Method for forming two thin conductive films isolated...

Details Method for forming two thin conductive films isolated... Method for forming two thin conductive films isolated...

2002-02-27

2003-09-23

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

C385S002000, C385S008000

Reexamination Certificate

active

06625361

ABSTRACT:

This application relies for priority upon Korean Patent Application No. 2001-78667, filed on Dec. 12, 2001, the contents of which are herein incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the fields of fiber optics, and more particularly, to a method for forming two thin conductive films that can be used as electrodes for poling on a single mode fiber or a multimode fiber.
2. Description of the Related Art
A crystal material such as LiNbO
3
or BaTiO
3
has been widely used as the optical nonlinear material. An amorphous material has no inversion symmetry, which means vanishing electro-optic and second order nonlinear coefficients. However, since amorphous silica glass materials were reported to have electrooptic and nonlinear optical properties by poling them under several different conditions, a lot of researches and progresses have been focused on the optoelectronic devices made of glass materials, for a possibility of integrating all glass, or specifically all fiber, active devices into a system. It has been suggested that packaging cost and optical power loss can be significantly reduced by making all the active devices out of glasses or fibers and integrating them into a system. Modulation speed and transmission rate can also be enhanced by integrating the active devices made of glasses or fibers into a system.
To induce and enhance the second order effect on a glass material, two techniques are well-known, thermal poling and UV poling. In thermal poling, a high electric field of a typical value of ~100 V/&mgr;m is applied to a glass material with heating to a desired temperature, typically 250° C. to 300° C., then the material is allowed to cool to room temperature with the high electric field still applied. In UV poling, a glass material is irradiated by UV light, typical wavelength of 150 nm to 400 nm, with the high electric field applied. Both methods, thermal and UV poling, may be applied together to a glass material at the same time.
To apply a high electric field to a single mode fiber, a typical diameter of 125 &mgr;m, requires well-tailored techniques and clever designing of electrodes to be located as close as allowed to the core of a fiber, and also requires isolation schemes to allow ~100 V/&mgr;m high electric field without breakdown.
The prior arts introduce several different designs to locate electrodes in appropriate positions with desired patterns [U.S. Pat. No. 5,617,499]. With reference to
FIGS. 1A through 1D
, a first electrode
12
is formed on a wafer
11
. A polyimide
13
is applied in order to attach a D-shape fiber
10
to the wafer
11
with the flat side down. Then, as a dielectric insulator, a thick polyimide layer
14
is deposited. The upper parts of the polyimide layer
14
and the fiber
10
are polished and a polished side is formed on the opposite side of the flat side of the fiber
10
. A second electrode
15
is formed on the polished side. If the electric field is applied to the fiber
10
through the first electrode
12
and the second electrode
15
, the electro-optical coefficient is induced.
Though adequate in some respects, these kinds of prior arts, which consist of the processes of the permanent attachment of the fiber to a substrate, polishing, depositing a polyimide layer as a dielectric insulator, and spin coating, have inevitable shortcomings. The permanent attachment of the fiber
10
to a substrate
11
requires additional packaging and handling costs for later use of a device and also reduces compactness. Depositing electrodes
12
and
15
not on the fiber surface itself, but on the polished surface and the substrate may result in an unstable and non-uniform electric field, which is caused mainly by the imperfectness of the polished surface and the polyimide glue layer
13
between the fiber
10
and the substrate
11
, which may also be a cause of a lack of reproducibility. Polishing a fiber is a difficult and costly process because polishing accuracy must be high due to the small size of the fiber. This is especially difficult and costly for long fiber distances. Depositing a polyimide layer
14
as a dielectric insulator also has a shortcoming that the polyimide layer
14
allows some amount of leakage current, especially in a high temperature under a high voltage. In our experiment we suffered a serious problem where extended ends
18
and
19
of the fiber
10
away from the substrate
11
were really weak such that the spin-coating was hardly allowable. Here, the extended ends of the fiber are necessary pieces that are used for splicing with external fiber ends.
FIG. 2
shows another prior art to induce and enhance electro-optic and nonlinear effects on a fiber
30
[U.S. Pat. No. 5,966,233]. The fiber
30
has two holes running parallel to a core
33
with pre-designed distances, where the two holes accommodate two thin electrode wires
31
and
32
inserted from each end of the fiber. Through the electrode wires
31
and
32
, the electric field is applied to the fiber
30
for UV or thermal poling, or both together. According to the method, the electro-optical coefficient was obtained to the value of maximum 6 pm/V that is practicable enough to facilitate the configuration of a nonlinear element using a silica fiber. Coupling the fiber to other fibers is difficult because the electrodes
31
and
32
must exit the fiber, preventing direct butt coupling or fusion splicing to other fibers. It is also very difficult to push the fragile electrode wires into the end of the fiber, henceforth the manufacturing cost and time are significant drawbacks. Inserting electrode wires from each end of the fiber means that the modulation frequency, if embodied into a modulator, is limited to low values since a high-speed traveling wave geometry is not possible.
Prior art U.S. Pat. No. 5,966,233 disclosing the design shown in
FIG. 3
suggests some improvements to the art in
FIG. 2
, removing the problem of inserting electrode wires into the holes, thereby making possible fusion splicing to other fibers and allowing a high-speed traveling wave geometry. In this method, grooves
64
and
65
are formed on the surface of a fiber
60
along the length direction of the fiber. Electrodes
69
are placed on the grooves
64
and
65
. The electric field is applied through the electrodes for poling.
Both the prior arts shown in
FIGS. 2 and 3
, however, still have a serious drawback that a patterned electrode such as a periodic pattern to meet the quasi-phase matching condition is very difficult to realize. Up to the present time, as far as we know, there have been no publications reporting that a significant result of nonlinear effects, such as Second Harmonic Generation (SHG), Difference Frequency Generation (DFG) or Sum Frequency Generation (SFG), four wave mixing etc., was obtained using a patterned electrode under the schemes shown in FIG.
2
and FIG.
3
. One shortcoming related to the prior arts shown in FIG.
2
and
FIG. 3
is that the diameters of their fibers, ~300 &mgr;m, to accommodate two electrode wires are much larger than a standard single mode fiber, ~125 &mgr;m, meaning that, although it is not impossible, the fusion splicing of their fibers with the standard single mode fiber is very difficult and allows a lot of power loss.
SUMMARY OF THE INVENTION
To solve the problems described above, it is an object of the present invention to provide a method for forming two thin conductive films that can be used as electrodes for poling on an optical fiber, addressing the problems of prior arts.
To achieve the above object, in accordance with the present invention, two conductive films isolated electrically from each other are deposited on the surface of a fiber that may be used as an optoelectronic device in an optical communication system. According to the present invention, D-shape fibers are attached into grooves on a silicon wafer substrate, and photoresist is used as glue. The wafer is prepared by forming grooves by

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