Balanced Self-Mixing Laser Detection for Common-Mode Noise Suppression
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Solution Overview
Problem
Self-mixing interferometry (SMI) optical sensors face challenges in noise suppression, particularly from ambient noise, laser/photodetector nonlinearity noise, driver noise, laser relative intensity noise, and optical sensor system noise, which affect the accuracy of spatial information retrieval.
Innovation Solution
The integration of a pair of photodetectors with a semiconductor laser, where photocurrents with opposite SMI excess phases are combined to remove common-mode noise, facilitating balanced detection and reducing signal distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single photodetector is used for SMI sensing, then the device structure is simple, but noise from ambient light, laser RIN, and system components degrades measurement precision
Solution Approach 1:
The single photodetector is segmented into two separate photodetectors (first photodetector and second photodetector) that are laterally offset from each other. Each photodetector detects light at a different lateral position, enabling differential detection that suppresses common-mode noise while maintaining structural simplicity through lateral rather than vertical segmentation.
Solution Approach 2:
The patent introduces a beam splitter as an intermediary optical component that divides the incident light into two separate paths, directing different portions of the light to the first and second photodetectors respectively. This intermediary enables the noise suppression function without requiring complex direct coupling between the photodetectors.
2Measurement precision
If photodetectors are vertically stacked to achieve balanced detection, then noise suppression is improved, but manufacturing precision requirements increase due to alignment constraints
Solution Approach 1:
The patent transitions from vertical stacking (z-dimension alignment) to lateral offset arrangement (x-y plane positioning) of the photodetectors. This dimensional change eliminates the need for precise vertical alignment while maintaining the balanced detection capability, as the lateral offset can be achieved with standard photodetector array positioning tolerances.
3Measurement precision
If balanced detection with laterally offset photodetectors is implemented, then common-mode noise is suppressed and signal-to-noise ratio improves, but the optical system complexity increases
Solution Approach 1:
The beam splitter is designed as a universal optical component that simultaneously performs multiple functions: dividing the incident light into two paths, directing light to laterally offset photodetectors, and maintaining optical path length equivalence. This multi-functionality achieves balanced detection without requiring separate components for each function, thereby limiting the increase in overall system complexity.
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 suppresses common-mode noise, enhancing the signal-to-noise ratio and bringing the SMI signal closer to the quantum limit of coherent detection, thereby improving the accuracy of spatial information retrieval.
Implementation Method 1
a semiconductor laser may generate and emit electromagnetic radiation from a resonant cavity of the semiconductor laser
Implementation Method 2
a third MQW structure operable to generate a second photocurrent responsive to detecting a second emission of the semiconductor laser
Implementation Method 3
self-mix the generated and returned electromagnetic radiation within the resonant cavity, and produce an SMI signal
Data Source
AI summary
An optical sensor system includes a set of epitaxial layers formed on a semiconductor substrate. The set of epitaxial layers defines a semiconductor laser having a first multiple quantum well (MQW) structure. Electromagnetic radiation is generated by the first MQW structure, emitted from the first MQW structure, and self-mixed with a portion of the emitted electromagnetic radiation that is returned to the first MQW structure. The set of epitaxial layers also defines a second MQW structure operable to generate a first photocurrent responsive to detecting a first emission of the semiconductor laser, and a third MQW structure operable to generate a second photocurrent responsive to detecting a second emission of the semiconductor laser. The optical sensor system also includes a circuit configured to generate a self-mixing interferometry (SMI) signal by combining the first photocurrent and the second photocurrent.