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Optical gain in planar waveguides

Description of optical waveguides is commonly restricted to propagation at distances much larger than the width of the waveguide core and therefore only guided modes are taken into account in theory. Effects connected with leaking of the waves into the substrate may be, however, very important for possible applications in microelectronics. In this paper, we overview our model of photoluminescence of an active planar waveguide and we extend it in order to describe mode formation during propagation in the waveguide. We also include optical amplification into our model and derive formula for description of the Variable Stripe Length (VSL) method widely used for measurement of the net gain coefficient. We demonstrate necessity for corrections of VSL results since their straightforward interpretation may be misleading.