High power superluminescent light-emitting diodes
Superluminescent light-emitting diodes (SLEDs) are optoelectronic devices which combine different aspects of laser diode and LED performance. The device utilises a ridge waveguide structure to amplify spontaneously emitted light. Steps are taken to suppress feedback by minimising facet reflectivity. The result is a high-power, directional light source with a broad, smooth emission spectrum. The combination of high-power, high spatial coherence and low temporal coherence initially saw SLEDs used in specialist applications such as sources for fibre-optic gyroscopes, optical coherence tomography and optical memory readout. The emergence of GaN based SLEDs in recent years has seen SLEDs mooted as potential sources for a range of emerging applications such as LiDAR, high resolution OCT and machine vision as well as for everyday applications like displays and car headlamps. This work focusses on the development of GaN based surface emitting SLEDs. The devices use an integrated turning mirror at either end of the ridge waveguide to divert light downward through the transparent substrate. The turning mirror in combination with an antireflection coating applied to the back side of the chip are effective in suppressing the formation of Fabry-Perot modes in the device. This allows for high optical powers to be attained without sacrificing spectral quality. The device provides 2.2 W of optical peak power under pulsed operation, a record for a SLED device. At this maximum output power, the emission spectrum has a FWHM of 6 nm. The far field characteristic indicates a divergence of 7°x15° from a single output. A model is presented which assists in the optimisation of SLEDs. Simulated L-I curves and spectra allow for the behaviour of devices to be predicted prior to fabrication. This enables the user to estimate the optimum device characteristics such as length, epitaxial structure, and facet reflectivity without going through the costly and time-consuming process of device fabrication. The optimisation of several key process steps are described. The development of an ohmic Pd p-contact allowed for the demonstration of simultaneously driven SLED arrays. An inductively coupled plasma etch was developed to provide smooth, angled sidewalls for the integrated GaN turning mirror. Further work should be done to develop a suitable package for the device to dissipate the heat generated and allow for higher duty cycles and CW operation. The integration of optics such as micro-lenses or colour converting phosphors onto the back of the chip should also be investigated.
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