Abstract:
Aligning an optical fibre with an optical device, component or package such as an optical transducer or sensor is disclosed, A housing for an optical device is provided having a tapered surface to receive a correspondingly tapered portion of a ferrule in which a fibre is mounted. The tapered surfaces, which may for example be frusta-conical or conical, enable the optical device and ferrule to be quickly and precisely aligned. An optical device may be provided in a cavity which may be sealed by the mating tapered surfaces. The optical device may also be provided on a mounting bracket with flexible supports connected to the housing to accommodate thermal expansion of the housing and maintain alignment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a national stage application under 35 U.S.C. §371(c) of prior-filed, co-pending, PCT application serial number PCT/US2013/069564, filed on Nov. 12, 2013, which claims priority to Great Britain Patent Application Serial No. 1222628.8, filed on Dec. 14, 2012 and titled OPTICAL FIBRE ALIGNMENT. All of the above-listed applications are herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the invention relate to aligning an optical fibre with an optical device, component or package such as an optical transducer or sensor. 
         [0003]    The alignment of an optical device to an optical fibre, which energises and/or measures a response of the optical device, is critical to its performance. Failure to align the two successfully would result in an unacceptable loss of signal. 
         [0004]    U.S. Pat. No. 4,699,456 discloses an assembly for aligning an optical fibre to a semiconductor laser. An optical fibre is threaded through a tube and soldered to it. The tube is slid into a flange provided on a wall of a housing containing the semiconductor laser. A seal is formed between the flange and the tube by applying pressure via a cap. The alignment of the fibre to the laser is adjusted by tightening or loosening the cap while monitoring the light output from the fibre. 
         [0005]    However, conventional fibre alignment using manual calibration and positional adjustment is time consuming, expensive, prone to error and impractical in many applications. Furthermore, when used at elevated temperatures and pressures, thermal expansion of components may lead to further misalignment. 
       SUMMARY 
       [0006]    According to a first aspect of the present invention there is provided a housing for an optical device. The housing has a tapered surface to be engaged with a correspondingly tapered portion of a ferrule in which a fibre is arranged to be mounted. Through the use of the tapered surface, such as a conical surface, which may provide a circular concentric reference geometry, the ferrule may be self-aligned with an optical device in the housing overcoming the need for manual fibre alignment adjustment and providing faster and more precise fibre alignment. 
         [0007]    A mounting bracket may be used to mount an optical device to the housing. In an embodiment, the mounting bracket has flexible supports to be attached to the housing. The flexible supports may act as hinges to isolate an optical device to be mounted in the bracket from expansion and contraction of the housing which may be exposed to various operating temperatures during use, e.g. 0° C. and below to 400° C. and above, preventing radial thermal strain in the housing from being transferred to a mounting surface on which an optical device may be arranged to be provided. The flexible supports may also maintain alignment of the optical device and fibre as the housing thermally expands and contracts in use enabling it to be used over large ranges of temperature and pressure. 
         [0008]    The ferrule is, in an embodiment, secured to the housing to provide a reliable compressive seal at the mating tapered surfaces. 
         [0009]    According to a second aspect of the present invention there is provided a method of aligning a fibre with an optical device having a housing with a tapered surface. The method comprises engaging the tapered surface of the housing with a corresponding tapered portion of a ferrule having a fibre mounted therein. The ferrule may be secured to the housing to generate a compressive pressure seal at the mating tapered surfaces of the housing and the ferrule to seal a cavity in the housing for an optical device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Examples of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0011]      FIG. 1  shows an example of a housing and a corresponding ferrule of an example of the present invention; 
           [0012]      FIG. 2  shows an example of a mounting bracket to mount an optical device to a housing; 
           [0013]      FIG. 3  shows a more detailed example of a housing and a corresponding ferrule; 
           [0014]      FIG. 4  is an enlarged view of a portion of  FIG. 3 ; 
           [0015]      FIG. 5  is an exploded perspective view of the arrangement of an optical device and mounting bracket with respect to a housing; 
           [0016]      FIG. 6  is an assembled perspective view of an optical device and mounting bracket attached to a housing and 
           [0017]      FIG. 7  is an exploded partly cut away perspective view of a ferrule arrangement to be inserted into a housing. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows a housing  1  for an optical device  2  such as a sensor. In this example the optical device is provided in a cavity  3  in the housing  1 . A surface  4  of the optical device  2  in this example is arranged to be exposed to a fluid to be tested to be able to measure a property of the fluid, such as its pressure. The housing  1  has a tapered surface  5  to receive a ferrule  6  with a correspondingly tapered portion  7  to be fitted therein and to present an end of a fibre  8  arranged to be mounted in the ferrule  6 . Although the housing tapered surface  5  and corresponding portion  7  of the ferrule  6  are shown as being conical or frusto-conical shaped, they could include any suitable tapered surface to enable the ferrule  6  to be precisely received within the housing to provide precise and reliable alignment of a fibre  8  with an optical device  2 . The tapered surface  5  of the housing  1  is shown as being concave in  FIG. 1  with the corresponding tapered surface  7  of the ferrule  6  being convex. However, the tapered surface  5  of the housing  1  could be convex and the corresponding surface  7  of the ferrule  6  could be concave. 
         [0019]      FIG. 2  shows an example of a mounting bracket  10  that may be used to mount an optical device  2  to the housing  1  to accommodate thermal expansion of the housing  1  whilst still maintaining alignment of the optical device  2  with the fibre  8 . The mounting bracket  10  has two flexible supports  11  to be joined to the housing  1 . The flexible supports  11  act as hinges when the housing  1  expands and contracts when exposed to various operating temperatures, eg 0° C. and below to 400° C. and above, to prevent radial thermal strain in the housing  1  from being transferred to a mounting surface  12  on which the optical device  2  is arranged to be provided. This example of the mounting bracket  10  also includes two stiffening beams  13  arranged along the sides between the flexible supports  11 . The stiffening beams  13  prevent or reduce any bending of the base  12  as a result induced bending moments which could also be caused by thermal expansion and contraction of the housing  1  to which the mounting bracket  10  is, in use, attached. 
         [0020]    The example of the mounting bracket  10  in  FIG. 2  includes a window or hole  14  through which electromagnetic signals may be passed between the optical device  2  and the fibre  8 . The window  14  could be any appropriate size and could occupy most of the base  12  of the mounting bracket  10  resulting in a supporting ledge around the inner edge of the base  12 . 
         [0021]    The housing  1  may be secured to the ferrule  6  by any convenient means, such as by threads in or at the ends of the tapered surfaces  5 ,  7  or in appropriate flanges attached to one or both of the housing  1  and ferrule  6  for example. The housing  1  is, in an embodiment, secured to the ferrule  6  sufficiently securely to generate a compressive pressure seal at the mating tapered surfaces  5 ,  7 . The seal has been found to be suitable for use at high pressures and temperatures, such as 1000 bar and 400° C. 
         [0022]      FIGS. 3 and 4  show a more detailed example of the present invention. In  FIG. 3  the housing  1  is in the form of a pressure bulkhead with an internal cavity  3 , containing a resonant silicon sensing element  2  supported via the strain-relieving mounting bracket  10 . The internal cavity  3  in this example is bounded by a fusion welded isolation diaphragm  20  and fluid filled, for example with oil  21  under vacuum via a central fibre aperture  22 . The fluid in the cavity  3  is transparent to the appropriate wavelength electromagnetic radiation provided to/from the optical device  2  that is used. A ferrule assembly, consisting of an optical fibre  8  with a gold buffer  23  soldered or brazed into the metal ferrule  6 , with a frusto-conical integral sealing face  7 , is located into the tapered surface  5  of the housing  1  providing axial, angular and separation alignment to the sensing element target  2 . A retaining screw  24  and backing spacer  25  are used to hold the ferrule  6  in place via a matching thread in the bulkhead  1 , generating a compressive metal to metal pressure seal at the bulkhead mating coned surface  5  and sealing the fluid cavity. 
         [0023]    The bulkhead  1 , diaphragm  20  and locking screw  24  may be made from a metal suitable for the operating environment conditions and process media in which the housing is to be used (typically a stainless steel, nickel alloy or titanium/titanium alloy for pressure sensing applications). 
         [0024]      FIG. 5  shows the arrangement of an optical device  2  and mounting bracket  10  in a cavity  3  of a housing  1 . The mounting bracket  10  may be made from a low thermal expansion metal (e.g. Nickel/Iron alloy) to provide a close thermal match to the optical device  2 , which may be a silicon sensing element. It provides a stiff and stable base on which to attach the optical device  2 , more particularly with a suitable frit or die attachment adhesive that is not sensitive to the thermal expansion of the surrounding structure of the bulkhead  1 . This stability of the mounting bracket  10  may be achieved by the use of the flexible mounting legs  11  that may be resistance welded or bonded  15  to the bulkhead  1 . Alternatively or additionally stiffening beams  13  are provided along the sides of the base  12 . The legs  11  act as hinges to prevent radial thermal strain in the bulkhead  1  being transferred to the mounting surface  12  and the beams  13  prevent any resultant bending of the base  12  as a result of the induced bending moments.  FIG. 6  shows the optical device  2  and mounting bracket  10  provided in the housing  1  with the mounting legs  11  attached  15  to the housing  1 . 
         [0025]    In an embodiment, there is a close thermal expansion match between the fibre  8  and ferrule  6  where they are joined, for example at a brazed joint, therefore a low thermal expansion ferrule  6  is used (e.g. Nickel/Iron alloy) with a high thermal expansion backing spacer  25  (e.g. Aluminium/Bronze Alloy or 300 series Stainless) as shown in  FIG. 7 . The spacer  25  serves the purpose of a thermal expansion compensator that counteracts any discrepancy in thermal expansion between the bulkhead  1  and ferrule  6  and maintains the axial compressive force at the coned surfaces  5 ,  7 . 
         [0026]    As also shown in  FIG. 7 , retaining screw  24  is arranged to be secured to the housing  1 , in this example via corresponding threads in each of the screw  24  and housing  1 , to generate a compressive pressure seal at the mating tapered surface  7  of ferrule  6  and the corresponding tapered surface  5  of the housing  1  sealing the fluid cavity  3 . 
         [0027]    The integrity of the pressure bulkhead  1  and the fluid cavity seal is provided over the operating temperature range to maintain both pressure containment and isolation of the sensing element  2 . However, access to the cavity  3  is required for the optical fibre  8  and the fluid fill process. Any openings or feedthroughs therefore represent a potential containment integrity failure path. To maintain a lean design and minimise this risk, the mounting of the optical fibre  8  has been combined with the cavity seal mechanism as explained above. 
         [0028]    During the assembly process, the housing  1  and optical device  2  need to be placed in both a weld chamber and an oil back-fill vacuum chamber to assemble the oil filled cavity  3 . These chambers have limited space and it would therefore prove impractical to have a length of optical fibre already attached to the optical device  2 . As the ferrule  6  is arranged to be provided with the fibre  8  and to seal with the housing  1  via the tapered surfaces  5 ,  7  after the housing  1  with cavity  3  has been assembled, this problem is overcome. 
         [0029]    By using correspondingly tapered surfaces, more particularly conical or frusto-conically shaped, providing concentric reference geometry on the bulkhead housing  1  and fibre ferrule  6 , the critical alignment of the fibre  8  and target  2  is addressed purely by design. Therefore the need for a manual alignment process and all associated time, cost and potential sources of error/yield loss are eliminated. 
         [0030]    The combination of the fluid fill port and subsequent optical fibre attachment allows for a robust assembly process, where all welding operations and the vacuum fill process, are performed without the presence of a length of delicate and expensive optical fibre. The critical high risk processes in this assembly are performed up front allowing the functionality of the sensor to be verified and failed units scrapped prior to completion with the attachment of the ferrule to provide a seal. 
         [0031]    Many variations may be made to the examples described above whilst still falling within the scope of the invention. For example, the ferrule  6  may be attached to the housing  1  to compress the mating tapered surfaces  5 ,  7  against each other in any desired way. The tapered surface  5  of the housing  1  is shown as being concave in  FIG. 1  with the corresponding tapered surface  7  of the ferrule  6  being convex. However, the tapered surface  5  of the housing  1  could be convex and the corresponding surface  7  of the ferrule  6  could be concave.