The Study On Modulation And Application Of The Modified Optical Vortices

Optical vortex is a particular light field that posses helical phase wavefront and orbital angular momentum. The unique nature of vortex field is expected to lead to important potential application in many areas including optical micromanipulation, atomic optics, space optical communication and so on. Especially in the last ten years, the study on optical vortex has become a new research field in the modern optics. In this application prospect, the production, modulation, detection and application mechanism of optical vortices have become a focal issues urgently needed to research. Different from Laguerre-Gaussian mode, in 2007, Kotlyar et al proposed an other solution form of optical vortices field-Hypergeometric mode. On the basis of this theory, this thesis focuses on the generation, sidelobes suppression and application of optical vortices. The major content and result are shown as follows:1. We proposed a simple, economical and reliable technique for fabricating a spiral phase plate (SPP) in a quartz substrate to generate optical vortex with a unit topological charge at the wavelengths of 632.8nm. The spiral phase plate is first formed in the photoresist by direct laser writing lithography and then transferred into the quartz substrate by inductively coupled plasma etching. The performance of the fabricated SPP is verified by using beam intensity distribution, which is in agreement with the theoretical calculation result. The interference measurement suggests that we have succeeded to generate optical vortex with a unit topological charge with the fabricated SPP. We also study the Fraunhofer diffraction of a plane wave of circular cross section by a spiral phase mask based on Spatial Light Modulator.2. We proposed a simple and accurate method for calculating the optimal width of an annular spiral phase plate (SPP) to generate optical vortices with sidelobes suppression and analyzed its properties. It is shown that sidelobes can be sharply suppressed when the ratio of inner and outer radii of an annular SPP equals to that of the principal ring and the first sidelobe diffracted by a circular SPP with the same topological charge n. Moreover, the ratio of inner and outer radii of the optimal annular SPP only depends on the topological charge n, while it is not affected by the incident wavelength and size of the SPP. This proposed approach is helpful for designing or setting an optimal annular SPP in many applications. Such as designing multi-ring structure of optical vortices with same or different topological charges and angular momentum directions in dynamic multi-optical tweezers as well as improving the bandwidth of the free space optical communication.3. We propose a generalized approach to producing optical vortices with suppressed sidelobes using a variable Bessel like function added to the conventional spiral phase plate (SPP) and establishes explicit relations between the radial modulation and the optimal sidelobes expression effect in the Bessel-like modulation technique. Experimental verifications are implemented by a phase-only spatial light modulator (SLM). Comparing with prior techniques, the proposed method has advantages in terms of wide topological charge coverage with variable Bessel like analytical functions and the unchanged primary ring size as compared to the conventional one. Furthermore, it verifies that both central and outskirt ring areas of the phase plate resulted in sidelobes in the diffraction pattern and the corresponding structural dimensions can be determined quantitatively.4. We propose a technique for modulating and detecting optical vortices based on the principal-sidelobe ring relationship, where topological charges are determined by radius ratios of the principal ring and the modified first sidelobe. The method is immune to harassments from alignment or phase matching between the encoded beams and the decoding element. Moreover, it is demonstrated that the radius ratio and corresponding radial modulation parameters are independent to the incident wavelength and the size of spiral phase mask.5. We proposed a novel spiral phase filter, called the Bessel-like modulated spiral phase filter to improve the output image contrast of radial Hilbert transform, and analyzed the analytical point spread function of the image processing system. The theoretical simulation shown that the Bessel-like modulated spiral phase filter possesses some advantages in comparison with the conventional spiral phase filter as for high contrast edge enhancement with high resolution.