Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2002-04-24
2004-03-02
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06700655
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber characteristic measuring device which produces a pulse of light incident to an optical fiber as an object to be measured and measures the characteristic of the optical fiber according to the returning light emitted from the optical fiber.
2. Description of Related Art
FIG. 5
is a block diagram showing a structure of an example of a conventional optical fiber characteristic measuring device. A light of constant frequency of &ngr;0 which is emitted from the light source
1
is incident on an incident port
4
i
of a first optical directional coupler
4
. The first optical directional coupler
4
has an incident port
4
i
and two emitting ports
4
t
1
and
4
t
2
. The first optical directional coupler
4
separates the light incident on the incident port
4
i
into 2 directions and emits the light from the two emitting ports
4
t
1
and
4
t
2
.
The light which is emitted from the emitting port
4
t
1
of the first optical directional coupler
4
is incident on the light pulse generating device
5
. The light pulse generating device
5
is specifically an electro-optical switch. The light pulse generating device
5
extracts the light pulse from the incident light by turning the switch on and off and emits the extracted light pulse.
The light pulse emitted from the light pulse generating device
5
is incident on a light amplifier
6
. The light amplifier
6
amplifies the incident light pulse to a predetermined level and emits the amplified light pulse. The light pulse emitted from the light amplifier
6
is incident on an incident port
7
i
of an optical switch
7
. The optical switch
7
has three ports such as an incident port
7
i
, an emitting/incident port
7
ti
, and an emitting port
7
t
, and emits the light pulse which is incident on the incident port
7
i
from the emitting/incident port
7
ti
. The optical switch
7
also emits the returning light which is incident on the emitting/incident port
7
ti
from the emitting port
7
t.
The emitting/incident port
7
ti
of the optical switch
7
is connected to an end
9
a
of the optical fiber
9
as an object to be measured via an optical connector
8
. Therefore, the light pulse emitted from the emitting/incident port
7
ti
of the optical switch
7
is incident on an end
9
a
of the optical fiber
9
via an optical connector
8
. The returning light which is emitted from the end
9
a
of the optical fiber
9
is incident again on the emitting/incident port
7
ti
of the optical switch
7
, and is further emitted from the emitting port
7
t
of the optical switch
7
.
The returning light emitted from the emitting port
7
t
of the optical switch
7
is incident to the incident port
10
i
1
of the second light directional coupler
10
. The second light directional coupler
10
has two incident ports such as
10
i
1
and
10
i
2
and two emitting ports such as
10
t
1
and
10
t
2
. To the incident port
10
i
2
of the second light directional coupler
10
, the light (hereinafter called “reference light”) emitted from the emitting port
4
t
2
of the first light directional coupler
4
is incident. Consequently, the second light directional coupler
10
combines the wave of the returning light which is incident from the incident port
10
i
1
and the wave of reference light which is incident from the incident port
10
i
2
. The second light directional coupler
10
further separates the combined light into two directions, and emits the lights from the two emitting ports
10
t
1
and
10
t
2
.
Both of the combined lights emitted from the two emitting ports
10
t
1
and
10
t
2
of the second light directional coupler
10
are received by balance receiving photodiode PD
11
. The balance receiving photodiode PD
11
converts the combined lights which is received to an electric signal (beat signal) and outputs the converted electric signal (beat signal). The beat signal which is output by the balance light receiving photodiode PD
11
is input to an amplifier
12
. The amplifier
12
amplifies the input beat signal to a predetermined level and sends the amplified beat signal to a mixer
13
.
The mixer
13
mixes the beat signal sent from the amplifier
12
and an RF signal generated by a signal generating circuit
14
, and outputs the mixed signal. A control circuit
15
controls the signal generating circuit
14
and determines the frequency &ngr;r of the RF signal generated by the signal generating circuit
14
. The frequency &ngr;r of the RF signal is set to a value which is close to 10.8 GHz as a shifting amount by the Brillouin scattering.
A low pass filter
16
inputs the mixed signal which is output by the mixer
13
, removes high frequency component which is included in the mixed signal which is input, passes only low frequency component, and outputs a difference signal which is a low frequency component. The amplifier
17
amplifies the difference signal which is output by the low pass filter
16
to a predetermined level, and outputs the amplified difference signal. The signal process section
18
inputs the difference signal which is output by the amplifier
17
, performs various signal treatment on the inputted difference signa, and determines the characteristic of the optical fiber
9
.
Next, the operation of the optical fiber characteristic measuring device is explained. The light with the frequency of &ngr;0 emitted from the light source
1
is sent to the light pulse generating circuit
5
via the light directional coupler
4
. Then, the light pulse generating circuit
5
extracts the light pulse with the frequency of &ngr;0 from the light which is sent.
The light pulse emitted from the light pulse generating circuit
5
is incident on the end
9
a
of the optical fiber
9
via an optical amplifier
6
, an optical switch
7
, and an optical connector
8
. When the incident light pulse is transmitted in the optical fiber
9
, Brillouin scattering, Rayleigh scattering, and reflection occur at several points in the optical fiber
9
, then the returning light including the Brillouin scattered light, Rayleigh scattered light, and reflected light return to the end
9
a
from such several points. The returning light is emitted from the end
9
a.
The returning light emitted from the end
9
a
of the optical fiber
9
and including the Brillouin scattered light is incident again on the emitting/incident port
7
ti
of the optical switch
7
via the optical connector
8
, and is further emitted from the emitting port
7
t
. The returning light emitted from the emitting port
7
t
of the optical switch
7
and including the Brillouin scattered light is incident on the incident port
10
i
1
of the second light directional coupler
10
. Additionally, to another incident port
10
i
2
of the second light directional coupler
10
, the reference light emitted from the emitting port
4
t
2
of the first light directional coupler
4
with a frequency of &ngr;0.
The second light directional coupler
10
mixes the wave of the Brillouin scattering light with frequency of &ngr;0±&ngr;B and the wave of reference light with frequency of &ngr;0. Consequently, resonance occur because the frequencies of these lights are so close that interference is caused. The frequency of the resonance is represented as the difference between the frequency of Brillouin scattering light such as &ngr;0±&ngr;B and the frequency of the reference light such as &ngr;0. Therefore the frequency of the resonance becomes &ngr;B.
When the mixed light in which the resonance of which frequency is &ngr;B occurs is received by the balance receiving photodiode PD
11
, the balance receiving photodiode PD
11
outputs the beat signal having the resonance of which frequency is &ngr;B. The beat signal which is output by the balance receiving photodiode PD
11
and has the resonance of which frequency is &ngr;B is input to the mixer
13
via the amplifier
12
. An RF signal of which frequency is &ngr;r which is generated by the signal generating circu
Sakairi Yoshiyuki
Uchiyama Haruyoshi
Ando Electric Co. Ltd.
Nguyen Tu T.
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