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The parametric characterization of a nonreturn-to-zero (NRZ) to return-to-zero (RZ) data format converter based on the external optical NRZ injection of a TO-46-can packaged vertical-cavity surface-emitting laser (VCSEL) directly modulated by a 10 GHz electrical pulse is demonstrated. The electrical-pulse modulation induced gain switching of the VCSEL is initiated under external optical NRZ data injection that increases the relaxation oscillation frequency of the homemade VCSEL from 2.2 to 7.4 GHz, thereby enabling its electrical modulation bandwidth up to 10 GHz. The external NRZ injection reduces the lasing threshold and enlarges the modulation depth of the VCSEL so that the converted RZ data pulsewidth can be shortened to 27 ps with a slightly increased peak-to-peak negative frequency chirp of 4.3 GHz (corresponding to a chirp parameter of 122 MHz/ps). The chirp and bit error rate (BER) display strong correlations with the injection power and the biased current of the VCSEL. With external injection, the receiving power required for achieving a BER below 10-9 at 10 Gbit/s is -19.5 dBm, and a power penalty of 16 dB is observed when the dc bias of the electrical-pulse modulated VCSEL is decreased by only 10% from the threshold condition.