Optical waveguide-based vortex beam generator and method of manufacturing the same

Abstract

This invention discloses a vortex beam generator based on an optical waveguide and its fabrication method, belonging to the field of integrated photonics device fabrication technology. A pretreated lithium yttrium fluoride crystal is used as the substrate material; helical traces are written into the interior of the lithium yttrium fluoride crystal to form a helical cladding structure, obtaining a single-mode optical waveguide; etching is performed on the surface of the lithium yttrium fluoride crystal corresponding to the emitting end of the single-mode optical waveguide to obtain a helical zone plate, thus completing the fabrication of the vortex beam generator. Compared with traditional vortex beam generation methods, this invention integrates the single-mode optical waveguide and the helical zone plate into the same crystal, resulting in a small device size, high integration density, and strong stability. The fabrication method has wide applicability and potential application value in the field of integrated photonics.

Description

Technical Field

[0001] This invention relates to the field of integrated photonic device fabrication technology, and in particular to a vortex beam generator based on an optical waveguide and its fabrication method. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] A vortex beam exhibits a helical phase distribution and a ring-shaped intensity distribution, with zero intensity at its center. From the observation plane, the beam center appears as a dark nucleus, a dark center, due to the singularity of the phase distribution at the center. This phase uncertainty at the center causes coherent destructive phase shifts in the optical field, resulting in zero intensity amplitude at the center of the vortex beam. Because of its helical phase wavefront and intrinsic orbital angular momentum, the vortex beam shows great potential for applications in terabitum-level data transmission, multidimensional coding in quantum computing, and optical micromanipulation. In recent years, vortex beams have been widely used in particle manipulation, super-resolution microscopy, high-capacity communication, biomedical research, and astronomical observation.

[0004] In recent decades, methods for generating vortex beams based on different optical theories have been proposed, including geometric methods, spiral phase plates, spiral zone plates, spatial light modulators, Q-plates, metasurfaces, and hybrid optical elements. These methods have greatly expanded the theoretical and practical applications of vortex beams. For example, J. Arlt et al. generated vortex beams using geometric methods [Opt. Commun. 177, 297-301 (2000)]; W. M. Lee et al. and E. Brasslet et al. generated vortex beams using spiral phase plates [Opt. Lett. 29, 1796-1798 (2004)][Appl. Phys. Lett. 97, 211108 (2010)]; Z. Tian et al. generated vortex beams using spiral zone plates [IEEE Photonics]. Technol. Lett. 28, 2299-2302 (2016)]; G. Gibson et al., NRHeckenberg et al., and J. Arlt et al. generated vortex beams using spatial light modulators [Opt. Express 12, 5448-5456 (2004)][Opt. Lett. 17, 221-223 (1992)][J. Mod. Opt. 45, 1231-1237 (1998)]; X. Wang et al. generated vortex beams using Q-plates. Vortex beams were generated [Appl. Phys. Lett. 110, 181101 (2017)]; S. Shrestha et al., M. Papaioannou et al. and S. Bajt et al. generated vortex beams using metasurfaces and hybrid optical elements [Light-Sci. Appl. 7, 85 (2018)][Light-Sci. Appl. 7, 17157 (2018)][Light-Sci. Appl. 7, 17162 (2018)].

[0005] An optical waveguide is a region of high-refractive-index medium surrounded by a low-refractive-index medium. Due to the principle of total internal reflection, optical waveguides can confine light waves to micrometer or even nanometer scales, thereby controlling their transmission direction. Optical waveguides have the advantages of high integration and high stability, and are the basic components of integrated optical circuits. They can realize the integration of different waveguide structures on the same substrate material, or the integration of waveguides with other optical components and structures, to obtain powerful integrated photonic chips, integrated optorheological chips, etc.

[0006] However, the inventors discovered that existing vortex beam generators based on optical waveguides suffer from drawbacks such as large size, low integration, and poor stability. Furthermore, traditional vortex beam generation methods are highly demanding in terms of environmental requirements and lack controllability; for example, they must be performed in a cleanroom environment, otherwise the optical performance of the waveguide will be affected. Therefore, how to fabricate a stable and easily implementable miniature vortex beam generator has become one of the urgent problems to be solved in the current technology. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a vortex beam generator based on optical waveguides and its fabrication method. By combining optical waveguides and helical zone plates, the device achieves high integration and has wide applications in integrated optical circuits. Compared with traditional vortex beam generation methods, it exhibits higher stability and controllability.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0009] The first aspect of this invention provides a method for fabricating a vortex beam generator based on an optical waveguide, comprising the following steps:

[0010] Pretreated lithium yttrium fluoride crystals were used as the substrate material.

[0011] A single-mode optical waveguide is obtained by writing helical traces capable of forming a helical cladding structure inside the lithium yttrium fluoride crystal.

[0012] The surface of the lithium yttrium fluoride crystal corresponding to the single-mode optical waveguide emission end is etched to obtain a spiral zone plate, thus completing the fabrication of the vortex beam generator.

[0013] Preferably, the pretreatment process is as follows: the lithium yttrium fluoride crystal is cut into six sides, and then the six sides of the lithium yttrium fluoride crystal are optically polished and cleaned.

[0014] Preferably, the writing method is femtosecond laser writing.

[0015] Preferably, the etching method is focused ion beam etching.

[0016] Preferably, the specific steps of the writing process are as follows: focusing a femtosecond laser onto the bottom surface of a lithium yttrium fluoride crystal, performing laser writing from the bottom surface in a spiral trajectory, and stopping the burning at a predetermined distance from the top surface of the lithium yttrium fluoride crystal, thereby forming a spiral cladding structure inside the lithium yttrium fluoride crystal. 。

[0017] Preferably, the direction of the helical cladding structure is perpendicular to the bottom surface of the lithium yttrium fluoride crystal.

[0018] Preferably, the specific etching steps are as follows: focusing the ion beam onto the corresponding yttrium lithium fluoride crystal surface at the light wave output end for etching.

[0019] Preferably, the etched spiral zone plate is coaxial with the optical waveguide.

[0020] Preferably, the femtosecond laser has a wavelength of 800 nanometers, a pulse repetition frequency of 1 kilohertz, a pulse width of 75 femtoseconds, a writing speed of 100 micrometers / second, and a laser pulse energy of 300 nanojoules.

[0021] Preferably, the distance between two adjacent threads on the optical waveguide with the spiral cladding structure is 3 to 12 micrometers, and the end face of the optical waveguide is circular with a diameter of 20 micrometers.

[0022] Preferably, a gold sputtering operation is performed on the surface of the lithium yttrium fluoride crystal before focused ion beam etching, wherein the gold sputtering thickness is 20 nanometers.

[0023] Preferably, the accelerating voltage of the focused ion beam etching is 30kV and the beam current is 9.3nA.

[0024] Preferably, the phase function of the spiral zone plate is obtained by multiplying the radial Hilbert phase function and the Fresnel zone plate phase function.

[0025] A second aspect of the present invention provides a vortex beam generator based on an optical waveguide, wherein the vortex beam generator is a vortex beam generator obtained by any of the above-described methods for preparing an optical waveguide-based vortex beam generator.

[0026] The above one or more technical solutions have the following beneficial effects:

[0027] This invention provides a method for fabricating a vortex beam generator based on an optical waveguide. The method combines an optical waveguide with a spiral zone plate to generate a vortex beam. The fabrication method is simple, highly integrated, practical, and easy to promote.

[0028] This invention provides a method for fabricating a vortex beam generator based on an optical waveguide. The optical waveguide is fabricated using femtosecond laser micromachining technology, which is simple, fast, and does not require a cleanroom environment. The refractive index change and waveguide mode of the writing region can be controlled by adjusting the writing parameters. The size of the waveguide end face can also be precisely controlled by adjusting the spiral trajectory, which has extremely high controllability and operability.

[0029] This invention provides a vortex beam generator based on an optical waveguide, which has the advantages of high waveguide stability and good optical performance; the vortex beam generator also has extremely high integration and practicality, and has high application value in the field of integrated photonics. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0031] Figure 1 The process flow diagram for fabricating a micro vortex beam generator based on an optical waveguide is shown in Embodiment 1 of the present invention.

[0032] Figure 2This is a schematic diagram of the process for fabricating a lithium yttrium fluoride crystal cladding waveguide structure by femtosecond laser helical writing on the ac surface of a lithium yttrium fluoride crystal according to Embodiment 1 of the present invention.

[0033] Figure 3 This is a schematic diagram of the process for fabricating a lithium yttrium fluoride crystal cladding waveguide by femtosecond laser spiral writing on the bc plane of a lithium yttrium fluoride crystal according to Embodiment 1 of the present invention.

[0034] Figure 4 This is a schematic diagram of the fabrication process of a helical zone plate etched by focused ion beam on the ac surface of a lithium yttrium fluoride crystal according to Embodiment 1 of the present invention.

[0035] Figure 5 This is a schematic diagram of the fabrication process of a helical zone plate etched by focused ion beam on the bc surface of a lithium yttrium fluoride crystal according to Embodiment 1 of the present invention.

[0036] Figure 6 This is a schematic diagram illustrating the process of generating a vortex beam using a vortex beam generator based on an optical waveguide on the ac surface of a lithium yttrium fluoride crystal according to Embodiment 1 of the present invention.

[0037] Among them: 1. Femtosecond laser focusing objective, 2. Femtosecond laser, 3. Lithium yttrium fluoride crystal, 4. Laser writing trace, 5. Cladding optical waveguide, 6. Focused ion beam system, 7. Ion beam, 8. Helical zone plate, 9. Focused vortex beam, 10. Focusing lens, 11. Incident laser. Detailed Implementation

[0038] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0039] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0040] Example 1:

[0041] Embodiment 1 of the present invention provides a method for fabricating a vortex beam generator based on an optical waveguide, such as... Figure 1 As shown, it includes the following steps:

[0042] Step 1: Cut the lithium yttrium fluoride crystal 3 into six sides. After cutting, perform optical polishing and cleaning on the six sides of the lithium yttrium fluoride crystal 3. Use the pretreated lithium yttrium fluoride crystal as the substrate material.

[0043] Step 2: A helical trace capable of forming a helical cladding structure is written into the interior of the lithium yttrium fluoride crystal using a femtosecond laser writing method to obtain a single-mode optical waveguide.

[0044] Step 3: The surface of the lithium yttrium fluoride crystal corresponding to the single-mode optical waveguide emission end is etched using focused ion beam etching to obtain a spiral zone plate, thus completing the fabrication of the vortex beam generator.

[0045] Step 4: The laser is coupled into the vortex beam generator through the waveguide transmitter end, and a vortex beam is generated through the helical zone plate.

[0046] In step 1, the lithium yttrium fluoride crystal 3 is cut to a size of 10(a)×2(b)×8(c) cubic millimeters.

[0047] In step 2, the specific steps of the writing process are as follows: A femtosecond laser is focused onto the bottom surface of a lithium yttrium fluoride crystal, and laser writing is performed from the bottom surface in a spiral trajectory. The burning process is stopped at a predetermined distance from the top surface of the lithium yttrium fluoride crystal, forming a spiral cladding structure inside the lithium yttrium fluoride crystal, such as... Figure 2 As shown.

[0048] Femtosecond laser micromachining is an effective method for fabricating optical waveguides in transparent dielectric materials, offering numerous advantages such as simple and rapid fabrication processes that do not require a cleanroom environment. The waveguides exhibit high stability and excellent optical performance. Furthermore, the refractive index change and waveguide mode in the written region can be controlled by adjusting the writing parameters, providing extremely high controllability and operability. Lithium yttrium fluoride (YDF) crystal is an excellent matrix material for fabricating optical waveguides. However, due to the inherently perfect crystal structure of YDF, laser writing can cause lattice disruption, leading to a decrease in refractive index. This invention employs femtosecond laser helical processing to form a helical cladding structure within the YDF crystal. The cladding structure has a low refractive index, while the region enclosed by the cladding structure has a relatively high refractive index, thus enabling the construction of a single-mode helical cladding waveguide. Research results show that the cladding waveguide formed by femtosecond laser helical writing in YDF crystal exhibits excellent beam confinement capabilities.

[0049] In this embodiment, the femtosecond laser 2 is focused onto the 8(c) mm × 2(b) mm bottom surface of the lithium yttrium fluoride crystal 3 using the femtosecond laser focusing objective 1, such as... Figure 3As shown, writing is performed along the a-axis of the lithium yttrium fluoride crystal 3 in a helical trajectory. The laser writing trace 4 stops 300 micrometers from the upper surface. The area enclosed by the helical structure is the cladding waveguide 5. The femtosecond laser 2 used has a wavelength of 800 nanometers, a pulse repetition frequency of 1 kilohertz, a pulse width of 75 femtoseconds, a writing speed of 100 micrometers / second, and a laser pulse energy of 300 nanojoules. The distance between two adjacent helical traces on the helical cladding waveguide is 3 to 12 micrometers, and in this embodiment, it is 3 micrometers. The waveguide end face is circular with a diameter of 20 micrometers, and the waveguide can achieve single-mode transmission of 632.8 nm laser. The direction of the helical cladding structure is perpendicular to the bottom surface of the lithium yttrium fluoride crystal. Afterwards, the waveguide end face located on the 8(c) mm × 2(b) mm bottom surface of the lithium yttrium fluoride crystal is polished and cleaned again.

[0050] In step 3, the specific etching steps are as follows: Since the crystal material is not conductive, a gold sputtering operation is performed on the surface of the lithium yttrium fluoride crystal before focused ion beam etching. The gold sputtering thickness is 20 nanometers. The ion beam is focused onto the corresponding surface of the lithium yttrium fluoride crystal at the light waveguide emission end for etching, such as... Figure 4 As shown.

[0051] Focused ion beam etching (FIE) is an advanced micro / nano fabrication technique that requires no mask and is widely used in the fabrication of submicron structures. In FIE systems, liquid ions are accelerated and focused by a focusing system before acting on the target material. This allows for micro / nano fabrication operations such as etching, deposition, and ion implantation on the target material. Research results show that spiral zone sheets fabricated using FIE can generate focused vortex beams.

[0052] In this embodiment, the focused ion beam 7 is focused onto the 8(c) mm × 2(b) mm upper surface of the lithium yttrium fluoride crystal 3 using the focused ion beam system 6 for etching, such as... Figure 5 As shown, the etched helical zone plate 8 is coaxial with the single-mode cladding waveguide 5. The etching was performed using a focused ion beam 7 with an accelerating voltage of 30 kV and a beam current of 9.3 nA. The resulting helical zone plate is coaxial with the optical waveguide. The helical zone plate 8 has a diameter of 120 micrometers, a topological charge of 1, a focal length of 350 micrometers, and a wavelength of 632.8 nanometers. The gold layer on the upper surface was then cleaned. The fabricated vortex beam generator was thus obtained.

[0053] In this embodiment, the phase function of the spiral zone plate is obtained by multiplying the radial Hilbert phase function and the Fresnel zone plate phase function.

[0054] The radial Hilbert phase function is given by the following equation:

[0055] H p (r, φ) = exp(ipφ)

[0056] The Fresnel zone phase function is given by the following equation:

[0057] F zp (r, φ) = exp(-i2π / λf)

[0058] The phase function of the spiral zone plate is given by the following equation:

[0059] SZP p (r, φ) = H p (r, φ)*F zp (r, φ) = exp(ipφ - i2π / λf)

[0060] Where p represents the topological charge number, r and φ both represent polar coordinates, λ represents the wavelength, and f represents the focal length of the Fresnel zone plate.

[0061] In step 4, the incident laser 11 of 632.8 nm is coupled from the light wave inlet to the single-mode cladding waveguide 5 using the focusing lens 10, and a focused vortex beam 9 can be obtained at the focal point of the spiral zone plate 8 at the outlet.

[0062] Example 2:

[0063] Embodiment 2 of the present invention provides a vortex beam generator based on an optical waveguide. The vortex beam generator is obtained by the fabrication method of the vortex beam generator based on an optical waveguide described in Embodiment 1. The fabrication method of the vortex beam generator based on an optical waveguide includes the following steps:

[0064] Step 1: Cut the lithium yttrium fluoride crystal 3 into six sides. After cutting, perform optical polishing and cleaning on the six sides of the lithium yttrium fluoride crystal 3. Use the pretreated lithium yttrium fluoride crystal as the substrate material.

[0065] Step 2: A helical trace capable of forming a helical cladding structure is written into the interior of the lithium yttrium fluoride crystal using a femtosecond laser writing method to obtain a single-mode optical waveguide.

[0066] Step 3: The surface of the lithium yttrium fluoride crystal corresponding to the single-mode optical waveguide emission end is etched using focused ion beam etching to obtain a spiral zone plate, thus completing the fabrication of the vortex beam generator.

[0067] Step 4: The laser is coupled into the vortex beam generator through the light wave guide end, and a vortex beam is generated through the helical zone plate.

[0068] The steps and methods involved in the apparatus of Embodiment 2 above correspond to those in Embodiment 1. For specific implementation details, please refer to the relevant description section of Embodiment 1.

[0069] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for fabricating a vortex beam generator based on an optical waveguide, characterized in that, Includes the following steps: Pretreated lithium yttrium fluoride crystals were used as the substrate material. A single-mode optical waveguide is obtained by writing helical traces capable of forming a helical cladding structure inside the lithium yttrium fluoride crystal. Etching is performed on the surface of the lithium yttrium fluoride crystal corresponding to the single-mode optical waveguide output end to obtain a spiral zone plate, thus completing the fabrication of the vortex beam generator; The etched spiral zone plate is coaxial with the optical waveguide.

2. The method of claim 1, wherein the method further comprises: The pretreatment process is as follows: the lithium yttrium fluoride crystal is cut into six sides, and then the six sides of the lithium yttrium fluoride crystal are optically polished and cleaned.

3. The method of claim 1, wherein the method further comprises: The writing method is femtosecond laser writing.

4. The method for fabricating a vortex beam generator based on an optical waveguide as described in claim 1, characterized in that, The etching method is focused ion beam etching.

5. The method of claim 3, wherein the method further comprises: The specific steps of the writing process are as follows: a femtosecond laser is focused on the bottom surface of a lithium yttrium fluoride crystal, and laser writing is performed from the bottom surface in a spiral trajectory. The burning is stopped at a preset distance from the top surface of the lithium yttrium fluoride crystal, so that a spiral cladding structure is formed inside the lithium yttrium fluoride crystal.

6. The method of claim 5, wherein the method further comprises: The direction of the helical cladding structure is perpendicular to the bottom surface of the lithium yttrium fluoride crystal.

7. The method of claim 4, wherein the method further comprises: The specific etching steps are as follows: focusing the ion beam onto the corresponding yttrium lithium fluoride crystal surface at the light wave output end for etching.

8. The method of claim 5, wherein the method further comprises: The femtosecond laser has a wavelength of 800 nanometers, a pulse repetition frequency of 1 kilohertz, a pulse width of 75 femtoseconds, a writing speed of 100 micrometers / second, and a laser pulse energy of 300 nanojoules.

9. The method of claim 6, wherein the method further comprises: The distance between two adjacent threads on the optical waveguide with the spiral cladding structure is 3 to 12 micrometers, and the end face of the optical waveguide is circular with a diameter of 20 micrometers.

10. The method for fabricating a vortex beam generator based on an optical waveguide as described in claim 7, characterized in that, A gold sputtering operation is performed on the surface of a lithium yttrium fluoride crystal before focused ion beam etching, and the gold sputtering thickness is 20 nanometers.

11. The method for fabricating a vortex beam generator based on an optical waveguide as described in claim 7, characterized in that, The accelerating voltage for focused ion beam etching is 30 kV, and the beam current is 9.3 nA.

12. The method for fabricating a vortex beam generator based on an optical waveguide as described in claim 1, characterized in that, The phase function of the spiral zone plate is obtained by multiplying the radial Hilbert phase function and the Fresnel zone plate phase function.

13. An optical waveguide-based vortex beam generator, comprising: The vortex beam generator is a vortex beam generator obtained by the fabrication method of the optical waveguide-based vortex beam generator according to any one of claims 1-12.

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