T Andersen

Supercontinuum generation in a photonic crystal fiber with two zero dispersion wavelengths

We demonstrate supercontinuum generation in a highly nonlinear photonic crystal fiber with two closely lying zero dispersion wavelengths. The special dispersion of the fiber has a profound influence on the supercontinuum which is generated through self-phase modulation and phasematched four-wave mixing and not soliton fission as in the initial photonic crystal fibers. The supercontinuum has high spectral density and is extremely independent of the input pulse over a wide range of input pulse parameters. Simulations show that the supercontinuum can be compressed to ultrashort pulses.

Supercontinuum generation by femtosecond single and dual wavelength pumping in photonic crystal fibers with two zero dispersion wavelengths

We investigate supercontinuum generation in photonic crystal fibers under femtosecond single and dual wavelength pumping experimentally and by numerical simulations. Details about the expansion of the blue but also the red side of the continuum due to cross-phase modulation (XPM) and transfer of energy to dispersive waves are revealed and experimentally confirmed. Additionally, simple guidelines are given to predicte the maximum bandwidth of supercontinuum generation only by the use of the dispersion curve of the fiber.

Continuous-wave wavelength conversion in a photonic crystal fiber with two zero-dispersion wavelengths.

We demonstrate continuous-wave wavelength conversion through four-wave mixing in an endlessly single mode photonic crystal fiber. Phasematching is possible at vanishing pump power in the anomalous dispersion regime between the two zero-dispersion wavelengths. By mixing appropriate pump and idler sources, signals in the range 500-650 nm are obtained in good accordance with calculated phasematching curves. The conversion efficiency from idler to signal power is currently limited to 0.3% by the low spectral density of the pump and idler sources at hand, but can be greatly enhanced by applying narrow line width lasers.

Systematic trends in atomic transition probabilities in neutral and singly-ionized zinc, cadmium and mercury

Abstract The beam-foil technique has been used to measure mean lives of excited levels in neutral and singly ionized zinc, cadmium, and mercury. In the neutral atoms, the lifetimes measured were in good agreement with those obtained by the Hanle-effect and phase-shift techniques, but discrepancies were found in some of the life-times measured by the delayed-coincidence method. For singly ionized zinc and cadmium, six mean lives have been measured, while seven were obtained in singly ionized mercury. For the ionized species, systematic trends show that some of the results recently obtained by the phase-shift method may be in error. The mean lives reported were converted to ; values for transitions for which reliable multiplet intensity ratios are available. The ; value systematics show that for the same type of spectral transition, identical ; values have been obtained within homologous atoms.

High repetition rate tunable femtosecond pulses and broadband amplification from fiber laser pumped parametric amplifier

We report on the generation of high energy femtosecond pulses at 1 MHz repetition rate from a fiber laser pumped optical parametric amplifier (OPA). Nonlinear bandwidth enhancement in fibers provides the intrinsically synchronized signal for the parametric amplifier. We demonstrate large tunability extending from 700 nm to 1500 nm of femtosecond pulses with pulse energies as high as 1.2 muJ when the OPA is seeded by a supercontinuum generated in a photonic crystal fiber. Broadband amplification over more than 85 nm is achieved at a fixed wavelength. Subsequent compression in a prism sequence resulted in 46 fs pulses. With an average power of 0.5 W these pulses have a peak-power above 10 MW. In particular, the average power and pulse energy scalability of both involved concepts, the fiber laser and the parametric amplifier, will enable easy up-scaling to higher powers.

Measurements of atomic lifetimes for neutral and ionized magnesium and calcium

Abstract The beam-foil excitation technique has been used to measure mean-lives of excited atomic states in MgI, MgII, CaI, and CaII. The mean-lives of the 3p 1P° states in MgI and the 4p 1P° state in CaI have been measured to be 2.2±0.2×10-9 sec and 6.2±0.5×10-9 sec, respectively. The discrepancies with some of the earlier published data for CaI have been attributed to the anomalously high cross section for broadening of the resonance line. For some of the doublets in MgII and CaII, mean-lives have been obtained for both transitions of the doublets, and the intensity ratios have been checked. Oscillator strengths have been evaluated and compared with theoretical predictions.

Excitation of Li and Na in collisions with He and Ne: measurements of Li I and Na I resonance-line emission and polarization

Total emission cross sections and polarizations have been measured for the 22S-22P, 22P-32D and 22P-32S Li I multiplets in Li-He, Ne collisions, and the 32S-32P and 32P-32D Na I multiplets in Na-He, Ne collisions in the 0.6-60 keV energy range. The excitation of the alkali resonance multiplet is found to be the dominant inelastic process for all four collision systems. The results are compared with recent calculations by Manique et al. (1977) based on a quasi-one-electron description using various model potentials. Good agreement is found for Li-He with a simple Hartree-Fock frozen-core potential. The agreement is fair for Li, Na-He calculations with a Baylis-type potential but poor for calculations based on Bottcher-model potentials. For Ne as target all three potentials overestimate by far the size of the cross section.

Atomic-Lifetime Measurements for Neutral and Ionized Sodium and Potassium

The beam-foil excitation technique has been used to measure mean lives of excited atomic states in Na i, Na ii, Na iii, K ii, and K iii. In agreement with earlier measurements, a mean life of 17.3±1.0×10−9 s has been obtained for the unresolved 3s–3p doublet transition in Na i. The mean lives of the 3p5 4p fine-structure levels in K ii have been measured and compared with earlier measurements in Ar i. Based on the observed mean lives, two spectral lines (2387 and 2394 A), previously assigned as Na ii lines, have been assigned as transitions in Na iii.

Radiative lifetimes of excited levels in three-electron systems: Ga I, Ge II, As III, Se IV and In I, Sn II, Sb III, Te IV

Comparison of theoretical and experimental lifetimes has been performed for excited levels in the Ga I and In I isoelectronic sequences in order to test the applicability of radiative lifetimes based upon semiempirical calculation methods. The theoretical lifetimes are obtained from (i) calculations using a single-parameter numerical Coulomb approximation, for which a consistent cut-off criterion has been developed, and (ii) published data based upon a parametrised Dirac radial equation. Twenty-five experimental lifetimes for Ge II, As III, Se IV and Sn II, Sb III, Te IV have been measured by means of the beam-foil technique. Good agreement is obtained between theoretical and experimental lifetimes for higher-lying levels, whereas deviations are present particularly for low-lying nd 2D levels.

The beam-foil excitation technique applied to heavy ion beams (20<Z<81)

The beam-foil excitation technique applied to heavy elements (20<Z<81) has been studied by means of a 600-kV heavy-ion accelerator, equipped with a universal ion source. The emission spectra of the foil-excited ion beams have been studied, and intensity ratios of resolved doublet and triplet components have been measured. A comparison with theoretical predictions and earlier experimental data indicates a lack of thermodynamical equilibrium at high initial ion energies. Scattering in the carbon foils has been studied by measuring angular distributions of penetrating radioactive 85Kr ion beams. The experimental reduced scattering angle shows satisfactory agreement with theoretical estimates, based on a Thomas-Fermi potential. In Zn, Sr I and Ba I, a false cascading tail has been observed for resonance transitions, giving rise to large systematic errors. It has been attempted to reveal the causes of this cascading by studying lifetimes as a function of initial energy and ion current.