Pressure sensitivity of a clad optical fiber

Experiments carried out by Bucaro et al. demonstrate that an optical-fiber acoustic wave detector with plastic coating several times its diameter exhibits greatly increased sensitivity compared with an uncoated fiber. In this paper we show that this effect should be expected because the plastic is much more compressible than the glass fiber. With a very thick coating of a Teflonlike plastic, the calculated longitudinal strain produced in the fiber by hydrostatic pressure is thirteen times larger than for an uncoated fiber. The effective phase change is increased by the much larger factor of 38 because the coated fiber expands laterally instead of contracting.