An ultrafast laser on a chip

EPFL researchers have built a photonic chip that rivals much larger laboratory lasers, delivers extremely short, high-energy optical pulses and could reshape technologies like medical diagnostics and optical atomic clocks.

Ultrafast lasers emit pulses lasting only a few hundred femtoseconds, or quadrillionths of a second. These flashes of light power applications from precision micromachining and eye surgery to optical frequency combs, the Nobel Prize-winning technology behind today’s most precise optical atomic clocks. Yet despite more than two decades of effort, ultrafast lasers have largely remained bulky, expensive systems confined to optical tables.

Now a team led by Professor Tobias J. Kippenberg at EPFL has brought them onto a photonic chip. Publishing in Nature, the researchers report the first integrated ultrafast laser that rivals table-top femtosecond lasers, delivering 1.05 nanojoules in pulses as short as 147 femtoseconds.

Photonic chips guide and process light in microscopic channels called waveguides patterned on a wafer, much like how electronic microchips route electricity. Already widely used in telecommunications, photonic chips have miniaturised complex functions that once required much larger systems.

“For more than 20 years, a high-pulse-energy femtosecond laser on chip was widely regarded as a holy grail of integrated photonics,” Kippenberg said. “Our result shows that it is not only possible, but that it can be achieved with a surprisingly elegant architecture that the integrated-photonics community had overlooked.”

An overlooked design

The EPFL team turned to a largely overlooked laser design known as the Mamyshev oscillator. In the laser cavity, a nonlinear waveguide sits between two optical filters that each let through a different slice of the colour spectrum. When a strong pulse travels through the waveguide, it broadens into a wider range of colours, allowing part of it to pass through both filters and keep circulating. Weak light does not broaden enough and is rejected.

“This design is especially attractive because it does not require any component that is difficult to make on this erbium-doped silicon nitride chip,” explained Zheru Qiu, a co-leading author of the paper.

Image caption: EPFL’s chip-based ultrafast laser operating in the laboratory test set-up. The device produces extremely short laser pulses directly on a photonic chip. Image credit: EPFL/Zheru Qiu.

Tiny laser, broad impact

On the chip, the 42 cm-long laser cavity can be folded into a space the size of a match head, far smaller than optical fibre-based lasers. Because these photonic chips can be manufactured at wafer scale, much like computer chips, more than 1000 laser cavities could be produced at once, opening a path towards much lower-cost ultrafast lasers for sensing, spectroscopy and metrology. “With kilowatt-level peak powers, the chip can drive demanding applications that have long depended on large, expensive laboratory lasers,” Qiu said.

The result could lead to portable and affordable tools for detecting pollutants, revealing hidden defects and performing medical diagnostics, while opening a path towards compact optical atomic clocks for future communication and navigation.

This is a modified version of a news item published by Ecole Polytechnique Fédérale de Lausanne (EPFL) under CC BY-NC-SA 4.0. This version is similarly licensed under CC BY-NC-SA 4.0. The original version of the news item can be accessed here.

Top image caption: Placed on a 1 CHF coin for scale, EPFL’s photonic chip shows how a laser architecture once confined to table-top systems can be shrunk to the millimetre scale. Image credit: EPFL/Zheru Qiu.