Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2000-04-11
2002-01-01
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06335788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical-fiber characteristics measuring apparatus which measures various characteristics of an optical fiber by emitting an optical pulse to the optical fiber and performing an optical heterodyne detection involving the combination of returned light from the optical fiber with local oscillation light.
2. Description of the Related Art
FIG. 3
is a block diagram illustrating the structure of an optical-fiber characteristics measuring apparatus according to the related art. The operation of this optical-fiber characteristics measuring apparatus will be described below. When a light source
31
emits coherent light
31
a
of a frequency f
0
to an optical directional coupler
32
, the coherent light
31
a
passes through the optical directional coupler
32
and enters an optical pulse generator
33
as coherent light
32
a
. The optical pulse generator
33
converts this coherent light
32
a
into pulse light
33
a
. It is to be noted that the coherent light
32
a
and the pulse light
33
a
have the same frequency as the frequency f
0
of the coherent light
31
a.
Next, an optical frequency converter
34
performs frequency conversion by shifting the frequency of the pulse light
33
a
by a predetermined frequency &Dgr;f and sends out coherent light
34
a
having a frequency “f
0
+&Dgr;f”. This pulse light
34
a
travels through an optical amplifier
35
, an optical switch
36
and an optical connector
37
and is emitted as pulse light
37
a
toward an optical fiber
38
to be measured. When this pulse light
37
a
enters the to-be-measured optical fiber
38
, reflection or scattering respectively produces reflected light or scattered light in accordance with the state in the to-be-measured optical fiber
38
. Part of the reflected light or scattered light travels as returned light
38
a
through the optical connector
37
and the optical switch
36
. Then, returned light
36
b
is emitted toward the balanced-light reception circuit
40
.
The balanced-light reception circuit
40
converts the returned light
36
b
into an electric signal through balanced-light reception with the coherent light
32
b
of the frequency f
0
emitted from the optical directional coupler
32
. Specifically, an optical directional coupler
41
combines the coherent light
32
b
and the returned light
36
b
, and a photoelectric converter
42
converts the combined optical signal into an electric signal which is in turn amplified by an electric signal
43
a
by an amplifier section
43
. This electric signal
43
a
is input to a signal processing section
46
through a low-pass filter
44
and an amplifier section
45
. The signal processing section
46
acquires various characteristics of the to-be-measured optical fiber
38
based on the input electric signal and processes this electric signal on the time axis to prepare the distribution on the distance axis of the to-be-measured optical fiber
38
.
According to the conventional optical-fiber characteristics measuring apparatus, as apparent from the above, the optical scheme using the optical frequency converter
34
shifts the frequency of the pulse light
37
a
to be input to the to-be-measured optical fiber
38
by the predetermined frequency &Dgr;f with respect to the frequency of the coherent light
31
a
. Then, the local oscillation light (coherent light
32
b
) and the returned light
36
b are combined, yielding a beat signal. The frequency &Dgr;f is set in accordance with the frequency of the returned light
36
b
in such a way that the frequency of the beat signal (i.e., the difference between the frequencies of the local oscillation light and the returned light) lies in an electrically processable range. Accordingly, backward scattered light, such as the Rayleigh scattered light and Brillouin scattered light, and reflected light, which is produced in the to-be-measured optical fiber
38
, can be detected as returned light.
The use of such an optical frequency conversion scheme requires that the optical frequency converter
34
should be constituted by an optical frequency shifter or by an optical ring comprising several optical components. This complicates the structure of the optical-fiber characteristics measuring apparatus. When an optical ring system is used, for example, while pulse light is travels along the optical ring, new pulse light cannot be input to the optical ring. This restricts the cycle period of the pulse light that is emitted from the optical ring, thus disabling fast measuring of the characteristics of the to-be-measured optical fiber. In addition, the frequency conversion increases the frequency of the pulse light, thereby restricting the pulse width of the pulse light.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an optical-fiber characteristics measuring apparatus that has a simple structure which does not require frequency conversion of pulse light to be input an optical fiber to be measured and does not restrict the cycle period of the pulse light, thereby ensuring fast measuring of the characteristics of the optical fiber using a fast optical output.
To achieve the above object, according to one aspect of this invention, there is provided an optical-fiber characteristics measuring apparatus for converting coherent light into pulse light, emitting the pulse light to an optical fiber, converting an optical signal acquired by balanced-light reception of returned light from the optical fiber and the coherent light into a first electric signal, and obtaining characteristics of the optical fiber from a frequency component of the returned light included in the first electric signal, which apparatus comprises signal generation means for generating a second electric signal having a frequency approximately coincident with a frequency of an optical signal to be detected in those optical signals included in the returned light; and mixing means for mixing the first electric signal and the second electric signal to thereby detect a frequency component of the optical signal to be detected.
According to this invention, as specifically described above, the frequency component of the desired optical signal is detected by producing a first electric signal by conversion of the optical signal that is acquired by the balanced-light reception of returned light and coherent light, producing a second electric signal whose frequency approximately matches with the frequency of the optical signal to be detected of optical signals included in returned light, and then mixing the first and second electric signals together. In the case of detecting the returned light by using a beat signal obtained by combining the returned light and local oscillation light (coherent light), therefore, the frequency component of the optical signal included in the returned light can be detected even if the frequency band of the signal processor for acquiring the characteristics of an optical fiber is not matched with the frequency component of the beat signal. This can ensure excellent coherent detection according to the frequency component of reflected light or any of various kinds of scattered lights contained in the returned light. Further, it is unnecessary to shift the frequency of the pulse light to be sent to an optical fiber, thus eliminating the need for a circuit, such as an optical frequency shifter or an optical ring system. This can help make the structure of the optical-fiber characteristics measuring apparatus simpler. Furthermore, there is no restriction on the cycle period of pulse light, so that the pulse light can be emitted in a shorter period, thereby ensuring fast measuring of the characteristics of the optical fiber.
In this optical-fiber characteristics measuring apparatus, the signal generation means may detect a spectrum of the optical signal to be detected by changing the frequency of the second electric signal over a spectrum width of the optical signal to be detected.
In this case, the spectrum
Kurashima Toshio
Uchiyama Haruyoshi
Ando Electric Co. Ltd.
Font Frank G.
Nguyen Tu T.
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