1. EE-566 Presentation Topic: Fiber Bragg Gratings
Presented By: Eric Glauber
Date: 10/29/03

2. Fiber Bragg Grating: Introduction
The Fiber Bragg Grating (FBG) is a fiber optic passive component exhibiting basic functional attributes of reflection and filtering.
FBG’s are relatively simple to manufacture, small in dimension, low cost and exhibit good immunity changing ambient conditions and EM radiation.
FBG’s have replaced bulk optic mirrors & beam splitters in equipment which increases system stability and portability.

3. Fiber Bragg Grating: Introduction
FBG’s are commercially used in the areas of Telecommunications and Sensors:
Telecommunications
Fiber Lasers
Fiber Amplifiers
Fiber Filters
Dispersion Compensators
Optical Fiber Phase Conjugator
WDM
Multiplexers
Demultiplexers
Sensors
Strain Sensors
Temperature Sensors
Chemical Sensors
Accelerometers

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. Fiber Bragg Grating: Manufacturing
1989 Meltz et. all.
Grating written into core by holographic side exposure method
Exposure with two beam interference pattern of UV light at 244nm
Focal spot is approx. rectangular, 4mm L X 125 mm W.

14. Fiber Bragg Grating: Manufacturing
Split Beam Interferometer Method
Insert Fig. 1, Meltz 1989

15. Fiber Bragg Grating: Manufacturing
Strongest gratings were written with244nm pulsed radiation with an average power of 4-20 mW.
Fibers:
Core dia micrometers
N.A
GeO2 doping of mol%
Bragg wavelength – nm
Application time:
Grating are observed to form quickly at power levels of 10mW or higher
Ie. 10 sec 23mW.

16. Fiber Bragg Grating: Manufacturing
Novel interferometer technique using a right angled prism.
Inherently more stable -because beams are perturbed similarly by any prism vibration.

17. Fiber Bragg Grating: Manufacturing
Phase Mask Technique.
UV is diffracted into –1,0,1 orders by relief grating.
Input mask is wavelength specific.
Different lB require different phase masks.
Silica plate with relief grating generated by e-beam exposure and plasma etching.
The two first orders undergo total internal reflection at the glass air interface of the rectangular prism and interfere on the bare fiber surface

18. Fiber Bragg Grating: Manufacturers
Advanced Optics Solutions GMBH
Blue Road Research
3M Optical OEM Systems
Alcatel Optronics
Boeing
Gould Fiber Optic
MPB Communications
OZ Optics Limited
TeraXion Inc.
Oxford Lasers Inc.
Thorlabs Inc.

19. Fiber Bragg Grating: Manufacturers
FBG custom design software:

20. Fiber Bragg Grating: Manufacturers
FBG order form:
Insert FBG order form

21. Fiber Bragg Grating: Manufacturers

22. Fiber Bragg Gratings - Application
Mach-Zehnder interferometric Band Pass Filter:
This arrangement has two identical FBG’s.
The relative phases of the two transmitted signals is adjusted to maximize the output from either port 3 or 4 and thus avoiding the 3-db loss of the couplers.
This arrangement is difficult to manufacture because the performance of the filter is strongly affected by the spectral characteristics of the two couplers. Also affecting the filter performance is any imbalance int the power splitting of the couplers or the relative phases of the signals.
Tap:
Light is emitted form the

23. Fiber Bragg Gratings - Application
Chirped Grating used for Dispersion Compensation:
The operating principle is that the grating reflects different wavelength components of the signal from different sections along the grating.
As shown below, short wavelengths are reflected first at the near end and longer wavelengths are reflected at a later time at the far end of the grating.
Shown below is a simple expression for the group delay dispersion of a linear chirped grating of length L.
Tap:
Light is emitted form the

24. Fiber Bragg Gratings - Application
Chirped Grating used for Dispersion Compensation:
Shown below is a system which transmits a 2 pico-second pulse which is compensated using a linearly chirped Bragg grating.
Shown to the lower right is a graph of the compensated and uncompensated signal.
Tap:
Light is emitted form the
Other more complicated grating structures are avaliable. For example cubic dispersion compensation is important in long distance, high bit rate transmission systems.

25. Fiber Bragg Gratings - Application
Mach-Zehnder DWM – Multiplexer / Demultiplexer:
This is the Mach-Zehnder arrangement for a WDM application.
Tap:
Light is emitted form the
Extraction of channel λk
Insertion of channel λk

26. Fiber Bragg Gratings - Application
Frustrated Coupler Drop Add Multiplexer:
This is composed of a mismatched coupler with a Bragg Grating written into one of the cores.
Power would not normally be transferred due to the mismatch of the core. With the Bragg grating, power transfer of the guided mode from port 1 to port 4 can happen if the sum of the propagation constants of the LP01 modes of each core satisfies the Bragg condition: (β01(λ12,1) +β01(λ12,2)) = 2Π/Λ
This cross coupling transfers the guided optical power at λ12 in core 1 into back propagating optical power in core 2.
Tap:
Light is emitted form the

27. Fiber Bragg Gratings - Application
Chemical Sensor:
FBG is blazed into the core at a tilt angle y.
The FBG Tap radiates a beam out of the core and cladding at an angle gB.
Tap:
Light is emitted form the

28. Fiber Bragg Gratings - Application
Chemical Sensor (Continued):
FBG Tap excites fluorescent layer which in turn emits light which is in turn collected by the core (lf1).
Insert Insert fig. 4, chem sensor

29. Fiber Bragg Gratings – Application, WDM
In WDM, each FBG sensor is assigned a portion of the source spectrum.
Enables quasi-distributed sensing of strain, temperature, chemical, etc….
No. of FBG is a function of:
Source profile width
Grating operational bandwidth
Insert Fig. 2, Kersey, 1997

30. Fiber Bragg Gratings – Application, WDM
Approx 20 strain sensors can be multiplexed along a single fiber (peak strains of +1000me)
Insert Fig. 2, Kersey, 1997

31. Fiber Bragg Gratings – Application, WDM
Time & Wavelength Division Multiplexing:
Insert Fig.9, Kersey, 1997
The literature discusses a 3x3 array (9 elements)
The literature states up to 100 element arrays are possible

32. Fiber Bragg Gratings – References
Othonos, Andreas and Kalli, Kyriacos, “Fiber Bragg Gratings – Fundamentals and Applications in Telecommunications and Sensing”, Artech House, Inc, 1999.
Hill, K.O., et al., “Photosensitivity in Optical Fiber Waveguides: Application to Reflection Filter Fabrication,” Appl. Phys. Letter, Vol.32 ( ) 1978.
 Kersey, Alan D., et al., “Fiber Grating Sensors,” Journal of Lightwave Technology, Vol. 15, No. 8, August 1997.
Maher, M.H. and E.G. Nawy, “Evaluation of Fiber Optic Bragg Grating Strain Sensor in High Strength Concrete Beams,” Fiber Optic Sensors for Construction Materials and Bridges, pp , Farhad Ansari, editor, Technical Publishing Company, Inc., 1998.
Pallas-Areny, R. and J.G. Webster, Sensors and Signal Conditioning, second Ed., John Wiley & Sons, Inc., 2001.
Meltz, G., W.W. Morey and W.H. Glenn, “Formation of Bragg Gratings in Optical Fibers by a Transverse Holographic Method,” Opt. Lett, 14, ( ) 1989.
 Webster, J.G., The Measurement Instrumentation and Sensors Handbook, RC Press, 1999.
Insert Fig.9, Kersey, 1997
The literature discusses a 3x3 array (9 elements)
The literature states up to 100 element arrays are possible