Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2001-10-12
2003-11-25
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06654104
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to measuring optical characteristics, such as chromatic dispersion characteristics of a DUT (Device Under Test) such as a fiber pair, and more specifically relays to measuring chromatic dispersion characteristics where independent measuring machines are connected tooth ends of the DUT.
2. Description of the Related Art
When light is transmitted over a long distance, losses are large if the light is transmitted only through an optical fiber. The loss can be prevented by using an optical fiber line where an optical fiber is combined with an optical amplifier (EDFA), which amplifies a light signal. The light amplifier allows light to pass in only a certain direction. For bi-directional communication, one optical fiber line for transmitting light in one direction, and another optical fiber line for transmitting light in a direction opposite to that one direction are combined as a cable. This cable is called one fiber pair.
FIG.
6
(
a
) shows a constitution of one fiber pair. An optical fiber line
110
is formed by combining an optical fiber
112
with optical amplifiers
114
. The optical fiber line
110
passes light in the right direction of FIG.
6
(
a
). The optical fiber line
120
is formed by combining an optical fiber
122
with optical amplifiers
124
. The optical fiber line
120
passes light in the left direction of FIG.
6
(
a
). The optical fiber line
110
and the optical fiber line
120
form one fiber pair
100
a
. Two fiber pairs are shown in FIG.
6
(
b
). Fiber pairs
100
a
and
100
b
form two fiber pairs
100
.
FIG. 7
shows a constitution of a measuring system when chromatic dispersion characteristics of two fiber pairs are measured. A variable wavelength light source
202
is connected to one end, and an O/E (optical/electrical) converter
302
is connected to the other end of one fiber pair
100
a
included in the two fiber pairs
100
. A fixed wavelength light source
204
is connected to one end, and an O/E (optical/electrical) converter
304
is connected to the other end of one fiber pair
100
b
included in the two fiber pairs
100
. Optical modulators may be provided between the variable wavelength light source
202
and the fiber pair
100
a
and/or between the fixed wavelength light source
204
and the fiber pair
100
b.
For measuring the chromatic dispersion characteristic, the wavelength &lgr;x of variable wavelength light source
202
is swept (changed at a constant speed) while the wavelength &lgr;
0
of the fixed wavelength light source
204
is fixed. A phase comparator
306
measures a phase difference between an output signal from the O/E converter
302
and an output signal from the O/E converter
304
, thereby measuring the wave dispersion characteristic of two fiber pairs.
In a high capacity transmission line in a trunk line system, it may be possible to use two fiber pairs. However, only one fiber pair can be used in most of the lines which have already been laid down. Thus it is necessary to measure the chromatic dispersion characteristic of one fiber pair.
SUMMARY OF INVENTION
The measuring method for the chromatic dispersion characteristic described above does not apply to one fiber pair. This is because two lines which pass light in the same direction and comprise a line for passing the fixed wavelength light and a line for passing the variable wavelength light are not available in one fiber pair.
When the measuring method for the chromatic dispersion characteristic described above is applied to the two fiber pairs
100
, an error may be generated in a measured result. A difference between phases of light transmitted through the one fiber pair
100
a
and the one fiber pair
100
b
may change due to components which do not depend on wavelength when physical changes such as a temperature change or a stress change in the transmission line occur. In these cases, an error is generated in a measured result. It is desirable to use only one fiber pair instead of using two fiber pairs for measuring the chromatic dispersion characteristic.
The purpose of present invention is to provide an apparatus and the like for measuring the chromatic dispersion characteristic and the like with only one fiber pair.
According to the present invention, an optical characteristics measuring apparatus for measuring characteristics of light transmitted through a device under test may include: a variable wavelength light source for generating variable wavelength light, whose wavelength is variable a fixed wavelength light source for generating fixed wavelength light, whose wavelength is fixed a light modulating unit for modulating the variable wavelength light and the fixed wavelength light with a predetermined frequency a composite light generating unit for entering composite light composed of the variable wavelength light and the fixed wavelength light into the device under test a fixed wavelength component extracting unit for extracting the fixed wavelength light component from transmitted light, which is transmitted through the device under test and a variable wavelength component extracting unit for extracting the variable wavelength light component from the transmitted light.
According to the optical characteristics measuring apparatus constituted as described above, the fixed wavelength component extracting unit extracts the fixed wavelength light component from transmitted light transmitted through a device under test, and the variable wavelength component extracting unit extracts the variable wavelength light component from the transmitted light transmitted through the device under test. Thus, a phase difference of the variable wavelength light component is measured with the fixed wavelength light component as a reference. Since the phase difference is measured with one line for passing composite light, the measuring is possible when a device under test is one fiber pair.
According to the present invention, an optical characteristics measuring apparatus for measuring characteristics of light transmitted through a device under test includes: a variable wavelength light source for generating variable wavelength light, whose wavelength is variable a fixed wavelength light source for generating fixed wavelength light, whose wavelength is fixed a light modulating unit for modulating the variable wavelength light and the fixed wavelength light with a predetermined frequency and a composite light generating unit for entering composite incident light composed of the variable wavelength light and the fixed wavelength light into the device under test.
According to the present invention, an optical characteristics measuring apparatus for measuring characteristics of light transmitting device under test may include: a fixed wavelength component extracting unit for extracting a fixed wavelength light component from transmitted light, which is generated after light, which is composed of variable wavelength light whose wavelength is variable and fixed wavelength light whose wavelength is fixed, has been transmitted through the device under test and a variable wavelength component extracting unit for extracting the variable wavelength light component from the transmitted light.
In the present invention as described above the fixed wavelength light source may switch the wavelength of the fixed wavelength light.
In the present invention as described above, wherein the composite light generating unit may be a coupler.
In the present invention as described above, the variable wavelength component extracting unit may be an optical filter, which reflects the fixed wavelength light component, and transmits the variable wavelength light component, and the fixed wavelength component extracting unit may be a circulator, which has a first terminal for receiving the transmitted light, a second terminal for outputting the light received by the first terminal and for receiving inputted light, and a third terminal for outputting the light received by the second terminal, and wherein the second
Imamura Motoki
Kawazawa Toshio
Kimura Eiji
Advantest Corporation
Nguyen Tu T.
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