Reduction of intra-channel four-wave mixing using subcarrier multiplexing

Abstract

Intra-channel four-wave mixing is a major limitation to long-haul propagation at data rates of 40 Gb/s and above. The intra-channel four-wave mixing process transfers energy from ONEs to ZEROs and among the ONEs themselves, creating ghost pulses in the bit slots of the logical ZEROs and inducing amplitude jitter in the pulses that correspond to logical ONEs. Recently, several methods have been proposed for reducing the effects of intra-channel four-wave mixing, such as optimizing the design of the dispersion map, choosing appropriate amounts of pre- and post-dispersion compensation, and using Raman amplification. The effects of intra-channel nonlinear interactions between pulses have also been reduced in new high data-rate systems by using specialty fibers with low nonlinearity and an ultra-low average dispersion slope. However, when installed systems are upgraded to higher per-channel data rates it is more cost effective to modify the transmitter and receiver subsystems than to install new fiber or to redesign the dispersion map. In this paper we propose to reduce the effect of intra-channel four-wave mixing by using two subcarriers for each wavelength-division multiplexed (WDM) channel. This method works best for systems in which there is a substantial pulse overlap, such as one would encounter when upgrading an installed system from 10 to 40 Gb/s. The method relies on trading off decreased spectral efficiency for increased propagation distance. For example, in a prototypical system, when two subcarriers are used instead of a single carrier, single channel simulations show that by choosing the combined bandwidth of the two subchannels to be 50% larger than that of the single carrier channel, we can propagate about three times as far.