An interoferometric fiber optic sensing system uses three optical fibers. A
sensing optical fiber is applied to a structure to be monitored to detect
displacement or the like by changing its optical path length. A reference
optical fiber has a fixed optical path length. An adjustable length
optical fiber is controllably adjusted in its optical path length. The
three optical fibers form optical paths whose light outputs are caused to
interfere. The adjustable length optical fiber is adjusted until an
interference fringe appears. The quantity to be detected is derived from
the maximum of the interference fringe. Several sensing optical fibers can
be multiplexed; by staggering their optical path lengths, their
interference fringes can be separated sufficiently to resolve them.
