Abstract

A short fiber clamped at one end and free at the other has a resonant frequency that varies inversely with the square of its length. We have adjusted the length of 130 multimode optical fibers to logarithmically cover the speech acoustic spectrum from 100 Hz to approximately 5 KHz. The free ends of the fibers are coated with reflecting material. Light is injected into the clamped end of the fiber bundle, and the reflected signal is novelty-filtered. When all the fibers are at rest, no light exits from the novelty filter. However, the output of the fibers that are vibrating appears at the output of the novelty filter. The fibers are currently driven by a piezoelectric transducer. The transducer is in turn driven by a microphone or some other acoustic signal source. Thus, the spectrum of the acoustic signal appears at the novelty output. We currently do not maintain ordering of the frequencies; we merely bunch the fiber outputs for convenience. An iron-doped lithium niobate crystal is used to holographically record the spectrum of simple sounds that have stationary spectra.

© 1990 Optical Society of America