Major impairments in 40 Gb/s transmission The APRZ and PAPRZ modulation formats

1. Outline of presentation • Major impairments in 40 Gb/s transmission • The APRZ and PAPRZ modulation formats

2. Background Introduction • Modern society relies heavily on information exchange, and it does more and more so: data traffic is ever increasing • The IT industry is recovering, but mindful of the recent crush, investment iscareful and focus is on returns  operators need to increasecapacity and save money! • Today’s typical optical network infrastructure: point-to-point connections,lots of dark fibre installed, expensive OEO interfaces.  add wavelengthchannels,  remove OEOinterfaces  increase the channel bit rate (40 Gb/s)

3. Introduction 40-Gb/s Transmission – Challenges • Narrow pulses (broad spectrum)  higher filteringdistortion through the MUX/DEMUX  higher group-velocity dispersion (GVD) impairments  worse receiver sensitivity(increased detected noise)  higher polarisation-mode dispersion (PMD)impairments • Due to GVD, pulses overlap during most part of transmission  higher intra-channel non-linearimpairments

4. Introduction 40-Gb/s Transmission – GVD • Different spectral components travel at different speeds in the fibre • Narrow pulses have wide spectrum  they broaden quickly and cause ISI solution (even at 10G): dispersion-compensation modules (DCM) • GVD is not constant with wavelength (slope)  DCM in installed systems might not compensate for dispersion slope solution: per-channel compensation

5. Introduction 40-Gb/s Transmission – non-linear effects in OOK • Intra-channel Cross Phase Modulation (IXPM)  timing jitter not phase sensitive generates attraction/repulsion between pulses, at certain locations solution: optimise dispersion map (pre-compensation) • Intra-channel Four-Wave Mixing (IFWM) amplitude jitter & ghost pulses phase sensitive different contributions add coherently solution: phase modulation techniques

6. Intra-channel Four-Wave Mixing Introduction • Pulse propagation in optical fibres is governed by the • Non-linear Schrödinger Equation (NLSE):

7. Introduction Phase modulation techniques for OOK • In On-Off Keying (OOK) information carried by signal intensity • phase represents a degree of freedom for IFWM suppress • Several modulation formats that combine phase modulation with OOK: • Chirped Return-to-Zero (CRZ) • Carrier-Suppressed Return-to-Zero (CSRZ) • Alternate Mark Inversion (AMI) precoding • Differential Phase Shift Keying (DPSK) • Alternate-Chirp Return-to-Zero (ACRZ) • Alternate-Phase Return-to-Zero (APRZ)

8. * * * The APRZ and PAPRZ modulation formats 40-Gb/s Transmission – APRZ • APRZ modulation format and IFWM suppression • IFWM perturbation (ghost pulse or ampli- tude jitter) is the coherent sum of contributions coming from different pulse combinations. • IFWM contributions coming from different pulse combinations will differ in phase

9. Implementation of an APRZ transmitter The APRZ and PAPRZ modulation formats • A straightforward way to generate an APRZ signal is simply cascading a phase modulator to an RZ transmitter • square modulating wave  square-APRZ • sinusoidal modulating  sine-APRZ 

10. An APRZ variation: PAPRZ The APRZ and PAPRZ modulation formats • Modulating the phase at ¼ of the bit slot, instead of ½ generates a signal in which the phase of neighbouring bit pairs alternates between two values: Pairwise APRZ (PAPRZ) • advantage: lower modulation frequency needed • drawback: higher modulation depth needed (as will be seen)