Depth-discrimination-based focus error detection scheme for optical storage applications

A novel, differential focus error detection scheme for use in optical recording applications is described. The technique is based on the depth-discrimination property of confocal optical systems. A simple theoretical treatment is used to predict the form of the focus error signal, which is also investigated experimentally using a scanning optical microscope. The focus error response is found to exhibit a more than adequate lock-on range and gain, and is expected to be less sensitive to tracking feedthrough and other forms of cross-talk.