Direct time-of-flight distributed analysis of nonlinear forward scattering

Scattering is among the most common and widely employed optical phenomena. The spatially resolved analysis of scattering contributions supports distributed sensing of quantities of interest. While optical backscatter events are readily mapped using time-of-flight considerations, the distributed analysis of forward scattering represents a fundamental and long-standing challenge. Interest in distributed analysis of forward scattering has reawakened in recent years, toward optical fiber sensors based on forward-stimulated Brillouin scattering. Existing protocols for distributed analysis of forward Brillouin scattering rely on secondary backscattering mechanisms and mandate the noise-prone differentiation of collected data with respect to position. Here we report on the direct, distributed analysis of forward scattering. The combined contributions of forward-stimulated Brillouin scattering and Kerr effect four-wave mixing are resolved with respect to position along polarization-maintaining fibers. The concept is based on the characteristics of intermodal scattering in such fibers: Forward scattering is initiated by a pair of orthogonally polarized and copropagating pump waves and observed through the nonlinear polarization switching of a counterpropagating probe. Measurements distinguish between dissimilar fibers connected in series, and between air and water outside a polyimide-coated fiber section in a specific location. The measurement range was 1.1 km. The spatial resolution currently achieved is estimated as 60 m, limited by the lifetimes of forward Brillouin scattering. The results provide preliminary proof of concept for distributed forward Brillouin fiber sensors that do not require the differentiation of data.