Tantalum pentoxide waveguides and microresonators for VECSEL based frequency combs

Abstract

Tantalum pentoxide (Ta₂O₅) is a promising material for mass-producible, multi-functional, integrated photonics circuits on silicon, exhibiting robust electrical, mechanical and thermal properties, as well as good CMOS compatibility. In addition, Ta₂O₅ has been reported to demonstrate a non-linear response comparable to that of chalcogenide glass, in the region of 3-6 times larger than that of materials such as silica (SiO₂) or silicon nitride (Si₃N₄). In contrast to Si-based dielectrics, it will accept trivalent ytterbium and erbium dopant ions, opening the possibility of on-chip amplification. The high refractive index of Ta₂O₅ is consistent with small guided mode cross-section area, and allows the construction of micro-ring resonators. Propagation losses as low as 0.2 dB=cm have been reported. In this paper we describe the design of a planar Ta₂O₅ waveguides optimised for the generation of coherent continuum with near infrared pulse trains at kW peak powers. The Pulse Repetition Frequency (PRF) of the VECSEL can be tuned to a sub-harmonic of the planar micro-ring and the optical pump power applied to the VECSEL can be adjusted so that mode-matching of the VECSEL pulse train with the micro-ring resonator can be achieved. We shall describe the fabrication of Ta₂O₅ guiding structures, and the characterisation of their nonlinear and other optical properties. Characterisation with conventional lasers will be used to assess the degree of coherent spectral broadening likely to be achievable using these devices when driven by mode-locked VECSELs operating near the current state-of-art for pulse energy and duration.

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