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Intense ultrashort waveforms of light that can be produced with an exactly predetermined electromagnetic field are essential in a number of applications of extreme nonlinear optics, most prominently in laser-driven sources of high-energy attosecond radiation. Field reproducibility in each laser shot requires stabilization of the carrier-envelope phase. The authors analyze different schemes of phase-stable pulse amplification and identify constraints limiting the precision with which the phase can be maintained. Next, they describe a phase-stabilized laser system based on a 20-fs multipass Ti:sapphire amplifier supplemented with a fiber compression stage for producing pulses in the few-cycle regime. It is shown that the amplifier introduces only a slow millihertz phase drift and, therefore, can be seeded by a standard phase-stabilized oscillator. This residual phase drift is assigned primarily to the beam pointing instability and can be precompensated in the phase-control loop of the seed oscillator using a feedback signal from a phase detector placed in the amplifier output. The phase stability of the resultant 5-fs 400-/spl mu/J pulses at a 1-kHz repetition rate is subsequently independently verified by higher order harmonic generation, in which different carrier-envelope phase settings are shown, both theoretically and experimentally, to produce distinctly different spectral shapes of the XUV radiation. From a series of such spectral patterns, the authors succeed in calibrating the value of the carrier envelope phase (with a /spl plusmn//spl pi/ ambiguity), which in turn allows them to fully characterize the temporal structure of the electric field of the laser pulses. The estimated precision of the phase control on the XUV target is better than /spl pi//5, which reduces the timing jitter between the driving laser pulse and the XUV bursts to /spl sim/ 250 as and opens the way to generate stable isolated attosecond pulses.

Femtosecond laser-frequency comb techniques are vastly simplifying the measurement and synthesis of optical frequencies. A single mode-locked femtosecond laser, with its spectrum broadened by self-phase modulation in a microstructured or tapered nonlinear fiber, can produce millions of sharp laser lines in a precise evenly spaced grid spanning much of the visible and near-infrared spectrum. The absolute frequency of each line is determined by two observable radio-frequency signals. The pulse repetition rate gives the spacing of the comb lines and the rate at which the phase of the lightwave slips, relative to the intensity envelope from pulse to pulse determines the offset frequency by which each line is displaced from a precise integral multiple of the repetition frequency. This offset frequency can be measured most easily if the comb spans more than an optical octave so that one can observe a radio frequency beat note between the second harmonic of the infrared comb lines with the corresponding comb lines at the blue end. Such an optical-frequency synthesizer makes optical oscillations readily countable and provides the long-awaited compact optical clockwork for an all-optical clock.

Harmonics up to order 21 generated in Xe are coupled out of a 78-MHz enhancement cavity with a fused silica Brewster plate. An irreversible degradation of the plate, accelerated by the generated harmonics, is observed.

Nowadays the highly demanded state-of-the-art XUV and X-ray radiation sources with sufficient brilliance for scientific and industrial applications are synchrotrons and free electron lasers. Limitations due to the large sizes and costs of such facilities have been motivating research and development efforts of table-top sources which can offer a comparable radiation brilliance and beam quality.

We have explored the performance of 2 "dark fibers" of a commercial telecommunication fiber link for a remote comparison of optical clocks. These fibers establish a network in Germany that will eventually link optical frequency standards at PTB with those at the Institute of Quantum Optics (IQ) at the Leibniz University of Hanover, and the Max Planck Institutes in Erlangen (MPL) and Garching (MPQ). We demonstrate for the first time that within several minutes a phase coherent comparison of clock lasers at the few 10(-15) level can also be accomplished when the lasers are more than 100 km apart. Based on the performance of the fiber link to the IQ, we estimate the expected stability for the link from PTB to MPQ via MPL that bridges a distance of approximately 900 km.

We generated a series of harmonics in a xenon gas jet inside a cavity seeded by pulses from a Ti:sapphire mode-locked laser with a repetition rate of 10.8 MHz. Harmonics up to 19th order at 43 nm were observed with plateau harmonics at the microW power level. An elaborate dispersion compensation scheme and the use of a moderate repetition rate allowed for this significant improvement in output power of the plateau harmonics of 4 orders of magnitude over previous results. With this power level and repetition rate, high-resolution spectroscopy in the extreme ultraviolet region becomes conceivable. An interesting target would be the 1S-2S transition in hydrogenlike He+ at 60 nm.

Summary form only given. Here we demonstrate an extension of the dispersion sensitive method that combines its phase sensitivity with comb mode resolution, pushing its resolution limitations down to laser line width. The basic idea is to detune the resonator with respect to the driving frequency comb such that the cavity mode frequency spacing is and integer fraction like 10/9th of the frequency comb repetition frequency.

We demonstrate high order harmonic generation directly from a 110 MHz femtosecond oscillator using a build-up cavity. This is an important step towards high precision spectroscopy and high power laser sources in the XUV.

The development of high finesse resonators for broadband frequency combs enabled the extension of the versatile frequency comb technique into the extreme ultraviolet with all its implications. We describe new developments in this field.

We have measured the absolute frequency of the hydrogen 1S-2S two-photon resonance with an accuracy near 3 parts in 10/sup 12/ by comparing it with the 28/sup th/ harmonic of a methane-stabilized 3.39 /spl mu/m He-Ne-laser. A frequency mismatch of 2 THz at the 7/sup th/ harmonic is bridged with the help of a phase-locked chain of five optical frequency interval dividers.

At present most laser systems for generating phase-stabilized high-energy pulses are chirped pulse amplifier systems that involve the selection and subsequent amplification of pulses from a phase-stabilized seed oscillator. We investigate the effect of the picking process on the carrier envelope phase stability and how the phase noise of the picked pulse sequence can be estimated from the phase noise properties of the seed oscillator. All noise components from the original pulse train above the picking frequency are aliased into the picked pulse train and therefore cannot be neglected.

A new scheme for stabilizing the carrier-envelope phase (CEP) of a few-cycle laser pulse train is demonstrated. Self-phase modulation and difference-frequency generation in a single periodically-poled crystal allows CEP locking directly in the usable output.

Different approaches have been proposed and recently carried out experimentally to measure the value of the carrier-envelope phase (CEP) of a powerful (> 1 /spl mu/J) few-cycle pulse: generation of high harmonics and above-threshold ionization in gases. We demonstrate for the first time that electron emission from a gold surface induced by 4-fs 3 nJ pulse is sensitive to the timing of electric field oscillations with respect to the pulse peak and use this sensitivity for measuring the light field's oscillation phase at the pulse peak in low-energy laser pulses.

The paper discusses on the generation and full characterization of 4-fs phase-stabilized pulses as well as their temporal carrier-envelope phase (CEP) jitter, first experimental demonstration of a phase-sensitive detector on a base of photoemission effect from a solid target, and generation of a self-phase-stabilized signal based on a difference frequency generation.

We have used optical frequency interval dividers, optical frequency comb generators and mode-locked lasers to measure large optical frequency differences of up to 45.2 THz between laser frequencies. We have shown that the modes of a mode-locked laser are distributed uniformly in frequency space within the experimental limit of 3.0 parts in 10/sup 17/ and that the mode separation equals the pulse repetition rate within the experimental limit of 6.0 parts in 10/sup 16/. We applied these techniques to determine the absolute frequencies of the cesium D/sub 1/ line at 335 THz (895 nm), the hydrogen 1S-2S transition at 2466 THz (121 nm) and the frequency of a methane stabilized He-Ne laser at 88.4 THz (3.39 /spl mu/m) as well as the isotope shift of the 1S-2S frequency.

Summary form only given. We explore the possibilities of frequency combs generated by femtosecond lasers in ultra high precision spectroscopy on atomic hydrogen at the 10/sup -14/ level and we have done fundamental tests of the fs frequency comb technique down to the 10/sup -21/ level. Further applications arise in the time domain where now the phase slip between carrier and envelope of such pulses can be controlled.

We have shown that the modes of a femtosecond mode-locked laser are distributed uniformly in frequency space and can be used like a ruler to measure large optical frequency differences. To measure absolute optical frequencies we use nonlinear optics for the conversion into a large frequency interval. Unlike the complex harmonic frequency chains used in the past this new approach uses only a few laser sources (ideally just one) and is nevertheless capable of measuring almost any optical frequency with the same set up. We applied the new technique to determine the absolute frequencies of the cesium D/sub 1/ line at 335 THz, of several components in I/sub 2/ around 563 THz, the sharp /sup 115/In/sup +/5s/sup 21/S/sub 0/-5s5p/sup 3/P/sub 0/ transition at 1267 THz and the hydrogen 1S-2S transition at 2466 THz.

We have determined the frequency of the a/sub 1/ component of the P(54)32-0 transition of molecular iodine with an estimated relative standard uncertainty below 1/spl middot/10/sup -11/ using a phase-coherent optical frequency chain from the CH/sub 4/ frequency standard to the visible. This allows the realisation of the meter by visible frequencies with an uncertainty reduced by more than one order of magnitude compared to previous measurements (1999).

Summary form only given. The periodic pulse train emitted by a modelocked femtosecond laser can be viewed in the frequency domain as a comb of equidistant modes spaced by the pulse repetition rate. This broad frequency comb can be used like a ruler to phase coherently measure large differences between laser frequencies. With our Kerr-lens mode-locked Ti:sapphire laser (pulselength 73 fs, Coherent Inc., model Mira 900) we have shown that the modes of the femtosecond frequency comb are distributed uniformly in frequency space within the experimental limit.

6-fs, 1-MW laser pulses have been produced at the full repetition rate of a mode-locked laser upon simultaneous synthesization of phase-locked laser modes over the spectral range extending from 500 nm to 1100 nm, opening the door to the measurement and stabilization of the absolute phase of few-cycle light pulses by use time- and frequency-domain techniques, respectively.

We demonstrate a solid-state device capable of unambiguously measuring the carrier-envelope phase of ultrashort (4-femtosecond), low-energy (unamplified) laser pulses using photoelectron emission from a gold surface in the multiphoton regime.

Using a coherent nonlinear optical technique, measurement of the temporal evolution of the absolute phase of 6-fs light wave packets has been demonstrated, permitting the generation of intense, few-cycle light with precisely-known electric and magnetic fields.