In-Fibre Bragg Gratings
(FBGs)
? An in-fibre Bragg grating is constructed by
varying the refractive index of the core
lengthwise along the fibre,
? Light of the specified wavelength traveling
along the fibre is reflected from the grating
back in the direction from which it came,
? Wavelengths which are not selected are
passed through with little or no attenuation,
What is FBG?
Fiber Bragg Grating
~ 500 nm (Bragg grating)
~ 200 μm (Long-period grating)
125 μm
8 μm
1 mm to 1500 mm
Singlemode fibre
Core
Cladding
Regions with higher
refractive index than
that of core’s
It works like this
? resonant wavelengths are reflected back
toward the source
? non-resonant wavelengths are transmitted
through the device without loss,
? The centre wavelength is given by,
?? e f fn2?
Principle of Operation
The grating forms an electromagnetic resonant
circuit,Power from the forward direction is
coupled into the resonant circuit and then
reflected back,Non-resonant wavelengths are
not affected very much,
Parameters
? The grating period is the distance between
modulations of the refractive index in the grating,
? The grating length,
? The,modulation depth”,determined by the RI
contrast within the grating,
? The RI contrast profile,
Characteristics of In-Fibre
Bragg Gratings
? Centre Wavelength
– This is the wavelength at the centre of the
grating's reflection band,
? Bandwidth
– This is the width of the reflection band and
specifies the range of wavelengths reflected,
? Reflectance Peak
– This is a measure of the proportion of incident light
reflected at the centre wavelength,
Typical Reflection Spectra
(a) typical reflection spectrum of a 1 cm long FBG with
relatively low RI contrast,The height of the peak is 100%
reflection and the width of the reflection band is,2 nm; (b)
same grating with a stronger contrast,The reflection band
has been broadened; (c) same grating as part (a) but after
apodisation,Note the reflection peak is now not quite 100%,
Apodisation the grating
? Apodisation is a process of tapering the
strength of the grating at either end so
that the apparent RI change is gradual
rather than abrupt,The reflection band
of an apodised grating is shown in part
(c) of the figure,
Chirped FBGs
? A,chirp” is where you get a variation in the
period of the grating (and hence a variation in
its response to different wavelengths) along
the length of the grating,
– vary the period of the grating or
– vary the average RI of the grating,
Chirped Grating - Apodised
a good 100% reflection over the whole
reflected band,
Multiple FBGs in the Same
Fibre Section
? you can write many different FBGs
into the same section of fibre - one
on top of the other,Each grating will
then respond quite separately and
independently to light of its own
resonant wavelength,
Blazed FBGs
A blazed grating is constructed when the grating
is written at an oblique angle to the centre axis of
the core,The selected wavelength is reflected
out of the fibre,
Another usage is to equalise power across a
range of wavelengths for example to“flatten” the
response of an EDFA,
Phase-Shifted FBGs
? A,transmission fringe” is created in the centre
of the reflection band where light is transmitted
through the filter rather than being reflected,
Long-Period In-Fibre Bragg
Gratings (LPGs)
? Most FBGs are constructed as first
order gratings,That is the grating period
is the same as the centre wavelength of
the reflection band,
? A long-period grating is one where the
grating period is many hundreds or
thousands of times the resonant
wavelength,
Principle
? In LPG it couples forward guideing
mode into a cladding mode,Thus after
a while the coupled light leaves the
system and is lost,A long-period grating
then gives much the same effect as a
blazed grating - resonant wavelengths
are removed from the system,
Temperature Stability of In-
Fibre Bragg Gratings
Caused by expansion and contraction of the fibre with
temperature and consequent change in the spacing of
the RI variations in the core; or a variation in the RI of the
fibre itself with temperature,
The unpackaged grating,a total variation of about 1 nm
over a temperature range of 80° C!
FBG Packaging for Passive
Thermal Compensation
Writing the Grating
? The grating is written by exposing the
fibre to UV light,UV light (244 nm) is
able to make permanent modifications
in the refractive index of the core,
? The change in the refractive index is
very small indeed,An index change
of.0001 is sufficient to make an effective
grating,
Make the fibre core more sensitive
? to increase the level of germanium
dopant,
? this too far before mechanical effects in
the fibre stop you,Co-doping with boron
and/or aluminium helps make the core
more photosensitive,
?,Loading” the fibre with hydrogen
Interference Pattern Technique
Phase Mask Technique
Applications of FBGs
? Wavelength Stable Lasers
? Dispersion Compensation
? Wavelength Selection in WDM Systems
Using in Dispersion
Compensation
Chirped Bragg grating
? ? ?
? long ? short
Reflection Profiles of a Chirped FBG
(a) the grating without apodisation,
(b) the same grating after apodisation,
Using in OADM
Identical FBGs at ? 4
? 1,? 2,? 3,? 4,? 5
? 4 D
D ?
4
A
A
Reported Characteristics
Channel passband,0.1 - 0.5 nm
Channel spacing,0.7 - 1.6 nm
Channel isolation,20 - 30 dB
Insertion loss < 0.5 dB
Return loss > 15 dB
? 1,? 2,? 3,? 4,? 5
Using in OADM
Summary for FBGs
? Consists of a periodic stack of regions of high and low
refractive index along an optical fiber,
? Made by exposing the fiber to an interference pattern
of ultraviolet(UV) light,
? Narrow band,almost square wavelength response,
? Low temperature sensitivity with athermal
packaging(as low as 0.4pm/oc,For unpackaged
grating it is about 0.01nm/oc)
? Need to use fiber Mach-Zehnder configuration or
circulator to construct a demultiplexer
(FBGs)
? An in-fibre Bragg grating is constructed by
varying the refractive index of the core
lengthwise along the fibre,
? Light of the specified wavelength traveling
along the fibre is reflected from the grating
back in the direction from which it came,
? Wavelengths which are not selected are
passed through with little or no attenuation,
What is FBG?
Fiber Bragg Grating
~ 500 nm (Bragg grating)
~ 200 μm (Long-period grating)
125 μm
8 μm
1 mm to 1500 mm
Singlemode fibre
Core
Cladding
Regions with higher
refractive index than
that of core’s
It works like this
? resonant wavelengths are reflected back
toward the source
? non-resonant wavelengths are transmitted
through the device without loss,
? The centre wavelength is given by,
?? e f fn2?
Principle of Operation
The grating forms an electromagnetic resonant
circuit,Power from the forward direction is
coupled into the resonant circuit and then
reflected back,Non-resonant wavelengths are
not affected very much,
Parameters
? The grating period is the distance between
modulations of the refractive index in the grating,
? The grating length,
? The,modulation depth”,determined by the RI
contrast within the grating,
? The RI contrast profile,
Characteristics of In-Fibre
Bragg Gratings
? Centre Wavelength
– This is the wavelength at the centre of the
grating's reflection band,
? Bandwidth
– This is the width of the reflection band and
specifies the range of wavelengths reflected,
? Reflectance Peak
– This is a measure of the proportion of incident light
reflected at the centre wavelength,
Typical Reflection Spectra
(a) typical reflection spectrum of a 1 cm long FBG with
relatively low RI contrast,The height of the peak is 100%
reflection and the width of the reflection band is,2 nm; (b)
same grating with a stronger contrast,The reflection band
has been broadened; (c) same grating as part (a) but after
apodisation,Note the reflection peak is now not quite 100%,
Apodisation the grating
? Apodisation is a process of tapering the
strength of the grating at either end so
that the apparent RI change is gradual
rather than abrupt,The reflection band
of an apodised grating is shown in part
(c) of the figure,
Chirped FBGs
? A,chirp” is where you get a variation in the
period of the grating (and hence a variation in
its response to different wavelengths) along
the length of the grating,
– vary the period of the grating or
– vary the average RI of the grating,
Chirped Grating - Apodised
a good 100% reflection over the whole
reflected band,
Multiple FBGs in the Same
Fibre Section
? you can write many different FBGs
into the same section of fibre - one
on top of the other,Each grating will
then respond quite separately and
independently to light of its own
resonant wavelength,
Blazed FBGs
A blazed grating is constructed when the grating
is written at an oblique angle to the centre axis of
the core,The selected wavelength is reflected
out of the fibre,
Another usage is to equalise power across a
range of wavelengths for example to“flatten” the
response of an EDFA,
Phase-Shifted FBGs
? A,transmission fringe” is created in the centre
of the reflection band where light is transmitted
through the filter rather than being reflected,
Long-Period In-Fibre Bragg
Gratings (LPGs)
? Most FBGs are constructed as first
order gratings,That is the grating period
is the same as the centre wavelength of
the reflection band,
? A long-period grating is one where the
grating period is many hundreds or
thousands of times the resonant
wavelength,
Principle
? In LPG it couples forward guideing
mode into a cladding mode,Thus after
a while the coupled light leaves the
system and is lost,A long-period grating
then gives much the same effect as a
blazed grating - resonant wavelengths
are removed from the system,
Temperature Stability of In-
Fibre Bragg Gratings
Caused by expansion and contraction of the fibre with
temperature and consequent change in the spacing of
the RI variations in the core; or a variation in the RI of the
fibre itself with temperature,
The unpackaged grating,a total variation of about 1 nm
over a temperature range of 80° C!
FBG Packaging for Passive
Thermal Compensation
Writing the Grating
? The grating is written by exposing the
fibre to UV light,UV light (244 nm) is
able to make permanent modifications
in the refractive index of the core,
? The change in the refractive index is
very small indeed,An index change
of.0001 is sufficient to make an effective
grating,
Make the fibre core more sensitive
? to increase the level of germanium
dopant,
? this too far before mechanical effects in
the fibre stop you,Co-doping with boron
and/or aluminium helps make the core
more photosensitive,
?,Loading” the fibre with hydrogen
Interference Pattern Technique
Phase Mask Technique
Applications of FBGs
? Wavelength Stable Lasers
? Dispersion Compensation
? Wavelength Selection in WDM Systems
Using in Dispersion
Compensation
Chirped Bragg grating
? ? ?
? long ? short
Reflection Profiles of a Chirped FBG
(a) the grating without apodisation,
(b) the same grating after apodisation,
Using in OADM
Identical FBGs at ? 4
? 1,? 2,? 3,? 4,? 5
? 4 D
D ?
4
A
A
Reported Characteristics
Channel passband,0.1 - 0.5 nm
Channel spacing,0.7 - 1.6 nm
Channel isolation,20 - 30 dB
Insertion loss < 0.5 dB
Return loss > 15 dB
? 1,? 2,? 3,? 4,? 5
Using in OADM
Summary for FBGs
? Consists of a periodic stack of regions of high and low
refractive index along an optical fiber,
? Made by exposing the fiber to an interference pattern
of ultraviolet(UV) light,
? Narrow band,almost square wavelength response,
? Low temperature sensitivity with athermal
packaging(as low as 0.4pm/oc,For unpackaged
grating it is about 0.01nm/oc)
? Need to use fiber Mach-Zehnder configuration or
circulator to construct a demultiplexer
