Patent ID: 7164822

Claim:
A variable optical gain control device having an optical circuit made up of optical waveguide paths formed on a flat face of a substrate, said optical gain control device comprising: a first optical waveguide path having a first end and a second end and having a first input port at the first end; a second optical waveguide path, separate from the first optical waveguide path, having a first end and a second end and a refractive index controlling means therebetween; a third optical waveguide path and a fourth optical waveguide path, the third optical waveguide path having a second input port, the first and second input ports receiving an optical signal and excitation light, the third and fourth optical waveguide paths being disconnected from each other and isolated from the second optical waveguide path; a first directional coupler connected to the respective first ends of the first and second optical waveguide paths; and a second directional coupler connected to the respective second ends of said first and second optical waveguide paths; a third directional coupler connected to the third optical waveguide path and the first end of the first optical waveguide path for coupling the excitation light and the optical signal, the first directional coupler receiving from the third directional coupler and dividing between the first and second optical waveguide paths the coupled optical signal and the excitation light, the second directional coupler combining the optical signal and the excitation light from the first and second optical waveguide paths and outputting the same; and a fourth directional coupler connected to the fourth optical waveguide path and the second end of the first optical waveguide path for decoupling the combined excitation light and the optical signal output from the second directional coupler and outputting the decoupled optical signal, wherein said first and second optical waveguide paths are doped with a rare earth element for amplifying the optical signal by means of the excitation light, and together with the first and second directional couplers form a Mach-Zehnder circuit portion for controlling luminance intensity of the amplified optical signal.