The development of a compact mid-infrared (mid-IR) laser operating over a 2–10 μm spectral range has been a challenge for several decades. This range contains the atmospheric transparency window that allows for easy passage of radiation to the earth’s surface. Many important atmospheric constituents have absorption lines in the 2–10 μm “molecular fingerprint” region. Mid-IR lasers are thus suitable candidates for applications in space optical communications, as well as in remote sensing, trace gas analysis, laser surgery, and medical diagnosis. Lasers based on II–VI compounds doped with transition metals (such as Fe-doped binary and ternary chalcogenides) with a gain bandwidth of up to 50% of the central wavelength constitute a viable route for broadly tunable mid-IR coherent sources. They can provide very high power levels with good beam quality, but realizing these in practice has been a challenge. While some sources have been developed, the output energy levels have been unacceptably low. N.-S. Myoung, S.B. Mirov, and colleagues from the University of Alabama at Birmingham have now achieved energy scaling in a 4.3 μm Fe:ZnSe mid-IR laser at room temperature by optimizing the fabrication technique based on post-growth thermal diffusion of Fe in polycrystalline ZnSe.

As reported in the January 1st issue of Optics Letters (DOI:10.1364/OL.36.000094; p. 94), the researchers investigated Er:Cr:YSGG laser-pumped Fe:ZnSe lasing in a Fabry–Perot cavity in the temperature range of 236–300 K. Thermal diffusion of Fe was carried out in sealed quartz ampoules at 10^-5 Torr for 14 days at 1000°C, which resulted in a highly concentrated (Fe concentration 2 × 10¹⁹ cm^-3) gain element. High gain in the developed active medium ensured reduced oscillation build-up time, improved temporal overlap of the pump pulse (20 ns) and output oscillation (15 ns), and effective absorption of the pump pulse, leading to increased output energy. A fourfold increase in the output energy of the gain-switched Fe:ZnSe laser was obtained. The maximum laser output energy was 4.7 mJ at 4.3 μm and 236 K, and 3.6 mJ at 4.37 μm and 300 K (limited by available pump energy) with maximum obtained power of 0.3 MW at 4.3 μm. The researchers said that additional improvements can be achieved by further optimization of the laser cavity and also by using a pump with higher energy.