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Title: Doughnut-shaped beam generation in solid-state and fibre lasers
Author: Lin, Di
ISNI:       0000 0004 5921 4855
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2015
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This thesis focuses on developing new techniques for generating doughnut-shaped beams with a radial polarization state or a vortex phase front. An all-fibre-based pump beam conditioning element based on a tapered capillary fibre has been developed to provide a ring-shaped pump beam for end-pumped solid-state lasers. This unique pumping scheme provides a simple, efficient and robust approach for generating the doughnut-shaped first higher-order Laguerre-Gaussian (LG01) mode due to its optimized spatial overlap for the LG01 mode. However, the analysis of mode composition shows the doughnut-shaped LG01 mode is apt to be a ‘hybrid’ mode consisting of TEM01 modes with orthogonal orientations and LG01 modes with opposite handedness of helical phase front at axial modes with different frequencies. A novel mode selection element consisting of two nanoscale thickness aluminium strips has been developed by exploiting the fact that the standing wave intensity distribution for the LG01+ and LG01- modes inside a laser resonator are different. This scheme has been demonstrated for the first time in a diode-pumped Nd:YAG laser to generate an LG01 mode with controllable handedness of helical phase front at output powers ~1 W. Using the ring-shaped pumping scheme, direct generation of a radially-polarized beam has been successfully demonstrated in a Nd:YAG laser by exploiting the fact that the radially-polarized and azimuthally-polarized beams have slightly different spatial overlaps with the ring-shaped pump beam due to bifocusing within the laser crystal, thus resulting in a different threshold for each mode. The ring-shaped pumping scheme also provides flexibility in transverse mode selection and a significant reduction in adverse thermal lensing effects, offering the prospect of power scalability in end-pumped lasers. 14 W of linearly-polarized TEM00 mode output with a beam quality factor (M2) < 1.1 and 16 W of linearly-polarized doughnut-shaped LG01 mode with M2<2.2 were obtained in a Nd:YVO4 laser with a slope efficiency of 53% and 55%, respectively. The results showed a reduction in thermal lens strength by approximately 30% compared with traditional fibre-coupled laser diode end pumping. Furthermore, I have investigated extracavity conversion to radial polarization in both 1µm and 2µm regimes using continuously space-variant lambda/4 or lambda/2 waveplates (also called S-waveplates), fabricated by a recently developed femtosecond writing technique. In the one-micron wavelength regime, the lambda/2 S-waveplate can transform a linearly polarized Gaussian beam into a radially polarized beam with a conversion efficiency of ~75% and M2 of 2.7, whilst the lambda/4 S-waveplate can transform a circularly polarized LG01 mode into radial polarization with a conversion efficiency of ~86% and M2 of 2.9. The S-waveplate has a much better performance in longer wavelength regime due to reduced Rayleigh scattering. We demonstrate highly efficient conversion (~86%) to radial polarization in the 2µm band with wavelength tuning from 1950nm to 2100nm by using a 2µm lambda/2 S-waveplate. The resulting radially polarized beam had a M2 of 2.1 and a maximum polarization extinction ratio (PER) of 17.5dB at 2050nm with a variation of less than 3dB over the full wavelength range. I finally exploited the lambda/2 S-waveplate as an intracavity polarization-controlling element to directly excite radially-polarized TM01 mode operation in an Yb-doped fibre laser. The laser yielded ~32W of output power with a radially-polarized TM01 mode output beam at 1040nm with a slope efficiency of 66% and polarization purity of 95%. Moreover, I have demonstrated a simple technique for selectively generating the fundamental LP01 mode and the next higher order doughnut-shaped LP11 mode with a vortex phase front in an Yb-doped fibre laser. This approach exploits the difference in polarization behaviour of individual transverse modes due to transverse variation of birefringence in a few-moded fibre, allowing robust mode discrimination through the use of an appropriately aligned intracavity polarizing element. The laser yielded ~36W of output power for both transverse modes with slope efficiency of 74% and a mode purity of 95%.
Supervisor: Clarkson, William Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; TK Electrical engineering. Electronics Nuclear engineering