Prism Coupling of Light to Discrete Substrate Modes

Abstract

The most common coupling between a prism and a waveguide requires a gap between the prism and the waveguide . The modes in the waveguide are guided modes . We show that discrete substrate modes may be excited if the waveguide film is directly deposited onto the prism . These discrete substrate modes have many similarities to guided modes . IntroductionThere are three types of waveguide modes [1] . The best understood is of course the guided mode . This mode is the means for the propagation of light down an optical fibre . The other two mode types are substrate and air modes, collectively known as radiative modes . We will concentrate on substrate modes .The type of waveguide mode that exists in a given system is determined by the waveguide and the surrounding materials . In a planar configuration, the waveguide material is sandwiched between two other materials, nominally semi-infinitely thick . If the waveguide index of refraction is larger than those of the surrounding materials then guided modes may exist in the waveguide . The light in the waveguide propagates through the waveguide by total internal reflection (TIR) at waveguide surfaces .If either or both of the surrounding media have an index of refraction greater than the waveguide index of refraction, then substrate or air modes, respectively, occur . In neither case is propagation expected . The lack of TIR at one or both interfaces allows light in the waveguide to be transmitted to the surrounding media and hence lost for propagation . We show that substrate modes are discrete and not continuous .In section 2, an overview of the experimental apparatus is presented . Section 3 shows the discrete nature of substrate modes and other results . The mechanism for the discrete substrate modes is discussed in section 4, along with a discussion on the problems of fitting the experimental data to the Fresnel equations . A conclusion is then given in section 5 .