Colors and wavelengths about laser pointers from wikipedia.org

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Colors and wavelengths about laser pointers

Green
Green laser pointers^[5] appeared on the market circa 2000, and are the most common type of DPSS lasers (also called DPSSFD for "diode pumped solid state frequency-doubled"). They are more complicated than standard red laser pointers, because laser diodes are not commonly available in this wavelength range. The green light is generated in an indirect process, beginning with a high-power (typically 100–300 mW) infrared AlGaAs laser diode operating at 808 nm. The 808 nm light pumps a crystal of neodymium-doped yttrium aluminum vanadate (Nd:YVO₄) (or Nd:YAG or less common Nd:YLF), which lases deeper in the infrared at 1064 nm. The vanadate crystal is coated on the diode side with a dielectric mirror that reflects at 808 nm and transmits at 1064 nm. The crystal is mounted on a copper block, acting as a heat sink; its 1064 nm output is fed into a crystal of potassium titanyl phosphate (KTP), mounted on a heat sink in the laser cavity resonator. The orientation of the crystals must be matched, as they are both anisotropic and the Nd:YVO₄ outputs polarized light. This unit acts as a frequency doubler, and halves the wavelength to the desired 532 nm. The resonant cavity is terminated by a dielectric mirror that reflects at 1064 nm and transmits at 532 nm. An infrared filter behind the mirror removes IR radiation from the output beam (this may be omitted or inadequate in less-expensive "pointer-style" green lasers), and the assembly ends in a collimator lens.
Nd:YVO₄ is replacing Nd:YAG and Nd:YLF because of lower dependency on the exact parameters of the pump diode (therefore allowing for higher tolerances), wider absorption band, lower lasing threshold, higher slope efficiency, linear polarization of output light, and single mode output. For frequency doubling of higher power lasers, LBO is used instead of KTP. Newer lasers use a composite Nd:YVO₄/KTP crystal instead of two discrete ones.
Some green lasers operate in pulse or quasi-continuous wave (QCW) mode, to reduce cooling problems and prolong battery life.
The recent announcement^[6] of a direct green laser (not requiring doubling) promises much higher efficiencies and could foster the development of new color video projectors.
Because even a low-powered green laser is visible at night through Rayleigh scattering from air molecules, this type of pointer is used by astronomers to easily point out stars and constellations. Green laser pointers can come in a variety of different output powers. The 5 mW green laser pointers (class llla) are the safest to use, and anything more powerful is usually not necessary for pointing purposes since the beam is still visible in dark lighting conditions.^{[citation needed]}
Blue

Main article: Blue laser

Blue laser pointers usually have the same basic construction as DPSS green lasers. In 2006 many factories began production of blue laser modules for mass storage devices, and these were used in laser pointers too. These were DPSS type frequency-doubled devices. They most commonly emit a beam at 473 nm, which is produced by frequency doubling of 946 nm laser radiation from a diode-pumped Nd:YAG or Nd:YVO4 crystal (Nd-doped crystals usually produce a principal wavelength of 1064 nm, but with the proper reflective coating mirrors can be also made to lase at other "higher harmonic" non-principal neodymium wavelengths). For high output power BBO crystals are used as frequency doublers; for lower powers, KTP is used. The Japanese company Nichia controlled 80% of the blue laser diode market in 2006.^[7]
Some vendors are now selling blue laser pointers with claimed powers exceeding 1,500 mW. However, since the claimed-power of "laser pointer" products also includes the IR power still present in the beam (for reasons discussed below), comparisons on the basis of strictly visual-blue component from DPSS-type lasers remain problematic, and the information is often not available. Because of the higher neodymium harmonic used, and the lower efficiency of frequency-doubling conversion, the fraction of IR power converted to 473 nm blue laser light in optimally configured DPSS modules is typically 10–13%, about half that typical for green lasers (20–30%).^[8]
Blue lasers can also be fabricated directly with InGaN semiconductors, which produce blue light without frequency-doubling. 450 nm blue laser diodes are currently available on the open market. The devices are brighter for the same power than 405 nm violet laser diodes, since the longer wavelength is closer to the peak sensitivity of the human eye. Mass production of laser diodes for commercial devices like laser projectors have driven down prices.
Red and red-orange
These are the simplest pointers, as laser diodes are available in these wavelengths. The pointer is no more than a battery-powered laser diode. The first red laser pointers released in the early 1980s were large, unwieldy devices that sold for hundreds of dollars.^[4] Today, they are much smaller and generally cost very little. In recent years,^[when?] diode-pumped solid-state (DPSS) red laser pointers emitting at 671 nm became available. Although this wavelength can be obtained directly with an inexpensive laser diode, higher beam quality and narrower spectral bandwidth are achieved through DPSS

 Violet

Main article: Blue laser

Lasers emitting a violet light beam at 405 nm may be constructed with GaN (gallium nitride) semiconductors. This is close to ultraviolet, bordering on the very extreme of human vision, and can cause bright blue fluorescence, and thus a blue rather than violet spot, on many white surfaces, including white clothing, white paper, and projection screens, due to the widespread use of optical brighteners in the manufacture of products intended to appear brilliantly white. On ordinary non-fluorescent materials, and also on fog or dust, the color appears as a shade of deep violet that cannot be reproduced on monitors and print. A GaN laser emits 405 nm directly without a frequency doubler, eliminating the possibility of accidental dangerous infrared emission^{[citation needed]}. These laser diodes are mass-produced for the reading and writing of data in Blu-ray drives (although the light emitted by the diodes is not blue, but distinctly violet). As of September 2011^[update], 405 nm blue-violet laser diode modules with an optical power of 250 mW, based on GaN violet laser diodes made for Blu-ray disc readers, had reached the market from Chinese sources for prices of about US$60 including delivery.^[9]
At the same time, a few higher-powered (120 mW) 404–405 nm "violet" laser pointers have become available which are not based on GaN, but use DPSS frequency-doubler technology from 1 watt 808 nm GaAlAs infrared diode lasers. As with infrared-driven green laser pointers above, such devices are able to pop balloons and light matches, but this is as a result of an unfiltered high-power infrared component in the beam.^[10] See the section on hazards below, for the difficulties with frequency-doubled IR-pumped lasers.