ L ≤ 330 nm), a width of W (90 nm ≤ W ≤ 180 nm), and a period of P (P = 350 nm). These nanopillars form a single-unit structure. Using FDTD simulation software, we performed full-wavelength simulations for different orientation angles within the achromatic wavelength range of 580 nm to 690 nm. The geometric phase and transport phase of rectangular nanopillars of different sizes are studied. By selecting nanopillars of different sizes and placing them at different locations within an achromatic metasurface with different orientation angles, phase modulation is constructed based on their geometric phase. By setting rectangular nanopillars of different sizes, their transport phases can be used to construct chromatic aberration compensation phases. Thus, the phase required for the achromatic metasurface structure 3 is achieved. This makes the output light field of the fiber end face a perfect achromatic vortex beam.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] 1. The present invention proposes an all-fiber achromatic perfect vortex beam generating device, which has all the advantages of fiber photonic devices.

[0021] 2. The all-fiber achromatic perfect vortex beam generating device proposed in this invention uses ordinary commercial single-mode fiber and coreless fiber, without the need to draw special fiber separately, thus effectively controlling costs.

[0022] 3. The all-fiber achromatic perfect vortex beam generating device proposed in this invention can generate the required achromatic perfect vortex beam by designing an achromatic metasurface. Moreover, the ring radius of the achromatic perfect vortex beam generated within the designed achromatic wavelength range does not change with the incident wavelength, which has extremely high design flexibility. It makes up for the shortcomings of existing perfect vortex beams in practical applications and is conducive to meeting diverse application needs. Attached Figure Description

[0023] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the structure of an all-fiber achromatic perfect vortex beam generating device provided in an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram of the rectangular nanopillar structure that forms the achromatic metasurface provided in the embodiments of the present invention.

[0026] Figure 3 This invention provides 49 different sizes of titanium dioxide rectangular nanopillars and the transport phases they generate.

[0027] Figure 4 This is a diagram showing the intensity distribution of the emitted light field above the fiber end face provided in an embodiment of the present invention. Figures (a)-(c) are respectively... The output light field intensity distribution in the XZ plane is shown in Figures (d)-(f). Figures (g)-(i) show the output light field intensity distribution in the focal plane at Z=80μm above the fiber end, respectively.

[0028] Figure 5 Figures (a)-(c) show the intensity distribution of the emitted light field above the fiber end face provided in this embodiment of the invention. Figures (a)-(c) show the intensity distribution of the emitted light field (i.e., the perfect vortex beam) generated when incident with right-handed circularly polarized light at wavelengths λ=580nm, 635nm, and 690nm, respectively, in the XZ plane. Figures (d)-(f) show the intensity distribution of the emitted light field in the focal plane at Z=80μm above the fiber end, respectively. Figure (g) shows the normalized intensity distribution of the emitted light field along the X-axis in the focal plane at Z=80μm above the fiber end.

[0029] Figure 6 This is a diagram showing the intensity distribution of the emitted light field above the fiber end face provided in an embodiment of the present invention. Figures (a)-(c) show the intensity distribution of the emitted light field (i.e., achromatic perfect vortex beam) generated when incident with right-hand circularly polarized light at wavelengths λ=580nm, 635nm, and 690nm, respectively, in the XZ plane. Figures (d)-(f) show the intensity distribution of the emitted light field in the focal plane at Z=80μm above the fiber end, respectively. Figure (g) shows the normalized intensity distribution of the emitted light field along the X-axis in the focal plane at Z=80μm above the fiber end. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other instances obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] A schematic diagram of the all-fiber achromatic perfect vortex beam generator proposed in this invention is shown below. Figure 1 It consists of a single-mode fiber 1, a coreless fiber 2, and an achromatic metasurface structure 3. In this embodiment, commercially available visible single-mode fiber and commercially available coreless fiber are selected, along with an achromatic metasurface structure constructed from titanium dioxide rectangular nanopillars of the same height but different lengths and widths.

[0032] In this embodiment, the achromatic wavelength range is designed to be 580nm-690nm. Circularly polarized light with wavelengths of λ=580nm, 635nm, and 690nm is selected and coupled into the all-fiber device. The mode field radius of the transmitted optical field in the visible single-mode fiber is... The length of coreless optical fiber ,focal length Parameters of the axial pyramid The achromatic metasurface structure has a radius of 17 μm. The orientation angles of the titanium dioxide rectangular nanopillars at different locations in the achromatic metasurface structure are... ,like Figure 3 This study investigated 49 different sizes of titanium dioxide rectangular nanopillars and their transport phases. Titanium dioxide rectangular nanopillars with different lengths and widths were placed at different locations to ensure their transport phases met certain conditions. Ultimately, the desired achromatic metasurface structure was constructed. For example... Figure 4 The image shows right-handed circularly polarized light with an incident wavelength of λ = 690 nm. The achromatic metasurface generates a topological charge. When a perfect vortex beam is formed, the intensity distribution of the output beam at the fiber end in the XZ plane. Figure 4 (d)-(f) correspond to the intensity distribution of the beam in the focal plane at Z=80μm above the fiber end, respectively. Figure 4 (g)-(i) represent the phase distribution of the corresponding beam in the Z=80μm plane. It can be seen that the intensity loop radius at the same position above the fiber end is unaffected by topological charge, verifying that the generated beam is a perfect vortex beam.

[0033] Subsequently, we further simulated and verified the device's achromatic aberration capability. For example... Figure 5 The image shows a metasurface constructed using only single-sized rectangular titanium dioxide nanopillars (L=150nm, W=129nm) without considering color difference compensation. Required geometric phase Simulation results of perfect vortex beams generated when right-handed circularly polarized light with incident wavelengths λ=580nm, 635nm, and 690nm are presented. It can be seen that as the incident wavelength changes, the ring radius of the perfect vortex beam at the same position above the fiber end increases with increasing incident wavelength, indicating a significant chromatic aberration. Figure 6To account for chromatic aberration compensation, achromatic metasurface structures were constructed using rectangular titanium dioxide nanopillars of different sizes. Simulation results were then obtained for the generation of perfect vortex beams when right-handed circularly polarized light with incident wavelengths λ=580nm, 635nm, and 690nm. It can be seen that the ring radius of the perfect vortex beams generated at different wavelengths on the focal plane remains essentially unchanged, verifying the achromatic characteristics of the device.

[0034] The above description discloses only one preferred embodiment of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. A fully fiber-optic achromatic perfect vortex beam generating device, characterized in that: It consists of a single-mode fiber (1), a coreless fiber (2), and an achromatic metasurface structure (3); the coreless fiber (2) is fused to the single-mode fiber (1) and serves to expand the transmitted light field in the core of the single-mode fiber (1); the achromatic metasurface structure (3) is located on the end face of the coreless fiber (2); the achromatic metasurface structure (3) is composed of rectangular nanopillars with different lengths, widths, and heights; when circularly polarized light of different wavelengths is coupled into the device, the achromatic metasurface structure (3) performs phase modulation and chromatic aberration compensation on the light field emitted from the end face of the coreless fiber (2), thereby realizing the output of an achromatic perfect vortex beam at the fiber end.

2. The all-fiber achromatic perfect vortex beam generating device according to claim 1, characterized in that: The achromatic metasurface structure (3) is designed by using the geometric phase design principle to determine the orientation angle of the rectangular nanopillars at different positions, so as to achieve the phase modulation required for the maximum wavelength in the achromatic wavelength range.

3. The all-fiber achromatic perfect vortex beam generating device according to claim 1, characterized in that: The achromatic metasurface structure (3) utilizes rectangular nanopillars of different sizes to achieve different transmission phases. By designing the size of the rectangular nanopillars at different positions, the chromatic aberration compensation phase required in the achromatic wavelength range can be realized.

4. The all-fiber achromatic perfect vortex beam generating device according to claim 1, characterized in that: The achromatic metasurface structure (3) can be composed of a rectangular nanopillar array of all-dielectric material, which can be titanium dioxide, gallium nitride or silicon nitride.

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