Phase Modulation Techniques for On-Off Keying Transmission

At bit rates of the order of 40 Gb/s, on-off keying (OOK) transmission becomes severely impaired by non-linear effects, especially Intra-channel Four-Wave Mixing (IFWM). This paper reviews phase modulation techniques for the suppression of IFWM: in the Alternate-Phase Return-to-Zero (APRZ) and Pairwise APRZ (PAPRZ), the phase of the optical signal alternates between neighbouring bits or pairs of bits. APRZ and PAPRZ achieve increased non-linear tolerance by causing different IFWM contributions to interfere destructively, when proper phase-alternation amplitude is applied. In another technique, Asynchronous Phase Modulation (APM), the phase of the optical signal is modulated by an independent clock signal, at a frequency lower than the bit rate. APM achieves increased non-linear tolerance by a combination of destructive interference of IFWM contributions and suppression of the frequency-matching condition for the build-up of IFWM, when phase modulation with proper amplitude and frequency is applied. The effectiveness of the techniques is verified in a number of numerical simulations and experiments.