Designing High-Order Microring Filters with Ultra-Large Free Spectral Ranges

Introduction

Optical filters play a crucial role in various applications such as optical communications, imaging, quantum systems, and tunable lasers. In silicon photonics, Coupled Resonance Optical Waveguide (CROW) type filters have gained significant attention due to their flat-top response, compact footprint, and good fabrication tolerance. To enhance the wavelength range of operation in optical systems, researchers have been actively working on increasing the free-spectral-ranges (FSR) of CROW filters. This tutorial will explore a novel design approach using width evolution functions (WEFs) to create CROW filters with ultra-large FSRs, reaching over 49nm.

Design and Simulation

The proposed CROW filters utilize Euler spiral-shaped micro-rings (ESMs) coupled by Euler spiral-shaped bend directional couplers (BDCs), as shown in Figure 1(a). The ESMs are designed with four segments of Euler spirals conforming to equations (1) and (2) in the paper.

Fig. 1. a) Schematic of proposed CROW filters b) Comparison of traditional sinusoidal evolution function and our proposed approach, c) d) Mapping of FSR and propagtaion loss for ESMs with different radii.

To mitigate the transition radiation losses caused by the rapid curvature transition from the maximum radius (Rmax) to the minimum radius (Rmin), a special WEF (equation (3) in the paper) is introduced. This WEF facilitates a swift increase in waveguide width for bends with large radii and decreases the rate of change as the radii become sharper, as illustrated in Figure 1(b).

The widths of the ESMs are held constant at 400nm and 600nm for w1 and w2, respectively. By sweeping the radii, the optimal parameters for achieving the highest FSR with reasonable losses can be determined. Figure 1(c) and (d) show the simulated results of propagation loss and FSR for different radii combinations.

The bus waveguide design employs an Euler spiral shape with a gap width of 190nm, within the design rules of commercial silicon photonics foundries. The coupling between ESMs inside the filters is optimized using the transfer matrix method (TMM) to attain the largest bandwidth and smallest flat-top unevenness. Figure 2(a) illustrates the simulated results for the designed CROW filters from 3rd to 10th order.

Fig. 2. a) and b) Simulation data for the 3rd to 10th CROWs detailing their free spectral range (FSR) and flat-top single resonance response. c) Simulation outcomes for a 7-channel WDM system with 6.4 nm channel spacing and 3.5 nm bandwidth.

The large FSR of these filters can be utilized in a seven-channel wavelength (de)multiplexing (WDM) system with 3.5nm bandwidth and 6.4nm channel spacing, using the 5th order CROW design. Wavelength shifts within each channel are achieved passively by changing the width w1. Figure 2(b) shows the simulation results for such a WDM system, with crosstalk reaching -60dB in adjacent channels.

Fabrication and Measurement

The devices were fabricated at Applied Nano Tools Inc. using e-beam lithography on a 220nm thick silicon-on-insulator with a 2μm thick buried oxide and a 2.2μm thick top oxide cladding.

The transmission spectra of the fabricated 3rd to 10th order CROWs were measured using a continuous wavelength sweeping technique with a tunable laser and a power meter. Figure 3(a) shows the measured spectra in a 100nm wavelength range, with zoom-in views displaying a single resonance for each filter.

Fig. 3. a) Measured spectra of 3rd to 10th CROWs over a 100nm wavelength range, featuring zoom-ins of single resonances for each filter. b) to e) Fabrication tolerance tests for 3rd to 10th CROWs with ±15nm width variations, alongside error-free microscope images.

The measured FSRs of the different order filters varied from 48.91 to 49.22nm, in good agreement with simulations. The measured channel losses for the 3rd, 5th, 8th, and 10th order filters were 0.77 dB, 0.92 dB, 0.43 dB, and 1.73 dB, respectively. The corresponding in-band unevenness was measured at 0.86 dB, 0.61 dB, 0.80 dB, and 1.40 dB. Additionally, the measured 3dB bandwidths ranged from 3.61nm to 5.07nm. All filters exhibited an extinction ratio around 50dB, limited by the background light scattering in the substrate.

Fabrication Tolerance

The fabrication tolerance of the designs was tested by introducing width variations of ±15nm. Two additional sets for each filter type were fabricated with these variations. Figures 3(b)-(e) show the results, indicating small variations in in-band unevenness (around 1dB) and channel loss (around 1dB). Notably, the FSR of the filters experienced only a 0.7nm variation with a 15nm error for all four types of filters, demonstrating the design's robustness to small parameter variations.

Conclusion

This tutorial presented a novel design approach for Euler spiral-shaped micro-ring CROW type filters with ultra-wide FSRs of 49nm. High-order CROW filters, up to 10th order, were successfully fabricated and measured, exhibiting modest losses of approximately 1.7dB and an in-band unevenness of 1.405dB. The fabrication tolerance was experimentally tested with ±15nm fabrication error, further validating the design's robustness. These filters are suitable for constructing WDM systems in high-speed communications and for applications in integrated quantum photonics. Further improvements may be possible by refining the design and assessing fabrication quality using CMOS-capable photolithography foundries.

Reference

[1] Y. Qin and H. K. Tsang, "High-order Euler Adiabatic Microring Filters with Free-Spectral-Range of over 49nm," in Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China, 2024.