Fano-Like Resonances in Coupled Sagnac Interferometers Formed by a Self-Coupled Waveguide

TL;DR

The paper addresses achieving high-extinction, steep-slope Fano-like resonances in integrated photonics using a compact self-coupled-waveguide architecture that couples two Sagnac interferometers with a feedback path. It combines a scattering-matrix theoretical framework with experimental SOI fabrication to demonstrate tunable, high-contrast Fano resonances via coherent mode interference, including a transition between IIR/FIR/hybrid filtering behavior. Theoretical targets reach $ER \approx 63~\mathrm{dB}$, $SR \approx 948~\mathrm{dB/nm}$, and $IL \approx 0.57~\mathrm{dB}$, and experiments validate the resonance features albeit with some discrepancies due to measurement setup limitations. The work highlights a scalable, robust approach for high-performance filtering, sensing, and modulation in integrated photonics, with potential impact on DWDM systems and quantum photonics.

Abstract

We demonstrate Fano-like resonances in silicon-on-insulator (SOI) nanowire-based coupled Sagnac interferometers (SIs) formed by a self-coupled waveguide. By adjusting the reflectivity of the two SIs and coupling strength between them, we tailor coherent mode interference to achieve high-performance optical analogues of Fano resonance. The device is theoretically analyzed and experimentally fabricated on a SOI platform. Theoretical analysis predicts periodic Fano-like resonances with a high extinction ratio and a steep slope rate, arising from strong coherent optical mode interference within a compact resonator comprising two SIs and a connected feedback waveguide. Experimental results align with the theoretical model, validating the expected resonance behavior and confirming the effectiveness of the design. These findings underscore the potential of compact coupled SIs for generating Fano-like resonances, enabling broader applications in integrated photonics.

Figures (4)

• Figure 1: (a) Schematic illustration of Fano resonance. $|g\rangle$: ground state. $|d\rangle$: a discrete state. $|c\rangle$: a continuum of states. (b) Schematic configuration of the device consisting of coupled SIs formed by a self-coupled wire waveguide. The definitions of $t_i \; (i = 1{-}3), k_i \; (i = 1{-}3), \text{SI}_i \; (i = 1, 2), L_{cw}$ are provided in Table \ref{['tab:device_parameters']}. The inset shows the cross-section of the simulated fundamental TE mode at $\lambda = 1550~\text{nm}$. W: width. H: height. $n_g$: group index. PL: propagation loss. (c) Transmission spectrum of the device at Port 2, where $t_1 = t_3 = 0.88$, $t_2 = 0.98$, $L_{s_1} = L_{s_2} = 115~\mu$m, and $L_{\text{cw}} = 230~\mu$m. (d) Zoom-in view of (c) in the wavelength range of 1550.5 nm--1551.2 nm. $\Delta\lambda$: wavelength difference between the resonance peak and notch. ER: extinction ratio.
• Figure 2: Influence of the variation in coupling strength of directional couplers on the device spectral response. (a) Power transmission spectra for various $\Delta t$ at Port 2. (b) Calculated IL, ER, and SR as functions of $\Delta t$ for the Fano-like resonances in (a). (c) The transmission spectra in Figure 2(a) over a wider wavelength range of 1548 nm–1553 nm for three values of $\Delta t = 0$, $0.005$, and $0.01$. In (a)--(c), $\Delta t$ denotes a common offset applied to the field transmission coefficient of each of the three directional couplers ($t_i \rightarrow t_i + \Delta t$ for $i = 1\text{--}3$). (d)–(f) Power transmission spectra for different $t_i$ ($i = 1$–$3$), respectively. In (a)--(f), the “initial value” curves correspond to the baseline design with $t_1 = t_3 = 0.88$, $t_2 = 0.98$, $L_{s1} = L_{s2} = 115\,\mu\mathrm{m}$, and $L_{cw} = 230\,\mu\mathrm{m}$ (as in Fig. 1(c)). All other structural parameters are identical to those in Fig. 1(c), except for the ones being varied.
• Figure 3: (a) Designing directional couplers using two adjacent silicon wire waveguides. (i) and (ii) show the simulated mode profiles for the even and odd modes of the directional coupler, respectively. W: width. H: height. G: gap between the two waveguides in the directional coupler. (b) Macrograph of the coupling scheme utilizing angle-polished fibers and grating couplers fabricated on an SOI platform; (i) side view and (ii) top view of horizontally positioned fibers on the chip. AP-SMF: angle-polished single-mode fiber. AP-MMF: angle-polished multi-mode fiber. The devices shown in (b-ii) are replicated grating-coupled devices arranged in a column to improve yield and provide redundancy. (c) SEM images of the fabricated components: (i) the fully etched grating coupler consisting of two square arrays of rectangular holes, (ii) coupled SIs formed by a self-coupled wire waveguide, and (iii) a zoomed-in micrograph of the coupling region of the SI.
• Figure 4: (a) Experimental setup for transmission spectrum measurement using the tunable laser scanning method. TL: tunable laser. PC: polarization controller. (b) Transmission spectrum of a straight waveguide with a pair of grating couplers at both ends. The inset shows a macrograph of the top view of the coupling scheme for input and output of a straight waveguide, utilizing grating couplers and angle-polished fibers at both ends. AP-SMF: angle-polished single-mode fiber. AP-MMF: angle-polished multi-mode fiber. (c) Measured transmission spectrum of the device, fabricated based on the structural parameters for the horn-like spectral lineshape shown in Figure 2(c). (d-i) Measured transmission spectrum of the device, fabricated based on the structural parameters designed to achieve the Fano-like spectral lineshape shown in Figure 1(c). (d-ii) Zoomed-in view of (d-i) within the wavelength range of 1550.3 nm–1550.5 nm. Panels (c) and (d) show data measured from the devices using grating couplers and angle-polished fibers at both ends.