Significance Mid-infrared lasers operating in the wavelength region of 2.5-5.0 mu m, which overlap both the atmospheric window and the so-called molecular fingerprint region, have found a growing number of applications in defense, medical treatment, and advanced scientific research. Compared with other laser sources, rare-earth-ion-doped fiber lasers exhibit the advantages of power scalability, wavelength tunability, thermal management, and beam quality, and are thus a promising approach for generating high-power mid-infrared laser emission. Of the different types of mid-infrared rare-earth-doped fiber lasers, Er-doped fluoride fiber lasers are the most studied because they can be conveniently pumped by laser diodes and have a mature fiber material base. Er ions provide two significant mid-infrared emission bands in fluoride glass hosts, namely, similar to 2.8 and similar to 3.5 mu m, which correspond to I-4(11/2)-> I-4(13/2) and F-4(9/2)-> I-4(9/2) transitions, respectively. With the development of ZBLAN fibers and ZBLAN fiber-based devices, the performance of mid-infrared Er-doped ZBLAN fiber lasers has benefitted significantly. In this study, we review the recent research progress of 2.8 mu m and 3.5 mu m continuous-wave (CW) Er-doped fluoride fiber lasers, including improvements in output power scaling, efficiency, and wavelength extension. Progress For 2.8 mu m Er-doped fluoride fiber lasers, output power scaling has been mostly limited by the self-termination of I-4(11/2)-> I-4(13/2) transition induced by the long lifetime of the lower laser level. The most popular approach to depopulate the lower laser level is to use heavily Er-doped fluoride fibers (typically >7 %) via the strong energy upconversion transfer (ETU) process (Fig. 2). In a previous study, a record output power of 41.6 W was obtained using this approach in a dual-end pumped Er-doped fluoride fiber laser; to date, this remains the highest mid-infrared laser output power obtained from rare-earth-doped fibers (Fig. 3). However, the high quantum defect-induced heat accumulation subjects the fiber tip to thermal damage, particularly in this type of heavily doped fiber. To reduce the heat load, researchers proposed a 2.8 mu m/1.6 mu m cascaded lasing scheme, depopulating the lower laser level via I-4(13/2)-> I-4(15/2) transition instead of through phonon relaxation (Fig. 4). This method does not rely on the ETU process and thus allows for the use of lightly Er-doped fluoride fibers for output power scaling. Based on this approach, a 10 watt-level 2.8 mu m Er-doped fluoride fiber laser with a high efficiency of 50 % (as compared with absorbed 0.98 mu m pump power) was demonstrated (Fig. 5). In addition, Er ions have shown strong excited state absorption (ESA, I-4(13/2)-> I-4(9/2)) at 1.6-1.7 mu m, and the overlap with the ground state absorption (GSA) spectrum enables the Er-doped fluoride fiber to be directly pumped by the 1.6-1.7 mu m fiber laser (Fig. 6). Through this new pumping scheme, the efficiency of 2.8 mu m Er-doped fluoride fiber lasers has been increased to greater than 50 %. For 3.5 mu m Er-doped fluoride fiber lasers, the major milestone is the development of the 0.98 mu m+2 mu m dual-wavelength pumping scheme (Fig. 8), wherein the Er ions are first pumped at 0.98 mu m via GSA to provide initial ion accumulation at the I-4(11/2) level or virtual ground state (VGS) and are then pumped at 2 mu m to populate the F-4(9/2) level through VGS absorption (VGSA). This pumping scheme effectively addresses the bottleneck induced by ion accumulation in the longer-lived levels, based on which a 15 W record output power at 3.55 mu m has been obtained from an all-fiber Er-doped fluoride fiber laser (Fig. 9). The broad emission band of F-4(9/2)-> I-4(9/2) transition with the aid of a wavelength selector enables the operating wavelength of Er-doped fluoride fiber laser to be further extended. For example, a 2 W output at 3790 nm has been achieved with a fiber Bragg grating (FBG)-based all-fiber configuration. Continuous wavelength tuning over a 450 nm span (3.33-3.78 mu m) was also demonstrated using diffraction grating (Fig. 10). Recently, by optimizing the cavity arrangements, our group further extended the operating wavelength to 3810 nm, which is the longest wavelength achieved with Er-based lasers. In addition, single-frequency operation and pulsed laser emission have also been demonstrated in this long-wavelength region. Conclusions and Prospects With the development of laser diodes, soft glass fibers, and fiber devices, rare-earth-doped fiber-based mid-infrared laser sources have rapidly developed in recent years. Due to their advantages of pumping convenience and their use of fiber materials, Er-doped fluoride fiber lasers are some of the most widely researched lasers. Many studies have reported major breakthroughs in output power scaling and operating wavelength extension. Future development of Er-doped fluoride fiber lasers should focus on the following two directions. First, new fiber glass with a high damage threshold and broad transparent windows should be developed, and existing fiber fabrication techniques should be improved to provide a high-performance gain medium for mid-infrared lasers. Second, mid-infrared fiber-based devices should be further developed using FBG and signal/pump combiners. These devices are the means by which mid-infrared oscillators and amplifiers with all-fiber configurations can be produced. Through these approaches, we believe that mid-infrared fiber lasers will eventually progress from mere laboratory status to more practical uses, thereby promoting the development and progress of technology in the industrial, medical, defense, and related fields.
