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Industry Insights

Connecting The Dots With Rudy: Boosting Fiber Optic Links

Thursday, April 28, 2016 | by Rudy Montgelas

Boosting Fiber Optic Links


In the last CTD on transceivers, I mentioned that there is no doubt that the "need for speed", or faster data transmission rates, is an ever increasing requirement for tomorrow's data centers, local area networks and digital communication systems.  This CTD presents some of the ways the industry is going to increase the fiber data rates. 


Fiber Optic communication links are made up of optical and electrical components that all have to work together to provide the required overall system performance.  In other words, the whole link is only as good as its weakest link.  Using the right fiber with the right transceiver is critical to achieving the desired distance and bandwidth. 



Did you know that there is more than one approach to speed up a fiber link?

Modulation Approach: 


If you increase the modulation rate of the transceiver by turning the laser or LED light source on and off at a faster rate, you can "squeeze" more bits of information into the same time frame.  However, doing so comes at the expense of complicating transceiver electronics, requiring increased costs and higher energies. 

Encoding Scheme Approach: 

Another approach is to change the encoding scheme to allow the transmission to hold more information.  Typically, the digital data (ones and zeros) are represented by two light levels (either on or off) in a transmitted symbol. This is called non-return-to-zero on-off keying (NRZ OOK).  However, by providing additional light levels in the symbol, you can send more data in the same amount of time.  This is called PAM4 (pulse amplitude modulation).  PAM4 is an emerging method used to double the data rate of transceivers from 10 gigabits per second (Gbps) to 20 Gbps and beyond. 



Parallel Optics Approach:


By simply adding more fibers or "lanes", it is possible to multiply the effective data rate by the number of duplex fibers.  This is called "parallel optics" or Space Division Multiplexing (SDM).  For instance, a common way to migrate from two-fiber 10 Gbps systems to eight-fiber 40 Gbps is to use MTP®/MPO 12-fiber cable assemblies and QSFP series transceivers.  (The four middle fibers in the connector are not used).  Just so you know, the industry is also looking at 8-fiber MTP/MPO cable assemblies and components (Base-8) to save the requirement for the four additional unused fibers in the cables.




WDM Approach: 

Increasing the number of wavelengths on each individual fiber is an effective method to boost system transmission capacity.  Wavelength Division Multiplexing (WDM) has been used for many years for this purpose where different "colors" of light are transmitted over a single mode fiber for telephony and outside plant applications.  However, WDM had required expensive lasers and components, until recently, with the use of short wavelength VCSEL lasers and multi-mode optical fibers.


Cisco has developed a 40 Gbps bi-directional (BiDi) short wavelength system (40G-SR-BD) that transmits two different wavelengths in opposite directions over a pair of multi-mode OM3 or OM4 fibers.  The advantage is that the legacy duplex fiber cable plant can be used with the BiDi transceivers without the need to install additional fibers into the plant.


Wide band multi-mode fibers (WBMMF) have recently been developed to extend the reach of Short Wavelength Division Multiplexing (SWDM) multi-mode fiber systems and increase the transmission capacities from 10 Gbps to 40 Gbps and 100 Gbps.  These fibers are optimized for use with the VSCEL lasers operating in a range from 850nm to 950nm.  A SWDM industry alliance has been formed to promote a new TIA-492AAAE detail specification for WBMMF, and the TIA-568-3.D specification will include cable with WBMMF as a recommended media.
 

Where is all this going?
By combining these approaches is it possible to further speed up the fiber link. 

Moving forward, new developments in fiber optic transceivers, and new types of multi-mode optical fibers, will take advantage of all the methods discussed here to boost the transmission capacity of fiber optic links.  These developments will provide solutions that will continue to promote the use of multi-mode fiber as a cost effective media for data centers.