Optical fiber acoustic sensor

Studies have been made on the use of long length, low-loss optical fiber coils as direct acoustic sensors. When a fiber coil is exposed to a sinusoidal pressure variation in water two phase modulation processes are easily detected. The first is due to interference between directly transmitted and back and forth reflected beams. The second arises because of interference between two propagating modes in the fiber. Data on these two processes are discussed in detail. Experimental data is also presented on intensity modulation effects detected in several different multimode step index fibers.