Wall shear stress sensor based on the optical resonances of dielectric microspheres

Abstract

We report an optical wall shear stress sensor based on the whispering gallery mode (WGM) shifts of dielectric micro-resonators. The optical resonators are spheres with a typical diameter of several hundred microns and they serve as the sensing element. The wall shear force acting on a movable plate is transmitted mechanically to the microsphere. As a result of the applied force, the shape of the resonator is perturbed leading to a shift of the optical resonance (WGM). The one-dimensional wall shear stress is measured by monitoring these WGM shifts. Shape perturbations of the order of a nanometer can be detected with this optical method. The measurement resolution and range can be optimized by using dielectric sphere materials of different stiffness covering a wide range of flows. Prototype sensors using PDMS spheres have been built and validated in a laminar Poiseuille flow and in a plane wave acoustic tube.

A Packaged Whispering Gallery Mode Strain Sensor Based on a Polymer-Wire Cylindrical Micro Resonator

“…Therefore, in our experiments the strain range is limited so that WGMs shift is less than one FSR corresponding to 0.33% (3250 μɛ) axial elongation of the polymer wire. The spectral shift of the WGM resonant wavelength due to axial strain can be described as a combined effect of changing diameter and the changing refractive index of the resonator [8][9][10][11][12][13][14][15]23]:…”

“…There are many reports concerning WGM strain sensing using spherical [8][9][10][11][12][13][14] as well as bubble [15,16] MRs based on silica and polymer materials. Typically, an evanescent light coupling technique is used to excite the WGMs in such MRs.…”

Effect of Dangling Bonds on De-Poling Time for Polymeric Electric Field Optical Sensors

“…Tangentially coupling the light to the cavity is one of the easiest methods to let the light start to interrogate through the total internal reflection of the cavity [12,[16][17][18], as shown in Figure 1a. Recently, MDR micro-cavities have been used in applications including spectroscopy [19,20], micro-cavity laser technology [21], optical communications (switching [22], filtering [23], and multiplexing [24]), and sensor technologies [3,4,8,12,15,17,[25][26][27][28][29][30][31][32][33]. In this paper, the electric field imposes a force on the dielectric microsphere (commonly referred to as the electrostriction effect) leading to the deformation of the sphere.…”

Strain-induced spectral tuning of the whispering gallery modes in a cylindrical micro-resonator formed by a polymer optical fiber

“…The second assumption is that there is a measurable change in the refractive index brought about by the different mechanical stress and strain components within the resonator material. Taken together, the fractional change in the wavelength of the WGMs is [21][22][23][24][25][30][31][32] Δλ λ ΔD D Δn n ;…”

“…Measurement sensitivity can be improved by using dielectric materials with smaller elastic modulus, such as polymers. For example, Ioppolo et al [21][22][23][24] have demonstrated that solid, as well as hollow polymethyl-methacrylate (PMMA), microspheres have a higher strain sensitivity compared to those made of silica.…”