Patent ID: 6892000

Claim:
A tunable filter for launching incident light from an input side optical fiber into a diffraction grating through a condenser lens and emitting emission light from said diffraction grating through said condenser lens and outputting output light of a selected wavelength through an output side optical fiber, said tunable filter comprising: one optical fiber common to input and output provided between said input side optical fiber and said output side optical fiber, and said condenser lens, and optical circulator member provided between said input side optical fiber and said output side optical fiber, and said one common optical fiber; member for splitting incident light, and member for rotating a plane of polarization of one of the split light 90°, which are provided between the condenser lens and said diffraction grating, wherein light with the plane of polarization rotated 90° and the other light polarized and split are launched into the diffraction grating, and an angle of incident light with respect to said diffraction grating is adjustable.