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Fiber: Single vs. Multi Mode
Single Mode  vs.  Multi-Mode:   What’s the Difference?
 
Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the cable through to the other end.  The light source can either be a light-emitting diode (LED) or a laser.  The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable converts the pulses back into the digital ones and zeros of the original signal.

Even laser light shining through a fiber optic cable is subject to loss of strength, primarily through dispersion and scattering of the light, within the cable itself.  The faster the laser fluctuates, the greater the risk of dispersion. Light strengtheners, called repeaters, may be necessary to refresh the signal in certain applications.

While fiber optic cable itself has become cheaper over time - an equivalent length of copper cable cost less per foot but not in capacity.  Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable.Fiber optic cable connectors and the equipment needed to install them are still more expensive than their copper counterparts.

There are three types of fiber optic cable commonly used:  single mode, multimode and plastic optical fiber (POF).  Although fibers can be made out of transparent plastic, glass, or a combination of the two, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation.  Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links.  Because of the tighter tolerances required to couple light into and between single-mode fibers (core diameter about 10 micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.

Single Mode Fiber
A typical single-mode optical fiber, showing diameters of the component layers.
A typical single-mode optical fiber, showing diameters of the component layers.

Single Mode cable is a single strand (most applications use 2 fibers) of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission.  Single Modem fiber is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed - (single-mode on one single fiber).

Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more.  Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.   Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.

Multimode Fiber
The propagation of light through a multi-mode optical fiber.
The propagation of light through a multi-mode optical fiber.

Multi-Mode cable has a little bit bigger diameter, with common diameters in the 50-to-100 micron range for the light carry component (in the US the most common size is 62.5um). Most applications in which Multi-mode fiber is used, 2 fibers are used (WDM is not normally used on multi-mode fiber).  POF is a newer plastic-based cable which promises performance similar to glass cable on very short runs, but at a lower cost.

Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit to 275m to 2km) over medium distances.  Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or 1300nm.  Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers.  However, in long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fiber in new applications using Gigabit and beyond.
 
Which do I need?
 
Today, multimode fiber optic systems are lagging behind singlemode systems in terms of growth.  In addition to supporting high data throughput, singlemode systems are attractive because they are easy to upgrade and help to "future proof" installations.  Nonetheless, multimode fiber still gets plenty of coverage in any fiber optic cable guide.  Why?  Because multimode is still the fiber of choice for many applications.

For example, multimode fiber optic cable is well suited for systems that have short fiber optic links, such as Local Area Networks (LANs) and Storage Area Networks (SANs).

Multimode Advantages

Multimode fiber optic cable and components are less expensive and easier to work with than their singlemode counterparts.  This is due largely to the fact that the multimode fiber core is larger, and alignment tolerances are much less critical than they are for singlemode fiber.

Like singlemode, multimode fiber provides high bandwidth at high speeds, but transmission is limited to shorter distances than singlemode.  (In longer cable runs, the multiple paths of light in a multimode fiber tend to create signal distortion).

Standard multimode cable is made of glass fibers, usually 50-to-100 micron in diameter (most common is 62.5).  Multimode cable is also available as low-cost Plastic Optical Fiber (POF), which offers performance similar to glass cable for very short runs.

Singlemode Advantages

Generally, singlemode cable provides less signal attenuation, higher transmissions speeds, and up to 50 times greater transmission distance than multimode cable.  Singlemode cable can transmit data at terabits per second over 100km without requiring re-amplification of the signal.

Singlemode fiber typically has a diameter of only 8.3 to 10 microns, which is much narrower than multimode fiber which is usually 50 to 100 microns in diameter.  The small core of a singlemode fiber allows for the propagation of only one light wave, so there is no possibility of distortion due to overlapping light pulses.  Also, singlemode is more stable than multimode for systems that have branching devices, such as couplers.

Which to Choose?

When deciding whether to use multimode or singlemode fiber, a lot depends on a system's current and future bandwidth requirements.  As a general guide, think of multimode bit rate as being limited to 100Mbps over distances up to 40km, with shorter links allowing for bit rates up to 10Gbps.

If your system is comprised of relatively short fiber links and bandwidth requirements are not expected to exceed multimode capacity over the system's lifetime, then multimode may be the logical choice.  It is less expensive to purchase, install and maintain.



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