Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2000-06-23
2002-07-16
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06421117
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for optical time domain reflectometry (OTDR) on multi-mode optical fibers that is useful in surveillance of fiberoptics based transmission lines. The present invention particularly relates to an apparatus for optical time domain reflectometry on multi-mode fibers that detects the position of breaks in multi-mode optical fibers or measures the optical loss at connections or splices. The invention also relates to a light source section of an apparatus for optical time domain reflectometry on multi-mode optical fibers (which is hereunder referred to as an “apparatus for OTDR”) and a process for producing the light source section.
FIG. 3
shows the construction of an apparatus for OTDR. As shown, the apparatus comprises a light source section
10
which outputs optical pulses, a light-receiving section
26
for receiving the back scattering and Fresnel reflection of optical pulses that are output from the light source section
10
to a multi-mode optical fiber
24
under analysis, a processing section
28
for processing an electric signal that is output from the light-receiving section
26
, and a display section
30
for presenting the result of processing with the processing section
28
. These components make up a measurement system which is connected by a coupler
20
to a dummy fiber
22
having the multi-mode optical fiber
24
connected at an end.
In the arrangement described above, optical pulses output from the light source section
10
are passed through the coupler
20
to be launched into the dummy fiber
22
. At the same time, the back scattered light and Fresnel reflected light that are produced in the dummy fiber
22
and the multi-mode optical fiber
24
are passed through the coupler
20
to be received by the light-receiving section
26
. Upon receiving the back scattered light and Fresnel reflected light, the light-receiving section
26
outputs electric signals which are averaged in the processing section
28
and thereafter sent to the display section
30
for image display.
The structure of the light source section of a conventional apparatus for OTDR is shown in FIG.
4
. As shown, the light source section consists of a laser diode
11
, a lens
12
and a multi-mode optical fiber
13
. The multi-mode optical fiber
13
is selectable from two types which are typically 50 GI (with a core diameter of 50 &mgr;m) and 62.5 GI (with a core diameter of 62.5 &mgr;m). The laser diode
11
and the lens
12
are arranged such that the central axis
14
of the former aligns with the optical axis
15
of the latter. And, as shown in
FIG. 5
(which is a partial enlarged view of FIG.
4
), the lens
12
is coupled and fixed to the multi-mode optical fiber
13
in such a way that the central axis
16
of the latter coincides with the axis of the alignment. Indicated by
131
in
FIG. 5
is the core of the optical fiber
13
and the reference numeral
132
represents the cladding of the same optical fiber
13
.
In the light source section of the above-described conventional apparatus for OTDR, oscillated light from the laser diode is focused by the lens and launched into the multi-mode optical fiber. The laser diode, the lens and the multi-mode optical fiber are coupled and fixed as they are arranged such that the central axes of the laser diode and the multi-mode optical fiber align with the optical axis of the lens. It should be noted here that this arrangement may potentially introduce distortions in the waveforms of measurement with the apparatus for OTDR. To give an example, bubbles may be entrapped in the center of the core of a multi-mode optical fiber during manufacture and if this problem occurs, a measured optical loss is sometimes greater than what occurs to the same multi-mode optical fiber from the use of a light source section in steady-stated excitation.
The results of OTDR conducted on a multi-mode optical fiber using the light source section of the conventional apparatus for OTDR are described below with reference to FIG.
6
. The horizontal axis of the graph in
FIG. 6
plots the distance from the end face of the coupler
20
to the dummy fiber
22
and the vertical axis plots the reflection signal level. As the distance from the input end of the dummy fiber
22
to the light source section
10
(see
FIG. 3
) increases, the power of the incident light attenuates and the reflection signal level of the backscatter occurring in the dummy fiber
22
decreases progressively as indicated by a waveform
31
.
If the optical pulses passing through the dummy fiber
22
reach the connector CN
2
, part of them is reflected by the connector C
2
to produce a Fresnel reflection waveform
32
and the remaining pulses pass through the connector CN
2
to be launched into the multi-mode optical fiber
24
under analysis, thereby producing back scattered light. Due to the losses inherent in the optical fiber, the reflection signal level of this back scattered light decreases progressively as indicated by a waveform
41
. If the multi-mode fiber
13
in the light source section
10
is not kept excited in a steady state, the waveform from the multi-mode optical fiber
24
under analysis becomes distorted. Fresnel reflection occurs at the far end of the optical fiber
24
and this is observed as a waveform
42
.
In order to ensure that the light source section
10
of the apparatus for OTDR is brought to steady-state excitation, the oscillated light from the laser diode
11
must spread to be wider than the numerical aperture (NA) of the multi-mode optical fiber. To this end, the oscillated light is focused with a lens having the same NA as the multi-mode optical fiber so that it falls at the central axis of the multi-mode optical fiber. However, if the light issued from the laser diode is allowed to fall at the central axis of the multi-mode optical fiber, it is not excited in a steady state since the spot diameter of the beam cannot be increased (the incident light is prone to propagate near the central axis of the multi-mode optical fiber since it has a comparatively high refractive index).
To achieve steady-state excitation of the multi-mode optical fiber without having this problem, a dummy fiber extending over a distance of several kilometers is connected upstream of the multi-mode optical fiber under analysis and, at the same time, a cylindrical lens must be added to the focusing lens to produce a circular cross section of the laser beam. However, this simply increases the complexity of the optical system in the light source section of the apparatus for OTDR.
Another way to bring the multi-mode optical fiber
24
to steady-state excitation is by using a light-emitting diode. However, a light-emitting diode outputs such a small emission level that if it is used on the apparatus for OTDR, the required dynamic range cannot be assured, making it difficult to perform measurements on an optical fiber spanning a long distance.
If a laser diode is used as a means of causing steady-state excitation, the optical system in the light source section becomes unduly complicated as already mentioned above. To get around this difficulty, a high-power laser diode must be selected but then the overall construction of the light source in the apparatus for OTDR on multi-mode optical fibers becomes complicated, leading to higher manufacturing cost.
SUMMARY OF THE INVENTION
The present invention is accomplished under these circumstances and has as an object providing an apparatus for optical time domain reflectometry on multi-mode optical fibers which, in spite of its simple construction, can achieve artificial steady-state excitation of a multi-mode optical fiber and which is capable of correct measurements on the multi-mode optical fiber, along with better reproducibility of their result.
Another object of the present invention is to provide a light source section of the apparatus.
Yet another object of the present invention is to provide a process for producing the light source section.
The first object of the pres
Ando Electric Co. Ltd.
Fish & Richardson PC
Font Frank G.
Nguyen Tu T.
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