Water body temperature profile coherent measurement system and method based on pseudo-random code modulation

Abstract

The invention relates to a water body temperature profile coherent measurement system and method based on pseudo-random code modulation, and the method comprises the steps: enabling laser output by a laser seed source to firstly pass through an optical isolator and then pass through a first coupler, and enabling the laser to be divided into two parts of light beams which are respectively output to an electro-optic phase modulator and an acousto-optic modulator after passing through the first coupler; the electro-optic phase modulator, the optical circulator and the telescope are connected in sequence; the telescope is connected with the second coupler through the optical circulator; the acousto-optic modulator, the second coupler, the photoelectric detector, the amplifier, the dual-channel AD acquisition card and the computer are connected in sequence; the signal generator is connected with the acousto-optic frequency shifter; the arbitrary waveform generator is connected with the dual-channel AD acquisition card and the electro-optic phase modulator; and the external trigger is connected with the arbitrary waveform generator and the dual-channel AD acquisition card. The problem that high-range-resolution and high-precision water body profile temperature measurement is difficult to realize by a laser radar-based ocean temperature profile active detection technology is solved, and high-range-resolution and high-precision water body profile temperature information can be obtained by utilizing a pseudo-random code phase modulation technology and a heterodyne detection technology.

Description

Technical Field

[0001] The present invention relates to the technical field of lidar, and particularly to a water temperature profile measurement system and method with strong anti-interference ability, high-precision temperature, and high spatial resolution. Background Art

[0002] Laser remote sensing technology is the only technical means expected to obtain ocean temperature profiles over a large area. As an active optical detection means, it has the advantages of high spatial resolution, large measurement range, and fast response. It can obtain ocean temperature profile data over a large area and shows a broader application prospect.

[0003] Currently, ocean temperature measurement technologies are mainly divided into two categories, namely active detection and passive detection. Among them, although passive detection has a wide measurement range, it can only obtain sea surface temperature information and cannot effectively detect ocean profile temperature information. For the active detection technology of ocean temperature profiles based on lidar, its range resolution is usually inversely proportional to the pulse width. However, if the pulse width is too narrow, the echo energy will be too small, which is extremely disadvantageous for the detection depth. Therefore, it is necessary to unlock the coupling relationship between the pulse width and the range resolution. Summary of the Invention

[0004] Aiming at the problem that the active detection technology of ocean temperature profiles based on lidar is difficult to achieve high range resolution and high-precision water body profile temperature measurement, and can ensure a high signal-to-noise ratio and at the same time have a strong anti-interference ability, a coherent measurement system and method for water temperature profiles based on pseudo-random code modulation are proposed. The coherent measurement method using pseudo-random code phase modulation can unlock the relationship between the pulse width and the range resolution. This technology can achieve high range resolution measurement on the basis of reducing the peak power of the system by phase modulating the laser pulse, and its range resolution is determined by the modulation rate. Moreover, because the system adopts coherent detection, the system has a high signal-to-noise ratio and can extract Brillouin spectral information, opening up a new way for the accurate measurement of water temperature profiles.

[0005] The technical solution of the present invention is as follows:

[0006] A water temperature profile coherent measurement system based on pseudo-random code modulation, comprising a laser seed source, an optical isolator, a first coupler, an electro-optic phase modulator, an optical circulator, a telescope, a signal generator, an acousto-optic modulator, a second coupler, a photodetector, an arbitrary waveform generator, an external trigger, a computer, a dual-channel AD acquisition card, and an amplifier; the output end of the laser seed source is connected to the input end of the optical isolator, the output end of the optical isolator is connected to the input end of the first coupler, and the output end of the first coupler is respectively connected to the first input end of the electro-optic phase modulator and the first input end of the acousto-optic modulator; the output end of the electro-optic phase modulator is connected to the first port of the optical circulator, and the outgoing laser output from the second port of the optical circulator is emitted by the coaxial transceiver telescope. After the outgoing laser is diffusely reflected by the water body to be measured, it is received by the same telescope. The third port of the optical circulator is connected to the first input end of the second coupler, and the output end of the signal generator is connected to the input end of the acousto-optic modulator for providing a driving signal. The output end of the acousto-optic modulator is connected to the second input end of the second coupler. The laser echo signal light carrying the temperature information of the water body to be measured is collected by the same telescope and then undergoes a coherent effect with the local oscillator signal light output from the acousto-optic modulator in the coupler through the third port of the optical circulator; the output end of the second coupler is connected to the input end of the photodetector, the output end of the photodetector is connected to the input end of the amplifier, the output end of the amplifier is connected to the first input end of the dual-channel AD acquisition card, and the output end of the dual-channel AD acquisition card is connected to a computer with signal acquisition, processing, control, calculation, and display functions; the output end of the arbitrary waveform generator is respectively connected to the second input end of the dual-channel AD acquisition card and the second input end of the electro-optic phase modulator for providing the same pseudo-random code driving signal; the output end of the external trigger is respectively connected to the input end of the arbitrary waveform generator and the input end of the dual-channel AD acquisition card for ensuring the time synchronization of the arbitrary waveform generator and the dual-channel AD acquisition card.

[0007] Further, the laser output by the laser seed source first passes through an optical isolator and then through a first coupler. After passing through the first coupler, it is divided into two parts; most of the laser passes through an electro-optic phase modulator and an optical circulator in sequence and is then emitted by a transmitting telescope; a small part of the laser is frequency-shifted by an acousto-optic modulator to serve as a local oscillator signal light; the telescope receives the echo signal light generated by the diffuse reflection of the water body to be measured and sends the diffuse reflection echo signal light of the water body to be measured into a second coupler through the optical circulator. The local oscillator signal light is also sent into the second coupler. The coherent light of the echo signal light and the local oscillator signal light output by the second coupler sequentially pass through a photodetector and an amplifier and are then collected by a dual-channel AD acquisition card. At the same time, the dual-channel AD acquisition card also collects the drive signal sent by an arbitrary waveform generator and makes different time delays for the drive signal; the output signal of the dual-channel AD acquisition card enters a computer. Since the water body will undergo continuous scattering under laser irradiation, the echo signal light received by the telescope is scattered light and also continuous light, and the local oscillator signal light is also continuous light. Therefore, the light entering the second coupler is all continuous light; in the computer, the coherent signal output by the amplifier is multiplied by the sequentially shifted drive signals and undergoes a fast Fourier transform to obtain the coherent light spectrum at different depths, and after processing, the Brillouin frequency shift f is obtained. brillouin 。

[0008] Preferably, the laser seed source is a 532 nm single-longitudinal-mode fiber continuous seed source.

[0009] Preferably, the electro-optic phase modulator is a lithium niobate electro-optic phase modulator.

[0010] Preferably, the first coupler is a 10:90 optical coupler, where 10% of the light enters the acousto-optic modulator and is used as the local oscillator light; 90% of the light enters the electro-optic phase modulator, and the electro-optic phase modulator performs phase modulation according to a pseudo-random code drive signal and is emitted through the telescope.

[0011] Preferably, the telescope is a telescope system with a common optical axis for transmission and reception.

[0012] Preferably, the signal generator is a sine signal generator.

[0013] A method for coherent measurement of water temperature profile based on pseudo-random code modulation is implemented based on the above-described coherent measurement system for water temperature profile based on pseudo-random code modulation, and includes the following steps:

[0014] Step 1: The laser output by the laser seed source first passes through an optical isolator and then through a first coupler. After passing through the first coupler, it is divided into two parts; most of the laser passes through an electro-optic phase modulator and an optical circulator in sequence and is then emitted by the telescope; a small part of the laser is frequency-shifted by an acousto-optic modulator to serve as a local oscillator signal light;

[0015] Step 2: The laser emitted by the telescope enters the water body to be measured. The water body to be measured returns the echo signal light. The telescope receives the echo signal light of the water body to be measured and sends the echo signal light of the water body to be measured into the second coupler through the optical circulator. The local oscillator signal light is also sent into the second coupler. The coherent light of the echo signal light and the local oscillator signal light output by the second coupler passes through the photodetector and the amplifier in sequence, and is collected by the dual-channel AD acquisition card. At the same time, the dual-channel AD acquisition card also collects the pseudo-random code driving signal generated by the arbitrary waveform generator and makes different time delays to the pseudo-random code driving signal; the output signal of the dual-channel AD acquisition card enters the computer. Due to the continuous scattering of the water body, the scattered light received by the receiving telescope is continuous light, and the local oscillator light is also continuous light. Therefore, the light entering the second coupler is continuous light; in the computer, the coherent signal output by the amplifier is multiplied by the driving signal that is sequentially shifted and undergoes a fast Fourier transform to obtain the coherent light signal spectrum at different depths. This coherent light signal spectrum, that is, the peak signal, is contributed by the scattering signals of seawater at different depths; the reference light f o -f aom is mixed with the Brillouin scattering signal f o -f brillouin . The photodetector outputs the difference frequency signal of the two. The peak of the spectrum is located at the difference frequency of the driving frequency f aom of the acousto-optic modulator and the Brillouin frequency shift f brillouin , that is, f brillouin -f aom ; Adding f aom to this frequency can obtain the Brillouin frequency shift f brillouin ;

[0016] Step 3: Substitute the Brillouin frequency shift f brillouin into Professor Fry's empirical formula: T(S, v B ) = t0 + t1(v B -7.5) + t2(v B -7.5) 2 + t3(v B -7.5) 3 + t4(v B -7.5) 6 + S[t5 +

[0017] t6(v B -7.5) + t7(v B -7.5) 2 + t8(v B -7.5) 3 , where S is the salinity of the water body in units of 1‰; T is the water temperature in degrees Celsius; v B = f brillouinis the Brillouin frequency shift, in GHz; t0 = 23.5, t1 = 65.5, t2 = 75, t3 = 252, t4 = 1100, t5 = -0.402, t6 = -0.287, t7 = -0.902, t8 = -5.5; assuming the water salinity is known, the water temperature T can be deduced from the Brillouin frequency shift f brillouin obtained by calculation.

[0018] The beneficial effects of the present invention are as follows:

[0019] 1. The system coherently amplifies the weak scattered signal containing Brillouin spectrum information through a specific signal processing method, greatly improving the system's detection ability for weak signals, enabling more accurate detection of the water body profile temperature information, and avoiding measurement errors or information loss caused by overly weak signals.

[0020] 2. The system operates in a state of wide pulse width and low peak power, significantly reducing the requirement for the system's peak power. It reduces equipment losses caused by high power and is also conducive to the miniaturization and portability design of the system.

[0021] 3. The pseudo-random code modulation technology is adopted in the wide pulse width, ensuring that the system has excellent range resolution. It can more finely divide different depth levels of the water body temperature profile, obtain more detailed temperature distribution information, and is of great significance for studying the vertical structure and temperature change law of the water body.

[0022] 4. Different pseudo-random codes are modulated during the repeated detection process, ensuring the consistency of the measured signals, making the multiple measurement results highly comparable and reliable. At the same time, it also ensures the randomness of the noise, which is conducive to better suppressing noise interference during the signal processing process and improving the measurement accuracy. Description of the Drawings

[0023] Figure 1 is the overall structural block diagram of the coherent measurement system for the water body temperature profile with pseudo-random code phase modulation provided by the embodiment of the present invention.

[0024] Reference Signs in the Drawings:

[0025] 1. Laser seed source; 2. Optical isolator; 3. First coupler; 4. Electro-optic phase modulator; 5. Optical circulator; 6. Telescope; 7. Signal generator; 8. Acousto-optic modulator; 9. Second coupler; 10. Photoelectric detector; 11. Arbitrary waveform generator; 12. External trigger; 13. Computer; 14. Dual-channel AD acquisition card; 15. Amplifier. Detailed Embodiments

[0026] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and the detailed implementation manner and specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0027] Overall idea:

[0028] A coherent measurement system for water temperature profile based on pseudo-random code modulation includes a laser seed source. The laser output by the laser seed source first passes through an optical isolator and then through a first coupler. After passing through the first coupler, the laser is divided into two parts of light beams that are respectively output to an electro-optic phase modulator and an acousto-optic modulator; the electro-optic phase modulator, an optical circulator, and a telescope are connected in sequence; the telescope is connected to a second coupler through the optical circulator; the acousto-optic modulator, the second coupler, a photodetector, an amplifier, a dual-channel AD acquisition card, and a computer are connected in sequence; a signal generator is connected to an acousto-optic frequency shifter; an arbitrary waveform generator is connected to the dual-channel AD acquisition card and the electro-optic phase modulator; an external trigger is connected to the arbitrary waveform generator and the dual-channel AD acquisition card. The present invention also provides a method for coherent measurement of water temperature profile by pseudo-random code modulation. The present invention jointly utilizes pseudo-random code phase modulation technology and heterodyne detection technology. The system operates at a wide pulse width and low peak power. Through coherent detection and fast Fourier transform algorithm, high-precision water profile temperature information can be obtained with high range resolution.

[0029] Specific implementation:

[0030] A coherent measurement system for water temperature profile based on pseudo-random code modulation includes a laser seed source, an optical isolator, a first coupler, an electro-optic phase modulator, an optical circulator, a telescope, a signal generator, an acousto-optic modulator, a second coupler, a photodetector, an arbitrary waveform generator, an external trigger, a computer, a dual-channel AD acquisition card, and an amplifier;

[0031] The output end of the laser seed source is connected to the input end of the optical isolator, the output end of the optical isolator is connected to the input end of the first coupler, and the output end of the first coupler is respectively connected to the first input end of the electro-optic phase modulator and the first input end of the acousto-optic modulator;

[0032] The output end of the electro-optic phase modulator is connected to the first port of the optical circulator. The outgoing laser output from the second port of the optical circulator is emitted by a transceiver coaxial telescope. After the outgoing laser is diffusely reflected by the water body to be measured, it is received by the same telescope. The third port of the optical circulator is connected to the first input end of the second coupler. The output end of the signal generator is connected to the input end of the acousto-optic modulator to provide a driving signal. The output end of the acousto-optic modulator is connected to the second input end of the second coupler. The laser echo signal light carrying the water temperature information to be measured is collected by the same telescope and coherently interacts with the local oscillator signal light output by the acousto-optic modulator in the coupler through the third port of the optical circulator;

[0033] The output end of the second coupler is connected to the input end of the photodetector, the output end of the photodetector is connected to the input end of the amplifier, the output end of the amplifier is connected to the first input end of the dual-channel AD acquisition card, and the output end of the dual-channel AD acquisition card is connected to a computer with signal acquisition, processing, control, calculation and display functions;

[0034] The output end of the arbitrary waveform generator is respectively connected to the second input end of the dual-channel AD acquisition card and the second input end of the electro-optic phase modulator to provide the same pseudo-random code driving signal; the output end of the external trigger is respectively connected to the input end of the arbitrary waveform generator and the input end of the dual-channel AD acquisition card to ensure the time synchronization of the arbitrary waveform generator and the dual-channel AD acquisition card.

[0036] Figure 1 It is the overall structural block diagram of the water temperature profile coherent measurement system based on pseudo-random code phase modulation provided in this embodiment. The water temperature profile coherent measurement system based on pseudo-random code phase modulation includes a laser seed source 1, an optical isolator 2, a first coupler 3, an electro-optic phase modulator 4, an optical circulator 5, a telescope 6, a signal generator 7, an acousto-optic modulator 8, a second coupler 9, a photodetector 10, an arbitrary waveform generator 11, an external trigger 12, a computer 13, a dual-channel AD acquisition card 14, and an amplifier 15;

[0037] The laser output by the laser seed source 1 first passes through the optical isolator 2 and then through the first coupler 3, and is divided into two parts after passing through the first coupler 3; most of the laser passes through the electro-optic phase modulator 4 and the optical circulator 5 in sequence and is emitted by the transmitting telescope 6; a small part of the laser is frequency-shifted by the acousto-optic modulator 8 to be used as the local oscillator signal light.

[0038] The telescope 6 receives the echo signal light generated by the diffuse reflection of the water body to be measured, and sends the diffuse reflection echo signal light of the water body to be measured into the second coupler 9 through the circulator 5. The local oscillator signal light is also sent into the second coupler 9. The coherent light of the echo signal light and the local oscillator signal light output by the second coupler 9 passes through the photodetector 10 and the amplifier 15 in sequence, and is collected by the dual-channel AD acquisition card 14. At the same time, the dual-channel AD acquisition card 14 also collects the drive signal sent by the arbitrary waveform generator 11 and makes different time delays for the drive signal; the output signal of the dual-channel AD acquisition card 14 enters the computer 13. Since the water body will undergo continuous scattering under laser irradiation, the echo signal light received by the telescope 6 is scattered light and also continuous light, and the local oscillator signal light is also continuous light. Therefore, the light entering the second coupler 9 is all continuous light. In the computer 13, the coherent signal output by the amplifier 15 is multiplied by the drive signal shifted in sequence and undergoes fast Fourier transform to obtain the coherent light spectrum at different depths. This coherent light spectrum (i.e., the peak signal) is contributed by the scattering signals of seawater at different depths. The reference light f o -f aom is mixed with the Brillouin scattering signal f o -f brillouin . The photodetector outputs the difference frequency signal of the two, and the peak of the spectrum is located at the difference frequency of the driving frequency f aom of the acousto-optic modulator and the Brillouin frequency shift f brillouin , that is, f brillouin -f aom . Adding f aom to this frequency can obtain the Brillouin frequency shift f brillouin .

[0039] The specific devices used in this embodiment are: the laser seed source 1 is a 532nm single-longitudinal-mode fiber continuous seed source; the electro-optic phase modulator 4 is a lithium niobate high-speed electro-optic phase modulator; the arbitrary waveform generator 11 has two output channels, which respectively provide drive signals for the electro-optic phase modulator 4 and pseudo-random code signals for the dual-channel AD acquisition card 14; the first coupler 2 is a 10:90 optical coupler. Among them, 10% of the light is frequency-shifted by the acousto-optic modulator 8 to be used as the local oscillator signal light; 90% of the light is phase-modulated according to the pseudo-random code signal by the electro-optic phase modulator 4 and the laser is emitted through the telescope 6; the telescope 6 is a telescope system with a common optical axis for emission and reception.

[0040] The present invention also provides a method for coherent measurement of water body temperature profile with pseudo-random code phase modulation, using the above-mentioned pseudo-random code modulated water body temperature profile coherent measurement system, including the following steps:

[0041] Step 1: The laser output from the laser seed source 1 first passes through the optical isolator 2 and then through the first coupler 3. After passing through the first coupler 3, it is divided into two parts; most of the laser passes through the electro-optic phase modulator 4 and the optical circulator 5 in sequence and is then emitted by the telescope 6; a small part of the laser is frequency-shifted by the acousto-optic modulator 8 and used as the local oscillator signal light.

[0042] Step 2: The laser emitted by the telescope 6 is incident on the water body to be measured. The water body to be measured returns the echo signal light. The telescope 6 receives the echo signal light of the water body to be measured and sends the echo signal light of the water body to be measured into the second coupler 9 through the optical circulator 5. The local oscillator signal light is also sent into the second coupler 9. The coherent light of the echo signal light and the local oscillator signal light output by the second coupler 9 passes through the photodetector 10 and the amplifier 15 in sequence and is then collected by the dual-channel AD acquisition card 14. At the same time, the dual-channel AD acquisition card 14 also collects the pseudo-random code drive signal generated by the arbitrary waveform generator and makes different time delays for the drive signal; the output signal of the dual-channel AD acquisition card 14 enters the computer 13. Due to the continuous scattering of the water body, the scattered light received by the receiving telescope 6 is continuous light, and the local oscillator light is also continuous light. Therefore, the light entering the second coupler 9 is all continuous light. In the computer 13, the coherent signal output by the amplifier 15 is multiplied by the sequentially shifted drive signals and undergoes a fast Fourier transform to obtain the coherent light signal spectrum at different depths. This coherent light signal spectrum (i.e., the peak signal) is contributed by the scattered signals of seawater at different depths. The reference light f o -f aom modulated by the acousto-optic modulator is mixed with the Brillouin scattering signal f o -f brillouin . The photodetector outputs the difference frequency signal of the two. The peak of the spectrum is located at the difference frequency of the acousto-optic modulator drive frequency f aom and the Brillouin frequency shift f brillouin , that is, f brillouin -f aom . Adding f aom to this frequency can obtain the Brillouin frequency shift f brillouin .

[0043] Step 3: Professor Fry (Fry, E.S., & Jones, R.L. (1965). Journal of the Acoustical Society of America, 37(1), 100 - 108.) has made an accurate fitting of the relationship between the Brillouin scattering frequency shift and temperature and salinity. Substitute the Brillouin frequency shift f brillouin into the empirical formula of Professor Fry: T(S, v B ) = t0 + t1(v B -7.5) + t2(v B -7.5) 2 + t3(v B-7.5) 3 +t4(v B -7.5) 6 +S[t5+t6(v B -7.5)+t7(v B -7.5) 2 +t8(v B -7.5) 3 , where S is the water salinity in units of 1‰; T is the water temperature in degrees Celsius; v B = f brillouin is the Brillouin frequency shift in units of GHz; t0 = 23.5, t1 = 65.5, t2 = 75, t3 = 252, t4 = 1100, t5 = -0.402, t6 = -0.287, t7 = -0.902, t8 = -5.5. Assuming the water salinity is known, the water temperature T can be deduced from the Brillouin frequency shift f brillouin by calculation.

[0044] The system provided by the present invention utilizes the pseudo-random code phase modulation technology and a high-sensitivity signal detection technology based on the principle of light wave interference. Through mixing the signal light with the local oscillator light, the heterodyne detection technology that converts the high-frequency optical signal to a low-frequency electrical signal for processing can, under the conditions of wide pulse width and low peak power, achieve high-sensitivity and high-range resolution for obtaining the water temperature profile information through coherent detection and the fast Fourier transform algorithm.

[0045] The system provided by the present invention can coherently amplify the weak scattering signal containing Brillouin spectrum information through signal processing methods such as filtering, FFT, and peak frequency extraction, and the sensitivity is significantly improved; the system operates under the conditions of wide pulse width and low peak power, greatly reducing the peak power required by the system; and the pseudo-random code modulation technology is used in the wide pulse width to ensure excellent range resolution; it ensures the consistency of the measured signal and the randomness of the noise; the Brillouin scattering spectrum is obtained through spectral analysis, and the profile temperature information is deduced according to the empirical formula. By processing the coherent signal in segments, the system can achieve high-range resolution and high-precision measurement of the water temperature profile.

[0046] The above embodiments only represent one implementation mode of the present invention, and the description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A water temperature profile coherent measurement system based on pseudo-random code modulation, characterized in that It includes a laser seed source, an optical isolator, a first coupler, an electro-optic phase modulator, an optical circulator, a telescope, a signal generator, an acousto-optic modulator, a second coupler, a photodetector, an arbitrary waveform generator, an external trigger, a computer, and a dual-channel AD acquisition card, and an amplifier; the output end of the laser seed source is connected to the input end of the optical isolator, the output end of the optical isolator is connected to the input end of the first coupler, and the output end of the first coupler is respectively connected to the first input end of the electro-optic phase modulator and the first input end of the acousto-optic modulator; the output end of the electro-optic phase modulator is connected to the first port of the optical circulator, and the outgoing laser output from the second port of the optical circulator is emitted by the coaxial transceiver telescope. After the outgoing laser is diffusely reflected by the water body to be measured, it is received by the same telescope. The third port of the optical circulator is connected to the first input end of the second coupler, and the output end of the signal generator is connected to the input end of the acousto-optic modulator to provide a driving signal. The output end of the acousto-optic modulator is connected to the second input end of the second coupler. The laser echo signal light carrying the temperature information of the water body to be measured is collected by the same telescope and then undergoes a coherent effect with the local oscillator signal light output by the acousto-optic modulator at the third port of the optical circulator in the coupler; the output end of the second coupler is connected to the input end of the photodetector, the output end of the photodetector is connected to the input end of the amplifier, the output end of the amplifier is connected to the first input end of the dual-channel AD acquisition card, and the output end of the dual-channel AD acquisition card is connected to a computer with signal acquisition, processing, control, calculation, and display functions; the output end of the arbitrary waveform generator is respectively connected to the second input end of the dual-channel AD acquisition card and the second input end of the electro-optic phase modulator to provide the same pseudo-random code driving signal; the output end of the external trigger is respectively connected to the input end of the arbitrary waveform generator and the input end of the dual-channel AD acquisition card to ensure the time synchronization of the arbitrary waveform generator and the dual-channel AD acquisition card.

2. The water temperature profile coherent measurement system based on pseudo-random code modulation according to claim 1, wherein The laser output from the laser seed source first passes through an optical isolator and then through a first coupler. After passing through the first coupler, it is divided into two parts; most of the laser passes through an electro-optic phase modulator and an optical circulator in sequence and is then emitted by a transmitting telescope; a small part of the laser is frequency-shifted by an acousto-optic modulator to serve as the local oscillator signal light; the telescope receives the echo signal light generated by the diffuse reflection of the water body to be measured and sends the diffuse reflection echo signal light of the water body to be measured into a second coupler through the optical circulator. The local oscillator signal light is also sent into the second coupler. The coherent light of the echo signal light and the local oscillator signal light output by the second coupler passes through a photodetector and an amplifier in sequence and is then collected by a dual-channel AD acquisition card. At the same time, the dual-channel AD acquisition card also collects the pseudo-random code driving signal sent by an arbitrary waveform generator and makes different time delays for the pseudo-random code driving signal; the output signal of the dual-channel AD acquisition card enters the computer. Since the water body will undergo continuous scattering under laser irradiation, the echo signal light received by the telescope is scattered light and also continuous light, and the local oscillator signal light is also continuous light. Therefore, the light entering the second coupler is all continuous light; in the computer, the coherent signal output by the amplifier is multiplied by the driving signal that is shifted in sequence and undergoes a fast Fourier transform to obtain the coherent light spectrum at different depths, and the Brillouin frequency shift f is obtained after processing brillouin .

3. The water temperature profile coherent measurement system based on pseudo-random code modulation according to claim 1, wherein The laser seed source is a 532 nm single-longitudinal-mode fiber continuous seed source.

4. The water temperature profile coherent measurement system based on pseudo-random code modulation according to claim 1, wherein The electro-optic phase modulator is a lithium niobate electro-optic phase modulator.

5. The water temperature profile coherent measurement system based on pseudo-random code modulation according to claim 1, wherein The first coupler is a 10:90 optical coupler, where 10% of the light enters the acousto-optic modulator and is used as the local oscillator light; 90% of the light enters the electro-optic phase modulator, and the electro-optic phase modulator performs phase modulation according to the pseudo-random code driving signal and is emitted through the telescope.

6. The water temperature profile coherent measurement system based on pseudo-random code modulation according to claim 1, characterized in that The telescope is a telescope system with a common optical axis for transmission and reception.

7. The water temperature profile coherent measurement system based on pseudo-random code modulation according to claim 1, characterized in that, The signal generator is a sine signal generator.

8. A method for coherent measurement of water temperature profile based on pseudo-random code modulation, which is implemented based on the water temperature profile coherent measurement system based on pseudo-random code modulation as described in any one of claims 1-7, characterized in that, It includes the following steps: Step 1: The laser output by the laser seed source first passes through the optical isolator and then through the first coupler. After passing through the first coupler, it is divided into two parts; most of the laser passes through the electro-optic phase modulator and the optical circulator in sequence and is then emitted by the telescope; a small part of the laser is frequency-shifted by the acousto-optic modulator and used as the local oscillator signal light. Step 2: The laser emitted by the telescope enters the water body to be measured. The water body to be measured returns the echo signal light. The telescope receives the echo signal light of the water body to be measured and sends the echo signal light of the water body to be measured into the second coupler through the optical circulator. The local oscillator signal light is also sent into the second coupler. The coherent light of the echo signal light and the local oscillator signal light output by the second coupler passes through the photodetector and the amplifier in sequence, and then is collected by the dual-channel AD acquisition card. At the same time, the dual-channel AD acquisition card also collects the pseudo-random code driving signal generated by the arbitrary waveform generator and makes different time delays for the pseudo-random code driving signal; the output signal of the dual-channel AD acquisition card enters the computer. Due to the continuous scattering of the water body, the scattered light received by the receiving telescope is continuous light, and the local oscillator light is also continuous light. Therefore, the light entering the second coupler is continuous light; in the computer, the coherent signal output by the amplifier is multiplied by the successively shifted driving signals and undergoes a fast Fourier transform to obtain the coherent light signal spectrum at different depths. This coherent light signal spectrum, that is, the peak signal, is contributed by the scattered signals of seawater at different depths; the reference light f o -f aom is mixed with the Brillouin scattering signal f o -f brillouin . The photodetector outputs the difference frequency signal of the two. The peak of the spectrum is located at the difference frequency of the driving frequency f aom of the acousto-optic modulator and the Brillouin frequency shift f brillouin , that is, f brillouin -f aom ; adding f aom to this frequency can obtain the Brillouin frequency shift f brillouin ; Step 3: Substitute the Brillouin frequency shift f brillouin into the empirical formula: T(S, v B ) = t0 + t1(v B - 7.5) + t2(v B - 7.5) 2 + t3(v B - 7.5) 3 + t4(v B - 7.5) 6 + S[t5 + t6(v B - 7.5) + t7(v B - 7.5) 2 + t8(v B - 7.5) 3 , where S is the water salinity in units of 1‰; T is the water temperature, in degrees Celsius; v B = f brillouin is the Brillouin frequency shift, in GHz; t0 = 23.5, t1 = 65.5, t2 = 75, t3 = 252, t4 = 1100, t5 = -0.402, t6 = -0.287, t7 = -0.902, t8 = -5.5; Assuming the water salinity is known, the water temperature T can be deduced from the Brillouin frequency shift f brillouin to obtain the water temperature T.

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