Abstract-Phase-sensitive optical time domain reflectometry ( OTDR ) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. Up to now, OTDRs have been used mostly for the measurement of sub-kHz vibrations, normally in the context of intrusion sensing. In this work, the authors present an experimental and theoretical description of a high-visibility OTDR and its performance when used for ultrasonic vibration measurements. The use of a semiconductor optical amplifier (SOA) in the setup allows to suppress coherent noise and also to improve the spectral response of the pump pulses. These two advantages greatly decrease the detected intra-band noise thus allowing frequency measurements in the limits set by the time of flight of the light pulses while maintaining the simplicity of the scheme, as no post-processing, extremely high coherence lasers or coherent detection methods are required. The sensor was able to measure vibrations of up to 39.5 kHz with a resolution of 5 m over a range which could go up to 1.25 km. This is the first time to our knowledge that a fully distributed measurement of ultrasonic waves was achieved. The statistical behavior of the system was also described theoretically and characterized experimentally.

In this work, the authors present an experimental and theoretical description of the use of first order Raman amplification to improve the performance of a Phase-sensitive optical time domain reflectometer (OTDR) when used for vibration measurements over very long distances. A special emphasis is given to the noise which is carefully characterized and minimized along the setup. A semiconductor optical amplifier (SOA) and an optical switch are used to greatly decrease the intra-band coherent noise of the setup and balanced detection is used to minimize the effects of RIN transferred from the Raman pumps. The sensor was able to detect vibrations of up to 250 Hz (close to the limits set by the time of flight of light pulses) with a resolution of 10 m in a range of 125 km. To achieve the above performance, no post-processing was required in the OTDR signal. The evolution of the OTDR signal along the fiber is also shown to have a good agreement with the theoretical model.

No abstract

This is a postprint version of the following published document:Filograno, M.L., Corredera, P., González-Herráez, M., Rodríguez-Plaza, M. and Andrés-Alguacil, A., "Wheel flat detection in high-speed railway systems using fiber Bragg