Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2001-07-10
2002-07-30
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06426792
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to measurement of chromatic dispersion characteristics of a DUT (Device Under Test) such as an optical fiber, and in particular to a technique for measuring dispersion characteristics without being influenced by the expansion and contraction of the DUT.
2. Description of the Related Art
When measuring chromatic dispersion characteristics of a device-under-test (DUT) such as an optical fiber and the like, it is desired that the measurement can be performed while excluding the influence of expansion and contraction of the DUT. A technique for measuring the DUT without being influenced by the expansion and contraction thereof is disclosed, for example, in Japanese Patent Laid-Open No. Hei 1-291141.
The construction of a measuring system is shown in FIG.
4
. As shown in
FIG. 4
, the measuring system is divided into a light source system
10
and a characteristic measuring system
20
. A variable-wavelength light source
12
of light source system
10
changes the wavelength, so that a light of wavelength &lgr;x (variable-wavelength light) is generated. A fixed-wavelength light source
13
fixes the wavelength, so that a light of wavelength &lgr;
0
(fixed-wavelength light) is generated. And, &lgr;0 is the wavelength at which the chromatic dispersion is minimized in an optical fiber
30
. The variable-wavelength light and fixed-wavelength light are modulated by an optical modulator
15
a
and an optical modulator
15
b,
respectively, to a frequency f, and multiplexed in an optical multiplexer
16
.
The light multiplexed in the optical multiplexer
16
is entered into the optical fiber
30
. The light transmitted through the optical fiber
30
is entered into an optical demultiplexer
21
of the characteristic measuring system
20
. The optical demultiplexer
21
divides the light transmitted through the optical fiber
30
into a light of wavelength &lgr;x and a light of wavelength &lgr;
0
. A photoelectric converter for measurement
22
a
and a photoelectric converter for reference
22
b
perform photoelectric conversion of the light of wavelength &lgr;x and the light of wavelength &lgr;0, respectively, and a phase comparator
24
detects the phase difference between outputs of the photoelectric converter for measurement
22
a
and the photoelectric converter for reference
22
b.
The transmitted light of wavelength &lgr;x is influenced by the chromatic dispersion, and the expansion and contraction of optical fiber
30
. The transmitted light of wavelength &lgr;
0
is influenced only by the expansion and contraction of optical fiber
30
. This is because &lgr;
0
is the wavelength at which the chromatic dispersion is minimized in the optical fiber
30
. Therefore, if the phase difference between the transmitted light of wavelength &lgr;x and the transmitted light of wavelength &lgr;
0
is detected, it is possible to exclude the influence caused by the expansion and contraction of optical fiber
30
.
SUMMARY OF INVENTION
However, in order to allow the optical demultiplexer
21
to divide the light transmitted through the optical fiber
30
into the light of wavelength &lgr;x and the light of wavelength &lgr;
0
, the wavelength &lgr;x and the wavelength &lgr;
0
should be somewhat separated. It is difficult for wavelength bands of wavelength &lgr;x and wavelength &lgr;
0
to be common. For example, the wavelength &lgr;x is from 1525 to 1635 nm and the wavelength &lgr;
0
is 1300 nm, so that the wavelength &lgr;x and the wavelength &lgr;
0
should be somewhat separated.
Therefore, the object of the present invention is to provide an apparatus that can perform measurement of chromatic dispersion, even if the wavelength of variable-wavelength light source and that of fixed-wavelength light source for reference are identical with each other.
According to the present invention described in claim 1, an apparatus for measuring optical characteristics of a device-under-test which transmits a light, includes: a variable-wavelength light source for generating a variable-wavelength light, the wavelength of which is variable; a fixed-wavelength light source for generating a fixed-wavelength light, the wavelength of which is fixed; a first optical modulation unit for casting an incident light for measurement on one end of the device-under-test, wherein the incident light for measurement is the variable-wavelength light subjected to intensity modulation to a predetermined frequency; a second optical modulation unit for casting an incident light for reference to the other end of the device-under-test, wherein the incident light for reference is the fixed-wavelength light subjected to intensity modulation to a predetermined frequency; a transmitted light obtaining unit for measurement that obtains a transmitted light for measurement, which is the incident light for measurement transmitted through the device-under-test; and a transmitted light obtaining unit for reference that obtains a transmitted light for reference, which is the incident light for reference transmitted through the device-under-test, wherein the apparatus measures the characteristics of device-under-test based on the transmitted light for measurement and the transmitted light for reference.
According to the optical characteristic measuring apparatus constructed as explained in the above, the variable-wavelength light is transmitted from the one end to the other end of the device-under-test, whereas the fixed-wavelength light is transmitted from the other end to the one end of the device-under-test. Therefore, it is possible to separate and obtain the variable-wavelength light and fixed-wavelength light transmitted through the device-under-test regardless of their wavelengths. Therefore, it is possible to measure the chromatic dispersion even if the wavelength of variable-wavelength light source is identical with the fixed-wavelength light source for reference.
According to the present invention described in claim 2, an apparatus for measuring optical characteristics of a device-under-test which transmits a light, includes: a transmitted light obtaining unit for measurement that obtains a transmitted light for measurement which is an incident light for measurement, transmitted through the device-under-test, wherein the incident light for measurement is a variable-wavelength light, the wavelength of which is variable, subjected to intensity modulation to a predetermined frequency and then cast on one end of the device-under-test; a transmitted light obtaining unit for reference that obtains a transmitted light for reference which is an incident light for reference, transmitted through the device-under-test, wherein the incident light for reference is a fixed-wavelength light, the wavelength of which is fixed, subjected to intensity modulation to a predetermined frequency and then cast on the other end of the device-under-test; a photoelectric conversion unit for measurement that performs photoelectric conversion of the transmitted light for measurement obtained by the transmitted light obtaining unit for measurement; a photoelectric conversion unit for reference that performs photoelectric conversion of the transmitted light for reference obtained by the transmitted light obtaining unit for reference; a phase comparison unit for detecting a phase difference between the phase of output of the photoelectric conversion unit for measurement and the photoelectric conversion unit for reference; and a characteristic calculation unit for calculating group delay characteristics or chromatic dispersion characteristics of the device-under-test using the phase difference.
The present invention described in claim 3, is an apparatus for measuring optical characteristics as claimed in claim 2, wherein the transmitted light obtaining unit for measurement includes a first terminal for measurement into which a light is entered, a second terminal for measurement from which the light entered into the first terminal for measurement exits and into which a light is also entered
Advantest Corporation
Font Frank G.
Nguyen Tu T.
LandOfFree
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