UCL Optical Tweezers Group:  3C00 Project on Computer Generated Holography

The aim of Seye's project was to produce Laguerre-Gaussian (LG) laser beams using computer generated holograms (CGH).  These beams have an interesting helical phase structure and an "optical vortex" on the axis.

The picture opposite can help visualise the LG beam structure - the balloons spiral around the column in four intertwined helices like the helical wavefront of the LG beam.

Seye with a helical balloon column found in UCL quad

The CGH that produces a Laguerre-Gaussian beam looks like a diffraction grating, but with a "fork" in the centre that gives rise to the optical vortex.

The CGH shown opposite has a single fork in the centre which produces an optical vortex with a topological charge of 1, that is the phase increases by 2p in a circular trajectory around the optical vortex, and the wavefront spirals once around the axis per wavelength.

An example of a CGH for making Laguerre-Gaussian beams

The diffracted orders from this "forked" grating have a dark "hole" in the centre, characteristic of LG beams.  This point is a phase singularity where the electric field (intensity) of the beam must be zero.

In the picture the beam in the centre is the straight through "zero-order" beam.  To either side are the first diffracted orders which carry a charge 1 optical vortex that causes the dark centre.  This was produced with the CGH shown on this page.

Some results from this project: Laguerre-Gaussian laser beams