Cancellation of reference update-induced 1/f noise in a chirped-pulse DAS

Abstract

Distributed acoustic sensors (DAS) perform distributed and dynamic strain or temperature change measurements by comparing a measured time-domain trace with a previous fiber reference state. Large strain or temperature fluctuations or laser frequency noise impose the need to update such a reference, making it necessary to integrate the short-term variation measurements if absolute strain or temperature variations are to be obtained. This has the drawback of introducing a 1/f noise component, as noise is integrated with each cumulative variation measurement, which is detrimental to the determination of very slow processes (i.e., in the mHz frequency range or below). This work analyzes the long-term stability of chirped-pulse phase-sensitive optical time-domain reflectometry (CP-ΦOTDR) with multi-frequency database demodulation (MFDD) to carry out “calibrated” measurements in a DAS along an unmodified SMF. It is shown that, under the conditions studied in this work, a “calibrated” chirped-pulse DAS (CP-DAS) with a completely suppressed reference update-induced 1/f noise component is achieved capable of making measurements over periods of more than 2 months with the same set of references, even when switching off the interrogator during the measurement.

Noise analysis of coherent and non-coherent detection in Φ-OTDR systems with chirped pulses

“…Still, it should be noted that the detection system might be more complex in this case. Nevertheless, these results provide opportunities to investigate the implementation of techniques for improving long-term stability 5 and to enhance measurement range or spatial resolution utilizing coding techniques such as pulse compression 8 , as the phase of the backscattered trace can be recovered using coherent detection. The expansion of the measurable range is a key goal in fields such as fiber optic submarine seismology, where the objective is to cover the greatest possible distance with a single interrogator.…”

“…Long-term stability is usually limited by drifts in the laser wavelength and by the 1/f noise component introduced when updating the reference trace. Whereas there are lasers with good wavelength stability, and techniques capable of avoiding the 1/f noise component 5 . These improvements cannot always be implemented in equipment suitable for field measurements.…”

“…These improvements cannot always be implemented in equipment suitable for field measurements. The reasons are: (i) a laser with such long-term wavelength stability increases considerably the cost; and (ii) the technique in 5 requires an initial sweep of the laser wavelength and consequently, tunable, or wide enough optical filters to cover the full sweep are needed. On the other hand, the measurement range of ΦOTDR schemes is mainly defined by the signal to noise ratio along the trace.…”

“…However, given the coherence of the source, a sufficiently narrowband filter must be tuned in with the central wavelength of the laser. Besides, a narrowband filter hinders the application of techniques to increase the long-term stability of the system 5 .…”

High resolution seafloor thermometry for internal wave and upwelling monitoring using Distributed Acoustic Sensing

“…standard DAS units are expected to suffer from noise increasing inversely proportional to frequency (1/f), our results show that this effect is not enough to impair the detection of the thermal oceanic signatures at the frequencies here considered. In a recent study [103], the possibility to suppress the 1/f noise was demonstrated, thus opening the way for a new generation of DAS systems robust for static sensing and capable of providing absolute temperatures over periods of months or longer.…”