Distributed temperature sensor using holmium-doped optical fiber and spread-spectrum techniques

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Abstract
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 attenuationmeasurements 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.