The FOA Reference For Fiber Optics - Measuring Loss http://www.thefoa.org/tech/ref/testing/test/loss.

html

The Fiber Optic Association,

Inc.
the non-profit professional society of fiber
optics

Table of Contents:
Topic: Measuring Loss in Fiber
The FOA Reference Guide To
Optics
Fiber Optics

Optical Fiber Testing - Loss and Attenuation Coefficient

For optical fiber, testing includes fiber geometry, attenuation and bandwidth. The
most fundamental parameter for optical fiber is geometry, since the dimensions of the
fiber determine its ability to be spliced and terminated to other fibers. The core
diameter, cladding diameter and concentricity are the most important factors on how
well one can connect or splice two fibers. Thus manufacturers work very hard to
control these parameters, including continuous testing throughout the manufacturing
process.

While testing diameter and concentricity may sound simple, measurements must be
made to submicron precision. The process is complicated by the fact that the material
is transparent and the dimensions are small enough to reach the limits of optical
measurements.

Fiber Types and Typical Specifications

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Core/Cladding Attenuation Coefficient

Fiber Type
Diameter 850 nm 1300 nm 1550 nm
0.2-1
POF 1 mm dB/meter
@650 nm
200/240
Step Index 6 dB/km
microns
Graded Index 50/125
3 dB/km 1 dB/km
Multimode microns
62.5/125
3 dB/km 1 dB/km
microns
85/125
3 dB/km 1 dB/km
microns*
100/140
3 dB/km 1 dB/km
microns*
0.4-0.5 0.2-0.3
Singlemode 9/125 microns
dB/km dB/km

* obsolete designs

Attenuation

The attenuation of the optical fiber is a

result of two factors, absorption and
scattering. The absorption is caused by the
absorption of the light and conversion to
heat by molecules in the glass. Primary
absorbers are residual OH+ and dopants
used to modify the refractive index of the
glass. This absorption occurs at discrete
wavelengths, determined by the elements
absorbing the light. The OH+ absorption is predominant, and occurs most strongly
around 1000 nm, 1400 nm and above1600 nm.

The largest cause of attenuation is scattering.Scattering occurs when light collides

with individual atoms in the glass and is anisotrophic. Light that is scattered at angles
outside the numerical aperture of the fiber will be absorbed into the cladding or
transmitted back toward the source Scattering is also a function of wavelength,
proportional to the inverse fourth power of the wavelength of the light. Thus if you
double the wavelength of the light, you reduce the scattering losses by 2 to the 4th
power or 16 times. Therefore , for long distance transmission, it is advantageous to
use the longest practical wavelength for minimal attenuation and maximum distance

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between repeaters. Together, absorption and scattering produce the attenuation curve
for a typical glass optical fiber shown above.

Fiber optic systems transmit in the "windows" created between the absorption bands
at 850 nm, 1300 nm and 1550 nm, where physics also allows one to fabricate lasers
and detectors easily. Plastic fiber has a more limited wavelength band, that limits
practical use to 660 nm LED sources.

Testing Fiber Attenuation

Measuring the fiber attenuation coefficient

requires transmitting light of a known
wavelength through the fiber and
measuring the changes over distance. The
conventional method, known as the
cutback method, involves coupling fiber to
the source and measuring the power out of
the far end. The fiber is then cut near the
source and power measured again. By
knowing the power at the source and end
of the fiber and the length of the fiber, its
attenuation coefficient can be determined
by calculating:

An alternative method of testing fiber, which may be easier in field measurements,

involves attaching a fiber pigtail to the source that has a connector on one end and a
temporary splice on the other end, similar to the loss measurement of terminated
cables (Chapter 4). This method introduces more uncertainty in the measurement
because of the loss of the splice coupled to the fiber under test, since it may not be
easy to accurately calibrate the output power of the pigtail. The best method is to use
a bare fiber adapter on the power meter to measure the output of the bare fiber, then
attach the splice. Alternately, have the splice attached on the pigtail and couple a
large core fiber to the pigtail with the splice and measure the power. The large core
fiber will minimize losses in the splice for accurate calibration.

Sources for Loss Measurements

On the test source, two factors must be controlled to minimize measurement

uncertainty, the spectral output and modal characteristics. The spectral output
characteristics obviously include wavelength, as seen in the spectral attenuation
curve, but may also include the spectral width. A wide spectral width source suffers
absorption over a larger range of wavelengths, making it more difficult to obtain

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precise data on spectral attenuation at any specific wavelength. Monochromators are

used as sources for spectral loss testing, since the spectral width of the source can be
controlled exactly.

For single wavelength measurements the source can be a fixed wavelength LED or
laser. Generally, attenuation measurements will be made with a source appropriate to
the fiber. Most multimode fiber systems use LED sources while singlemode fiber
systems use laser sources. Thus testing each of these fibers should be done with the
appropriate source. Lasers should not be used with multimode fiber, since coherent
sources like lasers have high measurement uncertainties in multimode fiber caused by
modal noise. The wide spectral width of LEDs sometimes overlap the singlemode
fiber cutoff wavelength (the lowest wavelength where the fiber supports only one
mode) at lower wavelengths and the 1400 nm OH: absorption band at the upper
wavelengths.

The additional absorption at either end of the LEDs spectral output may bias the
measurements of attenuation on singlemode fiber substantially. Tests from Bellcore
showed the effects of sources on measurements of singlemode fiber loss. The LED
spectrum covers from the singlemode cutoff wavelength around 1200 nm well into
the OH absorption band, while the laser concentrates all its power in an extremely
narrow spectral band where the fiber is actually used. Over the range covered by the
LED output, the fiber loss varies by 0.2 dB/km, ignoring the OH absorption band.
Bellcore tests showed an error of loss caused by the use of the LED of 0.034dB/km.

Even with laser sources, the loss varies substantially according to the wavelength of
the source. Again Bellcore tests showed a variation of loss of 0.05 dB/km with source
variations of 29 nm (1276 and 1305 nm), within the range of typical sources used in
the network. So testing should be done with sources as close to the system
wavelength as possible, especially with longer links. (Peters, Bellcore reference).

Modal Effects on Attenuation (This material is duplicated here)

In order to test multimode fiber optic cables accurately and reproducibly, it is

necessary to understand modal distribution, mode control and attenuation correction
factors. Modal distribution in multimode fiber is very important to measurement
reproducibility and accuracy.

What is "Modal Distribution" ?

In multimode fibers, some light rays travel straight down the axis of the fiber while all
the others wiggle or bounce back and forth inside the core. In step index fiber, the off
axis rays, called "higher order modes" bounce back and forth from core/cladding
boundaries as they are transmitted down the fiber. Since these high order modes
travel a longer distance than the axial ray, they are responsible for the dispersion that
limits the fiber's bandwidth.

In graded index fiber, the reduction of the index of refraction of the core as one
approaches the cladding causes the higher order modes to follow a curved path that

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is longer than the axial ray (the "zero order mode"), but by virtue of the lower index
of refraction away from the axis, light speeds up as it approaches the cladding and it
takes approximately the same time to travel through the fiber. Thus the "dispersion"
or variations in transit time for various modes, is minimized and bandwidth of the
fiber is maximized.

However, the fact that the higher order modes travel farther in the glass core means
that they have a greater likelihood of being scattered or absorbed, the two primary
causes of attenuation in optical fibers. Therefore, the higher order modes will have
greater attenuation than lower order modes, and a long length of fiber that was fully
filled (all modes had the same power level launched into them) will have a lower
amount of power in the higher order modes than will a short length of the same fiber.

This change in "modal distribution" between long and short fibers can be described
as a "transient loss", and can make big differences in the measurements one makes
with the fiber. It not only changes the modal distribution, it changes the effective core
diameter and numerical aperture also.

The term "equilibrium modal distribution" (EMD) is used to describe the modal
distribution in a long fiber which has lost the higher order modes. A "long" fiber is
one in EMD, while a "short" fiber has all its initially launched higher order modes.

What Does Fiber Modal Distribution Look Like ?

Modal distribution in a
multimode fiber depends
on your source, fiber,
and the intermediate
"components" such as
connectors, couplers and
switches, all of which
affect the modal
distribution of fibers
they connect. Typical
modal distributions for
various fiber optic
components are shown
here.

In the laboratory, a
lensed optical system can be used to fully fill the fiber modes and a "mode filter",
usually a mandrel wrap which stresses the fiber and increases loss for the higher
order modes, used to simulate EMD conditions. A "mode scrambler", made by fusion
splicing a step index fiber in the graded index fiber near the source can also be used
to fill all modes equally. If one has a proper optical system, one can control the
launch conditions to very specific levels as desired for the measurements being
performed.

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Relative Modal Distribution of Multimode Fibers: (Right)

A fully filled fiber means that all modes carry equal power, as shown by the line
across the top of the graph. A long length of fiber loses light in the higher order
modes faster, leading to the gently sloping "EMD" curve. Mode filtering strips off
the higher order modes, but provides only a crude approximation of EMD. The
microlensed LED , often thought to overfill the modes, actually couples most of its
power in lower order modes. The E-LED (edge-emitting LED) couples even more
strongly in the lower order modes. Connectors are mode mixers, since
misalignment losses cause some power in lower order modes to be coupled up to
higher order modes.

In an actual operating communications system, such controlled conditions obviously

do not exist. In fact some work presented by Corning at an EIA Standards meeting
shows how far the real world is from what we expected it to be.

It has been accepted as "common knowledge" that microlens LEDs (as used with
most multimode datacom systems) overfill fibers, and when we use them as test
sources, we are testing with an overfilled launch. Not so. Tests on microlens LEDs
indicate that they underfill compared to EMD. And edge-emitter LEDs (E-LED),
typical of the high speed emitters at 1300 nm, concentrate their power even more
into the lower order modes.

Other facts that come out of the Corning project shows that connectors mix some
power back into the higher order modes due to angular misalignment and switches
strip out higher modes . In a simulated FDDI system using 8 fiber optic switches and
20 pairs of connectors, with fiber lengths of 10 to 50 meters between them, the
majority of system power was concentrated in the lower order modes.

What conclusions can we draw ? The most significant conclusions is that it may not
be prudent to design datacom and LAN systems on the worst-case loss specifications
for connectors and switches. In actual operation, the simulated system exhibited
almost 15 dB less loss than predicted from worst case component specifications
(obtained with fully filled launch conditions). In most of today's high speed systems,
LEDs are too slow to be used as transmitters, so a special type of low cost 850 nm
laser called a VCSEL (vertical cavity surface-emitting laser) is used as a transmitter.
VCSELs couple light tightly into the core of a multimode fiber, similar ot a eLED in
the diagram above.

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And, when testing cables designed for low speed LED transmitter type systems, using
a LED source similar to the one used in the system and short launch cables may
provide as accurate a measurement as is possible under more controlled
circumstances, since the LED approximates the system source. For newer sytems
using VCSEL sources, one should use at least a LED with a mandrel wrap (see
below) or a commercially-available mode modifier.

The Effect on Measurements

If you measure the attenuation of a long fiber in EMD (or any fiber with EMD

simulated launch conditions) and compare it to a normal fiber with "overfill launch
conditions " (that is the source fills all the modes equally), you will find the
difference is about 1 dB/km, and this figure is the "transient loss". Thus, the EMD
fiber measurement gives an attenuation that is 1 dB per Km less than the overfill
conditions.

Fiber manufacturers use the EMD type of measurement for fiber because it is more
reproducible and is representative of the losses to be expected in long lengths of
fiber. But with connectors, the EMD measurement can give overly optimistic results,
since it effectively represents a situation where one launches from a smaller diameter
fiber of lower NA than the receive fiber, an ideal situation for low connector loss.

The difference in connector loss caused by modal launch conditions can be dramatic.
Using the same pair of biconic or SMA connectors, it is possible to measure 0.6 to
0.9 dB with a fully filled launch and 0.3 to 0.4 dB with a EMD simulated launch.
Which is a valid number to use for this connector pair's loss ?

That depends on the application. If you are connecting two fibers near a LED source,
the higher value may be more representative, since the launch cable is so short. But if
you are connecting to a cable one km away, the lower value may be more valid.

Mode Conditioners

There are three basic "gadgets" to condition the modal distribution in multimode
fibers :
mode strippers which remove unwanted cladding mode light,

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mode scramblers which mix modes to equalize power in all the modes, and
mode filters which remove the higher order modes to simulate EMD or steady
state conditions.

These devices are used to condition modal fill in multimode fiber to reduce
measurement uncertainty in testing loss or bandwidth. For more information on loss
testing, see Accuracy.

Cladding Mode Strippers

Cladding mode
strippers are used to
remove any light
being propagated in
the cladding to insure
that measurements include only the effects of the core. Most American fibers are
"self-stripping"; the buffer is chosen to have an index of refraction that will promote
the leakage of light from the cladding to the buffer. If you are using at least 1 meter of
fiber, cladding modes will probably not be a factor in measurements. One can easily
tell if cladding modes are a factor. Start with 10 meters of fiber coupled to a source
and measure the power transmitted through it. Cut back to 5 meters and then 4, 3, 2,
and 1 meter, measuring the power at every cutback. The loss in the fiber core is very
small in 10 meters, about 0.03 - 0.06 dB. But if the power measured increases
rapidly, the additional light measured is cladding light, which has a very high
attenuation, and a cladding mode stripper is recommended for accurate
measurements if short lengths of fiber must be used.

To make a cladding mode stripper, strip off the fiber's buffer for 2 to 3 inches (50 to
75 mm) and immerse the fiber in a substance of equal or higher index of refraction
than the cladding. This can be done by immersing the fiber in alcohol or mineral oil
in a beaker, or by threading the fiber through a common soda straw and filling the
straw with index matching epoxy or an optical gel (Note: stripping the buffer away
from the end of a fiber is easily done, using a chemical stripper. If the fiber cannot be
chemically stripped, like those with Teflon buffers, check with the fiber manufacturer
for instructions.) A caution. Do not stress the fiber after the mode stripper, as this
will reintroduce cladding modes, negating the effects of the mode stripper. Mode
stripping should be done last if mode scrambling and filtering are also done on a fiber
under test.

Mode Scramblers

Mode scrambling is an
attempt to equalize the
power in all modes,
simulating a fully filled launch. This should not be confused with a mode filter which
simulates the modal distribution of a fiber in equilibrium modal distribution (EMD).
Both may be used together sometimes however, to properly simulate test conditions.
Mode scramblers are easily made by fusion (or mechanical) splicing a short piece of
step index fiber in between two pieces of graded index fiber being tested. Simply

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attaching a step index fiber to a source as a launch cable before a reference launch
cable will also work. One can also use methods that produce small perturbations on
the fiber, such as running the fiber through a tube of lead shot. But these scramblers
are difficult to fabricate and calibrate accurately. In the laboratory, they are usually
unnecessary, since accurate launch optics are used to produce fully filled launch
conditions.

Mode Filters

Mode filters are used to selectively remove

higher order modes to attempt to simulate
EMD conditions, assuming that one starts
with fully filled modes. Higher order modes
are easily removed by stressing the fiber in a controlled manner, since the higher
order modes are more susceptible to bending losses.

The most popular mode filter is the "mandrel wrap", where the fiber is snugly
wrapped around a mandrel several times. The size of the mandrel and the number of
turns will determine the effect on the higher order modes. Other mode filters can be
made where the fiber is subjected to a series of gentle S bends, either in a form
or by wrapping around pins in a plate or by actually using a long length of fiber
attached to an overfilling source.

When Do You Use Them ?

Obviously, if you are working in the laboratory measuring fiber attenuation using a
lamp source and monochrometer, you probably need a combination of all of the
above. If you are using a LED or laser source, you might not need any of them, since
they greatly underfill the higher order modes. LEDs and lasers also are the same
mode fill as actual system sources, providing a proper simulation of actual operating
conditions without mode modifiers of any kind.

Testing SM Fiber

Testing single mode fiber is easy compared to multimode fiber. Singlemode fiber, as
the name says, only supports one mode of transmission for wavelengths greater than
the cutoff wavelength of the fiber. Thus most problems associated with mode power
distribution are no longer a factor. However, it takes a short distance for singlemode
fiber to really be singlemode, since several modes may be supported for a short
distance after connectors, splices or sources. Singlemode fibers shorter than 10 m
may have several modes. To insure short cables have only one mode of propagation,
one can use a simple mode filter made from a 4-6 inch loop of the cable.

Bending Losses

Fiber and cable are subject to additional losses as a result of stress. In fact, fiber
makes a very good stress sensor. However, this is an additional source of uncertainty
when making attenuation measurements. It is mandatory to minimize stress and/or

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stress changes on the fiber when making measurements. If the fiber or cable is
spooled, it will have higher loss when spooled tightly. It may be advisable to unspool
it and respool with less tension. Unspooled fiber should be carefully placed on a
bench

and taped down to prevent movement. Above all, be careful about how
connectorized fiber is placed. Dangling fibers that stress the back of the connector
will have significant losses.

(C)1999-2008, The Fiber Optic Association, Inc.

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