Mike S. Ferraro

Large-aperture multiple quantum well modulating retroreflector for free-space optical data transfer on unmanned aerial vehicles

We describe progress in the development of a multiple quan- tum well modulating retroreflector, including a description of recent dem- onstrations of an infrared data link between a small rotary-wing un- manned airborne vehicle and a ground-based laser interrogator using the device designed and fabricated at the Naval Research Laboratory (NRL). Modulating retroreflector systems couple an optical retroreflector, such as a corner cube, and an electro-optic shutter to allow two-way optical communications using a laser, telescope, and pointer-tracker on only one platform. The NRL modulating retroreflector uses a semiconductor-based multiple quantum well shutter capable of modula- tion rates greater than 10 Mbps, depending on link characteristics. The technology enables the use of near-infrared frequencies, which is well known to provide covert communications immune to frequency allocation problems. This specific device has the added advantage of being com- pact, lightweight, covert, and requires very low paper. Up to an order of magnitude in onboard power can be saved using a small array of these devices instead of the radio frequency equivalent. In the described dem- onstration, a Mbps optical link to an unmanned aerial vehicle in flight at a range of 100 to 200 feet is shown. Near real-time compressed video was also demonstrated at the Mbps level and is described.

Thermo-optic tuning and switching in SOI waveguide Fabry-Perot microcavities

Compact silicon-on-insulator (SOI) waveguide thermo-optically tunable Fabry-Perot microcavities with silicon/air Bragg mirrors are demonstrated. Quality factors of Q=4,584 are measured with finesse F=82. Tuning is achieved by flowing current directly through the silicon cavity resulting in efficient thermo-optic tuning over 2 nm for less than 50 mW applied electrical power. The high-Q cavities enable fast switching (1.9 mus rise time) at low drive power (<10 mW). By overdriving the device, rise times of 640 ns are obtained. Various device improvements are discussed.

Photonic microharp chemical sensors

We describe a new class of micro-opto-mechanical chemical sensors: A photonic microharp chemical sensor is an array of closely spaced microbridges, each differing slightly in length and coated with a different sorbent polymer. They are optically interrogated using microcavity interferometry and photothermal actuation, and are coupled directly to an optical fiber. Simultaneous measurements of the fundamental flexural resonant frequency of each microbridge allow the real-time detection and discrimination of a variety of vapor-phase analytes, including DMMP at concentrations as low as 17 ppb.

All-optical micromechanical chemical sensors

The authors describe experimental results from micromechanical resonators coated with a chemoselective polymer that detect chemical vapors from volatile organic compounds using all-optical interrogation. The shift in the resonant frequency of the gold microbeam is read out using photothermal actuation and microcavity interferometry. Response times of less than 5s are achieved for vapor concentrations as low as 60ppm using optical powers of a few megawatts.

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.

Free-space optical communication link across 16 kilometers over the Chesapeake Bay to a modulated retroreflector array

This paper presents the results of a successful bidirectional free-space optical link across 16 km to a modulated retroreflector array. The link was implemented at the Naval Research Laboratorys Chesapeake Bay Detachment laser test range. A 6-W cw 1550-nm class 1 M interrogation beam was used to illuminate an array of three modulated cats-eye retroreflectors located on a tower across the Chesapeake Bay on Tilghman Island. The modulated retroreflectors had a diameter of 16 mm and were arranged in a triangular pattern with a spacing of 30 cm. The interrogating terminal employed a 100-µrad divergence and a high-speed pointing and tracking system to maintain link alignment. Link testing occurred over 12 days in the months of September, October, and November of 2006. Topics presented in this paper include the link scenario for the 16-km free-space optical link, the link budget, and terminal designs, as well as link acquisition and performance. Link performance results presented include data transmission throughput, scintillation data, and pointing and tracking results.

Atmospheric turbulence effects measured along horizontal-path optical retro-reflector links.

The scintillation measured over close-to-ground retro-reflector links can be substantially enhanced due to the correlations experienced by both the direct and reflected echo beams. Experiments were carried out at China Lake, California, over a variety of ranges. The emphasis in this paper is on presenting the data from the 1.1 km retro-reflecting link that was operated for four consecutive days. The dependence of the measured irradiance flux variance on the solar fluence and on the temperature gradient above the ground is presented. The data are consistent with scintillation minima near sunrise and sunset, rising rapidly during the day and saturating at irradiance flux variances of ~10. Measured irradiance probability distributions of the retro-reflected beam are compared with standard probability density functions. The ratio of the irradiance flux variances on the retro-reflected to the direct, single-pass case is investigated with two data sets, one from a monostatic system and the other using an off-axis receiver system.

Modulating Retro-Reflector Lasercom Systems for Small Unmanned Vehicles

Lasercom, also known as free space optical (FSO) communication, has enjoyed a renewal of interest driven by increasing data rate requirements and the crowding of the RF spectrum, affecting both commercial and military sectors. Military communications must also deal with intentional or unintentional jamming, as well as frequency allocation restrictions, neither of which affects lasercom. The U.S. Naval Research Laboratory (NRL) has been conducting research on lasercom since 1998 with an emphasis on tactical applications. NRLs lasercom research has covered propagation studies in the maritime domain, component development, and systems demonstrations. NRL has developed both conventional lasercom systems and retro-reflecting systems for small platforms. This paper reviews some of the retro-reflecting work, discusses applications of lasercom in the areas of explosive ordnance disposal (EOD) unmanned ground vehicles (UGVs), and unmanned aerial vehicles (UAVs), and describes future directions.

Irradiance correlations in retro-reflected beams.

Communications links that utilize modulating retro-reflectors can make use of turbulence-induced fade information available at the remote data-signal terminal in order to optimize the data transfer rate. Experiments were conducted to measure the irradiance in both the direct and the retro-reflected beams. Both on-axis and off-axis components were recorded in order to further study the enhancement in the scintillation index observed in the retro-reflected beam. Measurements were made over a 1.8 km terrestrial range at AP Hill, Virginia. The degree of correlation of the received irradiance between the direct and double-passage beams is found to approach 90% on-axis and 70% outside of the Fresnel zone radius. The scintillation index in the retro-reflected beam is enhanced on-axis due to reciprocal optical paths. The measured scintillation indices, and the correlation of the retro-reflected beam with the direct beam, are compared with a point source, point scatterer, and point receiver model in the strong scintillation approximation.

45 Mbps cat's eye modulating retro-reflector link over 7 Km

Modulating retro-reflectors (MRR) allow free space optical links with no need for pointing, tracking or a laser on one end of the link. They work by coupling a passive optical retro-reflector with an optical modulator. The most common kind of MRR uses a corner cube retro-reflector. These devices must have a modulator whose active area is as large as the area of the corner cube. This limits the ability to close longer range high speed links because the large aperture need to return sufficient light implies a large modulator capacitance. To overcome this limitation we developed the concept of a cats eye MRR. Cats eye MRRs place the modulator in the focal plane of a lens system designed to passively retro-reflect light. Because the light focuses onto the modulator, a small, low capacitance, modulator can be used with a large optical aperture. However, the position of the focal spot varies with the angle of incidence so an array of modulators must be placed in the focal plane, In addition, to avoid having to drive all the modulator pixels, an angle of arrival sensor must be used. We discuss several cats eye MRR systems with near diffraction limited performance and bandwidths of 45 Mbps. We also discuss a link to a cats eye MRR over a 7 Km range.