A "mode" in Fiber Optic cable refers to the path in which light travels. Multimode cables have a larger core diameter than that of singlemode cables. This larger core diameter allows multiple pathways and several wavelengths of light to be transmitted. Singlemode cables have a smaller core diameter and only allow a single wavelength and pathway for light to travel. Multimode fiber is commonly used in patch cable applications such as fiber to the desktop or patch panel to equipment. Multimode fiber is available in two sizes, 50 micron and 62.5 micron. Singlemode fiber is typically used in network connections over long lengths and is available in a core diameter of 9 microns (8.3 microns to be exact).
Single Mode Fiber - Single mode fibers have small cores sizes; any power run through the cladding can be easily lost where fiber bends.
Advantages
Single mode fibers have an operating length of greater than five miles. They also have higher bandwidths than Multimode because of their lower fiber dispersion. Single mode fiber doesn’t have modal dispersion, modal noise, and other effects that come with multimode transmission; single mode fiber can carry signals at much higher speeds than multimode fibers. They are standard choice for high data rates or long distance span (longer than a couple of kilometers) telecommunications which use laser diode based fiber optic transmission equipment.
Disadvantages
Light from single mode fibers only travel down the center of the core. They also use laser diodes, which tend to be more complex and expensive than light emitting diodes (LEDs).Since single mode fiber’s core is so much smaller than a multimode fiber’s core, coupling light into single mode fiber requires much tighter tolerances than coupling light into the larger cores of multimode fiber. However, those tighter tolerances have proved achievable.
Single mode fiber components and equipment are also more expensive than their multimode counterparts, so multimode fibers are widely used in systems where connections must be made inexpensively and transmission distances and speeds are modest.
Multimode Fiber - The mode count depends on the size of the core and the numerical aperture. Because of the large core size, aligning spliced fibers is easier.
Advantages
Multimode fibers can use LEDs, which are cheaper and more durable than laser diodes. Power is also distributed into the core and cladding.
Disadvantages
Multimode fibers have an operating range of less than five miles. As the core size and numerical aperture increases, they also tend to lose bandwidth.
Questions
How do I know what type of fiber I need?
This is based on transmission distance to be covered as well as the overall budget allowed. If the distance is less than a couple of miles, multimode fiber will work well and transmission system costs (transmitter and receiver) will be in the $500 to $800 range. If the distance to be covered is more than 3-5 miles, single mode fiber is the choice. Transmission systems designed for use with this fiber will typically cost more than $1000 (due to the increased cost of the laser diode).
What is the difference between multimode and single mode fiber?
Multimode fiber has a relatively large light carrying core, usually 62.5 microns or larger in diameter. It is usually used for short distance transmissions with LED based fiber optic equipment. Single-mode fiber has a small light carrying core of 8 to 10 microns in diameter. It is normally used for long distance transmissions with laser diode based fiber optic transmission equipment.
Should I install single-mode or multimode fiber?
This depends on the application. Multimode fiber will allow transmission distances of up to about 10 miles and will allow the use of relatively inexpensive fiber optic transmitters and receivers. There will be bandwidth limitations of a few hundred MHz per Km of length. Consequently, a 10 mile link will be limited to about 10 to 30 MHz. For CCTV this will be fine but for high speed data transmission it may not be.
Single-mode fiber on the other hand will be useful for distances well in excess of 10 miles but will require the use of single-mode transmitters (which normally use solid-state laser diodes). The higher cost of these optical emitters mean that single-mode equipment can be anywhere from 2 to 4 times as expensive as multimode equipment.
What is the maximum distance fiber optic transmitters can operate at?
Normal transmission distances can vary from a fraction of a mile to 40 miles (60 Kilometers) or more. The maximum transmission distance depends on output optical power of the transmitter, the optical wavelength utilized, the quality of the fiber optic cable and the sensitivity of the optical receiver. In general single-mode based systems operate over longer distances than multimode systems.
I already have single-mode fiber installed, but I am only going a short distance. Can I use lower cost multimode equipment?
No. Multimode equipment will not launch (inject) enough light into a single-mode fiber since the light carrying core of this fiber is only 9 microns in diameter compared to 62.5 microns in diameter for multimode fiber. Unfortunately you must use single-mode equipment. If the fiber distance is short however, the cost for replacing the single-mode fiber with multimode fiber may be more economical than the higher cost for the single-mode electronics.
