The blu ray disc BD / Digital Versatile Disk DVD / CD compatibility with Polarized HOE Holographic Optical Element
In order to enable sufficient tilt margin, there is a thin 0.1mm cover layer in Blu-ray Disc. The Digital Versatile Disk DVD and Compact discs (CD) are difficult to reproduce by an optical pick up used only for BD because of the spherical aberration caused by the difference of cover layer thickness and wavelength between BD, DVD and CD. LG Electronics has developed the compatibility technology to reproduce DVD and CD with an optical BD blu-ray pick up. There are three functions implemented in the optical pick-up to achieve compatibility between these discs that have different transparent substrate thickness.
1) Keep sufficient free working distance of the objective lens.
2) Have means to compensate for spherical aberration.
3) Control the numerical aperture for each disc format.
To satisfy the need for backward compatibility, LGE has developed a BD/DVD/CD compatible pick-up with a single aspherical objective lens having a working distance of 0.54mm with NA 0.85, and a new polarized Holographic Optical Element HOE device. A birefringent material is used in order to make the HOE device. This material is sandwiched between two substrates and has the same refractive index as the bonding material for a certain polarization direction, but has a different refractive index for a perpendicular polarizing direction compared to the first polarizing direction.
The Holographic Optical Element HOE is designed not to affect the wave front of BD wavelength 405nm. But at wavelengths of DVD 650nm and CD 780nm, when their polarization direction is perpendicular to that of 405nm, there must be some phase distributions for both 650nm and 780nm wavelength beams due to the index difference between the birefringent and bonding materials. In the radial direction this HOE has a width variation of pitch and a depth in the surface of one substrate so as to control the phase distribution of the incident beam in the entrance pupil diameter for both CD and DVD. It also compensates for the spherical aberration generated by the thickness difference of the DVD and CD. To control the numerical aperture of CD and DVD, an aperture filter using an optical filter or diffraction type filter could be adopted in the HOE device to prevent the diffracted noise beam outside the effective diameter for both DVD and CD, even though its effect on the signal is not so critical.
It is possible to optimize and improve the optical efficiency for both DVD and CD system without any influence on BD system, when the non-diffracted beam for BD and the diffracted beams for DVD and CD are used. This HOE device is only available with linearly polarized light. In order to make the incident beam on the disk circularly polarized, a new hybrid HOE device is under development. This would remove the linearly polarized beam effect on the recording layer of Blu-ray disc by combining the polarized HOE with the wave plates. In pick-up, the beam shaping device for BD has been removed and a liquid crystal device is used to compensate for the cover layer thickness difference between single and dual layer BD blu ray disk. Therefore, both the HOE and the liquid crystal device have been put in the actuator. 23.3GB BD-RE disc, DVD and CD-ROM disc are reproduced by the newly developed BD/DVD/CD compatible pick-up.
For more details and online order, please visit www.tape4backup.com
About the Author: Shawn Paul
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