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.

由于氢氧火焰中预制棒的加工，羟基（OH）渗透到纤维预制棒的表面。通过用CO ₂激光对光学预制棒进行热成型，可以设计出具有凹凸边缘的复杂非圆形内包层形状的双包层光纤（DCF）。与机械技术不同，通过CO ₂激光进行热成型为制造具有各种内包层几何形状且不限于平坦边缘的DCF铺平了道路。但是，由于CO ₂的热物理效应激光在预成型坯表面上时，在成形过程中，OH基团仍然存在于拉伸纤维中，因为它们在预成型坯中扩散。OH基团会导致某些波长上的衰减，这些波长与DCF中通常泵浦的波长相当接近。在这项工作中，我们通过在由热成型光学预成型件绘制的DCF中实现低OH扩散来克服了这个问题。该方法基于在CO ₂之前用氢氟酸（HF）处理预成型件激光整形工艺，以去除被OH基团浸透的薄层。根据在水峰处距预制棒表面约600 µm的光衰减测量值，可以估算出OH的渗透深度。通过蚀刻仅200 µm的预成型件表面，然后用300 µm的烧蚀深度进行激光成形，可以将OH基团引起的衰减降低43％。与经蚀刻的预成型件绘制的DCF相比，经蚀刻的预成型件绘制的内衬形状DCF的表面散射低。测量是在从纯二氧化硅Heraeus F300棒拉制并涂有低折射率材料的多模光纤中进行的。