Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-01-22
2001-04-03
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S337000, C359S199200, C359S199200
Reexamination Certificate
active
06212001
ABSTRACT:
CROSS-REFERENCED TO RELATED APPLICATION
This application claims priority of European Patent Application No. 98300561.2, which was filed on Jan. 27, 1998.
TECHNICAL FIELD
The present invention generally relates to the field of optical transmission and particularly to a method and a system for controlling optical amplification in wavelength division multiplex optical transmission.
BACKGROUND OF THE INVENTION
Wavelength division multiplexing (WDM) is being introduced as a means of increasing the capacity of optical fiber transmission systems. In a WDM system, each individual fiber carries a number of optical signals having different wavelengths. When these optical signals are transmitted over long distances, periodic regeneration of the optical signals is necessary. Currently, this amplification is effected either by demultiplexing the different wavelengths and then converting the optical signals to corresponding electrical signals which are regenerated and then reconverted to optical signals or by using optical amplifiers, e.g. Erbium Doped Fiber Amplifiers (EDFA). Optical amplifiers do have the advantage of both relatively low cost and the ability to amplify all used wavelengths without the need for demultiplexing and optoelectronic regeneration.
WDM systems currently under development will have thirty or more channels, i.e., modulated optical signals with different wavelengths (known as Dense Wavelength Division Multiplexing, DWDM). These DWDM systems are demanding for optical amplifiers which, especially considering the cascadation of a plurality of optical amplifiers along the transmission path of the DWDM system, have only very limited tolerances in certain parameters. Among these parameters gain flatness and gain tilt are of special importance. Problems with gain tilt may arise from aging of the DWDM system, from temperature effects, from different attenuation slopes of fiber used to form the transmission path or from stimulated Raman scattering.
It is known that gain tilt and gain flatness of the optical amplifiers can be optimized by controlling the input power of the optical amplifier. In European Patent Application EP 0 637 148 A1, a WDM system is described wherein transmitters are used which have means for associating an identification signal with each transmitted wavelength and wherein each optical amplifier of the transmission path has means for determining from the identification signals the number of wavelengths present on the transmission path whereby to control the power of the different channels. The use of identification signals for each transmitted wavelength also allows for maintaining a power balance between different wavelength channels in order to maintain the necessary gain flatness. This is achieved by determination of the amplitudes of individual identification signals associated with the transmitted wavelengths.
The known WDM system has the disadvantage of associating an identification signal with each transmitted wavelength channel. In addition, it has the disadvantage that in WDM systems it is not guaranteed that every wavelength channel is present all the time. This causes problems if the amplitude of the individual identification signal normally associated with a missing wavelength channel is used to maintain gain flatness as described above.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method and a system for controlling optical amplification in wavelength division multiplex optical transmission. It is one aim of the inventive method under consideration to avoid the drawbacks known from the state of the art.
According to the principles of the invention, there is provided a system and method for controlling optical amplification of an optical transmission signal consisting of a plurality of modulated optical signals with different wavelengths for wavelength division multiplex optical transmission. In one illustrative embodiment, a first auxiliary optical signal is added having a given power level at a wavelength shorter than the shortest wavelength of the optical transmission signal. A second auxiliary optical signal is added having the same power level as the first auxiliary optical signal at a wavelength longer than the longest wavelength of the optical transmission signal. The auxiliary optical signals are detected from the optical transmission signal after optical amplification thereof, and the optical amplification of the optical transmission signal is controlled depending on the difference between the power levels of the detected auxiliary optical signals.
An advantage of the present invention is that the method and system described herein are insensitive to the absence of wavelength channels, e.g., the failure of certain wavelengths or optical signals forming the optical transmission signal. It is another advantage of the present invention that it allows to reduce the number of identification signals necessary.
The present invention will become more fully understood from the detailed description given hereinafter and further scope of applicability of the present invention will become apparent. However, it should be understood that the detailed description is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.
REFERENCES:
patent: 5909305 (1999-06-01), Kinoshita
patent: 6067187 (2000-05-01), Onaka et al.
patent: 91112133 (1991-07-01), None
patent: 95400053 (1995-01-01), None
patent: 96116724 (1996-10-01), None
patent: PCT/GB96/01884 (1996-08-01), None
Bode Dirk
Teichmann Bernd
Zhou Jianhui
Hellner Mark
Lucent Technologies - Inc.
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