Jonathan Bohbot

A real-time structural health monitoring (SHM) system of an airplane composite wing with adjustable damage is reported, where testing under realistic flight conditions is carried out in the controllable and repeatable environment of an industrial wind tunnel. An FBG-based sensing array monitors a debonded region, whose compromised structural strength is regained by a set of lockable fasteners. Damage tunability is achieved by loosening some of or all these fasteners. Real-time analysis of the data collected involves Principal Component Analysis, followed by Hotelling’s T-squared and Q measures. With previously set criteria, real-time data collection and processing software can declare the structural health status as normal or abnormal. During testing, the system using the Q measure successfully identified the initiation of the damage and its extent, while the T-squared one returned limited outcomes.

This paper investigates the so-far unexplored influence of the probes on the state of polarization (SOP) of the pump pulse in a distributed fiber sensor based on dual-probe Brillouin optical-time domain analysis (BOTDA). A theoretical framework, based on a vector formalism in the Stokes space, is proposed to describe the Brillouin interaction of a dual-probe BOTDA setup. Experimental results over a 50 km long fiber, supported by numerical simulations over a few km of a modeled birefringent fiber, demonstrate for the first time notable Brillouin-mediated attraction/repulsion actions from the probes on the SOP of the pump pulse. While the probes experience Brillouin-induced polarization transformation only during their relatively short interaction with the pump pulse, the effect of the probes on the pump SOP accumulates over the entire fiber length, becoming more significant with higher probe power. The opposite actions of the two (gain and loss) probes do not provide full compensation, since they are dissimilarly affected by the pump, and have slightly different associated Brillouin frequency shifts. The study highlights an overlooked limitation in BOTDA sensing, where the different influences of the probes on the pump SOP impair the commonly used method for polarization fading mitigation based on a polarization switch. This method involves launching a pair of orthogonally co-polarized probes (or pumps) propagating in the sensing fiber against a single-polarization pump (or single-polarization dual-sideband probes). This limitation becomes particularly relevant in advanced configurations using high probe powers, especially for long sensing ranges and high-performance applications.

Numerical solution of the involved governing equations confirm that the commonly used two orthogonal sets of gain- and loss-probes in BOTDA, differently affect the evolution of the pump state-of- polarization, thereby potentially compromising the minimization of polarization fading.

A Structural Health Monitoring assessment of a small aircraft demonstrator is done in real time using Principal Component Analysis based statistical methods. Real flying aircraft data is used to detect an overload caused by a High-G maneuver.

Slope-assisted Brillouin optical time domain analysis (SA-BOTDA) is a fast and distributed sensing technique that interrogates the Brillouin gain spectrum (BGS) near the middle of the linear region of its slope, allowing the measurement of high frequency vibrations. However, environmentally-induced deviations of the interrogation point from the rather small linear region around the optimum frequency position, result in harmonic distortion and changes in the slope value, which determines the gain-to-frequency conversion factor. Engineering the BGS by adding a loss probe, we propose a novel and improved version of SA-BOTDA, having a significantly wider drift tolerance for the interrogation point. Here, using a 14 ns pump pulse and a judicious choice of the frequency of the loss probe, we demonstrate an engineered BGS having a 70% wider frequency range (as compared with the conventional BGS), where the interrogation point can drift without compromising either the prescribed level of harmonic distortion or the given slope tolerance, further benefiting the flexibility and usage of SA-BOTDA.