Remote displacement measurement is demonstrated using a Fabry-Perot cavity with a multimode optical fiber link. The sensing cavity modulates, as a function of its length, the spectrum of a light-emitting diode (LED). The light returns via the fiber and is analyzed by a tunable reference cavity. A closed-loop control causes the reference cavity to track the sensing cavity length within 2 × 10-12 m. Displacement range is 2 × 10-6 m. The reference cavity length is measured interferometrically, using a laser, to obtain the sensing cavity length. Advantages of this sensing technique include compatibility with multimode fiber-optic components, high immunity to optical losses, and large dynamic range.
Glenn Beheim, "Remote displacement measurement using a passive interferometer with a fiber-optic link," Appl. Opt. 24, 2335-2340 (1985)