Laser diode area melting for high speed additive manufacturing of metallic components

Additive manufacturing processes have been developed to a stage where they can now be routinely used to manufacture net-shape high-value components. Selective Laser Melting (SLM) comprises of either a single or multiple deflected high energy fibre laser source(s) to raster scan, melt and fuse layers of metallic powdered feedstock. However this deflected laser raster scanning methodology is high cost, energy inefficient and encounters significant limitations on output productivity due to the rate of feedstock melting. This work details the development of a new additive manufacturing process known as Diode Area Melting (DAM). This process utilises customised architectural arrays of low power laser diode emitters for high speed parallel processing of metallic feedstock. Individually addressable diode emitters are used to selectively melt feedstock from a pre-laid powder bed. The laser diodes operate at shorter laser wavelengths (808 nm) than conventional SLM fibre lasers (1064 nm) theoretically enabling more efficient energy absorption for specific materials. The melting capabilities of the DAM process were tested for low melting point eutectic BiZn2.7 elemental powders and higher temperature pre-alloyed 17-4 stainless steel powder. The process was shown to be capable of fabricating controllable geometric features with evidence of complete melting and fusion between multiple powder layers

Gallium Nitride Super-Luminescent Light Emitting Diodes for Optical Coherence Tomography Applications

The role of biasing of absorber sections in multi-contact GaN ~400nm SLEDs is discussed. We go on to assess such devices for OCT applications. Analysis of the SLED emission spectrum allows an axial resolution of 6.0μm to be deduced in OCT applications

Modelling and device simulation of photonic crystal surface emitting lasers based on modal index analysis

We present a novel semi-analytical method utilising modal index analysis, for modelling the field resonances of photonic crystal surface emitting lasers (PCSELs). This method shows very good agreement with other modelling techniques in terms of mode calculations, with the added advantages of computational simplicity, the calculation of threshold gain, and rapid analysis of finite structures. We are able to model the effect of external lateral feedback and simulations indicate that the near-field peak can be electronically displaced and the threshold as well as the frequency can be controlled through external in-plane feedback, paving the way to dynamic control of PCSELs

Gallium nitride light sources for optical coherence tomography

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 application

Coherently coupled photonic-crystal surface-emitting laser array

The realization of a 1 × 2 coherently coupled photonic crystal surface emitting laser array is reported. New routes to power scaling are discussed and the electronic control of coherence is demonstrated

Development of a broadband superluminescent diode based on self-assembled InAs quantum dots and demonstration of high-axial-resolution optical coherence tomography imaging

We developed a near-infrared (NIR) superluminescent diode (SLD) based on self-assembled InAs quantum dots (QDs) and demonstrated high-axial-resolution optical coherence tomography (OCT) imaging using this QD-based SLD (QD-SLD). The QD-SLD utilized InAs QDs with controlled emission wavelengths as a NIR broadband light emitter, and a tilted waveguide with segmented electrodes was prepared for edge-emitting broadband electroluminescence (EL) spanning approximately 1–1.3 μm. The bandwidth of the EL spectrum was increased up to 144 nm at a temperature of 25 °C controlled using a thermoelectric cooler. The inverse Fourier transform of the EL spectrum predicted a minimum resolution of 3.6 μm in air. The QD-SLD was subsequently introduced into a spectral-domain (SD)-OCT setup, and SD-OCT imaging was performed for industrial and biological test samples. The OCT images obtained using the QD-SLD showed an axial resolution of ~4 μm, which was almost the same as that predicted from the spectrum. This axial resolution is less than the typical size of a single biological cell (~5 μm), and the practical demonstration of high-axial-resolution OCT imaging shows the application of QD-SLDs as a compact OCT light source, which enables the development of a portable OCT system

Three-dimensional finite-difference time-domain modelling of photonic crystal surface-emitting lasers

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. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Near-infrared and mid-infrared semiconductor broadband light emitters

Semiconductor broadband light emitters have emerged as ideal and vital light sources for a range of biomedical sensing/imaging applications, especially for optical coherence tomography systems. Although near-infrared broadband light emitters have found increasingly wide utilization in these imaging applications, the requirement to simultaneously achieve both a high spectral bandwidth and output power is still challenging for such devices. Owing to the relatively weak amplified spontaneous emission, as a consequence of the very short non-radiative carrier lifetime of the inter-subband transitions in quantum cascade structures, it is even more challenging to obtain desirable mid-infrared broadband light emitters. There have been great efforts in the past 20 years to pursue high-efficiency broadband optical gain and very low reflectivity in waveguide structures, which are two key factors determining the performance of broadband light emitters. Here we describe the realization of a high continuous wave light power of >20 mW and broadband width of >130 nm with near-infrared broadband light emitters and the first mid-infrared broadband light emitters operating under continuous wave mode at room temperature by employing a modulation p-doped InGaAs/GaAs quantum dot active region with a ‘J’-shape ridge waveguide structure and a quantum cascade active region with a dual-end analogous monolithic integrated tapered waveguide structure, respectively. This work is of great importance to improve the performance of existing near-infrared optical coherence tomography systems and describes a major advance toward reliable and cost-effective mid-infrared imaging and sensing systems, which do not presently exist due to the lack of appropriate low-coherence mid-infrared semiconductor broadband light sources

Whole-genome sequencing reveals host factors underlying critical COVID-19

Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genes—including reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)—in critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease

Polarization-pinning in substrate emission multi-mode vertical-cavity surface-emitting lasers using deep trenches

We investigated the stable polarization-pinning properties of substrate emission InGaAs-based 980 nm multi-mode vertical-cavity surfaceemitting lasers (VCSELs). For the multi-mode 40 um diameter aperture VCSELs, we introduced 30 lm wide, 9 lm depth deep trenches that are 15 lm away from the cavity aperture. The VCSELs with trench structure produced higher transverse-electric (TE) polarized light output power, as compared with transverse-magnetic (TM) polarized light output power, namely, the effective TM polarization suppression was realized. These trench-etched VCSELs exhibited a 7.5 dB orthogonal polarization suppression ratio with 16.8 mW of light output power at 60 mA of current injection. The dominant TE polarization distribution was observed in polarization-resolved near-field images of spontaneous and stimulated emission due to the induced strain by the etched trenches