Analysis of photo-darkening losses in high power Yb doped fiber lasers and amplifiers

Photodarkening(PD) in Yb-doped fiber is one of the power limiting factor for long-time operation in high power fiber lasers and amplifiers. In order to achieve Yb-doped fiber laser and amplifier with high output power, a steady-state rate equations considering the amplified spontaneous emission (ASE) and PD losses has been established. Thermal effects and output power characteristics of high power fiber laser and amplifier are investigated. Transverse and longitudinal temperature distributions in fibers have been calculated by solving thermal conduction equations. We theoretically analyzed the PD losses and temperature in Yb-doped fiber laser and amplifier, the simulation results show that with the PD losses increasing, then the laser power drop quickly and we find that higher doped concentration and larger PD losses will lead higher temperature, so we should choose suitable doped concentration fiber and decrease PD losses to achieve higher output power.