A method for detecting the lattice constant and period of a one-dimensional photonic crystal

Abstract

A kind of lattice constant and period detection method of one-dimensional photonic crystal belongs to photonic crystal structure parameter measurement technical field, method includes: spot position acquisition module with linearly polarized light incident to one-dimensional photonic crystal, obtains the spot position information of one-dimensional photonic crystal in continuous wavelength λ range;Displacement difference curve drawing module draws the measured IF displacement difference curve of one-dimensional photonic crystal according to the spot position information;Reflection spectrum measurement module measures the reflection spectrum data of one-dimensional photonic crystal in continuous wavelength TM, TE wave, and draws the measured reflectivity curve;Inversion calculation module substitutes the measured IF displacement difference curve and measured reflectivity curve into the evaluation function of one-dimensional photonic crystal lattice constant and period number, carries out inversion calculation, and finally outputs the parameter vector of one-dimensional photonic crystal when evaluation function is minimum.Effect: it can realize the accurate, nondestructive detection of the lattice constant and period number of one-dimensional photonic crystal on any substrate surface.

Description

Technical Field

[0001] This invention relates to the field of photonic crystal structure parameter measurement technology, and in particular to a method for detecting the lattice constant and period of a one-dimensional photonic crystal. Background Technology

[0002] Photonic crystals are a class of artificial optical structures with periodically arranged dielectric constants. They can achieve precise control over electromagnetic wave transmission through the photonic bandgap effect. Based on the spatial dimensional differences in the periodic modulation of the dielectric constant, they can be divided into three categories: one-dimensional, two-dimensional, and three-dimensional photonic crystals. Among them, one-dimensional photonic crystals are composed of two or more dielectric materials with different refractive indices stacked alternately. Their dielectric constant changes periodically only in a single spatial dimension, remaining uniform in the other two dimensions. This structure possesses unique photonic bandgap characteristics, where light waves within the bandgap cannot propagate within the structure and can only be suppressed by total internal reflection. Due to its advantages of simple structure, mature fabrication, and ease of integration, it has been widely used in various fields such as optical communication, photovoltaic cells, optical sensing, and new energy.

[0003] The core optical performance of one-dimensional photonic crystals is directly determined by key parameters such as the number of periods, the thickness of the dielectric layer, and the lattice constant. Accurate detection of these parameters is crucial for ensuring device performance. Currently, existing technologies for parameter detection in one-dimensional photonic crystals primarily employ elliptic polarization and cross-sectional morphology methods. Elliptic polarization suffers from inherent defects such as ambiguity in film thickness periods, easily leading to multiple interpretations of film thickness measurement results and hindering precise and unique parameter determination. Cross-sectional morphology methods characterize the sample's cross-sectional morphology using scanning electron microscopy (SEM) or transmission electron microscopy (TEM), and then combine this with image processing to measure the film period and layer thickness. However, this method requires destructive sample preparation, causing irreversible damage to the original sample. The tested sample cannot be reused, resulting in resource waste and failing to meet the requirements of online, non-destructive quality control in mass production.

[0004] Disadvantages of existing technologies: Existing one-dimensional photonic crystal parameter detection methods suffer from problems such as: multiple interpretations of detection results and low accuracy; and the need for destructive sample preparation, resulting in resource waste. Summary of the Invention

[0005] The present invention provides a method for detecting the lattice constant and period of a one-dimensional photonic crystal, which can realize accurate and non-destructive detection of the lattice constant and period number of a one-dimensional photonic crystal on any substrate surface.

[0006] To achieve the above objectives, this invention provides a method for detecting the lattice constant and period of a one-dimensional photonic crystal, which, crucially, includes the following steps:

[0007] Step 1: Construct a lattice constant and period detection system for a one-dimensional photonic crystal. This lattice constant and period detection system for a one-dimensional photonic crystal is equipped with a spot position acquisition module, a displacement difference curve plotting module, and an inversion calculation module connected in sequence. The inversion calculation module is also connected to a reflectance spectrum measurement module.

[0008] Step 2: The spot position acquisition module uses an IF displacement measurement device to incident linearly polarized light onto a one-dimensional photonic crystal, acquires the spot position information of the one-dimensional photonic crystal within a continuous wavelength λ range, and transmits it to the displacement difference curve plotting module.

[0009] Step 3: The displacement difference curve plotting module plots the incident wavelength-IF displacement difference relationship curve of the one-dimensional photonic crystal based on the spot position information, obtains the measured IF displacement difference curve of the one-dimensional photonic crystal, and then transmits it to the inversion calculation module.

[0010] Step 4: The reflection spectrum measurement module uses a spectral measurement device to measure the reflection spectrum data of the one-dimensional photonic crystal at continuous wavelength λ for transverse magnetic wave TM and transverse electric wave TE, plots the measured reflectance curve, and transmits it to the inversion calculation module.

[0011] Step 5: The inversion calculation module substitutes the measured IF displacement difference curve and the measured reflectivity curve into the evaluation function of the lattice constant and period number of the one-dimensional photonic crystal, performs inversion calculation, and finally outputs the parameter vector of the one-dimensional photonic crystal when the evaluation function is minimized. ;

[0012] Parameter vector =(Λ,N), where N represents the period number of the one-dimensional photonic crystal and Λ represents the lattice constant of the one-dimensional photonic crystal.

[0013] This invention uses an IF displacement measurement device to acquire the spot position information of a one-dimensional photonic crystal within a continuous wavelength λ range and plots the measured IF displacement difference curve. It also uses a spectral measurement device to measure the reflection spectra of transverse magnetic wave (TM) and transverse electric wave (TE) of the one-dimensional photonic crystal at continuous wavelength λ and plots the measured reflectivity curve. Finally, it constructs an evaluation function for the lattice constant and period number of the one-dimensional photonic crystal, and substitutes the measured IF displacement difference curve and the measured reflectivity curve into the evaluation function to calculate the period number and lattice constant of the one-dimensional photonic crystal through inversion.

[0014] Through the above design, by using incident light in a continuous wavelength band for measurement and selecting a joint characterization mode of IF displacement difference and reflection spectrum, the lattice constant and period number of one-dimensional photonic crystals can be solved simultaneously across multiple wavelengths. This avoids the shortcomings of existing spectroscopic methods that use reflectivity calculations at multiple wavelengths, which introduce large errors when performing analytical and fitting of thin film optical constants. This significantly improves the accuracy of solving for the lattice constant and period number of one-dimensional photonic crystals. This invention provides effective theoretical guidance and reference for the non-destructive testing of the lattice constant and period of one-dimensional photonic crystals, enabling accurate and non-destructive testing of the lattice constant and period number of one-dimensional photonic crystals on any substrate surface. It features good detection effect and strong applicability.

[0015] The full name of the IF displacement is Imbert-Fedorov displacement.

[0016] Preferably, in step 2, when measuring using the IF displacement measuring device, the substrate-one-dimensional photonic crystal layer-dielectric layer is used as the one-dimensional photonic crystal structure to be measured; the one-dimensional photonic crystal layer is a one-dimensional photonic crystal layer deposited on the substrate.

[0017] Preferably, in step 5, the evaluation function expressions for the lattice constant and period number of the one-dimensional photonic crystal are as follows:

[0018] (17);

[0019] in, For the evaluation function, This represents the total number of incident wavelengths. Let j be the reference point. This refers to the IF displacement difference value extracted from the measured IF displacement difference curve at the j-th reference point. The IF displacement difference under TE and TM polarization waves obtained from the inversion calculation; The reflectance of the j-th reference point is extracted from the measured reflectance curve. This represents the average reflectance under TM and TE polarization obtained from the inversion calculation. Let be the parameter vector of a one-dimensional photonic crystal.

[0020] As a preferred method, the inversion calculation process of the IF displacement difference under TE and TM polarization is as follows:

[0021] (1);

[0022] (2);

[0023] (3);

[0024] (4);

[0025] (5);

[0026] (6);

[0027] In this context, the subscript TM indicates a TM-polarized wave, and the subscript TE indicates a TE-polarized wave. For spatial IF displacement, For IF angular displacement, Indicates the spread angle of the incident beam. The angle at which the light is incident on the substrate. For the beam waist, Indicates the wavelength of the incident light. The IF displacement difference under TE and TM polarized waves obtained from the inversion calculation. This represents the total IF shift of the TM-polarized wave. This represents the total IF shift of the TE-polarized wave. Indicates the IF displacement coefficient. It is the virtual part. Represents the reflection coefficient. This represents the imaginary part of the IF displacement coefficient. This represents the real part of the IF displacement coefficient. This indicates the transmission distance of the reflected light.

[0028] As a preferred method, the inversion calculation process for the average reflectance under TM and TE polarization is as follows:

[0029] (7);

[0030] (8);

[0031] (9);

[0032] (10);

[0033] (11);

[0034] in, It is the characteristic matrix of a one-dimensional photonic crystal. The characteristic matrix of a high-refractive-index layer in a one-dimensional photonic crystal. The characteristic matrix of the low-refractive-index layer in a one-dimensional photonic crystal. The phase factor of the high refractive index layer. The phase factor for the low refractive index layer. For the number of periods, For the kth period, The optical transfer coefficient of the high refractive index layer. The optical transfer coefficient of the low-refractive-index layer. For the thickness of the high refractive index layer, The thickness of the low refractive index layer, The refractive index of the high refractive index layer, The refractive index of the low-refractive-index layer, The angle at which the light is incident on the substrate. Let be the refractive index of the substrate, and i be the imaginary part; Representing matrices respectively The four elements in;

[0035] For TM and TE polarized waves, the optical transfer coefficients of different layers The reflection coefficient r is expressed as:

[0036] (12);

[0037] (13);

[0038] (14);

[0039] (15);

[0040] in, , Indicates the basal layer. This represents a one-dimensional photonic crystal layer. Indicates the dielectric layer. This represents the optical transmission coefficient of the TM-polarized wave in layer t. This represents the optical transmission coefficient of the TE-polarized wave in layer t. Let be the dielectric constant of layer t. Let be the incident angle of layer t. The reflection coefficient of the TE-polarized wave. The reflection coefficient of the TM polarized wave; The reflectivity under TM polarization is... The reflectivity is denoted as TE polarized wave.

[0041] As a preferred option: the lattice constant of a one-dimensional photonic crystal The calculation expression is:

[0042] (16).

[0043] Preferably, the refractive index of the high refractive index layer is greater than 2.0, and the refractive index of the low refractive index layer is in the range of [1.3, 1.5].

[0044] Preferably, the incident angle and incident point are kept constant during the measurement of the IF displacement difference and reflection spectrum data.

[0045] The beneficial effects of this invention are:

[0046] (1) The continuous band incident light is used for measurement. The IF displacement difference and reflection spectrum joint characterization mode is selected to simultaneously realize the solution of the lattice constant and period of the one-dimensional photonic crystal in multiple wavelengths. This avoids the shortcomings of the existing spectroscopic method, which introduces large errors when performing analysis and fitting of thin film optical constants by solving the reflectivity in multiple wavelengths, and improves the solution accuracy.

[0047] (2) This invention provides effective theoretical guidance and reference for nondestructive testing of the lattice constant and period of one-dimensional photonic crystals, and has the characteristics of good detection effect and strong applicability. Attached Figure Description

[0048] Figure 1 This is a flowchart of the method of the present invention;

[0049] Figure 2 This is a schematic diagram of the one-dimensional photonic crystal structure and its IF shift in the embodiment;

[0050] Figure 3 This is a schematic diagram of the IF displacement measuring device in the embodiment;

[0051] Figure 4 These are the experimental and theoretical curve fitting diagrams of the IF displacement difference in the embodiments;

[0052] Figure 5 The figures shown are the experimental and theoretical curve fitting diagrams of reflectance in the embodiments. Detailed Implementation

[0053] The present invention will be further described in detail below with reference to the accompanying drawings and specific examples. The following embodiments or drawings are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0054] like Figure 1 As shown, a method for detecting the lattice constant and period of a one-dimensional photonic crystal includes the following steps:

[0055] Step 1: Construct a lattice constant and period detection system for a one-dimensional photonic crystal. This lattice constant and period detection system for a one-dimensional photonic crystal is equipped with a spot position acquisition module, a displacement difference curve plotting module, and an inversion calculation module connected in sequence. The inversion calculation module is also connected to a reflectance spectrum measurement module.

[0056] Step 2: The spot position acquisition module uses an IF displacement measurement device to incident linearly polarized light onto a one-dimensional photonic crystal, such as... Figure 2As shown. Then, the position information of the light spot of the one-dimensional photonic crystal in the continuous wavelength λ range is obtained and transmitted to the displacement difference curve plotting module;

[0057] Step 3: The displacement difference curve plotting module plots the incident wavelength-IF displacement difference relationship curve of the one-dimensional photonic crystal based on the spot position information, obtains the measured IF displacement difference curve of the one-dimensional photonic crystal, and then transmits it to the inversion calculation module.

[0058] Step 4: The reflection spectrum measurement module uses a spectral measurement device to measure the reflection spectrum data of the one-dimensional photonic crystal at continuous wavelength λ for transverse magnetic wave TM and transverse electric wave TE, plots the measured reflectance curve, and transmits it to the inversion calculation module.

[0059] During the measurement of the IF displacement difference and reflection spectrum data, the incident angle and incident point are kept constant.

[0060] Step 5: The inversion calculation module substitutes the measured IF displacement difference curve and the measured reflectivity curve into the evaluation function of the lattice constant and period number of the one-dimensional photonic crystal, performs inversion calculation, and finally outputs the parameter vector of the one-dimensional photonic crystal when the evaluation function is minimized. ;

[0061] Parameter vector =(Λ,N), where N represents the period number of the one-dimensional photonic crystal and Λ represents the lattice constant of the one-dimensional photonic crystal.

[0062] In step 2, when the IF displacement measuring device is used for measurement, the substrate-one-dimensional photonic crystal layer-dielectric layer is used as the one-dimensional photonic crystal structure to be tested; the one-dimensional photonic crystal layer is a one-dimensional photonic crystal layer deposited on the substrate.

[0063] In step 5, the evaluation function expressions for the lattice constant and period number of the one-dimensional photonic crystal are as follows:

[0064] (17);

[0065] in, For the evaluation function, This represents the total number of incident wavelengths. Let j be the reference point. This refers to the IF displacement difference value extracted from the measured IF displacement difference curve at the j-th reference point. The IF displacement difference under TE and TM polarization waves obtained from the inversion calculation; The reflectance of the j-th reference point is extracted from the measured reflectance curve. This represents the average reflectance under TM and TE polarization obtained from the inversion calculation. Let be the parameter vector of a one-dimensional photonic crystal.

[0066] The inversion calculation process of IF displacement difference under TE and TM polarization waves is as follows:

[0067] (1);

[0068] (2);

[0069] (3);

[0070] (4);

[0071] (5);

[0072] (6);

[0073] In this context, the subscript TM indicates a TM-polarized wave, and the subscript TE indicates a TE-polarized wave. For spatial IF displacement, For IF angular displacement, Indicates the spread angle of the incident beam. The angle at which the light is incident on the substrate. For the beam waist, Indicates the wavelength of the incident light. The IF displacement difference under TE and TM polarized waves obtained from the inversion calculation. This represents the total IF shift of the TM-polarized wave. This represents the total IF shift of the TE-polarized wave. Indicates the IF displacement coefficient. It is the virtual part. Represents the reflection coefficient. This represents the imaginary part of the IF displacement coefficient. This represents the real part of the IF displacement coefficient. This indicates the transmission distance of the reflected light.

[0074] The inversion calculation process for the average reflectance under TM and TE polarization is as follows:

[0075] (7);

[0076] (8);

[0077] (9);

[0078] (10);

[0079] (11);

[0080] in, It is the characteristic matrix of a one-dimensional photonic crystal. The characteristic matrix of a high-refractive-index layer in a one-dimensional photonic crystal. The characteristic matrix of the low-refractive-index layer in a one-dimensional photonic crystal. The phase factor of the high refractive index layer. The phase factor for the low refractive index layer. For the number of periods, For the kth period, The optical transfer coefficient of the high refractive index layer. The optical transfer coefficient of the low-refractive-index layer. For the thickness of the high refractive index layer, The thickness of the low refractive index layer, The refractive index of the high refractive index layer, The refractive index of the low-refractive-index layer, The angle at which the light is incident on the substrate. Let be the refractive index of the substrate, and i be the imaginary part; Representing matrices respectively The four elements in;

[0081] For TM and TE polarized waves, the optical transfer coefficients of different layers The reflection coefficient r is expressed as:

[0082] (12);

[0083] (13);

[0084] (14);

[0085] (15);

[0086] in, , Indicates the basal layer. This represents a one-dimensional photonic crystal layer. Indicates the dielectric layer. This represents the optical transmission coefficient of the TM-polarized wave in layer t. This represents the optical transmission coefficient of the TE-polarized wave in layer t. Let be the dielectric constant of layer t. Let be the incident angle of layer t. The reflection coefficient of the TE-polarized wave. The reflection coefficient of the TM polarized wave; The reflectivity under TM polarization is... The reflectivity is denoted as TE polarized wave.

[0087] Lattice constant of a one-dimensional photonic crystal The calculation expression is:

[0088] (16).

[0089] The high refractive index layer has a refractive index greater than 2.0, and the low refractive index layer has a refractive index range of [1.3, 1.5].

[0090] like Figure 3 As shown, the IF displacement measuring device is equipped with a laser, a half-wave plate, a first focusing lens, a first polarizer, a second focusing lens, a second polarizer, and a camera.

[0091] The laser emits incident laser light of a fixed wavelength, which passes through a half-wave plate, a first focusing lens, and a first polarizer to reach a one-dimensional photonic crystal. The one-dimensional photonic crystal reflects the incident laser light, which then passes through a second polarizer and a second focusing lens before being captured by a camera.

[0092] This embodiment uses a substrate-one-dimensional photonic crystal-medium structure model. Specifically, the one-dimensional photonic crystal is made of high-refractive-index titanium dioxide. Film and low refractive index silicon dioxide The film layers are alternately stacked on a BK7 glass substrate. In this embodiment, air is selected as the medium. If other materials are used as the medium, the corresponding refractive index value can be applied. The incident wavelength is selected as [350, 650] nm, and the incident angle is 65°. The solution domain is (0, 200] nm. The domain of N is (0, 100] nm, and the domain of N is (0, 5). Then, based on the incident wavelength-IF shift difference and incident wavelength-reflectivity obtained experimentally, these are substituted into the evaluation function. At the minimum value of the evaluation function, the number of periods N of the one-dimensional photonic crystal is found to be 2. It is 159.8 nm. The wavelength is 61.5 nm, therefore the lattice constant is 221.3 nm. From... Figure 4 and Figure 5 It can be seen that the IF displacement difference curve and reflectivity R curve obtained by fitting are almost consistent with the curves obtained by experiment, indicating that the one-dimensional photonic crystal structure parameters obtained by the present invention are reliable and effective.

[0093] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for detecting the lattice constant and period of a one-dimensional photonic crystal, characterized in that, Includes the following steps: Step 1: Construct a lattice constant and period detection system for a one-dimensional photonic crystal. This lattice constant and period detection system for a one-dimensional photonic crystal is equipped with a spot position acquisition module, a displacement difference curve plotting module, and an inversion calculation module connected in sequence. The inversion calculation module is also connected to a reflectance spectrum measurement module. Step 2: The spot position acquisition module uses an IF displacement measurement device to incident linearly polarized light onto a one-dimensional photonic crystal, acquires the spot position information of the one-dimensional photonic crystal within a continuous wavelength λ range, and transmits it to the displacement difference curve plotting module. Step 3: The displacement difference curve plotting module plots the incident wavelength-IF displacement difference relationship curve of the one-dimensional photonic crystal based on the spot position information, obtains the measured IF displacement difference curve of the one-dimensional photonic crystal, and then transmits it to the inversion calculation module. Step 4: The reflection spectrum measurement module uses a spectral measurement device to measure the reflection spectrum data of the one-dimensional photonic crystal at continuous wavelength λ for transverse magnetic wave TM and transverse electric wave TE, plots the measured reflectance curve, and transmits it to the inversion calculation module. Step 5: The inversion calculation module substitutes the measured IF displacement difference curve and the measured reflectivity curve into the evaluation function of the lattice constant and period number of the one-dimensional photonic crystal, performs inversion calculation, and finally outputs the parameter vector of the one-dimensional photonic crystal when the evaluation function is minimized. ; parameter vector =(Λ,N), where N represents the period number of the one-dimensional photonic crystal and Λ represents the lattice constant of the one-dimensional photonic crystal.

2. The method for detecting the lattice constant and period of a one-dimensional photonic crystal according to claim 1, characterized in that: In step 2, when the IF displacement measuring device is used for measurement, the substrate-one-dimensional photonic crystal layer-dielectric layer is used as the one-dimensional photonic crystal structure to be tested; the one-dimensional photonic crystal layer is a one-dimensional photonic crystal layer deposited on the substrate.

3. The method for detecting the lattice constant and period of a one-dimensional photonic crystal according to claim 1, characterized in that: In step 5, the evaluation function expressions for the lattice constant and period number of the one-dimensional photonic crystal are as follows: （17）； in, For the evaluation function, This represents the total number of incident wavelengths. Let j be the reference point. This refers to the IF displacement difference value extracted from the measured IF displacement difference curve at the j-th reference point. The IF displacement difference under TE and TM polarization waves obtained from the inversion calculation; The reflectance of the j-th reference point is extracted from the measured reflectance curve. This represents the average reflectance under TM and TE polarization obtained from the inversion calculation. Let be the parameter vector of a one-dimensional photonic crystal.

4. The method for detecting the lattice constant and period of a one-dimensional photonic crystal according to claim 3, characterized in that: The inversion calculation process of IF displacement difference under TE and TM polarization waves is as follows: （1）； （2）； （3）； （4）； （5）； （6）； In this context, the subscript TM indicates a TM-polarized wave, and the subscript TE indicates a TE-polarized wave. For spatial IF displacement, For IF angular displacement, Indicates the spread angle of the incident beam. The angle at which the light is incident on the substrate. For the beam waist, Indicates the wavelength of the incident light. The IF displacement difference under TE and TM polarized waves obtained from the inversion calculation. This represents the total IF shift of the TM-polarized wave. This represents the total IF shift of the TE-polarized wave. Indicates the IF displacement coefficient. It is the virtual part. Represents the reflection coefficient. This represents the imaginary part of the IF displacement coefficient. This represents the real part of the IF displacement coefficient. This indicates the transmission distance of the reflected light.

5. The method for detecting the lattice constant and period of a one-dimensional photonic crystal according to claim 3, characterized in that: The inversion calculation process for the average reflectance under TM and TE polarization is as follows: （7）； （8）； （9）； （10）； （11）； in, It is the characteristic matrix of a one-dimensional photonic crystal. The characteristic matrix of a high-refractive-index layer in a one-dimensional photonic crystal. The characteristic matrix of the low-refractive-index layer in a one-dimensional photonic crystal. The phase factor of the high refractive index layer. The phase factor for the low refractive index layer. For the number of periods, For the kth period, The optical transfer coefficient of the high refractive index layer. The optical transfer coefficient of the low-refractive-index layer. For the thickness of the high refractive index layer, The thickness of the low refractive index layer, The refractive index of the high refractive index layer, The refractive index of the low-refractive-index layer, The angle at which the light is incident on the substrate. Let be the refractive index of the substrate, and i be the imaginary part; Representing matrices respectively The four elements in; For TM and TE polarized waves, the optical transfer coefficients of different layers The reflection coefficient r is expressed as: （12）； （13）； （14）； （15）； in, , Indicates the basal layer. This represents a one-dimensional photonic crystal layer. Indicates the dielectric layer. This represents the optical transmission coefficient of the TM-polarized wave in layer t. This represents the optical transmission coefficient of the TE-polarized wave in layer t. Let be the dielectric constant of layer t. Let be the incident angle of layer t. The reflection coefficient of TE-polarized waves. The reflection coefficient of the TM polarized wave; The reflectivity under TM polarization is... The reflectivity is denoted as TE polarized wave.

6. The method for detecting the lattice constant and period of a one-dimensional photonic crystal according to claim 5, characterized in that: Lattice constant of a one-dimensional photonic crystal The calculation expression is: （16）。 7. The method for detecting the lattice constant and period of a one-dimensional photonic crystal according to claim 5, characterized in that: The high refractive index layer has a refractive index greater than 2.0, and the low refractive index layer has a refractive index range of [1.3, 1.5].

8. The method for detecting the lattice constant and period of a one-dimensional photonic crystal according to claim 1, characterized in that: During the measurement of the IF displacement difference and reflection spectrum data, the incident angle and incident point are kept constant.

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