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into dynamic control of asymmetric spectral responses via refractive index( RI) modulation. ElKabbash et al. [ 40 ] investigated Fano-resonant ultrathin-film optical coatings, enabling enhanced light – matter interactions and tunable spectral asymmetry in nanoscale photonic structures. Hu et al. [ 41 ] reported a laser-controlled Fano resonance sensing mechanism based on WGM coupling in eccentric-hole fibers integrated with azobenzene materials, achieving optically tunable sensitivity through photochemical modulation. Li et al. [ 42 ] developed a fiber Fabry – Perot interferometerbased Fano resonance coupler for whispering-gallery-mode resonators, improving coupling efficiency and enhancing the Q-factor through hybrid cavity interactions.
In 2022, Sun et al. [ 43 ] proposed a quasi-3D Fano resonance cavity integrated on the end-facet of an optical fiber for high signal-to-noise ratio“ dip-and-read” surface plasmon sensing. This configuration provides a compact and robust sensing platform capable of real-time refractive index monitoring, ideal for biosensing and chemical detection with excellent signal fidelity. Jiang et al. [ 44 ] introduced an electrically tunable Fano-like resonance in a graphene-coated fiber grating, enabling fast, reversible modulation through electrical biasing – a step toward reconfigurable photonic systems for sensing and communication. Zhu and Yin [ 45 ] investigated Fano-resonance-based optical fiber characteristics for blood glucose detection, leveraging sharp asymmetric spectral features for high RI sensitivity in biomedical sensing. Aman et al. [ 46 ] combined the Vernier effect with Fano resonance to realize an ultra-sensitive all-optical sensor, where the Vernier amplification and Fano asymmetry jointly enhanced detection precision. Kudashkin et al. [ 47 ] developed a self-stabilized whispering gallery mode resonator in active-core fibers using negative feedback to maintain spectral stability under environmental fluctuations.
In 2023, Gan et al. [ 48 ] reported a Fano-like spectrum with quasi-independent slope ratio tuning by coupling higher-order HE modes with WGMs, enabling fine control of spectral asymmetry and linewidth. Finally, Sakhabutdinov et al. [ 49 ] proposed a hybrid FBG – FP structure producing composite Fano-type resonances with sharp, tunable spectra, suitable for precision strain, temperature, and pressure measurements.
In 2024, Wang et al. [ 50 ] reported an all-optical modulation of Fano-like resonances in an apodized fiber Bragg grating using Er / Yb-doped fiber. The nonlinear gain dynamics of the doped medium enabled effective modulation of the Fano profile without external electronics, demonstrating a new route toward all-optical control and tunable photonic filtering. Chai et al. [ 51 ] investigated Fano resonance in a microcylinder – taper coupling system for liquid RI sensing based on the axial separation method. By precisely adjusting the taper – cavity distance, the system achieved strong interference between localized and continuum modes, allowing high-resolution detection of RI variations in liquids. La et al. [ 52 ] demonstrated a Bragg etalon-based optical fiber for optoacoustic detection. Robalinho et al. [ 53 ] introduced a phase-shifted FBG( PS-FBG), which was fabricated using selective pitch slicing. This enabled tuning of the Fano-like asymmetry spectrum, resulting in a wide-range vibrational sensor. Feng et al.
[ 54 ] proposed a simple scheme to generate multiple ultrahigh-slope Fano-like resonances via Mach – Zehnder interferometers( MZIs) cascaded with fiber gratings. The configuration achieved enhanced spectral discrimination and steep transmission slopes, suitable for high-resolution spectral sensing and wavelength filtering.
In 2025, Wu et al. [ 55 ] presented a method for the synchronous and controllable realization of Lorentzian, Fano, and electromagnetically induced transparency( EIT) lineshapes in an all-fiber configuration. This work demonstrated full optical tunability between distinct resonance profiles within a single platform, significantly expanding the flexibility of all-fiber photonic filters and sensors. Li et al. [ 56 ] explored Fano resonance in a whispering gallery mode microsphere resonator coupled with a femtosecond laser-drilled tapered fiber. The laser-drilled taper provided enhanced coupling precision and mechanical stability, resulting in sharper resonance lines and improved sensing repeatability. Wang et al. [ 57 ] developed a modulation-free laser frequency stabilization technique based on balanced detection of a common-path Fano resonance in an all – polarization-maintaining( PM) fiber cavity. This approach provided a compact and vibration-insensitive means of laser locking, eliminating the need for active modulation or complex feedback electronics. Li et al. [ 58 ] proposed a mechanism to enhance the dynamic range in microcavity sensing through nonlinear harmonic generation of Fano resonances. The introduction of higher-order nonlinear interactions broadened the measurable range while preserving spectral sensitivity, enabling versatile performance in high-intensity or nonlinear photonic environments. Finally, Zou et al. [ 59 ] investigated the resonantly driven nonlinear dynamics of soliton molecules in ultrafast fiber lasers. The study revealed that soliton molecule formation and evolution could be strongly influenced by Fano-like interference effects within the laser cavity, providing new insights into the control of ultrafast nonlinear dynamics and pulse shaping in fiber lasers. Figure 3 summarizes the principal fiber-optic configurations employed with this technique.
4 Fano resonance on fiber structures
As outlined in the historical overview, various structures were designed to induce Fano resonance through modal coupling within the fiber. This section will address these architectures and their operational principles. The three most common approaches are whispering gallery modes, microstructured fibers and interferometric systems. Recently, new configurations using FBG structures were published to excite the discrete mode.
4.1 Whispering gallery mode microcavities
Whispering gallery modes are waves capable of propagating around a concave surface and were first presented as acoustic waves in 1910 [ 60 ]. In 1961, an electromagnetic analog was also demonstrated [ 61 ]. The microcavities used for WGM present advantages such as a high Q-factor and a comparatively reduced mode volume [ 62 ]. When coupled