This letter proposes and experimentally demonstrates a simple scheme for generating 40-GBaud carrier-suppressed return-to-zero differential quadrature phase shift keying (CSRZ-DQPSK) modulation signals with tunable pulsewidths using two dual-parallel Mach–Zehnder modulators (DPMZMs). The duty cycle of the generated 40-GHz CSRZ-DQPSK pulse train is continuously tuned from 31% to 62%, with the full-width at half-maximum tuned from 7.8 to 15.5 ps, by electrically tuning the delay between the two sine-clock signals in one of the DPMZMs. Error-free performance is achieved after 320-km transmission.