A photonic crystal laser developed at Kyoto University. A laser beam with a diameter of 3 millimeters is emitted from the circular section at the center (Courtesy of Professor Susumu Noda).
Researchers in Japan are working to enable optical communication in space by using photonic crystals, artificial materials that can control the behavior of light.
A research team led by Kyoto University Distinguished Professor Susumu Noda, with KDDI Research, has successfully demonstrated terrestrial optical communication over a distance equivalent to roughly 60,000 kilometers.
The results were first reported in the British scientific journal Nature Photonics.
Optical communication is better suited than radio waves for transmitting large volumes of data. As its range is extended, the technology is also expected to be used in crewed lunar missions, which operate roughly 380,000 kilometers from Earth.
Undersea Cables to Lunar Links
Both light and radio waves are forms of electromagnetic radiation, but light has much higher frequencies and correspondingly shorter wavelengths.
Higher frequencies allow more data to be transmitted. This is why most communication between Japan and the United States across the Pacific relies on undersea cables made up of bundles of optical fibers.
In outer space, where the use of satellites and related technologies is expanding, the volume of data being exchanged is increasing rapidly.
Communication traffic is expected to grow not only in low Earth orbit and geostationary orbit, around 36,000 kilometers above Earth, but also between Earth and the Moon, some 380,000 kilometers away.
Next-Gen Optical Lasers
Conventional optical communication systems rely on a combination of semiconductor lasers, modulators, and amplifiers to overcome low output power. This adds weight and bulk, increases system complexity, and raises the risk of failure. It also limits their suitability for satellites and other space-based applications.
By contrast, photonic crystals are artificial materials that manipulate light at the nanometer scale, controlling its propagation through refraction, reflection, and wavelength.
Lasers based on photonic crystals have high luminance and resist diffusion. Despite their compact size and energy efficiency, they can match the performance of much larger lasers.
The Japanese research team developed a method to map digital signals in variations of light density, assigning "0" to lower frequencies (longer wavelengths) and "1" to higher frequencies (shorter wavelengths).
By adjusting the light's frequency to match the transmitted data, they were able to encode information efficiently.
Extending Light Across Space
In 2023, the team announced at ECOC 2023 — the world's largest international conference on optical communications — that they had successfully conducted ground-based experiments.
These experiments demonstrated optical communication over a distance equivalent to approximately 36,000 kilometers, from the ground to geostationary orbit.
However, the Moon lies much farther than geostationary orbit, at roughly 380,000 kilometers from Earth. To extend communication distances more than tenfold, it's necessary not only to increase frequency variations but also to maintain a consistent light intensity.
The research team focused on improving the photonic crystal. These crystals control how light propagates by adjusting factors such as the spacing of tiny holes etched into the material.
Photonic Crystals Take Off
By combining two types of photonic crystals with slightly different hole spacing and a semicircular design, the team created a circular crystal, 0.5 millimeters in diameter, capable of emitting laser light.
One semicircle was assigned to the high-frequency range, and the other to the low-frequency range. By adjusting the current distribution, they were able to vary the frequency and produce wavelength densities corresponding to the digital signals 0 and 1.
The design also amplified the frequency variation. Because the total current remained constant, the light intensity stayed stable, reducing noise. Ground experiments successfully simulated optical communication over a distance of roughly 60,000 kilometers.
While the current technology still falls short of reaching the Moon, the path forward has become clear.
In December, Noda was selected to receive the prestigious United Kingdom-based Rank Prize for scientific excellence, a globally recognized and highly respected award.
"Our goal is to demonstrate optical communication between satellites and the ground using photonic crystal lasers," he said, commenting on future developments.
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Author: Shinji Ono, The Sankei Shimbun
(Read this in Japanese)
