Fiber Optic Seismic Sensor Using Index-Matched Bonding
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Solution Overview
Problem
Traditional seismic sensors, such as geophones and MEMS accelerometers, have limitations in frequency range and operating temperature, which restrict their sensitivity and effectiveness for measuring small displacements and high-temperature applications.
Innovation Solution
A fiber optic seismic sensor with a dual cantilevered beam structure and a silicon frame, where an optical fiber is bonded to a borosilicate glass wafer, allowing for improved stability and optical performance, and the use of a bonding agent with a refractive index matching the optical fiber and glass wafer to enhance reflectance and reduce spurious reflections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional coil-type geophone sensors are used, then sensitivity to large oscillatory displacements is improved, but frequency range is limited
Solution Approach 1:
The patent replaces the traditional mechanical coil-type geophone sensor with an optical fiber-based interferometric sensor system. This substitution eliminates the mechanical bandwidth limitations of coil springs while maintaining sensitivity to displacement, enabling measurement across a broader frequency range from DC to several kHz.
Solution Approach 2:
The patent changes the fundamental measurement parameter from mechanical displacement of a mass-spring system to optical path length changes in an interferometric setup. This allows the sensor to respond to both static and dynamic displacements across a wide frequency range without being constrained by mechanical resonance characteristics.
2Adaptability or versatility
If MEMS accelerometers with integrated circuits are used, then measurement range and bandwidth are improved, but operating temperature is limited
Solution Approach 1:
The patent replaces electronic integrated circuits with an all-optical sensing system using fiber optics and interferometry. This eliminates the 75°C temperature limitation imposed by electronic components while maintaining the broad measurement range and bandwidth capabilities, enabling operation at temperatures exceeding 400°C.
3Device complexity
If optical fiber is bonded directly to glass wafer, then structural simplicity is improved, but spurious reflections increase
Solution Approach 1:
The patent introduces an index-matching bonding agent as an intermediary layer between the optical fiber and glass wafer. This bonding agent has a refractive index intermediate between the two materials, which reduces spurious reflections at the interfaces while maintaining the structural simplicity of the assembly.
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 sensor achieves enhanced sensitivity and stability, with improved modulation contrast and heterodyne gain, allowing for effective measurement of seismic changes over a broader frequency range and higher operating temperatures without the limitations of electronic integrated circuits.
Implementation Method 1
an interferometric micro-electro-mechanical system optical sensor to be used for seismic sensing
Implementation Method 2
Light is reflected into the optical fiber from the beam structure for measuring seismic changes
Implementation Method 3
bonding agent having an index of refraction between the refractive index of the fused silica optical fiber and the refractive index of the glass wafer
Data Source
AI summary
A dual cantilevered beam structure is attached to a silicon frame. An optical fiber extends from a borosilicate wafer bonded to the bottom of the frame. A second borosilicate wafer is bonded to the top of the frame. The bottom borosilicate wafer is bonded to the optical fiber with a bonding agent having an index of refraction between the refractive index of the fused silica optical fiber and the refractive index of the borosilicate wafer. In an embodiment, the bonding agent has a refractive index substantially similar to optical cement. Light is reflected into the optical fiber from the beam structure for measuring seismic changes.