Holmium doped fibre optimised for resonant cladding pumping

Summary form only given. Two-micron lasers are of great interest in a range of applications, from spectroscopy and free space communications, to polymer machining and laser surgery. They also provide an important stepping-stone for wavelength generation further into the mid-infrared via nonlinear frequency conversion. Holmium doped fibre laser systems offer particularly attractive properties as they allow access to the atmospheric transmission window between 2.1 and 2.25 μm. Recently, operational efficiencies for resonantly pumped Ho:silica fibre close to the quantum limit have been demonstrated for core pumped lasers [1]. To further power scale Ho:fibre lasers, development of cladding-pumped fibre configuration is necessary and indeed has enabled scaling of such fibres to >400 W cw average power [2]. Conventional polymer-coated double-clad fibres have high inner-cladding propagation loss at the ~1.95 μm pump wavelength, resulting from the presence of OH
^-
contamination and absorption in the polymer outer-cladding [3]. Here we report on optimisation of holmium all glass double-clad optical fibre in order to provide efficient operation for cladding pumping at 1.95μm.