Sven Brückner

Fiber Bragg grating inscription combining DUV sub-picosecond laser pulses and two-beam interferometry

The combination of fiber Bragg grating inscription with femtosecond laser sources and the usage of the Talbot interferometer setup not only gives access to the fabrication of Bragg gratings in new types of materials but also allows, at the same time, to keep the high flexibility of an interferometric setup in choosing the Bragg grating wavelength. Since the spatial and temporal coherence properties of the femtosecond laser source differ strongly from those of conventional laser sources, specific limits and tolerances in the interferometric setup have to be considered. Such limits are investigated on the basis of an analytical ray tracing model. The results are applied to tolerance measurements of fiber Bragg grating reflections recorded with a DUV sub-picosecond laser source at 262 nm. Additionally we demonstrate the wavelength versatility of the two-beam interferometer setup for femtosecond inscription over a 40 nm wavelength band. Inscription experiments in Al/Yb doped silica glasses are demonstrated as a prove for the access to non-photosensitive fibers.

Thermal regeneration of fiber Bragg gratings in photosensitive fibers

We report about a thermal regeneration of fiber Bragg gratings written in photosensitive fibers with nanosecond laser pulses. We observe a regenerative process in a highly photosensitive fiber without hydrogen loading which indicates a secondary grating growth in an optical fiber by thermal activation. This process is more temperature stable than the commonly known gratings produced by color center modifications. The writing conditions of such new type of gratings are investigated and the temperature behavior of these regenerated fiber Bragg gratings is analyzed. The application possibilities are in the field of high temperature sensor systems by making use of the combination of good spectral shape of a Type I grating with a Type II like temperature stability.

Multicore fiber with integrated fiber Bragg gratings for background-free Raman sensing

In the last years a variety of fiber optic Raman probes emerged, which are only partly suited for in vivo applications. The in vivo capability is often limited by the bulkiness of the probes. The size is associated with the required filtering of the probes, which is necessary due to Raman scattering inside the fibers. We employed in-line fiber Bragg gratings (FBG) as notch filter for the collection path and integrated them in a novel type of Raman probe. Multicore singlemode fibers (MCSMF) were designed and drawn integrating 19 singlemode cores to achieve better collection efficiency. A Raman probe was assembled with one excitation fiber and six MCSMF with inscribed FBGs as collection fibers. The probe was characterized regarding Raman background suppression, collection efficiency, and distance dependence. First Raman measurements on brain tissue are presented.

High force measurement sensitivity with fiber Bragg gratings fabricated in uniform-waist fiber tapers

Fiber Bragg gratings inscribed in the waist of tapered photosensitive fibers offer specific attractive properties for sensing applications. A small-diameter fiber reduces structural influences for imbedded fiber sensing elements. In the case of application as a force-sensing element for tensile forces, sensitivity scales inversely with the fiber cross-sectional area. It is therefore possible to increase force sensitivity by several orders of magnitude compared to Bragg grating sensors in conventionally sized fibers. Special requirements for such Bragg grating arrangements are discussed and experimental measurements for different fiber taper diameters down to 4 µm are presented.

Post-hydrogen-loaded draw tower fiber Bragg gratings and their thermal regeneration.

The idea of Bragg gratings generated during the drawing process of a fiber dates back almost 20 years. The technical improvement of the draw tower grating (DTG) process today results in highly reliable and cost-effective Bragg gratings for versatile application in the optical fiber sensor market. Because of the single-pulse exposure of the fiber, the gratings behave typically like type I gratings with respect to their temperature stability. This means that such gratings only work up to temperatures of about 300 °C. To increase temperature stability, we combined DTG arrays with hydrogen postloading and a thermal regeneration process that enables their use in high-temperature environments. The regenerated draw tower gratings are demonstrated to be suitable for temperatures of more than 800 °C.

Inscription of Fiber Bragg Grating Arrays in Pure Silica Suspended Core Fibers

Fiber Bragg grating (FBG) arrays in pure silica four-leaf-clover-shaped suspended core fibers were inscribed with two-beam interference and a deep UV femtosecond laser source. The target fibers are pure silica and do not contain any dopants, nor were they treated with hydrogen before grating inscription. The inscription method is appropriate to measure the effective mode field index of the fiber and allows confinement factors of the guided modes to be derived. Applications of such FBGs can be expected especially for fiber sensing.

Preparation of arsenic sulfide thin films for integrated optical elements by spiral bar coating

We report on the preparation of thin As35S65 films from their amine based solutions by a spiral bar coating technique on flexible PET substrates and the direct writing of subwavelength surface corrugated gratings with a period of 350 nm into these films by excimer laser interference lithography. The structural and optical properties of bar coated As35S65 glass are investigated in detail and compared with those of samples fabricated by vacuum thermal evaporation. The polarization dependent spectral filtering properties of the guided mode resonance devices built by the waveguiding thin films and the surface relief gratings written by a single laser pulse are presented finally.

Draw tower fiber Bragg gratings and their use in sensing technology

The idea of fabricating fiber Bragg gratings already during the drawing of a fiber dates back almost 20 years. The application of a transverse holographic writing method on a draw tower offers a promising solution for a highly effective Bragg grating production. Because of the high technology requirements it took more than 10 years to develop the method into a reliable process. During the last five years the improvements in the technical development enables cost effective industrial production of draw tower gratings (1DTG®). In this paper we report about new possibilities of the improved process with respect to the grating type (type I gratings, type II gratings), the coating type (2ORMOCER®, metals) and the fiber type and diameter (125μm, 80μm and below). Furthermore, we present examples for the application of draw tower fiber Bragg gratings in sensing technologies for medical applications.

Generation of flattop pump pulses for OPCPA by coherent pulse stacking with fiber Bragg gratings

We present a simple and robust pulse shaping device based on coherent pulse stacking. The device is embedded in a polarisation maintaining step index fiber. An input pulse is sent through a fiber optical circulator. Up to four pulse replicas are reflected by fiber Bragg gratings and interfere at the output. Temperature control allows tuning of the relative pulse phases of the sub-pulses. Additionally fine tuning of the sub-pulse amplitudes is demonstrated. We experimentally generated 235 ps and 416 ps long flattop pulses with rising and falling edges shorter than 100 ps. In contrast to other pulse shaping techniques the presented setup is robust, alignment free, provides excellent beam quality and is also suitable for pulse durations up to several nanoseconds.

Selective filling of metals into photonic crystal fibers

In this paper we present a method for the selective blocking and subsequent filling of metals into photonic crystal fibers. We derive a model which can predict the necessary duration of the filling process. With a melt and pump procedure we obtain single micron sized metal wires adjacent to the PCF core with aspect ratios of about 105. We will present a semi-analytical solution of the dispersion relation of a cylindrical metal wire in a dielectric and discuss the results with respect to surface plasmon polaritons. By comparision with finite element simulations of an unfilled photonic crystal fiber we will show that a coupling between a core mode and surface mode is possible at specific phase matching wavelengths. Furthermore, measurements of transmission spectra will be presented to confirm the mode coupling between the fundamental core mode and the surface plasmon polariton of order m = 3.