Customized Optical
Components, Systems
and Microsystems
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High precision IR glass molding
enables high volume
serial production of aspheres
and diffractives using
chalcogenide glasses
0.0 0.1 0.2 0.3
1.0
0.5
0.0
Receiver offset for
1, 2, 3, 4 in. lenses (m)
Normalized received
power (a.u.)
a)
b) - 3 dB
1 deg., 8 mm
1 deg., 1 in.
1 deg., 2 in.
1 deg., 3 in.
1 deg., 4 in.
16 deg., 8 mm
16 deg., 1 in.
16 deg., 2 in.
16 deg., 3 in.
16 deg., 4 in.
180 deg., 8 mm
180 deg., 1 in.
180 deg., 2 in.
180 deg., 3 in.
180 deg., 4 in.
FOV
180°
16°
1°
from scattering is very small and is almost
negligible until operating at more than
1 Gbit/s data rates.
Underwater system design
Communication system designers are interested in the ranges at which a system
can perform, as well as the size of lenses
needed and the field of view and point-
ing accuracy required to complete the
link between transmitter and receiver.
Underwater, these relationships are not
always intuitive.
Unlike the strongly forward-scattered
light in very clear water, in turbid water
the short mean free path greatly expands
the beam, allowing systems with wide-
field-of-view optics to efficiently capture
the photons that diffuse toward the receiv-
er. These situations can be modeled using
Monte Carlo simulations and compared
with the experiment (see Fig. 2). In tur-
bid water, the pointing requirements of
the system can be decreased.
Similarly, multipath dispersion—deter-
mined by increased path length as the pho-
nons scatter between particles—can be
computed and compared under different
transmitter-receiver geometries as a func-
tion of water quality. It turns out that for
the relatively short ranges expected for
underwater optical communications at
megabit rates, multipath dispersion is almost negligible. 1
At NCSU, we have found that optical
wireless systems can be very compact and
that error correction coding and adapting
the transmission rate as a function of range
can improve signal-to-noise by 6–8 dB (see
Fig. 3). 2 While the increased signal-to-noise
translates into increased range, error correction increases the robustness of the link.
FIGURE 2. In a 3.66 m tank used
to characterize underwater optical
communications (a), water quality is
controlled by injection of particulate and dye
to simulate the absorption and scattering of
seawater; simulations and experiments (b)
show that for harbor quality water at 15 m, it
is advantageous to control the field of view of
the receiver so that instead of just collecting
un-scattered light, multiple-scattered photons
will also contribute to the signal.
