Generation of multiple and ultra-high slope Fano-like resonances via MZI and cascaded fiber gratings

Abstract

The integration of fiber Bragg gratings (FBGs) into a Mach-Zehnder interferometer (MZI) enables the realization of asymmetric Fano lineshape in an all-fiber system, attributed to the interaction between the core mode of Bragg resonance and the continuum propagating mode in MZI. In this study, we propose a simple scheme for generation and tuning of multiple Fano-like resonances in a single fiber device by incorporating cascaded fiber Bragg gratings (FBGs), acting as a multi-discrete mode resonator, into an off-axis MZI. The theoretical simulation of the fiber structure with transfer matrix theory indicates two types of Fano-like resonance with reversed lineshapes is presented, depending on the sign of the phase difference between two arms of MZI. We further verify these findings by manipulating the phase difference of the structure with an optical delay line. The experimental results demonstrate that cascading FBGs of different wavelengths enable themselves as discrete mode providers, and realize the excitation of Lorentzian, Fano, and electromagnetically induced transparency (EIT) lineshape, synchronously, within the continuous mode envelope of MZI. Additionally, cascading two identical FBGs together can form a Fabry–Pérot (FP) cavity, and cause as much as 6 discrete asymmetric Fano-like resonances with slope rate up to 4.87×10⁴ dB/nm in an extremely narrow wavelength range of 0.2 nm. When the temperature modulation is applied on the system, the generated Fano-like resonances all undergo a periodic change, and consequently tunes Fano lineshapes with the Fano parameter q from -∞ to +∞. The achievement of multiple resonances and ultra-high slope of the Fano lineshape in a single fiber device may pave the way for high precision sensing, all-optical switching and signal processing.