Method for aberration correction of lens wavefront shaping femtosecond laser direct writing and application thereof

Abstract

The application discloses a kind of lens wavefront shaping femtosecond laser direct writing processing aberration method and application, belong to laser processing technical field, the method uses 4f optical lens system, by adjusting lens spacing correction laser wavefront to solve the problem of limited depth of field when femtosecond laser is carried out waveguide direct writing;The application is by designing the interval of the first lens compared to the second lens in 4f lens to thereby shape the wavefront of incident laser, aberration correction is realized at specific depth in material;By three-dimensional precision displacement platform, the depth of laser focal point in sample is matched with lens spacing, and focal point lengthening effect is weakened.Changing processing depth while adjusting 4f lens spacing, different layer depth aberration correction can be realized, so that femtosecond laser large-depth waveguide direct writing is realized.The application uses simple 4f lens group instead of expensive spatial light modulator, not only solves the aberration problem when different layer depth is processed, but also realizes the processing direct writing of large-depth waveguide.

Description

Technical Field

[0001] This invention belongs to the field of laser processing technology, specifically involving femtosecond laser direct writing processing of deep glass waveguides using lens wavefront aberration shaping. By changing the spacing of the lens group in the 4f system, the laser wavefront in front of the objective lens is modulated, and the depth of the focused laser spot within the sample is adjusted to match the modulated wavefront, thereby realizing direct writing processing of deep glass waveguides based on lens wavefront aberration shaping. Background Technology

[0002] As the most fundamental and crucial component in integrated photonic chips, the optical waveguide is a high-refractive-index region covered by a low-refractive-index cladding. Compared to methods such as planar lithography, ion diffusion, and ion / neutron implantation for fabricating optical waveguides, femtosecond laser direct writing technology offers high precision, 3D processing capabilities, and the ability to perform direct writing in almost any transparent medium, thus finding significant applications in quantum computing and simulation. Due to the refractive index difference between the sample and air, the unshaped femtosecond laser, when focused within the medium, elongates the focal spot, causing defocusing. This defocusing results in a narrow, elongated elliptical cross-section for the laser-written waveguide, and the material modification threshold varies with the focusing depth, preventing the fabrication of waveguides at great depths. This leads to differences in spatial coupling capabilities within the waveguide, significantly limiting the ability to create true 3D waveguide chips using femtosecond laser direct writing and hindering the improvement of chip integration density. Furthermore, the aforementioned defocusing effect also results in poor waveguide centrosymmetry, increasing coupling loss with optical fibers.

[0003] Currently, research groups in various countries mainly employ methods such as longitudinal direct writing, adding cylindrical lens groups or slit-shaped beams, and multiple scanning to improve the symmetry of waveguide cross-sections. However, these methods suffer from drawbacks such as short waveguide lengths, difficulty in fabricating curved circular waveguides, and the need to find optimal slit widths and optimal lens group focal lengths. The limitation on fabrication depth is generally mitigated through spatial beam modulation, which is costly, requires complex aberration compensation algorithms, and involves applying different phase compensation maps for different layer depths, further complicating waveguide fabrication. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention aims to provide a method for correcting aberrations by using lenses to shape the wavefront of a femtosecond laser, thereby enabling the fabrication of deep glass waveguides. This invention employs a 4f optical lens system, adjusting the lens spacing to correct the laser wavefront and solve the problems of defocusing and limited depth during direct writing of waveguides using femtosecond lasers. The principle of this invention is as follows: The spacing between the first and second lenses in the 4f lens system is designed to shape the wavefront of the incident laser, achieving aberration correction at a specific depth within the material. A three-dimensional precision displacement platform is used to match the depth of the laser focus within the sample with the lens spacing, reducing the focus elongation effect. Simultaneously, by adjusting the 4f lens spacing while changing the fabrication depth, aberration correction at different layer depths can be achieved, thus enabling direct writing of deep femtosecond laser waveguides. In this way, a simple 4f lens group replaces the expensive spatial light modulator, not only solving the aberration problem during fabrication at different layer depths but also realizing direct writing of deep waveguides.

[0005] This invention is achieved through the following technical solution:

[0006] A method for aberration correction using femtosecond laser direct writing with lens wavefront shaping, comprising the following specific steps:

[0007] Step 1: Leveling the sample stage;

[0008] The specific steps are as follows: First, the femtosecond laser emitted from the laser is incident on the first concave lens L1 and the first convex lens L2 in turn to expand the beam, increasing the beam size by 4 times. Then, it passes through the half-wave plate HWP and the Glan-Taylor prism GTL in sequence. After being reflected by the first mirror M1, the second mirror M2, the third mirror M3, and the fourth mirror M4, it passes through the first convex lens L3 in the 4f lens group mounted on the single-axis stepper motor LS, then through the second convex lens L4 in the 4f lens group, and finally through the reflection of the mirror M5 and the focusing of the objective lens OL before entering the glass sample surface on the sample stage. The illumination source LED is fixed on the frame of the mirror M5, so that the white light emitted by the illumination source LED enters the objective lens OL and is focused on the glass sample surface to illuminate the sample. The illumination light passes through the mirror M5 and is focused into the CCD. The CCD is connected to the computer to monitor the leveling process of the sample stage in real time.

[0009] Step 2: Establishment and verification of the L3 coordinate library for convex lens No. 1;

[0010] First, the origin coordinates of L3 are defined as the coordinates when the distance between the first convex lens L3 and the second convex lens L4 in the 4f lens group is 2f, and the direction of motion of the first convex lens L3 when it is close to the second convex lens L4 is defined as the positive Y-axis. Then, ZMAX is used to simulate the z-direction dimension of the focal point at different depths d1 inside the material. When the first convex lens L3 is at the origin, it is the uncorrected case. The coordinates of the first convex lens L3 are changed. When the z-direction length of the focal point decreases to 60% of the uncorrected case, the corresponding coordinate Y1 of the first convex lens L3 in the 4f lens group is recorded. Similarly, by changing different focusing depths d, the coordinates of the first convex lens L3 are changed to achieve aberration correction at different depths, thereby establishing a coordinate library for aberration correction of the first convex lens L3. Then, the Virtualab mid-field tracking model is used. When lens L3 is positioned at coordinate Y in the coordinate library of convex lens L3, the normalized energy distribution of the focal point at the corresponding depth d within the material is determined. The focus z-direction dimension is compared to the normalized energy distribution of the focal point when the position of convex lens L3 remains unchanged. If the z-direction dimension of convex lens L3 remains 60% of the uncorrected z-direction dimension under different coordinates, the accuracy of the coordinate library of convex lens L3 is verified, and a curve of the coordinates of convex lens L3 versus the correction depth is plotted, verifying the correction effect of the focal z-direction length. If there are points that are not 60% of the z-direction length, Virtualab is used again to perform simulation, find the corresponding correction coordinates at the depth, and replace the original points in the coordinate library.

[0011] Step 3: Direct waveguide writing at different depths;

[0012] First, based on the target writing depth d1, the calibration coordinate Y1 is found in the coordinate library of the first convex lens L3, and the first convex lens L3 is moved to the calibration coordinate Y1 using a single-axis stepper motor. Then, the objective lens is lowered to the target writing depth d1 using computer control. Next, the air-floating displacement platform is controlled to write the waveguide at depth d1. When the waveguide at depth d1 is written, the stepper motor is controlled to move the first convex lens L3 to the calibration coordinate Y2, while the objective lens is moved to depth d2. Then, the air-floating displacement platform is controlled to write the waveguide at depth d2. Thus, the direct writing fabrication of waveguides at different depths within the sample is achieved.

[0013] Further, the sample stage leveling process described in step one is as follows: Using the two mutually perpendicular sides of the sample as the X-axis and Y-axis, the motion platform is adjusted by the computer to draw lines on the X-axis, while simultaneously adjusting the X-direction leveling knob until the laser can scan out a uniform damage line. At this point, the X-direction leveling is complete. Similarly, the laser is used to draw lines on the Y-axis, while simultaneously rotating the Y-direction leveling knob to level the Y-direction. At this point, the laser can draw uniformly wide damage lines in both the X and Y directions, indicating that the sample stage is perpendicular to the focused laser beam, thus completing the sample stage leveling.

[0014] Further, the femtosecond laser in step one has a wavelength of 1030 nm, a pulse width of 239 fs, and a repetition rate of 1 MHz; the combination of the half-wave plate (HWP) and the polarizing beam splitter (PBS) can control the laser power, with a power of 300 mW used for leveling; the focal length of the first concave lens L1 is -8 cm, the focal length of the second convex lens L2 is 32 cm, the focal lengths of the first convex lens L3 and the second convex lens L4 in the 4f lens group are 10 cm, and the distance between the entrance pupil of the objective lens OL and the second convex lens L4 is 10 cm; the single-axis stepper motor LS is a linear stepper motor with a stroke of 45 cm, a maximum speed of 50 mm / s, and a positioning accuracy of 0.3 mm; the working distance of the objective lens OL is 5 mm, NA = 0.75, and a magnification of 40; the geometric centers of the objective lens OL, the reflecting mirror M, and the camera CCD element are located on the same vertical line, and the vertical distances from their geometric centers to the sample surface are Z and Z, respectively. OL = 5.25cm, Z M =10.50cm, Z CCD =21cm; the sample to be processed is Corning glass with dimensions of 2.5cm×2.5cm×0.5cm.

[0015] Furthermore, in step two, when the first convex lens L3 is at the origin 0cm, it is 20cm away from the second convex lens L4; in the ZMAX-simulated aberration correction coordinate library for the first convex lens L3, the correction depth d ranges from 1.1 to 3.0mm, with a step size of 0.1mm; based on the curve of the first convex lens L3 coordinates and the correction depth, the Y coordinate of the next convex lens L3 at the corresponding depth is obtained, with a corresponding coordinate range of 18.77 to -22.76cm.

[0016] Furthermore, in step three, the direct writing power of the waveguide is 650mW and the direct writing speed is 20mm / s; the length of the straight waveguide along the X-axis is 25mm; the depth d is greater than 1mm, at which point the phase difference correction effect using SLM is poor; after the direct writing is completed, the two end faces of the straight waveguide are finely polished, and then the end faces and modes of the waveguide are characterized.

[0017] On the other hand, the present invention also provides a method for processing aberrations by femtosecond laser direct writing of lens wavefront shaping to achieve the application of direct writing of waveguide structures at a large focusing depth.

[0018] Compared with the prior art, the advantages of the present invention are as follows:

[0019] (1) By using a simple 4f convex lens group to replace the expensive spatial light modulator system, direct writing of waveguide structures at great depths was realized, breaking through the problem that the cost of the processing system is proportional to the processing depth;

[0020] (2) By adjusting the distance between the two convex lenses in the 4f lens and modulating the femtosecond laser wavefront, the correction of femtosecond laser direct writing aberrations at different depths was achieved, avoiding the use of complex aberration correction algorithms and effectively solving the problem of femtosecond laser stretching the focal spot at great depths in the material, i.e., the defocusing problem.

[0021] (3) By controlling the synchronous movement of the stepper motor and the objective lens, the function of defocus correction at different depths within the sample is realized. Attached Figure Description

[0022] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0023] Figure 1 This is a schematic diagram of the optical path for femtosecond laser direct writing of a deep glass waveguide using lens wavefront aberration shaping according to the present invention.

[0024] Among them, Fs Laser - femtosecond laser, L1 - first concave lens, L2 - first convex lens, HWP - half-wave plate, GTL - Glan Taylor prism, LS - single-axis stepper motor, M1 - first reflector, M2 - second reflector, M3 - third reflector, M4 - fourth reflector, M5 - fifth reflector, L3 - first convex lens, L4 - second convex lens, OL - objective lens, CCD - camera, LED - illumination source, GS - Corning glass sample;

[0025] Figure 2 This invention provides an L3 coordinate library for aberration correction in femtosecond laser direct writing of deep glass waveguides for lens wavefront aberration shaping.

[0026] Figure 3 For machining at different depths, the corrected wavefront phase diagram of the objective lens before the entrance pupil is shown in the L3 corrected coordinate system of the No. 1 convex lens.

[0027] Figure 4The image shows the focal morphology before and after material aberration correction for femtosecond laser direct writing of a deep glass waveguide using lens wavefront aberration shaping, according to the present invention.

[0028] in, Figure 4 (a1)-(a15) represent the focal morphology of the femtosecond laser focused at different depths within the material when the first convex lens L3 is at the origin; Figure 4 (b1)-(b15) represent the focal morphology of the femtosecond laser focused at different depths within the material when the first convex lens L3 is in the corresponding corrected coordinates;

[0029] Figure 5 Optical microscope image of the waveguide end face during femtosecond laser direct writing of a deep glass waveguide for the purpose of lens wavefront aberration shaping according to the present invention.

[0030] in, Figure 5 (a1)-(b1) and (a2)-(b2) are microscopic images of the unshaped direct-write waveguide end face and mode field at depths of 1.5 mm and 2.5 mm, respectively; Figure 5 (c1)-(d1) and (c2)-(d2) are microscopic images of the direct-write waveguide end face and mode field after femtosecond laser wavefront shaping at a depth of 2.5 mm, respectively. Detailed Implementation

[0031] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0032] Example 1

[0033] Aberration correction for femtosecond laser direct writing at different depths was achieved using a 4f lens group.

[0034] The femtosecond laser wavefront in front of the objective lens can also be adjusted using a simple 4f lens group. Without the need for expensive SLMs and complex defocus compensation algorithms, the laser wavefront phase in front of the objective lens is altered by changing the distance between the front and rear lenses in the 4f lens group. Furthermore, by controlling the synchronous movement of the focal depth and the convex lens position, the defocusing problem during direct-write fabrication of waveguides at different layer depths using femtosecond lasers is solved, significantly reducing the difficulty of fabricating deep-embedded waveguides within materials.

[0035] The following are the specific steps for achieving femtosecond laser direct writing aberration correction at different depths using a 4f lens group:

[0036] (1) Leveling of the sample stage: The femtosecond laser used has a wavelength of 1030nm, a pulse width of 239fs, and a repetition rate of 1MHz; the focal length of the first concave lens L1 is -8cm, the focal length of the second convex lens L2 is 32cm, the focal lengths of the first convex lens L3 and the second convex lens L4 in the 4f lens group are 10cm, and the distance between the entrance pupil of the objective lens OL and the second convex lens L4 is 10cm; the single-axis stepper motor LS used is a linear stepper motor with a stroke of 45cm, a maximum speed of 50mm / s, and a positioning accuracy of 0.3mm; the working distance of the objective lens OL is 5mm, NA = 0.5, and the magnification is 40×. First, as Figure 1 As shown, the femtosecond laser emitted from the laser is expanded by the first concave lens L1 and the first convex lens L2, increasing the beam size by 4 times. It then passes sequentially through the half-wave plate HWP, the Glan Taylor prism GTL, the first reflecting mirror M1, the second reflecting mirror M2, the third reflecting mirror M3, the fourth reflecting mirror M4, the first convex lens L3, and the second convex lens L4. At this point, the first convex lens L3 is at the origin coordinates and does not modulate the laser wavefront, only acting as a convergent lens. Next, the laser is incident on the objective lens OL via the fifth reflecting mirror M5. The optical axis of the half-wave plate HWP is rotated so that the laser energy of the femtosecond pulse after propagating to the fifth reflecting mirror M5 and before the entrance pupil of the objective lens OL is 300mW. Subsequently, the objective lens OL focuses the laser onto the surface of the glass sample on the sample stage. Then, using the illumination light emitted by the LED light source fixed on the frame of the fifth reflecting mirror M5 and the CCD camera above the reflecting mirror M5, the leveling process of the sample stage can be monitored in real time on a computer. Using the two perpendicular sides of the sample as the X and Y axes, the motion platform is moved between the points (0, 0) and (2.5cm, 0) via a PC. Simultaneously, the X-axis leveling knob is adjusted until the laser can draw a uniform damage line 2.5cm long and 4μm wide. At this point, X-axis leveling is complete. Similarly, the laser is used to draw a line between the points (0, 0) and (0, 2.5cm), and the Y-axis leveling knob is used to level the Y-axis. At this point, the laser can draw uniformly wide damage lines in both the X and Y directions, indicating that the sample stage is perpendicular to the focused laser beam. The leveling of the sample stage is complete. The position of the sample stage is then fixed.

[0037] (2) Generation and verification of the coordinate system for convex lens L3: First, ZMAX was used to calculate the z-direction dimension of the focal point at depths ranging from 1.0 mm to 3.0 mm within the sample. The coordinates of convex lens L3 were varied; when the z-direction length of the focal point decreased to 60% of its uncorrected value, the coordinates corresponding to convex lens L3 in the 4f lens group were recorded, establishing the coordinate system as follows: Figure 2 The coordinate library for lens L3 is shown below; then, as shown... Figure 3As shown, the optical path is constructed using Virtualab to generate aberration-corrected laser wavefront phase maps of lens 1 at different L3 coordinates in front of the objective lens. Then, using Virtualab's field tracing function, the focal morphology at different depths within the material before and after laser wavefront shaping is simulated, as shown below. Figure 4 As shown. At this time, Virtualab can be used to verify whether the z-direction size of the shaped focal spot is 60% of the z-direction size when it is not rectified. The laser wavefront phase map is a grayscale image with a pixel size of 1024×1024 and a phase size range of [0, 2π].

[0038] (3) Waveguide direct writing at different depths:

[0039] First, based on the experimental design, the target structural processing depth d1 is determined, and the corresponding correction coordinate Y1 is found from the coordinate library of lens L3. The stepper motor LS is then controlled by a computer to move lens L3 to the corresponding correction coordinate Y1, while the objective lens OL is controlled to focus on depth d1. Next, the air-bearing displacement platform is controlled to directly write the waveguide at depth d1. After the waveguide structure processing at depth d1 is completed, L3 is moved to coordinate Y2 according to the L3 coordinate library to switch the depth d2 correction wavefront, while the objective lens OL is controlled to focus on depth d2. At this point, the laser correction wavefront before the entrance pupil of the objective lens OL switches from depth d1 correction to depth d2 correction. The processing depths d1 and d2 are 2.5 and 1.5 mm, respectively, corresponding to coordinates Y1 and Y2 of lens L3 as -1.13 and 14.96 cm, respectively. The movement speed of the stepper motor LS and the objective lens OL is 40 mm / s.

[0040] Example 2

[0041] Aberrations were corrected using a 4f lens group for direct-write waveguides at great depths, and their end-face and mode topography were compared with those of ordinary waveguides without wavefront shaping at the same depth. The specific steps are as follows:

[0042] (1) Leveling of the sample stage: Same as in Example 1.

[0043] (2) Direct writing of ordinary waveguide without wavefront shaping at great depth: First, rotate the optical axis of the half-wave plate HWP so that the laser power in front of the objective lens OL is 700mW; then, use the computer to move the stepper motor LS to place the first convex lens L3 at 0.0cm, and load the pre-written straight waveguide processing program on the computer to write the ordinary straight waveguide with a single focus along the X direction; the length of the straight waveguide being written is 25mm, and the depths are 2.5mm and 1.5mm respectively.

[0044] (4) Direct writing of aberration correction waveguides for lens wavefront shaping at great depths: First, rotate the optical axis of the half-wave plate HWP so that the laser power in front of the objective lens OL is 600mW; then, use the computer to move the stepper motor LS so that the first convex lens L3 is at -1.13cm and 14.96cm respectively, and load the pre-written straight waveguide processing program on the computer. Under the lens wavefront shaping conditions, write the aberration correction waveguides at the corresponding depths along the X direction respectively; the length of the straight waveguides written is 25mm, and the depth is 2.5mm and 1.5mm.

[0045] After processing, the end faces of the direct-write waveguides with and without wavefront shaping were polished, and their end face and mode characteristics were characterized. The results are as follows: Figure 5 As shown. By Figure 5 From (a1), (b1), (c1), and (d1), it can be seen that the wavefront shaping method using the 4f lens group in steps (1)-(4) corrected the focal elongation caused by the defocusing problem, thus improving the symmetry of the waveguide end face; Figure 5 As can be seen from (a2), (b2), (c2), and (d2), the mode field symmetry of the waveguide is improved. That is, the transmission loss and docking loss of the waveguide after wavefront shaping by the 4f lens group are reduced, which is beneficial to the fabrication of deep waveguide devices and thus promotes the development of 3D optical quantum chips.

[0046] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of 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 protection scope of the present invention.

[0047] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0048] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method of aberration correction for lens wavefront shaping femtosecond laser direct writing, characterized in that, The specific steps are as follows: Step one, leveling of the sample stage; The specific steps are as follows: first, the femtosecond laser emitted by the laser is incident into the first concave lens (L1) and the second convex lens (L2) in sequence to expand the beam, and the spot is expanded by 4 times, then passes through a half-wave plate (HWP) and a G-Lens (GTL) in sequence, and then is reflected by the first mirror (M1), the second mirror (M2), the third mirror (M3), and the fourth mirror (M4), and then passes through the first convex lens (L3) of the 4f lens group mounted on the single-shaft stepper motor (LS), and then passes through the second convex lens (L4) of the 4f lens group, and finally is reflected by the mirror (M5) and focused by the objective lens (OL) in sequence, and then is incident onto the glass sample surface on the sample stage; the illumination light source LED is fixed on the mirror frame of the mirror (M5), so that the white light emitted by the illumination light source LED is incident into the objective lens (OL) and focused on the glass sample surface to illuminate the sample; the illumination light is focused and imaged into the CCD after passing through the mirror (M5), and the CCD is connected with the computer, so as to monitor the leveling process of the sample stage in real time; Step two, establishment of the coordinate library of the first convex lens (L3) and verification of the correction effect; Firstly, the origin coordinates of the first convex lens (L3) are defined as the coordinates when the distance between the first convex lens (L3) and the second convex lens (L4) in the 4f lens group is 2f, and the movement direction of the first convex lens (L3) is defined as the positive direction of the Y axis when the first convex lens (L3) is close to the second convex lens (L4); then, the z direction size of the focal point at different depths d1 in the material is simulated by using ZMAX; when the first convex lens (L3) is at the origin, it is the uncorrected state, the coordinates of the first convex lens (L3) are changed, and when the z direction length of the focal point is reduced to 60% of the uncorrected state, the corresponding coordinates Y1 of the first convex lens (L3) in the 4f lens group are recorded; similarly, the coordinates of the first convex lens (L3) are changed to realize aberration correction at different depths, so as to establish the coordinate library of the aberration correction first convex lens; then, the normalized energy distribution of the focal point at the depth d in the material when the first convex lens (L3) is at the coordinates Y in the coordinate library of the first convex lens (L3) is simulated by using the field tracing simulation in Virtualab, and it is judged whether the z direction size is 60% of the uncorrected z direction size compared with the normalized energy distribution of the focal point when the position of the first convex lens (L3) is unchanged; if the z direction size of the first convex lens (L3) at different coordinates is 60% of the uncorrected z direction size, the accuracy of the coordinate library of the first convex lens (L3) is verified, the curve of the coordinates of the first convex lens (L3) and the correction depth is drawn, and the correction effect of the z direction length of the focal point is verified; if there is a point that is not 60% of the z direction length, the simulation is performed again by using Virtualab to find the correction coordinates at the corresponding depth, and the original point in the coordinate library is replaced; Step three, waveguide direct writing at different depths; First, according to the direct writing target depth d1, find the correction coordinates Y1 in the convex lens (L3) coordinate library, and move the convex lens (L3) to the correction coordinates Y1 by using the single-axis stepping motor; then, use the computer to control the objective lens to descend to the direct writing target depth d1; then, control the air floating displacement platform to directly write the waveguide at the depth d1; when the waveguide direct writing at the depth d1 is completed, move the convex lens (L3) to the correction coordinates Y2 at this time, and move the objective lens to the depth d2; then, use the control air floating displacement platform to directly write the waveguide at the depth d2; thereby, the waveguide direct writing preparation at different depths in the sample is realized.

2. The method of claim 1, wherein the lens wavefront shaping femtosecond laser direct writing is used to correct the aberration of the laser beam. The leveling process of the sample stage in step one is as follows: taking the two edges of the sample perpendicular to each other as the X-axis and the Y-axis, draw a line on the X-axis by adjusting the motion platform through the computer, and at the same time, adjust the X-direction leveling knob until the laser can scan a uniform damage line, at this time, the X-direction leveling is completed; similarly, draw a line on the Y-axis by using the laser, and at the same time, rotate the Y-direction leveling knob to level the Y-direction; at this time, the laser can directly write a uniform damage line in the X and Y directions, indicating that the sample stage is perpendicular to the focused laser beam, that is, the leveling of the sample stage is completed.

3. The method of claim 1, wherein the lens wavefront shaping femtosecond laser direct writing method is used to correct the aberration of the femtosecond laser direct writing. The wavelength of the femtosecond laser in step one is 1030 nm, the pulse width is 239 fs, and the repetition frequency is 1 MHz; the combination of the half-wave plate (HWP) and the polarization beam splitter (PBS) can control the laser power, and the power used in the leveling is 300 mW; the focal length of the first concave lens (L1) is -8 cm, the focal length of the second convex lens (L2) is 32 cm, the focal length of the first convex lens (L3) and the second convex lens (L4) in the 4f lens group is 10 cm, and the distance between the object lens (OL) entrance pupil and the second convex lens (L4) is 10 cm; the single-axis stepper motor (LS) is a linear stepper motor, the stroke is 45 cm, the maximum speed is 50 mm / s, and the positioning accuracy is 0.3 mm; the working distance of the object lens (OL) used is 5 mm, NA=0.75, and the magnification is ×40; the geometric centers of the object lens (OL), the mirror (M5), and the camera CCD element are located on the same vertical line, and the vertical distances from the geometric centers to the sample surface are Z OL =5.25 cm, Z M =10.50 cm, and Z CCD =21 cm; the size of the sample to be processed is 2.5 cm×2.5 cm×0.5 cm.

4. The method of claim 1, wherein the lens wavefront shaping femtosecond laser direct writing method is used to correct the aberration of the femtosecond laser direct writing. The convex lens (L3) in step two is at the origin 0 cm, and is 20 cm away from the convex lens (L4); the ZMAX simulation aberration correction convex lens (L3) coordinate library has a correction depth d in the range of 1.1 to 3.0 mm, and the step is 0.1 mm; according to the curve of the coordinates of the convex lens (L3) and the correction depth, the Y coordinates of the convex lens (L3) corresponding to the depth are obtained, and the corresponding coordinate range is 18.77 to -22.76 cm.

5. The method of claim 1, wherein the lens wavefront shaping femtosecond laser direct writing method is characterized by, The direct writing power of the waveguide in step three is 650 mW, and the direct writing speed is 20 mm / s; the waveguide is along the X-axis direction, and the length is 25 mm; the depth d is greater than 1 mm, at this time, the SLM correction aberration effect is poor; after the direct writing is completed, the two end faces of the waveguide are finely polished, and then the end face and the mode of the waveguide are characterized.

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