Differential carrier phase recovery for QPSK optical coherent systems with integrated tunable lasers.

The performance of a differential carrier phase recovery algorithm is investigated for the quadrature phase shift keying (QPSK) modulation format with an integrated tunable laser. The phase noise of the widely-tunable laser measured using a digital coherent receiver is shown to exhibit significant drift compared to a standard distributed feedback (DFB) laser due to enhanced low frequency noise component. The simulated performance of the differential algorithm is compared to the Viterbi-Viterbi phase estimation at different baud rates using the measured phase noise for the integrated tunable laser.

Impact of phase to amplitude noise conversion in coherent optical systems with digital dispersion compensation.

The impact of phase to amplitude noise conversion for QPSK, 16-QAM, and 64-QAM coherent optical systems are investigated with electronically-compensated chromatic dispersion (CD). The electronic equalizer is shown to convert the phase noise from the local oscillator (LO) to amplitude noise, limiting the amount of CD that can ideally be compensated digitally. The simulation results demonstrate that the performance of coherent systems can significantly be degraded with digitally compensated CD and LO phase noise. The maximum tolerable LO linewidth is also investigated for the different modulation formats and found to become increasingly stringent for longer transmission distance and higher symbol rate.

Intensity and phase measurements of asymmetric mode profiles and the transform in the near- to far-field transitions.

A fast and accurate measurement technique employing a calibrated CCD array is described to measure the intensity and phase distributions of the asymmetric mode profiles associated with optical waveguides. A Shack-Hartmaan wavefront sensor incorporated in the system provides the phase information. The transform describing the near-field (NF) to far-field (FF) transitions of the asymmetric mode profiles is investigated both experimentally and theoretically. The simulated NF to FF transitions using the transform are compared with the measured profiles at different positions from the end face of the waveguide. Good agreement is obtained between the measured and the computed profiles proving the accuracy of the measurement technique and also the transform used for propagation of the asymmetric mode profiles.