An optical system including an LDGU having a multi-element grating beam splitter and an optical source with an effective aperture for generating a radiation beam to be applied to a data track of an optical storage medium, an improved LDGU is described. The LDGU includes a baseplate having a plurality of registration pins and defining a plurality of clearance holes; a body having a ring portion which defines a central opening and having a plurality of registration openings; a ceramic header defining alignment openings and having a plurality of contact pins which are aligned to the clearance holes of the baseplate; at least two of the registration pins of the baseplate being aligned in the registration openings in the body and at least two of the registration pins being aligned in the alignment openings in the ceramic header; and a structure for mounting the grating beam splitter and the optical source on the ceramic header.