Vortex laser development employing an interferometric output coupler
Creators: name: Geberbauer, Jan Willem Taubin
Publisher: name: Imperial College London
Year: 2022

description: This thesis focuses on the design and implementation of new laser cavity designs for vortex mode generation. The vortex generation method is integrated into both solid-state laser cavities and systems using doped fibre gain mediums. Optical vortex laser beams have attracted a lot of attention due to their proposed applications in a wide range of industries from communication to particle manipulation to microscopy to material processing. More recently, vortex generation directly from the laser has attracted research due to the potential for higher purity, higher power and a compact system. In this work a modified Sagnac interferometer, dubbed the vortex output coupler (VOC), is integrated as the output coupler of a laser cavity and used to convert the fundamental Gaussian intracavity mode into a first order Laguerre Gaussian output. The VOC was first implemented into various solid-state cavities, with a Vanadate gain medium operating at 1064 nm, where vortex generation was successfully demonstrated. A record, from-the-source generated, vortex power of 31.3 W was achieved, with a laser slope efficiency of 62.5%. The mode purity was 95.2% and the M2 = 2.25. The handedness of the generated vortex was pure and switchable during operation. First order Hermite Gaussian modes with the same power were also demonstrated. The VOC was also shown to function in a pulsed cavity without any detrimental effects. It was found that the VOC has mode filtering properties, which helped maintain the a fundamental Gaussian in the cavity, despite mode mismatch between the pump beam and the fundamental Gaussian mode in the cavity. Fibre laser systems have the advantage of being compact, alignment insensitive and maintaining a close resemblance to the fundamental Gaussian mode through the use of a single mode fibre. To demonstrate the implementation versatility of the VOC and capitalise on its power scaling potential, the VOC was integrated into a non-polarisation maintaining fibre laser system as a bulk output coupler. An Ytterbium-doped gain fibre was used, operating at 1064 nm, which allowed for the same optics as in previous work to be used for the VOC. 5.08 W of vortex output power was achieved from the fibre laser system, with a mode purity and quality of 96.1% and M2 (X/Y) = 2.03/2.22, respectively. This system was also used as a first order vortex source for higher order vortex generation, using a spiral phase plate. Vortices with orbital angular momentum values of l = +2 and +3 were generated from the first order vortex (l = +1) input using spiral phase plates, which imparted +1 and +2 orbital angular momentum (helical phase ramps of 2π and 4π respectively). The VOC is made up of a beamsplitter and three turning mirrors, which are all high power damage threshold components. By choosing appropriate optical coatings for these components, considering wavelength and polarisation, the VOC can be implemented across the output spectrum making it incredibly versatile. The VOC is shown to function in a pulsed laser system, with a vortex pulse with duration 20 ns and energy 303 μJ shown in this work. The output mode can be switched between left and right vortex handedness and also between the first order Hermite Gaussian modes, all during operation. This pulsed operation and output mode versatility make it very interesting for material surface processing, particle levitation and manipulation, free-space communication and broadband, or ultrashort pulse, vortex generation. A VOC enhanced vortex laser can also be used as a high power and purity first order Hermite-Gaussian or Laguerre-Gaussain source for further conversion to higher order modes using other methods.