Hideki Takenaka

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.

Atmospheric turbulence-induced fading channel model for space-to-ground laser communications links

The fading channel model for generating a random time-varying signal based on the atmospheric turbulence spectrum for space-to-ground laser links is discussed. The temporal frequency characteristics of the downlink are theoretically derived based on the von Karman spectrum. The rms wind speed based on the Bufton wind model is used as the transverse wind velocity, which makes the simulation simple. The time-varying signal is generated as functions of the receiver aperture diameter and the rms wind speed. The simulated result of the time-varying signal is presented and compared with the gamma-gamma distribution based on the scintillation theory in a moderate-to-strong-turbulence regime.

Satellite-to-ground quantum-limited communication using a 50-kg-class microsatellite

The feasibility of satellite-to-ground quantum communication is demonstrated by using a microsatellite in low-Earth orbit. The quantum states are discriminated by a ground receiver with four photon-counters with a quantum bit error rate below 5%.

Experimental verification of fiber-coupling efficiency for satellite-to-ground atmospheric laser downlinks.

Optical communication is a high-capacity method that can handle considerable satellite data. When common-fiber optical devices such as optical fiber amplifiers based on single mode fibers are used in free-space laser communication systems, the laser beam has to be coupled to a single-mode fiber. Under atmospheric turbulence it would be difficult to make the required fiber coupling efficiency in satellite-to-ground laser propagation paths. A fast-steering mirror that can operate at high frequencies under atmospheric turbulence is fabricated, and its tracking performance is verified in real satellite-to-ground laser communication experiments. The measured fiber coupling loss of 10-19 dB in satellite-to-ground laser communication links under atmospheric turbulence shows good agreement with the predicted fiber coupling efficiency of 17 dB.

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.

Free-space optical channel estimation for physical layer security.

We present experimental data on message transmission in a free-space optical (FSO) link at an eye-safe wavelength, using a testbed consisting of one sender and two receiver terminals, where the latter two are a legitimate receiver and an eavesdropper. The testbed allows us to emulate a typical scenario of physical-layer (PHY) security such as satellite-to-ground laser communications. We estimate information-theoretic metrics including secrecy rate, secrecy outage probability, and expected code lengths for given secrecy criteria based on observed channel statistics. We then discuss operation principles of secure message transmission under realistic fading conditions, and provide a guideline on a multi-layer security architecture by combining PHY security and upper-layer (algorithmic) security.

First results of wavefront sensing on SOTA

For satellite to ground laser links, atmospheric turbulence is a major cause of impairments. The induced phase perturbations along the propagation path cause beam scintillation in the receiver plane and they can also severely compromise the coupling of the flux into a receiver of limited size. To address these impairments, dedicated mitigation strategies must be developed. This requires accurate understanding of the perturbation origin. Beam propagation models have demonstrated their ability to reproduce statistical characteristics of optical perturbations on a satellite to ground laser link for elevations as low as 20°. For smaller elevations, measurements performed on stars illustrated the limits of analytical approaches and the interest for end-to-end models. We report here the first propagation channel measurements performed on a LEO microsatellite with a Shack-Hartmann wavefront sensor (WFS). The laser beam at 976 nm provided by SOTA optical terminal have been analyzed with a Shack- Hartmann wavefront sensor located at Coudé focus of the French ground station (1,55 m MéO telescope) in July 2015. Wavefront characteristics and scintillation patterns recorded with the WFS are analyzed and compared to atmospheric turbulence perturbations model fed with in situ measurements of atmospheric parameters retrieved from GDIMM.

Polarization-Basis Tracking Scheme in Satellite Quantum Key Distribution

Satellite quantum key distribution is a promising technique that overcomes the limited transmission distance in optical-fiber-based systems. The polarization tracking technique is one of the key techniques in the satellite quantum key distribution. With free-space quantum key distribution between an optical ground station and a satellite, the photon polarization state will be changed according to the satellite movement. To enable polarization based quantum key distribution between mobile terminals, we developed a polarization-basis tracking scheme allowing a common frame of reference to be shared. It is possible to orient two platforms along a common axis by detecting the reference optical signal only on the receiver side with no prior information about the transmitters orientation. We developed a prototype system for free-space quantum key distribution with the polarization-basis tracking scheme. Polarization tracking performance was 0.092° by conducting quantum key distribution experiments over a 1 km free space between two buildings in a Tokyo suburb.

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.

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.