A device and method for simultaneous measurement of thickness and group refractive index of a birefringent crystal

Abstract

The application provides a thickness and group refractive index synchronous measurement device and method of a birefringent crystal, which comprises an optical interference device composed of a light source, a fiber coupler, a reference arm and a detection arm, and further comprises a spectrometer and an upper computer; when the thickness and group refractive index of the birefringent crystal are measured, the thickness and group refractive index of the crystal are modulated to generate an interference light signal, specifically: the light of the light source is divided into reference light and detection light through the fiber coupler, the reference light is reflected through the reference arm and re-enters the fiber coupler; the detection light is focused on the surface of the measured sample through the detection arm, is back-reflected and is transmitted to the fiber coupler through an optical path, is combined with the reflected reference light at the fiber coupler and interferes, the generated interference light signal is transmitted to the upper computer through the spectrometer after being transmitted through the fiber coupler and is used for measurement and analysis; the application provides a high-precision, rapid and simple measurement method for the optical speed parameter measurement of the birefringent crystal.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of non-contact optical measurement, in particular to a device and method for simultaneously measuring thickness and group refractive index of birefringent crystal. BACKGROUND

[0002] Refractive index is an optical parameter determined by the polarization of medium molecules, which reveals the interaction between light and matter and the structure of matter. In anisotropic crystal, the refractive index in different directions is different due to different crystal structures and spatial orientations, resulting in birefringence. Most optical functional crystals have birefringence, and birefringence is one of the key properties of optical functional crystals. It plays an indispensable role in angle phase matching of nonlinear optical crystals or adjusting the polarization of light. Measuring the birefringence of a crystal is an important parameter for characterizing the optical properties of anisotropic crystals and one of the most basic contents for measuring the physical properties of a crystal. Common methods for measuring refractive index mainly include angle measurement method, differential confocal method, image processing method, and interference method. However, the range and precision of refractive index measurement are generally limited by the conditions required by the test method itself, so the commonly used measurement methods have some shortcomings. For example, the minimum deviation angle method requires high precision of the test prism and is difficult to process. The image processing method has simple structure but low measurement precision and requires a high-sensitivity CCD camera. The differential confocal method requires repeated searching and accurate alignment of the position of the focused spot during measurement, which is tedious and time-consuming. The optical interference method is currently based on time-domain low-coherence measurement technology, which requires the refractive index of the measured sample to be known and a mechanical scanning mechanism to match the optical path in real time. The measurement time is long and the system is complex.

[0003] Current research on refractive index measurement mainly focuses on the measurement of refractive index of single refractive index materials, and the measurement of birefringent materials is less and mostly concentrated on the measurement of the maximum refractive index difference between ordinary light and extraordinary light. In addition, due to the existence of dispersion, the refractive index only considers the interaction between single-wavelength light beam and medium, while in broadband light system people also care about the comprehensive effect of broadband light beam and medium, i.e. the group refractive index of medium. There is almost no report on the measurement of the group refractive index of medium in China. In view of these problems and defects of the measurement method, this paper proposes a method for simultaneously measuring the thickness and group refractive index of birefringent crystal by broadband optical coherence. This method uses the principle of broadband optical frequency domain coherence, uses Fourier transform and energy spectrum correction method to obtain the optical path information of interference fringes, and simultaneously obtains the group refractive index and thickness of birefringent crystal by measuring the optical path change of the whole system before and after the sample. As an optical interference method, it saves the mechanical scanning mechanism compared with the traditional time-domain low-coherence measurement method, simplifies the complexity of the measurement system, improves the detection efficiency and accuracy. SUMMARY

[0004] The present invention provides a device and method for synchronously measuring the thickness and group refractive index of a birefringent crystal, providing a high-precision, fast and simple measurement method for measuring the light velocity parameters of the birefringent crystal.

[0005] The present invention adopts the following technical solutions.

[0006] A device for synchronously measuring the thickness and group refractive index of a birefringent crystal comprises an optical interference device consisting of a light source, an optical fiber coupler (2), a reference arm, and a detection arm, and further comprises a spectrometer and a host computer. When measuring the thickness and group refractive index of a birefringent crystal, the interference device modulates the thickness and group refractive index of the crystal to generate an interference light signal. Specifically, the light from the light source is divided into reference light and detection light via the optical fiber coupler, the reference light is reflected by the reference arm and re-enters the optical fiber coupler, the detection light is focused on the surface of a sample to be measured via the detection arm and is back-reflected, and then transmitted to the optical fiber coupler via an optical path, where it merges with the reflected reference light at the optical fiber coupler and interferes with the reflected reference light. The generated interference light signal is transmitted via the optical fiber coupler and then transmitted to the host computer via the spectrometer for measurement and analysis.

[0007] The light source is a low-coherence light source, and the detection arm is fixed on a height-adjustable lifting platform to focus the detection light spot on the sample to be tested.

[0008] The low-coherence light source is a superluminescent diode light source (1); and the optical fiber coupler is a 2x2 single-mode optical fiber coupler.

[0009] The reference arm is a fully enclosed sleeve structure, wherein the reference arm sleeve (4) contains two collimating lenses and a reflector, and the reference arm optical path structure adopts the form of collimation-parallel-focusing. An adjustable sleeve structure is used in the parallel light transmission link in the middle to facilitate the adjustment of the optical path of the reference arm; the reference light passes through the reference arm optical fiber APC connector (3), the reference arm collimating lens (5), the reference arm focusing lens (6), the reference arm reflector (7), and then is reflected along the original path.

[0010] The detection arm is a sleeve structure, and the detection arm sleeve (9) of the sleeve structure contains two detection arm collimating lenses (10) for collimating and parallelizing the detection light and then focusing it, so that the optical path structure is symmetrical with the detection arm; the detection light passes through the detection arm optical fiber APC connector (8), the detection arm collimating lens (10), the detection arm microlens (11) to the surface of the sample to be measured, and then is reflected along the original path.

[0011] The spectrometer comprises a reflection grating (17), a spectrometer collimating lens (18), a linear array CCD camera (15), and a spectrometer focusing lens (16); when the interference device measures the thickness and group refractive index of a birefringent crystal, the interference light signal entering the spectrometer is collimated by a fiber collimator and becomes parallel light to be irradiated on the grating, and the grating divides the broadband light into light of different diffraction directions according to wavelength. After being focused by the lens, the light of each wavelength is converged to different pixel units of the linear array CCD camera, and then the linear array CCD camera forms an image and transmits it to a host computer (14); the fiber collimator comprises a spectrometer focusing lens (18) placed in a spectrometer input end sleeve (19), a spectrometer optical fiber APC connector (20), and a fiber coupler is connected to the spectrometer optical fiber APC connector through an optical fiber to input the interference light signal to the spectrometer.

[0012] The spectrometer processes the collected interference light signal using Fourier transform technology, and calculates the thickness and group refractive index of the sample using a Hanning window energy center of gravity spectrum correction algorithm.

[0013] A method for synchronously measuring the thickness and group refractive index of a birefringent crystal, using the above-described apparatus for synchronously measuring the thickness and group refractive index of a birefringent crystal, is characterized in that: the method uses a fiber coupler to separate light from a light source into reference light and probe light; the probe light spot is focused on the surface of a sample to be measured and is back-reflected; the reflected light then merges with the reference light at the fiber coupler and interferes to form an interference light signal; the method utilizes the principle of broadband optical frequency domain coherence to convert the optical path information of each reflective layer of the birefringent crystal into interference fringes of corresponding frequencies; a fast Fourier transform and a Hanning window-based energy spectrum correction method are used to accurately calculate the optical path; and a two-step measurement method is used to compare the changes in the optical path before and after the sample is inserted into the measurement operation, thereby achieving accurate measurement of the crystal thickness and the group refractive index of O light and E light.

[0014] The measuring method is used for measuring lithium niobate wafers, wherein the sample to be measured is inserted in front of a reflector; specifically, the following steps are included:

[0015] Step S1, using a single-side polished frosted glass (13) as a reflector behind the sample to be measured, and using a detection arm to collect an interference signal from the reflector, wherein the interference signal includes optical path information of the reflector;

[0016] Step S2: Keep the positional relationship between the sample arm and the reflector unchanged, and place the double-sided polished lithium niobate wafer (12) with a Y-cut 0° direction between the reflector and the probe; then collect the interference signal after the sample is inserted. Therefore, the interference signal collected by the spectrometer is:

[0017]

[0018] Among them, the composition of the interference signal is divided into three parts: DC term, interference term, and self-correlation term, specifically:

[0019] 1. The reference mirror and each reflective layer of the sample directly reflect the signal;

[0020] 2. Interference signals between the reference mirror and each layer of the sample;

[0021] 3. The self-coherence signal of each layer of the sample; where s (k) is the spectral density function of the light source, I represents the reflectivity, subscripts M and S represent the reference mirror and sample respectively, subscripts n and m represent the nth and mth reflective layers in the sample arm; i is the imaginary number symbol, i.e. k is the reciprocal of wavelength λ, k = 1 / λ; ΔL refers to the optical path difference (OPD) between the sample reflector and the reference mirror;

[0022] Step S3: resample the interference signal sequence collected in step S2 in a wave number space uniform manner to obtain a signal sequence that changes strictly periodically with the wave number. Perform Fourier transform on the interference signal to obtain:

[0023]

[0024] Among them, G (τ) is the temporal coherence function of the light source, and the spectral density function of the light source are a Fourier transform pair; δ (τ) is the Dirac function;

[0025] Step S4: Since the time-domain spectrum information is truncated to a finite length, resulting in energy leakage, a spectrum correction method based on a Hanning window is used to accurately extract the frequency period of the interference signal; this greatly improves the measurement accuracy of the optical path to the nanometer or even sub-nanometer level; the Hanning window loaded in the interference signal is:

[0026] w Hn(n) =0.5-0.5·cos(2πn / N)

[0027] Where n = 1, 2, ..., N-1, N is the length of the Hanning window;

[0028] The Hanning window energy center correction algorithm is:

[0029]

[0030] Where i is the spectral line number, and G is the amplitude corresponding to the i-th spectral line.

[0031] Step S5: After accurately extracting the frequency period of the signal, the optical path of the interference signal is obtained. Then, the thickness of the wafer sample and the refractive index of the o-light and e-light in the sample are calculated based on the two-step measurement method. Then, the group refractive index of the sample is calculated based on the relationship between the refractive index and the group refractive index.

[0032] In step S5, the measurement principle of the crystal group refractive index is specifically as follows: based on the relationship between the group refractive index and the refractive index:

[0033]

[0034] Calculate the group refractive index of the sample o-light and the group refractive index of the sample e-light.

[0035] Compared with the prior art, the present invention has the following beneficial effects: Currently, most of the research on refractive index measurement focuses on the refractive index measurement research of single-refractive index materials, and the measurement research of birefringent index materials is relatively small and mostly focuses on the measurement of the maximum refractive index difference between ordinary light and extraordinary light. In addition, due to the existence of dispersion, the refractive index only considers the interaction between a single-wavelength light beam and the medium, and the measurement of the group refractive index of the medium is almost unreported in China. The present invention overcomes the above-mentioned defects, and based on the principle of broadband optical frequency domain coherence, adopts Fourier transform and energy spectrum correction method to obtain the optical path information of the interference fringes, and synchronously obtains the group refractive index and thickness of the birefringent crystal by measuring the optical path change of the overall system before and after the sample. Moreover, as an optical interference method, compared with the traditional time-domain low-coherence measurement method, it saves the mechanical scanning mechanism, simplifies the complexity of the measurement system, and improves the detection efficiency and detection accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0037] Attachment Figure 1 It is a schematic diagram of the structural principle of the present invention;

[0038] Attachment Figure 2 Schematic diagram of the principle of the two-step method for simultaneous measurement of optical path length and refractive index;

[0039] Attachment Figure 3 Schematic diagram of the FFT signal of the two-step measurement method for Y-cut 0° lithium niobate crystal;

[0040] In the figure: 1. Superluminescent diode light source; 2. Fiber coupler; 3. Reference arm fiber APC connector; 4. Reference arm sleeve; 5. Reference arm collimating lens; 6. Reference arm focusing lens; 7. Reflector; 8. Detection arm fiber APC connector; 9. Detection arm sleeve; 10. Detection arm collimating lens; 11. Detection arm microlens; 12. Lithium niobate wafer; 13. Reflector; 14. Computer; 15. Linear array CCD camera; 16. Spectrometer focusing lens; 17. Reflection grating; 18. Spectrometer focusing lens; 19. Spectrometer input sleeve; 20. Spectrometer fiber APC connector. DETAILED DESCRIPTION

[0041] As shown in the figure, a device for synchronously measuring the thickness and group refractive index of a birefringent crystal is provided. The device includes an optical interference device consisting of a light source, a fiber coupler 2, a reference arm, and a detection arm, and also includes a spectrometer and a host computer. When measuring the thickness and group refractive index of a birefringent crystal, the interference device modulates the thickness and group refractive index of the crystal to generate an interference light signal. Specifically, the light from the light source is divided into reference light and detection light by the fiber coupler. The reference light is reflected by the reference arm and re-enters the fiber coupler. The detection light is focused on the surface of the sample to be measured by the detection arm and is back-reflected, then transmitted to the fiber coupler via an optical path. It merges with the reflected reference light at the fiber coupler and interferes with it. The resulting interference light signal is transmitted through the fiber coupler and then transmitted to the host computer via the spectrometer for measurement and analysis.

[0042] The light source is a low-coherence light source, and the detection arm is fixed on a height-adjustable lifting platform to focus the detection light spot on the sample to be tested.

[0043] The low-coherence light source is a superluminescent diode light source 1; and the optical fiber coupler is a 2x2 single-mode optical fiber coupler.

[0044] The reference arm has a fully enclosed sleeve structure, and its reference arm sleeve 4 contains two collimating lenses and a reflector. The reference arm optical path structure adopts the collimation-parallel-focusing form, and an adjustable sleeve structure is used in the parallel light transmission link in the middle to facilitate the adjustment of the optical path of the reference arm; the reference light passes through the reference arm optical fiber APC connector 3, the reference arm collimating lens 5, the reference arm focusing lens 6 to the reference arm reflector 7 and then reflects along the original path.

[0045] The detection arm is a sleeve structure, and the detection arm sleeve 9 of the sleeve structure contains two detection arm collimating lenses 10, which are used to collimate the detection light and then focus it, so that the optical path structure is symmetrical with the detection arm;

[0046] The detection light passes through the detection arm optical fiber APC connector 8, the detection arm collimating lens 10, the detection arm microlens 11 to the surface of the sample to be measured and then reflects along the original path.

[0047] The spectrometer includes a reflection grating 17, a spectrometer collimating lens 18, a linear array CCD camera 15, and a spectrometer focusing lens 16. When the interferometer device measures the thickness and group refractive index of a birefringent crystal, the interference light signal entering the spectrometer is collimated by a fiber collimator and then becomes parallel light that is irradiated onto the grating. The grating separates the broadband light into light of different diffraction directions according to wavelength. After being focused by the lens, the light of each wavelength is converged onto different pixel units of the linear array CCD camera, and then the linear array CCD camera forms an image and transmits it to the host computer 14.

[0048] The fiber collimator includes a spectrometer focusing lens 18 and a spectrometer fiber APC connector 20 placed in a spectrometer input end sleeve 19. The fiber coupler is connected to the spectrometer fiber APC connector through an optical fiber to input an interference light signal to the spectrometer.

[0049] The spectrometer processes the collected interference light signal using Fourier transform technology, and calculates the thickness and group refractive index of the sample using a Hanning window energy center of gravity spectrum correction algorithm.

[0050] A method for synchronously measuring the thickness and group refractive index of a birefringent crystal, using the above-described apparatus for synchronously measuring the thickness and group refractive index of a birefringent crystal, is characterized in that: the method uses a fiber coupler to separate light from a light source into reference light and probe light; the probe light spot is focused on the surface of a sample to be measured and is back-reflected; the reflected light then merges with the reference light at the fiber coupler and interferes to form an interference light signal; the method utilizes the principle of broadband optical frequency domain coherence to convert the optical path information of each reflective layer of the birefringent crystal into interference fringes of corresponding frequencies; a fast Fourier transform and a Hanning window-based energy spectrum correction method are used to accurately calculate the optical path; and a two-step measurement method is used to compare the changes in the optical path before and after the sample is inserted into the measurement operation, thereby achieving accurate measurement of the crystal thickness and the group refractive index of O light and E light.

[0051] The measuring method is used for measuring lithium niobate wafers, wherein the sample to be measured is inserted in front of a reflector; specifically, the following steps are included:

[0052] Step S1: Use a single-side polished frosted glass 13 as a reflector behind the sample to be measured, and use a detection arm to collect an interference signal from the reflector. This interference signal contains the optical path information of the reflector.

[0053] Step S2: Keep the positional relationship between the sample arm and the reflector unchanged, and place the double-sided polished lithium niobate wafer (12) with a Y-cut 0° direction between the reflector and the probe; then collect the interference signal after the sample is inserted. Therefore, the interference signal collected by the spectrometer is:

[0054]

[0055] Among them, the composition of the interference signal is divided into three parts: DC term, interference term, and self-correlation term, specifically:

[0056] 1. The reference mirror and each reflective layer of the sample directly reflect the signal;

[0057] 2. Interference signals between the reference mirror and each layer of the sample;

[0058] 3. The self-coherence signal of each layer of the sample; where s (k) is the spectral density function of the light source, I represents the reflectivity, subscripts M and S represent the reference mirror and sample respectively, subscripts n and m represent the nth and mth reflective layers in the sample arm; i is the imaginary number symbol, i.e. k is the reciprocal of wavelength λ, k = 1 / λ; ΔL refers to the optical path difference (OPD) between the sample reflector and the reference mirror;

[0059] Step S3: resample the interference signal sequence collected in step S2 in a wave number space uniform manner to obtain a signal sequence that changes strictly periodically with the wave number. Perform Fourier transform on the interference signal to obtain:

[0060]

[0061] Among them, G (τ) is the temporal coherence function of the light source, and the spectral density function of the light source are a Fourier transform pair; δ (τ) is the Dirac function;

[0062] Step S4: Since the time-domain spectrum information is truncated to a finite length, resulting in energy leakage, a spectrum correction method based on a Hanning window is used to accurately extract the frequency period of the interference signal; this greatly improves the measurement accuracy of the optical path to the nanometer or even sub-nanometer level; the Hanning window loaded in the interference signal is:

[0063] w Hn(n) =0.5-0.5·cos(2πn / N)

[0064] Where n = 1, 2, ..., N-1, N is the length of the Hanning window;

[0065] The Hanning window energy center correction algorithm is:

[0066]

[0067] Where i is the spectral line number, and G is the amplitude corresponding to the i-th spectral line.

[0068] Step S5: After accurately extracting the frequency period of the signal, the optical path of the interference signal is obtained. Then, the thickness of the wafer sample and the refractive index of the o-light and e-light in the sample are calculated based on the two-step measurement method. Then, the group refractive index of the sample is calculated based on the relationship between the refractive index and the group refractive index.

[0069] In step S5, the measurement principle of the crystal group refractive index is specifically as follows: based on the relationship between the group refractive index and the refractive index:

[0070]

[0071] Calculate the group refractive index of the sample o-light and the group refractive index of the sample e-light.

[0072] Example:

[0073] The following embodiments are specifically cited to provide detailed descriptions as follows:

[0074] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the art to which this application belongs.

[0075] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0076] This embodiment provides a device for synchronously measuring the thickness and group refractive index of a birefringent crystal, including a superluminescent light source 1, a 2×2 single-mode fiber coupler 2, a birefringent crystal 12, a ground glass (reflector) 13, reference arms (3 to 7), detection arms (8 to 11), a host computer 14 and a spectrometer (15 to 20).

[0077] like Figure 1As shown, the superluminescent light source 1 emits broadband light with a center wavelength of 888.9 nm and a 10 dB bandwidth of 95.2 nm into the 2x2 single-mode fiber coupler 2, which is divided into reference light and probe light. The reference light is first collimated into parallel light by the round lens 5, focused by the round lens 6 placed behind, and irradiated onto the mirror 7, and then returned to the 2x2 single-mode fiber coupler 2. The probe light is first collimated into parallel light by the round lens 10, focused by the round lens 11 placed behind, and irradiated onto the birefringent crystal 12 and the frosted glass (mirror) 13, which occurs back reflection and returns to the 2x2 single-mode fiber coupler 2. The returned reference light and probe light meet at the 2x2 single-mode fiber coupler 2, and after coupling, form an interference beam. The interference beam is accessed into the spectrometer by the exit port of the 2x2 single-mode fiber coupler 2. The spectrometer is composed of two round lenses, a grating 17, and a line array camera 15. The interference beam is collimated into parallel light by the round lens 18, irradiated onto the grating 17, the grating 17 separates different wavelengths of light by spatial angle, and then converges to the photosensitive array of the line array camera 15 through the round lens 16; the interference signal is collected by the line array camera 15 after being split by the spectrometer, and the line array camera 15 converts the collected image into a digital signal input into the upper computer 14 for sample thickness and group refractive index analysis.

[0078] Figure 2 is the optical path and refractive index synchronous measurement principle diagram of the present embodiment. The principle of the two measurement methods is as follows: first step, only a frosted glass or mirror is placed in the sample arm as a sample, and the optical path ΔL OM of the frosted glass (mirror) is measured. OM OD1 OD2 The thickness and refractive index of the sample can be calculated by the refractive index calculation formula.

[0079] Figure 3 ​​is the FFT signal diagram of two-step measurement method of Y-cut 0° lithium niobate crystal in this embodiment. The cyan dashed line represents the initial optical path of the ground glass (mirror) obtained in the first step. The FFT processing result of the interference signal measured in the second step is represented by the red solid line in the figure. After inserting the sample, the FFT result appears many peak signals. By moving the lithium niobate wafer up and down, the peak signal position changes correspondingly. The mutual coherence signal is the peak signal position that does not change. The appearance of multi-order signal peaks is because the high reflectivity of the lithium niobate crystal surface caused by the large refractive index of the lithium niobate crystal leads to multiple back-and-forth reflections of the probe light in its interior. Each reflection will produce an interference signal and generate a multi-order signal peak. Because the distance traveled by each back-and-forth reflection is consistent, the spacing between each order of signal peak is the same. After clarifying the meaning of each signal peak, the optical path of the lithium niobate wafer can be calculated, and the thickness and group refractive index of the sample can be further calculated.

[0080] The embodiment provides a thickness and group refractive index synchronous measurement method of a birefringent crystal, including the following steps:

[0081] S1, using a single-side polished ground glass (mirror) as a measured sample, collecting an interference signal of the ground glass (mirror) by using a probe arm. The interference signal contains optical path information of the ground glass (mirror).

[0082] S2, keeping the positional relationship between the sample arm and the ground glass (mirror) unchanged, placing a double-side polished lithium niobate wafer in a Y-cut 0° direction between the ground glass (mirror) and the probe; and then collecting an interference signal after inserting the sample. Therefore, the interference signal collected by the spectrometer is:

[0083]

[0084] The composition of the interference signal can be divided into three parts: a direct current term, an interference term and a self-coherence term. 1. The direct reflection signals of the reference mirror and each reflection layer of the sample; 2. The interference signals of the reference mirror and each layer of the sample; 3. The self-coherence signals of each layer of the sample. In the formula, s (k) is the spectral density function of the light source, I represents reflectivity, the subscripts M and S represent the reference mirror and the sample respectively, the subscripts n and m represent the nth and mth reflection layers in the sample arm; i is an imaginary symbol, that is k is the reciprocal of the wavelength λ, k = 1 / λ; ΔL refers to the optical path difference (OPD) between the sample reflection layer and the reference mirror.

[0085] S3, uniformly re-sampling the wave number space of the interference signal sequence collected in S2 to obtain a signal sequence that strictly changes with the wave number. At this time, the Fourier transform of the interference signal can be obtained as:

[0086]

[0087] wherein G (τ) is the time coherence function of the light source, and is a Fourier transform pair with the spectral density function of the light source; delta (τ) is the Dirac function.

[0088] S4, due to the energy leakage caused by the time domain spectrum information being truncated to a finite length, the frequency period number of the interference signal is accurately extracted using a spectrum correction method of the energy center of the Hanning window. The measurement accuracy of the optical path can be greatly improved to the nanometer or even sub-nanometer level. The Hanning window loaded in the interference signal is:

[0089] w Hn(n) = 0.5-0.5·cos(2πn / N)

[0090] wherein n=1, 2, …, N-1, and N is the length of the Hanning window.

[0091] The Hanning window energy center correction algorithm is:

[0092]

[0093] wherein i is the spectral line number, and G is the amplitude corresponding to the i-th spectral line.

[0094] S5, after the frequency period number of the signal is accurately extracted, the thickness of the lithium niobate wafer sample in the Y-cut 0° direction and the refractive index of o light and e light in the sample are calculated according to the two measurement methods, and then the group refractive index of the sample is calculated through the relationship between the refractive index and the group refractive index.

[0095] The above is only a preferred embodiment of the present application, and is not intended to limit the present application in other forms. Any person skilled in the art can modify or change the above disclosed technical content to equivalent embodiments. However, any simple modification, equivalent change and modification made according to the technical essence of the present application to the above embodiments, without departing from the technical solution of the present application, still falls within the protection scope of the present application.

Claims

1. A method for synchronously measuring the thickness and group refractive index of a birefringent crystal, using an apparatus for synchronously measuring the thickness and group refractive index of a birefringent crystal, characterized in that: The method uses a fiber coupler to separate light from a light source into reference light and probe light. The probe light spot is focused on the surface of a sample to be measured and is reflected from the back. The reflected light then merges with the reference light at the fiber coupler and interferes to form an interference light signal. The method utilizes the principle of broadband optical frequency domain coherence to convert the optical path information of each reflective layer of a birefringent crystal into interference fringes of corresponding frequencies. Fast Fourier transform and a Hanning window-based energy spectrum correction method are used to accurately calculate the optical path. A two-step measurement method is used to compare the change in optical path before and after the sample is inserted into the measurement operation, thereby achieving accurate measurement of crystal thickness and the group refractive index of O light and E light. The measurement method is used for measuring lithium niobate wafers, and the sample to be measured is inserted in front of the reflector; specifically The following steps are included: Step S1, using a single-side polished frosted glass (13) as a reflector behind the sample to be measured, and using a detection arm to collect an interference signal from the reflector, wherein the interference signal includes optical path information of the reflector; Step S2: Keep the positional relationship between the sample arm and the reflector unchanged, and place a double-sided polished lithium niobate wafer with a Y-cut angle of 0° between the reflector and the probe; then collect the interference signal after inserting the sample; the interference signal collected by the spectrometer is: The interference signal is composed of three parts: DC term, interference term, and self-coherence term. Specifically, they are: direct reflection signal from the reference mirror and each reflective layer of the sample, interference signal from the reference mirror and each layer of the sample, and self-coherence signal from each layer of the sample. Where s (k) is the spectral density function of the light source, I represents the reflectivity, subscripts M and S represent the reference mirror and sample respectively, subscripts n and m represent the nth and mth reflective layers in the sample arm; i is the imaginary number symbol, i.e. k is the reciprocal of wavelength λ, k = 1 / λ; ΔL refers to the optical path difference OPD between the sample reflector and the reference mirror; Step S3: resample the interference signal sequence collected in step S2 in a wave number space uniform manner to obtain a signal sequence that changes strictly periodically with the wave number. Perform Fourier transform on the interference signal to obtain: Among them, G (τ) is the temporal coherence function of the light source, and the spectral density function of the light source are a Fourier transform pair; δ (τ) is the Dirac function; Step S4: Since the time-domain spectrum information is truncated to a finite length, resulting in energy leakage, a spectrum correction method based on a Hanning window is used to accurately extract the frequency period of the interference signal; this greatly improves the measurement accuracy of the optical path to the nanometer or even sub-nanometer level; the Hanning window loaded in the interference signal is: w Hn(n) =0.5-0.5·cos(2πn / N) Where n = 1, 2, ..., N-1, N is the length of the Hanning window; The Hanning window energy center correction algorithm is: Where i is the spectral line number, G is the amplitude corresponding to the i-th spectral line; Step S5: After accurately extracting the frequency period of the signal, the optical path of the interference signal is obtained. Then, the thickness of the wafer sample and the refractive index of the o-light and e-light in the sample are calculated based on the two-step measurement method. Then, the group refractive index of the sample is calculated based on the relationship between the refractive index and the group refractive index.

2. The method for synchronously measuring the thickness and group refractive index of a birefringent crystal according to claim 1, wherein: In step S5, the measurement principle of the crystal group refractive index is specifically as follows: based on the relationship between the group refractive index and the refractive index: Calculate the group refractive index of the sample o-light and the group refractive index of the sample e-light.

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