Er:Ta₂O₅ Microring Laser

• Er:Ta₂O₅ microring laser is a compact on-chip light source that integrates an erbium-doped tantalum oxide gain medium for efficient, single-mode operation at C-band wavelengths.
• It employs a hybrid microring-U-waveguide cavity with a customized Damascene process to achieve low propagation loss, high Q-factor, and robust Vernier-mode selection.
• Demonstrated metrics include a 53 dB SMSR, 2.76% slope efficiency, and temperature tunability, paving the way for scalable integration in silicon photonics.

The Er:Ta₂O₅ microring hybrid cavity single-mode laser is a monolithically integrated on-chip light source employing an erbium-doped tantalum oxide (Er:Ta₂O₅) gain medium within a microring resonator, coupled to a U-shaped waveguide on a silicon substrate. This structure leverages a customized Damascene fabrication process to achieve low propagation loss, high intrinsic Q-factor, robust single-mode selection via the Vernier effect, and efficient, tunable laser operation at telecommunications C-band wavelengths. The device demonstrates record performance in terms of slope efficiency, side-mode suppression, linewidth, and temperature tunability, enabling scalable integration of active and passive photonic elements on silicon platforms (Shui et al., 31 Jan 2026).

1. Context and Motivation

The demand for high-quality on-chip light sources in the 1.5 µm telecommunications band is central to the development of silicon photonics for optical communications, microwave photonics, and sensing applications. Erbium-doped oxide waveguides, specifically Er:Ta₂O₅, offer a unique combination of strong optical gain in the C-band (1500–1577 nm), high refractive index ($n ≈ 2.1$), low intrinsic loss, and compatibility with CMOS fabrication workflows. Conventional on-chip Er:Ta₂O₅ lasers have been hampered by low slope efficiency (typically $\lesssim 0.3\,\%$), poor single-mode performance, and low fabrication yield due to sidewall roughness and incomplete trench filling. Recent advances leverage a hybrid microring-U-waveguide cavity design and process innovations to overcome these challenges (Shui et al., 31 Jan 2026).

2. Device Architecture and Fabrication

2.1 Damascene Process for High-Quality Er:Ta₂O₅ Waveguides

• The device is fabricated on a silicon wafer with a 10 µm thermal SiO₂ undercladding.
• Submicron-deep (450 nm) and 1–3 µm wide trenches are defined using electron-beam lithography (EBL) and inductively coupled plasma (ICP) etching.
• Thermal reflow at approximately 1000 °C for one hour yields smooth trench sidewalls with approximately 110° tilt and sub-nanometer surface roughness.
• Er:Ta₂O₅ is deposited via magnetron sputtering at 200 °C (using Ta₂O₅:Er₂O₃, 99:1 wt%), followed by chemical mechanical polishing (CMP) for void-free planarization.
• Post-deposition annealing (∼800 °C) activates Er³⁺ ions for efficient emission.

2.2 Hybrid Cavity Geometry

• Microring Resonator:
  • Radius $R ≈ 30\,\mu$m, FSR$_\text{ring} ≈ 7.12$ nm (at 1556 nm), width $W_\text{ring} = 1.0\,\mu$m, height $h = 450$ nm.
  • Effective index $n_\text{eff} ≈ 2.05$, mode area $A_\text{eff} ≈ 0.8\,\mu$m².
• U-Shaped Gain Waveguide:
  • Width $W_\text{gain} = 3\,\mu$m, height $h = 450$ nm, length $L_\text{gain} ≈ 2$ mm.
  • Adiabatic taper from 3 µm to 1 µm over 100 µm suppresses higher-order modes; bends' radius $\geq 50\,\mu$m reduces bend loss.
• Coupling Regions:
  • Two symmetric points, gap $g ≈ 200$ nm, coupling length $\ell_c ≈ 25\,\mu$m.
  • Field-coupling coefficients $k_{1,2} ≈ 0.10$, $t_{1,2} ≈ 0.995$.
• The U-waveguide and microring form a dual-cavity configuration. The pump (1480 nm) is non-resonant, while the signal (∼1556 nm) is resonant in the cavities, with two couplers facilitating envelope filtering and loss balancing.

3. Material and Photonic Properties

• Propagation Loss and Quality Factor: Measured loss is $0.73$ dB/cm at ∼1530 nm. The intrinsic quality factor is $Q_i = 5.03\times10^5$ (3 dB linewidth $\Delta\lambda = 8.4$ pm).
• Mode Field Parameters: $n_\text{eff} ≈ 2.05$; $A_\text{eff} ≈ 0.8\,\mu$m²; effective mode volume $V_\text{mode} ≈ 150\,\mu$m³.
• Er³⁺ Spectroscopy: Upper-state $^4I_{13/2}$ lifetime $\tau_2 = 1.89$ ms, emission cross section $\sigma_e ≈ 0.8\times10^{-25}$ m², absorption cross section $\sigma_a ≈ 0.6\times10^{-25}$ m², consistent with erbium in glassy hosts.

4. Theoretical Principles

4.1 Resonator Performance Metrics

• Photon lifetime $\tau_p$ relates to $Q$ by $Q = \omega\tau_p$ with $\omega = 2\pi c/\lambda$.
• Intrinsic loss $\alpha$ and intrinsic $Q_i$:

$$Q_i = \frac{2\pi n_\text{eff}}{\alpha_\text{neper}\lambda}$$

• Threshold Pump Power: For a microring of volume $V_\text{mode}$, overlapping factor $\eta_\text{ov}$, and single-ended output coupling $\eta_\text{out}$, the threshold is:

$$P_\text{th} \approx \frac{n_\text{eff}h\nu_p V_\text{mode}}{\eta_\text{ov} \sigma_e \tau_2 \lambda Q_\text{tot}}$$

• Slope Efficiency:

$$\eta_s = \frac{dP_\text{out}}{dP_\text{pump}} \approx \eta_\text{ov} \frac{\sigma_e}{\sigma_e+\sigma_a} \frac{\lambda_p}{\lambda_s} \frac{Q_i}{Q_\text{tot}}$$

Experimentally, $\eta_s \approx 2.76\,\%$.

4.2 Vernier Effect and Mode Selection

• Two cavities with FSRs $FSR_1$ (ring) and $FSR_2$ (U-waveguide) produce enhanced transmission (longitudinal mode selection) when modes overlap:

$$FSR_\text{Vernier} \approx \frac{FSR_1 \cdot FSR_2}{|FSR_1 - FSR_2|}$$

• Transfer matrix (TMM) formalism describes the hybrid cavity, with round-trip field evolution governed by the cascaded 2×2 matrices for couplers and segments.

4.3 Spectral Properties

• Side-Mode Suppression Ratio (SMSR): $SMSR = 10\log_{10}(P_\text{main}/P_\text{side})$, measured at $53$ dB.
• Linewidth: Schawlow–Townes theory (with Henry’s $\alpha$-factor) sets the lower bound, but the experimental FWHM is $9.5$ pm (∼1.2 GHz), OSA-limited.

5. Experimental Characterization

• Spectral Output: Single-mode lasing at $\lambda=1556.27$ nm, with SMSR of $53.3$ dB within the Vernier envelope.
• Power Characteristics: On-chip pump coupling loss $\sim7.5$ dB/facet (1480 nm), output coupling loss $\sim6$ dB/facet (1550 nm). Threshold power $P_\text{th} \approx 3.3$ mW, slope efficiency $\eta_s = 2.76\%$, and maximum on-chip output $72.1$ µW at $29.2$ mW pump.
• Thermal Tuning: Using temperature control ($18^{\circ}$C to $68^{\circ}$C), the lasing wavelength shifts by $\sim0.55$ nm/10 °C. SMSR remains above $40$ dB except near $48^{\circ}$C, where mode competition arises. TMM predicts the thermal shift within $0.1$ nm, confirming close alignment with theory.

6. Applications and Integration Pathways

• Monolithic Integration: Er:Ta₂O₅ waveguides can integrate with passive Ta₂O₅ and Si₃N₄ components, supporting large-scale photonic circuit design on silicon.
• Wavelength-Division Multiplexing (WDM): Tuning of ring radii or coupling gaps enables multi-wavelength laser arrays or Vernier-limited banks for WDM sources.
• Process Scalability: The Damascene approach is CMOS-compatible, supporting wafer-scale production with low variability, customizable hybrid designs (e.g., multi-ring Vernier, MZI-enhanced Vernier tuning).
• Future Enhancements: Strategies to increase output power include high-reflectivity (Sagnac loop) input ports, elongated gain waveguides, and optimized fiber-chip interfaces. Narrower linewidths may be achieved by further increasing $Q_i$ (via enhanced sidewall smoothing or thicker films) and minimizing residual intrinsic losses.

7. Outlook and Impact

The Er:Ta₂O₅ microring hybrid-cavity single-mode laser delivers record slope efficiency, ultra-high SMSR, sub-GHz linewidth, and broad temperature tunability in a 6.2 × 2.3 mm² form factor. This enables scalable, high-performance on-chip light sources for next-generation silicon photonics and integrated optics, bridging the gap between monolithic active and passive photonic integration on tantalum oxide platforms (Shui et al., 31 Jan 2026).