393 research outputs found
Stationary resonances and mode density control in high index layers fully etched with a periodic microstructure
No full textTwo-dimensional photonic crystal material in fibre form
No full textPhotonic crystals are formed of periodically structured dielectric material, the pitch or period of the structure being of the order of the optical wavelength. A novel property of photonic crystal materials is they can be designed so as to exhibit photonic bandgaps, i.e. frequency ranges in which there are no propagating modes in the material. Some interesting consequences of such photonic band gaps occur for waves propagating out-of-plane in two-dimensionally materials. One possibility is to fabricate low-loss waveguides which guide soley by Bragg reflection. This could be done by using a 2-D structure which is effectively infinite in the third dimension, and which exhibits a bandgap in its transmission characteristics for waves which have a certain wavevector component beta =k along the structure. By purposefully introducing some kind of defect which is embedded in the crystal structure we can create a spatially localized region where such a wave can exist - a "defect state" appears in the band structure of the material. Light in this defect state would be unable to leak away from the defect through the crystal material, but would travel along the defect with propagation constant beta
Progress towards a photonic band gap fibre
No full textWe are currently developing an entirely new type of optical fibre that guides light by Bragg reflection instead of total internal reflection. The fibre will be made from pure silica, using conventional fibre drawing techniques to give a longitudinally invariant structure kilometres in length. In cross-section the fibre will include a hexagonal array of air holes with a pitch of 1-2µm. For a given optical frequency, there are ranges of axial wavevector B for which transverse propagation in this structure is forbidden: the photonic band gaps. It therefore acts as a totally reflecting "cladding" for such B values. A deliberate defect in the two-dimensional periodic structure (perhaps one hole is filled in, or is larger than its neighbours) provides a site for the localisation of light and so acts as the "core". The propagation of light along the defect can be engineered by modifying the unit cell and scale of the structure, giving rise to a number of unique applications
Full 2D photonic band gaps in silica/air structures
No full textFull 2-D photonic bandgaps are demonstrated for all polarisations in structures with refractive index contrast as small as that of silica and air. They occur for light propagating out of the transverse plane, with a longitudinal component of wave vector. A new type of optical fibre based on these structures is proposed
Two-dimensional photonic band-gap structures as quasi-metals
No full textBy considering waves that propagate out of the transverse plane, we show that common high index materials (eg GaAs) with a 2D array of air holes can act in some ways like a 3D photonic band-gap structure. In particular, we describe a dielectric "quasi-metal" that reflects all propagating light incident from free space
Photonic band structure of periodically etched high index waveguide layer:mode suppression in multimode waveguides
No full textThere is considerable current interest in using photonic band gap structures to control spontaneous emission and mode density in miniature laser resonators. In this paper, the photonic band structure and possible applications of resonant modes that occur within strongly modulated periodic dielectric films are reported. The structure analysed consists of a planar waveguide formed between planar sheets of perfect metal. Within the waveguide are alternating lines of high index dielectric and air. The guided modes in this structure (solid lines on figure) can be compared with those that would occur in a multimode waveguide with a homogeneous guiding layer of the same average index (dashed lines). At the Bragg condition a bandgap appears and this can be used to reduce the number of guided modes. From the figure it can be seen that at a normalised frequency of 4.2 there is a 13:1 mode reduction. In this way a fairly thick multi-mode waveguide can be made in which all but the highest order mode are suppressed, rendering the structure single-mode. The field microstructure of this mode shows that most of its intensity is a concentrated in the high index regions. This mode could be excited by rare-earth doping in the high index regions alone. This could have applications in single-mode waveguide lasers where an increased mode volume could be very beneficial
Pure silica single-mode fibre with hexagonal photonic crystal cladding
No full textPure silica fibres supporting guided modes were first investigated in the 1970's, the aim being to achieve low transmission losses. The huge success of chemical vapour deposition in producing extremely low loss fibre has largely superseded this early technology. We have recently revisited it in the context of photonic crystals, and report here the realisation of a new kind of pure silica microstructured optical fibre which supports a robust single mode. Photonic crystals are periodically microstructured materials with a pitch on the scale of the optical wavelength. They have recently been the subject of much interest because of their unusual optical properties, including their ability to support a full photonic band gap. Several research teams have reported fabricating two-dimensional photonic crystal material out of glasses using selective etching processes. However, such a fabrication process results in samples of at most a few millimetres in the third dimension. The photonic crystal fibre described here is formed by creating a hexagonal silica/air preform (including a deliberate defect to guide light) on a macroscopic scale and then reducing its size by several orders of magnitude by pulling it into an optical fibre (see Fig 1)
Photonic band structure of guided block modes in high index films fully etched through with periodic microstructure
No full textBy adapting the well-known 'zigzag' ray model for use with a periodic waveguide (i.e. replacing the plane wave rays with Bloch wave rays), we show that thin films of high refractive index, supported by a low index substrate and fully etched through with a periodic pattern, can support guided modes. From the dispersion relation of these guided Bloch modes, it is shown that the in-plane modal group velocity can be zero, suggesting applications in enhanced dipole-field interactions and control of spontaneous emission in waveguide lasers
All-silica single-mode optical fiber with photonic crystal cladding
No full textWe report the fabrication of a new type of optical waveguide: the photonic crystal fiber. It consists of a pure silica core surrounded by a silica-air photonic crystal material with a hexagonal symmetry. The fiber supports a single robust low-loss guided mode over a very broad spectral range of at least 458-1550 nm. Also see errata - http://eprints.soton.ac.uk/78010
The Drama of Reform: Theology and Theatricality, 1461–1553
No full textThis review considers The Drama of Reform by Tamara Atkin
- …
