Experimental observations of channel reciprocity in single-mode free-space optical links.

This article describes observations made during a recent series of single-mode lasercom experiments in which high-rate data transmission was demonstrated between a small aircraft and a ground station separated by distances up to 80 km. A significant result of the subsequent data analysis was the discovery of near-unity correlations between the signal fluctuations observed by power monitors at the two ends of the link. This evidence of reciprocity is presented, along with the description of a preliminary concept for utilizing this channel state information to improve link performance.

Propagation of data-modulated Gaussian beams through holographic optical elements.

This paper examines dispersion caused by diffraction through uniform volume holographic gratings. Of interest is the impact of this dispersion on the spatial and temporal fidelity of an optical communications signal. To this end, a holographic grating is illuminated by a Gaussian beam with 1/e(2) diameter large compared to the optical wavelength. Coupled-wave analysis is used to calculate the temporal response of the grating to transmitted symbols encoded in time as a train of Gaussian-shaped pulses. It is shown that temporal dispersion due to diffraction impacts bit-error performance, yielding increased power penalty for larger diffraction angles and beam diameters.

Emulation of dynamic wavefront disturbances using a deformable mirror.

Boundary-layer turbulence resulting from uneven airflow around window interfaces can impact airborne laser communications (lasercom). In the focal plane, these distortions can produce fast jitter and beam break-up, posing challenges for tracking and communications. We demonstrate an experimental emulator that reproduces aircraft aero-optical distortions using a deformable mirror. This boundary-layer emulator resides in a hardware testbed that experimentally mimics air-to-space lasercom links in a controlled, laboratory environment. The boundary-layer emulator operates in the 1.55-mum band and accurately recreates aero-optical distortions at a rate of 2 kilo-frames per second.

Experimental comparison of tracking algorithms in the presence of aircraft boundary-layer distortions for emulated free-space laser communication links.

We report experiments comparing different focal plane array (FPA) tracking algorithms for emulated laser communication links between an aircraft and spacecraft. The links include look-angle-dependent phase disturbances caused by boundary-layer turbulence replicated by using a deformable mirror. Impairments from platform jitter, atmospheric scintillation, and propagation delay are also included. We study a hyperhemispherical dome geometry that provides a large field of regard but generates boundary-layer turbulence. Results from experiments comparing peak and centroid FPA tracking algorithms in various environments show that power delivered to the optical fiber varies with algorithm and look angle. An improvement in steady-state fiber-coupled power of up to 1.0 dB can be achieved through appropriate choice of algorithm. In a real system, this advantage could be realized by implementing a tracking processor that dynamically changes its tracking algorithm depending on look angle and other parameters correlated to boundary-layer turbulence.

Optical phased array power penalty analysis.

This paper investigates the power penalty from optical phased arrays used for wide-angle beam steering of optical communication signals. The analysis studies the effect of aperture size, data rate, modulation format, and diffraction angle on digital lightwave signals. The results show increasing power penalties for larger angles, aperture sizes, and data rates. At a 10 masculine steering angle, 10-cm aperture, and for both on-off keying (OOK) and differential phase-shift keying (DPSK) the 2.5-Gb/s power penalty is approximately 1.0 dB, while at 10 Gb/s the penalty increases to 7.7 dB for OOK and 7.8 dB for DPSK.

Ultrasensitive and high-dynamic-range two-photon absorption in a GaAs photomultiplier tube.

We demonstrate improved efficiency and dynamic range for two-photon absorption at 1.5 microm in a photoelectron-counting GaAs photomultiplier tube. cw laser measurements show pure two-photon absorption occurring in the device from average powers of 1.3 microW to nearly 1 mW. We use the detector to implement an autocorrelator with peak-power x average-power sensitivity of 1.7 x 10(-4) (mW)(2) and discuss practical ways to reduce this figure of merit to as low as 1 x 10(-5) (mW)(2).