<p>A non-reciprocal unit and a parallel-surfaced flat birefringent plate are provided between incident side and outgoing side fiber collimators. The non-reciprocal unit is formed by disposing tapered birefringent plates on both sides of a 45<o> Faraday rotator consisting of a cylindrical permanent magnet and a magneto-optical element housed in the magnet. The optical axes of the parallel-surfaced flat birefringent plate and adjacent tapered birefringent plate are staggered from each other at 90<o>. The thickness of the paralle-surfaced birefringent plate is set equal to the sum of the thickness of the two tapered birefringent plates. <IMAGE></p>
