Ultra-sensitive free-space optical communication method demonstrated using a photonic crystal laser by KDDI and Kyoto University

KDDI Corporation, KDDI Research, and Kyoto University (a research group led by Professor Susumu Noda, Research Fellow Ryohei Morita, and Assistant Professor Takuya Inoue of the Graduate School of Engineering) have successfully demonstrated an ultra-sensitive free-space optical communication method using a photonic crystal laser that precisely controls light. The three organizations believe that this method will be useful for long-distance optical communications in space while ensuring a compact system size and low power consumption. This achievement was presented as a post-deadline paper at ECOC 2023, the world's largest international conference on optical communications held during October 1−5 where it was highly appreciated and discussed.

A photonic crystal laser can achieve a transmission power equal to or greater than that achieved with large devices such as optical fiber amplifiers, using only a single semiconductor device without the need for large equipment such as optical fiber amplifiers. Consequently, it is expected to be utilized for significant downsizing of and reducing the power consumption in communication systems.

The three organizations have been conducting research and development of free-space optical communications using the photonic crystal laser. In this study, they conducted experiments combining the frequency modulation of the photonic crystal laser and a coherent reception method. Generally, when direct current is injected into a semiconductor laser, the intensity of the output light from the semiconductor laser is modulated according to the current. During this process, the frequency of the output light is also modulated simultaneously.

The researchers actively utilized this phenomenon. On the transmitting end of the system, they operated the photonic crystal laser as a more efficient and higher power 'frequency modulator' than traditional intensity modulators. On the receiving end, they incorporated a coherent reception method, leveraging the narrow linewidth of the photonic crystal laser (characteristics closer to those observed at the ideal single frequency). Consequently, the free-space optical communication method, which is capable of receiving extremely weak light signals with ultra-high sensitivity.

Experiments were conducted to demonstrate this method. The researchers used a 0.5 Gbaud non-return-to-zero (NRZ) electrical signal to directly drive a photonic crystal laser and generate a high-power optical frequency-modulation signal. The optical signal was attenuated by a factor of 100 million and demodulated after coherent reception, and it was confirmed that the original NRZ signal was recovered.

The photonic crystal laser is expected to be used in free-space optical communications due to its compact size and low power consumption. The achievement of this study demonstrates capability for communication between low-earth orbit satellites and geostationary satellites (the distance between these satellites is approximately 36,000 kilometers) and is expected to drive future developments in this field.