CN113770515B - Method for resetting coupling coefficient of directional coupler by femtosecond laser secondary direct writing and application - Google Patents

Abstract

The invention discloses a method and application for resetting the coupling coefficient of a directional coupler by using femtosecond laser secondary direct writing, belonging to the technical field of laser processing, including cleaning of glass samples, construction of laser processing optical paths and leveling of samples to be processed ;Femtosecond laser direct writing device for the second time; firstly close the shutter and move the glass sample to the initial processing position; then rotate the half-wave plate to adjust the laser power; finally, the control software directly runs the pre-written processing program, in the In the processing area of the two waveguides in the coupling area, secondary direct writing is performed respectively; after the processing, the sample is polished to obtain two new waveguides with different coupling distances from the original waveguides; the processing area is distanced from the central axis of the original waveguides Within the area of 0μm-3μm above and below. The method of the invention can realize the repair and redistribution of the initial beam splitting ratio of the directional coupler, thereby solving the problem of manufacturing errors existing in the process of using the femtosecond laser to prepare the directional coupler and the integrated chip.

Description

A Method of Resetting Coupling Coefficient of Directional Coupler Using Femtosecond Laser Secondary Direct Writing and applications

technical field

The invention belongs to the technical field of laser processing, and specifically relates to modifying the coupling distance of a directional coupler in the first direct writing of the femtosecond laser by using the second direct writing of the femtosecond laser, so as to achieve the purpose of resetting the coupling coefficient of the initial directional coupler. By resetting the coupling coefficient of the initial device, the beam splitting ratio of the directional coupler is redistributed, which solves the inevitable manufacturing error problem in the process of preparing photonic devices using femtosecond lasers, and achieves basic unit devices and integrated chips. Manufacturing bug fix purposes.

technical background

In recent years, femtosecond laser direct writing technology has gradually become an important means of preparing photonic quantum integrated chips because of its true three-dimensional processing capability and the characteristics of fast maskless processing. The excellent computing power of photonic quantum integrated chips strongly depends on the manufacturing precision of basic unit devices such as directional couplers. However, affected by factors such as the complex interaction between pulsed laser and matter, laser power fluctuation, and environmental stability during processing, there are unavoidable manufacturing tolerances in these basic unit devices. In order to solve the above-mentioned problem of device manufacturing tolerance, two types of control schemes that can realize the reset of the splitting ratio of imperfect devices have been proposed. The first type is the electrode control scheme based on the thermo-optic effect or electro-optic effect of the substrate material. Although these dynamic control methods relying on electrodes have made great progress in resetting the beam splitting ratio of the directional coupler and realizing device repair, it is inevitable However, new problems such as limited chip integration and the need for additional power consumption have arisen. In order to achieve the purpose of repairing the beam splitting ratio of error devices without introducing additional power consumption and complicated external electrode control circuits, a static control scheme based on multiple scanning of femtosecond lasers to change the basic properties of waveguides has attracted attention. In the current method of static control, the femtosecond laser is used to scan one coupling arm of the directional coupler multiple times to change the propagation constant, resulting in the detuning of the propagation constants of the two coupling arms in the coupling area, so as to achieve the beam splitting ratio of the device. purpose of setting. This method of resetting the propagation constant of the waveguides in the directional coupler, although it can also reset the splitting ratio of the device, has two disadvantages: On the one hand, due to the detuning of the propagation constant, the energy in one of the waveguides cannot It is completely transferred to another waveguide, so it is difficult to realize the preparation of special devices with a beam splitting ratio of 0:1; on the other hand, due to the limited range of propagation constant detuning, it is difficult to achieve a wide range of device beam splitting ratios is reset. The above two shortcomings severely limit the application of this method in integrated chip error repair, so a new static control method is urgently needed to solve the problem of device error repair and improve the yield of integrated chips.

Contents of the invention

Aiming at the deficiencies of the prior art, the technical problem to be solved by the present invention is: to provide a method for resetting the coupling coefficient of the directional coupler by secondary direct writing with a femtosecond laser, so as to realize the restoration and redistribution of the initial beam splitting ratio of the directional coupler , so as to solve the manufacturing error problem existing in the process of using femtosecond laser to prepare directional coupler and integrated chip. For the two first waveguides and the second waveguides in the coupling region of the existing directional coupler, the femtosecond laser is used to write a new spliced waveguide directly in the area near the central axis of the first waveguide and the second waveguide respectively, and the two waveguides are obtained. A new combination of the third waveguide and the fourth waveguide with different coupling spacings is used to realize the resetting of the coupling coefficient and beam splitting ratio of the directional coupler. Since the spliced waveguide of the secondary direct writing overlaps the original first waveguide and the second waveguide highly, the two new combined third waveguides and fourth waveguides formed are actually the original first waveguide and the second waveguide and the second direct waveguide respectively. Written by splicing waveguide fusion.

The basic principle of resetting the coupling coefficient of the directional coupler involved in the present invention is: according to the coupling mode theory, the size of the coupling coefficient of the two waveguides is negatively correlated with the coupling distance, that is, the coupling coefficient decreases with the increase of the coupling distance; wherein, the coupling The spacing refers to the spacing between the geometric centers of the cross-sections of the two waveguides in the coupling area (ie, the spacing between the central axes). Therefore, the coupling coefficient of the directional coupler can be reset by changing the coupling distance between the two waveguides in the coupling area. In addition, since the beam splitting ratio of the directional coupler is directly related to two parameters, the coupling coefficient and the waveguide length in the coupling region, and the product of these two parameters satisfies the changing trend of a sinusoidal function. Therefore, under the condition that the coupling length of the directional coupler remains unchanged, the resetting of the splitting ratio of the directional coupler is realized by resetting the coupling coefficient.

The present invention realizes through following technical scheme:

A method for resetting the coupling coefficient of a directional coupler by secondary direct writing with a femtosecond laser, the specific steps are as follows:

(1) Cleaning of glass samples;

(2) The construction of laser processing optical path and the leveling of samples to be processed;

(3) Femtosecond laser direct writing device for the second time;

The specific steps are: first close the shutter, and move the glass sample in step (1) to the initial processing position; then turn the half-wave plate to adjust the laser power; finally, the control software directly runs the pre-written processing program, and Secondary direct writing is carried out in the processing area of the two waveguides in the area; after the processing, the sample is polished to obtain two new waveguides with different coupling spacing from the original waveguide; the processing area is up and down from the central axis of the original waveguide In the region of 0μm-3μm.

Further, the glass sample described in step (1) is a sample containing an initial directional coupler that has completed the first femtosecond laser processing; wherein, the first femtosecond laser processing parameters are: scanning speed 1mm/s-60mm /s; the laser power is 140mw-300mw, the length of the coupling area of the initial directional coupler is 0-20mm; the coupling distance d ₀ is 8μm-15μm.

Further, the laser processing optical path described in step (2) is specifically as follows: first, after the femtosecond laser is emitted by the femtosecond laser, the laser power is adjusted after passing through the half-wave plate and the polarization beam splitter; Reflection, the laser beam is expanded after passing through the first concave lens and the second convex lens; then the expanded laser beam passes through the diaphragm to select a laser beam with uniform energy distribution and the same spot diameter as the entrance size of the objective lens; then the laser beam passes through the second reflector After being reflected by the third mirror, the laser beam enters the objective lens; then the laser beam is focused on the surface or inside of the sample through the objective lens; There will be a part of the laser reflection in the interior. After the reflected laser passes through the objective lens, it passes through the third reflector, is reflected by the fourth reflector, and is imaged on the imaging camera after being converged by the third convex lens. According to the shape of the spot in the imaging camera, it can be judged After the objective lens is focused, the laser focus is on the surface or inside of the sample.

Further, the wavelength of the femtosecond laser described in step (2) is 500-1064nm, the pulse frequency is 100KHZ-2000KHz, the pulse width is 130fs-400fs, and the output power of the laser is 3W-20W; the multiple of the objective lens used is 20-100 times .

Further, the leveling of the sample to be processed described in step (2) is specifically:

Place the glass sample to be laser processed flat on the two-dimensional tilt platform, and the two-dimensional tilt platform is fixed at the center of the three-dimensional air-floating motion platform; adjust the XY axes of the air-floating motion platform so that the objective lens is at the upper left corner of the sample; then, adjust The Z-axis of the air-floating motion platform shortens the distance between the objective lens and the sample at intervals of 10 μm-100 μm, and observes the imaging camera until a "cross"-shaped reflection spot appears; then move the Y-axis of the air-floating motion platform, Make the objective lens directly above the lower left corner of the sample, and adjust the θx knob of the two-dimensional inclination adjustment platform to make the "cross"-shaped spot appear again in the imaging camera; repeatedly switch back and forth between the two vertices in the upper left corner and lower left corner, and judge the height according to the spot , adjust the θx knob repeatedly until a "cross"-shaped spot appears at the two vertices at the same time, which proves that the sample is horizontal in the Y-axis direction; similarly, repeatedly adjust the two vertices in the upper left corner and upper right corner to be at the same horizontal position; adjust the upper left corner and lower left corner at the same time After the three apexes in the corner and the upper right corner are at the same horizontal position; finally, move the air-floating motion platform to the apex in the lower right corner of the sample, and observe the state of the light spot in the imaging camera at this time. If the reflected light spot of the fourth apex is also in the "cross" The model proves that the entire sample is in a horizontal state and can be used for subsequent processing; wherein, the laser power P ₀ in the leveling process is 20mw-80mw.

Further, the method for determining the initial processing position in step (3) is specifically as follows:

After leveling, move the XY axis of the air bearing motion platform so that the objective lens is in the upper left corner of the sample; then move the X axis every 10μm-100μm until the "cross" type reflection spot in the imaging camera disappears, proving that the objective lens is in the X position of the sample. The edge position of the axis; similarly, move the Y axis every 10μm-100μm until the spot disappears, which proves that the objective lens is at the upper left corner of the sample; after the above two steps, the laser focus is at the upper left corner of the sample; at this time, Use the position of the upper left corner as the initial processing position, and set the motion platform software to reset the coordinates here to zero, that is, point (0, 0, 0).

Further, the power used in the second direct writing in step (3) is 140mw-300mw, the scanning speed is 1mm/s-60mm/s; the laser focus depth is 50um-1000um.

Further, the specific steps of polishing described in step (3) are as follows: remove the processed sample, use sandpaper to roughly polish the side of the sample where the end face of the waveguide is located, and the number of sandpaper is 100 mesh-7000 mesh; then place the sample Perform fine polishing on a polishing machine containing a polishing liquid, the particle size of the polishing liquid is 50nm-14um, and the polishing time for each side is 40 minutes to 300 minutes; after the sample is polished, wipe it clean and wait for the test.

The present invention also provides an application of resetting the coupling coefficient of the directional coupler by secondary direct writing of the femtosecond laser in resetting the beam splitting ratio of the device, which can realize the resetting of the initial beam splitting ratio of the directional coupler to any final beam splitting ratio place.

Compared with the prior art, the present invention has the following advantages:

(1) It can improve the integration of the chip; it is proposed to modify the coupling spacing of the directional coupler through the femtosecond laser secondary direct writing technology to realize the reconfiguration of the coupling coefficient, so as to achieve the purpose of resetting the beam splitting ratio of the device, which can solve the problem of in-chip The problem of device manufacturing error; this method can realize the restoration of the device beam splitting ratio as the existing electrode regulation method, but this method does not require additional electrodes, so it can improve chip integration and reduce unnecessary power consumption ;

(2) Three-dimensional chip repair can be carried out; compared with the current situation in which the electrode can only be prepared on the surface of the sample for shallow device regulation in the existing electrode regulation method, this method relies on the unique three-dimensional processing advantages of the femtosecond laser to achieve three-dimensional Fixed-point quantitative regulation of photon circuits;

(3) Full-cycle control is possible; compared with the existing static control method of the same type based on femtosecond laser multiple scanning, this method realizes the reset of the device beam splitting ratio by adjusting the coupling coefficient of the directional coupler , can realize the reset of the device from any initial beam splitting ratio to any final state beam splitting ratio, that is, can realize the full-cycle regulation of the device beam splitting ratio, and can meet the needs of any optical quantum chip.

Description of drawings

Fig. 1 is a schematic diagram of the laser processing optical path of the present invention using femtosecond laser secondary direct writing to reset the coupling coefficient of the directional coupler;

Fig. 2 is a schematic diagram of the principle of a method for resetting the coupling coefficient of a directional coupler using femtosecond laser secondary direct writing technology of the present invention;

Among them: the initial waveguide of the femtosecond laser direct writing for the first time is represented by a gray bar; the spliced waveguide of the femtosecond laser direct writing for the second time is represented by a dotted line; the initial waveguide and the spliced waveguide are fused to form a new combined waveguide in gray Superimposed dotted line representation. The coupling spacing of the initial device is defined by the central axis spacing of the initial first waveguide and the initial second waveguide as d ₀ , and the coupling length of the coupling region is L ₀ (a); , the device coupling spacing is defined by the central axis center of the combined new third waveguide and waveguide 4 as d ₁ (secondary direct writing inside the coupling region) (b) or d ₂ (secondary direct writing outside the coupling region), The coupling length is L ₀ (c);

Fig. 3 is a micrograph of waveguide end face and mode field overlap diagram of the coupling region before and after secondary direct writing of a second direct writing using femtosecond laser to reset the coupling coefficient of the directional coupler according to the present invention;

Among them: a is the end view of the two waveguides in the coupling region of the initial device, and the coupling distance is d ₀ ; b and c are the two direct writing in the coupling region after the second direct writing on the inside and outside of the initial coupling region, respectively. The end view of the root waveguide, the coupling spacing is d ₁ and d ₂ ; d, e and f are the mode field overlap diagrams of the coupling region corresponding to a, b and c respectively;

Fig. 4 is a change trend diagram of the coupling coefficient before and after the secondary direct writing of the present invention using femtosecond laser secondary direct writing to reset the coupling coefficient of the directional coupler;

Among them: the coupling spacing of the initial device is 8 μm, and the coupling coefficient is 0.89rad/mm; the modified coupling spacing is 5 μm-11 μm; the coupling coefficient is 0.47rad/mm-2.1rad/mm.

Fig. 5 is an application schematic diagram of resetting the beam splitting ratio of the directional coupler by using a method of resetting the coupling coefficient of the directional coupler by secondary direct writing of the femtosecond laser according to the present invention;

Among them: the coupling spacing of the initial device is 8 μm, and the coupling length is 3.5 mm, both of which are fixed values. The method of resetting the coupling coefficient by the secondary direct writing technology is used to control the splitting ratio of the device;

Fig. 6 is a test result diagram of resetting the beam splitting ratio of the directional coupler by using a femtosecond laser secondary direct writing method of the present invention to reset the coupling coefficient of the directional coupler;

Among them: the beam splitting ratio of the initial device is 46.1:53.9; the circular data points and square data points represent the results of secondary direct writing on the inside and outside of the coupling region of the initial device, respectively.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

Example 1

The coupling distance is one of the important parameters affecting the coupling coefficient, and the modification of the coupling distance in the directional coupler can reset the coupling coefficient of the device. In the case of the same processing parameters, all basic properties such as the size and cross-sectional shape of the two coupling arms in the directional coupler are basically the same, and the coupling spacing of the device can be considered as the central axis spacing of the two waveguides. Therefore, the problem of resetting the coupling coefficients of a directional coupler can be considered as a problem of reconstructing the geometric center of the waveguide. Using the femtosecond laser secondary direct writing method, the second spliced waveguide is directly written at the position where the central axis of the initial waveguide is very close, and the initial waveguide and the second spliced waveguide will be fused into a new combined waveguide. Since the combined waveguide is formed by the fusion of the original waveguide and the spliced waveguide, the center of the combined waveguide is the intermediate value of the geometric center of the original waveguide and the spliced waveguide, so the geometric center of the reconstructed combined waveguide is offset relative to the geometric center of the original waveguide , which realizes the reconstruction of the geometric center of the waveguide. Using this method to modify the coupling region of the initial device, a new coupling region composed of two new waveguides can be obtained. Since the new waveguide has a new geometric center, the coupling distance will occur due to the offset of the geometric center of the coupling region waveguide. Change. Furthermore, when the spliced waveguide is inside the coupling area, that is, when the geometric center of the new waveguide is biased to the inside of the coupling area, the coupling distance will decrease, but the coupling coefficient will increase; on the contrary, if the spliced waveguide is outside the coupling area, the geometric center of the new waveguide When it is biased to the outside of the coupling region, the coupling distance will increase and the coupling coefficient will decrease; in this way, the reconfiguration of the coupling coefficient of the device is realized.

A method for resetting the coupling coefficient of a directional coupler by secondary direct writing with a femtosecond laser, the specific steps are as follows:

(1) Prepare the sample to be processed:

Take a 25mm*25mm*0.7mm sample to be processed and put it on the filter paper, use tweezers to pick up the acetone cotton ball and the ethanol cotton ball to wipe the sample successively, and wipe along the same direction to avoid contamination of the sample; use tweezers Pick up the sample treated with acetone and ethanol, rinse it with deionized water once; finally use the ear washing ball to dry the sample along a fixed angle, and put it in a clean sample box for later use;

(2) Construction of laser processing optical path and determination of laser leveling power:

The femtosecond laser processing system is shown in Figure 1. The femtosecond laser wavelength is 1064nm, the pulse frequency is 2000KHz, the pulse width is 400fs, and the initial laser power is 3W. After the laser is emitted from the laser, the laser power is adjusted after passing through a half-wave plate and a polarizing beam splitter. The half-wave plate is placed on a rotatable stepping motor, and the stepping motor is rotated through software control to realize the rotation of the half-wave plate, thereby quantitatively adjusting the laser power after passing through the polarization beam splitter. After passing through the polarizing beam splitter, the laser beam is reflected by the first mirror, and the laser beam is expanded after passing through the first concave lens and the second convex lens. The focal length of the first concave lens is -7.5mm, and the focal length of the second convex lens is 30mm. After beam expansion, the laser spot diameter increases from 3mm to 12mm. The expanded laser beam passes through the aperture to select a laser beam with uniform energy distribution and the same spot diameter as the entrance size of the objective lens. The objective lens selected in this experiment is a 20x Olympus dry objective lens, and the entrance size of the objective lens is 7mm, so the diameter of the laser beam after passing through the diaphragm is also 7mm. Then the laser is reflected by the second reflector and the third reflector and then enters the objective lens; then the laser is focused on the surface of the sample through the objective lens; the sample is placed on the sample stage, which is connected with the air-floating motion platform and can move in three dimensions; the laser passes through the objective lens Part of the laser light will be reflected after converging on the surface or inside of the sample. The reflected laser light passes through the objective lens, passes through the third reflector, is reflected by the fourth reflector, passes through the third convex lens with a focal length of 50mm, and converges on the imaging camera for imaging. According to the shape of the light spot in the imaging camera, it can be judged whether the focus of the laser beam focused by the objective lens is on the surface or inside of the sample.

Because leveling needs to focus the laser on the sample surface for subsequent operations, and too high laser power will ablate the sample, so try to keep the leveling power as low as possible in the experiment. Specifically, place a power meter between the third mirror and the objective lens, adjust the stepper motor software, and control the rotation angle of the half-wave plate, so that the power meter reads 20mw as the leveling power.

(3) Leveling samples:

The principle of leveling is: according to the principle of determining a plane by three points, adjust the three vertices of the sample to be at the same level to make the sample level. The specific explanation is as shown in Figure 1. After the laser is focused by the objective lens, it will converge into a focal point. When the three-dimensional displacement motion platform is adjusted so that the focal point is just on the surface of the apex of the sample, the laser at the focal point is reflected by the sample surface and passes through the third convex lens. After converging, it enters the imaging camera. At this time, a "cross"-shaped spot will appear in the imaging camera, which proves that the laser is focused on the surface of the sample at this time; the knob for adjusting the inclination angle of the sample stage makes the reflected spots of the three vertices of the sample all " The "cross"-shaped spot proves that the three vertices are at the same level, that is, the leveling is over. According to the above principles, the specific experimental operation is as follows:

Fix the glass sample to be laser-processed on the sample holder above the air-floating motion platform; adjust the XY axes of the air-floating motion platform so that the objective lens is at the upper left corner of the sample; then, adjust the Z axis to make the distance between the objective lens and the sample every The interval of 100 microns is shortened once, and observe whether there is a reflection spot in the imaging camera; judge whether the focus position is above the surface (moving up) or below the surface (moving down) according to the shape of the spot; according to the central area of the reflection spot when the focus moves up The energy becomes weaker and the diameter becomes larger, and the number of reflection spot rings increases and the diameter becomes larger when the focus moves down. These two obvious criteria can make the distance between the objective lens and the sample controlled within the error range of plus or minus 1 micron, which is enough Meet the leveling requirements; according to the above judgment criteria, move the Z-axis of the motion platform so that the reflected light spot in the upper left corner just appears in the imaging camera as a "cross" type of light spot; subsequently move the Y-axis of the motion platform 23 cm, so that the objective lens is positioned at the lower left corner of the sample. Above, the sample may be uneven at this time, so the spot in the imaging camera is no longer a "cross" spot, and the spot judgment standard of moving the focus up and down is required to determine the distance between the objective lens and the lower left corner of the sample at this time. Larger or smaller, then adjust the θx knob of the two-dimensional inclination adjustment platform to make the "cross"-shaped spot appear again in the imaging camera; since the upper left corner of the sample may not be in the center of the adjustment platform, after adjusting the θx knob, the height of the upper left corner It has also changed, so it is necessary to move the Y-axis of the motion platform -23 cm to return to the upper left corner; at this time, the light spot in the upper left corner may also have changed, adjust the θx knob of the two-dimensional inclination bar platform again to make the "cross" appear again in the imaging camera ""-shaped light spot; then repeatedly switch back and forth between the two vertices in the upper left corner and lower left corner, and judge the height according to the light spot, and repeatedly adjust the θx knob until a "cross"-shaped light spot appears at the two vertices at the same time, proving that the sample is horizontal in the Y-axis direction ; Similarly, you can repeatedly adjust the two vertices of the upper left corner and the upper right corner to be at the same horizontal position; after adjusting the three vertices of the upper left corner, lower left corner and upper right corner to be at the same horizontal position at the same time, you can preliminarily consider the sample to be in a horizontal state; finally, move Move the platform to the vertex in the lower right corner of the sample, and observe the state of the light spot in the imaging camera at this time. If the reflected light spot at the fourth vertex is also in the "cross" shape, it proves that the entire sample is indeed in a perfect horizontal state and can wait for subsequent processing.

(4) Determine the initial processing position:

After leveling, move the XY axis of the moving platform so that the objective lens is at the upper left corner of the sample; then move the X axis every 100 microns until the "cross" type reflection spot in the imaging camera disappears, proving that the objective lens is at the edge of the sample X axis Position; Similarly, move the Y axis every 100 microns until the spot disappears, which proves that the objective lens is in the real upper left corner of the sample. At this time, take the position of the upper left corner as the initial processing position, and set the motion platform software to reset the coordinates here to zero, that is, point (0, 0, 0).

(5) Determine the laser processing power:

Place the power meter between the third reflector and the objective lens, and the software that controls the rotating stepper motor makes the half-wave plate rotate until the power meter reads 300mw; at this time, the power is used as the processing of the first and second direct writing power.

(6) Write the initial device directly for the first time:

The initial device is composed of a directional coupler whose coupling length is gradually increasing and other parameters are consistent; as shown in Figure 2 (a), the width of the directional coupler is 127 microns, the total length of the device is 25 cm, and the curvature The radius is 60 mm, the initial coupling spacing is 8 microns, and the direct writing speed of the device is 60 mm per second; according to the above specific parameters and requirements, the production of the processing program is completed; the written first processing program is loaded into the three-dimensional motion In the software of the platform, click Run to prepare the initial device.

(7) The second direct write device:

In order to realize the offset of the geometric center of the waveguide in the coupling region, the structures shown in Fig. 2(b) and (c) are adopted. The initial directional coupler of femtosecond laser direct writing for the first time consists of an initial first waveguide and an initial second waveguide, which are represented by dark gray areas in the figure. Use the femtosecond laser secondary direct writing technology to directly write the spliced waveguide at the position near the central axis of the first waveguide or the second waveguide, which is represented by a dotted line box in the figure, and the distance between the spliced waveguide and the initial waveguide central axis is Δd is ±( 0μm-3μm). Since the spliced waveguide is very close to the original first waveguide (2), the two waveguides are merged into a new combined third waveguide (4). It can be found that the geometric centers of the original waveguide and the spliced waveguide do not coincide, so the shape of the combined waveguide reconstructed by them will increase in the horizontal direction, that is to say, the lateral dimension of the combined waveguide increases compared with the original waveguide. It was further found that since the processing parameters of the spliced waveguide and the original waveguide are consistent, the geometric center of the combined waveguide is the intermediate value of the geometric center of the spliced waveguide and the original waveguide, that is, the geometric center of the combined waveguide is offset relative to the original waveguide. According to the above method, after the initial device is directly written by femtosecond laser twice, the geometric centers of the two waveguides in the coupling region are modified (shifted). Since the coupling spacing is defined according to the central axis spacing (geometric center spacing) of the two waveguides in the coupling area, the offset of the geometric center of the waveguide will directly cause the change of the coupling spacing, which further affects the change of the coupling coefficient; as shown in Figure 2 As shown in (b), when the spliced waveguide of the second direct writing is inside the coupling region, it is defined according to the geometric center of the two new combined waveguides, and the new coupling distance is d ₁ , which is significantly smaller than the initial distance d ₀ ; As shown in Figure 2(c), when the spliced waveguide of the second direct writing is outside the coupling region, the new coupling spacing is d ₂ , and the coupling spacing is significantly larger than the initial spacing d ₀ ; in the experiment, the second The root waveguide deviates from the first waveguide in the range of 0-3 microns, so the minimum value of d ₁ is 5 microns, and the maximum value of d ₂ is 11 microns; make the waveguide of the second direct writing and the initial waveguide of the first direct writing have the same length, the coupling area is completely composed of a new combined waveguide, and has a new coupling coefficient; according to the above method, the processing program for the second processing is written, that is, the program is based on the first program and the initial processing position. Locate the coordinates of the secondary direct writing, the scanning speed is still 60 mm per second, keep the first group as the control group, and perform secondary direct writing on the directional couplers of other groups; move the motion platform to (0, 0, 0) After clicking, click on the processing program to perform secondary processing.

(8) Polish the sample to be tested:

Take down the processed sample, hold the sample by hand, and polish the two sides of the sample where the end face of the waveguide is located. First use 400-grit sandpaper to quickly throw away the unnecessary position on the edge, and use one page of sandpaper on the two sides. Yes; then use 1000-mesh and 2000-mesh sandpaper for fine polishing; then use a polishing machine for fine polishing. The sample is placed on the fixture of the polishing machine and polished with a 50-nanometer polishing solution. The polishing time for each side is 40 minutes ; After polishing the sample, wipe it clean and wait for the test.

(9) Test samples:

Characterize the coupling region of the device before and after the second direct writing, and use a microscope to observe the waveguide end face of the coupling region. It is found that the coupling region of the initial device is composed of two initial waveguides separated from each other, as shown in Figure 3(a) As shown, the lateral dimensions of the initial first waveguide and the initial second waveguide are 5 microns, and the two vertical lines passing through the geometric centers of the two initial waveguides are called the initial waveguide centerline 1 and the initial waveguide centerline 2, respectively, and It is indicated by a thick vertical line in the figure. According to the definition of the coupling spacing (that is, the spacing between the geometric centers of the two waveguides in the coupling region), the coupling spacing of the initial device is the spacing between the initial waveguide centerline 1 and the initial waveguide centerline 2, denoted as d ₀ . In the experiment, the coupling pitch for the initial device is designed to be 8 microns, that is, d ₀ is equal to 8 microns. When using a method of resetting the coupling coefficient of the directional coupler by using the femtosecond laser secondary direct writing of the present invention, the effect is shown in Fig. 3(b) and Fig. 3(c). Let’s discuss Figure 3(b) first. The two initial waveguides written by the femtosecond laser for the first time are still represented by thick vertical lines, and the distance between their centers is still d0 as in Figure 3(a); The formation process is as follows: 3 microns near the center of the initial waveguide inside the coupling region (that is, the area between the initial centerline 1 and the initial waveguide centerline 2, and a distance of 3 microns from the initial centerline 1 or initial centerline 2 position), a spliced waveguide was prepared by using the femtosecond laser direct writing strategy for the second time; since the spliced waveguide and the initial waveguide are very close and only 3 microns, the initial waveguide and the spliced waveguide are fused into a new combined waveguide, and The combined waveguide still supports 810nm single-mode transmission; since the combined waveguide is composed of the original waveguide and the spliced waveguide, its lateral dimension is increased compared to the original waveguide, and its edge outline is depicted by an elliptical dashed line in Figure 3(b); Furthermore, since the processing parameters of the initial waveguide and the spliced waveguide are the same, the geometric center of the combined waveguide is the intermediate value of the geometric centers of the two waveguides, as shown in Figure 3(b) with combined waveguide centerline 1 and combined waveguide centerline 2 Describe the geometric center position of the combined waveguide (that is, the vertical centerline position of the centerline of the ellipse); it can be clearly seen that the centerline 1 of the combined waveguide and the initial centerline 1 are not coincident, that is to say, after the second femtosecond laser After direct writing, the geometric center of the waveguide in the coupling area has shifted. According to the coupling spacing is defined by the geometric center distance of the two coupling waveguides, then the new coupling spacing is d ₁ ; In the second direct writing, the combined waveguide centerline 1 and the combined waveguide centerline 2 are closer to the middle, so it can be clearly seen that the coupling distance d1 is smaller than the initial coupling distance d0; further, since the distance between the spliced waveguide and the initial third waveguide is microns, so The middle value is 1.5 microns, that is to say, the central line 1 of the composite waveguide is shifted 1.5 microns to the inner side of the coupling region compared with the central line 1 of the original waveguide. Similarly, the centerline 2 of the combined waveguide is also shifted inward by 1.5 microns. Then in case the geometric centers of the combined first waveguide and the combined second waveguide are simultaneously shifted inwards by 1.5 microns, the modified coupling spacing d1 is ₅ microns. According to the same method in Fig. 3(b), in Fig. 3(c), after using the femtosecond laser to directly write the spliced waveguide on the outside of the initial waveguide, the centerline of the combined waveguide is to the outside of the coupling region compared to the centerline of the initial waveguide Offset by 1.5 microns, the new coupling spacing _d2 increases to 11 microns. From Figure 3(a), (b) and (c), it can be clearly concluded that after the coupling region is modified, the coupling spacing can be reduced and increased; and after the coupling region spacing is modified, its influence on the coupling coefficient However, it is necessary to test the degree of mode field overlap in the coupling region. The 810nm laser is fed into the input port of the directional coupler, and the single-mode analyzer is used to observe the mode field overlap in the coupling region; as shown in Figure 3(d), the mode fields of the two waveguides of the initial device overlap The degree is 44.1%, which means that the mode field overlap degree of the initial device in Figure 3(a) is 44.1%; After the device is directly written twice, the mode field overlap degree of the two new waveguides of the device increases to 56.7%, which means that the mode field overlap degree of the modified device in Figure 3(b) is 56.7%; combined with Analyzing together the conclusion that the coupling spacing d1 is smaller than the coupling spacing d0 in Figure 3(b), it can be known that after the coupling spacing becomes smaller, the degree of mode field overlap of the two coupled waveguides in the coupling area becomes larger. According to the coupling mode theory , so that the oscillation frequency of the light wave energy in the two waveguides becomes faster, which can increase the coupling coefficient; similarly, as shown in Figure 3(f), the femtosecond laser secondary direct writing strategy is used to perform secondary After the second direct writing, the mode field overlap degree of the two new waveguides of the device is reduced to 24.4%, that is to say, the mode field overlap degree of the modified device in Figure 3(c) is 24.4%. The same analysis can be It is known that the smaller the degree of mode field overlap, the smaller the coupling coefficient will be; in summary, the use of femtosecond laser secondary direct writing method can change the geometric center position of the waveguide, further change the coupling distance, and further affect the mode field The degree of overlap finally causes a change in the coupling coefficient. The above is a theoretical discussion on how the coupling coefficient can be reset by the invention. How to obtain the specific experimental value after the coupling coefficient is reset requires a more accurate test; next, split the beams of multiple groups of directional couplers on the chip Ratio measurement, and fitting the data to get the results shown in Figure 4; the abscissa indicates the coupling spacing of the device after the secondary direct writing, and the different values of the abscissa indicate the splicing prepared by the femtosecond laser secondary direct writing strategy The position and spacing of the waveguide and the original waveguide are different. 8 microns is the coupling spacing of the initial device. When the spliced waveguide is inside or outside the coupling area and the distance is 3 microns, the coupling spacing has a minimum value of 5 microns or a maximum value of 11 microns (previously Introduced in detail); the ordinate is the coupling coefficient. According to the data points in the figure, it is not difficult to find that when the spliced waveguide of the secondary direct writing is inside the coupling area, the coupling distance d1 becomes smaller and the coupling coefficient increases to a maximum of 2.1rad/mm; on the contrary, the coupling coefficient decreases Small, the minimum can be reduced to 0.47rad/mm; after analysis, the coupling coefficient of the initial device can be reset in the range of 0.47-2.1rad/mm; in summary, it proves that the initial device can achieve coupling after secondary direct writing Coefficient reset, that is, the present invention provides a method for resetting the coupling coefficient of a directional coupler by using femtosecond laser secondary direct writing.

Embodiment 2 An application of using femtosecond laser secondary direct writing to reset the coupling coefficient of the directional coupler:

(1) Prepare the sample to be processed: Same as Example 1.

(2) Determining laser leveling power: Same as Embodiment 1.

(3) Leveling sample: same as embodiment 1.

(4) Determining the initial processing position: same as embodiment 1.

(5) Determining laser processing power: same as embodiment 1.

(6) Write the initial device directly for the first time:

The initial device is composed of a series of identical directional couplers with a fixed coupling length. The width of the directional couplers is 127 microns, the total length of the device is 25 cm, the radius of curvature of the turning area is 60 mm, and the initial coupling spacing is 8 microns. The device The direct writing speed is 60 millimeters per second; according to the variation trend of the beam splitting ratio of the initial device in Example 1, it can be known that when the coupling length is 3.5 mm, the initial beam splitting ratio of the device is about 50:50; according to the above specific parameters and requirements, complete the production of the processing program; load the written first processing program into the software of the three-dimensional motion platform, click to run, and prepare the initial device.

(7) The second direct writing to reset the splitting ratio of the device:

For the initial device prepared for the first time in Example 1, when the coupling length is fixed at 3.5 mm and the coupling spacing is fixed at 8 microns, the beam splitting ratio of the directional coupler is about 50:50; When the directional coupler is reprocessed, the beam splitting ratio of 50:50 can still be obtained in theory. However, due to the existence of manufacturing errors, although the beam splitting ratio of the subsequent processed directional coupler is very close to the ideal value, there will be some deviations. This deviation will affect the performance of the chip after the accumulation of multiple devices; in order to solve the above problems, the present invention proposes a method of resetting the coupling coefficient of the directional coupler by using femtosecond laser secondary direct writing to realize the directional coupler Reset the beam splitting ratio to restore it to the theoretical value; theoretically, the beam splitting ratio of the device is closely related to the coupling coefficient and the coupling length of the coupling region, and the product of the beam splitting ratio and the above two parameters satisfies the sine function squared trend. However, for a fabricated device, its coupling length cannot be changed, so the beam splitting ratio can be reset by changing the coupling coefficient. The method for resetting the coupling coefficient of the directional coupler has been shown in Embodiment 1, and now this method is used to realize the resetting of the beam splitting ratio of the directional coupler; When writing, the coupling coefficient is K1, and the initial coupling coefficient is K0; now a method that can change the device coupling coefficient multiple times is designed, as shown in Figure 5(a), a part of the coupling area inside the coupling area performs secondary The second direct writing, the modified part is K1, and the unmodified part is still K0; the coupling coefficient of the device coupling area is composed of K1 and K0, then the average coupling coefficient K01 of the device will be between K0 and K1, and with the With the gradual increase of the secondary direct writing length Ls, K01 will gradually approach K1; according to the above relationship between the coupling coefficient and the beam splitting ratio, every time the coupling coefficient is reset, the device beam splitting ratio will be reset, then using Femtosecond laser secondary direct writing can change the length of the secondary direct writing waveguide multiple times, and the coupling coefficient K01 can be reset multiple times, further realizing multiple resets of the device beam splitting ratio; similarly, as shown in Figure 5(b) It shows that the secondary direct writing is performed on a part of the coupling area outside the coupling area, the modified part is K2, and the unmodified part is K0; since K2 is less than K0, the average coupling coefficient K02 of the device will be between K2 and K0 Between, and as the secondary direct writing length Ls gradually increases, K02 will gradually approach K2; similarly, using femtosecond laser secondary direct writing to reset the coupling coefficient K02 multiple times, the device beam splitting ratio can be achieved multiple resets. In the experiment, the direct writing of secondary waveguides of different lengths is carried out at the position of micrometers away from the central axis of the initial first waveguide inside the coupling region, as shown in Figure 5(a); Secondary direct writing, as shown in Figure 5(b); write the processing program according to the above requirements, load the program in the software and start running.

(8) Polishing the sample to be tested: Same as in Example 1.

(9) Test samples:

In step (7), it is introduced that according to the processing parameters in Example 1, the initial beam splitting ratio of the device should be 50:50 in theory, but due to the existence of manufacturing errors, its specific beam splitting ratio is 46.1:53.9, such as Shown in C0 point among Fig. 6; In order to realize the repair of device beam splitting ratio, adopt a kind of method of the present invention to utilize femtosecond laser secondary direct writing to reset the coupling coefficient of directional coupler to carry out experiment; In Fig. 6 circular data The dots and square data points respectively represent the change trend of the splitting ratio of the device after the second direct writing inside and outside the coupling region; when the device is modified by the second direct writing inside and outside the coupling region, the coupling coefficient is reset, and the splitting ratio The beam ratio is also reset to a new value; it can be seen from the figure that after the coupling coefficient of a fabricated device is reset by femtosecond laser direct writing, the beam splitting ratio of the device can not only be restored to be very close to the ideal value 50:50, and can even be reset to any value including 0:100 and 100:0 equal beam splitting ratio; this method of resetting the coupling coefficient of the directional coupler by using femtosecond laser secondary direct writing is very important in realizing the device The aspect of resetting the beam splitting ratio has been well applied, which provides a good solution for solving the error problem existing in the preparation of optical quantum chips by femtosecond laser.

The preferred embodiment of the present invention has been described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the specific details of the above embodiment, 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, These simple modifications all belong to the protection scope of the present invention.

In addition, it should be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable way if there is no contradiction. The combination method will not be described separately.

In addition, various combinations of different embodiments of the present invention can also be combined arbitrarily, as long as they do not violate the idea of the present invention, they should also be regarded as the disclosed content of the present invention.

Claims (7)

1. A method for resetting the coupling coefficient of a directional coupler by femtosecond laser secondary direct writing is characterized by comprising the following specific steps:

(1) Cleaning a glass sample;

(2) Building a laser processing light path and leveling a sample to be processed;

(3) The femtosecond laser second direct writing device;

the method comprises the following specific steps: firstly, closing the optical gate, and moving the glass sample in the step (1) to an initial processing position; then, rotating the half-wave plate and adjusting the laser power; finally, the control software directly runs the pre-written processing program, and respectively carries out secondary direct writing in the processing areas of the two waveguides in the coupling area; polishing the sample after the processing is finished to obtain two new waveguides with different coupling distances from the original waveguides; the processing area is an area which is 0-3 mu m above and below the central axis of the original waveguide;

wherein, the glass sample in the step (1) is a sample which is already finished with the first femtosecond laser processing and comprises an initial directional coupler; wherein the first femtosecond laser processing parameters are as follows: the scanning speed is 1mm/s-60mm/s; the laser power is 140mw-300mw, the length of the coupling region of the initial directional coupler is 0-20mm, and the coupling distance d ₀ Is 8 to 15 mu m.

2. The method for resetting the coupling coefficient of the directional coupler by using femtosecond laser twice direct writing as set forth in claim 1, wherein the laser processing optical path in the step (2) is specifically: firstly, after the femtosecond laser is emitted by a femtosecond laser device, the laser power is adjusted after the femtosecond laser passes through a half-wave plate and a polarization beam splitter; then the laser beam is reflected by a first reflector, and the laser beam is expanded after passing through a first concave lens and a second convex lens; then, the expanded laser passes through a diaphragm to select laser beams with uniform energy distribution and the same spot diameter and size of the objective lens inlet; then the laser enters an objective lens after being reflected by a second reflector and a third reflector; then laser is focused on the surface or the interior of the sample through an objective lens; the sample is placed on a sample table, and the sample table is connected with an air floatation motion platform and can perform three-dimensional motion; the laser is converged on the surface or inside of the sample through the objective lens to reflect a part of the laser, the reflected laser passes through the third reflector after passing through the objective lens, is reflected by the fourth reflector, is converged through the third convex lens to form an image on the imaging camera, and the laser focus focused by the objective lens can be judged to be positioned on the surface or inside of the sample according to the appearance of light spots in the imaging camera.

3. The method for twice direct-writing resetting of the coupling coefficient of the directional coupler by using the femtosecond laser as claimed in claim 2, wherein the wavelength of the femtosecond laser is 500-1064nm, the pulse frequency is 100KHZ-2000KHz, the pulse width is 130fs-400fs, and the emergent power of the laser is 3W-20W; the objective lens multiple used is 20-100 times.

4. The method for resetting the coupling coefficient of the directional coupler by twice direct writing of the femtosecond laser as set forth in claim 1, wherein the method for determining the initial processing position in the step (3) is specifically as follows:

after leveling is finished, moving an XY axis of the air floatation motion platform to enable the objective lens to be positioned at the upper left corner of the sample; then moving the X axis once every 10-100μm until the cross-shaped reflection light spots in the imaging camera disappear, and proving that the objective lens is at the edge position of the X axis of the sample; similarly, moving the Y axis once at intervals of 10-100 mu m until the light spots disappear, and proving that the objective lens is located at the upper left corner of the sample; after the two steps, the laser focus is positioned at the uppermost left corner of the sample; at this time, the position of the upper left corner is taken as the initial processing position, and the motion platform software is set to zero the coordinates of the position, namely the (0,0,0) point.

5. The method for resetting the coupling coefficient of the directional coupler by the femtosecond laser secondary direct writing as set forth in claim 1, wherein the power used for the secondary direct writing in the step (3) is 140mw to 300mw, and the scanning speed is 1mm/s to 60mm/s; the laser focusing depth is 50um-1000um.

6. The method for twice direct-writing resetting of the coupling coefficient of the directional coupler by using the femtosecond laser as set forth in claim 1, wherein the polishing in the step (3) is specifically performed by the following steps: taking down the processed sample, and roughly polishing the side surface of the sample where the waveguide end surface is positioned by using abrasive paper, wherein the mesh number of the abrasive paper is 100-7000 meshes; then, placing the sample on a polishing machine containing polishing solution for fine polishing, wherein the granularity of the polishing solution is 50nm-14um, and the polishing time of each side is 40-300 minutes; and after the sample polishing is finished, wiping the sample clean and waiting for testing.

7. The use of the method of any one of claims 1-6 for resetting the coupling coefficient of the directional coupler using femtosecond laser direct writing twice in resetting the splitting ratio of the device.

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