Abstract:
An optical coupler is used to transmit optical data across a temporary connection. A housing is configured with a transparent interface window and positioned in front of a receiving optical coupler so that optical signals pass through the transparent interface window for reception by receiving optical coupler. The receiving optical coupler has a capability of accepting alignment errors caused by aligning the housing with a mating housing in an aquatic environment. Alignment is achieved via an alignment mechanism that mechanical positions the mating optical coupling components with sufficient alignment accuracy to permit the optical coupler to receive and transmit signals.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention is assigned to the United States Government. Licensing inquiries may be directed to Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; telephone 619-553-2778; email: T2@spawar.navy.mil. Reference Navy Case No. 100670. 
    
    
     BACKGROUND 
     The present disclosure relates to optical couplers useful for establishing repetitive optical connections, such as often in use on autonomous vehicles. 
     SUMMARY 
     An optical coupler capable of transmitting optical data is configured to include a first and second optical coupler component, each located within a separate housing either of which is capable of transmitting and/or receiving optical data and transmitting the received optical data through an optical cable. The optical coupler has a capability of continued functionality in the presence of an extended range of misalignment between the respective coupler component housings. Alignment of the housings is established by positioning the two housings in proximity to each other such that an integrated mechanical alignment mechanism is engaged thereby ensuring acceptable alignment tolerances for data transmission within prescribed operating parameters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing the effect of angular misalignment of a ball lens coupler assembly. 
         FIG. 2  is a diagram of an optical coupler used to transmit optical signals across a predetermined gap. 
         FIG. 3  is a schematic block diagram showing the data conversion operations using optical couplers of the present invention. 
         FIG. 4  is a diagram depicting one embodiment of a coupler assembly of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides a detailed description of optical data couplers that may be used for transmitting large quantities of digital data in a manner which allows data connections to be made autonomously and efficiently. The connection can be in any of a number of different operating environments, including but not limited to aqueous, such as water or other liquids, open air, air tight or subterranean, such as in the mining industry. For purposes of the following descriptions the primary example will be that of an underwater autonomous vehicle and a docking station, although the advantages of the present invention are equally applicable in numerous other settings as previously noted. 
     The physical communication layer is realized using multiple, bidirectional fiber optic data links. In one non-limiting example, a data coupler is capable of supporting a two independent optical channels for full duplex communication, in order to support protocol handshaking and re-transmit requests. Ideally the design should be readily expandable to more than two channels to support multiple, independent data streams using well-known wavelength multiplexing techniques. 
     The optical data coupler of the present invention is relatively insensitive to translational and angular misalignments in order to permit realistic manufacturing tolerances and to incorporate suitable clearances which are necessary to support automated coupling and uncoupling. The coupler is subjected to fouling which requires the use of imprecise tolerances in order to ensure optical coupling. This requirement is difficult to meet with conventional fiber optic connectors which require precise mechanical alignment in order to function properly. Additionally, it is desirable that the data coupler be tolerant of corrosion and fouling, such as would inevitably occur as a result of use of the device in a body of water such as the ocean, a lake, or river over extended periods of time. 
     Prior art optical data couplers have either required conversion of the optical signal into an electrical format or require that the polished faces (or images of the polished faces) of the individual optical fibers be accurately aligned in a pristine optical coupling environment. Conversion of the optical signal into an electrical format is often impractical for high bandwidth links and is expected to be even more difficult in the future, due to the trend of rapidly increasing bandwidths for fiber optic links. 
     Conventional connector designs requiring polished fiber ends also require that the ends of the individual optical fibers be accurately aligned in a pristine optical coupling environment. Such connector designs generally employ a reservoir of transparent oil and a series of wipers which remove contaminants from the oil in order to maintain a relatively benign environment for the optical coupling to take place. Such implementations are mechanically complex, expensive to purchase, exhibit excessive insertion and extraction forces during mating and de-mating, and is cycle limited as determined by the oil reservoir capacity requiring coupler replacement or reservoir replenishment on a regular basis. 
     A 1:1 magnification ratio is normally employed in an expanded beam fiber optic connector in order to provide minimum optical insertion loss at perfect alignment. The optical data coupler design according to the present disclosure is based upon an expanded-beam fiber optic connector in which the magnification ratio has been intentionally and significantly altered from 1:1. The configuration is such that the optical channel exhibits significantly increased insertion loss when the optics are perfectly aligned. However, with non-unity magnification, the resulting optical system provides a much increased range of alignment tolerances in both translational and angular error. Because the additional optical insertion loss is both constant and known it can be compensated for within an optoelectronic system by external means. 
     The insertion loss can be compensated for within the E/O system by incorporation of a more powerful transmitter, by use of a more sensitive receiver, or both. In practice, it is possible to construct an optical data coupler which is tolerant of several millimeters of translational misalignment and several degrees of angular misalignment and which is capable of transferring optical gigabit Ethernet data, error-free. These transmission rates can be achieved in murky seawater under fouled conditions. 
     Greatly decreased sensitivity to mechanical tolerances provides the data coupler designer freedom to incorporate the large clearances necessary to prevent physical binding due to corrosion and fouling during mating, which will always be present to some degree, especially in a system which is deployed for an extended period of time in water based environment. The teaching of the present invention also eliminates the necessity to manufacture matched highly accurate pairs of coupler components due to the fact that the optics are in effect de-tuned compared with perfect 1:1 imaging, and thus are much less sensitive to tolerances in general. Detuning makes a production ensemble of data couplers far more tolerant to manufacturing tolerances, as well. The transmitter portion of the coupler component&#39;s expanded optical beam provides robustness to any particulate matter that may be suspended in the water that occupies the gap between the two optical coupler components. As long as a portion of the transmitter&#39;s data light stream reaches the receiving fiber optic data link element of the mated optical coupler component the digital signal can be reconstituted. Typical data coupler designs having working safety margins of 100:1 (+20 dB) and even higher have been demonstrated. 
     Advantages of the presently disclosed technique include:
         The optical data coupler design is completely agnostic to the data format it supports (i.e., the coupling is entirely optical in nature with no dependency on signal bandwidth, spectrum, waveforms, or data formatting).   The optical data coupler design has a low sensitivity to particulate contamination and fouling because the information-carrying optical beam is spread over a large cross-sectional area (if the entire beam is not blocked some of the light will get through to the receiver, permitting error-free data recovery).   The optical data coupler design exhibits translational error tolerances on the order of several millimeters and angular error tolerances on the order of several degrees, which permits design of an optical data coupler incorporating predetermined mechanical clearance to accommodate anticipated debris and fouling for the planned operating environment while also permitting fully automated coupling and uncoupling operation.   The optical data coupler design is expandable to multiple independent optical channels by the physical duplication of a single channel (high channel-to-channel isolation is provided by spatially separating the light beams) and/or by incorporating wavelength division multiplexing.   The optical data coupler is relatively inexpensive to manufacture because its design is simple; conventional machining tolerances suffice for its components; and no precision grinding or unit-to-unit matching is necessary to achieve repeatable performance between arbitrary pairs of optical data couplers.       

     The receiver optics of the data coupler component is comprised of a ball lens, that has an effect similar to a simple magnifying glass, and is placed in front of the receiver element optic pigtail. When no optics at all are employed in front of the fiber face, the fraction of the optical flux captured by the receiving fiber is nominally equal to the ratio of the area of the core of the fiber to the area occupied by the collimated beam. This ratio, in general, is a very small number, and thus it corresponds to a very high insertion loss. To remedy this problem a high refractive index ball lens is placed in contact with the face of the receiving fiber such that its effective distance from the principal plane formed by its convex surfaces is less than one focal length from the fiber face (i.e. just like a magnifying glass). In effect the end of the receiving fiber is magnified, thereby causing it to capture proportionally more light from the transmitter beam. This comes at the expense of reduced angular tolerance because the magnified image will be proportionally more sensitive to angle; however, significant fiber face magnification is possible before angle sensitivity becomes a problem, in practice. 
     A ball lens has characteristics similar to those of a thick, low f-number spherical plano-convex lens placed less than one focal length away from the face of the optical fiber. It is possible to control how much magnification the ball lens performs by changing its index of refraction. 
     To measure the angular sensitivity of a given configuration the transmit and receive optics were mounted in a pair of four axis kinematic optical mounts fastened to an optical rail (tilt up-down, tilt right-left, translation in x, translation in y). The transmit optic was a commercial off-the-shelf 37 mm collimator which interfaces with an ordinary fiber-optic connector with a threaded body, having physical contact (FC/PC connector)—it produces an optical beam diameter at the 1550 nm transmitter wavelength which is approximately 6 mm when used with single-mode fiber. The receive optic was fabricated from another FC/PC connector which contained a 62.5 μm core multimode fiber pigtail mounted in an ordinary Type FC/PC bulkhead receptacle. The 2.5 mm diameter ball lens is pressed into the FC/PC receptacles such that the surface of the ball physically contacts the face of the FC/PC connector. The 2.5 mm ID (internal diameter) spring sleeve inside of the FC/PC receptacle holds the ball lens firmly in place. 
       FIG. 1  is a diagram showing the effect of angular misalignment of a ball lens in a collimated beam of light. A commercially available SF-8 (strain-free) ball lens was selected as having a “strong” index of refraction. It appears that up to +/−5° of angular misalignment is possible using the SF-8 ball lens as the receive optic (if translational error is zero because the stage translation was peaked for each angle). Up until a certain angle, in this case approximately 4°, the optical power transmitted through the ball lens remains fairly constant, in this case, with a power loss of −3.8 db to −5 dB. This shows an acceptable tolerance to angular misalignment. 
     In an effort to optimize the working dynamic range and reduce the mechanical footprint of the coupler, a 15.3 mm collimator was substituted for the (much larger) 37 mm transmit collimator. The smaller resulting beam diameter, 2.5 mm, increased the received signal proportionally without imposing a translational tolerance which would be particularly difficult to satisfy. The stronger signal strength means that much more angular and translational error can be tolerated before the signal drops below threshold and the receiver begins to make data errors. The angular limitation of the optical mounts becomes quite evident, as it was still possible to have a strong optical signals at the maximum angular deflection of 6°(−9 dB of increased optical insertion loss). 
       FIG. 2  is a diagram of an optical data coupler  201  using the described techniques. A first component of the optical data coupler receives an optical signal through optical cable  223 , which terminates at connector  225  with transmitting optical coupler element  227 . A transparent interface window plate  231 , which also functions as a pressure window is positioned in front of optical coupler  227 . 
     A second component, mechanically mated to the first component of the optical data coupler includes a transparent interface window plate  232  that is positioned near interface window plate  231 . A gap  237  is permitted to exist between interface window plates  231 ,  232 . On the other side of interface window plate  232  is connector  245 , which includes a receiving coupler element  249 . Receiving coupler element  249  is mounted to a receiving ferrule  251 . A ball lens  255  is located on the proximal portion of ferrule  251  that is closest to interface window plate  232 . Receiving coupler element  249  is connected to optical cable  263 . 
     Interface window plates  231 ,  232  provide protection from possible contamination or distortion attributable to the operating environment and may be removed in cases in which it is not required that the optical coupler  201  be sealed from the outside environment. In the case of a coupler used in an aqueous environment the sealing prevents water ingest. 
     Coupler  201  is configured so that an optical signal received through optical cable  223  is transmitted by optical coupler element  227 , through non-fiber media, is received at optical coupler element  249  and transmitted through optical cable  263 . In being transmitted by optical coupler element  227  and received at optical coupler element  249 , the optical signal passes across interface window plates  231 ,  232  and the gap  237 . 
     While gap  237  is described, coupler  201  can be used so that no gap or virtually no gap exists between window plates  231 ,  232 . Similarly, it is possible to construct coupler  201  without window plates  231 ,  232 , provided that protection from the environment is not required; however, it is contemplated that a change in environment may affect performance. 
     Receiving coupler element  249  is configured to permit angular and lateral misalignment of the first data coupler component with the optical coupler element  227  contained in the second optical data coupler component. In one example configuration, the maximum design misalignment is ±5°. In other configurations, misalignment of up to ±8° may be tolerated, although larger amounts of misalignment up to ±14.5° would still allow functionality but with significant optical power loss. By providing different types of fibers, or using lower refractive index ball lenses, even larger angles of misalignment are permitted. The apparatus can be configured to require alignment within narrower ranges, such as ±4°, ±2° and ±1°, with narrower specifications possible. Lateral misalignment is possible, as determined by the diameter of the beam transmitted by the transmit optical coupler. Ball lens  255  is used to provide good angular misalignment tolerance; however, other lens configurations are possible, provided that the signal can be transmitted to ball lens  255 . It is noted that the combination of lateral and angular misalignment may be cumulative, so a tolerable angular misalignment may be less tolerable with a significant lateral misalignment, and vice-versa. 
     The performance parameters described herein are given by way of non-limiting example. In the example, the performance is taken in an underwater ocean environment. 
       FIG. 3  is a schematic block diagram showing the data conversion operations used in optical data coupler  201 . The upper portion of the figure illustrates a first optical conversion circuit  311 , which includes first Ethernet interface  321  and first optical coupler subassembly  323 . First optical conversion circuit  311  is connected, at first optical coupler subassembly  323 , to first transmitting optical coupler element  337  and received at first optical receiving coupler element  339 , both elements are connected to first optical amplifier  341 . Also depicted is interface window plate  345 . 
     The lower part of  FIG. 3  depicts a similar arrangement, comprising a second optical conversion circuit  351 , which includes second Ethernet interface  361  and second optical coupler subassembly  363 . Second optical conversion circuit  351  is connected, at second optical coupler subassembly  363 , to second transmitting optical coupler element  377  and to second optical receiving coupler element  379  connected to second optical amplifier  381 . Also depicted is interface window plate  385 . Optical coupler element  337  transmits optical signals which are received by optical coupler element  379 . Similarly optical coupler element  377  transmits optical signals which are received by optical coupler element  339 , as described above in connection with optical coupler elements  227  and  249 . 
     The configuration of optical coupler  201  is such that data is converted at Ethernet interfaces  321 ,  361 , transmitted or received via optical coupler elements  337 ,  339 ,  377 ,  379 , through interface window plates  345 ,  385  and gap  391  which separates interface window plates  345 ,  385 . It should be noted that use of the optical amplifiers  341 ,  381  are depicted as representative configurations and could be alternatively incorporated through any feasible combination of dedicated or shared optical amplifiers within an optical coupler subassembly. It is also possible that no optical amplifier would be utilized and the optical data coupler could rely instead upon increased transmitter power output and/or receiver sensitivity in the Ethernet interfaces. 
     In one configuration, light from a laser data transmitter at a wavelength of 1270 nm propagates through a reinforced 9/125 μm single mode optical fiber terminated with an FC/PC fiber optic connector. The wavelength of transmission is selected in this example because 1270 nm is an optical wavelength at which the attenuation of the optical signal in water is relatively low and assumes the coupler is operating in a water environment. The connector attaches to a 11 millimeter focal length collimating telescope inside of the first optical data coupler (Thorlabs Model F220FC-C, produced by Thorlabs, Inc. of Newton, N.J.) where the light is collimated to form a 2.2 millimeter diameter optical beam. The beam of light shines through the first polycarbonate pressure window, through the gap, and through the second pressure window, and into the light collector in the second optical data coupler. A small portion of the incident light (about 2-5%) is captured by a 2.5 millimeter diameter ball lens (Align Optics Model L986991 produced by Align Optics Inc. of Sunrise, Fla.) in the second optical data coupler component and focused onto the core of the receiving fiber, a reinforced 62.5/125u multimode optical fiber terminated with an FC/PC connector. This fiber transfers the captured light to the data amplifier and on to a receiver where the digital data signal is extracted. Only a small area of the windows are used by each coupler pair to transmit and receive the light beam, and therefore, multiple data channels can be accommodated using a single set of pressure windows by simply spacing the channels physically. The light beams can be transmitted in the same direction or in opposite directions so that each optical data coupler component may be simultaneously transmitting and receiving data. 
     The angular misalignment tolerance can be increased significantly by incorporation of a multimode fiber optic receiver pigtail fiber in many applications. Angular misalignment tolerance as high as +/−6° has been demonstrated to be achievable empirically. Seawater in the gap between the transparent pressure windows exhibits high attenuation in the optical C Band (1550 nm nominal wavelength); however, the attenuation losses due to seawater can be minimized by making the gap very thin (&lt;1 mm) or operating at shorter optical wavelengths near 1310 nm (e.g., 1270 nm) where the attenuation of water is much lower.  FIG. 4  is a diagram showing a physical layout of mating an optical data coupler having components  405 ,  407  implementing the techniques that are described above for  FIGS. 2 and 3 . Optical data coupler component  405  includes coupler body  411 , having a data interface side  413 . A first interface window plate  417  is mounted on interface pin plate  421  on the data interface side  413 . Behind interface window plate  417  are a pair of transmit optical coupler elements  431 ,  432  and a pair of receive optical coupler elements  435 ,  436 . Interface pin plate  421  also includes a pair of alignment pins  441 , which mate with alignment sockets  451  on coupler component  407 . 
     Interface window plate  417  is mounted to coupler body  411  and is sealed against coupler body  411  to establish a watertight seal. Optical coupler elements  431 ,  432 ,  435 ,  436  and optical coupler elements  461 ,  462 ,  465 ,  466  are positioned so that, when coupler components  405  and  407  are mated via alignment mechanism  441  and  451 , transmit optical coupler elements  431 ,  432  are aligned with corresponding receive optical coupler elements  465 ,  466  in mating coupler component  407  and receive optical coupler elements  435 ,  436  are aligned with corresponding transmit optical coupler elements  461 ,  462  in mating coupler component  407 . The alignment is maintained by the engagement of alignment pins and sockets  441 ,  451  respectively. One or both optical coupler components may be articulated. If the optical data coupler of  FIG. 4  is used in an environment where sealing is important, the coupler components  405 ,  407  may be sealed with respect to the device it is attached to or integrated onto. This is depicted in  FIG. 4 , in which coupler component  405  is shown as affixed and sealed against an apparatus  481  by bellows  483 . 
     The techniques for alignment of coupler assemblies  405 ,  407 , are also given by way of non-limiting example and may be accomplished through any well known engagement mechanisms, such as the pin and socket depicted, interlocking fingers, or magnetically controlled such as via matched magnets. In general, the transmit optical couplers  431 ,  432  are aligned with corresponding receive optical couplers  461 ,  462  in order to provide transmission of optical data within the optical constraints of the optical couplers  431 ,  432 ,  435 ,  436 . 
     The optical data coupler design is based upon an asymmetric expanded optical beam concept which is intended to be relatively insensitive to translational and angular misalignment and tolerant of contamination and fouling on the optical surfaces. By way of non-limiting example, the couplers are brought into physical contact with alignment tolerances on the order of ±2.5 mm and angular tolerances on the order of +/−6° in order to establish a bidirectional optical circuit capable of supporting a bandwidth of several GHz. A movable mating coupler, attached to an expandable bellows  483  ( FIG. 4 ) may be normally retracted and extended upon anticipation of a device having a similar optical data coupler component. When data transfer is complete the bellows are depressurized, the optical data coupler component  405  is retracted Because the connection requires “zero force” to couple or un-couple, robotic equipment may utilize the optical data coupler described herein with greater confidence that the coupling process will have minimized risk of unintended “trapping” or damage of the robotic equipment due to faulty un-coupling, or failing to transmit data due to poor coupling. 
     The optical data coupler design has the advantage that, by incorporating a known excess insertion loss into the optics design (i.e., intentional mismatching of the transmitting and receiving optics magnification ratio), a corresponding relaxation in the required mechanical alignment tolerances for the completed data coupler results. 
     The known excess insertion loss is essentially constant, and because the excess insertion loss is constant, it can be compensated by increased transmitting power and/or increased receiving sensitivity. Compensation by increased transmitting power can be achieved by incorporation of either an erbium-doped amplifier (EDFA) or a semiconductor optical amplifier (SOA) in the transmitter fiber. Compensation by increased receiving sensitivity can be achieved by the use of an optical preamplifier in the receiver fiber or the use of a more sensitive avalanche photo detector (APD) instead of a photodiode p-n (PIN) detector. 
     The ability to compensate by increased transmitting power and increased receiving sensitivity is advantageous when used with discrete channel bidirectional communications, because this allows amplification to be performed at one side of the coupler. Thus, a device at one side of the coupler can provide increased transmitting power on its transmitting channel and provide increased receiving sensitivity on its receiving channel. 
     While bidirectional optical communication is described, it is also possible to use the disclosed techniques for transmitting optical data unidirectionally. When implementing unidirectional communications of optical data, control and request data in an opposite direction can be achieved by any convenient technique, such as by radio frequency or underwater ultrasound communications. Such unidirectional optical communications can be useful in applications in which large amounts of data are transmitted in one direction but not in the other direction. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.