Summary: We have developed a traveling-wave optical modulator using an n-i-n heterostructure fabricated on an InP substrate. The modulation characteristics are studied theoretically and experimentally. We obtained an extremely small π voltage (Vπ) of 2.2 V, even for a short signal-electrode length of 3 mm. We confirmed a wide frequency bandwidth and clearly open eye diagrams at 40 Gbit/s.