Patent ID: 7315676

Claim:
An optical modulator, comprising: a substrate having an electro-optical effect; first and second Mach-Zehnder type optical waveguides formed in series on said substrate; a first signal electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a clock signal; a second signal electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with a data signal; a first bias electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a bias voltage; a second bias electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with another bias voltage; a first coupler provided on the outgoing side of said first Mach-Zehnder type optical waveguide for fetching monitor light; a second coupler provided on the outgoing side of said second Mach-Zehnder type optical waveguide for fetching monitor light; a first monitor photodiode for detecting a light signal of the monitor light fetched by said first coupler; a second monitor photodiode for detecting a light signal of the monitor light fetched by said second coupler; a first bias voltage control section for controlling the bias voltage to be supplied to said first bias electrode section based on the light signal detected by said first monitor photodiode; and a second bias voltage control section for controlling the bias voltage to be supplied to said second bias electrode section based on the light signal detected by said second monitor photodiode; said first coupler including two input waveguides through one of which light from said first Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling said two input waveguides with a distance d 11 , and two output waveguides branched from said waveguide coupling element with a distance d 21 ; the distance d 11 in said waveguide coupling element of said first coupler being set to less than one half of width w 011 at a portion at which each of said input waveguides is connected to said waveguide coupling element; width w w11 of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w 011 .