Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2000-09-28
2003-08-05
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S343000, C359S341310, C372S006000
Reexamination Certificate
active
06603598
ABSTRACT:
BACKGROUND
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 can include one or more optical amplifiers, 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 DE/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. This pump radiation, when injected into the active fiber, excites the ions of the rare earth element, leading to gain in the core for an information bearing signal propagating along the fiber.
Rare earth elements used for doping typically include Erbium (Er), Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), Thulium (Tm) and Praseodymium (Pr). The particular rare earth element or elements used is determined in accordance with the wavelength of the input signal light and the wavelength of the pump light. For example, Er ions would be used for input signal light having a wavelength of 1.55 &mgr;m and for pump power having a wavelength of 1.48 &mgr;m or 0.98 &mgr;m; co-doping with Er and Yb ions, further, allows different and broader pump wavelength bands to be used.
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 Ser. No. 98110594.3 in the name of the Applicant proposes a thirty-two channels WDM optical transmission have optical characteristics that allow, in addition to the required wavelength selectivity, a predetermined spatial distribution of the light. If a micro-optic coupler is used, a focusing lens system able to provide the considered spatial distribution of the light is very difficult to implement. Therefore, the use of a double-cladding active fiber involves difficulties in achieving a high coupling efficiency between the pump source and the active fiber. Moreover, the considered micro-optic coupler has a relatively high insertion loss, greater than 1 dB at 1550 nm. 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 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 bulky.
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, for the excitation of Yb ions, 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-cladding 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-cladding Er/Yb fiber pumped by diode arrays at 980 nm is described, for example, in Minelly et al., “Diode-array pumping of Er
3+
/Yb
3+
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 to 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. This document discloses a fiber optic amplifier comprising a fiber with two concentric cores. Pump power provided by multi-mode sources couples transversely to the outer core (equivalent to an inner cladding) of the fiber through multi-mode fibers and mul
Di Pasquale Fabrizio
Gusmeroli Valeria Giuseppina
Oliveti Guido
Rossi Giacomo
Sacchi Giovanni
Black Thomas G.
Corning O.T.I. Inc.
Medlin Jennifer P.
Short Svetlana Z.
Sommer Andrew R.
LandOfFree
Optical amplifying unit and optical transmission system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical amplifying unit and optical transmission system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical amplifying unit and optical transmission system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3091902