Transcript Cisco Presentation Guide

Optical Fundamentals
Russ Gyurek
[email protected]
© 2003,
2001, Cisco Systems, Inc. All rights reserved.
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Agenda
• Introduction
• Safety
• Optical propagation in Fibers
• Attenuation & Dispersion
• Non Linearity
• SM Optical Fiber Types
• Summary, Q&A
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Modern Lightwave Eras
10000
Capacity (Gb/s)
1000
100
Optical networking
Wavelength Switching
Research Systems
10
Commercial Systems
1
Fiberization
Digitization
SONET rings and
DWDM linear
systems
0.1
1985
1990
1995
2000
Year
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Some terminology:
• Decibels (dB): unit of level (relative measure)
X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = 10-.3 = 0.501
Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is
power and represents loss or gain.
• Decibels-milliwatt (dBm) : Decibel referenced to a
milliwatt
X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW
• Wavelength (): length of a wave in a particular medium.
Common unit: nanometers, 10-9m (nm)
300nm (blue) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, &
1550nm
• Frequency (): the number of times that a wave is
produced within a particular time period. Common unit:
TeraHertz, 1012 cycles per second (Thz)
Wavelength x frequency = Speed of light   x  = C
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Some more terminology
• Attenuation = Loss of power in dB/km
The extent to which lighting intensity from the source is diminished as it passes
through a given length of fiber-optic (FO) cable, tubing or light pipe. This
specification determines how well a product transmits light and how much cable
can be properly illuminated by a given light source.
• Chromatic Dispersion = Spread of light pulse in
ps/nm-km
The separation of light into its different coloured rays.
• ITU Grid = Standard set of wavelengths to be used in Fibre Optic
communications. Unit Ghz, e.g. 400Ghz, 200Ghz, 100Ghz
• Optical Signal to Noise Ration (OSNR) = Ratio of optical
signal power to noise power for the receiver
• Lambda = Name of Greek Letter used as Wavelength
symbol ()
• Optical Supervisory Channel (OSC) = Management
channel
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dB versus dBm
• dBm used for output power and receive
sensitivity (Absolute Value)
• dB used for power gain or loss (Relative Value)
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ITU Wavelength Grid
1530.33 nm
0.80 nm
195.9 THz
100 GHz
1553.86 nm

193.0 THz

• ITU-T  grid is based on 191.7 THz + 100 GHz
• It is a standard for the lasers in DWDM systems
Freq (THz)
192.90
192.85
192.80
192.75
192.70
192.65
192.60
© 2003, Cisco Systems, Inc. All rights reserved.
ITU Ch
29
28
27
26
Wave (nm)
1554.13
1554.54
1554.94
1555.34
1555.75
1556.15
1556.55
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Bit Error Rate ( BER)
• BER is a key objective of the Optical
System Design
• Goal is to get from Tx to Rx with a BER <
BER threshold of the Rx
• BER thresholds are on Data sheets
• Typical minimum acceptable rate is 10 -12
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Optical Budget
Basic Optical Budget = Output Power – Input Sensitivity
Pout = +6 dBm
R = -30 dBm
Budget = 36 dB
Optical Budget is affected by:
Fiber attenuation
Splices
Patch Panels/Connectors
Optical components (filters, amplifiers, etc)
Bends in fiber
Contamination (dirt/oil on connectors)
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Power Budget with Power Penalties
Fiber Loss +
Splices +
Connectors +
Dispersion Penalties +
Fiber Nonlinearities Penalty +
Component Aging Penalties <
Power Budget = Launch Power – Receiver Sensitivity
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Glass Purity
Fiber Optics Requires
Very High Purity Glass
Window Glass
1 inch (~3 cm)
Optical Quality Glass
10 feet (~3 m)
Fiber Optics
9 miles (~14 km)
Propagation Distance Need to Reduce the
Transmitted Light Power by 50% (3 dB)
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Fiber Fundamentals
Attenuation
Dispersion
Nonlinearity
Distortion
It May Be a Digital Signal, but It’s Analog Transmission
Transmitted Data Waveform
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Waveform After 1000 Km
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Agenda
• Introduction
• Safety
• Optical propagation in Fibers
• Attenuation & Dispersion
• Non Linearity
• SM Optical Fiber Types
• Summary, Q&A
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A Few Words on Optical Safety
Think Optical Safety at ALL Times
Optical Power is INVISIBLE to the Human Eye
NEVER stare at an Optical Connector
Keep Optical Connectors Pointed AWAY FROM YOURSELF AND OTHERS
Glass (Fiber Cable) Can CUT and PUNCTURE
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Laser Classifications / Safety ICONS
Class 1
Lasers that are incapable of causing damage when the beam is directed into the eye
under normal operating conditions. These include helium-neon lasers operating at
less than a few microwatts of radiant power.
Class 2
SR and IR Optics, some LR
Lasers that can cause harm if viewed directly for ¼ second or longer. This includes
helium-neon lasers with an output up to 1 mW (milliwatt).
Class 3A
Many LR Optics, CWDM GBICS
Lasers that have outputs less than 5 mW. These lasers can cause injury when the
eye is exposed to either the beam or its reflections from mirrors or other shiny
surfaces. As an example, laser pointers typically fall into this class.
Class 3B
Some LR Optics, Amplifier Outputs
Lasers that have outputs of 5 to 500 mW. The argon lasers typically used in laser
light shows are of this class. Higher power diode lasers (above 5 mW) from optical
drives and high performance laser printers also fall into this class.
Class 4
Lasers that have outputs exceeding 500 mW. These devices produce a beam that is
hazardous directly or from reflection and can produce skin burn. Many ruby, carbon
dioxide, and neodymium-glass lasers are class 4.
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Protective Eyewear Available
Protective goggles or glasses
should be worn for all routine use
of Class 3B and Class 4 lasers.
Remember: Eyewear is
wavelength specific, a pair of
goggles that effectively blocks red
laser light affords no protection
for green laser light.
Do not use worn or scratched
glasses. Inspect at every use!
Laser Safety Equipment can be investigated in
greater detail at the following link:
http://www.lasersafety.co.uk/frhome.html
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Some Final Words on Optical
Equipment Safety
Remember: Optical cabling is constructed from strands of glass, about the
size of a human hair. Never handle exposed fiber strands unless you have the
proper training. Fiber fragments that become embedded under the skin can be
extremely painful, and are very difficult (in some cases impossible) to remove.
Remember: Optical equipment is typically powered from either 120vac or
48vdc. Always remove jewelry such as rings, watches, bracelets, etc…when
working with energized equipment. Care should be taken to never come in
contact with exposed electrical connections or buswork.
Remember: Be careful not to damage equipment through ESD (Electrostatic
Discharge). Ensure that all equipment is properly grounded, wrist straps are
used when removing modules with active components and always handle
equipment modules by their edges.
**Note: Many companies have documented ESD guidelines specific to their
environment. Please consult those documents for additional detail as
appropriate**.
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Agenda
• Introduction
• Safety
• Optical propagation in Fibers
• Attenuation & Dispersion
• Non Linearity
• SM Optical Fiber Types
• Summary, Q&A
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Analog Transmission Effects
Attenuation:
Reduces power level with distance
Dispersion and Nonlinearities:
Erodes clarity with distance and speed
Signal detection and recovery is an analog problem
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Fiber Geometry
Core
Cladding
• An optical fiber is made of
three sections:
The core carries the
light signals
The cladding keeps the light
in the core
The coating protects the glass
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Coating
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Fiber Dimensions
• Fiber dimensions are measured in µm
1 µm = 0.000001 meters (10-6)
1 human hair ~ 50 µm
Cladding
(125 µm)
Coating
(245 µm)
• Refractive Index (n)
n = c/v
n ~ 1.46
n (core) > n (cladding)
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Core
(8–62.5 µm)
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Geometrical Optics
• Light is reflected/refracted
at an interface
q1 = Angle of incidence
q1r = Angle of reflection
q2 = Angle of refraction
• Above qcritical=Sin-1(n2/n1),
all light is totally internally
reflected
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n2
n1
q2
q1 q1r
Snell’s Law
q1 = q1r
n1Sin q1 = n2Sin q2
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Propagation in Fiber
n2
q0
n1
Cladding
q1
Core
Intensity Profile
• Light propagates by total internal reflections
at the core-cladding interface
• Total internal reflections are lossless
• Each allowed ray is a mode
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Different Types of Fiber
n2
Cladding
• Multimode fiber
Core diameter varies
50 mm for step index
62.5 mm for graded index
Bit rate-distance product
>500 MHz-km
• Single-mode fiber
Core diameter is about 9 mm
Bit rate-distance product
>100 THz-km
© 2003, Cisco Systems, Inc. All rights reserved.
n1
n2
n1
Core
Cladding
Core
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Agenda
• Introduction
• Safety
• Optical propagation in Fibers
• Attenuation & Dispersion
• Non Linearity
• SM Optical Fiber Types
• Summary, Q&A
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Optical Spectrum
IR
UV
125 GHz/nm
Visible
• Light
Ultraviolet (UV)
Visible
Infrared (IR)
850 nm
980 nm
1310 nm
1480 nm
1550 nm
• Communication wavelengths
850, 1310, 1550 nm
Low-loss wavelengths
• Specialty wavelengths
980, 1480, 1625 nm
© 2003, Cisco Systems, Inc. All rights reserved.

1625 nm
C = x 
 (nanometers)
Frequency:  (terahertz)
Wavelength:
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Optical Attenuation
• Specified in loss per kilometer
(dB/km)
0.40 dB/km at 1310 nm
0.25 dB/km at 1550 nm
• Loss due to absorption
by impurities
1310
Window
1550
Window
1400 nm peak due to OH ions
• EDFA optical amplifiers
available in 1550 window
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Optical Attenuation
• Pulse amplitude reduction limits “how far”
• Attenuation in dB=10xLog(Pi/Po)
• Power is measured in dBm:
P(dBm)=10xlog(P mW/1 mW)
Examples
10dBm
10 mW
0 dBM
1 mW
-3 dBm
500 uW
-10 dBm
100 uW
-30 dBm
1 uW
)
Pi
P0
T
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T
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Types of Dispersion
• Chromatic Dispersion
Different wavelengths travel at different speeds
Causes spreading of the light pulse
• Polarization Mode Dispersion (PMD)
Single-mode fiber supports two polarization states
Fast and slow axes have different group velocities
Causes spreading of the light pulse
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Fiber Chromatic Dispersion
Characteristics
Dispersion ps/nm-km
Normal Fiber (SMF-28 or Equivalent)
Nondispersion Shifted Fiber (NDSF)
>95% of Deployed Plant
20
Wavelength

0
1310 nm
1550nm
Normal(ITU-T G.652)
Dispersion Shifted Fiber (DSF) (ITU-T G.653)
Nonzero Dispersion Shifted Fibers (NZDSF) (ITU-T G.655)
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A Snapshot on Chromatic Dispersion
Interference
• Affects single channel and DWDM systems
• A pulse spreads as it travels down the fiber
• Inter-symbol Interference (ISI) leads to
performance impairments
• Degradation depends on:
laser used (spectral width)
bit-rate (temporal pulse separation)
Different SM types
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Fiber Chromatic Dispersion (CD)
• The refractive index is wavelength dependent
• Different frequency-components of the optical
pulses travel at different speeds (the blue is faster
than red for anomalous dispersion where D > 0)
• As a result, we see pulse broadening and ISI
Red
z
z
z
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Blue
z
Transmission Fiber
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Limitations From Chromatic Dispersion
• Dispersion causes pulse distortion,
pulse "smearing" effects
• Higher bit-rates and shorter pulses are less
robust to Chromatic Dispersion
• Limits "how fast“ and “how far”
10 Gbps
60 Km SMF-28
t
40 Gbps
4 Km SMF-28
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Combating Chromatic Dispersion
• Use DSF and NZDSF fibers
(G.653 & G.655)
• Dispersion Compensating Fiber
• Transmitters with narrow spectral width
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Dispersion Compensating Fiber
• Dispersion
Compensating Fiber:
By joining fibers with CD of
opposite signs (polarity) and
suitable lengths an average
dispersion close to zero can
be obtained; the
compensating fiber can be
several kilometers and the
reel can be inserted at any
point in the link, at the
receiver or at the transmitter
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Dispersion Compensation
Cumulative Dispersion (ps/nm)
Total Dispersion Controlled
+100
0
-100
-200
-300
-400
-500
No Compensation
With Compensation
Distance from
Transmitter (km)
Dispersion Shifted Fiber Cable
Transmitter
Dispersion
Compensators
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Polarization Mode Dispersion (PMD)
• Caused by ovality of
core due to:
–Manufacturing process
–Internal stress (cabling)
–External stress (trucks)
• Only discovered in
the 90s
• Most older fiber is not
characterized for PMD
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Polarization Mode Dispersion (PMD)
Ey
nx
Ex
ny
Pulse As It Enters the Fiber
Spreaded Pulse As It Leaves the Fiber
• The optical pulse tends to broaden as it travels
down the fiber; this is a much weaker phenomenon
than chromatic dispersion and it is of some
relevance at bit rates of 10Gb/s or more
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Combating PMD
• Factors contributing to PMD
Bit Rate
Fiber core symmetry
Environmental factors
Bends/stress in fiber
Imperfections in fiber
• Solutions for PMD
Improved fibers
Regeneration
Follow manufacturer’s recommended installation
techniques for the fiber cable
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Agenda
• Introduction
• Safety
• Optical propagation in Fibers
• Attenuation & Dispersion
• Non Linearity
• SM Optical Fiber Types
• Summary, Q&A
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From Linear to Non Linear Propagation
•
As long as optical power within an
optical fiber is small, the fiber can be
treated as a linear medium
Loss and refractive index are independent of the
signal power
•
When optical power levels gets fairly
high, the fiber becomes a nonlinear
medium
Loss and refractive index depend on the optical
power
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Optical Fiber’s Nonlinear Index
Optical Pulse
Index of
Nonlinear
Light
Refraction Coefficient Intensity
Intensity
n = n0 + N2
Fast Phase
Velocity
Slow Phase
Velocity
Time
• Intensity of an optical pulse modulates the
index of refraction
• Nonlinearity scales as (channel power)2
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Effects of Nonlinearity
• A single channel’s pulses interact as they travel
Interference

Multiple channels interact as they travel
Interference
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Types of Nonlinearities
• Nonlinear index
Four-wave mixing

FWM
Self-phase modulation
Cross-phase modulation
• Stimulated scattering
Raman

Raman
Brillouin
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Four-Wave Mixing
1 2
Into Fiber
21-2 1
2 22-1
Out of Fiber
• Channels beat against each other to form
intermodulation products
• Creates in-band crosstalk that can not be filtered
(optically or electrically)
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FWM Example
Output Spectrum after 25 km of
Dispersion Shifted Fiber
-5
• FWM effects increase
geometrically with:
Number of channels
Spacing of channels
Optical power level
Power (dBm)
-10
Input Power = 3 mw/ch
-15
-20
-25
-30
-35
-40
1542 1543 1544 1545 1546 1547 1548
Wavelength (nm)
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FWM and Dispersion
Dispersion Washes Out FWM Effects
FWM Efficiency (dB)
0
D=0
-10
-20
D=0.2
-30
D=2
-40
D=17
-50
0.0
0.5
1.0
1.5
2.0
2.5
Channel Spacing (nm)
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Agenda
• Introduction
• Safety
• Optical propagation in Fibers
• Attenuation & Dispersion
• Non Linearity
• SM Optical Fiber Types
• Summary, Q&A
© 2003, Cisco Systems, Inc. All rights reserved.
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Types of Single-Mode Fiber
• SMF (standard, 1310 nm optimized, G.652)
Most widely deployed so far, introduced in 1986, cheapest
• DSF (Dispersion Shifted, G.653)
Intended for single channel operation at 1550 nm
• NZDSF (Non-Zero Dispersion Shifted, G.655)
– LS
For WDM operation in the 1550 nm region only
– TrueWave, FreeLight, LEAF, TeraLight…
Latest generation fibers developed in mid 90’s
For better performance with high capacity DWDM systems
–Low PMD ULH fibers
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Fiber Dispersion Characteristics
Normal fiber
Non-dispersion shifted fiber (NDSF) G.652
~95% of deployed plant
Dispersion (in ps/nm- km)
25
20
DS
NZDS+
NZDS-
SMF
15
10
5
0
-5
-10
DSF G.653
NZDSF G.655
-15
-20
1350 1370 1390 1410 1430 1450 1470 1490 1510 1530 1550 1570 1590 1610 1630 1650
Wavelength (in nm)
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Different Solutions for
Different Fiber Types
SMF
•Good for TDM at 1310 nm
(G.652)
•OK for TDM at 1550
•OK for DWDM (With Dispersion Mgmt)
DSF
•OK for TDM at 1310 nm
(G.653)
•Good for TDM at 1550 nm
•Bad for DWDM (C-Band)
NZDSF
•OK for TDM at 1310 nm
(G.655)
•Good for TDM at 1550 nm
•Good for DWDM (C + L Bands)
Extended Band
•Good for TDM at 1310 nm
(G.652.C)
•OK for TDM at 1550 nm
(suppressed attenuation
in the traditional water
peak region)
•OK for DWDM (With Dispersion Mgmt
•Good for CWDM (>8 wavelengths)
The primary Difference is in the Chromatic Dispersion Characteristics
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Agenda
• Introduction
• Safety
• Optical propagation in Fibers
• Attenuation & Dispersion
• Non Linearity
• SM Optical Fiber Types
• Summary, Q&A
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The 3 “R”s of Optical Networking
A Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations:
Pulse as It Enters the Fiber
Pulse as It Exits the Fiber
Loss of Energy
Shape Distortion
Phase Variation
Loss of Timing (Jitter)
(From Various Sources)
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t
ts Optimum
Sampling Time
t
ts Optimum
Sampling Time
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The 3 “R”s of Optical Networking
(Cont.)
The Options to Recover the Signal from Attenuation/Dispersion/Jitter
Degradation Are:
Pulse as It Enters the Fiber
Pulse as It Exits the Fiber
Re-Gen to Boost the Power
Re-Shape
DCU
Phase Variation
Phase Re-Alignment*
O-E-O
Re-Time
t
ts Optimum
Sampling Time
t
ts Optimum
Sampling Time
Re-gen, Re-shape and ts Optimum
Remove Optical Noise Sampling Time
t
*Simplification
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F0_5585_c2
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Back-up Slide(s)
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How Far Can I Go w/o Dispersion Comp?
Distance (Km) =
Specification of Transponder (ps/nm)
Coefficient of Dispersion of Fiber (ps/nm*km)
A laser signal with dispersion tolerance of 3400 ps/nm
is sent across a standard SMF fiber which has a Coefficient of
Dispersion of 17 ps/nm*km.
It will reach 200 Km at maximum bandwidth.
Note that lower speeds will travel farther.
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