Enhanced cladding pump absorption of ytterbium-doped double cladding fiber with internally modified cladding structures

We report an ytterbium-doped 20/400 µm double cladding fiber with an internally modified circular inner cladding. Through four fluorine-doped (F-doped) low refractive index quartz rods embedded into the inner cladding, the cladding pump absorption is considerably increased by 1.5 times. It is found that stimulated Raman scattering (SRS) threshold improves after using shorter fiber lengths. More than 2.1 kW laser output with good beam quality (M 2 = 1.38) has been obtained with a suitable pump power injected, and the slope efficiency of the all-fiber laser oscillator is about 75.4%, when pumped at 915 nm. The present results suggest that low refractive index quartz rods embedded into the inner cladding proves to be an effective way for enhanced cladding pump absorption even with a circular inner cladding, which facilitates fiber splicing and provides a novel and robust fiber design of industry grade ytterbium-doped double cladding fibers for high power fiber laser applications.