Group delay ripple reduction in cascaded ROADMs by adaptively modulated optical OFDM signals for Metro/Regional networks

The Group Delay Ripple (GDR) is investigated thoroughly, for the first time, for optical OFDM (OOFDM) transceivers using adaptive modulation in intensity modulation and direct detection (IM-DD) transparent Metro/Regional network systems incorporating cascaded reconfigurable optical add/drop multiplexers (ROADMs). The considered ROADMs are based on two optical filters, fibre-Bragg grating (FBG) and a wavelength-blocker (WB)/wavelength selective switch (WSS) device. It is shown that adaptive modulation can enhance the network tolerance not only to the GDR, but also to the pass-band loss ripple (LR) and frequencies dip, leading also to an improved system performance of up to about 60% in comparison with conventional identical modulation.