Grating-based Fiber Optic Sensors (FOSs), i.e. relying on Bragg Gratings (BGs), Long Period Gratings (LPGs), Tilted Fiber BGs (TFBGs), have seen a popularity in recent years for sensing applications, however, most of these are inscribed on Single-Mode Fibers (SMFs). Multi-Mode Fibers (MMF), on the other hand, offer new and different properties in grating design and performance characteristics compared to SMFs, since the spectral response may be tuned by core size, refractive index profile, numerical aperture, and mode coupling characteristics of the gratings. Also, MMFs can be readily coupled with inexpensive light sources and other optical components due to their large core and, thus, gratings in MMFs are preferred to yield lower cost systems. Moreover, in terms of sensing region, MMFs have a greater mode field surrounding the fiber when compared with SMFs, due to the larger core diameters of MMFs and, thus, even greater mode fields can be accessed with a smaller reduction of the fiber diameter which would have better mechanical robustness, when compared with gratings inscribed in SMFs. In this talk we present our latest research in BG structures inscribed in multi-mode optical polymer and glass fibers.
An optical fibre-based strain sensor embroidered to a functionalised carbon structure (FCS) that can be used for structural health monitoring (SHM) is introduced. The aim of the design is not only to monitor strain, but also to act as a structural strengthening mechanism in the target application. The integration of optical fibres on the FCS is achieved by “interweaving” the two elements on a polymer textile substrate in a grid formation using a specialised fabrication process. The thus obtained sensor was then characterised using a fibre optic Mach-Zehnder (MZ) interferometric setup where a variation in the fibre length, i.e. resulting from strain, would induce a variation in the interference pattern. To do so, two different functionalised skein samples (incorporating optical fibres) were subjected to varying elongation using a tensile testing machine by carefully incrementing the applied force. A good correlation between the applied force and measured length change was observed, showing the value of the dual-achievement of the proposed optical fibre-based mechanism in obtaining strain measurement while being utilised as a strengthening agent.
A Relative Humidity (RH) sensor incorporating a Long Period Grating (LPG) and a Fibre Bragg Grating (FBG) in a series configuration was developed and evaluated. The LPG was coated with polyimide allowing it to respond to humidity changes while the FBG is used for temperature compensation. The hybrid sensor was then tested over the change in RH from 20-95 % and temperature of 20-80 °C. The sensitivity of the hybrid sensor to temperature and RH were determined to be 9 pm/°C and 0.23 nm/%RH, with negligible hysteresis (<1% RH), showing the value of the dual LPG/FBG approach to optimize sensitivity.
The sensitivity of a Long Period Grating (LPG)-based refractive index (RI) sensor has been enhanced through an
appropriate coating of a thin polymer layer, with a higher RI than that of the cladding, on the LPG. The performance of
the coated sensor, together with a bare LPG, was evaluated as a chemical sensor when the sodium chloride concentration
in water varies from 0.01% to 2%. The corresponding attenuation band shifts of both sensors in response to the RI
change were observed. The experimental results confirmed that the coated LPG sensor has higher sensitivity than that of
the bare LPG sensor.