Phase Modulation Techniques for On-Off Keying in Optical Fibre Transmission.

Doctoral thesis, 2007

Increasing the channel bit rate is an efficient way to upgrade traffic capacity in installed transmission links. 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) – which limit the power at which the signal can be transmitted, and ultimately the transmission distance. This thesis introduces new techniques for the suppression of IFWM. In the modulation formats 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 techniques are studied analytically and by means of numerical simulations, and their properties are verified with transmission experiments, both in the laboratory and in installed transmission systems. It is shown that APRZ and PAPRZ can achieve a significant improvement in power tolerance, compared with currently used OOK modulation formats. It is also shown that APM can successfully increase the power tolerance of OOK transmission using standard modulation formats with low implementation cost such as RZ and NRZ. A patent has been granted for a generalisation of the PAPRZ scheme, and a patent application has been filed for the APM scheme.