A Blue Laser For Underwater Communication And Sensing

Researchers at EPFL have developed a chip-scale blue laser that combines stable light emission with rapid frequency control, opening possibilities for underwater communication, environmental sensing and quantum technologies.

Blue light has some very useful properties: it travels through water more easily than infrared light, which is widely used in today's optical communication systems. It also scatters strongly when it encounters tiny particles in the air. These properties make blue lasers attractive for underwater communication, pollution monitoring and scientific instruments.

But these applications demand precise control over the laser's light. For example, the frequency must remain stable enough to distinguish a useful signal from background noise, while also changing rapidly when needed to transmit information or make measurements.

Existing blue laser systems often rely on bulky optical components to achieve this precision. Shrinking them onto photonic chips, which guide and control light through microscopic structures, has proved difficult. At blue wavelengths, even tiny imperfections in these structures scatter light and reduce performance.

Keeping blue light stable on a chip
A team led by Tobias J. Kippenberg and Simone Bianconi at EPFL has now developed a compact blue laser that addresses these challenges. Published in Nature Photonics, the device combines low-noise operation with fast electronic control of its frequency

The researchers connected a conventional blue laser to a chip containing a microscopic ring. The ring selects a precise frequency of light and feeds some of it back into the laser, helping it produce a steadier beam.

To reduce light losses, the team made the chip's light-guiding structures exceptionally thin, only 25 nanometres thick. They also added components made from aluminium nitride, a material that changes shape when voltage is applied. This produces tiny mechanical stresses that alter how light travels through the ring, allowing the researchers to adjust the laser's frequency rapidly.

The resulting laser operated at a wavelength of approximately 461 nanometres. One configuration produced exceptionally stable blue light, with a linewidth below 30 kilohertz, a measure of how precisely the laser maintains its frequency. Another allowed the researchers to adjust the frequency over a range exceeding 900 megahertz, repeating these adjustments up to one million times per second.

From underwater communication to smoke detection
The researchers then tested two potential applications. First, they transmitted information through a 30-centimetre column of water by encoding a digital image of the EPFL logo into changes in the laser's frequency. The receiving system successfully reconstructed the image, demonstrating the principle of underwater communication using the device.

In a second laboratory experiment, the researchers used the laser to detect candle smoke in air. The smoke produced a clear change in the detected signal, while an infrared laser operating at 1,550 nanometres showed no detectable change under comparable conditions. The results demonstrate the blue laser's sensitivity to airborne particles, although the experiment did not measure their concentration.

These demonstrations show how compact blue lasers might support future underwater communication systems and instruments for monitoring airborne particles. Their precise frequency control also makes the technology relevant to atomic clocks and quantum computing, where stable blue light is essential.