[c-nsp] Sorta OT: MM GBICs

[Bill Wichers]

Normally multimode optics use LEDs, and singlemode optics use lasers. The
reason is that you NEED a laser to launch a particular mode properly into
the singlemode fiber, and the lasers can typically both produce much more
power as well as efficiently couple that power into the smaller core of
the singlemode fiber.

Multimode is made for short-n-cheap links, and an LED is both cheaper and
easier to drive than a laser (lasers normally use some feedback with a PIN
diode to control their power output). In multimode fiber, you have a
larger core and aren't concerned with the exact transmission mode, so an
LED is fine.

What will make your bet confusing is that there are now "crossover" GBICs
out there that use a laser to couple into multimode fiber (usually to
either provide increased reach, better BER on fast links (where modal
dispersion can cause problems -- especially at 10+ Gb/s)). There are also
GBICs that will work on BOTH singlemode AND multimode fiber (HP makes some
for their Procurve equipment that I frequently use). These typically use a
1300nm laser, which is typical for singlemode, but on multimode the longer
wavelength (relative to the more common 850nm usually used on MM fiber)
provides for less modal dispersion and thus allows a longer reach on
gigabit links for a given MHz/km rating of MM fiber.

So basic rules of thumb: LED for MM, Laser for SM, Laser for MM for longer
reach on faster links when using longer wavelengths (1300nm / 1550nm
usually), or when using an combination SM/MM GBIC.

     -Bill

> All,
>
> Please forgive the semi-unrelated post, but I have $20 riding on this...
>>=) Do MM GBICs use a laser, or an LED for data transmission?
>
> Replies off-list are fine...
>
> Thanks,
> Brant
>
>
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