James J. Foshee

Hybrid Optical RF Airborne Communications

The use of hybrid free-space optical (FSO)/radio-frequency (RF) links to provide robust, high-throughput communications, fixed infrastructure links, and their associated networks have been thoroughly investigated for both commercial and military applications. The extension of this paradigm to mobile, long-range networks has long been a desire by the military communications community for multigigabit mobile backbone networks. The FSO communications subsystem has historically been the primary limitation. The challenge has been addressing the compensation of propagation effects and dynamic range of the received optical signal. This paper will address the various technologies required to compensate for the effects referenced above. We will outline the effects FSO and RF links experience and how we overcome these degradations. Results from field experiments conducted, including those from the Air Force Research Laboratory Integrated RF/Optical Networked Tactical Targeting Networking Technologies (IRON-T2) program, will be presented.

Long distance laser communications demonstration

AOptix demonstrated a simulated air-to-air laser communications (laser-com) system over a 147Km distance by establishing a laser communication link between the islands of Hawaii and Maui. We expect the atmospheric conditions encountered during this demonstration to be representative of the worst seeing conditions that could be expected for an actual air to air link. AOptix utilized laser-com terminal incorporating Adaptive Optics (AO) to perform high speed tracking and aberration correction to reduce the effects of the seeing. The demonstration showed the feasibility of establishing high data rate point to point laser-com links between aircraft. In conjunction with Johns Hopkins University Applied Physics Laboratory networking equipment we were able to demonstrate a 40Gbit DWDM link, providing significantly more data throughput than is available using RF technologies. In addition to being very high data rate, the link demonstrates very low beam spread, which gives very high covertness, and a high degree of data security. Since the link is based on 1550nm optical wavelengths it is inherently resistant to jamming.

Demonstration of high-data-rate wavelength division multiplexed transmission over a 150-km free space optical link

A 150 km free-space optical (FSO) communication link between Maui (Haleakala) and Hawaii (Mauna Loa) was demonstrated by JHU/APL and AOptix Technologies, Inc. in September 2006. Over a 5 day period, multiple configurations including single channel 2.5 Gbps transmission, single channel 10 Gbps, and four wavelength division multiplexed (WDM) 10 Gbps channels for an aggregate data rate of 40 Gbps were demonstrated. Links at data rates from 10 to 40 Gb/s were run in excess of 3 contiguous hours. Data on the received power, frame synchronization losses, and bit error rate were recorded. This paper will report on the data transfer performance (bit error rates, frame synchronization issues) of this link over a 5 day period. A micropulse lidar was run concurrently, and on a parallel path with the FSO link, recording data on scattering loss and visibility. Comparisons between the state of the link due to weather and the data transfer performance will be described.

Demonstration of High Data Rate Wavelength Division Multiplexed Transmission over a 150 km Free Space Optical Link

A 150 km free-space optical (FSO) communication link between Maui (Haleakala) and Hawaii (Mauna Loa) was demonstrated by JHU/APL and AOptix Technologies, Inc. in September 2006. Over a 5 day period, multiple configurations including single channel 2.5 Gbps transmission, single channel 10 Gbps, and four wavelength division multiplexed (WDM) 10 Gbps channels for an aggregate data rate of40 Gbps were demonstrated. Links at data rates from 10 to 40 Gb/s were run in excess of 3 contiguous hours. Data on the received power, frame synchronization losses, and bit error rate were recorded.

Multiband phased array antennas for wireless communications

Need for exchanging a wide range of information among many users in a timely manner, has resulted in a rapid expansion of frequency utilization, in addition to bandwidth expansion. High resolution video, infrared sensors, and various types of radars now are expected to be readily exchanged among users, and these users can be located on the surface, in the air and these users can be expected to be not only mobile, but highly mobile. To exchange this high volume information reliably among these many users requires a high data rate and the use of directional antennas. The use of directional antennas would also tend to reduce the possibility of interference among the users and also reduce the RF terminal power consumption requirements. This paper describes a multi-functional phased array antenna design, which can operate at either Ku-Band or Ka-Band without the need for switching or reconfiguration; and can be readily switched to provide information in an timely manner to the various users.

A novel high-speed electro-optic beam scanner based on KTN crystals

We present a novel high-speed electro-optic beam scanner that provides a significantly improved scanning angle, angular resolution, and response time. Compared to conventional moving mirrors such as servo-controlled mirrors and galvanic mirrors, the demonstrated laser scanning device can improve the response time by 100 times. The presented device has many other unique features such as light weight, small dimension, low power consumption, and no-moving components, which are particularly suitable for airborne and space-borne applications. Electro-optic beam scanners are key components in advanced laser radars, laser communication systems. It also has important commercial applications in various fields such as display, printing, imaging, optical storage, optical communication, and so on.

Novel laser communications transceiver with internal gimbal-less pointing and tracking

This paper describes a novel laser communications transceiver for use in multi-platform satellite networks or clusters that provides internal pointing and tracking technique allowing static mounting of the transceiver subsystems and minimal use of mechanical stabilization techniques. This eliminates the need for the large, power hungry, mechanical gimbals that are required for laser cross-link pointing, acquisition and tracking. The miniature transceiver is designed for pointing accuracies required for satellite cross-link distances of between 500 meters to 5000 meters. Specifically, the designs are targeting Air Force Research Labs TechSat21 Program, although alternative transceiver configurations can provide for much greater link distances and other satellite systems. The receiver and transmitter are connected via fiber optic cabling from a separate electronics subsystem containing the optoelectronics PCBs, thereby eliminating active optoelectronic elements from the transceivers mechanical housing. The internal acquisition and tracking capability is provided by an advanced micro-electro-mechanical system (MEMS) and an optical design that provides a specific field-of-view based on the satellite clusters interface specifications. The acquisition & tracking control electronics will utilize conventional closed loop tracking techniques. The link optical power budget and optoelectronics designs allow use of transmitter sources with output powers of near 100 mW. The transceiver will provide data rates of up to 2.5 Gbps and operate at either 1310 nm or 1550 nm. In addition to space-based satellite to satellite cross-links, we are planning to develop a broad range of applications including air to air communications between highly mobile airborne platforms and terrestrial fixed point to point communications.

Terahertz parametric generation photonic band gap structure with negligible structural dispersion in the optical range

We present a photonic band gap (PBG) structure (or nonlinear photonic crystal) design for terahertz (THz) wave parametric generation, whose component materials have a small refractive index difference in the near infrared and a large index difference for THz waves. The structural dispersion of such a PBG structure is strong in the THz range but negligible in the optical range. The former allows the phase-matched pump wavelength to be placed in the near infrared to eliminate two-photon absorption of the pump and signal beams. The latter leads to a crystal layer fabrication tolerances of a few micrometers and traditional polishing methods are suitable for device fabrication. The added design flexibility also allows the use of the most efficient crystal orientations.

Terahertz wave generation by photonic bandgap materials

One dimensional photonic bandgap structures can be used to enhance the efficiency of nonlinear optical parametric processes. Structural dispersion can be used to achieve phase matching, and resonant effects can be used to increase the intensity of the pump beams in the nonlinear optical material. In this paper these ideas are used to derive for the a continuous wave or a quasicontinuous wave THz signal and show that an improvement of three orders of magnitude in the output intensity can be achieved for a structure involving as few as four layers of ZnTe.

Switched optical polymeric waveguide true-time-delay lines for wideband photonics phased array antennas

It has been realized that the lack of enabling technology of beam forming and steering devices significantly slows down the process of implementing wideband phased array antenna systems. In this paper, we present our research in developing an integrated electro-optic switched true-time-delay module as a boradband beam forming device for wideband phased array antennas. The unique feature of our approach is that both the true-time-delay waveguide circuit and electro-optic switching elements are monolithically integrated in a single substrate. As a result, this integration significantly reduces the device size while eliminating the most difficult packaging problem associated with the delicate interfaces between optical fibers and optical switches. Such a monolithic approach offers greater precision for the RF phase control than the fiber-delay-lines thanks to the sub-micrometer accuracy of lithography-defined polymeric waveguides. More important, the proposed optical switched true-time-delay network requires very low electrical power consumption due to the low power soncumption of electrically-switchable waveguide gratings. Furthermore, the electrically-switchable waveguide gratings have a very fast switching speed (<50 μm) that is at least 100 times faster than any existing commercial optical switching matrix. Photonic phased array antenna based on optical true-time delay lines offers improved performance and reduced weight and power consumption over existing parabolic dish antenna presently used for communications.