Two-octave spectral broadening of subnanojoule Cr:forsterite femtosecond laser pulses in tapered fibers

Spectral broadening of femtosecond Cr:forsterite laser pulses is enhanced due to the use of tapered fibers. Supercontinuum generation with unamplified subnanojoule femtosecond Cr:forsterite laser pulses is observed for the first time. With 40-fs 0.6-nJ pulses of 1.25-μm Cr:forsterite laser radiation coupled into a tapered fiber having a taper waist diameter of about 2 μm and a taper waist length of 90 mm, we observed the spectra spanning more than two octaves at the output of the fiber in the regime of anomalous group-velocity dispersion. This result opens the way for the creation of compact femtosecond Cr:forsterite laser plus tapered fiber systems for optical metrology and biomedical applications. Received: 23 October 2001 / Accepted: 16 January 2002 / Published online: 14 March 2002     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

Deep drilling of metals by femtosecond laser pulses

Results of recent investigations on deep drilling of metals by femtosecond laser pulses are reported. At high laser fluences, well above the ablation threshold, femtosecond lasers can drill deep, high-quality holes in metals without any post-processing or special gas environment. It is shown that for high-quality drilling of metals, the following processes are important: (1) laser-induced optical breakdown of air containing metal vapor and small metal particles (debris) generated by multi-pulse femtosecond laser ablation, (2) transformation of laser pulses into light filaments, and (3) low-fluence finishing. Received: 15 November 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/2788-100, E-mail: ch@lzh.de     

Spectral broadening of 40-fs Ti:sapphire laser pulses in photonic-molecule modes of a cobweb-microstructure fiber

Photonic-molecule modes of a cobweb-microstructure fiber allow efficient nonlinear optical spectral broadening of nanojoule femtosecond light pulses. Spectral widths of approximately 200 nm are achieved at the output of a 6-cm sample of such a fiber for 40-fs Ti:sapphire laser pulses with an energy of a few nanojoules coupled into photonic-molecule modes. Higher values of the group index and a lower group-velocity dispersion, attainable with higher-order photonic-molecule modes, allow the efficiency of spectral broadening of femtosecond laser pulses to be increased relative to the efficiency of spectral broadening in the fundamental photonic-molecule mode. Received: 9 June 2002 / Revised version: 29 June 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/939-3959, E-mail: zheltikov@top.phys.msu.su     

An all-solid-state laser system for generation of 100 μJ femtosecond pulses near 625 nm at 1 kHz

Frequency doubling the output of a high-power femtosecond Cr:forsterite regenerative amplifier with >50% conversion efficiency in a temperature-tuned noncritically phase-matched LBO crystal produces femtosecond pulses of >100 μJ energy in the visible range near 625 nm at a pulse duration of about 200 fs or >65 μJ at <170 fs. Received: 29 March 1999 / Revised version: 27 April 1999 / Published online: 24 June 1999     

Concatenation of plasma filaments created in air by femtosecond infrared laser pulses

We report the first observation of the attachment of two single plasma filaments created collinearly in the atmosphere by IR femtosecond laser pulses. The linked filamentary structure is electrically conductive and emits sub-THz radiation over its entire length. Concatenation is achieved only for a specific time ordering between the two initial laser pulses. The pulse producing the filament closer to the laser source must be retarded with respect to the other pulse. This special time ordering is attributed to the acceleration of light in a self-guided pulse. Received: 4 March 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +33-1/6931-9996, E-mail: stzortz@ensta.fr     

Travelling pulses in reaction-diffusion systems under global constraints

The influence of global inhibition on travelling pulses both in infinite and in finite periodic systems is investigated. Analytic investigation of the modified piecewise linear Rinzel-Keller-Model (RKM) shows a shrinking and slowing down of pulses due to global inhibition. Analytic results obtained are confirmed by numerical simulations. The investigation is completed by numerical simulations of the modified Oregonator model for the light-sensitive BZR. Received: 25 September 1997 / Revised: 9 January 1998 / Accepted: 9 January 1998     

Simultaneous measurement of pure-optical and thermo-optical nonlinearities induced by high-repetition-rate, femtosecond laser pulses: application to CS2

A simple experimental technique is presented capable of separating the contribution of purely optical Kerr effects from that of thermo-optical effects in the nonlinear response of materials under high-repetition-rate laser irradiation. The technique has been realized by combining the single-beam Z-scan method with the single-beam thermal lens measurement method. We demonstrate this technique by analysing the nonlinear response at 770 nm of CS₂ which exhibits cumulative thermal effects when irradiated by very intense femtosecond laser pulses at a 76-MHz repetition rate. Received: 3 November 1998 / Revised version: 4 January 1999 / Published online: 2 June 1999     

Generation of femtosecond pulses by two-photon pumping supercontinuum-seeded collinear traveling wave amplification in a dye solution

Visible femtosecond (fs) laser pulses have been obtained in a dye solution with a very simple traveling wave collinear configuration. A femtosecond Ti:sapphire laser (790 nm) pumps the dye solution by two-photon absorption and simultaneously generates supercontinuum, which seeds a light-amplification mechanism. Cross-correlation frequency-resolved optical gating measurements reveal a chirped structure in the dye pulse. The shortest pulse duration achieved is 170 fs and the overall energy efficiency of the process is typically 25%. Received: 22 March 2001 / Revised version: 4 July 2001 / Published online: 19 September 2001     

Gain dynamics of two-wavelength Ti:sapphire femtosecond laser

We report the theory and experiments of a two-wavelength bi-tuning femtosecond Ti:sapphire laser. The cross-mode-locked gain competition is described with numerical solution and operation of the laser is accomplished in which two trains of femtosecond pulses cross in the Ti:sapphire rod. For this reason, a cross-bearings technique is used. The laser resonator is a six-mirror bi-cavity, and two beams of pump lights cross in Ti:sapphire crystal. Two trains of sub-30 fs cross-mode-locked pulses have been generated with independent tunable wavelength when the pump power of the argon laser is approximately 5 W. As the design of the bi-cavity is fine, not only strong gain competition is overcome, but also stable output characteristics are performed. Received: 21 July 1998 / Revised version: 5 January 1999 / Published online: 30 July 1999     

All-reflective interferometric autocorrelator for the measurement of ultra-short optical pulses

We have developed a novel interferometric autocorrelator composed of only reflective elements, which functions as a beam splitter and an optical delay line. Analytical expressions are derived to give second-order autocorrelation functions and deconvolution factors for various conditions. The measurement of femtosecond laser pulses by interferometric autocorrelation is demonstrated in the visible region. The results are compared with those by calculation. Received: 9 December 2002 Revised version: 18 February 2003 / Published online: 5 May 2003 RID="*" ID="*"Present address: Department of Physics, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1207, Japan RID="**" ID="**"Corresponding author. Fax: +81-48/462-4682, E-mail: asuda@postman.riken.go.jp     

Enhanced spectral broadening of short laser pulses in high-numerical-aperture holey fibers

The influence of the structure of holey-fiber cladding on the spectral broadening of femtosecond laser pulses is experimentally studied. These experiments demonstrate that the spectral broadening of 70-fs pulses of 800-nm Ti:sapphire laser radiation transmitted through 2- and 3-μm-pitch holey fibers can be enhanced by a factor of about 1.5 by increasing the air-filling fraction of the fiber cladding from 16 up to 65%. Received: 23 April 2001 / Revised version: 18 June 2001 / Published online: 18 July 2001     

Pulse-parameter dependence of the configuration characteristics of a micro-structure in fused SiO2 induced by femtosecond laser pulses

The fabrication of a micro-structures in fused SiO₂ by femtosecond laser pulses was studied in the range of pulse duration 150∼500 fs, pulse energy 60 nJ to ∼7 μJ and wavelengths 400 nm and 800 nm. The characteristics of the cross section of the micro-structure were explained with the multiphoton-ionization and avalanche-ionization processes in the femtosecond laser-induced breakdown. The characteristics of the longitudinal length of the micro-structure relating to the direct-focusing and self-action effects are also discussed. Received: 10 January 2001 / Accepted: 7 April 2001 / Published online: 23 May 2001     

High-power intracavity frequency doubling of a Ti:sapphire femtosecond oscillator

We demonstrate intracavity frequency doubling of a standard femtosecond Ti:sapphire oscillator. The cavity is extended with a pair of focusing mirrors and a 0.5-mm-thick BBO crystal. We achieve a repetition rate of 50 MHz and simultaneously generate 22 mW of 55-fs pulses at 810 nm and 200 mW of 73-fs pulses at 405 nm, which corresponds to 4 nJ per pulse. We create a total of 330-mW, 405-nm light when pumping the Ti:sapphire crystal with 5.7 W from an Ar-ion laser, corresponding to a conversion efficiency of 5.7%. No saturation is found, which implies that higher outputs can be achieved with higher pump rates. Preliminary results from the use of blue pulses as pump in an optical parametric amplifier seeded by pulses from a photonic crystal fiber are presented. Received: 27 January 2003 / Revised version: 27 March 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +45-861/96199, E-mail: tva@chem.au.dk     

Measurement of filament length generated by an intense femtosecond laser pulse using electromagnetic radiation detection

We present a new method to measure the length of a filament induced by the propagation of intense femtosecond laser pulses in air. We used an antenna to detect electromagnetic pulses radiated from multipole moments inside the filament oscillating at the plasma frequency. The results are compared with the values detected from the backscattered fluorescence induced by multiphoton ionization of nitrogen molecules excited inside the filament. The values are found to be in good agreement. Received: 6 November 2002 / Revised version: 27 January 2003 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: shosseini@phy.ulaval.ca     

Femtosecond pulse shaping in the mid infrared by difference-frequency mixing

The generation of programmable complex femtosecond pulses in the mid infrared (3–10 μm) with high precision is reported. Designed pulse shapes in the near infrared (1–1.6 μm) are transferred to the mid infrared via difference-frequency mixing with a second infrared pulse spectrally narrowed in a zero-dispersion compressor. In particular, pulse sequences with variable relative phases have been obtained. The control of the pulse properties is achieved purely electronically, allowing for implementation into a feedback loop. Received: 12 December 2003 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-89/32905-200, E-mail: mcm@mpq.mpg.de     

Filamentation of femtosecond laser pulses in turbulent air

Formation and wandering of filaments in air are studied both experimentally and numerically. Filament-center deflections are collected from 1100 shots of 190-fs and 800-nm pulses in the plane perpendicular to the propagation direction. To calculate the filament wandering in air we have developed a model of powerful femtosecond laser pulse filamentation in the Kolmogorov atmospheric turbulence and employed the Monte Carlo method to model the propagation of several hundred laser pulses. Statistical processing of experimental and numerical data shows that filament-center displacements in the transverse plane obey the Rayleigh-distribution law. Parameters of the Rayleigh distribution obtained for numerical and experimental data are close to each other. Received: 23 May 2001 / Revised version: 26 September 2001 / Published online: 29 November 2001     

Proposal of high quality walk-off compensated sum frequency generation of ultra-short pulses

A multi-crystal and a frequency-selective group-velocity compensating schemes are proposed for the sum frequency-third harmonic generation of low power ultra-short pulses from a Ti:sapphire femtosecond laser. The mechanisms of compensation are analyzed accordingly. It is shown that the temporal walk-off can be effectively compensated without any dispersive element. By use of BBO, calcite crystals together with specially designed composite phase delay plates and dichroic beamsplitters, high quality third harmonic ultra-violet pulses within 300 nm can be generated by the two schemes.     

Simulation of Femtosecond Pulse Propagation through Hollow Fibre Filled with Noble Gases of Gradient Temperature

We propose a novel technique for generating intense few to mono-cycle femtosecond pulses. The simulation demonstrate that for the temperature difference of 300K, the spectrum of the output pulses is increased by 67% and the transform limited pulse width is reduced almost by half, compared with those obtained with hollow fibres in uniform temperature.     

Non-perturbative wave-packet calculations of time-resolved four-wave-mixing signals

We extend a method proposed by Seidner et al. [J. Chem. Phys. 103, 3998 (1995)] to extract directional terms from a time-dependent molecular polarization to obtain time-resolved four-wave-mixing (FWM) signals. Instead of employing perturbation theory, the total polarization induced in the molecular sample by the interaction with several femtosecond laser pulses is determined by solving the time-dependent Schr?dinger equation for the nuclear dynamics in coupled electronic states numerically exact. Repeating the calculation for several combinations of relative phases of the involved fields leads to a linear system of equations for the contributions emitted in different directions. Gas-phase I₂ molecules serve as a numerical example. Received: 22 September 1999 / Published online: 30 June 2000