Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-03-29
2003-07-08
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
Reexamination Certificate
active
06591041
ABSTRACT:
This application is based on Patent Application No. 2001-58828 filed Mar. 2, 2001 in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wide-bandwidth optical fiber coupler with less polarization dependency loss and less excessive loss, and a method and an apparatus for easily and stably fabricating the optical fiber coupler.
2. Description of the Related Art
An optical coupler with less wavelength dependency, e.g. a wide-bandwidth optical fiber coupler for branching and coupling a wide-bandwidth optical signal may enjoy wide utilization as various optical parts for use in general optical fields and micro optical fields and in optical communication and optical data processing.
Upon fabrication of such a wide-bandwidth optical fiber coupler, there is known a prior art method wherein one of two optical fibers possessing the same structural parameters of the optical fibers is previously heated and drawn to force the structural parameters of these two optical fibers to be different or two optical fibers possessing mutually different structural parameters of the optical fibers are used, melted and drawn in a twisted state of the fibers.
Referring to
FIGS. 29
to
31
, fabrication procedures of such a prior art wide-bandwidth optical fiber coupler is illustrated in schematically. Opposite ends of one optical fiber
1
is held on a pair of carrying blocks
2
. A coating is removed from a central portion of the optical fibers
1
located between these carrying blocks
2
as illustrated in
FIG. 29. A
distance between the pair of the carrying blocks
2
is increased while heating an exposed optical fiber strand
3
. The portion of the optical fiber strand
3
is preliminarily drawn, as illustrated in FIG.
30
. There are twisted a preliminarily drawn optical fiber
5
and the optical fiber
1
which optical fiber
1
is not preliminarily drawn and in which a coating thereof is only removed from a central portion thereof are twisted along that portions of the optical fiber strands. In this state, both ends of these optical fibers
1
and
5
are again held on the pair of the carrying blocks
2
. The distance between the pair of the carrying blocks
2
are increased while again heating the portions of the twisted optical fiber strands
3
with the burner
4
, as illustrated in FIG.
31
. The two optical fiber strands
3
are hereby melted and drawn at those twisted portions to obtain such an optical fiber coupler
6
as illustrated in FIG.
32
.
When a wide-bandwidth optical fiber coupler
6
with reduced wavelength dependency is fabricated, the foregoing prior art method where there are used two optical fibers with mutually different structural parameters requires that two kinds of optical fibers previously should be fabricated. This is not general in view of the production cost.
To solve this difficulty, the following method is exclusively employed: two optical fibers of the same kind are prepared, one of which is preliminarily drawn to change the parameters concerning the structure.
The prior art method illustrated in
FIGS. 29
to
31
, however, needs to preliminarily draw the optical fiber strand
3
by about 10 mm using the burner
4
. In the method, it is essentially impossible to finely control heating temperature for the optical fiber strand
3
in the unit of several tens of ° C. for example so as to prevent a wire diameter from being varied. In the prior art method, the wire diameter of a portion of the preliminarily drawn optical fiber strand
3
is narrowed, so that it is necessary to mutually twist the preliminarily drawn optical fiber strand
3
and the optical fiber strand
3
of the optical fiber
1
from which sheath is only simply removed for securely bringing them into close contact. As illustrated in
FIG. 23
, when the two optical fibers
1
and
5
are mutually twisted, it is necessary to apply tension uniformly to those two optical fibers
1
and
5
, so that those fibers should be twisted with subtle discretion. The prior art method therefore suffers from difficulty that work is complicated with deteriorated reproducibility. The prior art method further suffers from a difficulty that the optical fibers
1
and
5
are subjected to twisting, so that the optical fiber strand
3
suffers from larger polarization dependency owing to internal stress produced in itself, and allows polarization dependency loss (hereinafter, may be simply referred to PDL.) to be increased together with increased excessive loss.
Referring to
FIG. 33
, there is illustrated wavelength dependency of a coupling ratio, i.e. a branch ratio between a symmetrical optical fiber coupler A where an optical coupling portion is configured symmetrically and asymmetrical wide band optical fiber couplers B and C where an optical coupling portion is not symmetrical). For such a branch ratio of a general symmetrical optical fiber coupler in which parameters concerning the structures of two optical fibers constituting an optical fiber coupler A are same for each other, it varies periodically from 0 to 100% owing to the wavelength of an optical signal. In contrast, for wide band optical fiber couplers B and C possessing different parameters concerning the structures of two optical fibers constituting an optical coupling portion, a branch ratio is reduced to a specific value, 100% or less by combining optical fibers with outer diameters of clad portions thereof for example being respectively 115 &mgr;m and 125 &mgr;m. There is thereupon utilized a flat portion located in the vicinity of the maximum of the just-mentioned branch ratio. As illustrated in
FIG. 33
, the wide-bandwidth optical fiber coupler B has the branch ratio of 50% at wavelength of 1.4 &mgr;m and the wide-bandwidth optical fiber coupler C has the optical branch ratio of 20% at wavelength of 1.4 &mgr;m. It can be understood therefrom that a change in the branch ratio in the vicinity of the maximum branch ratio is more flattened than the wavelength characteristic of the symmetrical optical fiber coupler A would be.
For fabricating the aforementioned wide-bandwidth optical fiber coupler there is known a method proposed by International Application PCT/GB 86/00445. More specifically, one single mode optical fiber is preliminarily drawn, and the preliminarily drawn optical fiber and another optical fiber not preliminarily drawn are combined, melted and drawn to successfully obtain a wide-bandwidth optical fiber coupler
6
as illustrated in FIG.
34
and
FIG. 35
which illustrates a cross sectional structure viewed along XXXV—XXXV shown in FIG.
34
. In the resulting optical fiber coupler
6
, a propagation constant of the one single mode optical fiber
1
is altered with the aid of preliminary drawing to obtain different parameters from those concerning the structure of the optical fiber
5
not preliminarily drawn with the maximum branch ratio brought into a specific value less than 100%, and a flat wavelength characteristic in the vicinity of the maximum value of the branch ratio is utilized. In
FIG. 34
, designated at
7
a
is a core portion,
7
b
is a clad portion,
8
is preliminarily drawn portion, and
9
is a melted drawn portion.
The aforementioned prior art fabrication method for the wide-bandwidth optical fiber coupler
6
using the preliminary drawing, however, has a difficulty that uniform preliminary drawing is difficult. More specifically, uniform heating control for the preliminary drawn portion
8
is difficult to make impossible precision configuration control and hence to make it difficult to obtain a uniform outer diameter of the melted drawn portion
9
. The optical fibers
1
and
5
might thereupon been sometimes bent upon processing the resulting wide-bandwidth optical fiber coupler
6
. This might cause set value control for the branch ratio to be difficult and therefore accuracy of flatness of the branch ratio of the drawn optical fiber to be unsatisfactory with the very bad yield of the coupler.
To solve the a
Kobayashi Osamu
Tomaru Satoru
Uetake Takashi
Finnegan Henderson Farabow Garrett & Dunner LLP
NTT Advanced Technology Corporation
Ullah Akm E.
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