Long-range Raman distributed temperature sensor with high spatial and temperature resolution using graded-index few-mode fiber.

We designed and fabricated a graded-index few-mode fiber (GI-FMF) with large effective mode area and low intermodal dispersion for Raman distributed temperature sensor (RDTS) to simultaneously achieve high spatial and temperature resolution over long distance. In experiment, we measured the spatial and temperature resolution of the RDTS using different types of fibers under different launch conditions based on a commercially available RDTS system. By using the GI-FMF under the overfilled launch condition, we achieved a 1 °C temperature resolution with a spatial resolution of 1.13 m at the distance of 25 km. The spatial resolution using the standard MMF degraded to 2.58 m with only a 0.3 °C higher temperature resolution in comparison. As a result, the GI-FMF under the few-mode operation condition can provide a desirable temperature resolution comparable with that of the MMF with a negligible degradation on spatial resolution. Moreover, the RDTS using the GI-FMF under the quasi-single mode operation condition achieved a temperature resolution of 4.7 °C at the distance of 25 km with a 2.2 °C improvement and no degradation on spatial resolution compared with that using the standard SMF.