The invention provides an interpolation apparatus for improving the resolution of an optical encoder. The guide member has a pair of detectors arranged in the disk transfer direction with a predetermined distance therebetween so as to be brought into a detection state by a peripheral edge of a disk when the disk reaches such a position as to be clamped by the disk rotating section. The IC design is described in a high-level language by using the description of the leaf cells. The optical elements may include fibers and lenses. Such a frequency calibration apparatus may be incorporated with the lasers into a signal generator apparatus.