Research supports observation of attosecond pulses

Henry Kapteyn expressed his doubts in the June issue about our claim to have measured single 0.65-fs duration soft x-ray pulses. We appreciate LFW giving us this opportunity to highlight important aspects of the measurement that Dr. Kapteyn has overlooked.

A short pulse can be measured by cross-correlating it with any event of comparable duration. We use the quarter oscillation period (~0.6 fs) of a 750-nm laser wave. Dr. Kapteyn argues that it is impossible for us to differentiate a single pulse from a train of pulses if the train has the same periodicity as the laser wave and the light wave is perfectly periodic. We agree. However, our probing laser wave is far from perfectly periodic. It rises from 1/2 intensity to full intensity in just 3.5 femtoseconds or 1.5 periods. Furthermore, before it is used to measure the subfemtosecond pulse, it passes through a high-density gas jet. There it ionizes the atomic gas (a necessary corollary of producing the subfemtosecond pulse) and gains a blue frequency shift of >25% of its carrier frequency.

We use the subfemtosecond pulse to directly measure the frequency shift and it is clearly visible in Fig. 7 of our published research.1 Rise time is less than 1 fs. This is a critical observation that Dr. Kapteyn missed. Had the XUV pulse consisted of two or more subfemtosecond bursts separated by 1.3 fs, as Dr. Kapteyn suggests, then the apparent rise time of the blue shift would have exceeded 2 fs, even if the true rise time of the frequency shift were ~1 fs.

Dr. Kapteyn also notes that a smooth, structureless harmonic spectrum could, in principle, emerge from smearing effects arising over extended propagation rather than providing clear evidence for a single pulse. We agree, but under our experimental conditions, we do not believe that this happens. In our experiments, the harmonic spectrum produced by a few-cycle pulse is always observed to be continuous at the highest photon energies (cutoff range) and always exhibits a discrete harmonic structure at photon energies less than 80% of the cutoff energy (plateau range). We adjust the intensity of the pump laser pulse so that the spectral continuum in the cutoff range overlaps with the spectral range of our filter (at 90 eV) for the generation of a single subfemtosecond pulse.

Finally, we would like to take this opportunity to address a question that was not raised in Dr. Kapteyn's letter but has been asked by many peers: How can a single subfemtosecond pulse be observed without the carrier-envelope phase of the few-cycle laser pulse being stabilized? Numerical simulations of Nenad Milosevic and Thomas Brabec (co-authors of our published research) and of other researchers have delivered a simple and intuitive answer. The continuous spectrum in the cutoff range that is passed by the filter is produced exclusively by few-cycle pulses having an instantaneous electric field that peaks close to the pulse peak ("cosine" pulse). Pulses with other values of the carrier-envelope phase produce substantially weaker harmonic radiation at the highest harmonic photon energies observed, because the maximum electric field is significantly reduced in this latter case. Consequently, these pulses make little contribution to the measured electron signal in experiments performed with pulses exhibiting randomly varying carrier-envelope phase. Stabilization and proper setting of the carrier-envelope phase of few-cycle pulses is therefore expected to substantially improve the photoelectron yield in future attosecond pump-probe experiments.

Ferenc Krausz
Vienna University of Technology
[email protected]
Paul Corkum
National Research Council of Canada
[email protected]

REFERENCE

1.Hentschel et al., Nature 414, 509 (2001).

FBGs still offer economical model

In the article, "Thin films improve 50-GHz DWDM devices," the authors make the claim that thin-film filter technology has made strides that in many ways are better than fiber Bragg gratings (FBGs) in DWDM systems.

While it is certainly true that thin-film filters have come a long way in recent years, I think that FBGs still offer a more economical model in DWDM transmission/receive systems. The authors state that, "Fiber Bragg gratings, too, have their drawbacks. They can only be used in small-channel-count 50-GHz devices and suffer from high insertion loss and chromatic dispersion." At Ciena, we have been using FBGs in our 96-channel, 10-Gbit/s, 50-GHz-spaced systems for several years. Our next-generation transport DWDM system uses 25-GHz, ultra-low-dispersion FBGs to allow a carrier to pack in a whopping 192-channels-all in the C-band. While insertion loss and chromatic dispersions are critical specs that can affect a grating's performance, they are not impossible to overcome. Soon, Ciena will be moving toward 12-GHz spacing in the C-band utilizing FBG technology. While FBGs require lasers, optics, and an OSA for fabrication, the cost of, say, 2 m of fiber is far cheaper than GRIN lenses, thin-film filter, and coating technology. In addition, we've engineered a world-class grating fabrication facility at Ciena that allows us fast throughput and high yields through a repeatable process.

I find all the periodicals to which I subscribe from Pennwell are extremely informative.

Michael Boyd
Lead Optical Support Engineer
Gratings and Rainbow Manufacturing
Ciena Corp.
[email protected]

Laser TV projection is still a long way off

Your article on laser displays in entertainment ("Laser displays move from the amphitheater toward the living room," May 2002, p. 91) may give readers the mistaken impression that projecting large-scale television-type imagery with lasers is a "typical" application of the technology. If only it were so! A raster TV-projection is the most expensive and complex display that can be attempted with a laser projector. Although two or three companies are building laser-TV projectors, such devices remain pricey and are difficult to engineer from both an optical and mechanical perspective. They may find a home in ultraspecialized locations (theme parks, Las Vegas casinos), but don't expect to see them at your neighborhood theater any time soon. Most laser displays remain vector-based, which is great for beam shows, abstract imagery, and "outline-style" graphics (to see the very best of the genre, visit our web site: www.laserist.org/Laserist).

Manufacturers of full-color solid-state projectors such as the Schneider Showlaser mentioned in the article are to be commended for breaking new ground, but the big trend in the industry is the mushrooming growth of medium-powered green YAGs, which produce eye-popping beams seen in shows around the world. When a manufacturer successfully develops a solid-state blue laser that can be paired with existing solid-state green and red lasers, the market for affordable full-color laser projectors will really take off.

David Lytle
Editor, The Laserist
[email protected]

Correction

The Trumpf (Ditzingen, Germany) training facility in Plymouth, MI, was called a new facility in "Germany sets sights on educating U.S. customers" (June 2002, p. 162). The facility was actually established in 1995 and was expanded earlier this year.