Resolution Improvement of Laser Microphone with Miniature Multiple Reflection

A laser microphone using the self-coupling effect of laser diodes has a simple optical system, with a wide and flat frequency characteristic. However, due to the structure of the sensor section, detection of spherical sound waves has been difficult until now. In addition, simple miniaturization of the sensor parts of the laser microphone causes a decrease in sensitivity. To address these issues, a laser microphone sensor structure using multiple reflections from a pair of mirrors is considered. By using a multiple reflection sensor, the laser microphone retains sensitivity while achieving the desired miniaturization. When a multiple reflection sensor is used, the sensitivity improves up to approximately seven times, compared to simple miniaturization of the sensor part. When a sound source which generates spherical waves was used for measurement, the sound pressure distribution reconstruction achieved by the multiple reflection type laser microphone was the same as that of a conventional quarter inch sensor condenser microphone. It was found that the multiple reflection type sensor allowed downsizing of the sensor part while still being able to measure spherical sound waves.