A pulsed time-of-flight laser altimeter operates by transmitting a short laser pulse to an optical visible target and by detecting the reflected pulse by an optical detector. The measured distance is calculated from this flight time. So the accuracy of laser time interval is a basic beacon to measure its system. The technology of distance measurement is discussed. The shortage of traditional measurements is introduced. Using coarse and fine measurements together to meet the requirement of large range and high resolution. The following methods are described: analogue interpolation method,the Vernier method,tapped delay lines. Special attention has been paid to converters utilizing integrated delay lines for digital conversion of TIs,include using designs with phase-locked loop and delay-locked loop circuits in FPGA.