Laser Phase-Noise Cancellation in Chirped-Pulse Distributed Acoustic Sensors

Reaching pε/√Hz sensitivity in a distributed optical fiber strain sensor

“…Chirped-pulse φOTDR is also affected by phase-noise and equipment-induced jitter, which directly translate into an acoustic measurement over the whole fiber. However, it has been recently demonstrated that those effects can be readily compensated by detecting the undesired trace shifts from an unperturbed fiber region [9]. Having corrected the phase-noise and removed the sampling error, our system becomes limited by the accuracy of cross-correlation for TDE.…”

“…After that, the pulse is sent to the test fiber through a circulator. The sensing fiber has a length of ~1200 m, with a first section containing piezoelectric actuator (PZT), and a second ~1 km-long section to be used for phase-noise compensation [9]. This final section is introduced in a water bath to minimize any undesirable local temperature or air draft measurements.…”

Statistical Analysis of SNR in Chirped-pulse ΦOTDR

“…where  refers to correlation, arg max stands for the arguments of the maxima, nt are considered to be additive Gaussian noise in each signal within the correlation time window (recall that phase noise can be easily compensated in CP-ΦOTDR [7]). Assuming a wellconditioned power trace in terms of SNR,         and hence, the second and third terms in the second line of (1) can be considered as noise with equivalent power.…”

“…In this work, we present an analysis of the statistical properties of the SNR and sensitivity of CP-ΦOTDR. We show that the nature of the employed interrogation method permits perturbation quantification almost independently of the trace fading points, an SNR with significant lower variability (~10 dB) and only dependent on the trace optical noise (mainly amplitude noise, as the phase noise is mostly compensated [7]). Hence, we prove that CP-ΦOTDR offers not only a simpler interrogation procedure but also notably higher reliability in the obtained sensing measurements.…”

“…Besides, phase noise in the trace due to e.g. the limited coherence of the probe laser, can be measured and mitigated, allowing for high SNR measurements with relatively moderate coherence probe lasers [7].…”