Abstract: A device for controlling the lag angle of optical fiber as it is wound on a obbin prior to the fiber wound bobbin being placed in a missile and payed out during flight on the missile. The device includes a laser light source which is projected into the groove between the turns of fiber being wound on the bobbin. A hemispherical detector receives the reflected laser light as it tracks the light between the grooves and any difference in the light intensity that is received by the detector is amplified and is used to speed up or slow down a reversible, variable speed motor which controls the winding of the fiber on the bobbin.