The ElectroMagnetic Spectrum

Les Ondes Optiques

λ = 100 mm

λ = 1 mm

f = 3 GHz

f = 300 GHz

RADIO FREQUENCIES

MICRO WAVES

Riad Haïdar OPTICAL FREQUENCIES Propagation : optical fibers

ONERA Département d’Optique Théorique et Appliquée 2

Fiber propagation

Reflection & Refraction

ϕ2

n2 n2 n2

ϕ1

θ1

θ2

Snell’s law n1 sin ϕ1 = n2 sin ϕ 2 n1 cos θ1 = n2 cos θ 2

3

n2> Pertes typiques de 0,05 dB sur les fibres standard télécoms. 1.5

1.0

Rayleigh  R.Scatt ∝ 1    λ4  scattering 

IR band edge

UV absorption

Light insertion

electric arc

OH--peak 0.5

Micro Positioning

0.2

Light detection

Détection de Light lumière detection Détection de Micro lumière Positioning

0.16 dB/km

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Wavelength (µm) 7

8

2

Other Fiber losses (1)

Other Fiber losses (2) Courbures :

longitudinal D

>> Il y a modification des conditions de réflexion : un rayon totalement réfléchi dans un guide droit, peut s'échapper par réfraction lorsque le guide est courbé.

radial d

>> Les fibres monomodes tolèrent un rayon de courbure de l'ordre de 10 cm sans perte notable angular θ cores misaligning

>> les pertes croissent exponentiellement avec la courbure.

cores ellipticity 9

Numerical Aperture

Numerical Aperture

Multimode fiber θ0

n0 n0

θc

n2 n1

Critical angle:

10

cos θ c =

n2 n1

Maximum entrance angle:

sin θ 0,max =

if n1 ≈ n2 = n : 2

∆≡ n1 sin θ c n0

2

n1 − n2 n −n ∆n ≈ 1 2 = 2 n1 n 2n1 2

2

NA = n1 − n2 ≈ 2n ⋅ ∆n ≈ n 2∆ Numerical aperture:

NA ≡ n0 sin θ 0,max = n1 sin θ c = n1 1 − cos 2 θ c = n1 − n2 2

NA = 0.1 ⇒ θ 0,max ≈ 6°

2

11

12

3

Dispersion (intermodal)

Fiber types

L

n1

θc

n2

Tmin = L ⋅

n1 c

Tmax = L ⋅

n1 c ⋅ cosθ c

SM Single-Mode

cos θ c =

n2 n1

MM-SI Multi-Mode Step Index

MM-GI Multi-Mode Graded Index t

α  r  n(r ) = n1 1 − 2∆   a    

1/ 2

refractive index

13

14

Fiber classification (1)

Fiber classification (2)

MM-SI: Multi Mode - Step Index fiber

MM-GI: Multi Mode - Graded Index fiber

Core diameter (coeur)

50 - 400 µm

Core diameter

50 µm standard

Cladding (gaine)

125 (500) µm

Cladding

125 µm

2nd coating (2nde gaine)

250 - 1000 µm

2nd coating

200-1000 µm

NA (ouverture numérique)

0.16 - 0.5

NA

0.2 - 0.3

Attenuation

1 - 4 dB/km

Attenuation

1 dB/km (1300 nm)

Bandwidth

6 - 25 MHz.km

Bandwidth

150 MHz.km - 2 GHz.km

Application

Short distance, low cost

Application

Medium distance communication

limited bandwidth

LED/Laser sources

15

16

4

Fiber classification (3)

Modes & Rays waveguide

SM-SI: Single Mode - Step Index fiber Core diameter

3-10 µm

Cladding

50-125 µm

2nd coating

200-1000 µm

NA

~0.1 (not used)

Attenuation dB/km

0.20@1550nm - 0.4@1300nm

θ2

θ1

θ0

d

m=0

Bandwidth

>> 500 MHz.km

Application

Long distance communication

m=1

k x ,m =

m=2

(m + 1)π

 k x ,m    nk0 

θ m = arcsin

d

Lasers, standard fiber 17

18

Mode intensity profiles • Optical modes: d

Planar:

V=

Number of modes 2πd

λ

2

n1 − n2

2

• Number of modes in step-index fiber

Single-mode if V ≤ π 0

1

2

a

Fiber:

V=

2πa

λ

(

)

1  2πa  2 V2 2 M≈   n1 − n2 = 2 λ  2

2 2

n1 − n2

if V > 2.405

2

• Optical power in the cladding (gaine optique)

Single-mode if V ≤ 2.405

Pcladding P

≈

4 3 M

for large values of V

V number >> determines how many modes a fiber supports 19

20

5

Silica fibers – preform fabrication Gases in O2, He SiCl4 GeCl4 BBr3 POCl3

Fiber materials

preform Silica tube

Heating ring

• 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

furnace

Gases out Deposit

Diameter control Polymer coating solution Polymer curing

Modified chemical vapor deposition for preform fabrication

• Polymer optical fiber (POF)

Takeup reel Pulling drive

Pulling machine

– – – –

large core (multimode) large refractive index difference between core and cladding easy handling relatively high losses

21

22

Advantages of Optical communication

A note on dB and dBm • dB

• Huge bandwidth • Low loss • Low cost per bit

– optical signals:

P 10 log 1   P2 

– electrical signals:

V  I  VI  20 log 1  = 20 log 1  = 10 log 1 1  V I  2  2  V2 I 2 

• Small and light –

• Electrical isolation

Popt ∝ I el ∝ Pel

→ electrical dB = 2 x optical dB

• dBm

• No EMI (Lightning, interference)

– absolute power value (with 1 mW as reference)

• Security (no tapping)

– power level in dBm: 10 log P   1mW 

• Reliability 23

24

6

Transmission optique

Transmission optique

IV – Le Multiplexage

La solution : on combine Mux/Demux électronique et optique => le Wavelength Division Multiplexing WDM

Les réseaux optiques « classiques » sont bridés en débit :

Fibre optique

Fibre optique

nxλ Σλ

λ

n x 2 Gbits/s

2 Gbits/s

Multiplexeur/démultiplexeur électronique

Multiplexeur/démultiplexeur électronique

Conversion électronique/optique (laser) ou optique/électronique (détecteur)

Conversion électronique/optique (laser) ou optique/électronique (détecteur)

Demux/Mux optique 25

Polarization maintaining fibers

Losses in polymer optical fiber

BSiO2

non-cylindrical geometry

• Absorption loss in POF >>> Absorption loss in Silica fiber → search for low loss polymers • PMMA (Poly Methyl Metacrylate) • PS (Polystyrene) • FA (Fluoro acrylate) – Typical absorption levels: 100 dB/km – Low loss windows: several windows in the range 500-800 nm

and/or Ge - SiO2

stress induced anisotropy

Elliptic core

26

“Bow-tie”

• New material development: perfluorinated polymer → 50 dB/km from visible to 1600 nm • Core type

→ Modal birefringence → Little mode coupling → Maintenance of linear polarization

• Step index • Graded index 27

28

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