Fiber Mode Conditioning cable

Unanswered Question
Mar 28th, 2007

Can someone explain what a mode conditioning cable is used for?

I have this problem too.
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When a laser source (LX/LH GBIC/SFP for example) is applied to MMF, there's a relatively rare condition called Differential Mode Delay (DMD), that can cause tx/rx errors on fiber links.

A mode conditioning patch cable actually has a short SMF run spliced into the MMF Tx cable on each end of the patch cable. This allows the laser source to be condtioned prior to it's entry to the MMF fiber.

We have had this happen and remedied it by simply using diode sourced SX GBic/SFP etc.

But we did confirm the presence of DMD by installing mode conitioning patch cables first and then monitoring for errors. The line stayed clean, and we opted to go to SX optics modules over existing patch cabling rather than add mode conditioning cables as errors started to be realized.

And for a side note, the only instances of DMD we've had were across 8 trunk links that used laser sourced LX modules across 50um MMF. Just a note, each link was less than 100m, some DMD docs reference longer cable runs, but our testing showed different.

simpsoro2 Wed, 03/28/2007 - 08:46

SO, long story short if you are using an LX gbic/sfp on MMF you need a mode conditioning cable?

But not if you are using an SX gbic/sfp on MMF?

Not necessarily. If you are using laser source on MMF and you start seeing input/output errors on the links, then you may test for the occurrence of DMD by installing a mode-conditioning cable.

In my campus area network we have 42, 1Gb, MMF IDF trunk links to our core switches links, only 8 showed errors that where traceable to DMD, so we changed them to SX diode sources and the problem was solved.

If you had to have a bottom line statement you could say, all MMF links will use SX/Diode sourced optic transceivers. That way it's never an issue, and mode conditioning cables are $130+ each depending on connectors and length, that can add up.


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