Seabed micro-vibration measurement system and method based on optical interferometry

Abstract

The present invention discloses a system and method for measuring seabed microvibrations based on optical interference. The system includes a laser module, an interferometer module, a frequency doubling module, an optical path integration module, a photoelectric detection module, and a data processing module. The method comprises the following steps: S1: placing the interferometer in a seabed environment to begin measurement; S2: emitting a 1064nm infrared laser, which is divided into a reference light and a measurement light of equal optical path length, and irradiating the measurement light onto the interferometer to form a laser emission path; S3: arranging a first frequency doubling crystal on the laser return path to frequency-double the 1064nm infrared laser into a first green laser of 532nm; S4: arranging a second frequency doubling crystal on the laser return path to convert the unconverted 1064nm infrared laser in S3 into a second green laser of 532nm; S5: integrating the first and second green lasers and interfering with the reference light on the photoelectric detector to obtain the displacement change of seabed vibration. The present invention can detect seabed microvibrations below the micrometer level with high measurement accuracy.

Description

Technical Field

[0001] The present invention relates to the field of geophysical technology, in particular to the field of seismic exploration technology, and specifically to a seabed micro-vibration measurement system and method based on optical interference. Background Art

[0002] Seafloor microvibrations are tiny mechanical vibrations that occur on the seafloor. They are caused by a variety of factors, including waves, wind, crustal movement, groundwater flow, and other natural and human activities. These vibrations propagate across the seafloor and can cause tiny vibrations in the Earth.

[0003] Seafloor microvibrations are typically too small to be directly perceived, but they can be measured using high-precision instruments. They can be caused by a variety of natural phenomena, including seafloor geological activity, ocean currents, wave action, and marine biological activity. The study of seafloor microvibrations has important scientific and practical implications for understanding marine geophysical processes, monitoring seafloor resource extraction, and providing early warning of tsunamis and earthquakes.

[0004] The basic principle of optical interferometry is that when two or more coherent light waves meet in space, they interfere, forming alternating light and dark interference fringes. This phenomenon is very sensitive to tiny changes in the light waves and can therefore be used to measure tiny vibrations. The specific operation of measuring seafloor microvibrations based on optical interferometry is to place a portion of the measurement light path on the seafloor to capture microvibrations. When the seafloor microvibrations occur, the measurement light path changes, thereby changing the interference fringes.

[0005] However, in actual applications, there are problems with the measurement accuracy of optical interferometers due to factors such as the complexity of the seabed ring, uneven water quality, and water flow. Specifically, changes in dissolved matter, suspended matter, temperature, salinity and other factors in the seabed environment affect the propagation path and attenuation of the laser beam; at the same time, water flow can cause the laser beam to deviate, diffuse or distort, thereby affecting the measurement accuracy. Summary of the Invention

[0006] The present invention overcomes the deficiencies of the prior art and provides a seabed micro-vibration measurement system and method based on optical interference.

[0007] To achieve the above object, the technical solution adopted by the present invention is: a method for measuring seabed microvibration based on optical interferometry, comprising the following steps:

[0008] S1. Establish the interferometer, perform initial calibration on the interferometer, and then place the interferometer in the seabed environment to start measurement;

[0009] S2. Use a laser to emit an infrared laser with a wavelength of 1064 nm, and divide it into reference light and measurement light. Ensure that the optical path of the reference light and the measurement light are equal, and irradiate the measurement light to the interferometer to form a laser emission path;

[0010] S3, setting a first frequency doubling crystal on the laser return path to frequency-double the 1064nm infrared laser into a first green laser of 532nm through nonlinear optical effect;

[0011] S4, set a second frequency doubling crystal on the laser return path, guide the unconverted 1064nm infrared laser in S3 to the second frequency doubling crystal, and convert it into a second 532nm green laser;

[0012] S5. Integrate the first green laser and the second green laser in S3 and S4 to form a continuous green light output, and generate interference with the reference light on the photodetector, record the changes in the interference fringes in real time, and calculate the displacement changes of the seabed vibration.

[0013] In a preferred embodiment of the present invention, in S2, the specific steps of ensuring that the optical path lengths of the reference light and the measurement light are equal are:

[0014] S21, using the Michelson interferometer structure, the laser beam is divided into two optical paths: reference light and measurement light. The reference light is reflected by a fixed mirror and then enters the beam splitter. The measurement light passes through the beam splitter and irradiates the seabed, and then is reflected back from the seabed surface.

[0015] S22. Setting an optical path matcher in the reference light path, adjusting the reference light path by rotating the fine-tuning matcher, superimposing the reference light and the measurement light on the photodetector, and observing interference fringes in real time;

[0016] S23. By fine-tuning the matcher to keep the interference fringes stationary, the optical paths of the reference light and the measurement light are matched.

[0017] In a preferred embodiment of the present invention, in S3, the specific steps of forming the first green laser are:

[0018] S31, selecting a nonlinear optical crystal as a first frequency-doubling crystal, using a laser to emit an infrared laser with a wavelength of 1064 nm, focusing the 1064 nm infrared laser onto the first frequency-doubling crystal, wherein the light transmission range of the first frequency-doubling crystal is 355 nm-4500 nm;

[0019] S32. In the first frequency doubling crystal, the frequency of the 1064 nm infrared laser is doubled by the nonlinear optical effect to obtain a first green laser of 532 nm. The first green laser and the unconverted 1064 nm infrared laser are output through the first frequency doubling crystal together.

[0020] In a preferred embodiment of the present invention, in S4, the light transmission range of the second frequency doubling crystal is 1054 nm-1074 nm.

[0021] In a preferred embodiment of the present invention, in S4, the specific steps of obtaining the unconverted 1064 nm infrared laser are:

[0022] S41. At the output end of the first frequency-doubling crystal, use a beam splitter to separate the frequency-doubled 532nm green laser and the unconverted 1064nm infrared laser, direct the frequency-doubled 532nm green laser to a beam merging unit, and direct the unconverted 1064nm infrared laser to a second frequency-doubling crystal;

[0023] S42. Before being directed to the second frequency-doubling crystal, the unconverted 1064 nm infrared laser is pre-processed, the beam is shaped using a beam shaper, and then an optical filter is used to filter out unwanted wavelengths or stray light, and finally a focusing lens is used to focus the beam onto the second frequency-doubling crystal.

[0024] In a preferred embodiment of the present invention, in S5, the specific steps of forming a continuous green light output are:

[0025] S51, accurately locating the output position of the green light generated by the first frequency doubling unit and the second frequency doubling unit; using a beam combiner including a prism or a grating to combine the two green light beams into a single beam;

[0026] S52, adjusting the position and angle of the combiner to ensure that the two green light beams are aligned in space and time to maximize the efficiency of beam combining and reduce mode distortion;

[0027] S53. If the green light powers generated by the two frequency doubling units are different, an attenuator or a gain medium is used to match the powers of the two beams to prevent the brighter beam from overwhelming the darker beam during the merging process.

[0028] In a preferred embodiment of the present invention, in S5, the specific steps of calculating the displacement change of the seabed vibration are:

[0029] S54, interfering the continuous green light output with the reference beam on the photodetector. When the two beams of light meet on the photodetector, an interference pattern is formed on the detector due to the phase difference, which appears as alternating light and dark stripes.

[0030] S55, based on the displacement change of the collected interference fringes, calculate the fringes movement number: Where Δm is the number of interference fringes moved, λ is the wavelength of light, and the optical path difference (OPD) is calculated. According to OPD = 2·L·cos(θ), θ is the incident angle of the light beam. The optical path change (L) caused by seabed vibration is collected by the angle sensor and calculated.

[0031] The present invention also provides a seabed micro-vibration measurement system based on optical interference, comprising: a laser module, an interferometer module, a frequency doubling module, an optical path integration module, a photoelectric detection module, and a data processing module, characterized in that:

[0032] The laser module includes: a laser, an optical isolator and a beam splitter;

[0033] The interferometer module includes: an interferometer, a calibration unit and a seabed packaging unit;

[0034] The frequency multiplication module includes: a first frequency multiplication unit and a second frequency multiplication unit;

[0035] The optical path integration module includes: a beam merging unit and a beam adjusting unit;

[0036] The photoelectric detection module includes: a phase matching unit and a temperature adjustment unit;

[0037] The data processing module includes: a data acquisition card and a data transmission unit.

[0038] In a preferred embodiment of the present invention, the first frequency doubling unit includes a first frequency doubling crystal and a first focusing lens, and is used to frequency-double a portion of the 1064 nm infrared laser into a first green laser of 532 nm;

[0039] The second frequency doubling unit includes a second frequency doubling crystal and a second focusing lens, and is used to frequency-double the remaining 1064nm infrared laser into a second green laser of 532nm.

[0040] In a preferred embodiment of the present invention, the beam merging unit is used to merge the green light generated by the first frequency doubling unit and the second frequency doubling unit to form a single beam, that is, continuous green light output;

[0041] The beam adjustment unit is used to adjust and optimize the direction, intensity and pattern of the light beam to ensure that the light beam correctly enters the interferometer and photodetector and generates clear interference fringes.

[0042] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0043] (1) The present invention provides a seabed micro-vibration measurement system and method based on optical interference. The method utilizes laser interferometry technology. By utilizing infrared laser light emitted by a laser and converting it into green laser light through a frequency-doubling crystal, the method can perform extremely precise measurements of seabed micro-vibrations. By observing the changes in interference fringes, the displacement changes of the seabed can be inverted. The measurement accuracy is high and the detection of seabed micro-vibrations below the micron level can be achieved. Compared with traditional seabed seismic measurement methods, this method has a smaller measurement range but higher accuracy.

[0044] (2) The present invention uses a laser as a light source, which has high beam intensity and good beam concentration, enabling long-distance optical transmission. Compared with conventional light sources, the laser beam can transmit farther in seawater, which is beneficial for setting measurement points in deep sea areas. At the same time, the monochromatic nature of the laser light source can effectively filter out stray light and background light in the seawater, improving the signal-to-noise ratio and achieving higher-precision measurements.

[0045] (3) The present invention adopts optical interference technology for measurement. Compared with the prior art that uses seabed seismographs for measurement, the optical interference measurement method does not require the deployment of hardware such as sensor cables on the seabed. It uses optical non-contact methods for measurement, thereby avoiding the influence of seabed environment such as seawater pressure and ocean currents on the sensor, and improving the accuracy of the measurement results. The deployment is simpler and more convenient, and the maintenance cost is low. At the same time, it also avoids the influence of possible mechanical failures of the sensor on the measurement, creating conditions for long-term and real-time seabed micro-motion monitoring.

[0046] (4) The present invention uses frequency doubling technology to convert infrared laser frequency into visible light, effectively increasing beam intensity while overcoming the strong absorption of infrared light by seawater. Conversion of the measurement light frequency into visible light effectively increases the optical transmission distance in seawater, expands the measurement depth, and avoids the rapid attenuation of infrared light in seawater, facilitating long-distance optical transmission on the seabed.

[0047] (5) The present invention adopts a two-stage frequency doubling structure to fully utilize the power of the laser source and realize continuous light output, which solves the technical difficulty of short deep-sea optical transmission distance and difficulty in achieving long-distance high-precision measurement; the application of frequency doubling technology has promoted all-optical seabed micro-motion monitoring from theoretical feasibility to practical application level.

[0048] (6) The present invention utilizes a beam splitter to split the laser beam into two independent optical paths: a reference beam and a measurement beam. By setting up an optical path matcher to fine-tune the reference beam path, the reference and measurement beams are superimposed on the photodetector, thereby obtaining high-quality interference fringes that facilitate subsequent data processing and precise measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments described in the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive efforts.

[0050] Figure 1 is a system structure diagram of a preferred embodiment of the present invention;

[0051] Figure 2 is a flow chart of a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0052] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0053] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0054] The present invention provides a seabed micro-vibration measurement system based on optical interference, comprising: a laser module, an interferometer module, a frequency doubling module, an optical path integration module, a photoelectric detection module, and a data processing module.

[0055] The laser module includes:

[0056] Laser, used to generate infrared laser with a wavelength of 1064nm as the light source of the measurement system;

[0057] Optical isolator, used to isolate reflected light, prevent the laser from being affected by reverse light, and ensure the stable operation of the laser;

[0058] The beam splitter is used to split the laser light emitted by the laser into two beams, forming reference light and measurement light for interferometric measurement; the reference light and measurement light are both infrared lasers with a wavelength of 1064nm.

[0059] The specific structure of the laser is:

[0060] (1) Infrared semiconductor laser: As a pump source, it emits infrared light with a wavelength of 1064 nm and uses neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal as the laser material;

[0061] (2) Beam shaping system: Use a collimating lens such as a cylindrical lens to shape the pump beam into a Gaussian distribution to improve beam quality and reduce the heat-affected zone during focusing;

[0062] (3) Nonlinear crystals: KTP (potassium titanyl phosphate) or LBO (lithium triborate) are used as nonlinear optical crystals. These crystals have high nonlinear optical coefficients and can efficiently convert 1064nm infrared light into 532nm green light through second harmonic generation (SHG). At the same time, the crystals require precise temperature control to maintain phase matching conditions, which is usually achieved by placing a temperature control device such as a Peltier cooler or thermostat around the crystal.

[0063] (4) Resonant cavity: It includes two reflectors, one as the input mirror (partially transmittance 1064nm, high reflection 532nm), and the other as the output mirror (high transmittance 532nm); the function of the resonant cavity is to enhance the power density of infrared light, thereby improving the conversion efficiency of SHG.

[0064] (5) Filter: A filter is installed at the output end of the resonant cavity to filter out the unconverted 1064nm infrared light and only allow 532nm green light to pass;

[0065] (6) Output lens: An output lens is installed after the filter to adjust the divergence angle of the green light to ensure that the light beam meets the requirements of specific applications.

[0066] (7) Control and drive circuit: Use a microcontroller or computer control system to monitor and adjust the operating parameters of the laser, including pump power, crystal temperature, and resonant cavity stability.

[0067] Using a laser as a light source, with its high beam intensity and excellent beam concentration, enables long-distance optical transmission. Compared with conventional light sources, laser beams can travel further in seawater, facilitating the placement of measurement points in deep sea areas. Furthermore, the monochromatic nature of the laser light source effectively filters out stray and background light in the seawater, improving the signal-to-noise ratio and enabling higher-precision measurements.

[0068] The interferometer module includes:

[0069] An interferometer is used to receive the measuring light and form the optical path of the measuring light through its internal reflector, thereby generating and detecting the interference pattern of the laser and measuring the optical path difference;

[0070] Calibration unit, used to perform initial calibration of the interferometer to ensure measurement accuracy;

[0071] The seabed packaging unit is used to protect the interferometer from the influence of the seabed environment such as pressure, salinity, temperature changes, etc., to ensure its stable operation on the seabed.

[0072] The specific steps for initial calibration of the interferometer are:

[0073] (1) Ensure that the laboratory environment is stable, avoid vibration and temperature fluctuations, and use a reference light source to align the optical path of the interferometer to ensure that the reference light and the measurement light can produce a clear interference pattern on the photodetector. The reference light source is a calibration source with known properties, that is, a light source with stable frequency and phase, which is placed at the input end of the interferometer.

[0074] (2) By adjusting the reflectors or interferometer arms in the interferometer, the light emitted from the calibration source produces the expected interference pattern on the detector after passing through the interferometer;

[0075] (3) After obtaining a satisfactory interference pattern, it is necessary to record the interferometer parameters at this time, including the position and angle of the reflector; these parameters will serve as the baseline stability test for subsequent measurements: After alignment, the interferometer is subjected to a baseline stability test for a period of time to ensure that it can maintain a stable interference pattern in the absence of external interference.

[0076] Optical interferometry technology is used for measurement. Compared with the existing technology of using seabed seismographs for measurement, the optical interferometry measurement method does not require the deployment of sensor cables and other hardware on the seabed. It uses completely optical non-contact methods for measurement, thus avoiding the influence of seabed environment such as seawater pressure and ocean currents on the sensor, and improving the accuracy of the measurement results. The deployment is simpler and more convenient, with low maintenance costs. At the same time, it also avoids the impact of possible mechanical failures of the sensor on the measurement, creating conditions for long-term and real-time seabed micro-motion monitoring.

[0077] The subsea packaging unit is specifically:

[0078] (1) Pressure hull: A sturdy shell designed to withstand the high pressure environment of the seabed, made of stainless steel or aluminum alloy;

[0079] (2) Sealing system: Use sealing gaskets or O-rings to ensure that water and salt do not penetrate into the interferometer;

[0080] (3) Temperature control system: including heater or cooling system to maintain a stable temperature inside the interferometer;

[0081] (4) Corrosion-resistant materials: The packaging unit is constructed using corrosion-resistant materials to resist chemical erosion by seawater; the corrosion-resistant materials are seawater corrosion-resistant steel, duplex stainless steel, or titanium alloy;

[0082] (5) Cable interface: A sealed cable interface used to connect the interferometer to the data acquisition system and power supply.

[0083] The frequency multiplication module includes:

[0084] A first frequency doubling unit, comprising a first frequency doubling crystal and a first focusing lens, for frequency doubling a portion of the 1064 nm infrared laser into a first green laser of 532 nm;

[0085] The second frequency doubling unit includes a second frequency doubling crystal and a second focusing lens, and is used to frequency-double the remaining 1064nm infrared laser into a second green laser of 532nm.

[0086] The first frequency-doubling crystal and the second frequency-doubling crystal are media that use nonlinear optical effects to convert 1064nm infrared laser light into 532nm green laser light. The first frequency-doubling crystal uses β-barium borate (BBO), potassium dihydrogen phosphate (KDP), potassium dideuterium phosphate (DKDP), or LiNbO3 as a nonlinear optical crystal, and uses its secondary nonlinear optical effect to achieve frequency doubling of 1064nm infrared light into 532nm green light. The first focusing lens is arranged behind the first frequency-doubling crystal. The lens is made of quartz or sapphire material and is used to focus the first green light after frequency doubling by the crystal, and condense the light beam into a parallel beam for output.

[0087] Among them, the second frequency-doubling crystal also uses the same material as the first frequency-doubling crystal to achieve frequency doubling of the remaining 1064nm infrared light into a 532nm second green light; the second focusing lens is set behind the second frequency-doubling crystal to focus the generated second green light and output it as a parallel beam with the first green light.

[0088] The advantage of using 1064nm lasers for submarine microvibration measurements is that this wavelength has strong penetration in water and relatively low scattering losses. This means that 1064nm lasers can more effectively penetrate seawater, reducing energy loss during transmission and making longer-distance measurements possible.

[0089] However, when the laser is reflected back for interferometric measurement, the measurement accuracy is higher when using a laser with a wavelength of 532nm because the interference fringes of the short-wavelength laser are denser.

[0090] Frequency doubling technology, which converts infrared laser frequency into visible light, effectively increases beam intensity while overcoming the strong absorption of infrared light by seawater. Conversion of the measurement light frequency into visible light also significantly increases the optical transmission distance in seawater, extending the measurement depth and avoiding the rapid attenuation of infrared light in seawater, facilitating long-distance optical transmission on the seabed.

[0091] At the same time, a two-stage frequency doubling structure is adopted to fully utilize the power of the laser source and realize continuous light output, which solves the technical difficulty of short deep-sea optical transmission distance and difficulty in achieving long-distance high-precision measurement; the application of frequency doubling technology has enabled all-optical seabed micro-motion monitoring to be promoted from theoretical feasibility to practical application level.

[0092] The optical path integration module includes:

[0093] The beam merging unit is used to merge the green light generated by the first frequency doubling unit and the second frequency doubling unit to form a single beam, that is, continuous green light output;

[0094] The beam adjustment unit is used to adjust and optimize the direction, intensity and pattern of the light beam to ensure that the light beam enters the interferometer and photodetector correctly and produces clear interference fringes.

[0095] The specific steps for adjusting the direction, intensity and pattern of the beam are:

[0096] (1) Direction adjustment: Use the rotating optical axis adjuster to fine-tune the beam transmission direction so that it is precisely aligned with the entrance of the interferometer and photodetector to ensure the correct incidence of the beam;

[0097] (2) Intensity adjustment: The intensity ratio of the two beams is adjusted through the adjustable optical density filter to adapt to the dynamic range of the photodetector and optimize the signal quality;

[0098] (3) Mode and diameter adjustment: Use lenses or beam shapers to adjust the spot size, shape, and diameter of the beam to ensure that the beam fills the active area of ​​the interferometer and photodetector while avoiding beam shearing effects caused by overfilling.

[0099] For example: confirm the path that the green light needs to take after being output, including the position of the interferometer and photodetector; use a reflector or beam steering system to adjust the direction of the green light so that it accurately hits the entrance slit of the interferometer; if the intensity of the combined beam is too high or too low, use a variable attenuator or aperture to adjust the beam intensity to match the optimal operating range of the photodetector;

[0100] Next, a lens system is used to focus the green light, ensuring that the beam forms a sharp focus on the active areas of the interferometer and photodetector. If the beam pattern does not meet the requirements, a beam shaper or spatial light modulator is used to adjust the beam pattern to achieve a better interference effect. A vibration isolation table and fine-tuning device are used to reduce the impact of environmental vibration on beam quality.

[0101] Use a beam profiler or real-time monitoring system such as a CCD camera to observe the quality and interference pattern of the adjusted beam. After confirming that the direction, intensity, pattern, and focus of the beam are optimal, use locking screws to fix all optical components in place to prevent subsequent operation.

[0102] The photoelectric detection module includes:

[0103] Phase matching unit, used to ensure that the reference light and the measurement light have the same phase when interfering on the photodetector, thereby improving the contrast of the interference fringes and obtaining the best interference effect;

[0104] The temperature adjustment unit is used to compensate for the impact of temperature changes, maintain its temperature stability, and ensure measurement accuracy.

[0105] To ensure that the reference light and the measurement light have the same phase when they interfere with the photodetector, the specific steps are as follows:

[0106] (1) Use a calibration light source to calibrate the initial phase of the reference light and the measurement light to ensure that the phase difference between the two on the photodetector is zero or close to zero; use phase locking technology to ensure that the phase relationship between the reference light and the measurement light remains constant through a phase-locked loop (PLL);

[0107] (2) Real-time monitoring and adjustment: By monitoring the contrast or intensity changes of the interference fringes in real time, it is determined whether the phase difference between the reference light and the measurement light has changed. When there is a phase deviation, the optical path in the interferometer is adjusted using a piezoelectric actuator (PZT) to compensate for the phase difference.

[0108] The specific steps to compensate for the effects of temperature changes are:

[0109] (1) A temperature sensor is set inside the photoelectric detection module to monitor the temperature changes of the photoelectric detection module and its surrounding environment in real time;

[0110] (2) Based on the monitored temperature changes, the temperature compensation circuit or heating / cooling element is adjusted to compensate for the effect of temperature changes on the performance of the photodetector, so that the detector temperature is maintained within the range of ±0.1°C. For example, when the temperature rises, the power of the cooling element is increased to lower the temperature; when the temperature drops, the power of the cooling element is reduced or the power of the heating element is increased to raise the temperature;

[0111] (3) After temperature adjustment, the photoelectric detection module is calibrated and verified using a calibration light source to ensure that its performance meets the measurement requirements.

[0112] The data processing module includes:

[0113] The data acquisition card is used to receive the electrical signal output by the photodetector and convert it into a digital signal for subsequent processing.

[0114] Data transmission unit, used to transfer the collected data from the data acquisition card to a computer or dedicated processor for further processing and analysis.

[0115] The present invention also provides a method for measuring seabed microvibration based on optical interference, comprising the following steps:

[0116] S1. Establish the interferometer, perform initial calibration on the interferometer, and then place the interferometer in the seabed environment to start measurement;

[0117] S2. Use a laser to emit an infrared laser with a wavelength of 1064 nm, and divide it into reference light and measurement light. Ensure that the optical path of the reference light and the measurement light are equal, and irradiate the measurement light to the interferometer to form a laser emission path;

[0118] S3, setting a first frequency doubling crystal on the laser return path to frequency-double the 1064nm infrared laser into a first green laser of 532nm through nonlinear optical effect;

[0119] S4, set a second frequency doubling crystal on the laser return path, guide the unconverted 1064nm infrared laser in S3 to the second frequency doubling crystal, and convert it into a second 532nm green laser;

[0120] S5. Integrate the first green laser and the second green laser in S3 and S4 to form a continuous green light output, and generate interference with the reference light on the photodetector, record the changes in the interference fringes in real time, and calculate the displacement changes of the seabed vibration.

[0121] This method uses laser interferometry technology. By using the infrared laser emitted by the laser and converting it into green laser through a frequency-doubling crystal, it can perform extremely precise measurements of seabed micro-vibrations. By observing the changes in the interference fringes, the displacement changes of the seabed can be inverted. The measurement accuracy is high and it can achieve seabed micro-vibration detection below the micron level. Compared with traditional seabed seismic measurement methods, this method has a smaller measurement range but higher accuracy.

[0122] In the S1, a beam splitter is used to split the laser beam into two independent optical paths: reference light and measurement light. By setting up an optical path matcher to fine-tune the reference light path, the reference and measurement light beams are superimposed on the photodetector, resulting in high-quality interference fringes that facilitate subsequent data processing and precise measurement.

[0123] In S2, the specific steps to ensure that the optical path lengths of the reference light and the measurement light are equal are:

[0124] S21, using the Michelson interferometer structure, the laser beam is divided into two optical paths: reference light and measurement light. The reference light is reflected by a fixed mirror and then enters the beam splitter. The measurement light passes through the beam splitter and irradiates the seabed, and then is reflected back from the seabed surface.

[0125] S22. Setting an optical path matcher in the reference light path, adjusting the reference light path by rotating the fine-tuning matcher, superimposing the reference light and the measurement light on the photodetector, and observing interference fringes in real time;

[0126] S23. By fine-tuning the matcher to keep the interference fringes stationary, the optical paths of the reference light and the measurement light are matched.

[0127] In S3, the specific steps of forming the first green laser are:

[0128] S31, selecting a nonlinear optical crystal as a first frequency-doubling crystal, using a laser to emit an infrared laser with a wavelength of 1064 nm, focusing the 1064 nm infrared laser onto the first frequency-doubling crystal, wherein the light transmission range of the first frequency-doubling crystal is 355 nm-4500 nm;

[0129] S32. In the first frequency doubling crystal, the frequency of the 1064 nm infrared laser is doubled by the nonlinear optical effect to obtain a first green laser of 532 nm. The first green laser and the unconverted 1064 nm infrared laser are output through the first frequency doubling crystal together.

[0130] In S4, the transmittance range of the second frequency doubling crystal is 1054nm-1074nm.

[0131] In S4, the specific steps for obtaining the unconverted 1064 nm infrared laser are as follows:

[0132] S41. At the output end of the first frequency-doubling crystal, use a beam splitter to separate the frequency-doubled 532nm green laser and the unconverted 1064nm infrared laser, direct the frequency-doubled 532nm green laser to a beam merging unit, and direct the unconverted 1064nm infrared laser to a second frequency-doubling crystal;

[0133] S42. Before being directed to the second frequency-doubling crystal, the unconverted 1064 nm infrared laser is pre-processed, the beam is shaped using a beam shaper, and then an optical filter is used to filter out unwanted wavelengths or stray light, and finally a focusing lens is used to focus the beam onto the second frequency-doubling crystal.

[0134] In S5, the specific steps of forming a continuous green light output are:

[0135] S51, accurately locating the output position of the green light generated by the first frequency doubling unit and the second frequency doubling unit; using a beam combiner including a prism or a grating to combine the two green light beams into a single beam;

[0136] S52, adjusting the position and angle of the combiner to ensure that the two green light beams are aligned in space and time to maximize the efficiency of beam combining and reduce mode distortion;

[0137] S53. If the green light powers generated by the two frequency doubling units are different, an attenuator or a gain medium is used to match the powers of the two beams to prevent the brighter beam from overwhelming the darker beam during the merging process.

[0138] For example, a cube prism or waveguide combiner can be selected as a beam combiner and placed in the path of the two green light beams. Fine-tuning devices, such as precision adjustment mounts, can be used to fine-tune the two beams so that they are precisely aligned on the combiner. Specifically, adjustments must be made on the X, Y, and Z axes, including translation in the X and Y directions and rotation around the Z axis. The angle of the combiner can be adjusted so that the two green light beams interfere inside or on the surface of the combiner, producing a bright combined beam. Once the desired combining effect is achieved, the position of the combiner and the beams can be fixed using a magnetic base or mechanical fasteners.

[0139] In S5, the specific calculation steps of the displacement change of seabed vibration are:

[0140] S54, interfering the continuous green light output with the reference beam on the photodetector. When the two beams of light meet on the photodetector, an interference pattern is formed on the detector due to the phase difference, which appears as alternating light and dark stripes.

[0141] S55, based on the displacement change of the collected interference fringes, calculate the fringes movement number: Where Δm is the number of interference fringes moved, λ is the wavelength of light, and the optical path difference (OPD) is calculated. According to OPD = 2·L·cos(θ), θ is the incident angle of the light beam. The optical path change (L) caused by seabed vibration is collected by the angle sensor and calculated.

[0142] For example, in this application, the wavelength λ = 532 nm, and the interference fringes move Δm = 10 fringes during the measurement process, then OPD = 10 × 532 × 10 -9 m, L = OPD / 2 = 5 × 532 × 10 -9 m.

[0143] In another embodiment, after the interference pattern is formed, a high-speed camera system or a photoelectric detector is used to monitor the changes in the interference pattern in real time, collect real-time data of the interference pattern, and filter and Fourier transform the collected interference fringe change data to extract the displacement change information of the interference fringes; the displacement change of the seabed vibration is calculated based on the displacement change information of the interference fringes.

[0144] OPD(ω)=∫ -∞ ∞ OPD(t)·e -jωt dt, where ω is the angular frequency and j is the imaginary unit, satisfying j 2 = -1, e is the base of the natural logarithm, which is 2.71828, t is the time variable in seconds (s), dt is the small increment of time, used for integral expression; OPD(ω) is the Fourier transform of OPD(t), Where θ is the divergence angle of the light beam, and the displacement L(t) of the seabed vibration over time is calculated.

[0145] The above description is based on the ideal embodiment of the present invention. Based on the above description, relevant personnel can make various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the content of the specification and must be determined according to the scope of the claims.

Claims

1. A method for measuring seabed microvibration based on optical interferometry, characterized in that: The following steps are involved: S1. After the interferometer is initially calibrated, it is placed in the seabed environment and measurement begins. S2. The laser emits an infrared laser with a wavelength of 1064 nm, which is divided into a reference beam and a measurement beam. The optical paths of the reference beam and the measurement beam are ensured to be equal, and the measurement beam is passed through an interferometer to form a laser emission path. S3, passing the measuring beam through a first frequency doubling crystal on its laser return path, and frequency doubling the 1064 nm infrared laser to a first green laser of 532 nm through a nonlinear optical effect; S4, passing the unconverted 1064nm infrared laser in S3 through a second frequency doubling crystal on the laser return path of the measurement beam, thereby converting the unconverted 1064nm infrared laser in S3 into a second green laser of 532nm; S5, integrating the first green laser and the second green laser in S3 and S4 to form a continuous green light output, and generating interference with the reference light on the photodetector, recording the change of the interference fringes in real time, and calculating the displacement change of the seabed vibration; In S5, the specific steps for calculating the displacement change of the seabed vibration are: S54, interfering the continuous green light output with the reference beam on the photodetector. When the two beams of light meet on the photodetector, an interference pattern is formed on the detector due to the phase difference, which appears as alternating light and dark stripes. S55, based on the displacement change of the collected interference fringes, calculate the fringes movement number: Where Δm is the number of interference fringes moved, λ is the wavelength of light, and the optical path difference (OPD) is calculated. According to OPD = 2·L·cos(θ), θ is the incident angle of the light beam. The optical path change (L) caused by seabed vibration is collected by the angle sensor and calculated.

2. The method for measuring seabed microvibration based on optical interferometry according to claim 1, characterized in that: In S2, the specific steps for ensuring that the optical path lengths of the reference light and the measurement light are equal are: S21, using the Michelson interferometer structure, the laser beam is divided into two optical paths: reference light and measurement light. The reference light is reflected by a fixed mirror and then enters the beam splitter. The measurement light passes through the beam splitter and irradiates the seabed, and then is reflected back from the seabed surface. S22. Setting an optical path matcher in the reference light path, adjusting the reference light path by rotating the fine-tuning matcher, superimposing the reference light and the measurement light on the photodetector, and observing interference fringes in real time; S23. By fine-tuning the matcher to keep the interference fringes stationary, the optical paths of the reference light and the measurement light are matched.

3. The method for measuring seabed microvibration based on optical interferometry according to claim 1, characterized in that: In S3, the specific steps of forming the first green laser are: S31, selecting a nonlinear optical crystal as a first frequency-doubling crystal, using a laser to emit an infrared laser with a wavelength of 1064 nm, focusing the 1064 nm infrared laser onto the first frequency-doubling crystal, wherein the light transmission range of the first frequency-doubling crystal is 355 nm-4500 nm; S32. In the first frequency doubling crystal, the frequency of the 1064 nm infrared laser is doubled by the nonlinear optical effect to obtain a first green laser of 532 nm. The first green laser and the unconverted 1064 nm infrared laser are output through the first frequency doubling crystal together.

4. The method for measuring seabed microvibration based on optical interferometry according to claim 1, characterized in that: In the S4, the light transmission range of the second frequency doubling crystal is 1054nm-1074nm.

5. The method for measuring seabed microvibration based on optical interferometry according to claim 1, characterized in that: In said S4, the specific steps of obtaining the unconverted 1064 nm infrared laser are: S41. At the output end of the first frequency-doubling crystal, use a beam splitter to separate the frequency-doubled 532nm green laser and the unconverted 1064nm infrared laser, direct the frequency-doubled 532nm green laser to a beam merging unit, and direct the unconverted 1064nm infrared laser to a second frequency-doubling crystal; S42. Before being directed to the second frequency-doubling crystal, the unconverted 1064 nm infrared laser is pre-processed, the beam is shaped using a beam shaper, and then an optical filter is used to filter out unwanted wavelengths or stray light, and finally a focusing lens is used to focus the beam onto the second frequency-doubling crystal.

6. The method for measuring seabed microvibration based on optical interferometry according to claim 1, characterized in that: In S5, the specific steps of forming a continuous green light output are: S51, accurately locating the output position of the green light generated by the first frequency doubling unit and the second frequency doubling unit; using a beam combiner including a prism or a grating to combine the two green light beams into a single beam; S52, adjusting the position and angle of the combiner to ensure that the two green light beams are aligned in space and time to maximize the efficiency of beam combining and reduce mode distortion; S53. If the green light powers generated by the two frequency doubling units are different, an attenuator or a gain medium is used to match the powers of the two beams to prevent the brighter beam from overwhelming the darker beam during the merging process.

7. A seabed microvibration measurement system based on optical interferometry, based on the seabed microvibration measurement method based on optical interferometry according to any one of claims 1 to 6, comprising: Laser module, interferometer module, frequency doubling module, optical path integration module, photoelectric detection module, data processing module, characterized in that: The laser module includes: a laser, an optical isolator and a beam splitter; The interferometer module includes: an interferometer, a calibration unit and a seabed packaging unit; The frequency multiplication module includes: a first frequency multiplication unit and a second frequency multiplication unit; The optical path integration module includes: a beam merging unit and a beam adjusting unit; The photoelectric detection module includes: a phase matching unit and a temperature adjustment unit; The data processing module includes: a data acquisition card and a data transmission unit.

8. The seabed micro-vibration measurement system based on optical interferometry according to claim 7, characterized in that: The first frequency doubling unit includes a first frequency doubling crystal and a first focusing lens, and is used to frequency-double a portion of the 1064nm infrared laser into a first green laser of 532nm; The second frequency doubling unit includes a second frequency doubling crystal and a second focusing lens, and is used to frequency-double the remaining 1064nm infrared laser into a second green laser of 532nm.

9. The seabed micro-vibration measurement system based on optical interferometry according to claim 7, characterized in that: The beam merging unit is used to merge the green light generated by the first frequency doubling unit and the second frequency doubling unit to form a single beam, that is, continuous green light output; The beam adjustment unit is used to adjust and optimize the direction, intensity and pattern of the light beam to ensure that the light beam correctly enters the interferometer and photodetector and generates clear interference fringes.

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