Ultra-short pulse compression using photonic crystal fibre

A short section of photonic crystal fibre has been used for ultra-short pulse compression. The unique optical properties of this novel medium in terms of high non-linearity and relatively small group velocity dispersion are shown to provide an ideal platform for the standard fibre pulse compression technique used directly on the nano-Joule output pulses from a commercial laser system. We report an order of magnitude reduction of the pulse width to 25 fs FWHM but predict a substantially improved performance with a dedicated fibre design. Good agreement is obtained with a simple model for the spectral broadening in the fibre

Fast wavelength-tunable ultra-violet laser source for confocal Fura-2AM imaging

We report a novel wavelength-flexible laser source for three-dimensional ultra-violet imaging. Based on supercontinuum generation in photonic crystal fiber, the resultant broadband laser source extended from A = 331 nm into the visible region of the spectrum. Using an electronically-controlled filter wheel and filter set with a response time of approximately 50 ins, rapid wavelength selection was performed. The described scheme is capable of exciting the current range of ultra-violet-excited fluorophores and the simple and rapid wavelength control also provides a new approach for fast ratiometric imaging of Fura-2AM, facilitating an easy method of performing quantitative intracellular calcium concentration measurements

A novel technique for wide-field polarimetry with a radiotelescope array

We report the use of the Australia Telescope Compact Array (ATCA) to conduct polarimetric observations of the sky at 5 GHz. The ATCA is normally operated as an interferometer array, but these observations were conducted in a split array mode in which the antenna elements were used as single-dishes with their beams staggered to simultaneously cover a wide area of sky with a resolution of 10 arcmin. The linearly polarized sky radiation was fully characterized from measurements, made over a range of parallactic angles, of the cross correlated signals from the orthogonal linear feeds. We describe the technique and present a polarimetric image of the Vela supernova remnant made as a test of the method. The development of the techniques was motivated by the need for wide-field imaging of the foreground contamination of the polarized component of the cosmic microwave background signal.Comment: 13 pages, 2 figures, accepted for publication in A

Radicals of 0-regular algebras

We consider a generalisation of the Kurosh--Amitsur radical theory for rings (and more generally multi-operator groups) which applies to 0-regular varieties in which all operations preserve 0. We obtain results for subvarieties, quasivarieties and element-wise equationally defined classes. A number of examples of radical and semisimple classes in particular varieties are given, including hoops, loops and similar structures. In the first section, we introduce 0-normal varieties (0-regular varieties in which all operations preserve 0), and show that a key isomorphism theorem holds in a 0-normal variety if it is subtractive, a property more general than congruence permutability. We then define our notion of a radical class in the second section. A number of basic results and characterisations of radical and semisimple classes are then obtained, largely based on the more general categorical framework of L. M\'arki, R. Mlitz and R. Wiegandt as in [13]. We consider the subtractive case separately. In the third section, we obtain results concerning subvarieties and quasivarieties based on the results of the previous section, and also generalise to subtractive varieties some results for multi-operator group radicals defined by simple equational rules. Several examples of radical and semisimple classes are given for a range of fairly natural 0-normal varieties of algebras, most of which are subtractive

Torsion in the cohomology of congruence subgroups of SL(4,Z) and Galois representations

We report on the computation of torsion in certain homology theories of congruence subgroups of SL(4,Z). Among these are the usual group cohomology, the Tate-Farrell cohomology, and the homology of the sharbly complex. All of these theories yield Hecke modules. We conjecture that the Hecke eigenclasses in these theories have attached Galois representations. The interpretation of our computations at the torsion primes 2,3,5 is explained. We provide evidence for our conjecture in the 15 cases of odd torsion that we found in levels up to 31

VALUE OF IRRIGATION WATER IN THE MIDDLE ATLANTIC STATES: AN ECONOMETRIC APPROACH

Estimation of the economic value of irrigation water is complicated by a lack of data on the price or marginal cost of water. Through econometric estimation of an aggregate total value product function, this paper obtains marginal irrigation water value estimates for the Middle Atlantic region. Additionally, the impact of temperature and soil conditions on aggregate production within the region is estimated. Ridge regression and covariance analysis are employed to deal with problems of multicollinearity and simultaneous equation bias, respectively. Estimates indicate a substantial and growing return to irrigation within the region.Resource /Energy Economics and Policy,

Simulated Trading for Maryland's Nitrogen Loadings in the Chesapeake Bay

We investigate nutrient trading for point and non-point sources for the Bay Restoration Fund in Maryland. We demonstrate how to use the proceeds from the tax revenue to mimic a market by trading high-cost upgrades of sewage treatment plants for low-cost winter cover crops. Under an optimistic assumption about costs for non-point sources and naïve assumptions about the lag from planting cover crops to changes in nitrogen load, we calculate that 100 percent of abatement could be achieved at 56 percent of total costs, while in a pessimistic scenario, 100 percent of abatement could be could be achieved at 83 percent of total costs.Chesapeake Bay, cover crops, nitrogen abatement, nutrient trading, sewage treatment plants, trading ratios, water pollution, Environmental Economics and Policy,

SOME PRELIMINARY EVIDENCE ON SAMPLING OF ALTERNATIVES WITH THE RANDOM PARAMETERS LOGIT

Random utility models rely on the properties of the logistic distribution for ease of estimation, but this distribution implies the independence of irrelevant alternatives (IIA). The random parameters logit model offers a means of avoiding the IIA assumption as well as greater heterogeneity among agents, recreational anglers or beachgoers in the current application. A problem often encountered in the estimation of random utility models with many alternatives is the necessity of sampling alternatives or otherwise reducing the number of choices. Research has shown that in the random utility model, such changes in choice set still lead to consistent parameter estimates. However, with the random parameters logit, there is greater need to sample but no theoretical evidence that sampling is justified. In this paper we show the impact of sampling in a random parameters logit model. We find that sampling does not appear to change the parameter estimates substantially. We investigate two data sets: a study of beach use in the Chesapeake Bay and a study of marine recreational angling behavior for the Northeast of the U.S.Resource /Energy Economics and Policy,