G. Charmaine Gilbreath

Design and analysis of a diffraction-limited cat's-eye retroreflector

We describe a design for a modified, cats-eye retroreflector. The design is catadioptric, containing a single concave mirror and several lenses. This retroreflector design exhibits a unique combination of performance characteristics. It is diffraction limited over a large field angle while operating with a large aperture and numerical aperture. There is little vignetting of the optical beam, even at large field angles, providing good light return at all angles of incidence. It brings the light beam to a focus and allows access to the light near the focal plane, thus decoupling the size of the aperture from the size of devices used with the retroreflector. This final feature makes the design appealing for use with spatial light modulators, or other optical or electro-optical components.

45-Mbit/s cat’s-eye modulating retroreflectors

Modulating retroreflectors (MRRs) couple passive optical retroreflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing-acquisition-tracking system required on only one end of the link. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large-area MQW placed in front of the aperture of a corner cube. For a MQW modulator, the maximum modulation rate can range into the gigahertz, limited only by the RC time constant of the device. Most MRR systems have used corner-cube retroreflectors with apertures of about 1 cm, which require large, and hence high-capacitance, modulators. Thus data rates exceeding a few megabits per second are not possible. We describe a new kind of MQW MRR that uses a cats-eye retroreflector with the MQW in the focal plane of the cats-eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 45-Mbit/s free space link over a range of 7 km is demonstrated.

InGaAs Multiple Quantum Well Modulating Retro-reflector for Free Space Optical Communications"

Modulating retro-reflectors provide means for free space optical communication without the need for a laser, telescope or pointer tracker on one end of the link. These systems work by coupling a retro-reflector with an electro- optic shutter. The modulating retro-reflector is then interrogated by a cw laser beam from a conventional optical communications system and returns a modulated signal beam to the interrogator. Over the last few years the Naval Research Laboratory has developed modulating retro-reflector based on corner cubes and large area Transmissive InGaAs multiple quantum well modulators. These devices can allow optical links at speeds up to about 10 Mbps. We will discuss the critical performance characteristics of such systems including modulating rate, power consumption, optical contrast ratio and operating wavelength. In addition a new modulating retro-reflector architecture based upon cat s eye retroreflectors will be discussed. This architecture has the possibility for data rates of hundreds of megabits per second at power consumptions below 100 mW.

Modulating retroreflector architecture using multiple quantum wells for free-space optical communications

In this paper, we describe a demonstration using a Multiple Quantum Well modulator combined with an optical retroreflector which supported a high speed free space optical data link. Video images were transmitted over an 859 nanometer link at a rate of 460 kilo bits per second, where rate of modulation was limited by demonstration hardware, not the modulator. Reflection architectures for the modulator were used although transmission architectures have also been investigated but are not discussed in this paper. The modulator was a GaAs/Al0.3Ga0.7As quantum well which was designed and fabricated for use as a shutter at the Naval Research Laboratory. We believe these are the first results reported demonstrating a high speed free space optical data link using multiple quantum well shutters combined with retroreflectors for viable free space optical communications.

A comparison of optical turbulence models

The U.S. Navy has an interest in the use of laser systems for surface ships. Such systems must operate within a thin near-surface environment called the marine atmospheric surface layer. There exist substantial gradients in temperature and momentum within this layer which make turbulence a strong function of height. We are interested in robust and simple optical turbulence models that can be used to predict turbulence along near-horizontal paths. We discuss several different models that are based upon similarity theory, and we compare the models with field transmission data taken from both over-water and over-land propagation paths.

Measurements of intensity scintillations and probability density functions of retroreflected broadband 980-nm laser light in atmospheric turbulence

Intensity scintillation variances and intensity probability density functions (PDFs) were experimentally measured for broadband (2 nm), 980-nm laser light reflected by two or more corner cube retroreflectors as a function of retroreflector lateral spacing over a short (75 m) atmospheric optical path. The PDFs transitioned from broad double-peaked beta-shaped densities to lognormal as the retroreflector spacing was increased to exceed the optical fields lateral coherence length. Specific spacing for a given average atmospheric structural C n 2 eliminated interference between the light beams returned by the retroreflectors.

Spatial intensity correlation and aperture averaging measurements in a 20-mile retroreflected lasercom link

The Naval Research Laboratory has established a lasercom test bed across the Chesapeake Bay. The test bed uses a bi-static transmitter/receiver arrangement on the western shore of the Chesapeake Bay and various configurations of 5 cm retro-reflectors on the eastern shore to produce a 32 km retro-reflected lasercom test range. Experiments measuring the laser’s transverse spatial profile after propagation over the test range have been performed. These experiments use an InGaAs CCD to image the pupil plane of the 40 cm receiver telescope and a frame grabber to store contiguous images for analysis. Analysis of these image sequences allows measurement of transverse spatial correlations across the received beam after 32 km retro-reflected propagation of the beam. Various configurations and numbers of retro-reflectors were studied to investigate the impact of number and arrangement of retro-reflectors on the received beam’s spatial profile and spatial correlations. Additionally, since the CCD output is stored as a contiguous stream of images, analysis of these images’ intensity variance in time allows measurement of aperture averaging effects as a function of number of retro-reflectors and their geometry. Results from these experiments are presented.

Resolving the effects of rotation in early type stars

We review the theory of rotating stars, first developed 80 years ago. Predictions include a specific relation between shape and angular velocity and between surface location and effective temperature and effective gravity. Seen at arbitrary orientation rapidly rotating stars will display ellipsoidal shapes and possibly quite asymmetric intensity distributions. The flattening due to rotation has recently been detected at PTI and VLTI. With the increasing baselines available in the visible and the implementation of closure phase measurements at the NPOI it is now possible to search for the surface brightness effects of rotation. Roche theory predicts only large scale deviations from the usual centro-symmetric limb-darkened models, ideal when the stellar disks are only coarsely imaged as now. We report here observations of Altair and Vega with the NPOI using baselines that detect fringes beyond the first Airy zero in both objects. Asymmetric, non-classical intensity distributions are detected. Both objects appear to be rotating at a large fraction of their breakup velocity. Vega is nearly pole on, accounting for its low apparent rotational velocity. Altairs inclination is intermediate, allowing high S/N detection of all the predicted features of a Roche spheroid. We describe how these objects will test this fundamental theory and how Vegas role as a standard will need reinterpretation.

Analysis of the Naval Observatory Flagstaff Station 1-m telescope using annular Zernike polynomials

The Naval Observatory Flagstaff Station 1-m telescope is evaluated for the addition of adaptive optics capabilities to its instrumentation suite. Zernike decomposition of the optical system based on phase diversity measurements shows that the static optical aberrations are small enough that they will not degrade the performance of the deformable optical element. The analysis makes use of annular pupil Zernike polynomial reconstruction of the wavefront to accommodate the large obscuration in this telescope and compares this with the results from using filled circular Zernike polynomials.

Atmospheric Turbulence Studies of a 16 km Maritime Path

The Naval Research Lab (NRL) is currently operating a lasercom test facility (LCTF) across the Chesaepeake Bay between NRLs Chesapeake Bay Detachment (NRL-CBD) and NRL-Tilghman Island. This lasercom test facility has successfully demonstrated 32 km retro-reflected links at data rates up to 2.5 Gbps. Along with lasercom link studies, atmospheric characterization of the NRL-CBD to Tilghman Island optical path has been investigated. These studies range from passive optical turbulence monitoring based on angle-of-arrival measurements of a spotlights apparent motion, to intensity and angle-of-arrival measurements of a retro-reflected laser beam. Currently the LCTF is being upgraded from a retro-reflected link to a direct one-way link from NRL-CBD to NRL-Tilghman Island. Initial measurements of atmospheric turbulence effects in this one-way configuration have recently been performed. Results of these past and current atmospheric turbulence studies are presented.