Time-expanded phase-sensitive (TE-φ )OTDR is a distributed optical fiber sensing (DOFS) technique that takes advantage of the dual-frequency comb technology to offer distributed, dynamic, and high-spatial resolution measurements. The performance delivered by this recent approach is unmatched by any other DOFS, combining the high resolution of OFDR with the potential for long range and fast sampling of φ OTDR. In this contribution, we present an optimized TE-φ OTDR scheme with important improvements with respect to the traditional one. In particular, the new architecture uses electrooptical phase modulation instead of intensity modulation, increasing the energy-efficiency. Additionally, it employs an optical hybrid to double the spectral efficiency of the system, which in practical terms results in doubling the spatial resolution for the same interrogating comb bandwidth. The proposed architecture has been experimentally validated through a scheme providing 5 mm of spatial resolution, 80 m of range and 70 Hz sampling rate with a simple, compact and low-cost setup using field-programmable gate arrays (FPGA) and relatively low bandwidth photodetection (2 MHz).
Optical fiber sensors are becoming a prominent solution to perform shape sensing thanks to their interesting and well-known advantages such as flexibility, lightweight, sensitivity, etc. However, the different approaches commonly used to date, which include fiber Bragg gratings, OFDR or OTDR architectures, are not able to achieve a fully distributed and fast performance over relatively long ranges (e.g., tens of meters) and with fine (cm-scale) resolution. Here, we present a novel scheme to perform curvature sensing attaining all the previously mentioned features by application of time-expanded phase-sensitive (TE-Φ)OTDR technology. TE-ΦOTDR is a promising distributed sensing technique that delivers a performance ranging between that of OFDR and ΦOTDR. As a proof-of-concept, we interrogate three cores of a multicore fiber (MCF) using TE-ΦOTDR, attaining curvature sensing with 10 cm resolution over a maximum measurable range of 125 m and a sampling rate of 50 Hz. The implementation of shape sensing schemes with the performance provided by TE-ΦOTDR technology may open the door to new and interesting applications in civil engineering, medicine and seismology.
Any temperature change or strain acting on a section of the fiber induces a local variation of the refractive index. If the fiber is monitored by a chirped pulse φ-OTDR system, the variation of the refractive index causes a local shift of the backscattering trace and produces a change in the round trip time of the light coming from any further position of the fiber. While usually negligible, due to the high sensitivity of the chirped pulse φ-OTDR, in extreme occasions the distributed round trip time change may appear in the measurement as an undesired “virtual” perturbation. In this paper, we discuss and experimentally validate a mathematical model to account for (and eventually correct) the “virtual” perturbation.
Chirped pulse phase-sensitive optical time-domain reflectometry (chirped pulse Φ-OTDR) allows the interrogation of tens of kilometers of optical fiber with high sensitivity and linearity, but typically with spatial resolution limited to ten meters to ensure proper processing and signal-to-noise ratio (SNR). In this paper, we propose a method to increase the spatial resolution of a chirped pulse Φ-OTDR without reducing the pulse width. The improvement is achieved by adding an optical carrier to the input chirped pulse and by applying digital filtering to the measured backscatter traces. Experimental results validate the method, demonstrating a 10-fold resolution improvement with minimum impact on the measurement SNR.
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