Noise Figure in Optical Amplifiers With a Split-Band Architecture

This invention relates to broadband amplifiers for wavelength division multiplex optical communication networks and, more particularly, to the use of erbium-doped fiber amplifiers (EDFA).

Previous work reported by J. F. Massicott, et al, in Elect. Lett., 1990, vol. 26, pp. 1645-1646, has shown that optical gain can be obtained in erbium-doped silica fibers in the long wavelength range between 1570 and 1600 nanometers (L-band). It is more recently reported by Y. Sun, et al, in Proc. Opt. Amplifiers and Their Appl., PD2 Canada, July, 1997 and in Proc. ECOC., PDP 69, Edingburgh, U.K., September 1997, that by combining the gain in the conventional wavelength range between 1525 and 1565 nm (C-band) and the gain of the L-band, a bandwidth of 80 nm can be obtained using a split band EDFA structure.

The ultra wide band EDFA is physically divided into two (or more) sections. The first section includes a common amplification stage in order to achieve a good noise figure. In the second section the optical channels are split into separate parallel channels for amplification, one for the C-band and one for the L-band. Demultiplexing into the separate parallel channels may be accomplished by the combination of a circulator and a broad band fiber. A similar circulator and Bragg grating after the parallel channels can be used to re-multiplex the signals and an additional common output section to increase output power can be added, if desired.

While the common amplification stage improves noise figure, the improvement in the L-band is not as good as in the (-band because the gain of the first stage for the L-band is much lower than for the C-band. The loss associated with demultiplexing after the input gain stage increases the noise figure of the L-band more than the C-band. Accordingly, it would be advantageous to have a split-band optical amplifier architecture in which the input gain section yielded high gain for the L-band while maintaining high C-band gain.

This application incorporates by reference the disclosure of application Ser. No. 08/498112 filed May 15, 1998 entitled "Wide Band Optical Amplifier" and assigned to the assignee of the present invention. In accordance with the principles of the invention, in one illustrative embodiment thereof, a circulator is used in the input gain section, instead of the input isolator that was employed in the prior art. A broadband grating reflector is used after the input gain section to reflect the L-band signals so that the L-band signals pass through the common input gain section twice. The L-band signals are then directed by the circulator to a dedicated L-band gain section of the split structure. Meanwhile, the C-band signals pass through the reflector and move on to a dedicated C-band gain section of the split structure.

In another illustrative embodiment, a C-band broadband grating reflector is employed after the input gain section. C-band signals pass through the common input gain section twice, while the L-band signals and pump pass through the grating reflector and travel through the rest of the erbium-doped fiber.

The foregoing and other features of the present invention may become more apparent from a reading of the ensuing description when read together with the drawing, in which:

FIG. 1 shows a prior art ultra wide band erbium-doped fiber amplifier;

FIG. 2 shows one embodiment of the present invention; and

FIG. 3 shows an alternative embodiment of the invention.

Referring to FIG. 1, a prior art ultra wide band EDFA has a first common amplification section which includes an isolator 1, erbium-doped fiber 2, wavelength division multiplexer 3 and 980 nm counter-pump diode laser P.sub.I. The use of the counter-pump laser in the input gain section improves noise figure with sufficient pump power. After the input gain section, the optical channels are split (de-multiplexed) into two bands by circulator 4 and broad band fiber Bragg grating reflector 7. The C-band extends between approximately about 1525 and 1565 nm while the L-band covers between about 1570 and 1600 nm. In order to achieve the same gain for the C- and L-bands, the overall length of the erbium-doped fiber (11, 19, 29, 35, 41) for the L-band channel is longer than for the C-band channel because the gain per unit length for the L-band is lower than for the C-band.

After the input gain stage, the de-multiplexed C-band optical channel signals enter a dispersion compensating section in which gain is provided by wavelength division multiplexer 8 and 980 nm co-pump diode laser P.sub.C1, erbium-doped fiber 10 and isolator 12, followed by dispersion compensating element 14. Thereafter, the C-band signals encounter the gain equalization filter and large dynamic range (G.E.F. & L.D.R.) section which includes wavelength division multiplexer 16 and 980 nm co-pump diode laser P.sub.C2, erbium-doped fiber 18, isolator 20, gain equalization filter 22 and attenuator 24. Next, the C-band signals enter the power stage which includes wavelength division multiplexer 26 and 1480 nm diode laser co-pump P.sub.C4 followed by wavelength division multiplexer 28 and 980 nm laser co-pump P.sub.C3 ; erbium-doped fiber 30 and finally wavelength division multiplexer 32 and 1480 nm laser counter pump P.sub.C5.

In the L-band optical channel, after the de-mux stage the signals enter a dispersion compensating section which includes wavelength division multiplexer 9 and 980 nm co-pump laser P.sub.L1, erbium-doped fiber 11, isolator 13 and dispersion compensating filter 15. Next the L-band signals enter an equalization filter and large dynamic range section (G.E.F. & L.D.R.) which includes wavelength division multiplexer 17 and diode laser co-pump P.sub.L2 ; erbium-doped fiber 19; isolator 21; gain equalization filter 23 and attenuator 25. The power stage includes wavelength division multiplexer 27 and 980 nm co-pump P.sub.L3 ; erbium-doped fiber 29, isolator 31, wavelength division multiplexer 33 and 1480 nm co-pump P.sub.L4, isolator 37; wavelength division multiplexer 39 and 1480 nm co-pump P.sub.L5. Finally, both the C-band and L-band sections are re-multiplexed by broad band fiber Bragg grating reflector 43 and circulator 49.

As noted, while the common amplification stage (isolator 1, erbium-doped fiber 2, wavelength division multiplexer 3 and counter-pump laser P.sub.I) improves noise figure, the improvement in the L-band is not as good as in the C-band because the gain of the first stage for the L-band is much lower than for the C-band.

Referring now to FIG. 2, one embodiment of our invention is shown. In accordance with our embodiment, a common C-band and L-band optical input gain section now includes circulator 50, erbium-doped fiber 52, wavelength division multiplexer 54 and 980 nm counter pump P.sub.C7 and broad band grating reflector 56. The C-band signals pass through broad band grating reflector 56 while the L-band signals are reflected and pass through the common input gain section (wavelength division multiplexer and 980 nm diode laser counter pump 54, fiber 52, and circulator 50) a second time. The reflected L-band signals are then directed by the circulator 50 to the dedicated L-band gain section which begins with the dispersion compensating section comprising wavelength division multiplexer 9 and 980 nm diode laser co-pump P.sub.L1 ; erbium-doped fiber 11, isolator 13 and dispersion compensating element 15. The G.E.F. & L.D.R and Power sections of the L-band channel are similar to the corresponding sections of FIG. 1.

Meanwhile, the C-band signals, after passing through reflector 56, move on through the isolator 12 and dispersion compensating element 14 and the remaining G.E.F & L.D.R. and Power sections of the C-band gain section as in FIG. 1.

FIG. 3 shows another illustrative embodiment. In FIG. 3 the C-band optical channel is assumed to contain components substantially similar to those of FIG. 1. The L-band. and C-band signals pass through a common input section which includes circulator 70, wavelength division multiplexer 73 and 980 nm diode laser pump P.sub.L1, and erbium-doped fiber 75. The L-band signals pass through C-band broadband grating reflector 77 and continue on through erbium-doped fiber 79, isolator 13 and dispersion compensating element 15 and thence through the remainder of the L-band channel. The C-band signals, after passing through fiber 75 are reflected by reflector 77 and caused to pass through the common input section (erbium-doped fiber 75, wavelength division multiplexer 73 and circulator 70) a second time. From circulator 70 the C-band signals flow through the C-band section of the split structure. The L-band signals passing through fiber 75 also pass through grating reflector 77 and travel through another section of erbium-doped fiber 79 which is optimized to provide high gain in the L-band. Depending on design, a counter pump can be added before isolator 13 of FIG. 3. Then the L-band signals travel through the rest of the L-band optical channel in similar fashion to FIGS. 1 or 2.

What has been described is deemed to be illustrative of the principles of our invention. It should be understood that the C-band and L-band optical channels may each include additional (or fewer) wavelength division multiplexers, isolators, attenuators, pumps and erbium-doped fiber. The pumping direction can be changed and some of the components can be switched in position. One example is that in FIG. 2 the position of the Bragg reflector 56 can be interchanged with that of wavelength division multiplexer 54 and co-pump P.sub.C7. The same designs apply to other types of optical amplifiers also, such as fluoride fiber optical amplifiers. The components involved can be made with different technologies. For example, the broad band reflectors can be made with either fiber gratings or thin film technologies. The idea discussed above applies also to multiple reflectors for multiple bands. With more than two bands, two or more of the bands can share part of one or more gain stages. In other words, what we have described above for two bands could be done for more bands using multiple reflectors to improve flatness. Further and other modifications may be made by those skilled in the art without, however, departing from the spirit and scope of our invention.