Time-of-flight range detection using low-frequency intensity modulation of a cw laser diode: application to fiber length measurement

This paper describes a simple optical range detector in the time-of-flight (TOF) category using a cw laser diode emitting at 1.55 µm. The principle of operation combines TOF with frequency-modulated cw methods. The main originality of this method consists in measuring the time delay between two signals without using the time variable. The forward current of the laser diode is modulated by a low-frequency triangular signal. The emitted and received signals are compared, and the amplitude difference gives a readout signal whose peak-to-peak amplitude is proportional to the TOF. An application to optical fiber length measurement is given.