CN116148219A - Method and system for measuring tiny refractive index change rate based on vortex light interference - Google Patents
Method and system for measuring tiny refractive index change rate based on vortex light interference Download PDFInfo
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
- G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods
Abstract
The invention discloses a method and a system for measuring a tiny refractive index change rate based on vortex light interference. The invention uses two beams of vortex light with opposite topological charge signs as a reference beam and a measuring beam respectively, the measuring beam passes through a sample to be measured with a slowly-changing refractive index and then is converged with the reference beam into one beam to form a rotating petal-shaped interference pattern, and the rotation angular velocity of the petal-shaped pattern is in direct proportion to the refractive index change rate of the sample to be measured. The CCD is used for collecting and recording the interference pattern at a certain sampling frequency, the normalized cross-correlation method is used for processing the image collected by the CCD, and the time-frequency analysis method is introduced, so that the rotation speed of the interference pattern can be obtained, and the refractive index change rate of the sample to be measured is further obtained. The invention avoids errors caused by measuring the rotation angle of the interference pattern, can measure the change rate of the refractive index which changes at non-uniform speed, and can distinguish the direction of the change of the refractive index.
Description
Technical Field
The invention belongs to the field of optical sensing measurement, and particularly relates to a method and a system for measuring a tiny refractive index change rate.
Background
Vortex beams, also known as OAM (Orbital Angular Momentum) beams, were named from the first discovery by Allen et al in 1992 that beams with a helical phase factor exp (ilθ) were transmittedWith defined orbital angular momentum
Where l is the number of orbital angular momentum quanta, also called topological charge number (TC), and θ is the azimuth angle over the beam cross-section. The spin angular momentum different from the photon can only be taken +.>
The orbital angular momentum of the beam can theoretically be infinite, which expands a new degree of freedom for the application of the beam. Meanwhile, the center of the vortex beam generally has a phase singular point and an amplitude zero point due to the presence of the phase singular point. Because of these special properties, researchers have used vortex beams in a wide variety of fields in recent years, such as optical tweezers, optical manipulation, optical capturing, optical wrenches, microscopic imaging, quantum information processing, optical communications, and the like. Since the vortex beam has a rotationally symmetrical spiral phase structure, it can realize high-sensitivity sensing, and thus is also widely used for optical sensing measurement.
Vortex beams with opposite topological charges are interfered coaxially to generate petal-shaped interference patterns with rotationally symmetrical structures, and the method can be used for realizing high-sensitivity sensing measurement. However, most of the current sensing measurement based on petal-shaped interference patterns adopts a method for measuring the rotation angle of a light spot to estimate the variation of a physical quantity. In practical application, the center of the petal-shaped light spot is difficult to determine, and particularly, the shape of the light spot is deformed due to external disturbance. And only the magnitude of the change in the physical quantity can be estimated by the rotation angle of the interference pattern, and the rate of change in the physical quantity cannot be obtained.
Disclosure of Invention
The invention solves the technical problems: aiming at the defects in the vortex optical interference-based sensing measurement, a method and a system for measuring the tiny refractive index change rate based on the combination of vortex optical interference and time-frequency analysis are provided, and the method can avoid errors caused by the center of an estimated petal-shaped light spot and can measure the change rate of refractive index which changes at uniform speed and non-uniform speed; the system has simple structure and is easy to realize.
The invention aims at realizing the following technical scheme:
in a first aspect, the present invention provides a method for measuring a micro refractive index change rate based on vortex light interference, including the steps of:
step 1: the laser source emits a fundamental mode gaussian beam that is converted into a vortex beam using a spiral phase plate. The vortex beam is split into two beams by using a beam splitter, wherein one beam is used as a measuring beam, and the other beam is used as a reference beam. The measuring beam passes through the sample to be measured, and the refractive index of the sample to be measured changes slowly.
Step 2: and (3) utilizing a reflector and a beam splitter to make the parity of the reflection times of the reference beam and the measuring beam in the step (1) before reaching the CCD opposite, so that the topological charge signs of the reference beam and the measuring beam are opposite.
Step 3: and converging the reference beam and the measuring beam with opposite topological charge signs into one path, and coaxially interfering to form a rotating petal-shaped interference pattern.
Step 4: the interference pattern is acquired with a CCD at a sampling frequency.
Step 5: and taking the interference pattern acquired at the time of t=0 as a reference image, and calculating the correlation coefficient between each image acquired by the CCD and the reference image by using a normalized cross-correlation method to obtain a relation curve of the correlation coefficient changing along with time.
Step 6: and calculating the time-frequency distribution spectrum of the time change curve of the correlation coefficient by using a time-frequency analysis method to obtain the change frequency of the correlation coefficient at different moments.
Step 7: and calculating the rotation angular velocity of the interference pattern according to the relation between the change frequency of the correlation coefficient and the rotation angular velocity of the interference pattern.
Step 8: and calculating the refractive index change rate according to the relation between the rotation angular velocity of the interference pattern and the refractive index change rate of the sample to be detected.
Step 9: and judging the direction of refractive index change of the sample to be measured according to the rotation direction of the interference pattern in the measuring process.
According to the steps 5-9, the rotation angular velocity of the vortex optical interference pattern is calculated by innovatively adopting a normalized cross-correlation method and a time-frequency analysis method, so that the magnitude and the direction of the refractive index change rate of the medium are measured.
Further, in the step 4, according to nyquist sampling law, in order for the CCD to effectively sample the rotated interference pattern, the sampling frequency of the CCD must be greater than twice the correlation coefficient variation frequency, which depends on the refractive index variation rate of the sample to be measured. Therefore, the sampling frequency of the CCD can be reasonably selected according to the refractive index change rate of a sample to be tested, so that the requirement on the performance of a device is reduced, the waste is avoided, and the measuring range of the CCD can be enlarged under the condition that the performance of the device is allowed.
Further, in the step 5, the reference image may be replaced by an interference image acquired at any time during the measurement. For example, when interference or fault occurs at a certain moment in the measurement process, the previously acquired image is not available, and the interference pattern at the initial moment is not required to be measured again, so that the refractive index change rate of the sample to be measured after the interference occurs can be calculated.
Further, in the step 1, the measuring beam and the reference beam may be replaced by a vortex beam and a sphere beam, and the interference pattern generated in the step 3 is a spiral interference pattern. The method can be used for measuring not only two beams of vortex light with opposite topological charges, but also one beam of vortex light and one beam of Gaussian light. The interference pattern in this case is not petal-shaped but spiral, and both have a rotationally symmetrical structure.
Further, in the step 1, the refractive index of the sample to be measured is changed at a constant speed or is changed at a non-constant speed.
Further, the method is applicable to measuring the rate of change of a physical quantity that enables the phase of the measurement light to be changed, such as the speed of a slowly moving object.
In a second aspect, the present invention provides a system for measuring a micro refractive index change rate based on vortex light interference, comprising: the device comprises a laser light source (1), a spiral phase plate (2), a first beam splitter (3), a space attenuation plate (4), a second beam splitter (5), a first reflector (6), a second reflector (7), a sample to be tested (8), a third beam splitter (9), a polarizing plate (10), a lens (11), a CCD (12) and a computer (13);
the laser light source (1) is used for emitting a Gaussian beam of a fundamental mode;
the spiral phase plate (2) is used for converting the fundamental mode Gaussian beam into a vortex beam;
the first beam splitter (3) is used for splitting the vortex beam into two beams, one beam is used as reference light, and the other beam is used as measuring light;
the space attenuation sheet (4) is arranged behind the first beam splitter (3) and is used for adjusting the light intensity of the reference light or the measuring light so that the light intensity of the reference light and the measuring light before reaching the CCD (12) are equal;
the second beam splitting mirror (5) and the second reflecting mirror (7) are respectively used for changing the directions of the reference light and the measuring light;
the first reflecting mirror (6) is used for introducing an additional reflection, so that the parity of the reflection times of the reference light before reaching the CCD (12) and the measurement light is opposite, and the signs of the topological charges of the reference light before reaching the CCD (12) and the measurement light are opposite;
the sample (8) to be measured is a medium with a slowly-changing refractive index, and a light beam propagates through the medium;
the third beam splitter (9) is used for converging reference light and measuring light into one path to generate interference;
the polaroid (10) is used for enabling the polarization directions of the reference light and the measuring light to be consistent, and increasing the contrast of the interference pattern;
the lens (11) is used for focusing the generated interference pattern on the CCD (12);
-the CCD (12) is used for acquiring a rotating interference pattern;
the computer (13) is connected with the CCD (12) and is used for storing and processing interference patterns acquired by the CCD (12) and analyzing data.
Further, the spiral phase plate (2) may be replaced by a spatial light modulator loaded with phase holograms.
Further, the reference beam and the measuring beam must coaxially interfere after passing through the third beam splitter (9) to generate rotationally symmetrical petal-shaped interference patterns.
Further, the CCD (12) is connected with the computer (13) to perform image acquisition and data analysis at the same time, so that the measurement efficiency can be improved. Meanwhile, the new image can be collected, and the processed image can be deleted, so that the requirement on the storage capacity of a computer can be reduced.
The invention has the following beneficial effects:
(1) According to the invention, the rotation angular velocity of the petal-shaped interference pattern is calculated by adopting a normalized cross-correlation technology and a time-frequency analysis method, and the position of the light spot center in the interference pattern is not required to be determined, so that errors caused by estimating the light spot center when measuring the rotation angle of the interference light spot can be avoided.
(2) Compared with the traditional Fourier transform method, the method can measure the change rate of the refractive index of the sample to be measured at any moment in the change process in real time, whether at a constant speed or at a non-constant speed. The method can be further applied to measurement of the change rate of other physical quantities.
(3) Since the phase distribution of the vortex light on the cross section perpendicular to the optical axis has rotational symmetry, the interference pattern formed by the vortex light interference of two bundles of opposite topological charges and coaxial has rotational symmetry. Compared with the interference of two Gaussian beams, the invention can distinguish the direction of refractive index change according to the rotation direction of the petal-shaped interference pattern.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the description of the embodiments will be briefly described below, it being obvious that the drawings in the following description are only some embodiments of the present invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic diagram of an interference pattern of two beams of vortex light co-axial interference with opposite topological charge signs;
fig. 2 is a schematic diagram of the relationship between the correlation coefficient and the phase difference and the corresponding interference pattern, taking l= ±2 as an example;
FIG. 3 is a schematic diagram of a system for measuring the change rate of a micro refractive index based on vortex light interference according to the present invention;
FIG. 4 is a graph showing the uniform variation of refractive index of a sample to be measured according to embodiment 1 of the present invention;
FIG. 5 is a graph showing the variation of the correlation coefficient corresponding to the uniform variation of the refractive index of the sample to be measured according to embodiment 1 of the present invention;
FIG. 6 is a graph showing the results of time-frequency analysis of a correlation coefficient curve corresponding to uniform variation of refractive index according to embodiment 1 of the present invention;
FIG. 7 is a graph showing the variation of the non-uniform refractive index of a sample to be measured according to embodiment 2 of the present invention;
FIG. 8 is a graph showing the variation of the correlation coefficient corresponding to the non-uniform variation of the refractive index of the sample to be measured according to embodiment 2 of the present invention;
fig. 9 is a graph showing the result of time-frequency analysis of the correlation coefficient curve corresponding to the non-uniform variation of refractive index according to embodiment 2 of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are only some, but not all embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to fall within the scope of the invention.
The invention relates to a method for measuring a tiny refractive index change rate based on vortex light interference, which is briefly described below with reference to the accompanying drawings. A lager-gaussian beam is a commonly used type of vortex beam. For convenience, let the radial quantum number p=0 of the lager-gaussian beam, its complex amplitude distribution can be expressed as:
wherein r, θ, z are three parameters of the cylindrical coordinate system. C (C) pl For the normalization constant, l is the angular vector number of children, also called the topological charge number.
Is the radius of the beam, where w 0 For the base mode beam waist radius +.>
For rayleigh length, λ is wavelength. />
Is a Laguerre polynomial. Phi = (l+2p+1) arctan (z/z) R ) For the Gouy phase shift.
Two vortex light beams with opposite topological charges are used as a reference beam and a measuring beam, and the phase difference between the two light beams is as follows:
wherein, psi is the phase of vortex rotation,
phase delay introduced for the sample to be tested at time t, < >>
For the initial phase difference between the reference beam and the measuring beam when the sample to be measured is not placed, the symbols with the subscripts "-l" and "+l" respectively represent physical quantities corresponding to the eddy currents with topological charges-l and +l. Let Δψ=2mpi (m=0, 1,2, 3.) the angular position θ of the interference pattern speckle center can be found. Fig. 1 is a schematic diagram of petal-shaped interference patterns generated by coaxial interference of two bundles of vortex light with opposite topological charges, corresponding to l= ±1, l= ±2, l= ±5 and l= ±10 from left to right, respectivelyAn interferogram, wherein the color bars represent normalized intensity values. According to formula (2), it is possible to obtain:
when the refractive index of the sample to be measured changes,
and then changes, the size of the change is as follows:
wherein n (t) is the refractive index of the sample to be measured at the moment t, and L is the propagation length of the measuring light in the sample to be measured. n (0),
I.e. the sum of n values when t=0 +.>
Values. The equal sign two sides of the formula (4) can be obtained by differentiating t simultaneously: />
As can be seen from the combination of the formulas (3) and (5), the refractive index change rate
Rotational angular velocity with interference pattern->
Is in direct proportion to:
since the phase difference between the reference light and the measuring light changes, the angular position of the interference pattern bright spot changes accordingly, and the interference pattern rotates. And calculating the correlation coefficient between the interference pattern after rotation and the initial interference pattern by using a normalized cross-correlation method. When the phase difference changes by 2 pi, the interference pattern rotates by 2 pi/2 l angle, coincides with the initial interference pattern, and the correlation coefficient changes by one period and is equal to 1 again.
Fig. 2 is a schematic diagram of the relationship between the correlation coefficient and the phase difference, and the corresponding interferogram, taking l= ±2 as an example. Therefore, the frequency f of change of the correlation coefficient is proportional to the rotational angular velocity of the interference pattern:
when the refractive index of the sample to be measured changes at a non-uniform speed, the refractive index change rates at different moments are different, so that the change frequencies of the correlation coefficients at different moments are also different. At this time, a time-frequency analysis method is required to link the time domain and the frequency domain of the change of the correlation coefficient to obtain the frequency values at different moments. According to the formulas (6) and (7), the change frequency of the correlation coefficient can be obtained, and the refractive index change rate of the sample to be measured can be calculated.
Meanwhile, according to fig. 2, when the vortex light of l= -2 is adopted as the measurement light, the petal-shaped interference pattern rotates counterclockwise as the phase difference increases, which means that the refractive index increases, and the petal-shaped interference pattern rotates counterclockwise; conversely, the phase difference decreases, the refractive index decreases, and the petal-shaped interference pattern rotates clockwise. Therefore, the direction of the refractive index change can be determined from the rotation direction of the petal-shaped interference pattern.
As shown in fig. 3, a system for measuring a tiny refractive index change rate based on vortex light interference of the present invention mainly includes: the device comprises a laser light source 1, a spiral phase plate 2, a first beam splitter 3, a space attenuation plate 4, a second beam splitter 5, a first reflector 6, a second reflector 7, a sample 8 to be tested, a third beam splitter 9, a polaroid 10, lenses 11, CCD12 and a computer 13. The laser source 1 is used for generating a fundamental mode Gaussian beam, and the fundamental mode Gaussian beam is converted into a vortex beam after passing through the spiral phase plate 2. The vortex beam is divided into two beams by the first beam splitter 3, the transmitted light is used as reference light, and the reference light is reflected by the second beam splitter 5, the first reflector 6 and the third beam splitter 9 for three times and reaches the front of the polaroid 10; the reflected light is reflected by the second reflecting mirror 7 as measurement light, and undergoes two reflections before passing through the sample 8 to be measured and the third beam splitter 9 to reach the polarizing plate 10. Therefore, the reference light and the measurement light before reaching the polarizer 10 have opposite topological charge signs, and both have the same polarization direction after being adjusted by the polarizer 10. The position and angle of the third beam splitter 9 are adjusted so that the reference light and the measuring light are coaxially interfered to generate rotationally symmetrical petal-shaped interference patterns. The interference pattern is focused to a CCD12 via a lens 11. The spatial attenuation sheet 4 behind the first beam splitter 3 is adjusted to make the light intensity of the reference light and the measuring light which interfere approximately equal, and the contrast of the interference pattern is optimal. When the refractive index of the sample 8 to be measured is changed slowly, the petal-shaped interference pattern is rotated, and the interference pattern is recorded at a certain sampling frequency by the CCD12 and stored in the computer 13. And judging the increase or decrease of the refractive index according to the rotation direction of the petal-shaped interference pattern in the acquisition process. The computer 13 receives the data collected by the CCD12 and calculates the refractive index change rate of the sample 8 to be measured by a normalized cross correlation method and a time-frequency analysis method.
Example 1: the refractive index of the sample to be measured changes at a constant speed.
Fig. 4 is an embodiment of the present invention. In this embodiment, the refractive index of the sample to be measured changes slowly and at a constant speed, as shown in fig. 4. If the vortex light of l= ±2 is used as reference light and measuring light respectively, the correlation coefficient of the interference pattern at different time and the interference pattern at the initial time is calculated by adopting a normalized cross correlation method, and the result is shown in fig. 5. The time-frequency analysis method adopted by the embodiment of the invention is a short-time Fourier transform method, and the principle is as follows:
where f is the frequency, t and τ are time variables, g (t) is the signal to be processed, s (τ) is a window function, and STFT (t, f) represents the result of performing a short-time Fourier transform on the signal.
The correlation coefficient change curve shown in fig. 5 is subjected to short-time fourier transform, and the result is shown in fig. 6. According to fig. 6, the change frequencies of the correlation coefficient curves at different moments can be obtained, and the change frequencies of the correlation coefficient curves at different moments are equal because the refractive index of the sample to be measured changes at a uniform speed in the embodiment of the invention. Finally, the refractive index change rate of the sample to be detected can be calculated according to the formula (6) and the formula (7). This example demonstrates that the proposed method is effective in measuring the refractive index change rate of a sample having a uniform change in refractive index.
Real-time example 2: the refractive index of the sample to be measured changes at non-uniform speed.
Fig. 7-9 illustrate another embodiment of the present invention. In this embodiment, the refractive index of the sample to be measured changes slowly and non-uniformly, as shown in fig. 7. If the vortex light of l= ±2 is used as reference light and measuring light respectively, the correlation coefficient of the interference pattern at different time and the interference pattern at the initial time is calculated by adopting a normalized cross correlation method, and the result is shown in fig. 8. The time-frequency analysis method adopted in the embodiment of the invention is a short-time Fourier transform method, and the result is shown in FIG. 9 by performing short-time Fourier transform on the correlation coefficient change curve shown in FIG. 8. As can be seen from fig. 9, compared with the conventional fourier transform method, the short-time fourier transform method can obtain not only the frequency information of the correlation coefficient curve but also the time information of the correlation coefficient curve, which provides feasibility for measuring the refractive index change rate in real time and measuring the sample with non-uniform change of the refractive index. According to fig. 9, the change frequencies of the correlation coefficient curves at different moments can be obtained, and the change frequencies of the correlation coefficient curves at different moments are different due to the non-uniform change of the refractive index of the sample to be measured in the embodiment of the invention. Finally, the refractive index change rate of the sample to be detected can be calculated according to the formula (6) and the formula (7). Example 2 shows that the method provided by the invention can measure the refractive index change rate in the non-uniform refractive index change process in real time.
While the invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and substitutions can be made herein without departing from the scope of the invention as defined by the appended claims. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.
Claims (10)
1. A method for measuring a tiny refractive index change rate based on vortex light interference, which is characterized by comprising the following steps:
step 1: converting a fundamental mode Gaussian beam emitted by a laser source into a vortex beam by using a spiral phase plate, and dividing the vortex beam into two beams by using a beam splitter, wherein one beam is used as a measuring beam, and the other beam is used as a reference beam; the measuring beam passes through a sample to be measured, and the refractive index of the sample to be measured changes slowly;
step 2: the reflection times of the reference beam and the measuring beam in the step 1 before reaching the CCD are opposite in parity by using a reflector and a beam splitter, so that topological charge signs of the reference beam and the measuring beam are opposite;
step 3: converging the reference beam and the measuring beam with opposite topological charge signs in the step 2 into one path, and coaxially interfering to form a rotating petal-shaped interference pattern;
step 4: the CCD collects the petal-shaped interference patterns at a set sampling frequency;
step 5: taking an interference pattern acquired at the time of t=0 as a reference image, and calculating a correlation coefficient between each image acquired by the CCD and the reference image by adopting a normalized cross-correlation method to obtain a relation curve of the correlation coefficient changing along with time;
step 6: calculating the time-frequency distribution spectrum of the time change curve of the correlation coefficient by adopting a time-frequency analysis method to obtain the change frequency of the correlation coefficient at different moments;
step 7: calculating the rotation angular velocity of the interference pattern according to the relation between the change frequency of the correlation coefficient at different moments and the rotation angular velocity of the interference pattern in the step 3;
step 8: calculating to obtain the refractive index change rate according to the relation between the rotation angular velocity of the interference pattern and the refractive index change rate of the sample to be detected;
step 9: and judging the direction of refractive index change of the sample to be measured according to the rotation direction of the interference pattern in the measuring process.
2. The method for measuring the tiny refractive index change rate based on vortex light interference according to claim 1, wherein the method comprises the following steps: in the step 4, the sampling frequency of the CCD must be greater than twice the frequency of the change in the correlation coefficient, which depends on the refractive index change rate of the sample to be measured.
3. The method for measuring the micro refractive index change rate based on vortex light interference according to claim 1, wherein in the step 5, the reference image can be replaced by an interference image acquired at any time during the measurement.
4. The method for measuring the tiny refractive index change rate based on vortex light interference according to claim 1, wherein the method comprises the following steps: in the step 1, the measuring beam and the reference beam may be replaced by a vortex beam and a spherical beam, and the interference pattern generated in the step 3 is a spiral interference pattern.
5. The method for measuring a small refractive index change rate based on vortex light interference according to claim 1, wherein in the step 1, the refractive index of the sample to be measured is changed at a constant speed or is changed at a non-constant speed.
6. The method for measuring a minute refractive index change rate based on vortex light interference according to claim 1, wherein the method is adapted to measure a change rate of a physical quantity capable of changing a phase of measurement light.
7. A system for measuring a small refractive index change rate based on vortex light interference, the system comprising: the device comprises a laser light source (1), a spiral phase plate (2), a first beam splitter (3), a space attenuation plate (4), a second beam splitter (5), a first reflector (6), a second reflector (7), a sample to be tested (8), a third beam splitter (9), a polarizing plate (10), a lens (11), a CCD (12) and a computer (13);
the laser light source (1) is used for emitting a Gaussian beam of a fundamental mode;
the spiral phase plate (2) is used for converting the fundamental mode Gaussian beam into a vortex beam;
the first beam splitter (3) is used for splitting the vortex beam into two beams, one beam is used as reference light, and the other beam is used as measuring light;
the space attenuation sheet (4) is arranged behind the first beam splitter (3) and is used for adjusting the light intensity of the reference light or the measuring light so that the light intensity of the reference light and the measuring light before reaching the CCD (12) are equal;
the second beam splitting mirror (5) and the second reflecting mirror (7) are respectively used for changing the directions of the reference light and the measuring light;
the first reflecting mirror (6) is used for introducing an additional reflection, so that the parity of the reflection times of the reference light before reaching the CCD (12) and the measurement light is opposite, and the signs of the topological charges of the reference light before reaching the CCD (12) and the measurement light are opposite;
the sample (8) to be measured is a medium with a slowly-changing refractive index, and a light beam can pass through the medium;
the third beam splitter (9) is used for converging reference light and measuring light into one path to generate interference;
the polaroid (10) is used for enabling the polarization directions of the reference light and the measuring light to be consistent, and increasing the contrast of the interference pattern;
the lens (11) is used for focusing the generated interference pattern on the CCD (12);
-the CCD (12) is used for acquiring a rotating interference pattern;
the computer (13) is connected with the CCD (12) and is used for storing and processing interference patterns acquired by the CCD (12) and analyzing data.
8. The system for measuring the tiny refractive index change rate based on vortex light interference according to claim 7, wherein: the spiral phase plate (2) may be replaced by a spatial light modulator loaded with phase holograms.
9. The system for measuring the tiny refractive index change rate based on vortex light interference according to claim 7, wherein: the reference beam and the measuring beam must coaxially interfere after passing through a third beam splitter (9) to generate rotationally symmetrical petal-shaped interference patterns.
10. The system for measuring the tiny refractive index change rate based on vortex light interference according to claim 7, wherein: the CCD (12) is connected with the computer (13) and can perform image acquisition and data analysis at the same time, so that the measurement efficiency is improved.
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