Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2001-02-05
2003-07-01
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06587190
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system and method for measuring a chromatic dispersion in an optical fiber, and more particularly, a technology which enables an implementation of a system for an accurate measurement of a chromatic dispersion in both a long-distance optical fiber and a short-distance optical fiber such as an optical fiber device through a proposal of a measurement method for a chromatic dispersion in an optical fiber which is insensitive to an external environmental condition.
2. Description of the Related Art
The chromatic dispersion representing a wavelength dependency of a wave group velocity due to the property of matter and the structure of an optical waveguide is a characteristic of an optical fiber causing a temporal spreading of an optical pulse which allows transmission of data, and a controlled chromatic dispersion is one of main technologies in implementation of a very high-speed and high-capacity optical communication system.
Accordingly, there have been researched various methods for controlling and correctly measuring the chromatic dispersion of an optical fiber having a specific dispersion characteristic such as an optical fiber diffraction grating, a dispersion shifted fiber (hereinafter, referred to as “DSF”) or a dispersion flattened fiber (hereinafter, referred to as “DFF”).
Particularly, it is important to measure a chromatic dispersion in a long-distance single mode optical fiber which mainly acts as a transmission line for the control of the chromatic dispersion. Recently, there is a need for an accurate measurement of a chromatic dispersion characteristic for respective very short length of optical fiber device along with a development of an optical fiber device such as an optical fiber diffraction grating.
FIG. 1
is a schematic block diagram illustrating the construction of a system for measuring a chromatic dispersion in an optical fiber using a conventional optical fiber Raman laser.
Referring to
FIG. 1
, there is shown the chromatic dispersion measuring system which includes an optical fiber Raman laser
10
, a monochrometer
12
, a test optical fiber
14
, a photodetector
16
, an oscilloscope
18
, and an Nd:YAG pump laser
20
.
The Nd:YAG pump laser
20
further includes an M-L RF driver, a Q-S RF driver, and a digital delay generator.
This measurement technique is using the time-of-flight method.
First, an optical pulse is generated and the generated optical pulse passes through a test optical fiber
14
. The time delay difference between pulses having different wavelengths due to the chromatic dispersion in the test optical fiber is measured by a sampling oscilloscope
18
and a high speed photodetector
16
.
A silica optical fiber Raman laser
10
pumped with a mode locked and Q switched Nd:YAG pump laser (&lgr;=1.06 &mgr;m)
20
is used, it is possible to measure a relatively large wavelength band of 1.1~1.2 &mgr;m, but the necessity for the monochrometer
12
makes miniaturization of the chromatic dispersion measuring system difficult.
FIG. 2
is a schematic block diagram illustrating the construction of a system for measuring a chromatic dispersion in an optical fiber using a conventional semiconductor laser diode array.
A construction and work of the chromatic dispersion system shown in
FIG. 2
will be described hereinafter briefly.
Referring to
FIG. 2
, there is shown the chromatic dispersion measuring system which includes an electric pulse source, an InGaAsP semiconductor laser array
30
, a wavelength division multiplexer
32
, a test optical fiber
34
, a high speed photodetector
36
and an oscilloscope
38
.
The InGaAsP semiconductor laser array
30
is composed of six InGaAsP semiconductor lasers, which is driven with an electric pulse of 100 ps.
Like this, the use of the InGaAsP semiconductor laser array
30
enables miniaturization of the chromatic dispersion measuring system, but the necessity for the wavelength division multiplexer
32
results in an increase in a cost required for fabricating the chromatic dispersion measuring system.
Also, all the InGaAsP semiconductor lasers must be replaced according to a variation in a wavelength of a interest domain, which causes a difficulty in selecting a wavelength flexibly.
FIG. 3
is a schematic block diagram illustrating the construction of a system for measuring a chromatic dispersion in an optical fiber using a conventional phase shift measurement method.
A construction and work of the chromatic dispersion system shown in
FIG. 3
will be described hereinafter briefly.
Referring to
FIG. 3
, there is shown the chromatic dispersion measuring system which includes a light source
40
, a monochrometer
42
, a photodetector
44
, an amplifier, a vector electrometer
46
, a curve fitting section
48
, and a signal generator
50
.
The phase shift measurement method according to
FIG. 3
is a method in which an optical signal modulated with a sinusoidal wave instead of an optical pulse passes through a test optical fiber
34
in which a chromatic dispersion is measured, and then a phase shift for a wavelength caused due to the chromatic dispersion in the optical fiber is measured.
That is, when an optical signal modulated with a frequency f passes through the test optical fiber
34
in which a chromatic dispersion is measured and a phase shift before and after a passage of the optical signal through the test optical fiber
34
is measured according to a wavelength, the phase shift can be written as follows according to a group delay &tgr; g(&lgr;) per mode.
&PHgr;(&lgr;)=2
&pgr;f&tgr;g
(&lgr;)
Thus, such a phase shift according to a wavelength is measured and the measured phase shift undergoes a curve fitting process in the curve fitting section
48
so that the chromatic dispersion is estimated.
Accordingly, such a method has a disadvantage in that a phase shift according to a wavelength is measured, and then undergoes the curve fitting process again. Further, instead of a semiconductor laser an LED may be used as a light source
40
, which makes it possible to fabricate the chromatic dispersion measuring system at a low cost, but the necessity for the monochrometer
42
makes miniaturization of the chromatic dispersion measuring system difficult.
In addition, in case of the chromatic dispersion measuring system, it is difficult to separate the light source
40
modulated by the signal generator
50
and the vector electrometer
46
for detecting the optical signal which has passes through the test optical fiber
34
from each other, which causes a problem in a remote control of the chromatic dispersion measuring system.
FIG. 4
is a schematic block diagram illustrating the construction of a system for measuring a chromatic dispersion in an optical fiber using a conventional interferometer.
Referring to
FIG. 4
, there is shown the chromatic dispersion measuring system which includes a white light source
60
, a monochrometer
62
, a reference optical fiber
64
, a test optical fiber
66
, and photodetector
68
.
In an interferometric measurement method using the interferometer shown in
FIG. 4
, a Mach-Zehnder interferometer is used as a basic interferometer, and an input light is divided into two optical signals of an identical optical path. One of the two optical beams passes through the reference optical fiber
64
as a reference, and the other passes through the test optical fiber
66
.
The two beams (optical pulses) are synthesized. The chromatic dispersion characteristic in the optical fiber to be measured from an interference fringe of the synthesized beam.
In such a method, a very short optical fiber within 1 m is advantageous, but in many cases the white light source
60
is used and the necessity for the monochrometer
62
makes miniaturization of the chromatic dispersion measuring system difficult.
Moreover, there is also needed a data processing process for again calculating a chromatic dispersion from a group delay caused by the chromatic dispersion in the test optical fiber obtained
Chae Jung Hye
Lee Yong Tak
Bacon & Thomas PLLC
Kwangju Institute of Science & Technology
Nguyen Tu T.
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