Over the past few years, fiber optic cable has become more affordable and widely used. Fiber is ideal for high data-rate systems such as FDDI, multimedia, ATM, or any other network that requires the transfer of large, time-consuming data files. And the basic cables are multimode optic cables and single mode cables.

A Comparison Of Multimode Cables And Single Mode Cables

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 Duplex cables and Singlemode Simplex 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).

Most building cables had 62.5/125 micron multimode fibers for LANs or security systems, while outside plant cables were all single-mode fiber. For some time, we have been encouraging people to install hybrid cables with both multimode fibers for today and single-mode fibers for the future regardless of the fiber optic cable price.

If you are transmitting from a smaller fiber core to a larger one, it is not a problem since the larger fiber like large core optical fiber will collect all the light from the smaller one with minimal loss. But if you transmit light from a larger fiber to a smaller one, the light in the larger core will overfill the smaller core and large losses will occur. How big are the losses we are talking about? Coupling a multimode fiber to a single-mode fiber will cause about 20 dB loss. Connecting a 62.5 fiber to a 50 micron core fiber will cause 2 to 4 dB loss, depending on the type of source (laser or LED). In any case, it can be enough loss to prevent network equipment from working properly.

Both 50 micron and 62.5 micron multimode fibers have the same cladding diameter and can use the same connectors and termination processes, but testing still requires using the correct matching fiber optics patch cords or the measured loss will be too low by a few tenths of a dB in one direction (50 to 62.5), or 2 to 4 dB too high the other way (62.5 to 50.)

Needless to say, these mismatched fiber losses affect the end user the same way they affect the installer, creating excess connection loss that can cause systems to malfunction or have high error rates, causing an expensive and annoying service call. Unfortunately, there is no optical mating adapter that will match two dissimilar fibers—although it has been tried many times. There is no solution other than preventing mismatched fiber terminations.