SMF connections are also used in the latest FICON connections for mainframes. Older mainframe connections used larger cable cores in the 62.5 micron range. The chart below may help you with the differences.
Each connection type requires a specific GBIC (Gigabit interface converter), which is rated as a long-wave or short-wave laser. The Gigabit Interface Converter (GBIC) is where the cable plugs into the HBA card. Every HBA card has a GBIC that snaps into an opening in the card or is soldered into the card. The openings in the GBIC extend out of the back of the server, so you can plug in the cable. The GBIC houses the laser and electronics that convert the data inside your server into the light pulses that travel over the cables. GBICs are not used only in the HBA; they are used in every device in the SAN. Anywhere an optical cable needs to be plugged in, you will find a GBIC.
As for your question of whether one segment can be MMF and another SMF, the answer is yes, as long as the correct GBIC is used at the connection ends. Normally, this would be between two different switch ports. SMF connections are more expensive though, and are required only when the distance between devices are longer than supported by MMF connections. Therefore, there is no real advantage to using SMF unless you are connecting a mainframe via FICON, or need more distance between your devices and the switches. SMF connections are normally used between switches that connect data centers together into a single large fabric via a DWDM (Dense Wave Division Multiplexed) connection. These connections are sometimes referred to as "dark fiber", since it's harder to "see" the light if looking at the end of the cable. By the way, NEVER look into a fiber cable, since it WILL blind you.
This was first published in October 2004