Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-09-28
2003-05-13
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S024000, C385S147000
Reexamination Certificate
active
06563989
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber coupler which is made by arranging first and second single mode optical fibers in parallel, and elongating them upon fusion; and a single mode optical fiber for the optical fiber coupler.
2. Related Background Art
An optical fiber coupler is made by arranging first and second optical fibers in parallel and elongating them upon fusion.
FIG. 17
shows a configuration thereof. In the optical fiber coupler
1
shown in
FIG. 17
, light inputted from a first end
11
of a first optical fiber
10
is power-split in a fusion-elongated part
30
at a splitting ratio corresponding to its wavelength, and thus split light components are outputted from a second end
12
of the first optical fiber
10
and a second end
22
of the second optical fiber
20
, respectively. Here, the splitting ratio is 1:0 or 0:1 at some wavelengths. In this case, the light inputted from the first end
11
of the first optical fiber
10
is outputted from one of the second end
12
of the first optical fiber
10
and the second end
22
of the second optical fiber
20
. Such an optical fiber coupler
1
is used as an optical splitter for power-splitting light inputted to one terminal and outputting thus split light components from two terminals, an optical multiplexer for multiplexing two wave length bands of light inputted to different terminals and outputting thus multiplexed light from one terminal, an optical demultiplexer for demultiplexing two wavelength bands of light inputted to one terminal and outputting thus demultiplexed light components from different terminals, or the like.
For example, the optical fiber coupler
1
is used as an optical multiplexer in an optical fiber amplifier. In this case, the optical fiber coupler
1
multiplexes the signal light (e.g., in the 1.55-&mgr;m wavelength band) to be optically amplified by an optical amplifier medium included in the optical fiber amplifier and the pumping light (e.g., at a wavelength of 0.98&mgr;m) to be supplied to the optical amplifier medium. Specifically, when the signal light to be optically amplified is inputted to the first end
11
of the first optical fiber
10
whereas the pumping light outputted from the pumping light source is inputted to the first end
21
of the second optical fiber
20
, the optical fiber coupler
1
multiplexes the signal light and pumping light together and outputs thus multiplexed light from the second end
12
of the first optical fiber
10
toward the optical amplifier medium.
SUMMARY OF THE INVENTION
The inventors studied the prior art mentioned above and, as a result, have found problems as follows. Namely, it is required for such an optical fiber coupler
1
to minimize its excess loss. A case where light having a power P
11
is inputted to the first end
11
of the first optical fiber
10
in the optical fiber coupler
1
will be considered. Here, it is required to minimize the excess loss represented by the following expression:
Excess
Loss
(
dB
)
=
&LeftBracketingBar;
10
·
log
10
P
12
+
P
22
P
11
&RightBracketingBar;
(
1
)
where P
12
is the power of light outputted from the second end
12
of the first optical fiber
10
, and P
22
is the power of light outputted from the second end
22
of the second optical fiber
20
.
In an ideal optical fiber coupler, as shown in
FIG. 18A
, the power P
12
of light outputted from the second end
12
of the first optical fiber
10
and the power P
22
of the light outputted from the second end
22
of the second optical fiber
20
have respective waveforms substantially symmetrical to each other to a horizontal line. Also, the total of the light powers P
12
and P
22
is substantially equal to the power P
11
of the light inputted to the first end
11
of the first optical fiber
10
, thereby hardly yielding excess loss.
However, in order to propagate both of two wavelengths of light in a single mode, the cutoff wavelength of optical fibers constituting the optical fiber coupler is set shorter than the wavelength of light on the shorter wavelength side. Therefore, the confinement of light on the longer wavelength side into the core portion becomes weaker, so that the loss of light on the longer wavelength side increases when the optical fiber is bent at a smaller radius of curvature, whereby excess loss is likely to occur. When the confinement of light into the core portion is weak, the power of light is likely to spread outside, whereby the light may partly leak to the outside due to minute variations in the outer form caused by fluctuations in the heat of a heat source (heater or burner) or the like at the time of making the optical fiber coupler. As a result, the output power may fluctuate depending on the wavelength as shown in FIG.
18
B. In this case, the total of the power P
12
of the light outputted from the second end
12
of the first optical fiber
10
and the power P
22
of the light outputted from the second end
22
of the second optical fiber
20
becomes lower than the power P
11
of the light inputted to the first end
11
of the first optical fiber
10
, thereby yielding excess loss.
In order to reduce the excess loss in view of the foregoing, Japanese Patent Application Laid-Open No. HEI 7-301722, for example, discloses an optical fiber coupler made by using an optical fiber having a so-called double core type refractive index profile. Here, the double core type refractive index profile is one having, successively from the optical axis center, a first core region (with a refractive index n
1
), a second core region (with a refractive index n
2
), and a cladding region (with a refractive index n
3
), wherein the refractive indices have the relationship of n
1
>n
2
>n
3
in terms of magnitude.
However, though the optical fiber coupler disclosed in the above-mentioned publication intends to reduce the excess loss by using an optical fiber having a double core type refractive index profile, its degree of reduction in excess loss may not be sufficient.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical fiber coupler whose excess loss is fully reduced, and an optical fiber for the optical fiber coupler.
The present invention provides an optical fiber coupler made by arranging first and second single mode optical fibers in parallel, each operating a single mode transmission in a used all wavelength region, and elongating the first and second single mode optical fibers upon fusion; wherein each of the first and second single mode optical fibers has a cladding portion, placed about a core portion, having a refractive index gradually decreasing outward in a radial direction.
In the optical fiber coupler, since the refractive index of the cladding part of each of the first and second single mode optical fibers gradually decreases outward in the radial direction, the light spread to marginal areas of the core portion can return to the core portion without being emitted to the outside even when influenced by disturbances such as minute bending, whereby the excess loss can fully be reduced.
Also, the present invention provides an optical fiber coupler made by arranging first and second single mode optical fibers in parallel, each operating a single mode transmission in a used all wavelength region, and elongating the first and second single mode optical fibers upon fusion; wherein, in each of the first and second single mode optical fibers, letting r be the radial distance from the optical axis center, &Dgr;n(r) be the relative refractive index difference at the position r within a core portion with reference to the refractive index of a cladding portion placed about the core portion, &Dgr;n
peak
be the peak value of the relative refractive index difference &Dgr;n(r) at the position r
peak
, and a be the core radius, the relative refractive index difference &Dgr;n(r) satisfies the relationship of &Dgr;n(r)≦&Dgr;n
peak
[1−(r/
Ishikawa Shinji
Kiya Masayuki
Urano Akira
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
Ullah Akm E.
LandOfFree
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