Optical PAM-4 generation through polarization multiplexing in single-polarization single-mode VCSELs

PAM-4 modulation is considered a key enabler for scaling capacity in datacenter networks, due to its bandwidth efficiency and implementation simplicity. We propose a novel technique for optical generation of four-level signals by means of polarization multiplexing, leveraging the single-polarization operation of a double-mesa single-mode VCSEL with 20.5 GHz 3-dB bandwidth. The scheme obviates the need for linear electronic drivers and circumvents the requirement for driving the VCSEL in its linear regime. Experimental results verify the scheme, exhibiting BER within FEC standards at 28 Gb/s. Comparison with an electrical PAM-4 modulated VCSEL demonstrates superior performance of the proposed concept.