Monday, December 27, 2010

Nie lah Laser Hijau Yang Menjadi Isu Hangat Sekarang.

Green laser pointer

Green laser pointers 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 due to 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 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.

A frequency-doubled green laser pointer, showing internal construction. Cells and electronics lead to a laser head module (see lower diagram) This contains a powerful 808 nm IR diode laser that pumps a Nd:YVO4 laser crystal, that in turn outputs 1064 nm light. This immediately is doubled inside a non-linear KTP crystal, resulting in green light at the half-wavelength of 532 nm. This beam is expanded and infrared-filtered. In inexpensive lasers the IR filter is inadequate, or is omitted.

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