Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-03-01
2004-03-09
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
Reexamination Certificate
active
06704134
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier that amplifies a signal light based on Raman amplification function, an optical amplifying repeater and an optical transmission apparatus that are constituted by using the optical amplifier. More particularly, the present invention relates to an optical amplifier that collectively amplifies a signal light, in which signal lights having a plurality of wavelengths different from each other are wavelength division multiplexed (hereinafter referred to as a WDM signal light), and an optical amplifier that comprises a flattening function of a gain for each signal light.
2. Description of Related Art
It is known that Raman amplification function is generated by allowing a pumping light having a specified wavelength to be incident on a signal light that propagates in an optical fiber transmission path, and thereby an optical amplification of the signal light can be performed (for example, refer to Japanese Patent Laid-open No.10 (1998)-022931 gazette). The optical amplification by Raman amplification is performed by allowing the pumping light having the specified wavelength to be incident on the optical fiber transmission path in an opposite direction to a progress direction of the signal light that propagates in the optical fiber transmission path.
Incidentally, the optical amplification utilizing Raman amplification function could be utilized independently. However, in most cases, it is normally used in combination with the optical amplification which has a rare-earth element doped optical fiber as an amplification medium (hereinafter referred to as a normal optical amplification). In an optical amplifying repeater arranged midway or at an output end of the optical fiber transmission path, and in a front optical amplifier arranged within an optical reception terminal station, the pumping light for performing Raman amplification is incorporated in addition to the normal optical amplification in order to improve the gain.
However, when the optical amplification is performed on the WDM signal light by Raman amplification function, a difference (deviation) occurs in optical output levels of each signal light included in the WDM signal light after the optical amplification, since Raman amplification itself has a wavelength dependency of the gain. With regard to the difference of the optical output level, amounts of the difference are added in the multi-step optical amplification and repeating transmission system, resulting in large influences to a transmission characteristic.
Generally, an output wavelength characteristic of the optical amplifier as described above is designed in a state such that the WDM signal light to be input does not have the deviation in the optical output level depending on a wavelength. However, the WDM signal light, on which the optical amplification was performed by Raman amplification function generated in the optical fiber transmission path connected to an input portion of the optical amplifier, further undergoes the normal optical amplification. Therefore, at the time when the WDM signal light is input to the optical amplifier, when the deviation exists in optical input levels among the signal lights included in the WDM signal light, the deviation is further increased by the normal optical amplification. As a result, a desired output wavelength characteristic cannot be obtained.
The gain by Raman amplification is different from each other depending on the wavelength of the signal light and having the wavelength dependency, and the wavelength dependency itself is dependent on the wavelength of a pumping light source. Accordingly, there is a problem of difficulties in controlling the wavelength characteristic of the signal light uniformly, due to scattering of the wavelength at the time of manufacturing the pumping light source, a wavelength fluctuation according to difference in a pumping light intensity and a temperature at which the pumping light source is used, and the like.
SUMMARY OF THE INVENTION
The object of the present invention is to maintain uniformity in the gain even when the optical amplification is performed on the WDM signal light by the optical amplification utilizing Raman amplification function. Moreover, another object of the present invention is to make the optical output level of each signal light included in the WDM signal light to be uniform.
The object of the optical transmission apparatus of the present invention is, to perform a stable optical transmission such that a large deviation does not occur in the optical output level of each signal light, even in the case where the WDM signal light is transmitted.
To solve the foregoing problems, the optical amplifier of the present invention comprises: a first pumping light source for Raman amplification, which outputs a first pumping light having a first wavelength; a second pumping light source for Raman amplification, which outputs a second pumping light having a second wavelength; and a wavelength multiplexer for Raman amplification, which allows the first and second pumping lights to be incident on the optical fiber transmission path where the signal light propagates. The signal light is Raman amplified in the optical fiber transmission path by the first and second pumping lights. A propagation direction of the first and second pumping lights may be backward or may be forward (the same direction) to that of the signal lights.
Although the signal light may be applied to a single signal light, it is, specifically, the WDM signal light, in which the signal lights having a plurality of wavelengths different from each other are wave length division multiplexed. In other words, the optical amplifier of the present invention, with regard to the pumping light for performing Raman amplification, allows two pumping lights having the wavelengths different from each other to be incident on the optical transmission path.
In the above-described constitution, to reduce the influence on the WDM signal light from the wavelength dependency of the gain, which occurs when the optical amplification by Raman amplification is performed, the first wavelength is the wavelength to allow the signal light to be Raman amplified such that the wavelength having the peak gain by Raman amplification becomes shorter than the wavelength of the signal light included in the WDM signal light. On the other hand, the second wavelength is the wavelength to allow the signal light to be Raman amplified such that the wavelength having the peak gain by Raman amplification becomes longer than the wavelength of the signal light included in the WDM signal light.
Herein, a gradient showing the relation between the wavelength and the gain in Raman amplification by the first pumping light and a gradient showing the relation between the wavelength and the gain in Raman amplification by the second pumping light are in opposite directions from each other. The optical output level and the wavelength of the first and second pumping lights are respectively set such that the gains for signal lights included in the WDM signal light, on which Raman amplification was performed by the first and second pumping lights, become substantially equal to each other.
In the above-described constitution, the first and second pumping light sources for Raman amplification can be constituted of: first and second laser diodes that oscillate first and second lights respectively; and first and second fiber gratings, which are arranged in a previous step of the first and second laser diodes, that selectively transmit the lights having the first and second wavelengths in a specified ratio and reflect residual light. Alternatively, the first and second pumping light sources for Raman amplification can be also constituted of: the first and second laser diodes that oscillate the first and second lights respectively; and first and second optical filters, which are arranged in a previous step of the first and second laser diodes, that selectively transmit the l
Black Thomas G.
Hughes Deandra M.
McGinn & Gibb PLLC
NEC Corporation
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