The advent of optical coherence tomography (OCT) has permitted high-resolution, non-invasive, in vivo imaging of the eye, skin and other biological tissue. The axial resolution is limited by source bandwidth and central wavelength. With the growing demand for short wavelength imaging, super-continuum sources and non-linear fibre-based light sources have been demonstrated in tissue imaging applications exploiting the near-UV and visible spectrum. Whilst the potential has been identified of using gallium nitride devices due to relative maturity of laser technology, there have been limited reports on using such low cost, robust devices in imaging systems.
A GaN super-luminescent light emitting diode (SLED) was first reported in 2009, using tilted facets to suppress lasing, with the focus since on high power, low speckle and relatively low bandwidth applications. In this paper we discuss a method of producing a GaN based broadband source, including a passive absorber to suppress lasing. The merits of this passive absorber are then discussed with regards to broad-bandwidth applications, rather than power applications. For the first time in GaN devices, the performance of the light sources developed are assessed though the point spread function (PSF) (which describes an imaging systems response to a point source), calculated from the emission spectra. We show a sub-7μm resolution is possible without the use of special epitaxial techniques, ultimately outlining the suitability of these short wavelength, broadband, GaN devices for use in OCT applications.
In this paper we report a hybrid quantum well (QW) and quantum dot (QD) structure to achieve a broad spontaneous
emission and gain spectra. A single quantum well is introduced into a multi-layer stack of quantum dots, spectrally
positioned to cancel the losses due to the second excited state of the dots. Attributed to the combined effect of QW and
QDs, we show room temperature spontaneous emission with a 3dB bandwidth of ~250 nm and modal gain spanning over
~300 nm. We describe how this is achieved by careful design of the structure, balancing thermal emission from the QW
and transport/capture processes in the QDs. We will also compare results from a QD-only epitaxial structure to describe
how broadband gain/emission can be achieved in this new type of structure.
In this paper we report on the multi-section gain and absorption analysis of strain engineered molecular beam epitaxy
(MBE) grown GaAs and InGaAs capped bilayers. The InGaAs capped bilayer quantum dot (QD) lasers extends the
room temperature lasing wavelength to 1.45 μm. The spectral measurement of gain demonstrates that net modal gain is
achieved beyond 1.5 μm at room temperature. Analysis of the temperature and current density dependence gain
characteristics of a GaAs capped bilayer sample indicate that the temperature sensitivity of threshold current around
room temperature is due to phonon assisted thermal escape of carriers from the QDs.
We review the development of high performance, short wavelength (3 μm < λ < 3.8 μm) quantum cascade lasers (QCLs)
based on the deep quantum well InGaAs/AlAsSb/InP materials system. Use of this system has enabled us to demonstrate
room temperature operation at λ ~ 3.1 μm, the shortest room temperature lasing wavelength yet observed for InP-based
QCLs. We demonstrate that significant performance improvements can be made by using strain compensated material
with selective incorporation of AlAs barriers in the QCL active region. This approach provides reduction in threshold
current density and increases the maximum optical power. In such devices, room-temperature peak output powers of up
to 20 W can be achieved at λ ~ 3.6 μm, with high peak powers of around 4 W still achievable as wavelength decreases to