Broadband self-imaging device based on combined waveguide induced zigzag gradient scalar potential, preparation method and application

Abstract

The invention discloses a broadband self-imaging device based on combined waveguide induced zigzag gradient scalar potential, a preparation method and application, and belongs to the technical field of laser processing, the broadband self-imaging device comprises n periodically arranged imaging units, each imaging unit comprises m combined waveguides, and the coupling distance between the adjacent combined waveguides is d; each combined waveguide is formed by splicing two waveguides with circular cross sections at a center-to-center distance a; in a single imaging unit, the center distance a of the combined waveguides is gradually increased from the first waveguide to the mth waveguide at a fixed step length, so that the propagation constant beta of the combined waveguides is in gradient distribution, a propagation constant gradient delta beta is formed in each imaging unit, Bloch oscillation can be triggered when optical signals are transmitted in the imaging units, and when the optical signals are transmitted to LBloch, the propagation constant gradient delta beta is not changed. And the wave packet shrinks to a light field distribution state which is the same as that of the initial input light signal, so that self-imaging is realized. The periodically distributed zigzag gradient potential is introduced into the waveguide array by using the combined waveguides with different center distances, the self-imaging waveguide array can be expanded infinitely and periodically, and large-scale and multi-channel parallel signal transmission can be realized.

Description

Technical Field

[0001] The present invention belongs to the field of laser processing technology, and specifically relates to using femtosecond laser direct writing technology to prepare combined waveguides with different center spacings in glass materials, so that there are differences in propagation constants between the waveguides, thereby introducing a gradient propagation constant distribution in the waveguide array, and finally realizing the preparation of a broadband self-imaging device with a sawtooth gradient scalar potential by designing the periodic variation of the center spacing of the combined waveguides in the waveguide array. Background Art

[0002] With the rapid development of information technology, integrated photonic chips have attracted widespread attention from researchers around the world. As the most basic and important component in integrated photonic chips, optical waveguides (hereinafter referred to as "waveguides") are high-refractive-index regions covered by low-refractive-index claddings. They not only guide the transmission of light and serve as a bridge between different devices, but also utilize the exponentially decaying evanescent field appearing in the optical waveguide cladding to generate coupling between adjacent optical waveguides, exchange and transfer energy, and play a major role in the merging and splitting of optical signals. Although this near-field coupling of optical waveguides can be used to implement directional couplers or frequency dividers, it can also cause crosstalk between adjacent waveguides, resulting in discrete diffraction in the waveguide array, which severely limits the development of high-integration and scalability of integrated photonic chips. Waveguide array self-imaging is a technology that achieves precise reconstruction of optical signals through carefully designed optical waveguide arrays. By controlling the propagation and coupling characteristics of light, it can accurately reproduce the input light field at a specific location. This technology shows broad application prospects in the fields of high-density photonic integration, optical imaging and information processing, and can achieve low crosstalk and high-density optical signal transmission on photonic integrated chips.

[0003] Currently, researchers have achieved self-imaging in waveguide arrays by using femtosecond laser direct writing to create segmented waveguides, introducing specific phase shifts; They have also achieved self-imaging by cascading straight waveguides with sinusoidally curved waveguides to form metalenses within the waveguide array; and they have also achieved self-imaging by introducing specific inter-waveguide coupling to make the waveguide array meet the conjugate condition. However, these methods still have drawbacks such as complex design and high precision requirements. These methods require careful design of parameters such as the amplitude of the curved waveguides and the coupling spacing, making them difficult to apply on a large scale. Summary of the Invention

[0004] In response to the deficiencies in the prior art, the present invention provides a broadband self-imaging device, preparation method, and application based on a combined waveguide-induced sawtooth gradient scalar potential. The present invention processes combined waveguides with different center spacings a to obtain different propagation constants β, thereby introducing a gradient propagation constant periodic distribution in the waveguide array to induce a sawtooth gradient potential, thereby achieving waveguide array self-imaging. The structure is simple, easy to implement, and can be applied on a large scale. The main principle of the present invention is that by utilizing combined waveguides with different center spacings, a propagation constant gradient ▽β (gradient force F) is introduced into the waveguide array. When the light signal is transmitted in the waveguide array, it triggers a Broadach oscillation, that is, the light wave exhibits a breathing pattern of periodic expansion and contraction under the action of the transverse gradient force. The oscillation period of the breathing pattern is L Bloch Meet L Bloch =2π / ▽β, the width of the expanded wave packet A Bloch By A Bloch =4κ / ▽β|sin(▽β*z / 2))|, where ▽β is the gradient of the propagation constant, κ is the coupling coefficient between waveguides, and z is the distance the optical signal travels in the waveguide. Bloch / 2, and the wave packet shows a symmetrical lateral broadening with the incident waveguide as the center. When the optical signal is transmitted L Bloch When the coupling coefficient is constant, the oscillation period and maximum width of the wave packet decrease with the gradient of the propagation constant. Further keep the coupling coefficient κ unchanged and increase the propagation constant gradient This not only significantly improves the self-imaging effect of the waveguide array, but also significantly reduces the length of the waveguide array and the maximum width of the light field wave packet, which is conducive to improving its integration density on integrated photonic chips. In addition, by periodically varying the center spacing a of the combined waveguides in the waveguide array and introducing a periodic sawtooth-shaped propagation constant, the waveguide array can be extended infinitely, which is conducive to large-scale applications.

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

[0006] In a first aspect, the present invention provides a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential, comprising n periodically arranged imaging units (n≥1), each imaging unit comprising m combined waveguides (m≥2), the coupling spacing of adjacent combined waveguides being d; each combined waveguide being formed by splicing two mutually parallel circular cross-section waveguides with a center spacing a, and satisfying a single-mode transmission condition a≤b, wherein b is the maximum center spacing of the two circular cross-section waveguides during single-mode transmission; within a single imaging unit, the center spacing a of the combined waveguides increases from the first to the mth waveguide in a fixed step size, so that the propagation constant β of the combined waveguide is gradient-distributed, thereby forming a propagation constant gradient within each imaging unit. When the optical signal is transmitted in the imaging unit, it triggers Bloch oscillation, and its oscillation period is Optical signal is transmitted to L Bloch When , the wave packet shrinks to the same light field distribution state as the initial input light signal, thus achieving self-imaging.

[0007] Furthermore, the maximum difference in combined waveguide cross-sections at the boundaries of adjacent imaging units is 2.5 μm, and the maximum difference in propagation constants is 2.45 mm. -1 , to suppress cross-unit optical signal transmission.

[0008] Furthermore, the coupling spacing d is at least 9 μm, and the device can be infinitely periodically extended in space.

[0009] Furthermore, the propagation constant β of the combined waveguide is linearly related to the center spacing a, and the slope k is determined by experimental fitting of a directional coupler;

[0010] The propagation constant gradient Among them, a m is the center distance of the mth combined waveguide in a single imaging unit.

[0011] In a second aspect, the present invention further provides a method for preparing a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential, which specifically comprises the following steps:

[0012] Step 1: Level the sample stage;

[0013] Use femtosecond laser to scratch the damage line on the sample surface, and adjust the platform inclination angle to make the damage line width uniform;

[0014] Step 2: Determine the maximum center-to-center distance b between the two circular cross-section waveguides when the combined waveguide is transmitting in single mode;

[0015] Machining a composite waveguide with increasing center spacing a; after polishing the composite waveguide, testing it with a mode analyzer to determine the maximum center spacing b during single-mode transmission;

[0016] Step 3: Determine the propagation constant gradient parameter;

[0017] Multiple sets of directional couplers with the same coupling spacing d but different coupling lengths l are fabricated, wherein the center spacing between the two circular cross-section waveguides in the combined waveguides serving as the two arms of the coupler in each set of directional couplers is different. After fabrication, the samples are polished, and the transmittance of the directional couplers with different coupling lengths is measured to obtain a transmittance-coupling length curve. The coupling coefficient κ and the propagation constant difference Δβ between the two arms of the combined waveguide are fitted according to the following coupled-mode formula. A linear fit is then performed based on the propagation constant difference Δβ between the two arms of the combined waveguide to obtain a β-a linear relationship and a slope k.

[0018]

[0019] Where κ is the coupling coefficient, Δβ is the propagation constant difference, and l is the coupling length. is the initial phase;

[0020] Step 4: Preparation of waveguide array self-imaging device;

[0021] In the femtosecond laser direct writing optical waveguide processing system, the processing direction is the X axis, the waveguide processing sequence direction is the Y axis, and the sample depth direction is the Z axis. First, the processing position of the first combined waveguide of the first imaging unit is taken as the coordinate origin (0, 0), and the coordinates of the mth combined waveguide in the first imaging unit are (0, (m-1) d), thereby obtaining the first imaging unit; wherein, the center distances between the two circular waveguides of the combined waveguide in each imaging unit are a1, a2, a3, a4...a respectively. m Then, rotate the optical axis of the half wave plate (HWP) in front of the laser, adjust the laser power before the objective lens entrance pupil, and start processing. The processing length of all combined waveguides is L Bloch , Thus, a broadband self-imaging device consisting of n imaging units is prepared, where the propagation constant gradient is

[0022] Furthermore, step one specifically includes the following:

[0023] First, use acetone and ethanol to clean the surface of the sample to be processed and wipe off grease and dust; secondly, the femtosecond laser emitted by the laser passes through a half-wave plate (HWP) and a polarization beam splitter (PBS) and is reflected by the first reflector M1. The incident light beam is then shaped into an elliptical Gaussian beam by a cylindrical concave lens CL1 and a cylindrical convex lens CL2. The shaped light beam is then cut into a slender strip-shaped light spot by a slit with adjustable width. After passing through the second reflector M2, it enters the objective lens OL and is focused and incident on the surface of the sample to be processed on the sample stage; an illumination light source LED is fixed on the frame of the second reflector 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 reflector M2 and is reflected by the third reflector M3 and focused into 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 side;

[0024] Finally, using the two perpendicular sides of the sample as the X and Y axes, the motion platform was adjusted by the PC to move between (0, 0) and (2.5 cm, 0). At the same time, the X-direction leveling knob was adjusted until the laser could scan a uniform damage line with a length of 2.5 cm and a width of 4 μm. At this point, the X-direction leveling was completed. Similarly, the laser was used to draw a line between (0, 0) and (0, 2.5 cm) and the Y-direction leveling knob was used to level the Y direction. At this point, the laser could directly write a uniform width damage line in both the X and Y directions, indicating that the sample stage was perpendicular to the focused laser beam. The sample stage leveling was complete and the position of the sample stage was fixed.

[0025] Furthermore, step 2 specifically includes the following contents:

[0026] First, after the sample stage is leveled, the optical axis of the half-wave plate (HWP) in front of the laser is rotated to adjust the laser power in front of the objective lens entrance pupil. Next, the MATLAB processing program is loaded and run in the three-dimensional displacement platform control software. By controlling the processing depth, the power of the processing laser, and the speed of laser direct writing and changing the center spacing a of the combined waveguides, combined waveguides with different center spacings a can be processed. Finally, after processing, the sample is polished, and the combined waveguides with different center spacings a are tested using a mode analyzer to determine the maximum center spacing b of the combined waveguides when the combined waveguides are transmitting in single mode.

[0027] Furthermore, in step three, the directional couplers are divided into Q groups, and the difference in the center spacing between the two circular cross-section waveguides in the two-arm combined waveguides of each group of directional couplers is Δa; and the center spacing continuity is satisfied between two adjacent groups: the center spacing of the first arms of the latter group is equal to the center spacing of the second arms of the previous group, the center spacing of the first arms of the first group is 0, and the center spacing of the second arms of the last group is less than the maximum spacing b; the coupling length l of each group of directional couplers ranges from 0.5 to 6.5 mm, and the coupling spacing d is the same.

[0028] In a third aspect, the present invention also provides an application of a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential to transmit light intensity coded signals on an integrated photonic chip.

[0029] Compared with the prior art, the advantages of the present invention are as follows:

[0030] (1) The present invention utilizes femtosecond laser direct writing of combined waveguides, introduces a periodic distribution of gradient propagation constants in the waveguide array by changing the center spacing of the combined waveguides, and induces a sawtooth gradient potential, thereby achieving self-imaging of optical signals in the waveguide array;

[0031] (2) The present invention utilizes combined waveguides with different center spacings to introduce a periodically distributed sawtooth gradient potential in the waveguide array, which can infinitely periodically expand the self-imaging waveguide array, thereby facilitating large-scale, multi-channel parallel signal transmission.

[0032] (3) The designed self-imaging waveguide array has extremely strong robustness to the communication wavelength. In the wide spectrum range of 633-852nm, the energy recovery rate under single waveguide excitation is as high as over 94%. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly describes the drawings required for the specific embodiments or the description of the prior art. Similar elements or parts are generally identified by similar reference numerals throughout the drawings. Elements or parts in the drawings are not necessarily drawn to scale.

[0034] Figure 1 Schematic diagram of the optical path for preparing a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to the present invention;

[0035] Among them, HWP-half wave plate, PBS-polarization beam splitter, M1-first reflector, CL1-cylindrical concave lens, CL2-cylindrical convex lens, Slit-adjustable width slit, M2-second reflector, LED-illumination light source, OL-objective lens, M3-third reflector, L1-convex lens, CCD-camera, PC-computer;

[0036] Figure 2 Schematic diagram of the structure of a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to the present invention;

[0037] Among them, (a) is a structural diagram of a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential; (b) is a cross-sectional diagram of a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential;

[0038] Figure 3 It is the mode field photo and cross-sectional microscope photo of the single-mode combined waveguide;

[0039] The center spacing a of the single-mode combined waveguides increases from 0 μm to 2.5 μm with a step size of 0.5 μm, and the maximum center spacing b of the single-mode combined waveguides is 2.5 μm;

[0040] Figure 4 This is a structural diagram and test results diagram of a directional coupler based on a combined waveguide;

[0041] Among them, (a) is a structural schematic diagram of a directional coupler based on a combined waveguide. It can be seen that both arms of the directional coupler are combined waveguides, but the center spacing a between the two circular waveguides of the combined waveguides in the two arms is different, namely a1 and a2, so the propagation constants β of the two arms are different; (b) is the oscillation curve of the directional coupler; (c) is the relationship curve between the center spacing a of the combined waveguide and the propagation constant β obtained according to the oscillation curve of the directional coupler. Assuming that the propagation constant of the combined waveguide with a center spacing of 0 is 0, it can be seen that the propagation constant of the combined waveguide is linearly related to the center spacing a, and the slope k can be obtained by fitting.

[0042] Figure 5 The following are the mode field photos, simulated light field evolution diagrams, and the output port oscillation recovery energy ratio of the broadband self-imaging device based on the combined waveguide-induced sawtooth gradient scalar potential when light is passed through different input ports of the present invention, as well as the output port oscillation recovery energy ratio when light of different wavelengths is passed through a single port;

[0043] Among them, (a) is the propagation constant β of the 16 combined waveguides in the waveguide array relative to the combined waveguide with a center spacing of 0. It can be seen that the propagation constants of the 16 combined waveguides are periodically distributed; Figure (b) is the MATLAB simulated light field evolution diagram and experimental test mode field photo when the first combined waveguide in the waveguide array (the first combined waveguide of the first imaging unit) is passed through light; Figure (c) is the MATLAB simulated light field evolution diagram and experimental test mode field photo when the fifth combined waveguide in the waveguide array (the first combined waveguide of the second imaging unit) is passed through light; Figure (d) is the MATLAB simulated light field evolution diagram and experimental test mode field photo when the 11th combined waveguide in the waveguide array (the third combined waveguide of the third imaging unit) is passed through light; Figure (e) is the proportion of oscillation recovery energy at the output port for light of different wavelengths (633nm, 808nm, 852nm, 980nm and 1550nm) passed through a single port.

[0044] Figure 6 The simulated light field evolution diagram and experimental test mode field photograph of the broadband self-imaging device based on the combined waveguide induced sawtooth gradient scalar potential of the present invention when transmitting a light intensity coded signal;

[0045] Among them, (a) is a schematic diagram of the designed cascade waveguide input branch used to achieve signal encoding. It can be seen that light is passed through one input waveguide, and a ten-bit binary light intensity coding signal sequence 1101100011 is output at the output end of the cascade waveguide array; (b) is a MATLAB simulated light field evolution diagram and experimental test mode field photograph when light is passed through the coding signal input waveguide array; (c) is a MATLAB simulated light field evolution diagram and experimental test mode field photograph when the coding signal at the output end of the coding signal waveguide array in (b) is passed into a broadband self-imaging device based on the combined waveguide induced sawtooth gradient scalar potential. DETAILED DESCRIPTION

[0046] In order to clearly and completely describe the technical solution and specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings:

[0047] Example 1

[0048] like Figure 2 As shown in (a), this embodiment provides a broadband self-imaging device based on a sawtooth gradient scalar potential induced by a combination waveguide. The device includes four imaging units, namely imaging unit A, imaging unit B, imaging unit C and imaging unit D. Each imaging unit includes four combination waveguides, namely combination waveguide 1, combination waveguide 2, combination waveguide 3 and combination waveguide 4. The entire device has a total of 16 combination waveguides. The center spacing a of the combination waveguides of each imaging unit increases from the first to the fourth combination waveguide in the same step size, and the center spacing a of the fourth combination waveguide is less than the maximum center spacing b of the single-mode combination waveguide. Figure 2 (b) is a schematic diagram of the end face of the device. It can be seen that in the entire device, the coupling spacing between adjacent combined waveguides is d.

[0049] Example 2

[0050] Leveraging the advantages of femtosecond laser "true 3D" processing, two circular waveguides can be spliced ​​together with a certain center-to-center spacing a to form a combined waveguide. By varying the center-to-center spacing a, combined waveguides with different propagation constants can be produced. However, when the center-to-center spacing a is too large, the combined waveguide supports not only single-mode transmission but also multimode transmission. Therefore, it is necessary to determine the maximum value b of the center-to-center spacing a when the combined waveguide only transmits in single mode.

[0051] This embodiment provides a method for processing composite waveguides with different center spacings a using femtosecond laser direct writing technology. The specific steps are as follows:

[0052] (1) Leveling of the sample stage: Clean the surface of the sample to be processed (Corning glass); the wavelength of the femtosecond laser used is 1030nm, the pulse width is 239fs, and the repetition rate is 1MHz; the working distance of the objective lens OL used is 0.51mm, NA=0.75, and the magnification is ×40. First, as Figure 1 As shown, the femtosecond laser emitted by the laser passes through a half-wave plate HWP and a polarization beam splitter PBS with a high damage threshold; and is reflected by the first reflector M1 to a cylindrical lens system composed of a cylindrical concave lens CL1 and a cylindrical convex lens CL2, and then passes through a slit with a width of 0.85 mm. At this time, the light beam is shaped into a slender strip-shaped light spot by the cylindrical lens-slit; then, the laser is incident on the objective lens OL with the help of the second reflector M2, and 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 reflector M2 and before the entrance pupil of the objective lens OL is 50 mW; then, the objective lens OL focuses the laser and makes it incident on the surface of the glass sample on the sample stage; then, with the help of the illumination light emitted by the illumination light source LED fixed on the lens frame of the reflector M2 and the camera CCD to the right of the third reflector M3, the leveling process of the sample stage can be monitored in real time on the computer PC.

[0053] Using the two perpendicular sides of the sample as the X and Y axes, the motion platform was adjusted by a PC to move between (0, 0) and (2.5 cm, 0). Simultaneously, the X-direction leveling knob was adjusted until the laser could scan a uniform damage line 2.5 cm in length and 4 μm in width. At this point, X-direction leveling was complete. Similarly, a laser was used to draw a line between (0, 0) and (0, 2.5 cm) and the Y-direction leveling knob was used to level the sample in the Y direction. At this point, the laser could directly write a uniform damage line in both the X and Y directions, indicating that the sample stage was perpendicular to the focused laser beam. The sample stage was then leveled and fixed.

[0054] (2) Direct writing of the combined waveguide: First, focus the laser on 190 μm inside the sample; then, rotate the optical axis of the half-wave plate HWP so that the laser power before the entrance pupil of the objective lens OL is 490 mW; then, load the pre-written processing program of the combined waveguide with different center spacing on the computer PC, and process the two circular cross-section waveguides of the combined waveguide respectively; wherein, the processing speed of the two circular cross-section waveguides of the combined waveguide is the same, both of which are 40 mm / s, and the processing length of the combined waveguide is 25 mm, and the center spacing a of the two circular cross-section waveguides is adjusted to 0 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, and 3 μm for processing.

[0055] (3) Test of single and multi-mode combination waveguides: First, the input end face and output end face of the prepared combination waveguide are mechanically polished; then, the cross-sectional images of the combination waveguides with different center spacings a are observed and photographed using an optical microscope; then, using a precision displacement platform, 808nm vertical polarized light is coupled into the combination waveguide through an objective lens, and the output beam is collected at the output end of the combination waveguide using an objective lens. The modes of the output beams from the three different directions of lateral offset docking are detected using a mode analyzer. The test results are as follows: Figure 3 In the mode field photograph shown, the center spacing of the single-mode combined waveguide is 0μm-2.5μm, that is, the maximum center spacing b is 2.5μm. Below the mode field photograph is the corresponding combined waveguide cross-section photograph. It can be seen that as the center spacing a increases, the cross-section of the combined waveguide gradually becomes longer.

[0056] Example 3

[0057] A directional coupler based on a combined waveguide is fabricated using femtosecond laser direct writing technology. That is, both arms of the directional coupler are combined waveguides, and the center spacing a of the circular cross-section waveguides in the two arms is different, namely a1 and a2. By testing the transmittance of directional couplers with different coupling lengths l and fitting the oscillation curves, the propagation constant difference Δβ and the coupling coefficient κ of the combined waveguides in the two arms of the directional coupler can be obtained, and then the relationship curve between the propagation constant β of the combined waveguide and the center spacing a can be obtained.

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

[0059] (2) Direct writing of directional coupler: First, focus the laser on 190 μm inside the sample; then, rotate the optical axis of the half wave plate HWP so that the laser power before the entrance pupil of the objective lens OL is 490 mW; then, load the pre-written processing program of the directional coupler based on the combined waveguide on the computer PC, and process the combined waveguides of the two arms of the directional coupler respectively; wherein, according to the center spacing range of the single-mode combined waveguide explored in Example 2, the center spacing of the combined waveguides of the two arms of the direct writing directional coupler is selected as follows: 0 μm-0.5 μm, 0.5 μm-1.0 μm, 1.0 μm-1.5 μm, 1.5 μm-2.0 μm and 2.0 μm-2.5 μm, a total of five groups, the coupling spacing d between the two-arm combined waveguides is 12 μm, the coupling length l is 0.5-6.5 mm, the coupling area and the input / output waveguide are connected by an S-bend waveguide with a radius R=60 mm, and the direct writing speed is 40 mm / s.

[0060] (3) Determine the coupling coefficient of the combined waveguide with a coupling spacing of d = 12 μm and the relationship curve between the combined waveguide propagation constant β and the center spacing a: First, the input end face and the output end face of the processed sample are mechanically polished; then, using a precise displacement platform, the 808 nm vertical polarized light is coupled into the input arm of the directional coupler through the objective lens, and another objective lens is placed on the output side of the directional coupler. The focal length of the objective lens is adjusted so that the outgoing light beam can be focused; at the same time, two power meter probes are placed behind the objective lens. By adjusting the height and left and right directions of the objective lens, the two output light spots of the directional coupler are hit at the center of the probe, thereby realizing the transmittance measurement of the directional coupler; then, the measured transmittance curve is fitted using the origin software to obtain the propagation constant difference Δβ and coupling coefficient κ of the combined waveguide under different center spacings; finally, the combined waveguide propagation constant β is fitted and analyzed with the center spacing a, and the results are as follows: Figure 4 The fitting result shows that the coupling coefficient κ corresponding to the coupling spacing d = 12 μm is 0.148 mm. -1 ; From (b) of 4, it can be seen that the transmittance curves of the directional coupler composed of the combined waveguides with different center spacings a do not oscillate to 1, indicating that the propagation constant of the combined waveguide can be changed by changing the center spacing a of the combined waveguide; Figure 4 (c) shows that the propagation constant β of the combined waveguide is linearly related to the center spacing a of the combined waveguide. The propagation constant β increases almost linearly with the center spacing a. The slope k of the straight line in the figure is 0.985 mm. -1 / μm.

[0061] Example 4

[0062] A broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential was fabricated using femtosecond laser direct writing technology and its self-imaging effect was tested. The specific steps are as follows:

[0063] (1) Leveling of the sample stage: same as in Example 2.

[0064] (2) Preparation of a waveguide array self-imaging device: First, focus the laser at 190 μm inside the sample; then, rotate the optical axis of the half-wave plate HWP so that the laser power before the entrance pupil of the objective lens OL is 490 mW; then, load the pre-written processing program of the waveguide array self-imaging device on the computer PC, and process the combined waveguides in the waveguide array in sequence; using the device structure designed in Example 1, the waveguide array is divided into 4 imaging units, each imaging unit has 4 combined waveguides, and then according to the combined waveguide propagation constant β obtained in Example 2 and the center spacing a of the combined waveguides present a linear relationship, the center spacings (a1, a2, a3, a4) of the combined waveguides (1, 2, 3, 4) in each imaging unit are selected to be (0 μm, 0.8 μm, 1.6 μm, 2.4 μm), respectively, and the propagation constant gradient difference ▽β in each imaging unit is = k*a4 / (4-1) = 0.788 mm -1 , then the length of the combined waveguide is L Bloch =2π / ▽β=7.97mm; the direct writing speed of the combined waveguide is 40mm / s, and the coupling spacing d of the combined waveguide in the waveguide array is 12um;

[0065] (3) Test of the self-imaging effect of the waveguide array: After the processing is completed, the input end face and the output end face of the sample are mechanically polished; then the objective lens-chip-objective lens test system in Example 2 is used to test the splitting ratio, and vertically polarized light of different wavelengths (633nm, 808nm, 852nm, 980nm and 1550nm) is coupled into the first combined waveguide (the first combined waveguide of the first imaging unit), the fifth combined waveguide (the first combined waveguide of the second imaging unit) and the eleventh combined waveguide (the third combined waveguide of the third imaging unit) of the waveguide array through the objective lens, and the mode field energy of the output light is observed using a mode analyzer. The results are as follows: Figure 5 shown. Figure 5 (a) shows the distribution of the propagation constant β in the waveguide array. It can be seen that the propagation constant gradient ▽β reaches 0.788mm -1, and is distributed periodically. It can be seen from Figures 5(b), (c) and (d) that when 808nm light is introduced from the 1st combined waveguide (the first combined waveguide of the first imaging unit), the 5th combined waveguide (the first combined waveguide of the second imaging unit) and the 11th combined waveguide (the third combined waveguide of the third imaging unit) of the waveguide array respectively, the experimental test results are consistent with the MATLAB simulation light field evolution results. The light field distribution at the input end is reproduced at L=7.97mm, and the self-imaging effect is almost perfectly achieved. The oscillation recovery energy at the output end reaches 99.64%, 99.25% and 99.66% respectively; it can be seen from Figure 5(e) that in the wavelength range of 633-852nm, whether the light is introduced from the 1st, 5th or 11th combined waveguide, the proportion of the oscillation recovery energy at the output port is as high as more than 94%, indicating that the broadband self-imaging device based on the sawtooth gradient scalar potential induced by the combined waveguide of the present invention has extremely strong robustness to the communication wavelength.

[0066] Example 5

[0067] This embodiment provides an application of transmitting light intensity coded signals on an integrated photonic chip using a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential;

[0068] The specific steps are as follows:

[0069] (1) Leveling of the sample stage: same as in Example 2.

[0070] (2) Direct writing of the cascade waveguide input branch for light intensity signal coding: First, focus the laser at 190 μm inside the sample; then, rotate the optical axis of the half-wave plate HWP so that the laser power before the entrance pupil of the objective lens OL is 490 mW; then, load the pre-written coding signal transmission waveguide array processing program on the PC to process the coding signal transmission waveguide array. Among them, the coding signal is Figure 6 The cascade waveguide input branch shown in (a) is generated, light passes through the input port, and through the design of the coupling length, ten-bit light intensity encoding information 1101100011 is output at the output end; Figure 6 (b) shows the Matlab simulation light field evolution results of the cascade waveguide input branch and the experimentally measured light field distribution results at the output end, with the normalized output light intensity of each waveguide (0.954, 0.960, 0.0480, 0.876, 0.936, 0.000, 0.000, 0.00, 1.188, 1.038); in the program writing, the output end of the cascade waveguide input branch that generates the intensity-coded signal is connected between the 4th and 13th combination waveguides at the input end of the broadband self-imaging device.

[0071] (3) Test of the transmission of light intensity coding signal by broadband self-imaging device: After processing, the input end face and output end face of the sample are mechanically polished; then the transmission effect of the coding signal is tested using the objective lens-chip-objective lens test system in Example 2, and 808nm vertical polarized light is coupled into the input waveguide of the waveguide array through the objective lens, and the mode field energy of the output light is observed using a mode analyzer. The experimental test results are consistent with the MATLAB simulation light field evolution results as shown in Figure 2. Figure 6 As shown in (c), the normalized output light intensity of each waveguide (0.932, 1.113, 0.071, 1.092, 0.752, 0.013, 0.013, 0.015, 1.108, 0.890) is well maintained with the original input light intensity distribution.

[0072] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0073] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0074] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential, characterized in that: The invention comprises n periodically arranged imaging units, where n≥1, and each imaging unit comprises m combination waveguides, where m≥2, and the coupling spacing between adjacent combination waveguides is d; each combination waveguide is formed by splicing two mutually parallel circular cross-section waveguides with a center spacing a, and satisfies the single-mode transmission condition a≤b, where b is the maximum center spacing between the two circular cross-section waveguides in single-mode transmission; within a single imaging unit, the center spacing a of the combination waveguides increases from the first to the mth waveguide in a fixed step size, so that the propagation constant β of the combination waveguide is gradient distributed, thereby forming a propagation constant gradient within each imaging unit. When the optical signal is transmitted in the imaging unit, it triggers Bloch oscillation, and its oscillation period is Optical signal is transmitted to L Bloch When , the wave packet shrinks to the same light field distribution state as the initial input light signal, thus achieving self-imaging.

2. A broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to claim 1, characterized in that: The maximum difference in combined waveguide cross-section at the boundaries of adjacent imaging units is 2.5 μm, and the maximum difference in propagation constant is 2.45 mm. -1 , to suppress cross-unit optical signal transmission.

3. The broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to claim 1, characterized in that: The coupling spacing d is at least 9 μm, and the device can be infinitely periodically extended in space.

4. The broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to claim 1, wherein: The propagation constant β of the combined waveguide is linearly related to the center spacing a, and the slope k is determined by experimental fitting of a directional coupler; The propagation constant gradient Among them, a m is the center distance of the mth combined waveguide in a single imaging unit.

5. The method for preparing a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to claim 1, wherein: The specific steps include: Step 1: Level the sample stage; Use femtosecond laser to scratch the damage line on the sample surface, and adjust the platform inclination angle to make the damage line width uniform; Step 2: Determine the maximum center-to-center distance b between the two circular cross-section waveguides when the combined waveguide is transmitting in single mode; Machining a composite waveguide with increasing center spacing a; after polishing the composite waveguide, testing it with a mode analyzer to determine the maximum center spacing b during single-mode transmission; Step 3: Determine the propagation constant gradient parameter; Multiple sets of directional couplers with the same coupling spacing d but different coupling lengths l are fabricated, wherein the center spacing between the two circular cross-section waveguides in the combined waveguides serving as the two arms of the coupler in each set of directional couplers is different. After fabrication, the samples are polished, and the transmittance of the directional couplers with different coupling lengths is measured to obtain a transmittance-coupling length curve. The coupling coefficient κ and the propagation constant difference Δβ between the two arms of the combined waveguide are fitted according to the following coupled-mode formula. A linear fit is then performed based on the propagation constant difference Δβ between the two arms of the combined waveguide to obtain a β-a linear relationship and a slope k. Where κ is the coupling coefficient, Δβ is the propagation constant difference, and l is the coupling length. is the initial phase; Step 4: Preparation of waveguide array self-imaging device; In the femtosecond laser direct writing optical waveguide processing system, the processing direction is the X axis, the waveguide processing sequence direction is the Y axis, and the sample depth direction is the Z axis. First, the processing position of the first combined waveguide of the first imaging unit is taken as the coordinate origin (0, 0), and the coordinates of the mth combined waveguide in the first imaging unit are (0, (m-1) d), thereby obtaining the first imaging unit; wherein, the center distances between the two circular waveguides of the combined waveguide in each imaging unit are a1, a2, a3, a4...a respectively. m Then, rotate the optical axis of the half wave plate (HWP) in front of the laser, adjust the laser power before the objective lens entrance pupil, and start processing. The processing length of all combined waveguides is L Bloch , Thus, a broadband self-imaging device consisting of n imaging units is prepared, where the propagation constant gradient is 6. The method for preparing a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to claim 5, wherein: Step 1 specifically includes the following: First, use acetone and ethanol to clean the surface of the sample to be processed and wipe off grease and dust; secondly, the femtosecond laser emitted by the laser passes through the half-wave plate HWP and the polarization beam splitter PBS, and is reflected by the first reflector M1. Then, the incident light beam is shaped into an elliptical Gaussian beam by the cylindrical concave lens CL1 and the cylindrical convex lens CL2. The shaped light beam is then cut into a slender strip-shaped light spot by the slit Slit with adjustable width, enters the objective lens OL through the second reflector M2 and is focused and incident on the surface of the sample to be processed on the sample stage; an illumination light source LED is fixed on the frame of the second reflector 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 reflector M2, and is reflected by the third reflector M3 and focused into the camera CCD; the camera CCD is connected to the computer PC, so that the leveling process of the sample stage can be monitored in real time on the computer side; Finally, using the two perpendicular sides of the sample as the X and Y axes, the motion platform was adjusted by the PC to move between (0, 0) and (2.5 cm, 0). At the same time, the X-direction leveling knob was adjusted until the laser could scan a uniform damage line with a length of 2.5 cm and a width of 4 μm. At this point, the X-direction leveling was completed. Similarly, the laser was used to draw a line between (0, 0) and (0, 2.5 cm) and the Y-direction leveling knob was used to level the Y direction. At this point, the laser could directly write a uniform width damage line in both the X and Y directions, indicating that the sample stage was perpendicular to the focused laser beam. The sample stage leveling was complete and the position of the sample stage was fixed.

7. The method for preparing a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to claim 5, wherein: Step 2 specifically includes the following: First, after the sample stage is leveled, the optical axis of the half-wave plate (HWP) in front of the laser is rotated to adjust the laser power in front of the objective lens entrance pupil. Next, the MATLAB processing program is loaded and run in the three-dimensional displacement platform control software. By controlling the processing depth, the power of the processing laser, and the speed of laser direct writing and changing the center spacing a of the combined waveguides, combined waveguides with different center spacings a can be processed. Finally, after processing, the sample is polished, and the combined waveguides with different center spacings a are tested using a mode analyzer to determine the maximum center spacing b of the combined waveguides when the combined waveguides are transmitting in single mode.

8. The method for preparing a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential according to claim 5, wherein: In step 3, the directional couplers are divided into Q groups, and the difference in the center spacing between the two circular cross-section waveguides in the two-arm combined waveguides of each group of directional couplers is Δa; and the center spacing continuity is satisfied between two adjacent groups: the center spacing of the first arms of the latter group is equal to the center spacing of the second arms of the previous group, the center spacing of the first arms of the first group is 0, and the center spacing of the second arms of the last group is less than the maximum spacing b; the coupling length l of each group of directional couplers ranges from 0.5 to 6.5 mm, and the coupling spacing d is the same.

9. Application of a broadband self-imaging device based on a combined waveguide-induced sawtooth gradient scalar potential as claimed in claim 1 to transmit light intensity coded signals on an integrated photonic chip.

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