Patent ID: 7738794

Claim:
An optical transmitter comprising: a directly modulated laser; and a wavelength filter provided on a post-stage of said directly modulated laser, said wavelength filter comprising: a modulated light input port connected to said directly modulated laser and inputting modulated light output from said directly modulated laser; a filter transmitted light output port connected to an optical coupling system and outputting light having a wavelength included in a filter transmission band among said modulated light as filter transmitted light; and a filter cutoff light output port provided separately from said modulated light input port and said filter transmitted light output port and outputting light having a wavelength included in a filter cutoff band among said modulated light as filter cutoff light, wherein the peak of said filter transmission band is set on a shorter-wave side from the peak of the spectrum of modulated light output from said directly modulated laser, wherein said wavelength filter is a ring resonator type wavelength filter formed of a ring resonator, and said filter transmitted light output port and said filter cutoff light output port are located at an end opposite to said modulated light input port.