WHY THIS MATTERS IN BRIEF
As space based communication systems become more commonplace this record is a game changer for wireless communications.
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Researchers in California have achieved a breakthrough in Free-Space Optical (FSO) communications, transmitting data at a record-high 8.21 terabits per second without the use of fibre-optic cables. The experiment, published in Light: Science & Applications, demonstrates how advanced optical frequency “microcombs” can enable fibre-like performance through open air.
FSO systems use laser beams to transmit information wirelessly. The principle dates back to 1880 when Alexander Graham Bell demonstrated the photophone, sending speech via a beam of sunlight. Modern systems can already deliver up to 10 gigabits per second over about a kilometre and often outperform traditional radio-based wireless links. But atmospheric turbulence, beam drift and alignment issues have kept performance well below that of fibre, which remains the foundation of global internet infrastructure.
Fibre-optic networks currently achieve speeds of up to 402 terabits per second by using wavelength-division multiplexing (WDM) and advanced modulation techniques. For FSO to match that level, researchers need to adopt the same approaches while also addressing the added instability of open-air transmission.
The new study, led by Professor Chee Wei Wong at the University of California, Los Angeles, focused on micro resonator-based optical frequency combs, known as microcombs. These devices split a single laser into dozens or even hundreds of evenly spaced light frequencies. Each frequency can serve as a carrier channel, allowing many streams of data to be sent in parallel. In fibre-based tests, microcombs have enabled petabit-scale bandwidth.
For the latest demonstration, the team used a high-power platicon microcomb generated in a silicon nitride micro ring. The device produced over 100 coherent optical carriers spanning the C and L frequency bands. From these, 55 carriers were filtered, amplified and modulated using 16-state quadrature amplitude modulation (16QAM) at a rate of 20 gigabaud. Signals were transmitted in polarisation-diverse mode to double channel capacity.
The transmission crossed an outdoor free-space link of more than 160 metres. To maintain signal stability despite turbulence, the researchers employed an active beam-stabilisation system with a 10 kilohertz bandwidth, keeping alignment steady for more than 10 hours. They also introduced a “master–slave” carrier phase retrieval scheme to correct turbulence-induced intensity changes while reducing processing demands.
The result was an aggregated data rate of 8.21 terabits per second across all 55 carriers. In single-carrier trials, 35 channels delivered a combined 5.21 terabits per second. The team said the outcome shows that FSO can approach the performance levels of fibre-optic systems, while removing the need for physical cabling.
The researchers suggest that such technology could support a wide range of applications. Potential uses include high-speed optical interconnects inside data centres, metropolitan access networks where laying fibre is costly, and inter-satellite communication links in space.
More than a century after Bell’s photophone, the work highlights how photonics is again reshaping the future of communications. By integrating microcombs with coherent modulation in FSO systems, the study indicates a route towards compact, energy-efficient, fibreless networks capable of meeting surging global data demands.
The study, Terabits without fibres, by Zhenyu Xie and Qi-Fan Yang, was published on 2 July 2025 in Light: Science & Applications.
