Femtosecond pulsed lasers—which discharge light in ultrafast bursts enduring for a millionth of a billionth of a second—are robust tools employed in several applications from manufacturing and medicine, to sensing and precision measurements of time and space.
These lasers are generally high-end tabletop systems, which restricts their use in applications with size and power consumption limitations.
An on-chip femtosecond pulse source would unravel new applications in optical communications, astronomy, quantum and optical computing, and beyond. However, it has been difficult to incorporate tunable and extremely efficient pulsed lasers onto chips.
Currently, scientists from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have created a high-performance, on-chip femtosecond pulse source using an instrument that appears to have its roots in science fiction: a time lens.
Details of the study are published in Nature.
“Pulsed lasers that produce high-intensity, short pulses consisting of many colors of light have remained large. To make these sources more practical, we decided to shrink a well-known approach, used to realize conventional—and large—femtosecond sources, leveraging a state-of-the-art integrated photonics platform that we have developed,” said Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and the study’s senior author.
Importantly, our chips are made using microfabrication techniques like those used to make computer chips, which ensures not only reduced cost and size, but also improved performance and reliability of our femtosecond sources.
Marko Lončar, Senior Study Author and Tiantsai Lin Professor of Electrical Engineering, John A. Paulson School of Engineering and Applied Sciences, Harvard University
Conventional lenses, such as contact lenses or those seen in microscopes and magnifying glasses, bend light rays emanating from various directions by changing their phase so that they reach the same site in space—the focal point.
Time lenses, in their place, “bend” light rays in similar ways—but they change the phase of light beams in time instead of space. In this manner, various colors of light, which move at various speeds, are re-timed to reach the focal point simultaneously.
Visualize a car race, wherein each color of light is a diverse car. First, the time lens staggers the leave time of each car, then fixes their speed, so they reach the finish point at the same time.
To produce femtosecond pulses, the device created by the team employs a series of optical waveguides, modulators, couplers, and optical grating on the lithium niobate platform prepared by Lončar’s laboratory.
The researchers begin by transmitting a continuous-wave, single-color laser beam via an amplitude modulator that regulates the amount of light passing via the time-lens, a function akin to a conventional lens’ aperture. The light then spreads through the “bendy” area of the lens, a phase modulator in this case, where a frequency comb of diverse colors is produced.
Reverting to the car analogy, the phase modulator forms and then discharges the cars of diverse colors at various starting times.
Then, the last component of the laser is presented—a fishbone grating along the waveguide. The grating alters the speed of the various colors of light to arrange them all in line with each other, side by side, in the race so that they reach the end line (or focal plane) at the same time.
Since the device regulates how quickly various wavelengths travel and when they reach the focal plane, it efficiently alters the continuous, single-color laser beam into a broadband, high-intensity pulse source that can yield ultra-fast, 520 femtosecond bursts.
The device is extremely tunable, incorporated onto a 2 cm by 4 mm chip, and owing to the electro-optical properties of lithium niobate, necessitates considerably minimal power than tabletop systems.
“We’ve shown that integrated photonics offers simultaneous improvements in energy consumption and size,” said Mengjie Yu, a former postdoctoral fellow at SEAS and the study’s first author.
There’s no tradeoff here; you save energy at the same time you save space. You just get better performance as the device gets smaller and more integrated. Just imagine—in the future, we can carry around femtosecond pulse lasers in our pockets to sense how fresh fruit is or track our well-being in real time, or in our cars to do distance measurement.
Mengjie Yu, Study First Author and Former Postdoctoral Fellow, John A. Paulson School of Engineering and Applied Sciences, Harvard University
Yu is at present an Assistant Professor at the University of Southern California.
Going forward, the researchers aim to look for some of the applications for the laser as well as the time lens technology, including in lensing platforms such as telescopes as well as in quantum networking and ultrafast signal processing.
Harvard’s Office of Technology Development has safeguarded the intellectual property generated from the Loncar Lab’s advances in lithium niobate systems. Loncar is a cofounder of HyperLight Corporation, a startup that was formed to market integrated photonic chips based on particular innovations created in his laboratory.
The study was a partnership between Harvard, Columbia University, HyperLight, and Freedom Photonics.
The article’s co-authors included 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.
The study was supported by the Defense Advanced Research Projects Agency (HR0011-20-C-0137), the Army Research Office (W911NF2010248), the Office of Naval Research (N00014- 18-C-1043), and the Air Force Office of Scientific Research (FA9550-19-1-0376 and FA9550-20-1-0297).
Yu, M., et al. (2022) Integrated femtosecond pulse generator on thin-film lithium niobate. Nature. doi.org/10.1038/s41586-022-05345-1.