This PDF file contains the front matter associated with SPIE Proceedings Volume 6990, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

This communication introduces Next Generation Photonic Crystal Fibres (NextGenPCF) Project consortium, objectives and activities. The Project consortium, comprising leading European industries and research institutes, aims to advance the state of the art in photonic crystal fibre technology, and utilize the new fibres in key Biomedical, Telecom and Sensing applications. This Integrated project, launched in the scope of the EU "Information Society Technology (IST)" programme commenced on June 1st 2006 and is to run for 3 years.

In this paper, we present recent results concerning germanium doped highly non-linear photonic crystal fibres (HNLPCF). The finite element method is used to predict main fibre properties. The polarization behaviour of HNL-PCF is discussed in the case of disturbed numerical profiles and real fibres. Characteristics of an optimized germanium doped HNL-PCF are presented and future prospects for in-device integration are discussed as conclusion.

In this paper we report on the fabrication and characterization of hollow core photonic bandgap fibers that do not suffer from surface mode coupling. This enables low loss over the full spectral width of the photonic bandgap formed in the cladding. It also enables reduced dispersion slope, which is a key parameter for several applications of these fibers to high-power ultrashort pulse compression.

Photonic Crystal Fibers (PCF) based on special glasses with a high optical nonlinearity are of high interest for applications. High optical nonlinearity can be achieved by a large concentration of heavy elements in the fiber core. The dispersion characteristics can be tailored by an adapted microstructured cladding design. We have investigated two different fiber types based on highly germanium doped silica and on lanthanum silicate glass. The properties of these fiber types will be discussed and compared. Highly germanium doped PCFs with a Ge concentration of up to 36 mol% in the central rod have been prepared by a stack-and-draw technique from a silica capillary arrangement. Due to their compatibility with all-silica fibers, system integration is easily possible by low-loss splicing techniques. Microstructured fibers based on alternative La₂O₃-Al₂O₃-SiO₂-BaO-B₂O₃ glasses allow to further increase the optical nonlinearity compared to MCVD based high silica glasses. However, the application of such fibers is often limited by their transmission properties due to imperfect purity and homogeneity. We have achieved best fiber loss results of 1.2 dB/m at 1200 nm wavelength with glasses with a lanthanum oxide concentration of about 16 mol% and a moderate boron concentration. Other limiting factors caused by the thermochemical properties of the prepared glasses (such as low glass transition temperature, high thermal expansion coefficient, limited crystallization stability) can be partially overcome by modifications of the material composition, by suitable surface processing of preform components and by optimized drawing conditions.

One of the objectives of NextGenPCF European Union R&D project is to develop compact and low-cost white-light supercontinuum sources, based on the use of microchip lasers and air/silica microstructured optical fibres. In this paper, we present some experimental results obtained with doped or undoped highly nonlinear fibres. We also introduce and discuss some ways of getting blue/UV frequency generation, which is particularly useful for certain specific biomedical applications. Finally, the different methods proposed for supercontinuum generation are compared, in terms of spectral power density and spectrum range.

A lumped Raman amplifier for all-Raman long-haul and ultra-long haul optical communications systems based on highly non-linear Photonic Crystal Fiber (HNL-PCF) is proposed and demonstrated. Applications for such an amplifier are discussed, focusing on discrete loss compensation for L-Band all-Raman reconfigurable systems. The main specifications required for this and similar applications have been analyzed, and include Power Conversion Efficiency above 30%, Net gain of about 15dB, and output power in the range of 20-23 dBm. Additional specifications such as Noise Figure, Multi-Path Interference (MPI), and transient suppression are also considered. In order to achieve the required specifications, the HNL-PCF should exhibit high Raman efficiency and low attenuation at pump wavelengths of 1470-1500nm, resulting in a Figure of Merit (FOM) above 8 dB^-1W^-1. The splice loss of the HNL-PCF to conventional Single Mode Fiber is also shown to be critical, and should not exceed 0.5 dB. Initial samples of HNL-PCF have been characterized, and it has been demonstrated that high Raman efficiency and low splice loss are achievable, while further work is being carried out to increase the FOM. Finally, an experimental demonstration of 10Gb/s WDM transmission using a prototype Lumped Raman Fiber Amplifier based on HNL-PCF is presented.investigated and compared to similar VCSELs with etched mesa.

We demonstrate a Raman laser made from a grating-free highly-nonlinear photonic crystal fiber. The laser threshold power is lower than 600 mW and laser power characteristics recorded in experiments are accurately described from the usual simplest model dealing only with stationary evolutions of total optical powers. Experimental investigations of the spectral properties of our grating-free Raman fiber laser evidence that the shape of the Stokes power spectrum remains remarkably Gaussian whatever the incident pump power. Increasing the incident pump power induces a drift of the Stokes wavelength together with a broadening of the Stokes optical spectrum. Investigations on the role of light polarization on laser characteristics show that our grating-free Raman fiber laser behaves as a Raman laser made with a standard polarization maintaining fiber. At high pump power, the birth of the second-order Stokes wave induces a destabilization of the laser output with the emergence of self-oscillations of the optical powers which are explained from the interplay between counterpropagating pump and Stokes waves through stimulated Raman scattering.

We present a compact oxygen sensor based on hollow-core photonic crystal fibres (PCF). The compact design requires only a very small amount of gas. Both ends of the fibre are inserted into miniaturized vacuum adapters, which should result in an increased gas exchange and prevent fibre contamination. The detection of oxygen could be verified. Proper choice of the specific components (PCF, laser source) allows the detection of other gases.

The combination of the functionalities of Fiber Bragg Gratings (FBGs) and Photonic Crystal Fibers (PCFs) has unveiled new potential for FBG based sensors. The fabrication of FBGs in PCFs has been reported in literature. However, using dedicated PCFs to improve the sensitivity of FBG-based sensors has received only limited attention. In this report we therefore show how to eliminate some of the drawbacks of FBGs in conventional step-index fibers for sensor applications by exploiting the design flexibility of PCFs. The added value of PCFs stems from the ability to design an optical fiber in which an FBG acts as a sensor with a selective sensitivity, e.g. a sensor that is sensitive to strain but not to temperature. For this purpose we use a PCF with a birefringence on the order of 10^-3, which is one order of magnitude larger than for conventional birefringent fibers. The two FBG reflection peaks are therefore significantly separated from each other, e.g. 2 nm, which makes these FBGs suited for sensing purposes since both peaks can be unambiguously and accurately identified. As a conclusion we summarize the advantages and disadvantages of our approach to design and fabricate selective FBG-based sensors.

A fully-vectorial mode solver based on the finite element method is employed in a combination with the simplex method for the dispersion optimization of microstructured optical fibres made from highly nonlinear glasses. The nonlinear fibres are designed for FWM-based telecom applications such as parametric broadband amplification, wavelength conversion, ultra-fast switching and regeneration of optical signals. The optimization is carried out in terms of the zero dispersion wavelength, dispersion magnitude and slope, nonlinear coefficient and confinement loss in the wavelength range around 1550 nm and for the microstructured fibres made from lead oxide, bismuth oxide, tellurium oxide and chalcogenide glasses. We restrict our work to the index-guiding fibre structures with three, four and five rings of air holes. For most telecom applications a zero dispersion wavelength around 1550 nm is desirable, with a dispersion magnitude and slope as small as possible.

A modal solution approach based on the powerful, full-vectorial, H-field based finite element method (FEM) has been used to analyze the single mode operation of a PCF. Modal solutions of the fundamental modes of highly birefringent PCFs have been obtained. The FEM with perfectly matched layer condition has been used to characterize the leakage loss and the differential loss between the polarized modes of PCFs. The design of a single-mode single-polarization PCF has also been proposed.

The inscription of Bragg reflectors in commercial, all-silica microstructured optical fibres using picosecond and femtosecond 248nm laser radiation, will be presented. The research results reported herein aim to the investigation of the optimum photosensitivity regime and index engineering route, and its dependence upon the wavelength and intensity of the laser source, for the high yield inscription of Bragg reflectors in all-silica microstructured optical fibres. The ultraviolet laser source used was a 248nm, 5ps/500fs hybrid dye/excimer; while hydrogenated Blazephotonics ESM-12 and Crystal Fiber LMA-10 microstructured optical fibres were exposed. Refractive index evolution curves for both average and modulated index changes, as well as, thermal annealing results are presented and discussed. Refractive index changes of the order of 10^-4 were obtained for relatively low accumulated energy density doses (<18KJ/cm²). For the case of the 248nm femtosecond radiation, the underlying photosensitivity process was speculated to be that of two-photon absorption, however, significant contribution from single-photon processes related with hydrogen generated and oxygen pre-existing defects is also possible. The index engineering and thermal annealing results presented for the case of 248nm ps and fs radiation, are compared to Bragg grating inscriptions using 193nm, 10ns excimer laser radiation. Further, issues related to the spatial distribution of the ultraviolet laser energy density inside the fibre core for side-illumination are presented and discussed in conjunction with the refractive index growth data.

The use of high intensity femtosecond laser sources for inscribing fibre gratings has attained significant interest. The principal advantage of high-energy pulses is their ability for grating inscription in any material type without pre-processing or special core doping - the inscription process is controlled multi-photon absorption, void generation and subsequent local refractive index changes. The formation of grating structures in photonics crystal fibre has proven difficult, as the presence of holes within the fibre that allow wave-guidance impair and scatter the femtosecond inscription beam. Here we report on the consistent manufacture of long period gratings in endlessly single mode microstructure fibre and on their characterisation to external perturbations. Long period gratings are currently the subject of considerable research interest due to their potential applications as filters and as sensing devices, responsive to strain, temperature, bending and refractive index. Compared to the more mature fibre Bragg grating sensors, LPGs have more complex spectra, usually with broader spectral features. On the other hand they are intrinsically sensitive to bending and refractive index. Perhaps more importantly, the fibre design and choice of grating period can have a considerable influence over the sensitivity to the various parameters, for example allowing the creation of a bend sensor with minimal temperature cross-sensitivity. This control is not possible with FBG sensors. Here we compare the effects of symmetric and asymmetric femtosecond laser inscription.

We present a white-light interferometric method for measuring the wavelength dependence of the group index of a pure silica holey fiber. The method is based on the recording of a series of the spectral interferograms in a Mach-Zehnder interferometer with the fiber of known length placed in one of the interferometer arms and the other arm with adjustable path length. We measure the equalization wavelength as a function of the path length difference, or equivalently the group index dispersion. Subtracting the group dispersion of the optical components present in the interferometer along with the fiber, first we measure the wavelength dependence of the differential group index of the pure silica glass provided that that the light is guided by the outer cladding of the fiber. Second, we measure the wavelength dependence of the group effective index of the fundamental mode supported by the fiber provided that some of the recorded interferograms are also due to the mode.

We demonstrate that bending loss in large mode area photonic crystal fiber (LMA PCF) reveals a strong dependence of the amplitude and location of the loss peaks on the bend radius and on the fiber angular orientation. To do so we measured bending induced loss in LMA PCF as a function of wavelength in a broad spectral range (800-1500 nm) for different bend radii and for different angular orientations of the fiber with respect to the bending plane. The oscillations of bending loss with the bend radius and orientation are particularly well pronounced when the radiative component of the fundamental mode is reflected from the flat boundary of the holey cladding. We also show the good agreement between our measurement results and earlier simulations relying on a finite element method with perfectly matched layers and equivalent index model.

We report on the temperature response of FBGs recorded in pure PMMA and TOPAS holey fibres. The gratings are fabricated for operational use at near IR wavelengths, using a phase mask and a CW He-Cd laser operating at 325nm. The room temperature grating response is non-linear and characterized by quadratic behaviour for temperatures from room temperature to the glass transition temperature, and this permanent change is affected by the thermal history of the gratings. We also report the first FBG inscription in microstructured polymer optical fibres fabricated from TOPAS. This material is fully polymerized and has very low moisture absorption, leading to very good fibre drawing properties. Furthermore, although TOPAS is chemically inert and bio-molecules do not readily bind to its surface, treatment with Antraquinon and subsequent UV activation allows sensing molecules to be deposited in well defined spatial locations. When combined with grating technology this provides considerable potential for label-free bio-sensing.

Although singlemode fiber lasers become a mature technology, enhancements, in terms of output power, spatial beam quality, bend insensitivity are still required. A major trend is to increase the active core area to increase the thresholds of nonlinear effects while ensuring a transverse singlemode behavior. Actually, increasing the active ions' concentration is also demanded since it allows a drastic reduction of the fiber length, everything being equal. Two non-exclusive strategies are laid out to overcome fiber laser limitations. On the one hand, it is demonstrated that surrounding a highly multimode active core by a properly designed microstructured cladding, exhibiting specific resonant features, allows the fiber laser to be operated in the singlemode regime. On the other hand, a large mode area photonic bandgap fibre is shown to lead to a transverse singlemode fiber laser with very good lasing efficiency.

We demonstrate that second harmonic generation obtained in Ge-doped holey fibres can act as a seed for visible supercontinuum generation. This spectral enlargement is obtained by means of a double-pumping system. By using a microchip laser source delivering sub-nanosecond pulses at 1064 nm and a highly Ge-doped fibre, we obtain a second harmonic generation efficiency of 4.8 % after an optical poling process. A white light continuum extending on more than 250 nm is obtained in visible domain.

We describe an experiment in which a supercontinuum spectrum is generated by exciting the third-order mode of a highly nonlinear photonic crystal fiber (PCF). Our experiment consists of launching a train of femtosecond pulses into a 45-cm-long span of a PCF by means of an offset pumping technique that can selectively excite higher-order modes. For input wavelengths below 810 nm, the fiber was found to allow for the propagation of higher-order modes. When exciting the third-order mode we were able to generate an almost purely visible supercontinuum even with pulse energies below 100 pJ. Although the spectrum broadens on the short-wavelength side down to the blue region, no components at wavelengths larger than the pump wavelength were observed. The mechanism behind the spectral broadening is mainly ruled by soliton propagation leading to the generation of a blue-shifted dispersive wave. The fact that higher-order modes have a cut-off wavelength plays a fundamental role that accounts for the observed asymmetry of spectral broadening. Our experimental results are compared with the numerical solutions of the nonlinear Schrödinger equation. Good agreement between experimental and numerical results is found.

Supercontinuum generation (SCG) has been the subject of intense investigation during the last few years and its main mechanisms are now well understood. Focus has shifted towards tailoring the spectrum of the supercontinuum for specific applications. We experimentally investigate SCG with picosecond pumping in photonic crystal fibers with two closely spaced zero dispersion wavelengths. We couple parts of the output spectrum of the supercontinuum source back to the input in order to produce a gain of over 15 dB at some wavelengths. We use a variable time delay to optimize the overlap between the pump and the back seeded pulses and investigate how the delay and spectrum of the back seeded pulse affects the resulting supercontinuum spectrum.

Nonlinear propagation of femtosecond pulses in double core square lattice PCF made of multicomponent glass was investigated experimentally at excitation wavelength 1250 nm in the anomalous dispersion region. The obtained results expressed soliton fission and self frequency shift in the anomalous region, inspected by IR registration, with increasing complexity by increasing excitation energy. The visible registration, inspecting the normal dispersion region, exhibited soliton induced dispersive wave generation with blue shifting feature suggesting nonlinear phase change effect on the phase matching condition. The width of the overall spectral feature approached two octaves at approximately 10 nJ excitation energy in 6 cm long fiber sample. The knowledge about the evolving processes was extended by numerical simulation of the nonlinear propagation in the near IR region in reasonable correspondence with the experimental results. Furthermore, separate registration of the visible spectral features originating from the two fiber cores was ensured exhibiting significant differences between the multipeak spectra. The two core spectral content differences was possible to further alternate by rotation of the excitation polarization direction with application potential for polarization switched directional coupler accompanied by frequency conversion. Finally, single versus double core excitation conditions were compared. The double core excitation resulted in smoother spectral features, both in the case of IR and visible registration, at requirement at expense higher excitation energies needed for broadband supercontinuum generation.

In this paper we report on the fabrication of a micro-structured fiber made of in-house synthesized silicate glass, with a nonlinear Kerr refractive index of 4.0 10^-15 cm²/W. The micro-structured fiber uses three rings of holes around a slightly elliptical core with dimensions 2.6 μm x 3.4 μm. This fiber has a birefringence of about 10^-3 at 1.5 μm and zero dispersion wavelengths at 860 nm and 870 nm. Using this fiber we have demonstrated ultra broadband supercontinuum generation in the range 400 - 1600 nm for 19.5 cm fiber sample pumped with 100 fs pulses with central wavelength of 755 nm and energy of 2 nJ. Broadband generation of 200 nJ in the range 650-850 nm with pulse energy on the level of 0.5 nJ is also observed with the same structure.

Paper presents technology and some characteristic of manufactured suspended core microstructured optical fibers with cores undoped and doped with germanium dioxide. Manufactured fiber is very useful for evanescent wave sensors. Additionally on internal layers of holes thin (thickness about 28nm) silver layers were deposited. Those optical fibers probably will be very useful for surface plasmon resonance sensors. We introduced 6nm thick silver layers into holes of ordinary photonic crystal fibers 1m long.

We present a new method for measuring the wavelength dependence of phase modal birefringence in highly birefringent fibers. The method is based on application of a lateral pointlike force on the fiber and resolution of the spectral interference fringes. The displacement of the lateral force needed for one-period phase change of the spectral fringes at given wavelength is used to determine the beat length or the phase modal birefringence. In this paper, the new method is analyzed theoretically and experimentally. First, we model the spectral interferograms for two different locations of the lateral force with respect to the fiber end and show how the wavelength dependence of the phase modal birefringence can be retrieved from the interferograms. Second, we measure the wavelength dependence of phase modal birefringence in two different highly birefringent fibers and compare it with that resulting from the measurement of the wavelength dependence of group modal birefringence.

We propose a dual-core photonic quasicrystal fiber with six-fold symmetry that may be useful in compensating chromatic dispersion of a single-mode fiber. The geometry of the proposed photonic quasicrystal fiber is described and then the behaviors of effective index and chromatic dispersion are calculated according to the structural parameters. The dual-core fiber is composed of a pure silica inner core and an outer core that is formed by reducing the diameter of air holes in the third cladding layer. We investigate the dependence of the effective indices of inner core and outer core, the fundamental super mode, and the effect of chromatic dispersion on the structural parameters by using plane wave expansion method. The dual-core photonic quasicrystal fiber has a large negative chromatic dispersion value of approximately -2000 ps nm^-1 km^-1 over optical communication band around 1.5μm. Introducing quasicrystal structures in the dual-core optical fibers can improve the capability of dispersion compensation of the fibers significantly.

In recent years, hollow-core photonic bandgap fibers (HC-PBFs) have been demonstrated to be a promising technology for gas sensing. In particular, the long interaction path lengths available with these fibers are especially advantageous for the detection of weakly absorbing gases such as methane. In the near-infrared region, methane has the strongest absorption band, 2ν₃, at 1670 nm. However, HC-PBFs were not available until recently in this wavelength range and gas sensing devices based on HC-PBFs were previously made in the weaker band of 1300 nm. In this paper, we report the demonstration of a methane sensor based on a 1670-nm-band HC-PBF. A strong spectral feature, the R(6) manifold (near 1645 nm), was selected for sensing purposes as it shows a good signal-to-noise ratio. This absorption line is comprised of six energy transitions, strongly overlapped at our experimental conditions. For that reason, we applied a multiline algorithm that used information from the six transitions to fit the manifold. The goodness of the fitting was assessed measuring the concentration of different methane samples. With this method, a minimum detectivity of 10 ppmv for the system configuration was estimated.

This paper is oriented to present a short overview of chosen aspects in the area of Photonic Crystal Fibers, primarily nonlinear dispersion properties induced by fiber bending. This topic refers to applications of to microstructure optical components of high-speed transmission systems, such as dispersion compensator based on negative chromatic dispersion resulting from bending the fiber at certain radius. It is possible to achieve record-breaking negative dispersion (thousands of ps/nm/km) at large effective mode area - done without doping in the core. Although the viability of application negative CD to the compensation of group velocity is discussible (due to bending losses), there are conclusions about tuning the zero-dispersion wavelength and designing the fiber that would not be sensitive for bending over the C-band at office conditions. The study of effective indices for bending at different radii and for different fill fraction has been provided. The essential problem was to provide a study of CD profiles, analysis of the phase-matching while coupling between LP01 and cladding modes, defining the most optimal reference configuration ensuring the minimum negative CD with calculation of the sensitivity of optical nonlinearities to the deviation of bending radius or imprecision of structural parameters.

We have proposed a novel photonic crystal fibre (PCF), with three rings of air holes, that exhibit nearly zero ultra-flatted and negative chromatic dispersion, and low confinement loss at a wide telecommunication window. Key PCF fabrication parameters, such as the effects of air holes and their arrangements on the effective index, chromatic dispersion, effective mode area, non-linear coefficient and confinement losses have been analysed by use of full vectorial finite element method. Significant improvements of the PCF chromatic dispersion and confinement losses have been achieved when supplementary air holes have been incorporated.

We investigated theoretically and experimentally the polarimetric sensitivity to temperature of highly birefringent photonic crystal fibers (HB PCFs) in which the birefringence is induced by two large holes adjacent to the core. We carried out the sensitivity measurements in a broad spectral range (680-1550 nm) using a spectral domain interferometric method for two fibers with different pitch distance and hole diameter. Our results show that the polarimetric sensitivity to temperature in the investigated fibers is highly dispersive and crosses zero at specific wavelengths. Furthermore, we found good agreement between the measured and the calculated characteristics.