A directly modulated optical transmitter for use with a fiber optical communications system operating in the 1550 nm wavelength band exhibits very low chirp. The chirp inherently present in a directly modulated laser is cancelled by a phase modulator which optically modulates the directly modulated laser light beam by applying a 180.degree. phase delay to a split-off portion of the input radio frequency signal. This provides a low cost transmitter capable of operating in the 1550 nm band and with laser chirp effectively cancelled or substantially reduced, thereby avoiding distortions due to laser chirp interactions with the downstream optical fiber.