Analysis of diffraction grating induced distributed feedback for stable single mode quantum cascade lasers

Abstract

Since the first demonstration of quantum cascade lasers (QCLs) in 1994, there has been extensive research on their improvements as QCLs have potential applications in remote chemical sensing and pollution monitoring. By incorporating a distributed feedback (DFB) grating in a QCL structure, single-mode operation with narrower linewidth has been achieved. The coupling coefficient of a DFBQCL is one of the most important parameters to be considered for manipulating the behavior of the device. In this thesis, we have modeled this parameter using Coupled-Wave theory and observed its dependance on various design parameters such as temperature, grating depth, grating period, tooth angle, duty cycle, and grating shape. It has been found that the value of the coupling coefficient is greatly design-dependent. Since the higher its value, the better the performance of the DFB-QCL to select a single-mode, the design parameters can be changed to obtain a high value of the coupling coefficient. We have also developed a model to obtain the output optical power from a a DFB-QCL. A wide range of operating parameters have been varied to design a stable DFB-QCL for single-mode operation. It has been found that for every design of a DFB-QCL structure, there is an optimum grating depth, grating period and grating shape for which single-mode operation with high value of side-mode suppression ratio (SMSR) is possible.