---
title: Er:Ta₂O₅ Microring Laser
url: https://www.emergentmind.com/topics/er-ta2o5-microring-laser
type: topic
---

# Er:Ta₂O₅ Microring Laser

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 [2602.00695].

## 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 [2602.00695].

## 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 [2602.00695].

Source: https://www.emergentmind.com/topics/er-ta2o5-microring-laser