Tantalum pentoxide waveguide amplifier and laser for planar lightwave circuits

Abstract

A planar lightwave circuit (PLC) has been envisioned to provide a new generation of optical networks capable of delivering signal at high speed and bandwidth to the household. High index contrast (HIC) and optical gain in the same material system would substantially enhance integration of different optical devices in a small area and compensate for the losses in the system to realise low cost, dense multi-functional PLCs. This thesis investigates the use of tantala as a HIC material system for realising gain efficient Erbium doped waveguide amplifiers (EDWAs) and lasers to be used at 1.5µm wavelength, towards realising dense multi-functional PLCs.

Slab waveguides were fabricated by magnetron sputter deposition under optimised conditions of a powder pressed, Er:Ta₂O₅ target onto an oxidized silicon substrate. Optimised sputtering process yielded a Er:Ta₂O₅ thin film with a refractive index of 2.105 @ 1550 nm and a maximum erbium lifetime of 2.3 ms. Single mode rib waveguides were designed and the fabricated using photolithography and argon ion beam milling. A maximum propagation loss of 0.65 ± 0.05 dB/cm at 1600 nm was measured, the peak erbium absorption and emission cross-section was determined to be 4.8 ± 0.2 x 10^-21 cm² and 4.4 ± 0.2 x 10^-21 cm² respectively.

Numerical modeling of Er:Ta₂O₅ based EDWA predicted a maximum gain of 4 dB/cm at 200 mW pump power, in a 5.4cm long waveguide with an erbium concentration of 5.4 x 10²⁰ ions/cm³. Gain measurements were performed on a 2.3cm long rib waveguide with a erbium concentration of 2.7 x 10²⁰ ions/cm³, at a pump power of 200mW, and a net optical gain of 2.25dB/cm peaking at 1531.5 nm was measured in a 2µm wide waveguide. The pump threshold with respect to the launched pump power was measured to be as low as 4.5mW. The cavity was formed by affixing two mirrors at the end facets of the waveguides. Lasing was observed in a single longitudinal and transverse mode peaking between 1556 and 1560nm. The lasing threshold of 14mW with a slope efficiency of 0.3% was measured with respect to the launched power.

Finally, a feasibility study for inscribing sub-micron grating structures on the Er:Ta₂O₅ waveguides were carried out using interferometric ablation. Gratings inscribed with 23 mJ/cm² energy density and 1000 pulses yielded a maximum reflectivity of 11dB for TE polarisation at 1505nm. This feasibility study shows potential to realise integrated cavity line narrowed lasers and filters. Tantala has long been used for different photonic applications but gain at 1.5 μm is demonstrated for the first time. The results presented in the thesis demonstrate that tantala due to its HIC, net optical gain and other inherent properties that it possesses have the potential to realise low cost, compact PLCs for the short haul networks.

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Identifiers

ORCID for James S. Wilkinson: orcid.org/0000-0003-4712-1697

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