A green flashlight beam dives into a pool and reaches the bottom with barely any loss. Water is transparent to green light, which is why it's used for underwater communication. But to send video or sensor data, the light needs to be switched on and off very quickly — billions of times per second. Ordinary lasers are too slow and unstable for this.
Engineers took an ultra-thin plate of lithium tantalate crystal — a material that changes its properties under an electric field. This crystal acts like a faucet: it opens or shuts off the light flow on command. Unlike similar lithium niobate, the new material doesn't degrade under intense light and keeps the signal steady for a full hour. With this plate, they managed to transmit 112 billion bits per second over a distance of three meters underwater.
This modulator is the first to eliminate the need for direct laser modulation. Previously, the laser was 'yanked' directly, causing signal distortion at high speeds. Now the laser shines steadily, and a separate crystal handles the fast switching. It's like the difference between shouting while running and speaking calmly into a microphone: the voice stays clear, and the volume is controlled by an amplifier.
Green light emerges from the water again, but now it carries not just a beam, but a whole stream of data. A crystal plate the size of a fingernail has replaced bulky setups and opened the door for compact underwater robots that can transmit real-time video without wires.
Seawater absorbs red light most strongly, which is why everything looks blue-green at depths greater than 10 meters. That's why a green laser with a wavelength of 532 nanometers was chosen for underwater communication: it travels the farthest.
In James Cameron's film 'The Abyss', characters communicate with an underwater station via cable. The new technology allows video transmission without wires, as if the underwater world got its own Wi-Fi on green light.