A displacement sensor based on a mach-zehnder interferometer

Abstract

The application discloses a displacement sensor based on a Mach-Zehnder interferometer, comprising a laser, a beam splitter, an acousto-optic modulator, a first radio frequency driver, a first collimator, a first half-wave plate, a first mirror loaded on a measured object, a second collimator, a second half-wave plate, a second mirror loaded with a piezoelectric ceramic, a first polarization beam splitter prism, a third half-wave plate, a second polarization beam splitter prism, a balanced photodetector, a lock-in amplifier, a second radio frequency driver, a proportional-integral-derivative controller and a high-voltage amplifier. The application realizes self-adaptive phase locking by using a servo feedback system, and simultaneously uses a heterodyne modulation to form a modulation and demodulation module, so that the signal-to-noise ratio and stability of the sensor are improved. The background noise of the sensor is as low as 7.3 nm / √Hz, and a displacement signal with an amplitude of 24.2 nm is actually measured. The sensor can not only realize the calibration of displacement, but also can be used for the performance calibration of the expansion coefficient of materials.

Description

TECHNICAL FIELD

[0001] The present application relates to displacement sensors, in particular to a displacement sensor based on a Mach-Zehnder interferometer. BACKGROUND

[0002] Modern laser interferometry has experienced an evolution from the proposal of the basic theory to the modern high-precision application. From the early proposal of optical interference theory to the invention and application of laser, laser interferometer began to be applied to various precision measurement fields. At the same time, based on the excellent time coherence of laser, various laser interferometers were developed, including Michelson interferometer. These historical backgrounds and technical developments lay the foundation for the formation and application of laser interferometry, making it an indispensable tool in the field of modern precision measurement. However, the stability and measurement accuracy of the displacement measurement device based on the laser interferometer in the prior art still need to be further improved. SUMMARY

[0003] In view of the problems existing in the prior art, the purpose of the present application is to provide a displacement sensor based on a Mach-Zehnder interferometer with higher stability and measurement accuracy.

[0004] In order to achieve the above-mentioned application purpose, the present application provides the following technical scheme:

[0005] A displacement sensor based on a Mach-Zehnder interferometer includes a laser, a beam splitter, an acousto-optic modulator, a first radio frequency driver, a first collimator, a first half-wave plate, a first mirror mounted on a test object, a second collimator, a second half-wave plate, a second mirror mounted with piezoelectric ceramic, a first polarizing beam splitter, a third half-wave plate, a second polarizing beam splitter, a balanced photodetector, a lock-in amplifier, a second radio frequency driver, a proportional-integral-differential controller, and a high-voltage amplifier. The laser emits a laser beam, the beam splitter splits the emitted laser beam into a first laser and a second laser, and the acousto-optic modulator, driven by the first radio frequency driver, introduces a target frequency shift into the first laser. The first laser with the introduced target frequency shift passes through the first collimator and the first half-wave plate, is reflected by the first mirror, and then to the first polarizing beam splitter. The second laser... After passing through the second collimator and the second half-wave plate, the light is reflected by the second mirror to the first polarizing beam splitter, causing the first laser and the second laser to combine at the first polarizing beam splitter. The combined light then undergoes optical interference after passing through the third half-wave plate. The interference beam is split by the second polarizing beam splitter and then incident on the balanced photodetector. The balanced photodetector converts the detected optical signal into an electrical signal and outputs it, enabling displacement measurement based on the electrical signal from the balanced photodetector. The electrical signal from the balanced photodetector is also sent to the lock-in amplifier, which, driven by the second RF driver, demodulates the error signal. The error signal is phase-locked by the proportional-integral-differential controller, amplified by the second high-voltage amplifier, and input to the piezoelectric ceramic of the second mirror, causing the second mirror to move and locking the phase of the interference beam at π / 2.

[0006] Furthermore, the beam splitter is an optical fiber beam splitter, used to split the laser emitted by the laser into a first laser and a second laser with a 50 / 50 splitting ratio.

[0007] Furthermore, the acousto-optic modulator is an optical fiber acousto-optic modulator, which is connected to the first laser output port of the beam splitter.

[0008] Furthermore, the first collimator is a fiber collimator, used to convert the first laser from a fiber laser into a space laser.

[0009] Furthermore, the second collimator is an optical fiber collimator, which is connected to the second laser output port of the beam splitter via a first optical fiber, the length of which is the same as the length of the acousto-optic modulator.

[0010] Furthermore, the first radio frequency driver is used to output an 80MHz drive signal to drive the acousto-optic modulator to introduce an 80MHz frequency shift to the first laser.

[0011] Further, the second radio frequency driver is used for outputting a 80MHz driving signal.

[0012] Further, the balanced photodetector is composed of two photoelectric diodes with consistent optical response, each photoelectric diode is used for receiving an interference light beam, and the two photoelectric diodes convert the received interference light beams into electric currents and then subtract the electric currents to form a voltage signal output.

[0013] Further, the laser is a continuous wave narrow linewidth laser.

[0014] Further, the second polarization beam splitter divides the interference light beam into two interference light beams with a splitting ratio of 50 / 50.

[0015] Compared with the prior art, the present application has the following beneficial effects: firstly, a series of high-performance optoelectronic devices are used, including an acousto-optic modulator, a polarization beam splitter, a fiber beam splitter, a fiber collimator, a lock-in amplifier and a PID controller. The fiber beam splitter can divide the light beam into two beams with a splitting ratio of 50 / 50; the acousto-optic modulator introduces a radio frequency frequency shift for the laser, providing heterodyne modulation for subsequent demodulation work; the fiber collimator converts the light beam output by the optical fiber into a collimated light beam, improving the uniformity and quality of the light beam; the polarization beam splitter has accurate splitting ratio and high polarization state purity, which can improve the interference contrast of the interferometer; the lock-in amplifier can effectively suppress the interference of non-target frequencies, improving the detection sensitivity and detection accuracy of the signal; the PID controller can stabilize the phase-locked operating point, reduce overshoot and oscillation, and through adjusting the proportional, integral and differential parameters, the response speed and stability of the system can be optimized. The present application uses a servo feedback system to realize adaptive phase locking, and uses heterodyne modulation to form a modulation and demodulation module, which cooperatively improves the signal-to-noise ratio and stability of the device. The interference efficiency of the system is as high as 99.1%, and the interference contrast is 0.98. Secondly, the interference signal is collected by the balanced photodetector, and the balanced detection method can better suppress common-mode noise and improve the signal-to-noise ratio. Finally, the system has high-precision displacement resolution, and can be applied not only to the research of optical properties of substances, but also to the fields of precision machining and quantum precision measurement, such as measurement of material expansion coefficient, real-time dynamic monitoring of substance changes, etc. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 A structure schematic diagram of a displacement sensor based on a Mach-Zehnder interferometer is provided for the embodiment of the present application;

[0017] Figure 2 A structure schematic diagram of an experimental system of a displacement sensor based on a Mach-Zehnder interferometer is provided for the embodiment of the present application;

[0018] Figure 3The interference signal detected by the balanced photodetector of the present application;

[0019] Figure 4 The change from lock-off to lock-on of the interference signal detected by the balanced photodetector of the present application;

[0020] Figure 5 The noise performance of the error signal when the present application is locked;

[0021] Figure 6 The displacement variation resolved by the present application. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application.

[0023] As shown in the drawings, Figure 1 The displacement sensor based on the Mach-Zehnder interferometer provided by the embodiments of the present application comprises a laser 1, a beam splitter 2, an acousto-optic modulator 3, a first radio frequency driver 4, a first collimator 5, a first half-wave plate 6, a first mirror 7 loaded on a measured object, a second collimator 9, a second half-wave plate 10, a second mirror 11 loaded with a piezoelectric ceramic (PZT), a first polarization beam splitter 12, a third half-wave plate 13, a second polarization beam splitter 14, a balanced photodetector 15, a phase-locked amplifier 16, a second radio frequency driver 17, a proportional-integral-derivative controller (PID) 18, and a high-voltage amplifier 19.

[0024] The laser 1 is a continuous wave narrow linewidth laser, which is used to output a free space type continuous wave narrow linewidth 1064 nm laser. The beam splitter 2 is a fiber beam splitter, which is used to divide the laser emitted by the laser into a first laser and a second laser at a splitting ratio of 50 / 50. The first laser power and the second laser power are consistent.

[0025] The first radio frequency driver 4 is used to output a 80 MHz driving signal. The acousto-optic modulator 3 is a fiber type acousto-optic modulator, which is connected to the first laser output port of the beam splitter 2, and is used to introduce a 80 MHz frequency offset to the first laser under the driving of the first radio frequency driver 4. The first collimator 5 is a fiber collimator, which is used to convert the first laser from a fiber laser to a spatial laser. The first half-wave plate 6 is used to adjust the polarization state of the first laser. The first mirror 7 is used to reflect the first laser to the first polarization beam splitter 12. The first mirror 7 is loaded on the measured object, and when the measured object is displaced, the optical path of the first laser will change.

[0026] The second collimator 9 is a fiber collimator, connected to the second laser output port of the beam splitter 2 through the first optical fiber 8, the length of the first optical fiber 8 is consistent with the length of the acousto-optic modulator 3, used to compensate for the optical path variation introduced by the acousto-optic modulator 3. The second half-wave plate 10 is used to adjust the polarization state of the second laser, and the second laser is reflected to the first polarization beam splitter prism 12 through the second mirror 11, so that the first laser and the second laser are combined in the first polarization beam splitter prism, and the second laser in the combined light is consistent with the polarization state of the first laser after the third half-wave plate 13, thereby optical interference occurs, forming an interference beam. Among them, the beam splitter 2, the acousto-optic modulator 3, the first radio frequency driver 4, the first collimator, the first half-wave plate 6, the first mirror 7, the first optical fiber 8, the second collimator 9, the second half-wave plate 10, the second mirror 11, the first polarization beam splitter prism 12, and the third half-wave plate 13 together form a Mach-Zehnder interferometer.

[0027] The interference beam is divided into two interference beams at a splitting ratio of 50 / 50 in the second polarization beam splitter prism 14, and is incident into the balanced photodetector 15 respectively. The balanced photodetector 15 is composed of two photodiodes with consistent optical response, each photodiode is used to receive an interference beam, and the two photodiodes subtract the received interference beams after converting them into currents to form a voltage signal output, thereby suppressing the common-mode noise of the photoelectric system and improving the detection accuracy and signal-to-noise ratio. When the measured object is displaced, the optical path of the first laser will change, the first laser and the second laser will produce an optical path difference, the interference beam will change, and then the voltage signal output by the balanced photodetector 15 will change, so that the displacement measurement can be realized through the voltage signal output by the balanced photodetector 15.

[0028] The electrical signal of the balanced photodetector 15 is also sent to a phase-locked amplifier 16. A second radio frequency driver 17 is used to output a driving signal of 80MHz. The phase-locked amplifier 16 is used to input the driving signal of 80MHz as an input signal under the driving of the second radio frequency driver 17, input the electrical signal of the balanced photodetector 15 as an input signal, the frequency of the input signal is consistent with that of a reference signal, mix the input signal with the reference signal to obtain a signal containing a target frequency component, filter out high frequency components in the mixed signal by setting the parameters of a low-pass filter in the phase-locked amplifier 16, and thus obtain a demodulated error signal. The demodulated error signal is output to a proportional-integral-derivative controller 18 to obtain a feedback signal, which can lock the phase of the interference beam at π / 2, i.e. the maximum slope of the interference beam signal. After being amplified by a high-voltage amplifier 19, the feedback signal is output to the piezoelectric ceramic on the second mirror 11, which drives the second mirror 11 to move slightly, and thus adjusts the optical path of the second laser, so that the phase of the interference beam is locked at π / 2, and active phase locking is achieved, thereby improving the stability of the system. The phase-locked amplifier 16, the second radio frequency driver 17, the proportional-integral-derivative controller 18, and the high-voltage amplifier 19 together constitute a servo feedback system, which realizes high-precision demodulation and phase locking of the interference signal through closed-loop control.

[0029] The present application is verified by experiments, and the experimental system used in the experiments is shown in Figure 2 The object to be measured is replaced by a piezoelectric ceramic. In order to realize the slight displacement of the piezoelectric ceramic, a signal generator 21 outputs a triangular wave signal of 10Hz and 6V to a high-voltage amplifier 22, and then to the piezoelectric ceramic loaded on the first mirror 7. The piezoelectric ceramic will produce periodic movement under the driving of the triangular wave signal, so as to produce a periodic optical path difference between the first laser and the second laser. The amplification factor of the high-voltage amplifier 22 is about 37, so that the piezoelectric ceramic works in a safe voltage range. The balanced photodetector 15 is also connected to an oscilloscope 20, which is used to observe and collect the laser interference signal, so as to determine the displacement of the piezoelectric ceramic. The output end of the signal generator 21 is also connected to the oscilloscope 20, which is used to observe the information output by the signal generator 21.

[0030] The experimental results are shown in Figure 3 、 4 , 5, 6. Figure 3For the interference signal observed in the oscilloscope 20, that is, the electrical signal output by the balanced photodetector 15, the interference efficiency reaches 99.1%, and the interference contrast is 0.98. The interference signal detected by the balanced photodetector 15 is output to the lock-in amplifier 16 for demodulation, and the demodulated signal is directly connected to the oscilloscope 20 to observe the error signal after demodulation. The error signal is output to the proportional-integral-derivative controller 18, and the feedback signal output by the proportional-integral-derivative controller 18 is also connected to the oscilloscope 20, so that the feedback signal output by the proportional-integral-derivative controller 18 can be observed. Figure 4 For the change from lock off to lock on of the error signal, that is, the change when the servo feedback system is not working and working, it can be seen that the stability of the interference signal after locking is obviously improved, which reflects the advantage of the servo feedback system. Figure 5 For the noise performance of the system error signal when locked, the first proportional-integral-derivative controller 18 is connected to the oscilloscope 20 to observe, and the background noise of the sensor is calculated to be 7.3 nm / √Hz. The signal generator 21 outputs a 10 mV square wave signal to the piezoelectric ceramic of the first mirror 7, and the proportional-integral-derivative controller 18 is connected to the oscilloscope 20, so that Figure 6 The displacement measurement signal shown has a peak-to-peak value, that is, the displacement change amount that can be distinguished by the system, and an amplitude of 24.2 nm. It can be seen that the present application can detect very small displacement changes and is suitable for application scenarios that require high resolution. The sensor not only can realize displacement calibration, but also can be used for calibration of material expansion coefficients and the like.

[0031] It should be noted that in this paper, relational terms such as first and second are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0032] It should be understood that the above embodiments and descriptions in the specification are only principles, main features and advantages of the present application, and various changes and improvements can be made to the present application without departing from the spirit and scope of the present application, and these changes and improvements all fall within the scope of protection of the present application.

Claims

1. A displacement sensor based on a Mach-Zehnder interferometer, characterized in that: The application relates to a displacement measurement device, comprising a laser, a beam splitter, an acousto-optic modulator, a first radio frequency driver, a first collimator, a first half-wave plate, a first mirror loaded on a measured object, a second collimator, a second half-wave plate, a second mirror loaded with a piezoelectric ceramic, a first polarization beam splitter prism, a third half-wave plate, a second polarization beam splitter prism, a balanced photodetector, a lock-in amplifier, a second radio frequency driver, a proportional-integral-differential controller and a high-voltage amplifier, wherein the laser is used for emitting laser light, the beam splitter is used for splitting the laser light emitted by the laser into first laser light and second laser light, the acousto-optic modulator is used for introducing a target frequency offset into the first laser light under the driving of the first radio frequency driver, the first laser light with the target frequency offset is reflected by the first mirror to the first polarization beam splitter prism after passing through the first collimator and the first half-wave plate, the second laser light is reflected by the second mirror to the first polarization beam splitter prism after passing through the second collimator and the second half-wave plate, so that the first laser light and the second laser light are combined at the first polarization beam splitter prism, optical interference occurs after the combined light passes through the third half-wave plate, and the interference light beam is split by the second polarization beam splitter prism and then enters the balanced photodetector, the balanced photodetector is used for converting the detected optical signal into an electrical signal and outputting, so that displacement measurement is realized according to the electrical signal of the balanced photodetector, and the electrical signal of the balanced photodetector is also sent to the lock-in amplifier, the lock-in amplifier is used for demodulating an error signal under the driving of the second radio frequency driver, the error signal is phase-locked by the proportional-integral-differential controller, is amplified by the second high-voltage amplifier and then is input into the piezoelectric ceramic of the second mirror, so that the second mirror is driven to move and the phase of the interference light beam is locked at pi / 2.

2. The Mach-Zehnder interferometer based displacement sensor according to claim 1, characterized in that: The beam splitter is a fiber beam splitter, which is used for splitting the laser light emitted by the laser into the first laser light and the second laser light at a splitting ratio of 50 / 50.

3. The Mach-Zehnder interferometer based displacement sensor of claim 1, wherein: The acousto-optic modulator is a fiber type acousto-optic modulator, which is connected to the first laser light output port of the beam splitter.

4. The Mach-Zehnder interferometer based displacement sensor of claim 1, wherein: The first collimator is a fiber collimator, which is used for converting the first laser light from a fiber laser into spatial laser light.

5. The Mach-Zehnder interferometer based displacement sensor of claim 1, wherein: The second collimator is a fiber collimator, which is connected to the second laser light output port of the beam splitter through a first optical fiber, and the length of the first optical fiber is consistent with the length of the acousto-optic modulator.

6. The Mach-Zehnder interferometer based displacement sensor of claim 1, wherein: The first radio frequency driver is used for outputting a driving signal of 80 MHz to drive the acousto-optic modulator to introduce a frequency offset of 80 MHz into the first laser light.

7. The Mach-Zehnder interferometer based displacement sensor of claim 1, wherein: The second radio frequency driver is used for outputting a driving signal of 80 MHz.

8. The Mach-Zehnder interferometer based displacement sensor of claim 1, wherein: The balanced photodetector is composed of two photodiodes with consistent optical response, each photodiode is used for receiving an interference light beam, and the two photodiodes subtract the received interference light beams to form a voltage signal output after converting the received interference light beams into electric currents.

9. The Mach-Zehnder interferometer based displacement sensor of claim 1, wherein: The laser is a continuous wave narrow line width laser.

10. The Mach-Zehnder interferometer based displacement sensor of claim 1, wherein: The second polarization beam splitter prism splits the interference light beam into two interference light beams at a splitting ratio of 50 / 50.

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