Fabrication of a truly single mode, low loss and polarization maintaining HC-PCF is reported. This fiber has a 50 nm wide strictly single mode region with good polarization holding (h-parameter below 10-4 m-1) and low loss (<20 dB/km).

We present two low-loss 7-cell core hollow-core photonic crystal fibers (HC-PCF) with intrinsic single mode properties around 1550 nm. By reducing the number of surface modes within the bandgap these fibers can be operated close to the short wavelength bandgap edge. It is well known that by omitting a core tube in HC-PCF fabrication of a surface mode free bandgap can be achieved. We found by experimental as well as numerical, investigation that using a core tube with a wall thickness reduced to between 60-70 % is sufficient to have a surface mode free short wavelength bandgap edge. The transmission and mode properties of the fabricated fibers are examined experimentally and compared to numerical calculations.

ABSTRACT We present a new type of combiner based on a fused pump/signal fiber bundle. We obtain a record high signal-to-pump isolation of more than 46dB from a device optimized for high power counter directional pumping of Ytterbium-doped large mode area airclad fibers. The compact device combines more than 100W of light from 14 pcs. of 105µm NA=0.15 fibers into the pump cladding of an airclad fiber with a coupling efficiency of 90%. The signal light is delivered through the center of the combiner ensu ring the exceptional isolation. The high level of isolation, measured for a 60W amplifier configuration, is essential to ensure the reliability of the pump diodes in a high power pulsed system. The combiner is compatible with PM and non-PM systems and we demonstrate both a CW and a pulsed configuration. Keywords: Fiber optics components, double-clad fibers, high-power pulsed fiber lasers 1. INTRODUCTION Within the last few years, the Ytterbium-doped double-clad fiber amplifiers have developed from niche applications lasers into the high-power laser industry for material processing

A novel microstruct tired fibre has been created for use in an optical interconnection system. The fibre has low crosstalk with a high density of cores corresponding to 1150 channels/mm(2). A repeating pseudo-random binary sequence has been used to demonstrate a four-channel transmit/receive system using vertical cavity surface emitting lasers as both emitters and detectors.

Photonic crystal fibers provide increased range of mode-field diameters for passive and active fibers. At present, single mode photonic crystal fibers with mode field diameters ranging from sub-micron to beyond 40 /spl mu/m have been demonstrated. For a number of applications, it is desirable to introduce polarization maintaining properties of such fibers. In this presentation, we report on the latest development within this area and explain the design and characteristics of different types of photonic crystal fibers with both polarization-maintaining and polarizing properties.

We review the present state-of-the-art for air-guiding photonic bandgap fibers. Applications, loss mechanisms and future work is considered.

We report on the latest development within active photonic crystal fibers for high power lasers and amplifiers with special focus on how the fibers can be improved with both polarization-maintaining and polarizing properties. We describe rod-type fibers for which a record-high power extraction of 250W/m is achieved. Moreover. we describe how active characterization is used to optimize fibers for laser and amplifier sub-assemblies with respect to beam quality, efficiency and robustness. Finally, we illustrate how the fibers can be integrated with high NA tapers and passive air-clad fibers containing Bragg grating to form an all-fiber. alignment-free, high-power fiber laser subassembly.

We report on the laser properties of multicore photonic crystal fiber lasers. A stable phase locking of six- and seven-core structures through evanescent coupling is observed. Effective supermode selection is obtained by using both diffraction losses and the Talbot effect. A pure in-phase supermode is obtained (1.1 times diffraction limited). The laser operating in this mode has a slope efficiency of 70% with up to 44 W of output power. The modal area of the in-phase supermode multicore fiber is 1150 microm2, which makes it, to our knowledge, the single-mode fiber laser with the largest mode field area. In-phase laser action is stable when the fiber is bent.

A new class of hollow-core Bragg fibers1 composed of concentric cylindrical silica rings separated by nanoscale support bridges is presented.2, 3 These fibers are believed to be especially useful for high-power delivery of light within a broad wavelength range or at multiple wavelength bands. We experimentally observe theoretically predicted hollow-core confinement over an octave frequency range. The bandwidth of bandgap guiding in this type of Bragg fiber far exceeds that of other hollowcore fibers reported in the literature. With the record-low number of three rings of silica cladding layers, these Bragg fibers achieve a propagation loss of the order of 1 dB/m. The concept of hollowcore Bragg fibers, in which the fiber cladding is composed of cylindrical dielectric layers with alternating refractive indices, was first proposed in 1978.1 Cregan et al. demonstrated another class of hollow-core fibers, namely, photonic crystal fibers, in which the cladding structure is formed by creating a twodimensional array of airholes in a highindex material, typically silica.4 In general, the transmission coefficient through a planar Bragg reflector, which translates to a leakage coefficient of the Bragg fiber, depends exponentially on the number of layers.5, 6 We consider a specific Bragg fiber with a hollow-core radius of 10 μm. The fiber cladding is formed by three layers with a refractive index of 1.45 and average thickness of 370 nm, separated by 4.10m-thick air layers. In practice, support bridges must be introduced to separate the adjacent silica rings. Assuming mass conservation throughout the fiber pulling process, we estimate the support bridge thickness to be in the area of 45 nm, which is in reasonable agreement with the results obtained with scanning electron microscopes. Because the support bridges are much smaller than the wavelength of interest, we can, to a good approximation, neglect the presence of these support bridges and regard the region between the highindex silica layers as composed entirely of air. An interesting feature that will further inspire the development of this type of fiber is that, with only four silica layers, theoretical considerations predict that the fiber leakage loss can be reduced to less than 0.1dB/km. Also, the Bragg fiber supports low-loss modes (less than 1dB/m) in the 0.82–2.86m wavelength range, which is almost two octaves in frequency range, and also in a wavelength interval in which material losses are so high that one could indeed benefit from an air core of the fiber. T. Sorensen (thors@ com.dtu.dk), J. B. Jensen, T. P. Hansen and A. Bjarklev are with Research Center COM, Technical University of Denmark, Lyngby, Denmark. H. J. Deyerl is with the Abteilung fur Ionenphysik, Technische Universitat Chemnitz, Germany. Y. Xu, Y. Huang, M. Terrel, R. K. Lee and A. Yariv are with the California Institute of Technology, Pasadena, Calif. G. Vienne is with the Center for Advanced Research in Photonics, Chinese University of Hong Kong, Shatin, Hong Kong. C. Jakobsen, T. P. Hansen, J. Broeng and H. Simonsen are with Crystal Fibre A/S, Birkerod, Denmark. N. A. Mortensen is with the Department of Micro and Nanotechnology, Technical University of Denmark, Lyngby, Denmark.

For development of hollow-core transmission fibers, the realizable fibers lengths, bandwidth, characterization, and compatibility with standard technology are important issues. We report record-length air-guiding fiber, spectral properties, splicing, and optical time domain reflectometer (OTDR) measurements. Furthermore, spectral macrobending loss measurements for two different designs of air-core photonic bandgap fibers are presented. While bending loss is observed, it does not limit operation for all practical bending diameters (>5 mm).

We report on experimental studies of gas sensing using air-guiding photonic bandgap fibers. The photonic bandgap fibers have at one end been spliced to standard single mode fibers for ease of use and improved stability

We report on a single-mode photonic crystal fiber with attenuation and effective area at 1550 nm of 0.48 dB/km and 130 microm(2), respectively. This is, to our knowledge, the lowest loss reported for a PCF not made from VAD prepared silica and at the same time the largest effective area for a low-loss (< 1 dB/km) PCF. We briefly discuss the future applications of PCFs for data transmission and show for the first time, both numerically and experimentally, how the group velocity dispersion is related to the mode field diameter.

Recent development achievements in high power fiber laser designs and subassemblies have improved the performance and usability. Such achievements show the path for these fiber lasers out of the R&D labs and into industrial environments.

Modal cutoff is investigated experimentally in a series of high-quality nonlinear photonic crystal fibers. We demonstrate a suitable measurement technique with which to determine the cutoff wavelength and verify the technique by inspecting the near field of the modes that may be excited below and above the cutoff. We observe a double-peak structure in the cutoff spectra, which is attributed to splitting of the higher-order modes. The cutoff is measured for seven different fiber geometries with different pitches and relative hole sizes, and very good agreement with recent theoretical work is found.

We report on recent progress in the design and application of vertical-cavity surface-emitting lasers (VCSELs) for optical interconnect applications in the 850 nm emission wavelength regime. Ongoing work toward parallel optical interconnect modules with channel data rates of 10 Gbit/s is reviewed and performance results of flipchip integrated two-dimensional VCSEL arrays are presented. 10 Gbit/s speed as well as low thermal resistance of the lasers has been achieved. As a possible alternative to graded-index. multimode fibers, we show 10Gbit/s data transmission over 100m length of a novel, entirely undoped multimode photonic crystal fiber. The use of VCSELs with output powers in the 10mW range is demonstrated in a 16-channel free-space optical (FSO) module and VCSELs with even higher output power are shown to provide possible FSO connectivity up to data rates of 2.5 Gbit/s.

We numerically study the possibilities for improved large-mode-area endlessly single-mode photonic crystal fibers for use in high-power delivery applications. By carefully choosing the optimal hole diameter, we find that a triangular core formed by three missing neighboring air holes considerably improves the mode area and loss properties compared with the case with a core formed by one missing air hole. In a realized fiber we demonstrate an enhancement of the mode area by approximately 30% without a corresponding increase in the attenuation.

Quasi error-free 10Gbit/s data transmission is demonstrated over a novel type of 50 microm core diameter photonic crystal fi ber with as much as 100m length. Combined with 850 nm VCSEL sources, this fi ber is an attractive alternative to graded-index multi-mode fi bers for datacom applications. A comparison to numerical simulations suggests that the high bit-rate may be partly explained by inter-modal diffusion.

Both a scalar method and a fully vectorial method have been applied to modelling the bending losses in photonic crystal fibres (PCFs). Due to a decreasing index difference between the core and the cladding towards short wavelengths, an unusual loss mechanism is seen, that is a short-wavelength loss edge. This is verified by measurements. For large-mode area PCFs, the model gives a massive overestimate, never an underestimate.

An all-silica photonic crystal fiber with a core diameter of 15 mum was fabricated. Attenuation, dispersion and dispersion slope at 1550 nm were measured to be 2.2 dB/km, 27 ps/(km-nm) and 7.2middot10 -2 ps/(km-nm2), respectively