Hakki H. Refai

The transmission of multiple RF signals in free-space optics using wavelength division multiplexing

Free-space optics (FSO) is a technology that uses modulated optical lasers to transmit information in a line-of-sight path through the atmosphere. To date, the major focus of FSO research and development has been toward the transmission of digital data, mostly for “last mile” applications. This paper investigates the simultaneous transportation of multiple analog radio frequency (RF) signals over a single FSO link using wavelength division multiplexing (WDM) technology. Experimental measurements of optical peak power and signal-to-noise ratio (SNR) indicate the suitability of FSO links for supporting WDM applications.

Alignment and tracking of a free-space optical communications link to a UAV

This paper investigates the ability of a mechanical gimbal to perform alignment and tracking of a free-space optical communications link between a ground station and an unmanned aerial vehicle. The repeatability and accuracy of a mechanical gimbal are experimentally analyzed in order to generate probability distribution functions for the gimbals performance. A simulation portion investigates the amount of divergence present in the laser beam of the communications link in the presence of atmospheric turbulence along with the scintillation index of the laser beam and the probability of the signal fading below a required threshold. The data from the experimental and simulation portions are further analyzed to verify the ability of a gimbal to provide acquisition and tracking functions for a ground-to-air link along with the expected geometric loss associated with such a communications link.

Digital micromirror device for optical scanning applications

Recent mechanical and nonmechanical optical scanning devices do not meet the fast scanning requirements for contemporary and emerging applications and can only steer optical beams over relatively narrow angles. A variety of important applications require fast optical scanning devices that can steer laser beams rapidly to an arbitrary location and with no moving parts. We introduce a new optical scanning technique that can be used to collimate and steer optical beams for precision alignment in either 2-D free-space optical (FSO) communications links or image scanners. This nonmechanical technique is capable of rapidly redirecting the optical beams to arbitrary locations without greatly sacrificing other parameters such as aperture size, efficiency, and scanning range. A digital micromirror device (DMD) beam-steering system was successfully demonstrated and exhibited better performance results when compared with other available systems.

Scalable Upconversion Medium for Static Volumetric Display

Efficient up-conversion emitters are necessary to generate a full-color 3D display. Rare-earth, co-doped fluorides that efficiently convert near infrared diode laser emission to visible red, green, and blue light by sequential two photon absorption are necessary to accomplish this. An up-conversion medium for a 3D display, particularly the CSpace “static volumetric display,” can be fabricated by grinding rare earth-doped fluoride bulk crystals and then dispersing the resultant microcrystals within an index matched host. This in turn leads to a reduction in display cost, weight, and growing time, as well as facilitating display scalability. To demonstrate a scalable medium for the CSpace display, several rare earth-doped fluoride bulk crystals were ground into micro crystal powders and then dispersed in liquids of different refractive index, including 1.45, 1.456, 1.46, 1.464, 1.468, 1.47, 1.474, 1.476, 1.48, 1.484, and 1.49. Fluorescence strength and transmission measurements were taken. Different particle concentrations were tested and demonstrated as well, and the detailed experimental results are described. A real volumetric 3D image was constructed inside a prototype display medium of 40×40×10 mm3 using the CSpace display. A potential future solution is presented, and suggestions to improve the scalable medium are given.

Control system analysis for ground/air-to-air laser communications using simulation

Laser communication using free-space optics is an important element of the proposed US Department of Defense (DoD) Transformational Communication System. It offers data rates on the order of several Gigabits per second, resists electromagnetic jamming and interference, and enjoys low probability of intercept and detection. This paper describes an investigation, carried out using a computer simulation, to analyze the performance of the laser beam steering control system consisting of two gimbals, a fast steering mirror, the Global Positioning System (GPS), and attitude sensors. Two different application scenarios were simulated: ground-to-air and air-to-air. The simulation evaluated several control system parameters such as pointing accuracy, tracking capabilities and bandwidth requirements. The results of the simulation elucidated the impact of the system bandwidth on the performance and functionality of the ground or air-to-air laser communication system. The results also showed close correlation between pointing and tracking error with the uncertainties of the system due to GPS and attitude sensor measurements.

Omnidirectional free-space optical (FSO) receivers

Free-space optics (FSO) is a technology that utilizes modulated light beam to transmit information through the atmosphere. Line-of-sight connection between both FSO transceivers is a necessary condition to maintain continuous exchange of voice, video, and data information. To date, the primary concentration of mobile FSO research and development has been toward the accurate aligning between two transceivers. This study introduces an advanced FSO receiver that provides wider receiving angle compared with that of conventional FSO systems. We present data from measurements of optical power, which were very promising, and indicated that these advanced FSO receivers are suitable for FSO alignment applications and perform favorably with similar FSO receivers.

Optical tracking and auto-alignment transceiver system

Free-space optic (FSO) technology transmits information using a modulated light beam traveling through the atmosphere. In certain situations, when compared with wireless communication, it proves to be more effective by providing higher data rates at reduced size and cost. Current FSO technology transmits data, video, and voice communications at a bandwidth of up to 2.5 Gbps. FSO system transceivers rely on a line-of-sight connection to maintain a continuous exchange of data. Because atmospheric conditions can greatly affect FSO communications, facilitating continuous alignment requirements has been the primary concentration of mobile FSO research to date. This paper introduces a fully automatic, advanced FSO alignment system that has been tested to provide both continuous and simultaneous transmission and reception of data and modulated signal during mobile conditions. Building sway and aerial-to-base station communication were considered in the study.

A Tracking System for Mobile FSO

Free-space optics (FSO), or Optical Wireless, is an unlicensed line-of-sight technology that uses modulated lasers to transmit information through the atmosphere. By using light beams, FSO can transmit and receive data, voice, and video, information through the air. FSO provides data rates ranging from 100Mbps to 2.5Gbps. In most applications, FSO transceivers normally remain in a static location to ensure continuous line of sight and to maintain accurate alignment. One current challenge facing FSO technology is the desire to implement mobility. As a potential solution, this study introduces an auto-tracking system that will achieve and maintain alignment between two mobile FSO nodes. This auto-tracking system can be used in many different applications, such as reducing the time needed to achieve alignment of an FSO link, and maintaining a link between an aircraft and a stationary command post to exchange real-time video and data with high-speed laser communications. After link initiation, the auto-tracking system application will send steering commands back to the positioning gimbal. These steering commands are determined by feedback from Position Sensing Diodes (PSDs). The proposed FSO auto-tracking system provides optical beam steering and capturing mechanisms to provide tracking between two transceivers, either fixed or mobile. In this paper, we illustrate the feasibility of such a system and present experimental results for a source aligned with a PSD in a mobile environment.

The application of fiber optic wavelength division multiplexing in RF avionics

This paper demonstrates a successful application of wavelength division multiplexing (WDM) to the avionics environment to support analog RF signal transmission. We investigate the simultaneous transmission of four RF signals (channels) over a single optical fiber. These four analog channels are sequentially multiplexed and demultiplexed at different points along a fiber optic backbone to more closely emulate the conditions found onboard aircraft. We present data from the measurements of signal-to-noise ratio (SNR), transmission response (loss and gain), group delay that defines phase distortion, and dynamic range that defines nonlinear distortion. The data indicate that WDM is well-suited for avionic applications.

FSO tracking and auto-alignment transceiver system

Free-space optics (FSO) technology utilizes a modulated light beam to transmit information through the atmosphere. Due to reduced size and cost, and higher data rates, FSO can be more effective than wireless communication. Although atmospheric conditions can affect FSO communication, a line-of-sight connection between FSO transceivers is a necessary condition to maintain continuous exchange of data, voice, and video information. To date, the primary concentration of mobile FSO research and development has been toward accurate alignment between two transceivers. This study introduces a fully automatic, advanced alignment system that will maintain a line of sight connection for any FSO transceiver system. A complete transceiver system includes a position-sensing detector (PSD) to receive the signal, a laser to transmit the signal, a gimbal to move the transceiver to maintain alignment, and a computer to coordinate the necessary movements during motion. The FSO system was tested for mobility by employing one gimbal as a mobile unit and establishing another as a base station. Tests were performed to establish that alignment between two transceivers could be maintained during a given period of experiments and to determine the maximum speeds tolerated by the system. Implementation of the transceiver system can be realized in many ways, including vehicle-to-base station communication or vehicle-to-vehicle communication. This study is especially promising in that it suggests such a system is able to provide high-speed data in many applications where current wireless technology may not be effective. This phenomenon, coupled with the ability to maintain an autonomously realigned connection, opens the possibility of endless applications for both military and civilian use.