Research Progress of LD-Pumped 3 μm Er-Doped Solid-State Lasers

Significance 3 mu m 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 mu m wavelength is a more direct and efficient method. The radiation from energy level I-4(11/2) to I-4(13/2) of Er3+ ions produces 3 mu m 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 mu m 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 mu m 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 I-4(11/2) 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 mu m to produce a 3 mu m laser with a large quantum loss. Theoretically, the laser efficiency is limited to similar 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 mu m 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 mu m 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 mu m laser efficiency can be effectively improved due to the ETU process. Using laser materials with a low Er3+ doping concentration and cascading two transitions (I-4(11/2) -> I-4(13/2) -> I-4(15/2)) , where the first I-4(11/2) -> I-4(13/2) transition corresponds to the mid-IR similar to 3 mu m laser and the second I-4(13/2) -> I-4(15/2) corresponds to the eye-safe 1. 6 mu m 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 (I-4(13/2) -> I-4(15/2) ) effectively depletes the lower laser level and sustains a positive inversion, as required for a CW operation. Progress The 3 mu m 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 mu m and similar to 3 mu m cascading outputs, etc. At present, the 3 mu m 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 similar to 3 mu m 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 mu m 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 mu m 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 mu m 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 mu m 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 mu m wavelength is very promising, especially in the pulsed laser operation for producing high peak power and large pulse energy.

Automated summary

3 micron lasers with Er-doped gain media have wide applications and potential for high efficiency, with research focusing on optimizing doping concentration and material parameters for improved laser performance.