Method and device for measuring refractive index through broadband spectrum prism coupling

Abstract

The invention discloses a method for measuring refractive index by broadband spectrum prism coupling, which comprises the following steps: broadband spectrum light emitted by a broadband spectrum light source enters a semicircular prism at a certain angle, a to-be-measured sample is placed on the plane of the semicircular prism, and a certain pressure is applied to press the to-be-measured sample; reflected light of the plane of the semicircular prism enters the spectrograph, and the spectrograph collects a spectrum; spectral data is calculated through a broad spectrum prism coupling algorithm, and the refractive index of the material to be measured is obtained. The invention further discloses a broadband spectrum prism coupling measurement device which comprises the semicircular prism and a clamp for fixing the semicircular prism. The method has the advantages that the broadband spectrum light source is adopted, and one-time complete measurement can be obtained without rotating the angle adjusting table.

Description

Technical Field

[0001] The invention relates to the technical field of material refractive index measurement, and in particular to a method and device for measuring refractive index through wide spectrum prism coupling. Background Art

[0002] The prism coupler is an optical measuring instrument for measuring the refractive index of transparent materials or the parameters of surface waveguides. Its principle is based on the total reflection phenomenon of light and the refraction of light by prisms. When light propagates from one medium to another, if the incident angle is greater than the critical angle, the light will be totally reflected; if the incident angle is less than the critical angle, the light will be partially refracted into the material to be measured, and the power of the reflected light will decrease, and the reduction ratio is related to the incident angle. Therefore, the refractive index of the material to be measured can be measured according to the critical angle and the reflectivity when it is greater than the critical angle. In addition, the evanescent wave coupling of prism coupling can be used to measure the refractive index, depth and other parameters of the surface waveguide. When the incident angle is greater than the critical angle, if the material surface does not have a waveguide, the light will be totally reflected; if the material surface has a waveguide, at certain angles, the incident light will be coupled into the waveguide by the evanescent wave, thereby reducing the reflected light intensity. In addition, for multimode waveguides, the coupling angle will be greater than one, so the parameters of the waveguide can be calculated based on the incident angle and the reflected light intensity during coupling.

[0003] The prism coupling technology requires a large range and continuous adjustment of the incident angle of the light, and the measurement of the reflected light intensity at each incident angle, so the measurement speed is slow, generally more than a few minutes. This is due to the size and weight requirements of the prism. The device that rotates the prism generally chooses a stepper motor, servo motor, etc. that can bear a certain weight and can accurately position. Rotating the laser light source requires a larger and more precise mechanism, so it is generally not used. In addition, the light source of the prism coupling uses a visible or near-infrared monochromatic light source, such as near-monochromatic light filtered through a filter, or a laser light source. Therefore, an instrument can only measure the refractive index of one wavelength. If the refractive index of other wavelengths needs to be measured, the light source needs to be replaced, which is very troublesome. Summary of the invention

[0004] The technical problem to be solved by the present invention is that a known instrument can only measure the refractive index of one wavelength. If the refractive index of other wavelengths needs to be measured, the light source needs to be replaced, which is very troublesome.

[0005] The first purpose of the present invention is to improve the speed of prism coupling measurement by using a wide-spectrum light source and a prism coupling method to perform refractive index measurement.

[0006] The technical solutions adopted are as follows: A method for measuring refractive index by wide-spectrum prism coupling, the method comprising: using broadband spectrum light emitted by a wide-spectrum light source to enter a semicircular prism at a certain angle, placing a sample to be measured on the plane of the semicircular prism, and applying a certain pressure to compact it; the reflected light from the plane of the semicircular prism enters a spectrometer, and the spectrometer collects the spectrum; the spectrum data is calculated by a wide-spectrum prism coupling algorithm to obtain the refractive index of the material to be measured.

[0007] In the preferred embodiment of the technical solution of the present invention, the shortest wavelength of the spectrum of the wide spectrum light source is , the longest wavelength is , Should be greater than 100nm. Broad-spectrum light sources have a wide emission spectrum.

[0008] Preferably, the spectrum emitted by the wide-spectrum light source has polarization or non-polarization characteristics.

[0009] The second purpose of the present invention is to provide a wide-spectrum prism coupling measurement device, in which a wide-spectrum light source and a semicircular prism are coupled to perform refractive index measurement. A complete measurement can be obtained without rotating the angle adjustment table, thereby improving the speed of prism coupling measurement.

[0010] The technical solutions adopted are as follows: A wide spectrum prism coupling measurement device comprises a semicircular prism and a fixture for fixing the semicircular prism, wherein the semicircular prism comprises an arc surface for vertically receiving incident light and a plane for placing a sample to be measured, the fixture comprises a groove matching the semicircular prism, an incident window and an exit window are arranged at the top of the fixture corresponding to the arc surface of the semicircular prism, and the bottom of the fixture is opened to expose the plane of the semicircular prism.

[0011] Preferably, the top surface of the semicircular prism is an arc surface, the bottom surface is a plane, and the two side surfaces are semicircular.

[0012] Preferably, the clamp is composed of two vertical plates and a plurality of reinforcing edges connecting the two vertical plates, and a groove is formed between the vertical plates and the reinforcing edges to improve the mechanical strength of the clamp.

[0013] In a preferred embodiment of the technical solution of the present invention, positioning grooves for engaging the side surfaces of the semicircular prisms are provided on the inner side surfaces of the two vertical plates, thereby improving the clamping efficiency of the semicircular prisms.

[0014] Preferably, the semicircular prism is fixed to the groove by gluing, so as to improve the clamping strength of the semicircular prism and prevent it from slipping.

[0015] Preferably, the two vertical plates of the clamp are provided with screw holes for connecting with external mechanisms.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. The method of the present invention uses a wide spectrum light source, and a complete measurement can be obtained without rotating the angle adjustment table.

[0017] 2. The method of the present invention uses a wide spectrum light source with multiple wavelengths to obtain the refractive index of materials at multiple wavelengths.

[0018] 3. The device of the present invention adopts a semicircular prism. No matter at what angle the sample to be measured is placed, the incident light is perpendicular to the arc surface of the semicircular prism, so that no refraction occurs. The light is always at the center of the semicircle and a fixed point on the sample, avoiding changes in the measurement position; the reflected light is also perpendicular to the arc surface, so that different refracted light will not be generated due to dispersion, making it easier to collect a wide spectrum. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The principle block diagram of the method for measuring refractive index by wide spectrum prism coupling of the present invention; Figure 2 is an outline diagram of a semicircular prism of the present invention; Figure 3 It is a schematic diagram of the principle of light of the incident light source of the present invention entering the semicircular prism; Figure 4 is a first structural schematic diagram of the clamp in the present invention; Figure 5 is a second structural schematic diagram of the clamp in the present invention; Figure 6 It is a schematic diagram of the position of the semicircular prism after being installed by the fixture; Figure 7 for Figure 6 Exploded view of; Figure 8 It is a schematic diagram of the position of the sample to be tested above the semicircular prism; Fig. 9 for Figure 6 A cutaway view of Fig.10 A schematic diagram of the original spectrum collected and the spectrum normalization in an embodiment of the present invention; Fig.11 Schematic diagram of original measurement data and normalized data of BK7 glass in a specific embodiment; Fig.12 It is a schematic diagram of normalized data of BK7 and Schott glass in a specific embodiment; Wherein: 1-semicircular prism, 11-arc surface, 12-plane surface, 13-side surface, 2-fixture, 21-incident window, 22-exit window, 23-vertical plate, 231-positioning groove, 24-reinforcement edge, 3-sample to be tested. DETAILED DESCRIPTION

[0020] The technical solution of the present invention is described in detail below, but the protection scope of the present invention is not limited to the embodiments.

[0021] To make the content of the present invention more clearly understood, the following Figure 1 -Attached Fig.12 The specific implementation manner is further described.

[0022] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0023] like Figure 1 As shown, the present embodiment provides a method for measuring refractive index by wide-spectrum prism coupling: broadband spectral light emitted by a wide-spectrum light source is incident on a semicircular prism at a certain angle, a sample to be measured is placed on the plane of the semicircular prism, and a certain pressure is applied to compact it; reflected light from the plane of the semicircular prism enters a spectrometer, and the spectrometer collects the spectrum; the spectral data is calculated by a wide-spectrum prism coupling algorithm to obtain the refractive index of the material to be measured.

[0024] In the method of measuring refractive index by wide spectrum prism coupling of the present embodiment, the angle of the semicircular prism can be adjusted by the angle adjustment table; however, since the present invention adopts a wide spectrum light source, a complete measurement can be obtained without rotating the angle adjustment table, and the measurement time is only limited by the acquisition speed of the spectrometer. If a linear array high-speed spectrometer is used, the time can be shortened to less than one second, which greatly improves the measurement speed; if more precise measurements are required, or the refractive index of the material to be measured changes greatly, the angle adjustment table can be rotated to make the critical angle within the spectrum; since a wide spectrum light source with multiple wavelengths is used, the refractive index of materials with multiple wavelengths can be obtained. The above advantages make the method of the present invention have a great advantage over traditional methods.

[0025] In the method for measuring refractive index by wide spectrum prism coupling of this embodiment, the wide spectrum light source has a wide emission spectrum, and the shortest wavelength of the spectrum is , the longest wavelength is , It should be greater than 100nm. Light sources with these characteristics include arc lamps such as halogen tungsten lamps, xenon lamps, and mercury lamps, or white light LEDs, or supercontinuum laser light sources, or mode-locked ultrashort pulse frequency comb light sources.

[0026] The spectrum emitted by the broadband light source has polarized or non-polarized characteristics. When the spectrum has polarized characteristics, the refractive index of birefringent materials can be measured; when the spectrum has non-polarized characteristics, orthogonal polarization can be selected through a broadband polarizer to measure the refractive index of birefringent materials.

[0027] like Figure 2 As shown, a wide spectrum prism coupling measurement device comprises a semicircular prism 1 and a fixture 2 for fixing the semicircular prism. The semicircular prism 1 in the wide spectrum prism coupling measurement device does not have straight edge features such as triangular prisms; the semicircular prism 1 comprises an arc surface 11 for vertically receiving incident light and a plane 12 for placing a sample to be measured, specifically: the top surface of the semicircular prism 1 is an arc surface 11, the bottom surface is a plane 12, and the two side surfaces 13 are both semicircular.

[0028] like Figure 3 As shown, since the light source of the present invention is a wide-spectrum light source, if a straight-edge prism is used, the incident light source will produce different refraction angles due to dispersion, thereby causing many problems in the measurement position, reflected light collection, etc. The semicircular prism 1 of the present invention, because no matter at what angle the sample to be measured is placed, the incident light is perpendicular to the arc surface, so that no refraction occurs, and the light is always at the center of the semicircle and a fixed point on the sample, avoiding changes in the measurement position. Moreover, the reflected light is also perpendicular to the arc surface, so that different refracted light will not be produced due to dispersion, making it easier to collect a wide spectrum. In addition, the material used in the semicircular prism should have a larger refractive index, such as rutile.

[0029] like Figure 4 As shown, the semicircular prism 1 is clamped by a fixture 2, and the fixture 2 is made of a material such as metal, plastic, resin, etc. that has a certain strength and is easy to process.

[0030] The fixture 2 has a groove that fits the semicircular prism 1 and has two windows for incident and exiting. The bottom of the fixture has a window to expose the plane 12 of the semicircular prism 1 to fit the sample 3 to be tested.

[0031] like Figure 4 and 5 As shown, the fixture 2 is composed of two vertical plates 23 and a plurality of reinforcing edges 24 connecting the two vertical plates, and a groove is formed between the vertical plates 23 and the reinforcing edges 24. The semicircular prism 1 is clamped in the groove, and the semicircular prism 1 can be fixed in the groove by mechanical clamping, gluing, etc.; the groove is formed between the vertical plates 23 and the reinforcing edges 24, which further improves the mechanical strength of the fixture.

[0032] like Figure 5 As shown, the inner side surfaces of the two vertical plates 23 are provided with positioning grooves 231 for engaging with the side surfaces of the semicircular prism 1, so as to further improve the installation efficiency.

[0033] like Figure 6 and Figure 7 As shown, in this embodiment, the sample 3 to be tested is fitted with the plane 12 of the semicircular prism 1, and a support is provided below the sample 3 to be tested, and a certain pressure is applied by air flotation, springs, etc. to press the sample 3 to be tested. The fixing method of the sample 3 to be tested described here is known to those skilled in the art.

[0034] like Figure 8 As shown, in this embodiment, the fixture 2 can be placed upside down so that the sample 3 to be tested is above the semicircular prism 1. Similarly, the fixture 2 can be placed sideways so that the light is transmitted in a horizontal plane.

[0035] like Figure 6 As shown, specifically, the fixture 2 is connected to the angle adjustment mechanism by a mechanical connection method, and the fixture 2 and the angle adjustment mechanism are fixed by screw holes provided on two vertical plates 23 on the fixture 2. The angle adjustment mechanism is composed of a stepper motor, a servo motor, etc., and can read the current angle. The structure of the angle adjustment mechanism and the angle display and reading described here are all known to those skilled in the art.

[0036] In the method of measuring refractive index by wide spectrum prism coupling of this embodiment, the spectrometer is implemented by a scanning spectrometer, a linear array fixed spectrometer, a Fourier transform spectrometer, etc. Preferably, a linear array fixed spectrometer with a fast scanning speed is used. The spectrometer collects the light emitted by the wide spectrum light source and reflected by the prism plane. to spectral information.

[0037] Furthermore, in the method of measuring refractive index by wide-spectrum prism coupling of this embodiment, since the spectrum emitted by the wide-spectrum light source is not flat, the reflectivity value cannot be directly obtained, and the spectrum needs to be calibrated and normalized. The steps for implementing the above calibration and normalization are: (1) collecting the original spectrum of the wide-spectrum light source, and (2) dividing the measured spectrum by the original spectrum according to the wavelength.

[0038] The method for collecting the original spectrum of the broadband light source can adopt one of the following methods: ① pre-split the broadband light source through a fiber beam splitter, and one of the beams enters the spectrometer for spectrum collection; ② use a beam splitter to reflect the spectrum of part of the intensity, and enter the spectrometer for collection through spatial deflection; ③ do not place the sample, rotate the prism to a larger angle of incidence, and to The spectrum is totally reflected and the original reflected spectrum is collected.

[0039] The preferred method of the present invention is ①. Fig.10 As shown, the original spectrum Original is obtained by method ①, and then the normalized spectrum Normalized is obtained by dividing the measured spectrum by the original spectrum. Specific embodiment: The method of the present invention is further described by taking BK7 glass as an example.

[0041] The broadband light source in this example uses to A supercontinuum laser light source is provided, and the broad spectrum light source is split by an optical fiber. One beam enters a semicircular prism 1 after being collimated to collect the measurement spectrum; and the other beam enters an optical fiber spectrometer to collect the original spectrum.

[0042] The semicircular prism 1 in this example is made of rutile material. The refractive index of rutile is known. The data of the semicircular prism 1 can be fitted according to formulas (1)-(4). The parameters related to prism coupling can be given by the Fresnel formula: For the critical angle, we have (1) in, is the critical angle, , are the refractive indices of the media where the incident light and the outgoing light are located, respectively.

[0043] Reflectivity for P and S polarization and , respectively (2) (3) in, is the complex amplitude of the reflectivity of P polarization, is the complex amplitude of the reflectivity of S polarization, is the electric field intensity of the P-polarized reflected light, is the electric field intensity of the P-polarized incident light, is the electric field intensity of the S-polarized reflected light, is the electric field intensity of the S-polarized incident light, is the refractive index of the incident medium, is the refractive index of the refracting medium, is the incident angle of the measurement, is the measured refraction angle.

[0044] And the angles of incidence and refraction follow Snell's law: (4) The prism coupling process can be calculated according to the above formulas (1)-(4), and the refractive index and other parameters can be inferred.

[0045] Place the BK7 glass on the plane of the semicircular prism and press it tightly against the surface. Calculate and select the appropriate incident angle for measurement according to formula (1) and rotate it into place; in this embodiment, the incident angle for measurement is Select the critical angle Nearby, including the critical angle.

[0046] Turn on the broadband light source and spectrometer to start measuring. Fig.11 As shown, the original spectrum BK7 Original is obtained, and then the normalized spectrum BK7 Normalized is obtained by dividing the measured spectrum by the original spectrum.

[0047] The original spectral data collected by the spectrometer is as follows Fig.11 As shown in the BK7 Original. Fig.11 As shown, when the original spectrum is less than about 0.6μm, the refractive index of rutile is much greater than that of BK7 due to dispersion, and the semicircular prism plane produces total reflection with a reflectivity of 1.0. On the contrary, when the spectrum is greater than about 0.6μm, the semicircular prism plane is not in the total reflection range, and the reflectivity changes with wavelength.

[0048] Specifically, during measurement, the angle between the semicircular prism 1 and the incident light from the light source is adjusted, that is, In the embodiment of the present invention, The choice of is obtained according to the above measurement angle through formula (1); generally speaking, the measurement angle of incidence is selected At the critical angle First, select a wavelength In to between, and Closer Calculate wavelength The critical angle . Rotate the semicircular prism so that the incident angle is the critical angle , and then take measurements.

[0049] Replace the sample material with Schott glass. Repeat the above measurement steps and obtain the normalized data as shown below: Fig.12 As shown. It can be seen that the critical angles of Schott glass and BK7 glass There is a difference. This difference reflects the difference in the refractive index of the material. After obtaining the normalized data, the curve fitting is performed using formulas (1)-(4). to The refractive index.

[0050] The above embodiments are only for illustrating the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for measuring refractive index by wide spectrum prism coupling, characterized in that: The method comprises the following steps: broadband spectrum light emitted by a broadband spectrum light source is incident into a semicircular prism at a certain angle, a sample to be tested is placed on the plane of the semicircular prism, and a certain pressure is applied to compact the sample; reflected light from the plane of the semicircular prism enters a spectrometer, and a spectrum is collected by the spectrometer; spectrum data is calculated by a broadband spectrum prism coupling algorithm to obtain the refractive index of the material to be tested.

2. The method for measuring refractive index by wide spectrum prism coupling according to claim 1, characterized in that: The shortest wavelength of the spectrum of the wide spectrum light source is , the longest wavelength is , Should be greater than 100nm.

3. The method for measuring refractive index by wide spectrum prism coupling according to claim 1, characterized in that: The spectrum emitted by the wide spectrum light source has polarization or non-polarization characteristics.

4. The wide spectrum prism coupling measurement device of the method for measuring refractive index by wide spectrum prism coupling according to any one of claims 1 to 3, characterized in that: The invention comprises a semicircular prism (1) and a fixture (2) for fixing the semicircular prism, wherein the semicircular prism (1) comprises a circular arc surface (11) for vertically receiving incident light and a plane (12) for placing a sample to be tested, and the fixture (2) comprises a groove that matches the semicircular prism, and an incident window (21) and an exit window (22) are arranged at the top of the fixture (2) corresponding to the circular arc surface (11) of the semicircular prism (1), and the bottom of the fixture (2) is open to expose the plane (12) of the semicircular prism (1).

5. The wide spectrum prism coupling measurement device according to claim 4, characterized in that: The top surface of the semicircular prism (1) is an arc surface (11), the bottom surface is a plane surface (12), and the two side surfaces (13) are both semicircular.

6. The wide spectrum prism coupling measurement device according to claim 4, characterized in that: The clamp (2) is composed of two vertical plates (23) and a plurality of reinforcing edges (24) connecting the two vertical plates, and a groove is formed between the vertical plates (23) and the reinforcing edges (24).

7. The wide spectrum prism coupling measurement device according to claim 6, characterized in that: The inner side surfaces of the two vertical plates (23) are both provided with positioning grooves (231) for engaging with the side surfaces of the semicircular prism (1).

8. The wide spectrum prism coupling measurement device according to claim 6, characterized in that: The semicircular prism (1) is glued and fixed to the groove.

9. The wide spectrum prism coupling measurement device according to claim 6, characterized in that: The two vertical plates (23) of the clamp (2) are provided with screw holes for connecting to an external mechanism.

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