The neodymium-YAG laser

Laser action can be induced in Nd ions embedded in a suitable solid matrix. Several matrices, including some special glasses, are suitable but one of the most frequently used is yttrium aluminium garnet (Y3AI5O12), which is referred to as YAG. 

A krypton arc lamp may be used for CW pumping or a flashlamp for much higher power pulsed operation. 

The Nd YAG rod is a few centimetres long and contains 0.5 to 2.0 per cent by weight of Nd. In pulsed operation the peak power of each pulse is sufficiently high for generation of second, third or fourth harmonics at 533 nm, 355 nm and 266 nm, respectively, using suitable crystals. 

---

Figure 8.16 illustrates the energy levels of Nd in yttrium aluminium garnet (Y3AI5O12), which are involved in the laser action of this crystal (known as the neodymium YAG laser). Describe the processes that occur when the laser is working. 

Fig. 1. Some useful laser sources in the visible and ultraviolet spectrum. For infrared lasers and also a more complete listing of laser lines see, for example, ref. 1. YAG (II) and YAG (III) signify the second and third harmonics, respectively, of the neodymium YAG laser. KDP, potasium dihydrogen phosphate, and KPB, potassium pentaborate, are frequency doubling crystals.

Geyer O. Management of large, leaking, and inadvertent filtering blebs with the neodymium YAG laser. Ophthalmology 1998 105 983-987. 

FIGURE 20.19 The neodymium-YAG laser operates between two excited states of the neodymium ions. 

In Figure 20.19 we sketch the operation of the neodymium-YAG laser. The laser medium is a crystal of yttrium-aluminum-garnet doped with neodymium ions. This laser has the advantage over the ruby laser that the laser action occurs between two excited states, and the population inversion is consequently easier to maintain. The Nd-YAG laser is widely used in science and technology. One major application is to pump the so-called dye lasers in which the medium is intensely colored dye molecules (usually with conjugated double bonds) dissolved in... 

Most of the interest in Y3AI5O12 (YAG) has been in its application as a host material for rare-earth ion lasers. As of 1981, Kaminskii, in his book Laser Crystals, lists 45 rare-earth ion lasers with YAG as the host material. The neodymium YAG laser is, beyond a doubt, the most popular solid-state laser in existence today (of the 45 lasers listed by Kaminskii, 23 are with neodymium). 

Neodymium and YAG Lasers. The principle of neodymium and YAG lasers is very similar to that of the ruby laser. Neodymium ions (Nd +) are used in place of Cr + and are often distributed in glass rather than in alumina. The light from the neodymium laser has a wavelength of 1060 nm (1.06 xm) it emits in the infrared region of the electromagnetic spectrum. Yttrium (Y) ions in alumina (A) compose a form of the naturally occurring garnet (G), hence the name, YAG laser. Like the ruby laser, the Nd and YAG lasers operate from three- and four-level excited-state processes. 

The term solid-state laser refers to lasers that use solids as their active medium. However, two kinds of materials are required a host crystal and an impurity dopant. The dopant is selected for its ability to form a population inversion. The Nd YAG laser, for example, uses a small number of neodymium ions as a dopant in the solid YAG (yttrium-aluminum-gar-net) crystal. Solid-state lasers are pumped with an outside source such as a flash lamp, arc lamp, or another laser. This energy is then absorbed by the dopant, raising the atoms to an excited state. Solid-state lasers are sought after because the active medium is relatively easy to handle and store. Also, because the wavelength they produce is within the transmission range of glass, they can be used with fiber optics. 

Neodymium and its compounds have a number of important uses. One is in a kind of laser known as a neodymium yttrium aluminum garnet (Nd YAG) laser. A laser is a device for producing very bright and focused light of a single color. The Nd YAG laser is used for treating bronchial cancer and certain eye disorders. The bronchi are air tubes that lead into the lungs. 

All these problems can be prevented as much as possible by using modern lasers as light source for Raman spectroscopy, for example, Neodymium-YAG laser. The intensity of the light can often be around 0.5 Watts. This laser device is installed in most modern Raman spectrometers. 

Variations of the above apparatus are used in a number of laboratories. Several experiments utilize a type II KDP crystal to generate the third harmonic at 353 nm instead of the fourth harmonic at 264 nm. To obtain a higher repetition rate at a sacrifice in temporal resolution, the glass laser can be replaced with a mode-locked neodymium YAG laser. 

In metabolic cytometry, cells are incubated with a substrate that is tagged with a highly fluorescent dye we prefer tetramethylrhodamine because of its excellent spectroscopic properties and its compatibility with the frequency-doubled neodymium YAG laser. This substrate is prepared at high concentration and undergoes chromatographic purification to eliminate fluorescent impurities. 

For the highest peak powers (at lower-pulse repetition rates and average powers), the dye laser is pumped (and mixed) with harmonics of the g-switched neodymium-YAG laser. Higherpeakpowers resultfrom a more efficient pump wavelength (green, 532 run) for rhodamine dyes, coupled with a shorter pump pulse length (of 5-9 nsec). 

In the NIR side, the best lanthanides are 5dterbium, erbium, and neodymium (it is used in the Nd YAG laser, see Section 3.1.5). Holmium is less investigated but shows a very interesting emission spectrum with a red peak and an NIR peak. 

The Nd-YAG laser is one of the most widely used solid-state lasers. The lasing medium consists of neodymium ion in a host crystal of yttrium aluminum garnet. This system offers the advantage of being a four-level laser, which makes it much easier to achieve population inversion than with the ruby laser. The Nd-YAG laser has a very high radiant power output at 1064 nm, which is usually frequency doubled (see page 175)... 

This is in contrast to lasers based on mby or neodymium in glass, which operate at much lower pulse-repetition rates. Nd YAG lasers are often operated as frequency-doubled devices so that the output is at 532 nm. These lasers are the most common type of soHd-state laser and have dominated sohd-state laser technology since the early 1970s. Nd YAG lasers having continuous output power up to 1800 W are available, but output powers of a few tens of watts are much more common. 

The light source for excitation of Nd YAG lasers may be a pulsed flashlamp for pulsed operation, a continuous-arc lamp for continuous operation, or a semiconductor laser diode, for either pulsed or continuous operation. The use of semiconductor laser diodes as the pump source for sohd-state lasers became common in the early 1990s. A variety of commercial diode-pumped lasers are available. One possible configuration is shown in Figure 8. The output of the diode is adjusted by composition and temperature to be near 810 nm, ie, near the peak of the neodymium absorption. The diode lasers are themselves relatively efficient and the output is absorbed better by the Nd YAG than the light from flashlamps or arc lamps. Thus diode-pumped sohd-state lasers have much higher efficiency than conventionally pumped devices. Correspondingly, there is less heat to remove. Thus diode-pumped sohd-state lasers represent a laser class that is much more compact and efficient than eadier devices. 

Neodymium-doped yttrium-aluminum garnet is among the most commonly applied laser material and has broad application (neodymium-YAG). 

---