A Novel Pulse Compression Scheme in Coherent OTDR Using Direct Digital Synthesis and Nonlinear Frequency Modulation

We report on an experimental demonstration of a novel scheme for enhancing the spatial resolution of coherent OTDR using chirped pulse compression based on Direct Digital Synthesis (DDS) of the frequency modulation of probing pulses. We show that DDS can be used to generate linear frequency modulated (LFM) optical pulses with high fidelity and enables programmable customization of the frequency modulation laws of pulse waveforms for advanced pulse compression employing matched filtering with nonlinear frequency modulated (NLFM) pulses. We demonstrate the effectiveness of the method in significantly suppressing ambiguity in pulse compression by confirming the clear separation of events spaced by 50 cm at ~1.13 km using 1.2-µs pulses. Thanks to the use of rigorously defined, DDS-generated NLFM waveforms with desired features, error-inducing sidelobes in the pulse autocorrelation function are suppressed by more than ~20 dB, offering an improvement of ~16 dB compared to conventional LFM pulses.