For fiber laser which is used before 1980s, a pump laser is directly irradiated to the fiber core for pumping (fiber laser with direct cre-pumping). Although the damage threshold of silica glass, the main component used in optical fiber, is as high as ~2 GW/cm2 @CW, high power pumping of SMF by the direct core-pumping was difficult in terms of both the technique and material (especially when spatial-output LD is used), since the core diameter is very small. In the case of using multi-mode fiber (MMF), the direct core-pumping distorts the beam quality, and is not usable for the processing, which requires high power and a good beam quality.
In 1988, a method for realizing high power fiber laser with a good beam quality was suggested. The fiber laser was double clad fiber (DCF) laser. DCF laser became common so fast. Nowadays, DCF is a mainstream method for pumping high power fiber laser. DCF can be adapted for large mode area fiber.