Four-Level Lasers

Four-level lasers offer a distinct advantage over tlieir tliree-level counterjiarts, (figure C2.15.5). The Nd YAG system is an excellent example of a four-level laser. Here tlie tenninal level for tlie laser transition, 2), is unoccupied tlius resulting in an inverted state as soon as any atom is pumped to state 3. Solid-state systems based on tliis pumping geometry dominate tlie marketplace for high-power laser devices. 

Figure C2.15.5. (a) A four-level laser energy level diagram and (b) tire Nd YAG system.

A further advantage is the higher efficiency of the alexandrite laser because of its being a four-level laser. In the illustration in Figure 9.2(c), level 4 is a vibronic level and level 3 the zero-point level of the T2 state. Level 2 is a vibronic level of the 2 state and level 1 the zero-point level. Because of the excited nature of level 2 it is almost depopulated at room... 

Figure 9.23 Schematic arrangement of energy levels and transitions in a four-level laser. Internal transitions are marked with dotted lines.

Dye lasers are usually pumped by another laser, and a selection of around 25 dyes typically provides coverage of a wide region from 350 to 900 nm. Organic dye lasers are four-level lasers, even though only two electronic levels may be used (Figure 1.19). 

General Principles Two-, Three- and Four-level Lasers... 

The mechanism consists of using a conventional energy source (flash-lamp or other) to excite atoms or molecules from the ground state to some excited state, so that an inversion of population occurs in the Arrhenius55 sense this is usually best understood in a three-level laser, although two-level lasers are also discussed (see Problem 10.10.1), and many are four-level lasers (see Fig. 10.14). 

The trivalent neodymium ion is a good example of a four-level laser system. The trivalent Nd3+ system is illustrated in Fig. 12.12 in a schematic fashion. 

Lasing action has been observed with other rare earths. The lower laser level in holmium is close to the ground state and can operate as a four level laser at liquid nitrogen temperatures. Erbium is similar to holmium. With other rare earths, alternate wavelengths can be obtained. Both Ho and Er operate in a region greater than 1.5 pm. Erbium can also... 

Pumping-emission cycle for a typical four-level laser. 

SO undergoes rapid relaxation to the ground state, maintaining the population inversion (whereupon the laser process can recommence). Neodymium is thus said to act as a four-level laser. 

Figure 14.13 Schematic energy-level diagram for four-level laser such as Nd-YAG. Non-radialive processes are shown as black arrows.

Describe the advantage of a four-level laser system over a three-level type. 

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