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Monolithic tuneable quantum cascade lasers

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  • Ta Engineering (General). Civil Engineering (General)
  • Tk Electrical Engineering. Electronics Nuclear Engineering
  • Design
  • Physics


This thesis is concerned with the design, fabrication and characterisation of monolithic tuneable quantum cascade lasers (QCLs), which are suitable for tuneable diode laser based absorption spectroscopy and polarisation dependent spectroscopy in the mid-infrared wavelength range. All investigations and device development work were carried out using the QCL structure based on strain-compensated Ga0.331In0.669As/Al0.659In0.341As grown on an InP substrate that emits light around 4500 nm wavelength. To make the QCLs electrically tuned, two laser designs were investigated: the double ring quantum cascade laser based on the Vernier-tuning effect, and the integrated tuneable birefringent waveguide utilising current controlled birefringence in quantum-wells. The key advantage of the Vernier tuning effect based the double ring laser design is that it can facilitate both a single mode and wide-tuning range operation. The Vernier tuning enhancement factor associated with the coupled waveguide is responsible for a wide-tuning range observed in double ring configuration. However, the tuning range is limited by the available gain bandwidth (i.e. FWHM of spontaneous spectra) in the material and the maximum obtainable index change of the tuner ring. Theoretically, the tuning range of 155 nm was estimated for the double ring quantum cascade laser (DRQCL) design employed in this thesis. However, experimentally, a single mode (~19 dB single sideband suppression ratio) and tuning range of 59 nm which covers almost half the bandwidth were observed. For the first time in the history of the QCL, a research into the design, fabrication and characterisation of integrated polarisation mode convertors (PMCs) has been carried out. The PMC design is based upon etching trenches, using the RIE lag effect, of sub-wavelength dimensions into one side of a waveguide in order to achieve an asymmetric cross-sectional profile, resulting in a waveplating effect. This thesis presents such PMCs integrated with QCLs that emit 69% TE light with the polarisation angle of 65 degree from one facet and a pure TM light emitted from the other facet using a 256 μm long PMC design (design D2). An integrated tunable birefringent waveguide (ITBW) consisting of two PMCs with a differential phase shift (DPS) section between them. To probe the birefringence operation, a sub-threshold electroluminescence was employed to investigate the single pass operation of the ITBW. A theory based on the electro-optic properties of birefringence in QCL waveguides was used combined with a Jones-matrix based description to gain an understanding of the electroluminescence results. With the QCL operating above threshold, polarisation and wavelength tuning of the signal output was demonstrated. By comparing the sub-threshold electroluminescence and active polarisation angle measurement result with the Jones matrix model, the material birefringence (no DPS current), 4n, was estimated to be around 0.005 for the QCL employed in this work. However, single mode emission was not observed and 24 nm discontinuous tuning was recorded. Despite this, using a QCL incorporating an ITBW device, active polarisation control over 45 degree was demonstrated, and currently, to the best of the authors knowledge there has been no other QCL device that is capable of electronically controlling the output polarisation.

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