Optical Microphone Photoacoustic Sensor for Compact Gas Detection
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Solution Overview
Problem
Existing photoacoustic sensors are bulky and complex due to the size requirements of commercial MEMs microphones, which necessitate larger diaphragms and additional space for internal amplifiers, leading to increased size and noise interference from ambient signals.
Innovation Solution
A photoacoustic sensor utilizing an optical microphone with a pressure-sensitive membrane and a semiconducting laser, where the membrane is positioned to minimize noise interference by adjusting the radiation frequency based on ambient noise signatures, and an active valve with a speaker controls gas access, allowing for smaller form factor and reduced noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If commercial MEMs microphones are used to sense pressure waves, then measurable capacitance can be obtained, but the diaphragm size must be at least 1.5 mm×1.5 mm×1 mm which increases the overall sensor size
Solution Approach 1:
The patent replaces the mechanical capacitive sensing system with an optical detection system. A laser beam is directed through the diaphragm onto a photodetector, converting mechanical displacement into optical signal changes. This substitution eliminates the need for large diaphragms required by capacitive sensing while maintaining measurement precision.
Solution Approach 2:
The patent changes the detection parameter from capacitance to optical transmission/absorption. By monitoring changes in light transmission through the diaphragm rather than electrical capacitance, the system achieves sensitive pressure wave detection with significantly reduced diaphragm dimensions.
2Reliability
If an internal amplifier is added to enhance voltage signal levels, then output signals reach millivolt levels above ambient noise, but additional area is required which increases device complexity
Solution Approach 1:
The patent replaces the electrical amplification system with an optical detection system. The photodetector directly converts optical signals from the laser into electrical signals with sufficient voltage levels, eliminating the need for internal amplifiers and associated circuitry while maintaining signal reliability above ambient noise.
3Measurement precision
If larger diaphragms are used in MEMs microphones, then measurable capacitance is achieved, but the overall sensor complexity and size increase
Solution Approach 1:
The patent replaces the complex capacitive measurement system with a simpler optical measurement system. The laser-photodetector configuration provides direct conversion of diaphragm displacement into measurable electrical signals without requiring complex capacitance measurement circuitry, thereby reducing overall device 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
The optical microphone design achieves millivolt output signals without internal amplification, reducing sensor size and complexity while effectively filtering out ambient noise, enabling precise gas detection and analysis.
Implementation Method 1
Photoacoustic sensors may be used to detect sample gases based on the tendency of molecules of sample gases, when exposed to certain frequencies of radiant energy, to absorb the energy and reach higher levels of molecular vibration and rotation thereby to reach a higher temperature and pressure. When the radiant energy is amplitude modulated, the resulting fluctuations in energy available for absorption produce corresponding temperature and pressure fluctuations.
Implementation Method 2
direct coherent light toward a pressure-sensitive membrane and causing the membrane to direct reflected light back toward the semiconducting laser to mix the reflected light with the coherent light within a cavity of the semiconducting laser such that a voltage level of a p-n junction within the semiconducting laser changes
Data Source
AI summary
Some embodiments are directed to a photoacoustic sensor. The photoacoustic sensor may comprise: a gas cell with an opening; a light source to generate to radiate a sample gas within the gas cell; an optical microphone to detect the sample gas within the gas cell; and a membrane aligned with the opening of the gas cell to permit sample gas to enter the gas cell. The optical microphone includes a semiconducting laser. The semiconducting laser includes a p-n junction within a cavity of the semiconducting laser. The optical microphone further includes a pressure-sensitive membrane that receives coherent light emitted from the semiconducting laser and directs reflected light back toward the semiconducting laser. During operation of the optical microphone, the pressure-sensitive membrane flexes in response to acoustic pressure waves. The phase of the reflected light is dependent upon a distance of the pressure-sensitive membrane from an aperture of the semiconducting laser.