Study on laser microphone using self-couping effect of semiconductor laser for sensitivity improvement

In conventional microphone a diaphragm is used for sound wave detection. However, diaphragm limits the detectable frequency range or sound pressure range. A laser microphone which is applying self-coupling effect of LD for sound wave detection is developed. It has flat and wide frequency response by utilizing the self-coupling effect. From experiment results, laser microphone has nearly flat frequency response from 40Hz to 50kHz. However, the SN ratio decreases in low sound pressure. Therefore, actions for sensitivity improvement are subjected. Sound wave collection using parabolic sound reflector improves sensitivity. However, frequency characteristics are arisen in audible band. Light path extension using right angle prism improves sensitivity. Minimum detectable sound pressure decreases from 54dB to 47dB with light path extension.