Bandwidth increase has always been an important area of research in communications. A novel multiplexing
technique known as Spatial Domain Multiplexing (SDM) has been developed at the Optronics Laboratory of
Florida Institute of Technology to increase the bandwidth to T-bits/s range. In this technique, space inside
the fiber is used effectively to transmit up to four channels of same wavelength at the same time.
Experimental and theoretical analysis shows that these channels follow independent helical paths inside the
fiber without interfering with each other. Multiple pigtail laser sources of exactly the same wavelength are
used to launch light into a single carrier fiber in a fashion that resulting channels follow independent helical
trajectories. These helically propagating light beams form optical vortices inside the fiber and carry their
own Orbital Angular Momentum (OAM). The outputs of these beams appear as concentric donut shaped
rings when projected on a screen. This endeavor presents the experimental outputs and simulated results for
a four channel spatially multiplexed system effectively increasing the system bandwidth by a factor of four.
A novel multiplexing technique known as Spatial Domain multiplexing (SDM) has been developed in recent
years and offers many advantages over its counterparts. With multiple channel transmission of the same
wavelength over a single multimode carrier fiber, SDM increases the data capacity by multiple folds. Input
channels are launched at appropriate input angles to produce skew ray propagation. The output of the system
when projected on a screen is observed as concentric rings. These SDM beams carry orbital angular momentum.
Experiments show that two input sources with the same launch conditions, but opposite topological charge take
different helical paths inside the transmission fiber. Consequently the shadow of a straight wire does not remain
straight. Instead, it is displaced by a specific distance. This endeavor presents a model of such a system by
analyzing the shadow distortion, using principles of geometric optics. Experimentally obtained shadow
displacement results are quantified and then compared to the model. We also show that when two channels with
opposite topological charges are transmitted with same launch conditions, their orbital angular momenta are
equal and opposite. As a result orbital Angular momentum based multiplexing can be used to add another
degree of freedom to photons.
An optical encryption technique based on polarization property of light is proposed. Many techniques using polarization
beam splitters to encrypt the signal have been proposed earlier. They are based on splitting and interference of two light
signals, namely message and noise. Only by placing suitable mirrors and a second beam splitter in a suitable position,
the two signals are reconstructed at the output. In this paper we report the fiber optic version of a polarization based
encryption technique that also has the potential to double the data carrying capacity of the fiber. Using polarization
dependent couplers in the fiber optic cables, we are able to achieve encryption of the light signal. Theoretical analysis
and simulated results are also presented.
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