High-speed all-optical data-format conversion and logic gate from a synchronously modulated Fabry-Pérot laser diode (FPLD) below threshold condition is demonstrated by using external injection-induced nonlinear threshold reduction effect of the FPLD. A pulsed pseudoreturn-to-zero (PRZ) data with maximum extinction ratio of 14.5 dB is obtained by externally seeding the 9.953-Gb/s (OC-192) nonreturn-to-zero (NRZ) data stream to optically gain-switch the FPLD. At optimized radio frequency and optical injecting powers of 24 and 4 dBm, the root mean square timing jitter and the duty-cycle (pulsewidth) of the PRZ eye pattern are determined as 0.4 ps and 45% (44 ps), respectively. The bit-error rate (BER) can be as low as 10-13 under the 4-dBm external injecting power. A 1.5-dB power penalty with the BER of <10-9 is measured at a bit rate of 9.953 Gb/s. Highest NRZ-to-PRZ conversion rate of 12.5 Gb/s is achieved by increasing biasing current of the FPLD.