A method and system for measuring small refractive index changes based on vortex optical interferometry

Abstract

The present invention discloses a method and system for measuring the rate of change of a small refractive index based on vortex light interference. The present invention utilizes two beams of vortex light with opposite topological charge signs as a reference beam and a measuring beam respectively. After the measuring beam passes through a sample to be measured whose refractive index changes slowly, it merges with the reference beam into one beam to form a rotating petal-shaped interference pattern. The rotational angular velocity of the petal-shaped pattern is proportional to the rate of change of the refractive index of the sample to be measured. The interference pattern is collected and recorded using a CCD at a certain sampling frequency, the image collected by the CCD is processed using a normalized cross-correlation method, and a time-frequency analysis method is introduced to obtain the rotational velocity of the interference pattern, and then the rate of change of the refractive index of the sample to be measured is obtained. The present invention avoids the error introduced by measuring the rotation angle of the interference pattern, can measure the rate of change of the refractive index that changes non-uniformly, and can distinguish the direction of the refractive index change.

Description

Technical Field

[0001] The present invention belongs to the field of optical sensing measurement, and in particular relates to a method and system for measuring minute refractive index change rate. Background Art

[0002] Vortex beams are also called OAM (Orbital Angular Momentum) beams. Their name comes from the fact that Allen et al. first discovered in 1992 that a beam with a spiral phase factor exp(ilθ) has a certain orbital angular momentum during transmission. Where l is the orbital angular momentum quantum number, also known as the topological charge number (TC), and θ is the azimuth angle on the beam cross section. The orbital angular momentum of a light beam can theoretically be infinite, which opens up new degrees of freedom for light beam applications. Furthermore, due to the presence of phase singularities, vortex beams typically have phase singularities and amplitude zeros at their centers. Due to these unique properties, researchers have applied vortex beams to a wide range of fields in recent years, including optical tweezers, optical manipulation, optical trapping, optical wrenches, microscopy, quantum information processing, and optical communications. Because vortex beams have a rotationally symmetric spiral phase structure, they enable highly sensitive sensing and are therefore widely used in optical sensing measurements.

[0003] Coaxial interference of vortex beams with opposite topological charges produces a petal-shaped interference pattern with a rotationally symmetric structure, which can be used to achieve high-sensitivity sensing measurements. However, most current sensing measurements based on petal-shaped interference patterns use the method of measuring the rotation angle of the light spot to estimate the change in the physical quantity. In practical applications, the center of the petal-shaped light spot is difficult to determine, especially when the shape of the light spot is deformed by external disturbances. Moreover, the rotation angle of the interference pattern can only estimate the magnitude of the change in the physical quantity, but not the rate of change. Summary of the Invention

[0004] The technology of the present invention solves the problem: In response to the above-mentioned shortcomings in the vortex light interferometer sensing measurement, a method and system for measuring the rate of change of small refractive index based on the combination of vortex light interferometer and time-frequency analysis are provided. The method can avoid the error caused by estimating the center of the petal-shaped light spot, and can measure the rate of change of the refractive index with uniform and non-uniform changes; the system has a simple structure and is easy to implement.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] In a first aspect, the present invention provides a method for measuring a small refractive index change rate based on vortex optical interferometry, comprising the following steps:

[0007] Step 1: A laser source emits a fundamental mode Gaussian beam, which is converted into a vortex beam using a spiral phase plate. A beam splitter then splits the vortex beam into two beams, one serving as the measurement beam and the other as the reference beam. The measurement beam passes through the sample under test, causing the sample's refractive index to slowly change.

[0008] Step 2: Using a reflector and a beam splitter, the reference beam and the measurement beam described in step 1 undergo opposite numbers of reflections before reaching the CCD, thereby making the topological charges of the reference beam and the measurement beam have opposite signs.

[0009] Step 3: The reference beam and the measurement beam with opposite topological charge signs are merged into one path to cause coaxial interference, thereby forming a rotating petal-shaped interference pattern.

[0010] Step 4: Use CCD to collect interference patterns at a certain sampling frequency.

[0011] Step 5: Using the interference pattern collected at time t=0 as the reference image, the normalized cross-correlation method is used to calculate the correlation coefficient between each image collected by the CCD and the reference image, and a curve showing the relationship between the correlation coefficient and time is obtained.

[0012] Step 6: Calculate the time-frequency distribution spectrum of the time variation curve of the correlation coefficient using a time-frequency analysis method to obtain the variation frequency of the correlation coefficient at different moments.

[0013] Step 7: Calculate the rotational angular velocity of the interference pattern based on the relationship between the frequency of change of the correlation coefficient and the rotational angular velocity of the interference pattern.

[0014] Step 8: Calculate the refractive index change rate based on the relationship between the rotational angular velocity of the interference pattern and the refractive index change rate of the sample to be measured.

[0015] Step 9: Determine the direction of change in the refractive index of the sample to be measured based on the rotation direction of the interference pattern during the measurement process.

[0016] According to steps 5-9, the present invention innovatively uses the normalized cross-correlation method combined with the time-frequency analysis method to calculate the rotational angular velocity of the vortex light interference pattern, thereby measuring the magnitude and direction of the medium refractive index change rate.

[0017] Furthermore, in step 4, according to the Nyquist sampling theorem, in order for the CCD to effectively sample the rotating interference pattern, the CCD sampling frequency 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 CCD sampling frequency can be reasonably selected based on the refractive index variation rate of the sample to be measured, thereby reducing the requirements for device performance and avoiding waste. At the same time, it can also increase the measurement range of the present invention under the condition that the device performance allows.

[0018] Furthermore, in step 5, the reference image can be replaced with an interference image acquired at any point during the measurement process. For example, if interference or a fault occurs at some point during the measurement process, rendering the previously acquired image unusable, there is no need to re-measure the interference pattern at the initial moment; the refractive index change rate of the sample under test after the interference occurs can still be calculated.

[0019] Furthermore, in step 1, the measurement beam and reference beam can be replaced by a vortex beam and a spherical beam. In this case, the interference pattern generated in step 3 is a spiral interference pattern. This method can be used not only with two vortex beams of opposite topological charge, but also with a vortex beam and a Gaussian beam. In this case, the interference pattern is not petal-shaped, but spiral, and both have a rotationally symmetric structure.

[0020] Furthermore, in step 1, the refractive index of the sample to be measured changes at a uniform rate or changes at a non-uniform rate.

[0021] Furthermore, the method is applicable to measuring the rate of change of a physical quantity that can cause the phase of the measurement light to change, such as the speed of a slowly moving object.

[0022] In a second aspect, the present invention provides a system for measuring the rate of change of minute refractive index based on vortex optical interferometry, comprising: a laser light source (1), a spiral phase plate (2), a first beam splitter (3), a spatial attenuation plate (4), a second beam splitter (5), a first reflector (6), a second reflector (7), a sample to be measured (8), a third beam splitter (9), a polarizer (10), a lens (11), a CCD (12), and a computer (13);

[0023] The laser light source (1) is used to emit a fundamental mode Gaussian beam;

[0024] The spiral phase plate (2) is used to convert the fundamental mode Gaussian beam into a vortex beam;

[0025] The first beam splitter (3) is used to split the vortex light beam into two beams, one beam is used as reference light, and the other beam is used as measurement light;

[0026] The spatial attenuation plate (4) is placed behind the first beam splitter (3) and is used to adjust the intensity of the reference light or the measurement light so that the intensity of the reference light and the measurement light before reaching the CCD (12) are equal;

[0027] The second beam splitter (5) and the second reflector (7) are used to change the directions of the reference light and the measurement light respectively;

[0028] The first reflector (6) is used to introduce an extra reflection, so that the parity of the reflection times of the reference light and the measurement light before reaching the CCD (12) is opposite, thereby making the signs of the topological charges of the reference light and the measurement light before reaching the CCD (12) opposite;

[0029] The sample to be measured (8) is a medium with a slowly changing refractive index, and the light beam propagates through the medium;

[0030] The third beam splitter (9) is used to merge the reference light and the measurement light into one path to cause interference;

[0031] The polarizer (10) is used to make the polarization directions of the reference light and the measurement light consistent, thereby increasing the contrast of the interference pattern;

[0032] The lens (11) is used to focus the generated interference pattern onto the CCD (12);

[0033] The CCD (12) is used to collect the rotating interference pattern;

[0034] The computer (13) is connected to the CCD (12) and is used for storing and processing the interference pattern collected by the CCD (12) and performing data analysis.

[0035] Furthermore, the spiral phase plate (2) can be replaced by a spatial light modulator loaded with a phase hologram.

[0036] Furthermore, the reference beam and the measuring beam must coaxially interfere after passing through the third beam splitter (9) to generate a rotationally symmetrical petal-shaped interference pattern.

[0037] Furthermore, the CCD (12) is connected to the computer (13) to simultaneously perform image acquisition and data analysis, thereby improving measurement efficiency. At the same time, new images can be acquired while processed images are deleted, thereby reducing the requirement for computer storage capacity.

[0038] The present invention has the following beneficial effects:

[0039] (1) The present invention uses normalized cross-correlation technology and time-frequency analysis method to calculate the rotational angular velocity of the petal-shaped interference pattern, without having to determine the position of the center of the light spot in the interference pattern. Therefore, it can avoid the error introduced by estimating the center of the light spot when measuring the rotation angle of the interference spot.

[0040] (2) The present invention uses a time-frequency analysis method to link the changing frequency of the petal-shaped interference pattern with the time domain. Compared to the traditional Fourier transform method, the present invention can measure the rate of change of the refractive index of the sample under test at any moment in real time, whether it is uniform or non-uniform. The method can also be extended to measure the rate of change of other physical quantities.

[0041] (3) Because the phase distribution of vortex light on a cross section perpendicular to the optical axis is rotationally symmetric, the interference pattern formed by the interference of two coaxial vortex light beams with opposite topological charges is rotationally symmetric. Compared to using two Gaussian light beams for interference, the present invention can discern the direction of refractive index change based on the rotation direction of the petal-shaped interference pattern. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0043] Figure 1 Schematic diagram of the interference pattern of two vortex light beams with opposite topological charge signs coaxially interfering;

[0044] Figure 2 Schematic diagram of the relationship between the correlation coefficient and the phase difference and the corresponding interference pattern, taking l = ±2 as an example;

[0045] Figure 3 Schematic diagram of a system for measuring minute refractive index changes based on vortex optical interferometry according to the present invention;

[0046] Figure 4 A curve showing a uniform change in the refractive index of the sample to be tested provided in Example 1 of the present invention;

[0047] Figure 5 The correlation coefficient change curve corresponding to the uniform change of the refractive index of the sample to be tested provided in Example 1 of the present invention;

[0048] Figure 6 This is a graph showing the result of time-frequency analysis of the correlation coefficient curve corresponding to the uniform change of the refractive index provided in Example 1 of the present invention;

[0049] Figure 7 A curve showing a non-uniform change in the refractive index of a sample to be tested provided in Example 2 of the present invention;

[0050] Figure 8The correlation coefficient change curve corresponding to the non-uniform change of the refractive index of the sample to be tested provided in Example 2 of the present invention;

[0051] Figure 9 This is the result of time-frequency analysis of the correlation coefficient curve corresponding to the non-uniform change of the refractive index provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0052] The following is a clear and complete description of the technical solutions in the embodiments of the present invention, in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0053] The following, in conjunction with the accompanying drawings, briefly describes a method for measuring small refractive index changes based on vortex light interferometry. A Laguerre-Gaussian beam is a commonly used vortex beam. For convenience, assuming the radial quantum number p of the Laguerre-Gaussian beam is 0, its complex amplitude distribution can be expressed as:

[0054]

[0055] Among them, r, θ, z are the three parameters of the cylindrical coordinate system. pl is the normalization constant, and l is the angular quanta number, also known as the topological charge. is the radius of the beam, where w0 is the waist radius of the fundamental mode, is the Rayleigh length and λ is the wavelength. is the Laguerre polynomial. φ=(l+2p+1)arctan(z / z R ) is the Gouy phase shift.

[0056] Two vortex beams with opposite topological charges are used as the reference beam and the measurement beam. The phase difference between the two beams is:

[0057]

[0058] Where ψ is the phase of the vortex light, is the phase delay introduced by the sample to be measured at time t, is the initial phase difference between the reference and measurement beams without the sample being measured. The subscripts "-l" and "+l" denote the quantities corresponding to vortex light with topological charges of -l and +l, respectively. Setting Δψ = 2mπ (m = 0, 1, 2, 3, ...), we can determine the angular position θ of the center of the interference pattern's bright spot. Figure 1Schematic diagram of the petal-shaped interference pattern produced by the coaxial interference of two vortex beams with opposite topological charges. From left to right, the interference patterns correspond to l = ±1, l = ±2, l = ±5, and l = ±10, respectively. The color bar represents the normalized intensity value. According to equation (2), we can obtain:

[0059]

[0060] When the refractive index of the sample to be measured changes, It changes accordingly, and the magnitude of the change is:

[0061]

[0062] Where n(t) is the refractive index of the sample to be measured at time t, and L is the length of the measuring light propagating in the sample to be measured. That is, the value of n at t = 0 and Value. Differentiating both sides of the equation (4) with respect to t yields:

[0063]

[0064] Combining equations (3) and (5), we can see that the refractive index change rate is The angular velocity of the interference pattern Proportional to:

[0065]

[0066] As the phase difference between the reference and measurement beams changes, the angular position of the interference pattern's bright spot shifts, causing the interference pattern to rotate. The normalized cross-correlation method is used to calculate the correlation coefficient between the rotated interference pattern and the initial interference pattern. For every 2π change in the phase difference, the interference pattern rotates by 2π / 2l, coinciding with the initial interference pattern. The correlation coefficient changes by one cycle, returning to 1.

[0067] like Figure 2 The figure shows the relationship between the correlation coefficient and the phase difference, taking l = ±2 as an example, and the corresponding interference pattern. Therefore, the frequency f of the correlation coefficient is proportional to the angular velocity of the interference pattern:

[0068]

[0069] When the refractive index of the sample under test changes at a non-uniform rate, the rate of change varies at different times, and therefore the frequency of change of the correlation coefficient also varies at different times. In this case, a time-frequency analysis method is needed to link the time domain and frequency domain of the correlation coefficient change to obtain the frequency values at different times. Using equations (6) and (7), the frequency of change of the correlation coefficient can be obtained, and the rate of change of the refractive index of the sample under test can be calculated.

[0070] At the same time, according to Figure 2 As shown in the figure, when vortex light with l = -2 is used as the measurement light, the petal-shaped interference pattern rotates counterclockwise as the phase difference increases, indicating that the refractive index increases and the petal-shaped interference pattern rotates counterclockwise. Conversely, when 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 based on the rotation direction of the petal-shaped interference pattern.

[0071] like Figure 3 As shown, the present invention provides a system for measuring small refractive index changes based on vortex light interferometry, which mainly includes: a laser light source 1, a spiral phase plate 2, a first beam splitter 3, a spatial attenuation plate 4, a second beam splitter 5, a first reflector 6, a second reflector 7, a sample to be measured 8, a third beam splitter 9, a polarizer 10, a lens 11, a CCD 12, and a computer 13. The laser light source 1 is used to generate a fundamental mode Gaussian beam, which is converted into a vortex beam after passing through the spiral phase plate 2. The vortex beam is split into two beams by the first beam splitter 3. The transmitted light serves as the reference light and is reflected three times by the second beam splitter 5, the first reflector 6, and the third beam splitter 9 before reaching the polarizer 10. The reflected light serves as the measurement light and is reflected by the second reflector 7, passes through the sample to be measured 8 and the third beam splitter 9, and reaches the polarizer 10, undergoing two reflections in total. Therefore, the topological charge signs of the reference light and the measurement light before reaching the polarizer 10 are opposite. After being adjusted by the polarizer 10, the two have the same polarization direction. Adjust the position and angle of the third beam splitter 9 so that the reference light and the measurement light interfere coaxially, generating a rotationally symmetrical petal-shaped interference pattern. The interference pattern is focused to CCD12 via lens 11. Adjust the spatial attenuation plate 4 behind the first beam splitter 3 so that the intensities of the interfering reference light and the measurement light are roughly equal, and the contrast of the interference pattern is optimal. When the refractive index of the sample 8 to be tested changes slowly, the petal-shaped interference pattern rotates, and the interference pattern is recorded by CCD12 at a certain sampling frequency and saved to computer 13. Based on the rotation direction of the petal-shaped interference pattern during the acquisition process, it is determined whether the refractive index increases or decreases. While receiving the data collected by CCD12, the computer 13 calculates the refractive index change rate of the sample 8 to be tested using the normalized cross-correlation method and the time-frequency analysis method.

[0072] Example 1: The refractive index of the sample to be tested changes at a uniform rate.

[0073] Figure 4 This is an embodiment of the present invention. In this embodiment, the refractive index of the sample to be measured changes slowly and uniformly, such as Figure 4 If the vortex light with l = ±2 is used as the reference light and the measurement light respectively, the normalized cross-correlation method is used to calculate the correlation coefficient between the interference pattern at different times and the interference pattern at the initial time, and the result is as follows: Figure 5 The time-frequency analysis method adopted in the embodiment of the present invention is the short-time Fourier transform method, and its principle is as follows:

[0074]

[0075] Where f is the frequency, t and τ are time variables, g(t) is the signal to be processed, s(τ) is the window function, and STFT(t,f) represents the result of short-time Fourier transform of the signal.

[0076] right Figure 5 The correlation coefficient change curve shown in the figure is subjected to short-time Fourier transform, and the result is as follows Figure 6 As shown. Figure 6 The frequency of change of the correlation coefficient curve at different times can be obtained. Since the refractive index of the sample to be measured in the embodiment of the present invention changes at a uniform rate, the frequency of change of the correlation coefficient curve at different times is equal. Finally, the refractive index change rate of the sample to be measured can be calculated according to equations (6) and (7). This example proves that the method proposed by the present invention can effectively measure the refractive index change rate of a sample with a uniform refractive index change.

[0077] Real-time example 2: The refractive index of the sample to be measured changes at a non-uniform rate.

[0078] Figure 7-9 This is another embodiment of the present invention. In this embodiment, the refractive index of the sample to be measured changes slowly and non-uniformly, such as Figure 7 If the vortex light with l = ±2 is used as the reference light and the measurement light respectively, the normalized cross-correlation method is used to calculate the correlation coefficient between the interference pattern at different times and the interference pattern at the initial time, and the result is as follows: Figure 8 The time-frequency analysis method used in the embodiment of the present invention is the short-time Fourier transform method. Figure 8 The correlation coefficient change curve shown in the figure is subjected to short-time Fourier transform, and the result is as follows Figure 9 As shown. Figure 9 It can be seen that compared with the traditional Fourier transform method, the short-time Fourier transform method can not only obtain the frequency information of the correlation coefficient curve, but also the time information of the correlation coefficient curve, which provides feasibility for real-time measurement of the refractive index change rate and measurement of samples with non-uniform refractive index changes. Figure 9 The changing frequencies of the correlation coefficient curves at different moments can be obtained. Since the refractive index of the sample to be measured in the embodiment of the present invention changes at a non-uniform rate, the changing frequencies of the correlation coefficient curves at different moments are different. Finally, the refractive index change rate of the sample to be measured can be calculated according to equations (6) and (7). Example 2 shows that the method proposed by the present invention can measure the refractive index change rate in a process of non-uniform refractive index change in real time.

[0079] The above description is merely a partial list of specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for measuring small refractive index changes based on vortex optical interferometry, characterized in that: The method comprises the following steps: Step 1: A spiral phase plate is used to convert the fundamental mode Gaussian beam emitted by the laser light source into a vortex beam. A beam splitter then splits the vortex beam into two beams, one of which serves as a measurement beam and the other as a reference beam. The measurement beam passes through the sample to be measured, causing the refractive index of the sample to be measured to slowly change. Step 2: Using a reflector and a beam splitter, the reference beam and the measurement beam in step 1 are made to have opposite parity in the number of reflections before reaching the CCD, thereby making the topological charges of the reference beam and the measurement beam have opposite signs; Step 3: The reference beam and the measurement beam with opposite topological charge signs in step 2 are combined into one path to generate coaxial interference, forming a rotating petal-shaped interference pattern. Step 4: The CCD collects the petal-shaped interference pattern at a set sampling frequency; Step 5: Using the interference pattern collected at time t=0 as the reference image, the normalized cross-correlation method is used to calculate the correlation coefficient between each image collected by the CCD and the reference image, and a curve showing the relationship between the correlation coefficient and time is obtained; Step 6: Calculate the time-frequency distribution spectrum of the time variation curve of the correlation coefficient using a time-frequency analysis method to obtain the variation frequency of the correlation coefficient at different moments; Step 7: Calculate the rotational angular velocity of the interference pattern based on the relationship between the changing frequency of the correlation coefficient at different moments and the rotational angular velocity of the interference pattern described in step 3; Step 8: Calculate the refractive index change rate based on the relationship between the rotational angular velocity of the interference pattern and the refractive index change rate of the sample to be measured; Step 9: Determine the direction of change in the refractive index of the sample to be measured based on the rotation direction of the interference pattern during the measurement process.

2. The method for measuring a small refractive index change rate based on vortex optical interferometry according to claim 1, wherein: In step 4, the sampling frequency of the CCD must be greater than twice the frequency of correlation coefficient change, and the frequency of correlation coefficient change depends on the refractive index change rate of the sample to be measured.

3. The method for measuring a small refractive index change rate based on vortex light interferometry according to claim 1, wherein: In step 5, the reference image can be replaced by an interference image collected at any time during the measurement process.

4. The method for measuring minute refractive index change rate based on vortex optical interferometry according to claim 1, wherein: In step 1, the measuring beam and the reference beam can be replaced by a beam of vortex light and a beam of spherical light. In this case, the interference pattern generated in step 3 is a spiral interference pattern.

5. The method for measuring minute refractive index change rate based on vortex optical interferometry according to claim 1, wherein: In step 1, the refractive index of the sample to be measured changes at a uniform rate or at a non-uniform rate.

6. The method for measuring minute refractive index change rate based on vortex light interferometry according to claim 1, wherein: The method is suitable for measuring the rate of change of a physical quantity that can change the phase of measurement light.

7. A system for measuring minute refractive index changes based on vortex optical interferometry for implementing the method of claim 1, characterized in that: The system comprises: a laser light source (1), a spiral phase plate (2), a first beam splitter (3), a spatial attenuation plate (4), a second beam splitter (5), a first reflector (6), a second reflector (7), a sample to be measured (8), a third beam splitter (9), a polarizer (10), a lens (11), a CCD (12), and a computer (13); The laser light source (1) is used to emit a fundamental mode Gaussian beam; The spiral phase plate (2) is used to convert the fundamental mode Gaussian beam into a vortex beam; The first beam splitter (3) is used to split the vortex light beam into two beams, one beam is used as reference light, and the other beam is used as measurement light; The spatial attenuation plate (4) is placed behind the first beam splitter (3) and is used to adjust the intensity of the reference light or the measurement light so that the intensity of the reference light and the measurement light before reaching the CCD (12) are equal; The second beam splitter (5) and the second reflector (7) are used to change the directions of the reference light and the measurement light respectively; The first reflector (6) is used to introduce an extra reflection, so that the parity of the reflection times of the reference light and the measurement light before reaching the CCD (12) is opposite, thereby making the signs of the topological charges of the reference light and the measurement light before reaching the CCD (12) opposite; The sample to be measured (8) is a medium with a slowly changing refractive index, through which the light beam can propagate; The third beam splitter (9) is used to merge the reference light and the measurement light into one path to cause interference; The polarizer (10) is used to make the polarization directions of the reference light and the measurement light consistent, thereby increasing the contrast of the interference pattern; The lens (11) is used to focus the generated interference pattern onto the CCD (12); The CCD (12) is used to collect the rotating interference pattern; The computer (13) is connected to the CCD (12) and is used for storing and processing the interference pattern collected by the CCD (12) and performing data analysis.

8. The system for measuring minute refractive index changes based on vortex optical interferometry according to claim 7, characterized in that: The spiral phase plate (2) can be replaced by a spatial light modulator loaded with a phase hologram.

9. The system for measuring minute refractive index changes based on vortex optical interferometry according to claim 7, characterized in that: The reference beam and the measuring beam must coaxially interfere with each other after passing through the third beam splitter (9) to generate a rotationally symmetrical petal-shaped interference pattern.

10. The system for measuring minute refractive index changes based on vortex optical interferometry according to claim 7, characterized in that: The CCD (12) is connected to the computer (13) to simultaneously perform image acquisition and data analysis, thereby improving measurement efficiency.

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