Device and method for high-precision measurement of tiny refractive index change

Abstract

The invention belongs to the field of optical measurement, and particularly relates to a device and a method for high-precision measurement of tiny refractive index change. The invention aims to provide a device capable of efficiently measuring the change of the refractive index. The device comprises a laser light source, a half-wave plate, a beam splitter prism, a polarizer, a reflector, a sample to be detected, a photoelastic modulator, a photoelectric detector, a signal acquisition and digital phase locking module and a computer. Laser is split into reference light and measuring light, the measuring light and a sample act to generate phase change, the two light beams are combined and then periodic phase modulation is applied through the photoelastic modulator, the photoelectric detector converts an optical signal into an electric signal, the signal acquisition and digital phase locking module synchronously acquires and processes the signal and transmits the signal to the computer, and the computer reversely deduces the refractive index variation. Through precise polarization regulation and control and modulation and phase-locking phase-source design, the measurement precision and the anti-interference capability are improved, the response is fast, the structure is simple and easy to integrate, and the device and the method are suitable for micro refractive index change detection in multiple fields such as biosensing and material characterization.

Description

Technical Field

[0001] This invention relates to the field of optical measurement, and more particularly to a device and method for high-precision measurement of minute refractive index changes. Background Technology

[0002] Precise measurement of minute refractive index changes is a core technology in fields such as biosensing, optical material characterization, and environmental trace detection. For example, in the detection of biomolecular interactions, the binding of target molecules to the sensing interface causes minute changes in the refractive index of the medium (typically as low as 10). -6 The precise detection of this change (on the order of magnitude) directly determines the measurement accuracy of molecular interaction dynamics parameters. In the fabrication of optical functional materials, the minute refractive index inhomogeneities corresponding to the internal stress distribution also require high-precision measurement to achieve early defect identification. Therefore, high-sensitivity and high-stability measurement of minute refractive index changes has become a key requirement for technological development in related fields.

[0003] Existing methods for measuring minute refractive index changes mainly include the Michelson interferometry, fiber Bragg grating method, and electro-optic modulation interferometry. Among these, the Michelson interferometry is susceptible to external interference such as environmental vibrations and temperature drift, making it difficult to achieve high measurement accuracy. -5 The fiber Bragg grating method relies on the wavelength shift detection of the grating, and its resolution is limited by the accuracy of the spectrometer, and its response speed is relatively slow. Even with the use of elastic-optical modulation related measurement devices, existing schemes suffer from problems such as simple design of optical path polarization control links and low polarization state matching between reference light and measurement light, resulting in insufficient signal-to-noise ratio of interference signals. At the same time, the driving signal of elastic-optical modulation and the reference signal of phase-locked loop processing are not from the same source, introducing additional phase noise, which further restricts the improvement of measurement accuracy. Summary of the Invention

[0004] The purpose of this invention is to provide a device and method for high-precision measurement of minute refractive index changes, and to provide a device capable of efficiently measuring refractive index changes.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a device for high-precision measurement of minute refractive index changes, comprising a laser source, a half-wave plate, a non-polarizing beam splitter, a first polarizer, a second polarizer, a first mirror, a second mirror, a sample to be tested, a polarizing beam splitter, an elastic-optical modulator, an analyzer, a photodetector, a signal acquisition and digital phase-locked loop module, and a computer. The elastic-optical modulator includes an elastic-optical crystal and an LC resonant high-voltage driving circuit. The LC resonant high-voltage driving circuit converts an external driving electrical signal into periodic mechanical stress. The elastic-optical crystal is a crystal whose refractive index changes periodically after being subjected to periodic mechanical stress. The signal acquisition and digital phase-locked loop module includes a field-programmable gate array (FPGA) and an acquisition unit. The FPGA is electrically connected to the acquisition unit, and the acquisition unit is signal-connected to the photodetector. The acquisition unit converts the optical signal output by the photodetector into an electrical signal. The FPGA is electrically connected to the LC resonant high-voltage driving circuit. The array is used to output control signals to the LC resonant high-voltage drive circuit; the field-programmable gate array is also electrically connected to the computer, which is used to transmit data to the computer, and the computer is used to send instructions to the field-programmable gate array; the laser source, half-wave plate, and non-polarizing beam splitter are arranged sequentially along the optical path, and the two optical output ends of the non-polarizing beam splitter are connected to the optical input ends of the first polarizer and the second polarizer, respectively; the optical output end of the first polarizer is connected to the optical incident end of the first mirror, and the optical output end of the second polarizer, the optical incident end of the second mirror, the optical incident end of the sample to be tested, and the optical exit end of the sample to be tested are arranged sequentially along the optical path; the optical exit end of the first mirror and the optical exit end of the sample to be tested are both connected to the optical input end of the polarizing beam splitter; the optical output end of the polarizing beam splitter, the optical input end of the elastic modulator, the optical input end of the analyzer, and the optical input end of the photodetector are arranged sequentially along the optical path.

[0006] The laser source is a linearly polarized light output source with monochromaticity greater than a first preset threshold. The laser source is used to provide the incident light required for measurement.

[0007] The half-wave plate is a phase adjustment element with a light intensity transmittance greater than a second preset threshold. The non-polarizing beam splitter is used to split the light intensity according to a fixed splitting ratio. The beam splitting process is independent of the polarization angle of the incident light. After splitting, the polarization states of the two beams are consistent with the incident light.

[0008] The first polarizer, the second polarizer, and the analyzer are all high extinction ratio polarizers. The first polarizer is used to adjust the polarization direction of the reference light, the second polarizer is used to adjust the polarization direction of the measurement light, and the analyzer is used to adjust the polarization direction of the interference light after beam combining. The reference light is the beam that is transmitted to the first polarizer after the incident light has been split by the non-polarizing beam splitter.

[0009] The photoelastic modulator is a long rod-shaped structure driven by a single piezoelectric quartz crystal, and the photoelastic crystal is an isotropic material. The LC resonant high-voltage drive circuit receives the electrical signal output from the field programmable gate array (FPGA). The electrical signal output from the FPGA serves as the modulation drive signal for the photoelastic modulator and as a reference signal for signal acquisition and digital phase-locked loop (PLL) processing.

[0010] The acquisition unit is used to synchronously acquire the DC and AC components of the electrical signal. The field-programmable gate array (FPGA) is used to perform digital phase-locked loop (PLL) processing on the acquired signal. The FPGA is also used to register the DC component data and transmit the registered DC component data and the PLL-processed data to the computer in a coordinated manner.

[0011] This invention also provides a method for high-precision measurement of minute refractive index changes. Based on the aforementioned apparatus for high-precision measurement of minute refractive index changes, the method includes: S1, a laser source emits linearly polarized light, which is then incident on a non-polarizing beam splitter after being adjusted by a half-wave plate. The non-polarizing beam splitter splits the linearly polarized light into a reference beam and a measurement beam; S2, the reference beam, after having its polarization direction adjusted by a first polarizer, is incident on a first reflecting mirror and, after being reflected by the first reflecting mirror, is incident on a polarizing beam splitter; the measurement beam, after having its polarization direction adjusted by a second polarizer, is incident on a second reflecting mirror and, after being reflected by the second reflecting mirror, is incident on a... The sample to be tested undergoes a phase change due to its interaction with the sample and is then incident on a polarizing beam splitter. The reference beam and the measurement beam are combined at the polarizing beam splitter to form interference light. S3: The interference light is periodically phase modulated by an elastic-optical modulator, and its polarization direction is adjusted by an analyzer before being incident on a photodetector. The photodetector converts the interference light into an electrical signal. S4: The electrical signal is transmitted to the acquisition unit of the signal acquisition and digital phase-locked loop module. After conversion by the acquisition unit, it is transmitted to a field-programmable gate array (FPGA) for digital phase-locked loop processing. The computer calculates the refractive index change of the sample to be tested based on the processed data.

[0012] In step S2, the polarization direction of the reference light after being adjusted by the first polarizer is perpendicular to the polarization direction of the measurement light after being adjusted by the second polarizer, and the phase change of the measurement light is determined by the refractive index distribution of the sample to be tested.

[0013] In step S3, the periodic phase modulation of the interference light by the elastic-optical modulator is achieved based on the characteristics of the Bessel function. The periodic phase modulation is used to establish a quantitative correlation between the phase change of the interference light and the refractive index change of the sample to be tested.

[0014] In step S4, digital phase-locked loop processing is used to extract the amplitude of multiple harmonics in the AC component. Based on the correlation between the amplitude of multiple harmonics and the phase change, the computer inversely calculates the amount of refractive index change of the sample to be tested.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the device provided in the embodiments of this application, the sample to be tested only needs to be connected to the optical path between the second reflecting mirror and the polarizing beam splitter. No special modification or fixation of the sample is required. It is suitable for different forms of test media such as liquids and solids. At the same time, the core components such as the laser source and polarizer are all general optical devices, which are easy to replace and adjust according to different measurement requirements (such as different wavelengths). It can be widely used in the measurement of small refractive index changes in multiple fields such as biosensing, material characterization, and environmental monitoring.

[0016] 2. The electrical signal output from the field-programmable gate array (FPGA) in the device serves simultaneously as the modulation drive signal for the photoelectric modulator and the reference signal for digital phase-locked loop (PLL) processing. This ensures that the modulation and PLL signals originate from the same source, eliminating additional phase noise introduced by signals from different sources. Simultaneously, the non-polarizing beam splitter maintains the polarization state of the incident light during beam splitting, and the coordination between the half-wave plate and the high extinction ratio polarizer further reduces the interference of environmental vibrations and temperature drift on the optical path polarization state, enabling the device to maintain stable measurement performance even under complex operating conditions. In the signal acquisition and digital PLL modules, the acquisition unit synchronously acquires the DC and AC components of the electrical signal. The FPGA performs rapid digital PLL processing and coordinates data transmission, avoiding the delay problem of traditional step-by-step acquisition. The photoelectric modulator adopts a long rod structure driven by a single piezoelectric experiment. The refractive index response speed of its photoelectric crystal is faster than that of the electro-optic modulation element, reducing the response time of the entire measurement process to the millisecond level. Furthermore, the storage of DC component data provides a basis for subsequent data verification, improving the reliability of the measurement results. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a device for high-precision measurement of minute refractive index changes provided in an embodiment of the present invention.

[0018] In this array, 1 is a laser source, 2 is a half-wave plate, 3 is a non-polarizing beam splitter, 4 is a first polarizer, 5 is a second polarizer, 6 is a first reflector, 7 is a second reflector, 8 is the sample to be tested, 9 is a polarizing beam splitter, 10 is a light-elastic modulator, 11 is a polarizer, 12 is a photodetector, 13 is a signal acquisition and digital phase-locked loop module, and 14 is a computer. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] For example, refer to Figure 1This application provides a device for high-precision measurement of minute refractive index changes, including a laser source 1, a half-wave plate 2, a non-polarizing beam splitter 3, a first polarizer 4, a second polarizer 5, a first reflector 6, a second reflector 7, a sample to be tested 8, a polarizing beam splitter 9, an elastic-optical modulator 10, an analyzer 11, a photodetector 12, a signal acquisition and digital phase-locked loop module 13, and a computer 14.

[0021] The optical-elastic modulator 10 includes an optical-elastic crystal and an LC resonant high-voltage drive circuit. The LC resonant high-voltage drive circuit converts external driving electrical signals into periodic mechanical stress. The optical-elastic crystal is a crystal whose refractive index changes periodically after being subjected to periodic mechanical stress. The signal acquisition and digital phase-locked loop module 13 includes a field-programmable gate array (FPGA) and an acquisition unit (AD). The FPGA is electrically connected to the acquisition unit, and the acquisition unit is signal-connected to the photodetector 12. The acquisition unit converts the optical signal output by the photodetector 12 into an electrical signal. The FPGA is also electrically connected to the LC resonant high-voltage drive circuit and outputs control signals to the LC resonant high-voltage drive circuit. The FPGA is also electrically connected to a computer 14, which transmits data to the computer 14, and the computer 14 sends instructions to the FPGA.

[0022] The laser source 1, the half-wave plate 2, and the non-polarizing beam splitter 3 are arranged sequentially along the optical path. The two optical output ends of the non-polarizing beam splitter 3 are connected to the optical input ends of the first polarizer 4 and the second polarizer 5, respectively. The optical output end of the first polarizer 4 is connected to the optical incident end of the first reflector 6. The optical output end of the second polarizer 5, the optical incident end of the second reflector 7, the optical incident end of the sample to be tested 8, and the optical output end of the sample to be tested 8 are arranged sequentially along the optical path. The optical output ends of the first reflector 6 and the optical output ends of the sample to be tested 8 are both connected to the optical input end of the polarizing beam splitter 9. The optical output end of the polarizing beam splitter 9, the optical input end of the elastic modulator 10, the optical input end of the analyzer 11, and the optical input end of the photodetector 12 are arranged sequentially along the optical path.

[0023] The linearly polarized light emitted from the laser source 1 is adjusted by the half-wave plate 2 and then split into a reference beam and a measurement beam by the non-polarizing beam splitter prism 3. The reference beam is polarized by the first polarizer 4 and then reflected by the first mirror 6 to the polarizing beam splitter 9. The measurement beam is polarized by the second polarizer 5 and then reflected by the second mirror 7. It then interacts with the sample 8 to be tested, producing a phase change related to the refractive index distribution. Subsequently, it is incident on the polarizing beam splitter prism 9 and combined with the reference beam to form interference light. The interference light is periodically phase modulated by the photoelectric modulator 10 based on the characteristics of the Bessel function, and the polarization direction is adjusted by the analyzer 11. The interference light is then converted into an electrical signal by the photodetector 12. The electrical signal is converted by the acquisition unit of the signal acquisition and digital phase-locked loop module 13, and then digitally phase-locked by the field programmable gate array (synchronously extracting the amplitude of multiple harmonics). The processed data is transmitted to the computer 14. The computer 14 uses the correlation between the amplitude of multiple harmonics and the phase change to infer the amount of refractive index change of the sample to be tested, thus completing the high-precision detection of minute refractive index changes.

[0024] In some embodiments, the laser source 1 is a linearly polarized light output source with monochromaticity greater than a first preset threshold, and the laser source 1 is used to provide the incident light required for measurement. For example, the laser source 1 is a helium-neon laser source with a wavelength of 632.8 nm, which has excellent monochromaticity and high stability.

[0025] In some embodiments, the half-wave plate 2 is a phase-modulating element with a light intensity transmittance greater than a second preset threshold, which does not significantly attenuate or distort the light intensity of the incident light. For example, the light intensity transmittance should be close to 100%, thereby ensuring the precise response and linear conversion of the elastic modulator 10 to the phase change caused by the change in refractive index. The non-polarizing beam splitter 3 is used to achieve light intensity beam splitting at a fixed splitting ratio (e.g., 1:1), and the beam splitting process is independent of the polarization angle of the incident light. It will not cause the transmission / reflection light intensity ratio to shift due to the difference in polarization state. After beam splitting, the polarization states of the two beams are consistent with the incident light.

[0026] In some embodiments, the first polarizer 4, the second polarizer 5, and the analyzer 11 are all high extinction ratio polarizers. The first polarizer 4 is used to adjust the polarization direction of the reference light, the second polarizer 5 is used to adjust the polarization direction of the measurement light, and the analyzer 11 is used to adjust the polarization direction of the interference light after beam combining. The reference light is the beam that is transmitted to the first polarizer 4 after the incident light is split by the non-polarizing beam splitter 3.

[0027] For example, the first polarizer 4, the second polarizer 5, and the analyzer 11 all adopt calcite Glan Taylor type polarizers with an extinction ratio better than 100000:1. By configuring this high extinction ratio polarization device, the linear response characteristics of the light intensity signal to the phase change can be ensured, providing a stable optical basis for subsequent phase detection and signal analysis. Furthermore, the polarization axes of the first polarizer 4, the second polarizer 5, and the analyzer 11 are set to angles of 90°, 0°, and 45° relative to the fast modulation axis of the photoelastic modulator, respectively.

[0028] The first reflecting mirror 6 and the second reflecting mirror 7 are used to change the propagation path of light and create an optical path difference, and reflect the reference light and the measurement light to the polarization beam splitter 9. The polarization beam splitter 9 can precisely combine the reference orthogonal polarized light, which has been optically calibrated and carries the phase reference, and the probe orthogonal polarized light, which has been modulated by photoelasticity and carries the refractive index information of the sample to be tested, according to the polarization direction, to ensure the strict polarization orthogonality of the two interference beams.

[0029] In some embodiments, the photoelastic modulator 10 is a long rod-shaped structure driven by a single piezoelectric quartz crystal, and the photoelastic crystal is an isotropic material. For example, the photoelastic crystal is an isotropic fused silica, and the resonant frequency is set to 50kHz. The LC resonant high-voltage drive circuit receives the electrical signal output by the field programmable gate array. The electrical signal output by the field programmable gate array serves as the modulation drive signal of the photoelastic modulator and as a reference signal for the signal acquisition and digital phase-locked loop module 13 to perform digital phase-locked loop processing.

[0030] For example, the acquisition unit is used to synchronously acquire the DC component and AC component in the electrical signal, the field-programmable gate array is used to perform digital phase-locked processing on the acquired signal, the field-programmable gate array is also used to register the DC component data, and the registered DC component data and the phase-locked data are collaboratively transmitted to the computer.

[0031] The electrical signal output by the field-programmable gate array (FPGA) is transmitted to the LC resonant high-voltage drive circuit of the photoelastic modulator as the modulation drive signal of the photoelastic modulator. On the other hand, it serves as a reference signal for the signal acquisition and digital phase-locked loop (PLL) module to perform digital phase-locked loop (PLL) processing. The acquisition unit in the signal acquisition and PLL module synchronously acquires the DC and AC components of the electrical signal. The FPGA performs PLL processing on the acquired signal, registers the DC component data, and transmits the registered DC component data and the PLL-processed signal data together to the computer to provide complete data support for the subsequent calculation of the refractive index change.

[0032] The present invention also provides a method for high-precision measurement of minute refractive index changes, based on the above-described apparatus for high-precision measurement of minute refractive index changes, the method comprising: S1. The laser source emits linearly polarized light, which is then adjusted by a half-wave plate and incident on a non-polarized beam splitter. The non-polarized beam splitter splits the linearly polarized light into a reference beam and a measurement beam.

[0033] S2. The reference light is polarized by the first polarizer and then directed to the first mirror. After being reflected by the first mirror, it is incident on the polarization beam splitter. The measurement light is polarized by the second polarizer and then directed to the second mirror. After being reflected by the second mirror, it is directed to the sample to be tested. After being affected by the sample to be tested and undergoing a phase change, it is incident on the polarization beam splitter. The reference light and the measurement light are combined at the polarization beam splitter to form interference light.

[0034] For example, in step S2, the polarization direction of the reference light after being adjusted by the first polarizer is perpendicular to the polarization direction of the measurement light after being adjusted by the second polarizer, and the phase change of the measurement light is determined by the refractive index distribution of the sample to be tested.

[0035] S3. After the interference light is periodically phase modulated by the photoelectric modulator, its polarization direction is adjusted by the analyzer and then it is incident on the photodetector. The photodetector converts the interference light into an electrical signal.

[0036] For example, in step S3, the periodic phase modulation of the interference light by the elastic-optical modulator is achieved based on the characteristics of the Bessel function. The periodic phase modulation is used to establish a quantitative correlation between the phase change of the interference light and the refractive index change of the sample to be detected.

[0037] S4. The electrical signal is transmitted to the acquisition unit of the signal acquisition and digital phase-locked loop module. After being converted by the acquisition unit, it is transmitted to the field programmable gate array for digital phase-locked loop processing. The computer calculates the refractive index change of the sample to be tested based on the processed data.

[0038] For example, in step S4, digital phase-locked loop processing is used to extract the amplitude of multiple harmonics in the AC component. Based on the correlation between the amplitude of multiple harmonics and the phase change, the computer inversely calculates the amount of refractive index change of the sample to be tested.

[0039] In some embodiments, the fast modulation axis of the photoelectric modulator in the reference arm (an independent optical path branch through which the reference light after beam splitting by the non-polarizing beam splitter passes sequentially, including a first polarizer and a first reflector, used to provide a stable phase reference for measurement and counteract the influence of environmental interference on the measurement) is calibrated to be collinear with the horizontal direction; the polarization axes of the first polarizer, the polarization axis of the analyzer at the rear end of the polarizing beam splitter, and the polarization axis of the second polarizer are at 90°, 45°, and 0° to the fast modulation axis of the photoelectric modulator, respectively. The above measurement device uses Jones vectors and Jones matrices to quantitatively describe and mathematically calculate the polarization state and polarization state transmission changes of the optical signal in each optical component. The Jones vector of the incident light after passing through the half-wave plate is: (1) In the above formula The Jones vector of the incident light after passing through the half-wave plate.

[0040] First, let's analyze the reference arm path. The Jones matrix through the unpolarized beam splitter is: (2) In the above formula The Jones matrix is ​​obtained through a non-polarizing beam splitter.

[0041] The Jones matrix through the first biasing device is: (3) In the above formula Let be the Jones matrix passing through the first biasing device.

[0042] The Jones matrix through the polarizing beam splitter is: (4) In the above formula This is the Jones matrix passing through the polarizing beam splitter.

[0043] Therefore, the Jones vector of the emitted light from the reference arm is: (5) In the above formula The Jones vector of the emitted light from the reference arm.

[0044] Analysis of the measuring arm (an independent optical path branch through which the measuring light, after being split by the non-polarizing beam splitter in the device's optical path, sequentially passes; this branch includes a second polarizer, a second mirror, and the sample to be tested, used to cause the measuring light to interact with the sample and generate a phase change, subsequently combining with the reference light to form interference light, providing the core signal source for measuring the refractive index change of the sample) reveals the following Jones matrix through the non-polarizing beam splitter: (6) In the above formula The Jones matrix is ​​obtained through a non-polarizing beam splitter.

[0045] The Jones matrix obtained through the second biasing device is: (7) In the above formula The Jones matrix is ​​obtained by passing through the second biasing device.

[0046] The Jones matrix of the sample to be tested is: (8) In the above formula Let B be the Jones matrix of the sample to be tested, where B is the phase delay of the sample to be tested.

[0047] The Jones matrix through the polarizing beam splitter is: (9) In the above formula This is the Jones matrix passing through the polarizing beam splitter.

[0048] The Jones vector of the emitted light from the measuring arm is: (10) In the above formula The Jones vector is the output light from the measuring arm.

[0049] The Jones vectors of the measuring arm and the reference arm are combined after the polarizing beam splitter, so the Jones vector of the output light is: (11) In the above formula The Jones vector is the output light from the measuring arm.

[0050] Following the polarizing beam splitter, the optical modulator and analyzer are placed in sequence. The Jones matrices of the optical modulator and analyzer are as follows: (12) (13) In the above formula, in the above formula Here is the Jones matrix of the elastic modulator, where This is the phase delay of the elastic-optical modulator. , The phase modulation amplitude of the elastic modulator. It is the modulation angular frequency of the elastic modulator.

[0051] For an elastic-optical modulator, the piezoelectric crystal converts an external driving electrical signal into periodic mechanical stress or strain, which acts on a crystal medium with elastic-optical properties (such as fused silica, lithium niobate, etc.), causing a periodic change in the refractive index within the crystal, thereby altering the polarization state, phase, or amplitude of the transmitted light. After the beam is combined, the output light passes sequentially through the elastic-optical modulator and the analyzer. The Jones vector of the output light is: (14) In the above formula This is the Jones vector of the output light.

[0052] Substituting all equations (1) to (13) into equation (14), we can obtain the Jones vector of the output light as follows: (15) The light intensity passing through the photodetector can be obtained using the light intensity formula: (16) In the above formula The light intensity is the light intensity passing through the photodetector.

[0053] Then, by expanding the first kind of Bessel series, we can obtain the following equation (17): in, and These represent the light intensities emitted from the reference arm and the measuring arm, respectively. , , , as well as These are the 0th, 1st, 2nd, 3rd, and 4th order Bessel series, respectively. It can be seen that the amplitudes of the first, second, third, and fourth harmonic signals obtained through digital phase-locked loop in the AC term are respectively: (18) (19) (20) (twenty one) To obtain the phase delay of the sample to be tested, a ratio R can be defined: (twenty two) In the above formula It is a defined ratio.

[0054] The ratio R is obtained by the ratio of the first harmonic to the second harmonic of the AC term. Substituting equations (18) and (19) into equation (22) yields: (twenty three) The phase delay of the sample to be tested can be obtained using the inverse trigonometric function formula: (twenty four) Based on the relationship between phase retardation and optical path difference, we can obtain: (25) in The optical path difference between the reference arm and the measuring arm. The wavelength of light in a vacuum. Expanding equation (24) yields: (26) Where n is the refractive index of the sample to be tested, and d is the thickness of the sample to be tested.

[0055] According to equation (26), the refractive index formula of the sample to be tested can be obtained as follows: (27) Because the measurement speed of the elastic modulator is very fast, it can only measure the change in refractive index over a period of time, hence equation (28): Refractive index change It can be derived by recording the difference in refractive index changes over a period of time: (29) In the above formula The change in refractive index over a period of time, where and They are respectively and The refractive index at a given time.

[0056] Combining formulas (27), (28), and (29), we can obtain: (30) This allows us to measure the change in refractive index over a period of time.

[0057] The field-programmable gate array (FPGA) in the signal acquisition and digital phase-locked loop (PLL) module generates a fundamental frequency reference signal while simultaneously outputting the input signal to the LC resonant high-voltage drive circuit of the optical modulator. This FPGA multiplies the reference signal with the acquired AC signal, performs accumulation processing, and transmits the result to the computer. Simultaneously, the FPGA controls the acquisition and storage of DC signals and transmits the stored DC signal data to the computer. The computer calculates the phase delay amplitude of the sample using the ratio of the first harmonic amplitude to the second harmonic amplitude. After storing and displaying this phase delay amplitude, the current refractive index of the sample is demodulated based on the real-time acquired phase delay. Furthermore, the FPGA can simultaneously complete the data phase-locked loop processing for other harmonic channels; this measurement device can provide a reference for judging the system's operating status and performance by monitoring the second harmonic term.

[0058] In the description of this specification, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0059] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those 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 determined by the scope of the claims.

Claims

1. A device for high-precision measurement of minute refractive index changes, characterized in that, The system includes a laser source (1), a half-wave plate (2), a non-polarizing beam splitter (3), a first polarizer (4), a second polarizer (5), a first mirror (6), a second mirror (7), a sample to be tested (8), a polarizing beam splitter (9), an elastic-optical modulator (10), an analyzer (11), a photodetector (12), a signal acquisition and digital phase-locked loop module (13), and a computer (14). The elastic-optical modulator (10) includes an elastic-optical crystal and an LC resonant high-voltage driving circuit, which converts the external driving electrical signal into a high-voltage driving circuit. The periodic mechanical stress, wherein the elasto-optic crystal is a crystal whose refractive index changes periodically after being subjected to the periodic mechanical stress; the signal acquisition and digital phase-locked loop module (13) includes a field-programmable gate array (FPGA) and an acquisition unit, wherein the FPGA is electrically connected to the acquisition unit, and the acquisition unit is signal-connected to the photodetector (12), wherein the acquisition unit is used to convert the optical signal output by the photodetector (12) into an electrical signal; the FPGA is electrically connected to the LC resonant high-voltage drive circuit, wherein the FPGA is used to transmit signals to the LC resonant high-voltage drive circuit. Output control signal; the field programmable gate array is also electrically connected to the computer (14), the field programmable gate array is used to transmit data to the computer (14), and the computer (14) is used to send instructions to the field programmable gate array; the laser source (1), half-wave plate (2), and non-polarizing beam splitter (3) are arranged sequentially along the optical path, and the two optical output ends of the non-polarizing beam splitter (3) are respectively connected to the optical input ends of the first polarizer (4) and the second polarizer (5); the optical output end of the first polarizer (4) is connected to the first reflector ( The light path of the light incident end of the second polarizer (5), the light incident end of the second reflector (7), the light incident end of the sample to be tested (8), and the light emitting end of the sample to be tested (8) are arranged in sequence along the light path; the light emitting end of the first reflector (6) and the light emitting end of the sample to be tested (8) are connected to the light input end of the polarizing beam splitter (9); the light output end of the polarizing beam splitter (9), the light input end of the light modulator (10), the light input end of the analyzer (11), and the light input end of the photodetector (12) are arranged in sequence along the light path.

2. The device for high-precision measurement of minute refractive index changes according to claim 1, characterized in that, The laser source (1) is a linearly polarized light output source with monochromaticity greater than a first preset threshold. The laser source (1) is used to provide the incident light required for measurement.

3. The device for high-precision measurement of minute refractive index changes according to claim 1, characterized in that, The half-wave plate (2) is a phase adjustment element with a light intensity transmittance greater than the second preset threshold. The non-polarizing beam splitter (3) is used to achieve light intensity beam splitting according to a fixed beam splitting ratio. The beam splitting process is independent of the incident light polarization angle. After beam splitting, the polarization states of the two beams are consistent with the incident light.

4. The device for high-precision measurement of minute refractive index changes according to claim 1, characterized in that, The first polarizer (4), the second polarizer (5), and the analyzer (11) are all high extinction ratio polarizers. The first polarizer (4) is used to adjust the polarization direction of the reference light, the second polarizer (5) is used to adjust the polarization direction of the measurement light, and the analyzer (11) is used to adjust the polarization direction of the interference light after beam combining. The reference light is a beam that is transmitted to the first polarizer (4) after the incident light is split by the non-polarizing beam splitter (3).

5. The device for high-precision measurement of minute refractive index changes according to claim 1, characterized in that, The photoelastic modulator (10) is a long rod structure driven by a single piezoelectric quartz crystal, and the photoelastic crystal is an isotropic material; the LC resonant high voltage drive circuit receives the electrical signal output by the field programmable gate array, and the electrical signal output by the field programmable gate array serves as the modulation drive signal of the photoelastic modulator, and at the same time as the reference signal for the signal acquisition and digital phase-locked loop module (13) to perform digital phase-locked loop processing.

6. The device for high-precision measurement of minute refractive index changes according to claim 1, characterized in that, The acquisition unit is used to synchronously acquire the DC component and AC component in the electrical signal. The field programmable gate array is used to perform digital phase-locked processing on the acquired signal. The field programmable gate array is also used to register the DC component data and transmit the registered DC component data and the phase-locked data together to the computer (14).

7. A method for high-precision measurement of minute refractive index changes, based on the apparatus for high-precision measurement of minute refractive index changes according to any one of claims 1-6, characterized in that, Includes the following steps: S1. A laser source (1) emits linearly polarized light. The linearly polarized light is adjusted by a half-wave plate (2) and then incident on a non-polarizing beam splitter (3). The non-polarizing beam splitter (3) splits the linearly polarized light into a reference beam and a measurement beam. S2. The reference beam is polarized by a first polarizer (4) and then incident on a first mirror (6). After being reflected by the first mirror (6), the beam is incident on a polarizing beam splitter (9). The measurement beam is polarized by a second polarizer (5) and then incident on a second mirror (7). After being reflected by the second mirror (7), the beam is incident on the sample to be tested (8). The sample to be tested (8) causes a phase change, and the beam is then incident on the sample. The reference light and the measurement light are combined at the polarization beam splitter (9) to form interference light; S3, the interference light is subjected to periodic phase modulation by the elastic light modulator (10), and the polarization direction is adjusted by the analyzer (11) and incident on the photodetector (12). The photodetector (12) converts the interference light into an electrical signal; S4, the electrical signal is transmitted to the acquisition unit of the signal acquisition and digital phase-locked loop module (13), and after being converted by the acquisition unit, it is transmitted to the field programmable gate array for digital phase-locked processing. The computer (14) calculates the refractive index change of the sample (8) to be tested based on the processed data.

8. The method for high-precision measurement of minute refractive index changes according to claim 7, characterized in that, In step S2, the polarization direction of the reference light after being adjusted by the first polarizer (4) is perpendicular to the polarization direction of the measurement light after being adjusted by the second polarizer (5), and the phase change of the measurement light is determined by the refractive index distribution of the sample to be tested.

9. The method for high-precision measurement of minute refractive index changes according to claim 7, characterized in that, In step S3, the periodic phase modulation of the interference light by the elastic-optical modulator (10) is achieved based on the characteristics of the Bessel function. The periodic phase modulation is used to establish a quantitative correlation between the phase change of the interference light and the refractive index change of the sample to be tested (8).

10. The method for high-precision measurement of minute refractive index changes according to claim 7, characterized in that, In step S4, the digital phase-locked loop (PLL) process is used to extract the amplitude of multiple harmonics in the AC component. The computer (14) uses the correlation between the amplitude of multiple harmonics and the phase change to infer the amount of refractive index change of the sample (8) to be tested.

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