CN107290063B - Seawater temperature measuring method and system - Google Patents
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Abstract
The invention discloses a seawater temperature measuring method and system, and relates to the technical field of ocean temperature remote sensing. The method comprises the following steps: dividing laser into coherent local oscillation light and detection light, and focusing the detection light to a water area with a specified depth; receiving backward Rayleigh Brillouin scattered light from a water area with a specified depth; mixing backward Rayleigh Brillouin scattering light with local oscillator light, and outputting an intermediate frequency signal after photoelectric conversion; and extracting Brillouin frequency shift from the intermediate frequency signal, and calculating according to the seawater temperature inversion model to obtain the seawater temperature of the water area with the specified depth. The method can rapidly and accurately acquire Brillouin frequency shift and underwater vertical temperature distribution, and has the characteristics of low cost, simple and convenient measurement, good real-time performance and good noise resistance.
Description
Technical Field
the invention relates to the technical field of ocean temperature remote sensing, in particular to a seawater temperature measuring method and system.
background
brillouin scattering is inelastic scattering in which photons interact with a medium, and when brillouin scattering occurs, the photons are swallowed by collision to add self energy, or the self energy is lost to generate a phonon. The acousto-optic energy exchange resulting from such collisions causes a change in the frequency of the scattered light. On the frequency spectrum, the change of the optical frequency is represented by that the brillouin scattering peak is divided into a stokes peak and an anti-stokes peak, the positions of the brillouin scattering peaks are symmetrically distributed on two sides of the laser central frequency, and a certain offset is generated relative to the laser central frequency. This offset is named brillouin frequency shift. This spectral distribution of brillouin scattering is related to the energy of the acoustic phonon itself, and can be linked to medium particle free path and the like. When brillouin scattering occurs, it is not the medium particles that interact with the photons, but rather the density of the medium fluctuates, which is related to the physical properties of the medium, such as temperature, etc. Thus, the brillouin scattering spectrum can reflect physical properties of the medium, such as sound velocity, temperature, and the like. Brillouin frequency shift, which is a spectral characteristic parameter, is linked with physical properties of a medium, so that brillouin scattering based on laser is widely applied to the fields of distributed temperature optical fiber sensors, ocean temperature remote sensing and the like, and is used for measuring environmental physical parameters.
in the ocean temperature remote sensing application, the Brillouin scattering spectrum measurement technology adopted by the laser radar system based on Brillouin scattering mainly comprises the following 2 types: the edge detection technique and the F-P Fabry-perot (Fabry-perot) scanning interferometer technique, in which the cost of the edge detection technique is high and the scanning time of the F-P scanning interferometer is long, are both limited in practical applications.
Disclosure of Invention
The invention aims to provide a seawater temperature measuring method, and the seawater temperature measuring system can quickly and accurately acquire Brillouin frequency shift and underwater vertical temperature distribution and has the characteristics of low cost, simplicity and convenience in measurement, good real-time performance and good noise resistance.
the invention provides a seawater temperature measuring method, which comprises the following steps:
Dividing laser into coherent local oscillation light and detection light, and focusing the detection light to a water area with a specified depth;
receiving backward Rayleigh Brillouin scattered light from a water area with a specified depth;
mixing backward Rayleigh Brillouin scattering light with local oscillator light, and outputting an intermediate frequency signal after photoelectric conversion;
and extracting Brillouin frequency shift from the intermediate frequency signal, and calculating according to the seawater temperature inversion model to obtain the seawater temperature of the water area with the specified depth.
On the basis of the above technical solution, mixing the backward rayleigh brillouin scattered light with the local oscillation light and outputting an intermediate frequency signal after photoelectric conversion includes: and mixing the backward Rayleigh Brillouin scattering light with the local oscillator light by using a balanced heterodyne method and generating an intermediate frequency signal.
On the basis of the technical scheme, the frequency range of the intermediate frequency signal is 7GHz-8 GHz.
On the basis of the technical scheme, the laser is pulse laser.
on the basis of the above technical solution, extracting the brillouin frequency shift from the digital signal comprises:
Carrying out Fast Fourier Transform (FFT) on the digital signal, and obtaining a pulse echo power spectrum by using a periodogram method;
Accumulating the pulse echo power spectrums to obtain an accumulated power spectrum;
and calculating to obtain Brillouin frequency shift from the accumulated power spectrum by using a nonlinear fitting algorithm.
On the basis of the technical scheme, the seawater temperature inversion model comprises the following steps:
T(S,νB)＝a0+a1(νB-7.5)+a2(νB-7.5)2+a3(νB-7.5)3+a4(νB-7.5)6+
S[a5+a6(νB-7.5)+a7(νB-7.5)2+a8(νB-7.5)3]
Wherein, T is temperature, a i is constant, i is 1,2, … …,8, v B is Brillouin frequency shift, and S is salinity in seawater.
The present invention also provides a seawater temperature measuring system, comprising:
The laser emitting unit is used for generating laser, dividing the laser into coherent local oscillation light and detection light and focusing the detection light to a water area with a specified depth;
a light receiving unit for receiving backward rayleigh brillouin scattered light from a water area of a specified depth;
The photoelectric conversion unit is used for mixing backward Rayleigh Brillouin scattering light and local oscillation light, and outputting an intermediate frequency signal after photoelectric conversion;
And the data processing unit is used for extracting Brillouin frequency shift from the intermediate frequency signal and calculating and obtaining the seawater temperature of the water area with the appointed depth according to the seawater temperature inversion model.
On the basis of the technical scheme, the laser emitting unit comprises a laser, a beam splitter, a beam control and shaper, the laser is used for generating laser, the beam splitter is used for dividing the laser into local oscillation light and detection light, and the beam control and shaper is used for focusing the detection light to a water area with a specified depth.
on the basis of the above technical solution, the photoelectric conversion unit includes:
The balance detector is used for respectively receiving the local oscillator light and the backward Rayleigh Brillouin scattering light, mixing the backward Rayleigh Brillouin scattering light and the local oscillator light, and outputting an intermediate frequency signal after photoelectric conversion;
an amplifier for amplifying the intermediate frequency signal;
And the acquisition card is used for carrying out digital sampling on the amplified intermediate frequency signals.
On the basis of the technical scheme, the seawater temperature inversion model comprises the following steps:
T(S,νB)＝a0+a1(νB-7.5)+a2(νB-7.5)2+a3(νB-7.5)3+a4(νB-7.5)6+
S[a5+a6(νB-7.5)+a7(νB-7.5)2+a8(νB-7.5)3]
Wherein, T is temperature, a i is constant, i is 1,2, … …,8, v B is Brillouin frequency shift, and S is salinity in seawater.
Compared with the prior art, the invention has the following advantages:
(1) The method and the system for measuring the seawater temperature can rapidly and precisely obtain the Brillouin frequency shift and underwater vertical temperature distribution, and have the characteristics of low cost, simplicity and convenience in measurement, good real-time performance and good noise resistance.
(2) The influence of shot noise in local oscillation light on the heterodyne detection signal-to-noise ratio can be overcome by adopting a balanced heterodyne method, the signal-to-noise ratio of the intermediate frequency signal is improved, and the noise immunity is further improved.
(3) The seawater channel is a random channel, so that the obtained backward Rayleigh Brillouin scattering signal has strong fluctuation, the accumulation of a plurality of pulse echo power spectrums can reduce noise, and the signal-to-noise ratio is further improved.
drawings
FIG. 1 is a flow chart of a method for measuring seawater temperature according to an embodiment of the present invention;
Fig. 2 is a schematic diagram of a seawater temperature measuring system according to an embodiment of the present invention.
Detailed Description
The invention is based on the laser heterodyne technology, adopts pulse laser to be divided into detection light and local oscillator light after being split by a beam splitter, the detection light generates scattering in a water area with specified depth after being shaped by optical expanding and converging beams, and the backward Rayleigh Brillouin scattering light is guided into a balance detector after being received by a telescope to be mixed with the local oscillator light. Based on an optical heterodyne detection method, namely, by using an optical coherence superposition principle, local oscillator light with different frequencies and backward Rayleigh Brillouin scattering light are superposed, the backward Rayleigh Brillouin scattering light serving as a high-frequency light wave signal is converted into an intermediate-frequency envelope signal, a mixed differential signal enters a low-noise amplifier for electrical amplification to ensure the dynamic range of the differential signal, and then an acquisition card is used for carrying out analog-digital (AD) sampling and is sent to a computer for subsequent data processing, so that the measurement of Brillouin frequency shift and temperature inversion are realized.
the invention is described in further detail below with reference to the figures and the embodiments.
Referring to fig. 1, an embodiment of the present invention provides a method for measuring a temperature of seawater, including the following steps:
s1, dividing laser into coherent local oscillation light and detection light, and focusing the detection light to a water area with a specified depth. The laser is divided into 2 paths of light after passing through an optical beam splitter, and one path of light is used as detection light and is incident into a water area with specified depth after being subjected to light beam control and beam expanding and converging shaping so as to detect the temperature information of the water area; the other path is used as local oscillation light and is mixed with backward Rayleigh Brillouin scattering light received from a water area with a specified depth. The laser is preferably a pulsed laser with a central wavelength of 532 nm.
and S2, receiving backward Rayleigh Brillouin scattered light from a water area with a specified depth.
And S3, mixing the backward Rayleigh Brillouin scattering light with the local oscillator light, and outputting an intermediate frequency signal after photoelectric conversion.
Specifically, mixing the backward rayleigh brillouin scattered light with the local oscillation light and outputting an intermediate frequency signal after photoelectric conversion includes: and mixing the backward Rayleigh Brillouin scattering light with the local oscillator light by using a balanced heterodyne method and generating an intermediate frequency signal. The core device of the balance heterodyne method is a balance detector. Two channels are arranged in a balanced detector, two photodiodes with completely similar characteristics are used for respectively performing photoelectric conversion, optical signals output after backward Rayleigh Brillouin scattering light and local oscillation light are mixed enter the two channels respectively, one channel is added with a delay line or a Mach-Zehnder interferometer is used for adjusting the phase reverse bias of the other channel, and a differential amplifier is used at the rear end for amplifying differential mode signals and inhibiting common mode signals. After the output signals of the two channels are combined, the noises are completely balanced, and only the difference signal of the two channels is greatly amplified and output, so that the photoelectric conversion amplifying characteristic is good. The frequency range of the intermediate frequency signal is 7GHz-8 GHz.
the influence of shot noise in local oscillation light on the heterodyne detection signal-to-noise ratio can be overcome by adopting a balanced heterodyne method, the signal-to-noise ratio of the intermediate frequency signal is improved, and the noise immunity is further improved.
And S4, extracting Brillouin frequency shift from the digital signal, and calculating according to the seawater temperature inversion model to obtain the seawater temperature of the water area with the specified depth.
in step S4, the extracting the brillouin frequency shift from the digital signal includes:
s4.1, performing Fast Fourier Transform (FFT) on the digital signal, and obtaining a pulse echo power spectrum by using a periodogram method;
S4.2, accumulating the power spectrums of the pulse echoes to obtain an accumulated power spectrum;
And S4.3, calculating the Brillouin frequency shift from the accumulated power spectrum by using a Levenberg-Marquardt nonlinear fitting algorithm.
The seawater channel is a random channel, so that the obtained backward Rayleigh Brillouin scattering signal has strong fluctuation, the accumulation of a plurality of pulse echo power spectrums can reduce noise, and the signal-to-noise ratio is further improved.
The seawater temperature inversion model is as follows:
T(S,νB)＝a0+a1(νB-7.5)+a2(νB-7.5)2+a3(νB-7.5)3+a4(νB-7.5)6+
S[a5+a6(νB-7.5)+a7(νB-7.5)2+a8(νB-7.5)3]
Wherein T is temperature, a i is constant, i is positive integer, v B is Brillouin frequency shift, and S is salinity in seawater, wherein the salinity is 35 per thousand (world ocean average salinity).
it is noted that this seawater temperature inversion model is not only related to the brillouin frequency shift, but also to the salinity S in the seawater. In the inversion with this model, salinity S was treated as a known quantity of 35% o (world ocean average salinity).
a i is shown in Table 1:
TABLE 1 constant a i
| ai | |
| i＝0 | 23.5 |
| i＝1 | 65.5 |
| i＝2 | 75 |
| i＝3 | 252 |
| i＝4 | 1100 |
| i＝5 | -0.402 |
| i＝6 | -0.287 |
| i＝7 | -0.902 |
| i＝8 | -5.5 |
The method and the system for measuring the seawater temperature can rapidly and precisely obtain the Brillouin frequency shift and underwater vertical temperature distribution, and have the characteristics of low cost, simplicity and convenience in measurement, good real-time performance and good noise resistance.
Referring to fig. 2, an embodiment of the present invention further provides a seawater temperature measuring system, which includes a laser emitting unit 1, a light receiving unit 2, a photoelectric conversion unit 3, and a data processing unit 4.
the laser emitting unit 1 is used for generating laser, and preferably adopts pulse laser which has stronger energy than continuous laser, can penetrate deeper seawater, divides the laser into coherent local oscillation light and detection light, and focuses the detection light to a water area with specified depth. Specifically, the laser emitting unit 1 includes a laser 11, a beam splitter 12, and a beam control and shaper 13, where the laser 11 is configured to generate laser light, the beam splitter 12 is configured to split the laser light into local oscillation light and probe light, and the beam control and shaper 13 is configured to focus the probe light to a water area with a specified depth.
The laser light source is used as a device for providing optical energy and plays a very important role in the whole set of measuring system. For ocean remote sensing, as the lidar belongs to an active remote sensing device, the laser wavelength plays a very important role in the performance of the whole set of lidar because seawater has different absorptivities to light in different bands. The laser with proper wavelength is selected as the light source, so that the power and the cost required by the laser can be effectively reduced, the penetrability of the laser to seawater can be effectively improved, and the remote sensing depth is increased. On the optical absorption spectrum line of seawater, a transmission window exists in a blue-green light wave band of 480nm to 580nm, and the seawater has the lowest light absorption rate in the wavelength range. If the laser wavelength is selected in this band, its attenuation coefficient will be less than 0.01, and its attenuation length can reach more than 100 meters. In order to meet the detection depth requirement, the measuring system preferably adopts a pulse laser with the central wavelength of 532nm, and compared with continuous laser, the pulse laser has larger energy and can penetrate deeper seawater. In addition, the single pulse energy of the pulse laser is 650mJ, the pulse repetition frequency is 10Hz, and the minimum pulse width is about 2 ns.
the beam control and shaper 13 may comprise a beam control module consisting of a series of optical lens groups for controlling the probe light and a beam expanding and shaping module. The beam expanding, converging and shaping module connected with the light beam control module mainly comprises a beam expanding lens and a focusing lens and is used for focusing the detection light in a water area with a specified depth. After pulse laser generated by the laser is divided into monochromatic and coherent detection light and local oscillation light by the beam splitter, the detection light is expanded by the beam control module and the beam expander and then focused to a water area with a specified depth by the focusing lens for detecting the temperature of the seawater at the depth.
The light receiving unit 2 is for receiving backward rayleigh brillouin scattered light from a water area of a specified depth. The light receiving unit 2 mainly comprises a telescope 21 and an optical lens group 22, wherein the telescope 21 is used for receiving backward Rayleigh Brillouin scattered light of a water area with a specified depth, and then the backward Rayleigh Brillouin scattered light is guided into the photoelectric conversion unit 3 after the light path is adjusted by the series of optical lens groups 22 for beam control and shaping.
The photoelectric conversion unit 3 is used for mixing backward rayleigh brillouin scattering light and local oscillation light and outputting an intermediate frequency signal after photoelectric conversion, the photoelectric conversion unit 3 is a core part of the measurement system, and the photoelectric conversion unit 3 comprises a balance detector 31, an amplifier 32 and a capture card 33.
The balance detector 31 is configured to receive the local oscillator light and the backward rayleigh brillouin scattered light, mix the backward rayleigh brillouin scattered light and the local oscillator light, and output an intermediate frequency signal (electrical signal) after photoelectric conversion, where the intermediate frequency signal is a brillouin frequency shift to be measured. The photoelectric detector is one of key devices for laser reception, and according to the optical heterodyne detection technology, if local oscillation light contains shot noise, the signal-to-noise ratio of an output intermediate frequency signal is reduced, and in order to overcome the influence of the local oscillation light shot noise, a balanced heterodyne technology, that is, a balanced detector adopted by the measurement system, is generally adopted. Two channels are arranged in a balanced photoelectric detector, two photodiodes with completely similar characteristics are used for respectively performing photoelectric conversion, optical signals output after backward Rayleigh Brillouin scattering light and local oscillation light are mixed respectively enter the two channels, a delay line is added into one channel, or a Mach-Zehnder interferometer is used for adjusting the phase reverse bias of the other channel, a differential amplifier is used at the rear end for amplifying differential mode signals and inhibiting common mode signals. After the output signals of the two channels are combined, the noises are completely balanced, and only the difference signal of the two channels is greatly amplified and output, so that the photoelectric conversion amplifying characteristic is good. In addition, the balanced detector 31 converts the high-frequency backward rayleigh brillouin scattered light into a lower-frequency intermediate-frequency signal, which facilitates the digital data sampling by the acquisition card 33.
The amplifier 32 is used for amplifying the intermediate frequency signal, and the amplifier 32 preferably uses a low noise amplifier, and the low noise amplifier electrically amplifies the intermediate frequency signal output by the balanced detector 31, so as to ensure the dynamic range of the intermediate frequency signal.
The acquisition card 33 is used for digitally sampling the amplified intermediate frequency signal and sending the digital signal to the data processing unit 4 for back-end data processing, and the acquisition card 33 may be an analog-digital (AD) chip.
The data processing unit 4 is used for extracting Brillouin frequency shift from the intermediate frequency signal and calculating and obtaining the seawater temperature of a water area with a specified depth according to the seawater temperature inversion model. The data processing unit 4 mainly comprises a computer, digital signals output by a collecting card 33 of the photoelectric conversion system 3 are transmitted to the computer for power spectrum estimation and Brillouin frequency shift calculation, and then temperature calculation is performed by combining a seawater temperature inversion model.
The seawater temperature inversion model is as follows:
T(S,νB)＝a0+a1(νB-7.5)+a2(νB-7.5)2+a3(νB-7.5)3+a4(νB-7.5)6+
S[a5+a6(νB-7.5)+a7(νB-7.5)2+a8(νB-7.5)3]
Wherein T is temperature, a i is constant, i is 1,2, … …,8, v B is Brillouin frequency shift, and S is salinity in seawater, wherein 35 per thousand (world ocean average salinity) is taken.
the present invention is not limited to the above-described embodiments, and it will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the principle of the present invention, and such modifications and improvements are also considered to be within the scope of the present invention. Those not described in detail in this specification are within the skill of the art.
Claims (9)
1. A seawater temperature measuring method is characterized by comprising the following steps:
dividing laser into coherent local oscillation light and detection light, and focusing the detection light to a water area with a specified depth;
Receiving backward Rayleigh Brillouin scattered light from a water area with a specified depth;
Mixing backward Rayleigh Brillouin scattering light and local oscillator light by using a balanced heterodyne method, and outputting an intermediate frequency signal after photoelectric conversion, wherein optical signals output after mixing the backward Rayleigh Brillouin scattering light and the local oscillator light respectively enter two channels arranged in a balanced detector, photodiodes with completely similar characteristics on the two channels are respectively subjected to photoelectric conversion, a delay line is added into one channel, or a Mach-Zehnder interferometer is used for adjusting the phase reverse bias of the other channel, a differential amplifier is used at the rear end for amplifying a differential mode signal and inhibiting a common mode signal;
And extracting Brillouin frequency shift from the intermediate frequency signal, and calculating according to the seawater temperature inversion model to obtain the seawater temperature of the water area with the specified depth.
2. The seawater temperature measuring method of claim 1, wherein: the frequency range of the intermediate frequency signal is 7GHz-8 GHz.
3. the seawater temperature measuring method of claim 1, wherein: the laser is a pulsed laser.
4. The seawater temperature measurement method of claim 3, wherein extracting the Brillouin frequency shift from the digital signal comprises:
Carrying out Fast Fourier Transform (FFT) on the digital signal, and obtaining a pulse echo power spectrum by using a periodogram method;
accumulating the pulse echo power spectrums to obtain an accumulated power spectrum;
And calculating to obtain Brillouin frequency shift from the accumulated power spectrum by using a nonlinear fitting algorithm.
5. The seawater temperature measurement method of claim 1, wherein the seawater temperature inversion model is:
T(S,νB)＝a0+a1(νB-7.5)+a2(νB-7.5)2+a3(νB-7.5)3+a4(νB-7.5)6+S[a5+a6(νB-7.5)+a7(νB-7.5)2+a8(νB-7.5)3]
Wherein, T is temperature, a i is constant, i is 1,2, … …,8, v B is Brillouin frequency shift, and S is salinity in seawater.
6. a seawater temperature measurement system, comprising:
the laser emitting unit is used for generating laser, dividing the laser into coherent local oscillation light and detection light and focusing the detection light to a water area with a specified depth;
A light receiving unit for receiving backward rayleigh brillouin scattered light from a water area of a specified depth;
the photoelectric conversion unit is used for mixing backward Rayleigh Brillouin scattering light and local oscillator light and outputting an intermediate frequency signal after photoelectric conversion, the photoelectric conversion unit comprises a balance detector, optical signals output after the backward Rayleigh Brillouin scattering light and the local oscillator light are mixed enter two channels arranged in the balance detector respectively, photodiodes with completely similar characteristics on the two channels are used for photoelectric conversion respectively, one channel is added with a delay line or is provided with a Mach-Zehnder interferometer to adjust the phase reverse bias of the other channel, and a differential amplifier is used at the rear end to amplify a differential mode signal and inhibit a common mode signal;
And the data processing unit is used for extracting Brillouin frequency shift from the intermediate frequency signal and calculating and obtaining the seawater temperature of the water area with the appointed depth according to the seawater temperature inversion model.
7. the seawater temperature measurement system of claim 6, wherein: the laser emission unit comprises a laser, a beam splitter and a beam control and shaper, the laser is used for generating laser, the beam splitter is used for dividing the laser into local oscillation light and detection light, and the beam control and shaper is used for focusing the detection light to a water area with a specified depth.
8. the seawater temperature measurement system of claim 6, wherein: the photoelectric conversion unit includes:
The balance detector is used for respectively receiving the local oscillator light and the backward Rayleigh Brillouin scattering light, mixing the backward Rayleigh Brillouin scattering light and the local oscillator light, and outputting an intermediate frequency signal after photoelectric conversion;
An amplifier for amplifying the intermediate frequency signal;
and the acquisition card is used for carrying out digital sampling on the amplified intermediate frequency signals.
9. The seawater temperature measurement system of claim 6, wherein: the seawater temperature inversion model is as follows:
T(S,νB)＝a0+a1(νB-7.5)+a2(νB-7.5)2+a3(νB-7.5)3+a4(νB-7.5)6+S[a5+a6(νB-7.5)+a7(νB-7.5)2+a8(νB-7.5)3]
wherein, T is temperature, a i is constant, i is 1,2, … …,8, v B is Brillouin frequency shift, and S is salinity in seawater.
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| CN108051108A (en) * | 2017-11-30 | 2018-05-18 | 华中科技大学鄂州工业技术研究院 | A kind of seawater temp measuring system and method based on low pulse energy Gao Zhongying laser |
| CN108088481A (en) * | 2017-11-30 | 2018-05-29 | 华中科技大学鄂州工业技术研究院 | One kind is based on the contactless multi-parameter laser measurement system in flow field and method |
| CN111142121B (en) * | 2019-12-20 | 2022-04-08 | 自然资源部第二海洋研究所 | Brillouin scattering temperature measurement laser radar system based on two-stage virtual image phase array |
| CN114353989A (en) * | 2022-01-13 | 2022-04-15 | 江苏海洋大学 | Seawater temperature remote sensing system based on laser scattering |
| CN114485988B (en) * | 2022-01-18 | 2023-03-17 | 江苏海洋大学 | Underwater temperature remote measuring system based on Raman spectrum |
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| US4948958A (en) * | 1989-08-01 | 1990-08-14 | Gte Government Systems Corporation | Remote subsurface water temperature measuring apparatus with brillouin scattering |
| CN100504309C (en) * | 2007-09-30 | 2009-06-24 | 南京大学 | Brillouin optical time domain reflection measuring method based on quick fourier transform |
| CN102589748B (en) * | 2012-03-09 | 2013-11-27 | 华北电力大学（保定） | Environmental temperature measurement method based on optical fiber Rayleigh and Brillouin principle |
| CN103175629B (en) * | 2013-03-11 | 2014-12-10 | 哈尔滨工业大学（威海） | Method for fast measuring sea water temperature |
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