We report a new fiber-Ti:LiNbO3waveguide coupling loss reduction method and its application to modulators for phase shift keying (PSK) coherent systems. The titanium/magnesium double diffusion method, which symmetrizes the waveguide depth index profile and reduces the coupling loss by Mg diffusion after Ti-diffused waveguide formation, is proposed. A fiber-to-fiber total insertion loss of 0.5 dB for a 1-cm-long waveguide has been obtained using this method. The method was successfully applied to phase modulators by using the tapered MgO thickness diffusion which reduces the insertion loss without increasing the drive voltage. Ti:LiNbO3guided-wave phase modulators with 1.1-dB total insertion loss, 6.2- and 8.5-V modulation voltage at and 1.55 μm, respectively, and 4-GHZ bandwidth, have been obtained. A PSK heterodyne differential detection experiment at 1.2 Gbit/s was carried out successfully using one of the phase modulators described above.