Tests of Lorentz invariance using a microwave resonator

The frequencies of a cryogenic sapphire oscillator and a hydrogen maser are compared to set new constraints on a possible violation of Lorentz invariance. We determine the variation of the oscillator frequency as a function of its orientation (Michelson-Morley test) and of its velocity (Kennedy-Thorndike test) with respect to a preferred frame candidate. We constrain the corresponding parameters of the Mansouri and Sexl test theory to delta-beta + 1/2 = (1.5+/-4.2) x 10(-9) and beta-alpha - 1= (-3.1+/-6.9) x 10(-7) which is of the same order as the best previous result for the former and represents a 30-fold improvement for the latter.     

Optical-fiber pulse rate multiplier for ultralow phase-noise signal generation

In this Letter we report on an all optical-fiber approach to the synthesis of ultralow-noise microwave signals by photodetection of femtosecond laser pulses. We use a cascade of Mach-Zehnder fiber interferometers to realize stable and efficient repetition rate multiplication. This technique increases the signal level of the photodetected microwave signal by close to 18 dB. That in turn allows us to demonstrate a residual phase-noise level of -118 dBc/Hz at 1 Hz and -160 dBc/Hz at 10 MHz from a 12 GHz signal. The residual noise floor of the fiber multiplier and photodetection system alone is around -164 dBc/Hz at the same offset frequency, which is very close to the fundamental shot-noise floor.     

Engineered biosynthesis of novel polyenes: a pimaricin derivative produced by targeted gene disruption in Streptomyces natalensis.

BACKGROUND: The post-polyketide synthase biosynthetic tailoring of polyene macrolides usually involves oxidations catalysed by cytochrome P450 monooxygenases (P450s). Although members from this class of enzymes are common in macrolide biosynthetic gene clusters, their specificities vary considerably toward the substrates utilised and the positions of the hydroxyl functions introduced. In addition, some of them may yield epoxide groups. Therefore, the identification of novel macrolide monooxygenases with activities toward alternative substrates, particularly epoxidases, is a fundamental aspect of the growing field of combinatorial biosynthesis. The specific alteration of these activities should constitute a further source of novel analogues. We investigated this possibility by directed inactivation of one of the P450s belonging to the biosynthetic gene cluster of an archetype polyene, pimaricin. RESULTS: A recombinant mutant of the pimaricin-producing actinomycete Streptomyces natalensis produced a novel pimaricin derivative, 4,5-deepoxypimaricin, as a major product. This biologically active product resulted from the phage-mediated targeted disruption of the gene pimD, which encodes the cytochrome P450 epoxidase that converts deepoxypimaricin into pimaricin. The 4,5-deepoxypimaricin has been identified by mass spectrometry and nuclear magnetic resonance following high-performance liquid chromatography purification. CONCLUSIONS: We have demonstrated that PimD is the epoxidase responsible for the conversion of 4,5-deepoxypimaricin to pimaricin in S. natalensis. The metabolite accumulated by the recombinant mutant, in which the epoxidase has been knocked out, constitutes the first designer polyene obtained by targeted manipulation of a polyene biosynthetic gene cluster. This novel epoxidase could prove to be valuable for the introduction of epoxy substituents into designer macrolides.     

Uniform oscillations of supercontinua

Uniform oscillations of supercontinua, generated with microstructured fibre, have been observed and investigated. The oscillations are uniform in the sense that the optical power through the fibre changes with a regular and periodic waveform and the spectral bandwidth of the transmitted light changes correspondingly. The oscillations are continuous over periods of hours and the spectral width ranges from the octave-spanning condition (～310-THz wide at 20 dB below the maximum) to zero broadening of the input light. The corresponding contrast ratio of the fluctuating output light signal is as high as 0.75. Silica photonic crystal fibre with hexagonal air-hole structure and core diameter of 2.0 μm was used for the supercontinuum generation. The oscillations are produced when at least 15 mm of fibre is allowed to extend from the mount of the fibre launcher. Arguments in terms of thermal behaviour are used in an attempt to explain the fluctuations of light through the microstructured fibre. This appears to be the first occasion in which such behaviour has been reported.     

Efficient detection and control of the carrier-envelope offset frequency in a self-referencing optical frequency synthesizer

An efficient means of isolating and detecting the carrier-envelope offset frequency signal in an optical frequency synthesizer is demonstrated. The technique uses spliced and connectorized fiber for comb broadening, a periodically poled KTP crystal for doubling 1064-nm light in the supercontinuum, and a laser-line filter at 532 nm for comb-section selection. The technique produces an offset frequency with a 40-dB signal-to-noise ratio in a resolution bandwidth of 100 kHz with as little as 55 GW/cm² of peak pulse intensity inside the fiber. The strong signal-to-noise ratio helps realize an offset frequency signal with frequency instability of 1 mHz at 1 s when controlled through feedback to the optical power driving the femtosecond laser. PACS 06.30.Ft; 06.20.Fn; 42.62.-b; 42.60.-v     

Power-insensitive side locking for laser frequency stabilization

We present a simple modification of the traditional method of locking the laser frequency to the side of an atomic spectral line. We achieve first-order power insensitivity at arbitrary intensity and frequency and in this way eliminate one of the major drawbacks of the traditional method. A similar approach could also be used in locking a laser to a Fabry-Perot cavity.     

How to realize uniform three-dimensional ellipsoidal electron bunches

Uniform three-dimensional ellipsoidal distributions of charge are the ultimate goal in charged particle accelerator physics because of their linear internal force fields. Such bunches remain ellipsoidal with perfectly linear position-momentum phase space correlations in any linear transport system. We present a method, based on photoemission by radially shaped femtosecond laser pulses, to actually produce such bunches.     

Compression of subrelativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffraction

We demonstrate the compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching by inverting the positive space-charge-induced velocity chirp. This inversion is induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs.