On-Chip Erbium-Doped Tantalum Oxide Microring Hybrid Cavity Single-Mode Laser

We demonstrate a high-performance, single-mode Er:Ta2O5 microring laser monolithically integrated on a silicon platform via a customized Damascene process. The Er:Ta2O5 gain medium exhibits a low propagation loss of 0.73 dB/cm and a high intrinsic Q-factor of 5.03 x 105. By utilizing a hybrid cavity_consisting of a microring coupled to a U-shaped waveguide at two symmetric points_we exploit the Vernier effect to achieve robust longitudinal mode selection. Under a non-resonant 1480 nm pumping scheme, the laser yields a side_mode suppression ratio (SMSR) of 53.3 dB and a narrow linewidth of 9.5 pm. A slope efficiency of 2.76 % is achieved_the highest reported to date for Er:Ta2O5 lasers_with a lasing threshold of 3.3 mW. Furthermore, stable single-mode tuning is demonstrated across a temperature range of 18_68 celsius, consistently aligning with theoretical transfer matrix models. This work provides a scalable pathway for high-efficiency, tunable on-chip light sources, bridging the gap for monolithic active-passive integration on the tantalum oxide photonic platform.

💡 Research Summary

The paper reports a monolithically integrated erbium‑doped tantalum oxide (Er:Ta₂O₅) microring laser fabricated on a silicon wafer using a customized Damascene process. A 1 µm‑wide, 450 nm‑thick Er:Ta₂O₅ waveguide is formed by sputtering a Ta₂O₅:Er₂O₃ (99:1 wt %) target at 200 °C, followed by CMP planarization and high‑temperature annealing for Er³⁺ activation. The resulting film exhibits an ultra‑low propagation loss of 0.73 dB cm⁻¹ and an intrinsic quality factor Q ≈ 5.03 × 10⁵ near 1530 nm, corresponding to a 3‑dB resonance bandwidth of 8.4 pm.

The core of the device is a hybrid resonant cavity composed of a microring resonator coupled at two symmetric points to a U‑shaped waveguide. The microring (3 µm wide, 450 nm thick) contains the gain region and adiabatically tapers to a 1 µm width to suppress higher‑order modes, ensuring fundamental TE operation. The U‑shaped waveguide provides a second cavity with a distinct free spectral range (FSR). The mismatch between the two FSRs creates a Vernier envelope that selects a single longitudinal mode from the broad Er³⁺ gain band (1520–1580 nm). This Vernier‑based spectral filtering, together with precise phase matching and gain‑loss balance, yields robust single‑mode operation.

A non‑resonant pump at 1480 nm is launched via lensed fibers (≈6 dB/facet coupling loss at 1550 nm, 7.5 dB/facet at 1480 nm). The pump overlaps the signal mode with >90 % efficiency in the curved sections, while remaining non‑resonant, which makes the laser insensitive to pump linewidth and power fluctuations. The transfer matrix method (TMM) is employed to model the combined cavity, reproducing the experimentally observed Vernier transmission peaks and their temperature‑induced shifts.

Performance metrics are impressive: the laser reaches threshold at ~3.3 mW of coupled pump power, delivers a slope efficiency of 2.76 % (the highest reported for Er:Ta₂O₅ lasers), and produces an on‑chip output power of 72.14 µW at a pump power of 29.24 mW (single‑ended). The emission wavelength is centered at 1556.27 nm with a full‑width at half‑maximum (FWHM) of 9.5 pm, limited by the optical spectrum analyzer resolution, implying an even narrower intrinsic linewidth. Side‑mode suppression ratio (SMSR) reaches 53.3 dB, confirming excellent mode discrimination.

Temperature tuning from 18 °C to 68 °C (10 °C steps) shows a systematic red‑shift of the lasing wavelength from ~1530 nm to ~1555 nm, driven by the thermo‑optic effect and thermal expansion of the hybrid cavity. SMSR remains above 40 dB across most of the range, with a transient multi‑peak regime around 48 °C where gain‑loss imbalance allows simultaneous lasing of several Vernier peaks. The measured wavelength shifts agree well with TMM simulations, though minor discrepancies arise from nonlinear thermal gradients not captured in the static model.

The authors discuss pathways to further improve output power: integrating high‑reflectivity components (e.g., Sagnac loops) at the input port, extending the gain‑region length, and optimizing fiber‑to‑chip coupling to reduce the ~6–7 dB facet loss. They also suggest refining waveguide geometry to further narrow the intrinsic linewidth and exploring wafer‑scale production enabled by the Damascene flow.

In summary, this work demonstrates a high‑efficiency, low‑loss, single‑mode Er:Ta₂O₅ microring laser with unprecedented slope efficiency and SMSR, leveraging a Vernier‑enhanced hybrid cavity and a silicon‑compatible Damascene fabrication route. The results provide a scalable platform for on‑chip light sources in the 1.55 µm telecom band, bridging active and passive photonic integration on tantalum oxide and opening avenues for dense, low‑power photonic circuits.