Fellows Ralph Jimenez and Henry Kapteyn and their groups recently helped develop optical technology that will make femtosecond laser experiments much simpler to perform, opening the door to using such lasers in many more laboratories. The technology, which employs reflection grisms as laser pulse compressors, has been patented and is now available commercially. A reflection grism consists of a metal reflection gratings mounted on one face of a prism. Properly designed, they can compensate for dispersion (with efficiencies of up to 90%) from hundreds of meters of glass (fiber) path and withstand high average powers. They are also easy to align.

Reflection grisms are a major improvement over traditional transmission grisms whose low efficiency made them less than ideal for dispersion compensation and pulse compression. With high-efficiency reflection grisms, it’s a whole new ballgame. With them, laser light first diffracts from the grating and then propagates through the prism. It then refracts out of the prism at an air-glass interface tilted at a large angle with respect to the grating. Precise design of this exit angle is responsible for the high diffraction efficiency of the new grisms.

High efficiency is a key reason why grisms make good pulse compressors. It means they can be sized and aligned to precisely oppose the pulse stretching and asymmetric distortions that naturally occur when a laser pulse travels through a dispersive material such as glass. For instance, all optical materials in the 700–1100 nm wavelength range cause group-delay dispersion (GDD), which is linear with frequency and third-order dispersion (TOD), a delay that is quadratic with frequency. Both GDD and TOD are positive in sign. Pulse compressors made of reflection grisms can be designed to cause negative GDD and TOD in the same proportions as occur in optical materials.

Inserted into a laser’s optical path, pulse compressors employing reflection grisms ensure that a laser pulse reaching its destination looks just like the high-quality pulse leaving the laser — fully compressed, with high-peak power. Research associate Emily Gibson, graduate student David Gaudiosi, Fellow Margaret Murnane, and colleagues from the Colorado School of Mines, Cornell University, and Horiba Jobin Yvon, Inc. worked with Jimenez and Kapteyn to develop the new high-efficiency pulse compressors.

The new technology was applied to downchirped-pulse amplification (DPA), a relatively new technique for producing very high-average-power amplified laser pulses. The original technique stretched the pulses with gratings, amplified them in a Ti:sapphire crystal, and compressed them by sending them through 1–2 m of glass. The new grism-based technique stretches the pulses with negative GDD gratings, amplifies them with a specially designed cryogenically cooled Ti:sapphire crystal and multipass amplifier, and compresses them through 120 cm of positive GDD glass. It can produce pulses as short as 35 fs.

In related efforts, the researchers showed they could use a pair of reflection grisms spaced 1.2 cm apart to fully compensate for the dispersion in 10 m of optical fiber. This result suggests that reflection grisms could be useful in multiphoton microscopy and other applications that rely on fiber delivery of femtosecond pulses.—Julie Phillips

Reference:
Emily A. Gibson, David M. Gaudiosi, Henry C. Kapteyn, and Ralph Jimenez, Optics Letters, 31, 3363 (2006).

David M. Gaudiosi, Etienne Gagnon, Amy L. Lytle, Julie L. Fiore, Emily A. Gibson, Steve Kane, Jeff Squier, Margaret M. Murnane, Henry C. Kapteyn, Ralph Jimenez, and Sterling Backus, Optics Express, 14, 9277 (2006).

Steve Kane, Jeff Squier, Ralph Jimenez, Lyuba Kuznetsova, Frank Wise, Henry Kapteyn, and Bruno Touzet, Laser Focus World, 43, 95 (2007).

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