A fiber rotator mechanism for rotating a portion of an optical waveguide, specifically an optical fiber, about a longitudinal axis thereof comprises a motor having a tubular rotor through which the fiber extends, in use, and to which the fiber is secured, directly or indirectly. An optical fiber may be secured by means of a device which also compresses the optical fiber to induce a required birefringence, conveniently by means of a spring-loaded clamping device or a ferrule of shape memory material.