Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1998-10-16
2001-01-09
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S334000
Reexamination Certificate
active
06172803
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical amplifier and a transmission system which uses it, which are necessary in an optical fiber transmission system and optical signal processing system.
DESCRIPTION OF RELATED ART
The structure of an optical amplifier of the related technology used in an optical fiber transmission system is shown in FIGS.
23
~
25
.
FIGS. 23
,
24
, and
25
show respectively the first, second, and third structures of the optical amplifiers of the related technology.
In
FIG. 23
, the optical amplifier
23
-
1
comprises an amplifier
23
-
2
and a gain equalizer
23
-
3
. This optical amplifier
23
-
1
is connected to transmission fibers
23
-
4
and
23
-
5
. Signal beams with a plurality of wavelengths are incident on this optical amplifier
23
-
1
, and amplified. This amplifier
23
-
2
comprises a gain medium
23
-
6
(a rare-earth element doped fiber or waveguide), an excitation light source
23
-
7
, and an optical part
23
-
8
(multiplexer for excitation light and signal beam, a light isolator, etc.) disposed on the pre-stage of a gain medium
23
-
6
, and an optical part
23
-
9
(optical isolator, etc.) disposed on the post-stage of the gain medium
23
-
6
(see Citation Massicott et al., Electron. Lett., vol. 26, No. 20, pp. 1645-1646, 1990).
The gain characteristics of the optical amplifier
23
-
1
whose structure is shown in
FIG. 23
are shown in FIGS.
26
A~
26
C.
FIG. 26A
shows the wavelength dependency of the gain of the gain medium
23
-
6
. In
FIG. 26A
, the peak value of the gain is 30 dB, the gain-flattened bandwidth (for example, the 3 dB gain-reduction bandwidth) is 10 nm. The loss of the gain equalizer
23
-
3
is shown in FIG.
26
B. The peak value of this loss is about 10 dB. The value obtained by subtracting the loss of
FIG. 26B
from the gain of
FIG. 26A
is the gain of the optical amplifier
23
-
1
, and this is shown in FIG.
26
C. For simplification, the loss of the optical part
23
-
8
and the optical part
23
-
9
are ignored. By using the gain equalizer
23
-
3
, the gain-flattened bandwidth is increased by about 30 nm. In this manner, as long as the signal beam wavelength intervals are equal, if the gain-flattened bandwidth is widened, it is an advantage that signal beams of more wavelengths (and therefore more channels) can be amplified with an identical gain.
FIG. 24
has the same gain characteristics as
FIG. 23
, but compared to
FIG. 23
, this structure of an optical amplifier has lower noise. The difference between this figure and
FIG. 23
is that in this figure two excitation light sources
23
-
7
and
24
-
3
with different excitation light wavelengths are used. The wavelength of the excitation light which is output by excitation light source
24
-
3
is shorter than the wavelength of the excitation light output by excitation light source
23
-
7
, and the upper part of the gain medium
23
-
6
(with respect to the input direction of the signal beam) is excited to a higher population inversion state in comparison to
FIG. 23
(see Citation Massicott et al., Electron. Lett., vol. 28, No. 20, pp. 1924-1925, 1992).
FIG. 25
is an optical amplifier with a structure analogous to the structure of the present invention, although the widening of the bandwidth of the gain was not planned. The amplifier is divided into a pre-stage (amplifier
25
-
2
) and a post-stage (amplifier
25
-
3
), and a band restricting optical filter or a dispersion compensator is disposed therebetween. The signal beam is generally a single wavelength. When a band limiting optical filter is used, because the gain medium is divided into two stages, degradation of the amplification characteristics due to laser oscillation or amplified spontaneous emission light is not incurred, and a high gain is possible. When using a dispersion compensator, it is possible to eliminate degradation of the signal to noise ratio due to loss in the dispersion compensator (see Citation Masuda et al., Electron. Lett., vol. 26, No. 10, pp. 661-662, 1990).
In the structures shown in FIG.
23
and
FIG. 24
, flattened-gain dependence of the flat-gain bandwidth and equalizer loss dependency of the optical amplifier saturation power are shown respectively in FIG.
9
A and FIG.
9
B. In
FIG. 9A
, the flattened-gain bandwidth decreases along with the increase in the flattened-gain, and the flattened-gain is limited to 30 dB because of amplification characteristics degradation due to laser oscillation and amplified spontaneous emission light. In contrast, in
FIG. 9B
, the optical amplifier saturation output power remarkably decreases along with the increase in the equalizer loss. However, the drawback occurs that in obtaining a wide flattened-gain bandwidth, it is difficult to obtain a wide flattened-gain bandwidth while maintaining a large optical amplifier saturation output power because of a necessarily large equalizer loss.
The object of the present invention is to resolve these problems, and provide a wide bandwidth optical amplifier.
SUMMARY OF THE INVENTION
In order to obtain the above-described object, the present invention provides an optical amplifier provided with a split gain medium wherein a long gain medium using a rare-earth doped fiber as the gain medium is partitioned into two or more stages, two or more amplifiers which include excitation light sources which output excitation light such that the effective excitation wavelength of this gain medium is 1.53 &mgr;m, and a gain equalizer which is effective for a wide wavelength band of a gain medium disposed between each amplifier. In this manner, compared to the related technologies, the effect is obtained that the gain-flattened band is wide, and it is possible to realize a high saturation output, low noise optical amplifier.
In addition, the present invention provides a Raman amplifier provided with a high nonlinear fiber or a dispersion compensation fiber as a Raman amplifier medium, and carries out Raman amplification by this Raman amplifier medium, and a rare-earth element doped fiber amplifier which makes a rare-earth doped fiber the amplification medium. In this manner, the gain bandwidth is flattened and it is possible to structure a broadband lumped constant optical amplifier.
In addition, the present invention provides an optical transmission system with an optical amplifier as a structural component provided with a Raman amplifier which carries out Raman amplification by a dispersion-compensation fiber wherein a parameter which compensates the dispersion of the transmission path is set, and a rare-earth doped fiber amplifier which uses a rare-earth fiber as an amplifier medium. In this manner, when using a dispersion-compensation fiber as Raman amplifier medium, it is possible to realize large capacity wavelength division multiplex optical transmission because it is possible to compensate the dispersion of the transmission path.
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H. Masuda, et al., “Ultra-wideband Optical Amplification with 3dB Bandwidth of 65 nm using a gain-Equalized Two-stage Erbium-doped Fiber Amplifier and Raman Amplification,” Electronics Letters, vol. 33, No. 9, pp. 753-754, Apr. 24th, 1997.
H. Masuda, et al., “Wideband, Gain-flattened, Erbium-doped Fiber Amplifiers with 3dB Bandwidths of >50 nm”, Electronics Letters, vol. 33, No. 12, pp. 1070-1071, Jun. 5th, 1997.
H. Masuda, et al., “High Gain Two-stage Amplification with Erbium-dop
Aida Kazuo
Kawai Shingo
Masuda Hiroji
Suzuki Ken'ichi
Hellner Mark
Nippon Telegraph and Telephone Corporation
Pennie & Edmonds LLP
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