Optical communications – Transmitter and receiver system – Plural stations
Reexamination Certificate
2002-08-09
2004-11-09
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter and receiver system
Plural stations
C398S011000, C398S018000, C398S177000
Reexamination Certificate
active
06816683
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a supervisory system for monitoring an optical amplification repeater unit in an optical communications network to which wavelength division multiplexing (WDM) technology is applied.
2. Description of the Related Art
Recently, an optical communications network operated using the wavelength division multiplexing technology (WDM) has been frequently taken up in many studies and developments. In the optical communications network, a large capacity data transmission can be realized using a wavelength-multiplexed optical signal.
On the other hand, a fault in an optical amplification repeater unit is a serious problem in the optical communications network. Therefore, an optical transmission system using the optical amplification repeater unit requires a unit for monitoring the state of the optical amplification repeater unit.
A state of the optical amplification repeater unit to be monitored refers to an optical input/output power, an exciting power, the temperature of an exciting laser diode (LD), a bias current of the exciting LD, etc.
A conventional optical transmission system has a point-to-point type transmission line in which a terminal station A is connected to another terminal station B one to one through a plurality of optical repeater amplifiers REP
1
through REP
4
as a point-to-point type transmission line as shown in FIG.
1
. The system normally includes at least one optical wavelength multiplexing/demultiplexing unit; a plurality of terminal stations connected to at least one optical wavelength multiplexing/demultiplexing unit through an optical transmission line; and a plurality of optical repeater units positioned in the optical transmission line, wherein the second terminal station receives a supervisory signal (SV signal) transmitted from the first terminal station of the plurality of terminal stations to the optical repeater unit through the optical wavelength multiplexing/demultiplexing unit, and the received supervisory signal is transferred to the third terminal station through the optical wavelength multiplexing/demultiplexing unit. Otherwise, the system includes at lease one optical wavelength multiplexing/demultiplexing unit; a plurality of terminal stations connected to at least one optical wavelength multiplexing/demultiplexing unit through an optical transmission line; and a plurality of optical repeater units positioned in the optical transmission line, wherein the second terminal station receives a supervisory signal (SV signal) transmitted from the first terminal station of the plurality of terminal stations to the optical repeater unit through the optical wavelength multiplexing/demultiplexing unit; the received supervisory signal is transferred to the third terminal station through the optical wavelength multiplexing/demultiplexing unit.
In such a supervisory system in the above described optical transmission system, a supervisory signal (a command signal to an optical amplification repeater unit) is transmitted from the terminal station A to the optical amplification repeater unit REP
1
. In such a conventional 1-channel communications (1-wave transmission), the optical amplification repeater unit REP
1
which receives the supervisory signal performs control corresponding to the received supervisory signal, and transmits a response signal containing the state information of the optical amplification repeater unit to the adjacent optical amplification repeater unit REP
2
for the supervisory signal to be sent to the terminal station B. Thus, the supervisory signals are sequentially transmitted.
A system for transmitting a supervisory signal and a response signal to an optical amplification repeater unit can be a superposing system for superposing the supervisory signal (command signal for controlling the optical amplification repeater unit) into a data signal (main signal) to be transmitted, or a wavelength division multiplexing system for wavelength-multiplexing the supervisory signal having a wavelength different from that of the main signal.
FIGS. 2A through 2C
show an example of the operation of the optical amplification repeater unit in the superposing system. As shown in
FIG. 2A
, for example, the main signal M of 2.5 through 20 Gb/s is modulated by a supervisory signal SV of 1 through several tens MHz to generate a superposing signal, and transmitted to the optical fiber transmission line OP.
The optical amplification repeater unit branches an optical signal obtained by modulating the intensity of the main signal M using the supervisory signal SV, by the branch unit BR as shown in FIG.
2
B. The branched optical signal is converted into an electrical signal by the optical receiving element PD such as a photo-diode, etc.
Only the supervisory signal SV is extracted through the filter FIL from among the electrical signals converted by an optical receiving element PD, and led to the control circuit CONT. The control circuit CONT controls an optical amplification repeater unit according to the extracted supervisory signal SV, and outputs a response signal SV′ having a predetermined frequency corresponding to the state of the optical amplification repeater unit. This response signal SV′ drives an exciting laser diode LD.
The gain of the optical fiber amplifier AMP is proportional to an exciting power. Therefore, the exciting power proportional to the response signal SV′ is output from the exciting laser diode LD, and the intensity of the optical signal input to the optical fiber amplifier AMP is modulated in proportion to the response signal SV′. Through this operation, the response signal SV′ from the optical amplification repeater unit can be superposed on the main signal M input as shown in
FIG. 2C
, and transmitted to the next optical amplification repeater unit from the optical amplification repeater.
On the other hand, various developments have recently been made to realize wavelength division multiplexing communications, and it is considered that the wavelength division multiplexing system (WDM system) will also be adopted in the future optical communications network. In WDM transmission, a plurality of channels can be transmitted through a single fiber. Therefore, a plurality of terminal stations can be connected by demultiplexing and multiplexing an optical signal.
In 1-wave transmission, an amplitude modulating method is used when an SV signal is transmitted. However, in the WDM transmission in which a plurality of terminal stations are connected, it becomes difficult to perform supervisory operation in the above described method on the following grounds.
(1) The modulation rate of the SV signal becomes smaller because of a lack of channels of a main signal produced by multiplexing/demultiplexing the main signal on which the SV signal is amplitude-modulated, thereby disabling the reception at a repeater unit.
(2) In multiple-repeater transmission, accumulated noise lights make the ratio of the power of a signal light to the entire optical power smaller. As a result, the modulation rate of the SV signal superposed on the signal light naturally becomes smaller.
(3) The propagation time of a signal light depends on the wavelength through the wavelength dispersion of a fiber. Therefore, the SV signal superposed on each signal light deviates in phase, thereby lowering the modulation rate.
(4) If the modulation rate in transmission from a terminal station is set to a high level in consideration of the modulation rate lowered in (1), (2), and (3) above, then the modulation of the SV signal affects the main signal.
FIGS. 3A and 3B
show an example of an operation performed by the optical amplification repeater unit in the system in which the supervisory signal SV having a different wavelength from that of the main signal M is wavelength-multiplexed.
FIG. 3A
shows an optical spectrum and shows that the supervisory signal SV having a different wavelength from that of the main signal M is wavelength-multiplexed.
As shown i
Harasawa Shin-ichirou
Osaka Takeo
Terahara Takafumi
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