Free-space optical communications research and demonstrations at the U.S. Naval Research Laboratory.

Free-space optical communication can allow high-bandwidth data links that are hard to detect, intercept, or jam. This makes them attractive for many applications. However, these links also require very accurate pointing, and their availability is affected by weather. These challenges have limited the deployment of free-space optical systems. The U.S. Naval Research Laboratory has, for the last 15 years, engaged in research into atmospheric propagation and photonic components with a goal of characterizing and overcoming these limitations. In addition several demonstrations of free-space optical links in real-world Navy applications have been conducted. This paper reviews this work and the principles guiding it.

Adaptive optics performance model for optical interferometry.

The optical interferometry community has discussed the possibility of using adaptive optics (AO) on apertures much larger than the atmospheric coherence length in order to increase the sensitivity of an interferometer, although few quantitative models have been investigated. The aim of this paper is to develop an analytic model of an AO-equipped interferometer and to use it to quantify, in relative terms, the gains that may be achieved over an interferometer equipped only with tip-tilt correction. Functional forms are derived for wavefront errors as a function of spatial and temporal coherence scales and flux and applied to the AO and tip-tilt cases. In both cases, the AO and fringe detection systems operate in the same spectral region, with the sharing ratio and subaperture size as adjustable parameters, and with the interferometer beams assumed to be spatially filtered after wavefront correction. It is concluded that the use of AO improves the performance of the interferometer in three ways. First, at the optimal aperture size for a tip-tilt system, the AO system is as much as ~50% more sensitive. Second, the sensitivity of the AO system continues to improve with increasing aperture size. And third, the signal-to-noise ratio of low-visibility fringes in the bright-star limit is significantly improved over the tip-tilt case.

Vega is a rapidly rotating star.

Vega, the second brightest star in the northern hemisphere, serves as a primary spectral type standard. Although its spectrum is dominated by broad hydrogen lines, the narrower lines of the heavy elements suggested slow to moderate rotation, giving confidence that the ground-based calibration of its visible spectrum could be safely extrapolated into the ultraviolet and near-infrared (through atmosphere models), where it also serves as the primary photometric calibrator. But there have been problems: the star is too bright compared to its peers and it has unusually shaped absorption line profiles, leading some to suggest that it is a distorted, rapidly rotating star seen pole-on. Here we report optical interferometric observations that show that Vega has the asymmetric brightness distribution of the bright, slightly offset polar axis of a star rotating at 93 per cent of its breakup speed. In addition to explaining the unusual brightness and line shape peculiarities, this result leads to the prediction of an excess of near-infrared emission compared to the visible, in agreement with observations. The large temperature differences predicted across its surface call into question composition determinations, adding uncertainty to Vega's age and opening the possibility that its debris disk could be substantially older than previously thought.

Operating modes of a charge-transfer-plate liquid-crystal phase modulator.

New results for an electron-beam-addressed liquid-crystal phase modulator are presented. Local, as well as global, control of liquid-crystal molecular reorientation is demonstrated, and the results of five different operating modes are discussed. Linear phase modulation with depths up to 30pi are demonstrated with this device, as are two-dimensional arrays of computer-generated spherical lenses with actively variable focal lengths. The spherical lenses were written without the use of specialized transfer functions to compensate for device nonlinearities. Applications are discussed in the context of adaptive optics in spaceborne systems.

Suppression of photorefractive beam fanning using achromatic gratings.

We show that achromatic grating techniques, using the 488- and 514.5-nm lines of the argon-ion laser, can suppress beam fanning while still allowing two-beam coupling to occur. The suppression is studied as a function of the ratio of the two colors used to form the achromatic grating.

Photorefractive effects in tantalum-doped potassium niobate.

Photorefractive gain is measured in poled 1% tantalum-doped potassium niobate (KNbO(3)). Two-wave mixing experiments are presented which exploit the r(42) coefficient of KNbO(3) by employing nonsymmetric geometries which rotate the grating vector away from the optic axis. The effect of this rotation on the exponential gain coefficient is shown. Experimental results are compared to an analytic expression derived for this parameter. The experiments were performed using an argon laser stabilized at lambda = 514.5 nm. The light was extraordinarily polarized, and the beams were incident on the b-face of the crystal. The absorption and the response time of the crystal were measured. Losses due to scattering and phase conjugation were also investigated.

Absorption holograms made with mode-stabilized laser diodes for use in a satellite communications link.

An experimental technique is presented for constructing absorption holograms in IR-sensitive material with mode-stabilized laser diodes. The combination of an extremely sensitive recording medium and high power available from the diode is exploited to permit minimum exposure times. This prevents the inherent modal instability of the laser diode from becoming a problem while the interference pattern is recorded. Diffraction efficiencies into the first order were measured to be greater than 14% for small construction angles, indicating some phase effects. These absorption holograms will be used as the first step in a two-step process to produce holographic optical elements for use with laser diodes in a satellite communications link.