Abstract:
Quantum cascade lasers (QCLs) are unique unipolar semiconductor devices based on intersubband
transitions between the conduction subbands in complex heterostructures. At
present, QCLs are the only semiconductor lasers that can operate continuous wave at and
above room temperature in the mid-infrared spectral range, which makes them well suited
for various applications including the realization of compact, ultra-sensitive, trace-gas sensors
based on absorption spectroscopy. To this purpose up to now, mostly only distributed
feedback (DFB) single-mode devices and external cavity lasers have been used. However,
DFB lasers have relatively narrow tuning range, smaller or equal to about 1% of the wavelength,
which limits their usefulness for spectroscopic investigations. By contrast, external
cavity lasers are mechanically tuned and require additional optical components, which increase
size and also limits tuning speed. In this thesis, we propose a multi-segment QCL
that can provide multi-color widely tunable output for the purpose of multiple trace gas
detection simultaneously. We develop a comprehensive theoretical model to design and
analyze the output characteristics of a multi-segment QCL.
In the proposed multi-segment QCL, the cavity is divided into multiple electrically isolated
segments along its length and different bias voltages are applied to the segments so that
each segment emits at a different wavelength and we get a multi-color output from a single
QCL cavity. The output wavelengths depend on the input bias voltage, injected current,
and temperature of the heat sink connected to each segment. The output mode and its gain
is calculated considering the overall gain the mode receives from all the segments while
propagating through the cavity.
In this work, the gain, gain spectrum, and the modes supported by a Fabry-P´erot cavity are
discussed in detail and the equations introduced are used to numerically model the output
behavior of a multi-segment QCL. The developed model has been applied to three different quantum mechanical QCL structures each emitting at different wavelengths. It is found
that a multi-segment QCL cavity produces a multi-color output if the modes created in the
segments receive enough gain to overcome the losses of the cavity. An additional mode
often also emerges receiving an overall gain greater than the segment modes. The output
modes can be tuned by changing the input bias and also the segment lengths of the device.
Mode tuning can also be obtained by changing the temperature of the individual segments.
Temperature tuning can be achieved under uniform bias and also under variable segment
bias both.
