An optical encoder for measuring rotation is provided. The optical encoder includes an optical disk having a diffraction grating track and an index track. The index track being disposed radially inward from the diffraction grating track. The diffraction grating track having a plurality of equally spaced lines that create an alternating light/dark pattern. The index track includes a pattern with at least two sequences, the at least two sequences being equally spaced about the diameter of the index track, each of the sequences having at least one mark and each of the sequences having a different number of marks from the other sequences.