Mid-infrared (4?5 μm) radiation lies in the atmospheric transmission window and has broad application prospects in fields such as atmospheric remote sensing, environmental monitoring, and space communications. Compared with chemical lasers, nonlinear frequency conversion lasers, and other means of obtaining mid-infrared lasers, solid Fe: ZnSe lasers have the advantages of compact volume and high energy, offering a new way to achieve high-energy mid-infrared laser output. This study presents a high energy mid-infrared solid Fe∶ZnSe laser. We use the Er∶YAG laser as the pump laser and design an Fe∶ZnSe laser system whose crystal temperature can be controlled. The work performance of the Fe∶ZnSe laser is studied at different temperatures. In addition, Fe∶ZnSe laser spectra are obtained at different temperatures.

The Fe²⁺∶ZnSe crystal is sensitive to temperature, which causes a temperature quenching effect at higher temperatures and affects the laser efficiency. When the temperature is above 100 K, the lifetime of the laser upper level decreases rapidly with an increase in temperature, from 60 μs at 77 K to 360 ns at 294 K. To improve the lifetime of the laser upper-level, we use a liquid nitrogen Dewar temperature controller. The Fe∶ZnSe crystals are placed in a low-temperature vacuum chamber. A 2.94-μm Er∶YAG laser with axial pumping is incident on the crystal surface. The maximum output energy of the Er∶YAG laser is 3 J, and its pulse width is 50?300 μs, which comprises multiple spike pulses with a duration of several hundred nanoseconds. The resonant cavity is formed by a flat input mirror M1 and flat output coupler M2 with a cavity length of 50 mm. The input mirror M1 exhibits >98% transmittance for the pump laser and >99.5% reflectivity for the Fe²⁺∶ZnSe laser, whereas the output coupler M2 exhibits >99.9% reflectivity for the pump laser and 70% reflectivity for the Fe∶ZnSe laser. The energy density of the pump light incident on the cavity can be adjusted by changing the optical interval of the telescope. An iris is used to adjust the size of the pump spot incident on the Fe∶ZnSe crystal; in addition, it is used to suppress transverse parasitic oscillations. Previous research has shown that smaller pump spots can suppress transverse parasitic oscillations and improve laser efficiency. In this study, three Fe∶ZnSe crystals are grown via the vertical Bridgman method and simultaneously doped during growth with a higher doping uniformity. The crystal size of crystal #1 is 20 mm×20 mm×4 mm, with a doping concentration of 5×10¹⁸ cm^-3. Crystals #2 and #3 have the same size, and their doping concentrations are 0.9×10¹⁸ cm^-3 and 4.5×10¹⁸ cm^-3, respectively.

At 79 K, the maximum output energy of the Fe∶ZnSe laser is 1.04 J with the slope efficiency of 36.4% and optical-to-optical conversion efficiency of 37.8% at a pump energy of 2.75 J [Figs. 2(a) and(b)]. Figure 2(a) shows the output energy and slope efficiency of different Fe∶ZnSe crystals. Because of the difference in the doping concentration and gain length, the absorption and slope efficiencies of the crystals are different. The total absorptivities of crystals #1 and #3 are similar; therefore, the slope efficiency of the Fe∶ZnSe laser is also similar. Owing to the low doping concentration of crystal #2, the total absorption of the pump light is only 69%; therefore, the laser slope efficiency is lower than those of the other two crystals. The temporal profiles of the Fe∶ZnSe laser are shown in Figs. 2(c)?(f) and Figs. 3(a) and (b); it can be observed that the Fe∶ZnSe laser waveform remains strongly correlated with the pump laser waveform, and the width of a single spike pulse shortens with the increase in temperature. The output spectrum of the Fe∶ZnSe laser is shown in Fig. 3(d). The output spectrum redshifts with increasing temperature, and the tunable range widens.

In this study, a low-temperature Fe∶ZnSe laser is fabricated using an Er∶YAG laser as the pump energy source. The Fe∶ZnSe laser has the potential to produce large amounts of energy in the mid-infrared region. At 79 K, the output energy of the Fe∶ZnSe laser is 1.04 J with a slope efficiency of 36.4% and an optical-to-optical conversion efficiency of 37.8% at a pump energy of 2.75 J; the wavelength of the Fe∶ZnSe laser is 4.1 μm. At the thermoelectric cooling temperature of 240 K, the energy of the Fe∶ZnSe laser is 50 mJ with a wavelength of 4.4 μm and pump energy of 500 mJ. The Fe∶ZnSe laser introduced in this study has many potential applications in mid-infrared fields, such as environmental monitoring and laser communication, which provides a basis for further miniaturization and fabrication of wavelength-tunable Fe∶ZnSe lasers.