Patent ID: 6870675

Claim:
A variable optical attenuator comprising: a plane-parallel birefringent element for separation/synthesis which separates light beams having orthogonal polarization directions on the same optical path, said birefringent element synthesizing light beams on different optical paths; a light converging lens disposed adjacent to said birefringent element for separation/synthesis; a reflecting mirror disposed at a focal point of said lens opposite to said birefringent element for separation/synthesis with respect to said light converging lens; a variable polarization rotating device including a basic film Faraday element whose Faraday rotational angle varies depending on a variable synthetic magnetic field, and magnetic field applying device which applies external magnetic fields to said basic film Faraday element from two or more directions to thereby vary a synthetic magnetic field thereof; a fixed polarization rotating device including a compensating film Faraday element having different Faraday rotational direction from that of said basic film Faraday element, and a magnetic field applying device which applies external magnetic fields to said compensating film Faraday element from one or more directions; and an input port and an output port which are disposed at the end of said birefringent element for separation/synthesis; said variable polarization rotating device and said fixed polarization rotating device being disposed at any positions between said birefringent element for separation/synthesis and said reflecting mirror, said variable polarization rotating device controlling the rotational angle of the polarization direction of incident light beams from said input port, to thereby control the amount of reflected output light to said output port, and relationships: if θ 1 ≦61 degrees the 32−θ 1 ≦θ 2 ≦−3, and if θ 1 ≧61 degrees, then 32−θ 1 ≧θ 2 ≧58−θ 1 being satisfied where θ 1 is a maximum rotational angle achieved by said basic film Faraday element, and θ 2 is a maximum rotational angle achieved by said compensating film Faraday element.