Waveguide lasers in ytterbium doped tantalum pentoxide for integrated photonic circuits

Aghajani, Armen (2015) Waveguide lasers in ytterbium doped tantalum pentoxide for integrated photonic circuits. University of Southampton, Physical Sciences and Engineering, Doctoral Thesis, 134pp.

Record type: Thesis (Doctoral)

Abstract

This thesis investigates ytterbium doped tantalum pentoxide as a material system for realising waveguide lasers on silicon substrate, as a basis to provide the next generation of mass-producible, low-cost planar devices with many interlocking photonic circuits for multi-functionality.

Numerical modelling of symmetrical rib waveguide in Ta₂O₅ was carried out finding an optimum waveguide design for a near-circular mode profile with maximum confinement of light within the core with various etch depths and rib widths. A numerical study investigating the feasibility of integrated Kerr lens elements for future mode-locking was carried out, predicting that a Kerr lens slab with a length of 20 µm with input and out waveguides was able to achieve a 9% modulation depth for input intensity of 6 GW/m².

Thin films were fabricated by RF magnetron sputtering onto a silicon substrate with 2.5 µm silica layer from a powder pressed Yb:Ta₂O₅ target, with shallow rib waveguides realised using photolithography and ion beam milling. The excited-state lifetime of Yb:Ta₂O₅ was measured to be 0.25 ± 0.03 ms, and peak emission and absorption cross-sections were determined to be 2.9 ± 0.7×10^-20 cm² and 2.75 ± 0.2×10^-20 cm² respectively, with the fluorescence spectrum giving a broadband emission from 990 nm to 1090 nm.

Finally a 10.8 mm long waveguide laser in Ta₂O₅doped with ~ 6.2×10²⁰ Yb atoms/cm³ of ytterbium oxide was demonstrated with a laser cavity formed from a combination of high reflective mirrors with output couplers or reflections from the bare end-facets with Fresnel reflectivity of 12%. In a 5.4 µm wide waveguide, lasing was observed between 1015 nm and 1030 nm when end-pumped with a 977 nm laser diode with the highest output power of 25 mW at a wavelength of 1025 nm with an absorbed pump power of 120 mW for a cavity formed by a high reflector mirror and a bare end-facet at the output. In this case, the absorbed pump power threshold and slope efficiency were measured to be ~ 30 mW and ~ 26% respectively. The results presented in this thesis demonstrate that tantalum pentoxide has great potential for mass-producible, integrated optical circuits on silicon using conventional CMOS fabrication technologies.