Coherent Lidar Pixel Intensity Modulation for SNR Optimization
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Solution Overview
Problem
Coherent Lidar systems face challenges in achieving high-resolution, video-rate imaging due to limited time for distance measurements at each point, and existing parallel detection methods struggle with signal saturation and uniform signal-to-noise ratio across pixels.
Innovation Solution
A coherent Lidar imaging system with a temporally modulated laser source, a matrix of pixels, and an optical system that spatially separates and recombines reference and object beams, allowing for individual pixel intensity modulation to optimize signal-to-noise ratio and prevent saturation, using a feedback loop to adjust transmittance values for each pixel.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the entire scene is simultaneously illuminated by a diverging laser beam with parallel photodetection across the scene, then the acquisition rate of distance images can be increased, but the coherent imager configuration is not easily scalable to a large number of pixels and the local oscillator reference beam intensity is constant for all pixels causing signal saturation
Solution Approach 1:
The patent divides the scene into multiple depth ranges and uses multiple photodetector arrays, each dedicated to detecting signals from specific depth ranges. This segmentation allows the system to handle a large number of pixels by distributing them across multiple specialized detectors, improving scalability while maintaining high acquisition rates for simultaneous full-scene illumination.
Solution Approach 2:
The patent introduces variable optical elements that dynamically adjust the intensity of the local oscillator reference beam for different pixels based on their signal strength requirements. This dynamic adjustment prevents signal saturation in pixels receiving strong reflections while maintaining sufficient signal intensity for pixels with weaker reflections, resolving the contradiction between constant intensity simplicity and adaptive intensity performance.
2Ease of operation
If a constant intensity local oscillator reference beam is used for all pixels, then the system is simpler to implement, but pixels receiving strong reflected signals become saturated causing information loss
Solution Approach 1:
The patent applies different local oscillator reference beam intensities to different pixels based on their individual signal characteristics. Each pixel or group of pixels can have its reference beam intensity independently adjusted, allowing strong signal pixels to receive lower intensity reference beams (preventing saturation) while weak signal pixels receive higher intensity reference beams (improving signal-to-noise ratio).
3Measurement precision
If sequential scanning is used to create a complete image of the scene, then point-by-point measurements can be performed, but it is difficult to achieve video frame rate acquisition for high resolution images because the time available for distance measurement at each point is very short
Solution Approach 1:
The patent segments the detection task by dividing the scene into multiple depth ranges and assigning different photodetector arrays to detect signals from specific depth ranges. This allows parallel processing of multiple spatial regions simultaneously, achieving video frame rate acquisition for high-resolution images while maintaining accurate distance measurements through specialized detection for each depth segment.
Solution Approach 2:
The patent uses multiple photodetector arrays that each detect a portion of the total scene, with each array optimized for specific depth ranges. This partial detection approach across multiple parallel channels achieves complete scene coverage at video frame rates, as each detector performs measurements on its assigned portion simultaneously rather than sequentially scanning the entire scene.
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
Enables high-resolution, video-rate 3D imaging with improved signal-to-noise ratio and reduced saturation across pixels, allowing for efficient detection and processing of large numbers of pixels simultaneously.
Implementation Method 1
A coherent lidar system is a system in which part of the coherent illuminating light source is deflected in order to be used as an amplifier of the signal backscattered by the scene once it has been illuminated by the rest of the undeflected beam
Implementation Method 2
The interference of these two waves is detected by a photodetector PD, and the electrical signal at the detector's output exhibits an oscillating term called the beat signal
Implementation Method 3
The Doppler frequency shift of the backscattered wave is a function of the radial velocity v of the target T
Implementation Method 4
an optical imaging system having an optical axis and producing an image of the scene by imaging an object beam reflected by the scene onto the pixels of the detection device
Implementation Method 5
a fraction of the object beam reflected by said scene and illuminating a pixel being called the pixel image beam
Data Source
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AI summary
The invention relates to a coherent lidar imaging system (30, 35) comprising: - a laser source (SL) - a detection device (Det), - a first optical device, - an optical imaging system (lm) - a second optical device (D2) the photodetector component of a pixel being configured to generate a detected pixel signal (Spix), the detected pixel signal having an intensity called total pixel intensity (Itot/pix), the splitter having a first variable transmittance (T1) identical for all pixels and modulable, the second optical device further comprising at least one intensity modulator (IM, IMIij) adapted to modulate an intensity of each reference pixel beam by applying a modulable pixel transmittance (xij), - the coherent lidar imaging system further comprising a processing unit (UT) configured to apply a first transmittance value (T1) and, for each pixel, a pixel transmittance value (xij),said values being determined via a feedback loop and according to an optimization criterion, the optimization criterion comprising obtaining for each pixel a total pixel intensity lower than a threshold intensity (Is), and obtaining an improved signal-to-noise ratio (SNR).