Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2001-11-30
2004-03-30
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06714290
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical communications system for transmitting and receiving an optical signal bidirectionally through a single common fiber-optic cable. More particularly, the present invention relates to a method for measuring the far-end reflectance of a fiber-optic cable used in a digital communications system capable of high-speed transport, such as IEEE1394 and USB2.
2. Description of the Related Art
A conventional optical communications technique using a fiber-optic cable(s) will be described.
FIGS. 6A and 6B
are schematic diagrams for explaining a one-directional communications method using a fiber-optic cable.
In the one-directional communications method shown in
FIG. 6A
, information is transported as an optical signal from a first transceiver
1
to a second transceiver
2
via a fiber-optic cable
3
. A transmitter
1
a
of the first transceiver
1
and a receiver
2
b
of the second transceiver
2
are connected to the fiber-optic cable
3
.
Conversely, when information is transported as an optical signal from the second transceiver
2
to the first transceiver
1
via a fiber-optic cable
3
, as shown in
FIG. 6B
, a transmitter
2
a
of the second transceiver
2
and a receiver
1
b
of the first transceiver
1
are connected to the fiber-optic cable
3
.
Note that when an optical signal is transported only from the first transceiver
1
to the second transceiver
2
, the receiver
1
b
of the first transceiver
1
and the transmitter
2
a
of the second transceiver become unnecessary. Conversely, when an optical signal is transported only from the second transceiver
2
to the first transceiver
1
, the receiver
2
b
of the second transceiver
2
and the transmitter
1
a
of the first transceiver
1
become unnecessary.
To achieve both optical signal transport from the first transceiver
1
to the second transceiver
2
, and optical signal transport from the second transceiver
2
to the first transceiver
1
, as shown in
FIG. 6C
, the transmitter
1
a
of the first transceiver
1
and the receiver
2
b
of the second transceiver
2
are connected to each other via a single fiber-optic cable
3
while the transmitter
2
a
of the second transceiver
2
and the receiver
1
b
of the first transceiver
1
are connected to each other via another single fiber-optic cable
4
.
Therefore, in the conventional one-directional optical communications method, two fiber-optic cables are required for full-duplex communications in which an optical signal can be transmitted and received bidirectionally between a pair of transceivers.
Hereinafter, a full-duplex communications method capable of transmitting and receiving an optical signal through a single fiber-optic cable will be described.
FIG. 7A
is a schematic diagram for explaining a method for transmitting and receiving an optical signal bidirectionally through a single fiber-optic cable.
In this conventional bidirectional communications method, a single fiber-optic cable
13
is connected to a first transceiver
11
and a second transceiver
12
.
The transceivers
11
and
12
comprise connectors
11
a
and
12
a
, respectively. A plug (not shown) is provided at each of end faces
13
a
and
13
b
of the fiber-optic cable
13
, and is connected to each of the connectors
11
a
and
12
a
of the respective transceivers
11
and
12
.
FIG. 7B
is a schematic diagram showing the connectors
11
a
and
12
a
of the respective transceivers
11
and
12
and the end faces
13
a
and
13
b
of the fiber-optic cable
13
.
An optical signal is transmitted from the first transceiver
11
to the second transceiver
12
in the following manner. The optical signal is applied from the transmitter
11
b
of the transceiver
11
via the connector
11
a
to the end face
13
a
of the fiber-optic cable
13
. This optical signal is introduced into the fiber-optic cable
13
and transmitted to the second transceiver
12
. The optical signal is applied from the end face
13
b
of the fiber-optic cable
13
connected to the connector
12
a
of the second transceiver
12
to the receiver
12
c
of the second transceiver
12
.
Similarly, when an optical signal is transmitted from the second transceiver
12
to the first transceiver
11
, the optical signal transmitted via the fiber-optic cable
13
from the transmitter
12
b
of the second transceiver
12
is applied to the receiver
11
c
of the first transceiver
11
.
In this case, for example, an optical signal emitted by the transmitter
11
b
of the first transceiver
11
is transmitted through the fiber-optic cable
13
to reach the receiver
12
c
of the second transceiver
12
. In this case, however, part of the optical signal is reflected by the end faces
13
a
and
13
b
of the fiber-optic cable
13
.
FIGS. 8A and 8B
are schematic diagrams for explaining the reflection of an optical signal by the end faces
13
a
and
13
b
of the fiber-optic cable
13
.
As shown in
FIG. 8A
, the end faces
13
a
and
13
b
of the fiber-optic cable
13
are connected to the connectors
11
a
and
12
a
of the first and second transceivers
11
and
12
, respectively. When an optical signal is transmitted from the first transceiver
11
to the second transceiver
12
, as indicated by arrows C shown in
FIG. 8B
, a part of an optical signal incidnet to the fiber-optic cable
13
is reflected by the end face
13
a
(near-end reflection), and as indicated by arrows D shown in
FIG. 8B
, a part of an optical signal outgoing from the fiber-optic cable
13
is reflected by the end face
13
b
(far-end reflection). The optical signals reflected by the near-end face
13
a
and the far-end face
13
b
of the fiber-optic cable
13
are transported along with the original optical signal which is transmitted from the transmitter
12
b
of the second transceiver
12
to the receiver
11
c
of the first transceiver
11
. In this case, the reflected optical signal presents noise on the optical signal.
Therefore, it is important to measure how much an optical signal is reduced by the near- and far-end reflections.
A far-end reflectance representing a reduction in an optical signal due to far-end reflection is calculated, for example, in the following manner. An optical signal is emitted from the end face
13
b
of the fiber-optic cable
13
into air, allowing far-end reflection. The amount of light of the optical signal received by the receiver
12
c
is measured. On the other hand, the end face
13
b
of the fiber-optic cable
13
is immersed in a liquid matching oil having the same refractive index as that of the core of the fiber-optic cable
13
so that far-end reflection does not occur at the end face
13
b
of the fiber-optic cable
13
. In this situation, the amount of light of an optical signal received by the receiver
12
c
is measured. The far-end reflectance is calculated based on the two measured amounts of light of optical signals.
A plug or the like is attached to an end of the fiber-optic cable
13
, which is connected to a connector. When the end face
13
b
of the fiber-optic cable
13
is immersed in a liquid matching oil, the matching oil is likely to penetrate between the plug and a core of the fiber-optic cable
13
. Therefore, when a number of fiber-optic cables
13
are measured for far-end reflectance, the matching oil has to be removed from the end face
13
b
of each fiber-optic cable
13
, whereby the working efficiency is reduced.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, a method for measuring a far-end reflectance of a fiber-optic cable, comprises the steps of connecting an end face of the fiber-optic cable to a transceiver comprising a transmitter for transmitting an optical signal and a receiver for receiving an optical signal, transmitting an optical signal from the transmitter of the transceiver and receiving the optical signal reflected by the other end face of the fiber-optic cable, and measuring a first amount of light of the reflected optical signal, where the other
Nguyen Tu T.
Sharp Kabushiki Kaisha
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