Continuous - wave operation at 2 . 1 , um of a diode - laser - pumped , Tm - sensitized Ho : Y 3 AI 50 12 laser at 300 K

The 2.1-/im 5I7-518 laser transition in Ho3+ is attractive because of its long upper laser level lifetime, of the order of 5 msec, and its eye-safe wavelength. However, this laser typically is cooled to liquid-N2 temperatures to achieve continuous-wave (cw) or high-repetition-rate operation because its lower laser level is in the ground-state manifold, a few hundred inverse centimeters above the ground state, and therefore is populated at room temperature. Despite the three-level nature of this transition, we initiated a study of diodelaser pumping because the long upper laser level lifetime may permit larger energy storage than Nd:YAG for the same continuous pump power and because of the interest in eye-safe wavelengths for coherent radar and medical applications. We recently reported the first cw, diode-laserpumped operation on this transition at room temperature' by codoping a Ho:YAG crystal with Tm3 +. Here, the Tm3+ ion is pumped by a diode laser followed by energy transfer to populate the Ho 517 upper laser level. Cw operation at room temperature was previously achieved in Ho3+-doped garnets codoped with Cr3+ and Tm3+ by pumping with a Kr+ laser into the Cr3+ absorption bands.2' 3 Energy is transferred to Tm3 + and then to the Ho3 + 5I7 upper laser level. Pulsed room-temperature operation also is possible under flash-lamp pumping6 by codoping7 the crystal with other ions such as Er3+, Tm3+, and Cr3+ to absorb a larger fraction of the pump power and achieve higher pumping densities. In this Letter we report in more detail on 300-K operation in Ho:YAG codoped with Cr3+ and Tm3 +. Diode-laser pumping at 77 K was demonstrated in Ho:YAG. 8 Diode-laser-pumped operation of the 946nm transition in Nd:YAG with a similar three-level nature at 300 K was reported recently, indicating that population in the lower laser level is not a fundamental difficulty.9 Our work is the first reported demonstration of the use of codoping for lowering the threshold and obtaining room-temperature operation of a three-level laser system under diode-laser pumping. The work also points to the possibility of overcoming the large quantum defect by an energy-transfer process. Figure 1 shows the energy-level diagram and pumping scheme. The Tm3+ 3F 4 manifold is pumped by the AlGaAs diode laser. This is followed by either radiative and nonradiative relaxation to the H4 state or a cross-relaxation process between adjacent ions, Tm( 3 F4 3 H4 )-Tm( 3 H 6 3 H4 ), which converts one F4 excited state into two H4 states. For high enough Tm3+ doping densities this cross-relaxation process can be efficient and lead to an overall pump quantum efficiency of nearly 2. There is fast spatial energy migration among the Tm3+ ions, subsequent energy transfer to the Ho 5I7 energy level, and finally laser action on the Ho 5I7-518 transition.2 In diode-laser pumping Cr3+ plays no active role, but it is convenient to have Cr3+ in the crystal to allow for pumping into the 4T2 Cr3+ absorption band by a higher-power source such as a Rhodamine 6G (R6G) dye laser or a Kr+ laser. The laser scheme for pumping with one of these sources is similar to that for diode pumping except for an additional Cr3+ to Tm3+ energy-transfer step.2