Transfer of optical orbital angular momentum under nonreciprocity-reciprocity amplification conversion

Magnet-free optical nonreciprocity has significant applications in quantum communication, quantum networks, and optical information processing. In this research, considering a degenerate two-level thermal atomic system with the Doppler effect of thermal atoms, the nonreciprocal amplification (NRA) of dual-path degenerate four-wave mixing (FWM) signals is achieved under the action of a co-propagating pumping field. On this basis, spatially multiplexed multiple FWM processes are formed by introducing another counter-propagating pumping field, thereby achieving the reciprocal amplification (RA) of the dual-channel FWM signals. Furthermore, by using multiple sets of spiral phase plates to load spiral phases on the signal light and the pumping light respectively, higher-order Laguerre-Gaussian vortex beams carrying different optical orbital angular momentum (OAM) are generated and participate in the FWM process, achieving the transfer of the OAM of the pumping light to the amplified FWM fields. Simultaneously, using the Mach-Zehnder interferometer, the conservation characteristics of the OAM of each FWM signal in the NRA-RA conversion are further analyzed. Furthermore, experimental results demonstrate that in the multiple FWM process induced by a pair of counter-propagating pump fields, the OAM of the amplified FWM signal in each channel varies with that of the pump field. However, the overall process maintains the OAM conservation. This study provides a feasible solution for expanding the channel capacity using OAM based on NRA-RA system, showing that the OAM has potential application prospects in achieving high-capacity optical communication and multi-channel signal processing.