Abstract:We demonstrate the generation of wideband frequency sweeps using a semiconductor laser in an optoelectronic feedback loop. The rate and shape of the optical frequency sweep is locked to and determined by the frequency of a reference electronic signal, leading to an agile, high coherence swept-frequency source for laser ranging and 3-D imaging applications. Using a reference signal of constant frequency, a transformlimited linear sweep of 100 GHz in 1 ms is achieved, and real-time ranging with a spatial resolution of 1.5 mm is demonstrated. Further, arbitrary frequency sweeps can be achieved by tuning the frequency of the input electronic signal. Broadband quadratic and exponential optical frequency sweeps are demonstrated using this technique.

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Widespread adoption of superconducting technologies awaits the discovery of new materials with enhanced properties, especially higher superconducting transition temperatures T c . The unexpected discovery of high T c superconductivity in cuprates suggests that the highest T c s occur when pressure or doping transform the localized and moment-bearing electrons in antiferromagnetic insulators into itinerant carriers in a metal, where magnetism is preserved in the form of strong correlations. The absence of this transition in Fe-based superconductors may limit their T c s, but even larger T c s may be possible in their isostructural Mn analogs, which are antiferromagnetic insulators like the cuprates. It is generally believed that prohibitively large pressures would be required to suppress the effects of the strong Hund's rule coupling in these Mn-based compounds, collapsing the insulating gap and enabling superconductivity. Indeed, no Mn-based compounds are known to be superconductors. The electronic structure calculations and X-ray diffraction measurements presented here challenge these long held beliefs, finding that only modest pressures are required to transform LaMnPO, isostructural to superconducting host LaFeAsO, from an antiferromagnetic insulator to a metallic antiferromagnet, where the Mn moment vanishes in a second pressure-driven transition. Proximity to these charge and moment delocalization transitions in LaMnPO results in a highly correlated metallic state, the familiar breeding ground of superconductivity. correlated electron systems | electronic delocalization transition S uperconductivity with high transition temperatures T c was first found near an electron delocalization transition (EDT) in the cuprates, and subsequently in systems as diverse as quasi-two dimensional organic layer compounds (1), heavy fermions (2, 3), and endohedrally doped fullerides (4). One obstacle to achieving a higher T c in the Fe-based superconductors may be that the parent compounds are metallic (5-7), albeit with quasiparticle mass enhancements (8) that suggest varying degrees of proximity to an EDT (9-11). So far no insulating parent compounds have been identified that can, by analogy to the cuprates, be doped to achieve higher superconducting transition temperatures. It is possible that the recently isolated K 2 Fe 4 Se 5 (12) and La 2 O 2 Fe 2 OðSe; SÞ 2 (13) phases may prove to be the first compounds of this type. In contrast, isostructural Mn-based compounds often have large insulating gaps and ordered moments (14, 15), suggesting their suitability as possible parent compounds. At present there are no known Mn-based superconductors, however, and it is generally believed that the Hund's rule coupling in Mn compounds is prohibitively strong, so that doping will not reduce the overall scale of the correlations to the point at which superconductivity may become possible. The electronic structure calculations and X-ray diffraction measurements presented here show how the interplay of Hund's rule interactions with incr...

We propose a volume holographic data storage scheme that employs counterpropagating reference and image beams and wavelength multiplexing for page differentation.This method is compared with that based on angular multiplexing. A reduction in holographic cross talk is predicted. Further cross-talk reduction that is due to sidelobe suppression is observed in experiments by using photorefractive crystals and the proposed orthogonal data storage.Optical data storage in volume holograms has been an exciting prospect since the early days of holography.' This is due mostly to the theoretical storage capacity of -V/Al bits in a volume V. This, as an example, translates to 8 X 1012 bits in a 1-cm3 volume at A = 0.5 ,.m.Early attempts at exploiting this potential have been disappointing.t"10 Even though hundreds of holograms have been successfully stored in a single volume in these experiments, the data content of the individual holograms was minimal, and, therefore, so was the overall volumetric information density. This was due partly to material limitations"1", 2 but, more fundamentally, to the intolerably high cross talk resulting from the angular multiplexing method used to record the large number of holograms.'-' In this traditional method, many twodimensional images are recorded sequentially, each with its unique angularly multiplexed reference wave. Recently, however, phase-encoded or spatially orthogonal reference beams have been used to multiplex many holograms."31 4 These approaches, nonetheless, are variations of the basic angular multiplexing technique and differ fundamentally from the orthogonal wavelength-multiplexed storage technique proposed here.To appreciate the nature of the problem we recall that the basic property, and virtue, of holography is the essentially uniform distribution of the recorded information throughout the physical volume of the recording medium. We assert here the self-evident, but heretofore ignored, fact that it is equally important to distribute the information throughout the whereSuch a grating can be identified by the point K in KXIKZK space. If the wave E, is spatially modulated, say by image information, the corresponding K point becomes a surface whose extent (or solid angle) is determined by the smallest feature size of the information. Such a surface can be labeled by the reference wave used to record it and is shown as K(rl)ing in Fig. 1 along with the responsible kr$t. The same figure also shows the surface K Lting that is due to a second image recorded with a second, angle-multiplexed reference wave kr.2et Reconstruction of, say, image 2 by illuminating the stored holograms with a wave along kr(2} will inevitably lead to undesirable parasitic scattering

We describe and characterize a narrow bandwidth volume holographic optical filter operating at the Krypton transition line (1s2-2p8) at 1547.82 nm, which corresponds to the center wavelength of the proposed International Telecommunications Union wavelength standard. A reflectivity of 98% and a bandwidth full width at half-maximum of 0.18 nm are measured. The filter exhibits clean sideholes with a −20 dB optical response 0.5 nm away from the peak. The filter can be fabricated and operated with an absolute wavelength precision better than 0.005 nm.