Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-09-25
2003-09-16
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
Reexamination Certificate
active
06621620
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplification system using Raman amplification.
2. Description of the Related Arts
As a high-capacity technology, attention is being given to a wavelength division multiplexing (WDM: Wavelength Division Multiplexing) technology that allows a multiplicity of optical signals having different wavelengths to be transmitted through a single optical fiber. Moreover, in a WDM transmission system with the use of such a wavelength division multiplexing technology, a Raman amplification has been introducing to extend one span (the opposed node-to-node) interval longer.
Here, the Raman amplification means a technology to amplify signal lights, using an induction Raman diffusion phenomenon, one of non-linear effects within an optical fiber. When a signal light and an excitation light having shorter wavelength than that of the signal light for its specific Raman shift amount are inputted into an optical fiber, the signal light will be amplified within the optical fiber.
In other words, the excitation light having shorter wavelength than that of the signal light for its specific Raman shift amount allows a dipole to be generated within the optical fiber. And, optical amplification will be carried out, when a light whose wavelength is the same as that of the signal light is radiated, while returning to the normal order, after the energy is lost for the specific number of oscillations of the excitation light, with the signal light passing through.
FIGS. 1A
to
1
C show examples of a system with the use of such a Raman amplification.
FIGS. 1A
to
1
C also show optical amplifiers (optical AMPS)
1
and
2
to be placed in the optical amplification system at each node (including the terminal station node and the repeater node) of one span-to-one span connected with a transmission path
3
.
Generally, the optical amplifiers
1
and
2
are classified into a post amplifier that serves as a power amplifier when being placed at a transmitter of the terminal station node, and a preamplifier that amplifies weak signals when being placed at a receiver of the terminal station node. Moreover, when the optical amplifier is placed at the middle point of a circuit, in short, at a repeater node, it is classified as an in-line amplifier.
In a system shown in
FIG. 1A
, a Raman excitation light source (LS)
4
is placed on the side of the optical amplifier
1
, and the excitation light is sent out to an optical transmission path
3
in the same direction as the propagation direction of the main signal light. This mode is called a forward excitation. In a system shown in
FIG. 1B
, a Raman excitation light source
5
is placed on the side of the optical amplifier
2
, and a Raman excitation light is sent out to the optical transmission path
3
in the reverse direction of the propagation direction of the main signal light. This mode is called a backward excitation. In addition, in a system shown in
FIG. 1C
, the Raman excitation light sources
4
and
5
are placed on the side of the optical amplifiers
1
or on the side of the amplifier
2
, respectively, and the Raman excitation light is sent out to the optical transmission path
3
in the same or in the reverse direction of the propagation direction of the main signal light. This mode is called a two-way excitation.
Here, out of the three excitation modes, in the backward excitation (
FIG. 1B
) and the two-way excitation (FIG.
1
C), the Raman excitation light is radiated in, in the reverse direction of the propagation direction of the main signal light. In such a case, at the upstream optical amplifier
1
, the Raman excitation light will be radiated in from its output side.
Usually, on an optical amplifier, a laser ray having a power of 0 dBm through +20 dBm will be outputted to the optical transmission path
3
to be connected from the output area through a connector. Due to this reason, in consideration of the safety of a person who handles the device, the optical amplifier has a function (Laser Safety function) to reduce the output of the optical amplifier, detecting a Fresnel reflection at the location where the connector coming out of the output area of the optical amplifier.
However, as described above, in the backward excitation (
FIG. 1B
) and the two-way excitation (FIG.
1
C), the Raman excitation light will be radiated in from the output side of the optical amplifier
1
. Therefore, even if the connector is properly seated in place, due to the input of the Raman excitation light from the downstream, on the optical amplifier, a control function will be ON to reduce the output of the optical amplifier, misrecognizing the inputted light as a Fresnel reflection light to be generated if the connector is removed. In order to avoid such a failure, in normal operation, the Laser Safety function will be masked (stopped).
The masking treatment of the Laser Safety Function in such a normal operation will be controlled as follows, using the opposed circuit (transmission path).
FIG. 2
illustrates the masking treatment of the Laser Safety Function, and in this drawing, opposing optical amplification systems A and B are connected with the optical transmission path (OTP)
3
having opposing transmission paths
30
and
31
.
Assuming that the optical amplification systems A and B would be the end station nodes, optical amplifiers
1
and
10
correspond to the post amplifiers, and optical amplifiers
2
and
20
correspond to the preamplifiers. Also, if the optical amplification systems A and B would be the relay nodes, optical amplifiers
1
,
2
,
10
and
20
correspond to the in-line amplifiers.
The optical amplification systems A and B are connected with the transmission path
30
in the downward direction (direction from the optical amplifier
1
to the optical amplifier
2
) and the transmission path
31
in the upward direction (direction from the optical amplifier
10
to the optical amplifier
20
). The masking treatment of the Laser Safety Function will be controlled in the following procedure.
S
1
: If there is no trouble in the optical transmission path
3
(downward direction transmission path
30
), the smooth passage of signals will be checked on the side of the downstream optical amplifier
2
.
S
2
: The result of checking the smooth passage of signals will be transmitted to the optical amplifier
10
on the side of the transmission path
31
in the upward direction opposing to the optical amplification system B.
S
3
: In addition, from the optical amplifier
10
, the check information will be sent to the optical amplifier
20
on the downstream side of the optical amplification system A through the upward direction transmission path
31
, with the use of SV (monitor) signals.
S
4
: When the optical amplifier
20
for the optical amplification system A receives the check information, the information will be transmitted to the optical amplifier
1
on the side of the opposed circuit.
S
5
: By this procedure, to the optical amplifier
1
, control is carried out to mask the Laser Safety function.
Moreover, on the systems as illustrated in
FIG. 3
, the following shows examinations on emergency procedures when the opposing transmission path
31
is in trouble.
S
10
: When the opposing transmission path
31
is OFF, S
11
: At the optical amplifier
20
for the optical amplification system A, OFF state of the SV (monitor) signal will be detected.
S
12
: The information that the SV (monitor) signal is in the OFF state will be transmitted to the side of the optical amplifier
1
.
S
13
: Thus, the masking treatment of the Laser Safety Function of the optical amplifier
1
will be released, and the optical amplifier
1
will be controlled to continuously send out laser lights. However, in such a case, by releasing the masking treatment of the Laser Safety Function, the optical amplifier
1
will regard the Raman excitation light from the downstream side (side of the optical amplifier
2
) as the reflection light to be generated when the connector is
Black Thomas G.
Fujitsu Limited
Hughes Deandra M.
Staas & Halsey , LLP
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