1. Fiber Bragg Gratings

2. Periodic refraction index change
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
Periodic refraction index change
(Gratings)
Plastic jacket

3. 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
Reflection spectrum
reflect
Transmission spectrum
trans. in 
 n (refraction index difference)
r = 2neff 

4. 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.
‘The sinusoidal modulation of the index of refraction in the core due to spatial variation in the writing beam gives rise to a refractive index grating that can be used to couple energy of the fundamental guide mode to various guided and lossy modes.

5. Fiber Bragg Grating: Theory
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 Wavelength

6. Fiber Bragg Grating: Theory
The Bragg Condition is the result of two requirements:
Energy Conservation: Frequency of incident radiation and reflected radiation is the same.
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: lB = 2L neff
The grating reflects the light at the Bragg wavelength (lB)
lB is a function of the grating periodicity (L) and effective index (neff).
Typically; lB= 1.5 mm, L = 0.5mm
The Bragg wavelength is approx. 2-3 x the periodicity.
1300 – 1500nm (infrared)

7. 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

8. Fiber Bragg Grating: Theory
The shift in Bragg Wavelength with strain and temperature can be expressed using:
DlB = 2nL({1-(n2/2)[P12 – n(P11 + P12)]}e
+ [a + (dn/dT)/n]DT
Where:
e = applied strain
Pi,j = Pockel’s coef. of the stress-optic tensor
n = Pisson’s ratio
a = coef. of thermal expansion
DT = temperature change
[P12 – n(P11 + P12)] ~ 0.22
The shift in Bragg Wavelength is approximately linear with respect to strain and temperature.
Delta-Bragg-Wvlgth/Bragg-Wvlgth = Delta-Grating/Grating + Delta-index/index
1) When fiber is strained, Grating period inc. and index of refraction decreases. Thus they have contrasting effects on Bragg Wavelength.
2)With a perturbation, typically the change in grating index of refraction has the largest effect.

9. Fiber Bragg Grating: Theory
The measured strain response at a constant temperature is found to be:
(1/lB)dlB/ de = 0.78 x 10-6me-1
Sensitivity Rule of thumb at lB = 1300nm:
0.001nm/me

10. Fiber Bragg Grating: Theory
The measured temperature response at a constant strain is found to be:
(1/lB)dlB/ dT = 6.67 x 10-6 oC-1
Sensitivity Rule of thumb at lB = 1300nm:
0.009nm/ oC

11. 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׀/2v

12. 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)

13. Chirped FBG 0 0 f2 f1 f3 f1 f2 f3 Chirped FBG Dispersion comp. at
Incident f1 f2 f3
Chirped FBG
Reflected 0
Dispersion comp. at
Relative
Time
Delay (ps) 0
Wavelength (nm)
Linearly Chirped
Dispersion = dT/d  (ps/nm)

14. 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.

15. 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

16. 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

17. Types of Fiber Gratings
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
Slanted fiber gratings
The gratings are created with an angle to the transmission axis
Gain flattening filter

18. Typical FBG Production Procedures
Select
Proper
fiber H2
loading
Laser
writing
Annealing
Athermal
packaging
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

19. 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 ….

20. Possible Use of FBG in System
ITU FBG filter
Pump stabilizer & Gain flattening filter
Wave locker
Dispersion compensation filter
E/O
Dispersion control
EDFA
Multiplexer
OADM
Demux
EDFA
Switch
ITU FBG filter
Tunable filter
Pump stabilizer & Gain flattening filter
Monitor sensor
ITU FBG filter
Monitor

21. ITU FBG Filter for DWDM ... ... Multiplexer De-multiplexer l1, l2 … ln
FBG at l1 l1 l2
Circulator
FBG at l2 l3
FBG at l3
...
Multiplexer
l1, l2 … ln
FBG at l1 l1 l2
Circulator
FBG at l2 l3
FBG at l3
...
De-multiplexer

22. ITU FBG Filter for OADM Circulator Circulator FBG Incoming signal
Outgoing signal
Through signal
Dropped signal
Circulator
Circulator
FBG
Added signal

23. Dispersion Compensation Filter
circulator
Dispersed
pulse
Chirped FBG

24. FBG and Dispersion Compensationl5 l4 l3 l2 l1 t t
Fiber
Dispersion t t l5 l4 l3 l2 l1
FBG Disp.
Comp.

25. Pump Laser Stabilizer Focal lens Fiber 980 Stabilizer + - Pump Laser
spectrum

26. Gain Flattening Filter
Gain profile
GFF profile
Output