Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2001-07-10
2003-02-11
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06519028
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to the measurement of chromatic dispersion characteristics of a DUT (Device Under Test) such as an optical fiber, and in particular to the measurement which can obtain synchronization of a variable-wavelength light source and a phase comparator with a high precision, by providing the variable-wavelength light source at one end of the DUT and the phase comparator at the other end of the DUT.
2. Description of the Related Art
The construction of a measuring system which measures chromatic dispersion characteristics of the DUT such as an optical fiber is shown in
FIG. 8. A
light source system
100
is connected to one end of an optical fiber
300
and a measuring system
200
is connected to the other end of the optical fiber
300
. The light source system
100
has a variable-wavelength light source
102
and an optical modulator
104
. The measuring system
200
includes a photoelectric (OLE) converter
202
and a phase comparator
204
.
In measuring chromatic dispersion characteristics, the variable-wavelength light source
102
changes the wavelength ëx of generated light. The light generated by the variable-wavelength light source
102
is modulated by a modulation frequency Fm in the optical modulator
104
and inputted to the optical fiber
300
. The light transmitted through the optical fiber
300
is converted into an electric signal in the photoelectric (OLE) converter
202
. The phase comparator
204
measures a phase difference between a phase of an electric signal and a phase which is to be a reference with respect to the electric signal. Group delay (GD) can be calculated from the phase difference and modulation frequency Fm. Chromatic dispersion (CD) can be calculated by differentiating group delay by the wavelength of the group delay. In addition, the frequencies of ëx and Fm are communicated to the measuring system
200
.
Waveforms of light generated by the light source system
100
and light received by the measuring system
200
are schematically shown in FIGS.
9
(
a
)-
9
(
b
). FIG.
9
(
a
) shows the waveform of light generated by the light source system
100
. FIG.
9
(
b
) shows the waveform of light received by the measuring system
200
. Time delay t
0
added to the light generated by the light source system
100
makes a light to be received by the measuring system
200
. However, for simplicity, the drawing shows as if there is no discrepancy of phases between the light generated by the light source system
100
and the light received by the measuring system
200
. The time delay t
0
is L/(c
) {t
0
=L/(c
)}, where L is length of optical fiber
300
, c is velocity of light, and n is refraction index of optical fiber
300
. In addition, t
0
is increased as the length of optical fiber increases. For example, the length of optical fiber in a submarine cable and the like is about 10000 km, and to is up to 50 ms.
As shown in FIGS.
9
(
a
)-
9
(
b
), in the light received by the measuring system
200
, the time delay t
0
is generated. Therefore, if the light source system
100
changes a wavelength directly after a light of a certain wavelength is generated, it becomes impossible to know ë x (wavelength of light generated by the variable wavelength light source
102
) corresponding to the light received by the measuring system
200
.
Therefore, the wavelength ë x of light generated by the light source system
100
is fixed from t
0
to t
1
.
FIG. 10
shows a method for changing the wavelength of light generated by the light source system
100
. Firstly, a light, the wavelength of which is ë 0 from time
0
to t
1
, ë 1 from time t
1
to 2t
1
, and so on, is generated. That is, the wavelength of light is changed in a step form.
Here, the variable-wavelength light source
102
cannot perform measurement of wavelength while continuously changing the waveform, even if it had a function which renders it possible to continuously sweep the wavelength. This is because it is impossible to exactly known ë x (wavelength of light generated by the variable-wavelength light source
102
) corresponding to the light received by the measuring system
200
. That is, it is impossible to obtain a synchronization of light source system
100
and measuring system
200
. Therefore, the wavelength of light is changed in the step form and measured.
SUMMARY OF INVENTION
However, if the wavelength of light is changed in the step form and measured, the time required for measuring is longer than that required in the case of continuously sweeping the wavelength. Moreover, if wavelength changing values (&lgr;2-&lgr;1, &lgr;1-&lgr;0, . . . ) are not taken so high to a certain extent, the measuring time takes too long. Therefore, it is impossible to improve the resolution of wavelength.
Therefore, the object of the present invention is to provide a technique for measuring characteristics, such as chromatic dispersion and the like, by making it possible to continuously sweep the wavelength of light source.
According to the present invention, an apparatus for measuring optical characteristics of a device-under-test which transmits light, includes: a variable-wavelength light source for generating a variable-wavelength light, the wavelength of which is variable, having an identification waveform at the time when the wavelength is changing, wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing; an optical modulation unit for modulating the variable-wavelength light at a predetermined frequency and then inputting it to the device-under-test; and an identification waveform detection unit for detecting the identification waveform in the transmitted light transmitted through the device-under-test.
According to the apparatus for measuring optical characteristics constructed as explained above, since the time when the identification waveform detection unit detects the identification waveform is the time when the waveform starts to change, it is possible to obtain a synchronization between an incidence side and an exit side of a device-under-test using the time when the identification waveform is detected. Accordingly, it is possible to obtain the synchronization between an incidence side and an exit side of a device-under-test, even if the wavelength of light source is continuously swept.
According to the present invention, an apparatus for measuring optical characteristics of a device-under-test which transmits light, includes: a variable-wavelength light source for generating a variable-wavelength light, the wavelength of which is variable, having an identification waveform at the time when the wavelength is changing, wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing; and an optical modulation unit for modulating the variable-wavelength light at a predetermined frequency and then inputting it to the device-under-test.
According to the present invention, an apparatus for measuring optical characteristics of a device-under-test which transmits light, includes: an identification waveform detection unit for detecting identification waveform in a transmitted light which is an incident light transmitted through the device-under-test, wherein the incident light is a variable-wavelength light, the wavelength of which is variable, having in the form of the identification waveform at the time when the wavelength is changing, and wherein the identification waveform is distinguishable from a normal waveform before and after the time when the wavelength is changing.
The present invention described above, is an apparatus for measuring optical characteristics, wherein the identification waveform is a waveform different from the normal waveform in wavelength.
The present invention described above, is an apparatus for measuring optical characteristics, wherein the identification waveform is a waveform different from the normal waveform in output condi
Imamura Motoki
Kimura Eiji
Advantest Corporation
Lowe Hauptman & Gilman & Berner LLP
Nguyen Tu T.
LandOfFree
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