Single Mode vs. Multimode Optical fiber

What is Optical fiber?

The glass portion of a fiber optic cable – no jacketing or strength members included.?An optical fiber is made up of a light carrying core surrounded by cladding.?The cladding prevents light from escaping the core, effectively keeping the signal moving down the glass.

The difference of single mode and multimode fiber

Single mode fiber: a fiber featuring a small light-carrying core of about 9 micrometers (μm) in diameter.?For reference, a human hair is closer to 100 μm. The core is surrounded by a cladding that brings the overall diameter of the optical fiber to 125 μm.

Multimode fiber: a fiber with a core of 50 μm or above.?A larger core means multiple modes (or rays of light) can travel down the core simultaneously. Just like single mode, the core is surrounded by a cladding that brings the overall diameter of the optical fiber to 125 μm.

Common Misconceptions

It’s important to remember that (without the visual acuity of Superman) there is no way to distinguish between single mode and multimode optical fibers with the naked eye.?As noted above, standard optical fibers have cladding around the core that brings the diameter of the optical fiber itself to 125 μm.?When you put a connector on an optical fiber, you are primarily seeing the cladding and any integral protective coating, like SSF? polymer.

The terms “single mode” and “multimode” also have no relation to the number of optical fibers in the fiber optic cable you are running.?It’s possible to have a cable containing 144 single mode optical fibers, and it’s also possible to have a cable containing 144 multimode optical fibers.

What is Single Mode used for?

Single mode’s minuscule core limits dispersion, so higher bandwidth signals can be sent over a longer distance.?Sending data over the ocean floor??Single mode would be the cable for you. In general, single mode is the cable of choice for installations above about 300 m (1000 ft)– up to 100km until the signal needs to be re-amplified. This makes it ideal in WAN applications that require a signal to go further distances with higher payloads than other mediums can manage.

Conversely, in multimode fiber, light travels down the core, bouncing off the cladding as it goes.?As each beam of light has an individual path, each will reach the end of the optical fiber at different times.?This spread is modal dispersion, and it creates limits on data and distance.?For OM3 multimode, 10 Gbs can be sent a maximum of about 300 meters or 1000 feet before the signal becomes indistinguishable.

Single Mode vs Multimode Fiber Distance

It’s known that single mode fiber is suitable for long-distance applications, while multimode optical fiber is designed for short-distance runs. Then when it comes to single mode vs multimode fiber distance, what’s the quantifiable differences?

From the chart, we can see that 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. In addition, OS2 single mode fiber could support longer distances in 40G and 100G links, which is not listed in the table.

How to choose Single mode or multimode?

The answer to this comes down primarily to budget and applications.?Single mode cable requires single mode transceivers, and those tend to be far more expensive than multimode equivalents.?The difference in electronics can bring single mode system costs far above those of multimode, even if the per foot cost of single mode cable is low.?This is one of the primary reasons we’ll generally recommend multimode before single mode fiber in lower-distance applications.

However, there are still times when single mode may be recommended for short cable runs. It depends on the installation!

Don’t Mix and Match

Just as it’s important to note that you can’t mix OM1 and OM4, also note that single mode and multimode are not interchangeable.?Single mode electronics and connectors only work with single mode fiber, and multimode, likewise, only works with multimode.?This is due to the difference in core diameters between fiber types, as well as light wavelengths used for transmission.

Both single mode and multimode fibers provide excellent solutions for durable, high bandwidth installations.?Being aware of the differences between the two types of fiber will allow you to select the fiber most appropriate for your installation and data requirements.