Method and apparatus for measuring refractive index based on laser interferometry
By setting accelerometers at both ends of the area to be measured, vibration information is acquired and the length of the area to be measured is calculated. The refractive index and vibration information are separated, which solves the problem of low measurement accuracy in traditional methods and realizes high-precision refractive index measurement.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- INST OF SEMICONDUCTORS - CHINESE ACAD OF SCI
- Filing Date
- 2024-11-29
- Publication Date
- 2026-05-29
AI Technical Summary
Traditional methods for measuring refractive index based on laser interferometry fail to effectively separate refractive index and vibration information in a moving platform environment, resulting in low measurement accuracy.
By setting accelerometers at both ends of the area to be measured, vibration information is acquired and the length of the area to be measured is calculated using acceleration. The refractive index and vibration information are separated, and the phase information is de-wrapped using the arctangent phase demodulation method, thereby improving the measurement accuracy.
It enables high-precision refractive index measurement in a moving platform environment, improves the signal-to-noise ratio, reduces noise and distortion, and improves the accuracy of measurement results.
Smart Images
Figure CN122108521A_ABST
Abstract
Description
Technical Field
[0001] This disclosure relates to the technical field of optical precision measurement, and more specifically, to a method and apparatus for measuring refractive index based on laser interferometry. Background Technology
[0002] With the improvement of underwater vehicle noise reduction technology, the success rate of underwater vehicle detection technology based on traditional acoustics has been greatly reduced. However, underwater vehicle detection based on laser interferometry to measure the refractive index of the vehicle's wake has the advantages of fast response speed, high safety, and high detection accuracy, and has promising development prospects.
[0003] In traditional methods for measuring refractive index using laser interferometry, the refractive index information is included in the optical path length nD term of the photocurrent signal phase, where n is the refractive index and D is the probe region length. However, most of these studies are based on laboratory environments, assuming that the probe region length D is constant, and do not consider the influence of platform vibration on the demodulation process of laser interferometry, resulting in low accuracy of the demodulated refractive index. Summary of the Invention
[0004] In view of this, the present disclosure provides a method and apparatus for measuring refractive index based on laser interferometry.
[0005] This disclosure provides a method for measuring refractive index based on laser interferometry, comprising: calculating phase information based on photocurrent signals; calculating phase change rate based on phase information; acquiring a first acceleration and a second acceleration at the same moment as the phase information; wherein the first acceleration and the second acceleration are acceleration values at both ends of the region to be measured along the beam propagation direction; calculating the length of the region to be measured based on the first acceleration and the second acceleration; and calculating the refractive index of the region to be measured based on the phase change rate and the length of the region to be measured.
[0006] According to an embodiment of this disclosure, calculating the length of the region to be measured based on a first acceleration and a second acceleration includes: calculating a first velocity and a second velocity based on the first acceleration and the second acceleration respectively; and calculating the length of the region to be measured based on the first velocity and the second velocity.
[0007] According to an embodiment of this disclosure, calculating the length of the region to be measured based on a first speed and a second speed includes: calculating the difference between the first speed and the second speed; and calculating the length of the region to be measured based on the difference between the first speed and the second speed.
[0008] According to embodiments of this disclosure, calculating phase information based on a photocurrent signal includes: preprocessing the photocurrent signal to obtain an AC signal; calculating encapsulation phase information based on the AC signal; and calculating phase information based on the encapsulation phase information.
[0009] According to embodiments of this disclosure, preprocessing a photocurrent signal to obtain an AC signal includes: amplifying the photocurrent signal; and filtering out the DC component from the amplified photocurrent signal to obtain an AC signal.
[0010] According to embodiments of this disclosure, calculating package phase information based on an AC signal includes: splitting the AC signal into two signals; mixing one of the two signals with a cosine signal to obtain a first signal; mixing the other of the two signals with a sine signal to obtain a second signal; and performing arctangent phase demodulation on the first signal and the second signal to obtain package phase information.
[0011] According to an embodiment of this disclosure, mixing two signals with a cosine signal and a sine signal respectively to obtain a first signal and a second signal includes: mixing the two signals with a cosine signal and a sine signal respectively; wherein the frequency of the cosine signal and the frequency of the sine signal are the same; and filtering the two mixed signals to obtain the first signal and the second signal.
[0012] According to embodiments of this disclosure, the method further includes: calculating the rate of change of refractive index of the region to be measured based on the refractive index of the region to be measured.
[0013] Another aspect of this disclosure provides an apparatus for measuring refractive index based on laser interferometry, for implementing a method for measuring refractive index based on laser interferometry. The apparatus includes: a first reflector disposed at one end of the length of the region to be measured; a first accelerometer disposed on the first reflector; a second reflector disposed at the other end of the length of the region to be measured; and a second accelerometer disposed on the second reflector.
[0014] According to embodiments of this disclosure, the apparatus for measuring refractive index based on laser interferometry further includes: a laser source; a first polarizing beam splitter, with its first end connected to the laser source; a second polarizing beam splitter, with its first end connected to the first polarizing beam splitter; a lens, with its first end connected to the second end of the second polarizing beam splitter; a waveplate, with its first end connected to the second end of the lens and its second end connected to a first reflecting mirror; a third reflecting mirror, with its first end connected to the second end of the first polarizing beam splitter; an acousto-optic frequency shifter, with its first end connected to the second end of the third reflecting mirror; a beam splitter, with its first end connected to the second end of the acousto-optic frequency shifter and its second end connected to the third end of the second polarizing beam splitter; and a photodetector, with its first end connected to the beam splitter.
[0015] According to embodiments of this disclosure, the length of the region to be measured is calculated by obtaining the acceleration at both ends of the region to be measured, thereby separating the refractive index information from the length of the region to be measured. This solves the problem of refractive index and displacement coupling in traditional measurement, and achieves high-precision measurement of refractive index.
[0016] According to embodiments of this disclosure, vibration information is obtained by setting accelerometers at both ends of the area to be measured, and the separation of refraction and vibration information is achieved based on the vibration information. Attached Figure Description
[0017] The above and other objects, features and advantages of this disclosure will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:
[0018] Figure 1 A flowchart illustrating a method for measuring refractive index based on laser interferometry according to an embodiment of the present disclosure is shown schematically.
[0019] Figure 2 This schematically illustrates a flowchart of the prior art for obtaining phase information based on arctangent phase demodulation.
[0020] Figure 3 A schematic diagram of a device for measuring refractive index based on laser interferometry according to an embodiment of the present disclosure is shown.
[0021] Figure 4 A diagram illustrating a comparison of the rate of change of refractive index for processing the same signal according to an embodiment of the present disclosure and in the prior art is provided.
[0022] Figure 5 The diagram schematically illustrates a signal-to-noise ratio comparison between the present disclosure embodiment and the prior art for processing the same signal.
[0023] Explanation of reference numerals in the attached figures:
[0024] 1. Laser source; 2. First polarizing beam splitter; 3. Second polarizing beam splitter; 4. Lens; 5. Waveplate; 6. First reflecting mirror; 7. First accelerometer; 8. Second accelerometer; 9. Second reflecting mirror; 10. Third reflecting mirror; 11. Acousto-optic frequency shifter; 12. Beam splitter; 13. Photodetector. Detailed Implementation
[0025] The embodiments of the present disclosure will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the disclosure. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the present disclosure for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known systems and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of the present disclosure.
[0026] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this disclosure. The terms “comprising,” “including,” etc., as used herein indicate the presence of features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.
[0027] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.
[0028] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).
[0029] Figure 1 A flowchart illustrating a method for measuring refractive index based on laser interferometry according to an embodiment of the present disclosure is shown.
[0030] like Figure 1 As shown, this disclosure provides a method for measuring refractive index based on laser interferometry, comprising:
[0031] S101. Calculate the phase information based on the photocurrent signal. The expression for the phase information is:
[0032] ,
[0033] in, This represents the initial value of the wrapping phase. λ is the length of the region to be measured, and λ is the laser wavelength. The refractive index value of the region to be measured;
[0034] S102. Calculate the phase change rate based on the phase information. The expression for the phase change rate, differentiated with respect to the phase information, is as follows:
[0035] ;
[0036] S103. Acquire the first acceleration and the second acceleration at the same moment as the phase information; wherein, the first acceleration and the second acceleration are the acceleration values at both ends of the region to be measured along the beam propagation direction; the expression for the first acceleration is:
[0037] ,
[0038] The expression for the second acceleration is:
[0039] ,
[0040] Where A1 and A2 are the amplitudes of the first and second accelerometers, respectively, and f1 is the frequency of the two accelerometers.
[0041] S104. Calculate the length of the area to be measured based on the first acceleration and the second acceleration. ;
[0042] S105. Calculate the refractive index of the region to be measured based on the phase change rate and the length of the region to be measured. .
[0043] In the embodiments of this disclosure, since most traditional studies are based on laboratory environments, the length D of the light detection region is assumed to be constant, and the influence of platform vibration on the demodulation process of laser interferometry is not considered. However, in practical applications using a moving platform, the vibration of the platform itself will also cause changes in the detection region D. When both the refractive index n and the length D of the detection region change, the commonly used phase demodulation method, because it ignores the influence of the length D of the detection region, results in low accuracy of the demodulated refractive index n.
[0044] Based on this, the embodiments of this disclosure obtain the acceleration values at both ends of the length of the region to be measured, and obtain the vibration displacement based on the acceleration values to separate the vibration information and the refractive index information, thereby improving the measurement accuracy of the refractive index.
[0045] According to embodiments of this disclosure, calculating the length of the region to be measured based on a first acceleration and a second acceleration includes: calculating a first velocity based on the first acceleration and the second acceleration respectively. Second speed .
[0046] The difference between the first velocity and the second velocity is calculated based on the first velocity and the second velocity. The expression for the difference is:
[0047] ,
[0048] Where B is the amplitude of the velocity change, and Δv is the velocity change value of the area to be measured. ;
[0049] Integrating the difference between the first and second velocities yields the length of the region to be measured. The expression for the length of the region to be measured is:
[0050] ,
[0051] Where D0 is the initial value of the length of the area to be measured when there is no vibration, and C is the amplitude of the length change.
[0052] In the embodiments of this disclosure, the length of the region to be measured is... , bring into Solving for the refractive index yields the result. The expression for the refractive index is:
[0053] ,
[0054] Where C1 is a constant.
[0055] According to embodiments of this disclosure, calculating phase information based on a photocurrent signal includes: preprocessing the photocurrent signal to obtain an AC signal; calculating encapsulation phase information based on the AC signal; and calculating phase information based on the encapsulation phase information.
[0056] According to embodiments of this disclosure, preprocessing the photocurrent signal to obtain an AC signal includes: amplifying the photocurrent signal; and filtering out the DC component from the amplified photocurrent signal to obtain the AC signal. The expression is:
[0057] ,
[0058] in, The conversion coefficient of the photoelectric converter. The amplitude of the electric field intensity corresponding to the reference light, To measure the amplitude of the electric field intensity corresponding to the light, The frequency of the acousto-optic frequency shifter.
[0059] In embodiments of this disclosure, the photocurrent signal is preprocessed to retain only the useful AC signal while removing noise and DC bias.
[0060] According to embodiments of this disclosure, calculating package phase information based on an AC signal includes: splitting the AC signal into two signals; mixing one of the two signals with a cosine signal to obtain a first signal; mixing the other of the two signals with a sine signal to obtain a second signal; and performing arctangent phase demodulation on the first signal and the second signal to obtain package phase information.
[0061] According to an embodiment of this disclosure, mixing two signals with a cosine signal and a sine signal respectively to obtain a first signal and a second signal includes: mixing the two signals with a cosine signal and a sine signal respectively; wherein the frequency of the cosine signal and the frequency of the sine signal are the same; and filtering the two mixed signals to obtain the first signal and the second signal.
[0062] In the embodiments of this disclosure, the AC signal is divided into two equal signals, and the two signals are multiplied by a cosine signal and a sine signal respectively. After low-pass filtering, a first signal I and a second signal Q are obtained. Then, the two signals are divided, and the arctangent is calculated to obtain the phase information. The expression for the phase information is as follows:
[0063] ,
[0064] Where f0 is the frequency of the numerical oscillator. The frequency of the acousto-optic frequency shifter.
[0065] Due to the range characteristics of the arctangent function, the calculated phase will be wrapped, requiring an unwrapping operation to obtain the final phase. Typically, f0 and They have the same frequency.
[0066] According to embodiments of this disclosure, the method further includes: calculating the rate of change of refractive index of the region to be measured based on the refractive index of the region to be measured.
[0067] In embodiments of this disclosure, the rate of change of refractive index is obtained by differentiating the refractive index. The expression for the rate of change of refractive index is:
[0068] .
[0069] In the embodiments of this disclosure, the rate of change of refractive index is obtained by differentiating the refractive index, and the accuracy of the refractive index measurement is determined based on the rate of change of refractive index.
[0070] Figure 2 The flowchart illustrating the prior art of obtaining signal phase information based on arctangent phase demodulation is shown.
[0071] like Figure 2 As shown, in the prior art, the obtained photocurrent signal is demodulated based on the arctangent phase to obtain the signal phase information. The current signal is converted into a digital signal by an analog-to-digital converter. The digital signal is divided into two paths, one of which is connected to the sin(ω) generated by the local oscillator. c The signal is mixed with t) and then low-pass filtered to obtain the Q signal. Another path is mixed with the cos(ω) signal generated by the local oscillator. c t) The signal is mixed and then low-pass filtered to obtain the I signal. The I and Q signals are divided, and the arctangent is calculated to obtain the phase information of the encapsulation. Then, the unwrapping algorithm is used to obtain the phase information. This phase information includes vibration displacement. When calculating the refractive index, the vibration information was not separated, resulting in low accuracy of the measured refractive index.
[0072] Figure 3A schematic diagram of a device for measuring refractive index based on laser interferometry according to an embodiment of the present disclosure is shown.
[0073] like Figure 3 As shown, another aspect of this disclosure provides an apparatus for measuring refractive index based on laser interferometry, for implementing a method for measuring refractive index based on laser interferometry. The apparatus includes: a first reflector 6 disposed at one end of the length of the region to be measured; a first accelerometer 7 disposed on the first reflector 6; a second reflector 9 disposed at the other end of the length of the region to be measured; and a second accelerometer 8 disposed on the second reflector 9.
[0074] According to embodiments of this disclosure, the apparatus for measuring refractive index based on laser interferometry further includes: a laser source 1; a first polarizing beam splitter 2, with its first end connected to the laser source; a second polarizing beam splitter 3, with its first end connected to the first polarizing beam splitter; a lens 4, with its first end connected to the second end of the second polarizing beam splitter; a waveplate 5, with its first end connected to the second end of the lens and its second end connected to the first reflecting mirror; a third reflecting mirror 10, with its first end connected to the second end of the first polarizing beam splitter; an acousto-optic frequency shifter 11, with its first end connected to the second end of the third reflecting mirror; a beam splitter 12, with its first end connected to the second end of the acousto-optic frequency shifter and its second end connected to the third end of the second polarizing beam splitter; and a photodetector 13, with its first end connected to the beam splitter.
[0075] In the embodiments of this disclosure, the laser beam emitted from the laser source is split into a reference beam and a measurement beam by a first polarizing beam splitter at a splitting ratio of 1:3. The reference beam is then generated by an acousto-optic frequency shifter. The frequency is shifted, and another beam of measurement light, after passing through the area to be measured, is reflected again by the second and first reflecting mirrors. After passing through two quarter-wave plates, the polarization state of the measurement light is consistent with that of the reference light. Subsequently, the measurement light passes through the second polarizing beam splitter, and the beam splitter undergoes heterodyne interference with the reference light. The optical signal, after fine interference by a photodetector, is converted into a photocurrent signal. Vibration information is acquired by setting accelerometers on the first and second reflecting mirrors. Based on the acquired vibration information, the refractive index information and vibration information are separated by solving differential equations, achieving high-precision measurement of the refractive index.
[0076] In one possible embodiment of this disclosure, simulation is performed using Matlab, assuming the frequencies of the two accelerometers are 20Hz. Z The vibration displacement is 0.0005m, the initial value D0 of the length of the area to be measured is 0.1m, and the expression for the length of the area to be measured is:
[0077] ,
[0078] The refractive index signal frequency is 40Hz, and the refractive index change is 0.0005RIU. Assume the average refractive index of seawater is... The expression for refractive index is:
[0079] .
[0080] The refractive index change rate of this embodiment is compared with the assumed refractive index change rate and the refractive index change rate obtained without using the demodulation algorithm of this embodiment. The demodulation algorithm not used in this embodiment indicates that the length of the measured region is a fixed value, the vibration displacement and refractive index are coupled together, and there is no demodulation algorithm for separation; that is, only a demodulation algorithm is used. Figure 2 The phase is calculated by arctangent demodulation.
[0081] The results are as follows Figure 4 As shown, by comparing the demodulation algorithm using this invention with that not using this invention, it can be seen that the peak-to-peak value in the time domain using the demodulation algorithm of this invention is closer to the set rate of refractive index change, and the accuracy of the measured refractive index is higher. Figure 5 As can be seen, the demodulation algorithm of this embodiment improves the signal-to-noise ratio by 8.49 dB, which can effectively reduce noise and distortion, making the measurement results more accurate.
[0082] Those skilled in the art will understand that the features described in the various embodiments and / or claims of this disclosure can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this disclosure. In particular, the features described in the various embodiments and / or claims of this disclosure can be combined and / or combined in various ways without departing from the spirit and teachings of this disclosure. All such combinations and / or combinations fall within the scope of this disclosure.
[0083] The embodiments of this disclosure have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of this disclosure. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. The scope of this disclosure is defined by the appended claims and their equivalents. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of this disclosure, and all such substitutions and modifications should fall within the scope of this disclosure.
Claims
1. A method for measuring refractive index based on laser interferometry, characterized in that, include: Phase information is calculated based on the photocurrent signal; Calculate the phase change rate based on the phase information; Acquire the first acceleration and the second acceleration at the same moment as the phase information; wherein the first acceleration and the second acceleration are the acceleration values at both ends of the region to be measured along the beam propagation direction; The length of the region to be measured is calculated based on the first acceleration and the second acceleration. The refractive index of the region to be measured is calculated based on the phase change rate and the length of the region to be measured.
2. The method for measuring refractive index based on laser interferometry according to claim 1, characterized in that, Calculating the length of the region to be measured based on the first acceleration and the second acceleration includes: Calculate the first velocity and the second velocity based on the first acceleration and the second acceleration, respectively; The length of the region to be measured is calculated based on the first speed and the second speed.
3. The method for measuring refractive index based on laser interferometry according to claim 2, characterized in that, Calculating the length of the region to be measured based on the first velocity and the second velocity includes: Calculate the difference between the first speed and the second speed based on the first speed and the second speed; The length of the region to be measured is calculated based on the difference between the first velocity and the second velocity.
4. The method for measuring refractive index based on laser interferometry according to claim 1, characterized in that, The calculation of phase information based on the photocurrent signal includes: The photocurrent signal is preprocessed to obtain an AC signal; Calculate the package phase information based on the AC signal; The phase information is calculated based on the package phase information.
5. The method for measuring refractive index based on laser interferometry according to claim 4, characterized in that, Preprocessing the photocurrent signal to obtain the AC signal includes: The photocurrent signal is amplified. The DC component of the amplified photocurrent signal is filtered out to obtain the AC signal.
6. The method for measuring refractive index based on laser interferometry according to claim 4, characterized in that, Calculating package phase information based on the AC signal includes: The AC signal is split into two signals; One of the two signals is mixed with a cosine signal to obtain the first signal; The other of the two signals is mixed with a sine wave signal to obtain a second signal; The package phase information is obtained by performing arctangent phase demodulation on the first signal and the second signal.
7. The method for measuring refractive index based on laser interferometry according to claim 6, characterized in that, The two signals are mixed with a cosine signal and a sine signal respectively to obtain a first signal and a second signal, including: The two signals are mixed with a cosine signal and a sine signal respectively; wherein the frequency of the cosine signal and the frequency of the sine signal are the same. The two mixed signals are filtered to obtain the first signal and the second signal.
8. The method for measuring refractive index based on laser interferometry according to claim 1, characterized in that, Also includes: The rate of change of the refractive index of the region to be tested is calculated based on the refractive index of the region to be tested.
9. A device for measuring refractive index based on laser interferometry, characterized in that, The apparatus for implementing the method according to any one of claims 1-8, the apparatus comprising: A first reflecting mirror is disposed at one end of the length of the area to be measured; The first acceleration sensor is mounted on the first reflector; A second reflecting mirror is disposed at the other end of the length of the area to be measured; The second acceleration sensor is mounted on the second reflector.
10. The apparatus for measuring refractive index based on laser interferometry according to claim 9, characterized in that, Also includes: Laser source; The first polarizing beam splitter has its first end connected to the laser source. The second polarizing beam splitter has its first end connected to the first polarizing beam splitter. The lens, with its first end connected to the second end of the second polarizing beam splitter; A waveplate, with its first end connected to the second end of the lens, and the second end of the waveplate connected to the first reflecting mirror; The third reflecting mirror has its first end connected to the second end of the first polarizing beam splitter. The first end of the acousto-optic frequency shifter is connected to the second end of the third reflecting mirror; The first end of the beam splitter is connected to the second end of the acousto-optic frequency shifter, and the second end of the beam splitter is connected to the third end of the second polarizing beam splitter. The photodetector has its first end connected to the beam splitter.