Multi-element grating beam splitter with a reflection grating element for use in front facet substraction

A method of providing a front facet detector signal in an optical system, said optical system having an optical source for generating a radiation beam to be applied to a data track of an optical storage medium, said method comprising the steps of:

• applying said radiation beam to a multi-element grating beam splitter, such that a first portion of said radiation beam passes through said multi-element grating beam splitter, and a second portion of said radiation beam is reflected from a reflection grating in said multi-element grating beam splitter;
• applying said first portion of said radiation beam to said data track;
• applying said second portion of said radiation beam to a front facet detector to generate said front facet detector signal;
• separating a first and a second portion of a return beam resulting from application of said radiation beam to said data track, in first and second grating elements of said grating beam splitter, respectively, along at least one plane substantially parallel to a reference plane defined by an optical axis of said radiation beam and a tangent to said data track;
• and
• separating a third and a fourth portion of said return beam in third and fourth grating elements of said grating beam splitter, respectively, along planes substantially perpendicular to said reference plane.