a beam generation unit providing a linearly polarized light beam including a beam source for generating the linearly polarized light beam and a first polarization angle adjusting unit for setting polarization angle of the linearly polarized light beam, wherein when the optical system performs a writing operation in the holographic mode, the linearly polarized light beam has a first and a second linearly polarized light beam component, wherein when the optical system performs a reproducing operation in the holographic mode, the linearly polarized light beam has the first linearly polarized light beam component, and wherein when the optical system is in the CD mode or the DVD mode, the linearly polarized light beam has the second linearly polarized light beam component;

a beam splitter for respectively reflecting and transmitting the first and the second linearly polarized light beam component of the linearly polarized light beam from the beam generation unit, a polarization states of the first and the second linearly polarized light beam component being orthogonal to each other;

an optical sensor;

a first beam path through which the first linearly polarized light beam component reflected by the beam splitter propagates to serve as a reference beam for writing a holographic digital data to a holographic storage medium and reproducing the holographic digital data from the holographic storage medium;

a second beam path for accepting the second linearly polarized light beam component transmitted through the beam splitter, the second light beam component serving as a signal beam for the writing operation of the holographic mode and a beam source for reproducing a CD or a DVD digital data, wherein a beam carrying digital data reproduced from CD or DVD propagates along the second beam path and is reflected by the beam splitter toward the optical sensor;

a second polarization angle adjusting unit for adjusting a polarization angle of the second linearly polarized light component based on the mode of the optical system, the second polarization angle adjusting unit being provided on the second beam path, wherein when the optical system performs the writing operation in the holographic mode, the second polarization angle adjusting unit changes the polarization angle of the second linearly polarized light beam component by 90°; and

an optical device for selectively adjusting a numerical aperture of the second linearly polarized light beam component in the CD mode or the DVD mode and modulating the second linearly polarized light beam component into the signal beam for the holographic digital data in the holographic mode, the optical device being provided on the second beam path.