A focus servo system for an optical disk drive includes an optical pick-up unit comprising at least one photodetector. At least one analog to digital converter is configured to digitize signals received from the at least one photodetector. At least one processor is configured to execute a digital servo algorithm to process the versions of the digitized signals to calculate a focus error signal (FES) from the versions of the digitized signals and to indicate a focus open condition based on the FES and the versions of the digitized signals.