What’s the Blu-ray Disc

Comparison of laser wavelengths and NA

The NA of the Blu-ray lens is 0.85, as compared to 0.6 for DVD. Although the beam spot size can be reduced by increasing the NA, it was no easy task to manufacture a lens with a large NA.
In conventional optical discs, the NA of the objective lens is small. Because of this, molded single-lenses are used. It is possible to increase the NA by using a multi-lens system, similar to the lens systems used in microscopes. However, this makes the lens larger and heavier, making it unsuitable for use with optical discs.
It takes advanced manufacturing techniques, and especially molding technologies, to reduce the NA of a single lens. Panasonic has developed improved manufacturing technologies that produce a Blu-ray objective lens with an NA of 0.85.

Why Is the Cover Layer Only 0.1-mm Thick?
The third key technology for achieving the large data storage capacity of the Blu-ray Disc is one for reducing the thickness of the disc's cover layer.The cover layer is provided to protect the recording layer of the disc from scratches and dust.
Let's take a look at the cross sections of the DVD and Blu-ray Discs.
Cross sections of DVD and Blu-ray Discs
On the Blu-ray Disc, the distance between the disc surface and the signal recording surface (the thickness of the cover layer) is 0.1 mm, as compared to about 0.6 mm for the DVD disc.
Because optical discs are made of plastic, a certain amount of warping cannot be avoided. However, this warping changes the incident angle of the laser beam on the disc surface, which distorts the shape of the beam spot. This prevents stable data recording and playback. In the Blu-ray Disc, the NA of the lens is increased to reduce the size of the laser beam spot.
However, the larger the NA becomes, the greater the spot distortion becomes due to the warping of the disc. To solve this distortion problem, it was necessary to reduce the thickness of the cover layer.
Achieving a thin cover layer with uniform thickness at a low cost required the development of new disc manufacturing technologies. Panasonic successfully developed a method to produce discs with a 0.1-mm cover layer by using a resin spin coating process.
The new manufacturing method suppresses the effect of disc warping to achieve reliable recording and playback.

A cover layer that is 0.1 mm thick enables an objective lens with an NA of 0.85, to minimize spot distortion caused by disc warping.

The Blu-ray Disc has the following three basic optical specifications: a wavelength of 405 nm (nanometers), an objective lens numerical aperture (NA) of 0.85, and a transparent protective layer (cover layer) thickness of 0.1 mm.