High sensitivity detection of chemicals based on sinusoidally apodized structured grating assisted liquid filled directional coupler

The grating has a significant role in sensing applications. Similarly, the grating-assisted coupler has excellent potential in chemical sensing applications. The power coupling between two closely coupled waveguide couplers can be significantly tuned by incorporating grating between them. The grating has been taken of silica material with sinusoidal shape in variation. The grating layer is assumed to be embedded within the sensing layer while considering a changeable effective refractive index depending on the sensing layer substances. In the present paper, grating assisted directional coupler has been numerically analysed using its own developed MATLAB-based algorithm of finite difference method (FDM) scheme. FDM method has been applied to solve the Eigenvalue equation to obtain allowed Eigenvalues and corresponding Eigen vectors (TE and TM cases). In FDM, the analysis domain has been fine discretized into the mesh of 1-D equal spacing for reasonable accurate computation results. In experimental validation, Fibre Bragg grating (FBG) has been suspended between two high refractive index coupler regions, which act as a power coupling zone. Also, the coupling length has been changed from 5 to 20 m for tuning purposes and then optimized for grating parameters viz. length, period, etc. The whole structure is 2-Dimensional (x and y directions) with invariant in the y-direction.