Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-01-27
2002-09-10
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S337000
Reexamination Certificate
active
06449070
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength-multiplexed optical transmission system in which optical signals having different wavelengths are multiplexed and transmitted, and more particularly to an optical transmission device applicable to such a system.
A wavelength-multiplexed optical transmission system is known as a large-capacity transmission system. The wavelength-multiplexed optical transmission system includes terminal equipment or devices receiving and transmitting optical signals, optical cross-connect devices, add/drop devices and optical amplifiers. The optical cross-connect devices perform an add and drop operation on an optical signal and a wavelength interchanging operation. The wavelength-multiplexed transmission system is required to avoid collision of optical signals having the same wavelength and to manage wavelengths used.
2. Description of the Related Art
FIG. 1
schematically illustrates a wavelength-multiplexed optical transmission system.
The system includes terminal devices
201
and
202
, an optical cross-connect device
203
, an optical amplifier
204
, and optical transmission paths
205
. The terminal devices
201
and
202
multiplex optical signals of wavelengths &lgr;
1
-&lgr;n, and transmit and receive multiplexed optical signals. The optical cross-connect device
203
is equipped with the functions of setting cross-connects from incoming paths to outgoing paths and performing the add/drop operation on the optical signals. The function of setting the cross-connects include a wavelength conversion of optical signals. The cross-connect device
203
can add or drop an optical signal having a wavelength&lgr;x. Although only one optical cross-connect device
203
is illustrated in
FIG. 1
, a plurality of optical cross-connect devices can be connected in accordance with the size of the network. An ADM (Add/Drop Module) may be used in which an optical filter is utilized to drop and add optical signals.
The optical amplifier
204
is provided in the optical cross-connect device
203
or in a repeater provided in the optical transmission paths
205
at given intervals. The optical amplifier
204
amplifies the wavelength-multiplexed optical signal as per se. For example, the optical amplifier
204
employs a rare-earth doped optical fiber, which receives a received optical signal and an exciting (pumping) light, so that the received optical signal can be amplified. The level of the amplified optical output signal can be controlled by controlling the power of the exciting light.
Generally, the optical amplifier
205
is controlled by an automatic level control (ALC) so as to obtain a constant optical output level. The automatic level control controls a current that flows in a semiconductor laser generating the exciting light. If the optical signal having a certain wavelength contained in the wavelength-multiplexed optical signal is broken, the optical signals of wavelengths other than the above certain wavelength are excessively amplified in order to obtain the constant optical output level. Hence, there is an increased possibility that the interference between the wavelengths may be increased or error in receipt due to level variations in the respective wavelengths on the receive side may be increased. In contrast, when an increased number of wavelengths is used, the levels for the wavelengths that are already used are decreased in order to maintain the optical output level at constant.
With the above in mind, the wavelength-multiplexed optical signal is demultiplexed into the respective optical signals, and monitors such as photodiodes are provided to the respective wavelengths. The monitors are used to determine whether the respective wavelengths are now in use. Hence, it is possible to determine whether each of the optical signals having the respective wavelengths has been broken down. Further, it is possible to detect a situation in which an optical signal of another wavelength is started to be used. The number of wavelengths that are in use can be obtained and thus the automatic level control of the optical amplifier
205
can be performed based on the number of wavelengths that are in use.
The optical signals are modulated by using frequencies respectively provided to the wavelengths thereof, and are multiplexed before transmission. The optical amplifier
205
is equipped with a monitor converting an optical signal into an electric signal, and filters respectively provided to the modulation frequencies. The optical signals are detected by the filters in order to detect the breakdown of an optical signal and an event in which an optical signal is started-to be used. Hence, the automatic level control of the optical amplifier
205
can be performed based on the number of wavelengths that are in use.
When the system is started to operate, the terminal devices
201
and
202
inform the optical amplifier
205
of the number of wavelengths to be used by means of control information. The optical amplifier
205
performs the automatic level control based on the number of wavelengths to be used. A method has been proposed in which if there is a wavelength which is started to be used or stopped while the system is working, the optical amplifier
205
is informed of the presence of the above wavelength by the control information. Hence, the optical amplifier
205
receives the control information and performs the automatic level control based on a change of the number of wavelengths.
However, the prior art described above has the following disadvantages. In the case where the optical amplifier
205
employs the automatic level control using the monitors for the respective wavelengths, as an increased number of wavelengths is used, the optical amplifier
205
has an increased circuit size. This increases the cost. In the case where the optical amplifier
205
employs the filters, only one monitor is required to convert the optical signal into the electric signal. However, the optical amplifier
205
requires the filters equal in number to the frequencies for modulation. This increases the circuit size and the production cost.
The arrangement is less expensive in which the terminal devices
201
and
202
inform the optical amplifier
205
of the number of wavelengths to be used by the control information. However, the optical amplifier
205
is required to have the functions of receiving and discriminating the control information, identifying the wavelengths that are in use by means of software, and performing the automatic level control based on the number of wavelengths that are in use. However, a considerable long time is required to execute the software process. Further, it is difficult to automatically change the levels at the same time as the number of wavelengths is changed. Hence, the level of the received level may instantaneously be increased or decreased. Such an abrupt change in the level of the optical signal may increasingly cause a burst error on the reception side.
In the wavelength-multiplexed optical transmission system including the optical cross-connect device
203
, a plurality of terminal devices are mutually connected via the cross-connect device
203
. Hence, it is difficult to inform all the optical amplifiers
205
of the number of wavelengths that are in use. Particularly, the arrangement of the optical cross-connect devices
203
performing the wavelength change and route switching makes it possible to ensure the stable automatic level control of the optical amplifiers
205
. If the wavelength-multiplexed optical transmission system has an increased scale and a complex configuration, different terminal devices may use optical signals of the same wavelength. In such a case, the optical signals collide with each other, and the normal transmission is no longer ensured.
In the wavelength-multiplexed optical transmission system having a plurality of terminal devices, optical signals of the same wavelength may collide with each other in the optical cross-connect device or the l
Fujitsu Limited
Negash Kinfe-Michael
Staas & Halsey , LLP
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