The study of LPFG-based sensors and their possible applications in chemical sensing is presented. Using sucrose solution and sodium chloride solution as examples, we demonstrate that substantial enhancement of RI sensitivity and limit of detection for chemical solution concentration sensing can be achieved by using an LPFG sensor with its cladding surface modified by gold nanoparticles.2.?Principle of refractive index sensingThe LPFG with periods ranging from several hundred microns to several millimeters couples incident light guided by a fundamental mode in the core to different forward-propagating cladding modes of high diffraction order m in an optical fiber, which decay rapidly due to the radiation from scattering losses.
The coupling of the light into the cladding region generates a set of resonant bands centered at wavelength��m in the transmission spectrum of the fiber. The resonance wavelengths ��m of an attenuation band are solutions of the following phase matched conditions [12]:��m=[ncoeff?ncl,meff]��=��neff��(1)where �� is the period of grating, and ncoeff is the effective RI of the fundamental core mode at the wavelength of ��m , which is also dependent on the core RI and cladding RI. Also ncl,meff is the effective RI of the mth radial cladding mode (m =2,3,4,��) at the wavelength ��m, which also is a function of cladding RI and in particular the RI of the surrounding medium ns . Note that both indices are dependent on the temperature and the strain experienced by the fiber.
The spectral properties of individual cladding modes are determined by the fiber structure and may be observed through their associated attenuation bands.
When the concentration GSK-3 or the RI of the surrounding medium changes also ncl,meff changes and a wavelength shift can be obtained in the transmission spectrum. The wavelength shift can be linearly related to the concentrations of the solution under test. An LPFG can be very sensitive to the changes in temperature and deformations by fiber imperfections, loading, and bending, which also produce a noticeable wavelength shift in loss peaks.
Therefore, to precisely measure variations of concentrations or RI changes, temperature Dacomitinib changes and deformations must be compensated or avoided.Since the grating period is unaltered under the effect of a change in ambient RI and assuming that the RI of the core mode remains unchanged by ns, the influence of variation in the RI around the cladding of an LPFG is expressed by:(d��dns)m=(d��dncl,meff)(dncl,meffdns)(2)The spectral sensitivity, defined as, d��dns is relevant to each of the measurands and contributes to effective wavelength change of the mth cladding mode.