[c-nsp] SX optics on single-mode fiber?

[Bill Wichers]

> Provider just pulled large bundle of fiber for some gigabit cross
> connects (<200 feet). However it turns out they determined I wanted
> single-mode fiber, though since distance is so short I intend on just
> using SX GBIC's.
>
> So, will this work at all? Are  there attentuator/conditioning cables
> that should be used? Any references I've found are for running
> higher-quality (ie. LH/LX) optics on multimode, not the other way
> around...

Yes, it can sometimes work, but is not recommended. You *might* be able to
just plug right in and have it work on such a short run, but maybe not. SX
optics are designed to couple (usually LED) light into a multimode fiber
core of 50 or 62.5 um, while LH/LX optics are designed to couple laser
light into singlemode core of about 9 um. The result is that using SX
optics with singlemode (~ 9 um core) fiber results in very inefficient
coupling into the fiber which has the effect of more loss and a greatly
reduced useable distance. I have successfully used LH optics on multimode
fiber before (but only for quick'n'dirty tests with a patch cord), but not
the other way around.

There are such things as 'mode conditioning' patch cords that are intended
for this sort of thing, and they *might* be useful to you over this
relatively short distance. Not sure on cost since I've never used them.

My recommendation would be to just pull some multimode fiber along the
singlemode you have and use that for your SX optics. Materials-wise it
will be cheaper than the added cost of the LH optics, although the labor
to install it might make it cheaper in the long run to just purchase LH
GBICs. You *might* need optical attenuators on such a short run of fiber
if you use LH optics though.

I would stay away from trying to run SX optics on the singlemode fiber
unless you are either short on time and it works temporarily for you, or
if the link is not at all important in your network.

     -Bill

*****************************
Waveform Technology
UNIX Systems Administrator