Dimitar Kolev

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.

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.

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.

Adaptive optics results with SOTA

Next generation satellite-to-ground laser link systems, either for telemetry or satcom will request very high data rate. This goal could be achieved with a manageable cost if the benefits from the fibered technologies are reaped. For downlink the wave therefore needs to be coupled into a single mode fiber. Due to atmospheric turbulence its spatial coherence is compromised. This causes strong coupling losses that result into deep fadings. To cope with this, adaptive optics (AO) is envisaged. Thanks to a real time compensation of atmospheric induced optical path differences, it might enable to reach average coupling efficiencies as high as 0,5 (3 dB average losses). AO is now a mature technology, mostly brought to market by astronomy or biomedical applications. Usual correction bandwidth and available flux to perform the wavefront measurement are rather small (typically 50 Hz bandwidth and tenth of photons per subaperture and per frame). The specificity of AO for LEO satellite to ground optical links resides into higher requested bandwidth, optimal operation for a wide variety of atmospheric conditions (daytime and nighttime) with potentially low elevations that might severely affect wavefront sensing due to scintillation. In addition to this the performance criterion of the correction is different from usual imaging applications, with appropriate constraints on coupling statistics and temporal characteristics. To address these specificities, AO dimensioning approach needs to be adapted and consolidated by in situ measurements. We report here the first AO results on a LEO microsatellite as far as we know. The AO bench located at Coudé focus of the MéO telescope, designed for imaging applications, is used to correct for optical aberrations on a 976 nm laser beam provided by SOTA terminal. AO performances are investigated and confronted to state of the art performance evaluations for satellite to ground laser links.

Current status of research and development on space laser communications technologies and future plans in NICT

The National Institute of Information and Communications Technology (NICT) has successfully conducted several laser communication experiments between geostationary earth orbit (GEO) and low earth orbit (LEO) satellites and optical ground stations. To date other organizations have also conducted many space laser communication demonstrations worldwide and the time has come when space laser communications can be used as operational systems. The NICT has recently carried out the first-ever successful data transmission from a 50-kg class micro-satellite via laser communication links. This paper presents recent activities on space laser communications in the NICT including the organizations future plans for next generation space laser communication research aiming to achieve 10 Gbps-class and 40 Gpbs-class laser communications at GEO and LEO distances.

Received-Power Fluctuation Analysis for LEO Satellite-to-Ground Laser Links

In this paper, the received-power fluctuations downlink data from two different low Earth orbit satellite experiments are analyzed. The first set of data was gathered from the Optical Inter-orbit Communications Engineering Test Satellite project. The data from the Small Optical TrAnsponder were gathered for two different laser transmitters with different parameters. The measured scintillation indices and their relation to the elevation angle are shown and the increasing number of higher scintillation index values at high elevation angles is discussed in terms of azimuth-axis slew rate and tracking and pointing errors. Furthermore, the received-power fluctuation histograms were fitted to a few theoretical probability density functions looking for the best fit to describe the satellite-to-ground link performance.

SOTA OPTICAL DOWNLINKS TO DLR’S OPTICAL GROUND STATIONS

Optical Satellite Downlinks have gathered increasing attention in the last years. A number of experimental payloads have become available, and downlink experiments are conducted around the globe. One of these experimental systems is SOTA, the Small Optical Transponder, built by the National Institute of Information and Communications Technology (NICT). This paper describes the downlink experiments carried out from SOTA to the German Aerospace Center’s Optical Ground Stations located in Oberpfaffenhofen, Germany. Both the Transportable Optical Ground Station (TOGS) as well as the fixed Optical Ground Station Oberpfaffenhofen (OGS-OP) are used for the experiments. This paper will explain the preparatory work, the execution of the campaign, as well as show the first results of the measurements.

Design status of the development for a GEO-to-ground optical feeder link, HICALI

Recently, satellite broadband communication services using Ka-band are emerging all over the world, some of them with capacities in excess of 100 Gbps. However, as the radio bandwidth resources become exhausted, high-speed optical communication can be used instead to achieve ultra-broadband communications. The National Institute of Information and Communications Technology (NICT) in Japan has more than 20 years of experience in R&D of space laser communications, with important milestones like ETS-VI (Engineering Test Satellite VI), OICETS, and SOTA. We are currently developing a laser-communication terminal called “HICALI”, which goal is to achieve 10 Gbps-class space communications in the 1.5-μm band between Optical Ground Stations (OGSs) and a next generation high-throughput satellite (called ETS-IX) with a hybrid communication system using radio and optical frequencies, which will be launched into a geostationary orbit in 2021. The development of test and a breadboard model for HICALI has been conducted for several years and we are now carrying out an engineering model as well as designing the OGSs segment. In this paper, we describe concepts and current design status of the HICALI system.

Satellite-to-ground optical communications using small optical transponder (SOTA) – received-power fluctuations

In this paper, experimental results related to the received-power fluctuations in a LEO satellite-to-ground laser-communication link using the Small Optical TrAnsponder (SOTA) are presented. The data is compared to the theoretical expectations based on the Hufnagel-Valley model. The discrepancies in the results are discussed by spectrum analysis and by a comparison with the Differential Image Motion Monitor (DIMM) measurements of the Fried parameter. Finally, the experimentally-derived coherence time is shown and discussed.

Development of a breadboard model of space laser communication terminal for optical feeder links from Geo

National Institute of Information and Communications Technology (NICT) has a long history of the R&D of space laser communications.