**2. Necessity of mixed WDM transmission system**

With the explosive growth in demand for capacity in national, regional, and metropolitan optical networks, high spectral efficiency wavelength division multiplexing transmission becomes essential. Current WDM optical transport systems are primarily based on 10 Gbit/s channels that are modulated with on – off – keying on a 50 GHz channel grid. Capacity upgrade of these systems calls for 40 Gbit/s wavelength channels to be carried in the same system "in [5,9]". To achieve this, several technical challenges need to be resolved.
