Patent Publication Number: US-2015085352-A1

Title: Optical amplifier for space-division multiplexing

Description:
BACKGROUND 
     1. Field 
     The present invention relates to optical communication equipment and, more specifically but not exclusively, to optical amplifiers. 
     2. Description of the Related Art 
     This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art. 
     An optical amplifier is a device that amplifies an optical signal directly in the optical domain without converting the optical signal into a corresponding electrical signal. Optical amplifiers are widely used, for example, in the fields of optical communications and laser physics. 
     One type of an optical amplifier is a doped-fiber amplifier, with a well-known example being the Erbium-doped fiber amplifier (EDFA). In operation, a signal to be amplified and a pump beam are applied to the doped fiber. The pump beam excites the doping ions, and amplification of the signal is achieved by stimulated emission of photons from the excited dopant ions. 
     Another type of an optical amplifier is a Raman amplifier, which relies on stimulated Raman scattering (SRS) for signal amplification. More specifically, when a signal to be amplified and a pump beam are applied to an optical fiber made of an appropriate material, a lower-frequency signal photon induces SRS of a higher-frequency pump photon, which causes the pump photon to pass some of its energy to the vibrational states of the fiber material, thereby converting the pump photon into an additional signal photon. The pump beam may be coupled into the fiber in the same direction as the signal (co-directional pumping) or in the opposite direction (contra-directional pumping). 
     SUMMARY OF SOME SPECIFIC EMBODIMENTS 
     Disclosed herein are various embodiments of an optical amplifier, e.g., suitable for amplifying space-division multiplexed signals received from an optical fiber transmission line. In an example embodiment, the optical amplifier comprises a doped multi-core optical fiber configured to receive an optical pump beam in a manner that causes the optical pump energy to be transferred into the optical waveguide cores of the doped multi-core optical fiber while being guided along the length of the multi-core optical fiber. The optical amplifier further comprises two optical couplers placed at the ends of the doped multi-core fiber, with each optical coupler having a respective plurality of optical waveguide cores optically coupled to the optical waveguide cores of the doped multi-core optical fiber. The spatial arrangement of the optical waveguide cores at the input end of the first optical coupler may be configured for low-loss intake of the optical energy from an input fiber optical transmission line. The spatial arrangement of the optical waveguide cores at the output end of the first optical coupler and the spatial arrangement of the optical waveguide cores at the input end of the second optical coupler match the spatial arrangement of the optical waveguide cores in the doped multi-core fiber. The spatial arrangement of the optical waveguide cores at the output end of the second optical coupler may be configured for low-loss transfer of the optical energy into an output fiber optical transmission line. In various embodiments, each of the input and output fiber optical transmission lines can be selected from a set including a multimode optical fiber, a multi-core optical fiber, and a fiber-optic cable. The doped multi-core optical fiber can be configured to amplify optical signals via a stimulated-emission process or a stimulated Raman-scattering process. 
     According to one embodiment, provided is an apparatus comprising: a first rare-earth doped multi-core optical fiber having a first plurality of optical waveguide cores, each optical waveguide core configured to guide and amplify a respective portion of optical power received through an input end thereof to generate a respective amplified light signal at an output end thereof; a first three-dimensional optical waveguide device configured to end-couple an input optical fiber transmission line to the input end of the first rare-earth doped multi-core optical fiber; and a second three-dimensional optical waveguide device configured to end-couple light from the output end of the first rare-earth doped multi-core optical fiber into an output optical fiber transmission line. 
     According to another embodiment, provided is an apparatus comprising: an input port configured to end-connect to a first optical fiber transmission line; an output port configured to end-connect to a second optical fiber transmission line; a first doped multi-core optical fiber having a first plurality of optical waveguide cores, each configured to amplify, using a first optical pump beam, a respective portion of optical power received through the input port to generate a respective amplified portion of the optical power, wherein the first doped multi-core optical fiber is configured to direct the amplified portions of the optical power generated by different ones of the first plurality of optical waveguide cores toward the output port; and a first optical coupler coupled between the input port and the first doped multi-core optical fiber. The first optical coupler comprises a second plurality of optical waveguide cores, each configured to guide the respective portion of the optical power received through the input port toward the respective one of the first plurality of optical waveguide cores. A spatial arrangement of the second plurality of optical waveguide cores at an input end of the first optical coupler is different from a spatial arrangement of the second plurality of optical waveguide cores at an output end of the first optical coupler. 
     According to yet another embodiment, provided is an apparatus comprising: an input port configured to end-connect to a first optical transmission line; an output port configured to end-connect to a second optical transmission line; a first doped multi-core optical fiber having a first plurality of cores, each configured to amplify, using a first optical pump beam, a respective portion of optical power received through the input port to generate a respective amplified portion of the optical power, wherein the first doped multi-core optical fiber is configured to direct the amplified portions of the optical power generated by different ones of the first plurality of cores toward the output port; and a first optical coupler coupled between the first doped multi-core optical fiber and the output port. The first optical coupler comprises a second plurality of cores, each configured to guide the respective one of the amplified portions of the optical power generated by different ones of the first plurality of cores toward the output port. A spatial arrangement of the second plurality of cores at an input end of the first optical coupler is different from a spatial arrangement of the second plurality of cores at an output end of the first optical coupler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, features, and benefits of various embodiments of the invention will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which: 
         FIG. 1  shows a block diagram of an optical amplifier according to an embodiment of the disclosure; 
         FIG. 2  shows a cross-sectional view of a multi-core optical fiber that can be used in the optical amplifier of  FIG. 1  according to an embodiment of the disclosure; 
         FIGS. 3A-3C  show a three-dimensional optical waveguide device that may form the optical end-couplers used in the optical amplifier of  FIG. 1  according to an embodiment of the disclosure; 
         FIG. 4  shows a block diagram of an optical amplifier according to another embodiment of the disclosure; and 
         FIG. 5  shows a block diagram of a gain-equalizing filter that can be used in the optical amplifier of  FIG. 4  according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Some embodiments disclosed herein may benefit from the subject matter of U.S. patent application Ser. No. 1______/______,______, filed on the same date as the present application, by Roland Ryf, Nicolas Fontaine, and David Neilson, attorney docket reference 814846-US-NP, entitled “COMPACT TWO-STAGE OPTICAL AMPLIFIER,” which is incorporated herein by reference in its entirety. 
     Herein, a three-dimensional optical waveguide device has multiple optical waveguide cores embedded in an optical cladding and connecting first and second faces of the device. In such a device, the lateral spatial arrangement of, at least, two of the optical waveguide cores varies between the first and second faces. 
     A multimode or multi-core optical fiber can provide a higher transmission capacity than a single-mode fiber by means of space-division multiplexing (SDM), wherein different spatial modes of the multimode or multi-core optical fiber are populated with different modulated optical signals or different combinations of a given set of modulated optical signals. Long-distance transport of SDM signals over multimode and/or multi-core optical fibers can greatly benefit from the use of optical amplifiers that enable the optical gain for each spatial and/or wavelength channel to be individually controlled and/or adjusted, e.g., to ensure that the corresponding optical-transport link has favorable signal-transport characteristics. 
     For example, different spatial modes of a multimode fiber (e.g., LP modes in a weakly guiding cylindrical fiber or waveguide) are generally subjected to different degrees of attenuation over the same fiber length, with the higher-order modes typically being subjected to stronger attenuation than the lower-order modes. However, a desired characteristic of an optical-transport link is often about a 0-dB net gain for all populated spatial modes. This dB number implies that it is often beneficial when signal attenuation in the optical-transport link is substantially canceled or compensated to a significant degree by signal amplification in the corresponding optical amplifier(s) at the proximal and/or distal end of the optical-transport link. 
     Some features of spatial mode multiplexing in multimode and multi-core optical fibers are described, e.g., in U.S. Patent Application Publication No. 2013/0070330 and U.S. Pat. No. 8,320,769, both of which are incorporated herein by reference in their entirety. 
     Some of the above-mentioned and other pertinent problems in the art may be addressed by some of the embodiments of an optical amplifier, which are disclosed herein. In one embodiment, such an optical amplifier can be used, e.g., to amplify SDM optical signals, which are transported through a multimode or multi-core optical fiber before and/or after optical amplification in the optical amplifier. In various embodiments, such an optical amplifier can be configured to amplify optical signals via a stimulated-emission process or a stimulated Raman-scattering (SRS) process. Advantageously, some embodiments of the optical amplifier disclosed herein may provide a cost-effective solution for long-distance optical transport of SDM signals. 
     Some embodiments of an optical amplifier disclosed herein can also be adapted for amplifying optical signals transported by a fiber-optic cable comprising a plurality of bundled single-mode or multimode fibers, e.g., using an amplifier design conceptually similar to that used for multi-core optical fibers. 
       FIG. 1  shows a block diagram of an optical amplifier  100  according to an embodiment of the disclosure. Optical amplifier  100  has an SDM input port  102  and an SDM output port  162 , each of SDM ports  102 ,  162  being configured to be connected to a respective optical fiber SDM transmission line. In various embodiments, an optical SDM transmission line can be a multimode fiber, a multi-core optical fiber, or a fiber-optic cable comprising a plurality of bundled optical fibers. 
     In some embodiments, SDM input port  102  and SDM output port  162  may be designed for being connected to different types of transmission lines. For example, in one embodiment, SDM input port  102  may be designed for being connected to a multimode fiber, while SDM output port  162  may be designed for being connected to a fiber-optic cable or to a multi-core optical fiber. As another example, in an alternative embodiment, SDM input port  102  may be designed for being connected to a multi-core optical fiber, while SDM output port  162  may be designed for being connected to a fiber-optic cable or to a multimode fiber. As yet another example, in yet another alternative embodiment, SDM input port  102  may be designed for being connected to a fiber-optic cable, while SDM output port  162  may be designed for being connected to a multi-core optical fiber or to a multimode optical fiber. 
     Optical amplifier  100  performs optical signal amplification using a doped multi-core optical fiber  140 . In one embodiment, multi-core optical fiber  140  comprises a plurality of doped cylindrical optical waveguide cores surrounded by a first (e.g., inner) cylindrical cladding. The first cylindrical cladding is further surrounded by a second (e.g., outer) cylindrical cladding. The refractive indices of the optical waveguide cores and the first and second claddings satisfy the following condition: 
       n c &gt;n 1 &gt;n 2   (1)
 
     where n c  is the refractive index of the material of the optical waveguide cores; n 1  is the refractive index of the material of the first optical cladding; and n 2  is the refractive index of the material of the second optical cladding. Due to this condition and the spacings between the various optical waveguide cores and between the optical waveguide cores and the boundaries of the optical claddings, multi-core optical fiber  140  has (i) a first set of guided modes, wherein each mode substantially is a guided mode of a respective one of the optical waveguide cores, and (ii) a second set of guided modes, wherein each mode substantially is a guided mode of the first cladding. An example embodiment of multi-core optical fiber  140  is described in more detail below in reference to  FIG. 2 . 
     Optical amplifier  100  has a pump laser  130  configured to apply optical-pump energy to multi-core optical fiber  140 , e.g., by illuminating the terminus of the first cladding on a first end face  138  of the multi-core optical fiber. Due to the condition expressed by Eq. (1), the second cladding of multi-core optical fiber  140  may cause the optical-pump energy to be guided along the length of the first cladding while part of said guided optical-pump energy is transferred into the optical waveguide cores. The optical-pump energy transferred into the optical waveguide cores may provide some of the energy source for optical signal amplification therein, e.g., via stimulated emission or stimulated Raman scattering. In one embodiment, an optional pump-stop filter  146  can be used to block further downstream propagation of the residual optical-pump energy (if any) exiting a second end face  142  of multi-core optical fiber  140 . Pump-stop filter  146  however, typically should not substantially attenuate the amplified optical signals that exit multi-core optical fiber  140  so that those signals pass and will propagate toward the downstream portion of optical amplifier  100 . 
     Optical amplifier  100  also may include a free-space isolator  120 ,  150  at one or both ends of multi-core optical fiber  140 . An optional additional free-space isolator (not explicitly shown in  FIG. 1 ) can be placed between pump laser  130  and an optical beam combiner (e.g., a dichroic mirror)  134 . Typically, free-space isolators, also known as Faraday optical isolators, are implemented as magneto-optic devices that preferentially transmit light along a single direction, thereby shielding the upstream optics from back reflections. Back reflections may be detrimental because they can create instabilities in light sources and increase the level of optical noise. In some cases, intense back-reflected light can even permanently damage some optical components. 
     The incoming optical signals received through SDM input port  102  or different portions of the incoming optical signals received through SDM input port  102  are spatially rearranged using an optical coupler  110  to generate a plurality of spatially rearranged optical signals  112 . Optical signals  112  are then coupled into multi-core optical fiber  140  such that each of optical signals  112  is coupled into a respective one of the optical waveguide cores in multi-core optical fiber  140 . When the first cladding of multi-core optical fiber  140  is optically pumped by pump laser  130 , optical signals  112  are optically amplified in the respective optical waveguide cores, e.g., as described above, to generate a plurality of amplified optical signals  152 . An optical coupler  160  then spatially rearranges amplified optical signals  152  to generate outgoing optical signals that exit optical amplifier  100  through SDM output port  162 . An example embodiment of an optical coupler that can be used as optical coupler  110  and/or optical coupler  160  is described in more detail below in reference to  FIGS. 3A-3C . 
       FIG. 2  shows a cross-sectional view of a multi-core optical fiber  200  that can be used as multi-core optical fiber  140  according to an embodiment of the disclosure. Illustratively, multi-core optical fiber  200  is shown as having seven doped optical waveguide cores  202   1 - 202   7 . Doped optical waveguide core  202   1  is located substantially on the center axis of multi-core optical fiber  200 . Doped optical waveguide cores  202   2 - 202   7  are located at the apices of a hexagon centered on the center axis of multi-core optical fiber  200 . One of ordinary skill in the art will understand that other lateral spatial arrangements of the doped optical waveguide cores are also possible. For example, another arrangement may have a number of optical waveguide cores that is different from seven and/or have the doped optical waveguide cores arranged in a geometric pattern that is different from a centered hexagon. 
     Doped optical waveguide cores  202   1 - 202   7  are surrounded by an inner cladding  204 . Inner cladding  204  may be further surrounded by an outer cladding  206 . The refractive indices of doped optical waveguide cores  202   1 - 202   7 , inner cladding  204 , and outer cladding  206  satisfy the condition expressed by Eq. (1). 
       FIGS. 3A-3C  show a three-dimensional optical waveguide device  300  that can be used as optical coupler  110  or  160  according to an embodiment of the disclosure. More specifically,  FIG. 3A  shows a three-dimensional view of optical waveguide device  300 .  FIG. 3B  shows a first end view of three-dimensional optical waveguide device  300  looking at an end face  310  thereof.  FIG. 3C  shows a second end view of three-dimensional optical waveguide device  300  looking at an end face  320  thereof. 
     Three-dimensional optical waveguide device  300  comprises a three-dimensional (3D) monolithic structure having multiple optical waveguide cores designed to provide optical end-coupling between (i) multi-core optical fiber  200  (see  FIG. 2 ) and (ii) a multimode optical fiber connected to SDM input port  102  or SDM output port  162  (see  FIG. 1 ). When three-dimensional optical waveguide device  300  operates as optical coupler  110  (i.e., is configured between SDM input port  102  and multi-core optical fiber  140 ,  FIG. 1 ), the optical signals flow from end face  310  to end face  320 . When optical coupler  300  operates as optical coupler  160  (i.e., is configured between SDM output port  162  and multi-core optical fiber  140 ,  FIG. 1 ), the optical signals flow from end face  320  to end face  310 . 
     Referring to  FIG. 3A , three-dimensional optical waveguide device  300  comprises a block  308  of an optical cladding material that surrounds seven optical waveguide cores  302   1 - 302   7 . Each of optical waveguide cores  302   1 - 302   7  has an approximately circular cross-section in any plane parallel to the XY coordinate plane. Optical waveguide cores  302   1 - 302   7  are packed relatively closely together at end face  310  (also see  FIG. 3B ). The separation between optical waveguide cores  302   1 - 302   7  gradually (e.g., adiabatically) increases along the Z-coordinate axis between end faces  310  and  320 , e.g., as indicated in  FIG. 3A . Cores  302   1 - 302   7  are separated from one another by relatively large distances at end face  320  (also see  FIG. 3C ). Note that, for clarity, only optical waveguide cores  302   1 ,  302   2 ,  302   4 , and  302   5  are shown within the body of optical cladding block  308  in the see-through view shown in  FIG. 3A , while optical waveguide cores  302   3 ,  302   6 , and  302   7  are not explicitly shown therein. 
     Referring to  FIG. 3B , to couple light from an input multimode fiber connected to SDM input port  102  into optical waveguide cores  302   1 - 302   7  of three-dimensional optical waveguide device  300 , optical amplifier  100  employs a first set of imaging optics (not explicitly shown in  FIG. 1 ) configured to image the end face of the input multimode fiber onto end face  310 , e.g., such that the optical waveguide core of the input multimode optical fiber forms an image on end face  310  indicated in  FIG. 3B  by a dashed circle  304 . The diameters of optical waveguide cores  302   1 - 302   7  and the magnification/demagnification of the imaging optics may be selected such that the coupling losses are kept to a relatively low (e.g., close to a minimum possible) value. The gradual increase in the separation between optical waveguide cores  302   1 - 302   7  within the body of cladding block  308  may ensure that further optical losses within the cladding block, e.g., through radiation modes of the individual optical waveguide cores, are relatively low. As a result, most of the light coupled into optical waveguide cores  302   1 - 302   7  at end face  310  may be guided by the optical waveguide cores and exit three-dimensional optical waveguide device  300  through end face  320 . 
     In reference to both  FIGS. 2 and 3C , the geometric arrangement of optical waveguide cores  302   1 - 302   7  at end face  320  of three-dimensional optical waveguide device  300  may substantially match the geometric arrangement of doped optical waveguide cores  202   1 - 202   7  at the corresponding end face of multi-core optical fiber  200 . This substantial geometric match may enable optical amplifier  100  to use a second set of imaging optics (not explicitly shown in  FIG. 1 ) to image end face  320  onto the corresponding end face of multi-core optical fiber  200 , e.g., such that each of optical waveguide cores  302   1 - 302   7  ( FIG. 3C ) is imaged onto a respective one of doped optical waveguide cores  202   1 - 202   7  ( FIG. 2 ). As a result, the light leaving optical waveguide cores  302   1 - 302   7  through end face  320  can be efficiently transferred into doped optical waveguide cores  202   1 - 202   7  for optical amplification therein. 
     At the back end of multi-core optical fiber  200 , two additional sets of imaging optics (not explicitly shown in  FIG. 1 ) and another instance (physical copy) of three-dimensional optical waveguide device  300  can similarly be used to couple amplified optical signals generated by multi-core optical fiber  200  into an output multimode fiber connected to SDM output port  162  ( FIG. 1 ). More specifically, a first additional set of imaging optics can be used to image the back-end face of multi-core optical fiber  200  onto end face  320  of three-dimensional optical waveguide device  300  configured as optical coupler  160  ( FIG. 1 ). This imaging will cause efficient transfer of light from doped optical waveguide cores  202   1 - 202   7  of multi-core optical fiber  200  into optical waveguide cores  302   1 - 302   7  of three-dimensional optical waveguide device  300 . A second additional set of imaging optics can then be used to image end face  310  of three-dimensional optical waveguide device  300  configured as optical coupler  160  onto the end face of the output multimode fiber connected to SDM output port  162 , e.g., in the manner indicated by dashed circle  304  in  FIG. 3B . This imaging will cause efficient transfer of light from optical waveguide cores  302   1 - 302   7  of three-dimensional optical waveguide device  300  into the optical waveguide core of the output multimode optical fiber. 
     The optical couplers  110  and  160  may or may not be nominally identical to one another, as in the above-described example. More specifically, optical coupler  110  can be designed to provide efficient light transfer from the specific input multimode optical fiber, multi-core optical fiber, or fiber-optic cable used at SDM input port  102 , through a respective set of free-space imaging optics, and into the doped optical waveguide cores of the specific embodiment of multi-core optical fiber  140  in optical amplifier  100 . Similarly, optical coupler  160  can be designed to provide efficient light transfer from the doped optical waveguide cores of the specific embodiment of multi-core optical fiber  140 , through a respective set of free-space imaging optics, and into the specific output multimode optical fiber, multi-core optical fiber, or fiber-optic cable used at SDM output port  162 . As such, alternative embodiments of optical couplers  110  and  160  used in alternative embodiments of optical amplifier  100  may be different from three-dimensional optical waveguide device  300  ( FIG. 3 ) and/or from each other. Three-dimensional optical waveguide devices for optical couplers  110  and  160  can be manufactured, e.g., as described in U.S. Pat. No. 8,270,788 and/or U.S. Patent Application Publication No. 2012/0039567, both of which are incorporated herein by reference in their entirety. Suitable 3D multi-optical-core waveguide devices for optical couplers  110  and  160  may also be commercially obtained, e.g., from Optoscribe Ltd. of Livingston, West Lothian, Scotland, UK. 
     Alternative geometric patterns in which optical waveguide cores may be arranged at end face  310  can be selected, e.g., from a set including but not limited to: (i) a honeycomb-like pattern; (ii) a linear array of optical waveguide cores; (iii) a rectangular array of optical waveguide cores; (iv) an array of optical waveguide cores arranged on a circle; (v) an array of optical waveguide cores arranged on two or more concentric circles; and (vi) a non-symmetric or irregular pattern. Alternative geometric patterns in which optical waveguide cores may be arranged at end face  320  can similarly be selected from these and other suitable alternatives. Within the same three-dimensional optical waveguide device  300 , the respective arrangements of optical waveguide cores at end faces  310  and  320  may differ from one another in at least one of: (i) the separation distance between at least two of the optical waveguide cores, (ii) the diameter of at least one optical waveguide core, and (iii) the geometric patterns in which the optical waveguide cores are arranged at the end faces. In some embodiments, the number of optical waveguide cores at end face  310  may differ from the number of optical waveguide cores at end face  320 . In such embodiments, some of optical waveguide cores have a branched topology, wherein an optical waveguide core is split into two or more optical waveguide cores inside optical cladding block  308 . 
       FIG. 4  shows a block diagram of an optical amplifier  400  according to another embodiment of the disclosure. Optical amplifier  400  differs from optical amplifier  100  ( FIG. 1 ) in that it is a two-stage amplifier, with the two stages being labeled in  FIG. 4  as Stage  1  and Stage  2 , respectively. However, optical amplifier  400  employs some of the same components as optical amplifier  100 , as indicated by the common numerical labels used in both  FIGS. 1 and 4 . The description of these (reused) components is not repeated here. Instead, the description of optical amplifier  400  that follows outlines the differences between optical amplifiers  400  and  100  and focuses on the additional components present in optical amplifier  400  but not in optical amplifier  100 . 
     Each of Stages  1  and  2  includes a respective optical signal monitor  470 . Optical signal monitor  470   1  in Stage  1  is configured to receive (attenuated) copies of spatially rearranged optical signals  112  from an optical splitter  412  located between optical coupler  110  and free-space isolator  120 . In an example embodiment, optical splitter  412  has a signal-splitting ratio of about 1% to about 99%, which causes about 1% of the light from each of optical signals  112  to be tapped off and redirected toward optical signal monitor  470   1 . Optical signal monitor  470   2  in Stage  2  is configured to receive (attenuated) copies of amplified optical signals  152  from an optical splitter  452  located between free-space isolator  150  and optical coupler  160 . In an example embodiment, optical splitter  452  is similar to optical splitter  412  in terms of its signal-splitting, which causes about 1% of the light from each of optical signals  152  to be tapped off and redirected toward optical signal monitor  470   2 . 
     By processing the light received from optical splitter  412 , optical signal monitor  470   1  measures the total signal intensity and optionally the spectrum of each of optical signals  112 . The measurement results are then provided, via an electrical signal  472   1 , to a controller  480 . By similarly processing the light received from optical splitter  452 , optical signal monitor  470   2  measures the total signal intensity and optionally the spectrum of each of optical signals  152 . Optical signal monitor  470   2  similarly provides the measurement results to controller  480 , via an electrical signal  472   2 . 
     In an example embodiment, an optical signal monitor  470  comprises means for spectrally dispersing the received light (e.g., a diffraction grating) and an array of photo-detectors (e.g., a charge-coupled device, CCD) configured to measure the intensity of the spectrally dispersed light in a wavelength-sensitive manner. Different surface areas of the photo-detector array can be used for receiving and measuring different signals, as known in the art. Such a configuration enables optical signal monitor  470  to perform the above-indicated spectral and intensity measurements in a signal-specific manner. 
     Based on the measurement results received from optical signal monitors  470   1  and  470   2 , controller  480  can determine and monitor over time the effective optical gain in optical amplifier  400  in a spatial-channel- and wavelength-specific manner. As used herein, the term “spatial channel” refers to a series of optical elements in optical amplifier  400  optically coupled to one another and operating to transform a respective one of optical signals  112  into a respective one of amplified optical signals  152 . An example spatial channel in optical amplifier  400  comprises (i) a doped optical waveguide core of multi-core optical fiber  140   1 , (ii) a corresponding sub-channel of a gain-equalizing filter  444 , and (iii) a corresponding doped optical waveguide core of multi-core optical fiber  140   2 . 
     Controller  480  can use control signals  482   1 ,  482   2 , and  484 , e.g., to adjust the respective optical gains of different spatial channels of optical amplifier  400  as appropriate or necessary. For example, in response to control signal  482   1 , pump laser  130   1  may change the output wavelength and/or intensity of the pump beam applied to the inner cladding of doped multi-core optical fiber  140   1 . In response to control signal  482   2 , pump laser  130   2  may similarly change the output wavelength and/or intensity of the pump beam applied to the inner cladding of doped multi-core optical fiber  140   2 . Note that, during at least some time periods, pump lasers  130   1  and  130   2  may output pump beams having different respective wavelengths. In response to control signal  484 , gain-equalizing filter  444  may change signal attenuation in its sub-channels, e.g., as described below in reference to  FIG. 5 . Thus, using control signals  482   1 ,  482   2 , and  484 , controller  480  can adjust the operating parameters of pump lasers  130   1  and  130   2  and gain-equalizing filter  444  to achieve and maintain the desired optical-gain characteristics for optical amplifier  400 . 
     In some embodiments, gain-equalizing filter  444  is configured to individually control gain equalization for each spatial and/or wavelength channel. For example, the gain correction or equalization imposed by gain-equalizing filter  444  as a function of wavelength may depend on the signal&#39;s input power and/or be directed at correcting the effects of possible small variations between the characteristics of different doped optical waveguide cores  202  ( FIG. 2 ). 
       FIG. 5  shows a block diagram of a gain-equalizing filter  500  that can be used as gain-equalizing filter  444  according to an embodiment of the disclosure. Gain-equalizing filter  500  is illustratively shown as being connected between multi-core optical fibers  140   1  and  140   2  ( FIG. 4 ) and configured to receive control signal  484  (also see  FIG. 4 ). One of ordinary skill in the art will understand that gain-equalizing filter  500  can also be used in other alternative configurations. 
     Gain-equalizing filter  500  comprises a plurality of variable optical attenuators  510   1 - 510   N . For example, an embodiment of gain-equalizing filter  500  suitable for being connected between two multi-core optical fibers  200  ( FIG. 2 ) corresponds to N=6 or 7. The number N may be the number of spatial channels in optical amplifier  400  or may be the number of spatial channels in optical amplifier  400  minus one. Thus, N can be any positive integer greater than one in various embodiments. The attenuation in each of optical attenuators  510   1 - 510   N  can be changed, e.g., using a respective one of control signals  484   1 - 484   N  generated by controller  480  based on electrical signals  472   1  and  472   2  ( FIG. 4 ). 
     In operation, each of optical attenuators  510   1 - 510   N  receives a respective optical signal from a respective doped optical waveguide core of multi-core optical fiber  140   1 . In response to a respective one of control signals  484   1 - 484   N , each of optical attenuators  510   1 - 510   N  applies a respective desired degree of attenuation to the received optical signal to generate a respective attenuated optical signal. The signal attenuation imposed in each of optical attenuators  510   1 - 510   N  may be substantially wavelength-independent so that all wavelengths in the received optical signal are attenuated in optical attenuator  510  by substantially the same dB value. 
     Gain-equalizing filter  500  may further comprise a wavelength-balancing filter  520  configured to receive the attenuated optical signals from optical attenuators  510   1 - 510   N , filter these signals, and couple each of the resulting filtered optical signals into a respective doped optical waveguide core of multi-core optical fiber  140   2 . Unlike optical attenuators  510   1 - 510   N , wavelength-balancing filter  520  filters the received optical signals in a wavelength-dependent manner. One example spectral transfer function may cause wavelength-balancing filter  520  to impose greater attenuation on shorter wavelengths, e.g., when optical amplifier  400  needs to provide higher optical gain for longer wavelengths. An alternative spectral transfer function may cause wavelength-balancing filter  520  to impose greater attenuation on longer wavelengths, e.g., when optical amplifier  400  needs to provide higher optical gain for shorter wavelengths. The wavelength-balancing filter  520  may apply a suitable spectral transfer function to cause the optical gain of optical amplifier  400  to exhibit desired spectral characteristics. For example, one possible desired spectral characteristic might be a flat (constant, wavelength-independent) optical gain. An alternative desired spectral characteristic might be an optical gain that gradually increases with wavelength. Yet another alternative desired spectral characteristic might be an optical gain that gradually decreases with wavelength. In an example embodiment, wavelength-balancing filter  520  can employ free-space optical elements, such as lenses and filter plates. 
     In an alternative embodiment, each optical attenuator  510  can be an attenuator that can provide the attenuation in a wavelength-dependent manner and is configurable to set and/or change the imposed attenuation for each wavelength individually. In such an embodiment, wavelength-balancing filter  520  is optional and can be removed from gain-equalizing filter  500 . 
     According to an embodiment disclosed above in reference to  FIGS. 1-5 , provided is an optical amplifier (e.g.,  100 ,  FIG. 1 ;  400 ,  FIG. 4 ) suitable for amplifying space-division multiplexed signals, the optical amplifier comprising: an input port (e.g.,  102 ,  FIG. 1 ) configured to end-connect to a first optical fiber transmission line; an output port (e.g.,  162 ,  FIG. 1 ) configured to end-connect to a second optical fiber transmission line; a first doped multi-core optical fiber (e.g.,  140 ,  FIG. 1 ) having a first plurality of optical waveguide cores (e.g.,  202 ,  FIG. 2 ), each configured to amplify, using a first optical pump beam, a respective portion (e.g.,  112 ,  FIG. 1 ) of optical power received through the input port to generate a respective amplified portion of the optical power, wherein the first multi-core optical fiber is configured to direct the amplified portions of the optical power generated by different ones of the first plurality of optical waveguide cores toward the output port; and a first optical coupler (e.g.,  110 ,  FIG. 1 ;  300 ,  FIG. 3 ) coupled between the input port and the first doped multi-core optical fiber. The first optical coupler comprises a second plurality of optical waveguide cores (e.g.,  302 ,  FIG. 3 ), each configured to guide the respective portion (e.g.,  112 ,  FIG. 1 ) of the optical power received through the input port toward the respective one of the first plurality of optical waveguide cores. A spatial arrangement of the second plurality of optical waveguide cores at an input end (e.g.,  310 ,  FIG. 3B ) of the first optical coupler is different from a spatial arrangement of the second plurality of optical waveguide cores at an output end (e.g.,  320 ,  FIG. 3C ) of the first optical coupler. 
     In some embodiments of the above optical amplifier, the first optical fiber transmission line is a multimode fiber, a multi-core optical fiber, or a fiber-optic cable; and the second optical transmission line is a multimode fiber, a multi-core optical fiber, or a fiber-optic cable. 
     In some embodiments of any of the above optical amplifiers, the first transmission line and the second transmission line are transmission lines of different respective types. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises at least one of: an end portion of the first optical fiber transmission line connected to the input port; and an end portion of the second optical fiber transmission line connected to the output port. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises a first laser (e.g.,  130 ,  FIG. 1 ) configured to generate the first optical pump beam. 
     In some embodiments of any of the above optical amplifiers, the spatial arrangement of the second plurality of optical waveguide cores at the input end of the first optical coupler is different from the spatial arrangement of the second plurality of optical waveguide cores at the output end of the first optical coupler in at least one of: a separation distance between at least two of the optical waveguide cores; a diameter of at least one of the optical waveguide cores; respective geometric patterns in which the optical waveguide cores are arranged; and respective total numbers of the optical waveguide cores. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises a second optical coupler (e.g.,  160 ,  FIG. 1 ;  300 ,  FIG. 3 ) coupled between the first doped multi-core optical fiber and the output port. The second optical coupler comprises a third plurality of optical waveguide cores (e.g.,  302 ,  FIG. 3 ), each configured to guide the respective one of the amplified portions (e.g.,  152 ,  FIG. 1 ) of the optical power generated by different ones of the first plurality of optical waveguide cores toward the output port. A spatial arrangement of the third plurality of optical waveguide cores at an input end (e.g.,  320 ,  FIG. 3C ) of the second optical coupler is different from a spatial arrangement of the third plurality of optical waveguide cores at an output end (e.g.,  310 ,  FIG. 3B ) of the second optical coupler. 
     In some embodiments of any of the above optical amplifiers, the spatial arrangement of the third plurality of optical waveguide cores at the input end of the second optical coupler is different from the spatial arrangement of the third plurality of optical waveguide cores at the output end of the second optical coupler in at least one of: a separation distance between at least two of the optical waveguide cores; a diameter of at least one of the optical waveguide cores; respective geometric patterns in which the optical waveguide cores are arranged; and respective total numbers of the optical waveguide cores. 
     In some embodiments of any of the above optical amplifiers, the first doped multi-core optical fiber comprises (i) a first optical cladding (e.g.,  204 ,  FIG. 2 ) that laterally surrounds the first plurality of optical waveguide cores and (ii) a second optical cladding (e.g.,  206 ,  FIG. 2 ) that laterally surrounds the first optical cladding. The optical amplifier is configured to couple the first optical pump beam into the first optical cladding. The second optical cladding is configured to guide the first optical beam along the first optical cladding to cause optical energy of the first optical beam to be transferred into the first plurality of optical waveguide cores. 
     In some embodiments of any of the above optical amplifiers, the first optical coupler further comprises a monolithic block (e.g.,  308 ,  FIG. 3A ) of an optical cladding material that laterally surrounds the second plurality of optical waveguide cores. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises a second doped multi-core optical fiber (e.g.,  140   2 ,  FIG. 4 ) having a third plurality of optical waveguide cores (e.g.,  202 ,  FIG. 2 ), each configured to further amplify, using a second optical pump beam, the respective amplified portion of the optical power received from the respective one of the first plurality of optical waveguide cores to generate a respective further-amplified portion of the optical power, wherein the second multi-core optical fiber is configured to direct said further-amplified portions of the optical power generated by different ones of the third plurality of optical waveguide cores toward the output port. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises a second optical coupler (e.g.,  160 ,  FIG. 4 ;  300 ,  FIG. 3 ) coupled between the second doped multi-core optical fiber- and the output port. The second optical coupler comprises a fourth plurality of optical waveguide cores (e.g.,  302 ,  FIG. 3 ), each configured to guide the respective one of the further-amplified portions (e.g.,  152 ,  FIG. 4 ) of the optical power generated by different ones of the third plurality of optical waveguide cores toward the output port. A spatial arrangement of the fourth plurality of optical waveguide cores at an input end (e.g.,  320 ,  FIG. 3C ) of the second optical coupler is different from a spatial arrangement of the fourth plurality of optical waveguide cores at an output end (e.g.,  310 ,  FIG. 3B ) of the second optical coupler. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises a second laser (e.g.,  130   2 ,  FIG. 4 ) configured to generate the second optical pump beam. 
     In some embodiments of any of the above optical amplifiers, the second doped multi-core optical fiber comprises (i) a first optical cladding (e.g.,  204 ,  FIG. 2 ) that laterally surrounds the third plurality of optical waveguide cores and (ii) a second optical cladding (e.g.,  206 ,  FIG. 2 ) that laterally surrounds the first optical cladding. The optical amplifier is configured to couple the second optical pump beam into the first optical cladding. The second optical cladding is configured to guide the second optical beam along the first optical cladding to cause optical energy of the second optical beam to be transferred into the third plurality of optical waveguide cores. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises: a first optical monitor (e.g.,  470   1 ,  FIG. 4 ) configured to individually measure intensities of the respective portions of the optical power received through the input optical port; a second optical monitor (e.g.,  470   2 ,  FIG. 4 ) configured to individually measure intensities of said further-amplified portions of the optical power generated by the third plurality of optical waveguide cores; a gain-equalizing optical filter (e.g.,  444 ,  FIG. 4 ) coupled between the first doped multi-core optical fiber and the second doped multi-core optical fiber; and a controller (e.g.,  480 ,  FIG. 4 ) configured to receive measurement results (e.g.,  472   1  and  472   2 ,  FIG. 4 ) from the first optical monitor and the second optical monitor and, in response to the received measurement results, to cause the gain-equalizing optical filter to individually change attenuations applied to the amplified portions of the optical power generated by the first plurality of optical waveguide cores. 
     In some embodiments of any of the above optical amplifiers, the gain-equalizing optical filter comprises: a plurality of variable optical attenuators (e.g.,  510 ,  FIG. 5 ), each configured to attenuate the respective one of the amplified portions of the optical power generated by the first plurality of optical waveguide cores; and a balancing optical filter (e.g.,  520 ,  FIG. 5 ) configured to change spectral composition of the amplified portions of the optical power generated by the first plurality of optical waveguide cores. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises: a first laser (e.g.,  130   1 ,  FIG. 4 ) configured to generate the first optical pump beam; and a second laser (e.g.,  130   2 ,  FIG. 4 ) configured to generate the second optical pump beam, wherein the controller is further configured to cause (i) the first laser to change at least one of intensity and spectral composition of the first optical pump beam and (ii) the second laser to change at least one of intensity and spectral composition of the second optical pump beam, in response to the received measurement results. 
     In some embodiments of any of the above optical amplifiers, the controller is further configured to cause the first optical pump beam and the second optical pump beam to have different spectral compositions, in response to the received measurement results. 
     In some embodiments of any of the above optical amplifiers, the optical amplifier further comprises: a first set of imaging optics configured to image a proximate end of the first optical transmission line onto the input end of the first optical coupler; and a second set of imaging optics configured to image the output end of the first optical coupler onto an input end of the first doped multi-core optical fiber. 
     In some embodiments of any of the above optical amplifiers, the spatial arrangement of the second plurality of optical waveguide cores at the output end of the first optical coupler is nominally identical to a spatial arrangement of the first plurality of optical waveguide cores at an input end of the first doped multi-core optical fiber (e.g., as shown in  FIGS. 2 and 3C ). 
     According to another embodiment disclosed above in reference to  FIGS. 1-5 , provided is an optical amplifier (e.g.,  100 ,  FIG. 1 ;  400 ,  FIG. 4 ) suitable for amplifying space-division multiplexed optical signals, the optical amplifier comprising: an input optical port (e.g.,  102 ,  FIG. 1 ) configured to end-connect to a first optical fiber transmission line; an output optical port (e.g.,  162 ,  FIG. 1 ) configured to end-connect to a second optical fiber transmission line; a first doped multi-core optical fiber (e.g.,  140 ,  FIG. 1  or  4 ) having a first plurality of optical waveguide cores (e.g.,  202 ,  FIG. 2 ), each configured to amplify, using a first optical pump beam, a respective portion (e.g.,  112 ,  FIG. 1 ) of optical power received through the input port to generate a respective amplified portion of the optical power, wherein the first multi-core optical fiber is configured to direct the amplified portions of the optical power generated by different ones of the first plurality of optical waveguide cores toward the output port; and a first optical coupler (e.g.,  160 ,  FIG. 1 ;  300 ,  FIG. 3 ) coupled between the first doped multi-core optical fiber and the output optical port. The first optical coupler comprises a second plurality of optical waveguide cores (e.g.,  302 ,  FIG. 3 ), each configured to guide the respective one of the amplified portions (e.g.,  152 ,  FIG. 1  or  4 ) of the optical power generated by different ones of the first plurality of optical waveguide cores toward the output port. A spatial arrangement of the second plurality of optical waveguide cores at an input end (e.g.,  320 ,  FIG. 3C ) of the first optical coupler is different from a spatial arrangement of the second plurality of optical waveguide cores at an output end (e.g.,  310 ,  FIG. 3B ) of the first optical coupler. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. 
     Contra-directional pumping is not limited to Raman optical amplifiers and can be used with other amplifier types when deemed beneficial. 
     Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the inventions pertain are deemed to lie within the principle and scope of the invention as expressed in the following claims. 
     Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range. 
     It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of the inventions may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims. 
     The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures. 
     Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence. 
     The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
     Also for purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements. 
     The various present inventions may be embodied in other specific apparatus and/or methods. The described embodiments are to be considered in all respects as only illustrative and not restrictive. In particular, the scope of the inventions is indicated by the appended claims rather than by the description and figures herein. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     The functions of the various elements shown in the figures, including any functional blocks labeled as “processors” or “controllers,” may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. Moreover, explicit use of the term “computer,” “processor,” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. 
     The description and drawings merely illustrate the principles of the inventions. It will thus be appreciated that those of ordinary skill in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the inventions and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the inventions and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the inventions, as well as specific examples thereof, are intended to encompass equivalents thereof.