The method of using the continuously adjustable terahertz optical asymmetrical demultiplexer (TOAD) structure to tailor the duty ratio of return to zero (RZ) code is proposed. And the dependence of the additional window on the recovery time of semiconductor optical amplifier (SOA) in the TOAD loop is analyzed. For 2.5 Gb/s RZ code, experiments are demonstrated to compress the duty ratio from 50% (200 ps) to 25% (100 ps) and then 10% (40 ps), and broaden from 25% to 50%. The experiments indicate that with the TOAD structure, the duty ratio can be broadened up to 100% (RZ to NRZ), and can be narrow to picosecond or even sub-picosecond, if the switch window of the TOAD is tuned narrower.