[c-nsp] Sorta OT: MM GBICs

[Will Hargrave]

Bill Wichers wrote:
> 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.

> 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.

This used to be the case until gigabit speeds came along. LEDs can't be 
modulated quickly enough at those speeds (there were probably a variety 
of other engineering problems which made them unsuitable). Fortunately 
VCSEL laser diodes came along and are pretty cheap due their ease of 
semiconductor-style fabrication.

eg http://www.finisar.com/optics/FTR-8519-3D.php is an SX 850nm GBIC 
designed for multimode use which uses a VCSEL. I think Cisco OEM Finisar 
optics so that'd be the roughly the same product as you'd buy from Cisco.

The longer range 1310nm/1550nm/etc transceivers use different laser types.