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This paper describes the feasibility analysis of using Holmium-doped single-mode fiber and spread-spectrum techniques to monitor a temperature distribution in the cryogenic range. An optimized holmium-doped fiber was realized and thermally characterized. For probing the sensitive fiber we realized an optical time domain reflectometer using pseudorandom sequences to improve the dynamic range at the operating wavelength and to obtain both rapid response time and high spatial resolution. The system, tested over a fiber length of 240 m, provides temperature dependent fiber attenuation measurements in good agreement with the transmission results. With a 10 MHz pseudorandom binary sequence the system detects changes of 24 °C with a spatial resolution of 30 m.

We propose a multisensor network where the refractive index modulators are distributed in cascade along the optical fibre. Reflections at fibre glass-monitoring medium interfaces provide a train of time multiplexed pulses. Backscattered radiation in the fibre serves as a reference channel for the individual sensors which can be designed for temperature. pressure, chemical composition measurements. AbstractWe propose a multisensor network where the refractive index modulators are distributed in cascade along the optical fibre. Reflections at fibre glass-monitoring medium interfaces provide a train of time multiplexed pulses. Backscattered radiation in the fibre serves as a reference channel for the individual sensors which can be designed for temperature, pressure, chemical composition measurements.