Gain equalization in c+l erbium-doped fiber amplifiers

Abstract

Techniques are provided for improving gain equalization in C-band and L-band ("C+L") erbium-doped fiber amplifiers (EDFAs). For example, C-band and L-band amplification sections of a C+L EDFA can be separated and configured in a parallel arrangement or a serial arrangement. For both parallel and serial arrangements, the C-band and L-band amplification sections can share a common gain flattening filter (GFF), or each amplification section can include and employ a separate GFF. Also, in some examples, an "inter-stage" L-band GFF can be positioned before or upstream of the L-band amplification section, such that the L-band optical signal is gain-equalized or flattened before being amplified in the L-band amplification section.

Description

Technical Field

[0001] Embodiments of this disclosure relate to the field of optical communication systems. More specifically, this disclosure relates to at least improving gain equalization in C-band and L-band (“C+L”) erbium-doped fiber amplifiers (EDFAs) used in optical communication systems. Background Technology

[0002] Long-range optical communication systems (such as submarine optical communication systems) are typically affected by signal attenuation caused by various factors, including scattering, absorption, and bending. To compensate for attenuation, these long-distance systems may include a series of optical amplifiers spaced along the signal transmission path and configured to amplify or enhance the optical signal in a manner that allows for reliable detection at the receiver. The number of optical amplifiers positioned along the path may vary depending on the length of the transmission path.

[0003] In long-range optical communication systems, optical amplifiers commonly used are erbium-doped fiber amplifiers (EDFAs), which consist of erbium-doped fiber (a rare earth element) that can be excited by lasers (e.g., in the 980 nm and 1480 nm wavelength regions) to enhance the intensity of certain wavelengths of incoming optical signals. Typically, EDFAs are used in the C-band (approximately 1525 nm to 1565 nm) and L-band (approximately 1568 nm to 1610 nm) optical wavelength bands, where transmission loss in the telecommunications fiber is likely to be the lowest compared to other wavelength bands (e.g., O-band, E-band, and S-band).

[0004] The wavelength-dependent gain of the EDFA output is known. Therefore, when an optical signal, such as a wavelength division multiplexed (WDM) optical signal, is amplified by an EDFA, some wavelengths may be amplified more than others. To reconcile the inconsistencies in wavelength amplification, a gain-flattening filter (GFF) can be used to restore all wavelengths in the optical signal to approximately the same intensity; this is commonly known as gain equalization or gain flattening.

[0005] Various types of GFFs can be used in EDFA design. Generally, a key objective in amplifier design is to reduce the total number of passive components (such as optical isolators) to achieve painstakingly designed power efficiency. Another important design objective is to reduce component-related losses, which minimizes the total loss at the amplifier output. Specific system designs more relevant to long-range submarine communications may have performance limited by the available amplifier pump power. In power-constrained systems, improving power efficiency and reducing losses in the amplifier are crucial for increasing overall transmission capacity and reducing cost. For example, short-period Bragg grating filters (SP-BGFs) can be used for gain flattening in EDFAs due to their high resolution in terms of loss distribution. However, a drawback of SP-BGFs is that they require additional optical isolation (and therefore additional optical isolators) at both the input and output of each filter to address the high back reflection problem.

[0006] Other types of GFFs can also be used in EDFAs, such as slanted Bragg grating filters (S-BGF), thin film filters (TFF), and long-period grating filters (LP-GF). While these filters do not require optical isolators at both the filter input and output (such as SP-BGF), a disadvantage of using S-BGF and LP-GF may be their physical length, which may exceed practical constraints and / or require more space in the repeater. Furthermore, due to the limited loss shape resolution and the limited ability to track the steep slope of the gain curve associated with TFF (as well as S-BGF and LP-GF), TFF (as well as S-BGF and LP-GF) may not achieve sufficient accuracy in gain equalization, especially in wideband applications such as C+L EDFAs. Summary of the Invention

[0007] Techniques are provided for improving gain equalization in C-band and L-band (C+L) erbium-doped fiber amplifiers (EDFAs). For example, the C-band and L-band amplification sections of 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 amplification sections can share a common gain flattening filter (GFF), or each amplification section can include and employ a separate GFF. Furthermore, in some examples, an "interstage" L-band GFF can be located before or upstream of the L-band amplification section, such that the L-band optical signal is gain equalized or flattened before the L-band is amplified by the L-band amplification section.

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

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

[0010] In another embodiment, an apparatus may include a C-band amplification section, an L-band amplification section, and a gain flattening filter (GFF) for performing C-band and L-band gain equalization. The C-band amplification section may include at least a first erbium-doped fiber (EDF) for amplifying C-band optical signals, and the L-band amplification section may include at least a second EDF for amplifying L-band optical signals, wherein the C-band and L-band amplification sections are configured in a series or parallel arrangement. Attached Figure Description

[0011] Figure 1 An example optical communication system is shown.

[0012] Figure 2 The first example parallel scheme of C+L EDFA is shown.

[0013] Figure 3 A second example parallel scheme for C+L EDFA is shown.

[0014] Figure 4 The first example serial scheme of C+L EDFA is shown.

[0015] Figure 5 An example gain plot is shown.

[0016] Figure 6 A second example serial scheme for C+L EDFA is shown.

[0017] Figure 7 A third example serial scheme for C+L EDFA is shown.

[0018] Figure 8 The fourth example serial scheme of C+L EDFA is shown.

[0019] Figure 9 The fifth example serial scheme of C+L EDFA is shown.

[0020] Figure 10 An example noise figure (NF) plot is shown.

[0021] Figure 11 An example of reduced protection bands is shown.

[0022] Figure 12 An example parallel scheme using reflective GFF is shown.

[0023] Figure 13 An example serial scheme using a reflective GFF is shown. Detailed Implementation

[0024] This invention relates to improving gain equalization or flattening in a C+L erbium-doped fiber amplifier (EDFA) for optical communication systems by separating the C-band and L-band amplification sections of a C+L EDFA according to various arrangements. According to one example, the C-band and L-band amplification sections of the C+L EDFA can be configured in a parallel arrangement (also referred to as a "parallel scheme"). According to another example, the C-band and L-band sections of the C+L EDFA can be configured in a serial arrangement (also referred to as a "serial scheme").

[0025] As will be further described below, at least two variations of the parallel scheme are possible. In a first variation, the C-band and L-band amplification sections of the C+L EDFA can share a common GFF. In a second variation, the C-band amplification section may include and can be coupled to separate C-band GFFs, and the L-band amplification section may include and can be coupled to separate L-band GFFs. Similarly, for the serial scheme, the C-band and L-band amplification sections of the C+L EDFA can share a common GFF, and further, the C-band and L-band amplification sections may include separate corresponding C-band and L-band GFFs.

[0026] In another embodiment, the serial scheme can be configured to include inter-stage guard filters (GFFs). For example, an L-band GFF can be arranged or located between the C-band and L-band amplification sections. Furthermore, one or more blocking filters can be integrated into the parallel and / or serial scheme of the C+LEDFA to at least reduce or minimize the guard band during the transition from the C-band to the L-band.

[0027] The invention will now be described more fully below with reference to the accompanying drawings, in which preferred embodiments of the invention are illustrated. 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, the same reference numerals consistently refer to the same elements.

[0028] Refer to the attached diagram. Figure 1 An exemplary bidirectional optical communication system 101 is shown, which can use high-bandwidth optical fiber to transmit large amounts of data over long distances. Bidirectional data transmission can be implemented by constructing fiber pairs within an optical cable, with each fiber pair transmitting one or more channels (e.g., wavelength division multiplexing channels).

[0029] As shown in the figure, the optical communication system 101 may include terminals 103 and 105 connected by two unidirectional optical paths 111 and 121, which together form a bidirectional fiber pair. Optical path 111 can transmit information in one direction (e.g., to the right) from transmitter 113 at terminal 103 to receiver 115 at terminal 105. Optical path 121 can transmit information in the other direction (e.g., to the right) from transmitter 125 at terminal 105 to receiver 123 at terminal 103. With respect to terminal 103, optical path 111 is the outgoing path, and optical path 121 is the incoming path. Optical path 111 may include optical fibers 117-1 to 117-n and optical amplifiers 119-1 to 119-n, and optical path 121 may include optical fibers 127-1 to 127-n and optical amplifiers 129-1 to 129-n. One or more of optical amplifiers 119-1 to 119-n and 129-1 to 129-n can be EDFAs, such as C+L EDFAs. It is understood that in some examples, transmitter 113 and receiver 123 can be housed together as transponders at terminal 103, and similarly, transmitter 115 and receiver 125 can be housed together as transponders at terminal 105.

[0030] Optical path pairs (e.g., optical paths 111, 121) can be configured as a set of amplifier pairs 119-1 to 119-n and 129-1 to 129-n within repeaters 131-1 to 131-n, which are connected by fiber pairs 117-1 to 117-n and 127-1 to 127-n that can be included in fiber optic cables along with fibers supporting additional path pairs. Each repeater 131 may include a pair of amplifiers 119, 129 for each path pair and may include additional amplifiers for additional path pairs. Optical amplifiers 119, 129 may utilize EDFAs or other rare-earth-doped fiber amplifiers, Raman amplifiers, or semiconductor optical amplifiers (SOAs). Coupling paths 133-1 to 133-n may be coupled between optical paths 111, 121, for example, in one or more of repeaters 131-1 to 131-n. It is understood that, as used in this article, the term “couple” broadly refers to any connection or connecting, coupling, link, or link, whether direct or indirect, limited or wireless, and does not necessarily mean that the coupled components or elements are directly connected to each other.

[0031] Although exemplary embodiments of the optical communication system 101 have been shown and described, variations of the optical communication system 101 are also within the scope of this disclosure. The optical communication system 101 may include, for example, more optical path pairs and more or fewer repeaters. Alternatively, the optical communication system 101 may not include any optical amplifiers, or may include an optical pump power supply adapted to implement optical gain through Raman amplification within the optical fiber connecting the repeaters instead of an optical amplifier.

[0032] Furthermore, it is understood that a transmitter, receiver, repeater (including transmitter and receiver) or any other suitable device for transmitting and receiving data may include at least one memory and one or more processors (e.g., CPU, ASIC, FGPA, any conventional processor, etc.) to execute instructions stored in the memory.

[0033] Figure 2An example parallel configuration 200 of a C+L EDFA 201 according to an embodiment is shown. The C+L EDFA 201 includes at least a C-band amplification section 202 (e.g., a C-band amplification stage) and an L-band amplification section 212 (e.g., an L-band amplification stage), as shown in the dashed box. As illustrated, the C-band and L-band amplification sections 202 and 212 are configured in a parallel arrangement. This can be understood as an arrangement with two or more separate and independent optical signal paths for the C-band and L-band amplification sections, for example, amplification in the respective sections is performed on separate paths, and therefore, amplification in one section does not affect the other section, and vice versa. It is understood that some degree of interaction between the C-band and L-band amplification sections is possible due to the limited performance of bandwidth-splitting components (such as C / L WDMs 226 and 234). C-band segment 202 includes EDF-C 204, which is an erbium-doped fiber (which may also be referred to as a C-band EDF) configured to amplify C-band (e.g., the C-band wavelength of a C-band optical signal) using light pumped in the 980 nm wavelength region. Similarly, L-band segment 212 includes EDF-L 214, which is also an erbium-doped fiber (which may be referred to as an L-band EDF) amplifying L-band (e.g., the L-band wavelength of an L-band optical signal) using 980 nm wavelength region pump light (which may also be referred to herein as “980 nm pump light”).

[0034] In parallel scheme 200, when a wavelength division multiplexing (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 is understood that the optical isolator can be a passive magneto-optical device that allows light to travel in only one direction and can be used to protect the light source from back reflections or signal interference that may occur downstream of the isolator. The terms "downstream" and "upstream" can be understood herein to refer broadly to the position of the components relative to different components as viewed from the direction of the optical signal flow. The optical signal can then pass through C / L WDM 226, which can be a splitter (or any other suitable component) configured to split, decouple, or separate the C-band and L-band into corresponding C-band and L-band optical signals.

[0035] Pump light from the 980nm wavelength region pump laser diode 228 can be combined with the C-band wavelength of the C-band signal using a 980WDM-C230 (e.g., a combiner), which can then be amplified or enhanced by an EDF-C 204. On the L-band side, the 980nm pump light can be combined with the L-band wavelength of the L-band signal via a 980WDM-L 232 (e.g., a combiner), and then enhanced by an EDF L 212. Each of the amplified C-band and L-band signals can then be combined by a C / L WDM 234 (which can be a combiner or any other suitable combining, multiplexing, or coupling component), passed through an optical isolator 236, and input to a GFF 238, which can be a non-reflective GFF. The GFF 238 can be considered a common GFF used for gain flattening of the C-band and L-band wavelengths in the enhanced C+L signal. The amplified and gain-flattened signal can then be output at output node 240. The advantage of the parallel scheme 200 lies in the use of two optical isolators 224 and 236, which, for example, increases the power efficiency of the C+L EDFA 201. The use of only two optical isolators is possible because a non-reflective GFF technique is used in scheme 200 instead of a reflective GFF.

[0036] C+L EDFA uses two different parallel schemes, 1200 and 1220, respectively, employing reflective GFFs. Figure 12 As shown, and in order to combine schemes 1200 and 1220 with those using non-reflective GFFs Figure 2 The purpose of comparing with Scheme 200 is described herein. In both Schemes 1200 and 1220, the C-band and L-band segments share a common reflective GFF, where at least two isolators are required due to reflections from the GFF, one upstream and one downstream. An isolator near the input node is needed to avoid performance loss due to backscattering from the span and other possible reflections in the C+L EDFA (although operation is possible without such an isolator). Moreover, the taps at the input and output nodes can be three-port devices, and the 980nm wavelength region pump source can be a single laser diode, where power separation is performed between the C-band and L-band via a fixed passive or tunable splitter, where each C-band and L-band can have separate 980nm pump sources (which can be single or multiple laser diodes).

[0037] Figure 3An example parallel configuration 300 of EDFA 301 according to an embodiment is shown. From input node 330 up to the EDF-C 304 and EDF-L 314 components, parallel configuration 300 can be identical to parallel configuration 200. As shown, the difference between the two configurations 300 and 200 is that parallel configuration 300 uses at least two separate GFFs, for example, a C-GFF 306 arranged or coupled in the C-band amplification section 302 and an L-GFF 316 arranged or coupled in the L-band amplification section 312. As further shown, isolator 305 can be arranged or coupled between EDF-C 304 and C-GFF 306, and isolator 315 can be arranged or coupled between EDF-L 314 and L-GFF 316.

[0038] In parallel scheme 300, similar to scheme 200, the C-band optical signal can be amplified or enhanced by EDF-C 304 using 980nm pump light, and the L-band optical signal can be enhanced by EDF-L 314 using 980nm pump light. In C-band segment 302, the amplified C-band passes through isolator 305 and then through C-GFF 306. In left-band segment 212, the amplified L-band passes through isolator 315 and then through L-GFF 316. Subsequently, the gain-flattened C-band and L-band can be combined by C / L WDM 322 to output the amplified and gain-flattened signal at output node 324.

[0039] According to the example, at least isolators 305 and 315, C-GFF 306, L-GFF 316, C / L WDM 322, and fiber optic taps can be packaged, configured, or arranged together as a hybrid assembly 330, as shown in the dashed box. At least in this respect, the hybrid assembly 300 can advantageously allow for the easy and efficient integration of separate C-band and L-band GFFs into existing amplifier designs or schemes. For example, because the configurations of parallel schemes 200 and 300 are identical up to the C-band and L-band EDFs, at least C / L WDM 234, isolator 236, GFF 238, and optical taps (arranged downstream of GFF 238) in parallel scheme 200 can be replaced by the hybrid assembly 330 to easily, efficiently, and effectively convert parallel scheme 200 into parallel scheme 300.

[0040] Figure 4An example serial configuration 400 of a C+L EDFA 401 according to an embodiment is shown. The C+L EDFA 401 includes a C-band amplification section 402 (e.g., a C-band amplification stage) and an L-band amplification section 412 (an L-band amplification stage), as shown in the dashed box. As shown, the C-band and L-band amplification sections 402 and 412 are configured in a serial or series arrangement. This can be understood as an arrangement with a single signal path for both the C-band and L-band amplification sections, rather than a parallel arrangement with two or more independent paths. For example, in a serial or series arrangement, the C-band and L-band amplification sections are connected such that amplification of the upstream section affects the downstream section. The C-band amplification section 402 includes at least an EDF-C 404 configured to amplify the C-band using 980nm pump light provided by a pump laser diode 434. The L-band amplification section includes at least the EDF-L 414 configured to amplify the L-band.

[0041] For example, when an optical signal (e.g., a C+L optical signal) is input or received at input node 422, the signal can pass through an optical tap and then through an optical isolator 424, where a 980 nm pump light is combined or multiplexed with the C-band of the optical signal such that the C-band wavelength is amplified or enhanced by EDF-C 404. While amplifying the C-band, the signal can be input to a C / L WDM 426, which can be a beamsplitter configured to separate, decouple, or separate the C-band and L-band of the optical signal.

[0042] As shown, the separated C-band signal (which has been amplified by the EDF-C 404 as described above) can then be input to the C / L WDM 428. The separated L-band signal can be combined with 980nm pump light and input to the EDF-L414, which can be configured to amplify or enhance the L-band. The enhanced L-band signal can also be input to the C / LWDM 428.

[0043] A C / L WDM 428 (e.g., a combiner) can be configured to combine or multiplex enhanced C-band and L-band signals and input the enhanced C+L signal to an optical isolator 430. After passing through isolator 430, the signal is then provided to a common GFF 432 for flattening or equalizing the gain of the enhanced C+L signal, which then passes through an optical tap and is output via output node 433. Similar to the parallel scheme 200, the serial scheme 400 also implements two optical isolators 424 and 430, thereby advantageously increasing the power efficiency of the C+L EDFA 401. Similarly, it is possible to use only two optical isolators because a non-reflective GFF technique is used instead of a reflective GFF in scheme 400. Figure 13 The diagram illustrates a serial scheme 1300 for a C+L EDFA using a reflective GFF, and describes how to connect scheme 1300 with a non-reflective GFF. Figure 4 This paper compares the scheme with the 400 scheme and cites the serial scheme. The above refers to... Figure 12 The description of the common-reflection GFF is provided, and it is also applicable to the use of at least two isolators (due to reflections from the GFF), taps, and a 980nm wavelength region pump source. Figure 13 .

[0044] Figure 5 An example gain diagram 500 of a serial scheme 400 of a C+L EDFA 401 according to an embodiment is shown. For example, the gain diagram shown represents the C-band and L-band gain with a span loss of 10 dB. As described above, the C-band segment 402 and L-band segment 412 of the serial scheme 400 share a common GFF 432.

[0045] like Figure 5 As shown, the C-band EDF (e.g., EDF-C 404) provides the required 10 dB amplification in the C-band. Furthermore, because at least the C-band amplification stage is positioned upstream of the L-band amplification stage in the serial scheme 400, the C-band EDF can provide additional amplification in the L-band, for example, ranging from approximately 1 dB to 8 dB. In this example, the additional amplification advantageously allows the serial scheme 400 to function as a two-stage amplifier for the L-band, which at least improves the overall power efficiency of the C+L EDFA 401.

[0046] Figure 6An example serial scheme 600 of the C+L EDFA 601 according to an embodiment is shown. As shown, serial scheme 600 can be the same as serial scheme 400 from the input node up to the EDF-L 614 component. However, the difference between schemes 400 and 600 is the use of two separate GFFs (e.g., C-GFF 620 for gain flattening the C-band and L-GFF 622 for gain flattening the L-band).

[0047] As further shown, isolators 624, 626, C-GFF 620, L-GFF 622, C / L WDM 628, and optical tap 630 can be arranged or packaged together to form a hybrid assembly 633, as indicated by the dashed box. As described above, for example regarding... Figure 3 The hybrid component 633 advantageously allows for the easy and efficient integration of separate C-band and L-band GFFs 620 and 622 into existing amplifier designs or schemes, such as serial scheme 400. Since the configurations of serial schemes 400 and 600 are identical from the input node to the EDF-L 614, the hybrid component 633 can simply and effectively replace at least the C / L WDM428, isolator 430, common GFF 432, and optical taps.

[0048] In the serial scheme 600, the C-band of the C+L signal is enhanced by EDF-C604 in the C-band amplification section 602, and then the C+L signal is separated or split by C / L WDM 634, such that the enhanced or amplified C-band signal is input to isolator 624 and gain-flattened by C-GFF 620. The L-band signal separated from the C+L signal by C / LWDM 634 can be enhanced or amplified by EDF-L614 in the L-band amplification section 612 via 980nm pump light, which can then be fed into optical isolator 626 and then gain-flattened by L-GFF 622. The equalized and / or flattened C-band and L-band can be combined using C / L WDM 628, and the combined C+L signal can then pass through optical tap 630, and the signal can then be output at output node 640.

[0049] Figure 7 An example serial scheme 700 of the C+L EDFA 701 according to an embodiment is shown. The serial scheme 700 is substantially similar to the serial scheme 600 of the C+L EDFA 601 (e.g., two separate GFFs (C-GFF 702 and L-GFF 704) for the C+L EDFA 701), except that the isolator placement is different.

[0050] As shown, for example, optical isolator 706 can be placed, arranged, or coupled in front of, behind, or upstream of EDF-L 708 to perform optical isolation of the L-band signal before it is amplified or enhanced. Furthermore, optical isolator 710 can be placed behind, in front of, or downstream of C / L WDM 712 to isolate the amplified or combined C+L signal (e.g., where the C-band and L-band are amplified and combined) via optical isolator 710.

[0051] Figure 8 An example serial configuration 800 of the C+L EDFA 801 according to an embodiment is shown. Serial configuration 800 is substantially similar to serial configuration 700 of the C+L EDFA 701 described above (e.g., users of separate C-band and L-band GFFs), except for a significant difference: the L-band GFF (e.g., L-GFF 802) is placed, arranged, or coupled before, after, or upstream of the L-band amplifier (e.g., EDF-L 804). At least in this respect, L-GFF 802 can be considered an “interstage” GFF, meaning that the L-band GFF is located between the C-band and L-band amplification segments or stages.

[0052] Because, for example, the EDF-C 806 provides additional amplification in the L-band (e.g., ranging from approximately 1 to 8 dB), at least one advantage of the interstage L-GFF 802 of the serial scheme 800 could be, for example, filtering certain wavelengths of the L-band signal before amplifying the L-band signal using the EDF-L 804, which will be described in further detail below. Moreover, the advantages of the interstage L-band GFF 802 located between the C-band and L-band amplification sections are similar to the benefits of the intermediate stage GFF in a dual-stage C-band or L-band EDA, which is an improvement in power efficiency in the C+L EDFA 801.

[0053] Figure 9 An example serial configuration 900 of a C+L EDFA 901 according to an embodiment is shown. Serial configuration 900 is substantially similar to serial configuration 600 of a C+L EDFA 601, except that, similar to serial configuration 800, the inter-stage GFF (e.g., L-GFF 902) is located between the C-band and L-band amplification sections. As shown, L-GFF 902 is placed, arranged, or coupled in front of, behind, or upstream of EDF-L904, which is an L-band EDF.

[0054] The additional L-band amplification (e.g., ranging from approximately 1 to 8 dB) provided by the EDF-C 906 before the optical signal is separated into separate C-band and L-band signals by the C / L WDM 908 can be flattened or equalized by the interstage L-GFF 902 before the separated L-band signal is amplified using the EDF-L 904. Similar to the serial scheme 800, the advantage of the interstage GFF configuration is that it improves the overall power efficiency of the C+L EDFA 901. Furthermore, the interstage GFF 902 advantageously provides gain tilt correction and other advantages, which will be described in further detail below.

[0055] Figure 10 Example noise figure (NF) diagram 1000 is shown for a serial scheme (e.g., serial scheme 800, serial scheme 900) with interstage GFFs according to embodiments. As shown in NF diagram 1000, for example, interstage GFFs can advantageously provide gain tilt correction or tilt filter functionality. It will be understood that gain tilt refers to the slope of the optical gain over a certain wavelength window, which can be an indicator of the gain flatness of the amplifier.

[0056] Without a gain tilt filter, as shown in Figure 1010, the additional gain in the L-band provided by the C-band EDF can result in a lower average inversion, a higher noise figure, and may also require a physically longer L-band EDF. With a tilt filter, the L-band noise figure is lower, as shown in curve 1020, and further, the tilt filter can be considered a “lossless” filter with losses at shorter L-band wavelengths, thus reducing the length of the L-band EDF by at least approximately 6% and increasing the noise figure by approximately 0.06 dB. It is understood that other C+LEDFA components, besides the interstage GFF, can support or provide tilt filter functionality, such as C / L WDM combiners.

[0057] Figure 11 An example guard band reduction 1100 for a C+L EDFA 1101 according to an embodiment is shown. Generally, C / L WDM combiners may have limited resolution, which can result in a wide guard band (e.g., 3 to 5 nm) without the use of a blocking filter. It is understood that at bandwidth transition points, a guard band may be needed to avoid crosstalk between the C-band and L-band.

[0058] like Figure 11As shown, blocking filters 1102 and 1104 can be arranged immediately before or after the C / L WDM 1106, such that the filters can receive and perform guard band reduction on the amplified C-band and L-band before the C / L WDM 1106 combines the C-band and L-band. As further shown in dashed 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. When no blocking filter is applied, the final width of the combined C-band and L-band guard band reduction is smaller (e.g., approximately 1 nm) compared to the width of the combined band guard band. For example, the blocking filter can be an integrated thin-film filter (TFF) or other suitable type of filter or component. Therefore, one advantage of integrating the blocking filter in the C+L EDFA is the reduction in insertion loss.

[0059] As explained above, the parallel and serial C+L EDFA schemes and their configuration variations described above are advantageous in many respects. For example, when using a non-reflective GFF or a non-reflective gain flattening technique instead of a reflective GFF, both parallel and serial schemes provide improved power efficiency of the EDFA by minimizing the use of passive EDFA components such as optical isolators. Moreover, both parallel and serial schemes with a common GFF and those with separate C-band and L-band GFFs share a similar architecture, which allows the output architecture of the scheme to be included in a hybrid component for easy and efficient integration into a C+L EDFA. Furthermore, as described above, the interstage GFF variation in the serial scheme provides at least the following: (i) improved power efficiency depending on the total amplifier gain and background loss of the GFF, for example, a reduction in total pump power of approximately 0.5 dB for 10 dB span loss and 0.6 dB GFF background loss; (ii) gain flattening and / or gain flattening with gain equalization for shortening the L-band EDF and improving average inversion and reducing NF; and (iii) tilt correction or tilt filter functionality. Additionally, blocking filters can be used to narrow or reduce the width of the guard band in the combined C-band and L-band to increase the effective transmission bandwidth.

[0060] It is understood that the parallel and serial C+L EDFA schemes described above can be arranged in various different configurations. For example, the order or sequence of the C-band and L-band amplification stages can be switched in serial schemes, etc., and are not limited to any particular configuration or in any other way.

[0061] This document discloses novel and inventive techniques for improving gain equalization in C+L EDFAs. This disclosure is not limited in scope to the specific embodiments described herein. In fact, various other embodiments and improvements of this disclosure, besides those described herein, will be apparent to those skilled in the art from the foregoing description and accompanying drawings.

[0062] Therefore, such other embodiments and modifications are intended to fall within the scope of this disclosure. Furthermore, although this disclosure has been described herein in a specific context and for a specific purpose in a specific setting, those skilled in the art will recognize that its usefulness is not limited thereto, and that this disclosure can be advantageously implemented in any number of settings for any number of purposes. Therefore, the claims set forth below should be interpreted in accordance with the full breadth and spirit of the disclosure described herein.

Claims

1. An apparatus for improving gain equalization in an optical amplifier, comprising: C-band amplification section; L-band amplification section; A first gain flattening filter (GFF) is used to perform C-band gain equalization, and the first GFF is coupled and positioned after the C-band amplification section; as well as A second GFF, different from the first GFF, is used to perform L-band gain equalization. The second GFF is coupled and positioned after the L-band amplification section. The C-band amplification section includes at least a first erbium-doped fiber (EDF) for amplifying C-band optical signals. The L-band amplification section includes at least a second EDF for amplifying the L-band optical signal; A first optical isolator is disposed or coupled between the first EDF and the first GFF in the C-band amplification section; as well as A second optical isolator is disposed or coupled between the second EDF and the second GFF in the L-band amplification section. The C-band amplification section and the L-band amplification section are arranged in parallel. At least the first optical isolator and the second optical isolator, as well as the first GFF and the second GFF, form a hybrid assembly.

2. The apparatus according to claim 1, further comprising: The third optical isolator is used to receive input signals; as well as A separator is used to separate the input signal into the C-band optical signal and the L-band optical signal.

3. The apparatus of claim 1, wherein the first GFF and the second GFF are respectively coupled to or arranged after or downstream of the first EDF and the second EDF, such that the first GFF and the second GFF perform gain equalization on the amplified C-band signal and the L-band signal, and the apparatus further includes a combiner for combining the amplified and gain-equalized C-band and L-band signals for output.

4. The apparatus of claim 1, wherein the first EDF and the second EDF use 980 nm pump light provided by a laser diode to perform amplification.

5. An apparatus for improving gain equalization in an optical amplifier, comprising: C-band amplification section; L-band amplification section; A first gain flattening filter (GFF) is used to perform C-band gain equalization, and the first GFF is coupled to the C-band amplification section; as well as A second GFF, different from the first GFF, is used to perform L-band gain equalization, and is coupled to the L-band amplification section. The C-band amplification section includes at least a first erbium-doped fiber (EDF) for amplifying C-band optical signals. The L-band amplification section includes at least a second EDF for amplifying the L-band optical signal, and The C-band amplification section and the L-band amplification section are arranged in series. A separator is used to separate the input signal into the C-band optical signal and the L-band optical signal; A first optical isolator, wherein the first optical isolator and the splitter are arranged or coupled between the first EDF and the first GFF in the C-band amplification section; and A second optical isolator is disposed or coupled between the second EDF and the second GFF in the L-band amplification section. At least the first optical isolator and the second optical isolator, as well as the first GFF and the second GFF, form a hybrid assembly.

6. The apparatus according to claim 5, further comprising: A third optical isolator is used to receive the input signal, and The third optical isolator is arranged or coupled in front of or upstream of the separator.

7. The apparatus of claim 5, wherein the serial arrangement includes the C-band amplification section arranged or coupled before or upstream of the L-band amplification section, and wherein the first GFF and the second GFF are respectively coupled to and arranged after or downstream of the first EDF and the second EDF, such that the first GFF and the second GFF perform gain equalization on the amplified C-band and L-band signals.

8. The apparatus according to claim 5, further comprising: A combiner used to combine amplified C-band optical signals and L-band optical signals; The second GFF is arranged or coupled between the first optical isolator and the second EDF of the L-band amplification section, with the second GFF located in front of or upstream of the second EDF, such that the L-band gain equalization is performed on the L-band optical signal before the second EDF amplifies the L-band optical signal.

9. The apparatus of claim 5, wherein the second GFF is arranged or coupled in front of or upstream of the second EDF, such that the L-band gain equalization is performed on the L-band optical signal before the second EDF amplifies the L-band optical signal, and The second GFF is an interstage GFF and is configured to provide gain tilt correction.

10. The apparatus according to claim 5, further comprising: First blocking filter; as well as Second blocking filter, The guard band corresponding to the amplified C-band optical signal is reduced or decreased by the first blocking filter, and The guard band corresponding to the amplified L-band optical signal is reduced or decreased by the second blocking filter.

11. The apparatus of claim 5, wherein the first EDF and the second EDF use 980 nm pump light provided by a laser diode to perform amplification, and wherein the apparatus is a C+LEDFA arranged or implemented in a long-range optical communication system.

12. An apparatus for improving gain equalization in an optical amplifier, comprising: C-band amplification section; L-band amplification section; as well as Gain flattening filters (GFFs) are used to perform C-band and L-band gain equalization, and The C-band amplification section includes at least a first erbium-doped fiber (EDF) for amplifying C-band optical signals. The L-band amplification section includes at least a second EDF for amplifying the L-band optical signal. A first optical isolator is disposed or coupled between the first EDF and the first GFF in the C-band amplification section; as well as A second optical isolator is disposed or coupled between the second EDF and the second GFF in the L-band amplification section. The C-band amplification section and the L-band amplification section are configured in a series or parallel arrangement, and At least the first optical isolator and the second optical isolator, as well as the first GFF and the second GFF, form a hybrid assembly.

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