Self-Mixing Laser Microphone for High-SNR Sound Pickup
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Solution Overview
Problem
Conventional MEMS microphones face limitations in improving signal-to-noise ratio (SNR) due to high squeeze-film damping and the pull-in effect between the silicon diaphragm and back electrode, which restricts sensitivity and acoustic overload point improvement.
Innovation Solution
A laser microphone design with a diaphragm, laser device, control circuit, self-mixing signal obtaining apparatus, and signal processing circuit, where the laser device emits light to the diaphragm, and the feedback light signal interferes in a resonant cavity to generate a self-mixing light signal, with a controlled drive current to stabilize at the highest sensitivity and reduce phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a MEMS capacitor structure is used with small distance between diaphragm and back electrode, then sensitivity is improved, but high squeeze-film damping is introduced causing high mechanical noise
Solution Approach 1:
The patent replaces the traditional MEMS capacitive sensing mechanism with an optical sensing system. A laser beam reflects off the diaphragm surface, and optical interference patterns detect diaphragm vibration. This substitution eliminates mechanical noise from the back electrode and squeeze-film damping effects, while maintaining high sensitivity through optical interference measurement.
Solution Approach 2:
The patent introduces an optical field as an intermediary between the sound wave and the detection system. The laser beam serves as a mediator that interacts with the diaphragm without direct mechanical contact, converting mechanical vibration into optical signal changes that can be measured without introducing additional mechanical noise.
2Measurement precision
If distance between silicon back electrode and silicon diaphragm is reduced, then sensitivity is improved, but pull-in effect limits further distance reduction
Solution Approach 1:
The patent eliminates the mechanical back electrode structure by using optical reflection from the diaphragm surface itself. The laser reflects off the diaphragm's reflective coating, and the optical path length changes with diaphragm vibration. This removes the pull-in effect constraint entirely, as there is no opposing electrical force between electrodes.
3Ease of manufacture
If conventional MEMS microphone structure is used, then manufacturing is simplified, but signal-to-noise ratio improvement is limited
Solution Approach 1:
The patent integrates multiple functions into the diaphragm structure: it serves as both the sound-sensitive membrane and the optical reflector. The diaphragm's reflective coating enables optical sensing while maintaining acoustic functionality. This multi-functionality approach achieves high SNR without significantly complicating manufacturing, as the reflective coating can be applied using standard thin-film deposition techniques.
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 laser microphone achieves a higher signal-to-noise ratio, enhancing voice recognition sensitivity and long-distance sound pickup quality by improving coupling efficiency and maintaining stability through controlled drive current modulation.
Implementation Method 1
the feedback light signal interferes with laser in a resonant cavity of the laser device to obtain a self-mixing light signal
Implementation Method 2
the reflected laser beam carries sound information of vibration of the vibration sound film, and the partially reflected laser beam is mixed with laser in the VCSEL resonant cavity to generate a laser self-mixing interference effect
Implementation Method 3
the silicon diaphragm can sense air vibration generated by sound waves and vibrate accordingly
Data Source
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AI summary
An embodiment of this application provides a laser microphone, including a diaphragm, a laser device, a control circuit, a self-mixing signal obtaining apparatus, and a signal processing circuit. The laser device is configured to emit light to the diaphragm and receive a feedback light signal from the diaphragm, and the feedback light signal interferes with laser in a resonant cavity of the laser device to obtain a self-mixing light signal. A distance between the laser device and the diaphragm ranges from 30 to 300 µm. The control circuit is connected to the laser device, and is configured to drive and control the laser device to emit light. The self-mixing signal obtaining apparatus is connected to the laser device, and is configured to obtain a target voltage signal related to the self-mixing light signal. The signal processing circuit is connected to the self-mixing signal obtaining apparatus, and is configured to receive the target voltage signal output by the self-mixing signal obtaining apparatus and process the target voltage signal into an audio voltage signal. The laser microphone has a high signal-to-noise ratio, such that a voice recognition rate and an awakening rate can be increased, and long-distance sound pickup effect is improved. An embodiment of this application further provides a terminal including the laser microphone.