Comment on “optimum depth of the information pit on the data surface of a compact disk”

In a recent paper [1] Li et al. investigated the question of optimum pit depth in optical disc read-out. Data on such a disc (e.g. CD or DVD) is encoded in a spiral track of pits of varying length and read out by scanning a diffraction limited spot along this spiral track. When the scanning spot is over land all light is reflected and collected by the objective lens. The resulting intensity measured at the photodetector is near maximum. When the scanning spot is over a pit light is scattered away from the aperture of the objective lens, and as a result the measured intensity at the photo-detector will be lower than for the land case. The question is how deep the pits should be in order to maximize the modulation between the land and pit signal. In reference [1] and in a previous paper of similar character [2] the authors calculated the interference between two axially displaced point sources that are imaged by a thin lens to two points close to the detector plane. This results in an interference term that varies as cos k 1þM T , with MT being the lateral magnification of the lens and k 1⁄4 2 /l, where l is the wavelength. Minimum intensity (optimized pit depth) is obtained when