Electrostrictive optical resonators for non-contact displacement measurement.

This paper describes a non-contact transduction mechanism for the measurement of linear displacements that is based on the electrostrictive properties of a polymeric optical resonator. The spherical resonators, with a diameter of ∼1 mm and an average optical quality factor of ∼106, are made using a commercially available polymer (Super Soft Plastic-Manufacturing Company). The spherical resonator is immersed in a homogeneous electric field that is generated by applying a voltage difference between two metallic plates. One of the plates is fixed, whereas the other one is movable. By changing the distance between the plates, the electric field intensity changes, leading to a variation of the mechanical forces (electrostrictive effect) acting on the resonator. This effect, in turn, leads to a change in the morphology of the optical resonator and therefore to a shift of its optical resonances. By tracking the shift of the optical modes, it is possible to determine the displacement of the movable plate. Our results indicate a sensitivity ranging from 0.008 to 0.642 pm/µm with a resolution on the order of a few hundreds of nanometers.

Untethered photonic sensor for wall pressure measurement.

In this Letter, we study a novel untethered photonic wall pressure sensor that uses as sensing element a dome-shaped micro-scale laser. Since the sensor does not require any optical or electrical cabling, it allows measurements where cabling tends to be problematic. The micro-laser is made by a mixture of Trimethylolpropane Tri(3-mercaptopropionate), commercial name THIOCURE and Polyethylene (glycol) Diacrylate (PEGDA) mixed with a solution of rhodamine 6G. Two different volume ratios between the THIOCURE and the PEGDA are studied, since different ratios lead to different mechanical properties. In addition, two different sensor configurations are presented: (i) sensor coupled to a membrane, that allows differential wall pressure measurement and (ii) sensor without membrane that allows absolute wall pressure measurement. The sensitivity plots are presented in the paper for both sensor configurations and polymer ratios.

High data rate transient sensing using dielectric micro-resonator.

An approach to high-speed tracking of optical mode shifts of microresonators for wide-bandwidth sensing applications is presented. In the typical microresonator sensor, the whispering gallery optical modes (WGM) are excited by tangentially coupling tunable laser light into the resonator cavity, such as a microsphere. The light coupling is achieved by overlapping the evanescent field of the cavity with that of a prism or the tapered section of a single-mode optical fiber. The transmission spectrum through the fiber is observed to detect WGM shifts as the laser is tuned across a narrow wavelength range. High data rate transient-sensing applications require the tuning of the diode laser at high repetition rates and tracking of the WGM shifts. At high repetition rates, the thermal inertia prevents appropriate tuning of the laser, thus leading to smaller tuning ranges and waveform distortions. In the present paper, the laser is tuned using a harmonic (rather than ramp or triangular) waveform, and its output is calibrated at various input frequencies and amplitudes using a Fabry-Perot interferometer to account for the tuning range variations. The WGM shifts are tracked by performing a modified cross-correlation method on the transmission spectra. Force sensor experiments were performed using ramp and harmonic waveform tuning of the diode laser with rates up to 10 kHz. Results show that the harmonic tuning of the laser eliminates the high-speed transient thermal effects. The thermal model developed to predict the laser tuning agrees well the experiments.

Effect of wall pressure and shear stress on embedded cylindrical microlasers.

In this paper, we carried out numerical experiments to study the effect of the shear stress and the wall pressure on the optical mode shift of two embedded cylindrical microlasers. The optical cavities (laser) are encapsulated in a slab that is clamped at the bottom surface while the other sides of the slab are free-stress boundaries. When a uniform shear stress and pressure is applied on the top surfaces of the slab, the morphology of the optical resonators are perturbed. This leads to a shift in the optical modes [commonly referred to as the whispering gallery mode (WGM)] of the resonators. The effect of the geometry (size and position of the optical cavities) and materials properties on the optical mode shift are studied. The results show a linear dependency of the WGM shift on the applied external pressure. In addition, the optical mode shift is slightly dependent on the geometry and the material properties. The effect of the shear stress on the WGM shift shows a quadratic dependency and this nonlinearity is strongly dependent on the position of the resonators within the slab. The studies also show that the proposed configuration could be used as a sensor for simultaneous measurements of wall pressure and shear stress.

Dome-shaped whispering gallery mode laser for remote wall temperature sensing.

In this paper, we carried out experiments to investigate dome-shaped microlaser based on the whispering gallery modes for remote wall temperature sensing. The dome-shaped resonator was made of Norland blocking adhesive (NBA 107) doped with a solution of rhodamine 6G and ethanol. Two different configurations are considered: (i) resonator placed on top of a thin layer of 10:1 polydimethylsiloxane (10:1 PDMS), and (ii) resonator encapsulated in a thin layer of 10:1 PDMS. The microlaser was remotely pumped using a Q switch Nd:YAG laser with pulse repetition rate of 10 Hz, pulse linewidth of 10 ns, and pulse energy of 100 µJ/cm². The excited optical modes showed an average optical quality factor of 104 for both configurations. In addition, the measurements showed sensitivity to temperature of ~0.06 nm/°C and a resolution of 1°C for both configurations. This sensitivity was limited by the resolution of the experimental setup used in these studies.

Effect of angular velocity on sensors based on morphology dependent resonances.

We carried out an analysis to investigate the morphology dependent optical resonances shift (MDR) of a rotating spherical resonator. The spinning resonator experiences an elastic deformation due to the centrifugal force acting on it, leading to a shift in its MDR. Experiments are also carried out to demonstrate the MDR shifts of a spinning polydimethylsiloxane (PDMS) microsphere. The experimental results agree well with the analytical prediction. These studies demonstrated that spinning sensor based on MDR may experience sufficient shift in the optical resonances, therefore interfering with its desirable operational sensor design. Also the results show that angular velocity sensors could be designed using this principle.

Development of whispering gallery mode polymeric micro-optical electric field sensors.

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy, micro-cavity laser technology, optical communications as well as sensor technology. The WGM-based sensor applications include those in biology, trace gas detection, and impurity detection in liquids. Mechanical sensors based on microsphere resonators have also been proposed, including those for force, pressure, acceleration and wall shear stress. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies. A candidate application of this sensor is in the detection of neuronal action potential. The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 µm) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 10(6)). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.

Magnetorheological polydimethylsiloxane micro-optical resonator.

We investigate the possibility of using magnetorheological polydimethylsiloxane (MR-PDMS) spheres as micro-optical resonators. In particular, the effect of a magnetic field on the whispering gallery modes (WGM) of these resonators is studied. The applied field induces mechanical deformation, causing shifts in the WGM. The microspheres are made of PDMS with embedded magnetically polarizable particles. An analysis is carried out to estimate the WGM shifts induced by an external magnetic field. An experiment is also carried out to demonstrate the magnetic field-induced WGM shifts in an MR-PDMS microsphere. The results indicate that MR-PDMS microspheres can be used as high-Q-factor tunable optical cavities with potential applications in sensing.

Tuning of whispering gallery modes of spherical resonators using an external electric field.

In this paper we investigate the electrostriction effect on the whispering gallery modes (WGM) of polymeric microspheres and the feasibility of a WGM-based microsensor for electric field measurement. The electrostriction is the elastic deformation (strain) of a dielectric material under the force exerted by an electrostatic field. The deformation is accompanied by mechanical stress which perturbs the refractive index distribution in the sphere. Both strain and stress induce a shift in the WGM of the microsphere. In the present, we develop analytical expressions for the WGM shift due to electrostriction for solid and thin-walled hollow microspheres. Our analysis indicates that detection of electric fields as small as ~500V/m may be possible using water filled, hollow solid polydimethylsiloxane (PDMS) microspheres. The electric field sensitivities for solid spheres, on the other hand, are significantly smaller. Results of experiments carried out using solid PDMS spheres agree well with the analytical prediction.

Micro-optical force sensor concept based on whispering gallery mode resonators.

A micro-optical force sensor concept based on the morphology-dependent shifts of optical modes of dielectric microspheres is investigated. The optical resonances, commonly referred to as the whispering gallery modes (WGM), were excited by evanescently coupling light from a tunable diode laser using a tapered single-mode fiber. A compressive force applied to the sphere induces a change in both the shape and the index of refraction of the sphere leading to a shift in WGM. By tracking the shifts, the force magnitude is determined using solid silica as well as solid and hollow Polymethyl-methacrylate (PMMA) microsphere resonators. A measurement sensitivity as high as dlambda/dF=7.664 nm/N was demonstrated with a 960 mum hollow PMMA sphere.