Note: Descriptions are shown in the official language in which they were submitted.
CA 02288349 2002-O1-21
75998-32
1
LONG PERIOD OPTICAL FIBER GRATING FILTER DEVICE
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a long
period optical fiber grating filter device, and in
particular, to a temperature compensating long period
optical fiber grating filter device which shows no coupling
shift characteristics with respect to a temperature change.
2. Description of the Related Art
An optical fiber grating is generally used as a
filter for selecting an optical signal at a specific
wavelength propagating along a core. The optical fiber
grating can eliminate or reflect light at a specific
wavelength by inducing a periodic change in a refractive
index of an optical fiber using an ultraviolet (UV) laser.
Optical fiber gratings are categorized into short period
optical fiber gratings and long period optical fiber
gratings.
The short period optical fiber gratings reflect
only light at a specific wavelength signal in filtering,
whereas the long period optical fiber gratings couple a core
mode in which an optical signal propagates along the core of
an optical fiber to a cladding mode in the same propagating
direction. Long period
CA 02288349 1999-11-02
-2-
optical fiber gratings of a period ranging from several tens of ~m to several
hundreds of ~m are used as a gain flattening filter in an EDFA (Erbium Doped
(-~ fiber Amplifier) due to its capability of removing light at an intended
wavelength
by shifting light in a core mode to a cladding mode in the same propagating
direction.
The long period optical fiber gratings are fabricated by varying a
refractive index in the core of an optical fiber sensitive to UV radiation for
every
predetermined period. The refractive index increases in a core portion exposed
1 (> to the UV radiation and is not changed in a core portion experiencing no
UV
exposure, resulting in a periodic change in the refractive index along the
longitudinal axis of the optical fiber.
The long period optical fiber gratings are sensitive to temperature and its
1 ~ optical characteristics are influenced by an ambient refractive index of
an optical
fiber cladding. Micro bending of the optical fiber significantly influences
the
central wavelength and extinction ratio of the long period optical fiber
gratings,
which are determined by coupling between a core mode and a cladding mode.
20 A recoating exhibiting stable optical characteristics against influences of
an extel-nal environment is required for use of the long period optical fiber
gratings. The external environment factors are temperature, moisture, dust
introduction, and micro cracks and micro bending of an optical fiber.
2 > Coupling occurs in a long period optical fiber grating filter device when
the phase matching condition of Eq. 1 is satisfied.
f'cn -f cm) = 2?L' ..... (1)
A
P8950ST3(~~~ off- o~T71 ~ ~I o~~~ ) (38267/1999)
CA 02288349 1999-11-02
-3-
where p « is a propagation constant in a core mode, ~;~"~ IS a propagation
constant in an m-order cladding mode, and A is a grating period.
If /~ = 2~r ~ (n is a refractive index and ~, is a wavelength),
~"~~ _ 'i ~ { )
n~.~, - ra~~ -
Light at a wavelength can be shifted to a cladding mode by determining
the grating period n and a refractive index difference (n~." - n~;~) .
The refractive difference is obtained by appropriately irradiating a UV-
sensitive optical fiber with UV light. That is, the optical fiber is masked
with a
mask with a specific grating period A and UV light is projected onto the mask.
fl'ien, the optical fiber reacts to the UV radiation in such a way that the
refractive
1 ~ index of a core increases and a coupling wavelength increases to a long
wavelength. In order to obtain an intended spectrum (i.e., intended coupling
wavelength and extinction ratio) of the long period optical fiber grating
filter
device, the UV light should be projected for an appropriate time, accurately
controlling a masking period.
The coupling wavelength of the thus-fabricated optical fiber gratings is
influenced by temperature. A shift in the coupling wavelength with respect to
a
temperature change is determined by variations in a refractive index and
lengthwise thermal expansion with the temperature change. This can be
expressed as
d~,~"'> d~~"'~ do d~,~'"~ d~
dT do dT + d~ dT
P8950ST3(~.rs~ o~ o~T~l ~ ~l~ o~~l ) (38267/1999)
I
CA 02288349 2002-O1-21
75998-32
_4_
inhere T is temperature.
When a long period optical fiber grating filter device is fabricated of a
~_~neral communication optical fiber or distribution shifted optical fiber.
~ii""~ do d~r~n d~ d~.~"'' dr1
- is larger than - by several tens of times and thus dA ~T is
cln c!T ~ d,'1 dT
ne~,lected. For example, the coupling wavelength of Flexcor 1060 of Corning
shifts by ~nm per 100°C. In a typical distribution shifted optical
fiber, a
coupling wavelength shifts by 0.3nm per 100°C with respect to
lengthwise
wpansion and by ~nm per 100°C with respect to a refractive index
change.
I i ~ Temperature stability of about 0.3nm per 100°C is required for a
gain flattening
f i l ter being one of applications of a long period optical fiber grating
filter in an
e~ctual applied system.
For compensating a temperature change, a refractive index distribution in
I ~ an optical fiber is designed or the grating period of the optical fiber is
selected so
that in Eq. 3 has a negative value in prior art. Alternatively, B,_O; is
c!.\
added to the optical fiber to get ~T = 0.
d~.~""
If ~~ < 100~m in a general long period optical fiber grating filter, d~~
(~ is a negative value in the conventional method of controlling the
refractive index
d;~ 'm,
of the filter by setting d,, to a negative value. When A = 40q.m, the
dependence of wavelen~h on temperature in the Flexcor 1060 fiber is 0.1 ~-
f n-~snm! 100°C but a i.''"' mode is in a I .1 ~m region, thus
deviating from a
~ommunicat~on region.
*Trademark
CA 02288349 2002-O1-21
75998-32
A temperature compensating long period optical
fiber grating filter device is disclosed in detail in Korea
Application No. 99-8332 entitled "Temperature Compensating
Long Period Optical Fiber Grating Filter", filed by the
5 present applicant.
While a recoating of the long period optical fiber
grating filter in the above application is formed of a
material the refractive index of which increases with
temperature, the refractive index of a general recoating,
especially a polymer recoating decreases due to thermal
expansion at an increased temperature. Therefore, when
recoating a long optical fiber grating filter formed of a
general optical fiber, a long wavelength shift effect of the
recoating adds to a long wavelength shift characteristic of
the long optical fiber grating filter and thus a particular
recoating material reducing a refractive index should be
used. This recoating material is yet to be developed.
SUGARY OF THE INVENTION
It is, therefore, an object of the present
invention to provide a temperature compensating long period
optical fiber grating filter device which shows no coupling
shift characteristics with respect to a temperature change.
It is a further object of the present invention to
provide a temperature compensating long period optical fiber
grating filter device which is resistant against moisture
and soft enough to prevent micro bending.
To achieve the above objects, there is provided a
long period optical fiber grating filter device comprising:
a core having long period optical fiber gratings formed
CA 02288349 2002-O1-21
75998-32
6
therein at predetermined periods; a cladding surrounding the
core; a coating covering a cladding portion free from the
long period optical fiber gratings; a recoating covering a
cladding portion having the long period optical fiber
gratings; a core/cladding refractive index changing portion
where a coupling wavelength has a negative wavelength shift
range with respect to a temperature change according to the
amount of a dopant added to the core; and a
cladding/recoating refractive index changing portion where a
refractive index decreases at an increased temperature and a
coupling wavelength has a positive wavelength shift range;
wherein the dopant includes B203 and Ge02 and the sum of
coupling wavelength shifts caused by a refractive index
increased according to the amount of Ge02 and by a
refractive index decreased according to the amount of B203
has a negative wavelength shift value.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and
advantages of the present invention will become more
apparent from the following detailed description when taken
in conjunction with the accompanying drawings in which:
FIG. lA is a perspective view of a long period
optical fiber grating filter device packaged;
FIG. 1B is a perspective view of the long period
optical fiber grating filter with a recoating removed;
FIG. 1C is a sectional view of the long period
optical fiber grating filter device with the recoating
removed;
CA 02288349 2002-O1-21
75998-32
6a
FIGS. 2A to 2D are graphs showing a coupling
wavelength shift with respect to an ambient refractive index
of a cladding;
FIG. 3 is a graph showing a coupling wavelength
shift with respect to a change in the ambient refractive
index of the cladding;
FIG. 4 is a graph showing a coupling wavelength
shift with respect to the ambient refractive index of the
cladding when it is smaller than the refractive index of the
cladding;
CA 02288349 1999-11-02
_7_
FIG. SA is a graph showing a refractive index variation with temperature
of a recoating when it is formed of a general polymer material;
FIG. SB is a graph showing a refractive index variation with temperature
of a recoating when it is formed of silicon resin;
FIG. 6 is a graph showing a coupling wavelength shift with respect a
temperature change in a recoating material;
FIG. 7 is a graph showing a refractive index variation with temperature at
different dopant concentrations in an optical fiber core;
FIG. 8 is a graph showing a wavelength dependence on temperature at
I 0 d i ffercnt dopant concentrations in the optical fiber core;
FIG. 9 is a graph showing a temperature compensation effect of a long
period optical fiber grating filter device according to the present invention;
FIG. l0A is a graph showing a temperature dependence of a general long
period optical fiber grating device with a recoating removed;
1 ~ FIG. l OB is a graph showing a temperature dependence of the general
long period optical fiber grating filter device with the recoating;
FIG. 11 is a graph showing a temperature dependence of the long period
optical fiber grating filter device according to the present invention; and
FIG. 12 is a sectional view of the long period optical fiber grating filter
20 device according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A preferred embodiment of the present invention will be described
2s hereinbelow with reference to the accompanying drawings. In the following
description, well-known functions or constructions are not described in detail
since they would obscure the invention in unnecessary detail.
An optical fiber coating is removed for a predetermined length to forni
P8950ST3( '~L-~.~. o~ o~-~r-~~ ~ ~)- o~~~) (28267/1999)
CA 02288349 2002-O1-21
75998-32
_g_
long period optical fiber Gratings in an optical fiber. Then, the long period
«ptical fiber gratings are formed on the exposed portion using a LJV' laser
and an
amplitude mask. The uncoated long period optical fiber gratings are influenced
l,v an external environment including temperature, moisture, dust, micro
cracks,
and micro bending and thus needs protection to prevent a change in optical
characteristics.
Furthermore, a plurality of long period optical fiber gratings formed
along the length of an optical fiber for a predetermined period function as a
filter
I t~ tar coupling a core mode to a cladding mode. Therefore, the refractive
index of
:~ recoatin~ material should be considered. -
As shown in FIGS. lA, 1B, and 1C, a packaged long period optical fiber
~~ratin~ filter device 100 includes a core 10 having long period optical fiber
i ~ ~_ratings formed thereon at every predetermined periods, a cladding I
surrounding the core 10, a coating 14 surrounding the cladding 12, and a
recoating 18 coated on the long period optical fiber gratings 16. A recoating
is
applied to a portion from which the coating 14 is removed to protect the long
period optical fiber Gratings 16.
?(l
In FIG. 1 C, arrows indicating a wavelength propagating direction denotes
coupling from a core mode to a cladding mode in the long period optical fiber
~;ratin~ filter device. The thickness of an arrow indicates the intensity of
light at
a wavelength.
,;
An optical signal at a central wavelength traveling in a fundamental
~_uide mode in the core 10 is scattered at a refractive index changing
portion, that
is. in the lone period optical fiber gratings 16. As the scattered light is
coupled
to the cladding 12, light at a wavelength satisfying a phase matching
condition is
CA 02288349 1999-11-02
-9-
coherently reinforced. The light goes outside the cladding 12 and the long
period optical fiber grating filter device 100 acts as a wavelength dependent
attenuator.
The intensity of the light traveling in the fundamental guide mode is
reduced while passing through the long period optical fiber gratings 16, as
indicated by a decrease in the thickness of an arrow, and the intensity of the
light
at the wavelength coupled to the cladding 12 is increased as indicated by an
i ncrease in the thickness of arrows.
An external condition of the cladding I2, namely air has a refractive
index of 1. If the cladding 12 is recoated with a material with a refractive
index
n after formation of the long period optical fiber gratings 16, a coupling
condition
is changed and thus a coupling wavelength is shifted to a long or short
1 > w avelength.
FIGS. 2A to 2D are graphs showing shifts of a coupling wavelength with
respect to an ambient refractive index of the cladding.
20 FIG. 2A is a graph showing an optical transmittance characteristic when
an ambient refractive index (the refractive index of air) of the cladding
surrounding the long period optical fiber gratings is 1.
FIG. 2B is a graph showing an optical transmittance characteristic when
the ambient refractive index of the cladding is 1.400. It is noted that an
optical
transmittance is increased and a coupling wavelength shifts to a short
wavelength
by about 4.8nm, as compared to the graph of FIG. 2A.
FIG. 2C is a graph showing an optical transmittance characteristic when
P8950ST3(-Q-~~ o~ o~-~r71 ~~l- o~~l) (~S267/1999)
CA 02288349 1999-11-02
- 10-
the ambient refractive index of the cladding is 1.448. The coupling wavelength
shifts to a short wavelength by l6.Snm, as compared to FIG. 2A.
FIG. 2D is a graph showing an optical transmittance characteristic when
tl~e ambient refractive index of the cladding is 1.484. The coupling
wavelength
shifts to a long wavelength, as compared to FIG. 2A.
If the ambient refractive index of the cladding increases from 1 but is
smaller than the refractive index of the cladding, the coupling wavelength
shifts
I () to a short wavelength, as shown in FIGs. 2B and 2C. On the other hand, if
the
ambient refractive index of the cladding exceeds the refractive index of the
cladding, the coupling wavelength shifts to a long wavelength, as shown in
FIG.
2 D. If the ambient refractive index of the cladding is equal to the
refractive
index of the cladding, a full reflection condition is released and a coupling
peak
I ~ disappears.
FIG. 3 is a graph showing a coupling wavelength shift with respect to a
change in the ambient refractive index of the cladding. The coupling
wavelength shifts to a short wavelength as the ambient refractive index
increases
2() From 1.0, the coupling peak disappears when the ambient refractive index
is equal
to the refractive index of the cladding, and then the coupling wavelength
shifts to
a long wavelength when the ambient refractive index exceeds the refractive
index
of the cladding.
2 ~ FIG. 4 is a graph showing a coupling wavelength shift with respect to a
change in the ambient refractive index of the cladding when the ambient
refractive index is smaller than the refractive index of the cladding.
Refernng to
FIG. 4, as the ambient refractive index decreases, the coupling wavelength
shifts
to a long wavelength, only if the ambient refractive index is smaller than the
P8950ST3(~-~~ o~ o~T~~ ~~1~ o~~~) (38267/1999)
CA 02288349 1999-11-02
-11-
refractive index of the cladding.
The results shown in FIGS. 2A to 4 are described in detail in a thesis by
the present inventor "Displacement of the Resonant Peaks of a Long period
Fiber
(hating Induced by a Change of Ambient Refractive Index", 1997 Optics Letters,
December 1, 1997/VoI. 22, No. 23.
FIG. ~A is a graph showing a change in the refractive index of a general
recoating material with respect to a temperature change, and FIG. SB is a
graph
( () slowing a change in the refractive index of silicon resin taken as an
example of
tl~e general recoating material, with respect to a temperature change.
Referring to FIG. ~A, a general recoating material, that is, a polymer
experiences thermal expansion at an increased temperature and has a reduced
1 > retractive index. Referring to FIG. SB, silicon resin also experiences
thermal
expansion at an increased temperature and has a reduced refractive index.. The
refractive index variation with temperature of the silicon resin is -2.4x10-
2/100°C.
FIG. 6 is a graph showing a coupling wavelength shift of a recoating
20 material with respect to a temperature change. It is noted from the drawing
that
the coupling wavelength shifts to a long wavelength as the refractive index of
the
recoating material decreases with a temperature increase. The shift of the
coupling wavelength to a long wavelength implies that it has a positive
wavelength shift range.
~>
FIG. 7 is a graph showing a coupling wavelength shift with respect to a
tcmperaW re change at a different concentration of a dopant added to an
optical
Iiber core. Temperature compensation by adding Bz03 and GeO, as dopants to
a core is disclosed in detail in EP 0 800 098 A2 entitled "Optical Waveguide
P8950ST3(~ ~~ off- o~T 71 ~ ~)- 0~ ~1 ) (38267/1999)
75998-32
CA 02288349 2002-O1-21
-12-
Grating and Production Method Thereof '. As shown in FIG. 7, with B,O, more
than GeO,, the long period optical fiber gratings have a negative wavelength
shift
range when temperature increases. That is, a refractive index variation with
mmperature has a negative value. In the present invention, a temperature
change is compensated by setting the wavelength shift range of the coupling
wavelength to a negative value in the long period optical fiber gratings and
to a
positive value in a recoating material.
For example, if 20mo1% of GeO, and 1 ~mol% of B,03 are added to the
core, a change in the refractive index of the long period optical fiber
gratings
formed on the core with respect to a temperature change has a negative value
and
thus the coupling wavelength has a negative wavelength shift range. This is
illustrated in FIG. 8.
FIG. 8 is a graph showing a shift of the coupling wavelength to a short
wavelength at an increased temperature when the amount of B=O; is larger than
that of GeO, in the core and the long period optical fiber gratings are not
recoated.
In FIG. S, the coupling wavelength shifts to a short wavelength when
temperature
increases. This implies that the coupling wavelength in the long period
optical
?() fiber grating filter device has a negative wavelength shift range.
FIG. 9 is a graph showing a long wavelength shift effect of a recoating
material like silicon resin at an increased temperature in the long period
optical
trber grating filter device and temperature compensation resulting from a
short
wavelength shift effect produced by use of B,03 more than GeO~. Reference
numeral 1 indicates a shift of the coupling wavelength to a long wavelength
due
to a refractive index changing portion of the cladding%recoating according to
a
temperature change, and reference numeral 3 indicates a shift of the coupling
wavelength to a short wavelength due to a refractive index changing portion of
CA 02288349 2002-O1-21
X5998-'2
-13-
the coreicladding according to a temperature change.
The long wavelength shift and the short wavelength shift of the coupling
wavelength concurrently occur in the long period optical fiber grating filter
;i~vice, thereby achieving temperature compensation in the present invention,
as
indicated by reference numeral 2.
FIGS. l 0 A and l OB are graphs showing wavelength shifts with respect to
a temperature change in the cases that a general long period optical fiber
orating
I c> filter device showing no short wavelength shift effect in a core is not
recoated
,end is recoated with silicon resin, respectively.
FIG. 8 is a graph showing a wavelength shift with respect to a
temperature change when the long period optical fiber grating filter device of
the
present invention is not recoated while it has a negative wavelength shift
range
with B.O; more than Ge0= used. FIG. 11 is a graph showing a wavelength shift
with respect to a temperature change when the long period optical fiber
grating
filter device of the present invention is recoated with silicon resin while it
has a
ne~,ative wavelen~h shift range with B,03 more than GeO, used.
Temperature compensation of the present invention will be described
hereinbelow by comparing FIGS. l0A and lOB showing the conventional
technology with FIGS. 8 and 9 according to the present mvennon.
As shown in FIG. 1 OA, when the general long period optical fiber grating
f i l ter device is not recoated, the coupling wavelength shifts to a long
wavelength
as temperature increases, and a temperature dependence of the wavelength is
about ~.08nmi 100°C.
CA 02288349 1999-11-02
- 14-
In FIG. lOB, when the general long period optical fiber grating filter
device is recoated with silicon resin, the coupling wavelength shifts to a
long
wavelength at an increased temperature, and a temperature dependence of the
wavelength is about lOnm/100°C.
It can be noted from FIGS. l0A and lOB that recoating the general long
period optical fiber gratings with silicon resin incurs a synergy between a
long
wavelength shift effect of the optical fiber core and the long wavelength
shift
effiect of silicon resin to thereby further the long wavelength shift effect.
That is,
1 () temperature dependence is further increased.
In FIG. 8, when the optical fiber core includes BZ03 more than GeO, and
the long period optical fiber grating filter device is not recoated in the
present
invention, the coupling wavelength shifts to a short wavelength at an
increased
l > temperature, and a temperature dependence of the wavelength is about
-4.7nm/ 100°C.
In FIG. 11, when the optical fiber core includes Bz03 more than GeO,
and the long period optical fiber grating filter device is recoated with
silicon resin
?0 in the present invention, a short wavelength effect of the core and a long
wavelength shift effect of the recoating material concurrently occur, thereby
compensating for a temperature change. As a result, there is no change in the
coupling wavelength with respect to a temperature change. Here, a temperature
dependence of the wavelength is about 0.7nm/100°C.
The thus-fabricated long period optical fiber grating filter device of the
present invention is shown in FIG. 12. Reference numeral 120 denotes a core
with B,O~ more than Ge02, reference numeral 122 denotes a cladding
surrounding the core 120, and reference numeral 126 denotes a plurality of
long
P8950ST3(-Q-rs~'o o~~ 7~ ~ ~}~ ~1-~l ) (38267/1999)
CA 02288349 1999-11-02
-15-
period optical fiber gratings formed along the length of the core 120.
Reference
numeral 128 denotes a silicon resin recoating which covers the long period
optical fiber gratings 126.
It can be concluded that if a coupling wavelength shifts within a positive
wavelength shift range at an increased temperature by using Bz03 more than
GeO,, in an optical fiber core, and a refractive index decreases with an
increase in
temperature and the coupling wavelength shifts within a positive range in a
rccoating, a temperature change can be compensated for without little coupling
1 (> wavelength shift.
As described above, the long period optical fiber grating filter device
according to the present invention includes a core where a coupling wavelength
shifts within a negative range at an increased temperature according to the
1 > amount of a dopant added, and a recoating where a refractive index
decreases
with the temperature increase and the coupling wavelength shifts within a
positive range. Thus, the coupling wavelength shift of the long period optical
fiber gratings attributed to a temperature change can be compensated for, and
temperature compensation thereof is facilitated.
2 ~)
While the invention has been shown and described with reference to a
certain preferred embodiment thereof, it will be understood by those skilled
in the
art that various changes in form and details may be made therein without
departing from the spirit and scope of the invention as defined by the
appended
2~ claims.
ps9sosT3l~-~~ o~ ~~-T~l ~y o~~l) (3az~ot~~9~
