Ron Spittel

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.

Optical switch based on a fluid-filled photonic crystal fiber Bragg grating

We report the implementation of an in-fiber optical switch by means of filling a fluid into the air holes of a photonic crystal fiber with a fiber Bragg grating. Such a switch can turn on/off light transmission with an extinction ratio of up to 33 dB within a narrow wavelength range (Bragg wavelength) via a small temperature adjustment of +/-5 degrees C. The switching function is based on the temperature-dependent coupling between the fundamental core mode and the rod modes in the fluid-filled holes resulting from the thermo-optic effect of the filled fluid.

Nanoparticle Layer Deposition for Plasmonic Tuning of Microstructured Optical Fibers

Plasmonic nanoparticles with spectral properties in the UV-to-near-IR range have a large potential for the development of innovative optical devices. Similarly, microstructured optical fibers (MOFs) represent a promising platform technology for fully integrated, next-generation plasmonic devices; therefore, the combination of MOFs and plasmonic nanoparticles would open the way for novel applications, especially in sensing applications. In this Full Paper, a cost-effective, innovative nanoparticle layer deposition (NLD) technique is demonstrated for the preparation of well-defined plasmonic layers of selected particles inside the channels of MOFs. This dynamic chemical deposition method utilizes a combination of microfluidics and self-assembled monolayer (SAM) techniques, leading to a longitudinal homogeneous particle density as long as several meters. By using particles with predefined plasmonic properties, such as the resonance wavelength, fibers with particle-adequate spectral characteristics can be prepared. The application of such fibers for refractive-index sensing yields a sensitivity of about 78 nm per refractive index unit (RIU). These novel, plasmonically tuned optical fibers with freely selected, application-tailored optical properties present extensive possibilities for applications in localized surface plasmon resonance (LSPR) sensing.

A miniature temperature high germanium doped PCF interferometer sensor

We report in this paper a high thermal sensitivity (78 pm/°C) modal interferometer using a very short Photonic Crystal Fiber stub with a shaped Germanium doped core. The Photonic Crystal Fiber is spliced between two standard fibers. The splice regions allow the excitation of the core and cladding modes in the PCF and perform an interferometric interaction of such modes. The device is proposed for sensitive temperature measurements in transmission, as well as in reflection operation mode with the same high temperature sensitivity.

Thermo-Optic Switching Effect Based on Fluid-Filled Photonic Crystal Fiber

We report a thermo-optic switching effect with a high extinction ratio of 30 dB by means of filling a fluid into air holes of a solid-core photonic crystal fiber (PCF). Such an effect can perform a turn on-off operation of the transmitted light via a small temperature adjustment of ±10°C. The switching function attributes to the absorption of the filled fluid in combination with the interaction between the core mode and the excited ¿fluid rod¿ modes, resulting from the thermo-optic effect of the filled fluid.

Curvature-induced geometric momenta: the origin of waveguide dispersion of surface plasmons on metallic wires

We show that the propagation of surface plasmon polaritons (SPPs) on metallic wires is governed by two solely curvature-induced geometric momenta, leading to a significant modification of the waveguide dispersion, i.e. a change of their phase velocity. By quantifying the azimuthal momentum and superimposing two planar SPPs of opposite helicity, we find an analytic expression for the dispersion of guided SPPs. This expression shows excellent agreement with numerical simulations and allows explaining fundamental SPP properties such as waveguide dispersion.

Analysis of the multifilament core fiber using the effective index theory

Multifilament core (MFC) fibers are a new type of microstructured fiber recently introduced. We investigate their properties using finite element modeling and show that the equivalent step index fiber based on moments theory does not provide similar properties. We propose an effective index theory based on the fundamental space filling mode which allows to predict the MFC properties using a semi-analytical modeling. Good resistance to bending is thus attributed to increased core effective index due to the high index filaments.

Polymer-Filled Silica Fibers as a Step Towards Electro-Optically Tunable Fiber Devices

Microstructured fibers offer the possibility of being filled with unconventional materials in order to influence the propagation properties in an optical fiber waveguide. We have studied the filling of microstructured silica fibers with electro-optic polymers. For this purpose a new electro-optic polymer formulation has been developed which is well suited for the complete filling of capillaries in microstructured optical fibers. Such filling could be of special interest for all-solid bandgap fibers thanks to the high refractive index of electro-optic polymers compared with silica, for fiber integrated electro-optical modulation, or for tunable fiber Bragg gratings. We demonstrate waveguiding with such filled fibers, investigate the integration of electrodes for poling and prove the possibility of long-period grating inscription in such fibers.

A semi-analytical model for the approximation of plasmonic bands in arrays of metal wires in photonic crystal fibers.

We present a highly efficient semi-analytical and straightforward-to-implement model for the determination of plasmonic band edges of metallic nanowire arrays inside photonic crystal fibers. The model relies on the approximation of the hexagonal unit cell by a circle and using particular boundary conditions, showing an accurate agreement with finite element simulations. The model reduces simulation time by a factor of 100, thus representing an efficient tool for structure design. It further allows the calculation of all relevant modes in the system by slight changes of the entries in a 4 × 4 matrix.

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.