Spatial decoherence of broad-area vertical-cavity surface-emitting laser and their implementation in low-speckle applications

It is hard to move around in modern day society without stumbling across a laser application of some sort. Lasers have proven to be very polyvalent, their implementation ranging from medecine (e.g. eye surgery), to technological gadgets (e.g. DVD and blue-ray in gaming consoles), all the way to cutting through stainless steel or detecting product defaults in industry. Since the development of the first laser proto-types in the early 1960's, a diverse spectrum of different laser types has been developed, each of which distinguishes itself based on its emission wavelength(s) and beam properties. A laser type that has certainly gained attention in the last decades is the semiconductor laser. Due to a high efficiency, compact design and low production costs, semiconductor lasers have taken the foreground in many laser applications. However, in some applications, such as illumination and projection systems, the use of lasers is not yet well developed. This may appear strange, since semiconductor lasers can boast interesting properties such as a wall-plug efficiency above 50% [1, 2], long lifetime, focusability and color pureness, all of which would make these lasers excellent candidate light sources. The laser's limiting factor that overshadows these advantages is known as the speckle phenomenon. Speckle occurs whenever a coherent light source (e.g. laser) illuminates an optically rough surface. It manifests itself as a disturbing visual granularity in the image of the illuminated object. Semiconductor lasers are, typically speaking, sources of coherent light. However, TONA researchers have been able to achieve spatially incoherent emission employing a Broad-Area Vertical-Cavity Surface-Emitting Laser (BA-VCSEL). When pumping a BA-VCSEL with high amplitude electrical pulses, a strong decrease of spatial coherence is observed. Simultaneously the far field intensity transforms drastically from a multi-mode structure to a Gaussian profile. This unique emission regime is uncommon for lasers, and can be useful for applications that require low spatial coherence. In this work we investigate both the origin of the spatial decoherence of BA-VCSELs, and its use in low speckle applications. We clarify why and how this spatial decoherence only occurs under strict pumping conditions. The required ingredients to reach this emission regime are a fast resonant wavelength shift together with a strong transverse gradient in the effective cavity length. In depth understanding of the mechanism that causes the VCSEL's dynamic change of emission properties allows us to determine the emission regime's boundaries, and control its onset. Understanding of the BA-VCSEL's low spatial coherence and of speckle reduction techniques have lead us to implement a spatially incoherent BA-VCSEL in a realistic image projection setup. As a result we achieve strong speckle reduction, and are able to match the experiments with theoretical speckle contrast values. Another laser based application that typically suffers from speckle is metrology based on optical line generators. These line generators are commonly used devices that illuminate objects in order to verify their exact shape (for e.g. quality control). In these systems, speckle leads to reduced measurement precision. We design a low speckle line generator based on a spatially incoherent BA-VCSEL. More in particular, we experimentally evaluate and compare different line generating confi