Parallel near-ultraviolet（NUV）beam, 387nm femtosecond laser pulses with linear and circular polarisations were used to inscribe high efficiency Volume Bragg Gratings (VBGs) in clinical grade poly(methyl)methacrylate (PMMA) with the aid of a Spatial Light Modulator (SLM). Large, high quality VBGs with dimensions of 5mm x 5mm in size with (1-7)mm thickness and 20µm pitch were created at high speed, fabricated in 36 minutes, showing a first order diffraction efficiency 1 >94% with 4mm thickness. This is the highest diffraction efficiency observed to date in the undoped polymer, PMMA. Linear polarisation produced a higher refractive index contrast than circular polarisation, was found to be due to polarisation dependent non-linear filamentation, initiated through self-focusing. Using the threshold for supercontinuum, the ratio of critical power for self-focusing in PMMA was measured to be Pc lin/Pc circ 1.5, in accord with theoretical expectations. The pulse energy uniformity of the multiple beams was also dramatically improved by a camera based feedback system which was integrated into a LabVIEW based hologram calculation and display system, reducing the non-uniformity of parallel NUV and NIR beams significantly, thus improving the inscription process.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:664390 |
| Date | January 2014 |
| Creators | Ye, Lingyi |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://livrepository.liverpool.ac.uk/2011959/ |
Page generated in 0.0084 seconds