Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2002-06-27
2003-06-24
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S349000
Reexamination Certificate
active
06583924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier for use in optical fiber communication systems and optical signal processing systems.
2. Description of the Related Art
The basic configuration of a conventional optical amplifier is shown in FIG.
22
. This optical amplifier is comprised by two amplifying sections having different gain band regions (L-amplifying section and S-amplifying section), a divider, and a combiner, and attempts to broaden the operational bandwidths by coupling the two gain band regions in the wavelength domain. (refer to “Broadband and gain-flattened amplifier composed of a 1.55 &mgr;m-band and a 1.58 &mgr;m-band Er
3+
-doped fiber amplifier in a parallel configuration”, M. Yamada et. al., IEE Electronics Letters, Vol. 33, No. 8, 1977, pp710-711) (Reference 1).
Various types of dividers and combiners are utilized in constructing such optical amplifiers, and they can be classified as a dielectric multi-layer filter or a combination of fiber grating in association with circulator. Those based on dielectric multi-layer filter are reported in the following references.
“Broadband and gain-flattened amplifier composed of a 1.55 &mgr;M-band and a 1.58 &mgr;m-band Er
3+
-doped fiber amplifier in a parallel configuration”, M. Yamada et. al., IEE Electronics Letters, vol. 33, no. 8, 1977, pp710-711. (Reference 1)
Japanese Unexamined Patent Application, First Publication, No. Hei 10-229238, Publication Date, Aug. 25, 1998. (Reference 2)
Japanese Unexamined Patent Application, First Publication, No. Hei 11-204859, Publication Date, Jul. 30, 1999. (Reference 3)
International Application Published under the Patent Cooperation Treaty, PCT/US98/16558, “Optical amplifier apparatus”, International publication date, Apr. 8, 1999, International Publication Number WO 99/17410. (Reference 4)
Those based on a combination of circulator and fiber grating are reported in the following references.
European Patent Application, EP 0 883 218 A1, “Wide band optical amplifier”, Publication date Sep. 12, 1998. (Reference 5)
“A gain-flattened ultra wide band EDFA for high capacity WDM optical communication systems”, Y. Sun et. al., Technical Digest of ECOC'98, pp.53-54, 1998. (Reference 6)
The following reference does not specify what type of device is used.
Japanese Unexamined Patent Application, First Publication, No. Hei 4-101124, Publication Date, Apr. 2, 1992. (Reference 7)
All of the above references assume that the wavelength division properties are either perfect or present no particular problems.
However, as will be shown with specific examples in the following, wavelength division properties of these devices are not perfect, and problems are encountered depending on the manner and conditions of applying the amplifiers.
First, the most common problem of such amplifiers is encountered when the dividers and combiners are made of dielectric multi-layer filters.
FIGS. 1A and 1B
show the configuration of a conventional optical amplifier, where
FIG. 1A
shows a design based on dielectric multi-layer filters of the long wavelength transmission type (L-type) for the divider and combiner, represented by L-divider (
3
) and L-combiner (
4
), and
FIG. 1B
shows a design based on dielectric multi-layer filters of the short wavelength transmission type (S-type) for the divider and combiner, represented by S-divider (
5
) and S-combiner (
6
).
The amplifying section has a gain medium and a pumping section for excitation, and examples of such optical amplifiers are rare-earth doped fiber amplifier, fiber Raman amplifier and semiconductor laser amplifier. The rare-earth doped fiber amplifiers include erbium-doped fiber amplifier and the like, and according to “Wideband erbium-doped fibre amplifiers with three-stage amplification”, H. Masuda et. al., IEE Electronics Letters, vol. 34, no. 6, 1998, pp567-568 (Reference 8), it is advantageous when such an amplifier has a gain equalizer to broaden the region of flat gain because such an amplifier can produce a large total gain bandwidth.
In optical communication systems, optical amplifiers are generally designed to receive wavelength-multiplexed light signals, and in optical signal processing systems for instruments and the like, optical amplifiers are generally designed to receive wavelength- multiplexed light signals or single wavelength light signals.
FIGS. 2A and 2B
show configurations of the divider, where
FIG. 2A
represents an L-type dielectric multi-layer filter (L-divider), and
FIG. 2B
represents an S-type dielectric multi-layer filter (S-divider). The L-divider receives input light containing a short-wavelength &lgr;s and a long wavelength &lgr;l in the common port (c), and transmits a long wavelength &lgr;l from the transmission port (l) and reflects a short wavelength &lgr;s from the reflection port (s). On the other hand, the S-divider receives input light containing a short-wavelength &lgr;s and a long wavelength &lgr;l in the common port (c), and reflects a long wavelength light &lgr;l from the reflection port (l) and transmits a short wavelength light &lgr;s from the transmission port (s).
FIGS. 3A and 3B
show configurations of the combiner, where
FIG. 3A
represents an L-type dielectric multi-layer filter (L-combiner), and
FIG. 3B
represents an S-type dielectric multi-layer filter (S-combiner). The L-combiner receives input light containing a short-wavelength &lgr;s from the reflection port (s) and a long wavelength &lgr;l from the transmission port (l), and outputs light containing a long wavelength &lgr;l and a short wavelength &lgr;s from the common port (c). On the other hand, the S-combiner receives input light containing a short-wavelength &lgr;s from the transmission port (s) and a long wavelength &lgr;l from the reflection port (l), and outputs light containing a long wavelength &lgr;l and a short wavelength &lgr;s from the common port (c).
Referring to
FIGS. 1A and 1B
, the dividers (L- and S-dividers)
3
and
5
, perform the steps described above, and divide the multiplexed signal light containing a long wavelength &lgr;l and a short wavelength &lgr;s into a long wavelength signal light &lgr;l and a short wavelength signal light &lgr;s, which are input into the respective amplifying sections (L- and S-amplifying sections)
1
and
2
, and are combined in the combiners
4
and
6
and multiplexed light signals are thus output.
However, there are problems in the performance of the light amplifiers described above, which will be explained in the following.
FIGS. 4A and 4B
show gain spectra obtained in the amplifying sections (L- and S-amplifying sections)
1
and
2
, where
FIG. 4A
shows an overall view of the gain region while
FIG. 4B
shows details of gains in the vicinities of the wave boundaries (wavelengths &lgr;tr-s to &lgr;tr-l) of the L- and S-amplifying sections. In
FIG. 4B
, wavelengths in the L-amplifying section are denoted by 1 l* and those in the S-amplifying section are denoted by &lgr;s*, and the peak gains in the L-, S-amplifying sections are denoted by G while the wavelength-specific gains for &lgr; l*, &lgr;s* are denoted by G*.
FIGS. 5A and 5B
show loss spectra in the L-divider, and show the losses relating to a transmission loss between ports c and s, and the same between ports c and l (refer to FIGS.
2
A and
2
B), which are denoted respectively by Lcs1 and Lcl1 where 1 indicates that the losses are related to long wavelengths.
FIG. 5A
shows the loss in the overall view of the gain region and
FIG. 5B
shows the details of the loss in the wave boundary. For both Lcs1 and Lcl1, the loss becomes larger as the wavelength of the signal waves moves away from the respective boundary wavelengths (wavelengths &lgr;tr-s to &lgr;tr-l) into fringes of the respective gain regions.
However, in the long wavelength side of the loss spectrum (i.e., at the &lgr;tr-l end), the loss Lcs1 between the port s (for reflected light) and the common port c is limited to a certain constant value, because of the contribution from resi
Kani Jun-ichi
Masuda Hiroji
Takachio Noboru
Black Thomas G.
Nippon Telegraph and Telephone Corporation
Sommer Andrew R
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