Patent ID: 8503072

Claim:
A gain-clamped semiconductor optical amplifier comprising: a gain area for amplifying guided light, said gain area including a multimode interference optical waveguide; a pair of reflectors disposed such that said gain area is disposed between said pair of reflectors; a first port for receiving external signal light; a second port for emitting light which has been amplified by said gain area; and third and fourth ports for emitting clamped light generated by said reflectors; wherein: said multimode interference optical waveguide has a waveguide end face connected to said first port through a first single mode waveguide and connected to said third port through a second single mode waveguide, and another waveguide end face connected to said second port through a third single mode waveguide and connected to said fourth port through a fourth single mode waveguide; one of said reflectors is disposed in said second single mode waveguide, and the other of said reflectors is disposed in said fourth single mode waveguide; said first and fourth single mode waveguides are disposed respectively on opposite end sections of said multimode interference optical waveguide; said second and third single mode waveguides are disposed respectively on opposite end sections of said multimode interference optical waveguide; said first and third single mode waveguides propagate said signal light, and said second and fourth single mode waveguides propagate said clamped light; a part of one of said second and said fourth single mode waveguides comprises a gain adjusting area for adjusting an oscillation threshold gain of said clamped light; and when a current is injected into or a voltage is applied to said gain adjusting area, said gain adjusting area produces a gain of said clamped light or attenuates said clamped light, thereby adjusting said oscillation threshold gain of said clamped light.