2 Components of WDM Networks 2.1 Optical Fibers ... - BU

Optical Networks

Poompat Saengudomlert

Session 2

Building Blocks of Optical Networks:Optical Fibers & Optical Couplers

P. Saengudomlert (2018) Optical Networks Session 2 1 / 16

2 Components of WDM Networks2.1 Optical Fibers

Key benefits of optical fiber as transmission medium(compared to metallic cable)

Lower loss

⇒ larger transmission distance

Higher bandwidth

⇒ larger transmission bit rate, future-proofed technology

Smaller and lighter

More immune to electromagnetic interference (EMI)


Typically made of silica

Commonly used fibers have core and cladding layers.

cladding

core50125

cladding

core

10125

multimode fiber cross section single-mode fiber cross section

Light travels along a fiber core through total internal reflection.

cladding

core

mode 1mode 2


Reflection and Refraction

Refractive index: ratio between light speed in vaccumm and lightspeed in material

Light reflection and refraction

incident vector(propagation direction

of incident light)

reflected vector

unit-norm vectornormal to boundary

refracted/transmittedvector

ϕI = ϕR (law of reflection)

nI sinϕI = nT sinϕT (Snell law)

nI > nT : possible to have ϕT = 90o

The corresponding ϕI is the critical angle ϕC = arcsinnTnI


Numerical Aperture

Typically, ncore ≈ 1.5, ncladding/ncore ≈ 0.99 ⇒ ϕC ≈ 82o

cladding

core

air

At air-core interface, maximum θ yielding ϕ > ϕC is acceptance angle

nair sin θmax = ncore sin(90o−ϕC ) = ncore cosϕC =

√n2core − n2cladding� ��

numerical aperture (NA)

The higher the NA, the easier it is to couple light into fiber.


Signal Degradation in Optical Fibers

Three types of signal degradation

Attenuation or loss⇒ low signal-to-noise ratio (SNR) at receiver

Dispersion⇒ pulse spreading and inter-symbol interference (ISI)

Fiber nonlinearity: scattering, Kerr effect, four wave mixing (FWM)⇒ reduced transmit power, pulse spreading, and crosstalk

Still cannot minimize all three at the same time!


Attenuation/Loss

Loss parameter αatt is usually expressed in dB/km.

P(L): signal power at distance L (in km) from transmitter

P(L)dBm = P(0)dBm − αattL

Three major low-loss windows

850 nm1300 nm1550 nm (most popular for WDM)

The 40-nm band (called C-band) around 1550 nm is widely used forlong distance transmission in WDM networks due to the availability ofoptical amplifiers.


Fiber Loss Characteristics

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

Wavelength

Opt

ical

loss

(dB

/km

)

8

7

6

5

4

3

2

1

0

Early 1980s

ModernFiber

Late1980s

Firs

t Win

dow

Sec

ond

Win

dow

Third

Win

dow

(L b

and)

Third

Win

dow

(C b

and)

Loss of single-mode fiber (www.cisco.com)


Dispersion

Large core diameter yields multiple propagation modes⇒ multimode fiber⇒ intermodal dispersion

cladding

core

mode 1mode 2

Dispersion: Different signal components travel at different speeds.

Intermodal dispersion⇒ pulse spreading⇒ inter-symbol interference (ISI)


ISI

es

Inputpulses

Distance along fiber →

Puls

e sh

apes

and

am

plitu

de

Outputpattern

ISI

pulses

ISI leads to high bit error rate (BER).

Most commercial transmission systems rely on sampling andthreshold-based detection. No complicated equalization is done atGbps rates.


Chromatic Dispersion

Small fiber core⇒ single propagation mode⇒ single-mode fiber⇒ no intermodal dispersion; chromatic dispersion dominant

Chromatic dispersion: Different frequency components travel atdifferent speeds.

Chromatic dispersion parameter D (typically in ps/nm/km)

D =1

L

dτ

dλ

L: fiber length (in km)τ : propagation delay (in ps)λ: wavelength of light (in nm)


Dispersion Characteristics

Waveguidedispersion

Materialdispersion

Totaldispersion

1.1 1.2 1.3 1.4 1.5 1.6 1.7

- 20

- 10

0

10

20

30

Dispersion,

(ps/(nm-km))

D

Wavelength

Normaldispersion

Anomalousdispersion

Material dispersion: ncore and ncladding are λ-dependent.Waveguide dispersion: Core/cladding power distribution (neff) isλ-dependent.


Fiber Refractive Index Profile

Can change chromatic dispersion through changing refractive index profile,i.e. changing waveguide dispersion.

(a) Step-index fiber (b) DSF (c) DCF

Dispersion-shifted fiber (DSF) has zero dispersion and low loss at1550 nm.

Dispersion compensation fiber (DCF) has dispersion of opposite signscompared to regular fiber to cancel dispersion.


Nonlinear Effects

At high transmit power (a few mW) and bit rate (> 2.5 Gbps),a transmitted signal is affected by fiber nonlinearity.

Scattering: power loss due to interaction with molecules

Kerr effects: neff is intensity dependent

Four-wave mixing (FWM): intermodulation product of fi , fj , fk ,yielding “fourth” frequencies at

±fi ± fj ± fk


Dispersion vs. Nonlinear Effects

Trade-off between dispersion and nonlinear effects.

For WDM, zero dispersion ⇒ high crosstalk from FWM

Distancefromcorecenter

Refractiveindex

Distancefromcorecenter

refr

activ

e index

distance from core center distance from core center

common NZ-DSF LEAF

Non-zero DSF (NZ-DSF) tolerates some dispersion to reduce FWM.

Large effective area fiber (LEAF) is a NZ-DSF that spreads opticalpower more evenly in fiber, reducing nonlinear effects.


2.2 Optical Couplers

A coupler is used to combine/split optical signals.

Simple to make by fusing 2 (or more) fibers together.

Relationship between input and output powers

[Pout,1

Pout,2

]= γ

[α 1− α

1− α α

] [Pin,1

Pin,2

]

α: coupling ratioγ: excess loss

Cannot do lossless combining.

Each input leads to outputs different in phase by π/2 rad.


Documents

2 Components of WDM Networks 2.1 Optical Fibers ... - BU