Optical waveguides – Accessories – External retainer/clamp
Reexamination Certificate
2001-10-12
2004-09-28
Nguyen, Thong (Department: 2872)
Optical waveguides
Accessories
External retainer/clamp
C385S136000
Reexamination Certificate
active
06798969
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fiber arrays used to fix one or more optical fibers in a line, and to connect the one or more optical fibers to an optical element, as well as to methods for fabricating such fiber arrays and optical devices using such fiber arrays.
2. Description of the Related Art
Conventional fiber arrays are mainly used to couple optical fibers to wave-guides of optical devices. As shown in a top view in FIG.
3
(
a
), the corresponding lateral view in FIG.
3
(
b
), and in FIGS.
4
(
a
)-(
c
), the end faces of a fiber array and a waveguide substrate are optically fixed by an end face adhesive A, and the end faces of the optical fibers are up to about 10 &mgr;m removed from the end face of the waveguide D. As shown in FIG.
4
(
a
), which is a front view, and in FIG.
4
(
c
), which is a cross-section taken from above, stripped fibers
8
are placed in a V-groove substrate
1
, and a fiber fixing substrate
2
fixes the stripped fibers
8
in the center of the V-grooves. The stripped fibers
8
are fastened by a peripheral adhesive B disposed around the stripped fibers
8
, so that the gap between the V-grooves and the stripped fibers
8
, in other words the periphery of the fibers, is filled with the peripheral adhesive B. Furthermore, coated fibers
9
are housed by an upper substrate
3
, to which they are fixed with an adhesive. It should be noted that the upper substrate
3
is sometimes not needed, since the fiber fixing substrate
2
is used to fix the stripped fibers
8
securely in the center of the V-grooves
7
.
The V-groove substrate
1
includes an upper plane
4
and a lower plane
6
that is recessed by a step
5
from the upper plane
4
. V-grooves
7
are formed by mechanical machining using a grinding stone or the like, or by Si etching to house the stripped fibers
8
in the upper plane
4
. The height of the step
5
is set to about half the diameter of the coated portion of the optical fiber. This height is adjusted such that the stripped fibers
8
are placed in the center of the V-grooves
7
, when the coating of the optical fibers is put on the lower plane
6
. Moreover, the fiber fixing substrate
2
is fastened tightly to the upper side of the upper plane
4
to fix the stripped fibers
8
in the center of the V-grooves
7
. Then, grooves are formed in the lower side of the upper substrate
3
, and the upper substrate
3
is placed on the lower plane
6
of the V-groove substrate
1
.
As a method for fabricating such a fiber array, first the upper substrate
3
is placed on the lower plane
6
of a V-groove substrate
1
, and the end face of the upper substrate
3
is adhered tightly to the step
5
of the V-groove substrate
1
. Thus, both parts are positioned with respect to longitudinal direction. At the same time tunnel-shaped holes are formed by the lower plane
6
and the grooves in the lower surface of the upper substrate
3
. If the optical fibers are introduced from the outside into these holes, then the stripped optical fibers
8
are matched with the centers of the V-grooves
7
. Here, the coating housing portion is machined so that the grooves in the lower surface precisely match the coating of the fibers. Consequently, once the optical fibers are inserted, they are maintained in that state. Then after the fiber fixing substrate
2
is placed on the upper plane
4
of the V-groove substrate
1
to hold down the stripped fibers
8
, a thermosetting or UV-setting resin adhesive B is injected and filled around the stripped fibers
8
to fix them adhesively.
Next, after polishing the surface joined to the waveguide D, both end faces of the fiber array and the waveguide D substrate are optically fixed with the end face adhesive A.
A problem is that over the passage of years, the peripheral adhesive B swells and its volume expands when the adhesive B is disposed around the fibers in the fiber array. If it protrudes out to the front, pressing against the end face adhesive A as shown in FIG.
4
(
c
), a strong exfoliation stress acts on the adhesion interface between the end face adhesive A and the fiber end faces. It may lead to exfoliations
8
b
. More specifically, as shown in FIG.
4
(
b
), the spaces for the fibers have the shape of triangular prisms. The peripheral adhesive B around the fibers protrudes from the portions corresponding to the three vertices in longitudinal direction while enclosing the fibers. As a result, stress concentrates locally on the adhesion surface where the fiber end face borders against the end face adhesive A. The fiber end face easily exfoliates from the end face adhesive A. Even at a microscopic level, if the core portion
8
a
of the fibers exfoliates, such exfoliations immediately cause light reflections, leading to a deterioration of the transmission signal from the light source. The volume taken up by the peripheral adhesive B is small when compared to the total volume of fiber array and waveguides. However, since it is disposed around the fibers, it can lead to the tremendous problem of light reflection by merely causing local exfoliation.
Furthermore, if the extent of the exfoliation of the fiber core is increased, other signal losses of the transmission light besides reflection may occur, which can also cause severe problems.
Moreover, if the fiber array and waveguide are coupled by “butt joints,” in which the end faces are placed against one another, the volume increase of the peripheral adhesive B due to its aging creates forces around the fibers. This force directly expands the end face and may lead to deterioration of the adhesion and becomes a cause for exfoliation. Thus, in the case of butt joints, there is the risk of reflection and transmission light loss as well.
In order to perform an accelerated aging test, a fiber array housing 48 fibers was produced, which had a total longitudinal length of 12 mm, and in which the length of the distance over which the fibers were fixed in the V-grooves was 4 mm. For the peripheral adhesive B used in the assembly, an epoxy adhesive was selected that had a curing shrinkage ratio of 2%, a water absorption ratio of 0.5%, a thermal expansion coefficient of 1×10
4
, and a Shore D85 hardness. When this sample fiber array was subjected to an accelerated aging test of 85° C/85%RH for 2 weeks, the change of the protrusion length of the adhesive in the direction protruding from the end face was 0.1 to 1&mgr;m. This is for a measurement under ordinary temperatures, but it seems that at elevated temperatures, an even larger protrusion may occur due to the influence of the thermal expansion. It seems that this protrusion occurs because of the swelling, which is aggravated by thermal expansion at elevated temperatures. This change is affected not only by the water absorption ratio, but also by the shrinkage due to curing, the length of the portion over which the fiber is fixed in the V-groove, and the adhesion structure of V-groove substrate and fiber fixing substrate. It was found that it differs depending on the adhesives used, the structure of the fiber array and the fabrication conditions.
It is an object of the present invention to solve these conventional problems, and to prevent the occurrence of light reflections due to an increased volume of the peripheral adhesive B around the fibers.
SUMMARY OF THE INVENTION
To achieve these objects, according to a first aspect of the present invention, a fiber array housing a stripped fiber in a V-groove of a V-groove substrate is provided. The stripped fiber is fixed in the V-groove by a fiber fixing substrate, and is adhered by a peripheral adhesive B disposed around the stripped fiber. An end face of the peripheral adhesive B is recessed with respect to an end faces of the fiber. Thus, the end face of the peripheral adhesive B does not protrude from the fiber end face, even when the volume increases due to swelling by aging. As a result, the deformation stress at the entire adhesion interface is relieved and local exfoliations of the fiber end f
Fukuyama Masashi
Matsumoto Akira
Burr & Brown
Lavarias Arnel C.
NGK Insulators Ltd.
Nguyen Thong
LandOfFree
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