Chenhao Wan

Compact and high-resolution optical orbital angular momentum sorter

A compact and high-resolution optical orbital angular momentum (OAM) sorter is proposed and demonstrated. The sorter comprises a quadratic fan-out mapper and a dual-phase corrector positioned in the pupil plane and the Fourier plane, respectively. The optical system is greatly simplified compared to previous demonstrations of OAM sorting, and the performance in resolution and efficiency is maintained. A folded configuration is set up using a single reflective spatial light modulator (SLM) to demonstrate the validity of the scheme. The two phase elements are implemented on the left and right halves of the SLM and connected by a right-angle prism. Experimental results demonstrate the high resolution of the compact OAM sorter, and the current limit in efficiency can be overcome by replacing with transmissive SLMs and removing the beam splitters. This novel scheme paves the way for the miniaturization and integration of high-resolution OAM sorters.

Experimental generation of complex optical fields for diffraction limited optical focus with purely transverse spin angular momentum

We demonstrate a method to generate complex optical fields at the pupil plane of a high numerical aperture (NA) objective lens for the creation of diffraction limited optical focus with purely transverse spin angular momentum. The complex optical fields are analytically deduced through reversing the radiated patterns from two electric dipoles, which are located at the focal point of the high NA lens and oscillate respectively in x- and z- directions with phase difference of π/2. The derived fields can be experimentally created with a vectorial optical field generator. Using the Richard-Wolf vectorial diffraction theory, the electric fields within the focal region are calculated to evaluate the intensities and polarization distributions of the tightly focused beams corresponding to both the theoretical and experimentally generated pupil fields and the results clearly demonstrate the validity of the proposed technique.

Tightly focused optical field with controllable photonic spin orientation

The spin angular momentum of photons offers a robust, scalable and high-bandwidth toolbox for many promising applications based upon spin-controlled manipulations of light. In this work, we develop a method to achieve controllable photonic spin orientation within a diffraction limited optical focal spot produced by a high numerical aperture objective lens. The required pupil field is found analytically through reversing the radiation patterns from two electric dipoles located at the focal point of the lens with orthogonal oscillation directions and quadrature phase. The calculated pupil fields are experimentally generated with a vectorial optical field generator. The produced photonic spin orientations are quantitatively evaluated by their spin densities according to the tightly focused electric fields calculated by Richard-Wolf vectorial diffraction theory to demonstrate the validity and capability of the proposed technique.

Tailoring optical orbital angular momentum spectrum with spiral complex field modulation

We report the generation of prescribed optical orbital angular momentum (OAM) spectrum with spiral complex field modulation. Both symmetric and asymmetric OAM spectrums are generated with a vector beam generator and measured with a hybrid conformal mapper. Three methods for OAM spectrum generation ranging from the pure spiral phase modulation, the spiral amplitude and phase modulation, to the spiral phase and polarization modulation are demonstrated. The OAM spectrum generation with spiral complex field modulation may find applications in optical trapping and high-speed data communication.

Precise transverse alignment of spatial light modulator sections for complex optical field generation

Based on the properties of the dove prism and the Fourier optics approach, the coordinate relationships among four spatial light modulator (SLM) sections in a vectorial optical field generator are derived and experimentally verified. Taking the coordinate system of the first SLM section as a reference, the coordinate displacements between the first section and subsequent sections are determined via employing specially designed four-quadrant patterns, which enable the visualization of the degree of freedom controlled by each SLM section. A complex optical field could be accurately generated through combining the derived coordinate relationships and pre-compensation of the measured coordinate displacements. Several typical complex optical fields are experimentally generated to demonstrate the validity of the proposed transverse alignment method.

Compact and high-resolution sorting of optical orbital angular momentum states

We propose and demonstrate a compact and high-resolution optical orbital angular momentum (OAM) sorter that consists of a quadratic fan-out mapper and a dual-phase corrector positioned in the pupil plane and the Fourier plane respectively. The novel scheme paves the way for the miniaturization and integration of high-resolution OAM sorters.

Precise transverse alignment of a vectorial optical-field generator for complex optical field generation

Based on the Fraunhofer diffraction theory and the properties of dove prism, the coordinate relationships among the four spatial light modulator (SLM) sections in a vectorial optical field generator (VOF-Gen) are derived and experimentally verified. Taking the coordinate system of the first SLM section as reference, the coordinate displacements between the first SLM section and the subsequent ones are measured through employing the specially designed four-quadrant patterns with varying shifts of the corresponding cross point. Complex optical field could be accurately generated by combining the derived coordinate relationships and pre-compensation of the measured coordinate displacements. A typical complex optical field is generated and the experimental results demonstrate the validity of the proposed transverse alignment method for the VOF-Gen.

High efficiency geometric-phase polarization fan-out grating on silicon

We report the design, fabrication and characterization of a 1-by-5 geometric-phase polarization fan-out grating for coherent beam combining at 1550 nm. The phase profile of the grating is accurately controlled by the local orientation of the binary subwavelength structure instead of the etching depth and profile empowering the grating to be more tolerant to fabrication errors. Deep-UV interference lithography on silicon offers an inexpensive, highly efficient and high damage threshold solution to fabricating large-area fan-out gratings than electron beam lithography (EBL) and photoalignment liquid crystals. The theoretical and experimental diffraction efficiency of the grating is 87% and 85.7% respectively. Such a fan-out grating may find application to high-power beam combining in the infrared regime.