Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2000-04-18
2002-04-02
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06366348
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a optical fiber distortion measuring apparatus and optical fiber distortion measuring method which detect back scatter light generated by directing an optical pulse into an assessed optical fiber, and based on the results of this detection, obtain the amount of distortion in the assessed optical fiber.
2. Background Art
When a distortion is generated at a position in an optical fiber, the frequency distribution (spectra) of the Brillouin scattering light generated at that position when a light pulse is directed into the optical fiber is shifted by an amount proportional to the amount of distortion, when compared with the case in which a distortion is not present.
Optical fiber distortion measuring apparatuses which measure the amount of distortion in an optical fiber which is the subject of the measurement (an assessed optical fiber) using this principle were conventionally known.
FIG. 5
is a block diagram showing an example of a conventional optical fiber distortion measuring apparatus. This apparatus comprises a light source
1
, an optical coupler
2
, a light frequency conversion circuit
3
, a light pulse output circuit
4
, an optical coupler
5
, a light receiving circuit
7
, an amplifier circuit
8
, an A/D conversion circuit
9
, a signal processing unit
10
, a curve approximating unit
11
, a peak frequency detecting unit
12
, a distortion amount calculating unit
13
, and a display unit
14
. The operation of the optical fiber distortion measuring apparatus having the structure described above will be explained.
(1) Measurement of the Time Change Waveform
The optical fiber distortion measuring apparatus shown in
FIG. 5
obtains the time change waveform shown in
FIG. 6
by directing an optical pulse from one end of the assessed optical fiber. In
FIG. 6
, the horizontal axis indicates time from the application of the light pulse. Here, the time from the application of the light pulse corresponds to the distance from the input end of the assessed optical fiber
6
to each position in the assessed optical fiber
6
. Furthermore, the vertical axis shows the intensity of the Brillouin scattering light generated at each position.
The measurement operation of the time change waveform described above by the optical fiber distortion measuring apparatus shown in
FIG. 5
will be explained. In
FIG. 5
, light source
1
generates a constant light (CW light) and a series of light pulses having a constant difference in light frequency from the constant light. The constant light generated by light source
1
is directed into the light receiving circuit
7
via optical coupler
2
, while the series of light pulses generated by light source
1
are directed to the light frequency conversion circuit
3
via optical coupler
2
.
Light frequency conversion circuit
3
conducts a frequency shift of the light frequency of the series of light pulses generated by light source
1
, and converts them to a predetermined light frequency &ngr;. Then, light pulse output circuit
4
outputs one light pulse from among the series of light pulses having a light frequency of &ngr;, and the outputted light pulse is directed into the assessed optical fiber
6
via optical coupler
5
.
When the optical pulse is directed into the assessed optical fiber
6
, Brillouin scattering light is generated at each position in assessed optical fiber
6
. The Brillouin scattering light generated at each position in assessed optical fiber
6
is successively directed into light receiving circuit
7
via optical coupler
5
while being delayed by an amount of time proportional to the distance from the input end of the assessed optical fiber
6
to each position.
Using the constant light (CW light) generated by light source
1
, light receiving circuit
7
successively conducts the coherent detection of the Brillouin scattering light generated at each position in assessed optical fiber
6
, and outputs electrical signals proportional to the intensity of each Brillouin scattering light.
Amplifier circuit
8
amplifies the electrical signal outputted by light receiving circuit
7
, and A/D conversion circuit
9
conducts the A/D conversion of the electrical signals amplified by amplifier circuit
8
.
Signal processing unit
10
first conducts signal processing such as noise removal, logarithmic conversion, and the like with respect to the electrical signal values which were A/D converted, and then conducts plotting such that the electrical signal values are correlated with the amount of time elapsed from the application of the light pulse (that is to say, the distance from the input end of the assessed optical fiber), and generates the time change waveform shown in FIG.
6
. By means of the above processing, the Brillouin scatting light time change waveform is obtained in the case in which a light pulse having a light frequency &ngr; is inputted.
(2) Calculation of the Amount of Distortion
Next, the method for calculating the amount of distortion of the assessed optical fiber
6
will be explained. When the amount of distortion of the assessed optical fiber
6
is calculated, the optical fiber distortion measurement apparatus shown in
FIG. 5
repeats the operations described in (1) above while successively altering, by a specified value, the light frequency &ngr; of the light pulse inputted into the assessed optical fiber
6
, using the frequency conversion circuit
3
. By means of this, the time change waveform, an example of which is shown in
FIG. 6
, is obtained with respect to a plurality of light frequencies.
FIG. 7
is a three-dimensional graph showing an example of time change waveforms relating to a plurality of light frequencies. In the figure, the horizontal axis indicates the light frequency &ngr; of the light pulse inputted into the assessed optical fiber
6
, while the vertical axis indicates the intensity of the Brillouin scattering light, and the axis which intersects both these axis at right angles (the angled axis) indicates the time from the input of the light pulse (the distance from the input end of the assessed optical fiber
6
; that is to say, the position within the assessed optical fiber
6
). In other words, the coordinate plane formed by the vertical axis and the angled axis in
FIG. 7
corresponds to the coordinate plane shown in FIG.
6
.
Furthermore,
FIG. 8
is a graph in which the three-dimensional graph shown in
FIG. 7
is sectioned at a certain distance D along the angled axis (the distance from the input end of the assessed optical fiber). In other words,
FIG. 8
is a waveform (spectrum waveform) showing the frequency distribution (spectra) of the Brillouin scattering light at distance D.
When a spectrum waveform (see
FIG. 8
) is obtained at this certain distance D by means of this processing, then the curve approximating unit
11
shown in
FIG. 5
applies the data shown by the spectrum waveform to a second-order formula, and produces an approximate curve (a second-order curve) of the spectrum waveform.
Then, the peak frequency determining unit
12
differentiates this approximate curve, and determines a light frequency indicating the maximum value of the intensity of the Brillouin scattering light (the peak frequency &ngr;p).
Finally, distortion amount calculating unit
13
substitutes the peak frequency &ngr;p determined by the peak frequency calculating unit
12
into the formula (1) shown below, and calculates the amount of distortion &egr;.
&egr;=(&ngr;p−&ngr;b)/(&ngr;b×K) (1)
&ngr;b: peak frequency when there is no distortion (characteristic value for assessed optical fiber
6
)
K: distortion coefficient By means of the processing described above, the amount of distortion &egr; at a position within the assessed optical fiber
6
(at a distance D from the input end) is determined and is displayed in display unit
14
.
However, in the conventional optical fiber distortion measuring apparatus described above, when the amount of distortion of an assessed o
Kurashima Toshio
Sato Yasushi
Uchiyama Haruyoshi
Ando Electric Co. Ltd.
Font Frank G.
Nguyen Sang H.
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