Continuous and wide-tunable laser sources in the mid-infrared range of 3?5 μm have attracted significant attention in fields such as spectral analysis, remote sensing, medical treatment, environmental monitoring, and optoelectronic countermeasures. Currently, the primary approach to achieve broadly tunable lasers in this wavelength range is nonlinear frequency conversion methods such as those using optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs). These methods have advantages, such as a wide tuning range and technological maturity. However, they typically require high-performance near-infrared pulsed lasers or narrow-linewidth lasers as pump sources, leading to challenges such as an extensive system volume, relatively complex resonators, low optical conversion efficiency, and high costs. In recent years, increasing attention has been paid to schemes that directly generate tunable mid-infrared lasers, including semiconductor quantum cascade lasers and mid-infrared oscillators based on mid-infrared laser gain media. Among them, the Fe∶ZnSe crystal exhibits broad absorption and emission spectra, large absorption and emission cross-sections, as well as low phonon energy, making it one of the best candidate materials for directly generating broadly tunable mid-infrared laser sources in the 3?5 μm spectral range. Since the first Fe∶ZnSe mid-infrared laser was reported by Adams et al. in 1999, many studies have been conducted on Fe∶ZnSe lasers. However, stable continuous-wave (CW) and wavelength-tunable Fe∶ZnSe lasers have seldom been domestically reported beyond 4 μm in the mid-infrared wavelength. In this study, pumped by a homemade Er∶Y₂O₃ ceramic laser, CW and wideband tunable Fe∶ZnSe lasers are demonstrated in the 3?5 μm spectral range.

According to the Füchtbauer?Ladenburg (F?L) equation, the emission cross-section of a Fe∶ZnSe crystal is influenced by its fluorescence spectrum and spontaneous emission lifetime. Moreover, it has been demonstrated that there is a noticeable redshift in the central wavelength of the fluorescence spectrum of the Fe∶ZnSe crystals with increasing temperature. Therefore, the emission cross section of the Fe∶ZnSe crystals is temperature-dependent. By controlling the operating temperature of the Fe∶ZnSe crystals, it is possible to achieve a temperature-induced gain spectrum shift, enabling a wavelength-tunable output of the Fe∶ZnSe laser. In the experiment, the Er∶Y₂O₃ ceramic gain medium has a length of 10 mm and a diameter of 1 mm. The atomic fraction of doped Er³⁺ of the sample is 7%. The two end faces of the ceramic are laser-grade-polished and plated with antireflection films in the 3 μm wavelength band. A compact two-mirror plano?plano resonator is employed for the laser oscillation. The pumping source is a fiber-coupled semiconductor laser with a maximum output power of 100 W centered at 976 nm. The gain medium is mounted on a heat sink and directly water-cooled to remove the heat accumulated during pumping. The temperature of cooling water is maintained at 15 ℃. An output power of 3.77 W with a central wavelength of 2740 nm is obtained using an Er∶Y₂O₃ ceramic laser. Then, the 2740 nm laser is collimated by a convex lens F3. After being reflected by the two flat mirrors, M2 and M3, it is further focused on the Fe∶ZnSe laser resonator as the pump light through a lens F4. The Fe∶ZnSe crystal used in the experiment is 6.5 mm long and has a cross-section of 3 mm×3 mm, with an Fe²⁺ concentration of approximately 5×10¹⁸ cm^-3. The Fe∶ZnSe laser resonator consists of a plano-concave input mirror (M4) with a curvature radius of 100 mm and a plano-plano output mirror (OC2). The output mirror has a transmissivity of approximately 5% in the 4?5 μm wavelength range. The total length of the laser resonator is approximately 68 mm. Based on the ABCD matrix, the laser beam waist radius at the Fe∶ZnSe crystal position is calculated to be ~258 μm. To achieve an effective CW laser operation, it is necessary to cool the Fe∶ZnSe crystal to ensure a sufficiently long upper-level lifetime. In this study, a low-temperature vacuum chamber cooled with liquid nitrogen is designed. The Fe∶ZnSe crystals are wrapped in an indium foil and mounted on a copper heat sink. The copper heat sink is installed on a Dewar inside the vacuum chamber. The vacuum chamber is equipped with CaF₂ window plates on both sides. These window plates are coated with broadband antireflection coatings in the mid-infrared range, ensuring a transmissivity of over 96% for both pump and laser wavelengths.

First, the CW laser characteristics of the Fe∶ZnSe crystal are studied under liquid-nitrogen cooling at 103 K. In the experiment, the maximum output power of the Er∶Y₂O₃ ceramic laser is set to 3.0 W to ensure stable operation of the pumping source. The corresponding pump power incident on the Fe∶ZnSe crystal is approximately 2.17 W owing to the Fresnel reflection losses and absorption of the optical components. The output power of the Fe∶ZnSe laser is measured using a thermal sensor power meter. The laser threshold for the incident pump power is approximately 231 mW. The laser output power exhibites an approximately linear increase with the incident pump power. When the pump power reaches 2.17 W, a CW laser output of 352 mW is obtained, and the slope efficiency of the incident pump power is approximately 19.2%. The laser spectra are measured using a mid-infrared spectrometer. It exhibits a single-peak structure with a central wavelength of 4.17 μm and a spectral bandwidth of approximately 35 nm. Subsequently, the wavelength tuning performance modulated by the temperature of the Fe∶ZnSe laser is investigated. As the temperature of the Fe∶ZnSe crystal increases, the central wavelength of the laser shifts from 4170 nm at 103 K to 4553 nm at 173 K, resulting in a tuning range of 383 nm. In the experiment, the laser successfully achieves a wide spectral tuning range. When the temperature of the Fe∶ZnSe crystal exceeds 173 K, the upper-level lifetime of the Fe∶ZnSe crystal quickly decreases. Therefore, longer wavelengths are not used in this experiment.

By employing a homemade Er∶Y₂O₃ ceramic laser as a pump source, stable CW and wideband tunable Fe∶ZnSe lasers are demonstrated. An average output power of 352 mW is obtained at 4170 nm under liquid-nitrogen cooling to 103 K. Furthermore, the wavelength-tuning performance of the Fe∶ZnSe laser modulated by temperature is investigated, and a continuous tuning bandwidth of 383 nm (4170?4553 nm) is successfully achieved in the experiment.