Generating ultrashort light pulses requires careful control of the light’s dispersion: Phase velocity depends on frequency, and because a real pulse contains a spread in frequency, it will broaden as it travels through an optical medium. Simple, inexpensive sources of sub-picosecond pulses, soliton lasers consist primarily of a laser diode and an optical fiber. They mitigate spreading by balancing it against Kerr focusing—the narrowing of a pulse caused when light’s electric field alters the medium’s refractive index—so each pulse travels as a soliton, and its duration remains unchanged. (For more on Kerr focusing, see Physics Today, August 2001, page 17.)
Soliton lasers are attractive because of their simple construction, but they can’t achieve the high energies of techniques such as chirped-pulse amplification (see Physics Today, December 2018, page 18). That’s because the energy E and duration τ vary inversely, so shortening a pulse only increases its energy so much.
Now Antoine Runge and coworkers at the University of Sydney, with collaborators at Macquarie University and Nokia Bell Labs, have overcome that limitation. Their new pure-quartic soliton laser uses a spatial light modulator (SLM) to manipulate the light’s dispersion relation to allow for higher energy pulses.
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