Monolithic coupled-cavity laser diodes\ud for bio-sensing applications
This thesis describes an investigation into the potential of coupled-cavity semiconductor lasers for bio-sensing applications. This has involved the design and development of a fabrication process for a novel micro-fluidic coupled-cavity laser sensor. The efficiency of the etched inner laser facets of this device have been identified as a key determinant of the device behaviour. The multi-section gain characterization technique has been used to measure the efficiency of these facets to be η = 0.48 ± 0.13.\ud Perturbation of the optical coupling between the two laser sections of the device can induce a wavelength shift in the laser output of Δλ = 20 ± 5 Å. This wavelength change is attributed to the difference in the threshold gain requirements of the coupled-cavity and individual cavity modes of the device. A multi-mode travelling wave rate equation model has been used to predict that the size of this effect can be maximized by optimizing the cavity lengths of the device. For the AlGaInP quantum well material used in this work the coupling effect is maximized by using the shortest cavity lengths possible that can still achieve laser action.\ud The utility of including a segmented contact system to the coupled-cavity design has also been investigated. This modification enables wavelength tuning via the gain lever effect and self-pulsation through saturable absorption. A wavelength tuning range of Δλ = 1.2 ± 0.2 nm has been measured for a single cavity laser with a segmented contact length ratio of 4:1. This tuning behaviour has been attributed to the carrier density dependence of the net modal gain peak. Rate equation modelling has been used to interpret the self-pulsation behaviour of the segmented contact device and to demonstrate how optical pumping of a saturable absorber can increase the sensitivity of the coupled-cavity device.
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