Tantalum Pentoxide Integrated Photonics: A Promising Platform for Low-Loss Planar Lightwave Circuits with Low Thermo-Optic Coefficients

Abstract

Photonic integrated external laser cavities are transformative components in laser frequency stabilization and linewidth narrowing applications. A key challenge in contemporary photonic integration is to realize low-loss waveguides while balancing the thermo-optic response of the cavities. Tantalum pentoxide (Ta₂O₅) has emerged as a promising photonic platform due to its low thermo-optic coefficient (TOC) and low material loss. Incorporating Ta₂O₅/SiO₂ waveguides into a photonic circuit requires trade-offs among waveguide loss, device footprints, thermorefractive noise, optical I/O efficiency, and other desired functionalities. In this work, we present a Ta₂O₅ photonic platform that emphasizes low propagation loss and low thermo-optic sensitivity. We demonstrate that the intrinsic Q factors of these resonators exceed 10⁶, corresponding to a propagation loss of 0.27 dB/cm at 1550 nm, using our CMOS-compatible fabrication technique. The temperature-dependent wavelength shift (TDWS) of the Ta₂O₅ resonators is merely 9 pm/K. The thermorefractive frequency noise of the Ta₂O₅ microresonators is predicted to be half smaller than that of Si₃N₄ resonators. Several key building blocks of integrated external laser cavities were demonstrated, including high-Q resonators, self-coupled microresonators, Vernier ring resonators, Sagnac loop mirrors, edge couplers, and Y-branch splitters, which contribute to a comprehensive suite of planar lightwave components on the Ta₂O₅ platform. A narrow-linewidth hybrid-integrated laser has been demonstrated, utilizing a distributed feedback (DFB) laser diode self-injection locked to a Ta₂O₅ microresonator. A fundamental linewidth of 1.6 kHz has been obtained. The distinctive optical characteristics of the Ta₂O₅ waveguides could have a broader impact on high-capacity, temperature-robust, and multifunctional photonic integrated circuits, which are essential for the next-generation photonic computing systems, quantum photonic circuits, and beyond.