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?
Track pitch: 0.32 mm Capacity: 25GB
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.
What does a lens with a large numerical aperture (NA) mean?
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.
