Patent Publication Number: US-2012027365-A1

Title: Optical communication device and method of assembling the same

Description:
CLAIM OF PRIORITY 
     This application claims benefit of Japanese Patent Application No. 2010-167850 filed on Jul. 27, 2010, which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to an optical communication device in which a light receiving unit, including a casing and a light receiving element received in the casing, is position-adjusted and attached to a housing and light traveling along an optical path, such as an optical fiber, is received by the light receiving element, and relates to a method of assembling the same. 
     2. Description of the Related Art 
     In an optical communication device including a light receiving element, a light receiving unit including a casing and the light receiving element received in the casing is attached to a housing, an optical fiber serving as an optical path is mounted on the housing, and the optical fiber applies light to the light receiving element. 
     Japanese Unexamined Patent Application Publication No. 9-211258 discloses an optical communication module in which an opening is formed in a housing mounted with an optical fiber and a casing supporting a light receiving element is fitted and attached to the opening. Japanese Unexamined Patent Application Publication No. 2005-164971 discloses an optical module in which a holding member is attached to a side surface of a housing, the holding member has a through-hole, and a casing supporting a light receiving element is fitted and secured to the through-hole. 
     In such an optical communication device, the casing supporting the light receiving element has to be positioned relative to the housing and be secured thereto such that the optical axis of light applied from the optical path including the optical fiber coincides with the light receiving element as much as possible. 
     Since the optical communication module disclosed in Japanese Unexamined Patent Application Publication No. 9-211258 has a structure in which the casing supporting the light receiving element is fitted and secured to the opening in the housing, the casing can be moved in a direction intersecting the center line of the casing to adjust the light receiving element such that the light receiving element coincides with the optical axis of light transmitted through the optical fiber. It is, however, difficult to perform adjustment for moving the light receiving element in a direction along the optical axis and adjustment for changing an angle at which the optical axis faces the light receiving element. 
     In the optical module disclosed in Japanese Unexamined Patent Application Publication No. 2005-164971, when the holding member attached to the side surface of the housing is moved in a direction orthogonal to the optical axis, the position of the light receiving element can be adjusted in the direction intersecting the optical axis. It is, however, difficult to adjust the position of the light receiving element in the direction along the optical axis and an angle of inclination (hereinafter, “inclination angle”) of the light receiving element relative to the optical axis. 
     In the optical module disclosed in Japanese Unexamined Patent Application Publication No. 2005-164971, in order to adjust the position of the light receiving element in the direction along the optical axis or an angle by which the light receiving element is inclined relative to the optical axis, a plurality of holding members having different thicknesses or a plurality of holding members having through-holes at different angles have to be provided, an optimum holding member has to be selected from among them, and after that, assembly is done. Disadvantageously, the provision of the holding members having different thicknesses or the holding members having the through-holes at different angles results in an increase in manufacturing cost. Management of the holding members is complicated and an operation of selecting any of the holding members and attaching the selected member is also complicated. Disadvantageously, the number of steps of adjusting the attachment position of the light receiving element is increased. 
     SUMMARY 
     An optical communication device includes a metal housing, a casing having a metal flange, a light receiving element supported by the casing, a metal holder that fixes the casing to the housing, and an optical path that guides light to the light receiving element. The holder has a through-hole, an abutting portion surrounding one open end of the through-hole, and a welded portion surrounding the other open end of the through-hole, the welded portion including a projection whose cross-sectional area is smaller as the corresponding cross-section is closer to the tip of the projection. The holder and the flange are welded to each other through the partly melted projection pressed against the flange such that unmelted part of the projection adjusts an inclination angle of the abutting portion of the holder relative to the center line of the casing. The holder is fixed to the housing such that the abutting portion is in contact with the housing and an angle at which the light receiving element faces the optical path is determined on the basis of the inclination angle. 
     Another aspect provides a method of assembling an optical communication device including a metal housing, a casing having a metal flange, a light receiving element supported by the casing, a metal holder that fixes the casing to the housing, and an optical path that guides light to the light receiving element, the holder having a through-hole, an abutting portion surrounding one open end of the through-hole, and a welded portion surrounding the other open end of the through-hole, the welded portion including a projection whose cross-sectional area is smaller as the corresponding cross-section is closer to the tip of the projection. The method includes the steps of (a) while pressing the projection against the flange, supplying current to the holder and the flange to melt the projection in order to weld the holder to the flange, (b) adjusting an inclination angle of the holder relative to the flange during welding, and (c) bringing the abutting portion into contact with the housing to fix the holder to the housing such that an angle at which the light receiving element faces the optical path is set on the basis of the inclination angle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a optical communication device according to an embodiment of the present invention; 
         FIG. 2  is a diagram explaining an operation of adjusting a light receiving element to be attached to a housing; 
         FIG. 3A  is a sectional view of a holder; 
         FIG. 3B  is an enlarged sectional view of part of the holder; 
         FIG. 4  is a diagram explaining an operation of welding the holder to a flange; 
         FIG. 5  is a sectional view of the holder welded to the flange; and 
         FIG. 6  is a graph illustrating the relationship between coupling efficiency and an angle deviation between an optical path and the light receiving element. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
       FIG. 1  illustrates an optical communication device  1  according to an embodiment of the present invention. The optical communication device  1  includes a housing  2  made of metal, such as ferritic stainless steel. The housing  2  has a fiber attachment face  2   a,  serving as an outer face on the right side in  FIG. 1 . The housing  2  further has a fiber mounting hole  3  which inwardly extends from the fiber attachment face  2   a.    
     One end of an optical fiber  11  is held by a ferrule  12 . The ferrule  12  is held by a ferrule holder  13  made of ferritic stainless steel. While one end of the ferrule  12  is fitted in the fiber mounting hole  3 , a flange  13   a  of the ferrule holder  13  is in tight contact with the fiber attachment face  2   a.  The ferrule holder  13  is secured to the housing  2  by, for example, laser welding. 
     The one end, which is beveled and indicated by  14 , of the optical fiber  11  extending from one end of the ferrule  12  is exposed in an internal space  5  of the housing  2 . An optical path L 1  passing through the center of the optical fiber  11  is slightly bent by passing the beveled end face of the optical fiber  11  and then enters the internal space  5 . 
     The housing  2  further has a light-emission attachment face  2   b,  serving as an outer face on the left side in  FIG. 1 , and an optical-path hole  4  which inwardly extends from the light-emission attachment face  2   b.    
     A light-emission holder  15  is attached to the light-emission attachment face  2   b.  The light-emission holder  15  is made of metal, such as ferritic stainless steel. A lens barrel  16  holding a converging lens  17  is secured inside the light-emission holder  15 . The light-emission holder  15  holds a light emitting unit  18 . The light emitting unit  18  includes a casing  18   a  made of metal, such as ferritic stainless steel, and a light emitting element  19 , such as a laser diode, supported in the casing  18   a.    
     Adjustment is performed such that the light-emission holder  15  is moved in a direction orthogonal to a light emission axis L 2  while being in contact with the light-emission attachment face  2   b  of the housing  2 , thus allowing the light emission axis L 2  to coincide with the optical path L 1 . After the adjustment, the light-emission holder  15  is secured to the housing  2  by, for example, laser welding. 
     The housing  2  has a first light-receiving attachment face  2   c,  serving as the upper face thereof in  FIG. 1 , and a second light-receiving attachment face  2   d,  serving as the lower face thereof in  FIG. 1 . A first light receiving holder  21  is fixed to the first light-receiving attachment face  2   c.  The first light receiving holder  21  is made of metal, such as ferritic stainless steel. A first light receiving unit  22  is fastened to the first light receiving holder  21 . The first light receiving unit  22  includes a casing  23  made of metal, such as ferritic stainless steel, a light receiving element  24 , such as a PIN photodiode, and a light receiving lens  25  such that the light receiving element  24  and the light receiving lens  25  are held in the casing  23 . 
     The first light receiving holder  21  is fixed to the casing  23  by welding. The first light receiving holder  21  is fixed to the housing  2  by welding such that the first light receiving holder  21  is in tight contact with the first light-receiving attachment face  2   c.    
     A second light receiving holder  26  is firmly attached to the second light-receiving attachment face  2   d.  The second light receiving holder  26  is made of metal, such as ferritic stainless steel. A second light receiving unit  27  is fixed to the second light receiving holder  26 . The second light receiving unit  27  includes a casing  28  made of metal, such as ferritic stainless steel, a light receiving element  29 , such as a PIN photodiode, and a light receiving lens  30  such that the light receiving element  29  and the light receiving lens  30  are held in the casing  28 . 
     The second light receiving holder  26  is fixed to the casing  28  by welding. The second light receiving holder  26  is fixed to the housing  2  by welding such that the holder is in tight contact with the second light-receiving attachment face  2   d.    
     Two wavelength separation filters  33  and  34  are arranged in the internal space  5  of the housing  2 . The two wavelength separation filters  33  and  34  are fixed to a support  35  disposed in the internal space  5 . 
     To transmit data in the optical communication device  1 , laser light emitted from the light emitting element  19  of the light emitting unit  18  is modulated based on transmission data and is then transmitted into the optical fiber  11 . 
     The optical communication device  1  is capable of receiving wavelength division multiplexed laser light. A light component in a first waveband transmitted through the optical fiber  11  is separated by the first wavelength separation filter  33 . The separated light component travels along a first optical path branch L 3  and is then received by the light receiving element  24  of the first light receiving unit  22 . After that, a received signal is demodulated. A light component in a second waveband transmitted through the optical fiber  11  is separated by the second wavelength separation filer  34 . The separated light component travels along a second optical path branch L 4  and is then received by the light receiving element  29  of the second light receiving unit  27 . After that, a received signal is demodulated. 
     In the optical communication device  1  illustrated in  FIG. 1 , data transmitted as a light component separated by the first wavelength separation filter  33  is image data and data transmitted as a light component separated by the second wavelength separation filer  34  is communication data other than the image data. 
     When the first light receiving unit  22  is attached to the housing  2 , it is necessary to perform adjustment such that the light receiving element  24  is positioned on the center line of the first optical path branch L 3 . Similarly, when the second light receiving unit  27  is attached to the housing  2 , it is necessary to perform adjustment such that the light receiving element  29  is positioned on the center line of the second optical path branch L 4 . In addition, it is necessary to change the fixed position of the first light receiving unit  22  in the vertical direction in  FIG. 1  in order to adjust the positions of the light receiving element  24  and the light receiving lens  25  on the first optical path branch L 3  so that received light is focused on the light receiving element  24  to form an image. The same applies to attachment of the second light receiving unit  27 . 
     Since a signal transmitted as a light component received through the first light receiving unit  22  is image data, the density and transmission rate of data to modulate light are high. In addition to positioning the light receiving element  24  of the first light receiving unit  22  on the center line of the first optical path branch L 3 , therefore, it is necessary to adjust an angle at which the first light receiving unit  22  faces the first optical path branch L 3 . 
     Referring to  FIG. 2 , a tolerance in a rotation direction (a direction) in which the ferrule  12  holding the optical fiber  11  is rotated during attachment, the accuracy of dimension of the support  35 , a tolerance of attachment of the first wavelength separation filter  33  relative to the support  35 , and the like are accumulated to cause a tolerance of an inclination angle β 1  relative to the center line of the first optical path branch L 3 . On the other hand, in the first light receiving unit  22 , for example, a tolerance of an attachment position of the light receiving element  24  and a tolerance of attachment of the light receiving lens  25  in the casing  23  cause an angle tolerance β 2  between an optimum optical axis L 5  for light receiving through the light receiving element  24  and the center line O 1  of the casing  23 . 
     Accordingly, when the first light receiving unit  22  is attached to the housing  2 , an angle deviation up to β 1 +β 2  occurs between the light-receiving optical axis L 5  for the light receiving element  24  and the first optical path branch L 3 . 
       FIG. 6  illustrates coupling efficiency plotted against the angle deviation. When “1” indicates the power of light received by the light receiving element  24  of the first light receiving unit  22  when the angle deviation is zero, the coupling efficiency is the ratio of received light power upon occurrence of an angle deviation to received light power of “1”. Referring to  FIG. 6 , when an angle deviation of two degrees occurs, the coupling efficiency falls to about 93%. 
     When the first light receiving unit  22  for detecting light modulated based on image data is attached to the housing  2 , therefore, the inclination of the first light receiving unit  22  has to be adjusted so that the angle deviation is eliminated. 
     The optical communication device  1  according to the present embodiment includes the first light receiving holder  21  having a structure as illustrated in  FIGS. 3A and 3B . 
     Referring to  FIG. 3A , the first light receiving holder  21  has a cylindrical outer face  21   a  and a through-hole  21   b  which extends through the thickness of the first light receiving holder  21 . The through-hole  21   b  has a circular cross-section. The center of the through-hole  21   b  and that of the outer face  21   a  are positioned on a common center line O 2 . A portion surrounding one open end of the through-hole  21   b  serves as an abutting portion  21   c  to abut against the first light-receiving attachment face  2   c  of the housing  2 . A portion surrounding the other open end thereof serves as a portion (hereinafter, “welded portion”)  21   d  to be welded to a flange  23   a  provided for the casing  23  of the first light receiving unit  22 . The abutting portion  21   c  and the welded portion  21   d  are parallel to each other and are flat faces orthogonal to the center line O 2 . 
     As illustrated in  FIG. 3A , a projection  21   e  projects from the welded portion  21   d.  The projection  21   e  is continuously provided on the whole circumference (360°) of a circle locus whose center coincides with the center line O 2 . Referring to  FIG. 3B , the projection  21   e  is tapered such that the cross-sectional area is smaller as the corresponding cross section is farther away from the flat face, serving as the welded portion  21   d.  In other words, when the projection  21   e  is cut along a plane including the center line O 2 , the width of the projection  21   e  is thinner as the cut plane is farther away from the flat face, serving as the welded portion  21   d.    
     The inner diameter φ of the through-hole  21   b  of the first light receiving holder  21  is about 4 to about 10 mm. The height, indicated at h, of the projection  21   e  is in the range of about 0.1 to about 0.6 mm. A portion of the projection  21   e  to face the flange  23   a  has an angle θ in the range of about 20 to about 45 degrees. The projection  21   e  is sharply shaped such that the width t of the tip is equal to or greater than 0.05 mm, preferably, equal to or greater than 0.02 mm. 
       FIG. 4  illustrates a welding operation when the first light receiving unit  22  is fixed to the first light receiving holder  21 . 
     The first light receiving holder  21  is made of a conductive metallic material, such as ferritic stainless steel, capable of being resistance-welded. The casing  23  of the first light receiving unit  22  includes the flange  23   a  and a cap  23   b  for receiving the light receiving element  24  and the light receiving lens  25 . At least the flange  23   a  is made of the conductive metallic material, such as ferritic stainless steel, capable of being resistance-welded. 
     Referring to  FIG. 4 , resistance welding equipment includes a lower electrode  31  and an upper electrode  32 . The upper face of the lower electrode  31  serves as a facing face  31   a.  The lower electrode  31  has a clearance hole  31   b  which is positioned in the center of the electrode and vertically extends through the thickness of the electrode. The lower face of the upper electrode  32  serves as a facing face  32   a.  The upper electrode  32  has a clearance hole  32   b  which is positioned in the center of the electrode and vertically extends through the thickness of the electrode. 
     The first light receiving unit  22  is set such that terminals extend through the clearance hole  31   b  of the lower electrode  31  and the rear face of the flange  23   a  is in tight contact with the facing face  31   a  of the lower electrode  31 . The cap  23   b  extends through the through-hole  21   b  of the first light receiving holder  21  such that the tip of the projection  21   e  on the first light receiving holder  21  abuts against the front face of the flange  23   a.  The inner diameter of the through-hole  21   b  is slightly larger than the outer diameter of the cap  23   b,  such that the cap  23   b  can be inclined relative to the center line O 2  of the first light receiving holder  21 . 
     The upper end of the cap  23   b  is received in the clearance hole  32   b  of the upper electrode  32 . The facing face  32   a  of the upper electrode  32  faces the abutting portion  21   c  of the first light receiving holder  21 . The inner diameter of the clearance hole  32   b  is slightly larger than the outer diameter of the cap  23   b,  such that the upper electrode  32  can be inclined relative to the center line O 1  of the cap  23   b.    
     In the welding operation, the lower electrode  31  is moved closer to the upper electrode  32 , the projection  21   e  of the first light receiving holder  21  is pressed against the flange  23   a  of the casing  23 , and a voltage is applied between the lower electrode  31  and the upper electrode  32  to supply current to the first light receiving holder  21  and the casing  23 . The current is concentrated to the contact between the projection  21   e  and the flange  23   a.  The projection  21   e  is partly melted by Joule heat, thus welding the first light receiving holder  21  to the casing  23 . 
     As illustrated in  FIG. 4 , a slight angle γ is provided between the facing face  31   a  of the lower electrode  31  and the facing face  32   a  of the upper electrode  32 . The upper electrode  32  presses the first light receiving holder  21  until the facing face  32   a  of the upper electrode  32  is brought into tight contact with the abutting portion  21   c  of the first light receiving holder  21 . By this welding operation, the first light receiving holder  21  can be welded to the casing  23  such that the abutting portion  21   c  of the first light receiving holder  21  is inclined at the angle γ relative to a plane perpendicular to the center line O 1  of the casing  23 , as illustrated in  FIG. 5 . 
     During the welding operation, the distance between the lower electrode  31  and the upper electrode  32  is changed while current is being supplied therebetween, the distance, indicated at H, between the front face of the flange  23   a  and the abutting portion  21   c  of the first light receiving holder  21  after welding may be adjusted in the range corresponding to the height h of the projection  21   e.    
     In an operation of assembling the optical communication device  1 , as illustrated in  FIG. 2 , an inclination direction of the first optical path branch L 3  set in the housing  2  and the inclination angle β 1  thereof are measured. In the first light receiving unit  22 , the inclination angle β 2  between the center line O 1  of the casing  23  and the optical axis L 5  and a focal length on the optical axis L 5  for forming an image on the light receiving element  24  are measured. 
     The inclination angle γ of the abutting portion  21   c  necessary for canceling an angle error is obtained on the basis of the measured inclination angles. In the resistance welding equipment, the angle γ is provided between the facing face  31   a  of the lower electrode  31  and the facing face  32   a  of the upper electrode  32 . 
     An optimum value of the distance H between the front face of the flange  23   a  and the abutting portion  21   c  is calculated on the basis of the measured focal length. The distance between the facing face  31   a  and the facing face  32   a  to be adjusted when the lower electrode  31  is moved close to the upper electrode  32  is set on the basis of the calculated optimum value. 
     When the first light receiving holder  21  is welded to the first light receiving unit  22  on the basis of the above-described settings, the angle γ of the abutting portion  21   c  relative to the line perpendicular to the center line O 1  of the casing  23  (or the front face of the flange  23   a ) can be set and the distance H between the front face of the flange  23   a  and the abutting portion  21   c  can be set as illustrated in  FIG. 5 . 
     So long as the welding operation is performed on the basis of the above-described settings, it is rare that the projection  21   e  is squashed to be fully melted after the first light receiving holder  21  is welded and fixed to the first light receiving unit  22 . In most cases, at least part of the projection  21   e  retains its original form without being melted such that the unmelted part of the projection  21   e  remains between the flange  23   a  and the welded portion  21   d  of the first light receiving holder  21 . 
     As regards a method of providing the angle γ between the facing face  31   a  of the lower electrode  31  and the facing face  32   a  of the upper electrode  32 , the lower electrode  31  is fixed in the resistance welding equipment and an adjustment mechanism capable of attaching the upper electrode  32  such that the axis of the upper electrode  32  is slightly inclined is provided. Alternatively, the upper electrode  32  is fixed and an inclination adjustment mechanism is provided for a table on which the lower electrode  31  is mounted. Alternatively, a plurality of upper electrodes  32  having facing faces  32   a  inclined at different angles γ may be provided and any of the upper electrodes  32  may be selected and used. 
     Preferably, the projection  21   e  includes a ring-shaped continuous structure. The projection  21   e  may include segments successively arranged on a cylinder. 
     After the first light receiving holder  21  is welded and fixed to the first light receiving unit  22  as illustrated in  FIG. 5 , the cap  23   b  of the casing  23  is inserted into the housing  2  and the abutting portion  21   c  of the first light receiving holder  21  is brought into tight contact with the first light-receiving attachment face  2   c  of the housing  2  as illustrated in  FIG. 1 . At this time, the inclination angle γ of the first light receiving unit  22  and the position of the first light receiving unit  22  in a direction along the first optical path branch L 3  are determined. The first light receiving holder  21  is finely adjusted in the direction orthogonal to the first optical path branch L 3  so that the light receiving element  24  is positioned on a light receiving path on which light travels along the first optical path branch L 3 . After that, the first light receiving holder  21  is fixed to the housing  2  by, for example, laser welding. Thus, the position of the light receiving element  24  on the first optical path branch L 3  and the inclination angle of the element  24  relative to the first optical path branch L 3  can be appropriately set. 
     The second light receiving holder  26  and the second light receiving unit  27  in  FIG. 1  can also be adjusted and incorporated in a manner similar to the first light receiving holder  21  and the first light receiving unit  22 . When the second light receiving unit  27  receives light modulated based on data having a relatively low density and low transmission rate, only the distance H between the flange and the abutting portion of the second light receiving holder  26  may be adjusted without the inclination angle γ illustrated in  FIG. 5  being adjusted. 
     The optical communication device according to the present embodiment of the present invention is not limited to the wavelength division multiplex type provided with the plurality of light receiving units  22  and  27  as illustrated in  FIG. 1 . The optical communication device may include only the light receiving unit  22  attached to the housing. Alternatively, the device may be a light-receive-only module that does not include the light emitting unit  18 . The optical path may include an optical fiber or may include no optical fiber.