Fano resonance in coupled microspheres with a tapered Bragg grating for dual-parameter detection

The Fano resonance phenomenon demonstrates a huge advantage in high-sensitivity sensing applications because of its sharp variation in the phase and amplitude and has thus attracted wide attention. In this work, two polymethyl methacrylate (PMMA) microspheres are fixed together via UV glue to realize Fano resonance. A tapered fiber Bragg grating is utilized to support and fix the two-sphere system, which couples the input light and the signal light at the same time. With the advantages of the high thermo-optic coefficient and the hygroscopic expansion property of PMMA and UV glue, the Fano lineshape demonstrates excellent sensing performance in temperature and relative humidity (RH) cases, showing sensitivities of 124.8 pm/°C and 113.1 pm/%RH, respectively. To solve the cross-sensitivity in dual-parameter detection, a tapered Bragg grating is utilized as the compensating element because of its relatively small temperature sensitivity of 10.7 pm/°C and the lack of response on the RH. Such a compact fiber-optic sensor with Fano resonance possesses remarkable potential for dual-parameter sensing applications.