A method for calibrating the cutting shape of an acousto-optic tunable filter

Abstract

The acousto-optic tunable filter (AOTF) is an electrically controlled tunable spectral spectrometer. The calibration of its acousto-optic crystal cut affects the design of the spectral detection system. The existing calibration method adopts the method of fitting the parameters of the multi-incident wavelength tuning curve. The calibration process changes the wavelength of the incident light, obtains the tuning relationship under the principle of parallel tangents by actual measurement, and then solves the crystal cut parameters by fitting. The process is complicated, the conditions are harsh, and there is an error caused by not considering the refractive index dispersion. Therefore, the present invention proposes a method for calibrating the cut of an acousto-optic tunable filter. By measuring the matching ultrasonic frequency under the conditions of fixed wavelength and different polar angle incidence, the minimum root mean square error between the measured data and the simulation data is used as the evaluation index to solve the crystal cut parameters. Compared with the existing method, the present invention can completely calibrate the three characteristic cut angles, has the characteristics of less required instruments, simple optical path structure, high precision, etc., and makes up for the defect of the existing method that ignores the refractive index dispersion.

Description

Technical Field

[0001] The present invention relates to the technical field of acousto-optic tunable filtering, and in particular to a crystal cutting calibration method for an acousto-optic tunable filter (AOTF). Background Art

[0002] An AOTF is a spectral spectrometer based on the principle of acousto-optic diffraction. It offers advantages such as high spectral resolution, arbitrarily adjustable wavelength and fast tuning speed, and the absence of a moving mechanism. It is widely used in spectral detection, polarization imaging, notch filtering, beam deflection, and spatial filtering. There are two main design approaches for common AOTF devices: collinear and non-collinear. In the collinear design, the interacting light and sound waves propagate in the same direction; in the non-collinear design, the light and sound waves propagate in different directions. Non-collinear AOTF devices offer several advantages that extend their application, such as a larger angular aperture and the ability to select a wider range of materials with high acousto-optic figures of merit. Tellurium dioxide (TeO2) crystals are typically considered excellent acousto-optic crystals, covering a spectral range of 350-4500 nm. TeO2 crystals are birefringent media with optical anisotropy, which requires consideration in theoretical modeling.

[0003] Crystal cut is a key characteristic of AOTF devices, determining their spectral and spatial responses and influencing the optical design of spectral imaging detection systems. Complex fabrication processes can lead to discrepancies between designed and actual crystal cut parameters. Therefore, using only designed crystal cut parameters for system analysis and design can lead to inaccurate results. Therefore, AOTF device crystal cut calibration is a crucial step in spectral detection system design.

[0004] To address this issue, the patent (CN101706362A) proposes a method for calibrating the parameters of an acousto-optic tunable filter using frequency tuning curve fitting. However, this method can only calibrate the ultrasonic cutting angle of the crystal cut, and the test conditions are relatively harsh, with poor operability and robustness. The patent does not address the relevant test accuracy. Summary of the Invention

[0005] The purpose of the present invention is to provide an acousto-optic tunable filter crystal cutting calibration method based on the incident polar angle-ultrasonic frequency matching relationship, which can be used to completely calibrate the actual cutting parameters of AOTF devices.

[0006] In order to solve the above technical problems, the technical solution of the present invention is as follows: a method for calibrating the crystal cutting of an acousto-optic tunable filter based on the incident polar angle-ultrasonic frequency matching relationship is proposed; this method first establishes a precise theoretical model between the crystal cutting parameters of the AOTF device and the external calibration parameters; based on this model, a calibration parameter testing device is designed to obtain the calibration data of the AOTF external incident light polar angle and the matching tuning frequency; finally, based on the minimum root mean square error criterion, the actual crystal cutting parameters can be inverted, thereby realizing the precise calibration of the three characteristic cutting angles corresponding to the crystal cutting of the AOTF device.

[0007] The calibration method proposed in the present invention mainly includes the following steps:

[0008] (1) Establish an accurate theoretical model of AOTF parameters;

[0009] (2) Obtaining calibration parameter groups such as incident polar angle and matching tuning frequency through the test device;

[0010] (3) traverse and input the AOTF crystal cutting parameters to obtain the theoretical calibration parameter group;

[0011] (4) Calculate the root mean square error between the measured data set and the theoretical data set, and make judgments based on the minimum root mean square error criterion;

[0012] (5) Output the crystal cutting parameters of the AOTF device to be tested. This parameter group corresponds to the crystal cutting parameters input in step (3) when the minimum root mean square error is obtained.

[0013] Compared with the existing AOTF crystal cutting calibration method, the advantages of the present invention are: (1) the method can completely calibrate the three characteristic cutting angles of the AOTF device crystal cutting data; (2) for TeO2 crystals, precise expressions are used in the modeling process and can be solved analytically, which has the advantage of high precision; (3) the designed calibration method does not rely on complex test conditions (such as: satisfying the parallel tangent principle), and has the advantages of simple operation and high robustness. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will be further described below with reference to the accompanying drawings:

[0015] Figure 1 This is a flow chart of the calibration method;

[0016] Figure 2 This is a schematic diagram of the crystal cutting parameters of the AOTF device;

[0017] Figure 3 Schematic diagram of forward and reverse calibration modes;

[0018] Figure 4 Schematic diagram of the layout of the acousto-optic action wave vector;

[0019] Figure 5 Schematic diagram of the test device;

[0020] Figure 6 is the root mean square error distribution diagram between the measured data group and the theoretical data group;

[0021] Figure 7 The data graph of the calibration results is shown in Figure 1. The data points are the measured data set, and the theoretical curve is the theoretical data set corresponding to the calibration results.

[0022] Table 1 is a table of constant parameters involved in the modeling and analysis process of the present invention; DETAILED DESCRIPTION

[0023] The following is a detailed description of the method for calibrating the profile of an acousto-optic tunable filter proposed by the present invention with reference to the accompanying drawings.

[0024] like Figure 1 As shown in the figure, the calibration method proposed in this invention mainly includes three key steps: model establishment, calibration test and parameter inversion. The main goal is to fully calibrate the three characteristic cutting angles of the AOTF device crystal cutting data. like Figure 2 shown.

[0025] First, let's focus on model building. The precise theoretical model mainly involves two analysis processes: (1) refraction process: solving the refraction angle in the crystal based on the incident light angle; (2) acousto-optic diffraction process: solving the matching tuning frequency based on the wave vector layout in the crystal. First, analyze the refraction process, and the relevant parameters are as follows: Figure 3 As shown. The refraction effect of the incident surface satisfies the refraction law:

[0026] n0sinθ0=n1sinθ1, (1)

[0027] Among them, θ0 and θ1 are the incident light polar angle in air and the refracted light polar angle in the crystal, n0 and n1 are the refractive index of air and the actual refractive index in the crystal, respectively. TeO2 crystal is anisotropic, and its actual refractive index satisfies:

[0028]

[0029]

[0030] The superscripts o and e are used to distinguish ordinary (ordinary-o) light from extraordinary (extraordinary-e) light. o With n e is the principal refractive index of the TeO2 crystal. λ is the wavelength of the incident light. θ2 is the angle between the refracted light wave vector in the crystal and the crystal axis

[001] . The conversion relationship between θ2 and θ1 in the polar angle plane is:

[0031]

[0032] in is the angle between the incident surface of the calibration device and the crystal axis

[110] , and n=1 corresponds to forward calibration, and n=2 corresponds to reverse calibration. Compared with the actual crystal cutting data of AOTF device The relationship between them is:

[0033]

[0034] This formula shows that two calibrations, forward and reverse, are required to obtain complete AOTF device crystal cutting data, such as Figure 3 As shown. If x=tanθ2,a1=sin 2 θ0,a2=(n e ) 2 , a3=(n o ) 2 , After combining equations (1)-(3), we can get:

[0035]

[0036] This equation is a quadratic equation, so the value of x is easy to solve. At this time, after combining equations (1)-(5), the relationship between the external incident light polar angle, the crystal internal refraction polar angle and the crystal cutting data can be obtained:

[0037]

[0038] This formula can be solved analytically.

[0039] Further analysis of the acousto-optic diffraction process shows that the acousto-optic diffraction effect of AOTF is often solved by wave vector layout, such as Figure 4 When momentum matching is satisfied, the diffraction efficiency is the highest. At this time, the incident light wave vector in the wave vector layout is Sonic Vector and diffracted light wave vector A closed triangle is formed between them, namely Among them, the light wave vector satisfies And the endpoints A and B of the light wave vector are located on the o / e light wave vector ellipsoid surface and satisfy:

[0040]

[0041] The sound wave vector satisfies:

[0042]

[0043] where fα is the tuning frequency, V α is the ultrasonic phase velocity, and its value satisfies:

[0044]

[0045] Where V 110 and V 001 are the shear wave velocities of the acoustic wave along the corresponding axes.

[0046] According to the wave vector layout, the coordinates of point A (x A ,y A ) and tanθ2=y A / x A And the wave vector ellipsoid equation (7), it is easy to get the coordinates of point A:

[0047]

[0048] Then we need to solve the coordinates of point B (x B ,y B ) can be solved by the coordinates of the intersection of the AB line equation and the diffracted light wave vector ellipsoid equation. Wherein, for both the forward mode and the reverse mode, the AB line equation is:

[0049]

[0050] Further combining the wave vector ellipsoid equation (7) we can obtain:

[0051]

[0052] This is a quadratic equation. It is easy to get around point B, and we can solve it. Further combined with formula (8), the matching tuning frequency f can be solved α Combining formulas (7)-(12), this process establishes the relationship from the refraction angle θ2 in the crystal to the tuning frequency f α and is related to the incident wavelength and the ultrasonic cutting angle, that is:

[0053] f α =F(θ2,λ,θ α ). (13)

[0054] Combining equations (6) and (13), we can obtain:

[0055]

[0056] This formula establishes an accurate model between the external incident light polar angle and the matching tuning frequency. The constant parameters in the solution process are shown in Table 1. From this formula, it can be seen that when the incident light wavelength λ and the external polar angle θ0 are known, the AOTF device crystal cutting parameters can be effectively calibrated by measuring its matching ultrasonic frequency.

[0057] Further attention is paid to the calibration test aspect. Based on this model, the present invention designs a calibration parameter test device, as shown in the figure. The device consists of a monochromatic laser of known wavelength, a polarizer, a precision rotating stage, and a photodetector. In the example of the present invention, during the specific implementation, the attenuator uses a cross-polarizer to control the incident light intensity, and the AOTF device to be tested is placed on the precision rotating stage. Another polarizer is used as a polarizer to control the polarization state of the incident light entering the AOTF device. The photodetector uses a visible light camera. Compared with a power meter, it can simultaneously detect the diffracted light emitted by the AOTF device and the transmitted full-color light, which helps to avoid measurement errors caused by unstable laser power and polarization state. At the same time, as analyzed above, this calibration method requires flipping the incident surface and the exit surface of the AOTF device and calibrating them separately to achieve comprehensive calibration of the three characteristic cutting angles corresponding to the crystal cut of the AOTF device. During the calibration process, the calibration data that needs to be recorded mainly includes the external incident light polar angle and the matching tuning frequency data set. In specific implementation, it is necessary to first ensure that the incident light is perpendicular to the AOTF incident surface by checking whether the laser reflected laser point coincides with the emission point, and record the scale value of the precision rotation stage at this position as the reference zero point. Other external incident light angles are adjusted based on this reference value. The tuning frequency F that matches each external incident light angle θ0 is m It can be obtained by frequency sweeping, corresponding to the peak diffraction efficiency, and the present invention adopts a fourth-order polynomial fitting method to solve this value.

[0058] Finally, regarding the parameter inversion, based on the collected measured calibration data set and multiple sets of theoretical calculation data sets, the present invention needs to calculate the root mean square error (RMSE) between them respectively as follows:

[0059]

[0060] Where N is the number of sampling points, F m is the measured matching tuning frequency corresponding to each external incident light polar angle θ0(i). Figure 6 The RMSE value distribution of forward and reverse calibration under different input theoretical cutting data is shown respectively. Finally, based on the minimum RMSE principle, the crystal cutting data in forward and reverse modes can be inverted. In the case of forward calibration, and In the case of reverse calibration, we can get and Theoretically, the ultrasonic cutting angle θ under positive and negative calibration conditions is α Should remain unchanged, that is The measured results show that the ultrasonic cutting angle θ under positive and negative calibration conditions α The difference is 0.002°, which shows a very high calibration accuracy (better than 0.01°). The present invention takes the average of the two calibrations as the actual ultrasonic cutting angle value, and further combines it with formula (4) to obtain the complete AOTF crystal cutting parameters: θ α =6.485° and θ β =4.638°. The AOTF device used in the present invention is manufactured by China Electronics Technology Group Corporation, and its model number is SGL100-400 / 850-20LG-K. Figure 7 The measured data set and the theoretical data curve based on the calibration parameters are displayed. The difference between the measured data and the theoretical data is very small, which further demonstrates that the present invention has the advantage of high precision.

[0061] The contents not described in detail in the specification of the present invention belong to the prior art known to those skilled in the art.

[0062] Table 1

[0063]

Claims

1. A method for calibrating the profile of an acousto-optic tunable filter, characterized in that: This method requires flipping the incident surface and the exit surface of the AOTF device and calibrating them separately. The matching ultrasonic frequency of a fixed wavelength light source under different incident polar angles is sampled. The minimum root mean square error between the experimental data and the simulation data is used as the evaluation index. The three characteristic cutting angles of the AOTF device crystal cut are traversed and solved for fitting.

2. The method for calibrating the profile of an acousto-optic tunable filter according to claim 1, wherein: Based on the establishment of an accurate theoretical model of the relationship between the crystal cutting parameters of AOTF devices and the external calibration parameters, an external calibration parameter testing device was designed.

3. The method for calibrating the profile of an acousto-optic tunable filter according to claim 1, wherein: The specific calibration steps of this method are: (1) Establish an accurate theoretical model of AOTF parameters; (2) obtaining a calibration parameter set of incident polar angle and matching ultrasonic frequency through a test device; (3) traverse and input the AOTF crystal cutting parameters to obtain the corresponding theoretical calibration parameter group; (4) Calculate the root mean square error between the measured data set and the theoretical data set, and make judgments based on the minimum root mean square error criterion; (5) Output the crystal cutting parameters of the AOTF device to be tested. This parameter group corresponds to the crystal cutting parameters input in step (3) when the minimum root mean square error is obtained.

4. The method for calibrating the profile of an acousto-optic tunable filter according to claim 1, wherein: An accurate theoretical model for the relationship between the crystal cutting parameters of AOTF devices and external calibration parameters can achieve accurate analytical solution of relevant parameters for tellurium dioxide (TeO2) acousto-optic crystals.

5. The method for calibrating the profile of an acousto-optic tunable filter according to claim 2, wherein: The external calibration parameter test device consists of a monochromatic laser, a polarizer, the AOTF device to be tested, a precision rotating stage and a photodetector. The wavelength of the monochromatic laser must be known, the precision rotating stage can accurately read the value, and the photodetector can use a power meter or camera with a corresponding wavelength range.

6. The method for calibrating the profile of an acousto-optic tunable filter according to claim 1, wherein: The three characteristic cutting angles of the AOTF device crystal are: (1) The angle between the incident surface of the crystal and the crystal axis [110] It is referred to as incident cut angle; (2) Angle θ between the crystal ultrasonic transducer surface and the crystal axis [001] α , referred to as ultrasonic cutting angle; (3) The angle θ between the crystal exit surface and the crystal incident surface β , referred to as the exit cut angle.

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