Consequences of Stress
Stress
Optical Communications Systems
Bending Loss and Reliability in Optical Fibres
Increased Loss in the
Fibre
Increased Probability of
Failure
Bending Loss in Fibres
yAt a bend the propagation conditions alter and light rays which would
propagate in a straight fibre are lost in the cladding.
yMacrobending, for example due to tight bends
Attenuation:
Bending Loss
yMicrobending, due to microscopic fibre deformation, commonly
caused by poor cable design
Microbending is commonly
caused by poor cable
design
Macrobending is commonly
caused by poor installation
or handling
Ray Diagram View of
Macrobending
Mode Field View of
Macrobending
yRecall that macrobending is caused typically by poor handling or installation.
yMode field view is more accurate but harder to visualise, a must for singlemode
yRay diagram view used with multimode fibre provides approximate explanation.
yIn a fibre a wavefront perpendicular to the direction of travel must be maintained
yAt a sharp bends light rays which propagate by TIR on straight fibre are lost into
the cladding.
yResult is optical power loss and thus attenuation.
yAt a sharp bend the outer part of the mode field must travel faster than the inner part
to maintain the wavefront
yThus outer part of mode field may be forced to travel faster than the velocity of light in
the material
yAs this is not possible the energy in the outer part of the mode field is lost through
radiation
Cladding
At a bend loss
Loss of a
portion of the
mode field at a
sharp bend
occurs where
TIR fails
Power lost via
radiation from
cladding
Core
Mode field
Macrobending in Multimode
Fibre
yCritical radius is the bend radius below which loss increases rapidly
yCritical radius of curvature Rc for multimode fibre is given approximately by:
Rc =
3 n 12 λ
4π n 12 − n 22
Macrobending in Singlemode
Fibre
yIn a singlemode fibre as the spot size or mode field radius (MFR) increases the
loss at a bend increases
yQualitatively this is because a greater proportion of the mode field is lost if the
MFR is larger
yFull analysis of loss is complex and beyond the scope of current discussions
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Low MFR = Lower Loss
Larger MFR = Higher Loss
Cladding
yLoss can be reduced by using larger refractive index differences
Power lost via
radiation from
cladding
Core
yFor a given bend radius a larger NA will result in a lower Rc and thus lower loss
yWhile Rc is influenced by wavelength it is found that above Rc the loss is not a
a strong function of wavelength (multimode fibre only)
Cladding
Mode field
More power lost
via radiation
from cladding
Core
Mode field
Quantifying Macrobending in
Singlemode Fibre (I)
Quantifying Macrobending in
Singlemode Fibre (II)
Influence of Mac# on loss in dB/m at 1320 nm
yMacrobending can be characterised in SM fibres by the empirical formula:
Loss = exp 8.5 - 519 x Dmm
1
λ x Mac#
3
dB/m
yThe Mac# (Macrobending Number) is a function of the MFR and the "effective fibre
cutoff wavelength λce":
Mac# = 2 x MFR
λ ce
Quantifying Macrobending in
Singlemode Fibre (III)
yThe higher the operating wavelength above the cutoff wavelength the
lower the V-value
yA lower V-value means a larger MFR
ySo for longer wavelengths the MFR and thus the loss increases
yThus the loss due to bending can be expected to increase at 1550
nm relative to 1330 nm
yTypical Mac#'s in singlemode fibre are 8-9 and >10 in so called weakly
guiding fibres
Quantifying Macrobending in
Singlemode Fibre (IV)
Influence of wavelength on loss in dB/m for a Mac# of 9
Bending Loss Tests for Cables
Microbending in Fibres
yMinimum bend radius for a cable is typically 10 to 20 times the outer
diameter of the cable.
yCommon value used in Cabling Standards is 15 times the cable diameter
Microbending in Fibres
yMore critical than macrobending
yDue to processing rather than
mishandling.
yLoss can occur due to distortion of
the
core
cladding
interface,
induced by manufacture or poor
cable design
Fibre Reliability
yFibre is intrinsically very reliable in a benign environment
yFew documented failure mechanisms
yMost failures are caused by poor cable choice, poor installation or accidental
damage
yIntrinsic tensile fibre strength exceeds that of an equivalent steel wire
Fibre Reliability
yTheoretical strength is 20 GPa (2,900,000 Psi)
yDue to surface defects such as cracks strength in practice is much lower, typically 5
GPa (725 kPsi)
Fibre showing
surface cracks
and flaws
(exaggerated)
1kPa = 0.145 Psi
Fibre Proof Testing
Crack and Flaw Growth
yWeak fibres are those with large surface defects after production
yAll produced fibres are proof tested after production
yTypical proof test stress is three times normal service maximum
yFailure occurs when under stress a crack grows to some critical dimension
yCrack growth is depended on the so-called fatigue susceptibility parameter, "n"
yLarger values of n mean faster crack growth, shorter lifetime
yStress accelerates crack growth
Simplified proof
test apparatus
yMoisture and high temperatures also accelerate crack growth and reduce
lifetime
Minimum Time to Failure
Fibre Failure Examples (I)
yPhoto shows an end view of a
failed fibre
yMost important parameter for cable designers
yMagnification is 2000x
yThe time to failure tf is given by:
yAssume cable is under a constant stress "s"
yFailure caused by small flaw
on the fibre surface
yTwo distinct areas visible:
•
•
Smooth area near flaw were
crack propagated quickly but
cleanly
Jagged area were fibre failed
completely
tf =
As
-n
yA is constant and n is the fatigue susceptibility parameter (15 to 50 for glass,
typically 20)
yAs stress grows the time to failure drops rapidly
Problem:
For n = 20 develop an argument to show that a stress "s" applied for 1
second is equivalent to a stress of 0.35s applied for 40 years
Effect of Moisture and
Temperature
Proof Testing Results
Proof test stress
Maximum flaw
size
Predicted lifetime at
maximum service stress
Effect of Moisture
yMoisture does not penetrate silica glass, so it does not affect propagation
yPresence of water as OH ions on the fibre surface accelerates crack growth
Typical industry
test
50 kPsi
(0.35 GPa)
2.3 micron
30 Years
Higher reliability
tests
100 kPsi
(0.7 GPa)
0.7 micron
>>100 Years
yThis process is called stress corrosion
yMoisture protection is important in fibre cables
Effect of Temperature
yHigher proof test stress means longer lifetime
yAt 90 degrees centigrade the fatigue susceptibility parameter is significantly
higher than that at 25 degrees
yBut higher stress means more fibres are rejected, lower yield/higher cost
yFibre strength decreases by 25% at 90 degrees compared to 25 degrees
yLifetimes assume no moisture ingress and normal temperatures
yHigh tensile strength and zero moisture ingress cables are essential at elevated
temperatures