Wide Gain Bandwidth C-Band Fiber Amplifier

Abstract

A fiber amplifier, specifically configured to provide gain over a wide range of the C-band spectral range (i.e., preferably providing a gain bandwidth of at least 42 nm in the range of 46-48 nm), utilizes a combination of a specially designed discrete Raman amplifier and a high inversion level EDFA to extend the gain bandwidth of a conventional EDFA C-band optical amplifier while keeping gain fluctuations below acceptable values. The EDFA provides high inversion level operation, and the dedicated discrete Raman amplifier (sDRA) element has specific parameters (dispersion, length, effective area) selected to maintain operation in the "small gain" regime, while also extending the long wavelength edge of the gain bandwidth and reducing gain fluctuations caused by EDFA components.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 725,029, filed on August 30, 2018, and is incorporated herein by reference. Technical Field

[0003] The present invention relates to fiber-based optical amplifiers, and more particularly to erbium-doped fiber amplifiers (EDFAs) configured to provide gain over a wide range of the C-band spectral range using specially designed fiber Raman amplifiers to extend the gain bandwidth and reduce gain fluctuations to acceptable levels. Prior art

[0004] EDFA is widely deployed in fiber optic communication systems due to its superior performance in terms of high power conversion efficiency (PCE) and low noise figure (NF). Typical prior art C-band EDFA operates in the optical wavelength range of 1528-1565nm (also expressed as a gain bandwidth of the order of about 37nm). Next, for applications such as telecommunications, it is important to increase the gain bandwidth of the C-band EDFA to at least 42nm, and preferably to 46-48nm (i.e., extending the wavelength range to about 1525-1573nm). This additional 10nm or so of bandwidth over which an acceptable level of gain can be achieved allows for increased capacity of the transmission system, reduced system cost, allows for the use of adaptive signal modulation formats, etc.

[0005] While the EDFA gain bandwidth can be extended by, for example, varying the dopant concentration in the gain fiber, the result is a significant increase in the non-uniformity of the amount of gain provided at different individual wavelengths (known as "gain fluctuation"), possibly to a level where conventional gain flattening filters (GFFs) cannot compensate for the gain variation. This approach to extending the gain bandwidth necessarily reduces the power conversion efficiency (PCE) and increases the noise figure (NF), both of which are contrary to desired system requirements.

[0006] Another approach to extending the gain bandwidth of C-band optical amplifiers is to utilize conventional discrete Raman amplifiers (DRAs) in place of EDFAs. While providing an extended gain bandwidth, DRAs are known to have lower PCEs and higher NFs than EDFAs, and are also susceptible to other technical issues, such as multipath interference (MPI) associated with double Rayleigh scattering and nonlinear effects associated with the relatively small effective area of ​​the optical fiber used in the DRA structure. Distributed Raman amplifiers (which utilize in-situ transmission fiber in place of a separate "gain fiber" as provided in discrete Raman amplifiers) and various types of "hybrid" fiber amplifiers based on a combination of EDFAs with distributed Raman amplifiers have been found to still exhibit practical problems associated with installation. For example, there are known issues associated with splicing and safety issues and reliance on in-situ transmission fiber as the amplification medium for distributed Raman amplifiers. Summary of the invention

[0007] The present invention addresses the need that exists in the art and relates to fiber-based optical amplifiers, and more particularly, to erbium-doped fiber amplifiers (EDFAs) that are specifically configured to provide gain over a large range of the C-band spectral range using a combination of specially designed discrete Raman amplifiers and high inversion level EDFAs to extend the gain bandwidth of the C-band optical amplifier while keeping the gain fluctuation below acceptable values.

[0008] According to the principles of the present invention, a fiber-based wide gain bandwidth C-band optical amplifier is proposed, which utilizes an EDFA (operating at a high inversion level) that is implemented by a specialized discrete Raman amplifier (sDRA) element with specific parameters that are selected to increase the gain at the long wavelength edge of the C-band while also reducing the gain fluctuations caused by the EDFA operating at a relatively high inversion level. The sDRA component of the present invention is maintained in a "small gain" state (e.g., an average gain of no more than 4 dB) for example by using a relatively short length of optical gain fiber. In particular, it has been found that a length of less than about 6 km (preferably less than 4 km, and more particularly in the range of about 2-4 km) maintains the gain within the "small gain" state. In addition to or instead of using a small length of optical gain fiber, the sDRA can be configured to operate in the "small gain" state by controlling the pump power (i.e., relatively low power) and / or the Raman gain coefficient of the optical fiber itself.

[0009] An exemplary embodiment of the present invention takes the form of a fiber-based C-band optical amplifier, including an erbium-doped fiber amplifier (EDFA) module and a discrete Raman amplifier (DRA) module. The EDFA module is configured to operate at a high inversion level and includes a section of erbium-doped fiber responsive to pump light from a first laser pump source operating at a power level sufficient to achieve a high inversion level. The DRA module includes a section of Raman optical gain fiber for receiving as input one or more optical signals propagating at a wavelength within the C-band range, and a second laser pump source for providing pump light to the optical gain fiber section. This section of Raman optical gain fiber is selected to have a combination of length, Raman gain coefficient, and pump power that keeps the DRA operating in a small gain state (e.g., less than 7 dB). The combination of the EDFA module and the DRA module provides a gain bandwidth of at least 42 nm in the C-band wavelength range of 1525-1573 nm.

[0010] Another exemplary embodiment of the present invention relates to a method for amplifying an optical signal propagating at a wavelength in a C-band range of 1525-1573 nm, comprising the steps of: (1) applying a plurality of optical input signals operating in the C-band range as input to a discrete Raman amplifier (DRA), the DRA being configured to exhibit a tilted gain profile, wherein the gain increases for longer wavelengths, the gain is no greater than 6.6 dB for the long wavelength edge of 1573 nm, and the average gain is no greater than 4 dB for the entire C-band wavelength range; (2) providing an amplified optical output signal from the sDRA as a small gain state output, the amplification following the tilted gain profile; and (3) applying the amplified optical output signal from the DRA as input to an erbium-doped fiber amplifier (EDFA) operating at an inversion level of at least 0.69, the combination of the EDFA and the DRA producing a gain bandwidth of at least 42 nm.

[0011] Other and further embodiments and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Referring now to the accompanying drawings,

[0013] Figure 1 An exemplary wide gain bandwidth C-band optical amplifier formed in accordance with the present invention is shown;

[0014] Figure 2 Included Figure 1 A graph of gain as a function of wavelength for a dedicated discrete Raman amplifier used in the amplifier shown;

[0015] Figure 3An alternative embodiment of the present invention is shown, wherein a wide gain bandwidth C-band optical amplifier utilizes a high aluminum ratio EDFA and a discrete Raman amplifier based on a positive dispersion gain fiber;

[0016] Figure 4 A set of graphs containing gain as a function of wavelength for a dedicated DRA, a high aluminum ratio EDFA, and an exemplary inventive amplifier formed by a combination of a dedicated DRA and a high aluminum ratio EDFA;

[0017] Figure 5 A comparative gain profile graph of an EDFA and an amplifier of the present invention (including a combination of a dedicated DRA and a high aluminum ratio EDFA) operating under conditions necessary to provide increased bandwidth, showing the reduction in gain ripple and the expansion of gain bandwidth provided by the amplifier configuration of the present invention;

[0018] Figure 6 Another embodiment of a wide gain bandwidth C-band optical amplifier formed according to the present invention is shown;

[0019] Figure 7 include Figure 6 Noise figure (NF) plots of the two components of the amplifier (EDFA and sDRA), and the overall NF associated with their combination; and

[0020] Figure 8 Another embodiment of a wide gain bandwidth C-band optical amplifier formed in accordance with the present invention is shown. DETAILED DESCRIPTION

[0021] Typical prior art EDFAs known for amplifying optical signals in the C-band provide acceptable gain only in a limited band between 1528 nm and 1565 nm (i.e., a gain bandwidth of approximately 37 nm). The present invention aims to extend this gain bandwidth to a value of at least 42 nm, and preferably in the range of 46-48 nm, providing gain at wavelengths of 1525-1573 nm.

[0022] As will be described below, an EDFA operating at a relatively high inversion level (e.g., not less than about 0.69) is used to generate the majority of the total gain, while a dedicated discrete Raman amplifier (sDRA) with specific Raman gain fiber properties (in terms of length, and possibly in terms of dispersion and / or effective area) is used to augment the gain spectrum of the high inversion level EDFA, such that the combination has a wide bandwidth and exhibits gain fluctuations that can be controlled by a conventional gain flattening filter (GFF). Since the EDFA has a fixed wavelength range, the dedicated DRA is able to extend the wavelength range over which gain is achieved (extending on the long wavelength edge).

[0023] Figure 1An exemplary C-band optical amplifier 10 formed in accordance with the principles of the present invention is shown, which uses a high inversion level EDFA on the output side to provide most of the gain, and a dedicated DRA (sDRA) on the input side to extend the long wavelength edge of the gain bandwidth and ensure that gain fluctuations can be handled. Figure 1 As shown, the C-band optical amplifier 10 is responsive to a plurality of optical input signals operating at wavelengths within the C-band range of 1525-1573 nm.

[0024] The amplifier 10 is shown to include an EDFA module 12, which includes a section of erbium-doped fiber 14 and a pump light source 16. An optical coupler 18 (e.g., a WDM coupler) is used to introduce pump light into the erbium-doped fiber 14, wherein in the configuration of the amplifier 10, the pump light propagates in the same direction as the input signal (i.e., co-directional pumping). The pump light source 16 includes a laser diode operating at a wavelength of about 980 nm, and the source 16 is configured to provide a relatively high output power. The relatively high output power is used to ensure that most of the erbium ions within the doped fiber 14 are "inverted", thereby providing a significant portion (i.e., most) of the gain when interacting with the input signal passing through the doped fiber 14. For the purposes of the present invention, an inversion level of 0.69 (i.e., 69% of the erbium ions are inverted to a higher energy state) has been found to be acceptable. An inversion level of the order of about 0.72 is considered preferred.

[0025] In accordance with the present invention, the ability to extend the gain bandwidth of a fiber-based C-band optical amplifier is provided by including a specially designed discrete Raman amplifier (sDRA) in the overall amplifier structure. Figure 1 , an exemplary sDRA 20 is shown, which includes a relatively short section of optical fiber 22 (having a length L of less than about 6 km, preferably in the range of 2-4 km). The optical fiber 22 (hereinafter referred to as "Raman gain fiber 22") is reversely pumped by a semiconductor laser 24 operating at a pump wavelength in the range of about 1464-1472 nm. A second WDM coupler 26 is used to inject the pump light into the output of the optical fiber 22, so that the pump light propagates in the opposite direction relative to the input optical signal, thereby providing a "reverse pumping" configuration. In some cases, the Raman gain fiber 22 is configured to have a relatively small effective area (A eff ), about 20 μm 2 .

[0026] In order to achieve low noise figure (NF) and minimize nonlinear impairments from sDRA 20, Raman gain fiber 22 needs to exhibit high Raman gain coefficient, low attenuation at both pump and signal. Figure 2The gain profile of an exemplary sDRA 20 is plotted in FIG. As shown, its profile is tilted toward the long wavelength end of the C-band, providing a maximum gain of no more than 6.6 dB at the longest wavelength of 1573 nm. In contrast to conventional distributed Raman amplifiers in which relatively long transmission fibers (e.g., tens of kms) are used as gain media, the "discrete" Raman amplifiers used in accordance with the present invention are self-contained modules coupled to existing transmission fibers. Thus, for discrete Raman amplifiers, the amount of gain provided can be controlled by, for example, selecting the length and effective area (or, pump power and Raman gain coefficient) of the Raman gain fiber suitable for the purpose. According to the present invention, it is intended to maintain the sDRA 20 in a "small gain" regime for operation (i.e., a gain of no more than about 7.0 dB at the long wavelength end of the C-band range). The length L of the Raman gain fiber suitable for this purpose may be as short as 6 km, with a length of the order of 2-4 km preferably ensuring operation in the small gain regime. Preferably, the sDRA 20 has an average gain value of 4.0 dB (e.g., from 1.5 dB at 1525 nm to 6.6 dB at 1573 nm). By maintaining the gain of the sDRA 20 at this relatively low level, its combination with an EDFA operating at an inversion level of at least 0.69 results in a gain bandwidth of at least 42 nm with an acceptable level of gain fluctuation.

[0027] Figure 3 An alternative embodiment of the present invention is shown, in which a C-band optical amplifier 30 with an extended gain bandwidth includes an EDFA 32 that exhibits an increased gain level by using an aluminum and erbium co-doped gain fiber. A high power pump source 34 is used to provide 980 nm pump light, and a first WDM 36 is used to couple the pump light to the input of the high aluminum ratio gain fiber 32. Figure 1 As with the illustrated embodiment, a co-directional pumping configuration is preferred so that the maximum level of pump power interacts with the maximum power of the propagating optical signal.

[0028] According to this embodiment of the invention, the EDFA 32 includes a section of gain fiber 34 co-doped with erbium and aluminum (preferably formed to have a high aluminum content). The presence of the aluminum dopant is known to increase the gain level achieved while maintaining a relatively low noise figure (NF). In particular, the concentration of aluminum is known to have a significant effect on the gain spectrum of an erbium-doped fiber. As is well known in the art, the absorption level at 1480 nm is a very sensitive measure of changes in the aluminum co-doping concentration in the erbium-doped fiber. The absorption at 1480 nm (denoted as α 1480nm ) and peak absorption (denoted as α 峰 ) is defined as the "aluminum ratio" and is selected as a measurable parameter to distinguish between different aluminum doping levels and monitor spectral uniformity / consistency. For the purposes of the present invention, high aluminum ratios (e.g., α1480nm / α 峰 About 0.446, more broadly, greater than about 0.44, and in some cases greater than about 0.46) allows the amplifier to produce additional gain while operating at a desired high inversion level while also maintaining a desired low NF and wide gain bandwidth.

[0029] Figure 4 The gain profile of an exemplary high aluminum ratio EDFA (eg Figure 3 The EDFA 32 shown (in Figure 4 1)). The use of a high aluminum ratio EDFA is shown to provide a gain of greater than 15 dB (i.e., a gain bandwidth of about 35 nm) in the wavelength range of 1525 to about 1560 nm. For this type of high aluminum ratio EDFA, it is difficult to extend the gain bandwidth well beyond this 35 nm range through fiber design, amplifier design, or changes in glass composition without significantly compromising gain flatness, NF, and power conversion efficiency (PCE).

[0030] Return to reference Figure 3 In the illustrated embodiment, the amplifier 30 includes an sDRA 40 formed by a short length (i.e., 6 km or less) of "positive dispersion" optical fiber 42 (hereinafter referred to as "Raman gain fiber 42"). A Raman pump source 44 is used as the source of pump light, and a second WDM coupler 46 is used to couple the Raman pump light to the output of the positive dispersion Raman gain fiber 42. Although Figure 3 4 is shown as a preferred counter-pumping configuration, but it should be understood that the sDRA 40 can also be configured in a co-pumping configuration, especially if the relative intensity noise (RIN) of the pump source 44 is sufficiently low.

[0031] Dispersion is a measure of the difference in propagation speeds of different wavelengths through an optical fiber. "Positive dispersion" specifically refers to a particular fiber composition in which longer wavelength light propagates faster than shorter wavelength light (as opposed to conventional "negative dispersion" fiber where the opposite effect occurs). Positive dispersion is desirable in this particular application (although it is not typically used in discrete Raman amplifiers) to avoid nonlinear inter-channel and intra-channel impairments typically found in coherent transmission systems. For the purposes of the present invention, positive dispersion values ​​of at least 10 ps / nm*km at 1550 nm are preferred, but not required. In the broadest sense, as long as a Raman gain fiber exhibits non-zero dispersion, it will properly serve as a "nonlinear" element to extend the gain bandwidth of a C-band amplifier.

[0032] Figure 4The gain associated with the sDRA 40 (curve II) is also shown in FIG. 3 , which also shows the total amplifier gain from the combination of the high aluminum ratio EDFA 32 and the sDRA 40, which is shown to produce a total average gain of 20 dB and a gain bandwidth of 48 nm. A conventional gain flattening filter (GFF) can be used to compensate for the 4.7 dB gain fluctuation.

[0033] Figure 5 A graph is included to better illustrate the improved performance achieved by using a dedicated DRA (sDRA) in conjunction with a high aluminum ratio EDFA to provide a gain bandwidth approaching 48 nm in the C-band operating wavelength range. Figure 5 Curve A in FIG. 1 is the gain profile associated with a high aluminum ratio EDFA operating as a C-band amplifier. By using a sufficiently high power pump input (e.g., 600 mW of pump power), the high aluminum ratio EDFA can be driven to exhibit a gain bandwidth close to the desired 48 nm value, but this occurs at the expense of introducing significant fluctuations in the gain. Figure 5 As shown, the associated fluctuations are on the order of 7.3 dB, which results in unacceptably low PCE values ​​and is beyond the ability to correct using typical gain flattening filters.

[0034] Figure 5 Curve B of FIG. 1 shows the performance improvement achieved by combining a dedicated DRA (having the above features) with a high aluminum ratio EDFA. As shown, the gain bandwidth is now further extended along the long wavelength edge of the C-band, and the gain ripple is reduced to a value of 4.74 dB (a ripple that can be compensated by using a conventional gain flattening filter).

[0035] Figure 6 Another fiber-based C-band optical amplifier formed according to one or more embodiments of the present invention is shown. Figure 3 As described above, an exemplary optical amplifier 60 is shown as including a high aluminum ratio EDFA 32 and a sDRA 40. Figure 6 In the embodiment of the present invention, a gain flattening filter (GFF) 62 disposed at the output of the EDFA 32 is also shown. Using techniques known in the art, the GFF 62 is capable of reducing the residual gain fluctuation of 4.74 dB to a value of, for example, about 0.2-0.3 dB, thereby providing a substantially uniform gain over the entire bandwidth of the C-band optical amplifier 60. An optical isolator is also included as a component of the C-band optical amplifier 60 to reduce the amount of noise generated by the reflected optical signal. As shown, isolators 64, 66, and 68 are disposed at the input of the sDRA 40, the input of the EDFA 32, and the output of the EDFA 32, respectively.

[0036] In addition to using optical isolators, specific parameters of the sDRA 20, 40 can be designed to maintain a relatively low noise figure (NF) of the fiber-based optical amplifier of the present invention. In particular, the NF is determined by the following equation:

[0037]

[0038] Among them G sDRA is the average gain component associated with the sDRA 20 , 40 .

[0039] Figure 7 The calculated total noise figure of amplifier 60 at C-band is plotted along with the sDRA 40 (NF sDRA ) and EDFA 32(NF EDFA ) is associated with the noise figure component. EDFA When looking at the graph of the 40A / D ratio, it is clear that the high Al ratio EDFA 32 contributes only a small part of the total NF (because the amplifier is specifically configured to operate at high inversion levels), which remains essentially below the value of 3.5 dB. The main NF contribution comes from the sDRA 40, where the NF sDRA The graph of sDRA 40 shows a maximum NF of 4.8 dB at the long wavelength edge of 1473 nm. However, the NF from this dedicated discrete Raman amplifier is relatively low compared to the NF of a conventional discrete Raman amplifier. This is because the sDRA 40 operates under "small gain" conditions due to its special characteristics, including, for example, the short length of the Raman gain fiber 42 (i.e., the average gain from the sDRA 40 is only about 4.0 dB). Other special characteristics can also maintain the "small gain" operation of the sDRA 40, such as the use of low power pumps and the inclusion of gain fibers with relatively low Raman gain coefficients. In addition, the use of a counter-pumping arrangement further reduces the overall noise of the Raman amplifier by preventing the relative intensity noise (RIN) of the pump source from being transferred to the propagating optical signal (which occurs in a co-pumped Raman amplifier).

[0040] Therefore, to achieve low NF operation, the net gain from the sDRA 40 should be small, and the characteristics of the Raman gain fiber 42 (and possibly the pump source 44) controlled accordingly. Multipath interference (MPI) is also expected to be small since it operates in a small gain regime.

[0041] In many embodiments, the sDRA element of the amplifier of the present invention will be operated in a low signal power state, which is located on the input side of the EDFA element. Therefore, in these embodiments, it is expected that the nonlinear impairment from the sDRA will be relatively small. In fact, when looking at the overall NF curve, it is clear that within the 48nm bandwidth where there is a sufficient amount of gain, the maximum level remains below 5.5dB.

[0042] However, there are other applications where the EDFA component can be positioned as an "input" amplifier, while the sDRA component is arranged at the output of the EDFA component. In these cases, either the transmission system is not affected by the nonlinearity present in the sDRA component, or the nonlinear length of the sDRA is too short to introduce significant nonlinearity.

[0043] Figure 8 An exemplary wide gain bandwidth optical amplifier 80 of the present invention having this particular structure is shown. As shown, multiple input signals are coupled into an EDFA 82 operating at a high inversion level using, for example, a high power pump source 84. The gain fiber 86 may include only erbium dopants, or may be co-doped with aluminum to provide additional gain levels as described above. The amplified output from the EDFA 82 is then applied as an input to an sDRA 90, which is specifically configured to extend the long wavelength end of the gain bandwidth as described above, as well as to reduce the amount of fluctuations in the gain bandwidth produced by the EDFA 82. That is, the characteristics of the Raman gain fiber 92 (in terms of length, dispersion, effective area, Raman gain coefficient) that may control the power level of the pump source 94 are used to maintain the operation of the sDRA 90 in a "small gain" state, while also providing improvements to the amplified output from the EDFA 82 in terms of extending the gain bandwidth (e.g., up to about 48 nm) and reducing fluctuations. In effect, Figure 8 The configuration can eliminate the need for an additional GFF at the output of the amplifier.

[0044] Although the foregoing invention has been described in terms of the above embodiments, many variations are possible. Therefore, modifications and changes such as those suggested above (but not limited thereto) are considered to be within the scope of the appended claims.

Claims

1. A C-band optical amplifier based on optical fiber, comprising: An erbium doped fiber amplifier EDFA module operating at a high inversion level, the EDFA module comprising a section of erbium doped fiber and a first laser pump source for providing pump light having a power level sufficient to achieve the high inversion level, wherein one or more optical input signals propagating through the EDFA module at a wavelength within the C-band spectral range are amplified due to the presence of the pump light; as well as A discrete Raman amplifier (DRA) module, the DRA module comprising a section of optical gain fiber and a second laser pump source, the section of optical gain fiber being used to receive as input one or more optical signals propagating at a wavelength within the C-band range, the second laser pump source being used to provide pump light to the section of optical gain fiber, the section of optical gain fiber having a length selected to maintain operation in a small gain state, generating a gain within the range of 1.5-7.0 dB, wherein the combination of the EDFA module and the DRA module provides a gain bandwidth of at least 42 nm within the C-band wavelength range of 1525-1573 nm.

2. The fiber-based C-band optical amplifier according to claim 1, wherein: The length of erbium doped optical fiber is aluminum co-doped and has a defined aluminum ratio.

3. The fiber-based C-band optical amplifier according to claim 2, wherein: The length of erbium doped optical fiber has an aluminum ratio of at least 0.

440.

4. The fiber-based C-band optical amplifier according to claim 1, wherein: The first laser pump source operates at an output power of more than 350 mW, so that the EDFA module exhibits an erbium ion inversion level greater than 0.

69.

5. The fiber-based C-band optical amplifier according to claim 1, wherein: The small gain state of the DRA module is defined as an average gain of no greater than 4dB.

6. The fiber-based C-band optical amplifier according to claim 1, wherein: The DRA module includes a section of Raman gain fiber having a length no greater than 6 km.

7. The fiber-based C-band optical amplifier according to claim 6, wherein: The Raman gain optical fiber includes a section of positive dispersion optical fiber, and the section of positive dispersion optical fiber exhibits a positive dispersion value within the C-band wavelength range.

8. The fiber-based C-band optical amplifier according to claim 1, wherein: The optical amplifier also includes a gain flattening filter coupled to the output of the EDFA module.

9. The fiber-based C-band optical amplifier according to claim 1, wherein: The EDFA module is configured as a co-propagating amplifier, and further comprises an optical wavelength division multiplexer WDM arranged at an input of the EDFA module for multiplexing the pump light with the propagating optical signal.

10. The fiber-based C-band optical amplifier according to claim 1, wherein: The DRA module is configured as a counter-propagating amplifier and further comprises an optical wavelength division multiplexer WDM arranged at an output of the DRA module, the WDM receiving pump light from a second pump source at an input port and directing the pump light backwards along a propagation direction of the optical input signal.

11. The fiber-based C-band optical amplifier of claim 1, wherein: The DRA module is arranged before the EDFA module so that the amplified output signal from the DRA module is applied as an input signal to the EDFA module.

12. The fiber-based C-band optical amplifier of claim 1, wherein: The EDFA module is disposed before the DRA module such that an amplified output signal from the EDFA module is applied as an input signal to the DRA module.

13. A method of amplifying an optical signal propagating at a wavelength within the wavelength range of 1525-1573 nm, the method comprising: applying a plurality of optical input signals operating in the wavelength range of 1525-1573 nm as inputs to a discrete Raman amplifier DRA, the DRA being configured to exhibit a sloped gain profile wherein the gain increases for longer wavelengths, exhibiting a gain greater than 1.5 dB and no greater than 6.6 dB for the long wavelength edge of 1573 nm, and an average gain no greater than 4 dB for the entire C-band wavelength range; providing an amplified optical output signal from the DRA as a small gain state output, the amplification following the sloped gain profile; as well as The amplified optical output signal from the DRA is applied as input to an erbium doped fiber amplifier EDFA operating at an inversion level of at least 0.69, the combination of the DRA and the EDFA producing a gain bandwidth of at least 42 nm in the C-band range.

14. The method according to claim 13, wherein: The DRA includes a length of Raman gain fiber having a defined length L of less than 6 km, and the plurality of optical input signals are applied as inputs to the Raman gain fiber.

15. The method according to claim 14, wherein: The DRA includes a pump source, and the method further includes directing a low power pump wave through a length of positive dispersion gain fiber in a counter-propagation direction relative to a propagation direction of the plurality of optical input signals.

16. The method according to claim 14, wherein: The Raman gain fiber includes a section of positive dispersion fiber.

17. The method according to claim 13, wherein: The EDFA includes a high aluminum ratio EDFA using a gain fiber co-doped with erbium and aluminum.

18. The method according to claim 13, wherein: The combination of the DRA and high inversion level EDFA provides a gain bandwidth on the order of 46 nm to 48 nm.

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