Distributed strain sensing using slope assisted BOTDA based on virtual Brillouin gain spectrum synthesized by multi-frequency light

We experimentally demonstrate a fiber optic distributed strain sensor based on slope assisted Brillouin optical time domain analysis (SA-BOTDA) using virtual Brillouin gain spectrum (BGS) generated by multi-frequency light. Generally, strain along an optical fiber can be measured by using Brillouin scattering, because BGS has a linear dependence on the fiber strain. While conventional Brillouin fiber optic strain sensors sweep the probe light frequency to measure the BGS, it limits the measurement time and requires precise control of probe frequency. In contrast, the recently proposed method of SA-BOTDA measures the strain in real time from the power change of fixed frequency probe light that has been amplified by Brillouin amplification. However, because the conventional SA-BOTDA has a nonlinear response to the strain, it has to compensate the nonlinearity or only use BGS with relatively narrow range of linear slope region. In the last study, we proposed virtual BGS that has a relatively wide linear slope region and confirmed the principle. In this paper, we experimentally generate virtual BGS that has more than 100 MHz of linear slope region by using 12-GHz spacing five-frequency pump and probe. The strain distribution along the fiber is successfully measured in real time.