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Even in the age of high bandwidth and huge network demands, there are many companies that still have legacy network systems that communicate in lower bandwidth.
The infrastructure of these companies is based on standard 62.5u-multimode fiber. When the time comes to expand the network, the dilemma the network manager has involves whether to install the nearly obsolete 62.5u-fiber or 50u laser-optimized OM3 or OM4 type fiber.
In my opinion, mixing 62.5u and 50u fiber types is not recommended; however, if the network speeds are 10/100 Mbps, then it is possible to successfully use OM3/OM4 type glass with 62.5u transceivers. Comparing the two fiber types is like comparing two garden hoses. When the water flows from the small hose to the big one, most of the water will go through; however, when the water flows from the big hose to the small one, some of the water will spill. The same concept occurs with the two fiber types—signal flow from the 62.5u to the 50u will incur a loss of about 4.7dB. For 10/100 Mbps signals, which are LED-based, the optical receiver can still read the optical signal even with a loss of 4.7dB.
So, what is the conclusion? It is this—LED-based systems can tolerate the loss of the fiber mismatch, provided that the transceiver power is sufficient to endure the added loss in the channel. For bandwidth of 1 Gbps and above, a VCSEL (vertical-cavity surface-emitting laser) system is used. In this case you should pay close attention to the loss budget.
If the loss (it should be smaller than the LED-based system) is within the application budget, than it is possible to mix the two types of fibers together for VCSEL systems.