Mach–Zehnder (MZ) waveguide interferometers integrated on SOI (silicon on insulator) for 1.3 &mgr;m operation are studied on the basis of the large cross‐section single‐mode rib waveguide condition and the free‐carrier plasma dispersion effect in Si wafer direct bonding SOI by back‐polishing. And the MZ interferometers are fabricated by using KOH anisotropic etching. Their insertion losses and modulation depths are measured to be 4.81 dB and 98%, respectively, at the wavelength of 1.3 &mgr;m when a forward bias voltage applied to ap+njunction is 0.95 V and the active zone length of the MZ interferometers is 816.0 &mgr;m. ©1995 American Institute of Physics.