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.

This paper introduces and numerically investigates a special optical fiber cable with zero temperature-induced phase shift. The cable structure consists of stacked layers of two materials with opportune mechanical, thermal, and geometrical properties. This structure allows adjusting the thermal-induced strain to the fiber, resulting in a broad tunability of the bare thermal expansion, including the negative range. By a proper choice of materials, the thickness of each layer, and the radius of the cable, the induced thermal strain can fully compensate for the thermo-optic effect, resulting in a complete temperature insensitivity of the phase shift. This cable may be of great interest in the sensing fields in all those applications where the temperature compensation is critical, such as in low-frequency distributed acoustic sensing. Moreover, it could be relevant for a wide range of telecom applications that require precise thermal control.

Distributed acoustic sensors cumulatively measure fiber variations, introducing a 1/f noise that makes long-term measurements impossible. We present an “absolute” measurement method that allows accurate single-shot measurements at frequencies from <0.1 μHz to acoustic frequencies.

Chirped-pulse phase sensitive (CP-Φ) OTDR is a distributed sensing technology that allows for quantitative measurement of strain and temperature along an optical fiber by simply direct detection of the Rayleigh backscattering. Typically, chirped pulses have a linear frequency modulation covering few GHz. Backscattered traces must be amplified before detection, which introduces noise and limits the signal-to-noise ratio (SNR) and, therefore, the maximum measurable range. To increase the SNR, an optical filter is usually placed before photodetection aimed at reducing broadband optical noise caused by amplified spontaneous emission. However, narrow-band filters (e.g., 10 GHz bandwidth) are not easily compatible with multi-wavelength approaches, used to improve the long-term stability. Furthermore, in practice it is not straightforward to find narrowband optical filters that continuously match the central frequency of the laser, considering laser wavelength drifts. In this study, the influence of the optical filter bandwidth on the range in CP-ΦOTDR is theoretically investigated for two types of photodetection: direct and coherent. The results show that when using coherent detection, the SNR does not depend on the filter bandwidth. Therefore, it is possible to achieve an equivalent measurement range by using a wide optical filter (e.g., 100 GHz) as compared to that obtained when using direct detection with a narrowband filter. This finding suggests that coherent detection can be used to increase the range in CP-ΦOTDR and could be compatible with the use of multi-wavelength techniques to improve the long-term stability for applications such as civil engineering and seismology.

This study compares noise and signal-to-noise ratio (SNR) in direct detection and coherent detection fiber-based distributed acoustic sensing (DAS) systems. Both detection schemes employ the dynamic analysis of Rayleigh-backscattered light in phase-sensitive optical time-domain reflectometry (ΦOTDR) systems. Through theoretical and experimental analysis, it is determined that for photodetection filters with a sufficiently narrow bandwidth, the SNR performance of both detection schemes is comparable. However, for filters with poor selectivity, coherent detection was found to exhibit superior performance. These findings provide crucial guidelines for the design of high-performance time-domain DAS systems.