We experimentally investigate a gain‐switched Fabry–Perot semiconductor laser externally injection seeded with a narrow linewidth continuous wave laser. We determine the dependence of pulse width, side‐mode suppression ratio, and timing jitter on seeding power and seeding wavelength. As external seeding power is increased up to a certain value, we observe negligible change in pulse width, a linear increase in side‐mode suppression ratio, and a linear decrease in timing jitter. Exceeding a certain value leads to substantial pulse broadening. By tuning the seeding wavelength across a chirped mode of the laser, we observe the lowest timing jitter and the largest side‐mode suppression when the seed wavelength is aligned with the center of the chirped spectrum of a desired mode. ©1995 American Institute of Physics.