Download Lecture 16 - University of St Andrews

Lecture 16 – Joining Fibres and Analysing
Links
Optical Fibres and Telecommunications – Real World Networks
Introduction
• Where are we?
• Joining optical fibres
– Mechanical splices
– Fusion splicing
– Connectors
• OTDR
• Power budgets
Optical Fibres and Telecommunications – Real World Networks
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Last time
• Semiconductor optic amplifiers
• FP and TW designs
– Bandwidth considerations
– Gain ripple
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Cross talk and cross saturation
Polarisation dependent gain
Gain clamping
Real amplifiers
Introduction to real world networks
Optical Fibres and Telecommunications – Real World Networks
Joining Optical Fibres
• Optical fibres are usually made in spans of a few 10’s km.
• This is obviously not a great enough distance in many
cases.
• Fibres must be joined together.
• Matching similar fibres is done by ‘splicing.’
• Matching dissimilar fibres, or fibres with other components
(eg. amplifiers) can be done using connectors.
• Aim is to keep losses as low as possible through this
process.
• First step is normally fibre stripping and cleaving.
Optical Fibres and Telecommunications – Real World Networks
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Cable structure
• Stripping involves removing the coating from the optical fibre.
• Can be done chemically, mechanically or thermally.
• Next step is cleaving the fibre.
Optical Fibres and Telecommunications – Real World Networks
Fibre cleaving
• Edge of fibre is scored or nicked with a metal / diamond blade.
• Fibre is then broken under tension.
• Very good quality, flat faces can be achieved.
• Best possible results are achieved with polishing.
Picture credit: http://www.tpub.com/neets/tm/108-3.htm Optical Fibres and Telecommunications – Real World Networks
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Fibre Splicing
• In a fibre splice two bare fibre ends are joined together.
• Losses can occur in a variety of ways (intrinsic and
extrinsic losses).
• Can either use mechanical (direct butting) or fusion
splicing.
• Loss can be ~ 0.1dB/Splice.
• Careful preparation of the fibre is required.
Optical Fibres and Telecommunications – Real World Networks
Intrinsic Splice Losses
Core Diameter Mismatch
2r2
2r1
Numerical Aperture Mismatch
Loss = 10 log10((r2/r1))2
2θ2
Mode Field Diameter Mismatch
ω1
2θ1
Loss = 10 log10((NA2/NA1))2
ω2
Loss = 10 log10(4/(ω2/ω1+ω1/ω2)2)
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Intrinsic Splice Losses II
• Intrinsic losses occur due to differences in the two
fibres being spliced.
• Can have a combination of these effects.
• Worst case scenario – splicing single mode to
multimode fibre. Loss > 20dB !
• Have to ensure the fibres are well matched.
Optical Fibres and Telecommunications – Real World Networks
Extrinsic Splice Losses
Lateral Misalignment
All formulas are for single mode fibre !
x
End Separation
Loss = 10log10(exp(-x/w0)2)
z
Angular Misalignment
Loss = 10log10[1/(S2+1)]
where: S=λz/(2πnw0)
θ
Loss = 10log10(exp(-nπw0sin(θ)/λ)2)
Optical Fibres and Telecommunications – Real World Networks
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Extrinsic Splice Losses & Reflection Losses
• Extrinsic losses are caused by imperfections that can be
eliminated.
• Misalignment losses are illustrated in the previous slide.
• Also problems with bad cleaves, dirt, errors in fibre
fabrication.
• Reflection losses are caused by the air space that may
remain between the fibres. Total loss is called the Fresnel
loss.
• Loss=-10log10(1-R) where is given by the Fabry-Perot
reflection of the airspace, length z:
– R=R1+R2-2(R1R2)0.5cos(4πz/λ)
– R1, R2 are the reflections of the fibre/air interfaces.
Optical Fibres and Telecommunications – Real World Networks
Making Splices - Mechanical
v-groove
• Fibres are aligned in specially machined
v-groove.
• Index matching fluid applied to fibre
tips.
• Splice is covered.
• Fibres are placed in a capillary tube.
• Index matching fluid can be applied.
• Fibre rotated until maximum signal
power observed
• Use mechanical splices where a
relatively low number of splices
required.
• Relatively low skill level required.
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Making Splices – Fusion Splicing
Graphic Credit: www.corningcablesystems.com
Optical Fibres and Telecommunications – Real World Networks
Aligning fusion splices
Can use video
camera to align
splice.
Or use output power – bend loss !!
Graphic
GraphicCredit:
Credit:
www.corningcablesystems.com
www.corningcablesystems.com
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Fusion Movies
Fusion Splice
Alignment Process
Movies from: http://floti.bell.ac.uk/MATHSPHYSICS/splicing.htm
Optical Fibres and Telecommunications – Real World Networks
Factors affecting a good splice.
Graphic Credit: www.corningcablesystems.com
Splice loss can be <0.1dB !!!
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Connectors
• In some situations cleaving is not suitable.
• eg. Temporary joins, joining to standardised
equipment.
• In this case use connectors – ‘Pulg and Play.’
• Tend to suffer from higher losses than good quality
cleaves.
Picture credit: http://www.tpub.com/neets/tm/108-9.htm
Optical Fibres and Telecommunications – Real World Networks
Making a connector
• Stripped optical fibre is place in ceramic capillary.
• Fibre is then glued into place.
• Fibre tip is polished back to the top of the ferrule.
Picture credit: http://www.tpub.com/neets/tm/108-9.htm
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Connector Losses
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Loss occurs in much the same way as for splices.
Need to ensure connectors are kept clean.
Connector loss normally ~0.5-1dB.
Staff need little training in operation.
Can add to network costs.
Optical Fibres and Telecommunications – Real World Networks
Analysing the link - OTDR
• In order to characterise our link and find connector / splice
losses we can use Optical Time Division Reflectometry
(OTDR).
• This technique sends pulses of light down the fibre and
looks at the reflected and backscattered light from the fibre.
• This information allows a plot of intensity versus distance to
be performed for the link.
• Can use this information to perform fibre link analysis over
distances of 100’s of km.
• Broken fibres are easily spotted.
Optical Fibres and Telecommunications – Real World Networks
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OTDR Plot
Optical Fibres and Telecommunications – Real World Networks
Understanding OTDR I
Single fibre with no
splices / connectors
Dead Zone
Beginning of a fibre
Dead zone gives short distance limit.
Picture Credit www.agilent.com
Optical Fibres and Telecommunications – Real World Networks
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Understanding OTDR II
End of the fibre
Fibre Break
Picture Credit www.agilent.com
Optical Fibres and Telecommunications – Real World Networks
Understanding OTDR III
Fusion Splice
Connector
Picture Credit www.agilent.com
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Building a Network – The Power Budget
• In order for any fibre-based system to work, it’s essential
that enough power is received at the receiver to allow
signal detection.
• The amount of power received ABOVE the receiver
sensitivity is called the System Margin.
• If the power received is less than the sensitivity then the
amount of power BELOW the sensitivity is called the
System Deficit.
• Note that there is sometimes sign confusion over the
system deficit.
• In order to calculate an optical power budget, we must take
into account all of the sources of power and loss in the
system.
Optical Fibres and Telecommunications – Real World Networks
Point to Point Link
Tx
Rx
LED Transmitter P1550nm=-20dBm
30km Fibre in 10km spans: Loss = 0.2dB/km. Splice Loss = 0.2dB
Connector loss = 0.8dB
Receiver sensitivity = -30dBm
Calculate the system margin.
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Point to Point Link II
Rx
Tx
Power in:
-20dBm
Connector loss:
-0.8dB
Fibre loss:
30x0.2dB=-6dB
Splice loss:
2x0.2dB =-0.4dB
Connector Loss:
-0.8dB
System Margin:
-2828-(-30)dBm
-20dBm
-20.8dBm
-26.8dBm
-27.2dBm
-28.0dBm
2dB
System will function, but system margin is low – need to aim for 5-10dB
Optical Fibres and Telecommunications – Real World Networks
Point to Point Link III
Rx
Tx
It is now decided to increase the link to 60km – what is the
system margin ?
Power in:
-20dBm
-20dBm
Connector loss:
-0.8dB
-20.8dBm
Fibre loss:
60x0.2dB=-12dB
-32.8dBm
Splice loss:
5x0.2dB =-1dB
-33.8dBm
Connector Loss:
-0.8dB
-34.6dBm
System Deficit:
-34.634.6-(-30)dBm -4.6dB
System Cannot Operate !!!
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Point to Point Link IV
Rx
Tx
One answer could be to replace the LED with a laser.
Output Power = -5dBm.
Power in:
-5dBm
-5dBm
Connector loss:
-0.8dB
-5.8dBm
Fibre loss:
60x0.2dB=-12dB
-17.8dBm
Splice loss:
5x0.2dB =-1dB
-18.8dBm
Connector Loss:
-0.8dB
-19.6dBm
System Margin:
-19.619.6-(-30)dBm 10.4dB
This is much healthier!
Optical Fibres and Telecommunications – Real World Networks
Homework1:
a. Calculate the system margin (deficit) for a 300km fibre link,
made up of 10km fibres (Loss=0.25dB/km). Signals go through
two connector pairs in a patch panel at each end. Assume
splice loss=0.1dB/splice. Laser power = 0.0dBm. Receiver
sensitivity =-32.0dBm.
b. In order to improve performance, it is decided to place
amplifiers with a gain of 30dB at the 100km and 200km points.
The amplifiers are placed in the link using one connector at
each end (loss=0.8dB/connector.) Calculate the system
margin (deficit) in this case.
1. Example from Understanding Fiber Optics, Hecht Optical Fibres and Telecommunications – Real World Networks
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Conclusions
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Joining and cleaving optical fibres
Factors affecting splices
Fusion and mechanical splices
Connectors
OTDR plots
Power budgets
– System margin
– System deficits
• Power budget calculations
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