Analysis of nonlinear fitting methods for distributed measurement of temperature and strain over 36 km optical fiber based on spontaneous Brillouin backscattering

We experimentally analyzed two different nonlinear fitting methods for a Brillouin optical time domain reflectometery (BOTDR) system. By measuring the spontaneous Brillouin frequency and comparing a Lorentzian fit with a Voigt fit, we confirmed that the accuracy of the Lorentzian fit was higher than that of the Voigt fit. We also performed long-range distributed measurements using coherent detection system with a 36 km long-range optical fiber. The extinction ratio of optical pulse was improved by employing an acousto-optic modulator (AOM). As a result, we could successfully measure the Brillouin frequency shift (BFS) using a Lorentzian fit algorithm in order to calculate applied temperature and strain in the optical fiber.