Generation of a Radially Polarised Beam in a Solid-State Laser Using an Intracavity Spatially Variant Waveplate

Radially polarised laser sources are of considerable interest due to their applications in areas such as laser processing, particle manipulation and high-resolution imaging. Previous approaches for generating radially polarised beams have made use of extra-cavity approaches such segmented waveplates [1] , along with intra-cavity methods such as grating mirrors [2] or the use of birefringent crystals [3] . An alternative approach is to use a femtosecond laser-written spatially variant waveplate (S-waveplate) to a convert linearly polarised beam to a radially polarised beam. S-waveplates have the attraction that they can produce radially-polarised beams with very high polarisation purity [4] , but, in the past, have suffered from very high scattering loss greatly limiting their usefulness. However, recent research has led to the discovery of a new type of ultrafast-laser-written birefringent modification allowing the writing of S-waveplates with better than an order-of-magnitude lower loss [5] . This latest development opens up the possibility of using S-waveplates as polarisation selecting elements in a solid-state laser to allow direct generation of desirable polarisation distributions.