Patent ID: 8306429

Claim:
An optical ultrasonic microphone, which is a microphone for receiving a sound wave from an ambient space filled with a gas, the optical ultrasonic microphone comprising: a base that is provided with (i) an opening through which the sound wave is received from the ambient space and (ii) an acoustic waveguide for transmitting the sound wave received by the opening; a light source for applying light; an optical acoustic propagation medium portion held by the base, the optical acoustic propagation medium forming at least one portion of the acoustic waveguide through which the sound wave from the acoustic waveguide and the light from the light source are propagated; a light detecting means for detecting the light emitted from the optical acoustic propagation medium portion and for detecting a modulation of the light caused by the sound wave; and an operation unit for finding a sound pressure from results of the detection by the light detecting means, wherein, between a density ρ a and a sound velocity C a of the gas with which the acoustic waveguide is filled, a density ρ n and a sound velocity C n of an optical acoustic propagation medium of the optical acoustic propagation medium portion are allowed to satisfy the following expression: (ρ a /ρ n )<( C n /C a )<1 and wherein the operation unit finds the sound pressure from the result of the detection by the light detecting means according to the following expression: P = - C n 2 × ρ n × n n - 1 ⁢ Δ ⁢ ⁢ L L = - C n 2 × ρ n × n n - 1 ⁢ 1 L ⁢ ∫ v ⁢ ⅆ t where P is the sound pressure, ρ n is the density of the optical acoustic propagation medium, C n is the sound velocity of the optical acoustic propagation medium, L is the distance in which the sound wave and the light interfere with each other in the optical acoustic propagation medium, ΔL is a displacement output of the light detecting means, and ν is a velocity output of the light detected by the light detecting means.