There are increasing demands for mid-infrared lasers at 3-5 μm, which overlaps with the transparency window of atmosphere, for their potential applications in various fields, including laser surgery, spectroscopy, infrared countermeasures, environmental monitoring, and laser communication. There are lots of approaches to achieve lasers at 3-5 μm band, which can be roughly divided into two major categories. The first category is based on population inversion, namely the linear method, which includes HF/DF gas lasers, semiconductor cascade lasers, fiber lasers, and solid-state lasers (typically Fe∶ZnSe or Fe∶ZnS lasers). The second category is based on the nonlinear effect, including optical parametric oscillators (OPO, typically using PPLN and ZGP crystals as nonlinear media), difference frequency generation (DFG), and frequency doubling. Compared to these lasers, the Fe∶ZnSe or Fe∶ZnS lasers, emitting at the mid-infrared range of 4-5 μm, enjoy several advantages, including high efficiency, wide wavelength-tuning range, and compactness of optical cavity. As a consequence, lots of efforts have been made in the development of Fe∶ZnSe/Fe∶ZnS lasers in the past decade. For some practical applications, continuous-wave (CW) Fe∶ZnSe/Fe∶ZnS lasers are required. Several CW Fe∶ZnSe lasers, all of which operates at the wavelengths of over 4 μm, have been demonstrated by using various CW pumping sources, including the Cr:ZnSe laser, Er:YAG laser, Er:ZBLAN fiber laser, and Er:Y₂O₃ laser. Limited by matured pump sources at ~3 μm, CW Fe∶ZnSe lasers were seldom domestically reported. In this paper, a CW Fe∶ZnSe laser at ~4 μm is demonstrated by using a self-developed continuous-wave Er-doped fiber laser at 2.8 μm.

The pump source used in our study is a continuous-wave Er-doped fiber laser at ~2.8 μm developed in our lab. It has a maximum output power of about 4 W. The gain medium, i.e., the Fe∶ZnSe crystal is 3.5 mm in length and has a cross section of 10 mm×10 mm, with the Fe²⁺ ion concentration of 1.0×10¹⁹ cm^-3. For obtaining effective CW lasing, the crystal is wrapped in a piece of indium foil and clamped to a copper mount cooled to ～77 K by liquid nitrogen in a cryostat, owing to the lifetime of the upper laser level in a Fe∶ZnSe crystal being as short as 370 ns at room temperature while around 57 μs at 77 K. Longer lifetime of upper laser level makes continuous-wave emission much easier. The faces of the gain crystal are anti-reflection coated at 2.7-4.8 μm. Windows of the cryostat are 3 mm CaF₂ plates with AR coated at 2.7-4.8 μm. In the laser cavity arrangement, the feedback is a special coated CaF₂ plano-concave mirror, while the output mirror is another CaF₂ plano-concave mirror with a coupling ratio of ~35%. The radius of each cavity mirror is identical at 50 mm. A dichroic mirror is placed with an incidence angle of ~45° to separate the pump beam and output laser beam. An uncoated CaF₂ lens with a focal length of 50 mm is used to couple the pump beam into the crystal. Unlike the previous demonstrations of CW Fe∶ZnSe lasers, the pump direction in our experiment is counter-pumping, i.e., the pump beam and laser beam propagate in opposite directions.

An optical spectrum analyzer is used to monitor the lasing of the 4 μm signal. When the pump power is increased to about 0.4 W, there is a little peak in captured spectrum. Subsequently, fixing the pump power slightly higher than the threshold, we adjust the cavity mirrors and the coupling lens to maximize the output power. The output power as a function of pump power is recorded and shown in Fig. 3. The maximum power is 0.97 W, which is limited by the pump capability. The slope efficiency is fitted to be ~38.6%, which nearly approaches the limited efficiency of the current laser. The measured laser spectra are shown in Fig. 4. The central wavelength shifts from 3747.1 nm at low output power to 3773.3 nm at high output power with a signal to noise ratio (SNR) of as high as 40 dB, which is also named with “red-shift” and is common in free-running solid-state lasers. The wavelength in this laser is approximately 3.8 μm. The laser spot indicates that the beam profile has a desirable fundamental Gaussian distribution.

A watt-level high efficiency 3.8 μm mid-infrared all-solid-state continuous wave laser is demonstrated in this study. The output power of 0.97 W with a central wavelength at 3.8 μm with a slope efficiency of 38.6% from Fe∶ZnSe crystal is obtained by employing a self-developed continuous-wave Er-doped fiber laser at 2.8 μm as the pump source. The pump source is employed under liquid nitrogen cooling, and the beam profile has a desirable fundamental Gaussian distribution.