Broadband Waveguide Grating Couplers with Over 100 nm Bandwidth for Silicon Photonics

Introduction

Waveguide grating couplers (GCs) are widely used in silicon photonics for coupling light to and from optical fibers. They offer several advantages, including large fabrication tolerance, flexibility in positioning on the chip surface, compatibility with wafer-scale testing, and relaxed fiber alignment tolerances. However, a key limitation of conventional GCs is their relatively narrow optical bandwidth, typically around 30-40 nm, due to the inherent wavelength dependence of Bragg diffraction.

Many applications, such as wavelength division multiplexed communications, optical frequency comb generation, and broadband optical sensors, require GCs with much wider optical bandwidths. In this tutorial, we present a novel approach to significantly increase the optical bandwidth of waveguide GCs to over 100 nm while maintaining high coupling efficiency and compatibility with standard photolithography fabrication processes.

Concept and Design

The key idea behind the proposed broadband waveguide grating coupler (WBGC) is to employ a center-symmetric grating structure, as illustrated in Figure 1. This structure can be viewed as a combination of multiple cavities around the central symmetry axis. By carefully designing the cavity lengths and their intercoupling strengths, we can engineer the spectral resonances and dispersions to achieve a broad, flat-topped optical passband.

Figure 1: Schematic of the cross-section of the WBGC.

Each cavity in the WBGC has a different resonant wavelength due to its unique length. By precisely tuning the intercoupling between these cavities, the overall structure can exhibit a broadband response resulting from the superposition of the individual cavity resonances. Furthermore, the multiple-cavity design enables a double-pass effect, effectively shortening the grating length and increasing the optical bandwidth, as the bandwidth scales inversely with the grating length.

The center-symmetric structure also allows engineering the diffracted beam's power profile to match the fiber mode, facilitating high coupling efficiency. Additionally, the carefully designed dispersion characteristics of the WBGC can compensate for the waveguide dispersion, further enhancing the overall bandwidth.

Optimization and Fabrication

To realize the WBGC design, the authors employed a genetic algorithm optimization method, constraining the algorithm to produce a center-symmetric grating structure. The fitness parameter used in the optimization was the product of the coupling efficiency and the 1 dB bandwidth, evaluated using 2D finite-difference time-domain (FDTD) simulations.

The optimized WBGC was designed for a silicon-on-insulator (SOI) wafer with a 220 nm thick top silicon layer and a 2 μm thick buried oxide layer. The polysilicon layer thickness was 160 nm, and the etch depth was 230 nm, with the polysilicon layer fully etched and the crystalline silicon waveguide layer shallowly etched by 70 nm. These parameters were compatible with standard multi-project wafer (MPW) fabrication processes offered by commercial foundries, without requiring any process customization.

Figure 2 shows the final optimized structural parameters of the WBGC, including the width of the grooves and teeth for each grating period. The minimum feature size was set to 181 nm to comply with the foundry's design rules. Most of the other feature sizes were larger than 181 nm, ensuring robust fabrication using deep UV photolithography.

Figure 2: Final optimized structural parameters of the WBGC.

The optimized WBGC design was simulated using 3D FDTD simulations, predicting a coupling efficiency of -2.91 dB at 1565 nm and a remarkable 1 dB bandwidth of 109 nm, as shown in Figure 3. The compact footprint of the focusing WBGC, measuring 42 μm × 24 μm, makes it suitable for high-density integration and multi-core fiber interfacing.

Figure 3: Coupling efficiency spectra of the optimized structure.

Experimental Results

The WBGC was fabricated using 193 nm deep-ultraviolet (DUV) lithography in the multi-project wafer fabrication service provided by IMEC. The authors received about 20 dies and measured the coupling efficiency and bandwidth of different chips.

The best experimentally measured coupling efficiency was -3.0 dB (without index matching gel), with a 1 dB bandwidth of 108 nm, from 1519 nm to 1627 nm. The best 1 dB bandwidth measured was 112 nm, with a coupling efficiency of -3.59 dB. The mean peak coupling efficiency across the set of 16 gratings was -3.35 dB, and the mean 1 dB optical bandwidth was 105.6 nm.

Figure 4(a) from the paper compares the simulated and experimental transmission spectra of the WBGC, showing good agreement. Figure 4(b) presents the coupling efficiency and bandwidth results for different chips across the wafer, illustrating the fabrication tolerance and potential yield in large-volume manufacturing.

Figure 5 shows microscope images of the fabricated focusing WBGC, highlighting its compact footprint and suitability for high-density integration.

Figure 4. (a) Simulated and experiment test transmission spectra of the WBGC, inset shows the measured FP ripples. (b) The coupling efficiency and bandwidth test results for different chips.

Figure 5. Microscope image of the WBGC. (a) Input grating. (b) Output grating.

Conclusion

The proposed WBGC design achieves state-of-the-art performance in terms of optical bandwidth and coupling efficiency while maintaining compatibility with standard photolithography fabrication processes. The center-symmetric structure, combined with cavity resonance engineering and dispersion compensation, enables a remarkable 1 dB bandwidth of over 100 nm and a high coupling efficiency of -3.0 dB.

This wideband and flat-topped transmission characteristic makes the WBGC suitable for various applications, such as coarse wavelength division multiplexing (CWDM) transceivers, integrated optical frequency comb sources, broadband integrated optical spectrometers, and other wideband applications. Moreover, the large tolerance to fabrication variations ensures high yield in large-volume production.

The authors have demonstrated a novel conceptual approach to significantly enhance the optical bandwidth of waveguide grating couplers, validated by experimental implementation using a commercial foundry service. This work represents a significant advancement in the field of silicon photonics and paves the way for the realization of high-performance, wideband integrated photonic devices and systems.

Reference

[2] X. Zhou and H. K. Tsang, "Photolithography Fabricated Broadband Waveguide Grating Couplers With 1 dB Bandwidth Over 100 nm," in IEEE Photonics Journal, vol. 16, no. 1, pp. 1-6, Feb. 2024, Art no. 6600206, doi: 10.1109/JPHOT.2023.3344361.