Remote sensing, spectroscopy, medicine and military countermeasures are among the applications that would benefit from efficient, compact mid-infrared lasers. Fiber lasers have proved both efficient and compact sources in the near-IR, but because dopants that lase in the mid-IR often require bulky, nondiode pump sources, those few mid-IR fiber lasers developed so far have not been particularly efficient or compact.Figure 1. When lasing at 2840 nm, a holmium laser can be pumped from the ground level directly to the upper laser level. Depending on the wavelength of the resonator feedback, lasing can occur at either 2840 or 2100 nm.Holmium-doped fiber lasers lase in the 2- to 3-μm spectral region, but several serious problems with these devices have prevented their adoption into many practical applications. One was the long spontaneous lifetime of the lower laser level of the 2.9-μm transition (5I7, Figure 1), approximately 3 ms. Several years ago, Stuart D. Jackson at the University of Sydney in Australia showed that co-doping a Ho-doped fiber laser with praseodymium could depopulate the lower level through a series of multi-photon decay processes. Pumping a Pr:Ho:fiber laser with an 1100-nm Yb:fiber laser, he produced up to 2.5 W at 2.9 μm.Now Jackson and his university colleagues have increased the practicality of the Ho:fiber laser even further by demonstrating that it can be directly pumped with a diode laser rather than with the diode-pumped fiber laser he used in the earlier experiments. They took advantage of recently developed high-power, high-brightness diode lasers based on highly strained InGaAs quantum wells that produce output at 1148 nm. Holmium absorption (5I8 to 5I6) is significantly stronger at 1148 nm than at the 1100-nm wavelength of the previous fiber laser pump. Figure 2. The holmium-rich fluoride fiber produced more power than the praseodymium-rich fiber. (The output power (Pout) is plotted here as a function of launched pump power (PL), and L is the length of fiber in each laser.) Reprinted with permission of Optics Letters.The scientists experimented with three types of fiber — two fluoride (ZBLAN) and one silica — and with several lengths of each type. Jackson explained that they selected resonator optics that forced the fluoride fiber lasers to lase on the 2.9-μm transition (5I6 to 5I7) and the silica fiber lasers to lase on the 2.1-μm transitions (5I7 to 5I8).None of the fibers were co-doped with a sensitizer to enhance absorption of pump energy into the holmium; such sensitizers bring with them disadvantageous energy transfers that increase laser threshold and reduce energy-storage efficiency. The doping levels of the two fluoride fibers were markedly different: The first was doped with 1000 parts per million (ppm) molar holmium and 20,000 ppm molar praseodymium; the second with 30,000 ppm molar holmium and 3000 ppm molar praseodymium. All three fibers had only a single cladding and could be efficiently core-pumped by the high-brightness InGaAs diode lasers. All the lasers based on these fibers oscillated in a single transverse mode. Figure 3. The 2.1-μm output from the silica fiber laser resulted from a transition in the holmium ion different from the 2.9-μm output from the fluoride fiber lasers. Reprinted with permission of Optics Letters.Of the two fluoride fibers, the one with relatively high holmium doping generated more power (Figure 2), but its spectrum was shifted to longer wavelengths than that of the fiber with relatively high praseodymium doping (Figure 3). Jackson said that the reason is the praseodymium depopulation of the lower laser level (5I7) in the praseodymium-rich fiber. Population buildup in the lower Stark levels of 5I7 in the holmium-rich fiber forced the laser transition to terminate on higher Stark levels, increasing its wavelength.However, in this case at least, the praseodymium doping seemed to do more harm than good, as indicated by the lower power of the praseodymium-rich fiber in Figure 2. The scientists believe that the diminished performance may result from a parasitic absorption or from ion-ion interactions caused by the praseodymium doping.The slope efficiency of the silica fiber was approximately the same as that of the better of the two fluoride fibers, but the total output was lower (Figure 2). Pump-induced excited- state absorption from the upper laser level (5I7) of the 2.1-μm transition reduced the slope efficiency of the silica fiber laser from that previously observed with 1100-nm pumping.Optics Letters, Sept. 1, 2007, pp. 2496-2498.