Fiber Bragg gratings as key components in telecommunication, fiber lasers, and sensing systems usually rely on the Bragg condition for single mode fibers. In special applications, such as in biophotonics and astrophysics, high light coupling efficiency is of great importance and therefore, multimode fibers are often preferred. The wavelength filtering effect of Bragg gratings in multimode fibers, however is spectrally blurred over a wide modal spectrum of the fiber. With a well-designed all solid multicore microstructured fiber a good light guiding efficiency in combination with narrow spectral filtering effect by Bragg gratings becomes possible.
In the present work, the use of a single mode fiber (SMF) with high Germanium doped core as temperature sensor is studied. The fiber core consists of a 1.1 μm highly germanium doped step index waveguide surrounded by a pedestal of 3.5 μm diameter. The outer diameter of the fiber is 125 μm. A short stub of ~2 mm is used in the fabrication of the interferometer. The highly germanium doped fiber is spliced between two standard SMF. In one of the splices both fibers are ideally aligned, in the other splice a small misalignment between the fibers is done. An annealing process is made for 5 hours at 850°C which results in a good operation stability up to 700°C. A wavelength shift as a function of temperature of 76 pm/°C is reported. To demonstrate the interferometer efficiency, a fiber Bragg grating is written in the highly germanium doped core and tested for temperature response. A temperature sensitivity of 13pm/°C was demonstrated. The interferometer fabrication requires only a few and easy steps. Due to the standard splices made between the fibers, the device is robust. We believe that the sensor may be used under harsh environmental conditions, since it shows a high sensitivity and a small size in combination with great robustness.
In this work, the use of a photonic crystal fiber (PCF) with a highly Germanium (Ge) doped core is exploited as temperature sensor for the first time (to our knowledge). The PCF has an outer diameter of 125 μm and consists of a microstructured cladding with an average pitch and hole diameter of Λ=4.6 μm and d=1.0 μm, respectively. A short PCF stub (~2.0 mm) is used for the preparation of an interferometer. The PCF is spliced between single mode fibers (SMF), meaning that the PCF holes are fully collapsed in the splicing region while the Ge-doped core is still present. The splice parameters were changed to make a short collapse region of (200±30) μm. The first splice is used to excite the fundamental core mode and multiple higher order cladding modes by applying a core-to-core offset. The second splice acts as spatial filter to detect only the light which is guided in and near the core. The interferometer is heated up to 500°C and the total wavelength shift with the temperature variation found to be 74 pm/°C which is more than 5 times higher than a fiber Bragg grating at 1550 nm (13 pm/°C). The PCF interferometer preparation requires only a few steps, cleaving and splicing the fibers. The short length, the high thermal sensitivity and stability of the structure make the device attractive for many sensing applications including high temperature ranges.
The combination of Raman spectroscopy with fiber optic probes enables analyzing the biochemical composition of tissues without markers in a non-destructive way. A small diameter (1 mm) fiber optic probe with one excitation fiber, 11 detection fibers and integrated filters (Emvision, USA) was recently coupled to a Raman spectrometer (Kaiser Optical Systems) to study excised arteries ex vivo and rabbit arteries in vivo. The current contribution introduces a novel fiber optic Raman probe with in-line fiber Bragg gratings (FBGs) as notch filter in the collection path. Multi-core single-mode fibers (MCSMF) were drawn integrating 19 and 61 single-mode cores to improve collection efficiency. Raman probes were assembled with one fiber for excitation and six MCSMF with inscribed FBGs for collection. The diameter of the 6 around 1 geometry can be reduced down to 0.375 mm. Background suppression, collection efficiency and distance dependence of the probes were characterized and first Raman measurements are presented. The advantages of the novel probes are discussed and further applications to Raman-on-chip detection schemes are described.
The accuracy of the recently presented<sup>1</sup> 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.
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
Microstructured optical fibers (MOFs) as a novel type of light guiding media typically combine structural elements with
very different chemical and optical behavior, e.g. silica - air, silica - high refractive index glasses. The applicative
potential is very manifold: devices for telecommunication, nonlinear optics, sensing devices, fiber based gas lasers, etc.
We report about preparation and characterization of selected total internal reflection (TIR) guiding MOFs: Air Clad
Fiber, Suspended Core Fiber and heavy metal oxide (HMO) glass core MOFs. We fabricated Air Clad Fibers with
extreme air fraction. The bridge width of about 0.13 μm corresponds to a numerical aperture (NA) of about 0.6.
Suspended core fibers for evanescent sensing were prepared by pressurized drawing of arrangements of three and four
capillaries. By inflating the cavities the NA was increased up to 0.68. Material combined MOFs were prepared for
nonlinear application (e.g. supercontinuum generation) with lanthanum aluminum silicate glass core. Thermochemical
and optical behaviors of high nonlinear core glass candidates were investigated for alumina concentration up to 20 mol%
and lanthanum oxide concentration up to 24 mol% in silica matrix. The manufactured HMO glass core MOF with a
La<sub>2</sub>O<sub>3</sub> concentration of 10 mol% shows a similar background loss level like the unstructured HMO glass fiber about
Silica based microstructured holey fibers offer the possibility for filling with unconventional fiber materials. Of
special interest are chalcogenide glasses due to their high refractive index and their nonlinear optical properties.
We demonstrate two types of fibers: an index guiding fiber type with high-index glass core and silica cladding and
a fiber with silica core surrounded by a periodic, hexagonal high-index glass structure giving antiresonant guiding
properties. We prepared such fibers filled with arsenic sulphide glass and arsenic selenide glass by a pressurized
infiltration technique. The manufacturing process is modelled on the basis of viscous glass flow parameters and
is compared with experimental results obtained from the filled fibers. The propagation and spectral transmission
properties of such fibers are measured and discussed.
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 10<sup>5</sup>.
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
<i>m</i> = 3.
Microstructured optical fibers offer different possibilities for infiltration with unconventional fiber materials. By this way
the propagation properties of the guided light can be modified in a very flexible way and new functionality in sensing or
modulation can be introduced in optical fiber structures.
We report about spectral transmission behavior and influence on chromatic dispersion of index guiding microstructured
fibers (MOFs) in terms of material effects of the light propagating core, geometric parameters of the microstructured
cladding and preparation parameters. Two core compositions were investigated pure silica and silica-germania glass with
maximum 36 mol% GeO<sub>2</sub>. The MOFs with large pitch (>5 μm) were manufactured by single step technique. Small pitch
MOFs were prepared by dual step method. They show a relatively high OH absorption. The dual step prepared silicagermania
MOFs show a more than one order of magnitude higher hydroxide contamination compared to similar silica
MOFs. This result seems to be caused by the higher permeation of hydroxide groups in silica-germania glass compared
to silica. Simulations show that the red shift of the zero dispersion wavelength (ZDW) caused by high germanium doping
can be compensated by a holey cladding structure with medium up to large ratios of d/Λ.
We have investigated different possibilities to fill a Photonic Crystal Fibers (PCF) with an electro-optic (EO) polymer.
As the EO polymers are generally solution processed, the solvents have to be removed from the holes of the PCF after
filling with the liquid polymer, leaving a solid polymer layer. Repeated filling is required to create a multilayer of the EO
polymer to fill about 80% of the volume of the holes. The remaining volume can be filled with a epoxy monomer and
cured. Because of time consuming repeated steps in the solvent processing, solvent free single step filling processes are
also presented. Polability of these different systems and their final attainable properties are compared. Considering the
high refractive index of the polymer materials, possible applications e.g. for antiresonant guiding with variations of the
transmission bandgaps are discussed.