Singlemode and Multimode fibre cabling 1

[ADB100]

I had a wierd experience recently with a customers Fibre cabling which I couldn't understand why it worked.
We had what I thought was a singlemode fibre infrastructure, connected to this were some LAN switches and 1000BaseLX/LH GBIC's using singlemode fibre patch cables. When connected the links came up and we could pass traffic, becasue this was early on in the project we didn't do any soak tests or throughput tests etc. A few days in it was discovered that the infrastructure cabling was all multimode. There was then a mad rush to re-install the infrastructure with singlemode.
What I couldn't understand was why the links came up? I am sure I have done this previously and links just wouldn't come up. I assume there is a physical reason why this worked (although it may have been generating errors).

Andy

 

[phoneguy55]

Isnt multimode fiber orange and single mode yellow...

 

[ADB100]

The bore diameter of the glass is what makes the fibre singlemode or multimode. Multimode is usually 50 or 62.5 microns in diameter, whilst singlemode is 9 microns. This means that with singlemode only one principle mode can travel down the fibre which minimizes dispersion of light pulses, increasing the distance the signal can be sent. Multimode allows more than one mode of light to travel down it and it suffers more dispersion than singlemode reducing the distance that the light will travel.

What I can't understand is why it worked?

If this is a cross section of single mode:
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Then multimode would be something like this:
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If you line up one fibre to the other I can't understand why it would work - i.e.
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NOt sure if my illustrations will work.... But basically a tiny bore butted up against a large one 9-microns next to 50-microns.

Andy

 

[ve3slw]

Andy,
You almost had it correct in your sketch. The key is that the 2 cores line up exactly due to the outside diameter of both types of fiber being 125 micron.
Your source entered the multimode and essentially spread out in the larger core. The working link would be due to a few factors including power of your signal and the attenuation of the installed multimode link.

 

[oldtimerbob]

What I don't understand is why they replaced the multi mode.

Single mode is normally used for the longer distances. Since he doesn't mention that aspect I can only state:

Would it not have been better to replace the single with the multi?

 

[ADB100]

In reality they didn't need to replace the infrastructure Multimode with Singlemode, just the patch cables. However they did.....

ve3slw - I understand how it would work with the laser going from 9-microns to 50-microns, but what about the other way around? Wouldn't the light be in the wrong place?

Andy