Blu-ray Disc (BD) is a next-generation video recording standard that supports crystal-clear, HD video to be recorded without any loss of image quality. In creating this technology, Sony first had to develop a blue-violet semiconductor laser capable of providing the required recording density on an optical disc. The blue-violet laser used in BD equipment emits light with a wavelength in the 400nm-410nm range. This short wavelength enables data to be recorded and played back at high densities. The key feature of BD technology is its ability to store five times as much data on a 12cm optical disc as a red laser DVD system.
What is a Semiconductor Laser?
Apart from semiconductor lasers, there are also solid-state lasers based on ruby, glass and other materials, gas lasers, which use helium, neon, argon and other gases, and pigment-based liquid lasers. Semiconductor lasers have relatively simple structures compared with these other types. They emit laser light when an electric current is applied to the p-n junction of a compound semiconductor. Sony began to produce semiconductor lasers in 1984, using the metal organic chemical vapor deposition (MOCVD) method of growing crystals. This advance paved the way for the subsequent adoption of music CDs and MDs.
Figure 1 shows the structure of a blue-violet semiconductor laser. Semiconductors are divided into types according to the way in which electrons are retained. An N-type-or excess electron-semiconductor holds an abundance of electrons, while a P-type–or excess hole–semiconductor has a surplus of holes to accept electrons. The LED emits light at the junction when these two materials are joined together with the cathode connected to the P-type and the anode to the N-type.
A semiconductor laser must be capable not only of emitting light, but also of maintaining a stable light at the wavelength required for a particular purpose. This is achieved by sandwiching an MQW light-emitting layer between the P-type and N-type semiconductors. In Sony's blue-violet laser, the cladding layers for the P-type and N-type are made from aluminum gallium nitride (AlGaN). By optimizing the structure around the MQW light-emitting layer, Sony has been able to achieve excellent light-emitting efficiency.
A key challenge on the path to the development of a working blue-violet laser was durability. A semiconductor laser produces heat when emitting light, and this can degrade the materials and shorten the life of the device. Earlier semiconductor lasers had sapphire substrates, but the poor heat transfer efficiency of this material limited the scope for improvements in semiconductor life. Sony radically redesigned the semiconductor substrate and used gallium nitride, which has excellent heat conductivity, instead of sapphire. By improving the efficiency with which laser heat is transferred from the semiconductor, Sony was able to achieve a dramatic improvement in product life.
Laser Wavelengths (Colors) and Storage Densities
Data storage densities on optical discs are influenced by the wavelength and output characteristics of the laser beam. By reducing the wavelength of the laser beam, it is possible to focus the beam on a smaller spot, thereby increasing the data recording density.
The world's first semiconductor laser had an oscillation wavelength of 840nm and produced infrared light. In 1975, Sony achieved room-temperature continuous-wave operation of an AlGaAs infrared laser with a wavelength of 780nm. This device helped to ensure the popularity of the music CD by enabling 650-700MB of data to be recorded on a 12cm disc. In 1985 Sony succeeded in developing the world's first red AlGalnP laser capable of room-temperature continuous-wave operation at 650nm. It was now possible to record a full-length movie on a single 12cm DVD with a capacity of 4.7GB per layer.
With an extremely short wavelength of 405nm, the blue-violet semiconductor laser was developed to provide even higher recording densities than the DVD. By developing the blue-violet semiconductor, Sony succeeded in creating the BD format, which can store 25GB on a single-layer disc or 50GB on a double-layer disc. This is sufficient to record over two hours of HD video at 1920x1080i resolution.
