Semiconductor Review: Specials Magazine

Ultrashort laser pulses are the shortest technological events ever created. But that doesn’t mean they’re too exotic to be useful. They now have numerous applications, ranging from ophthalmology and materials characterization to micromachining. They are even used to study ultrafast events in atoms and molecules—provided that the pulse is shorter than the event. But measuring the pulse itself, to confirm its extreme brevity, proved to be quite a challenge, as there is no shorter event available to measure it. For decades, the best that could be done was “autocorrelation,” which provided only a blurry black-and-white snapshot of the pulse in time. Worse, when pulses had unstable intensity shapes vs. time, autocorrelation had a notorious artifact, called the “coherent artifact,” which yielded an erroneously short pulse. As instability is almost a way of life with ultrafast lasers, this was quite problematic. The complete artifact-free temporal measurement of such pulses became a famous unsolved—perhaps even unsolvable—problem. But, in a truly inspired insight, Rick Trebino solved it. He noted that, by measuring a novel type of spectrogram of the pulse that used the pulse to gate itself in time and then employing sophisticated phase-retrieval mathematics, borrowed from astronomical imaging, he could completely reconstruct the pulse in time. This yielded a high-resolution full-color image of the pulse in time. He called his device Frequency-Resolved Optical Gating, or FROG. Features that had once seemed immeasurable—few-femtosecond intensity and phase variations—came into sharp focus. Even better, FROG doesn’t suffer from the coherent artifact. Indeed, because it provides so much more information in its measurement vs. time and frequency, FROG also clearly indicates the presence of pulse-shape instability or confirms its absence. The Instrument that Made Ultrafast Accurate The earliest FROG devices were groundbreaking in capability, but complex (like most other optical devices), requiring the steady hands and deep expertise of trained experimentalists. So, Rick then introduced a very clever, highly simplified version of FROG that he called GRENOUILLE (the French word for “frog”). GRENOUILLE reduces the entire FROG apparatus to an alignment-free monolithic block of only four simple optics plus a camera. The resulting device never misaligns, even in a hostile environment. Although compact in size, it is remarkably effective, operating multi- or single shot and yielding the pulse nearly instantaneously. For laser labs accustomed to highly complex apparatuses, painstaking daily realignment procedures and tediously slow data collection, this was revolutionary. With his wife, Linda, Rick founded Swamp Optics (of course; frogs live in swamps) to bring these breakthroughs to the research and engineering communities. GRENOUILLE gave the ultrafast optics community a truly convenient way to know their pulses to a quintillionth of a second.