Brian Stadler

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.

Optical communications in atmospheric turbulence

Recent experiments conducted under the Optical RF Communications Adjunct program demonstrate and validate the viability of hybrid free space optical communications links in heavy atmospheric turbulence. Long range air-to-mountain link closures were established under extreme atmospheric turbulence. The system implemented adaptive mechanisms such as adaptive optics, an optical automatic gain controller, forward error correction coding, and link-level retransmission to achieve low packet error rates for long distance links with heavy turbulence. The system, experiments, and results are presented and comparisons are made to statistical prediction models.

Free space optical communications: coming of age

Information superiority, where for the military or business, is the decisive advantage of the 21st Century. While business enjoys the information advantage of robust, high-bandwidth fiber optic connectivity that heavily leverages installed commercial infrastructure and service providers, mobile military forces need the wireless equivalent to leverage that advantage. In other words, an ability to deploy anywhere on the globe and maintain a robust, reliable communications and connectivity infrastructure, equivalent to that enjoyed by a CONUS commercial user, will provide US forces with information superiority. Assured high-data-rate connectivity to the tactical user is the biggest gap in developing and truly exploiting the potential of the information superiority weapon. Though information superiority is much discussed and its potential is well understood, a robust communications network available to the lowest military echelons is not yet an integral part of the force structure, although high data rate RF communications relays, e.g., Tactical Common Data Link, and low data SATCOM, e.g, Ku Spread Spectrum, are deployed and used by the military. This may change with recent advances in laser communications technologies created by the fiber optic communications revolution. This paper will provide a high level overview of the various laser communications programs conducted over the last 30 plus years, and proposed efforts to get these systems finally deployed.

Optical RF Communications Adjunct

The capacity to integrate RF and free space optical hybrid communications now feasible given advances in adaptive optics and optical automated gain control. The ORCA program is developing on operationally capable of highly reliable hybrid communications. This paper provides an overview of the ORCA systems and discusses some of the key developments in making the systems a reality.

Hybrid optical radio frequency airborne communications

Optical RF Communications Adjunct Program flight test results provide validation of the theoretical models and hybrid optical radio frequency (RF) airborne system concepts developed by the Defense Advanced Research Projects Agency and the U.S. Air Force Research Laboratory. Theoretical models of the free-space optical communications (FSOC), RF, and network components accurately predict the flight test results under a wide range of day and night operating conditions. The FSOC system, including the adaptive optics and optical modem, can operate under high turbulence conditions. The RF and network mechanisms of Layer 2 retransmission and failover provide increased reliability, reducing end-to-end packet error rates. Overall the test results show that stable, long-range FSOC is possible and practical for near-term operations.

Observations of atmospheric effects for FALCON laser communication system flight test

Free-space optical communication terminals have been designed and extensively tested in various configurations. The FALCON terminals are designed to operate on large unmanned airborne vehicles (UAVs) or piloted aircraft. They provide a secure, two-way air-to-air and air-to-ground data link. In the latest flight test a successful 132km link was established. The beacon lasers operated at half of their available power, which was sufficient to establish and maintain link for the full flight track. The data and beacon links remained locked for approximately 30 minutes during which both aircraft turned, banked, and experienced air turbulence. This demonstration proved that laser communications is possible with tip-tilt correction as the primary control system compensation. It further demonstrated that compact, low cost free-space optical communications are now available for test and evaluation of operational scenarios.

Optical RF communications adjunct: Coming of age

The concept of Free Space Optical (FSO) communications has been around since the late 1960s.This paper will describe some recent experimental results that demonstrate and validate hybrid FSO/RF communication links as viable components in a tactical high data rate network. In particular, we will describe air-mountain link closure up to ranges of 200 km under heavy atmospheric turbulence. These links were operated at low packet and bit error rates, with occasional link outages. Like the internet, we used retransmission to minimize the effect of these outages on link throughput. For example, when the FSO system is running, ORCA uses the RF system for retransmission to improve link efficiency. In addition, we will discuss potential losses created by the aircraft aero-optics effects. Finally, we will show comparison between model predictions and experimental data that suggest we can predict link performance if the atmospheric turbulence conditions are known.

Visualizing aero-optic interactions about a nose-mounted turret

One aspect of the propagation-physics challenge associated with airborne, free-space, optical communications (FSOC), for example, is the characterization and mitigation of link losses due to aero-optic interactions. That is, air-density gradients due to compressibility effects in turbulent boundary layers, separated flows, and freeshear flows can disturb the wavefront in the near field of the transceiver. To better understand these aero-optical mechanisms, a model of a nose-mounted, FSOC transceiver recently was placed in a compressible-flow wind tunnel, and the resulting wavefront degradations, as a function of flow scenario, were recorded. High-speed, time-resolved movies of the aero-optic disturbances have been realized, using a Schlieren-imaging technique, and a very-highframe-rate camera. Discrete, vortical structures (amid otherwise-irregular shedding) were seen to emerge and convect past the clear aperture. The frequencies of these disturbances have been estimated from the movies, and these have been compared with high-speed, time-resolved wavefront reconstructions. Losses of -3.5 dB (for the case of Mach - 0.45 at 10 kft, side view, and λ - 1.55 μm, for example), and disturbance frequencies of - 1200 Hz (and higher) were observed. The system-level impact of the resulting wavefront degradations will be discussed.

Characterization of an optical phased array for use in free space optical communication antennas

Liquid Crystal Optical Phased Arrays (LCOPA) capable of steering optical beams over large angles require very large number of individually addressable electrodes that can be reduced by grouping the electrodes into periodic pattern to modulate phase profiles with consequent stepwise phase corrections made by an additional LCOPA. Such phase ramp-corrector configuration allows for reductions in the total number of the addressed electrodes and results in lower costs of development and manufacturing of LCOPA devices. Characterization of the device made by Teledyne Scientific for an experimental RF/EO antenna has been accomplished. Issues concerning optical beam steering efficiency, incident angle dependency and transparent electrodes alignment were investigated.

Liquid Crystal Based Electro-Optic Bragg Gratings for Laser Beam Attenuations

This paper provides research progress in the development of fast electro-optic gratings based on liquid crystals for laser beam attenuations. The electro-optic phase grating is formed by the phase separation of ~100nm liquid crystals droplets from a polymerizing organic matrix using holographic interference technique. The formed grating separates the incident laser beam into the output beams: the transmitted and diffracted beam, whose intensities can be electrically adjusted through electro-optic effect. The fast electro-optic gratings have a very fast electro-optic response time of 50 microseconds with diffraction efficiency above 99.8%. Optical receivers used in FSO have a limited dynamic range and there is a need for in-line variable attenuators to keep the signal levels from overloading the receiver. These attenuators should be continuous, provide sufficient attenuation, and also provide a low insertion loss for weak signal reception. The use of electro-optic Bragg gratings is one solution to meet the requirements for an in-line attenuator for FSO.