Patent ID: 7864410

Claim:
An optical active device having a plurality of optical amplifying sections cascaded on a path for propagating input light with a predetermined wavelength, said optically active device comprising: a front-stage optical amplifying section, positioned on the upstream side as seen in a propagating direction of input light in optical amplifying sections adjacent to each other selected from said plurality of optical amplifying sections, said front-stage optical amplifying section including, at least, a front-stage amplification optical fiber doped with ytterbium as an optically active material, and a front-stage pumping light source for supplying said front-stage amplification optical fiber with pumping light for pumping ytterbium by way of a multiplexer, the pumping light containing at least a wavelength component in a band of 0.98 Î¼m; a rear-stage optical amplifying section, positioned on the downstream side as seen in the propagating direction of input light in said selected optical amplifying sections adjacent to each other; said rear-stage optical amplifying section including, at least, a rear-stage amplification optical fiber doped with ytterbium as an optically active material, and a rear-stage pumping light source for supplying said rear-stage amplification optical fiber with pumping light for pumping ytterbium by way of a multiplexer; an optical isolator provided between said front-stage amplification optical fiber and said rear-stage amplification optical fiber, said optical isolator blocking backward light having the same wavelength as light to be amplified and passing through part of light in the band of 0.98 Î¼m; and a deterioration preventing structure configured, without including said optical isolator, so as to prevent said front-stage pumping light source in said front-stage optical amplifying section from deteriorating a performance, wherein said deterioration preventing structure including one of a structure such that, in backward ASE light generated in said rear-stage amplification optical fiber and directed from said rear-stage optical amplifying section to said front-stage optical amplifying section, part of a backward ASE component in the band of 0.98 Î¼m is not directly incident on said front-stage pumping light source by way of the multiplexer in said front-stage optical amplifying section, and a structure attenuating a power of the part of the backward ASE component in the band of 0.98 Î¼m until falling below a level that does not influence the performance deterioration of said front-stage pumping light source before the part of the backward ASE light is incident on said multiplexer in said front-stage optical amplifying section.