Single-shot distributed temperature and strain tracking using direct detection phase-sensitive OTDR with chirped pulses

Abstract

So far, the optical pulses used in phase-sensitive OTDR (ΦOTDR) were typically engineered so as to have a constant phase along the pulse. In this work, it is demonstrated that by acting on the phase profile of the optical pulses, it is possible to introduce important conceptual and practical changes to the traditional ΦOTDR operation, thus opening a door for new possibilities which are yet to be explored. Using a ΦOTDR with linearly chirped pulses and direct detection, the distributed measurement of temperature/strain changes from trace to trace, with 1mK/4nε resolution, is theoreticaly and experimentaly demonstrated. The measurand resolution and sensitivity can be tuned by acting on the pulse chirp profile. The technique does not require a frequency sweep, thus greatly decreasing the measurement time and complexity of the system, while maintaining the potential for metric spatial resolutions over tens of kilometers as in conventional ΦOTDR. The technique allows for measurements at kHz rates, while maintaining reliability over several hours.

Chirped-Pulse Phase-Sensitive Reflectometer Assisted by First-Order Raman Amplification

“…1. It is based on a chirped-pulse ΦOTDR scheme [3]- [5] but including a continuous wave (CW) Raman pump. The Raman pump is applied at the two ends of the sensing fiber aimed at compensating the energy loss of both the probe pulse and the reflected trace along the fiber, and consequently achieving larger measurement ranges.…”

“…The 2.5 ns rise/fall times and extinction ratio (ER) of >50 dB of the SOA allow us to generate 100 ns-width quasi-square-shaped pulses with high ER. This leads to a spatial resolution of 10 m in the phasesensitive reflectometer scheme [1], [3]. The ramp signal is controlled so that the resulting linearly chirped pulse has a total spectral content of 630 MHz.…”

“…In contrast, in chirpedpulse ΦOTDR a refractive index change simply translates into a proportional temporal shift ∆t in the corresponding section of the power trace. The relationship between ∆n and ∆t is given by [3]- [5]:…”

“…Thus, the system allows for the quantitative measurement of distributed temperature or strain changes with no need for a frequency scan or the application of coherent detection methods. More information about the fundamentals of this sensor can be found in a previous work by the same authors [3].…”

“…Particularly, DOFS based on phase-sensitive optical timedomain reflectometry (ΦOTDR) are commonly used for vibration detection with potential applications in pipeline surveillance and perimeter protection [2]. Recent advances in this technology have allowed to perform high-resolution, quantitative dynamic temperature or strain variation measurements in a simple, robust and time-effective manner [3]- [5], based on the use of linearly chirped probe pulses. This novel technique is named chirped-pulse phase-sensitive optical time-domain reflectometry (chirped-pulse ΦOTDR).…”