The brightness of laser is defined by the following equation,
where P is the light power, w0 is the radius of beam waist, and θ is a half angle of far-field beam divergence. Provided the brightness of pumping light, BPump, and the brightness of laser light, BLaser, the ratio of BLaser to BPump is smaller than 1 for the direct core-pumping fiber laser, while it is given by the following equatio for the clad-pumping fiber laser,
where ra, rb is core radius, inner clad radius, respectively, and we have θa = arcsinNAcore, and θb = arcsinNAinner-clad (NAcore is NA of core, and NAinner-clad is NA of inner clad). For a typical clad-pumping Yb fiber fiber laser, BLaser/BPump can be larger than 5000.