Optical Communications  
：Free-Space Optical Communications

At the Space Communication Systems Laboratory, we conduct research and development of optical communication systems that take into account:

• Installation on a wide variety of platforms, ranging from small drones to large satellites
• Application to diverse optical communication scenarios, from fixed ground links to mobile platforms
• Communications under various conditions and distances, from short-range links to long-distance links extending into space

In order to achieve optimal solutions while accommodating these diverse requirements, NICT is organizing and defining the functions that optical communication terminals should possess. Based on this approach, we are currently conducting research on two types of terminals.

These two types of optical communication terminals are referred to as the Full Transceiver (FX) and the Simple Transponder (ST).

• Full Transceiver (FX): A terminal equipped with the necessary functions for high-speed, long-distance, and bidirectional links
• Simple Transponder (ST): A terminal in which some functions of the FX are reduced or constrained in terms of size, mass, and power consumption

In addition, the transmission rates of the optical modulators/demodulators for these terminals are assumed as follows:

• Variable transmission rates ranging from 0.6 to 10 Gbps to adapt to diverse channel characteristics
• Realization of ultra-high-speed, large-capacity optical communications at hundreds of Gbps to several Tbps by utilizing multiple optical signals with different wavelengths

Through these research efforts, we aim to realize an advanced information and communications network that seamlessly connects the ground and space.

These terminals have been validated through several field trials, including a 2024 demonstration using the 10 Gbps modem with the two terminals separated by 7.4 km in daylight and under strong atmospheric turbulence. More recently, in 2025, the same test was repeated with the 2 Tbps prototype modem, achieving the first-ever Tbps-class link using compact lasercom terminals suitable for deployment on moving platforms such as HAPS and satellites.

Press Release:"World's First Successful 2 Tbit/s Free-Space Optical Communication Using Small Optical Terminals Mountable on Satellites and HAPS" (2025)