Orbital angular momentum of light has drawn a lot of interest in the last few years. Orbital angular momentum is carried by vortex beams with helical phase fronts which are characterized by an azimuthally dependent phase factor. Orbital angular momentum transferred from vortex beams initiates rotation in trapped dielectric particles making it an efficient tool for an array of applications based on optical manipulation. Orbital angular momentum is therefore implemented as a multiplexing method for increasing data transmission capacity in optical fiber and free-space communication systems.
The operation of the micro-ring emitter is to couple the confined whispering gallery mode of the micro-ring resonator to a free-space propagated orbital angular momentum implementing a second-order Bragg grating. However, the intensity profile of the emitted beam in the far field has been identified as a series of concentric annuli that is the interference pattern produced by the scattered field of every grating component. This profile renders it difficult to collect and fully utilize the energy emitted. In other words, beam coupling into an orbital angular momentum is not perfect.
Researchers led by professor Xinlun Cai from Sun Yat-sen University in China proposed and demonstrated the fabrication of an improved emitter based on multi-turn Archimedean spiral waveguide which would emit low divergence and high directivity vortex beams. The Archimedean spiral was proposed to replace the closed cavity of the micro-ring, which would have enabled spatial radial modulation of the source in a bid to concentrate the power into a ring with far less side lobes and stronger intensity. Their work is now published in Optics Letters.
The authors adopted a highly directional vortex beam emitter consisting of an input waveguide, a 3-turn Archimedean spiral adiabatic wave guide designed with angular gratings for radiating the vortex beams and an extended Archimedean spiral adiabatic inverse taper for dissipating the remaining light. The width of the spiral waveguide was tapered along the azimuthal direction in a bid to increase the effective refractive of the TE mode so that the angular velocity of light remained constant.
The team placed the grating elements on the inner sidewall of the waveguide with constant angular intervals. The spiral emitter was set to operate at a wavelength of 1550 nm and a 30µm parameter to control the starting position of the spiral. This left an enough space for a multi-turn spiral. The authors also fixed the distance between successive turns at 1.2µm in order to ensure low coupling between the adjacent spiral turns.
For comparison, the authors also prepared a 1-turn spiral emitter with a similar starting radius as the 3-turn device. The near field of the 3-turn device was observed to be slightly larger as compared to that of a 1-turn device. After propagating to far field, the researchers observed that the radiation intensity of the 3-turn device was concentrated within the 10 deg diffraction cone. On the other hand, for the 1-turn device, there was a large intensity distribution outside the 10 deg cone. These results confirmed that directivity of the radiation from the orbital angular momentum device could be improved by adding a multi-turn wave-guide.
The emission efficiency of the 3-turn emitter was approximately 3% considering that the weak grating radiated less light power. In addition, by sufficiently designing the waveguide width, the emitter generated vortex beams with high orbital angular momentum purity. The value of the orbital angular momentum could be adjusted by varying the wavelength of the injected light. The radiation of the spiral emitter was observed to have a small divergence angle as well as high directivity than single-ring emitter, therefore, giving a better option for efficient orbital angular momentum receivers and transmitters.
Shimao Li,1 Wen Yu,1 Laura Meriggi,2 Qingsheng Xiao,1 Zhichao Nong,1 Xinlun Cai,1,* Marc Sorel,2and Siyuan Yu1,3. High-directional vortex beam emitter based on Archimedean spiral adiabatic waveguides. Vol. 42, No. 5 / March 1 2017 / Optics Letters.Show Affiliations
1State Key Laboratory of Optoelectronic Materials and Technologies and School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China
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