Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2002-01-25
2003-07-15
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06594005
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a chromatic dispersion distribution measurement apparatus (wavelength dispersion distribution measurement apparatus) for measuring a chromatic dispersion distribution (wavelength dispersion distribution) in an optical device to be measured, such as an optical fiber, and a method for the same. Moreover, the present invention relates to a storage medium for storing a program for calculating a chromatic dispersion distribution in an optical device to be measured.
2. Description of Related Art
In recent years, in order to satisfy the demands for high speed information communications, optical communication systems using optical fibers have been constructed. One of the factors in preventing the high speed signal transmission and the long transmission distance in the above optical communication systems, is the chromatic dispersion. The chromatic dispersion is a phenomenon caused by varying the speeds of lights transmitted in a medium, with the wavelength of the light. In the construction of the optical communication systems, it is necessary to grasp the chromatic dispersion characteristic in detail.
A chromatic dispersion distribution measurement apparatus for measuring the chromatic dispersion is shown in, for example, Japanese Patent Application Publication No. Tokukai-Hei 10-83006 (corresponding to the U.S. Pat. No. 5,956,131 and the European Patent Application No. 0819926A2). In the publication, the chromatic dispersion distribution measurement apparatus measures the dispersion distribution in a longitudinal direction of a fiber to be measured, as follows. Two lights having different wavelengths from each other are inputted into the fiber to be measured. A specific wavelength component is extracted by an optical bandpass filter from a four-wave mixed light caused by the interaction between these two lights. A light having the extracted specific wavelength component is inputted into an Optical Time Domain Reflectometer (OTDR).
The four-wave mixing (FWM) is a phenomenon caused by the non-linearity of a plurality of lights having different wavelengths from each other in an optical fiber. For example, when two lights have wavelengths &lgr;
1
and &lgr;
2
respectively, a wavelength &lgr;
3
of a light (Stokes light) caused by this phenomenon and a wavelength &lgr;
4
of a light (anti-Stokes light) caused by the phenomenon satisfy the following equation (1).
&lgr;
2
−&lgr;
1
=&lgr;
1
−&lgr;
4
=&lgr;
3
−&lgr;
2
(1)
The chromatic dispersion distributions which are measured by using the OTDR generally vary with each optical fiber to be measured. Moreover, a chromatic dispersion value is marked with a constant sign (positive (+) or negative (−) sign) (hereinbelow, referred to as “sign”) in spite of wavelengths of the inputted lights. That is, the sign of a chromatic dispersion value depends on an optical fiber to be measured. By using a former chromatic dispersion distribution measurement apparatus, only an absolute value of a chromatic dispersion value can be measured. The sign of a chromatic dispersion value is judged by using another apparatus.
With reference to
FIGS. 1 and 7
, a chromatic dispersion distribution in a long distance optical cable, which is measured by a former chromatic dispersion distribution measurement apparatus, is explained.
FIG. 1
is a view showing a schematic structure of a long distance optical cable
200
as an optical device to be measured.
FIG. 7
shows a chromatic dispersion distribution in the long distance optical cable
200
, which is shown without marking each chromatic dispersion value with the sign. The chromatic dispersion distribution shown without marking each chromatic dispersion value with the sign, is measured by a chromatic dispersion distribution measurement apparatus. The chromatic dispersion distribution shown without marking each chromatic dispersion value with the sign, is also measured as an interim data by an optical fiber chromatic dispersion distribution measurement apparatus
100
to which the present invention is applied. The distribution will be explained in detail later.
The long distance optical cable
200
is constructed by jointing three optical fibers H
1
, H
2
and H
3
. A range of the optical fiber H
1
in the longitudinal direction of the long distance optical cable
200
is 0 to 20 km. A range of the optical fiber H
2
is 20 to 40 km. A range of the optical fiber H
3
is 40 to 60 km. The chromatic dispersion value marked with the sign (hereinbelow, referred to as “signed chromatic dispersion value”), of the optical fiber H
1
is +17 (ps
m/km). The signed chromatic dispersion value of the optical fiber H
2
is −4 (ps
m/km). The signed chromatic dispersion value of the optical fiber H
3
is −50 (ps
m/km).
As shown in
FIG. 7
, a chromatic dispersion distribution in the long distance optical cable
200
, which is measured by a former chromatic dispersion distribution measurement apparatus is expressed by using each absolute value of the chromatic dispersion value of the optical fibers H
1
to H
3
. In detail, the absolute value of the chromatic dispersion value of the optical fiber H
1
, which is denoted by B
1
is “17 (ps
m/km)”. The absolute value of the chromatic dispersion value of the optical fiber H
2
, which is denoted by B
2
is “4 (ps
m/km)”. The absolute value of the chromatic dispersion value of the optical fiber H
3
, which is denoted by B
3
is “50 (ps
m/km)”.
However, there was a problem as explained below in such a former chromatic dispersion distribution measurement apparatus. The chromatic dispersion distribution in a long distance optical cable constructed by jointing a plurality of optical fibers, which is measured by a former chromatic dispersion distribution measurement apparatus, is expressed by using each absolute value of the chromatic dispersion value of the optical fiber. Moreover, each chromatic dispersion value of this chromatic dispersion distribution is outputted as a positive (+) value in spite of the optical fibers. Therefore, it is difficult to measure correctly a chromatic dispersion distribution in the long distance optical cable and an accumulated chromatic dispersion value of the long distance optical cable, in consideration of each sign of the chromatic dispersion value.
For example, when the accumulated chromatic dispersion value of the long distance optical cable
200
is calculated in accordance with the measurement result shown in
FIG. 7
, which is measured by a former chromatic dispersion distribution measurement apparatus, the calculating result is the sum of the products of each absolute value of the chromatic dispersion value and the length of each optical fiber. That is, the accumulated chromatic dispersion value is “17×20+4×20+5×20=1420 (ps
m)”. However, the correct accumulated chromatic dispersion value of the long distance optical cable
200
(the sum of the products of each signed chromatic dispersion value and the length of each optical fiber) is “17×20+(−4)×20+(−50)×20=−740 (ps
m)”. Therefore, the calculated accumulated chromatic dispersion value is different from the correct accumulated chromatic dispersion value.
SUMMARY OF THE INVENTION
In order to solve the above-described problems, an object of the present invention is to provide a chromatic dispersion distribution measurement apparatus which can measure a chromatic dispersion value marked with a positive sign or a negative sign, and to provide a measurement method which can measure a chromatic dispersion value marked with a positive sign or a negative sign. Moreover, another object is to provide a storage medium storing a program which can calculate a chromatic dispersion value marked with a positive sign or a negative.
That is, in accordance with a first aspect of the present invention, a chromatic dispersion distribution measurement apparatus for measuring a
Aoki Shoichi
Ichikawa Akio
Ando Electric Co. Ltd.
Nguyen Tu T.
Oliff & Berridg,e PLC
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