The Blu-ray Disc is an optical disc format that was created to meet the needs of high-definition video by storing a huge amount of data.

The Blu-ray Disc has the same dimensions as CD and DVD discs, with a 12-cm diameter, but...

...it features an amazing data storage capacity.

While a DVD disc stores up to 4.7 GB (gigabytes) of data on one side, the Blu-ray Disc can hold as much as 25 GB on a single side, giving it more than five times the storage capacity.

Because each recording layer of a single-side, dual-layer Blu-ray Disc is capable of storing 25 GB of data, the total capacity is an astonishing 50 GB per disc. In other words, one Blu-ray Disc can store as much data as ten DVD discs.

How does the Blu-ray Disc achieve such a large data storage capacity?

The Blu-ray Disc is 12 cm in diameter.
To efficiently record a large amount of data in a disc having the same dimensions as CD and DVD discs, it was necessary to develop different technologies from those used in CDs and DVDs.

There are three key points in these technologies.

Blue* laser
*While the Blu-ray laser is also known as a blue-violet laser, we simply call it a blue laser here.

Objective lens with a large numerical aperture (NA)

0.1-mm transparent protective layer (cover layer)

We use a laser beam to read and write information from and to the Blu-ray Disc, but the wavelength of the Blu-ray Disc is different from those that we use for CD and DVD discs.

These are enlarged views of disc recording surfaces. Can you tell which photo shows which disc surface?

As viewed from left to right, the photos show a Blu-ray Disc, a DVD disc and a CD disc. The sections that appear black in these photos are pits where information is recorded. A comparison of these photos shows that the pits, and the intervals between them, are smaller and denser on the Blu-ray Disc. This allows the Blu-ray Disc to store data at a higher density.

While the track pitch for recording signals is 0.74 µm (micrometers) on the DVD disc, it is a narrower 0.32 µm on the Blu-ray Disc. One micrometer equals 1/1,000 of a millimeter. Given that the average thickness of a strand of hair is about 0.1 mm, or 100 µm, more than 300 tracks can fit into the width of a single strand of hair on the Blu-ray Disc. As these figures indicate, the Blu-ray Disc stores a huge amount of data in a very small area.

The blue, red and yellow circles on the above photos represent the sizes of the laser beam spots formed on the disc surfaces.

Blu-ray uses a blue laser, which has a shorter wavelength than a red laser. This reduces the size of the laser spot on the disc. Combined with an objective lens that has a larger numerical aperture (abbreviated as NA), which indicates the lens' focusing power, Blu-ray further reduces the size of the laser beam spot on the disc recording surface.

The NA is expressed by nsinθ (n is the refraction index, which takes a value of 1 in air). The larger the NA, the greater the θ value (θ1 > θ2) and the smaller the laser beam spot.

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.

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.

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.

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.

These are the ultimate requirements for producing a practical laser, optical pickup, and disc. Optical disc engineers combined their efforts to optimize these specifications, and created the Blu-ray format as a mainstream, high-density recording media for the years ahead.

Blu-ray is a next-generation disc format. The single-side, dual-layer disc structure is also an important factor for enabling the immense data storage capacity of 50 GB. Next, a Panasonic specialist in this field will describe this structure in detail.