Femtosecond laser manufacturing method of optical waveguide with total positive refractive index or total negative refractive index change in single crystal

Abstract

The invention discloses a femtosecond laser manufacturing method of a total positive refractive index or total negative refractive index change optical waveguide in a single crystal, and relates to the technical field of integrated photons. The method specifically comprises the following steps: adjusting the transverse intensity distribution of a femtosecond laser beam through a slit shaping and oil-immersed objective lens technology to obtain a shaped femtosecond laser beam, and then focusing the femtosecond laser beam into a transparent single crystal material; a positive refractive index change area is constructed to form a waveguide core layer in similar photonic crystal arrangement or a negative refractive index change area is constructed to form a waveguide cladding in similar photonic crystal arrangement through a mode of multiple femtosecond laser direct writing, and then a Type-I type core layer waveguide or a Type-II type cladding waveguide is formed. By adopting the method disclosed by the invention, the waveguide with high refractive index contrast ratio, high precision and low loss is formed in the transparent single crystal material, and the section of the waveguide can be customized into any shape. The method is suitable for various active / passive transparent single crystal materials, and has a wide application prospect in the fields of integrated optical amplifiers and light quantum chips.

Description

Technical Field

[0001] This invention relates to a method for manufacturing femtosecond lasers in a single-crystal optical waveguide with varying all-positive or all-negative refractive indices, belonging to the field of integrated photonics technology. Background Technology

[0002] Optical waveguides are core components of photonic integrated circuits, and their performance directly affects the device's loss, gain, and integration density. Transparent single-crystal materials (such as YAG, LuAG, and LiNbO3) are ideal waveguide matrix materials due to their low optical loss, high thermal conductivity, and high damage threshold. However, traditional waveguide fabrication techniques (such as ion implantation and photolithography) suffer from drawbacks when applied to transparent single-crystal materials, including complex processes, inability to perform three-dimensional fabrication, and high costs. While femtosecond laser direct writing (FLDW) technology offers simple and low-cost options for three-dimensional fabrication, it is primarily used for preparing hybrid optical waveguides (typically with both positive and negative refractive index modifications), making it difficult to achieve high processing precision. Therefore, there is an urgent need for a femtosecond laser fabrication method that can precisely control the refractive index distribution in transparent single-crystal materials and fabricate low-loss optical waveguides with a core layer exhibiting all positive refractive index variations or a cladding layer exhibiting all negative refractive index variations. Summary of the Invention

[0003] To address the shortcomings of related technologies, this invention provides a femtosecond laser manufacturing method for optical waveguides with all-positive or all-negative refractive index variations in a single crystal. This method offers advantages such as high precision, low loss, and customizable cross-sectional shapes for waveguide fabrication. It solves the problems of low refractive index control precision and high loss when fabricating optical waveguides with all-positive or all-negative refractive index variations in transparent single-crystal materials.

[0004] The purpose of this invention is to provide a method for manufacturing femtosecond lasers in a single-crystal optical waveguide with either all positive or all negative refractive index variations, specifically including the following steps: (1) The transverse intensity distribution of the femtosecond laser beam is adjusted by slit shaping and oil immersion objective lens technology to obtain the shaped femtosecond laser beam, and the shaped femtosecond laser beam is focused into the interior of the transparent single crystal material.

[0005] (2) A waveguide core layer with a photonic crystal-like arrangement is formed by constructing a positive refractive index change region through multiple femtosecond laser direct writing, or a waveguide cladding with a photonic crystal-like arrangement is formed by constructing a negative refractive index change region, thereby forming a Type-I core waveguide or a Type-II cladding waveguide.

[0006] Preferably, the femtosecond laser manufacturing method for the optical waveguide with all positive or all negative refractive index in the single crystal is characterized in that the slit width of the slit shaping in step (1) is 0.4~0.8mm; and the numerical aperture NA of the oil-immersed objective is ≥1.

[0007] Preferably, the transparent single-crystal material in step (1) is YAG or Er.3+ :YAG、Yb 3+ :YAG、Er 3+ One of LuAG and LiNbO3 crystals.

[0008] Preferably, the conditions for femtosecond laser direct writing in step (2) are: laser wavelength of 343nm, 515nm, 800nm, or 1030nm, pulse width of 30fs to 1ps, and repetition frequency of 1kHz to 1MHz.

[0009] Preferably, the femtosecond laser direct writing in step (2) is performed 3 to 66 times; the adjacent spacing of the multiple femtosecond laser direct writings is 0.5 to 1.9 μm.

[0010] Preferably, the cross-sectional shape of the waveguide in step (2) is programmable and can be one of the following: circular, triangular, square, annular, or symmetrical double-lobed.

[0011] A waveguide with a symmetrical double-lobed cross-section means that there are two separate symmetrical high-refractive-index regions or two separate light-transmitting regions on the cross-section; a waveguide with the cross-sectional shape described in this invention can support LP. 11 High-order mode transmission.

[0012] LP 11 The mode is a first-order linearly polarized high-order mode in a weakly guided optical waveguide. The transverse light intensity is symmetrically distributed in two lobes. There is a dark line in the center in the angular direction. There is no dark ring in the radial direction. It has one degeneracy in the angular direction. The light field is symmetrically distributed along the orthogonal azimuth angle. The two lobes are out of phase.

[0013] After selecting the laser wavelength and repetition frequency, the pulse energy and scanning speed are adjusted using conventional process optimization methods. A single scan is performed, and the morphology and refractive index changes of the cross-sectional modified region are observed, thereby determining the optimal process window for a specific material and waveguide type.

[0014] Mechanism of the invention: This invention suppresses longitudinal optical field energy diffusion by adjusting the lateral intensity distribution of the femtosecond laser beam and the number of direct writing operations using a slit, as well as through the synergistic effect of adjacent spacing design. Simultaneously, it utilizes a high numerical aperture immersion oil objective and refractive index matching oil (n... oil ~=1.51), eliminating spherical aberration. Densely stacked to form a uniform positive refractive index modulated photonic crystal-like waveguide core region or a negative refractive index modulated photonic crystal-like waveguide cladding region, the structural schematic diagram of the active / passive transparent single-crystal optical waveguide fabrication system of this invention is shown below. Figure 1 (a) The present invention uses simulated field intensity distribution of femtosecond laser beams before and after slit shaping and corresponding measured cross-sectional diagrams of the modified region as shown in the figure. Figure 1 As shown in (b).

[0015] The beneficial effects of this invention are: This invention achieves a precise improvement in refractive index in various passive and active transparent single-crystal materials, including YAG, LuAG, and LiNbO3, by adjusting the lateral intensity distribution of the femtosecond laser beam and the number of femtosecond laser direct writing operations through synergistic effects of adjacent spacing design. This results in a refractive index difference of up to 10 between the waveguide core and cladding. -2 The magnitude of the difference is significantly higher than the highest refractive index difference induced by traditional femtosecond lasers in direct-write waveguides within glass, thus greatly enhancing the ability to confine the optical field. Optical waveguides fabricated using this technology exhibit excellent transmission performance, with low transmission and insertion losses in the 1550nm communication band. They also possess a spatial processing resolution of up to 700nm, allowing for flexible design of arbitrary waveguide cross-sections such as circular, triangular, square, annular, and symmetrical double-lobed shapes, supporting stable transmission of the fundamental and higher-order modes. Furthermore, the fabricated waveguides achieve polarization insensitivity with high mode roundness (≥94%), with polarization-dependent losses below 0.1dB, significantly enhancing the device's environmental adaptability and signal fidelity. This invention provides an efficient, flexible, and high-performance fabrication scheme for integrated optical devices, with broad application prospects in optical communication, optical sensing, and quantum optics. Attached Figure Description

[0016] Figure 1 This diagram illustrates the structure of the active / passive transparent single-crystal optical waveguide fabrication system of the present invention, along with simulated field strength distributions before and after beam shaping, and measured cross-sectional views of the modified region. Figure 1 (a) is a schematic diagram of the optical waveguide fabrication system; Figure 1 (b) The top row, from left to right, shows the simulated field intensity distribution under the conditions of all positive refractive index before beam shaping, all negative refractive index after beam shaping, and the bottom row shows the measured cross-sectional view of the modified region under the corresponding conditions.

[0017] Figure 2 Er prepared in Example 1 of this invention 3+ The polarization-dependent transmittance test results of a circular cross-section optical waveguide with a fully positive refractive index variation in doped YAG single crystal.

[0018] Figure 3 Er prepared in Example 1 of this invention 3+ Cross-sectional view (a) and measured near-field mode view (b) of a circular cross-section optical waveguide with a fully positive refractive index variation in doped YAG single crystal.

[0019] Figure 4 Er prepared in Example 2 of this invention 3+ Cross-sectional view (a) and measured near-field mode view (b) of a circular cross-section optical waveguide with a fully positive refractive index variation of doped LuAG single crystal.

[0020] Figure 5 The cross-sectional view (a) and the measured near-field mode view (b) of the circular cross-section core waveguide with full positive refractive index variation of LiNbO3 single crystal prepared in Example 3 of the present invention are shown.

[0021] Figure 6 Yb prepared in Example 4 of this invention 3+ Cross-sectional view (a) and measured near-field mode view (b) of a circular cross-section cladding optical waveguide with a fully negative refractive index variation in doped YAG single crystal.

[0022] Figure 7 The following are the design and measured diagrams of the core waveguide cross-section with customizable all-positive refractive index variation prepared in Example 5 of this invention, as well as the measured diagram of the near-field mode. Detailed Implementation

[0023] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments. In the embodiments and comparative examples of this invention, unless otherwise specified, all chemical reagents used were commercially available analytical grade reagents.

[0024] Example 1 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.45 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto Er 3+ YAG single crystal ( <111> The atomic percentage doping concentration is 2.5%, and the size is 8×8×1mm. 3 Inside, eliminate spherical difference.

[0025] (2) A waveguide core layer with submicron spatial resolution positive refractive index variation region was constructed by 37 femtosecond laser direct writing to form a photonic crystal-like arrangement. The laser center wavelength was 1030 nm, the pulse width was 213 fs, the repetition frequency was 1 MHz, the pulse energy was 113 nJ, linearly polarized light was used for output, the scanning speed was 1 mm / s, and the adjacent spacing of the femtosecond laser direct writing was 1.1 μm, thus forming a waveguide core layer with submicron spatial resolution positive refractive index variation region. 3+ : A circular cross-section optical waveguide with a fully positive refractive index variation prepared in YAG single crystal.

[0026] This embodiment is in Er 3+ Cross-sectional view of a circular core optical waveguide with a fully positive refractive index variation fabricated in a YAG single crystal, as shown below. Figure 3As shown in (a), the waveguide is a circular waveguide with a diameter of 9.7 μm, and the corresponding near-field mode at 1550 nm is as follows. Figure 3 As shown in (b), Figure 3 (b) shows that the waveguide cross-section forms a central bright spot, exhibiting a typical Gaussian or Gaussian-like light pattern. The polarization-dependent transmittance test results for this Type-I circular waveguide are as follows: Figure 2 As shown, the Type-I circular cross-section Er prepared in this embodiment 3 + The YAG single-crystal optical waveguide exhibits significant polarization insensitivity. The polar coordinate profiles of transmittance under TE polarization (transverse electric polarization) and TM polarization (transverse magnetic polarization) completely coincide, and there are no significant fluctuations in polarization-dependent transmittance across the entire angular range. This demonstrates that the geometric symmetry of the circular core layer effectively eliminates polarization-preferred transmission phenomena. The waveguide exhibits extremely low polarization-dependent loss, meeting the polarization compatibility requirements for 1550nm optical transmission, with a corresponding polarization-dependent loss of approximately 0.1dB. In this embodiment, the refractive index difference between the waveguide core layer and cladding is approximately 10. -2 .

[0027] This embodiment achieves a refractive index difference of 10 between the waveguide core and cladding by adjusting the lateral intensity distribution of the femtosecond laser beam and the number of femtosecond laser direct writing operations through the synergistic effect of adjacent spacing design. -2 The magnitude of the difference is significantly higher than the highest refractive index difference induced by traditional femtosecond lasers in direct-write waveguides within glass, thus greatly enhancing the ability to confine the optical field. The optical waveguide fabricated based on this technology exhibits excellent transmission performance, with extremely low transmission loss in the 1550nm communication band. It also allows for flexible design of various waveguide cross-sectional shapes, supporting stable transmission of the fundamental mode and higher-order modes. Furthermore, the fabricated waveguide achieves polarization insensitivity with high mode roundness (≥94%), and polarization-dependent loss is approximately 0.1dB, significantly enhancing the device's environmental adaptability and signal fidelity. This embodiment provides an efficient, flexible, and high-performance fabrication scheme for integrated optical devices, with broad application prospects in optical communication, optical sensing, and quantum optics.

[0028] Example 2 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.45 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto Er 3+ LuAG single crystal ( <111> The atomic percentage doping concentration is 2.5%, and the size is 8×8×1mm. 3Inside, eliminate spherical difference.

[0029] (2) A waveguide core layer with submicron spatial resolution positive refractive index variation region was constructed by 19 femtosecond laser direct writing operations to form a photonic crystal-like arrangement. The laser center wavelength was 1030 nm, the pulse width was 213 fs, the repetition frequency was 1 MHz, the pulse energy was 113 nJ, linearly polarized light was used for output, the scanning speed was 1 mm / s, and the adjacent spacing of the femtosecond laser direct writing was 1.1 μm, thus forming a waveguide core layer with submicron spatial resolution positive refractive index variation region. 3+ : A circular cross-section optical waveguide with a fully positive refractive index variation prepared in LuAG single crystal.

[0030] This embodiment is in Er 3+ Cross-sectional view of a circular core optical waveguide with varying positive refractive index fabricated in LuAG single crystal, as shown below. Figure 4 As shown in (a), the waveguide is a circular waveguide with a diameter of 7.6 μm, and the corresponding near-field mode at 1550 nm is as follows. Figure 4 As shown in (b), Figure 4 (b) This shows that the waveguide cross-section forms a central bright spot, exhibiting a typical Gaussian or Gaussian-like spot. The Type-I circular cross-section Er prepared in this embodiment... 3+ The LuAG single-crystal optical waveguide exhibits significant polarization insensitivity. The polar coordinate profiles of transmittance for TE and TM polarization completely coincide, and there are no significant fluctuations in polarization-dependent transmittance across the entire angular range. This demonstrates that the geometric symmetry of the circular core layer effectively eliminates polarization-preferred transmission phenomena. The waveguide exhibits extremely low polarization-dependent loss, meeting the polarization compatibility requirements for 1550nm optical transmission. The polarization-dependent transmittance test results for this Type-I circular waveguide show a polarization-dependent loss of approximately 0.1dB. In this embodiment, the refractive index difference between the waveguide core and cladding is approximately 10. -2 .

[0031] This embodiment achieves a refractive index difference of 10 between the waveguide core and cladding by adjusting the lateral intensity distribution of the femtosecond laser beam and the number of femtosecond laser direct writing operations through the synergistic effect of adjacent spacing design. -2 The magnitude of the difference is significantly higher than the highest refractive index difference induced by traditional femtosecond lasers in direct-write waveguides within glass, thus greatly enhancing the ability to confine the optical field. The optical waveguide fabricated based on this technology exhibits excellent transmission performance, with extremely low transmission loss in the 1550nm communication band. It also allows for flexible design of various waveguide cross-sectional shapes, supporting stable transmission of the fundamental mode and higher-order modes. Furthermore, the fabricated waveguide achieves polarization insensitivity with high mode roundness (≥94%), and polarization-dependent loss is approximately 0.1dB, significantly enhancing the device's environmental adaptability and signal fidelity. This embodiment provides an efficient, flexible, and high-performance fabrication scheme for integrated optical devices, with broad application prospects in optical communication, optical sensing, and quantum optics.

[0032] Example 3 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.5 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto a z-cut LiNbO3 single crystal (size 4.5 × 4.5 × 0.4 mm). 3 Inside, eliminate spherical difference.

[0033] (2) A waveguide core layer with submicron spatial resolution positive refractive index variation region was constructed by 61 femtosecond laser direct writing to form a photonic crystal-like arrangement. The laser center wavelength was 1030 nm, the pulse width was 213 fs, the repetition frequency was 10 kHz, the pulse energy was 150 nJ, linearly polarized light was used for output, the scanning speed was 5 mm / s, and the adjacent spacing of the femtosecond laser direct writing was 1.25 μm, thereby forming a circular cross-section core waveguide with full positive refractive index variation prepared in LiNbO3 single crystal.

[0034] The cross-sectional diagram of the circular core waveguide with a fully positive refractive index variation prepared in LiNbO3 single crystal in this embodiment is shown below. Figure 5 As shown in (a), the waveguide is a circular waveguide with a diameter of 10 μm, and the corresponding near-field mode at 1550 nm is as follows. Figure 5 As shown in (b), Figure 5 (b) This shows that the waveguide cross-section forms a central bright spot, exhibiting a typical Gaussian or Gaussian-like light spot. The Type-I circular cross-section LiNbO3 single-crystal optical waveguide prepared in this embodiment exhibits significant polarization insensitivity. The polar coordinate profiles of the transmittance for TE and TM polarization completely coincide, and the polarization-dependent transmittance shows no significant fluctuations across the entire angular range, proving that the geometric symmetry of the circular core layer effectively eliminates the polarization-preferred transmission phenomenon. The waveguide's polarization-dependent loss is extremely low, meeting the polarization compatibility requirements for 1550nm optical transmission. The polarization-dependent transmittance test results for this Type-I circular waveguide show that its corresponding polarization-dependent loss is approximately 0.1dB. The refractive index difference between the waveguide core layer and cladding in this embodiment is approximately 10. -2 .

[0035] This embodiment achieves a refractive index difference of 10 between the waveguide core and cladding by adjusting the lateral intensity distribution of the femtosecond laser beam and the number of femtosecond laser direct writing operations through the synergistic effect of adjacent spacing design. -2The magnitude of the difference is significantly higher than the highest refractive index difference induced by traditional femtosecond lasers in direct-write waveguides within glass, thus greatly enhancing the ability to confine the optical field. The optical waveguide fabricated based on this technology exhibits excellent transmission performance, with extremely low transmission loss in the 1550nm communication band. It also allows for flexible design of various waveguide cross-sectional shapes, supporting stable transmission of the fundamental mode and higher-order modes. Furthermore, the fabricated waveguide achieves polarization insensitivity with high mode roundness (≥94%), and polarization-dependent loss is approximately 0.1dB, significantly enhancing the device's environmental adaptability and signal fidelity. This embodiment provides an efficient, flexible, and high-performance fabrication scheme for integrated optical devices, with broad application prospects in optical communication, optical sensing, and quantum optics.

[0036] Example 4 A method for manufacturing a femtosecond laser for a single-crystal optical waveguide with a completely negative refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.4 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto Yb 3+ YAG single crystal ( <111> The atomic percentage doping concentration is 10%, and the size is 5×5×1mm. 3 Inside, eliminate spherical difference.

[0037] (2) A submicron spatially resolved negative refractive index variation region was constructed by 66 femtosecond laser direct writing to form a waveguide cladding with a photonic crystal-like arrangement. The laser center wavelength was 1030 nm, the pulse width was 223 fs, the repetition frequency was 1 MHz, the pulse energy was 230 nJ, linearly polarized light was used for output, the scanning speed was 5 mm / s, and the adjacent spacing of the femtosecond laser direct writing was 1.1 μm, thus forming a waveguide cladding with a submicron spatial resolution negative refractive index variation region. 3+ : A circular cross-section cladding optical waveguide with full negative refractive index variation prepared in YAG single crystal.

[0038] This embodiment is in Yb 3+ Cross-sectional diagram of a circular cladding waveguide with a fully negative refractive index variation prepared in a YAG single crystal, as shown below. Figure 6 As shown in (a), the waveguide is a circular waveguide with a diameter of 20 μm, and the corresponding near-field mode at 1550 nm is as follows. Figure 6 As shown in (b), Figure 6 (b) This shows that the waveguide cross-section forms a central bright spot, exhibiting a typical Gaussian or Gaussian-like light spot. The Type-II circular cross-section Yb prepared in this embodiment... 3+The YAG single-crystal optical waveguide exhibits significant polarization insensitivity. The polar coordinate profiles of transmittance for TE and TM polarization completely coincide, and there are no significant fluctuations in polarization-dependent transmittance across the entire angular range. This demonstrates that the geometric symmetry of the circular cladding effectively eliminates polarization-preferred transmission phenomena. The waveguide exhibits extremely low polarization-dependent loss, meeting the polarization compatibility requirements for 1550nm optical transmission. The polarization-dependent transmittance test results for this Type-II circular waveguide show a polarization-dependent loss of approximately 0.1dB. In this embodiment, the refractive index difference between the waveguide cladding and the core layer is approximately 10. -2 .

[0039] This embodiment achieves a refractive index difference of 10 between the waveguide core and cladding by adjusting the lateral intensity distribution of the femtosecond laser beam and the number of femtosecond laser direct writing operations through the synergistic effect of adjacent spacing design. -2 The magnitude of the difference is significantly higher than the highest refractive index difference induced by traditional femtosecond lasers in direct-write waveguides within glass, thus greatly enhancing the ability to confine the optical field. The optical waveguide fabricated based on this technology exhibits excellent transmission performance, with extremely low transmission loss in the 1550nm communication band. It also allows for flexible design of various waveguide cross-sectional shapes, supporting stable transmission of the fundamental mode and higher-order modes. Furthermore, the fabricated waveguide achieves polarization insensitivity with high mode roundness (≥94%), and polarization-dependent loss is approximately 0.1dB, significantly enhancing the device's environmental adaptability and signal fidelity. This embodiment provides an efficient, flexible, and high-performance fabrication scheme for integrated optical devices, with broad application prospects in optical communication, optical sensing, and quantum optics.

[0040] Example 5 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.45 mm. A 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto a YAG single crystal ( <111> Dimensions: 5×5×0.5mm 3 Inside, eliminate spherical difference.

[0041] (2) A waveguide core layer with submicron spatial resolution positive refractive index variation region was constructed by direct writing with femtosecond lasers 37 times to form a photonic crystal-like arrangement. The laser center wavelength was 1030 nm, the pulse width was 213 fs, the repetition frequency was 1 MHz, the pulse energy was 123 nJ, linearly polarized light was used for output, the scanning speed was 1 mm / s, and the adjacent spacing of the femtosecond laser direct writing was 1.1 μm. Thus, a triangular, square, annular, and symmetrical double-lobed cross-section core waveguide with full positive refractive index variation was prepared in YAG single crystal.

[0042] The cross-sectional diagram of the optical waveguide with a fully positive refractive index variation prepared in YAG single crystal in this embodiment is shown below. Figure 7 As shown, in this embodiment, multiple trajectories are arranged to form corresponding cross-sectional shapes. The cross-sections prepared in this embodiment have triangular, square, annular, and double-lobed shapes (i.e., supporting LP). 11 The core waveguide (with all-positive refractive index variation of the mode) corresponds to the 1550nm near-field mode as follows: Figure 7 As shown, Figure 7 The waveguide cross-sections fabricated in this embodiment all exhibit a central bright spot, displaying typical Gaussian or Gaussian-like light patterns. Polarization-dependent transmittance tests of the aforementioned waveguides show that their corresponding polarization-dependent losses are all ≤0.1dB. The refractive index difference between the waveguide core and cladding in this embodiment is approximately 10. -2 This embodiment demonstrates how, through the synergistic effect of adjusting the lateral intensity distribution of the femtosecond laser beam and the number of femtosecond laser direct writing operations, as well as the design of adjacent spacing, a core-layer optical waveguide with a fully positive refractive index variation and different cross-sectional shapes was successfully fabricated, achieving a refractive index difference of 10 between the waveguide core and cladding. -2 The magnitude of the difference is significantly higher than the highest refractive index difference induced by traditional femtosecond lasers in direct-write waveguides within glass, thus greatly enhancing the ability to confine the optical field. Optical waveguides fabricated using this technology exhibit excellent transmission performance, with extremely low transmission loss in the 1550nm communication band. Furthermore, various waveguide cross-sectional shapes can be flexibly designed to support stable transmission of the fundamental mode and higher-order modes. In addition, the fabricated waveguides achieve polarization insensitivity with high mode roundness (≥94%) and polarization-dependent loss below 0.1dB, significantly enhancing the device's environmental adaptability and signal fidelity. This demonstrates flexible cross-sectional customization capabilities, suitable for complex photonic integrated circuits.

[0043] Comparative Example 1 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.3 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto a YAG single crystal ( <111> Dimensions: 5×5×0.5mm 3 Inside, eliminate spherical difference.

[0044] (2) A waveguide core layer with a submicron spatial resolution positive refractive index variation region was constructed by direct writing with femtosecond lasers 37 times, forming a photonic crystal-like arrangement. The laser center wavelength was 1030 nm, the pulse width was 213 fs, the repetition frequency was 1 MHz, the pulse energy was 123 nJ, linearly polarized light was used for output, the scanning speed was 1 mm / s, and the adjacent spacing of the femtosecond laser direct writing was 1.1 μm, thus forming a circular cross-section optical waveguide with a full positive refractive index variation fabricated in YAG single crystal.

[0045] The cross-section of the circular core waveguide with a fully positive refractive index variation fabricated in YAG single crystal, as shown in this comparative example, has a diameter of approximately 10 μm. The corresponding 1550 nm near-field mode output indicates that this waveguide has poor optical field confinement. The polarization-dependent transmittance test results for this Type-I circular waveguide show a polarization-dependent loss of approximately 0.3 dB. The refractive index difference between the core and cladding of this comparative waveguide is approximately 10 μm. -3 In this comparative example, due to the use of a smaller slit width (0.3 mm), the light diffraction efficiency changes, and the light field distribution in the horizontal and vertical directions has not yet reached equilibrium, resulting in a smaller refractive index difference in the induced structure and an increase in optical waveguide loss.

[0046] Comparative Example 2 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.9 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto a YAG single crystal ( <111> Dimensions: 5×5×0.5mm 3 Inside, eliminate spherical difference.

[0047] (2) A waveguide core layer with a submicron spatial resolution positive refractive index variation region was constructed by direct writing with femtosecond lasers 37 times, forming a photonic crystal-like arrangement. The laser center wavelength was 1030 nm, the pulse width was 213 fs, the repetition frequency was 1 MHz, the pulse energy was 123 nJ, linearly polarized light was used for output, the scanning speed was 1 mm / s, and the adjacent spacing of the femtosecond laser direct writing was 1.1 μm, thus forming a circular cross-section optical waveguide with a full positive refractive index variation fabricated in YAG single crystal.

[0048] The cross-section of the circular core waveguide with a fully positive refractive index variation fabricated in YAG single crystal, as shown in this comparative example, has a diameter of approximately 10 μm. The corresponding 1550 nm near-field mode output indicates that this waveguide has poor optical field confinement. The polarization-dependent transmittance test results for this Type-I circular waveguide show a polarization-dependent loss of approximately 0.2 dB. The refractive index difference between the core and cladding of this comparative waveguide is approximately 10 μm. -3 In this comparative example, due to the use of a larger slit width (0.9 mm), the light diffraction efficiency changes, and the light field distribution in the horizontal and vertical directions is still not in equilibrium, resulting in a smaller refractive index difference in the induced structure and an increase in optical waveguide loss.

[0049] Comparative Example 3 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.5 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto a YAG single crystal ( <111> Dimensions: 5×5×0.5mm 3 Inside, eliminate spherical difference.

[0050] (2) A waveguide core layer with a submicron spatial resolution positive refractive index variation region was constructed by direct writing with femtosecond lasers 37 times, forming a photonic crystal-like arrangement. The laser center wavelength was 1030 nm, the pulse width was 213 fs, the repetition frequency was 1 MHz, the pulse energy was 123 nJ, linearly polarized light was used for output, the scanning speed was 1 mm / s, and the adjacent spacing of the femtosecond laser direct writing was 0.1 μm, thereby forming a circular cross-section optical waveguide with a full positive refractive index variation prepared in YAG single crystal.

[0051] The cross-section of the fully positive refractive index-varying circular core waveguide fabricated in YAG single crystal in this comparative example shows that the waveguide is a circular waveguide with a size of 10 μm. The polarization-dependent transmittance test results for this Type-I circular waveguide show that its polarization-dependent loss is approximately 0.3 dB. The refractive index difference between the core and cladding of this comparative waveguide is approximately 10. -3 This comparative example uses a small femtosecond laser direct writing adjacent spacing (0.1μm), resulting in the coexistence of positive and negative refractive indices, a decrease in effective refractive index difference, and an increase in waveguide loss.

[0052] Comparative Example 4 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.5 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto a YAG single crystal ( <111> Dimensions: 5×5×0.5mm 3 Inside, eliminate spherical difference.

[0053] (2) A waveguide core layer with submicron spatial resolution positive refractive index variation region was constructed by 37 femtosecond laser direct writing to form a photonic crystal-like arrangement. The laser center wavelength was 1030nm, the pulse width was 213fs, the repetition frequency was 1MHz, the pulse energy was 123nJ, linearly polarized light was used for output, the scanning speed was 1mm / s, and the adjacent spacing of the femtosecond laser direct writing was 2μm. Thus, a circular cross-section optical waveguide with full positive refractive index variation was formed in YAG single crystal.

[0054] The cross-section of the fully positive refractive index-varying circular core waveguide fabricated in YAG single crystal in this comparative example shows that the waveguide is a circular waveguide with a size of 10 μm. The polarization-dependent transmittance test results for this Type-I circular waveguide show that its polarization-dependent loss is approximately 0.2 dB. The refractive index difference between the core and cladding of this comparative waveguide is approximately 10. -3 In this comparative example, the large adjacent spacing (2μm) used for direct writing with femtosecond laser resulted in some materials not being induced to have a positive refractive index, leading to a decrease in the effective refractive index difference in the core waveguide region and an increase in waveguide loss.

[0055] Comparative Example 5 A method for fabricating a femtosecond laser in a single-crystal optical waveguide with all-positive refractive index variation includes the following steps: (1) The transverse intensity distribution of the femtosecond laser beam was adjusted by slit shaping and oil immersion objective techniques to suppress longitudinal optical field energy diffusion. The slit width for slit shaping was 0.5 mm, and a 100x oil immersion objective with an NA of 1.25 and an oil refractive index of 1.51 was used to obtain the shaped femtosecond laser beam. The shaped femtosecond laser beam was then focused onto a YAG single crystal ( <111> Dimensions: 5×5×0.5mm 3 Inside, eliminate spherical difference.

[0056] (2) A waveguide core layer with submicron spatial resolution positive refractive index variation region is constructed by one femtosecond laser direct writing to form a photonic crystal-like arrangement. The laser center wavelength is 1030nm, the pulse width is 213fs, the repetition frequency is 1MHz, the pulse energy is 123nJ, the linearly polarized light output is used, the scanning speed is 1mm / s, and the adjacent spacing of the femtosecond laser direct writing is 1.1μm, thereby forming a circular cross-section core waveguide with full positive refractive index variation prepared in YAG single crystal.

[0057] The cross-section of the fully positive refractive index-varying circular core waveguide fabricated in YAG single crystal in this comparative example shows that the waveguide is a circular waveguide with a size of ~1μm. The polarization-dependent transmittance test results for this Type-I circular waveguide show that its polarization-dependent loss is approximately 10dB. The refractive index difference between the core and cladding of this comparative waveguide is approximately 10 dB. -2 Because this comparative example only performs one femtosecond laser direct writing, the waveguide size is too small, which means it cannot support 1550nm wavelength light guiding, resulting in high waveguide loss.

[0058] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for manufacturing a femtosecond laser in a single crystal waveguide with either all positive or all negative refractive index variations, characterized in that, Specifically, the following steps are included: (1) The transverse intensity distribution of the femtosecond laser beam is adjusted by slit shaping and oil immersion objective lens technology to obtain the shaped femtosecond laser beam, and the shaped femtosecond laser beam is focused into the interior of the transparent single crystal material. (2) A waveguide core layer with a photonic crystal-like arrangement is formed by constructing a positive refractive index change region through multiple femtosecond laser direct writing, or a waveguide cladding with a photonic crystal-like arrangement is formed by constructing a negative refractive index change region, thereby forming a Type-I core waveguide or a Type-II cladding waveguide.

2. The femtosecond laser manufacturing method for a single-crystal optical waveguide with all positive or all negative refractive index variations according to claim 1, characterized in that, The slit width for slit shaping in step (1) is 0.4~0.8mm; the numerical aperture NA of the oil-immersion objective is ≥1.

3. The method for manufacturing femtosecond lasers using a single-crystal optical waveguide with either all positive or all negative refractive index variations according to claim 1, characterized in that, The transparent single-crystal material in step (1) is YAG or Er. 3+ :YAG、Yb 3+ :YAG、Er 3+ One of LuAG and LiNbO3 crystals.

4. The femtosecond laser manufacturing method for a single-crystal optical waveguide with all positive or all negative refractive index variations according to claim 1, characterized in that, The conditions for femtosecond laser direct writing in step (2) are: the laser center wavelength is one of 343nm, 515nm, 800nm, or 1030nm, the pulse width is 30fs to 1ps, and the repetition frequency is 1kHz to 1MHz.

5. The femtosecond laser manufacturing method for a single-crystal optical waveguide with all positive or all negative refractive index variations according to claim 1, characterized in that, Step (2) involves 3 to 66 femtosecond laser direct writing operations; the spacing between adjacent femtosecond laser direct writing operations is 0.5 to 1.9 μm.

6. The femtosecond laser manufacturing method for a single-crystal optical waveguide with all positive or all negative refractive index variations according to claim 1, characterized in that, The cross-sectional shape of the waveguide in step (2) is programmable and can be one of the following: circular, triangular, square, annular, or symmetrical double-lobed.

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