Abstract
We propose optical injection locking (OIL) to enable compensation of the inter-channel nonlinear phase noise, which is dominated by cross-phase modulation (XPM). In this paper, injection locking is used to create a local oscillator for a homodyne receiver from a residual carrier. The locking is fast enough to follow XPM-phase distortion, but slow enough to reject the signal bands, which are spaced slightly away from the pilot. The homodyne receiver thus partially cancels XPM, as it is common to the signals and the pilot. An experimental 7-channel WDM system gives 1-dB (0.7-dB) improvement in the peak Q of the center channel, for QPSK (16-QAM) modulated OFDM subcarriers, and increased the transmission reach by 320 km. The optimum performance was achieved at an injection ratio of -45 dB, with the injected power as low as -24.5 dBm.
Highlights
The transmission reach of optical communication systems is restricted by nonlinearities in optical fibers [1]
Dar et al showed that this inter-channel nonlinear interference noise (NLIN) cannot strictly be considered as circularly symmetric (CS) Gaussian noise, but can be dominated by phase noise in systems with many short-spans: the CS Gaussian noise model is only appropriate when longer distances are covered by fewer spans [8]
We propose using optical injection locking (OIL) to identify the XPMinduced fluctuations from the residual carrier, which can be cancelled in a coherent receiver
Summary
The transmission reach of optical communication systems is restricted by nonlinearities in optical fibers [1]. Inter-channel nonlinear interference, e.g. from cross-phase modulation (XPM) and four-wave mixing (FWM), are difficult to model with significant precision for effective compensation, based on full-field digital propagation [4]; in optimized laboratory experiments, there are clear benefits from compensating inter-channel nonlinear interference [5]. Digital methods based on virtual propagation are computationally intensive for inter-channel nonlinear interference, due the need to process extremely wide-bandwidth signals. They are compromised when channels are added or dropped in optically routed networks [4]. The XPM model developed in [10] indicates a reduction in the bandwidth of inter-channel phase modulation by dispersion-induced walk-off between the WDM channels, which depends on the channel spacing. [12], shows that the XPM bandwidth for 50-GHz channel spacing with QPSK modulation is
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