Optical Directional Coupler for MEMS Displacement Readout
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Solution Overview
Problem
Conventional MEMS devices using electrical readout for inertial navigation sensors face issues with crosstalk and noise pickup from harsh environments, leading to degraded performance and small capacitance changes, necessitating an alternative method to accurately determine displacement of a vibrating proof mass.
Innovation Solution
The implementation of an optical directional coupler with waveguides and photodetectors to detect changes in optical power, allowing for the determination of proof mass displacement through evanescent coupling, which is proportional to inertial forces applied, thereby enhancing sensor accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If electrical readout (capacitive sensing) is used to sense displacement of the vibrating proof mass, then the device complexity is reduced and ease of manufacture is improved, but the measurement precision is degraded due to crosstalk and noise pickup from harsh environment
Solution Approach 1:
The patent replaces the electrical readout system with an optical sensing system. Specifically, it uses an optical directional coupler with waveguides and photodetectors to sense the displacement of the vibrating proof mass, substituting electrical fields with optical fields to eliminate crosstalk and noise pickup issues inherent in electrical sensing methods
Solution Approach 2:
The patent introduces an optical intermediary (the optical directional coupler and waveguides) between the vibrating proof mass and the detection system. This intermediary converts the mechanical displacement into optical signal changes through evanescent field coupling, providing isolation from electrical interference while maintaining measurement capability
2Reliability
If electrical readout is used, then the device structure is simplified, but the reliability is degraded due to noise pickup from harsh environment
Solution Approach 1:
The patent replaces electrical sensing with optical sensing to improve reliability in harsh environments. The optical directional coupler uses evanescent field coupling between waveguides to detect displacement, which is inherently immune to electromagnetic interference and noise pickup that plagues electrical readout systems in harsh conditions
Solution Approach 2:
The patent creates an optically isolated measurement environment using the waveguide structure. The evanescent field coupling occurs within the confined optical modes of the waveguides, effectively shielding the measurement from external electromagnetic interference and creating an 'inert' optical environment that rejects harsh environmental noise
3Measurement precision
If optical directional coupler with evanescent coupling is used to determine displacement, then the measurement precision is improved, but the device complexity increases
Solution Approach 1:
The patent segments the optical sensing function into distinct modular components: light sources, waveguides with specific coupling regions, photodetectors, and signal processing circuits. This segmentation allows each component to be optimized independently and facilitates integration while managing overall system complexity
Solution Approach 2:
The optical directional coupler structure serves multiple functions simultaneously: it guides light, creates evanescent field coupling for sensing, and provides signal modulation. This multi-functionality reduces the need for separate components and helps manage device complexity despite the enhanced measurement precision capabilities
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
This approach improves the accuracy and reliability of determining inertial forces by reducing noise interference and increasing the sensitivity of displacement measurement, leading to better performance in harsh environments.
Implementation Method 1
the light beam is at least partially evanescently coupled between the first waveguide and the second waveguide in the central region
Implementation Method 2
the first photodetector is configured to detect a first optical power in the light beam propagating through the first waveguide
Data Source
AI summary
A microelectromechanical systems (MEMS) device comprises an optical directional coupler comprising: a first waveguide having a first and a second end, wherein a light beam is introduced into the first end; a second waveguide having a third and a fourth end, wherein the light beam is evanescently coupled between the two waveguides in the central region; a first photodetector to detect first optical power in the light beam at the second end; and a second photodetector to detect second optical power in the light beam at the fourth end; a vibrating proof mass adjacent to the coupler in a first direction from the coupler, wherein when inertial forces are applied to the MEMS device in a second direction, the proof mass moves in the first direction; a processor to determine the displacement of the proof mass from the coupler as a function of the first and the second optical power.