Efficient approach to designing low OH diffusion in the thermally shaped double-clad fibers

Abstract

Hydroxyl groups (OH) penetrate the surface of the fiber preforms as a result of preform processing in the oxy-hydrogen flame. Designing double-clad fibers (DCFs) that have a complex noncircular inner-cladding shape with concave and convex edges are possible by shaping their optical preforms thermally with a CO2 laser. Unlike the mechanical technique, the thermal shaping via CO2 laser has paved the way for fabricating DCFs with various inner-cladding geometries that are not limited to the flat edge. However, due to the thermo-physical effects of the CO2 laser on the preform surface, the OH groups are still present in the drawn fibers as they were diffused in the preform while shaping. The OH groups cause attenuation on certain wavelengths that lay rather close to the commonly pumping wavelengths in DCFs. In this work, we have overcome this issue by approaching low OH diffusion in DCFs drawn of thermally shaped optical preforms. This approach was based on treating the preforms with hydrofluoric (HF) acid prior the CO2 laser shaping process in order to remove a thin layer soaked with OH groups. The OH penetration depth was estimated based on the optical attenuation measurement at water peak to be about ~600 µm from preform surface. By etching just 200 µm of the preform surface followed by laser shaping with ablation depth of 300 µm can reduce the attenuation attributed to OH groups by 43%. Surface scattering of the inner-clad shaped DCFs drawn of the etched preforms was shown to be low compared to the DCFs drawn of ground preforms. The measurements were carried out in multimode fibers drawn from pure silica Heraeus F300 rods and coated with low index material.