Unconventional spiral phase elements : design and application

Abstract

Optical vortex (OV), also known as helical beam and optical singularity, is a specific mode of laser beam having a phase term of exp(ilφ), here l stands for the topological charge of the OV and φ is the azimuthal angle of the polar coordinates. This phase structure results in an intensity zero and a phase singularity at the center of the beam. Some previous researchers have been interested in the generation, analysis and application of OV, and the spiral phase optical components involved are computer generated holograms (CGH), spiral phase plates (SPP), spatial light modulators(SLM), et cetera. Properties of conventional spiral phase elements have been well studied and various advanced applications have been introduced, which include laser trapping and manipulation of microscopic particles, the vortex coronograph, spiral interferometry, spiral phase contrast, and free‐space vortex communication. The focus of this thesis is to further extend the advantages as well as eliminate the drawbacks of ordinary spiral phase elements by modifying the conventional optical components. The unconventional designs of the spiral phase elements and the improvement of performance discussed in this thesis include coaxial hollow spiral phase plate (HSPP), raised‐cosine‐modulated spiral phase filter, Dammann vortex grating. Furthermore, analysis has been performed for fractional and quantized multilevel SPPs. Spiral interferometry is an elegant technique to identify the phase structure without elevation/depression confusion. HSPP is implemented in a 4‐f system to perform spiral interferometry coaxially. In the HSPP structure, the reference beam passes through the central hollow hole without any phase modulation, while the outer portion of the plate performs the same modulation function for the higher spatial frequency beam as an ordinary SPP. This provides a very compact optical component, which could be implemented in conventional microscopic systems.