World’s First 3D-Mode-Locked Laser

Doctoral student Logan Wright (l.) and Frank Wise, at Cornell University developing and testing new kind of mode-locked lasers. (Source: Cornell U.)

When you bang on a simple drumhead, it will most cer­tainly make a noise, but you probably wouldn’t call that noise a musical note. “You hit it, and it generates all of these vibra­tions in random relation­ships to each other. As a result, you get a thud,” said Frank Wise, the Samuel B. Eckert Pro­fessor of Engi­neering in the Department of Applied and Engi­neering Physics. Why does a drum make a thud, when clarinets and guitars can produce notes and chords? The difference is dimen­sionality: In a narrow tube or taut string, oscil­lations of the air within the tube or of the string are one-dimen­sional. In contrast, a drumhead is two-dimen­sional, and its vibra­tions usually are more compli­cated.

“But imagine if all of the vibrations of the drumhead were locked together,” he said. “Then you’d get a different thing – a note.” Essen­tially that is what Wise and doctoral student Logan Wright have done with their instru­ment of choice: the laser. Also contri­buting was Demetrios Christo­doulides, professor of optics and photonics at the Univer­sity of Central Florida. Wise and Wright have submitted a patent appli­cation for this poten­tially revo­lutionary laser method. The modes of a laser reso­nator are three-dimen­sional with variations along the axis of the resonator as well as two trans­verse dimen­sions – up and down, and side to side.

But the common laser – such as that found in a laser pointer – restricts opera­tion to a single trans­verse mode. “Nothing is changing in the transverse dimen­sions,” Wise said, “so it’s essen­tially a one-dimen­sional device.” That doesn’t mean the other patterns of light aren’t there, and Wise’s group has proven the ability to lock those other wave­lengths together. And not only are all the wave­lengths locked, but so are the different three-dimen­sional spatial patterns of laser light. “It’s locking of time and space,” Wise said.

“Light at high inten­sities behaves differently from what we’re used to,” Wright said. “We were able to design the laser so that this mode-locking happens by itself – inter­actions between those modes actually cause the light in the laser to self-organize.” Wright calls this “the world’s first three-dimen­sional mode-locked laser,” but what could it be good for? A few things, said Wise and Wright – who both were quick to point out that what they’ve done is demon­strate a concept, not invent a useful new tool. “It’s going to take a lot more work for that,” Wise said.

Potential appli­cations include super high-power lasers, since the light within the laser can be more spread out and therefore the peak inten­sity can be much lower and control­ling the emittance of the beam so as to create a specific pattern in space-time. In theory, this could be used to first catalyze a reaction in a compli­cated molecule with a pulse of light, then record details of that reaction with another pulse a few femto­seconds later. “This could be useful for studying reactions of very complex mole­cules, such as are found in biolo­gical systems,” Wise said.

Wise and Wright believe their idea could push forward a tech­nology that’s pretty much been locked in one mode for more than 50 years, since the laser’s inven­tion. “Just as you’d be surprised to hear a note come out of a drum,” Wise said, “people will be surprised to hear a spatio­temporal pulse come out of a laser. We’re using degrees of freedom that hadn’t been used before.” (Source: Cornell U.)

Reference: L. G. Wright et al.: Spatiotemporal mode-locking in multimode fiber lasers, Science 358, 94 (2017); DOI: 10.1126/science.aao0831

Link: School of Applied and Engineering Physics (F. W. Wise), Cornell University, Ithaca, USA


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