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

摘要

Quantum Cascade Lasers (QCL's) are unipolar semiconductor lasers offering theudpotential for low cost, high power, laser sources emitting in the mid- and far- infrared.udSingle-mode devices are required for spectroscopic and imaging applications becauseudof their narrow emission linewidth and are achieved through distributed feedbackud(DFB) designs where gratings are incorporated in or close to the active region. Thisudthesis describes design and fabrication technologies for single-mode yieldudimprovements and improved wavelength targeting of QC lasers.udMid-infrared, single-mode laser designs are developed utilising novel fabricationudprocesses and designs in the indium phosphide (InP) based material system. Lasersudwithout facet coatings, with single-mode yields up to 80% and side mode suppressionudratios (SMSR) greater than 25dB are observed. Metalised surface gratings, buriedud(overgrown) gratings and lateral gratings are defined using inductively coupledudplasma (rCP) etching to produce DFB lasers operating above room temperature forudwavelengths near to lOllm. A deep etched lateral grating quantum cascade DFB isuddemonstrated for the first time in the InP material system.udWaveguide modelling demonstrates accurate methods to predict the grating couplingudstrength of the DFB lasers with good agreement to experimental results. The emissionudwavelength (A. -IOllm) is found to be highly dependent on the laser ridge width withudthe experimental shift in wavelength found to be much greater than that predicted. Audridge width and temperature dependence on emission wavelength is utilised in anudarray device where a continuous tuning range in excess of 230nm is observed.udii udFinally, an increase in thermal conductance and a symmetric far-field profile areudobserved for lasers (A. - 4J.lm) with narrow ridge widths. The lasers were designed forudimproved heat extraction from the active region and a beam quality factor of M2 ::::: 1.udAlmost identical lateral and vertical far-field profiles with high duty cycle operation atudthermoelectric temperatures are observed for a ridge waveguide laser that isudapproximately 5 J.lm wide.