Patent ID: 7123403

Claim:
An optical signal-to-noise ratio improving method, comprising: dividing an input optical signal in a predetermined polarization state into a first polarization optical component and a second polarization optical component orthogonal to the first polarization optical component with the first polarization optical component and the second polarization optical component having a predetermined power ratio; supplying the first polarization optical component to a first gain device, a gain of said first gain device saturating at a first value; supplying the second polarization optical component to a second gain device, a gain of said second gain device saturating at a second value less than the first value; combining output light from the first gain device and output light from the second gain device; and improving an extinction ratio of the combined optical signal to be greater than or equal to a predetermined value by utilizing a change of a polarization state of the combined optical signal dependent on power of the input optical signal, and a polarized light filtering effect of a polarizer arranged at a later stage of the optical signal-to-noise ratio improving device.