Scanning LiDAR Descan Compensation for Angular Walk-Off
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Solution Overview
Problem
Conventional LIDAR systems suffer from signal degradation due to angular walk-off of target signals at high scanning mirror velocities, especially in single-mode fibers, limiting their measurement range and efficiency.
Innovation Solution
The system employs frequency modulation and coherent detection with polarized light separation, using a quarter wave plate to separate optical detection from the primary source fiber, allowing larger optical detectors and minimizing optical losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the scanning mirror angular velocity is increased to achieve higher frame rates, then the framerate is improved, but the target signal from distant objects is severely degraded due to angular walk-off at the fiber tip
Solution Approach 1:
The patent divides the optical detection path into two separate paths: a source path using single-mode fiber and a detection path using multi-mode fiber or waveguide. This segmentation allows each path to be optimized independently, with the source path maintaining high speed and the detection path accommodating larger core diameter for better coupling efficiency at high angular velocities.
Solution Approach 2:
The patent introduces a polarization beam splitter as an intermediary component that separates the local oscillator signal from the target return signal based on their polarization states. This allows the detection path to receive combined signals from both paths without interference, resolving the coupling efficiency problem at high scan speeds.
2Volume of moving object
If single-mode fiber is used for coaxial light emission and detection, then the system achieves compactness, but the coupling efficiency decreases severely at high angular velocities due to angular walk-off
Solution Approach 1:
The patent segments the optical paths into separate source and detection paths with different fiber types. The source path uses single-mode fiber for compactness, while the detection path uses multi-mode fiber or waveguide with larger core diameter to maintain coupling efficiency at high angular velocities, thus resolving the contradiction between compactness and coupling efficiency.
3Measurement precision
If frequency modulation LIDAR systems are used to provide simultaneous range and velocity measurement, then the measurement capability is improved, but significant losses occur in the beam's return path requiring higher average beam output power
Solution Approach 1:
The patent uses a polarization beam splitter as an intermediary to separate the local oscillator signal from the target return signal. This allows the detection path to receive combined signals from both paths without interference, reducing energy losses in the return path and enabling simultaneous range and velocity measurement with lower average beam output power requirements.
4Measurement precision
If the fiber diameter is decreased to use single-mode fiber, then the system achieves better spatial resolution, but the angular walk-off effect becomes more severe decreasing coupling efficiency
Solution Approach 1:
The patent segments the optical detection function from the source fiber by using separate multi-mode fiber or waveguide in the detection path. This allows the source path to use single-mode fiber for high spatial resolution while the detection path uses larger core diameter fibers to maintain coupling efficiency at high angular velocities, resolving the contradiction between spatial resolution and coupling efficiency.
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 enhances measurement range and immunity to crosstalk, improving framerate and detection capabilities while maintaining compactness and stability in varying environmental conditions.
Implementation Method 1
The light leaving the system can be polarized with a quarter wave plate (QWP). After the QWP, a portion of the light can be reflected back toward the system as a local oscillator (LO)
Implementation Method 2
Because the combined signal is polarized, it can be reflected from a polarization beam splitter to one or more optical detectors that are separate from the laser source
Implementation Method 3
Example implementations of the present disclosure are directed to an improved scanning LIDAR system. Example implementations of the present disclosure are based on a type of LIDAR that uses frequency modulation (FM) and coherent detection
Implementation Method 4
Example implementations of the present disclosure are based on a type of LIDAR that uses frequency modulation (FM) and coherent detection
Implementation Method 5
Fast-scanning mirrors are the primary components used to illuminate a scene in most conventional LIDAR systems
Implementation Method 6
Light emission and detection from target reflections are done coaxially, typically via a single mode fiber
Data Source
Figure 1A~1B
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AI summary
A light detection and ranging (LIDAR) apparatus is provided that includes a laser source configured to emit a laser beam in a first direction. The apparatus includes lensing optics to pass a first portion of the laser beam in the first direction toward a target, return a second portion of the laser beam into a return path as a local oscillator signal, and return a target signal into the return path. The apparatus also includes a quarter- wave plate to polarize the laser beam headed in the first direction and polarize the target signal returned through the lensing optics. The apparatus also includes a polarization beam splitter to pass non-polarized light through the beam splitter in the first direction and reflect polarized light in a second direction different than the first direction, wherein the polarization beam splitter further enables interference between the local oscillator signal and the target signal to generate a mixed signal. The apparatus also includes an optical detector to receive the mixed signal. In an embodiment, optical circuits 300 include multiple fiber outputs 302 and multiple optical detectors 304. The multiple fiber outputs 302 and optical detectors 304 may provide multiple data points during a single time interval. Accordingly, fewer rotations of fast scanning mirrors may provide additional data. For example, the optical circuits 300 may include a 1/4 wave plate 308 to polarize light, lensing 310 to provide reflected light, and the like. The alignment of the polarized beam splitter 306 may be set such that the fiber outputs 302 and optical detectors 304 are aligned when returned light is reflected. There may be multiple fiber outputs 302 and multiple optical detectors 304, but also multiple polarized beam splitters 306. The signals received at each of the optical detectors 304 may be analyzed separately to generate distance or velocity data at a point. The fiber outputs 302 may provide a laser beam at different wavelengths. Because the beams have interfered before the optical detectors and the detection path is decoupled from source path, the optical detectors can be larger core fibers or waveguides, silicon- based optical detectors, or other types of optical detectors that can sense the combined signal.