Fibre Optic Cable Types

Although single mode fibre (SMF) and multimode fibre (MMF) optic cable types are widely used in diverse applications, the differences between single mode fiber and multimode fiber optic cables are still confusing. 

Single Mode Vs Multimode Fibre Optic Cable

Single mode means the fiber enables one type of light mode to be propagated at a time. While multimode means the fiber can propagate multiple modes. The differences between single mode and multimode fiber optic cable mainly lie in fiber core diameter, wavelength & light source, bandwidth, color sheath, distance and cost.

Single mode fiber core diameter is much smaller than multimode fiber. Its typical core diameter is 9 µm even if there are others available. And multimode fiber core diameter is 50 µm and 62.5 µm typically, which enables it to have higher "light gathering" ability and simplify connections. The cladding diameter of single mode and multimode fiber is 125 µm.

Wavelength & Light Source

Due to the large core size of multimode fiber, some low-cost light sources like LEDs (light-emitting diodes) and VCSELs (vertical cavity surface-emitting lasers) that works at the 850nm and 1300nm wavelength are used in multimode fiber cables. While the single mode fiber often uses a laser or laser diodes to produce light injected into the cable. And the commonly used single mode fiber wavelength is 1310 nm and 1550 nm.

Color Sheath

According to the TIA-598C standard definition, for non-military applications, single mode cable is coated with yellow outer sheath, and multimode fiber is coated with orange or aqua jacket.

Fibre Transmission Distance

It’s known that single mode fiber is suitable for long-distance applications, while multimode optical fiber is designed for short-distance runs. Single mode fiber distance is much longer than that of multimode fiber cables at the data rate from 1G to 10G, but OM3/OM4/OM5 multimode fiber supports a higher data rate. Because multimode optical fiber has a large core size and supports more than one light mode, its fiber distance is limited by modal dispersion which is a common phenomenon in multimode step-index fiber. While single mode fiber is not. That’s the essential difference between them.

Notes

·         Single mode fiber and multimode fiber cable have their own advantages on cost and applications. There is no such thing that single mode optical fibers are better than multimode ones. Just choosing the best-fit one for your applications is ok.

·         Multimode fiber and single mode fiber have different core sizes, and the number of light modes that they transmit is also different. If you mix the two fibers, or connect them together directly, you’ll lose a large amount of optical loss, resulting in a link flapping or being down. Keep in mind that never mix different types of cabling randomly.

·         Large optical loss will occur if a multimode transceiver is connected with single mode fiber. However, the opposite will work. For example, 1000BASE-LX single mode SFP can work on multimode fiber cable by using mode conditioning fiber cable. Sometimes, fiber media converters also can be used to solve such problems between single mode transceivers and multimode transceivers.

·         When making a decision between single mode and multimode fiber cables, the first factor to consider is the fiber distance which you need actually. For example, in a data center, multimode fiber cables are enough for the distance of 300-400 meters. While in applications that require distance up to several thousands of meters, the single mode fiber is the best choice. And in applications that can use single mode and multimode fiber, other factors like cost and future upgrade requirements should be taken into consideration for your choice.