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     

High-efficiency optical compression of Ti:sapphire laser pulses at 800 nm using a silver-coated hollow fiber

A 50 cm silver coated hollow fiber with inner diameter of 250 μm and filled with argon has been used to compress optical pulses from a Ti:sapphire laser at 800 nm. Input pulses with energy of 250 μJ and duration of 110 fs were used and compressed pulses with energy of 220 μJ and duration of 20 fs were generated by using a prism compressor. Numerical and experimental results are compared. There is good agreement between the measured beam diameters of the hollow-fiber output pulse and the calculated values obtained from propagation of the HE₁₁ mode into free space. For comparison, a similar uncoated fused-silica hollow fiber was also used to obtain 20 fs compressed pulses with an energy of 190 μJ. Received: 7 September 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-780/492-1811, E-mail: mohebbi@ee.ualberta.ca     

Soliton-like pulse-shaping mechanism in passively mode-locked surface-emitting semiconductor lasers

We discuss a mechanism that allows the formation of nearly transform-limited soliton-like pulses in passively mode-locked optically pumped external-cavity surface-emitting semiconductor lasers. It involves the interplay of positive dispersion and the nonlinear index changes in gain medium and saturable absorber, while ordinary solitons are based on dispersion and the Kerr effect. The obtained quasi-soliton pulses share some of the properties of ordinary solitons (in particular, their stability and near bandwidth-limited nature), while other properties are different. In particular, the pulse duration scales with the square root of the cavity dispersion, and an excessive drift of the laser wavelength must be avoided by proper design. Received: 23 April 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1059, E-mail: Paschotta@iqe.phys.ethz.ch     

Wavelength-agile fiber laser using group-velocity dispersion of pulsed super-continua and application to broadband absorption spectroscopy

A swept-wavelength source is created by connecting four elements in series: a femtosecond fiber laser at 1.56 μm, a non-linear fiber, a dispersive fiber and a tunable spectral bandpass filter. The 1.56-μm pulses are converted to super-continuum (1.1–2.2 μm) pulses by the non-linear fiber, and these broadband pulses are stretched and arranged into wavelength scans by the dispersive fiber. The tunable bandpass filter is used to select a portion of the super-continuum as a scan-wavelength output. A variety of scan characteristics are possible using this approach. As an example, an output with an effective linewidth of approximately 1 cm^-1 is scanned from 1350–1550 nm every 20 ns. Compared to previous scanning benchmarks of approximately 1 nm/μs, such broad, rapid scans offer new capabilities: a gas sensing application is demonstrated by monitoring absorption bands of H₂O, CO₂, C₂H₂ and C₂H₆O at a pressure of 10 bar. Received: 5 August 2002 / Revised version: 23 September 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +1-608/265-2316, E-mail: ssanders@engr.wisc.edu     

Self-starting five optical cycle pulse generation in Cr<Superscript>4+</Superscript>:YAG laser

We report stable self-starting near-transform-limited 26-fs pulses at 250-mW output power from a Kerr-lens mode-locked (KLM) Cr⁴⁺:YAG laser using chirped mirrors in combination with prisms. The highest output power achieved in KLM regime was 600 mW at 55-fs pulse duration. The experimental results agree well with the results of theoretical analysis with respect to KLM self-starting ability and stability against continuous-wave and multi-pulse operation. Parameter ranges for stable transform-limited KLM pulses as well as the shortest achievable pulse durations are established. Using an InGaAs-InP semiconductor saturable absorber mirror we could obtain self-starting 57-fs pulses at the average output power of 200 mW. Two-photon absorption was found to be the main mechanism favoring the multiple-pulse operation. Received: 2 May 2002 / Revised version: 16 July 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +43-1/58801-38799, E-mail: sorokina@tuwien.ac.at     

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     

Laser-induced nano-patterning by means of interference subpatterns generated by microspheres

Interference patterns generated by a regular lattice of SiO₂ microspheres on a transparent support are used for the surface patterning of polyimide (PI) foils. Using 248 nm excimer-laser radiation, thousands to millions of holes with a full width at half maximum (FWHM) of 160±40 nm can be generated with a single laser pulse. Received: 1 March 2002 / Accepted: 4 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, E-mail: dieter.baeuerle@jku.at     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu     

Dispersion and alignment of carbon nanotubes in polycarbonate

Dispersion and alignment of carbon nanotubes in thermoplastic polymers such as polycarbonate have been studied. Dispersion was accomplished by mixing in a conical twin-screw extruder and alignment was carried out using a fiber-spinning apparatus. The effects of mixing time and fiber draw rates on dispersion and alignment were investigated. Uniform dispersions were produced with relatively short residence times in the extruder. Excellent alignment of carbon nanotubes in nanocomposite filaments was obtained when the fiber draw rate was greater than 70 m/min. The ability to closely control the dispersion and alignment of carbon nanotubes in polymers is expected to lead to the development of nanocomposites with desirable electronic and structural properties. Received: 7 January 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-508/233-5521, E-mail: Michael.Sennett@natick.army.mil     

Phase-stabilized 4-fs pulses at the full oscillator repetition rate for a photoemission experiment

Compressing pulses of a mode-locked extended-cavity Ti:sapphire laser using a standard single-mode fiber and tilted-front-interface chirped mirrors yields phase-stabilized 4-fs, 3-nJ light pulses at the full, 24-MHz, repetition rate. The demonstrated source paves the way towards exploring interactions sensitive to the carrier-envelope-offset phase, such as, for example, photoemission from solid targets at moderate intensities (∼10¹² W/cm²). Received: 13 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +43-1/58801-38799, E-mail: apolonski@tuwien.ac.at     

Geometrical optical modeling of femtosecond setups having angular dispersion

A reliable and efficient method based on a geometrical optical approach is presented to model the propagation of ultrashort pulses in optical systems. It is shown that the commonly used method to determine the group delay of the spectral components of a pulse from their geometrical optical path lengths is only correct for aberration-free optical systems. In the case of systems with angular dispersion and imaging errors, a correction to the path values obtained from ray-tracing calculations must be applied, since for specific systems neglect of the correction causes significant errors. A technique for performing this correction is presented. Two optical arrangements used for the generation and detection of tunable THz radiation by the femtosecond tilted-pulse-front excitation technique are analyzed to demonstrate the indispensability of the correction. Received: 29 July 2002 / Revised version: 22 October 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +36-72/501-571, E-mail: kozma@fizika.ttk.pte.hu     

Synchronously pumped femtosecond optical parametric oscillator of congruent and stoichiometric MgO-doped periodically poled lithium niobate

A synchronously pumped femtosecond optical parametric oscillator based on congruent MgO-doped periodically poled lithium niobate (c-MgO:PPLN) is reported. The system, operating at room temperature, was pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler waves were tuned from 870 nm to 1.54 μm and 1.58 to 5.67 μm, respectively, by changing the pump wavelength, the grating period or the cavity length. Pumped by 1.1 W of 755 nm laser radiation, the OPO generated 310 mW of 1080 nm radiation. This signal output corresponds to a total conversion efficiency of 50%. Without dispersion compensation the OPO generated phase-modulated signal pulses of 200 fs duration. Besides the OPO of c-MgO:PPLN, an OPO of stoichiometric (s) MgO:PPLN was investigated. Because of the reduced sensitivity to photorefractive damage, both crystals allowed efficient OPO operation at room temperature. Received: 19 August 2002 / Revised version: 11 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: andres@physik.uni-kl.de     

Effects of a prepulse on laser-induced EUV radiation conversion efficiency

The influence of prepulses on the conversion efficiency (CE) of laser radiation into 13 nm extreme ultraviolet (EUV) radiation, resulting from lithium-like oxygen ions from O20 μm water droplets, was investigated. The laser pulse durations in the experiment ranged from 200 fs to 120 ps. Applying prepulses preceding the main pulse at various delays of up to 11 ns, it was shown that the CE increases differently for each measured pulse duration. The strongest dependence on the introduction of a prepulse was observed for 2 ps laser pulses with a 20 mJ laser pulse energy. The EUV CE was improved by a factor of 15 by the introduction of a prepulse. Calculations on the atomic physics of oxygen ions and simulations of the laser–plasma interaction revealed the influence of the prepulse on the EUV yield. Received: 25 October 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-3641/947-202, E-mail: duesterer@ioq.uni-jena.de     

100-W average-power, high-energy nanosecond fiber amplifier

We report on the fiber-based amplification of a Q-switched Nd:YAG thin-disk laser. At repetition rates between 3 and 50 kHz output powers up to 100 W are generated. Pulse energies up to 4 mJ, with diffraction-limited beam quality, are generated in a 30-μm Yb-doped large-mode-area fiber, furthermore pulse energies up to 8 mJ are achieved from a multimode fiber amplifier. Received: 8 May 2002 / Revised version: 10 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-3641/65-7680, E-mail: Jens.Limpert@uni-jena.de     

Adaptive shaping of two-cycle visible pulses using a flexible mirror

We present a double-pass non-collinear optical parametric amplifier (NOPA) that delivers 4-fs visible–near-infrared pulses. The paper discusses geometrical and temporal properties of the NOPA configuration, which allow us to broaden the gain band of simultaneous parametric amplification in excess of 250 THz. The key elements of the bandwidth enhancement include (i) the use of a broadband second-harmonic pump, (ii) optimization of the incidence angle of all monochromatic spectral components of the pump in order to produce a favorable offset of the corresponding phase-matching curves, and (iii) selection of the chirp rate and time delay between the stretched pump and seed pulses. We next devise a two-stage compressor that incorporates a flexible mirror for adaptive pulse shaping and develop a simple and trustworthy feedback loop based on a one-dimensional spectral measurement. Our rapid numerical algorithm for adaptive control of the flexible membrane is found superior to more complex search routines that are less resistant to the fluctuations of the laser intensity. The automated optimization procedure results in the generation of two-cycle pulses with a carrier wavelength at ∼600 nm. The absence of deep modulation on the parametrically amplified spectrum in combination with the adaptive phase correction lead to a high quality of the temporal profile and allow concentrating 90% of the pulse energy within only a 7.5-fs time window. Received: 8 April 2002 / Revised version: 26 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Present address: Institut für Photonik, Technische Universit?t Wien, Gusshausstrasse 27, 1040 Vienna, Austria RID="**" ID="**"Corresponding author. Fax: +81-3/5841-4165, E-mail: kobayashi@phys.s.u-tokyo.ac.jp     

Fine tuning of the higher-order dispersion of a prismatic pulse compressor

The wavelength-dependent thermal refractive index gives an extra degree of freedom for adjustment of higher-order dispersion of a prismatic pulse compressor. The effect allows of fine tuning of both intracavity and extracavity dispersion of ultra-fast oscillators. The calculations have been carried out using a new and rather handy formalism, which describes the operation of a prism pulse compressor as a linear function of the ratio of the total glass path in the prisms and the prism apex distance. The validity of theoretical calculations is supported by experimental evidence. Received: 16 April 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +36-62/544658, E-mail: osvay@physx.u-szeged.hu     

Additive-pulse mode locking of a diode-pumped Nd:KGd(WO4)2 laser

Self-starting additive-pulse mode locking of a diode-pumped Nd:KGd(WO₄)₂ laser is demonstrated for the first time. An output power of 0.85 W is achieved with 2.3-ps pulses at a repetition frequency of 76.5 MHz. The shortest pulse generated had a duration of 1.9 ps. Received: 2 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +44-141/5482891, E-mail: a.major@phys.strath.ac.uk     

Ionization spectrum broadening and frequency blue-shift of high-intensity femtosecond laser pulses in gas-filled capillary tubes

. We report on the experimental and theoretical study of spectrum transformation and frequency blue-shift of femtosecond laser pulses with intensities up to 2×10¹⁶ W/cm², propagating in glass capillary tubes under gas ionization. Monomode optical guiding with 45% transmission efficiency is demonstrated in a 100-μm-diameter, 20-cm length capillary. A broadening of the initial spectrum as much as several initial spectrum widths is achieved. Besides the broadening, the mean frequency of the output radiation in the spectrum experiences a blue-shift of up to several initial spectrum widths, caused by the non-stationary, non-linear process of gas ionization. Our numerical simulations, in the form of a simple one-dimensional model for the propagation of intense laser pulses in gas-filled capillaries, are in good qualitative agreement with the experimental results. These simulations show the possibility of significant compression of an output pulse in a simple compression scheme (e.g. a piece of silica glass with normal dispersion), which is very important for obtaining laser pulses with few optical cycles at the millijoule energy level. Received: 25 September 2001 / Revised version: 6 December 2001 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +7-8312/363-792, E-mail: dekart@ufp.appl.sci-nnov.ru     

Sub-ps laser microstructuring of soft X-ray Mo/Si multilayer gratings

The sub-picosecond laser microstructuring of multilayer gratings is presented in this paper. A micromachining system operating with a 0.5 ps KrF laser at 248 nm was used to etch grating structures with a groove width of 1–2 μm in Mo/Si and Si/Mo multilayers. Atomic force microscopy, scanning electron microscopy and X-ray reflectivity were used to characterize the microetched patterns. The ω-scans around the 1st Bragg maximum show symmetric satellites up to 3rd order, with positions corresponding to the grating period. The use of sub-picosecond laser pulses minimizes the thermally affected zone and enhances the quality of the etched features. Short pulse laser processing is advantageous for the fabrication of high spatial resolution microstructures required in X-ray optics. Received: 21 May 2002 / Accepted: 19 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Email: dpapa@iesl.forth.gr     

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