CD & Laser Optics

A CD is made from polycarbonate plastic. The program area is 86.05 cm².

• 1.2 millimetres (0.047 in) thick and weighs 15–20 grams.
• Pit size is ~100 nm deep by 500 nm wide, 850 nm to 3.5 µm in length.
• Distance between the tracks, the pitch, is 1.6 µm.
• The length of the recordable spiral is (86.05 cm2 / 1.6 µm) = 5.38 km.
• Scanning speed of 1.2 m/s. Playing time is 74 minutes, or 650 MB of data on a CD-ROM.
• CD uses 780 nm (near infrared) semiconductor laser
• Binary data representation uses non-return-to-zero, inverted method - a change from pit to land or land to pit indicates a one, while no change indicates a series of zeros.
• Data encoding / decoding uses eight-to-fourteen modulation - data to be stored is first broken into 8-bit blocks (bytes). Each 8-bit block is translated into a corresponding 14-bit codeword using a lookup table.
• Error detection and error correction uses cross-interleaved Reed-Solomon code (or CIRC) - adds to every three data bytes one redundant parity byte.

DVD & Blu-ray

• DVD pit size is 400nm. Blu-ray pit size is 150nm.
• DVD track pitch is 740 nm. Blu-ray track pitch is 320 nm.
• DVD uses 650 nm red laser. Blu-ray Disc uses 405 nm "blue" (actually in violet range) laser directly (without frequency doubling or other nonlinear optical mechanisms) from GaN (gallium nitride) laser diodes.

Laser Optics:

The optics may include a collimating lens, diffraction grating (to produce the three beams in a three beam pickup), beamsplitter prism or mirror, turning mirror (for horizontally mounted optics), and focusing (objective) lens.

Typical CD laser optics put out about 0.3 to 1 mW at the objective lens though the diodes themselves may be capable of up to 4 or 5 mW (max). For the Blu-ray player, the output power is 40mW - 100mW. Blu-ray burner uses higher power, its output is over 500mW.

The minimum "spot size" on which a laser can be focused is limited by diffraction, and depends on the wavelength of the light and the numerical aperture of the lens used to focus it. By decreasing the wavelength, increasing the numerical aperture from 0.60 to 0.85, and making the cover layer thinner to avoid unwanted optical effects, the laser beam can be focused to a smaller spot, which effectively allows more information to be stored in the same area.

Blue laser pointers have the same basic construction as DPSS (Diode-pumped solid-state) green lasers. They emit light at 473 nm (sometimes reported as 474 nm), which is produced by frequency doubling of 946 nm laser radiation from a diode-pumped Nd:YAG or Nd:YVO4 crystal. The principal wavelength of Neodymium-doped crystals is 1064 nm. With proper reflective coating mirrors, it can be made to lase at other non-principal neodymium wavelengths, such as the 946 nm. As with green DPSS lasers, use of a 1000 mW IR diode usually results in approximately 300 mW of visible blue light.

For high output power BBO (Beta barium borate (β-BaB₂O₄)) crystals are used as frequency doublers; for lower powers, KTP (Potassium titanyl phosphate (KTiOPO₄)) is used. Output powers available are up to 1000 mW, but this usually is the total output including the infrared.

A few higher-powered (120 mW) 404–405 nm "violet" laser pointers have become available which are not based on GaN, but uses DPSS frequency-doubler technology starting from 1 watt 808 nm gallium arsenide infrared diode lasers being directly doubled, without a longer-wave neodymium laser interposed between diode laser and doubler-crystal. As with all high powered lasers, such devices are able to pop balloons and light matches.