This is the first high resolution measurement of the electron energy spectrum from high intensity above-threshold ionization (ponderomotive energy » ionization potential ^>hv) at intensities up to 7x 10 15 W/cm 2 . In contrast to previous work, we have used a short pulse and a large focal spot along with low gas densities to minimize the effect of ponderomotive forces and collisions. Existing models do not agree well with electron energy spectra for either linearly or circularly polarized light. The electron temperature is too hot for the implementation of 820 nm multiphoton ionized recombination type XUV laser schemes. PACS numbers: 32.80.RmWe report the first high resolution measurement of the electron energy distribution from high intensity abovethreshold ionization (ATI) at intensities up to 7xl0 15 W/cm 2 with a short pulse. In the short pulse regime [1-4] the pulse is over before the electrons have been disturbed by the ponderomotive force: The electrons are therefore characteristic of the ionization intensity and not of the excitation geometry. Previous measurements of electron spectra in the so-called "long pulse regime" were dominated by the ponderomotive acceleration of the electrons, which is their quiver response to the oscillating electromagnetic field in the intensity gradient at the laser focus [1-6]. The measurements reported here are at intensities 30 times more intense than those previously published and allow us to test models of ATI in the high intensity limit when ATI models should be most valid, viz., ponderomotive energy »ionization potentialshv. ATI electron energy spectra from both linearly and circularly polarized light were measured. While the threshold intensities for multiphoton ionization agree with models of ATI [7-9], the electron spectra from linear or circular polarization differ markedly from theory. The predicted preponderance of slow electrons from linearly polarized ATI is not observed. This result is particularly noteworthy in light of recently proposed x-ray laser schemes that use multiphoton ionization to produce high ion stages in a relatively cold electron distribution [10-12].These measurements are valuable because the ATI electron energy distributions contain minimal ponderomotive force contributions, despite the fact that the ponderomotive energy is much higher than the ionization potential of the parent atom. To achieve this, we require both a short pulse and a large focus. Our laser source is a colliding-pulse mode-locked laser [13] centered at 820 nm, amplified in a Ti-sapphire chirped-pulse amplifier [14]. It is capable of delivering 50 mJ pulses of 180 fs duration at 5 Hz. Laser pulses are focused with a 1 m lens into a vacuum chamber to a Gaussian waist of 0)0 = 35 jum. After the lens the laser beam propagates almost entirely in vacuum. The electron energy spectra are measured with a field-free time-of-flight (TOF) spectrometer with a detection cone angle of 2° (3.8 x 10 ~3 sr collection solid angle) at the focus. Electrons are accelerated at the end of the TOF ...

We describe the construction and operation of a 109-nm, photoionization-pumped, single-pass laser in Xe III. The laser is pumped by soft x rays emitted from a laser-produced plasma in a traveling-wave geometry. Using a 3.5-J, 300-psec, 1064-nm laser pump pulse, we measure a small-signal gain coefficient of 4.4 cm(-1) and a total small-signal gain of exp(40). The laser is fully saturated and produces an output energy of 20 microJ in a beam with 10-mrad divergence.

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A polarization-diverse subsystem based on periodically poled lithium niobate waveguides is used as an optical phase conjugator for compensation for linear and nonlinear distortion. We show successful transmission formats of 13 x 40 Gbit/s non-return-to-zero mixed with 6 x 10 Gbit/s non-return-to-zero and 40-Gbit/s duobinary over 8 x 100 km of standard single-mode fiber. A single phase conjugator is used to conjugate all data formats, including the alternative duobinary format, simultaneously.

We report 2-Hz operation of a single-pass 109-nm laser with a small-signal gain of exp 33 and a saturated output energy of 1 ,J. The laser is based on an oblique-incidence, laser-produced-plasma pumping geometry and requires only 500 mJ of 1064-nm energy in a O.5-nsec pump pulse. We use the laser to produce a two-slit interference pattern and demonstrate a focusable intensity of greater than 10 9 W/cm 2 .