Interferometer Noise Cancellation for Optical Sensors
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Solution Overview
Problem
Optical sensors using interferometers face limitations in signal-to-noise ratio (SNR) due to laser frequency and intensity noise, particularly in compact and mobile applications like microphones, where existing stabilization techniques are bulky and ineffective against relative intensity noise.
Innovation Solution
A device comprising a measurement interferometer responsive to physical parameter changes and a reference interferometer insensitive to these changes, with a signal processor generating a difference output signal to cancel noise, allowing for increased SNR and compact design. The operating points of the interferometers are optimized for linear transmission, and tuning mechanisms adjust refractive index, spacing, or wavelength to achieve this.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Fabry-Perot interferometer or etalon is used to stabilize laser frequency, then frequency fluctuations are compensated, but the device becomes bulky and expensive
Solution Approach 1:
The patent extracts the noise compensation function from a separate stabilization device and integrates it into the measurement system by using a reference interferometer that directly measures laser noise, which is then subtracted from the measurement signal. This eliminates the need for bulky external stabilization equipment.
Solution Approach 2:
The reference interferometer and measurement interferometer are combined into a single integrated device that simultaneously performs both noise measurement and physical parameter measurement. The reference interferometer is configured to be unresponsive to the physical parameter while the measurement interferometer responds to it, allowing differential noise cancellation.
2Reliability
If conventional laser stabilization is used, then frequency noise is reduced, but relative intensity noise cannot be compensated
Solution Approach 1:
The reference interferometer serves multiple functions: it measures both frequency fluctuations and intensity fluctuations of the laser source. By detecting the interference pattern changes in the reference interferometer, both types of noise are captured and can be compensated in the measurement signal.
Solution Approach 2:
The system uses feedback by continuously monitoring the reference interferometer output and using this information to compensate for noise in the measurement interferometer signal. The reference interferometer provides real-time noise information that is fed back to correct the measurement signal.
3Volume of moving object
If optical sensors are made compact for mobile applications, then portability is improved, but SNR deteriorates due to inability to compensate laser noise
Solution Approach 1:
The reference interferometer is nested within the same device housing as the measurement interferometer, with both interferometers integrated into a compact structure. The reference interferometer can be positioned adjacent to or overlapping with the measurement interferometer path, allowing space-efficient design while maintaining noise compensation functionality.
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 significantly enhances the SNR of optical sensors by isolating noise from the measurement signal, enabling compact and robust optical microphones suitable for mobile communications with high stability and resistance to shock and wind noise.
Implementation Method 1
light from a laser is coupled into an interferometer, which is influenced by changes in the physical parameter to produce corresponding changes in the interference pattern
Implementation Method 2
The etalon converts frequency fluctuations into intensity fluctuations, which can be detected by a photodetector
Implementation Method 3
These changes in the interference pattern manifest as changes in intensity, which can be detected by a photodetector
Data Source
AI summary
A device comprising measurement (7) and reference (3) interferometers is disclosed. Each interferometer is configured to receive light from the same light source (1) and to emit light to respective detectors (6) and has a respective operating point. The measurement interferometer (7) is configured to respond to variations in a physical parameter by varying the intensity of light emitted, whereas the reference interferometer (3) is configured to be unresponsive to variations in the physical parameter. The device further comprises a signal processor for generating a differential output signal depending on respective output signals generated by the detectors (6).