Polarisation effects in optical microcoil resonators

Optical microcoil resonators (OMRs) fabricated by wrapping a microfibre around a rod to allow evanescent coupling between adjacent turns as in Fig 1. (a) have recently attracted much interest due to their high Q-factor and large extinction ratios resonances, low input and output coupling losses, large evanescent field and compactness [1,2], with applications such as sensing [3] and signal processing [4]. However, theoretical models published so far have neglected polarisation effects, and hence in order to develop a more detailed understanding we have modelled the OMR with polarisation-dependent coupled mode equations in the linear [5] and nonlinear regimes

Theoretical study of noise reduction of NRZ signals using nonlinear broken micro-coil resonators

Nonlinear microcoil resonators are extremely attractive devices for nonlinear optics; however, due to their high-Q values, their use at high speeds is limited. In this letter, we analyze a simple way of increasing their bandwidth, namely breaking the fiber in several places, and show that the resulting device is suitable for noise reduction in realistic systems. Simulations show that an in-line broken resonator can significantly reduce the impact of amplitude noise on the bit-error rate of nonreturn-to-zero signals

Modelling applications of photonic bandgap fibres

Photonic crystal fibres (PCFs)[1] are one of the most exciting developments in the field of photonics that has emerged in recent years. Not only have they already led to cheap all-fibre high brightness white light sources and have sparked a renaissance in the field of nonlinear optics but they also have the potential to dramatically change the next generation of telecommunication systems. PCFs can be split into two categories, the first have a solid core and guide light by modified total internal reflection, while the second photonic bandgap fibres (PBF) guide light by photonic bandgap effects and typically have a low index core compared to the cladding. Also of interest are "arrow" fibres which have a solid core and guide light due to the arrangement of high index defects in the cladding. In this paper we will be concentrating on designing and manipulating the properties of PBFs. etc..

Improved design of a DFB Raman fibre laser

A Raman fibre laser based on phase shifted DFB structures is modelled for the first time. Using parameters of realistic devices, the model predicts low-threshold and highly-efficient laser output. The change of position and width of the phase shift were found to have a substantial impact on laser performanc

These difficult days: Mission church reactions to Bantu education in South Africa 1949-56.

Mission education in South Africa became increasingly untenable in the 1940s because mission churches lacked the finances to be able to maintain a national system of educational provision and because Africans were questioning mission education which they increasingly considered institutionally racist. This crisis led the Nationalist Government to appoint an education commission to make recommendations leading to the reform of 'native education'. This commission argued that mission education was incompatible with the ordering of society envisaged by the government. It suggested that the state should wrestle control of 'native education' from missionaries, enabling it to fashion a policy more in sympathy with apartheid. In 1953 the Bantu Education Act was passed which withdrew state aid from mission schools and made their continued registration dependent on the approval of the Minister for Native Affairs. The Catholic Church, able to raise the necessary finance, decided to continue running alt their schools privately. This decision was based purely on religious doctrine and meant that the Church taught Bantu Education in alt of its schools. Of the remaining denominations only the American Board had the resources to maintain a single privately funded school. Aside from a few politicised figures in the Church of the Province of South Afiica who refused to lease their schools, claiming that to do so was to compromise with apartheid, church leaders leased their schools to the government. This decision was made because Church leaders considered that Bantu Education, despite their abhorrence to its ultimate aims, provided better opportunities to children than an 'education' picked up on the streets. This decision was made with little, if any, consultation with Africans (parents, teachers or students) because church governing structures were undemocratic and racially exclusive, denying Afiicans any meaningful representation. Ironically, the decision to lease was in accord with the wishes of most Africans who reluctantly accepted the fact that Bantu Education, in the absence of any realistic alternative, was better than no education

Simulations ofthe 100kW TJNAF FEL using a step-tapered undulator

The Thomas Jefferson National Accelerator Facility (TJNAF) free electron laser (FEL) can be upgraded to operate at 100kW average power in the near future using a configuration that recirculates the electron beam to recover energy. It is important to extract the maximum energy from the electron beam in a pass through the undulator while inducing the minimum amount of exhaust energy spread. A larger energy extraction reduces the requirement for a large recirculating current, while a smaller exhaust energy spread allows the intense electron beam to be recirculated without damaging components. To improve FEL performance, we explore the use of the step-tapered undulator, which alters the resonance condition halfway through the undulator. Short pulses complicate the desired interaction. Comparisons are made to the conventional periodic and linearly-tapered undulators.The authors are grateful for the support of the Office of Naval Research, Thomas Jefferson National Accelerator Facility, and contributions of Dave Douglas of TJNAF

Simulations of the TJNAF FEL with tapered and inversely tapered undulators

Experiments using the TJNAF FEL have explored the operation with both tapered and inversely tapered undulators. We present here numerical simulations using the TJNAF experimental parameters, including the effects of taper. Singlemode simulations show the effect of taper on gain. Multimode simulations describe the evolution of short optical pulses in the far infrared, and show how taper affects single-pass gain and steady-state power as a function of desynchronism. A short optical pulse presents an ever-changing field strength to each section of the electron pulse so that idealized operation is not possible. Yet, advantages for the recirculation of the electron beam can be explored.The authors are grateful for support by the Naval Postgraduate School

Overproduction of cosmic superstrings

We show that the naive application of the Kibble mechanism seriously underestimates the initial density of cosmic superstrings that can be formed during the annihilation of D-branes in the early universe, as in models of brane-antibrane inflation. We study the formation of defects in effective field theories of the string theory tachyon both analytically, by solving the equation of motion of the tachyon field near the core of the defect, and numerically, by evolving the tachyon field on a lattice. We find that defects generically form with correlation lengths of order M_s^{-1} rather than H^{-1}. Hence, defects localized in extra dimensions may be formed at the end of inflation. This implies that brane-antibrane inflation models where inflation is driven by branes which wrap the compact manifold may have problems with overclosure by cosmological relics, such as domain walls and monopoles.Comment: 31 pages, 16 figures, JHEP style; References added; Improved discussion of initial condition

Caustic Formation in Tachyon Effective Field Theories

Certain configurations of D-branes, for example wrong dimensional branes or the brane-antibrane system, are unstable to decay. This instability is described by the appearance of a tachyonic mode in the spectrum of open strings ending on the brane(s). The decay of these unstable systems is described by the rolling of the tachyon field from the unstable maximum to the minimum of its potential. We analytically study the dynamics of the inhomogeneous tachyon field as it rolls towards the true vacuum of the theory in the context of several different tachyon effective actions. We find that the vacuum dynamics of these theories is remarkably similar and in particular we show that in all cases the tachyon field forms caustics where second and higher derivatives of the field blow up. The formation of caustics signals a pathology in the evolution since each of the effective actions considered is not reliable in the vicinity of a caustic. We speculate that the formation of caustics is an artifact of truncating the tachyon action, which should contain all orders of derivatives acting on the field, to a finite number of derivatives. Finally, we consider inhomogeneous solutions in p-adic string theory, a toy model of the bosonic tachyon which contains derivatives of all orders acting on the field. For a large class of initial conditions we conclusively show that the evolution is well behaved in this case. It is unclear if these caustics are a genuine prediction of string theory or not.Comment: 23 pages, 5 figures; accepted for publication in JHEP. Revised derivation of eikonal equation for the DBI action. Added comments concerning the relationship between p-adic string theory and tachyon matter. Added second example of inhomogeneous evolution in p-adic string theory. Misleading statements concerning caustic-free evolution removed, references adde