Behaviour of Poynting Vector for Guided Modes of Dielectric Optical Waveguides and Its Usefulness in the Design of Evanescently Coupled Optical Devices

In electrical and electronic engineering, the flow of electromagnetic energy is a fundamental physical quantity that is usually estimated by Poynting vector. In this paper we plot and discuss about the spatial and temporal evolution of Poynting Vector for the guided modes of a three layer dielectric optical waveguide. We emphasise that knowledge of the evolution of Poynting vector is very important in order to understand the operation of optical devices such as optical sensors and directional couplers, which are based on the principle of evanescent coupling. We show and report in this paper for the first time, that in the cladding region the decaying factor by which the instantaneous Poynting vector decays exponentially away from the core, is different from that for the time averaged Poynting vector. This decaying factor gives an estimate of the distance up to which the instantaneous Poynting vector is significant outside the core in the cladding region of the waveguide. This is a very useful design parameter for the fabrication of evanescently coupled optical devices. The results are generated for a symmetric slab optical waveguide in which a Si core surrounded by SiO
₂
cladding (Silicon-on-Insulator, commonly known as SOI) is considered at an operating wavelength of 1.55 μm.