Fiber laser to achieve mid-infrared laser output, the first step is to select the appropriate fiber matrix material. In near-infrared fiber lasers, quartz glass matrix is the most common fiber matrix material with very low transmission loss, reliable mechanical strength and excellent stability. However, because of the high phonon energy (1150 cm-1), quartz fiber can not be used for mid-infrared laser transmission. In order to achieve low loss transmission of mid-infrared laser, we need to re-select other fiber matrix materials with lower phonon energy, such as sulfide glass matrix or fluoride glass matrix. Sulfide fiber has the lowest phonon energy (about 350 cm-1), but it has the problem that the doping concentration cannot be increased, so it is not suitable for use as a gain fiber to generate mid-infrared laser. Although the fluoride glass substrate has a slightly higher phonon energy (550 cm-1) than the sulfide glass substrate, it can also achieve low-loss transmission for mid-infrared lasers with wavelengths less than 4 μm. More importantly, the fluoride glass substrate can achieve a high rare earth ion doping concentration, which can provide the gain required for mid-infrared laser generation, for example, the most mature fluoride ZBLAN fiber for Er3+ has been able to achieve a doping concentration of up to 10 mol. Therefore, fluoride glass matrix is the most suitable fiber matrix material for mid-infrared fiber lasers.

Recently, the team of Professor Ruan Shuangchen and Professor Guo Chunyu at Shenzhen University developed a high-power femtosecond pulse fiber laser composed of 2.8μm mode-locked Er:ZBLAN fiber oscillator, single-mode Er:ZBLAN fiber preamplifier and large-mode field Er:ZBLAN fiber main amplifier.

Based on the self-compression and amplification theory of mid-infrared ultra-short pulse controlled by polarization state and numerical simulation work of our research group, combined with nonlinear suppression and mode control methods of large-mode optical fiber, active cooling technology and amplification structure of double-ended pump, the system obtains 2.8μm ultra-short pulse output with an average power of 8.12W and a pulse width of 148 fs. The international record of the highest average power achieved by this research group was further refreshed.

Figure 1 Structure diagram of Er:ZBLAN fiber laser based on MOPA structure
The structure of the femtosecond laser system is shown in Figure 1. The single-mode double-clad Er:ZBLAN fiber of 3.1 m length was used as the gain fiber in the preamplifier with a doping concentration of 7 mol.% and a core diameter of 15 μm (NA = 0.12). In the main amplifier, a double clad large mode field Er:ZBLAN fiber with a length of 4 m was used as the gain fiber with a doping concentration of 6 mol.% and a core diameter of 30 μm (NA = 0.12). The larger core diameter makes the gain fiber have lower nonlinear coefficient and can withstand higher peak power and pulse output of larger pulse energy. Both ends of the gain fiber are fused to the AlF3 terminal cap.