Parallel multiplexing in optical backscatter reflectometry by the use of nano-particles doped optical fiber

Optical Backscatter Reflectometer (OBR), based on Optical Frequency Domain Reflectometry principles can transform a simple and cheap single mode fiber in an efficient spatially distributed (over the fiber length) sensor of temperature and strain variation. Nevertheless, the use of OBR is limited to function with a single sensing fiber. Connecting multiple fibers in parallel can be problematic. The scattering level of each fiber is of the same magnitude so that the backscattering cannot be discriminated. Unfortunately, particular medical applications, such as the guided insertion of needle or medical catheters, can benefit of multiple fiber sensors mounted in parallel. The adopted solution of switching between different sensors in different time frames if feasible, but it significantly reduces the interrogation frequency. In this work a new solution for overcoming this issue, by the use of a high scattering nano-particles doped fiber (NPDF), is proposed. This fiber presents a random distributed pattern of magnesium oxide nanoparticles, whose size varies between 20 to 100 nm, in the core. Its backscattering is 50 dB larger than a standard single mode fiber. The use a NPDF segment spliced to a standard single mode pigtail, with different lengths, such that the NPDF position corresponds to a pigtail on the other fibers, permits to connect them in parallel. Thus, the OBR can spatially resolve the NPDF high backscattering, since the single mode pigtail scattering is irrelevant. Experiments have shown positive results in the terms of temperature and strain discrimination.