UV written Bragg gratings

UV-written Bragg gratings

Morten Ibsen

~ Outline ~

•

Introduction to Bragg gratings.

– Fundamentals.

– History.

•

Application areas of Bragg gratings.

– Dispersion-free gratings for add-drop filtering in high-speed

systems.

– Gratings for optical code division multiple access (O-CDMA)

systems.

– Short gratings for dispersion management in high-speed systems.

•

Future trends.

~ Bragg grating fundamentals ~

•

What is a Bragg grating?.

– A periodic or almost periodic structure consisting of a variation of

~ Bragg grating fundamentals ~

•

What is a Bragg grating?.

– A small reflection (Fresnel reflection) from each low-high (high-low)

refractive index transition.

9 2

2

10 2

) (

)

( ₋

≈ ⎟ ⎠ ⎞ ⎜

⎝ ⎛

+ =

⎟⎟ ⎠ ⎞ ⎜⎜

⎝ ⎛

+ +

+ − =

n n

n n

n n

n n n r

δ

δ

δ

~ Bragg grating fundamentals ~

•

What does it do?.

– Coupling of a forward propagating core-mode to a backward

propagating core-mode.

– Acts as a band-rejection filter passing all wavelengths that are not

~ Bragg grating fundamentals ~

•

Parameters related to a Bragg grating.

– Strong overall reflection is achieved when each of the reflected

contributions add in-phase (phase coherence/matching).

~ Bragg grating fundamentals ~

•

Parameters related to a Bragg grating.

– n_eff ~ 1.455 in silica.

– “Short” period grating to operate in lowest order mode (m=1) with

Bragg wavelength λ_B ~ 1550nm, Λ~500nm.

– Typical index changes, δn ~ 10-5 _{– 10}-3_.

~ Bragg grating history ~

•

1978

: First observation of photo-induced fibre Bragg grating.

– Discovered by a coincidence.

– Fibre was exposed to 514.5nm light.

~ Bragg grating history ~

•

1989

: First demonstration of Bragg grating inscription of a

wavelength different to the writing-beam wavelength.

– Initial grating demonstrations was believed to be a two-photon

process (based on work done by Lam and Garside in 1980-81).

– UV-light at 257nm was used.

G.Meltz et al., Optics Lett., 14, p. 823, 1989.

) sin(

) sin( 2

φ λ λ

φ λ

write eff

B

write

n ⋅

= ⇓

~ Bragg grating history ~

•

1993

: First demonstration of practical Bragg grating inscription.

– Phase-masks makes for stable and repeatable grating inscription.

K.O. Hill et al., Appl. Phys. Lett., 62, p. 1035, 1993.

2

core fibre

Λ

=

~ Historical background ~

•

(Canadian innovative excellence).

•

IEEE/OSA for his pioneering contributions to fibre-optic technology.

•

Morey awarded the Rank Prize for Opto-electronics for the invention

and development of practical Fibre Bragg Gratings.

~ Bragg grating design ~

•

Parameters that can be altered or controlled in a Bragg grating.

ch.

period/pit

Grating

)

(

index.

refractive

Effective

)

(

phase.

Grating

)

(

.

modulation

amplitude

index

Refractive

)

(

z

z

n

z

z

A

eff

Λ

θ

)

(

)

(

)

(

ν

ν

ν

out

T

E

E

=

function

transfer

Grating

)

(

ν

~ Grating applications ~

•

Applications of Bragg gratings in systems.

– Telecommunications systems.

• Transmitter-sources/Source-stabilisation.

• Multiplexing/de-multiplexing (add-drop filtering) at high bit-rates.

• Gain-equalisation.

• Dispersion-management.

• Encryption.

• Header-recognition.

– Sensing systems.

• Temperature and strain monitoring.

λ _λ

λ _λ

λ _λ

λ

λ

λ

λ

M

u

lt

ip

le

x

e

r

D

e

-m

ul

ti

pl

e

x

er

EDFA

DC

EDFA EDFA EDFA

Application of Bragg gratings

WADM WADM

Tx( ) Rx( )

Tx Tx

Rx Rx

Tx( ) Rx( )

Tx( ) Rx( )

Tx( ) Rx( )

Tx( ) Rx( )

Š

Š Š

Š Š Š Š

Š Š

Š

Š Š Š

~ Bragg grating design ~

•

Fourier theory

can give a good first approximation to the

spectral response of a Bragg grating.

– Wave-vector response

∫

∞

∞ −

=

A

z

e

dz

F

π

κ

(

)

2

1

)

(

~ Bragg grating design ~

•

Single-channel.

– Uniform grating.

1545.4 1545.5 1545.6 1545.7 1545.8 1545.9 -45

-40 -35 -30 -25 -20 -15 -10 -5 0 5

Reflection [d

B]

Wavelength [nm]

Constant.

)

(

Constant.

)

(

.

controlled

Not

)

(

Constant.

)

(

z

z

n

z

z

A

eff

Λ

~ Bragg grating design ~

•

Single-channel design using Fourier theory.

– Bragg grating with square spectral response (square filter).

~ Bragg grating design ~

•

Single-channel.

– Apodised grating.

Constant. ) ( Constant. ) ( . controlled Not ) ( Tapered. ) ( z z n z z A eff Λ θ

~ Bragg grating design ~

•

Fourier theory can only be used for the precise design of

gratings when the reflectivity is low (<50%).

– Grating strength

•

When

κ

#

L

1

(higher reflectivity) Fourier theory can no

longer provide an accurate design tool. Inverse-scattering

(backward design) techniques become necessary.

•

Layer-peeling inverse-scattering techniques can add

functionality to a given design.

– The grating response is inverted in the time-domain.

– Based on causality.

– Layer-by-layer building of the Bragg grating with full phase-control.

– Directional design – asymmetric designs.

1

≤

⋅

L

~ Application of Bragg gratings ~

•

Increase in data-traffic requires more bandwidth

~ Add-drop gratings ~

•

Apodised Bragg gratings.

– High bandwidth utilisation.

• Uniform reflection in the stop-band (ΔR<0.5dB).

• Large sidelobe suppression (R_sidelobes <-30dB).

• Sufficient “drop” function (>30dB).

L_gr 0 R e fr ac ti v e i n dex m odul a ti on Grating position

1542.0 1542.4 1542.8 -50 -40 -30 -20 -10 0 R e fl e c ti o n [d B ]

W avelength [nm ]

~ Dispersion from Bragg gratings ~

•

Out-of-band dispersion

from “standard” gratings.

– Operating near a bandgap.

– Affects adjacent channels and can impose a limitation to the number of channels passing the grating.

– Proportional to grating-strength (κL).

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -40

-30 -20 -10 0

T

rans

m

is

s

io

n [

d

B

]

Wavelength [nm]

0 20 40 60 80 100 120 140 160

κ

2

T

ide

-d

el

ay

[p

s

]

0

z

δ

-κ

κ

L_gr

0

~ Dispersion from Bragg gratings ~

•

In-band dispersion

from “standard” gratings.

– Induced by the apodisation-process of the grating.

• Different penetration into the grating as a function of detuning.

– Affects channels to be dropped and added.

– Proportional to grating-length (L), the longer the grating the higher

the dispersion for constant κ.

0

z

δ

-κ

κ

L_gr 0

Grating position -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-40 -30 -20 -10

0 κ

2

R

e

fl

e

c

ti

o

n

[

d

B]

Wavelength [nm]

0 20 40 60 80 100 120 140 160

Ti

m

e

-d

e

la

y

[p

s

~ Dispersion from Bragg gratings ~

•

In-band dispersion.

– Added term to the BWU. • 1) Reflection-bandwidth. • 2) Transmission-bandwidth.

• 3) Dispersion-limited bandwidth.

– Dispersion-limited bandwidth is bit-rate dependant.

– Tolerable dispersion in systems

• 2.5Gbit/s ~ 15000ps/nm (~1000km)

• 10Gbit/s ~ 1000ps/nm (~60km)

~ Dispersion-free gratings ~

•

Index profile designed from layer-peeling inverse-scattering.

– Directional design.

0 20 40 60 80 100 -600

-400 -200 0 200

Wrong side input Right side input

C oup ling c o e ff ic ie n t [m -1 ]

Grating position [mm]

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 200 300 400 500 600 700 800 T im e d e la y [ p s]

Wavelength detuning [nm]

-6 -4 -2 0 2 4 6

"Right"-sided input delay "Wrong"-sided input delay

1dB reflection bandwidth

R e fl e c tio n [d B ]

•

Only positive index-modulation

is used.

– When there is a change in sign of the index-modulation a discrete

phase-shift of π is inserted.

0 2 4 6 8 10 12 -600

-400 -200 0 200

Coupling coef

ficient

[m

-1 ]

Grating position [cm]

0 2 4 6 8 10 12 0

100 200 300 400 500 600 700 800 900 1000

"Wrong side" "Right side"

π-phaseshifts

Coupling coefficient [m

-1 ]

Grating position [cm]

~ Dispersion-free gratings ~

~ Dispersion-free gratings ~

•

Dispersion-free gratings vs standard apodised gratings in a

10Gbit/s NRZ system.

1545.6 1545.8 1546.0 1546.2 1546.4 1546.6 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 Re flection [dB] Wavelength [nm] 0 50 100 150 200 250 300 350 400 450 500 550 Tim e de la y [ps]

1545.6 1545.8 1546.0 1546.2 1546.4 1546.6 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 Reflection [dB] Wavelength [nm] 13 12 11 10 8 6 4 -log(BER)

1541.8 1542.0 1542.2 1542.4 1542.6 1542.8 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 Reflec tion [ d B ] Wavelength [nm] 0 50 100 150 200 250 300 350 400 450 500 550 Ti m e de la y [ p s]

1541.8 1542.0 1542.2 1542.4 1542.6 1542.8 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 Re fle c tio n [d B] Wavelength [nm] 13 12 11 10 8 6 4 4 -log(BER)

~ Dispersion-free gratings ~

•

Dispersion-free gratings vs standard apodised gratings in a

40Gbit/s

RZ system (100GHz grid).

~ OCDMA applications ~

•

Pulse encoding/decoding using superstructured gratings.

– Multiple users having same carrier frequency but a unique code/key. – Principle: A short pulse is spread in time in accordance with the

refractive index profile of an encoding grating. A correctly decoded pulse is detected as the autocorrelation function of the code.

Circulator Circulator

Decoder

h(t) [∝A(x)] h(t) ⊗ δ(t)

h(-t) [∝A(-x)] h(-t) ⊗[h(t) ⊗ δ(t)]

δ(t)

~ OCDMA applications ~

•

Pulse encoding/decoding using super-structured gratings.

EDFA1 (strain-tunable) (strain-tunable) Decoder Encoder1 EDFA2 Pulses in Diagnostics EFRL 25km SMF MOD 0.5-2.5Gbit/s LCFBG (strain-tunable) Encoder2 50:50 Coupler 50:50 Coupler Interference signal Wavelength (nm)

1552 1554 1556 1558

R e fl ec ti v it y ( 1 0 d B /di v ) (b) 0 π Wavelength (nm)

1552 1554 1556 1558

R e fl e c ti v it y ( 1 0 d B /d iv ) (a) 0 π Wavelength (nm)

1552 1554 1556 1558 1560

R e fl e c ti v it y (1 0 d B /d iv ) Wavelength (nm)

1552 1554 1556 1558 1560

R e fl ect iv it y ( 1 0dB /d iv )

Code 1

Code 2

Time (ps)

-400 -200 0 200 400

Intensity (arb. un.)

~ OCDMA applications ~

•

Pulse encoding/decoding using super-structured gratings.

Wavelength (nm)

1548 1549 1550 1551 1552 1553 1554 1555 1556 1557

R e fl e c ti v it y ( 1 0 d B /d iv

) Wavelength (nm)

1548 1549 1550 1551 1552 1553 1554 1555 1556 1557

R e fl ec ti v it y ( 1 0 d B /d iv )

0 2 4 6 8

Length (cm) P h as e S h if ts 0 π 0.5π 1.5π Wavelength (nm)

1548 1549 1550 1551 1552 1553 1554 1555 1556 1557

R e fl e c ti v it y ( 1 0 d B /d iv

) Wavelength (nm)

1548 1549 1550 1551 1552 1553 1554 1555 1556 1557

R e fl ec ti v it y ( 1 0 d B /d iv )

0 2 4 6 8

Length (cm) P h as e S h if ts 0 π 0.5π 1.5π Time (ps)

-800 -400 0 400 800

In te n s it y ( a rb . u n .) 0 1 Time (ps)

-800 -400 0 400 800

In te n s it y (a rb . u n .) 0 1

Tim e (ps)

-800 -40 0 0 400 80 0

In te n s it y ( a rb . u n .) 0 1 Input Pulse Q1 Q2 Time (ps)

-800 -400 0 400 800

In te n s it y ( a rb . u n .) 0 1 Q2:Q1* Q1:Q1* Time (ps)

-800 -400 0 400 800

In te n s it y ( a rb . u n .) 0 1 Time (ps)

-800 -400 0 400 800

In te n s it y (a rb . u n .) 0 1

Tim e (ps)

-800 -40 0 0 400 80 0

In te n s it y ( a rb . u n .) 0 1 Input Pulse Q1 Q2 Time (ps)

-800 -400 0 400 800

~ Dispersion management ~

~ Dispersion-slope compensation ~

•

Broadband Bragg gratings for pure third-order dispersion

compensation.

0 1 2 3 4 5 6 7 8 0

500 1000 1500 2000 2500

3000 Experimental profile

π - phaseshifts

Grating position [mm]

-3000 -2000 -1000 0 1000 2000 3000

Input/output direction

Design profile

Coupling coefficient

κ

, [m

-1 ]

•

BWU

=0.66

– -1dB bw=3nm.

– -30dB bw=4.5nm.

•

R

=75%.

– Uniform grating pitch!!.

•

Only

positive index-modulation is used.

M.Ibsen and R. Feced, OFC’2002, PD paper FA7, 2002.

. / 2 20ps nm D

− = ∂ ∂

~ Dispersion-slope compensation ~

•

Broadband Bragg gratings for pure third-order dispersion

compensation.

~ Bragg grating trends ~

•

There has been a growing demand for devices which can be

tuned or re-configured.

– Full C-band coverage of filters and possibly between bands, L-band

to C-band forexample.

– Tunable transmitter lasers.

– Dynamic dispersion equalisers.

– Completely re-configurable devices.

~ Tunable grating devices ~

•

Electrically re-configurable OCDMA encoder/decoder.

M.R. Mokhtar et al., OFC’2002, paper ThGG54

Wolfram Wires 5 mm chip

Variable

Potentiometers To DC

+ve

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