Nicolas Perlot

Overview of the inter-orbit and the orbit-to-ground laser communication demonstration by OICETS

The experiment results on the inter-orbit laser communications between OICETS and a geostationary satellite and the results of two kinds of orbit-to-ground laser communications between OICETS and ground stations are summarized. The geostationary satellite for the inter-orbit demonstrations is the European Space Agencys geostationary satellite, ARTEMIS, and the ground stations for the orbit-to-ground demonstrations are of the National Institute of Information, and Communications Technology (NICT) in Japan and the German Aerospace Center (DLR), respectively. The descriptions of those experiments contain some statistically analyzed results as well as data samples measured during the demonstrations. The authors present the overview of these demonstration progresses and discuss on the results.

Experimental verification of optical backhaul links for high-altitude platform networks: Atmospheric turbulence and downlink availability

Optical backhaul downlinks from high-altitude platforms (HAPs) are investigated. An experiment demonstrated the advantages of optical links: a small and lightweight terminal with low power consumption was launched to the stratosphere and data transmitted down to a ground station at a rate of 1.25 Gbit/s: Owing to the chosen system parameters and the high budget margin, disturbing turbulence effects did not decrease the link performance. The scientific aspect of the experiment was to study turbulence effects in order to design future systems with higher transmission performance. On the day of the experiment, measured scintillation and wavefront distortions were minimal in the morning. The best atmospheric conditions were observed about 3 h after sunrise with a peak of the atmospheric coherence length r0 at 16 cm. An r0 of 4 cm was measured as the worst case before sunrise and later during the day. This trend could also be observed for power- and intensity scintillation index. The latter changed from 0.28 (best case) to 1.12. For small scintillation index a lognormal intensity probability density function was measured. Apart from the robust intensity modulation scheme with direct detection which was used for the trial, future improved systems could benefit from a coherent transmission scheme. According to the r0 measurements and further simulations on heterodyne efficiency it turned out that the aperture size can be decreased from 40 to 10 cm without any significant change in the link margin. Future stratospheric optical links between HAPs or links from platforms to satellites will not suffer from cloud blockage but it remains an issue for up/downlinks to a ground station. This can be mitigated by ground-station diversity. Four optical ground stations in the southern part of Europe can lead to an availability of over 98%. The separation distance of the ground stations is about 900 km with a negligible correlation of cloud cover. A change of wavelength from the employed 1.55 to a wavelength around 11 microns with minimum cloud attenuation would increase the link availability for thin clouds.

Multiple wavelength free-space laser communications

Free-space optical communications systems in the atmosphere, based on intensity modulation and direct detection, are heavily affected by fading caused by turbulence cells of varying scale and motion. Several data sets of fading measurements under different scenarios have been recorded demonstrating this effect. In this paper we introduce a form of free-space laser communications involving a source operating on several wavelengths. The goal is to overcome atmospheric interference on a communications link. We have performed simulations using the DLR PILab Matlab toolbox. These indicate the extent to which the turbulence and beam properties interact. Experimental investigations are planned. Further properties are also taken into account, including the choice of appropriate laser bandwidth and wavelengths, the effect of atmospheric absorption from aerosols and molecular absorption lines, as well as effects of atmospheric structure on beam propagation. Possible scenarios for application of this scheme will be presented as well.

Turbulence-induced fading probability in coherent optical communication through the atmosphere

To assess the coherent detection of an optical signal perturbed by atmospheric turbulence, the loss in the mean signal-to-noise ratio (SNR) is usually invoked although it constitutes a limited description of the signal fluctuations. To produce statistical distributions of the SNR, we generate random optical fields. A 5/3-power law for the phase structure function is considered. The benefit of a wavefront tilt correction is assessed. Based on the 1%-probability fade, an optimum receiver size is found. For phase fluctuations only, a similarity between the signal distribution and the beta distribution is observed. Phase and amplitude are assumed independent, and the influence of amplitude perturbations is assessed with a scintillation index of 2. Turbulence impairments are compared for a coherent receiver and a direct-detection receiver.

Evaluation of the Scintillation Loss for Optical Communication Systems with Direct Detection

In optical communications through the atmosphere, the evaluation of a link feasibility often requires the quantification of the scin- tillation penalty in terms of power loss. To find how much additional op- tical power is needed to reach the bit-error-rate BER requirements, the optical-power fluctuations must be characterized as well as the response of the receiver to those fluctuations. In the present analysis, the direct- detected optical power is assumed to be either lognormal or gamma- gamma distributed. To account for the dynamics of the atmospheric channel, a distinction is made between short-term and long-term BERs. For a simple On-Off Keying OOK modulation, expressions of scintilla- tion losses are given for different system requirements. Specifically, an upper bound is set to any of the three following quantities: the long-term BER, the probability of having a too-high short-term BER, or the mean time during which the short-term BER is too high. Results show that, without any fade mitigation, losses under moderate scintillation are con- siderable. Finally, a simple code-word approach shows how scintillation losses can be reduced by channel coding.

Aperture averaging: theory and measurements

Atmospheric laser communications using direct-detection systems do suffer from severe degradation caused by scintillation. Because the atmospheric cut-off frequency can be as low as 100 Hz, temporal averaging is not applicable in high-speed communications. The simplest way of reducing fading is to increase the receiver size and to take advantage of aperture averaging. Spatial and temporal variations of the received intensity have to be investigated in order to predict the efficiency of aperture averaging. This paper reviews briefly the theory of spatial averaging that characterizes the direct-detected optical power. For comparison purposes, results of measurements are presented. These measurements consist of recorded pupil intensity patterns for a scenario with known turbulence profile. Statistics derived from measurement data are compared with theoretical second-order statistics.

Experimental verification of the limits of optical channel intensity reciprocity

Optical data links through the atmosphere suffer from turbulence-induced signal scintillation. In a coaxially-symmetric bidirectional link scenario, the variations of the axial intensities at both ends are correlated. This relation can be used as an inherent feedback mechanism, with negligible delay, to enhance the capacity of the transmission system. By experiment, we show the correlation coefficient of both received signals can reach values close to one over long atmospheric distances, provided the receiver apertures are smaller than specific intensity speckle structures, while the correlation reduces gradually with larger apertures. This allows transmission capacity to be optimized with adaptive transceiver systems that take into account the degree of correlation.

Results of the optical downlink experiment KIODO from OICETS satellite to optical ground station Oberpfaffenhofen (OGS-OP)

Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center (DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2° and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the experiment and highlights the results of the measurement trials.

Measurements of the Beam-Wave Fluctuations over a 142-km Atmospheric Path

An optical link has been established between the Canary Islands La Palma and Tenerife. A 1064-nm transmitting laser was located on La Palma whereas a BPSK communication receiver and measurement instruments were installed in ESAs OGS on Tenerife. Beside the demonstration of a high-data-rate coherent signal transmission, the goal of the experiment was to measure the effects of the atmosphere on the beam propagation in order to estimate its impact on optical links. In particular, wavefront distortions have been investigated by means of a DIMM instrument and scintillation was observed by imaging the pupil of the OGS telescope on a CCD camera. Strong scintillation was observed during all the experiment with scintillation peaks at sunsets and sunrises, and saturation at about noon. Because of the narrowness of the beam (15-μrad divergence), beam wander has been a serious issue. Statistical results are compared with theory. Recommendations regarding the specifications of optical coherent systems in such detrimental conditions are given.

Numerical simulations of beam propagation through optical turbulence for high-altitude platform crosslinks

In this paper we introduce a simulation method for modelling clear-air atmospheric turbulence effects for long horizontal stratospheric paths. Divergence angles of several hundred microradians in combination with link distances up to 800 km require to adapt the appropriate resolution of the transverse optical field along the path. For this purpose, we implemented a propagation method in Cartesian coordinates. We choose two reference scenarios for high-altitude platform crosslinks and discuss the influence of simulation parameters to the derived results. Finally a method for computation of temporal IM/DD-time signals form simulated intensity matrices is presented.