Yoshisada Koyama

Polarization measurements through space-to-ground atmospheric propagation paths by using a highly polarized laser source in space

The polarization characteristics of an artificial laser source in space were measured through space-to-ground atmospheric transmission paths. An existing Japanese laser communication satellite and optical ground station were used to measure Stokes parameters and the degree of polarization of the laser beam transmitted from the satellite. As a result, the polarization was preserved within an rms error of 1.6 degrees, and the degree of polarization was 99.4+/-4.4% through the space-to-ground atmosphere. These results contribute to the link estimation for quantum key distribution via space and provide the potential for enhancements in quantum cryptography worldwide in the future.

Optical terminal for NeLS in-orbit demonstration

This paper presents outline of the optical terminal for Next-generation LEO System (NeLS) in-orbit demonstration, which will be conducted as part of Phase 2 of NeLS project. Two small satellites are assumed to launch into GTO orbit changing distance between them from 500km to 3000km. Acquisition and tracking experiments with a star or planet and 2.4Gbps data transmission between two SmartSat is also planned. The design of optical terminal is briefly presented.

LEO-to-ground polarization measurements aiming for space QKD using Small Optical TrAnsponder (SOTA)

Quantum communication, and more specifically Quantum Key Distribution (QKD), enables the transmission of information in a theoretically secure way, guaranteed by the laws of quantum physics. Although fiber-based QKD has been readily available since several years ago, a global quantum communication network will require the development of space links, which remains to be demonstrated. NICT launched a LEO satellite in 2014 carrying a lasercom terminal (SOTA), designed for in-orbit technological demonstrations. In this paper, we present the results of the campaign to measure the polarization characteristics of the SOTA laser sources after propagating from LEO to ground. The most-widely used property for encoding information in free-space QKD is the polarization, and especially the linear polarization. Therefore, studying its behavior in a realistic link is a fundamental step for proving the feasibility of space quantum communications. The results of the polarization preservation of two highly-polarized lasers are presented here, including the first-time measurement of a linearly-polarized source at λ = 976 nm and a circularly-polarized source at λ = 1549 nm from space using a realistic QKD-like receiver, installed in the Optical Ground Station at the NICT Headquarters, in Tokyo, Japan.

Expanded laser communications demonstrations with OICETS and ground stations

The restarted OICETS-ground laser communications experiments are introduced. The events are sequentially summarized from the launch of OICETS to the end-of-life, where the reopened experiments started from October 2008. In the period, the satellite-ground laser communications campaign with the four optical ground stations of DLR, ESA, JPL and NICT are conducted from April 2009 to September 2009. The open pointing characteristics of OICETS measured in those trials show that the performance remains almost the same as before in 2006. The average rate of the link establishments through the whole period is about 0.6 due to the weather conditions. The viewable periods of OICETS from the four ground stations are analyzed as an example. The result indicates that the satellite could be accessible once an hour from at least one of the four ground stations, which implies a possibility of a LEO satellite-ground quasi-continuous connection.

Experiment plan for a small optical transponder onboard a 50 kg-class small satellite

50kg-class satellites can reduce production cost and time compared to larger satellites. On the other hand, the communication capacity in radio frequency for 50 kg-class satellites is limited, however it can be improved by using optical communications. The National Institute of Information and Communications Technology (NICT) has begun to develop a Small Optical TrAnsponder (SOTA) onboard a 50kg-class satellite, which project is called the Space Optical Communications Research Advanced Technology Satellite (SOCRATES). We will introduce the experiment plan for SOTA onboard the 50 kg-class small satellite.

Components development for NeLS optical terminal

Next-Generation LEO System (NeLS) Research Center is now conducting continuous effort to demonstrate feasibility of key technologies for optical inter-satellite links in space. Evaluation of critical components for the NeLS optical terminal, such as Wide-range FPM, RX-collimator combined with a fine tracking sensor and devices for optical receiver, were carried out using trial models. In this paper, performance evaluation results are presented including mechanical environmental test and radiation test.

SOTA: Small Optical Transponder for micro-satellite

NICT initiated R & D activities of Small Optical Transponder (SOTA) for micro-satellites to demonstrate attractive features of optical technology. Development of the SOTA started based on the bread board model (BBM). Design review of BBM revealed the necessity of engineering model (EM) for critical technologies of 2-axes gimbal and receiving optics. Evaluation of the EM carried out and final design of the SOTA was fixed. This paper describes design progress of the SOTA.

In-orbit verification of small optical transponder (SOTA): evaluation of satellite-to-ground laser communication links

Research and development of space optical communications is conducted in the National Institute of Information and Communications Technology (NICT). The NICT developed the Small Optical TrAnsponder (SOTA), which was embarked on a 50kg-class satellite and launched into a low earth orbit (LEO). The space-to-ground laser communication experiments have been conducted with the SOTA. Atmospheric turbulence causes signal fadings and becomes an issue to be solved in satellite-to-ground laser communication links. Therefore, as error-correcting functions, a Reed-Solomon (RS) code and a Low-Density Generator Matrix (LDGM) code are implemented in the communication system onboard the SOTA. In this paper, we present the in-orbit verification results of SOTA including the characteristic of the functions, the communication performance with the LDGM code via satellite-to-ground atmospheric paths, and the link budget analysis and the comparison between theoretical and experimental results.

Research and development of free-space laser communications and quantum key distribution technologies at NICT

Since 2006, ground-to-satellite laser communication experiments have been successfully performed using intensity modulation/direct detection (IMDD) between the National Institute of Information and Communications Technology (NICT) optical ground station located in Tokyo and a low earth orbit (LEO) satellite. With the advent of coherent binary phase-shift keying (BPSK) receivers in orbit by a German satellite, it is now absolutely imperative to establish interoperability between the different optical communications systems. At NICT, an optical receiver that can demodulate both IMDD and coherent optical signals for free-space laser communications has been developed that can recover the carrier phase after homodyne detection by means of digital signal processing (DSP). A transportable optical ground station has also been developed for site-diversity purposes in order to increase accessibility between terrestrial and space systems. A new NICT mid-term research plan has started since April 2011 and this paper presents research and development in free-space laser communications technologies at NICT.

Current development status of Small Optical TrAnsponder (SOTA) for satellite-ground laser communications

Current development status of the small optical transponder (SOTA) to be installed into a small satellite is described, where the breadboard model, the engineering model and the protoflight model are respectively introduced. The tracking performance is estimated to show that the angular error is low enough in comparison to the divergence angle of the transmitted beam.