Functionalization of optical fibers by selective laser etching for sensing applications

Selective laser etching (SLE) is a promising subtractive manufacturing technology to create highly precise 3-dimensional structures in glass. The process itself is based on two main steps. First, the designed structure is transferred inside the glass by a fs-pulsed laser. Subsequently, the sample is placed in a lye where only the pre-structured material is removed. The employed liquid for the post-processing step depends on the used glass type. We developed this process for the functionalization of standard optical glass fibers. This happens mainly in two ways, i.e. direct and indirect modification of the fiber. For the latter, lens-like structures were written as molds in fused silica substrates. Subsequently, the mold is filled with photoresist, the fiber is dipped into the mold and the structure is cured by external UV-illumination. During the direct structuring process of the fiber, a part of the fused silica cladding is modified by a fs-laser and subsequently etched within a KOH-solution. Removing a sufficient part of the cladding gives access to the fiber core which can be functionalized by adding, i.e., molecular switches on that area, enabling multi-parameter optical fiber sensing or further integration of optical filter structures on top of the fiber core, by employing two-photon polymerization, respectively.