Coherent Detection : A Key Enabler for Next-Generation Optical Transmission Systems ?

While today's WDM optical networks are mostly based on 10 Gbit/s data, modulated according to the nonreturn-to-zero (NRZ) format, networks at 40 Gbit/s and beyond will likely use more complex modulation formats, possibly involving more than 1 bit/symbol. For these formats, conventional quadratic detection techniques appear less practical, while the coherent detection technique looks more appealing. The latter provides the real and imaginary parts of the signal, at the expense of a larger complexity and cost. These drawbacks should be weighted by the promises offered by the technique when combined with advanced digital signal processing (DSP). DSP not only solves some severe implementation issues, but also holds in store a tremendous potential against fibre impairments. We will demonstrate experimentally that a coherent receiver involving DSP can mitigate distortions from chromatic dispersion, from polarisation mode dispersion and from narrow optical filtering, even after several thousand kilometres of fibre, thereby paving the way for higher-capacity, longer-reach transparent optical networks.