Optical waveguides – Optical transmission cable
Reexamination Certificate
2000-12-15
2002-07-16
Ngo, Hung N. (Department: 2839)
Optical waveguides
Optical transmission cable
C385S123000, C385S024000
Reexamination Certificate
active
06421484
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber transmission line employable as an optical transmission line in a wavelength division multiplexing (WDM) transmission system, and an optical cable including the same.
2. Related Background Art
A WDM transmission system enables high-speed, large-capacity optical communications by utilizing a plurality of signal channels. The optical transmission line laid in each repeating section of the WDM transmission system is desired to have an excellent transmission characteristic in a signal wavelength region (e.g., 1.55-&mgr;m wavelength band). Therefore, as an optical transmission line having an excellent signal transmission characteristic, optical fibers whose transmission characteristic changes along the longitudinal direction thereof have been proposed.
For example, a conventional optical fiber transmission line described in T. Naito, et al., “1 Terabit/s WDM Transmission over 10,000 km,” ECOC 99, PD2-1 (1999) (first literature) is constituted by a single-mode optical fiber (an optical fiber having a positive chromatic dispersion) positioned on the upstream side in the signal-advancing direction and a dispersion-compensating optical fiber (an optical fiber having a negative chromatic dispersion) positioned on the downstream side. In the 1.55-&mgr;m wavelength band, the single-mode optical fiber has a positive chromatic dispersion and a relatively large mode field diameter. On the other hand, the dispersion-compensating optical fiber has a negative chromatic dispersion and a relatively small mode field diameter in the 1.55-&mgr;m wavelength band, and is likely to generate nonlinear optical phenomena in general.
In the conventional optical fiber transmission line described in the above-mentioned first literature, signals successively propagate through the single-mode optical fiber and dispersion-compensating optical fiber. Though the signals propagating through the single-mode optical fiber have a high power, nonlinear optical phenomena are restrained from occurring since the single-mode optical fiber has a relatively large mode field diameter. The signals lower their power while propagating through the single-mode optical fiber, and the signals having lowered their power reaches the dispersion-compensating optical fiber. As a consequence, the occurrence of nonlinear optical phenomena is sufficiently suppressed even when the signals propagate through the dispersion-compensating optical fiber having a relatively small mode field diameter. Further, since the single-mode optical fiber and dispersion-compensating optical fiber have chromatic dispersions different from each other, the cumulative chromatic dispersion of the optical fiber transmission line as a whole will be kept low if the ratio of their lengths is designed appropriately. Thus, an optical fiber transmission line in which a single-mode optical fiber and a dispersion compensating optical fiber are successively disposed along the signal-advancing direction can effectively restrain the transmission quality from deteriorating due to nonlinear optical phenomena and cumulative chromatic dispersion.
On the other hand, the optical fiber transmission line disclosed in U.S. Pat. No. 5,894,537 (second literature) is a unitary optical fiber in which a plurality of positive dispersion portions having a positive chromatic dispersion and a plurality of negative dispersion portions having a negative chromatic dispersion are alternately arranged adjacent each other along its longitudinal direction. The occurrence of nonlinear optical phenomena, such as four-wave mixing in particular, is suppressed when the absolute value of chromatic dispersion in each of the positive and negative dispersion portions is set greater, and the deterioration in transmission quality caused by cumulative chromatic dispersion is suppressed when the absolute value of mean chromatic dispersion in the optical fiber transmission line as a whole is set lower.
SUMMARY OF THE INVENTION
The inventors have studied the conventional techniques mentioned above and, as a result, have found problems as follows. Namely, the conventional optical fiber transmission line described in the first literature is required to change its transmission characteristic along the longitudinal direction while yielding a desirable value of mean transmission characteristic in the optical fiber transmission line as a whole. Consequently, each optical fiber constituting this optical fiber transmission line is restricted in terms of its transmission characteristic or its length. That is, the conventional optical fiber transmission line is required to have a small mean chromatic dispersion as a whole, whereby it is necessary that the ratio of respective lengths of the single-mode optical fiber and dispersion-compensating optical fiber constituting the optical fiber transmission line be set to a predetermined value.
The conventional optical fiber transmission line described in the above-mentioned second literature is also required to have a small mean chromatic dispersion as a whole, whereby it is necessary that the ratio of respective lengths of the positive and negative dispersion portions be set to a predetermined value.
Even when an optical fiber transmission line designed so as to yield a desirable mean transmission characteristic as a whole is made, if an end portion of this optical fiber transmission line is cut off, then the mean transmission characteristic of thus cut optical fiber transmission line as a whole may not attain the desirable value. When the process of making an optical cable from an optical fiber transmission line such as that mentioned above is concerned, for example, both end portions of each optical fiber are cut off until a desirable condition is obtained in each of steps of welding water-pressure-resistant copper tubes, extruding sheaths, and the like in the case of the optical fiber transmission line. The fiber lengths (hereinafter referred to as “cut lengths”) of both end portions cut in such individual steps amount to several hundreds of meters.
Between before and after the cutting, the mean transmission characteristic of the optical fiber transmission line as a whole changes according to the cut length. In a transmission line in which a chromatic dispersion with a large absolute value locally occurs, such as the optical fiber transmission lines described in the above-mentioned first and second literatures in particular, the change in mean transmission characteristic (mean chromatic dispersion) in the optical fiber transmission line as a whole between before and after the cutting is large.
Concerning the optical fiber transmission line described in the above-mentioned first literature, the problem mentioned above will be explained specifically with reference to a case of making a submarine optical cable having a cross-sectional structure shown in
FIG. 1A
by way of example. Here
FIG. 1A
is a view showing the cross-sectional structure of the submarine optical cable, whereas
FIG. 1B
is a view showing the cross-sectional structure of the optical fiber unit included in the submarine optical cable. On the other hand,
FIGS. 2A
to
2
D are views showing respective changes in the length of optical fiber transmission line in individual steps of making the optical cable.
As shown in
FIG. 1A
, a three-part metal tube
310
, a high-tension steel twisted wires
320
, a copper tube
330
, and an insulating plastic layer
340
are successively disposed on the outer periphery of an optical fiber unit
300
holding a plurality of optical fiber transmission lines, so as to construct the optical cable. Here, as shown in
FIG. 1B
, the optical fiber unit
300
has a structure in which a plurality of optical fiber transmission lines
410
are secured about a tension member
420
by way of a buffer layer (unit filler resin)
430
.
Before bundling, each optical fiber transmission line
410
is, as shown in
FIG. 2A
, constituted by a single-mode optical fiber
412
(SMF: Single-Mode o
Ishisaki Hiroshi
Nishimura Masayuki
Tanaka Shigeru
McDermott & Will & Emery
Ngo Hung N.
Sumitomo Electric Industries Ltd.
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