Design and fabrication technologies for single-mode quantum cascade lasers.

Quantum Cascade Lasers (QCL's) are unipolar semiconductor lasers offering the
potential for low cost, high power, laser sources emitting in the mid- and far- infrared.
Single-mode devices are required for spectroscopic and imaging applications because
of their narrow emission linewidth and are achieved through distributed feedback
(DFB) designs where gratings are incorporated in or close to the active region. This
thesis describes design and fabrication technologies for single-mode yield
improvements and improved wavelength targeting of QC lasers.
Mid-infrared, single-mode laser designs are developed utilising novel fabrication
processes and designs in the indium phosphide (InP) based material system. Lasers
without facet coatings, with single-mode yields up to 80% and side mode suppression
ratios (SMSR) greater than 25dB are observed. Metalised surface gratings, buried
(overgrown) gratings and lateral gratings are defined using inductively coupled
plasma (rCP) etching to produce DFB lasers operating above room temperature for
wavelengths near to lOllm. A deep etched lateral grating quantum cascade DFB is
demonstrated for the first time in the InP material system.
Waveguide modelling demonstrates accurate methods to predict the grating coupling
strength of the DFB lasers with good agreement to experimental results. The emission
wavelength (A. -IOllm) is found to be highly dependent on the laser ridge width with
the experimental shift in wavelength found to be much greater than that predicted. A
ridge width and temperature dependence on emission wavelength is utilised in an
array device where a continuous tuning range in excess of 230nm is observed.
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Finally, an increase in thermal conductance and a symmetric far-field profile are
observed for lasers (A. - 4J.lm) with narrow ridge widths. The lasers were designed for
improved heat extraction from the active region and a beam quality factor of M2 ::::: 1.
Almost identical lateral and vertical far-field profiles with high duty cycle operation at
thermoelectric temperatures are observed for a ridge waveguide laser that is
approximately 5 J.lm wide.