Laser Seismometer Phase Modulation Noise Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional seismic sensors face limitations in measuring small displacements, velocities, and low frequencies due to high noise levels, particularly 1/f noise and kT noise, which reduce sensitivity and accuracy at low frequencies.
Innovation Solution
A laser seismometer that measures displacement directly using a phase modulated optical signal, employing a beamsplitter and phase modulator to generate a combined interfered signal, which is then processed to eliminate noise and provide high sensitivity across a wide bandwidth, including low frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional seismic sensors (geophones or MEMS) are used, then the device complexity and cost are reduced, but the measurement precision and sensitivity at low frequencies deteriorate due to high noise levels
Solution Approach 1:
The patent replaces conventional mechanical sensing elements (geophones, MEMS capacitive sensors) with an optical interferometric system. The measurement is performed by detecting phase changes in light reflected from the proof mass, eliminating the need for mechanical-to-electrical transduction that introduces 1/f noise and kT noise. This substitution of measurement physics fundamentally removes the dominant noise sources while maintaining high sensitivity at low frequencies.
Solution Approach 2:
The patent changes the measurement parameter from electrical voltage (in conventional sensors) to optical phase. By using laser light with coherent properties and detecting phase modulation caused by proof mass displacement, the system achieves ultra-low noise performance. The optical domain provides inherently lower noise floors compared to electrical domains, enabling detection of extremely small displacements at low frequencies.
2Adaptability or versatility
If conventional seismic sensors are used, then the bandwidth response is limited, but the device complexity remains low
Solution Approach 1:
The patent replaces mechanical bandwidth limitations with optical domain advantages. The interferometric measurement system has no inherent mechanical resonance that limits bandwidth, allowing flat response from DC to very high frequencies. The optical system's natural properties enable wide bandwidth operation without the trade-offs inherent in mechanical sensor designs.
3Measurement precision
If seismometers are used instead of geophones, then the measurement precision improves, but the ease of operation and cost worsen significantly
Solution Approach 1:
The patent creates a new class of sensors that combine the high precision of optical interferometry with the ruggedness and simplicity of geophones. By using a simple retroreflector attached to a robust proof mass and measuring with an optical beam, the system achieves seismometer-level precision without the complexity, fragility, and cost of conventional seismometers. The optical path can be routed externally, allowing easy deployment in various environments.
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 system achieves high sensitivity and wide bandwidth measurements down to very low frequencies, overcoming noise issues and improving detection capabilities for seismic surveys and geological feature detection.
Implementation Method 1
the first beam strikes and reflects off of a stationary proof mass
Implementation Method 2
recombined at the beamsplitter with the reflected light from the first beam to generate a combined interfered signal
Implementation Method 3
The phase modulator changes a phase of the reflected second beam by changing an index of refraction of a material of the phase modulator
Implementation Method 4
changing an index of refraction of a material of the phase modulator
Data Source
AI summary
A laser seismometer may measure the change in a phase modulated optical signal. Unlike traditional interferometers, the laser phase is first modulated by a radio frequency (RF) source, which is then demodulated following detection to provide the signal of interest. The net result is a direct measurement of displacement with the effects of amplitude noise eliminated via limiting and the effects of 1/f phase noise (frequency drift, etc.) eliminated by self-interfering the signal. Because the signal-to-noise ratio (SNR) of the optical signal is strong, the technique provides a strong measure of the displacement and avoids the extremely low voltages and associated problems of traditional sensors.