Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-09-21
2003-04-29
Hellner, Mark (Department: 2733)
Optical: systems and elements
Optical amplifier
Optical fiber
Reexamination Certificate
active
06556346
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 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. EP patent application No. 98110594.3 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.
Several methods have been proposed to improve the system performances, for example in terms of amplification gain and amplification bandwidth.
One technique for improving the system performances consists in co-doping an erbium-doped amplification fiber with ytterbium (Yb). Co-doping an active fiber with erbium and ytterbium not only broadens the pump absorption band from 800 nm to 1100 nm, offering greater flexibility in selection of the pump wavelength, but also greatly increases the ground state absorption rate due to the higher absorption cross section and dopant solubility of ytterbium. The ytterbium ions absorb much of the pump light and the subsequent cross relaxation between adjacent ions of erbium and ytterbium allows the absorbed energy to be transferred to the erbium system. As described in Grubb et al., “+24.6 dBm output power Er/Yb co-doped optical amplifier pumped by diode-pumped Nd:YLF laser”, Electronics Letters, 1992, 28, (13) pp. 1275-1276, and in Maker, Ferguson, “1.56 &mgr;m Yb-sensitized Er fibre laser pumped by diode-pumped Nd:YAG and Nd:YLF lasers”, Electronics Letters, 1988, 24, (18), pp. 1160-1161, the co-doping technique may be applied to efficiently excite fiber amplifiers and lasers through direct pumping in the long wavelength tail of ytterbium absorption spectrum. This pumping is preferably performed by means of diode-pumped solid state lasers, for example 1047 nm Nd:YLF lasers or 1064 nm Nd:YAG lasers.
Using an erbium and ytterbium co-doped amplification fiber to amplify communication signals is further described in European patent application EP 0 803 944 A2 and in U.S. Pat. No. 5,225,925. EP 0 803 944 A2 refers to a multistage Er-doped fiber amplifier (EDFA) operating in the wavelength band 1544-1562 nm and comprising a first stage that includes Er and Al and a second stage that includes Er and a further rare earth element, for example Yb. Such multistage EDFA can have advantageous characteristics in the cited wavelength band over the all-erbium amplification systems, e.g. a relatively wide flat gain region, and relatively high output power, without significant degradation of the noise figure. However, the Applicant noted that the amplifier proposed in EP 0 803 944 A2 offers no advantages in terms of number of transmitted channels, the amplification bandwidth being still limited to the relatively narrow (and largely exploited) 1544-1562 nm band. Furthermore, the Er/Yb second stage is pumped by means of a diode-pumped Nd-doped fiber laser emitting at 1064 nm. This pump source, largely used for the excitation of mono-modal amplification fiber, is relatively expensive and cumbersome.
U.S. Pat. No. 5,225,925 relates to an optical fiber for amplifying or sourcing a light signal in a single transverse mode. The fiber comprises a host glass doped with erbium (Er) and a sensitizer such as ytterbium (Yb) or iron (Fe). Preferably the host glass is a doped silica glass (e.g. phosphate or borate doped). The Applicant noted that U.S. Pat. No. 5,225,925 proposes an amplification fiber that, due to the shape of its gain curve, is particularly adapted for the transmission of a single channel at 1535 nm but is not suitable for WDM transmissions. Moreover, such an amplification fiber is adapted to be pumped by means of a diode-pumped Nd-doped fiber laser that has the above mentioned disadvantages.
Neither EP 0 803 944 A2 nor U.S. Pat. No. 5,225,925 address amplification by an Er/Yb co-doped optical amplifier of a signal in a wavelength band different from the transmission band around 1550 nm.
An improvement of Er/Yb amplification fibers has been obtained by means of the cladding pumping technique, which consist in pumping the active fiber in an inner cladding region surrounding the core, instead that directly in the core. Cladding pumping is a technique that allows high power broadstripe diodes and diode bars to be employed as efficient, low cost and small dimension pump sources for double-clad rare earth doped single-mode fibers. Output powers ranging from several hundred milliwatts to several tens of watts may be attained by this technique. A double-clad Er/Yb fiber pumped by diode arrays at 980 nm is described, for example, in Minelly et al., “Diode-array pumping of Er3+/Yb3+ co-doped fibre lasers and amplifiers”, IEEE Photonics Technology Letters, 1993, 5, (3), pp. 301-303. The erbium-ytterbium co-doped scheme enables much higher ground state absorption for erbium in the band about 980 nm than singly-doped erbium fibers, resulting in much shorter optimum length. The technique of inserting the pump radiation into a portion of the fiber external the core (which can be identically identified as an inner cladding or an outer core) is also described, for example, in PCT patent application WO 95/10868 and in U.S. Pat. No. 5,696,782.
Several methods have also been proposed 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-paten
Di Pasquale Fabrizio
Meli Fausto
Sacchi Giovanni
Turolla Silvia
Corning O.T.I.SpA
Hellner Mark
Medlin Jennifer
Short Svetlana Z.
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