High-precision ocean optical parameter detection system based on wedge-shaped cavity

Abstract

The invention discloses a high-precision ocean optical parameter detection system based on a wedge-shaped cavity. A collimation optical unit focuses a laser radar echo signal and then shapes the laser radar echo signal into parallel beams; the interference narrow-band light filtering unit filters background light mixed in signals, and the transmitted signals are divided into a horizontal polarization component and a vertical polarization component through the polarization beam splitter. The point detector performs echo power detection on the vertical polarization component; the multi-beam interference discriminator is used for the horizontal polarization component to generate multi-beam interference to form a light and dark linear spectrum; the light beam shaping unit is used for shaping bright and dark linear spectrums, and the high-sensitivity detector array adopts a linear array channel to detect spectral shapes of elastic spectrums and non-elastic spectrums. And the comprehensive control and data processing unit calculates a diffusion attenuation coefficient by using an echo power detection result of the point detector, and calculates a 180-degree backscattering coefficient of the water body by using spectral shapes of an elastic spectrum and a non-elastic spectrum. According to the method, the overlapping problem of the elastic scattering spectrum and the inelastic scattering spectrum can be solved, and the inversion precision of optical parameters such as the backscattering coefficient of the water body is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of hyperspectral laser radar and relates to a detection system based on multi-beam interference of a wedge-shaped cavity, which can detect the optical parameter profile of the ocean. Background Art

[0002] After the laser interacts with the water body, elastic scattering and inelastic scattering will occur. Elastic scattering includes Mie scattering and Rayleigh scattering. This type of scattering is mainly caused by the combined effect of particles and water molecules in the water body. The central wavelength of the elastic scattered light does not shift in frequency relative to the original laser wavelength. Inelastic scattering includes Brillouin scattering and Raman scattering. This type of scattering is only caused by molecular scattering in the water body. The central wavelength of inelastic scattering will shift in frequency relative to the laser wavelength. The proportion of Rayleigh component in elastic scattering is extremely low, and elastic scattering can be regarded as scattering of particles. Since the Brillouin scattering cross section of the water body is larger than that of Raman scattering, the echo energy is strong, the wavelength is very close to the elastic scattering wavelength, and the molecular scattering signal energy ratio of the Brillouin scattering signal is strongly correlated with the optical parameters of the water body, the measurement of elastic scattering and Brillouin scattering can be used to invert the optical parameters of the water body.

[0003] Usually, the Brillouin frequency shift of blue-green laser caused by water is 7-8 GHz. The measurement of this tiny frequency shift requires the use of high spectral resolution laser radar. The core component of high spectral resolution laser radar is a high-precision discriminator. Common discriminators include atomic and molecular vapor absorption cell, Michelson interferometer, Fabry-Pérot etalon, etc. The atomic and molecular vapor absorption cell only has a high spectral filtering effect on specific wavelengths according to the absorption peak of specific atoms or molecules, and the transmittance function form of each spectrum band is determined by the atomic and molecular characteristics. The method of regulating the transmittance function of the discriminator is relatively limited. The Michelson interferometer is a dual-beam interference device. It can preliminarily distinguish elastic scattering from inelastic scattering by dichotomy, but in fact elastic scattering and inelastic scattering overlap in the frequency domain. This dichotomy can establish an equation to analyze the influence of the overlapping area, and it is necessary to add an energy detection channel for calibration. The Fabry-Pérot (FP) etalon can be used as a filter when the wedge angle of the two plates is zero. It also distinguishes elastic scattering and inelastic scattering by dichotomy, and it is also necessary to add an energy detection channel for calibration. Summary of the invention

[0004] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art, to provide a high-precision detection method and system for ocean optical parameters based on a wedge-shaped cavity, to use a wedge-shaped cavity for spectral splitting, to simultaneously obtain the spectral shapes of an elastic scattering spectrum and an inelastic scattering spectrum in a single detection, and to have strong real-time performance; to perform waveform decomposition of the elastic scattering spectrum and the inelastic scattering spectrum through spectrum calculation and analysis, and to calculate the energy transmittance of the elastic scattering spectrum and the inelastic scattering spectrum respectively, to solve the overlapping problem of the elastic scattering spectrum and the inelastic scattering spectrum, and to improve the inversion accuracy of optical parameters such as the water body backscattering coefficient.

[0005] The technical solution of the present invention is: a high-precision detection system for ocean optical parameters based on a wedge-shaped cavity, including a collimating optical unit, an interference narrow-band filter unit, a polarization beam splitter, a multi-beam interference frequency discriminator, a beam shaping unit, a high-sensitivity detector array, a point detector, and an integrated control and data processing unit, wherein:

[0006] Collimation optical unit: focuses the laser radar echo signal and then shapes it into a parallel beam before sending it to the interference narrow-band filter unit;

[0007] Interference narrow-band filter unit: filters out the background light mixed in the laser radar echo signal, and sends the transmitted laser radar echo signal to the polarization beam splitter;

[0008] Polarization beam splitter: splits the input laser radar echo signal into horizontal polarization component and vertical polarization component, sends the horizontal polarization component to the multi-beam interference frequency detector, and sends the vertical polarization component to the point detector;

[0009] Point detector: detects the echo power of the vertical polarization component and transmits the echo power detection result to the data processing unit;

[0010] Multi-beam interference frequency discriminator: It provides multi-beam interference for horizontal polarization components to form bright and dark line spectra and send them to the beam shaping unit;

[0011] A beam shaping unit is used to shape the light and dark linear spectrum, so that the long side direction of the light and dark stripes of the spectrum is parallel to the long side direction of the pixel of the high-sensitivity detector array and the length is equal, and the width direction of the spectrum is parallel to the short side direction of the light and dark stripes and just covers the width of the high-sensitivity detector array;

[0012] High-sensitivity detector array: uses N×1 linear array channels to detect the spectral shapes of elastic and inelastic spectra, and sends the results to the integrated control and data processing unit; N represents the total number of detector pixels, N is a positive integer and N≥3;

[0013] Integrated control and data processing unit: Calculate the diffuse attenuation coefficient using the echo power detection results of the point detector; Calculate the 180° backscattering coefficient of the water body using the spectral shapes of the elastic spectrum and inelastic spectrum detected by the high-sensitivity detector array.

[0014] Preferably, the multi-beam interference discriminator is a wedge-shaped cavity, which is composed of two glass plates placed opposite to each other and maintaining an angle, and there is a vacuum gap between the two plates.

[0015] Furthermore, the integrated control and data processing unit calculates the 180° backscattering coefficient of the water body using the spectral shapes of the elastic spectrum and the inelastic spectrum detected by the high-sensitivity detector array, including:

[0016] S1. Collect the input spectrum signal of the high-sensitivity detector array and obtain the actual power measurement value J(υ M ), where υ M = -FSR / 2+(2M-1)FSR / (2N) is the frequency shift of the center frequency of the laser echo detected by the Mth pixel relative to the center frequency of the emitted laser, M is a positive integer and M=1, 2, ..., N, N is the total number of pixels in the high-sensitivity detector array;

[0017] S2, the theoretical power calculation value of the input spectrum is J1(υ M )=T B ( M )β B +T p ( M )(β-β B ),in represents the transmittance of the molecular scattering spectrum, Represents the transmittance of the particle scattering spectrum, β B is the 180° backscattering coefficient of water molecules caused by inelastic scattering, β B =2.4×10 -4 m -1 Sr -1 , β is the total water body 180° backscattering coefficient, letters B and r are the inelastic scattering component and elastic scattering component respectively, Γ B and Γ r are the inelastic scattering spectrum line width and elastic scattering laser line width, υ is the laser echo frequency, and υ B is the frequency shift of laser inelastic scattering relative to the laser emission frequency, * represents convolution, FP(υ) is the transmittance function of the multi-beam interferometer frequency discriminator, R is the reflectivity of the two glass plates of the wedge-shaped cavity, and FSR is the free spectral range of the wedge-shaped cavity;

[0018] S3, the actual power measurement value and theoretical power calculation value from the second channel to the Nth channel are respectively compared with the actual power measurement value and theoretical power calculation value of the first channel, and N-1 equations {J1(υ M ) / J1(υ1)=J(υ M ) / J(υ1)}, and obtain the 180° backscattering coefficients of N-1 water bodies respectively is the ratio of the actual power measurement value of the Mth pixel to the actual power measurement value of the first pixel;

[0019] S4. Average the 180° backscattering coefficients of N-1 water bodies to obtain the 180° backscattering coefficient of the water body.

[0020] Furthermore, the integrated control and data processing unit calculates the diffuse attenuation coefficient using the echo power detection result of the point detector, including: substituting the 180° backscattering coefficient β(z) of the water body into the laser depth measurement equation P(z)=X0β(z)exp(-2K d (z)z), the diffuse attenuation coefficient K at depth z is calculated d (z) = -ln(P(z) / X0 / β(z)) / 2z; where n is the refractive index of water, X0 is a constant related to the laser emission power, transmission medium, and detection system, and P(z) is the laser echo power obtained by the point detector.

[0021] Preferably, the wedge-shaped cavity is placed in a closed vacuum chamber with a light-transmitting window, and the temperature of the vacuum chamber is maintained at 30°C ± 0.2°C.

[0022] Preferably, the included angle between the two glass plates of the wedge-shaped cavity is λ0 / 2 / D, where λ0 is the central wavelength of the emitted laser and D is the light transmission diameter of the multi-beam interference discriminator.

[0023] Preferably, the inner sides of the two opposite glass plates of the wedge-shaped cavity are coated with a high-reflection film for the central wavelength of the emitted laser, and the reflectivity is not less than 90%.

[0024] Preferably, the transmittance bandwidth of the interference narrow-band filter unit is ±0.1 nm, the peak-to-peak transmittance is above 98%, and the cut-off band transmittance is above OD6.

[0025] Preferably, the high-sensitivity detector array adopts a photomultiplier tube array or an avalanche diode array.

[0026] Preferably, the point detector is a photomultiplier tube or an avalanche diode.

[0027] The advantages of the present invention compared with the prior art are:

[0028] (1) The system of the present invention can be used at any wavelength and has a wider wavelength working range than the atomic and molecular vapor absorption cell;

[0029] (2) The ocean optical parameter detection system based on the dual-beam interference scheme of the Michelson interferometer and FP etalon filters out the elastic scattering part in the echo signal, but due to the overlap between elastic scattering and inelastic scattering, there is a large error in the subsequent optical parameter inversion. The spectral resolution of the system of the present invention is higher, and the spectral information of the elastic scattering and inelastic scattering components is detected at the same time, and mutual calibration is performed through multi-channel detection, thereby solving the overlap problem;

[0030] (3) Compared with the ocean optical parameter detection scheme in the laser energy domain, such as the Fernald method, the system of the present invention does not require the assumption of the laser radar ratio and can simultaneously invert the backscattering coefficient and diffuse attenuation coefficient of the water body through high-resolution analysis of the laser spectrum. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The figure is a block diagram of the composition principle of the system of the present invention. DETAILED DESCRIPTION

[0032] like Figure 1 As shown, the present invention is a high-precision detection system for ocean optical parameters based on a wedge-shaped cavity, comprising a collimating optical unit 1, an interference narrow-band filter unit 2, a polarization beam splitter 3, a multi-beam interference frequency discriminator 4, a beam shaping unit 5, a high-sensitivity detector array 6, a point detector 7, and an integrated control and data processing unit 8.

[0033] The laser radar echo signal scattered back by the seawater first enters the collimating optical unit 1. The collimating optical unit 1 focuses the laser radar echo signal first and then shapes it into a parallel beam and then sends it to the interference narrow-band filter unit 2. The interference narrow-band filter 2 filters out the background light mixed in the echo signal, and the transmitted echo signal enters the polarization beam splitter 3. The polarization beam splitter 3 divides the echo signal into a horizontal polarization component and a vertical polarization component. The horizontal polarization component passes through the multi-beam interference frequency discriminator 4, and the output light and dark stripes can completely cover the high-sensitivity detector array 6 through the beam shaping unit 5. The high-sensitivity detector array 6 uses NⅩ1 linear array channels to detect the spectral shape of the elastic spectrum and the inelastic spectrum, and transmits the results to the comprehensive control and data processing unit 8. The vertical polarization component directly passes through the point detector 7 for echo power detection, and the results are transmitted to the data processing unit 8. The data processing unit 8 performs detection timing control, as well as data acquisition, processing and inversion.

[0034] The transmittance bandwidth of the interference narrowband filter unit 2 is ±0.1 nm, the peak-to-peak transmittance is above 98%, and the cut-off band transmittance is above OD6.

[0035] The multi-beam interference discriminator 4 is a wedge-shaped cavity, which is composed of two glass plates placed opposite to each other and maintaining an angle. The inner sides of the opposite surfaces of the two plates are coated with a high-reflection film for the central wavelength of the emitted laser, and the reflectivity is not less than 90%. There is a vacuum gap between the two plates. The value of the angle between the two plates is λ0 / 2 / D, where λ0 is the central wavelength of the emitted laser, and D is the light-passing diameter of the multi-beam interference discriminator 4. The wedge-shaped cavity is placed in a closed vacuum cavity with a light-passing window. The temperature of the vacuum cavity is controlled to be maintained at 30℃±0.2℃ by active temperature control of a semiconductor refrigerator combined with water cooling, so that the transmittance curve of the wedge-shaped cavity to the laser spectrum is stable. The input of the multi-beam interference discriminator 4 is the echo of the horizontal polarization component output by the polarization beam splitter 3. This part of the echo just covers the light-passing window of the multi-beam interference discriminator 4 through beam expansion or contraction, and outputs light and dark linear stripes.

[0036] The beam shaping unit 5 shapes the light and dark line spectrum output by the multi-beam interference discriminator 4, making the long side direction of the light and dark stripes of the spectrum parallel to the long side direction of the pixel of the high-sensitivity detector array 6 and equal in length, and the spectrum width direction is parallel to the short side direction of the light and dark stripes, and the spectrum width direction is adjusted to just cover the width of the high-sensitivity detector array 6.

[0037] The high-sensitivity detector array 6 is a photomultiplier tube array or avalanche diode array of N×1 scale, where N is a positive integer and N≥3. The point detector 7 uses a photomultiplier tube or avalanche diode.

[0038] The integrated control and data processing unit 8 receives the light emitting synchronization signal of the laser emission system, starts the high-speed data acquisition of the point detector 7 and the high-sensitivity detector array 6 for a total of (N+1) channels, and then completes the storage of multi-channel data. The stored data is used to calculate the optical parameters such as the backscattering coefficient and diffuse attenuation coefficient of the water body. The data of the point detector 7 can be used to calculate the diffuse attenuation coefficient, and the detection data of the high-sensitivity detector array 6 is used to calculate the 180° backscattering coefficient of the water body.

[0039] The calculation methods of 180° backscattering coefficient and diffuse attenuation coefficient of water body include:

[0040] S1, collect the spectral signal input by the high-sensitivity detector array 6, and obtain the actual power measurement value J (υ M ), M It is the frequency shift of the center frequency of the detection laser echo corresponding to the Mth pixel relative to the center frequency of the transmitting laser, M is a positive integer and M=1, 2, ..., N, N is the total number of pixels of the high-sensitivity detector array 6.

[0041] S2. Based on the multi-beam interference theory, the transmittance function of the multi-beam interference discriminator 4 is calculated. Where R is the reflectivity of the two plates of the wedge-shaped cavity of the multi-beam interference discriminator 4, and FSR is the free spectral range of the wedge-shaped cavity. The calculated value of the output spectral property obtained by theoretical derivation is J1(υ M )=T B ( M )β B +T p ( M )(β-β B ), where υ M =-FSR / 2+(2M-1)FSR / (2N).

[0042] represents the transmittance of the molecular scattering spectrum, Represents the transmittance of the particle scattering spectrum, β B represents the 180° backscattering coefficient of water molecules caused by inelastic scattering, which is 2.4×10 -4 m -1 Sr -1 , β represents the total water body 180° backscattering coefficient, letters B and r represent the inelastic scattering component and elastic scattering component respectively, Γ B and Γ r is the inelastic scattering spectrum line width and elastic scattering laser line width, υ is the laser echo frequency, υ B is the frequency shift of laser inelastic scattering relative to the laser emission frequency, * represents convolution, N represents the total number of detector pixels, and M represents the Mth detector pixel.

[0043] S3. The measured values ​​and calculated values ​​from the second channel to the Nth channel are respectively compared with the measured values ​​and calculated values ​​of the first channel, and (N-1) equations {J1(υ M ) / J1(υ1)=J(υ M ) / J(υ1)}(M=2,…N), and obtain the 180° backscattering coefficients of (N-1) water bodies respectively here is the ratio of the power measurement value of the Mth pixel to the power measurement value of the first pixel.

[0044] S4. Average the 180° backscatter coefficients of (N-1) water bodies to obtain the 180° backscatter coefficient of the water body.

[0045] S5. The backscatter coefficient β of water body is a function of the water depth z. The minimum step value of z is Δz=cΔt / (2n), where c is the speed of light in vacuum, Δt is the minimum time interval corresponding to high-speed sampling, and n is the refractive index of water body. Based on the LiDAR bathymetric equation P(z)=X0β(z)exp(-2K d(z)z), where X0 is a constant related to the laser emission power, transmission medium, and detection system, β(z) is the 180° backscattering coefficient of the water body, and P(z) is the laser echo power obtained by the point detector 7. Substituting the 180° backscattering coefficient β(z) calculated by S4 into the laser depth measurement equation, the diffuse attenuation coefficient K at depth z can be calculated. d (z)=-ln(P(z) / X0 / β(z)) / 2z.

[0046] The contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art.

Claims

1. A high-precision detection system for ocean optical parameters based on a wedge-shaped cavity, characterized by: It comprises a collimating optical unit (1), an interference narrow-band filter unit (2), a polarization beam splitter (3), a multi-beam interference frequency detector (4), a beam shaping unit (5), a high-sensitivity detector array (6), a point detector (7), and an integrated control and data processing unit (8), wherein: Collimating optical unit (1): focusing the laser radar echo signal first and then shaping it into a parallel beam and then sending it to the interference narrow-band filter unit (2); Interference narrow-band filter unit (2): filters out background light mixed in the laser radar echo signal, and transmits the transmitted laser radar echo signal to the polarization beam splitter (3); Polarization beam splitter (3): splits the input laser radar echo signal into a horizontal polarization component and a vertical polarization component, sends the horizontal polarization component to the multi-beam interference frequency detector (4), and sends the vertical polarization component to the point detector (7); Point detector (7): performs echo power detection on the vertical polarization component and transmits the echo power detection result to the data processing unit (8); Multi-beam interference frequency detector (4): for causing multi-beam interference of horizontal polarization components to form a bright and dark line spectrum which is sent to the beam shaping unit (5); A beam shaping unit (5) is used to shape the light and dark linear spectrum, so that the long side direction of the light and dark stripes of the spectrum is parallel to the long side direction of the pixel of the high-sensitivity detector array (6) and the length is equal, and the width direction of the spectrum is parallel to the short side direction of the light and dark stripes and just covers the width of the high-sensitivity detector array (6); High-sensitivity detector array (6): uses N×1 linear array channels to detect the spectral shapes of elastic spectrum and inelastic spectrum, and sends the results to the integrated control and data processing unit (8); N represents the total number of detector pixels, N is a positive integer and N≥3; The integrated control and data processing unit (8) calculates the diffuse attenuation coefficient using the echo power detection result of the point detector (7); and calculates the 180° backscattering coefficient of the water body using the spectral shapes of the elastic spectrum and the inelastic spectrum detected by the high-sensitivity detector array (6).

2. According to claim 1, a high-precision detection system for ocean optical parameters based on a wedge-shaped cavity is characterized in that: The multi-beam interference frequency discriminator (4) is a wedge-shaped cavity, which is composed of two glass plates placed opposite to each other and maintaining an angle, and a vacuum gap is formed between the two plates.

3. The high-precision detection system for ocean optical parameters based on a wedge-shaped cavity according to claim 2, characterized in that: The integrated control and data processing unit (8) calculates the 180° backscattering coefficient of the water body using the spectral shapes of the elastic spectrum and the inelastic spectrum detected by the high-sensitivity detector array (6), including: S1, collect the input spectrum signal of the high-sensitivity detector array (6), and obtain the actual power measurement value J (υ M ), where υ M = -FSR / 2+(2M-1)FSR / (2N) is the frequency shift of the center frequency of the laser echo detected by the Mth pixel relative to the center frequency of the emitted laser, M is a positive integer and M=1, 2, ..., N, N is the total number of pixels in the high-sensitivity detector array (6); S2, the theoretical power calculation value of the input spectrum is J1(υ M )=T B ( M )β B +T p ( M )(β-β B ),in represents the transmittance of the molecular scattering spectrum, Represents the transmittance of the particle scattering spectrum, β B is the 180° backscattering coefficient of water molecules caused by inelastic scattering, β B =2.4×10 -4 m -1 Sr -1 , β is the total water body 180° backscattering coefficient, letters B and r are the inelastic scattering component and elastic scattering component respectively, Γ B and Γ r are the inelastic scattering spectrum line width and elastic scattering laser line width, υ is the laser echo frequency, and υ B is the frequency shift of the laser inelastic scattering relative to the laser emission frequency, * represents convolution, FP(υ) is the transmittance function of the multi-beam interferometer (4), R is the reflectivity of the two glass plates of the wedge-shaped cavity, and FSR is the free spectral range of the wedge-shaped cavity; S3, the actual power measurement value and theoretical power calculation value from the second channel to the Nth channel are respectively compared with the actual power measurement value and theoretical power calculation value of the first channel, and N-1 equations {J1(υ M ) / J1(υ1)=J(υ M ) / J(υ1)}, and obtain the 180° backscattering coefficients of N-1 water bodies respectively is the ratio of the actual power measurement value of the Mth pixel to the actual power measurement value of the first pixel; S4. Average the 180° backscattering coefficients of N-1 water bodies to obtain the 180° backscattering coefficient of the water body.

4. The high-precision detection system for ocean optical parameters based on a wedge-shaped cavity according to claim 3 is characterized in that: The integrated control and data processing unit (8) calculates the diffuse attenuation coefficient using the echo power detection result of the point detector (7), including: substituting the 180° backscattering coefficient β(z) of the water body into the laser depth measurement equation P(z)=X0β(z)exp(-2K d (z)z), the diffuse attenuation coefficient K at depth z is calculated d (z) = -ln(P(z) / X0 / β(z)) / 2z; wherein n is the refractive index of the water body, X0 is a constant related to the laser emission power, transmission medium, and detection system, and P(z) is the laser echo power obtained by the point detector (7).

5. The high-precision detection system for ocean optical parameters based on a wedge-shaped cavity according to claim 2, characterized in that: The wedge-shaped cavity is placed in a closed vacuum cavity with a light-transmitting window, and the temperature of the vacuum cavity is maintained at 30°C±0.2°C.

6. The high-precision detection system for ocean optical parameters based on a wedge-shaped cavity according to claim 5, characterized in that: The value of the angle between the two glass plates of the wedge-shaped cavity is λ0 / 2 / D, where λ0 is the central wavelength of the emitted laser and D is the light transmission diameter of the multi-beam interference frequency discriminator (4).

7. The high-precision detection system for ocean optical parameters based on a wedge-shaped cavity according to claim 5, characterized in that: The inner sides of the two opposite glass plates of the wedge-shaped cavity are plated with a high-reflection film for the central wavelength of the emitted laser, and the reflectivity is not less than 90%.

8. The high-precision detection system for ocean optical parameters based on a wedge-shaped cavity according to claim 2, characterized in that: The interference narrow-band filter unit (2) has a transmittance bandwidth of ±0.1 nm, a peak-to-peak transmittance of more than 98%, and a cut-off band transmittance of more than OD6.

9. The high-precision detection system for ocean optical parameters based on a wedge-shaped cavity according to claim 2, characterized in that: The high-sensitivity detector array (6) adopts a photomultiplier tube array or an avalanche diode array.

10. The high-precision detection system for ocean optical parameters based on a wedge-shaped cavity according to claim 2, characterized in that: The point detector (7) adopts a photomultiplier tube or an avalanche diode.

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