Low-jitter GHz excitation synchronization and calibration time-strobe relocking phase reconstruction system and method

Abstract

The application provides a low-jitter GHz excitation synchronization and calibration delay stroboscopic relocking phase reconstruction system and method, comprising an optoelectronic reference clock extraction module, a clock shaping module, a programmable delay / phase offset module, a delay calibration and lookup table module, a coherent GHz excitation synthesis module, a synchronization monitoring module, an acquisition interface module and a relocking phase reconstruction module. The optoelectronic module extracts a reference clock from a femtosecond pulse train, inputs the shaped clock into a delay module, and generates a GHz excitation locked with an optical comb after the delay. The calibration module obtains the real time corresponding to the delay code as the sampling time axis. The reconstruction module constructs a complex demodulation kernel according to the real time, performs matched filtering or least squares solving on the sequence, and obtains the full-field vibration amplitude and phase. The application is suitable for stroboscopic-excitation synchronization measurement in the MHz to tens of GHz frequency band.

Description

Technical Field

[0001] This invention relates to the fields of ultra-high frequency micro-vibration measurement and femtosecond laser synchrotron electronics, specifically, to a low-jitter GHz excitation synchronization and calibration delay stroboscopic phase-locked reconstruction system and method. More particularly, it relates to a low-jitter GHz excitation synchronization and calibration delay stroboscopic phase-locked reconstruction system and method based on a femtosecond optical comb.

[0002] This invention can be used for femtosecond stroboscopic digital holography, femtosecond pulse interferometry vibration measurement, pump-probe measurement, optoelectronic synchronous RF excitation, full-field vibration measurement of micro / nano electromechanical systems, modal diagnosis of acoustic devices, and ultra-high frequency phase synchronization testing. It is especially suitable for measuring the full-field amplitude, phase, and modal distribution of MEMS / NEMS, FBAR, BAW, XBAR, SAW, PMUT, optoacoustic coupling devices, phononic crystals, and quantum acoustic devices operating in the frequency band of hundreds of MHz to tens of GHz. Background Technology

[0003] With the development of 5G / 6G RF front-ends, on-chip acoustics, quantum acoustics, and high-frequency sensing, an increasing number of micro / nano electromechanical devices are operating in the GHz band. The vibration periods of these devices can be as low as tens of picoseconds or even shorter, and the vibration amplitudes typically drop to the picometer or femtometer level. Traditional electrical testing can obtain overall information such as S-parameters, impedance spectra, and resonant frequencies, but it is difficult to reveal the true acoustic modes, local energy leakage, stray modes, and boundary scattering within the device. Therefore, measurement techniques capable of achieving high sensitivity, non-contact, and full-field imaging in the GHz band are needed.

[0004] Existing measurement techniques for GHz-level sound fields or micro-vibration fields mainly include scanning laser Doppler vibration measurement, scanning homodyne or heterodyne interferometry, ultrafast pump-probe measurement, Brillouin scattering measurement, synchrotron radiation X-ray diffraction imaging, microwave impedance microscopy, and scanning electron microscopy. These methods typically rely on complex systems, have long measurement times, and are difficult to directly obtain quantitative full-field displacement of engineering devices under operating conditions. Stroboscopic holographic measurement methods can use area array cameras to record full-field interferometric images of the device under test at a specific vibration phase, representing an important route for realizing dynamic full-field measurement of micro- and nano-devices. The dynamic time-delay sampling and spatiotemporal joint processing method for stroboscopic holographic micro-vibration measurement, through controllable delay stepping of the synchrotron pulse light and combining time-dimensional single-frequency extraction and spatial-dimensional filtering, can achieve the extraction of picometer-level full-field vibration signals from MEMS devices at the 100MHz level. However, when the measurement object further enters the GHz to tens of GHz frequency band, the device vibration period is shortened to hundreds of picoseconds, tens of picoseconds or even shorter. The time gate width, trigger jitter and phase noise of the nanosecond or sub-nanosecond pulse source will significantly limit the stroboscopic stabilization accuracy. Therefore, it is necessary to introduce picosecond or femtosecond lasers as stroboscopic light sources with sufficiently narrow pulse widths.

[0005] Femtosecond laser pulse trains exhibit a stable sequence of short pulses in the time domain and a uniformly spaced optical frequency comb in the frequency domain. Each femtosecond pulse can be considered as an extremely narrow time sampling gate. When the electrical excitation signal of the device under test is strictly coherently locked with the femtosecond pulse train, the device is in the same or controllably changing vibration phase at the moment each optical pulse arrives, thus enabling stroboscopic freezing of ultra-high frequency vibrations under long-exposure camera conditions.

[0006] Unlike nanosecond pulsed lasers, the pulse cycle within a femtosecond oscillator is determined by the mode-locked cavity and cannot typically be triggered on demand by external electronic signals. Therefore, in femtosecond stroboscopic measurements, the synchronization direction is not achieved by triggering optical pulses with an electrical signal, but rather by generating an electrical reference clock from a femtosecond optical pulse train, which is then used to synthesize the excitation signal. If the synchronization electronic link cannot maintain sufficiently low additional jitter, or if the high-frequency excitation phase cannot be precisely controlled relative to the femtosecond pulse train, stroboscopic freezing will cause phase ambiguity and amplitude averaging, directly reducing the accuracy of picometer-level vibration measurements.

[0007] Existing stroboscopic holography techniques can modulate weak vibration signals into single-frequency components in a sequence dimension through dynamic time-delay sampling, and reduce noise by combining time-domain single-frequency extraction and spatial filtering. However, existing solutions are geared towards the hundreds of MHz range or use externally triggerable pulsed light sources, and their delay steps are often treated as ideally equal intervals. When the measurement bandwidth extends to several GHz to tens of GHz, on the one hand, femtosecond lasers have stable repetition frequency characteristics, making external triggering methods unusable; on the other hand, picosecond-level delay errors at ultra-high frequencies will be amplified into significant phase errors. Programmable delay lines, direct digital frequency synthesizers, phase shifters, comparators, transmission lines, connectors, temperature drift, and PLL phase synchronization processes can all introduce nonlinear delays and residual jitter. In high-frequency measurements, this will result in main spectral line collapse, spectral leakage, amplitude compression, and phase deviation.

[0008] Therefore, there is an urgent need for a synchronous electronics and coding demodulation method that can use a femtosecond optical comb as the master clock, generate GHz excitation with low jitter, precisely control the stroboscopic phase, and directly incorporate the actual delay calibration results into the full-field complex vibration reconstruction. This method should be compatible with both femtosecond holographic full-field measurements and point-type pulse interferometry or other ultra-high frequency measurement systems that require femtosecond optoelectronic synchronization.

[0009] Patent application CN112763083A discloses a device and method for measuring waveform parameters of a picosecond-level pulse generator, including a microwave signal source, a data acquisition and analysis module, and a matching terminal. The signal output terminal of the microwave signal source is connected to the data acquisition and analysis module. The 10MHz reference signal output terminal of the microwave signal source is connected to the matching terminal in sequence through a power divider, an ultrafast pulse generator, an adapter, and an electro-optic sampling probe. The power divider is connected to a femtosecond laser through a laser repetition frequency locking module. The spatial femtosecond laser excitation output from the optical output terminal of the femtosecond laser is placed in the gap of the electro-optic sampling probe. The signal output terminal of the electro-optic sampling probe is connected to the data acquisition and analysis module through a polarization beam splitter, a balanced photodetector, and a current amplifier. However, this patent cannot completely solve the existing technical problems, nor can it meet the needs of this invention. Summary of the Invention

[0010] To address the shortcomings of existing technologies, the purpose of this invention is to provide a low-jitter GHz excitation synchronization and calibration delay stroboscopic phase-locked loop reconfiguration system and method.

[0011] The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to the present invention includes: Femtosecond photoelectric reference clock extraction module: used to extract an electrical reference signal coherent with the frequency of the femtosecond laser pulse from the femtosecond laser pulse train; Low-noise clock shaping module: Its input terminal is connected to the output terminal of the femtosecond photoelectric reference clock extraction module, and it is used to shape the electrical reference signal into a low-jitter digital clock. Programmable delay / phase offset module: Its input terminal is connected to the output terminal of the low-noise clock shaping module, and it is used to apply a controllable relative delay or phase offset to the digital clock to obtain a reference clock; Coherent GHz excitation synthesis module: Its input is connected to the output of the programmable delay / phase offset module, and it is used to generate a GHz excitation signal that is phase-locked with the femtosecond optical comb based on the reference clock that has been delayed or phase-off. Delay calibration and lookup table module: It is connected to the programmable delay / phase offset module and is used to measure and store the correspondence between delay control codes and actual delay times to generate a delay lookup table; Synchronization status monitoring module: Its input terminal is connected to the output terminal of the coherent GHz excitation synthesis module, and it is used to monitor the phase relationship between the reference clock, the delayed reference clock, and the GHz excitation signal; Holographic sequence acquisition interface module: It is connected to the programmable delay / phase offset module and the synchronization status monitoring module, and is used to control the image acquisition device to acquire stroboscopic image sequences according to the switching of delay control codes and phase synchronization status; Non-equal interval phase-locked reconstruction module: Its input end is connected to the holographic sequence acquisition interface module and the delay calibration and lookup table module, and is used to determine the real sampling time corresponding to each frame of image according to the delay lookup table, and perform non-equal interval phase-locked reconstruction of the stroboscopic image sequence accordingly.

[0012] Preferably, the femtosecond photoelectric reference clock extraction module includes a high-speed photodetector, a radio frequency bandpass filter, and a low-noise amplifier connected in sequence; The high-speed photodetector is used to convert a femtosecond laser pulse train into an electrical signal containing the laser repetition frequency and its harmonics; the radio frequency bandpass filter is used to select a preset harmonic component from the electrical signal; and the low-noise amplifier is used to improve the signal-to-noise ratio of the reference signal and provide a zero-crossing slope.

[0013] Preferably, the low-noise clock shaping module includes an AC coupling network, a bias network, an impedance termination, and a high-speed comparator; The input of the AC coupling network is connected to the output of the femtosecond photoelectric reference clock extraction module, the output of the AC coupling network is connected to the input of the bias network, the output of the bias network is connected to the input of the high-speed comparator, and the impedance termination is set at the input of the high-speed comparator. The AC coupling network is used to isolate the DC component of the input signal, the bias network is used to provide a DC bias voltage for the high-speed comparator, the impedance termination is used to achieve input impedance matching, and the high-speed comparator is used to compare the reference signal after AC coupling and biasing with a stable threshold to output a shaped digital clock.

[0014] Preferably, the programmable delay / phase offset module includes a digital delay chip, an analog delay circuit, a delay-locked loop, a digital phase interpolator, an RF phase shifter, a vector modulator, an adjustable transmission line, a direct digital frequency synthesizer (DDS) phase accumulator, or a combination thereof. The digital delay chip, analog delay circuit, delay-locked loop, and digital phase interpolator are used to apply a time delay to the input signal, and the radio frequency phase shifter, vector modulator, adjustable transmission line, and DDS phase accumulator are used to apply a phase shift to the input signal. One or more of the digital delay chip, analog delay circuit, delay-locked loop, digital phase interpolator, radio frequency phase shifter, vector modulator, adjustable transmission line, and DDS phase accumulator are connected in series between the input and output terminals of the programmable delay / phase shift module. The programmable delay / phase shift module is controlled by the delay or phase shift control code output by the controller, and switches the delay value or phase shift value in each sampling step or each group of sampling steps.

[0015] Preferably, the coherent GHz excitation synthesis module includes an integer N phase-locked loop, a fractional N phase-locked loop, a broadband frequency synthesizer, a direct digital frequency synthesizer (DDS), a direct digital synthesis-mixer chain, a comb harmonic selector, an active frequency multiplier, a passive frequency multiplier, an injection-locked oscillator, an opto-mixer, or a combination thereof. The integer N-phase-locked loop, fractional N-phase-locked loop, wideband frequency synthesizer, DDS, and direct digital synthesis-mixer chain are used to generate a radio frequency signal of a preset frequency based on the input reference clock. The comb harmonic selector, active frequency multiplier, passive frequency multiplier, injection-locked oscillator, and opto-mixer are used to perform frequency expansion or frequency transformation on the radio frequency signal. One or more of the integer N-phase-locked loop, fractional N-phase-locked loop, wideband frequency synthesizer, DDS, direct digital synthesis-mixer chain, comb harmonic selector, active frequency multiplier, passive frequency multiplier, injection-locked oscillator, and opto-mixer are connected in series between the input and output of the coherent GHz excitation synthesis module. The target excitation frequency generated by the coherent GHz excitation synthesis module satisfies... or The coherent frequency relationship, where N is an integer or rational number. Frequency offset This represents the repetition frequency of the femtosecond laser pulse train.

[0016] Preferably, the delay calibration and lookup table module obtains calibration data through high-speed oscilloscope edge statistics, lock-in amplifier, vector network analyzer phase slope measurement, IQ mixer phase measurement, time interval analyzer, sampling oscilloscope, dual-channel comparison, cross-correlation algorithm, or phase noise analyzer; the calibration results are stored as a delay lookup table, piecewise polynomial, spline function, temperature compensation model, or multidimensional calibration matrix; the delay lookup table is represented as... Where D is the delay control code, T is the temperature, V is the power supply or input amplitude, f is the reference frequency or excitation frequency, and ch is the channel number.

[0017] Preferably, for the k-th sampling step, the delay calibration and lookup table module determines the delay control code based on the delay control code. The actual sampling time is obtained by looking up the table. Or obtain the equivalent phase within the vibration period. ;in, This represents the actual sampling time of the k-th sampling step. To delay, This refers to the initial time offset caused by the system's initial phase or a fixed delay. This represents the equivalent sampling phase of the k-th sampling step within the vibration period. The target vibration angular frequency, This is the initial phase or fixed phase offset of the system.

[0018] Preferably, when the object of measurement is a two-dimensional holographic complex field sequence, let... Let be the pixel complex field value of the k-th frame after single-frame holographic reconstruction, reference correction, background subtraction, or normalization, and let the target vibration angular frequency be . Then the non-equal interval complex-locked phase reconstruction module constructs a non-equal interval complex-locked phase core. And calculate the matched filtering result: ;in These are the weighting coefficients. The normalization coefficient is... For the non-equal-interval complex-locked phase-locked kernel corresponding to the k-th frame, i is the imaginary unit. It is a natural exponential function. The amplitude of the full-field complex vibration obtained from the reconstruction.

[0019] Preferably, the non-equal-interval complex phase-locked reconstruction module uses a least-squares approach to establish an observation model for each pixel. Alternatively, a matrix model can be established that includes the target frequency, DC term, drift term, harmonic term, and noise term. The complex vibration coefficients are solved using weighted least squares, regularized least squares, robust regression, non-uniform DFT, NUFFT, Kalman filtering, or Bayesian estimation; among these, For the observation value of the kth sampling point, The complex amplitude coefficient of the positive frequency component. The complex amplitude coefficient of the negative frequency component. This is either a DC term or a constant bias term. For the noise term at the k-th sampling point, The vector formed by the observations of all sampling points. For the observation matrix, Let be the vector of model parameters to be solved. This is the noise vector.

[0020] The low-jitter GHz excitation synchronization and calibration delay stroboscopic phase-locked reconfiguration method provided by the present invention, applied to the aforementioned low-jitter GHz excitation synchronization and calibration delay stroboscopic phase-locked reconfiguration system, includes the following steps: Step S1: Extract a reference clock coherent with the repetition frequency from the femtosecond pulse train; Step S2: Perform low-noise filtering, amplification, and comparison shaping on the reference clock to obtain a low-jitter digital reference clock; Step S3: Apply a programmable delay or phase shift to the low-jitter digital reference clock; Step S4: Based on the reference clock that has been delayed or phase-shifted, generate a GHz excitation signal that is phase-locked with the femtosecond optical comb through coherent frequency synthesis; Step S5: Generate a delay lookup table based on the calibration results of each delay control code; Step S6: Apply the GHz excitation signal to the device under test according to the preset delay code sequence, and simultaneously acquire the stroboscopic image sequence; Step S7: Convert the delay lookup table into the actual sampling time or equivalent sampling phase corresponding to each frame of the image; Step S8: Based on the actual sampling time or equivalent sampling phase, construct a non-equal interval complex phase-locked kernel; Step S9: Perform phase-locked reconstruction on each pixel or each measurement point according to the non-equal interval phase-locked core to obtain the amplitude and phase of the target vibration.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses a femtosecond optical comb as a unified time reference to synchronize the optical sampling pulse and the GHz electrical excitation to the same coherent clock system, avoiding phase drift between the external free-running radio frequency source and the femtosecond optical pulse, and can realize coherent synchronization locking between the femtosecond pulse laser and the device vibration, and achieve stable fixation of the high-speed vibration state. (2) The present invention enables femtosecond stroboscopic phase scanning to be accurately and quickly realized by applying a programmable picosecond delay to the reference clock, directly attaching the phase to the generated frequency signal, and controlling the phase shift of the generated frequency signal. When using a femtosecond laser with a stable repetition frequency, it is still possible to achieve fast step modulation of the sampled phase so as to separate the effective vibration signal from the environmental noise in the time and frequency domain. (3) The present invention explicitly incorporates the nonlinearity of the delay link, codeword deviation and channel difference into the sampling model through the delay calibration and lookup table module, so that it no longer destroys the stroboscopic coding as an error term, but becomes the real time axis of complex phase-locked reconstruction; (4) Compared with the ideal equal-step phase-locked method, the non-equal interval complex phase-locked reconstruction of the present invention can significantly reduce amplitude compression, phase shift and spectral leakage in high-frequency measurements, and is particularly suitable for femtosecond stroboscopic measurements above 10 GHz or with non-negligible delay residuals. (5) The present invention can adopt a streaming reconstruction architecture. After each frame of hologram reconstruction is completed, it is multiplied by the corresponding complex phase-locked kernel and accumulated to the complex amplitude buffer. There is no need to store all the original images and intermediate complex field data for a long time. It is suitable for high frame rate, large field of view and GPU / FPGA / multi-threaded online processing. (6) The present invention does not limit the specific frequency synthesizer, delay chip, comparator, camera or holographic optical path, and is compatible with off-axis holography, coaxial phase-shift holography, white light interference, point pulse interference, pump-probe and scanning measurement systems, and has strong adaptability. Attached Figure Description

[0022] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the overall architecture of the photoelectric synchronization and phase modulation system for femtosecond stroboscopic measurement according to the present invention; Figure 2 This is a schematic diagram of the phase relationship when a femtosecond pulse train is used as the main time reference and coherent GHz excitation is generated in reverse. Figure 3 This is a schematic diagram of the femtosecond optical comb reference clock extraction and low-jitter shaping link; Figure 4 A schematic diagram of a structure for generating a phased-controlled GHz excitation signal using an input reference clock delay offset and phase-locked loop frequency multiplication; Figure 5 A schematic diagram of a structure for generating a femtosecond optical synchronous phased excitation signal using phase shift at a GHz radio frequency output terminal; Figure 6 A schematic diagram of a structure for generating femtosecond optical synchronous phased excitation signals using a DDS and optional external frequency multiplication or mixing links; Figure 7 This represents the intent of the calibration lookup between the phase control code and the actual sampled phase. Figure 8 This is a schematic diagram of the process of stroboscopic coding and non-equal interval complex phase-locked reconstruction based on calibration phase. Detailed Implementation

[0023] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0024] This invention addresses the problems in existing femtosecond stroboscopic measurement systems, such as the difficulty in maintaining low-jitter coherent locking between GHz excitation and femtosecond optical combs, unequal sampling intervals in high-frequency stroboscopic sampling due to programmable delay nonlinearity, and amplitude and phase errors in traditional equal-step phase-locked reconstruction at frequencies above GHz. It proposes a low-jitter GHz excitation synchronization and calibration delay stroboscopic phase-locked reconstruction method and system based on femtosecond optical combs.

[0025] The objectives of this invention include: first, converting the repetition frequency of a femtosecond pulse train or its harmonics into a low-noise electrical reference clock with high fidelity; second, applying a programmable and calibrable picosecond-level time offset to the reference clock; third, coherently synthesizing an excitation signal from the offset reference clock or applying a corresponding phase shift to the excitation signal while maintaining its phase controllability; fourth, establishing a lookup table between the delay control code and the actual sampling time; and fifth, constructing a non-equal-interval complex-locked phase nucleus using the actual sampling time to achieve accurate reconstruction of the full-field vibration amplitude and phase.

[0026] To achieve the above objectives, the present invention relates to a low-jitter GHz bandwidth excitation synchronization system based on a femtosecond optical comb, comprising a femtosecond optoelectronic reference clock extraction module, a low-noise clock shaping module, a programmable delay / phase offset module, a delay calibration and lookup table module, a coherent GHz excitation synthesis module, a phase synchronization and status monitoring module, an acquisition and control interface module, and a calculation and processing module.

[0027] The femtosecond photoelectric reference clock extraction module is used to obtain the laser repetition frequency from the femtosecond laser pulse train. A coherent electrical reference signal. Preferably, the module includes a high-speed photodetector inside or outside the femtosecond laser, an RF bandpass filter, a low-noise amplifier, a power divider, and an impedance matching network. The photodetector converts the femtosecond pulse train into a signal containing... The electrical signal and its harmonics; the radio frequency bandpass filter selects the fundamental frequency, harmonics or preset harmonic components from the electrical signal; the low noise amplifier improves the signal-to-noise ratio of the reference signal and provides sufficient zero-crossing slope for subsequent comparison shaping.

[0028] Preferably, the repetition frequency of the femtosecond optical pulse train is 1MHz to 1GHz, more preferably 10MHz to 250MHz; preferably, the femtosecond laser that generates the femtosecond optical pulse sequence has an adjustable repetition frequency function; the reference signal output by the femtosecond optoelectronic reference clock extraction module can be one or more of the following: sine wave, limited sine wave, square wave, differential clock, LVDS, CML, LVPECL, PECL or CMOS clock.

[0029] The low-noise clock shaping module is used to shape the reference signal into a digital clock with steep edges, low jitter, and low common-mode noise. Preferably, the module includes an AC coupling network, a bias network, an impedance termination, a high-speed comparator, a limiting amplifier, a clock buffer, or a level shifter. By performing a zero-crossing comparison on the bandpass-filtered reference sine signal at a stable threshold, the timing information of the femtosecond optical pulse train can be mapped to low-jitter electrical edges.

[0030] The programmable delay / phase shift module is used to apply a controllable relative delay to the digital clock to achieve scanning of the vibration phase of the device under test relative to the sampling time of the femtosecond optical pulse. Preferably, the module includes a digital delay chip, an analog delay circuit, a delay-locked loop, a digital phase interpolator, an RF phase shifter, a vector modulator, an adjustable transmission line, a DDS phase accumulator, or a combination thereof. The programmable delay / phase shift module outputs delay or phase shift control codes from the controller. And switch the delay value in each sampling step or each group of sampling steps.

[0031] The delay calibration and lookup table module is used to measure and store the delay control code D and the actual delay. The module establishes the correspondence between these parameters. Calibration data can be obtained through high-speed oscilloscope edge statistics, lock-in amplifiers, vector network analyzer phase slope measurement, IQ mixer phase measurement, time interval analyzer, sampling oscilloscope, dual-channel comparison, cross-correlation algorithm, or phase noise analyzer. Calibration results are stored as delay lookup tables, piecewise polynomials, spline functions, temperature compensation models, or multidimensional calibration matrices.

[0032] The coherent GHz excitation synthesis module is used to coherently multiply the delayed or phase-shifted reference clock to the target excitation frequency. Preferably, the module includes an integer N phase-locked loop, a fractional N phase-locked loop, a broadband frequency synthesizer, a DDS, a direct digital synthesis-mixer chain, a comb harmonic selector, an active frequency multiplier, a passive frequency multiplier, an injection-locked oscillator, an opto-mixer, or a combination thereof. The target excitation frequency can satisfy... Or other frequency relationships coherent with the femtosecond optical comb, where N is an integer or rational number. This is the frequency offset used for equivalent slow scans or heterodyne measurements. This is the electrical reference clock frequency obtained after photoelectric detection and clock shaping.

[0033] Preferably, when the coherent GHz excitation synthesis module uses a PLL-type scheme, the phase-locked loop (PLL) has a phase synchronization or phase reset function, enabling it to realign the phase of the output GHz excitation to the input reference clock after the delay control code switch. The phase synchronization or phase reset function can be implemented through the PLL's synchronization port, frequency divider reset, triggered vector modulation, gated restart, or phase detection feedback.

[0034] Preferably, when the coherent GHz excitation synthesis module uses a DDS-type scheme, the numerically controlled oscillator or phase accumulator in the DDS should have a phase synchronization function or an initial phase zeroing calibration function relative to the input clock to achieve initial phase alignment with the input reference clock. More preferably, the DDS used has an output phase adjustment function, thereby partially replacing the function of the programming delay / phase offset module.

[0035] The phase synchronization and status monitoring module is used to monitor the phase relationship between the femtosecond reference clock, the delayed reference clock, the GHz excitation signal, and the image acquisition trigger. Preferably, this module includes an I / Q mixer, a power divider, a directional coupler, a phase detector, a lock-in amplifier, a spectrum analysis unit, a low-speed ADC, an FPGA, an MCU, or a computer acquisition card. The system can determine whether the current stroboscopic state meets the acquisition requirements based on the monitored I / Q output, phase residual, amplitude residual, or unlock flag, and can perform phase compensation, resynchronization, or removal of abnormal frames.

[0036] The acquisition and control interface module is used to establish a timing relationship between delay control code switching, GHz excitation phase synchronization, sample stabilization, and image exposure. Preferably, this module sends synchronization control signals to the camera, image acquisition card, excitation source, PLL, delay chip, aperture, shutter, or host computer, so that each delayed sampling step corresponds to one or more holograms, interferometric images, or photoelectric sampling data.

[0037] The computational processing module is used to perform calibration delay stroboscopic coding and non-equal interval complex phase-locked reconstruction based on the delay lookup table and the acquired data. For the k-th sampling step, the system uses the delay control code... The actual sampling time is obtained by looking up the table. Or obtain the equivalent phase within the vibration period. The aforementioned and It can be determined by the system's initial phase, the fixed delay of the transmission link, a reference frame, or calibration measurements. Among these, This represents the actual sampling time of the k-th sampling step. To delay, This refers to the initial time offset caused by the system's initial phase or a fixed delay. This represents the equivalent sampling phase of the k-th sampling step within the vibration period. The target vibration angular frequency, This is the initial phase or fixed phase offset of the system.

[0038] When the object of measurement is a two-dimensional holographic complex field sequence, let Let be the pixel complex field value of the k-th frame after single-frame holographic reconstruction, reference correction, background subtraction, or normalization, and let the target vibration angular frequency be . The computational processing module then constructs a non-equal-interval complex locked phase core. And calculate the matched filtering result: ,in These are the weighting coefficients. The normalization coefficient is... For the non-equal interval complex-locked phase core corresponding to the k-th frame, The imaginary unit, It is a natural exponential function. The amplitude of the full-field complex vibration obtained from the reconstruction. The modulus corresponds to the vibration amplitude distribution, and the argument corresponds to the vibration phase distribution.

[0039] When the sampling phase distribution is uneven, or there are missing frames, abnormal frames, or multiple frequency components, the non-equal-interval complex phase-locked reconstruction can adopt a least-squares form. An observation model is established for each pixel. Alternatively, a matrix model can be established that includes the target frequency, DC term, drift term, harmonic term, and noise term. The complex vibration coefficients are solved using weighted least squares, regularized least squares, robust regression, non-uniform DFT, NUFFT, Kalman filtering, or Bayesian estimation. For the observation value of the kth sampling point, The complex amplitude coefficient of the positive frequency component. The complex amplitude coefficient of the negative frequency component. This is either a DC term or a constant bias term. For the noise term at the k-th sampling point, The vector formed by the observations of all sampling points. For the observation matrix, Let be the vector of model parameters to be solved. This is the noise vector.

[0040] Preferably, the calculation processing module can introduce multi-dimensional compensation for the delay lookup table based on temperature, power supply, level, frequency, codeword transition history, or board-level channels. The lookup table can be represented as... Where T is temperature, V is power supply or input amplitude, f is reference frequency or excitation frequency, and ch is channel number. The system can insert calibration steps before, after, or during measurement to update the lookup table or perform post-correction on the measurement results.

[0041] This invention also relates to a calibration delay stroboscopic coding and non-equal interval phase-locked loop reconstruction method, comprising the following steps: S1, extracting a reference clock coherent with the repetition frequency from a femtosecond pulse train; S2, performing low-noise filtering, amplification, and comparison shaping on the reference clock; S3, applying a programmable delay or phase offset to the shaped clock; S4, generating an excitation signal of a given frequency through phase-locked loop frequency multiplication or equivalent coherent frequency synthesis; S5, generating a delay lookup table used in a single acquisition based on the calibration results of each delay control code; S6, driving the device under test according to a preset delay code sequence and acquiring a stroboscopic image sequence; S7, converting the delay lookup table into the actual sampling time or sampling phase of each frame; S8, constructing a non-equal interval phase-locked loop core based on the actual sampling time; S9, performing phase-locked loop reconstruction on each pixel or each measurement point to obtain the amplitude and phase of the target vibration.

[0042] With the development of RF front-end filters, on-chip acoustic devices, high-frequency MEMS / NEMS resonators, piezoelectric ultrasonic transducers, and quantum acoustic devices, an increasing number of micro / nanomachinery devices are operating in the GHz to tens of GHz frequency band. The vibration periods of these devices are typically as low as hundreds of picoseconds, tens of picoseconds, or even shorter, while the vibration amplitudes can drop to the picometer or femtometer level. Traditional electrical testing methods can obtain overall response information such as impedance spectrum, S-parameters, resonant frequency, and quality factor, but they are difficult to directly reveal the actual vibration modes, local energy leakage, stray modes, boundary scattering, and process defects within the device. Therefore, a synchronous excitation and measurement control method capable of non-contact, coherent, and tunable phase sampling of device vibration in the GHz band is needed.

[0043] Stroboscopic optical measurement utilizes short pulses of light to illuminate the device under test at predetermined times, overcoming camera frame rate limitations and enabling equivalent slow sampling of high-frequency periodic vibrations. The applicant has previously proposed a stroboscopic holographic micro-vibration measurement method based on dynamic time-delay sampling and spatiotemporal joint processing, which can obtain the full-field vibration amplitude and phase of high-frequency MEMS devices through time-delay stepping and sequence-dimensional phase-locked extraction. However, in such schemes, the pulse source can typically be triggered by an external electrical signal, or at least the pulse emission time can be changed by an external delayed trigger signal. Therefore, the system can generate multiple delayed laser trigger signals around an external radio frequency signal. In contrast, femtosecond lasers, in a stable mode-locked operating state, have a definite pulse repetition frequency and pulse arrival time. Their repetition frequency is mainly determined by the laser resonator length and mode-locked state. They typically cannot have their emission time changed pulse by pulse by pulse by external signal, as is the case with triggered pulsed laser diodes, nor can the phase relationship between the femtosecond pulse train and the radio frequency excitation be arbitrarily modulated directly by an external electrical signal.

[0044] Therefore, when stroboscopic holographic measurement is further extended to the GHz to tens of GHz frequency band, requiring the use of femtosecond laser pulse trains as ultrashort-time sampling gates, the system synchronization relationship must be changed from the original invention's RF signal-driven or triggered pulse source to one that uses the femtosecond pulse train as the master time reference and generates coherent RF excitation in reverse. In other words, a low-jitter reference clock needs to be extracted from the femtosecond laser pulse train or its repetition frequency electrical signal, and a GHz-level RF excitation signal that is strictly coherently locked to the femtosecond laser pulse needs to be generated based on this reference clock. Simultaneously, without altering the stable operating state of the femtosecond laser, precise phase shifting or phase modulation of the GHz-level RF excitation signal is required, thereby indirectly changing the position of the device under test's vibration phase relative to the femtosecond optical pulse sampling time.

[0045] To address the aforementioned problems, this invention proposes a photoelectric synchronization and phase modulation system and method for femtosecond stroboscopic measurements. This invention uses a femtosecond laser pulse train or its repetition frequency electrical signal as the master time reference to generate a GHz-level radio frequency excitation signal that is highly synchronized with the femtosecond optical pulses. This ensures a strict coherent relationship between the periodic vibration of the MEMS / NEMS device under test driven by the GHz radio frequency signal and the femtosecond optical pulse train. Therefore, during camera exposure, when each femtosecond optical pulse reaches the device surface, the device's vibration state is as close to the same transient position as possible. This allows for stroboscopic freezing of ultra-high frequency vibrations under long exposure conditions, effectively achieving a transient exposure effect with a femtosecond-level time gate.

[0046] Furthermore, this invention performs precise phase modulation on the GHz-level radio frequency excitation signal, enabling the sampling position of the femtosecond optical pulse relative to the device's vibration period to be changed according to a preset method. Through programmable delay at the reference end, phase-locked loop frequency multiplication, phase shift at the GHz output end, I / Q vector modulation, DDS phase bias, external frequency multiplication, mixing, or combinations thereof, the radio frequency excitation signal applied to the device under test (DUT) can generate a repeatable, calibrable, and adjustable phase shift relative to the femtosecond pulse detection time. Thus, while maintaining stable mode-locking and a fixed repetition frequency of the femtosecond laser, the system can still acquire the transient response of the DUT at different vibration phases, providing multi-phase sampling data for subsequent acquisition of the full-field vibration amplitude and phase distribution.

[0047] Meanwhile, this invention generates a vibration calculation sequence with a specific time series relationship through precise phase modulation. Addressing the phase nonlinearity, temperature drift, amplitude-phase imbalance, and inter-channel differences present in actual devices such as delay chips, RF phase shifters, I / Q modulators, DDS, frequency multipliers, mixers, amplifiers, cables, and probes, this invention introduces a pre-calibrated nonlinear phase lookup table or equivalent delay lookup table, and uses the actual sampled phase for non-equidistant phase-locked loop processing during reconstruction. Therefore, even if the phase control code and the actual sampled phase are not ideally linear or ideally equidistant, the full-field vibration amplitude and phase of the device under test can still be accurately and efficiently extracted, avoiding amplitude compression, phase deviation, spectral leakage, and modal misjudgment caused by phase mismatch.

[0048] Firstly, such as Figure 1 As shown, this embodiment of the invention provides a photoelectric synchronization and phase modulation method for femtosecond stroboscopic measurement. The method includes: extracting an electrical reference signal coherent with the pulse repetition frequency from a femtosecond laser pulse train; filtering, amplifying, comparing, shaping, and level-shifting the electrical reference signal to obtain a low-jitter digital reference clock; generating a GHz-level radio frequency excitation signal coherently locked to the femtosecond pulse train based on the low-jitter digital reference clock; performing controllable phase shifting or phase modulation on the GHz-level radio frequency excitation signal; calibrating the correspondence between the phase control code and the actual sampled phase; and performing non-equidistant complex phase-locked reconstruction on the holographic image sequence, complex field sequence, or other optical sampling sequence acquired by the femtosecond stroboscopic measurement according to the actual sampled phase to obtain the vibration amplitude and phase information of the device under test.

[0049] Secondly, such as Figure 1 As shown, this embodiment of the invention provides a phase calibration and non-equal-interval complex phase-locked loop processing method for femtosecond stroboscopic measurements. The method includes: obtaining the nonlinear correspondence between the phase modulation control code and the actual sampled phase, and constructing a phase lookup table or an equivalent delay lookup table; calibrating the true phase corresponding to each sampled sequence based on the lookup table; and performing non-equal-interval complex phase-locked loop processing on the holographic image sequence, complex field sequence, or other optical sampled sequences acquired by the femtosecond stroboscopic measurements according to the calibrated true sampled phase, thereby accurately reconstructing the vibration amplitude and phase information of the device under test.

[0050] like Figure 2As shown, the overall architecture of this invention uses a femtosecond laser pulse train as a unified time reference. A femtosecond stroboscopic detection is formed along the optical measurement path, and a coherent GHz excitation is formed along the electrical synchronization path. A definite phase relationship between the two is established through phase modulation and calibration reconstruction. The femtosecond laser pulse train serves as an optical sampling gate to illuminate the device under test (DUT), and after photoelectric detection and clock shaping, it enters the radio frequency (RF) synchronization link. The RF synchronization link generates a GHz excitation signal coherently locked to the femtosecond pulse train and applies it to the DUT. The phase modulation module changes the excitation phase according to a preset control sequence, enabling the femtosecond laser pulses to sample different vibration transients. The calibration and reconstruction module performs non-equidistant phase-locked processing on the acquired sequence based on a measured phase lookup table, thereby obtaining the full-field vibration amplitude and phase. Thus, Figure 1 Correspondingly, the overall inventive architecture of this invention is shown, from femtosecond optical reference, GHz synchronous excitation, phase-precise modulation to calibration complex phase-locked reconstruction.

[0051] This invention, through the aforementioned structure and method, incorporates the advantages of low timing jitter and extremely short time gate width of femtosecond optical pulses into the GHz radio frequency excitation synchronization process, establishing a definite, stable, and adjustable phase relationship between the electrical drive phase of the device under test and the femtosecond optical sampling time. This invention achieves high-precision phase freezing under GHz conditions, high accuracy in multi-phase sampling, and reliable full-field complex vibration reconstruction. It is applicable to the measurement of MEMS / NEMS resonators, bulk acoustic wave devices, surface acoustic wave devices, piezoelectric transducers, phononic crystal structures, on-chip acoustic devices, and other high-frequency dynamic structures.

[0052] To enable those skilled in the art to more clearly understand the technical solutions provided by the embodiments of the present invention, the technical solutions provided by the embodiments of the present invention will be described in detail below through specific embodiments. Embodiment 1 illustrates a femtosecond optical comb reference clock extraction and low-jitter shaping link; Embodiment 2 illustrates a femtosecond optical synchronization phased array signal generation method using input reference clock offset; Embodiment 3 illustrates a femtosecond optical synchronization phased array signal generation method using RF output phase offset; Embodiment 4 illustrates a femtosecond optical synchronization phased array excitation signal generation method using a direct digital frequency synthesizer and an optional external frequency multiplier; Embodiment 5 illustrates a stroboscopic coding and non-equidistant phase-locked loop reconstruction method based on calibrated phase. The above embodiments can be implemented individually or in combination according to the target frequency, phase resolution, output power, number of channels, measurement speed, and system cost.

[0053] It should be noted that in different embodiments, the same or similar modules may be represented by the same reference numerals. The photodetectors, filters, amplifiers, comparators, delay chips, phase-locked frequency synthesizers, DDS, I / Q modulators, RF phase shifters, frequency multipliers, mixers, RF amplifiers, phase detectors, acquisition cards, and computing modules mentioned in the following description can all be equivalently replaced according to the target frequency, phase noise, output power, bandwidth, number of channels, interface type, and system cost. The scope of protection of this invention is defined by the claims and is not limited by specific device models, specific operating frequencies, specific communication interfaces, or specific circuit parameters.

[0054] Example 1: Femtosecond optical comb reference clock extraction and low jitter shaping link.

[0055] like Figure 3 As shown, this embodiment provides a specific implementation for generating a low-jitter reference clock from a femtosecond optical comb pulse train. The femtosecond laser outputs a pulse train with a repetition frequency of approximately 50MHz. This pulse train can be converted into an electrical pulse signal by an integrated photodetector within the laser or an external high-speed photodetector. This electrical pulse signal contains the repetition frequency in the frequency domain. And its multiple harmonics.

[0056] The output signal from the photodetector first enters a passive or active bandpass filter. Preferably, the center frequency of the bandpass filter is set at... Nearby, the passband covers the laser repetition frequency tuning range, and out-of-band suppression of high-order harmonics, low-frequency noise, and DC drift. For femtosecond lasers with a repetition frequency of approximately 50 MHz, a multi-order Butterworth bandpass filter centered around 47 MHz to 53 MHz can be used. More preferably, such as... Figure 2 As shown in this example, in order to reduce phase jitter noise, a bandpass filter with a center around 200MHz, i.e., 4 times the repetition frequency, can be selected.

[0057] The filtered near-sinusoidal reference signal enters a low-noise amplifier. The low-noise amplifier has a low noise figure and sufficient linear range to improve the zero-crossing slope of the reference clock signal. Preferably, in The output amplitude at the terminal can reach hundreds of millivolts to several volts peak value, thereby converting the input noise of the subsequent comparator into a smaller time jitter.

[0058] The low-noise amplified reference signal is input to a high-speed comparator via an AC coupling and bias network. The comparator converts the near-sinusoidal reference signal into a digital clock in LVDS, CML, PECL, LVPECL, or CMOS format. Preferably, the comparator input is configured with... To reduce reflections, the differential output terminals are impedance matched to the corresponding level standards to maintain edge integrity.

[0059] The digital reference clock obtained from the above link inherits the short-time stability of the femtosecond optical comb repetition frequency and converts the time information of the femtosecond optical pulse train into electrical edges. This reference clock can be used as input to subsequent programmable delay modules, or as a common reference for phase-locked frequency synthesizers, camera triggers, acquisition controllers, or synchronization monitoring modules.

[0060] Example 2: Generation using a femtosecond optical synchronization phase signal offset from the input reference clock.

[0061] like Figure 4 As shown, this embodiment provides an implementation method for generating phase-controllable radio frequency excitation through programmable delay and frequency locking. A low-jitter digital reference clock from Embodiment 1 enters the programmable delay chip. The programmable delay chip may have multiple binary weighted delay units or a subdivided phase interpolation structure, and the controller outputs the delay control code D through a parallel bus, SPI, I2C, or GPIO.

[0062] Under each delay control code D, the delay module output has a delay relative to the input reference clock. The clock signal is then delayed and converted to a PECL, LVPECL, CML, or other suitable input level for the frequency synthesizer, and fed into the reference port of the phase-locked loop frequency synthesizer. Internally, the frequency synthesizer forms a closed loop through a phase detector, loop filter, VCO, and frequency divider to ensure the output frequency meets the specified requirements. Or other coherent frequency relationships. Preferably, a phase-locked loop (PLL) core-type frequency synthesizer is used. Utilizing the closed-loop characteristics within the PLL-type frequency synthesizer and a specific loop filter design, targeted suppression of phase noise in a specific frequency band can be achieved. This closed-loop relationship can compress time jitter noise introduced in the preceding stages due to photodetector noise, comparator noise, etc., thereby optimizing the synchronization accuracy and stability between the output RF signal and the femtosecond optical pulse. In this preferred configuration, whenever the input reference clock is generated... The delay step will then output the frequency. It will generate corresponding The phase stepping is used to achieve phase control of the output signal.

[0063] In a preferred embodiment, With a repetition frequency of approximately 50 MHz for a femtosecond laser, and N taking an integer from 10 to 600, an RF excitation signal of approximately 0.5 GHz to 30 GHz is generated. For some GHz MEMS devices, N can also be a fraction or generated using a mixing method. This is used for heterodyne or equivalent slow scan measurements.

[0064] Preferably, the frequency synthesizer has a phase synchronization function. When the delay control code D is switched, the controller sends a phase synchronization trigger to the frequency synthesizer, causing its internal frequency divider, phase accumulator, or synchronization logic to realign to the reference clock after the current delay. This ensures that the picosecond-level delay offset on the low-frequency reference clock is deterministically transmitted to the GHz output excitation phase, rather than being destroyed by the randomness of the PLL's initial phase.

[0065] To obtain the true delay lookup table, the system calibrates the programmable delay module before measurement. During calibration, the input reference clock frequency is fixed, and delay control codes are set one by one. The time difference between the output clock and the input clock or the reference channel is measured. Measurements can be performed using edge interpolation averaging of a dual-channel high-speed oscilloscope, phase-frequency slope of a lock-in amplifier, output phase of an IQ mixer, or cross-correlation calculations.

[0066] The data obtained from calibration is formed A lookup table is used to store absolute delay, relative delay, correction residual, standard deviation, and environmental parameters, as actual delays may include fixed delay, codeword nonlinearity, binary weight deviation, inter-channel differences, and temperature drift. The controller or computational processing module directly calls this lookup table during subsequent sampling and reconstruction. Instead of assuming an ideal linear relationship between delay and codeword.

[0067] Because there is a relationship between the phase offset and the harmonic frequency and the delay. Therefore, when the output frequency is very high, picosecond-level delay differences can cause significant phase shifts. Especially when large delays are required, drift in the delay step size can significantly cause phase mismatch. Therefore, it is preferable to implement stability control for the delay device in multiple aspects, such as temperature control and drive voltage stabilization control, to ensure that the pre-calibrated delay code lookup table is applicable.

[0068] Given the limited upper limit of the output frequency of commonly used phase-locked loop (PLL) frequency synthesizers, it is preferable to additionally connect a frequency synthesizer with a frequency converter in series at the output of the PLL frequency synthesizer. The rate-multiplication and frequency-multiplication subsystem ensures that the final synthesized output frequency reaches the specified frequency. Since the external series subsystem has a fixed system transfer function, the output frequency-doubled signal and the femtosecond optical pulse sequence can still be locked into a definite and controllable phase relationship.

[0069] Example 3: Generation of femtosecond optical synchronization phase signal using phase offset of RF output terminal.

[0070] This embodiment presents a synchronous phase control implementation method different from Embodiment 2. For example... Figure 5As shown, in this embodiment, the programmable delay or phase shift module is not located before the reference input of the phase-locked frequency synthesizer, but rather after the GHz output of the phase-locked frequency synthesizer, frequency multiplier chain, frequency mixer chain, or other RF signal generation module. The low-jitter reference clock from the femtosecond laser pulse train first undergoes phase-locked frequency multiplication, direct frequency synthesis, and frequency mixing or frequency multiplication to generate a GHz RF signal coherently locked to the femtosecond pulse train. Subsequently, a phase control module located at the RF output performs a controllable phase shift on the GHz RF signal to obtain a synchronous excitation signal with a preset phase relationship relative to the femtosecond pulse detection time.

[0071] The RF output phase control module can be implemented using an RF vector modulator, I / Q modulator, digitally controlled phase shifter, analog electrically controlled phase shifter, adjustable transmission line, switching delay line, distributed transmission line phase shifting network, vector synthesis network, reflective phase shifting network, or a combination thereof. Preferably, the RF output phase control module has a phase control code P that can be set by a controller. The controller can set the phase control code P through SPI, I2C, parallel bus, DAC voltage, GPIO control word, or host computer communication interface, so that the RF output signal generates a programmable phase shift while maintaining a frequency-locked relationship.

[0072] In one implementation, the phase-locked frequency synthesizer outputs a radio frequency signal that is coherently related to the repetition frequency of the femtosecond pulse, the frequency of which can be expressed as: This can also be expressed as other preset coherent frequencies obtained through mixing, frequency multiplication, or frequency division. After the radio frequency signal is input to an I / Q modulator or radio frequency vector modulator, it is decomposed into an in-phase component I and a quadrature component Q; the controller determines the target phase... Weights are set for the I and Q channels to ensure the output RF signal meets a preset phase shift. Preferably, the weights can be set... Alternatively, a pre-calibrated I / Q control lookup table can be used to compensate for non-ideal factors such as modulator amplitude-phase imbalance, DC bias, insufficient port isolation, and amplitude-phase variation. In the k-th sampling step, the in-phase component control weight of the I / Q modulator or vector modulator is used to control the in-phase amplitude of the output RF signal; In the k-th sampling step, the amplitude coefficient of the output radio frequency signal is used to control the overall amplitude of the signal. In the k-th sampling step, the quadrature component control weights of the I / Q modulator or vector modulator are used to control the quadrature amplitude of the output RF signal.

[0073] In another embodiment, the RF output phase control module is a digitally controlled phase shifter, an analog electronically controlled phase shifter, or a combination thereof. The controller sends a phase control code P to the digitally controlled phase shifter, or outputs a control voltage, control current, or digital-to-analog conversion signal to the analog electronically controlled phase shifter, causing the RF output signal to generate a phase shift corresponding to the control quantity. The numerically controlled phase shifter can achieve discrete phase stepping, suitable for multi-phase stroboscopic sampling; the analog electronically controlled phase shifter can achieve continuous or quasi-continuous phase adjustment, suitable for subdivided phase scanning or phase error compensation. Preferably, the relationship between the control codes of the numerically controlled phase shifter and the analog electronically controlled phase shifter and the actual phase offset is obtained through pre-calibration and stored as a phase lookup table or an equivalent delay lookup table. All of the above-mentioned phase offset methods can be used to ensure that the device under test is in different controllable vibration phases when the femtosecond pulse arrives.

[0074] Compared to the method in Embodiment 2, which introduces a delay at the low-frequency reference and transmits it to the GHz output via phase-locked loop (PLL) frequency multiplication, this embodiment directly performs phase shifting at the GHz output. Therefore, it does not require the PLL frequency synthesizer to re-execute phase synchronization triggering after each phase switch, nor does it require the preceding reference clock delay to cover a large time range. In applications requiring rapid phase scanning, fine-grained phase steps, or separate phase settings for multiple RF output channels, this embodiment offers advantages such as flexible structure, fast phase switching speed, and ease of multi-channel expansion.

[0075] To obtain the correspondence between the GHz output phase control code and the actual sampled phase, this embodiment also sets up a calibration process. During calibration, the femtosecond pulse repetition frequency and the RF output frequency are fixed, the phase control code P is set one by one, and the phase difference of the RF output signal relative to the femtosecond pulse train, the electrical reference clock, or another reference RF channel is measured. The measurement can be implemented using a sampling oscilloscope, a real-time oscilloscope, a vector network analyzer, a phase noise analyzer, a lock-in amplifier, an IQ mixer, a down-conversion receiver, or a digital cross-correlation algorithm. For situations where direct measurement is difficult at higher frequencies, the GHz RF signal can be first mixed with a coherent local oscillator to an intermediate frequency or low frequency, and then its phase change can be measured and converted to obtain the equivalent sampled phase.

[0076] The calibration data forms a phase control lookup table. The lookup table may include parameters such as phase control code, corresponding RF output phase, equivalent sampling phase, equivalent time delay, amplitude variation, residual phase error, repeatability standard deviation, temperature, power supply voltage, and RF output frequency. Since actual RF phase shifters, I / Q modulators, or adjustable transmission lines typically exhibit phase nonlinearity, amplitude fluctuations, frequency correlation, temperature drift, and inter-channel differences, subsequent stroboscopic sampling and phase-locked loop reconstruction preferably directly utilize the measured lookup table, without assuming an ideal linear relationship between the phase control code and the actual sampling phase.

[0077] Example 4: Generation of femtosecond optical synchronization phase signal using direct digital synthesis and external frequency doubling.

[0078] This embodiment provides a method for generating phase-controllable radio frequency excitation signals using a direct digital frequency synthesizer (DDS). For example... Figure 6 As shown, in this embodiment, the low-jitter reference clock generated in Embodiment 1 is first processed by clock shaping, frequency division, frequency multiplication, or phase-locked loop (PLL) cleaning, and then used as the system clock of the DDS or a coherent reference for its system clock. The DDS outputs an RF signal that satisfies a preset coherence relationship with the femtosecond pulse train based on the frequency tuning word FTW and the phase offset word POW, and achieves programmable phase offset of the excitation signal relative to the femtosecond pulse detection time by changing the phase offset word POW.

[0079] In one implementation, the system clock frequency of the DDS satisfies an integer multiple, fractional multiple, or phase-locked loop coherent relationship with the femtosecond pulse repetition frequency. The DDS output frequency can be determined by the frequency tuning word FTW, expressed as follows: and the femtosecond pulse repetition frequency The preset coherence relation is satisfied. While keeping the frequency tuning word FTW constant, the controller changes the phase offset word POW of the DDS, causing a phase shift in the DDS output signal. Because the phase accumulator and phase biaser inside the DDS are digitally deterministic, the phase offset can be repeatedly set between different sampling states, thereby allowing the device under test to be in different controllable vibration phases when the femtosecond pulse arrives.

[0080] In a preferred embodiment, the DDS output terminal is further provided with an external frequency multiplier, a harmonic selection filter, an RF amplifier, a mixer, or a combination thereof, for extending the DDS output frequency to the GHz or tens of GHz band. When the multiplication factor of the external frequency multiplier is M, the final RF output frequency can be expressed as: In an ideal situation, the phase shift at the DDS output... After frequency doubling, the corresponding formation is formed The radio frequency phase shift. Therefore, if the target phase needs to be obtained at the final radio frequency output... The controller can set the DDS phase offset to approximately Alternatively, the calibrated phase control lookup table can be directly invoked to compensate for fixed phase deviations and non-ideal phase errors caused by the frequency multiplier chain, filters, amplifiers, and connecting cables.

[0081] Preferably, the DDS has synchronous reset, phase clearing, continuous phase update, or deterministic IO update functions. When the controller switches the frequency tuning word FTW or the phase bias word POW, the DDS phase accumulator, phase register, or output update logic can be re-aligned with the femtosecond optical synchronization reference via a synchronous trigger signal. This avoids the uncertainty of the DDS start-up phase or update timing from disrupting the deterministic relationship between the femtosecond pulse detection timing and the RF excitation phase. For scenarios using multiple DDS channels or multiple RF outputs, multi-channel coherent phase control can be achieved through a common system clock, a common synchronous reset signal, and an inter-channel phase calibration table.

[0082] Because the DDS output may contain non-ideal factors such as quantization spurious signals, clock phase noise, digital-to-analog conversion nonlinearity, and variations in image frequency and amplitude with codeword, and external frequency multiplier chains may introduce phase noise amplification, harmonic selection errors, and amplitude-phase coupling, subsequent stroboscopic sampling and phase-locked loop reconstruction preferably use the measured lookup table directly, rather than assuming an ideal linear relationship between the DDS phase bias word and the final RF sampling phase, to achieve stability of the digital phase-locked loop method. Furthermore, the phase stability between the final GHz synchronization excitation signal and the femtosecond pulse train can be improved through bandpass filtering, low-phase-noise clock sources, spurious-optimized frequency planning, output amplitude normalization, and temperature stabilization control.

[0083] Example 5: Calibration delay stroboscopic coding combined with femtosecond off-axis interference and non-equal interval complex-locked phase reconstruction.

[0084] This embodiment presents a stroboscopic coding and complex phase-locked loop reconstruction method based on a measured phase lookup table. For example... Figure 7 As shown, the device under test (DUT) generates periodic vibrations driven by a GHz excitation signal coherently locked to a femtosecond optical pulse train. The femtosecond optical pulse train acts as a sampling gate, repeatedly sampling the same or controllably varying vibration phase during camera exposure in a manner with a stable phase relationship to the GHz excitation signal. This phase relationship can be generated by reference delay, GHz output phase shift, I / Q vector modulation, DDS phase bias, or a combination thereof.

[0085] The controller follows a preset phase control sequence Switch the phase modulation module. The phase control code... It can be a programmable delay code, RF phase shifter control code, I / Q weight code, DDS phase bias word, switch delay line codeword, or other control quantity capable of changing the GHz excitation phase. Each phase control code This corresponds to one or more image acquisition steps. The acquired raw hologram undergoes off-axis Fourier reconstruction, phase reference correction, background subtraction, normalization, or complex field extraction to obtain the complex field for each frame or each set of sampling states. .

[0086] like Figure 7 As shown in the calculation results below, compared with the assumption of uniform phase shift, the use of calibration-based non-equal interval complex phase-locked reconstruction can significantly improve the reconstruction accuracy of the amplitude and phase of the whole field vibration. Especially when the measurement frequency reaches above 10 GHz, it will effectively compensate for the significant phase mismatch caused by the picosecond-level drift of the phase shift word.

[0087] like Figure 8 As shown, the actual sampled phase corresponding to the k-th frame is obtained according to the pre-calibrated phase lookup table. Or obtain an equivalent time delay And convert to .in, The target GHz excitation angular frequency, This refers to the fixed phase bias introduced by the optical path, cable, RF amplifier, phase shifter, probe, sample response, and phase reference definition. The phase can be determined through reference sample measurement, baseline channel measurement, frequency sweep fitting, phase zeroing, or overall offset correction after phase expansion. Therefore, subsequent reconstruction directly uses the measured sampled phase. However, it does not assume that there is an ideal equal interval between the phase control code and the actual sampling phase.

[0088] For single-frequency steady-state vibration, the calculation and processing module constructs a complex phase-locked kernel for each pixel. And perform weighted summation. ,in It is the imaginary unit. If If it is a real-valued displacement sequence, then Represents the complex amplitude of the positive frequency component of the target frequency; if If it is a holographic complex field sequence, then This can represent the complex vibration components after reference state correction. Finally, we take... The vibration amplitude is obtained, and then... The vibration phase is obtained.

[0089] When there are issues such as uneven sampling phase distribution, missing phase control codes, abnormal frames, low-frequency drift, amplitude variations with phase control codes, or harmonic interference, a matrix solution method can be used. A matrix solution is established for each pixel. ,in It can represent low-order drift terms or slowly varying background terms. is the amplitude coefficient of the cosine component. is the amplitude coefficient of the sinusoidal component. This is either a DC bias or a constant term. Write all samples as... and through Solve it in its complex form, where This is the matrix transpose symbol. It is derived from A, B, or the complex number system. The amplitude and phase at the target frequency are obtained. The weight matrix W can be determined based on the hologram signal-to-noise ratio, phase calibration error, abnormal frame criteria, RF output amplitude variation, or camera exposure stability.

[0090] For situations involving multiple target frequencies, harmonic responses, or nonlinear harmonics, the following can be used: Matrix expansion to include The multi-order phase basis, in which , where m is the harmonic order and M is the maximum harmonic order. The full-field complex amplitude values ​​of multiple vibration components can be obtained simultaneously through multi-frequency least squares, sparse regularization, non-uniform discrete Fourier transform, or NUFFT.

[0091] To reduce memory usage, this embodiment employs streaming processing. The system completes single-frame holographic reconstruction and adjusts the reconstruction based on the corresponding real sampling phase of each hologram acquired. Generate complex locked phase nuclei Then multiply the reconstruction result by Then accumulated to the global complex amplitude cache and weight cache The original image and intermediate complex field are then extracted and released. After acquisition, only the cached results need to be normalized to obtain the final full-field vibration amplitude map and phase map.

[0092] It should be noted that the phase lookup table in this embodiment can be derived from the reference end delay calibration described in Embodiment 2, the GHz output end phase shift calibration described in Embodiment 3, or the DDS phase offset calibration and its external frequency multiplication or mixing link calibration described in Embodiment 4. As long as the actual sampling phase corresponding to each sampling state can be obtained... or equivalent sampling time The non-equal interval phase-locked reconstruction method described in this embodiment can be used to extract the amplitude and phase of the full-field vibration.

[0093] It should be noted that the synchronous electronics and phase modulation system of this invention does not depend on a specific holographic optical path, microscopic imaging structure, or type of device under test. Regardless of whether the femtosecond pulse probe light employs off-axis holographic interferometry, common-path interferometry, heterodyne interferometry, homodyne interferometry, fiber interferometry, pump-probe reflection measurement, or other non-contact optical measurement methods, as long as the measurement method can utilize the femtosecond pulse to sample the transient response of the device under test at a defined time, it can be combined with the femtosecond optical synchronous phased excitation signal generation method of this invention. Accordingly, this invention can be used for MEMS / NEMS resonators, bulk acoustic wave devices, surface acoustic wave devices, piezoelectric transducers, phononic crystal structures, and on-chip acoustic devices, as well as other high-frequency dynamic measurement objects requiring femtosecond stroboscopic sampling and coherent RF excitation.

[0094] Those skilled in the art will understand that, in addition to implementing the system, apparatus, and their modules provided by this invention in purely computer-readable program code, the same program can be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system, apparatus, and their modules provided by this invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; alternatively, modules for implementing various functions can be considered both software programs implementing the method and structures within the hardware component.

[0095] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system, characterized in that, include: Femtosecond photoelectric reference clock extraction module: used to extract an electrical reference signal coherent with the frequency of the femtosecond laser pulse from the femtosecond laser pulse train; Low-noise clock shaping module: Its input terminal is connected to the output terminal of the femtosecond photoelectric reference clock extraction module, and it is used to shape the electrical reference signal into a low-jitter digital clock. Programmable delay / phase offset module: Its input terminal is connected to the output terminal of the low-noise clock shaping module, and it is used to apply a controllable relative delay or phase offset to the digital clock to obtain a reference clock; Coherent GHz excitation synthesis module: Its input is connected to the output of the programmable delay / phase offset module, and it is used to generate a GHz excitation signal that is phase-locked with the femtosecond optical comb based on the reference clock that has been delayed or phase-off. Delay calibration and lookup table module: It is connected to the programmable delay / phase offset module and is used to measure and store the correspondence between delay control codes and actual delay times to generate a delay lookup table; Synchronization status monitoring module: Its input terminal is connected to the output terminal of the coherent GHz excitation synthesis module, and it is used to monitor the phase relationship between the reference clock, the delayed reference clock, and the GHz excitation signal; Holographic sequence acquisition interface module: It is connected to the programmable delay / phase offset module and the synchronization status monitoring module, and is used to control the image acquisition device to acquire stroboscopic image sequences according to the switching of delay control codes and phase synchronization status; Non-equal interval phase-locked reconstruction module: Its input end is connected to the holographic sequence acquisition interface module and the delay calibration and lookup table module, and is used to determine the real sampling time corresponding to each frame of image according to the delay lookup table, and perform non-equal interval phase-locked reconstruction of the stroboscopic image sequence accordingly.

2. The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to claim 1, characterized in that, The femtosecond optoelectronic reference clock extraction module includes a high-speed photodetector, an RF bandpass filter, and a low-noise amplifier connected in sequence. The high-speed photodetector is used to convert a femtosecond laser pulse train into an electrical signal containing the laser repetition frequency and its harmonics; the radio frequency bandpass filter is used to select a preset harmonic component from the electrical signal; and the low-noise amplifier is used to improve the signal-to-noise ratio of the reference signal and provide a zero-crossing slope.

3. The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to claim 1, characterized in that, The low-noise clock shaping module includes an AC coupling network, a bias network, an impedance termination, and a high-speed comparator. The input of the AC coupling network is connected to the output of the femtosecond photoelectric reference clock extraction module, the output of the AC coupling network is connected to the input of the bias network, the output of the bias network is connected to the input of the high-speed comparator, and the impedance termination is set at the input of the high-speed comparator. The AC coupling network is used to isolate the DC component of the input signal, the bias network is used to provide a DC bias voltage for the high-speed comparator, the impedance termination is used to achieve input impedance matching, and the high-speed comparator is used to compare the reference signal after AC coupling and biasing with a stable threshold to output a shaped digital clock.

4. The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to claim 1, characterized in that, The programmable delay / phase offset module includes a digital delay chip, an analog delay circuit, a delay-locked loop, a digital phase interpolator, an RF phase shifter, a vector modulator, an adjustable transmission line, a direct digital frequency synthesizer (DDS) phase accumulator, or a combination thereof. The digital delay chip, analog delay circuit, delay-locked loop, and digital phase interpolator are used to apply a time delay to the input signal, and the radio frequency phase shifter, vector modulator, adjustable transmission line, and DDS phase accumulator are used to apply a phase shift to the input signal. One or more of the digital delay chip, analog delay circuit, delay-locked loop, digital phase interpolator, radio frequency phase shifter, vector modulator, adjustable transmission line, and DDS phase accumulator are connected in series between the input and output terminals of the programmable delay / phase shift module. The programmable delay / phase shift module is controlled by the delay or phase shift control code output by the controller, and switches the delay value or phase shift value in each sampling step or each group of sampling steps.

5. The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to claim 1, characterized in that, The coherent GHz excitation synthesis module includes an integer N phase-locked loop, a fractional N phase-locked loop, a broadband frequency synthesizer, a direct digital frequency synthesizer (DDS), a direct digital synthesis-mixer chain, a comb harmonic selector, an active frequency multiplier, a passive frequency multiplier, an injection-locked oscillator, an opto-mixer, or a combination thereof. The integer N-phase-locked loop, fractional N-phase-locked loop, wideband frequency synthesizer, DDS, and direct digital synthesis-mixer chain are used to generate a radio frequency signal of a preset frequency based on the input reference clock. The comb harmonic selector, active frequency multiplier, passive frequency multiplier, injection-locked oscillator, and opto-mixer are used to perform frequency expansion or frequency transformation on the radio frequency signal. One or more of the integer N-phase-locked loop, fractional N-phase-locked loop, wideband frequency synthesizer, DDS, direct digital synthesis-mixer chain, comb harmonic selector, active frequency multiplier, passive frequency multiplier, injection-locked oscillator, and opto-mixer are connected in series between the input and output of the coherent GHz excitation synthesis module. The target excitation frequency generated by the coherent GHz excitation synthesis module satisfies... or The coherent frequency relationship, where N is an integer or rational number. Frequency offset This represents the repetition frequency of the femtosecond laser pulse train.

6. The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to claim 1, characterized in that, The delay calibration and lookup table module obtains calibration data through high-speed oscilloscope edge statistics, lock-in amplifier, vector network analyzer phase slope measurement, IQ mixer phase measurement, time interval analyzer, sampling oscilloscope, dual-channel comparison, cross-correlation algorithm or phase noise analyzer. The calibration results are stored as a delay lookup table, a piecewise polynomial, a spline function, a temperature compensation model, or a multidimensional calibration matrix; the delay lookup table is represented as follows: Where D is the delay control code, T is the temperature, V is the power supply or input amplitude, f is the reference frequency or excitation frequency, and ch is the channel number.

7. The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to claim 1, characterized in that, For the k-th sampling step, the delay calibration and lookup table module uses the delay control code. The actual sampling time is obtained by looking up the table. Or obtain the equivalent phase within the vibration period. ;in, This represents the actual sampling time of the k-th sampling step. To delay, This refers to the initial time offset caused by the system's initial phase or a fixed delay. This represents the equivalent sampling phase of the k-th sampling step within the vibration period. The target vibration angular frequency, This is the initial phase or fixed phase offset of the system.

8. The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to claim 1, characterized in that, When the object of measurement is a two-dimensional holographic complex field sequence, let Let be the pixel complex field value of the k-th frame after single-frame holographic reconstruction, reference correction, background subtraction, or normalization, and let the target vibration angular frequency be . Then the non-equal interval complex-locked phase reconstruction module constructs a non-equal interval complex-locked phase core. And calculate the matched filtering result: ;in These are the weighting coefficients. The normalization coefficient is... For the non-equal-interval complex-locked phase-locked kernel corresponding to the k-th frame, i is the imaginary unit. It is a natural exponential function. The amplitude of the full-field complex vibration obtained from the reconstruction.

9. The low-jitter GHz excitation synchronization and calibration delay stroboscopic reconfiguration system according to claim 8, characterized in that, The non-equal interval phase-locked reconstruction module establishes an observation model for each pixel using the least squares method. Alternatively, a matrix model can be established that includes the target frequency, DC term, drift term, harmonic term, and noise term. The complex vibration coefficients are solved using weighted least squares, regularized least squares, robust regression, non-uniform DFT, NUFFT, Kalman filtering, or Bayesian estimation; among these, For the observation value of the kth sampling point, The complex amplitude coefficient of the positive frequency component. The complex amplitude coefficient of the negative frequency component. This is either a DC term or a constant bias term. For the noise term at the k-th sampling point, The vector formed by the observations of all sampling points. For the observation matrix, Let be the vector of model parameters to be solved. This is the noise vector.

10. A low-jitter GHz excitation synchronization and calibration delay stroboscopic phase-locked loop reconstruction method, characterized in that, The low-jitter GHz excitation synchronization and calibration delay stroboscopic phase-locked reconfiguration system applied to any one of claims 1 to 9 includes the following steps: Step S1: Extract a reference clock coherent with the repetition frequency from the femtosecond pulse train; Step S2: Perform low-noise filtering, amplification, and comparison shaping on the reference clock to obtain a low-jitter digital reference clock; Step S3: Apply a programmable delay or phase shift to the low-jitter digital reference clock; Step S4: Based on the reference clock that has been delayed or phase-shifted, generate a GHz excitation signal that is phase-locked with the femtosecond optical comb through coherent frequency synthesis; Step S5: Generate a delay lookup table based on the calibration results of each delay control code; Step S6: Apply the GHz excitation signal to the device under test according to the preset delay code sequence, and simultaneously acquire the stroboscopic image sequence; Step S7: Convert the delay lookup table into the actual sampling time or equivalent sampling phase corresponding to each frame of the image; Step S8: Based on the actual sampling time or equivalent sampling phase, construct a non-equal interval complex phase-locked kernel; Step S9: Perform phase-locked reconstruction on each pixel or each measurement point according to the non-equal interval phase-locked core to obtain the amplitude and phase of the target vibration.

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