Page 342 - Optical Communications Essentials

Test and Measurement

332 Chapter Nineteen

where n fiber and n air are the refractive indices of the fiber core and air, respect-
ively. A perfect fiber end reflects about 4 percent of the power incident on it.
However, since fiber ends generally are not polished perfectly and perpendicu-
lar to the fiber axis, the reflected power tends to be much lower than the max-
imum possible value.
Two important performance parameters of an OTDR are dynamic range and
measurement range. Dynamic range is defined as the difference between the
initial backscattered power level at the front connector and the noise level after
3 min of measurement time. It is expressed in decibels of one-way fiber loss.
Dynamic range provides information on the maximum fiber loss that can be
measured and denotes the time required to measure a given fiber loss. Thus it
often is used to rank the capabilities of an OTDR. A basic limitation of an OTDR
is the tradeoff between dynamic range and resolution. For high spatial resol-
ution, the pulse width has to be as small as possible. However, this reduces the
signal-to-noise ratio and thus lowers the dynamic range. For example, a 100-ns
pulse width allows a 24-dB dynamic range, whereas a 20-µs pulse width
increases the dynamic range to 40dB.
Measurement range deals with the capability of identifying events in the link,
such as splice points, connection points, or fiber breaks. It is defined as the max-
imum allowable attenuation between an OTDR and an event that still enables
the OTDR to accurately measure the event. Normally, for definition purposes,
a 0.5-dB splice is selected as the event to be measured.

19.6.2. Fiber fault location
In addition to measuring attenuation and component losses, an OTDR can be
used to locate breaks and imperfections in an optical fiber. The fiber length L
(and hence the position of the break or fault) can be calculated from the time
difference between the pulses reflected from the front and far ends of the fiber.
If this time difference is t, then the length L is given by

ct
L (19.2)
2n 1
where n 1 is the core refractive index of the fiber. The factor 2 accounts for the
fact that light travels a length L from the source to the break point and then
another length L on the return trip.

19.7. Multifunction Optical Test Systems
For laboratory, manufacturing, and quality-control environments, there are
instruments with exchangeable modules for performing a variety of measure-
ments. Figure 19.8 shows an example from EXFO, which includes a basic modu-
lar mainframe and an expansion unit. The mainframe is a Pentium-based unit
that coordinates data compilation and analyses from a variety of test instru-
ments. This test system can control external instruments having RS-232

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