Abstract: The present invention relates to an optical sensor of air borne acoustic waves. The sensor comprises means for producing mutually coherent optical sampling and reference beams, which may be combined to form an intermediate frequency carrier, the sampling beam being exposed to the acoustic field, in which acoustic wave induced density variations occur. These density variations produce a variation in the index of refraction and thereupon a phase modulation of the sampling beam. This phase modulation may be recovered by an optical detector and a phase detector as an electrical signal representative of the acoustic signal. The invention has application to security systems.

Abstract: A method for sensing acoustic energy as the energy passes from a more dense to a less dense medium while compensating for energy level loses that may have occurred along the acoustic path of the acoustic energy. The method may be used to replace a multi-sensor system for detecting acoustic energy. The method includes permitting the acoustic energy to progress from a first transmitting medium to a second transmitting medium, the first transmitting medium being more dense than the second transmitting medium and the first medium being contiguous with the second medium along a predetermined interface; disposing an optical sensor in the second medium so that at least a portion of the acoustic energy imposes a modulation proportional to the frequency components of the acoustic energy on a sampling light emitted by the optical sensor; and detecting at least a portion of the sampling light having modulation in response to the portion of the acoustic energy.