Active fiber-optic opto-acoustic detector

Abstract

An opto-acoustic detector includes a fiber laser having a cavity which includes an optical fiber having an active medium portion doped with a gain material so that the detector may be used in high frequency, high density arrays. The cavity is bounded by a frequency selective reflector and a broadband or frequency selective reflector adapted to be acoustically coupled to a medium supporting ultrasound radiation (i.e. acoustic waves). In one embodiment, the frequency selective reflector is a diffraction grating placed external to the optical fiber. In another embodiment, the frequency selective reflector is a narrowband reflector placed directly in the optical fiber. An acoustic signal enhancing feature which increases sensitivity of the detector is provided by utilizing a reflective membrane having a thickness substantially less than the acoustic wavelength, .lambda., as the broadband optical reflector. The membrane is displaced from a free end of the optical fiber in the cavity and a laser beam exiting therefrom is focused on the membrane with a spot size on the order of the optical fiber core diameter. In this way, the sensing area of the detector is essentially the cross-sectional area of the fiber core which is typically comparable to or less than .lambda..sup.2 even at ultrasound operating frequencies greater than 500 MHz. The reduced element size permits closely spaced optical detectors enabling high density arrays for high frequency imaging.