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.
We investigate the beam divergence in far-field region, diffraction loss and optical confinement factors of all-semiconductor
and void-semiconductor photonic-crystal surface-emitting lasers (PCSELs), containing either
InGaP/GaAs or InGaP/air photonic crystals using a three-dimensional FDTD model. We explore the impact
of changing the PC hole shape, size, and lattice structure in addition to the choice of all-semiconductor or
void-semiconductor designs. We discuss the determination of the threshold gain from the diffraction losses,
and explore limitations to direct modulation of the PCSEL.
980 nm GaAs-based photonic crystal surface emitting lasers containing all semiconductor GaAs/InGaP and GaAs/air photonic crystals (PC) inside their cavity are theoretically investigated. We use a combination of an average index approach and optical coupled mode theory to optimize the PC interaction with optical modes of the laser waveguide and draw guidelines for design of PCSELs based on a range of material systems and operating wavelengths. Results show that the all-semiconductor PC provides a higher coupling with the optical mode in most cases.
In this work we report on the simulation of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs). We simulate an etched mesa structure (substrate emission) with the substrate acting as the current spreading layer. The effect of contact misalignment on the carrier distribution within the active element is explored and confirms the validity of the model in describing the carrier distribution. We go on to discuss the effects of the substrate thickness and trench depth on the intensity profile. Simulation results show that a thicker substrate and a trench partially etched into the substrate may improve the intensity profile in future devices.