optical transmission fundamental

Cisco Confidential 1© 2010 Cisco and/or its affiliates. All rights reserved.

Optical Transmission Fundamental

v1, 1-Dec

© 2010 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2

Optical Transmission Fundamentals

• What is DWDM?

• Optical Basics

• Optical Fiber & Impairments

• Non Linear Effects

• Optical Transmission

• Erbium Doped Fiber Amplifiers

• OTN Basics

• DWDM Technology

• DWDM Network Topologies

• Optical Transmission Systems Network Design

• DWDM Software


• DWDM systems use optical devices to combine the output of several optical transmitters

Optical

fiber pair

TX

Optical

transmittersOptical

receivers

TX

TX

TX

RX

RX

RX

RX

Transmission

DWDM devices


• One traffic channel per fiber pair

• 40 x 10 Gbps channels, 80 fibers

Traditional TDM Model

STM-64 TxSTM-64 Rx

STM-64 Tx STM-64 Rx

STM-64 TxSTM-64 Rx

STM-64 Tx STM-64 Rx

STM-64 TxSTM-64 Rx

STM-64 Tx STM-64 Rx

STM-64 TxSTM-64 Rx

STM-64 Tx STM-64 Rx

STM-64 TxSTM-64 Rx

STM-64 Tx STM-64 Rx

STM-64 TxSTM-64 Rx

STM-64 Tx STM-64 Rx

STM-64 TxSTM-64 Rx

STM-64 Tx STM-64 Rx

STM-64 TxSTM-64 Rx

STM-64 Tx STM-64 Rx


Dense Wavelength Division Multiplexing DWDM

• Multiple traffic channels on a fiber pair

• Each channel transmitted on a different wavelength/color prevents

channel interference and allows them to be separated at the receiving end

• 40 x 10 Gbps channels, 2 fibers

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Rx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx

STM-64 Tx


ITU Wavelength Grids

1530.33 nm 1553.86 nm0.80 nm

195.9 THz 193.0 THz100 GHz

1530.33 nm 1553.86 nm0.40 nm

195.9 THz 193.0 THz50 GHz

• ITU-T l grids are based on 191.7 THz + 100 GHz or + 50 GHz

• It is a standard for the channels in DWDM systems

lWavelength

Frequency

100GHz Grid

50GHz Grid

lWavelength

Frequency


DS-1

DS-3

OC-1

OC-3

OC-12

OC-48

FiberSONET

ADM

TDM and DWDM Comparison

TDM (SONET/SDH)Takes sync or async signals and

multiplexes them to a single

higher optical bit rate.

Uses E/O or O/E/O conversion.

DWDMTakes multiple optical signals

and multiplexes them onto a

single fiber.

If required O/E/O conversion at

ingress for wavelength

conversion.

OC-48

OC-192

OC-768

GE

ESCON/FC

ATM

OEO

OEO

OEO

Fiber

DWDM

Mux


DWDM CWDM

Application Long Haul Metro

Amplifiers Typically EDFAs Almost Never

# Channels Up to 80 Up to 8

Channel Spacing 0.4 nm 20nm

Distance Up to 3000km Up to 80km

Spectrum 1530nm to 1560nm 1270nm to 1610nm

Filter Technology Intelligent Passive


Section 2: Optical Transmission Fundamentals

What is DWDM?

• Optical Basics

• Optical Fiber & Impairments




• OTN Basics

• DWDM Technology



• DWDM Software


Optical communication wavelength bands in the InfraRed:

• 850 nm over Multimode fiber

• 1310 nm over Singlemode fiber

• C-band:1550 nm over Singlemode fiber

• L-band: 1625 nm over Singlemode fiber

UltraViolet InfraRed

850 nm 1310 nm 1550 nm 1625 nm

l

Optical Spectrum

Visible


Wavelength and Frequency

• Wavelength (Lambda l) of light: in optical communications normally measured in nanometers, 10–9m (nm)

• Frequency () in Hertz (Hz): normally expressed in TeraHertz (THz), 1012 Hz

• Converting between wavelength and frequency:

Wavelength x frequency = speed of light l x = C

C = 3x108 m/s

For example: 1550 nanometers (nm) = 193.41 terahertz (THz)


Optical Power and dBm

• The optical power of a signal can be measured in milliwatts (mW)

• dBm is the optical power expressed in decibels relative to one milliwatt

• Power in dBm = 10 log10 [Optical power (mW)/1mW]

• Examples:

Optical Power mW Optical Power dBm

0.1 mW -10 dBm

1.0 mW 0 dBm

2.0 mW +3 dBm

10 mW +10 dBm

100 mW +20 dBm



What is DWDM?

Optical Basics

• Optical Fiber




• OTN Basics

• DWDM Technology



• DWDM Software


Fiber Development Timeline

Invention of

first

low-loss

optical fiber

1970

Introduction

of Corning

62.5/125 um

multimode

fiber

1976

1983

1985

1986

Introduction

of Corning

SMF-21 fiber

Introduction

of Corning

SMF/DS

dispersion

shifted fiber

Introduction of

Corning SMF-

28 fiber

1986

Introduction

of Corning

50/125 um

fiber

1994

Introduction of

Corning SMF-

LS non-zero

dispersion

shifted fiber

1998

Introduction of

Corning LEAF

non-zero

dispersion

shifted fiber

with large

effective area

Introduction

of Lucent

TrueWave

non-zero

dispersion

shifted fiber

1993

Introduction of

Lucent TrueWave

RS reduced

slope non-zero

dispersion

shifted fiber

1998


Fiber Geometry and Dimensions

• The core carries the light signals

• The refractive index difference between core & cladding confines the light to the core

• The coating protects the glass

Coating

250 microns

An optical fiber is comprised of three sections:

Cladding

125 microns

Core

SMF 8 microns


n2

n1

Cladding

Core

Propagation in Singlemode Fiber

• Light is weakly guided through index difference between core and cladding n2-n1

• Single mode is transmitted

• Mode field travels in core and cladding

Intensity Profile


Transmission Degradation

Attenuation, two primary loss mechanisms

• Absorption loss due to impurities

• Scattering loss due to refractive index fluctuations

Chromatic dispersion:

• Wavelengths travel at different speeds (refractive index function of l)

• Smears pulses because lasers are not perfectly monochromatic

Polarization mode dispersion (PMD):

• Light travels in two orthogonal modes

• If core is nonsymmetric, different modes travel at different speeds

• Issue at high bit rates such as 10 Gbps and higher

Nonlinear effects

• Prevalent at higher signal powers


Insertion Loss and Attenuation in Decibels (dB)

• The Insertion Loss or Attenuation between transmitter and receiver is expressed by the difference between the transmitted and received power

• Attenuation expressed in decibels (dB) is a negative gain, calculated by

10 x log10 Prx/Ptx (dB)

• If half the power is lost, this is 3 dB

• Example: Attenuation = 30 dB means transmitter power is 1000 times the receive power

Transmitter Receiver

Transmit Power = Ptx (mW) Receive Power = Prx (mW)

Lossy optical

component


Optical Fiber Attenuation dB/km

• Fiber attenuation expressed in dB/km, calculated by

10 log10 (Ptx/Prx)/L

• Example:

A fiber of 10 km length has Pin = 10 μW and Pout = 6 μW

Its loss expressed in dB is

Fiber loss = 10 log10(10/6) = 2.2 dB

And expressed in dB/km = 0.22 dB/Km

Transmitter Receiver

Transmit Power = Ptx (μW or mW) Receive Power = Prx (μW or mW)

Length = L km


Optical Fiber Attenuation

• Attenuation specified in loss per kilometer (dB/km)

0.40 dB/km @ 1310 nm, 0.25 dB/km @ 1550 nm

• Loss due to absorption by impurities, 1400 nm peak due to OH (water) ions

• Rayleigh scattering loss, fundamental limit to fiber loss

1550

window

1310

windowRayleigh scattering loss

Fundamental mode

Bending lossOH Absorption Loss


Optical Bands

• O-band: 1260 - 1360 nm

• S-band: 1460 - 1530 nm

• C-band: 1530 - 1565 nm

• L-band: 1565 - 1625 nm

L-b

an

d

C-b

an

d

S-b

an

d

O-b

an

d


Fiber Chromatic Dispersion

Wavelength l

Dis

per

sio

n

ps/

nm

-km

20

0

1310 nm 1550nm

• Chromatic dispersion causes a broadening in time of the input signal as it

travels down the length of the fiber.

• The phenomenon occurs because the optical signal has a finite spectral

width, and different spectral components will propagate at different speeds

along the length of the fiber.

• The cause of this velocity difference is that the index of refraction of the fiber

core is different for different wavelengths.

• This is called material dispersion and it is the dominant source of chromatic

dispersion in single-mode fibers.

Variation of Chromatic

Dispersion with

wavelength for Standard

SingleMode fiber

(>95% of installed fiber)


Fiber Dispersion CharacteristicsStandard SingleMode Fiber

>95% installed fiber

Non-zero dispersion shifted fibers (NZDSF)

Lower dispersion in 1550nm window

Wavelength l

Dis

per

sio

n

ps/

nm

-km

20

0

1310 nm 1550nm

1530 1540 1550 1560nm

+2

+4

- 2

- 4

Corning LS

Corning DSF

Dis

per

sio

n (

ps/

nm

-k

m)

Lucent TW+Corning Leaf


Dispersion Limitation

• Dispersion limitation is defined by the dispersion tolerance of the transmitter

and the receiver

• Total dispersion is calculated from the fiber dispersion characteristics and the

fiber length for any channel or traffic path

• The effect of fiber dispersion should be taken into account in the power

budget as the dispersion penalty budget

• If any channel hit the dispersion limit, the dispersion should be compensated

or the channel signal should be regenerated (O-E-O)

• Doubling of bit rate results in an increase of dispersion penalty of up to four

times


Dispersion Limited Transmission Distances

Distance (Km) =Dispersion Tolerance of Transponder (ps/nm)

Coefficient of Dispersion of Fiber (ps/nm*km)

Transmission Rate Modulation formatDispersion Tolerance

Distance

2.5 Gb/sExternal

Modulation20,000 ps/nm/km ~ 1,100 km

2.5 Gb/s Direct Modulation 2,400 ps/nm/km 140 km

10 Gb/sExternal

Modulation1,200 ps/nm/km 70 km

40 Gb/sExternal

Modulation200 ps/nm/km 12 km

• Dispersion limited transmission distances over SMF fiber (17 ps/nm/km):


Effect of Chromatic Dispersion

• In fiber the different frequency components of the signal propagate at different speeds

• The effect is signal distortion and intersymbol Interference, the penalty is “eye-closure”

• Can be compensated for by the use of Dispersion Compensation

Eye opening

FOLDING

Tx bit sequence Eye diagram

no dispersion


Combating Chromatic Dispersion

• Dispersion generally not an issue below 10Gbps

• Narrow spectrum laser sources (external modulation) and low chirp* laser sources reduce dispersion penalty. With broad/chirped sources the different spectral components of the source will see different dispersions thus broadening the pulse in time

• New fiber types (NZ-DSF) greatly reduce effects

• Dispersion compensation techniques

• Dispersion compensation fiber

• Dispersion compensating optical filters

• Dispersion Compensating Units (DCU) generally placed in mid-stage access of EDFA to alleviate DCU insertion loss

• *Chirp: frequency of launched pulse changes with time


Dispersion Compensating Unit (DCU)

• Dispersion Compensating Fiber:

DCUs use fiber with chromatic dispersion of opposite sign/slope and of suitable length to bring the average dispersion of the link close to zero.

The compensating fiber can be several kilometers in length, the DCU are typically inserted after each span

• Use of dispersion compensating fiber to combat dispersion

Cisco Confidential© 2010 Cisco and/or its affiliates. All rights reserved. 30

The primary difference is in the Chromatic Dispersion Characteristics

Applications for the Different Fiber Types

Good for TDM at 1310 nm

OK for TDM at 1550 nm

OK for DWDM (With Dispersion Mgmt

Good for CWDM (>8 wavelengths)

Extended Band

(G.652.C)

(suppressed attenuation

in the traditional water

peak region)



Good for DWDM (C + L Bands)

NZDSF

(G.655)



Bad for DWDM (C-Band)

DSF

(G.653)


OK for TDM at 1550

OK for DWDM (With Dispersion Mgmt)

SMF

(G.652)


Polarization Mode Dispersion (PMD)• PMD causes a broadening in time of the optical signal

• In an ideal optical fiber, the core has a perfectly circular cross-section. In this

case, the fundamental light mode has two orthogonal polarizations (orientations

of the electric field) that travel at the same speed through the fiber

• Birefringence (index of refraction variation between two polarization axis) arises

due to random imperfections and asymmetries, causes broadening of the optical

pulse due to the two orthogonal polarization states traveling at different speeds

n1

n2

n1 > n2 refractive index difference due

to mechanical stress


• The “PMD coefficient”, with units of ps/km1/2, indicates the rate at which PMD builds up along the fiber length

• Limits optical reach in high-speed transmission systems

• Typical PMD tolerance

2.5 Gbps: typically 40 ps

10 Gbps: typically 10 ps

40 Gbps: typically 2.5 ps (can be larger dependant onmodulation format)

• Power penalty due to PMD (1-2 dB)

PMD & Bit Rate Dependence


Link PMD:

• Individual fibers have higher PMD values than when concatenated in a link

• The PMD link value determines the statistical upper limit for system PMD

PMD Limited Transmission Distances

Transmission Rate Distance

2.5 Gb/s 1,000,000 km

10 Gb/s 62,500 km

40 Gb/s 3,906 km

Transmission Rate Distance

2.5 Gb/s 40,000 km

10 Gb/s 2,500 km

40 Gb/s 156 km

ELEAF: PMD spec <0.1 ps/km1/2, PMD Link Value of <0.04 ps/km1/2

Leads to PMD limited system length of:

Old SMF: PMD spec <0.5 ps/km1/2, PMD link value of <0.2 ps/km1/2

Leads to PMD limited system length of:

Examples:


Combating PMD

• Not an issue at 2.5 Gbps

• 2000+ Km at 10 Gbps on typical fiber

• Increase system robustness with Forward Error Correction (FEC) and optimized transmitter modulation formats

• Deploy PMD-optimized fibers

• Use PMD Compensation (PMDC) (e.g. electronic post processing in 40/100G Optical Module DSP)



What is DWDM?

Optical Basics

Optical Fiber




• OTN Basics

• DWDM Technology



• DWDM Software


Fiber Non Linear Effects

• As long as the optical power density within the optical fiber core is

low, the fiber can be considered 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 are dependent on the optical power

• High channel count, high bit rate, long reach systems require

higher per channel powers making them susceptible to non-linear

effects


Fiber Non Linear Effects

• Single channels non-linear effects

• Self Phase Modulation (SPM)

• Stimulated Brilliouin Scattering (SBS)

• Multi channel effects

• Four Wave Mixing (FWM)

• Cross Phase Modulation (XPM)

• Stimulated Raman Scattering (SRS)


Optical Fiber’s Nonlinear Index

Inte

nsit

y

Time

Slow Phase

VelocityFast Phase

Velocity

Optical Pulse

n = n0 + N2

Index of

Refraction

Nonlinear

Coefficient

Light

Intensity

• Non-linearity arises (excluding scattering NLEs) from the modulation of the refractive index of the fiber through the interaction of the high optical power

• Intensity of an optical pulse modulates the index of refraction

• Nonlinearity scales as (channel power)2


Self Phase Modulation (SPM)

• Self Phase Modulation is a single channel effect

• Through the non-linear index, as earlier mentioned, the signal

intensity variation of a channel modulates the fiber’s local refractive

index

• Therefore different parts of the optical signal see different refractive

indexes, and therefore different phase velocities

• The resultant effect on the signal depends on fiber dispersion

• For Dispersion < 0, SPM can add on to chromatic dispersion and

increase temporal broadening of the optical pulses, thus reducing the

dispersion tolerance of the system

• For Dispersion > 0, SPM can narrow the optical pulse and thus

alleviate chromatic dispersion pulse broadening


No SPM, just Dispersion

SPM + Dispersion < 0

SPM + Dispersion > 0

Self Phase Modulation (SPM)


Cross Phase Modulation (XPM)

• Cross Phase Modulation is a multi-channel effect

• Through the non-linear index adjacent channels also modulate the fiber’s local refractive index and therefore modulate the phase of the channel under consideration

• The effect of XPM is to act as a crosstalk penalty

• Increasing channel spacing reduces XPM because dispersion increases and the individual pulse streams “walk away” from each other

• Optimized dispersion compensation mapping can also reduce the effect.


Out of Fiber

1 221-2 22-11 2

Into Fiber

Four-Wave Mixing (FWM)

• Channels beat against each other to form intermodulation products

• Creates in-band crosstalk that can not be filtered (optically or

electrically)


Output Spectrum after 25 km of Dispersion Shifted Fiber

Four Wave Mixing Example

Wavelength (nm)

-5

-10

-15

-20

-25

-30

-35

-40

1542 1543 1544 1545 1546 1547 1548

Input Power = +4 dBm/chP

ow

er

(dB

m)


Channel Spacing (nm)

FW

M E

ffic

iency (

dB

)

0.0 0.5 1.0 1.5 2.0 2.5

-50

-30

-10

0

-20

-40

D= 0 ps/nm

D= 17 ps/nm

D= 2 ps/nm

D= 0.2 ps/nm

• FWM effect efficiency strongly dependant on dispersion

• With higher dispersion and greater channel spacing effect negated

• Dispersion Shifted fiber with disp zero in C-band exhibits high FWM penalty

• Uneven channel spacing can reduce effect because intermodulation products

do not fall on channels

Combating Four Wave Mixing

2( )

*( )

*FWM

eff

P nP

A D


Stimulated Raman Scattering (SRS)

• Effect and consequences

• SRS causes a signal wavelength to behave as a “pump” for longer wavelengths. Energy is transferred from the shorter to longer wavelengths

• Thus the shorter wavelengths are attenuated by this process and longer wavelengths amplified

• SRS takes place in the transmission fiber

• SRS (Raman) Amplification

• SRS can be used for amplification in the transmission fiber. Using Raman pumps it is possible to implement a distributed Raman amplifier

f fTransmission Fiber


-50

-45

-40

-35

-30

-25

-20

-15

-10

1528 1532 1536 1540 1544 1548 1552 1556 1560

Wavelength (nm)

Sp

ectr

um

(d

B)

Stimulated Raman Scattering (SRS)

• Impact of SRS in a DWDM system


Section 2: Optical Transmission Fundamentals

What is DWDM?

Optical Basics

Optical Fiber

Non Linear Effects



• OTN Basics

• DWDM Technology



• DWDM Software


Fabry-Perot Laser

• Spectrally broad linewidth

• Unstable center/peak wavelength

• Characteristic of low-cost SR/IR optics

Distributed Feedback Laser (DFB)

• Dominant single wavelength

• Tighter wavelength control

• Can be externally modulated

• Necessary for DWDM transmission

lc

l

Power

l

Powerlc

Non-DWDM Laser

Characteristic

DWDM Laser

Characteristic

Laser Characteristics


Laser Modulation

• Direct modulation

• Directly varying the laser drive current with the information stream to produce a varying optical output power, “1” and “0”

• Thermal difference between “1” and “0” state creates wavelength shift, induces spectral broadening of the laser spectrum… “Chirping”

• Spectrally broad, chirped signal has low dispersion tolerance

• External modulation

• High-speed system to minimize undesirable effects, such a chirping

• Modulation achieved through

• separate device, for example Lithium Niobate Mach-Zehnder interferometer

• or integral part of the laser transmitter, electro-absorption

• Spectrally narrow signal has high dispersion tolerance


Electrical

Signal in

Direct Modulation

• Laser diode’s bias current is

modulated with signal input to

produce modulated optical output

External Modulation

Iin

Optical

Signal out

Electrical

Signal inDC Iin

Mod. Optical

Signal

Unmodulated

Optical Signal

• The laser diode’s bias current is stable

• External modulator operates as a fast

shutter to generate a modulated optical

signal from the electrical input

External

Modulator

Laser Modulation


Common Modulation Formats

Each modulation format has advantages and disadvantages.

• IM-OOK NRZ: Intensity Modulation – On Off Keying Non Return to Zero

• RZ: return to Zero

• ODB: Optical Duobinary

• (D)PSK: (Differential) Phase Shift Keying

• (D)QPSK: (Differential) Quadrature Phase Shift Keying

• PM-(D)QPSK: Polarization Multiplexing (D)QPSK

0 1 0 1 1 0

0 0 0

Time

NR

ZR

Z

( )RxE t

( )IxE t

x̂

0

1

( )RxE t

00

11

10 01

( )IxE t

(D)QPSK(D)PSKIM-OOK


OSNR• Measures the degree of impairment when

the optical signal is carried by an optical transmission system that includes optical amplifiers.

• Optical Signal to Noise Ratio, expressed in dB, is given by the following:

OSNR=10 x log(Psig/N) + log (Bm/ Br )

where:

Psigis the optical signal power (mW)

Bm is the resolution bandwidth (nm)

Nis the noise power measured in Bm (mW)

Br is the reference optical bandwidth, typically chosen to be 0.1 nm

• Typical OSNR value in 0.5 nm resolution bandwidth is >10 dB


TX RX

With no noise

With no Inter Symbol Interference

BER=0 independent of power

• BER is a key objective of Optical System Design

BER is the number of erroneous bits received divided by the total number of bits transmitted over a stipulated period

• Goal is to get from the Tx to Rx with a BER less than the BERthreshold of the Rx

• Typical minimum acceptable system BER is 10-12 (10-15 with Forward Error Correction)

Bit Error Rate (BER)


Optical Budget

Optical Budget is affected by:

Fiber attenuation

Splices

Patch Panels/Connectors

Optical components (filters, amplifiers, etc)

Bends in fiber

Contamination/dirt on connectors

Link Optical Budget = Ptx – Prx

Where: Ptx = Transmitter output power

Prx = Receiver input sensitivity to achieve required BER performance

Ptx = +3 dBm Prx = -26 dBm

Budget = 29 dB


Eye Diagram

• The vertical eye opening shows the ability to distinguish between a 1 and a 0 bit

• The horizontal opening gives the time period over which the signal can be sampled

FOLDING

Tx bit sequence


Decision Threshold

“1” Level

“0” Level

What causes bit errors:

• Noise introduced through receivers and amplifiers

• Pulse shape distortion introduced through dispersion and non-linear effects

These contribute to errors in bit detection when determining if a bit is a “1” or a “0”

Bit Errors in Signal Transmission



What is DWDM?

Optical Basics

Optical Fiber

Non Linear Effects

Optical Transmission


• OTN Basics

• DWDM Technology



• DWDM Software


• Erbium Doped Fiber Amplifiers (EDFA)

• Operating range: C-band: 1530 to 1565 nm

L-band: 1605 to 1625nm

• Gain up to 30 dB, 1000x amplification for small signals

• High output saturation power up to +27 dBm, 500 mW

• Low signal distortion and cross-talk

• Optically Transparent

Signal format and Bit rate independent

Erbium Doped Fiber Amplifier (EDFA)


Energy = h .

Fundamental State

Excited State

Pump Photon

at 980 nm

Fundamental State

Transition to a lower energy state

Metastable State

Telecom signal

photon at 1550 nm

Energy = h .

• The photon generated by the decay of the

Erbuim ion back to Its fundamental state is in

phase with the signal photon that initiated the

Stimulated Emission

+ =Amplified Telecom

Signal

Photon at 1550 nm

= Erbium Ions

EDFA Gain Mechanism


Signal

Input

980 or 1480 nm

Pump Laser

Erbium

Doped

Fiber

Amplified

Signal

Output

Isolator

WDM Coupler

for pump and

signal

Isolator

EDFA Components

• Gain though high power pump laser(s) at either 980nm or 1480nm pumping

into the absorption bands of the erbium ions

• Input and output isolators stop the EDFA “lasing” due to reflected power

passing back through EDFA

• WDM coupler efficiently combines pump and signal wavelengths

Basic EDFA

configuration


Gain and Decibels (dB)

• Gain can be expressed by the ratio of Pout/Pin

• Gain is measured more conveniently in dB , calculated by

10 log10 Pout/Pin

• If the power is doubled by an amplifier, this is +3 dB

• Example: Pout/Pin = 50, Gain = 17 dB

AmplifierPinPout


EDFA Modes of Operation

AMP

Gain 14dB

Total Output Power : +2dBm

Per channel

output power

-1dBm

Per channel

input power

-15dBm

AMP

Total Output Power Constant : +2dBm

Total Output

Power +2dBm

Per channel

power -4dBmPer channel

power -15dBm

AMP

Gain Stays Constant : Gain 14dB

Total Output

Power +5dBm

Per channel

power -1dBmPer channel

power -15dBm

Constant Gain Mode Constant Power Mode

Total Input Power : -12dBm


• Automatically corrects amplifier gain for capacity change, ageing effects, operating conditions

• Keep traffic working after network failures

• Prevent BER degradation due to network degrade

EDFA Modes of Operation

• For DWDM applications Constant Gain mode is preferred

• Constant Power mode suitable for single channel applications


EDFA Gain Spectrum

Ch1C

ha

nn

el

Po

we

rCh40

EDFA non-flat

gain spectrum

Non-flat amplified

signal spectrum

Pump bands

Gain band

• Erbium absorption and

emission lines.

• The multiple emission

lines gives rise to the

broad spectrum of the

EDFA


Channel Power Evolution Through EDFA


EDFA OSNR Degradation

• EDFAs are the source of noise, Amplified Spontaneous Emission noise

(ASE) in a system

• The difference between the optical power of a channel and the noise

power is called the Optical Signal to Noise Ratio, OSNR

• Between EDFAs, the OSNR stays constant

• The lower the input power to the EDFA the lower the OSNR at the output

• The only way to recover OSNR is via an OEO Regeneration.

• OSNR is tracked on a per channel basis, each channel will have a

different OSNR

Every optical interface (line card, Transponder etc) has a minimum

OSNR specification that must be met



What is DWDM?

Optical Basics

Optical Fiber

Non Linear Effects


Erbium Doped Fiber Amplifiers

• OTN Basics

• DWDM Technology



• DWDM Software


• Optical Transport Network (OTN) is a group of DWDM optical transport standards:

G.709 (digital wrapper) defines hierarchy and frame structures.

G.872 defines architecture.

G.798 defines management functions, etc.

• OTN is a transparent protocol that is very similar to SONET/SDH:

Similar frame structure.

Similar OAM&P capabilities.

Similar protection schemes.

• The main differences between OTN and SONET/SDH are:

Forward error correction (FEC) for DWDM transmission

Higher bit rate than SONET (100G and beyond)

Multiple layers of performance monitoring


Framing and Rates


• OTN defined a fixed “hierarchy” of payloads from OTU1 (2.5G) to OTU3 (40G). Now ODU0 (1G) and OTU4 (100G) are being added.

• OTN started as a pure wrapper around WDM client signals to improve reach and manageability.

• Recently it has developed into a complex multiplexing structure that makes OTN a good choice for a transport network as it can carry multiple types of traffic—data, voice, or video—with a common framing structure, while still allowing for service-level guarantees, management, monitoring, and error correction.

Frame Payload (OPU)

ODU-0 (coming) 1,238,954 kbps

OTU-1 2,488,320 kbps

OTU-2 9,995,276 kbps

OTU-3 40,150,519 kbps

OTU-4 (coming) 104,355,975 kbps

Payload


• ITU simultaneously defined an ODU0 at 1.25 Gbps to carry GigE

• This supplants ODU1 (2.5 Gb/s) as the fundamental TS size

• ODU4 is divided into 80, 1.25 Gb/s Time Slots

• ITU also defined an ODUflex container, of which ODU2e is the first

Clients and Mappings


OPU

ODU

OTU OTU OTU

OT

ND

WD

M

OCh

OMS

OTS OTS OTS



What is DWDM?

Optical Basics

Optical Fiber

Non Linear Effects



OTN Basics

• DWDM Technology



• DWDM Software


OA OADM OA

LTE

LTE

LTE

OEO

OEO

LTE

LTE

LTE

DWDM Building Blocks

OEO

OEO

Optical

Amplifier

Optical

Add/Drop

DWDM

MultiplexerDWDM

Demultiplexer

ITU Line

Card

Grey Line

Cards

Transponder Transponder


Transponder O-E-O Convertor• Converts the wide spectrum client laser input into a DWDM ITU compliant

channel

• Converts the DWDM channel into a client compliant output signal

• Down side is that they add significant cost

lc

l

Power

l

Powerlc

OEO

TRANSPONDER

ITU Wavelength specific

DWDM trunk channel Broad spectrum, broad range

wavelength client signal


Externally Modulated Transponder

l l

l1 l1l1

Splitter

Splitter

Laser

Alarm Unit

Electrical

Optical

Optical Optical

Optical

Receiver and

RF Amplifier

Mach-Zehnder or

Electro-Absorption

External Modulator

Optical


DWDM Filter Elements: Multiplexers & Demultiplexers

• DWDM Multiplexing

MUX combines multiple channels

into a single fiber

• DWDM Demultiplexing

DEMUX separates each channel

at the output

l1, l2, l3

l1

l2

l3

DWDM

Mux

l1

DWDM

Demux

l2

l3


DWDM Filter Elements: Optical Add Drop Multiplexer OADM

• Optical Add-Drop Multiplexer

• OADM Modules allow Add-Drop of specific channels in a DWDM system

• Requires careful channel management and forecasting

• Multiple part numbers require multiple sparing

Drop

Channel

Add

Channel

Drop and

Insert


DWDM Filter Elements: Reconfigurable Optical Add Drop Multiplexer ROADM

• Operational Simplicity

• Remotely configurable

• Per wavelength SW

provisioning and management

• Simple cabling

• Faster Deployment

• No re-engineering when

capacity is exceeded as in fixed

OADM

• Increased Reliability

• Network requires fewer manual

touches

• Software configuration reduces

erroneous cabling errors

De-

MuxMux

Mux

Optical

Space

Switch

De-

mux

A

D

M



What is DWDM?

Optical Basics

Optical Fiber

Non Linear Effects



OTN Basics

DWDM Technology



• DWDM Software


Network Topologies

EDFATerminal TerminalOADM

Linear Network

EDFA

Closed Ring

(No Hub)

Amplified OADM

OADM

ROADM

Multiidegree

ROADMs

Mesh Network

Hub

OADM

EDFA

Open Ring

(Hub Ring)

ROADM


ROADM Based DWDM Networks

• ROADM Based Architecture

• Operationally efficient: Plan network once

• Dynamic traffic pattern: All nodes can talk to all nodes day one

• Minimal OPEX and CAPEX for growth and improved network performance

O

Improved

Opex Efficiency

• Fixed OADM Based Architecture

• Operationally inefficient: Re-plan network

every time a new services is added

• Fixed traffic pattern: certain sites can

only communicate with certain other sites

• Painful CAPEX and OPEX: Extensive

man hours to retune the network

OO

O

OO

O

O

O

R

R

R

R

R

R

R

R

1-8ch OADM2° ROADM


ROADM

West

ROADM

East

ROADM

What is a ROADM?

ROADM is an optical Network Element able to Add/Drop or Pass through any wavelength

– A ROADM is typically composed by 2 line interfaces and 2 Add/Drop interfaces

Typical ROADM implementations have Add/Drop interfaces dedicated to a direction

– As a side-effect, if it is required to reconfigure the connection to drop the channel from a different side the new channel is sent to a different physical port: this would require to manually change the cabling of any connected client equipment

ROADM

West

ROADM

East

Directional ROADM

Line

WestLine

East

Add/Drop

WestAdd/Drop

East

Line

WestLine

East

Add/Drop

WestAdd/Drop

East


Multi-Degree ROADMs for Mesh Applications

WXC

MUX

DMX

B

P

WX

C

MU

X

DM

X

B

P

WXC

MUX

DMX

P

B

WX

C

MU

X

DM

X

P

B

• ROADMs can be extended to multi-degree

• 8 degree node using wavelength cross connects


DWDM Mesh Benefits

2° ROADM

Mesh ArchitectureA–Z provisioning—data follows fiber topology

more efficient use of fiber

Better load balancing increases capacity

Shorter distance = less regeneration

Eliminate transponders

More options for service & protect paths

4 Transponders

Eliminated

Capacity Increase, Efficient Fiber Usage, Increased Availability

Physical RingsPhysical RingsPhysical Rings

OEO ring interconnect 2° -8° ROADM

Ring-Based ArchitectureTraffic must follow ring topology, constricted

Inefficient traffic routing increase regeneration

Costly transponders for OEO ring interconnects

Single choice for service path & protect path


Optical Channel Protection Schemes

Path & Equipment Protection

(Client protection)

Path & Equipment Protection

(Y-Cable protection)

Wo

rkin

g p

ath

Pro

tectio

n p

ath

TX

P

TX

P

TX

P

TX

P

Wo

rkin

g p

ath

Pro

tectio

n p

ath

TX

P

TX

P

TX

P

TX

P

Path Protection

(Splitter protection)

Wo

rkin

g p

ath

Pro

tectio

n p

ath

TX

PP

TX

PP



What is DWDM?

Optical Basics

Optical Fiber

Non Linear Effects



OTN Basics

DWDM Technology

DWDM Network Topologies


• DWDM Software


Power Limited Network Design

• Transmission distance limited by receiver power level

• Assuming channel power is in linear range so non-linear effects can be ignored

• Transmission distance limitation is limited by

Loss

Chromatic dispersion

PMD


Receiver Bit Error Rate

Receiver Power

BER

10-14

10-12

10-10

10-8

-22-21

-20 -18-23-24 -17

10-6

10-4

Target BER < 10-12

Min. required PRX

without fiber

TX/RX BER

without fiber

10G Xenpak BER curve

-19

In a power limited network the BER is

dependant only on receiver input power

BER over fiber leads to penalties

Target BER < 10-12 without Forward Error

Correction FEC

Target BER < 10-15 with FEC


Chromatic Dispersion Limitations

• Limit due to Chromatic Dispersion

• Typical CD tolerance at a power penalty of 2dB

2.5 Gbps: Direct Modulation typically 2400 ps/nm

External Modulation typically 20,000 ps/nm

10 Gbps: typically 1200 ps/nm

40 Gbps: typically 200 ps/nm (can be larger dependant onmodulation format)

Distance (Km) =Dispersion Tolerance of Transponder (ps/nm)

Coefficient of Dispersion of Fiber (ps/nm*km)


PMD Limitations for Fiber

• Limit due to Polarization Mode Dispersion

• Typical PMD tolerance

2.5 Gbps: typically 40 ps

10 Gbps: typically 10 ps

40 Gbps: typically 2.5 ps (can be larger dependant onmodulation format)

• Power penalty due to PMD (1-2 dB)

• Need to account for fiber and system components PMD

Distance (Km) =PMD Tolerance of Transponder (ps)

PMD Coefficient of Fiber (ps/km1/2)


Example: Link Design with Booster

+5 dBm/ch

Rx: -19dBm

90 km SMF @ 0.23 dB/km

~21 dB, ~1600 ps/nm

10G Xenpak spec: Tx: +3 to -1dBm, Rx min: -21dBm (0ps/nm)

CD tolerance: +1600ps/nm @ 2dB penalty.

From calculations require Rx power of >-19dBm

Mux

Dem

ux

TX RX

-16 dBm/ch

3 dB loss

-1dBm +5dBm-16dBm

1600ps/nm0ps/nm

-19dBm

1600ps/nm0ps/nm

3dB loss

Tx: -1dBm min

Time

Domain

Meets receiver

minimum power

requirement


OSNR Limited Network Design

• Transmission distance limited by receiver OSNR limitation

• No power limitation through use of receive side pre-amplifier

• OSNR limited designs generally contain links with multiple EDFAs

(or very long spans)… EDFAs add noise!

• Non-linear effects (NLE) become important

• Transmission distance limitation is limited by

Minimum receiver OSNR requirement

Chromatic dispersion

Non-linear effects

PMD


Link Design with Line Amplifiers10G Xenpak spec: Tx: +3 to -1dBm, Rx min: -21dBm (0ps/nm)

CD tolerance: +1600ps/nm @ 2dB penalty

OSNR min: 16dB (0.5nm resolution)

-1dBm +2dBm

0ps/nm

Time

Domain

OSNR: 18dB

Rx: -9dBm

Meets receiver

minimum OSNR

and power

requirement

+2dBm/ch

TX RX

Tx: -1dBm min

Mux

Dem

ux

DCU

-1600

ps/nm25dB 25dB

DCU

-1600

ps/nm

+2dBm/ch-23dBm/ch -23dBm/ch

OSNR= 21dB

Noise

OSNR= 18dB

Noise

OSNR= 35dB

Noise

-23dBm

1600ps/nm

+2dBm

0ps/nm

-23dBm

1600ps/nm

+2dBm

0ps/nm

OSNR= 35dB

Noise

OSNR= 21dB

Noise

8dB

3 dB loss



What is DWDM?

Optical Basics

Optical Fiber

Non Linear Effects



OTN Basics

DWDM Technology

DWDM Network Topologies

Optical Transmission Systems Network Design

• DWDM Software


• Modern systems compensate real-time for variations in the network

Gain Equalization

Amplifier Control

Automatic Node Setup

Automatic Power Control

• Allows for less truck rolls and maintenance windows


Why Per-Channel Optical Power Equalization

• For amplifiers to operate correctly, all channels must be equalized in power.

• If channel powers are not equal, more gain will go to the higher powered channels.

• Channel power is inherently unequal due to different insertion losses, different

paths (add path vs. express/pass-through), etc.

• Controlling the optical power of each channel in an optical network is required.

AMP

AMP

Optical Power Equalized Channels

Channels with Unequal Optical Power

OADM Without Power Equalization

Express Path

Add/Drop

Path


OADM Without Power Equalization

Express Path

Add/Drop

Path

AMP AMP

OADM With Power Equalization

Express Path

Add/Drop

Path

AMP AMP


• DWDM set-up

Paper/e-documents transfer—truck rolls!

• ONS 15454 MSTP set-up

From network design straight to installation!

Remote

Operation


Express Path

Add/Drop Path

AMP AMP

T3

T Target Power

T2

T1

VOA

T4

ANS Target

Powers

Per Channel

Power

T1 +2dBm

T2 -16dBm

T3 -9dBm

T4 +2dBm

Express Path VOA

Constant AttenuationL1

L2

L3

Loss dB

L1 (Express

Drop)

2.5dB

L2 (Per Ch

Add)

5.0dB

L3 (Express

Add)

2.5dB

L4 (Per Ch

Drop)

5.5dB

VOA dB

Express Path

VOA

6dB

Add VOA N/A (depends upon

laser TX power

Drop VOA 12.5dB (Start point)

Add/Drop VOA

Constant Power

L4

• Target Power comes from design tool or Measured Span Loss Values from System

• Loss values are measured and stored in the OADM(s) / ROADM(s)

• Constant Attenuation VOA’s set via ANS software logic

• Constant Power VOA’s set to close loopLossL


• Automatically corrects amplifier power/gain for capacity change, ageing effects, operating conditions

• Keep traffic working after network failires

• Prevent BER due to network degrade

• Keep constant either power or gain on each amplifier

• No truck rolls

• No troubleshooting required

• No operation complexity

APC

No Human Intervention Required


Dro

p M

ux

VOAPhoto-

Diode

lx

Rx

Network Element

Tx

VOA

Photo-

Diode

lx

VOA

Photo-

Diode

Optical Add/Drop Module (OADM)

AD

D M

ux

Express Path

AMP

Dro

p M

ux

VOAPhoto-

Diode

lx

Rx

Network Element

Tx

VOA

Photo-

Diode

lx

VOA

Photo-

Diode


Add/Drop

Channel

AD

D M

ux

Express Path

AMP

16dB

Target Power

+2dBm

Target Power

+2dBm

Target Power -9dBm

Target Power -15dBm

Loss 5.5 dB

Loss 5.5 dB

Gain

11dB

Gain

14dB

11.5 dB

TX Power +5dBm

Amplifiers don’t readjust since they

are in Gain Control Mode

VOA

Photo-

Diode

lx

TX Power +3.2dBm

AMP 1 Power AMP 2 Power

Total Input Power -6dBm Total Input Power -11dBm

Per Channel Input Power -9dBm Per Channel Input Power -14dBm

Total Output Power +5dBm Total Output Power +3dBm

Per Channel Output Power +2dBm Per Channel Output

Power

0dBm


Dro

p M

ux

VOAPhoto-

Diode

lx

Rx

Network Element

Tx

VOA

Photo-

Diode

lx

VOA

Photo-

Diode


AD

D M

ux

Express Path

AMP

Dro

p M

ux

VOAPhoto-

Diode

lx

Rx

Network Element

Tx

VOA

Photo-

Diode

lx

VOA

Photo-

Diode


Add/Drop

Channel

AD

D M

ux

Express Path

AMP

16dB

Target Power

+2dBm

Target Power

+2dBm

Target Power -9dBm

Target Power -15dBm

Loss 5.5 dB

Loss 5.5 dB

Gain

11dB

Gain

16dB

11.5 dB

TX Power +5dBm

APC Corrects the Gain Setting on

AMP 2 to maintain a per channel

power of + 2dBm

VOA

Photo-

Diode

lx

TX Power +3.2dBm

AMP 1 Power AMP 2 Power

Total Input Power -6dBm Total Input Power -11dBm

Per Channel Input Power -9dBm Per Channel Input Power -14dBm

Total Output Power +5dBm Total Output Power +5dBm

Per Channel Output Power +2dBm Per Channel Output

Power

+2dBm


• ALS is required to decrease the risk of laser damage to the human eye

• The complete sequence of events is completed within 1s as required by IEC 825-2

• This is not possible on passive dwdm systems

OSCM

OPT-BST Node B

East side

OPT-PRE

P

P

OSCM

OPT-BST

Node A

West side

OPT-PRE

Fiber cut

Amplifier Automatic

Lasers Shutdown

Payload (LOS-P) & OSC

(LOS-O) detected

1

1

Loss Of Signal (LOS) is

declared1

Amplifier Automatic

Lasers Shutdown

P

P

Amplifier Automatic

Lasers Shutdown

Payload (LOS-P) & OSC

(LOS-O) detected1

1

Loss Of Signal (LOS) is

declared 1

Amplifier Automatic

Lasers Shutdown

LOS-O is detected

OSCM Automatic

Laser Shutdown

LOS-O is detected

OSCM Automatic

Laser Shutdown

Thank you.

optical transmission fundamental

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