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This paper describes an optical range finder in the time-of-flight category using a CW laser diode emitting 6 mW at 1.55 p. m and modulated by a triangular signal in a frequency band ranging between 100 Hz and 50kHz. The forward current of the laser diode is modulated. The emitted and received signals are compared and the amplitude difference gives a new signal whose peak-to-peak amplitude is proportional to the time-of-flight. It permits distance measurements from l m to a few tens of meters with an accuracy of about 2 centimeters. It uses only standard components developed for telecommunications; the signal processing is simplified compared with other categories of range finder. The use of a modulation by low frequency linear slopes allows a direct modulation of the forward current of the laser diode without degrading its performance. Being based on the intensity modulation the range finder operation is independent of the source spectrum profile and width. Consequently, the principle of the device can be easily adapted to the microwave field or to acoustics. Furthermore, the use of low frequency modulation avoids the phenomena of dispersion due to the propagation medium, and of profile distortion of the signal due to the capacitive effects in the electronic devices. The authors anticipate that using a new class of laser diode (300mW: wavelength = 1.48 micron) developed as pumping diodes should yield improved performance by increasing the effective range of the range finder to a few hundreds of meters.