Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-04-23
2002-05-14
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S337400
Reexamination Certificate
active
06388801
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical amplification apparatus for amplifying signal light by utilizing Raman amplification and a controlling method thereof. More particularly, present invention relates to an optical amplification apparatus for achieving the improvement of noise characteristics of the overall optical amplification apparatus by taking influences of noise light generated by Raman amplification into consideration, and a controlling method thereof.
RELATED ART
Demands for information have been increased drastically in recent years with the progress of Internet technologies. A greater capacity and the formation of more flexible networks have been required in a trunk type optical transmission system in which an information capacity is integrated. A WDM optical transmission system for transmitting wavelength division multiplexed (WDM) signal light obtained by multiplexing a plurality of optical signals having different wavelengths is one of the most effective means that can cope with such a system demand. In a conventional WDM optical transmission system, an optical fiber amplifier using an optical fiber doped with a rare earth element such as erbium (Er) is utilized, as an optical repeater. By utilizing broadband characteristics of this optical fiber amplifier, one optical fiber can realizes WDM optical transmission for repeating and transmitting optical signals of a plurality of wavelengths.
To further increase the capacity and to extend the distance and the repeating interval in the WDM optical transmission system as described above, means for compensating for the degradation of S/N in the transmission system becomes necessary. For this purpose, it is effective to use means for supplying excitation light to a transmission path, to perform distributed Raman amplification of the transmission p path by utilizing an amplification operation using the effect of stimulated Raman scattering, so that the repeating loss is equivalently reduced, in addition to an existing optical amplification repeating transmission system.
FIG. 8
is a structural diagram showing the outline of a WDM optical transmission system using distributed Raman amplification, which has been proposed heretofore.
In the WDM optical transmission system in
FIG. 8
, a transmission path
3
connects a transmission station (Tx)
1
and a reception station (Rx)
2
, and a plurality of optical repeaters
4
are arranged on the transmission path
3
with predetermined intervals so that WDM signal light is transmitted and repeated from the transmission station
1
to the reception station
2
. Each optical repeater
4
includes an optical amplification apparatus constituted by combining a DRA (Distributed Raman Amplifier) with an EDFA (Erbium-Doped Fiber Amplifier). In this DRA, excitation light for Raman amplification (hereinafter called “Raman excitation light”) generated in an excitation light source is supplied through an optical coupler to the transmission path
3
connected to the transmission station side, and the WDM signal light propagated through the transmission path
3
is subjected to distributed Raman amplification. The WDM signal light subjected to the distributed Raman amplification is input to the EDFA, to be amplified to a necessary level, and is again output to the transmission path
3
. With such a WDM optical transmission system, since the loss in the transmission path
3
in each repeating segment is decreased due to distributed Raman amplification, transmission characteristics of the WDM signal light can be improved.
Noise characteristics of the optical amplification apparatus constituted by combining the DRA with the EDFA and used for such a WDM optical transmission system are affected not only by the noise figure (NF) of the EDFA but also by noise light generated by the Raman amplification. The noise light resulting from Raman amplification is generated also when only Raman excitation light is incident to an amplification medium under a state where signal light is not input, and is generally called “Raman scattering light due to pumping light”. Here, noise light generated in the DRA is called “Amplified Spontaneous Raman Scattering (ASS) light” in contrast with Amplified Spontaneous Emission (ASE) light generated in the EDFA.
To improve the noise characteristics of the optical amplification apparatus and to further improve the transmission characteristics, it is necessary to reduce the noise figure of the overall optical amplification apparatus by taking the influences of ASS light into consideration. To improve the noise characteristics of the optical amplification apparatus, technologies for reducing independently the noise figure of the EDFA have been studied in the past, but specific considerations taking the influences of ASS into account have not been made.
The present invention has been made in view of the problems described above, and it is an object of the present invention to provide an optical amplification apparatus for achieving the improvement of noise characteristics by controlling an amplification operation by assuming a noise figure of an overall optical amplification apparatus while taking influences of noise light resulting from Raman amplification into account, and a controlling method of such an optical amplification apparatus.
DISCLOSURE OF THE INVENTION
To accomplish the object described above, an optical amplification apparatus utilizing Raman amplification according to the present invention comprises: first optical amplifying means for Raman amplifying signal light propagated through a Raman amplification medium by supplying excitation light to the Raman amplification medium; and second optical amplifying means for amplifying the signal light output from the first optical amplifying means, wherein the optical amplification apparatus further comprises: target value setting means for setting a target value for minimizing a noise figure of the overall optical amplification apparatus as to input light power of said second amplifying means; and excitation light controlling means for controlling an excitation light supply condition of the first optical amplifying means in accordance with the target value set by the target value setting means.
According to this construction, input light power of the second optical amplifying means to which Raman amplified signal light is input, is taken into specific consideration, and its target value is set by the target value setting means.
The input light power target value of the second optical amplifying means minimizes the noise figure of the overall optical amplification apparatus constituted by combining the first and second amplifying means. As the excitation light supply condition of the first optical amplifying means is adjusted by the excitation light controlling means in accordance with the set target value, actual input light power of the second optical amplifying means is so controlled as to coincide with the target value. Consequently, the noise characteristics of the overall optical amplification apparatus, that takes the influences of noise light due to Raman amplification into consideration, can be optimized by the control inside its own apparatus, and an optical amplification apparatus having excellent noise characteristics can be realized.
As one aspect of the optical amplification apparatus described above , the target value setting means may include an excitation light power detecting section for detecting excitation light power supplied to the Raman amplification medium, and a computing section for computing noise light power by the first optical amplifying means in accordance with a detection result of the excitation light power detecting section, and setting an input light power target value of the second optical amplifying means for minimizing the noise figure of the overall optical amplification apparatus on the basis of the computed noise light power and on the basis of noise characteristics of the second optical amplifying means.
According to this aspect, in the computing section, th
Hayashi Etsuko
Sugaya Yasushi
Tanaka Tomoto
Fujitsu Limited
Hellner Mark
Staas & Halsey , LLP
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