SELECTIVE   PUMPING

       It is evident that the pumping method must ensure that the upper lasing level is populated while the lower level is relatively empty. For this reason, pumping mechanisms must be designed with forethought. Consider the helium–neon laser. Helium is used as an intermediary that absorbs energy from the electrical discharge (by electron collision) and transfers it, quite selectively, to the upper lasing level of neon, allowing that level to populate while the lower level receives no energy from helium. The lower level will certainly be populated thermally (and to a larger extent than the upper level), but at over 18 eV above ground level, the thermal population of the level will be negligible.

         Other examples of selective pumping exist in solid-state lasers such as the YAG. Examining the absorption spectrum of YAG in Figure 1.1, we see a large absorption band in the red and near-infrared regions of the spectrum around 750 and 800 nm. High powered lamps using inert gases are employed to pump this rod optically. Consider the output spectra of various lamps that may be used for pumping, as shown in Figure 1.2. Xenon lamps, the most common used for optically pumped

Figure 1.1. Absorption spectrum of YAG. (Courtesy of Perkin Elmer OptoElectronics.)

lasers, show an output rich in the blue and green end of the spectrum, where YAG does not absorb light particularly well. Most of the energy output by the xenon lamp in this region is wasted since the YAG rod cannot absorb it. Krypton lamps, on the other hand, have a rich output precisely in the region where YAG absorbs, around

Figure 1.2. Flash lamp spectra. (Courtesy of Perkin-Elmer OptoElectronics.)

750 to 850 nm. Krypton lamps are a more efficient pump source than xenon lamps for YAG rods and are hence used in most commercial lasers of this type. Xenon lamps may still be used as a pump source, but are not as efficient.

        Like the YAG, the ruby laser has defined absorption bands, but these are in the yellow–green region of the spectrum, where xenon lamps have much higher output than krypton lamps, hence xenon lamps are a better match as a pump lamp. Later in the chapter we revisit YAG and ruby lasers.