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.

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.

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.

An all-fibre based Raman-on-chip setup is introduced which enables analysis of solutions and trapped particles without microscopes or objectives. Beside the novel quartz microfluidic chip, innovative multi-core single-mode fibres with integrated fibre Bragg gratings are used for detection. The limit of quantitation is 7.5 mM for urea and 2.5 mM for nicotine with linear Raman spectroscopy. This is an improvement of more than two orders of magnitude compared with previous fibre-based microfluidic Raman detection schemes. Furthermore, our device was combined with optical traps to collect Raman-on-chip spectra of spherical polymer beads.

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°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. An index-guiding PCF may be transformed into an all-solid photonic bandgap fiber (PBF) by filling highindex material into the air holes. Such an invertible transformation in the fiber types (PCF/PBF) has led to many switching applications [3][4][5][6]. These switches, however, usually turn on/off the light transmission over a broad wavelength range and cannot perform the switching function within a narrow wavelength range. In this Letter, we demonstrate a thermo-optic in-fiber optical switch produced by filling immersion oil into the air holes of a solid-core PCF in combination with an FBG. Such a switch can turn on/off light transmission within a narrow wavelength range (Bragg wavelength) as the result of the coupling between the fundamental core mode and the LP 01 -like mode in the fluid rods. A solid-core Ge-doped PCF (IPHT-252b5) with a core diameter of 4.1 m was employed in our experiments. Air holes of the PCF have an average diameter of 3.5 m and are arranged in a hexagonal pattern with an average pitch of 4.2 m. Other parameters of the PCF are listed in Table 1 of [7]. The PCF has a low transmission loss within a broad wavelength range, as shown in Fig. 1(a). The central region of the fiber core with a diameter of about 0.5 m was doped with a high concentration of germanium ͑36 mol.% ͒. The calculated refractive indices of the Ge-doped core and of the pure silica ͑SiO 2 ͒ cladding are Ϸ1.5039 and 1.4538, respectively, at a wavelength of 830 nm.As shown in Fig. 1(b), an FBG with a reflectivity of 45%, a Bragg wavelength of 829.76 nm, and a grating length of Ϸ6 mm was inscribed in the Ge-doped small-core PCF by the use of a 248 nm KrF excimer laser [8]. One end of the PCF with the FBG was spliced to a standard single-mode fiber (SMF) with a splice loss of Ϸ1.0 dB using the arc fusion splicing technique as reported in [9]. Another end of the PCF with a length of 300 mm was cleaved at a distance of 5 mm from the FBG and then placed in an immersion oil (n = 1.482 at room temperature; http:// www.niepoetter.de). Thus the immersion oil was filled into the air holes of the PCFs by means of the well-known capillarity action. The actual fluid-filled PCF including the FBG had a total length of Ϸ150 mm.Then we investigated the response of the fluidfilled FBG to the temperature change using a semiconductor Peltier cooler, a temperature controller (THORLABS TED200), and a specifically adapted FBG interrogation system [7]. As shown in Fig. 2(a), in the case of the temperatur...