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OCTOBER 22–26, 2017
Sheraton® Atlanta Hotel
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FREE-SPACE OPTICAL COMMUNICATIONS continued
when frame rates are reduced and com-
pression algorithms are used. Optical
wireless, on the other hand, can easily
transmit at 1 to 5 Mbit/s using light-emit-
ting diodes (LEDs) and at gigabit data
rates using diode lasers (see Fig. 1). There
have been recent efforts to increase the
bandwidth of LEDs that may open additional opportunities for higher-data-rate,
Going deep with optics
Structures permanently placed on the
seabed can be serviced by underwater
fiber-optic cables that deliver very high
bandwidth communications at gigabit
rates between subsea nodes. However,
compared to guided modes in optical fi-
bers, the underwater FSO communica-
tions proposition is more complicated.
Terrestrial FSO systems have bulk
optics to minimize beam divergence,
good pointing and tracking systems,
and multi-wavelength adaptive optics
to handle atmospheric turbulence. Such
systems can currently provide gigabit data
rates over kilometer-length scales. These
capabilities have been greatly accelerated
by leveraging eye-safe 1. 55 µm lasers, er-
bium fiber amplifiers, and detectors de-
veloped for fiber-optic communications.
Because near-infrared (near-IR) wavelengths are strongly absorbed by water,
a challenge for underwater communications has been finding bright blue-green
lasers and LEDs. Fortunately, high-bright-
ness gallium-nitride (GaN)-based LEDs
and diode lasers now provide inexpensive
light sources for small underwater platforms, while frequency-doubled near-IR
lasers provide high peak powers for larger pulsed systems.
A second challenge is that during daylight hours for shallow depths, the sun is
a very bright interfering light source that
limits underwater detector performance,
so there is a need for good optical filtering and wide dynamic range detectors.
Finally, compared to terrestrial FSO
communications, turbulence is not the
dominating factor in undersea commu-
nications systems. Instead, absorption
and scattering are the major issues to
be addressed. In clear-blue ocean water, maximum transmission is achieved
at blue wavelengths (405–440 nm), with
the absorption coefficient approximately
0.017 m- 1. In coastal waters, the absorption coefficient is about twice as much—
approximately 0.033 m- 1, with the transmission maximum shifting to the green
(510–530 nm). In the most turbid, yel-low-matter-rich coastal water, the absorption is approximately 0.291 m- 1 with the
minimum absorption in the yellow.
Based on these values, the expected transmission range for optical wireless communications in the ocean is
150–200 m in very clear ocean waters,
50–75 m in ordinary ocean waters, and
only a few meters in turbid harbor water.
Unlike acoustics, the multipath dispersion