The broadening of the pulse reduces the bandwidth of the system and can cause detection errors at the receiver. The difference in arrival time between modes within a pulse is known as differential mode delay (DMD). In DMD testing, high-powered laser pulses are transmitted in small steps across the entire core of the fiber. Only a few modes are excited at each step, and their arrival times are recorded. The DMD of the fiber is the difference between the earliest and the latest arrival times of all modes at all steps. The lower the differential mode delay, the higher the bandwidth of the fiber.
Help is available
There still is no medium available today that affords the flexibility fiber optic transmission offers. Fiber is one of the only true mediums that allow you to easily change the type of modem on either end and accommodate a completely different signal type. Whether you're taking advantage of an existing in-use fiber or an unused dark fiber, companies such as ComNet offer audio, video and data fiber optic transmission equipment as well as fiber optic Ethernet transmission products that allow fiber to be used to it greatest potential. The tests outlined above can give you a competitive advantage in using all the resources available to you. Finding installed optical fiber in place in your next installation can be like discovering buried treasure.
John Nave is an application engineer for ComNet and Frank "Skip" Haight is the vice president of Marketing. ComNet is based in Danbury, Conn. Visit www.comnet.net.