[show abstract][hide abstract] ABSTRACT: Silica-based fiber tips are used in a variety of spectroscopic, micro- or nano-scopic optical sensor applications and photonic micro-devices. The miniaturization of optical sensor systems and the technical implementation using optical fibers can provide new sensor designs with improved properties and functionality for new applications. The selective-etching of specifically doped silica fibers is a promising method in order to form complex photonic micro structures at the end or within fibers such as tips and cavities in various shapes useful for the all-fiber sensor and imaging applications. In the present study, we investigated the preparation of geometrically predefined, nanoscaled fiber tips by taking advantage of the dopant concentration profiles of highly doped step-index fibers. For this purpose, a gas phase etching process using hydrofluoric acid (HF) vapor was applied. The shaping of the fiber tips was based on very different etching rates as a result of the doping characteristics of specific optical fibers. Technological studies on the influence of the etching gas atmosphere on the temporal tip shaping and the final geometry were performed using undoped and doped silica fibers. The influence of the doping characteristics was investigated in phosphorus-, germanium-, fluorine- and boron-doped glass fibers. Narrow exposed as well as protected internal fiber tips in various shapes and tip radiuses down to less than 15 nm were achieved and characterized geometrically and topologically. For investigations into surface plasmon resonance effects, the fiber tips were coated with nanometer-sized silver layers by means of vapour deposition and finally subjected to an annealing treatment.
[show abstract][hide abstract] ABSTRACT: We describe a novel, fully symmetrical deposition method, based on a sputtering technique and use of a gold ring target for deposition of a gold layer uniform around the taper waist. With such a circular symmetric coating, the plasmon resonance effect in the fiber taper becomes completely independent of the polarization of the illuminating light. We have measured the complex refractive index of such sputtered gold layers. On this basis, model calculations have been performed to describe the plasmon resonance as a function of taper waist diameter, gold layer thickness, analyte interaction length, and analyte refractive index. Optimized parameters especially for measurement in aqueous solutions are derived from these theoretical calculations. Experimental results are shown to be in good agreement with the theoretical analysis.
[show abstract][hide abstract] ABSTRACT: In this Letter, we present the technology of fiber Bragg grating (FBG) inscription in highly birefringent (HB) few-mode microstructured fibers (MSFs) with two different (nanosecond and femtosecond) lasers in a Talbot interferometer setup. The spectral characteristics of FBGs written in the core region of the investigated fiber, with particular modes represented by dual peaks, are presented and discussed. Furthermore, we calculate the fundamental fiber parameters (mode effective refractive index and phase modal birefringence) from the spectral characteristics and show very good agreement with the performed numerical fiber characterization. We expect the results of our experiments to be very useful in future development of FBG sensors based on novel HB MSFs, with enhanced strain sensitivity of higher-order modes.
[show abstract][hide abstract] ABSTRACT: Photodarkening and photobleaching effects in the case of UV femtosecond-pulse exposure of optical
fibres are investigated. We evaluate the existence of loss equilibrium states in Yb-doped and Yb-free
fibres. Supposing that parasitic VIS to NIR losses induced by fibre Bragg grating (FBG) inscription can
also be addressed by a photobleaching treatment, we find grating absorption remarkably reduced by a
post-exposure treatment. We also present photobleaching applied directly after FBG inscription with UV
femtosecond pulses to improve the wavelength and power stability of a monolithic FBG-based fibre laser
as a noteworthy alternative to subsequent thermal treatment.
[show abstract][hide abstract] ABSTRACT: The accuracy of the recently presented1 equivalent step index
approximation of multifilament core fibers is analyzed in terms of the
effective refractive index, mode field area and bending losses of the
fundamental mode. A modified Vparameter for this class of fibers as well
as a single-mode condition is proposed. By comparison with a
full-vectorial finite element method it is shown that the relative
deviation of the effective refractive index and the mode field area are
in the magnitude of 1 %. No significant decrease of bending losses is
found for multifilament core fibers.
[show abstract][hide abstract] ABSTRACT: The paper describes the implementation of fiber Bragg gratings inscribed by femtosecond laser pulses with a wavelength of 400 nm. The use of a Talbot interferometer for the inscription process makes multiplexing practicable. We demonstrate the functionality of a three-grating multiplexing sensor and the temperature stability up to 1200 °C for a single first-order Bragg grating.
[show abstract][hide abstract] ABSTRACT: We have realized a modified time-delay spectrometer based on a step-chirped fiber Bragg grating array. This method allows simultaneous spectral and temporal characterization of pulsed light sources in the nanosecond regime, which can also be applied to the investigation of single pulses. With a spectral resolution in the 100 pm range, pulse spectrograms are measured and exemplarily used to explore the emission behavior of a wavelength-stabilized laser diode directly modulated in the nanosecond range.
[show abstract][hide abstract] ABSTRACT: We present a novel approach to directly measure the bend loss of individual modes in few-mode fibers based on the correlation filter technique. This technique benefits from a computer-generated hologram performing a modal decomposition, yielding the optical power of all propagating modes in the bent fiber. Results are compared with rigorous loss simulations and with common loss formulas for step-index fibers revealing high measurement fidelity. To the best of our knowledge, we demonstrate for the first time an experimental loss discrimination between index-degenerated modes.
[show abstract][hide abstract] ABSTRACT: Polarization instability (PI) of ultrashort light pulses, giving rise to vectorial supercontinuum generation, is demonstrated using a subwavelength-core, highly birefringent, normally dispersive optical fiber. The evolution of ultrashort pulses in the regime of PI is shown to radically differ from polarization-instability dynamics of cw fields and longer laser pulses. As the peak power of the laser field decreases along the propagation path due to dispersion-induced pulse stretching, the Poincaré-sphere map of field dynamics is shown to evolve from the behavior typical of PI in the highly nonlinear regime toward the beating dynamics of uncoupled polarization modes, characteristic of low field intensities and cw fields.
[show abstract][hide abstract] ABSTRACT: Besides well-known applications of fiber Bragg gratings (FBGs) in sensing and telecommunication, current research focuses also on an integration of gratings in actively doped fibers in order to realize high-power monolithic fiber lasers. The inscription of Bragg gratings directly into the active laser material has been achieved successfully, but the influence of the different dopants in rare-earth-doped fibers on the inscription process has usually failed to be considered. In this paper we report on a systematic investigation of Bragg grating growth behavior during inscription with UV and VIS fs pulses in various actively doped fibers. Furthermore we present an analysis of the initial grating growth in active fibers with different rare-earth dopant concentration under different femtosecond laser exposure conditions. A positive influence of cerium doping on grating growth and photosensitivity has been found. Ytterbium doping has been found to decelerate grating growth, meaning decreased photosensitivity to femtosecond pulses. The specific doping type seems to play an important role in the fabrication of high-reflective Bragg gratings in rare-earth-doped fibers.
[show abstract][hide abstract] ABSTRACT: We report on the in-core fiber temperature measurements in Yb2O3-doped aluminum silicate fibers during optical pumping using UV femtosecond-induced fiber Bragg gratings (FBGs). The FBG containing fiber samples show a different temperature behavior during pumping before and after thermal annealing, caused by absorption centers generated during grating inscription. These absorption centers can be removed completely by thermal annealing. In this way, remaining temperature rise during pumping due to the quantum defect of the Yb3+ ions is determined depending on the Yb concentration in the range 0.14–0.46 mol% and the pump wavelength.
[show abstract][hide abstract] ABSTRACT: We report on the possibilities of nanoscale optical fibers with all-normal dispersion behavior for pulse-preserving and coherent supercontinuum generation at deep ultraviolet wavelengths. We discuss the influence of important parameters such as pump wavelength and fiber diameter, for both optical nanofibers and nanoscale suspended-core optical fibers. Simulations reveal that by appropriate combination of fiber geometry and input pulse parameters, intensive spectral components well below 300 nm are generated. In addition, the impact of preceding taper transitions used for input coupling purposes is discussed in detail.
[show abstract][hide abstract] ABSTRACT: Design and realization of fiber Bragg grating sensor systems are described for implementation of structural health monitoring in energy plants. Application examples in conventional, renewable, and nuclear energies demonstrate their potential in advanced energetics.
[show abstract][hide abstract] ABSTRACT: Since the first presentation of selectively metal filled photonic
crystal fibers (PCFs) in 2008, a lot of work and effort has been put in
the understanding of propagation characteristics of such fibers which
can be utilized as filters or polarizers. A semi-analytical model for
the implicit description of the effective refractive index of surface
plasmon polaritons propagating (SPPs) along the metal wires has been
developed and coupling of fiber core modes to such surface modes has
been confirmed experimentally. In this work we will present a method for
the fabrication of selectively metal filled photonic crystal fibers and
derive the dispersion equation for micron sized wires in silica. We will
present a ray-optical approximation of SPPs based on the dispersion of a
planar dielectric-gold interface which leads to a full-analytical
equation for the prediction of cutoff wavelengths of the SPPs.
[show abstract][hide abstract] ABSTRACT: The properties of optical fibers can significantly be influenced by
intrinsic stress. It is well known that these stresses are caused by
various reasons, e.g. the variations in the thermal expansion coecient
of the differently doped regions in the fiber. The so called thermal
stresses are only dependent on the composition of the fiber and not on
its preparation history. Another main reason for stress in the final
fiber is the mechanical force that is applied during the fiber drawing
process. It generates so called mechanical stress that depends on the
composition of the fiber and the thermal history of the fiber
fabrication process. Using a non-destructive polarimetric system, we are
able to measure the intrinsic stress state in optical fibers as well as
in their preforms. Knowing on the one hand the thermal induced stresses
in the preform of a fiber and on the other hand the final stress state
in the fiber itself, we are able to differentiate between the two kinds
of stress. In this paper we present results of stress measurements on
optical ber preforms and fibers. We show, that the measured stress
profile in the preform matches the theoretically assumed stress profile
for thermal stress very well. Moreover we used this preform to draw
fibers under different drawing conditions represented in a large
difference in the applied force during the fiber drawing. We present the
stress results for these differently fabricated fibers and show how huge
the effect of the drawing tension can be. We find that for high drawing
forces, the stress state can be reversed in comparison to the thermal
stresses that are induced by the material composition. Due to the fact
that stress on the one hand has a strong effect on the mechanical
properties of glass and modifies the refractive index, this can lead to
signicant effects on the fiber stability and modal behaviour. Finally,
we present a way to compensate the additionally induced mechanical
stress, which is for example a very good possibility to increase the
stress birefringence in polarization maintaining (PM) fibers with panda
structure. We compare the mechanical stress states of such Panda Fibers
after their fabrication with the state after an additional high
temperature step. We clearly find that it is possible to improve the
birefringence of these fibers using appropriate preparation steps.
[show abstract][hide abstract] ABSTRACT: The combination of fiber Bragg grating arrays integrated in a soft
plastic tube is promising for high resolution manometry (HRM) where
pressure measurements are done with high spatial resolution. The
application as a medical device and in vivo experiments have to be
anticipated by characterization with a measurement setup that simulates
natural conditions. Good results are achieved with a pressure chamber
which applies a well-defined pressure with a soft tubular membrane. It
is shown that the proposed catheter design reaches accuracies down to 1
mbar and 1 cm.
[show abstract][hide abstract] ABSTRACT: We report on the inscription of FBGs in rare earth doped optical fibers,
the reduction of inherent absorption effects in the FBGs and the
FBG-based temperature measurement within the core of actively doped
fiber samples during core pumping. Besides a temperature increase due to
the quantum defect of Yb-ions a change in temperature during pumping was
observed and fits qualitatively well to the parallel measured
[show abstract][hide abstract] ABSTRACT: Silica-based fiber tips are used in a variety of spectroscopic, micro-
or nanoscopic optical sensor applications. The efficiency of such
measurement systems is dependent on the tip geometry and the diameter of
the tapered fiber end. In the present study we investigated the
preparation of geometrically predefined, nanoscaled fiber tips by taking
advantage of the dopant concentration profiles of highly doped
step-index fibers. For this purpose, a gas phase etching process using
hydrofluoric acid (HF) vapor was applied. The shaping of the fiber tips
was based on very different etching rates as a result of the doping
characteristics of specific optical fibers. Technological studies on the
influence of the etching gas atmosphere on the temporal tip shaping and
the final geometry were performed using undoped and doped silica fibers.
The influence of the doping characteristics was investigated in
phosphorus-, germanium-, fluorine- and boron-doped glass fibers. Narrow
exposed as well as protected internal fiber tips in various shapes and
tip radiuses down to less than 15 nm were achieved and characterized
geometrically and topologically. For investigations of surface plasmon
resonance effects the fiber tips were coated with nanometer-sized silver
layers by means of vapour deposition and finally subjected to an
[show abstract][hide abstract] ABSTRACT: We investigated the effect of modification of silica with high concentrations of germanium up to 36mol% as well as with highly
polarizable dopants (e.g., barium and lanthanum) on optical behavior of microstructured fibers. The goal was to investigate
the influence of doping on several properties like fiber attenuation, supercontinuum generation and birefringence in microstructured
optical fibers (MOF).
Optical and Quantum Electronics 04/2012; 39(12):1057-1069. · 0.99 Impact Factor