Fig. 2: Optical spectrum of downstream modulated and upstream unmodulated optical sidebands.

Optical spectrum after OC2 (OC2, optical circulator 2), consisting of 8 downstream modulated optical sidebands with 30 Gb/s PAM4 (PAM4, four-level pulse amplitude modulation) signals in both x- and y-polarisations and 8 upstream unmodulated optical sidebands.