Abstract:
The systems and methods described herein provide a reconfigurable, long-range, optical modem-based underwater communication network. In particular, the network provides a low power, low cost, and easy to deploy underwater optical communication system capable of being operated at long distances. Optical modem-based communication offer high data rate, omni-directional spatial communication in the visual spectrum. The omni-directional aspect of communication is advantageous because precise alignment of communication units may not be required. The optical modems may be deployed via unmanned underwater vehicles (UUVs) and physically connected by tethers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/430,081 filed Jan. 5, 2011, entitled “Systems and methods for establishing an underwater optical communication network,” hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     There is an increasing demand for long-range communication systems to connect seafloor observatories spread over large portions of the earth-ocean system. Ocean observatories are deployed at the seafloor and connected by cable or buoy to surface networks. These seafloor observatories may be located many hundreds of kilometers off the coast of the nearest shore station and may be positioned at depths of over 5000 meters. Typically, the observatory has one or more network connections to shore stations through which data collected from the seafloor observatory may be passed onto the Internet and which may serve seafloor instruments with power and control commands. However, these are dedicated connections and do not offer any options for reconfigurable long-range connection of a seafloor observatory to another seafloor observatory, or another shore station. 
     Accordingly, there is a need for an easy to deploy underwater communication system that is readily reconfigurable. 
     SUMMARY 
     The systems and methods described herein provide a reconfigurable, long-range, optical modem-based underwater communication network. In particular, the network provides a low power, low cost, and easy to deploy underwater optical communication system capable of being operated at long distances. Optical modem-based communication offers high data rate, and omni-directional spatial communication in the visual spectrum. The omni-directional aspect of communication is advantageous because precise alignment of communication units may not be required. The optical modems may be deployed by unmanned underwater vehicles (UUVs) and physically connected by a tether (e.g., a light-weight fiber optic cable). 
     In one aspect, the systems and methods described herein provide for establishing an underwater optical communication link between two underwater cabled observatories that are located in two different locations. A first optical modem may be mechanically coupled by a tether to a second optical modem. The first optical modem may be disposed within an optical range of a first cabled observatory, and the second optical modem may be disposed within an optical range of a second cabled observatory. An underwater optical connection between the first cabled observatory and the second cabled observatory may be established through the first optical modem, the tether and the second optical modem. Each optical modem may communicate with the cabled observatory by diffusing electromagnetic radiation of wavelength in the optical spectrum between about 300 nm to about 800 nm. 
     In some embodiments, at least one of the first and second optical modems is disposed using an underwater vehicle. 
     In some embodiments, the optical modem communicates with a cabled observatory using a communication protocol, e.g., time division multiple access (TDMA), code division multiple axis (CDMA), space division multiple access (SDMA), frequency division multiple access (FDMA) or any other suitable communication protocol. 
     In some embodiments, methods are provided for detecting a fault in the underwater optical connection and determining if the fault occurs at one or more of the cabled observatories, the optical modems, and connecting tethers. 
     In some embodiments, a detected fault in the underwater optical connection is corrected by disposing a plurality of optical modems in place of a faulty node. For example, an underwater optical connection may be initially established between a first and a second cabled observatory via a first optical modem (disposed near the first observatory) physically connected by a tether to a second optical modem (disposed near the second observatory). If a fault is detected in the tether, the fault may be corrected by disposing a plurality of optical modems between the first optical modem and the second optical modem. If the fault is detected in the first optical modem, the fault may corrected by disposing a plurality of optical modems between the first cabled observatory and the second optical modem. If the fault is detected in the second optical modem, the fault may be corrected by disposing a plurality of optical modems between the first optical modem and the second cabled observatory. The disposed plurality of optical modems are placed within their respective optical ranges such that they form a continuous optical communication link between both ends of the plurality. 
     In some embodiments, each optical modem communicates on at least two different communication channels. For example, an optical modem may communicate with a first optical modem on a first communication channel, and communicate with a second optical modem on a second communication channel. 
     In some embodiments, a detected fault in the underwater optical connection is corrected by disposing a third optical modem within a communication range of the first optical modem, and disposing a fourth optical modem within a communication range of the second optical modem, where the third and fourth optical modems may be mechanically coupled by a tether. 
     In some embodiments, a detected fault in the underwater optical connection is corrected by disposing a third optical modem within a communication range of the first cabled observatory, disposing a fourth optical modem within a communication range of the second cabled observatory and retrieving the first and second optical modems that were part of the detected fault in the underwater optical connection. For example, if the fault were detected in a tether, a first optical modem connected to the tether, a second optical modem connected to the tether, and the tether itself may be retrieved. 
     In some embodiments, establishing the optical data connection includes determining an optical range of the first cabled observatory, and determining an optical range of the second cabled observatory. 
     In some embodiments, the optical communication network is extended by disposing a third optical modem within an optical range of the second cabled observatory, and disposing a fourth optical modem within an optical range of a third cabled observatory that has a location different from the first and second observatories. 
     In some embodiments, the optical range of an optical modem is at least about 100 meters. 
     In one aspect, the systems and methods described herein provide for an underwater vehicle to establish an underwater optical communication link between a first cabled observatory and a second cabled observatory. The underwater vehicle may include two optical modems, mechanically coupled by a tether. Each optical modem may include a transmitter having at least one optical source capable of emitting electromagnetic radiation of wavelength in the optical spectrum between about 300 nm to about 800 nm, and a diffuser capable of diffusing the electromagnetic radiation and disposed in a position surrounding a portion of the at least one source for diffusing the electromagnetic radiation in a plurality of directions. 
     In some embodiments, the tether includes a fiber optic cable, copper cable, or any other suitable type of cable. 
     In some embodiments, each optical modem includes at least two optical sources. A first optical source may be configured to emit electromagnetic radiation at a wavelength different from a second optical source. 
     In some embodiments, the underwater vehicle is configured to dispose an optical modem from an underside portion of the vehicle, and retrieve a disposed optical modem into the underside portion of the vehicle. 
     In some embodiments, an underwater optical modem is integrated into the underwater vehicle. 
     In one aspect, the systems and methods described herein provide for a system to establish an underwater optical communication link between two cabled observatories. The system may include a first underwater vehicle including an integrated first optical modem, a second underwater vehicle including an integrated second optical modem, and a tether mechanically coupled to the first and second optical modems of each underwater vehicle. Each optical modem may include a transmitter having at least one optical source capable of emitting electromagnetic radiation of wavelength in the optical spectrum between about 300 nm to about 800 nm, and a diffuser capable of diffusing the electromagnetic radiation and disposed in a position surrounding a portion of the at least one source for diffusing the electromagnetic radiation in multiple directions. 
     In some embodiments, the first underwater vehicle is configured to dispose a third optical modem from an underside portion of the underwater vehicle, and retrieve the disposed third optical modem into the underside portion of the first underwater vehicle. The third optical modem may be mechanically coupled to an integrated first optical modem of the first underwater vehicle. 
     In some embodiments, the second underwater vehicle is configured to dispose a fourth optical modem from an underside portion of the underwater vehicle, and retrieve the disposed fourth optical modem into the underside portion of the second underwater vehicle. The fourth optical modem may be mechanically coupled to an integrated second optical modem of the second underwater vehicle. 
     In some embodiments, an optical modem is disposed on the forward surface of the underwater vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following figures depict certain illustrative embodiments of the systems and methods described herein in which like reference numerals refer to like elements. These depicted embodiments may not be drawn to scale and are to be understood as illustrative and not as limiting in any way. 
         FIG. 1  illustrates an arrangement of first and second cabled observatories, and an underwater vehicle in the process of establishing an underwater optical communication link in accordance with some illustrative embodiments; 
         FIGS. 2A-2B  illustrate an underwater vehicle disposing a first optical modem near a first cabled observatory in accordance with some illustrative embodiments; 
         FIG. 3  illustrates an underwater vehicle disposing a second optical modem near a second cabled observatory in accordance with some illustrative embodiments; 
         FIG. 4  illustrates an underwater vehicle including an integrated optical modem establishing an optical connection with a second cabled observatory in accordance with some illustrative embodiments; 
         FIG. 5  illustrates an example of an underwater optical modem in accordance with some illustrative embodiments; 
         FIG. 6  illustrates optical ranges of optical modems in accordance with some illustrative embodiments; 
         FIGS. 7A-7C  illustrates a fault in an optical communication link between first and second cabled observatories, and methods for re-establishing the optical communication link in accordance with some illustrative embodiments; 
         FIGS. 8A-8E  illustrate detection of a fault in an underwater optical communication network and methods for re-establishing an underwater optical communication link within in the network in accordance with some illustrative embodiments; 
         FIG. 9  illustrates an underwater optical communication network made up of a plurality of underwater optical modems and underwater vehicles in accordance with some illustrative embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The systems and methods described herein will be described with reference to certain illustrative embodiments. However, the systems and methods described herein is not to be limited to these illustrated embodiments which are provided merely for the purpose of describing the systems and methods and are not to be understood as limiting in anyway. 
     In many aspects, the systems and methods described herein include an optical modem-based underwater communication network for connecting a plurality of cabled ocean observatories having optical modems. The observatories may be deployed at certain locations underwater on the seafloor. An unmanned underwater vehicle (UUV) or similar underwater vehicle may navigate to a first observatory, determine an optical range of the first observatory&#39;s optical modem, and deploy a first optical modem within the optical range. It may be difficult to deploy the optical modem accurately within the optical range from the water surface. The UUV is better suited to navigate to the observatory and deploy the optical modem accurately within the observatory&#39;s optical range. The optical modem may be shaped or weighted such that when deployed it lands in a particular orientation. The optical modem may be tethered to the UUV via a tethering system, e.g., a micro-tethering system. The UUV may pay out a tether with the aid of the micro-tethering system as it navigates towards a second observatory. The micro-tethering system uses lightweight fiber optic cable which may allow the UUV to pay out a tether over a distance of greater than 10 km, or even 100 km. 
     On arriving at the second observatory, the UUV may determine an optical range of the second observatory&#39;s optical modem and deploy a second optical modem. The second optical modem may be connected via a tether to the first optical modem deployed at the first observatory. As such, the UUV may establish a communication link between the first and second observatories through the first and second optical modems connected via the tether. In some embodiments, the UUV is adapted to retrieve the first and second optical modems, and redeploy them as necessary at any of the plurality of observatories. Consequently, the UUV may establish a reconfigurable communication link between the plurality of observatories as required. In some embodiments, the UUV includes the second optical modem integrated on-board and itself lands within the optical range of the second observatory to establish a communication link. The UUV and its integrated optical modem may move to another observatory as required, and provide a reconfigurable communication link between various observatories in this manner. 
       FIG. 1  illustrates an arrangement of first and second cabled observatories, and an underwater vehicle in the process of establishing an underwater optical communication link in accordance with some illustrative embodiments. Illustrated in  FIG. 1  are a first cabled observatory  110 , a first buoy  115 , a second cabled observatory  120 , a second buoy  125 , and a UUV  140 . The UUV  140  includes an integrated optical modem  138  and a deployable optical modem  130 . The optical modem  130  is mechanically coupled to the integrated optical modem  138  via tether  135 . 
     The first and second cabled ocean observatories may be submerged under a water body at a desired depth, resting on a sea floor or suspended in the body of water. As referred to herein, the terms “cabled ocean observatory” and “cabled observatory” may be used interchangeably. The cabled ocean observatory may be designed around either a surface buoy or a submarine fiber optic/power cable connecting one or more seafloor nodes. In some embodiments, an underwater observatory maybe a stand-alone unit that is not connected to another communication unit by a tether or a cable. The stand-alone underwater observatory may include an independent power source such as a battery to operate independently. As referred to herein, the term “seafloor node” may refer to an underwater communication unit that includes an optical modem or any other suitable communication device. The observatory may also include sensors and optical imaging systems to measure and record ocean phenomena. A cabled observatory may be connected to a surface buoy, one or more seafloor nodes by a cable, a surface ship, or a station on land. The cabled observatory  110  is connected to the surface buoy  115  by a cable  116 , and second cabled observatory  120  is connected to a second surface buoy  125  by a second cable  126 . In some embodiments, the cable includes a tether as described in further detail below. The cabled observatory may include an optical modem, which will be described in further detail below in reference to  FIG. 5 . The cabled observatory  110  includes an integrated optical modem  118 , and the second cabled observatory  120  includes an integrated optical modem  128 . As illustrated in  FIG. 1 , in some embodiments, the optical modem is oriented with a hemispherical diffuser downwards. It should be understood that in some embodiments, the optical modem may be oriented upwards, sideways, or any other suitable direction. 
     An optical connection may be formed between the two cabled observatories by UUV  140 . As illustrated, the UUV  140  includes an integrated optical modem  138 . The UUV  208  may determine a first optical range of the integrated optical modem at the first observatory. For example, the UUV may determine the first optical range by navigating toward the first cabled observatory until information is successfully exchanged between the integrated optical modem  138  of the UUV and the integrated optical modem  118  of the first cabled observatory, via a modulated optical signal. After determining the first optical range, the UUV may dispose a first optical modem  130  within the determined first optical range. The disposed optical modem  130  may be mechanically coupled to the integrated optical modem  138  by a tether  135 . As referred to herein, the term “mechanically coupled” may be defined as a connection enabled by any number of connectors or cables. For example, a first end of the tether may be physically connected to the optical modem  130 , and a second end of the tether may be connected to the integrated optical modem  138  through a bulkhead connector within the UUV  140 . The underside  136  of UUV  140  may include a compartment that may store optical modem  130  during transit before disposal. 
     After disposing the optical modem  130  within a first optical range of the first cabled observatory, UUV  140  may navigate toward second cabled observatory  120  until a second optical range of the second observatory is determined. Upon determining the second optical range, the UUV  140  may maintain a position within the second optical range, and complete an optical communication link between the first cabled observatory and the second cabled observatory. In some embodiments, UUV  140  disposes a second optical modem within an optical range of a second cabled observatory  120 , further described below in reference to  FIG. 3 . 
       FIGS. 2A-2B  illustrate an underwater vehicle  208  disposing a first optical modem  210  near a first cabled observatory  202  in accordance with some illustrative embodiments. In  FIG. 2A , cabled ocean observatory  202  is submerged under a water body at a desired depth. Cable  206  extends from observatory  202  towards the surface buoy or one or more seafloor nodes. Observatory  202  includes optical modem  204  for underwater optical communication, having an optical range  214 . UUV  208  passes by observatory  202  and disposes optical modem  210  within the optical range of optical modem  204 . Optical modem  210  is tethered to UUV  208  via tether  212 , which may include a fiber optic cable. The tethering system may be a micro-tethering system of the type described in commonly-owned U.S. Pat. No. 7,621,229, entitled “Systems and Methods for Tethering Underwater Vehicles”. Once optical modem  210  is in range of optical modem  204 , a communication link may be established automatically. As such, UUV  208  establishes a connection with observatory  202  via optical modems  204  and  210 . 
     In  FIG. 2B , UUV  208  begins to move away from observatory  202  and pays out tether  212  such that the optical modem  210  and UUV  208  remain tethered. For example, UUV  208  may receive instructions to navigate from observatory  202  to a second observatory (not shown). The instructions may be pre-programmed into UUV  208 , or may be received by remote control. For example instructions may be received from a cable connecting UUV  208  to a surface ship (not shown), or may be received from optical communication with observatory  202 . 
       FIG. 3  illustrates underwater vehicle  208  disposing a second optical modem  310  near a second cabled observatory  302  in accordance with some illustrative embodiments. In  FIG. 3 , UUV  208  arrives at another cabled ocean observatory  302 , which includes an optical modem  304  having an optical range  314 . UUV  208  deploys another optical modem  310  within the optical range of optical modem  304 , allowing optical modem  310  to be in communication with observatory  302 . Since optical modem  310  is attached to the other end of tether  212 , it is also in communication with optical modem  210  from  FIGS. 2A and 2B . As such, an optical modem-based underwater communication network is established between cabled ocean observatories  202  and  302  through optical modems  210  and  310  connected via tether  212 . The optical modems may have batteries making them self-powered. The optical modems may conserve energy by using a modest duty cycle and low power tether. 
     In some embodiments, UUV  208  may include an undercarriage portion  207  that may hold one or more underwater optical modems in transit before deployment or after retrieval. In some implementations, the undercarriage portion may include a mechanical arm, not shown, used to dispose an optical modem on the sea floor. In some implementations, the UUV may include magnets used to dispose or retrieve the optical modem. For example, the mechanical arm may include electromagnets that may be used to hold an underwater optical modem during retrieval or disposal from the undercarriage portion. 
     In addition to providing low power communication (e.g., transmission power of about 400 mW for a range about 100 meters at a data rate of about 10 Mbit/sec, or transmission power of about 25 mW for a range about 100 meters at a data rate of about 1 Mbit/sec), fiber optic cable also may be advantageous over, for example, copper cable, since signals travel along them with minimal loss and are immune to electromagnetic interference. Furthermore, fiber optic cable may be easier to deploy underwater due to lighter weight. For example, commonly-owned U.S. Pat. No. 7,621,229, entitled “Systems and Methods for Tethering Underwater Vehicles”, describes an optical fiber having a cross-section diameter of about 250 microns and the weight of about 11 km of the optical fiber in water is only about 173 g. However, the fiber optic cable may require protective covering capable of guarding against corrosion and damage from, e.g., fish bites. The fiber optic cable may be surrounded by a protective jacket layer made of, for example, plastic, or any other suitable material. Fiber optic cable  212  may extend for distances greater than 50 km, 100 km, or 200 km. Tether  212  may include a bundle of a plurality fiber optic wires. 
       FIG. 4  illustrates an underwater vehicle  408  including an integrated optical modem  410  establishing an optical connection with a second cabled observatory in accordance with some illustrative embodiments.  FIG. 4  depicts an alternative embodiment of the UUV and attached optical modem described in  FIG. 3 . In  FIG. 4 , UUV  408  arrives at cabled ocean observatory  302 , with optical modem  304  having optical range  314 . UUV  408  is different from UUV  308  because instead of deploying an optical modem, UUV  408  includes an optical modem  410  integrated into the body of the UUV. UUV  408  lands within the optical range of observatory  302 , and established a communication link with optical modem  304  of observatory  302 . Similar to the embodiment depicted in  FIG. 3 , UUV  408  has a tether  212  attached to optical modem  310  at the other end of the fiber optic cable. As such, an optical modem-based underwater communication network is established between cabled ocean observatories  202  and  302  through optical modems  210  and  310  connected via tether cable  212 . 
     The depicted embodiment of UUV  408  may allow for easy reconfiguration of the optical modem-based underwater communication network. For example, UUV  408  may alternately deploy optical modem  410  between two or more cabled observatories. Since optical modem  410  is integrated in UUV  408 , there is no issue of retrieving the optical modem  410  after deployment. In some embodiments, optical modem  310  is retractable into the body of UUV  408  for easy redeployment of both ends of the optical modem-based communication link. The depicted embodiment of UUV  408  may be advantageous in circumstances where the optical modem needs to be oriented vertically or at a certain angle, as opposed to being deployed horizontally on the seafloor. 
       FIG. 5  illustrates an underwater optical modem in accordance with some illustrative embodiments. In  FIG. 5 , a close-up view of the optical modem is shown. Optical modem  500  includes a plurality of sources  504  disposed within a diffuser  502 , which is of hemispherical shape having an inner curved surface  506  and an outer curved surface  508 . Sources  504  may be disposed within hemispherical diffuser  502  such that they are located inside inner curved surface  506 . In some embodiments, not shown, sources  504  are disposed in hemispherical diffuser  502  such that they are protruding from inner curved surface  506  towards outer curved surface  508 . In one embodiment, source  504  includes light emitting diodes and diffuser  502  includes a lightly scattered TiO2/Silicone dome. Each of sources  504  may be individually controllable such that one or more sources  504  is used to emit electromagnetic radiation. In certain implementations, the emitted electromagnetic radiation is directed along one or more directions based at least in part on the operation of one or more sources  504 . Power consumption may be controlled by the operating a specific number of sources  504  as required. 
     Because optical modem  500  may diffuse electromagnetic radiation in multiple directions, to avoid cross-talk and collision during communications, the optical modems may communicate using protocols such as time division multiple access (TDMA), code division multiple axis (CDMA), space division multiple access (SDMA) and frequency division multiple access (FDMA), or any other suitable protocol. Each optical modem may also be capable of communicating on a plurality of different optical communication channels. As referred to herein, the term “optical communication channel” may be defined as a different carrier wavelength of light. For example, a first optical modem and second optical modem may communicate using a first wavelength. The second optical modem and a third optical modem may communicate using a second wavelength different form the first wavelength, to avoid cross-talk of communications between the first and second optical modems and the second and third optical modems. 
     In  FIG. 6 , optical ranges  604  and  608  are illustrated for optical modems  602  (similar to optical modem  410 ) and  606  (similar to optical modem  210 ). The optical range of each optical modem may be greater than 100 m. Optical modems are advantageous when compared with, e.g., inductively-coupled modems, since no physical contact is required to establish a communication link. As soon as an optical modem is in range of another optical modem, a communication link may be established automatically. Additionally, inductively coupled modems may suffer from signal attenuation in a conductive medium such as salt water. Radio frequency modems may also suffer from signal attenuation in conductive media. However, optical modems do not suffer from such drawbacks. Examples of suitable underwater optical modems may be found in commonly-owned U.S. patent application Ser. No. 11/348,726, entitled “Systems And Methods For Underwater Optical Communication” which is hereby incorporated by reference herein in its entirety. 
     In some embodiments, an underwater optical modem-based communication network may be established using un-tethered optical modems. A UUV may deploy a sequence of optical modems between a first cabled observatory and a second cabled observatory. The first optical modem may be deployed such that it is in optical range of the first cabled observatory. The second optical modem may be deployed such that it is communication with the first optical modem, and so on and so forth. The last optical modem may be deployed such that it is in communication with the second seafloor observatory, as well as the previously deployed optical modem. As such, an optical communication link may be established between the first and the second seafloor observatory via multiple hops of optical modems in communication with each other. Since the range of an optical modem is limited, such an optical communication link may be preferable for short-range applications. The network may be readily reconfigurable if the UUV is adapted to retrieve the deployed optical modems and redeploy them as necessary. In some embodiments, the optical modems may be deployed to establish communication between three or more seafloor observatories. For example, if three seafloor observatories were placed in triangular formation, a single optical modem deployed at the center of the triangle may be sufficient for establishing a communication link between any two of the seafloor observatories. 
     Although, described with reference to optical modems  210 ,  310 , and  410 , it should be noted that the underwater optical modem-based communication system described in  FIGS. 1-6  may be used in conjunction with any and all types of sensors, underwater vehicles such as UUVs, remotely operated vehicles (ROVs), and autonomous underwater vehicles (AUVs), cabled or tethered observatories, devices buried under the sea bed, tools and logging devices used underwater and under the surface of the seabed such as ROTs. 
       FIGS. 7A-7C  illustrate an exemplary application of a reconfigurable underwater optical communication network. In particular  FIGS. 7A-7C  illustrate detection of a fault in an optical communication link between first and second cabled observatories, and methods for re-establishing the optical communication link in accordance with some illustrative embodiments. Shown in  FIG. 7A  is an example of an underwater optical communication network including a first cabled observatory  710 , second cabled observatory  720 , first optical modem  730 , second optical modem  740  and a tether  750  mechanically coupled to the first and second optical modems. In  FIG. 7B , a fault  754  has been detected in the optical network. As referred to herein, the term “fault” may refer to a malfunctioning component of the optical communications network. The fault may occur at a tether, optical modem, or any other suitable component of the optical communication network. For example, the fault is a severed tether  752 . 
     In some embodiments, the fault is repaired and the optical communication link is re-established by disposal of optical modems near a first optical modem proximate to the fault and second optimal modem proximate to the fault. As illustrated in  FIG. 7B , first optical modem  730  and second optical modem  740  are proximate to fault  754 . In some implementations, to re-establish the optical communication link between cabled observatories  710  and  720 , a UUV  770  may deploy a third optical modem  760  within an optical range of optical modem  730  and deploy a fourth optical modem  768  within an optical range of optical modem  740 . The third and fourth optical modems may be mechanically coupled by a tether  765 . Third and fourth optical modems may be configured to land on the seafloor with the diffuser and light sources pointed toward optical modems  730  and  740 , respectively. UUV  770  may include a mechanical arm  773 , as described above in reference to  FIG. 3 . 
     As illustrated in  FIG. 7C , in some implementations, one of the disposed optical modems is a deployable unit  785 , and another of the disposed optical modems is an optical modem integrated into a UUV  780 . The deployable optical modem  785  and integrate optical modem of UUV  780  may be mechanically coupled by tether  788 . UUV  780  may navigate to optical modem  730  and dispose optical modem  785  within an optical range of optical modem  730 . UUV  780  may then navigate to and establish an optical connection with optical modem  740 , thereby re-establishing a connection between optical modems  730  and  740 , and cabled observatories  710  and  720 . 
       FIGS. 8A-8E  illustrate detection of a fault in an underwater optical communication network and methods for re-establishing an underwater optical communication link within in the network in accordance with some illustrative embodiments. As illustrated in  FIG. 8A , an underwater optical communications network may include a first cabled observatory  802 , a second cabled observatory  804 , and a plurality of optical modems  810 ,  820 ,  830  and  840 . Optical modem  810  is mechanically coupled to optical modem  820  via tether  815 , and optical modem  830  is mechanically coupled to optical modem  840  via tether  835 . It should be understood that an optical modem may be a stand-alone unit or integrated as part of a UUV as described in reference to  FIGS. 2A-B ,  3  and  4 . 
       FIG. 8B  illustrates a fault  817  that has occurred in the optical communication network at tether  815 . A first optical modem  810  and second optical modem  820  may be determined to be proximate to the fault by network analysis. For example, each optical modem may have an associated unique network address. To determine a fault in the network, each optical modem in the network may be polled for activity and connectivity to other optical modems by one or more network controllers, which may be located within the cabled observatories, surface buoy, or any other suitable communication unit in the optical communication network. For example, a network controller may store a mapping of the optical modems in the network. The network controller may determine that there is a fault in the network at an optical modem if controller stops receiving verification of activity from a particular optical modem. For example, the network may ping or transmit a short message addressed to each optical modem and poll for an acknowledgement or response from the addressed optical modem. If there is no response from the optical modem, the network may determine that there is a fault at the optical modem. To determine first and second optical modems proximate to the fault, a network controller may retrieve the stored mapping of the optical modems in the network, and determine which optical modems of the network last communicated directly with the faulty modem. 
     In some implementations, the fault may be in a tether. The network controller may determine that the tether is faulty by determining that a communication path that included the tether is no longer operating. The network controller may determine a first and second optical modem proximate to the faulty tether by examining a stored network mapping and determining which optical modems last communicated across a communication path using the tether. 
       FIG. 8C  illustrates a method for re-establishing an optical connection in the network. Either of the first or second optical modems proximate to the fault, or both, may be moved, such that the first and second optical modems are still within an optical range of each other, and still within an optical communication of other optical modems in the network to re-establish the optical communication link between the first and second cabled observatories  802  and  804 . For example, optical modem  810  may be moved such that it is within an optical range of cabled observatory  802  and optical modem  820 . 
     In some embodiments, the optical modems in the communication network may be spaced too far apart for the method of  FIG. 8C  to be effective for re-establishing an optical communication link. For example, the distance between cabled observatory  802  and optical modem  820  may be greater than the optical ranges of the observatories  810  and  820 . In such scenarios, it may be possible to re-establish the optical communication link by disposing optical modems  850  and  860 , mechanically coupled to each other by a mechanical tether, between optical modems  810  and  820 . Optical modem  850  may be disposed within an optical range of optical modem  810 , and optical modem  860  may be disposed within an optical range of optical modem  820 . 
     In some implementations, optical modems may be deployed to replace the optical modems proximate to the fault. For example, the optical modem  850  may be disposed within an optical range of cabled observatory  802 , and the optical modem  860  may be disposed within an optical range of optical modem  830 . Optical modems  810  and  820  may then be retrieved for repair. For example, if optical modems  810  and  820  are integrated into a UUV, the UUV may be returned to the surface. If optical modems  810  and  820  are standalone units, they may be retrieved by a UUV (not shown), and returned to the surface of the body of water for repair. 
     In some implementations, a plurality of optical modems may be deployed between the optical modems  810  and  820 , proximate to the fault  817 . The plurality of deployed optical modems may not be mechanically coupled by any tether but instead operate in a hopping optical communication link. As illustrated in  FIG. 8E , optical modems  850  and  860  may be deployed between optical modem  810  and  820 . At least one of the plurality of optical modems may be within an optical range of a first optical modem proximate to the fault, e.g.  810 , and at least one of the plurality of optical modems may be within an optical range of a second optical modem proximate to the fault, e.g.  820 . Each of the disposed optical modems may be within an optical range of at least one other disposed optical modem to maintain the hopping network. As described above in reference to  FIG. 5 , each optical modem may diffuse electromagnetic radiation in a plurality of directions. To avoid cross-talk among the plurality of modems, different collision avoidance protocols may be used, including TDMA, CDMA, FDMA, SDMA or any other suitable protocol as described above. In addition, each modem may communicate on a plurality of optical channels, such as a different wavelength of electromagnetic radiation. 
       FIG. 9  illustrates an underwater optical communication network made up of a plurality of underwater observatories, optical modems and vehicles in accordance with some embodiments of the present systems and methods described herein. The underwater optical communication network may be established using the methods and systems described herein. Illustrated in  FIG. 9  are a plurality of underwater observatories ( 910 ,  920 ,  930 ,  940 ,  950  and  960 ), a plurality of stand-alone underwater optical modems ( 913 ,  914 ,  932 , 934 ,  974 , and  972 ), and a plurality of underwater vehicles ( 936 ,  970 ,  980 ,  992 ,  994 ). Also illustrated in  FIG. 9  are various tethers ( 917 ,  933 ,  935 ,  973 ,  983 , and  993 ), that mechanically couple various optical modems. Cables ( 905 ,  915 ,  925 , and  926 ) are illustrated that may connect underwater observatories to surface buoys, underwater observatories to other underwater observatories, or an underwater vehicle to a surface ship  900 . 
     Illustrated within  FIG. 9  are various configurations of underwater observatories. In a first configuration, a cabled underwater observatory  910  may be connected via cable  915  to a surface buoy  912 , which may reside at the surface of the water. In a second configuration, a cabled underwater observatory  920  may be connected via cable  925  to a surface buoy  912 , which may reside at the surface of the water. Cabled observatory  920  may be connected via cable  926  to an underwater observatory  930 . In a third embodiment, an underwater observatory may be a stand-alone unit, as illustrated by underwater observatory  940 ,  950  and  960 . 
     An optical communication network may be established between the plurality of underwater observatories. Stand-alone underwater optical modem  913  may be disposed within an optical range of underwater observatory  910 , stand-alone underwater optical modem  914  may be disposed within an optical range of underwater observatory  940 . A tether  917  may mechanically couple underwater optical modem  913  to underwater optical modem  914 . underwater optical modem  913  and underwater optical modem  914  may be deployed using a UUV as described above in reference to  FIG. 3 . 
     The network may be extended to include a plurality of nodes. As referred to herein, the term “node” may be defined as an underwater optical modem that is part of an optical communication network. underwater optical modem  932  may be deployed by a UUV  936  within an optical range of underwater observatory  930 . underwater optical modem  934  may also be deployed by UUV  936  at a location different from underwater optical modem  932  to facilitate connection to other underwater optical communication links. underwater optical modem  934  may be mechanically coupled to underwater optical modem  932  by tether  933  and to UUV  936  by tether  935 . UUV  936  may include an integrated optical modem that enables it to communicate with nodes in the optical communication network. For example, UUV  936  may navigate to a location within an optical range of underwater optical modem  913 , and establish a an optical connection with underwater optical modem  913 , thereby establishing an optical communication link between underwater observatories  910 ,  920 ,  930 , and  940 . 
     Faults in the underwater optical communication network may be repaired by reconfiguring nodes in the network. For example, a fault may be detected in tether  926 , breaking the optical communication link between underwater observatory  920  and underwater observatory  930 . To re-establishing an optical communication link between underwater observatory  920  and underwater observatory  930 , optical modems may be deployed at nodes in the network that are connected to the underwater observatory  920  and underwater observatory  930 . For example, UUV  994  and UUV  992  may each include an integrated optical modem, that may be mechanically coupled to each other by tether  993 . UUV  994  may navigate to and establish an optical connection with underwater observatory  920 , and UUV  992  may navigate to and establish an optical connection with underwater optical modem  934 . An optical communication link may be formed between underwater observatory  930  and underwater observatory  920  through UUV  992  and UUV  994 . In some embodiments, each of UUV  992  and UUV  994  is configured to deploy an optical modem (not shown), that is mechanically coupled by a tether to an integrated optical modem. For example, UUV  992  may be configured to deploy a first optical modem that is mechanically coupled by a tether to an optical modem integrated with UUV  992 , which is also mechanically coupled to the integrated optical modem of UUV  994  by a tether  993 . In some embodiments, the UUV  994  is configured to deploy a second optical modem that is mechanically coupled by a tether to the integrated optical modem of UUV  994 , and also mechanically coupled to the integrated optical modem of UUV  992 , and the first optical modem that is deployable from UUV  992 . 
     In some embodiments, optical connections may be formed to stand-alone underwater observatories. For example, UUV  980  may deploy underwater optical modem  985  within an optical range of underwater optical modem  934 . UUV  980  may include an integrated optical modem and navigate to stand-alone underwater observatory  950 . The integrated optical modem of UUV  980  may be mechanically coupled to underwater observatory  985  by tether  983 . UUV may be connected to a surface ship  900  by a cable  905 . The cable  905  may enable remote control of underwater vehicle  980 . 
     In some embodiments, optical connections may be formed by deploying a set of stand-alone optical modems. For example, UUV  970  may deploy underwater optical modem  974  within an optical range of  985 , and deploy underwater optical modem  972  within an optical range of stand-alone underwater observatory  960 . UUV  970  may deploy the underwater optical modem  972  by using a set of connectors  975 , as described above in reference to  FIG. 3 . Underwater optical modem  972  and underwater optical modem  974  may be connected by tether  973 . 
     As illustrated in  FIG. 9 , a plurality of different nodes may connected in a linear or a non-linear arrangement. As referred to herein, the term “linear arrangement” may refer to a series of optical modems that may be connected in a non-branching chain. For example, the series of underwater optical modems  914 ,  913 ,  936 ,  934  and  932  may be considered a linear arrangement. As referred to herein, the term “non-linear”arrangement may refer to an arrangement of optical modems that include branches. For example, the collection of underwater optical modems  972 ,  974 ,  980 ,  985 ,  934  and  932  may form a branched arrangement that extend from underwater optical modems  934 ,  974  and  985  as a nexus. 
     Those skilled in the art will know or be able to ascertain using no more than routine experimentation, many equivalents to the embodiments and practices described herein. For example, the illustrative embodiments discuss the use of UUVs, but other underwater vehicles such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) may be used with the systems and methods described herein. Accordingly, it will be understood that the systems and methods described are not to be limited to the embodiments disclosed herein, but is to be understood from the following claims, which are to be interpreted as broadly as allowed under the law. 
     Variations, modifications, and other implementations of what is described may be employed without departing from the spirit and scope of the systems and methods described herein. More specifically, any of the method, system and device features described above or incorporated by reference may be combined with any other suitable method, system or device features disclosed herein or incorporated by reference, and is within the scope of the contemplated systems and methods described. The systems and methods may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative, rather than limiting of the systems and methods described. The teachings of all references cited herein are hereby incorporated by reference in their entirety.