Dynamics of long-period gratings tuned with internal fiber electrodes.

The physics of electrically switched long-period grating in a twin-hole fiber with internal electrodes is studied. Dynamic measurements for the two polarizations show how the grating spectra shift in time due to the mechanical stress and heat transfer in the core and the cladding.

Birefringence switching of Bragg gratings in fibers with internal electrodes.

A fiber Bragg grating was written in a side-hole fiber with internal metal alloy electrodes. The initial geometrical birefringence of this fiber gives rise to two Bragg resonances separated by 43 pm. Nanosecond risetime current pulses of up to 23 A were applied to the metal electrode, which heated and expanded rapidly. This caused mechanical stress in the fiber on a nanosecond scale, resulting in a negative shift of the Bragg wavelength peak for the fast axis mode, and positive but smaller shift for the slow axis mode. The fast change increased the peak separation to approximately 143 pm, corresponding to an increase in birefringence from 4.0 x 10(-5) to 1.3 x 10(-4). Both peaks subsequently experienced a red-shift due to the relaxation of mechanical stress and the increasing core temperature transferred from the metal in many microseconds. Simulations give accurate description of the experimental results.

Nanosecond switching of fiber Bragg gratings.

A FBG was written in a two-hole fiber with internal alloy electrodes. Nanosecond high current pulses cause metal expansion, increase birefringence and tune the gratings with a response time of 29 ns. This short length, low loss, all-spliced high-speed wavelength switching devices described here has potential use in Q-switching fiber laser.

An RNA polymerase II complex containing all essential initiation factors binds to the activation domain of PAR leucine zipper transcription factor thyroid embryonic factor.

Transcription initiation of protein-encoding genes involves the assembly of RNA polymerase II and a number of general transcription factors at the promoter. A mammalian RNA polymerase II complex containing all of the components required for promoter-specific transcription initiation can be isolated by immunopurification with a monoclonal antibody directed against the cyclin-dependent kinase CDK7, a subunit of the general transcription factor TFIIH. In vitro transcription by this immunopurified RNA polymerase II complex is effectively stimulated by thyroid embryonic factor (TEF), a basic leucine zipper transcription factor. Thus, the RNA polymerase II complex must also contain components required for activated transcription that interact with the transactivation domain of TEF. This conjecture was verified by affinity selection experiments in which the TEF transcription activation domain was used as a bait. Indeed, an RNA polymerase II complex containing all of the accessory proteins required for transcription initiation can be enriched by its affinity to recombinant proteins containing the TEF transactivation domain. These results are compatible with a mechanism by which TEF can recruit an RNA polymerase II holoenzyme to the promoter in a single step.

High-reflectivity fiber-optic bandpass filter designed by use of the iterative solution to the Gel'fand-Levitan-Marchenko equations.

A fiber Bragg grating bandpass filter has been investigated. The profile of the 3.1-cm-long grating was synthesized for high reflectivity and low dispersion by use of the iterative solution to the Gel'fand-Levitan-Marchenko equations. A grating designed according to this synthesis was written into a boron-codoped germanosilica fiber. The measured spectral response was in good agreement with simulations. The achieved in-band reflection was 97%, and the passband ripple was only 0.06 dB.

Nondestructive method for measuring the degree of cluster-induced quenching in er(3+)-doped waveguides and fibers.

Ion clustering is one of the major dissipative processes in the operation of Er(3+)-doped planar waveguide amplifiers and heavily doped fiber amplifiers. We propose and demonstrate a nondestructive method for measuring the fraction of erbium ions in doped planar waveguides and fibers that are quenched by ultrafast parasitic energy transfer in ion pairs or clusters. The method is based on unsaturable absorption measurements in the 980-nm absorption band. By combining transmission data at resonance with the corresponding data for radiation outside the absorption band, the influence of background and coupling losses can be eliminated.

The two PAR leucine zipper proteins, TEF and DBP, display similar circadian and tissue-specific expression, but have different target promoter preferences.

The two highly related PAR basic region leucine zipper proteins TEF and DBP accumulate according to a robust circadian rhythm in liver and kidney. In liver nuclei, the amplitude of daily oscillation has been estimated to be 50-fold and 160-fold for TEF and DBP, respectively. While DBP mRNA expression is the principal determinant of circadian DBP accumulation, the amplitude of TEF mRNA cycling is insufficient to explain circadian TEF fluctuation. Conceivably, daily variations in TEF degradation or nuclear translocation efficiency may explain the discrepancy between mRNA and protein accumulation. In vitro, TEF and DBP bind the same DNA sequences. Yet, in co-transfection experiments, these two proteins exhibit different activation potentials for two reporter genes examined. While TEF stimulates transcription from the albumin promoter more potently than DBP, only DBP is capable of activating transcription efficiently from the cholesterol 7 alpha hydroxylase (C7alphaH) promoter. However, a TEF-DBP fusion protein, carrying N-terminal TEF sequences and the DNA binding/dimerization domain of DBP, enhances expression of the C7alphaH-CAT reporter gene as strongly as wild-type DBP. Our results suggest that the promoter environment, rather than the affinity with which PAR proteins recognize their cognate DNA sequences in vitro, determines the promoter preferences of TEF and DBP.

Tension increase correlated to refractive-index change in fibers containing UV-written Bragg gratings.

A strong axial tension increase induced by UV laser radiation is observed in the cores of single-mode optical fibers containing Bragg gratings, independently of the initial core stress. The induced index modulation of the gratings is linearly correlated to stress changes with a slope of (0.8 +/- 0.2) x 10(-4) mm(2)/kg. The phenomenon can be explained by a structural change of the glass in the fiber core into a more compact configuration.

The role of the transcriptional activator protein DBP in circadian liver gene expression.

DBP, a liver-enriched transcriptional activator protein of the leucine zipper protein family, accumulates according to a very strong circadian rhythm (amplitude approx. 1000-fold). In rat parenchymal hepatocytes, the protein is barely detectable during the morning hours. At about 2 p.m., DBP levels begin to rise, reach maximal levels at 8 p.m. and decline sharply during the night. This rhythm is free-running: it persists with regard to both its amplitude and phase in the absence of external time cues, such as daily dark/light switches. Also, fasting of rats for several days influences neither the amplitude nor the phase of circadian DBP expression. Since the levels of DBP mRNA and nascent transcripts also oscillate with a strong amplitude, circadian DBP expression is transcriptionally controlled. While DBP mRNA fluctuates with a similar phase and amplitude in most tissues examined, DBP protein accumulates to high concentrations only in liver nuclei. Hence, at least in nonhepatic tissues, cyclic DBP transcription is unlikely to be controlled by a positive and/or negative feedback mechanism involving DBP itself. More likely, the circadian DBP expression is governed by hormones whose peripheral concentrations also oscillate during the day. Several lines of evidence suggest a pivotal role of glucocorticoid hormones in establishing the DBP cycle. Two genes whose mRNAs and protein products accumulate according to a strong circadian rhythm with a phase compatible with regulation by DBP encode enzymes with key functions in cholesterol metabolism: HMG-coA reductase is the rate-limiting enzyme in cholesterol synthesis; cholesterol 7-alpha hydroxylase performs the rate-limiting step in the conversion of cholesterol to bile acid.(ABSTRACT TRUNCATED AT 250 WORDS)