What is Dispersion?

Transcription

What is Dispersion?

Fiber Characterization for High Speed Networks
Tony Lowe
Exfo America
1
© 2009
2008 EXFO Electro-Optical Engineering Inc. All rights reserved.
Introduction
Telecommunications service providers, due to increased demand for
information, video conferences and downloading of music and movies,
are facing continuously growing bandwidth demands in all network
areas, from long-haul to access.
The Telecommunications Industry Association (TIA) states that as of
2007, there was 13.1 million miles of fiber installed. A large portion of
the installed fiber is old and exhibit physical characteristics that may
limit their ability to transmit high-speed signals.
How do service providers deal with the need for increased bandwidth?
How do determine the limitations in our fiber plant?
2
© 2009
Ways to Increase Bandwidth
More fiber
 Extremely expensive, especially when high Bandwidth is required
More WDM
 Most systems are designed with 50GHz, and are more than 50% lit up.
Bit-Rate
(Urban Areas)
Not a lot of place for growth
 40G with improving economics as line-card cost decreases
 Develop new modulation schemes (DPSK, DQPSK)
 Offers 4x Bandwidth
3
Mike Andrews
© 2008 EXFO Electro-Optical Engineering Inc. All rights reserved.
Exploring the Limitations of the Fiber
What is fiber characterization?
Fiber characterization is the evaluation of installed fiber against a specified requirement.
What is the ITU-T G.650.3 idea of fiber characterization?
“A comprehensive suite of measurements that is carried out on an optical fiber cable link to
determine the key performance attributes of that link which may effect current of future
applications that operate over that link.”
What does fiber characterization tell us?
It gives information about the capability of the network for the current application
It provides data for future upgrades since legacy fiber will eventually be
unable to support higher bit rates or additional wavelengths
When is fiber characterization usually performed?
Normally done to commission new optical fiber cable links
May be carried out to satisfy service level agreements
May be done to verify attributes of older links that my be used at higher bit rates
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Fiber Characterization
What tests make up fiber characterization?
IEC G.650.1 and G.650.3 defines these tests make up fiber
characterization:
Insertion Loss
Optical Return Loss (including reflectance)
Connector End Face Inspection
OTDR Trace
Chromatic Dispersion (CD)
Polarization Mode Dispersion (PMD)
5
Introduction
Due the increase demand for information and more bandwidth over the past 30 years, network operators
have been forced to upgrade their systems and install new hardware. In doing this, we have realized that
there are limitations in the fiber due to physical properties of the material.
Since the early 1970’s, we have seen this growth develop due to:
1. The introduction of low loss fiber (0.2 dB/km now)
2. The development of the erbium-doped fiber amplifier (EDFA) to extend the signals
generated, but that also introduced us to chromatic dispersion (CD)
3. Once it was determined that CD could be compensated for, higher bit rate signals began
to be transmitted and they discovered polarization mode dispersion (PMD). At that time, the
bit rates were so low, this was considered to be ―not significant‖. It was not until bit rate
systems generating 10 Gbps that it became ―significant‖.
4. The development of dispersion shifted fibers (DSF) also helped with the CD issue, but it
was discovered that since the core size in DSF was smaller than standard fiber, it made is
more sensitive to core ellipticities. That in turn resulted in fibers with high birefringence and
consequently, high PMD.
5. Wavelength-division multiplexing (WDM) was also introduced to increase the utilization
of the fiber bandwidth. Multiple lower bit rate systems on one fiber was the goal and this
helped avoid the PMD issue. But, that introduced another issue. Due to CD, four wave mixing
(FWM), which induces crosstalk between channels, was discovered. NZDSF fibers created.
6. Now, to avoid or work around the physical issues in the fiber, advanced modulation
schemes have been developed (DPSK and DQPSK for example). Example: sending a 40Gig
signal down the fiber like 4 X 10Gig
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Testing Credibility
Testing requirements are changing. Technicians are now expected to get more
information from their OTDR traces and dispersion results. Just knowing the fiber
length is not enough. Manufacturers, Engineers and Managers need to be able to
interpret the measurements made by the OTDR.
The data obtained from the test equipment must be:
Credible
Capable of Documentation
Technicians need to be able to look at the data obtained by the OTDR and dispersion
equipment and make decisions based on the fact that:
1. Optical loss min/max windows at the receiver are tighter making even the smallest macrobend
unacceptable
2. Lasers and networks becoming less tolerable with optical return loss.
3. Dirty connectors and poor reflectance events can take a network down.
4. Attenuation levels in older cables must be realized for WDM and high speed networks.
5. Launch conditions at the OTDR can jeopardize the credibility of the trace data
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Optical Loss Testing
Once a fiber span is built, an optical loss test is done to ensure that adequate
signal strength is available at the receiver. But before a loss test is done, at the
engineering stage, a loss budget will be calculated based on the minimum
output of the transmitter and the minimum sensitivity of the receiver.
Typical Fiber Link
Fusion
Splice
Bend
Connector
Pair
Tx
Fiber
End
Rx
Mechanical
Splice
Maximum output: -3 dBm
Minimum output: -9 dBm
There are two sources of loss
that make up the total loss
number.
Crack
Maximum input: -3 dBm
Minimum input: -20 dBm
Attenuation
Insertion
Loss
Optical
Loss
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Optical Return Loss (ORL)
The measurement of ORL is becoming more important in the characterization of
optical networks as the use of WDM and high speed data increases. These
systems use lasers that have a lower tolerance for reflectance.
ORL is a measure taken from one end of the total energy reflected back to the source
by all the interfaces due to a variation of the index of refraction (IOR), breaks,
voids, backscatter, etc., created inside a component or along a link.
It is expressed as a positive value.
Power in
(dBm)
Power
Reflected
(dBm)
Optical
Return
Loss (dB)
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Optical Return Loss (ORL)
What contributes to ORL?
Rayleigh
Backscatter
Fresnel
Backreflection
Optical Return
Loss (ORL)
Rayleigh backscattering: intrinsic to the fiber and cannot be completely
eliminated.
Fresnel backreflections: caused by different network elements (mainly
connectors and components) with air/glass or glass/glass interfaces
and can always be improved by special care or better design.
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ORL Summary
Poor ORL can cause:
1.
2.
3.
4.
5.
Strong fluctuations in laser output power
Instability in the laser due to temperature increase
Receiver interference
Lower signal to noise ratio
Higher BER
OC-48
2.5Gig
• 24 dB
OC-192
10Gig
• 27 dB
OC-768
40Gig
• 30 dB
FTTX Video
• 32 dB
The higher the ORL value, the better the network will perform
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OTDR Definition and Overview
OTDR – Optical Time Domain Reflectometer
How does the OTDR acquire data and create a trace?
OTDRs launch short duration light pulses into a fiber and then measures, as a function of
time after the launch, the optical signal returned to the instrument. As the optical pulses
propagate along the fiber, they encounter reflecting and scattering sites resulting in a fraction
of the signal being reflected back in the opposite direction.
Raleigh scattering and Fresnel reflections are physical causes for this behavior. By
measuring the arrival time and amplitude of the returning light, the locations and
magnitudes of faults can be determined and the fiber link can be characterized.
The OTDR has been used, and is today by many, to test the fiber length and determine if
there are any broken fibers in the span.
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The Complete Fiber Characterization Suite
13
What is Dispersion?
What is the definition of dispersion?
Dispersion is spreading or broadening.
“All types of dispersion degrade the modulation-phase relationships between optical signals
reducing information carrying capacity through pulse broadening.”
Designers of optical networks must cope with three basic forms of dispersion:
Intermodal (Multimode)
Chromatic Dispersion - CD
Polarization Mode Dispersion - PMD
14
CD
Ensuring Network Integrity
Chromatic
Dispersion
15
Index of Refraction (IOR)
The velocity at which light travels through in a material is determined
by the refractive index of that material. The refractive index (n)
represents the ratio of the velocity of light in a vacuum to the
velocity of light in a material.
c
_
n= v
Speed of light in a vacuum (299,792,458 meters/sec)
Speed of light in the material
Water
1.33
Diamond
2.4
1310 nm
1.4677
1550 nm
1.4682
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Chromatic Dispersion
Chromatic Dispersion is a mathematical way of describing the fact that different wavelengths
travel at different speeds in optical fiber.
It takes into account the fact that laser sources are spectrally thin, but not monochromatic.
This means that the input contains several wavelength components traveling at different
speeds causing the pulse to spread (disperse).
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Fabry-Perot Laser Output
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All fiber has the property that the speed (IOR) an optical pulse
travels depends on its wavelength. This is caused by several
factors including material dispersion and wave guide dispersion.
The net effect is that if an optical pulse contains multiple
wavelengths (colors), then the different colors will travel at
different speeds and arrive at different times, smearing the
received optical signal.
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Cause 1: Material Dispersion
In a prism, material
dispersion (a wavelengthdependent refractive index)
causes different colors to
refract at different angles,
splitting white light into a
rainbow.
In optical fiber, material
dispersion (a wavelengthdependent refractive index)
causes different colors to
travel at different speeds,
leading to blurring of optical
pulses. The longer the link,
the greater the blurring.
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Cause 2: Waveguide Dispersion
Waveguide dispersion is caused by the wavelength dependent
relationship between the optical wave and the core (mode field)
diameter as well as the difference of refractive index between the
core and the cladding
If you could view a photon travelling
through space from the front or back and
could “see” the field - you would see that it
has a diameter. This field “diameter” at
1310 nm is ~8.25 microns – hence the
core size for standard single-mode optical
fiber.
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Dispersion and Inter-Symbol Interference (ISI)
Inter-symbol interference
(ISI) in pulse transmission due
to chromatic dispersion – is
caused by overlap between
sequential symbols or bits.
The more wavelengths a
pulse contains (meaning wider
spectral width), the more it
can be impacted, overall, by
Chromatic Dispersion.
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(ps/nm-km)
Chromatic Dispersion - Optical Fibers
+
+
dispersion unshifted G.652
+

dispersion shifted G.653

non-zero
dispersion





non-zero dispersion shifted G.655


(nm)
When you look at this chart, it shows two interesting pieces of information:
1. This shows the level of typical dispersion per km for the specific wavelength.
Example: G.652 fiber has typical +17 ps/nm/km at 1550 nm.
2. This graph also shows where there is zero dispersion at which wavelength.
Example: G.652 fiber has zero dispersion around 1310 nm.
Is this information presented on the GUI of the test gear? Absolutely
23
Chromatic Dispersion – Relative Group Delay
Given that even closely space wavelengths travel at slightly different speeds in the optical fiber, it is
inevitable that digital pulses will tend to spread out over time (disperse). Too much dispersion results in
adjacent pulses overlapping. This leads to an inability to recover the data at the optical receiver.
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Chromatic Dispersion - Compensation
Chromatic dispersion can be adjusted by adding specialty optical fiber or special Bragg Grating devices that
help regroup the original wavelengths and somewhat reshape the pulse. Adding dispersion compensation
to optical fiber routes allows operators to send faster data rates over longer distances before chromatic
dispersion adversely affects the data.
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GUI shot – FTB-5700
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Fiber Types and CD at 1550 nm
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A few CD Facts
Chromatic Dispersion is Linear, and fairly predictable
Data Capacity
Feature Richness
• Standard fiber has between 15-20 ps/nm/km in the C-Band
• The OC-192 limit is around 1176ps/nm (60 - 80km)
• The 10GigE limit is 738ps/nm (37.5 – 50km)
• The OC-768 limit is 150ps/nm (7.5 – 9km)
•So what do we do if we exceed the CD limit?
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Chromatic Dispersion Compensation
Fortunately, CD is stable, predictable and controllable. Since this is true,
CD can be compensated. The most used compensation is dispersion
compensation fiber (DCF). It is inserted into the link at regular intervals to
minimize the overall CD.
These DCF spools are adequate to solve the CD issue for one channel,
but when you have a DWDM system, it is difficult to compensate for all
the channels. This created the need for tunable compensation modules.
29
Short Links, long travelling distances
Deployment
Service Assurance
Data Capacity
Feature Richness
Design
30
Dispersion Compensation
How does “Dispersion Mapping” help the customer?

Dispersion compensation must be managed per wavelength as opposed to using
broadband compensators.

Formerly, broadband compensating fibers could be used that were adequate for 1 to 10
Gbit/sec traffic since tolerances were relatively high.
With the much more stringent dispersion thresholds of 40 Gigabit/sec, a broadband
compensator cannot adequately compensate lambdas near the “edges” of the DWDM
spectrum.

31
Solutions for complex topology dispersion issues
• For chromatic dispersion, since a wavelength can come and go to/from different networks, compensating at the
receiver end proves extremely complex, if not impossible. This is why ROADMs typically have integrated
dispersion-compensating fibers (DCF) after the line amplifiers and before the switch itself. This ensures that all
wavelengths, whatever their origin and destination, are relatively well compensated for. (Audet, et al.)
• Longer links with several cascaded ROADMs may require an additional tunable compensator at the receiving
site to make up for the leftover dispersion caused by the inexact specifications of the DCF across all
wavelengths and by the contribution of the ROADM itself.
• Different possible paths for a
single lambda imply that
consideration must be given to
link/section compensation
rather than route compensation.
• It is, therefore, essential
that link dispersion
characteristics be fully
understood.
32
A few CD Facts, a step further…
Very simple conclusions
Design
Deployment
Service Assurance
Data Capacity
Feature Richness
Travelled optical distances can be great.
Every section can (and will) be part of different links
You will be above CD danger zones
Test every section required for “dispersion mapping”
Having a good dispersion map means good DCM
33
PMD
Polarization
Mode
Dispersion
34
Polarization Mode Dispersion - PMD
Polarization mode dispersion (PMD) is a form of modal dispersion
where two different polarizations of light in a waveguide, which
normally travel at the same speed, travel at different speeds
due to random imperfections and asymmetries, causing
random spreading of optical pulses. Unless it is compensated,
which is difficult, this ultimately limits the rate at which data
can be transmitted over a fiber.
35
Polarization Mode Dispersion
Polarization Mode Dispersion (PMD) is a consequence of certain physical properties of optical fiber that
result in distortion of optical pulses. These distortions result in somewhat variable dispersion of the optical
pulse over time as well as a reduction in peak power.
36
Polarization Mode Dispersion – the same but different
• While it is somewhat similar to CD in that it is a type of dispersion, it is random (stochastic) and will be
impacted by physical parameters such as temperature, wind load, ice load, etc. This means that PMD as
measured on spooled fiber will be different from PMD measured in the field on the same fiber after
installation. Therefore, PMD must be measured in situ – that is, after installation.
•Optical Fibers manufactured before approximately 1994 were most likely NOT designed with PMD
mitigation in mind.
37
Polarization Mode Dispersion - Causes
Fiber defects
Manufacturing (Rare)
Installation generated –
twists, strains, bends.
Environmental
Constraints
Geometric
Internal Stress
Heat
Lateral Pressure
Bend
Wind
(aerial fibers)
38
Polarization Mode Dispersion (PMD)
Design
Perfect Fiber
Deployment
Service Assurance
Single « section » of
Bad Fiber
39
PMD and Differential Group Delay—The Causes

Asymmetries in fiber during fiber manufacturing and/or stress
distribution during cabling, installation and/or servicing create fiber
local birefringence. (Birefringence, a double-refraction phenomenon in
which an unpolarized beam of light is divided into two beams with
different polarization vectors and relative velocities, meaning that two
orthogonally polarized photons will “see” different refractive indices.)

A "real-world" long fiber is an addition of these randomly distributed
locally birefringent portions.
The “cloud” of photons shrinks and expands in size as it passes
through different sections with different characteristics. At the end of
the link the difference in arrival times of the first and last photons in
the “cloud” yields the differential group delay (DGD) due to PMD.
40
Factoring In the Environment
What happens if we rotate a section of fiber?
(Imagine a section of aerial fiber whipping in
the wind!)
41
Answer 
The Principal State of Polarization (PSP) is constantly changing
thereby changing the Differential Group Delay at random rates and
amounts.
Testing aerial cable for PMD can be a challenge. One important fact: The IEC
document 61280-4-4 states that out of the 6 recognized methods of measuring
PMD in aerial cables, the Interferomety Method is the only that can be used
(EXFO 5500B uses this method). The Fixed Analyzer is not accepted.
Measurements can be disrupted if the link is vibrating as in the case of aerial
cables and the optical properties of the fiber can change within the time used to
measure the data for calculating individual DGD values.
This past December, another document, FOTP-243, was approved stating that
the SOP (State-Of-Polarization) Scrambling Method was accepted as well
(FTB-5700). EXFO’s products are approved for aerial PMD measurements.
42
PMD for 99.999% probability that the tolerable broadening will
correspond to 10% of the bit period on the average with a mean
power penalty of 1 dB
Bit rate
(Gbit/s)
Average DGD*
(ps)
2.5
40
10
10
40
2.5
43
Facts on Polarization Mode Dispersion
Is stochastic (Random
occurrence with
measureable
distribution.)
Is not linear
Polarization Mode
Dispersion:
Is affected by the
environment
Cannot be easily
compensated
44
© 2009
So, what are the choices if PMD is high?
Find another
fiber
Install another
fiber
PMD too high for
high speed
Test and hope
100K USD per
mile
Re-route traffic
Weeks and
months of
planning
High cost
Cancel the
project
Root cause
analysis
No super
services
Competitors step
up to the plate
Solve the problem at the source of the
problem by replacing smaller sections that
contribute most to high PMD
45
Single-Ended CD/PMD Analyzer
Single Ended CD/PMD measurements
FTB-5700
Aerial Cable - IEC FOTP-243
40Gig Ready
Testing Range: Up to 140 km
46
Mainframe CD and PMD analyzers – long-haul plus amplified links.
Design
FTB-5500B/FTB-5800
Deployment
Service Assurance
Test Through EDFA’s
100 Gig Ready
Ideal for Aerial Fiber - IEC 61280-4-4
Suitable for All Networks
47
FTB-5600 Distributed PMD Analyzer
The industry’s first distributed PMD analyzer
Locates fiber sections contributing
high PMD
Enables customer to isolate and
replace bad sections
Can help identify small changes that
can help boost network performance
48
Dispersion mitigation – Why Improved Modulation Techniques help!
2 x 20 Gbit/sec
40 Gbit/sec
40 Gbit/sec
Here’s an example!
• Data stream is divided into two each “half bit rate” paths and then superimposed (modulated) onto the
same “carrier” while being separated by phase or polarization. This yields reduced overall spectral width
(fewer wavelengths!) after the LASER is modulated. (Anyone remember heterodyning?)
• Further reductions in spectral width may be made by dividing the data into 4 or 8 reduced bit rate signals
and then superimposing on the carrier via a combination of phase and polarization separated components.
• All signals are resolved (detected) and recombined at the receiver to yield the original 40 Gbit/sec data
stream.
• Reducing overall spectral width means that dispersion thresholds for lower throughput signals may be
used instead of the more stringent 40 Gbit/s criteria. For example, it is possible to apply thresholds for 18
Gbit/sec to certain 40 Gbit/sec optical data streams because of the chosen modulation method (DQPSK). 49
Why Reduced Spectral Width Helps
 There is a direct correlation between bandwidth (data speed) and

spectral width; increasing bandwidth increases spectral width of
modulated LASER output.
Uncompensated Chromatic Dispersion limits for specific data rates:
 16640ps/nm for 2.5GB/s
 1040ps/nm for 10GB/s,
 65ps/nm for 40GB/s
 Reducing spectral width of a 40 GB/s signal to something near the
same as a 10 GB/s signal means the less stringent limit may be used!
 Cost savings in per-lambda modules,
 Extend link/route length without adverse impact
 Simplified network design requirements!
50
Fiber Characterization Testing
Optical Loss
Optical Return
Loss
OTDR
Chromatic
Dispersion
Polarization
Mode
Dispersion
51
Fiber Characterization Testing
52
Questions?
Thank you
Tony Lowe
Technical Sales Specialist
[email protected]
53

What is Dispersion?

Transcription

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