Orbital angular momentum Talbot array illuminator based on detour phase encoding
Nie Fangsong, Jiang Meiling, Zhang Mingsi, Cao Yaoyu, Li Xiangping     
Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Photonics Technology, Jinan University, Guangzhou, Guangdong 510632, China

Overview: Orbital angular momentum (OAM) beam with helical phase distribution has demonstrated important applications in information optics, optical storage, laser processing, super-resolution, optical trapping, and optical manipulation. These exceptional achievements heavily rely on the development of OAM micro-devices that can precisely manipulate optical fields of demand. As such functional components gradually reach out to large-scale production for practical applications from the laboratory-scale researches, more requirements are raised for producing OAM beams with equal properties in batches. At present, there are varied methods to generate OAM beams, for example, spiral phase plate method, variable spiral plate method, hologram folk grating method, and spatial light modulator method. However, the above methods are mostly focused on generating a single OAM beam, which overlooks the needs of fostering multi-focus array light field that is highly desirable for novel functions in numerous studies. How to readily realize focused OAM arrays beams over a large area remains a tough challenge from concept to implementation. In this paper, based on fractional Talbot effect, we have designed a planar optical device which can generate periodic array of focused orbital angular momentum beam. The phase distribution of the devised structure contains two parts: the focusing lens phase distribution and the spiral vortex phase distribution. According to detour phase encoding, the phase distribution calculated by fractional Talbot effect is implemented on the planar optical device by discretizing the phase distribution with arrayed phase-control units. The multi-level phase distribution is transformed to the lateral displacement of the rectangular bars from the center of each unit cell, which is proportioned to the phase shift as designed. The focusing property of this optical device with periodic square and hexagonal structures are simulated by finite difference time domain (FDTD). The intensity distribution and phase profile of each single focused light beam in the illumination plane are virtually identical. With changing the arrangement of the phase-regulation unit from square to hexagonal Talbot array, the symmetry of the intensity distribution for the focused light spot with vortex phase distribution changes accordingly. The symmetry of the hexagonal Talbot array is higher than the square counterpart. This optical device with explicit advantages of being easy to fabricate, splice, duplicate, and integrate can efficiently prop up the generation of high-quality large-area array-type OAM beams for widely spreading applications in optical trapping, optical manipulation, optical fabrication, and other fields.

Supported by National Natural Science Foundation of China (61605061, 61875073), the Natural Science Foundation of Guangdong Province (2016A030313088), and Guangdong Provincial Innovation and Entrepreneurship Project (2016ZT06D081)