A novel transmitter for 100 Gbit-Ethernet applications is proposed, based on the serial cascade of two 50 Gbit/s inverse-return-to-zero (also known as dark soliton) transmitters based on Mach-Zehnder modulators. The proposed transmitter and demultiplexer system uses commercially available components optimised for 40 Gbit/s applications. A 2.9 dB penalty at 100 Gbit/s is obtained using a single-stage OTDM demultiplexer and a preamplified receiver.