Kristian Hougaard

Photonic crystal fibers

Photonic crystal fibers having a complex microstructure in the transverse plane constitute a new and promising class of optical fibers. Such fibers can either guide light through total internal reflection or the photonic bandgap effect, In this paper, we review the different types and applications of photonic crystal fibers with particular emphasis on recent advances in the field.

Dispersion Properties of Photonic Crystal Fibers - Issues and Opportunities

The dispersion, which expresses the variation with wavelength of the guided-mode group velocity, is one of the most important properties of optical fibers. Photonic crystal fibers (PCFs) offer much larger flexibility than conventional fibers with respect to tailoring of the dispersion curve. This is partly due to the large refractive-index contrast available in silica/air microstructures, and partly due to the possibility of making complex refractive-index structures over the fiber cross section. We discuss the fundamental physical mechanisms determining the dispersion properties of PCFs guiding by either total internal reflection or photonic bandgap effects, and use these insights to outline design principles and generic behaviours of various types of PCFs. A number of examples from recent modeling and experimental work serve to illustrate our general conclusions.

Coupling reducing k-points for photonic crystal fiber calculations

Abstract When describing localized electromagnetic modes in dielectric waveguides by the planewave method, a supercell geometry must necessarily be adopted. We demonstrate in the present work that the convergence of the calculations with respect to supercell size depends strongly on the choice of the transverse Bloch wave vector, k . We describe a method to derive k -points that minimize the coupling between repeated images of the guided modes in real space. Calculations have been done for a quadratic and a triangular photonic crystal fiber structure. With the new coupling reducing (CR) k -points, the convergence of the eigenfrequencies for both the fundamental and second order modes with respect to supercell size is considerably improved. The general approach outlined may also be applied in the case of three-dimensional photonic crystal structures.

Photonic crystal structures in sensing technology

Photonic crystal materials and waveguides have since their appearance in 1987 attracted very much attention from the scientific community. From being a more academia discipline, new components and functionalities have emerged, and photonic crystals have today started to enter the field of commercial devices. Especially the photonic crystal fiber (PCF) with its lattice of air holes running along the length of the fiber has matured, and the technology provides a large variety of novel optical properties and improvements compared to standard optical fibers. With respect to optical sensors, the photonic crystal structures have several important properties. First of all the wavelength-scale periodically-arranged material structures provide completely new means of fabricating tailored optical properties either using modified total internal reflection or the photonic bandgap effect. Secondly, the new materials with numerous micro- or even nano-scale structures and voids allow for superior mode control, use of polarization properties, and even more a the potential of close interaction between optical field and the material under test. The present paper will be using the example of the relatively mature photonic crystal fiber to discuss the fundamental optical properties of the photonic crystals, and recent examples of their use as optical sensors will be reviewed.

Photonic crystal fibers; fundamental properties and applications within sensors

Since the first experimental demonstration of a photonic crystal fiber (PCF) in 1996 by Knight et al. the optical properties and the fabrication of such fibers have attracted significant attention. The fiber structure with a lattice of air holes running along the length of the fiber provides a large variety of novel optical properties and improvements compared to standard optical fibers. The stack-and-pull procedure used to manufacture PCFs is a highly flexible method offering a large degree of freedom in the fabrication of PCFs with specific characteristics. A few of the remarkable optical properties of silica based PCFs are described and their applications within sensors are summarized.

Coupling to photonic crystal fibers

In this work we have analyzed the correspondence between the fundamental mode of PCFs and Gaussian modes as a function of frequency, pitch, and air hole size. Such analysis provides insight into design space regions of PCFs, where low-loss coupling to standard fibers may be obtained.

Microbending in photonic crystal fibres - an ultimate loss limit?

Microbending losses are for the first time estimated in index-guiding photonic crystal fibres, and comparisons with standard step-index fibres are made. The results indicate that typical photonic crystal fibres are significantly less sensitive (one order of magnitude smaller loss) towards microbending than standard optical fibres.

Very low zero-dispersion wavelength predicted for single-mode modified-total-internal-reflection crystal fibre

A zero-dispersion wavelength (λZD) below 600 nm has been predicted for a pure silica modified-total-internal-reflection (M-TIR) single-mode crystal fibre with a 1 µm pitch. The fibre is single mode from at least 250 nm, i.e., from well below λZD. Both a method for designing single-mode crystal fibres using an anti-guiding structure and a fast calculation scheme for investigating the structure for different pitch sizes are proposed.

Photonic crystal fibers: a variety of applications

Photonic crystal fibres having a microstructured air-silica cross section offer new optical properties compared to conventional fibres. These include novel guiding mechanisms, new group velocity dispersion properties and new non-linear possibilities.

Are hollow-core fibers attractive for high-power fiber lasers?

Silica-based hollow-core photonic bandgap (HC-PBG) fibers are of interest for high-power laser applications, due to the possibility of guiding the majority of the optical power in air, thus suppressing nonlinearities and the limitations set by the breakdown threshold of silica. In this contribution, we study numerically the laser-induced damage threshold in HC-PBG fibers as function of core size and cladding air-filling fraction, and compare to a typical silica-core large-mode area (LMA) fiber. Remarkably, the HC-PBG fibers yield no significant improvement over the LMA reference, indicating that radically new design ideas will be needed for HC-PBG fibers to be competitive as active components in a high-power laser system.