An underwater ranging system and method based on dual-comb pulse coding

Abstract

The present invention discloses an underwater ranging system and method based on dual-comb pulse coding. The system includes a dual-comb underwater ranging unit that combines and interferes dual-comb signals, including P-reference pulse light and P-measurement pulse light, with P-local oscillator pulse light to achieve accurate underwater distance measurement using the dual-comb. A pulse coding ranging unit encodes the P-polarized light and S-polarized light of the P-reference light pulse and P-measurement light pulse in the dual-comb signal, respectively, to obtain S-coded reference pulse light and S-coded measurement pulse light. The S-coded reference pulse light and the S-coded measurement pulse light are combined to complete the pulse-coded underwater distance measurement. An underwater refractive index correction unit corrects the refractive index after performing two measurements in air and underwater. The present invention can expand the unambiguous measurement range and achieve high-precision, high-sampling-rate, accurate underwater distance measurement.

Description

Technical Field

[0001] The present invention relates to the field of underwater ranging technology using pulse coding detection technology, and in particular to an underwater ranging device and detection method based on dual-comb pulse coding. Background Art

[0002] Underwater target detection is a core technical means and universal tool for understanding and exploring the ocean. Currently, underwater ranging technology is indispensable for underwater engineering construction, seabed surveying, underwater target search and rescue, and positioning. With the continuous advancement of information technology, fields such as topographic and geomorphological exploration, oceanographic traceability and metrology, and underwater guidance and navigation have placed higher demands on underwater ranging. Furthermore, when measuring distance through underwater light waves, the group refractive index, a fundamental issue in underwater detection, directly affects the results. Generally speaking, correction for the underwater group refractive index can be achieved through direct and indirect measurement methods. For example, the Abbe refractometer uses the principle of total reflection, and the direct measurement method uses the principle of interferometry to correct the refractive index. Indirect measurement methods require fitting calculations based on extensive experimental data to obtain empirical formulas (generally expressed as a function of temperature, salinity, and wavelength). For example, the refractive index of pure water is calculated using the Harvey formula, while the Quan-Fry formula is used for seawater.

[0003] As a new type of laser light source, the femtosecond optical frequency comb not only features narrow pulse width, broad spectrum, and high peak power, but also possesses excellent time-frequency metrology properties. This has excellent theoretical advantages and application potential for underwater ranging. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing technology, the present invention proposes an underwater ranging system and method based on dual-comb pulse coding. With the optical frequency comb as the basis, the design of the underwater distance measurement system and method is realized through pulse coding and underwater refractive index correction, thereby expanding the unambiguous range of measurement and realizing high-precision and high-sampling rate underwater distance measurement.

[0005] The present invention is achieved by utilizing the following technical solutions:

[0006] An underwater ranging system based on dual-comb pulse coding, the system includes a signal pulse laser SL and a local oscillator pulse laser LO, a dual-comb ranging unit, a pulse coding ranging unit and a refractive index correction unit; wherein:

[0007] The signal pulse laser SL emits a stable, equally spaced signal pulse sequence through the first collimator CL1, which is then split into P-polarized light and S-polarized light after passing through the first half-wave plate HWP1 and the first polarization beam splitter PBS1.

[0008] The local oscillator pulse laser LO emits a pulsed light which passes through a second collimator CL2, a second half-wave plate HWP2 and a second polarization beam splitter PBS2 and is then divided into a P-local oscillator pulse light and an S-local oscillator pulse light.

[0009] The dual-comb distance measuring unit includes a first beam splitter BS1, a third polarization beam splitter PBS3, a sixth beam splitter BS6, a seventh beam splitter BS7, a third beam splitter BS3, a measuring reflector MM2 and a local oscillator pulse laser LO; the P polarized light is divided into two pulse lights, namely, a P-reference pulse light and a P-measurement pulse light, after passing through the third polarization beam splitter PBS3 and the sixth beam splitter BS6. The P-reference pulse light is reflected to the seventh beam splitter BS7 after passing through the third beam splitter BS3 and a section The underwater distance L propagates to the measuring reflector MM2, and is reflected back to the third beam splitter BS3 by the measuring reflector MM2 along the original path. In the seventh beam splitter BS7, it is combined and interfered with the P-reference pulse light and the P-local oscillator pulse light emitted by the local oscillator pulse laser LO. The interference signal is collected and processed by the second balanced detector BPD2, the low-pass filter LPF and the signal processing module SP. After the interference fringes are Hilbert transformed, the time interval Δt based on the dual-comb interferometry measurement is obtained, thereby realizing the accurate measurement of the underwater distance of the dual-comb.

[0010] The pulse code ranging unit includes an acousto-optic modulator (AOM), a second reflector M2, a first beam splitter BS1, a third polarization beam splitter PBS3, a fifth collimator CL5, a third beam splitter BS3, a measurement reflector MM2, a fourth polarization beam splitter PBS4, a sixth collimator CL6, and a 2×1 fiber coupler FC. An S-polarized light is incident on the acousto-optic modulator (AOM) and encoded, and the S-coded polarized light obtained after passing through the second reflector M2 and the first beam splitter BS110 is split into an S-coded reference pulse light and an S-coded measurement pulse light. Light; the S-coded reference pulse light is reflected by the third polarization beam splitter PBS3 to the fifth collimator CL5, and the S-coded measurement pulse light, after passing through the third beam splitter BS3, propagates through a section of underwater distance L and is reflected back to the third beam splitter BS3 by the measurement reflector MM2 along the original path. The S-coded measurement pulse light passes through the fourth polarization beam splitter PBS4 and the sixth collimator CL6 and is combined with the S-coded reference pulse light in the 2×1 fiber coupler FC to achieve underwater distance measurement based on pulse coding and obtain a rough underwater distance measurement;

[0011] The refractive index correction unit includes a second beam splitter BS2, a first reflector M1, a reference reflector RM1, a target reflector MM1, and a fifth beam splitter BS5; another path of S polarized light is incident from the first reflector M1 to the fourth beam splitter BS4. 14The Michelson interferometer composed of the reference reflector RM1 and the target reflector MM1 combines and interferes with the S local oscillator pulse light emitted by the local oscillator pulse laser LO, and the interference fringes are received by the first balanced detector BPD1; when measuring, it is necessary to measure once in air and once in water respectively, so as to obtain the optical path difference L in air a The optical path difference L in water w .

[0012] An underwater ranging method based on dual-comb pulse coding, the method specifically comprising the following steps:

[0013] Step 1: Lock the frequency and phase of the two lasers based on the atomic clock;

[0014] Step 2: Split the light into P-polarized light and S-polarized light at the first polarization beam splitter PBS1;

[0015] Step 3-1: Complete dual-comb measurement based on P-polarized light. This specifically includes the following processing:

[0016] At the first beam splitter BS1, it is divided into P-reference pulse light and P-measurement pulse light;

[0017] The P-reference pulse light passes through the first beam splitter BS1, the third polarization beam splitter PBS3, and the sixth beam splitter BS6, and then reflects to the seventh beam splitter BS7. The P-measurement pulse gloss passes through the first beam splitter BS1, the third beam splitter BS3, and the measurement reflector MM2, and then passes through the first polarization beam splitter PBS4 and is incident to the seventh beam splitter BS7.

[0018] In the seventh beam splitter BS7, the two pulse lights, the P-reference pulse light and the P-measurement pulse light, are combined and interfered with the P-local oscillator pulse light emitted by the local oscillator pulse laser LO.

[0019] The interference fringes are received by the second balanced detector BPD2, and the interference fringes are subjected to Hilbert transform to obtain the time interval Δt measured based on the dual-comb interferometry.

[0020] Step 3-2: Complete pulse coding underwater ranging and underwater refractive index correction based on S-polarized light; this includes the following processing:

[0021] The S-polarized light obtained after passing through the first polarization beam splitter PBS1 and the second beam splitter BS2 is incident on the acousto-optic modulator AOM;

[0022] Pulse encoding of S-polarized light in an acousto-optic modulator (AOM);

[0023] The S-coded polarized light obtained after encoding is split into S-coded reference pulse light and S-coded measurement pulse light by the second reflector M2 and the first beam splitter BS1 for rough measurement of underwater distance;

[0024] The detection signal is received by the photoelectric detector PD to obtain the rough distance;

[0025] Step 3-3: Perform refractive index correction by measuring the refractive index twice in air and water respectively;

[0026] The process of refractive index correction in air involves:

[0027] After passing through the first polarization beam splitter PBS1 and the second beam splitter BS2, the other S-polarized light is incident on the Michelson interferometer after passing through the first reflector M1.

[0028] The reflected light from the reference reflector RM1 and the target reflector MM1 is combined and interfered with the S local oscillator pulse light emitted by the local oscillator pulse laser LO at the fifth beam splitter BS5;

[0029] The interference fringes are received by the first balanced detector BPD1, and the optical path difference L in the air is obtained based on the interference fringes. a ;

[0030] The process of performing refractive index correction underwater includes:

[0031] After passing through the first polarization beam splitter PBS1 and the second beam splitter BS2, the other S-polarized light is incident on the Michelson interferometer after passing through the first reflector M1.

[0032] The reflected light from the reference reflector RM1 and the target reflector MM1 is combined and interfered with the S local oscillator pulse light emitted by the local oscillator pulse laser LO at the fifth beam splitter BS5;

[0033] The interference fringes are received by the first balanced detector BPD1, and the interference fringes are used to obtain the underwater optical path difference L w ;

[0034] Complete the underwater refractive index correction and obtain the corrected underwater refractive index n w =L w ·n g / L a ;

[0035] Step 4: Calculate the underwater distance L to be measured, as shown in the following formula:

[0036]

[0037] l NAR =c / (2n w ·f r )

[0038] Where c is the speed of light in vacuum, Δt is the time interval between the interference fringes of the dual-comb, and Δfr is the repetition frequency difference between the signal pulse laser SL and the local oscillator pulse laser LO, f r is the repetition frequency of the signal pulse laser SL, k is an integer, l NAR is the unambiguous range of measurement, n w is the corrected underwater refractive index.

[0039] Compared with the prior art, the present invention can achieve the following beneficial technical effects:

[0040] 1) The system cleverly combines dual-comb underwater measurement, pulse coding, and underwater refractive index correction to achieve high-precision, high-sampling-rate underwater absolute distance measurement, providing an excellent measurement solution for underwater detection and sensors.

[0041] 2) While maintaining accuracy and sampling rate in underwater distance measurement, the unambiguous range of measurement is expanded, enabling one-time large-scale, high-precision underwater measurement.

[0042] 3) The refractive index correction unit is used to provide real-time feedback of refractive index parameters during detection. The module can be applied to different water areas, ensuring high-precision underwater measurements. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is an optical path diagram of an underwater ranging system based on dual-comb pulse coding according to the present invention;

[0044] Figure 2 This is a flow chart of an underwater ranging method based on dual-comb pulse coding according to the present invention;

[0045] Figure 3 Schematic diagram of the dual-comb interference experiment results; (3a) Amplitude curve of the dual-comb interference wave, (3b) Normalized amplitude;

[0046] Figure 4 Schematic diagram of the pulse code interference experiment results; (4a) the amplitude curve of the pulse code interference wave, (4b) the normalized amplitude;

[0047] The reference numerals are as follows:

[0048] 1. Atomic clock 2. Signal pulse laser SL

[0049] 3. Local oscillator pulse laser LO 4. First half wave plate HWP1

[0050] 5. Second half-wave plate HWP2 6. First polarization beam splitter PBS1

[0051] 7. Second beam splitter BS2 8. Acousto-optic modulator AOM

[0052] 9. Second reflector M2 10. First beam splitter BS1

[0053] 11. Third beam splitter BS3 12. Measuring reflector MM2

[0054] 13. First reflector M1 14. Fourth beam splitter BS4

[0055] 15. Reference reflector RM1 16. Target reflector MM1

[0056] 17. Fifth beam splitter BS5 18. Second reflector M3

[0057] 19. Fourth collimator CL4 20. Balanced detector BPD1

[0058] 21. Third polarization beam splitter PBS3 22. Fourth polarization beam splitter PBS4

[0059] 23. Fifth collimator CL5 24. Sixth collimator CL6

[0060] 25. Fiber coupler FC 26. Seventh collimator CL7

[0061] 27. Photodetector PD 28. Second polarization beam splitter PBS2

[0062] 29. Third collimator CL3 30. Sixth beam splitter BS6

[0063] 31. Seventh beam splitter BS7 32. Fourth reflector M4

[0064] 33. Second balanced detector BPD2 34. Low-pass filter LPF

[0065] 35. First collimator CL1 36. Second collimator CL2

[0066] 37. Signal processing module SP 38. Added long optical fiber

[0067] 39. Water Tank DETAILED DESCRIPTION

[0068] The technical solution will be described in detail below with reference to the accompanying drawings and embodiments.

[0069] like Figure 1 Figure 2 shows the optical path of an underwater ranging system based on dual-comb pulse coding according to the present invention. The system mainly consists of three parts: a dual-comb underwater ranging unit for precise ranging, a pulse coding ranging unit for coarse ranging, and a refractive index correction unit.

[0070] The specific implementation process is as follows:

[0071] The two laser sources, the signal pulse laser SL 2 and the local oscillator pulse laser LO 3, are well locked to the atomic clock 1 to ensure the stability of the optical frequency comb.

[0072] The signal pulse laser SL2 emits a series of stable, equally spaced signal pulses through the first collimator CL135. After passing through the first half-wave plate HWP14 and the first polarization beam splitter PBS16, the pulses are split into P-polarized light (transmitted light) and S-polarized light (reflected light). The P-polarized light is used for dual-comb underwater ranging, while the S-polarized light is used for encoding and refractive index correction. The S-polarized light is then split into two S-polarized light paths after passing through the second beam splitter BS27: one path is used for the pulse code ranging unit, and the other is used for the refractive index correction unit. The local oscillator pulse light emitted by the local oscillator pulse laser LO 3 is divided into P-local oscillator pulse light (second polarization beam splitter PBS228 - sixth beam splitter BS630 section) and S-local oscillator pulse light (i.e., second polarization beam splitter PBS228 - third collimator CL329 - fourth collimator CL419 - fifth beam splitter BS517 section) after passing through the second collimator CL236, the second half-wave plate HWP25, and the second polarization beam splitter PBS228.

[0073] In the dual-comb underwater ranging unit, the P-polarized light is split into two pulsed beams, a P-reference pulse light and a P-measurement pulse light, after passing through the first beam splitter BS110. The P-reference pulse light, after passing through the third polarization beam splitter PBS321 and the sixth beam splitter BS630, is reflected to the seventh beam splitter BS731. The P-measurement pulse light, after passing through the third beam splitter BS311, travels an underwater distance L to reach the measurement reflector MM212, where it is reflected back to the third beam splitter BS311 along the same path. In the seventh beam splitter BS731, it is then combined and interfered with the P-reference pulse light and the P-local oscillator pulse light emitted by the local oscillator pulse laser LO 3, achieving underwater distance measurement using the dual-comb system. The interference signal is collected and processed by the second balanced detector BPD233, the low-pass filter LPF 34, and the signal processing module SP 27.

[0074] In the pulse coding underwater ranging unit, one channel of S-polarized light is incident on the acousto-optic modulator AOM 8 (here, the acousto-optic modulator AOM acts as an optical switch. By quickly controlling the optical switch, a part of the light pulses pass through, while the other part of the light pulses do not pass through, thereby completing the encoding of the light pulses) to realize pulse coding. Specifically, the acousto-optic modulator quickly controls the optical switch so that a part of the light pulses pass through, while the other part of the light pulses do not pass through, thereby completing the encoding of the S-polarized light in the dual-comb signal. The encoding sequence is {1,0,1,0,1,0,1,0,1,0,1,0,1,0,1,1,0,0,1,1,0,0,1,1,0}, and the period is set to 0.5ms, that is, a pulse coding sequence is generated every 0.5ms. Equivalently, the underwater ranging system theoretically obtains an unambiguous range of about 55 kilometers. NAR1 (l NAR1 =c·t p / (2·n w )≈55km), where c is the speed of light in vacuum, t p is the time interval between two coding sequences, n w is the underwater refractive index. Compared with the 3.01m dual-comb ranging without coding, NAR Compared with the above, a significant improvement has been achieved. The S-coded polarized light obtained after encoding is split into S-coded reference pulse light and S-coded measurement pulse light after passing through the second reflector M29 and the first beam splitter BS110. Similarly, the S-coded reference pulse light is reflected by the third polarization beam splitter PBS321 into the fifth collimator CL523. The S-coded measurement pulse light, after passing through the third beam splitter BS311, propagates through a section of underwater distance L (third beam splitter BS311-measurement reflector MM212 section) and is reflected back to the third beam splitter BS311 by the measurement reflector MM212 along the original path. Then, the S-coded measurement pulse light passes through the fourth polarization beam splitter PBS422 and the sixth collimator CL624 and is combined and interfered with the S-coded reference pulse light in the 2×1 fiber coupler FC 25 to complete the pulse-coded underwater distance measurement and obtain a rough underwater distance. A length of optical fiber is added between the sixth collimator CL624 and the fiber coupler FC25 because the pulse-coded signal is slightly longer and to prevent signal aliasing during measurement. Pulse coding is performed by a pulse coding module 39, which consists of a first polarization beam splitter PBS16, a second beam splitter BS27, an acousto-optic modulator AOM8, a second reflector M29, and a first beam splitter BS110.

[0075] The dual-comb underwater ranging system and the pulse code underwater ranging system share common components: the first beam splitter BS110–third polarization beam splitter PBS321 and the first beam splitter BS110–third beam splitter BS311–measurement reflector MM212–third beam splitter BS311–fourth polarization beam splitter PBS422 segments. During measurement, since the polarization directions of P-polarized light and S-polarized light are perpendicular to each other, interference between the signals is eliminated. Furthermore, the third polarization beam splitter PBS321 and the fourth polarization beam splitter PBS422, both of which have excellent extinction ratios, are used to receive both the dual-comb underwater ranging signal and the pulse code signal. Since P-polarized light can only be transmitted but not reflected by polarization beam splitters, the P-reference pulse light directly passes through the third polarization beam splitter PBS321 and enters the sixth beam splitter BS630. Similarly, the P-measurement pulse light directly passes through the fourth polarization beam splitter PBS422 and enters the seventh beam splitter BS731. The S-coded reference pulse light is reflected by the third polarization beam splitter PBS311 into the fifth collimator CL523 , and the S-coded measurement pulse light is reflected by the fourth polarization beam splitter PBS422 into the sixth collimator CL624 .

[0076] In the underwater refractive index correction unit, measurements are primarily performed using S-polarized light. After passing through the second beam splitter BS27, another path of S-polarized light is incident on the Michelson interferometer via the first reflector M113. This light is then reflected back along the same path by the reference reflector RM115 and the target reflector MM116, located in the water tank 39, to the fifth beam splitter BS517, where it is used as the S-polarized light for refractive index correction. This S-polarized light is then combined and interfered with the S-polarized light emitted by the local oscillator pulse laser LO 3, producing interference fringes. The specific algorithm for the underwater refractive index correction unit is as follows:

[0077] Two measurements are performed in air and underwater to obtain the optical path difference L in air a The optical path difference L under water w , then through the two optical path differences and the air group refractive index n g Complete the correction of underwater refractive index, underwater refractive index n w =L w ·n g / L a The unit can be applied to different water areas, ensuring high-precision underwater measurement;

[0078] The underwater refractive index is obtained according to the refractive index correction module, and the precise measured distance L is calculated as shown in the following formula:

[0079]

[0080] Where c is the speed of light in vacuum, Δt is the time interval between the dual-comb interference fringes (i.e., the time interval between the two peaks after the reference interference fringes and the measured interference fringes are Hilbert transformed), and Δf r is the repetition frequency difference between the signal pulse laser SL and the local oscillator pulse laser LO, f r is the repetition frequency of the signal pulse laser SL, k is an integer, l NAR is the unambiguous range of measurement, l NAR =c / (2n w ·f r ), assuming the repetition frequency f r 36.5MHz, l NAR About 3.01m.

[0081] When measuring distance, the high repetition rate of an optical frequency comb, typically ranging from hundreds of MHz to over GHz, results in a narrow unambiguous detection range. However, this invention incorporates pulse coding technology to encode the optical frequency comb pulses, while simultaneously ensuring detection accuracy and enabling long-distance measurement.

[0082] Here, an acousto-optic modulator (AOM) is used to implement optical pulse coding, where the coding result is as follows: Figure 2 Here, the coded sequence is {1,0,1,0,1,0,1,0,1,0,0,1,0,1,1,0,1,0,0,1,1,0,0,1,1,0}, and the period is set to 0.5ms, that is, a pulse coding sequence is generated every 0.5ms. Equivalently, the underwater ranging system theoretically obtains an unambiguous range of about 55 kilometers. NAR1 (l NAR1 =c·t p / (2·n w )≈55km). Compared with the 3.01m dual-comb ranging without coding NAR Compared with the previous results, it has been significantly improved.

[0083] like Figure 2 As shown, a flow chart of an underwater ranging method based on dual-comb pulse coding of the present invention.

[0084] Step 1: Lock the frequency and phase of the two lasers based on the atomic clock;

[0085] Step 2: Split the light into P-polarized light and S-polarized light at the first polarization beam splitter PBS16;

[0086] Step 3-1: Complete dual-comb measurement based on P-polarized light. This specifically includes the following processing:

[0087] At the first beam splitter BS110, it is divided into P-reference pulse light and P-measurement pulse light;

[0088] The P-reference pulse light passes through the first beam splitter BS110, the third polarization beam splitter PBS321, and the sixth beam splitter BS630 and is reflected to the seventh beam splitter BS731. The P-measurement pulse light passes through the first beam splitter BS110, the third beam splitter BS311, and the measurement reflector MM212 and is reflected, and then passes through the first polarization beam splitter PBS422 and is incident to the seventh beam splitter BS731.

[0089] In the seventh beam splitter BS731, the two pulsed lights, the P-reference pulse light and the P-measurement pulse light, are combined and interfered with the P-local oscillator pulse light emitted by the local oscillator pulse laser LO 3.

[0090] The interference fringes are received by the second balanced detector BPD233, and after performing Hilbert transform on the interference fringes, the time interval Δt based on the dual-comb interferometry is obtained;

[0091] Step 3-2: Complete pulse coding underwater ranging and underwater refractive index correction based on S-polarized light; this includes the following processing:

[0092] The S-polarized light obtained after passing through the first polarization beam splitter PBS16 and the second beam splitter BS27 is incident on the acousto-optic modulator AOM 8;

[0093] Pulse encoding of S-polarized light in the acousto-optic modulator AOM 8;

[0094] The S-coded polarized light obtained after encoding is split into S-coded reference pulse light and S-coded measurement pulse light by the second reflector M29 and the first beam splitter BS110, and is used to roughly measure the underwater distance;

[0095] The detection signal is received by the photoelectric detector PD 27 to obtain a rough distance measurement;

[0096] Step 3-3, perform refractive index correction, in air and underwater respectively:

[0097] The process of refractive index correction in air involves:

[0098] The other S-polarized light obtained after passing through the first polarization beam splitter PBS16 and the second beam splitter BS27 passes through the first reflector M113 and is incident on the Michelson interferometer. The Michelson interferometer consists of a fourth beam splitter BS414, a reference reflector RM115 and a target reflector MM116.

[0099] The reflected light from the reference reflector RM115 and the target reflector MM116 is combined and interfered with the S local oscillator pulse light emitted by the local oscillator pulse laser LO 3 at the fifth beam splitter BS517;

[0100] The interference fringes are received by the first balanced detector BPD120, and the interference fringes are used to obtain the optical path difference L in the air. a ;

[0101] The process of performing refractive index correction underwater includes:

[0102] The other S-polarized light obtained after passing through the first polarization beam splitter PBS16 and the second beam splitter BS27 passes through the first reflector M113 and is incident on the Michelson interferometer. The Michelson interferometer consists of a fourth beam splitter BS414, a reference reflector RM115 and a target reflector MM116.

[0103] The reflected light from the reference reflector RM115 and the target reflector MM116 is combined and interfered with the S local oscillator pulse light emitted by the local oscillator pulse laser LO 3 at the fifth beam splitter BS517;

[0104] The interference fringes are received by the first balanced detector BPD120, and the interference fringes are used to obtain the underwater optical path difference L w ;

[0105] Then, by the two optical path differences and the air group refractive index n g Complete the correction of underwater refractive index, underwater refractive index n w =L w ·n g / L a ;

[0106] Step 4: Calculate the precise underwater distance L to be measured, as shown in the following formula:

[0107]

[0108] Where c is the speed of light in vacuum, Δt is the time interval between the dual-comb interference fringes (i.e., the time interval between the two peaks after the reference interference fringes and the measured interference fringes are Hilbert transformed), and Δf r is the repetition frequency difference between the signal pulse laser SL and the local oscillator pulse laser LO, f r is the repetition frequency of the signal pulse laser SL, k is an integer, l NAR is the unambiguous range of measurement, l NAR =c / (2n w ·f r ), assuming the repetition frequency f r 36.5MHz, l NAR About 3.01m.

[0109] like Figure 3The following is a schematic diagram of the dual-comb interferometry experimental results. Among them, (3a) is the interference fringe amplitude curve; (3b) is the normalized result curve after the Hilbert change of the interference fringe amplitude.

[0110] like Figure 4 Shown are the experimental results of pulse coding.

[0111] In summary, the present invention uses dual-comb technology and pulse coding technology as basic principles, combined with the constructed refractive index correction module to correct the refractive index of water in real time, extending the unambiguous range of measurement and correcting the refractive index in real time without sacrificing accuracy and sampling rate, thereby achieving one-time high-precision, high sampling rate, and long-distance underwater long-distance measurement with a large unambiguous range.

[0112] Specifically, the dual-comb underwater measurement method, pulse coding technology, and underwater refractive index correction module are combined. When measuring underwater distance, the dual-comb detection technology and pulse coding technology are combined to expand the non-ambiguous range of measurement without losing accuracy and sampling rate, thereby achieving one-time large-scale and high-precision underwater measurement. During underwater detection, a refractive index correction module is built so that it can provide real-time feedback of refractive index parameters during detection. This module is applicable to different water areas, ensuring high-precision underwater measurement. This system can achieve high-precision, high-sampling-rate underwater absolute distance measurement, providing a good detection solution for underwater detection and sensors.

[0113] The present invention provides examples of the models of various devices as follows:

[0114] The atomic clock model is Microsemi 8040.

[0115] The model of the signal pulse laser SL is Menlo System-515.

[0116] The model of the local oscillator pulse laser LO is Menlo System-515.

[0117] The first and second half-wave plates HWP1 and HWP2 are Thorlabs / WPH05M-514.

[0118] The first to fourth polarization beam splitters PBS1, PBS2, PBS3, and PBS4 are of Thorlabs / PBS251 model.

[0119] The first to seventh beam splitters BS1, BS2, BS3, B S4 , BS5, BS6, and BS7 is Thorlabs / BS013.

[0120] The model of the acousto-optic modulator AOM is GH-3110-120.

[0121] The first to fourth reflecting mirrors M1, M2, M3 and M4 are of Thorlabs / BB1-E02 model.

[0122] The reference mirror RM1 is Thorlabs model number / BB1-E02.

[0123] The target mirror MM1 and the measurement mirror MM2 are Thorlabs / BB1-E02.

[0124] The first to fifth collimators CL1 , CL2 , CL3 , CL4 and CL5 are of Thorlabs / F110APC-532 model.

[0125] The first and second balanced detectors BPD1 and BPD2 are of Thorlabs / PDB230A.

[0126] The photodetector PD is Thorlabs / APD430A.

[0127] The model of the fiber optic coupler FC is Thorlabs / PN530R5F1.

[0128] The model of the low-pass filter LPF is Mini-Circuits.

[0129] Unless otherwise specified, there is no restriction on the models of other devices, as long as the device can complete the above functions.

[0130] The above description is merely an embodiment of the present application and is not intended to limit the scope of protection claimed in the present invention. It will be apparent to those skilled in the art that any modifications, equivalent substitutions, or variations made without departing from the spirit and principles of the present invention fall within the scope of protection of the present invention as defined by the appended claims. It will be understood by those skilled in the art that the accompanying drawings are merely schematic diagrams of preferred embodiments, and that the serial numbers of the embodiments of the present invention are for descriptive purposes only and do not represent the merits of the embodiments.

Claims

1. An underwater ranging system based on dual-comb pulse coding, characterized in that: The system includes a signal pulse laser SL and a local oscillator pulse laser LO, a dual-comb ranging unit, a pulse coding ranging unit and a refractive index correction unit; wherein: The signal pulse laser SL emits a stable, equally spaced signal pulse sequence through the first collimator CL1, which is then split into P-polarized light and S-polarized light after passing through the first half-wave plate HWP1 and the first polarization beam splitter PBS1. The local oscillator pulse light emitted by the local oscillator pulse laser LO is divided into P-local oscillator pulse light and S-local oscillator pulse light after passing through the second collimator CL2, the second half-wave plate HWP2 and the second polarization beam splitter PBS2; The dual-comb ranging unit includes a first beam splitter BS1, a third polarization beam splitter PBS3, a sixth beam splitter BS6, a seventh beam splitter BS7, a third beam splitter BS3, and a measuring reflector MM2; the P polarized light is divided into two pulse lights, namely, a P-reference pulse light and a P-measurement pulse light, after passing through the first beam splitter BS1: the P-reference pulse light passes through the third polarization beam splitter PBS3 and the sixth beam splitter BS6 and is reflected to the seventh beam splitter BS7, and the P-measurement pulse light passes through the third beam splitter BS3 and passes through an underwater distance L The light propagates to the measuring reflector MM2, and is reflected back to the third beam splitter BS3 by the measuring reflector MM2 along the original path. In the seventh beam splitter BS7, it is combined and interfered with by the P-reference pulse light and the P-local oscillator pulse light emitted by the local oscillator pulse laser LO. The interference signal is collected and processed by the second balanced detector BPD2, the low-pass filter LPF and the signal processing module SP. After the interference fringes are Hilbert transformed, the time interval Δt based on the dual-comb interferometry measurement is obtained, thereby realizing the accurate measurement of the underwater distance of the dual-comb. The pulse code ranging unit includes an acousto-optic modulator (AOM), a second reflector M2, a first beam splitter BS1, a third polarization beam splitter PBS3, a fifth collimator CL5, a third beam splitter BS3, a measurement reflector MM2, a fourth polarization beam splitter PBS4, a sixth collimator CL6, and a 2×1 fiber coupler FC. An S-coded polarized light is obtained after being incident on the acousto-optic modulator (AOM) and encoded. The S-coded polarized light is then split into an S-coded reference pulse light and an S-coded measurement pulse light after passing through the second reflector M2 and the first beam splitter BS1. The S-coded reference pulse light is reflected by the third polarization beam splitter PBS3 to the fifth collimator CL5. After passing through the third beam splitter BS3, the S-coded measurement pulse light propagates through a distance L underwater and is reflected back to the third beam splitter BS3 by the measurement reflector MM2 along the original path. The S-coded measurement pulse light passes through the fourth polarization beam splitter PBS4 and the sixth collimator CL6 and is combined with the S-coded reference pulse light in the 2×1 fiber coupler FC to achieve underwater distance measurement based on pulse coding and obtain a rough underwater distance. The refractive index correction unit includes a second beam splitter BS2, a first reflector M1, a reference reflector RM1, a target reflector MM1, and a fifth beam splitter BS5; another path of S polarized light is incident from the first reflector M1 to a Michelson interferometer composed of a fourth beam splitter BS4, a reference reflector RM1, and a target reflector MM1, and is combined and interfered with the S local oscillator pulse light emitted by the local oscillator pulse laser LO, and the interference fringes are received by the first balanced detector BPD1; during measurement, it is necessary to measure once in air and once in water, respectively, to obtain the optical path difference L in air a The optical path difference L in water w .

2. The underwater ranging system based on dual-comb pulse coding according to claim 1, characterized in that: The acousto-optic modulator (AOM) implements pulse coding, and the coding sequence is {1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0}, generating a pulse coding sequence every 0.5 ms.

3. An underwater ranging method based on dual-comb pulse coding, characterized in that: The method specifically comprises the following steps: Step 1: Lock the frequency and phase of the two lasers according to the atomic clock; Step 2: Split the light into P-polarized light and S-polarized light at the first polarization beam splitter PBS1; Step 3-1: Complete dual-comb measurement based on P-polarized light. This specifically includes the following processing: At the first beam splitter BS1, it is divided into P-reference pulse light and P-measurement pulse light; The P-reference pulse light passes through the first beam splitter BS1, the third polarization beam splitter PBS3, and the sixth beam splitter BS6, and then reflects to the seventh beam splitter BS7. The P-measurement pulse gloss passes through the first beam splitter BS1, the third beam splitter BS3, and the measurement reflector MM2, and then passes through the first polarization beam splitter PBS4 and is incident to the seventh beam splitter BS7. In the seventh beam splitter BS7, the two pulse lights, the P-reference pulse light and the P-measurement pulse light, are combined and interfered with the P-local oscillator pulse light emitted by the local oscillator pulse laser LO. The interference fringes are received by the second balanced detector BPD2, and the interference fringes are subjected to Hilbert transform to obtain the time interval Δt measured based on the dual-comb interferometry. Step 3-2: Complete pulse coding underwater ranging and underwater refractive index correction based on S-polarized light; this includes the following processing: The S-polarized light obtained after passing through the first polarization beam splitter PBS1 and the second beam splitter BS2 is incident on the acousto-optic modulator AOM; Pulse encoding of S-polarized light in an acousto-optic modulator (AOM); The S-coded polarized light obtained after encoding is split into S-coded reference pulse light and S-coded measurement pulse light by the second reflector M2 and the first beam splitter BS1 for rough measurement of underwater distance; The detection signal is received by the photoelectric detector PD to obtain the rough distance; Step 3-3: Perform refractive index correction by measuring the refractive index twice in air and water respectively; The process of refractive index correction in air involves: After passing through the first polarization beam splitter PBS1 and the second beam splitter BS2, the other S-polarized light is incident on the Michelson interferometer after passing through the first reflector M1. The reflected light from the reference reflector RM1 and the target reflector MM1 is combined and interfered with the S local oscillator pulse light emitted by the local oscillator pulse laser LO at the fifth beam splitter BS5; The interference fringes are received by the first balanced detector BPD1, and the optical path difference L in the air is obtained based on the interference fringes. a ; The process of performing refractive index correction underwater includes: After passing through the first polarization beam splitter PBS1 and the second beam splitter BS2, the other S-polarized light is incident on the Michelson interferometer after passing through the first reflector M1. The reflected light from the reference reflector RM1 and the target reflector MM1 is combined and interfered with the S local oscillator pulse light emitted by the local oscillator pulse laser LO at the fifth beam splitter BS5; The interference fringes are received by the first balanced detector BPD1, and the interference fringes are used to obtain the underwater optical path difference L w ; Complete the underwater refractive index correction and obtain the corrected underwater refractive index n w =L w ·n g / L a , n g is the group refractive index of air; Step 4: Calculate the underwater distance L to be measured, as shown in the following formula: <h2 style=";text-align:left;direction:ltr">l<h2 style=";text-align:left;direction:ltr"> NAR <h2 style=";text-align:left;direction:ltr"> =c / (2n<h2 style=";text-align:left;direction:ltr"> w <h2 style=";text-align:left;direction:ltr"> ·f<h2 style=";text-align:left;direction:ltr"> r <h2 style=";text-align:left;direction:ltr"> ) Where c is the speed of light in vacuum, Δt is the time interval between the interference fringes of the dual-comb, and Δf r is the repetition frequency difference between the signal pulse laser SL and the local oscillator pulse laser LO, f r is the repetition frequency of the signal pulse laser SL, k is an integer, l NAR is the unambiguous range of measurement, n w is the corrected underwater refractive index.

4. The underwater ranging method based on dual-comb pulse coding according to claim 3, characterized in that: The acousto-optic modulator (AOM) implements pulse coding, and the coding sequence is {1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0}, generating a pulse coding sequence every 0.5 ms.

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