Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2001-02-28
2003-02-11
Moskowitz, Nelson (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S337300, C359S341330, C359S341500
Reexamination Certificate
active
06519080
ABSTRACT:
It is an object of the present invention to provide an optical amplifying unit to be used for optical telecommunications. The invention also relates to an optical transmission system, more particularly a wavelength division multiplexing (WDM) optical transmission system, which uses the above-mentioned optical amplifying unit. The optical amplifying unit of the invention is also adapted to be used in analog applications, for example for CATV systems.
In WDM optical transmission systems, transmission signals including several optical channels are sent over a same line (that includes at least an optical amplifier) by means of wavelength division multiplexing. The transmitted channels may be either digital or analog and are distinguishable because each of them is associated with a specific wavelength.
Present-day long-distance high-capacity optical transmission systems use optical fiber amplifiers that, differently from previously used electronic regenerators, do not need OE/EO conversion. An optical fiber amplifier includes an optical fiber of preset length, having the core doped with one or more rare earths so as to amplify optical signals by stimulated emission when excited by pump radiation.
Optical fibers doped with erbium (Er) have been developed for use as both optical amplifiers and lasers. These devices are of considerable importance since their operating wavelength coincides with the third window for optical fiber communications, around 1550 nm.
Patent application EP 964275 in the name of the Applicant proposes a thirty-two channels WDM optical transmission system that uses erbium-doped fiber amplifiers (EDFAs) in the wavelength bands 1529-1535 nm and 1541-1561 nm.
Patent application EP 897205 A2 in the name of Fujitsu Limited describes a device comprising an erbium-doped fiber optical amplifier, and first and second optical filters operatively connected to the optical amplifier to suppress the wavelength dependence of gain in the gain bands 1.52-1.54 &mgr;m and 1.54-1.58 &mgr;m, the optical amplifier being pumped in a 0.98 &mgr;m band or in a 1.48 &mgr;m band.
Several methods have been proposed to improve the system performances in terms of amplification bandwidth in order to increase the number of channels to be transmitted. One way to increase channel numbers is to narrow the channel spacing. However, narrowing channels spacing worsens nonlinear effects such as cross-phase modulation or four wave mixing, and makes accurate wavelength control of the optical transmitters necessary. Applicant has observed that a channel spacing lower than 50 GHz is difficult to achieve in practice do to the above reasons.
Another way to increase the channel number is to widen the usable wavelength bandwidth in the low loss region of the fiber. One key technology is optical amplification in the wavelength region over the conventional 1550 nm transmission band. In particular, the high wavelength band around 1590 nm, and precisely between 1565 nm and 1620 nm, is a very promising band for long-distance optical transmissions, in that a very high number of channels can be allocated in that band. If the optical amplifier for the 1565-1620 nm band must deal with a high number of channels, the spectral gain characteristics of such amplifier are fundamental to optimize the system's performances and costs. The use of the 1590 nm transmission wavelength region of erbium-doped fiber amplifiers in parallel to the 1530 and 1550 wavelength regions, is attractive and has been recently considered. As an additional advantage, by employing the 1590 nm wavelength region it is possible to use dispersion-shifted fiber (DSF) for WDM transmissions without any degradation caused by four-wave mixing.
Several articles in the patent and non-patent literature address amplification by erbium-doped fiber amplifiers in the high wavelength transmission band (from 1565 nm up to 1620 nm).
U.S. Pat. No. 5,500,764 relates to a SiO
2
—Al
2
O
3
—GeO
2
single-mode optical fiber (having a length between 150 m and 200 m) doped with erbium, pumped by 1.55 &mgr;m and 1.47 &mgr;m optical sources and adapted to amplify optical signals between 1.57 &mgr;m and 1.61 &mgr;m.
Ono et al., “Gain-Flattened Er
3+
-Doped Fiber Amplifier for a WDM Signal in the 1.57-1.60 &mgr;m Wavelength Region”, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 9, No. 5, May 1997, pp. 596-599, disclose a gain-flattened Er
3+
-doped silica-based fiber amplifier for the 1.58 &mgr;m band WDM signal; different fiber lengths were tested and the authors found that 200 m was the optimum length of EDF (Erbium-Doped Fiber) for constructing an EDFA with high gain and low noise.
Masuda et al., “Wideband, gain-flattened, erbium-doped fibre amplifiers with 3 dB bandwidths of >50 nm”, ELECTRONICS LETTERS, Jun. 5, 1997, Vol. 33, No. 12, pp. 1070-1072, propose a scheme with two-stage erbium-doped fibres and an intermediate equalizer, obtaining a 52 nm band (1556-1608 nm) for a silicate erbium-doped fiber amplifier and a 50 nm band (1554-1604 nm) for a fluoride erbium-doped fiber amplifier; in the case of a silicate erbium-doped fiber amplifier, the two stages include a 50 m EDF and a 26 m EDF, respectively. In the proposed experiment, the two EDFs are pumped by 1480 nm laser diode sources via dichroic mirror-type wavelength-selective couplers.
Jolley et al., “Demonstration of low PMD and negligible multipath interference in an ultra flat broad band EDFA using a highly doped erbium fiber”, “Optical Amplifiers and their Applications” Conference, Vail, Colo., Jul. 27-29 1998, TuD2-1/124-127 proposes a broad band EDFA which amplifies signals in the 1585 nm band using 45 m of erbium fiber and reaching a maximum external power of more than +18.3 dBm at 1570. The active fiber is bidirectionally pumped by a 980 nm laser diode and a 1480 nm laser diode.
The Applicant has observed that the pump wavelength is an important parameter for the design of optical amplifiers in the considered band, since it influences the amplifier's performances in terms of gain efficiency and noise figure. This influence is not observable in conventional amplifiers in the 1550 nm band, in which the effects of the pump wavelength choice are prevalently on the spectral shape of the gain curve.
F. A. Flood and C. C. Wang, “980-nm Pump-Band Wavelengths for Long-Wavelength-Band Erbium-Doped Fiber Amplifiers”, IEEE Photonics Technology Letters, Vol. 11, No. 10, Oct, 1999, attests the importance of a careful choice of pump wavelengths to ensure optimum amplifier performance in the long-wavelength band (L-band) and shows the dependency of output signal power and backward amplified spontaneous emission (ASE) power on 980-nm band pump wavelength and input signal power for a L-band EDFA. In particular, this article demonstrates that tuning pump wavelength ±30-nm away from the 980-nm absorption peak provides 3-5 dB improvement in pump-to-signal conversion.
The Applicant has tackled the problem of providing an optical amplifier for the L-band with improved performances with respect to the known amplifiers above described.
The Applicant has first observed that 1480-nm pumping determines best performances with respect to 980-nm pumping in the considered amplifiers in terms of power conversion efficiency and quantum conversion efficiency. For power EDFA's, the power conversion efficiency (PCE) can be defined as the ratio:
PCE
=
P
s
out
-
P
s
in
P
p
in
where P
s
in
, P
s
out
and P
p
in
are the signal power at the input and at the output of the amplifier and the pump power at the input of the amplifier, respectively, while the quantum conversion efficiency (QCE) can be defined by:
QCE
=
φ
s
out
-
φ
s
in
φ
p
in
=
λ
s
λ
p
PCE
,
where &phgr;
p
in
, &phgr;
s
in
and &phgr;
s
out
are the input or output pump and signal photon fluxes (&phgr;
p,s
x
=P
p,s
x
/hv
p,s
). The maximum possible value for the QCE is unity, which corresponds to the case where all pump photons are effectively converted into signal photons.
The Applicant has then found that, in the L-
Di Pasquale Fabrizio
Gurnari Paolo
Sacchi Giovanni
Vavassori Paolo
Corning O.T.I. SpA
Moskowitz Nelson
Short Svetlana Z.
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