CN107290063A - A kind of ocean temperature measuring method and system - Google Patents

Abstract

The invention discloses a seawater temperature measurement method and system, and relates to the technical field of seawater temperature remote sensing. The method comprises the following steps: dividing the laser light into coherent local oscillator light and probe light, and focusing the probe light to a specified depth water area; receiving back Rayleigh Brillouin scattered light from the specified depth water area; The Brillouin scattered light is mixed with the local oscillator light, and the intermediate frequency signal is output after photoelectric conversion; the Brillouin frequency shift is extracted from the intermediate frequency signal, and the seawater temperature in the designated depth water area is calculated according to the seawater temperature inversion model. The invention can acquire the Brillouin frequency shift and the underwater vertical temperature distribution quickly and with high precision, and has the characteristics of low cost, simple and convenient measurement, good real-time performance and good noise resistance.

Description

Method and system for measuring seawater temperature

technical field

The invention relates to the technical field of ocean temperature remote sensing, in particular to a seawater temperature measurement method and system.

Background technique

Brillouin scattering is an inelastic scattering in which photons interact with a medium. When Brillouin scattering occurs, a photon will annex a phonon to increase its own energy through collision, or lose its own energy to generate a phonon. The exchange of acousto-optic energy generated by this collision will cause a change in the frequency of the scattered light. In the frequency spectrum, this light frequency change is reflected in the fact that the Brillouin scattering peaks are divided into Stokes peaks and anti-Stokes peaks, and their positions are symmetrically distributed on both sides of the laser center frequency, and relative to the laser center There is a certain shift in frequency. This shift is named the Brillouin shift. The spectral distribution of Brillouin scattering is related to the energy of the acoustic phonon itself, and can be related to the free path of the medium particle. When Brillouin scattering occurs, it is not the medium particles that interact with the photons, but the density fluctuations of the medium, which are related to the physical properties of the medium, such as temperature. Therefore, the Brillouin scattering spectrum can reflect the physical properties of the medium, such as sound velocity, temperature and so on. The Brillouin frequency shift as a characteristic parameter of the spectrum is gradually being used to study and measure the physical properties of the medium because of its connection with the physical properties of the medium. For example, the Brillouin scattering based on laser is widely used in distributed temperature Optical fiber sensors, ocean temperature remote sensing and other fields are used to measure environmental physical parameters.

In the application of ocean temperature remote sensing, the Brillouin scattering spectroscopy measurement technology adopted by the Brillouin scattering-based lidar system mainly falls into the following two categories: edge detection technology and F-P Fabry-Pérot scanning interferometer Among them, the cost of edge detection technology is relatively high, and the scanning time of F-P scanning interferometer is relatively long, both of which are limited in practical application.

Contents of the invention

In view of the defects existing in the prior art, the main purpose of the present invention is to provide a seawater temperature measurement method, and another purpose of the present invention is to provide a seawater temperature measurement system, which can quickly and accurately obtain the Brillouin frequency shift As well as underwater vertical temperature distribution, it has the characteristics of low cost, simple measurement, good real-time performance and good noise resistance.

The invention provides a method for measuring seawater temperature, comprising the following steps:

Divide the laser light into coherent local oscillator light and probe light, and focus the probe light to the specified depth of water;

Scatter light to Rayleigh Brillouin after receiving from the specified depth of water;

Mix the backward Rayleigh Brillouin scattered light with the local oscillator light and output the intermediate frequency signal after photoelectric conversion;

The Brillouin frequency shift is extracted from the intermediate frequency signal, and the seawater temperature of the water area at a specified depth is calculated according to the seawater temperature inversion model.

On the basis of the above technical solution, mixing the backward Rayleigh Brillouin scattered light with the local oscillator light and outputting the intermediate frequency signal after photoelectric conversion includes: using the balanced heterodyne method to convert the backward Rayleigh Brillouin scattered light Mix with local oscillator light and generate intermediate frequency signal.

On the basis of the above technical solution, the frequency range of the intermediate frequency signal is 7GHz-8GHz.

On the basis of the above technical solution, the laser is a pulsed laser.

On the basis of the above technical solutions, extracting Brillouin frequency shift from digital signals includes:

Perform fast Fourier FFT transform on the digital signal, and use the periodogram method to obtain the pulse-echo power spectrum;

Accumulating multiple pulse echo power spectra to obtain an accumulated power spectrum;

The Brillouin frequency shift is calculated from the accumulated power spectrum using a nonlinear fitting algorithm.

On the basis of the above technical solution, the seawater temperature inversion model is:

T(S,ν _B )＝a ₀ +a ₁ (ν _B -7.5)+a ₂ (ν _B -7.5) ² +a ₃ (ν _B -7.5) ³ +a ₄ (ν _B -7.5) ⁶ +

S[a ₅ +a ₆ (ν _B -7.5)+a ₇ (ν _B -7.5) ² +a ₈ (ν _B -7.5) ³ ]

Where, T is the temperature, a _i is a constant, i=1,2,...,8, ν _B is the Brillouin frequency shift, and S is the salinity in seawater.

The present invention also provides a seawater temperature measurement system, comprising:

A laser emitting unit, which is used to generate laser light, divide the laser light into coherent local oscillator light and probe light, and focus the probe light to a specified depth of water;

a light receiving unit, which is used to receive and scatter light to Rayleigh Brillouin from a specified depth of water;

A photoelectric conversion unit, which is used to mix the backward Rayleigh Brillouin scattered light and the local oscillator light and output an intermediate frequency signal after photoelectric conversion;

The data processing unit is used to extract the Brillouin frequency shift from the intermediate frequency signal, and calculate and obtain the seawater temperature in the water area at a specified depth according to the seawater temperature inversion model.

On the basis of the above technical solution, the laser emitting unit includes a laser, a beam splitter, a beam control and a shaper, the laser is used to generate laser light, the beam splitter is used to divide the laser light into local oscillator light and probe light, the beam control and Shapers are used to focus the probe light to a specified depth of water.

On the basis of the above technical solution, the photoelectric conversion unit includes:

A balanced detector, which is used to respectively receive the local oscillator light and the backward Rayleigh Brillouin scattered light, mix the backward Rayleigh Brillouin scattered light with the local oscillator light and output an intermediate frequency signal after photoelectric conversion;

an amplifier, which is used to amplify the intermediate frequency signal;

The acquisition card is used for performing digital sampling on the amplified intermediate frequency signal.

On the basis of the above technical solution, the seawater temperature inversion model is:

T(S,ν _B )＝a ₀ +a ₁ (ν _B -7.5)+a ₂ (ν _B -7.5) ² +a ₃ (ν _B -7.5) ³ +a ₄ (ν _B -7.5) ⁶ +

S[a ₅ +a ₆ (ν _B -7.5)+a ₇ (ν _B -7.5) ² +a ₈ (ν _B -7.5) ³ ]

Where, T is the temperature, a _i is a constant, i=1,2,...,8, ν _B is the Brillouin frequency shift, and S is the salinity in seawater.

Compared with prior art, advantage of the present invention is as follows:

(1) The local oscillator light with different frequencies and the backward Rayleigh Brillouin scattered light are superimposed to generate an intermediate frequency signal. Based on the optical heterodyne detection method, that is, the principle of optical coherent superposition (spectrum detection) is used to realize the detection of the Brillouin frequency. Compared with the traditional signal receiving and processing system, the seawater temperature measurement method and system of the embodiment of the present invention can obtain the Brillouin frequency shift and the underwater vertical temperature distribution quickly and with high precision, and has the advantages of low cost, simple measurement, real-time Good performance and good noise immunity.

(2) The balanced heterodyne method can overcome the influence of the shot noise in the local oscillator on the signal-to-noise ratio of the heterodyne detection, improve the signal-to-noise ratio of the intermediate frequency signal, and further improve the noise resistance.

(3) Since the seawater channel is a random channel, the obtained backward Rayleigh Brillouin scattering signal has strong fluctuations. Accumulation of multiple pulse echo power spectra can reduce noise and further improve the signal-to-noise ratio.

Description of drawings

Fig. 1 is the flow chart of the seawater temperature measurement method of the embodiment of the present invention;

Fig. 2 is a schematic diagram of a seawater temperature measurement system according to an embodiment of the present invention.

detailed description

The invention is based on the laser heterodyne technology. The pulsed laser beam is divided into the detection light and the local oscillator light after being split by the beam splitter. After the scattered light is received by the telescope, it is introduced into the balance detector and mixed with the local oscillator light. Based on the optical heterodyne detection method, that is, using the principle of optical coherent superposition, the local oscillator light with different frequencies and the backward Rayleigh Brillouin scattered light are superimposed, and the backward Rayleigh Brillouin scattered light as a high-frequency light wave signal is converted It is an intermediate frequency envelope signal, and the differential signal after mixing enters the low noise amplifier for electrical amplification to ensure the dynamic range of the differential signal, and then the acquisition card performs analog-to-digital AD sampling and sends it to the computer for subsequent data processing to realize the Burri Abyss frequency shift measurement and temperature inversion.

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Referring to Fig. 1, an embodiment of the present invention provides a seawater temperature measurement method, comprising the following steps:

S1. Divide the laser light into coherent local oscillator light and probe light, and focus the probe light to the specified depth of water. The laser beam is divided into two paths by the optical beam splitter, one path is used as the detection light after beam control and beam expansion and shaping, and then enters the water area at a specified depth to detect the temperature information of the water area; The back Rayleigh Brillouin scattered light received in deep water is mixed. The laser preferably has a pulsed laser with a center wavelength of 532 nm.

S2. Scattering light to Rayleigh Brillouin after receiving from the specified depth of water.

S3. Mixing the backward Rayleigh Brillouin scattered light with the local oscillator light and outputting an intermediate frequency signal after photoelectric conversion.

Specifically, mixing the backward Rayleigh Brillouin scattered light and the local oscillator light and outputting the intermediate frequency signal after photoelectric conversion includes: using the balanced heterodyne method to perform the backward Rayleigh Brillouin scattered light and the local oscillator light Mix and generate IF signal. The core device of the balanced heterodyne method is the balanced detector. The balanced detector has two built-in channels, and uses two photodiodes with completely similar characteristics to perform photoelectric conversion respectively, and the output optical signals after mixing the Rayleigh Brillouin scattered light and the local oscillator light enter the two channels respectively, among which Add a delay line to one path, or use a Mach-Zehnder interferometer to adjust the phase reverse bias of one path, and use a differential amplifier at the back end to amplify the differential-mode signal and suppress the common-mode signal. After the output signals of the two channels are combined, the noise is completely offset, and only the difference signal between the two is greatly amplified and output, so it has good photoelectric conversion and amplification characteristics. The frequency range of the IF signal is 7GHz-8GHz.

The balanced heterodyne method can overcome the influence of the shot noise in the local oscillator light on the signal-to-noise ratio of the heterodyne detection, improve the signal-to-noise ratio of the intermediate frequency signal, and further improve the noise resistance.

S4. Extract the Brillouin frequency shift from the digital signal, and calculate and obtain the seawater temperature in the specified depth water area according to the seawater temperature inversion model.

In step S4, extracting the Brillouin frequency shift from the digital signal includes:

S4.1 Perform fast Fourier FFT transform on the digital signal, and use the periodogram method to obtain the pulse echo power spectrum;

S4.2 Accumulate multiple pulse echo power spectra to obtain an accumulated power spectrum;

S4.3 Use the Levenberg-Marquardt nonlinear fitting algorithm to calculate the Brillouin frequency shift from the accumulated power spectrum.

Since the seawater channel is a random channel, the obtained backward Rayleigh Brillouin scattering signal has strong fluctuations. Accumulating the power spectrum of multiple pulse echoes can reduce noise and further improve the signal-to-noise ratio.

The seawater temperature inversion model is:

T(S,ν _B )＝a ₀ +a ₁ (ν _B -7.5)+a ₂ (ν _B -7.5) ² +a ₃ (ν _B -7.5) ³ +a ₄ (ν _B -7.5) ⁶ +

S[a ₅ +a ₆ (ν _B -7.5)+a ₇ (ν _B -7.5) ² +a ₈ (ν _B -7.5) ³ ]

Among them, T is the temperature, a _i is a constant, i is a positive integer, ν _B is the Brillouin frequency shift, and S is the salinity in seawater, where 35‰ (the average salinity of the world's oceans) is taken.

It should be noted that the seawater temperature inversion model is not only related to the Brillouin frequency shift, but also related to the salinity S in seawater. When using this model for inversion, the salinity S is treated as a known quantity of 35‰ (the average salinity of the world's oceans).

a _i See Table 1:

Table 1: Constants a _i

a _i 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 intermediate frequency signal is generated by superimposing the local oscillator light with different frequencies and the backward Rayleigh Brillouin scattered light. Based on the optical heterodyne detection method, that is, the principle of optical coherent superposition (spectrum detection) is used to realize the measurement of the Brillouin frequency shift. , compared with the traditional signal receiving and processing system, the seawater temperature measurement method and system of the embodiment of the present invention can obtain the Brillouin frequency shift and the underwater vertical temperature distribution quickly and with high precision, and has the advantages of low cost, simple measurement, good real-time performance, Good noise immunity.

Referring to FIG. 2 , the embodiment of the present invention also provides a seawater temperature measurement 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 to generate laser light. In the present invention, pulsed laser light is preferably used. Compared with continuous laser light, pulsed laser light has stronger energy and can penetrate deeper seawater. Laser light is divided into coherent local oscillator light and probe light. , and focus the detection light to the specified depth of water. Specifically, the laser emitting unit 1 includes a laser 11, a beam splitter 12, a beam control and shaper 13, the laser 11 is used to generate laser light, the beam splitter 12 is used to divide the laser light into local oscillator light and probe light, and the beam control and The shaper 13 is used to focus the probing light to a specified depth of water.

As a device for providing optical energy, the laser light source plays a very important role in the entire measurement system. For ocean remote sensing, since lidar is an active remote sensing device, the laser wavelength plays a very important role in the performance of the entire lidar, because seawater has different absorption rates for light in different bands. Choosing a laser with a suitable wavelength as a light source can not only effectively reduce the power and cost of the laser, but also effectively improve the penetration of the laser into seawater and increase the depth of remote sensing. On the optical absorption line of seawater, there is a transmission window in the blue-green light band from 480nm to 580nm, and seawater has the lowest light absorption rate in this 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 requirements of detection depth, the measurement system preferably uses a pulsed laser with a center wavelength of 532nm. Compared with continuous lasers, pulsed lasers have greater energy and can penetrate deeper seawater. In addition, the single pulse energy of this pulsed laser is 650mJ, the pulse repetition frequency is 10Hz, and the minimum pulse width is about 2ns.

The beam control and shaper 13 may include a beam control module and a beam expansion and shaping module. The beam control module is composed of a series of optical lens groups for controlling the probe light. The beam expansion and shaping module connected with the beam control module is mainly composed of a beam expansion lens and a focusing lens, which are used to focus the detection light on the specified depth of water. After the pulsed laser light generated by the laser is divided into monochromatic, coherent probe light and local oscillator light by the beam splitter, the probe light is first expanded by the beam control module and the beam expander, and then focused by the focusing lens to a specified depth of water for detection Seawater temperature at that depth.

The light receiving unit 2 is used to receive and scatter light to Rayleigh Brillouin from the specified depth water area. The light receiving unit 2 is mainly composed of a telescope 21 and an optical lens group 22. The telescope 21 is used to receive the Rayleigh Brillouin scattered light in the specified depth water area, and then a series of optical lens groups 22 adjust the optical path to control and shape the light beam. introduced into the photoelectric conversion unit 3 .

The photoelectric conversion unit 3 is used to mix the backward Rayleigh Brillouin scattered light with the local oscillator light and output an intermediate frequency signal after photoelectric conversion. The photoelectric conversion unit 3 is the core part of the measurement system. The photoelectric conversion unit 3 includes a balance Detector 31, amplifier 32 and acquisition card 33.

The balance detector 31 is used to respectively receive the local oscillator light and the backward Rayleigh Brillouin scattered light, mix the backward Rayleigh Brillouin scattered light with the local oscillator light and output an intermediate frequency signal (electric signal ), the intermediate frequency signal is the Brillouin frequency shift to be measured. The photodetector is one of the key devices for laser reception. According to the optical heterodyne detection technology, if the local oscillator light contains shot noise, the signal-to-noise ratio of the output intermediate frequency signal will decrease. In order to overcome the influence of the shot noise of the local oscillator light, usually Adopt balanced heterodyne technology, that is, the balanced detector used in this measurement system. The balanced photodetector has two built-in channels, and two photodiodes with completely similar characteristics are used to perform photoelectric conversion respectively, and the output optical signals after the Rayleigh Brillouin scattered light is mixed with the local oscillator light enter the two channels respectively, One of them is added with a delay line, or a Mach-Zehnder interferometer is used to adjust the phase reverse bias of one, and a differential amplifier is used at the back end to amplify the differential-mode signal and suppress the common-mode signal. After the output signals of the two channels are combined, the noise is completely offset, and only the difference signal between the two is greatly amplified and output, so it has good photoelectric conversion and amplification characteristics. In addition, the balanced detector 31 converts the high-frequency backward Rayleigh Brillouin scattered light into a lower-frequency intermediate frequency signal, which is convenient for the acquisition card 33 to perform digital data sampling.

The amplifier 32 is used to amplify the intermediate frequency signal. The amplifier 32 is preferably a low noise amplifier. The low noise amplifier electrically amplifies the intermediate frequency signal output by the balanced detector 31 to ensure the dynamic range of the intermediate frequency signal.

The acquisition card 33 is used to digitally sample the amplified intermediate frequency signal and send it to the data processing unit 4 for back-end data processing. The acquisition card 33 can be an analog-to-digital AD chip.

The data processing unit 4 is used to extract the Brillouin frequency shift from the intermediate frequency signal, and calculate and obtain the seawater temperature in the specified depth water area according to the seawater temperature inversion model. The data processing unit 4 is mainly composed of a computer, and the digital signal output by the acquisition card 33 of the photoelectric conversion system 3 is transmitted to the computer for power spectrum estimation and Brillouin frequency shift calculation, and then combined with the seawater temperature inversion model for temperature calculation.

The seawater temperature inversion model is:

T(S,ν _B )＝a ₀ +a ₁ (ν _B -7.5)+a ₂ (ν _B -7.5) ² +a ₃ (ν _B -7.5) ³ +a ₄ (ν _B -7.5) ⁶ +

S[a ₅ +a ₆ (ν _B -7.5)+a ₇ (ν _B -7.5) ² +a ₈ (ν _B -7.5) ³ ]

Among them, T is the temperature, a _i is a constant, i=1,2,...,8, ν _B is the Brillouin frequency shift, S is the salinity in seawater, here we take 35‰ (the average salinity of the world ocean) .

The present invention is not limited to the above-mentioned embodiments. For those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications are also considered protection of the present invention. within range. The content not described in detail in this specification belongs to the prior art known to those skilled in the art.

Claims (10)

1. A seawater temperature measurement method, is characterized in that, comprises the following steps: Divide the laser light into coherent local oscillator light and probe light, and focus the probe light to the specified depth of water; Scatter light to Rayleigh Brillouin after receiving from the specified depth of water; Mix the backward Rayleigh Brillouin scattered light with the local oscillator light and output the intermediate frequency signal after photoelectric conversion; The Brillouin frequency shift is extracted from the intermediate frequency signal, and the seawater temperature of the water area at a specified depth is calculated according to the seawater temperature inversion model. 2. The method for measuring seawater temperature as claimed in claim 1, wherein mixing the backward Rayleigh Brillouin scattered light with the local oscillator light and outputting the intermediate frequency signal after photoelectric conversion comprises: using a balanced heterodyne method The backward Rayleigh Brillouin scattered light is mixed with the local oscillator light to generate an intermediate frequency signal. 3. The seawater temperature measurement method according to claim 2, characterized in that: the frequency range of the intermediate frequency signal is 7GHz-8GHz. 4. The seawater temperature measuring method according to claim 1, characterized in that: the laser is a pulsed laser. 5. seawater temperature measuring method as claimed in claim 4, is characterized in that, extracting Brillouin frequency shift from digital signal comprises: Perform fast Fourier FFT transform on the digital signal, and use the periodogram method to obtain the pulse-echo power spectrum; Accumulating multiple pulse echo power spectra to obtain an accumulated power spectrum; The Brillouin frequency shift is calculated from the accumulated power spectrum using a nonlinear fitting algorithm. 6. seawater temperature measuring method as claimed in claim 1, is characterized in that, described seawater temperature inversion model is: T(S,ν _B )＝a ₀ +a ₁ (ν _B -7.5)+a ₂ (ν _B -7.5) ² +a ₃ (ν _B -7.5) ³ +a ₄ (ν _B -7.5) ⁶ + S[a ₅ +a ₆ (ν _B -7.5)+a ₇ (ν _B -7.5) ² +a ₈ (ν _B -7.5) ³ ] Where, T is the temperature, a _i is a constant, i=1,2,...,8, ν _B is the Brillouin frequency shift, and S is the salinity in seawater. 7. A seawater temperature measurement system, characterized in that, comprising: A laser emitting unit, which is used to generate laser light, divide the laser light into coherent local oscillator light and probe light, and focus the probe light to a specified depth of water; a light receiving unit, which is used to receive and scatter light to Rayleigh Brillouin from a specified depth of water; A photoelectric conversion unit, which is used to mix the backward Rayleigh Brillouin scattered light and the local oscillator light and output an intermediate frequency signal after photoelectric conversion; The data processing unit is used to extract the Brillouin frequency shift from the intermediate frequency signal, and calculate and obtain the seawater temperature in the water area at a specified depth according to the seawater temperature inversion model. 8. The seawater temperature measurement system as claimed in claim 7, wherein the laser emitting unit comprises a laser, a beam splitter, a beam control and a shaper, the laser is used to generate laser light, and the beam splitter is used to divide the laser light into Local oscillator light and probe light, beam steering and shaper are used to focus the probe light to the specified depth of water. 9. The seawater temperature measurement system according to claim 7, wherein the photoelectric conversion unit comprises: A balanced detector, which is used to respectively receive the local oscillator light and the backward Rayleigh Brillouin scattered light, mix the backward Rayleigh Brillouin scattered light with the local oscillator light and output an intermediate frequency signal after photoelectric conversion; an amplifier, which is used to amplify the intermediate frequency signal; The acquisition card is used for performing digital sampling on the amplified intermediate frequency signal. 10. The seawater temperature measurement system according to claim 7, characterized in that: the seawater temperature inversion model is: T(S,ν _B )＝a ₀ +a ₁ (ν _B -7.5)+a ₂ (ν _B -7.5) ² +a ₃ (ν _B -7.5) ³ +a ₄ (ν _B -7.5) ⁶ + S[a ₅ +a ₆ (ν _B -7.5)+a ₇ (ν _B -7.5) ² +a ₈ (ν _B -7.5) ³ ] Where, T is the temperature, a _i is a constant, i=1,2,...,8, ν _B is the Brillouin frequency shift, and S is the salinity in seawater.