The study and application of optical vortices have gained significant prominence over the last two decades. An interesting challenge remains the determination of the azimuthal index (topological charge) ℓ of an optical vortex beam for a range of applications. We explore the diffraction of such beams from a triangular aperture and observe that the form of the resultant diffraction pattern is dependent upon both the magnitude and sign of the azimuthal index and this is valid for both monochromatic and broadband light fields. For the first time we demonstrate that this behavior is related not only to the azimuthal index but crucially the Gouy phase component of the incident beam. In particular, we explore the far field diffraction pattern for incident fields incident upon a triangular aperture possessing non-integer values of the azimuthal index ℓ. Such fields have a complex vortex structure. We are able to infer the birth of a vortex which occurs at half-integer values of ℓ and explore its evolution by observations of the diffraction pattern. These results demonstrate the extended versatility of a triangular aperture for the study of optical vortices.

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Optical cooling in an all fiber system using fiber laser pumps and cooling fibers doped with rare earth ions has been investigated both theoretically and experimentally. A 2% Tm doped germanate glass was selected from glasses with different Tm concentrations 0.5, 1, 2, 3, 4, 5, 6, 8 and 10% wt for fabrication of the cooling fiber. A high efficiency, single mode Tm-doped fiber laser has been built to pump a Tm-doped fiber cooler. The cooling experiments done in a vacuum chamber show indications that cooling has occurred in the fiber. A theoretical framework to understand the nature of cooling in this laser cooling system has been developed which highlights the cooling power available as a function of various material and fiber parameters including background loss and absorption saturation effects in the cooling fiber. Cooling characteristics, with special emphasis on the fiber's saturation behavior, have been studied using theoretical models of Tm 3+ -doped glass (4-level models) and Tm 3+ doped KLa(WO 4 ) 2 crystals (20-level model).