You cant use multimode fiber with single mode modules and vice versa right? Just want to be sure.

Single-mode fiber works with multi-mode and single-mode modules, but multi-mode fiber only works with multi-mode modules, if I recall correctly.

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'Can' and 'should' are two separate questions. IMO it's bad form mixing fiber types, you're just adding another potential point of failure. If you can't help it, you can't help it, but if it can be avoided I would recommend it.

Singlemode has a 6 - 10 micron sized core. Since it has a laser not LED light source, it needs the smaller size for less scatter during transmission.

Multimode has a 50 - 62.5 micron sized core. This is because of the LED light source that has a larger light diameter but is of course limited.

Because of the larger core diameter on multimode, it isn't recommended to shoot singlemode light source more then 75 feet because of light scatter and reflection.

Same goes for Singlemode. Becuase of the smaller glass diameter you will have issues w/ the larger LED light source shooting through the smaller diameter glass.

Now 10 Gig multimode light sources are rated @ 50 micron and a laser source is used during light transmission, but this still isn't as optimized as a Single Mode light source as the maximum distance for 10gig over Multimode is rated at 300 meters w/ minimum 90 degree bends (as this increase light reflection and scatter). 10gig single mode can transmit over 60Km at the 6 to 10 micron sized core.

So in short, it isn't recommended to mix glass types w/ light sources.

Hope this helps!

Someone correct me if Im wrong, but arent some MM fibres also made from (I guess) super clear plastics, and not glass?

Perhaps its only short patch leads, but Im pretty sure I heard/read that somewhere.

Thanks guys. I knew it wouldnt be good practice to mix them. We dont have any multimode drops available so my boss just wants me to use the single mode but I thought it would just be a complete waste since its only going like 100 feet.

That is correct, Multimode is made from plastics, not glass. Some multi-count cables (12 count, etc) may be made from glass from the higher-end manufacturers (corning, etc). But most of the time yeah, they are plastic.

Thanks for this info rfn_jhardy, very useful.