This PDF file contains the front matter associated with SPIE Proceedings Volume 9886, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

In this work, we present experimental results of characterization of the developed holmium-doped silica-based optical fibers with holmium ions concentrations in the range from 1000 to 10000 ppm. The fibers were fabricated by the modified chemical vapor deposition and solution doping method. They were characterized in terms of their spectral attenuation, refractive index profile, and especially performance in fiber laser. Simultaneously, two different fiber laser setups were tested. In the first one, holmium-doped fiber in Fabry-Perot configuration was pumping by in house developed thulium-doped fiber laser in ring arrangement. In the second one, bulk-optic pump-coupling configuration, consisted of a commercially available thulium fiber laser emitting at 1940 nm and system of lenses and mirrors was used. We have focused on comparison of laser output powers, slope efficiencies, and laser thresholds for individual holmiumdoped fiber in these different laser arrangements. Finally, the application of the developed fiber in subpicosecond fiber laser with graphene-based saturable absorber for mode-locking operation was investigated.

In the last few years Yb-doped double cladding fibers have become the key component for the development of reliable and high-performance lasers. Despite an effective cooling of the fiber medium, a significant heat load is generated when high pump power is involved, which alters the mode propagation characteristics, causing unwanted coupling among the modes and destroying the output beam quality. This work presents a new tool for the analysis of the amplification and modal properties of Yb-doped double-cladding fibers, which comprises a full-vector modal solver, based on the finite-element method, an amplifier model and a thermal one. Simulation results, shown for two large pitch fiber designs, both in co-propagating and counter-propagating pumping schemes, have demonstrated the influence of the generated heat load on the overlap integral and on the power evolution of the guided modes.

Fiber Bragg grating (FBG) inscription methods based on femtosecond laser sources are becoming increasingly popular owing to the (usually) non-linear nature of the index modification mechanism and to the resulting advantages. They allow, for example, fabricating fiber gratings that can survive temperatures exceeding 700°C, which can be an asset in the domain of fiber sensing. However applying femtosecond laser based grating fabrication to microstructured optical fibers (MOFs) can be challenging due to the presence of the air holes in the fiber cladding. The microstructured cladding not only impedes light delivery to the core in most cases, but also causes a non-uniform intensity distribution in the MOF core. To deal with these challenges we present a modeling approach that allows simulating how the reflectivity of the grating and the nature of the index modulation are affected by the inscription conditions. We rely on transverse coupling simulations, empirical data and coupled mode analysis to model the induced index change and the resulting grating reflectivity. For IR femtosecond grating inscription we show that due to the intensity redistribution in the core region, irreversible Type II index changes can be induced in a MOF at laser peak intensities below the Type II threshold for step-index fibers. The resulting non-uniform induced index change has repercussions on the reflection spectrum of the grating as well. Our coupled mode analysis reveals, for example, that although the average index change in the core region can be high, the partial overlap of the core mode with the index change region limits the reflectivity of the grating.

The large-pitch fiber design and its fundamental operating principle – the delocalization of higher-order modes – have enabled the current performance records of ultrashort-pulse fiber laser systems. However, further average power scaling is limited by transverse mode instabilities. This effect has been linked to the average thermal load inside the active fiber and, consequently, it is strongly enhanced for short fiber amplifiers that are typically used for ultrashort-pulse amplification. Recently, it has also been discovered that photo-darkening is an additional, very important heat source in an active fiber that can be as strong as the quantum defect. A reduction of the Yb-doping concentration in high-power active fibers is a promising way to mitigate this problem. In this contribution we propose a new large-pitch design which has an increased delocalization of higher-order modes and, at the same time, allows for a larger core to clad ratio that enables reducing the Yb-ion concentration while maintaining a similar amplification efficiency. A comprehensive laser simulation that includes modal properties, thermo-optical effects and photo-darkening is used to highlight the important benefits of this design. Additionally, this novel and compact fiber design can be a promising unit-cell for a multi-core fiber, which should ultimately enable the development of multi-kW average power and multi-mJ pulse energy ultrashort-pulse fiber lasers in the near future.

In this work, we design a highly nonlinear noncircular core photonic crystal fiber (HNL-PCF) for the generation of a supercontinuum (SC) at 1.3 μm having minimum anomalous dispersion and using many nonlinear effects by introducing self-phase modulation (SPM), self-steepening and Raman effects. The proposed geometry of the HNL-PCF is composed of six rings of air-holes and silica as a background material for the core. Using the vectorial Finite Element method (FEM) with a perfectly matched layer (PML), the proposed HNL-PCF is numerically modeled for determining its characteristics as Group Velocity Dispersion (GVD) and nonlinear properties. After optimizing the properties of the proposed HNL-PCF (GVD= - 0.95 ps²/km; γ= 55.45 [W.km]^-1 around 1.3 μm), the SC is generated by solving the nonlinear Schrodinger equation (NLSE), that contains different parameters of the cited nonlinear effects, with split-step Fourier method (SSFM). The introducing of this different effects in our work allows to generate a SC of spectral bandwidth SBW=260 nm at 1,3 μm using only 1.89 mm long of PCF.

In this paper, we propose a design of a multimode step index fiber where the first six modes can propagate simultaneously. We investigate the propagation of six strongly coupled groups of modes which are very important for spatial division multiplexing (SDM) optical communications. We solve numerically the six coupled Manakov equations and we find that fundamental solitons propagating in different groups of modes travelling with the same speed due to spectral shifts for each soliton. This phenomenon is known as soliton trapping and is a consequence of the intermodal nonlinear coupling based on cross phase modulation. This fiber is very promising to increase the capacity of SDM systems by more than a tenfold factor compared to single mode systems.

In this work, we demonstrate the use of particularly characterised multicore optical fibres (MCFs) to devise compact, compellingly simple, ultrasensitive interferometric sensors which are capable of sensing single or multiple physical parameters. Generally, our devices operate in reflection mode and consist of a few centimetres of MCF fusion spliced to standard single-mode optical fibre (SMF). The tools and instrumentation needed to fabricate our devices are a conventional fibre cleaver and a fusion splicing machine. We demonstrate a highly-sensitive bending sensor (inclinometer) with a MCF with three strongly coupled cores which is capable of distinguishing multiple bending or inclination orientations, and also a force sensor based on MCF with seven coupled cores. In both cases the devices are interrogated with a low-power LED and a miniature spectrum analyser. Bending or force on the MCF induces drastic changes of the supermodes, their excitation, and consequently, on the reflected spectrum (interference pattern).

We present research into the use of femtosecond lasers to develop optical waveguides inscribed in the cladding of singlemode, silica optical fibre (SMF28). The waveguides are inscribed near to the fibre core, coupling light into them evanescently and so behaving as traditional couplers. By carefully controlling the laser parameters we are able to inscribe cladding waveguides with no evidence of damage through ablation. We show that this flexible inscription method can be used as an enabling technology to couple light from single-core fibres to new multi-core optical fibres, and in this work specifically to 4-core fibre. The SMF28 fibre is fusion spliced to the multi-core fibre and using the femtosecond laser we inscribe bridging waveguides from the centrally located single mode fibre core to a selected offset core of the 4-core fibre. To demonstrate the efficiency of the method and the possibility of making new kinds of optical fibre sensors, we inscribe a fibre Bragg grating (FBG) in one of the four fibre cores. The light reflected from the FBG is coupled back to the SMF28 core via bridging waveguide and we recovered the reflection spectrum of the grating using a commercial high-resolution spectrometer.

Herein we present the experimental implementation of a power coupling device that combines the technology of tapered, multicore microstructured optical fibres (MOFs) with ferrofluidic overlayers. Power coupling between different cores of a tapered, multicore MOF is demonstrated, utilizing magneto-refraction effects induced by magnetic field stimulus into a ferrofluidic outcladding surrounding of the multicore optical fibre taper. By tapering the multicore all-solid MOF to a specific diameter, the excitation of all the adjacent cores through the central one is achieved. Transmission spectra measurements of the individual cores proved that light coupling between fiber cores can be manipulated by magnetic field stimulus. We anticipate that such a type of magneto-tunable power-coupling photonic device can find applications in optical magnetometry, imaging and optical communications.

Due to the limitation of the lens effect of the optical fibre and the inhomogeneity of the laser fluence on different cores, it is still challenging to controllably inscribe different fibre Bragg gratings (FBGs) in multicore fibres. In this article, we reported the FBG inscription in four core fibres (FCFs), whose cores are arranged in the corners of a square lattice. By investigating the influence of different inscription conditions during inscription, different results, such as simultaneous inscription of all cores, selectively inscription of individual or two cores, and even double scanning in perpendicular core couples by diagonal, are achieved. The phase mask scanning method, consisting of a 244nm Argon-ion frequencydoubled laser, air-bearing linear transfer stage and cylindrical lens and mirror setup, is used to precisely control the grating inscription in FCFs. The influence of three factors is systematically investigated to overcome the limitations, and they are the defocusing length between the cylindrical lens and the bare fibre, the rotation geometry of the fibre to the irritation beam, and the relative position of the fibre in the vertical direction of the laser beam.

Herein, we demonstrated a T-shaped whispering gallery modes (WGMs) excitation system including a tapered single mode fiber (SMF), a tapered microstructured optical fiber (MOF) and a BaTiO₃ microsphere for efficient light coupling and routing between the two fibers. The BaTiO₃ microsphere is semi-immersed into the capillary of a tapered MOF, while the tapered SMF is placed perpendicularly to MOF in a contact with equatorial region of the microsphere. Based on that, three channels joined by the microsphere are formed, and excitation and measurement of WGMs is possible either using the SMF or the MOF taper. The measured WGMs spectra reveal light routing along Q-factors between 4500 and 6100, along with scattering signal with all three fiber ports and parities.

We present the results of investigations regarding laser micro-structuring of single mode optical fibres by direct access of the fibre end face and compare this with inscription in planar samples. We combine a high numerical aperture objective and femtosecond laser radiation at visible wavelengths to examine the spatial limits of direct writing and structuring at the surface of the optical fibre. We realise a number of interesting devices from one- and two-dimensional grating structures, to Bessel, Airy and vortex beam generators. We show the versatility of this simple but effective inscription method, where we demonstrate classic multiple slit diffraction patterns and patterns for non-diffracting beams, confirming that the flexible direct write method using femtosecond lasers can be to produce binary masks that can lead to beam shaping using a method that is applicable to all types of planar samples and through fine control of laser parameters to multi-mode and singlemode optical fibres.

Hydrogen sensing technology by definition necessitates high accuracy, rapid response time, and durability. Thin film Pd has demonstrated excellent use in this field owing to large sensitivity and fast detection time. Interaction with hydrogen causes a crystallographic phase transition of the Pd lattice resulting in expansion. Subsequently repeated hydrogen loading cycles increases mechanical stress on the Pd lattice and thus leads to delamination of the hydrogen sensitive layer. By alloying Pd with Y, it is possible to mitigate the unwanted phase transition thereby significantly improving durability. We present the first optical fibre surface plasmon resonance (OFSPR) hydrogen sensor based on a multilayer Ag/SiO₂/PdY deposited on the unclad core of a silica optical fibre. In this submission, we investigated the spectral influence of fibre numerical aperture in addition to Ag and SiO₂ thickness within the multilayer. Sensor sensitivity and figure of merit were found to reach a maximum when a fixed Ag thickness was paired with a set of corresponding SiO₂ thicknesses. We demonstrate that changing the thickness of one of these layers alters the optimal thickness of the other. We present a figure by which an array of optimal sensing structures can be determined. The largest sensor figure of merit in this study was found to be 0.062732, and was produced using Ag = 50nm, and SiO₂ = 70nm. This sensor operates with sensitivity of 17.57nm to 4% hydrogen, detection accuracy of 0.014282nm^-1, and operated at a spectral centre of 524.09nm.

This paper reports the first demonstration of a silica fibre Bragg grating (SOFBG) embedded in an FDM 3-D printed housing to yield a dual grating temperature-compensated strain sensor. We also report the first ever integration of polymer fibre Bragg grating (POFBG) within a 3-D printed sensing patch for strain or temperature sensing. The cyclic strain performance and temperature characteristics of both devices are examined and discussed. The strain sensitivities of the sensing patches were 0.40 and 0.95 pm/με for SOFBG embedded in ABS, 0.38 pm/με for POFBG in PLA, and 0.15 pm/με for POFBG in ABS. The strain response was linear above a threshold and repeatable. The temperature sensitivity of the SOFBG sensing patch was found to be up to 169 pm/°C, which was up to 17 times higher than for an unembedded silica grating. Unstable temperature response POFBG embedded in PLA was reported, with temperature sensitivity values varying between 30 and 40 pm/°C.

The work described in this paper involved two different material fibre Bragg grating (FBG) arrays, investigating their performance as quasi-distributed sensors by capturing the vibrating response of a free-free metal beam close to its resonance frequencies. A six meter length of low-loss, gradient-index, multimode CYTOP fibre and of SMF-28 were used for the inscription of multiple FBG sensors using a femtosecond laser inscription method. The FBG arrays were multiplexed in the wavelength domain using a high-speed commercial demodulator, from which we recovered wavelengthand time-dependent displacement information. We compared the vibration response of the two arrays and using a novel computation algorithm we extract the first mode shape of the free-free metal beam that was exited at its first resonance frequency using a vibrating force.

An automated fs-laser machining procedure was developed to engrave circumferential grooves into the cladding of optical fibres. The grooves are positioned centrally to type I fibre Bragg gratings (FBG) and form locally micro structured FBGs. The grooves realized so far were ~30μm deep and were 48 μm to 200 μm long. These devices show the occurrence of a phase shifted spectrum when axial stress is applied. It is shown in this paper that this property can be used to achieve higher force sensitivities when compared to conventional FBGs. These devices are advantageous for the investigation of tissue by indentation-type elasticity measurements. An experimental and theoretical investigation of the dependence of the force sensitivity on the length of the structure is reported.

Optical microfibers functionalized with surface microstructures, have attracted great attentions and been widely used in various optical devices. In this work, we demonstrated a technique to fabricate microstructures on polymer coated optical microfibers via ultraviolet inscription. Optical microfibers were firstly functionalized with ultraviolet photosensitivity by surface PMMA jackets. Microstructures were fabricated on optical microfibers via point-by-point ultraviolet inscription since the photo-etching in PMMA jackets. As an illustration, a 2-mm-long microfiber long period grating (MLPG) with a pitch of 80 μm was inscribed. The diameter of optical microfiber is 5.4 μm and a resonant dip of 15dB was observed at 1377 nm. The MLPG showed a high sensitivity of strain and axial force, i.e., -1.93 pm/με and -867 μN/nm. Sealed by a PMMA housing, the temperature sensitivity of MLPG could be enhanced from -12.75 pm/°C to -385.11 pm/°C. This technique demonstrates ultraviolet inscription of MLPGs and also provides an approach to fabricate microstructures on optical microfiber via ultraviolet exposure.

Opto-acoustic imaging is a growing field of research in recent years, providing functional imaging of physiological biomarkers, such as the oxygenation of haemoglobin. Piezo electric transducers are the industry standard detector for ultrasonics, but their limited bandwidth, susceptibility to electromagnetic interference and their inversely proportional sensitivity to size all affect the detector performance. Sensors based on polymer optical fibres (POF) are immune to electromagnetic interference, have lower acoustic impedance and a reduced Young’s Modulus compared to silica fibres. Furthermore, POF enables the possibility of a wideband sensor and a size appropriate to endoscopy. Micro-structured POF (mPOF) used in an interferometric detector has been shown to be an order of magnitude more sensitive than silica fibre at 1 MHz and 3 times more sensitive at 10 MHz. We present the first opto-acoustic measurements obtained using a 4.7mm PMMA mPOF Bragg grating with a fibre diameter of 130 μm and present the lateral directivity pattern of a PMMA mPOF FBG ultrasound sensor over a frequency range of 1-50 MHz. We discuss the impact of the pattern with respect to the targeted application and draw conclusions on how to mitigate the problems encountered.

In this paper, both non-annealed and annealed trans-4-stilbenemethanol-doped step-index polymer optical fibers were photo-inscribed using a 325 nm HeCd laser with two different beam power densities reaching the fiber core. In the high density regime where 637 mW/mm² are used, the grating reflectivity is stable over time after the photo-writing process but the reflected spectrum is of limited quality, as the grating physical length is limited to 1.2 mm. To produce longer gratings exhibiting more interesting spectral features, the beam is enlarged to 6 mm, decreasing the power density to 127 mW/mm². In this second regime, the grating reflectivity is not stable after the inscription process but tends to decay for both kinds of fibers. A fortunate property in this case results from the possibility to fully recover the initial reflectivity using a post-inscription thermal annealing, where the gratings are annealed at 80 °C during 2 days. The observed evolutions for both regimes are attributed to the behavior of the excited intermediate states between the excited singlet and the ground singlet state of trans- and cis-isomers as well as the temperature-dependent glassy polymer matrix.

The annealing effects on strain and stress sensitivity of polymer optical fibre Bragg grating sensors after their photoinscription are investigated. PMMA optical fibre based Bragg grating sensors are first photo-inscribed and then they were placed into hot water for annealing. Strain, stress and force sensitivity measurements are taken before and after annealing. Parameters such as annealing time and annealing temperature are investigated. The change of the fibre diameter due to water absorption and the annealing process is also considered. The results show that annealing the polymer optical fibre tends to increase the strain, stress and force sensitivity of the photo-inscribed sensor.

Safety in civil aviation is increasingly important due to the increase in flight routes and their more challenging nature. Like other important systems in aircraft, fuel level monitoring is always a technical challenge. The most frequently used level sensors in aircraft fuel systems are based on capacitive, ultrasonic and electric techniques, however they suffer from intrinsic safety concerns in explosive environments combined with issues relating to reliability and maintainability. In the last few years, optical fiber liquid level sensors (OFLLSs) have been reported to be safe and reliable and present many advantages for aircraft fuel measurement. Different OFLLSs have been developed, such as the pressure type, float type, optical radar type, TIR type and side-leaking type. Amongst these, many types of OFLLSs based on fiber gratings have been demonstrated. However, these sensors have not been commercialized because they exhibit some drawbacks: low sensitivity, limited range, long-term instability, or limited resolution. In addition, any sensors that involve direct interaction of the optical field with the fuel (either by launching light into the fuel tank or via the evanescent field of a fiber-guided mode) must be able to cope with the potential build up of contamination – often bacterial – on the optical surface. In this paper, a fuel level sensor based on microstructured polymer optical fiber Bragg gratings (mPOFBGs), including poly (methyl methacrylate) (PMMA) and TOPAS fibers, embedded in diaphragms is investigated in detail. The mPOFBGs are embedded in two different types of diaphragms and their performance is investigated with aviation fuel for the first time, in contrast to our previous works, where water was used. Our new system exhibits a high performance when compared with other previously published in the literature, making it a potentially useful tool for aircraft fuel monitoring.

We study in detail the macrobending effects in a wide transmission bandwidth (~200nm) 19 cell hollow-core photonic bandgap fiber operating at 1550nm. Our results indicate low bend sensitivity over a ~130nm wide interval within the transmission window, with negligible loss (<0.1dB) for bending radii down to 5mm. The “red shift” and “blue shift” of the bandgap edge have been observed at the short and long wavelength edges, respectively. The cutoff wavelengths where air-guiding modes stop guiding can be extracted from the bending loss spectra, which matches well with the simulated effective refractive index map of such fiber.

A novel experimental set-up for measuring the spectral dependency of light-guidance of specialty non-active multimodefibers will be introduced. Light coupling into the test fiber is realized and controlled with a micro-structured single mode (SM) fiber and an image-system based on a microscope objective The far- and near-field profiles of the SM-fiber will be shown. The inverse far field method is modified and improved by using three wavelengths simultaneously under the same input conditions; the coupling conditions into the test-fiber and the far- and near-field at fiber output are observed with cameras. The numerical aperture (NA) and mode-conversion or focal-ratio-degradation (FRD) are measured in respect to wavelength at three wavelengths in the VIS region. For the analysis, the patterns are captured at varying exposure times to increase the dynamic range and finally analyzed using image processing methods. Characteristic parameters, such as skew mode propagation and ray-conversion, of circular and non-circular MM-fibers will be discussed, taking the surface roughness into account.

Fabrication of Ytterbium-doped active fibers with different designs, compositions and high Yb concentration has attracted an intense interest. For making highly Yb-doped fibers, co-dopants like phosphorous (P) and aluminum (Al) are also employed in order to modify refractive index and increase Yb solubility, avoiding clusters and phase segregations. Indeed, Yb-clustering results in quenching effects and increased propagation losses due to energy transfer between clustered ions. Therefore, the chemical composition and phase homogeneity of the fiber core have key influences on the performance of an active fiber. However, conventional fabrication techniques such as MCVD (modified chemical vapor deposition) and OVD (outside vapor deposition) are approaching the limit. In this contribution, we have developed an approach for fabrication of such active fibres based on granulated silica derived from the sol-gel process. The advantage of this method is the fabrication of active fibers with high dopant contents and homogeneity. Here, using high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in atomic scale, we report the direct, nano-scale and atomic-resolution observation of individual Yb dopant and co-dopant (i.e. Al, P) atoms for different fabricated fibers. The chemical mapping from STEM-EDX shows an extremely homogeneous distribution of the dopants and co-dopants in nano-scale for our fabrication protocol. However in atomic resolution, we also identified the possible Yb clusters in the range of 10 atoms within the core structure. The size, structure, and distribution of these clusters are determined with an Yb-atom detection efficiency of almost 100% by STEM.

The applications of fibre lasers demand for increasing power. Limits are set by various nonlinear effects. Leakage channel fibres (LCF) are one approach to this problem. With this type of fibre, most nonlinear effects can, in principle, be mitigated simultaneously by increasing the mode field area and by maintaining the single mode regime. For its implementation, we propose to use the powder-in-tube preform technique. While the microstructure consists of commercial pure silica rods, the surrounding is filled with index-raised aluminum-doped silica oxide granulate. For the fabrication of the latter, we tested two different methods. For the first one, the oxide precursors were mixed in pure powder form. In the other method, the material was produced with the helps of the sol-gel process, where the mixing takes place in liquid phase, thus resulting in an expected improved homogeneity. Prior to the fabrication of a prototype, their feasibility has been tested with the help of a finite-difference method simulation tool (Lumerical MODE Solutions). Two such fibres have been fabricated according to this results. The influence of the granulate mixing method and of the grain size on the homogeneity in refractive index has been tested. Although the produced fibres do not yet show the desired performance, the produced prototypes prove that LCFs can indeed be realised with this approach.

Novel special optical fibers nowadays can take advantage of several new preform production techniques. During the last years we have devoted our attention to the granulated silica method. It is one of the variants of the powder-in-tube technique and potentially offers a high degree of freedom regarding the usable dopants, the maximum possible dopant concentration, the homogeneity of the dopants, the geometry and minimal refractive index contrast. We developed and refined an approach for the production of doped granulated silica material based on the sol-gel process. Here, we present material analysis results of an ytterbium (Yb) doped, aluminum (Al) and phosphorous (P) co-doped glass on the basis of our sol-gel glass based granulated silica method as well as first measurements of two LMA fibers obtained from this material. For the material analysis we used advanced analysis techniques, such as HAADF-STEM and STEM-EDX spectroscopy to determine the composition of the material and the distribution of the dopants and the codopants. The chemical mapping of the STEM-EDX shows an extremely homogeneous distribution of the dopants and co-dopants in nano-scale. Based on self-made LMA fibers, we measured the refractive index contrast of the sol-gelbased granulated silica derived core compared to the pure silica cladding. In addition we quantified optical characteristics such as the emission and absorption spectrum. The measured upper state lifetime of the optical active dopant ytterbium was 0.99ms, which in turn confirms the homogeneous distribution of the Yb atoms. The propagation losses were determined to be 0.2dB/m at 633nm and 0.02414dB/m at1550nm.

We report on the first polarization maintaining single-mode fiber that delivers a flat-top intensity profile at 1050 nm. A high quality fundamental flat mode was obtained. We showed that our fiber can be considered as single-mode in practice with low confinement losses. Its birefringence was measured to be 0.6x10^-4, and the PER was measured at more than 20 dB even for a 20 m fiber long. Strategies to enhance this birefringence preserving the flat top profile and the singlemode behaviour as well are also discussed.

While hollow core-photonic crystal fibres are now a well-established fibre technology, the majority of work on these speciality fibres has been on designs with a single core for optical guidance. In this paper we present the first dual hollow-core anti-resonant fibres (DHC-ARFs). The fibres have high structural uniformity and low loss (minimum loss of 0.5 dB/m in the low loss guidance window) and demonstrate regimes of both inter-core coupling and zero coupling, dependent on the wavelength of operation, input polarisation, core separation and bend radius. In a DHC-ARF with a core separation of 4.3 μm, we find that with an optimised input polarisation up to 65% of the light guided in the launch core can be coupled into the second core, opening up applications in power delivery, gas sensing and quantum optics.

We describe the fabrication of a polycarbonate (PC) micro-structured polymer optical fiber (mPOF) and the writing of fiber Bragg gratings (FBGs) in it to enable strain and temperature measurements. We demonstrate the photosensitivity of a dopant-free PC fiber by grating inscription using a UV laser. We further show that PC Bragg gratings can be extended up to at least 3% without affecting the initial functionality of the micro-structured fiber. The response of PC FBGs to temperature up to 125°C is also investigated. Polycarbonate has good mechanical properties and its high temperature resistance might extend the range of application of polymeric FBGs.

We develop an analytical approach for finding of self-similar shape of an optical pulse at its propagation in a medium with non-instantaneous nonlinear absorption. The main feature of such pulse shape is asymmetric. Moreover, this mode of optical pulse propagation takes place only for a chirped pulse in contrast to well-known soliton solution of nonlinear Schrödinger equation (or set of such equations). Therefore, we discuss a new type of self-similar mode of laser pulse propagation: self-similar chirped pulse. An existence of this pulse takes place for certain distance of its propagation. We derive a relation between the problem parameters and shape of pulse and its chirp which are necessary for an occurrence of the self-similar mode for optical pulse propagation in a medium under consideration. The developed solution is confirmed by computer simulation results.

As it is well-known, THz TDS is a modern tool for the detection and identification of substance. Often, in real conditions a substance under identification is covered by various materials (paper sheet, napkins, rag, and et.al). Therefore, the identification occurs for a substance covered by disordered structure, which acts for THz radiation as disordered photonic structure. In standard THz TDS method the substance detection carries out using a comparison of spectrum of a substance under consideration with spectra of the substances from database. Thus, an investigation of spectral medium response covered by disordered structure is very important for security and screening problem. Moreover, what we will see if we analyze a response from disordered structure without any dangerous substance? This question is a key one for practical application. Using computer simulation, we investigate below a propagation of laser pulse with a few cycles in a linear layered structure with random fluctuation of either layer dielectric permittivity or layers thicknesses or both characteristics of this structure. The process under consideration is described by 1D Maxwell’s equations. We show that a spectrum of pulse either reflected from substance or transmitted through substance depends in strong way from a number of random realization and fluctuating parameters of layered structure and an observer can see various absorption frequencies corresponding to dangerous substances. Nevertheless, we discuss one of possible ways for overcoming the influence of disordered structure on the observed spectrum.

We report development of suspended-core silica and lead-silicate microstructured optical fibers for detection of liquids and supercontinuum generation. Theoretical analysis of effective mode area, dispersion curve, nonlinear coefficient and mode-field overlap is presented. For a specific lead-silicate fiber we determinated the zero dispersion wavelength at 1113 nm with a nonlinear coefficient of 1321 W^-1km^-1. For detection of liquids both silica and lead-silicate fibers are found to be suitable in different refractive index ranges 1.38-1.44 and 1.68-1.74 respectively. Coupling efficiency into all studied fibers over 40%; fiber attenuation was measured by the cut-back technique and is approximately 2 dB/m for silica glass fibers and over 3 dB/m for lead-silicate fibers.

The stress sensitivity of polymer optical fibre (POF) based Fabry-Perot sensors formed by two uniform Bragg gratings with finite dimensions is investigated. POF has received high interest in recent years due to its different material properties compared to its silica counterpart. Biocompatibility, a higher failure strain and the highly elastic nature of POF are some of the main advantages. The much lower Young’s modulus of polymer materials compared to silica offers enhanced stress sensitivity to POF based sensors which renders them great candidates for acoustic wave receivers and any kind of force detection. The main drawback in POF technology is perhaps the high fibre loss. In a lossless fibre the sensitivity of an interferometer is proportional to its cavity length. However, the presence of the attenuation along the optical path can significantly reduce the finesse of the Fabry-Perot interferometer and it can negatively affect its sensitivity at some point. The reflectivity of the two gratings used to form the interferometer can be also reduced as the fibre loss increases. In this work, a numerical model is developed to study the performance of POF based Fabry-Perot sensors formed by two uniform Bragg gratings with finite dimensions. Various optical and physical properties are considered such as grating physical length, grating effective length which indicates the point where the light is effectively reflected, refractive index modulation of the grating, cavity length of the interferometer, attenuation and operating wavelength. Using this model, we are able to identify the regimes in which the PMMA based sensor offer enhanced stress sensitivity compared to silica based one.

Refractive index profile measurement is a key instrument for characterization of optical properties of preforms, which are used for drawing of high-quality optical fibers. Common industrial optical preform analyzers have been designed for measurement of simple symmetric structures such as step-index or graded-index preforms with refractive index close to the silica (n=1.457 at 633 nm). However, these conditions are usually far from more complex structures used in fiber lasers or in fiber sensor area. Preforms for the drawing of advanced optical fibers, such as Bragg, microstructure or photonic crystal fibers, are usually constituted from stacks with non-symmetric internal structure or composed of alternating layers with high refractive index contrasts. In this paper we present comparison of refractive index profile measurements of simple as well as complex structures with high refractive index differences simulating the Bragg structures. Commercial Photon Kinetics 2600 preform analyzer was used for the refractive index profile measurements. A set of concentrically arranged silica tubes was welded to form a complex preforms. Free space between the tubes was filled by immersion with varying refractive indices to simulate the Bragg structure. Up to three tubes were used for the analysis and the refractive indices of immersion were changed from 1.4 to 1.5. When refractive index of immersion was independently measured the structure of preform was defined. Profiles of these “known” structures were compared to measured data processed by originally proposed algorithm. The work provides an extension of issues of refractive index profile measurements in non-symmetric complex silica structures by a commercial preform analyzer and proposes more convenient methods of numeric data processing.

We propose and demonstrate a technique for cascading a microsphere and an abrupt taper together in a standard singlemode fiber. The proposed microsphere-taper cascading structured microfiber (MTCSM) was fabricated by fusion splicing and electric-arc discharge. Exposing the MTCSM segment to increasing temperatures results in a significant shift of the transmission notches towards longer wavelengths with a slope of approximately 17.24 pm/°C∇ from 35°C to 170°C, and the linearity is 99.8%. Due to its compact size and all-fiber configuration, the proposed sensor has advantages of good mechanical strength, simplicity and low-cost fabrication, such new device would find potential applications in communication and sensing fields.

Polymer cladding temperature of active fiber in lasing regime is important parameter as it allows determination of fiber core temperature that in turn effects laser generation and amplification efficiency. Besides polymer cladding has much lower temperature damage threshold comparing to fused silica. For example, 200 degrees Kelvin overheating of the polymer cladding can result in fiber degradation. In present paper we introduce novel and simple method for precise temperature measurement of active fibers cladding under conditions of laser generation and amplification. Dependence of longitudinal temperature distribution along active fibers on optical pump power can be determined. This method employs measurement of temperature dependent electrical resistance of the metal wire being in thermal contact with fiber polymer cladding. The wire is reeled on the active fiber segment. Under lasing or amplification conditions the polymer cladding of the active fiber is heated together with coiled metal wire resulting in its electrical resistance change. By measuring resistance variation one can determine the temperature of the given fiber section.

We determine the radial profile of the photoelastic constant C(r) in two single mode and one multimode polymer optical fibers (POFs), all fabricated from polymethylmethacrylate (PMMA). To determine C(r) we first determine the retardance of the laterally illuminated fiber submitted to a known tensile stress uniformly distributed over the fiber cross-section. Then we determine the inverse Abel transform of the measured retardance to finally obtain C(r). We compare two algorithms based on the Fourier theory to perform the inverse transform. We obtain disparate distributions of C(r) in the three fibers. The mean value of C(r) varies from -7.6×10^-14 to 5.4×10^-12 Pa^-1. This indicates that, in contrast to glass fibers, the radial profile of the photoelastic constant can considerable vary depending on the type and treatment of POFs, even when made from similar materials, and hence the photoelastic constant should be measured for each type of POF.

In this study our main objective is to design new Erbium-doped fiber amplifier (EDFA) configurations to obtain higher gain and lower noise figure (NF) values using an input signal -35 dBm at the wavelength of 1550 nm and the pump power 14 mW at the wavelength of 1480 nm. Utilizing Giles and Desurvire model, we proposed miscellaneous Single Pass (SP) and Double Pass (DP) EDFA configurations different in the means of pump signal direction where the two level system model equations are solved numerically in Matlab. The physical properties of the simulated EDFA configurations such as gain, noise factor, population density and amplified spontaneous emission were studied under various pumping directions.

The refractive index distribution in the core-cladding region of an optical fiber plays an important role in determining the transmission and dispersion properties of the waveguide. The refracted near-field technique (RNF) is among the most widespread techniques used for measuring the refractive index profile of optical fibers and is based on illuminating the end-facet of a fiber with a focused beam whose vertex angle greatly exceeds the acceptance angle of the fiber, which is immersed in an index matching liquid. What one observes are then the refracted unguided rays rather than the guided rays. Nevertheless, the standard refracted near-field technique cannot be applied to a wide range of optical fibers e.g. if their shapes are not axially symmetric. In this work we demonstrate a modified method which allows 2-D imaging of the refractive index profile and thereby overcoming the axial symmetric limitation of the standard RNF. The new system is operating at 630 nm and based on the same principle of the RNF, but the optical path is reversed so that the light at the fiber end-facet is collected by an objective lens and detected by a CCD camera. The method does not require scanning over the fiber end-facet. Thus the system is faster and less sensitive to vibrations and external conditions compared to the standard RNF, furthermore it allows averaging to improve the signal to noise ratio. The spatial resolution of the system is determined by the numerical aperture of the objective and by the resolution of the CCD camera. To calibrate the setup, a reference multi-step index fiber provided by National Physical Laboratory was used.

Ultrafast laser owns extreme small beam size and high pulse intensity which enable spatial localised modification either on the surface or in the bulk of materials. Therefore, ultrafast laser has been widely used to micromachine optical fibres to alter optical structures. In order to do the precise control of the micromachining process to achieve the desired structure and modification, investigations on laser parameters control should be carried out to make better understanding of the effects in the laser micromachining process. These responses are important to laser machining, most of which are usually unknown during the process. In this work, we report the real time monitored results of the reflection of PMMA based optical fibre Bragg gratings (POFBGs) during excimer ultraviolet laser micromachining process. Photochemical and thermal effects have been observed during the process. The UV radiation was absorbed by the PMMA material, which consequently induced the modifications in both spatial structure and material properties of the POFBG. The POFBG showed a significant wavelength blue shift during laser micromachining. Part of it attributed to UV absorption converted thermal energy whilst the other did not disappear after POFBG cooling off, which attributed to UV induced photodegradation in POF.

We report the evaluation of one long period grating (LPG) and one fiber Bragg grating (FBG) under gamma irradiation. The LPG was produced by the melting-drawing method based on CO2 laser assisted by a micro-flame and was engraved in a commercial single mode fiber SMF28 from Corning, grating length 25 mm, grating pitch of 720 μm. After the manufacturing of the grating, the fiber was re-coated with Acrylate and the grating was inserted into special ceramic case transparent to gamma radiation. The FBG is commercialized by Technica SA, and it is written in SMF-28 optical fiber (λ= 1546 nm; grating length of 12 mm; reflectivity > 80 %; bandwidth – BW @3 dB < 0.3 nm; side lobe suppress ratio – SLSR >15 dB; Acrylate recoating). By on-line monitoring of the LPG wavelength deep with an optical fiber interrogator during the irradiation exposure and pauses, both the irradiation induced shift (maximum 1.45 nm) and the recovery (in the range of 200 pm) phenomena were observed. Temperature sensitivity of the LPS was not affected by gamma irradiation.

In this paper, we present the first results regarding the on-line monitoring of gamma-ray exposure effects on a commercial multi-mode perfluorinated polymer optical fiber (PF-POF), type GigaPOF-50SR from Chromis Fiberoptics. Our focus was to evaluate on-line the radiation induced attenuation (RIA) over a wide spectral range (320 nm – 1700 nm), in order to assess the fiber’s radiation hardness and its possible use in radiation detection. An Ocean Optics QE65000 high sensitivity spectrometer and a StellarNet near-IR spectrometer were used to cover the spectral ranges 200 nm – 1μm and 900 nm – 1.6 μm, respectively. Electron paramagnetic resonance was used to monitor the recovery of the irradiation induced centers at room temperature. The study indicated that the optical fiber can be used as radiation monitor at low dose rates by measuring the attenuation in the UV, while higher dose rates irradiation can be observed by RIA monitoring at specific wavelengths in the visible spectral range.