Horizontal line-of-sight turbulence over near-ground paths and implications for adaptive optics corrections in laser communications

Abstract

Atmospheric turbulence over long horizontal paths perturbs phase and can also cause severe intensity scintillation in the pupil of an optical communications receiver, which limits the data rate over which intensity-based modulation schemes can operate. The feasibility of using low-order adaptive optics by applying phase-only corrections over horizontal propagation paths is investigated. A Shack-Hartmann wave-front sensor was built and data were gathered on paths 1 m above ground and between a 1- and 2.5-km range. Both intensity fluctuations and optical path fluctuation statistics were gathered within a single frame, and the wave-front reconstructor was modified to allow for scintillated data. The temporal power spectral density for various Zernike polynomial modes was used to determine the effects of the expected corrections by adaptive optics. The slopes of the inertial subrange of turbulence were found to be less than predicted by Kolmogorov theory with an infinite outer scale, and the distribution of variance explained by increasing order was also found to be different. Statistical analysis of these data in the 1-km range indicates that at communications wavelengths of 1.3 mum, a significant improvement in transmitted beam quality could be expected most of the time, to a performance of 10% Strehl ratio or better.

Relay-Assisted Quantum-Key Distribution Over Long Atmospheric Channels

“…Different adaptive optics systems with fully [21]- [23] and partially [24], [25] compensation can be used for this purpose. In a fully compensated adaptive optics system, the scale of the operating elements (sub-aperture of the wave-front sensor and the actuator of the deformable mirror) matches the scale of the atmospheric correlation length which is inversely related to the link length [23], [24]. Since the proposed relay-assisted system divides the link range into shorter hops, adaptive optics can be implemented relatively easier with larger, therefore fewer, elements.…”

Wireless Optical Communications Through the Turbulent Atmosphere: A Review

“…The main technique to counterbalance the degrading effects of receiving the optical wave off-axis, is by the combination of fast steering mirrors and adaptive optics algorithms (Levine et al, 1998;Tyson, 2002;Weyrauch & Vorontsov, 2004).…”

Optical Communication Using Subcarrier PSK Intensity Modulation Through Atmospheric Turbulence Channels

“…When R A (0) is normalized, the probability distribution of a sample at any time instant is determined by one parameter defined as the scintillation level σ. The power spectral density was measured in [25] for near-ground horizontal turbulent paths of 1 and 2.5 km using an unmodulated laser beam. The bandwidth of the measured power spectral density was less than 1 kHz.…”

“…The PSK signal is upconverted to an intermediate frequency (IF) f c . Because the bandwidth of the scintillation process A(u, t) is only a few kilohertz [25], f c can be chosen high enough to guarantee that the first term and the second term in either (10) or (21) do not overlap in the frequency domain. With the current RF technology, f c can be large enough to support several gigabits per second.…”

“…The atmospheric turbulence is relatively slow [25] compared with the data rates in optical wireless systems. This means the scintillation is almost the same over successive bits.…”