Optical phase stabilization is required for operation of a Mach-Zehnder-Interferometer (MZI) used as an intensity modulator. The stabilization has been achieved both by tuning the wavelength of the light source and by tuning the phase in an integrated-optic phase modulator, which is incorporated in the MZI. A phase stability of about 3 · 10~3 rad has been achieved. Furthermore, intensity modulation for data transmission is realized by use of the phase modulator, too. Thus a phase/amplitude (PM/AM)-conversion technique is applied.