Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2002-03-01
2004-05-18
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
Reexamination Certificate
active
06738180
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement of gain flatness of a Raman amplifier using a wavelength multiplexed pump light source in a wavelength-division multiplexing transmission system.
2. Description of the Related Art
An optical fiber using silica as its main component and containing germanium in its core is widely used. It is known that, in a Raman amplifier using this optical fiber, maximum Raman gain value is obtained at a wavelength spaced apart by approximately 100 nm to the long wavelength side from the pump light source wavelength. An attempt has been made, utilizing this phenomenon, to amplify a wavelength-division multiplexing signal (hereinafter referred to as WDM signal) using a plurality of light sources of different wavelengths (hereinafter referred to as wavelength multiplexed pump light sources). To prevent pump lightwave on the longest wavelength side of the wavelength multiplexed pump light source from overlapping signal lightwaves on the shortest wavelength side of the WDM signal, it is necessary for the maximum wavelength band of the wavelength multiplexed pump light source to be approximately 100 nm.
Japanese Patent Application Laid-open No. 2000-98433 discloses that it is necessary to improve the flattening of the Raman gain wavelength property over a wide band in order to effect wavelength-division multiplexing transmission of signal lightwaves, and that it is necessary to take into account the wavelength arrangement of the wavelength multiplexed pump light source.
More specifically, regarding the arrangement of a plurality of pump light sources of different wavelengths, when the wavelength interval between adjacent pump lightwaves is less than 6 nm, it is impossible to secure a margin obtained by adding the band width of a combiner which does not involve an increase in the insertion loss of the combiner due to crosstalk of the combiner when combining pump lightwaves of different wavelengths to the band width of the pump light source. When the wavelength interval exceeds 35 nm, a reduction in gain which is so great as to be unsuitable for wavelength-division multiplexing transmission occurs near the center of the total Raman gain band width of the respective Raman gains generated from adjacent wavelengths, so that it is necessary for the wavelength interval of the pump lightwave to be in the range of 6 nm to 35 nm.
Further, there has been disclosed a technique in which, in order to make the wavelength interval of adjacent pump light sources as small as possible so that the total gain flatness may not increase, the pump lightwave is divided into lightwaves for forward pump and lightwaves for backward pump; for example, the pump light wavelength interval respectively belonging to the forward pump and backward pump is approximately 6 nm, and pump light wavelengths &lgr;
2
, &lgr;
4
, . . . respectively belonging to the backward pump are arranged between pump light wavelengths &lgr;
1
, &lgr;
3
, . . . belonging to the forward pump, whereby the pump light wavelength interval of the Raman amplifier is made less than 6 nm to realize a dense pump light wavelength arrangement, making it possible to realize a Raman amplifier so that the difference between maximum and minimum of the Raman gain wavelength property of the Raman amplifier, that is, the gain flatness is so small as to allow dense WDM transmission.
In the Raman amplifier disclosed in Japanese Patent Application Laid-open No. 2000-98433, forward pump and backward pump are treated independently; assuming that the combining position of forward pump lightwave is A and that the combining position of backward pump lightwave is B, the longitudinal section A→B of the optical fiber where stimulated Raman scattering is generated by the forward pump lightwave and the longitudinal section B→A of the optical fiber where stimulated Raman scattering is generated by the backward pump lightwave are common to each other except for the pump direction.
That is, in the invention as disclosed in Japanese Patent Application Laid-open No. 2000-98433, an amplifier is formed so that gain flatness is so small as to allow dense WDM transmission by using a forward pump light source and a backward pump light source, Raman amplification being effected in the same section in the optical transmission system.
However, any practical Raman amplifier has a certain amount of gain deviation within the gain band even if it is the Raman amplifier with small gain flatness as disclosed in Japanese Patent Application Laid-open No.2000-98433.
That is, regarding the Raman amplifier in which forward pump and backward pump are combined with each other, even in the case of an amplifier with small gain flatness, when Raman amplifiers of the same gain wavelength property are used in a plurality of stages, the wavelength determining the maximum value of the Raman gain wavelength property and the wavelength determining the minimum value thereof are the same in each Raman amplifier, so that each time the amplification stages are passed, the maximum values and minimum values of the Raman gain wavelength property are accumulated, and the deviation of the Raman gain wavelength property increases, with the result that there is a large difference in power between the channels, resulting in a rather poor degree of flatness. For example, when inputting a WDM signal to an optical amplifier using an erbium doped optical fiber (EDF), or after outputting it from the optical amplifier, it is necessary to compensate for the power for each channel by a means like an equalizer.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an optical transmission system characterized in that: a plurality of Raman amplifiers including a plurality of pump light sources of different pump wavelengths are used; the longitudinal section of the optical fiber where stimulated Raman scattering is generated differs depending upon the plurality of Raman amplifiers; and the plurality of Raman amplifiers mutually compensate for the respective Raman gain wavelength properties.
That is, according to the first aspect of the present invention, there is provided an optical transmission system in which, at an output point for a wavelength division multiplexing signal of the optical transmission system using a plurality of stages of Raman amplifiers, the total Raman gain flatness of the plurality of Raman amplifiers smaller than the cumulative flatness on the assumption that every amplifier has the same gain profile of one of the plurality of Raman amplifier.
According to a second aspect of the present invention, in the first aspect of the invention, there is provided an optical transmission system characterized in that the plurality of Raman amplifiers are designed to be of at least two types of pump wavelength sets, and when there exist a plurality of Raman amplifiers of the same pump wavelengths which use the same kind of amplifier fibers, they include different set gains.
According to a third aspect of the present invention, in the first or second aspect of the invention, there is provided an optical transmission system characterized in that: at least a first Raman amplifier and a second Raman amplifier are used as the plurality of Raman amplifiers; a wavelength band where the Raman gain wavelength property of the first Raman amplifier exhibits an upward convex curve including a maximum value of Raman gain of G
Amax
at a wavelength of &lgr;
Amax
and a wavelength band where the Raman gain wavelength property of the second Raman amplifier exhibits a downward convex curve including a minimum value of Raman gain of G
Bmin
at a wavelength of &lgr;
Bmin
overlap with each other; a wavelength band where the Raman gain wavelength property of the first Raman amplifier exhibits an downward convex curve including a minimum value of Raman gain of G
Amin
at a wavelength of &lgr;
Amin
and a wavelength band where the Raman gain wavelength property of the second Raman amplifier exhibits an
Emori Yoshihiro
Hirasawa Takeshi
Kado Soko
Black Thomas G.
Hughes Deandra M.
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