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摘要
Significance: 3 μm laser sources have a broad application prospect in the fields of national defense and security, biomedicine, spectral analysis, and so on. Compared with the nonlinear frequency conversion and semiconductor laser technologies, laser-diode pumping for Er-doped lasers to emit 3 μm wavelength is a more direct and efficient method. The radiation from energy level 4I11/2 to 4I13/2 of Er3+ ions produces 3 μm lasing. Since the lifetime of the upper level is shorter than that of the lower level, it is difficult to realize a continuous laser operation due to the "self-termination effect" from the classic laser theory. However, due to the inherent complex energy transfer process and Stark sub-level splitting of the Er3+ doped system, an efficient 3 μm laser continuous wave operation has been realized in many Er-doped matrices such as Er: YAG, Er: YLF, and Er: Y2O3. The main known problems linked with Er-doped 3 μm lasers are self-termination and large quantum losses. If the Er3+ ions are doped into low phonon matrix, the probability of non-radiative transition is reduced and the fluorescence decay time at the upper laser level 4I11/2 is prolonged. This fact results in a low probability of self-termination. To reduce the self-termination effect in high phonon matrix, a high Er-doping level is required. With the increase of doping level, the spacing between Er3+ ions is shortened, which is beneficial to ion-ion energy transfer. However, using the high doped active medium affects thermal conductivity, which makes the thermal management of the laser system more difficult. The Er-doped laser gain medium is pumped by a semiconductor laser of 0.97 μm to produce a 3 μm laser with a large quantum loss. Theoretically, the laser efficiency is limited to ~33%, which means that two-thirds of the pump power is wasted and deposited into the gain medium as parasitic heat, resulting in degradation of laser performance. Researchers have established a mathematical model to estimate the theoretical limit of the emission efficiency of 3 μm Er: YAG laser, and they have found a simple analytical expression for the emission efficiency, which shows that the theoretical quantum efficiency can reach 59.8% due to the existence of an ETU process. So far, the maximum efficiency of 3 μm lasers with Er3+-doped gain media in experiments has reached to 50% in Er: LiYF with doping concentration (atomic fraction) of 15 %. This result proves that with an optimized doping concentration, the 3 μm laser efficiency can be effectively improved due to the ETU process. Using laser materials with a low Er3+ doping concentration and cascading two transitions (4I11/2→4I13/2 →4I15/2), where the first 4I11/2→4I13/2 transition corresponds to the mid-IR ~3 μm laser and the second 4I13/2 →4I15/2 corresponds to the eye-safe 1.6 μm spectral region, respectively, provide a number of important benefits, including the increased overall efficiency of optical output and thermal management. In addition, the second eye-safe transition (4I13/2 →4I15/2) effectively depletes the lower laser level and sustains a positive inversion, as required for a CW operation. Progress: The 3 μm laser performance in various Er-doped host materials is summarized (Table 1), including continuous wave output, Q-switched pulse output, mode-locked pulse output, and 1.6 μm and ~3 μm cascading outputs, etc. At present, the 3 μm laser products are commercially available with Er: YAG and Er: YLF as gain media. Due to the strong heat generation inside Er: YAG with 50% atomic fraction, the side-pumping design is carried out, which results in a poor beam quality. Sesquioxides, which have a low phonon energy and high thermal conductivity, have immerged as a promising laser host material for ~3 μm laser operation in recent years. An efficient laser operation could be obtained with the sesquioxides at a low Er3+ concentration. With 2%(atomic fraction) Er: Y2O3 ceramics as laser gain media, researchers have obtained a 14 W laser output at 2.8 μm and cryogenically cooled temperature of 77 K (Fig. 4). In 2016, the output power is further increased to 24 W with a slope efficiency of 24% under the liquid nitrogen cooled condition. Research group from Jiangsu Normal University has obtained a 3.8 W laser output at 2.7 μm at room temperature with 7% (atomic fraction) Er: Y2O3 ceramic, and the output power has been increased to more than 10 W. Conclusion and Prospects: With its recent breakthrough in terms of output power and laser efficiency, the erbium-doped 3 μm laser has become an object of intense scientific research. With the improvement of high-quality low phonon energy laser gain media, especially the development of ceramic gain media, the 3 μm laser performance can be further improved by optimizing the wavelength and spectral linewidth of pump sources, doping concentration of Er-doped laser gain media, and laser cavity parameters. Compared with semiconductor laser and nonlinear frequency conversion technologies, the laser-diode pumped Er-doped laser emitting 3 μm wavelength is very promising, especially in the pulsed laser operation for producing high peak power and large pulse energy.
