Extended-range optical time domain–reflectometry system at 165??µm based on delayed Raman amplification

We describe both theoretical and experimental results obtained in an investigation of a new technique for increasing the dynamic range of $1.65-\mathit{\mu }\mathrm{m}$ optical time domain–reflectometry (OTDR) systems. The technique utilizes delayed Raman amplification of a $1.65-\mathit{\mu }\mathrm{m}$ signal pulse by a $1.53-\mathit{\mu }\mathrm{m}$ pump pulse. Amplification occurs when the two pulses overlap, and this position is determined by the initial delay between the pulses and the fiber dispersion. An increase in dynamic range of 17.5 dB has been observed, and the OTDR backscattered Rayleigh signal was detected up to 100 km. No significant noise penalty is introduced as a result of the directionality of the Raman gain.