Kyoto University develops photonic-crystal laser for long-distance space communications

A research group led by Associate Professor Takuya Inoue, Distinguished Professor Susumu Noda, Lecturer (part-time) Ryohei Morita, Professor Menaka De Zoysa, and Program-Specific Associate Professor Kenji Ishizaki of the Graduate School of Engineering and the Institute for Advanced Study, Kyoto University, in collaboration with KDDI Research Inc. and Chitose Institute of Science and Technology, has successfully developed a new photonic-crystal laser. It is capable of modulating oscillation frequency with high efficiency and high speed for applications in long-distance free-space communications, including space optical communications. This achievement represents a major step toward realizing ultra-compact, lightweight, and highly efficient satellite-mounted optical transmitters, as the photonic-crystal laser can replace optical transmitters composed of numerous optical devices. The results were published in the online edition of Nature Photonics.

In next-generation mobile communication systems, the range of communication is expected to expand both on the ground and into space. Even now, communications between low Earth orbit satellites at altitudes below 2,000 kilometers are useful in areas with insufficient ground communication infrastructure. During disasters, geostationary orbit satellites at altitudes of 36,000 kilometers enable real-time transmission of large volumes of data from Earth observation satellites. However, such long-distance optical communications in space require optical transmitters that combine semiconductor lasers, external modulators, large optical amplifiers, transmission lenses, and other components. Because many optical devices are needed, the systems are large, and it is costly to ensure reliability. In tandem, they suffer from poor energy efficiency.

The research group has previously achieved high output power of 1 to 50 watts in continuous operation by cleverly creating a structure of photonic crystals. In September 2022, they achieved high-speed transmission of optical signals without amplifiers or transmission lenses by using a watt-class photonic-crystal laser. However, when attempting to transmit over 380,000 kilometers between Earth and the Moon, methods that modulate light intensity resulted in the optical energy being attenuated to less than one hundred millionth of the transmitted level. As signal discrimination is difficult, the propagation distance is limited to approximately 2,000 kilometers.

Therefore, research and development proceeded based on the principle that frequency changes can be detected even from weak light. This is accomplished by modulating the frequency of transmitting optical signals and causing the attenuated optical signals to interfere with reference light.

Photonic-crystal lasers normally can only output a fixed frequency. In this study, the research group succeeded in developing a photonic-crystal laser that can output lasers of different frequencies on both sides of the photonic crystal by finely adjusting the structure of only half of the photonic crystal (changing the distance between crystal holes by about 0.01%). Through varying the magnitude of current flowing to the left and right, the frequency of the output laser can be adjusted.

In communication experiments simulating propagation loss in space, they demonstrated that communication could be established even when light was attenuated approximately 2 to 3 times stronger than conventional photonic-crystal lasers. This makes intersatellite communication over propagation distances of approximately 60,000 kilometers feasible. The research group is advancing improvements in device design to achieve even longer distances. By enlarging and optimizing photonic-crystal devices, space communications over 380,000 kilometers between the Moon and Earth could become possible.

Inoue commented: "What I find interesting about this research is that simply opening the spacing of holes by a truly tiny amount can dramatically extend the communication distance. It's very fascinating that controlling the nanometer scale produces effects over tens of thousands of kilometers. I believe there is still tremendous potential in photonic-crystal lasers. As we work to realize space communications, I want to advance research to control the physics of photonic-crystal lasers and the physics of the nanoscale world for practical applications."

Journal Information
Publication: Nature Photonics
Title: Frequency-modulated high-power photonic-crystal surface-emitting lasers for long-distance coherent free-space optical communications
DOI: 10.1038/s41566-025-01782-2