LASER COMPONENTS are the UK distributor for Holo-Or, who manufacture vortex lens diffractive optics. The vortex lens is a unique optic that converts a Gaussian beam into a doughnut shaped energy ring, which is used in applications such as solar coronagraphs, high resolution microscopy, optical tweezers for particle trapping and manipulation, lithography and quantum optics.
Each vortex lens is wavelength specific. Figure 1 depicts the winding “staircase” surface structure which is comprised of spiral, or helical, phase steps whose purpose it is to control the phase of the transmitted beam. The total etching depth from the top to bottom of the “staircase” is a function of the design wavelength and the substrate’s optical index.
Generally, this depth is of the same order of magnitude as the design wavelength. To magnify the intensity of the energy ring one increases the number of staircases in a 360º turn, the so called topological charge m.
These vortex lenses require a collimated single mode Gaussian input beam, which it converts into a TEM01 axially symmetrical mode. In a typical set up, a spatial filter is used to increase the beam size, which has 2 advantages: firstly, a larger beam reduces the sensitivity of the output to DOE alignment tolerances, and secondly, it will also result in the ability to produce a smaller vortex spot.
To achieve the best results it is important to align the beam precisely in the plane of the nominal working distance, also ensuring the use of high quality beams, spatial filter and beam expanders contribute to the stability of the system.
