In this paper, using full vector modal field analysis, we present physical parameter sensing, e.g., temperature, axial strain, bulk refractive index, and affinity sensing characteristics of counterpropagating cladding modes in optical fibers. Counterpropagating cladding modes ( H E m n ) are considered to be excited by resonant power coupling from the fundamental core mode using a suitably designed fiber Bragg grating (FBG). We show that for such couplings, the bandwidths of reflection spectra are much smaller than those obtained for conventional core mode reflecting FBGs. Next, we also show that the reflection bandwidth increases linearly with increasing grating strength, whereas it decreases exponentially with increasing grating length. The reflection spectra of such FBGs, as a function of different ambient physical perturbation parameters, are obtained, and then the grating sensitivity corresponding to those parameters is established. Interestingly, we have noticed that despite having the same evanescent field in the analyte region, unlike the long-period grating assisted co-propagating cladding modes ( H E m n ), the same cladding modes while propagating counter to the core mode are weakly sensitive to changes in the ambient refractive index. The reason behind this observation is discussed.

We propose and analyze a counterpropagating cladding mode assisted tunable frequency Fabry–Perot interferometer formed by a Bragg grating (BG) cavity in a liquid crystal coated planar optical waveguide. A full vector modal analysis has been used to obtain the transmission spectra of the individual Bragg reflectors, and the cavity effects have been incorporated by employing a suitable phase matrix. We show that the cavity resonances that appear from two fiber BGs forming a resonator can be efficiently explained by incorporating appropriate phase shifts in one BG grating period. We further show that utilizing the cladding mode evanescent field, a liquid crystal overlay can be used to tune the cavity resonance over the entire free-spectral range of the cavity transmission spectra. Our study should find application in designing highly tunable integrated optical Fabry–Perot interferometers.