Patent Publication Number: US-2023163555-A1

Title: Optical amplifier modules

Description:
FIELD 
     The present disclosure generally relates to optical amplifier modules, including for example erbium-doped fiber amplifiers (EDFAs). More particularly, but not exclusively, the present disclosure relates to controlling power of different optical signals with a common pump laser or source in an optical amplifier module. 
     BACKGROUND 
     Unless otherwise indicated herein, the materials described herein are not prior art to the claims in the present application and are not admitted to be prior art by inclusion in this section. 
     Some datacenters include routers, switches, or other transmission chassis configured to receive multiple communication modules that convert data between the optical and electrical domains, with optical signals generally being used for transmission between switches and routers and electrical signals generally being used internally on the switches and routers. Optical loss between two interconnected routers and switches may be significant enough that optical amplifiers may be deployed before, after or both before and after fiber spans to compensate the optical loss. 
     The subject matter claimed herein is not limited to implementations that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some implementations described herein may be practiced. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential characteristics of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     In one embodiment, an optical amplifier module includes a booster optical amplifier configured to increase optical power of a first optical signal, a preamp optical amplifier configured to increase optical power of a second optical signal, and a pump laser optically coupled to the booster optical amplifier and the preamp optical amplifier. The pump laser is configured to provide a booster power to the booster optical amplifier and a preamp power to the preamp optical amplifier. The preamp power is effective to induce a gain in optical power to provide a target optical power of the second optical signal from the preamp optical amplifier, and the booster power is dependent on the preamp power. 
     In another embodiment, an optical amplifier module includes a booster optical amplifier configured to increase optical power of a first optical signal, a preamp optical amplifier configured to increase optical power of a second optical signal, and a pump laser optically coupled to the booster optical amplifier and the preamp optical amplifier. The pump laser is configured to provide a power output to increase optical power of the first optical signal and the second optical signal. The preamp optical amplifier is configured to receive, from the power output, a first amount of power effective to induce a gain in optical power to provide a targeted optical power of the second optical signal from the preamp optical amplifier. The booster optical amplifier is configured to receive, from the power output, a second amount of power corresponding to a difference between the power output and the first amount of power. 
     In still another embodiment, a method includes determining an optical power gain to impart to a first optical signal by a first preamp optical amplifier to achieve a target optical power value of the first optical signal; providing from a first optical pump laser a first power level to the first preamp optical amplifier effective to impart the determined optical power gain to the first optical signal; and providing from the first optical pump laser to a first booster optical amplifier a second power level dependent on the first power level as provided to the first preamp optical amplifier, the first booster optical amplifier configured to increase optical power of a second optical signal. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which 
         FIG.  1    illustrates an example operating environment in which some embodiments described herein may be implemented; 
         FIG.  2    is a block diagram of an example pluggable bidirectional optical amplifier module that may be implemented in the operating environment of  FIG.  1   ; 
         FIGS.  3 A- 3 C  include two perspective views and an end view of an example pluggable bidirectional optical amplifier module that may be implemented in the operating environment of  FIG.  1   ; 
         FIG.  4    illustrates an example optical amplifier that may be implemented in the pluggable bidirectional optical amplifier module of  FIG.  2 - 3 C ; and 
         FIG.  5    is a block diagram of a portion of an optical network including a pair of optical amplifier modules. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments described herein include a pluggable bidirectional optical amplifier module (hereinafter “pluggable amplifier”) for efficient datacenter interconnection. The pluggable amplifier may be optically bidirectional, e.g., it may provide optical amplification in each of two opposing transmission directions. The pluggable amplifier may have attributes such as a mechanical form factor to plug in directly into coherent transceiver cages/slots on routers, switches, or other transmission chassis used in a datacenter. 
     The pluggable amplifier may include a preamp optical amplifier and a booster optical amplifier, each of which may include an erbium-doped fiber amplifier (EDFA). Each of the preamp and booster optical amplifiers may use an operational wavelength range in an optical spectrum of the EDFA that has a relatively flat gain spectrum. As such, expensive and space inefficient gain flattening filter (GFFs) may be omitted from the pluggable amplifier. 
     In some embodiments, the pluggable amplifier may have a host interface that supports or is adapted from a Common Management Interface Specific (CMIS) Rev 2.0 (or other revisions). The host interface may implement register mapping on a serial interface common to transceiver shelves for digital diagnostics and management purposes. 
     Reference will now be made to the drawings to describe various aspects of example embodiments of the invention. It is to be understood that the drawings are diagrammatic and schematic representations of such example embodiments, and are not limiting of the present invention, nor are they necessarily drawn to scale. 
       FIG.  1    illustrates an example operating environment  100  in which some embodiments described herein may be implemented. The environment  100  includes a datacenter switch fabric made up of one or one or more interconnected switches or routers.  FIG.  1    specifically illustrates two switches  102 ,  104  optically interconnected to each other where the switch  102  has various intra-switch optical connections as well. 
     Each of the switches  102 ,  104  includes multiple cages or slots  106  to receive multiple pluggable communication modules (hereinafter “communication modules”), such as an optoelectronic transceiver or transponder module. In particular, each cage or slot  106  is configured to receive any of the communications modules  108 . Only some of the cages or slots  106  and some of the communications modules  108  are labeled in  FIG.  1    for simplicity. 
     Each of the switches  102 ,  104  specifically includes  32  cages or slots  106  as illustrated, although there may be more or fewer cages or slots  106  in other embodiments. Each cage or slot  106  is configured to receive an OSFP-compliant communication module  108  in the example of  FIG.  1   . An OSFP-compliant communication module is a communication module that is compliant with the OSFP multisource agreement (MSA). In other embodiments, the communication modules  108  may be compliant with other communication module MSAs, such as the QSFP56-DD MSA, the CFP8 MSA, or other MSA. All of the cages or slots  106  may have the same form factor, e.g., may conform to the same MSA. The cages or slots  106  and the communication modules  108  generally conform to the same MSA. 
     In some embodiments, a pluggable bidirectional optical amplifier module (hereinafter “pluggable amplifier”) may have a same form factor as the communication modules  108  and the cages or slots  106 . Accordingly, one or more of the pluggable amplifiers as described herein may be plugged into any of the cages or slots  106  of the switches  102 ,  104 . One or more of the pluggable amplifiers as described herein may also be implemented in a dedicated chassis with amplifier line cards plugged in, or in a dedicated and relatively large box form factor. 
     Such pluggable amplifiers may be configured to optically amplify optical signals transmitted from one switch or router to another switch or router before or after a span of optical fiber that interconnects the two switches/routers to compensate for insertion loss. An optical amplifier placed upstream of the span may be referred to as a booster optical amplifier, while an optical amplifier placed downstream of the span may be referred to as a preamp optical amplifier. 
     In comparison to such pluggable amplifiers, the communication modules  108  are generally configured to convert outbound electrical signals from a host, such as the switches  102 , to outbound optical signals, and to convert inbound optical signals to inbound electrical signals for the host. 
       FIG.  2    is a block diagram of an example pluggable bidirectional optical amplifier module  200  (hereinafter “pluggable amplifier 200”) that may be implemented in the operating environment  100  of  FIG.  1   , arranged in accordance with at least one embodiment described herein. The pluggable amplifier  200  may generally include a preamp optical amplifier  202  (hereinafter “preamp  202 ”), a booster optical amplifier  204  (hereinafter “booster  204 ”), and a housing  206 . The preamp  202  and the booster  204  may be doped-fiber amplifiers that have a flat gain spectrum across an operational wavelength range, although other variations are possible. The operational wavelength range may be about 1542 nanometers to 1554 nanometers, although other variations are possible. In one form, a doped-fiber amplifier of the preamp  202  may have a flat gain spectrum across the operational wavelength range when operated at an inversion of at least 0.7 and a target gain of at least 15 decibels (dB). However, other variations are contemplated. The pluggable amplifier  200  may additionally include a line-side port  208 , a local-side port  210 , a host interface  212 , and a register table  214 . 
     The preamp  202  is configured to amplify optical signals traveling in a first direction through the pluggable amplifier  200 . In particular, inbound line-side optical signals  216 A from a line side  218  may be optically amplified by the preamp  202  and output to a local side  220  as outbound local-side optical signals  222 B. 
     The booster  204  is configured to amplify optical signals traveling in a second direction through the pluggable amplifier  200 . The second direction may be opposite to the first direction. In particular, inbound local-side optical signals  222 A from the local side  220  may be optically amplified by the booster  204  and output to the line side  218  as outbound line-side optical signals  216 B. 
     The pluggable amplifier  200  also includes a pump laser  205  which is optically coupled to the preamp  202  and the booster  204 . The pump laser  205  is structured to provide a booster power to the booster  204  and a preamp power to the preamp  202 . In one form, the preamp power is effective to induce a gain in optical power in the optical signal  216 A which provides a target optical power of the optical signal  222 B from the preamp  202 . In one aspect of this form, the booster power provided to the booster  204  is dependent on the preamp power, and the gain in optical power induced in the optical signal  222 A will ultimately depend on the power provided by the pump laser  205  to the preamp  202 . Similarly, the optical power of the optical signal  216 B will depend on the power provided by the pump laser  205  to the preamp  202 . In one form, the booster power may be the same as the preamp power. 
     In the illustrated form, an optical splitter  207  is positioned between the pump laser  205  and the booster  204  and preamp  202 . The optical splitter  207  may be, for example, a 50/50 splitter. In addition, the pluggable amplifier  200  also includes a single power monitor  209  associated with the booster  204 . The power monitor  209  may be configured to determine the optical power of the optical signal  222 A at an input of the booster  204 , the optical power of the optical signal  216 B at the output of the booster  204 , or both. The determined optical power of the optical signal associated with the booster  204  may be provided to another, remotely spaced pluggable amplifier  200  and the pluggable amplifiers  200  may be configured to exchange network information with one another through a virtual supervisory channel (VSC). The determined optical power of the optical signal associated with the booster  204  may be used to determine the preamp power provided by the pump laser  205  for inducing a gain in the optical power in the optical signal to a achieve a target optical power in the optical signal provided by the preamp  202 . The pluggable amplifier  200  may also include a single power monitor  211  associated with the preamp  202 . The power monitor  211  may be configured to determine the optical power of the optical signal  216 A at an input of the preamp  202 , the optical power of the optical signal  222 B at the output of the preamp  202 , or both. 
     The pluggable amplifier  200  may also include a pump driver operatively coupled to the pump laser  205 . The pump driver may be configured to output an electrical drive signal to drive the pump laser  205 , and a modulator may be operatively coupled to the pump driver and configured to output a modulation signal to the pump driver to modulate the electrical drive signal. In one aspect, the modulation signal appears as an envelope on at least one of the optical signal  222 B provided by the preamp  202  and the optical signal  216 B provided by the booster  204 . The amplifier  200  may also include a demodulator which is operatively coupled to the preamp  202 , the booster  204 , or both, and configured to receive as an input an electrical version of the optical signal and to recover from the electrical version of the optical signal network information superimposed on the optical signal. 
     The inbound and outbound local-side optical signals  222 A,  222 B (collectively “local-side optical signals  222 ”) may be exchanged between the pluggable amplifier  200  and one or more local communication modules, e.g., communication modules that are installed in the same switch, router, or other datacenter transmission chassis as the pluggable amplifier  200 . In comparison, the inbound and outbound line-side optical signals  216 A,  216 B (collectively “line-side optical signals  216 ”) may be exchanged between the pluggable amplifier  200  and one or more remote pluggable amplifiers or communication modules, e.g., pluggable amplifiers or communication modules that are installed in a different switch, router, or other datacenter transmission chassis as the pluggable amplifier  20 . 
     The housing  206  is configured to at least partially enclose the preamp  202  and the booster  204 . The housing  206 , and more generally the pluggable amplifier  200 , may have a mechanical form factor that is compliant with a pluggable communication module MSA such as the OSFP MSA or other suitable MSA. 
     Each of the line-side port  208  and the local-side port  210  may be formed in or by the housing  206  and may be configured to receive a fiber optic connector to optically couple the pluggable amplifier  200  to one or more optical fibers terminated by the fiber optic connector. The line-side port  208  may include a duplex line-side port or two discrete line-side ports. Similarly, the local-side port  210  may include a duplex line-side port or two discrete local-side ports. 
     The line-side port  208  and the local-side port  210  may be configured to receive fiber optic connectors with the same or different form factors. In an example, the line-side port  208  may include a duplex port configured to receive a duplex LC connector while the local-side port  210  may include a duplex port configured to receive a duplex CS connector. 
     Although not illustrated in  FIG.  2   , the pluggable amplifier  200  may include a printed circuit board (PCB) at least partially enclosed within the housing  206  with one or more integrated circuits (ICs) or other components mounted thereon. For example, the preamp  202 , the booster  204 , pump laser  205  (or one or more components thereof such as a modulator, demodulator, or other components), a controller, the register table  214 , the host interface  212 , or other components may be mounted to or at least partially included on or in the PCB. The PCB may also include an edge connector to communicatively couple the pluggable amplifier  200  to a host device, such as the switch  102 ,  104  of  FIG.  1   . 
     The host interface  212  may provide a communication interface between the pluggable amplifier  200  and the host device for digital diagnostics and management purposes. The PCB edge connector may be included in the host interface  212 . The host interface  212  may include a serial interface common to that used between communication modules (such as the communication modules  108  of  FIG.  1   ) and host devices (such as the switches  102 ,  104  of  FIG.  1   ). For example, the host interface  212  may include a serial interface commonly used to communicate network and control data, as opposed to customer or payload data, between host devices and communication modules, such as inter-integrated circuit (I2C), management data input/output (MDIO), 1-wire, or other suitable serial interface. 
     The host interface  212  may be based on or may implement a Common Management Interface Specification (CMIS) Rev 4.0, other version of CMIS, other management interface, or modification(s) thereof. Accordingly, the host interface  212  may support some or all of the functionality of CMIS Rev 4.0 or other management interface. In some embodiments, the management interface upon which the host interface  212  is based may be modified to include a register table, e.g., the register table  214 , for ease of deployment. The register table  214  may be directly accessible from the host device through the host interface  212 . 
     The host interface  212  may include a paged module interface, which may be familiar to datacenter administrators already familiar with communication module management. The pluggable amplifier  200  may present through the host interface  212  as a two-lane transceiver or other communication module where one lane represents the line side  218  and the other lane represents the local side  220 . The host interface  212  may include a single Module State Machine and a single Data Path State Machine. 
     The host interface  212  may be as compliant as can be realized given the underlying pluggable amplifier  200  is a dual EDFA as opposed to a communication module such as a transceiver. The pluggable amplifier  200  may advertise one “application” with no pertinent control set parameters through the host interface  212 . On boot, the pluggable amplifier  200  may automatically activate this application. 
     The host interface  212  may support one or more pages of CMIS Rev 4.0. For example, the page(s) supported by the host interface  212  may include one or more of Lower Page (Control and Status Essentials), Upper Page 00h (Administrative Information), Upper Page 01h (Advertising), Upper Page 02h (Module and Lane Thresholds), Upper Page 10h (Lane and Data Path Control), Upper Page 11h (Lane Status), and Upper Page 9Fh (CDB Messaging). “Banking” may be supported by the host interface  212 . Upper pages 10h and 11h may implement one bank in the host interface  212 , consistent with two lanes. Upper page 9Fh may implement two banks in the host interface  212 , consistent with two independent Command Data Block engines. 
     The host device may detect the presence of the pluggable amplifier  200  using the same methods as it does communication modules. The host device may retrieve monitor values and sense alarms as it does for communication modules. The host device need not issue commands to start up the pluggable amplifier  200 . Instead, it may simply raise the ResetL digital input and the LPMode digital input. Raising the ResetL digital input may allow the pluggable amplifier  200  to come out of reset. Raising the LPMode digital input may allow the pluggable amplifier  200  to transition to full power. 
     The host interface  212  may include some custom interface aspects, implemented in a compliant way. For example, the CMIS Rev 4.0 specification assumes that all lanes are symmetric and only provides for a single set of alarm thresholds for optical receive (RX) and transmit (TX) powers. Some embodiments of the host interface  212  described herein include a second set of “alternate” thresholds because the two lanes, e.g., EDFAs included in each of the booster  204  and the preamp may not be symmetric. 
       FIGS.  3 A- 3 C  include two perspective views and an end view of an example pluggable bidirectional optical amplifier module  300  (hereinafter “pluggable amplifier 300”) that may be implemented in the operating environment  100  of  FIG.  1   , arranged in accordance with at least one embodiment described herein. The pluggable amplifier  300  may include or correspond to the pluggable amplifier  200  of  FIG.  2   . 
     For example, as illustrated, the pluggable amplifier  300  includes a housing  302  with line-side and local-side ports  304 ,  306  that may include or correspond to the housing  206  with the line-side and local-side ports  208 ,  210  of  FIG.  2   . The pluggable amplifier  300  additionally includes a PCB  308  with edge connector  310  that may include or correspond to the PCB and edge connector discussed with respect to  FIG.  2   . 
     The housing  302  includes a top shell  302 A (omitted from  FIG.  3 B ) and a bottom shell  302 B that cooperate to at least partially enclose one or more components inside the housing  302 , such as the PCB  308 , preamp and booster optical amplifiers, and other components. The line-side port  304  may include a duplex port configured to receive a duplex LC connector while the local-side port  306  may include a duplex port configured to receive a duplex CS connector. 
       FIG.  4    illustrates an example optical amplifier  400  that may be implemented in the pluggable amplifier  200 ,  300  of  FIGS.  2 - 3 C , arranged in accordance with at least one embodiment described herein. The optical amplifier  400  may include or correspond to the preamp  202  or the booster  204  of  FIG.  2    or other preamp or booster optical amplifiers described herein, provided that the pump laser  410  described below is shared between the preamp  202  and the booster  204 . 
     The optical amplifier  400  may include an input  402 , an input optical splitter and photodiode  404 , an input optical isolator  406  (hereinafter “input isolator  406 ”), an optical combiner  408 , a pump laser  410 , a doped-fiber amplifier  412 , an output optical isolator  414  (hereinafter “output isolator  414 ”), an output optical splitter and photodiode  416  (or in some forms an optical combiner), and an output  418 . The optical amplifier  400  may be configured to operate in the C-band, the L-band, or both the C-band and the L-band. 
     An input optical splitter of the input optical splitter and photodiode  404  may receive an optical signal (e.g., an input signal) from the input  402 . The input optical splitter may split and provide the input signal to an input photodiode of the input optical splitter and photodiode  404  and to the optical combiner  408  through the input isolator  406 . The input optical splitter may equally or unequally divide the optical power level of the input signal between optical paths to, respectively, the input photodiode and the optical combiner  408  such that the input photodiode and the optical combiner  408  may receive the input signal at divided optical power levels. 
     The input photodiode of the input optical splitter and photodiode  404  may generate an electrical signal from which supervisory message data included in an envelope of the optical signal may be demodulated. Input power of the optical signal may alternatively or additionally be determined from the electrical signal generated by the input photodiode. The determined input power may be used in a gain control algorithm of the optical amplifier  400  generally in both feedforward and for input level for feedback. Alternatively or additionally, the determined input power may be used as an input in determining and setting gain of the optical amplifier  400  when coupled with information received in the supervisory message data with regard to launch power from an opposite end of a fiber span coupled to the input  402 . When used for feed forward control of the pump laser  410 , the electrical signal generated by the input photodiode may be referred to as an electrical feed forward control signal. An electrical drive signal applied to the pump laser  410  may be at least partially determined based on the feed forward control signal. The drive signal of the pump laser  410  may alternatively or additionally be determined based on an electrical feedback control signal generated by an output photodiode of the output optical splitter and photodiode  416 . The drive signal may determine an optical gain of the optical amplifier  400 . 
     The pump laser  410  may receive the drive signal from a pump driver (not shown in  FIG.  4   ) and may generate a laser signal based on the drive signal. In some embodiments, the pump laser  410  may generate the laser signal as an optical signal representative of the drive signal. In these and other embodiments, the optical power level of the laser signal may be based on a current level of the drive signal. 
     The optical combiner  408  may receive, through the input isolator  406 , the input signal at the divided optical power level from the input optical splitter of the input optical splitter and photodiode  404 . In addition, the optical combiner  408  may receive the laser signal, which may also be referred to as the pump power which activates the gain medium, from the pump laser  410 . The optical combiner  408  may combine the input signal and the laser signal into a combined signal provided to the doped-fiber amplifier  412 . In some embodiments, the optical power level of the combined signal may be equal to a sum of the optical power levels of the input signal and the laser signal received by the first optical combiner  408 . In other embodiments, the optical power level of the combined signal may be greater than the optical power level of the input signal or the laser signal individually but less than the sum of these optical power levels. 
     The doped-fiber amplifier  412  may receive the combined signal from the optical combiner  408 . The doped-fiber amplifier  412  may generate an output signal as an optical signal based on the combined signal. In some embodiments, the doped-fiber amplifier  412  may be configured to apply a gain to the combined signal such that the output signal is generated as an amplified version of the combined signal. In these and other embodiments, the combined signal may drive the doped-fiber amplifier  412 . 
     An output optical splitter of the output optical splitter and photodiode  416  may receive the output signal from the doped-fiber amplifier  412 , e.g., via the output isolator  414 , and split it in two. In particular, the output optical splitter may divide the optical power level of the output signal between the output  418  and the output photodiode of the output optical splitter and photodiode  416 . In these and other embodiments, the output optical splitter may equally or unequally divide the optical power level of the output signal. 
     The output photodiode of the output optical splitter and photodiode  416  may generate an electrical feedback control signal to implement feedback control of the pump laser  410  based upon which the drive signal applied to the pump laser  410  may be at least partially determined. As previously discussed, the drive signal of the pump laser  410  may be determined based on both the feed forward control signal and the feedback control signal. 
     With combined reference to  FIGS.  2  and  4   , each of the preamp  202  and the booster  204  may include the same or similar or other configuration as the optical amplifier of  400  of  FIG.  4   . In some embodiments, the preamp  202  and the booster  204 , whether implemented as two instances of the optical amplifier  400  or in some other configuration, may accommodate variable span loss from 0 to 20 decibels (dB) and may support the data associated with eight channels, e.g., communication modules at each end of the span. As described in more detail below, the eight channels may be limited to a relatively narrow operational wavelength (or channel) range within the C-band or L-band and the operational wavelength range may have a relatively flat gain spectrum. 
     Typical multi-channel optical amplifiers are designed to work for a full band, such as the full C-band or full L-band. To support a full band and a high optical gain, a gain flattening filter (GFF) is typically required to reduce the wavelength (or channel)-dependent gain variation fundamental to the doped-fiber amplifier within each of the preamp and the booster. To keep gain ripple low over a variable gain, a variable optical attenuator (VOA) is typically needed. The GFF, VOA, or other components may be placed before, between or after one or more gain coils of each doped-fiber amplifier. Placing these components before the gain coil may increase a noise figure (NF) and decrease OSNR. Placing these components between the gain coils may require that there be at least 2 gain coils (and all the associated components). Placing these components after the gain coils may require higher pump power to reach the same output power. Inclusion of these components in an amplifier module increases costs of the amplifier module and may exceed available space of a desired package. For example, it may be difficult or impossible to fit all of the foregoing components in a pluggable bidirectional optical amplifier module that has a mechanical form factor that is compliant with the OSFP MSA. 
     Accordingly, in some embodiments, each of the optical amplifiers  400  implemented herein, such as the preamp  202  and the booster  204 , may have an operational wavelength range that is a subset of the C-Band or the L-Band and that has a relatively flat gain spectrum. The flat gain spectrum has reduced tilt or ripple within the operational wavelength range compared to non-flat gain spectra. Ripple of the gain spectrum within the operational wavelength range may be defined as a difference at a given target gain between a maximum wavelength-dependent gain within the operational wavelength range and a minimum wavelength-dependent gain within the operational wavelength range. The flat gain spectrum may have reduced rippled such that no GFF is needed. In comparison, tilt of the gain spectrum within the operational wavelength range may be defined as a difference at a given target gain between a maximum wavelength-dependent gain within the operational wavelength range and a minimum wavelength-dependent gain within the operational wavelength range of a linear fit to the gain spectrum. 
     The flat gain spectrum may be obtained by operating the doped-fiber amplifier  412  at an appropriate inversion. In some embodiments, the appropriate inversion is an inversion that is higher than that usually applied to a doped-fiber amplifier when not trying to impart a flat gain spectrum to the doped-fiber amplifier. The inversion of the doped-fiber amplifier  412  may depend on the optical power level of the combined signal received from the optical combiner  408 . Accordingly, the inversion of the doped-fiber amplifier  412  may be controlled by the drive signal applied to the pump laser  410 . 
     In some embodiments, the flat gain spectrum may have reduced temperature-dependent gain within the operational wavelength range compared to non-flat gain spectra. 
     Because the tilt is reduced within the operational wavelength range compared to non-flat gain spectra, the VOA may be omitted. The absence of the VOA may improve optical performance, e.g., OSNR, of the optical amplifier  400  compared to an optical amplifier with a VOA as it eliminates passive loss from the VOA and associated taps and the intended attenuation that the VOA provides to keep the gain flat. The OSNR of the optical amplifier  400  may thereby be dramatically improved at least at low gain. 
     In some embodiments, the booster  204  may be or operate as a fixed gain amplifier and the preamp  202  may be or operate as a variable gain amplifier. Because of the simplicity of the design of the doped-fiber amplifier with flat gain that may be implemented in each of the booster  204  and the preamp  202  according to some embodiments, there is no actual difference between the two in some embodiments such that either or both of the booster  204  and the preamp  202  may be operated as a variable gain amplifier. 
     In addition, boosters and preamps in different pluggable amplifiers, e.g., at opposite ends of a span, may communicate with each other and exchange network information through a VSC as described elsewhere herein. Accordingly, overall link performance may be optimized. For example, gain at each of the booster and preamp at opposite ends of the span may be set to reduce electrical power consumption, e.g., in low span loss cases. In this example, the booster may operate at a lower gain and thus lower output power. Overall ripple and gain shape of a link made up of the booster and preamp at opposite ends of the span may be independent of how the gain is distributed between them. 
     In some embodiments, the relatively narrow operational wavelength range of the doped-fiber amplifier  412  may coincide with a region of a gain spectrum of the doped-fiber amplifier that is relatively flat to eliminate the need for a GFF. Alternatively or additionally, the operational wavelength range may coincide with minimum effect from temperature-dependent spectral gain change to eliminate the need for a coil heater or other temperature control. Over the operational wavelength range, the tilt associated with the target gain may be small enough that for a link that includes a booster and preamp at opposite ends of a span, the ripple associated with 20 dB gain is tolerable. This may be accomplished by pre-biasing the tilt negatively at the highest gain so that the lowest gain is approximately the same in the positive direction. 
     Further details of the foregoing operational aspects of the amplifier  400 , as well as the relatively narrow operational wavelength (or channel) range within the C-band or L-band and the operational wavelength range having a relatively flat gain spectrum in which the amplifiers described herein may be used may be found in U.S. Pat. Application No. 16/812,186 filed on Mar. 6, 2020. The contents of this application are incorporated herein by reference in their entirety. 
       FIG.  5    is a high level diagram of a portion of an optical communication network  10 , in particular illustrating a pair of optical amplifier nodes  12 ,  14  formed in accordance with the present invention to utilize pump modulation for bidirectional signaling of supervisory message data (as well as, perhaps, other non-customer network management commands and messages) between the amplifier nodes. The amplifier nodes  12 ,  14  of  FIG.  5    may be implemented in pluggable form and may therefore include or correspond to, e.g., the pluggable amplifiers  200 ,  300  of  FIG.  2 - 3 C  and may be referred to individually as the first node  12  or the second node  14 . 
     In  FIG.  5   , a first optical fiber  16  is used to support the “west-to-east” transmission of optical signals from the first node  12  to the second node  14 , and a second optical fiber  18  is used to support the “east-to-west” transmission of optical signals from the second node  14  to the first node  12 . The term “bidirectional” as applied to supervisory messages as described herein refers to the establishment of signal paths for supervisory messages to travel in each direction between a given pair of amplifier nodes. These bidirectional supervisory messages may propagate along (unidirectional) optical fibers  16  and  18 ; the combination of optical fibers  16  and  18  thus forming the bidirectional link for the transmission of supervisory messages between a pair of adjacent amplifier nodes. 
     As with the conventional operation of an amplifier node as discussed above, customer signals that enter the first node  12  are passed through an optical booster amplifier  20  to boost the power in these signals before being coupled into the optical fiber  16  and transmitted along to the second node  14 . The optical amplifier  20  may include or correspond to the booster optical amplifiers described elsewhere herein and may be in the form of a doped-fiber amplifier, including a section of rare-earth doped gain fiber for example. Also shown in this view is a pump source  22  (e.g., a laser diode that operates at a known pump wavelength, such as 980 nm for Er-doped fiber) that is optically coupled to the optical amplifier  20 . A pump driver (not shown) may be used to provide an input drive signal to the pump source  22  and energize the pump source  22 . An optical combiner  24  may receive and combine the incoming optical signals, and their combination along with the pump light may be provided as the input to the optical booster amplifier  20 , where the presence of the pump light results in amplification (increasing the power level) of the customer signals. The first node  12  also includes a preamp optical amplifier  26  which is also optically coupled to the pump source  22 . The preamp optical amplifier  26  may include or correspond to the booster optical amplifiers described elsewhere herein and may be in the form of a doped-fiber amplifier, including a section of rare-earth doped gain fiber for example. An optical splitter  30  may split the optical signal which exits from the preamp optical amplifier  26 . 
     An amplified optical signal provided by the first node  12  may propagate along the optical fiber  16  and be received by the second node  14 . The second node  14  includes an optical preamp amplifier  32  to boost the power in these signals before exiting the preamp amplifier  32 . An optical splitter  33  may split the optical signal which exits from the preamp optical amplifier  32 . The preamp optical amplifier  32  may include or correspond to the booster optical amplifiers described elsewhere herein and may be in the form of a doped-fiber amplifier, including a section of rare-earth doped gain fiber for example. Also shown in this view is a pump source  34  (e.g., a laser diode that operates at a known pump wavelength, such as 980 nm for Er-doped fiber) that is optically coupled to the optical amplifier  32 . A pump driver (not shown) may be used to provide an input drive signal to the pump source  34  and energize the pump source  34 . The second node  14  also includes a booster optical amplifier  36  which is also optically coupled to the pump source  34 . The booster optical amplifier  36  may include or correspond to the booster optical amplifiers described elsewhere herein and may be in the form of a doped-fiber amplifier, including a section of rare-earth doped gain fiber for example. An optical combiner  38  may receive and combine the incoming optical signals, and their combination along with the pump light may be provided as the input to the optical booster amplifier  36 , where the presence of the pump light results in amplification (increasing the power level) of the customer signals. An amplified optical signal provided by the second node  14  may propagate along the optical fiber  18  and be received by the first node  12 . 
     As indicated above, the booster optical amplifier  20  and the preamp optical amplifier  26  in the first node  12  share a common pump source  28 , and the preamp optical amplifier  32  and the booster optical amplifier  36  in the second node  14  share a common pump source  34 . As illustrated in  FIG.  5    for example, an optical signal may have different strengths at various different locations A, B, C, D, E, F, G and H in the optical communication network  10 . Regarding the second node  14  for example, an optical signal may have a target optical power at location D for example. The power of the optical signal received by the booster amplifier  20  at the location A may be communicated to the preamp optical amplifier  32 , and the amount of power provided by the preamp optical amplifier  32  to induce a gain in optical power of the optical signal to provide a target optical power at the location D may be determined based on the information provided by the booster amplifier  20 . Similarly, in this configuration, the preamp optical amplifier  32  or the pump source  34  may target an appropriate output power so that the gain of the booster amplifier  20 , the span between the first node  12  and the second node  14 , and the preamp amplifier  32  achieves the target optical power at the location D. Since the booster amplifier  36  at the second node  14  is also optically coupled to the pump source  34 , the power provided to the booster amplifier  36  is dependent on the power provided to the preamp amplifier  32 . If a 50/50 splitter is positioned between the pump source  34  and the booster amplifier  36  and the preamp amplifier  32  for example, then the power provided to the booster amplifier  36  and the preamp amplifier  32  is the same. The gain induced in the power of an optical signal by the booster amplifier  36  between the locations E and F, for example, will be whatever it turns out to be dependent on the power provided by the pump source  34  to achieve the target optical power of the optical signal at the location D. 
     Regarding the first node  12  for example, an optical signal may have a target optical power at location H for example. The power of the optical signal received by the booster amplifier  36  may be communicated to the preamp optical amplifier  26 , and the amount of power provided by the preamp optical amplifier  26  to induce a gain in optical power of the optical signal to provide a target optical power at the location H may be determined based on the information provided by the booster amplifier  36 . Similarly, in this configuration, the preamp optical amplifier  26  or the pump source  28  may target an appropriate output power so that the gain of the booster amplifier  36 , the span between the second node  14  and the first node  12 , and the preamp amplifier  26  achieves the target optical power at the location H. Since the booster amplifier  20  at the first node  12  is also optically coupled to the pump source  28 , the power provided to the booster amplifier  20  is dependent on the power provided to the preamp amplifier  26 . If a 50/50 splitter is positioned between the pump source  28  and the booster amplifier  20  and the preamp amplifier  26  for example, then the power provided to the booster amplifier  20  and the preamp amplifier  26  is the same. The gain induced in the power of an optical signal by the booster amplifier  20  between the locations A and B, for example, will be whatever it turns out to be dependent on the power provided by the pump source  28  to achieve the target optical power of the optical signal at the location H. 
     Unless specific arrangements described herein are mutually exclusive with one another, the various implementations described herein can be combined to enhance system functionality or to produce complementary functions. Likewise, aspects of the implementations may be implemented in standalone arrangements. Thus, the above description has been given by way of example only and modification in detail may be made within the scope of the present invention. 
     With respect to the use of substantially any plural or singular terms herein, those having skill in the art can translate from the plural to the singular or from the singular to the plural as is appropriate to the context or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. A reference to an element in the singular is not intended to mean “one and only one” unless specifically stated, but rather “one or more.” Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the above description. 
     In general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general, such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that include A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B, and C together, etc.). Also, a phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to include one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.” 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.