Fiber optic sensor applications using Fabry-Perot interferometry

Abstract

Fabry and Perot first discussed their interferometry concepts in 1898. Over the years, use of the concept has found a wide variety of applications. The availability of solid state sources and optical fiber has further broadened the potential number of applications for interferometry of all types. When considering optical fiber interferometry, one naturally tends to consider single mode operation. Coherent light sources tend to make for an easier conceptual design, and can offer advantages in sensitivity and resolution. Some examples of single mode fiber Fabry Perot interferometers are the work of Matsumoto who described an acoustic sensing diaphragm, and by Lee and Taylor who utilize an in-line fiber interferometer to measure temperature. The work carried out at NetriCor, however, utilizes a multimode Fabry-Perot resonator. This has a number of advantages which makes for a very cost effective trade-off. Features of a multimode interferometer include: * Efficient coupling from long lived LED sources. The mean time to failure for a common LED is approximately an order of magnitude higher for a laser. * Adequate sensitivity for most applications. For most industrial applications, the ultimate in sensitivity is not the predominant issue. * Readily available and inexpensive components. * Easy fiber termination. Although great strides have been made in both single mode and multimode fiber termination technology, the requirements on a multimode termination tend to be less stringent. * Inexpensive sensing element. The sensing element, as will be described later, can be made by 194 / SPIE Vol. 1267 Fiber Optic Sensors IV(1990) semiconductor techniques and produce inexpensively in large quantities. * Common readout. With the technique described, a large variety of sensors can be made to utilize the same and often interchangeable instrumentation.