This PDF file contains the front matter associated with SPIE Proceedings Volume 8426, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

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 annealing treatment.

Here, we demonstrate the use of a colloidal CdSe:Te quantum dots suspension as active liquid-core in a specially designed optical element, based on a double-clad optical fiber structure. The liquid-core fiber was realized by filling the hollow core of a capillary and waveguiding of the core was ensured by using a liquid host that exhibits a larger refractive index than the cladding material of the capillary. Since the used capillary possessed a cladding waveguide structure, we obtained a liquid-core double-clad structure. To seal the liquid-core fiber and e.g. prevent the formation of bubbles, we developed a technique based on SMA connectors. The colloidal CdSe:Te quantum dots were excited by cladding-pumping using a pump laser at 532nm operating in the continuous-wave regime. We investigated the photoluminescence emitted from the colloidal CdSe:Te quantum dots suspension liquid-core and guided by the double-clad fiber structure. We observed a red shift of the (core) emission, that depends on the liquid-core fiber length and the pump power. This shift is due to the absorption of unexcited colloidal quantum dots and due to the waveguiding properties of the core. Here we report a core photoluminescence output power of 79.2μW (with an integrated brightness of ≈ 215.5 W/cm²sr ). Finally, we give an explanation, why lasing could not be observed in our experiments when setup as a liquid-core fiber cavity.

In this work, we demonstrate the formation of Poly-dimethylsiloxane (PDMS) films inside the holes of conventional silica photonic crystal fiber. The index guiding properties of the new PDMS-layer/Silica structures were investigated and optimized numerically using FDTD analysis. Films with thicknesses ranging from 100nm to 1μm were formed using different PDMS solution concentrations and different solution movement speeds. The thickness of films was very uniform along almost all the deposition length as indicated by Scanning Electron Microscopy (SEM) images and micro- Raman mapping.

Size, shape and location of the air holes allow to tailor microstructured fibre (MSF) parameters in a very wide range way beyond classical fibres what opens up many possibilities for various applications. Additionally, the propagation parameters of MSF can be actively tuned when the air-holes are filled with different gases, liquids (e.g., liquid crystals) or solid materials (e.g., polymers). The mode confinement in such a filled MSF can be affected by temperature dependent refractive index of material filling the fibre. This idea puts forward a new type of components for creating novel fibre devices such as switches, attenuators and others. Variable optical attenuators (VOAs) play an important role in optical communications as equalizers for dynamic channel power and wavelength division multiplexing in a transmission system. Controlling and monitoring of optical power are also necessary in sensing applications, and especially, in optical systems which require high power laser operation or critical temperature threshold monitoring. Various types of VOA have been developed based on different mechanisms, such as bending loss control, light leaking from the fibre cladding, temperature tuning of the polymer incorporated into the tapered microstructured fibre or electrical tuning of the liquid crystal layers. In this paper we would like to discuss the highly dynamic VOA based on a tuneable microstructured fibre filled with different chemical mixtures used as an on/off temperature switch. Furthermore, the technology of low loss coupling and splicing of the applied MSF with a standard single mode fibre has been developed. Therefore, in the proposed application an optical signal can be transmitted to and from the switch by a standard telecom fibre which considerably reduces transmission losses and allows for the use of standard off-the-shelf components reducing costs of the overall system.

One of the recent methods of grating inscription is based on inducing an array of highly localized index changes in the silica core of a fiber by tightly focused high intensity laser pulses. There were already several reports of such point-by-point gratings in conventional step index fibers. Applying this technique to photonic crystal fibers (PCFs) is still not straightforward. The main reason is the air holed microstructure which distorts the wave front of the inscribing laser beam and counters the focusing of the light in the core. We propose a new concept of microstructure-assisted grating inscription in photonic crystal fibers by introducing a focusing microstructure in the cladding of the fiber. We designed special types of photonic crystal fibers with a photonic crystal Mikaelian lens (PCML) in their cladding. Such a PCML is the implementation of a conventional Mikaelian or gradient index lens in a photonic crystal lattice. The effective index variation in a PCML is achieved via a variable air hole radius. In a fiber that is equipped with a PCML the inscribing laser beam can be tightly focused to the fiber core. This concept allows increasing optical power densities in the core of a PCF by an order of magnitude. We present a numerical model of a PCF with a PCML designed for 800 nm wavelength 125 femtosecond laser pulses.

Infiltration of glass matrices inside Photonic Crystals Fibres (PCFs) for achieving photonic bandgap (PBG) guidance and expand devices development capabilities has been recently demonstrated. Herein, we report the fabrication of an all-solid PBG guiding PCF by suction-assisted infiltration of molten silver-metaphosphate (AgPO₃) glass into the air capillaries of a commercial solid core PCF. The relatively low viscosity of the AgPO₃ glass melt permitted infiltration at ~ 700 °C inside an annealing oven apparatus by applying suction with the use of a standard mechanical vacuum pump, while its low glass transition temperature of ~ 190 °C allows structural relaxations at temperatures close to ambient and the formation of high quality glass strands inside the silica structure of the PCF. The AgPO₃/silica PCF was characterized by means of its transmission spectrum that showed PBG guidance over the measurement range (350-1650nm). The effect of the AgPO₃ glass photosensitivity on the guiding properties of the AgPO₃/silica PCF was explored by employing a 355nm, 150 ps laser irradiation. The exposure gave rise to a photo-induced enhancement of the transmission-to-stop-band extinction ratio by ~60 dB/cm as well as bandwidth tuning. Numerical calculations of the transmission spectra of the AgPO₃/silica PCF have been performed for confirming the experimental results and modelling the photo-induced variation of the two-glass fibre transmission. We believe that the fabrication of the AgPO₃/silica PBG fibre constitutes a strong base for the development of new in-fibre sensing and scattering-based devices, by exploiting the high photosensitivity of silver and its specific plasmon absorption properties.

We report a linear response optical refractive index (RI) sensor, which is fabricated based on a micro-channel created within a Fabry Perot (F-P) cavity by chemical etching assisted by femtosecond laser inscription. The experimental results show the F-P resonance peak has a linear response with the RI of medium and the measuring sensitivity is proportion to the length of micro-channel. The sensor with 5 μm -long micro-channel exhibited an RI sensitivity of 1.15nm/RIU and this sensitivity increased to 9.08nm/RIU when widening the micro-channel to 35μm. Furthermore, such micro-channel FP sensors show a much broader RI sensing dynamic range (from 1.3 to 1.7) than other reported optical fiber sensors.

The numerically optimized Ge-doped photonic crystal fiber long-period gratings operate around the dispersion turning point on the phase matching curve of the coupled modes. This special type of LPG, referred to as a Turn-Around-Point (TAP) LPG, can be employed for evanescent broadband absorption spectroscopy or optical intensity-based refractometry. The numerical optimization of a PCF-LPG utilizes the finite element method for PCF modal analysis and the simplex downhill method to minimize the objective function based on target-specific PCF properties. For gas and aqueous analytes infiltrated into PCF's air holes, the TAP PCF-LPG's periods are shorter than those achievable with a CO2 laser LPG inscription, and therefore the use of a femtosecond laser is supposed. The transmission spectrum of a TAP PCF-LPG is highly sensitive to variations in PCF geometrical parameters. The effects of imprecision in PCF fabrication on the LPG's transmission spectra can be mitigated with a stronger refractive index modulation, which can be achieved easier in a Ge-doped PCF than in a puresilica PCF. Moreover, germanium doping allows to precisely define the grating area for maximizing the coupling coefficient. Potential and limitations of TAP LPGs inscribed with a femtosecond laser into Ge-doped PCFs for evanescent chemical sensing will be evaluated.

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 photodarkening evolution.

A small and simple hydrostatic pressure sensor using fiber Bragg grating sensor for liquid level sensing is reported. The working principle of the sensor head design is based on transferring hydrostatic radial pressure to axial strain to the FBG. An FBG written in a fiber of diameter 50μm has been used for the measurement. The experimental result shows that sensitivity of the sensor can reach 23pm/cm of liquid column. The sensor can be useful in applications that involved with less hydrostatic pressure, like a tank with inflammable liquid in a fuel gas station.

The possibility of manufacturing highly birefringent (HB) microstructured optical fibers (MOF) made these fiber types very attractive for use in sensing applications. In contrary to traditional optical fibre sensors, properly designed MOF based components do not need temperature compensation as their birefringence remains insensitive to temperature changes. Furthermore the polarimetric strain sensitivity can significantly increase (even two orders of magnitude according to our previously reported results) for higher order modes, as their mode maxima get closer to the holey region of the fiber, hence are subjected to higher strain distribution. In this paper we present the results of numerical modeling of the propagation conditions in the HB dual-mode MOF including effective refractive index, confinement losses and birefringence calculations. Furthermore we show and discuss the spectral characteristics of fiber Bragg grating (FBG) structures written in the dedicated fiber with two technologies (with a nanosecond and femtosecond UV laser sources). A comparison of the theoretical and experimental values of effective refractive index and birefringence of the fundamental and second order modes is also included. We show the preliminary results of the fabricated structures strain response measurements and discuss ideas of increasing the structures strain sensitivity.

We report a grating-less, in-fibre magnetometer realised in a polymethylmethacrylate (PMMA) microstructured optical fibre that has been infiltrated using a hydrocarbon oil based ferrofluid. The lossy magnetic fluid has been infiltrated by capillarity action into the microcapillaries of the fiber cladding, resulting in a generation of a short cut-off band located in the vicinity of 600nm. When the magnetic field is applied perpendicular to the fiber axis, the ferrofluid undergoes refractive index and scattering loss changes, modulating the transmission properties of the infiltrated microstructured fibre. Spectral measurements of the transmitted signal are reported for magnetic field changes up to 1300Gauss, revealing a strong decrease of the signal near its bandgap edge proportionally with the increase of the magnetic field. Instead, when the magnetic field is applied with respect to the rotational symmetry the fibre axis, the sensor exhibits high polarisation sensitivity for a specific wavelength band, providing the possibility of directional measurements.

An intensity curvature sensor using a Photonic Crystal Fiber (PCF) with three coupled cores is proposed. The three cores were aligned and there was an air hole between each two consecutive cores. The fiber had a low air filling fraction, which means that the cores remain coupled in the wavelength region studied. Due to this coupling interference is obtained in the fiber output even if just a single core is illuminated. A configuration using transmission interrogation, which used a section fiber with 0.08 m of PCF as the sensing head, and a configuration using reflection interrogation, which used a section fiber with 0.13 m of PCF as the sensing head, were characterized and compared for curvature sensing. When the fiber is bended along the plane of the cores, one of the lateral cores will be stretched and the other compressed. This changes the coupling between the three cores, changing the optical power intensity. The sensibility of the sensing head was strongly dependent on the direction of bending, having its maximum when the bending direction was along the plane of the cores. A maximum curvature sensitivity of 1.8 dB.m was demonstrated between 0 m and 2.8 m.

Suspended core fiber tapers with different cross sections (from 70μm to 120μm diameter) were produced by filament heating. Before obtaining the taper, the spectral behavior of the suspended core fiber presents multimode interference. When the taper is made an intermodal interference is observed. This effect is clearly visible for high taper reduction. The spectral response of the microtaper inside the suspended core fiber is similar to a beat of two interferometers. The microtaper was subjected to strain, and an increase of sensitivity with the reduction of the transverse area was observed. When the taper was immersed in liquids with different refractive indices or subjected to temperature variations, no spectral change occurred.

Microstructured fibres (MSFs) reveal unique properties including endlessly single-mode operation from ultraviolet to infrared wavelengths, very high birefringence or nonlinearity, very large or very small effective mode field area, and many others. The size, shape and the location of the air holes allow for tailoring MSF parameters in a very wide range, way beyond the classical fibres, what opens up the possibilities for various applications. Due to their advantages MSFs obtain growing attention for their perspectives in sensing applications. Different MSF sensors have already been investigated, including interferometric transducers for diverse physical parameters. Until now, there have not been any publications reporting on the sensing applications of MSF Mach-Zehnder interferometers, targeting the mechanical measurements of vibrations, dynamic or static pressure, strain, bending and lateral force. Moreover, a critical feature opening the prospective of optical fibre transducer to successfully accomplish a particular sensing task remains its cross-sensitivity to temperature. Studied MSF is made of pure silica glass in the entire cross-section with a hexagonal structure of the holes. Consequently, there is no thermal stress induced by the difference in thermal expansion coefficients between the doped core region and the pure silica glass cladding, in contrast to standard fibres. In this paper we present the experimental comparison of mechanical and temperature sensitivities of Mach- Zehnder interferometer with replaceable FC connectorized sensing fibre arm, such as: off-the-shelf endlessly single mode MSF or standard telecom single mode fibre. Experimental results clearly show very low cross-sensitivity to temperature of studied MSF compared with standard fibre. Additionally, microstructured fibre Mach-Zehnder interferometer with standard FC receptacles allows using different fibres as sensors with the same device. Moreover, investigated interferometer consumes in total extremely low electric power (< 20 mW) due to the implementation of exceptionally effective data analysis electronics and VCSEL as the light source.

A procedure for the preparation of optically homogeneous glass for fiber preforms through sintering of coarse oxide particles and further processing of the resultant glass, including several drawing and stacking steps, is described. Reducing the pressure to 10^-2 Torr during sintering considerably reduced the amount of gas bubbles in Yb/Al-doped silica glass and decreased the background loss to 100 dB/km after the third drawing-stacking-consolidation cycle. For comparison, a fiber singly doped with alumina was fabricated by the same procedure as above. The level of wavelength- independent losses in that fiber was 65 dB/km.

We report the first experimental demonstration of an optical fiber supporting a fundamental mode with flattened intensity profile around 1050 nm. The design has been defined through intensive numerical simulations by paying a special attention to the constraints imposed by the fabrication process. We show that the fabricated fiber presents a single-mode behaviour.

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.

In this work we present the results of our recent studies on up-conversion phenomena in erbium doped TZN glasses. The set of five samples, of concentrations ranging from 1000 to 37500 ppm, has been carefully investigated by means of highly resolved laser spectroscopy, with specific attention concentrated on possible up-conversion processes resulting in red, green and violet emission under semiconductor lasers pumping. The excitation-dependant luminescence spectra together with fluorescence dynamics profiles enabled analysis of processes responsible for observed behaviour of up-converted emission.

The paper compares the absorption and emission properties of bulk glasses prepared by sintered in an iridium crucible and optical fibers fabricated by the powder-in-tube method. Both the bulk glasses and fibers were prepared from identical mixtures. The emission properties of the bulk samples and fibers were similar, while the "gray losses" in the fibers were an order of magnitude lower than those in the crucible melted glasses.

Two spectral interferometric techniques employing a supercontinuum source are used for dispersion characterization of birefringent microstructured and specialty optical fibers over a broad spectral range (e.g. 500-1600 nm). First, a technique employing an unbalanced Mach-Zehnder interferometer is used for measuring the chromatic dispersion and zero-dispersion wavelength of one polarization mode supported by a microstructured optical fiber. Second, a technique employing a tandem configuration of a Michelson interferometer and a fiber under test is used for measuring the group modal birefringence dispersion of the fiber and the chromatic-dispersion difference as a function of wavelength. From these measurements, the chromatic dispersion and the zero-dispersion wavelength of the other polarization mode supported by the microstructured optical fiber are retrieved. We revealed from four measurements the dependence of the zero-dispersion wavelength on the geometry of air-silica microstructured optical fiber. We also measured by the second technique the zero-chromatic-dispersion difference wavelength for elliptical-core optical fibers. We revealed from four measurements that the dispersion parameter can be tuned by the fiber geometry.

Dispersion characteristics are the fundamental properties of microstructured fibres (MSFs) with respect to the nonlinear applications. The changes of fibre dispersion may strongly influence the whole chain of diverse nonlinear effects resulting in supercontinuum generation (SG). Transferring the experience from the topics related to tailoring different properties of MSFs to investigate the potential design freedom of dispersion opens novel possibilities of building the customized, all-fibre broadband and bright light sources. The silica nonlinear microstructured fibres, as presented in this paper, become compatible with standard fibre components and technologies (e.g. splicing, connectorization etc). Supercontinuum generated in a small-core MSF is a very interesting nonlinear phenomenon from application-oriented point of view. A development of tailored dispersion of highly non-linear silica MSF offers us the possibility of constructing a customized broadband light source. Therefore, in the paper we present a theoretical and experimental investigation of dispersion characteristics of several different MSFs. Our studies are leading to the development of adapted dispersion properties, allowing construction of customized supercontinuum sources. All fibre, white light interferometry set-up, resulting in extremely high precision measurement of chromatic dispersion, is demonstrated, together with fully computer controlled fringe pattern analysis. Constructed set-up permitted comparison of chromatic dispersion measurements of microstructured fibres with modified fibre cross-section dimensions during the production process. High correlation between modelling and measured data gives possibility to control dispersion level in manufacturing process. Additionally, precisely designed and measured chromatic dispersion, especially around the zero dispersion wavelength, enables superior estimation of MSF nonlinear effects.

As the demand for high power fiber-coupled violet laser systems increases existing problems remain. The typical power of commercially available diode lasers around 400 nm is in the order of 100 to 300 mW, depending on the type of laser. But in combination with the small core of single-mode fibers reduced spot sizes are needed for good coupling efficiencies, leading to power densities in the MW/cm² range. We investigated the influence of 405 nm laser light irradiation on different fused silica fibers and differently treated end-faces. The effect of glued-and-polished, cleaved-and-clamped and of cleaved-and-fusion-arc-treated fiber end-faces on the damage rate and behavior are presented. In addition, effects in the deep ultra-violet were determined spectrally using newest spectrometer technology, allowing the measurement of color centers around 200 nm in small core fibers. Periodic surface structures were found on the proximal end-faces and were investigated concerning generation control parameters and composition. The used fiber types range from low-mode fiber to single-mode and polarization-maintaining fiber. For this investigation 405 nm single-mode or multi-mode diode lasers with 150 mW or 300 mW, respectively, were employed.

Microstructured optical fibers are increasingly used in optical fiber sensing applications such as for example optical fiber based structural health monitoring. In such an application the fiber may experience substantial mechanical loads and has to remain functional during the entire lifetime of the structure to be monitored. The resistance to different types of mechanical loads has therefore to be characterized in order to assess the maximum stress and strain that a fiber can sustain. In this paper we therefore report on the extensive set of tensile tests and bending experiments that we have conducted both on microstructured optical fibers with an hexagonal air hole lattice and on standard optical fibers. We use Weibull statistics to model the strength distribution of the fibers and we follow a fracture mechanics approach in conjunction with microscopic observations of the fractured end faces to study crack initiation and propagation in both types of fibers. We show that the failure strain of microstructured fibers is about 4.3% as obtained with tensile tests, compared to 6.7% for reference fibers. Although the mechanical strength of microstructured optical fibers is lower than that of the standard fibers it is still adequate for these fibers to be used in many applications.

A new type of dispersion flattened photonic fiber is presented. The fiber has an improved threefold symmetry core that consists of a silica core surrounded by three low-index regions and three air-holes. It can be observed from numerical simulation employing the full-vectorial finite difference frequency domain method that nearly-zero ultra-flattened dispersion can be obtained over the wavelength range of 1250-1700 nm. All fiber's parameters are found to be non-immune to imperfections of geometry. An attention should though be paid to the potential fabrication process. The chromatic dispersion behavior with fabrication tolerances of 1 % and 2 % has been numerically demonstrated. Finally, fiber designs with five different hole-to-hole spacing (pitch) have been proposed. Each of the proposed fibers exhibits remarkable chromatic dispersion properties, such as nearly-zero ultra-flattened dispersion over wide wavelength range or zero dispersion at the wavelength of 1550 nm.

We are arrived in this work to apply the SC-FEM to PCF to determine the modal field distribution and other important characteristics as normalized frequency, numeric aperture and chromatic dispersion according to the optogeometric parameters of the fiber. We could vanish the chromatic dispersion in the PCF at many low wavelengths because of its large degree of liberty.

We present a design of a photonic crystal fiber for high power laser and amplifier applications. Our fiber comprises a core with a diameter larger than 60 μm and exhibits single mode operation when the fiber is bent around a 10 cm radius at a wavelength of 1064 nm. Single mode guidance is enforced by the high loss of higher order modes which exceeds 80 dB/m whereas the loss of the fundamental mode (FM) is lower than 0.03 dB/m. The fiber can therefore be considered as an active medium for compact high power fiber lasers and amplifiers with a nearly diffraction limited beam output. We also analyze our fiber in terms of tolerance to manufacturing imperfections. To do so we employ a statistical design methodology. This analysis reveals those crucial parameters of the fiber that have to be controlled precisely during the fabrication process not to deteriorate the fiber performance. Finally we show that the fiber can be fabricated according to our design and we present experimental results that confirm the expected fiber performance.

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.

A hexangular lattice dual-concentric-core photonic crystal fiber is proposed, which is composed of an inner core to be formed by missing a central air-hole, an outer ring core to be produced by reducing the size of the air-holes of the third ring and the double cladding circle air-holes along the direction of fiber length. Based on the full vector finite element method with anisotropic perfectly matched layers, its dispersion, leakage loss and mode field area are numerically investigated. Numerical results indicate that the proposed fiber shows large negative dispersion, strong confinement ability of guide mode, large effective mode area and low leakage loss and low sensitivity to the structure parameters. And the wavelength of high negative dispersion value can be adjusted by artificially choosing the parameters of the proposed PCF, such as Λ, d₁ and f. The optimal design parameters with Λ=1.2μm, f=0.92, d₁=0.52μm for proposed PCF are obtained to achieve ultra-narrowband negative dispersion value for dispersion compensation. For the optimal design, the dispersion value reaches as high as -3400 ps·km^-1 nm^-1 and the dispersion slope value is between -1000~ -6000 ps·km^-1 nm^-2 over C band (1.53-1.565μm). At wavelength of 1.55μm, the leakage loss is closed to 10^-2 dB·m^-1 and the corresponding area of effective mode is 36μm².

Photonic crystal fibers (PCFs) offer great design flexibility as their internal microstructure can be tailored to achieve a wide range of optical guiding properties adapted to many different applications. Fiber Bragg grating fabrication in such fibers is now extensively investigated to enable new fiber sensor and all-fiber laser applications. Grating writing in PCF is not necessarily straightforward. This is due, to a large extent, to the air hole microstructure in the fiber cladding that impedes the inscribing beam intensity to reach the fiber core in sufficient amounts. This issue is more pronounced for multi-photon absorption based grating inscription techniques, for which the intensity of the light reaching the core is crucial to induce the desired refractive index change. We performed a numerical study of transverse light propagation through the cladding to the core for various hexagonal lattice PCFs. A numerical tool based on commercial FDTD software was developed for that purpose. To assess the influence of the PCF microstructured cladding, we defined a figure of merit to quantify the amount of laser light reaching the core: the "transverse coupling efficiency" (TCE). We studied the influence of the hexagonal lattice parameters, in particular the air hole radius and pitch, on the energy reaching the core for various angular orientations of the fiber with respect to the impinging laser beam. We conducted this study for ultraviolet and infrared femtosecond laser sources. As a result we have identified favorable PCF lattice parameters and a fiber orientation that would allow efficient femtosecond grating inscription. We show that the microstructure of a PCF can not only have a limiting, but also a constructive influence on the laser energy reaching the core of the fiber and thus on the efficiency with which gratings can be inscribed.

The terahertz (THz) frequency region of the electromagnetic spectrum is located between the traditional microwave spectrum and the optical frequencies, and offers a significant scientific and technological potential in many fields, such as in sensing, in imaging and in spectroscopy. Waveguiding in this intermediate spectral region is a major challenge. Amongst the various THz waveguides suggested, metal-clad plasmonic waveguides and specifically hollow core structures, coated with insulating material are the most promising low-loss waveguides used in both active and passive devices. Optical power splitters are important components in the design of optoelectronic systems and optical communication networks such as Mach-Zehnder Interferometric switches, polarization splitter and polarization scramblers. Several designs for the implementation of the 3dB power splitters have been proposed in the past, such as the directional coupler-based approach, the Y-junction-based devices and the MMI-based approach. In the present paper a novel MMI-based 3dB THz wave splitter is implemented using Gold/polystyrene (PS) coated hollow glass rectangular waveguides. The H-field FEM based full-vector formulation is used here to calculate the complex propagation characteristics of the waveguide structure and the finite element beam propagation method (FE-BPM) and finite difference time domain (FDTD) approach to demonstrate the performance of the proposed 3dB splitter.

We have numerically investigated the guiding properties in a hybrid polymer/silica photonic crystal fiber (PCF). We considered poly-dimethylsiloxane (PDMS) as the infused polymer into the air-holes of PCF and we present how the modal properties of the fiber are affected due to PDMS inclusions. We numerically calculated the guiding and thermal properties of the hybrid structure in terms of the effective index, single-mode operation, confinement loss, numerical aperture (NA), effective modal area (EMA) and fraction of power into the polymer-filled cladding for different relative hole sizes, d/Λ (0.35-0.75) of the hybrid PCF whereas direct comparison with a conventional air-filled PCF is also shown. Further investigation of EMA, NA and fraction of power in the cladding with respect to thermal variations is also reported for a range of temperatures from 0°C to 100°C.

In this paper, an all-fiber Brillouin laser ring cavity using a 3-m-long suspended-core chalcogenide As₃₈Se₆₂ fiber is reported for the first time to our knowledge. For a nonresonant ring cavity with no servo-locking, a laser threshold power of 37 mW and an efficiency of 26 % were obtained for a fiber having a core diameter of 5 μm. The linewidth of the Brillouin fiber laser and the pump laser were respectively measured to be below 4 kHz, the resolution of our autocorrelator, and 250 kHz, thus showing the linewidth-narrowing nature of the Brillouin laser. This result paves the way to compact Brillouin lasers with low threshold power and good spectral purity. A full experimental Brillouin characterization is also reported. We measured a Brillouin gain spectrum of 14.2 MHz, a Brillouin gain coefficient of 5.6x10^-9 m/W and a Brillouin frequency shift of 7.95 GHz in our fiber.

We report a detailed implementation of a 2-D finite element method that is applied to calculate the stimulated Brillouin scattering (SBS) characteristics in highly nonlinear tellurite microstructured fibers. Our analysis is firstly made in four air-holes different tellurite microstructured fibers and compared to silica and chalcogenide fibers. Such fibers have drawn much interest because of their capacity of increasing the SBS gain. A Brillouin gain coefficient, gB, of 1 10^-10 m/W is found around the acoustic frequency of 8 GHz. Then, we used real scanning electron microscope images of small core highly nonlinear tellurtie fibers and compared the SBS parameters to numerical predicted results. Good agreement is found between the SBS gain coefficients and the frequency shifts.

Phase matching curves for parametric generation in four wave mixing (FWM) processes of different types are studied experimentally and numerically for a polarization maintaining photonic crystal fiber (PCF) pumped by a tunable continuous wave (CW) ytterbium doped fiber laser near 1 μm. Parametric frequency shifts up to 100 THz for scalar and pump-divided vector FWM processes are observed providing generation of idler wave with wavelengths as short as 765 and 758 nm for the two processes respectively. Explicit analytical solutions for scalar and polarization phase matching in vicinity of the zero dispersion wavelength have been also deduced. They are based on phase mismatch Taylor series expansion taking into account the polarization contribution. A good quantitative agreement between experimental and calculated frequency shifts is demonstrated.

We present the development of a large core multimode photonic crystal fibre with hyperspectral transmission that covers the visible, near and (in part) mid infra-red wavelength ranges (400-6500 nm). We have optimised the composition of a heavy metal-oxide glass based on the PbO-Bi₂O₃-Ga₂O₃ system modified with Nb₂O₅, Ta₂O₅, SiO₂, GeO₂, BaO, CdO, Na₂O and K₂O. The optimised glass shows good transmission up to 6 μm as well as good rheological properties that permits multiple thermal processing steps in an optical drawing tower without crystallisation. The selected glass is synthesized inhouse and has been used for fibre development. We have fabricated a multi-mode photonic crystal fibre with an effective mode area of 295 μm². The photonic cladding is composed of 8 rings of air holes with a fill factor of 0.46. The transmission of a hyperspectral spectrum is experimentally verified using a broadband source. The attenuation of the fibre and its sensitivity to bending losses is presented.

A novel block copolymer material is applied successfully to a low cost Polymer Optical Fiber- POF for sensing applications. The copolymer consists of two blocks, one hydrophilic and sensitive to polar substances and the other hydrophobic and sensitive to hydrocarbons. Prediction of polymer's behavior in the presence of analytes has been successfully verified experimentally. The existence of two different blocks allows for the detection of a wide variety of agents and this response could be further engineered and enhanced by suitable adjusting the ratio of the two blocks. Using methanol as polymer's solvent PMMA POF successfully coated without deteriorating its properties. Functionalized U-bent POFs were employed as sensing tips overcladded with the sensitive material using a simple dip coating technique. Due to material's high transition temperature (well above 120°degC) the overlayers were very stable and environmentally robust. The developed sensors characterized and exhibited very fast response in ammonia, humidity, toluene and benzene, with full operational reversibility.

In this paper we report the in-house synthesis of optical grade PMMA suitable for fiber development and fabrication of a large core micro-structured polymer optical fiber (mPOF). We have designed an mPOF with a core area of 580 μm² and single mode performance at a wavelength of 650 nm. The photonic cladding is composed of 3 rings of air holes with a filling factor of 0.58 ensuring in practice a single mode performance at the design wavelength of 650 nm. The designed mPOF fiber was fabricated using the stack and draw technique, however some deformation of the structure of the photonic cladding has been observed during final stage of fiber drawing. The influence of this development imperfection on the overall fiber performance has been modeled. Finally the optical properties of the fabricated fiber were measured and a comparison between these and the modeled properties was made.

An increasing interest in making sensors based on fiber Bragg gratings (FBGs) written in polymer optical fibers (POFs) has been seen recently. Mostly microstructured POFs (mPOFs) have been chosen for this purpose because they are easier to fabricate compared, for example, to step index fibers and because they allow to tune the guiding parameters by modifying the microstructure. Now a days the only technique used to write gratings in such fibers is the phase mask technique with UV light illumination. Despite the good results that have been obtained, a limited flexibility on the grating design and the very long times required for the writing of FBGs raise some questions about the possibility of exporting POF FBGs and the sensors based on them from the laboratory bench to the mass production market. The possibility of arbitrary design of fiber Bragg gratings and the very short time required to write the gratings make the point-by-point grating writing technique very interesting and would appear to be able to fill this technological gap. On the other end this technique is hardly applicable for microstructured fibers because of the writing beam being scattered by the air-holes. We report on the design and realization of a microstructured polymer optical fiber made of PMMA for direct writing of FBGs. The fiber was designed specifically to avoid obstruction of the writing beam by air-holes. The realized fiber has been used to point-by-point write a 5 mm long fourth order FBG with a Bragg wavelength of 1518 nm. The grating was inspected under Differential Interferometric Contrast microscope and the reflection spectrum was measured. This is, to the best of our knowledge, the first FBGs written into a mPOF with the point-by-point technique and also the fastest ever written into a polymer optical fiber, with less than 2.5 seconds needed.

A new type of fibre-optic biochemical concentration sensor based on a polymer optical fibre Bragg grating (POFBG) is proposed. The wavelength of the POFBG varies as a function of analyte concentration. The feasibility of this sensing concept is demonstrated by a saline concentration sensor. When polymer fibre is placed in a water based solution the process of osmosis takes place in this water-fibre system. An osmotic pressure which is proportional to the solution concentration, will apply to the fibre in addition to the hydraulic pressure. It tends to drive the water content out of the fibre and into the surrounding solution. When the surrounding solution concentration increases the osmotic pressure increases to drive the water content out of the fibre, consequently increasing the differential hydraulic pressure and reducing the POFBG wavelength. This process will stop once there is a balance between the osmotic pressure and the differential hydraulic pressure. Similarly when the solution concentration decreases the osmotic pressure decreases, leading to a dominant differential hydraulic pressure which drives the water into the fibre till a new pressure balance is established. Therefore the water content in the polymer fibre - and consequently the POFBG wavelength - depends directly on the solution concentration. A POFBG wavelength change of 0.9 nm was measured for saline concentration varying from 0 to 22%. For a wavelength interrogation system with a resolution of 1 pm, a measurement of solution concentration of 0.03% can be expected.

The inscription of Bragg gratings has been demonstrated in PMMA-based polymer optical fibre. The water affinity of PMMA can introduce significant wavelength change in a polymer optical fibre Bragg grating (POFBG). In polymer optical fibre losses are much higher than with silica fibre. Very strong absorption bands related to higher harmonics of vibrations of the C-H bond dominate throughout the visible and near infrared. Molecular vibration in substances generates heat, which is referred to as the thermal effect of molecular vibration. This means that a large part of the absorption of optical energy in those spectral bands will convert into thermal energy, which eventually drives water content out of the polymer fibre and reduces the wavelength of POFBG. In this work we have investigated the wavelength stability of POFBGs in different circumstances. The experiment has shown that the characteristic wavelength of a POFBG starts decreasing after a light source is applied to it. This decrease continues until equilibrium inside the fibre is established, depending on the initial water content inside the fibre, the surrounding humidity, the optical power applied, and the fibre size. Our investigation has shown that POFBGs operating at around 850 nm show much smaller wavelength reduction than those operating at around 1550 nm in the same fibre; POFBGs with different diameters show different changes; POFBGs powered by a low level light source, or operating in a very dry environment are least affected by this thermal effect.

This work is focused on the selected aspects of designing of microstructured POF (mPOF) with relatively large core, limited modal dispersion and improved resistance to bending losses, discussed in the context of its possible application in FTTH systems. The calculations confirmed the possibility of effective controlling both, the propagation and macrobending losses, as well as manipulation on the number of modes and modal area. The careful theoretical analysis allowed to design a series of geometries supporting the propagation of limited number of modes and, simultaneously, relatively large mode area together with limited bending losses.

We report the fabrication of tellurite composite microstructured optical fiber (CMOF) with ultra-flattened zero dispersion (±3 ps/nm/Km) over 200nm band. To obtain this dispersion profile together with high nonlinearity, one ring of air holes and two layers of glass cladding are employed in the tellurite CMOF. The core of fiber is made of TeO₂-Li₂O-WO₃ -MoO₃-Nb₂O₅ (TLWMN) tellurite glass which possesses high linear and nonlinear refractive indices. The refractive index (n) at 1544nm and nonlinear refractive index (n2) of TLWMN glass is 2.08 and 3.78×10^-11 esu, respectively. TeO₂-ZnO-Na₂O-La₂O₃ (TZNL) glass with n of 1.96 at 1544 nm and TeO₂-ZnO-Li₂O-Na₂O-P₂O₅ (TZLNP) glass with low refractive index n of 1.63 at 1544 nm are used as the first cladding and the second cladding, respectively. Six small air holes are located between the core and the first glass cladding. Such kind of fiber with ~1.7 μm core and ~0.6 μm air holes are fabricated by a rod-in-tube method. The chromatic dispersion of the fiber is calculated by the fully vectorial finite difference method (FV-FDM) and becomes (±3 ps/nm/Km) in the wide range from 1.53 μm to 1.72 μm. And the nonlinear coefficient of present fiber is about 3.47 m^-1W^-1 which is much higher than that of silica MOFs. Furthermore, broad and flattened supercontinuum generation is demonstrated in 30-cm-long fiber with femtosecond laser pumping at 1557 nm. This kind of fiber has promising potential in nonlinear applications owing to the high nonlinearity and flattened dispersion profile.

This paper presents the design, fabrication process, and experimental evaluation of a high-sensitivity, all-silica, all-fiber, micro machined Fabry-Perot strain-sensor. This sensor has a short Fabry-Perot cavity and thus allows for the application of low-resolution spectral interrogation systems; in our case the commercial white light signal interrogator was used. The fabrication process includes the design and production of special sensor-forming optical-fiber. This fiber includes a central titanium-doped region, a phosphorus doped-ring surrounding a titanium doped region, and pure silica cladding in order to produce the proposed sensor, two sections of sensor forming fiber are cleaved and etched in a HF/IPA solution. The phosphorus-doped region etches at a considerably higher rate than the other fiber-sections, and thus creates a deep gutter on the cleaved fibers frontal surface. The titanium-doped region etches at a rate that is, to some extent, higher than the etching-rate of pure silica, and thus creates a slightly retracted surface relative to the pure silica fiber-cladding. The etched fibers are then re-spliced to create an all-silica strain sensor in "double configuration", which has a section of etched sensor-forming fiber on both sides. Thus this sensor has a long active length, whilst the length of the Fabry-Perot cavity can be adjusted by a titanium-doping level. The central titanium-doped region also creates a waveguide structure that is used to deliver light to the cavity through one of the fibers. The proposed fabrication process is cost-effective and suitable for high-volume production. The greatest achievement of the depicted in-line strain sensor is the extension of its active sensor length, which is more than 50 times greater than the sensor-cavity's length, and is thus approximately 50 times more sensitive to strain. This sensor also exhibits low-intrinsic temperature sensitivity.

This paper presents an all-optically controlled, all-fiber optical path-length modulator. The presented system takes advantage of the heating effect induced within vanadium-doped fiber through laser excitation. It can be applied in various applications, for example in white-light interferometry. The system consists of a Michelson interferometer with vanadium-doped fiber in one arm, a 980 nm excitation high-power laser diode, and 1310/1550 nm signal sources or channels. Due to the spectral-absorption properties of vanadium ions in silica, the absorbed optical power emitted by the 980 nm source is mostly converted via a non-radiative relaxation process into heat within the vanadium-doped fiber. A rise of fiber core temperature causes the fiber core refractive index to change and consequently, a change in the optical path difference of the interferometer. The extinction laser diode operates in pulse mode for continuous scanning of the white-light interferometer. The vanadium-doped fiber is, therefore, periodically heated and self-cooled. The optical path difference of the scanning interferometer is simultaneously measured using a high-coherence source that provides the needed reference trace. The achieved modulated optical path is over 150 μm, with a system time constant of below 1 s. This all-optical configuration of the scanning interferometer allows for the remote and electrically passive control of the optical path length differences in various fiber-optic systems. In particular, the proposed design would be suitable as an interrogation system for various sensors, where an absolute optical path length variation/measurement is required.

In this paper, we report on the design, implementation and performance issues of solid-core microstructured optical fibers (MOFs) having two types of asymmetry introduced intentionally into the typical triangular cladding configuration. First adaptation represents MOF with a large core shifted for the pitch value from its usual location in the center of the lattice. Second variation includes regular structure with several peripheral air holes omitted on purpose to organize the 'incomplete cladding' design. Fiber core dimensions range from 12.5 to 35 μm. The results of investigating properties of guided modes, transmission loss and macrobending resistance are presented. Whereas the structure with several missing air holes in the cladding negligibly differs from the regular MOF structure, the fiber with a shifted core reveals some essential preferences. This fiber exhibits practical fundamental mode operation with a great beam quality within the expanded transmission spectra. The ultimate spectral widening is about 300 nm, which is possible due to a comparatively high air filling fraction (diameter-to-pitch ratio is larger than 0.60) that helps to improve fiber bend performance. Robust single-mode guidance originates from the enhanced higher order mode loss mechanism and consequent differential mode attenuation factor. Minimal optical losses equal to 5 dB/km at λ = 1550 nm in the single-mode regime.

We present our progress in the production of ytterbium (Yb) doped optical fibers fabricated by two variants of the granulated aluminophosphosilicate method. We show advantages and disadvantages of mixing rare earth and aluminophosphosilicate granulated oxides directly (variant 1) or by using the sol-gel method to produce doped granulate material (variant 2). For both methods we studied the effects of varying the dopant concentrations and of introducing iterative melting and milling procedures. In particular, the sol-gel based method eases the inclusion of P2O5 and thus, in combination with Al₂O₃, higher dopant concentration of Yb and Er are possible. Sintering the sol-gel material at high temperature eliminated bubbles in the core. We fabricated optical fibers that, piecewise, between individual strong scatterers, exhibited attenuation losses as low as 0.35dB/m. For our comparative study we determined volume percentage and distribution of chemical elements in the fabricated fiber glasses by the analytical technique of Energy-Dispersive X-ray, Electro Probe Microanalysis and the degree of crystallization by X-Ray Diffraction analysis. Furthermore we measured fluctuations of the refractive index profile and scattering losses of the fiber core.

The development of all-solid photonic crystal fibers for nonlinear optics is an alternative approach to the air-glass solid core photonic crystal fibers. The use of soft glasses ensures a high refractive index contrast (>0.1) and a high nonlinear coefficient of the fibers. In addition, the manipulation of the subwavelength structure of the core of a photonic crystal fiber allows significant modification of its dispersion characteristics and efficient generation of supercontinuum with various femtosecond and nanosecond sources. The development of all-solid photonic crystal fiber allows very accurate control of all the parameters of the developed fiber in very good agreement with the design criteria. In this paper, we report on the dispersion management capabilities in all-solid photonic crystal fibers with nanostructured cores using thermally matched glasses, which can be jointly processed using the stack-and-draw fiber fabrication technology. We consider a photonic crystal fiber made of the high index lead-silicate glass SF6 and the in-house synthesized low index silicate glass NC21. The NC21 glass plays the role of low index inclusion in the photonic cladding and a nano-inclusion in the core of the fiber. The final dispersion profile of the photonic crystal fiber is determined by the low index nano-inclusion in the core with diameter in the range 100-500nm. The dispersion profiles are modeled for a theoretical structure and for the developed fiber. Supercontinuum generation is expected and numerically confirmed for the developed fiber in the range 1150-1500nm with flatness below 1dB. The fiber is dedicated for supercontinuum generation with 1550nm laser sources.

Designing of all in-line fiber optic systems with a supercontinuum light source gives some issues. The use of a standard single mode fiber (SMF) as an input do not secure single mode transmission in full wavelength range. In the paper, the experimental results of the tested hybrid fiber optic coupler were presented. It was manufactured by fusing a standard single mode fiber (SMF28) and a photonic crystal fiber (PCF). The fabrication process is based on the standard fused biconical taper technique. Two types of large mode area fibers (LMA8 and LAM10 NKT Photonics) with different air holes arrangements were used as the photonic crystal fiber. Spectral characteristics within the range of 800 nm - 1700 nm were presented. All process was optimized to obtain a mode conversion between SMF and PCF and to reach a single mode transmission in the PCF output of the coupler.

The small size of the core (about 1 μm) of the suspended-core optical fibres gives rise to evanescent wave in the surrounding channels. That process allows efficient coupling between light and liquid introduced into the channels and application of the fibre for analytic purposes. In the presented work, the channels of a suspended-core fibre were filled with water and aqueous solutions of oxazine 725 perchlorate and their absorption spectra were measured. While the spectra of the water-filled fibre were consistent with the Lambert-Beer law, the absorptivity of the fibre filled with oxazine 725 perchlorate solution demonstrated an increased sensitivity caused by aggregation of the oxazine molecules, independent on the fibre length.

Photonic crystal fibres (PCF) and more commonly microstructure fibres, remain interesting and novel fibre types and when suitably designed can prove to be "ideal" for sensing applications, as the different geometrical arrangement of the air holes alters their optical wave-guiding properties, whilst also providing tailored dispersion characteristics. This impacts the performance of grating structures, which offer wavelength encoded sensing information. We undertake a study on different air hole geometries and proceed with characterization of fibre Bragg and long period gratings, FBG and LPG, respectively that have been inscribed (using either a femtosecond or ultraviolet laser system) within different designs of microstructured fibre that are of interest for sensing applications.

Microfluidics are important micro-scale devices that can be used to manipulate very small volumes of fluids on the order of nano- to femto-litres. The control and sorting of nano-particles is a primary goal using this technology. There is particular interest in the use of microstructure optical fibres for the transfer of fluids, whereby the guided light interacts with a fluid in the region of the air-hole structure. We study the fluid transport capabilities of microstructure fibres with cross sections containing circular or elliptical holes, considering the effects of flow rates, fluid viscosity, and the channel diameter. The role of heat flux is considered in relation to the fluid characteristics. We solve the time-dependent Navier- Stokes equations and the convection-diffusion equation.