ABSTRACT
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.
ABSTRACT
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.
