Communications Specialties Application Note

CALCULATING TRANSMISSION

DISTANCE FOR FIBER OPTIC

EQUIPMENT

MARKET: All

DOC. NUMBER: T-21

One of the many advantages of fiber optic trans-

mission is that it can transmit video, audio, and data

over much farther distances than traditional coax

cable and twisted pair wire. The exact distance that

can be supported by any given system is a function

of many factors, including type of cable being used,

frequency of the signal transmission, the bandwidth

of the fiber, and the number of splices and connec-

tors used across the entire transmission distance.

This TECHnique will help to clarify some of these

issues and provide some guidance in determining

what type of fiber optic equipment is necessary for

transmission over a given distance.

Fibers with wider core diameters are called

multi-

mode

fiber. These types of fibers allow for multiple

modes of light to pass along their core. The two most

common sizes of multimode fiber are 62.5/125 and

50/125. The first number represents the “core” di-

ameter size measured in microns and the second

number indicates the diameter of the fiber’s “clad-

ding” – the outer coating that surrounds the core and

keeps light within the fiber.

Frequency of Transmission:

When calculating the

maximum distance a given fiber optic transmission

system can support, it is also necessary to consider

the frequency at which the fiber optic signal will be

transmitted. The higher the frequency, the greater

distance the system will be able to support. For mul-

timode systems, commonly used frequencies are

850 and 1300 nanometers. For single-mode sys-

tems, 1300 and 1550 nanometers are standard.

Fiber Bandwidth:

Bandwidth of fiber is described

in MHz per kilometer. As the length of fiber increases,

the bandwidth decreases proportionally. For example,

a fiber that can support 500 MHz bandwidth at a dis-

tance of one kilometer will only be able to support

250 MHz at 2 kilometers and 100 MHz at 5 kilometers.

Due to the way in which light passes through them,

single-mode fiber has an inherently higher bandwidth

than multimode fiber. For an explanation of why this

is the case, refer to CSI’s Educational Guide “Intro-

duction to Fiber Optics.” Typical fiber bandwidth range

from hundreds of MHz per km for multimode fibers

to thousands of MHz per km for single-mode fibers.

Splices and Connectors:

The maximum distance

a system can support will also be a function of how

many splices and connectors are used across the

entire transmission distance. Each splice or connec-

tor that the optical signal must pass through causes

some signal loss to occur. The exact amount of loss

depends on the types of connectors or splices used,

as well as how well they are installed. Today, the most

common connector types in use are

and

FCPC

and the most common splices are

mechanical

splices

and

fusion splices.

For an explanation of

ypes of Fiber Optic Cable:

Most fiber optic trans-

mission systems are designed to work with one

of two types of fiber optic cable:

single-mode

multimode.

The glass core in the center of single-mode fiber

is thinner than in multimode fiber, typically 8 to 10

microns (millionths of a meter) in diameter. In fact,

it is so thin that only a single “mode” of light can be

transmitted along its core. As a rule, single-mode

fiber can transmit signals over a farther distance

than multimode fiber. For an explanation of why, re-

fer to CSI’s Educational Guide entitled “Introduction

to Fiber Optics.”

Fiberlink and TECHniques are trademarks of Communications Specialties, Inc.

Communications Specialties Application Note

Page 2:

CALCULATING TRANSMISSION DISTANCE FOR FIBER OPTIC

EQUIPMENT

each, refer to CSI’s Educational Guides “Introduc-

tion to Fiber Optics” and “Fiber Optic Cables and

Connectors.” As a general rule, it is safe to calcu-

late a loss budget of .5 to .75dB for each splice and

connector used. However, more exact specifications

can be obtained from the manufacturer of the fiber

and connectors.

Calculating a System’s Transmission Distance:

When designing a fiber optic system, all the above

parameters must be taken into consideration. To de-

termine the system’s total optical loss, calculate the

sum of the cable loss (as a function of desired trans-

mission distance), splice loss and connector loss.

Then, for safety measures, add an additional 3dB.

Compare this figure with the allowable optical loss

budget of the receiver to be used within the trans-

mission system. If the loss level is acceptable, and

if the fiber’s bandwidth is adequate to pass the type

of signal desired, then you can feel confident that

the system can transmit over the distances desired.

The following charts might be helpful in determin-

ing how much optical loss will occur when using

different fiber types, transmission frequencies and

fiber bandwidths over various transmission distances.

Remember – the ultimate goal when designing a

fiber optic system is to ensure that enough light

reaches the receiver. If this does not happen, the

entire system will not operate properly.

To assure maximum system performance, it is

necessary to use high quality methods of splicing

fiber optic cables and installing fiber optic connec-

tors. Pre-terminated cables and splicing kits are

available from Communications Specialties.

•

All Fiberlink transmission systems

•

Fiber optic cables, assemblies, termination and

splicing kits from Communications Specialties

•

Educational Guide:

An Introduction to

Fiber Optics

•

Educational Guide:

An Introduction to Fiber

Optic Cable and Connectors

Fiberlink and TECHniques are trademarks of Communications Specialties, Inc.