Experimental Investigation of the Optical Noise Sensing Capabilities of the Brillouin Fiber Ring Amplifier

An optical noise sensing scheme based on the high gain Brillouin fiber ring amplifier is investigated experimentally. The sensing mechanism is the result of the close relationship between the guided acoustic eigenmodes in the fiber and the resultant Brillouin scattering of an externally fed optical signal. Since these eigenmodes are thermal and vibrational in origin, and light scattering due to them is quite weak, they constitute a broadband noise source centered around the incident optical signal frequency. The Brillouin fiber ring amplifier is designed to have the capability to listen to such optical noise in the fiber, when the latter is subjected to ambient physical disturbances of a mechanical and thermal nature. By properly designing the system power pack, the fiber layout and sensing scheme, the proposed system can be robust and fault tolerant. Noise sensing is inherently multimeasurand, as opposed to traditional fiber-optic sensors, which are usually single-measurand. The proposed scheme can thus render sensing considerably more economical. This experimental feasibility study will thus concentrate on exploring the low noise sensing capability of the Brillouin fiber ring amplifier in a two-ring format, powered by a single laser. The first ring generates a Stokes wave to serve as the noise carrying signal in a fiber, while the second ring by varying GeO/sub 2/ doping generates a Stokes wave at slightly shifted wavelength, sufficiently apart from the signal Stokes as not to amplify it. The latter ring is thus designed to amplify only the Brillouin noise, which is normally about 6 x 10/sup -5/ of signal power. The high gain and low internal noise of the fiber ring are the promising features of this unique noise amplifier. The increasing usage of embedded fibers on structures to monitor structural integrity has prompted many sensing schemes, but none so far has demonstrated robustness and fault tolerance as required for practical application. Our proposed sensing scheme is expected to fill the gap.