Coherent Lidar Signal Processing for Noise Reduction

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Solution Overview

Problem

Coherent lidars face challenges in reducing photonic shot noise originating from the detection of the local-oscillator signal, which limits their sensitivity and detection performance.

Innovation Solution

A method involving the generation of first and second beat signals through interference between a local-oscillator signal and a backscattered signal, with their cross-correlation processed in the frequency domain to determine spectral-density values, and averaging these values to decrease noise and enhance signal-to-noise ratio, compatible with balanced detection architectures.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If conventional spectral density processing is used, then the measurement process is simple, but the noise floor remains high limiting sensitivity

Engineering Contradiction:

ImprovesensitivityVSAvoidprocessing complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent divides the spectral density estimation into multiple independent segments (first spectral density estimate, second spectral density estimate, etc.) that are processed separately and then combined. This segmentation allows noise reduction through averaging while keeping each individual processing step relatively simple, resolving the contradiction between measurement precision and processing complexity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent combines multiple spectral density estimates (first, second, and potentially more) through averaging to produce a final spectral density value. This merging of multiple measurements reduces the noise floor and improves sensitivity while distributing the processing load, thereby improving measurement precision without proportionally increasing overall complexity.

Inventive Principle:

Principle #5Merging (Combining)

2Measurement precision

If multiple spectral density estimates are averaged, then the noise floor decreases improving sensitivity, but the processing time and complexity increase

Engineering Contradiction:

Improvenoise floorVSAvoidprocessing time

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The patent uses a limited number of spectral density estimates (at least two, potentially more) averaged together rather than requiring an excessive number of measurements. This partial action approach achieves significant noise floor reduction and sensitivity improvement while keeping processing time reasonable, resolving the contradiction between measurement precision and time loss.

Inventive Principle:

Principle #16Partial or excessive action

3Reliability

If photonic shot noise from local-oscillator detection is reduced, then detection performance improves, but the system complexity increases

Engineering Contradiction:

Improvedetection performanceVSAvoidsystem complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent replaces complex hardware modifications with a signal processing approach. Instead of changing the physical detection system to reduce photonic shot noise, the invention uses mathematical processing (averaging multiple spectral density estimates) to achieve the same effect, thereby improving detection performance without significantly increasing system complexity.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

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 effectively decreases the noise floor, increasing the sensitivity and detection range of the lidar by reducing both the variance and average noise, thereby improving the contrast and signal-to-noise ratio.

Implementation Method 1

each beat signal being generated by interference between a local-oscillator signal generated by the coherent source and a signal backscattered by a target illuminated by the lidar

Methodology Applied

Scientific EffectInterference: Interference

Implementation Method 2

The interference of these two waves is detected by a photodetector D, and the electrical signal output from the detector has an oscillating term named the beat signal Sb

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Data Source

PatentUS11243307B2Method for processing a signal from a coherent lidar in order to reduce noise and related lidar system

Publication Date: 2022.02.08 THALES SA

AI summary

A method for processing a signal from a coherent lidar includes a coherent source, the method comprising steps consisting of: generating a first beat signal and a second beat signal, using respectively a first detection assembly and a second detection assembly for a plurality of n time intervals, determining n respective values of spectral density using a transform in the frequency domain of the cross-correlation between the first and second beat signals, determining a mean value of the spectral density using said n values of spectral density, determining a piece of location information on the target using the mean value of said spectral density.