Quantized tuneable Helix Phase Plates - Design and Experiment

Optical helices or vortices recently gained significance in applications like optical tweezing, high resolution microscopy or astronomy. The helical wave front is in most of the cases generated by static DOEs or for higher versatility by spatial light modulators. Tuneable helix phase plates based on the Alvarez-Lohmann principle consist of two static DOEs. If those DOEs are rotated relative to each other around the optical axis the steepness of the helix can be tuned. However as a side effect of that approach, the resulting wave front shows a section where the phase is inverse to the desired one. The size of this section is proportional to the rotation angle. We present a quantization method to overcome this disadvantage and to generate a tuneable full 360° phase vortex beam. . Elements designed with this approach were fabricated as four level DOE. We introduce our approach theoretically and compare it to experimental results, estimating possible residual effects.