Optical waveguide grating and method of making the same

Details Optical waveguide grating and method of making the same Optical waveguide grating and method of making the same

2000-07-13

2003-06-17

Robinson, Mark A. (Department: 2872)

Optical waveguides

With optical coupler

Input/output coupler

C385S141000

Reexamination Certificate

active

06580854

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide grating formed in an optical waveguide such as an optical fiber, and a method of making the same.
2. Related Background Art
Various applications of optical waveguide gratings having a periodic refractive index changing area (grating area) along the optical axis of an optical waveguide to optical filters and the like in optical communication systems have conventionally been studied. Among others, those having a relatively long grating period of several hundreds of micrometers are known as long-period grating (see, for example, A. M. Vengsarkar, et al., J. Lightwave Tech., Vol. 14, No. 1, pp. 58-65, 1996), and their use in gain equalizers, band-stop filters, and the like is expected. Since characteristics of such a long-period optical waveguide grating have been known to vary with changes in temperature, temperature characteristics of optical waveguide gratings have been analyzed (see, for example, J. B. Judkins, et al., OFC '96, PD-1, 1996).
That is, in an optical waveguide grating, SiO
2
is used as a main ingredient of the optical waveguide, and GeO
2
is generally added thereto in order to form a core region, which is a light-propagating area of the optical waveguide, and periodically generate a refractive index modulation within the core region, so as to form a grating area. Since the change in refractive index with respect to temperature, i.e., the temperature dependence of refractive index, is greater in GeO
2
than in SiO
2
, the temperature dependence of refractive index in the core region and that in its surrounding cladding region differ from each other. As a result, in a long-period optical waveguide grating formed in such an optical waveguide, the temperature dependence of effective refractive index for core-propagating light and that for cladding-mode light differ from each other, whereby peak wavelengths would vary when temperature changes.
The temperature dependence of each of the respective refractive indices of SiO
2
glass, GeO
2
glass, and B
2
O
3
glass has been known (O. V. Mazurin, et al., “Handbook of Glass Data,” Elsevier, 1985), and there has been known a technique based thereon in which, when an optical waveguide is a silica type optical fiber, its core region is co-doped with Ge element and B element, so as to lower the temperature dependence of characteristics of the optical waveguide grating (K. Shima, et al., OFC'97, FB2, 1997).
SUMMARY OF THE INVENTION
However, since the technique disclosed in the above-mentioned publication of Shima, et al., is based on the result of investigation of temperature dependence of refractive indices in three kinds of oxides (SiO
2
, GeO
2
, and B
2
O
3
) disclosed in the above-mentioned publication of O. V. Mazurin, et al., the optimal doping ratio can be adjusted only by combinations of these oxides, whereby the freedom in designing is limited. Also, since the infrared absorption edge of B
2
O
3
is located on the shorter wavelength side from the respective infrared absorption edges of SiO
2
and GeO
2
, the light absorption in the wavelength band of 1.55 &mgr;m will increase by one digit or more if an optimal amount of B
2
O
3
is added to the optical waveguide. For example, an optical fiber having a relative refractive index difference of about 1% in which the doping ratio of Ge element and B element have been optimized yields an absorption loss of 24 dB/km, whereby it would not be applicable to practical use. Hence, there has been no development of optical waveguide gratings having a low temperature dependence and low absorption loss in the wavelength band of 1.55 &mgr;m, which is used for optical communications.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical waveguide grating having a low temperature dependence and yielding a low absorption loss in the wavelength band of 1.55 &mgr;m, and a method of making the same.
Here, the temperature dependence of the peak wavelength &lgr;
m
of mode coupling in an optical waveguide grating having a grating period of &Lgr; is represented by the following expression:
ⅆ
λ
m
ⅆ
T
=
Λ
⁡
(
ⅆ
n
01
ⅆ
T
-
ⅆ
n
m
ⅆ
T
)
(
1
)
Here, n
01
is the effective refractive index of core-propagating light, n
m
is the effective refractive index of the m-th order cladding mode light, and T is the absolute temperature. Namely,
ⅆ
n
01
ⅆ
T
is the temperature dependence of the core-propagating light, whereas
ⅆ
n
m
ⅆ
T
is the temperature dependence of the m-th order cladding mode light. As can be seen from the above-mentioned expression, if the temperature dependence of effective refractive index for core-propagating light and that for cladding mode light can be caused to match each other, the temperature dependence of the peak wavelength &lgr;
m
in the optical waveguide grating can be lowered.
From such a viewpoint, the inventors have studied the temperature dependence of refractive indices of various glass-forming oxides. The glass-forming oxides studied here are constituted by four kinds, i.e., pure SiO
2
, SiO
2
doped with 10% of GeO
2
, SiO
2
doped with 10% of B
2
O
3
, and SiO
2
doped with 10% of P
2
O
5
. For each of the glass-forming oxides, the refractive index difference &Dgr;n with reference to pure SiO
2
, and the temperature dependence of refractive index dn/dT were studied. The results are listed in the following Table 1.
TABLE 1
Refractive Indices of Glass-Forming Oxides
and Their Temperature Dependence
Difference of
&Dgr;n
dn/dT
dn/dT from SiO
2
Pure SiO
2
0
1.380 × 10
−5
0
90% SiO
2
-10% GeO
2
+0.952%
1.434 × 10
−5
+0.054 × 10
−5
90% SiO
2
-10% B
2
O
3
−0.289%
1.051 × 10
−5
−0.329 × 10
−5
90% SiO
2
-10% P
2
O
5
+0.780%
1.077 × 10
−5
−0.303 × 10
−5
As can be seen from Table 1, as with B
2
O
3
, the difference in temperature dependence dn/dT of P
2
O
5
from pure SiO
2
is negative. Further, since P
2
O
5
has an infrared absorption edge located on the longer wavelength side from that of B
2
O
3
, the absorption loss in the wavelength band of 1.55 &mgr;m would not deteriorate when it is added to silica glass. Consequently, the inventors have found it possible to lower the absorption loss in the wavelength band of 1.55 &mgr;m while yielding a temperature dependence dn/dT substantially matching that of pure SiO
2
if P
2
O
5
, in place of B
2
O
3
, is added to SiO
2
together with GeO
2
.
The optical waveguide grating in accordance with the present invention is based on this finding, and is formed in an optical waveguide, mainly composed of SiO
2
, having a cladding region around a core region, having an area where a refractive index of the core region changes periodically along the optical axis direction; wherein the core region is co-doped with GeO
2
and P
2
O
5
.
As a consequence of such a configuration, the temperature dependence dn/dT of the optical waveguide grating can substantially match that of pure SiO
2
without adding B
2
O
3
, which may enhance the absorption loss, or while reducing the amount of addition thereof. Hence, while the absorption loss in the wavelength band of 1.55 &mgr;m is suppressed, the temperature dependence of characteristics of the optical waveguide grating is lowered.
The molar doping amount of P
2
O
5
in the core region is preferably {fraction (1/15)} to 1 times, more preferably 0.6 to 1 times that of GeO
2
. According to the inventors' findings, the temperature dependence and absorption loss can favorably be lowered if the molar doping ratio is set as such. Also, since the doping amount can be selected from such a range, it increases the design flexibility, and it is easy to manufacture.
Alternatively, the core region may further be doped with B
2
O
3
, such that the sum of respective molar doping amounts of P
2
O
5
and B
2
O
3
is {fraction (1/15)} to 1 times the molar doping amount of GeO
2
, w

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