The impact of laterally coupled grating microstructure on effective coupling coefficients

Abstract

Lithographic fabrication may be used to define laterally coupled gratings of high refractiveindex contrast on waveguide ridges, eliminating the need for regrowth steps in suchdistributed feedback lasers. These may be made more amenable to fabrication byemploying higher-order gratings. Reliable exploration of the laser design space requires thatthe radiating partial waves be accurately incorporated in numerical simulations. We modifythe coupled-mode approach to fully consider the two-dimensional cross section, analyzingrectangular, sinusoidal, triangular and trapezoidal grating shapes. Effective couplingcoefficients are determined for grating orders from first to third. We show that, by tailoringthe grating microstructure, effective coupling coefficients up to double that of a 0.5 dutycycle rectangular grating can be achieved. The actual grating microstructure of anas-fabricated grating was analyzed and its effective coupling coefficient predicted as. This was found to be in excellent agreement with the value extracted from theamplified spontaneous emission spectrum, .