Patent Publication Number: US-2018054036-A1

Title: Efficient pumping of an array of optical amplifiers

Description:
BACKGROUND 
     Field 
     The present disclosure relates to optical amplifiers. 
     Description of the Related Art 
     This section introduces aspects that may help facilitate a better understanding of the disclosure. 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 an optical pump beam are applied to the doped fiber. The optical pump beam excites the doping ions, and amplification of the optical 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 an optical signal to be amplified and an optical pump beam are applied to a fiber made of a suitable 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. 
     SUMMARY OF SOME SPECIFIC EMBODIMENTS 
     Disclosed herein are various embodiments of an array of optical amplifiers that recycles the unused pump power of some or all constituent amplifiers thereof, thereby beneficially improving pump-power utilization therein compared to that of conventional optical amplifiers. In an example embodiment, different amplifiers of the array can be configured to receive approximately equal pump power and be used to independently amplify different respective optical signals. In various embodiments, the unused pump power can be recycled using one or more optical couplers and/or optical paths that appropriately interconnect different amplifiers of the array. Some embodiments have one or more optical loops configured to operate as a ring laser that regenerates pump light in response to the unused pump power being coupled thereto. Some embodiments provide a spectral gain profile suitable for amplifying WDM signals in at least some of the constituent amplifiers of the array. 
     According to one embodiment, provided is an apparatus comprising: a plurality of optical amplifiers, each configured to: amplify a respective optical signal in response to receiving pump light at a respective input pump port thereof; and output a respective unused portion of the pump light through a respective output pump port thereof; and a set of one or more optical couplers that connect the plurality of optical amplifiers in a series. Each optical coupler of the set is connected between a respective preceding optical amplifier of the series and a respective next optical amplifier of the series and is configured to: split the respective unused portion of the pump light that exits the respective output pump port of the preceding optical amplifier into a respective first light beam and a respective second light beam; apply the respective first light beam to the respective input pump port of the respective next optical amplifier; and direct the respective second light beam to bypass the respective next optical amplifier. 
     According to another embodiment, provided is an apparatus comprising: an optical coupler having first and second input ports and first and second output ports; a first optical amplifier having an input pump port and an output pump port, the input pump port of the first optical amplifier connected to receive pump light from the first output port of the optical coupler, and the output pump port of the first optical amplifier connected to apply an unused portion of the pump light to the first input port of the optical coupler; and a second optical amplifier having an input pump port connected to receive pump light from the second output port of the optical coupler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, features, and benefits of various disclosed embodiments 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 that can be used in various embodiments; 
         FIG. 2  shows a block diagram of two arrayed optical amplifiers according to an embodiment; 
         FIG. 3  shows a block diagram of two arrayed optical amplifiers according to another embodiment; 
         FIG. 4  shows a block diagram of two arrayed optical amplifiers according to yet another embodiment; 
         FIGS. 5A-5B  show block diagrams of arrayed optical amplifiers according to an embodiment; and 
         FIGS. 6-9  illustrate several alternative pumping schemes for the arrayed optical amplifiers shown in  FIG. 5  according to respective alternative embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of an optical amplifier  100  that can be used in various embodiments. Optical amplifier  100  is a four-port device that includes an optical gain medium  120  operatively connected between optical couplers  110   1  and  110   2  as indicated in  FIG. 1 . In an example embodiment, optical gain medium  120  comprises a length of optical fiber made of an appropriate material that causes optical-signal amplification therein in response to receiving optical pump power. Depending on the particular embodiment, optical-signal amplification in optical gain medium  120  can occur via stimulated emission or stimulated Raman scattering (SRS). 
     Optical coupler  110   1  is a 2×1 coupler that has an optical pump port  106   1  and an optical signal port  108   1 . Optical coupler  110   2  is a 1×2 coupler that similarly has an optical pump port  106   2  and an optical signal port  108   2 . In some embodiments, optical coupler  110   2  can be a nominal copy of optical coupler  110   1 . 
     In an example embodiment, an optical coupler  110  can be implemented using a dichroic mirror  112  that is substantially transparent to the pump light and is highly reflective for the signal light. In operation, dichroic mirror  112  of optical coupler  110   1  passes through the pump light applied to optical pump port  106   1 , thereby coupling the pump light into optical gain medium  120 . Dichroic mirror  112  of optical coupler  110   2  similarly passes through the residual pump light received from optical gain medium  120 , thereby coupling the residual pump light out of optical amplifier  100  through optical pump port  106   2 . The pump light is partially depleted in optical gain medium  120 , e.g., due to the transfer of optical power therefrom to the optical signal that is being amplified in optical amplifier  100 . This depletion can be quantified using a coefficient α (&lt;1) that provides a measure of the unused portion of the pump power that passes through amplifier  100 , from optical pump port  106   1  to optical pump port  106   2 . For example, if the pump power applied to optical pump port  106   1  is P, then the unused pump power that exits optical amplifier  100  through optical pump port  106   2  is αP, as indicated in  FIG. 1 . 
     In different embodiments, the optical signal to be amplified in optical amplifier  100  may be coupled into optical gain medium  120  in the same direction as the pump light for co-directional pumping or in the opposite direction for contra-directional pumping. For example, for co-directional pumping, the optical signal to be amplified can be applied to optical signal port  108   1 , and the corresponding amplified optical signal exits optical amplifier  100  through optical signal port  108   2 . For contra-directional pumping, the optical signal to be amplified can be applied to optical signal port  108   2 , and the corresponding amplified optical signal exits optical amplifier  100  through optical signal port  108   1 . In both cases, dichroic mirrors  112  of optical couplers  110   1  and  110   2  operate to appropriately direct the optical signals between optical signal ports  108   1  and  108   2 , through optical gain medium  120 . 
       FIG. 2  shows a block diagram of an optical amplifier  200  according to an embodiment. For illustration purposes and without undue limitation, amplifier  200  is described below in reference to an embodiment in which amplifier  200  includes two arrayed amplifiers  100 , which are labeled in  FIG. 2  as  100   A  and  100   B , respectively. Optical amplifiers  100   A  and  100   B  are interconnected in amplifier  200  to recycle at least some of the unused pump energy, but otherwise can be configured to independently amplify two different respective optical signals. The four ports of optical amplifier  100   A  are labeled in  FIG. 2  using the same labels as in  FIG. 1 , but with an additional subscript letter “A” appended thereto. The four ports of optical amplifier  100   B  are similarly labeled in  FIG. 2  using an additional subscript letter “B.” From the provided description, a person of ordinary skill in the art will understand how to make and use alternative embodiments of amplifier  200  by similarly arraying other suitable optical amplifiers instead of optical amplifiers  100   A  and  100   B . 
     Amplifier  200  can also include an optical pump source  210 . In various embodiments, optical pump source  210  may include one or more of: (i) one or more lasers; (ii) a frequency-comb source; and (iii) a continuous broadband light source. 
     Optical amplifiers  100   A  and  100   B  are parts of an amplifier array  220  in which these optical amplifiers are connected to optical pump source  210  and to one another using a 2×2 optical coupler  224 . More specifically, the two input ports (labeled IN 1 , IN 2 ) and the two output ports (labeled OUT 1 , OUT 2 ) of the 2×2 optical coupler  224  are connected as follows. A feedback path (e.g., optical fiber or waveguide)  230  connects optical pump port  106   2A  of optical amplifier  100   A  to input port IN 1 . Input port IN 2  is connected to receive the pump light generated by optical pump source  210 . Output port OUT 1  is connected to feed the pump light to optical pump port  106   1A  of optical amplifier  100   A . Output port OUT 2  is connected to feed the pump light to optical pump port  106   1B  of optical amplifier  100   B . 
     In some embodiments, optical coupler  224  can be tunable to change the ratio of optical-power transfer from an input port to the output ports OUT 1  and OUT 2  in response to an appropriate control signal  212  received from an external electronic controller. More specifically, when optical power P is applied to input port IN 1 , the portions of the optical power transferred to output ports OUT 1  and OUT 2  may be P 1  and P 2 , respectively, with the ratio P 1 /P 2  being changeable, for example, in the range between approximately 0.1 and 10. Due to the insertion loss of optical coupler  224 , the sum (P 1 +P 2 ) may be smaller than P. A person of ordinary skill in the art will understand, that such tunable optical coupler  224  may also exhibit similar power-transfer characteristics for power transfer from input port IN 2  to output ports OUT 1  and OUT 2 . When P 1 /P 2 =1, both optical amplifiers  100   A  and  100   B  of amplifier array  220  receive approximately equal pump power at optical pump ports  106   1A  and  106   1B , respectively. 
     Feedback path  230  helps to improve utilization of the pump power in amplifier  200  by coupling back into amplifier array  220  the unused portion of the pump power that exits optical amplifier  100   A  through optical pump port  106   2A . As a result, amplifier  200  may advantageously exhibit better pump-power utilization characteristics than two separately pumped amplifiers  100 . For example, to achieve a certain optical gain for the optical signals applied to the corresponding signal ports  108 , each of separately pumped optical amplifiers  100  ( FIG. 1 ) may need the pump power P to be applied to its optical pump port  106   1 , for the total pump power  2 P for the two amplifiers. For comparison, to achieve the same optical gain for the optical signals applied to the corresponding signal ports  108  of optical amplifiers  100   A  and  100   B  in amplifier  200  ( FIG. 2 ), optical pump source  210  only needs to apply to input port IN 2  the pump power that is approximately (2−α) P. Therefore, the ratio (η) of the pump-power utilization factor corresponding to two separately pumped optical amplifiers  100  and the pump-power utilization factor corresponding to amplifier  200  can be estimated as follows: 
       η=2/(2−α)&gt;1  (1)
 
     In an example embodiment in which α=0.5, Eq. (1) gives an estimate of the ratio η as being approximately 1.33. A person of ordinary skill in the art will appreciate that this value of η represents a 33% improvement in the pump-power utilization for amplifier  200  compared to that of two separately pumped optical amplifiers  100 . 
       FIG. 3  shows a block diagram of an optical amplifier  300  according to another embodiment. Similar to amplifier  200  ( FIG. 2 ), amplifier  300  includes amplifier array  220 . For better clarity of depiction, the four optical signal ports  108  of amplifier array  220  and control signal  212  are not explicitly shown in  FIG. 3  (see  FIG. 2 ). The presence of feedback path  230  in amplifier  300  is optional, which is indicated in  FIG. 3  using the dashed line that depicts feedback path  230  therein. Instead of or in addition to feedback path  230 , amplifier  300  has a feedback path (e.g., optical fiber or waveguide)  330  that connects optical pump port  106   2B  of amplifier array  220  and a gain element  310 , as indicated in  FIG. 3 . Gain element  310  is further connected to input port IN 2  of amplifier array  220 . 
     Gain element  310  is different from optical gain medium  120  ( FIG. 1 ) in that gain element  310  operates to provide optical gain for pump light, whereas optical gain medium  120  operates to provide optical gain for signal light. In an example embodiment, gain element  310  provides sufficient optical gain for the pump light to: (i) offset optical losses, such as the above-mentioned pump-light depletion in optical gain media  120  of amplifier array  220 , and (ii) cause amplifier  300  to also function as a ring laser that regenerates the used-up pump light. In various embodiments, gain element  310  can be implemented using one or more of the following: (i) an electrically pumped solid-state (e.g., semiconductor) optical amplifier; (ii) an optically pumped optical amplifier; and (iii) an optically pumped doped-fiber amplifier. A corresponding (e.g., electrical or optical) pump source for gain element  310  is represented in  FIG. 3  by an arrow  308 . 
     In an example embodiment, the ring laser of amplifier  300  comprises an optical loop  302  that includes gain element  310 , a portion of the 2×2 optical coupler  224  of amplifier array  220  (see  FIG. 2 ), optical amplifier  100   B  of amplifier array  220  (see  FIG. 2 ), and feedback path  330 . Feedback path  330  may include one or both of an optical isolator  332  and an optical filter  334 . Optical isolator  332  operates to suppress counterclockwise light circulation through optical loop  302 . Optical filter  334  operates to spectrally limit the optical gain spectrum of the ring laser to a desired spectral band. In some embodiments, optical filter  334  can be an integral part of gain element  310 . 
     In embodiments in which feedback path  230  is present in amplifier  300 , the ring laser of amplifier  300  further comprises an optical loop  304  that includes another portion of the 2×2 optical coupler  224  of amplifier array  220 , optical amplifier  100   A  of amplifier array  220 , and feedback path  230  (also see  FIG. 2 ). Similar to feedback path  330 , feedback path  230  in amplifier  300  may include one or both of an additional optical isolator and an additional optical filter (not explicitly shown in  FIG. 3 ) that are functionally similar to optical isolator  332  and optical filter  334 , respectively. Optical loops  302  and  304  are optically coupled to one another through the 2×2 optical coupler  224  of amplifier array  220  (also see  FIG. 2 ). 
     Feedback paths  230  and  330  operate to improve utilization of the pump power in amplifier  300  by coupling back into the amplifier the unused portions of the pump power that exit amplifier array  220  through optical pump ports  106   2A  and  106   2B , respectively. 
       FIG. 4  shows a block diagram of an optical amplifier  400  according to yet another embodiment. Amplifier  400  is a modification of amplifier  300  ( FIG. 3 ) in which optical loop  304  is replaced by an optical loop  404 . In an example embodiment, optical loop  404  comprises a gain element  410 , a portion of the 2×2 optical coupler  224  of amplifier array  220  (see  FIG. 2 ), optical amplifier  100   A  of amplifier array  220  (see  FIG. 2 ), and a feedback path  430 . Feedback path  430  connects optical pump port  106   2A  of amplifier array  220  and gain element  410  and may include one or both of an optical isolator  432  and an optical filter  434 . 
     In some embodiments of amplifier  400 , gain element  410 , optical isolator  432 , and optical filter  434  can be nominal copies of gain element  310 , optical isolator  332 , and optical filter  334 , respectively. In such embodiments, optical loops  302  and  404  form a ring laser that generates pump light in the spectral band corresponding to the (common) passband of optical filters  334  and  434 . 
     In some other embodiments of amplifier  400 , optical filter  434  can have a spectral passband that is different from (e.g., does not overlap with or is spectrally shifted with respect to) the spectral passband of optical filter  334 . Gain element  410  may also be different from gain element  310 , e.g., by being capable of generating light having wavelengths within the spectral passband of optical filter  434  in response to being pumped by a corresponding electrical or optical pump source  408 . In such embodiments of amplifier  400 , optical loops  302  and  404  of the corresponding ring laser operate to generate pump light of different respective wavelengths. Such embodiments may be useful for amplification of optical wavelength-division-multiplexed (WDM) signals in amplifier array  220 , e.g., because more-efficient amplification of different WDM components in optical amplifier  100  ( FIG. 1 ) may occur in response to pump light of different respective wavelengths. 
       FIGS. 5A-5B  show block diagrams of an optical amplifier  500  having (N+2) arrayed constituent amplifiers according to an embodiment, where N is a positive integer. More specifically,  FIG. 5A  shows an overall block diagram of amplifier  500 .  FIG. 5B  shows a block diagram of a constituent amplifier  510   i , where the index “i” can be 1, . . . , N. Two additional constituent amplifiers are parts of the amplifier array  220  that is connected to amplifiers  510   i  as indicated in  FIG. 5A  (also see  FIG. 2 ). The latter two amplifiers are labeled in  FIG. 5A  as the (N+1)-th OA and (N+2)-th OA, respectively. The amplifier array that includes the N arrayed amplifiers  510   i  and the amplifier array  220  is labeled in  FIG. 5A  as  501 . In some alternative embodiments, the amplifier array  220  can be replaced by one or two additional (e.g., serially connected) amplifiers  510 . In some other alternative embodiments, the amplifier array  220  is optional and can be removed. 
     For illustration purposes and without undue limitation,  FIG. 5A  shows an embodiment corresponding to N≧3. A person of ordinary skill in the art will understand that embodiments corresponding to N=1 and N=2 are also possible. In general, the amplifier architecture illustrated by  FIGS. 5A-5B  can be used to implement an optical amplifier having three or more arrayed amplifiers, each of which can be used to independently amplify a respective different optical signal. 
     Referring to  FIG. 5B , amplifier  510   i  comprises a 2×2 optical coupler  224   i , an optical amplifier  100   i , and an optional variable optical attenuator (VOA)  530   i . The 2×2 optical coupler  224   i  is a nominal copy of the 2×2 optical coupler  224  already described above in reference to  FIG. 2 . The optical amplifier  100   i  is a nominal copy of optical amplifier  100  already described above in reference to  FIG. 1 . The four ports of optical amplifier  100   i  are labeled in  FIG. 5B  using the same labels as in  FIG. 1 , but with an additional subscript letter “i” appended thereto. Variable optical attenuator  530   i  is a conventional variable optical attenuator that can change the attenuation imposed on the pump light passing therethrough in response to an appropriate control signal  528   i  received from an external electronic controller. In various embodiments, control signals  212   i  and  528   i  can be generated by the same electronic controller or by different respective electronic controllers. The control signal can also be applied mechanically and/or manually, e.g., during a manual factory calibration process. Fixed optical couplers and/or attenuators can alternatively be used, e.g., if no dynamic control of the couplers/attenuators is desired for a particular embodiment. 
     In some embodiments, variable optical attenuator  530   i  can be replaced by an optical gain element that is similar to gain element  310  ( FIG. 3 ) or  410  ( FIG. 4 ). 
     Amplifier  510   i  has four optical pump ports that are labeled  502   i ,  504   i ,  506   i , and  106   2i , respectively. Ports  502   i  and  504   i  are input ports. Ports  506   i  and  106   2i  are output ports. The four ports of the 2×2 optical coupler  224   i  are connected as follows. Input port IN 1  of the 2×2 optical coupler  224   i  is connected to the optical pump port  502   i . Input port IN 2  of the 2×2 optical coupler  224   i  is connected to the optical pump port  504   i . Output port OUT 1  of the 2×2 optical coupler  224   i  is connected to optical pump port  106   1i  of optical amplifier  100   i . Output port OUT 2  is connected to the optical pump port  506   i  by way of the optional variable optical attenuator or gain element  530   i . 
     In operation, the 2×2 optical coupler  224   i  causes the pump power received by amplifier  510   i  at input ports  502   i  and  504   i  to be divided into two portions. The first portion is applied to optically pump amplifier  100   i  through optical pump port  106 E thereof. The second portion bypasses amplifier  100   i  and is directed to the optical pump port  506   i  by way of the optional variable optical attenuator  530   i . The first portion is partially depleted in optical amplifier  100   i  e.g., due to the transfer of optical power therefrom to the optical signal that is being amplified in that optical amplifier. The corresponding residual pump power exits optical amplifier  100   i  through optical pump port  106   2i . 
     Referring back to  FIG. 5A , amplifier array  501  has four optical pump ports that are labeled  502   1 ,  504   1 ,  106   2A , and  106   2B , respectively. Ports  502   1  and  504   1  are input ports. Ports  106   2A  and  106   2B , are output ports. In the embodiment shown in  FIG. 5A , input port  504   1  is connected to receive the pump power generated by an optical pump source  210 , example embodiments of which have been described above in reference to  FIG. 2 . In various alternative embodiments, the optical pump ports  502   1 ,  504   1 ,  106   2A , and  106   2B  of amplifier array  501  can be connected to various external pump sources and/or each other, e.g., as described in more detail below in reference to  FIGS. 6-9 . 
     Output ports  506   1  and  106   21  of amplifier  510   1  are connected to input ports  504   2  and  502   2 , respectively, of amplifier  510   2 . Output ports  506   2  and  106   22  of amplifier  510   2  are connected to the input ports of the next amplifier  510 . Input ports  502   N  and  504   N  of amplifier  510   N  are connected to the output ports of the preceding amplifier  510 . Output ports  106   2N  and  506   N  of amplifier  510   N  are connected to input ports IN 1  and IN 2 , respectively, of the amplifier array  220  used in amplifier  500 . For better clarity of depiction, the optical signal ports  108  and control signals  212  and  528  of the various constituent amplifiers in amplifier array  501  are not explicitly shown in  FIG. 5A  (see  FIGS. 2 and 5B ). 
     For each of the (N+2) optical amplifiers  100  used in optical amplifier  500  to receive approximately equal input pump power, an embodiment having the following example features can be used. None of variable optical attenuators  530   i  is present in amplifiers  510   1 - 510   N , or the attenuation is set to ˜0 dB. The (N+2) optical couplers  224  used in amplifier array  501  are configured to have the coupling ratios in accordance with the following numerical pattern:
         (1) ½, for optical coupler  224  used in amplifier array  220 ;   (2) ⅔, for optical coupler  224   N  used in amplifier  510   N ;   (3) (N+2−j)/(N+3−j), for optical coupler  224   j  used in amplifier  510   j , where j=3, 4, . . . , N−1;   (4) N/(N+1), for optical coupler  224   2  used in amplifier  510   2 ; and   (5) (N+1)/(N+2), for optical coupler  224   1  used in amplifier  510   1 .       

     If it is desired for at least some of the (N+2) optical amplifiers  100  used in optical amplifier  500  to receive different respective pump powers, then the optical couplers  224  and variable optical attenuators or gain elements  530  can be reconfigured accordingly, e.g., using the corresponding control signals  212  and  528 , respectively. In some embodiments, optical couplers  224  and variable optical attenuators or gain elements  530  of amplifier array  501  can be used to dynamically adjust the optical gain of the various constituent amplifiers of optical amplifier  500 , e.g., as deemed appropriate or necessary for the amplification of the respective optical signals applied thereto. A person of ordinary skill in the art will understand that optical amplifier  500  is advantageously capable of providing significant pump-power savings with respect to (N+2) separately pumped optical amplifiers  100 . 
       FIG. 6  shows a block diagram of an optical amplifier  600  according to an embodiment. Similar to optical amplifier  500  ( FIG. 5A ), optical amplifier  600  has amplifier array  501  and optical pump source  210 . In addition, optical amplifier  600  has a feedback path  230  that connects optical pump ports  106   2A  and  502   1  of amplifier array  501  as indicated in  FIG. 6 . Feedback path  230  helps to improve utilization of the pump power in amplifier  600  by coupling back into amplifier array  501  the unused portion of the pump power that exits the amplifier array through optical pump port  106   2A . 
       FIG. 7  shows a block diagram of an optical amplifier  700  according to an embodiment. Similar to optical amplifier  500  ( FIG. 5A ), optical amplifier  700  has amplifier array  501  and optical pump source  210 . In addition, optical amplifier  700  has an optical pump source  710  connected to optical pump port  502   1  of amplifier array  501  as indicated in  FIG. 7 . In an example embodiment, optical pump sources  210  and  710  can generate pump light of different respective wavelengths. Such an embodiment may be useful for amplification of optical WDM signals in amplifier array  501 , e.g., because more-efficient amplification of different WDM components in individual optical amplifiers  100  of the amplifier array may occur in response to pump light of different respective wavelengths. 
       FIG. 8  shows a block diagram of an optical amplifier  800  according to an embodiment. Similar to optical amplifier  600  ( FIG. 6 ), optical amplifier  800  includes amplifier array  501 . However, the presence of a feedback path  230  in optical amplifier  800  is optional, which is indicated in  FIG. 8  using the dashed line that depicts feedback path  230  therein. Optical amplifier  800  further differs from optical amplifier  600  ( FIG. 6 ) in that optical pump source  210  is replaced by a feedback path  330  and a gain element  310  that are connected to amplifier array  501  as indicated in  FIG. 8 . 
     In an example embodiment, gain element  310  provides sufficient optical gain for the pump light to cause optical amplifier  800  to also function as a ring laser that regenerates the pump light. The ring laser of amplifier  800  comprises an optical loop  802  that includes gain element  310 , a portion of amplifier array  501 , and feedback path  330 . Feedback path  330  may include one or both of an optical isolator  332  and an optical filter  334 . Optical isolator  332  operates to suppress counterclockwise light circulation through optical loop  802 . Optical filter  334  operates to spectrally limit the optical gain spectrum of the ring laser to a desired spectral band. 
     In embodiments in which feedback path  230  is present in optical amplifier  800 , the ring laser of optical amplifier  800  further comprises an optical loop  804  that includes another portion of amplifier array  501  and feedback path  230 . Optical loops  802  and  804  are optically coupled to one another through the 2×2 optical couplers  224  of amplifier array  501  (also see  FIGS. 2 and 5A-5B ). Feedback paths  230  and  330  operate to improve utilization of the pump power in optical amplifier  800  by coupling back into amplifier array  501  the unused portions of the pump power that exit the amplifier array through optical pump ports  106   2A  and  106   2B , respectively. 
       FIG. 9  shows a block diagram of an optical amplifier  900  according to an embodiment. Optical amplifier  900  is a modification of optical amplifier  800  ( FIG. 8 ) in which optical loop  804  is replaced by an optical loop  904 . In an example embodiment, optical loop  904  comprises a gain element  410 , a portion of amplifier array  501 , and a feedback path  430 . Feedback path  430  connects optical pump port  106   2A  of amplifier array  501  and gain element  410  and may include one or both of an optical isolator  432  and an optical filter  434 . 
     In some embodiments of optical amplifier  900 , gain element  410 , optical isolator  432 , and optical filter  434  can be nominal copies of gain element  310 , optical isolator  332 , and optical filter  334 , respectively. In such embodiments, optical loops  802  and  904  form a ring laser that generates pump light in the spectral band corresponding to the (common) passband of optical filters  334  and  434 . 
     In some other embodiments of optical amplifier  900 , optical filter  434  can have a spectral passband that is different from the spectral passband of optical filter  334 . Gain element  410  may also be different from gain element  310 , e.g., by being capable of generating light having wavelengths within the spectral passband of optical filter  434 . In such embodiments of optical amplifier  900 , optical loops  802  and  904  of the corresponding ring laser operate to generate pump light of different respective wavelengths. Such embodiments may be useful for amplification of optical WDM signals in amplifier array  501 , e.g., because more-efficient amplification of different WDM components in individual optical amplifiers  100  of the amplifier array may occur in response to pump light of different respective wavelengths. 
     According to an example embodiment disclosed above in reference to  FIGS. 1-9 , provided is an apparatus comprising: a plurality of optical amplifiers (e.g.,  100   i ,  100   A ,  FIGS. 1, 2, 5 ), each configured to: amplify a respective optical signal (e.g., received through  108   1  or  108   2 ,  FIG. 1 ) in response to receiving pump light at a respective input pump port thereof (e.g.,  106   1 ,  FIG. 1 ); and output a respective unused portion of the pump light through a respective output pump port thereof (e.g.,  106   2 ,  FIG. 1 ); and a set of one or more optical couplers (e.g.,  224   k , where k=2, . . . , N, and/or  224  in  220 ,  FIG. 5 ) that connect the plurality of optical amplifiers in a series. Each optical coupler of the set is connected between a respective preceding optical amplifier of the series (e.g.,  100   k-1 ,  FIG. 5 ) and a respective next optical amplifier of the series (e.g.,  100   k  or  100   A  in  220 ,  FIG. 5 ) and is configured to: split the respective unused portion of the pump light that exits the respective output pump port of the preceding optical amplifier into a respective first light beam (e.g., directed through OUT 1 ,  FIG. 2 or 5B ) and a respective second light beam (e.g., directed through OUT 2 ,  FIG. 2 or 5B ); apply the respective first light beam to the respective input pump port of the respective next optical amplifier; and direct the respective second light beam to bypass the respective next optical amplifier. 
     As used herein, the term “light beam” should be construed to cover both free-space light beams and guided light beams that propagate through an optical fiber or waveguide. 
     In some embodiments of the above apparatus, the set comprises at least two optical couplers. 
     In some embodiments of any of the above apparatus, the set comprises: a first optical coupler (e.g.,  224   k ,  FIG. 5 ) having a first coupling ratio; and a second optical coupler (e.g.,  224  in  220 ,  FIG. 5 ) having a different second coupling ratio. 
     In some embodiments of any of the above apparatus, the optical couplers of the set have respective coupling ratios that cause the plurality of optical amplifiers to receive approximately (e.g., within ±10%) equal pump power at the respective input pump ports thereof. 
     In some embodiments of any of the above apparatus, the set comprises: a first optical coupler having first and second output ports (e.g., OUT 1 , OUT 2 ,  FIG. 5B ), the first output port configured to output the respective first light beam, the second output port configured to output the respective second light beam; and a second optical coupler having first and second input ports (e.g., IN 1 , IN 2 ,  FIG. 5B , or IN 1 , IN 2  in  220 ,  FIG. 5A ), the first input port connected to the first output port of the first optical coupler by way of one (e.g.,  100   i ,  FIG. 5 ) of the plurality of optical amplifiers, and the second input port connected to the second output port of the first optical coupler. 
     In some embodiments of any of the above apparatus, the apparatus further comprises a variable optical attenuator (e.g.,  530   i ,  FIG. 5B ) connected between the second output port of the first optical coupler and the second input port of the second optical coupler. 
     In some embodiments of any of the above apparatus, the apparatus further comprises an optical gain element (e.g., in place of  530   i ,  FIG. 5B ) connected between the second output port of the first optical coupler and the second input port of the second optical coupler. 
     In some embodiments of any of the above apparatus, the second optical coupler has first and second output ports (e.g., OUT 1 , OUT 2 ,  FIG. 5B , or OUT 1 , OUT 2  in  220 ,  FIG. 5A ). 
     In some embodiments of any of the above apparatus, the first output port of the second optical coupler is connected to the respective input pump port of another one (e.g.,  100   A  in  220 ,  FIG. 5A ) of the plurality of optical amplifiers. 
     In some embodiments of any of the above apparatus, the second output port of the second optical coupler is connected to an input pump port of an additional optical amplifier (e.g.,  100   B  in  220 ,  FIG. 5A ). 
     In some embodiments of any of the above apparatus, at least one optical coupler of the set is tunable to change a coupling ratio thereof in response to a control signal (e.g.,  212   i ,  FIG. 5B ) received from an electronic controller. 
     In some embodiments of any of the above apparatus, the apparatus further comprises a feedback path (e.g.,  230 ,  FIG. 6 ;  430 ,  FIG. 9 ) that connects the respective output pump port of a last optical amplifier (e.g.,  100   A  in  501 ,  FIG. 6 or 9 ) of the series and the respective input pump port of a first optical amplifier (e.g.,  100   1  in  501 ,  FIG. 6 or 9 ) of the series. 
     In some embodiments of any of the above apparatus, the feedback path includes an optical gain element (e.g.,  410 ,  FIG. 9 ) capable of generating pump light. 
     In some embodiments of any of the above apparatus, the apparatus further comprises a ring laser (e.g., having  904 ,  FIG. 9 ) that includes the feedback path. 
     In some embodiments of any of the above apparatus, the apparatus further comprises a first pump source (e.g.,  210 ,  FIG. 6 or 7 ) configured to feed pump light into the respective input pump port of a first optical amplifier (e.g.,  100   1  in  501 ,  FIG. 6 or 9 ) of the series. 
     In some embodiments of any of the above apparatus, the apparatus further comprises an additional optical coupler (e.g.,  224   1 ,  FIG. 5 ) having a first input port (e.g., IN 2 ,  FIG. 5B ), a second input port (e.g., IN 1 ,  FIG. 5B ), and an output port (e.g., OUT 1 ,  FIG. 5B ), wherein: the first input port of the additional optical coupler is connected to the first pump source; and the output port of the additional optical coupler is connected to the respective input pump port of the first optical amplifier of the series. 
     In some embodiments of any of the above apparatus, the apparatus further comprises a second pump source (e.g.,  710 ,  FIG. 7 ) connected to the second input port of the additional optical coupler to feed pump light into the output port of the additional optical coupler. 
     In some embodiments of any of the above apparatus, the second pump source is configured to generate pump light that has a different wavelength than the pump light generated by the first pump source. 
     In some embodiments of any of the above apparatus, the apparatus further comprises a feedback path (e.g.,  230 ,  FIG. 6 ) that connects the respective output pump port of a last optical amplifier (e.g.,  100   A  in  501 ,  FIG. 6 or 9 ) of the series and the second input port of the additional optical coupler. 
     According to another example embodiment disclosed above in reference to  FIGS. 1-9 , provided is an apparatus comprising: an optical coupler (e.g.,  224 ,  FIG. 2 ) having first and second input ports (e.g., IN 1 , IN 2 ,  FIG. 2 ) and first and second output ports (e.g., OUT 1 , OUT 2 ,  FIG. 2 ); a first optical amplifier (e.g.,  100   A ,  FIG. 2 ) having an input pump port (e.g.,  106   1A ,  FIG. 2 ) and an output pump port (e.g.,  106   2A ,  FIG. 2 ), the input pump port of the first optical amplifier connected to receive pump light from the first output port of the optical coupler, and the output pump port of the first optical amplifier connected to apply an unused portion of the pump light to the first input port of the optical coupler; and a second optical amplifier (e.g.,  100   B ,  FIG. 2 ) having an input pump port (e.g.,  106   1B ,  FIG. 2 ) connected to receive pump light from the second output port of the optical coupler. 
     In some embodiments of the above apparatus, the apparatus further comprises a pump source (e.g.,  210 ,  FIG. 2 ) configured to apply generated pump light to the second input port of the optical coupler. 
     In some embodiments of any of the above apparatus, the first optical amplifier is configured to amplify a first optical signal (e.g., received through  108   1A  or  108   2A ,  FIG. 2 ) in response to receiving pump light at the input pump port thereof. 
     In some embodiments of any of the above apparatus, the second optical amplifier is configured to amplify a different second optical signal (e.g., received through  108   1B  or  108   2B ,  FIG. 2 ) in response to receiving pump light at the input pump port thereof. 
     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 if deemed beneficial. 
     Co-directional pumping is not limited to EDFAs and can be used with other amplifier types if deemed beneficial. 
     Various suitable rare-earth doped fiber amplifiers can be used to implement optical amplifiers  100  in various alternative embodiments. 
     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.