Abstract
Optical grating structures are basic elements in filters, DFB/DBR lasers. Coupled-mode theory is often used in the analysis of these devices. The mode patterns in the coupledmode analysis do not vary along waveguide axis[1]. The periodic perturbations therefore only change the amplitudes of the ideal modes. This approximation is, however, valid only if the grating perturbation is weak. Otherwise, the ideal mode approximation will become questionable. The problem is more serious for the TM modes because the ideal modes do not satisfy the boundary conditions at the interface. The coupling coefficients evaluated based on the ideal-modes are not correct even for small grating height[2]. Moreover, as the grating perturbations increases, the possible radiation loss due to the scattering at the surface grating will also become inevitable. The complicated expressions of the radiation modes is required in the coupled-mode analysis to assess the radiation loss[3].
© 1992 Optical Society of America
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