Optical PAM-4/PAM-8 generation via dual-Raman process in Rydberg atoms

A scheme of optical four-level pulse amplitude modulation (PAM-4) is proposed based on dual-Raman process in Rydberg atoms. A probe field counter-propagates with a dual-Raman field which drives the ground and the excited states transition, respectively, and the Rydberg transition is driven by a microwave (MW) field. A gain peak appears in the probe transmission and is sensitive to the MW field strength. Optical PAM-4 can be achieved by encoding an MW signal and decoding the magnitude of a probe signal. Simulation results show that the differential nonlinearity and the integral nonlinearity of the proposed scheme can be reduced by 5 times and 6 times, respectively, compared with the counterparts of previous scheme, and the ratio of level separation mismatch is close to the ideal value 1. Moreover, the scheme is extended to optical PAM-8 signal, which may further improve the spectral efficiency.