A high-precision ocean optical parameter detection system based on a wedge cavity

Abstract

A high-precision detection system for ocean optical parameters based on a wedge-shaped cavity. A collimating optical unit focuses and shapes the laser radar echo signal into a parallel beam. An interference narrowband filter removes background light mixed in the signal, and the transmitted signal is separated into horizontally polarized and vertically polarized components by a polarization beam splitter. A point detector detects the echo power of the vertically polarized component. A multi-beam interference discriminator generates multi-beam interference for the horizontally polarized component, forming a light and dark linear spectrum. A beam shaping unit shapes the light and dark linear spectrum, and a high-sensitivity detector array uses linear array channels to detect the spectral shapes of the elastic and inelastic spectra. An integrated control and data processing unit uses the echo power detection results of the point detector to calculate the diffuse attenuation coefficient and uses the spectral shapes of the elastic and inelastic spectra to calculate the 180° backscattering coefficient of the water body. This invention can solve the problem of overlapping elastic and inelastic scattering spectra, improving the inversion accuracy of optical parameters such as the water backscattering coefficient.

Description

Technical Field

[0001] This invention belongs to the field of hyperspectral lidar technology and relates to a multi-beam interferometry detection system based on a wedge cavity, which can detect the optical parameter profile of the ocean. Background Technology

[0002] When a laser interacts with water, elastic and inelastic scattering occur. Elastic scattering includes Mie and Rayleigh scattering, primarily caused by the interaction of particles and water molecules in the water. The center wavelength of the elastically scattered light does not shift relative to the original laser wavelength. Inelastic scattering includes Brillouin and Raman scattering, caused solely by molecular scattering in the water. The center wavelength of inelastic scattering undergoes a frequency shift relative to the laser wavelength. The Rayleigh component is extremely low in elastic scattering, which can be considered as scattering by particles. Because the Brillouin scattering cross-section of water is larger than that of Raman scattering, the echo energy is stronger, and the wavelength is very close to that of elastic scattering. Furthermore, the proportion of molecular scattering energy in the Brillouin scattering signal is strongly correlated with the optical parameters of the water body. Therefore, measurements of elastic and Brillouin scattering can be used to invert the optical parameters of water bodies.

[0003] The Brillouin shift of blue-green laser light induced by water is typically between 7 and 8 GHz. Measuring this minute shift requires high-spectral-resolution lidar. The core component of high-spectral-resolution lidar is a high-precision frequency discriminator. Common discriminators include atomic and molecular vapor absorption cells, Michelson interferometers, and Fabry-Pérot etalons. Atomic and molecular vapor absorption cells, based on the absorption peaks of specific atoms or molecules, provide high-spectral filtering only for specific wavelengths. Furthermore, the transmittance function of each spectral band is determined by the atomic and molecular characteristics, limiting the methods for adjusting the discriminator's transmittance function. Michelson interferometers, being two-beam interferometers, can initially distinguish between elastic and inelastic scattering using a dichotomy method. However, elastic and inelastic scattering overlap in the frequency domain. This dichotomy method can establish an equation to analyze the influence of the overlapping region, but requires additional energy detection channels for calibration. The Fabry-Pérot (FP) etalon, with its two plates having a zero wedge angle, can be used as a filter. It also distinguishes between elastic and inelastic scattering using a dichotomy method and similarly requires additional energy detection channels for calibration. Summary of the Invention

[0004] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide a high-precision detection method and system for marine optical parameters based on a wedge cavity. The wedge cavity is used for spectral dispersion, and the spectral shapes of elastic scattering spectrum and inelastic scattering spectrum can be obtained simultaneously in a single detection, with strong real-time performance. By calculating and analyzing the spectrum, the elastic scattering spectrum and inelastic scattering spectrum are decomposed into waveforms, and then the energy transmittance of the elastic scattering spectrum and inelastic scattering spectrum are calculated separately. This can solve the overlap problem of elastic scattering spectrum and inelastic scattering spectrum and improve the inversion accuracy of optical parameters such as water backscattering coefficient.

[0005] The technical solution of this invention is: a high-precision detection system for marine optical parameters based on a wedge cavity, comprising a collimating optical unit, an interferometric narrowband filter unit, a polarization beam splitter, a multi-beam interferometric frequency discriminator, a beam shaping unit, a high-sensitivity detector array, a point detector, and an integrated control and data processing unit, wherein:

[0006] Collimating optical unit: The laser radar echo signal is first focused and then shaped into a parallel beam before being sent to the interference narrowband filter unit;

[0007] Interferometric narrowband filter unit: filters out the background light mixed in the lidar echo signal, and sends the transmitted lidar echo signal into the polarization beam splitter;

[0008] Polarization beam splitter: splits the incoming lidar echo signal into a horizontal polarization component and a vertical polarization component. The horizontal polarization component is sent to a multi-beam interferometer frequency discriminator, and the vertical polarization component is sent to a point detector.

[0009] Point detector: performs echo power detection on the vertical polarization component and transmits the echo power detection results to the data processing unit;

[0010] Multi-beam interference discriminator: provides multi-beam interference for horizontal polarization components, forming bright and dark line spectra that are sent to the beam shaping unit;

[0011] Beam shaping unit: Shapes the bright and dark line spectrum so that the long side of the bright and dark stripes of the spectrum is parallel to the long side of the pixels of the high-sensitivity detector array and has the same length, and the width of the spectrum is parallel to the short side of the bright and dark stripes and just covers the width of the high-sensitivity detector array.

[0012] High-sensitivity detector array: The array uses an N×1 linear channel 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: Calculates the diffuse attenuation coefficient using the echo power detection results of the point detector; calculates the 180° backscattering coefficient of the water body using the spectral shapes of the elastic and inelastic spectra detected by the high-sensitivity detector array.

[0014] Preferably, the multi-beam interference frequency discriminator is a wedge-shaped cavity, consisting of two glass plates placed opposite each other and maintaining an included angle, with 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 and inelastic spectra detected by the high-sensitivity detector array, including:

[0016] S1. Acquire the input spectral signal of the high-sensitivity detector array and obtain the actual power measurement value J(υ) of the input spectral shape. 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, where M is a positive integer and M=1,2,…,N, and N is the total number of pixels in the high-sensitivity detector array;

[0017] S2. The theoretical power calculation value of the input spectral shape is obtained through theoretical derivation: J1(υ M ) = T B (υ M )β B +T p (υ M )(β-β B ),in The transmittance represents the molecular scattering spectrum. β represents the transmittance of the particulate scattering spectrum. B β is the backscattering coefficient of water molecules at 180° caused by inelastic scattering. B =2.4×10 -4 m -1 Sr -1 β is the total water backscattering coefficient at 180°, B and r are the inelastic and elastic scattering components, respectively, and Γ B and Γ r These represent the linewidths of the inelastic scattering spectrum and the laser linewidth of the elastic scattering laser, respectively, where υ is the laser echo frequency. B Let ν represent the frequency shift of laser inelastic scattering relative to the laser emission frequency, * denotes convolution, and FP(υ) be the transmittance function of the multibeam interferometer frequency discriminator. R is the reflectance of the two glass plates of the wedge cavity, and FSR is the free spectral range of the wedge cavity;

[0018] S3. Ratio the actual power measurement and theoretical power calculation values ​​from the 2nd to the Nth channel to the actual power measurement and theoretical power calculation values ​​of the first channel, respectively, and establish N-1 equations {J1(υ M ) / J1(υ1)=J(υ M ) / J(υ1)}, respectively, to obtain the 180° backscattering coefficients of N-1 water bodies. This 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 coefficients of the water bodies.

[0020] Furthermore, the integrated control and data processing unit calculates the diffuse attenuation coefficient using the echo power detection results of the point detector, including: substituting the 180° backscattering coefficient β(z) of the water body into the laser depth sounding equation P(z)=X0β(z)exp(-2K d (z)z), calculate the diffuse attenuation coefficient K at depth z. d (z)=-ln(P(z) / X0 / β(z)) / 2z; where n is the refractive index of the 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 inside a sealed vacuum chamber with a light-transmitting window, and the temperature of the vacuum chamber is maintained at 30℃±0.2℃.

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

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

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

[0025] Preferably, the high-sensitivity detector array is 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 this invention compared to the prior art are:

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

[0029] (2) A marine optical parameter detection system based on a Michelson interferometer and FP etalon dual-beam interferometry scheme filters out the elastic scattering component in the echo signal. However, due to the overlap between elastic and inelastic scattering, the subsequent optical parameter inversion has a large error. The system of this invention has higher spectral resolution, detects the spectral information of both elastic and inelastic scattering components, and performs mutual calibration through multi-channel detection, thereby solving the overlap problem.

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

[0031] Figure 1 This is a block diagram illustrating the composition principle of the system of the present invention. Detailed Implementation

[0032] like Figure 1 As shown, the present invention discloses a high-precision detection system for marine optical parameters based on a wedge cavity, comprising a collimating optical unit 1, an interference narrowband 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 lidar echo signal scattered back from seawater first enters the collimating optical unit 1. The collimating optical unit 1 focuses and shapes the lidar echo signal into a parallel beam before sending it to the interference narrowband filter unit 2. The interference narrowband filter 2 filters out the mixed background light in the echo signal, and the transmitted echo signal enters the polarization beam splitter 3. The polarization beam splitter 3 splits the echo signal into a horizontal polarization component and a vertical polarization component. The horizontal polarization component passes through the multi-beam interferometric discriminator 4, and the output bright and dark fringes are completely covered by the beam shaping unit 5 to reach the high-sensitivity detector array 6. The high-sensitivity detector array 6 uses an NX-1 linear array channel to detect the spectral shapes of the elastic and inelastic spectra and sends the results to the integrated 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 sent 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 ±0.1nm, the peak-to-peak transmittance is above 98%, and the cutoff band transmittance is above OD6.

[0035] The multi-beam interferometer frequency discriminator 4 is a wedge-shaped cavity composed of two glass plates placed opposite each other at an angle. The inner surfaces of the opposing faces of the two plates are coated with a high-reflectivity film targeting the center wavelength of the emitted laser, with a reflectivity of no less than 90%. A vacuum gap lies between the two plates. The angle between the two plates is λ0 / 2 / D, where λ0 is the center wavelength of the emitted laser and D is the optical transmission diameter of the multi-beam interferometer frequency discriminator 4. The wedge-shaped cavity is placed within a sealed vacuum cavity with an optical transmission window. The temperature of the vacuum cavity is maintained at 30℃±0.2℃ through active temperature control using a semiconductor cooler combined with water cooling, ensuring a stable transmittance curve for the laser spectrum. The input to the multi-beam interferometer frequency discriminator 4 is the echo of the horizontal polarization component output from the polarization beam splitter 3. This echo, after beam expansion or contraction, precisely covers the optical transmission window of the multi-beam interferometer frequency discriminator 4, outputting bright and dark linear fringes.

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

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

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

[0039] The methods for calculating the 180° backscattering coefficient and diffuse attenuation coefficient of water bodies include:

[0040] S1. Acquire the spectral signal input to the high-sensitivity detector array 6, and obtain the actual power measurement value J(υ) of the output spectral shape. M ), υ M The frequency shift of the center frequency of the probe laser echo corresponding to the Mth pixel relative to the center frequency of the emitted laser is M, where M is a positive integer and M = 1, 2, ..., N, and N is the total number of pixels in the high-sensitivity detector array 6.

[0041] S2. The transmittance function of the multi-beam interferometer frequency discriminator 4 is calculated based on the multi-beam interferometry theory. Where R is the reflectivity of the two plates of the wedge cavity in the multi-beam interferometric discriminator 4, and FSR is the free spectral range of the wedge cavity. Theoretical derivation yields the calculated value of the output spectral characteristics as J1(υ). M ) = T B (υ M )β B +T p (υ M )(β-β B ), where υ M = -FSR / 2+(2M-1)FSR / (2N).

[0042] The transmittance represents the molecular scattering spectrum. β represents the transmittance of the particulate scattering spectrum. B The coefficient representing the 180° backscattering of water molecules due to inelastic scattering is taken as 2.4 × 10⁻⁶. -4 m -1 Sr -1 β represents the total water backscattering coefficient at 180°, B and r represent the inelastic and elastic scattering components, respectively, and Γ B and Γ r Here, υ represents the linewidth of the inelastic scattering spectrum and the linewidth of the elastic scattering laser, and υ represents the laser echo frequency. B The frequency shift of laser inelastic scattering relative to the laser emission frequency is represented by *, where * represents convolution, N represents the total number of detector pixels, and M represents the Mth detector pixel.

[0043] S3. Ratio the measured and calculated values ​​from the 2nd to the Nth channel to the measured and calculated values ​​of the first channel, and establish (N-1) equations {J1(υ M ) / J1(υ1)=J(υ M ) / J(υ1)}(M=2,…N), respectively, to obtain the 180° backscattering coefficients of (N-1) water bodies. here It 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° backscattering coefficients of (N-1) water bodies to obtain the 180° backscattering coefficients of the water bodies.

[0045] S5. The backscattering coefficient β of the 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 the water body. Based on the lidar depth measurement 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 backscattering coefficient of the water body at 180°, and P(z) is the laser echo power acquired by point detector 7. Substituting the backscattering coefficient β(z) of the water body at 180° calculated by S4 into the laser depth sounding 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 this specification are common knowledge to those skilled in the art.

Claims

1. A high-precision detection system for marine optical parameters based on a wedge cavity, characterized in that: It includes a collimating optical unit (1), an interference narrowband 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), wherein: Collimating optical unit (1): The laser radar echo signal is first focused and then shaped into a parallel beam before being sent to the interference narrowband filter unit (2); Interference narrowband filter unit (2): filters out the mixed background light in the lidar echo signal, and the transmitted lidar echo signal is sent to the polarization beam splitter (3); Polarization beam splitter (3): splits the incoming lidar echo signal into a horizontal polarization component and a vertical polarization component. The horizontal polarization component is sent to the multi-beam interference discriminator (4), and the vertical polarization component is sent to the point detector (7). Point detector (7): performs echo power detection on the vertical polarization component and transmits the echo power detection results to the data processing unit (8); Multi-beam interference discriminator (4): provides multi-beam interference for horizontal polarization components, forming bright and dark line spectra that are sent to the beam shaping unit (5); Beam shaping unit (5): Shapes the bright and dark linear spectrum so that the long side direction of the bright and dark stripes of the spectrum is parallel to the long side direction of the pixels of the high-sensitivity detector array (6) and the length is equal, and the spectral width direction is parallel to the short side direction of the bright and dark stripes and just covers the width of the high-sensitivity detector array (6). High-sensitivity detector array (6): The N×1 linear array channel is used to detect the spectral shape of elastic and inelastic spectra and the results are sent 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; Integrated control and data processing unit (8): Calculates the diffuse attenuation coefficient using the echo power detection results of the point detector (7); Calculates the 180° backscattering coefficient of the water body using the spectral shape of the elastic and inelastic spectra detected by the high-sensitivity detector array (6); The multi-beam interference frequency discriminator (4) is a wedge-shaped cavity, consisting of two glass plates placed opposite each other and maintaining an included angle, with a vacuum gap between the two plates; The integrated control and data processing unit (8) calculates the 180° backscattering coefficient of the water body using the spectral shapes of the elastic and inelastic spectra detected by the high-sensitivity detector array (6), including: S1. Acquire the input spectral signal of the high-sensitivity detector array (6) and obtain the actual power measurement value J(υ) of the input spectral shape. 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, where M is a positive integer and M=1,2,…,N, and N is the total number of pixels in the high-sensitivity detector array (6); S2. The theoretical power calculation value of the input spectral shape is obtained through theoretical derivation: J1(υ M ) = T B (υ M )β B +T p (υ M )(β-β B )in The transmittance represents the molecular scattering spectrum. β represents the transmittance of the particulate scattering spectrum. B β is the backscattering coefficient of water molecules at 180° caused by inelastic scattering. B =2.4×10 -4 m -1 Sr -1 β is the total water backscattering coefficient at 180°, B and r are the inelastic and elastic scattering components, respectively, and Γ B and Γ r These represent the linewidths of the inelastic scattering spectrum and the elastic scattering laser, respectively, where υ is the laser echo frequency. B Let $\frac{ ... R is the reflectance of the two glass plates of the wedge cavity, and FSR is the free spectral range of the wedge cavity; S3. Ratio the actual power measurement and theoretical power calculation values ​​from the 2nd to the Nth channel to the actual power measurement and theoretical power calculation values ​​of the first channel, respectively, and establish N-1 equations {J1(υ M ) / J1(υ1)=J(υ M ) / J(υ1)}, respectively, to obtain the 180° backscattering coefficients of N-1 water bodies. This 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 coefficients of the water bodies.

2. The high-precision marine optical parameter detection system based on a wedge cavity according to claim 1, characterized in that: The integrated control and data processing unit (8) calculates the diffuse attenuation coefficient using the echo power detection results of the point detector (7), including: substituting the 180° backscattering coefficient β(z) of the water body into the laser depth sounding equation P(z)=X0β(z)exp(-2K d (z)z), calculate the diffuse attenuation coefficient at depth z. Where n is the refractive index of the water, X0 is a constant related to the laser emission power, transmission medium, and detection system, and P(z) is the laser echo power acquired by the point detector (7).

3. The high-precision marine optical parameter detection system based on a wedge cavity according to claim 1, characterized in that: The wedge-shaped cavity is placed inside a sealed vacuum chamber with a light-transmitting window, and the temperature of the vacuum chamber is maintained at 30℃±0.2℃.

4. The high-precision marine optical parameter detection system based on a wedge cavity according to claim 3, characterized in that: The angle between the two glass plates of the wedge cavity is λ0 / 2 / D, where λ0 is the center wavelength of the emitted laser and D is the light transmission diameter of the multi-beam interference discriminator (4).

5. A high-precision marine optical parameter detection system based on a wedge cavity according to claim 3, characterized in that: The inner sides of the two opposing glass plates of the wedge-shaped cavity are coated with a high-reflectivity film for the center wavelength of the emitted laser, with a reflectivity of not less than 90%.

6. The high-precision marine optical parameter detection system based on a wedge cavity according to claim 1, characterized in that: The interference narrowband filter unit (2) has a transmittance bandwidth of ±0.1nm, a peak transmittance of over 98%, and a cutoff transmittance of over OD6.

7. A high-precision marine optical parameter detection system based on a wedge cavity according to claim 1, characterized in that: The high-sensitivity detector array (6) adopts a photomultiplier tube array or an avalanche diode array.

8. The high-precision marine optical parameter detection system based on a wedge cavity according to claim 1, characterized in that: The point detector (7) is a photomultiplier tube or an avalanche diode.

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