Coherent Receiver for FMCW LiDAR Phase Noise Suppression
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Solution Overview
Problem
FMCW LiDAR systems face degraded signal-to-noise ratio (SNR) due to overlapping phase noise from positive and negative images in the reference beat frequency, which affects the accuracy of range and velocity estimation.
Innovation Solution
A coherent reference receiver with a 90° optical hybrid is used to suppress the negative frequency image by combining the outputs of the hybrid receiver, and signal processing schemes are applied to correct for imperfections and account for up/down chirped signals, improving the SNR and accuracy of phase noise estimation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single or balanced coherent detector is used in the reference optical path, then the system structure is simplified, but the phase noise from positive and negative images overlaps, degrading the quality of phase noise estimation and reducing target SNR
Solution Approach 1:
The patent divides the detection function into four separate photodetectors instead of using a single or balanced detector. Each photodetector processes a specific quadrature component (0°, 90°, 180°, 270°) of the optical signal, allowing separate processing of positive and negative frequency images. This segmentation enables the system to suppress the negative image contribution through differential processing, thereby improving phase noise estimation quality without excessive complexity increase.
Solution Approach 2:
The patent transitions from single-dimensional detection (single detector) to four-dimensional detection (four photodetectors measuring different quadrature phases). By adding the phase dimension as a new measurement axis, the system can distinguish between positive and negative frequency images that overlap in the single-detector case. This dimensional expansion allows for image suppression techniques that improve measurement precision.
2Reliability
If a 90° optical hybrid receiver is used to suppress the negative frequency image, then the target SNR is improved, but the device complexity and signal processing requirements increase
Solution Approach 1:
The patent combines the functions of phase modulation, quadrature detection, and image suppression into a unified 90° optical hybrid receiver structure. The hybrid receiver merges multiple optical paths with different phase shifts (0°, 90°, 180°, 270°) into a single detection system, allowing simultaneous extraction of all quadrature components. This merging achieves image suppression and SNR improvement while consolidating what would otherwise require separate systems.
Solution Approach 2:
The 90° optical hybrid receiver acts as an intermediary device between the optical signal source and the photodetectors. It transforms the incoming optical signal into four quadrature components that can be processed independently, mediating the conversion from optical domain to electrical domain in a way that enables negative image suppression. The hybrid receiver's internal phase-shifting mechanisms serve as intermediaries that separate the overlapping frequency images before detection.
3Measurement precision
If signal processing schemes are applied to correct for imperfections of the 90° optical hybrid receiver, then the measurement accuracy is improved, but the processing complexity and computational requirements increase
Solution Approach 1:
The patent implements feedback-based calibration and correction algorithms that use known reference signals to characterize and compensate for imperfections in the 90° optical hybrid receiver. By continuously monitoring the system response and adjusting processing parameters based on measured deviations, the system maintains high measurement accuracy despite hardware imperfections. The feedback loop enables real-time correction of phase imbalances, amplitude mismatches, and other non-idealities.
Solution Approach 2:
The patent employs parameter adjustment and optimization techniques to compensate for receiver imperfections. By dynamically adjusting detection parameters, gain settings, and phase offsets based on calibration data and operating conditions, the system maintains optimal measurement precision. Parameter changes include adjusting the weighting of different quadrature components, modifying the reference signal characteristics, and optimizing the integration time to account for hardware variations.
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
The solution enhances the SNR and accuracy of range and velocity information by effectively suppressing the negative image of the reference beat frequency, leading to improved target detection in FMCW LiDAR systems.
Implementation Method 1
a 90° optical hybrid to receive a portion of the optical beam along the reference path and a local oscillator (LO) signal to generate a first, a second, a third and a fourth output signal
Implementation Method 2
a first photodetector to receive the first and the second output signal to generate a first mixed signal. The coherent receiver further includes a second photodetector to receive the third and the fourth output signal to generate a second mixed signal
Data Source
AI summary
A LiDAR system includes an optical source to emit an optical beam along a target path towards a target and a reference path and a coherent receiver disposed in the reference path to produce a first mixed signal comprising a first portion of a reference beat signal and a second mixed signal comprising a second portion of the reference beat signal. The LiDAR system further includes a processor to combine the first mixed signal and the second mixed signal to generate a combined reference signal, wherein a negative image of a reference beat frequency signal produced by the optical beam and a local oscillator (LO) signal is suppressed to estimate a phase noise of the optical source to determine at least one of range or velocity information of the target.