Ultrahigh scan-rate quasi-distributed acoustic sensing system using array match interrogation

Abstract

Inspired by compressed sensing techniques, a method for significantly enhancing the maximum allowable scan rate in quasi-distributed acoustic sensing (Q-DAS) is described and studied. Matching the scan parameters to the interrogated array facilitates orders of magnitude improvement in the scan rate and a corresponding increase in the maximum slew rate (SR) of differential phase variations which can be measured without ambiguity. The method is termed array matched interrogation (AMI). To improve the method’s SNR, maximum number of sensing sections and maximum range, the interrogation pulse can be replaced by a perfect periodic autocorrelation (PPA) code. This version of the method is referred to as coded array matched interrogation (C-AMI). The implementation of C-AMI is not trivial and requires special design rules which are derived and tested experimentally. The design rules ensure that the ‘folding’ of the returning peaks of the Q-DAS array into a scan period, which is much shorter than the fiber's roundtrip time, will not lead to overlaps. The method demonstrated a scan rate of 20 times higher than the common limit and measurement of unprecedented slew-rate of 10.5 ×106 rad/s.

Ultrasonic long range underwater acoustic sensing: going beyond the standard pulse repetition rate

“…is the interrogation rate increment factor. 3 In AMI-based interrogation, the noise floor is a result of shot-noise and hence its mean power over 75, 000 segments is relatively flat.…”

“…Taking advantage of the sparse impulse response of a Q-DAS array in order to increase the acoustic BW has been demonstrated with an unordered array of reflectors and with an ordered array of reflectors and methodical rules for finding interrogation rates which greatly surpass the standard limit were established. 3,4 In the case of an ordered array of M discrete reflectors, equally separated from one another by the round trip time, T rt , AMI scanning parameters (T ± scan ) and pulse width are determined from:…”

“…Our group demonstrated Q-DAS at ultrahigh interrogation rates combined with Perfect Periodic Autocorrelation (PPA) codes. 3,4 The method involves accurate tailoring of the code parameters to the configuration of the FBG's array and is thus named Coded Array Matched Interrogation (C-AMI). In this work we tested the method in underwater conditions for the first time and successfully measured the propagation of bursts of 95kHz ultrasound pulses along a 20m test pool.…”