In recent years, double clads rare-earth doped fiber lasers and amplifiers have been widely used in the fields of industry, medicine, and military for their high power, high reliability, low cost, high beam quality, etc. With the laser power increasing and applied ranges extending, the photo-darkening of Yb3+-doped fiber and active radiation hardening of Tm3+-doped fiber have gained more and more attentions.
Wuhan National Laboratory for Optoelectronics fiber laser technology group (FLTG) Professor Li Jinyan and doctor Zhao Nan firstly studied the 793 nm pump induced photo-bleaching of photo-darkened Yb3+-doped fibers. Compared to LDs with other bleaching wavelengths, the commercial 793 nm LDs are economical pump sources with higher power and various pigtail sizes, and more suitable for the photo-bleaching of PD in double-clad Yb-doped high power fiber lasers. The measurements showed that over 68% PD loss was mitigated. Furthermore, the fiber shows repeatable processes of PD and photo-bleaching when pumped alternately with 915 and 793 nm LDs. Besides, simultaneous pump with 915 and 793 nm LDs also effectively suppressed the PD loss by 80%. All the above indicates that the 793 nm LDs will play very important roles in power stability and recovery of Yb-doped fiber lasers.
Under the guidance of Professor Jin-yan Li and Professor Neng-li Dai, Doctor Ying-bin Xing from Wuhan National Laboratory for Optoelectronics fiber laser technology team has studied the pump bleaching effects of thulium-doped silica fiber under gamma-ray irradiation in the range 50Gy-675Gy. It is found that the higher dose results in the more recovery time, the higher RIA and the lower fiber slope efficiency. The 793nm pump source can bleached the irradiated TDF significantly and the extent of the recovery is also positively correlated with dose. From the experiments and calculation, it follows that the photo-bleaching may be more effective to retrieve the fiber performance than the thermal bleaching. This work can be a good reference for radiation hardening of thulium-doped fiber used in adverse circumstances.
Both works were published in the Optics Express of Optical Society of America (OSA)(2015)( Vol.23, No.19, pp.25272-25278 and Vol.23, No.19, pp.24236-24245) entitled with “793 nm pump induced photo-bleaching of photo-darkened Yb3+-doped fibers” and “Active radiation hardening of Tm-doped silica fiber based on pump bleaching”. The works were financially supported by the National High-Technology Research and Development Program of China (Grant No. 2013AA031501), the National Science Foundation of China (Grant No. 61378070) and Director Fund of WNLO.
Fig. 1. (a) Absorption spectra of the fiber. The inset displays an enlargement of the spectra between 650 nm and 850 nm; (b) evolution of photo-darkening under 915 nm pump and photo-bleaching under 793 nm pump.
Fig.2 Output power of thulium-doped fiber laser versus time under multi-dose
Fig.3 Slope efficiencies of thulium-doped fibers before and after bleaching