Gain equalization in C+L erbium-doped fiber amplifiers.

Abstract

To provide a technique for improving gain equalization in C-band and L-band ("C+L") erbium-doped fiber amplifier (EDFA).SOLUTION: For example, C-band and L-band amplification sections in a C+L EDFA may be split and configured to be placed in parallel or in series. For both parallel and series arrangements, the C-band and L-band amplification sections may share a common gain flattening filter (GFF), or may be used with a separate GFF for each amplification section. Also, in some examples, the L-band GFF between stages is located in front of or upstream of the L-band amplification section to provide the L-band optical signal gain equalization or flattening before amplifying the L-band in the L-band amplification section.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] Embodiments of the present invention relate to the field of optical communication systems, and more particularly to improving gain equalization in at least C-band and L-band ("C+L") Erbium-Doped Fiber Amplifiers (EDFAs) used in optical communication systems. [Background technology]

[0002] Long-haul optical communication systems, such as undersea optical communication systems, are typically affected by signal attenuation due to a variety of factors, including scattering, absorption, and bending. To compensate for attenuation, these long-haul systems may include a series of optical amplifiers spaced along the signal transmission path and configured to amplify or strengthen the optical signal for reliable detection at a receiver. Depending on the length of the transmission path, the number of optical amplifiers positioned along the path may vary.

[0003] An optical amplifier commonly used in long-distance optical communication systems may be an erbium-doped optical fiber amplifier (EDFA), which includes an optical fiber doped with erbium (a rare earth element). The erbium-doped optical fiber can be pumped by lasers, for example, in the 980 nm and 1480 nm wavelength regions, to increase the intensity of specific wavelengths of the input optical signal. EDFAs are typically used in the C-band (approximately 1525 nm to 1565 nm) and the L-band (approximately 1568 nm to 1610 nm) optical wavelength bands, and are considered to have the lowest transmission loss in communication optical fibers compared to other wavelength bands, such as the O-band, E-band, and S-band.

[0004] EDFAs are known to output wavelength-dependent gain. This means that when an optical signal, such as a wavelength division multiplexed (WDM) signal, is amplified by an EDFA, some wavelengths may be amplified more than others. To adjust for wavelength amplification mismatch, a gain flattening filter (GFF) can be used to restore the overall wavelength of the optical signal to approximately the same intensity, which is commonly known as gain equalization or gain flattening.

[0005] Various types of GFFs can be used in EDFA designs. Generally, an important goal in amplifier design is to reduce the total number of passive components, such as optical isolators, and improve the power efficiency of the design. Another important design goal is to reduce component-related losses, thereby minimizing the total loss at the amplifier output. Certain system designs, particularly those related to long-distance submarine communications, may have performance constrained by the available amplifier pump power. In power-constrained systems, improving power efficiency and reducing amplifier loss are crucial to improving total transmission capacity while reducing cost. For example, due to the high resolution of the loss profile, short-period Bragg grating filters (SP-BGFs) can be used to achieve gain flattening of EDFAs. However, SP-BGFs have the disadvantage of requiring additional optical isolation at the input and output of each filter to address the issue of high back reflection, thus necessitating the addition of optical isolators.

[0006] Other types of GFFs can be used with EDFAs, such as slanted Bragg grating filters (S-BGFs), thin film filters (TFFs), and long period grating filters (LP-GFs). These filters do not require optical isolators at both the input and output of the filter, as do SP-BGFs. However, the drawback of using S-BGFs and LP-GFs is their physical length, which may exceed practical limits and / or require more space than repeaters. Furthermore, due to the limited loss profile resolution and tracking ability for steep gain curves associated with TFFs, S-BGFs, and LP-GFs, TFFs, S-BGFs, and LP-GFs may not achieve sufficient accuracy in the gain equalization process, especially in wideband applications such as C+LEDFAs. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides techniques for improving gain equalization in C-band and L-band (C+L) erbium-doped optical fiber amplifiers (EDFAs). For example, the C-band and L-band amplifier sections in a C+L EDFA can be separated and configured in a parallel or serial arrangement. For both parallel and serial arrangements, the C-band and L-band amplifier sections can share a common gain-flattening filter (GFF), or a separate GFF can be provided and used for each amplifier section. In some examples, an "interstage" L-band GFF is located before or upstream of the L-band amplifier section to gain-flatten or equalize the L-band optical signal before the L-band is amplified in the L-band amplifier section. [Means for solving the problem]

[0008] In one embodiment, an apparatus may include a C-band amplifier section, an L-band amplifier section, a first gain-flattening filter (GFF) for performing gain equalization in the C-band, and a second GFF for performing gain equalization in the L-band separately from the first GFF, wherein the first GFF is coupled to the C-band amplifier section and the second GFF is coupled to the L-band amplifier section. The C-band amplifier section may include at least a first erbium-doped fiber (EDF) for amplifying an optical signal in the C-band, and the L-band amplifier section may include at least a second EDF for amplifying an optical signal in the L-band, wherein the C-band amplifier section and the L-band amplifier section are configured in parallel.

[0009] In another embodiment, an apparatus may include a C-band amplifier section, an L-band amplifier section, a first gain-flattening filter (GFF) for performing gain equalization in the C-band, and a second GFF for performing gain equalization in the L-band separately from the first GFF, wherein the first GFF is coupled to the C-band amplifier section and the second GFF is coupled to the L-band amplifier section. The C-band amplifier section may include at least a first erbium-doped optical fiber (EDF) for amplifying optical signals in the C-band, and the L-band amplifier section may include at least a second EDF for amplifying optical signals in the L-band, wherein the C-band amplifier section and the L-band amplifier section are configured to be arranged in series.

[0010] In a further embodiment, the apparatus may include a C-band amplifier section, an L-band amplifier section, and a gain-flattening filter (GFF) for performing C-band and L-band gain equalization. The C-band amplifier section may include at least a first erbium-doped optical fiber (EDF) for amplifying an optical signal in the C-band, and the L-band amplifier section may include at least a second EDF for amplifying an optical signal in the L-band, wherein the C-band amplifier section and the L-band amplifier section are configured in a series or parallel arrangement. [Brief explanation of the drawings]

[0011] [Figure 1] 1 illustrates an exemplary optical communication system. [Figure 2] 1 shows a first exemplary paralleling scheme in a C+L EDFA. [Figure 3] 10 illustrates a second exemplary paralleling scheme in a C+L EDFA. [Figure 4] 1 shows a first exemplary series configuration for a C+L EDFA. [Figure 5] 1 shows an exemplary gain diagram. [Figure 6] 1 shows a second exemplary series configuration for a C+L EDFA. [Figure 7] 10 shows a third exemplary series configuration for a C+L EDFA. [Figure 8] 10 shows a fourth exemplary series configuration for a C+L EDFA. [Figure 9] 10 illustrates a fifth exemplary series configuration for a C+L EDFA. [Figure 10] 1 shows an exemplary Noise Figure (NF) plot. [Figure 11] 1 illustrates an exemplary guard band reduction. [Figure 12] 1 illustrates an exemplary parallel scheme using a reflective GFF. [Figure 13] 1 illustrates an exemplary serial scheme using a reflective GFF. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention relates to improving gain equalization or flattening in at least C+L Erbium-doped fiber amplifiers (EDFAs) used in optical communication systems by dividing at least the C-band and L-band amplifier sections of the C+L EDFA according to various configurations. According to one example, the C-band and L-band amplifier sections of the C+L EDFA may be configured in a parallel configuration (also known as a "parallel configuration"). According to another example, the C-band and L-band sections of the C+L EDFA may be configured in a serial configuration (also known as a "cascade configuration").

[0013] As will be further explained below, there may be at least two variations for the parallel approach. In a first variation, the C-band and L-band amplifier sections in the C+L EDFA may share a common GFF. In a second variation, the C-band amplifier section may include or be coupled to a separate C-band GFF, and the L-band amplifier section may include or be coupled to a separate L-band GFF. Similarly, for the serial approach, the C-band and L-band amplifier sections in the C+L EDFA may share a common GFF, and further, the C-band and L-band amplifier sections may include separate C-band and L-band GFFs, respectively.

[0014] In another embodiment, the series arrangement may be configured with an inter-stage GFF. For example, an L-band GFF may be disposed or located between the C-band and L-band amplifier sections. Also, one or more blocking filters may be integrated into the parallel and / or series arrangement of C+L EDFAs so that the guard band at least in the transition section from C-band to L-band is reduced or narrowed.

[0015] The present invention will now be described more fully with reference to the drawings, in which preferred embodiments of the invention are shown. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like reference numerals refer to like elements throughout.

[0016] 1 illustrates an exemplary bidirectional optical communication system 101 capable of transmitting large amounts of data over long distances using high-bandwidth optical fibers. Bidirectional data transmission can be achieved by establishing optical fiber pairs within an optical cable and transmitting one or more channels, such as wavelength division multiplexed channels, per fiber pair.

[0017] As shown in the figure, optical communication system 101 may include terminals 103 and 105 connected by two unidirectional optical paths 111, 121, which together form a bidirectional fiber pair. Optical path 111 may transmit information in one direction, e.g., rightward, from a transmitter 113 in terminal 103 to a receiver 115 in terminal 105. Optical path 121 may transmit information in the other direction, e.g., rightward, from a transmitter 125 in terminal 105 to a receiver 123 in terminal 103. With respect to terminal 103, optical path 111 is the outgoing optical path, and optical path 121 is the incoming optical path. Optical path 111 may include fibers 117-1 to 117-n and optical amplifiers 119-1 to 119-n, and optical path 121 may include fibers 127-1 to 127-n and optical amplifiers 129-1 to 129-n. One or more of the optical amplifiers 119-1 through 119-n and 129-1 through 129-n may be an EDFA, such as a C+L EDFA, and it should be understood that in some examples, the transmitter 113 and the receiver 123 may be housed together as a repeater at terminal 103, and similarly, the transmitter 115 and the receiver 125 may be housed together as a repeater at terminal 105.

[0018] Optical path pairs, e.g., optical paths 111 and 121, may be configured into a set of amplifier pairs 119-1 to 119-n and 129-1 to 129-n within repeaters 131-1 to 131-n, which may connect to fiber pairs 117-1 to 117-n and 127-1 to 127-n housed in a fiber cable along with the fiber supporting additional path pairs. Each repeater 131 may include a pair of amplifiers 119 and 129 for each path pair, and may also include additional amplifiers for additional path pairs. The optical amplifiers 119 and 129 may use EDFAs or other rare-earth doped optical fiber amplifiers, Raman amplifiers, or semiconductor optical amplifiers (SOAs). Coupled paths 133-1 to 133-n may be coupled, for example, between optical paths 111 and 121 in one or more of repeaters 131-1 to 131-n. It should be noted that the term "coupled" as used herein broadly refers to any connection, coupling, link, or connection, direct, indirect, limited, or wireless connection, and does not necessarily imply that coupled components or elements are directly connected to each other.

[0019] Although an exemplary embodiment of optical communication system 101 has been shown and described, variations of optical communication system 101 are within the scope of the present invention. Optical communication system 101 may include, for example, more optical path pairs and more or fewer repeaters. Alternatively, optical communication system 101 may not include any optical amplifiers, or may instead include optical pumping power sources suitable for implementing optical gain through Raman amplification in fibers connecting to the repeaters.

[0020] It should be understood that a transmitter, a receiver, a repeater including a transmitter and a receiver, or any other device used for data transmission and reception includes at least one memory device and one or more processors, such as a CPU, an AS and IC, an FGPA, any conventional processor, etc., that are capable of executing instructions stored in the memory device.

[0021] FIG. 2 illustrates an exemplary parallel configuration 200 of a C+L EDFA 201 according to an embodiment. As indicated by the dotted line box, the C+L EDFA 201 includes at least a C-band amplifier section 202, e.g., a C-band amplifier stage, and an L-band amplifier section 212, e.g., an L-band amplifier stage. As shown, the C-band and L-band amplifier sections 202 and 212 are configured in a parallel arrangement, which should be understood as an arrangement having two or more separate and independent optical signal paths for the C-band and L-band amplifier sections. For example, amplification in each section is performed on a separate path, and therefore amplification in one section does not affect the other section, and vice versa. It should be understood that there may be some interaction between the C-band and L-band amplifier sections due to the limited performance of the bandwidth division components, e.g., the C / L WDMs 226 and 234. The C-band section 202 includes an EDF-C 204, which is an erbium-doped optical fiber (also referred to as a C-band EDF) that uses pumping light in the 980 nm wavelength region to amplify the C-band, e.g., the C-band wavelength of a C-band optical signal. Similarly, the L-band section 212 includes an EDF-L 214, which is also an erbium-doped optical fiber (also referred to as an L-band EDF) that uses pumping light in the 980 nm wavelength region (also referred to as "980 nm pumping light" in this specification) to amplify the L-band, e.g., the L-band wavelength of an L-band optical signal.

[0022] In parallel system 200, when a wavelength division multiplexed (WDM) optical signal, e.g., a C+L optical signal, is received at input node 222, the signal first passes through optical isolator 224. It should be understood that an optical isolator may be a passive magneto-optical device configured to allow light to travel only in one direction and is used to guard the light source from possible back reflections or signals downstream of the optical isolator. It should be understood that the terms "downstream" and "upstream" herein broadly refer to the location of components relative to the location of different components in terms of the direction of optical signal flow. The optical signal then passes through C / L WDM 226, which may be a splitter or any other suitable component configured to split, decouple, or separate the C and L bands.

[0023] The pump light from the pump laser diode 228 in the 980 nm wavelength region can be combined with the C-band wavelength of the C-band signal through a 980 WDM-C 230, e.g., a combiner, and then amplified or enhanced by an EDF-C 204. On the L-band side, the 980 nm pump light can be combined with the L-band wavelength of the L-band signal through a 980 WDM-L 232, e.g., a combiner, and then enhanced by an EDF-L 212. The amplified C-band and L-band signals can be combined by a C / L WDM 234, pass through an optical isolator 236, and input to a GFF 238, which can be a combiner or any other suitable combining, multiplexing, or combining component, and which is a non-reflective GFF. The GFF 238 can be regarded as a common GFF for gain-flattening the C-band and L-band wavelengths of the enhanced C+L signal. The amplified and gain-flattened signal can then be output at output node 240. An advantage of parallel scheme 200 is the use of two optical isolators 224 and 236, for example, improving the power efficiency of C+L EDFA 201. The use of non-reflective GFF technology in scheme 200, rather than reflective GFF, allows for the use of only two optical isolators.

[0024] Figure 12 shows two different parallel schemes 1200 and 1220, respectively, using reflective GFFs in a C+L EDFA, and is described herein to compare schemes 1200 and 1220 with scheme 200 of Figure 2, which uses a non-reflective GFF. In both schemes 1200 and 1220, the C-band and L-band sections share a common reflective GFF, requiring at least two optical isolators to address reflections from the GFF, one upstream and one downstream of the GFF. While operational, an optical isolator is required near the input node to avoid performance loss due to backscattering caused by the span and other possible reflections in the C+L EDFA. Also, the taps at the input and output nodes may be three-port devices, and the pump source in the 980 nm wavelength region may be a single laser diode, and power separation between the C-band and L-band may be achieved via a defined passive or tunable splitter, with separate 980 nm pump sources for the C-band and L-band, which may be one or multiple laser diodes.

[0025] 3 illustrates an exemplary paralleling scheme 300 of an EDFA 301 according to an embodiment. The paralleling scheme 300 from the input node 330 to the EDF-C 304 and EDF-L 314 components may be the same as the paralleling scheme 200. As shown, the difference between the two schemes 300 and 200 is that the paralleling scheme 300 uses at least two separate GFFs, for example, one located or coupled to a C-GFF 306 in the C-band amplifier section 302 and one located or coupled to an L-GFF 316 in the L-band amplifier section 312. Additionally, as shown, an optical isolator 305 may be located or coupled between the EDF-C 304 and the C-GFF 306, and an optical isolator 315 may be located or coupled between the EDF-L 314 and the L-GFF 316.

[0026] In parallel system 300, similar to system 200, the C-band optical signal can be amplified or strengthened by EDF-C 304 using 980 nm pump light, and the L-band optical signal can be strengthened by EDF-L 314 using 980 nm pump light. In the C-band section 302, the amplified C-band passes through optical isolator 305 before passing through C-GFF 306. In the left band section 212, the amplified L-band passes through optical isolator 315 before passing through L-GFF 316. The gain-flattened C-band and L-band may then be combined by C / L WDM 322 to output an amplified and gain-flattened signal at output node 324.

[0027] According to an example, as shown by the dotted line box, at least optical isolators 305 and 315, C-GFF 306, L-GFF 316, C / L WDM 322, and a fiber tap are encapsulated and configured or arranged together as hybrid component 330. At least in this respect, hybrid component 300 advantageously enables separate C-band and L-band GFFs to be easily and effectively integrated into existing amplifier designs or schemes. For example, because the parallel schemes 200 and 300 configurations are the same up to the C-band and L-band EDFs, in parallel scheme 200, at least C / L WDM 234, optical isolator 236, GFF 238, and an optical tap located downstream of GFF 238 can be replaced by hybrid component 330 to easily, efficiently, and effectively switch from parallel scheme 200 to parallel scheme 300.

[0028] FIG. 4 illustrates an exemplary series configuration 400 of a C+L EDFA 401 according to an embodiment. As indicated by the dotted line box, the C+L EDFA 401 includes a C-band amplifier section 402, e.g., a C-band amplifier stage, and an L-band amplifier section 412, e.g., an L-band amplifier stage. As shown, the C-band and L-band amplifier sections 402 and 412 are configured in a series or serially connected arrangement, which may be understood as an arrangement having a single signal path for the C-band and L-band amplifier sections rather than a parallel arrangement of two or more independent paths. For example, in a series or serially connected arrangement, the C-band amplifier section and the L-band amplifier section are connected, and amplification in the upstream section affects the downstream section. The C-band amplifier section 402 includes an EDF-C 404 configured to amplify the C-band using at least 980 nm pumping light provided by a pump laser diode 434. The L-band amplifier section includes an EDF-L 414 configured to amplify at least the L-band.

[0029] For example, when an optical signal, e.g., a C+L optical signal, is input or received by input node 422, the signal may pass through an optical tap and then through optical isolator 424, where the 980 nm pump light and the C-band of the optical signal are combined or multiplexed so that the C-band wavelengths are amplified or enhanced by EDF-C 404. When amplifying the C-band, the signal is input to C / L WDM 426, which may be a C-band and L-band splitter configured to separate, decouple, or split the optical signal.

[0030] As shown, the separate C-band signal, having been amplified by the EDF-C 404 described above, is then input to the C / L WDM 428. The separate L-band can be combined with 980 nm pump light and then input to the EDF-L 414, which may be configured to amplify or enhance the L-band. The enhanced L-band signal may be input to the C / L WDM 428.

[0031] The C / L WDM 428, e.g., a combiner, may be configured to combine or multiplex the enhanced C-band and L-band signals and input the enhanced C+L signal into an optical isolator 430. After passing through the optical isolator 430, the signals are fed into a common GFF 432, which uses flattening or equalization to gain the enhanced C+L signal, and the signal then passes through an optical tap before outputting at an output node 433. Similar to the parallel scheme 200, the serial scheme 400 advantageously improves the power efficiency of at least the C+L EDFA 401 by implementing two optical isolators 424 and 430. Similarly, the use of non-reflective GFF technology in scheme 400, rather than a reflective GFF, allows the use of only two optical isolators. FIG. 13 shows a series scheme 1300 for a C+L EDFA using a reflective GFF, and is used herein to compare the scheme 1300 with the scheme 400 of FIG. 4, which uses a non-reflective GFF.

[0032] 5 illustrates an exemplary gain diagram 500 for a series system 400 of C+L EDFAs 401 according to an embodiment. For example, the illustrated gain diagram shows gain for the C-band and L-band with a 10 decibel (dB) span loss. As mentioned above, the C-band section 402 and the L-band section 412 of the series system 400 share a common GFF 432.

[0033] 5, a C-band EDF, e.g., EDF-C 404, provides the required 10 dB amplification at the C-band. Also, because at least the C-band amplification stage is configured upstream of the L-band amplification stage in series system 400, the C-band EDF can provide additional amplification at the L-band, e.g., in the range of approximately 1 dB to 8 dB. In some examples, the additional amplification allows series system 400 to advantageously function as a two-stage amplifier at the L-band, which at least improves the overall power efficiency of C+L EDFA 401.

[0034] 6 illustrates an exemplary series scheme 600 of C+L EDFAs 601 according to an embodiment. As shown in the drawing, the series scheme 600 from the input node to the EDF-L 614 component may be the same as the series scheme 400. However, the distinction between schemes 400 and 600 lies in the two separate GFFs, e.g., C-GFF 620 for gain-flattening the C-band and L-GFF 622 for gain-flattening the L-band.

[0035] Additionally, as shown by the dotted line box, optical isolators 624 and 626, C-GFF 620, L-GFF 622, C / L WDM 628, and optical tap 630 are configured or encapsulated together to form hybrid component 633. As described above, for example, with reference to FIG. 3 , hybrid component 633 advantageously allows separate C-band and L-band GFFs 620 and 622 to be easily and effectively integrated into existing amplifier designs or schemes, such as series scheme 400. Because the configurations of series schemes 400 and 600 are the same from the input node to EDF-L 614, hybrid component 633 can switch at least C / L WDM 428, optical isolator 430, common GFF 432, and optical tap in a simple and effective manner.

[0036] In the serial system 600, the C-band in the C+L signal is strengthened by the EDF-C 604 in the C-band amplifier section 602, and then the C+L signal is separated or split by the C / L WDM 634, and the strengthened or amplified C-band signal is input to the optical isolator 624 and gain-flattened by the C-GFF 620. The L-band signal separated from the C+L signal by the C / L WDM 634 is strengthened or amplified via 980 nm pump light by the EDF-L 614 in the L-band amplifier section 612, and then conveyed to the optical isolator 626 and gain-flattened by the L-GFF 622. The equalized and / or flattened C-band and L-band can be combined using the C / L WDM 628, and the combined C+L signal passes through the optical tap 630, and the signal can then be output from the output node 640.

[0037] 7 illustrates an exemplary series system 700 of C+L EDFAs 701 according to an embodiment. The series system 700 is almost similar to the series system 600 of C+L EDFAs 601, except for a different placement of optical isolators, e.g., two separate GFFs, C-GFF 702 and L-GFF 704, are used in the C+L EDFA 701.

[0038] As shown, for example, an optical isolator 706 may be placed, disposed, or coupled before, after, or upstream of the EDF-L 708 to perform optical isolation of the L-band before the L-band signal is strengthened or amplified. Also, an optical isolator 710 may be placed after, before, or downstream of the C / L WDM 712 to perform isolation on the C+L signal strengthened or amplified and combined by the optical isolator 710, e.g., the C-band and L-band are amplified and combined.

[0039] 8 illustrates an exemplary series system 800 of C+L EDFAs 801 according to an embodiment. The series system 800 is almost similar to the series system 700 of C+L EDFAs 701 described above, except for the notable distinction that the L-band GFF, e.g., L-GFF 802, is left, placed, or coupled before, after, or upstream of the L-band amplifier, e.g., EDF-L 804, and is, for example, the user of separate C-band and L-band GFFs. In this respect at least, the L-GFF 802 can be considered an “inter-stage” GFF, meaning that the L-band GFF is between the C-band and L-band amplification sections or stages.

[0040] For example, the EDF-C 806 provides additional amplification in the L-band, e.g., in the range of approximately 1 to 8 dB, and the inter-stage L-GFF 802 in the series configuration 800 may have at least one advantage of filtering out specific wavelengths of the L-band before amplifying the L-band signal using, e.g., the EDF-L 804, as will be described in more detail below. Additionally, an advantage of the inter-stage L-band GFF 802 located between the C-band and L-band amplification sections is to improve power efficiency in the C+L EDFA 801, similar to the advantages of an intermediate-stage GFF in a two-stage C-band or L-band EDA.

[0041] 9 illustrates an exemplary series system 900 of C+L EDFAs 901 according to an embodiment. The series system 900 is mostly similar to the series system 600 of C+L EDFAs 601, except that the series system 900 is located between the C-band and L-band amplifier sections, similar to the inter-stage GFF, e.g., L-GFF 902, of the series system 800. As shown, the L-GFF 902 is left, positioned, or coupled before, after, or upstream of the EDF-L 904, which is an L-band EDF.

[0042] Before the optical signal is separated into separate C-band and L-band signals by the C / L WDM 908, the additional amplification of the L-band provided by the EDF-C 906, e.g., in the range of approximately 1 to 8 dB, is flattened or equalized by the inter-stage L-GFF 902 before amplifying the separate L-band signal in the EDF-L 904. As with the serial configuration 800, an advantage of the inter-stage GFF configuration is improved overall power efficiency of the C+L EDFA 901. The inter-stage GFF 902 can also advantageously provide gain tilt compensation and other benefits, which will be described in more detail below.

[0043] 10 illustrates an exemplary noise figure (NF) graph 1000 of a series arrangement of inter-stage GFFs according to an embodiment, e.g., series arrangement 800, series arrangement 900. As illustrated in NF graph 1000, for example, the inter-stage GFFs can advantageously provide gain tilt compensation or tilt filter functionality. It should be understood that gain tilt refers to the slope of optical gain over a particular wavelength window, which can be an indicator of the gain flatness of an amplifier.

[0044] As shown in Figure 10, if the gain tilt filter function were not provided, the additional gain in the L-band provided by the C-band EDF could result in lower average inversion, a higher noise figure, and a physically longer L-band EDF might be required. As shown in curve 1020, the tilt filter function results in a relatively low noise figure in the L-band. Furthermore, the tilt filter can be considered a "lossless" filter with loss at the relatively short L-band wavelengths, thereby reducing the length of the L-band ED by at least about 6% and improving the noise figure by about 0.06 dB. It should be understood that, except for the inter-stage GFF, other C+L EDFA components can support or provide the tilt filter function, such as a C / L WDM combiner.

[0045] 11 illustrates an exemplary guard band reduction 1100 for a C+L EDFA 1101 according to an embodiment. Typically, a C / L WDM combiner may have limited resolution, resulting in wide guard bands, e.g., 3 to 5 nm, without the use of blocking filters. It should be understood that guard bands may be required at bandwidth transitions to avoid crosstalk between the C and L bands.

[0046] As shown in FIG. 11 , blocking filters 1102 and 1104 are positioned immediately before or after C / L WDM 1106, so that the filters receive and perform guard band reduction on the amplified C and L bands before combining them in C / L WDM 1106. Furthermore, as indicated by the dotted box 1110, for example, the guard band of the C band can be reduced or shortened using blocking filter 1102. Similarly, blocking filter 1104 can reduce or shorten the guard band of the L band. If blocking filters were not used, the final guard-reduced width of the combined C and L bands would be reduced, e.g., by approximately 1 nm, compared to the guard band width of the combined bands. For example, blocking filters can be used or integrated with thin-film filters (TFFs) or other suitable types of filters or components. Therefore, one advantage of integrating blocking filters into the C+L EDFA is reduced insertion loss.

[0047] As shown in FIG. 11 , blocking filters 1102 and 1104 are positioned immediately before or after C / L WDM 1106, so that the filters receive and perform guard band reduction on the amplified C and L bands before combining them in C / L WDM 1106. Furthermore, as indicated by the dotted box 1110, for example, the guard band of the C band can be reduced or shortened using blocking filter 1102. Similarly, blocking filter 1104 can reduce or shorten the guard band of the L band. If blocking filters were not used, the final guard-reduced width of the combined C and L bands would be reduced, e.g., by approximately 1 nm, compared to the guard band width of the combined bands. For example, blocking filters can be used or integrated with thin-film filters (TFFs) or other suitable types of filters or components. Therefore, one advantage of integrating blocking filters into the C+L EDFA is reduced insertion loss.

[0048] As described above, the parallel and series C+L EDFA architectures and their variations are advantageous in many ways. For example, when using a non-reflective GFF or non-reflective gain-flattening techniques rather than a reflective GFF, both the parallel and series architectures offer improved EDFA power efficiency by minimizing passive EDFA components, such as optical isolators. Furthermore, both the parallel and series architectures with a common GFF and the parallel and series architectures with separate C-band and L-band GFFs share a similar architecture, allowing the output architecture of the architecture to be incorporated into hybrid components for easy and efficient integration with a C+L EDFA. Furthermore, as described above, the variations in GFFs between stages in the series architecture provide at least the following three benefits: (i) (ii) (iii) (i) improved power efficiency depending on the total amplifier gain and background loss of the GFF, e.g., for a 10 dB span loss and a 0.6 dB GFF background loss, the total pump power is reduced by approximately 0.5 dB; (ii) gain flattening with gain flattening and / or gain equalization for shortening the L-band EDF, improving average inversion, and reducing NF; and (iii) slope compensation or slope filter function. Furthermore, to increase the effective transmission bandwidth, a cutoff filter can be used to narrow or reduce the guard band width of the combined C-band and L-band.

[0049] It should be understood that the parallel and series C+L EDFA configurations described above can be arranged in a variety of different configurations, for example, the order or permutation of the C-band and L-band amplification stages can be switched in a series configuration, etc., and are not limited to any particular configuration or any other form.

[0050] Disclosed herein are novel and inventive techniques for improving gain equalization in C+L EDFAs. The disclosure is not intended to be limited in scope by the specific embodiments described herein. Indeed, various other embodiments and modifications of the disclosure, in addition to those described herein, will be apparent to those skilled in the art from the foregoing description and drawings.

[0051] Accordingly, such alternative embodiments and modifications are intended to be included within the scope of the present disclosure. Moreover, while the present disclosure has been described herein in particular environments, for particular purposes, and in the context of particular implementation paradigms, those skilled in the art will recognize that the present disclosure is not limited in its usefulness and that the present disclosure may be advantageously implemented in any number of environments and for any number of purposes. Accordingly, the claims of the present disclosure should be construed in light of the full breadth and spirit of the present disclosure as described herein.

Claims

1. a C-band amplifier section including at least a first erbium-doped optical fiber (first EDF) for amplifying an optical signal in the C-band; an L-band amplifier section including at least a second EDF for amplifying an L-band optical signal; a first gain flattening filter (first GFF) used for performing C-band gain equalization, coupled to and arranged after the C-band amplifier section; a second GFF, separate from the first GFF, used to perform L-band gain equalization, the second GFF being coupled to and disposed after the L-band amplifier section; a first optical isolator disposed between or coupled to the first EDF and the first GFF in the C-band amplification section; a second optical isolator disposed between or coupled to the second EDF and the second GFF in the L-band amplification section; the C-band amplifier section and the L-band amplifier section are configured in a parallel arrangement; The apparatus, wherein at least the first and second optical isolators and the first and second GFFs form a hybrid component.

2. a third optical isolator for receiving the input signal; 10. The apparatus of claim 1, further comprising a splitter for splitting the input signal into the C-band optical signal and the L-band optical signal.

3. The first GFF and the second GFF are coupled or disposed after or downstream of the first EDF and the second EDF, respectively, so that the first GFF and the second GFF perform gain equalization on the amplified C-band and L-band signals; and 3. The apparatus of claim 1, further comprising a combiner for combining the amplified and gain-equalized C-band and L-band signals for output convenience.

4. 4. The apparatus according to claim 1, wherein the first and second EDFs perform amplification using 980 nm pumping light provided by a laser diode.

5. a C-band amplification section including at least a first erbium-doped optical fiber (first EDF) for amplifying a C-band optical signal; an L-band amplifier section including at least a second EDF for amplifying an L-band optical signal; a first GFF coupled to the C-band amplifier section, the first GFF being used to perform C-band gain equalization; a second GFF, separate from the first GFF, used to perform L-band gain equalization, the second GFF coupled to the L-band amplifier section; a splitter for splitting an input signal into said C-band optical signal and an L-band optical signal; a first optical isolator disposed or coupled together with the splitter between the first EDF and the first GFF of the C-band amplification section; a second optical isolator disposed between or coupled to the second EDF and the second GFF in the L-band amplification section; the C-band amplifier section and the L-band amplifier section are configured in a series arrangement; The apparatus, wherein the second GFF is coupled to or positioned after or downstream of the second EDF, such that the second GFF performs gain equalization on the amplified L-band optical signal.

6. a third optical isolator adapted to receive the input signal; 6. The apparatus of claim 5, wherein the third optical isolator is located or coupled before or upstream of the splitter.

7. the series arrangement includes the C-band amplifier section being arranged before or upstream of or coupled to the L-band amplifier section; and 7. The apparatus of claim 5, wherein the first GFF is coupled or located after or downstream of the first EDF, such that the first GFF performs gain equalization on the amplified C-band optical signal.

8. a splitter for splitting an input signal into said C-band optical signal and an L-band optical signal; a combiner for combining the amplified C-band optical signal and the L-band optical signal; a first optical isolator disposed between or coupled to the splitter and a second EDF of the L-band amplifier section; a second optical isolator disposed or coupled after or downstream of the combiner so as to be located between the combiner and an output node; 8. The apparatus according to claim 5, wherein the second GFF is disposed or coupled between the first optical isolator and a second EDF of the L-band amplification section, and the second GFF is located before or upstream of the second EDF, such that the second EDF performs L-band gain equalization on the L-band optical signal before amplifying the L-band optical signal.

9. a first blocking filter; a second blocking filter; a guard band corresponding to the amplified C-band optical signal is narrowed or reduced by the first blocking filter; The apparatus according to any one of claims 5 to 8, wherein a guard band corresponding to the amplified L-band optical signal is narrowed or reduced by the second blocking filter.

10. The first and second EDFs perform amplification using 980 nm pumping light provided by a laser diode; and The apparatus of any one of claims 5 to 9, which is a C+L EDFA deployed or implemented in a long-haul optical communication system.

11. a C-band amplification section including at least a first erbium-doped optical fiber (first EDF) for amplifying a C-band optical signal; an L-band amplifier section including at least a second EDF for amplifying an L-band optical signal; a first GFF coupled to the C-band amplifier section, the first GFF being used to perform C-band gain equalization; a second GFF, separate from the first GFF, used to perform L-band gain equalization, the second GFF coupled to the L-band amplifier section; a splitter for splitting an input signal into said C-band optical signal and an L-band optical signal; a combiner for combining the amplified C-band optical signal and the L-band optical signal; a first optical isolator disposed between or coupled to the splitter and the second EDF of the L-band amplifier section; a second optical isolator disposed or coupled after or downstream of the combiner so as to be located between the combiner and an output node; the C-band amplifier section and the L-band amplifier section are configured in a series arrangement; the second GFF is disposed or coupled between the first optical isolator and the second EDF of the L-band amplification section, and the second GFF is located before or upstream of the second EDF, such that the second EDF performs L-band gain equalization on the L-band optical signal before amplifying the L-band optical signal.

12. a C-band amplification section; an L-band amplification section; a first gain flattening filter (first GFF) coupled to the C-band amplifier section, the first GFF being used to perform C-band gain equalization; a second GFF, separate from the first GFF, used to perform L-band gain equalization, the second GFF coupled to the L-band amplifier section; a first blocking filter; a second blocking filter; the C-band amplification section includes at least a first erbium-doped optical fiber (first EDF) for amplifying a C-band optical signal; the L-band amplification section includes at least a second EDF for amplifying the L-band optical signal; the C-band amplifier section and the L-band amplifier section are configured in a series arrangement; a guard band corresponding to the amplified C-band optical signal is narrowed or reduced by the first blocking filter, and a guard band corresponding to the amplified L-band optical signal is narrowed or reduced by the second blocking filter.

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