Variable optical gain control device

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A variable optical gain control device is capable of amplifying an optical signal while controlling its luminous intensity. The device is made up of optical waveguide paths which are formed on a flat face of a substrate and includes an input directional coupler for combining an input optical signal and a pump laser light; a luminous intensity control portion in the form of a Mach-Zehnder interferometer for controlling the luminous intensity of the optical signal combined with the pump laser light; and an output directional coupler for separating the pump laser light from the resulting optical signal. The luminous intensity control portion is doped with a rare earth element so that the optical signal is amplified by means of the pump laser light, while a heater is applied to one of the optical waveguide paths forming the Mach-Zehnder interferometer to control the luminous intensity of the optical signal.