Patent ID: 7379226
Filing Date: 2008-05-27
Classification: G02F

Abstract:
1. A variable optical attenuator comprising: a separation/combination birefringent element of a parallel plane type that separates lights of orthogonal polarization directions on a same optical path and combines lights on different optical paths; a light converging lens; a mirror placed on the opposite side of the light converging lens from the separation/combination birefringent element and at a focal position of the lens; variable polarization rotation means and fixed polarization rotation means placed at positions in between the separation/combination birefringent element and the mirror; and an input port and an output port provided on an end face of the separation/combination birefringent element, wherein the variable polarization rotation means comprises a base film Faraday element of which a Faraday rotation angle varies according to a variable combined magnetic field, and variable magnetic field applying means that applies two or more magnetic fields of different directions externally to the base film Faraday element and can vary a combined magnetic field thereof, wherein the fixed polarization rotation means comprises a compensation film Faraday element of which a Faraday rotation angle hardly varies and a Faraday rotation direction is the same as that of the base film Faraday element, and a hollow cylindrical permanent magnet that applies a magnetic field externally to the compensation film Faraday element, and wherein the compensation film Faraday element is placed in the cylindrical permanent magnet and the base film Faraday element is placed outside the cylindrical permanent magnet such that both the Faraday elements are magnetized in directions different from each other, and by controlling the rotation angles of the polarization directions with the variable polarization rotation means, the amount of reflected output light is controlled.