Coupling coefficient measurement method based on identical waveguide array

Abstract

The invention discloses a coupling coefficient measurement method based on an identical waveguide array, and belongs to the field of optical waveguide property measurement and test, and the method comprises the steps: building a waveguide array coupling coefficient measurement system; testing an evolution result of the waveguide array; reversely deducing a coupling coefficient through a simulation experiment; according to the invention, based on the consistency of the evolution process of photons in a waveguide array and quantum walking in continuous time, an identical single-mode waveguide array is adopted, according to a physical model of quantum walking in continuous time, output probability amplitudes under different evolution lengths are measured, and a least square method is combined to evaluate simulation and test results; and finally obtaining the coupling coefficient of the waveguide. The method is low in requirement on experimental equipment, small in workload and less in required sample width; and the evolution condition of the light in the waveguide array can be accurately predicted and analyzed.

Description

Technical Field

[0001] The invention belongs to the field of measuring and testing optical waveguide properties, and in particular relates to a coupling coefficient measuring method based on an identical waveguide array. Technical Background

[0002] As the most basic and important component in integrated photonic chips, optical waveguides can guide light beams to transmit efficiently in a certain direction. They are not only the basic units of on-chip integrated components, but also the link between integrated components. The coupling coefficient can describe the interaction strength of the light field between adjacent waveguides in the waveguide array. By determining the coupling coefficient, the propagation path, mode distribution and light intensity changes of light in the waveguide array can be accurately predicted and analyzed, which is crucial for the design and optimization of photonic devices based on waveguide arrays. At present, various research groups mainly use far-field imaging technology, Mach-Zehnder interferometer, transmission spectrum measurement and other methods to determine the coupling coefficient of the waveguide array, but the above methods have problems such as poor measurement accuracy, high equipment cost, easy interference from external factors resulting in large errors, and low efficiency. Summary of the invention

[0003] In view of the shortcomings of the prior art, the present invention provides a new waveguide coupling coefficient measurement method based on an identical waveguide array, which is suitable for measuring the coupling coefficient of waveguides and waveguide arrays. Based on the consistency between the evolution process of photons in the waveguide array and the continuous-time quantum walk, the present invention adopts an identical single-mode waveguide array, and according to the physical model of the continuous-time quantum walk, the Hamiltonian H of the system is:

[0004]

[0005] Where κ is the coupling coefficient between adjacent waveguides, a n represents the annihilation operator of the photon in the nth waveguide, represents the photon generation operator in the nth waveguide, and β0 is the propagation constant of a single waveguide. Based on the Heisenberg equation of motion We can get:

[0006]

[0007] in, is the column vector composed of all waveguide generation operators, z is the propagation distance of photons in the waveguide, is the form of the system's Hamiltonian in matrix representation. The matrix form is:

[0008]

[0009] The solution of the above formula is:

[0010]

[0011] in, is the initial probability amplitude distribution. Due to the intrinsic propagation term It only contributes a same phase factor and does not affect the propagation result. The Hamiltonian is only related to the coupling coefficient κ. The simulation and test results are evaluated by combining the least square method, and finally the coupling coefficient κ of the waveguide is obtained.

[0012] The present invention is achieved through the following technical solutions:

[0013] A coupling coefficient measurement method based on an identical waveguide array, the specific steps are as follows:

[0014] (1) Build a measurement system for the waveguide array coupling coefficient;

[0015] The specific steps are as follows: the measurement system includes a light source, a polarization beam splitter PBS, a camera CCD, a half wave plate HWP, mirrors M1 and M2, objective lenses OL1 and OL2, a waveguide array to be measured, a translation stage, an illumination light source LED, a beam profiler and a computer PC; the light emitted from the light source can be adjusted to any linear polarized light after passing through PBS and HWP, and then enters the objective lens OL1 after being collimated by M1 and M2; the waveguide array to be measured is placed on the translation stage; the height and rotation angle of the beam profiler are adjusted so that its center position is the same as the sample height and perpendicular to the direction of the sample output end; the CCD is used to observe the alignment of the OL1 output light and the waveguide array;

[0016] (2) Test the waveguide array evolution results;

[0017] The specific steps are as follows: first, turn on the light source and LED, and adjust the translation stage with the assistance of CCD to couple the light into the input end of the waveguide array; second, adjust the position of the beam profiler so that the output light spot is located at the center of the beam profiler; then, adjust the output power of the light source to maximize the light intensity received by the beam profiler and save the data; next, change the waveguide array evolution length z, repeat the above measurement steps, and measure the light intensity under different evolution lengths 2;

[0018] (3) Inverse calculation of coupling coefficients through simulation experiments;

[0019] The specific steps are as follows: First, extract the centroid position of each spot in the array evolution result in step (2), and use Gaussian function to fit it to obtain the position, shape and intensity of each spot; then, combine the Hamiltonian and the evolution formula to simulate the evolution results of the waveguide array with different coupling coefficients κ in MATLAB, and use the least squares method and the spot peak intensity information to evaluate, and use the determination coefficient R in statistics to obtain the position, shape and intensity of each spot. 2 As an evaluation indicator, R 2The corresponding κ at the maximum is the coupling coefficient of the waveguide array.

[0020] Furthermore, in step (1), the waveguide array to be tested is a 1×N type fully identical dimensional waveguide array with output ends on the same side, the number of array input ports is 1, and the number of output ports is N, such as 1×2, 1×5 or 1×7, etc. The sample width required for a single waveguide array is (N-1) times the coupling spacing.

[0021] Furthermore, the waveguide array evolution length in step (2) is 0.1-25 mm, and the number of arrays is 2-8.

[0022] Furthermore, the changing of the waveguide array evolution length z in step (2) is achieved by processing multiple arrays with different evolution lengths z, or by using a diamond wire cutting machine to perform multiple cuts from the array output end to obtain arrays with different evolution lengths z, and the subsequent array width is calculated according to the former.

[0023] Furthermore, the position, shape and intensity of the extracted light spot in step (3) specifically include the following contents:

[0024] First, read the test results to remove the influence of background noise on the spot energy, then sum the columns, compress them into one-dimensional data and reduce the influence of stray light, use the findpeaks function in MATLAB to extract the peak value and peak width of the spot, and classify them according to the peak value. If the peak value is lower than 1500, one-dimensional Gaussian fitting is used, otherwise two-dimensional Gaussian fitting is used. The form of the one-dimensional Gaussian function is:

[0025]

[0026] Among them, the fitting parameter A is the peak intensity of the spot, x0 is the center of the spot, σ x is the spot width;

[0027] The two-dimensional Gaussian function has the form:

[0028]

[0029] Among them, the fitting parameter A is the peak intensity of the spot, (x0, y0) is the center of the spot, (σ x , σ y ) is the spot width, thus obtaining the position, shape and intensity of each spot.

[0030] Furthermore, in step (3), the evolution of the waveguide array is simulated, and the calculation interval of the coupling coefficient κ in the Hamiltonian is [0.2, 0.8] mm -1 , the calculation step is limited to 0.0001, the evolution length is 5-25mm, R 2 The calculation formula is:

[0031]

[0032] Among them, SSE is the sum of squares of the errors between the simulation data and the Gaussian fitting data, and SST is the sum of squares of the differences between the Gaussian fitting data and its mean.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] (1) Compared with the traditional waveguide array coupling coefficient measurement method, this method has low requirements for experimental equipment, small workload, and requires less sample width;

[0035] (2) This method can accurately predict and analyze the evolution of light in a waveguide array. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0037] Figure 1 A flow chart of a coupling coefficient measurement method based on an identical waveguide array provided by the present invention;

[0038] Figure 2 It is a schematic diagram of the structure of the measurement system of the present invention;

[0039] Among them, PBS represents polarization beam splitter, CCD represents camera, HWP represents half wave plate, M1 and M2 represent reflectors, OL1 and OL2 represent objective lenses, LED represents illumination source, and PC represents computer;

[0040] Figure 3 Schematic diagram of a 1×7 waveguide array in Example 1 of the present invention;

[0041] Among them, the number of waveguides in the array is 7, the coupling spacing d is 11μm, and the coupling length L is 5.2-7mm;

[0042] Figure 4 It is a schematic diagram of the test results of the 1×7 waveguide array evolution of the present invention;

[0043] Among them, (a), (b), (c), and (d) are the mode field photos when the coupling length is 5.2, 5.8, 6.4, and 7 mm, respectively;

[0044] Figure 5 It is a schematic diagram of the simulation evolution result of the 1×7 waveguide array of the present invention;

[0045] Figure 6 The comparison diagram of Gaussian fitting results and simulation evolution results of the 1×7 waveguide array of the present invention is shown in FIG. Figure 4 ;

[0046] Figure 7 This is a diagram for evaluating the coupling coefficient of a 1×7 waveguide array of the present invention;

[0047] Figure 8 It is a schematic diagram of the test results of the 1×5 waveguide array evolution of the present invention;

[0048] Among them, (a) and (b) are the mode field photos when the coupling length is 1 and 1.5 mm respectively;

[0049] Fig. 9 It is a schematic diagram of the simulation evolution result of the 1×5 waveguide array of the present invention;

[0050] Fig.10 The comparison diagram of Gaussian fitting results and simulation evolution results of the 1×5 waveguide array of the present invention is shown in FIG. Figure 8 ;

[0051] Fig.11 This is a diagram for evaluating the coupling coefficient of the 1×5 waveguide array of the present invention. DETAILED DESCRIPTION

[0052] In order to clearly and completely describe the technical solution and its specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings of the specification:

[0053] Example 1

[0054] This embodiment is described by taking a 1×7 type identical waveguide array with a coupling spacing of 11 μm as an example.

[0055] This embodiment provides a coupling coefficient measurement method based on an identical waveguide array, and the flow chart is as follows: Figure 1 The measurement device used is as shown in Figure 2 As shown, combined Figure 1 and Figure 2 The method of this embodiment is described, and the specific steps are as follows:

[0056] (1) Measurement system construction;

[0057] like Figure 2The figure shows the measurement system of the waveguide array coupling coefficient. The measurement system includes a light source, a polarization beam splitter PBS, a camera CCD, a half wave plate HWP, mirrors M1 and M2, objective lenses OL1 and OL2, a waveguide array to be measured, a translation stage, an illumination light source LED, a beam profiler and a computer PC. The light emitted by the light source is adjusted to vertical V polarized light after passing through PBS and HWP, and is focused by objective lens OL1 and enters the incident end of the waveguide array after being collimated by M1 and M2. The emitted light is focused by OL2 and received by the beam profiler;

[0058] (2) Test the waveguide array evolution results;

[0059] First, place the waveguide array to be tested on the translation stage, turn on the light source and LED, and adjust the translation stage with the assistance of CCD to allow light coupling to enter the input end of the waveguide array. At this time, place an infrared fluorescent card after OL2, and an obvious light spot can be observed on it, indicating that the rough alignment of the light source and the waveguide has been completed; secondly, fine-tune the position of the translation stage to make the light spot as bright as possible under naked eye observation; next, adjust the position of the beam profiler so that the light spot is located at the center of the beam profiler; then, adjust the output power of the light source to maximize the light intensity received by the beam profiler and save the data; then, repeat the above steps several times, and the final waveguide array evolution test results are as follows: Figure 4 As shown;

[0060] (3) Inverse calculation of coupling coefficients through simulation experiments;

[0061] First, the waveguide array test results in step (2) were imported into MATLAB. The HDF5 specification was used to read the raw data with a threshold lower limit intensity of 1% to remove the interference of background noise on the spot energy test, and the column sum was performed to reduce the influence of stray light while compressing it into one-dimensional data. Then, the findpeaks function was used to extract the peak value and peak width of the spot, and the spot was classified according to the peak value. If the peak value was lower than 1500, a one-dimensional Gaussian fitting was used, otherwise a two-dimensional Gaussian fitting was used. Then, combined with the physical model of Hamiltonian evolution, the coupling coefficient κ calculation interval was set to [0.2, 0.4] mm. -1 , limiting the calculation step to 0.0001 and the evolution length to 8.5 mm, simulating the evolution of the waveguide array, the results are as follows Figure 5 As shown in Figure 2, as the evolution length increases, the incident light energy is gradually coupled from the input end into the remaining waveguides of the array. Next, the Gaussian fitting results are evaluated with the simulation results using the least squares method. 2 The corresponding κ when it is the maximum is the coupling coefficient of the waveguide array, and the result is as follows Figure 6-7 As shown in the figure, the spot position and peak intensity of the Gaussian fitting results are basically consistent with the simulation results. In the 1×7 waveguide array with a coupling spacing of 11μm, the coupling coefficient of the waveguide array is 0.2928mm -1.

[0062] In this embodiment, the laser light source used is 808 nm, and light sources of other wavelength bands may also be used.

[0063] In this embodiment, the polarization state of the light source used is vertical V polarized light, and other polarized light may also be used.

[0064] In this embodiment, the process for preparing the 1×7 type identical waveguide array is femtosecond laser direct writing technology, and other processes may also be used.

[0065] Example 2

[0066] This embodiment is described by taking a 1×5 type identical waveguide array with a coupling spacing of 9 μm as an example, and the specific steps are as follows:

[0067] (1) Measurement system construction: same as in Example 1;

[0068] (2) Test waveguide array evolution results: same as in Example 1;

[0069] (3) Simulation experiment reverse calculation of coupling coefficient: Same as in Example 1, where the calculation interval of the coupling coefficient κ is set to [0.5, 0.7] mm -1 , limit the calculation step size to 0.0001, the evolution length to 5mm, and simulate the evolution results of the waveguide array. The results are as follows Fig. 9 Then, the Gaussian fitting results were evaluated with the simulation results using the least squares method, R 2 The corresponding κ when it is the maximum is the coupling coefficient of the waveguide array, and the result is as follows Figure 10-11 As shown in the figure, the spot position and peak intensity of the Gaussian fitting results are basically consistent with the simulation results. In the 1×5 waveguide array with a coupling spacing of 9μm, the coupling coefficient of the waveguide array is 0.621mm -1 .

[0070] In this embodiment, the sample width used by a single waveguide array is only 36μm, while the method of measuring the DC splitting ratio of directional couplers with different coupling lengths usually requires testing five DCs. Due to the large turning radius in its structure, the sample width consumed by a single DC can reach 127μm. The width spacing between different devices to achieve no crosstalk is the same. In this example, the device spacing of 60μm can ensure that the device has no crosstalk. The sample width used in the embodiment of the present invention is 132μm, while the sample width consumed by the latter is 875μm. This method can effectively reduce the workload and occupy a significantly smaller area, which is conducive to improving the integration of photonic chips.

[0071] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0072] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0073] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A coupling coefficient measurement method based on an identical waveguide array, characterized in that: The specific steps are as follows: (1) Build a measurement system for the waveguide array coupling coefficient; The specific steps are as follows: the measurement system includes a light source, a polarization beam splitter PBS, a camera CCD, a half wave plate HWP, mirrors M1 and M2, objective lenses OL1 and OL2, a waveguide array to be measured, a translation stage, an illumination light source LED, a beam profiler and a computer PC; the light emitted from the light source can be adjusted to any linear polarized light after passing through PBS and HWP, and then enters the objective lens OL1 after being collimated by M1 and M2; the waveguide array to be measured is placed on the translation stage; the height and rotation angle of the beam profiler are adjusted so that its center position is the same as the sample height and perpendicular to the direction of the sample output end; the CCD is used to observe the alignment of the OL1 output light and the waveguide array; (2) Test the waveguide array evolution results; The specific steps are as follows: first, turn on the light source and LED, and adjust the translation stage with the assistance of CCD to couple the light into the input end of the waveguide array; second, adjust the position of the beam profiler so that the output light spot is located at the center of the beam profiler; then, adjust the output power of the light source to maximize the light intensity received by the beam profiler and save the data; next, change the waveguide array evolution length z, repeat the above measurement steps, and measure the light intensity under different evolution lengths z; (3) Inverse calculation of coupling coefficients through simulation experiments; The specific steps are as follows: first, extract the centroid position of each light spot in the array evolution result in step (2), and use Gaussian function to fit it to obtain the position, shape and intensity of each light spot; then, combine the Hamiltonian and the evolution formula to simulate the evolution results of the waveguide array with different coupling coefficients κ in MATLAB, and use the least squares method and the light spot peak intensity information to evaluate, and use the determination coefficient R in statistics to calculate the shape and intensity of each light spot. 2 As an evaluation indicator, R 2 The corresponding κ at the maximum is the coupling coefficient of the waveguide array.

2. A coupling coefficient measurement method based on an identical waveguide array as claimed in claim 1, characterized in that: In step (1), the waveguide array to be tested is a 1×N type fully identical dimensional waveguide array with output ends on the same side, the number of array input ports is 1, and the number of output ports is N, such as 1×2, 1×5 or 1×7, etc. The sample width required for a single waveguide array is (N-1) times the coupling spacing.

3. A coupling coefficient measurement method based on an identical waveguide array as claimed in claim 1, characterized in that: The waveguide array evolution length in step (2) is 0.1-25 mm, and the number of arrays is 2-8.

4. A coupling coefficient measurement method based on an identical waveguide array as claimed in claim 1, characterized in that: The changing of the waveguide array evolution length z in step (2) is achieved by processing multiple arrays with different evolution lengths z, or by using a diamond wire cutting machine to perform multiple cuts from the array output end to obtain arrays with different evolution lengths z, and the subsequent array width is calculated according to the former.

5. A coupling coefficient measurement method based on an identical waveguide array as claimed in claim 1, characterized in that ,, the position, shape and intensity of the extracted light spot described in step (3) specifically include the following contents: First, read the test results to remove the influence of background noise on the spot energy, then sum the columns, compress them into one-dimensional data and reduce the influence of stray light, use the findpeaks function in MATLAB to extract the peak value and peak width of the spot, and classify them according to the peak value. If the peak value is lower than 1500, one-dimensional Gaussian fitting is used, otherwise two-dimensional Gaussian fitting is used. The form of the one-dimensional Gaussian function is: Among them, the fitting parameter A is the peak intensity of the spot, x0 is the center of the spot, σ x is the spot width; The two-dimensional Gaussian function has the form: Among them, the fitting parameter A is the peak intensity of the spot, (x0, y0) is the center of the spot, (σ x , σ y ) is the spot width, thus obtaining the position, shape and intensity of each spot.

6. A coupling coefficient measurement method based on an identical waveguide array as claimed in claim 1, characterized in that: In step (3), the evolution of the waveguide array is simulated, and the calculation interval of the coupling coefficient κ in the Hamiltonian is [0.2, 0.8] mm -1 , the calculation step is limited to 0.0001, the evolution length is 5-25mm, R 2 The calculation formula is: Among them, SSE is the sum of squares of the errors between the simulation data and the Gaussian fitting data, and SST is the sum of squares of the differences between the Gaussian fitting data and its mean.

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