Optical whispering-gallery modes (WGMs) have been extensively investigated in solid micro-cavities of various geometries and materials demonstrating impressive quality (Q) factors. The peculiarity of WGMs supported by such solid structures is that they can be excited via evanescent-wave coupling while resonant light travels along closed paths at the boundary between the surface of the resonator and the surrounding environment. Here, we use micro-cavities made directly from small, vertically-suspended liquid droplets realizing excitation of their whispering-gallery modes by free-space laser beams and demonstrating laser-frequency locking on corresponding optical resonances with various liquids for sensing applications. From direct cavity photon lifetime measurements, we show intrinsic optical Q-factors > 10<sup>7</sup> for highly-transparent liquid polymers in the visible, that may be limited by scattering due to thermal-induced surface distortions and residual optical absorption. On the other hand, the interaction between light and mechanical motion is also investigated in these droplets. Based on our recent experimental results, liquid microresonators exhibit interesting properties that potentially allow for optical stimulation of mechanical vibrations.
In this work, we analyze the possibility of exploiting the recently demonstrated super-resonant intracavity coherent absorption effect for quantitative sensing applications. The method relies on a system of two nested resonators (ring and Fabry-Perot) which allows to increase the radiation-matter interaction pathlength by a factor proportional to the product of the two resonator’s individual finesse coefficients. In this way, it is possible to dramatically increase the interaction of intracavity radiation with a weakly absorbing sample without need of an ultra-high finesse optical cavity. Here, we propose a measurement strategy that allows to exploit the super-resonant enhancement provided by the nested-resonators system and cancel out the noise connected with the radiation source at the same time, and compare it to a conventional cavity-enhanced absorption measurement.
We present a new generation of compact and rugged mid-infrared (MIR) difference-frequency coherent radiation sources referenced to fiber-based optical frequency comb synthesizers (OFCSs). By coupling the MIR radiation to high-finesse optical cavities, high-resolution and high-sensitivity spectroscopy is demonstrated for CH<sub>4</sub> and CO<sub>2</sub> around 3.3 and 4.5 μm respectively. Finally, the most effective detection schemes for space-craft trace-gas monitoring applications are singled out.
We present a simple and effective set-up to exploit the enhancement of passive optical cavities that are directly made from a liquid droplet. The optical resonances, corresponding to the so-called whispering-gallery modes (WGMs), are excited by a focused free-space beam edge-coupling scheme. Very narrow resonances are observed, both in the visible and near-infrared spectral regions, with quality (Q) factors ranging from 10<sup>5</sup> to 10<sup>7</sup> and beyond. Different methods for interrogation via frequency locking of a laser source to the WGM are shown. Locking of a diode laser to the equatorial modes of a liquid droplet resonator is demonstrated at 1560 nm and 663 nm. This approach makes high-performance optical sensing directly feasible in liquid samples with a number of advantages in view of their application for detection and quantification of bio-molecules.
We describe a novel strain sensor based on a fiber Bragg grating (FBG) ring resonator. The spectrum of this resonator consists of equally-spaced resonances, exhibiting a frequency splitting proportional to the reflectivity of the intracavity FBG. A strain applied to the FBG causes a shift of the bragg wavelength, and thus a variation of the resonance splitting. The splitting is insensitive to the thermal/strain noise affecting the fiber in the region outside the FBG. The sensitivity and resolution of the sensor are analyzed in detail, showing that a subpicostrain resolution can be easily achieved with an optimized setup.
A novel generation of sensors of strain, temperature, absolute and relative molecular
concentration is reported. Such devices, based on 1-D photonic structures, rely on ultrastable laser sources,
referenced to a fiber-based optical frequency comb synthesizer (OFCS). In particular, recent advances in the
realization of two complementary laser sensors are presented. One is a spectroscopic facility which exploits
frequency mixing in a periodically-poled LiNbO<sub>3</sub> crystal to generate highly coherent (a few hundred kHz
linewidth) infrared radiation tunable in the 2.9-3.5 micron wavelength range. Such radiation can be coupled
to high-finesse enhancement cavities to detect trace amounts of gases, including rare isotopes in natural
abundance. The other system, making use of fiber Bragg grating components, provides strain and
temperature sensing with extremely high sensitivities (about 100 fε, i.e. 10<sup>-13</sup> ΔL/L). Due to the remoteness
guaranteed by the fiber coupling, these two systems can both be used in difficult environments and inserted
in a multi-parametric network for real-time and continuous monitoring of large areas. Prospects for
application in volcanic areas are also discussed.