Diode lasers generate red light, but their performance at elevated temperatures degrades so badly that they are not suitable for laser projection displays. Figure 1. Blue pump light, incoming from the right, pumps the Pr:LiYF4 laser on the right, which lases in the red. The orange light visible is spontaneous emission from the praseodymium. The seemingly violet light scattered from the lens is probably the result of red and blue light on the photographic film. Frequency-doubled infrared lasers offer an alternative, but a solid-state laser that lases directly in the red spectral region would have the advantage of simplicity. Recently, scientists at Keio University in Yokohama, Japan, demonstrated a diode-pumped Pr:LiYF4 laser that lased directly in the red at 639 nm (Figure 1). Figure 2. The red-emitting laser was pumped with a blue GaN diode laser. Reprinted with permission of Optics Letters (R= reflectivity, T= transmission, LD = laser diode). Pumped with a GaN diode laser from Nichia Corp. of Tokushima, Japan, the solid-state laser was configured in a straightforward linear resonator (Figure 2). A series of aspheric and cylindrical lenses collimated the 444-nm output of the diode laser and focused it into the 4-mm-long laser crystal, but these lenses accounted for an approximate 10 percent loss of the pump light. When temperature-controlled with a Peltier cooler, the diode produced up to 500 mW of blue pump power, but at the higher end, it generated a secondary peak at 447 nm that was not useful for pumping. Figure 3. The laser’s output did not decrease significantly at temperatures encountered in laser projection displays. Reprinted with permission of Optics Letters (η= slope efficiency). At room temperature, the laser produced as much as 112 mW from 334 mW of absorbed pump power and, significantly, the performance did not degrade much at higher temperatures. Laser projector temperatures typically reach 320 K or higher, but the output of the Pr:LiYF4 laser decreased only slightly as its temperature reached 350 K or even 380 K (Figure 3). Optics Letters, Sept. 1, 2007, pp. 2493-2495.