Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-01-12
2003-06-10
Kim, Robert H. (Department: 2882)
Optical waveguides
With optical coupler
Input/output coupler
C385S124000
Reexamination Certificate
active
06577790
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical device used in a communication system in which optical fibers are utilized. More particularly, the present invention relates to an optical device having rod lenses.
2. Related Background Art
Recently, due to the explosive growth of the internet, there is a strong demand for increase in capacity of an optical fiber communications network. For the purpose of increasing the capacity, the development of WDM (Wavelength Division Multiplexing) communication has been accelerated. In the WDM communication, since lights with slightly different wavelengths convey individual pieces of information, an optical function element such as a filter, an isolator, or the like with excellent wavelength selectivity is required in an optical device.
The optical device often has a configuration such that light leaving an end face of an optical fiber from which light outgoes (hereinafter referred to as an “output fiber” throughout the present specification) is changed to a parallel beam by a collimator, and the parallel beam then is allowed to pass through a flat plate component with a filter or isolator function and is condensed by a condenser lens again to enter an end face of an optical fiber from which light enters (hereinafter referred to as an “input fiber” throughout the present specification). A rod lens with a refractive index distribution in its radial direction, a glass ball lens, a pressed aspherical lens, or the like is used as the collimator or the condenser lens. In view of the shape and aberration correction, the rod lens can be used most easily.
FIG. 1
is a schematic view showing an optical system of an optical device having rod lenses. Generally, as shown in
FIG. 1
, end faces (facing rod lenses) of an output fiber
1
and an input fiber
2
are processed to have slopes with a tilt angle of 6° to 8° to prevent crosstalk caused by reflected light (the tilt angle of the end face of the output fiber
1
is indicated as &thgr;
FA
, and that of the end face of the input fiber
2
as &thgr;
FB
). For the same reason, the end faces (facing the optical fibers) of a first rod lens
3
and a second rod lens
4
also are processed to have slopes (the tilt angle of the end face of the first rod lens
3
is indicated as &thgr;
PA
, and that of the end face of the second rod lens
4
as &thgr;
PB
). The output fiber
1
and the first rod lens
3
are positioned to oppose each other with a suitable air space W
A
being provided therebetween, and the second rod lens
4
and the input fiber
2
are positioned to oppose each other with a suitable air space W
B
being provided therebetween. In order to reduce the loss due to reflected light, the spaces between the output fiber
1
and the first rod lens
3
and between the second rod lens
4
and the input fiber
2
may be filled with a transparent liquid or solid with a refractive index close to that of the optical fibers and the rod lenses in some cases. An optical function element such as a filter, an isolator, or the like is placed between the rod lenses
3
and
4
(i.e. in a space L).
Generally, a single-mode fiber is used as an optical fiber for communication, and therefore a beam leaving the fiber is a Gaussian beam. In the present specification, the light ray with a highest intensity at the symmetrical center of the Gaussian beam is referred to as a “center light ray”. In order to increase the coupling efficiency between the optical fibers
1
and
2
shown in
FIG. 1
, the respective optical fibers
1
and
2
and rod lenses
3
and
4
are required to be positioned so that the following conditions (1) to (3) are satisfied.
(1) A beam leaving the output fiber
1
is focused at a focal point on the end face of the input fiber
2
.
(2) The numerical aperture NA at the focal point is the same as that of the input fiber
2
.
(3) The path of the center light ray entering the input fiber
2
coincides with the optical axis of the input fiber
2
.
Furthermore, it is desirable that the respective rod lenses
3
and
4
have a numerical aperture NA at least 1.5 times to 2 times the numerical aperture NA of the optical fibers so as to transmit the Gaussian beam without causing eclipse in actual use. Naturally, various aberrations in the working wavelengths should be corrected sufficiently in the rod lenses
3
and
4
.
As shown in
FIG. 1
, however, when the optical axes of all the optical fibers and rod lenses are aligned, it becomes difficult to satisfy the conditions described above due to the presence of many processed faces with slopes. Consequently, either or both of
(4) the shift of the focal point from the optical axis of the input fiber
2
, or/and
(5) the tilt of the center light ray with respect to the optical axis of the input fiber
2
is/are caused and thus the coupling efficiency is decreased. Table 1 below shows specific design values in the case where optical axes of all the optical fibers and rod lenses are aligned and the tilt angles (&thgr;
FA
, &thgr;
FB
, &thgr;
PA
, and &thgr;
PB
) are set to be 8° uniformly (Reference Example 1), as an example. In this case, the “shift of the focal point” is very small, but the “tilt of the center light ray” is great, namely 2.85°, resulting in low coupling efficiency, namely 77.3% (−1.118 dB). As shown in
FIG. 13
, therefore, it is required to make the optical axis of the input fiber
2
and center light ray coincide by making corrections in the input fiber
2
(correcting the tilt angle, positions in X and Y directions, and the like). Reference Example 2 (see Table 1 below) was obtained through a correction of the tilt angle in Reference Example 1. In Reference Example 2, the coupling efficiency is improved to 98.28% (−0.075 dB).
TABLE 1
Ref. Ex. 1
Ref. Ex. 2
Outgoing-Side Fiber End Face
8.00°
8.00°
Tilt &thgr;
FA
Outgoing-Side Space W
A
0.2351 mm
0.2351 mm
Outgoing-Side Rod Lens End Face
8.00°
8.00°
Tilt &thgr;
QA
Outgoing-Side Rod Lens Length
4.4329 mm
4.4329 mm
Z
A
Lens Interval L
2.00 mm
2.00 mm
Incident-Side Rod Lens Length Z
B
4.4329 mm
4.4329 mm
Incident-Side Rod Lens End Face
8.00°
8.00°
Tilt &thgr;
QB
Incident-Side Space W
B
0.2340 mm
0.2340 mm
Incident-Side Fiber End Face
8.00°
8.00°
Tilt &thgr;
FB
&thgr;
QA0
4.55°
4.55°
&thgr;
QB0
4.55°
4.55°
(&thgr;
QA
+ &thgr;
QB
) − (&thgr;
QA0
+ &thgr;
QB0
)
−9.11°
−9.11°
&thgr;
3A
0.43°
0.43°
&thgr;
3B
0.43°
0.43°
&thgr;
3A
− &thgr;
3B
0.00°
0.00°
Shift between Center Light Ray
0.00005 mm
0.00005 mm
and Optical Axis &Dgr;Y
Tilt of Center Light Ray with
−2.850°
By the tilting
respect to Optical Axis
of the fibers,
TLA
the tilt is set
to be 0°.
Coupling Efficiency
0.7730
0.9828
(−1.118 dB)
−0.0753 dB)
Ref. Ex. = Reference Example
The same correction also can be made by the tilting or shifting of the output fiber
1
and the respective rod lenses
3
and
4
, individually.
However, it takes time to correct the positions (in the X and Y directions) and the tilt angles of the optical fibers and the rod lenses, which causes cost increase.
Therefore, in view of the assembly of an optical device, for instance, as shown in
FIG. 2
, it is desirable to hold the output fiber
1
and the input fiber
2
with ferrules
5
with the same outer diameter as those of the first and second rod lenses
3
and
4
and to insert the input fiber
2
, the second rod lens
4
, the first rod lens
3
, and the output fiber
1
into a single sleeve
6
, sequentially. In this case, it is not possible to carry out the “position shift” and “tilt angle correction”, but it is possible to adjust the positions of the respective optical fibers
1
and
2
by pulling and inserting them in a Z axis direction. In
FIG. 2
, numeral
8
indicates an optical function element.
In order to obtain high coupling efficiency in the configuration shown in
FIG. 2
, an optical system is required, which is designed so that the above-mentioned conditions are satisfied through the adjust
Kim Robert H.
Merchant & Gould P.C.
Nippon Sheet Glass Co. Ltd.
Wang George
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