Accelerometers Based on Fabry-Perot Interferometers
The design of a navigation-grade silicon accelerometer is investigated. By utilizing an optical detection system, a small sensor with high sensitivity and wide bandwidth can be created. The sensor will be highly resistant to RFI and EMI, as well as temperature fluctuations, making it ideal for applications in harsh environments, e.g. aerospace, automotive, nuclear, industrial, etc.
A Fabry-Perot interferometer (FPI), consisting of two parallel plates with reflective inner surfaces, serves as the main building block for the accelerometer. One of the FPI's transparent plates is fixed and the other is suspended and can move due to acceleration.
Inside the FPI, two mirrors form a cavity with an optical resonance that depends on the distance between the plates. At the resonant wavelengths, all of the incident light energy is transmitted through the FPI, and intensity peaks will therefore occur. If the space between the mirrors is changed, e.g. due to acceleration, the wavelengths of the intensity peaks will change. Thus, by detecting the shift in wavelength of the transmitted light, the acceleration can be acquired.
Extension to Distributable Sensor Arrays
By taking advantage of the optical properties of Fabry-Perot interferometers, such sensors can be used in the creation of simple, highly sensitive sensor arrays. If the plates of the FPI are made using high quality notch filter mirrors, they will be reflective only in a small segment of the spectrum and filter light only in that region - the rest of the light will pass through untouched. If a number of similar FPI's are connected by an optical fiber, 'downstream' sensors that are each reflective in different regions can use this light. This encodes the acceleration signals on different wavelengths in a way similar to optical networks. The wavelength transmitted in each reflective region, or channel, is continuously monitored resulting in simultaneous acceleration measurements from each sensor. For a simple system with three channels and three FPI sensors, one can imagine a rainbow with three colors with each color continuously changing hue. In our system, one needs only to note the hue of each color to know the acceleration of the sensor. Distributing the micro-FPI's about a large structure tells us the accelerations throughout the structure.