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Between a high-altitude platform (HAP) in the stratosphere and a satellite, high-data-rate optical links are conceivable but the challenges are still not clearly identified. We thus review the main requirements to properly establish such links. The design of a HAP-LEO link is in several aspects different from that of a HAP-GEO link. Among others, we consider the possible limitations caused by atmospheric propagation, the Sun and the environment of the HAP terminal. Signal transmission appears markedly more difficult when the optical beam must propagate near the HAP horizon or below.

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All optical clock recovery has been experimentally demonstrated by an actively mode-locked fiber ring based on the semiconductor optical amplifier (SOA) from a 10-Gb/s NRZ data stream. The system exploits the cross-gain modulation (XGM) nonlinear effect in SOA, and a Fabry-Perot etalon suitably inserted in the loop improves the mode-locking stabilization and efficiency. © 2004 Wiley Periodicals, Inc