Phase- coherent comparison of two optical frequency standards over 146 km using a telecommunication fiber link

We have explored the performance of two "dark fibers" of a commercial telecommunication fiber link for a remote comparison of optical clocks. The two fibers, linking the Leibniz University of Hanover (LUH) with the Physi-kalisch-Technische Bundesanstalt (PTB) in Braunschweig, are connected in Hanover to form a total fiber length of 146 km. At PTB the performance of an optical frequency standard operating at 456 THz was imprinted to a cw trans-fer laser at 194 THz, and its frequency was transmitted over the fiber. In order to detect and compensate phase noise related to the optical fiber link we have built a low-noise optical fiber interferometer and investigated noise sources that affect the overall performance of the optical link. The frequency stability at the remote end has been measured using the clock laser of PTB's Yb+ frequency standard operating at 344 THz. We show that the frequency of a frequency-stabilized fiber laser can be transmitted over a total fiber length of 146 km with a relative frequency uncertainty below 1E-19, and short term frequency instability given by the fractional Allan deviation of sy(t)=3.3E-15/(t/s)

Phase-coherent synthesis of optical frequencies and waveforms

Precision phase control of an ultrawide-bandwidth optical-frequency comb has produced remarkable and unexpected progress in both areas of optical-frequency metrology and ultrafast optics. A frequency comb (with 100 MHz spacing) spanning an entire optical octave (>300 THz) has been produced, corresponding to millions of marks on a frequency "ruler" that are stable at the Hz level. The precision comb has been used to establish a simple optical clock based on an optical transition of iodine molecules, providing an rf clock signal with a frequency stability comparable to that of an optical standard, and which is superior to almost all conventional rf sources. To realize a high-power cw optical frequency synthesizer, a separate, widely tunable single-frequency cw laser has been employed to randomly access the stabilized optical comb and lock to any desired comb component. Carrier-envelope phase stabilization of few-cycle optical pulses has recently been realized. This advance in femtosecond technology is important for both extreme non-linear optics and optical-frequency metrology. With two independent femtosecond lasers, we have not only synchronized their relative pulse timing at the femtosecond level, but have also phase-locked their carrier frequencies, thus establishing phase coherence between the two lasers. By coherently stitching the optical bandwidth together, a "synthesized" pulse has been generated with its 2nd-order autocorrelation signal displaying a shorter width than those of the two "parent" lasers

A ‘VISUAL’ APPROACH TO MENANDER - (A.K.) Petrides Menander, New Comedy and the Visual. Pp. xii + 322, ills. Cambridge: Cambridge University Press, 2014. Cased, £65, US$99. ISBN: 978-1-107-06843-8.

High-energy X-rays and gamma-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.Comment: This is an article from a monograph on the physics of solar flares, inspired by RHESSI observations. The individual articles are to appear in Space Science Reviews (2011

Reconstructing the environmental conditions experienced by early modern humans at Tam Pà Ling (northeast Laos) using higher plant wax biomarkers

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