Research progress of erbium fluoride-doped fiber lasers in the mid-infrared band with high power and high efficiency
summary
With its high output power, high beam quality and stable and compact system structure, the mid-infrared fiber laser light source with rare earth ion doped fiber as the gain medium has broad application prospects in the fields of medical treatment and cutting-edge scientific research. The erbium-doped fluoride fiber system has two important mid-infrared emission bands of 2.8 μm and 3.5 μm, which has become an important research direction in the field of mid-infrared laser technology due to its convenience of compatibility with semiconductor laser direct pumping and mature fiber material foundation. In this paper, the recent research progress of mid-infrared continuous-wave erbium-doped fluoride fiber lasers is systematically reviewed, and the representative research results on the improvement of output power and efficiency and the expansion of operating wavelength are reviewed and reviewed, and the future development trend of mid-infrared erbium-doped fluoride fiber lasers is briefly prospected around the bottleneck constraints at this stage.
abstract
significance
Mid-infrared lasers operating in the 2.5‒5.0 μm wavelength region overlap with atmospheric windows and so-called molecular fingerprint regions and are increasingly used in defense, medical, and advanced scientific research. Compared to other laser sources, rare earth ion-doped fiber lasers are a promising method for generating high-power mid-infrared laser emissions due to their advantages such as power scalability, wavelength tunability, thermal management, and beam quality. Among the different types of mid-infrared rare earth-doped fiber lasers, erbium-doped fluoride fiber lasers are the most studied because they can be conveniently pumped by laser diodes and have a mature fiber material base. The Er ions provide two important mid-infrared emission bands, ~2.8 μm and ~3.5 μm, in the fluoride glass body, corresponding to the transitions of 4I11/2→4I13/2 and 4F9/2→4I9/2 transitions, respectively. With the development of ZBLAN fibers and ZBLAN fiber devices, the performance of mid-infrared erbium-doped ZBLAN fiber lasers has been significantly improved. In this study, we review the latest advances in 2.8 μm and 3.5 μm continuous-wave (CW) erbium-doped fluoride fiber lasers, including improvements in output power scaling, efficiency, and wavelength expansion.
progress
For 2.8 μm erbium-doped fluoride fiber lasers, the output power scaling is mainly affected by the transition caused by the long lifetime of 4I11/2→4I13/2 by lower laser levels. The most popular method to reduce lower laser levels is to use heavily doped erbium fluoride fibers (typically >7%) through the High Energy Upconversion Transfer (ETU) process (Figure 2). In a previous study, a record output power of 41.6 W was obtained using this method in a double-ended pumped erbium-doped fluoride fiber laser; To date, this is still the highest mid-infrared laser output power ever obtained from a rare-earth-doped fiber (Figure 3). However, the thermal accumulation induced by high quantum defects subjectes the fiber tip to thermal damage, especially in this type of heavily doped fiber. To reduce the heat load, the researchers proposed a 2.8 μm/1.6 μm cascade laser protocol with a 4I13/2→4I15/2 transition rather than by phonon relaxation (Figure 4). This method is not dependent on the ETU process and therefore allows for output power scaling with lightly erbium-doped fluoride fibers. Based on this approach, a 10-watt class 2.8 μm erbium-doped fluoride fiber laser with up to 50% efficiency compared to the 0.98 μm pump power absorbed was demonstrated (Figure 5). In addition, the Er ions exhibit strong excited state absorption (ESA, 4I13/2→4I9/2) and overlap with the ground state absorption (GSA) spectrum, enabling erbium-doped fluoride fibers to be directly pumped by a 1.6–7 μm fiber laser (Figure 6). With this new pumping scheme, the efficiency of the 2.8 μm erbium-doped fluoride fiber laser has increased by more than 50 percent.
For the 3.5 μm erbium-doped fluoride fiber laser, the main milestone was the development of a 0.98 μm + 2 μm dual-wavelength pumping protocol (Figure 8), in which Er ions are first pumped at 0.98 μm by GSA to be absorbed by VGS at the 4I 11/2 level or virtual ground state (VGS), followed by pumping at a speed of 2 μm to fill the 4F9/2 level by VGS absorption (VGSA). This pumping scheme effectively addresses the bottleneck caused by the accumulation of ions at long-lifetime levels, and on top of that, an all-fiber erbium-doped fluoride fiber laser achieves a record output power of 15 W at 3.55 μm (Figure 9). The wide emission band 4F9/2→4I9/2 can further extend the operating wavelength of the erbium-doped fluoride fiber laser with the help of the wavelength selector. For example, with an all-fiber configuration based on a fiber Bragg grating (FBG), a 2 W output can be achieved at 3790 nm. Continuous wavelength tuning over a span of 450 nm (3.33–78 μm) was also demonstrated using a diffraction grating (Figure 10). Recently, by optimizing the cavity arrangement, our group further extended the working wavelength to 3810 nm, which is the longest wavelength achieved by an ER-based laser. In addition, single-frequency action 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 optical fiber devices, mid-infrared laser sources based on rare earth doped fibers have developed rapidly in recent years. Due to their advantages of easy pumping and the use of fiber materials, erbium-doped fluoride fiber lasers are one of the most widely studied. Many studies have reported major breakthroughs in output power scaling and operating wavelength expansion. In the future, the development of erbium-doped fluoride fiber lasers should focus on the following two directions. First, new glass fibers with high damage thresholds and wide transparent windows should be developed, and existing fiber manufacturing techniques should be improved to provide high-performance gain media for mid-infrared lasers. Second, devices based on mid-infrared fibers should be further developed using FBGs and signal/pump combiners. These devices are a means of producing mid-infrared oscillators and amplifiers with all-fiber configurations. With these approaches, we believe that mid-infrared fiber lasers will eventually evolve from a purely laboratory state to a more practical use, driving technological advancement and advancement in industrial, medical, defense, and related fields.
