Ultrafast Measurement Leaps Forward
Brent D. Johnson
Ultrafast lasers have applications in telecommunications, microlithography and genetic sequencing. However, their intensity and phase vs. time have always been difficult to measure.
In addition, femtosecond pulses are often plagued with spatial chirp, in which the wavelength varies spatially across the beam. Consequently, many applications require characterization of not only these attributes, but often the beam spatial profile as well. The collective diagnostic devices required would fill an optical table and would be very expensive.
Rick Trebino of Swamp Optics LLC has developed an alternative: a frequency-resolved optical gating, or Frog. The technology accurately characterizes the full intensity and phase of the pulse vs. time and frequency, yielding both the spatial profile and the beam spatial chirp.
"Autocorrelators measure intensity of the pulse, but [they give] a smeared-out version of the pulse," Trebino said. Frog sees the structure in the intensity and phase of the pulse by splitting it in two. One pulse is variably delayed, and both beams pass through a nonlinear crystal before they are spectrally resolved.
Although thin nonlinear crystals generate the second harmonic of all colors in the forward direction, thicker crystals disperse them at an angle, serving as a sort of prism. A thicker crystal simplifies the optical configuration and alignment of the instrument for characterizing femtosecond pulses. Courtesy of Swamp Optics.
Trebino discovered that he could split the beam with a single Fresnel biprism rather than combining a beamsplitter, delay line and beam-combining optics. He also found that a thick second-harmonic-generation crystal disperses the output beam in angle, thereby serving simultaneously as a second-harmonic-generation crystal and spectrometer. The arrangement is so simple that it never needs alignment, he said.
Trebino calls this new device "Grenouille," which is French for frog. The current Grenouille, available from Swamp Optics, measures pulses 50 to 300 fs in length and covers the range of most commercial Ti:sapphire lasers. A new infrared version will measure pulses over the 1- to 2-µm range, and an extremely short pulse version will measure pulses as short as 10 to 15 fs.
David N. Fittinghoff of Lawrence Livermore National Laboratory has applied the technology to a thomson scattering project designed to produce x-rays for materials science studies. His unit, actually a demo model, measures 150- to 200-fs pulses out of an 800-nm Ti:sapphire laser system used to drive a photo-gun for a linear accelerator. He said the Grenouille was very easy to connect and align.
"All you have to do is point the light into it. It measures intensity and phase of the pulse," he said, adding that, "This one is by far the cheapest. You turn it on, capture the trace and run the code."
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