Generally, we use multimode fiber (MMF) patch cables for multimode equipment while single-mode fiber (SMF) patch cables for single-mode equipment, like MMF patch cord for 1000BASE-SR transceiver and SMF patch cord for 1000BASE-LR transceiver. But there are occasions when we cannot directly plug multimode patch cord into some 1G/10G electronics in 1/10 Gigabit Ethernet Applications. Typical cases are where 1000BASE-LX/LH, 10GBASE-LX4 and 10GBASE-LRM transceivers are used over legacy multimode fiber cables. In such situations, mode conditioning patch cord (MCP) is required to be used to avoid the loss brought by directly connecting these devices with multimode patch cables. This article will introduce what mode conditioning patch cable is, when to use it and some cautions in using it.
A mode conditioning patch cord is a special fiber patch cable that combines multimode fiber cable and single-mode fiber cable with calibration. It is a duplex multimode fiber patch cord constructed with a length of single-mode to multimode offset fiber connection part at the start of one of the two legs. Mode conditioning patch cables are 62.5/125µm or 50/125µm fiber cables assembled with fiber connectors on both ends, such as LC-LC, LC-SC, SC-ST, etc. Here’s a figure of a OM1 LC-LC mode conditioning patch cable.
Figure 1: mode conditioning patch cord LC-LC.
The calibrated offset fiber is the key part of the mode conditioning cable. When a highly concentrated single-mode fiber optic light is sent into the center of the multimode fiberglass, the transmitting laser pulse disseminates over time causing bit errors in the transmission signal. This effect is called differential mode delay (DMD). Differential mode delay results in reduced bandwidth over distance. By permitting the only mode laser launch to become offset from the center of the multi-mode fiber, the mode conditioning patch cord cuts down on the effect of such differential mode delay.
As mentioned at the beginning, mode conditioning patch cords are utilized when 1000BASE-LX/LH, 10GBASE-LX4 and 10GBASE-LRM transceivers are used over legacy multimode fiber cables. Because these transceivers have to operate for both single-mode and multimode fiber. They are equipped with single-mode laser source of 1300/1310 nm, which is different from the LED source that legacy multimode fiber cable can tolerate.
In Gigabit Ethernet networks, when we deploy 1000BASE-LX/LH switches, routers, or optical transceivers in existing multimode system, we need to use mode conditioning patch cable. In the 10 Gigabit Ethernet networks, even though there’s choice to shift to 850 nm laser-optimized 50/125µm OM3 or OM4 multimode fiber cables, some users still want to remain the legacy fiber distributed data interface (FDDI)-grade, OM1, or OM2 multimode fiber cabling instead of rebuilding the network with OM3/OM4. Then 10GBASE-LX4 and 10GBASE-LRM modules can be the right path to 10G multimode on the premise that mode conditioning patch cord is used.
Mode conditioning patch cord is not the same with common fiber patch cables so there are some cautions when using mode conditioning patch cable.
The first thing is the installation requirements when used with fiber optic transceivers. At one end of the patch cable, the single-mode fiber connector must be plugged into the transmit side of the transceiver, and connect its adjacent multimode fiber connector to the receive side of the transceiver. At the other end of the patch cable, the two multimode fiber connectors are connected to the cable plant. Repeat the steps for the other transceiver at the other side of the link. So we usually use mode conditioning patch cables in pairs.
Figure 2: install mode conditioning patch cord.
The second thing is that mode conditioning patch cord can only be used with 1300/1310nm 1000BASE-LX/LH, 10GBASE-LX4 and 10GBASE-LRM modules, it should never be used with 850nm optics, including 1000BASE-SX and 10GBASE-SR transceivers.
Besides, mode conditioning patch cord should not be used for applications over laser-optimized OM3/OM4 or any newer multimode fiber cables. If there’s need to use 1000BASE-LX/LH, 10GBASE-LX4 and 10GBASE-LRM modules but no willing to use mode conditioning cable, one alternative is to add attenuators on the transmitting side to protect the link and the devices; another way is to use single-mode fiber patch cable for short reaches.
In simple terms, mode conditioning patch cord is a duplex multimode fiber patch cable that includes a length of single-mode fiber. Mode conditioning cable is typically required in applications where 1000BASE-LX/LH, 10GBASE-LX4 and 10GBASE-LRM modules are used over OM1/OM2 multimode fiber cables. Due to its unique design, there are some things to be noticed when using these cables and they have been introduced in the article.
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