An optical pick-up including a first objective lens having a numerical aperture (NA) suitable for a BD, a second objective lens having an NA suitable for an HD-DVD, a light source to emit light beams suitable for the BD and the HD-DVD, a first beam splitter to transmit and reflect the emitted beams emitted to be selectively guided to one of the first and second objective lenses, a grated diffraction element to split the emitted beams into a zeroth order main beam and ±1st order sub-beams that can be projected on a single track of the BD and the HD-DVD in series, a first optical detector to receive a beam reflected by and returned from the BD, and a second optical detector to receive a beam reflected by and returned from the HD-DVD, wherein the first and second optical detectors are positioned at similar distances from the first beam splitter.