optical fiber _material science10

8/7/2019 Optical Fiber _material science10

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Optical Fiber



What is an optical fiber?

• Optical fibers are very fine fibers of glass.

• Usually glass “core” ---- size -----roughly 50 micrometres (diameter)

glass "cladding" ---- size ------ 120 micrometres (diameter).

• They make use of total internal reflection to confine light within the

core of the fiber.



Structure of a Fibre

• The core has a higher refractive index than the cladding. --- TIR.

• The cladding is not just a mere covering but controls critical angle.

• Optical Fibres are optical waveguides. So optical fibres can beused to make light bend round corners



What is an optical fiber (continued)• Fiber ---- Buffer coating (for protection ) ---- PVC loose

tube (fiber to flex and bend) ---------- Kevlar yarn

strength member (absorbs most of the strain) ---------

Finally, a PVC outer jacket (prevents from moisture).



The Optical Fiber: Other Types of Fibers• Single Mode fiber---- small core diameter ------ Glass,

Silica, ----- data transmission for long distance --- losses

are less

• Multi mode fiber ------- Large core diameter ----- Glass,silica ----- high power lasers and sensors, such as

medical laser-surgery

• All-plastic fibers (multi mode)

– Useful for very short data links with in equipment because it may

be used with relatively inexpensive LEDs.– An isolation system for use as part of a high voltage power

supply.



Standard Single-Mode (SM)

Fiber Fiber core

SiO2+ GeO2

Ø 10 μm

n ≅ 1.443

SiO2 Cladding

Ø 125 μm

n ≅ 1.44

Primary coating (soft)Ø 400 μm

Secondary coating (hard)

Ø 1 mm



Optical Fibers in brief

• An optical fiber consists of a high-index glasscore in a low-index glass sheath

• When light tries to leave the high-index core at

a shallow angle, it experiences total internalreflection

• Light bounces endlessly through the core and

emerges from the end of the fiber

• If the glass is pure and perfect enough, the

light may travel for many kilometers throughthe fiber



History of Fiber optics

During 1930, ideas were developed with this fiberoptic such as transmitting images through a fiber.

• During the 1960s, Lasers were introduced asefficient light sources

• In 1970s , All plastic fibers experienced excessive

optical loss. This motivated the scientists to developglass fibers.

• Applications such as medical environment to the

broadcasting industry. It is used to transmit voice,

television, images and data signals through small

flexible threads of glass or plastic.



The idea was developed in 1930 then why it

is implemented in 1970-1980• Interest in the use of light as a carrier for

information grew in the 1960's with the advent of

the laser as a source of coherent light.• Initially the transmission distances were very

short, but as manufacturing techniques for very

pure glass arrived in 1970, it became feasible touse optical fibres as a practical transmissionmedium.

• At the same time developments in semi-conductor light sources and detectors meant thatby 1980 world wide installation of fibre opticcommunication systems had been achieved.



Reflection & refraction

n2<n1

n1

θ1 θ1

ϕ1

θ2

ϕ2

Snell’s law

2211 sinsin ϕ ϕ nn =

2211 coscos θ θ nn =

n2<n1

n1

θ1= θc

ϕc

Critical angle

1

2sinnn

c =ϕ

1

2cos

n

nc =θ

n2<n1

n1 ϕ1 >ϕc

Total internal

reflection



Total Internal Reflection



Propagation of light in optical fiber

θA= Acceptance angle

φC φC



Show that Numerical Aperture

1

2sinn

nc =ϕ Critical angle:

Maximum entrance angle:r A

n

nθ θ sinsin

0

1max, =

Multimode fiber

n1

n2

θA

n0

n0

2

2

2

1

2

111max,0 sin1cossinsin nnnnnnNA ccr A −=−===≡ ϕ ϕ θ θ

1

21

2

1

2

2

2

1

21

2

: if

n

nn

n

nn

nnn

−≈

−≡Δ

=≈Δ≈−= 2

2

2

2

1 nnnNA

°≈⇒= 61.0 max,ANAIf θ

φcθr

2

2

2

10 sin nnnNA A −== θ

Where ∆ is called fractional refractive index change



Modes & Rays

waveguide

d

θ2 θ1 θ0

m=0 m=2m=1

( )d

mk mx

π 1,

+= ⎟⎟⎠

⎞⎜⎜⎝

⎛ = −

0

,1sinnk

k mx

mθ



V-parameter

• V number: determines how many modes a

fiber supports

• Single-mode fiber:

( ) NAa

nna

V

λ

π

λ

π 22 2

2

2

1 =−=

405.2≤V



Number of modes

• Number of modes in step-index fiber

( ) 2

2

2

1 22

2

2

1

2V

nna

M =−⎟⎠

⎞

⎜⎝

⎛ ≈λ

π



Dispersion in Fiber Optics

• Dispersion occurs when photons from the same

light pulse take slight different paths along the

optical fiber.• Because some paths will be longer or shorter

than other paths.

• The photons will arrive at different times thussmearing the shape of the pulse.



Dispersion Continued …

Normal fiber optic cable is called

multimode because photons can takedifferent paths along it.

The more expensivemonomode

fibre opticovercomes dispersion by having a core so

thin that the light can only take one path

along it.



Attenuation loss

Pin Pout

kmdBP

P

L out

in / log10

10 ⎟⎟⎠

⎞⎜⎜⎝

⎛ =α

Example

Pin=5mW Pout=30µW

L=40 km

Determine Attenuation per unit length α ( dB/km)

Answer: 0.56 dB/km

Attenuation loss

per unit length

L



Types of Optical Fiber

• Glass fiber

– Step Index Fiber • Single mode (SM Fiber)

• Multimode (MM Fiber)

– Graded Index Fiber

• Plastic fiber



Step Index Single Mode

GRIN Multimode fiber

Step Index Multimode fiber

Types of Optical Fiber

Optical fibers are the actual media that guides the light



refractiveindex

SMSingle-Mode

MM-SI

Multi-ModeStep Index

MM-GIMulti-Mode

Graded Index

n1 = ncore(r=0)

n2 = nclada = Core radius

r = 0 at centre

Stepfunction

Step

function

nclad

n(r)

( )

1 / 2

1

2 2

1 22 2

1

1 2

( )( ) , .2

r n r n fo r r a

a

n nn r n fo r r an

α ⎡ ⎤⎛ ⎞= − Δ <⎢ ⎥⎜ ⎟⎝ ⎠⎢ ⎥⎣ ⎦

−= ≥ =∆



Optical communication systems

First Generation, ~1975, 0.8 μm

MM-fiber, GaAs-laser or LED

Second Generation, ~1980, 1.3 μm, MM & SM-fiber

InGaAsP FP-laser or LED

Third Generation, ~1985, 1.55 μm, SM-fiber

InGaAsP DFB-laser, ~ 1990 Optical amplifiers

Fourth Generation, 1996, 1.55 μm

WDM-systems

1.80.8 1.0 1.2 1.4 1.60.9 1.1 1.3 1.5 1.7Wavelength (μm)

At t e nu a t io n

2 dB/cm

Th d t f fib ti



The advantages of fiber optic over

wire cable

• Thinner

• Higher carrying capacity

• Less signal degradation

• Light signal

• Low power

• Flexible

• Non-flammable• Lightweight

Di d t f fib ti



Disadvantage of fiber optic over

copper wire cable• Optical fiber is more expensive per meter

than copper • Optical fiber can not be join together as

easily as copper cable. It requires trainingand expensive splicing and measurement

equipment.



Fiber materials

• Silica glass fiber – starting material: pure silica (SiO2) in the form of

fused quartz (amorphous)

– modification of refractive index by addition of

impurities• lowering refractive index : B2O3, F

• raising refractive index : P2O5, GeO2

• Polymer optical fiber (POF)– large core (multimode)

– large refractive index difference between core and

cladding– easy handling

–



Communication Issues

• Light must remain together during passage

– Dispersion and path differences are bad

– Use laser light (monochromatic)

– Use low-dispersion glass at its best wavelength

– Use narrow (single-mode) fiber • Light attenuates during the trip

– Use low-loss glass

– Amplify the light periodically (Use repeaters)– Use fiber laser amplifiers



Electromagnetic spectrum

• Optical communication wavelength: λ =1500 nm

corresponds to

ν = c/λ ≈ 200 THz = 200.000 GHz

• 1% = 2 THz = 2000 GHz

• EDFA-bandwidth 30 nm ≈ 4 THz



1.80.8 1.0 1.2 1.4 1.60.9 1.1 1.3 1.5 1.7

Wavelength (μm)

At

tenuation (d

B/km)

0.2

0.5

1.0

1.5

0.16 dB/km

Rayleigh

scattering

IR band edge

OH--peak

UVabsorption

0.70.6

Fiber attenuation (SiO2)

optical fiber _material science10

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