Optical fibre grating refractometers for resin cure monitoring

Abstract

Integrated Optical Fibre (IOF) is a passive optical platform that directly integrates optical fibre to a planar substrate, illustrated in Fig 1(a). It possess advantages associated with optical fibre such a low propagation loss, whilst enabling planar functionality commonplace with integrated optics. Planarization is uniquely achieved through a modified Flame Hydrolysis Deposition (FHD) technique that forms a robust glass alloy between the fibre and substrate. The binding medium is of optical quality and resistant against common solvents, chemicals and elevated temperatures of up to 1000 o C. Furthermore as fibre can be brought seamlessly on-off chip there is no need for glues or cumbersome coupling arrangements, which are typically a point of mechanical weakness when monitoring harsh environments. Thus far, developments in IOF have largely utilised either the environmental stability [1] and/or physical robustness [2] of the platform and not the optical quality of the binding medium. This work reports for the first time and evanescent field based refractometer using IOF, in which the optical mode directly interacts with the deposited FHD glass and the external environment. The concept for design is based largely on so termed thinned-FBGs [3], in which an optical fibre is wet etched using buffered HF. Through applying a similar geometry to the IOF format one can achieve external refractive index sensitivity, but with an enhanced mechanical integrity and capacity for integration. Fig. 1 (a) a cross-section profile of an Integrated Optical Fibre with SMF-28. (b) A cross-section profile of an Integrated Optical Fibre with a wet etched SMF-28 fibre (fibre 8 µm in diameter). (c) The optical response of a Bragg grating in construct (b), under exposure to Cargille refractive index matching liquids. This work reports a proof-of-concept refractometer design, fabricated through wet etching SMF-28 fibre in a buffered HF solution then applying it in an IOF format. For the device depicted in Fig 1 (b), etching is continued until a diameter of 8 µm is reached. This etched fibre is then subsequently transferred and layered-up onto a silicon wafer that has a thick thermally grown oxide (15 µm). The purpose of the oxide is to act as an optical buffer layer (clad), ensuring the final device maintains a guided mode. To form the IOF and maintain as much evanescent field exposure to the external environment a 0.5 µm FHD layer is deposited and consolidated at 1250 o C, leaving the cross sectional geometry shown in Fig 1 (b). The FHD recipe is a silicate, doped with boron and phosphorus that once consolidated has a refractive index of 1.4452 (at a wavelength of 1553 nm). Changes in external refractive index were inferred through use of a Direct UV Written fibre Bragg grating [2]. The grating was inscribed at a fluence of 30kJcm-2 into the fibre core, after the FHD had been consolidated. To enhance UV photosensitivity the chip was hydrogen loaded at 120 bar for 7 days. Cargille refractive index matching liquids (series AA and AAA) were ...

Monitoring the production of FRP composites: A review of in-line sensing methods

“…A multi-point flow detection system comprised of consecutive bare and grated regions along a single fiber was developed and experimentally validated; resin arrival was successfully detected on all bare regions. Buggy et al [46] employed two Fresnel OFRs operating in different wavelengths to monitor the curing of an epoxy resin. The measured refractive index increased rapidly after an initial decrease, indicating correlation with viscosity.…”

Curing Monitoring of Graphite/Epoxy Laminated Composite Material by Using Embedded Corrugated Long-Period Fiber Grating Sensor

“…When the curvature was within the range of 0-2 m −1 , the resonance amplitude depth increased linearly and the position of the resonance wavelength remained unchanged, at a sensitivity of 5.065 dBm −1 . In 2007, Buggy et al (9) monitored variations in refractive index during resin curing by employing three fiber optic sensors, namely the LPFG, tilted Bragg gratings, and the Fresnel refractometer. Their results revealed that the refractive indices measured using the LPFG and the Fresnel refractometer were highly consistent.…”

Optical Fibre Long-Period Gratings Functionalised with Nano-Assembled Thin Films: Approaches to Chemical Sensing

“…As the coupling wavelength and efficiencies are dependent upon the properties of the cladding modes, the resonance features in the TFBG transmission spectrum exhibit sensitivity to surrounding refractive index [10,11]. Analysis of the transmission spectrum facilitates measurements with resolution of 10 -4 RIU [11].…”

“…As the coupling wavelength and efficiencies are dependent upon the properties of the cladding modes, the resonance features in the TFBG transmission spectrum exhibit sensitivity to surrounding refractive index [10,11]. Analysis of the transmission spectrum facilitates measurements with resolution of 10 -4 RIU [11]. The use of thin film coatings of refractive index higher than that of the cladding on both polished FBGs and TFBGS has been shown to allow the RI range over which the devices show their highest sensitivity to the tuned to lower values [10,12,13].…”