Note: Descriptions are shown in the official language in which they were submitted.
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A PROCESS AND SYSTEM FOR MANUFACTURING STABLE
FIBER BRAGG GRATINGS (FBGs)
FIELD OF THE INVENTION
The present invention relates to telecommunications, sensors, and related
areas.
Particularly, the present invention relates to the field of fiber optics.
Still particularly, the present invention relates to a manufacturing process
for
highly stabilized Fiber Bragg Gratings (FBGs).
DEFINITIONS
In this specification, the following terms have the following definitions as
given alongside. These are additions to the usual definitions expressed in the
art.
A Fiber Bragg Grating (FBG) is a distributed Bragg reflector constructed
in a segment of an optical fiber that reflects particular wavelengths of
light,
known as Bragg wavelength, and transmits all others. A fiber Bragg grating
can therefore be used as an inline optical filter to block certain
wavelengths,
or as a wavelength-specific reflector.
FBG Growing is the process of inscribing a periodic variation of refractive
index into the core of an optical fiber, thereby creating an FBG by impinging
the optical fibers with intense ultraviolet (UV) rays produced by laser
sources.
An Excimer Laser (also termed as an exciplex laser) is a form of ultraviolet
laser which is commonly used in eye surgery and semiconductor
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manufacturing. The term excimer is the short form of 'excited dimer', while
exciplex is the short form of 'excited complex'. An excimer laser typically
uses a combination of an inert gas (argon, krypton, or xenon) and a reactive
gas (fluorine or chlorine). Under the appropriate conditions of electrical
stimulation, a pseudo-molecule called an excimer (or in case of noble gas
halides, exciplex) is created, which can only exist in an energized state and
can give rise to laser light in the ultraviolet range.
Exposure Duration is the time for which the optical fiber is exposed to the
UV rays during the growth of an FBG.
Exposure Intensity is the intensity of the UV rays impinging on the optical
fiber during the growth of an FBG.
Exposure Conditions is the term used to define the different combinations
of various parameters required for growing an FBG on a photosensitive
material including the exposure intensity, exposure duration, wavelength of
the UV laser rays, pulse energy of the UV laser rays and repetition rate of
the UV laser rays.
Refractive Index (or index of refraction) of a medium is a measure of how
much the speed of light (or other waves such as sound waves) is reduced
inside the medium. The refractive index, n, of a medium is defined as the
ratio of the velocity, c, of a wave phenomenon such as light or sound in
vacuum to its velocity, vi,, in the medium itself as given by:
n = ¨
VP
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Reflectivity of a surface is the fraction of the radiation incident on the
surface which is reflected by the surface.
Defects in an FBG: When the UV rays interact with the fiber during the
FBG growing process, the energy of the UV photons is transferred to the
fiber resulting in a change in the structure of the fiber. This change in
structure is called a defect.
Activation Energy: The minimum energy required to re-transform a
decayed defect to its original state is known as the activation energy.
Growth Characteristics of an FBG include the reflectivity of the FBG, the
refractive index modulation, the saturated refractive index modulation, the
Bragg wavelength and the residual temperature as a function of exposure
duration.
Decay Characteristics of an FBG include the normalized refractive index
change, defect transformation rate and defect demarcation energy.
Defect Energy Distribution is the graph which shows the relationship
between defect density and defect demarcation energy.
Scaling Factor is the factor by which the growth phase defect energy
distribution of an FBG is to be sized to arrive at the decay phase defect
energy distribution of the FBG.
BACKGROUND OF THE INVENTION AND PRIOR ART
Fiber Bragg gratings are created by "inscribing" (another term used for
inscribing is writing) the periodic variation of refractive index into the
core
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of optical fibers using intense ultraviolet (UV) sources such as excimer
lasers. Special germanium-doped silica fibers are used in the manufacture of
fiber Bragg gratings. The germanium-doped fibers are photosensitive
wherein the refractive index of the core changes with exposure to UV light,
with the amount of change depending on the exposure intensity and
duration.
The reflected wavelength from the grating, called the Bragg wavelength
(A,B), is defined by the relationship A,B = 2nA, where n is the effective
refractive index of the grating in the fiber core and A is the grating period.
When the UV radiation interacts with the fiber during the FBG growing
process, the energy of the UV photons is transferred to the fiber resulting in
a change in the structure of the fiber via transformation of defects and hence
modifying the refractive index in the exposed region as compared to the
unexposed regions of the fiber. The defects so created in the fiber structure
are not completely stable and end up decaying with different time constants
and of different amplitudes. The minimum energy required to re-transform a
defect to its original state is known as the 'Activation energy' Ea. Based on
such a definition, the UV-induced defects can be broadly classified into two
types:
1) Shallow activation energy defects: These are defects that re-
transform to their original state with the application of relatively low
energy (supplied thermally for a short time or through accumulation
of thermal energy over longer time). The transformation process
sustains until all the defects that have activation energy lower than
demarcation energy (Eda) have been quenched. The demarcation
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energy is defined as the energy corresponding to the desired lifetime
at the field temperature of the application it is used for. The FBG can
be stabilized by a process called 'Annealing', which involves heating
the grating to high temperatures until the defects with activation
energy lower than the demarcation energy are quenched. The recipe
for this annealing process (the specific annealing temperature and
time) are decided based on the results obtained via accelerated aging
experiments.
2) Deep activation energy defects: These defects have activation
energy higher than the demarcation energy and are relatively stable
during the desired lifetime of the FBG. These defects are conserved
even after the above annealing process, and hence are critical for the
functionality of the FBG in the desired application.
Some applications of FBGs including telecommunication applications have
very critical requirements. One example of such a requirement can be given
as: the optical performance characteristics such as the insertion loss, Bragg
wavelength and the like should be within the specified range when subjected
to environmental tests simulating the field conditions. As such, thermal
stability of FBGs written in photosensitive fibers is of critical importance
for
the devices to perform reliably within the specifications over a long period
of time. Typically, this is ensured by annealing the gratings at an elevated
temperature (for example, 150 deg C) for a short time (few minutes
typically). High-temperature annealing of the FBGs written in photo
sensitive fibers results in both the grating strength reduction and the shift
in
the Bragg wavelength. As a result, it is also important to quantify the
grating
strength reduction and the wavelength shift resulting from the annealing
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process before specifying the device performance characteristics. In
addition, an optimal annealing process minimizes the performance
degradation over the gratings' life. Thus, for
the refractive index
corresponding to the exposed region of the fiber to remain stable for a long-
period of time, a critical requirement is stabilizing the UV-induced change in
the refractive index.
After the FBG growing, the FBG has to be stabilized by removing some
parts of it for improving the grating usability. To understand and optimize
the defects, a sample grating is subjected to accelerated aging experiments,
which may be through Iso-Thermal Annealing (ITA), Iso-Chronal Annealing
(ICA) or a combination of both. The results of these experiments are used to
obtain the defect details. The annealing methodology and recipe for the other
gratings fabricated in the same batch are decided based on the above defect
details. The FBG is then annealed to remove the shallow defects. As the
accelerated aging process is a lengthy step, considerable amount of time and
money are spent towards the stabilization of the grating.
Several attempts have been made to manufacture stabilized FBGs. The
following are certain disclosures related to different stabilization
techniques
for FBGs.
PCT application W00184191A2 published on 08.11.2001 discloses an
apparatus for measuring environmental parameters comprising an optical
fiber-based sensor having thermally-induced diffraction gratings which are
stable at very high temperatures for many hours. The diffraction gratings are
formed in an optical fiber by exposure to light from an infrared laser and
they do not degrade at high temperatures. The optical fiber-based sensor is
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positioned within a high temperature environment having a parameter
desired for measurement. The light source directs light into the optical fiber-
based sensor. A detector measures the differential diffraction of the light
output from the optical fiber-based sensor and determines a value of the
environmental parameter based, at least in part, upon a known correlation
between the differential diffraction and the environmental parameter. The
diffraction gratings used in the apparatus disclosed in W00184191A2
requires non-standard fabrication processes which increases the cost of
manufacturing.
PCT application W003005082 published on 16.01.2003 discloses a method
and a device for tuning a Bragg grating in an optical fiber. Tuning of the
grating is obtained by applying current to at least one longitudinal, internal
electrode arranged along the core of the fiber. When current is passed
through the electrode, thermal expansion occurs which in turn produces a
stress on the fiber core. At the same time, the temperature of the core is
increased. This leads to an electrically controlled tuning of the Bragg
grating. The disclosure in W003005082 deals only with the tuning of
gratings and not with permanent correction of FBGs which is needed for the
production of stabilized FBGs with tight tolerance levels.
United States patent application US20030133658 published on 17.07.2003
discloses a Bragg grating tuning method and apparatus. The Bragg grating is
tuned with a heater which is used to adjust the temperature of the
semiconductor substrate on which the grating is written using an optical
beam. Again, the disclosure in US20030133658 deals only with the tuning
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of gratings and not with permanent correction of FBGs which is needed for
the production of highly stabilized FBGs with tight tolerance levels.
United States patent application US20040161195 published on 19.08.2004
discloses a system and method for manufacturing FBGs. The different steps
followed in the manufacturing process are: a) UV-writing an FBG in an
optical fiber; b) monitoring characteristic data of the FBG; and c) generating
a controlled complex temperature profile along the FBG with a heating
means according to the characteristic data for providing an accurate
controlled annealing process of the FBG, thereby providing an accurate
trimming. The main drawback of the system and method disclosed in
US20040161195 is that it requires a series of isochronal annealing steps with
increasing temperature, thereby resulting in increasing the manufacturing
cost considerably.
United States patent US7142292 published on 28.11.2006 discloses a
method for improving optical properties of a Bragg grating having a spatial
refractive index profile along a propagation axis. The method includes the
following steps: i) characterizing defects of the spatial refractive index
profile of the Bragg grating by measuring optical properties of the grating,
reconstructing the spatial refractive index profile of the grating based on
these measured optical properties and comparing the reconstructed spatial
refractive index profile with a target spatial refractive index profile; ii)
calculating an average index correction to the spatial refractive index
profile
as a function of the defects characterized in step i; and iii) applying this
average index correction to the Bragg grating by controlling the light source
characteristics and period of writing. The defects characterized in step i are
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period defects, apodization defects or both. But the method disclosed in
US7142292 requires the reconstruction of the spatial refractive index profile
of the grating for providing the necessary correction from the measured
optical properties which makes the manufacturing process very complicated.
There is therefore felt a need for a process and a system for manufacturing
highly stable FBGs, wherein:
= the defects are stabilized based on the grating growth process itself
without going through the elaborate accelerated aging studies;
= the gratings which may be determined to be unusable based on the
writing data can be discarded without further processing;
= the tight tolerance requirements of optical communication and sensor
applications can be met; and
= the knowledge of decay phase defect energy distribution is obtained
without the accelerated aging experiments, thereby reducing the
manufacturing cost and time considerably.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a manufacturing process
and system for high quality FBGs.
It is another object of the present invention to avoid going through expensive
and time consuming accelerated aging studies used to characterize the decay
behaviour of FBGs.
It is still another object of the present invention to meet the tight
tolerance
requirements of optical communication and sensor applications.
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It is still another object of the present invention to discard unusable FBGs
based on the writing data without further processing.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a process for
manufacturing stable Fiber Bragg Gratings (FBGs) using different types of
photo sensitive fiber materials under different exposure conditions, said
FBGs having specific growth and decay characteristics, said process
comprising the following steps:
= growing an FBG on a selected photo sensitive fiber material by
exposing said fiber material to Ultra Violet (UV) laser rays produced
by a laser source under predetermined exposure conditions defined by
selected combinations of exposure duration, exposure intensity,
wavelength of said UV laser rays, pulse energy of said UV laser rays
and repetition rate of said UV laser rays;
= monitoring the growth of said FBG to determine the different growth
characteristics thereof including the reflectivity of the FBG, the
refractive index modulation, the saturated refractive index
modulation, the Bragg wavelength and the residual temperature as a
function of exposure duration;
= determining the growth phase defect energy distribution of said FBG
using said monitored growth characteristics;
= deducing the decay phase defect energy distribution of said FBG by
scaling said growth phase defect energy distribution of said FBG by a
scaling factor determined by a step of comparing said FBG with an
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FBG grown on a similar photo sensitive fiber material under exposure
conditions similar to said predetermined exposure conditions;
= obtaining the percentage of the shallow activation energy defects and
the deep activation energy defects in said compared FBG from said
deduced decay phase defect energy distribution;
= analyzing said percentage of the shallow activation energy defects by
comparing it with a threshold value for determining whether said
compared FBG is to be retained or discarded;
= analyzing said deduced decay phase defect energy distribution of said
retained FBG to determine the annealing temperature and annealing
time; and
= annealing said retained FBG using said determined annealing
temperature for said determined annealing time to remove all shallow
activation energy defects and to obtain a stable, high quality FBG.
Typically, the process for manufacturing stable FBGs includes:
i. a step of creating a database populated with the growth and decay
characteristics of FBGs grown on different types of photo
sensitive fiber materials under different exposure conditions
according to the following steps:
= growing an FBG on a selected photo sensitive fiber
material by exposing said fiber material to Ultra Violet
(UV) laser rays produced by a laser source under
predetermined exposure conditions defined by selected
combinations of exposure duration, exposure intensity,
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wavelength of said UV laser rays, pulse energy of said UV
laser rays and repetition rate of said UV laser rays;
= monitoring the growth of said FBG to determine the
different growth characteristics thereof including the
reflectivity of the FBG, the refractive index modulation,
the saturated refractive index modulation, the Bragg
wavelength, and the residual temperature as a function of
exposure duration;
= determining the growth phase defect energy distribution of
said FBG using said monitored growth characteristics ;
= carrying out accelerated aging experiments on said FBG to
obtain the decay characteristics including the normalized
refractive index change, defect transformation rate, and
defect demarcation energy, and thereby determining the
decay phase defect energy distribution; and
= obtaining a scaling factor between decay phase defect
energy distribution and growth phase defect energy
distribution; and
ii. a step of providing a comparator adapted to compare said FBG
being manufactured with an FBG grown on a similar photo
sensitive fiber material under exposure conditions similar to said
predetermined exposure conditions and retrieving the scaling
factor corresponding to said compared FBG from said database.
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Typically, the step of growing said FBG includes a step of impinging said
photo sensitive fiber material with UV rays produced from an excimer laser
source.
Typically, the step of growing said FBG includes a step of exposing said
photo sensitive fiber material to said UV rays until the refractive index
change of said photo sensitive fiber material reaches saturation.
Typically, the step of growing said FBG includes a step of controlling the
spatial distribution of the exposure intensity by a photo mask.
Typically, the step of growing said FBG includes a step of controlling the
spatial distribution of the exposure intensity by a diffractive phase photo
mask.
Typically, the step of monitoring the growth of said FBG includes a step of
said FBG using radiations emitted by a compact broadband light source.
Typically, the step of monitoring the growth of said FBG includes a step of
analyzing the rays reflected from said FBG by an optical spectrum analyzer.
In accordance with the present invention, there is provided a system for
manufacturing stable Fiber Bragg Gratings (FBGs) comprising: i) an FBG
growing mechanism having a UV laser source adapted to produce UV rays
directed to impinge on a photo sensitive fiber material under predetermined
exposure conditions controlled by a photo mask, thereby growing an FBG;
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ii) an FBG stabilizing mechanism; and iii) an FBG annealing mechanism,
said FBG stabilizing mechanism comprising:
= a monitoring mechanism adapted to monitor different growth
characteristics of said FBG including the reflectivity of the FBG, the
refractive index modulation, the saturated refractive index
modulation, the Bragg wavelength and the residual temperature as a
function of exposure duration;
= a comparator adapted to:
i. compare said FBG grown on said photo sensitive fiber material
with an FBG grown on a similar photo sensitive fiber material
under exposure conditions similar to said predetermined
exposure conditions; and
ii. obtain the scaling factor corresponding to said compared FBG;
and
= an analyzing mechanism adapted to:
i. obtain the growth phase defect energy distribution of said FBG
using said monitored growth characteristics;
ii. deduce decay phase defect energy distribution of said compared
FBG by scaling said growth phase defect energy distribution
with said scaling factor;
iii. obtain the percentage of the shallow activation energy defects
and the deep activation energy defects in said compared FBG
using said deduced decay phase defect energy distribution;
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iv. analyze said percentage of shallow activation energy defects by
comparing it with a threshold value for determining whether
said compared FBG is to be retained or discarded; and
v. analyze said deduced decay phase defect energy distribution of
said retained FBG to determine the annealing temperature and
annealing time.
Typically, said FBG stabilizing mechanism co-operates with a database
populated with the growth characteristics, decay characteristics and the
scaling factor of FBGs grown on different types of photo sensitive fiber
materials under different exposure conditions.
Typically, said predetermined exposure conditions include exposure
conditions selected from a group consisting of different combinations of the
exposure duration, exposure intensity, wavelength of said UV rays, pulse
energy of said UV rays and repetition rate of said UV rays.
Typically, said monitoring mechanism comprises a compact broadband light
source adapted to produce radiations directed to fall on said FBG.
Typically, said monitoring mechanism comprises an optical spectrum
analyzer adapted to analyze the rays reflected from said FBG.
Typically, said comparator is adapted to co-operate with said database to
compare said FBG grown on said photo sensitive fiber material with an FBG
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grown on a similar photo sensitive fiber material under exposure conditions
similar to said predetermined exposure conditions.
Typically, said comparator is adapted to retrieve the scaling factor
corresponding to said compared FBG from said database.
In accordance with yet another aspect of the present invention, there is
provided an FBG manufactured in accordance with the process which is
substantially described herein above.
In accordance with yet another aspect of the present invention, there is
provided an FBG manufactured by the system which is substantially
described herein above.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The manufacturing process steps of FBGs in accordance with this invention
is now described with the help of accompanying drawings, in which:
Figure 1 illustrates a flow chart of the manufacturing process to obtain a
highly stabilized FBG;
Figure 2 illustrates a block diagram of the system used for the manufacture
of a highly stabilized FBG;
Figure 3 illustrates defect distributions calculated from FBGs in different
photosensitive fibers during growth phase; and
Figure 4 illustrates defect distributions calculated from FBGs in different
photosensitive fibers during decay phase.
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DETAILED DESCRIPTION OF THE INVENTION
The drawings and the description thereto are merely illustrative of a
manufacturing process and system to obtain a highly stabilized FBG in
accordance with this invention and only exemplify the process and system of
the invention and in no way limit the scope thereof.
The present invention relates to a manufacturing process and system to
produce high quality Fiber Bragg Gratings by calculating the decay
behaviour of the FBGs from their growth and annealing the grown FBG
under a temperature for a time decided on the basis of the analysis done on
the growth characteristics. This process also excludes the need for expensive
and time consuming accelerated aging testing experiments.
Figure 1 illustrates a flow chart of the manufacturing process to obtain a
highly stabilized FBG. The different steps involved in the manufacturing
process are explained with respect to Figure 1 as given below.
Creating a database is the first step of the FBG manufacturing process as
represented by the reference numeral 102. The database is used to store the
growth and decay characteristics of the FBGs grown on different types of
photo sensitive fiber materials under different exposure conditions. During
the creation of the database in step 102, the decay characteristics and defect
details corresponding to the growth characteristics of an FBG are stored by
establishing proper relationships with each other. The different steps
involved in creating the database are:
= growing an FBG on a selected photo sensitive fiber material by
exposing the fiber material to Ultra Violet (UV) laser rays produced
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by a laser source under predetermined exposure conditions defined by
selected combinations of exposure duration, exposure intensity,
wavelength of the UV laser rays, pulse energy of the UV laser rays
and repetition rate of the UV laser rays;
= monitoring the growth of the FBG to determine the different growth
characteristics including the reflectivity of the FBG, the refractive
index modulation, the saturated refractive index modulation, the
Bragg wavelength, and the residual temperature as a function of
exposure duration;
= determining the growth phase defect energy distribution of the FBG
using the monitored growth characteristics;
= carrying out accelerated aging experiments on the FBG to obtain the
decay characteristics including the normalized refractive index
change, defect transformation rate, and defect demarcation energy,
and thereby determining the decay phase defect energy distribution;
and
= obtaining a scaling factor between decay phase defect energy
distribution and growth phase defect energy distribution.
The abovementioned steps are explained in detail under the section
'Experimental Details'.
The next step in the process is growing an FBG as represented by the
reference numeral 104 on a photo sensitive fiber material by exposing the
fiber material to Ultra Violet (UV) laser rays produced by a laser source
under predetermined exposure conditions defined by selected combinations
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of exposure duration, exposure intensity, wavelength of the UV laser rays,
pulse energy of the UV laser rays and repetition rate of the UV laser rays. A
typical growing process involves inscription of periodic variation of
refractive index into the core of the photo sensitive fiber using intense UV
radiations obtained from UV lasers, typically excimer lasers. Typically,
special germanium-doped silica fibers are used in the manufacture of fiber
Bragg gratings. The refractive index of its core change on exposure to UV
light, with the amount of change depending on the exposure intensity and
duration. The fiber material is exposed to the UV rays until the refractive
index change of the material reaches saturation. Typically, photo masks are
placed between the UV light sources and the photosensitive fibers. Photo
masks control the values of exposure duration and said exposure intensity by
a diffractive phase photo mask. The intensity distribution determined by the
photo masks determines the grating structure based on the transmitted
intensity of light striking the fibers.
The next step is monitoring the growth of the FBG as represented by the
reference numeral 106. Monitoring is done to determine the different growth
characteristics including the reflectivity of the FBG, the refractive index
modulation, the saturated refractive index modulation, the Bragg wavelength
and the residual temperature as a function of exposure duration. The
monitoring is done by exposing the FBG to the rays emitted by a compact
broadband light source. The rays reflected from the FBG are then analyzed
by an optical spectrum analyzer as a function of the duration of exposure of
the FBG to the rays emitted by the compact broadband light source.
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After monitoring the growth of the FBG, the defect demarcation energy, Ed
is calculated from the values of the growth characteristics including the
reflectivity (R) and Bragg wavelength (km measured as a function of time,
the normalized index change (T), the initial defect transformation rate (k10),
and the residual temperature increase (ATr) in the fiber during the growing
process using the equation (1) given below as:
Ed = k B(To + AT.). In (ki t), equation (1)
where is the Boltzmann constant, 'T0' is the initial temperature and 't' is
the exposure time. The normalized index change (1) can then be obtained as
a function of the demarcation energy (Ed) for the grating as given by the
equation (2) shown below.
1+ Ao exp(flEd) equation (2)
where 'A0' and 'JO' are the fit parameters.
The growth phase defect energy distribution, (g(E)) is then determined. This
step is represented by the reference numeral 108. The main step involved for
obtaining the defect energy distribution is the calculation of the mean
activation energy by differentiating the equation (2),
1+ Ao expOEd with respect to the demarcation energy of the defect, Ed.
In step 110, the FBG is compared with an FBG grown on a similar photo
sensitive fiber material under exposure conditions similar to the
predetermined exposure conditions which is stored in the database using a
comparator to obtain the scaling factor corresponding to the compared FBG.
In step 112, the decay phase defect energy distribution of the compared FBG
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is deduced by scaling the growth phase defect energy distribution with the
scaling factor. The decay phase defect energy distribution indicates the
percentage of the shallow activation energy defects and the deep activation
energy defects present in the grown FBG. The percentage of shallow
activation energy defects is obtained from the decay phase defect energy
distribution in step 114.
In step 116, the percentage of shallow activation energy defects obtained
from the deduced decay phase defect energy distribution is compared with a
threshold value for determining whether the compared FBG is to be retained
or discarded. In case the shallow defect percentage is greater than a
threshold value (typically 3-15%), the grating can be discarded without
further processing.
In step 118, the deduced decay phase defect energy distribution of the
retained FBG is analyzed to determine the annealing temperature and
annealing time for removing the shallow activation energy defects. Finally,
the retained FBG is annealed in step 120 using the determined annealing
temperature for the determined annealing time to obtain a stable FBG. The
resulting FBG will be of high quality with tight tolerance required for the
present day telecommunication applications.
In accordance with another aspect of the present invention, a block diagram
of the system provided to execute the manufacturing process described
above is illustrated in Figure 2. The system comprises an FBG growing
mechanism, an FBG stabilizing mechanism and an FBG annealing
mechanism.
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The FBG growing mechanism 202 has a UV laser source adapted to produce
UV rays directed to impinge on a photo sensitive fiber material under a
predetermined exposure condition controlled by a photo mask, thereby
growing an FBG.
The FBG stabilizing mechanism comprises a database 208 for storing the
growth and decay characteristics and the scaling factor for different types of
commercially available and used photo sensitive fiber materials under
different exposure conditions. The other important components of the FBG
stabilizing mechanism are a monitoring mechanism 204, a comparator 206
and an analyzing mechanism 210 which are explained in detail as given
below.
The monitoring mechanism 204 monitors the different growth characteristics
of the FBG including the reflectivity of the FBG, the refractive index
modulation, the saturated refractive index modulation, the Bragg wavelength
and the residual temperature as a function of exposure duration. Typically,
the monitoring mechanism comprises a compact broadband light source used
to produce light rays directed to fall on the FBG and an optical spectrum
analyzer used to analyze the rays reflected from the FBG.
A comparator 206 compares the FBG with an FBG stored in the database
208 grown on similar photo sensitive fiber material exposure conditions
similar to the predetermined exposure conditions and obtains the scaling
factor corresponding to the compared FBG from the database 208.
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An analyzing mechanism 210 is provided to communicate with the
comparator and to:
i. obtain the growth phase defect energy distribution of the FBG
using the monitored growth characteristics;
ii. deduce decay phase defect energy distribution of the compared
FBG by scaling the growth phase defect energy distribution
with the scaling factor;
iii. obtain the percentage of the shallow activation energy defects
and the deep activation energy defects in the compared FBG
using the deduced decay phase defect energy distribution;
iv. analyze the percentage of shallow activation energy defects by
comparing it with a threshold value for determining whether the
compared FBG is to be retained or discarded; and
v. analyze the deduced decay phase defect energy distribution of
the retained FBG to determine the annealing temperature and
annealing time.
An FBG annealing mechanism 212 then anneals the retained FBG using the
determined annealing temperature for the determined annealing time to
remove the shallow activation energy defects and to obtain a stable FBG 214
which is highly stabilized and has very tight tolerance as required by the
present day telecommunication applications. In accordance with the process
and system of the present invention, the annealing time and annealing
temperature are thus obtained from the growth data, thereby obviating the
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cumbersome and time-consuming accelerated aging procedure for every
batch. Using this process and system for manufacturing FBGs, a cost saving
of 20 - 30 % can be achieved.
Experimental Details
The typical steps involved in the step of creating the database 102 (as
illustrated in Figure 1) for storing the growth and decay characteristics of
the
FBGs are described below:
Three photosensitive fibers from different vendors (NewportTM F-SBG-15,
CorActiveTM UVS-652 and NufernTM GF1) were inscribed with Bragg
gratings using ultraviolet radiations from KrF excimer lasers (BraggStarTM
500, Lambda Physik) operating at 248 nm with 2.5 mJ pulse energy and 200
Hz repetition rate. The gratings were fabricated using diffractive phase
masks (1070nm period, Avensys) which transmit less than 5% of the zero-
order. The grating growth was monitored in the reflection mode as a
function of exposure time using a compact broadband light source (DL-BX9,
Denselight), and an optical spectrum analyzer (IMON400-E, Ibsen). The
photosensitive fibers were typically exposed until the index change reached
saturation. The typical exposure time and the saturated index change along
with the other results of the experiments are tabulated below:
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Cutoff
Photosensi Numerical Wave FBG Exposure Saturate
Dopants
tive Fiber Aperture length ID Time(sec) d On
(nm)
Newport Ge02/B 1180 NP20
0.12 26 4x10-4
F-SBG-15 co-doped 80 0-4
CorActive 1200 CA20
Ge02 0.14 680 3.9x10-4
UVS-652 75 0-1
Nufem 1260 GF20
Ge02 0.13 455 3.5x10-4
GF1 80 0-3
From the analysis of the results, Reflectivity (R) and Bragg Wavelength (4)
measured as a function of time, the normalized index change (II), the initial
defect transformation rate (k10), and the residual temperature increase (AT,.)
in the fiber during the writing process were calculated. From the
abovementioned values, the demarcation energy of the defects was
calculated using the equation Ed = k B(To + AT,),In(ki t) where 1,9' is
the
Boltzmann constant, 'To' is the initial temperature, and 't' is the exposure
time. The normalized index change (ri) was then plotted as a function of the
demarcation energy (Ed) for the gratings fabricated in the different
photosensitive fibers as shown by I + Ao
expOEdwhere 'A0' and 13 are
the fit parameters. The defect energy distribution, (g(E)) during the growth
phase was then calculated by differentiating the above curve with respect to
Ed. The mean activation energy of defects was seen to be in the range of 0.5-
0.7 eV, which was consistent with the theoretical estimations.
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To determine the energy distribution of the decay phase, accelerated aging
experiments were performed on the above gratings. Specifically, Iso-thermal
accelerated annealing (ITA) within Iso-chronal accelerated annealing (ICA)
approach was followed. Such an approach combines the best features of both
ITA and ICA, providing a cross-referencing mechanism that improves the
confidence in the decay analysis. The accelerated aging experiments
consisted of annealing the test FBG at temperatures starting from 100 C in
steps of 75 C until the grating decayed to <5% reflectivity. As part of the
ICA routine, two different gratings were annealed for 5 minutes and 500
minutes respectively and their reflectivities were observed after each
interval. During the 500 minutes annealing, the FBG reflectivity data was
continuously observed and subsequently used for ITA analysis. Finally, the
ITA and the ICA results were correlated to deduce the decay phase defect
energy distribution. The scaling factor is also determined by finding out the
factor with which the growth phase defect energy distribution is to be sized
to arrive at the decay phase defect energy distribution.
The gratings fabricated in the three photosensitive fibers at 200 Hz pulse
repetition rate of the excimer laser were analyzed. The gratings fabricated in
the F-SBG-15 fiber (B co-doped) were found to grow relatively quickly and
had mean activation energy of 0.55 eV. Such gratings were found to decay
relatively quickly i.e., the mean activation energy deduced from the
accelerated aging experiments was lower compared to the gratings in the
other two fibers. Moreover, the energy distribution obtained through the
decay analysis for gratings fabricated in the other two photosensitive fibers
were also found to be roughly consistent with the energy distribution
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obtained during the growth phase. Figure 3 illustrates defect distributions
calculated from FBGs in different photosensitive fibers during growth phase.
Figure 4 illustrates defect distributions calculated from FBGs in different
photosensitive fibers during decay phase.
The results obtained from the experiments were found to be in accordance
with the theory postulated by B. Poumellec in 'Journal of Non-Crystalline
Solids 239 (1998) 108-115' which tells that the period for which a Fiber
Bragg Grating remains stable depends upon two factors:
1) the initial rate of transformation of the defects; and
2) the temperature at which the grating is grown.
The analysis is extended in a similar fashion to a variety of commercially
available and used photosensitive fiber materials under different exposure
conditions and the database is created and stored with each of their growth
and decay characteristics including reflectivity, Bragg wavelength,
normalized refractive index change, defect transformation rate, defect
demarcation energy, defect activation energy, residual temperature increase
and other relevant parameters including the scaling factor.
TECHNICAL ADVANCEMENTS
= The manufacturing process disclosed in the present invention helps in
the development of high quality FBGs in lesser time; those can meet
the tight tolerance requirements of optical communication and sensor
applications.
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= This process helps to avoid performing expensive and time consuming
annealing experiments used to test and stabilize the decay behaviour
of FBGs.
= This process helps in discarding the gratings which may be
determined to be unusable based on the writing data without further
processing.
= The manufacturing and maintenance costs of FBGs can be reduced
considerably using this process.
While considerable emphasis has been placed herein on the particular
features of this invention, it will be appreciated that various modifications
can be made, and that many changes can be made in the preferred
embodiments without departing from the principles of the invention. These
and other modifications in the nature of the invention or the preferred
embodiments will be apparent to those skilled in the art from the disclosure
herein, whereby it is to be distinctly understood that the foregoing
descriptive matter is to be interpreted merely as illustrative of the
invention
and not as a limitation.
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