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.
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.
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.
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.
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.
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.
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.
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.