Hartmut Bartelt

Institut für Photonische Technologien, Jena, Thuringia, Germany

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Publications (263)261.49 Total impact

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    ABSTRACT: The interaction between longitudinal acoustic waves and fiber Bragg gratings in suspended-core optical fibers is numerically investigated. The fiber core size and the air hole size are varied, and the mechanical and grating properties are simulated by means of the finite element method and the transfer matrix method, respectively. Changes of the effective index, confinement factor, silica area, strain and wavelength shift induced by the acoustic wave are evaluated, and the resultant side lobe reflectivity is estimated. A side lobe reflectivity increase of 66% compared to standard fibers is estimated, which allows reducing the modulation index or the grating length in as much as 75%. Besides, the larger reduction of the required acoustic power for achieving the acousto-optic modulation points out to more efficient modulator devices in suspended-core fibers.
    Optics Communications 06/2015; 344. · 1.54 Impact Factor
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    ABSTRACT: The propagation properties of optical fibers can be significantly influenced by intrinsic stress. These effects are often undesired but in some cases essential for certain applications, e.g. in polarization maintaining (PM) fibers. In this paper, we present systematic studies on the influence of the fiber drawing process on the generated stress and demonstrate an approach to significantly increase the stress induced birefringence of PM-fibers. It is shown that the thermal stress caused by the material composition is superimposed with the mechanical stress caused by the fiber fabrication process. This intrinsic stress has a strong effect on the optical and mechanical properties of the glass and thus influences the fiber stability and modal behavior. By applying a thermal annealing step, the mechanical stress due to the fiber drawing process can be canceled. It is shown that this annealing step compensates the stress reducing influence of the drawing process on the birefringence of PM-fibers with panda structure. The comparison of the intrinsic stress states after fabrication with the state after the additional high temperature annealing step clearly shows that it is possible to improve the overall birefringence of panda fibers using appropriate preparation steps.
    Optical Materials. 02/2015;
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    ABSTRACT: We demonstrate amplification experiments using a very large mode area Yb-doped doubleclad fiber with 100 μm aluminum-cer codoped core and 440 μm pump cladding realized by high aluminum codoping. The material for core and pump cladding was fabricated by reactive powder sinter technology. A high numerical aperture (NA) of the pump cladding with NA = 0.21 and a low one of the core with NA = 0.084 could be realized. Using a 0.55 m short fiber sample as the main amplifier in a three-stage ns pulsed fiber master oscillator power amplifier system we achieved 3 ns, 2 mJ output pulses with 360 kW peak power limited by the available pump power. Stimulated Raman scattering effects and amplified spontaneous emission were successfully suppressed.
    Laser Physics Letters 11/2014; 12(1):015103. · 2.96 Impact Factor
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    ABSTRACT: A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950°C for more than 24 h without degradation in reflectivity.
    Optics Express 11/2014; 22(22). · 3.53 Impact Factor
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    ABSTRACT: We discuss a fiber-integrated refractive index sensor with strongly improved detection performance. The resonator has been implemented by means of focused-ion beam milling of a step index fiber and shows a sensitivity of about 1.15µm/RIU. Coating the resonator walls led to a strongly improved mirror reflectivity by a factor of about 26. Design rules for device optimization and a detailed mathematical analysis are discussed, revealing that the sensor operates as an optimized Fabry-Perot resonator. We also show that the performance of such kind of Fabry-Perot sensors is, in general, limited by the detection limit function – a quantity depending on the cavitiy’s finesse and on the measurement capabilities used.
    Optics Express 10/2014; 22(21). · 3.53 Impact Factor
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    ABSTRACT: In this letter, we investigate the inscription of fiber Bragg gratings in the visible spectral range using deep ultraviolet femtosecond laser exposure and two-beam interferometry. The properties of first-order reflection gratings and third-order gratings for use in the visible wavelength range are compared. Stronger gratings have been achieved for first-order reflecting Bragg gratings compared with third-order gratings. We demonstrate a fiber Bragg grating with a grating period of 226 nm and a filtering efficiency of more than 30 dB.
    IEEE Photonics Technology Letters 08/2014; 26(16):1653-1656. · 2.18 Impact Factor
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    ABSTRACT: We present a method to tune the emission wavelength of pulsed fiber lasers using stepchirped fiber Bragg grating arrays as filters. First experimental results demonstrate excellent spectral characteristics and a tuning range of 18 nm.
    Advanced Photonics, OSA Congress, Barcelona; 07/2014
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    ABSTRACT: We present results for combining fiber lasers with a total average power of 5.1kW into a 50μm output fiber. The heating of the free-standing 7x1 glass structure is only 12K/kW.
    Advanced Photonics, OSA Congress, Barcelona; 07/2014
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    ABSTRACT: We report on a Yb-doped large-core fiber laser at 976 nm prepared by powder-sinter technology. Wavelength-locked single-mode operation is achieved with 10 W out of a standard single-mode fiber.
    Advanced Photonics, OSA Congress, Barcelona; 07/2014
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    ABSTRACT: In this Letter we present, for the first time to our knowledge, the results of fiber Bragg grating (FBG) inscription in a novel microstructured multicore fiber characterized by seven single-mode isolated cores. A clear Bragg reflection peak can be observed in all of the 7 cores after one inscription process with a KrF nanosecond laser in a Talbot interferometer set up. We furthermore perform a numerical analysis of the effective refractive indices of the particular modes and compare it with the FBG inscription results. An experimental analysis of the strain and temperature sensitivities of all of the Bragg peaks is also included.
    Optics Letters 06/2014; 39(12):3571-3574. · 3.39 Impact Factor
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    ABSTRACT: We demonstrate a distributed measurement technique to observe temperature changes along pumped Yb-doped fibers. This technique is based on an array of fiber Bragg gratings acting as a temperature sensor line. The Bragg gratings are inscribed directly into the Yb-doped fiber core using high-intensity ultrashort laser pulses and an interferometric setup. We studied the temperature evolution in differently co-doped Yb fibers during optical pumping and identified different effects contributing to the observed temperature increase. We found that preloading of fibers with hydrogen supports the formation of Yb2+ during UV irradiation and has a large impact on fiber temperature during pumping. The proposed technique can be applied to investigate the homogeneity of pump absorption in active fibers and to support spatially resolved photodarkening measurements.
    Optical Engineering 06/2014; 53(6):066101. · 0.96 Impact Factor
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    ABSTRACT: The regeneration of fibre Bragg gratings (FBG) is supposed to be seeded by the relief of the in-frozen core cladding stress of the fibre. Earlier results lead to the assumption that the diffusion process of small atoms or molecules through the glass matrix is sufficient to seed regeneration of an FBG. To prove this theory, a regeneration experiment was performed with hydrogen loaded, pristine and out-diffused SMF28 fibers, respectively. For the FBG in an out-diffused fibre, the fibre was stored at room temperature after hydrogen loading to get rid of the total amount of hydrogen before FBG inscription. The FBG inscription experiments confirmed that the diffusion of hydrogen changes the fibre intrinsic stresses. From all three kinds of SMF28 fibers, one FBG was annealed with a stepwise temperature profile for regeneration. Only the FBG in hydrogen loaded fibre regenerated, what implies that the formation of hydroxyl groups is necessary for the regeneration process. The reduction of the in-frozen stress due to the hydrogen diffusion, in contrast, was not sufficient to seed regeneration.
    Proc. SPIE. 9157, 23rd International Conference on Optical Fibre Sensors; 06/2014
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    ABSTRACT: Focused ion beam technology is combined with chemical etching of specifically designed fibers to create Fabry-Perot interferometers. Hydrofluoric acid is used to etch special fibers and create microwires with diameters of 15 μm. These microwires are then milled with a focused ion beam to create two different structures: an indented Fabry-Perot structure and a cantilever Fabry-Perot structure that are characterized in terms of temperature. The cantilever structure is also sensitive to vibrations and is capable of measuring frequencies in the range 1 Hz - 40 kHz.
    Optics Express 06/2014; 22(11):13102-13108. · 3.53 Impact Factor
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    ABSTRACT: A miniaturized design of a fibre-optic based total temperature probe is presented which is intended to be used for aircraft engine testing. The FBG-based probes were experimentally characterized in the harsh environment of an open jet test bench at air stream velocities up to Mach numbers of 0.8 and they showed similar performance as conventional sensing techniques. The reduction of the size of the transducer was possible by using small sized FBG-based temperature sensors. Due to the smaller probe design and thinner signal transmission cables, less distortion of the air stream is expected with the fibre-optic sensor, when compared to conventional instrumentation. This allows a higher density of instrumented sensors and thus a higher spatial resolution of the measurement. This is, to our knowledge, the first demonstration of total temperature measurements at high air stream velocities with FBG-based transducers.
    Proc. SPIE. 9157, 23rd International Conference on Optical Fibre Sensors; 06/2014
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    ABSTRACT: Microstructured optical fibers (MOF) sometimes also referred to as photonic crystal fibers (PCF) have been a subject of extensive research for over a decade. This is mainly due to the fact that by changing the microstructure geometry (e.g. distribution and size of the air-holes) fiber properties can be significantly modified to better fit specific applications. In this manuscript we present a novel fiber design with three large air-holes neighboring the core and report on how the air-hole diameter influences the effective refractive index strain sensitivity. As direct measurement of the effective refractive index change may be complex and challenging, we propose to use fiber Bragg gratings (FBG) in our sensing set up. The Bragg wavelength is a function of the effective refractive index, hence the external strain changes can be monitored through the Bragg wavelength shift with a simple optical spectrometer. Furthermore we also include an analysis of the fibers temperature sensitivity.
    SPIE Photonics Europe; 05/2014
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    ABSTRACT: Photonic crystal fibers (PCF), sometimes also referred to as microstructured fibers (MSF), have been a subject of extensive research for over a decade. This is mainly due to the fact that by changing the geometry and distribution of the air holes the fiber properties can be significantly modified and tailored to specific applications. In this paper we present the results of a numerical analysis of the influence of the air-hole distribution on the sensitivity of the propagated modes' effective refractive index to externally applied longitudinal strain. We propose an optimal strain sensitive fiber design, with a number of fibers drawn and experimentally evaluated to confirm the theoretical results. Furthermore as the direct measurement of the effective refractive index change may be complex and challenging in field environment, we propose to use fiber Bragg gratings (FBG) in our sensing set-up. As the Bragg wavelength is a function of the effective refractive index, the external strain changes can be monitored through the Bragg wavelength shift with a simple optical spectrometer. Moreover, since the PCF is also optimized for low-loss splicing with standard single mode fiber, our novel sensor head can be used with standard off-the-shelf components in complex multiplexed sensing arrays, with the measured signal transmitted to and from the sensor head by standard telecom fibers, which significantly reduces costs.
    02/2014;
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    ABSTRACT: We report on a tapered large-core Yb fiber laser operating at 976 nm emission wavelength. It was realized using a high numerical aperture large-core fiber with 126 um core diameter, which was fabricated by powder-sinter technology and shows a very homogeneous step-index profile. The end of the fiber is tapered down to match a single-mode fiber containing a fiber Bragg grating. Using the benefits of core-pumping and the feedback of the spliced fiber Bragg grating, we achieved efficient pump light absorption and wavelength stable 976 nm lasing with single-mode performance. We could demonstrate 10 W laser power out of a 10µm fiber core with a slope efficiency of 31% with respect to the launched pump power. The presented device is well-suited for fiber-coupled pumping of amplifiers for high peak power.
    Laser Physics Letters 02/2014; 11(4). · 2.96 Impact Factor
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    ABSTRACT: A simple optical inclinometer based on a phase-shifted Bragg grating in a taper configuration is proposed. Two phase-shifted fiber Bragg gratings were fabricated using a DUV femtosecond laser technique in the taper region, with taper waist diameters of 30 μm and 50 μm. Both sensing heads were compared based on their response to angle and strain. Whereas the higher diameter sensor yielded a higher sensitivity to curvature (23.8 ± 0.3 pm/degree), the lower diameter one was more sensitive to strain (8.94 ± 0.04 pm/με).
    IEEE Photonics Technology Letters 02/2014; 26(4):405. · 2.18 Impact Factor
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    ABSTRACT: In this paper we report on an alternative technique for the preparation of ytterbium (Yb)-doped silica fibers and their characteristics compared to the conventional modified chemical vapor deposition (MCVD) process in combination with solution doping and powder sinter technology (REPUSIL). In the case of the technique applied here, the active core diameter in the preform can be significantly increased via the deposition of Yb and the most important codopant, aluminum (Al), in the gas phase through the high-temperature evaporation of the Yb chelate compound and Al chloride in the MCVD process. The prepared preform shows a homogenous distribution of the refractive index and dopant concentration. The background loss of the drawn fiber was measured to be 25 dB km−1 at 1200 nm. Efficient lasing up to 200 W, showing a slope efficiency of about 80%, was demonstrated, which is comparable to fibers made via MCVD/solution doping and the REPUSIL technique.
    Laser Physics 02/2014; 24(3). · 1.03 Impact Factor
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    ABSTRACT: In a highly Ge-doped photosensitive fiber co-doped with phosphorus and fluorine and with a doped inner cladding, we observed, during annealing experiments, an anomalous variation of the FBG strength as a function of temperature and time. This characteristic only occurred when the fiber had been loaded with hydrogen. During a stepwise temperature increase from room temperature to 1200 °C with 15 min dwell times, the gratings showed, after an initial thermal decay, in two distinct temperature ranges a significant increase in reflectivity with time. One range with an increase in reflectivity was found between 850 °C and 1000 °C. The behavior of the FBGs in this range was very similar to that known from chemical composition or regenerated gratings. Another temperature range with increasing reflectivity was found between 500 °C and 700 °C, followed by a strong decrease in reflectivity at temperatures between 750 °C and 800 °C. Both regimes of regeneration might be associated with changes in the core and the doped inner cladding of this type of fiber. At different temperatures the drift behavior of λB and ΔnAC was investigated. In specific temperature regions a positive wavelength drift combined with a decrease in ΔnAC has been observed. This is in contradiction to the standard thermal decay model and seems to be a characteristic of regenerated gratings.
    Optics Communications 02/2014; · 1.54 Impact Factor

Publication Stats

1k Citations
261.49 Total Impact Points

Institutions

  • 2007–2014
    • Institut für Photonische Technologien
      Jena, Thuringia, Germany
  • 2003–2013
    • Friedrich-Schiller-University Jena
      • • Abbe Center of Photonics (ACP)
      • • Department of Applied Physics
      Jena, Thuringia, Germany
  • 2008–2011
    • Free University of Brussels
      • Applied Physics and Photonics (TONA)
      Bruxelles, Brussels Capital Region, Belgium
    • Universität Paderborn
      • Department of Chemistry
      Paderborn, North Rhine-Westphalia, Germany
  • 2009–2010
    • Universität Osnabrück
      Osnabrück, Lower Saxony, Germany
    • Wroclaw University of Technology
      • Institute of Physics
      Wrocław, Lower Silesian Voivodeship, Poland
  • 2002
    • High-Tech Institute
      North Carolina, United States