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Sugar concentration in floral nectars is an assessment required in several diverse fields of application. The widely used analysis, consisting of nectar extraction with a microcapillary and sugar concentration measurement with a light refractometer, is not reliable when the nectar is secreted in small quantities, unextractable with a microcapillary. Ancillary methods adopted in such cases are destructive, rather complicated and often provide much less precise and accurate results. The microscopic-size, low cost and biocompatibility of optical fibers were exploited to deliver light directly inside the flower with minimal invasiveness and measure instantaneously the refractometric properties of the nectar without extracting it. After comparing the new and old methods using two known nectariferous species, the new approach was validated on Primula palinuri, whose nectar is unextractable with microcapillaries. The fiber-optic probe was able to measure the nectar refractive index in P. palinuri flowers making it possible to highlight a previously undetected significant trend of the sugar concentration throughout the long anthesis of the single flowers. Changes in nectar concentrations are similar in both longistylous and brevistylous flowers. The fiber-optic refractometer is an advancement of light refractometer analysis. Further customization of the laboratory set-up into portable equipment will boost applications.

Miniaturization of Fourier transform spectrometers must necessarily deal with the problem of the mechanical scan of an interferometer, which is the core element of this technology. Several small-scale FT spectrometers with no moving parts have been demonstrated over the last years, although they all rely on advanced integrated photonics technology. We have shown previously that spectrum of an unknown radiation can be accurately measured with the simplest of the optofluidic systems: a droplet evaporating on a partially reflective substrate. This principle allows to devise a miniaturized FT spectrometer with a spontaneous evaporation-driven scan. Here, we show that such optofluidic spectrometer can be employed for the readout of a plasmonic nanostructured optical fiber sensor, allowing to devise a plasmonic sensing setup where either radiation source, sensor and readout spectrometer are all integrated in an optical fiber. The system performance is analogous to that of a macroscopic plasmonic setup, but it can be obtained with a dramatically reduced cost and dimensions.

Starting from a recently developed model based on the generalized Lorenz-Mie theory, we simulate and experimentally investigate the resonance spectrum and the scattered light field of whispering-gallery modes (WGMs) in a liquid droplet excited with a free-space visible laser beam. Depending on the illumination configuration and the spectral region, we observe WGMs with diverse quality factors and coupling efficiencies. Through imaging of scattering patterns, we show a robust agreement between theory and experiment. Our study provides an insight of the free-space optical coupling phenomenon, opening pathways for a controlled excitation of ultrahigh-quality WGMs in a plethora of liquid and solid optical microresonators for sensing and imaging applications.

The subsoil consolidation in the archaeological sites or in areas with historic existing buildings (e.g. Pompeii) needs the use of ground improvement techniques that must guarantee the safeguarding of such relevant areas and structures. One of the best approaches in such cases is low pressure grouting. An experimental multidisciplinary study was carried out with the aim to design and characterize a sustainable chemical grout sodium silicate-based catalysed by NaHCO3. The evolution from sol-to-glassy phase of the grouts was carefully investigated, furthermore the effectiveness of the grout has been verified either by lab tests or by real-scale field trials. The experimental findings suggested that it is possible to properly control the viscosity and gel-time of the grout modulating the amount of catalyst without affecting the final consolidation performances. Furthermore, the results at real-scale validate the safety of the above approach in cultural heritage applications, since it allowed a complete permeation of the soil without worsening its final properties. (C) 2019 Elsevier Ltd. All rights reserved.

Small-scale Fourier transform spectrometers are rapidly revolutionizing infrared spectro-chemical analysis, enabling on-site and remote sensing applications that were hardly imaginable just few years ago. While most devices reported to date rely on advanced photonic integration technologies, here we demonstrate a miniaturization strategy which harnesses unforced mechanisms, such as the evaporation of a liquid droplet on a partially reflective substrate. Based on this principle, we describe a self-operating optofluidic spectrometer and the analysis method to retrieve consistent spectral information in spite of the intrinsically non-reproducible droplet formation and evaporation dynamics. We experimentally realize the device on the tip of an optical fiber and demonstrate quantitative measurements of gas absorption with a 2.6 nm resolution, in a 100 s acquisition time, over the 250 nm span allowed by our setup’s components. A direct comparison with a commercial optical analyzer clearly points out that a simple evaporating droplet can be an efficient small-scale, inexpensive spectrometer, competitive with the most advanced integrated photonic devices., Compact spectrometers can be useful in many applications and many sophisticated architectures have been proposed. In this work, the authors show that with an evaporating droplet on a fiber tip, spectrometry can be robustly and accurately performed with a simple and passive microfluidic system.

Surface-plasmon resonance (SPR) sensors represent a formidable technology for molecular biology and bioanalytics applications. Here, we devise a new interrogation architecture that transforms a standard SPR chip into an optical-heterodyne clock detector comparable with the best SPR instruments. The key ingredients are (i) the conversion of refractive-index changes on the SPR chip into frequency shifts of an oscillatory electronic signal (ii) a differential probe & reference optical-cavity scheme that cancels out most source/detector noise contributions. A rigorous characterization of the sensor performance with the integration time demonstrates a refractive-index resolution on the 10−8-RIU level over a 1-s timescale using a bare, unstabilized SPR element. A bio-sensing test with a functionalized surface interfaced with a microfluidic flow-cell is carried out, showing detection of streptavidin via covalent bond down to concentrations of 90 fg/mm2.

Improving the sensing capabilities of miniaturized probes nanofabricated on the tip of optical fibers is crucial for a large number of sensing applications in remote and harsh environ-ments. While most research efforts are directed towards developing improved fabrications techniques or more sensitive structures, we show that the performance of these sensors can be significantly enhanced by designing an appropriate interrogation strategy. In our scheme, a nanostructured plasmonic fiber probe is included in an optical fiber resonator and interrogated with a single-mode laser. A wavelength shift of the plasmonic resonance generates a mismatch of the resonator optical impedance, which is monitored and actively compensated by an optoelectronic gain element. We characterize the technique using samples with different refractive index and compare its performance to a conventional sensing scheme. Our cavity-enhanced configuration leads to a 50-fold improvement of the probe’s refractive index resolution (corresponding to a detection limit of 10−5 RIU, the lowest reported so far for this type of sensors). The demonstrated measurement scheme can be devised with any fiber-optic sensor that responds with a resonance shift or with an attenuation of the transmitted/reflected intensity.

A droplet optical resonator is a unique environment to investigate light-matter interaction and optomechanics in liquids. Here, we report on light pressure effects derived from whispering gallery modes excited in a liquid-polymer droplet micro-resonator by free-space laser scattering. From the nonlinear resonance spectrum observed in the visible, we provide evidence of photon pressure exerted at the liquid-air boundary and quantify it with a coherent physical model. Our findings pave the way to studies on micro-rheology and nonlinear optofluidics, where droplets serve as miniature liquid laboratories.(C) 2021 Optical Society of America

Droplets are very simple physical systems whereby surface tension shapes liquids into ideal optical and mechanical cavities. Stimulated Brillouin scattering (SBS) is the strongest optical nonlinearity in dielectrics and can generate acoustic waves with unrivalled efficiency, typically at GHz rates [1]. Basically, SBS in dielectrics is driven by the refractive-index modulation induced in the material by the light field itself. Forward Brillouin scattering allows much lower vibrations rates than in the backward direction (< 100 MHz), thereby reducing material viscosity losses [2]. Until recently, forward SBS opto-mechanics has been demonstrated only with solid whispering-gallery-mode (WGM) micro-resonators [3, 4]. In liquids, we observed such effects using oil droplets [5]. In our work, we excite optical WGMs in vertically-suspended liquid-polymer droplet microresonators using free-space laser beams (Fig. 1) [5, 6]. Droplets are made from a low-viscosity liquid polymer with radii ranging between 140 and 150 μm. Their optical quality factor is around 107 and is limited by optical absorption by the liquid material [7]. Cavity-enhanced Brillouin scattering stimulate surface-acoustic waves circumferentially propagating in a single droplet that thus turns into a high-quality hypersound-laser micro-mechanical oscillator, in the 60–70 MHz range. Analysing the beat note between the pump and Stokes-scattered light fields, we investigate its mechanical finesse and acoustic amplitude as a function of the pump optical power. Then, its short-term and long-term frequency stability is characterized with a time-domain and Allan variance analysis [8]. The stimulated generation of acoustic waves on microlitre-nanolitre droplets gives momentum to new optical schemes for characterization of material viscous-elastic properties, laboratory investigation of atmospheric phenomena and mass sensing for analysis of biological fluids based on ultrasound-hypersound coherent generation and detection. New experiments and simulations are currently underway in order to study nonlinear properties and sensing capabilities of other liquid materials using different optical wavelengths.

We provide here an overview of passive optical micro-cavities made of droplets in the liquid phase. We focus on resonators that are naturally created and suspended under gravity thanks to interfacial forces, illustrating simple ways to excite whispering-gallery modes in various slow-evaporation liquids using free-space optics. Similar to solid resonators, frequency locking of near-infrared and visible lasers to resonant modes is performed exploiting either phase-sensitive detection of the leakage cavity field or multiple interference between whispering-gallery modes in the scattered light. As opposed to conventional micro-cavity sensors, each droplet acts simultaneously as the sensor and the sample, whereby the internal light can detect dissolved compounds and particles. Optical quality factors up to 107–108 are observed in liquid-polymer droplets through photon lifetime measurements. First attempts in using single water droplets are also reported. These achievements point out their huge potential for direct spectroscopy and bio-chemical sensing in liquid environments. Finally, the first experiments of cavity optomechanics with surface acoustic waves in nanolitre droplets are presented. The possibility to perform studies of viscous-elastic properties points to a new paradigm: a droplet device as an opto-fluid-mechanics laboratory on table-top scale under controlled environmental conditions.

This paper reports proof-of-principle of an optical fiber Bragg grating (FBG) operating as an electric field sensor. The system is based on a semiconductor diode laser, emitting around 1577.5 nm that interrogates the fiber sensor around its maximum reflectivity point. The application of a strong electric field causes a small variation of the optical path length of the waveguide due to electrostriction and thus perturbs the grating peak reflectivity wavelength. Calibration and dynamic range tests are performed using a parallel-plate capacitor to create a variable electric field (1-100 kV/cm). A great potential is pointed out in view of possible field applications for voltage graduation in power generators.

Droplets are very simple physical systems, whereby surface tension shapes liquids into ideal opto-mechanical devices. This has recently enabled low-viscosity liquid samples to serve as miniature acoustic resonators harnessing optical generation of bulk vibrations, capillaries, or surface waves. Uniquely, a simple room-temperature pendant droplet can be activated as a hypersound-laser emitter when illuminated by a free-space, low-power visible laser thanks to stimulated Brillouin scattering of optical and acoustic whispering-gallery modes. Here, we demonstrate continuous operation of a liquid polymer opto-mechanical resonator and characterize its quality factor and long-term frequency stability. Our results point to the feasibility of all-liquid micro-mechanical oscillators working in the 50-100 MHz range. The stimulated generation of high-quality surface waves on nanoliter droplets gives momentum to new optical schemes for characterization of material viscous-elastic properties, laboratory investigation of atmospheric phenomena, and mass sensing for direct analysis of biological fluids based on ultrasound-hypersound coherent generation and detection. (C) 2018 Optical Society of America

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 7 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.

Liquid droplets are ubiquitous in nature wherein surface tension shapes them into perfect spheres with atomic-scale smooth surfaces. Here, we use stable droplets that cohost equatorial acoustical and optical resonances phase matched to enable the exchange of energy and momentum between sound and light. Relying on free-space laser excitation of multiple whispering-gallery modes, we harness a triple-resonant forward Brillouin scattering to stimulate optomechanical surface waves. Nonlinear amplification of droplet vibrations in the 60-70 MHz range is realized with spectral narrowing beyond the limit of material loss, thereby activating the droplet as hypersound-laser emitter.

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 CH4 and CO2 around 3.3 and 4.5 μm respectively. Finally, the most effective detection schemes for space-craft trace-gas monitoring applications are singled out.

Microresonators have undergone an impressive development in the last decade, opening new pathways to nonlinear optics, laser stabilization, spectroscopy and sensing. Here we present our recent results on linewidth narrowing of quantum cascade lasers and chemical sensing using crystalline solid and liquid whispering-gallery-mode resonators.

Liquid droplet whispering-gallery-mode microresonators open a new research frontier for sensing, optomechanics and photonic devices. At visible wavelengths, where most liquids are transparent, a major contribution to a droplet optical quality factor is expected theoretically from thermal surface distortions and capillary waves. Here, we investigate experimentally these predictions using transient cavity ring-down spectroscopy. With our scheme, the optical out-coupling and intrinsic loss are measured independently while any perturbation induced by thermal, acoustic and laser-frequency noise is avoided thanks to the ultra-short light-cavity interaction time. The measurements reveal a photon lifetime at least ten times longer than the thermal limit and indicate that capillary fluctuations activate surface scattering effects responsible for light coupling. This suggests that droplet microresonators are an ideal optical platform for ultra-sensitive spectroscopy of highly transparent liquid compounds in nano-liter volumes.

We demonstrate a new optical rheometer where a fiber Bragg grating sensor (FBG) detects the periodical strain caused by the oscillations of a string tightened within a sample fluid. We show that the viscoelastic moduli of complex fluids can be obtained instantaneously from the FBG optical backreflection signal, without need of any current flow in the wire or magnetic field that could affect the sample chemistry. After a validation with known-viscosity solutions, the technique is employed to monitor the phase transition of a sol-gel compound. The results provide an insight in the early-stage gelation dynamics that cannot be obtained with traditional rheometers, and allow to clearly identify the gel point as the intersection of the viscoelastic moduli. (C) 2017 Elsevier B.V. All rights reserved.

The capability of optical resonators to extend the effective radiation-matter interaction length originates from a multipass effect, hence is intrinsically limited by the resonator’s quality factor. Here, we show that this constraint can be overcome by combining the concepts of resonant interaction and coherent perfect absorption (CPA). We demonstrate and investigate super-resonant coherent absorption in a coupled Fabry-Perot (FP)/ring cavity structure. At the FP resonant wavelengths, the described phenomenon gives rise to split modes with a nearly-transparent peak and a peak whose transmission is exceptionally sensitive to the intracavity loss. For small losses, the effective interaction pathlength of these modes is proportional respectively to the ratio and the product of the individual finesse coefficients of the two resonators. The results presented extend the conventional definition of resonant absorption and point to a way of circumventing the technological limitations of ultrahigh-quality resonators in spectroscopy and optical sensing schemes.

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 105 to 107 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.

Simulations of molecular spectral profiles recorded in the Dicke narrowing regime by off-axis high-finesse optical cavities are presented. The influence of experimental parameters on the transmitted lineshapes is investigated on the basis of a model that takes into account the effect of the cavity transfer function. In particular, the collisional narrowing pressure range is found to shrink until disappearance as the molecular absorption increases. It is also shown that fitting the measured lineshapes with a pure Galatry function, i.e. neglecting instrumental effects, may lead to discrepancies of several tens percent in the extracted line parameters.

A technique for the selective characterization of the losses of the transverse modes propagating in a multimode fiber is presented. The method is based on the implementation of a fiber ring cavity which introduces a frequency discrimination among the resonances of the different modes, thus allowing to investigate them separately. Each mode is excited by locking the laser frequency on the corresponding cavity resonance, and its losses are measured from the average photon lifetime with a ring-down technique. Two different distributions of transverse modes, obtained by controlling the injection of the multimode fiber at the cavity input port, are investigated. The technique is proposed as a complementary approach for the characterization of multimode fiber propagation and intermodal power exchange effects.

In this paper we theoretically investigate a ring resonant cavity obtained by closing on itself a pi-shifted fiber Bragg grating, to be used for refractive index sensing applications. Differently from a conventional pi-shifted fiber Bragg grating, the spectral structure of this cavity is characterized by an asymmetric splitting doublet composed by a right side resonance having an asymmetric Fano profile and a left side resonance having a symmetric Lorentzian profile. The right side resonance shows a narrower and sharper peak than all the other kinds of resonance achievable with both conventional ring resonators and pi-shifted fiber Bragg gratings. A reduction of the resonant linewidth with respect to a conventional pi-shifted Fiber Bragg grating and a fiber ring resonator, having the same physical parameters, is theoretically proved, achieving up to five orders of magnitude improvement with respect to the usual ring resonator. Due to these resonance features, the pi-shifted Bragg grating ring resonator results suitable for RI sensing applications requiring extremely narrow resonances for high resolution measurements. In particular, by assuming a refractive index sensing to detect the presence of sugar in water, the sensor can show a theoretical resolution better than 10(-9) RIU. (C)2015 Optical Society of America

We report on a portable, widely-tunable, mW-power mid-infrared spectrometer based on difference frequency generation (DFG) in a periodically-poled lithium-niobate crystal, realized in a compact and robust design. The analytical performance for real-time monitoring of natural-abundance trace-gases in ambient air is evaluated, pointing out the possibility of field applications. In a direct-absorption scheme, a minimum detectable concentration of 3 ppb×Hz^-1/2 is demonstrated around 3.3 ?m for methane at atmospheric pressure. The sensitivity is further improved by using a two-tone frequency modulation spectroscopy (TTFMS) technique that provides an enhancement of a factor 100 in the signal-to-noise ratio, thus yielding a minimum absorption coefficient of 5.3×10^-9×cm-1×Hz^-1/2.

We present the development of a mid-infrared tunable coherent radiation source, based on difference-frequency generation (DFG). The signal and pump beams coming from two cw rare-earth-doped amplifiers are mixed into a periodically-poled LiNbO3 crystal to produce radiation in the 2.9-3.5 ¼m spectral range. For the first time to our knowledge, an output power of a few mW has been achieved. We show that such a high power can be exploited for high-resolution investigation of molecular absorption spectra. Saturation Lamb-dips have been observed for a number of ro-vibrational transitions belonging to the CH4 ½3 fundamental band.

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.

We report the theoretical description and the experimental demonstration of an optical resonator formed by inserting a Fiber Bragg Grating (FBG) in a closed fiber loop. The spectral characteristics of such a resonator strongly depend on the reflectivity of the FBG. In the wavelength region where the FBG reflectivity R is negligible, the system behaves like a conventional ring resonator. On the other hand, when R is not vanishing, a split-mode structure can be observed, associated to the degeneracy removal of two counterpropagating resonant modes. The magnitude of the mode splitting can be used to sense small variations of the FBG physical parameters, such as length, temperature or group index. An example of strain sensing with this setup is reported, showing that the mode splitting is sensitive to a mechanical strain applied to the FBG, while it is almost insensitive to a strain applied to any other point of the resonator. This peculiar feature allows to perform cavity-enhanced, local strain measurements with a reduced sensitivity to environmental perturbations, which represents an important improvement in the framework of the fiber-optic sensors. (C)2013 Optical Society of America

We demonstrate a strain sensor with very high sensitivity in the static and low frequency regime based on a fiber ring cavity that includes a ? phase-shifted fiber Bragg grating. The grating acts as a partial reflector that couples the two counter-propagating cavity modes, generating a splitting of the resonant frequencies. The presence of a sharp transition within the ? phase-shifted fiber Bragg grating's spectral transmittance makes this frequency splitting extremely sensitive to length, temperature, and the refractive index of the fiber in the region where the grating is written. The splitting variations caused by small mechanical deformations of the grating are tracked in real time by interrogating a cavity resonance with a locked-carrier scanning-sideband technique. The measurable strain range and bandwidth are characterized, and a resolution of 320 p?/Hz1/2 at 0 Hz is experimentally demonstrated, the highest achieved to date with a fiber Bragg grating sensor. © 2014 Optical Society of America

Liquid droplets suspended by the tip of a thin wire, a glass capillary, or a needle form high-Q optical resonators, thanks to surface tension. Under gravity equilibrium conditions, the maximum drop diameter is approximately 1.5 mm for paraffin oil (volume ∼ 0.5 μL) using, for instance, a silica fiber with 250 μm thickness. Whispering gallery modes are excited by a free-space near-infrared laser that is frequency locked to the cavity resonance. The droplet cavity serves as a miniature laboratory for sensing of chemical species and particles.

We report on a method for surface plasmon resonance (SPR) refractive index sensing based on direct time-domain measurements. An optical resonator is built around an SPR sensor, and its photon lifetime is measured as a function of loss induced by refractive index variations. The method does not rely on any spectroscopic analysis or direct intensity measurement. Time-domain measurements are practically immune to light intensity fluctuations and thus lead to high resolution. A proof of concept experiment is carried out in which a sensor response to liquid samples of different refractive indices is measured. A refractive index resolution of the current system, extrapolated from the reproducibility of cavity-decay time determinations over 133 s, is found to be about 10(-5) RIU. The possibility of long-term averaging suggests that measurements with a resolution better than 10(-7) RIU/√Hz are within reach.

We report a detailed theoretical and experimental study of fiberoptic cavities under broadband excitation by mode-locked laser combs. We calculate the effects of fiber dispersion on the cavity transmission. For any integer ratio between the comb repetition rate and cavity free spectral range, the theoretical resonant output spectrum exhibits a narrow group of resonant teeth, surrounded by minor, unevenly spaced resonances. Also, the central resonance can be rapidly and precisely tuned over the entire comb span by only acting on its repetition rate. Experimental observations are provided by a single-mode fiber ring and a telecom-wavelength comb laser. The resulting spectral pattern agrees very well with our theoretical prediction, allowing a thorough characterization of the cavity dispersion and opening new perspectives for comb spectroscopy in dielectric resonators. © 2013 Optical Society of America.

In this Letter, we report the theoretical model and the experimental evidence of a mode-splitting cloning effect due to the resonant coupling between modes having different polarizations in weakly birefringent fiber Bragg grating (FBG) ring resonators. This modal coupling depends on the fiber birefringence and the FBG reflectivity. In the ideal case of the absence of birefringence, a single split-mode resonant structure can be observed in the resonator transmission spectrum due to the degeneracy removal of the two counter-propagating modes. In the presence of FBG birefringence, a secondary split doublet resulting in a clone of the initial one is generated. The described effect can be exploited for spectroscopic-sensing applications based on more complex split-mode dynamics. (C) 2016 Optical Society of America

A fiber Bragg grating (FBG)-coupled ring laser sensor is demonstrated. In the proposed configuration the interrogating source, the sensing head and the readout instrument are integrated in a single fiber-optic device. An FBG inserted within a bidirectional fiber ring couples the two counterpropagating modes of the cavity, generating a splitting of the resonant wavelengths proportional to the FBG reflectivity. When the cavity gain is brought beyond threshold, the two peaks of the split resonances simultaneously lase, leading to a beat note in the emission spectrum whose frequency tracks any small shift of the FBG reflectivity spectrum. Such a beat note can be simply monitored by a frequency counter, without the need for an optical spectrometer, allowing to significantly reduce size and costs of the sensor setup. The sensing performance compares well to the state-of-the-art thermomechanical fiber sensors. (C) 2016 Optical Society of America

We present a simple and effective method for frequency locking a laser source to a free-space-coupled whispering-gallery- mode cavity. The scheme relies on the interference of spatial modes contained in the light scattered by the cavity, where low-and high-order modes are simultaneously excited. A dispersion-shaped signal proportional to the imaginary component of the resonant optical field is simply generated by spatial filtering of the scattered light. Locking of a diode laser to the equatorial modes of a liquid droplet resonator is demonstrated using this scheme, and its performance is compared to the Pound-Drever-Hall technique. This new approach makes laser-frequency locking straightforward and shows a number of advantages, including robustness, low cost, and no need for sophisticated optical and electronic components. (C) 2016 Optical Society of America

In this paper, we analyze the sensitivity enhancement attainable by combining a wavelength modulation (WM) technique to integrated cavity output spectroscopy (ICOS), pointing out how the spectrometer’s parameters and the acquisition strategy affect the detection noise in both techniques. We point out that WM-ICOS is mainly limited by the slow scan rate that it requires, compared to regular ICOS. Nevertheless, according to our analysis, WM can still appreciably improve the SNR of an ultrasensitive ICOS system, if the cavity transmission is so low that the detector noise is not negligible. In light of these considerations, we directly compare the performance of ICOS and WM-ICOS in a high sensitivity ambient-air methane detection experiment, finding a good agreement with the theoretical influence of the various spectrometer parameters.

We report on recent development of IR spectrometers based on non-linear optical generation and quantum-cascade lasers. Frequency stabilization and referencing to optical frequency-comb synthesizers is described. Their characteristics for spectroscopic as well as metrological applications are pointed out. The potential of the combination of mid-IR sources with enhancement cavities for high-resolution and sensitive measurements of gas spectra is illustrated. Finally, we describe wave-front engineering of long wavelength beams for spatial control and imaging applications.

The electric quadrupole fundamental (v=1

High-sensitivity spectroscopy of methane around 3 microm was carried out by means of a 5.5-mW cw difference-frequency generator in conjunction with a high finesse cavity in off-axis alignment. By cavity-output integration a minimum detectable absorption coefficient of 5.7*10-9 cm-1Hz-1/2 was achieved, which compares well with results already reported in the literature. Detection of methane in natural abundance was also performed in ambient air, for different values of total pressure, allowing direct concentration measurements via evaluation of the integrated absorbance of the spectra. In particular, at atmospheric pressure, a minimum detectable concentration of 850 parts per trillion by volume (pptv)*Hz-1/2 was demonstrated.

We report the realization and characterization of a tunable, two-dimensional Talbot array illuminator for mid-infrared (MIR) wavelengths. A phase array, prepared by deposing tin-doped indium oxide electrodes on a square-lattice-geometry poled LiNbO3 sample, is illuminated by a difference-frequency generator emitting at 3 mu m. Then, combining the electro-optic with the Talbot effect allows generation of a variety of light patterns under different values of distance and external electric field. Several potential applications with great relevance to the MIR spectral region are discussed.

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 LiNbO3 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 fe, i.e. 10-13 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.

We present and experimentally test a simple model for difference frequency generation (DFG) in periodically-poled crystals with gaussian pumping beams. Focusing of input beams originates several non-collinear quasi-phase-matching configurations of the interacting wavevectors, which contribute to the idler output field. In this picture, we accurately describe a number of effects, such as the occurrence of annular idler intensity profiles and the asymmetric trend of DFG power vs temperature. Finally, we quantitatively test the model by means of an indirect measurement of the crystal poling period.

The transmission spectrum of a ring resonator enclosing a ?-phase shifted fiber Bragg grating (?-FBG) shows a spectral feature at the Bragg wavelength that is much sharper than resonance of the ?-FBG alone, and that can be detected with a simple integrated cavity output technique. Hence, the resolution of any sensor based on the fitting of the ?-FBG spectral profile can be largely improved by the proposed configuration at no additional fabrication costs and without altering the sensor robustness. A theoretical model shows that the resolution enhancement attainable in the proposed closed-loop geometry depends on the quality factor of the ring resonator. With a commercial grating in a medium-finesse ring, a spectral feature 12 times sharper than the ?-FBG resonance is experimentally demonstrated. A larger enhancement is expected in a low-loss, polarization maintaining setup.