Fiber Bragg Gratings

Fiber Grating• Fiber grating is made by periodically changing

the refraction index in the glass core of the fiber. The refraction changes are made by exposing the fiber to the UV-light with a fixed pattern.

Glass core

Glass cladding Plastic jacket Periodic refraction index change(Gratings)

Fiber Grating Basics• When the grating period is half of the input light

wavelength, this wavelength signal will be reflected coherently to make a large reflection.– The Bragg Condition

r = 2neff

in

Reflection spectrum

reflect

Transmission spectrum

trans.

n (refraction index difference)

Fiber Bragg Grating: Theory

1978 – Hill et. all• Phenomenon of photosensitivity in optical

fibers• Exposed Ge-doped core fibers to intense light

at 488 or 514 nm• Induced permanent refractive index changes

to the core.

• FBG is a longitudinal periodic variation of the index of refraction in the core of an optical fiber.

• The spacing of the variation is determined by the wavelength of the light to be reflected.

Bragg

Bragg

Fiber Bragg Grating: Theory

The Bragg Condition is the result of two requirements:1. Energy Conservation: Frequency of incident radiation and reflected radiation is the

same.2. Momentum Conservation: Sum of incident wave vector and grating wave vector

equal the wave vector of the scattered radiation. K + ki = kf

The resulting Bragg Condition is: B = 2neff

• The grating reflects the light at the Bragg wavelength (B) • B is a function of the grating periodicity () and effective index (neff). • Typically; B= 1.5 m, = 0.5m

Fiber Bragg Grating: Theory

• The spectral component reflected (not transmitted) typically has a bandwidth of 0.05 – 0.3 nm.

A general expression for the approximate Full Width Half Maximum bandwidth of a standard grating is given by (S = grating parameter (.5 to 1), N = numbers of grating pains):

Δλ =λ B S( (Δn/2n0)2 + (1/N)2 )1/2

1570 1572 1574 1576 1578

-40

-30

-20

-10

0

Lo

ss in

dB

Wavelength in nm

Reflection Transmission

Fiber Bragg Grating: Theory

• The shift in Bragg Wavelength with strain and temperature can be expressed using:

B = 2n({1-(n2/2)[P12 – (P11 + P12)]}+ [ + (dn/dT)/n]T

Where: = applied strainPi,j = Pockel’s coef. of the stress-optic tensor = Pisson’s ratio = coef. of thermal expansion T = temperature change[P12 – (P11 + P12)] ~ 0.22

• The shift in Bragg Wavelength is approximately linear with respect to strain and temperature.

Fiber Bragg Grating: Theory

• The measured strain response at a constant temperature is found to be:

(1/B)B/ = 0.78 x 10-6-1

• Sensitivity Rule of thumb at B = 1300nm:0.001nm/

Fiber Bragg Grating: Theory

• The measured temperature response at a constant strain is found to be:

(1/B)B/ T = 6.67 x 10-6 oC-1

• Sensitivity Rule of thumb at B = 1300nm:0.009nm/ oC

Fiber Bragg Grating: Theory

Fiber Bragg Grating: Theory – Blazed Grating

• Bragg grating planes are tilted at an angle to the fiber axis.• Light which otherwise would be guided in the fiber core, is coupled

into the loosely bound, guided cladding or radiation modes.• The bandwidth of the trapped out light is dependent on the tilt angle

of the grating planes and the strength of the index modulation.• As shown above, the vector diagram is a result of the conservation of

momentum and conservation of energy requirement. The results of applying the law of cosines yealds: Cos(θb) = ׀K 2/׀ v

Fiber Bragg Grating: Theory – Chirped Grating

• Bragg grating has a monotonically varying period as illustrated above.• These gratings can be realized by axially varying either the period of

the grating or the index of refraction of the core or both.• The Bragg Condition becomes: λB = 2neff(z)Λ(z)• The simplest type of chirped grating is one which the grating period

varies linearly with axial length: Λ(z) = Λ0 + Λ(z)

f3Incident

Reflected

Chirped FBG

Relative Time

Delay (ps)

Wavelength (nm)

Linearly Chirped0

Dispersion comp. at

f2f1f3f1

f2

0

Dispersion = dT/d (ps/nm)

Chirped FBG

Creating Gratings on Fiber• One common way to make gratings on fiber is using

Phase Mask for UV-light to expose on the fiber core.

Characteristics of FBG• It is a reflective type filter

– Not like to other types of filters, the demanded wavelength is reflected instead of transmitted

• It is very stable after annealing– The gratings are permanent on the fiber after proper

annealing process– The reflective spectrum is very stable over the time

• It is transparent to through wavelength signals– The gratings are in fiber and do not degrade the

through traffic wavelengths, very low loss• It is an in-fiber component and easily integrates

to other optical devices

Temperature Impact on FBG• The fiber gratings is generally sensitive to

temperature change (10pm/°C) mainly due to thermo-optic effect of glass.

• Athermal packaging technique has to be used to compensate the temperature drift

1533.8

1534.0

1534.2

1534.4

1534.6

1534.8

1535.0

1535.2

-5 15 35 55 75

Temperature (℃)

Cen

ter

Wav

elen

gth

(n

m)

Athermal

Normal

Types of Fiber Gratings

TYPES CHARACTERS APPLICATIONS

Simple reflective gratings

Creates gratings on the fiber that meets the Bragg condition

Filter for DWDM, stabilizer, locker

Long period gratings

Significant wider grating periods that couples the light to cladding

Gain flattening filter, dispersion compensation

Chirped fiber Bragg gratings

A sequence of variant period gratings on the fiber that reflects multiple wavelengths

Gain flattening filter, dispersion compensation

Slanted fiber gratings

The gratings are created with an angle to the transmission axis

Gain flattening filter

Typical FBG Production Procedures

SelectProperfiber

H2loading

Laserwriting

AnnealingAthermalpackaging

Testing

Different FBG requires different specialty fiber

Increase photo sensitivity for easier laser writing

Optical alignment & appropriate laser writing condition

Enhance grating stability

For temperature variation compensation

Spec test

Current Applications of FBG• FBG for DWDM• FBG for OADM• FBG as EDFA Pump laser stabilizer• FBG as Optical amplifier gain flattening filter• FBG as Laser diode wavelength lock filter• FBG as Tunable filter• FBG for Remote monitoring• FBG as Sensor• ….

Possible Use of FBG in System

MultiplexerDispersion control EDFA

OADM

SwitchEDFADemux

ITU FBG filter Dispersion

compensation filter

Pump stabilizer & Gain flattening

filter

ITU FBG filter

Tunable filter

ITU FBG filter

Pump stabilizer & Gain flattening

filter

E/O

Wave locker

Monitor

Monitor sensor

ITU FBG Filter for DWDM

1, 2 … nFBG at 1

1 2

Circulator CirculatorFBG at 2

3

CirculatorFBG at 3

...

1, 2 … nFBG at 1

1 2

Circulator CirculatorFBG at 2

3

CirculatorFBG at 3

...

Multiplexer

De-multiplexer

ITU FBG Filter for OADM

Circulator Circulator

FBG

Through signal

Dropped signal Added signal

Outgoing signalIncoming signal

Dispersion Compensation Filter

Dispersedpulse

circulator

Chirp

ed

FBG

FBG and Dispersion Compensation

FiberDispersion

FBG Disp.Comp.

t t

t t

5

4

3

2

1

5

4

3

2

1

Pump Laser Stabilizer

980

spectrum

Focal lens

Fiber 980 Stabilizer

+

-Pump Laser

Gain Flattening Filter

1 5 0 0 1 5 2 0 1 5 4 0 1 5 6 0 1 5 8 0 1 6 0 0W av e len g th (n m )

-1 5

-1 0

-5

0

5

1 0

1 5

2 0

Gai

n (d

B)

Gain profile

GFF profile

Output