Underwater range measurement system and method based on tunable chirped pulse interferometry

Abstract

The application discloses an underwater distance measurement system and method based on tunable chirped pulse interference, and a tunable optical frequency comb generates an optical frequency comb pulse signal with a variable repetition frequency, which is transmitted through a monitoring light path, a reference light path and a measurement light path respectively, the monitoring light path uses a photodetector and a frequency counter to monitor the repetition frequency of the optical frequency comb pulse signal in real time; a long optical fiber is used as a reference arm in the reference light path to generate a reference pulse signal; a long optical fiber stabilization unit is further arranged in the reference light path; the measurement light path generates a measurement chirped pulse signal; the reference light path and the measurement light path interfere to generate an interference signal, high-resolution spectral image high-speed acquisition of a spectral signal is carried out based on a calibrated CCD camera, and an underwater target absolute distance measurement value is obtained. Compared with the prior art, the application realizes accurate, fast and highly stable underwater distance measurement.

Description

Technical Field

[0001] This invention belongs to the field of underwater laser ranging, and in particular relates to an underwater distance measuring device and method based on pulse interferometry. Background Technology

[0002] Absolute distance measurement is fundamental to geometric metrology and represents basic research in marine metrology. Pulsed lasers, with their high efficiency and precision, are widely used in underwater optical detection. Optical frequency combs, derived from laser mode-locking technology, offer stable transmission in underwater environments with their wide spectrum and high peak power, and are currently widely used in high-precision underwater detection. Spectroscopic interferometry utilizes the wide spectrum of optical frequency combs, employing a wavelength-based measurement mode to obtain the detection distance using the phase information of the interference spectrum, thus achieving high-precision absolute distance measurement. However, due to directional ambiguity in the phase portion of the interference spectrum, it is difficult to distinguish the relative positions of the reference target and the measured target. Currently, this ambiguity is typically eliminated by adding a dispersive device, using the broadened reference pulse and the measurement pulse to interfere, thus generating chirped pulse interferometry. Traditional chirped pulse interferometry can achieve a unique determination of the measured distance, but it requires an additional dispersive device to the measurement system. Furthermore, limited by the resolution of the spectrometer, the chirped pulse interferometry range is only at the centimeter level, resulting in undetectable measurement "dead zones" in the measurement path, making it difficult to achieve arbitrary distance measurements. Meanwhile, both traditional spatial dispersive spectrometers and Fourier transform spectrometers are limited by their internal mechanical components, making it impossible to rapidly acquire spectral signals in high-resolution mode and achieve a balance between measurement speed and resolution. Therefore, overcoming the limitations of spectrometers and improving the efficiency of chirped pulse interferometry based on optical frequency combs to achieve efficient and high-precision underwater ranging is a key problem that needs to be solved. Summary of the Invention

[0003] To address the limitations of chirped pulse interferometry based on optical frequency combs, an underwater distance measurement device and method based on tunable chirped pulse interferometry is proposed. This method utilizes a tunable optical frequency comb with repetitive frequency scanning combined with a long fiber delay line to extend the range of synthesized wavelengths. A detection system built with a CCD camera is used to acquire spectral interference signals at high speed, thereby covering the entire distance equivalent to the pulse-to-pulse length. This overcomes the shortcomings of traditional spectrometers in detection speed and enables rapid underwater arbitrary distance measurement.

[0004] This invention is achieved using the following technical solution:

[0005] An underwater distance measurement system based on tunable chirped pulse interferometry includes: a tunable optical frequency comb light source, a correlation optical path, and an underwater target; the tunable optical frequency comb generates an optical frequency comb pulse signal with a variable repetition frequency; the correlation optical path includes a monitoring optical path, a reference optical path, and a measurement optical path; the optical frequency comb pulse signal is transmitted through the monitoring optical path, the reference optical path, and the measurement optical path, respectively; a photodetector and a frequency counter are set in the monitoring optical path, and the repetition frequency of the optical frequency comb pulse signal is obtained in real time by using the photodetector and the frequency counter; a long optical fiber connected to a piezoelectric ceramic is set in the reference optical path, and the long optical fiber serves as a reference arm to generate a reference pulse signal; the underwater distance measurement system based on tunable chirped pulse interferometry includes: a tunable optical frequency comb light source, a correlation optical path, and a measurement optical path; the tunable optical frequency comb light source generates an optical frequency comb pulse signal with a variable repetition frequency; the correlation optical path includes a monitoring optical path, a reference optical path, and a measurement optical path ... The reference optical path is further equipped with a long fiber stabilization unit based on a servo feedback system to stabilize the length of the long fiber. This unit includes a continuous wave laser, a long fiber connected to piezoelectric ceramics, an acousto-optic modulator, a signal generator, a photodetector, a phase meter, a low-pass filter, and a phase-locked loop. The measurement optical path is incident on the underwater target, and the measurement pulse signal transmitted underwater is obtained after reflection from the underwater target, generating a measurement chirped pulse signal. The reference optical path and the measurement optical path interfere and generate an interference signal. A CCD camera calibrated based on the repetition frequency of the optical frequency comb pulse signal performs high-resolution spectral image acquisition of the spectral signal at high speed to obtain the absolute distance measurement value of the underwater target, as shown in the following formula:

[0006]

[0007] Among them, Lc P ix is ​​the calibration distance per unit pixel, L cPix =L cal / (P w1 -P w2 Pw1 and Pw2 are the pixel positions of the two widest stripes during the calibration process, and n g Let L be the group refractive index of water. cal To calibrate the distance, f rep Where N is the repetition frequency of the optical frequency comb, c is the speed of light in a vacuum, and P is the repetition frequency of the optical frequency comb. shift This represents the pixel offset of the widest stripe at the repetition frequency. During calibration, the repetition frequency scan causes the pulse position to change linearly, and the calibration distance is shown in the following formula:

[0008]

[0009] Where, Δf rep For the repetition frequency difference, f rep1 f rep2 n represents the repetition frequency of the optical frequency comb corresponding to the two widest fringes during the calibration process. g Let be the group refractive index of water, and c be the speed of light in a vacuum.

[0010] The interference signal is acquired by a high-speed spectral detection unit, which further includes a grating, a cylindrical mirror, and a CCD camera. The grating and cylindrical mirror disperse and shape the interference signal before projecting it onto the CCD camera.

[0011] The continuous-wave laser signal emitted by the continuous-wave laser serves as the light source signal for the long-fiber stabilization unit. It is transmitted to a reference arm formed by a long optical fiber connected to a piezoelectric ceramic and a measurement arm formed by an acousto-optic modulator. The reference arm signal is collimated by a collimator to produce spatial light, its direction is changed by a mirror, and it is then combined with the measurement arm signal at a spatial light beam splitter. The photodetector acquires the heterodyne interference signal between the reference arm and the measurement arm. The phase meter uses the output of the signal generator as a reference to detect phase fluctuations in the heterodyne interference signal. The fluctuation error signal is transmitted through a low-pass filter into a phase-locked loop, generating a driving voltage V. p ; Utilizing driving voltage V p To achieve servo feedback control of piezoelectric ceramics.

[0012] An underwater distance measurement method based on tunable chirped pulse interferometry includes:

[0013] The repetition frequency of the optical frequency comb pulse signal is obtained in real time by monitoring the photodetector and frequency counter set in the monitoring optical path; a reference pulse signal is generated by using a long optical fiber connected to piezoelectric ceramics as a reference arm in the reference optical path; a long optical fiber stabilization unit based on servo feedback is set in the reference optical path to stabilize the length of the long optical fiber; the measurement optical path is incident on the underwater target, and the measurement pulse signal transmitted underwater is obtained after reflection from the underwater target, generating a measurement chirped pulse signal; interference occurs between the reference optical path and the measurement optical path to generate an interference signal; a high-resolution spectral image of the spectral signal is acquired at high speed by a CCD camera calibrated based on the repetition frequency of the optical frequency comb pulse signal to obtain the absolute distance measurement value of the underwater target, as shown in the following formula:

[0014]

[0015] Among them, L cPix L is the calibration distance per unit pixel. cPix =L cal / (P w1 -P w2 ), P w1 P w2 n represents the pixel positions of the two widest stripes during the calibration process. g Let f be the group refractive index of water. rep Where N is the repetition frequency of the optical frequency comb, c is the speed of light in a vacuum, and P is the repetition frequency of the optical frequency comb. shift This represents the pixel offset of the widest stripe at the repetition frequency.

[0016] During the calibration process, the repeated frequency scanning causes the pulse position to change linearly, and the calibration distance is shown in the following formula:

[0017]

[0018] Where, Δf rep For the repetition frequency difference, f rep1 f rep2 n represents the repetition frequency of the optical frequency comb corresponding to the two widest fringes during the calibration process. g Let be the group refractive index of water, and c be the speed of light in a vacuum.

[0019] The interference signal is acquired by a high-speed spectral detection unit, and the interference signal is dispersed and shaped by the grating and cylindrical mirror in the high-speed spectral detection unit before being projected onto a CCD camera.

[0020] The continuous-wave laser signal emitted by the continuous-wave laser serves as the light source signal for the long-fiber stabilization unit. It is transmitted to a reference arm formed by a long optical fiber connected to a piezoelectric ceramic and a measurement arm formed by an acousto-optic modulator. The reference arm signal is collimated by a collimator to produce spatial light, its direction is changed by a mirror, and it is then combined with the measurement arm signal at a spatial light beam splitter. The photodetector acquires the heterodyne interference signal between the reference arm and the measurement arm. The phase meter uses the output of the signal generator as a reference to detect phase fluctuations in the heterodyne interference signal. The fluctuation error signal is transmitted through a low-pass filter into a phase-locked loop, generating a driving voltage V. p ; Utilizing driving voltage V p To achieve servo feedback control of piezoelectric ceramics.

[0021] The beneficial effects and advantages of this invention are as follows:

[0022] 1) The inherent physical properties of water introduce chirped pulses into optical frequency comb interferometry, eliminating the need for additional dispersive elements, achieving unique determination of the distance result, and eliminating directional ambiguity in distance measurement;

[0023] 2) A spectral detection system was built using a CCD camera, which simultaneously achieved high-speed and high-resolution acquisition of interferometric spectral images of spectral interference signals, thus compensating for the insufficient detection speed of the spectrometer.

[0024] 3) By combining a tunable optical frequency comb with a long fiber delay line and a repetitive frequency scanning, the range of synthesized wavelengths is expanded, thereby covering the entire distance equivalent to the pulse-to-pulse length, eliminating the measurement "dead zone", and realizing arbitrary distance measurement underwater. Attached Figure Description

[0025] Figure 1This is a schematic diagram of an underwater distance measurement system based on tunable chirped pulse interferometry according to the present invention;

[0026] Figure 2 This is a diagram showing the calibration results of the CCD camera system of the present invention;

[0027] Figure 3 This is a schematic diagram illustrating the length locking effect of a 30m long optical fiber based on a long optical fiber stabilization unit.

[0028] Figure 4 Spectral results of underwater measurements at different distances;

[0029] Figure 5 A schematic diagram of underwater arbitrary distance measurement results.

[0030] Figure label:

[0031] 1. Tunable optical frequency comb; 2. Monitoring optical path; 21. Photodetector; 22. Frequency counter; 23. Collimator; 3. Reference optical path; 31. Piezoelectric ceramic; 32. Long optical fiber; 321. Continuous wave laser; 322. Acousto-optic modulator; 323. Signal generator; 324. Photodetector; 325. Phase meter; 326. Low-pass filter; 327. Phase-locked loop; 328. Mirror; 329. Spatial beam splitter; 330. Collimator; 4. Measurement optical path; 41. Circulator; 42. Collimator; 5. Underwater target; 51. Optical window; 52. Water tank; 6. Grating; 7. Cylindrical mirror; 8. CCD camera; 9. Spatial beam; 10. Collimator. Detailed Implementation

[0032] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] like Figure 1 As shown, this invention discloses an underwater distance measurement system based on tunable chirped pulse interferometry. The system includes a tunable optical frequency comb 1 connected by optical fibers, a correlation optical path, and an underwater target. The tunable optical frequency comb generates optical frequency comb pulse signals with a variable repetition frequency. The correlation optical path includes a monitoring optical path, a reference optical path, and a measurement optical path. The optical frequency comb pulse signals are transmitted through the monitoring optical path, the reference optical path, and the measurement optical path, respectively.

[0034] The tunable optical frequency comb 1 generates an optical frequency comb pulse signal with a variable repetition frequency. This signal is divided into three paths: the first path is a monitoring optical path 2, in which a photodetector 21 and a frequency counter 22 are installed. The repetition frequency of the optical frequency comb pulse signal is obtained in real time using the photodetector 21 and the frequency counter 22. The photodetector 21 is connected to a collimator 23 to collimate the spatial beam output. The second path is a reference optical path 3, in which a long optical fiber 32 connected to a piezoelectric ceramic (PZT) 31 is installed. The long optical fiber 32 serves as a reference arm to generate a reference pulse signal. The third path is a measurement optical path 4, which is incident through an optical window 51 onto an underwater target 5 in a water tank 52 located in an underwater environment. The measurement pulse signal is obtained after reflection from the underwater target 5. The reference optical path 3 has a fixed optical path and does not experience pulse width changes. The measurement optical path is transmitted underwater, and the strong dispersive properties of the water cause the measurement optical signal to broaden, resulting in a chirping phenomenon and generating a measurement chirped pulse signal. The reference and measurement optical paths are combined to generate interference signals. Grating 6 and cylindrical mirror 7 are then used to disperse and shape the interference signals before projecting them onto a CCD camera 8 for high-speed spectral image acquisition. Grating 6, cylindrical mirror 7, and CCD camera 8 constitute a high-speed spectral detection unit, which acquires high-resolution spectral images of the signals at high frame rates. If necessary, the combined beam signal received by grating 6 is a collimated spatial beam emitted through collimator 10. A spatial beam 9, transmitted before and after the cylindrical mirror 7, exists between grating 6 and CCD camera 8.

[0035] The system operates as follows: Pulse signals emitted from a tunable optical frequency comb are transmitted via optical fiber and split into three optical paths for signal transmission: The first path, the monitoring path, receives the emitted laser signal using a photodetector and transmits it to a frequency counter to monitor the repetition frequency of the laser source. The second path, the reference path, transmits the emitted signal to a long optical fiber connected to a piezoelectric ceramic (PZT) 31, using it as a reference arm to generate a reference pulse signal. The third path, the measurement path, transmits the emitted signal to the underwater target using a circulator 41, outputting spatial light to the underwater target. If necessary, the spatial beam is collimated by a collimator 42. The spatial light is then incident underwater through an optical window 51, reflected by the underwater target 5, and the chirped pulse passes through the circulator 41 again to generate a measurement chirped pulse signal. The reference pulse signal and the measurement pulse signal are combined and transmitted to the high-speed spectral detection unit. The grating 6 disperses the signal and the cylindrical mirror 7 shapes it. Finally, the signal is projected onto the CCD camera 8. The CCD camera detection system is calibrated by repetitive frequency tuning, thereby enabling high-speed signal acquisition for spectral detection.

[0036] The tunable optical frequency comb 1 achieves cavity length adjustment by relying on the displacement stage in the cavity, generating an optical frequency comb pulse signal with a variable repetition frequency, thereby realizing linear scanning within the laser annular cavity.

[0037] The long optical fiber, used as a delay line, combined with the repetition frequency scanning range of the optical frequency comb, can cover the pulse interval. Therefore, the combination of the tunable optical frequency comb and the long optical fiber delay line can effectively improve the measurement range of the device, eliminate the measurement "dead zone" in traditional methods, and realize arbitrary distance measurement underwater. However, the long optical fiber is easily affected by environmental noise, resulting in length jitter. Therefore, a servo feedback method is used in the measurement device to stabilize its length.

[0038] The long fiber stabilization unit based on servo feedback is used to stabilize the length of a long fiber. In this invention, the long fiber stabilization unit is designed and implemented by a Mach-Zehnder heterodyne interferometer. The unit includes a continuous wave laser 321, a long fiber 32 connected to a piezoelectric ceramic 31, an acousto-optic modulator 322, a signal generator 323, a photodetector 324, a phase meter 325, a low-pass filter 326, and a phase-locked loop 327. The continuous-wave laser signal emitted by the continuous-wave laser 321 serves as the light source signal for the long fiber stabilization unit. It is transmitted to a reference arm formed by the long fiber 32 connected to the piezoelectric ceramic 31 and a measurement arm formed by the acousto-optic modulator 322. The reference arm signal is collimated by the collimator 330 for spatial light output. Depending on the specific application, the spatial light direction is changed by the reflector 328. The reference arm signal is combined with the measurement arm signal at the spatial light beam splitter 329. The photodetector 324 collects the heterodyne interference signal between the reference arm and the measurement arm. The phase meter 325 uses the output of the signal generator 323 as a reference to detect phase fluctuations in the heterodyne interference signal. The fluctuation error signal is transmitted through a low-pass filter 326 into the phase-locked loop 327, ultimately generating a driving voltage V. p Using the driving voltage V p Servo feedback control of the piezoelectric ceramic (PZT) 31 is achieved. Therefore, as the driving voltage V... p The piezoelectric ceramic (PZT) 31 micro-displacement is adjustable, and the length of the long optical fiber remains stable in a disturbed underwater environment. The acousto-optic modulator has a driving frequency of 80MHz.

[0039] In summary, to accurately estimate the mapping coefficient between a unit pixel and the calibration distance in an underwater environment, a repetition frequency tuning method is used to calibrate the CCD camera detection system, eliminating the dependence on the positioning accuracy of traditional displacement stage calibration methods. First, the generated chirped pulse interference spectrum is divided into two parts. One part is observed and measured using a commercial spectrometer; the other part is projected onto a grating for dispersion, then laser-shaped by a cylindrical mirror, and finally projected onto the CCD camera for observation and acquisition of the fringes. Two repetition frequencies, 100.723MHz and 100.774MHz, are selected to generate the chirped pulse interference fringes. An aperture is used to reduce background brightness, minimize the influence of ambient light, and improve the contrast of the acquired fringes. Based on the grayscale information of the acquired image, interpolation fitting is performed on the widest fringe for pixel correction of the optical frequency. The calibration results are attached. Figure 2 As shown, the pixel offset is 58, corresponding to a calibration distance of 1108.9 μm. Therefore, the mapping coefficient L between a unit pixel and the calibration distance is... cPix It can be calculated as 19.1 μm / pixel.

[0040] The relevant principles are as follows:

[0041] The interference spectral intensity of the chirped pulse interference signal based on the optical frequency comb is expressed as:

[0042]

[0043] Where I0(f) is the power spectrum of the reference pulse, n(f) is the refractive index of water, f is the light frequency, c is the speed of light in vacuum, 2n(f) / c represents the time delay between the reference pulse and the measurement pulse (let τ = 2n(f) / c), and l is the fractional part of the actual measurement distance L, L = N / 2·L pp +l, N = floor(2L / L) pp ), where N is an integer, L pp The pulse-to-pulse length can be expressed as L. pp =c / (n g f rep ), n g Let f be the group refractive index of water. rep This is the repetition frequency of the optical frequency comb.

[0044] According to Harvey's formula, the refractive index of water is expressed as n(f) = n(f0) + 2πα(f - f0), where f0 is the reference frequency and α is the linear coefficient. Therefore, the phase of the interference spectral intensity in formula (1) can be expressed as:

[0045]

[0046] From formula (2), it can be seen that φ(f,l) and f are quadratic functions, therefore the generated chirped pulse interference fringes oscillate at an unstable modulation frequency. For a fixed measurement target, when the reference mirror is moved by a known length Δl, it is assumed that the wavelength position corresponding to the widest fringe moves from f1 to f2. Correspondingly, before and after the movement, the optical path changes from 2n(f1)L to 2n(f2)L. Applying Harvey's formula, the optical path difference before and after the movement can be calculated as Δl=4πα(f1-f2)L. Therefore, the distance to be measured is expressed as:

[0047]

[0048] In the formula, f shift Let η be the frequency shift of the widest fringe, and let η be the mapping relationship between the frequency shift and the spatial distance, where η = 4παL. As can be seen from formula (3), by analyzing the frequency shift of the widest fringe and estimating the value of parameter η, the underwater distance can be accurately determined.

[0049] The value of parameter η is determined through a calibration system. Calibration is achieved using an adjustable repetition frequency, eliminating the influence of mechanical module displacement. When L is at L... pp Around / 2, the widest fringe varies back and forth with linear scanning of the repetition frequency. To reduce the influence of environmental noise, we use information from two adjacent peaks to determine the position of the widest fringe in the interference spectrum. Specifically, we use the pixel positions P1 and P2 of two adjacent peaks. r The widest stripe P can be calculated. w The precise pixel position is P w =(P1+P r ) / 2. During the calibration process, the repetitive frequency scan causes the pulse position to change linearly; therefore, the calibration distance calculation formula is expressed as:

[0050]

[0051] In the formula, Δf rep This represents the repetition frequency difference. Since the calibration distance maintains a linear relationship with the displacement of the widest fringe, therefore L... cPix L is the calibration distance per unit pixel. cPix =L cal / (P w1 -P w2 ), where P w1 P w2 These are the pixel positions of the two widest stripes during the calibration process. Therefore, for the spectral detection system of a CCD camera, the absolute distance calculation formula of equation (3) is expressed as:

[0052]

[0053] In the formula, P shiftThis represents the pixel offset of the widest stripe at the repetition frequency.

[0054] like Figure 3 The diagram illustrates the length-locking effect of a 30m long optical fiber based on a long-fiber stabilization unit. Data results show that the active servo system effectively stabilizes the length of the long optical fiber, and the corresponding phase-locked loop also reduces the fluctuation of the error signal. The feedback servo system starts operating at approximately 600s, and both the error signal and fiber length fluctuations are significantly suppressed. The voltage difference caused by the error is approximately 1.8V, equivalent to half the wavelength of a continuous-wave laser. Therefore, we can optimize the length variation to a standard deviation of 7.6nm, corresponding to a length drift of 10... -10 The relative jitter of the level.

[0055] Verification Experiment: An underwater target was fixed on a long rail for measurements at different distances. To constrain the rail, the spatial light was kept at a constant height. Within the repetition frequency range, an appropriate repetition frequency was selected to obtain interference spectral images. The measurement spectral results at underwater positions of 1m and 3m are shown below. Figure 4 As shown in the figure, the interference image was acquired by a CCD spectral detection system, enabling rapid acquisition. The position of the widest fringe varies depending on the selected repetition frequency at different measurement locations. Furthermore, the peak power of the interference fringes decreases due to water attenuation. Collimation and beam height vary with increasing measurement distance, causing the modulation depth of the interference fringes to be affected by optical power fluctuations. Measurements were performed at different locations of 1m, 2m, 3m, 4m, and 5m, with 10 repeated experiments at each distance. The underwater measurement results at arbitrary distances are shown in the figure. Figure 5 As shown, the experimental results agree well with the theoretical analysis. The tunable chirped pulse interferometry overcomes the resolution limitations of spectrometers, enabling arbitrary underwater distance measurement. Furthermore, the dynamic information from the optical frequency comb ensures that the interferogram yields a unique distance calculation result. Within a 5m range, the measurement results are better than 59.8μm.

[0056] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. An underwater distance measurement system based on tunable chirped pulse interferometry, characterized in that, The system includes a tunable optical frequency comb light source, a correlation optical path, and an underwater target. The tunable optical frequency comb generates an optical frequency comb pulse signal with a variable repetition frequency. The correlation optical path includes a monitoring optical path, a reference optical path, and a measurement optical path. The optical frequency comb pulse signal is transmitted through the monitoring optical path, the reference optical path, and the measurement optical path, respectively. A photodetector and a frequency counter are installed in the monitoring optical path to monitor and obtain the repetition frequency of the optical frequency comb pulse signal in real time. A long optical fiber connected to a piezoelectric ceramic is installed in the reference optical path, serving as a reference arm to generate a reference pulse signal. A servo feedback-based long optical fiber stabilization unit is further installed in the reference optical path to stabilize the length of the long optical fiber. This unit includes a continuous wave excitation... The system comprises an optical transducer, a long optical fiber connected to a piezoelectric ceramic, an acousto-optic modulator, a signal generator, a photodetector, a phase meter, a low-pass filter, and a phase-locked loop (PLL). The continuous-wave laser signal emitted by the continuous-wave laser serves as the light source signal for the long optical fiber stabilization unit. It is transmitted to a reference arm formed by the long optical fiber connected to the piezoelectric ceramic and a measurement arm formed by the acousto-optic modulator. The reference arm signal is collimated by a collimator, its spatial light direction is changed by a mirror, and it is then combined with the measurement arm signal by a spatial light beam splitter. The photodetector acquires the heterodyne interference signal between the reference arm and the measurement arm. The phase meter uses the output of the signal generator as a reference to detect phase fluctuations in the heterodyne interference signal. The fluctuation error signal is transmitted through a low-pass filter into the PLL to generate a driving voltage. V p ; Utilizing driving voltage V p Servo feedback control of piezoelectric ceramics is achieved; the measurement optical path is incident on the underwater target, and the measurement pulse signal transmitted underwater is obtained after reflection by the underwater target, generating a measurement chirp pulse signal; the reference optical path and the measurement optical path interfere and generate an interference signal; a CCD camera calibrated based on the repetition frequency of the optical frequency comb pulse signal performs high-resolution spectral image acquisition of the spectral signal at high speed to obtain the absolute distance measurement value of the underwater target, as shown in the following formula: in, The calibration distance is per unit pixel. , , These are the pixel positions of the two widest stripes during the calibration process. Let be the group refractive index of water. To calibrate the distance, This is the repetition frequency of the optical frequency comb. It is an integer. The speed of light in a vacuum. This represents the pixel offset of the widest stripe at the repetition frequency.

2. The underwater distance measurement system based on tunable chirped pulse interferometry according to claim 1, characterized in that, During the calibration process, the repeated frequency scanning causes the pulse position to change linearly, and the calibration distance is shown in the following formula: in, For the difference in repetition frequency, , The repetition frequency of the optical frequency comb corresponding to the two widest stripes during the calibration process. Let be the group refractive index of water. It is the speed of light in a vacuum.

3. The underwater distance measurement system based on tunable chirped pulse interferometry according to claim 1, characterized in that, The interference signal is acquired by a high-speed spectral detection unit, which further includes a grating, a cylindrical mirror, and a CCD camera. The grating and cylindrical mirror disperse and shape the interference signal before projecting it onto the CCD camera.

4. A method for underwater distance measurement using the underwater distance measurement system based on tunable chirped pulse interferometry according to any one of claims 1 to 3, characterized in that, include: The repetition frequency of the optical frequency comb pulse signal is obtained in real time by monitoring the photodetector and frequency counter set in the optical path. A reference pulse signal is generated by using a long optical fiber connected to a piezoelectric ceramic in the reference optical path as a reference arm; The reference optical path is used to set up a long fiber stabilization unit based on servo feedback to stabilize the length of the long fiber. The measurement pulse signal is obtained by incident the measurement optical path onto the underwater target and reflected by the underwater target, and then transmitted underwater. This generates a measurement chirp pulse signal. The reference optical path and the measurement optical path interfere with each other to generate an interference signal. Based on the repetition frequency calibration of the optical frequency comb pulse signal, a CCD camera performs high-resolution spectral image acquisition of the spectral signal at high speed to obtain the absolute distance measurement value of the underwater target, as shown in the following formula: in, The calibration distance is per unit pixel. , , These are the pixel positions of the two widest stripes during the calibration process. Let be the group refractive index of water. This is the repetition frequency of the optical frequency comb. It is an integer. The speed of light in a vacuum. This represents the pixel offset of the widest stripe at the repetition frequency.

Images