Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-01-27
2002-05-07
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
Reexamination Certificate
active
06384964
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical gain equalizer for flattening gain wavelength dependency of an optical amplifier used for optical transmissions, etc., a method for producing the optical gain equalizer, an optical amplifier utilizing the optical gain equalizer, and a wavelength multiplexed light transmission system utilizing the optical amplifier.
BACKGROUND OF THE INVENTION
A wavelength multiplexing system (optical wavelength division multiplexed transmission system) is available as a system to achieve high bit rate transmissions in optical transmission systems. The system is such that optical signals of a plurality of wavelengths different from each other are multiplexed and transmitted in a single optical transmission line consisting of, for example, optical fibers. Various research has been carried out on a wavelength multiplexed light transmission system utilizing the system.
Optical semiconductor amplifiers in which optical semiconductors are utilized have been conventionally researched. Recently, an optical fiber type optical amplifier has been researched, in which a rare metal doped optical fiber such as an erbium doped optical fiber, etc., is used as an amplifying medium, and such an optical fiber type optical amplifier has rapidly been achieved for practical applications. Such an optical amplifier can collectively amplify light of wavelengths in a gain wavelength band. Therefore, it is highly expected that, by applying such an optical amplifier to the wavelength multiplexed light transmission system, a high bit rate and long haul transmission system is achieved.
However, in both an optical fiber type optical amplifier and an optical semiconductor amplifier, the gain thereof has a wavelength dependency. If such a wavelength multiplexed light as described above is collectively made incident into an optical amplifier since the gain size differs, depending on wavelengths of light incident into an optical amplifier, the intensity of light outputted from the optical amplifier will differ, depending on the wavelengths. Due to differences in the outputted light intensity resulting from the wavelengths, a problem of crosstalk between the respective wavelengths occurs. Also, if the intensity of light outputted from an optical amplifier differs, another problem occurs in setting the receiving level, whereby the reception level of a wavelength multiplexed light receiving portion which receives the outputted light must be established at different values depending on, for example, the wavelength of the received light.
Therefore, in order to compensate a gain wavelength dependency of such an optical amplifier, such a method has been proposed, in which a transmission light filter type optical gain equalizer to flatten the gain wavelength dependency of the optical fiber type optical amplifier is produced by a combination of Fabry-Perot etalon filters, and the optical gain equalizer is inserted into an optical fiber type optical amplifier. The method is described in, for example, Japanese Laid-open Patent Publication No. 289349 of 1997. The proposed method flattens gains of an optical amplifier by expanding a gain curve of the optical amplifier with respect to signal light wavelengths to a Fourier series and combining an etalon filter having since-wave type loss characteristics of the same amplitude and cycminimumhose of the since-wave type loss characteristics obtained by the expansion.
OBJECT AND SUMMARY OF THE INVENTION
However, the following problems exist in the method proposed above. That is, the equalizing characteristic T if etalon is expressed by expressions (1), (2) and (3), where the reflection index is R, the incident angle is &thgr; (where the incident angle perpendicularly incident with respect to the filter surface is 0), the refractive index of a filter substrate is n, the thickness of the filter substrate is d, the light speed is c, and the wavelength of the incident light is frequency f. The equalizing characteristic T of etalon is different in a waveform from respective cosine-wave components expanded in the Fourier series. Thus, since, in the abovementioned method, an optical gain equalizer is in an attempt to be formed by an expansion which is not inherently mathematically guaranteed, there remain components which are not compensated even though the gain wavelength dependency of an optical amplifier is attempted to be compensated.
T
=
-
10
×
log
10
[
1
+
m
i
sin
2
(
2
π
f
2
m
2
)
]
(
1
)
m
1
=
4
R
(
1
-
R
)
2
(
2
)
m
2
=
2
nd
1
-
sin
2
θ
/
n
2
c
(
3
)
A waveform of any optional form can be expressed in the form of the cosine- or sine-wave infinite series. Therefore, it is considered that, if a very large number of etalon filters are used although the gain wavelength dependency of an optical amplifier cannot be completely compensated, characteristics close to the since-wave loss characteristics obtained by expanding the abovementioned Fourier series can be obtained. Actually, however, it is impossible to form an optical gain equalizer by a very large number of etalon filters which are nearly infinite. For convenience in production, the number of etalon filters is four, at most, which is the limit in view of production. Accordingly, in actuality, the since-wave loss characteristics cannot be obtained by the method proposed above, wherein it is also impossible to only effectively compensate the gain wavelength dependency of an optical gain amplifier.
That is, since, in the abovementioned prior art method, infinite term components obtained by expanding the gain curve of an optical amplifier with respect to signal light wavelengths are in an attempt to be achieved by definite terms (that is, an attempt to be achieved by etalon filters, the usage number of which is limited), the gain wavelength dependency cannot be effectively compensated.
Further, in the case of Fourier series expansion, parameters of expanding terms differ in wavelength cycles selected in an attempt to compensate by an optical gain equalizer, that is, basic cycles of the Fourier series. Accordingly, a remarkably great amount of labor is required in order to determine design matters of etalon, which are obtained by using the Fourier series expansion.
The present invention was developed to solve the abovementioned problems and shortcomings in the prior art methods. It is therefore a first object of the invention to provide an optical gain equalizer which is capable of effectively compensating a gain wavelength dependency of an optical amplifier by a simple method, and a method for producing the same optical gain equalizer. It is a second object of the invention to provide an optical amplifier device having almost no gain wavelength dependency by proposing such an optical gain equalizer, and further it is a third object of the invention to provide a wavelength multiplexed light transmission system which is capable of suppressing the wavelength dependency of light intensity at the receiving side, by using an optical amplifier device in which the optical gain equalizer is used.
In order to achieve the above objects, the invention has the characteristic structures described below. That is, a first aspect of a method for producing an optical gain equalizer according to the invention is featured in that, where it is assumed that a loss wavelength characteristic which completely compensates the gain wavelength dependency of an optical amplifier device in the predetermined set range of wavelengths including at minimum a usage range of wavelengths is the ideal loss wavelength characteristic, N (N: a positive integer) wavelengths different from each other, which are optionally selected in the usage range of wavelengths is &lgr;i (i is an integer which increases sequentially from 1 like 1, 2, 3, . . . N), and design values of parameters which determine the loss wavelength characteristic of an optical component are a
1
, . . . , a
m
having a nonlinear coupling, the respective parameters a
1
Aso Osamu
Irie Yuichiro
Mizuno Kazuyo
Namiki Shu
Ozawa Shoichi
Hughes Deandra M.
Lacasse & Associates
Tarcza Thomas H.
The Furukawa Electric Co. Ltd.
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