Frequency-Modulated Coherent Lidar Signal Processing
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Solution Overview
Problem
Existing pulsed lidar systems for measuring wind profiles are limited by their size and weight, and they require high peak power, which is not feasible with photonic integrated circuits (PICs). Additionally, frequency-modulated continuous-wave lidar systems struggle with signal processing in diffuse targets, leading to ambiguities in frequency representation.
Innovation Solution
A signal processing method for a coherent lidar system with a periodically frequency-modulated coherent source, which involves decomposing the beat signal into intervals, determining elementary and average power spectral densities, and using lower and upper frequency bounds to calculate distance and velocity information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pulsed lidar is used for wind profile measurement, then distance and velocity measurement capability is achieved, but system size and weight increase due to fiber amplifiers and fiber components
Solution Approach 1:
The patent replaces the mechanical fiber amplifier system with a photonic integrated circuit (PIC) based frequency-modulated continuous-wave lidar system. This substitution eliminates bulky fiber components and fiber amplifiers, achieving compact system integration while maintaining wind profile measurement capability through frequency modulation and coherent detection techniques
Solution Approach 2:
The patent transitions from pulsed operation with high peak power to frequency-modulated continuous-wave operation with low peak power. This parameter change enables the use of PICs instead of fiber amplifiers, significantly reducing system size and weight while preserving the ability to measure wind profiles through Doppler shift analysis
2Weight of moving object
If frequency-modulated continuous-wave lidar is used, then system size is reduced and low peak power is achieved, but signal processing ambiguities occur in diffuse targets
Solution Approach 1:
The patent introduces a local oscillator as an intermediary reference signal that mixes with the backscattered light from diffuse targets. This heterodyne detection scheme converts the optical frequency information into electrical beat signals, resolving the signal processing ambiguities and enabling accurate extraction of distance and velocity information from frequency-modulated continuous-wave lidar returns
Solution Approach 2:
The patent replaces direct detection methods with coherent detection using a local oscillator. This substitution transforms the detection mechanism to preserve phase and frequency information, eliminating signal processing ambiguities in diffuse targets while maintaining the benefits of low peak power and compact PIC-based architecture
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 method allows for the isolation of backscatter from multiple fluid layers, enabling accurate telemetric/velocimetric measurements while maintaining low peak power, and it provides a wind profile along the lidar axis, suitable for various applications including sniper support and wind turbine optimization.
Implementation Method 1
a beat signal being generated by a photodetector from the interference between an optical signal, called local oscillator... and an optical signal backscattered by a moving fluid
Implementation Method 2
a periodically frequency-modulated coherent source... the local oscillator frequency consisting of the sum of an average value and a modulation frequency resulting from the modulation of the source
Implementation Method 3
The Doppler frequency shift νDop of the backscattered wave is a function of the radial velocity v of the target T
Data Source
AI summary
A method for processing a signal from a coherent lidar comprising a periodically frequency-modulated coherent source (L), the method includes the following steps: A decomposing each modulation period indexed j into a plurality of intervals indexed i, and determining, for each interval Iij, an elementary power spectral density DSP(i,j) of the beat signal over the interval, B determining an average power spectral density over j DSP(i), C determining a lower frequency bound of the average power density DSP(i) and an upper frequency bound, D determining a distance dk(i) and a velocity of the fluid vk(i) from the lower and upper bounds.