Patent ID: 7050216

Claim:
A variable optical filter comprising: a first polarizer; a first phase difference element operable to generate a phase difference; a variable Faraday rotator operable to impart a variable Faraday rotation; a second phase difference element operable to generate a phase difference: and a second polarizer, wherein said polarizers, said variable Faraday rotator, and said phase difference elements are arranged in the mentioned order along the optical axis, wherein when the angles formed by the optic axes of said first and second phase difference elements and the transmitted polarization azimuth of the first polarizer are assumed respectively to be φ 1 and φ 2 , these φ 1 and φ 2 are set to different angles, wherein when the phase differences generated by the first and second phase difference elements are assumed to be Δ 1 and Δ 2 respectively, at least one of the phase differences is (2n+1)λ/4, where n is an integer and λ is an arbitrary wavelength within a given transmission band, and wherein when the center wavelength in the given transmission band is assumed to be λ 0 , the phase difference Δ 1 of the first phase difference element is 13/4λ 0 ≦Δ 1 ≦49/4λ 0 , the phase difference Δ 2 of the second phase difference element is 13/4λ 0 ≦Δ 2 ≦49/4λ 0 , and the variations in the mean insertion loss are 0.5 dB or less at the transmission bandwidth of 40 nm and over the wavelength slope from −5 dB to +5 dB.