Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-08-02
2002-07-02
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S125000, C385S126000, C385S127000, C385S128000
Reexamination Certificate
active
06415089
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber applicable to a transmission line in optical communications, and an optical transmission system including this optical fiber.
2. Related Background Art
Conventionally, as a transmission line in optical communications, standard single-mode optical fibers having a zero-dispersion wavelength in a 1.3-&mgr;m wavelength band (1280 nm to 1320 nm) have mainly been utilized. The transmission loss resulting from the main material (silica) of such an optical fiber has been known to become the lowest in a 1.55-&mgr;m wavelength band (1530 nm to 1565 nm). In addition, optical fiber amplifiers using an Er-doped optical fiber can amplify light in the 1.55-&mgr;m wavelength band at a high efficiency. For such a reason, dispersion-shifted optical fibers designed so as to have a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band are applied to transmission lines in wavelength division multiplexing (WDM) communications for transmitting a plurality of wavelengths of signal light. As for a light source for sending out signal light, device technologies for enabling light in the 1.3-&mgr;m wavelength band and light in the 1.55-&mgr;m wavelength band to be outputted have conventionally been established.
SUMMARY OF THE INVENTION
The inventors have studied the prior art mentioned above and, as a result, found problems as follows. Namely, in the case where light in the 1.3-&mgr;m wavelength band is transmitted while a dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band is used as an optical transmission line, the absolute value of dispersion becomes so large that WDM communications cannot be carried out in a wide band. Also, when signal light in the 1.55-&mgr;m wavelength band is transmitted through such a dispersion-shifted optical fiber, the absolute value of dispersion becomes so small that four-wave mixing, which is one of nonlinear optical phenomena, is likely to occur. In the case where light in the 1.3-&mgr;m wavelength band is transmitted while a standard single-mode optical fiber having a zero-dispersion wavelength in the 1.3-&mgr;m wavelength band is used as an optical transmission line, on the other hand, the absolute value of dispersion becomes so small that four-wave mixing, which is one of nonlinear optical phenomena, is likely to occur. Also, when signal light in the 1.55-&mgr;m wavelength band is transmitted through such a single-mode optical fiber, the absolute value of dispersion becomes so large that WDM communications cannot be carried out in a wide band.
For this matter, attempts have been made to develop optical fibers for suppressing the occurrence of dispersion over a wide wavelength band (see, for example, K. Okamoto et al., “Zero total in single-mode optical fibers over an extended spectral range,” Radio Science, Volume 17, Number 1, pages 31-36, January-February 1982). For example, an optical fiber having a low dispersion value over a wide wavelength band has been proposed by yielding a large relative refractive index difference of 2.4% between its cladding region and core region and a small diameter of 3.5 &mgr;m in the core region. However, it is difficult to make such an optical fiber having a very large relative refractive index difference between the cladding region and core region, and its transmission loss is large. In an optical fiber whose core region has a smaller diameter, on the other hand, the effective area becomes smaller, and nonlinear optical phenomena are likely to occur.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical fiber which enables efficient transmission of both of signal light in the 1.3-&mgr;mwavelength band and signal light in the 1.55-&mgr;m wavelength band, and an optical transmission system including the same.
The optical fiber according to the present invention is an optical fiber which enables efficient transmission of both of signal light in the 1.3-&mgr;m wavelength band and signal light in the 1.55-&mgr;m wavelength band, the optical fiber having only one zero-dispersion wavelength within a wavelength range of 1.20 &mgr;m to 1.60 &mgr;m and having a positive dispersion slope at the zero-dispersion wavelength. Here, this zero-dispersion wavelength lies within a wavelength range of 1.37 &mgr;m to 1.50 &mgr;m sandwiched between the 1.3-&mgr;m wavelength band and the 1.55-&mgr;m wavelength band. Also, the above-mentioned dispersion slope preferably has an absolute value of 0.10 ps
m
2
/km or less at the above-mentioned zero-dispersion wavelength (preferably 0.06 ps
m
2
/km or less at a wavelength of 1.55 &mgr;m), and monotonously changes (e.g.,monotonously increases) at least in a wavelength range of 1.30 &mgr;m to 1.55 &mgr;m.
Thus, since this optical fiber has a zero-dispersion wavelength within the wavelength range of 1.37 &mgr;m to 1.50 &mgr;m including a wavelength of 1.38 &mgr;m at which an increase in transmission loss caused by OH absorption is seen, dispersion occurs to a certain extent in the vicinity of the 1.3-&mgr;m wavelength band and in the vicinity of the 1.55-&mgr;m wavelength band. As a consequence, the optical fiber comprises a structure in which four-wave mixing is hard to occur even when the signal light in the 1.3-&mgr;m wavelength band and the signal light in the 1.55-&mgr;m wavelength band propagate therethrough.
In the case where a thulium-doped fiber amplifier having an amplification band in a 1.47-&mgr;m wavelength band is utilized, the zero-dispersion wavelength is more preferably set within a wavelength range of 1.37 &mgr;m to 1.43 &mgr;m. It is because of the fact that the transmission band can further be widened if the zero-dispersion wavelength is aligned with a skirt of the OH absorption peak (1.38 &mgr;m). In the case where the above-mentioned OH absorption peak is kept low by dehydration processing or the like, so as to utilize the wavelength band including the wavelength of 1.38 &mgr;m as its signal light wavelength band, on the other hand, the zero-dispersion wavelength may be set within a wavelength range of longer than 1.45 &mgr;m but not longer than 1.50 &mgr;m in order to intentionally generate dispersion in the above-mentioned wavelength band.
In the optical fiber, while the dispersion slope monotonously increases, the absolute value of the dispersion slope at its zero-dispersion wavelength is 0.10 ps
m
2
/km or less, and the dispersion slope at a wavelength of 1.55 &mgr;m is preferably 0.06 ps
m
2
/km or less, whereby the dispersion in the 1. 3-&mgr;m wavelength band and the dispersion in the 1.55-&mgr;m wavelength band are homogenized. Here, each of the absolute value of dispersion in the 1.3-&mgr;m wavelength band and the absolute value of dispersion in the 1.55-&mgr;m wavelength band is 6 ps
m/km or more but 12 ps
m/km or less.
As mentioned above, the optical fiber according to the present invention realizes efficient optical communications in both of the 1.3-&mgr;m wavelength band and the 1.55-&mgr;m wavelength band. From the viewpoint of guaranteeing a single mode, the case where the cutoff wavelength is 1.3 &mgr;m or shorter while the transmission line length is several hundreds of meters or less is preferable since only the ground-mode light can propagate in each of the 1.3-&mgr;m wavelength band and the 1.55-&mgr;m wavelength band. Also, in view of the dependence of cutoff wavelength on distance, no practical problem occurs in optical transmission over a relatively long distance (a transmission line length of several kilometers or less) even if the cutoff wavelength is 1.45 &mgr;m or shorter (in the case where it is longer than the signal light wavelength). From the viewpoint of reducing the bending loss, on the other hand, there are cases where the bending loss increases remarkably when the cutoff wavelength is shorter than 1.0 &mgr;m. As a consequence, the cutoff wavelength is preferably 1.05 &mgr;m or more, more preferably 1.30 &mgr;m or more.
Fu
Kato Takatoshi
Sasaoka Eisuke
Tanaka Shigeru
Doan Jennifer
Sumitomo Electric Industries Ltd.
Ullah Akm E.
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