45 GHz bandwidth travelling wave electrode Mach-Zehnder modulator with integrated spot size converter

Mach-Zehnder interferometers (MZI) with capacitively loaded travelling wave electrodes (TWEs) have the potential for data rates beyond 40 Gb/s. They are not only suitable for on-off keying but also for phase modulation schemes like differential phase shift keying (DPSK) and return-to-zero (RZ)-DPSK which have received increased attention, recently. However, in order to reduce the packaging costs for a commercial product, to enhance reliability and decrease insertion loss, a spot size converter (SSC) has to be integrated for an efficient fibre/chip coupling without deteriorating the high-frequency characteristics of the modulator. This paper reports on the design, fabrication and experimental characterisation of the SSC integrated MZI modulator showing a bandwidth of 45 GHz.