4962319 Remote subsurface water temperature measuring apparatus with brillouin scattering

Abstract

Apparatus is disclosed for remotely sensing the unknown subsurface temperature T s of a bulk transparent medium, such as ocean water, using a cw laser beam and a pulsed laser beam. In one embodiment the cw output of a continuous laser is split into two sub-beams. An optical amplifier periodically amplifies the first such sub-beam thereby generating an intense pulsed laser beam having a plurality of pulses with the same wavelength as the cw laser beam. The intensity level of the pulses is sufficient to produce stimulated Brillouin scattering when focussed into the ocean water. The pulsed laser beam is directed into the water thereby generating an intense return phase-conjugate beam propagating along the path of the pulsed laser beam but in the opposite direction thereto. The phase-conjugate beam and the second sub-beam are mixed at the cathode of a photodetector to produce a heterodyne frequency proportional to the temperature T s . A frequency measuring instrument converts the heterodyne frequency into a temperature value equal to T s . Since the cw beam in always present at the photodetector for mixing with the return phase-conjugate pulses, the path lengths of the cw laser beam and the pulsed laser beam may be unequal.