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.
