Pulse Waveform Manipulation in FDM-OTDR for Suppressing Inter-Channel Crosstalk

The inter-channel crosstalk (XT) in frequency division multiplexing coherent optical time domain reflectometry (FDM-OTDR) is described theoretically. The inter-channel XT degrades the spatial resolution of the OTDR trace at the position, where the reflectivity changes precipitously, and is dependent on the frequency channel spacing, pulse waveform, and weighting function of the signal processing. We show that the deterioration of the OTDR trace can be fully simulated by using impulse response analysis, and that apodization of the probe pulse shape and weighting function effectively suppress the inter-channel XT. Moreover, we propose a novel nonlinear self-phase modulation (SPM) suppression scheme to adapt apodized probe pulse in our experiment. The experimental results for the impulse response, OTDR trace, and SPM suppression are discussed.