Method and apparatus for focus error compensation having path length independence

摘要

A method and apparatus for generating a focus error signal from a beam of light reflected from an optical surface, is shown to include a beamsplitter for splitting the reflected light beam into first and second beams, first and second detectors for generating first and second detection signals representative of light on said detectors, a first lens having a first focal length for focusing the first beam at a first focal point, wherein the first detector is positioned in the convergent portion of the focused first beam so that the ratio of the distance of the first detector from the first lens to the first focal length is a first value, a second lens having a second focal length, different from the first focal length for focusing the second beam at a second focal point, wherein the second detector is positioned in the convergent portion of the focused second beam so that the ratio of the distance of the second detector from the second lens to the second focal length is a second value equal to the first value, and a differencing circuit for subtracting the signals generated by the first and second detectors thereby generating a focus error signal. Such apparatus and method has excellent rejection of beam pattern noise, i.e., spatial variations of optical power unrelated to the state of focus of the beam and is suitable for incorporation in a split optics information system.