New Way to Generate Ultra-Short Bursts of Light

Alireza Marandi and Marc Jankowski prepare to carry out experiments at the optical bench to create new sources for femtosecond pulses. (Source: L.A. Cicero)

Although critical for varied appli­cations, such as cutting and welding, surgery and trans­mitting bits through optical fiber, lasers have some limi­tations as they only produce light in limited wave­length ranges. Now, researchers from the Ginzton Lab at Stanford Univer­sity have modified optical para­metric oscil­lators, to overcome this obstacle. Until now, these lesser-known light sources have been mostly confined to the lab because their setup leaves little room for error – even a minor jostle could knock one out of alignment. However, following a counter­intuitive decision, the researchers may have found a solu­tion to this weakness that could lead to smaller, lower-cost and more effi­cient sources of light pulses.

They demonstrate a new way to produce femto­second pulses in desirable wave­length ranges using this light source. The technology could poten­tially lead to better detection of pollu­tants and diseases by merely scanning the air or someone’s breath. The light source consists of an initial step where pulses of light from a tradi­tional laser are passed through a special crystal and converted into a wave­length range that’s difficult to access with conven­tional lasers. Then, a series of mirrors bounce the light pulses around in a feedback loop. When this feedback loop is syn­chronized to the incoming laser pulses, the newly converted pulses combine to form an increa­singly strong output.

Tradi­tionally, people could not convert much of the initial light pulses into the desired output with such a contrap­tion. But to be effective in real-world appli­cations, the group had to bump up that percen­tage. “We needed higher conver­sion effi­ciency to prove it was a source worth studying,” said Alireza Marandi, a staff member in the Ginzton Lab. “So we just said, ‘OK, what are the knobs we have in the lab?’ We turned one that made the mirrors reflect less light, which was against the standard guide­lines, and the conversion effi­ciency doubled.”

Cranking up the power in a conven­tional design usually results in two unde­sirable outcomes: The pulses lengthen and the conver­sion effi­ciency drops. But in the new design, where the researchers signi­ficantly decreased the reflec­tivity of their mirrors, the opposite occurred. “We were thinking about this regime based on the standard design guide­lines, but the behavior we would see in the lab was different,” said Marc Jankowski, graduate student in the Ginzton Lab. “We were seeing an improve­ment in perfor­mance, and we couldn’t explain it.”

After more simu­lations and lab experi­ments, the group found that the key was not just making the mirrors less reflec­tive but also lengthe­ning the feedback loop. This lengthened the time it took for the light pulses to complete their loop and should have slowed them too much. But the lower reflec­tivity, combined with the time delay, caused the pulses to interact in unex­pected ways, which pulled them back into synchroni­zation with their incoming partners.

This unanti­cipated synchroni­zation more than doubled the bandwidth of the output, which means it can emit a broader span of wave­lengths within the range that is difficult to access with conven­tional lasers. For appli­cations like detecting molecules in the air or in a person’s breath, light sources with greater bandwidth can resolve more distinct molecules. In principle, the pulses this system produces could be compressed to as short as 18 femto­seconds, which can be used to study the behavior of molecules.

The decision to reduce the mirror reflec­tivity had the surprising consequence of making a formerly per­snickety device more robust, more efficient and better at produ­cing ultra-short light pulses in wave­length ranges that are difficult to access with traditional lasers. The next challenge is designing the device to fit in the palm of a hand.

“You talk with people who have worked with this techno­logy for the past 50 years and they are very skeptical about its real-life appli­cations because they think of these reso­nators as a very high-finesse arrange­ment that is hard to align and requires a lot of upkeep,” said Marandi. “But in this regime of operation these require­ments are super-relaxed, and the source is super-reliable and doesn’t need the extensive care required by standard systems.”

This newfound design flexi­bility makes it easier to minia­turize such systems onto a chip, which could lead to many new appli­cations for detecting molecules and remote sensing. “Sometimes you completely reshape your under­standing of systems you think you know,” Jan­kowski said. “That changes how you interact with them, how you build them, how you design them and how useful they are. We’ve worked on these sources for years and now we’ve gotten some clues that will really help bring them out of the lab and into the world.” (Source: Stanford U.)

Reference: M. Jankowski et al.: Temporal Simultons in Optical Parametric Oscillators, Phys. Rev. Lett. 120, 053904 (2018); DOI: 10.1103/PhysRevLett.120.053904

Link: Edward L. Ginzton Laboratory, Stanford Univ., Stanford, USA

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