Optical microphone

1. An optical microphone comprising:

a light source;

a first polarizer for allowing linearly-polarized light, of light output from the light source, to pass therethrough;

a second polarizer for allowing linearly-polarized light having a different polarization plane from the first polarizer to pass therethrough;

a sound-receiving section including an opening, and an acoustic medium having a smaller sound velocity than an air, wherein an acoustic signal entering through the opening propagates through the acoustic medium, the sound-receiving section being arranged so that the linearly-polarized light having passed through the first polarizer passes through the acoustic medium so as to cross a path along which the acoustic signal propagates and enters the second polarizer; and

a photodetector for converting an intensity of light having passed through the second polarizer to an electric signal,

wherein between the first polarizer and the second polarizer, the linearly-polarized light having passed through the first polarizer is given different phase shifts in two orthogonal directions which are each different from a polarization direction.

2. The optical microphone of claim 1 , further comprising:

an elliptically-polarized light generation section arranged between the first polarizer and the second polarizer for converting the linearly-polarized light to elliptically-polarized light,

wherein the linearly-polarized light is given different phase shifts in the two orthogonal directions by the elliptically-polarized light generation section.

3. The optical microphone of claim 2 , wherein the elliptically-polarized light generation section includes at least one of a wave plate, a liquid crystal and an optical crystal having an optical anisotropy.

4. The optical microphone of claim 2 , wherein the elliptically-polarized light generation section is located between the first polarizer and the sound-receiving section.

5. The optical microphone of claim 2 , wherein the elliptically-polarized light generation section is located between the second polarizer and the sound-receiving section.

6. The optical microphone of claim 2 , wherein an optical axis of the elliptically-polarized light generation section and an optical axis of the second polarizer are orthogonal to each other.

7. The optical microphone of claim 1 , wherein the acoustic medium has an optical anisotropy, and the linearly-polarized light is given different phase shifts in the two orthogonal directions by the optical anisotropy.

8. The optical microphone of claim 7 , wherein the optical anisotropy is generated by a residual stress of the acoustic medium.

9. The optical microphone of claim 7 , wherein the sound-receiving section further includes a pressure-giving section for giving a static pressure to the acoustic medium, and the optical anisotropy is generated by the static pressure.

10. The optical microphone of claim 1 , wherein an optical axis of the first polarizer and an optical axis of the second polarizer are orthogonal to each other.

11. The optical microphone of claim 1 , further comprising a signal processing section for amplifying an electric signal converted by the photodetector.

12. The optical microphone of claim 11 , wherein the signal processing section includes a direct-current component removing section for removing a direct-current component from the electric signal output from the photodetector.

13. The optical microphone of claim 12 , wherein:

the signal processing section further includes a differential amplifier; and

the differential amplifier amplifies and outputs a difference between the electric signal output from the photodetector and an output of the direct-current component removing section.

14. The optical microphone of claim 1 , wherein the acoustic medium is a dry silica gel.