[Paper Review] High Performance Optical Filters Using Three Waveguide Coupled Sagnac Loop Reflectors

Hamed Arianfard, Jiayang Wu+2 more|arXiv (Cornell University)|Mar 13, 2021

Photonic and Optical DevicesEngineering168 references135 citations

TL;DR

This paper proposes three waveguide-coupled Sagnac loop reflectors (3WC-SLRs) as a highly versatile platform for integrated photonic filters, leveraging coherent mode interference to achieve ultrahigh extinction ratios, steep slope rates, and diverse filter responses. The design enables high-performance optical Fano resonances, mode-splitting with low free spectral range, and classical Butterworth, Chebyshev, Bessel, and elliptic filters, validated through detailed parameter and tolerance analysis for practical deployment.

ABSTRACT

We theoretically investigate advanced multi-functional integrated photonic filters formed by three waveguide coupled Sagnac loop reflectors (3WC-SLRs). By tailoring the coherent mode interference, the spectral response of the 3WC-SLR resonators is engineered to achieve diverse filtering functions with high performance. These include optical analogues of Fano resonances that yield ultrahigh spectral extinction ratios (ERs) and slope rates, resonance mode splitting with high ERs and low free spectral ranges, and classical Butterworth, Bessel, Chebyshev, and elliptic filters. A detailed analysis of the impact of the structural parameters and fabrication tolerances is provided to facilitate device design and optimization. The requirements for practical applications are also considered. These results theoretically verify the effectiveness of using 3WC-SLR resonators as multi-functional integrated photonic filters for flexible spectral engineering in diverse applications.

Motivation & Objective

• To develop a multi-functional integrated photonic filter platform capable of achieving diverse spectral responses with high performance.
• To address the limitations of existing resonator-based filters by introducing a three-waveguide coupled Sagnac loop architecture for enhanced spectral engineering flexibility.
• To enable practical deployment by analyzing structural parameter sensitivity and fabrication tolerances.
• To demonstrate the realization of classical optical filters (Butterworth, Chebyshev, Bessel, elliptic) and exotic responses (Fano resonance, mode splitting) in a single integrated platform.
• To validate the design through theoretical modeling using the scattering matrix method on a CMOS-compatible SOI platform.

Proposed method

• The 3WC-SLR resonator architecture employs three Sagnac loop reflectors (SLRs) coupled via bus waveguides, forming either a zig-zag or parallel configuration to enable complex coherent mode interference.
• Coherent mode interference is tailored by independently controlling the reflectivity (ts) and coupling strength (tb) of the SLRs and waveguide couplers.
• The scattering matrix method is used to model the spectral response, enabling accurate simulation of interference effects across all configurations.
• The design leverages CMOS-compatible silicon-on-insulator (SOI) waveguides with a group index of 4.3350 (TE mode) and propagation loss of 2.4 dB/cm.
• Filter responses are engineered by adjusting waveguide lengths (LSLR, Li) and coupling parameters to achieve target spectral functions.
• Theoretical analysis includes performance evaluation of Fano resonance, mode splitting, and classical filter responses using standard filter design principles.

Experimental results

Research questions

• RQ1 Can three-waveguide-coupled Sagnac loop reflectors (3WC-SLRs) achieve high-performance optical filtering with enhanced flexibility compared to 2WC-SLRs?
• RQ2 To what extent can coherent mode interference in 3WC-SLRs be tuned to realize diverse spectral responses, including Fano resonance and classical filter shapes?
• RQ3 What are the impacts of structural parameters (e.g., coupling strength, waveguide length) and fabrication tolerances on filter performance and robustness?
• RQ4 Can 3WC-SLRs simultaneously support multiple filtering functions—such as Fano resonance, mode splitting, and classical Butterworth/Chebyshev/Bessel/elliptic filters—within a single compact platform?
• RQ5 What are the practical design requirements for deploying 3WC-SLRs in real-world applications such as optical communications and sensing?

Key findings

• The 3WC-SLR platform achieves optical Fano resonances with ultrahigh extinction ratios (ERs) and steep slope rates, enabling high-sensitivity applications.
• Resonance mode splitting is realized with high ERs and low free spectral range (FSR), enabling compact, high-Q devices suitable for dense-wavelength division multiplexing (DWDM).
• Classical filter responses—including Butterworth, Chebyshev, Bessel, and elliptic—were successfully synthesized in the optical domain using the 3WC-SLR architecture.
• Theoretical analysis confirms that the 3WC-SLR design is robust under fabrication tolerances, with performance maintained across realistic parameter variations.
• The parallel and zig-zag configurations offer distinct advantages: the former enables hybrid FIR/IIR filtering, while the latter provides enhanced interference control for complex spectral shaping.
• The platform is compatible with CMOS fabrication, enabling scalability, low loss, and mass producibility for practical photonic integrated circuits.