Patent ID: 7593448

Claim:
A semiconductor laser module comprising: a distributed-feedback laser having a cavity with a cavity length; a current monitor that monitors leaks of a laser beam from the distributed-feedback laser; and a lens that guides the laser beam to an optical fiber, wherein when a predetermined transmission loss, a predetermined number of channels, and a predetermined modulation factor per channel are given, the distributed-feedback laser is configured to have the cavity length satisfy a condition that a distortion is less than a predetermined distortion level and a carrier-to-noise ratio is more than a predetermined value based on a relation between transmission loss, number of channels, modulation factor per channel, and the cavity length, and wherein the cavity length of the distributed-feedback laser is configured to have an optimal cavity-length with which a value obtained by multiplying a distortion difference from the predetermined distortion by a carrier-to-noise ratio difference from the predetermined carrier-to-noise ratio is maximum.