Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2002-07-01
2004-03-23
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06710863
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatuses and methods for measuring characteristics of optical fibers in length directions based on stimulated Brillouin scattering effects that occur in optical fibers.
2. Description of the Related Art
Recently, optical fibers are frequently used as information transmission media to secure high-speed transmission for large amounts of information. In order to secure satisfactory communication qualities, it is necessary to periodically perform measurements on characteristics of optical fibers in length directions. For example, measurements are performed to locate faults or defects that actually occur in optical fibers or that may likely occur. Specifically, OTDR (i.e., Optical Time Domain Reflectometer) measurement techniques are provided to measure characteristics (e.g., distortions) of optical fibers. That is, light pulses are input into one ends of optical fibers, wherein measurement is performed with respect to backward scattering light that occur in optical fibers during propagation of light pulses therethrough.
Since the OTDR measurement techniques are capable of specifying distorted positions of optical fibers, they are applicable to optical fiber sensors and the like that measure temperature distributions in environments for facilitating optical fibers as well as distributions of physical values such as distortions. In order to perform maintenance and management with respect to large-scale structures such as dams and embankments, it is necessary to detect distortions of large-scale structures. In this case, large-scale structures are wired with optical fibers whose characteristics such as distortions are measured by optical fiber sensors. Recently, it is strongly demanded to develop high-performance optical fiber sensors, having high spatial resolutions, which can specify distorted positions of optical fibers as accurately as possible.
To cope with the above demand, there is provided a measurement apparatus that performs measurement based on stimulated Brillouin scattering effects induced in ‘measured’ optical fibers. Specifically, stimulated Brillouin scattering effects occur in optical fibers in which probe beams are input into one ends while pump beams are input into other ends.
FIG. 5
is a block diagram showing an example of the measurement apparatus using stimulated Brillouin scattering effects. Herein, reference numeral
100
designates a light source that comprises a semiconductor laser
101
and a signal generation circuit
102
. The signal generation circuit
102
performs frequency modulation or phase modulation on laser beams output from the semiconductor laser
101
, thus generating modulation signals. Reasons why the frequency modulation or phase modulation is performed on laser beams output from the semiconductor laser
101
will be described later. Briefly speaking, however, the frequency modulation or phase modulation is required to determine positions of correlation peaks that can be clearly recognized between probe light L
11
and pump light L
12
, which are input into a measured optical fiber
107
from different ends respectively. Reference numeral
103
designates an optical coupler or branch that provides two branches with respect to laser beams output from the light source
100
.
That is, laser beams of the first branch from the optical branch
103
are input into a light modulator
104
, wherein they are subjected to modulation to shift light frequencies thereof. Due to the modulation of the light modulator
104
, sidebands are caused to occur with respect to the center wavelength of laser beams. The light modulator
104
comprises a microwave generator
105
and a light intensity modulator
106
. The light modulator
104
modulates laser beams to produce sidebands in order to cause stimulated Brillouin scattering effects in the measured optical fiber
107
. The microwave generator
105
generates microwaves for frequency shifting, which are imparted to laser beams output from the optical branch
103
. The light intensity modulator
106
produces sidebands having frequency differences, which match frequencies of microwaves generated by the microwave generator
105
, with respect to the center frequency of laser beams input thereto. Incidentally, the microwave generator
105
can vary the frequency of microwaves output therefrom. The light intensity modulator
106
outputs the probe light L
11
, which is input into one end of the measured optical fiber
107
. Specifically, the lower sideband is used for the probe light L
11
.
The optical branch
103
also provides laser beams of the second branch, which are input to a light delay
108
. That is, the light delay
108
delays incoming laser beams with respect to time in order to delay the pump light L
12
, which is input into the other end of the measured optical fiber
107
. Due to the provision of the light delay
108
, a prescribed delay time is set between the probe light L
11
and the pump light L
12
. Delayed laser beams output from the light delay
108
are supplied to the other end of the measured optical fiber
107
via an optical branch
109
as the pump light L
12
.
The probe light L
11
propagate through the measured optical fiber
107
from one end to the other end. The optical branch
109
branches off the output light of the measured optical fiber
107
having light frequency bands containing the frequency band of the probe light L
11
. The intensity of the probe light L
11
may be influenced by stimulated Brillouin scattering effects that occur in the measured optical fiber
107
. A light wavelength filter
110
has a filtering characteristic to allow transmission of only the lower sideband, within the light output from the optical branch
109
, therethrough. A light detector
111
detects light power of the lower sideband that is isolated by the optical wavelength filter
110
.
In the measurement apparatus having the aforementioned configuration shown in
FIG. 5
, laser beams that are subjected to frequency modulation or phase modulation and that are output from the light source
100
are supplied to the optical branch
103
, which in turn provides laser beams of the first branch that are input into the light modulator
104
. In the light modulator
104
, laser beams are modulated (in intensity) to provide the probe light L
11
whose light frequency can be varied. The probe light L
11
is incident on one end of the measured optical fiber
107
. In addition, the optical branch
103
provides laser beams of the second branch that are delayed by the prescribed delay time in the light delay
108
and that are then incident on the other end of the measured optical fiber
107
via the optical branch
109
as the pump light L
12
.
Both the probe light L
11
and the pump light L
12
are respectively produced based on the same laser beams that are modulated in frequency or phase in the same light source
100
. Therefore, the probe light L
11
and the pump light L
12
, which are input into the measured optical fiber
107
from opposite ends respectively, are mutually influenced by each other to periodically cause correlation peaks. At each position showing a correlation peak, a ‘constant’ light frequency difference appears between the probe light L
11
and the pump light L
12
, which may be amplified in light intensity due to stimulated Brillouin scattering effects.
At other positions other than the positions of correlation peaks, the probe light L
11
and the pump light L
12
may be normally varied in light frequencies, so that the probe light L
11
may not be affected by Brillouin amplification and will be substantially unchanged in light intensity. Therefore, it can be said that the gain of the probe light L
11
may be greatly caused by Brillouin amplification at the positions of correlation peaks.
The probe light L
11
whose gain is caused by Brillouin amplification is output from the other end of the measured optical fiber
107
, from which it is supplied to the optical branch
109
. Then, the probe lig
Hotate Kazuo
Kannou Motokatsu
Barth Vincent P
Darby & Darby
Hotate Kazuo
Rosenberger Richard A.
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