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The most important laser types include solid-state [1] , semiconductor [2] , fiber [3] , excimer [4] , and carbon dioxide lasers [5] . Depending on type and operating mode, lasers can emit pulsed [6] or continuous-wave [7] monochromatic [8] light over the full width of the visible spectrum and beyond [9] .
Solid state lasers have a solid gain medium and, in this broad sense, include also semiconductor and fiber lasers, which are usually treated as separate types due to their significance. The gain medium is a single crystal or glass doped with rare earth or transition metal ions. The very common Nd:YAG laser uses a crystalline rod made from synthetic yttrium aluminum garnet (Y3Al5O12) doped with neodymium (Nd). For higher quality light, a yttrium orthovanadate (YVO4) crystal is used. Nd and Y-doped glasses are used in very high-power laser systems for inertial confinement fusion research (see also Fusion, Note 3).
The smallest and cheapest laser with abundant applications in consumer electronics and research is the inconspicuous laser diode (LD), a semiconductor laser similar to a small light-emitting diode (LED) (compare also with transistor and diode). Both, laser diodes and LEDs, are remarkably sophisticated products resulting from a long history of semiconductor development fueled by quantum physics and material sciences. Despite their apparent likeness, the two devices differ in type of semiconductors and dopants, power conversion efficiency (30% for LD vs. 70% for LED), and light output (collimated and coherent (LD) vs. dispersed and incoherent (LED)).
Fiber lasers have a glass optical fiber doped with erbium (Er) or other rare earth elements as the lasing medium. Like in optical telecommunications cables, the light is guided along the flexible fiber, which is covered by two coaxial claddings. Pumping of the central lasing fiber is being achieved by light from laser diodes which is fed into the inner cladding. The fiber laser is closely related to the non-lasing erbium-doped fiber amplifier (EDFA) which plays a key role in modern trans-oceanic cables.
Carbon dioxide lasers are working with a mixture of CO2, N2, and He gases. Electric discharges cause vibration of the N2 molecules, which in turn are transferred to the CO2 molecules, which are then amenable to stimulated infrared photon emission.
The energy of a single pulse can vary from picojoules (pJ) to kilojoules (kJ), the pulse duration can be shortened from milliseconds to attoseconds (10-18 s), and the repetition rate of pulses can range from 1 per second (1 Hz) to hundreds of Terahertz (1012 Hz), depending on type of laser and technology used (see encyclopedia for technologies such as Q-switching, mode locking, and high harmonic generation). Short pulses result in very high power peaks at modest pulse energy and modest average laser power, e.g., a pulse with the duration of 1 microsecond, energy of 1 joule, and repetition rate of 10 hertz, has a peak power of 1 megawatt (1 J / 10-6 s = 106 J/s = 1 MW), while the laser's average power is only 0.1 watt (1 J × 10 Hz = 1 J / 10 s = 0.1 W).
A gas laser was the first laser with continuous wave emission, a mode more difficult to achieve than pulsed emission. The continuous pumping is usually effected by electrical gas discharge.
Lasers do not emit true monochromatic light, but normally have a spectral linewidth of a few kHz, less than a billionth of visible light's frequency band). In addition to the near-monochromatic property, laser light also has other features of coherence, including fixed phase and polarization.
The wavelength of common types of lasers ranges from about 1500 nm (far infrared) to 450 nm (ultraviolet). Dye lasers were the first tunable lasers able to cover the visible spectrum (400-700 nm) through the use of a variety of fluorescing organic dyes (tunable diodes and Ti-sapphire lasers largely replaced dye lasers). So-called free-electron lasers, installed at some large research facilities, can produce narrow-width x-ray beams, down to a wavelength of less than 1 nm.