Patent Publication Number: US-9893834-B2

Title: Locally powered optical communication network

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This U.S. patent application is a continuation of, and claims priority under 35 U.S.C. § 120 from, U.S. patent application Ser. No. 14/601,558, filed on Jan. 21, 2015, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to locally powered optical communication networks, as by using power sources along a transoceanic fiber optic cable. 
     BACKGROUND 
     Submarine optical communication cables are optical cables laid on the sea bed connecting land-based stations. The optical communication cables carry signals across the ocean allowing for cable communication over the sea between different continents. Each transoceanic submarine optical cable includes multiple (up to eight) pairs of fibers; each pair has one fiber in each direction. The submarine optical communication cable is divided into multiple sections, each section connected to the other section by a submarine repeater. As the signals are carried across the ocean, the signals may lose some of their power. Each submarine repeater comprises multiple Erbium Doped Fiber Amplifiers (EDFA), one amplifier for the signal in each direction of each fiber. Each EDFA has a gain sufficient to compensate for the loss experienced by the signal during its propagation in the previous section of fiber. A typical length of cables (i.e., fiber) section between repeaters is about 60 km. Atypical Trans-Pacific cable with a length of 10,000 km has about 150-180 repeaters. Therefore, the repeaters ensure that the land-based station receiving the signal, from another land-based station, understands the signal. In other words, the repeaters increase the power of the signal to compensate for any loss during the signal propagation. The increased power translated to higher capacity because the clearer and more precise the optical signal is, the more complex signals may be sent from one land-based station to another. 
     Referring to  FIG. 1 , an optical communication system  10  includes first and second trunk terminals  110 ,  120  (also referred to as stations) coupled to a communication trunk  102 . The communication trunk may include one or more repeaters  150 . The repeaters  150  are powered by a constant current, typically 1 Ampere by power feeding equipment, e.g., power source  112  located by the shore. Due to nonzero electrical resistivity of copper, even with large copper area conductor having a resistance as low as 1 Ohm/kilometer, power feeding voltage drops by 60 Volt at each section of cable, so that about half of power feeding voltage is lost due to heat dissipation in copper for Trans-Pacific cable. In some examples, each power source  112  can provide power feed voltage of up to 15 kilovolts. Further increase of power feeding voltage to higher that 15 kV may result in a fault during cable operation, which has a lifetime of 25 years. The power feeding equipment  112  powers the repeaters  150  by a power cable  113 , such as a copper cable. With half of power feeding voltage lost due to heat dissipation in copper cable, and due to large number of repeaters,  150 - 180 , voltage drop at each repeater is limited to below 50 Volt. Typical output power launched into a submarine fiber is 17 dBm (50 mW) for each direction. Assuming highly efficient conversion from electrical into optical of 30% in diode laser and 10% efficiency of EDFA pumped by diode laser, each EDFA requires about 2 Volts power drop at constant current of 1 A. Thus 50V power feed voltage per repeater limits the number of EDFAs in the repeater to 20-25, i.e., supporting compensation of loss for not more than 10-12 fiber pairs. Most of subsea cables today have six fiber pairs. Thus, the power feeding of repeaters from the shores limits further growth of capacity of trans-oceanic submarine cables. 
     SUMMARY 
     The present disclosure addresses the limitations of conventional transoceanic fiber optic cables without the drawback of limited power and bandwidth. One mechanism for overcoming the current limitations is the use of power sources along the communication trunk that extends across an ocean. 
     One aspect of the disclosure provides an optical system for a locally powered optical communication network. The optical system includes a first trunk terminal emitting an optical signal, a second trunk terminal receiving the optical signal, a communication trunk, an intermediate unit and a power source. The communication trunk is disposed along a floor of a body of water and couples the first trunk terminal to the second trunk terminal. The communication trunk transmits the optical signal from the first trunk terminal to the second trunk terminal. The intermediate unit is connected to the communication trunk between the first and second trunk terminals. The intermediate unit receives the emitted optical signal from the first trunk terminal, amplifies the received optical signal and sends the amplified optical signal to the second trunk. The power source is connected to and powers the intermediate unit. The power source is located at or near a surface of the body of water. 
     Implementations of the disclosure may include one or more of the following optional features. In some implementations, the intermediate unit includes a laser powered by the power source and emits an optical output and an optical combiner in communication with the laser and the first and second trunk terminals. The optical combiner includes receiving the optical signal from the first trunk terminal and the optical output from the laser, amplifies the optical signal by combining the optical signal from the first trunk terminal and the optical output from the laser and outputs the amplified signal to the second trunk terminal. 
     In some examples, the optical combiner includes an optical amplifier, an optical combiner/splitter, or an optical add-drop multiplexer. The system may further include an optical fiber connecting the optical combiner and the laser and transmits the optical output from the laser to the optical combiner. The optical combiner may include a wavelength splitter to enable Raman Amplification in submarine fiber or an Erbium Doped Fiber Amplifier. The power source may power the laser. 
     The system may include an electrical power cable coupling the power source and the intermediate unit and powers the intermediate unit. The intermediate unit may include an optical communication amplifier or optical regenerator to improve quality of optical communication signal. The power source may include a wind power source, a wave power source, a solar power source, a thermoelectric power source, or a fuel power source. 
     In some examples, the system includes a branching unit disposed along the communication trunk and couples a branch terminal to the communication trunk. The branching unit includes an optical add-drop multiplexer including a first filter filtering a first band of wavelengths of a communication spectrum for a first communication segment and a second filter filtering a second band of wavelengths of the communication spectrum for a second communication segment. The second band of wavelengths overlap the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band, the overlap band having a variable size. The first band of wavelengths may include a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment. 
     The first band of wavelengths may include an entire overlap band of wavelengths for the first communication segment and the second band of wavelengths excludes the overlap band of wavelengths from the second communication segment. The overlap band of wavelengths may include common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths. The first filter and/or the second filter may provide a fixed sized overlap band of wavelengths of the communication spectrum. The first filter and/or the second filter may be adjustable to provide a variable sized overlap band of wavelengths of the communication spectrum. Filtering may include adding, dropping, and/or reusing wavelengths. 
     Another aspect of the disclosure provides a method for a locally powered optical communication network. The method includes emitting an optical signal from a first trunk terminal into a communication trunk disposed along a floor of a body of water. The communication trunk couples the first trunk terminal to a second trunk terminal and transmits the optical signal from the first trunk terminal to the second trunk terminal. The method further includes receiving the optical signal at an intermediate unit between the first trunk terminal and the second trunk terminal. The intermediate unit is connected to the communication trunk between the first and second trunk terminals. The intermediate unit is powered by a power source located at or near a surface of the body of water. The method further includes amplifying the received optical signal at the intermediate unit and sends the amplified optical signal from the intermediate unit to the second trunk terminal. 
     In some implementations, amplifying the received optical signal includes combining an optical output from a laser of the intermediate unit with the received optical signal. The method may further include, at the intermediate unit, optical regeneration to improve quality of optical signal. For example, an optical regenerator may convert the received optical signal to an electrical signal, process the electrical signal, and convert the electrical signal to the amplified optical signal. The intermediate unit may further include a laser powered by the power source and emitting an optical output and an optical combiner in communication with the laser and the first and second trunk terminals. The optical combiner may include receiving the optical signal from the first trunk terminal and the optical output from the laser, amplifying the optical signal by combining the optical signal from the first trunk terminal and the optical output from the laser and outputting the amplified signal to the second trunk terminal. The power source may include one of a wave power source, a solar power source, a thermoelectric power source, or a fuel power source. 
     In some examples, the method includes filtering, at a branching unit in communication with the intermediate unit, a first band of wavelengths of a communication spectrum for a first communication segment and filtering, at the branching unit, a second band of wavelengths of the communication spectrum for a second communication segment. The second band of wavelengths overlap the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band. The overlap band has a variable size. The first band of wavelengths may include a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment. The overlap band of wavelengths may include common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths. 
     Yet another aspect of the disclosure provides a second method for a locally powered optical communication network. The method includes receiving an optical signal at an intermediate unit connected to a communication trunk disposed along a floor of a body of water, the intermediate unit coupling a first trunk terminal to a second trunk terminal. The method further includes amplifying the received optical signal at the intermediate unit by injecting light into the communication trunk. The intermediate unit includes an optical combiner connected to the communication trunk and a laser in optical communication with the optical combiner and sends an optical output to the optical combiner. The method further includes powering the laser using a power source in communication with the intermediate unit and is located at or near a surface of the body of water. 
     In some examples, amplifying the received optical signal includes combining an optical output from the laser of the intermediate unit with the received optical signal. The method further includes, at the intermediate unit, coupling the laser pump light having a frequency different than the signal frequency so that the laser pump may amplify the signal through a process of stimulated Raman scattering. The laser may have a frequency higher than the signal frequency, enabling efficient Raman amplification. The laser may be located at or near the power source or at or near the optical combiner. The power source may include a wave power source, a solar power source, a thermoelectric power source, or a fuel power source. 
     In some implementations, the method includes filtering, at a branching unit in communication with the intermediate unit, a first band of wavelengths of a communication spectrum for a first communication segment and filtering, at the branching unit, a second band of wavelengths of the communication spectrum for a second communication segment. The second band of wavelengths overlap the first band of wavelengths in an overlap band of wavelengths with no guard band between the first band and the second band, the overlap band having a variable size. The first band of wavelengths includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment. The overlap band of wavelengths includes common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths. The optical combiner may include a Raman Amplifier or a high power Erbium Doped Fiber Amplifier. 
     The details of one or more implementations of the disclosure are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a prior art schematic view of an optical communication system having on shore power sources. 
         FIGS. 2 and 3  are a schematic view of an exemplary optical communication system having power sources at or near the ocean surface along the communication trunk. 
         FIG. 4  is a schematic view of an exemplary optical communication system having power sources at or near the ocean surface along the communication trunk and having one or more branching units. 
         FIG. 5  is a schematic view of an exemplary optical communication system having power sources at or near the ocean surface along the communication trunk where the repeater is separated from the electrical portion. 
         FIG. 6  is a schematic view of an exemplary optical communication system having power sources at or near the ocean surface along the communication trunk where the combiner/splitter is separated from the electrical portion. 
         FIG. 7  is a schematic view of an exemplary optical communication system having power sources at or near the ocean surface along the communication trunk and having one or more branching units connecting to one or more High Altitude Communication Devices. 
         FIG. 8A  is a perspective view of an exemplary communication balloon of the global-scale communication system that includes the optical communication system. 
         FIG. 8B  is a perspective view of an exemplary satellite of the global-scale communication system that includes the optical communication system. 
         FIG. 9  is a schematic view of a network comprising one or more communication systems interconnected. 
         FIG. 10  is a schematic view of an exemplary arrangement of operations for a method of optical communication. 
         FIG. 11  is a schematic view of an exemplary arrangement of operations for a method of optical communication. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 2 , an optical communication system  100  includes first and second trunk terminals  110 ,  120  (also referred to as stations) coupled to a communication trunk  102 . The coupling may be any connection, link or the like by which signals carried by one system element are imparted to the “coupled” element. The coupled elements may not necessarily be directly connected to one another and may be separated by intermediate components or devices that may manipulate or modify the signals. The communication trunk  102  may include a plurality of optical cable segments  102 ,  102   a - n  (e.g., optical submarine cables) that carry optical signals  105  on associated optical channels/wavelengths λ. 
     Each cable segment  102  may include one or more sections of fiber optic cable including optical fiber pairs and one or more repeaters  150  to provide a transmission path for bi-directional communication of optical signals  105  between the first and second trunk terminals  110 ,  120 . The system  100  may be configured as a long-haul system, e.g., having a length between at least two of the terminals  110 ,  120  of more than about 600 km, and may span a body of water, e.g., an ocean and having a length of 6,000-12,000 km. 
     The repeater(s)  150  may include any optical amplifier configuration that compensates for signal attenuation on the transmission path. For example, one or more repeaters  150  may be configured as an optical amplifier, such as an erbium doped fiber amplifier (EDFA), a Raman amplifier, or a hybrid Raman/EDFA amplifier. Optical amplifiers are devices that amplify an optical signal directly without converting the signal to an electrical signal first. An optical amplifier may be considered a laser without an optical cavity. Doped fiber amplifiers (DFAs) are optical amplifiers that use a doped optical fiber (i.e., an optical fiber containing a dopant, which is a trance impurity element that is inserted into a substance (in very low concentration) to alter the substance&#39;s optical properties) as a gain medium to amplify the optical signal. The signal to be amplified and a pump laser are multiplexed into the doped fiber, and the signal is amplified through interaction with doping ions. EDFA is the most common example of DFAs, where the core of a silica fiber is doped with trivalent erbium ions and may be efficiently pumped with a laser at wavelengths of 980 nm or 1,480 nm and still exhibit gain in the 1,550 nm region. In a Raman amplifier, the signal is intensified by Raman amplification, which is based on the stimulated Raman scattering (SRS) phenomenon, when a lower frequency signal photon induces the inelastic scattering of a higher-frequency pump photon in an optical medium in the non-linear regime. This results in another signal photon being produced, with the extra energy between pump and signal frequencies passed to the vibrational states of the medium, allowing for optical amplification. Therefore, the Raman amplification produces nonlinear interaction between the signal and a pump laser within the optical fiber (unlike the EDFA). The main advantage of Raman amplification is its ability to provide distributed amplification within each fiber segment  102 , which increases the length of the fiber segment  102  before the next amplifier  150 . A system bandwidth may coincide with the usable bandwidth of the optical amplifiers within the system  100 . Each repeater  150  is locally powered through a local power source  160  (instead of the on shore power source  112  as shown in  FIG. 1 ), which eliminates the physical limitations shown and described with respect to  FIG. 1 . By adding a local power source  160  that powers each repeater  150 , the optical communication system  100  increases the number of fiber pairs each cable trunk includes, for example, previously each cable trunk included 5-6 fiber pairs, which may be increased to 10 times to 100 times by adding the local power source  160 . In some examples, the repeaters  150  are spaced out for redundancy so that if one repeater  150  fails, the system  100  does not fail. In addition, power sources  160  powering adjacent repeaters  150  to the failed repeater  150 , may compensate for the failed repeater  150  by increasing the power supplied to the working adjacent repeaters  150 . In another example, one or more repeaters  150  may be configured as an optical communication repeater (also known as optical-electrical-optical (OEO)) that is used to regenerate an optical signal by converting it to an electrical signal, processing the electrical signal and then reconverting the electrical signal to an optical signal and outputting (transmitting) the converted optical signal. The optical communication repeaters are used to extend the reach of optical communications links by overcoming loss due to attenuation of the optical fiber and distortion of optical signal. 
     The local power source  160  provides power to the repeater  150  through a power cable  104  connecting the power source  160  and the repeater  150 . The local power source  160  may be a solar powered battery, or based on wave energy, or based on fossil fuel, or a combination of both. Other examples of power are possible as well. 
     Referring to  FIG. 3 , in some implementations, the power source(s)  160  are located on or near the sea floor. Such power sources  160  may generate power based on wave motion, temperature gradients, or other suitable methods of power generation. Wave power is the transport of energy by ocean surface waves, and the capture of that energy to generate power to provide electricity, distill water, pump water, or any other uses. In some examples, the power source  160  is positioned between the ocean bed and the ocean surface, such that a boat may reach the power source  160  to maintain the power source  160 . 
     Referring to  FIG. 4 , in some implementations, one or more branch terminals  130  are coupled to the communication trunk  102  between the first and second trunk terminals  110 ,  120  by corresponding branching units  140 . A branching unit  140  may be an OADM (optical add-drop multiplexers) branching unit. OADMs used in optical communication networks are capable of removing wavelength channels from multiple wavelength signals and adding channels to those signals. Moreover, one or more repeaters  150  and linking optical cables  102  may couple the branch terminal  130  to its corresponding branching unit  140 . The system  100  may therefore be configured to provide bi-directional or uni-directional communication of optical signals  105  between terminals  110 ,  120 ,  130 . 
     Branching units  140  enable the function of capacity redirection between express paths for express wavelengths λ E  (e.g., from Station A to Station B) and add/drop paths (e.g., from Station A to Station C and/or Station B to Station C). This can be done, for example, by simultaneously adding/dropping a band of wavelengths λ A  at each OADM  140 . The terms “add/drop,” “adding/dropping,” and “added/dropped” refer to either the operation of adding one or more wavelengths λ, dropping one or more wavelengths λ, or adding wavelengths λ and dropping others. Those terms are not intended to require both add and drop operations, but are also not intended to exclude add and drop operations. The terms are merely used as a convenient way to refer to either adding or dropping or both adding and dropping operations. 
     In general, the branching units  140  may add and drop channels λ to/from the communication trunk  102 . In some implementations, a wavelength division multiplexing (WDM) signal  105  originates at one or more of the terminals  110 ,  120 ,  130 , and the branching units  140  may be configured either to pass some channels λ through the branching units  140  to travel uninterruptedly through the communication trunk  102  from an originating trunk terminal  110 ,  120  to a receiving trunk terminal  110 ,  120  or other branching unit  140 . The branching units  140  may add or drop one or more other channels λ to/from the branch terminals  130 . For example, a WDM signal  105  originating at the first trunk terminal  110  may include information occupying one or more channels λ. Likewise, a WDM signal  105  originating at the branch terminal  130  may occupy one or more channels λ. Both WDM signals  105  may be transmitted to the branching unit  140  that passes certain channels λ therethrough from the originating first trunk terminal  110  along the communication trunk  102  without interruption to the second trunk terminal  120 . The branching unit  140  may be configured to drop, i.e., extract information from, one or more channels λ originating from the first trunk terminal  110  and pass the dropped channels λ to the branch terminal  130 . The branching unit  140  may also be configured to add, i.e., insert information on, certain channels λ originating from the branch terminal  130  to the WDM signal  105  originating from the first trunk terminal  110  and pass the resulting WDM signal  105  (that includes the added information) onto the second trunk terminal  120 . In some examples, the WDM signal  105  originating from the first trunk terminal  110  is fully terminated at the branching unit  140 , in which case only the added information from the branch terminal  130  would be passed onto the second trunk terminal  120 . Other branching units  140  may similarly pass through, add, and/or drop certain channels λ. 
     Any branching unit  140  may be disposed in an undersea environment and may be seated on the ocean floor. Additionally or alternatively, the branching unit  140  may be in a terrestrial environment and may be co-located at the same central office as the branch terminal  130 . The communication trunk  102  may thus span between beach landings, or may provide a terrestrial connection between two terminal stations. The OADM  140  may include one or more tunable/adjustable filters  142  that provide a traffic mix of express wavelengths λ E  and add/drop wavelengths λ A  in the signal  105 . 
     Multiple terminals/stations  110 ,  120 ,  130  share optical bandwidth of the same fiber pair by separating the whole spectrum into bands B using optical filters in the OADMs  140 . A band B includes two or more wavelengths λ (also referred to as channels) residing spectrally adjacent to one another. By adding/dropping one or more bands B of signal wavelengths λ at each OADM  140 , only signals  105  having wavelengths λ adjacent to the spectral edges of the band are affected by asymmetry penalties and high loss. The term “spectral edge” refers to the wavelength λ contained within a band of wavelengths λ that is immediately adjacent to a wavelength λ not included within that particular band B of wavelengths λ. None of the signals  105  having wavelengths λ within the added/dropped band experience this spectral distortion. 
     In some implementations, the repeater  150  has a local power source  160  connected to the repeater  150 . A type of connection to the repeater  150  may depend on the type of repeater  150 . Referring to  FIGS. 2-4 , the repeater  150  is an active repeater  150   a  in need of power, the connected power supply  160  powers the repeater  150   a  though a power cable  104 . However, referring to  FIGS. 5-7 , if the repeater  150  is a passive repeater  150   p  (a repeater  150   p  without any electronics to power the repeater  150   p ), the connected power supply  160  delivers power though a power cable  104  to a laser  170 , which in turn delivers light to the passive repeater  150   p  through an optical fiber  102 . The system  100  of  FIGS. 5-7 , separates the electronic layer, i.e., the power source  160  and the laser  170  from the optical layer, i.e., the passive repeater  150   p . Therefore, in instances where the power source  160  or the laser  170  requires maintenance, they are positioned at a location where they can be easily reached. The laser  170  is a device that generates light though a process of optical amplification based on the stimulated emission of electromagnetic radiation. The laser  170  emits light coherently, which allows the laser  170  to be focused to a tight spot. For example, spatial coherence allows a laser beam (light) to stay narrow over long distances. 
     Referring to  FIG. 5 , the optical communication system  100  includes first and second trunk terminals  110 ,  120  (also referred to as stations) coupled to the communication trunk  102   c . The coupling may be any connection, link or the like by which signals carried by one system element are imparted to the “coupled” element. The coupled elements may not necessarily be directly connected to one another and may be separated by intermediate components or devices that may manipulate or modify the signals. The communication trunk  102  may include a plurality of optical cable segments  102 ,  102   a - n  (e.g., optical submarine cables) that carry optical signals  105  on associated optical channels/wavelengths λ o . 
     Each cable segment  102  may include one or more sections of fiber optic cable including optical fiber pairs and one or more passive repeaters  150   p  to provide a transmission path for bi-directional communication of optical signals  105  between the first and second trunk terminals  110 ,  120 . The system  100  may be configured as a long-haul system, e.g. having a length between at least two of the terminals  110 ,  120  of more than about 600 km, and may span a body of water, e.g., an ocean and having a length of 6,000-12,000 km. 
     The passive repeater(s)  150   p  may include any optical amplifier configuration that compensates for signal attenuation on the transmission path. For example, one or more repeaters  150  may be configured as an optical amplifier, such as an erbium doped fiber amplifier (EDFA), a Raman amplifier, or a hybrid Raman/EDFA amplifier. The passive repeater  150   p  combines the communication trunk  102   c  and the pump fiber  102   p . The pump fiber  102   p  delivers a laser beam (having optical energy) to excite the doped optical fiber of the passive repeater  150   p  (e.g., to excite the erbium ions). The EDFA passive repeater  150   p  combined a relatively high-powered beam of light from the pump fiber  102   p  with an input signal from the communication trunk  102   c  using a wavelength selective coupler. The input communication signal of the communication trunk  102   c  and the high-powered beam of light of the pump fiber  102   p  must be at significantly different wavelengths. The pump fiber  102   p  output light having a wavelength within the absorption bandwidth of the Erbium (Er 3+  doped fiber) of the passive repeater  150   p . The combined light (from the pump fiber  102   p  and the communication trunk  102   c ) is guided into a section of fiber within the passive repeater  150   p  with erbium ions included in the core of the fiber. This causes the high-powered beam of light from the pump fiber  102   p  to excite the erbium ions to their higher-energy state. When the photons of the communication signal of the communication trunk  102   c  at different wavelength from the high-powered beam of light of the pump fiber  102   p  meet the excited erbium atoms, the erbium atoms give up some of their energy to the communication signal of the communication trunk  102   c  and return to their lower-energy state. The erbium gives up its energy in the form of additional photons, which are in the same phase and direction as the signal being amplified, which is the communication signal from the communication trunk  102   c . Therefore, the signal is amplified only in its direction of travel. Selection of a pump wavelength λ p  of the pump fiber  102   p  is based on the erbium Er′ absorption spectrum and the low loss of power of the fiber used to deliver the pump signal to the passive repeater  150   p . In some example, the wavelength range of the pump wavelength λ p  is within the 1400 nm and less than 1500 nm. By eliminating the power cable  113  from the power source  112  located on the shore ( FIG. 1 ), the system  100  can increase the number of fibers in the communication trunk  102  to include 100 up to 1,000 fibers, and in some instances more. Therefore, each local power source  160  powers the 100 up to 1,000 fibers by supplying the laser  170  with a power of 1 kW (kilowatt) up to 10 kW. 
       FIG. 6  is similar to  FIG. 5  except that the passive amplifier  150   p  is replaced with wavelength splitter  152 . As shown, the splitter  152  enables Raman amplification in the transmission fiber by combining Raman pump light with wavelength lambdaP with signal light at wavelength lambda0. Splitter  152  is positioned at the sea bed and is in communication with the communication trunk  102   c  and the pump optical fiber  1021 . The laser  170  may be a Raman laser  170   r , which is a specific type of laser in which the fundamental light-amplification mechanism is simulated Raman scattering (this is different than conventional lasers, which rely on simulated electronic transitions to amplify light. Raman lasers  170   r  are optically pumped; however, the pumping does not produce a population inversion like conventional lasers. Instead, pump photons are absorbed and immediately re-emitted as lower-frequency laser light photons by simulated Raman scattering. The difference between the two photon energies is fixed and corresponds to a vibrational frequency of the gain medium. Thus, it is possible to produce arbitrary laser-output wavelengths by choosing pump-laser wavelength accordingly. (In conventional lasers, the possible laser output wavelengths are determined by the emission lines of the gain material.) 
     The Raman pump light having a Raman pump wavelength λ R  is injected in the communication trunk  102   c  every 60 to 100 kilometers. The Raman pump wavelength λ R  is shifted from the transmission wavelengths λ o  by energy of molecular vibrations in glass. In this case, complete separation of the optical layer, i.e., the splitter  152  and the electrical layers, i.e., the power source  160  and the Raman laser  170   r  is achieved. The splitter  152 , which is also a passive optical layer, has a long lifetime because no additional elements are mounted on the cable  102 . Thus, the setup shown in  FIG. 6  extends the lifetime of the cable  102  by more than 25 years because the splitter  152  comprises passive wavelength splitter. Moreover, in some examples, the Raman amplifier  152  when pumped by several Raman lasers  170   r  each having a different frequency may extend the amplifier bandwidth between two to three times compared to C-Band of 4 THz. 
     Referring back to  FIGS. 2-6 , the system  100  allows for simpler operation and maintenance of the equipment, since all active electronics, such as the local power sources  160  and the lasers  170  are located near the surface of the ocean. In addition, if the system  100  needs to be upgraded by adding more fiber pairs, then additional communication trunks  102 ,  102   c  are added without interfering with the existing communication trunks  102 ,  102   c , and additional pump cables  102   p  are also added without interfering with the existing pump fiber (cable)  102   p . Thus the existing cables  102  remain untouched during an upgrade to expand the system  100  bandwidth. Moreover, in this case, an additional laser  170  (not shown) is added to support the additional fibers  102 . 
     Referring to  FIGS. 7-8B , in some implementations, a communication system  100   a  similar to the system previously described additionally includes High Altitude Communication Devices (HACD)  200  and gateways  300  (such as, a source ground station, a destination ground station, or linking ground stations). An HACD  200  is a device released into the earth&#39;s atmosphere. HACD  200  may refer to a communication balloon  200   a  or a satellite  200   b  in Low Earth Orbit (LEO) or Medium Earth Orbit (MEO) or High Earth Orbit (HEO), including Geosynchronous Earth Orbit (GEO). The HACD  200  includes an antenna  207  that receives a communication  20  from a source ground station  300  and reroutes the communication signal to a destination ground station  300 . The HACD  200  also includes a data processing device  210  that processes the received communication  20  and determines a path of the communication  20  to arrive at the destination ground station  300 . The system  100  may include communication balloons  200   a , satellites  200   b , or a combination of both. 
     As shown, one or more local power sources  160  may include a gateway  300  that provides communication with HACD device  200  and other gateways, providing redundant connections through HACDs  200  and communication trunks  102 . The local power source  160  and the gateways  300  act as a POP (point of presence) in the sea to provide high bandwidth connection to HACDs  200  network. 
     In some implementations, one or more gateways  300  is a trunk terminal  130  communicating with the communication trunk  102   c  between the first and second trunk terminals  110 ,  120  by corresponding branching units  140 , which also acts as repeaters. As previously described with respect to  FIG. 4 , a branching unit  140  may be an OADM (optical add-drop multiplexers) branching unit. OADMs used in optical communication networks are capable of removing wavelength channels from multiple wavelength signals and adding channels to those signals. Moreover, one or more repeaters  150  and linking optical cables  102  may couple the branch terminal  130  to its corresponding branching unit  140 . The system  100  may therefore be configured to provide bi-directional or uni-directional communication of optical signals  105  between terminals  110 ,  120 ,  130 . 
     Referring to  FIG. 8A , the communication balloons  200   a  include a balloon  204  (e.g., sized about 49 feet in width and 39 feet in height), an equipment box  206   a , and solar panels  208 . The equipment box  206   a  includes a data processing device  210  that executes algorithms to determine where the high-altitude balloon  200   a  needs to go, then each high-altitude balloon  200   a  moves into a layer of wind blowing in a direction that will take it where it should be going. The equipment box  206   a  also includes batteries to store power and a transceiver (e.g., antennas  207 ) to communicate with other balloons  200   a , internet antennas on the ground or gateways  300 . The communication balloons  200   a  also include solar panels  208  that power the equipment box  206   a . In some examples, the solar panels  208  produce about 100 watts in full sun, which is enough to keep the communication balloons  200   a  running while charging the battery and is used during the night when there is no sunlight. When all the high-altitude balloons  200   a  are working together, they form a balloon constellation. In some implementations, users on the ground have specialized antennas that send communication signals to the communication balloon  200   a  eliminating the need to have a source or destination ground station  300 . The communication balloon  200   a  receiving the communication  20  sends the communication  20  to another communication balloon  200   a  until one of the communication balloons  200   a  is within reach of a destination ground station  320  that connects to the local internet provider and provides service to the user via the network of balloons  200   a.    
     A satellite  200   b  is an object placed into orbit around the earth and may serve different purposes, such as military or civilian observation satellites, communication satellites, navigations satellites, weather satellites, and research satellites. The orbit of the satellite  200   b  varies depending in part on the purpose the satellite  200   b  is being used for. Satellite orbits may be classified based on their altitude from the surface of the earth as Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and High Earth Orbit (HEO). LEO is a geocentric orbit (i.e., orbiting around the earth) that ranges in altitude from 0 to 1,240 miles. MEO is also a geocentric orbit that ranges in altitude from 1,200 mile to 22,236 miles. HEO is also a geocentric orbit and has an altitude above 22,236 miles. Geosynchronous Earth Orbit (GEO) is a special case of HEO. Geostationary Earth Orbit (GSO, although sometimes also called GEO) is a special case of Geosynchronous Earth Orbit. 
     Multiple satellites  200   b  working in concert form a satellite constellation. The satellites  200   b  within the satellite constellation may be coordinated to operate together and overlap in ground coverage. Two common types of constellations are the polar constellation and the Walker constellation, both designed to provide maximum earth coverage while using a minimum number of satellites  200   b.    
     Referring to  FIG. 8B , a satellite  200   b  includes a satellite body  206   b  having a data processing device  210 , similar to the data processing device  210  of the communication balloons  200   a . The data processing device  210  executes algorithms to determine where the satellite  200   b  is heading. The satellite  200   b  also includes an antenna  207  for receiving and transmitting a communication  20 . The satellite  200   b  includes solar panels  208  mounted on the satellite body  206   b . The solar panels  208  provide power to the satellite  200   b . In some examples, the satellite  200   b  includes rechargeable batteries used when sunlight is not reaching and charging the solar panels  208 . 
     When constructing a system  100  from multiple HACDs  200 , it is sometimes desirable to route traffic over long distances through the system  100  by linking one HACD  200  to another. For example, two satellites  200   b  may communicate via inter-satellite links and two balloons  200   a  may communicate via inter-balloon links. Such inter-device (satellite  200   b  or balloon  200   a ) linking IDL is useful to provide communication services to areas far from source and destination ground stations  300  and may also reduce latency and enhance security (fiber optic cables may be intercepted and data going through the cable may be retrieved). This type of inter-device communication is different than the “bent-pipe” model, in which all the signal traffic goes from a ground-base gateway  300  to a satellite  200   b , and then directly down to a user on earth or vice versa. The “bent-pipe” model does not include any inter-device communications; instead, the satellite  200   b  acts as a repeater. In some examples of “bent-pipe” models, the signal received by the satellite  200   b  is amplified before it is re-transmitted; however, no signal processing occurs. In other examples of the “bent-pipe” model, part or all of the signal may be processed and decoded to allow for one or more of routing to different beams, error correction, or quality-of-service control; however no inter-device communication occurs. 
     In some implementations, long-scale HACD constellations (e.g., balloon constellation or satellite constellations) are described in terms of a number of planes or groups (not shown), and the number of HACDs  200  per plane. HACDs  200  within the same plane maintain the same position relative to their intra-plane HACD  200  neighbors. However, the position of an HACD  200  relative to neighbors in an adjacent plane varies over time. 
     Inter-device link (IDL) eliminates or reduces the number of HACDs  200  to gateway hops, which decreases the latency and increases the overall network capabilities. Inter-device links allow for communication traffic from one HACD  200  covering a particular region to be seamlessly handed over to another HACD  200  covering the same region, where a first HACD  200  is leaving the first area and a second HACD  200  is entering the area. 
     Referring to  FIG. 9 , in some implementations, multiple terminals/stations  110 ,  120 ,  130  are connected via one or more communication trunks  102 . The communication trunk  102  may include a plurality of optical cable segments  102 ,  102   a - n  (e.g., optical submarine cables) that carry optical signals  105  on associated optical channels/wavelengths λ. In some examples, the multiple terminals/stations  110 ,  120 ,  130  are connected via HACDs  200  and gateways  300  (as described with respect to  FIGS. 7 and 8 ). In some examples, one or more switches are places within the network redirect a communication when a portion of the network requires maintenance. Therefore, the communication is goes through a different path until maintenance is complete. The optical switch may be mounted at local power sources  160  or nearby for operational simplicity. 
       FIG. 10  is a schematic view of an exemplary arrangement of operations for a method  1000  of optical communication that includes at block  1002 , emitting an optical signal  105  from a first trunk terminal  110  into a communication trunk  102 ,  102   c  disposed along a floor of a body of water. The communication trunk  102 ,  102   c  couples the first trunk terminal  110  to a second trunk terminal  120  and transmits the optical signal  105  (having a wavelength λ) from the first trunk terminal  110  to the second trunk terminal  120 . At block  1004 , the method  1000  further includes receiving the optical signal  105  at an intermediate unit  151  between the first trunk terminal  110  and the second trunk terminal  120 . The intermediate unit  151  is connected to the communication trunk  102  between the first and second trunk terminals  110 ,  120 . The intermediate unit  151  is powered by a power source  160  located at or near a surface of the body of water for easy access to the power source when the power source  170  requires maintenance. At block  1006 , the method  1000  further includes amplifying the received optical signal  105 , received from the communication trunk  102   c  at the intermediate unit  151  and at block  1008 , sending the amplified optical signal  105  from the intermediate unit  151  to the second trunk terminal  120 . 
     In some implementations, amplifying the received optical signal  105  received from the communication trunk  102   c  includes combining an optical output  105  from a laser  170  via the pump fiber  102   p  of the intermediate unit  151  with the received optical signal  105 . The method  1000  may further include, at the intermediate unit  151  (e.g., using an optical communication repeater), converting the received optical signal  105  to an electrical signal, processing the electrical signal and converting the electrical signal to the amplified optical signal  105 . In some examples, the intermediate unit may include a laser  170  powered by the power source  160  and emitting an optical output  105  through a power pump  102   p , and an optical combiner  150  in communication with the laser  170  and the first and second trunk terminals  110 ,  120 . The optical combiner  150  may include receiving the optical signal  105  from the first trunk terminal  110  and the optical output  105  from the laser  170 , amplifying the optical signal  105  by combining the optical signal  105  from the first trunk terminal  110  and the optical output  105  from the laser  170  and outputting the amplified signal  105  to the second trunk terminal  120 . The power source  160  may include one of a wave power source, a solar power source, a thermoelectric power source, or a fuel power source. 
     In some examples, the method  1000  includes filtering, at a branching unit  140  in communication with the intermediate unit  151 , a first band of wavelengths λ E  of a communication spectrum for a first communication segment and filtering, at the branching unit  140 , a second band of wavelengths λ A  of the communication spectrum for a second communication segment. The second band of wavelengths overlap the first band of wavelengths λ E  in an overlap band of wavelengths with no guard band between the first band λ E  and the second band λ A . The overlap band has a variable size. The first band λ E  of wavelengths may include a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths λ A  includes a remaining fraction of the overlap band of wavelengths for the second communication segment. The overlap band of wavelengths may include common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths. 
       FIG. 11  is a schematic view of an exemplary arrangement of operations for a method  1100  of optical communication that includes, at block  1102 , receiving an optical signal  105  at an intermediate unit  151  connected to a communication trunk  102   c  disposed along a floor of a body of water, the intermediate unit  151  coupling a first trunk terminal  110  to a second trunk terminal  120 . At block  1104 , the method  1100  further includes amplifying the received optical signal  105  at the intermediate unit  151  by injecting light (e.g., optical signal from a laser  170 ) into the communication trunk  102   c . The intermediate unit  151  includes an optical combiner  150  connected to the communication trunk  102   c  and a laser  170  in optical communication with the optical combiner  150  and sends an optical output  105  to the optical combiner  150 . At block  1106 , the method  1100  further includes powering the laser  170  using a power source  160  in communication with the intermediate unit  151  and is located at or near a surface of the body of water. 
     In some examples, amplifying the received optical signal  105  from the communication trunk  102   c  includes combining an optical output  105  from the laser  170  of the intermediate unit  151  with the received optical signal  105  from the communication trunk  102   c . The laser  170  may be located at or near the power source  160  or at or near the optical combiner. The power source  160  may include a wave power source, a solar power source, a thermoelectric power source, or a fuel power source. 
     In some implementations, the method  1100  includes filtering, at a branching unit  140  in communication with the intermediate unit  151 , a first band of wavelengths λ E  of a communication spectrum for a first communication segment and filtering, at the branching unit  140 , a second band of wavelengths λ A  of the communication spectrum for a second communication segment. The second band of wavelengths λ A  overlap the first band of wavelengths λ E  in an overlap band of wavelengths with no guard band between the first band λ E  and the second band λ A , the overlap band having a variable size. The first band of wavelengths λ E  includes a first fraction of the overlap band of wavelengths for the first communication segment and the second band of wavelengths includes a remaining fraction of the overlap band of wavelengths for the second communication segment. The overlap band of wavelengths includes common wavelengths between a spectral edge of the first band of wavelengths and a spectral edge of the second band of wavelengths. The optical combiner may include a Raman Amplifier or a high power Erbium Doped Fiber Amplifier. 
     Various implementations of the systems and techniques described here can be realized in digital electronic and/or optical circuitry, integrated circuitry, specially designed ASICs (application specific integrated circuits), computer hardware, firmware, software, and/or combinations thereof. These various implementations can include implementation in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device. 
     These computer programs (also known as programs, software, software applications or code) include machine instructions for a programmable processor, and can be implemented in a high-level procedural and/or object-oriented programming language, and/or in assembly/machine language. As used herein, the terms “machine-readable medium” and “computer-readable medium” refer to any computer program product, non-transitory computer readable medium, apparatus and/or device (e.g., magnetic discs, optical disks, memory, Programmable Logic Devices (PLDs)) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions as a machine-readable signal. The term “machine-readable signal” refers to any signal used to provide machine instructions and/or data to a programmable processor. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, while the concepts disclosed herein are illustrated for submarine networks where the Branching Unit with OADM is not easily accessible and replaceable, this disclosure is applicable to non-subsea (i.e., terrestrial) networks as well. Moreover, the concept of flexible add/drop by using an overlap band Bo is extensible to dimensions other than spectrum sharing. Any other dimensions that have inherent orthogonality can be used for the flexible add/drop using an overlap band Bo, such as time division multiplexing, space division multiplexing using multi core fibers or many mode fibers, polarization division multiplexing. Accordingly, other implementations are within the scope of the following claims.