Chirped-pulse-amplification circuits for fiber amplifiers, based on chirped-period quasi-phase-matching gratings

A new type of compact chirped-pulse-amplification circuit for high-power amplification of femtosecond pulses in an optical fiber is demonstrated. This circuit is based on a novel pulse compressor, chirped-period quasi-phase-matching gratings in electric-field-poled lithium niobate. The main advantages of this circuit are simplicity, the small number of components, compactness, and wavelength conversion of Er-doped fiber output to the technologically important 780-nm wavelength region.     

High-spin states in99Tc

The level scheme of⁹⁹Tc was studied byγ-ray spectroscopy with the⁹⁶Zr(⁶Li, 3n γ)⁹⁹Tc reaction. New levels were established and spins and parities assigned.⁹⁹Tc is compared with^{93, 95, 97}Tc, with the neighbouring even-even Mo-isotopes and with the results of an IBFM calculation.     

Extended-ion approximation and pressure shifts inF-band energies

Pressure variation of maximumF-band absorption energies in the halides of lithium, sodium and potassium has been investigated employing the extended-ion approximation for the calculation of theF-electron energy eigenvalues and using values of local compressibility in the neighbourhood of theF-centres which include the effect of vacancy and pressure. The results obtained agree with the experimental results.     

Wave Node Dynamics and Revival Symmetry in Quantum Rotors

Symmetries and dynamics of wave nodes in space and time expose principles of quantum theory and its relativistic underpinning. Among these are key principles behind recently discovered dephasing and rephasing phenomena known as revivals. A reexamination of basic Eberly revivals, Berry “quantum fractal” landscapes, and the “quantum carpets” of Schleich and co-workers reveals a simple Farey arithmetic and C_n-group revival structure in one of the earliest quantum wave models, the Bohr rotor. These principles may be useful for interpreting modern time-dependent rovibrational spectra. The nodal dynamics of the Bohr rotor is seen to have a quasi-fractal structure similar to that of earlier systems involving chaotic circle maps. The fractal structure is an overlay of discrete versions of Bohr's rotor model. Each N-point Bohr rotor acts like a base-N quantum “odometer” which performs rational fraction arithmetic. Such systems may have applications for optical information technology and quantum computing.     

EXCITED STATES AND PHOTOBIOLOGICAL PROPERTIES OF POTENTIAL DNA CROSS-LINKING AGENTS, THE BENZODIPYRONES

Abstract —Potential DNA cross-linking agents, benzo{1,2-b:4,5-b'}dipyran-2,8-dione ( cis -benzodipyrone) and benzo{1,2-b:4,5-b'}dipyran-2,7-dione ( trans -benzodipyrone), have been synthesized. The excited triplet states of these reagents resemble those of other coumaryl derivatives in their spectroscopic assignments and characteristics and thus their reactivites. In particular, trans -benzodipyrone is significantly more reactive toward pyrimidine bases in solution than the other coumaryl derivatives examined, by virtue of its possessing two reactive pyrone C=C bonds within the molecular framework. The unique reactivity of trans -benzodipyrone, with its two reactive pyrone bonds across the long molecular axis, appears to be reflected in its ability to photoinduce mutation in certain Bacillus subtilis strains. This preliminary study provides data that will be useful in designing experiments to elucidate the molecular mechanisms for the biological activities of these compounds.     

PHOTOREACTIVITY OF HYDROXYPSORALENS AND THEIR PHOTOBIOLOGICAL EFFECTS IN BACILLUS SUBTILIS*

Abstract— Although psoralen and many substituted psoralens are potent skin-photosensitizing agents, hydroxypsoralens are not. A satisfactory molecular interpretation of this structural specificity has been given in terms of dissociation of the hydroxyl group of 5- and 8-hydroxypsoralens in their excited states. The dissociation process in the S₁ state effectively competes with S₁→T₁ intersystem crossing, thus reducing the photoreactive T₁ population. The T₁ states of the anions are more delocalized than those of neutral psoralens so that they are less reactive toward photocycloaddition with pyrimidine bases of DNA. The lack of significant phosphorescence of hydroxypsoralens in ionizing solvent or in the presence of base at low temperatures (14–77 K.) indicates ineffective S₁→T₁ and/or effective T₁→S₀ intersystem crossing. These factors make hydroxypsoralens unreactive, electronically and kinetically, as skin photosensitizers, which are known to react with DNA. In correlation with the hydroxypsoralens' spectroscopic characterization, they are also found to be ineffective or less effective photosensitizers in Bacillus subtilis.     

Ultrawide tunable Er soliton fiber laser amplified in Yb-doped fiber

A Raman-shifted and frequency-doubled high-power Er-fiber soliton laser for seeding an efficient high-power Yb fiber femtosecond amplifier is demonstrated. The Raman-shifted and frequency-doubled Er-soliton laser is tunable from 1.00 to 1.070microm and produces bandwidth-limited 24-pJ pulses at a repetition rate of 50 MHz with a FWHM pulse width of 170 fs at 1.040microm . The Yb(3+) amplifier has a slope efficiency of 52% and generates 3-ps linearly chirped pulses with an average power of 0.8 W at 1.05microm . After pulse compression, 74-fs bandwidth-limited pulses with an average power of 0.4 W and a pulse energy of 8 nJ are generated.     

Diagrammatic structure of the general coupled cluster equations

The general formula for the number of diagrammatic terms occurring in the T_n equation within a particular coupled cluster model is derived. Both the antisymmetrized and Goldstone diagrams are considered. In addition to the full coupled cluster equation approximate approaches are discussed, and for each the general formula for the number of terms is given. Analogous expressions are presented for the number of diagrammatic terms contributing to the elements of the transformed Hamiltonian [Hbar] = e^?T He^T .     

Frequency doubling of femtosecond erbium-fiber soliton lasers in periodically poled lithium niobate

We report efficient frequency doubling of passively mode-locked femtosecond erbium-fiber lasers. Quasi-phase-matched second-harmonic generation in periodically poled lithium niobate is used to generate 8.1 mW of 190-fs (FWHM), 90-pJ pulses at 777 nm with a conversion efficiency greater than can be obtained with existing birefringently phase-matched nonlinear materials. A dispersion-compensation-free soliton oscillator generating transform-limited 230-fs (FWHM) pulses at 1554 nm is used as a pump laser.