Ruby Laser Three-Level Lasers

The first laser produced was the ruby laser, invented in 1960. Rubies are crystals of aluminum oxide (corundum, AI2O3), containing about 0.5% chromium ions Cr3+, as substitution impurities, CrA, and laser action, as well as color, is entirely due to these 

3The multiplicity is written as a superscript to the spectroscopic symbol, representing the energy of the 

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Figure C2.15.4. (a) A three-level laser energy level diagram and (b) the ruby system.

Fig. 3.31. The ruby three-level laser. The pumping nansiiion is 1, the lasing transition 3. The nonradiaiive transition 2 is fast relative to the radiative inverse of 1. The level notations are for Cr +. The orders of magnitude of the relevant rates are p( T2 - Aj) = 10 s .

We have already mentioned the difficulties involved with a three-level laser in which the laser transition is terminated in the continuously well populated ground state. The successful operation of the ruby laser relies on the very favourable combination of broad absorption bands and the long upper-state lifetime allowing the storage of energy. In a 4-level laser., a final level, which is not the ground state, is used. The basic diagram is shown in Fig. 8.9a. 

Figure 4.7 Energy levels in lasers three-level and four-level systems. Schemes for obtaining population inversion in optically pumped systems (a) three-level (ruby) laser (b) four-level (Nd laser) (c) four-level (dye laser). From Ref. [l,c, p. 121].

Photochromic materials change their color upon illumination. In some cases, a quasi-stationary concentration of an exdted state has strong absorption bands, such as triplet-triplet transitions in many aromatic hydrocarbons, or the doublet-doublet transitions of chromium(in) in ruby operating as a three-level laser. In other cases, the color is due to exdted states of the reversible or irreversible product of a photochemical reaction. Many organic molecules can undergo such reactions, like rearrangement between cis- and iran -isomers around double... 

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 efficiency of a ruby laser is less than 0.1 per cent, typically low for a three-level laser. 

The properties of the ruby and Nd-YAG laser are summarized in Table 32.10. The Cr transition is a three-level... 

FIGURE 12.3 Three-level ruby laser. The levels shown are ligand field levels. Level 1 is the A ground state level 2 is the " Ti and Tj excited states, and level 3 is the excited state. 

Solid State Lasers. The first successful laser, and one that is still used, is a three-level device in which a ruby crystal is the active medium. Ruby is primarily AhO, but contains approximately 0.05% chroraium(III) distributed among the aluminum(III) lattice sites, which accounts for the red coloration. The chromium(III) ions are the active lasing material. In early lasers, the ruby was machined into a rod about 4 cm long and 0.5 cm in diameter. A flash tube (often a low-pressure xenon lamp) was coiled around the cylinder to produce intense flashes of light (A = 694.3 nm). Beeause the flashlamp was pulsed, a pulsed beam was produced. Continuous-wave (CW) ruby sources are now available. 

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