Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-11-09
2004-02-03
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S033000, C385S124000
Reexamination Certificate
active
06687435
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical grating coder; and, more particularly, to a chirped optical fiber grating coder implementing predetermined codewords for use in an OCDMA (Optical Code Division Multiple Access) system, a fabricating method and an apparatus therefor.
2. Description of the Related Art
An optical fiber grating is generally fabricated by using the refractive index variation of a core of an optical fiber when a core doped with Germanium (Ge) is exposed to an ultraviolet light. A short-period optical fiber grating is typically referred to an optical fiber grating. The short-period optical fiber grating has a grating period of 0.5 &mgr;m, approximately, and a length less than 10 mm. The short-period optical fiber grating may be used as a reflection filter for filtering light wavelengths, external reflection mirrors of an optical fiber laser and a semiconductor laser, an optical fiber grating Fabry-Perot etalon, and the like.
A fabricating method of an optical fiber grating is generally classified into two: one is to form an interference fringe induced by the interference of ultraviolet lights and to imprint the interference fringe into a core of an optical fiber; and the other, which is gaining popularity, is to utilize a phase mask creating an interference pattern of the ultraviolet lights in the fabrication process of the optical fiber grating.
FIG. 1
shows an apparatus for fabricating a short-period optical fiber grating by utilizing a phase mask according to the prior art. The apparatus
100
of
FIG. 1
comprises a light source
101
, a convex lens
102
, a phase mask
104
, and a single-mode optical fiber
106
that is typically used in a telecommunication system. In
FIG. 1
, in order to measure the transmission characteristics of a short-period fiber grating
112
imprinted into the fiber
106
, an optical spectrum analyzer
106
and an optical amplifier
110
are also provided in the apparatus
100
.
The light source
101
may be one of a KrF excimer laser (248 nm), a second harmonic laser (244 nm) of an Ar laser, and a fourth harmonic laser (265 nm) of an Nd:YAG laser. The light source
101
irradiates a coherent ultraviolet light beam to the fiber
106
. The ultraviolet light beam reaches on the fiber
106
through the convex lens
102
and the phase mask
104
. In practice, after the diffraction by the phase mask
104
, a zero-order beam has been suppressed to less than 5% of incident light beams. Light beams exiting the phase mask
104
are the diverging plus-one and minus-one orders, each of which contained typically more than 30% of the incident light beams. The diffracted light of the plus-one and minus-one orders form interference fringes prior to reaching on the fiber
106
. The interference fringes are imprinted into the fiber
106
to produce the short-period fiber grating
112
.
A Ge-doped or a Ge- and B(boron)-doped optical fiber is commonly used in the fabrication of the short-period fiber grating
112
. The fiber
106
may be the Ge-doped or the Ge- and B-doped optical fiber. As is well known in the art, it is required to perform a hydrogen (H
2
) treatment on the fiber
106
in order to form the short-period fiber grating
112
thereon.
FIG. 2
presents an apparatus for fabricating a chirped fiber grating according to the prior art. The apparatus
200
of
FIG. 2
comprises a light source
201
, a concave lens
202
, a convex lens
204
, a phase mask
206
, a single-mode optical fiber
208
for use in the telecommunication system. Similar to the optical amplifier
110
and the optical spectrum analyzer
108
of
FIG. 1
, an optical amplifier
210
and an optical spectrum analyzer
212
are also provided for measuring the transmission characteristics of a chirped fiber grating
214
.
As seen from
FIG. 2
, a grating period of the chirped fiber grating
214
is gradually changed along the axis of the fiber
208
different to that of the short-period fiber grating
112
shown in FIG.
1
. The chirped fiber grating
214
induces a time difference to light beams irradiated from the optical amplifier
210
to the fiber
208
depending on the wavelengths of the light beams. Using the time difference induced by the chirped fiber grating
214
, it is possible to compensate the dispersion of the light beams from the optical amplifier
210
propagating the fiber
208
depending on the dispersion characteristics thereof, so that the chirped fiber grating
214
may be suitable for an ultrahigh-speed optical communication system.
In order to use the chirped fiber grating
214
as an optical grating coder in an OCDMA (Optical Code Division Multiple Access) system, it is required to connect to each other short-period fiber gratings (or chirped fiber gratings), each of which has a different grating period, or to fabricate a super-structural fiber grating having a length enough for implementing desired codewords.
Specifically, after forming one short-period fiber grating on an optical fiber, by moving the optical fiber to a predetermined direction in order to prevent from overlapping the short-period fiber grating formed on the optical fiber with another short-period fiber grating to be formed thereon, a chirped optical fiber grating coder consist of a plurality of the short-period gratings having different grating periods is fabricated enough for implementing desired codewords. To use the process described above, however, would call for an additional procedure of alternating a plurality of phase masks during the fabrication of the chirped optical fiber grating coder. Further, the length of the chirped optical fiber grating coder fabricated would be elongated as a number of the short-period fiber gratings for implementing the desired codewords are increased.
Consequently, there are some drawbacks that the conventional fabrication process of the chirped optical fiber grating coder takes much time and a plurality of phase masks is necessary.
SUMMARY OF THE INVENTION
It is, therefore, an objective of the present invention to provide a chirped optical fiber grating coder implementing predetermined codewords for use in an OCDMA system, a fabricating method and an apparatus therefor.
In accordance with one aspect of the present invention, there is provided an optical fiber grating coder comprising predetermined codewords imprinted into an optical fiber in the form of a plurality of striations through an apparatus including a light source, an amplitude mask, and a phase mask.
In accordance with another aspect of the present invention, there is provided a method for fabricating an optical fiber grating coder, comprising the steps of: a) providing a light source; b) providing an amplitude mask designed for predetermined codewords; c) providing a phase mask; d) providing an optical fiber; e) exposing the optical fiber to a light beam irradiated from the light source passing sequentially through the amplitude and phase masks; and f) forming an optical fiber grating coder on the optical fiber, including the predetermined codewords in the form of a plurality of stripes.
In accordance with still another aspect of the present invention, there is provided an apparatus for fabricating an optical fiber grating coder, comprising: a light source; an amplitude mask designed for predetermined codewords; a phase mask; and an optical fiber.
REFERENCES:
patent: 5367588 (1994-11-01), Hill et al.
patent: 5604829 (1997-02-01), Bruesselbach
patent: 5881186 (1999-03-01), Starodubov
patent: 6240224 (2001-05-01), Reekie et al.
patent: 6314221 (2001-11-01), Riant et al.
Choi Sang Sam
Koo Hyun Deok
Lee Sang Bae
Doan Jennifer
Korea Institute of Science and Technology
Ullah Akm Enayet
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