Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-01-27
2001-09-18
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S199200
Reexamination Certificate
active
06292289
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method, device, and system for transmitting a supervisory optical signal.
2. Description of the Related Art
In recent years, information demand has rapidly increased in concert with the development in multimedia network, and an arterial optical transmission system integrating an information capacity is demanded to further increase a transmission capacity and form a flexible network. At present, wavelength division multiplexing (WDM) is the most effective technique responding to such a system demand, and the commercialization of WDM is now being pursued actively mainly in North America. A method of supervising a system adopting WDM is an indispensable function in operating the system. The present invention provides such a supervising method and its associated device suitable for a system adopting WDM.
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put into practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for amplifying an optical signal or signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and a pumping unit for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) has been developed to amplify signal light having a wavelength band of 1.55 &mgr;m where the loss in a silica fiber is minimum. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 &mgr;m band or a 1.48 &mgr;m band, a gain band including a wavelength band of 1.55 &mgr;m can be obtained.
Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain main signal light (WDM signal light), which is output to an optical fiber transmission line. At a receiving end, the main signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data (a main signal) is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity by a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain of the optical amplifier which characteristic is represented by a gain deviation or gain tilt. For example, in a typical EDFA, it is known that a gain deviation is produced at wavelengths near 1.55 &mgr;m. If the gain deviations in a plurality of cascaded EDFAs are accumulated, an optical SNR (signal-to-noise ratio) in a channel included in a low-gain band is degraded. Accordingly, to allow high-quality transmission, it is preferable to flatten the wavelength characteristic of gain of an optical amplifier.
On the other hand, to increase the number of WDM channels, it is effective to broaden the bandwidth of a gain band (a band where gain is generated) of an optical amplifier. For the purpose of such bandwidth broadening, there has been proposed a device including two optical amplifiers arranged in parallel, wherein one of the two optical amplifiers amplifies optical signals having wavelengths in a shorter-wavelength band and the other optical amplifier amplifies optical signals having wavelengths in a longer-wavelength band.
In any case, one optical amplifier or two optical amplifiers connected in parallel is/are provided in an optical repeater, and a plurality of such optical repeaters are arranged along an optical fiber transmission line.
To perform supervisory control of each optical repeater and the other system components, a supervisory optical signal is transmitted. For example, a supervisory optical signal for providing information on the number of WDM channels in operation is transmitted from a transmitting end to each optical repeater. In each optical repeater, a reference level to be used in ALC (automatic output level control) for maintaining an output level per channel constant is set according to the supervisory optical signal received. Some conventional methods for transmitting a supervisory optical signal will now be described with reference to
FIGS. 1
to
3
.
FIG. 1
is a block diagram of a conventional system for transmitting a supervisory optical signal. This system includes a first terminal device
2
as a transmitting end, a second terminal device
4
as a receiving end, an optical fiber transmission line
6
placed between the terminal devices
2
and
4
, and a plurality of optical repeaters
8
arranged along the optical fiber transmission line
6
.
The terminal device
2
includes an optical transmitter (TX)
10
for outputting an optical signal of one channel or WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths as main signal light, and an optical supervisory circuit (OSC)
12
for outputting a supervisory optical signal. It is now assumed that the optical transmitter
10
outputs main signal light obtained by WDM in a conventional band (referred to as “1.55 &mgr;m band” or “C band”) defined by the range of 1.53 to 1.56 &mgr;m, provided that WDM is applied to this system. The main signal light output from the optical transmitter
10
and the supervisory optical signal output from the optical supervisory circuit
12
are multiplexed by a wavelength coupler
14
and then supplied to the optical fiber transmission line
6
.
Each optical repeater
8
includes an optical amplifier
16
for amplifying the main signal light. The main signal light and the supervisory optical signal transmitted by the optical fiber transmission line
6
are separated from each other by a wavelength coupler
18
. The main signal light from the wavelength coupler
18
is supplied to the optical amplifier
16
, and the supervisory optical signal from the wavelength coupler
18
is supplied to an optical supervisory circuit
20
. The optical supervisory circuit
20
regenerates a supervisory signal as an electrical signal according to the supervisory optical signal received to perform processing based on the supervisory signal and update the supervisory signal. The supervisory signal updated is converted into a supervisory optical signal, which is in turn supplied to a wavelength coupler
22
. In the wavelength coupler
22
, the supervisory optical signal updated by the optical supervisory circuit
20
and the main signal light amplified by the optical amplifier
16
are multiplexed to be output to the optical fiber transmission line
6
. The processing based on the supervisory signal to be performed in the optical supervisory circuit
20
includes setting of a reference level to be used in ALC in the optical amplifier
16
according to the supervisory signal or updating the supervisory signal according to a monitored value of the gain of the optical amplifier
16
, for example.
The second terminal device
4
includes a wavelength coupler
24
for demultiplexing
Chikama Terumi
Sugaya Yasushi
Fujitsu Limited
Hellner Mark
Staas & Halsey , LLP
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