Characterizing isolated attosecond pulses from hollow-core waveguides using multi-cycle driving pulses.

The generation of attosecond-duration light pulses using the high-order harmonic generation process is a rapidly evolving area of research. In this work, we combine experimental measurements with careful numerical analysis, to demonstrate that even relatively long-duration, 15 fs, carrier-envelope-phase (CEP) unstabilized near-infrared (NIR) pulses can generate isolated attosecond extreme-ultraviolet (EUV) pulses by the dynamically-changing phase matching conditions in a hollow-core waveguide geometry. The measurements are made using the laser-assisted photoelectric effect to cross-correlate the EUV pulse with the NIR pulse. A FROG CRAB analysis of the resulting traces (photoelectron signal versus photoelectron energy and EUV-NIR delay) is performed using a generalized projections (GP) algorithm, adapted for a wide-angle photoelectron detection geometry and non-CEP stabilized driving laser pulses. In addition, we performed direct FROG CRAB simulations under the same conditions. Such direct simulations allow more freedom to explore the effect of specific pulse parameters on FROG CRAB traces than is possible using the automated GP retrieval algorithm. Our analysis shows that an isolated pulse with duration of approximately 200 attoseconds can result from CEP unstabilized, high intensity approximately 15 fs multi-cycle driving pulses coupled into a hollow-core waveguide filled with low-pressure Argon gas. These are significantly longer driving pulses than used in other experimental implementations of isolated attosecond pulses.

Absolute-frequency measurements with a stabilized near-infrared optical frequency comb from a Cr:forsterite laser.

A frequency comb is generated with a chromium-doped forsterite femtosecond laser, spectrally broadened in a dispersion-shifted highly nonlinear fiber, and stabilized. The resultant evenly spaced comb of frequencies ranges from 1.1 to beyond 1.8 microm. The frequency comb was referenced simultaneously to the National Institute of Standards and Technology's optical frequency standard based on neutral calcium and to a hydrogen maser that is calibrated by a cesium atomic fountain clock. With this comb we measured two frequency references in the telecommunications band: one half of the frequency of the d/f crossover transition in 87Rb at 780 nm, and the methane v2 + 2v3 R(8) line at 1315 nm.

420-MHz Cr:forsterite femtosecond ring laser and continuum generation in the 1-2-micrometre range.

We demonstrate a chromium-doped forsterite femtosecond ring laser that generates 30-fs pulses at a 420-MHz repetition rate with nearly 500 mW of average power. The compact solid-state design and broad spectral output make this laser attractive for telecommunications applications in the 1.3-1.5-micrometre region. Additional spectral broadening of the laser output in highly nonlinear optical fiber leads to octave-spanning spectra ranging from 1.06 to 2.17 micrometre. The octave is reached at a level of 18 dB below the peak. The underlying optical frequency comb can be linked to existing optical frequency standards.