Miniature, high performance, low-cost fiber optic microphone

Abstract

A small, high performance fiber optic microphone has been designed, fabricated, and tested. The device builds on a previous design utilizing a thin, seven-fiber optical probe, but now adds a micromachined 1.5μm thick silicon diaphragm active element. The resulting sensor head is thin (several millimeters) and light, and the overall microphone system is less expensive than conventional microphones with comparable performance. Measurements in the laboratory using a standard free-field technique at high frequencies, an enclosed calibrator at lower frequencies, and pseudostatic pressure changes demonstrate uniform broadband response from near dc (0.01 Hz) up to near 20 kHz. The measured microphone internal noise is nearly flat over this band and does not exhibit noticeable levels of 1∕f noise. Over the audible portion of this band, the minimum detectable pressure is determined to be 680μPa per root Hz with further reductions possible using lower noise∕higher power light sources and∕or improvements in the diaphragm. In contrast to conventional high-performance microphones, there is no need for preamplifier packages close to the relatively small sensor head resulting in much lower acoustic scattering cross sections. This attribute, together with high performance, low cost, and immunity to emi, makes the microphone ideal for multielement array applications.