Ultra-compact three-coupler based on combined waveguide, preparation method and application

Abstract

This invention discloses an ultra-compact triple coupler based on a combined waveguide, its fabrication method, and its application, belonging to the field of laser processing technology. The triple coupler includes a central media waveguide, a first combined waveguide, a second combined waveguide, and a third combined waveguide. The first and second combined waveguides are each composed of two circular cross-section waveguides spliced ​​together with a certain center-to-center distance 'a', and connected to the coupling region waveguide through a conical waveguide. The center-to-center distance 'a' is defined as the distance between the centers of the two circular cross-section waveguides. When the combined waveguide transmits in single-mode, the maximum center-to-center distance between the two corresponding circular cross-section waveguides is defined as the upper limit 'b' of the center-to-center distance for single-mode transmission. The central media waveguide is a circular cross-section waveguide that exists only in the coupling region. The first combined waveguide is composed of a first port region waveguide, a first conical waveguide, and a first coupling region waveguide connected sequentially. The second combined waveguide is composed of a second port region waveguide, a second conical waveguide, and a second coupling region waveguide connected sequentially. The third combined waveguide is a circular cross-section waveguide, considered as two circular cross-section waveguides spliced ​​together with a center-to-center distance. Based on this design, the introduction of S-bend waveguides within the device is avoided, while compact and controllable evanescent field coupling can be achieved, thus realizing an ultra-compact tricoupler in space. This effectively solves the problem of large cross-sectional dimensions of traditional tricouplers and is conducive to improving their integration on integrated photonic chips.

Description

Technical Field

[0001] This invention belongs to the field of laser processing technology, specifically involving the fabrication of combined waveguides with different center spacings in glass materials using femtosecond laser direct writing technology, so that there are differences in propagation constants between waveguides, thereby suppressing the coupling effect between waveguides over short distances, and finally realizing the fabrication of an ultra-compact three-dimensional three-coupler by designing a three-dimensional spatial structure of a three-coupler. Technical Background

[0002] With the continuous development of integrated optics, integrated photonic chips are increasingly sought after by researchers worldwide. Compared to traditional waveguide fabrication processes such as ion implantation, thin film deposition, and planar lithography, femtosecond laser direct writing offers advantages such as no need for masks, high processing precision, and the ability to perform "true three-dimensional" fabrication, making it the only reliable method for fabricating three-dimensional coupled devices. Among numerous three-dimensional waveguide devices, the tritter, as an important component for on-chip energy transfer, signal exchange, and power distribution, has been applied in quantum optics to realize multiphoton interference experiments. However, due to the relatively small change in effective refractive index of femtosecond laser-written waveguides (10⁻¹⁰),... -4 -10 -3 This results in a large bending radius (~40mm) for the triple coupler to avoid bending losses, leading to a larger device size. Therefore, improving its performance through design and optimization of its structure, such as increasing the splitting ratio and reducing insertion loss, is crucial for enhancing the performance of the triple coupler and even for the practical application of femtosecond laser direct-write photonic circuits. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention aims to provide a method for realizing a tritter based on combined waveguide (CWG) controlled coupling. By fabricating combined waveguides with different center-to-center spacing 'a', different propagation constants 'β' are obtained, thereby achieving an ultra-compact tritter with short-distance coupling and solving the problem of large device size. The main principle is to introduce a difference in propagation constant between waveguides by changing their cross-sections, thus suppressing the coupling effect between waveguides. Furthermore, replacing the S-bend waveguide with waveguides of different cross-sectional shapes as input / output terminals makes the entire tritter linear, significantly reducing its cross-sectional size. In addition, four circular cross-section waveguides are introduced within the coupling region of the device, with the central waveguide acting as the medium, allowing light to couple from the incident waveguide to the other waveguides. This design avoids the introduction of S-bend waveguides within the device while achieving compact, controllable evanescent field coupling, thus realizing an ultra-compact tritter in space. This effectively solves the problem of large cross-sectional size in traditional tritters and is beneficial for improving their integration density on integrated photonic chips.

[0004] This invention is achieved through the following technical solution:

[0005] An ultra-compact triple coupler based on combined waveguides includes a central medium waveguide 1, a first combined waveguide 2, a second combined waveguide 3, and a third combined waveguide 4. The first combined waveguide 2 and the second combined waveguide 3 are both composed of two circular cross-section waveguides spliced ​​together with a certain center-to-center distance 'a', and are connected to the coupling region waveguide through a conical waveguide. The center-to-center distance 'a' is defined as the distance between the centers of the two circular cross-section waveguides. When the combined waveguides transmit in single-mode, the maximum center-to-center distance between the two corresponding circular cross-section waveguides is defined as the upper limit 'b' of the center-to-center distance for single-mode transmission. The central medium waveguide 1 is a circular cross-section waveguide existing only in the coupling region. The first combined waveguide 2 is composed of a first port region waveguide 5, a first conical waveguide 6, and a first coupling region waveguide 7 connected sequentially. The second combined waveguide 3 is composed of a second port region waveguide 8, a second conical waveguide 9, and a second coupling region waveguide 10 connected sequentially. The third combined waveguide 4 is a circular cross-section waveguide, considered as two circular cross-section waveguides spliced ​​together with a center-to-center distance '0'.

[0006] Furthermore, the first port region waveguide 5 is formed by splicing two circular cross-section waveguides with a center-to-center distance b; the first coupling region waveguide 7 is connected to the first port region waveguide 5 through the first conical region waveguide 6; the second port region waveguide 8 is formed by splicing two circular cross-section waveguides with a center-to-center distance b / 2; the second coupling region waveguide 10 is connected to the second port region waveguide 8 through the second conical region waveguide 9; since the waveguide cross-sections of the port region waveguides (4, 5, 8) of the three couplers are different, there are differences in propagation constants between them, and coupling will be suppressed; in addition, by adjusting the direct writing speed, the propagation constants between the central medium waveguide 1 and the coupling region waveguides (4, 7, 10) of the coupling region of the three couplers are made the same, so that effective coupling transmission can be performed in the coupling region.

[0007] On the other hand, the present invention also provides a method for fabricating an ultracompact triple coupler based on a combined waveguide, the specific steps of which are as follows:

[0008] (1) Leveling the sample stage;

[0009] The specific steps are as follows: First, the surface of the sample to be processed is cleaned with acetone and ethanol to remove grease and dust. Second, the femtosecond laser emitted from the laser passes through a half-wave plate (HWP) and a polarizing beam splitter (PBS) before being reflected by the first mirror M1. The incident beam is then shaped into an elliptical Gaussian beam by a cylindrical concave lens CL1 and a cylindrical convex lens CL2. The shaped beam is then cut into a thin strip of light by an adjustable slit Slit, and after passing through the second mirror M2 and being focused by the objective lens OL, it is incident on the surface of the sample to be processed on the sample stage. An illumination source LED is fixed on the frame of the second mirror M2. When the white light emitted by the illumination light is incident on the objective lens OL and focused on the surface of the glass sample, the sample will be illuminated. The illumination light passes through the second mirror M2 and is reflected by the third mirror M3 before being focused onto the camera CCD. The camera CCD is connected to a computer PC, so that the leveling process of the sample stage can be monitored in real time on the computer.

[0010] (2) Determine the direct writing speed and coupling coefficient-coupling spacing curve at different depths;

[0011] The specific steps are as follows: First, after leveling the sample stage, rotate the optical axis of the half-wave plate (HWP) in front of the laser to adjust the laser power in front of the objective lens entrance pupil; then, load the MATLAB processing program in the three-dimensional displacement platform control software, and according to the designed three-coupler structure, based on the spatial positions of the central medium waveguide 1 and the third combined waveguide 4, as well as the central medium waveguide 1 and the first coupling region waveguide 7, directly draw a series of cross-layer deep directional couplers with different coupling lengths L within the sample (i.e., the two arms of the directional coupler are at different depths according to different coupling spacings d), wherein the two arms of the first part of the directional coupler are at different depths. The spatial positions of the two arms of the second part of the directional coupler correspond to the spatial positions of the central medium waveguide 1 and the third combined waveguide 4, respectively. The spatial positions of the two arms correspond to the spatial positions of the central medium waveguide 1 and the first coupling region waveguide 7. By adjusting the direct writing speed of each arm at different depths, the interlayer waveguide properties are made the same when the scanning power is constant, so that the transmission ratio of the cross-layer deep directional coupler can oscillate to 1. This achieves the direct writing speed that makes the waveguide properties of the two arms of the cross-layer deep directional coupler the same at different depths. Furthermore, by measuring the transmission ratio of the cross-layer deep directional coupler, the transmission ratio-coupling length L curve is obtained. Finally, according to the coupling mode formula:

[0012]

[0013] Where κ is the coupling coefficient, Δ is the propagation constant, and L is the coupling length. As the initial phase, the coupling coefficient κ corresponding to different coupling spacings d can be fitted, and then the coupling coefficient κ-coupling spacing d curve can be obtained. This curve is used to determine the coupling spacing between the central medium waveguide 1, the third combined waveguide 4, the first coupling zone waveguide 7 and the second coupling zone waveguide 10 in the subsequent fabrication of the three couplers.

[0014] (3) Determine the required coupling spacing for the three couplers;

[0015] The specific steps are as follows: First, the distance between every two (4-5, 4-8, 5-8) of the three port waveguides of the triple coupler is defined as the coupling spacing D between the input / output port waveguides of the triple coupler; the distance between the central medium waveguide 1 and the third combined waveguide 4, and between the central medium waveguide 1 and the second coupling region waveguide 10 of the triple coupler are defined as the coupling spacing d1; the distance between the central medium waveguide 1 and the first coupling region waveguide 7 of the triple coupler is defined as the coupling spacing d2; then, an experiment is designed to determine the minimum coupling between the port waveguides. Small spacing, and use this spacing as the coupling spacing D between the port area waveguides; load the pre-written combined waveguide fabrication program, and use the direct writing speed of step (2) to fabricate three sets of cross-layer depth combined waveguide combinations (4-5, 5-8, 4-8) in the port area waveguides respectively; then, test the coupling situation within the three sets of waveguides in the direct writing, so as to determine the minimum coupling spacing D between the port area waveguides without coupling, that is, the coupling spacing between the input / output port area waveguides of the three coupler; finally, according to the geometric structure of the designed three coupler, use the formula:

[0016]

[0017] Calculate the coupling distance d1 between the central medium waveguide 1 and the third combined waveguide 4, and between the central medium waveguide 1 and the second coupling region waveguide 10 of the three coupler; then, through the coupling coefficient κ-coupling distance d curve obtained in step (2), make the coupling coefficient κ values ​​of the two curves equal, so as to finally determine the coupling distance d2 between the central medium waveguide 1 and the first coupling region waveguide 7 of the three coupler.

[0018] (4) Fabrication of the three-coupler;

[0019] The specific steps are as follows: First, take the processing position of the central medium waveguide 1 as the origin of the coordinate system (0, 0), then the coordinates of the second combined waveguide 3 are (-d1*cos60°, -d1*sin60°), the coordinates of the third combined waveguide 4 are (d1*cos60°, -d1*sin60°), and the coordinates of the first combined waveguide 2 are (0, d2). Next, rotate the optical axis of the half-wave plate (HWP) in front of the laser, adjust the laser power in front of the objective lens entrance pupil, and according to the various coupling spacings D, d1, d2 required for the preparation of the three couplers determined in step (3), use the direct writing speed obtained in step (2) to directly write out the waveguides at different layer depths: central medium waveguide 1, first combined waveguide 2, second combined waveguide 3 and third combined waveguide 4, thereby preparing the three couplers.

[0020] Furthermore, the femtosecond laser in step (1) has a wavelength of 1030 nm, a pulse width of 200-400 fs, and a repetition rate of 0.5 MHz-2 MHz; the half-wave plate HWP is mounted on an electrically controlled rotary stepper motor, and the rotation angle is precisely controlled by a computer, so that the combination of HWP and polarization beam splitter PBS can achieve laser power control.

[0021] Furthermore, the processing depth control range in step (2) is 190-220μm (the depth of the sample upper surface is 0μm), and the coupling length L control range is 0-5mm.

[0022] Further, the cross-layer deep directional coupler described in step (2) is divided into two parts. In the first part, the waveguide connection line of the coupling region of the fixed-depth arm and the variable-depth arm forms a 60° angle with the depth direction. In the second part, the waveguide connection line of the coupling region of the fixed-depth arm and the variable-depth arm forms a 0° angle with the depth direction. The two fixed-depth arms have the same depth. After processing, the waveguide end face of the sample is polished and the transmittance is tested to obtain the relationship curve between the coupling length and the transmittance, as well as the relationship curve between the coupling spacing and the coupling coefficient.

[0023] Further, in step (3), in the coupling of the three sets of combined waveguides, the third combined waveguide 4 and the first port area waveguide 5 form a 30° angle in the depth direction; the second port area waveguide 8 and the first port area waveguide 5 form a -30° angle in the depth direction; the centers of the third combined waveguide 4 and the second port area waveguide 8 are at the same depth; wherein, the first port area waveguide 5 is formed by splicing two circular waveguides at 90°, and the second port area waveguide 8 is formed by splicing two circular waveguides at 30°; after the processing is completed, the waveguide end face of the sample is polished and the mode field is tested, so as to determine the minimum coupling distance D between the port area waveguides without coupling.

[0024] Further, in the three-coupler described in step (4), the first combined waveguide 2 and the central medium waveguide 1 form a 0° angle in the depth direction; the processing position of the second combined waveguide 3 forms a -60° angle in the depth direction with the central medium waveguide 1; the processing position of the third combined waveguide 4 forms a 60° angle in the depth direction with the central medium waveguide 1; the direct writing speeds of the first combined waveguide 2, the second combined waveguide 3 and the third combined waveguide 4 are the same as the direct writing speeds of the first port area waveguide 5, the second port area waveguide 8 and the third combined waveguide 4 in step (3); the depth and direct writing speed of the central medium waveguide 1 in the coupling region are consistent with the depth and direct writing speed of the solid-deep arm of the cross-layer deep directional coupler in step (2); the coupling length is 0.15-5.55mm; after processing, the waveguide end face of the sample is polished and the mode field is tested to obtain the beam splitting situation of the three-coupler.

[0025] Thirdly, the present invention also provides an application of an ultra-compact triple coupler based on a combined waveguide in quantum interference measurement and quantum metrology.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) This invention constructs combined waveguides with different center spacings by using the waveguide cross section as the degree of freedom to adjust the propagation constant, thereby adjusting the propagation constant between waveguides and realizing the control of waveguide coupling at close range;

[0028] (2) Compared with the traditional triple coupler, the structure of the ultra-compact triple coupler can be constructed by using a combined waveguide and inserting a central medium waveguide at the coupling region of the triple coupler, which can effectively reduce its cross-sectional area and improve its integration on the integrated photonic chip. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the optical path for fabricating a combined waveguide using a cylindrical lens-slit modulated femtosecond laser to achieve coupling control and fabricate an ultra-compact triple coupler according to the present invention.

[0030] Among them, HWP - half-wave plate, PBS - polarizing beam splitter, M1 - first mirror, CL1 - cylindrical concave lens, CL2 - cylindrical convex lens, Slit - adjustable width slit, M2 - second mirror, LED - illumination source, OL - objective lens, M3 - third mirror, L1 - convex lens, CCD - camera, PC - computer.

[0031] Figure 2 This is a schematic diagram of a three-coupler structure based on a combined waveguide design according to the present invention;

[0032] Among them, (a) is a structural diagram of a traditional triple coupler; (b) is a port cross-sectional diagram of a traditional triple coupler; (c) is a cross-sectional diagram of the coupling region of a traditional triple coupler; (d) is a structural diagram of an ultra-compact triple coupler based on a combined waveguide, in which the S-bend waveguide in the traditional triple coupler is replaced by a combined waveguide, making the triple coupler linear; (e) is a cross-sectional diagram of the port region of the ultra-compact triple coupler, showing that the waveguide cross-sections of the three port regions are different, thus introducing a propagation constant difference between the waveguides; (f) is a cross-sectional diagram of the coupling region of the ultra-compact triple coupler, showing that a central medium waveguide 1 is inserted in the center of the coupling region. Under the same coupling spacing d = 8 μm, the traditional triple coupler has a port spacing Dis = 45 μm and a cross-sectional area of ​​approximately 876 μm due to the presence of the S-bend waveguide. 2 The cross-sectional area of ​​the ultra-compact tricoupler is approximately 84 μm. 2, The waveguide cross section is reduced to one-tenth of its original size. (g) is a schematic diagram of the combined waveguide structure. The cross section of the waveguide gradually decreases along the Z-axis and then gradually increases. The propagation constant β of the waveguide also changes accordingly.

[0033] Central medium waveguide 1, first combined waveguide 2, second combined waveguide 3, third combined waveguide 4, first port region waveguide 5, first cone region waveguide 6, first coupling region waveguide 7, second port region waveguide 8, second cone region waveguide 9, and second coupling region waveguide 10;

[0034] Figure 3 The diagram shows the structure and test results of the two-part cross-layer deep directional coupler of the present invention;

[0035] Wherein, (a) is the oscillation curve of the first part of the directional coupler; (b) is the oscillation curve of the second part of the directional coupler; (c) is the relationship curve between the coupling spacing d and the coupling coefficient κ obtained from the oscillation curves of the two parts of the directional coupler; where the solid line and the dashed line represent the relationship between the coupling coefficient and the coupling spacing of the first part and the second part of the directional coupler, respectively.

[0036] Figure 4 This invention illustrates the coupling characteristics of three combined waveguides with a coupling spacing D = 13.9 μm between the waveguides in the port region.

[0037] Among them, (a) shows the coupling between waveguides 4 and 5 in the port region; (b) shows the coupling between waveguides 5 and 8 in the port region; (c) shows the coupling between waveguides 4 and 8 in the port region. It can be seen that under this coupling spacing, the three sets of combined waveguides achieve complete coupling isolation.

[0038] Figure 5 These are mode field photographs of the ultra-compact tricoupler of the present invention when light passes through different input ports;

[0039] Among them, (a) is a mode field photograph when port area 4 is open to light; (b) is a mode field photograph when port area 5 is open to light; and (c) is a mode field photograph when port area 8 is open to light. Detailed Implementation

[0040] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0041] Example 1

[0042] We used femtosecond laser direct writing technology to fabricate deep directional couplers (DC) across layers, explored the direct writing speed at different depths, and obtained the coupling coefficient-coupling spacing curve.

[0043] Taking advantage of the "true three-dimensional" processing of femtosecond lasers, two cross-layer deep directional couplers with different coupling lengths L are directly written into the sample. Different direct writing speeds are adopted according to the direct writing depth corresponding to different coupling spacings, and the transmittance is tested and fitted. When the transmittance can oscillate to 1, it means that the corresponding direct writing speed at that depth has been found, and finally the coupling coefficient-coupling spacing curve is obtained.

[0044] The specific steps are as follows:

[0045] (1) Leveling the sample stage: Clean the surface of the sample (Corning glass) to be processed; the femtosecond laser used has a wavelength of 1030nm, a pulse width of 239fs, and a repetition rate of 1MHz; the objective lens OL has a working distance of 0.51mm, NA = 0.75, and a magnification of 40. First, as... Figure 1As shown, the femtosecond laser emitted from the laser is passed through a half-wave plate HWP and a polarizing beam splitter PBS with a high damage threshold; it is then reflected by the first mirror M1 to a cylindrical lens system consisting of a concave cylindrical lens CL1 and a convex cylindrical lens CL2, and then passes through a slit with a width of 0.85 mm. At this point, the beam is shaped into a thin strip of light by the cylindrical lens-slit system; next, the laser is incident on the objective lens OL by the second mirror M2. The optical axis of the half-wave plate HWP at the laser exit is rotated by computer control, so that the laser energy behind the mirror M2 and in front of the entrance pupil of the objective lens OL is 210 mW; then, the objective lens OL focuses the laser and incident it on the surface of the glass sample on the sample stage; then, the leveling process of the sample stage can be monitored in real time on a computer PC by the illumination light emitted by the LED light source fixed on the frame of the mirror M2 and the camera CCD on the right side of the third mirror M3. Using the two perpendicular sides of the sample as the X and Z axes, the motion platform is moved between the points (0, 0) and (2.5cm, 0) via a PC. Simultaneously, the X-axis leveling knob is adjusted until the laser can draw a uniform damage line 2.5cm long and 4μm wide. At this point, X-axis leveling is complete. Similarly, the laser is used to draw a line between (0, 0) and (0, 2.5cm), and the Z-axis leveling knob is used to level the Z-axis. At this point, the laser can draw uniformly wide damage lines in both the X and Z directions, indicating that the sample stage is perpendicular to the focused laser beam. The sample stage leveling is complete. The sample stage is then fixed in its position.

[0046] (2) Direct writing of the cross-layer deep directional coupler: First, rotate the optical axis of the half-wave plate HWP so that the laser power in front of the objective lens OL entrance pupil is 410mW; then, load the pre-written cross-layer deep directional coupler processing program on the PC and process the two parts of the directional coupler respectively; wherein, the coupling distance between the two arms of the directly written directional coupler is d = 5, 6, 7.5, 8 and 9μm, the coupling length L is 0.5-4mm, and the coupling area is connected to the input / output straight waveguide by an S-bend waveguide with a radius R = 50mm; the depth of the fixed deep arm is fixed at 205μm, and the corresponding direct writing speed is 70mm / s, the first part of the directional coupler In the first part of the directional coupler, the waveguide connection line of the coupling region of the variable depth arm and the fixed depth arm forms a 60° angle with the depth direction, while in the second part of the directional coupler, the waveguide connection line of the coupling region of the variable depth arm and the fixed depth arm forms a 0° angle with the depth direction. Different direct writing speeds are used according to different coupling spacing corresponding to different direct writing depths. The direct writing speeds of the variable depth arm in the first part of the directional coupler are 71.5 mm / s, 72 mm / s, 72.5 mm / s, 72.7 mm / s, and 73 mm / s, respectively, while the direct writing speeds of the variable depth arm in the second part of the directional coupler are 67 mm / s, 65 mm / s, 63 mm / s, 61.5 mm / s, and 59.5 mm / s, respectively.

[0047] (3) Determining the direct writing speed and coupling coefficient-coupling distance curves at different depths: First, the input and output ends of the processed sample were mechanically polished; then, using a precision displacement platform, 808nm vertically polarized light was coupled into the input arm of the directional coupler through an objective lens. Another objective lens was placed on one side of the output end of the directional coupler, and the focal length of this objective lens was adjusted to focus the output beam; simultaneously, two power meter probes were placed behind this objective lens, and by adjusting the height and left-right direction of the objective lens, the two output beams of the directional coupler were made to hit the center of the probes, thereby realizing the transmission ratio measurement of the cross-layer deep directional coupler; next, the measured transmission ratio curve was fitted using Origin software to obtain the coupling coefficient κ under different coupling distances; finally, the relationship between the coupling distance d and the coupling coefficient κ obtained in the two cross-layer deep directional couplers was fitted and combined into one figure, as shown in the figure below. Figure 3 As shown. By Figure 3 As shown in (a) and (b), the test results and fitting results of the transmission ratio versus coupling length curve are basically consistent, and the transmission ratio of these cross-layer deep directional couplers can oscillate to 1. This indicates that by adjusting the direct writing speed of the two arms of the directional coupler at different depths, the waveguide properties of the two arms can be made the same, thus determining the waveguide direct writing speed at different depths; Figure 3 (c) It can be seen that the curves showing the relationship between the coupling coefficients κ1 and κ2 of the two cross-layer deep directional couplers and the coupling spacing almost overlap, and the test results are in good agreement with the fitting results. Therefore, the direct writing speeds at different depths were determined, and the curves showing the relationship between the coupling coefficients and the coupling spacing were obtained.

[0048] This invention provides an application of an ultracompact tricoupler based on combined waveguides in quantum interferometry and quantum metrology. The ultracompact tricoupler based on combined waveguides has wide applications in quantum interferometry and quantum metrology. By using a three-port beam splitting structure instead of the traditional two-port coupling, its ability to generate path-entangled states can be effectively improved, thus reducing the complexity of optical quantum chips in terms of the number of optical components. Simultaneously, the compactness of the tricoupler based on combined waveguides makes it a fundamental building block for future integrated optical chips to realize complex networks, and it is expected to open up new perspectives in many research areas of quantum information, such as fundamental quantum mechanical testing with increasing photon counts, quantum state engineering, quantum sensing, and quantum simulation.

[0049] Example 2

[0050] The coupled waveguide group based on the combined waveguide was fabricated using femtosecond laser direct writing technology, and the coupling of each group was tested to determine the required coupling spacing of the three couplers. The specific steps are as follows:

[0051] (1) Leveling of the sample stage: Same as in Example 1.

[0052] (2) Direct writing of waveguide combination: First, rotate the optical axis of the half-wave plate HWP so that the laser power in front of the objective lens OL entrance pupil is 410mW; then, load the pre-written combination waveguide coupling judgment processing program on the computer PC, and process three sets of cross-layer deep waveguide combinations with a coupling spacing D of 13.9μm respectively; among them, each combination consists of two combination waveguides: the first group is the combination of port area waveguides 4-5, with center spacing of 0μm and 3.6μm respectively; the second group is the combination of port area waveguides 5-8, with center spacing of 1.8μm and 3.6μm respectively; the third group is the combination of port area waveguides 4-8, with center spacing of 0μm and 1.8μm respectively, and the coupling length is 0.2-6.2mm. The direct writing depth of the first port region waveguide 5 is 197 μm, and the direct writing speed is 61.5 mm / s; the processing positions of the third combined waveguide 4 and the second port region waveguide 8 are at angles of 30° and -30° respectively with respect to the depth direction, and the direct writing speed of both is 72.7 mm / s; wherein, the first port region waveguide 5 is composed of two circular cross-section waveguides spliced ​​at 90°, and the second port region waveguide 8 is composed of two circular cross-section waveguides spliced ​​at 30°.

[0053] (3) Determining the required coupling spacing of the three couplers: After processing, the input and output ends of the sample are mechanically polished; then, the coupling control test between the combined waveguides is performed using the test system in Example 1. 808nm vertically polarized light is coupled into any one of the three waveguide combinations through the objective lens, and then focused onto the mode analyzer through the objective lens on the other side of the sample. The mode field on the computer screen shows the coupling energy exchange between the waveguides. The results are as follows: Figure 4 As shown. By Figure 4As shown in (a), (b), and (c), by constructing port waveguides (4, 5, 8) with different center spacings, the waveguide cross-section can be controlled, thereby controlling the propagation constant between waveguides and effectively suppressing coupling between waveguides at close range. In the figures, when the spacing between the two combined waveguides is 13.9 μm, it can be seen that light only propagates within the incident waveguide and does not couple into the other waveguide. This determines the shortest coupling spacing of 13.9 μm for complete isolation between the two combined waveguides, achieving coupling control based on combined waveguides. It also determines the coupling spacing between the input / output port waveguides, D = 13.9 μm, for the subsequent fabrication of the three-coupler. Subsequently, based on the geometry of the three-coupler, the coupling spacing d1 = 8 μm between the central medium waveguide 1 and the third combined waveguide 4, and between the central medium waveguide 1 and the second coupling waveguide 10, is calculated. The value of κ1 can then be determined using the coupling coefficient κ1 and coupling spacing d1 curve obtained in Example 1. To achieve better coupling effect with the three couplers, the coupling coefficient κ2 = κ1 in the direction of waveguide 1-7 in the coupling region is set. Thus, the coupling distance d2 = 7.97 μm between the central medium waveguide 1 and the first coupling region waveguide 7 is determined by the relationship curve between the coupling coefficient κ2 and the coupling distance d2.

[0054] Example 3

[0055] An ultracompact triple coupler based on a combined waveguide was fabricated using femtosecond laser direct writing technology, and its beam splitting was tested. The specific steps are as follows:

[0056] (1) Leveling of the sample stage: Same as in Example 1.

[0057] (2) Fabrication of the three-coupler: First, rotate the optical axis of the half-wave plate HWP so that the laser power in front of the objective lens OL entrance pupil is 410mW; then, load the pre-written three-coupler fabrication program on the PC; since the port waveguides (4, 5, 8) are close together, fabricating three three-couplers with the same fabrication parameters but different input ports can facilitate subsequent testing; a central medium waveguide 1 with a center spacing of 0μm and a propagation constant consistent with the propagation constant of the coupling region waveguides (4, 7, 10) is directly written at the coupling region position of the three-coupler; in order to reduce mode mismatch loss, the waveguides inside the coupling region and the ports outside the coupling region are connected by a tapered waveguide, the length of which is L taper =1.5mm; Based on the coupling spacings D, d1, and d2 required for fabricating the triple coupler as determined in Example 2, the waveguides at different layer depths of the triple coupler are fabricated using the direct writing speed obtained in Example 1, thereby fabricating the triple coupler. The coupling length of the coupling region of the triple coupler is 0.15-5.55mm.

[0058] (3) Spectrometer ratio test of the three-coupler: After processing, the input and output surfaces of the sample were mechanically polished; then, the spectrometer ratio was tested using the objective-chip-objective testing system in Example 1. Vertically polarized light of 808nm was coupled into any input port of the three-coupler through the objective lens, and the mode field energy of the output light was observed using a mode analyzer. The results are as follows: Figure 5 As shown. By Figure 5 As shown in (a), (b), and (c), good uniform beam splitting is achieved when light is transmitted from the port waveguides (4, 5, and 8) respectively. Specifically, when light is input from ports (4, 5, and 8), the output beam splitting ratios of the three-coupler are 0.35:0.34:0.31, 0.34:0.33:0.33, and 0.35:0.33:0.32, respectively. Furthermore, the mode field diagram shows that the three ports of the three-coupler have different cross-sectional shapes, resulting in different propagation constants for the waveguides, thus achieving coupling isolation between the waveguides. Compared to traditional three-couplers, the ultra-compact three-coupler based on combined waveguides successfully replaces the S-bend waveguide in the traditional three-coupler, reducing the cross-sectional area to one-tenth of the original, and enabling better on-chip integration.

[0059] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0060] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable way without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0061] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A combination waveguide-based ultra-compact tri-coupler, characterized in that, The system includes a central medium waveguide (1), a first combined waveguide (2), a second combined waveguide (3), and a third combined waveguide (4). The first combined waveguide (2) and the second combined waveguide (3) are both composed of two circular cross-section waveguides spliced ​​together with a certain center spacing a, and connected to the coupling zone waveguide through a conical waveguide. The center spacing a is defined as the distance between the centers of the two circular cross-section waveguides. When the combined waveguide transmits in single mode, the maximum center spacing between the two corresponding circular cross-section waveguides is defined as the upper limit b of the center spacing for single-mode transmission. The central medium waveguide (1) is a circular cross-section waveguide that exists only in the coupling zone. The first combined waveguide (2) is composed of a first port zone waveguide (5), a first conical waveguide (6), and a first coupling zone waveguide (7) connected in sequence. The second combined waveguide (3) is composed of a second port zone waveguide (8), a second conical waveguide (9), and a second coupling zone waveguide (10) connected in sequence. The third combined waveguide (4) is a circular cross-section waveguide, which is considered to be composed of two circular cross-section waveguides spliced ​​together with a center spacing 0. The first port area waveguide (5) is formed by splicing two circular cross-section waveguides with a center-to-center distance b; the first coupling area waveguide (7) is connected to the first port area waveguide (5) through the first conical waveguide (6); the second port area waveguide (8) is formed by splicing two circular cross-section waveguides with a center-to-center distance b / 2; the second coupling area waveguide (10) is connected to the second port area waveguide (8) through the second conical waveguide (9); since the waveguide cross-sections of the port area waveguides (4, 5, 8) of the three couplers are different, there is a difference in propagation constant between them, and coupling will be suppressed; in addition, by adjusting the direct writing speed, the propagation constants between the central medium waveguide (1) and the coupling area waveguides (4, 7, 10) of the coupling area of ​​the three couplers are the same, thereby achieving effective coupling transmission in the coupling area.

2. The method of claim 1, wherein the method further comprises: The specific steps are as follows: (1) Leveling the sample stage; The specific steps are as follows: First, the surface of the sample to be processed is cleaned with acetone and ethanol to remove grease and dust. Second, the femtosecond laser emitted from the laser passes through a half-wave plate and a polarizing beam splitter, and is reflected by the first reflecting mirror M1. The incident beam is then shaped into an elliptical Gaussian beam by a cylindrical concave lens CL1 and a cylindrical convex lens CL2. The shaped beam is then cut into a thin strip of light by an adjustable slit Slit, and enters the objective lens OL through the second reflecting mirror M2 for focusing before being incident on the surface of the sample to be processed on the sample stage. An illumination source LED is fixed on the frame of the second reflecting mirror M2. When the white light emitted by the illumination light enters the objective lens OL and is focused on the surface of the glass sample, the sample will be illuminated. The illumination light passes through the second reflecting mirror M2 and is reflected by the third reflecting mirror M3 before being focused onto the camera CCD. The camera CCD is connected to a computer PC, so that the leveling process of the sample stage can be monitored in real time on the computer. (2) Determine the direct writing speed and coupling coefficient-coupling spacing curve for different depths; The specific steps are as follows: First, after the sample stage is leveled, rotate the optical axis of the half-wave plate in front of the laser and adjust the laser power in front of the objective lens entrance pupil; then, load the MATLAB processing program in the three-dimensional displacement platform control software, and according to the designed three-coupler structure, according to the spatial positions of the central medium waveguide (1) and the third combined waveguide (4), as well as the central medium waveguide (1) and the first coupling region waveguide (7), directly draw a series of cross-layer deep directional couplers with different coupling lengths L in the sample, that is, the two arms of the directional coupler are at different depths according to different coupling spacings d, wherein the spatial positions of the two arms of the first part of the directional coupler are... Corresponding to the spatial positions of the central medium waveguide (1) and the third combined waveguide (4), the spatial positions of the two arms of the second part of the directional coupler correspond to the spatial positions of the central medium waveguide (1) and the first coupling region waveguide (7); by adjusting the direct writing speed of the two arms at different depths, the interlayer waveguide properties are the same when the scanning power is constant, so that the transmission ratio of the cross-layer deep directional coupler can oscillate to 1, thereby obtaining the direct writing speed that makes the waveguide properties of the two arms of the cross-layer deep directional coupler the same at different depths; and by measuring the transmission ratio of the cross-layer deep directional coupler, the transmission ratio-coupling length L curve is obtained; finally, according to the coupling mode formula: ； wherein, κ is a coupling coefficient, is a propagation constant, L is a coupling length, is an initial phase, the coupling coefficient κ corresponding to different coupling distances d is fitted, and then the coupling coefficient κ-coupling distance d curve is obtained, which is used to determine the coupling distance between the center medium waveguide (1), the third combined waveguide (4), the first coupling region waveguide (7) and the second coupling region waveguide (10) in the subsequent preparation of the three-coupler. (3) Determine the required coupling spacing for the three couplers; The specific steps are as follows: First, the distance between every two (4-5, 4-8, 5-8) of the three port waveguides of the three coupler is defined as the coupling spacing D between the input / output port waveguides of the three coupler; the distance between the central medium waveguide (1) and the third combined waveguide (4), and between the central medium waveguide (1) and the second coupling waveguide (10) of the three coupler is defined as the coupling spacing d1; the distance between the central medium waveguide (1) and the first coupling waveguide (7) of the three coupler is defined as the coupling spacing d2; then, an experiment is designed to determine the distance between the port waveguides. The minimum spacing for coupling is determined, and this spacing is used as the coupling spacing D between the port area waveguides; a pre-written combined waveguide fabrication program is loaded, and the direct writing speed of step (2) is used to fabricate three sets of cross-layer depth combined waveguide combinations (4-5, 5-8, 4-8) in the port area waveguides respectively; then, the coupling situation within the three sets of waveguides in the direct writing is tested to determine the minimum coupling spacing D between the port area waveguides, that is, the coupling spacing between the input / output port area waveguides of the three coupler; finally, according to the geometry of the designed three coupler, the formula is used: ； The coupling distance d1 between the central medium waveguide (1) and the third combined waveguide (4) and between the central medium waveguide (1) and the second coupling region waveguide (10) of the three coupler is calculated; then, by using the coupling coefficient κ-coupling distance d curve obtained in step (2), the coupling coefficient κ values ​​of the two curves are made equal, so as to finally determine the coupling distance d2 between the central medium waveguide (1) and the first coupling region waveguide (7) of the three coupler; (4) Fabrication of the three-coupler; The specific steps are as follows: First, take the processing position of the central medium waveguide (1) as the origin of the coordinate system (0, 0), then the coordinates of the second combined waveguide (3) are (-d1*cos60°, -d1*sin60°), the coordinates of the third combined waveguide (4) are (d1*cos60°, -d1*sin60°), and the coordinates of the first combined waveguide (2) are (0, d2). Next, rotate the optical axis of the half-wave plate HWP in front of the laser, adjust the laser power in front of the objective lens entrance pupil, and according to the various coupling spacings D, d1, d2 required for the preparation of the three couplers determined in step (3), use the direct writing speed obtained in step (2) to directly write out the waveguides at different layer depths: the central medium waveguide (1), the first combined waveguide (2), the second combined waveguide (3) and the third combined waveguide (4), thereby preparing the three couplers.

3. A method of fabricating a combination waveguide based ultra-compact tri-coupler as claimed in claim 2, wherein, The femtosecond laser in step (1) has a wavelength of 1030nm, a pulse width of 200-400fs, and a repetition rate of 0.5MHz-2MHz. The half-wave plate HWP is mounted on an electrically controlled rotary stepper motor, and the rotation angle is precisely controlled by a computer, so that the combination of HWP and polarization beam splitter PBS can achieve laser power control.

4. The method of claim 2, wherein the method further comprises: The processing depth adjustment range in step (2) is 190-220μm, and the coupling length L is 0-5mm.

5. The method of claim 2, wherein the method further comprises: The cross-layer deep directional coupler described in step (2) is divided into two parts. In the first part, the waveguide connection line of the coupling region of the fixed-depth arm and the variable-depth arm forms a 60° angle with the depth direction. In the second part, the waveguide connection line of the coupling region of the fixed-depth arm and the variable-depth arm forms a 0° angle with the depth direction. The two fixed-depth arms have the same depth. After processing, the waveguide end face of the sample is polished and the transmittance is tested to obtain the relationship curve between coupling length and transmittance, as well as the relationship curve between coupling spacing and coupling coefficient.

6. A method of fabricating a combination waveguide based ultra-compact tri-coupler as claimed in claim 2, wherein, In step (3), among the three sets of combined waveguide coupling, the third combined waveguide (4) and the first port area waveguide (5) form a 30° angle in the depth direction; the second port area waveguide (8) and the first port area waveguide (5) form a -30° angle in the depth direction; the centers of the third combined waveguide (4) and the second port area waveguide (8) are at the same depth; wherein, the first port area waveguide (5) is formed by splicing two circular waveguides at 90°, and the second port area waveguide (8) is formed by splicing two circular waveguides at 30°; after the processing is completed, the waveguide end face of the sample is polished and the mode field is tested to determine the minimum coupling distance D between the port area waveguides without coupling.

7. A method of fabricating a combination waveguide based ultra-compact tri-coupler as claimed in claim 2, wherein, In the three-coupler described in step (4), the first combined waveguide (2) and the central medium waveguide (1) form a 0° angle in the depth direction; the processing position of the second combined waveguide (3) forms a -60° angle in the depth direction with the central medium waveguide (1); the processing position of the third combined waveguide (4) forms a 60° angle in the depth direction with the central medium waveguide (1); the direct writing speeds of the first combined waveguide (2), the second combined waveguide (3) and the third combined waveguide (4) are the same as the direct writing speeds of the first port area waveguide (5), the second port area waveguide (8) and the third combined waveguide (4) in step (3); the depth and direct writing speed of the central medium waveguide (1) in the coupling area are consistent with the depth and direct writing speed of the solid-deep arm of the cross-layer deep directional coupler in step (2); the coupling length is 0.15-5.55mm; after processing, the waveguide end face of the sample is polished and the mode field is tested to obtain the beam splitting situation of the three-coupler.

8. The application of the ultracompact triple coupler based on combined waveguides as described in claim 1 in quantum interferometry and quantum metrology.

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