Experimental demonstration on improvement of photo-darkening in tandem-pumped Yb-doped optical fiber lasers

It is previously reported that the photo-darkening (PD), which means the background loss increases due to the generation of defect center during the laser operation, becomes worse as the higher the population inversion of Yb ions. Therefore, the tandem-pumped configuration with a relatively low excitation density has been theoretically predicted that PD can be alleviated, but there is no experimental demonstration yet. In this paper, photo-darkening of tandem-pumped fiber lasers was experimentally measured and compared with that of diode-pumped fiber lasers. Test-fiber for PD measurement in this paper is Yb-doped fiber (YDF) having a Yb-doped core of 20 µm in diameter, cladding of silica 400 µm in diameter which is widely used in construction of high power fiber lasers. The optimal length for YDF is theoretically analyzed using commercial software RP Fiber Power (RP Photonics Consulting GmbH) including the population inversion per unit length and Yb3+ excitation level along the fiber. To accelerate PD, the YDF was core-pumped which results in high population inversion, and a laser diode at 633 nm was used as a probe source. The transmitted 633 nm probe power for a 1018 nm tandem-pumped YDF was decreased gradually and it reduced by 1.9 % after 2 hours. On the other hand, the transmitted probe power for a 976 nm diode -pumped YDF was reduced by 18.5 % after 2 hours. The transmitted loss at 633 nm probe source in the tandem-pumped YDF is 9.7 times smaller than that of the diode-pumped one. Therefore we confirmed experimentally that the tandem-pumping can significantly mitigate the loss due to photo-darkening in the fiber laser.