This PDF file contains the front matter associated with SPIE Proceedings Volume 7357, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

We demonstrate a low loss microstructured fiber (MOF) with a high nonlinear glass core and silica holey cladding. The substitution of mostly used silica as core material of microstructured fibers by lanthanum oxide glass promises a high nonlinear conversion efficiency for supercontinuum (SC) generation. The glass composition is optimized in terms of thermochemical and optical requirements. The glass for the MOF core has a high lanthanum oxide concentration (10 mol% La₂O₃) and a good compatibility with the silica cladding. This is performed by adding a suitable alumina concentration up to 20 mol%. The lanthanum oxide glass preform rods were manufactured by melting technique. Besides purity issues the material homogeneity plays an important role to achieve low optical loss. The addition of fluorides allows the better homogenization of the glass composition in the preform volume by refining. The minimum attenuation of an unstructured fiber drawn from this glass is about 0.6 dB/m. It is mostly caused by decreasing of scattering effects. The microstructured silica cladding allows the considerable shifting of dispersive behavior of the MOF for an optimal pump light conversion. The MOF shows zero dispersion wavelengths (ZDW) of 1140 nm (LP₀₁ mode) and 970 nm (LP₁₁ mode). The supercontinuum generation was investigated with a 1064 nm pump laser (650 ps). It shows a broad band emission between 500 nm and 2200 nm.

We present the design of the chalcogenide (As₂S₃) glass nanofibers with the nonlinear tellurite glass as the cladding material. Both of these glasses have high nonlinearity. We show that the simple step index structure with tellurite cladding has the normal dispersion in the telecommunication window for any value of the core diameter, ranging from the sub-wavelength size to a few micro-meters. The dispersion achieved is flat so as to be applicable to dispersion compensating devices. However, the dispersion can not be tailored to zero. So we propose the photonic crystal fiber (PCF) structure, which has six circular identical air holes introduced in the tellurite cladding around the chalcogenide core. The air holes introduced cause the decrease in the effective cladding index. This structure shows anomalous dispersion in the telecommunication band with two zero dispersion wavelengths. With further optimization the structure can show zero flattened dispersion. We study the effect of the various design parameters - the chalcogenide core diameter, the air hole diameter, and the pitch, that is the distance between the centers of two neighbouring air holes, which is same as the distance between the center of the core and any of the air holes. We optimize the design to achieve zero slope (flattened) at the zero dispersion wavelength, 1.55 μm. The thermal characteristics of tellurite glass match with the chalcogenide making the fabrication of the chalcogenide core PCF with the tellurite glass as a cladding material feasible. We also present the calculation of the nonlinearity of the PCF.

The paper reports preparation and applicative aspects of two types of index guiding microstructured fibers (MOFs) with germanium doped cores. The first fiber type has a solid core with graded germanium profile. It shows a high photosensitivity compared to pure silica MOFs. We inscribed high-quality Bragg gratings with a reflectivity of 73% without hydrogen loading. The solid core germanium doped MOF was spliced with standard silica fiber. The minimum splice loss was about 1 dB at 1550 μm wavelength. A more complex MOF type was prepared with germanium doped holey core in a silica holey cladding. The germanium doped core area includes seven holes in hexagonal arrangement with equal diameter and pitch sizes. The holey core propagates a large area annulus mode. We show the suitability of this MOF for chemical gas sensing by filling the core cavities with hydrocarbon analytes.

We demonstrate dispersion-engineering of microstructured polymer optical fibres (mPOFs) made of poly(methyl methacrylate) (PMMA). A significant shift of the total dispersion from the material dispersion is confirmed through measurement of the mPOF dispersion using white-light spectral interferometry. The influence of strong loss peaks on the dispersion (through the Kramers-Kronig relations) is investigated theoretically. It is found that the strong loss peaks of PMMA above 1100 nm can significantly modify the dispersion, while the losses below 1100 nm only modify the dispersion slightly. To increase the nonlinearity of the mPOFs we investigated doping of PMMA with the highly-nonlinear dye Disperse Red 1. Both doping of a PMMA cane and direct doping of a PMMA mPOF was performed.

Y-shaped microstructured optical fibers (MOF) are gaining increasing attention due to its simplicity, since the cross section of this fiber is formed by a solid-core and only three large air holes in the cladding. In the fabrication process, in order to provide a Ge-doped core, a step-index multimode Ge-doped fiber can be inserted in the interstitial hole between the three capillaries. We have found that the filling of a Ge-doped Y-shaped MOF with a liquid of refractive index higher than the index of silica but lower than the index of the Ge-doped core produces a cutoff of the fundamental mode. This cutoff is very sensitive to small changes of the relative refractive index values of the liquid, as well as of in the silica and the Ge-doped core. Finally, we use the Ge-doped fiber to inscribe fiber bragg gratings (FBG) and see the spectral shift when the grating is put down strain and temperature changes.

The model describes solitons as relativistic particles, the visible light of the continuum as the result of Cerenkov radiation and the infrared light of the continuum as the result of material recoil. The model is applied on super continua generated by a 1064 nm mode locked 2-10 ps pump source launched into microstructure fibres with zero dispersion wavelength in the near infrared. The model predicts that the leading soliton of a pulse train dominates the self frequency shifting of a train of solitons with Tera Hertz repetition rate. The mechanism is responsible for very efficient super continuum generation in certain combinations of ps pulse sources and microstructure fibre designs.

Supercontinuum generation in a highly Ge-doped core Y-shaped microstructured optical fiber using long pump pulses of 9 ns duration at 1064 nm is reported. The generation of nonlinear effects takes advantage of the large nonlinear refractive index and Raman gain of the Ge-doped core, as well as the air hole structure that surrounds the core. The fiber is easy to fabricate due to its simple structure and shows good compatibility with standard fibers. Although the fiber was pumped in normal dispersion far from the zero dispersion wavelength, flat and smooth supercontinuum in the fundamental mode from 550 nm to beyond 1750 nm was generated with a value of fiber length and pump peak power product of 11.7 kW•m.

We recognize some photonic-crystal-fiber structures, made up of soft glass, that generate ultrawide (over an octave), very smooth and highly coherent supercontinuum spectrum when illuminated with femtosecond pulsed light around 1.55 μm. The design of soft-glass microstructured fiber geometry with nearly ultraflattened, positive and low dispersion is crucial to accomplish the above goals.

We present the results of broadband dispersion measurement of a two-mode birefringent holey fiber (BHF). First, a spectral interferometric technique employing an unbalanced Mach-Zehnder interferometer with the fiber in the test arm is used for measuring the wavelength dependence of the group effective index of the fundamental mode supported by the fiber. Second, a spectral interferometric technique employing a tandem configuration of a Michelson interferometer and the BHF under test is used for measuring the group modal birefringence dispersion for two lowest-order linearly polarized (LP) modes supported by the BHF. The data measured over a broad spectral range are fitted to polynomials to obtain the dispersion of the phase modal birefringence for both LP modes. We reveal that the results are in agreement with a general model of birefringence in air-silica BHFs.

We have excited both LP₀₁ and LP₁₁ modes using a high magnification objective lens (60×) in a nonlinear photonic crystal fibre (PCF) of core diameter 2.2μm and simultaneously detected the modes using low coherence interferometry. We placed the nonlinear PCF of length ~11cm in one arm of an interferometer, and then interfered the output with light in the reference arm onto a photodetector via a single mode collection fibre positioned at a point in a near-field image of the fibre endface. More than one fringe packet was observed in the interferogram, indicating the presence of two modes propagating in the fibre core. To uniquely identify the dispersion curves we need to know which mode corresponds to each fringe packet in the interferogram. In the same experimental setup we replaced the photodetector with a digital CCD camera to record the 2-D interference pattern across the image as function of group delay. A Fourier analysis technique was used to compute the intensity and phase of the mode field patterns corresponding to the various interferograms. Using this technique we can simultaneously measure the group velocity dispersion and the mode profile with phase information of the modes excited in a multimode PCF.

We present the results of measurement of birefringence dispersion in elliptical-core fibers. The measurement is performed over a broad wavelength range by two different spectral interferometric techniques. First, a technique employing a tandem configuration of a Michelson interferometer and an optical fiber under test is used for measuring the group modal birefringence for two linearly-polarized modes supported by the fiber. Second, a method of a lateral point-like force acting on the fiber and based on spectral interferometry is used for measuring the phase modal birefringence at one wavelength for the fundamental mode only. The measured value is combined with the dispersion of the group modal birefringence to obtain the phase modal birefringence over a broad wavelength range. We measured dispersion of the birefringences for two different elliptical-core fibers and resolved for example that the maximum of the group modal birefringence for the fundamental mode is at a certain wavelength, which depends on the parameters of the fiber.

The paper presents a theoretical analysis of the dynamic characteristics of an ytterbium-doped high power fiber laser. The proposed analysis focuses on the last stage of amplification, characterized by a photonic crystal fiber. For the intrinsic characteristics of this stage, an ytterbium-doped large mode area fiber is necessary to obtain great efficiency. Photonic crystal fibers present one of the most useful option due to the good thermal properties, large mode area, high damage threshold and a high threshold for the nonlinear effect such as Raman and Brillouin scattering. The dynamic behaviour of an Ytterbium-doped rodlike photonic crystal fiber for pulsed laser is numerically investigated through a "Reservoir" model. Theoretical results demonstrate the effect of the pulse duration and the frequency repetition rate on the amplifier characteristics. Through the numerical model, the optimum length and pump power have been obtained in order to achieve high output peak power. Moreover, an analysis on the temporal evolution and shape of the signal pulses at the amplifier output is carried out by exploiting the dynamic model.

Optical fiber sources have experienced a massive growth over the past ten years principally due to the compactness, robustness and good spatial quality of such systems. Fiber sources now cover a large spectrum from visible to near infrared helped on this point by the development of microstructured fibers (MOFs). A particular class of MOFs also called hollow-core photonic crystal fibers (HC-PCFs) offers to get rid of silica's absorption thanks to band gap guidance and therefore to extend transmission range of silica fibers. We propose here two all-fiber architectures based on HCPCFs in view to generate mid infrared wavelengths by amplification of spontaneous Raman scattering (SRS) in gaseous medium. We report on design, fabrication and characterization of two kinds of HC-PCF matching the architecture needs.

A chalcogenide optical fiber of special design is proposed to convert a short-wavelength IR radiation (around 2 μm) up to second transparency window of atmospheric air (around 4.5 μm) by degenerate four-wave mixing. The fiber supports a small core surrounded by three large air holes. The zero-dispersion wavelength is shifted down to 2 μm in this fiber by properly tailoring geometry of the fiber core. We demonstrate by solving the nonlinear Schrödinger equation that efficient wavelength-conversion can be obtained by pumping the fiber with a Tm:SiO₂ pulsed fiber laser.

We fabricated leakage channel microstructured crystalline fiber (MCF) from solid solution AgCl_xBr_1-x for middle IR with one ring of six rods and large mode field area 13 600 μm² for the first time. Experiments proved that MCF is effectively single mode at 10.6 μm wavelength, which corresponds to numerical simulation. Measured optical losses are 8 dB/m.

We report some of our recent progress in the area of Bragg grating writing in photonic crystal fibres (PCFs). The various challenges that PCFs present are discussed and the methods used to overcome these challenges are presented. The fabrication of highly-durable type-IIa gratings in highly nonlinear photonic crystal fibre is demonstrated, the rotational variance of grating inscription is also investigated through both experiments and numerical modeling. In other experiments we fabricate a narrow-linewidth distributed feedback (DFB) laser in erbium-doped PCF, achieving stable, single-mode and CW operation. The potential of such a DFB PCF in sensing applications is assessed by accurately measuring an absorption line of acetylene gas.

Combining the functionalities of fiber Bragg gratings (FBGs) and microstructured optical fibers (MOFs) offers promising technological perspectives in the field of optical fiber sensors. Indeed, MOFs could overcome some of the limitations of FBGs in conventional fibers for sensor applications. The added value of MOFs 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 directional strain but not to temperature. For this purpose we use a MOF with a phase modal birefringence on the order of 8×10^-3. A FBG in this MOF yields two Bragg peak wavelengths, with a wavelength separation that depends on the phase modal birefringence of the MOF. We characterize these FBGs for transversal loads on a bare fiber and compare the results with simulated sensitivities. Then, we embed the sensor in a composite coupon and measure the response of the Bragg peak wavelengths as a function of the applied transversal pressure on the composite material. This allows drawing conclusions on the advantages of FBGs in MOFs for sensing applications.

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 preprocessing or special core doping. In the field of fibre optical sensing LPGs written in photonic crystal fibre have a distinct advantage of low temperature sensitivity over gratings written in conventional fibre and thus minimal temperature cross-sensitivity. Previous studies have indicated that LPGs written by a point-by-point inscription scheme using a low repetition femtosecond laser exhibit post-fabrication evolution leading to temporal instabilities at room temperatures with respect to spectral location, strength and birefringence of the attenuation bands. These spectral instabilities of LPGs are studied in photonic crystal fibres (endlessly single mode microstructure fibre) to moderately high temperatures 100°C to 200°C and their performance compared to fusion-arc fabricated LPG. Initial results suggest that the fusion-arc fabricated LPG demonstrate less spectral instability for a given constant and moderate temperature, and are similar to the results obtained when inscribed in a standard single mode fibre.

The fabrication of Type IIA Bragg reflectors in a highly Ge-doped microstructured optical fiber using 193nm, 10ns laser radiation, is reported. The fiber exposed was manufactured in a five-air-ring design in order to ensure both a small central core cross-section and a large cladding area with high air fraction, while the maximum germanium content of the embedded core-socket is 36mol%. Finite-difference time-domain simulations revealed that the fiber supports two strongly guided modes. The refractive index evolution curves for both average and modulated index changes obtained for the two guiding modes, exhibited a Type IIA photosensitivity behaviour. Average refractive index changes of the order of 10^-3 were recorded, under exposures of 215mJ/cm² energy density per pulse. The refractive index evolution curves denoted that the two guiding modes do not exhibit the same index engineering behaviour due to sufficiently different overlap with the induced perturbations. The above finding was also confirmed during the thermal annealing of the gratings inscribed. The photosensitivity effects observed, as well as, post-exposure grating amplification effects are discussed with respect to the Type IIA stress and compaction model. Finally, preliminary data related to the inscription of Type IIA gratings in the same fibre utilizing 248nm, 500fs radiation are also presented.

Long period gratings were written step-by-step in microstructured poly(methyl methacrylate) (PMMA) fibre using a continuous wave HeCd laser at 325nm irradiating the fibre with a power of 1mW. The grating had a length of 2 cm and a period of 1mm. A series of cladding mode coupling resonances were observed throughout the spectral region studied of 600 to 1100nm. The resonance wavelengths were shown to be sensitive to both surrounding refractive index and the water content of the polymer fibre.

The origin and the behavior of the birefringence of solid-core air-silica microstructured fibers is described with the help of a simple approximate model. The first two modes of three different types of fibers are studied. Numerical results, obtained from both finite element and boundary integral methods calculations, are presented to support the validity of the model and to delineate its limits.

Rod-type photonic crystal fibers are large mode area double-cladding fibers with an outer diameter of few millimeters which can provide important advantages for high-power lasers and amplifiers. Numerical studies have recently demonstrated the guidance of higher-order modes in these fibers, which can worsen the output beam quality of lasers and amplifiers. In the present analysis a sectioned core doping has been proposed for Ybdoped rod-type photonic crystal fibers, with the aim to improve the higher-order mode suppression. A full-vector modal solver based on the finite element method has been applied to properly design the low refractive index ring in the fiber core, which can provide an increase of the differential overlap between the fundamental and the higher-order mode. Then, the gain competition among the guided modes along the Yb-doped rod-type fibers has been investigated with a spatial and spectral amplifier model. Simulation results have shown the effectiveness of the sectioned core doping in worsening the higher-order mode overlap on the doped area, thus providing an effective single-mode behavior of the Yb-doped rod-type photonic crystal fibers.

For most telecom nonlinear applications a high effective nonlinearity, low group velocity dispersion with a low dispersion slope and a short fibre length are the key parameters. Combining photonic crystal fibre (PCF) technology with highly nonlinear glasses could meet these requirements very well. We have performed dispersion optimization of PCFs made from selected nonlinear glasses with a solid core and small number of hexagonally arrayed air holes. The optimization procedure employs the Nelder-Mead downhill simplex algorithm. For the modal analysis of the photonic crystal fibre structure a fully-vectorial mode solver based on the finite element method is used. We have obtained two types of dispersion optimized nonlinear PCF designs: PCFs of the first type are single-mode and highly nonlinear with a small and flattened dispersion in the 1500-1600 nm range. These PCF structures have air holes hexagonally arrayed in from 3 to 5 rings, however, their dispersion characteristics are very sensitive to variations in structural parameters. PCFs of the second type are two-ring PCFs with larger multi-mode cores. They have fundamental mode's zero dispersion wavelength around 1550 nm with non-zero moderate dispersion slopes which are less sensitive to structural variation. It is supposed that this alternative PCF design will be easier to fabricate. The effects of fabrication imprecision on the dispersion characteristics for both PCF designs are demonstrated numerically and discussed in the context of nonlinear telecom applications.

Photonic crystal fibres (PCFs) with elliptical air-holes located in the core area that exhibit high birefringence, low losses, enhanced effective mode area, and low chromatic dispersion across a wide wavelength range have been presented. The effects of bending on birefringence, confinement losses and chromatic dispersion of the fundamental mode of the proposed PCFs have been thoroughly investigated by employing the full vectorial finite element method (FEM). Additionally, localization of higher order modes is presented. Also, effects of angular orientation on bending loss have been reported. Significant improvement on key propagation characteristics of the proposed PCFs are demonstrated by carefully altering the desired air hole diameters and their geometries and the hole-to-hole spacing.

We study guided acoustic wave Brillouin scattering (GAWBS) in several photonic crystal fibers (PCF) with different kind of air-hole microstructure and we show this effect is enhanced only for a few acoustic phonons. The results of our numerical simulations based on a finite element method reveal that these acosuti waves emitted in the GHz range are indeed trapped within the air-hole microstructure, in good agreement with experimental observations. The periodic wavelength-scale air-hole microstructure of solid-core PCFs can indeed drastically alter the transverse elastic waves distribution and therefore forward Brillouin scattering compared to what is commonly observed in conventional all-silica fibers. We show additionnally that the elasto-optic diffraction coefficient and the transverse acousto-optic field overlap are maximum for these acoustic waves. For the most intense GAWBS modes, we investigate the scattering efficiency and temperature dependence of the fundamental phonon frequency for sensing applications.

We have thoroughly studied and modelled many important aspects for the realization of gas-light interactions in suspended-core fibres. The fraction of the optical field propagating in holes could be calculated from the fibre geometry to predict the total absorption for a given molecular absorption line and fibre length. In addition, the gas diffusion into the fibre holes could be modelled to precisely anticipate the filling time for a given fibre geometry and length. This was experimentally validated by preparing several samples of suspended-core fibres showing various lengths. These samples were filled with acetylene at low pressure (< 50 mbar) and were hermetically and permanently sealed by fusion splicing each fibre end to a plain single-mode silica fibre. The adequacy between the modelling and the experimental results turned out to be excellent. Several physical parameters essential for the fibre characterization could be extracted from a set of measurements, sketching a specific metrological approach dedicated to this type of fibre. Finally, applications and advanced experiments that can be specifically carried out using these fibres are discussed.