High-Q Ultracompact Silicon Photonic WGMR

• The paper’s primary contribution is demonstrating an ultracompact WGMR design that leverages open-path geometry and high-efficiency mode converters to achieve a loaded Q of 1.78×10^5 and a six-fold footprint reduction compared to conventional MRRs.
• Utilizing asymmetric directional couplers and broadband adiabatic mode converters, the design achieves near-unity modal transmission with reflectivities exceeding 97.9%, enabling robust on-chip photon recirculation.
• The compact device footprint facilitates dense integration in silicon photonic circuits, bolstering applications in WDM filtering, reconfigurable signal routing, and emerging quantum photonic systems.

Ultracompact high loaded-Q silicon photonic whispering-gallery mode resonators (WGMRs) are integrated optical devices that achieve efficient, low-loss recirculation of light in extremely small footprints by leveraging advanced geometries, modal engineering, and state-of-the-art mode conversion within planar photonic circuits. Distinct from traditional closed-loop designs, these WGMRs exploit novel open-path geometries and multi-mode spatial domain routing, resulting in unprecedented integration densities and optical performance metrics suitable for next-generation photonic systems.

1. Device Architecture and Mode Multiplexing

Recent advances in ultracompact high loaded-Q WGMRs have shifted from the conventional paradigm of closed circular loops to open curved waveguide paths enabled by spatial mode multiplexing. In this configuration, light is launched into a single-mode bus waveguide, where an asymmetric directional coupler (ADC) partitions optical power from the fundamental TE₀ mode into a higher-order TE₁ mode that propagates along a multi-mode curved waveguide section. At both ends of this section, broadband adiabatic mode converters (AMCs) provide high-efficiency (≥97.9% reflectivity, mode conversion efficiency up to 99.98%) transformation between TE₀ and TE₁ states.

This architecture forms a reentrant, unidirectional recirculating cavity in the “spatial-mode domain” despite remaining a physically non-closed structure. Unlike standard microrings, this approach eliminates the need for 360° waveguide loops and large-radius bends, directly reducing the device's spatial footprint (Xiong et al., 15 Oct 2025).

2. Optical Performance Metrics and Scaling

The fabricated ultracompact WGMR exhibits a measured loaded quality factor ($Q_{loaded}$) of $1.78 \times 10^5$ at a resonance wavelength $\lambda_0 = 1554.3\,\mathrm{nm}$, with a narrow free spectral range (FSR) of approximately $1.051\,\mathrm{nm}$. Lorentzian fits to individual resonances yield full-width at half-maximum (FWHM) values as low as $8.7$-$10.3\,\mathrm{pm}$, confirming the device's high spectral selectivity. The effective device footprint is $0.00137\,\mathrm{mm}^2$, which is at least six times smaller than that of standard closed-loop microring resonators (MRRs), while the Q-factor is two orders of magnitude higher than typical photonic crystal resonators in similar scales.

The loaded Q-factor is determined as:

$Q = \frac{\lambda_0}{\Delta\lambda}$

where $\Delta\lambda$ is the FWHM of the resonance. The cavity finesse is given by $F = \mathrm{FSR} / \Delta\lambda$, with measured values (e.g., $F \approx 120.8$) indicating high energy storage relative to linewidth.

3. Mode Converter-Based Photonic Routing

Central to the open-path WGMR is the use of ultra-broadband, low-loss AMCs and ADCs. The AMCs at both ends of the recirculating region function as modal reflectors and transformers, allowing light to convert between TE₀ and TE₁ states with minimal backscattering and loss. The ADC offers controlled coupling efficiency between the bus and the resonator path. Simulation and experimental metrics for these elements confirm near-unity modal transmission ($\sim$99.99%) and robust operation across telecommunication bands. This photonic router configuration achieves reentrant photon recycling and enables high photon lifetime even in a straight or slightly curved waveguide (Xiong et al., 15 Oct 2025).

These mode-converting photonic routers are fabricated using deep-ultraviolet (DUV) lithography, facilitating a minimum feature size down to $\sim$135\,nm and making the approach fully compatible with silicon photonics foundry platforms.

4. Comparison with Conventional WGMRs and Alternative Resonator Types

When benchmarked against traditional MRRs, which typically require closed-loop topologies and large-area bends (such as Euler bends for minimal loss), the open-path WGMR achieves marked dimensional reduction, simplifies cascading of multiple devices on-chip, and reduces fabrication complexity. Bending losses, photon leakage, and minimum bend radii constraints confine the scalability of MRR arrays, whereas open-path WGMRs side-step these limitations by recirculating optical power in the spatial-domain.

In comparison to photonic crystal nanobeam cavities (loaded Q $\lesssim 1.7 \times 10^3$ in some cases (Xiong et al., 15 Oct 2025)), the open-path WGMR offers at least 100$\times$ greater Q-factor in a similarly compact or smaller footprint. The dependence on high-index contrast and tight feature control often renders PhC cavities vulnerable to fabrication variation; in contrast, mode converter-based recirculation is robust to typical process deviations.

5. Applications and System-Level Implications

Ultracompact open-path high-Q WGMRs are suited for advanced on-chip photonic functions where parallel integration of dense filter banks, high-density add-drop arrays, and minimal device-to-device separation is demanded. Specific application areas include:

• Wavelength-division multiplexing (WDM) add-drop filter arrays, where the small footprint allows dense channel packing;
• Reconfigurable signal routing, leveraging thermo-optic tuning (measured tuning efficiency $\sim 13.1\,\mathrm{pm}/\mathrm{mW}$) and low power requirements;
• Sensing and nonlinear optics, where high Q and small cavity volume boost sensitivity and nonlinear interaction strength;
• High-bandwidth optical interconnects and emerging quantum photonic processor arrays, which benefit from the design’s scalability and integration density.

Device arrays can be configured without large U-bent waveguides due to the reentrant spatial mode recycling, further reducing on-chip area and design complexity.

6. Mathematical Description and Physical Limitations

The open-path WGMR follows standard resonance and mode-matching theory:

• The resonance condition is captured by $Q = \lambda_0 / \Delta\lambda$.
• Finesse is $F = \text{FSR} / \Delta\lambda$.
• Modal conversion efficiency for AMCs is quantified via reflection and transmission S-parameters (with simulated reflectivity $>97.9\%$ and transmission $>99.98\%$).

Potential limitations include:

• The required precision for optimal AMC and ADC design, as device Q relies on minimal mode-conversion loss and low crosstalk.
• Thermo-optic crosstalk between neighboring elements, manifesting as local resonance extinction fluctuations; proposed mitigation involves integrating auxiliary microheaters.
• Bandwidth and polarization limits are intrinsic to the designed modal conversion basis (primarily TE₀/TE₁ pairs).

These constraints are, however, generally less restrictive than those in high-Q photonic crystal or narrow waveguide-based ring designs, given the relaxed feature size sensitivity and minimized physical loop path.

7. Outlook and Impact

The introduction of ultracompact, high loaded-Q silicon photonic WGMRs based on open-path, mode-multiplexed recirculation constitutes a significant step forward in photonic integration. By addressing both the inverse scaling of Q with physical footprint and the complexity limits imposed by closed-loop geometry, these devices support a new regime of dense, high-performance photonic signal processing, filtering, sensing, and multiplexing on silicon platforms. The compatibility with CMOS fabrication processes and robust tolerance to dimensional variation bode well for industrial-scale deployment in future optical and quantum information processing systems (Xiong et al., 15 Oct 2025).