Coherent WDM, towards > 1 bit/s/Hz information spectral density

Andrew D. Ellis*, Fatima C.G. Gunning

*Corresponding author for this work

    Research output: Contribution to journalConference articlepeer-review


    Many approaches to achieving high information spectral density (ISD), have been reported recently. The standard non-retum-to-zero (NRZ) format, which offers a base line performance around 0.4 bit/s/Hz, may be enhanced using a variety of techniques, including: pre-filtering within the transmitter, multi-level modulation formats and polarisation interleaving or multiplexing. These techniques either increase the information per channel (multi-level formats and polarization multiplexing) or minimise interferometric cross talk (pre-filtering and polarisation interleaving) and result in ISDs around 0.8 bit/s/Hz. Combinations of these techniques have been used to provide ISDs of up to 1.6 bit/s/Hz. In this paper we propose a new technique, which we call Coherent WDM (CoWDM), to increase the ISD of NRZ binary coded signals in a single polarisation from 0.4 to 1 bit/s/Hz whilst simultaneously eliminating the need for pre-filters within the transmitter. Phase control within the transmitter is used to achieve precise control of interferometric cross talk. This allows the use of stronger demultiplexing filters at the receiver, and provides optimum performance when the bit rate equals the channel spacing, giving an ISD of 1 bit/s/Hz. This interference control may be achieved by controlling the phase of each laser individually with optical phase locked loops, or by replacing the typical bank of lasers with one or more coherent comb sources, and encoding data using an array of modulators that preserves this relative optical phase. Since optical filtering is not required in the transmitter, stronger optical filters may be used to demultiplex the individual WDM channels at the receiver, further reducing cross talk.

    Original languageEnglish
    Article number76
    Pages (from-to)482-490
    Number of pages9
    JournalProceedings of SPIE - The International Society for Optical Engineering
    Publication statusPublished - 3 Jun 2005
    EventOpto-Ireland 2005: Optoelectronics, Photonic Devices, and Optical Networks - Dublin, Ireland
    Duration: 4 Apr 20056 Apr 2005


    • High Speed Transmission
    • Optical Demultiplexing
    • Optical Filtering
    • Optical Multiplexing
    • Phase control
    • Spectral Efficiency
    • Wavelength Division Multiplexing


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