DC02 Giovanni Vacirca - CEA

Schedule-driven Non-Destructive Evaluations (NDE) are carried out during structure/equipment’ life to detect major degradations endangering safety and impairing service availability. In addition to NDE, Structural Health Monitoring (SHM) involves the use of in-situ sensing systems and algorithms to evaluate structure worthiness. Condition-based maintenance (CBM) is therefore progressively emerging with the aim to anticipate failure modes and reduce indirect cost and production/service downtime. NDE/SHM make an increasing use of Optical Fiber Sensors (OFS) because of their intrinsic advantages (distributed and quasi-distributed measurements along a single fibre strand, small size, electromagnetic-immunity, etc.). Among OFS, Fibre Bragg Gratings (FBGs) are photowritten on singlemode telecommunication fibres, providing both wavelength-Division Multiplexing (WDM) and high spatial localization (mm). FBGs are tiny sensors (150 µm in diameter, possibly less), already successfully implemented within concrete, metallic and composite materials, along with connection procedures for interrogation during operational or maintenance phases. FBG sensors provide in situ strain (and temperature) measurement and give valuable data about internal strain change due to fatigue and defects propagation inside the material. FBGs are also used for acoustic sensing in association with piezoelectric actuators or laser ultrasonics (US) in order to perform tomography reconstruction and identify defects (delamination etc.) inside complex high-value structures. Commercially available FBG readout units rely upon WDM or Optically Frequency-Domain Reflectometry (OFDR). WDM-based units are limited in capacity (several sensors) but may reach high scan rate (MHz). Conversely, OFDR-based units are limited in scan rate (several tens of Hz) but may accommodate large number of sensors (typically up to 2000). Tomography with acoustic techniques requires both high capacity and high scan rate with the aim to improve quality of image reconstruction. Optical Time-Stretch (OTS, also called Wavelength-to-Time Mapping (WTM)) is a time-domain technique that has potential to improve both capacity and scan rate and to open the way to efficient tomography reconstruction processes. The principle of OTS is to use highly-dispersive optical media providing WTM of pulse light signals. The Bragg shift is the inferred from the delay time between the pulse reflected by the FBG and a reference one. Engineers in Shock Physics already use OTS with high-bandwidth oscilloscopes (40 GHz) and high fibre length (20 km) as dispersive medium. As capacity is trading-off with scan rate, the readout device is usually tailored to every end-user demand.

Keywords: Fiber Bragg Gratings (FBGs), Optical Time Stretch (OTS), Wavelength-to-Time Mapping (WTM), Structural Health Monitoring (SHM), Tomography.

Research fields: Measurement technology, Optronics, Control engineering