Overview of Fiber Optic Instrumentation
Testing fiber optic components and cable
plants requires making
several measurements with the most common measurement parameters
listed in the Table below. Optical power, required for measuring
source power, receiver power and, when used with a test source, loss or
attenuation, is the most
important parameter and is required for almost every fiber optic
test. Backscatter and wavelength measurements are the next most
important and bandwidth or dispersion are of lesser importance.
Measurement or inspection of geometrical parameters of fiber are
essential for fiber manufacturers. And troubleshooting installed
cables and networks is required.
Fiber Optic Testing Requirements
|
Test Parameter |
Instrument |
|
Optical Power
(Source Output, Receiver Signal Level) |
Fiber Optic Power Meter |
|
Attenuation or Loss of Fibers, Cables & Connectors
(Insertion Loss) |
FO Power Meter & Source or
OLTS (optical loss test set) |
|
Source Wavelength, Spectral Width |
FO Spectrum Analyzer |
|
Backscatter For Loss, Length and Fault Location) |
Optical Time Domain Reflectometer(OTDR) |
|
Fault Location |
OTDR,
Visual Cable Fault Locator |
|
Bandwidth / Dispersion
(MM:Modal & Chromatic,
SM: Chromatic and Polarization Mode) |
Dedicated Bandwidth Testers |
| Reflectance |
OTDR, OCWR (Optical Continuous Wave Reflectometer) |
Fiber Geometry
(Core and cladding diameter, concentricity, etc.) | Various mechanical and optical inspection tools |
Standard Test Procedures
Most test procedures for fiber optic component specifications
have been standardized by national and international standards
bodies, including TIA in the US and ISO/IEC internationally.
Procedures for measuring absolute optical power, cable and connector
loss and the effects of many environmental factors (such as temperature,
pressure, flexing, etc.) are covered in these procedures. Here is a listing of some TIA standards.
In order to perform these tests, the basic fiber optic instruments
are the FO power meter, test source, OTDR, optical spectrum analyzer
and an inspection microscope. These and some other specialized
instruments are described below.
Fiber Optic Power Meters
Fiber
optic power meters measure the average optical power out of an optical fiber. Power meters typically consist of a solid state
detector (silicon for short wavelength systems, germanium or InGaAs
for long wavelength systems), signal conditioning circuitry and
a digital display of power. To interface to the large variety
of fiber optic connectors in use, some form of removable connector
adapter is usually provided.
Power meters are calibrated to read in dB
referenced to one milliwatt of optical power. Some meters offer a
relative dB
scale also, useful for loss measurements since the reference value may
be set to "0 dB" on the output of the test source. Occasionally, lab
meters may also measure in
linear units (milliwatts,
microwatts and nanowatts.) Since all semiconductor detectors have a
sensitivity that varies with the wavelength of the light it is
measuring, power meters are calibrated at the typical wavelengths used
in fiber optics, 850, 1300 and 1550 nm. Meters for POF systems are
usually calibrated at 650 and 850 nm, the wavelengths used in POF
systems.
Power meters cover a very broad dynamic range, over 1 million
to 1 or 60 dB. Although most fiber optic power and loss measurements
are made in the range of 0 dBm to -50 dBm, some power meters offer
much wider dynamic ranges. For testing analog CATV systems or
fiber amplifiers, on needs special meters with extended high power
ranges up to +20 dBm (100 mW). Although no fiber optic systems
operate at very low power, below about -50 dBm, some lab meters
offer ranges to -70 dBm or more, which can be useful in measuring
optical return loss or spectral loss characteristics with a monochromator
source.
Power meters measure the time average of the optical power,
not the peak power, so the meters are sensitive to the duty cycle
of an input digital pulse stream. One can calculate peak power
if one knows the duty cycle of the input, by dividing the average
power by the duty cycle. For most loss measurements, one uses
a test source with CW (steady state) or 2 kHz pulsed output. As
long as the source modulation doesn't change, no compensation
needs to be made. When testing link transmitter power or receiver
sensitivity, it is necessary to establish a standard test pattern,
generally a 50% duty cycle, called a square wave, to allow accurate
measurement of transmitter output or receiver sensitivity.
FO power meters have a typical measurement
uncertainty of +/-5%,
when calibrated to transfer standards provided by national standards
laboratories like the US National Institute of Standards and Technology
(NIST). Sources of errors are the variability of coupling efficiency
of the detector and connector adapter, reflections off the shiny
polished surfaces of connectors, unknown source wavelengths (since
the detectors are wavelength sensitive), nonlinearities in the
electronic signal conditioning circuitry of the FO power meter
and detector noise at very low signal levels. Since most of these
factors affect all power meters, regardless of their sophistication,
expensive laboratory meters are hardly more accurate that the
most inexpensive handheld portable units. Meters should be recalibrated
frequently by labs with NIST traceable calibration systems. (Photo courtesty Advanced Fiber Solutions)
Fiber Optic Test
Sources
In
order to make measurements of optical loss or attenuation in fibers,
cables and connectors, one must have a test source
as well as a FO power meter. The test source must be chosen for compatibility
with the type of fiber in use (singlemode or multimode with the
proper core diameter) and the wavelength desired for performing
the test. Most sources are either LED's or lasers of the types
commonly used as transmitters in actual fiber optic systems, making
them representative of actual applications and enhancing the usefulness
of the testing. Some tests, such as measuring spectral attenuation
of fiber requires a variable wavelength source, which is usually
a tungsten lamp with a monochromator to vary the output wavelength.
Typical wavelengths of sources are 650 or 665 nm (plastic fiber),
820, 850 and 870 nm (short wavelength glass fiber ) and 1300 or 1310 nm and
1550 nm (long wavelength ). LED's are typically used for testing
multimode fiber and lasers are used for singlemode fiber, although
there is some crossover, especially in high speed LANs which
use multimode fiber with lasers and the testing of short singlemode
jumper cables with LED's. The source wavelength can be a critical
issue in making accurate loss measurements on long links, since attenuation
of the fiber is wavelength sensitive especially at short wavelengths.
Thus all test sources should be calibrated for wavelength.
Test sources almost always have fixed
connectors. Hybrid test jumpers with
connectors compatible with the source on one end and the connector
being tested on the other must be used as reference cables. This may
affect the type of reference setting mode used for loss testing.
Other source-related factors affecting
measurement accuracy
are the stability of the output power and the modal distribution
launched into multimode fiber. For extremely accurate measurements, the
source may need optical feedback stabilization to maintain output
power at a precise level for long times required for some measurements.
Industry standards have requirements or recommendations on the modal
output of test sources for multimode fiber that are aimed at the
manufacturers of the test sources. Mode scramblers, filters and
strippers may be required to
adjust the modal distribution in the fiber to approximate actual
operating conditions. (Photo courtesty Advanced Fiber Solutions)
Optical Loss Test
Sets/Test Kits
The
optical loss test set is an instrument formed by the combination
of a fiber optic power meter and source which is used to measure
the loss of fiber, connectors and connectorized cables. Early
versions of this instrument were called attenuation meters. A
test kit has a similar purpose, but is usually comprised of separate
instruments and includes accessories to customize it for a specific
application, such as testing a FO LAN, telco or CATV.
The
OLTS may have several optional features that affect its use. Some have
individual source outputs and meter inputs like a separate power meter
and test source, but may have two wavelengths from one source output
(MM: 850/1300, SM:1310/1550.) Some offer bidirectional testing on a
single fiber and some have two bidirectional ports. Some manufactures
of premises copper cabling testers offer modules to convert these
testers to an OLTS, allowing fiber and copper testing with one
instrument.
The combination OLTS instrument which
contains both a meter and source may be less convenient than an
individual source and power
meter, since the ends of the fiber and cable
are usually separated by long distances, which would require two
OLTSs instead of one source and one meter. An OLTS often has a single
port for bidirectional measurements also. This port usually has a fixed
connector which may cause problems when testing cable plants with
connector styles different than those on the instrument, requiring a 2-
or 3-cable reference for loss testing which may not meet industry
standards. The bidirectional port may also have problems meeting
standards for modal power distribution in multimode fibers. Ask the
OLTS manufacturer about these issues before purchasing an
instrument. (Photo courtesty EXFO)
Optical Time
Domain Reflectometer
The
optical time domain reflectometer (OTDR) uses the phenomena of
fiber backscattering to characterize fibers and installed cables, find faults and optimize
splices. Since scattering is one of the primary loss factors in
fiber (the other being absorption), the OTDR can send out into
the fiber a high powered pulse and measure the light scattered
back toward the instrument. The pulse is attenuated on the outbound
leg and the backscattered light is attenuated on the return leg,
so the returned signal is a function of twice the fiber loss and
the backscatter coefficient of the fiber.
If one assumes the backscatter coefficient is constant, the
OTDR can be used to measure loss as well as locate fiber breaks,
splices and connectors. In addition, the OTDR gives a graphic
display of the status of the fiber being tested. And it offers
another major advantage over the source/FO power meter or OLTS,
in that it requires access to only one end of the fiber.
The uncertainty of the OTDR measurement is heavily dependent
on the backscatter coefficient, which is a function of intrinsic
fiber scattering characteristics, core diameter and numerical
aperture. It is the variation in backscatter coefficient that
causes many splices to show a "gain" instead of the
actual loss. OTDRs must also be matched
to the fibers being tested in both wavelength and fiber core diameter
to provide accurate measurements. Thus many OTDRs have modular
sources to allow substituting a proper source for the application.
While most OTDR applications involve finding faults in installed
cables or verifying splices, they are very useful in inspecting
fibers for manufacturing faults. Development work on improving
the short range resolution of OTDRs for LAN applications and new
applications such as evaluating connector return loss promise
to enhance the usefulness of the instrument in the future.
OTDRs come in three basic versions. Full size OTDRs offer the
highest performance and have a full complement of features like
data storage, but are very big and high priced. MiniOTDRs provide
the same type of measurements as a full OTDR, but with fewer features
to trim the size and cost. Fault finders use the OTDR technique,
but greatly simplified to just provide the distance to a fault,
to make the instruments more affordable and easier to use. (Photo courtesty Corning Cable Systems)
Visual Cable
Tracers and Fault Locators
Many
of the problems in connection of fiber optic networks are related
to making proper connections. Since the light used in systems
is invisible, one cannot see the system transmitter light. By
injecting the light from a visible source, such as a LED or incandescent
bulb, one can visually trace the fiber from transmitter to receiver
to insure correct orientation and check continuity besides. The
simple instruments that inject visible light are called visual
fault locators.
If a powerful enough visible light ,such as a HeNe or visible
diode laser is injected into the fiber, high loss points can be
made visible. Most applications center around short cables such
as used in telco central offices to connect to the fiber optic
trunk cables. However, since it covers the range where OTDRs are
not useful, it is complementary to the OTDR in cable troubleshooting.
This method will work on buffered fiber and even jacketed single
fiber cable if the jacket is not opaque to the visible light.
The yellow jacket of singlemode fiber and orange of multimode
fiber will usually pass the visible light. Most other colors,
especially black and gray, will not work with this technique,
nor will most multifiber cables. However, many cable breaks, macrobending
losses caused by kinks in the fiber , bad splices etc. can be
detected visually. Since the loss in the fiber is quite high at
visible wavelengths, on the order of 9-15 dB/km, this instrument
has a short range, typically 3-5 km.
Fiber Identifiers
Telco technicians often need to identify a
fiber in a splice closure or at a patch panel. If one carefully bends a
singlemode fiber enough to cause loss, the
light that couples out can also be detected by a large area detector.
A fiber identifier uses this technique to detect a signal in the
fiber at normal transmission wavelengths. These instruments usually
function as receivers, able to discriminate between no signal,
a high speed signal and a 2 kHz tone. By specifically looking
for a 2 kHz "tone" from a test source coupled into the
fiber, the instrument can identify a specific fiber in a large
multifiber cable, especially useful to speed up the splicing or
restoration process.
Fiber identifiers can be used with both buffered fiber and
jacketed single fiber cable. With buffered fiber, one must be
very careful to not damage the fiber, as any excess stress here
could result in stress cracks in the fiber which could cause a
failure in the fiber anytime in the future.
Measuring Fiber Bandwidth
Although fiber has a very high bandwidth,
some applications
actually approach its limits, requiring performance evaluation. Two
factors limit multimode fiber bandwidth: modal dispersion and
chromatic dispersion. Long singlemode links require concern over
chromatic dispersion or polarization-mode dispersion. Specialized
instruments are available for testing each of these specifications but
are expensive and rarely used outside the laboratory.
O/E and E/O Converters
Optical to electrical (O/E) and electrical to optical (E/O)
converters have other uses besides testing fiber bandwidth. O/E
converters can be used with high speed oscilloscopes to analyze
pulses in fiber optic links to see if the waveforms are of the
proper shape. This means measuring rise and fall times of the
pulse and the depth of modulation (the difference between the
peak power of the pulse and the lowest power reached between pulses.
They can be used for testing lasers and LEDs used in transmitters
and link dispersion in long links. E/O converters are used to
test receivers for bandwidth and margin, usually in conjunction
with a bit error rate tester and attenuator.
Optical Continuous Wave Reflectometers (OCWR)
The OCWRor reflectance tester was originally proposed as a special purpose instrument
to measure the reflectance or optical return loss of connectors installed on
patchcords or jumpers. Unfortunately, its purpose became muddled
between conception and inception. As actual instruments came on
the market, they had much higher measurement resolution than appropriate
for the measurement uncertainty (0.01 dB resolution vs. 1 dB uncertainty),
leading to much confusion on the part of users as to why measurements
were not reproducible. In addition, several instruments were touted
as a way to measure the optical return loss of an installed cable
plant, obviously in ignorance of the fact that they would also
be integrating the backscatter of the fiber with any reflections
from connectors or splices. Since the measurement of return loss
from a connector can be made equally well with any power meter,
laser source and calibrated coupler, and an OTDR is the only way
to test installed cable plants for return loss, the OCWR has seen little
use in fiber optic testing.
Optical Fiber Analyzers
There are many parameters of optical fiber that require testing
by the manufacturer. These include attenuation (as a function
of source wavelength), bandwidth/dispersion, numerical aperture
and all the physical dimensions such as core and cladding diameter,
ovality, and concentricity. Automated laboratory instruments are
available to measure all these parameters automatically, but many
fiber manufacturers prefer to build their own. The most difficult
part of fiber measurements is the fact that subtle differences
in test setup and instrumentation can cause differences in measured
values.
Visual Inspection
with Microscopes
Cleaved
fiber ends prepared for splicing and polished connector ferrules
require visual inspection to find possible defects. This is accomplished
using a microscope which has a stage modified to hold the fiber
or connector in the field of view. Fiber optic inspection microscopes
vary in magnification from 30 to 800 power, with 30-100 power
being the most widely used range. Cleaved fibers are usually viewed
from the side, to see breakover and lip. Connectors are viewed
end-on or at a small angle to find polishing defects such as scratches.
Fiber Optic Talksets
While technically not an measuring instrument, FO talksets
are useful for FO installation and testing. They transmit voice
over fiber optic cables already installed, allowing technicians
splicing or testing the fiber to communicate effectively. Talksets
are especially useful when walkie-talkies and telephones are not
available, such as in remote locations where splicing is being
done, or in buildings where radio waves will not penetrate.
The way to use talksets most effectively is to set up the talksets
on one fiber (or pairs appropriate) and leave them there while
all testing or splicing work is done. Thus, there will always
be a communications link between the working crew, which facilitates
deciding which fibers to work with next. The continuous communications
capability will greatly speed the process.
Recent developments in talksets include talksets for networking
multi-party communications, especially helpful in restoration,
and system talksets for use as intercoms in installed systems.
There are also combination testers
and talksets.
There are no standards for the way talksets communicate. Some
use simple AM transmission, some FM and some proprietary digital
schemes. Thus no two manufacturers' talksets can communicate with
each other. Bellcore has addressed this matter in a technical
advisory that proposes a FM method at 80 and 120 kHz, but it will
take years before a standard has been set and manufacturers offer
compatible instruments.
Attenuators
Attenuators are used to simulate the loss of long fiber runs
for testing link margin in network simulation in the laboratory
or self-testing links in a loopback configuration. In margin testing,
variable attenuators are used to increase loss until the system
has a high bit error rate. For loopback testing, an attenuator
is used between a single piece of equipment's transmitter and
receiver to test for operation under maximum specified fiber loss.
If systems work in loopback testing, they should work with a proper
cable plant. Thus many manufacturers of network equipment specify
a loopback test as a diagnostic/troubleshooting procedure.
Attenuators can be made by gap loss, or a physical separation
of the ends of the fibers, inducing bending losses or inserting
calibrated optical filters. Both variable and fixed attenuators
are available, but variable attenuators are usually used for testing.
Fixed attenuators may be inserted in the system cables where distances
in the fiber optic link are too short and excess power at the
receiver causes transmission problems.
Reference Test
Jumper Cables and Mating Adapters
In order to test cables in an insertion loss test,
one needs to establish test conditions. This requires reference launch jumper
cables to connect the test source to the cable under test and
receive cables to connect the fiber optic power meter. For accurate
measurements, the launch and receive cables must be made with
fiber and connectors matching the cables to be tested and terminated carefully to ensure low loss. To provide
reliable measurements, launch and receive cables must be in good
condition and kept very clean. They can easily be tested against each other to insure
their performance. Connector mating adapters are used
to connect the cables under test to the launch and receive cables.
Only the highest performance bulkhead splices should be used,
and their condition checked regularly, since they are vitally
important in obtaining low loss connections.
Additional Reading
Testing Installed Cable Plants
Accuracy of fiber optic measurements
