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Peter Schweiger, Agilent Technologies, Photonics Measurement Division
Optical Time Domain Reflectometer (OTDR) remains a fundamental tool for measuring
fiber links, but the product offering has changed as both the available technology and the fiber network under test has changed.
Available products can be grouped into three classes
1. Break locators and fault finders
2. Mainstream Mini-OTDR’s
3. Optical Characterization Platforms
To choose, first evaluate your needs and the skill of the intended users honestly. Questions to ask:
1. Are you Installing or Maintaining fiber?
2. If Maintenance, is finding the location of the fault the main task?
3. If Installation, do you need measure more than loss and length? E.g. Connector quality, dispersion, Optical Return Loss?
For simple maintenance, a simple or low cost OTDR is a good fit. They are easy to use, require the lowest possible investment and some even provide total link loss and optical return loss values.
If you are doing complex installation of high speed networks than you are probably looking for an Optical Characterization Platform. These allow modules, either to be plugged in or better yet snapped on and give the most flexibility, speed and performance.
If you are in between the cases above, welcome to 80% of the users. You need a regular Mini-OTDR based system. There are many products on the market and choosing the best fit requires both a look through the key spec’s, and some hands on time.
What is an Optical Time
Domain Reflectometer (OTDR)
See
picture
Figure
1. Simplified Block Diagram and Examples of Events on an OTDR
Figure 1 shows a simplified block diagram and an example of how
different “events” appear on an OTDR display or “trace”.
An OTDR sends pulses of light into a fiber and
digitizes the reflected backscatter signal coupled back to its
detector. Using the fiber’s refractive index, it converts time into
distance over a scale of 100m to beyond 200km. It can measure distance
to faults very accurately.
The need for speed
Whether 100m or 10km long, the increased
deployment of fibers needs to be tested in less time with fewer
people.
The key specification for an OTDR is dynamic range. It’s the dB
difference between the initial power level reflected from the fiber
under test and the value equal to the noise floor of the detector, it
is specified at the OTDR’s largest pulse width when making a
measurement for 3 minutes.
This figure in dB determines both how far you can
measure and how fast an instrument can obtain a clear trace.
Every dB counts. In fact 1db extra dynamic range over another unit can
mean an increase in speed and productivity of up to 2X.
Most products on the market are grouped into performance ranges and
can be segmented into dynamic range classes of the nearest 5dB. Table
1outlines the dramatic effect of dynamic range vs. averaging time to
acquire a good trace.
Table 1.
Dynamic
Range
vs. Average Time to Acquire a Trace
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Link length
Test time
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<50 km
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100 km
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150 km
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200+km
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180 sec
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30dB
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35dB
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40dB
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45dB
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40 sec
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35dB
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40dB
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45dB
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10 sec
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40dB
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45dB
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2 sec
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45dB
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(See
Picture if not clear)
If you are driving hours to measure 2 short
fibers, you don’t need to pay the premium for a 45dB OTDR. But if
you are installing Metropolitan fiber networks with 864 fibers. The
time difference can add up to weeks and a faster OTDR can pay for
itself in the very first cable measured. Dynamic range is key, but
Ergonomics, storage and other spec’s should influence your decision.
Table 2. Key Product
Specifications and
their importance
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Dynamic
Range
(See
Picture)
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Determines range and speed. Serious
products range from 30-45db for Single mode (SM) testing,
18-34dB for Multimode (MM) fiber testing. Compare
products at the same pulsewidth.
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Reflection Accuracy.
(See
Picture)
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Reflections or Reflectances are
caused by connectors, mechanical splices, and breaks. High speed
networks require small reflectances
. Look for a product that measures accurately across a realistic
range like 14-60dB with an accuracy of +-2dB.. A great test is
to see if an open 100m from the unit reads 14dB.
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Loss Accuracy
And linearity
(See Picture)
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Determines total link loss and individual
splice loss accuracy. Measuring fusion splices takes high
dynamic range and repeatability. Measure a splice a few times
and evaluate the instruments repeatability.
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Battery
Life –hrs
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Look for 4+ hours and fast charge times,
does the unit indicate actual time remaining or simply display a
guess-O-gauge?
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Size and Weight
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Products exist from 1kg -9kg. In today’s
crowded POP’s which unit will fit? The smaller the unit, the
lower shipping costs are too. If you want to hold it while
measuring and you’ll need a unit <4kg.
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Trace storage
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What is the internal and optional memory in
the product. Does it support flash cards? Do you want a hard
drive in an instrument that could be dropped?
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User interface
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50% of users love touchscreen. 50% prefer
reliable keys. Many users? Choose an OTDR with a variety of
interfaces. A keyboard input port is a must if you are
documenting lots of fibers.
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Expandability
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Just enough test or fit for future? Empty
frames can cause extra weight and complexity. Inquire if a built
in source is an option or standard.
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Available Wavelengths
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A single wavelength allow fault location,
Dual sources can show changes in loss at different wavelengths
to detect fiber bending.
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Service and Support
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Asking what an extended warranty would cost
is a great indication of the reliability of a unit and
efficiency of the repair organization. Can you get on-site
training for your unit?
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Consider the whole product and all its supplied
accessories and you’ll make a decision that fits your whole
process better.
Buy as much dynamic range as you can afford. It
makes everything possible.
Whatever you buy. Learn it. Only a small
percentage of users actually take advantage of all the time saving
features in modern units. Don’t buy a 200hp car and keep it in 1st
gear. Read the manual or ask a professional to teach you some driving
tip. www.otdr.com
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