CN115200692A - Pulse laser interference vibration measurement method based on phase-locked amplification - Google Patents
Pulse laser interference vibration measurement method based on phase-locked amplification Download PDFInfo
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Abstract
A pulse laser interference vibration measurement method based on phase-locked amplification adopts a same-source continuous wave radio frequency signal as an excitation signal to excite mechanical vibration of a sample to be measured and as a local oscillation input signal of a frequency mixer respectively; ultrafast pulse laser is converted into an electric frequency comb signal through a fast photoelectric detector, a vibration interference measurement signal carrying out-of-plane vibration information of a sample is formed through a Michelson interferometer, and the vibration interference measurement signal is input into a phase-locked amplifier as a signal to be measured after passing through a low-speed photoelectric detector; and mixing the electric frequency comb signal with the local oscillator input signal, inputting the mixed signal into a phase-locked amplifier as a reference signal, demodulating the signal to be detected in the phase-locked amplifier, and extracting the amplitude and phase information of the mechanical vibration of the sample to be detected. The invention converts the coherent ultrahigh frequency mechanical vibration into a low-frequency signal through optical down-mixing, and then dynamically extracts the low-frequency signal by utilizing a phase-locked amplification technology, thereby obviously reducing the noise level, realizing high-precision phase-sensitive absolute amplitude detection, and ensuring the minimum detectable amplitude to be about 55 femtos.
Description
Technical Field
The invention relates to a technology in the field of laser vibration measurement, in particular to a pulse laser interference vibration measurement method based on phase-locked amplification.
Background
The laser interference measurement method has wide and successful application in the aspect of representation of micro-acoustic devices, can image displacement fields and phase fields generated by vibration of the resonator in the normal operation process of the resonator, and provides contents in the aspects of intuitive vibration modes, modal mixing, dissipation paths and the like. At present, continuous wave laser scanning homodyne and heterodyne interferometry are mainstream, and can simultaneously obtain the absolute amplitude and phase information of out-of-plane vibration of each test point, so as to realize the phase-sensitive mapping of the surface vibration profile with high resolution. However, the use in the ultra high frequency region (> 3 GHz) is very challenging as they are susceptible to more or less electromagnetic interference from the surrounding environment.
The prior art is limited by the following two points in the practical application process:
one is as follows: crosstalk between the receive and signal, loss in the transmission cable, and noise in the photodetector all increase significantly with increasing frequency, causing the tiny signals to be overwhelmed and difficult to detect.
And the second step is as follows: the amplitude of the vibration of the micromechanical resonator decreases sharply with increasing frequency, and the best displacement resolution achievable by continuous wave laser interferometry is about picometers (10) -12 m) level, difficult to control amplitude at femto meter (10) -15 m) order of magnitude. However, the performance characterization of such devices is crucial to high and new technologies such as 5G wireless communication and quantum information generation, storage and transmission.
Disclosure of Invention
The invention provides a pulse laser interference vibration measurement method based on phase-locked amplification aiming at the defects of low signal-to-noise ratio and low displacement resolution ratio existing in an ultrahigh frequency band in the prior art, wherein coherent ultrahigh frequency mechanical vibration is converted into a low-frequency signal through optical down-mixing, and then the low-frequency signal is dynamically extracted by utilizing the phase-locked amplification technology, so that the noise level is obviously reduced, high-precision phase-sensitive absolute amplitude detection can be realized, and the minimum detectable amplitude is about 55 femtos.
The invention is realized by the following technical scheme:
the invention relates to a pulse laser interference vibration measurement method based on phase-locked amplification, which adopts homologous continuous wave radio frequency signals as excitation signals to excite the mechanical vibration of a sample to be measured and as local oscillation input signals of a frequency mixer respectively; ultrafast pulse laser is converted into an electric frequency comb signal through a fast photoelectric detector, a vibration interference measurement signal carrying out external vibration information of a sample is formed through a Michelson interferometer, and the vibration interference measurement signal is input into a phase-locked amplifier as a signal to be measured after passing through a low-speed photoelectric detector; and mixing the electric frequency comb signal and the local oscillator input signal, inputting the mixed signal serving as a reference signal into a phase-locked amplifier, demodulating the signal to be detected in the phase-locked amplifier, and extracting the amplitude and phase information of the mechanical vibration of the sample to be detected.
The tooth space of the electric frequency comb signal is adjustable by tuning the repetition frequency f of the ultrafast pulse laser light source p Is achieved.
The vibration interferometry signal is generated by the following method:
step 1) ultrafast pulse laser is input into a Michelson interferometer and is divided into reflected beams with mutually orthogonal polarization directions through a polarization beam splitter, namely a measuring arm and a transmitted beam, namely a reference arm, wherein the measuring arm is projected to the surface of a sample generating mechanical vibration and then is reflected;
in step 1, the phase of the reflected light beam from the surface of the sample is modulated by mechanical vibration, and carries out-of-plane vibration information of the sample to be detected, wherein the frequency of the mechanical vibration is equal to the frequency f of the excitation signal e So that the frequency component of the reflected light beam is determined by the resonant frequency mf of each step of the pulsed laser p (m =1,2,3.) and vibration excitation frequency f e Is determined by optical mixing.
Step 2) reflecting the reference arm at the movable reference mirror;
and 3) the reflected light beams generated in the steps 1 and 2 pass through a lambda/4 wave plate twice, the polarization directions are changed by 90 degrees at the same time, the reflected light beams are overlapped at the original polarization beam splitter, and then the reflected light beams are interfered under the action of a polarizing plate to finally generate a vibration interference measurement signal.
The signal to be measured is obtained by vibrationThe interferometric signal is input to a slow-speed photodetector with slow response, and the frequency of the signal is f e And the nth pulse laser harmonic nf closest to it p Difference frequency component f formed therebetween d It is given.
The bandwidth of the fast photoelectric detector is 12.5GHz, and the bandwidth of the low-speed photoelectric detector is 10MHz; under the same input optical power, the equivalent noise power of the low-speed photoelectric detector with the 10MHz bandwidth is at least 10 times lower than that of the fast photoelectric detector with the 12.5GHz bandwidth.
The reference signal is obtained by tuning the repetition frequency f of the pulsed laser p To make its higher harmonic frequency nf p Close to the vibration excitation frequency f e Then mixing the two to form a beat frequency signal f capable of implementing measurement in low frequency band b 。
The signal to be measured and the reference signal have the same frequency, i.e. | nf p -f e |。
Technical effects
The invention uses a repetition frequency f p The adjustable ultrafast pulse laser converts GHz ultrahigh frequency mechanical vibration into a difference frequency signal f of several MHz through an optical down-mixing technology d The difference frequency signal f d The low-speed photoelectric detector can be used for measurement; the obviously reduced noise level enables the technology to achieve unprecedented displacement resolution when ultrahigh frequency mechanical vibration measurement is carried out, and high-precision dynamic extraction of femto-meter mechanical vibration displacement information can be realized by combining a phase-locked amplification technology; the technology can directly measure the device in the running state under the typical power level, and has the characteristics of simple and convenient operation, low power consumption and the like.
Drawings
FIG. 1 is a schematic diagram of the operation of the method;
FIG. 2 is a schematic diagram of an embodiment electrical frequency comb;
FIG. 3 is a graph showing the measurement results of the examples;
in the figure: the system comprises an ultrafast pulse laser light source 1, a non-polarization beam splitter 2, a fast photoelectric detector 3, a polarization beam splitter 4, a first lambda/4 wave plate 5, a second lambda/4 wave plate 6, a movable reference mirror 7, a microscope objective lens 8, a sample to be detected 9, a three-dimensional scanning table 10, a distributor 11, a signal source 12, a mixer 13, a low-pass filter 14, a plane mirror 15, a polarizing plate 16, a low-speed photoelectric detector 17 and a phase-locked amplifier 18.
Detailed Description
As shown in fig. 1, the present embodiment relates to a phase-locked amplification-based pulse laser interference vibration measurement method, which includes:
converting a continuous wave radio frequency signal sent by a signal source 12 into two paths of same signals through a distributor 11, wherein one path is used as an excitation signal to excite the mechanical vibration of a sample 9 to be detected, and the other path is used as a local oscillation input signal of a frequency mixer 13;
secondly, pulsed light emitted by the ultrafast pulse laser source 1 is firstly divided into two beams of light with the same polarization by the non-polarization beam splitter 2, one beam enters the Michelson interferometer to form a vibration interference measurement signal, and the other beam enters the fast photoelectric detector 3 to be converted into an electrical frequency comb signal as shown in the figure 2;
the ultrafast pulse laser light source 1 has tunable repetition frequency f p Therefore, the tooth space of the electric frequency comb signal formed after the conversion of the fast photoelectric detector 3 is adjustable.
The generation of the vibration interferometry signal comprises the following steps:
1) The light beam entering the Michelson interferometer is firstly divided into a measuring arm and a reference arm with mutually orthogonal polarization directions by a polarization beam splitter 4;
2) The measuring arm is the return beam of the first reflected light beam generated by the polarizing beam splitter 4 after it has been projected by the microscope objective 8 onto the surface of the sample undergoing mechanical vibrations having a frequency equal to the frequency f of the excitation signal e The phase of the return beam is modulated by the mechanical vibration, so that the phase carries out-of-plane vibration information of the sample to be measured, and the frequency component of the return beam is the resonance frequency mf of each order of the pulse laser p (m =1,2,3.) and vibration excitation frequency f e Optical mixing determination of (a);
3) The reference arm is the reflected beam of the first transmitted beam at the movable reference mirror 7 produced by the polarizing beam splitter 4;
4) The measuring arm and the reference arm pass through the lambda/4 wave plates 5 and 6 twice, the polarization directions are changed by 90 degrees at the same time, the polarization directions are overlapped at the original polarization beam splitter 4, then the interference is generated after the action of the polaroid 16, and finally a vibration interference measuring signal is generated.
Step three, a vibration interference measurement signal generated by the michelson interferometer passes through the low-speed photoelectric detector 17 and then enters the phase-locked amplifier 18 as a signal to be measured, and an electric frequency comb signal output by the fast photoelectric detector 3 and a local oscillation input signal generated by the signal source 12 pass through the mixer 13 and the low-pass filter 14 and then enter the phase-locked amplifier 18 as a reference signal;
the signal to be measured is generated in such a way that the vibration interference measurement signal passes through the slow-response low-speed photoelectric detector 17 and is output as f e And the nth pulse laser harmonic nf closest to it p Difference frequency component f formed therebetween d 。
In particular, the sample emits a continuous wave radio frequency signal f at the signal source 12 e Excited to vibrate, and the vibration information is superposed to the frequency f p The ultrafast pulse laser source 1 emits light, and the phase of the light is modulated on the measuring arm reflected by the polarization beam splitter 4; the superimposed signal is reflected by the sample again back to the polarization beam splitter 4 where it meets the reference arm reflected by the movable reference mirror 7, and a vibration interferometry signal is generated by the polarizer 16; since the low-speed photodetector 17 cannot respond to the high-frequency light component, the output voltage contains only f e And the nth pulse laser harmonic nf closest to it p Difference frequency component f formed therebetween d Obtaining the signal to be measured
Wherein: u shape s And
respectively, the amplitude and phase of the voltage change caused by the mechanical vibration of the sample to be measured.
The reference letterBy tuning the repetition frequency f of a pulsed laser p To make the higher harmonic frequency nf of a certain order p (i.e. corresponding to a tooth in the electrical frequency comb) as close as possible to the vibration excitation frequency f e Then mixing the two to form a beat frequency signal f capable of measuring in a low frequency band b After passing through a low pass filter 14, the output voltage
Wherein: u shape r Is the magnitude of the reference voltage and,
is the phase of the radio frequency excitation signal.
The signal to be measured and the reference signal entering the lock-in amplifier 18 have the same frequency and the magnitude is equal to | nf p -f e |。
Step four, demodulating the signal to be detected in the lock-in amplifier 18, and extracting the amplitude and phase information of the mechanical vibration of the sample to be detected, which specifically comprises: the in-phase component of the signal to be detected and the reference signal is amplified and detected in phase-locked mode, and then the voltage is output
Wherein: due to U r And
it is known, therefore, to derive from the output result of the lock-in amplifier U representing the amplitude and phase of the mechanical vibrations s And
through specific practical experiments, under the environment of room temperature and atmospheric pressure, a 10mW continuous wave radio frequency signal is used for exciting the mechanical vibration of a sample, 100 muW ultrafast pulse laser power is used, and the light source repetition frequency f is adjusted p Tuning between 50MHz and 52.5MHz, performing frequency sweep measurement on the bulk acoustic wave resonator to be measured in the range of 1GHz to 12GHz, setting the step length to be 50MHz, and measuring the displacement of the resonance peak of 6.5GHz to be about1pm。
As shown in fig. 3, the vibration measurement result of the bulk acoustic wave resonator to be measured with the resonance frequency around 6.5GHz shows: when the amplitude is larger than 55fm, the amplitude variation of about 10fm can be detected along with the variation of the excitation power, which proves that the invention can realize extremely excellent measurement precision and sensitivity.
Compared with the prior art, the method has the advantages of obviously reducing the background noise level, improving the displacement measurement sensitivity to dozens of femto meters, meeting the technical requirement of performing precise measurement on ultrahigh frequency micro vibration signals, being capable of directly measuring devices in normal operation under a typical power level, being simple and convenient to operate, low in power consumption and the like.
The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (5)
1. A pulse laser interference vibration measurement method based on phase-locked amplification is characterized in that homologous continuous wave radio frequency signals are respectively used as excitation signals to excite mechanical vibration of a sample to be measured and used as local oscillation input signals of a frequency mixer; ultrafast pulse laser is converted into an electric frequency comb signal through a fast photoelectric detector, a vibration interference measurement signal carrying out-of-plane vibration information of a sample is formed through a Michelson interferometer, and the vibration interference measurement signal is input into a phase-locked amplifier as a signal to be measured after passing through a low-speed photoelectric detector; mixing an electric frequency comb signal with a local oscillator input signal, inputting the mixed signal into a phase-locked amplifier as a reference signal, demodulating a signal to be detected in the phase-locked amplifier, and extracting amplitude and phase information of mechanical vibration of a sample to be detected;
the bandwidth of the fast photoelectric detector is 12.5GHz, and the bandwidth of the low-speed photoelectric detector is 10MHz.
2. The phase-locked amplification-based pulsed laser interference vibration measurement method according to claim 1It is characterized in that the tooth space of the electric frequency comb signal is adjustable by tuning the repetition frequency f of the ultrafast pulse laser light source p Is achieved.
3. The phase-locked amplification-based pulsed laser interference vibration measurement method according to claim 1, wherein the vibration interference measurement signal is generated by:
step 1) ultrafast pulse laser is input into a Michelson interferometer and is divided into reflected beams with mutually orthogonal polarization directions through a polarization beam splitter, namely a measuring arm and a transmitted beam, namely a reference arm, wherein the measuring arm is projected to the surface of a sample generating mechanical vibration and then is reflected; the phase of the reflected beam from the sample surface is modulated by mechanical vibration with frequency equal to the frequency f of the excitation signal and carries out-of-plane vibration information of the sample to be measured e So that the frequency component of the reflected light beam is determined by the resonant frequency mf of each step of the pulsed laser p And vibration excitation frequency f e M =1,2, 3.;
step 2) reflecting the reference arm at the movable reference mirror;
and 3) the reflected light beams generated in the steps 1 and 2 pass through a lambda/4 wave plate twice, the polarization directions are changed by 90 degrees at the same time, the reflected light beams are overlapped at the original polarization beam splitter, and then the reflected light beams are interfered under the action of a polarizing plate to finally generate a vibration interference measurement signal.
4. The phase-locked amplification based pulsed laser interference vibration measurement method as claimed in claim 1, wherein the signal to be measured is obtained by inputting a vibration interference measurement signal into a slow-response low-speed photodetector, and the frequency of the signal is f e And the nth pulse laser harmonic nf closest to it p Difference frequency component f formed therebetween d Giving out;
the reference signal is obtained by tuning the repetition frequency f of the pulsed laser p So that its higher harmonic frequency nf p Close to the excitation frequency f of the vibration e Then mixing the two to form a beat frequency signal f capable of measuring in a low frequency band b ；
The signal to be measured and the reference signal have the same frequency, i.e. | nf p -f e |。
5. The phase-locked amplification-based pulse laser interference vibration measurement method according to any one of claims 1 to 4, which comprises the following steps:
converting a continuous wave radio frequency signal sent by a signal source into two paths of same signals through a distributor, wherein one path of same signals is used as an excitation signal to excite the mechanical vibration of a sample to be detected, and the other path of same signals is used as a local oscillation input signal of a frequency mixer;
secondly, pulsed light emitted by the ultrafast pulse laser source is firstly divided into two beams of light with the same polarization through a non-polarization beam splitter, one beam of light enters a Michelson interferometer to form a vibration interference measurement signal, and the other beam of light enters a fast photoelectric detector to be converted into an electrical frequency comb signal;
the ultrafast pulse laser light source has tunable repetition frequency f p Therefore, the tooth space of the electric frequency comb signal formed after the conversion of the rapid photoelectric detector is adjustable;
step three, a vibration interference measurement signal generated by the Michelson interferometer passes through a low-speed photoelectric detector and then enters a phase-locked amplifier as a signal to be measured, and an electric frequency comb signal output by the quick photoelectric detector and a local oscillation input signal generated by a signal source pass through a mixer and a low-pass filter and then enter the phase-locked amplifier as a reference signal;
the generation mode of the signal to be detected is as follows: continuous wave radio frequency signal f emitted by sample at signal source e Excited to vibrate, and the vibration information is superposed to the frequency f p The ultrafast pulse laser source emits light, and the phase of the light is modulated on the measuring arm reflected by the polarization beam splitter; the superposed signal is reflected back to the polarization beam splitter by the sample again, and after being converged with the reference arm reflected by the movable reference mirror, the superposed signal is generated into a vibration interference measurement signal through a polarizing film; since the low-speed photodetector cannot respond to the high-frequency light component, the output voltage contains only f e And the nth pulse laser harmonic nf nearest to it p Difference frequency component f formed therebetween d Obtaining the signal to be measured
Wherein: u shape s And
respectively the amplitude and the phase of the voltage change caused by the mechanical vibration of the sample to be tested;
the reference signal is obtained by tuning the repetition frequency f of the pulsed laser p To make the higher harmonic frequency nf of a certain order p As close as possible to the vibration excitation frequency f e Then mixing the two to form a beat frequency signal f capable of implementing measurement in low frequency band b After passing through a low-pass filter, the output voltage
Wherein: u shape r Is the magnitude of the reference voltage and,
is the phase of the radio frequency excitation signal;
step four, demodulating the signal to be detected in the phase-locked amplifier, and extracting the amplitude and phase information of the mechanical vibration of the sample to be detected, which specifically comprises the following steps: the in-phase components of the signal to be detected and the reference signal are subjected to phase-locked amplification detection, and then the voltage is output
Wherein: due to U r And
it is known, therefore, to derive from the output result of the lock-in amplifier U representing the amplitude and phase of the mechanical vibrations s And
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