Practical use of fiber Bragg gratings (FBGs) inscribed in multimode fibers (MMFs), especially in step-index (SI-) MMFs, is limited due to their unstable reflection spectra (RS). In this work, the physical mechanisms for RS instabilities occurring in Bragg reflectors inscribed in MMFs are considered theoretically, and methods to ensure RS stabilization are proposed and evaluated. They are based on mechanical modulation of MMF and on the use of long lengths of intermediate MMFs. These stabilization methods are experimentally investigated for FBGs inscribed in standard 50/125 and 105/125 SI-MMFs. Reduction of relative standard deviation of reflection peaks' amplitudes from 30-80% down to 1-4% is demonstrated. The efficiency of stabilized interrogation of SI-MMF FBGs is further confirmed by applying a strain test. Effective RS stabilization is also demonstrated for Bragg reflectors inscribed in specialty MMFs including those of no-core polypropylene light guide, silica graded-index MMF and polymer perfluorinated CYTOP graded-index fiber.

An optical fiber sensor is described for monitoring asphaltene content in crude oil samples, utilizing a no-core fiber segment spliced between two multimode fibers. Light transmitted through the no-core fiber section is attenuated due to the presence of scattering and absorbing effects in the surrounding medium. The sensor assesses asphaltene precipitation in a crude oil/n-heptane solution, correlating asphaltene concentration with the resulting transmission loss. Tests on four crude oil samples from different extractions and oil deposits, with asphaltene content ranging from 1% to 8.4% w/w, revealed a distinct transmission loss per sample and an exponentially increasing loss trend with regard to asphaltene content. The recorded transmission loss (after 1 hour of precipitation), measured with the non-core optical fiber sensor, ranged from 1.9 dB to 8.5 dB for the aforementioned asphaltene content values.

The adoption of polymer optical fibers in a great number of sensing and telecom applications has prompted the study of their photosensitivity using laser radiation [1]. Polypropylene (PP) is a now emerging optical material which is characterised by good optical transmission in the visible and telecom bands, yet of excellent chemical durability, tensibility and high potential for being used in the THz wavelength band. Herein, we investigate the photosensitivity of polypropylene no-core optical fibers using 248nm excimer laser radiation, while performing Bragg grating recording. In the same time, a new photosensitisation approach is presented by means of toluene in-diffusion in the PP matrix, for augmenting photosensitivity yield at the specific irradiation wavelength [2]. Toluene exhibits higher optical absorption than the PP matrix at the wavelength of 248nm, also acting as a plasticiser into the polymeric matrix, improving macromolecular mobility, thus, laser induced structural modifications.

The impact of K+ ion-exchange into the photoelastic properties (as those are manifested into the Pockels' coefficients) of borosilicate glass fibers, is studied in this manuscript, while employing whispering gallery mode resonation technique at the 1.5 mu m wavelength band. The spectral shift results of polarization resolved whispering gallery mode resonances measured in the cylindrical borosilicate glass optical fibers examined, are experimentally investigated and numerically simulated, for monitoring strain introduced TE/TM birefringence, and estimating the corresponding Pockels' coefficients. The experimental results presented well predict the Pockels' coefficients of the pristine borosilicate glass (p(11) = 0.119 +/- 0.008 and p(12) = 0.2120 +/- 0.014), as well as strain-optical coefficients, and simultaneously reveal that for 11h of ion-exchange the above p(1i) coefficients undergo a substantial decrease of similar to 22 %; longer ion-exchange times recover the Pockels' coefficients to higher values. The results obtained are discussed while considering the physical origins of Pockels' coefficients with respect to the glass coordination and molecular polarizability of K+ ion; also in accordance with Young modulus measurements obtained in ion-exchanged planar samples. The results of this study denote that K+ ion-exchange processing can be used as an efficient method for tuning the photoelasticity of glass materials.

Fiber Bragg grating (FBG) technology is one of the most developed and industrially integrated among the broader fiber optic sensors domain. FBGs inscribed in single-mode fibers are used in several applications due to their stable and reliable single-peak reflection spectrum. On the other hand, FBGs inscribed in multimode fibers (MMFs) demonstrate complex multipeak reflection spectrum (RS), deteriorating their applicability. Thanks to the formation of mode groups, the problem is not that complicated in the case of graded-index (GI) MMFs due to well separated and relatively stable peaks in the FBG reflection spectrum. Oppositely, multimode (MM) step-index (SI) FBGs demonstrate complex and unstable RS with highly overlapping peaks. Furthermore, the effects of intermodal interference and mode coupling cause significant RS modifications even when the FBG is isolated from external perturbations, and only the MMF of the interrogation scheme is disturbed (e.g. heated). The above render the use of MM-SI-FBGs impractical, especially, in the case of polymer fibers (which are highly-MM SI fibers in the most cases), where inscription of MM FBGs is the only available solution; the above outlines the problem of FBG stabilization being of high importance [1].

The utilization of polypropylene (PP) as an optical material offers a number of attractive features, such as biocompatibility, high tensibility, and high chemical stability, rendering PP suitable for use with a great number of organic and inorganic solvents. In this work, we present the use of Bragg reflectors inscribed in PP no-core optical fibers in probing the indiffusion of organic solvents (OSs) such as xylene and toluene. We show that PP fibers keep being operable after longterm immersion in OS (up to 24 hours), while PP no-core optical fiber Bragg reflectors demonstrate gigantic red-shift (approximate to 31 nm) after their immersion into toluene and xylene OSs, for characteristic times of 24 h. The transduction behavior is described using polymer matrix swelling and refractive index changes due to molecular in-diffusion.

The accurate monitoring of volatile organic compounds (VOCs) at low detection limits is critical for biomedical applications, as many VOCs serve as biomarkers for early disease detection and environmental exposure to harmful substances. This study presents the development and performance evaluation of a compact, easily produced, and low-cost, optical fiber sensor (OFS) for the detection of VOCs at parts per billion (ppb) levels. A 32-mu m cavity formed at the end face of a standard optical fiber, using a commercially available photopolymerizable resin, acts as a sensing Fabry-Perot (FP) interferometer registering the presence of methanol, ethanol, and isopropanol vapors with distinctive temporal and spectral response. A clear correlation between the molar mass of the examined compounds and the recorded temporal/refractive response of the sensor is established. The performance of the sensor was demonstrated in the detection of 200 ppb of methanol, while a potential limit of detection (LOD) for methanol vapor was estimated of the order of 1 ppb. Preliminary experiments in exhaled breath monitoring confirmed the ability of the sensor to perform breath alcohol content (BrAC) measurements.

Drone spraying operations are increasingly integrated into agricultural practices and play a crucial role in Precision Agriculture. However, their broader adoption is still limited due to concerns related to droplet drift effects. Herein, an optical fiber sensing probe based on a long period grating is employed for monitoring droplet dispersion during drone spraying operations. Tau he sprayed micro-droplets dissipate onto the exposed cladding area of the long period grating, introducing amplitude changes and wavelength shifts of the characteristic attenuation spectral notch. Open field trials, over a tarmac area, and an olive tree orchard have been performed to obtain realistic implementation data while correlating the spectral changes obtained from the optical fiber probes with droplet coverage area figures measured with conventional methods. Normalised amplitude changes of up to 17% have been registered for the transmission notch of long period grating sensors when exposed to droplet coverage areas of similar to 5.9%, for open field spraying. Furthermore, the operation of the sensor was confirmed for distances up to 22 m away from the drone spraying path in olive tree orchard. Preliminary investigations indicate that a spraying volume per area of 0.07 ml/100 cm(2) detected through droplet dissipation of the optical fiber sensor during drone spraying operations in an olive tree orchard, is adequate for tracing common pesticide active ingredients used in pest management, well below their Wipe Limit figure.

A resin cavity at the end face of a single mode optical fiber, monitors the presence of methanol, ethanol and isopropanol vapors with distinct wavelength shift and with ppb sensitivity.

Bragg grating reflectors are inscribed and characterized in toluene loaded, polypropylene lightpipes using 248nm, excimer laser radiation. Refractive index changes of the order of ~6.6x10-4 are introduced in the polymer matrix, through single photon-absorption.

Inscription and characterization of Bragg reflectors in polypropylene (PP) no-core optical fibers using 248-nm KrF excimer laser radiation are presented for the first time. A new photo-sensitization approach is adopted by means of pre-loading the PP no-core fibers using toluene in-diffusion, for increasing Bragg reflector recording yield. The Bragg reflector inscription process was monitored online, for both pristine and toluene loaded PP no-core fibers under identical exposure conditions, revealing that toluene loaded samples leaded to threefold stronger reflectors and average refractive index changes of the order of 1.1x10-3. The Bragg reflectors were characterized for their response to strain (1.62 pm/με), and temperature (-145 pm/℃), while negligible sensitivity to water diffusion was recorded. The photosensitivity mechanism of the toluene in-diffused PP no-core optical fibers using 248-nm KrF excimer laser radiation is also discussed.

A review of optical fiber sensors utilizing tilted Bragg gratings, long-period gratings, or Fabry-Perot interferometer coupled with metal oxide or polymeric sensing overlays/cavities for the detection of methanol, ethanol, isopropanol and acetone vapors.

We present the inscription and characterization of fiber Bragg gratings (FBGs) in polypropylene coreless cylindrical fibers. Polypropylene material offers several advantages, such as strong chemical resistance, biocompatibility, and high tensibility. Therefore, polypropylene FBGs can be useful for sensing in chemically aggressive environments and in biomedical applications. The coreless, cylindrical polypropylene waveguides used in these experiments had a diameter of 150 mu m, typical length up to 20 cm, and a refractive index of 1.49. The inscription was performed in the 1550 nm transparency window by using a phase mask technique and 193 nm excimer laser radiation. Inscribed FBGs demonstrated complex multi-peak reflection spectra due to highly multi-mode nature of the polypropylene waveguides. Due to a high attenuation of the polypropylene, the maximum waveguide FBG interrogation length -in reflection- was 6 cm. Gratings characterization demonstrated a strain sensitivity of 0.9 pm/mu epsilon, a temperature sensitivity of -60.4 pm/degrees C and humidity-insensitive behavior.

We present a zinc oxide (ZnO) out-cladding, overlaid optical fiber Bragg grating sensor, for the detection of vapors of common alcohols and acetone at concentrations lower than 25 ppm while operating at room temperature (RT). The optical fiber sensing results indicate a chemostriction effect occurring in the ZnO layer when exposed to volatile organic compounds (VOCs), which in turn induces shifts in the cladding, and most importantly, in the core confined, Bragg mode. The sensor exhibits a maximum sensitivity of $\sim $ 1 pm/ppm to ethanol vapors, with exposure to other alcohol vapors (isopropanol and methanol) showing lower sensitivities; also, response to acetone vapors was traced at $\sim $ 0.5 pm/ppm. X-ray diffraction (XRD) measurements of the ZnO nanolayer revealed that, in saturated ethanol vapors atmosphere, the polycrystalline ZnO film undergoes a contraction by 0.6% of the interplanar distance corresponding to the (002) crystalline direction, denoting the chemostrictive effect through an underlying piezotronic mechanism. XRD measurements and optical fiber sensing data are further correlated by numerical simulations carried out, so to study the strain interactions of the ZnO layer with the silica glass optical fiber.

Silk fibroin is an important biomaterial for photonic devices in wearable systems. The functionality of such devices is inherently influenced by the stimulation from elastic deformations, which are mutually coupled through photo-elasticity. Here, we investigate the photo-elasticity of silk fibroin employing optical whispering gallery mode resonation of light at the wavelength of 1550 nm. The fabricated amorphous (Silk I) and thermally-annealed semi-crystalline structure (Silk II) silk fibroin thin film cavities display typical Q-factors of about 1.6 × 104. Photo-elastic experiments are performed tracing the TE and TM shifts of the whispering gallery mode resonances upon application of an axial strain. The strain optical coefficient K’ for Silk I fibroin is found to be 0.059 ± 0.004, with the corresponding value for Silk II being 0.129 ± 0.004. Remarkably, the elastic Young’s modulus, measured by Brillouin light spectroscopy, is only about 4% higher in the Silk II phase. However, differences between the two structures are pronounced regarding the photo-elastic properties due to the onset of β-sheets that dominates the Silk II structure.

Whispering gallery mode (WGM) resonation has been used for tracing modal distribution in thermally poled glass hetero-cavities {Korakas, 2022 #7516}, or for measuring the photo-elasticity (Pockels' coefficients) in cylindrical glass [2], or polystyrene [3] cavities. Photo-elasticity is a significant physical quantity correlating the molecular refractivity of a material with its mechanical properties (Poisson ratio and Young modulus), which can shed light on its molecular structure.

We describe an optical fiber sensor for detection of ammonia vapors employing a fluorinated graphene-like overlayer on a tilted Bragg grating. Exploiting the laser-mediated explosive synthesis and transfer (LEST) of graphene (Gr) flakes a thin film of few-layer turbostratic graphene flakes doped with F atoms (~ 3.3 at. %) are deposited on the fiber at the location of the grating. The response of the sensor was investigated for NH4OH vapors while for reference, the effect of H2O was also monitored at identical conditions. Under increasing vapor pressure of NH4OH, wavelength shift is recorded not only in the cladding modes but also for the fundamental Bragg mode, indicating that the effect is not solely due to changes in the optical parameters of the overlayer. The monitored wavelength shift is initially negative turning to positive when vapor saturation is reached. Furthermore, there is a distinct difference in the magnitude of the monitored shifts with the higher order mode exhibiting 2.5x higher values compared to the Bragg mode. The study is ongoing and will also include overlayers of pure LEST Gr and LEST Gr decorated with Six nanoparticles.

The detection of volatile organic compounds (VOCs) is a major market and research domain, where optical sensing technologies have shown promising advances [1]. Accordingly, ZnO has been extensively used as a transduction material in the sensing of VOCs in the vapour phase, while employing both resistive [2] and optical [3] detection methodologies. ZnO also exhibits significant piezoelectric properties, which in turn have been recently used in the development of piezotronic [4] self-powered, sensing and actuating devices.

A sensing probe is presented for the detection of 2,2,2-trifluoroethanol in the vapor phase, while using poly(vinylidene fluoride (PVDF) thin films, overlaid onto tilted optical fiber Bragg gratings. The 2,2,2-trifluoroethanol sensor operates in the 1.5 μm band, in transmission mode, where the signal of both the core and cladding modes is monitored. Best detectivities obtained are 2 ppm for 2,2,2-trifluoroethanol vapors in ambient atmosphere, for typical response times of 50 min. The sensing probe presented - based on PVDF transductor - shows limited reversibility after being used in the tracing of 2,2,2-trifluoroethanol vapors; its subsequent exposure to nitrogen flow, partly reverses its spectral behavior back to the starting point, denoting the involvement of mechanisms other than physisorption into the underlying transduction. The actual sensing mechanism of 2,2,2-trifluoroethanol vapors while using thin PVDF films is currently under investigation.