I. Introduction atellite optical communication system with laser is a key technology for future high speed huge throughput space communication system. Engineering Testing Satellite VI (ETS-VI) launched in 1991 carrying Laser Communication Equipment (LCE) developed by Communication Research Laboratory (current National Institute of Information and Communications Technology (NICT)). In 2005, NICT have achieved the laser communication between low earth orbit (LEO) satellite and ground with rate of 50Mbps in 0.8 micrometers laser. Both of the experiment is the frontier work in space laser communication research. Current our research and development target is orbital demonstration and practical implementation of satellite communication technique with 1.5 micrometers wavelength laser. The 1.5 micrometers laser is the light source used for ground base fiber communications. The fiber communication technique is developed rapidly for application of broadband communications such as Internet and high quality commercial device is expected to be available in low cost. For rapid progress and practical implementation of space optical communication technique, application of the optical communication technology in 1.5 micrometers is the efficient strategy and crucial approach. Laser communication is still newly growing technology even in ground base utility, some devices and components are not easily available as space qualified one. Therefore orbital demonstration of devices, components, and systems are important issue to confirm feasibility of space use under thermal vacuum and radiation environment in orbit. Furthermore, in view point of communication application, we have to understand propagation property and establish a propagation model to adopt new wavelength as communication medium. There are no enough understanding of propagation property of 1.5 micrometers laser between LEO and ground, understanding the propagation property is a critical topic for utilization of the wavelength for satellite communication. Based on understandings of the propagation property, communication system including modulation, coding, and error correction should be studied and designed. For the understanding and design of the communication system, demonstration and communication experiment in orbit is the essential action.

NICT has developed Small Optical TrAnsponder (SOTA) for demonstration of satellite laser communication with 1.5 micrometers wavelength in LEO. SOTA is mounted on SOCRATES satellite which is 50kg class very small satellite with three axis attitude control system developed by Advanced Engineering Services Co., Ltd,(Tsukuba, Japan). SOCRATES satellite with SOTA is launched on 24 May 2014 by H2-A rocket. After initial check-out of the satellite and SOTA, experiment of fundamental research for future satellite laser communication is now successfully on-going. This experiment is the first demonstration in real space environment on 1.5 micrometers satellite laser communication in LEO.