Khan, Abid; (1995) Vertical cavity surface emitting lasers: Design, characterisation and integration. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

Vertical cavity surface emitting lasers (VCSELs) are semiconductor lasers with extremely short (~1 wavelength) vertical optical cavities, the cavity being defined by distributed Bragg reflectors (DBRs). These lasers have emission properties distinct from normal waveguide in-plane devices (IPLs). This thesis presents theoretical and experimental results on strained QW InGaAs/GaAs/AlGaAs based VCSELs and integrated laser/ modulator devices. Optical pumping techniques are used to study the sub-threshold emission characteristics of VCSEL (microcavity) structures. These show that the optical properties of the microcavity dictate the structures emission properties. In particular an incomplete overlap of the QW spontaneous emission spectrum with the mirror high reflectivity stop-band results in the emission of side-modes. Current injected VCSELs, with threshold currents below 5mA and operating between 940nm and 980nm, are also studied. It is demonstrated that graded-index separate-confinement heterostructures may be used to increase the lasers power output and (electrical-optical) conversion efficiency. We present devices exhibiting more than 50mW output. Through the use of short-period superlattices, inserted into the DBR mirrors, we have achieved threshold voltages below 5 volts. Other methods for the reduction of series resistance within DBRs are investigated. In particular, the use of post-growth annealing of the DBRs. Results show a 50[percent] decrease in the series resistance with the reflectivity of the DBR remaining high. The devices are successfully modelled by using a transfer matrix approach that allows for the emission spectra, threshold currents and power outputs to be calculated. The thesis ends by presenting a novel integrated VCSEL and modulator. This device utilises a new reflection-modulator which attains a very high reflectivity. By modulating the (modulator) voltage the cavity photon lifetime of the laser is changed. This scheme is shown, theoretically, to result in improved high speed performance relative to a current modulated device.

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