Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-06-24
2002-04-09
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S199200, C359S337110, C359S337130
Reexamination Certificate
active
06369938
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention generally relates to a light amplifier for a wavelength division multiplexed (WDM) optical transmission system, and more particularly to a light amplifier having a two-stage configuration which eliminates a wavelength-dependence of the gain of the light amplifier.
Recently, an optical communications network has increasingly been used in practice. Nowadays, it is required that the optical communications network cope with multi-media networking. A WDM system is more attractive, particularly in terms of an increase in the transmission capacity. In order to realize the WDM system, it is necessary to use a multi-wavelength light amplifier capable of amplifying a wavelength division multiplexed signal. It is required that such a multi-wavelength light amplifier does not have wavelength-dependence of the gain, which is further required not to be changed due to a variation in the power of the input light.
A light amplifier is known which has an optical fiber doped with a rare-earth element and directly amplifies the input light. There has been some activity in the development of a multi-wavelength light amplifier which amplifies a wavelength division multiplexed light signal including signal components having different wavelengths (channels).
However, normally, the rare-earth-element doped fiber amplifier has a very narrow range in which the gain thereof does not have the wavelength-dependence. In this regard, nowadays, there is no available light amplifier which can practically be used for the WDM system. That is, there is no available light amplifier which does not have wavelength-dependence of the gain, which is not changed due to a variation in the power of the input light. Particularly, the wavelength-dependence of the gain, which takes place when the input power changes, deteriorates the signal-to-noise ratio with respect to a particular signal. This prevents the multi-wavelength light amplifier from being used in practice.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide a multi-wavelength light amplifier in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a multi-wavelength light amplifier which does not have wavelength-dependence of the gain, which is not changed due to a variation in the power of the input light.
The above objects of the present invention are achieved by a multi-wavelength light amplifier comprising: a first-stage light amplifier which has a first light amplifying optical fiber amplifying a light input; a second-stage light amplifier which has a second light amplifying optical fiber amplifying a first light output from the first-stage light amplifier; and an optical system which maintains a second light output of the second-stage light amplifier at a constant power level. The first-stage and second-stage light amplifiers have different gain vs wavelength characteristics so that the multi-wavelength light amplifier has no wavelength-dependence of a gain.
The above multi-wavelength light amplifier may be configured as follows. The first-stage light amplifier comprises a first pump source which pumps the first light amplifying optical fiber so as to have a first gain vs wavelength characteristic in which, as a wavelength of light to be amplified becomes shorter, a gain of the first-stage light amplifier becomes higher. The second-stage light amplifier comprises a second pump source which pumps the second light amplifying optical fiber so as to have a second gain vs wavelength characteristic in which, as a wavelength of light to be amplified becomes longer, a gain of the first-stage light amplifier becomes higher.
The above multi-wavelength light amplifier may be configured as follows. The first-stage light amplifier comprises a first pump source which pumps the first light amplifying optical fiber so as to have a first gain vs wavelength characteristic having a first linear gain slope. The second-stage light amplifier comprises a second pump source which pumps the second light amplifying optical fiber so as to have a second gain vs wavelength characteristic having a second linear gain slope. A combination of the first and second linear gain slopes results in a flat gain vs wavelength characteristic of the multi-wavelength light amplifier.
The above multi-wavelength light amplifier may further comprise an optical filter which emphasizes the gain vs wavelength characteristic of the first-stage light amplifier.
The above multi-wavelength light amplifier may further comprise an optical filter which compensates for a difference between the gain vs wavelength characteristics of the first-stage light amplifier and the second-stage light amplifier.
The above multi-wavelength light amplifier may be configured as follows. The optical filter is provided so as to follow the first-stage light amplifier. The first-stage light amplifier comprises a first pump source which pumps the first light amplifying optical fiber so as to have a first gain vs wavelength characteristic having a first linear gain slope. The second-stage light amplifier comprises a second pump source which pumps the second light amplifying optical fiber so as to have a second gain vs wavelength characteristic having a second linear gain slope. The optical filter emphasizes the first linear gain slope to provide an emphasized first linear gain slope. A combination of the emphasized first linear slope and the second linear gain slope results in a flat gain vs wavelength characteristic of the multi-wavelength light amplifier.
The above multi-wavelength light amplifier may be configured as follows. The optical filter is provided so as to follow the first-stage light amplifier. The first-stage light amplifier comprises a first pump source which pumps the first light amplifying optical fiber so as to have a first gain vs wavelength characteristic having a first linear gain slope. The second-stage light amplifier comprises a second pump source which pumps the second light amplifying optical fiber so as to have a second gain vs wavelength characteristic having a second linear gain slope. The optical filter compensates for the difference between the first and second linear gain slopes so that a flat gain vs wavelength characteristic of the multi-wavelength light amplifier can be obtained.
The above multi-wavelength light amplifier may be configured as follows. The first-stage light amplifier has a first AGC (automatic gain control) system so that a ratio of the input light and the first light output is constant. The second-stage light amplifier has a second AGC system so that a ratio of the first light output and the second light output is constant.
The above multi-wavelength light amplifier may be configured as follows. The first-stage light amplifier has an AGC (automatic gain control) system so that a ratio of the input light and the first light output is constant. The second-stage light amplifier has an automatic power control (APC) system so that the second light amplifying optical fiber is pumped at a predetermined constant power level.
The above multi-wavelength light amplifier may be configured as follows. The first-stage light amplifier has an AGC (automatic gain control) system so that a ratio of the input light and the first light output is constant. The second-stage light amplifier has an automatic level control (ALC) system so that the second light output is maintained at a predetermined constant power level.
The above multi-wavelength light amplifier may be configured as follows. The first AGC system comprises first means for detecting a first level of the light input and a second level of the first light output and pumping the first light amplifying optical fiber so that a ratio of the first and second levels is maintained at a first predetermined constant value. The second AGC system comprises second means for detecting a third level of the first light output and a fourth level of the second light output and pumping the second light am
Chikama Terumi
Kinoshita Susumu
Sugaya Yasushi
Takeda Miki
Fujitsu Limited
Moskowitz Nelson
Staas & Halsey , LLP
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