Light powers more of the modern world than people realize. The internet runs through fiber optic cables. Satellites talk to ground stations with laser links. Medical sensors and quantum experiments depend on precisely controlled beams. The catch is that those beams weaken as they travel, and the devices used to boost them back up — optical amplifiers — have traditionally been bulky, power-hungry, or both.
A team at Stanford has now built a compact optical amplifier roughly the size of a fingertip that can increase light intensity by about 100 times while drawing only a few hundred milliwatts of power. The device, described in the journal Nature, is small enough to operate from a battery and integrate onto a chip alongside other electronics — including, potentially, the ones inside a laptop or smartphone.
"We've demonstrated, for the first time, a truly versatile, low-power optical amplifier, one that can operate across the optical spectrum and is efficient enough that it can be integrated on a chip," said Amir Safavi-Naeini, the study's senior author and an associate professor of physics at Stanford. "That means we can now build much more complex optical systems than were possible before."
The energy recycling trick
Optical amplifiers work much like the amplifiers in a stereo, except they boost light instead of sound. Traditional chip-scale versions need significant power to operate, which limits how and where they can be deployed.
The Stanford team got around that with what doctoral student and co-first author Devin Dean called an "energy recycling trick." The amplifier uses a separate, intense beam of light — called a pump — to transfer energy into the signal being amplified. The cleverer the pump system, the more efficient the amplifier.
In this device, the pump light is generated inside a resonator: a structure in which the light circulates in a continuous loop, much like a racetrack. As it laps around, it builds in intensity, then transfers that intensity to the signal being amplified. Less pump energy goes to waste; more of it goes into useful amplification.
"By recycling the energy of the pump that powers this amplifier, we made it more efficient, and this doesn't come at a cost to its other properties," Dean said.
Quieter, broader, smaller
Like their audio cousins, optical amplifiers tend to add unwanted noise when they boost signals. The Stanford design keeps that added noise unusually low. It also operates across a wider band of wavelengths than typical chip-scale amplifiers, which means more data can be carried over a single device with less interference.
The combination — strong amplification, low power, low noise, broad bandwidth — is what makes the result notable. Each of those properties is achievable individually; getting all four at once on a chip-sized footprint has been the hard problem.
What it could power
Because the amplifier is both compact and energy-efficient, the team sees it slotting into a wide range of devices. The most obvious application is data communications, where light-based transmission keeps expanding to handle global internet traffic. But the same building block could improve biosensors that look for trace molecules, new compact light sources for spectroscopy, and the precision optical systems used in quantum computing and sensing.
"When you can do that, then the possibilities are really quite broad because they are so small that you can mass produce them and power them with batteries," Dean said.
The work was supported in part by DARPA, NTT Research, and the National Science Foundation, with co-authors including doctoral students Taewon Park, Sam Robison, Alexander Hwang, Luke Qi and Jason Herrmann, applied physics professor Martin Fejer, and postdoctoral fellow Hubert Stokowski.
The device is not yet in any consumer product. But it nudges integrated photonics — long promised as the optical equivalent of the silicon chip revolution — meaningfully closer to that future.
