Abstract: Methods and apparatuses negate a measured mechanical twist of an optical fiber by introducing a counteracting twist to bring the net mechanical twist closer to zero, which reduces the fiber's polarization mode dispersion (PMD). A spool of optical fiber having undesirable mechanical twist is mounted and fiber is drawn from the end of the spool to impart a specified counteracting mechanical twist. Additionally, the spool may be controllably rotated by a control system while optical fiber is drawn there from, allowing the system to generate a precise amount of counteracting mechanical twist in the fiber.

Abstract: An apparatus for directly measuring an angular displacement of an actuator arm relative to a reference position is disclosed, wherein the actuator arm is rotatable about a fixed axis. The apparatus includes a reflection-type diffraction grating, a light source, a transmission-type diffraction grating and a detector. The reflection-type diffraction grating is mounted on the actuator arm. The light source emits a generally rectangularly-shaped laser beam that is aligned to strike the reflection-type diffraction grating to produce reflected beams. The transmission-type diffraction grating, through which the reflected beams pass, causes the reflected beams to converge and form an interference pattern. The detector is positioned to detect the interference pattern and generate a signal representative of the angular displacement of the actuator arm relative to the reference position.