We propose a novel InP-based traveling-wave electrode Mach-Zehnder modulator. It has an n-i-n isotype heterostructure to reduce both electrical signal loss and the optical loss caused by the <i>p</i>- type cladding layer. This device provides a large modulation bandwidth of more than 40 GHz. We have also developed a compact
push-pull driven modulator module. We obtained error-free operation for a 40-Gbit/s NRZ signal in a push-pull configuration with a very low driving voltage of 1.3 V<sub>pp</sub>. We also confirmed that the modulator has low chirp characteristics by demonstrating a 100-km SMF transmission with a penalty of less than 1.5 dB for a 10-Gbit/s NRZ signal.