With the rapid growth of Internet services and broadband telecommunications, it is indispensable to increase the capacity of optical communication systems so as to support the huge bandwidth requirements in the future. There are mainly two ways to increase the capacity of optical communication systems. One way is to increase the number of multiplexing channels, such as wavelength division multiplexing, polarization division multiplexing, and mode division multiplexing, etc. The other way is to improve coding efficiency over a channel, such as amplitude modulation, phase modulation and quadrature amplitude modulation, etc.
Besides the conventional techniques, orbital angular momentum (OAM) has attracted great attention recently. OAM can be used as a multiplexing way as well as a coding way. The OAM coding approaches reported are quite similar, in which one code only includes one OAM state, so the largest bit number is increased by lg2N for N OAM states. In fact, the coding efficiency of state-of-the-art has the potential to be increased further if considering the hybrid of multiple OAM states, which means that one code may include multiple OAM states with intrinsic orthogonality.
Several researchers in Wuhan National Lab for Optoelectronics, including Prof. Xinliang Zhang, Prof. Jianji Dong, and Mr. Hailong Zhou, etc. have put forward a hybrid coding method of multiple OAM states based on the inherent orthogonality. Using concentric ring resonator array by electro-optic tuning, different OAM states can be emitted, which represent multi-bits. Assume there are N concentric ring resonators and every ring can emit two OAM beams, in our approach, the coding method can contribute N-1 bits for N OAM states. This new coding method can tremendously improve the coding efficiency compared to the conventional way, and our coding method of OAM can be used together with other modulation (amplitude and phase modulation).
This work has been on-line published in Optics Letters on 3 February 2014. Relative work was partially supported by the National Basic Research Program of China (Grant No. 2011CB301704), the Program for New Century Excellent Talents in Ministry of Education of China (Grand No. NCET-11-0168), a Foundation for Author of National Excellent Doctoral Dissertation of China (Grand No. 201139), and the National Natural Science Foundation of China (Grand No. 11174096).