Abstract:
A fiber optic feedthrough includes a housing defining a cylindrical bore, the housing fitting hermetically into an aperture of a wall. A first collimator sealed within the bore receives output from an optical fiber located on a first side of the wall, and a second collimator also sealed within the bore, optically aligned with the first collimator, receives the signal and provides it to a second optical fiber located on a second side of the wall. The polarization extinction ratio of an optical signal traveling through the feedthrough is maintained at a value of at least about 12 dB, and the hermeticity of the feedthrough across the wall is better than about 10 −6  atm.cc.sec −1 .

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/945,796, filed Jun. 22, 2007, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to a fiber optic connections, and more particularly to passing a fiber through a wall. 
       BACKGROUND 
       [0003]    It is often necessary to pass an optical fiber through a wall, such as the wall of a vessel or package containing electronic components. When the wall separates two regions with significantly different pressure, such as an evacuated region or the inside of a vessel, from the atmosphere, the fiber needs to pass through the wall without allowing gas leakage. Traditional methods involve stripping off a portion of the fiber cladding in the region of the fiber that passes through the wall, filling in the stripped portion with an epoxy layer, and positioning and sealing the epoxy-coated portion in an aperture passing through the wall. In order to provide a good hermetic seal a tight mechanical attachment is usually required. But such a tight attachment tends to induce stress on the optical fiber, and this stress can degrade the quality of the fiber transmission through the wall. In particular, the stress can degrade the polarization extinction ratio (PER). 
       SUMMARY 
       [0004]    The described embodiment features a fiber optic feedthrough that maintains high hermeticity and polarization extinction ratio. 
         [0005]    In general in one aspect a fiber optic feedthrough includes: a housing defining a bore, the housing being adapted to fit hermetically into an aperture of a wall; a first collimator configured to receive an output from a first optical fiber located on a first side of the wall, the first collimator sealed within a portion of the bore adjacent to a first end of the bore; and a second collimator configured to provide an input to a second optical fiber located on a second side of the wall, the second collimator sealed within a portion of the bore adjacent to a second end of the bore, the second collimator being optically aligned with the first collimator. The polarization extinction ratio of an optical signal traveling from the first optical fiber through the first and second collimators to the second optical fiber is maintained at a value of at least 12 dB, and the hermeticity of the feedthrough across the wall is better than about 10 −6  atm.cc.sec −1 . 
         [0006]    The inventions may also feature one or more of the following. The first collimator and the second collimator each include a lens. The surface of the bore defines a cylinder. An angular offset between an axis defined by the first collimator and an axis defined by the second collimator is less than about one milliradian. The first and second collimators can be rotated about an axis parallel to an axis defined by the bore. The first collimator is rotationally aligned with the second collimator to within about one degree. The polarization extinction ratio is maintained at a value of at least 20 dB. The hermeticity of the feedthrough across the wall is better than about 10 −7  atm.cc.sec −1 , and can be better than about 10 −8  atm.cc.sec −1 . 
         [0007]    In general, in another aspect, a vessel includes: a wall, the wall being capable of separating a first fluid having a first pressure and a second fluid having a second pressure, the first pressure being greater than the second pressure; a housing defining a hollow bore, the housing being adapted to fit hermetically into an aperture defined by the wall; a first collimator configured to receive an output from a first optical fiber located in the first volume, the first collimator sealed within a portion of the bore adjacent to a first end of the bore; and a second collimator configured to provide an input to a second optical fiber located in the second volume, the second collimator sealed within a portion of the bore adjacent to a second end of the bore, the second collimator being optically aligned with the first collimator, the housing and the first and second collimators comprising a fiber optic feedthrough, wherein a polarization extinction ratio of an optical signal traveling between the first volume and the second volume through the fiber optic feedthrough is maintained at a value of at least 12 dB. The vessel may be a spacecraft, and the wall may be capable of separating a volume of gas at a pressure of about one atmosphere from the vacuum of outer space. The vessel may be a submarine, and the fluid outside a wall of the submarine may be water. The vessel may be a pressure vessel for use in a chemical plant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an illustration of a fiber optic feedthrough, according to the described embodiment. 
           [0009]      FIG. 2  is an illustration of a demountable fiber optic feedthrough. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    A method and apparatus described here for making a hermetic fiber feedthrough uses an expanded beam coupling. The fiber passes through a barrier, such as a wall, that divides a higher pressure side and a lower pressure side. The wall can be the wall of an evacuated vessel or package, or the wall of a spacecraft. In the case of a spacecraft, the wall divides regions of about 0 atm and 1 atm, although the feedthrough can be used in other contexts. 
         [0011]      FIG. 1  illustrates the described embodiment  100  of a fiber optic feedthrough embedded in an aperture of a wall  102  separating lower pressure region  104  from higher pressure region  106 . Housing  108  of the feedthrough is made of a hard material, such as Kovar or stainless steel, and may be cylindrical, with an outer diameter that fits securely into a cylindrical aperture in wall  102 , and a hollow bore that houses the optics. The outer surface of the housing may define a square, rectangular, or other cross section, and the hollow bore may be cylindrical or, alternatively, have a square, rectangular, or other cross section. On the low pressure side of the wall, housing  108  is enclosed in rubber boot  110  to protect protruding optical elements and to secure the incoming optical fiber. First fiber  112  enters feedthrough  100  through an aperture in rubber boot  110 , and terminates at beam expander  114 , which provides an output to first collimator  116 . Collimator  116  includes lens  118 , shown in the figure as a discrete lens aligned to the fiber in a tube. Alternatively, the lens may be fused to the fiber, using, for example, a Lightpath technique. In this technique a graded index lens is directly fused to the silica fiber to provide a high strength and low loss joint. The resulting assembly is then used as a single monolithic component. After passing through lens  118 , the output from first fiber  112  passes through inner cylindrical region  120  of housing  110  to second collimator  122 , which is substantially the same as first collimator  116 . The diameter of inner cylindrical region  120  is about 3 mm, and is accurately machined so as to act as a mechanical boresight that aligns second collimator  122  with first collimator  116 . The distance between the first and second collimators depends on the thickness of wall  102 ; in described embodiment  100 , the distance between the first lens and the second lens is about 30 mm, and each collimator is about  10  mm long. Second collimator  122  provides output to second fiber  124 , which exits feedthrough  100  in higher pressure region  106 . 
         [0012]    All the elements of described embodiment  100  are soldered in place so that none of the optical elements are exposed to ambient gas, such as, for example, the atmosphere. Such an arrangement can achieve hermeticity values of better than 10 −6  atm.cc.sec −1  and typically in a range of 10 −6  to 10 −9  atm.cc.sec −1 . In many practical situations it is desirable for the hermeticity to be at least as good as 10 −7  atm.cc.sec −1  or even 10 −8  atm.cc.sec −1 , which the described embodiment can provide. Such situations include operation in the vacuum environment of space, where hermeticity levels of about 10 −9  atm.cc.sec −1  are required to maintain an acceptable gaseous atmosphere inside the enclosure for a period of at least about five years. 
         [0013]    The outer diameter of the collimator and the inner diameter of the bore is manufactured to a tolerance of 10 microns or better to insure proper alignment of the collimators. In addition the collimator can be inserted far enough into the bore to assure that the angular offset is negligible. For example, with an insertion distance of 10 mm, an angular offset of about a milliradian can be achieved. Lateral offsets are negligible since the device is in collimated space, and the collimated, expanded beam is not sensitive to lateral displacements. 
         [0014]    In most fiber applications, it is important to maintain the PER. As mentioned above, stress on a fiber degrades the PER. Using fiber optic feedthrough  100 , this kind of PER degradation is almost completely obviated since at no point is a fiber subjected to significant stress. Thus feedthrough  100  maintains a PER of about 20 dB as the optical signal travels between a lower pressure region and a higher pressure region. 
         [0015]    Feedthrough  100  also enables the collimators to rotate relative to each other, and in order maintain the polarization angle and PER of the optical signal, the collimators are rotationally aligned with each other to an accuracy of at least about 1 degree. This adjustment is significant because the fibers maintain polarization, i.e., the light emerging from a fiber is polarized at a well-defined polarization angle. Since a light-accepting fiber is also polarized at a well defined angle, the two fibers should be rotationally aligned to within about one degree or better, otherwise a portion of the polarized light from the emitting fiber is launched into the orthogonal axis of the receiving fiber, resulting in degradation of the PER. 
         [0016]    A PER as low as 12 dB is acceptable for many applications, especially in cases where the principal concern is the total amount of power in the correct polarization state and the power in the other state is not detrimental to the application. At a PER of 12 dB, about 94% of the power is in the correct polarization state. An example is the combining of differently polarized beams to achieve higher pump powers. By contrast, in cases where light in the wrong polarization state induces system noise, for example in a communications system, then a PER of 20 dB or higher, corresponding to 1% or less of the power in the wrong state, is required. Such a PER is easily achieved with the described hermetic fiber feedthrough. For comparison, the PER supported by an unstressed fiber is about 30 dB. 
         [0017]    A demountable feedthrough is achieved by inserting a hermetic window between the two collimators. This allows one or both collimators to be removed from the housing without allowing gas to pass from the higher pressure region into the lower pressure region.  FIG. 2  illustrates a demountable feedthrough with window  202  hermetically fitted into bore  120 . The window is tilted to prevent reflections from interfering with the beam. The window material includes sapphire, silica, or crown glass, such as Schott BK7. For applications in which the light is in the mid-infrared region (2-10 microns), the window material includes a zinc selenide, germanium, or chalcogenide glass. 
         [0018]    In this way a simple, cost-effective, high transmission, high PER fiber feedthrough can be realized at minimal development cost. By comparison, designing and developing techniques to window strip a fiber and apply a highly hermetic seal without imposing sufficient stress to the fiber to degrade the polarization is a lengthy, iterative procedure. 
         [0019]    One application involves using the described embodiment to pass a fiber through the wall of a spacecraft, where high hermeticity is required between the pressurized cabin inside the craft and the very low pressures of space. In another application, polarized fibers are passed through the walls of a pressure vessel in a marine environment, such as, for example, passing signals from sensors through the walls of a submarine. A further application is to pass polarization-maintaining fibers through the wall of a pressure vessel, such as used in a chemical plant, to extract data from distributed temperature and pressure sensors inside the vessel. The inside of the pressure vessel may be a gas or liquid at high pressure. The outer surface of the pressure vessel wall may be in contact with gas at a lower pressure, such as the atmosphere, or with a liquid within which the vessel is immersed. The pressures may be reversed, with the vessel containing a fluid at low pressure within a higher pressure fluid in contact with the outer wall of the vessel. 
         [0020]    Other aspects, modifications, and embodiments are within the scope of the following claims.