Experimental demonstration of a PDM QPSK real-time burst mode coherent receiver in a packet switched network

We achieve full real-time operation of a 28 Gb/s burst-mode PDM-QPSK coherent receiver capable of recovering symbol timing and polarizations. The receiver is demonstrated with temporal sub wavelength switching in a four-node WDM optical packet network testbed. Introduction Optical packet networks make an efficient use of wavelength division multiplexing (WDM) infrastructures, by allowing sub-wavelength switching and statistical multiplexing [1]. The massive success of coherent technologies in circuit-switched networks leaves limited doubt that packet switched networks would profit from the introduction of coherent technologies. Investigations of applications to optical packet networks have been reported, mostly based on off-line results [2-6]. Recently, we have tackled one of the specific issues of burst-mode coherent receivers, namely packet-by-packet polarization demultiplexing [7]. In packet switched optical networks, packets coming from different transmitters arrive at the receiver with different temporal alignments with respect to the timing of the analog to digital converters. Moreover, the receiver and transmitter clock could differ, or drift apart in time. It is therefore essential to recover the symbol timing of the optical packets at the receiver. In this paper, we expand our preliminary results of [7]. We implement a novel algorithm in our field programmable gate arrays (FPGAs) for symbol timing recovery, and thereby demonstrate full real-time operation of a coherent receiver at 28 Gb/s.