The Blu-ray Disc BD system uses blue light and a high numerical aperture (NA) of 0.85. A cover layer thickness of 0.1 mm is chosen to secure the disk tilt margin. DVD recorders are so popular that BD recorders are expected to be backward-compatible and allow reading and recording of DVDs, despite the fact that their optical parameters are very different. The cover layer thickness, the light wavelength and the NA of DVD are 0.6 mm, 650 nm and 0.6 respectively.

In a BD and DVD compatible pickup, red light for DVD also passes through the diffractive optical element DOE. Higher light power efficiency is needed for the optical system for realizing recoding. A BD-DVD compatible lens converge parallel blue and red rays, which is needed to stabilize tracking servo control. The DOE pattern and refractive lens surfaces are divided into inner and outer regions. The inner regions are designed to focus the second-order diffracted parallel blue ray through the 0.1 mm cover layer and the first-order diffracted parallel red ray through the 0.6 mm cover layer.

The light power distributions of focused spots of red and blue parallel rays using the assembled compatible lens were observed. The spot sizes of red light through a 0.6 mm substrate were 0.56 micron x 0.58 micron by full width at half maximum (FWHM) and those of blue light through a 0.1 mm substrate were 0.27 micron x 0.28 micron by FWHM, demonstrating that a compatible lens can converge both red and blue parallel rays.

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