Increasing Throughput for Optical Communications

 Modern Optical Systems

Since their inception into communications systems in 1970, fiberoptic cables have revolutionized communications and the transfer of information.  Not only do they provide for faster propagation of encoded information (traveling at fractions of the speed of light), optical systems are able to carry more information in a single optical fiber.  In electrical systems, using even the most sophisticated transmitting techniques, copper wires can attain transmission rates of approximately 200 kb/s.  By comparison, with the optical systems of twenty years ago, it was already possible to reach transmission rates of 500 Mb/s.  Thus, even twenty years ago, fiberoptic cables increased the throughput of information transfers by more than 25 times.

(Graphic-bar graph showing max. throughput of copper vs. max. throughput of optic relate the two both in terms of information/second and capacity, like number of phone conversations possible simultaneously)

Due to the ever-increasing need for faster transfers and an increase in communications traffic, there is a constant need to increase the capacity of optical fibers.  Several important developments have allowed current optical fibers to increase their throughput by 80 times compared to the 500 Mb/s fibers of the 1980s.

The Future of Optical Systems

As mentioned in the discussion on DWDM, though wavelength multiplexing is capable of significant expansion, it will ultimately be limited by the number of channels that it can practically handle.  Since each channel must have its own transmitted signal and receptor, it could become extremely impractical to manage if the number of channels grows too large.  One estimate predicts that DWDM will become impractical after approximately 100 channels which will yield a throughput of not much more than 1 Tb/s for each optical fiber.

An alternative to DWDM for increasing throughput is time-division multiplexing (TDM).  Whereas WDM sends all channels simultaneously through the fiber, TDM sends only one laser pulse at a time.  Each laser signal sent through the fiber has a unique pulse period.  Although DWDM sounds infinitely better at first glance, TDM has great potential for expanding its capacity.  Instead of being limited by the amount of space in the fiber and the separate components needed for each channel, TDM is limited only by the speed of the electronics which process the signal.  TDM channels are differentiated by individual periods of time for a particular pulse.  Thus, with optical components that can send, receive, and process extremely short pulses of light, TDM systems could easily surpass DWDM systems.

The following categories outline the numerous advantages of using a TDM system and how it can be applied for extremely high throughput systems with the use of ultra-fast light pulses.