Distributed optical fibre sensing at 1.65μm using a Q-switched fibre laser

It is becoming increasingly vital to monitor telecommunication links during operation and installation process. By using a high peak power source and the optical time domain reflectometry (OTDR) technique operating at the wavelength region of 1.6μm, it is possible to monitor conventional C-band Erbium-doped fibre amplifier (EDFA) systems whilst transmitting data, and to characterise losses at the higher wavelengths of extended bandwidth systems designed around the L-band EDFA systems. We describe a compact design based on Raman shifting the output of an Erbium-doped Q-switched fibre laser operating at 1.5μm for obtaining a pulsed source at 1.6μm. This source was used for an OTDR measurement and also as a source for a 1.65μm Raman-based distributed temperature sensor, in contrast to distributed temperature sensors normally operating at 1.5μm. OTDR measurements at 1.65μm provide more accurate determination of macro and micro-bend losses than at 1.5μm as such losses increase with wavelength. The temperature measurement extracted from the anti-Stokes Raman signal at 1.5μm was made over a sensing range of 10.1km, with a spatial resolution of 10m and temperature resolution of 4°C.