A novel scheme to generate binary optical dark return-to-zero (RZ) signals with tunable duty cycle and extinction ratio is proposed, through modifying the process of precoding, modulating and coding. The optical dark return-to-zero signals are easily generated only by using a dual-arm Mach-Zehnder LiNbO3 modulator along with an electrical signal time-delay device. It is demonstrated experimentally that the optical dark return-to-zero signals can be directly measured by a conventional binary intensity modulation-direct detection (IM-DD) receiver. When the electrical signal time-delay device at 2.5 Gbit/s is adjusted, the changes of optical spectra, the bit-error rate curves, and the eye diagrams of the optical dark return-to-zero signal for duty of cycles 0.25, 0.35, 0.60 and 0.80 are obtained experimentally. Moreover, optical dark return-to-zero signals generated by this novel scheme can be used as labels, in optical label switched networks.