[Abstract] Bi-directional ground-to-satellite laser communication experiments between the optical ground station developed by the National Institute of Information and Communications Technology (NICT) located in Koganei of downtown Tokyo and a low earth orbit (LEO) satellite, the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) called “Kirari”, were successfully performed in March, May and September, 2006. The Kirari Optical Communication Demonstration Experiments with the NICT optical ground station (KODEN) was conducted in cooperation between the Japan Aerospace Exploration Agency (JAXA) and NICT. The ground-to-OICETS laser communication experiment is the first in-orbit demonstration with respect to the LEO satellite. Phase-1 laser communication experiment was conducted in March, 2006 and the downlink bit error ratio (BER) of around 1e-5 was measured. Phase-2 laser communication experiment was done in May, 2006 but there was no significant improvement due to bad weather. The uplink and downlink BERs of 1e-4 ~ 1e-7 were simultaneously measured in Phase-3 in September, 2006 with an uplink of 2 Mbps and a downlink of 50 Mbps, respectively. Eighteen trials over the course of three months were conducted during the test campaign. There were eleven successful passes out of all the trials. Acquisition and tracking of the OICETS satellite were always successful when partly clear skies were predominant, showing the optical link was successfully established by 100 % without bad weather conditions. For the uplink, multi-beam laser transmission was used from the optical ground station to the OICETS satellite in order to reduce the intensity fluctuation of the optical signal due to atmospheric turbulence. The fluctuation of the received signal power was well minimized by using four-beam laser transmission. The applicability of the onboard optical terminal was demonstrated, aiming not only for geostationary earth orbit (GEO)-LEO links but also for ground-to-LEO optical links. This paper presents the results of Phase-1, Phase-2 and Phase-3 KODEN experiments.

We present a spatial acquisition scheme based on a fish-eye lens and a sinusoidal amplitude grating that can acquire laser beacons in an ultrawide field of view for atmospheric optical links. In this scheme, an incoming laser beacon can be imaged into three diffraction spots. The laser direction and the wavelength can be measured from the center position of the spots and the distance between two adjacent spots, respectively. The total optical transfer function can be described by the combined effect of the acquisition system itself and the spatial coherence degradation due to atmospheric turbulence. Spot spread and spot dancing effects are both considered for long-term exposure and short-term exposure. We investigate how root mean square errors of the measured laser direction and wavelength vary with the ratio of the pupil diameter D2 to atmospheric coherence length r0, without consideration of aberrations of the fish-eye lens. The results show that with the limitation of r0 the performance cannot be infinitely improved by enlarging D2. When considering aberrations of the fish-eye lens, the imaged spot is gradually blurred and deteriorated with the increase of the field angle, since aberrations are enlarged with the increase of the field angle. We study how the measured errors of laser direction and wavelength vary with r0 and incident angles (with different aberrations). A severe turbulence with a small value of r0 and a large incident angle will cause a bad acquisition performance.

We investigated the performance of mode diversity reception of a polarization-division-multiplexed (PDM) signal with few-mode-fiber (FMF) coupling for high-speed free-space optical communications under atmospheric turbulence. Optical propagation through eigenmodes of a FMF yields coupling between different linearly polarized (LP) modes in orthogonal polarizations, which causes power imbalance and loss of the orthogonality of multiplexed signals within each individual LP mode. Due to this phenomenon, the architecture of mode diversity combining affects the receiver performance. We numerically simulated the power fluctuation coupled to each LP mode after atmospheric propagation and FMF propagation in the condition of an optical downlink from a low-Earth-orbital satellite to the ground. We found that full receiver-side multiple-input multiple-output (Rx-MIMO) architecture in three-mode diversity reception improved the performance by 5 dB compared with selection combining (SC) of signals decoded individually in LP modes, and that it mitigated the required transmitted power by 6 dB compared with reception with single mode fiber (SMF) coupling. We also experimentally confirmed in three-mode diversity reception of a 128 Gb/s PDM-quadrature phase-shift keying with a diffuser plate as a turbulence emulator, that full Rx-MIMO with adaptive filters could work under severe fading and that it outperformed SC.

We investigate the use of differential phase-shift keying (DPSK) in multiwavelength spatial diversity transmitters for mitigation of atmospheric fading. By selecting the transmitter wavelengths to coincide with the transmission peaks of the delay-line interferometer in the receiver, the only modification required to a standard DPSK receiver is a wider optical filter. We examine the wavelength separation required to minimize penalties from beating between the wavelengths and find that a separation of twice the data rate is sufficient for a four-wavelength system with narrow filtering. We also demonstrate a reduction in scintillation loss for a four-wavelength DPSK system in a fading channel.

We investigate the power penalty caused by excess amplified spontaneous emission in an optically preamplified receiver for use with a multi-wavelength spatial diversity transmitter to mitigate atmospheric fading.

We investigate the power penalty caused by excess amplified spontaneous emission in an optically preamplified receiver for a communications link where a multiwavelength spatial diversity transmitter is used to mitigate atmospheric fading. We compare measured receiver sensitivity for a four-wavelength 10.7-Gb/s nonreturn-to-zero on-off-keyed system to theory using both the Gaussian noise approximation and chi square statistics.

We demonstrate a single-polarization cladding-pumped Er : Yb optical amplifier using a dual-pass design with a Faraday rotator mirror and nonpolarization-maintaining gain fiber. Over a 25-nm range centered at 1562 nm, the amplifier output exceeds 1 W and maintains a polarization extinction ratio greater than 22 dB. We investigate enhanced relative intensity noise inherent in the dual-pass configuration and discuss scaling to higher powers.

To achieve rapid establishment of free-space optical (FSO) links, we previously proposed an ultrawide field-of-view (FOV) acquisition scheme, which combines the fisheye lens and Voigt anomalous dispersion optical filter. In this paper, we focus on the angle detection accuracy of this ultrawide FOV acquisition scheme, which is one of primary factors for rapid establishment of FSO links. The positioning process is composed of two parts. One is the spot location, which acquires the position of the imaging spot of the incident beam. The other is the mapping from spot location to incident angle, with which the position of imaging spot can be turned into the angles of the incident beam. For the first part, an improved centroid locating algorithm is present to realize the subpixel accuracy of spot location, and theory and simulation are carried out to demonstrate the improved accuracy of this algorithm. For the second part, we propose the fisheye lens calibration algorithm, which integrates the fisheye lens projection model and natural neighbor interpolation algorithm. In addition, by combining with the measured data, the angle detection accuracy of this method can be improved gradually with the increased number of the measured data. The simulation results indicate that this new method can provide high-precision positioning of the laser beams, and the maximum angle detection error of this ultrawide FOV acquisition scheme can be less than 0.054° (0.94 mrad).

The initial verification results of the in-orbit free-space laser communication functions equipped on the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) are presented. The operation system for OICETS is introduced with some characteristics on the radio-frequency communication links which connects OICETS to the ground facilities and satellites. The verification procedures of optical functions are carried out with a scenario that begins with measurements of optical devices' responses and moves to confirmations of automatic sequences in acquisition and tracking stars. Finally, all the OICETS functions required for the in-orbit laser communications with ARTEMIS are found to be in good conditions. We describe here the story of the initial verifications with typical examples measured by these tests.

The first bi-directional laser communications demonstration between the optical ground station developed by the National Institute of Information and Communications Technology (NICT) located in Koganei, Tokyo and the Optical Inter-orbit Communication Engineering Test Satellite (OICETS) was successfully conducted in March, May, and September, 2006. The Kirari Optical communication Demonstration Experiments with the NICT optical ground station (KODEN) were jointly conducted by the Japan Aerospace Exploration Agency (JAXA) and NICT. Data from the uplink and downlink optical communication links were analyzed. For the downlink, the scintillation index agreed well with the theoretical results calculated based on the strong fluctuation theory. The aperture averaging effect was the dominant factor in reducing the variation of the downlink signals. The probability density functions as a function of elevation angles were measured and compared with the theoretical model, showing good agreement. For the uplink, the scintillation index disagreed with the calculated results based on the strong fluctuation theory. The multiple beam effect of the uplink transmission with large beams will have an additional reduction factor, which will help to establish ground-to-satellite laser communication links in the future. Four laser beams transmitted from the optical ground station to the OICETS satellite also helped to reduce the optical signal's intensity fluctuation due to atmospheric turbulence.

The first bi-directional laser communication demonstration between an optical ground station and the Optical Inter-orbit Communication Engineering Test Satellite (OICETS) was successfully conducted in March, May, and September, 2006, with an uplink of 2 Mbps and a downlink of 50 Mbps. The optical ground station, located in Koganei, Tokyo, Japan, is operated by the National Institute of Information and Communications Technology (NICT), Japan. Four laser beams were transmitted from the optical ground station to the OICETS satellite in order to reduce the optical signal's intensity fluctuation due to atmospheric turbulence. The optical scintillation as a function of the number of beams and the frequency response were measured, and the uplink and downlink laser transmission results were obtained.

The advantages of optical communications as compared to radiowave (RF) communications include broader bandwidth, larger capacity, lower power consumption, more compact equipment, higher security against eavesdropping, better protection against interference, and the absence of regulatory restrictions (Hyde & Edelson, 1997). Moreover, the demand for high-data-rate transmission from spaceborne observation platforms is steadily increasing (Toyoshima, 2005a). Free-space optical (FSO) communications systems are expected to play an important role in providing such high-data-rate communications, and optical technologies for satellite networks are expected to revolutionize space system architecture (Chan, 2003). For terrestrial optical wireless communications, a transmitter with a 3 × 3 square array of vertical-cavity surface-emitting lasers (VCSELs) was evaluated in Parand et. al. (2003). Multiple-input multiple-output (MIMO) systems were presented based on optical code-division multiple-access (OCDMA), imaging diversity receivers, and white LEDs (Hamzeh et. al., 2004; Djahani et. al., 1999; Zeng et. al., 2009; Minh et. al., 2009). For longrange free-space laser communications, however, maintaining a line of sight between transceivers is particularly difficult because of the small divergence angle of the laser beams. Minimizing the requirements for the tracking system and ensuring the steady operation of the onboard optical terminal are therefore important for realization of commercial applications. FSO links are also well known for their susceptibility to adverse weather conditions such as cloud and fog, which pose great challenges to link availability. For terrestrial FSO links, RF links can be backed up to ensure continuous availability (Wu et. al., 2007). Optical terminals for long-distance communications tend to have large mass because optical tracking systems require mechanically movable parts for coarse laser pointing and tracking. Reductions in mass, power, and volume can decrease interference with other missions on satellites. Non-mechanical movable architecture is extremely attractive for robust and lifelong operation of an optical terminal in orbit. The small satellite community still uses 9.6kbps communication links by employing ham radio communications due to resource constraints in nano-class satellites (Nakaya et al., 2003; Miyashita et al., 2006). Compact terminals can be used in nano-class satellites that have a mass of the order of a few tens of kilograms. There is also a significant advantage concerning frequency-licensing problems faced by satellites, and in this regard optical frequency carriers will be of great use to the small satellite community. Research and development at National Institute of Information

The R&D activities and current status in NICT on space laser communications are reported, where it is shown the goal and scenario originating from the satellite-ground laser communication demonstrations with ETS-VI since 1994 for two decades. The experiences obtained in the demonstrations have been inherited to the experiments with OICETS in 2006. The experiments using the satellite are on going in 2008. Among these demonstrations, a laser terminal with combination of key technologies was experimentally produced. For the next space laser communication system, we have started the next version development technologies, on which trial manufactures are currently in progress. They have been (and will be) implemented and tested at the ground station.

The European Space Agency has built an optical ground station sited at the Observatorio del Teide operated by the Instituto de Astrofísica de Canarias. This station, equipped with a 1m telescope, has a multipurpose configuration for in-orbit commissioning and checkout of laser communication payloads. Since November 2001, the bidirectional link with satellite ARTEMIS has been established in more than 80 successful sessions. In this paper, we analyze the influence of turbulence parameters on the performance of communications in the bidirectional ground to space laser communication experiments. The link performance observed in the satellite-to-ground channel showed average bit error rates of 1E-6 over long durations (20 minutes), however in some occasions BER's of at least 10-9 -10-10 over durations of 5 to 30 minutes were observed. The behavior of the Bit Error Rate measurements performed in different turbulence conditions is characterized.

The European Space Agency (ESA) has launched the geostationary data-relay satellite ARTEMIS with one of its payloads being a laser communication terminal (LCT). The LCT is used within the semiconductor-laser intersatellite link experiment (SILEX) to receive Earth observation data transmitted from a similar LCT onboard the SPOT-4 satellite. To support SILEX, ESA has also reached an agreement with the Instituto de Astrofisica de Canarias (JAC) to build the Optical Ground Station (OGS), in the Teide Observatory ofthe IAC. ARTEMIS and the OGS are an ideal and unique test-bed to study and characterise laser beam propagation through atmospheric turbulence. Theoretical models of laser beam propagation through atmospheric turbulence have been reviewed and developed, to predict the performance of the optical links from the propagation and communication point of view. Special effort has been invested in modelling the uplink effects. Optical link experiments have been planned in detail, to gather the necessary data required to be statistically representative, to compare the results with theoretical predictions and to validate and adjust the theoretical models. This comparison will pave the way towards the implementation of deep-space laser communication links. The first results ofthese experiments, presenting the theoretical models, analysing separately downlink and uplink measurements, and comparing the data with the theoretical predictions, are presented.

The European Space Agency (ESA) has built an optical ground station (OGS) for commissioning and checkout of its laser communication payloads in orbit. The first such payload is the laser communication terminal (LCT) onboard ARTEMIS, ESA's latest data-relay and telecommunication satellite in geostationary orbit. ARTEMIS is now routinely relaying Earth observation data sent via a similar LCT from the low-earth orbiting satellite SPOT-4 to a ground station in Toulouse. This paper focuses on bidirectional space to ground laser communication experiments, which have been performed between the OGS and ARTEMIS. ESA's interest in laser communication is first briefly explained, then the design of the ground and space terminals is introduced, the pointing, acquisition and tracking strategies are explained and a summary of all laser links performed so far is given. Experimental uplink and downlink results are presented in terms of temporal irradiance behavior and link statistics. The uplink irradiance behavior is investigated with changing number of transmit beams. Finally, ESA's future activities and upgrades planned for the OGS are discussed.

The European Space Agency (ESA) geostationary data-relay satellite ARTEMIS started its operation in February 2003, after reaching its final position in the geostationary orbit. Since then, ESA and the Instituto de Astrofísica de Canarias (IAC) have carried out routinely bidirectional optical link experiments between ARTEMIS and the Optical Ground Station (OGS), installed in the Teide Observatory of the IAC in the Canary Islands, Spain. The main purpose of such experimental campaigns is to characterise and model the optical links performance from the propagation and communication points of view, under different atmospheric turbulence conditions. The statistical results presented in this paper cover the uplink and downlink performance, including scintillation, fade and surge statistics, intensity distributions and spectral analysis. The effect of using different number of transmitted beams and different divergences is also considered. Additionally, the results are correlated with the atmospheric turbulence conditions, in terms of profiles of the index of refraction structure constant, isoplanatic angle, seeing and wind profiles, measured in most of the cases simultaneously with the laser communication experiments

SummaryThe KODEN is a first in-orbit laser communication demonstration between a low earth orbit satellite called OICETS/Kirari and an optical ground station at the National Institute of Information and Communications Technology (NICT) in Koganei, Tokyo. Optical links could always be established even under atmospheric turbulence when partly clear skies were predominant in March, May, and September, 2006. The uplink and downlink bit error ratios were measured to be 10−7–10−3. This achievement is a milestone for future high-data-rate satellite communications.ZusammenfassungDas Akronym KODEN steht für die erste erfolgreiche Demonstration eines Laserkommunikationsexperiments zwischen dem Satelliten OICETS/Kirari in erdnaher Umlaufbahn (LEO) und der optischen Bodenstation des staatlichen Institutes für Information und Kommunikation (NICT) in Koganei, Tokio. Trotz atmosphärischer Turbulenzen konnte die optische Verbindung stets hergestellt werden, unter vorwiegend wolkenfreiem Himmel in den Monaten März, Mai und September 2006. Die Fehlerbitraten der beiden Kommunikationskanäle lag zwischen 10−7–10−3. Das Experiment stellt einen Meilenstein in der Entwicklung zukünftiger Satellitenkommunikationssyteme mit hohen Datenraten dar.

Spatial acquisition is essential for the establishment of atmospheric optical links. The detection probability in the acquisition process can be degraded by atmospheric-turbulence-induced scintillation. We present an aperture-array acquisition scheme to suppress this scintillation noise. The aperture array is composed of N receiving elements, each containing an aperture to receive the optical signal, an optical filter to reject the background radiation, and a charge-coupled device (CCD) to detect the optical signal. The mathematical model of the long-term average detection probability (LTADP) for the aperture-array acquisition is derived based on the lognormal distribution in turbulent atmosphere, when the CCD sample time is shorter than scintillation characteristic time. In this case, the average signal count and the detection probability in the CCD sample time are both random variables; therefore, the probability density of the average signal count needs to be considered and the LTADP can be calculated based on this probability density. The simulation results show that this aperture-array acquisition scheme can suppress scintillation effectively and enhance the LTADP when the one-aperture signal-to-noise ratio is fixed.

Since the European Space Agency (ESA) geostationary data-relay satellite ARTEMIS started its operation in February 2003, ESA and the Instituto de Astrofisica de Canarias (IAC) have carried out routinely bidirectional optical link experiments between ARTEMIS and the Optical Ground Station (OGS), installed in the Teide Observatory of the IAC in the Canary Islands, Spain. The experiments aimed at characterizing statically and dynamically the performance of the optical downlinks and uplinks in different atmospheric turbulence conditions, together with the development and testing of appropriate theoretical models to predict the link performance. An overview of the OGS and additional facilities on top the IAC Teide Observatory is given, as well as a summary of the statistical results on propagation and communication for different experimental configurations, including different number of transmitting subapertures and divergence in the uplink. Key parameters, like scintillation and fade and surge statistics, are correlated with theoretical predictions and an analysis of the far field pattern is included. The results of the deep space uplink experiments between the OGS and ESA satellite SMART-1 are also presented. Finally ESA free space optical communication programs are summarised, including optical payloads on board different satellites.