Pump linewidth requirements for processing dispersion-altered DQPSK signals using FWM

• In a four-wave mixing wavelength conversion scheme, the effect of the phase noise transfer from the pump laser to the idler wave can be detrimental especially of the idler has been altered through group velocity dispersion.
• The coupling of the laser phase noise to amplitude noise can degrade the bit error rate performance of the idler as the idler propagates over the second half of the optical link.
• The paper estimates the OSNR penalty for a given pump linewidth to convert DQPSK signals at baud rates of 10, 28 and 56 Gbaud.
• The results show that pump linewidths below 1 MHz are needed to convert signals DQPSK signals. The results are pertinent for considering optical phase conjugation.
• The results indicate that while FWM would be suitable to be implemented as wavelength converters in optical networks where the propagation distances are less than 500 km; care will be needed to find appropriate pump lasers with sub-MHz linewidth in order to process signals using FWM-based mid-span spectral inversion where the propagation distances could exceed 1000 km.

We report on a potentially deleterious issue regarding the four-wave mixing (FWM)-based processing of dispersion-altered signals. We estimate the baudrate-dependent pump linewidth tolerances by calculating the extra optical signal to noise ratio (OSNR) penalty with respect to the propagation distance. We find that the issue is not important for 10 Gbaud differential quadrature phase shift keying (DQPSK) signals, though for 28 Gbaud (and 56 Gbaud) DQPSK signals we find that the pump linewidth requirements to implement FWM based optical signal processing needs to be in the sub-MHz range in order to avoid excessive OSNR penalties for the case of dispersion-altered signals. These results are pertinent for systems employing FWM, which could be all—optical wavelength converters for packet switching or mid-span spectral inversion techniques.