Research progress of Er:ZBLAN fiber lasers at the wavelength of 3 μm
Zhang Xin, Shu Shili, Tong Cunzhu     
State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, Jilin 130033, China

Overview: Laser emitting at the wavelength of 3 μm has great demand for a wide range of scientific and technological applications, including military, medicine and communication. The laser emitting at this special wavelength can be generated by using crystals, glass, semiconductors, and gases as gain media. Compared with these gain media, Er doped ZBLAN (Er:ZBLAN) fiber used as gain media for 3 μm laser has larger surface area and volume ratio, which is conductive to heat dissipation. Its special waveguide structure is also conductive to high beam quality. And it can be pumped by 976 nm diode. Therefore, the 976 nm pumped Er:ZBLAN fiber is a common method to realize laser emitting at 3 μm. In this paper, the recently research progress of 3 μm Er:ZBLAN fiber laser is reviewed from both continuous and pulsed directions. For CW 3 μm Er:ZBLAN fiber laser, spatial coupling and all-fiber structure are two main methods for power scaling. Spatial coupling is a common and easy to realize method, but the end face of Er:ZBLAN fiber is easily damaged due to thermal accumulation and deliquescence. However, all-fiber structure does not need to consider the damage of the end face caused by thermal accumulation and the coupling efficiency is higher than that of spatial coupling. It is reported that only the University of Laval has realized the all-fiber structure emitting at 3 μm based on fluoride fiber Bragg grating, and recently the power has been further increased to 41.6 W. The fluoride fiber Bragg grating is the key device for all-fiber structure to achieve this high power. So the research of fluoride fiber device is important for the development of Er:ZBLAN fiber laser. For pulsed 3 μm Er:ZBLAN fiber laser, Q-switched and mode-locked are two main methods to realize Er:ZBLAN fiber laser pulse emmiting. Active and passive Q-switched has been used to the accomplish the Q-switched Er:ZBLAN fiber laser. Compared to the passive Q-switched method, the active Q-switched can get higher peak power. In order to accomplish femtosecond 3 μm Er:ZBLAN fiber laser, the mode-locked method was also used, including nonlinear polarization evolution and saturable absorber.

At present, the power of both CW and pulsed 3 μm Er:ZBLAN fiber laser still have a large room for improvement. The multi-stage pulse amplification can rise laser energy, especially for femtosecond 3 μm Er:ZBLAN fiber laser. In order to breakthough the power limit of single laser, the fiber combining will be the best choice to improve the power of CW and pulsed 3 μm Er:ZBLAN fiber laser.

Supported by National Natural Science Foundation of China (61790584)