I have a fiber patch panel with the installed fibers 50/125 OM4. I have bought an OM2: FO patch cord, duplex, LC to LC MM OM2 50/125 µ, 1 m Length 1m
cable which works between 2 switches but not through the fiber in the wall. The diameters of the fiber are the same in OM2 and OM4. When I link the 2 switches together with the cable I bought they work fine, not with the patch panel.
Is OM4 for single mode fiber only?
It's generally not advisable to mix different fiber grades. While OM3 and OM4 mix pretty well, a mix of OM2 and OM3/OM4 can severly limit the reach of the fiber. If you're close to the maximum reach it might just cause too much attenuation. Better get the correct OM4 patch.
However, you might also want to check if the crossovers are all correct - each patch cable and each deployed fiber is supposed to have a crossover. You always need an odd number of crossovers for a run to work. Short-wave optics are usually visible to the eye (or an electronic camera), so its easy to locate the transmitter side.
OMx stands for multi-mode fiber (MMF). FDDI and OM1 used large diameter fiber (62.5 µm core) and are obsolete. OM2 is "LED optimized" (legacy) and doesn't mix too well with OM3 or OM4 ("laser optimized"). OM5 is pretty new and optimized for short-wave WDM (40+ Gbit/s).
Single-mode fiber (SMF) grades are OS1 (legacy) and OS2 (current). These mix well but OS1 has much higher attenuation, leading to shorter reach.
Generally, you can't mix SMF with MMF without high power loss penalties = severly reduced reach.
