Optical fiber transmission of multipleQ-switch Nd: YAG laser pulses with microsecond interpulse separations

Up to sixQ-switch pulses with interpulse separations of between 2 and 100 s are extracted during single flashlamp pulses of a Nd:YAG laser (1.06 m) by opening the Pockels cell in up to six stages. The interpulse separations and output energies are adjustable for the individual pulses of the 10 Hz repetitive laser bursts. Measurements of maximum fiber transmission and stimulated backscattering are reported for 1 mm diam fused-silica step-index fibers.     

A simple current pulsed low-pressure CO2 laser with passive Q-switching

We report on a simple and stable pulse tunable CO₂ laser suitable for many investigations in the region around 10m. The pulsed discharge when combined with the passiveQ-switching technique provides pulses of 100 ns duration with a peak power of few kilowatts.Due to the interest in pulsed low-pressure CO₂ lasers as useful irradiation sources, extensive investigations have been carried out. In particular, various Q-switching techniques have been developed to generate short CO₂ laser pulses [1–4].     

Study ofγ radiation from100Pd decay

This paper presents a study of the rays from the electron capture decay of 3.63d ¹⁰⁰Pd. Singles and coincidence measurements provided an accurate determination of-ray intensities and an improved knowledge of the¹⁰⁰Rh level scheme. Experimental evidence supports excitation of levels in¹⁰⁰Rh at 86-, 136- and 154 keV, not previously reported from the decay of¹⁰⁰Pd.On leave from Departamento de Fisica, Comision Nacional de Energia Atomica, Buenos Aires, Argentina     

Generation of short light pulses under the stimulated Brillouin scattering in fibers with an optimal feedback

The generation of short light pulses (1 ns) in single mode fibers under pumping by wide laser pulses (of a microsecond duration) due to the backward stimulated Brillouin scattering (SBS) is numerically investigated. The influence of the acoustic diffraction is taken into account. The cases of acoustic waveguide and anti-waveguide fibers are considered. For an acoustic anti-waveguide fiber, a dependence of overlap integral S on the acoustic mode number n has a sharp peak in the region of n 100. Computer simulations have demonstrated the energy conversion of the pump wave into short pulses of the signal (Stokes) wave in the case of synchronous pumping. The optimal length of the fiber should be approximately equal to the half-length of the pump pulse. The bypass time of the Stokes pulse of the optimal circuit fiber and the feedback loop must be equal to the repetition period of the pump pulse. An importance of acoustic mode structure of the fiber for the process of forming pulse train in shown. We have found that the acoustic anti-waveguide fibers with a small core (a < 3 m) can be preferable for obtaining the stable train of compressed pulses.     

Sub-picosecond UV-laser ablation of Ni films

Laser ablation of thin Ni films on fused silica by 0.5 ps KrF-excimer-laser pulses at 248 nm is reported. The onset of material removal from different film thicknesses (0.1, 0.3, 0.6 and 1.0 m) was measured in a laser ionization time-of-flight mass spectrometer by the amount of Ni atoms vs laser fluence. Significant amounts of metal atoms are already evaporated at laser fluences around 20 mJ/cm², a threshold up to 100 times smaller compared to the one for 14 ns pulses. In contrast to ns laser pulses, the ablation threshold for 0.5 ps pulses is independent of the film thickness. These results reflect the importance of thermal diffusion in laser ablation of strongly absorbing and thermally good conducting materials and prove that for ablation with short pulses, energy loss to the bulk is minimized.     

High-repetition-rate hard X-ray generation with sub-millijoule femtosecond laser pulses

Hard X-ray radiation is generated at 1-kHz repetition rate on metal targets using 30-fs sub-millijoule laser pulses. Spinning-disc targets of medium-Z (Ti, Cr, Fe, Ni, Cu, Zn and Mo) and high-Z (Ta) metals are investigated. For medium-Z targets, characteristic K-shell emission (K and K) is observed in the 4–20 keV energy range in addition to a broadband bremsstrahlung background. Whereas in former studies similar results have been obtained by applying laser pulses in the tens-to-hundreds-of-millijoules range, we observe characteristic X-ray generation even at applied pulse energies as low as 100 J. The well-defined emission wavelength, the high intensity and the high brightness of this radiation makes this source a promising tool for time-resolved experiments with high-repetition-rate (10 kHz) small-scale table-top laser systems . PACS 42.65.-k; 52.38.Ph; 52.50.Jm     

Actively mode-locked 1.3 and 1.55 μm InGaAsP diode lasers

Active mode-locking of uncoated InGaAsP diode lasers having an external diffraction grating cavity was investigated experimentally. A high frequency r.f. signal and short-duration electrical pulses were used to drive the lasers. The pulse duration was measured by an ultrafast streak camera. Pulses as short as 13 ps at 1.3m and 29 ps at 1.55m were generated at a repetition rate of 1 GHz. The reason for obtaining broader pulses from the 1.55m laser which had the same structure as the 1.3m diode laser is explained.Formerly with GEC Hirst Research Centre, Wembley, Middlesex, UK.     

Weyl quantization and metaplectic representation

The formal expansion defining the twisted exponential of an element of the Lie algebra _n ( ⁿ Sp(2, )) can be summed and this result is used to explicitly obtain the classical function u _t corresponding to an evolution operator associated to a quantum Hamiltonian belonging to the above mentioned Lie algebra.Then, by applying the Weyl quantization procedure to u _t we get a representation of the group W _n ( ⁿ Sp(2, )) in terms of integral operators, the kernels of which are expressed by means of the classical action. The family u _t being only locally defined, it must be considered as a distribution on the classical phase space in order to get the metaplectic representation.Membres du Centre National de la Recherche Scientifique.     

Tunable sub-100 femtosecond dye-laser pulses generated with a nanosecond pulsed pumping

Subpicosecond pulses at a fixed wavelength produced with a low-Q cavity dye laser pumped by a single, nanosecond laser (Q-switched Nd:YAG) are converted into tunable high-power sub-100 femtosecond pulses by generation, spectral selection, amplification and compression of a supercontinuum. The tunable, chirped, high-energy pulses obtained are compressed with a prism pair. Energies up to 50 J in sub-100 fs pulses were obtained in the 540 to 650 nm range using 40 mJ of the Nd: YAG-laser pumping pulses at 532 nm. The whole sub-100 fs system including the low-Q dye laser uses only one Nd:YAG laser as a pump source.     

Flow effects on jet profiles and multiplicities

We study the effects of low- p_T collective flow on radiative energy loss from high- p_T partons traversing the QCD medium created in high-energy nucleus-nucleus collisions. We illustrate this idea through three examples. Due to longitudinal flow, jet profiles at the LHC present marked asymmetries in the -plane, and widths in and of particle distributions associated with a high- p_T trigger at RHIC become different. Finally, transverse flow implies an increase of high- p_T v₂ at RHIC.Arrival of the final proofs: 17 March 2005PACS: 25.75.Ld - 24.85. + p - 25.75.Gz Present address: Departamento de Física de Partículas and Instituto Galego de Altas Enerxías, Facultade de Física, Campus Sur, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain.     

Femtosecond-pulse laser ablation of human corneas

A femtosecond pulse laser in the visible spectral region shows promise as a potentially new powerful corneal sculpting tool. It combines the clinical and technical advantages of visible wavelengths with the high ablation quality observed with nanosecond-pulse excimer lasers at 193 nm. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs and 7 ns, and centre wavelengths at 615 nm and 600 nm, respectively, both focused to an area of the order of 10^–5 cm², have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30–100 m range from the excision edge at all fluences above the ablation threshold of F _th60 J cm^–2 (I _th9 GW cm^–2). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterised by damage zones of less than 0.5 m were observed at all fluences above ablation threshold of F _th1 J cm^–2 or I _th3 TW cm^–2 (3×10¹² W cm^–2). The transparent cornea material can be forced to absorb ultrashort pulses of extremely high intensity. The fs laser generates its own absorption by a multiphoton absorption process.     

The effects of slippage and diffraction in long wavelength operation of a free electron laser

The Free-Electron Laser user facility FELIX produces picosecond optical pulses in the wavelength range of 5–110m. The proposed installation of a new undulator with a larger magnetic period would allow extension towards considerably longer wavelengths. This would result in the production of extremely short, far-infrared pulses, with a duration of a single optical period or even less. In order to investigate the pulse propagation for free-electron lasers operating in the long wavelength limit, a three-dimensional simulation code was developed. Using the FELIX parameters, with the addition of a long-period undulator, the effects of slippage, diffraction losses, changes in the filling factor, as well as the effects of the optical cavity geometry were studied for wavelengths up to 300m, with electron pulses in the ps regime. It is shown that slippage effects are less restrictive for long wavelength operation than the increasing losses due to optical beam diffraction.     

Quantization on the Virasoro group

The quantization of the Virasoro group is carried out by means of a previously established group approach to quantization. We explicitly work out the two-cocycles on the Virasoro group as a preliminary step. In our scheme the carrier space for all the Virasoro representations is made out of polarized functions on the group manifold. It is proved that this space does not contain null vector states, even forc1, although it is not irreducible. The full reduction is achieved in a striaghtforward way by just taking a well defined invariant subspace _{(c, h)} , the orbit of the enveloping algebra through the vacuum, which is irreducible for any value ofc andh. _{(c, h)} is a proper subspace of the space of polarized functions for those values ofc andh for which the Kac determinant is zero. We give the local version of these group representations as well as the associated classical phase space structures, i.e., symplectic form and Noether invariants.Research partially supported by the Conselleria de Cultura de la Generalitat Valenciana, the Plan de formación del Personal investigador, the Comision Interministerial de Ciencia y Tecnologia (CICYT) and the British Council     

Generation of picosecond and subpicosecond light pulses with saturable absorbers

Single light pulses, generated by a mode-locked Nd-glass laser, were shortened with saturable absorbers of low initial transmissionT ₀. The pulse duration was reduced from 8 to 2.6 ps after a single pass through a dye cell ofT ₀=10^–7. Light pulses as short as 0.5 ps were observed after five transits through an absorberamplifier system. Detailed calculations of the stationary and the transient situation (with respect to the dye relaxation time) are presented to demonstrate optimum conditions for the pulse shortening.     

L'asymptotique de Weyl pour les bouteilles magnétiques

Nous prouvons une formule pour le comportement asymptotique de la fonctionN() de dénombrement des valeurs propres de l'opérateur de Schrödinger avec un champ magnétique qui tend vers l'infini `a l'infini de ^d . La preuve utilise un résultat précis sur l'estimation des valeurs propres pour un champ magnétique constant dans un cube de ^d.     

Experimental demonstration of a photonic-crystal-fiber optical diode

Two cascaded hollow-core photonic-crystal fibers with slightly shifted, but still overlapping, transmission peaks are shown to function as an optical diode for ultra-short laser pulses. Submicrojoule 100-fs Ti:sapphire laser pulses with a spectrum falling within the passband of one of the fibers, but outside the passband of the second fiber, experience spectral broadening due to self-phase modulation in the first fiber. A part of this self-phase-modulation-broadened spectrum is then transmitted through the second fiber. Identical short pulses propagating in the opposite direction are blocked by the second fiber with a shifted passband. A forward-to-backward signal ratio exceeding 40 is achieved with the created photonic-crystal fiber diode for 0.9-J, 100-fs pulses of 800-nm Ti:sapphire laser radiation. PACS 42.65.Wi; 42.81.Qb     

High-energy long-pulse Nd:YAG picosecond laser

A long train of actively mode-locked pulses is obtained from a negative feedback controlled Nd:YAG laser. The 70s pulse train contains up to 0.1 J energy and the duration of the picosecond pulses is 120 ps. The laser is operated at a repetition rate of 20 Hz. ActiveQ-control of the cavity generates a short pulse train of duration 50 to 70 ns. An excellent peak-to-background intensity ratio of the correlation function for the picosecond pulses in the train of 5×10³ is reported.     

Four-wave mixing in alkali halide crystals and aqueous solutions

Noncollinear phase-matched nonresonant four-photon frequency mixing _p+ _p– _{L s} in crystals and aqueous solutions of LiCl, CsCl, KF, and KI is studied. The concentration of the aqueous solutions is varied between 0.5 mol/l and saturation. Picosecond laser pulses of a mode-locked Nd-glass laser are applied as pump pulses. The energy conversion of laser light at frequency _L to frequency _S is measured and the nonlinear susceptibilities ⁽³⁾ are calculated. The dependence of the hyperpolarizabilities on concentration is analysed and gives information on the solute-solvent interaction.     

Pulsed operation of a liquid metal ion source

Short ion pulses (100 ns FWHM) have been produced from an In liquid metal ion source (LMIS) of the needle type by superimposing short voltage pulses (500 ns) onto a subcritical extraction field. Extractor-pulsed LMIS are thought to be useful as primary ion sources in time-of-flight (TOF) mass spectrometers, particularly for space applications, where extended charge life time and reduced power consumption are of great importance.     

Narrow-band seeding of an optical parametric amplifier in the femtosecond regime

Nearly transform limited femtosecond pulses tunable between 2.56 and 3.16 m have been generated by optical parametric amplification. The single stage parametric amplifier is pumped by a tunable high power femtosecond Ti:sapphire laser system at 1 kHz repetition rate and seeded by quasi-continuous wave (cw) radiation from the Q-switched Nd:YLF laser used to pump the regenerative amplifier. The 100 fs idler pulses are shorter than the pump pulses. The mechanism of the achieved pulse compression is discussed and experimental results are compared with numerical simulations.