  Optical vortices generically arise when optical beams are combined. Recently,
we reported how several laser beams containing optical vortices could be
combined to form optical vortex loops, links and knots embedded in a light beam
(Leach et al 2004). Here, we describe in detail the experiments in which vortex
loops form these structures. The experimental construction follows a
theoretical model originally proposed by Berry and Dennis, and the beams are
synthesised using a programmable spatial light modulator and imaged using a CCD
camera.
