SPM-induced spectral compression of picosecond pulses in a single-mode Yb-doped fiber amplifier

In a fiber amplifier, spectral compression due to self-phase modulation is demonstrated for ultrashort pulses. We report the generation of near-transform-limited picosecond pulses with peak powers of several kW at a repetition rate of 74 MHz and diffraction-limited beam quality in a Yb-doped fiber amplifier when seeding with a negative chirped pulse. Received: 17 September 2001 / Revised version: 22 November 2001 / Published online: 17 January 2002     

Two-color pump-probe ionization spectroscopy of gaseous water enhanced by preliminary impulsive Raman excitation

We use sub-10-fs pulses at 400 nm and 15-fs pulses at 800 nm to ionize water molecules and their isotopomers HDO and D₂O in a pump–probe scheme. Pulses are generated via spectral broadening of 25-fs pulses of a 1-kHz Ti:sapphire amplifier system by self-phase modulation in a noble-gas-filled hollow waveguide and subsequent compression using chirped mirrors. At this time scale vibronic excitation of the first bending mode of water in the electronic ground state by impulsive Raman scattering is possible (e.g. the fundamental bending mode of H₂O: t_vib=20 fs). The effect of this pre-excitation on the ionization rate is shown. Received: 14 May 2001 / Revised version: 24 August 2001 / Published online: 19 September 2001     

Reduction of the spectral width and beam divergence of a BBO-OPO by using collinear type-II phase matching and back reflection of the pump beam

We report on the reduction of the divergence and the spectral width of the output of a 355 nm pumped pulsed optical parametric oscillator (OPO) of β-barium borate (BBO). Detailed theoretical investigations indicated that type-IIphase matching in combination with double-passing the pump beam should simultaneously reduce the spectral width and the divergence of the OPO output beam. These predictions are confirmed in the experiments reported in this paper. In fact the bandwidth is reduced by more than a factor of 20 to less than 0.1 nm. Simultaneously the divergence of the OPO waves is reduced in the phase matching plane by more than a factor of 5 to 1.0 mrad. The small divergence and the reduced bandwidth allows efficient frequency doubling of the 5-ns-long visible OPO signal pulses into the UV. Doubling in a 7-mm-long BBO crystal provided conversion efficiencies of up to 35%. Received: 21 August 2000 / Revised version: 11 December 2000 / Published online: 21 March 2001     

Generation of high-quality 10-femtosecond laser pulses with intracavity spatial and spectral control

Spatial and spectral control, using an intracavity capillary and a slit, is applied to improve the output pulse quality of a Ti:sapphire laser. Satellite-free 10-fs optical pulses with a smooth spectral and spatial profile have been generated. Employing a root-mean-square formalism for pulse characterization, spatial, spectral and temporal intensity distributions are analyzed for laser pulses with a duration as short as three to four optical cycles. Received: 11 June 2001 / Published online: 18 July 2001     

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     

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     

Broadband four-wave optical parametric amplification in bulk isotropic media in the ultraviolet

We report on four-wave optical parametric amplification of the ultrashort ultraviolet light pulses in bulk fused silica and CaF₂. Exact phase-matching in these isotropic media is achieved by means of non-collinear interaction with cylindrical beam focusing. Four-wave optical parametric amplifier efficiently operates in the UV spectral range with 1-ps laser pulses, delivering amplified signal energy exceeding 50 μJ using millijoule pump pulses in the visible (527 nm). Results of scanning of the parametric gain profile suggest that broad amplification bandwidth as wide as ∼20 nm (at FWHM) under these experimental settings is achieved, which might support amplification of sub-10-fs ultraviolet pulses with central wavelength around 330 nm. It is also shown experimentally and verified theoretically that the parametric gain profile exposes a distinct inhomogeneity and its bandwidth notably broadens due to effects of self- and cross-phase modulation imposed by the intense pump beam.     

Modifications to the lidar equation due to nonlinear propagation in air

We study how the well-known lidar equation is affected by the use of ultra-short, high-power laser pulses. Because of the self-focusing and self-guiding, the overlap function ξ, representing the reduction fraction of the signal resulting from geometrical effects inside the experimental system, needs to be reconsidered. The losses due to multi-photon ionisation in the filament entail a heavy weakening of the return signal. We also investigate the contribution of the white-light components generated by self-phase modulation. Received: 2 January 2001 / Revised version: 8 June 2001 / Published online: 18 July 2001     

A dispersionless Michelson interferometer for the characterization of attosecond pulses

A novel application of a free-standing transmission grating as a beam splitter in a Michelson-type interferometer is described. The arrangement can operate in the XUV and soft X-ray spectral region and, therefore, it is well suited for the characterization of attosecond pulses. Using ray-tracing codes, we have analyzed three different setups in which spherical mirrors are employed in conjunction with the transmission grating and have investigated in detail their dispersive characteristics. It is shown that such an arrangement can be made to exhibit group-delay dispersion of ∼1 as² while it provides two co-propagating and converging beams. Received: 12 October 2001 / Revised version: 17 December 2001 / Published online: 7 February 2002     

Coherent enhancement of broadband frequency up-conversion in BBO crystal by shaping femtosecond laser pulses

An optimal feedback control of broadband frequency up-conversion in BBO crystal is experimentally demonstrated by shaping femtosecond laser pulses based on genetic algorithm, and the frequency up-conversion efficiency can be enhanced by ∼16%. SPIDER results show that the optimal laser pulses have shorter pulse-width with the little negative chirp than the original pulse with the little positive chirp. By modulating the fundamental spectral phase with periodic square distribution on SLM-256, the frequency up-conversion can be effectively controlled by the factor of about 17%. The experimental results indicate that the broadband frequency up-conversion efficiency is related to both of second harmonic generation (SHG) and sum frequency generation (SFG), where the former depends on the fundamental pulse intensity, and the latter depends on not only the fundamental pulse intensity but also the fundamental pulse spectral phase.     

Coherent anti-Stokes Raman scattering velocimetry with nearly degenerate pumps

The time evolution of the anti-Stokes signal produced from the non-linear interaction of a short Stokes pulse and two long pump pulses that are nearly degenerate in frequency has been investigated. It is shown that this approach allows us to specify the accuracy of CARS (coherent anti-Stokes Raman scattering) velocimetry and to extend the range of operation of the method. In addition, an original optical scheme capable of delivering short visible pulses with good spatial and spectral properties is reported. The optical bench has been used for the characterisation of a low-pressure laminar Mach-10 flow. Received: 24 October 2001 / Revised version: 8 January 2002 / Published online: 14 March 2002     

Intense few-cycle laser pulses from self-compression in a self-guiding filament

Sub-10-fs-pulses are generated by self-compression in a noble gas filament. Using input pulses from a Ti:sapphire amplifier system with an energy of about 1.5 mJ at a repetition rate of 3 kHz and a pulse duration of 30 fs self-compressed sub-10-fs pulses with energies of about 0.3 mJ have been generated. These pulses are characterized with spectral phase interferometry for direct electrical-field reconstruction (SPIDER). Depending on the laser parameters, we observe a significant change in the chirp of the white-light. The spectral distribution of the outcoming beam profile is measured to distinguish the white-light core from the surrounding halo.     

Directional coupling of optical signals by odd dark beams with mixed phase dislocations

Numerical simulations on the evolution of step-screw and edge-screw optical phase dislocations in bulk saturable self-defocusing nonlinear media are presented, with emphasis on their ability to induce steering waveguides for signal beams (pulses). Two schemes for directional coupling of such signals, both ensuring a reasonable coupling-to-cross-talk efficiency ratio, are investigated. The parameters useful for performance optimization of the couplers are discussed. Received: 19 July 2000 / Revised version: 5 February 2001 / Published online: 27 April 2001     

Optical pumping and modulation techniques with a molecular Ramsey–Bordé interferometer

Using the example of a matter-wave interferometer with K₂ molecules, we present different methods to simplify the observed interference structures within a hyperfine pattern for improving phase measurements of the interference structure. We consider optical pumping for depletion of specific components to simplify the observed spectra. In connection with a lock-in amplifier, amplitude modulation as well as frequency modulation of laser beams in the interferometer are investigated for better detection of the interference pattern. As a follow-up treatment, digital filtering is demonstrated. Received: 22 December 2000 / Revised version: 30 April 2001 / Published online: 18 July 2001     

Passive mode locking with slow saturable absorbers

We give a comprehensive overview on passive mode locking of solid-state lasers with slow saturable absorbers, based on analytical and numerical calculations. For picosecond lasers, we present a simple equation to estimate the obtained pulse duration and compare the results to those for mode locking with fast saturable absorbers. We also discuss how much shorter the pulse duration can be compared to the absorber recovery time and present a simple rule. The effect of self-phase modulation is found to be qualitatively different compared to the case of a fast saturable absorber, and the effect of phase changes in the absorber is also discussed. Finally, we discuss various issues concerning soliton mode-locked lasers. Received: 20 July 2001 / Revised version: 28 September 2001 / Published online: 7 November 2001     

Experimental study of pulse compression in a side-pumped Yb-doped double-clad mode-locked fiber laser

We have developed a side-pumped passively mode-locked Yb-doped double-clad fiber laser emitting sub-picosecond pulses at around 1.05 μm. Mode locking is achieved through a polarization additive-pulse mode-locking technique and compression is obtained with a grating pair inserted in the cavity. We have investigated the compression properties of this laser. High-energy pulses are emitted and different behaviors are observed, such as sideband generation, secant-like or Gaussian-like pulse emission. Received: 7 May 2001 / Revised version: 13 November 2001 / Published online: 17 January 2002     

A laser system providing tunable, narrow-band radiation from 35 nm to 2 μm

We describe a laser system that readily provides radiation tunable from 2 μm in the infra-red to 35 nm in the extreme ultraviolet spectral range. The broad spectral range is covered through a range of non-linear processes such as Raman shifting and high-order harmonic generation. Pulses with duration of tens of picoseconds are obtained. The relative bandwidth of the radiation is δλ/λ=10^-4, comparable with what can be achieved by using high-resolution monochromators at state-of-the-art synchrotron beamlines. We discuss different methods for characterising the radiation in this wide wavelength regime. We also discuss the capabilities of the system from the measured parameters. Received: 12 December 2000 / Revised version: 8 March 2001 / Published online: 27 April 2001     

Pulse self-compression in normally dispersive bulk media

We experimentally demonstrate that high-power femtosecond pulses can be compressed during the nonlinear propagation in the normally dispersive solid bulk medium. The self-compression behavior was detailedly investigated under a variety of experimental conditions, and the temporal and spectral characteristics of resulted pulses were found to be significantly affected by the input pulse intensity, with higher intensity corresponding to shorter compressed pulses. By passing through a piece of BK7 glass, a self-compression from 50 to 20 fs was achieved, with a compression factor of about 2.5. However, the output pulse was observed to be split into two peaks when the input intensity is high enough to generate supercontinuum and conical emission.     

Characterization of a high efficiency, ultrashort pulse shaper incorporating a reflective 4096-element spatial light modulator

We demonstrate pulse shaping via arbitrary phase modulation with a reflective, 1 × 4096 element, liquid crystal spatial light modulator (SLM). The unique construction of this device provides a very high efficiency when the device is used for phase modulation only in a prism based pulse shaper, namely 85%. We also present a single shot characterization of the SLM in the spatial domain and a single shot characterization of the pulse shaper in the spectral domain. These characterization methods provide a detailed picture of how the SLM modifies the spectral phase of an ultrashort pulse.     

A 9.5-W 82-MHz-repetition-rate picosecond optical parametric generator with cw diode laser injection seeding

We report on an injection-seeded 9.5-W 82-MHz-repetition-rate picosecond optical parametric generator (OPG) based on a 55 mm long crystal of periodically poled lithium niobate (PPLN) with a quasi-phase-matching (QPM) grating period of 29.75 μm. The OPG is excited by a continuously diode pumped mode-locked picosecond Nd:YVO₄ oscillator-amplifier system. The laser system generates 7 ps pulses with a repetition rate of 82.3 MHz and an average power of 24 W. Without injection-seeding the total average output power of the OPG is 8.9 W, which corresponds to an internal conversion efficiency of 50%. The wavelengths of the signal and idler waves were tuned in the range 1.57–1.64 μm and 3.03–3.3 μm, respectively, by changing the crystal temperature from 150 °C to 250 °C. Injection seeding of the OPG at 1.58 μm with 4 mW of single frequency continuous-wave radiation of a distributed-feedback (DFB) diode laser increases the OPG output to 9.5 W (53% conversion efficiency). The injection seeding increases the pulse duration and reduces the spectral bandwidth. When pumped by 10 W of 1.06 μm laser radiation, the duration of the signal pulses increased from 3.6 ps to 5.5 ps while the spectral bandwidth is reduced from 4.5 nm to 0.85 nm. Seeding thus improved the time-bandwidth product from 1.98 to a value of 0.56, much closer to the Fourier limit. Received: 29 April 2002 / Published online: 8 August 2002