Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-11-13
2003-12-09
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S123000, C385S126000
Reexamination Certificate
active
06661958
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber and a nonlinear optical fiber, an optical amplifier and wavelength converter using the same, and a method of making an optical fiber.
2. Related Background Art
In general, it has been known that various nonlinear optical phenomena such as stimulated Raman effect and four-wave mixing occur in a medium when light having a high intensity (high optical density) propagates through the medium. These nonlinear optical phenomena also occur when light is transmitted through an optical fiber. Such nonlinear optical phenomena in the optical fiber can be used for optical amplification, wavelength conversion, and the like (see International Publication WO99/10770).
SUMMARY OF THE INVENTION
The nonlinearity of an optical fiber is represented by the nonlinear coefficient &ggr; in the following expression:
&ggr;=(2&pgr;/&lgr;)×(
N
2
/A
eff
)
where &lgr; is the wavelength of light, N
2
is the nonlinear refractive index in the optical fiber at &lgr;, and A
eff
is the effective area of the optical fiber at &lgr;. This expression indicates that the nonlinear coefficient &ggr; can be made greater if the concentration of GeO
2
added into the core of the optical fiber is enhanced so as to raise the nonlinear refractive index N
2
while the relative refractive index difference between the core and cladding is increased so as to reduce the effective area A
eff
.
When the configurational condition mentioned above is employed so as to increase the nonlinear coefficient &ggr;, however, the cutoff wavelength &lgr;
c
of the optical fiber may become longer. When the four-wave mixing occurring in the optical fiber is used for carrying out wavelength conversion, in particular, it is necessary that the wavelength of excitation light be located near the zero-dispersion wavelength of the optical fiber. In the above-mentioned configuration, by contrast, the cutoff wavelength &lgr;
c
becomes longer than the zero-dispersion wavelength, so that no single mode can be attained, whereby the efficiency of wavelength conversion decreases.
In recent years, in order to widen the wavelength band of signal light used in optical transmission systems, the use of not only the amplification band of EDFA usually employed as an optical amplifier, but also S band in which wavelength ranges from 1.45 to 1.53 &mgr;m, which is located on the shorter wavelength side of the former band, has been under consideration. For the S band, the EDFA is very difficult to be used since its amplification band is outside thereof, where by few effective amplifiers exist. If a Raman amplifier is to be used with a highly nonlinear optical fiber, the cutoff wavelength &lgr;
c
becomes longer than the wavelength of excitation light, which is about 1.3 to 1.5 &mgr;m, whereby the efficiency in Raman amplification decreases.
For overcoming the foregoing problems, it is an object of the present invention to provide an optical fiber or nonlinear optical fiber exhibiting a shorter cutoff wavelength while having a sufficient nonlinearity, an optical amplifier and wavelength converter using the same, and a method of making an optical fiber.
For achieving such an object, the optical fiber in accordance with the present invention is characterized in that (1) it comprises, at least, a core region having a maximum refractive index value of n
1
; a first cladding region, disposed at an outer periphery of the core region, having a minimum refractive index value of n
2
(where n
2
<n
1
); and a second cladding region, disposed at an outer periphery of the first cladding region, having a maximum refractive index value of n
3
(where n
2
<n
3
<n
1
); and that (2) it has, as characteristics with respect to light having a wavelength of 1.55 &mgr;m, an effective area of 11 &mgr;m
2
or less, a cutoff wavelength &mgr;
c
of at least 0.7 &mgr;m but not exceeding 1.6 &mgr;m at a fiber length of 2 m, and a nonlinear coefficient of at least 18/W/km.
This optical fiber does not use a single-cladding structure but a double-cladding structure in which first and second cladding regions are disposed at the outer periphery of the core region. As a consequence, the cutoff wavelength &lgr;
c
can sufficiently be shortened even when, in order to increase the nonlinear coefficient &ggr;, the concentration of GeO
2
added into the core is enhanced so as to raise the nonlinear refractive index, or the relative refractive index difference between the core and cladding is increased so as to reduce the effective area A
eff
. Also, this configuration can make the dispersion slope negative.
Here, as for the cladding structure, one or more other cladding regions each having a predetermined refractive index value and a width may be formed between the above-mentioned first and second cladding regions.
The nonlinear optical fiber in accordance with the present invention is the above-mentioned optical fiber characterized in that it utilizes a nonlinear optical phenomenon exhibited when a predetermined wavelength of light is fed therein. When the high nonlinearity in the optical fiber is actively utilized, a nonlinear optical fiber, applicable to various purposes, having a favorable characteristic can be obtained.
The optical amplifier in accordance with the present invention comprises (a) the above-mentioned nonlinear optical fiber having a cutoff wavelength &lgr;
c
; and (b) an excitation light source for supplying excitation light having a predetermined wavelength &lgr;
p
(where &lgr;
c
<&lgr;
p
) to the nonlinear optical fiber with respect to signal light having a wavelength &lgr;
s
fed into the nonlinear optical fiber; wherein (c) a nonlinear optical phenomenon exhibited in the nonlinear optical fiber is utilized for optically amplifying the signal light.
Thus configured optical amplifier is utilizable as a Raman amplifier using the stimulated Raman effect occurring in the nonlinear optical fiber. Also, thus configured nonlinear optical fiber can make the cutoff wavelength &lgr;
c
shorter than the wavelength &lgr;
p
of the excitation light (pumping light), whereby optical amplification can be carried out with a high efficiency in a single mode.
The wavelength converter in accordance with the present invention comprises (a) the above-mentioned nonlinear optical fiber having a cutoff wavelength &lgr;
c
; and (b) an excitation light source for supplying excitation light having a predetermined wavelength &lgr; (where &lgr;
c
<&lgr;
p
) to the nonlinear optical fiber with respect to signal light having a wavelength &lgr;
s
(where &lgr;
c
<&lgr;
s
) fed into the nonlinear optical fiber; wherein (c) a nonlinear optical phenomenon exhibited in the nonlinear optical fiber is utilized for converting the wavelength of the signal light so as to output converted light having a wavelength &lgr;
s
′ (where &lgr;
c
<&lgr;
s
′).
Thus configured wavelength converter is utilizable as a wavelength converter using the four-wave mixing occurring in the nonlinear optical fiber. Also, thus configured nonlinear optical fiber can make the cutoff wavelength &lgr;
c
shorter than each of the wavelengths of signal light, converted light, and excitation light, whereby wavelength conversion can be carried out with a high efficiency in a single mode. Further, the signal light can keep a favorable transmission characteristic without being affected by mode dispersion.
The method of making an optical fiber in accordance with the present invention comprises (1) a first step of preparing a core glass rod to become a core region made of SiO
2
doped with a predetermined amount of GeO
2
by synthesizing glass by VAD or OVD method and extending thus synthesized glass so as to attain a predetermined outer diameter; (2) a second step of preparing a first cladding glass pipe to become a first cladding region made of SiO
2
doped with a predetermined amount of F by synthesizing glass by VAD or OVD method and extending thus synthesized glass so as to attain a predetermined inner dia
Hirano Masaaki
Okuno Toshiaki
Onishi Masashi
McDermott & Will & Emery
Palmer Phan T. H.
Sumitomo Electric Industries Ltd.
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