Strain-enabled tuning of whispering gallery modes in a polymer capillary microresonator

We demonstrate strain-enabled, reversible spectral tuning of whispering gallery modes (WGMs) in a polymethyl methacrylate (PMMA)-based capillary micro resonator. The WGMs were excited using evanescent wave coupling via an adiabatic fiber taper. A finite element method (FEM) model was developed to investigate strain-induced deformation in polymer capillaries, from which the corresponding wavelength tuning characteristics were numerically derived. Experimental results reveal that the polymer capillary resonator shows greater strain sensitivity than a solid polymer cylindrical resonator. A maximum strain sensitivity of 0.82 pm/με was experimentally demonstrated using a polymer capillary resonator (PCR) with an outer diameter of 100 μm and wall thickness of 25 μm. Discrepancies associated with varying strain ranges were also investigated. A linear WGM spectral tuning of up to 21.67 nm was achieved over a strain range of 30,000 με, with a discrepancy of 1.8 nm between forward and reverse cycles. No hysteresis was observed up to 4800 με, which is identified as the practical operating range. The device exhibits relatively high strain sensitivity, and the achieved tuning range exceeds those reported for both silica- and PMMA-based cylindrical resonators.