New device opens doors to applications in communications, quantum computing and astronomy

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Femtosecond pulsed lasers – which emit light in ultra-fast bursts lasting a millionth of a billionth of a second – are powerful tools used in a range of applications from medicine and manufacturing to detection and precision measurements of space and time. Today, these lasers are typically expensive benchtop systems, which limits their use in applications with size and power consumption restrictions.

An on-chip femtosecond pulse source would open up new applications in quantum and optical computing, astronomy, optical communications and beyond. However, it has been difficult to integrate tunable and highly efficient pulsed lasers on chips.

Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed an on-chip high-performance femtosecond pulse source using a tool that looks straight out of science -fiction: a temporal lens.

The research is published in Nature.

“Pulsed lasers that produce short, high-intensity pulses composed of many colors of light remained important,” said Marko Lon?ar, Tiantsai Lin Professor of Electrical Engineering at SEAS and lead author of the study. “To make these sources more practical, we decided to scale down a well-known approach used to realize conventional – and large – femtosecond sources, taking advantage of a state-of-the-art integrated photonics platform that we have developed. , our chips are manufactured using microfabrication techniques like those used to manufacture computer chips, which not only ensures reduced cost and size, but also improved performance and reliability of our femtosecond sources.

Traditional lenses, such as contact lenses or those found in magnifying glasses and microscopes, distort rays of light coming from different directions by changing their phase so that they reach the same location in space – the focal point.

Time lenses, on the other hand, “bend” light beams in the same way, but they change the phase of the light beams in time rather than in space. In this way, different colors of light, which travel at different speeds, are resynchronized so that they each reach the focal plane at the same time.

Imagine a car race, in which each color of light is a different car. First, the time lens staggers each car’s start time, then adjusts their speed so they arrive at the finish line at the same time.

To generate femtosecond pulses, the team’s device uses a series of optical waveguides, couplers, modulators and optical gratings on the lithium niobate platform developed by the Lon?ar lab .

The team begins by passing a single-color, continuous-wave laser beam through an amplitude modulator that controls the amount of light passing through the temporal lens, a function similar to an aperture in a conventional lens. The light then travels through the “curved” part of the lens, a phase modulator in this case, where a frequency comb of different colors is generated. Going back to the car analogy, the phase modulator creates and then releases cars of different colors at different start times.

Then the final component of the laser comes into play – a herringbone grating along the waveguide. The network changes the speed of different colors of light to align them with each other, neck and neck in the race, so that they reach the finish line (or focal plane) at the same time

Because the device controls how fast the different wavelengths travel and when they reach the focal plane, it effectively transforms the continuous single-color laser beam into a source of high-bandwidth, high-speed pulses. intensity that can produce ultra-fast bursts of 520 femtoseconds.

The device is highly tunable, integrated on a 2cm by 4mm chip, and due to the electro-optical properties of lithium niobate, requires significantly reduced power compared to tabletop products.

“We have shown that integrated photonics offers simultaneous improvements in power consumption and size,” said Mengjie Yu, a former postdoctoral fellow at SEAS and first author of the study. “There are no compromises here; you save power at the same time as you save space. You just get better performance as the device becomes smaller and more integrated. Just imagine: in the future, we may carry femtosecond pulse lasers in our pockets to detect how fresh fruit is or track our well-being in real time, or in our cars to measure distance.”

Yu is currently an assistant professor at the University of Southern California.

Next, the team aims to explore some of the applications of the laser itself and time lens technology, including in lens systems like telescopes as well as in ultrafast signal processing and quantum networks.

Harvard’s Office of Technology Development has protected intellectual property arising from Loncar Laboratory’s innovations in lithium niobate systems. Loncar is a co-founder of HyperLight Corporation, a startup that was launched to commercialize integrated photonic chips based on certain innovations developed in his lab.

The research was a collaboration between Harvard, HyperLight, Columbia University and Freedom Photonics.

The article was co-authored by David Barton, Rebecca Cheng, Christian Reimer, Prashanta Kharel, Lingyan He, Linbo Shao, Di Zhu, Yaowen Hu, Hannah R. Grant, Leif Johansson, Yoshitomo Okawachi, Alexander L. Gaeta and Mian Zhang.

It was supported by the Defense Advanced Research Projects Agency (HR0011-20-C-0137), Army Research Office (W911NF2010248), Office of Naval Research (N00014-18-C-1043), and Air Force Office of Scientific Research (FA9550-19-1-0376 and FA9550-20-1-0297).

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