A digital image of EPFL’s logo crossed a 30-centimeter column of water on a beam of blue light, then appeared again at the other end. The test was a first step toward using a compact laser to send information underwater—and it showed how carefully controlled light can also help detect particles in the air.
Researchers at EPFL in Lausanne developed a chip-scale blue laser that combines steady light with rapid control over its frequency, or the rate at which its light waves repeat. Blue light can travel through water more easily than infrared light, which is common in optical communication systems. It also scatters strongly off tiny airborne particles. Those qualities could make blue lasers useful for underwater links and tools that monitor pollution. But such uses need light stable enough to separate a signal from background noise, with the ability to change frequency quickly when sending data or taking measurements.
The team, led by Tobias J. Kippenberg and Simone Bianconi, built the device by connecting a conventional blue laser to a photonic chip. A tiny ring on the chip selects a precise light frequency and sends some of that light back into the laser, helping steady its output. Making a blue laser this compact is challenging: small imperfections can scatter blue light and weaken the beam. To reduce losses, the researchers made the chip’s light-guiding structures just 25 nanometers thick. They also used aluminum nitride, a material that changes shape when voltage is applied. The resulting small stresses let the team alter how light travels through the ring and tune the laser quickly.
The laser operated at a wavelength of about 461 nanometers. In one setup, its light had a linewidth below 30 kilohertz. Linewidth is a measure of how narrowly a laser holds its frequency; a smaller value means steadier light. In another setup, the researchers shifted the frequency across a range greater than 900 megahertz, and could repeat those adjustments up to one million times per second.
To test underwater communication, the team encoded a digital image of the EPFL logo in changes to the laser’s frequency and sent it through the water. The receiving system reconstructed the image. In a separate laboratory test, the laser detected candle smoke as a clear change in its signal. Under comparable conditions, an infrared laser at 1,550 nanometers showed no detectable change. The experiment demonstrated sensitivity to airborne particles, but did not measure how much smoke was present.
The results, published in Nature Photonics in 2026, are demonstrations rather than a finished communication or pollution-monitoring system. Still, they show that precise, quickly adjustable blue light can be made on a small chip and used in both water and air tests. The same control may also matter for atomic clocks and quantum computing, where stable blue light is useful. The next steps will determine how the technology performs beyond these laboratory demonstrations.