An optical time-domain reflectometer is an optoelectronic instrument used to characterize
an optical fiber. An OTDR is the optical equivalent of an electronic time domain reflectometer.
It injects a series of optical pulses into the fiber under test. It also extracts, from
the same end of the fiber, light that is scattered or reflected back from points along the fiber.
The strength of the return pulses is measured and integrated as a function of time, and
plotted as a function of fiber length.
Description OTDR is short for Optical Time-Domain Reflectometer
An OTDR may be used for estimating the fiber length and overall attenuation, including
splice and mated-connector losses. It may also be used to locate faults, such as breaks,
and to measure optical return loss. To measure the attenuation of multiple fibers, it is
advisable to test from each end and then average the results, however this considerable extra
work is contrary to the common claim that testing can be performed from only one end
of the fiber. In addition to required specialized optics
and electronics, OTDRs have significant computing ability and a graphical display, so they may
provide significant test automation. However, proper instrument operation and interpretation
of an OTDR trace requires special technical training and experience.
OTDRs are commonly used to characterize the loss and length of fibers as they go from
initial manufacture, through to cabling, warehousing while wound on a drum, installation and then
splicing. The last application of installation testing is more challenging, since this may
involve extremely long distances, or multiple splices spaced at short distances, or fibers
with different optical characteristics joined together. OTDR test results are often stored
in case of later fiber failure or warranty claims. Fiber failures can be very expensive,
both in terms of repair cost, and service loss.
OTDRs are also commonly used for fault location on installed systems. In this case, reference
to the OTDR trace made at the time of installation is useful to determine where changes have
occurred. Use of an OTDR for fault finding may require an experienced operator able to
correctly judge the appropriate instrument settings to locate a problem accurately. This
is particularly so in cases involving long distance, closely spaced splices or connectors,
or PONs. OTDRs are available with a variety of fiber
types and wavelengths, to match common applications. In general, OTDR testing at longer wavelengths,
such as 1550 nm or 1625 nm, can be used to identify fiber attenuation caused by fiber
problems, as opposed to the more common splice or connector losses.
The dynamic range of an OTDR is limited by a combination of optical pulse output power,
optical pulse width, input sensitivity, and signal integration time. Higher optical pulse
output power, and better input sensitivity, combine directly to improve measuring range,
and are usually fixed features of a particular instrument. However optical pulse width and
signal integration time are user adjustable, and require trade-offs which make them application
specific. A longer laser pulse improves dynamic range
and attenuation measurement resolution at the expense of distance resolution. For example,
using a long pulse length, it may be possible to measure attenuation over a distance of
more than 100 km, however, in this case, an optical event may appear to be over 1 km
long. This scenario is useful for overall characterisation of a link, but would be of
much less use when trying to locate faults. A short pulse length will improve distance
resolution of optical events, but will also reduce measuring range and attenuation measurement
resolution. The "apparent measurement length" of an optical event is referred to as the
"dead zone". The theoretical interaction of pulse width and dead zone can be summarised
as follows:
The OTDR "dead zone" is a topic of much interest to users. Dead zone is classified in two ways.
Firstly, an "Event Dead Zone" is related to a reflective discrete optical event. In this
situation, the measured dead zone will depend on a combination of the pulse length, and
the size of the reflection. Secondly, an "Attenuation Dead Zone" is related to a non-reflective
event. In this situation, the measured dead zone will depend on a combination of the pulse
length. A long signal integration time effectively
increases OTDR sensitivity by averaging the receiver output. The sensitivity increases
with the square root of the integration time. For example, if the integration time is increased
by 16 times, the sensitivity increases by a factor of 4. This imposes a practical sensitivity
limit, with integration times of seconds to a few minutes.
The dynamic range of an OTDR is usually specified as the attenuation level where the measured
signal becomes lost in the detection noise, for a particular combination of pulse length
and signal integration time. This number is easy to deduce by inspection of the output
trace, and is useful for comparison, but not in practice, since at this point, the measured
values are random. So the practical measuring range is smaller, depending on required attenuation
measurement resolution. When an OTDR is used to measure the attenuation
of multiple joined fiber lengths, the output trace can incorrectly show a joint as having
gain, instead of loss. The reason for this is that adjacent fibers may have different
backscatter coefficients, so the second fiber, with the same level of light travelling through
it, reflects more light than the first fiber. If the OTDR is placed at the other end of
this same fiber pair, it will measure an abnormally high loss at that joint. However if the two
signals are then combined, the correct loss will be obtained. For this reason, it is common
OTDR practice to measure and combine the loss from both ends of a link, so that cable joint,
and end to end loss, can be measured accurately. The theoretical distance measuring accuracy
of an OTDR is extremely good, since it is based on software and a crystal clock with
an inherent accuracy of better than 0.01%. This aspect does not need subsequent calibration
since practical cable length measuring accuracy is typically limited to about 1% due to: cable
length is not the same as the fiber length, the speed of light in the fiber is known with
limited accuracy, and cable length markers have limited accuracy.
An OTDR excels at identifying the existence of unacceptable point loss or return loss.
Its ability to accurately measure absolute end-to-end loss or return loss can be quite
poor, so cable acceptance usually includes an end-to-end test with a light source and
power meter, and optical return loss meter. Its ability to exactly locate a hidden cable
fault is also limited, so for fault-locating, it may be augmented with other localised tools
such as a red laser fault locator, clip-on identifier, or "Cold Clamp" optical cable
marker. Reliability and quality of OTDR equipment
The reliability and quality of an OTDR should be based on its accuracy, measurement range,
ability to resolve and measure closely spaced events, measurement speed, and ability to
perform satisfactorily under various environmental extremes and after various types of physical
abuse. In addition to cost, the instrument should be rated on features provided, size,
weight, and ease of use. Accuracy is defined as the correctness of
the measurement. The measurement range of the OTDR is defined
as the maximum attenuation that can be placed between the instrument and the event being
measured, for which the instrument will still be able to measure the event within acceptable
accuracy limits. Instrument resolution is a measure of how
close two events can be spaced and still be recognized as two separate events. The duration
of the measurement pulse and the data sampling interval create a resolution limitation for
OTDRs. The shorter the pulse duration and the shorter the data sampling interval, the
better the instrument resolution, but the shorter the measurement range. Resolution
is also often limited when powerful reflections return to the OTDR and temporarily overload
the detector. When this occurs, some time is required before the instrument can resolve
a second fiber event. Some OTDR manufacturers use a “masking” procedure to improve resolution.
The procedure shields or “masks” the detector from high-power fiber reflections, preventing
detector overload and eliminating the need for detector recovery.
Industry requirements for the reliability and quality of OTDRs are in GR-196, Generic
Requirements for Optical Time Domain Reflectometer Type Equipment.
Types of OTDR-like test equipment The common types of OTDR-like test equipment
are: Full-feature OTDR
Hand-held OTDR Fiber Break Locator
RTU in RFTSs The equipment is summarized below, and detailed
in GR-196, Generic Requirements for Optical Time Domain Reflectometer Type Equipment.
Full-feature OTDR Full-feature OTDRs are traditional, optical
time domain reflectometers. They are feature-rich and usually larger, heavier, and less portable
than either the hand-held OTDR or the fiber break locator. Despite being characterized
as large, their size and weight is only a fraction of that of early generation OTDRs.
Often a full-feature OTDR has a main frame that can be fitted with multi-function plug-in
units to perform many fiber measurement tasks. Larger color displays are common. The full-feature
OTDR often has a greater measurement range than the other types of OTDR-like equipment.
Often it is used in laboratories and in the field for difficult fiber measurements. Most
full-feature OTDRs are powered from AC and/or a battery.
Hand-held OTDR and Fiber break locator Hand-held OTDRs and fiber break locators are
designed to troubleshoot fiber networks in a field environment, often using battery power.
The two types of instruments cover the spectrum of approaches to fiber optic plant taken by
communication providers. Hand-held, inexpensive OTDRs are intended to be easy-to-use, light-weight,
sophisticated OTDRs that collect field data and perform rudimentary data analysis. They
may be less feature rich than full-feature OTDRs. Often they can be used in conjunction
with PC-based software to perform data collection and sophisticated data analysis. Hand-held
OTDRs are commonly used to measure fiber links and locate fiber breaks, points of high loss,
high reflectance, end-to-end loss, and Optical Return Loss.
Fiber break locators are intended to be low-cost instruments specifically designed to determine
the location of a catastrophic fiber event, e.g., fiber break, point of high reflectance,
or high loss. The fiber break locator is an opto-electronic tape measure designed to measure
only distance to catastrophic fiber events. In general, hand-held OTDRs and fiber break
locators are lighter and smaller, simpler to operate, and more likely to employ battery
power than full-feature OTDRs. The intent with hand-held OTDRs and fiber break locators
is to be inexpensive enough for field technicians to be equipped with one as part of a standard
tool kit.
Remote Test Unit The RTU is the testing module of the RFTS
described in GR-1295, Generic Requirements for Remote Fiber Testing Systems. An RFTS
enables fiber to be automatically tested from a central location. A central computer is
used to control the operation of OTDR-like test components located at key points in the
fiber network. The test components scan the fiber to locate problems. If a problem is
found, its location is noted and the appropriate personnel are notified to begin the repair
process. The RFTS can also provide direct access to a database that contains historical
information of the OTDR fiber traces and any other fiber records for the physical fiber
plant. Since OTDRs and OTDR-like equipment have many
uses in the communications industry, operating environments vary widely, both indoors and
outdoors. Most often, however, these test sets are operated in controlled environments,
accessing the fibers at their termination points on fiber distribution frames. Indoor
environments include controlled areas such as central offices, equipment huts, or Controlled
Environment Vaults. Use in outside environments is rarer, but may include use in a manhole,
aerial platform, open trench, or splicing vehicle.
OTDR Data Format In the late 1990s, OTDR industry representatives
and the OTDR user community developed a unique data format to store and analyze OTDR fiber
data. This data was based on the specifications in GR-196, Generic Requirements for Optical
Time Domain Reflectometer Type Equipment, referenced above. The goal was for the data
format to be truly universal, in that it was intended to be implemented by all OTDR manufacturers.
OTDR suppliers developed the software to implement the data format. As they proceeded, they identified
inconsistencies in the format, along with areas of misunderstanding among users.
From 1997 to 2000, a group of OTDR supplier software specialists attempted to resolve
problems and inconsistencies in what was then called the “Bellcore” OTDR Data Format.
This group, called the OTDR Data Format Users Group, made progress. Since then, many OTDR
developers continued to work with other developers to solve individual interaction problems and
enable cross use between manufacturers. In 2011, Telcordia decided to compile industry
comments on this data format into one document entitled, SR-4731, Optical Time Domain Reflectometer
Data Format. This Special Report summarizes the state of the Bellcore OTDR Data Format,
renaming it as the Telcordia OTDR Data Format. The data format is intended for all OTDR-related
equipment designed to save trace data and analysis information. Initial implementations
require standalone software to be provided by the OTDR supplier to convert existing OTDR
trace files to the SR-4731 data format and to convert files from this universal format
to a format that is usable by their older OTDRs. This file conversion software can be
developed by the hardware supplier, the end user, or a third party. This software also
provides backward compatibility of the OTDR data format with existing equipment.
The SR-4731 format describes binary data. While text information is contained in several
fields, most numbers are represented as either 16-bit or 32-bit signed or unsigned integers
stored as binary images. Byte ordering in this file format is explicitly low-byte ordering,
as is common on Intel® processor-based machines. String fields are terminated with a zero byte
“\0”. OTDR waveform data are represented as short, unsigned integer data uniformly
spaced in time, in units of decibels times 1000, referenced to the maximum power level.
The maximum power level is set to zero, and all waveform data points are assumed to be
zero or negative, so that the minimum power level in this format is -65.535 dB, and the
minimum resolution between power level steps is 0.001 dB. In some cases, this will not
provide sufficient power range to represent all waveform points. For this reason, the
use of a scale factor has been introduced to expand the data point power range.
See also Optical return loss
External links BBC News: Work begins to repair severed net
This article incorporates public domain material from the General Services Administration
document "Federal Standard 1037C". References