Fiber characterization

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Fiber characterization
Gwenn Amice Senior Applications
engineer gamice_at_exfo.com
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First thing first Connectors cleaning and
inspection
Follow approved equipment and procedures
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Testing Loss and ORL
The fiber inspection probe your best friend!
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dB vs. dBm

• dB
• Measure IL and ORL
• Relative value
• Defined as 10Log(Po/Pi)
• e.g.
• -3dB is 50 loss
• -10dB is 90 loss, 10 pass
• -30dB is 99.9 loss, 0.1 pass

• dBm
• Measure optical power
• Absolute value
• Defined as 10Log(P), P in mW
• 3dBm is 2mW
• 10dBm 10mW
• 30dBm is 1000mW, or 1W

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Reflectance vs. Connectors
Testing Loss and ORL
weather proof
SC/APC
SC/UPC
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ORL (dB)
Testing Loss and ORL

• ORL comes from the amount of energy lost within
  the components and fiber due to back reflections.
• We use the term ORL when referring to the amount
  of energy returned by a section or an entire
  link.
• ORL is expressed as a positive value.

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ORL (dB)
Testing Loss and ORL
Some examples
Link ORL 35 dB
Section 2 and 3 ORL 45 dB
Connector reflectance -50 dB
Mechanical splice reflectance -50 dB
Connector reflectance -55 dB
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ORL Testing
Optical Return Loss Testing is vital to ensure
the system is not reflecting too much power.
Exfo provides several test solutions for Optical
Return LOSS.
ITU Recommended ORL OC-3 20 dB OC-12 20 / 24
dB OC-48 24 dB OC-192 27 dB FTTx 32 dB with
Analog Video
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What is LOSS Testing?

• Optical Loss Testing is a test performed to
  determine the loss in energy within a device or
  fiber
• The Loss is a result of attenuation from
  absorption, scattering, microbending,
  macrobending, connectors, splices, and
  discontinuities in the fiber span or device.

insertion loss dB 5 dBm Pin dBm
insertion loss dB 5 dBm 1 dBm
insertion loss dB 5 dBm 1 dBm 4 dB of
LOSS!
insertion loss dB Pout dBm Pin dBm
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Fiber Intrinsic Loss
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Fiber Loss
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Basic Testing -Loss Budget Optical Return Loss
(ORL)

• Prior to each test session, a reference must be
  taken

• The side-by-side referencing method is
  recommended as it gives a better accuracy.
• The detectors used during the side-by-side
  reference, are the same used during FasTest
  testing sessions
• Requires both units at the same location
• The LoopBack referencing is used when units are
  at remote location
• The external detector is used which adds an extra
  uncertainty

Side-by-side referencing
Loopback referencing
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Basic Testing -Loss Budget Optical Return Loss
(ORL)

• IL and ORL testing using the FOT-930

Step 1 Side-by-side referencing
Master test jumper
Master test jumper
Bulkhead adapter
Step 2 Automated IL ORL bidirectional testing
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Result in FOT-930
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What does an OTDR do?

• Provides a detailed map of each fiber
• Gives location and type of each event
• Loss and Reflection reading from each event
• Attenuation of each span
• Total Reflection of the system (ORL)

See notes for details
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Reflectometry theory

• Rayleigh Backscattering
• Comes from the Natural reflection of the fiber
• The OTDR will use the Rayleigh back reflections
  to measure fibers attenuation (dB/Km)
• Back reflection level around -75 dB
• Higher wavelength will be less attenuated by the
  Rayleigh Backscatter

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Reflectometry theory

• Fresnel back reflections
• Will come from abrupt changes in the IOR, ex
  (glass/air)
• Fiber break, mechanical splice, bulkheads,
  connectors
• Will show as a spike on the OTDR trace
• UPC reflection is typically -55dB and APC -65dB
  (as per ITU)
• Fresnel reflections will be approximately 20 000
  times higher than fibers backscattering level
• Will create a  Dead Zone  after the reflection

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OTDR specifications limitations - Pulse width
vs. Dead zones and Dynamic range

• Short pulses will give a better resolution but
  less dynamic range

Two connectors 3 meters apart
End of link (patch panel)
Connectors are measured for distance and marked
as separate events
5ns pulse
End of fiber is not reached due to low power of
short pulses
Long pulses will give a better dynamic range but
less resolution
Connectors are  merged  and identified as one
event
30ns pulse
End of fiber is reached and located when using a
larger pulse
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Reflectometry theory - Loss in fiber is
wavelength-dependent
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Singlemode gainers
When testing fibers with different geometry (due
to mismatch of fiber types or due to
manufacturing process variability) excess loss
and gainers can be seen on OTDR traces.
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Echos on OTDR traces
Echos are more frequent in multimode because of
high reflectance connectors.
In this example the echo is located at twice the
distance of reflectance 2.
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Pulse suppressor box (PSB)
Link budget FUT (excluding connectors A and B)
OTDR
A
B
FUT
Link budget connector A FUT
OTDR
A
PSB
B
FUT
Link budget connector A FUT connector B
OTDR
A
PSB
B
FUT
PSB
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Introduction to the chromatic dispersion concept
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Reflective index f(?)
The speed of the light in the optical fibre
cannot be higher than the speed of the light in a
vacuum. This ratio is named Reflective index n
c / v c Speed of the light in a vacuum
2.99792458 10 exp 8 m/s v Typically around
2.068 10 exp 8 m/s n around 1.45 as a function
of the wavelength
Ref http//www.fiber-optics.info
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Chromatic dispersion issuesDifferent wavelengths
different velocities
The slope of this
?? zero dispersion wavelength
Gives this
Pulse delay (ps)

S0 slope at zero dispersion
??(nm)
?
Chromatic Dispersion (ps/nm km)
_
??
??(nm)
zero dispersion wavelength
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Fiber Attenuation and Dispersion
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Different sources Differents behaviors

• Light sources finite spectral power
  distribution
• Source wavelengths do not propagate at the same
  velocity (group velocity) as they see different
  material and index structure and arrive at
  different times
• A pulse transmitted in such medium suffers a
  spread, a dispersion, limiting the
  transmission bandwidth.

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Chromatic or intra-modal dispersion

• A pulse transmitted in such medium suffers a
  broadening, a dispersion, limiting the signal
  transmission bandwidth.

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Dispersion in a transmission
OC-12 gt633Mbps gt T 1608 ps gt 10160ps
GIGE gt1.25Gbps gt T 800ps gt 1080ps
OC-48 gt2.488Gbps gt T 402 ps gt 1040ps
OC-192 gt9,953Gbps gt T 100.5 ps gt 1010ps
10GigE WAN gt9.953Gbps gt T 100.5 ps gt
1010ps 10GigE LAN gt10.3Gbps gt T 97
ps gt 109.7ps OTN gt10.709Gbps gt T 93.4
ps gt 109.3ps OC-768 gt39.808Gbps gt T
25.1 ps gt 102.5ps
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Dispersion in a transmission
A pulse is send in a SMF28 fiber The source a DFB
laser Linewith7pm, central wavelength 1550
nm, The network is a wdm network with 7
EDFA Fiber 6100Km, CD17ps/nm.km_at_1550nm
A the laser output, the delay is t1Depend of
the opto-electronic t2 will be
t1(17ps/nm.km 7pm 6 100Km) 71.4ps!! It
means that the information will be somewhere _at_
/- 36ps around the synchronisation clock If you
transmit an OC-12 the dispersion will be around
2 OC-48 gt 9 border line !!! OC-192 gt
36 Forget it !! Without CD compensation the
maximum _at_ OC-192 is 140 Km with G.652 fiber (99
of the core network)
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Next generation networks challenges
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Tolerance to CD versus Bit Rate
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DSF Fiber

• ?0 1547.754 nm

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Compensating fiber (DCM)
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Introduction to the PMD concept
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Polarization

• This electric field can be described
• horizontal linear polarization state along the
  X-Z plane
• vertical linear polarization state along the Y-Z
  plane
• For polarized light, these two modes are
  propagated through the fibre. They are orthogonal
  and do not interfere. As they propagate, however,
  they become separated.

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Power Splitting Ratio

• If the polarization of the source is at an angle
  ?
• Relative to the PSP, the intensities of the two
  modes, E0x and E0y, are equal to
• E0x E0 cos ?
• E0y E0 sin ?

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Polarization Mode Dispersion PMD
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DGD
The PSPs components do not travel at the same
speed along the axis of propagation (Z) in
relation with the physical characteristics of the
fibre (geometry, variation of the IOR, etc.).
This delay is called Differential Group Delay
DGD. As they travel at different speed, we use to
call them Fast and slow axis or modes.
T
t
z,t
slow axis
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PMD Transmission EffectsRandom Mode Coupling
b birefringence ps/km L fiber length km h
coupling length km
PMD increases with square root of distance
t
Long cabled fiber deployed in the field is an
example
z,t
Dt
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PMD and DGD
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Which Fiber is better ?
PMD Max. Coefficient depends also on the cable
manufacturer

• Source John Peters, Ariel Dori, and Felix
  Kapron, Bellcore

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Which Fiber is better ?
PMD Max. Coefficient depends also on the age of
the cable

• Source John Peters, Ariel Dori, and Felix
  Kapron, Bellcore

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Dispersion in a transmission

• OC-12 gt633Mbps gt T 1608 ps gt 10160ps
• GIGE gt1.25Gbps gt T 800ps gt 1080ps
• OC-48 gt2.488Gbps gt T 402 ps gt 1040ps
• OC-192 gt9,953Gbps gt T 100.5 ps gt 1010ps
• 10GigE WAN gt9.953Gbps gt T 100.5 ps gt
  1010ps
• 10GigE LAN gt10.3Gbps gt T 97 ps gt
  109.7ps
• OTN gt10.709Gbps gt T 93.4 ps gt 109.3ps
• OC-768 gt39.808Gbps gt T 25.1 ps gt 102.5ps

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PMD effects on the eye diagram
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Digital transmissionsPMD specifications
Proposed PMD coefficient for a 99.994
probability that the power penalty will be less
than 1 dB for 0.1 of the bit period
Bit rate (Gb/s)
Average DGD (ps)
PMD coefficient 400 km fiber (ps/km½)
2.5 10 20 40
40 10 5 2.5
? 2.0 ? 0.5 ? 0.25 ? 0.125(or 25 km with 0.5
ps/km1/2)
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Digital and Analog TransmissionsPMD
specifications

• PMD Max. Coefficient depends on transmission type
  (analog or digital) and transmission speed
• Distance (Km)1/2 x PMD Coefficient PMD Total

Source John Peters, Ariel Dori, and Felix
Kapron, Bellcore
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EXFO Fiber Characterization Solution

• With the Exfo FTB-200 Universal Test System
  complete
• Fiber Characterization can be performed easily.

• FTB200 Modular Test Platform
• OTDR Module
• Loss Module and handheld unit
• Single ended CD/PMD Module

• CD PMD Testing
• Single ended
• Fast and reliable
• Cost effectivecompact

• OTDR Testing
• Length Measurement
• Splice/connector characterization
• Macro-bend testing
• 15 second test time per fiber

• Loss/ORL Testing
• Power Loss Measurement
• ORL Measurement
• 10 second test time

Full Physical Layer Testing Suite Using the
FTB-200 Modular Test Set
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EXFO Fiber Characterization Solution

• With the Exfo FTB-400 Universal Test System
  complete
• Fiber Characterization can be performed easily.

• FTB400 Modular Test Platform
• OTDR Module
• Loss Module and handheld unit
• CD Module and Source
• PMD Module and existing Source

• OTDR Testing
• Length Measurement
• Splice/connector characterization
• Macro-bend testing
• 15 second test time per fiber

• Loss/ORL Testing
• Power Loss Measurement
• ORL Measurement
• 10 second test time

• PMD Testing
• Enter Length Measurement
• PMD Measurement

• CD Testing
• Input length measurement from OTDR
• CL Band

Full Physical Layer Testing Suite Using the
FTB-400 Modular Test Set
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Thank you!