CommScope® Enterprise Data Center Design Guide - Public ...

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Historically, LEDs have been the preferred choice for short distance/multimode fiber systems
and have operated at data rates of 10 and 100megabits per second for the commonly used
Ethernet protocols. Fibre Channel, ATM and ESCON can also operate with LEDs over
multimode fiber at low data rates. LEDs have a significant limitation, however, in that their
maximum data rate output is limited to 622 Mb/s, requiring the use of more expensive
electronics to run data rates of 1 Gb/s or higher. Although single-mode quality lasers could be
utilized, the additional cost made research into a new option desirable.
Lasers
A Laser (Light Amplification by the Stimulated Emission of Radiation) generates light by a
process called stimulated emission, where an outside source is required to active the process.
With semiconductor lasers, an electric current is passed through the laser material to excite the
atoms into a higher energy state. As the atoms drop back into the lower state, they release their
energy as photons, or light.
The laser is a diode, similar to the description of LEDs above, with “p” and “n” regions, but the laser
requires stimulated emission, not spontaneous. Light energy must be extracted from the recombining
electrons before they can spontaneously emit light. This requires a laser resonator, higher drive
currents than those used in LEDs and confinement of both the excitation and the generated light.
Fabry-Perot (FP), also know as edge-emitting, lasers are relatively simple and low cost to make.
Hence they are commonly used for short range applications. A FP laser cavity is designed as
a set of parallel mirrors on opposite ends of the semiconductor chip that the light can resonate
(bounce) between to stimulate light emission from excited atoms. One edge has a coating that
will reflect most of the light back into the semiconductor. The other edge is left without the coating,
to allow only one place for the light to exit as the laser beam; hence the name edge-emitting.
There are other lasers on the market, typically used for long-reach applications, well beyond
distances seen within the data center.
Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does
not work, whether due to bad contacts or poor material growth quality, the production time and
the processing materials have been wasted. Although the manufacturing costs of lasers are low,
the waste rate can cause unpredictable manufacturing yield.
Vertical Cavity Surface Emitting Lasers
Developed in the late 1980s, Vertical Cavity Surface Emitting Lasers (VCSELS) have several
advantages during production when compared with the production process of edge-emitting lasers.
Operating at the 850 nanometer (nm) wavelength, VCSELs emit energy in spikes that tend to inject
light into a small subset of available modes within a fiber, and these spikes can be somewhat
unpredictable and variable over time. The output profile can vary greatly between manufacturers,
and from VCSEL to VCSEL within the same manufacturing lot. This has created the need for new
testing procedures to evaluate the bandwidth of optical fibers when using a VCSEL as the source.
Lasers Reveal DMD Problems
LED
10Gb/s
DMD only slightly degrades performance
Power in high DMD modes relatively low
Pulse detectable as one
DMD causes bit errors
Power concentrated in 2 modes w/ high delay
Causes split pulse
10 Gb/s
Bit Period