The analysis of noises in a fiber optic microphone

The general formulations to evaluate the pressure detection limit determined by various noise sources within the fiber optic microphone are developed including the shot and thermal noises of the opto-electronic detector. The formulations to calculate the optical power level of the thermal radiation from the sensing membrane and optical fibers, which will degrade the microphone performance by increasing the detector shot noise, particularly at high temperatures, are developed as a function of the temperature along the optical fibers as well as the microphone parameters. These factors determine the limit that the optical power output from the optical fiber microphone must exceed in order to perform pressure detection properly within the intended temperature range. The eventual pressure detection limit is evaluated based on the thermally generated membrane random vibration while the conditions to measure this noise effect are also established. The analysis provides a general approach to design the fiber optic microphones for a particular application and for a wide temperature range. Calculations illustrating our approach based on a typical fiber optic microphone design are also included.