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
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
* 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