Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2000-09-28
2002-08-27
Moskowitz, Nelson (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S341330, C372S070000
Reexamination Certificate
active
06441954
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical fiber amplifier and, in particular, to a method for varying an overall scale factor of the gain spectrum of the optical fiber amplifier without substantially changing the shape of the gain spectrum. Thus, while in a conventional erbium doped fiber amplifier (EDFA), the magnitude and shape of the gain spectrum are completely coupled, the present invention provides a technique for decoupling the magnitude and shape of the gain spectrum of an EDFA.
BACKGROUND OF THE INVENTION
Achieving a uniform or flat gain spectrum is desirable for optical amplifiers used in wavelength-multiplexed communication systems. The gain flatness of an EDFA can be optimized by using host glass materials for the erbium that produce flat gain spectra (e.g., aluminum codoped silica or fluoride glasses such as ZBLAN) and operating the amplifier at an average inversion that provides optimally flat gain in the spectral region of interest (C. R. Giles and D. J. D. Giovanni, “Spectral dependence of gain and noise in erbium-doped fiber amplifiers,”
IEEE Photonics Technology Letters,
vol. 2, pp. 797-800, 1990). The gain flatness can further be improved through the use of gain flattening filters (M. Tachibana, R. I. Laming, P. R. Morkel, and D. N. Payne, “Erbium-doped fiber amplifier with flattened gain spectrum,”
IEEE Photonics Technology Letters,
vol. 3, pp. 118-120, 1991). All of these techniques, however, only provide the optimum gain flatness at a single gain value (i.e., gain at any particular wavelength). It is well known that if the gain of an EDFA is changed by changing the inversion (e.g., by changing the relative pumping rate), the gain changes in a well defined spectrally dependent manner (C. R. Giles and D. J. D. Giovanni, “Spectral dependence of gain and noise in erbium-doped fiber amplifiers,”
IEEE Photonics Technology Letters,
vol. 2, pp. 797-800, 1990; J. Nilsson, Y. W. Lee, and W. H. Choe, “Erbium doped fibre amplifier with dynamic gain flatness for WDM.,”
Electronics Letters,
vol. 31, pp. 1578-1579, 1995). As a result, if a conventional EDFA is used in an application where its gain needs to be different from the flattest gain of the amplifier, its gain spectrum will show excess normalized gain ripple ((maximum gain—minimum gain)/minimum gain as calculated on the wavelength band of interest).
An example of how this can be a problem is provided by an optically amplified fiber transmission system where one needs to support fiber spans shorter than those for which the amplifier is designed. It is typically impractical to have separate amplifiers custom designed for each fiber span. Therefore, one is either forced to have an amplifier with a distorted gain spectrum or to add enough loss to the system that the design gain is actually needed from the amplifier. The latter uses more optical power than a redesigned amplifier would and has inferior noise performance, even if the loss is added between stages of an EDFA (Y. Sugaya, S. Kinoshita, and T. Chikama, “Novel configuration for low-noise and wide-dynamic-range Er-doped fiber amplifier for WDM systems,” in Optical Amplifiers and their Applications, 1995 OSA Technical Digest Series, Vol. (Optical Society of America, Washington, D.C.) 158-161).
It is an object of the present invention to provide a novel technique which enables one to change an overall scale factor of a gain spectrum of an optical amplifier such as an EDFA without substantially changing the shape of the gain spectrum. More generally, it is an object of the invention to provide a technique for decoupling the magnitude and shape of the gain spectrum of an EDFA or other optical amplifier. (The two are completely coupled in a conventional EDFA.) Thus, using the present invention, it is possible to provide an EFDA in a span which is shorter than the design span without adding attenuation, by controlling the scale factor of the gain spectrum.
Gain can be obtained in an EDFA when some fraction of the erbium dopant ions are excited into the metastable
4
I
13/2
state. A signal band photon (wavelength typically 1525-1600 nm) incident upon an excited ion can stimulate the release of a photon identical to itself and cause the erbium ion to return the
4
I
15/2
ground state. Silica based EDFAs used in telecommunication systems are typically pumped at wavelengths near (+/−~25 nm) 1480 nm or 980 nm. As shown in
FIG. 1
, the former directly excites the ions from the ground state to the metastable level, whereas the latter makes use of the auxiliary
4
I
11/2
state. Other pump bands are possible, but lower power conversion efficiency is typically found in 650 nm and 800 nm bands due to excited state absorption (ESA) and consequently they are not used in commercial systems.
The gain spectrum of an EDFA can be approximately written as
G
dB
(&lgr;)=10 log
10
(
e
)&Ggr;(&lgr;)
L
EDF
[{overscore (N
2
+L )}&sgr;
e
(&lgr;)−
{overscore (N
1
+L )}&sgr;
a
(&lgr;)] (1)
where G
dB
is the amplifier gain in decibels, L
EDF
is the total length of the erbium-doped fiber (EDF) within the amplifier, &sgr;
e
and &sgr;
a
are the emission and absorption cross sections, respectively, and N
1
and N
2
are average populations (ions per unit volume) of the ground state and metastable levels, respectively (C. R. Giles and E. Desurvire, “Modeling erbium-doped fiber amplifiers,”
Journal of Lightwave Technology,
vol. 9, pp. 271-283, 1991). The local fractional average inversion (i.e., inversion averaged over a cross section of the fiber at a particular axial point) is calculated as
n
i
_
=
1
L
∫
0
L
ⅆ
z
′
∫
0
doping
radius
r
ⅆ
r
N
i
(
r
,
z
′
)
/
[
N
1
(
r
,
z
′
)
+
N
2
(
r
,
z
′
)
]
{
i
=
1
,
2
}
.
(
2
)
which will be convenient for re-writing the form of Eqn. (1) below.
EDFAs are typically used such that nearly all erbium ions are in the metastable
4
I
13/2
level (level 2) or the ground state
4
I
15/2
. This is because efficiency reducing ESA is likely to be a problem if this is not the case. When pumping in the 980 band, ions are predominantly moved from the ground state to the
4
I
11/2
level. In silica this state has a short lifetime on the order of 10 &mgr;s and the ions undergo non-radiative decay to the metastable state. Because the lifetime of the
4
I
11/2
level is so much shorter than that of the metastable (at the power levels typically encountered in commercial communication systems) the population of this level is typically negligible. In low phonon energy glasses (i.e., glasses with a phonon energy significantly lower than in silica) such as ZBLAN, the
4
I
11/2
level has a lifetime on the order of 10 ms which is a large fraction of that of the metastable. Furthermore, there is an ESA within the 980 nm pump band which results in the excitation of an ion from the
4
I
11/2
to the
4
I
7/2
state and this process can reduce the efficiency of the amplifier. As a result, EDFAs made out of low phonon energy glasses have typically not been pumped in the 980 nm band. Recently, there has been work at finding 980 nm band wavelengths which would provide efficient amplification primarily for obtaining a high inversion to get a low noise figure (good noise performance) (M. Yamada, Y. Ohishi, T. Kanamori, H. Ono, S. Sudo and M. Shimizu, “Low-noise and gain-flattened fluoride-based Er
3
+-doped fiber amplifier pumped by 0.97 &mgr;m laser diode,”
Optics Letters,
vol. 33, pp. 809-810, 1997; M. Yamada, Y. Ohishi, T. Kanamori, S. Sudo and M. Shimizu, “Low-noise and gain-flattened fluoride-based Er
3
+-doped fiber amplifier pumped by 0.97 &mgr;m laser diode,”
Optics Letters,
vol. 22, pp. 1235-1237, 1997). Good power conversion efficiencies have yet to be demonstrated, though, so cost effective 980 nm band pumping has yet to be proven. Since N
1
+N
2
is approximately equal to the total number of active erbium ions in the amplifier
Corning Incorporated
Moskowitz Nelson
Short Svetlana Z
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