Simulation and Fabrication of a Spiral Phase Plate for Generation of Optical Vortices
An optical vortex is any configuration of an electromagnetic field which is radially symmetric and has a value of zero at the center (colloquially referred to as a “doughnut mode”). Such a configuration is generated when a beam of light twists in a helical motion about its axis of propagation; this creates a point in the center of the beam known as a phase singularity, a point where infinitely many phases destructively interfere which results in zero intensity. In addition, due to the twisting motion, beams of this type carry orbital angular momentum, and as a result have useful applications in optical trapping, STED microscopy, and optical communications. However, the purpose of this project is to present methods of simulating the intensity distribution of an optical vortex generated by a spiral phase plate (SPP), including imperfections within the plate and beam misalignment, and a brief overview of the process of fabrication of such phase plates. A SPP imparts a phase shift on an incident Gaussian beam, increasing linearly from 0 to 2 pi around the azimuthal coordinate. The result at the output is a specific type of optical vortex known as a Kummer beam, or a Hypergeometric-Gaussian mode, as beams of this type can be described by a hypergeometric series. A simulation was conducted in MATLAB to calculate the expected far-field intensity distribution with an FFT algorithm, generated by a corresponding phase plate fabricated from AZ nLOF 2070 photoresist with UV etching using a micro-pattern generator. The experimental intensity distribution for a two-level phase plate closely matched the simulation near the zero point, but varied as the distance from the center increased. Since only one phase plate was fabricated and tested during the course of the project, it was undetermined if optical properties of the photoresist were not being considered, such as internal reflections or possible unwanted diffraction. Further experimentation is required, and as the simulation improves it will serve as a useful aid for predicting the expected intensity distribution from any spiral phase plate with any incident beam configuration.