Method for preparing short-wavelength I-type waveguide in sapphire by using femtosecond laser and application of short-wavelength I-type waveguide

Abstract

The invention discloses a method for preparing a short-wavelength I-type waveguide in sapphire by using femtosecond laser and application thereof, and belongs to the technical field of laser processing, and the method comprises the following steps: step 1, shaping a slit to obtain a circularly distributed three-dimensional focal field; 2, performing slit shaping to obtain a circularly distributed three-dimensional focal field; 3, preparing a short-wavelength I-type waveguide; according to the method, a slit shaping technology is adopted to realize light beam shaping, a processing light spot is adjusted, a waveguide cross section is regulated and controlled, and specific refractive index change is directly induced in sapphire, so that a light guide area of the waveguide is only limited to be right below a processing track, and preparation of the waveguide with the shortest wavelength of 405nm is realized through a single modification line. The method is simple and efficient in preparation process and high in reliability, the prepared waveguide is compact in structure, and the integration level of the femtosecond laser direct writing photon chip can be greatly improved.

Description

Technical Field

[0001] The present invention belongs to the field of laser processing technology, and in particular relates to a method for preparing a short-wavelength type I waveguide in sapphire by using a femtosecond laser and an application thereof. Background Art

[0002] With the rapid development of artificial intelligence and big data technologies and the widespread adoption of high-speed internet, photonic integrated chips are playing an increasingly important role in modern technology. Optical waveguides, as the most fundamental and critical component in integrated photonic chips, are the core structure for optical signal transmission and processing. However, with the continuous expansion of application scenarios, optical waveguides face higher performance requirements. In particular, they must operate stably in challenging environments such as high temperature and pressure, strong electromagnetic interference, high humidity, and high corrosion. Furthermore, short-wavelength optical waveguides are of irreplaceable importance in many cutting-edge fields. In biomedical imaging, short-wavelength optical waveguides enable high-resolution imaging of cells and tissues. In quantum information processing, short-wavelength optical waveguides can be used to construct efficient quantum information transmission channels, helping to improve the transmission efficiency and stability of quantum bits. In miniaturized optoelectronic devices, short-wavelength optical waveguides can be used to manufacture highly integrated optical sensors and optical chips. Their short wavelength advantage significantly improves device performance and functional density, meeting the growing demand for miniaturization.

[0003] In the short-wavelength range, material selection is crucial, as the material's absorption, refractive index, and nonlinear properties significantly affect waveguide performance. For example, traditional silicon-based waveguides are limited at short wavelengths due to material absorption. Sapphire crystals exhibit high transparency from the ultraviolet to near-infrared spectrum. This property allows for efficient transmission of short-wavelength light in waveguides without significant losses due to material absorption. Furthermore, sapphire crystals exhibit excellent physical and chemical stability, including high hardness, high-temperature resistance, and corrosion resistance, making them widely used in extreme applications such as military industry and aerospace. However, these properties also make sapphire processing extremely difficult. While chemical vapor deposition, photolithography, and ion exchange processes each have their own unique advantages, they are limited to two-dimensional surfaces and struggle to achieve complex three-dimensional structures. Femtosecond lasers, however, possess extremely high peak power and can focus energy to any location in transparent materials, enabling flexible three-dimensional direct writing. Therefore, femtosecond laser direct writing technology is a reliable method for fabricating sapphire waveguides. Waveguides written by femtosecond lasers are generally categorized as Type I, Type II (dual linear waveguides), and Type III (depressed cladding waveguides). The so-called Type I waveguide refers to an optical waveguide in which the refractive index of the material increases in the modified area at the focus of the femtosecond laser, thereby forming a waveguide core area. The refractive index of the core area of ​​the optical waveguide is higher than that of the cladding area. In crystal materials, the refractive index change induced by femtosecond lasers is relatively complex. For other crystal materials such as sapphire, it is usually difficult to achieve a stable and uniform refractive index increase (Δn>0) area, and Type I waveguides cannot be prepared. On the contrary, these crystal materials are more likely to have a refractive index decrease (Δn<0) after femtosecond laser irradiation, making them more suitable for the preparation of depressed cladding waveguides or stress-induced double-line waveguides.

[0004] While femtosecond laser processing of sapphire waveguides has made significant progress, successfully fabricating various structures, including photonic crystal-like waveguides, dual-line waveguides, and depressed cladding waveguides, using existing femtosecond lasers to fabricate short-wavelength, single-mode sapphire waveguides remains challenging due to the small size and high refractive index requirements. Furthermore, the large size of single-mode sapphire waveguides fabricated using existing techniques reduces the integration density of on-chip photonic devices. These challenges make it difficult to meet the high-density, low-loss, and short-wavelength transmission requirements of integrated photonic chip waveguides. Summary of the Invention

[0005] To address the shortcomings of existing short-wavelength, single-mode sapphire waveguide fabrication techniques, the present invention provides a method for fabricating short-wavelength, Type I waveguides in sapphire using a femtosecond laser, and its application. This method employs slit shaping technology to achieve beam shaping, adjust the processing spot, and manipulate the waveguide cross-section. This method directly induces a specific refractive index change within the sapphire, limiting the waveguide's light-guiding region directly below the processing trajectory. Using a single modified line, waveguides with wavelengths as short as 405 nm can be fabricated. This method boasts a simple, efficient, and highly reliable fabrication process, resulting in a compact waveguide structure that will significantly increase the integration density of femtosecond laser direct-writing photonic chips.

[0006] The present invention is achieved through the following technical solutions:

[0007] In a first aspect, the present invention provides a method for fabricating a short-wavelength type I waveguide in sapphire using a femtosecond laser, comprising the following steps:

[0008] Step 1: Slit shaping to obtain a circularly distributed three-dimensional light focal field;

[0009] The femtosecond laser emits low single pulse energy and passes through a cylindrical lens system consisting of a cylindrical concave lens CL1 and a cylindrical convex lens CL2, a half-wave plate HWP1, a polarization beam splitter PBS, a first reflector M1, and a half-wave plate HWP2, and then is shaped into a long and thin strip of light through a slit. The laser is then reflected by a second reflector M2 to the objective lens OL, and the laser energy before the objective lens entrance pupil is controlled by adjusting the optical axis direction of the half-wave plate HWP1.

[0010] The polarization of the laser is monitored using a polarization beam splitter, and the polarization direction of the femtosecond laser before the objective lens entrance pupil is adjusted using a half-wave plate (HWP2) so that the laser polarization direction is perpendicular to the scanning direction. The position of the laser center is determined using a fluorescent card, and the angle and position of the slit are observed and adjusted using a CCD so that the center of the slit coincides with the center of the laser.

[0011] The objective lens OL focuses the shaped laser onto the sample surface, so that the focal field of the objective lens is distributed in a triangular shape inside the sample along the laser scanning direction;

[0012] Step 2: Determine the processing parameters that minimize the waveguide transmission loss;

[0013] Focus the shaped laser on the preset processing position of the sample, and directly write the waveguide by adjusting the laser processing power, scanning speed and number of scans; load the pre-written MATLAB processing program, and control the movement of the precision three-dimensional motion platform according to the written MATLAB program;

[0014] The waveguide end face was characterized using an optical microscope to screen the processing parameters of the single-mode waveguide. Based on the processing parameters of the single-mode waveguide, the transmission loss of waveguides of different lengths was tested using the truncation method. The processing power, scanning speed, and number of scans were adjusted to obtain the lowest transmission loss parameters.

[0015] Step 3: Prepare short-wavelength type I waveguide;

[0016] Repeat step 1 with a new sample to perform slit shaping;

[0017] Using the minimum transmission loss parameter obtained in step 2, the laser focus is controlled to a set depth inside the sample, and a single-mode type I waveguide is prepared by multiple single-line scanning.

[0018] The phase delay of the light guiding region of the waveguide is measured, and the refractive index change is calculated based on the phase delay, wherein the light guiding region of the waveguide is located below the laser modified region and has a higher refractive index.

[0019] Furthermore, in step 1, the wavelength of the femtosecond laser is 515 nm, the pulse width is 197 fs, and the repetition frequency is 0-2 MHz; the maximum single pulse energy of the femtosecond laser is 0.4 mJ, the numerical aperture of the objective lens OL is 0.75, and the magnification is 40×; the sample to be processed is a 25 mm × 25 mm × 5 mm sapphire; the half-wave plate HWP1 is mounted on an electrically controlled knob stepper motor, and the rotation angle is precisely controlled by a computer, so that the combination of HWP1 and the polarization beam splitter PBS can achieve laser power control.

[0020] Furthermore, in step 2, the laser processing power is 30-130 mW, and the scanning speed is 0.1-15 mm / s;

[0021] The number of scans is 3 or 5, and the test wavelength of waveguide transmission loss is 510 nm.

[0022] Furthermore, in step 2, the truncation method is used to obtain the transmission loss of the waveguide by testing the insertion loss of waveguides of different lengths, which specifically includes the following:

[0023] Insertion loss is calculated using the formula IL = PL * L + C, where PL is the waveguide transmission loss (dB / cm), L is the waveguide length, and C is the coupling loss (dB).

[0024] Multiple groups of insertion loss values ​​were measured by changing the waveguide length L, and the slope PL and intercept C were obtained by linear fitting.

[0025] Furthermore, in step three, the laser focus depth is 170-190 μm below the sample surface.

[0026] Furthermore, in step 3, the refractive index change calculation formula is:

[0027] in: is the phase change, λ is the wavelength of light, Δn is the refractive index change, and L is the length of the medium through which the light travels.

[0028] Furthermore, in step 3, for the processing parameters with the lowest transmission loss, the light-guiding area with a high refractive index formed below the processing track is compared with the unprocessed sapphire, and the refractive index increase is 6 to 8×10 -4 .

[0029] In a second aspect, the present invention also provides an application of a method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser in preparing a short-wavelength type I photonic device. The short-wavelength type I photonic device includes a short-wavelength type I waveguide with different degrees of bending, a Y-type beam splitter with an arbitrarily adjustable splitting ratio, and a directional coupler DC device.

[0030] Furthermore, the bending degree of the curved waveguide is controlled by the horizontal distance L and the vertical distance S, with S set to 80 μm and 100 μm, and L set to 5-7 mm;

[0031] The beam splitting ratio of the Y-type beam splitter is achieved by adjusting the length of the coupling region.

[0032] In a third aspect, the present invention also provides a method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser for application in preparing single-mode waveguides of different wavelength bands.

[0033] Compared with the prior art, the advantages of the present invention are as follows:

[0034] (1) The present invention utilizes femtosecond laser slit shaping technology to prepare a low-loss type I waveguide inside sapphire through single-line modification. This method is simple and reliable, greatly reduces the size, and effectively solves the problem of complex process and low integration of femtosecond laser preparation of low-loss waveguide inside sapphire material.

[0035] (2) The sapphire type I waveguide prepared by the present invention is more stable and can meet the needs of various complex environments. The transmission loss is as low as 1dB / cm at a pass wavelength of 510nm, which gives it unique advantages in specific spectral applications and is conducive to improving the overall performance and integration of the femtosecond laser direct writing on-chip three-dimensional photonic chip. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly describes the drawings required for the specific embodiments or the description of the prior art. Similar elements or parts are generally identified by similar reference numerals throughout the drawings. Elements or parts in the drawings are not necessarily drawn to scale.

[0037] Figure 1 This is a schematic diagram of the optical path of the present invention for efficiently preparing a type I photonic device in sapphire material using a femtosecond laser;

[0038] Among them, CL1-cylindrical concave lens, CL2-cylindrical convex lens, HWP1-half wave plate, PBS-polarization beam splitter, M1-first reflector, HWP2-half wave plate, Slit-adjustable width slit, M2-second reflector, OL-objective lens, M3-third reflector, L1-convex lens, CCD-camera;

[0039] Figure 2 Here are the light intensity distribution diagrams before and after the slit cutting, the microscope end-view diagram of the processed waveguide trajectory, and the refractive index distribution diagram along the dotted line in the end-view diagram;

[0040] Among them, (a1) is the light intensity before slit cutting, (a2) is the morphology of the waveguide end face, and (a3) ​​is the refractive index distribution along the dotted line in the waveguide end face diagram;

[0041] (b1) is the light intensity before and after the slit is trimmed, (b2) is the morphology of the waveguide end face, and (b3) is the refractive index distribution along the dotted line in the waveguide end face diagram;

[0042] Figure 3 It is the curve of insertion loss changing with the cross-sectional size of the light-guiding area;

[0043] Figure 4 This is the transmission loss curve of the waveguide at different depths, scan times, power and speed;

[0044] Among them, (a) shows the results of varying processing power, scanning speed, and number of scans at a depth of 170 μm; (b) shows the results of varying processing power, scanning speed, and number of scans at a depth of 180 μm; (c) shows the results of varying processing power, scanning speed, and number of scans at a depth of 190 μm.

[0045] Figure 5 The prepared curved waveguide structure and the insertion loss curves of different curved structures;

[0046] Among them, (a) is a schematic diagram of waveguides with different curved structures; (b) is the insertion loss curve of waveguides with different curved structures;

[0047] Figure 6 The figure shows a microscope top view of the Y-branch device, the mode field of the output port of the Y-branch device, and the prepared DC oscillation curve.

[0048] Among them, (a) is the top view of the Y-branch device; (b) is the output port mode field of the Y-branch device; (c) is the DC oscillation curve under different coupling spacing and coupling lengths prepared.

[0049] Figure 7 To prepare the end face image of multi-band single-mode waveguide and the mode field image of TE polarized light based on splicing technology. DETAILED DESCRIPTION

[0050] In order to clearly and completely describe the technical solution and specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings:

[0051] Example 1

[0052] This embodiment provides a method for fabricating a short-wavelength, low-loss Type I waveguide in sapphire using a femtosecond laser through slit shaping technology. This method utilizes a femtosecond laser uniform scanning method and adopts different processing power, scanning speed, and scanning times to fabricate a straight waveguide. A curve of insertion loss versus waveguide length is obtained through truncation testing, thereby fabricating a Type I waveguide with low transmission loss. The specific steps are as follows:

[0053] (1) Light path adjustment: The light path adjustment of the femtosecond laser direct writing processing system used in this embodiment follows two basic principles, namely, central incidence and original path return; specifically, the following steps are included:

[0054] First, turn on the processing platform laser and preheat it to stabilize the laser output. The laser's output wavelength is 515nm, its pulse width is 197fs, and its repetition rate is 1MHz. The objective lens OL used has an NA of 0.75 and a magnification of 40×. Adjust the first reflector M1 so that the center of the light beam enters the objective lens OL, and adjust the second reflector M2 so that the light beam can return along its original path. Then, place the slit, use a fluorescent card to determine the position of the laser center, and use the CCD to observe and adjust the angle and position of the slit so that the center of the slit coincides with the center of the laser.

[0055] (2) Leveling the sample stage, specifically including the following steps:

[0056] First, if Figure 1As shown, the femtosecond laser emitted by the laser is expanded by the first concave lens CL1 and the second convex lens CL2. The center of the light beam passes through the center of the half-wave plate HWP1 and the polarization beam splitter PBS and passes through the reflector M1, the half-wave plate HWP2 and the slit Slit in sequence. At this time, the light beam is shaped into a slender strip-shaped light spot by the cylindrical lens-slit; then, the laser is incident on the objective lens OL with the help of the second reflector M2. The optical axis of the half-wave plate HWP1 at the laser exit is rotated by computer control to adjust the laser energy behind the reflector M2 and before the entrance pupil of the objective lens OL. The objective lens OL focuses the laser and then makes it incident on the surface of the glass sample on the sample stage; then, with the help of the illumination light emitted by the fixed illumination light source LED and the camera CCD to the right of the reflector M3, the leveling process of the sample stage can be monitored in real time on the computer PC. To focus the femtosecond laser on the glass sample surface, adjust the Z axis of the precision three-dimensional motion platform so that the objective lens (OL) descends to the working distance. Meanwhile, observe the CCD imaging during the descent of the objective lens (OL). After finding the sample surface, begin leveling the sample stage, and use the CCD imaging system to monitor the leveling process in real time. The specific leveling process is as follows: define the two perpendicular sides of the sample as the X-axis and Y-axis. Use the computer to adjust the precision three-dimensional motion platform in the X direction, and adjust the X-direction leveling knob at the same time. Repeat this process until the CCD imaging system displays a uniform minimum reflection spot on the glass sample surface when the femtosecond laser moves in the X direction. At this point, X-direction leveling is complete. Similarly, make the sample stage horizontal in the Y direction. Finally, use the computer to adjust the three-dimensional motion platform so that the laser moves clockwise (or counterclockwise) along the edge of the glass sample. The CCD imaging system displays a uniform minimum reflection spot on the glass sample surface, indicating that the sample stage is perpendicular to the focused laser beam, indicating that the sample stage is leveled.

[0057] (3) Fabrication of low-loss type I waveguides by in-situ multiple uniform-speed scanning of femtosecond lasers;

[0058] First, the laser pulse repetition frequency was adjusted to 1 MHz, and the half-wave plate HWP1 was rotated to maintain a laser power of 80 mW before the entrance pupil of the objective lens (OL). The laser was focused 180 μm into the sample. Next, a processing program was loaded onto a PC, causing the femtosecond laser to scan relative to the 3D displacement stage at a speed of 0.5-5 mm / s, with a scan length of 27 mm and a processing depth of 180 μm. Similarly, the laser output power, laser pulse repetition frequency, and laser focus depth were sequentially varied, and Type I waveguides were fabricated with scans ranging from 1 to 5 times. Finally, the resulting sample end faces were polished to nanometer-level roughness, and the insertion loss and output mode field of the waveguides fabricated with different laser single pulse energies, scan speeds, and scan times were measured.

[0059] (4) Characterization of Type I waveguide performance and transmission loss obtained using the truncation method;

[0060] First, the end face of the sample was polished. The optical microscope photo of the waveguide end face confirmed that after the slit shaping, the processed waveguide changed and a fixed light-guiding area was generated. Usually, the light field without any beam shaping will have a vertical line structure on the processed end face, which cannot form a good light-guiding area. Figure 2 After the slit shaping, the energy of the laser focus is broadened in the horizontal direction, and the processed end face has a triangular structure, forming a good light-guiding area at the lower end, as shown in (a2). Figure 2 As shown in (b2).

[0061] The morphology of the stress-modified waveguides was evaluated by optical microscopy. Figure 2 (b2) shows that the induced modified region is divided into two parts, the upper region being black and the lower region being light gray. This layered feature also exists under other processing parameters. The stress-induced compression increases the refractive index below the processed track, generating a stress-induced waveguide. The black region above the processed track indicates a negative refractive index difference relative to the unmodified glass, while the gray region below the processed track indicates a positive refractive index difference relative to the unmodified glass.

[0062] The phase delay of the sample was converted into visible light and dark contrast through phase contrast microscopy. The phase delay of the processing track before and after slit shaping was measured. The phase difference along the dotted line distribution in the microscope end face images (a2) and (b2) was calculated respectively. The refractive index change of different modified areas was calculated according to the formula. The refractive index change calculation formula is:

[0063] in: is the phase difference, λ is the wavelength of light, Δn is the refractive index change, and L is the length of the medium through which the light passes. Assuming the refractive index change of the bulk material is 0, Figure 2 (a3) and (b3) show the slit before and after shaping, respectively. Figure 2The refractive index change distribution of the dotted path in (a2) and (b2). The processing track without any beam shaping shows a negative refractive index change (Δn<0). Although there is a positive refractive index change, the amount of positive refractive index change is very small and cannot form a good light-guiding area. After slit shaping, the focus energy distribution changes from a longitudinally elongated ellipse to a triangular distribution. A triangular end face shape can be prepared inside the sapphire material. The processing track also shows a negative refractive index change, but below the processing track, compared with other areas, only this position clearly shows a larger positive refractive index change, forming a stable light-guiding area. Type I waveguide means that the focused femtosecond laser pulse causes a uniform increase in the refractive index at the focus (Δn>0). The scanning track is a waveguide, which can easily write three-dimensional optical waveguide devices directly in the medium. Type I-like waveguides, on the other hand, are fabricated using a femtosecond laser pulse that induces a negative refractive index change (Δn < 0) at the focal point. This stress causes the refractive index to increase below the machining track, leaving the light-guiding region confined to the scanning track. Similar to Type I waveguides, complex modified structures are not required to increase the core refractive index; a single-line modification can create an effective light-guiding region within the material. Therefore, slit shaping can be used to fabricate Type I-like waveguides within sapphire.

[0064] The main principle is that after the circular Gaussian light beam is shaped by the slit and focused by the lens into the interior of the sapphire material, its focal energy distribution changes from a longitudinally elongated ellipse to a triangular distribution, and a triangular end face shape can be prepared inside the sapphire material. Through a phase contrast microscope, the phase delay of the sample is converted into a visible light and dark contrast, the phase delay is measured, the phase difference between the waveguide and the area around the waveguide is calculated, and the refractive index change is calculated. The material in the focal area undergoes damaging modifications, including the destruction of the lattice structure and volume expansion, which leads to a decrease in the refractive index of the damaged area and the formation of a low refractive index area. The volume expansion of the damaged area will exert mechanical stress on the surrounding undamaged crystals. The stress changes the refractive index of the surrounding material through the photoelastic effect, so that a stable, high refractive index light guiding area is formed below the processing track.

[0065] The shape of the processed region produced by slit shaping technology in different crystal materials depends primarily on the material's crystal structure, the laser-material interaction mechanism, and the optimization of processing parameters. In isotropic materials (such as quartz glass), circular waveguide cross-sections can be easily achieved. However, in anisotropic materials (such as sapphire), the shape of the processed region may be affected by the crystal orientation, resulting in a triangular or other specific shape. When a femtosecond laser is focused within a sapphire crystal, the laser energy primarily causes localized modification of the material through thermal effects. Due to the anisotropy of sapphire, the rate of heat diffusion varies in different directions, causing the shape of the processed region to be affected by the crystal orientation. Due to sapphire's hexagonal structure, the material response is more pronounced in certain directions, making the processed region more susceptible to expansion in these directions. The rate of heat diffusion varies in different directions. In some directions, heat diffusion is faster, resulting in a more pronounced expansion of the processed region in these directions, resulting in a triangular cross-section. By adjusting parameters such as laser pulse energy, scanning speed, and focal position, it is possible to induce a region of refractive index variation with a specific shape in the sapphire crystal.

[0066] from Figure 2 It can be seen from the microscope image that the strong stress areas in the vertical direction indicate that the asymmetric vertical stress generated by femtosecond laser exposure has a preferential orientation. These stress areas also indicate the local densification of the material, which is partly responsible for the change in refractive index. Similar stress-affected areas have been found in other crystals. The dense waveguide area formed outside the modified track is also formed by femtosecond laser exposure, which produces stress-induced waveguides. From the test results, it can be seen that the modified area below the processing track can guide light, and the modified area has a significant dependence on the polarization state, that is, the TE mode exhibits low-loss light-guiding characteristics in this area, while the TM mode cannot guide light. The variation of insertion loss with the cross-sectional size of the light-guiding area was then explored, such as Figure 3 As shown, within a specific cross-sectional size range, the insertion loss of the waveguide will be significantly reduced, and the loss of the waveguide can be controlled by adjusting the cross-sectional size of the light-guiding area.

[0067] Then, a 510nm laser is used to test the mode field and loss of the straight waveguide respectively. Next, through the three-axis precision alignment fiber optic test platform, the 510nm horizontally polarized light (H light) is coupled into the input end of the photonic chip through a 450-600nm optical fiber, and a 50× objective lens is used to collect the light beam emitted from the output end of the photonic chip. The waveguide mode is recorded and judged through a mode analyzer, the single-mode waveguide is recorded, and the output power of the single-mode waveguide is measured. The insertion loss calculation formula is used: Insertion loss = -10×lg (output power / input power), the output power represents the maximum output power of each waveguide obtained by the power meter, and the input power represents the laser power emitted by the optical fiber without the waveguide; then, the truncation method is used to measure the transmission loss of the waveguide, and a diamond cutting wire is used to cut from the output end of the photonic chip, with a cutting length of 2mm. The newly cut waveguide end face is polished, and the insertion loss is continued to be tested, and the above steps are repeated 4-6 times. The experimental data is linearly fitted according to IL=PL*L+C to obtain the linear intercept C and slope PL. The experimental results are shown in the figure. Figure 4 As shown in the figure, the experimental results show that when the waveguide transmission loss is the lowest at 1dB / cm, the processing power is 80mW, the repetition rate is 1MHz, the processing speed is 1mm / s, the number of scans is 5, and the depth is 190μm. This shows that reducing the scan speed and increasing the number of scans will reduce the waveguide loss, but using an even lower scan speed or further increasing the number of scans will cause the waveguide mode to become multimode. Thus, all the optimal processing parameters for preparing low-loss Type I waveguides have been determined, including laser single pulse energy, scan speed, and number of scans.

[0068] Therefore, by shaping the slit and controlling parameters such as the femtosecond laser scanning speed, processing power and scanning speed, a short-wavelength, low-loss Type I waveguide can be prepared inside the sapphire material, greatly improving the performance and integration of integrated photonic chips.

[0069] Example 2

[0070] This embodiment provides a method for fabricating a type I curved waveguide using a femtosecond laser. The specific steps are as follows:

[0071] (1) Optical path adjustment: same as in Example 1.

[0072] (2) Leveling of the sample stage: same as in Example 1.

[0073] (3) Femtosecond laser fabrication of type I curved waveguide: First, the design of the curved waveguide adopts Figure 5 The curved waveguide structure is designed as shown. The lateral position y(x) of the waveguide at a distance x along the bending direction is described by the following expression:

[0074]

[0075] Among them, S is the vertical distance, which is set to 80 and 100 μm, and L is the horizontal distance of different sizes, which is set to 5-7 mm, such as Figure 5 (a) shows a top view of waveguides with different bending curvatures.

[0076] The curvature is given by:

[0077]

[0078] Among them, S is the vertical distance, which is set to 80 and 100 μm, L is different horizontal distances, which is set to 5-7 mm, and r is the bending radius.

[0079] Afterwards, the optimal laser-processed type I waveguide parameters were obtained through Example 1. By changing the length in the horizontal and vertical directions and controlling the curvature radius of the S-bend, a computer (PC) was used to control the three-dimensional mobile platform to produce type I waveguides with different curved structures.

[0080] (4) Type I bend waveguide loss test: First, the prepared sample is polished. Using a six-axis precision alignment fiber test platform, 509nm horizontally polarized light (H light) is coupled into the input end of the photonic chip through a 450-600nm optical fiber. A 50× objective lens is used to collect the light beam emitted from the output end of the photonic chip, and the output power of the type I bend waveguide is measured. The insertion loss calculation formula is: Insertion loss = -10×lg (output power / input power). The output power represents the maximum output power of each waveguide obtained by the power meter, and the input power represents the laser power emitted from the fiber without waveguide. Figure 5 (b) shows the insertion loss of the curved waveguide at different S and L lengths. As the bending radius increases, the turning radius decreases and the total insertion loss of the waveguide decreases.

[0081] In summary, based on the above processing methods and processing parameters, low-loss curved waveguides can be produced, providing a basis for the preparation of complex and stable integrated photonic devices.

[0082] Example 3

[0083] This embodiment provides a method for preparing a short-wavelength, low-loss type I photonic device using a femtosecond laser; the pre-processing preparation steps are as described in Example 1 (1)(2), and then the single-mode waveguide parameters with the lowest loss in Example 1 are used to process the photonic chip by repeated scanning of a slit-shaped femtosecond laser, and the end face of the photonic chip is polished. First, a Y-type beam splitter with an arbitrarily adjustable beam splitting ratio is successfully prepared. Figure 6 As shown in (a), the beam splitting ratio of the Y beam splitter is controlled by controlling the length of the coupling region. The beam splitting ratio of the Y beam splitter is tested using a 510nm laser. Figure 6(b) shows the mode fields of the output ports with different splitting ratios. Then, based on this, the same method was used to prepare directional couplers (DC) with different coupling spacings and different coupling lengths. The splitting ratio of DC was tested using a 510nm laser. The power of the two output ports of DC was recorded simultaneously with two identical powers. Finally, the transmittance curve was obtained, as shown in the figure below. Figure 6 The DC fabricated by this method can achieve beam splitting control of any proportion. Combined with the stability and high-temperature resistance of sapphire materials, it improves the integration of photonic chips and enables effective light modulation. When constructing complex three-dimensional structures, it can effectively reduce signal attenuation and noise interference, ensuring that photonic chips can maintain efficient and stable operation under high-density integration conditions.

[0084] Example 4

[0085] This embodiment provides a method for preparing single-mode waveguides of different wavelength bands by splicing technology; using the single-mode waveguide parameters with the lowest loss in Example 1, horizontal splicing of different horizontal lengths is performed to change the overall size of the waveguide to meet the requirements of different light-transmitting bands, such as Figure 7 As shown in the waveguide end face diagram. When the waveguide is not spliced, the waveguide can meet the short wavelength single-mode light transmission of 405 and 510nm. When the waveguide is horizontally spliced, by adjusting the number of waveguide splices and increasing the horizontal length of the waveguide, single-mode light transmission at wavelengths of 808 and 980nm can be achieved, as shown in Figure 7 The waveguide mode field is shown in the image. Fabricating waveguides that meet different optical transmission bands is of great significance for meeting diverse application requirements, improving system integration and flexibility, expanding the application scope of photonic technology, and enhancing the performance and efficiency of photonic devices.

[0086] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0087] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0088] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser, characterized in that: The specific steps include: Step 1: Slit shaping to obtain a circularly distributed three-dimensional light focal field; The femtosecond laser emits low single pulse energy and passes through a cylindrical lens system consisting of a cylindrical concave lens CL1 and a cylindrical convex lens CL2, a half-wave plate HWP1, a polarization beam splitter PBS, a first reflector M1, and a half-wave plate HWP2, and then is shaped into a long and thin strip of light through a slit. The laser is then reflected by a second reflector M2 to the objective lens OL, and the laser energy before the objective lens entrance pupil is controlled by adjusting the optical axis direction of the half-wave plate HWP1. The polarization of the laser is monitored using a polarization beam splitter, and the polarization direction of the femtosecond laser before the objective lens entrance pupil is adjusted using a half-wave plate (HWP2) so that the laser polarization direction is perpendicular to the scanning direction. The position of the laser center is determined using a fluorescent card, and the angle and position of the slit are observed and adjusted using a CCD so that the center of the slit coincides with the center of the laser. The objective lens OL focuses the shaped laser onto the sample surface, so that the focal field of the objective lens is distributed in a triangular shape inside the sample along the laser scanning direction; Step 2: Determine the processing parameters that minimize the waveguide transmission loss; Focus the shaped laser on the preset processing position of the sample, and directly write the waveguide by adjusting the laser processing power, scanning speed and number of scans; load the pre-written MATLAB processing program, and control the movement of the precision three-dimensional motion platform according to the written MATLAB program; The waveguide end face was characterized using an optical microscope to screen the processing parameters of the single-mode waveguide. Based on the processing parameters of the single-mode waveguide, the transmission loss of waveguides of different lengths was tested using the truncation method. The processing power, scanning speed, and number of scans were adjusted to obtain the lowest transmission loss parameters. Step 3: Prepare short-wavelength type I waveguide; Repeat step 1 with a new sample to perform slit shaping; Using the minimum transmission loss parameter obtained in step 2, the laser focus is controlled to a set depth inside the sample, and a single-mode type I waveguide is prepared by multiple single-line scanning. The phase delay of the light guiding region of the waveguide is measured, and the refractive index change is calculated based on the phase delay, wherein the light guiding region of the waveguide is located below the laser modified region and has a higher refractive index.

2. The method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser as claimed in claim 1, wherein: In step 1, the wavelength of the femtosecond laser is 515nm, the pulse width is 197fs, and the repetition rate is 0-2MHz; the maximum single pulse energy of the femtosecond laser is 0.4mJ, the numerical aperture of the objective lens OL is 0.75, and the magnification is 40×; the sample to be processed is a 25mm×25mm×5mm sapphire; the half-wave plate HWP1 is installed on an electric control knob stepper motor, and the rotation angle is precisely controlled by a computer, so that the combination of HWP1 and the polarization beam splitter PBS can achieve laser power control.

3. The method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser as claimed in claim 1, wherein: In step 2, the laser processing power is 30-130 mW, and the scanning speed is 0.1-15 mm / s; The number of scans is 3 or 5, and the test wavelength of waveguide transmission loss is 510 nm.

4. The method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser according to claim 1, wherein: In step 2, the truncation method is used to obtain the transmission loss of the waveguide by measuring the insertion loss of waveguides of different lengths. The specific contents include the following: Insertion loss is calculated using the formula IL = PL * L + C, where PL is the waveguide transmission loss, L is the waveguide length, and C is the coupling loss. Multiple groups of insertion loss values ​​were measured by changing the waveguide length L, and the slope PL and intercept C were obtained by linear fitting.

5. The method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser as claimed in claim 1, wherein: In step 3, the laser focus depth is 170-190 μm below the sample surface.

6. The method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser as claimed in claim 1, wherein: In step 3, the refractive index change calculation formula is: in: is the phase change, λ is the wavelength of light, Δn is the refractive index change, and L is the length of the medium through which the light travels.

7. The method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser as claimed in claim 1, wherein: In step 3, for the processing parameters with the lowest transmission loss, the high refractive index light guide area formed under the processing track is compared with the unprocessed sapphire, and the refractive index increase is 6 to 8×10 -4 .

8. Application of the method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser as claimed in claim 1 in preparing a short-wavelength type I photonic device, characterized in that: The short-wavelength type I photonic device includes a short-wavelength type I waveguide with different bending degrees, a Y-type beam splitter with an arbitrarily adjustable beam splitting ratio, and a directional coupler DC device.

9. Use of the method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser as claimed in claim 8 in preparing a short-wavelength type I photonic device, characterized in that: The degree of bending of the curved waveguide is controlled by the horizontal distance L and the vertical distance S, with S set to 80μm and 100μm, and L set to 5-7mm; The beam splitting ratio of the Y-type beam splitter is achieved by adjusting the length of the coupling region.

10. Use of the method for preparing a short-wavelength type I waveguide in sapphire using a femtosecond laser as claimed in claim 1 in preparing single-mode waveguides of different wavelength bands.

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