Fano Resonance In An Add/Drop Interferometer

Ring resonators have numerous potential applications in sensors, modulators, filters,laser systems, optical buffers and interferometers, etc. The line shape of conventionalsingle ring resonator is a symmetric Lorentz dip in the vicinity of resonance. Recently,Fano resonance, originating from the interference between a resonance pathway andcoherent background pathway, which is asymmetric near resonance, has attractedworldwide research attention due to its potential application in high-sensitive sensing andlow power optical switching. In this paper, an alternative structure based on add/dropfilter to achieve Fano resonance is proposed and the tunability by controlling thestructural parameters is investigated.A single ring resonator is a basic building block to achieve versatile functions.Firstly, the transmission characteristics of add/drop filter are theoretically andexperimentally investigated. Based on the transitive and dispersive characteristic ofadd/drop filter, Fano resonance is achieved due to the interference between two resonantpathways by introducing a coherent light to the add port of add/drop filter. Based on thetransfer matrix, the explicit expressions of add/drop interferometer are deduced.The transmission spectra of add/drop interferometer is plotted according to thetransmittance and horizontally mirror-symmetric line shapes in the drop port and throughport are simultaneously obtained. The effect of coupling coefficient of input coupler andcouplers between ring resonator and bus waveguide and the phase difference between thetwo input lights to the resonator on the transmission characteristics is studied and tunableFano resonance is achieved. When the phase difference between the two input lights isodd times of π/2, the slope of Fano resonance achieves maximum; however, when thephase difference is integral times of π, the spectrum displays Lorentz dip and doublesfree spectral range of conventional single ring resonator. By configuring the add/dropinterferometer in fiber system, the spectrum is measured. The experimental results agreewell with the theoretical results.The add/drop interferometer can be utilized as a temperature sensor. The sensitivityexpressions of two sensing methods: resonance wavelength shift and output intensityvariation at fixed wavelength are deduced. The sensing ability of macro/micro-resonatorbased add/drop interferometer configured by Silica and polydimethyl siloxane (PDMS) isdiscussed. Due to the temperature-sensitive characteristics of PDMS, it can obtain highersensitivity and lower detection limit; however, the detection range is much smaller thanthat based on silica. In addition, it proves that macroresoantor, such as fiber resonator is unsuitable in temperature sensing application due to the small detection range.