2022, Pereira, Diana, Bierlich, Jörg, Kobelke, Jens, Ferreira, Marta S.

The measurement of physical parameters is important in many current applications, since they often rely on these measurands to operate with the due quality and the necessary safety. In this work, a simple and robust optical fiber sensor based on an antiresonant hollow square core fiber (HSCF) is proposed to measure simultaneously temperature, strain, and curvature. The proposed sensor was designed in a transmission configuration where a segment of HSCF, with a 10 mm length, was spliced between two single mode fibers. In this sensor, a cladding modal interference (CMI) and a Mach-Zehnder interference (MZI) are enhanced along with the antiresonance (AR) guidance. All the present mechanisms exhibit different responses towards the physical parameters. For the temperature, sensitivities of 32.8 pm/°C, 18.9 pm/°C, and 15.7 pm/°C were respectively attained for the MZI, AR, and CMI. As for the strain, sensitivities of 0.45 pm/μϵ, -0.93 pm/μϵ, and -2.72 pm/μϵ were acquired for the MZI, AR and CMI respectively. Meanwhile, for the curvature measurements, two regions of analysis were considered. In the first region (0 m-1 - 0.7 m-1) sensitivities of 0.033 nm/m-1, -0.27 nm/m-1, and -2.21 nm/m-1 were achieved, whilst for the second region (0.7 m-1 - 1.5 m-1) sensitivities of 0.067 nm/m-1, -0.63 nm/m-1, and -0.49 nm/m-1 were acquired for the MZI, AR and CMI, respectively.

2022, Santos, João P., Bierlich, Jörg, Kobelke, Jens, Ferreira, Marta S.

In this work, a hybrid sensor based on a silica capillary in a balloon-like shape for simultaneous measurement of displacement and temperature is proposed for the first time, to the best of our knowledge. The sensor is fabricated by splicing a segment of a hollow core fiber between two single mode fibers (SMF) and by bending the fiber in a balloon shape with the capillary at the top-center position. In a transmission scheme, the SMF-capillary-SMF configuration excites an antiresonant (AR) guidance and the balloon shape enhances a Mach-Zehnder interferometer (MZI). The different responses of the interferometers to external displacement and temperature variations are conducive to a hybrid application of the sensor for simultaneous measurement of these parameters. Experimental results show that, for a capillary length of 1.2 cm and a balloon length of 4 cm, AR is insensitive to displacement and its sensitivity to temperature is 14.3 pm/°C, while the MZI has a sensitivity to displacement of 1.68 nm/mm and twice the sensitivity of AR to temperature, of 28.6 pm/°C. The proposed fiber sensor consists of only one sensing element in one configuration exciting two interferometers at the same time, which makes it of simple fabrication as well as low cost.

2016, Monteiro, Catarina S., Ferreira, Marta S., Silva, Susana O., Kobelke, Jens, Schuster, Kay, Bierlich, Jörg, Frazão, Orlando

A curvature sensor based on an Fabry-Perot (FP) interferometer was proposed. A capillary silica tube was fusion spliced between two single mode fibers, producing an FP cavity. Two FP sensors with different cavity lengths were developed and subjected to curvature and temperature. The FP sensor with longer cavity showed three distinct operating regions for the curvature measurement. Namely, a linear response was shown for an intermediate curvature radius range, presenting a maximum sensitivity of 68.52 pm/m-1. When subjected to temperature, the sensing head produced a similar response for different curvature radii, with a sensitivity varying from 0.84 pm/°C to 0.89 pm/°C, which resulted in a small cross-sensitivity to temperature when the FP sensor was subjected to curvature. The FP cavity with shorter length presented low sensitivity to curvature.

2021, Schaarschmidt, Kay, Kobelke, Jens, Nolte, Stefan, Meyer, Tobias, Schmidt, Markus A.

We provide a correction due to an erroneous repetition rate of one of the laser systems (90 fs pulse duration) in our previously published paper [Opt. Express28, 25037 (2020)10.1364/OE.399771].

2020, Schaarschmidt, Kay, Kobelke, Jens, Nolte, Stefan, Meyer, Tobias, Schmidt, Markus A.

Third harmonic generation in a circular liquid core step-index fiber filled with a highly transparent inorganic solvent is demonstrated experimentally using ultrafast pump pulses of different durations in the telecom domain for the first time. Specifically we achieve intermodal phase matching to the HE13 higher order mode at the harmonic wavelength and found clear indications of a non-instantaneous molecular contribution to the total nonlinearity in the spectral broadening of the pump. Spectral power evolution and efficiency of the conversion process is studied for all pulse parameters, while we found the greatest photon yield for the longest pulses as well as an unexpected blue-shift of the third harmonic wavelength with increasing pump power. Our results provide the basis for future studies aiming at using this tunable fiber platform with a sophisticated nonlinear response in the context of harmonic generation. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

2020, Jiang, Shiqi, Förster, Ronny, Plidschun, Malte, Kobelke, Jens, Ando, Ron Fatobene, Schmidt, Markus A.

Three-dimensional (3D) tracking of nano-objects represents a novel pathway for understanding dynamic nanoscale processes within bioanalytics and life science. Here we demonstrate 3D tracking of diffusing 100 nm gold nanosphere within a water-filled optofluidic fiber via elastic light scattering-based position retrieval. Specifically, the correlation between intensity and position inside a region of a fiber-integrated microchannel has been used to decode the axial position from the scattered intensity, while image processing-based tracking was used in the image plane. The 3D trajectory of a diffusing gold nanosphere has been experimentally determined, while the determined diameter analysis matches expectations. Beside key advantages such as homogenous light-line illumination, low-background scattering, long observation time, large number of frames, high temporal and spatial resolution and compatibility with standard microscope, the particular properties of operating with water defines a new bioanalytical platform that is highly relevant for medical and life science applications. © 2020 Shiqi Jiang et al., published by De Gruyter. 2020.

2021, Ferreira, Marta S., Bierlich, Jörg, Kobelke, Jens, Pinto, João L., Wondraczek, Katrin

Three different types of strain and temperature sensors based on negative curvature hollow core fiber (NCHCF) are proposed. Each sensor is produced by splicing a small section of the NCHCF between two sections of single mode fiber. Different types of interferometers are obtained simply by changing the splicing conditions. The first sensor consists on a single Fabry-Perot interferometer (FPI). The remaining two configurations are attained with the same sensing structure, depending on its position in relation to the interrogation setup. Thus, a double FPI or a hybrid sensor, the latter being composed by an FPI and a Michelson interferometer, are formed. The inline sensors are of submillimeter size, thus enabling nearly punctual measurements.

2022, Leich, Martin, Müller, Robert, Unger, Sonja, Schwuchow, Anka, Dellith, Jan, Lorenz, Adrian, Kobelke, Jens, Jäger, Matthias

The hybrid approach of combining a Tm:YAG laser crystal with an amorphous fused silica tube is investigated to evaluate the suitability of the resulting crystal-derived fibers for efficient double-clad fiber lasers. The fabrication process and fiber properties of these Tm fibers are investigated, focusing on the dependence of the active fiber properties on the incorporated Tm3+ concentration. Crystal rods with different doping concentrations (TmxY1-x)3Al5O12 (x = 0.02, 0.05 and 0.08) were used as starting core material for fiber drawing. The investigated fibers are mechanically stable and result in a fairly homogenous and amorphous core glass with optical absorption and emission spectra that are similar to conventional Tm:Al doped silica fibers. Regarding laser properties with 790 nm cladding pumping, we could achieve a maximum slope efficiency of 47% with an output power of 4 W. The fiber laser results are compared to a conventionally fabricated double-clad Tm fiber prepared by Modified Chemical Vapor Deposition and solution doping. To the best of our knowledge, we demonstrate the highest laser output and the highest efficiency obtained from a Tm:YAG crystal-derived fiber.

2018, Baghdasaryan, Tigran, Geernaert, Thomas, Chah, Karima, Caucheteur, Christophe, Schuster, Kay, Kobelke, Jens, Thienpont, Hugo, Berghmans, Francis

It is common belief that photonic crystals behave similarly to isotropic and transparent media only when their feature sizes are much smaller than the wavelength of light. Here, we counter that belief and we report on photonic crystals that are transparent for anomalously high normalized frequencies up to 0.9, where the crystal’s feature sizes are comparable with the free space wavelength. Using traditional photonic band theory, we demonstrate that the isofrequency curves can be circular in the region above the first stop band for triangular lattice photonic crystals. In addition, by simulating how efficiently a tightly focused Gaussian beam propagates through the photonic crystal slab, we judge on the photonic crystal’s transparency rather than on isotropy only. Using this approach, we identified a wide range of photonic crystal parameters that provide anomalous transparency. Our findings indicate the possibility to scale up the features of photonic crystals and to extend their operational wavelength range for applications including optical cloaking and graded index guiding. We applied our result in the domain of femtosecond laser micromachining, by demonstrating what we believe to be the first point-by-point grating inscribed in a multi-ring photonic crystal fiber.