Double clad fiber lasers open the new world of high power lasers for industrial applications with high beamquality power delivering. The physics in the absorption process of double clad fiber is described related to thespatial hole burning effect. The homogeneous absorption is essential to achieve high saturation power. Thebasic physics of fiber lasers is described in a manner of general physics of lasers. A fiber-embedded disk in aside pumping scheme is proposed from the basic principle. The fiber disk laser is promising for the kW outputfrom a single fiber with multimode core. A photonic fiber is introduced as a future model of high power andhigh performance fiber lasers.
