Overview of Fiber Optic Sensors Over the past twenty years two major product revolutions have taken place due to the growth of the optoelectronics and fiber optic communications industries. The optoelectronics industry has brought about such products as compact disc players, laser printers, bar code scanners and laser pointers. The fiber optic communication industry has literally revolutionizedthe telecommunication industry by providing higher performance, more reliable telecommunication links with ever decreasing bandwidth cost. This revolution is bringing about the benefits of high volume production to component users and a true information superhighway built of glass. In parallel with these developments fiber optic sensor [1-6] technology has been a major user of technology associatedwith the optoelectronic and fiber optic communication industry. Many of the components associated with these industries were often developed for fiber optic sensor applications. Fiber optic sensor technology in turn has often been driven by the development and subsequent mass production of components to support these industries. As component prices have fallen and quality improvements have beenmade, the ability of fiber optic sensors to displace traditional sensors for rotation, acceleration, electric and magnetic field measurement, temperature, pressure, acoustics, vibration, linear and angular position, strain, humidity, viscosity, chemical measurements and a host of other sensor applications, has been enhanced. In the early days of fiber optic sensor technology most commerciallysuccessful fiber optic sensors were squarely targeted at markets where existing sensor technology was marginal or in many cases nonexistent. The inherent advantages of fiber optic sensors which include their ability to be lightweight, of very small size, passive, low power, resistant to electromagnetic interference, high sensitivity, wide bandwidth and environmental ruggedness were heavily used tooffset their major disadvantages of high cost and unfamiliarity to the end user. The situation is changing. Laser diodes that cost $3000 in 1979 with lifetimes measured in hours now sell for a few dollars in small quantities, have reliability of tens of thousands of hours and are used widely in compact disc players, laser printers, laser pointers and bar code readers. Single mode optical fiber thatcost $20/m in 1979 now costs less than $0.10/m with vastly improved optical and mechanical properties. Integrated optical devices that were not available in usable form at that time are now commonly used to support production models of fiber optic gyros. Also, they could drop dramatically in price in the future while offering ever more sophisticated optical circuits. As these trends continue, theopportunities for fiber optic sensor designers to produce competitive products will increase and the technology can be expected to assume an ever more prominent position in the sensor marketplace. In the following sections the basic types of fiber optic sensors that are being developed will be briefly reviewed followed by a discussion of how these sensors are and will be applied.
Basic Conceptsand Intensity Based Fiber Optic Sensors Fiber optic sensors are often loosely grouped into two basic classes referred to as extrinsic or hybrid fiber optic sensors, and intrinsic or all fiber sensors. Figure 1 illustrates the case of an extrinsic or hybrid fiber optic sensor. Light Modulator Input Fiber Output Fiber
Figure 1. Extrinsic fiber optic sensors consist ofoptical fibers that lead up to and out of a "black box" that modulates the light beam passing through it in response to an environmental effect.
In this case an optical fiber leads up to a "black box" which impresses information onto the light beam in response to an environmental effect. The information could be impressed in terms of intensity, phase, frequency, polarization, spectral content or...