a system for phase detection in page - based optical data storage involves a number of features that may be used individually or in combination to provide optimizations and to facilitate the heterodyne detection of the modulated readout signal beam . in one aspect , alignment sensors provide signals indicative of angular misalignment between the heterodyne reference beam and the signal beam , and a feedback control system aligns the reference beam with respect to the signal beam . in another aspect , a differential detection approach is utilized . in yet another aspect , a scanning approach is used wherein the reference beam is scanned over approximately a wavelength of phase difference to cause interference fringes to shift over the photodetector pixel array . each pixel detects the signal at the phase value that cancels out localized variations of the wavefront , thus increasing the tolerance of the system to phase errors and to angular misalignments .