A non-contact laser Doppler seismometry robust optomechanical system and method

Abstract

This invention belongs to the field of geophysical exploration instruments and laser measurement technology, and relates to a non-contact laser Doppler seismometry robust optomechanical system and method. The system consists of an LDV optical unit, a two-dimensional micro-rotator actuator, a variable field-of-view optical structure, a photoelectric detection and analog front-end, an analog-to-digital conversion and digital processing unit, and a data output acquisition system. The method includes constructing a heterodyne interferometric optical path; building a common-path optical antenna and a variable field-of-view coupling structure; configuring photoelectric detection and sampling links; performing modulation and echo quality assessment control in the digital domain; achieving bright spot locking and re-acquisition control based on the echo quality function; outputting data and performing necessary control compensation. This invention, through the combination of a variable effective field of view, echo quality closed-loop locking, and re-acquisition of lost scans, significantly reduces the probability of phase demodulation interruption caused by deep speckle fading under naturally rough surface conditions, improving the continuity and usability of vibration records in seismic exploration scenarios.

Description

Technical Field

[0001] This invention belongs to the field of geophysical exploration instruments and laser measurement technology, specifically relating to a non-contact laser Doppler seismometry robust optomechanical system and method, and particularly to a non-contact laser Doppler seismometry robust optomechanical system and method for rough surfaces. Background Technology

[0002] Seismic exploration is a crucial method for detecting underground geological structures and identifying oil, gas, and mineral resources. Seismic exploration and engineering seismometry typically employ contact geophone arrays to acquire surface vibration signals. This method requires physically coupling (inserting) the geophones into the surface soil. However, in seismic exploration scenarios with complex terrain, such as mountainous areas, deserts, remote uninhabited areas, swamps, or muddy conditions caused by rain and snow, practical engineering problems arise, including extremely difficult geophone deployment, low deployment efficiency, difficulty in guaranteeing coupling effectiveness, and challenges in achieving rapid, unmanned data acquisition.

[0003] Laser Doppler Vibrometry (LDV) is an instrument that uses a laser to illuminate the surface of a vibrating object and obtains vibration information by detecting the Doppler frequency shift of the reflected or scattered light. Current LDVs are primarily used for industrial structural vibration measurement or remote vibration measurement. Their basic optical path typically employs a heterodyne Mach-Zehnder interferometer structure. Specifically, the laser consists of a measurement beam and a reference beam. The reference arm introduces a fixed beat frequency carrier wave via an acousto-optic frequency shifter. The measurement arm illuminates the target and receives the echo. The two beams interfere at the detector to generate a beat frequency electrical signal, which is then demodulated in the electrical domain to obtain the vibration information. In scenarios with highly reflective targets or where reflective films can be applied, the system can obtain stable echoes and achieve continuous demodulation.

[0004] Non-contact seismic exploration utilizes methods such as laser Doppler vibration measurement to achieve non-contact seismic measurement. It can serve as a substitute or supplement to seismic detectors for non-contact vibration / velocity / displacement measurements on the surface during seismic exploration. It holds promise for applications in vehicle-mounted towing, rapid deployment, and complex terrain scenarios, reducing deployment difficulty and improving acquisition efficiency. However, in practical engineering applications, laser Doppler vibration meters face significant physical challenges. The targets of seismic exploration are optically rough surfaces such as soil, rock, or asphalt pavement. The application of reflective film also faces the same problems as traditional detector deployment, making it difficult to apply the film at every detector point. When coherent laser light illuminates a rough surface, the laser emitted from the LDV undergoes random interference on rough surfaces such as soil and gravel, often resulting in echoes exhibiting strong scattering and speckle statistical characteristics. Due to surface movement, minor shifts in the measurement platform, or atmospheric turbulence, echo signals experience dramatic intensity fluctuations, sometimes even signal loss, generating severe speckle noise. This results in non-physical "peaks" in the measurement data, masking weak deep seismic reflection signals. Rough surfaces can be considered as a superposition of numerous tiny scattering units; the echo electric field is the coherent vector sum of multiple scattering components, spatially forming alternating bright and dark spots. When the receiving system, including the receiving aperture, effective field of view, and mode matching coupled to the single-mode fiber, happens to acquire a combination of dark spots or near-phase cancellation at a certain moment, the echo amplitude drops sharply, interference contrast decreases, and the carrier-to-noise ratio exceeds the demodulation threshold. This leads to phase jumps, velocity spikes, or demodulation interruptions. This phenomenon directly undermines the requirements of seismic data acquisition for continuous time series, stable amplitude and phase characteristics, and controllable sampling rates.

[0005] To facilitate understanding from a theoretical perspective, a basic signal model for heterodyne LDV is given. Let the laser center frequency be: Where c is the speed of light. A fixed frequency shift is introduced into the reference arm to represent the laser wavelength. A beat frequency carrier is constructed. The reference light and the measurement echo can be represented as follows:

[0006] ;

[0007] .

[0008] After the two beams of light are superimposed at the detector, the light intensity is expanded to obtain the following: For the carrier frequency difference term:

[0009] ;

[0010] in, Under ideal single scattering or stable reflection conditions, and Stable and demodulatorable; however, on rough surfaces, the echo can be considered as a superposition of multiple scattering components:

[0011] .

[0012] The equivalent amplitude and phase of the corresponding difference frequency term are given by vector sum. Decision. Due to various and With changes in spatial location and minute geometric perturbations, Random fluctuations will occur; when the vector sum approaches zero, the equivalent... The signal size decreases significantly, causing a sharp drop in the carrier-to-noise ratio of the beat frequency signal. This leads to a threshold crossing in the demodulator, manifesting as a phase jump or disconnection.

[0013] Meanwhile, strategies such as polarization diversity, multi-channel diversity, and dual-wavelength diversity have been developed to address the speckle dropout problem. However, most of these efforts remain focused on the diversity aspect itself or are geared towards general seismic measurement scenarios, and some methods are quite expensive. At the same time, optimization research on optical antennas, such as Cassegrain receiving antennas, primarily focuses on coupling efficiency, image quality, and long-distance light reception capabilities, without developing a system-level closed-loop control method with "echo quality-phase continuity" as the core feedback. Seismic measurement requires maintaining phase / velocity waveform continuity and amplitude-phase fidelity in the low-frequency band, while allowing for engineering trade-offs between deployment efficiency and robustness. The aforementioned research approaches are often independent, lacking a collaborative working method tailored to the specialized scenario of "earthquake measurement."

[0014] In summary, in non-contact laser Doppler seismometry, the speckle statistics of the scattered echoes from rough surfaces cause random fluctuations in the amplitude and phase of the echoes coupled to the coherent receiving channel. This leads to a significant decrease in interference fringe contrast and carrier-to-noise ratio within a short period, resulting in unstable or even interrupted phase / frequency demodulation of the heterodyne carrier, leading to dropped lines, missing data, or severe noise spikes. Therefore, there is an urgent need to develop a robust optomechanical system for seismometry on rough surfaces, capable of significantly reducing the dropped line rate, maintaining continuous phase demodulation, and meeting seismic bandwidth requirements under rough surface conditions. Summary of the Invention

[0015] The purpose of this invention is to provide a non-contact laser Doppler seismometry robust optomechanical system and a non-contact laser Doppler seismometry method to solve the problem of achieving long-term continuous and repeatable phase / velocity demodulation output in seismometry on rough surfaces by configuring optical structure parameters and using a control method based on echo quality, without relying on or minimizing additional treatments such as surface film application and compaction.

[0016] This invention is achieved through the following technical solution:

[0017] A non-contact laser Doppler seismometry robust optomechanical system consists of an LDV optical unit, a two-dimensional micro-rotation actuator, a variable field-of-view optical structure, a photoelectric detection and analog front-end, an analog-to-digital conversion and digital processing unit, and a data output acquisition system.

[0018] The LDV optical unit is used to generate laser light, construct the front-end heterodyne interference basic optical path of the reference optical path and the measurement optical path, and complete the echo transmission and reception separation after the measurement light propagates back and forth, outputting the measurement beam, the reference optical signal and the echo measurement optical signal;

[0019] The two-dimensional micro-rotation actuator is positioned on the common transmission path of the measurement light and the echo light, and is used to synchronously deflect the transmitted measurement beam and the echo receiving beam.

[0020] The variable field-of-view optical structure is positioned after the two-dimensional micro-rotation actuator and is used to emit the measurement beam to the rough ground surface and to receive, converge, shape, and adjust the field of view of the scattered echo from the rough ground surface.

[0021] The photoelectric detection and simulation front end is used to perform beam combining interference between the reference optical signal from the LDV optical unit and the echo measurement optical signal output from the LDV optical unit transceiver separation node, and to complete photoelectric conversion, balanced detection, analog filtering and analog amplification.

[0022] The analog-to-digital conversion and digital processing unit is used to sample, digitally demodulate, evaluate echo quality, determine status, and generate control signals from the electrical signals output by the analog front end, and output actuator control signals and field-of-view control signals.

[0023] The data output acquisition system is used to receive at least one of velocity data and displacement data output by the analog-to-digital conversion and digital processing unit, and to complete display, recording, and subsequent seismic data acquisition and processing.

[0024] Furthermore, the LDV optical unit includes a fiber laser, a polarization-maintaining fiber end, a collimation component, a pre-stage beam splitter component, a reference light frequency shifter component, a transmit / receive separation component, and a reference light extraction component. The fiber laser is a narrow-linewidth continuous laser, and its output end is connected to the free-space optical path through the polarization-maintaining fiber end. The polarization-maintaining fiber end is used to ensure the stability of the polarization state of the output light. The collimation component is located after the polarization-maintaining fiber end and is used to shape the diverging laser output from the polarization-maintaining fiber into a collimated laser beam. The pre-stage beam splitter component uses a PBS beam splitter to split the collimated laser into two paths: a measurement beam and a reference beam. The reference light frequency shifter component includes an acousto-optic frequency shifter and a reflector or folding mirror in the reference optical path. The transmit / receive separation component includes a quarter-wave plate and a PBS, located at the common return node of the measurement beam and the echo beam, to enable the emitted measurement beam and the echo measurement beam to form a separable polarization relationship, and to extract the echo beam from the common measurement optical path after it returns to the LDV optical unit.

[0025] Furthermore, the two-dimensional micro-rotation actuator is positioned on the common optical path of the measurement beam output path and the echo receiving path, for synchronous adjustment of the transmission and reception directions; the two-dimensional micro-rotation actuator includes a two-dimensional galvanometer or a MEMS dual-axis reflector, as well as a matching drive module and position feedback module; the two-dimensional micro-rotation actuator receives the measurement beam from the LDV optical unit and guides the measurement beam to the variable field of view optical structure at a set angle; the two-dimensional micro-rotation actuator also receives the echo beam received by the variable field of view optical structure and returns it back to the transceiver separation component in the LDV optical unit along a predetermined common return path;

[0026] The variable field-of-view optical structure is positioned between the two-dimensional micro-rotation actuator and the target object to complete the transmitting antenna, receiving antenna, and field-of-view adjustment. The variable field-of-view optical structure consists of a transmitting antenna, a receiving antenna, an aperture mechanism, and necessary coupling lenses or image relay lenses.

[0027] Furthermore, the photoelectric detection and analog front end is used to receive two optical signals and complete interferometric detection and analog signal conditioning. The photoelectric detection and analog front end includes a beam combiner, a balanced photodetector, a transimpedance amplifier circuit, an analog bandpass filter circuit, and an analog gain adjustment circuit. The beam combiner is used to spatially combine the reference optical signal from the LDV optical unit and the echo measurement optical signal output from the LDV optical unit's transceiver separation component into a coaxial interference beam. The balanced photodetector is located after the beam combiner and is used to convert the interference optical signal into a differential analog electrical signal, while suppressing common-mode intensity noise and improving the effective carrier-to-noise ratio of the beat frequency signal.

[0028] Furthermore, the analog-to-digital conversion and digital processing unit is used to perform digital processing and control logic operations from analog signals to digital velocity / displacement data; the analog-to-digital conversion and digital processing unit includes an ADC, a digital demodulation terminal, an echo quality evaluation function module, and a state machine and control logic module;

[0029] The data output acquisition system is used to receive velocity / displacement data output by the analog-to-digital conversion and digital processing unit, and to complete data storage, display, transmission, or data interface with the seismic acquisition host. The data output acquisition system can be an independent data logger, a host computer acquisition terminal, a field seismic acquisition station, or other equipment with data receiving and storage functions.

[0030] A seismometry method using a non-contact laser Doppler seismometry robust optomechanical system includes the following steps:

[0031] S1. Construction of the heterodyne interference optical path;

[0032] S2. Construct a common-path optical antenna for transmitting and receiving and a variable field-of-view coupling structure;

[0033] S3. Configure photoelectric detection and sampling links;

[0034] S4. Demodulation and echo quality assessment and control are performed in the digital domain;

[0035] S5. Bright spot locking and offline re-acquisition control based on echo quality function;

[0036] S6 outputs velocity / displacement data and performs necessary control compensation.

[0037] Further, in step S1, a narrow-linewidth continuous laser is selected as the light source, and linearly polarized laser is output through a polarization-maintaining fiber. After collimation, the laser enters the beam splitting structure and is divided into a measurement arm and a reference arm. The reference arm is introduced into a fixed beat frequency carrier by an acousto-optic frequency shifter, and then sent to the beam combining position after being refracted by a reflector; the measurement arm continues to propagate forward as a ground-illuminating beam, forming a heterodyne interference basic optical path.

[0038] Step S2: The laser beam output from the measuring arm is introduced into the transceiver common optical antenna. The transmitting end adopts a collimated emission structure or an equivalent transmitting antenna so that the laser beam can illuminate the target area on the ground within a predetermined ranging range. The receiving end adopts a reflective telescope structure. A variable field of view mechanism is set in the receiving coupling section.

[0039] Further, in step S3, the measurement echo reflected back from the rough surface is guided to a beam combiner via a receiving antenna, a polarization separation structure, and a common return path. This beam is then spatially combined with the reference light formed in step S1, thereby generating a heterodyne beat frequency signal at the detector. The combined interference light is then fed into a balanced photodetector, converting the optical beat frequency signal into a differential analog electrical signal. This differential analog electrical signal undergoes bandpass filtering, gain adjustment, and noise suppression at an analog front-end before being sent to an ADC for high-speed sampling. This process converts the optical phase change caused by the vibration of the rough surface into a differential beat frequency electrical signal that can be processed in the digital domain, providing a basis for subsequent... Demodulation, phase unrolling, and velocity / displacement recovery are provided as inputs.

[0040] Further, in step S4, the digital beat frequency signal output by the ADC is input to the digital demodulation terminal for quadrature demodulation to obtain in-phase and quadrature components. The instantaneous amplitude and instantaneous phase are then calculated, and the phase is expanded to obtain a continuous phase. Based on the continuous phase, at least one of the target's velocity and displacement along the beam direction is calculated. Simultaneously, the short-time statistics of amplitude and phase are updated within a preset time window to construct the echo quality function. .

[0041] Further, in step S5, the variable field-of-view optical structure is set to a large field-of-view working state in the capture state, and the two-dimensional micro-rotation actuator performs a limited range of angle scanning within the target area. During the scanning process, the analog-to-digital conversion and digital processing unit continuously calculates... Value, and determine the current Has the preset locking threshold been reached? This achieves the switching to a locked state, switching the variable field-of-view optical structure to a small field-of-view operating state. Simultaneously, it controls the two-dimensional micro-rotation actuator to perform micro-disturbance search and hill-climbing updates near the current bright spot, causing the actuator to continuously move towards... The direction of increase is fine-tuned; if in the locked state, The connection drops below the preset disconnection threshold. And lasting longer than the preset time If the current bright spot lock fails or there is a significant risk of disconnection, the system automatically exits the lock state and re-enters the scanning and re-acquisition process, searching for a new high-quality echo region within a limited angle range according to a preset trajectory; once a suitable echo region is found... Exceed The position is locked again, and the field of view is contracted again;

[0042] In step S6, under locked state, at least one of the velocity data and displacement data of the ground surface along the beam direction is continuously output by the digital demodulation terminal and transmitted to the data output acquisition system; for the small control actions introduced by the actuator during the bright spot locking or re-acquisition process, the actuator control signal is recorded synchronously and converted into a geometric compensation term when needed, and subtracted from the demodulation phase.

[0043] Compared with the prior art, the beneficial effects of the present invention are:

[0044] Compared to existing schemes that rely on fixed light spots or simply surface coating / compaction to improve reflection, this invention, through a combination of variable effective field of view, echo quality closed-loop locking, and recapture after a missed scan, significantly reduces the probability of phase demodulation interruption caused by deep speckle fading under rough surface conditions, thus improving the continuity and availability of vibration records. The large field of view setting in the capture state improves the probability of initial bright spot capture; the small field of view and coupling optimization in the locked state improve coherence and interference contrast, thereby enhancing phase continuity; and the scan recapture triggered by the missed scan criterion enables automatic recovery, avoiding time loss and inconsistencies caused by manual realignment. Through optional polarization diversity, multi-channel diversity, or dual-wavelength diversity, the missed scan rate can be further reduced and the adaptability to different surface reflection characteristics can be improved. The continuous velocity / displacement time series output by this invention can be directly used in seismic data acquisition and subsequent processing workflows, such as gather construction, filtering, and imaging, making LDV closer to a non-contact seismograph sensor that can be engineering-replaced by geophones. Attached Figure Description

[0045] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 This is a main block diagram of the non-contact laser Doppler seismometry robust optomechanical system of the present invention;

[0047] Figure 2 This is a flowchart of the system control logic.

[0048] Figure 3 This is a flowchart of the steps of the non-contact laser Doppler seismometry method of the present invention;

[0049] Figure 4 This is a schematic diagram of the non-contact laser Doppler seismometry robust optomechanical system of the present invention. Detailed Implementation

[0050] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, the accompanying drawings show only the parts relevant to the present invention, and not all of the structures.

[0051] This invention addresses the problem of unstable speckle echoes generated under coherent light on rough surfaces such as gravel, soil, concrete, and asphalt. It proposes a robust seismometry optical-mechanical system and method with an engineering-featured variable receiving field of view, bright spot locking, and recapture capability, enabling LDV to stably and continuously output ground vibration velocity / displacement information without relying on or with minimal additional processing such as ground film application.

[0052] This invention proposes a system that combines feasible optical structure design, echo quality-driven closed-loop control, and offline scan reacquisition. The overall system still utilizes a mature heterodyne LDV framework as its optical base, consisting of a laser source, beam splitter and frequency shifter modules, transceiver optical antennas, beam combiner and photodetector, analog front-end, analog-to-digital sampling, and digital demodulation. The innovation of this invention lies not in the heterodyne interference structure itself, but in: 1. Introducing a variable effective field of view / variable coupling condition optomechanical structure into the receiving link, enabling the system to switch between "acquisition" and "lock" operating states; 2. Using the echo quality function as feedback to drive a small angle / lateral displacement actuator in the transceiver common path, achieving speckle and bright spot locking and continuous phase tracking; 3. Automatically triggering scan reacquisition when the quality index drops below the demodulation threshold, searching for new bright spot operating points and resuming continuous demodulation.

[0053] (1) Variable field-of-view receiving antenna and variable coupling mechanism. In seismic exploration scenarios, a trade-off needs to be made between "collecting as much echo energy as possible" and "maintaining the purity / contrast of the coherent receiving mode". This invention adopts a reflective telescope structure at the receiving end, such as a Cassegrain reflective telescope structure, to obtain a larger aperture and a longer equivalent focal length, thereby improving the ability to collect long-distance scattered echoes; a variable aperture or zoom group is set in the subsequent coupling section to realize the switching or continuous adjustment of the effective field of view (FOV) or equivalent numerical aperture (NA). The system includes at least two operating states: the acquisition state uses a larger effective field of view (e.g., on the order of 0.3° to 1.0°, specifically determined by the system focal length and aperture) to increase the probability of capturing bright spots under unknown speckle patterns; the locking state uses a smaller effective field of view (e.g., on the order of 0.05° to 0.2°) to suppress off-axis scattering components, reduce speckle aliasing and improve interference contrast, thereby improving phase demodulation stability. The variable field of view can be achieved through an aperture switching disk driven by a stepper motor, a continuously variable aperture, or by moving a zoom coupling group formed by two sets of lenses. In engineering implementation, it is necessary to ensure that the aperture position and the conjugate relationship between the coupling lens / fiber end face are stable, and to ensure that each position is repeatedly positioned through mechanical limiting and calibration.

[0054] (2) Bright Spot Locking Closed Loop Based on Echo Quality Function. This invention arranges a two-dimensional micro-rotation mechanism, such as a galvanometer or MEMS scanning mirror, in the common transmit / receive optical path, causing its rotation to act simultaneously on both the transmit and receive directions, avoiding geometric mismatch introduced by only changing the transmit or receive direction. On the ground, the micro-rotation manifests as a tiny lateral movement of the light spot, thereby changing the sampled speckle position. This invention uses the micro-rotation degree of freedom as the actuated quantity, actively locking the light spot with the demodulation capability of the echo signal as the target.

[0055] To transform the concept of "bright spot / dark spot" into a usable controllable quantity, this invention defines an echo quality function. , It can be composed of one or more of the following: the amplitude of the beat frequency signal, the carrier-to-noise ratio, the interference contrast, and the short-time phase stability. (Using I...) Taking demodulation as an example, suppose the beat frequency bandpass signal is quadrature demodulated to obtain the in-phase component. Orthogonal components Then the instantaneous amplitude:

[0056] ;

[0057] Instantaneous phase:

[0058] .

[0059] In the time window Intra-calculable mean amplitude Amplitude Standard Deviation Phase increment Standard deviation And construct, for example:

[0060] ;

[0061] in, , , As weight, To prevent small amounts from being divided by zero, This is a normalized scale. The system control objective is to maximize... Or at least keep The threshold is above the lock-in threshold, thus ensuring continuous and stable phase demodulation.

[0062] In terms of control strategy, closed-loop optimization can be achieved by using small-perturbation comparison and hill-climbing update. Specifically, from the current perspective... Calculation Subsequently, in , , , Calculate the corresponding mass value at the location, and select to make The direction of increase updates the angle. Typically, the measurement ranges from a few microradians to tens of microradians to ensure that the movement scale does not cross the bright spot scale. The control update period can be selected on the order of milliseconds, making it much faster than the change in the dominant frequency of the seismic signal. This allows for continuous tracking of speckle and bright spots without significantly modulating the seismic signal, ensuring that the light spot remains in the high-quality speckle and bright spot region, thereby significantly reducing the probability of line drops and maintaining phase continuity.

[0063] It should be noted that when the two-dimensional micro-rotation mechanism is used for bright spot locking and recapture, the geometric change it introduces is mainly manifested as a small lateral displacement of the light spot on the measured rough surface, rather than a radial distance change along the laser line of sight. Since laser Doppler vibrometers are sensitive to the radial optical path rate of change but not to lateral displacement, this type of micro-rotation primarily alters the speckle sampling position and does not introduce an equivalent Doppler frequency shift of the same order of magnitude as the target vibration. In engineering implementation, the micro-rotation angle change is typically on the order of microradians, and the equivalent optical path change it introduces on the target distance scale is much smaller than the optical path change caused by seismic vibration; the corresponding equivalent Doppler frequency shift is mainly concentrated in the extremely low frequency band, which can be separated from the seismic signal frequency band in the frequency domain or filtered out as platform attitude drift in digital demodulation. Therefore, the bright spot locking and scanning recapture process will not produce observable spurious modulation of the seismic vibration signal, thus affecting the inversion of the true vibration information of the surface. In one implementation, the actuator control signal can be synchronously recorded at the digital demodulation end, and the geometric optical path change it causes can be subtracted from the demodulation phase as a compensation term to further eliminate residual low-frequency terms that may be introduced by actuator micro-motion.

[0064] (3) Scan re-acquisition mechanism triggered by disconnection. The bright spot locking loop can converge quickly and maintain stability near the local optimum. However, due to sudden deep fading, abrupt changes in orientation, or environmental disturbances... During a sharp descent, localized uphill climbs may become trapped in low-quality zones and struggle to recover. Therefore, this invention introduces a state machine: capture state, lock state, and recapture state, to perform a rapid "region search" within a defined range near the current angle to find new usable bright spot areas.

[0065] Set disconnection threshold With lock threshold , And set the disconnection detection time. When the system detects Continue to exceed If it is determined that the bright spot lock cannot be maintained and there is a risk of disconnection, immediately switch to scan and reacquisition, and switch the receiving field of view to acquisition mode. The scan is performed within a limited range near the current angle (e.g., ±0.2 to ±0.5 milliradians, specifically determined by the spot size, ranging, and actuator capabilities), traversing candidate angle points according to a preset trajectory (e.g., a circular or Lissajous trajectory), and calculating at each candidate point. Once discovered Immediately use this angle as the new locking starting point, return to the locked state, and switch the receiving field of view to the small field of view setting of the locked state. It is important to emphasize that scanning is not the normal operating mode, but only used when... The scan is triggered after falling below the threshold and persisting for a period of time; during normal seismic measurement, the bright spot locking closed loop continues to operate, only slightly following the continuous evolution of the speckle, thus avoiding frequent scan triggers. Therefore, this invention uses scanning for situations caused by severe disturbances. During a sharp decline, the system "quickly searches for available bright spots" instead of averaging the regional signal over a long period, avoiding disruption of the seismic equivalence of individual points. Output data during the scan can be marked as invalid or post-processed on the host computer to prevent scan modulation contamination of the seismic signal. This mechanism transforms manual realignment into a reproducible, automated process, meeting the self-healing requirements of vehicle-mounted towing or long-term field operations.

[0066] (4) Optional Enhancement Module. To further reduce the probability of dropped connections and expand the applicable surface range, this invention allows the integration of diversity and enhancement modules as optional implementations within the above core framework. Polarization diversity can be achieved by splitting the echo into two orthogonal polarization paths via PBS for separate detection and demodulation, and then performing optimal or weighted synthesis in the digital domain; multi-channel reception can be achieved by sampling different spatial speckles through multiple coupled channels or multiple detectors, and real-time selection. Higher channel output; dual-wavelength diversity allows simultaneous measurement and fusion of two similar wavelengths to improve stability by utilizing the decorrelation of speckle at different wavelengths. The above modules do not change the main theme of this invention: using echo quality as feedback for bright spot locking and re-acquisition in case of line loss.

[0067] (5) Digital Demodulation and Control Implementation. The system uses balanced photoelectric detection and analog bandpass filtering to obtain the beat frequency electrical signal, which is then sampled by an ADC and processed digitally. Digital processing can be implemented on an FPGA or equivalent high-speed hardware platform, including quadrature demodulation, amplitude and phase calculation, phase expansion, quality function calculation, state machine and actuator control output. The phase-to-velocity / displacement conversion relationship is as follows:

[0068] ;

[0069] in, This represents the continuous phase after expansion. The control output of the two-dimensional micro-rotor actuator can drive the galvanometer / motor using DAC analog voltage or PWM + power amplifier. The key to the control closed loop is to ensure that the actuator response bandwidth matches the control update cycle, and to avoid mechanical shock through limiting and soft start.

[0070] like Figure 1 , Figure 4 As shown, the non-contact laser Doppler seismometry robust optomechanical system of the present invention consists of an LDV optical unit, a two-dimensional micro-rotation actuator, a variable field-of-view optical structure, a photoelectric detection and analog front end, an analog-to-digital conversion and digital processing unit, and a data output acquisition system.

[0071] The LDV optical unit is used to generate laser light, construct the front-end heterodyne interference basic optical path of the reference optical path and the measurement optical path, and complete the echo transmission and reception separation after the measurement light propagates back and forth, thereby outputting the measurement beam, reference optical signal and echo measurement optical signal to the system respectively.

[0072] The two-dimensional micro-rotation actuator is positioned on the common transmission path of the measurement light and the echo light, and is used to synchronously deflect the transmitted measurement beam and the echo receiving beam.

[0073] The variable field-of-view optical structure is positioned after the two-dimensional micro-rotation actuator and is used to emit the measurement beam onto the rough ground surface and to receive, converge, shape, and adjust the field of view of the scattered echoes from the rough ground surface.

[0074] The photoelectric detection and simulation front end is used to perform beam combining interference between the reference optical signal from the LDV optical unit and the echo measurement optical signal output from the LDV optical unit transceiver separation node, and to complete photoelectric conversion, balanced detection, analog filtering and analog amplification.

[0075] The analog-to-digital conversion and digital processing unit is used to sample, digitally demodulate, evaluate echo quality, determine status, and generate control signals from the analog front-end output electrical signals, and output actuator control signals and field-of-view control signals.

[0076] The data output acquisition system is used to receive at least one of velocity data and displacement data output by the analog-to-digital conversion and digital processing unit, and to complete display, recording or subsequent seismic data acquisition and processing.

[0077] The LDV optical unit preferably includes a fiber laser, a polarization-maintaining fiber end, a collimation component, a pre-stage beam splitter, a reference light frequency shifter, a transmit / receive separation component, and a reference light extraction component. The fiber laser is preferably a 1550nm narrow-linewidth continuous laser, with its output end connected to a free-space optical path via the polarization-maintaining fiber end. The polarization-maintaining fiber end is an integral part of the LDV optical unit, used to ensure the stability of the output light's polarization state and reduce the impact of polarization drift on heterodyne interference demodulation. The collimation component is located after the polarization-maintaining fiber end and is used to shape the divergent laser output from the polarization-maintaining fiber into a collimated laser beam. The pre-stage beam splitter preferably uses a PBS or other equivalent beam splitter, its function being to split the collimated laser into two paths: a measurement beam and a reference beam. The measurement beam is output as the working beam to the two-dimensional micro-rotator, while the reference beam enters the reference light frequency shifter. The reference light frequency shifting component preferably includes an acousto-optic frequency shifter and a reflector or folding mirror in the reference optical path. The reflector is part of the reference optical path structure in the LDV optical unit and is used to adjust the propagation direction of the reference optical path, compress the structural size, and guide the frequency-shifted reference light to the photoelectric detection and simulation front end. The transceiver separation component preferably includes a quarter-wave plate and a PBS, which are set at the common return node of the measurement light and the echo light. This allows the transmitted measurement light and the echo measurement light to form a separable polarization relationship, and after the echo returns to the LDV optical unit, it is exported from the common measurement optical path. Specifically, the transmitted measurement light is converted into circularly polarized light after passing through the quarter-wave plate before or during the transceiver common path section, and is then directed towards the target object via a two-dimensional micro-rotator and a variable field-of-view optical structure. After the echo returns to the LDV optical unit along the original common path, it passes through the quarter-wave plate again, causing its polarization state to change relative to that during transmission. It is then separated from the transmitted measurement optical path at the PBS, guided into the echo measurement light output channel, and finally sent to the photoelectric detection and simulation front end. The LDV optical unit therefore outputs three types of optical signals related to heterodyne interference: one is the measurement beam sent to the two-dimensional micro-rotator and ultimately illuminating the target object; another is the reference light signal directly fed into the photoelectric detection and analog front end; and the third is the echo measurement light signal extracted by the transceiver separation component after the measurement light has propagated back and forth. It should be noted that the so-called heterodyne interference optical path is not limited to a single module, but is jointly constituted by the pre-stage beam splitting, frequency shifting, reference light formation, and echo transceiver separation sections in the LDV optical unit, and the beam combining and detection sections in the photoelectric detection and analog front end. Specifically, the LDV optical unit is responsible for forming the reference light, sending the measurement light, and extracting the echo measurement light, while the photoelectric detection and analog front end is responsible for realizing the final interference between the reference light and the echo measurement light and extracting the electrical signal.

[0078] The two-dimensional micro-rotation actuator is positioned on the shared optical path of the measurement beam output path and the echo reception path, and is used to synchronously adjust the transmission and reception directions. The two-dimensional micro-rotation actuator preferably includes a two-dimensional galvanometer, a MEMS dual-axis mirror, or other reflective actuators capable of achieving dual-degree-of-freedom micro-angle adjustment, along with a corresponding drive module and position feedback module. The two-dimensional micro-rotation actuator receives the measurement beam from the LDV optical unit and guides the measurement beam to the variable field-of-view optical structure at a set angle. Simultaneously, the two-dimensional micro-rotation actuator also receives the echo beam received by the variable field-of-view optical structure and directs it back along a predetermined common return path to the transceiver separation component in the LDV optical unit. Because the two-dimensional micro-rotation actuator operates on the shared portion of the transmission and reception optical paths, its angle change is synchronous with transmission and reception, enabling bright spot locking within the target area, micro-range position search, and re-acquisition scanning after line loss without altering the reference optical path structure. The two-dimensional micro-rotation actuator itself does not perform photoelectric conversion, nor does it perform reference light formation and polarization separation. Its function is to change the spatial orientation of the measurement light and the echo light, so that the sampling position of the system can be actively selected in the speckle field of the rough surface.

[0079] The variable field-of-view optical structure is positioned between the two-dimensional micro-rotating actuator and the target object, serving as a transmitting antenna, receiving antenna, and field-of-view adjustment function. Preferably, the variable field-of-view optical structure comprises a transmitting antenna, a receiving antenna, an aperture mechanism, and necessary coupling lenses or image-receiving lenses. In a preferred embodiment, the transmitting antenna is a transmitting optical antenna composed of two collimating lenses, used to further shape the measurement light from the two-dimensional micro-rotating actuator and project it onto the rough surface at a predetermined divergence angle; the receiving antenna is preferably a Cassegrain reflector telescope structure, used to improve the collection efficiency of the echo light and the effective receiving aperture. The Cassegrain reflector telescope structure is the receiving antenna part of the variable field-of-view optical structure, typically composed of a primary mirror, a secondary mirror, and corresponding mounting support structures. The primary and secondary mirrors together constitute a reflective receiving telescope system, converging the scattered echoes returned from the rough surface and guiding them back to the common transmitting and receiving optical path where the two-dimensional micro-rotating actuator is located. The aperture mechanism, positioned at the aperture location of the receiving antenna or its conjugate aperture location, is a component of the variable field-of-view optical structure and is used to adjust the system's effective receiving field of view and equivalent numerical aperture. By changing the aperture opening, it can switch between a "capture state" and a "locked state": in the capture state, the aperture opening is larger to expand the effective receiving field of view and increase the probability of capturing usable bright spots in the random speckle field; in the locked state, the aperture opening is smaller to suppress off-axis scattering components, reduce speckle aliasing, and improve interference contrast and phase demodulation stability. The aperture mechanism can be implemented using a stepper motor-driven mechanical aperture, an aperture switching disk, or an equivalent variable field-of-view mechanism.

[0080] The photoelectric detection and analog front-end receives two optical signals and performs interferometric detection and analog signal conditioning. This module preferably includes a beam combiner, a balanced photodetector, a transimpedance amplifier circuit, an analog bandpass filter circuit, and an analog gain adjustment circuit. The beam combiner spatially combines the reference optical signal from the LDV optical unit and the echo measurement optical signal output from the LDV optical unit's transceiver separation component into a coaxial interference beam, thereby forming a beat frequency interference signal on the subsequent detector. The beam combiner can be implemented using an optical fiber coupler, a beam splitter prism, a polarization beam combiner, or other equivalent optical beam combiners; it belongs to the photoelectric detection and analog front-end, not to the two-dimensional micro-rotator actuator or variable field-of-view optical structure. The balanced photodetector is located after the beam combiner and converts the interference optical signal into a differential analog electrical signal, while suppressing common-mode intensity noise and improving the effective carrier-to-noise ratio of the beat frequency signal. The balanced photodetector is a core component of the photoelectric detection and analog front-end. The differential current signal output by the balanced photodetector is amplified by transimpedance and converted into a voltage signal. Then, it is filtered by an analog bandpass filter circuit to remove irrelevant low-frequency noise and high-frequency spurious terms. Finally, it is amplified by an analog gain adjustment circuit to a suitable amplitude range for analog-to-digital sampling and output as an analog signal to the analog-to-digital conversion and digital processing unit.

[0081] The analog-to-digital conversion and digital processing unit (ADC) is used to complete all digital processing and control logic operations from analog signals to digital velocity / displacement data. This module preferably includes an ADC, a digital demodulation terminal, an echo quality evaluation function module, and a state machine and control logic module. The ADC is a front-end component of the ADC, and its function is to perform high-speed sampling and conversion of the analog beat frequency electrical signal from the photoelectric detection and analog front-end into a digital sequence. The digital demodulation terminal is also a component of the ADC, and it is preferably implemented by an FPGA, DSP, or other high-speed digital processor. It is used to perform quadrature demodulation of the digital beat frequency signal output by the ADC. Amplitude and phase recovery, phase unwrapping, and velocity / displacement calculation are performed. The echo quality evaluation function module is set after or implemented in parallel with the digital demodulation end, and is used to calculate the echo quality evaluation function based on parameters such as amplitude, carrier-to-noise ratio, and short-time phase stability obtained from demodulation. The state machine and control logic module is connected to the echo quality evaluation function module, according to... Whether the value reaches the preset lock threshold, disconnection threshold, and duration condition, corresponding actuator control signals and field-of-view control signals are generated. The actuator control signal is sent to the 2D micro-rotator actuator to control it to perform bright spot locking, local hill-climbing search, or scan re-acquisition after disconnection. The field-of-view control signal is sent to the aperture mechanism or equivalent variable field-of-view mechanism in the variable field-of-view optical structure to switch the field of view range between the acquisition and lock states. After digital demodulation and control logic processing, the analog-to-digital converter and digital processing unit outputs at least one of the final velocity data and displacement data to the data output acquisition system.

[0082] The data output acquisition system is used to receive velocity / displacement data output by the analog-to-digital converter and digital processing unit, and to perform data storage, display, transmission, or data interface with the seismic acquisition host. This data output acquisition system can be a standalone data logger, a host computer acquisition terminal, a field seismic acquisition station, or other equipment with data receiving and storage functions. It is preferably connected to the analog-to-digital converter and digital processing unit via a digital interface to ensure the integrity and time synchronization of the output velocity / displacement data.

[0083] In summary, in this embodiment, the polarization-maintaining fiber end, fiber laser, collimation assembly, pre-stage beam splitter assembly, reference light frequency shifting assembly, mirror in the reference optical path, and quarter-wave plate and PBS used for transmit / receive separation in the transmit / receive separation assembly all belong to the LDV optical unit; the two-dimensional galvanometer or MEMS biaxial mirror and its drive feedback assembly belong to the two-dimensional micro-rotation actuator; the two-piece transmitting antenna, Cassegrain reflective receiving antenna, and aperture mechanism belong to the variable field of view optical structure; the beam combiner and balanced photodetector belong to the photoelectric detection and analog front end; and the ADC and digital demodulation end belong to the analog-to-digital conversion and digital processing unit. The above modules form a complete measurement link according to the following connection relationship: The LDV optical unit outputs a measurement beam to the two-dimensional micro-rotation actuator, which guides the measurement beam into the variable field-of-view optical structure. The variable field-of-view optical structure emits the measurement beam onto the rough ground surface. The scattered echo returning from the rough ground surface is received by the variable field-of-view optical structure and transmitted back to the two-dimensional micro-rotation actuator. It then returns along the common transmit / receive path to the transmit / receive separation component in the LDV optical unit, where it is separated from the common measurement optical path by a quarter-wave plate and PBS, and then sent to the photoelectric detection and analog front end. At the same time, the LDV optical unit outputs a reference beam to the photoelectric detection and analog front end. The photoelectric detection and analog front end combines the reference beam with the echo measurement beam and converts it into an analog electrical signal. The analog electrical signal enters the analog-to-digital conversion and digital processing unit to complete sampling, demodulation, quality assessment, and control logic operations. The analog-to-digital conversion and digital processing unit outputs actuator control signals and field-of-view control signals to the two-dimensional micro-rotation actuator and the variable field-of-view optical structure, respectively, and outputs the final velocity / displacement measurement data to the data output acquisition system.

[0084] like Figure 2 , Figure 3 As shown, the seismometry method of the non-contact laser Doppler seismometry robust optomechanical system of the present invention includes the following steps:

[0085] S1. Construction of the heterodyne interference optical path:

[0086] A narrow-linewidth continuous-wave laser is selected as the light source, preferably a 1550nm fiber laser, and linearly polarized laser is output through a polarization-maintaining fiber to reduce the impact of polarization drift on the stability of heterodyne interference. After collimation, the laser enters a beam-splitting structure and is divided into a measurement arm and a reference arm. The reference arm is introduced with a fixed beat frequency carrier via an acousto-optic frequency shifter, and then refracted by a mirror before being sent to the beam-combining position; the measurement arm continues to propagate forward as the ground illumination beam. Preferably, a quarter-wave plate and a PBS are incorporated into the measurement arm to complete the polarization shaping of the emitted beam and the polarization separation of the return beam, ensuring that the measurement light has a polarization state suitable for illuminating a rough surface upon exiting, and that the returning echo can enter the receiving link along a predetermined return path. These steps form a stable heterodyne interference basic optical path, where the reference light provides a fixed beat frequency reference, and the measurement light illuminates the rough surface and carries the surface vibration information back. The purpose of this step is to establish the heterodyne beat frequency conditions required for subsequent velocity / displacement demodulation, providing a stable reference for phase recovery.

[0087] S2. Construct a common-path optical antenna for transmitting and receiving and a variable field-of-view coupling structure:

[0088] The laser beam output from the measuring arm is introduced into a common transmitting and receiving optical antenna. The transmitting end employs a collimated output structure or an equivalent transmitting antenna, enabling the laser beam to illuminate the target area on the ground within a predetermined ranging range. The receiving end uses a reflective telescope structure, preferably a Cassegrain reflective receiving antenna, to improve the ability to receive light from echoes scattered from rough surfaces. A variable field-of-view mechanism is provided in the receiving coupling section. This mechanism can be implemented using a switchable aperture, a continuously variable aperture, or a zoom coupling group, used to switch between the effective field of view or the equivalent numerical aperture between at least two operating states: one is the acquisition state, corresponding to a larger effective field of view, to increase the probability of detecting usable bright spot echoes under random speckle conditions; the other is the locked state, corresponding to a smaller effective field of view, to suppress off-axis scattering components, reduce speckle aliasing, and improve interference contrast. This step enables the system to simultaneously possess the contradictory yet necessary capabilities of "easy acquisition" and "stable locking," which is a key difference between this invention and fixed field-of-view receiving schemes.

[0089] S3. Configure the photoelectric detection and sampling link:

[0090] The measurement echo reflected back from the rough ground surface is guided to a beam combiner via a receiving antenna, polarization separation structure, and common return path. This beam is then spatially combined with the reference light formed in step S1, creating a heterodyne beat frequency signal at the detector. The combined interference light is fed into a balanced photodetector, converting the optical beat frequency signal into a differential analog electrical signal. This differential analog signal undergoes bandpass filtering, gain adjustment, and noise suppression at an analog front end before being fed into an ADC for high-speed sampling. The ADC sampling rate should match the selected beat frequency carrier and the expected Doppler sideband width, while retaining sufficient digital processing margin. The purpose of this step is to convert the optical phase change caused by the vibration of the rough ground surface into a beat frequency electrical signal that can be processed in the digital domain, providing a basis for subsequent... Demodulation, phase unrolling, and velocity / displacement recovery are provided as inputs.

[0091] S4. Perform demodulation and echo quality assessment in the digital domain:

[0092] The digital beat frequency signal output from the ADC is input to the digital demodulation terminal, and quadrature demodulation is preferably performed using an FPGA or other high-speed digital processing platform to obtain the in-phase component. and orthogonal components Further calculation of instantaneous amplitude and instantaneous phase And expand the phase to obtain a continuous phase. ; Calculate at least one of the target's velocity and displacement along the beam direction based on the continuous phase, and simultaneously, within a preset time window The short-time statistics of amplitude and phase are updated internally to construct the echo quality function. The aforementioned The ability to characterize whether the current echo signal is in a stable demodulation state can be composed of one or more of the following: beat frequency signal amplitude, carrier-to-noise ratio, interference contrast, and short-time phase stability.

[0093] S5. Bright spot locking and re-acquisition control based on echo quality function:

[0094] After the system is powered on, it preferably enters the capture state by default. At this time, the variable field-of-view optical structure is set to a large field-of-view operating state, and a two-dimensional micro-rotation actuator performs a limited-range angular scan within the target area to search for available high-quality echo regions. During the scanning process, the analog-to-digital conversion and digital processing unit continuously calculates... Value, and determine the current Has the preset locking threshold been reached? .when Reaching or exceeding When the system has captured a bright spot region suitable for stable demodulation, it switches to a locked state. Once locked, the system switches the variable field-of-view optical structure to a small field-of-view operation. Simultaneously, it controls the two-dimensional micro-rotation actuator to perform small perturbation searches and hill-climbing updates near the current bright spot, ensuring the actuator continuously moves towards... The direction of increase is finely adjusted to keep the measurement spot as close as possible to the high-quality speckle and bright spot region, maintaining continuous phase demodulation. In the locked state, The connection drops below the preset disconnection threshold. And lasting longer than the preset time If the current bright spot lock fails or there is a significant risk of disconnection, the system automatically exits the lock state and re-enters the scanning and re-acquisition process, searching for a new high-quality echo region within a limited angle range according to a preset trajectory; once a suitable echo region is found... Upon reaching the desired position, the system re-enters the locked state and re-shrinks the field of view. Through this state switching process, the system achieves closed-loop control from initial acquisition and bright spot locking to self-recovery after a connection loss.

[0095] S6. Output velocity / displacement data and perform necessary control compensation:

[0096] In the locked state, the digital demodulation terminal continuously outputs at least one of the velocity and displacement data along the beam direction of the ground surface and transmits it to the data output acquisition system for subsequent seismic gather construction, filtering, and imaging processing. For minor control actions introduced by the actuator during bright spot locking or recapture, the actuator control signal can be recorded synchronously and converted into a geometric compensation term when needed, subtracted from the demodulated phase, to further suppress residual low-frequency components that may be introduced by actuator micro-motion. Finally, the system achieves continuous operation of non-contact laser Doppler seismometry under rough surface conditions through a cyclical working mode of "capture—lock—disconnection determination—scan recapture—relock".

[0097] Example 1

[0098] Taking 1550nm heterodyne LDV seismometry as an example, the heterodyne interferometric optical path is first constructed. A 1550nm fiber laser is selected as the light source, and polarization-maintaining fiber is used for output to reduce polarization drift. After the light exits from the polarization-maintaining fiber end, it passes through a collimating lens and exits into free space. A quarter-wave plate is then added to convert the linear polarization to circular polarization. Subsequently, a PBS is used to split the light into a measurement arm and a reference arm. An acousto-optic frequency shifter is connected in series in the reference arm to introduce a fixed beat frequency. For example, a range of 20MHz to 100MHz is selected, specifically matched to the ADC sampling rate and digital bandwidth, and the reference light is guided to the beam combiner position via a mirror. The measuring arm passes through another PBS and is then converted to circular polarization by a quarter-wave plate to reduce the sensitivity of the polarization state to changes in the incident angle or surface normal. The beam is emitted through the transmitting antenna to illuminate the ground surface, and the echo is collected by the receiving antenna and enters the beam combiner through the PBS to interfere with the reference light.

[0099] Secondly, a common-path optical antenna for transmitting and receiving, coupled with a variable field of view, is constructed. The transmitting end can employ a collimated emission structure to cover the target distance range; the receiving end uses a Cassegrain reflector telescope structure to obtain a larger entrance pupil and higher light-gathering capability. A variable aperture or zoom group is set in the receiving coupling section to form at least two effective field-of-view settings: acquisition mode and locked mode. In the acquisition mode, the aperture opening is larger, allowing more scattering angles to enter the coupling section, thereby increasing the probability of capturing bright spot echoes under random speckle conditions; in the locked mode, the aperture opening is reduced to suppress off-axis scattering components and improve interference contrast. The aperture mechanism can be driven by a stepper motor and combined with mechanical limits to ensure repeatable positioning; if a continuously variable aperture or zoom group is used, the control quantities (aperture or group spacing) should be mapped to the equivalent field of view / NA using a calibration table.

[0100] Next, the photoelectric detection and sampling link is configured. The combined interference light is output to a balanced photodetector to improve the effective carrier-to-noise ratio of the beat frequency signal and suppress common-mode noise. The differential output is then sent to the ADC for sampling after bandpass filtering and amplification. The ADC sampling rate should meet the bandwidth requirements of the beat frequency and the expected Doppler sideband, and allow for digital filtering margin.

[0101] Next, demodulation and control are implemented in the digital domain. Quadrature demodulation is performed on the sampled signal to obtain the in-phase component. Orthogonal components Calculate the amplitude With phase And expand the phase to obtain a continuous phase. .according to Calculation speed With displacement and with time windows right and The short-time statistics are updated, and the echo quality function is calculated. At the control level, the system enters capture mode by default after power-on: a large field of view is set, the actuator points to the center of the target area, and calculations are performed. And determine whether the lock threshold has been reached. When Reaching or exceeding At this time, the system switches to locked state: the field of view shrinks and a bright spot locking closed loop is initiated. The closed loop outputs control input to the two-dimensional micro-rotation mechanism using a small-disturbance comparison and hill-climbing update method, so that... Maintaining it in the high-value region ensures continuous phase demodulation. If in the locked state... Break And continue to exceed Then proceed to scan and recapture: scan within a limited angle range according to a preset trajectory, and calculate in real time. And seeking new satisfaction The system then switches to the locked state and shrinks the field of view again. This process is repeated cyclically, enabling the system to automatically recover from speckle line drops.

[0102] In terms of engineering parameter adjustment, The sample size should be sufficient to stably calculate the statistic, but it should not be too long to avoid slowing down the response time. and Statistical analysis can be conducted on different land surfaces through preliminary experiments. Distribute and select; The scanning step should be matched with the spot size and speckle-related scale to avoid crossing the effective bright spot; the scanning range should cover an area sufficient to find adjacent bright spots, while avoiding being too large to cause excessive re-acquisition time. Actuator control needs to be set with amplitude and rate limits to avoid mechanical shock; if the system is used in a vehicle or vibration platform, the rigidity of the optomechanical structure and actuator installation should be ensured to prevent the control loop from being amplified by structural resonance.

[0103] To further improve robustness, polarization diversity or multi-channel diversity can be added to the above framework. The basic approach is to demodulate each channel independently and calculate... Then, the optimal output or weighted synthesis is performed, and the optimal result can be used in the feedback of the closed-loop control to ensure that the actuator always converges toward a higher quality channel state.

[0104] It will be understood by those skilled in the art that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A contactless laser Doppler seismically robust optomechanical system, characterized by: It consists of an LDV optical unit, a two-dimensional micro-rotation actuator, a variable field-of-view optical structure, a photoelectric detection and analog front-end, an analog-to-digital conversion and digital processing unit, and a data output acquisition system. The LDV optical unit is used to generate laser light, construct the front-end heterodyne interference basic optical path of the reference optical path and the measurement optical path, and complete the echo transmission and reception separation after the measurement light propagates back and forth, outputting the measurement beam, the reference optical signal and the echo measurement optical signal; The two-dimensional micro-rotation actuator is positioned on the common transmission path of the measurement light and the echo light, and is used to synchronously deflect the transmitted measurement beam and the echo receiving beam. The variable field-of-view optical structure is positioned after the two-dimensional micro-rotation actuator and is used to emit the measurement beam to the rough ground surface and to receive, converge, shape, and adjust the field of view of the scattered echo from the rough ground surface. The photoelectric detection and simulation front end is used to perform beam combining interference between the reference optical signal from the LDV optical unit and the echo measurement optical signal output from the LDV optical unit transceiver separation node, and to complete photoelectric conversion, balanced detection, analog filtering and analog amplification. The analog-to-digital conversion and digital processing unit is used to sample, digitally demodulate, evaluate echo quality, determine status, and generate control signals from the electrical signals output by the analog front end, and output actuator control signals and field-of-view control signals. The data output acquisition system is used to receive at least one of velocity data and displacement data output by the analog-to-digital conversion and digital processing unit, and to complete display, recording, and subsequent seismic data acquisition and processing. The two-dimensional micro-rotation actuator is positioned on the common optical path of the measurement beam output path and the echo receiving path, and is used to synchronously adjust the transmission and reception directions. The two-dimensional micro-rotation actuator includes a two-dimensional galvanometer or a MEMS dual-axis reflector, as well as a matching drive module and position feedback module. The two-dimensional micro-rotation actuator receives the measurement beam from the LDV optical unit and guides the measurement beam to the variable field of view optical structure at a set angle. The two-dimensional micro-rotation actuator also receives the echo beam after it has been received by the variable field of view optical structure, and causes it to return to the transceiver separation component in the LDV optical unit along a predetermined common return path. The variable field-of-view optical structure is positioned between the two-dimensional micro-rotation actuator and the target object to complete the transmitting antenna, receiving antenna, and field-of-view adjustment. The variable field-of-view optical structure consists of a transmitting antenna, a receiving antenna, an aperture mechanism, and a coupling lens or a relay lens.

2. A non-contact laser Doppler seismic robust optical machine system according to claim 1, characterized in that: The LDV optical unit includes a fiber laser, a polarization-maintaining fiber end, a collimation component, a pre-stage beam splitter component, a reference light frequency shifter component, a transmit / receive separation component, and a reference light extraction component. The fiber laser is a narrow-linewidth continuous laser, and its output end is connected to the free-space optical path through the polarization-maintaining fiber end. The polarization-maintaining fiber end is used to ensure the stability of the polarization state of the output light. The collimation component is located after the polarization-maintaining fiber end and is used to shape the diverging laser output from the polarization-maintaining fiber into a collimated laser beam. The pre-stage beam splitter component uses a PBS beam splitter to split the collimated laser into two paths: a measurement beam and a reference beam. The reference light frequency shifter component includes an acousto-optic frequency shifter and a mirror or folding mirror in the reference optical path. The transmit / receive separation component includes a quarter-wave plate and a PBS, located at the common return node of the measurement beam and the echo beam, to enable the emitted measurement beam and the echo measurement beam to form a separable polarization relationship, and to extract the echo beam from the common measurement optical path after it returns to the LDV optical unit.

3. A non-contact laser Doppler seismic robust optical machine system according to claim 1, characterized in that: The photoelectric detection and analog front end is used to receive two optical signals and complete interferometric detection and analog signal conditioning. The photoelectric detection and analog front end includes a beam combiner, a balanced photodetector, a transimpedance amplifier circuit, an analog bandpass filter circuit, and an analog gain adjustment circuit. The beam combiner is used to spatially combine the reference optical signal from the LDV optical unit and the echo measurement optical signal output from the LDV optical unit's transceiver separation component into a coaxial interference beam. The balanced photodetector is located after the beam combiner and is used to convert the interference optical signal into a differential analog electrical signal, while suppressing common-mode intensity noise and improving the effective carrier-to-noise ratio of the beat frequency signal.

4. A non-contact laser Doppler seismic robust optical machine system according to claim 1, characterized in that: The analog-to-digital conversion and digital processing unit is used to perform digital processing and control logic operations from analog signals to digital velocity / displacement data; the analog-to-digital conversion and digital processing unit includes an ADC, a digital demodulation terminal, an echo quality evaluation function module, and a state machine and control logic module; The data output acquisition system is used to receive velocity / displacement data output by the analog-to-digital conversion and digital processing unit, and to complete data storage, display, transmission, or data interface with the seismic acquisition host; the data output acquisition system can be any one of an independent data logger, a host computer acquisition terminal, or a field seismic acquisition station.

5. The non-contact laser Doppler seismometer robust optical system seismometer method of claim 1, wherein, Includes the following steps: S1. Construction of the heterodyne interference optical path; S2. Construct a common-path optical antenna for transmitting and receiving and a variable field-of-view coupling structure; S3. Configure photoelectric detection and sampling links; S4. Demodulation and echo quality assessment and control are performed in the digital domain; S5. Bright spot locking and offline re-acquisition control based on echo quality function; S6, output velocity / displacement data and perform control compensation.

6. The non-contact laser Doppler seismometer robust optical system seismometer method of claim 5, wherein: Step S1: A narrow linewidth continuous laser is selected as the light source, and linearly polarized laser is output through a polarization-maintaining fiber. After collimation, the laser enters the beam splitting structure and is divided into a measurement arm and a reference arm. The reference arm is introduced into a fixed beat frequency carrier through an acousto-optic frequency shifter, and then sent to the beam combining position after being folded by a reflector. The measurement arm continues to propagate forward as a beam illuminating the ground, forming a heterodyne interference basic optical path. Step S2: The laser beam output from the measuring arm is introduced into the transceiver common optical antenna. The transmitting end adopts a collimated emission structure or an equivalent transmitting antenna so that the laser beam can illuminate the target area on the ground within a predetermined ranging range. The receiving end adopts a reflective telescope structure. A variable field of view mechanism is set in the receiving coupling section.

7. The seismic method of claim 5, wherein the non-contact laser Doppler seismic robust optical machine system is characterized by: In step S3, the measurement echo reflected back from the rough ground is guided to the beam combiner through the receiving antenna, polarization separation structure, and common return path, and spatially combined with the reference light formed in step S1, thereby forming a heterodyne beat frequency signal at the detector; the combined interference light is sent to a balanced photodetector to convert the optical beat frequency signal into a differential analog electrical signal; the differential analog electrical signal is then subjected to bandpass filtering, gain adjustment, and noise suppression by the analog front end, and then sent to the ADC for high-speed sampling; This allows the optical phase change caused by vibrations of a rough surface to be converted into a differential beat frequency electrical signal that can be processed in the digital domain, providing input for subsequent demodulation, phase unwrapping, and velocity / displacement recovery.

8. The seismic method of claim 5, wherein: Step S4: Input the digital beat frequency signal output by the ADC into the digital demodulation terminal for quadrature demodulation to obtain the in-phase component and the quadrature component. Further calculate the instantaneous amplitude and instantaneous phase, and expand the phase to obtain the continuous phase. Based on continuous phase calculations, at least one of the target's velocity and displacement data along the beam direction is calculated. Simultaneously, short-time statistics of amplitude and phase are updated within a preset time window to construct the echo quality function. .

9. The non-contact laser Doppler seismometer robust optical system seismometer method of claim 5, wherein: Step S5: Enter the capture state and set the variable field-of-view optical structure to a large field-of-view working state. The two-dimensional micro-rotation actuator performs a limited range of angle scanning within the target area. During the scanning process, the analog-to-digital conversion and digital processing unit continuously calculates... Value, and determine the current Has the preset locking threshold been reached? This achieves the switching to a locked state, switching the variable field-of-view optical structure to a small field-of-view operating state. Simultaneously, it controls the two-dimensional micro-rotation actuator to perform micro-disturbance search and hill-climbing updates near the current bright spot, causing the actuator to continuously move towards... The direction of increase is fine-tuned; if in the locked state, The connection drops below the preset disconnection threshold. And lasting longer than the preset time If the current bright spot lock fails or there is a significant risk of disconnection, the system automatically exits the lock state and re-enters the scanning and re-acquisition process, searching for a new high-quality echo region within a limited angle range according to a preset trajectory; once a suitable echo region is found... Exceed The position is locked again, and the field of view is contracted again; Step S6: In the locked state, the digital demodulation terminal continuously outputs at least one of the velocity data and displacement data of the ground surface along the beam direction and transmits it to the data output acquisition system. For the minor control actions introduced by the actuator during bright spot locking or recapture, the actuator control signal is recorded synchronously and converted into a geometric compensation term when needed, and subtracted from the demodulation phase.

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