Atmospheric noise signals are fundamental limitation of free-space optical communications, as the decrease in signal-to-noise ratio reduces the range and/or bandwidth of the link. In this paper we consider the limitations that this imposes, and investigate the use of discrete wavelet transformation (DWT) to overcome them. Simulations are performed to validate the use of the DWT in the demodulation of the analog data in the presence of noise. Results of the experiments are presented.

Based on wavelet transformation and adaptive Wiener filtering, a new method was presented by the authors to perform synchronization and detection of binary data from a free-space optical (FSO) signal. It was shown prevlously that the Haar wavelet with a fixed scale is an excellent choice for this purpose. The output of the filter was zero mean and was closely related to the derivative of the binary data. In this effort, an analysis of the prior work is presented to obtain the probability of bit error using a Bayesian ternary hypotheses testing. The analysis also results in determining optimum thresholds for the detection of binary data. Simulation experiments are performed and presented to validate the results of the theoretical analysis. (c) 2007 Society of Photo-Optical Instrumentation Engineers.

Free-space optical communication is often desired between two nodes having different capabilities. Using a modulating retroreflector (MRR) shifts most of the power, weight, and pointing requirements to one end of the link, allowing the other end to be extremely small, low-power, and requiring only rough pointing (to within +/-15 degrees). For a 5 Mb/s link at 2 km, our entire MRR package including drive electronics weighs only 8.5 g and requires 60 mW. This same device has also been arrayed to further relax the pointing requirements. In the simplest MRR link, an unmodulated (CW) laser interrogates a MRR comprised of an absorptive modulator and a retroreflector. If the interrogation beam is within the retroreflector's field of view (FOV), the beam will return to the interrogator with data impressed on it. In this discussion, we present the range of MRRs developed since 1998 at the Naval Research Laboratory (NRL). NRL's MRRs include both corner cube retroreflector and cat's eye retroreflector types, both individually and in arrays. Each variation has unique characteristics which may be beneficial or disqualifying in different situations. Size, weight, FOV, power consumption, cost, speed × range product, modulator type, ruggedness, time-to-market, and requirement for angle division multiplexing are all determining factors in MRR design and retroreflector choice. We compare strengths, weaknesses, and give current link performance data for several different systems. Links demonstrated include shore-to-shore, boat-to-shore, and UAV-to-ground over a wide range of distances and data rates.

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 cat's-eye retroreflector with the MQW in the focal plane of the cat's-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.

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 cat's eye MRR. Cat's 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 cat's eye MRR systems with near diffraction limited performance and bandwidths of 45 Mbps. We also discuss a link to a cat's eye MRR over a 7 Km range.

An eye-safe free-space optical communications link at 5 Mbps over a distance of about one kilometer using quantum well based modulating retro-reflectors is described. Atmospheric effects and the effects of modulator performance are characterized.

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. © 2001 Society of

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.

The vacuum beat wave accelerator (VBWA) is discussed and design parameters for proof-of-principle experiment are presented. The VBWA utilizes two focused laser beams of differing wavelengths to generate a beat wave that can impart a net acceleration to charged particles. Theory and simulations show that the single-stage energy gain of the VBWA is limited by diffraction of the laser beams, particle slippage in phase and velocity, and radial walk-off. In the simulations the particles are synchronous with the beat wave for a short interval of time and the energy gain has the nature of an impulse delivered near the focal region. Simulations also show that the problem of radial walk-off may be ameliorated by using a converging beam of particles. For terawatt-level laser beams, with wavelengths 1 mu m and 0.5 mu m, and a 4.5 MeV finite-emittance electron beam the energy can be increased to 12.5 MeV in a non-synchronous interaction over a distance of under 4 mm, with a peak acceleration gradient > 15 GeV/m and an estimated trapping fraction of similar to 1 %.

Experiments were conducted in the ELMO Bumpy Torus (EBT) from 1973 until 1984. A number of papers have been published concerning various aspects of experiments during the final two years of operation. The paper summarizes the final experimental conclusions and discusses those issues which remain unresolved. The main conclusions are as follows: (1) measurements of the width of the hot electron rings showed them to be wider than previously suspected, diluting their diamagnetism and negating their ability to locally reverse the magnetic field gradient; (2) two-dimensional plots of the plasma potential revealed open potential contours in the C-mode and closed potential contours near the plasma centre in the T-mode; (3) an in–out asymmetry in the plasma potential was always observed, giving rise to electric fields which drove convective plasma loss.

The data system for the advanced toroidal facility (ATF) is a logical extension of the system used for the Impurity Studies Experiment ISX-B. Based on the ISX-B concepts and hardware, it extends them to provide support for long-pulse or steady-state operation of ATF. The system is now network based rather than shared memory based and can dynamically add or remove processors. The hardware consists of a pair of VAX 8700 CPUs and an assortment of MicroVAX front ends. This hardware acquires all data, generates all control waveforms, and provides trending and logging services for the safety interlock and control systems. These systems are based on programmable logic controllers (PLCs) which are independent of the data system. The software consists of three major components: a master scheduling and monitoring system (SAMS) that provides synchronizing services and allocates files and memory; a signal-based data and file management package (DMG) that provides data access services; and the ORNL CAMAC driver package. All diagnostics are supported by data acquisition and analysis software (either generic or specialized) that is based on and shared by these services.

An eight-shot pneumatic pellet injection system has been developed for plasma fueling of the tokamak fusion test reactor (TFTR). The active cryogenic mechanisms consist of a solid hydrogen extruder and a rotating pellet wheel that are cooled by flowing liquid-helium refrigerant. The extruder provides solid hydrogen for stepwise loading of eight holes located circumferentially around the pellet wheel. This design allows for three different pellet diameters: 3.0 mm (three pellets), 3.5 mm (three pellets), and 4.0 mm (two pellets) in the present configuration. Each of the eight pellets can be shot independently. Deuterium pellets are accelerated in 1.0-m-long gun barrels with compressed hydrogen gas (at pressures from 70 to 105 bar) to velocities in the range 1.0–1.5 km/s. The pellets are transported to the plasma in an injection line that incorporates two stages of guide tubes with intermediate vacuum pumping stations. A remote, stand-alone control and data-acquisition system is used for injector and vacuum system operation. The eight-shot injection system has been installed and operated on TFTR. The design features, operation, and performance characteristics of the system are described.

The radio frequency test facility (RFTF) at Oak Ridge National Laboratory, used to test and evaluate high-power ion cyclotron resonance heating (ICRH) systems and components, is monitored and controlled by a multicomponent computer system. This data-acquisition and control system consists of three major hardware elements: (1) an Allen-Bradley PLC-3 programmable controller, (2) a VAX 11/780 computer, and (3) a CAMAC serial highway interface. Operating in LOCAL as well as REMOTE mode, the programmable logic controller (PLC) performs all the control functions of the test facility. The VAX computer acts as the operator’s interface to the test facility by providing color mimic panel displays and allowing input via a trackball device. The VAX also provides archiving of trend data acquired by the PLC. Communications between the PLC and the VAX are via the CAMAC serial highway. Details of the hardware, software, and the operation of the system are presented in this paper.

The Oak Ridge National Laboratory is currently developing a deuterium pellet injector for installation on the tokamak fusion test reactor (TFTR). This paper describes the design and development of a stand-alone data acquisition and control system for that device. Major elements of the hardware are an Allen–Bradley PLC 2/30 programmable logic controller, a MicroVAX-II computer using the VMS operating system, CAMAC data acquisition and communication equipment, and special-purpose controllers for temperature and for the sequencing of pellet firing valves. The PLC performs all actual control actions and acquires data pertinent to those actions. The MicroVAX receives the data acquired by the PLC, displays it for the operator, prompts for and processes requests for action from the operator, and informs the PLC of those requests. The primary purpose of this paper is to describe the software operating in the MicroVAX, including the system architecture, major tasks, and ancillary and background tasks.

The repeating pneumatic hydrogen pellet injector, which was developed at the Oak Ridge National Laboratory (ORNL), has been installed and operated on the Tokamak Fusion Test Reactor (TFTR). The injector combines high-speed extruder and pneumatic acceleration technologies to propel frozen hydrogen isotope pellets repetitively at high speeds. The pellets are transported to the plasma in an injection line that also serves to minimize the gas loading on the torus; the injection line incorporates a fast shutter valve and two stages of guide tubes with intermediate vacuum pumping stations. A remote, stand-alone control and data acquisition system is used for injector and vacuum system operation. In early pellet fueling experiments on TFTR, the injector has been used to deliver deuterium pellets at speeds ranging from 1.0 to 1.5 km/s into plasma discharges. First, single large (nominal 4-mm-dia) pellets provided high densities in TFTR (1.8 x 10/sup 14/ cm/sup -3/ on axis); after conversion to smaller (nominal 2.7-mm-dia) pellets, up to five pellets were injected at 0.25-s intervals into a plasma discharge, giving a line-averaged density of 1 x 10/sup 14/ cm/sup -3/. Operating characteristics and performance of the injector in initial tests on TFTR are presented.

The transient waveform system described in this report is designed to acquire analog waveforms from the ELMO Bumpy Torus (EBT) diagnostic experiments. Pressure, density, synchrotron radiation, etc., are acquired and digitized with a Kinetic Systems TR812 transient recorder and associated modules located in a CAMAC crate. The system can simultaneously acquire, display, and transmit sets of data consisting of identification parameters and up to 1024 data points for 1 to 64 input signals (frequency range = 0.01 pulse/s to 100 kHz) of data every one or more minutes; thus, it can run continuously without operator intervention. The data are taken on a VAX 11/780 and transmitted to a data base on a DECSystem-10. To aid the programmer in making future modifications to the system, detailed documentation using the Yourdon structural methods has been given.