Optomechanical low-temperature measuring system and method based on nano-film thermal strain

Abstract

The application relates to an optomechanical low-temperature temperature measurement system and method based on nanometer film thermal strain, wherein the temperature measurement system comprises a laser light source, a polarization modulation assembly, a beam splitting assembly, a measurement assembly, a reference assembly, a beam combining assembly and a feedback control unit; the temperature measurement method comprises the following steps: performing frequency spectrum analysis on the vibration displacement signal to obtain a vibration characteristic frequency, using a low-temperature refrigeration device to realize accurate control of the nanometer film environmental temperature in a wide temperature range of 77K to 300K, and calculating the environmental temperature according to a pre-established corresponding relationship between the vibration characteristic frequency and the temperature. The temperature measurement method constructs a partitioned linear relationship between the temperature and the optomechanical response signal, does not depend on material energy levels or electrical characteristics, can significantly simplify the temperature calibration process, and combines the frequency-temperature relationships of the independent calibrations of various modes to establish temperature measurement, so that the overall calibration error is reduced.

Description

Technical Field

[0001] This invention belongs to the technical field of low-temperature temperature measurement methods, specifically relating to an optomechanical low-temperature temperature measurement system and method based on the thermal strain of nanofilms. Background Technology

[0002] Temperature measurements in low-temperature environments typically employ resistance thermometers, semiconductor diode thermometers, and superconducting quantum interference device (SQU) thermometers. Resistance thermometers require electrical contact with the object being measured, which can easily introduce self-heating errors in measurements of weak heat capacity or microstructures, and multi-point calibration is necessary to achieve measurement accuracy. Semiconductor diode thermometers exhibit significant nonlinearity over a wide temperature range, and their calibration process is complex. While SQU thermometers can be used as primary thermometers, they rely on specific low-temperature operating conditions and complex electromagnetic environments.

[0003] To avoid thermal disturbances caused by electrical contact, existing technologies include non-contact temperature measurement methods based on optical readout, such as radiation thermometry. However, these methods typically rely on radiation intensity calibration, and the signal is weak at low temperatures, making them unsuitable for precise temperature measurement of microscale structures.

[0004] Meanwhile, mechanical resonant structures affected by temperature changes have been used in physical sensing, but existing technologies mainly read the resonant response through external driving, and no measurement method has been developed to directly characterize the ambient temperature using the intrinsic vibration frequency induced by thermal stress. Therefore, there are still shortcomings in temperature measurement over a wide temperature range. Thus, it is necessary to propose a temperature measurement method that requires no electrical contact, no external excitation, and is applicable to a wide temperature range. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an optomechanical low-temperature temperature measurement system and method based on nanofilm thermal strain, which solves the problems of existing thermometers requiring electrical contact, complex calibration, and poor low-temperature measurement stability.

[0006] The technical solution of the present invention is as follows: A photomechanical cryogenic temperature measurement system based on nanofilm thermal strain includes a laser source, a polarization modulation component, a beam splitter component, a measurement component, a reference component, a beam combiner component, and a feedback control unit. The laser source outputs a laser beam, which is then polarized by a polarization modulation component and incident on a beam splitter. The incident laser beam is split into a measurement beam and a reference beam. The measurement beam propagates along the measurement component, and the reference beam propagates along the reference component. Then, the measurement beam and the reference beam are combined by a beam combiner and sent to the feedback control unit.

[0007] According to a preferred embodiment of the present invention, the polarization modulation component includes a neutral density filter and a half-wave plate A; the laser beam output from the laser source is attenuated by the neutral density filter and then the polarization direction of the laser beam is adjusted by the half-wave plate A.

[0008] According to a preferred embodiment of the present invention, the beam splitting component is a beam splitting cube A; the beam splitting cube A splits the laser beam after passing through the polarization modulation component into a measurement beam propagating along the principal optical axis and a reference beam propagating along the vertical direction.

[0009] According to a preferred embodiment of the present invention, the measuring component includes a polarizing beam splitter cube B, a quarter-wave plate, a short-focus lens, and a nanofilm; the nanofilm is disposed in the controlled temperature environment of the cryogenic refrigeration equipment to be measured, and the measuring beam passes sequentially through the polarizing beam splitter cube B and the quarter-wave plate, and is then focused by the short-focus lens onto the center of the nanofilm, and is then reflected by the nanofilm and collected by the quarter-wave plate and the polarizing beam splitter cube B.

[0010] According to a preferred embodiment of the present invention, the reference assembly includes a first reflector, a second neutral density filter, and a second reflector; the reference beam passes sequentially through the first reflector, the second neutral density filter, and the second reflector, with the first reflector serving as the reference mirror.

[0011] According to a preferred embodiment of the present invention, the beam combining assembly includes a half-wave plate B, a polarizer, a 50:50 beam splitter, and a third reflector. The measurement beam after passing through the measurement assembly has its polarization state adjusted by the half-wave plate B, and the reference beam after passing through the reference assembly has its polarization state adjusted by the polarizer. After the polarization states of the measurement beam and the reference beam are the same, the measurement beam and the reference beam are transmitted and reflected by the 50:50 beam splitter and reflected by the third reflector to form two interference spots with equal intensity and opposite phase.

[0012] According to a preferred embodiment of the present invention, the feedback control unit includes a balanced detector, a data acquisition system, and a piezoelectric ceramic drive controller. The two interference spots are detected by the balanced detector and digitally processed by the data acquisition system. The data acquisition system uses the calculated differential interference signal as a negative feedback signal and adjusts the length of the interference arm where the reference mirror is located through the closed loop of the piezoelectric ceramic drive controller to ensure that the optical path difference between the two interference arms is stably locked at the zero differential interference signal position.

[0013] According to a preferred embodiment of the present invention, the thickness of the nanofilm is on the order of hundreds of nanometers, and the nanofilm has an internal stress tension of 80-250 MPa and a fundamental mode intrinsic vibration frequency of less than 100 kHz.

[0014] The nanofilm of this invention has an internal stress tension of 80-250 MPa, which improves the quality factor and frequency stability of the device, thereby achieving better temperature resolution and measurement stability. Based on the size and material parameters of the nanofilm, its fundamental mode eigenfrequency is below 100 kHz. Within the target temperature range, the eigenfrequency of the nanofilm exhibits a stable response relationship with temperature, specifically showing continuous frequency change without abrupt changes, while having small frequency drift and a stable quality factor. This nanofilm has good thermal stability.

[0015] According to a preferred embodiment of the present invention, the laser source is a single-mode continuous laser with a coherence length > 2 meters. The wavelength of the laser beam output by the laser source is 785 nm.

[0016] According to a preferred embodiment of the present invention, the cryogenic refrigeration device is a liquid nitrogen thermostat.

[0017] According to a preferred embodiment of the present invention, the liquid nitrogen thermostat is vacuum-sealed and connected to a vacuum chamber. The sample rod is supported in the vacuum chamber by a metal wire shock absorber. A 3 mm central hole is provided at the sample attachment point of the sample rod. A copper ring is connected to the central hole of the sample rod by N-grease. The outer diameter of the copper ring is 10 mm and the inner diameter is 2.5 mm.

[0018] According to a preferred embodiment of the present invention, the nanofilm is connected to a copper ring via a square ring silicon wafer, thereby forming a suspended window structure; the outer square of the square ring silicon wafer has a side length of 5 mm, and the inner square has a side length of 2 mm.

[0019] In this invention, the nanofilm forms a suspended window structure, which generates thermal strain and causes changes in internal tension when the ambient temperature changes; the sample rod is provided with a central hole to help check whether the laser beam is incident on the nanofilm; different sizes of suspended window structures have different measurement sensitivities and temperature ranges, and the size of the suspended window structure needs to be selected according to the temperature measurement requirements and the highest frequency of measurement.

[0020] According to a preferred embodiment of the present invention, the vacuum chamber is provided with high-transmittance windows for 785nm wavelength laser beams on both sides along the direction of the measurement beam.

[0021] According to a preferred embodiment of the present invention, the short focal length lens is positioned in front of the high-transmittance window of the vacuum chamber to focus the laser beam within a 60 μm area at the center of the nanofilm and measure the vibration information at the center of the nanofilm. The displacement gradient and phase change of the fundamental mode vibration mode are more intense at the center of the nanofilm, and the center point of the nanofilm is also the position with the strongest optical coupling. Therefore, optical focusing can effectively improve the sensitivity of the nanofilm vibration measurement.

[0022] The present invention also provides a temperature measurement method for the above-mentioned optomechanical cryogenic temperature measurement system based on the thermal strain of nanofilms.

[0023] A photomechanical cryogenic measurement method based on the thermal strain of nanofilms, using the aforementioned temperature measurement system, includes the following steps: S1. The laser source outputs a laser beam. The beam splitter A divides the laser beam into a measurement beam that propagates along the principal optical axis and a reference beam that propagates in the perpendicular direction. The measurement beam is reflected by a nanofilm, while the reference beam is reflected by a reference mirror. The nanofilm-reflected beam and the reference mirror-reflected beam are reflected and transmitted by the same 50:50 beam splitter, forming two interference spots with equal intensity and opposite phase. The two interference spots are respectively connected to the two photosensitive areas of the balanced detector. S2. Based on the control of the LabVIEW program, the balanced detector continuously collects the intensity of the reflected light from the nanofilm and the reference mirror, and calculates the average value of each light intensity. The system measures the intensity-optical path difference curve of the single-path interference light and calculates the coherence to verify the system's interference state. After adjusting the system coherence to above 0.8, differential interference signals are collected. The differential interference signal detected by the balanced detector is used as a negative feedback signal to adjust the length of the interference arm where the reference mirror is located in a closed loop, and the optical path difference between the two interference arms is stably locked at the zero differential interference signal, that is, the optimal operating point for the interference curve sensitivity. The differential interference signals are continuously collected as a set of time-domain measurement signals for subsequent data processing.

[0024] S3. The collected time-domain measurement signals are processed by segmented windowing, Fourier transform is performed segment by segment, and the Fourier transform results of all segments are ensembled and averaged to finally obtain the vibration spectrum of the nanofilm. Based on the vibration spectrum of the nanofilm, Lorentz curve fitting is performed on the characteristic vibration peaks to obtain the vibration peak position parameters.

[0025] S4. Using the vibration peak position parameter as the measured characteristic frequency, and combining it with the system temperature-frequency calibration curve, the corresponding temperature value is obtained; multiple sets of temperature data are obtained through multiple measurements, the average value of the multiple sets of temperature data is calculated as the final temperature measurement result of the system, and the standard deviation of the multiple sets of temperature data is calculated as the system temperature measurement error.

[0026] The technical features and beneficial effects of this invention are as follows: 1. The temperature measurement method of the present invention utilizes the intrinsic vibration generated by the temperature-induced change in internal stress of the nanofilm, which eliminates the need for an external driving signal and reduces measurement disturbance.

[0027] 2. The temperature measurement method of this invention adopts optical displacement readout to achieve non-contact temperature measurement. The laser power of less than 5uW avoids the heat load caused by laser self-heating and is suitable for a wide temperature measurement range from liquid nitrogen temperature to room temperature.

[0028] 3. The temperature measurement method of the present invention establishes a interval linear relationship between temperature and photomechanical response signal, which does not depend on material energy levels or electrical properties, and can significantly simplify the temperature calibration process; the frequency-temperature relationship of each mode independently calibrated is combined to establish temperature measurement, thereby reducing the overall calibration error. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the optomechanical low-temperature temperature measurement system based on nanofilm thermal strain of the present invention; Figure 2 The measurement results of the fundamental mode vibration frequency of the nanofilm in Example 1 of this invention in the low-temperature region; Figure 3 The measurement results of the fundamental mode vibration frequency of the nanofilm in Example 1 of this invention in the room temperature range; Figure 4 The measurement results of the higher-order vibrational frequencies of the nanofilm (1,2) mode in the low-temperature region in Example 1 of this invention; Figure 5 The measurement results of the higher-order vibrational frequencies of the (2,2) mode of the nanofilm in Example 1 of this invention in the low-temperature region; Figure 6 The measurement results of the higher-order vibration frequencies of the (1,3) mode of the nanofilm in Example 1 of this invention in the low-temperature region; Figure 7 This is the temperature dependence result of the fundamental mode vibration frequency of the nanofilm in Example 1 of the present invention over a wide temperature range; Figure 8 This is the linear fitting result of the temperature-dependent linear region of the fundamental mode vibration frequency of the nanofilm in Example 1 of the present invention; Figure 9 The results of frequency-temperature measurements in the temperature range of 78K to 210K in Embodiment 1 of the present invention; Figure 10 This is a test diagram of the oscillation frequency of a standard oscillator using the optomechanical cryogenic temperature measurement system based on nanofilm thermal strain in Experiment Example 1 of this invention. Figure 11 This is the sensitivity result of temperature measurement of the fundamental mode vibration frequency of the nanofilm in Experiment Example 2 of the present invention; The components are: 1. Laser source, 2. Neutral density filter one, 3. Half-wave plate A, 4. Beam splitter cube A, 5. Polarizing beam splitter cube B, 6. Quarter-wave plate, 7. Short focal length lens, 8. Nanofilm, 9. Liquid nitrogen thermostat, 10. Mirror one, 11. Neutral density filter two, 12. Mirror two, 13. Half-wave plate B, 14. Polarizer, 15. 50:50 beam splitter, 16. Mirror three, 17. Balance detector, 18. Data acquisition system. Detailed Implementation

[0030] The present invention will be further described below with reference to embodiments, but is not limited thereto. The described embodiments are some embodiments of the present invention. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. Unless otherwise specified in the embodiments of the present invention, all techniques existing in the art can be used.

[0032] Example 1 like Figure 1 As shown, an optomechanical cryogenic temperature measurement system based on the thermal strain of a nanofilm includes a laser source 1, a polarization modulation component, a beam splitting component, a measurement component, a reference component, a beam combining component, and a feedback control unit. The laser source 1 outputs a laser beam, which is then polarized by a polarization modulation component and incident on a beam splitter component. The incident laser beam is split into a measurement beam and a reference beam. The measurement beam propagates along the measurement component, and the reference beam propagates along the reference component. Then, the measurement beam and the reference beam are combined by a beam combiner component and sent to the feedback control unit.

[0033] The polarization modulation component includes a neutral density filter 2 and a half-wave plate A3; the laser beam output from the laser source 1 is attenuated by the neutral density filter 2, and the polarization direction of the laser beam is adjusted by the half-wave plate A3.

[0034] The beam splitter is a beam splitter cube A4; the beam splitter cube A4 splits the laser beam after polarization modulation into a measurement beam that propagates along the principal optical axis and a reference beam that propagates in the perpendicular direction.

[0035] The measurement components include a polarization beam splitter cube B5, a quarter-wave plate 6, a short-focus lens 7, and a nanofilm 8. The nanofilm 8 is placed in the controlled temperature environment of the cryogenic refrigeration equipment to be measured. The measurement beam passes through the polarization beam splitter cube B5 and the quarter-wave plate 6 in sequence, and is then focused by the short-focus lens 7 and incident on the center of the nanofilm 8. After being reflected by the nanofilm 8, it is collected by the quarter-wave plate 6 and the polarization beam splitter cube B5.

[0036] The reference assembly includes a reflector 10, a neutral density filter 21, and a reflector 22. The reference beam passes through the reflector 10, the neutral density filter 21, and the reflector 22 in sequence. The reflector 10 serves as the reference mirror, and its position is adjusted by a piezoelectric control displacement stage.

[0037] The beam combining assembly includes a half-wave plate B13, a polarizer 14, a 50:50 beam splitter 15, and a reflector 16. The measurement beam after passing through the measurement assembly has its polarization state adjusted by the half-wave plate B13, and the reference beam after passing through the reference assembly has its polarization state adjusted by the polarizer 14. After the polarization states of the measurement beam and the reference beam are the same, the measurement beam and the reference beam are transmitted and reflected by the 50:50 beam splitter 15 to form two interference spots with equal intensity and opposite phase.

[0038] The feedback control unit includes a balance detector 17, a data acquisition system 18, and a piezoelectric ceramic drive controller. The two interference spots are detected by the balance detector 17 and digitally processed by the data acquisition system 18. The data acquisition system 18 uses the calculated differential interference signal as a negative feedback signal and adjusts the length of the interference arm where the reference mirror is located through the closed loop of the piezoelectric ceramic drive controller to ensure that the optical path difference between the two interference arms is stably locked at the zero differential interference signal position. The two interference arms refer to the reflection path of the nanofilm 8 and the reflection path of the reference mirror. The reflection path of the nanofilm 8 is as follows: the laser sequentially passes through the beam splitter cube A4, the polarization beam splitter cube B5, the quarter-wave plate 6, the short focal length lens 7, the nanofilm 8, and after reflection by the nanofilm 8, it sequentially passes through the short focal length lens 7, the quarter-wave plate 6, the polarization beam splitter cube B5, the half-wave plate B13, and the 50:50 beam splitter 15. The reflection path of the reference mirror is as follows: the laser sequentially passes through the beam splitter cube A4, the reference mirror, and after reflection by the reference mirror, it sequentially passes through the beam splitter cube A4, the neutral density filter II 11, the reflector II 12, the polarizer 14, and the 50:50 beam splitter 15.

[0039] The silicon nitride nanofilm 8 has a thickness on the order of hundreds of nanometers; the nanofilm 8 has an internal stress tension of 80–250 MPa and a fundamental mode intrinsic vibration frequency of less than 100 kHz.

[0040] The nanofilm 8 of the present invention has an internal stress tension of 80-250 MPa, which improves the quality factor and frequency stability of the device, thereby achieving better temperature resolution and measurement stability. Based on the size and material parameters of the nanofilm 8, its fundamental mode eigenfrequency is below 100 kHz. Within the target temperature range, the eigenfrequency of the nanofilm 8 exhibits a stable response relationship with temperature, specifically, the frequency change is continuous and without abrupt changes, while having a small frequency drift and a stable quality factor. The nanofilm 8 has good thermal stability.

[0041] Laser source 1 is a single-mode continuous laser with a coherence length greater than 2 meters. The wavelength of the laser beam output by laser source 1 is 785 nm.

[0042] The cryogenic refrigeration equipment is a liquid nitrogen thermostat.

[0043] The liquid nitrogen thermostat 9 is vacuum-sealed with a vacuum chamber. The sample rod is supported inside the vacuum chamber by a metal wire shock absorber. A 3 mm central hole is opened at the sample attachment point of the sample rod. A copper ring is connected to the central hole of the sample rod by N grease. The outer diameter of the copper ring is 10 mm and the inner diameter is 2.5 mm.

[0044] The nanofilm 8 is connected to the copper ring via a square ring silicon wafer, forming a suspended window structure; the outer square of the square ring silicon wafer has a side length of 5 mm, and the inner square has a side length of 2 mm.

[0045] In this invention, the nanofilm 8 forms a suspended window structure, which generates thermal strain and causes changes in internal tension when the ambient temperature changes; the sample rod is provided with a central hole to help check whether the laser beam is incident on the nanofilm 8; different sizes of suspended window structures have different measurement sensitivities and temperature ranges, and the size of the suspended window structure needs to be selected according to the temperature measurement requirements and the highest frequency of measurement.

[0046] High-transmittance windows for 785nm wavelength laser beams are set on both sides of the vacuum chamber along the direction of the measurement beam.

[0047] A short focal length lens 7 is placed in front of the high-transmittance window of the vacuum chamber to focus the laser beam onto a 60μm area at the center of the nanofilm 8 to measure the vibration information at the center of the nanofilm 8. The displacement gradient and phase change of the fundamental mode vibration mode are more intense at the center of the nanofilm 8, and the center point of the nanofilm 8 is also the position with the strongest optical coupling. Therefore, focusing the light can effectively improve the sensitivity of the vibration measurement of the nanofilm 8.

[0048] A photomechanical cryogenic measurement method based on the thermal strain of nanofilms, using the aforementioned temperature measurement system, includes the following steps: S1. System Debugging and Temperature Measurement: First, the ambient temperature of the nanofilm 8 was precisely controlled. After pre-evacuating the liquid nitrogen thermostat chamber, a low liquid nitrogen flow rate was set, and the PID parameters were adjusted to control the heating power to ensure that the sample rod of the liquid nitrogen thermostat 9 maintained a low vibration state and precise temperature control, reducing vibration interference caused by liquid nitrogen flow and boiling. Second, the laser was focused to the center of the nanofilm 8, and the two interference spots of the balanced probe were adjusted to be of equal phase. The half-wave plate B13 was adjusted so that the transmitted and reflected light power of the light reflected from the nanofilm 8 through the 50:50 beam splitter was equal. The polarizer 14 was adjusted so that the transmitted and reflected light power of the light reflected from the mirror 12 through the 50:50 beam splitter gradually approached and became equal. Finally, one interference spot was observed, and the direction and power of the reflection from the nanofilm 8 and the mirror 12 were adjusted until the power was equal, less than 5uW, and with good coherence.

[0049] Laser source 1 outputs a laser beam. After the laser beam's intensity is attenuated by neutral density filter 2, it passes through half-wave plate A3 in the optical path to adjust the beam's polarization direction. The polarization-modulated laser beam is then incident on beam splitter A4, which splits the incident laser beam into a measurement beam propagating along the principal optical axis and a reference beam propagating perpendicularly. The measurement beam continues to propagate along the principal optical axis, passing sequentially through polarization beam splitter B5 and a quarter-wave plate. After being focused by a short-focus lens 7 with a focal length of 75mm, it illuminates the center of a nanofilm 8 installed inside a liquid nitrogen thermostat 9. The reflected light from the nanofilm 8 passes sequentially through a quarter-wave plate, polarization beam splitter B5, and half-wave plate B13. After the reference beam is emitted from the beam splitter, it propagates along the reference optical path and passes sequentially through the reference mirror (reflector 10), neutral density filter 11, reflector 12, and polarizer 14. The measurement beam and the reference beam are adjusted to have the same polarization state through half-wave plate B13 and polarizer 14 to achieve the most perfect interference. The reflected light from the nanofilm 8 and the reflected light from the reference mirror are reflected and transmitted through the same 50:50 beam splitter. The transmitted light of the measurement beam through the 50:50 beam splitter 15 and the reflected light of the reference beam through the 50:50 beam splitter 15 are simultaneously reflected by reflector 16, forming two interference spots with equal intensity and opposite phase. The two interference spots are respectively connected to the two photosensitive areas of the balanced detector 17.

[0050] S2. The two interference spots are detected by a balanced detector 17 (avalanche photodiode) and digitized by a data acquisition system 18 with a sampling rate of 1 MHz. The balanced detector 17 continuously collects the intensity of the reflected light from the nanofilm 8 and the reference mirror for 30 seconds, and calculates the average value of each light intensity. The system measures the intensity-optical path difference curve of a single interference light and calculates the coherence to verify the interference state of the system. The system coherence is adjusted to be above 0.8. The coherence is calculated by the interference light intensity and the detection response coefficient, and the calculation formula is as follows: ; in, and These represent the maximum and minimum values ​​of the single-channel interference voltage acquired when the optical path difference of the reference mirror is changed. and The light intensities reflected to the detector by the nanofilm 8 and the reflector-10 are respectively. Let G be the detector response coefficient, and G be the detector gain coefficient.

[0051] Using the differential interference signal detected by the balanced detector 17 as the negative feedback signal, the length of the interferometer arm containing the reference mirror is adjusted in a closed loop to stably lock the optical path difference between the two interferometer arms at the optimal operating point of the interference curve sensitivity, wherein the optical path difference controls the maximum interference sensitivity to 0.66 V / nm.

[0052] LABVIEW program control automatically realizes differential interference signal acquisition and optical path difference control (on the one hand, the differential interference signal is automatically acquired through the data acquisition system 18 to filter out common mode noise; on the other hand, the piezoelectric ceramic drive controller finely adjusts the position of the piezoelectric control displacement stage according to the negative feedback of the differential interference signal, thereby adjusting the length of the interference arm where the reference mirror is located in a closed loop to ensure that the optical path difference between the two interference arms is stably locked near the zero differential interference signal, that is, the optimal operating point for the sensitivity of the interference curve).

[0053] A 1MHz sampling rate was used to continuously acquire differential interferometric signals for 10 seconds as a set of time-domain measurement signals. Multiple sets of time-domain measurement signals were acquired at 10K intervals from 78K to room temperature for subsequent data processing and temperature calibration.

[0054] S3. Obtaining the frequency domain curve: Perform Fourier transform on each group of acquired time-domain measurement signals. To obtain smooth and noise-resistant frequency domain data, the time-domain measurement signals are segmented, windowed, and Fourier transform is performed segment by segment. Then, the Fourier transform results of all segments are averaged to obtain the vibration spectrum of the nanofilm 8 in the frequency domain range of 0~500kHz. Figure 2 , Figure 3 The measurements of the 8 fundamental mode vibration frequencies of the nanofilm in this embodiment are shown in the low-temperature and room-temperature regions, respectively. The vibration frequency calculation is based on the following formula: ; Where ι represents the size of the nanofilm 8, σ(T) represents the tension of the nanofilm 8, and ρ represents the density of the nanofilm 8, approximately 3100 Kg / m³. 3 m=1,2,…,n=1,2,…, where m and n represent different vibrational modes of the nanofilm 8. For the fundamental mode m=n=1, the frequency should be below 100 kHz.

[0055] The frequency peak near 70 kHz is within the 8 fundamental mode frequency range of this nanofilm. Figures 4-6 The above are the measurement results of the higher-order vibration frequencies of the nanofilm 8 (1,2), (2,2), and (1,3) modes in the low-temperature region in this embodiment. The frequency domain of each vibration mode is compared with... The proportional relationship matches well.

[0056] S4. System Temperature Calibration: Figure 7 This presents the temperature dependence results of the vibrational frequencies of the eight fundamental modes of the nanofilm over a wide temperature range in this embodiment. Figure 7It was found that the vibration frequency of the nanofilm 8 exhibits a piecewise linear variation with temperature. Analysis suggests that this piecewise linearity may stem from the different temperature-dependent coefficients of thermal expansion between the window of the nanofilm 8 and the square ring silicon wafer in different temperature ranges, leading to changes in the internal stress state of the nanofilm 8 with temperature variations. Furthermore, within a wide temperature range of 78K to 210K, the vibration frequency of the nanofilm 8 shows an approximately linear relationship with temperature, with a temperature response sensitivity of approximately -122Hz / K. This indicates that the measurement method in this embodiment has high temperature response sensitivity and good temperature measurement performance over a wide temperature range (78K to 210K). Based on the relevant theoretical derivation of the thermal expansion of the nanofilm 8, the temperature calibration curve can be qualitatively represented by the following formula: ; in, This is the calibration coefficient.

[0057] Over a wide temperature range of 78K to 210K, system temperature calibration can be easily obtained directly from a few temperature measurement points. For example... Figure 8 As shown, a linear fit was performed on the frequency-temperature measurement results within the temperature range of 78 K to 210 K, yielding the calibration curve: T = (85120.23 - f) / 127.07 (K), where f is the vibration frequency obtained from the fitted measurement. After system calibration, the temperature is directly obtained from the measured frequency. For example, a measured frequency of 68537.97 Hz yields a measured temperature of 130.5 K. The average frequency error corresponding to this calibration curve is approximately 17.49 Hz, corresponding to a temperature error of approximately 138 mK.

[0058] Experimental Example 1 Example 1: The temperature measurement system tests the oscillation frequency of a standard oscillator. The feedback control unit adjusts the length of the interferometer arm containing the reference mirror via a piezoelectric ceramic drive controller, which can be connected to a function generator to achieve specific waveform driving. The function generator is set to a 1600Hz sine wave, with the amplitude set within the required range of the piezoelectric ceramic drive controller, to achieve 1600Hz frequency sinusoidal vibration of the reference mirror. The single-channel interference signal and differential interference signal from the balanced detector 17 are read, and the vibration spectrum is measured according to the aforementioned piecewise Fourier transform averaging method to obtain the vibration peaks of the standard oscillator, such as... Figure 10 As shown. By Figure 10 It is concluded that the temperature measurement system of Example 1 can achieve accurate measurement at a frequency of 1.6 kHz, and the detection efficiency of balanced detection is four times higher than that of single-channel interferometric detection.

[0059] Experimental Example 2 To obtain the sensitivity of the temperature measurement system in Example 1, a temperature interval of 200mK was set around 78K, and 3 to 5 sets of frequency curves were measured at each temperature. The average value and error bar of the measured frequency at each temperature were calculated. Figure 11 The results show the sensitivity of temperature measurement of the fundamental mode vibration frequency of the nanofilm 8. The temperature control accuracy of the liquid nitrogen thermostat 9 is less than 10 mK. Within a temperature interval of 200 mK, the error bars of the vibration frequency measurement of the nanofilm 8 are significantly distinguishable, demonstrating a temperature measurement sensitivity better than 200 mK.

Claims

1. A photomechanical cryogenic temperature measurement system based on nanofilm thermal strain, characterized in that, It includes a laser source, polarization modulation components, beam splitting components, measurement components, reference components, beam combining components, and a feedback control unit; The laser source outputs a laser beam, which is then polarized by a polarization modulation component and incident on a beam splitter. The incident laser beam is split into a measurement beam and a reference beam. The measurement beam propagates along the measurement component, and the reference beam propagates along the reference component. Then, the measurement beam and the reference beam are combined by a beam combiner and sent to the feedback control unit.

2. The optomechanical cryogenic temperature measurement system based on nanofilm thermal strain according to claim 1, characterized in that, The polarization modulation component includes a neutral density filter and a half-wave plate A; the laser beam output from the laser source is attenuated by the neutral density filter and then the polarization direction of the laser beam is adjusted by the half-wave plate A.

3. The optomechanical cryogenic temperature measurement system based on nanofilm thermal strain according to claim 2, characterized in that, The beam splitting component is a beam splitting cube A; the beam splitting cube A splits the laser beam after passing through the polarization modulation component into a measurement beam that propagates along the principal optical axis and a reference beam that propagates in the perpendicular direction.

4. The optomechanical cryogenic temperature measurement system based on nanofilm thermal strain according to claim 3, characterized in that, Includes one or more of the following conditions: a. The measurement component includes a polarizing beam splitter cube B, a quarter-wave plate, a short-focus lens, and a nanofilm; the nanofilm is placed inside the cryogenic refrigeration equipment to be measured. The measurement beam passes through the polarizing beam splitter cube B and the quarter-wave plate in sequence, and is then focused by the short-focus lens and incident on the center of the nanofilm. After being reflected by the nanofilm, it is collected by the quarter-wave plate and the polarizing beam splitter cube B. b. The reference assembly includes a first reflector, a second neutral density filter, and a second reflector; the reference beam passes sequentially through the first reflector, the second neutral density filter, and the second reflector, with the first reflector serving as the reference mirror.

5. The optomechanical cryogenic temperature measurement system based on nanofilm thermal strain according to claim 4, characterized in that, The beam combining assembly includes a half-wave plate B, a polarizer, a 50:50 beam splitter, and a third reflector. The measurement beam after passing through the measurement assembly has its polarization state adjusted by the half-wave plate B, and the reference beam after passing through the reference assembly has its polarization state adjusted by the polarizer. After the polarization states of the measurement beam and the reference beam are the same, the measurement beam and the reference beam are transmitted and reflected by the 50:50 beam splitter and reflected by the third reflector to form two interference spots with equal intensity and opposite phase.

6. The optomechanical cryogenic temperature measurement system based on nanofilm thermal strain according to claim 5, characterized in that, The feedback control unit includes a balanced detector, a data acquisition system, and a piezoelectric ceramic drive controller. The two interference spots are detected by the balanced detector and digitally processed by the data acquisition system. The data acquisition system uses the calculated differential interference signal as a negative feedback signal and adjusts the length of the interference arm where the reference mirror is located through the closed loop of the piezoelectric ceramic drive controller to ensure that the optical path difference between the two interference arms is stably locked at the zero differential interference signal position.

7. The optomechanical cryogenic temperature measurement system based on nanofilm thermal strain according to claim 6, characterized in that, The nanofilm is a silicon nitride nanofilm with a thickness on the order of hundreds of nanometers; the nanofilm has an internal stress tension of 80-250 MPa and a fundamental mode intrinsic vibration frequency of less than 100 kHz.

8. The optomechanical cryogenic temperature measurement system based on nanofilm thermal strain according to claim 7, characterized in that, The laser source is a single-mode continuous laser with a coherence length greater than 2 meters, and the wavelength of the laser beam output by the laser source is 785 nm.

9. The optomechanical cryogenic temperature measurement system based on nanofilm thermal strain according to claim 8, characterized in that, The cryogenic refrigeration equipment is a liquid nitrogen thermostat; the liquid nitrogen thermostat is vacuum-sealed and connected to a vacuum chamber, in which a sample rod is supported by a metal wire shock absorber; a 3 mm central hole is provided at the sample attachment point of the sample rod, and a copper ring is connected to the central hole of the sample rod by N-grease; the nanofilm is connected to the copper ring by a square ring silicon wafer, so that the nanofilm forms a suspended window structure.

10. A photomechanical cryogenic measurement method based on nanofilm thermal strain, comprising using the photomechanical cryogenic temperature measurement system based on nanofilm thermal strain as described in claim 9, characterized in that... Includes the following steps: S1. The laser source outputs a laser beam. The beam splitter A divides the laser beam into a measurement beam that propagates along the principal optical axis and a reference beam that propagates in the perpendicular direction. The measurement beam is reflected by a nanofilm, while the reference beam is reflected by a reference mirror. The nanofilm-reflected beam and the reference mirror-reflected beam are reflected and transmitted by the same 50:50 beam splitter, forming two interference spots with equal intensity and opposite phase. The two interference spots are respectively connected to the two photosensitive areas of the balanced detector. S2. Based on the control of the LABVIEW program, the balanced detector continuously collects the intensity of the reflected light from the nanofilm and the reflected light from the reference mirror, and calculates the average value of each light intensity; the system measures the intensity-optical path difference curve of the single-path interference light and calculates the coherence to check the system interference state. After adjusting the system coherence to above 0.8, the differential interference signal is collected. The differential interference signal detected by the balanced detector is used as the negative feedback signal. The length of the interferometer arm where the reference mirror is located is adjusted in a closed loop to stably lock the optical path difference between the two interferometer arms near the zero differential interference signal, which is the optimal operating point for the sensitivity of the interference curve. The differential interference signal is continuously acquired as a set of time-domain measurement signals for subsequent data processing. S3. The collected time-domain measurement signals are processed by segmented windowing, Fourier transform is performed segment by segment, and the Fourier transform results of all segments are ensembled and averaged to finally obtain the vibration spectrum of the nanofilm. Based on the vibration spectrum of the nanofilm, Lorentz curve fitting is performed on the characteristic vibration peaks to obtain the vibration peak position parameters. S4. Using the vibration peak position parameter as the measured characteristic frequency, and combining it with the system temperature-frequency calibration curve, the corresponding temperature value is obtained; multiple sets of temperature data are obtained through multiple measurements, the average value of the multiple sets of temperature data is calculated as the final temperature measurement result of the system, and the standard deviation of the multiple sets of temperature data is calculated as the system temperature measurement error.

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