Impact of four-wave mixing phase noise transfer on wavelength converted QPSK signals

Future high capacity wavelength division multiplexed (WDM) optical networks will rely on transmitting signals with information encoded on both the amplitude and phase of multiple optical carriers. To optimise the efficiency of these networks, they are likely to employ some level of wavelength switching [1], and a key function required by wavelength switching is wavelength conversion [1]. Wavelength conversion of phase modulated signals in wavelength switched optical networks will need to be undertaken using a coherent nonlinear process, such as four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) [2] employing a fast wavelength tuneable pump laser to ensure an optical channel can be dynamically routed to different wavelengths. A crucial issue that is likely to have a major impact on the wavelength conversion of such high capacity signals using FWM is the phase noise transfer to the converted signal. A detailed study of FWM wavelength converted continuous-wave signals in [3] showed that the phase noise transfer from the pump to the idler, increased the linewidth of the idler by a factor of 4 relative to the pump. Such a large line broadening is highly detrimental to the detection of phase-encoded signals, especially since the tuneable lasers that exhibit small wavelength switching times (<;50 ns) and would be employed in wavelength switched networks tend to have linewidths of the order of a few MHz [4]. Therefore estimating the linewidth limits that enable FWM wavelength conversion using this scheme is vital to select a tuneable pump laser with suitable linewidth.