Laser linewidth requirements for optical DQPSK systems

A differential quadrature phase-shift-keying (DQPSK) system is analyzed in the presence of additive optical noise and phase noise. In DQPSK, the impact of laser linewidth is frequently neglected: This letter aims to determine whether this assumption is valid. Using asymptotic techniques, we determine the effect that phase noise has on the bit-error rate (BER); this leads to a simple condition for when laser linewidth may be neglected. For a 10-GBd DQPSK system operating at a BER of 10^-3, a linewidth of less than 3 MHz is required for the signal-to-noise-ratio penalty to be less than 0.1 dB.