Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2002-10-25
2004-08-24
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Particular active medium
C359S341100, C372S040000
Reexamination Certificate
active
06781750
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of optical amplifiers, and, more particularly, relates to the field of optical amplifiers comprising a length of optical fiber having an active dopant which fluoresces in response to pump light.
2. Description of the Related Art
Erbium-doped fiber amplifiers (EDFAs) are used extensively in commercial optical communication systems, both as all-optical repeaters and pre-amplifiers. After traveling through long lengths of communication fiber (typically several tens of kilometers), the information-encoded signals are strongly attenuated by fiber loss, and it is the role of the erbium-doped fiber amplifier (EDFA) to amplify the signals to a reasonable power level. EDFAs have now reached the point where they have been optimized with respect to their energy efficiency to minimize their requirement for pump power (which is costly). EDFAs have further been optimized with respect to their noise performance such that noise figures approaching the best-case theoretical limit of 3 dB are now possible. The gain flatness of optimized EDFAs now exceeds a few tens of dB over tens of nanometers of bandwidth. EDFAs can now be designed so that their gain depends very little on the polarization of the input signals.
One area of EDFA research that is still very active is the gain bandwidth. The gain bandwidth parameter is important because it ultimately dictates the number of signals of different wavelengths that can be amplified by a given EDFA. The broader the bandwidth is, the larger the number of individual signals that can be amplified, and therefore the higher the bandwidth (bits of information per unit time) that can be carried by a single fiber. Because the host affects the spectroscopy of the erbium ions, a number of fiber host materials, including silica, fluorozirconate glasses, and chalcogenides, have been and continue to be investigated in an attempt to identify a host that will provide a larger gain bandwidth for the
4
I
13/2
→
4
I
15/2
transition of Er
3+
. In silica-based glasses, the bandwidth is generally divided into what are known as the C-band and the L-band. In approximate terms, the C-band is the portion of the optical spectrum below about 1,565 nanometers, while the so-called L-band is the portion of the optical spectrum above about 1,565 nanometers. In silica-based fibers, the total bandwidth of the combined C and L-bands is approximately 80 nanometers, although this figure has only been attained so far by concatenating two EDFAs. (See, for example, Y. Sun, et al., 80
nm ultra
-
wideband erbium
-
doped silica fibre amplifier, Electronics Letters,
Vol. 33, No. 23, November 1997, pp. 1965-1967.) The situation is similar with a fluorozirconate host. (See, for example, S. Kawai, et al.,
Wide bandwidth and long distance WDM transmission using highly gain
-
flattened hybrid amplifier, Proceedings of Optical Fiber Communication OFC'
99, Paper FC3, February 1999, pp. 56-58.) In tellurite glass fibers, the total bandwidth is also around 80 nanometers, but it can be accomplished with a single fiber. (See, for example, Y. Ohishi, et al.,
Optical fiber amplifiers for WDM transmission, NTT R
&
D,
Vol. 46, No. 7, pp. 693-698, 1997; and Y. Ohishi, et al.,
Gain characteristics of tellurite
-
based erbium
-
doped fiber amplifiers for
1.5-&mgr;
m broadband amplification, Optics Letters,
Vol. 23, No. 4, February 1998, pp. 274-276.
FIG. 1
illustrates an exemplary standard EDFA configuration
100
comprising an erbium-doped fiber (EDF)
110
. Optical signals are input to the erbium-doped fiber
110
via a first optical isolator
120
and a wavelength division multiplexing (WDM) coupler
122
. An optical pump signal from an optical pump source
124
is also input to the erbium-doped optical fiber
110
via the WDM coupler
122
. The amplified output signals from erbium-doped optical fiber
110
are output through a second optical isolator
126
. The optical isolators
126
,
120
are included to eliminate backward reflections into the erbium-doped fiber
110
from the output port and to eliminate backward reflections from the erbium-doped fiber
110
to the input port, respectively. The erbium-doped optical fiber
110
can be pumped in the forward direction, as illustrated in
FIG. 1
, or in the backward direction (not shown) or in both directions. Because of the broad nature of the fiber gain medium, the configuration of
FIG. 1
produces gain over a large bandwidth. For example, erbium-doped tellurite fibers and erbium-doped chalcogenide fibers have been used in the configuration of FIG.
1
. As set forth in Y. Ohishi, et al.,
Gain characteristics of tellurite
-
based erbium
-
doped fiber amplifiers for
1.5-
&mgr;m broadband amplification, Optics Letters,
Vol. 23, No. 4, February 1998, pp. 274-276, gain bandwidths of around 80 nanometers have been produced using a tellurite fiber.
When the fiber host is a silica-based glass, gain cannot be provided over the whole bandwidth (approximately 1,525 nanometers to approximately 1,610 nanometers) with a single fiber. Instead, gain needs to be produced over two adjacent spectral regions, and then the outputs resulting from the gain are combined. A generic method for achieving broader gain bandwidth is to use hybrid amplifiers, in which two or more amplifiers made of different hosts are concatenated. The amplifiers are designed such that they provide gain spectra that complement each other, thus producing a larger overall gain bandwidth than either one of them. This method was successfully demonstrated with a silica-based EDFA followed by a fluoride-based EDFA, which produced a 0.5-dB-bandwidth of 17 nanometers. (See, P. F. Wysocki, et al.,
Dual
-
stage erbium
-
doped, erbium/ytterbium
-
codoped fiber amplifier with up to +
26-
dBm output power and a
17-
nm flat spectrum, Optics Letters,
Vol. 21, November 1996, pp. 1744-1746.) More recently, a similar concept was applied to two fluoride-based EDFAs. (See, Y. Sun, et al., 80
nm ultra
-
wideband erbium
-
doped silica fibre amplifier, Electronics Letters,
Vol. 33, No. 23, November 1997, pp. 1965-1967.)
FIG. 2
illustrates an exemplary configuration
200
having two EDFAs
210
,
220
. One EDFA (the lower EDFA
210
) is designed to amplify the C-band (from approximately 1,525 nanometers to approximately 1,565 nanometers), and the other EDFA (the upper EDFA
220
) is designed to amplify the L-band (approximately 1,565 nanometers to approximately 1,620 nanometers). The two EDFAs
210
,
220
advantageously include respective pump sources (not shown) which are coupled to the erbium-doped fibers using respective WDM couplers, as illustrated in FIG.
1
. The input signals, which have different wavelengths &lgr;
i
spaced apart by a certain amount, are split into the two branches with a WDM coupler
230
and the amplified output signals are combined in an output coupler
232
. An input optical isolator
240
and an output optical isolator
242
operate as described above. Because of the WDM coupler
230
, signals with wavelengths less than approximately 1,565 nanometers are coupled into the lower branch to propagate to the C-band EDFA
210
, and signals with wavelengths greater than approximately 1,565 nanometers are coupled into the upper branch to propagate to the L-band EDFA
220
. (In practice, there is a narrow guard band between the C-band and the L-band to avoid overlapping signals in the two arms.) For example, a silica-based EDFA can be designed to have an L-band with a gain spectrum that is flat within 0.5 dB over the 1,568-nanometer to 1,602-nanometer range. The gain flatness is partly achieved, for example, by selecting the proper fiber length or with the use of filters. Both methods are well known in the art.
The C-band EDFA
210
and the L-band EDFA
220
can be made from the same erbium-doped fiber, or of different fibers, or of different host materials. The C-band and L-band EDFAs
210
,
220
may differ in their respective designs, particularly with respect to
Digonnet Michel J. F.
Feillens Yannick G.
Fejer Martin M.
Black Thomas G.
Cunningham Stephen
Knobbe Martens Olson & Bear LLP
The Board of Trustees of the Leland Stanford Junior University
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