Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2001-02-21
2003-09-02
Kim, Robert H. (Department: 2882)
Optical waveguides
With optical coupler
Particular coupling structure
C065S382000, C065S408000
Reexamination Certificate
active
06614961
ABSTRACT:
CLAIM OF PRIORITY
This application claims the benefit of priority of Korean Patent Application No. 2000-9812 filed Feb. 28, 2000.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a directional coupler and, more particularly, to a method of fabricating a fused-type directional coupler that can be easily made and has stable characteristics.
2. Description of the Prior Art
The function of an ordinary direction coupler is to enable light that is propagating in one optical fiber to diverge into two optical fibers (2×2 directional coupler) or a plurality of optical fibers (N×N directional coupler). The directional coupler is a basic optical device that is used in almost every optical communication system. The most common technique of fabricating a directional coupler is to fabricate a fused-type directional coupler. In some special instances, polished-type directional couplers are also used.
An optical fiber taper refers to an optical fiber to the part of which heat is applied and pulled to be extended, and this process is referred to as a tapering process. Fused-type directional couplers are fabricated by such tapering process.
FIGS. 1A through 1D
are diagrams illustrating the conventional method of fabricating a general 2×2 fused-type directional coupler.
As shown in
FIG. 1A
, approximately 1-2 cm of the jackets
110
of two optical fibers
100
are stripped in the middle to expose the claddings
120
. Subsequently, as shown in
FIG. 1B
, the exposed claddings
120
are put in contact, and the contact point C is heated using a torch
130
and at the same time extended in the longitudinal direction F-F′ of the optical fibers so that the contact point C is fused. At the same time, an optical signal is input to the optical fibers. The heating and extending of the optical fibers are stopped when a desired amount of splitting of light is obtained. As a result, as shown in
FIG. 1C
, it is possible to fabricate the waist C′ of the directional coupler formed by the fusion of two optical fiber tapers. The waist C′ is the most slender part of the directional coupler, yet has a uniform thickness and generates the strongest directional coupling. Subsequently, as shown in
FIG. 1D
, the fabrication of the directional coupler is completed by fixing the waist C′ with a quartz glass tube
140
and epoxy
150
in order to protect the waist C′ of the directional coupler. Typically, the two optical fibers are identical types, however, they can also be different from each other or one of them may be extended in advance in order to achieve the desired wavelength characteristics or mode-coupling ratio.
FIGS. 2A through 2E
are diagrams for illustrating the conventional method of fabricating a general polished-type directional coupler. As shown in
FIG. 2A
, a quartz block
200
having the shape of a rectangular hexahedron is prepared. Then, as shown in
FIG. 2B
, a groove
202
having a certain depth suitable for the thickness of the jacket of the optical fiber is made to form a fixing support
200
a
. Subsequently, as shown in
FIG. 2C
, epoxy is used to fix the optical fiber
210
in the groove
202
. Then, as shown in
FIG. 2D
, the block face fixing the optical fiber is polished to grind the cladding of the optical fiber in order to form a polished-type optical fiber
210
a
, so that a small part of the light propagating through the core of the optical fiber leaks. Then, as shown in
FIG. 2E
, the two blocks
200
a
and
200
a
′ fabricated as illustrated above are put in contact to join the cores of the two optical fibers
210
a
and
210
a
′, so that the directional coupler is completed. Such polished-type directional couplers are used in cases where the inherent birefringence (double refraction) axis present in the optical fiber core has to be adjusted. Examples of such cases include polarized directional couplers utilizing polarized optical fibers and mode-selective directional couplers utilizing an elliptical core two mode fiber (“TMF”). Such polished-type directional couplers are also used in variable directional couplers of which the coupling ratio is variable. However, polished-type directional couplers have a disadvantage that they are unstable against change of temperature or environment compared with all-fiber type directional couplers such as fused-type directional couplers, because polished-type directional couplers require use of index matching oil on the junction face in order to obtain effective directional coupling.
FIGS. 3A and 3B
are diagrams for illustrating the functions of mode-selective directional couplers. The directional couplers shown in
FIGS. 3A and 3B
have common general functions irrespective of the technique (fused-type or polished-type) used to fabricate them. The basic structure is such that a TMF
300
and a single mode fiber (“SMF”)
301
are joined at the coupling region
302
.
The normalized frequency is a characteristic value of an optical fiber. The normalized frequency V of an optical fiber can be calculated as follows:
V
=(2&pgr;/&lgr;)·&agr;·(n
co
2
−n
cl
2
)
1/2
Equation 1,
where &agr; is the radius of the core, &lgr; is the wavelength, and n
co
and n
cl
are the refractive indices of the core and the cladding, respectively.
Generally, the larger the normalized frequency V of an optical fiber is, the more modes can propagate through the optical fiber. When the normalized frequency V is between 0 and 2.405, the optical fiber becomes a SMF through which only the LP01 mode can propagate. When the normalized frequency V is between 2.405 and 3.83, the optical fiber becomes a TMF through which both the LP01 mode and the LP11 mode can propagate. When the normalized frequency V is less than about 1.4, the guidance of the fiber core becomes too weak and the electric field of the LP01 mode begins to spread out to the cladding region. In such case, if the cladding comes into contact with other material, leakage or mode coupling of light occurs. Considering the above characteristics, the normalized frequency V is typically adjusted to 0.8 for fused-type directional couplers fabricated utilizing general SMF.
Referring to
FIG. 3A
illustrating mode selection, when both the LP01 and LP11 modes
303
propagate through the TMF
300
, the LP01 mode propagates through the coupling region
302
to be output as the LP01 mode
304
of the output terminal TMF, whereas the LP11 mode of the TMF
300
is converted to the LP01 mode
305
of the SMF
301
. Also, as shown in
FIG. 3B
, when the LP01 mode
306
is incident to the SMF
301
, it is converted to the LP11 mode
307
of the TMF in the coupling region
302
.
It is preferable to have 100% efficiency in mode selection or mode conversion. The mode coupling ratio and the mode extinction ratio are two criteria from which the quality of a mode-selective directional coupler can be judged. Referring to
FIG. 3B
, the mode coupling ratio and the mode extinction ratio can be calculated as follows:
Mode Coupling Ratio=100×(
P
1/
P
0
) Equation 2,
Mode Extinction Ratio=10·log(
P
1/
P
2
) Equation 3,
where P
0
is the power of the LP01 mode
306
incident to the SMF
301
, P
1
is the mode-coupled power of the LP11 mode
307
in the output terminal TMF after propagating through the coupling region
302
, and P
2
is the mode-coupled power of the LP01 mode in the output terminal TMF. The mode coupling ratio and the mode extinction ratio are typically 90% and 20 dB, respectively, and larger values are preferable in both cases.
FIG. 4A
is a diagram for illustrating the conventional method of fabricating a polished-type mode-selective directional coupler. Referring to
FIG. 4A
, the SMF and the TMF are fabricated as polished blocks
400
and
401
as in general polished-type directional couplers. Then, the two blocks ±
400
and
401
are juxtaposed by using index matching oil and epoxy on the face
402
of their junction. Mode-selective
Kim Byoung Yoon
Song Kwang Yong
Yun Seok Hyun
Kao Chih-Cheng Glen
Kim Robert H.
Suzue Kenta
The Korea Advanced Institute of Science and Technology
Wilson Sonsini Goodrich & Rosati
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