Distributed temperature sensing via Brillouin-tailored optical fiber

We present Brillouin-tailored optical fiber for use in distributed temperature sensing systems. A manufactured fiber operates with two Brillouin modes that have been gain-equalized to within 0.5 dB, with frequency difference of 175 MHz. Some traditional distributed systems employ a heterodyne scheme to measure a temperature-dependent microwave frequency Stokes' shift (∼11 GHz at 1530 nm) imparted by Brillouin scattering. Realizing an RF detection scheme for the temperature distribution may include the development of an optical fiber engineered to have two gainequalized Brillouin frequencies. The two acoustic modes should respond differently to temperature variations, and thus the detection of their beat signal would provide temperature data. One approach investigated is to structure the core to have two or more dissimilar layers that are 'quasi-independent' such that their resulting Brillouin frequencies are far enough apart, and have a significantly dissimilar dependence on temperature. Gain equalizatio n between these two modes results from the proper tailoring of the overlap integrals with the optical mode. Our best results were achieved through core-cladding Brillouin-gain equalization via the reduction of Brillouin gain in the core of a tailored fiber. A linear temperature dependence of -1.1 MHz/°C was measured for the beat frequency of a developed fiber.