Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2002-03-11
2004-05-25
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
Reexamination Certificate
active
06741389
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to an optical transmission system and an optical transmission method for transmitting a signal light while amplifying the signal light by utilizing Raman amplification, and more particularly, to a technique for controlling the Raman amplification so as to suppress deterioration of transmission characteristics due to the nonlinear optical effect.
(2) Related Art
For example, in a wavelength-division multiplexing (WDM) optical transmission system for multiplexing a plurality of optical signals of mutually different wavelengths to transmit these multiplexed optical signals through a single optical fiber transmission path, it is important to densely multiplex optical signals of wavelengths (wavelength channels) as many as possible, so as to increase a transmission capacity of the optical transmission system. However, there is a possibility that, as a result of densely multiplexing the wavelength channels, the nonlinear optical effect, such as four wave mixing or cross phase modulation, occurs in the optical fiber transmission path, so that the inter-channel interference is caused by an influence of the nonlinear optical effect, to thereby deteriorate the transmission characteristics. In order to avoid such deterioration of transmission characteristics, it is effective to reduce an input light power to an optical fiber, to thereby suppress the occurrence of nonlinear optical effect. However, this brings a reduction of optical signal-to-noise ratio (OSNR), thereby causing a possibility of deteriorating the transmission characteristics.
As conventional technique to suppress the aforementioned deterioration of transmission characteristics, there has been proposed a technique, as disclosed in Japanese Unexamined Patent Publication No. 2000-330145, in which, in an optical repeater provided with a typical optical amplifier, a pumping light is supplied to an optical fiber transmission path that is connected to the optical repeater, to perform distributed Raman amplification, so that an output light power from the optical repeater, i.e., an input light power to an optical fiber transmission path at the latter repeating section, is reduced to thereby decrease the influence of nonlinear optical effect. Further, in Japanese Unexamined Patent Publication No. 2000-299522, there has been proposed an optical transmission system in which the distributed Raman amplification using an optical fiber transmission path as an amplifying medium is combined with discrete Raman amplification in an amplifying medium within an optical repeater. Such a system using both of the distributed Raman amplification and discrete Raman amplification is considered to be more effective to suppress deterioration of transmission characteristics due to the nonlinear optical effect to be caused in the optical fiber transmission path.
For optical transmission systems utilizing the Raman amplification, in addition to the above techniques, various techniques have been proposed. Japanese Unexamined Patent Publication No. 10-73852 discloses an optical repeating transmission system provided with optical components for Raman amplification within an optical repeater so as to compensate for a loss in transmission path. Further, Japanese Unexamined Patent Publication No. 10-22931 discloses an optical amplifying transmission system that Raman amplifies a signal light being propagated through a transmission path, by pumping lights of a plurality of wavelengths, to make a signal band to be a broad band. Moreover, Japanese Unexamined Patent Publication No. 2000-314902 discloses a distributed Raman amplifier in which a power difference is caused between a pumping light at a shorter wavelength side and a pumping light at a longer wavelength side, in order to solve the wavelength dependence of the OSNR. Additionally, Japanese Unexamined Patent Publication No. 2001-109025 has proposed a method for reducing a gain fluctuation in a Raman amplifier and for reducing a gain variation for each gain medium fiber.
Furthermore, Japanese Unexamined Patent Publication No. 11-84440 discloses an optical transmission system capable of providing a flat and wide gain band and of compensating for dispersion of transmission path, by combining a Raman amplifying section with a rare earth element doped fiber amplifying section. Further, Japanese Unexamined Patent Publication No. 2001-15845 discloses a technique to automatically control an output level to be constant in a case of performing the distributed Raman amplification.
Meanwhile, in the aforementioned optical transmission system that uses both of the distributed Raman amplification and discrete Raman amplification, it is effective to utilize, as an amplifying medium for causing the discrete Raman amplification within the optical repeater, a medium such as a dispersion compensation fiber having a larger non-linearity and a higher Raman amplifying efficiency, compared with a typical optical fiber transmission path to be connected to an optical repeater. In a case where the discrete Raman amplification is to be used using the dispersion compensation fiber, in order to effectively suppress the transmission characteristic deterioration due to the nonlinear optical effect caused in the entire system, it is necessary to design the system, taking account of not only the influence of nonlinear optical effect caused in the optical fiber transmission path and the like, but also the influence of nonlinear optical effect caused in the dispersion compensation fiber and the like within the optical repeater.
In the aforementioned prior art, although the transmission characteristic deterioration due to the nonlinear optical effect caused in the optical fiber transmission path is reducible, there is not considered the influence of nonlinear optical effect caused in the optical repeater within which the discrete Raman amplification is conducted. Therefore, there is a problem that it is difficult to reliably suppress the transmission characteristic deterioration to be caused in the entire system or each repeating section as a unit.
Concerning techniques for suppressing the nonlinear optical effect caused in an optical fiber transmission path, in Japanese Unexamined Patent Publication No. 10-200509, there is disclosed an optical transmission system that sets a gain per unit length of a distributed amplifying medium on an optical fiber transmission path, so that the maximum intensity of WDM signal light is less than a defined value for causing the nonlinear optical effect. Although this optical transmission system is not applied with the discrete Raman amplification in an optical repeater, the controlling method to be set here for suppressing the nonlinear optical effect considers only the nonlinear optical effect caused in the optical fiber transmission path. Thus, even by this system, it is also difficult to effectively suppress the nonlinear optical effect caused in the entire system or each repeating section.
There will be described in detail hereinafter the influence of nonlinear optical effect in an optical transmission system using both of the distributed Raman amplification and discrete Raman amplification, while showing a specific calculation example.
FIG. 22
is a block diagram showing an example of a WDM optical transmission system using both of the distributed Raman amplification and discrete Raman amplification.
In the WDM optical transmission system of
FIG. 22
, for example, optical signals of different wavelengths generated at a plurality of optical senders (OSs) are wavelength multiplexed by a multiplexer, and transmitted to an optical fiber transmission path comprising a single mode fiber (SMF), to be sent towards optical receivers (ORs). Here, as the WDM signal light to be transmitted, it is assumed to include optical signals of 40 waves (&lgr;1 to &lgr;40) arranged in a C-band (for example, a wavelength band between 1529 nm and 1561 nm) at intervals of 100 GHz. A plurality of optical repeaters are arranged on the optical f
Kumasako Junichi
Terahara Takafumi
Black Thomas G.
Fujitsu Limited
Hughes Deandra M.
Staas & Halsey , LLP
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