A scheme for generating a width-tunable optical pulse is proposed and experimentally demonstrated, which is based on the Mach–Zehnder modulator’s traveling wave modulation characteristics in the Sagnac loop. In the proposed scheme, the clockwise light is modulated, and the counterclockwise light remains unmodulated and acts as a continuous-wave light. By adjusting the polarization controller and the bias voltage of the modulator, the generation of tunable-width pulses can be achieved. Experimental results show that the full-width at half-maximum of pulses (20 GHz) can be tuned continuously from 10.97 to 25.06 ps, with corresponding duty cycles of 21.94% and 50.12%, which is consistent with the simulated results. An 80 to 10 Gbit/s optical time division multiplexing (OTDM) signal after 100 km transmission demultiplexing experiment has also been demonstrated with the time window provided by the proposed scheme. The experimental results demonstrate the applicability of this scheme for high-speed OTDM signal demultiplexing after long-distance transmission. Furthermore, by adding an additional photodetector and an electric bandpass filter to the proposed scheme, both demultiplexing and clock extraction can be achieved simultaneously.