Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-07-13
2002-11-26
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S028000, C385S029000, C385S126000
Reexamination Certificate
active
06487340
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical components applicable to optical communication systems and, more particularly, to an optical fiber grating element provided with a long-period grating in a multi-mode optical fiber, a production method thereof, and an optical fiber filter including the same.
2. Related Background Art
An optical fiber grating element provided with a long-period grating (LPG: Long-Period Grating) in a core is an optical component that couples core-mode light of a predetermined wavelength to cladding-mode light by the long-period grating to attenuate the light of the predetermined wavelength. In other words,the optical fiber grating element is the optical component that selectively transfers the power of the core-mode light of the predetermined wavelength to the cladding-mode light (for example, see A. M. Vengsarkar, et al., J. of Lightwave Tech., Vol. 14 (1996) pp58-64). Here the core-mode light is light propagating as being confined in the core region of the optical fiber. On the other hand, the cladding-mode light is light radiating from the core into the cladding of optical fiber. Such optical fiber grating elements are utilized as optical fiber filters or the like for selectively cutting off the core-mode light of the predetermined wavelength (loss wavelength) out of the core-mode light of a wavelength band in use having propagated in the optical fiber, in the fields of optical communications and the like.
Cladding modes mean high-order modes except for the fundamental mode when consideration is given to the entire region of the optical fiber specified by the cladding surface being the outermost layer or by the interface between the cladding and a coating layer covering the cladding. For example, in single-mode optical fibers, there exist the fundamental mode with consideration to propagation only in the core and the high-order modes with consideration to propagation in the entire region A of the optical fiber, as illustrated in FIG.
1
A. In the case of the single-mode optical fibers, therefore, change in the refractive indices of the surroundings around the cladding (the refractive indices of the air layer and the coating layer) will also cause a shift of the wavelength at which the coupling occurs from the core-mode light to the cladding-mode light, i.e., a shift of the loss wavelength in the long-period grating. There will also occur variations in attenuation factor of the core-mode light of the loss wavelength. Particularly, it is known that when the peripheral surface of the multi-mode optical fiber is covered with a resin having the refractive index close to glass, the high-order modes (cladding modes) disappear as illustrated in
FIG. 1B
(for example, see B. H. Lee, et al., OECC'98, 14P-50 and B. H. Lee, et al., Electronics Letters, Vol. 34 (1998) pp1129-1130) . For that reason, it was infeasible to cover the optical fiber grating element provided with the long-period grating in the multi-mode optical fiber, with a coating for the purpose of protection for the element.
In order to overcome this problem, the optical fiber grating element described in above B. H. Lee et al. comprises the long-period grating in the single-mode optical fiber having the index profile of dual shape core (DSC) structure the base of which is silica. Here the single-mode optical fiber of DSC structure is composed of a first core region of a refractive index n1, a second core region of a refractive index n2, and a cladding region of a refractive index n3 in the order named from the center of the optical axis (where n1>n2>n3). The first and second core regions of the single-mode optical fiber both are doped with GeO
2
and these first and second core regions are exposed to ultraviolet light spatially intensity-modulated, thereby obtaining the optical fiber grating element in which an index-modulated area or a grating is formed across these two regions. The optical fiber grating element, which is obtained by providing the long-period grating in the single-mode optical fiber as described, couples the core-mode light of the predetermined wavelength propagating in the first core region to the high-order mode (cladding-mode) light, so as to cut off the core-mode light of the predetermined wavelength.
This long-period grating is a grating that induces coupling (mode coupling) between the core mode propagating in the optical fiber and the cladding mode, as elucidated in U.S. Pat. No.5,703,978, and that is definitely discriminated from the short-period gratings that reflect the light centered about the predetermined wavelength. In the long-period grating, in order to achieve strong power conversion from the core mode to the cladding mode, the grating period (pitch) &Lgr; is set so that the difference between propagation constants of the core-mode light of the predetermined wavelength (loss wavelength) and the cladding-mode light becomes 2&pgr;/&Lgr;. Since the long-period grating acts to couple the core mode to the cladding mode in this way, the core-mode light attenuates in a narrow band centered around the predetermined wavelength (loss wavelength).
SUMMARY OF THE INVENTION
Inventors studied the above conventional techniques and found the following issues. First, it is extremely difficult to design and fabricate the optical fiber grating element with desired cutoff characteristics (loss wavelength and loss amount) by the technology described in the above documents of B. H. Lee et al. The reason is that changes of index in each of the first and second core regions based on the exposure to ultraviolet light are greatly affected by the conditions of the intensity of the radiant ultraviolet light, the exposure time, and so on, or by the conditions of a pretreatment of the optical fiber to be exposed to the ultraviolet light.
In addition, since it is difficult to predict the changes of index in each of the first and second core regions, it is extremely difficult to accurately control the fabrication of the optical fiber grating element so that both the loss wavelength (cutoff wavelength) and loss amount (cutoff amount) fall at designed values or within a designed range.
The present invention has been accomplished in order to solve the problems described above and an object of the invention is to provide an optical fiber grating element of structure permitting more precise design and fabrication, a production method thereof, and an optical fiber filter including the same.
An optical fiber grating element according to the present invention comprises a multi-mode optical fiber having a cutoff wavelength regarding to LP02-mode light on the longer wavelength side than a wavelength band in use, and a long-period grating provided in the multi-mode optical fiber, for selectively coupling fundamental LP01-mode light of a predetermined wavelength within the wavelength band in use to LP0m (m≧2)-mode light. Specifically, the multi-mode optical fiber comprises a first core region of a refractive index n1 extending along a predetermined axis, a second core region of a refractive index n2 (<n1) disposed on a periphery of the first core region, and a cladding region of a refractive index n3 (<n2) disposed on a periphery of the second core region, and the long-period grating is provided in the first core region surrounded by the second core region. Such a multi-mode optical fiber may take such structure that an intermediate core region is provided between the first and second core regions or such structure that a depressed region is further provided between the second core region and the cladding region. In either of the structures,the multi-mode optical fiber applied to the optical fiber grating element has such structure that the cladding region is provided so that a propagation region A of a high-order mode is spaced away from the interface between the peripheral surface of the fiber and a coating material, as illustrated in FIG.
2
.
Particularly, the optical fiber grating element according to the present invention is c
Enomoto Tadashi
Ishikawa Shinji
Shigehara Masakazu
Takushima Michiko
Doan Jennifer
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
Ullah Akm E.
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