New Method for Ultrafast Laser Pulses

Graphic depicting how specific frequencies of light emerge from the electronic background noise (blue) in a new ultrafast electro-optic laser. The vertical backdrop shows how these colors combine to create an optical frequency comb. (Source: D. Carlson, NIST)

Physicists at the National Institute of Standards and Tech­nology NIST have used common electronics to build a laser that pulses 100 times more often than conven­tional ultrafast lasers. The advance could extend the benefits of ultrafast science to new appli­cations such as imaging of biolo­gical materials in real time. The tech­nology for making electro­optic lasers has been around for five decades, and the idea seems alluringly simple. But until now researchers have been unable to elec­tronically switch light to make ultrafast pulses and eliminate electronic noise, or inter­ference.

NIST scientists developed a filtering method to reduce the heat-induced inter­ference that otherwise would ruin the consistency of elec­tronically synthesized light. “We tamed the light with an aluminum can,” project leader Scott Papp said, referring to the cavity in which the elec­tronic signals are stabilized and filtered. As the signals bounce back and forth inside something like a soda can, fixed waves emerge at the strongest fre­quencies and block or filter out other frequencies.

The conven­tional source of ultrafast light is an optical frequency comb. Combs are usually made with sophis­ticated mode-locked lasers, which form pulses from many different colors of light waves that overlap, creating links between optical and microwave frequencies. Inter­operation of optical and micro­wave signals powers the latest advances in communi­cations, time­keeping and quantum sensing systems. In contrast, the new electro-optic laser imposes microwave electronic vibra­tions on a continuous-wave laser operating at optical frequencies, effectively carving pulses into the light.

“In any ultrafast laser, each pulse lasts for, say, 20 femto­seconds,” David Carlson said. “In mode-locked lasers, the pulses come out every 10 nano­seconds. In our electro-optic laser, the pulses come out every 100 pico­seconds. So that’s the speedup here – ultrafast pulses that arrive 100 times faster or more.” “Chemical and biological imaging is a good example of the appli­cations for this type of laser,” Papp added. “Probing bio­logical samples with ultrafast pulses provides both imaging and chemical makeup infor­mation. Using our tech­nology, this kind of imaging could happen drama­tically faster. So, hyper­spectral imaging that currently takes a minute could happen in real time.”

To make the electro-optic laser, the researchers start with an infrared continuous-wave laser and create pulses with an oscil­lator stabi­lized by the cavity, which provides the equivalent of a memory to ensure all the pulses are iden­tical. The laser produces optical pulses at a micro­wave rate, and each pulse is directed through a microchip wave­guide structure to generate many more colors in the frequency comb.

The electro-optic laser offers unpre­cedented speed combined with accuracy and stabi­lity that are comparable to that of a mode-locked laser, Papp said. The laser was constructed using commer­cial telecommu­nications and micro­wave components, making the system very reliable. The combi­nation of reliability and accuracy makes electro-optic combs attractive for long-term measure­ments of optical clock networks or communi­cations or sensor systems in which data needs to be acquired faster than is currently possible. (Source: NIST)

Reference: D. R. Carlson et al.: Ultrafast electro-optic light with subcycle control, Science 361, 1358 (2018); DOI: 10.1126/science.aat6451

Link: Time and Frequency Division, National Institute of Standards and Technology NIST, Boulder, USA

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