A blue laser diode is a type of semiconductor laser that produces a concentrated beam of photons with wavelengths of about 400 to 500 nanometers — the area of the visible electromagnetic spectrum perceived as blue or violet by the human eye. Blue laser diodes are more difficult to make than many other types of laser, especially at higher power levels, but the short wavelength of blue light allows for greater precision. Blue laser diodes are used today for applications such as laser pointers and video projectors as well as in devices using high-definition optical discs, most prominently the Blu-ray format.
Like all laser diodes, a blue laser diode's light is produced by pumping energy into a semiconductor. In laser diodes, this is done with electric current, which distinguishes them from optically-pumped semiconductor lasers that use light. This causes the electrons in the semiconductor material to briefly rise in energy level. When the electrons revert to their original energy level again, the lost energy is released as photons, producing light. The light is then collimated by the laser's lens, focusing the photons produced in a single direction to produce a concentrated beam of light.
The color of the laser's light depends on the wavelength of the photons, which depends on the properties of the atoms or molecules comprising the gain medium. The most common gain medium material for a blue laser diode is gallium nitride (GaN), a crystalline semiconductor. Indium nitride (InN) is also used, as is indium gallium nitride, an alloy of the two.
The blue laser diode is the basis for several optical data storage formats, such as Blu-ray, China Blue High-Definition, and HD DVD. All optical discs, such as CD and DVDs, store information in a pattern of microscopic indentations that are read by a laser as the disc spins inside the disc player. CDs and DVDs use lasers that produce red light, which has a longer wavelength than blue light and consequently requires wider indentations in the disc in order to read it properly. The shorter wavelength of blue light allows a blue laser to accurately read smaller features on a disc, which means more indentations and thus more data can be included on a disk of the same size. This allows a disk designed to be read by a blue laser diode to fit about 25 gigabytes of data into a single disc layer, more than five times the capacity of a DVD.
The blue laser diode is a relatively recent innovation and did not enter commercial use until 2001. This was due to the difficulty of producing a suitable gain medium and the advances in materials science needed before the technology could become viable. It was primarily the product of research carried out in Poland and Japan, most prominently by Dr. Shuji Nakamura, a Japanese engineer and researcher, and Dr. Sylwester Porowski of the Polish Academy of Sciences.