Optical fiber basics in a nutshell

Optical fiber basics in a nutshell

Nuphar Lipkin, Lambda Crossing, Israel

Talk outline

(a taste of)

:

(Late 70-s: 1st _{phone lines, 1988: 1}st _{TAT, now: FTTH)}

• Optical communication systems- basic concepts,

advantages

• Physics of optical wave-guiding

• Optical fibers-

most demanding transmission WG-s, types, material requirements and limitations

• Optical communication systems

use light as the primary medium

to carry information

• The light is usually guided

through fibers (fiberoptic

technology)

• Most applications use Infrared

light (

1.55

,

1.3µm

)

Digital modulation “bit”, “bit rate”

Why optical communication?

Main advantage:

• Most cost effective way to

move huge amounts

*

of

information (voice, data)

quickly and reliably over

long distances

and:

• Light is insensitive to

electromagnetic interference

* ADSL: 6 Mbit/s down +640 kbit/s up to ~4-6 km

Fiber:2.5-10 Gbit/s per single channel, ~400 Gbit/s per single fiber, ~50km

FiberOptics vs. Free- space:

• no atmospheric interference

Wavelength division multiplexing

# of channels:

-width of transmission band (opto- electronics) - width of channel, channel separation (filters, wavelength stability of components)

WDM:

2-8 channels

DWDM:

16-320 channels

Typical:

40-80 channels

Different λ-s passing through the same medium do not

interfere with each other

(almost)

LAN

Performance

Price

LAN: Local area networks (within building/ vehicle)

optical communication?

Silica- predominant fiber material: transmission properties

in out

I

I

dB

A

(

)

=

10

log

Optical loss of fiber- grade Silica

(absorption + scattering)

C band: 1535- 1565nm (~50 channels)

Erbium doped amplifiers

Basic functions of an optical communication link

Amplifier span: 30-120 km Regenerator span: 50-600 km

• Multiplex (WDM)

Integrated/ free space optics

• Transmit

Silica fibers

• Amplify

(Electronic, Optical (e.g.-Erbium doped))

• Regenerate pulse

shape

(Electronic, Optical)

• De-multiplex

(Optical)

All- optical networks

(Potential) Advantages:

• No need to transfer back and forth to electronics

(speed, price)

• Low power consumption

• Immunity to electromagnetic interference, ionizing

radiation, temperature

Optical communication systems -Summary:

Optical communication systems

- Can carry huge amounts of information

- Many wavelengths (channels) through same fiber (WDM)

- Different ranges/ requirements: Long haul, metro, access, LAN - Silica fibers: wavelength bands, C- band (~1550nm)

- Basic functions of a communication link - Drive for all- optical networks

Light confinement in an optical waveguide

TIR critic ray

θ

_

θ

>

n

_core

> n

_cladding

• Light is

confined and

• Ray optic description:

Optical Waveguides: Slab waveguide

Low index n1 High index n2 Low index n3 Bottom Cladding Core Top Cladding

The simplest

waveguide-slab waveguide (dielectric

layer sandwich).

Light propagating in z

Layers infinite in (z,y)

Claddings semi- infinite in x

_n

Optical waveguides: Wave optic description

Solutions to Maxwell’s equations

satisfying the condition of field

continuity at the boundary

between the optical media

⇒ Only specific propagation

modes can propagate in the

waveguide. These modes are

characterized by their field intensity

distribution functions

Core Top cladd ing Substrate / Bottom c ladding 0

2

λ

π

ω

₌

=

c

k

Solutions:

modes

guided

for

1 0

kn

kn

<

β

_m

<

m= 0,1,2…

β-

Propagation constant in z

Mode definition:

Optical waveguides: Mode shape

• The number of confined modes, and the shape of modes

are a function of:

–

∆

n (core/ cladding)

– Waveguide dimensions (height for slab waveguide)

– Operation wavelength

• Higher

∆

n , larger waveguide dimensions (vs.

λ) ⇒ more

supported modes

Symmetric Slab WG: strong confinement

http://www.unice.fr/dess_ntic/Optique/Applets/guide_plan.html

• High

∆

n

• Large diamensions (vs. λ)

⇒ Strong

confinement-Symmetric Slab WG: weak confinement

Lower ∆n, longer λ

⇒

Weaker confinement- substantial field

Symmetric Slab WG: number of modes

1-st to 4-th modes of:

n

_s

=1.45

∆n=0.2

λ= 1.4

d=3um

A 5-th mode is not

supported by this

WG.

Physics of wave guiding -Summary:

-Optical communication systems - Physics of wave-guiding:

- Confinement, d~λ, n_core >n_cladding

- Ray optics description: total internal reflection

- Wave optics description: Guided modes- solution to Maxwell’s equations

- Slab waveguide example:

- High ∆n, large diamensions (vs. λ) ⇒ Stronger confinement, more supported modes

- Optical fibers:

- Material requirements - Silica fibers

Materials for optical fibers: considerations

•

High transparency at the working λ (depending on application)

• Mechanical strength

• Environmental durability during storage/ operation • Stability of optical and mechanical properties

• Drawing to a thin fiber with excellent uniformity • Cost of material, cost of manufacturing

Crystalline materials- scattering at grain boundaries, non-isotropic (sensitivity to light polarization)

Polymers- Stability of properties, durability, transparency

Silica fibers

Preform formation – Drawing

How to transmit light 10-100km away?

• Window glass: 1dB/cm (10% left after 10cm)

• Fibers from optical glasses (from mineral SiO2):

~1000dB/ km (10% left after 10m) Main loss factors:

• absorption (λ) by traces of Fe, Cu, V, Co, Ni, Mn, Cr ions

• Absorption of OH

• scattering (λ) from irregularities in the bulk and at the core/ clad interface

Contemporary silica fibers

• Breakthrough- Corning

~1970: Gaseous precursors

• SiCl

₄

+ O

₂

⇒

SiO

₂

+Cl

₂

•

SiCl₄- liquid at room temp. Boiling point: 58C. Impurities evaporate at higher temp.

•

Current performance: ~0.2

dB/km @1.55um (10% left

after 50km)

~ theoretical limit

Fundamental vibrations of Silica Rayleigh scattering Wavelength Loss (dB)

Loss vs. Wavelength of

fiber-grade Silica

Silica fibers: index profiles

Doping:

Ge- raises RI

F - lowers RI

Silica fibers: index profile realization

http://www.vislab.usyd.edu.au/photonics

MCVD: Gases react on the inner side of a

rotating tube. Energy source- heat, layered

deposition (cladding first), Collapse of

Optical fibers (silica fibers) -Summary:

- Optical communication systems - Physics of wave-guiding:

- Optical fibers:

- Silica fibers:

- Contemporary technology: Loss brought down close to the theoretical limit (Rayleigh scattering)

- Fabrication of preform from gaseous precursers - Controlling the Index profile through dopants

Fiber structures:

• Core diameter

(# of modes)

• Index profile

• Step index

• Index profile

Main Fiber Types:

• Multimode- step index

• Multimode- Graded index

• Single mode

Fiber types: Step index multimode

Multimode

Multimode fiber: Modal dispersion

The ability to transfer data in a multimode fiber is limited by modal

dispersion

Multi mode graded index fiber

Modes traveling closer to the cladding travel

through a medium with a lower index of refraction, and their velocity

increases

Used for short- range communication

Single mode fiber

Small core- more difficult light coupling, requires connections accuracy

Multi- mode limitations: -Compensation for modal dispersion not full

-Interference between modes (modal noise) -The technological difficulty to create a perfect index profile -Complicated and

expensive manufacturing -⇒ Single mode fiber

Chromatic dispersion

(wavelength- dependent speed*)

* Signal is not monochromatic- laser linewidth, fourier transform of modulation function, etc.

dispersion

Properties of Silica:

• Variation of the refractive index with frequency

• Speed of light in the dielectric=C/n

• n(λ)- the response of various polarization

mechanisms is frequency dependent (ability to respond to high frequency fields)

Dielectric constant vs. frequency

The dielectric constant is frequency

(wavelength) dependant

Constant

Dielectric

=

Chromatic dispersion: origins

•

Properties of Silica:

variation of the refractive index with

frequency

•

Mode shape:

the more confined the mode is the higher the

effective index

• The two contributions

cancel in standard single

mode step index fiber @

~1300nm

Single mode fiber

1300nm band-lowest Chromatic dispersion 1550 band- lowest loss (Rayleigh scattering)

zero- dispercion

and

minimum loss @ 1550nm

Un- expected problem- coherence between channels (wavelengths) can cause non- linear interactions at high laser intensities (4- wave mixing)

4 3 2 1

ν

ν

ν

ν

+

−

=

GHz GHz GHz) ( 200 ) 100 300 ( : channels 100GHz spaced evenly for e.g. + = − − + + ν ν ν ν

Noise on an existing channel

The target: Shift the zero- dispersion point to ~1550nm, where attenuation is lower.

Polarization maintaining (PM) fiber

• Breaking the degeneration between TE and TM modes

(and reducing the probability for intensity transfer)

by:

– Shape induced birefringence (non- round core) – Stress induced birefringence

• Coupling of light into the fiber- along one of the main axes

• Special connectors, alignment critical

For ER > 20dB φ < 6° For ER > 30dB φ < 1.8°

Optical fibers (silica fibers) -Summary:

- Optical communication systems - Physics of wave-guiding:

- Optical fibers:

- Silica fibers: - Types of fibers:

- Multimode fiber- easy light coupling, modal dispersion - Graded index multimode fiber- partial solution to modal

dispersion

- Single mode fiber

- Transmission limitations: attenuation, chromatic dispersion

- Controlling dispersion: shifting the zero-dispersion point to 1550nm

Polymeric fibers

dB/m !! • Loss much higher than Silica (O-C,

C-H bonds)

• Transmission fair in visible, worse in the infrared

• 650nm (red LED): 150dB/km for commercial fibers, 50dB/km in labs with fluorinated/ deuterated

polymers

• Used in short range communication (inside buildings/ vehicles), for image transmission and for lighting

• Standard plastic fiber: PMMA core (n=1.492), Cladding (n=1.402), diameter= 85um – 3mm

• Stability of optical properties,

Fibers for the Infra- red

• If the theoretical limit is Rayleigh scattering- why not longer

wavelength?

• Materials:

Fluorozirconate fibers (ZrF₄+ BaF₄, 0.4-5um, 24dB/km @ 2.6um

lab-1dB/km)

Chalcogenide glasses- S, Se compounds (3.3-11um, 0.7dB/m (m) @ 5.5um)

• Rare and expensive

• Viscosity too low for fiber drawing • Fibers demonstrate low strength • Environmental durability: sensitivity

to humidity, bases

Optical fibers -Summary:

-Optical communication systems - Physics of wave-guiding

- Optical fibers:

- Silica fibers

- Polymeric fibers, Infrared fibers- limited applications due to material shortcomings

Thank you

*

References (+ pictures) include: J. Hecht, “Understanding Fiber Optics”, Prentice Hall; 3 edition, 1998; R. G. Hunsperger, “Integrated optics”, Springer-Verlag, 3rd edition 1991; S. V. Kartalopoulos, “Introduction to DWDM”, Wiley-IEEE Press 1999