Distributed Acoustic Sensing Using Chirped-Pulse Phase-Sensitive OTDR Technology

Abstract

In 2016, a novel interrogation technique for phase-sensitive (Φ)OTDR was mathematically formalized and experimentally demonstrated, based on the use of a chirped-pulse as a probe, in an otherwise direct-detection-based standard setup: chirped-pulse (CP-)ΦOTDR. Despite its short lifetime, this methodology has now become a reference for distributed acoustic sensing (DAS) due to its valuable advantages with respect to conventional (i.e., coherent-detection or frequency sweeping-based) interrogation strategies. Presenting intrinsic immunity to fading points and using direct detection, CP-ΦOTDR presents reliable high sensitivity measurements while keeping the cost and complexity of the setup bounded. Numerous technique analyses and contributions to study/improve its performance have been recently published, leading to a solid, highly competitive and extraordinarily simple method for distributed fibre sensing. The interesting sensing features achieved in these last years CP-ΦOTDR have motivated the use of this technology in diverse applications, such as seismology or civil engineering (monitoring of pipelines, train rails, etc.). Besides, new areas of application of this distributed sensor have been explored, based on distributed chemical (refractive index) and temperature-based transducer sensors. In this review, the principle of operation of CP-ΦOTDR is revisited, highlighting the particular performance characteristics of the technique and offering a comparison with alternative distributed sensing methods (with focus on coherent-detection-based ΦOTDR). The sensor is also characterized for operation in up to 100 km with a low cost-setup, showing performances close to the attainable limits for a given set of signal parameters [≈tens-hundreds of pe/sqrt(Hz)]. The areas of application of this sensing technology employed so far are briefly outlined in order to frame the technology.

Long-Range Distributed Solar Irradiance Sensing Using Optical Fibers

“…In CP-φOTDR, the pulse-to-pulse frequency scanning is replaced by a single linearly chirped pulse probe signal [38,39]. Given a chirped-pulse bandwidth much larger than the transform-limited pulse bandwidth, any external temperature or strain induced changes in the fiber optical path will translate into a local time shift (at the corresponding optical trace position).…”

“…Consequently, CP-φOTDR sensors are not only able of detecting and localizing an external stimulus, but they are also able of quantifying them with high accuracy. In addition, CP-φOTDR ensures a consistent value of sensitivity along the length of the fiber, without being affected by the fading points (which are positions with nearly no visibility affecting conventional φOTDR traces) [38,39]. Therefore, CP-φOTDR sensors are extremely appealing for applications that need to quantify a certain physical parameter over large distances and that require a similar measuring quality in all points along the fiber, being widely implemented in areas going from security to seismology [33,34].…”

“…Therefore, CP-φOTDR sensors are extremely appealing for applications that need to quantify a certain physical parameter over large distances and that require a similar measuring quality in all points along the fiber, being widely implemented in areas going from security to seismology [33,34]. Currently, CP-φOTDR has shown to reach sub-mK resolutions over ranges of up to 100 km with metric spatial resolution and sampling rates of the order of 1 kHz [39], reaching more recently a record dynamic strain sensitivity of 10 −12 ε/ √ Hz (equivalent to 10 −6 K/ √ Hz) [43]. Working principle of chirped-pulses in phase-sensitive optical time domain reflectometry (CP-φOTDR): A linear frequency chirped pulse is sent into the optical fiber.…”

“…The experimental setup used in the proposed proof-of-concept demonstration of a distributed solar irradiance sensor is depicted in Figure 3. The sensor is a CP-φOTDR interrogator [38,39], whose operation has been already described in section 2. Working principle of chirped-pulses in phase-sensitive optical time domain reflectometry (CP-φOTDR): A linear frequency chirped pulse is sent into the optical fiber.…”

“…Recently, an advanced optical fiber reflectometry technique using chirped-pulses in phase-sensitive optical time domain reflectometry (CP-φOTDR) has been demonstrated to achieve sub-mK resolutions in distributed temperature detection, achieving fast (kHz) and reliable measurements over distances of up to 100 km with meter-scale spatial resolutions [38][39][40]. With these advances, it is now possible to develop a feasible and practical distributed surface solar irradiance sensor using optical fibers.…”