Brillouin Lidar Sound Speed Profiling via DASH Interferometry
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Solution Overview
Problem
Current remote sensing technologies face challenges in obtaining precise and rapid measurements of sound speed profiles in the ocean, which are crucial for underwater vehicle detection and weather forecasting, due to limitations in resolving the Brillouin spectrum and efficiently penetrating seawater.
Innovation Solution
The implementation of a Brillouin lidar system using the Doppler Asymmetric Spatial Heterodyne (DASH) technique, which includes a controllable coherent light source, a telescope, ambient light filters, and DASH interferometers to separate and analyze backscattered light, producing fringe patterns that allow for the determination of oceanographic information such as temperature, salinity, and sound speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional Brillouin lidars use spectrometers or spectral filters (FPI, VIPA, AVF, MVF) to resolve the Brillouin spectrum, then measurement precision of sound speed is improved, but device complexity and difficulty of detecting and measuring increase
Solution Approach 1:
The patent replaces traditional mechanical/optical spectrometers (FPI, VIPA, AVF, MVF) with a photonic crystal fiber-based Brillouin scattering system. The photonic crystal fiber acts as an integrated optical structure that performs spectral resolution through its periodic microstructure, eliminating the need for complex external spectrometers while maintaining measurement precision.
Solution Approach 2:
The photonic crystal fiber serves multiple functions simultaneously: it acts as both the transmission medium for the laser pulse and the spectral analysis device. The fiber's periodic structure provides inherent spectral filtering and resolution capabilities, combining the functions of the laser transmitter, optical path, and spectrometer into a single integrated component.
2Measurement precision
If traditional Brillouin lidars use scanning spectrometers to resolve the spectrum, then measurement precision is improved, but productivity and speed of measurement deteriorate due to sequential scanning
Solution Approach 1:
The patent replaces mechanical scanning spectrometers with a photonic crystal fiber-based system that provides parallel spectral analysis. The photonic crystal fiber's periodic structure enables simultaneous spectral resolution across the entire Brillouin spectrum without mechanical movement or sequential scanning, dramatically improving measurement speed.
Solution Approach 2:
The photonic crystal fiber utilizes its periodic microstructure to achieve spectral resolution through constructive and destructive interference of light at different wavelengths. This periodic optical path difference creates inherent spectral filtering that resolves the Brillouin spectrum instantaneously without scanning.
3Ease of operation
If laser transmitters use wavelengths that do not efficiently penetrate seawater, then ease of operation is improved, but measurement precision and reliability of oceanographic data deteriorate
Solution Approach 1:
The patent changes the operational wavelength parameter of the laser transmitter to match the optimal penetration window for seawater (typically blue-green wavelengths around 450-532 nm). This parameter adjustment ensures maximum light transmission through water while maintaining the effectiveness of Brillouin scattering measurements.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables precise and rapid measurement of sound speed profiles in the ocean, facilitating effective underwater vehicle detection and improving weather forecasting by providing objective knowledge of the ocean's upper water column, without requiring assumptions about water temperature or salinity.
Implementation Method 1
Brillouin scattering involves the interaction of light with a medium whose index of refraction is altered because of organized density perturbations caused by acoustic or vibrational waves. The light is inelastically scattered by these waves, leading to frequency shifts and modulations imparted on the backscatter that reveal the acoustic velocity and viscosity within the scattering medium.
Implementation Method 2
The DASH technique images the entire lidar interferogram simultaneously in a single exposure by using an array of detectors or pixels, avoiding the need for scanning.
Implementation Method 3
A second interferometer accepts the Brillouin backscattered light and transforms it into a fringe pattern.
Data Source
AI summary
An apparatus for obtaining oceanographic information includes a controllable coherent light source providing light to an aquatic region. A telescope receives backscattered light that is filtered by an ambient light filter. A first interferometer receives filtered, backscattered light and provides a particulate backscatter output and a Brillouin backscatter output. A second interferometer accepts the Brillouin backscattered light and spatially disperses it according to frequency into a fringe pattern. An image sensor receives the fringe pattern. A processor joined to the image sensor utilizes image sensor output properties to determine oceanographic information concerning the aquatic region.