For most OAM beam generators, the optical vortex’s radius, which can be tuned via varying transmission distance in free space manually or automatically, increases with the TC. The manual control of transmission distance is not precise, while automatic control is very complex and costly. In addition, it is very hard to freely control several OAM beams with different TCs simultaneously. This means that the OAM beam radius cannot be freely tuned for a fixed transmission system without changing the TC. Nevertheless, the TC of OAM beams characterizes the corresponding mode and the magnitude of optical torque. Especially in some cases, it is necessary to precisely control the beams size without changing the inherent nature of OAM beams (distinguished by TC), such as OAM demultiplexing, fiber coupling and optical particle manipulations. For OAM demultiplexing or fiber coupling, the OAM beams radius need be matched with the coherent receiver or the ring fiber to obtain maximum coupling efficiency. And in terms of optical particle manipulation, by freely tuning the OAM beam radius, we can manipulate the particle in radial direction without changing the optical torque.
Several researchers in Wuhan National Lab for Optoelectronics, including Prof. Xinliang Zhang, Prof. Jianji Dong, and Dr. Hailong Zhou, etc. have put forward a technique to manipulate beam size of certain OAM based on space diffraction compensation. It employs a 4f Fourier lens system containing a phase mask to generate an inverse spatial quadratic phase to compensate the space diffraction during free-space propagation. The OAM beams size can be controlled by designing the phase mask and the phase patterns can be generated by commercial SLMs. This technique of space diffraction compensation can be used to improve the receiving efficiency of OAM demultiplexing and ring fiber coupling for OAM beams, this technique also has potential to offer a flexible and programmable way to control the micro particles both in radial and azimuthal direction.
This work has been on-line published in Optics Express on 14 July 2014. Relative work was partially supported by the Program for New Century Excellent Talents in Ministry of Education of China (Grant No. NCET-11-0168), a Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant No. 201139), the National Natural Science Foundation of China (Grant No. 11174096). Full text can be viewed by
H. Zhou, J. Dong, S. Yan, Y. Zhou, L. Shi, and X. Zhang, "Manipulation of orbital angular momentum beams based on space diffraction compensation," Optics Express, vol. 22, pp. 17756-17761, 2014/07/28 2014.