Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-11-05
2002-10-08
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S141000, C385S142000, C385S144000, C372S006000, C359S341100, C359S341500
Reexamination Certificate
active
06463201
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a light amplification optical fiber utilized mainly for an optical communication system; and a light amplifier using the optical fiber.
2. Background Art
In recent years, a demand for communication by internet, etc. has suddenly increased, and an increase in the transmission capacity of an optical transmission passage is being actively discussed. In accordance with these facts, an increase in the capacity of a wavelength division multiplexing (WDM) optical transmission system, i.e. an increase in the number of channels and the expansion of a transmission band are progressing. In order to attain an increase in the capacity of a WDM optical transmission system, it is indispensable to increase the level of an output from a signal light amplifying light amplifier and attain the expansion of a transmission band, and a light amplifier as an important key device has increasingly attracted the technicians' attention.
With an increase in the number of channels for a WDM signal, total signal light input power increases, and higher saturation power has been demanded with respect to a light amplifier which is adapted to collectively increase the number of channels and signal light input power. Under the circumstances, the increasing of a light amplifier output has been forwarded by increasing the efficiency of a light amplification optical fiber and a pumping source output.
The discussing of the expansion of a gain band for a light amplifier is also being forwarded so as to expand a transmission band, and a light amplifier having a gain band of a wavelength larger than that a gain band in a related art light amplifier has been attained in practice by using an elongated light amplification optical fiber.
However, the above-mentioned increase in an output from and the length of a light amplification optical fiber has caused a new problem to arise, i.e., nonlinear phenomena which had not been observed in a related art light amplification optical fiber have occurred. The main nonlinear phenomena include four-wave mixing and cross phase modulation. Since the transmission quality of signal light is deteriorated due to these nonlinear effects, the suppression of the occurrence of nonlinearity has become necessary and indispensable.
The waveform distortion due to nonlinear phenomena can be estimated by the following equation (1).
&PHgr;
NL
=(2&pgr;/&lgr;)·(
n
2
/A
eff
)
L
eff
P
(1)
In this equation (1), L
eff
represents an effective length of an amplification optical fiber, A
eff
an effective core area n
2
a nonlinear refractive index, &lgr; a signal light wavelength, and P input light intensity.
It is understood from the equation (1) that, in order to suppress the occurrence of waveform distortion &PHgr;
NL
, it is preferable to (1) reduce the effective length of a fiber, and (2) minimize a nonlinear constant (n
2
/A
eff
).
In order to reduce a length of a fiber used, it is necessary to heighten a gain coefficient per unit length by increasing an amount of addition of a rare earth element. However, when the concentration of erbium is increased in an erbium-doped optical fiber, which is being put to practical use as a light amplifier for 1.55 &mgr;m band, a decrease in the amplification efficiency due to concentration quenching occurs.
In general, the concentration of erbium at which concentration quenching starts occurring is several hundred wtppm in the case of a pure quartz (SiO
2
) host, and it is said to be around 1000 wtppm even in the case of Al
2
O
3—SiO
2
host to which aluminum having a concentration quenching suppressing effect is co-doped (refer to, for example, R.I.LAMING, D.N.PAYNE, F.MELI, G.GRASSO, E.J.TARBOX, “SATURATED ERBIUM-DOPED FIBRE AMPLIFIERS”, OAA '90.Tech.Digest MB3.). Therefore, there is a limit to the reduction of the length of an erbium-doped optical fiber.
The increasing of an effective core area A
eff
may be mentioned as another nonlinearity suppressing method. In general, an effective area of an erbium-doped optical fiber has a value one digit smaller than that of a regular single-mode optical fiber (SMF). The reason for the above is to form excellent inverted population over the whole of a core by increasing a relative refractive index difference &Dgr; of a core portion with respect to a clad portion, and thereby heightening the pumping light density so as to improve the gain characteristics of the erbium-doped optical fiber with a high efficiency.
Furthermore, a light amplification optical fiber contains various kinds of large amounts of dopants in a core portion thereof as compared with a regular transmission optical fiber, and this makes it more difficult to control an effective core area.
A light amplification fiber contains many elements, for example, an element for improving the solubility of a rare earth element for melting the rare earth element in host glass, an element for increasing a refractive index of host glass, an element for varying gain wavelength characteristics thereof, an element for improving the amplification characteristics thereof, an element doped to regulate the viscosity thereof and the like besides rare earth elements constituting active substances. These elements, which are other than the rare earth elements, doped to a core portion are called co-dopants.
The addition concentration of these co-dopants is determined from the viewpoint of the optimization of the amplification characteristics of the light amplifier. Conjointly with the diversity of the co-dopants, the above matter makes it very difficult to control the refractive index.
The germanium has heretofore been used as a co-dopant for improving the refractive index of a core portion.
The germanium is effective not only in improving the refractive index but also in regulating the viscosity of a core portion. Such co-dopants as are mentioned above which are doped to a core portion of a light amplification optical fiber are all substances for improving the refractive index just as the germanium.
However, since the relative refractive index difference &Dgr; becomes high due to the addition of the co-dopants, it is difficult to suppress the nonlinearity of a light amplification optical fiber. It is necessary as previously mentioned to reduce the using length, i.e. effective length of a light amplification optical fiber for the purpose of suppressing the nonlinearity thereof. However, in order to meet this requirement, it is necessary to improve the gain coefficient per unit length. Therefore, the addition amounts of a rare earth element and a co-dopant have to be increased, and this causes the relative refractive index difference &Dgr; to further increase.
In view of these problems, a method of improving a limit level of addition concentration of erbium, and a new method of attaining a desired effective core area A
eff
without depending upon the addition concentration of co-dopants, such as a rare earth element, etc. for practically obtaining a nonlinear phenomena-suppressed amplification optical fiber have been demanded.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned circumstances, and provides a nonlinear phenomena-suppressed light amplification optical fiber, and an optical amplifier using the fiber.
DISCLOSURE OF THE INVENTION
According to an aspect of the present invention, the light amplification optical fiber has the following construction, i.e. this optical fiber has a core portion, and not less than one layer of clad portion surrounding an outer circumference of the core portion, erbium ions being doped to the core portion, at least one kind of rare earth element ions which are other than the erbium ions, and which have ion radius of not lower than 70% and not higher than 130% of that of the erbium ions, being further doped to the core portion.
According to another aspect of the present invention, the light amplification optical fiber has aluminum doped to the core portion, the number of the ions of the aluminum doped to the
Aiso Keiichi
Tashiro Yoshio
Yagi Takeshi
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
The Furukawa Electric Co. Ltd.
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