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.
In this paper, we describe progress in the development of the NRL Multiple Quantum Well modulating retro-reflector including a description of recent demonstrations of an infrared data link between a small rotary-wing unmanned airborne vehicle and a ground based laser interrogator using the NRL multiple quantum well modulating retro-reflector. Modulating retro-reflector systems couple an optical retro- reflector, 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 retro-reflector uses a semiconductor based multiple quantum well shutter capable of modulation rates up to 10 Mbps, depending on link characteristics. The technology enable the use of near-infrared frequencies, which is well known to provide covert communications immune to frequency allocation problems. The multiple quantum well modulating retro-reflector has the added advantage of being compact, lightweight, covert, and requires very low power. 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 demonstration, a Mbps optical link to an unmanned aerial vehicle in flight at a range of 100-200 feet is shown. Near real-time compressed video is also demonstrated at the Mbps level.
This paper reports progress on the development of a fast modulating retroreflector for a free space optical data link. A previous publication reported sustaining video over a 17 meter link using a multiple quantum well shutter with a diameter of 0.5 cm at a rate on the order of 0.5 Mbps, limited by the demonstration electronics. This work describes improvements in the device performance, which is on the order of 4 Mbps to 6 Mbps with a Bit Error Rates of 10-6 over a robust optical link. This device lends itself to an array configuration for long range applications and will clearly support T1 rates of 1.54 Mbps, and higher.
AlGaAs/GaAs Narrow Base Heterojunction Bipolar Transistors (NBHBTs) with 50 angstroms thick bases exhibit maximum small signal common emitter current gains hfe of 1400 at 300 K and 3000 at 80 K. The performance of the device is attributed to the superlattice graded emitter contact and a novel planar base access fabrication process. Low temperature measurements indicate that the maximum current gain increases exponentially with decreasing temperature until is saturates around 200 K, suggesting a tunnelling limited current transport mechanism.
The advantages of using Sb containing III-V compounds in bipolar type devices are discussed with recent experimental results of two different applications of GaAsSb in Heterojunction Bipolar Transistors (HBTs). The performance of a prototype AlGaAs/GaAsSb/GaAs double HBT (DHBT) that exhibits a current gain of five and a maximum collector current density of 5 X 104 A/cm2 and a pnp AlGaAs/GaAs HBT with a superlattice GaAsSb emitter ohmic contact, with specific contact resistivity of 5 +/- 1 X 10-7 (Omega) -cm2 across the sample, are examined.
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