The IEEE 802.3ba committee ratified the 40 Gigabit Ethernet standard and along with the general specification, defined a number of fiber optic interfaces. These standard interfaces attempted to satisfy a number of different objectives including support for MMF and SMF compatibility.
40G LR4 QSFP+
40GBASE-LR4 transceivers are most commonly deployed between data-center or IXP sites with single mode fiber.
40G SR4 QSFP+
40GBASE-SR4 transceivers are used in data centers to interconnect two Ethernet switches with 8 fiber parallel multimode fiber OM3/OM4 cables.
40G LR4 PARALLEL SINGLE MODE (PSM)
40G PSM transceivers are used to provide support for up to four 10Gb Ethernet connections on a QSFP+ port over single mode fiber at distances up to 10km.
40G LM4 QSFP+
40G LM4 transceivers provide 40Gb connections over a single pair of duplex OM3/OM4 MMF or SMF.
40G ER4 QSFP+
40G ER4 transceivers can be used to provide long distance 40Gb connections up to 40km.
40G BIDIRECTIONAL MMF QSFP+
40Gb BiDi transceivers support 40Gb connections over a single pair of MMF.