Novel dual-Brillouin-frequency optical fiber for distributed temperature sensing

We present an approach to distributed fiber-optic temperature sensing utilizing a dual-Brillouin-frequency optical fiber. Traditional distributed sensor systems employ a heterodyne detection scheme to measure a temperature-dependent microwave frequency Stokes' shift. Our approach toward realizing an RF, rather than microwave, detection scheme is the development of an optical fiber engineered to have two gain-equalized Brillouin requencies (dual-Brillouinfrequency fiber, or DBFF). The design goal is that the two acoustic modes respond differently to temperature variations, and thus the detection of their beat signal (in the RF) would provide temperature data. One approach is to structure the core to have two or more dissimilar layers that are 'quasi-independent' such that their resulting Brillouin frequencies have a dissimilar dependence on temperature. Proper tailoring of the overlap integrals with the optical mode results in gain equalization between resulting acoustic modes. A slightly different approach is presented, where two Brillouin frequencies are achieved through core-cladding Brillouin-gain equalization via the reduction of Brillouin gain in the core. Temperature sensing is then accomplished by the direct detection of the RF beat frequency between them (~175 MHz). A linear temperature dependence of -1.07 MHz/C was measured for the beat frequency of a tailored fiber.