Method for preparing low-loss sharp-bend waveguide by in-situ multiple modification using femtosecond laser

Abstract

The present invention discloses a method for preparing a low-loss sharp-bend waveguide by in-situ multiple modification using a femtosecond laser, which belongs to the field of laser processing technology. The method uses a femtosecond laser in-situ multiple modification to accurately optimize the problem of large bending loss in a 90° sharp-bend waveguide in a single direct writing. The main principle is as follows: by controlling a high-precision three-dimensional motion platform to focus the femtosecond laser inside the material, the femtosecond laser processing energy and the laser scanning speed are adjusted to achieve the preparation conditions of a single-mode waveguide; then, the waveguide performance is regulated by in-situ multiple modification using a femtosecond laser to prepare a 90° low-loss sharp-bend waveguide with different bending radii. In-situ multiple modification can make the structure of the material modified by the laser more compact, and the refractive index of the core layer of the waveguide is increased, which helps to suppress radiation loss; in-situ multiple modification can also drive the impurity ions in the waveguide to migrate to the waveguide boundary, reduce the scattering and absorption of light by the impurity ions, and reduce the absorption and scattering losses of the waveguide. The preparation process of this method is simple, the reliability is high, and it is simple and easy to implement.

Description

Technical Field

[0001] The invention belongs to the technical field of laser processing, and in particular relates to a method for preparing a low-loss sharp-bend waveguide by in-situ multiple modification using a femtosecond laser. Background Art

[0002] With the continuous development of integrated photonics, the preparation process of optical chips has continued to mature. Existing waveguide preparation processes include traditional photolithography, electron beam exposure, femtosecond laser direct writing and other processing technologies. Traditional photolithography and electron beam exposure technologies have disadvantages such as complex preparation process, long preparation cycle, and low three-dimensional integration efficiency. The femtosecond laser direct writing technology has promoted the rapid development of the three-dimensional photonic chip field with its advantages such as simple process, high efficiency, true three-dimensional, and strong material applicability. However, one of the key factors affecting the integration of femtosecond laser direct writing photonic chips is the excessive loss of the waveguide under sharp bend transmission. The reason for the excessive waveguide loss under sharp bend transmission is that the refractive index change of the femtosecond laser direct writing waveguide is small, resulting in large radiation loss, and the presence of impurity ions in the waveguide structure increases the scattering loss and absorption loss.

[0003] Existing technologies for reducing waveguide bending losses include using air cracks to increase the refractive index of the waveguide bend to achieve low-loss turns, but this method will destroy the integrity of the material and reduce the degree of integration. In addition, using femtosecond lasers to finely control the refractive index at the bend of the S-shaped waveguide can also effectively reduce waveguide losses, but this method requires multiple objective lenses to cooperate with processing, making the preparation process complicated and unable to solve the loss problem of 90° sharp bend waveguides, making it difficult to meet the needs of sharp bends in integrated photonic chip waveguides. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a method for preparing 90° sharp bend low-loss waveguides with different bending radii by in-situ multiple modification of femtosecond laser, and the in-situ multiple modification of femtosecond laser can accurately optimize the problem of large bending loss of 90° sharp bend waveguide in single direct writing. The main principle is as follows: by controlling the high-precision three-dimensional motion platform to focus the femtosecond laser inside the material, the femtosecond laser processing energy and laser scanning speed are adjusted to achieve the preparation conditions of single-mode waveguide; then the waveguide performance is regulated by in-situ multiple modification of femtosecond laser to prepare 90° low-loss sharp bend waveguides with different bending radii. In-situ multiple modification can make the material structure modified by laser in the material more compact, and the refractive index of the core layer of the waveguide is increased, which helps to suppress radiation loss; in-situ multiple modification can also drive the impurity ions in the waveguide to migrate to the waveguide boundary, reduce the scattering and absorption of light by the impurity ions, and reduce the absorption and scattering loss of the waveguide. Therefore, femtosecond laser can be used to perform multiple in-situ modifications to prepare low-loss waveguides with a small bending radius and a 90° sharp bend. This not only solves the problem of large bending loss in femtosecond laser direct writing, but also has a simple preparation process, high reliability, and is easy to implement. It will help to greatly improve the integration of femtosecond laser direct writing photonic chips.

[0005] The present invention is achieved through the following technical solutions:

[0006] The method for preparing a low-loss sharp-bend waveguide by multiple in-situ modification using a femtosecond laser specifically comprises the following steps:

[0007] Step 1: Build the optical path and level the sample stage. The specific steps are as follows:

[0008] The femtosecond laser emits a femtosecond laser with low single pulse energy, and the angle is adjusted so that the femtosecond laser passes through the center of the half wave plate HWP and the polarization beam splitter PBS device in sequence, and the first reflector M1 is adjusted so that the light beam passes through the center of the beam expander f1, and the beam of the laser output is expanded by the beam expander; the beam after the beam expansion passes through the second reflector M2 and is uniformly irradiated on the spatial light modulator SLM, and the light beam reflected by the spatial light modulator passes through the third reflector M3, the fourth reflector M4, the first convex lens f2 and the center of the second convex lens f3 in sequence, and finally is reflected by the sixth reflector M6 and focused by the objective lens OL in sequence and incident on the sample surface on the sample stage, and the laser spot is imaged in the CCD by adjusting the fifth reflector M5, and the CCD is connected to the computer PC, and the leveling process of the sample stage is monitored in real time by the computer PC;

[0009] Step 2: Determine the processing parameters of the in-situ multiple modified low-loss straight waveguide. The specific steps are as follows:

[0010] First, the focusing position of the laser in the sample is adjusted so that the light spot is focused inside the sample surface; then, the sample is prepared by direct writing using different laser single pulse energies and scanning speeds; after that, the end face of the obtained sample is polished to a nanometer-level roughness, and the transmission loss of the waveguide prepared with different laser single pulse energies and scanning speeds is tested respectively, and the processing parameters with the lowest straight waveguide transmission loss among the above different laser single pulse energies and scanning speeds are obtained;

[0011] Then, the straight waveguide samples with different modification times are processed respectively using the processing parameters of the straight waveguide with the lowest loss, and then the end faces of the obtained samples are polished;

[0012] Finally, the waveguide end faces with different modification times were characterized by optical microscope. The transmission loss and mode field size of the straight waveguide with different modification times were tested and characterized respectively. The modification times of the waveguide end faces with the second bright core boundary were selected based on the optical microscope photos of the waveguide end faces.

[0013] Step 3: In-situ multiple modification to prepare low-loss curved waveguide, specifically including the following steps:

[0014] First, a new sample is used to repeat the sample stage leveling operation in step one. Then, using the processing parameters of the low-loss straight waveguide obtained in step two, a computer is used to control the movement of the three-dimensional mobile platform on the XY plane to prepare 90° curved waveguides with different modification times and bending radii in the sample. The end face of the obtained sample is then polished and its bending loss is tested.

[0015] Furthermore, in step one, the central wavelength of the femtosecond laser is 1030±10nm, the pulse width is 290fs-20ps, the repetition frequency is 1Hz-1MHz, the pulse energy is 0-5μJ, the numerical aperture of the objective lens OL is 0.75, and the magnification is 40 for the air-space objective lens; the sample to be processed is a boroaluminosilicate glass of 25mm×25mm×7mm.

[0016] Furthermore, in step 1, the sample stage is leveled, and the specific steps are as follows:

[0017] First, adjust the Z-axis of the three-dimensional mobile platform to focus the laser spot on the upper surface of the sample to complete the adjustment of the Z-axis; make the sample stage reciprocate within the travel range along the X-axis direction of the three-dimensional mobile platform, and adjust the X-axis leveling knob where the sample is located until there is no obvious change in the morphology of the laser focus spot on the CCD, and then adjust the Z-axis to refocus the laser spot to complete the adjustment of the X-axis; similarly, make the Y-axis reciprocate within the travel range, and then adjust the Z-axis to refocus the laser spot and level the Y-axis; at this point, the entire leveling of the sample stage is completed.

[0018] Furthermore, in step 2, the focusing position of the laser in the sample is adjusted to 170 μm inside the sample.

[0019] Furthermore, in step 2, the energy of the single laser pulse is 210-290 nJ, and the scanning speed is 5-50 mm / s.

[0020] Furthermore, in step 2, the number of in-situ modifications is 1-16 times, and the test wavelength of waveguide transmission loss is 808±10 nm.

[0021] Furthermore, in step three, the processing parameters include laser single pulse energy, scanning speed and number of modifications.

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] (1) The present invention uses a femtosecond laser in-situ multiple modification technology to prepare a low-loss 90° sharp bend waveguide. The method is simple and reliable, avoiding the destruction of the integrity of the material or the complex process of replacing the objective lens and repeating the processing multiple times, and effectively solves the problem of complex process and low integration of femtosecond laser preparation of low-loss curved waveguide inside the material;

[0024] (2) The method of the present invention greatly reduces the bending radius and bending loss of the femtosecond laser direct writing 90° low-loss sharp bend waveguide at 808±10nm, reducing the bending loss of the existing 808±10nm waveguide with a bending radius of 30mm and a bending loss of 3.6dB / cm to a bending loss of 1.02dB / cm with a bending radius of 10mm, which is beneficial to improving the integration of the three-dimensional photonic chip on the femtosecond laser direct writing chip. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. 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 according to the actual scale.

[0026] Figure 1 A schematic diagram of a femtosecond laser direct writing processing system for preparing a 90° low-loss sharp bend waveguide according to the present invention;

[0027] Among them, HWP-half wave plate, PBS-polarization beam splitter, f1-4x beam expander, f2 and f3-4f system, SLM-spatial light modulator, M-reflector, OL-objective lens, CCD-camera, PC-computer;

[0028] Figure 2 The schematic diagram of the structure of the low-loss straight waveguide prepared based on multiple in-situ modifications of the present invention and the microscope photos of the end face of the waveguide after 1-16 in-situ modifications;

[0029] Among them, a is a schematic diagram of the change of waveguide shape and end face shape after in-situ modification once and multiple times; b is the morphological change of the waveguide end face after different multiple in-situ modifications;

[0030] Figure 3 The mode field diameter of the straight waveguide prepared by the in-situ multiple modification technology of the present invention at different modification times;

[0031] Among them, a is the mode field diameter of the TE polarized light mode profile in the horizontal and vertical directions under different modification times; b is the mode field diameter of the TM polarized light mode profile in the horizontal and vertical directions under different modification times; among them, the dark area is the 95% confidence band, and the light area is the 95% prediction band;

[0032] Figure 4 The transmission loss of the straight waveguide prepared by the in-situ multiple modification technology of the present invention at different modification times;

[0033] Among them, a is the transmission loss of the straight waveguide in TE polarized light under different modification times; b is the transmission loss of the straight waveguide in TM polarized light under different modification times; among them, the dark area is the 95% confidence band, and the light area is the 95% prediction band;

[0034] Figure 5 The bending loss of the 90° sharp bend waveguide at different modification times and bending radii prepared by the in-situ multiple modification technology of the present invention;

[0035] Where a is the bending loss of TE polarized light under different modification times and bending radius (L b ); b is the bending loss of TE polarized light under different modification times and bending radius (L b ); where the solid line represents the exponential fitting curve of waveguide bending loss at different bending radii. DETAILED DESCRIPTION

[0036] In order to clearly and completely describe the technical solution and its specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings of the specification:

[0037] Example 1: Preparation of low-loss straight waveguide using femtosecond laser in-situ multiple modification technology.

[0038] The mechanism of reducing waveguide loss by multiple in-situ modifications mainly includes: multiple laser modifications cause ion migration, resulting in an increase in three- and four-membered rings in the waveguide core layer, leading to a densified waveguide core structure, and then an increase in the refractive index, reducing the radiation loss of the curved waveguide. Multiple modifications can drive away impurity ions in the waveguide and reduce the absorption and scattering losses of the waveguide.

[0039] Specifically, this embodiment provides a method for preparing a 90° sharp bend low-loss waveguide with different bending radii by multiple in-situ modification using a femtosecond laser, and the specific steps are as follows:

[0040] (1) Light path adjustment: The light path adjustment of the femtosecond laser direct writing processing system adopted in this embodiment follows two basic principles, namely, central incidence and original path return; specifically, the following steps are included:

[0041] First, a femtosecond laser with a wavelength of 1030nm emits femtosecond laser, and the pulse width and repetition frequency of the femtosecond laser are 290fs and 1MHz respectively, so that the output laser pulse energy is around 50nJ. The center of the beam is made to pass through the center of the half-wave plate HWP and the polarization beam splitter PBS; the first reflector (M1) is adjusted to make the beam pass through the center of the 4x beam expander f1, and the beam expander is required to achieve concentric scaling of the beam;

[0042] After that, adjust the second reflector M2 so that the expanded light beam is evenly covered on the liquid crystal phased array panel of the spatial light modulator SLM; then repeatedly adjust the third reflector M3 and the fourth reflector M4 so that the light beam passes through the 4f system composed of the first convex lens f2 and the second convex lens f3; after the light beam passes through the 4f system, the spot diameter is reduced by 0.8 times, so that the light beam covers the entrance pupil of the objective lens (diameter 6mm);

[0043] Finally, the sixth reflecting mirror M6 is adjusted to make the center of the light beam incident into the objective lens OL, and the fifth reflecting mirror M5 is adjusted to make the focus morphology of the light spot appear in the CCD.

[0044] (2) Leveling the sample stage includes the following steps:

[0045] First, clean the sample and fix it on the sample stage, then use the adjusted optical path to level the sample stage; operate the Z axis of the three-dimensional mobile platform through the PC to make the light spot focused by the objective lens located on the sample surface; the specific adjustment method is to first make a rough adjustment of the Z axis up and down by several hundred microns, and then make a fine adjustment of a few microns until a clear focused light spot morphology appears in the CCD, and the Z axis adjustment is completed;

[0046] Secondly, make the sample stage reciprocate within a range of 2.5 cm along the X-axis direction, and adjust the X-axis leveling knob where the sample is located until there is no obvious change in the morphology of the laser focus spot, and then refocus the spot by adjusting the Z-axis position to complete the X-axis adjustment.

[0047] Similarly, the control software of the 3D mobile platform is used to reciprocate within a range of 2.5 cm in the Y-axis direction and level the Y-axis. At this point, the leveling of the sample stage is completed.

[0048] (3) In-situ multiple modification to prepare low-loss straight waveguides, specifically including the following steps:

[0049] First, the laser was focused at 170 μm inside the sample. Then, the single pulse energy of the laser was adjusted to 210 nJ using a half-wave plate (HWP) and a polarization beam splitter (PBS).

[0050] Then, a computer-controlled three-dimensional mobile platform was used to process the straight waveguide modified once at a scanning speed of 5, 10, 20, 30, 40, and 50 mm / s in the XY plane, and the writing length of the straight waveguide was 25 mm. After that, the laser single pulse energy was adjusted to 230, 250, 270, and 290 nJ in turn, and the straight waveguide modified once in situ was processed at a scanning speed of 5, 10, 20, 30, 40, and 50 mm / s respectively;

[0051] Finally, the end faces of the obtained samples were polished to nanometer-level roughness, and the transmission loss of the waveguides prepared with different laser single pulse energies and scanning speeds was tested to obtain the laser single pulse energy and scanning speed of the low-loss straight waveguide after one in-situ modification. After that, the straight waveguides modified 2-16 times in-situ were processed based on the determined laser single pulse energy and scanning speed.

[0052] (4) Characterization of straight waveguide performance and loss testing after multiple in-situ modifications, specifically including the following steps:

[0053] First, the end face of the sample was polished, and the change of the refractive index of the waveguide core was confirmed by the optical microscope photo of the waveguide end face. Generally, as the number of in-situ modifications increases, the refractive index of the waveguide core will gradually increase. When the refractive index of the waveguide core increases to the limit, the core size gradually increases and a second bright core boundary appears, such as Figure 2 As shown in (b). It should be noted that the increase in the refractive index of the waveguide core can reduce the mode field size of the transmitted light beam in the waveguide and reduce the loss of the waveguide. The appearance of the second bright core layer causes the size of the waveguide core to gradually increase, which in turn causes the mode field size to increase and the waveguide loss to increase. Therefore, after the number of scans increases, the increase in the refractive index of the waveguide core is in competition with the size of the second bright core layer. Only under the appropriate number of scans can the mode field size be controlled and the waveguide loss be reduced. Then, the 808nm loss test equipment is used to adjust the TE and TM polarized light to test the mode field size of the straight waveguide respectively. By Figure 3 From (a) and (b), it can be seen that the diameter size of the waveguide mode field shows a trend of first decreasing and then increasing with the increase in the number of modifications, and the mode field size is the smallest when the number of modifications is 9. This shows that after multiple modifications, the core refractive index of the waveguide is increased, which reduces the restriction of the waveguide on light and is beneficial to reducing losses. However, after the number of modifications is greater than 9 times, the mode field size of the waveguide gradually increases. This is because after multiple scans, the core refractive index of the waveguide has increased to the upper limit, and the second core boundary around the waveguide core layer gradually widens, which leads to an increase in the overall size of the waveguide, which is not conducive to the restriction of the waveguide on light and will increase the loss of the waveguide. Finally, the 808nm loss test equipment is used to adjust the TE and TM polarized light to test the transmission loss of the straight waveguide respectively. By Figure 4 It can be seen from (a) and (b) that the transmission loss of the straight waveguide first decreases and then increases after the number of modifications increases, and the transmission loss is relatively small when the number of modifications is 9. This is because too many modifications will cause the core boundary of the waveguide to widen and the waveguide loss to increase. Through multiple modifications, the transmission loss of the straight waveguide in the TE polarization mode can be optimized from 0.42dB / cm to 0.24dB / cm. Therefore, multiple in-situ modifications can effectively reduce the transmission loss of the waveguide. So far, all the optimal processing parameters for preparing low-loss straight waveguides have been obtained, including laser single pulse energy, scanning speed and number of modifications.

[0054] Example 2: A method for preparing 90° sharp-bend low-loss waveguides with different bending radii by in-situ multiple modification using a femtosecond laser.

[0055] The bending loss of the waveguide after a single modification is relatively high, which is due to the relatively high radiation loss, absorption loss and scattering loss of the waveguide as a whole. Radiation loss is usually caused by the conversion of the guided mode in the waveguide into a leakage mode or even a radiation mode after the waveguide is bent to a certain extent, thereby increasing the bending loss. After multiple in-situ modifications, the refractive index of the core layer of the waveguide increases, which helps to suppress radiation loss; absorption loss mainly comes from the absorption of light by the waveguide material itself and impurity ions; scattering loss is caused by defects in the material inside the waveguide. Multiple in-situ modifications can drive the impurity ions in the glass to migrate to the waveguide boundary, and the migration of impurity ions in the waveguide caused by multiple modifications reduces the absorption and scattering losses of the waveguide during the transmission of the light beam.

[0056] Specifically, this embodiment provides a method for preparing a 90° sharp bend low-loss waveguide with different bending radii by multiple in-situ modifications using a femtosecond laser. In order to highlight the effect of multiple modifications, different modification times were used for the experiment, and the specific steps are as follows:

[0057] (1) Optical path adjustment: same as in Example 1.

[0058] (2) Leveling of the sample stage: same as in Example 1.

[0059] (3) Preparation of low-loss 90° sharp bend waveguide by multiple in-situ modification using femtosecond laser: First, the optimal laser processing straight waveguide parameters, i.e., single pulse energy of 250 nJ and scanning speed of 40 mm / s, were obtained through Example 1. A 90° sharp bend waveguide with a bending radius of 10-21 mm was prepared by controlling a three-dimensional mobile platform with in-situ modification 1-9 times.

[0060] (4) Loss test of 90° sharp bend waveguide after multiple in-situ modifications: First, the prepared sample is polished. The transmission loss of the 90° sharp bend waveguide under different bending radii and modification times is tested by adjusting TE and TM polarized light using 808nm loss test equipment. Figure 5As can be seen from (a) and (b), the transmission loss of the 90° sharp bend waveguide with a bending radius of 10mm is significantly reduced after the number of modifications increases. In the TE polarization mode, the transmission loss of the 90° sharp bend waveguide with a bending radius of 10mm is reduced from 12.39dB / cm to 1.42dB / cm. In the TM polarization mode, the transmission loss of the 90° sharp bend waveguide with a bending radius of 10mm is reduced from 9.18dB / cm to 1.02dB / cm. In-situ multiple modifications significantly reduce the bending loss of the sharp bend waveguide, and can also significantly reduce the bending radius of the sharp bend waveguide. This shows that the in-situ multiple modification technology can effectively reduce the bending loss of the 90° sharp bend waveguide. The specific principle is confirmed in Example 1, that is, the core refractive index of the waveguide is optimized, which reduces the radiation loss of the waveguide; the literature also confirms that laser modification can optimize the distribution of impurity ions in the waveguide, which will reduce the scattering and absorption losses of the waveguide. The reduction in the bending radius and the reduction in bending loss of the 90° sharp bend waveguide will greatly improve the integration of photonic chips.

[0061] Therefore, the test results show that the small bending radius sharp bend low loss waveguide prepared by femtosecond laser in-situ multiple modification technology can reduce the bending radius to 10mm, and the minimum bending loss is only 1.02dB / cm. This method does not destroy the integrity of the material, the preparation process is simple, and it can effectively reduce the bending radius and bending loss of the waveguide, which is sufficient to meet the preparation of most high-integration photonic chips.

[0062] 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, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0063] 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 further describe various possible combinations.

[0064] In addition, 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 low-loss sharp-bend waveguide by multiple in-situ modifications using a femtosecond laser, characterized in that: The specific steps include: Step 1: Build the optical path and level the sample stage. The specific steps are as follows: The femtosecond laser emits a femtosecond laser with low single pulse energy, and the angle is adjusted so that the femtosecond laser passes through the center of the half wave plate HWP and the polarization beam splitter PBS device in sequence, and the first reflector M1 is adjusted so that the light beam passes through the center of the beam expander f1, and the beam of the laser output is expanded by the beam expander; the beam after the beam expansion passes through the second reflector M2 and is uniformly irradiated on the spatial light modulator SLM, and the light beam reflected by the spatial light modulator passes through the third reflector M3, the fourth reflector M4, the first convex lens f2 and the center of the second convex lens f3 in sequence, and finally is reflected by the sixth reflector M6 and focused by the objective lens OL in sequence and incident on the sample surface on the sample stage, and the laser spot is imaged in the CCD by adjusting the fifth reflector M5, and the CCD is connected to the computer PC, and the leveling process of the sample stage is monitored in real time by the computer PC; Step 2: Determine the processing parameters of the in-situ multiple modified low-loss straight waveguide. The specific steps are as follows: First, the focusing position of the laser in the sample is adjusted so that the light spot is focused inside the sample surface; then, the sample is prepared by direct writing using different laser single pulse energies and scanning speeds; after that, the end face of the obtained sample is polished to a nanometer-level roughness, and the transmission loss of the waveguide prepared with different laser single pulse energies and scanning speeds is tested respectively, and the processing parameters with the lowest straight waveguide transmission loss among the above different laser single pulse energies and scanning speeds are obtained; Then, the straight waveguide samples with different modification times are processed respectively using the processing parameters of the straight waveguide with the lowest loss, and then the end faces of the obtained samples are polished; Finally, the waveguide end faces with different modification times were characterized by optical microscope. The transmission loss and mode field size of the straight waveguide with different modification times were tested and characterized respectively. The modification times of the waveguide end faces with the second bright core boundary were selected based on the optical microscope photos of the waveguide end faces. Step 3: In-situ multiple modification to prepare low-loss curved waveguide, specifically including the following steps: First, a new sample is used to repeat the sample stage leveling operation in step 1; then, using the processing parameters of the low-loss straight waveguide obtained in step 2, a computer is used to control the movement of the three-dimensional mobile platform on the XY plane, and 90° curved waveguides with different modification times and bending radii are prepared in the sample; then, the end face of the obtained sample is polished and its bending loss is tested; In step 1, the central wavelength of the femtosecond laser is 1030±10nm, the pulse width is 290fs-20ps, the repetition frequency is 1Hz-1MHz, the pulse energy is 0-5μJ, the numerical aperture of the objective lens OL is 0.75, and the magnification is 40 air-space objective lens; the sample to be processed is 25mm×25mm×7mm boroaluminosilicate glass; In step 2, the laser single pulse energy is 210-290nJ, and the scanning speed is 5-50mm / s; In step 2, the number of in-situ modifications is 1-16 times, and the test wavelength of waveguide transmission loss is 808±10nm; In step three, the number of in-situ modifications of the curved waveguide is 1 to 9 times.

2. The method for preparing a low-loss sharp-bend waveguide by in-situ multiple modification using a femtosecond laser as claimed in claim 1, characterized in that: In step 1, the sample stage is leveled. The specific steps are as follows: First, adjust the Z axis of the three-dimensional mobile platform to focus the laser spot on the upper surface of the sample, and complete the adjustment of the Z axis; make the sample stage reciprocate along the X axis direction of the three-dimensional mobile platform within the travel range, and adjust the X axis leveling knob where the sample is located until there is no obvious change in the morphology of the laser focus spot on the CCD, and then adjust the Z axis to refocus the laser spot, that is, complete the adjustment of the X axis; similarly, make the Y axis reciprocate within the travel range, and then adjust the Z axis to refocus the laser spot, and level the Y axis; At this point, the leveling of the sample stage is completed.

3. The method for preparing a low-loss sharp-bend waveguide by multiple in-situ modifications using a femtosecond laser as claimed in claim 1, characterized in that: In step 2, the focusing position of the laser in the sample is adjusted to 170 μm inside the sample.

4. The method for preparing a low-loss sharp-bend waveguide by in-situ multiple modification using a femtosecond laser as described in claim 1, wherein in step three, the processing parameters include laser single pulse energy, scanning speed, and number of modifications.

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