B. Rácz

Group velocity dispersion in prisms and its application to pulse compression and travelling-wave excitation

Abstract In a beam propagating through a dispersive prism a delay of the pulse front occurs. A general expression relating the delay and the angular dispersion was found. Using a streak camera this delay was experimentally observed. In practical systems the delay varies from a few to several tens of picoseconds. A pulse compressor equivalent to the Treacy-compressor, based on this delay is described. A novel scheme for the excitation of a travelling-wave picosecond amplified spontaneous emission is also proposed.

Bandwidth limited amplification of 220 fs pulses in XeCl

Abstract Transform limited pulses of 200–250 fs duration at 308 ns generated by a specially designed excimer laser pumped dye laser system have been amplified in a XeCl gain module. Simultaneous study of the spectrum and the temporal behaviour of the amplified UV pulses showed bandwidth limitation by the gain spectrum of XeCl. This effect is strongly dependent on the operational condition of the amplifier; in a single pass amplification scheme 0.5 mJ, 250 fs pulses, in a double pass arrangement 5 mJ, 220 fs pulses were obtained, with excellent reproducibility.

Femtosecond pulse front tilt caused by angular dispersion

Femtosecond pulse fronts suffer a time delay across the beam when propagating through diffraction gratings or dispersive prisms. A general relation was found describing the tilt angle γ of the pulse front as, tanγ=λ d e/ d λ where d e/ d λ is the angular dispersion of the grating or prism and λ is the central wavelength of the pulse. The expression is valid for any spectral device having angular dispersion (e.g., Fabry-Perot interferometer, Lummer-Gehrcke plate, and Michelson echelon). The tilt angle is shown to have a close relation to the classical uncertainty principle.

Plume emission, shock wave and surface wave formation during excimer laser ablation of the cornea.

Excimer lasers are now used for corneal surgery; however, the physical processes occurring during photoablation of the cornea are incompletely understood. High speed laser-based photographic arrangement was constructed. The temporal resolution was better than 1 ns. The setup could work as a Schlieren arrangement, which is sensitive to the refractive index change caused by the shock wave propagating in the air above the eye. With minor changes the setup was converted into a shadowgraph, which could detect the ablation plume and the waves propagating on the surface of the eye. Due to the impact of the excimer laser pulse onto the surface of the cornea, a shock wave was generated in the air. The shadowgraph clearly showed the ejection of the ablated cornea. The ejection velocity of the plume was found to be over 600 m/s. It was shown for the first time that the recoil forces of the plume are generating a wave on the surface of the eye. The laser-based high speed photographic arrangement is a powerful arrangement in the study of physical effects occurring during photoablation of the cornea.

Group refractive index measurement by Michelson interferometer

Abstract A Michelson interferometer illuminated by a polychromatic light source was used to measure the time of flight of a light pulse (or intensity substructure) through a sample of optical material. In this way the group index of optical materials (fused silica and Schott glass F3) was measured with an accuracy of 10 -4 . The method can be also applied for measurement of the dispersive properties of optical fibers.

Femtosecond transient reflection from polymer surfaces during femtosecond UV photoablation

A subpicosecond KrF laser system (248 nm, 0.5 ps) was used as a light source for ablation of PMMA Mylar and Kapton. The time-dependent reflectivity of the light-induced plasma mirror as measured by 496 nm, 0.5 ps long probe pulses showed an increase of up to 94% with 0.4–1 ps rise time and 10–15 ps fall time. The highdensity plasma mirror shows perfect optical quality, and seems to be a promising light-controlled ultrafast switch for UV and visible light. The spectrum of the UV light reflected from the ablated spot is blue shifted by 0.5 nm and shows 1 nm broadening.

Comparative study of beam expanders used in nitrogen laser pumed dye lasers

Abstract Four methods of beam expanding in nitrogen laser pumped dye lasers were investigated, and compared. The most important parameters of beam expanders were considered i.e. the magnification, and the efficiency of feedback. Besides the optimalization of multiple-prism beam expanders, a new combined beam expander was studied, and optimalized. This combined, prism-grating beam expander produced high efficiency of feedback in a wide range of magnifications and easily produced single mode operation with 4% energy conversion efficiency.

Bandwidth limited amplification of 220 fs pulses in XeCl: Theoretical and experimental study of temporal and spectral behavior

Abstract The spectral structure of the gain-profile of XeCl was found to strongly influence the spectral and temporal characteristics of amplified ultrashort light pulses. A simple model was used to numerically study both effects as a function of amplifier length and input pulse energy. The results are in good agreement with experiments. With these data the performance of a XeCl amplifier was optimized for shortest pulse duration with minimum distortion.

Ultrashort pulse generation by distributed feedback dye lasers. I: Temporal characteristics

The N2-laser pumped distributed feedback dye laser is a new type of picosecond light source. A rate equation model describing the ultrashort pulse formation is presented. The temporal behavior of the distributed feedback dye laser predicted by the model has been compared with experimental results obtained with a streak camera system. The observed good agreement allows application of the theoretical model for the engineering of an inexpensive picosecond laser

Generation of 320 fs pulses with a distributed feedback dye laser

A new achromatic distributed feedback dye laser (DFDL) arrangement is described. The experimental conditions for subpicosecond pulse generation with the new device were investigated. For the first time, stable generation of subpicosecond pulses (τ≦350 fs) at 616 nm was achieved with a DFDL. The simultaneous spectral and autocorrelation measurements showed that the amplified DFDL pulses are nearly transform limited, having a pulse form close to the sech2 shape.