SNR enhancement in TE $-\emptyset$ OTDR via multi Nyquist zone averaging

Time-expanded phase-sensitive optical time-domain reflectometry
$(\text{TE}-\phi \text{OTDR})$
has become as a promising technology for those applications that require monitoring dynamic events with an elevated number of sensing points and high spatial resolution. Besides, the available spatial resolution is detached from the required detection bandwidth, in such a way that centimeter resolution can be readily achieved with sub-MHz detection bandwidth, instead of the GHz bandwidth typically required in traditional
$\phi$
OTDR scheme for the said resolution. However, in TE-OTDR, not only one spectrally compressed version of the fiber response is attained upon detection; multiple copies of nearly the same information are obtained in each Nyquist zone. In this contribution, we propose a technique to enhance the signal-to-noise ratio (SNR) of the detected waveforms by means of a smart averaging of the signal in each Nyquist zone. The SNR is hence enhanced without reducing the effective interrogation sampling rate. Experimental tests to corroborate the validity of our proposal have allowed us detect a temperature perturbation over 2 cm with a detection bandwidth of 5 MHz. By averaging 11 Nyquist zones, we have achieved an improvement of almost 4-fold the temperature measurement accuracy.