Textiles: ironing or smoothing – Smoothing machines – Platen presser
Reexamination Certificate
2001-01-05
2002-10-15
Palmer, Phan T. H. (Department: 2874)
Textiles: ironing or smoothing
Smoothing machines
Platen presser
C385S128000, C385S124000, C385S141000, C359S199200
Reexamination Certificate
active
06463684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical fiber gratings, and more particularly to a temperature-compensated long-period grating.
2. Description of the Related Art
An optical fiber grating is typically used as a filter for selecting an optical signal at a particular wavelength traveling along the core of an optical fiber. By combining fiber gratings in various arrangements, the optical fiber grating directs or reflects light at a particular wavelength to different output fibers. Basically, there are two types of gratings with varying reflective index periods inside the fiber: a short period grating and a long-period grating.
The short period gratings reflect light at a particular wavelength, whereas the long-period gratings couple light from a core mode to a cladding mode. Typically, long-period gratings with the period of tens of micrometers to hundreds of micrometers are used as a gain flattened filter for an erbium doped fiber amplifier (EDFA) as they can remove light at a particular wavelength by coupling light from the core mode to the coupling mode. These long-period gratings are fabricated by inducing a periodic change in the refractive index of the photosensitive area of the core. The refractive index of the fiber changes when exposed to the UV (Ultra Violet) light. Hence, a periodic change in the refractive index can be produced using the light exposure. Moreover, the long-period gratings are very sensitive to the temperature change as well as the ambient refractive index of the cladding. Furthermore, micro bending of the optical fiber has great influences over the peak wavelength and the extinction ratio of the long-period gratings.
Coupling between the core and cladding occurs when the long-period grating filter satisfies a phase matching condition given as:
β
CO
-
β
cl
(
m
)
=
2
π
Λ
,
(
1
)
wherein &bgr;
co
represents a propagation constant in the core mode, &bgr;
cl
(m)
represents a propagation constant in the cladding mode, and &Lgr; represents a grating period.
For
β
=
2
π
n
λ
(where n represents a refractive index and &lgr; represents a wavelength), Eq. (1) can be expressed as:
n
CO
-
n
cl
(
m
)
=
λ
Λ
.
(
2
)
Accordingly, light at a particular wavelength is coupled to the cladding mode, and it is represented by the difference between a grating period &Lgr; and a refractive index difference (n
co
−n
cl
(m)
).
The refractive index difference is readily achieved by appropriately irradiating an optical fiber to the UV light using a UV laser. The optical fiber exposed to the UV light is covered with a mask with a predetermined grating period &Lgr;, then the UV laser is projected onto the fiber via the mask in order to increase the refractive index of the core of the optical fiber. As the refractive index increases, the coupling wavelength becomes longer. For a desired spectrum, that is, a desired coupling wavelength and desired extinction ratio of a long-period grating device, the UV laser must be projected for an appropriate period of time while controlling the mask period precisely.
The coupling wavelength of the above long-period fiber gratings is affected by temperature changes. The shift of the coupling wavelength caused by temperature change is determined by temperature-induced changes in the refractive index and the thermal expansion along the length of the fiber. This relationship is represented, as below:
ⅆ
λ
(
m
)
/
ⅆ
T
=
(
ⅆ
Λ
/
ⅆ
T
)
(
δ
n
)
(
m
)
+
Λ
ⅆ
(
δ
n
)
(
m
)
/
ⅆ
T
=
(
ⅆ
λ
(
m
)
/
ⅆ
(
δ
n
)
(
m
)
)
(
ⅆ
(
δ
n
)
(
m
)
/
ⅆ
T
)
+
(
ⅆ
λ
(
m
)
/
ⅆ
Λ
)
(
ⅆ
Λ
/
ⅆ
T
)
,
(
3
)
wherein T represents temperature.
For a general silica optical fiber, d&lgr;
(m)
/dT is a 5-15 nm per 100° C. change in temperature. Here, temperature dependency can be decreased by setting d&lgr;
(m)
/d&Lgr; to a negative value or setting d(&dgr;n)
(m)
/dT to zero. In fabricating a long-period grating using a general optical fiber or distributed shifted fiber for communication, the second term of Eq. (3) on the right side is neglected as it is significantly less than the first term. For example, the coupling wavelength shifts by a 5 nm per 100° C. change in temperature when a fiber (i.e., Flexcor 1060 made by the Corning Co.) is used. When a distributed shifted-fiber is used, the lengthwise expansion shifts the coupling wavelength by only 0.3 nm per 100° C., whereas the refractive index shifts the coupling wavelength by 5 nm per 100° C. However, the temperature stability of about 0.3 nm per 100° C. is required for a gain flattened filter to be effective.
For temperature compensation, the refractive index profile of an optical fiber can be arranged differently, or the grating period can be selected such that the d&lgr;
(m)
/d&Lgr; in Eq. (3) becomes a negative value. Alternatively, B
2
O
3
can be added to make the dn/dT of Eq. (3) zero.
It is known that d&lgr;
(m)
/d&Lgr; becomes a negative value, by controlling the refractive index in a typical long-period fiber filter, when &Lgr;<100 &mgr;m. In the case of using fibers, i.e., Flexcor 1060 made by the Corning Co., temperature-dependent wavelength change is about a 0.15-0.45 nm per 100° C. change in temperature for &Lgr;=40 &mgr;m, but the &lgr;
(m)
mode is in the
1.1 &mgr;m region which falls outside the communication region.
d(&dgr;n)
(m)
/dT can be set to zero when
d&lgr;
(m)
/dT
=(
d&Lgr;/dT
)(&dgr;
n
)
(m)
+&Lgr;d
(&dgr;
n
)
(m)
/dT
(4),
wherein d&Lgr;/dT corresponds to the thermal expansion. When silica is used, a
SiO2
=5.5×10
7
/° C. and the contribution of d&Lgr;/dT to wavelength dependency becomes negligible, i,e., 0.1 nm or below per 100° C. Then, the effect of d(&dgr;n)
(m)
/dT can be expressed as:
d
(&dgr;
rt
)
(m)
/dT=d
(
n
01
−n
(m)
)/
dT≈d
(
n
core
−n
SiO2
)/
dT
(5).
Here, d(&dgr;n)
(m)
/dT can be zero by doping the core with appropriate amounts of GeO
2
(dn
Ge
/dT>dn
SiO2
) and B
2
O
3
(dn
B
/dT<dn
SiO2
/dT). For reference, the temperature compensation effect is disclosed in detail “Optical Waveguide Grating and Production Method Thereof”, EP 0 800 098 A2.
The applicant in this invention has filed two applications relating to a temperature-controlled long-period grating device. Both disclose controlling materials that may be considered in selecting recoating material to be applied around the cladding for achieving temperature-compensated fiber gratings. One method is disclosed in the Korea Application No. 1999-8332 filed with the Korean Patent Office. In this application, dn/dT of the core is provided to be greater than dn/dT of the cladding due to higher Ge concentration in the core, wherein the coupling peak of long-period gratings shifts to a long wavelength as temperature increases. Here, the requirement of a recoating material is that its refractive index increases with temperature to induce a short wavelength shift effect through the refractive index of a coating material. Another method is disclosed in Korea Application No. 1999-38267, which is filed by the present applicant. In this application, dn/dT of the core is less than dn/dT of the cladding by adding Ge/B. As temperature increases, the short wavelength effect takes place in the mode of the long-period gratings. The requirement of a coating material around the cladding includes that its refractive index increases with temperature to compensate the short wavelength shift effects, by using the long wavelength shift effect of the recoating material. In both methods, the initial refractive index of the coating material around the cladding must be less than the refractive index of the cladding in order to guide both the core mo
Cha Steve
Klauber & Jackson
Palmer Phan T. H.
Samsung Electronics Co,. Ltd.
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