
larger core size disappears. Therefore, more advanced alternatives or additional
fiber-design measures are required for extra LMA (e-LMA) fiber design. Known
examples in this regard include helical-core double-clad fibers [77] and HOM
suppression/conversion methods [78].
To allow the high-power MM pump light from laser diode bars to be efficiently
coupled into double-clad fiber, a high NA for the inner-clad “pump-core” is
critical. This is achieved through the use of a low-refractive-index glass preferably,
or polymer as the outer-clad material. In high-power applications, the polymer
near the inner–outer clad interface can easily burn or gradually ...