A novel highly sensitive displacement sensing method based on the intensity detection of the resonant spectrum, which arises from a fiber-based surface nanoscale axial photonics (SNAP) resonator, is proposed. By means of dissipative and dispersive coupling mechanisms, the changes of the relative position between the SNAP resonator and fiber taper not only bring the shift in a resonant wavelength, but also lead to the variation of the linewidth and extinction ratio of the whispering gallery mode (WGM) in spectrum. Instead of the wavelength shift, we utilize the extinction ratio changes to realize the displacement sensing, which is robust against lasing and microresonator frequency noise in the detecting system. Using the analytical expression of the transmission spectrum, the extinction ratio as a function of the displacement for different axial modes is obtained. It is proved that a large range and high resolution displacement sensor can be achieved by simultaneously tracking the extinction ratio of multiple axial modes. The fiber-based SNAP resonator can be fabricated into a probe-type sensor, making it potential and a powerful tool for many displacement sensing applications such as microstructure measurements in both aerospace and nano-lithography fields.
With the finite difference time domain (FDTD) method, whispering gallery modes (WGM) in a microsphere coated with
three layers of high, low, and high refractive index (RI) are simulated. In the simulation, the coupling system includes a
coating microsphere, a waveguide and a nanoscale gap separating the waveguide and the microsphere. A pulse with
ultra-wide bandwidth that spans over several resonant modes of the resonator is used for simulation. Via waveguide
coupling, the relative intensity spectra of the three layers and the transmission spectrum of the coupling system are
obtained. We investigate the effects of the waveguide RI and the thickness of the low-RI layer on resonance
characteristics. It is found that each of the two high-RI layers can sustain its own WGM if the values of RI and thickness
of the three layers are appropriate. Furthermore, the effect of the RI of the surrounding medium on resonance
characteristics is also studied. The simulation results show that a RI change of the surroundings will only change the
resonance wavelength of the outer layer, and will not affect the WGM of the inner layer. Such property makes it feasible
for a potential application in high-precision RI and temperature sensing.
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