Optical fibre long period grating based selective vapour sensing of volatile organic compounds

Abstract

The chemical sensing capabilities of a long period grating (LPG) coated with a functional material are explored. The LPG period and coating thickness are chosen such that the mode transition region, characteristic of LPGs coated with nanostructured coatings, corresponds with the phase matching turning point, ensuring optimum sensitivity. The functional material is a calixarene, which is shown to exhibit sensitivity to the aromatic compounds toluene and benzene, while being relatively insensitive to the aliphatic hydrocarbon hexane. A nanoscale cavity in the calixarene molecule entraps the VOC molecule, thereby altering the refractive index of the coating and influencing the transmission spectrum of the LPG. A sensitivity to toluene of 231 ppmv has been achieved, based on a spectrometer resolution of 0.3nm. This is a weak molecular interaction thereby ensuring the sensor gives a reversible response, when the VOC is removed. The recovery time is of the order of 15 seconds.

Highly sensitive volatile organic compounds vapour measurements using a long period grating optical fibre sensor coated with metal organic framework ZIF-8

“…Measurements of the wavelengths separation or width of the band, as will be shown in this work, allows to achieve higher sensitivity. This can be achieved by choosing an appropriate LPG period and coating thickness [29,30]. The cladding modes associated with the attenuation bands have been calculated through numerical modelling using the phase matching equation and weakly guided approximation to determine the effective refractive indices of the core and cladding modes and the known values of the central wavelengths and the grating period [31].…”

Fiber Optic Sensors Based on Nanostructured Materials

“…The contribution of James et al to this field has reached a particular relevance in the last years, with different approaches of optical fiber sensors based on coated LPGs [43][44][45]. Among other examples, they have created VOC detectors [46][47][48], chemical sensors [49] or interferometers [50] based on this technology. In Fig.…”

“…The shift of the attenuation bands to higher wavelengths as long as the concentration of ammonia is increasing can be clearly appreciated. Additionally, a great variety of coated-LPG-based sensors developed by other groups can be found in the bibliography [46][47][48][51][52][53].…”

Long Period Gratings in New Generation Optical Fibers

“…A particular dispersion behaviour of one of the cladding modes obtained for a specific grating period at a certain wavelength, the so-called turn around point (TAP), together with functional coatings of synthetic or biological nature, was exploited to obtain ultra-sensitivity for volatile organic compounds or biomolecules detection Z. Wang et al, 2009;Topliss et al, 2010). It should be noted that the number of biosensing applications with LPGs is rapidly growing and it can be foreseen www.intechopen.com that it will represent an area of major interest in coming years (Eggen et al, 2010;Smietana et al, 2011).…”