Abstract
An optical fibre chemical sensor that is insensitive to interfering parameters including temperature and surrounding refractive index is described. The sensor is based upon a Mach-Zehnder interferometer formed by a pair of identical cascaded long period gratings (LPGs), with the entire device coated with a mesoporous coating of silica nanoparticles. A functional material is infused only into the coating over the section of optical fibre separating the LPGs. The transmission spectrum of the device consists of a channeled spectrum arising from interference of the core and cladding modes within the envelope of the LPG resonance band. Parameters such as temperature, strain and surrounding refractive perturb the entire device, causing the phase of the channeled spectrum and the central wavelength of the envelope shift at the same rate. Exposure of the device to the analyte of interest perturbs only the optical characteristics of the section of fibre into which the functional material was infused, thus influencing only the phase of the channeled spectrum. Measurement of the phase of the channeled spectrum relative to the central wavelength of the envelope allows the monitoring of the concentration of the analyte with no interference from other parameters.
Highlights
Optical fibre long period grating (LPG) based devices offer interesting opportunities for sensing applications due to their inherent sensitivity to a range of environmental perturbations [1]
Where, under the assumption of a symmetrical refractive index modulation, λx represents the wavelength at which light is coupled to the LP0x cladding mode, ncore is the effective refractive index of the mode propagating in the core of the fibre, nclad(x) is the effective index of the LP0x cladding mode and Λ is the period of the LPG
The section of the coated optical fibre separating the LPGs was immersed into a solution of tetrakis-(4-sulfophenyl)porphine (TSPP), a functional material that we have shown previously to be suitable for use on the LPG sensing platform to develop ammonia sensors [3]
Summary
Optical fibre long period grating (LPG) based devices offer interesting opportunities for sensing applications due to their inherent sensitivity to a range of environmental perturbations [1]. As the phase and the resonance wavelength are both dependent upon the difference between the core and cladding mode effective indices, when the entire length of the cascaded LPG is perturbed both the resonance band central wavelength and the phase of the channelled spectrum change at the same rate [10] This effect can be seen when coating cascaded LPGs with nanoscale films [11]. Perturbation of the entire length of the device by environmental parameters such as temperature, strain and changes in the refractive index of the fluid containing the analyte causes the central wavelength of the resonance band envelope and the phase of the channelled spectrum to change at the same rate, assuming that the presence of the functional material does not alter the thermal and refractive index responses of the cladding modes.
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