The theoretical and experimental performance of guided-wave serrodyne phase modulators formed through indiffusion of Ti and LiNbO/sub 3/ substrates is discussed. Basic serrodyne theory is reviewed and a modified model of the serrodyne modulator incorporating effects of polarization, light-source coherence, device response time, and device-related intensity modulation is developed. Sources of modulation errors and harmonic suppression limitations imposed by each are identified. Experimental results showing greater than 50-dB suppression to 200-kHz ramp frequency and correspondence to expected limitations are then presented. Good agreement between theory and experiment is found. The results indicate that very high suppression, greater than 60 dB, at frequencies to several hundred kilohertz are possible with Ti-LiNbO/sub 3/ waveguide devices. >