Patent Publication Number: US-2017365971-A1

Title: Multi-span optical communications link having remote optically pumped amplifier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application Ser. No. 61/955,486 filed Mar. 19, 2014, which provisional patent application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Fiber-optic communication networks serve a key demand of the information age by providing high-speed data between network nodes. Fiber-optic communication networks include an aggregation of interconnected fiber-optic links. Simply stated, a fiber-optic link involves an optical signal source that emits information in the form of light into an optical fiber. Due to principles of internal reflection, the optical signal propagates through the optical fiber until it is eventually received into an optical signal receiver. If the fiber-optic link is bi-directional, information may be optically communicated in reverse typically using a separate optical fiber. 
     Fiber-optic links are used in a wide variety of applications, each requiring different lengths of fiber-optic links. For instance, relatively short fiber-optic links may be used to communicate information between a computer and its proximate peripherals, or between a local video source (such as a DVD or DVR) and a television. On the opposite extreme, however, fiber-optic links may extend hundreds or even thousands of kilometers when the information is to be communicated between two network nodes. 
     Long-haul and ultra-long-haul optics refers to the transmission of light signals over long fiber-optic links on the order of hundreds or thousands of kilometers. Typically, long-haul optics involves the transmission of optical signals on separate channels over a single optical fiber, each channel corresponding to a distinct wavelength of light using principles of Wavelength Division Multiplexing (WDM) or Dense WDM (DWDM). 
     Transmission of optical signals over such long distances using WDM or DWDM presents enormous technical challenges, especially at high bit rates in the gigabits per second per channel range. Significant time and resources may be required for any improvement in the art of high speed long-haul and ultra-long-haul optical communication. Each improvement can represent a significant advance since such improvements often lead to the more widespread availability of communications throughout the globe. Thus, such advances may potentially accelerate humankind&#39;s ability to collaborate, learn, do business, and the like, with geographical location becoming less and less relevant. 
     BRIEF SUMMARY 
     At least one embodiment described herein relates to the use of a remote optically pumped amplifier in a multi-span optical communications link. A backwards Raman pump module performs backwards Raman amplification in the optical communications span that contains the remote optically pumped amplifier. A residual amount of backwards Raman pump power is then used to power the remote optically pumped amplifier. The remote optically pumped amplifier may be located 40 to 120 kilometers in optical distance from the backwards Raman pump module such that at least three milliwatts of residual Raman pump power is received by the remote optically pumped amplifier. The Raman pump module may be a multi-pump Raman pump module. The optical communications link may be a terrestrial optical communications link in which the remote optically pumped amplifier is positioned in a splice protector box or other enclosure. 
     There may be multiple optical communications spans in the optical communications link that similarly contain a remote optically pumped amplifier that is powered by a backwards Raman pump module. A controller might control an amount of pump power provided by at least one of the pumps of the backwards Raman pump module, so as to at least partially compensate for optical signal strength versus wavelength variation introduced by the remote optically pumped amplifier and the backwards Raman pump module. The control may be based on a measure of the optical signal power versus wavelength variation at or proximate the backwards Raman pump module or downstream of the backwards Raman pump module. 
     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 features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features can be obtained, a more particular description of various embodiments will be rendered by reference to the appended drawings. Understanding that these drawings depict only sample embodiments and are not therefore to be considered to be limiting of the scope of the invention, the embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  schematically illustrates an example optical communications system in which the principles described herein may be employed; 
         FIG. 2  schematically illustrates a multi-span optical communications link in which remote optically pumped amplifiers are utilized in more than one optical communications spans; and 
         FIG. 3  illustrates a flowchart of a method for controlling optical signal power versus wavelength in a multi-span optical communications link. 
     
    
    
     DETAILED DESCRIPTION 
     Although the principles described herein are not limited to embodiments that have the advantages that will now be described, some embodiments described herein relate to a remote optically pumped amplifier in a multi-span optical communications link. A backwards Raman pump module performs backwards Raman amplification in the optical communications span that contains the remote optically pumped amplifier. A residual amount of backwards Raman pump power is then used to power the remote optically pumped amplifier. The remote optically pumped amplifier may be located 40 to 120 kilometers in optical distance from the backwards Raman pump module such that at least three milliwatts of residual Raman pump power is received by the remote optically pumped amplifier. The Raman pump module may be a multi-pump Raman pump module. The optical communications link may be a terrestrial optical communications link in which the remote optically pumped amplifier is positioned in a splice protector box or other enclosure. 
     There may be multiple optical communications spans in one communications link that similarly contain a remote optically pumped amplifier that is powered by a backwards Raman pump module. A controller might control an amount of pump power provided by at least one of the pumps of the backwards Raman pump module, so as to at least partially compensate for optical signal strength versus wavelength variation introduced by the remote optically pumped amplifier and the backwards Raman pump module. The control may be based on a measure of the optical signal power versus wavelength variation at or proximate the backwards Raman pump module or downstream of the backwards Raman pump module. 
       FIG. 1  schematically illustrates an example optical communications system  100  in which the principles described herein may be employed. In the optical communications system  100 , information is communicated between terminals  101  and  102  via the use of optical signals. For purposes of convention used within this application, optical signals travelling from the terminal  101  to terminal  102  will be referred to as being “eastern”, whereas optical signals traveling from the terminal  102  to the terminal  101  will be referred to as being “western”. Thus, the optical communications system  100  includes an eastern optical communications link leading from the terminal  101  to the terminal  102 , and a western optical communications link leading from the terminal  102  to the terminal  101 . 
     The terms “eastern” and “western” are simply terms of art used to allow for easy distinction between the two optical signals traveling in opposite directions. The use of the terms “eastern” and “western” does not imply any actual geographical relation of components in  FIG. 1 , nor to any actual physical direction of optical signals. For instance, terminal  101  may be geographical located eastward of the terminal  102 , even though the convention used herein has “eastern” optical signals traveling from the terminal  101  to the terminal  102 . 
     In one embodiment, the optical signals are Wavelength Division Multiplexed (WDM) and potentially Dense Wavelength Division Multiplexed (DWDM). In WDM or DWDM, information is communicated over each of multiple distinct optical channels called hereinafter “optical wavelength channels”. Each optical wavelength channel is allocated a particular frequency for optical communication. Signals that fall within the particular frequency will be referred to as respective optical wavelength signals. Accordingly, in order to communicate using WDM or DWDM optical signals over the eastern optical communications link, the terminal  101  may have “n” optical transmitters  111  (including optical transmitters  111 ( 1 ) through  111 ( n ), where n is a positive integer), each optical transmitter for transmitting over a corresponding eastern optical wavelength channel. Likewise, in order to communicate over the western optical link, the terminal  102  may have “n” optical transmitters  121  including optical transmitters  121 ( 1 ) through  121 ( n ), each also for transmitting over a corresponding western optical wavelength channel. 
     The principles described herein are not limited, however, to communications in which the number of eastern optical wavelength channels is the same as the number of western optical wavelength channels. Furthermore, the principles described herein are not limited to the precise structure of the each of the optical transmitters. However, lasers are an appropriate optical transmitter for transmitting at a particular frequency. That said, the optical transmitters may each even be multiple laser transmitters, and may be tunable within a frequency range. 
     As for the eastern channel for optical transmission in the eastern direction, the terminal  101  multiplexes each of the eastern optical wavelength signals from the optical transmitters  111  into a single eastern optical signal using optical multiplexer  112 , which may then be optically amplified by an optional eastern optical amplifier  113  prior to being transmitted onto a first optical fiber communications span  114 ( 1 ). 
     There are a total of “m” repeaters  115  and “m+1” optical fiber communications spans  114  between the terminals  101  and  102  in each of the eastern and western optical communications links. However, there is no requirement for the number of repeaters in each of the eastern and western optical communications links to be equal. In an unrepeatered optical communication system, “m” would be zero such that there is but a single optical fiber communications span  114 ( 1 ) and no repeaters between the terminals  101  and  102 . In a repeatered optical communications system, “m” would be one or greater. Each of the repeaters, if present, may consume electrical power to thereby amplify the optical signals. 
     The eastern optical signal from the final optical fiber communications span  114 ( m +1) is then optionally amplified at the terminal  102  by the optional optical amplifier  116 . The eastern optical signal is then demultiplexed into the various wavelength optical wavelength channels using optical demultiplexer  117 . The various optical wavelength channels may then be received and processed by corresponding optical receivers  118  including receivers  118 ( 1 ) through  118 ( n ). 
     As for the western channel for optical transmission in the western direction over the western optical communications link, the terminal  102  multiplexes each of the western optical wavelength signals from the optical transmitters  121  (including optical transmitters  121 ( 1 ) through  121 ( n )) into a single western optical signal using the optical multiplexer  122 . The multiplexed optical signal may then be optically amplified by an optional western optical amplifier  123  prior to being transmitted onto a first optical fiber communications span  124 (m+1). If the western optical channel is symmetric with the eastern optical channel, there are once again “m” repeaters  125  (labeled  125 ( 1 ) through  125 ( m )), and “m+1” optical fiber communications spans  124  (labeled  124 ( 1 ) through  124 (m+1)). Recall that in an unrepeatered environment, “m” may be zero such that there is only one optical fiber communications span  124 ( 1 ) and no repeaters  125  in the western channel. 
     The western optical signal from the final optical fiber communications span  124 ( 1 ) is then optionally amplified at the terminal  101  by the optional optical amplifier  126 . The western optical signal is then demultiplexed using optical demultiplexer  127 , whereupon the individual wavelength division optical channels are received and processed by the receivers  128  (including receivers  128 ( 1 ) through  128 ( n )). Terminals  101  and/or  102  do not require all the elements shown in optical communication system  100 . For example, optical amplifiers  113 ,  116 ,  123 , and/or  126  might not be used in some configurations. Furthermore, if present, each of the corresponding optical amplifiers  113 ,  116 ,  123  and/or  126  may be a combination of multiple optical amplifiers if desired. 
     Often, the optical path length between repeaters is approximately the same. The distance between repeaters will depend on the total terminal-to-terminal optical path distance, the data rate, the quality of the optical fiber, the loss-characteristics of the fiber, the number of repeaters (if any), the amount of electrical power deliverable to each repeater (if there are repeaters), and so forth. However, a typical optical path length between repeaters (or from terminal to terminal in an unrepeatered system) for high-quality single mode fiber might be about 50 kilometers, and in practice may range from 30 kilometers or less to 100 kilometers or more. That said, the principles described herein are not limited to any particular optical path distances between repeaters, nor are they limited to repeater systems in which the optical path distances are the same from one repeatered segment to the next. 
     The optical communications system  100  is represented in simplified form for purpose of illustration and example only. The principles described herein may extend to much more complex optical communications systems. The principles described herein may apply to optical communication systems in which there are multiple fiber pairs, each for communicating multiplexed WDM optical signals. Furthermore, the principles described herein also apply to optical communications in which there are one or more branching nodes that split one or more fiber pairs and/or optical wavelength channels in one direction, and one or more fiber pairs and/or optical wavelength channels in another direction. 
       FIG. 2  illustrates an optical communications link  200  that represents an example of one of the optical communications links of  FIG. 1 . The optical communications link  200  might be the eastern optical communications link or the western optical communications link of  FIG. 1 . There are six optical communications spans  201  through  206  illustrated in  FIG. 2  (i. “m” would equal 5 in this example optical communications span  200 ). This is for purposes of illustration only, as the principles described herein may apply to any multi-span optical communications link. There are powered amplifiers bordering each optical communications span. Thus, with there being six optical communications spans in this example, there are seven powered amplifiers  211  through  217 . 
     The powered amplifiers  211  and  217  are incorporated in terminals. For instance, if the optical communications link  200  is the eastern optical communications link of  FIG. 1 , then the amplifier  211  is incorporated within the terminal  101  of  FIG. 1  (such as the amplifier  113  of  FIG. 1 ), and the amplifier  217  is incorporated within the terminal  102  of  FIG. 1  (such as the amplifier  116  of  FIG. 1 ). On the contrary, if the optical communications link  200  is the western optical communications link of  FIG. 1 , then the amplifier  211  is incorporated within the terminal  102  of  FIG. 1  (such as the amplifier  123  of  FIG. 1 ), and the amplifier  217  is incorporated within the terminal  101  of  FIG. 1  (such as the amplifier  126  of  FIG. 1 ). 
     The powered amplifiers  212  through  216 , on the other hand, are incorporated within repeaters. For instance, if the optical communications link  200  is the eastern optical communications link of  FIG. 1 , then the amplifiers  212  through  216  would be incorporated within repeaters  115 ( 1 ) through  115 ( 5 ) respectively, with “m” being equal to 5 in  FIG. 1 . On the other hand, if the optical communications link  200  is the western optical communications link of  FIG. 1 , then the amplifiers  212  through  216  would be incorporated within repeaters  125 ( 5 ) through  125 ( 1 ), respectively. 
     In accordance with the principles described herein, one or more of the optical communications spans has incorporated a backwards Raman pump module. For instance, in the example of  FIG. 2 , four of the optical communications spans have a backward Raman pump module. Specifically, the optical communications spans  201 ,  202 ,  205  and  206  each have respective Raman pump modules  221 ,  222 ,  225  and  226  that respective backward Raman pump power  231 ,  232 ,  235  and  236  that counter-propagates against the optical signals and thus provides distributed backward Raman amplification of the optical signal within the respective optical communications spans. 
     In accordance with the principles described herein, each of at least one of the optical communications spans that has a corresponding backwards Raman pump module also has incorporated therein a remote optically pumped amplifier (ROPA) that receives a residual amount of backwards Raman pump power to thereby perform discrete optical amplification of the optical signal. For instance, in  FIG. 2 , the optical communications span  201  includes a remote optical pumped amplifier  241  that receives a residual amount of backwards Raman pump power  231  to thereby perform discrete optical amplification of the optical signal. In addition, the optical communications span  206  includes a remote optical pumped amplifier  246  that receives a residual amount of backwards Raman pump power  236  to thereby perform discrete optical amplification of the optical signal. 
     In one embodiment, one or more of the remote optically pumped amplifiers is positioned from 40 to 120 kilometers in optical distance from the backwards Raman pump module and receives at least three milliwatts of backwards Raman pump power from the corresponding backward Raman pump module. These parameters provide reasonably good likelihood that there is sufficient residual Raman pump power such that the remote optically pumped amplifier can still efficiently convert that power into amplification of the optical signal, while still permitting the remote optically pumped amplifier to provide significant additional distance in the optical communications span. 
     Whereas traditional remote optically pumped amplifiers are conventionally supplied by a single pump backwards Raman pump module (in which the wavelength of the pump optics is approximately 1480 nanometers), one or more of the backwards Raman pump modules of the optical communications link might be multi-pump modules. For instance, one or more of the Raman pump modules  221  and  226  may be multi-pump modules. The remainder of the Raman pump modules  222  and  225  may likewise be multi-pump modules. 
     Such multi-pump modules may more evenly perform backwards Raman amplification within the optical communications span across all frequencies of optical wavelength signals. However, the residual backwards Raman pump power received at the remote optically pumped amplifier might still be predominantly the same wavelength. This is because higher frequency pumps can be converted into lower frequency pumps also due to the principle of Raman conversion. The multi-pump Raman conversion results in higher pump power in the lower frequency pump at the ROPA than would be the case if there were only a single pump at lower frequency. Accordingly, the remote optically pumped amplifier, which is designed for most suitable amplification at that wavelength, may still operate efficiently. In one embodiment, a backwards Raman pump module comprises at least four distinct pump wavelengths with the longest pump wavelength being between 1470 and 1510 nanometers. 
     A filter may be provided to at least partially compensate for signal gain variation versus wavelength of the combined remote optically pumped amplifier and the backwards Raman pump module. For instance, the filter  251  may at least partially compensate for signal gain variation caused by the remote optically pumped amplifier  241  and the backwards Raman amplification caused by the backwards Raman pump  221  within the optical communications span  201 . In addition, the filter  256  may at least partially compensate for signal gain variation caused by the remote optically pumped amplifier  246  and the backwards Raman amplification caused by the backwards Raman pump  226  within the optical communications span  206 . Alternatively, one the optical filters  251  and  256  may be omitted, thereby relying on the other filter to at least partially compensate for signal gain variety caused by both sets of remote optically pumped amplifiers and backwards Raman pump modules. Alternatively, optical filters  251  or  256  may at least partially compensate for the signal gain variation caused by other elements in communications link  200 . Optical filters  251  and  256  may have a fixed spectral shape or may be dynamic which allows the spectral shape to be adjusted electronically. 
     The filter may be located within the remote optically pumped amplifier, or may be located within a powered node on either side of the optical communications span. For instance, the filter  251  may be located within the same assembly as the remote optically pumped amplifier  241 , or may be located within the same assembly as the amplifier  211  or the amplifier  212 . Likewise, the filter  256  may be located within the same assembly as the remote optically pumped amplifier  246 , or may be located within the same assembly as the amplifier  216  or the amplifier  217 . 
     The optical communications link  200  may be a terrestrial optical communications link, in which all of the optical communications spans  201  through  206  are terrestrial optical communications spans. Alternatively, the optical communications link  200  may be a submarine optical communications link, in which all of the optical communications spans  201  through  206  are submarine optical communications spans. The optical communications link  200  may also be a hybrid in which one or more of the optical communications spans  201  through  206  are terrestrial optical communications spans, and one or more of the optical communications spans  201  through  206  are submarine optical communications spans. 
     However, in some embodiments, the optical communications spans  201  and  206  are terrestrial optical communications spans. In this case, the remote optically pumped amplifiers  241  and  246  are located terrestrially in a non-submarine environment. The remote optically pumped amplifiers  241  through  246  may perhaps even be located within respective splice box  261  and  266 , which are normally used to protect splices of the optical fibers. For instance, optical fibers normally come in lengths of several kilometers. Thus, in order to connect optical fibers of tens or hundreds of kilometers, there are periodically splices in the optical fiber. Splice boxes typically are provided to protect the areas where one optical fiber is spliced to meet another optical fiber. Such splice boxes may be used to fit also the remote optically pumped amplifier. In some embodiments, enclosures  261  and  266  only house the remote optically pumped amplifiers and do not house span fiber splices. 
     The optical communications system  200  further includes a controller  252 .  FIG. 3  illustrates a flowchart of a method  300  for controlling optical signal power versus wavelength in a multi-span optical communications link. The controller  252  may perform the method  300  of  FIG. 3 , by first detecting optical signal power wavelength variation versus wavelength at a node in the optical communications span (act  301 ). The controller  252  then adjusts the pump power of at least one pump of a multi-pump backward Raman pump module that provides backwards Raman pump power (act  302 ). For instance, the controller  252  might adjust the pump power of one of the pumps in backwards Raman pump module  221 ,  222 ,  225  and  226 . This feedback pattern may be continued to provide more uniform gain across all optical wavelength signals within the optical communications link, despite the presence of remote optically pumped amplifier(s) and backward Raman amplification. 
     As an example, the controller  252  may monitor optical signal power versus wavelength at the assembly that contains the amplifier  212  (or any other node or amplifier downstream in the optical communications link  200  such as amplifiers  213  through  217 ), and adjust the pump power of one or more pumps of the backwards Raman pump module  221  accordingly. The controller  252  may monitor optical signal power versus wavelength at the assembly that contains the amplifier  217 , and adjust the pump power of one or more pumps of the backwards Raman pump module  226  accordingly. 
     Thus, an effective mechanism for incorporating remote optically pumped amplifiers into a multi-span optical communications link has been described. 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.