Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1998-02-18
2001-04-03
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C359S341430, C359S199200, C359S199200
Reexamination Certificate
active
06212311
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light signal transmission system employing an optical fiber, and in particular to a light signal transmission system including a rare-earth doped optical fiber amplifier controlled from a remote area. The rare-earth metal is, for example, Erbium.
2. Related Arts
A light signal transmission system is available for which an EDFA (Erbium Doped Optical Fiber Amplifier) is provided to facilitate the transmission of light signals through an optical fiber. In particular, a repeaterless light transmission system has recently become available in which excitation light produced by a excitation light source at a terminal station is supplied, by remote control, to an EDF (Erbium Doped Optical Fiber), that is optical amplification fiber, inserted into an optical fiber transmission path.
An example arrangement for a light signal transmission system using EDFAs is shown in FIG.
6
. In
FIG. 6
, light signals are transmitted along an optical fiber
300
into which EDFs
310
a
and
310
b
are inserted. Excitation light is supplied by excitation light sources
200
a
and
200
b,
which are semiconductor laser light sources, and light signals are amplified in proportion to the excitation level of the excitation light. For light excitation, the excitation light source
200
a
is connected by an optical fiber
320
a
to a optical coupler
400
a
and the optical fiber
300
, so that excitation light to excite the EDF
310
a
is transmitted along the optical fiber
300
in the same direction as light signals (this is called forward excitation); and the excitation light source
200
b
is connected by an optical fiber
320
b
to a optical coupler
400
b
and the optical fiber
300
, so that excitation light to excite the EDF
310
b
is transmitted along the optical fiber
300
in the direction opposite to the light signal transmission direction (this is called backward excitation). An optical isolator
330
is located at the position shown in FIG.
6
.
In the repeaterless light transmission system, the excitation light sources
200
a
and
200
b
are disposed at remote locations away from the EDFs
310
a
and
310
b
such as inside the two terminal stations connected at either end of the optical fiber
300
. The terminal stations control the respective excitation light sources
200
a
and
200
b
to excite the EDFs
310
a
and
310
b
in order to amplify light signals. Through this process, a repeaterless light transmission system is accomplished which does not require an optical amplification repeater device for repeating light through the optical fiber
300
which connects the terminal stations.
For such a repeaterless light transmission system, in the design stage the EDFs
310
a
and
310
b
are inserted into predetermined locations along the optical fiber
300
having a predetermined length, and the excitation light sources
200
a
and
200
b
output excitation light having a predetermined energy.
In an optical submarine communication system for which the repeaterless light transmission system is employed, when the optical fiber
300
laid on the seabed is cut for some reason, the cut portions must be pulled up to the sea surface and a new optical fiber inserted between them (this is hereinafter referred to as an “insertion”).
FIGS. 7A and 7B
are specific diagrams illustrating an insertion. In
FIGS. 7A and 7B
, the optical fiber
300
is provided on the seabed at a depth h. When the optical fiber is cut at the portion indicated by the x, as is shown in
FIG. 7A
, the cut portions are pulled up to the sea surface, and a new optical fiber
340
is used to join the cut portions together, as is shown in FIG.
7
B. At this time, for the optical fiber
340
a length of about 2.5 h is required, while taking into account a slight margin when the optical fiber is again deposited on the seabed.
When this insertion is performed, the length of the optical fiber between the two terminal stations is extended and is longer than the original designed length, and a transmission loss increase proportional to the length of the added portion of the optical fiber is incurred. Therefore, to compensate for the transmission loss, the signal light amplification rates for the EDFs
310
a
and
310
b
must be increased, and the excitation levels for the excitation light sources
200
a
and
200
b
must be manually re-adjusted.
SUMMARY OF THE INVENTION
It is, therefore, one object of the present invention to provide a light signal transmission system and a light signal transmission method for automatically adjusting the excitation levels of excitation light sources when an optical fiber transmission loss along a transmission path is changed due to an insertion.
To achieve the above object, a first arrangement for a light signal transmission system according to the present invention comprises:
a first optical fiber for transmitting a light signal sent from a first terminal station to a second terminal station;
a second optical fiber for transmitting a light signal sent from said second terminal station to said first terminal station;
a first optical amplification fiber, inserted along the route of said first optical fiber;
a second optical amplification fiber, inserted along the route of said second optical fiber;
a first excitation light source, being located away from said first optical amplification fiber, for supplying excitation light to said first optical amplification fiber;
a second excitation light source, being located away from said second optical amplification fiber, for supplying excitation light to said second optical amplification fiber;
first control means for controlling a first excitation level of said excitation light output from said first excitation light source in accordance with a difference between a first reception level of said light signal received by said first terminal station and a second reception level of said light signal received by said second terminal station; and
second control means for controlling a second excitation level of said excitation light output from said second excitation light source in accordance with said difference between said first reception level and said second reception level.
In addition to the first arrangement, a second arrangement for a light signal transmission system according to the present invention comprises:
first detection means for detecting said first reception level;
second detection means for detecting said second reception level; and
transmission means for transmitting said first and said second reception levels to said second and said first terminal stations respectively,
wherein said first and said second control means each compare said first reception level with said second reception level.
With the above described arrangement, the excitation level of the excitation light output by the excitation light sources is automatically adjusted in accordance with a difference in the reception levels at the first and the second terminal stations.
When, for example, the reception level for the light signal received by the first terminal station is lower than the reception level for the light signal received by the second terminal station, the second control means increases the excitation level for the second excitation light source; and when the reception level for the light signal received by the second terminal station is lower than the reception level for the light signal received by the first terminal station, the first control means increases the excitation level for the first excitation light source.
In addition, the first or second control means may control the excitation level of the first or second excitation light source, so that the reception levels of the light signals received by the first and the second terminal stations are equal.
Furthermore, to achieve the above object of the present invention, provided is a method for transmitting light signals from a first terminal station to a second terminal station through at least one pair of optical fibers, bot
Fujitsu Limited
Kang Juliana K.
Lee John D.
Staas & Halsey , LLP
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