As the use of fiber in premise networks continues to grow, so do the requirements for testing and certifying it. An optical time-domain reflectometer (OTDR) is an electronic-optical instrument used to characterize optical fibers. It locates defects and faults, and determines the amount of signal loss at any point in an optical fiber. This article describes how an OTDR works and the key specifications that should be considered when choosing an OTDR.

1. WHITE PAPER
Fiberstore White Paper | Introduction to OTDR 1
As the use of fiber in premise networks continues to grow, so do the requirements for testing and
certifying it. An optical time-domain reflectometer (OTDR) is an electronic-optical instrument used
to characterize optical fibers. It locates defects and faults, and determines the amount of signal loss
at any point in an optical fiber. This article describes how an OTDR works and the key specifications
that should be considered when choosing an OTDR.
How Does an OTDR Work?
An OTDR uses the effects of Rayleigh scattering and Fresnel reflection to measure the characteristics
of an optical fiber. By sending a pulse of light into a fiber and measuring the travel time (“time
domain”) and strength of its reflections (“reflectometer”) from points inside the fiber, it produces a
characteristic trace, or profile, of the length vs. returned signal level on a display screen. The
working principle of OTDR is based on Rayleigh scattering, a small fraction of light spreading in all
direction, which is caused when a light pulse encounters the faults and heterogeneity in optical fiber.
(as shown in the following picture.)
OTDR Selection Guide
Introduction to OTDR

2. WHITE PAPER
Fiberstore White Paper | Introduction to OTDR 2
When choosing an OTDR, it is important to select the specific OTDR performance and features
according to the required specifications listed below.
 Dynamic Range
The dynamic range of an OTDR determines how long of a fiber can be measured. The total optical
loss that an OTDR can analyze is mainly determined by the dynamic range. The dynamic range
affects the accuracy of the link loss, attenuation and far-end connector losses. Thus, having
sufficient dynamic range is really important. The manufacturers specify dynamic range in different
way. The higher the dynamic range, the longer the distance an OTDR can analyze.
 Dead Zones
Dead zone refers to the space on a fiber trace following a Fresnel reflection in which the high return
level of the reflection covers up the lower level of backscatter. To specify an OTDR’s performance, it
is important to analyze the dead zone and ensure the whole link is measured. Dead zones are
characterized as an event dead zone and an attenuation dead zone. Event dead zone refers to the
minimum distance required for consecutive reflective events to be “resolved” (for example, to be
differentiated from each other). Attenuation dead zone refers to the minimum distance required,
after a reflective event, for the OTDR to measure a reflective or non-reflective event loss.
 Resolution
There are two resolution specifications: loss (level), and spatial (distance). Loss resolution is the
ability of the sensor to distinguish between levels of power it receives. When the laser pulse gets
farther out in the fiber, the corresponding backscatter signal gets weaker and the difference
between backscatter levels from two adjacent measurement points becomes larger. Spatial
resolution is how close the individual data points that make up a trace are spaced in time (and
corresponding distance). The OTDR controller samples the sensor at regular time intervals to get the
data points. If it takes readings from the sensor very frequently, then the data points will be spaced
close together and the OTDR can detect events in the fiber that are closely spaced.
 Pass/Fail Thresholds
This is an important feature because a great deal of time can be saved in the analysis of OTDR traces
if the user is able to set pass/fail thresholds for parameters of interest (such as splice loss or
connector reflection). These thresholds highlight parameters that have exceeded a warning or fail
limit set by the user and, when used in conjunction with reporting software, it can rapidly provide
re-work sheets for installation/commissioning engineers.
 Post-Processing and Reporting
Report generation could be another major time saver. For example, some OTDRs with specialized
post-processing software allow fast and easy report generation, which might reduce the
post-processing time up to 90 percent. These reports also include bidirectional analyses of OTDR
traces and summary reports for high-fiber-count cables.
 Your Applications and Users
Some OTDRs are designed to test long distance optical fibers and some others to test short distance

3. WHITE PAPER
Fiberstore White Paper | Introduction to OTDR 3
optical fibers. For example, if you are to test premises fiber networks where short distance optical
fibers are installed, OTDRs designed for testing long distance optical fibers are not suitable. Besides,
knowing your users and the time it will cost is also necessary. Because some types of OTDRs are
easy to use and some others are complicated to set up.
When selecting an OTDR, you’re supposed to take all the above factors into consideration.
Fiberstore supplies a wide range of OTDRs available with various fiber types and wavelengths
(including single-mode fiber, multi-mode fiber, 1310nm, 1550 nm, 1625 nm, etc). They also supply
OTDRs of famous brands, such as JDSU MTS series, EXFO FTB series, YOKOGAWA AQ series and so
on. OEM portable and handheld OTDRs are available as well.
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W1B 3HH, United Kingdom
Tel: +44 2081441980
Notice: This document is for informational purposes only and does not set forth any warranty, expressed or
implied, concerning any equipment, equipment features, or service offered or to be offered by Fiberstore.
Fiberstore reserves the right to make changes to this document at any time, without notice, and assumes no
responsibility for its use. This information document describes features that may not be currently available.
Contact a Fiberstore sales team for information on feature and product availability.
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