Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
1999-09-21
2002-03-19
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
C359S345000
Reexamination Certificate
active
06359728
ABSTRACT:
It is an object of the present invention to provide a pump device for pumping an active fiber of an optical amplifier. In particular, it is an object of the present invention to provide a pump device for coupling a pump radiation to an an optical amplifier adapted to be used in an optical transmission system, for example a wavelength division multiplexing (WDM) optical transmission system. The invention also relates to an optical amplifier that uses the above mentioned pump device.
Conventional optical fiber amplifiers include active fibers having a core doped with a rare earth element. Pump power at a characteristic wavelength for the rare earth element, when injected into the active fiber, excites the ions of the rare earth element, leading to gain in the core for an information signal propagating along the fiber.
Rare earth elements used for doping typically include Erbium (Er), Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), 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.
Traditional pump sources include single mode laser diodes and multi-mode broad area lasers coupled to the active fiber over single mode and multi-mode pumping fibers, respectively, to provide the pump power. Single mode lasers provide low pump power, typically in the order of 100 mW. Broad area lasers, on the other hand, provide high pump power, in the order of 500 mW. These lasers of high output power, however, cannot efficiently inject light into the small core of a single mode fiber. Consequently, the use of high power broad area lasers requires the use of wide core and multi-mode fibers for pumping optical amplifiers. This non-active pumping fiber in turn typically inputs the pump power through a coupler and into the active fiber, for example into the inner cladding of a double-clad active fiber, acting as a multi-mode core for the pump power.
In a double-clad amplifier fiber, pump power is guided into the inner multi-mode cladding of the fiber from which it is transferred into a single mode core doped with an active dopant. The double-cladding fiber pumping mechanism is described for example in 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 multi-mode optical couplers. The pump power propagates through the outer core and interacts with the inner core to pump active material contained in the inner core. This pumping technique is also described in U.S. Pat. No. 5,291,501, which illustrates a mono-mode optical fiber with doped core and doped inner cladding.
A well-known basic amplifying system includes a multi-mode pump source coupled to an amplification fiber, for example an ErNb doped doubleclad fiber, via a conventional fused fiber wavelength division multiplexer (WDM) type coupler. WDM couplers behave as multi-mode couplers for the pump power and transmit the single mode signals along the amplification fiber substantially without coupling to the pump fiber. During the pumping operation, most of the outer modes of the pump power are transmitted to the amplification fiber, leaving the inner modes of the pump power unused. In the case of a multi-mode or a double cladding amplifier fiber, a fused fiber coupler has a theoretical coupling coefficient directly proportional to the ratio of the areas of the two fibers constituting the coupler itself. In an ideal case for two identical fibers, the coupling coefficient is approximately 50%, but in practice it is in the range of 45-48%. This means that only about 45-48% of the total pump power passes from the pumping fiber into the inner cladding of the double-clad active fiber, while the remaining 52-55% remains in the pumping fiber.
Some systems use two optical fibers having different diameter of cores to improve the coupling coefficient of the multi-mode coupler. However, such arrangements often lead to a waste of power due to the difficulty in matching the tapering of two cores of different size.
To increase the coupling efficiency, the Applicant has considered the possibility of using micro optic couplers. Micro optic couplers couple optical beams using a wavelength selective mirror and a focusing lens system. With this construction, micro optic couplers obtain much better coupling efficiencies than traditional WDM couplers, typically in the range of 89%. Applicant has remarked that micro optic couplers have several drawbacks that limit their use for pump coupling in fiber amplifiers. In particular, if a single micro optic coupler is used upstream of the active fiber so as to feed the pump radiation to the active fiber in a co-propagating direction, the transmission signals passing through the coupler undergo a power loss that is much higher than the loss introduced by a fused fiber coupler and that may be excessive (particularly in consideration of the fact that the signals undergo the attenuation before being amplified and this leads to an increase of the noise figure for the amplifier). Alternatively, if a single micro optic coupler is positioned down-line with respect to the active fiber so as to feed the pump radiation to the active fiber in a counter-propagating direction, the signal to noise ratio undergoes a reduction which again may be excessive. Moreover, due to high coupling efficiency achievable by using a micro optic coupler, and then to the high pump power fed into the fiber, an inhomogeneous distribution of the population inversion is produced along the fiber.
More recent systems have attempted to recover the lost pump power in a conventional fused fiber coupler by means of different pumping schemes using fused fiber couplers. In particular, different solutions have been proposed that include a second optical coupler in addition to a first optical coupler positioned according to the above-described single-coupler arrangement. The second coupler is positioned at the opposite end of the active fiber with respect to the first coupler and is coupled to the first coupler through a multi-mode pump fiber so as to receive the residual pump power (i.e. the fraction of the pump power that has not been directly fed to the active fiber by means of the first coupler). The second coupler is then adapted to couple the residual pump power to the same active fiber in a counter-propagating direction, or to a different active fiber in a copropagating direction. The proposed pumping schemes using the above-mentioned technique to recover the pump power include only couplers of the fused fiber type. The Applicant observed that the addition of a second fused fiber coupler does not significantly improve the total pump power transfer over the single-coupler system described above. In fact, the second coupler receives from the pump fiber prevalently internal modes left over by the first coupling operated by the first coupler, and the transfer of the internal modes into the active fiber is inefficient.
EP patent application No 97114622.0 in the name of the Applicant proposes a technique to improve the total coupling efficiency in the above two couplers pumping schemes. The improvement is obtained by interposing, along the pump fiber connection coupling the first and the second coupler, a mode scrambler, i.e. a device that operates a scrambling of the inner modes on the residual pump radiation so as to regenerate a high number of external modes that can be efficiently transferred into the active fiber through the second coupler. Under ideal circumstances, 50% of the pump power signal enters the active fiber at each of the couplers. This would lead to a total
Angellieri Marco
Castagnetti Roberta
Sacchi Giovanni
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hellner Mark
Pirelli Cavi e Sistemi S.p.A.
LandOfFree
Pump device for pumping an active fiber of an optical... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Pump device for pumping an active fiber of an optical..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Pump device for pumping an active fiber of an optical... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2828471