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- ItemFibre optic sensing system for monitoring of current collectors and overhead contact lines of railways(Göttingen : Copernicus Publ., 2017) Schröder, Kerstin; Rothhardt, Manfred; Ecke, Wolfgang; Richter, Uwe; Sonntag, André; Bartelt, HartmutFibre optic sensors are excellent tools to use for monitoring high-voltage current collectors. Because of their small cross section and electrical neutrality, they are easily integrated into the current collector strip and are well specialized for detection of high-speed load events. The conventional contact force measurement with four force sensors below the collector strips can also be simplified by using fibre optic force and acceleration sensors.
- ItemIntra-cavity measurement concept of dispersion properties with a tunable fiber-integrated laser(Philadelphia, Pa. : IOP Publ., 2019) Tiess, Tobias; Hartung, Alexander; Becker, Martin; Chojetzki, Christoph; Rothhardt, Manfred; Bartelt, Hartmut; Jäger, MatthiasThe dispersion properties of fibers depict a key characteristic to model the propagation of ultra-short pulses in waveguides. In the following, a new method is presented to directly measure the dispersion properties of fibers and optical components in the time domain. The analysis is based on pulse shape variations along the tuning range of a theta cavity fiber laser (TCFL) depending on the adjusted repetition rate. The automated measurement procedure, evaluating pulse symmetry, achieves a temporal sensitivity below 5 ps surpassing the resolution of the acquisition electronics. Exemplarily, two samples of Nufern PM980-XP fiber are investigated with an Yb-doped tunable TCFL retrieving the mean dispersion parameter D? by comparative measurements. The obtained results are compared to a reference method based on spectral interferometry. With deviations in D? between either approach of 0.3% and 1.3%, respectively, the results agree well within the measurement errors of the TCFL, verifying the presented concept. Due to the pulse formation process extending over multiple round trips, this approach achieves an enhanced sensitivity compared to competing direct temporal methods. Together with an alignment free operation, the fiber-integrated TCFL depicts a simple and robust concept showing potential in specific measurement scenarios such as in quality management. © 2019 Astro Ltd.
- ItemThermal tuning of a fiber-integrated Fabry-Pérot cavity(Washington, DC : Soc., 2021) Singer, Clemens; Goetz, Alexander; Prasad, Adarsh S.; Becker, Martin; Rothhardt, Manfred; Skoff, Sarah M.Here, we present the thermal tuning capability of an alignment-free, fiber-integrated Fabry-Pérot cavity. The two mirrors are made of fiber Bragg gratings that can be individually temperature stabilized and tuned. We show the temperature tuning of the resonance wavelength of the cavity without any degradation of the finesse and the tuning of the individual stop bands of the fiber Bragg gratings. This not only permits for the cavity’s finesse to be optimized post-fabrication but also makes this cavity applicable as a narrowband filter with a FWHM spectral width of 0.07 ± 0.02 pm and a suppression of more than -15 dB that can be wavelength tuned. Further, in the field of quantum optics, where strong light-matter interactions are desirable, quantum emitters can be coupled to such a cavity and the cavity effect can be reversibly omitted and re-established. This is particularly useful when working with solid-state quantum emitters where such a reference measurement is often not possible once an emitter has been permanently deposited inside a cavity.
- ItemTwo-Step-Model of Photosensitivity in Cerium-doped Fibers(Washington D.C. : Optical Society of America, 2019) Elsmann, Tino; Becker, Martin; Olusoji, Olugbenga; Unger, Sonja; Wondraczek, Katrin; Aichele, Claudia; Lindner, Florian; Schwuchow, Anka; Nold, Johannes; Rothhardt, ManfredThe photosensitivity of various cerium-doped fibers has been experimentally investigated for both excimer- and femtosecond-laser illumination. The results of single-pulse, few-pulse and multi-pulse inscription of fiber-Bragg-gratings with both laser systems and the thermal aging of those gratings demonstrated the restrictions of the conventional color center model for cerium-doped fibers. To explain the short-term stability of single-pulse gratings against long-term stability of multi-pulse gratings, an extension into a two-step-model was deduced.
- ItemNanofiber-based high-Q microresonator for cryogenic applications(Washington, DC : Soc., 2020) Hütner, Johanna; Hoinkes, Thomas; Becker, Martin; Rothhardt, Manfred; Rauschenbeute, Arno; Skoff, Sarah M.We demonstrate a cryo-compatible, fully fiber-integrated, alignment-free optical microresonator. The compatibility with low temperatures expands its possible applications to the wide field of solid-state quantum optics, where a cryogenic environment is often a requirement. At a temperature of 4.6 K we obtain a quality factor of (9.9 ± 0.7) × 106. In conjunction with the small mode volume provided by the nanofiber, this cavity can be either used in the coherent dynamics or the fast cavity regime, where it can provide a Purcell factor of up to 15. Our resonator is therefore suitable for significantly enhancing the coupling between light and a large variety of different quantum emitters and due to its proven performance over a wide temperature range, also lends itself for the implementation of quantum hybrid systems. © 2020 OSA - The Optical Society. All rights reserved.
- ItemIndependently tunable dual-wavelength fiber oscillator with synchronized pulsed emission based on a theta ring cavity and a fiber Bragg grating array(Washington D.C. : Optical Society of America, 2017) Tiess, Tobias; Becker, Martin; Rothhardt, Manfred; Bartelt, Hartmut; Jäger, MatthiasWe present a fiber-integrated laser enabling independent tuning of two emission wavelengths with a synchronized pulsed emission. The discrete tuning concept comprises a theta cavity fiber laser (TCFL), a fiber Bragg grating (FBG) array as a versatile spectral filter, facilitating tailored tuning ranges, and optical gating to control the emission spectrum. A novel electrical driving scheme uniquely enables independently tunable multi-wavelength emission from a single laser oscillator. Tunable dual-wavelength emission is experimentally investigated with a ytterbium (Yb)-doped TCFL using an FBG array with 11 gratings. Over a tuning range of 25 nm, 55 wavelength pairs have been demonstrated with high signal contrast (≈ 40 dB) and narrow linewidth (< 40GHz). Based on the demands of prospective applications, pulse synchronicity is studied with a fiber-based time-delay spectrometer (TDS) simultaneously measuring the joint temporal and spectral pulse properties down to a single-pulse analysis. Accordingly, tunable and fully synchronized dual-wavelength emissions have been verified by driving the TCFL with optimized electrical gating parameters. This unique operation mode achieved in a cost-efficient fiber-integrated laser design targets novel applications e.g. in nonlinear spectroscopy and biophotonics.
- ItemMultimode Fabry-Perot Interferometer Probe based on Vernier Effect for Enhanced Temperature Sensing(Basel : MDPI, 2019) Gomes, André D.; Becker, Martin; Dellith, Jan; Zibaii, Mohammad Ismail; Latifi, Hamid; Rothhardt, Manfred; Bartelt, Hartmut; Frazão, OrlandoNew miniaturized sensors for biological and medical applications must be adapted to the measuring environments and they should provide a high measurement resolution to sense small changes. The Vernier effect is an effective way of magnifying the sensitivity of a device, allowing for higher resolution sensing. We applied this concept to the development of a small-size optical fiber Fabry–Perot interferometer probe that presents more than 60-fold higher sensitivity to temperature than the normal Fabry–Perot interferometer without the Vernier effect. This enables the sensor to reach higher temperature resolutions. The silica Fabry–Perot interferometer is created by focused ion beam milling of the end of a tapered multimode fiber. Multiple Fabry–Perot interferometers with shifted frequencies are generated in the cavity due to the presence of multiple modes. The reflection spectrum shows two main components in the Fast Fourier transform that give rise to the Vernier effect. The superposition of these components presents an enhancement of sensitivity to temperature. The same effect is also obtained by monitoring the reflection spectrum node without any filtering. A temperature sensitivity of −654 pm/°C was obtained between 30 °C and 120 °C, with an experimental resolution of 0.14 °C. Stability measurements are also reported.
- ItemOptical Harmonic Vernier Effect: A New Tool for High Performance Interferometric Fiber Sensors(Basel : MDPI AG, 2019) Gomes, André D.; Ferreira, Marta S.; Bierlich, Jörg; Kobelke, Jens; Rothhardt, Manfred; Bartelt, Hartmut; Frazão, OrlandoThe optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry–Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.
- ItemGiant refractometric sensitivity by combining extreme optical Vernier effect and modal interference([London] : Macmillan Publishers Limited, part of Springer Nature, 2020) Gomes, André D.; Kobelke, Jens; Bierlich, Jörg; Dellith, Jan; Rothhardt, Manfred; Bartelt, Hartmut; Frazão, OrlandoThe optical Vernier effect consists of overlapping responses of a sensing and a reference interferometer with slightly shifted interferometric frequencies. The beating modulation thus generated presents high magnified sensitivity and resolution compared to the sensing interferometer, if the two interferometers are slightly out of tune with each other. However, the outcome of such a condition is a large beating modulation, immeasurable by conventional detection systems due to practical limitations of the usable spectral range. We propose a method to surpass this limitation by using a few-mode sensing interferometer instead of a single-mode one. The overlap response of the different modes produces a measurable envelope, whilst preserving an extremely high magnification factor, an order of magnification higher than current state-of-the-art performances. Furthermore, we demonstrate the application of that method in the development of a giant sensitivity fibre refractometer with a sensitivity of around 500 µm/RIU (refractive index unit) and with a magnification factor over 850.