Yttrium-Aluminum-Garnet (YAG) Lasers

Yttrium-Aluminum-Garnet (Y₃Al₅O₁₂), YAG, is a clear and transparent crystal that is most commonly used as the host crystal for neodymium impurity atoms in Nd:YAG lasers. Even though the lasing transition occurs in the Neodymium (Nd) ions, these lasers are often called YAG lasers. Typically 1 - 2% of the Y is replaced by Nd in these lasers. Similar to the ruby laser, here again the crystal not only hosts the atom, but also its broad absorption bands (in the presence of the Nd-impurity) effectively absorb optical radiation - mostly around 700 nm and 800 nm. This energy is then transferred to the impurity ions through non-radiative processes.

Similar to the level diagram for the ruby laser, the above diagram is a simplified representation of the transitions for the neodymium ion, Nd+++, that take place in the YAG laser. Again, the dashed arrows indicate non-radiative transitions, where the energy lost in them is transferred as heat to the crystal. But unlike the ruby laser, the Nd:YAG is a four-level laser and has much more efficient lasing transitions than the ruby laser. Because of this efficiency in population inversion it can be pumped to produce a wide variety of laser output energies and can be operated in the cw as well as pulsed modes. The lasing photon has a wavelength of 1064 nm, well in the IR range.

Another host used for Nd is plain glass, but its optical and thermal properties are not as desirable as YAG's. The trade off is that growing large YAG crystals is not easy. A typical YAG rod, which is drilled out of a crystal block, is smaller than 1 cm in diameter and from a few to 10 cm in length. Still, these lasers can produce high output powers by using several rods in tandem. Pulsed YAG lasers can produce high peak powers by Q-switching and/or by using several rods as amplifiers. When used as amplifiers, the rods are not coated for reflectivity to produce optical amplification in the rod. Instead, the output of the first rod (the oscillator) is used as a Q-switch to generate stimulated emission in the next rod (the amplifier), and so on.

In the cw mode YAG lasers are pumped either by an arc lamp or a semiconductor laser. Pumping with a diode laser is more efficient because the diode's wavelength can be chosen to closely match the absorption of the YAG. Flashlamps are often used for pulsed operation of these lasers. In this mode Q-switching the laser can produce ns pulses with relatively high peak powers. But, to reach very high powers, like that generated by the NOVA laser, many rod amplifications are necessary.

YAG lasers are extremely versatile. They are used in applications from welding and drilling to range finding. Because their output wavelength is not visible, YAG lasers are used in many applications that require secrecy; from military to security applications. It is now possible to convert IR radiation into the visible range using so called non-linear crystals (also known as second-harmonic-generators). When high density photons impinge on such a crystal, the non-linear properties of its index of refraction causes the crystal to absorb two long wavelength photons and generate, in their place, one photon with twice their energy and therefore half their wavelength. Using these crystals, the YAG laser wavelength can be shortened to 532 nm (1064 nm/2), which is seen by the eye as green light. Today's green-colored laser pointers are diode-pumped YAG lasers with doubling crystals.

Although we can change the wavelength of the YAG laser from IR to green (and even further into UV, via another second harmonic generation), this is not a continuous tuning. Other interesting solid-state lasers, called vibronic lasers, use effective bands as their lower lasing level. In this way these lasers can produce a tunable output; i.e. one with a continually changeable wavelength. The most commonly used laser of this type is the Ti:sapphire laser, which is an aluminum oxide crystal, the same as ruby's host, but doped with titanium instead of chromium atoms.