Fs-laser fabrication of photonic structures inside phospate glasses
Compact rare earth doped optical amplifiers and laser devices have emerged as a very important aspect of many optical communications devices. Advanced 3-D micromaching of sub-surface optical waveguides, amplifiers and laser devices using tightly focused femtosecond laser (fs) laser pulses is rapidly becoming an important alternative to standard industrial 2-D fabrication techniques, such as photolithography. Fs-laser micromachining is a technique that uses relatively simple equipment to achieve three-dimensional device fabrication with inexpensive software, all while avoiding the use of photolithography and clean room facilities. The photo inscription of waveguides, however, intrinsicaly depend on the bulk glass substate used for fabrication. Phosphate glasses are an important substrate because they can incorporate high concentrations of rare earth ions, making them an ideal host material for fabricating compact high gain waveguide lasers and amplifiers that operate in the telecommunications window. However, many phosphate glasses will demonstrate changes to the glass structure after fs-laser writing that are not ideal for the fabrication of waveguiding devices. My research focuses on how phosphate glasses, as well as specific phosphate glass compositions, will affect the fabrication of waveguiding structures using focused femtosecond laser pulses. In this project we examine the fundamental relationships between the initial composition of phosphate glasses and the structural changes associated with refractive index modification that result from fs-laser irradiation.
Fig. 1. Microscope images of fs-modified zinc phosphate glass written with fs-laser fluences of 19 J/cm2 (1) 30ZnO-10Al2O3-60P2O5 glass (2) 50ZnO-50P2O5 glass (3) 55ZnO-45P2O5 glass (4) 60ZnO-40P2O5 glass (5) 65ZnO-35P2O5; (a) white light images of the modification along the waveguide direction (b) Transmission white light images of the modification cross section (c) 660 nm transmission near field images (d) Waveguide near field output guiding profile for 60ZnO-40P2O5 glass (e) Waveguide far field output profile for 60ZnO-40P2O5 glass.
Changes to the localized glass network structure and the formation of refractive index change of phosphate glasses upon femtosecond laser irradiation can be studied using scanning confocal micro-Raman and fluorescence microscopy. Systematic changes in the Raman spectrum and the excited photoluminescence indicate atomic level changes to the phosphate network that depends on the femtosecond laser writing conditions and initial phosphate glass composition. Through this research, which is done in collaboration with Prof. R. K. Brow at MS&T, we have discovered a specific zinc polyphosphate glass composition that has demonstrated ideal micro-structure changes to the glass in that can be used to fabricate subsurface single-pass waveguide amplifiers using fs-laser waveguide writing techniques (Fig 1). The results demonstrate that the exact glass composition should be taken into account when fabricating waveguide devices in phosphate glasses, in order to both expand the fs-laser processing conditions and maximize favorable morphological changes for 3-D photonic devices.