Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2001-03-16
2002-12-03
Tarcza, Thomas H. (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S341300, C385S037000, C372S006000
Reexamination Certificate
active
06490078
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an optical fiber amplifier, an optical fiber laser, and an optical fiber for optical amplifier (OFA) applicable thereto as an optical amplification medium.
2. Related Background Art
An optical fiber for optical amplifier (OFA) has a function of amplifying signals propagating therethrough by pumping light supplied thereto, and is an optical device applicable as an optical amplification medium in an optical fiber amplifier, an optical fiber laser, and the like. In general, an OFA is a silica-based optical fiber and has a core region doped with a luminescent element for signal amplification. As this luminescent element, for example, a rare earth element, preferably Er can be used. An OFA to which Er, as a luminescent element, is added can amplify signals of a wavelength band of 1.55 &mgr;m when pumping light of a wavelength of 1.48 &mgr;m or 0.98 &mgr;m is supplied thereto. In such an OFA, increasing a signal gain is important, and some kinds of techniques for meeting this requirement have been proposed.
For example, an OFA disclosed as a conventional technology in Japanese Patent Laid-Open No. 135548/1998 (Document 1) has a core region having a refractive index of n
1
, an inner cladding region provided on the periphery of the core region and having a refractive index n
2
(<n
1
), and an outer cladding region provided on the periphery of the inner cladding region and having a refractive index n
3
(<n
2
). The core region is doped with a luminescent element. This optical fiber makes the pumping light propagate as light of an inner cladding mode (a mode in which the pumping light propagates while being confined in both the core region and inner cladding region), and makes signals as light of a core mode (a mode in which the signals propagate while being supported in the core region). Such an OFA will hereinafter be referred to as a cladding-pumped fiber.
Japanese Patent Laid-Open No 238883/1991 (Document 2) discloses a structure suitable for above-mentioned cladding-pumped fiber. It is described that, in this cladding-pumped fiber, pumping light can propagated as an inner cladding mode light, so that a coupling efficiency of the pumping light in the fiber is high. Further, since the pumping light also propagates through an inner cladding region containing no luminescent element and a transmission loss of the pumping light is small, a gain efficiency can be improved.
A cladding-pumped fiber disclosed in International Laid-open Publication PCT/EP96/04187 (Document 3) works to induce a mode coupling between an inner cladding mode and a core mode with respect to pumping light by employing a non-circular cross-sectional shape to the inner cladding region. Concerning the pumping light introduced into this cladding-pumped fiber, the inner cladding mode is more effectively coupled to the core modes, and a gain efficiency is thereby increased to a high level.
Further, in a cladding-pumped fiber disclosed in Japanese Patent Laid-Open No. 107345/1998 (Document 4), signals propagate with not only light of a fundamental mode (core mode) but also that of a high-order mode while being confined in a core region, and a signal gain is thereby increased only light of the fundamental mode is outputted to outside by a grating cutting of that of the high-order mode.
SUMMARY OF THE INVENTION
The inventors discussed the above-mentioned prior techniques to discover the following problems. Namely, in the cladding-pumped fiber of Documents 1 and 2, the pumping light of inner cladding mode cannot effectively pump the luminescent element added to the core region, thereby the increasing of a signal gain could not be sufficiently attained. It is difficult to control the manufacturing of a cladding-pumped fiber having a non-circular cross-sectional shape of an inner cladding region, and further the mechanical strength of such optical fiber is liable to decrease.
In the cladding-pumped fiber of Documents 3 and 4 through which signals propagate with not only light of a fundamental mode but also that of a high-order mode while being supported in a core region, it is difficult to completely cut off the high-order mode light by a grating. The leakage high-order light induces a high noise figure. Moreover, a splice loss between this cladding-pumped fiber and another optical fiber becomes large, and the pumping of the high-order mode occurs in the latter optical fiber, this matter also constitutes a noise figure.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide an OFA having a high gain efficiency, capable of being manufactured easily, having a high mechanical strength, not inducing a large splice loss with respect to any other optical fiber, and not inducing noise in a signal wavelength, an optical fiber amplifier and an optical fiber laser to both of which said OFA can be applied as an optical amplification medium.
An OFA according to the present invention is a silica-based optical fiber having a function of amplifying signals when pumping light is supplied thereto, and comprises a core region having a refractive index n
1
, an inner cladding region provided on the periphery of the core region and having a refractive index n
2
(<n
1
), and an outer cladding region provided on the periphery of the inner cladding region and having a refractive index n
3
(<n
2
). The core region has a structure ensuring a single mode with respect to the signals, and the inner cladding region has a structure ensuring a multi-mode with respect to the pumping light. A signal amplifying luminescent element is aided to at least a part of the core region.
In particular, the OFA according to the present invention comprises one or more mode coupling gratings, each allowing core mode light to pass therethrough and inducing a mode coupling between an inner cladding mode and a core mode with respect to the pumping light.
In the OFA, the pumping light initially introduced thereinto propagates as light of an inner cladding mode, and the inner cladding mode is then converted into a core mode by a mode coupling effect of the mode coupling grating. The pumping light of the core mode pumps the luminescent element added to at least the part of the core region of the OFA. On the other hand, signals propagate as light of the core mode and pass through the mode coupling grating while remaining its mode. Consequently, amplified signals are obtained from the OFA.
At least one of mode coupling gratings in the OFA according to the present invention may include a long-period grating which induces a mode coupling between an inner cladding mode and a core mode whose propagation direction is identical with that of the inner cladding mode. In this case, the period of the long-period grating is preferably 10 &mgr;m-400 &mgr;m. When the pumping light reaches a long-period grating, the inner cladding mode is converted into a core mode whose propagation direction is identical with that of the inner cladding mode by the mode coupling effect of the mode coupling grating.
At least one of mode coupling gratings in the OFA according to the present invention may include a tilted fiber Bragg grating which induces a mode coupling between an inner cladding mode and a core mode whose propagation direction is contrary to that of the inner cladding mode. In this case, an angle of inclination of the tilted fiber Bragg grating with respect to a predetermined axis, for example, an optical axis is preferably 0.5°-20°. When the pumping light reaches the tilted fiber Bragg grating, the inner cladding mode is converted into a core mode whose propagation direction is contrary to that of the inner cladding node by the mode coupling effect of this tilted fiber Bragg grating.
As clearly disclosed in U.S. Pat. No. 5,703,978, a long-period grating is a grating inducing a mode coupling between a core mode and a cladding mode propagating through an optical fiber, and clearly distinguished from a fiber Bragg grating whi
Endo Shinji
Enomoto Tadashi
Ishikawa Shinji
Kakui Motoki
Ohga Yuichi
Sommer Andrew R.
Sumitomo Electric Industries Ltd.
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