Tutorial on optical fibresTutorial on optical fibres

F. ReynaudIRCOM LimogesÉquipe optique

F. ReynaudIRCOM LimogesÉquipe optique

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E cra n : v isu a lisa t io n d u fa iscea u d iffra c té

F ib re o p tiq u e

/a

a (d ia m è tre c o e u r )

0 (d iam ètre d ’u n gra in )

D im en sio n

D irec tio n

0= 2 O N

~

C a s d ’u n e f ib re o p tiq u e m u ltim o d e

n 1

n 2

t

I

E la rg issem en t d e l’ im p u lsio n

t

Is

12

2 > > 1

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optical fibre structure optical fibre structure

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S te p in d e x p ro file

n

R a d iu s .

G ra d e d in d e x p ro f i le

n

R a d iu s .

1) Generalities1) Generalities

M e c h a n ic a l c o a t in g

C la d d in g C o re

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Silica fibres typical refractive index : 1,45 – 1,50Refractive index difference

Core diameter : 5 à 50 µm Cladding diameter : 125 à 500 µm

%1gaine

gainemaxcoeur

n

nnn

%1

maxcore

claddingnnn

n cladding

Refractive index profil

optical fibre manufacturingoptical fibre manufacturing

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G a z (o x y g è n e + d o p a n t)

s il ic e tu b e

H e a te r (1 7 0 0 ° C )

In tern a l co a tin g C o lla p se step

H e a te r (1 9 0 0 ° C )

C o re C la d d in g

P reform m a n u fac tu r in g b y M C V D p ro cess

In te r n a l c o a tin g

1) preform manufacturing1) preform manufacturing

2) Drawing process2) Drawing process

P refo rm

O v en

C o a tin g

P o ly m er iza tio n u s in g U -V

C a b esta n M eca n ica lTests

F ib re ro ll

F ib re d ia m eter sen so r )

D ra w in g sp eed serv o co n tro l

D raw in g p ro cess

(Modified Chemical Vapour Deposition). PCVD (Plasma Chemical Vapour Deposition)OVPO (Outside Vapour Phase Oxydation

1) Generalities1) Generalities

2) Propagation in optical fibres2) Propagation in optical fibres

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m a x

C a se o f a n o p tica l f ib re

n 1

n 2C la d d in g

C o re

Geometrical optics

1)isin(n

nlim

2

1

Snell Decartes law :

n

To ta l re flex io n

i1 i1

n 1

n 2

i1 > ilim

)isin(n)isin(n 2211

1)cos(n

nmax

2

1

)n

narccos(

1

2

Possibility to trap light beams in an high refractive index area surrounded by a low refractive index area

i1

n 1

n 2

i2

n

R efra ct io n

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)sin(2i

Wave theory

P la n a r w a v eg u id e w ith m irro rs

a

In ten s ity

P o s itio n

2) Propagation in optical fibres2) Propagation in optical fibres

First example planar mirror guide

Propagation without losses:Intensity = 0 on mirrors

a = n i with n = integer

Two directions interference between two plane waves

n

a

)sin(2

)a2

nsin(Arc

For each n

one propagation mode

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a

n 2

n 2

n 1

In ten s ité

P o s itio n

2) Propagation in optical fibres2) Propagation in optical fibres

)sin(2i

Propagation without losses:Intensity =0 close to the core/cladding interface

a+ 2 = n i with n = integer

Two directions interference between two plane waves

Second example

Planar dielectric waveguide

Wave theory

One mode n ))2a(2

nsin(Arc

One angle n solution of the equation

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2) Propagation in optical fibres2) Propagation in optical fibres

a

n 2

n 2

n 1

In ten s ité

P o s itio n

Second example

Planar dielectric waveguide

Wave theory

))2a(2

nsin(Arcn

Limited number of modeIf only one>>>monomode

)n

narccos(

1

2n

)n

narccos(

1

2n

max

0

solutions

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E x a m p les o f m o d es L P F ib re

L P 0 1 L P 11 L P 2 1 L P 0 2

L a rg e v a r ie ty o f d irec t io n

a (c o re d ia m e tre r )

0 (O ne spot d iam etre))

N ea r fie ld

~O p tica l f ib re

2) Propagation in optical fibres2) Propagation in optical fibresWave theory

3 D interference

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2) Propagation in optical fibres2) Propagation in optical fibres

Properties of the modal structure

Decomposition of any optical field on the mode basis

a

n 2

n 2

n 1

In ten s ité

P o s itio n

Wave theory

Same transverse field distribution at the input and output

Propagation = phase shift z

n is the propagation constant

n

z.jnnout

ne.emod.aEPropagation = phase shiftn

nnin emod.aE

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m u lt im o d e o p tica l fib re

n 1

n 2

t

I

P u lse sp rea d in g

t

Is

12

2 > > 1

In p u t p u lse

2) Propagation in optical fibres2) Propagation in optical fibres

depends upon

Dispersion

Mode in a multimode fibre

Modal or intermodal dispersion

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2) Propagation in optical fibres2) Propagation in optical fibres

Dispersion

Wavelength dependent

t

M o n o m o d e fib re

n 1

n 2

t

IIs

12

P u lse sp rea d in gIn p u t p u lse

Chromatic or intramodal dispersion

-10

-5

0

5

10

15

20

25

1200 1300 1400 1500 1600

Wavelength (nm)C

hro

mat

ic d

isp

ersi

on

(p

s/n

m.k

m)

G.6

52 (0

.08

ps/n

m2 .k

m)

G.6

53

ED

FA

ban

dw

idth

G.6

55

G.6

55

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N ea r fie ld

0

F a r f ie ld

m a x 0 = /

D iffra c t io n =

Tra n sfo rm F o u r ier

D im e n s io n s D ire c tio n s (fa r f ie ld )

/a

F o u r ie rTr a n sfo r m

N ea r fie ld

3) Determination of the mode number3) Determination of the mode number

M

N

O

M N = a

x

d x

O ’

F e n te é lé m en ta ire d e la r ge u rd x s itu é e à la d is ta n ce x d e O

O O ' = x sin

Diffraction properties

Basic rules

General caseMultimode beam

Monomode beam

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L a rg e v a r ie ty o f d irec tio n

screen : fa r fie ldv isu a lisa tio n

/a

a (c o re d ia m e te r )

0 (O ne spot d iam eter))

N ea r fie ld

F a r f ie ld

0= 2 N A

~O p tica l f ib re

F o u r ie rTr a n sfo r m

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3) Determination of the mode number3) Determination of the mode number

Core diameter /a

Numerical apertureNA

Number of spots or speckles Number of modes

2221

nn)sin(NA Case of an optical fibre

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3) Determination of the mode number3) Determination of the mode number

NA20

2

220

)NA2(44s

4

aS

2

2

2

2

)NA2(4

4a

s

SN

2)NA2(

S4

a2

2

SN

a (c o re d ia m e tre r )

0 (O ne spot d iam etre))

N ea r fie ldOne specklediameter surface

Fibre core

Number of degrees of freedom

diameter surface

a

Warning: N is wavelength dependent

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3) Determination of the mode number3) Determination of the mode number

Two examples

n2=1.450

a=8µm

rad12.0nn)sin(NA 2221

Monomode fibre

1S

N2

n1=1.455

a=50µm

n2=1.450

n1=1.462

rad19.0nn)sin(NA 2221

esmod60S

N2

Multimode fibre

Warning: N is wavelength dependent

@ = 1.3µm @ = 1.3µm

4) Characterisation of optical fibres4) Characterisation of optical fibres

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F ib re h o ld er

F ib re

D étec to r

2221

nn)sin(NA

2

21

21

nn

nnR

Numerical aperture

Refractive index distribution

L ig h t so u rce

R e flec ted p o w er

O p tica l fib ren(radius)

radius

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4) Characterisation of optical fibres4) Characterisation of optical fibres

First step

Detector

Launching assembly

Fibre length L

10/010 LII

)I

I(Log

L

10

z

0

dB)P

P(Log10Loss

out

in

Fibre losses

1000 1200 1300 1400 1500 1600

1.0

Loss

(d

B/k

m)

0.1

0.5

0.2

Wavelength (nm)

Transmission fibre loss (silica)

I2

I1

Second step

Detector

Fibre length d

Launching assembly

=0

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x

0

L o sses d B

x / 0

0 .1 0 .2 0 .3 0 .4 0 .5

0 .5

1

2

3

0 .2

1 .7

Connectors

Plug with a ceramic ferule

FCPC

E2000

body

Loss as function of The transverse position error

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5) Optical fibre implementation5) Optical fibre implementation

A b r a s io n C o lla g e

1

2

3

F u s io n E tir a g e

F ib re o p t. 1

F ib re o p t. 2

1

2

3

4

1

2

3

4

couplers

Fusion splicing

polishing Glued

From 2 to 2

From 2 to 8

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principal use>> optical fibre telecommunications

6) Application of optical fibers6) Application of optical fibers

V (t)

L a ser P h o to d io d e

V ’(t)

Very high bit rate 1 Tbit/sec Very low losses

The solution for long distance signal propagation

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S o u rce

F ib reo p tiq u e

F ib reo p tiq u e

T (x )

M esu re d e la p u issa n ce

S o u rce

F ib reo p tiq u e

R (x )

M esu re d e lap u issa n ce

Optical fibre sensors

TemperaturePressureRotationChemical concentration

6) Application of optical fibers6) Application of optical fibers

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Possibility to built interferometers

Mach Zehnder configuration

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6) Application of optical fibers6) Application of optical fibers

See next lecture

D etec to r

L a ser d io d e

M o n o m o d e fib re

B S )

S

F O 1 (L )

F O 2 (L )

I= 2 I [1 + co s(2 )]0

L

L

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7) Material and new optical fibers7) Material and new optical fibers

UV0.3µm Visible

Near IR2µm

Far IR10µm

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DGD

Slow PSP

Fast PSP

7) Material and new optical fibers7) Material and new optical fibers

Polarisation preserving fibers

Highly birefringent fibres

core

cladding

Stress area

Propagation

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7) Material and new optical fibres7) Material and new optical fibres

structure

Photonic crystal fibres

Monomode over a very large spectral domain