We experimentally demonstrate optical rotation and manipulation of microscopic particles by use of optical vortex beams with fractional topological charges, namely fractional optical vortex beams, which are coupled in an optical tweezers system. Like the vortex beams with integer topological charges, the fractional optical vortex beams are also capable of rotating particles induced by the transfer of orbital angular momentum. However, the unique radial opening (low-intensity gap) in the intensity ring encompassing the dark core, due to the fractional nature of the beam, hinders the rotation significantly. The fractional vortex beam's orbital angular momentum and radial opening are exploited to guide and transport microscopic particles.
We propose a method for producing a sequence of focused optical vortices along the propagation direction by using a spiral fractal zone plate. The generated beam possesses the optical vortices embedded at subsidiary foci as well as the major ones of the fractal zone plate. The experimental results are obtained in good agreement with the simulations.
A 1 x 16 optical power splitter with wide splitting angle, uniform outputs, and low excess loss is demonstrated. The 1 x 16 splitter comprising cascaded 1 x 2 splitters with arc-shaped branching waveguides is fabricated on the silicon-on-insulator (SOI) substrate. The gap between the branching waveguides is widened in a short propagation length such that influences of etch residues and air voids in the gap on the optical power uniformity are reduced significantly. The measured power uniformity of the 1 x 16 splitter is better than 0.3 dB at wavelength of 1550 nm.
We present a beam shaping technique in controlling the complex amplitude of an optical beam. The constraint on the amplitude of the output beam in the Gerchberg-Saxton algorithm is replaced with constraints both on the amplitude and phase of the output beam in the proposed method. The total areas of the constrained regions and free regions on the complex amplitude of the output beam in the proposed method are maintained. An output beam with arbitrary complex amplitude can be realized with the proposed method. The computing result from the proposed method is a phase-only distribution, which can be fabricated as diffractive optical element for higher diffraction efficiency. Both simulations and experiments are present and the effectiveness of the proposed method is verified.
As a proof of concept, we experimentally demonstrate multiplexing of free-space optical signals in multiple channels labeled with different states of orbital angular momentum. The multiplexing process is carried out by a dynamic liquid-crystal spatial light modulator, while the phase function is calculated by an iterative algorithm. A binary amplitude computer-generated hologram serves as a demultiplexer.
Higher-order Bessel beams have been demonstrated to have the ability to trap and rotate low- and high-index particles simultaneously [Phys. Rev. A 66, 063402 (2002)]. The rotation and trapping is caused by the presence of orbital angular momentum arising from its azimuthal phase variation (that changes at integer multiples of 2pi) and the concentric rings of the Bessel mode. We demonstrate for the first time to our knowledge a branch from the family of higher-order Bessel beams that has fractional azimuthal variation at its beam axis. This new family of laser beams has the ability to perform dynamic optical manipulation with dynamic control of a spatial light modulator. Furthermore, we take the opportunity to explore the propagation characteristics of higher-order Bessel beams for which the azimuthal phase changes at noninteger multiples of 2pi.
In this communication, the sub-micron size polycrystalline silicon (poly- Si) single mode waveguides are fabricated and integrated with SiON waveguide coupler by deep UV lithography. The propagation loss of poly-Si waveguide and coupling loss with optical flat polarization-maintaining fiber (PMF) are measured. For whole C-band (i.e., lambda approximately 1520-1565nm), the propagation loss of TE mode is measured to approximately 6.45+/-0.3dB/cm. The coupling loss with optical flat PMF is approximately 3.4dB/facet for TE mode. To the best of our knowledge, the propagation loss is among the best reported results. This communication discusses the factors reducing the propagation loss, especially the effect of the refractive index contrast. Compared to the SiO(2) cladding, poly-Si waveguide with SiON cladding exhibits lower propagation loss.
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