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