Abstract:
A bi-directional optical subassembly (BOSA) with a unique arrangement to set the WDM filter is disclosed. The BOSA includes the stem and the lens cap. The stem mounts both the LD and the PD. While, the lens cap provides, in addition to the condenser lens, a structure to set the WDM filter in a top center thereof. The WDM filter is set in the lens cap with a present angle to the primary surface of the stem when the lens cap is fixed to the stem. Assembling the lens cap with the stem, the relative optical alignment between the LD, the PD and the WDM filter is performed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to the following commonly-assigned U.S. patent applications: U.S. Ser. No. 12/430,520, entitled: BI-DIRECTIONAL OPTICAL MODULE INSTALLING LIGHT-EMITTING DEVICE AND LIGHT-RECEIVING DEVICE IN SIGNAL PACKAGE; and U.S. Ser. No. 11/905,505, entitled: BI-DIRECTIONAL OPTICAL MODULE WITH A POLARIZATION INDEPENDENT OPTICAL ISOLATOR; which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a bi-directional optical subassembly, in particular, the invention relates to a bi-directional optical subassembly (hereafter denoted as BOSA) that installs a semiconductor laser diode (hereafter denoted as LD) and a semiconductor photodiode (hereafter denoted as PD) in a common coaxial CAN package and also installs a wavelength division multiplexing (hereafter denoted as WDM) filter in the CAN package. 
         [0004]    2. Related Prior Art 
         [0005]    Recent optical access network represented by the fiber to the home (FTTH) has aggressively introduced the passive optical network (PON) system because the PON system may realize a higher speed and a larger capacity communication with a reasonable cost. The PON system uses a bi-directional communication on a single fiber, which may decrease a number of fibers to be installed in the system, and shares the single fiber with a plurality of subscribers. Thus, the PON system may reduce the cost of the communication system as that using the metal cables. According to the PON system, two or three optical signals each having wavelengths of 1.31 μm, 1.49 μm and/or 1.55 μm, respectively, propagate within the signal fiber. 
         [0006]    A BOSA practically installed within the PON system is necessary to couple the light emitted from the LD optically with the fiber and also to couple the other light provided from the single optical fiber with the PD. One type of the BOSA installs the LD and the PD within a coaxial CAN package in addition to a WDM filter that reflects the light coming from the LD toward the optical fiber, while transmits the other light provided from the optical fiber to the PD. Thus, the WDM filter in the primary surface thereof is set by 45° with respect to the primary surface of the package where the LD and the PD are mounted thereon. Various techniques to mount the WDM filter by the angle 45° have been proposed in prior arts. 
         [0007]    For instance, a Japanese Patent Application published as JP-2004-271921A has disclosed an arrangement shown in  FIG. 4A  where the LD  14  is mounted to the stem  13  through the sub-carrier  15  such that the optical axis thereof is aligned in parallel to the primary surface  13   a  of the stem  13 . The light coming from the LD  14  is reflected by the WDM filter  16  so as to enter the optical fiber F. That is, the WDM filter is assembled with the surface  16   a  of the sub-carrier  15  such that the primary surface  16   a  of the WDM filter at which the light coming from the LD  14  is reflected makes a preset angle with respect to the primary surface  16   a.    
         [0008]    The prior application mentioned above has also disclosed another arrangement within the package shown in  FIG. 4B . This modified arrangement forms a portion  13   b  extending from the primary surface  13   a  of the stem  13  to set the WDM filter thereon such that the light from the LD  14  on the sub-carrier  15  enters the optical fiber F. The portion  13   b  has an inclined surface  13   c  that makes a preset angle with respect to the primary surface  13   a , and the WDM filter  16  is set on this inclined surface  13   c.    
         [0009]    Even in respective arrangements shown in  FIGS. 4A and 4B , the WDM filter  16  is necessary to be firstly aligned in the angle thereof relative to the optical axis of the LD  14  so as to guide the light emitted from the LD  14  toward the optical fiber F, and to align the lens  12   a  secondly so as to enter the light reflected by the WDM filter  16  in the optical fiber F. The arrangement shown in  FIG. 4A  fixes the WDM filter  16  onto one surface  15   a  of the LD carrier  15 , accordingly, the first alignment above mentioned may be carried out by rotating the LD  14  on the LD sub-carrier  15 , and the second alignment may be performed by positioning the lens cap  12  on the primary surface  13   a  of the stem  13 . While, in the arrangement shown in  FIG. 4B , the first alignment may be done by rotating the sub-carrier  13  for the WDM filter  16  on the primary surface  13   a  and the second alignment may be also performed by adjusting the position of the lens cap  12  on the stem  13 . Two-steps alignment is inevitable in each case. 
       SUMMARY OF THE INVENTION 
       [0010]    An aspect of the present invention relates to an arrangement of a bi-directional optical subassembly that optically couples with an external optical fiber and comprises an LD, a PD, a WDM filter and a coaxial CAN package including a cap and a stem. The LD emits first light with a first wavelength toward an external fiber. The PD receives second light with a second wavelength different from the first wavelength provided from the external fiber. The WDM filter reflect the first light coming from the LD toward the external fiber; while, transmits the second light coming from the external fiber toward the PD. The LD, the PD, and the WDM filter are mounted on the primary surface of the stem and air-tightly sealed within a space formed by the cap and the stem. A feature of the present arrangement is that the WDM filter is attached to the cap with a preset angle to the primary surface of the stem. The cap of the present invention may have a hollow with a slant surface inclined with the preset angle to the primary surface of the stem, in which the WDM filter is attached to the slant surface of the hollow. 
         [0011]    Another aspect of the present invention relates to a method to assemble the bi-directional optical subassembly. The method includes steps of: (a) mounting the LD and the PD on the stem, (b) attaching the WDM filter to the cap, where the stem and the cap constitute a coaxial CAN package, (c) optically aligning the cap with the stem, and (d) fixing the cap to the stem. A feature of the invention is that the optical alignment of the cap includes steps of (c 1 ) viewing an image of an active layer of the LD through the WDM filter, (c 2 ) viewing an image of an sensitive are of the PD through the WDM filter, and (c 3 ) positioning the cap on the stem such that the image of the active layer of the LD overlaps with the image of the sensitive area of the PD on the WDM filter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
           [0013]      FIG. 1  is a partial cross section showing an embodiment of a bi-directional optical subassembly according to the present invention; 
           [0014]      FIG. 2  is a partial cross section showing another embodiment of a bi-directional optical subassembly according to the present invention; 
           [0015]      FIGS. 3A and 3B  illustrate a lens cap installed in the bi-directional optical subassembly shown in  FIGS. 1 and 2 ; and 
           [0016]      FIGS. 4A and 4B  are cross sections of a conventional BOSA. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]      FIGS. 1 and 2  illustrate a BOSA according to the present invention, in which  FIG. 1  is a cross section of the BOSA; while,  FIG. 2  is a partial cross section of the BOSA.  FIGS. 3A and 3B  each shows a lens cap  7   b  of the BOSA shown in  FIGS. 1 and 2 , in which  FIG. 3A  is a cross section of the lens cap  7   b ; while,  FIG. 3B  is a perspective view of the lens cap  7   b.    
         [0018]    The BOSA according to the present embodiment shown in  FIGS. 1 and 2  has the LD  2 , the PD  3  and the WDM filter  4  within the coaxial CAN package  5 . The LD  2  emits light with a wavelength of 1310 nm toward the external optical fiber, and is mounted on the primary surface  6   a  of the stem  6  through the heat sink  2   a . The LD  2  may be a type of the distributed feedback (DFB) LD. The optical axis of the LD  2 , along which the light emitted from the LD propagates, is substantially in parallel with the primary surface  6   a . The PD  3  receives light with a wavelength of, for instance, 1490 nm different from that of the light emitted from the LD  2 , and is also mounted on the primary surface  6   a  of the stem  6  through the sub-mount  3   a.    
         [0019]    The WDM filter  4  reflects the light coming from the LD  2  toward the external optical fiber; while, at the same time, the other light provided from the external optical fiber transmits toward the PD  3 . This WDM filter  4  is set within the CAN package  5  so as to make a preset angle to the primary surface  6   a  of the stem  6 . In a conventional BOSA as shown in  FIGS. 4A and 4B , the WDM filter is mounted on a surface  15   a  ( 13   c ) of the sub-carrier  15  ( 13   b ); while, the BOSA  1  according to the present embodiment has the WDM filter  4  is fixed to the lens cap  7 , which is one of specific features of the present BOSA  1 . 
         [0020]    The CAN package of the present embodiment includes the stem  6  with the primary surface  6   a  on which the LD  2  and the PD  3  are mounted and the lens cap  7  provided with a condenser lens  7   a . The lens cap  7  is assembled with the stem  6  to seal the devices; the LD  2 , the PD  3 , the WDM filter  4  and so on, air-tightly on the primary surface  6   a . The LD  2  is mounted on the heat sink  2   a , and the heat sink  2   c  is set on the mesa  6   c  that protrudes from the primary surface  6   a . Although not explicitly illustrated in  FIGS. 1 and 2 , the primary surface  6   a  mounts, in addition to the LD  2  and the PD  3 , a pre-amplifier IC for amplifying a signal generated by the PD  3  and bypass capacitors to reduce noise caused in power supply lines to the LD  2 , the PD  3  and the pre-amplifier. 
         [0021]    The stem  6  also provides a plurality of lead pins  8 . These lead pins  8  pass the through holes  6   d  as filling the gap with seal glass. One of lead pins  8   a  mounts the monitor PD  9  on a surface  8   a  in a top portion thereof. The monitor PD  9  monitors light emitted from the back facet of the LD  2 . In order to prevent the light reflected by the surface of the monitor PD  9  from returning the LD  2 , the surface  8   a  where the monitor PD  9  is mounted thereon is slightly tilted, by about 5° to 10°, with respect to the optical axis of the LD  2 . 
         [0022]    The lens cap  7 , as illustrated in  FIGS. 3A and 3B , includes the condenser lens  7   a  and the shell  7   b  to hold the lens  7   a . The shell  7   b  may be formed by, for instance, machining or stamping the metal sheet. The lens  7   a  is set in an aperture  7   c  on a top center portion of the shell  7   b  with a seal glass. The shell  7   b  is assembled with the stem  6  by the resistance welding. 
         [0023]    The ceiling of the shell  7   b  provides a hollow  7   d  with a slant surface  7   e  to attach the WDM filter  4  thereto. The slant surface  7   e  has an angle of 45° to the primary surface  6   a  when the lens cap  7   a  is assembled with the stem  6 . Thus, the WDM filter  4  in the surface  4   a  thereof may have the preset angle with respect to the optical axis of the LD  2 , that of the PD  3  and also that X of the lens  7   a  as it is attached to the slant surface  7   e . The WDM filter  5  may be attached on the slant surface  7   e  with a low-melting glass or an ultraviolet curable resin after the lens  7   a  is set in the aperture  7   c  with a seal glass. 
         [0024]    A BOSA with a WDM filter generally sets the WDM filter first and then aligns the LD such that the light emitted from the LD enters the optical fiber through the condenser lens. That is, two steps of optical alignments are necessary in the conventional BOSA. While, in the BOSA according to the present embodiment, because the WDM filter is attached to the shell  7   b  and this shell  7   b  is to be assembled with the stem  6 , the optical alignment of the LD  2  is necessary only to align the shell  7   b  on the stem  6 , which may effectively reduce a process time for the optical alignment. Specifically, the optical alignment for the LD  2  according to the present arrangement of the BOSA may be carried out by aligning the shell  7   b  so as to overlap the light-emitting layer of the LD  2  and the light-sensitive area of the PD  3  each viewed through the WDM filter  4  at the position where the lens is to be set. 
         [0025]    Thus, the BOSA  1  according to the present embodiment may decrease the process time to align the devices optically, which may effectively reduce the cost of the BOSA  1 . In the conventional BOSA shown in  FIGS. 4A and 4B , a device such as sub-carrier for the LD is necessary to mount the WDM filter on a slant surface of the device. Such a sub-carrier may be formed by grinding, polishing and so on. On the other hand, the present BOSA provides, to mount the WDM filter, the shell that is formed by machining or cutting. 
         [0026]    While there has been illustrated and described what are presently considered to be example embodiments of the present invention, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.