For decades, ultrafast lasers have been one of the most powerful tools in modern optics. Its pulses last just a few hundred femtoseconds, or one quintillionth of a second, and enable a variety of technologies, from precision manufacturing and eye surgery to optical frequency combs, a Nobel Prize-winning technology that powers the world’s most accurate optical atomic clocks.
Despite their importance, these lasers remain large and expensive systems that occupy entire optical tables.
Now, researchers led by Professor Tobias J. Kippenberg at EPFL have achieved a breakthrough that could dramatically scale down this technology. Writing in progress naturethe research team reports the first integrated ultrafast laser that rivals the performance of traditional tabletop femtosecond lasers. The device provides a pulse energy of 1.05 nanojoules and a short pulse duration of 147 femtoseconds, all from a photonic chip.
Introducing ultrafast lasers to photonic chips
Photonic chips use microscopic structures called waveguides etched into the wafer to manipulate light. In much the same way that electronic chips channel electrical signals, photonic chips channel and process light.
These chips are already widely used in communications and are helping to miniaturize many optical technologies that previously required much larger equipment.
“For more than 20 years, high-pulse-energy femtosecond lasers on a chip have been widely considered the holy grail of integrated photonics,” Kippenberg says. “Our results show that not only is it possible, but it can be achieved with a surprisingly elegant architecture that has been overlooked by the integrated photonics community.”
The overlooked effects of laser design
To accomplish this feat, the researchers employed a laser architecture known as a Mamishev oscillator, a design that has received relatively little attention in integrated photonics.
This system places a nonlinear waveguide between two optical filters, each of which transmits a different part of the optical spectrum. When a powerful laser pulse passes through a waveguide, the pulse spreads out over a wider range of colors. A portion of the expanded pulse passes through both filters and continues to circulate within the laser cavity.
Weaker light behaves differently. Because it doesn’t spread far enough, it is blocked by the filter and removed from the cycle.
“This design is particularly attractive because it does not require components that are difficult to fabricate on this erbium-doped silicon nitride chip,” explains Zheru Qiu, co-lead author of the paper.
According to Qiu, this design has another big advantage. Photonic chips confine light into very small waveguides, making it strongly interact with itself. In many laser architectures, these nonlinear effects can cause laser pulse instability. However, Mamishev oscillators are much less susceptible to these problems, making them particularly suitable for integrated photonic devices.
Small device, big potential
The length of the laser cavity is 42 centimeters, but it can be folded onto a chip that occupies almost the area of the match head. This makes it significantly smaller than traditional fiber-based ultrafast lasers.
Photonic chips can be manufactured at wafer scale using methods similar to those employed in computer chips, potentially allowing more than 1,000 laser cavities to be manufactured simultaneously. This manufacturing advantage has the potential to significantly reduce the cost of ultrafast lasers while expanding their availability in sensing, spectroscopy, and precision measurement applications.
“With kilowatt-level peak power, this chip can drive demanding applications that have long relied on large, expensive laboratory lasers,” Qiu said.
Researchers believe this technology could eventually lead to portable, affordable devices for detecting environmental contaminants, identifying hidden defects in materials, and performing medical diagnostics. It could also help pave the way for compact optical atomic clocks that could play a key role in future communication and navigation systems.
Researchers from the EPFL Institute for Electrical Microengineering and Dresden-Rossendorf-Helmholtzzentrum (HZDR) participated in the study.
