Magnetooptical effect based optical demodulation and high frequency surface acoustic field measurement system

Abstract

The application belongs to the field of magneto-optical and acousto-optical measurement technology, and discloses a magneto-optical effect and high-frequency surface acoustic field measurement system based on optical demodulation. The magneto-optical effect measurement system based on optical demodulation comprises a fiber broadband light source, an isolator, an optical intensity modulator 1, a circulator 1, a polarizer, a phase modulator, a function generator, a polarization maintaining optical fiber, a collimating mirror, a quarter-wave plate, a photodetector 1, a lock-in amplifier 1 and a low-temperature subsystem; the system guarantees the low-temperature environment of the sample through low-temperature design, and completes the extremely low-temperature high-precision magneto-optical Kerr measurement by using the interference magneto-optical Kerr measurement method. The high-frequency surface acoustic field measurement system based on optical demodulation comprises a spontaneous amplification radiation light source, an optical intensity modulator 2, a circulator 2, a self-focusing lens, a focusing lens 2, a photodetector 2 and a lock-in amplifier 2, and the system realizes the amplitude and phase delay of the high-frequency deformation caused by the surface acoustic wave at any point of the sample.

Description

Technical Field

[0001] This invention relates to the field of magneto-optical and acousto-optic measurement technology, specifically including a magneto-optical effect measurement system based on optical demodulation, and a high-frequency surface acoustic field measurement system based on optical demodulation. Background Technology

[0002] Surface acoustic waves (SAWs) are a type of sound wave that propagates on the surface of a solid. An interdigital transducer (IDT) is an electrical device that uses the inverse piezoelectric effect to convert alternating electromagnetic fields into surface acoustic waves for propagation. The center frequency of a surface acoustic wave... Depends on the forked period and the inherent sound velocity of the substrate Surface acoustic wave (SAW) devices utilize piezoelectric and inverse piezoelectric effects to propagate and receive signals, thus exhibiting strong resistance to electromagnetic interference. They are widely used as single-frequency filters in aerospace, radar detection, and other fields.

[0003] Currently, most surface acoustic wave (SAW) device measurements rely on interdigitated electrodes to convert acoustic signals into electrical signals for measurement. However, electrical measurements lack the ability to resolve the overall acoustic field distribution of the sample. Furthermore, SAW filters and other devices have complex structures, and in order to optimize device design, it is necessary to determine the acoustic field distribution of the sample.

[0004] The magneto-optical effect studies the interaction between magnetic materials and electromagnetic radiation, a phenomenon widely present in nature. The magneto-optical Kerr effect refers to the change in polarization state of linearly polarized light after reflection from the surface of a magnetic material. It can be understood as a phase difference, or Kerr angle, arising from the reflection of left- or right-handed polarized light.

[0005] The polarization state phenomenon caused by the magneto-optical effect is widely used to characterize magnetic materials, especially physical quantities such as magnetic domains and magnetization on magnetic surfaces. Generally speaking, the Kerr angle of magnetic materials is typically greater than... The phase transition temperature is between 10K and 300K. However, many new two-dimensional magnetic materials currently have a Kerr angle smaller than 10K. The phase transition temperature is less than 4K. Existing magneto-optical Kerr microscopes typically employ a spatial optical path, using an optical window in a cryogenic cavity for optical detection. However, blackbody radiation from this window makes it difficult to cool the sample below 5K. Furthermore, existing magneto-optical Kerr measurement devices generally use polarization spectroscopy to measure polarization state changes caused by phase transitions. This method suffers from poor signal-to-noise ratio when measuring small signals, making it difficult to improve accuracy to below 5K. Therefore, the measurement accuracy of currently commercially available magneto-optical Kerr microscopes is typically within [specific range missing]. The measurement temperature range is usually 5K-300K, which is insufficient to detect the properties of novel two-dimensional magnetic materials.

[0006] Fiber optic paths do not require additional optical windows, and fibers are almost thermally insulated at low temperatures, making them highly promising for applications in cryogenic environments. However, fiber optic measurements lack imaging or photographic capabilities. To achieve spatially resolved studies of magnetic phenomena at low temperatures, a cryogenic displacement stage is needed for scanning measurements; furthermore, the displacement stage generates heat during operation, affecting sample cooling and hindering precise quantum magnetic measurements. Interferometric measurements are more sensitive to minute phase changes, and the introduction of high-frequency phase modulation further improves accuracy and stability, offering superior accuracy for small Kerr angle measurements compared to polarization state measurement methods. However, high-frequency modulation electrical demodulation methods are often complex, susceptible to crosstalk, and introduce additional noise. Summary of the Invention

[0007] The present invention aims to provide a magneto-optical effect measurement system based on optical demodulation and a high-frequency surface acoustic field measurement system based on optical demodulation, so as to solve the problems in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A magneto-optical effect measurement system based on optical demodulation includes a fiber broadband light source, an isolator, an intensity modulator, a circulator, a polarizer, a phase modulator, a function generator, a polarization-maintaining fiber, a collimating lens, a quarter-glass slide, a photodetector, and a lock-in amplifier.

[0010] The fiber broadband light source is used to generate the initial optical signal. The isolator is used to isolate reflected light generated by other devices to ensure the stability of the fiber broadband light source's operating mode. The intensity modulator is used to adjust the intensity of the initial optical signal for optical mixing and demodulation. The circulator enables unidirectional transmission and control of the optical signal, maintains polarization stability, and serves as a key component in bidirectional optical communication transmission lines, enabling accurate and stable transmission and processing of the optical signal in the system. The optical signal adjusted by the intensity modulator enters the polarizer through the circulator.

[0011] The polarizer and phase modulator are used to modulate the beam, and the function generator is used to adjust the operating frequency of the intensity modulator and the phase modulator and to provide a clock synchronization signal.

[0012] The polarization-maintaining fiber has birefringence, allowing two linearly polarized lights with perpendicular polarization to propagate independently. It is used to connect the phase modulator and the collimating lens, enabling bidirectional transmission of the optical signal. The collimating lens is used to collimate the optical signal into parallel light, and the quarter-glass slide is used to convert linearly polarized light into circularly polarized light and to convert the reflected beam into linearly polarized light.

[0013] When the reflected light beam from the sample passes through the polarizer, interference occurs, generating an interference signal. The interference signal enters the photodetector through the circulator and is used to collect the interference signal. The lock-in amplifier is used to record the interference signal.

[0014] Furthermore, it also includes a cryogenic subsystem to ensure an extremely low temperature environment for the sample. The cryogenic subsystem includes a dilution refrigerator. The polarization-maintaining fiber, collimating lens, quarter glass slide, and sample are placed inside the vacuum chamber of the dilution refrigerator. The fiber broadband light source, isolator, intensity modulator, circulator, polarizer, phase modulator, function generator, photodetector, and lock-in amplifier are located outside the dilution refrigerator. The optical fibers inside and outside the dilution refrigerator are connected by a high-vacuum polarization-maintaining fiber feedthrough.

[0015] Furthermore, magneto-optical Kerr angle The calculation method is as follows:

[0016] Superheterodyne demodulation is performed using an intensity modulator to reduce the bandwidth requirements of the photodetector; a low-frequency detector is used to reduce measurement noise and improve measurement accuracy; intensity modulation is applied to reduce the influence of electromagnetic interference and improve measurement stability.

[0017] The incident light intensity modulated by the light intensity modulator , When the frequency is amplitude modulation, the final output of the interferometer containing amplitude modulation is:

[0018]

[0019] In the formula, For phase modulation frequency, Modulation depth; For a with The relevant frequency, when the frequency of the intensity modulator equal or At that time, a low-frequency detector is used to retain the DC and low-frequency components. Specifically:

[0020] hour,

[0021] hour,

[0022] In the formula, , The first and second order Bessel functions of the first kind are respectively, and the incident light intensity after modulation by the intensity modulator is... The second harmonic amplitude δ is much smaller than 1 to obtain a better signal-to-noise ratio and dynamic range;

[0023] Difference frequency obtained by using low-frequency locking amplitude and amplitude They are respectively:

[0024]

[0025]

[0026] Finally, the magneto-optical Kerr angle was obtained. The formula for calculation is:

[0027] .

[0028] Furthermore, the cryogenic subsystem also includes a distillation pan, a distillation heat shield, a mixing chamber pan, a mixing chamber cooling finger, an optical adjustment frame, a precision displacement stage, a sample heat shield, precision screws, a tension spring, and a sample holder. The optical adjustment frame, precision displacement stage, and sample heat shield are connected to the distillation pan via connectors. The precision displacement stage is connected to the sample holder via the distillation heat shield. The mixing chamber pan is connected to the sample holder via the mixing chamber cooling finger. The sample holder is connected to the optical adjustment frame via the precision screws and a tension spring. The sample is placed on the sample holder, and the precision displacement stage is used to scan the sample. During the scanning process, the precision displacement stage controller applies a high-voltage triangular wave signal to the piezoelectric ceramic in the displacement stage guide rail, causing it to actuate and thus generate displacement.

[0029] Furthermore, the distillation heat shield is a hollow brass tube.

[0030] Furthermore, the connector is a hollow brass rod.

[0031] Furthermore, the cooling finger of the mixing chamber is a thermally conductive copper wire.

[0032] The aforementioned magneto-optical effect measurement system based on optical demodulation is used to measure changes in the polarization state of light, in order to detect Kerr angles smaller than 1. Properties of two-dimensional magnetic materials with a phase transition temperature of less than 4K.

[0033] A high-frequency surface acoustic field measurement system based on optical demodulation includes a self-amplifying radiation source, a second light intensity modulator, a second circulator, a self-focusing lens, a second focusing lens, a second photodetector, and a second lock-in amplifier.

[0034] The system comprises the following components: a self-emitting amplifying light source to generate a broadband initial light signal and provide illumination; a second light intensity modulator to modulate the intensity of the light signal to support subsequent superheterodyne demodulation; a second circulator to control the unidirectional transmission of the light signal, guiding the incident light to the sample and the reflected light to the detection path; a self-focusing lens to collect the reflected light from the sample and detect changes in light intensity caused by the movement of the light spot; a second focusing lens to focus the incident light onto the sample surface, forming a small light spot to detect local acoustic waveform variations; a second photodetector to convert the received reflected light signal into an electrical signal; and a second lock-in amplifier to demodulate the electrical signal, extract the amplitude and phase information of the surface acoustic wave, and process the high-frequency signal to achieve imaging.

[0035] The above-described method for measuring surface acoustic wave imaging using a high-frequency surface acoustic field measurement system based on optical demodulation sets the horizontal distance between the optical centers of the self-focusing lens and the focusing lens to be... The distance from the center of the reflected light spot to the optical center of the self-focusing lens is obtained as follows: The intensity of the reflected light received by the self-focusing lens is denoted as . The maximum reflected light intensity is denoted as The tilt angle caused by surface acoustic waves at the incident photoelectric point This causes the reflected light spot to move. The distance the reflected light spot moves under the action of surface acoustic waves. The intensity of the reflected light received by the self-focusing lens will also become ,because Therefore, the change in light intensity caused by surface acoustic waves is:

[0036]

[0037] In the formula, Let t be the maximum tilt angle at the incident light point, and t be time. For the focal length of the focusing lens, For the frequency of SAW, This represents the phase distribution of SAW in the propagation space.

[0038] Furthermore, superheterodyne demodulation is performed using an electro-optic amplitude modulator; at this point, the reflected light intensity is... The formula for calculating the change in light intensity caused by surface acoustic waves is:

[0039]

[0040] By making The detection bandwidth is less than that of the second lock-in amplifier, in order to detect... The frequency signal, from the in-phase and quadrature components of lock-in amplifier two, through... as well as The amplitude and phase delay of the high-frequency deformation caused by the surface acoustic wave at any point on the sample are obtained; the sample is moved to obtain a scanning image of the SAW, and the imaging data is subjected to fast Fourier analysis to obtain the sound field distribution.

[0041] Beneficial effects of the technical solution:

[0042] 1. The present invention provides a magneto-optical effect measurement system based on optical demodulation. The low-temperature design ensures the low-temperature environment of the sample. The interferometric magneto-optical Kerr measurement method is used to complete the high-precision magneto-optical Kerr measurement. At the same time, the superheterodyne demodulation is used to reduce the measurement frequency to the medium frequency that is easy to measure in conventional laboratories, thus achieving the goal of high-precision magneto-optical Kerr measurement at extremely low temperatures.

[0043] 2. This invention provides a high-frequency surface acoustic field measurement system based on optical demodulation, which simultaneously characterizes the phase and amplitude of surface acoustic waves by visualizing the high-frequency sound field distribution. Furthermore, it utilizes optical superheterodyne demodulation to reduce the high-frequency sound field signal to an intermediate frequency that is easily measured in conventional laboratories. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of a magneto-optical effect measurement system based on optical demodulation in Embodiment 1 of the present invention;

[0045] Figure 2 This is a schematic diagram of the structure of the cryogenic subsystem based on the dilution refrigeration unit modified in Embodiment 1 of the present invention;

[0046] Figure 3 This is a schematic diagram illustrating the changes in the first and second harmonic signals with the modulation voltage in Embodiment 1 of the present invention;

[0047] Figure 4 This is a graph showing the changes in temperature and measurement accuracy of the mixing cavity disk under different incident light powers in Embodiment 1 of the present invention.

[0048] Figure 5 This is a graph showing the temperature change of the hybrid cavity disk during long-term displacement of the precision displacement stage under different magnetic fields in Embodiment 1 of the present invention.

[0049] Figure 6 This is a schematic diagram of a high-frequency surface acoustic field measurement system based on optical demodulation in Embodiment 3 of the present invention.

[0050] Figure 7 This is a graph showing the relationship between reflected light intensity, measurement signal amplitude, and eccentricity in Embodiment 3 of the present invention.

[0051] Figure 8 This is a SAW scan image from Embodiment 3 of the present invention.

[0052] In the figure, (a) is the in-phase component image of the lock-in amplifier output, (b) is the quadrature component image of the lock-in amplifier output, (c) is the relationship between the in-phase and quadrature components of the lock-in amplifier output as a function of position, as intercepted by the black dashed lines in (a) and (b); (d) is the FFT of (c), which can reflect the noise floor of its amplitude measurement.

[0053] The names of the corresponding labels in the attached diagram are:

[0054] 1. Broadband fiber optic light source; 2. Isolator; 3. Intensity modulator I; 4. Circulator I; 5. Polarizer; 6. Phase modulator; 7. Function generator; 8. Polarization-maintaining fiber; 9. Collimating lens; 10. Quarter glass slide; 11. Sample; 12. Photodetector I; 13. Lock-in amplifier I; 14. Distillation dish; 15. Distillation heat shield; 16. Mixing chamber disk; 17. Mixing chamber cold finger; 18. Optical adjustment frame; 19. Precision displacement stage; 20. Sample heat shield; 21. Precision screw; 22. Tension spring; 23. Self-emitting amplifying radiation source; 24. Intensity modulator II; 25. Circulator II; 26. Self-focusing lens; 27. Focusing lens II; 28. Photodetector II; 29. ​​Lock-in amplifier II. Detailed Implementation

[0055] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments:

[0056] Example 1: As Figure 1 As shown, a magneto-optical effect measurement system based on optical demodulation includes a fiber broadband light source 1, an isolator 2, an intensity modulator 3, a circulator 4, a polarizer 5, a phase modulator 6, a function generator 7, a polarization-maintaining fiber 8, a collimating lens 9, a quarter-glass slide 10, a photodetector 12, a lock-in amplifier 13, and a cryogenic subsystem.

[0057] Fiber optic broadband light source 1 is used to generate the initial optical signal. Isolator 2 is used to isolate reflected light generated by other devices to ensure the stability of the operating mode of fiber optic broadband light source 1. Intensity modulator 3 is used to adjust the intensity of the initial optical signal for optical mixing and demodulation. Circulator 4 enables unidirectional transmission and control of the optical signal, maintains polarization stability, and serves as a key component in bidirectional optical communication transmission lines, ensuring accurate and stable transmission and processing of the optical signal in the system. The optical signal adjusted by intensity modulator 3 enters polarizer 5 through circulator 4. Polarizer 5 and phase modulator 6 are used to modulate the beam, and function generator 7 is used to adjust the intensity modulator 3 and phase modulator 6. The phase modulator 6 operates at a specific frequency and provides a clock synchronization signal; the polarization-maintaining fiber 8 has birefringence characteristics, allowing two perpendicularly polarized linearly polarized lights to propagate independently, and is used to connect the phase modulator 6 to the collimating mirror 9, enabling bidirectional transmission of the optical signal; the collimating mirror 9 is used to collimate the optical signal into parallel light, and the quarter-glass slide 10 is used to convert linearly polarized light into circularly polarized light and the reflected beam into linearly polarized light; the reflected beam of the sample 11 interferes after demodulation, generating an interference signal, which enters the photodetector 12 through the circulator 4, and the photodetector 12 is used to collect the interference signal, while the lock-in amplifier 13 is used to record the interference signal;

[0058] The cryogenic subsystem includes a dilution refrigerator, a polarization-maintaining fiber 8, a collimating lens 9, a quarter glass slide 10, and a sample 11, all housed within the vacuum chamber of the dilution refrigerator. A fiber broadband light source 1, an isolator 2, an intensity modulator 3, a circulator 4, a polarizer 5, a phase modulator 6, a function generator 7, a photodetector 12, and a lock-in amplifier 13 are located outside the dilution refrigerator. The optical fibers inside and outside the dilution refrigerator are connected via a high-vacuum polarization-maintaining fiber 8.

[0059] The interference signal recorded by the lock-in amplifier 13 includes the amplitudes of the first harmonic and second harmonic signals of the modulation angular frequency; and based on the amplitudes of the first and second harmonic signals, the magneto-optical Kerr angle is calculated. The formula for calculation is:

[0060]

[0061] In the formula, For modulation frequency, and These are the amplitudes of the first harmonic signal and the second harmonic signal, respectively. , These are first-order and second-order Bessel functions of the first kind, respectively. Modulation depth;

[0062] This embodiment utilizes optical superheterodyne demodulation to address the issues of excessively high measurement frequency and excessively long optical fiber, enabling measurements to be performed using a conventional lock-in amplifier. By adding an electro-optic amplitude modulator before circulator 4, the light intensity is increased. ,in This is the modulation frequency of the amplitude modulator; at this point, the original harmonic signal becomes... ;make and The difference frequency in The magnitude can be determined by measuring lower frequencies. The frequency signal is used to obtain the first harmonic amplitude.

[0063] Similarly, change Make it and The difference frequency in With this level of measurement capability, the second harmonic amplitude can be measured at low frequencies. This embodiment, based on the above design, will measure the amplitude of the target frequency. High-frequency signal frequency shifted to The intermediate frequency (IF) can also be increased while maintaining the measurement frequency at the IF. This shortens the fiber length between the phase modulator 6 and the collimating lens 9, reducing interference caused by stress and other factors on the fiber. The first and second harmonic signals, after optical superheterodyne demodulation, such as... Figure 3 As shown.

[0064] like Figure 2 As shown, the cryogenic subsystem based on the modified dilution refrigerator includes a distillation plate 14, a distillation heat shield 15, a mixing chamber plate 16, a mixing chamber cooling finger 17, an optical adjustment frame 18, a precision displacement stage 19, a sample 11, a sample 11, a sample 11, a sample 11, a sample holder, and a sample holder. To achieve imaging, this embodiment uses the precision displacement stage 19 to scan the sample 11. During scanning, the precision displacement stage 19 controller applies a high-voltage triangular wave signal to the piezoelectric ceramic in the stage guide rail, causing it to actuate and thus generate displacement. The modification method is as follows:

[0065] First, the distillation heat shield 15 was modified by replacing part of the original sample distillation heat shield 15 with a hollow brass tube to connect the precision translation stage and the sample holder. The optical adjustment frame 18, the precision translation stage 19, and the distillation heat shield 15 surrounding the sample holder are connected to the distillation pan 14 (approximately 900 mK) by a custom-made hollow brass rod, ensuring good thermal contact and very low thermal resistance. The cooling power on the distillation pan 14 is much greater than the heating power generated by the friction of the precision translation stage 19. Therefore, this thermal connection method can ensure that the precision translation stage 19, the optical adjustment frame 18, and the distillation heat shield 15 are maintained at 900 mK.

[0066] Secondly, the mixing chamber 16 (with a minimum temperature of approximately 10 mK and a cooling capacity of 400 kWh at 100 mK) The sample holder is connected to the sample holder with a thermally conductive copper wire to ensure that the sample holder and sample 11 have a cold source with sufficient power to be cooled to below 100mK by the mixing chamber disk 16.

[0067] Meanwhile, the sample holder and the optical adjustment frame 18 are connected only by optical precision screws 21 and tension springs 22. Since the optical precision screws 21 and tension springs 22 are in point contact with the sample holder, the thermal connection between the sample holder and the optical adjustment frame 18 is poor, resulting in high thermal resistance. Therefore, the frictional heat generated by the displacement stage will not flow to the sample holder with higher thermal resistance, but will only flow to the distillation plate 14 with very low thermal resistance. This ensures that the sample 11 area can be cooled by the relatively small cooling power of the mixing chamber plate 16, while preventing the temperature from rising due to the frictional heat of the precision displacement stage 19.

[0068] Finally, the low-resistance cable transmitting the triangular wave signal is placed in tiered heat sinks on 50K and 4K cold plates, allowing the heat leakage from the cable to be released step-by-step across these two cold plates. Because the 50K and 4K cold plates have high cooling capacity, the cable can be cooled progressively from room temperature along the heat sinks until it reaches the point of direct electrical connection with the displacement stage, where it is cooled to the same temperature as the stage. This step-by-step cooling ensures that the vast majority of the heat leakage is cooled by the 50K and 4K cold plates, minimizing heat leakage from the low-resistance signal transmission cable to the displacement stage.

[0069] After the above low-temperature design, such as Figure 4 As shown, within the incident light intensity range required for conventional measurements, the temperature of the mixing chamber disk 16 can be maintained below 20 mK, while the measurement accuracy is better than 4. This demonstrates that the demodulation method described above can maintain high measurement accuracy even in extremely low temperature environments. Under different magnetic fields, when the translation stage moves over a wide range, the temperature of the mixing chamber disk 16 remains as follows: Figure 5 As shown, all values ​​are around 56 mK. This indicates that the low-temperature design based on this embodiment can guarantee the extremely low-temperature environment of sample 11.

[0070] In summary, by ensuring good thermal contact between the sample holder environment and the high-power low-temperature cold source (distillation plate 14), and by using stepwise heat sinks for the low-resistance cables that can leak heat to the environment, the low-temperature environment of the sample holder can be guaranteed. By ensuring good thermal contact between the sample holder and the low-power ultra-low-temperature cold source (mixing chamber plate 16), and by ensuring good thermal isolation between the low-temperature environment and the sample holder, the temperature of the sample holder and sample 11 can be kept stable at extremely low temperatures. In other words, this embodiment ensures the low-temperature environment of sample 11 through low-temperature design, achieves high-precision magneto-optical Kerr measurement using an interferometric magneto-optical Kerr measurement method, and reduces the measurement frequency to a mid-frequency that is easily measured in conventional laboratories using superheterodyne demodulation, thus achieving the goal of performing high-precision magneto-optical Kerr measurement at extremely low temperatures.

[0071] Example 2: A magneto-optical effect measurement system based on optical demodulation for measuring changes in the polarization state of light, in order to detect Kerr angles smaller than 1. Properties of two-dimensional magnetic materials with a phase transition temperature of less than 4K.

[0072] Example 3: As Figure 6 As shown, a high-frequency surface acoustic field measurement system based on optical demodulation includes a self-emitting amplifying radiation source 23, an intensity modulator 24, a circulator 25, a self-focusing lens 26, a focusing lens 27, a photodetector 28, and a lock-in amplifier 29.

[0073] Among them, the self-emitting amplifying radiation source 23 is used to generate a broadband initial light signal to provide an illumination source; the light intensity modulator 24 is used to modulate the intensity of the light signal to achieve light intensity modulation to support subsequent superheterodyne demodulation; the circulator 25 is used to control the unidirectional transmission of the light signal, guide the incident light to the sample and guide the reflected light to the detection path; the self-focusing lens 26 is used to collect the reflected light from the sample and detect the light intensity change caused by the movement of the light spot; the focusing lens 27 is used to focus the incident light onto the sample surface to form a small light spot to detect local acoustic waveform changes; the surface acoustic waves of the sample cause changes in the light reflection characteristics; the photodetector 28 is used to convert the received reflected light signal into an electrical signal; the lock-in amplifier 29 is used to demodulate the electrical signal, extract the amplitude and phase information of the surface acoustic waves, and process the high-frequency signal to achieve imaging.

[0074] Due to the undulations caused by surface acoustic waves In the PM range, much smaller than the SAW wavelength. Therefore, when the diameter of the incident light spot Less than half wavelength of SAW ,Right now At that time, the maximum tilt angle of the surface acoustic wave at the incident light point can be considered as... Therefore, the tilt angle at the incident light point can be written as ,in For the frequency of SAW, This represents the phase distribution of SAW in the propagation space.

[0075] Let the horizontal distance between the optical center of self-focusing lens 26 and the optical center of focusing lens 27 be... The distance from the center of the reflected light spot to the optical center of the self-focusing lens 26 is... The intensity of the reflected light received by the self-focusing lens 26 is denoted as The maximum reflected light intensity is denoted as .like Figure 7 The black square in the middle shows the actual measurement. The black curve is obtained by fitting the measured data with a Gaussian function. The tilt angle caused by the surface acoustic wave at the incident photoelectric point. This will cause the reflected light spot to move. ,like Figure 6 As shown, where This is the focal length of the focusing lens. The reflected light spot moves under the influence of surface acoustic waves. Then the intensity of the reflected light received by the self-focusing lens 26 will also become ,because Therefore, the change in light intensity caused by surface acoustic waves is:

[0076]

[0077] In the formula, Let t be the maximum tilt angle at the incident light point, and t be time. For the focal length of the focusing lens, For the frequency of SAW, This represents the phase distribution of SAW in the propagation space.

[0078] Therefore, when At its maximum, the change in light intensity caused by surface acoustic waves is also extremely large. This can be achieved by measuring different eccentricities. The change in light intensity can be obtained as follows Figure 7 The curve represented by the red origin in the diagram.

[0079] because Generally in the hundreds of megabytes The magnitude is much larger than the bandwidth of a typical lab-grade lock-in amplifier. This embodiment utilizes an electro-optic amplitude modulator for superheterodyne demodulation. At this point, the reflected light intensity is... Changes in light intensity caused by surface acoustic waves

[0080]

[0081] By making The detection bandwidth is less than that of the lock-in amplifier 29, in order to detect The frequency signal, from the in-phase and quadrature components of the lock-in amplifier 29, through... as well as The amplitude and phase delay of the high-frequency deformation caused by the surface acoustic wave at any point on the sample were obtained. Simultaneously, the sample was moved using a horizontal displacement stage to obtain a scanning image of the SAW. The imaging results are as follows: Figure 8 As shown in (a) and (b) in the figure.

[0082] Through the Figure 8 Fast Fourier analysis of the data in (a) yields... Figure 8 (d). From this spectrum result, the noise floor can be obtained as: , corresponding to less than The resolution of sound wave fluctuations.

[0083] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific technical solutions or characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A magneto-optical effect measurement system based on optical demodulation, characterized in that, It includes a fiber broadband light source, isolator, light intensity modulator I, circulator I, polarizer, phase modulator, function generator, polarization-maintaining fiber, collimating lens, quarter glass slide, photodetector I, and lock-in amplifier I; The fiber broadband light source is used to generate the initial optical signal. The isolator is used to isolate reflected light generated by other devices to ensure the stability of the fiber broadband light source's operating mode. The intensity modulator is used to adjust the intensity of the initial optical signal for optical mixing and demodulation. The circulator enables unidirectional transmission and control of the optical signal, maintains polarization stability, and serves as a component in the optical sensing circuit, allowing the optical signal to be transmitted in the system with low loss, accuracy, and without disturbance. The optical signal adjusted by the intensity modulator enters the polarizer through the circulator. The polarizer and phase modulator are used to modulate the beam, and the function generator is used to adjust the operating frequency of the intensity modulator and the phase modulator and to provide a clock synchronization signal. The polarization-maintaining fiber has birefringence, allowing two linearly polarized lights with perpendicular polarization to propagate independently. It is used to connect the phase modulator and the collimating lens to realize the transmission and reception of the sensing optical signal and provide the necessary delay for phase modulation. The collimating lens is used to collimate the optical signal into parallel light and to efficiently couple the reflected light from the surface of the sample under test into the fiber optic return detector. The quarter-glass slide is used to convert linearly polarized light into circularly polarized light and to convert the reflected beam into linearly polarized light. When the reflected light beam from the sample passes through the polarizer, interference occurs, generating an interference signal. The interference signal enters the photodetector through the circulator and is used to collect the interference signal. The lock-in amplifier is used to record the interference signal.

2. The magneto-optical effect measurement system based on optical demodulation according to claim 1, characterized in that, It also includes a cryogenic subsystem to ensure an extremely low temperature environment for the sample. The cryogenic subsystem includes a dilution refrigerator. The polarization-maintaining fiber, collimating lens, quarter glass slide, and sample are placed inside the vacuum chamber of the dilution refrigerator. The fiber broadband light source, isolator, light intensity modulator, circulator, polarizer, phase modulator, function generator, photodetector, and lock-in amplifier are located outside the dilution refrigerator. The optical fibers inside and outside the dilution refrigerator are connected by a high-vacuum polarization-maintaining fiber feedthrough.

3. The magneto-optical effect measurement system based on optical demodulation according to claim 2, characterized in that, The interference signal recorded by the lock-in amplifier includes the amplitude of the first harmonic signal and the amplitude of the second harmonic signal of the modulation angular frequency; and the magneto-optical Kerr angle is calculated based on the amplitude of the first harmonic signal and the amplitude of the second harmonic signal.

4. The magneto-optical effect measurement system based on optical demodulation according to claim 3, characterized in that, Magneto-optical Kerr angle The calculation method is as follows: Superheterodyne demodulation is performed using an intensity modulator to reduce the bandwidth requirements of the photodetector; a low-frequency detector is used to reduce measurement noise and improve measurement accuracy; intensity modulation is applied to reduce the influence of electromagnetic interference and improve measurement stability. The incident light intensity modulated by the light intensity modulator The final output of the interferometer, including amplitude modulation, is: In the formula, For phase modulation frequency, Modulation depth; For a with Related frequency, when equal or At that time, a low-frequency detector is used to retain the DC and low-frequency components. Specifically: hour, hour, In the formula, , The first and second order Bessel functions of the first kind are respectively, and the incident light intensity after modulation by the intensity modulator is... The second harmonic amplitude δ is less than 1 to obtain a better signal-to-noise ratio and dynamic range; Difference frequency obtained by using low-frequency locking amplitude and amplitude They are respectively: Finally, the magneto-optical Kerr angle was obtained. The formula for calculation is: 。 5. The magneto-optical effect measurement system based on optical demodulation according to claim 2, characterized in that, The cryogenic subsystem further includes a distillation pan, a distillation heat shield, a mixing chamber pan, a mixing chamber cooling finger, an optical adjustment frame, a precision displacement stage, a sample heat shield, precision screws, a tension spring, and a sample holder. The optical adjustment frame, precision displacement stage, and sample heat shield are connected to the distillation pan via connectors. The precision displacement stage is connected to the sample holder via the distillation heat shield. The mixing chamber pan is connected to the sample holder via the mixing chamber cooling finger. The sample holder is connected to the optical adjustment frame via the precision screws and a tension spring. The sample is placed on the sample holder, and the precision displacement stage is used to scan the sample. During the scanning process, the precision displacement stage controller applies a high-voltage triangular wave signal to the piezoelectric ceramic in the displacement stage guide rail, causing it to actuate and thus generate displacement.

6. The magneto-optical effect measurement system based on optical demodulation according to claim 5, characterized in that, The distillation heat shield is a hollow brass tube.

7. The magneto-optical effect measurement system based on optical demodulation according to claim 5, characterized in that, The connector is a hollow brass rod.

8. The magneto-optical effect measurement system based on optical demodulation according to claim 5, characterized in that, The mixing chamber cooling finger is a thermally conductive copper wire.

9. The magneto-optical effect measurement system based on optical demodulation according to claim 5 is used for measuring changes in the polarization state of light, to detect Kerr angles smaller than 1. Properties of two-dimensional magnetic materials with a phase transition temperature of less than 4K.

10. A high-frequency surface acoustic field measurement system based on optical demodulation, characterized in that, It includes a self-emitting large-radiation source, a second light intensity modulator, a second circulator, a second self-focusing lens, a second focusing lens, a second photodetector, and a second lock-in amplifier; The system comprises the following components: a self-emitting amplifying light source to generate a broadband initial light signal and provide illumination; a second light intensity modulator to modulate the intensity of the light signal to support subsequent superheterodyne demodulation; a second circulator to control the unidirectional transmission of the light signal, guiding the incident light to the sample and the reflected light to the detection path; a self-focusing lens to collect the reflected light from the sample and detect changes in light intensity caused by the movement of the light spot; a second focusing lens to focus the incident light onto the sample surface, forming a small light spot to detect local acoustic waveform variations; a second photodetector to convert the received reflected light signal into an electrical signal; and a second lock-in amplifier to demodulate the electrical signal, extract the amplitude and phase information of the surface acoustic wave, and process the high-frequency signal to achieve imaging.

11. The method for measuring surface acoustic wave imaging using a high-frequency surface acoustic field measurement system based on optical demodulation according to claim 10, characterized in that, Let the horizontal distance between the optical center of the self-focusing lens and the optical centers of the focusing lens be... The distance from the center of the reflected light spot to the optical center of the self-focusing lens is obtained as follows: The intensity of the reflected light received by the self-focusing lens is denoted as . The maximum reflected light intensity is denoted as The tilt angle caused by surface acoustic waves at the incident photoelectric point This causes the reflected light spot to move. ; The distance the reflected light spot moves under the action of surface acoustic waves The intensity of the reflected light received by the self-focusing lens will also become ,because Therefore, the change in light intensity caused by surface acoustic waves is: In the formula, Let t be the maximum tilt angle at the incident light point, and t be time. For the focal length of the focusing lens, For the frequency of SAW, This represents the phase distribution of SAW in the propagation space.

12. The method for measuring surface acoustic wave imaging using a high-frequency surface acoustic field measurement system based on optical demodulation according to claim 11, characterized in that, Superheterodyne demodulation is performed using an electro-optic amplitude modulator; the intensity of the reflected light is then... The formula for calculating the change in light intensity caused by surface acoustic waves is: By making The detection bandwidth is less than that of the second lock-in amplifier, in order to detect... The frequency signal, from the in-phase and quadrature components of lock-in amplifier two, through... as well as The amplitude and phase delay of the high-frequency deformation caused by the surface acoustic wave at any point on the sample are obtained; the sample is moved to obtain a scanning image of the SAW, and the imaging data is subjected to fast Fourier analysis to obtain the sound field distribution.

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