Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-03-03
2002-04-23
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337200
Reexamination Certificate
active
06377390
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical gain equalizer for eliminating level deviation of a wavelength division multiplexing light due to wavelength dependency of gain of an erbium doped optical fiber amplifier and wavelength dependency of a transmission system, and an optical amplifier and a wavelength-division multiplex transmitter both comprising such an optical gain equalizer.
2. Related Background Art
Long distance and great capacity optical transmission techniques which have remarkably been progressed in recent years are technically based on an optical fiber amplifier using a rare earth doped optical fiber and a wavelength division multiplexing (referred to as “WDM” hereinafter) transmission.
Nowadays, many optical fiber amplifiers using an orbium doped optical fiber have been developed and put to practical use in various fields, as well as the transmission systems. On the other hand, gain of the optical fiber amplifier has wavelength dependency. Accordingly, when the optical fiber amplifier is used in the WDM transmission system, and particularly when a number of optical fiber amplifiers are interconnected in a cascade manner, the wavelength dependency of the gain arises a problem. If the gain has the wavelength dependency, level deviation is generated in an amplified wavelength division multiplexing light, thereby deteriorating cross-talk between wavelengths and worsening setting of light receiving level of a receiver.
Although the optical fiber amplifier itself has been developed to minimize its gain wavelength dependency, in the WDM transmission system in which the amplifiers are interconnected in a multi stage manner, in order to eliminate the wavelength dependency of the gain of each optical fiber amplifier, optical gain equalizers are used.
Regarding the optical gain equalizer, although an equalizer using an etalon filter, an equalizer using an optical fiber coupler, an equalizer using an interference film filter comprised of a dielectric multilayer film, an equalizer using grating (fiber-type grating or grating provided on a glass substrate), an equalizer using a Mach-Zehnder type optical filter and the like have been developed, for several reasons (Japanese Patent Application No. 9-289349 (1997) describes kinds and problems of the optical gain equalizer), the equalizer using the etalon filter has been expected.
The etalon filter has a sinusoidal wave loss characteristic, so that loss characteristic having desired wavelength dependency can be created by combining a plurality of etalon filters having different characteristics. Thus, in the optical gain equalizer, as shown in
FIG. 9
, a plurality of etalon filters A having a sinusoidal wave loss characteristic of the same amplitude and period as those of the term obtained by Fourier series expansion of a gain curve for gain flattening are prepared, and these filters are arranged in cascade, so that a light (wavelength division multiplexing light) between optical fibers D and E is passed through the group of filters by using collimator lenses B, C, thereby completely eliminating the wavelength dependency of the gain.
In the optical gain equalizer obtained by combining the etalon filters A shown in
FIG. 9
, in principle, the greater the number of filters to be combined, the more complete elimination of the wavelength dependency of the gain. However, in consideration of manufacturing viewpoint and total permeation loss, the number of etalon filters which can be combined is limited to about four. Accordingly, it is difficult to eliminate the wavelength dependency of the gain completely, and substantial flattening becomes about 1 dB.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical gain equalizer in which gain wavelength dependency of an optical fiber amplifier can be compensated to an error of about 0.1 dB by utilizing etalon filters and in which deviation dependency of permeability is small and which can easily be manufactured. Another object of the present invention is to provide an optical fiber amplifier in which wavelength dependency of gain becomes small by using such an optical gain equalizer and a wavelength-division multiplex transmitter in which such optical fiber amplifiers in a multi-stage manner.
In an optical gain equalizer according to a first aspect of the present invention, a plurality of etalon filters and one or more fiber gratings or dielectric multilayer filters are arranged in cascade, and a beam of light externally applied is passed through the etalon filters and the one or more fiber gratings or dielectric multilayer filters and outputted to the outside, and the etalon filters have different free spectrum ranges.
In an optical gain equalizer according to a second aspect of the present invention, a plurality of etalon filters and one or more fiber gratings or dielectric multilayer filters are arranged in cascade, and a beam of light externally applied is passed through the etalon filters and the one or more fiber gratings or dielectric multilayer filters and outputted to the outside, and the etalon filters have different free spectrum ranges, and have a sinusoidal wave loss characteristic of the same amplitude and period as those of the term obtained by Fourier series expansion of the loss wavelength characteristic for gain flattening.
In an optical gain equalizer according to a third aspect of the present invention, a plurality of etalon filters and one or more fiber gratings or dielectric multilayer filters are arranged in cascade, and a beam of light externally applied is passed through the etalon filters and the one or more fiber gratings or dielectric multilayer filters and outputted to the outside, and the etalon filters have different free spectrum ranges, and the one or more fiber gratings or dielectric multilayer filters compensate the ripple component remaining as the difference between the loss wavelength characteristic for gain flattening and the loss wavelength characteristic owing to the etalon filters.
In an optical gain equalizer according to a fourth aspect of the present invention, a plurality of etalon filters and one or more fiber gratings or dielectric multilayer filters are arranged in cascade, and a beam of light externally applied is passed through the etalon filters and the one or more fiber gratings or dielectric multilayer filters and outputted to the outside, and the etalon filters have different free spectrum ranges, and have a sinusoidal wave loss characteristic of the same amplitude and period as those of the term obtained by Fourier series expansion of the loss wavelength characteristic for gain flattening, and the one or more fiber gratings or dielectric multilayer filters compensate the ripple component remaining as the difference between the loss wavelength characteristic for gain flattening and the loss wavelength characteristic owing to the etalon filters.
According to a fifth aspect of the present invention, an optical amplifying device comprises an optical gain equalizer in which a plurality of etalon filters and one or more fiber gratings or dielectric multilayer filters are arranged in cascade, and a beam of light externally applied is passed through the etalon filters and the one or more fiber gratings or dielectric multilayer filters and outputted to the outside, and the etalon filters have different free spectrum ranges; and a first optical amplifier for amplifying a wavelength division multiplexing light.
According to a sixth aspect of the present invention, an optical amplifying device comprises an optical gain equalizer in which a plurality of etalon filters and one or more fiber gratings or dielectric multilayer filters are arranged in cascade, and a beam of light externally applied is passed through the etalon filters and the one or more fiber gratings or dielectric multilayer filters and outputted to the outside, and the etalon filters have different free spectrum ranges, and have a sinusoidal wave loss characteristic of the same amplitude and period a
Aso Osamu
Irie Yuichiro
Mizuno Kazuyo
Namiki Shu
Ohta Ikuo
Hellner Mark
The Furukawa Electric Co. Ltd.
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