FIELD OF THE INVENTION 0001 The present invention relates to optical amplifiers having operating wavelengths longer than main emission peak wavelengths, and more particularly to erbium doped fiber and waveguide amplifiers operating in the long wavelength regime (1560-1620 nm), especially for wavelength division multiplexing (WDM) applications.

BACKGROUND OF THE INVENTION 0002 Conventional erbium doped fiber amplifiers (EDFA) have been extensively used in optical telecommunications as means to amplify weak optical signals in the third telecommunication window (near 1550 nm) between telecommunication links. Much work has been done on the design of these amplifiers to provide efficient performance, such as high optical gain and low noise figure. However, with the recent enormous growth of data traffic in telecommunications, owing to the Internet, intranets, and e-commerce, new optical transmission bandwidths are required to provide increased transmission capacity in dense wavelength division multiplexing (DWDM) systems. 0003 There are a few solutions to this demand. One proposed solution is to utilize new materials compositions as a host for the fiber gain medium (instead of silica) such as telluride, which may provide broader amplification bandwidth (up to 80 nm). However, the non-uniform gain shape and poor mechanical properties of telluride glass make these amplifiers difficult to implement in the telecom systems. Also, Raman amplifiers can be considered as an alternative solution to high bandwidth demand, since these amplifiers are capable of providing flexible amplification wavelength with a broad bandwidth. However, these amplifiers place restrictions on optical system architectures because of their required designs for efficient performance, such as long fiber length (>5 km), high pump power (>500 mW) and co-pumping configurations. On the other hand, relatively long erbium doped fibers (EDFs) may also provide amplification in the long wavelength range (1565-1625 nm) when they are used with high power pump sources. This range is commonly called L band. The conventional range, also known as C band is in the wavelength range between 1525-1565 nm. 0004 In principle, L band amplifiers take advantage of the fact that EDFs have higher emission cross-section than absorption cross-section at longer wavelengths. Therefore, for long EDFs, amplified spontaneous emission (ASE) becomes more emphasized at long wavelengths. However, there are still several issues for optimization of L band amplifiers for efficient performance. So far, reported performance of L band EDFAs has been inferior to that of C band EDFAs, with drawbacks as evidenced by higher noise figure (NF) and lower output power and gain. It would be beneficial to provide an L band amplifier with a low noise figure and high output power and gain.

BRIEF SUMMARY OF THE INVENTION 0005 The present invention also provides an L band optical amplifier. The optical amplifier comprises a signal line including an input, an output disposed optically downstream of the input and an amplifying gain medium optically disposed between the input and the output. The optical amplifier further comprises a laser optically connected to the first amplifying gain medium and a C band seed pump optically connected to the signal line between the input and the amplifying gain medium for directing C band light into the amplifying gain medium.

BRIEF DESCRIPTION OF THE DRAWINGS 0006 The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention. In the drawings: 0007 FIG. 1 is a schematic drawing of an L band amplifier according to a first embodiment of the present invention. 0008 FIG. 2 is a schematic drawing of an L band amplifier according to a second embodiment of the present invention. 0009 FIG. 3 is a graph showing measured gain and noise figures vs. input signal wavelength for the first embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION 0010 In the drawings, like numerals indicate like elements throughout. The present invention provides novel techniques and arrangements for improving the performance of L band EDFAs. In general, the present invention utilizes ASE in the C band to provide additional amplification capability in the amplifier. The ASE is generated during signal amplification by a separate C band seed pump. Components are defined to be optically connected when light signals can be transmitted between those components. 0011 A first embodiment of an L band amplifier 700 according to the present invention is shown schematically in FIG. 1. The amplifier 700 includes a signal line 702 which extends from an input Pin at one end of the amplifier 700 to an output Pout at another end of the amplifier 700. As used herein, the term optically downstream is defined to mean a direction along the signal line 702 from the input Pin toward the output Pout. The input Pin and the output Pout are optically connected to each other along the signal line 702 through the amplifier 700. Signal light Xs having at least one, and preferably, multiple wavelengths is transmitted through the amplifier 700 from the input Pin to the output Pout, from left to right as shown in FIG. 1. The wavelengths of the signal light S preferably range approximately from 1565 to 1625 nanometers, placing the signal light S in the L band. Those skilled in the art will recognize that the signal line 702 can be a fiber, a waveguide, or other light transmitting device. 0012 A C-L band multiplexer 710 is optically disposed in the signal line 702 between the input Pin and the output Pout. The C-L band multiplexer 710 optically connects a tunable C band seed pump 712 to the signal line 702 via a C band pump guide 714. Alternatively, an optical coupler (not shown) can be used instead of the C-L band multiplexer 710. A first optical isolator 720 is disposed in the signal line 702 optically downstream of the C-L band multiplexer 710. The first optical isolator 720 prevents backscattered light and other optical noise from traveling backward along the signal line 702, from the first optical isolator 710 toward the input Pin. 0013 A pump-signal multiplexer 730 is disposed along the signal line 702 optically downstream of the first optical isolator 720. The pump-signal multiplexer 730 couples a first end of a pump laser guide 732 to the signal line 702. A second end of the pump laser guide 732 is connected to a pump laser 734. Preferably, the pump laser 734 is a 980 nanometer laser which emits a pump signal P, although those skilled in the art will recognize that other wavelengths can be used as well. Also preferably, the pump laser 734 has an output power of at least 100 mW, although those skilled in the art will recognize that the pump laser 734 can have other output powers as well. 0014 A rare earth doped amplifying gain medium 740 is disposed along the signal line 702 optically downstream of the pump-signal multiplexer 730. Preferably, the rare earth is erbium, although those skilled in the art will recognize that other elements, including, but not limited to lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, thulium, ytterbium, lutetium, and combinations and blends thereof can be used. Although the amplifying gain medium 740 does not have a minimum or maximum length, those skilled in the art will recognize that the length of the amplifying gain medium 740 can be varied, in conjunction with different output powers of the pump laser 734, to provide different amplification gains and/or output powers. While the amplifying gain medium 740 is preferably a fiber, those skilled in the art will recognize that the amplifying gain medium 740 can also be a waveguide or other light transmitting device. 0015 A second optical isolator 750 is disposed along the signal line 702 optically downstream of the amplifying gain medium 740. The second optical isolator 750 prevents backscattered light and other optical noise from traveling backward along the signal line 702, from the second optical isolator 750 toward the amplifying gain portion 740. The second optical isolator 750 is optically connected to the output Pout of the amplifier 700. 0016 In operation, the signal light S having a wavelength band of approximately between 1565 and 1625 nanometers is injected into the amplifier 700 in a first direction at the input Pin. The signal light S is transmitted along the signal line 702 to C-L band multiplexer 710. The signal light S passes through the C-L band multiplexer 710 and along the signal line 702 to the first optical isolator 720. The signal light S passes through the first optical isolator 720 to the pump-signal multiplexer 730. 0017 The C band seed pump 712 generates a tunable C band light signal C, between 1530 nm and 1570 nm. The C band seed pump 712 can be tuned to generate an optimized C band seed wavelength for transmission toward the amplifying gain medium 740. The C band light signal S travels along the C band pump guide 714 to the C-L band multiplexer 720, where the C band light signal S enters the signal line 702. The C band light signal S then travels along the signal line 702 with the signal light s. 0018 The pump laser 734 transmits a 980 nanometer pump signal P along the pump laser guide 732 to the pump-signal multiplexer 730. At the pump-signal multiplexer 730, the signal light S and the C band light signal C are combined with the pump signal P emitted by the pump laser 734. The C band light signal C is amplified in the gain medium 740 and suppresses the backward ASE. The amplified C band light signal C, as well as the signal light S and the pump signal P, propagate through the amplifying gain medium 740. The amplified C band light signal C and any residual pump signal P excite the rare earth element in the amplifying gain medium 740, amplifying the signal light S. The C band light signal C does not significantly generate ASE in the C band because of the longer wavelength of the C band light signal C. As a result, backward ASE is significantly reduced and additional C band pumping by the C band seed is generated, resulting in greater amplification of the signal light S. 0019 A second embodiment of an L band amplifier 700 is shown schematically in FIG. 2. The second embodiment is similar to the first embodiment shown in FIG. 1, but with additional C band seed pumps 7121 through 712n optically connected to the signal line 702. Each C band seed pump 712, 7121 through 712n generate C band seed at a separate wavelength within the C C band. The multiple wavelengths of C band seed provide additional amplification of the signal light S over the C band seed provided by the single C band seed pump 712. 0020 The top two curves on the graph of FIG. 3 (solid square and solid circle) show measured gain vs. input signal wavelength for the first embodiment of the present invention. The third curve (open circle) shows measured gain vs. input signal wavelength for an amplifier using a Bragg grating to reflect C band ASE. The fourth curve (solid triangle) shows measured gain vs. input signal wavelength without any seed. The pump laser 734 used was a 980 nm pump, operating at approximately 180 mW. 0021 It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.