This PDF file contains the front matter associated with SPIE Proceedings Volume 7714, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

In this work, we propose to take advantage of properties of a Bragg fibre for optical sensing. The Bragg fibre exhibits three concentric high refractive index layers embedded in pure silica and surrounding a 35μm diameter core. A short (0.3 m long) piece of Bragg fibre slightly multimode, is used to elaborate an intermodal interferometer, the spectral response of which exhibits a fringe pattern that depends on the operating wavelength, which can therefore be used as a sensor. The two modes considered were found to be the fundamental LP01 and the high-order mode LP02. The sensor has been characterized in strain and temperature and presents a sensitivity of - 1.09 pm/με and 14.1 pm/°C respectively. The sensor demonstrated insensitivity to curvature thanks to well known Bragg fibre properties.

In this work, we demonstrate the possibility of fabricating short LPGs and rocking filters in highly birefringent Photonic Crystal Fiber using CO₂ laser. In our experiments both kinds of gratings were made in the same Boron doped highly birefringent PCF using similar exposure parameters. We also present the sensing capabilities of both fabricated gratings to temperature, strain and hydrostatic pressure by interrogation of the wavelength shifts at the different resonances.

This paper introduces a supercontinuum (SC) laser source emitting from 400 nm to beyond 1750 nm, with adjustable pulse repetition rate (from 250 kHz to 1 MHz) and duration (from ~200 ps to ~2 ns). This device makes use of an internally-modulated 1.06 μm semiconductor laser diode as pump source. The output radiation is then amplified through a preamplifier (based on single-mode Yb-doped fibres) followed by a booster (based on a double-clad Yb-doped fibre). The double-clad fibre output is then spliced to an air-silica microstructured optical fibre (MOF). The small core diameter of the double-clad fibre allows reducing the splice loss. The strongly nonlinear propagation regime in the MOF leads to the generation of a SC extending from the violet to the nearinfrared wavelengths. On the Stokes side of the 1.06 μm pump line, i.e., in the anomalous dispersion regime, the spectrum is composed of an incoherent distribution of quasi-solitonic components. Therefore, the SC source is characterised by a low coherence length, which can be tuned by simply modifying pulse duration, that is closely related to the number of quasi-solitonic components brought into play. Finally, the internal modulation of the laser diode permits to achieve excellent temporal stability, both in terms of average power and pulse-to-pulse period.

The increase of the output power in fiber lasers and amplifiers is directly related to the scaling of the core diameter. State of the art high power laser and amplifier setups are based on large mode area (LMA) photonic crystal fibers (PCF) exhibiting core diameters ranging from 40 μm up to 100 μm¹ (rod-type PCF). For instance, a two-stage femtosecond chirped pulse amplification (CPA) system based on 80 μm core diameter rod-type PCF was demonstrated generating 270 fs 100 μJ pulses2. Although highly suited to reach very large mode areas, this fiber design suffers some drawbacks such as high bend sensitivity (for core diameter equal to or larger than 40 μm³) and practical handling (cleaving, splicing, etc.) due to presence of air holes. As an alternative we have recently proposed all-solid photonic bandgap (PBG) Bragg fiber (BF) design⁴. Due to their waveguiding mechanism completely different from total internal reflection this type of fiber offers a very flexible geometry for designing waveguide structures with demanding properties (singlemodedness in large core configuration⁵, chromatic dispersion⁶, polarization maintaining⁷, low bend sensitivity⁸). During the last few years our interest was mainly focused on the realization of an active BF and scaling up the core diameter. We showed that, in principle, core diameters in excess of 50 μm can be reached⁹. As an example, an Yb-doped LMA BF with 20 μm core diameter was realized and single transverse mode operation in continuous wave (cw)⁹ and mode-locking¹⁰ oscillation regimes was demonstrated. Moreover, operation of two dimensional all-solid PBG fibers in laser and amplifier regimes was recently demonstrated^11-13. In this paper we report on the first demonstration of amplification of femtosecond pulses in LMA PBG BF. A single transverse mode was obtained and the BF allowed for generating 5 μJ 260 fs pulses in a system with a moderate stretching of 150 ps.

Photonic Crystal Fibers (PCF) play a crucial role for fundamental investigations such as acousto-optical interactions as well as for applications, such as distributed sensors. One limiting factor for these experiments is the fiber inhomogeneity owing to the drawing process. In this paper we study the effect of structural irregularities on both the backward and forward Brillouin scattering by comparing two PCFs drawn with different parameters, in order to minimize diameter fluctuations. We fully characterize their Brillouin properties including the backward Brillouin spectrum, the Brillouin threshold, a distributed measurement along the fibers and polarized Guided Acoustic Wave Brillouin Scattering (GAWBS). In the Brillouin spectrum we observe a single peak as in a singlemode fiber whereas former investigations have often shown a multiple peak spectrum in PCFs with small core. The theoretical and experimental values for the Brillouin threshold are in good agreement, which results from the single peak spectrum. By using a Brillouin echoes distributed sensing system (BEDS), we also investigate the Brillouin spectrum along the fiber with a high spatial resolution of 30 cm. Our results reveal a clear-cut difference between the distributed measurements in the two fibers and confirm the previous experiments. In the same way the GAWBS allows us to estimate the uniformity of the fibers. The spectra show a main peak at about 750 MHz, in accordance with theoretical simulations of the acoustic mode and of the elasto-optical coefficient. The fiber inhomogeneity impacts on the stability and the quality factor of the measured GAWBS spectra. We finally show that the peak frequency of the trapped acoustic mode is more related to the optical effective area rather than the core diameter of the PCF. Thus measuring the main GAWBS peak can be applied for the precise measurement of the effective area of PCFs.

Freestanding nanofibers with submicron diameter exhibit a strong waveguide dispersion and field localization caused by both the extreme small core diameter compared to common optical standard fibers and the large refractive index difference of Δn = 0.45 between the silica core and air cladding. These are promising characteristics for nonlinear applications like e.g. supercontinuum generation that require waveguides with large nonlinear coefficients and specific dispersion behavior in dependence on the pump wavelength. Optical nanofibers allow a shift of the zero dispersion wavelength and in addition anomalous dispersion down to a wavelength of about 460 nm. We report on deep ultraviolet broadband supercontinuum generation in optical nanofibers pumped with femtosecond pulses from a frequency doubled titan-sapphire oscillator at 400nm wavelength. Numerical simulations of the generated spectra show a remarkable broadening in the deep ultraviolet wavelength range below 250 nm after only a few millimeter of propagation distance, while a single recompressible pulse is maintained in the time domain. The spectrum can be influenced by nanofiber diameter, pulse duration, and pulse energy. In experiment these nanofibers are situated in an optical fiber taper configuration where a waist of constant submicron diameter is located between two taper transitions with varying diameter. For this reason the generated supercontinuum is not only influenced by the nanofiber diameter but by the taper transitions.

In this work, we report results of lead sulfide (PbS) quantum dots (QDs) luminescence spectra evolution during the QDs spread process around the core of silica micro-structured (MS) fibers. These QDs are excited, via evanescent field effect, with a 532nm or 785nm laser guided by the MS-fiber cores. The PbS-core QDs of different sizes (originally immersed in Toluene) with emission bands around 877 nm (PbS877), 1160 nm (PbS1160) and 1474 nm (PbS1474) were inserted inside the silica MS-fiber structure by using an N2 gas pressure system. The broadband luminescence spectra varying from around 1000 nm to 1600 nm were obtained by using QDs mixtures impregnated around MS-fiber core surfaces. This QDs spread technique and the PbS QDs broadband luminescence spectra results could have potential applications in optical amplifier,sensor and nonlinear optical fiber loop mirror devices.

The development of chalcogenide glasses fibers for application in the infrared wavelength region between 1 and 10 μm is a big opportunity. More particularly, the possibility to generate efficient non linear effects above 2 μm is a real challenge. We present in this work the elaboration and optical characterizations of suspended core microstructured optical fibers elaborated from the As₂S₃ chalcogenide glass. As an alternative to the stack and draw process a mechanical machining has been used to the elaboration of the preforms. The drawing of these preforms into fibers allows reaching a suspended core geometry, in which a 2.5 μm diameter core is linked to the fiber clad region by three supporting struts. The zero dispersion wavelength is thus shifted towards 2 μm. At 1.55 μm our fibers exhibit a dispersion around -250 ps/nm/km. Their background level of losses is below 0,5 dB/m. By pumping them at 1.55 μm with a ps source, we observe self phase modulation as well as Raman generation. Finally a strong spectral enlargement is obtained with an average output power of - 5 dbm.

Chalcogenide or heavy metal oxide glasses are well known for their good transparency in the mid-infrared (MIR) domain as well as their high nonlinear refractive index (n₂) tens to hundreds times higher than that of silica. We have investigated the nonlinear frequency conversion processes, based upon either stimulated Raman scattering (SRS) or soliton fission and soliton self-frequency shift (SSFS) in fibres made up with such highly nonlinear infrared transmitting glasses. First, SRS has been investigated in a chalcogenide As2S3 step index fibre. In the single pass configuration, under quasi continuous wave 1550 nm pumping, Raman cascade up to the forth Stokes order has been obtained in a 3 m long piece of fibre. The possibility to build a Raman laser thanks to in-fibre written Bragg gratings has also been investigated. A 5 dB Bragg grating has been written successfully in the core. Then, nonlinear frequency conversion in ultra-short pulse regime has been studied in a heavy metal oxide (lead-bismuth-gallium ternary system) glass photonic crystal fibre. Broadband radiation, from 800 nm up to 2.8 μm, has been obtained by pumping an 8 cm long piece of fibre at 1600 nm in sub-picosecond pulsed regime. The nonlinear frequency conversion process was assessed by numerical modelling taking into account the actual fibre cross-section as well as the measured linear and nonlinear parameters and was found to be due to soliton fission and Raman-induced SSFS.

Tellurite highly nonlinear microstructured fibers were fabricated by pumping a positive pressure of nitrogen gas into the holes of cane in the fiber drawing process. By adjusting the pump pressure to inflate the holes of the fiber, the microstructures were reshaped, and the chromatic dispersions were tailored. Two kinds of fiber were fabricated. One is an air-clad fiber with a 1 μm hexagonal core, which is the smallest core in this shape for the air-clad fiber. By changing the inflation pressure, the diameter ratio of holey region to core (DRHC) was changed in the range of 1-20. Fibers with DRHC of 3.5, 10, 20 were demonstrated. By increasing the DRHC, the zero dispersion wavelengths were shifted to the short wavelength and the confinement loss were decreased. Another is a complex microstructure fiber with a 1.8 μm core surrounded by four ring holes. The shape of the microstructure was reshaped so heavily by the inflation pressure that it is obviously different from the original shape in the cane. The correlations among pump pressure, hole size, surface tension and temperature gradient were investigated. The temperature gradient at the bottom of the preform's neck region was evaluated quantitatively. The chromatic dispersion of this fiber was compared with that of a step-index air-clad fiber. It was found that this fiber had a much more flattened chromatic dispersion. Supercontinuum generations were investigated by the pump of a 1557 nm femtosecond fiber laser. Intense third harmonic generations were obtained from the 1μm haxgonal core fiber. Broad and flattened spectrum was obtained from the complex microstructure fiber. This investigations show that, by using a positive pressure to reshape the microstructure and by controlling the fabrication conditions exactly, highly nonlinear soft glass fibers with desirable chromatic dispersion can be fabricated, and such fibers have interesting applications in highly nonlinear field such as THG and SC generation.

bre Bragg gratings have been inscribed in multimode microstructured polymer optical fibre (POF), with a core size of 50μm. The microstructured POF (mPOF) consists of a three ring hole structure and is made purely from poly(methyl methacrylate) (PMMA). In comparison to silica fibre, POF has a much smaller Young's modulus and a much greater breaking strain; additionally multimode fibre holds advantages of ease of handling and launching conditions. A linear strain sensitivity of 1.32 ± 0.01pm/με has been measured in the range 0 to 2% strain. The fibre drawing process leads to a degree of molecular alignment along the fibre axis. This alignment can be thermally annealed out; this can induce a permanent blue shift in the Bragg wavelength of a grating fabricated prior to annealing by up to 20 nm. Utilising this, wavelength demultiplexed gratings can be fabricated using a single phase mask. As an illustration of this we present for the first time wavelength division multiplexing of the spectral response of three Bragg gratings in POF within the C-band region. Complementing this work, a technique of splicing mPOF to step index silica fibre is described using UV curing optical adhesive, allowing characterisation of Bragg gratings fabricated in this fibre.

We investigated sensing properties of single mode poly methyl methacrylate (PMMA) microstructured polymer optical fibres (MPOF) with mechanically imprinted long period gratings (LPG). After preparation of the MPOF end-faces the samples were elongated with silica fibres. These samples were used to measure the influence of strain to the LPG wavelength which showed the viscoelastic nature of PMMA. We also measured the influence of temperature and humidity. The results show that MPOF LPGs are well suited for strain sensing. One MPOF LPG was stitched to a textile. Using this textile we measured a simulated respiratory motion.

We have systematically measured the differential stress-optic coefficient, ΔC, and Young's modulus, E, in a number of PMMA fibers drawn with different stress, ranging from 2 up to 27 MPa. Effect of temperature annealing on those parameters was also investigated. ΔC was determined in transverse illumination by measuring the dependence of birefringence on additional axial stress applied to the fiber. Our results show that ΔC in PMMA fibers has a negative sign and ranges from -4.5 to -1.5×10^-12 Pa^-1 depending on the drawing stress. Increase of the drawing stress results in greater initial fiber birefringence and lower ΔC. The dependence of ΔC and initial birefringence upon drawing stress is nonlinear and gradually saturates for higher drawing stress. Moreover, we find that ΔC is linearly proportional to initial fiber birefringence and that annealing the fiber has no impact on the slope of this dependence. On the other hand, no clear dependence was observed between the fiber drawing stress and the Young's modulus of the fibers as measured using microscopic digital image correlation with the fibers tensioned using an Instron tension tester.

The possibility of second-harmonic generation based on surface dipole and bulk multipole nonlinearities in silica nanowires is investigated numerically. Both circular and microstructured nanowires are considered. Phase matching is provided by propagating the pump field in the fundamental mode, while generating the second harmonic in one of the modes of the LP₁₁ multiplet. This is shown to work in both circular and microstructured nanowires, although only one of the LP₁₁ modes can be phase-matched in the microstructure. The prospect of obtaining large conversion efficiencies in silica-based nanowires is critically discussed, based on simulations of second-harmonic generation in nanowires with a fluctuating phase-matching wavelength. It is concluded that efficient wavelength conversion will either require strong improvements in the nanowire uniformity, or an increase of the second-order nonlinearity by at least an order of magnitude by use of a different base material, or highly polarizable surface coatings.

Yb-doped rod-type photonic crystal fibers are double-cladding large-mode-area fibers with the outer dimension of few millimeters. The higher-order mode suppression through gain filtering has been demonstrated in these fibers, by enlarging the core radius from 30 μm to 40 μm, while keeping fixed the doped-area dimension. Sectioned core doping, obtained by adding a low refractive index ring in the fiber core, has been taken into account, in order to design fibers with an effective single-mode behaviour. Moreover, the gain competition among the guided modes in the enlarged-core rod-type PCFs has been analyzed with a spatial and spectral amplifier model, showing the positive effect of the gain filtering in improving the fundamental mode amplification, to the detriment of the higher-order mode one. Comparisons have been made with the properties of rod-type fibers with 30 μm core radius, both with uniform and sectioned doping, in order to show the effectiveness of the down-doped ring in the enlarged core for the higher-order mode suppression.

A modal solution approach based on a rigorous full vectorial finite element method has been used to determine single mode single polarization properties of a bent highly birefringent fiber photonic crystal fiber. A design approach for the single mode single polarization design has been discussed.

We analyze a stability of optical soliton, which is propagated along the nonlinear layered structure (photonic crystal), with respect to perturbation of propagation direction. Soliton is located over a number of layers. Profile of soliton is found as a solution of corresponding eigenfunction problem for nonlinear Schrödinger equation with periodic coefficients. The stability of optical soliton is investigated for transverse perturbations with respect to propagation direction on the base of computer simulation. Three different types of soliton evolution depending on the amplitude of transverse perturbation are discussed.

An index-guiding solid core photonic crystal fibre (PCF) is numerically optimized to achieve through an inscribed long period grating (LPG) broadband coupling between the fundamental mode (LP₀₁) and the first-order symmetric cladding (LP₀₂-like) mode. The vectorial finite element method and the Nelder-Mead simplex method are used for the optimization of the PCF to get for LP₀₁-LP₀₂ mode coupling the phase matching curve with the dispersion turning point located in the center of a selected range of resonant wavelengths. The bandwidth of a LPG close to the dispersion turning point, where the resonance condition is nearly matched for multiple wavelengths, is large. An evanescent power overlap of the LP02 cladding mode with the PCF's air holes is an order of magnitude higher then that of the LP₀₁ mode. We optimize the PCF with only five rings of hexagonally arrayed air holes. By enlarging the air holes in the outmost ring the LP₀₂ mode confinement loss is reduced to a negligible value which allows to lengthen the cladding mode interaction length with an analyte infiltrated into the air holes.

Hollow-core microstructured fibres are designed for the short wavelength domains, either visible or ultra-violet ones. The experimental results confirm that kagomé-lattice antiresonant fibres are good candidate for this purpose. Thorough numerical modelling is carried out in order to determine the physical causes responsible for the loss level observed. From these computations the following conclusions are drawn: (i) the sole antiresonant core surround dictates the location of the transmission windows and (ii) the cladding bridges are sources of extra leakage from the core to the surrounding solid cladding. A straightforward model is therefore devised to determine accurately the loss level in this kind of structure by quasi-analytical calculus.

We have investigated different possibilities to fill a Photonic Crystal Fibers (PCF) with an electro-optic (EO) polymer. As the EO polymers are generally solution processed, the solvents have to be removed from the holes of the PCF after filling with the liquid polymer, leaving a solid polymer layer. Repeated filling is required to create a multilayer of the EO polymer to fill about 80% of the volume of the holes. The remaining volume can be filled with a epoxy monomer and cured. Because of time consuming repeated steps in the solvent processing, solvent free single step filling processes are also presented. Polability of these different systems and their final attainable properties are compared. Considering the high refractive index of the polymer materials, possible applications e.g. for antiresonant guiding with variations of the transmission bandgaps are discussed.

The design flexibility of photonic crystal fibers has accelerated the development of specialty optical fibers for a wide variety of applications. Optical fiber sensor applications for instance can benefit from this fiber technology. Fiber Bragg grating inscriptions in photonic crystal fibers have been reported with inscription setups that go from continuous-wave to femtosecond pulsed laser sources. However, the compatibility of the microstructures in these fibers with conventional ultraviolet inscription techniques was never before investigated in a broad range of (Germanium doped) fibers. We present UV laser induced dynamics of Bragg gratings growths in photonic crystal fibers with a hexagonal arrangement of 6 rings of airholes around a Germanium doped core region. The average refractive index increase and the refractive index modulation by the grating inscription process are compared for microstructures with several doping levels, airhole filling factors, airhole pitch distance and fiber orientation. We show how the parameters of the microstructure can influence the Bragg grating inscriptions. In addition we expand the range of fibers in which Bragg gratings, with reflection strengths that are useable for sensing purposes, can be inscribed to fibers with Germanium doping concentrations as low as 1.36 and 0.45 mol%.

This paper describes the practical demonstration of self-imaging, through the multimode interference effect, of the output of photonic crystal fibre (PCF) within a step-index multimode fibre (MMF). Self-imaging was found to occur at MMF lengths of 6 860 and 13 720 μm. A PCF-MMF-PCF chain was arranged with butt-couple connections at each interface. An overall transmission efficiency of 76% was measured. This proof of principle opens up the possibility to realize robust and efficient connectors for PCF.

In this work, the authors present an alternative Sagnac interferometer based on a suspended twin-core fibre. Using the suspended twin-core fibre, the pattern fringe is due to the different optical paths of the light in the two cores. In this case, the value of the refractive index difference between the two cores is ~10^-3, which indicates an advantage of this approach, namely the possibility to use a small length of the suspended twin-core fibre. The sensing configuration was characterized for strain, temperature, curvature and torsion, respectively.

We present a method to measure the complete field distribution emerging from photonic crystal fibers (PCFs). Assuming an invariant fiber cross-section, the eigenmodes of a microstructured optical fiber can be calculated numerically. These spatial modes build a complete set of orthogonal eigenfunctions. The modal decomposition of an arbitrary wave field guided by the fiber is therefore unique. We use an adapted computer-generated hologram to determine experimentally a single complex-valued mode coefficient describing the amplitude and the phase of a specific eigenmode. Angular multiplexing enables us to obtain simultaneously all mode coefficients for one polarization state. A second measurement with the orthogonal polarization allows the determination of the complete field information described by a coherent superposition of eigenmodes. Such a reconstructed near field distribution is compared to the measured intensity distribution and conformity is obtained. Applying this method to a multimode effective index guiding fiber, we investigate how bending of the PCF affects the modal composition at the fiber output for a wavelength of 1064 nm. Knowing the complete field in the fiber output plane, the field distribution in every free space plane can be calculated by numerical propagation techniques. Thus, the determination of the beam propagation ratio M2 can be virtually realized according to the ISO standard 11146 and allows the comparison of the beam quality for different bending radii.

Six photonic crystal fibers (PCFs) were characterized at NIT laboratory participating in COST Action 299 "FIDES", allowing for comparisons of properties and their dependence on fiber design. Samples tested included three nonlinear fibers with germanium doped core, two fibers with un-doped core and honeycomb photonic structure, and a "PANDAlike" PCF with a pair of large holes along an un-doped core. Tests included optical time domain reflectometer (OTDR) measurements, spectral loss, polarization mode dispersion (PMD) and its variations with temperature, fiber twist and axial strain. Elastooptic coefficient was measured for 2 fibers. Most samples exhibited high PMD, up to 3 ps/m. PMD was usually reduced by twisting the fiber, but twist sensitivity varied widely. The "PANDA-like" PCF, however, had PMD virtually unaffected by both twist and tensile strain; the latter property made it different from true PANDA fiber tested for comparison. Intensity of backscattering in each PCF was stronger compared to a standard telecom single mode fiber (SMF), by a factor up to 110x.

Photonic quasicrystals are unique structures having long-range order but no periodicity. It has been found that quasiperiodic structures give rise to unusual phenomena and properties that have not been observed in periodic structures. Recently, it has been reported that introducing quasi-periodic structures of microscopic air holes in optical fibers can give rise to a unique dispersion property such as almost zero ultra-flattened chromatic dispersion and large mode area dispersion compensating fiber. In this paper, we introduce a ring core photonic quasicrystal fiber (PQF) and its optical properties are theoretically analyzed. The chromatic dispersion properties of doped ring core PQF, are investigated along with their dependence on the proposed defect parameters using 3D full-vectorial Beam Propagation Method (BPM) and plane wave expansion method.

We present the results of interferometric measurements of chromatic dispersion of polarization modes in holey fibers over a broad spectral range (e.g. 500-1600 nm). First, a spectral interferometric technique employing an unbalanced Mach-Zehnder interferometer with a birefringent holey fiber in the test arm is used for measuring the wavelength dependence of the differential group effective index of the one of the polarization modes supported by the fiber. We apply a five-term power series fit to the measured data and by its differentiation we obtain the chromatic dispersion. Second, a spectral interferometric technique employing a tandem configuration of a Michelson interferometer and the holey fiber under test is used for measuring the group modal birefringence in the fiber. From the measurements, the differential group effective index and the chromatic dispersion of the other polarization mode supported by the fiber are retrieved. We confirmed that the measurement results agree well with that specified by the manufacturer. We also measured by these techniques the chromatic dispersion in other birefringent holey fiber.

We present a fibered master oscillator power amplifier system which allows generation of multi-μJ narrow-bandwidth few nanosecond pulses with active pulse shape control. Temporal shaping is demonstrated with sub-nanosecond resolution and high dynamic. In order to precompensate gain saturation, input optimal pulse shaping is calculated using a numerical model including ASE and feed-back algorithm.

In this paper, we proposed a bidirectional coupler implemented in a dual-core photonic crystal fiber (PCF) that has nematic liquid crystal filled holes in the cladding region. Light wave is guided in this PCF by total internal reflection (TIR) due to the refractive index contrast between soft glass and liquid crystal. Its coupling, birefringence and dispersion characteristics are evaluated. The coupler can be used to realize wavelength selective MUX-DEMUX as well as a polarization compensator for wavelength division multiplexing (WDM) application. Moreover the device demonstrates tunability with temperature change.

Photonic bandgap Bragg fibers are promising for designing large-mode-area structures owing to their high bend immunity. However, at a large core diameter, filtering of high-order modes (mainly, the LP₁₁ mode) becomes difficult, because the propagation constant of such modes is close to that of the fundamental LP₀₁ mode. In this paper, we demonstrate the possibility to suppress high-order modes in Bragg fibers by introducing low-index inclusions into the Bragg fiber core. Numerical analysis shows that an appropriate choice of the position and types of such inclusions allows one to increase the LP₁₁ mode radiation loss without increasing the optical loss of the fundamental LP₀₁ mode. The Bragg fiber with two B-doped and two F-doped rods in the core was fabricated and studied. The fundamental LP01 mode at 1064 nm had a mode-field area of about 340μm² and an optical loss below 0.2 dB/m at a bending radius of 15 cm. The LP₁₁ mode was not observed in both bent and straight fibers at this wavelength. Only the LP₂₁ mode was detected in a straight fiber; however, it was completely suppressed after propagating a length of 60 cm in a fiber bent with a radius <50 cm.

Coupling a photonic crystal fiber (PCF) to measuring instruments or optical subsystems is often done by splicing it to short lengths of single mode fiber (SMF) used for interconnections, as SMF is standardized, widely available and compatible with most fiber optic components and measuring instruments. This paper presents procedures and results of loss measurements during fusion splicing of five PCFs tested at NIT laboratory within activities of COST Action 299 "FIDES". Investigated silica-based fibers had 80-200 μm cladding diameter and were designed as single mode. A standard splicing machine designed for telecom fibers was used, but splicing procedure and arc power were tailored to each PCF. Splice loss varied between 0.7 and 2.8 dB at 1550 nm. Splices protected with heat-shrinkable sleeves served well for gripping fibers during mechanical tests and survived temperature cycling from -30°C to +70°C with stable loss. Collapse of holes in the PCF was limited by reducing fusion time to 0.2-0.5 s; additional measures included reduction of discharge power and shifting SMF-PCF contact point away from the axis of electrodes. Unfortunately, short fusion time sometimes precluded proper smoothing of glass surface, leading to a trade-off between splice loss and strength.

Photonic crystal fibers with kagome lattice are a particular kind of micostructured hollow-core fibers whose cross-section is characterized by a web of thin silica struts intersecting in a Star-of-David pattern. Such fibers show unusual properties, such as light confinement in the air-core in absence of a full photonic bandgap. The primary design parameter for such fibers is the strut thickness, which is responsible for the position and the width of the transmission bands. In this paper the guiding properties of hollow-core photonic crystal fibers with kagome lattice are investigated by means of a full-vector modal solver based on the finite element method. The fundamental mode effective index and confinement loss have been evaluated in a wide wavelength range, spanning from 300 nm to 1600 nm, accounting for the influence of the silica strut width on the transmission window. Moreover, the effects of selective alteration of the width and the shape of the silica struts surrounding the core have been analyzed. Simulation results show that the core-surrounding silica ring has the strongest effect on the transmission band, the loss level and the resonance wavelength position and, consequently, it should be carefully controlled during the fiber fabrication.

We report about spectral transmission behavior and influence on chromatic dispersion of index guiding microstructured fibers (MOFs) in terms of material effects of the light propagating core, geometric parameters of the microstructured cladding and preparation parameters. Two core compositions were investigated pure silica and silica-germania glass with maximum 36 mol% GeO₂. The MOFs with large pitch (>5 μm) were manufactured by single step technique. Small pitch MOFs were prepared by dual step method. They show a relatively high OH absorption. The dual step prepared silicagermania MOFs show a more than one order of magnitude higher hydroxide contamination compared to similar silica MOFs. This result seems to be caused by the higher permeation of hydroxide groups in silica-germania glass compared to silica. Simulations show that the red shift of the zero dispersion wavelength (ZDW) caused by high germanium doping can be compensated by a holey cladding structure with medium up to large ratios of d/Λ.