Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2002-06-13
2003-11-11
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S337000
Reexamination Certificate
active
06646792
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light amplifier and a light transmission system using the same, and more specifically to a light amplifier for a wavelength division multiplexing optical signal, so configured to control an amplification level in accordance with the result of the comparison of a predetermined level with the level of a signal partially branched from an amplified optical signal, and a light transmission system using the same.
In a wavelength division multiplexing (abbreviated to “WDM”) transmission system using an optical fiber amplifier, it is necessary to stably transmit a wavelength division multiplexing optical signal over a long distance. Generally, each optical fiber amplifier used in the wavelength division multiplexing transmission system is configured to output an optical signal having a constant level for each wavelength.
In the conventional optical fiber amplifier used in the wavelength division multiplexing transmission system, the amplification level of the light amplifier is controlled in such a manner that a portion of the output signal of the light amplifier is branched so that the level of the optical output signal is detected by a photo diode (abbreviated to “PD”), and an automatic power control (abbreviated to “APC”) is carried out to maintain the level of the optical output signal at a constant.
The construction of the conventional optical fiber amplifier mentioned above is illustrated in FIG.
4
. In
FIG. 1
, the reference number
1
designates an optical amplifier, and the reference number
2
indicates an optical divider/splitter. The reference number
6
denotes a photo diode, and the reference number
7
shows a comparator. The reference number
8
denotes a reference level generator, and the reference number
13
indicates an exciting light source. In addition, the reference number
10
denotes an optical signal input terminal, and the reference number
11
shows an optical signal output terminal. The reference number
12
designates a wavelength number information input terminal, which receives an actually transmitted wavelength number information through a monitoring and controlling line from a not-shown terminal station. Incidentally, the optical amplifier
1
is an erbium doped fiber amplifier (abbreviated to “EDFA”).
In the optical fiber amplifier shown in
FIG. 4
, the wavelength division multiplexing optical signal inputted to the optical signal input terminal
10
is amplified to a predetermined level by action of the optical amplifier
1
, and then, a portion of the optical output signal of the optical amplifier
1
is branched by action of the optical divider/splitter
2
, and the level of the optical output signal is detected by the photo diode
6
. The comparator
7
compares the detected level with a reference level which is generated in the reference level generator
8
in accordance with the wavelength number of the wavelength division multiplexing optical signal supplied from the wavelength number information input terminal
12
. On the basis of the result of this comparison, the level of the exciting light generated by the exciting light source
13
is controlled, with the result that the APC control is carried out to maintain the level of the optical output signal of the optical amplifier
1
at a constant, and therefore, the amplification level of the light amplifier is controlled.
In the conventional optical fiber amplifier, namely, since the level of the optical output signal outputted from the light amplifier is different dependently upon the wavelength number of a multiplexed optical signal, the transmitted wavelength number information is notified to the reference level generator
8
, and the detected level of the optical output signal is compared with the reference level corresponding to the wavelength number by action of the comparator
7
, so that level of the optical output signal per one wavelength is controlled to be maintained at a constant.
In addition, Japanese Patent Application Pre-examination Publication No. JP-A-10-51388 discloses another conventional light amplification device. This is so configured that a portion of the optical output signal of the light amplifier is branched into two paths, in one of which the level of the branched optical output signal is measured, and in the other of which the level of an optical signal (amplified spontaneous emission, abbreviated to “ASE”) obtained by removing a desired wavelength component from the optical output signal, is measured. Further, the amplification level of the light amplifier is controlled on the basis of a control signal (a signal of the desired wavelength component) obtained by subtracting the signal level of the other branched path from the signal level of the one branched path.
As mentioned above, on the other hand, since the wavelength division multiplexing transmission system using the optical fiber amplifier is required to stably transmit the wavelength division multiplexing optical signal over a long distance. A optical fiber which is a transmission line, involves a loss, and therefore, in order to stably transmit the optical signal over a long distance, it is necessary to compensate for the transmission loss in the optical fiber. For this purpose, a number of low-noise optical fiber amplifiers are located on the way of the optical fiber transmission path. The optical fiber amplifier amplifies the optical signal to a predetermined optical signal level and sends the amplified signal to the optical fiber transmission line. By repeating this operation, the long-distance light transmission is ensured.
However, in the above mentioned conventional optical fiber amplifier of the APC control, since the level of the optical output signal outputted from the optical fiber amplifier is different dependently upon the number of the wavelengths transmitted in a multiplexed manner, the control is carried out to maintain the level of the optical output signal per one wavelength in accordance with the transmitted wavelength number information. Therefore, the level of the ASE light generated in the light amplifier also changes with the change of the level of the optical output signal. On the other hand, since the range of the wavelength detectable to the photo diode for detecting the optical output signal is wide, the photo diode cannot discriminate how much the detected optical signal contains the ASE light. Accordingly, a problem is encountered in that the degree of deterioration of the optical output signal caused by the ASE light greatly changes dependently upon the number of wavelengths transmitted. In other words, the larger the number of the wavelengths transmitted is, the influence of the ASE light to the optical output signal is small, and the smaller the number of the wavelengths transmitted is, the influence of the ASE light to the optical output signal becomes remarkable.
In addition, since the light amplification device disclosed in JP-A-10-51388 mentioned above is targeted for the optical signal of a certain specific wavelength, when a multiplexed optical signal is inputted, it is impossible to measure the level of the ASE light of the other path mentioned above in a high fidelity. Furthermore, since the multiplexed optical signal is outputted without modification in the one path mentioned above, it is impossible to precisely grasp the level of the optical output signal of the light amplifier. As a result, a problem is encountered in that the amplification level of the light amplifier cannot be controlled.
Moreover, in the wavelength division multiplexing transmission system using the conventional optical fiber amplifier, a problem does not occur in the case that a number of wavelengths are transmitted, since the level of the optical output signal is sufficiently larger than the level of the ASE light. However, at the beginning of the introduction of a system, the number of wavelengths transmitted is small and becomes one in a certain case. In such a situation, the level of the optical output signal drops because
Hellner Mark
NEC Corporation
Young & Thompson
LandOfFree
Light amplifier and light transmission system using the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Light amplifier and light transmission system using the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Light amplifier and light transmission system using the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3132823