Abstract:
An optical transceiver module of the present invention includes: a light emitting device; a photodetector; and a shielding member covering the photodetector and having a hole for providing an optical path for a received beam incident to the photodetector; wherein the received beam incident to the photodetector forms an angle of less than 90 degrees with respect to a beam emitted from the light emitting device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical transceiver module, and more particularly to an optical transceiver module suitable for use in optical communications. 
         [0003]    2. Background Art 
         [0004]    Optical subscriber terminals for FTTH (Fiber to the Home) networks use an optical transceiver module that allows bidirectional transmission over a single optical fiber. Japanese Laid-Open Patent Publication No. 2004-264659 discloses such an optical transceiver module, which is characterized as follows. The module is a hermetically sealed package containing a light emitting device (or laser diode) and a photodetector (or photodiode). A light beam of one wavelength is emitted from the light emitting device and coupled into the optical fiber, while another light beam of a different wavelength is emitted from the optical fiber and coupled into the photodetector. These two light beams of different wavelengths are combined/separated (or muxed/demuxed) using the same lenses and diffractive optical elements. This configuration enables single-fiber bidirectional communications. Other conventional art includes Japanese Laid-Open Patent Publication No. 3-289826 (1991), 59-128508 (1984), 2001-345475, and 2000-89065. 
         [0005]    In an optical transceiver, the power ratio of the electrical signal input to the light emitting device relative to the electrical signal output from the photodetector is high (approximately 50 dB). Therefore, the electrical signal output from the photodetector tends to be affected by the electrical signal input to the light emitting device, resulting in interference called “crosstalk.” This problem of crosstalk is more significant in the case of single-package optical transceiver modules containing both a light emitting device and a photodetector, since these components are disposed in close proximity. To reduce crosstalk, the photodetector may be covered with or enclosed within a shielding member (a grounded metal member). This prevents external signals from reaching the photodetector. However, adding such a shielding member requires extra space in the module. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention has been devised to solve the above problems. It is, therefore, an object of the present invention to allow a single-package optical transceiver module containing a light emitting device and a photodetector to exhibit reduced crosstalk without increasing its component mounting space and hence its size, which leads to easy mounting of the device. 
         [0007]    According to one aspect of present invention, an optical transceiver module include a light emitting device, a photodetector, and a grounded metal member covering the photodetector and having a hole for providing an optical path for a received beam. The optical path extending between a light receiving point of the photodetector and a predetermined point along an optical path traveled by a beam emitted from the light emitting device. The optical path extending between the light receiving point and the predetermined point forms an angle of less than 90 degrees with respect to the optical path traveled by the beam emitted from the light emitting device. 
         [0008]    According to another aspect of the present invention, an optical transceiver module include a metal pin through which a light emitting device receives an input signal, and a metal pin through which a light-receive-side signal passes. The metal pin through which the light emitting device receives the input signal and the metal pin through which the light-receive-side signal passes penetrate through a metal stem. The circumferential surface of the metal pin through which the light emitting device receives the input signal is covered with a thicker material than the circumferential surface of the metal pin through which the light-receive-side signal passes. 
         [0009]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a diagram illustrating an optical transceiver module according to a first embodiment of the present invention; 
           [0011]      FIG. 2  is a diagram showing an exemplary configuration in which the optical transceiver module shown in  FIG. 1  is used for optical communications; 
           [0012]      FIG. 3  shows an optical transceiver module of the second embodiment; 
           [0013]      FIG. 4  is a diagram showing an optical transceiver module of the third embodiment; and 
           [0014]      FIG. 5  is a diagram showing an optical transceiver module of the fourth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0015]      FIG. 1  is a diagram illustrating an optical transceiver module according to a first embodiment of the present invention. 
         [0016]    Referring to  FIG. 1 , a stem  10  is a grounded metal member. A metal member  28  for mounting a light emitting device thereon is mounted on the stem  10 . As shown in  FIG. 1B , an light emitting device  12  is disposed on almost upper edge portion of the light-emitting device mounting member  28 . The light emitting device  12  is connected to two light-emitting device power supply pins  14  by lead wires  30 . The two light-emitting device power supply pins  14  are used to supply an input signal to the light emitting device  12 . The light-emitting device power supply pins  14  are fixed to the stem  10  by frit glass. 
         [0017]    A photodetector  16  is also mounted on the stem  10 . As shown in  FIG. 1B , the photodetector  16  is disposed substantially immediately below the light emitting device  12 . The photodetector  16  is connected to a preamp IC  18  which in turn is connected to two photodetector output pins  24  on the respective sides thereof. The photodetector output pins  24  are fixed to the stem  10  by frit glass. A capacitor  22  is connected to the above preamp IC  18 . It is a decoupling capacitor used to supply stable power to the preamp IC  18 . This capacitor is also connected to a preamp IC power supply pin  20  to receive power. The preamp IC power supply pin  20  is fixed to the stem  10  by frit glass. 
         [0018]    A shielding member  26  as shown in  FIG. 1C  is mounted on the stem  10 . It is made of a metal and is electrically coupled to the stem  10  which is grounded. The shielding member  26  has a pinhole  32  to provide an optical path for the received beam. According to the present embodiment, the pinhole  32  has a diameter of 100 μm. 
         [0019]    The diameter of the pinhole  32  is preferably large enough to allow the (entire) received beam to reach the photodetector  16 . The received beam has a divergence angle of approximately 8 degrees. According to the present embodiment, the light receiving surface of the photodetector  16  is spaced a distance of 0.3 mm from the inner surface of the top wall of the shielding member  26 , as shown in  FIG. 1B . In this case, the minimum required diameter of the pinhole  32  is 84 μm. In this example, the diameter of the pinhole  32  is 100 μm, as described above, to accommodate process variations. This arrangement of the present embodiment prevents the photodetector  16  from receiving unwanted light (i.e., light other than the beam received from the optical fiber). 
         [0020]    The shielding member  26  is provided to isolate the components on the light receiving side from those on the light emitting side, namely, the light emitting device  12 , the light-emitting device power supply pins  14 , and the lead wires  30 , to prevent electrical interference. Specifically, according to the present embodiment, the shielding member  26  is disposed to cover the photodetector  16 , the preamp IC  18 , the photodetector output pins  24 , the capacitor  22 , and the preamp IC power supply pin  20 . 
         [0021]      FIG. 2  is a diagram showing an exemplary configuration in which the optical transceiver module shown in  FIG. 1  is used for optical communications. This configuration includes a diffraction grating  34  that has a function to cause one wavelength λ 1  of light to travel straight and to diffract another wavelength λ 2  of light at an angle, which is referred to as a “diffraction angle.” According to the present embodiment, the diffraction angle of the diffraction grating  34  (at the wavelength λ 2 ) is smaller than 90 degrees, namely approximately 10 degrees. As shown in  FIG. 2 , an optical fiber  36  is disposed above the diffraction grating  34 . Although an optical communications system basically includes a lens(es) to collimate light, a description thereof will not be provided herein for brevity. 
         [0022]    The operation of the optical transceiver module of the present embodiment will now be described. The light emitting device  12  emits a transmission beam of wavelength λ 1 . This transmission beam (of wavelength λ 1 ) enters the diffraction grating  34  and propagates straight through the gating. After passing through the diffraction grating  34 , the transmission beam (of wavelength λ 1 ) enters an optical fiber  36  at an end face thereof. On the other hand, a received beam of wavelength λ 2  is emitted from the same end face of the optical fiber  36 . When the received beam (of wavelength λ 2 ) passes through the diffraction grating  34 , the received beam (of wavelength of λ 2 ) is diffracted and hence its travel direction is changed by an angle corresponding to the above diffraction angle of the diffraction grating  34 . The optical system shown in  FIG. 2  is configured such that the diffracted received beam (of wavelength of λ 2 ) passes through the pinhole  32  to reach the photodetector  16 . Thus, the photodetector  16  can receive the received beam of wavelength λ 2  from the optical fiber  36 . In this way, the system shown in  FIG. 2  provides single-fiber bidirectional communications. 
         [0023]    In an optical transceiver module, the power ratio of the signal (or light) emitted from the light emitting device  12  relative to the signal (or light) input to the photodetector  16  is high (approximately 50 dB). As a result, the high power signal input to the light emitting device  12  interferes with the signal output from the photodetector  16 . (This interference is referred to as “crosstalk.”) One factor in causing such crosstalk is spatial coupling between electromagnetic fields. An effective method for controlling, or reducing, crosstalk is to cover the photodetector  16  with a grounded metal member to shield the photodetector output signal from external electromagnetic waves. 
         [0024]    In the optical transceiver module of the present embodiment, the electromagnetic waves emitted from the light-emitting device power supply pins  14  and the lead wires  30  may cause crosstalk. It should be noted that reducing the distance between the light emitting device  12  and the photodetector  16  results in a reduction in the distances between the photodetector  16  and the light-emitting device power supply pins  14  and the lead wires  30 , leading to increased crosstalk. Therefore, in order to reduce crosstalk, the light emitting device  12  must be spaced a sufficient distance apart from the photodetector  16 . On the other hand, to reduce the component mounting space, the light emitting device  12  and the photodetector  16  are preferably disposed in close proximity. In the optical transceiver module of the present embodiment, the photodetector  6 , etc. are enclosed within the shielding member  26 , which allows the light emitting device  12  and the photodetector  16  to be disposed in close proximity (that is, allows reduction of the component mounting space) while reducing crosstalk. 
         [0025]    Although in the optical transceiver module of the present embodiment the shielding member  26  covers not only the photodetector  16  but also the capacitor  22 , the photodetector output pins  24 , the preamp IC power supply pin  20 , and the preamp IC  18 , the present invention is not limited to such a configuration. For example, in the case of an optical transceiver module in which capacitors and ICs are not disposed on the stem  10 , only the photodetector  16  and the components around it may be covered with the shielding member  26 . 
         [0026]    Further, although the diffraction grating  34  shown in  FIG. 2  has been described as having a diffraction angle of 10 degrees (at wavelength λ 2 ), it may have any diffraction angle less than 90 degrees, as described above, which still results in reduced component mounting space. 
       Second Embodiment 
       [0027]    A second embodiment of the present invention provides an optical transceiver module that has smaller component mounting space than the optical transceiver module of the first embodiment. 
         [0028]      FIG. 3  shows an optical transceiver module of the present embodiment. This optical transceiver module differs from that of the first embodiment in that it includes a stem  38  and a shielding member  40  instead of the stem  10  and the shielding member  26 . The stem  38  is a grounded metal member and has upwardly protruding stem protrusions  13 , as shown in  FIG. 3B . Each stem protrusion  13  surrounds a respective light-emitting device power supply pin  14  and has a height equal to or greater than the thickness of the shielding member. This means that the circumferential surfaces of the light-emitting device power supply pins  14  are covered with a thicker metal than the circumferential surfaces of the photodetector output pins  24 . It should be noted that the surfaces of the stem protrusions  13  in contact with the shielding member  40  form a flat plane together with the surface of the light-emitting device mounting member  28  on which the light emitting device  12  is mounted. 
         [0029]    The shielding member  40  has a top surface  44  and three side surfaces  46 ,  48 , and  50 , as shown in  FIG. 3C . Each side surface  46 ,  48 ,  50  meets the top surface  44 . The shielding member  40  also has a pinhole  32 , which has the same dimensions as described in connection with the first embodiment. The shielding member  40  is mounted on the stem  38  such that an open side  45  of the shielding member  40  is in contact with the stem protrusions  13 , and the shielding member  40  covers the photodetector  16 , the photodetector output pins  24 , the preamp IC  18 , the preamp IC power supply pin  20 , and the capacitor  22 . Thus, the shielding member  40  is electrically coupled to the stem  38  which is grounded. 
         [0030]    The optical transceiver module of the present embodiment operates in the same manner as the optical transceiver module of the first embodiment. 
         [0031]    In the case of an optical transceiver module having a restricted component mounting space, the light emitting device  12  and the photodetector  16  must be disposed in close proximity, which may prevent the shielding member  26  of the first embodiment from being mounted on the stem. Even in such a case, the shielding member  40  of the present embodiment may be able to be mounted on the stem, since the side ( 45 ) of the shielding member  40  facing the stem protrusions  13  has no side wall, that is, the open side  45  of the shielding member  40  is in contact with the stem protrusions  13 . Thus, the shielding member  40  allows the light emitting device  12  and the photodetector  16  to be disposed closer to each other, as compared to the shielding member  26  of the first embodiment. 
         [0032]    According to the present embodiment, the stem protrusions  13  block the propagation of the electromagnetic waves emitted from the light-emitting device power supply pins  14 . Further, the shielding member  40  and the stem protrusions  13 , which together cover the photodetector  16 , etc., block the electromagnetic waves emitted from the lead wires  30 . In this way, the shielding member  40  and the stem protrusions  30  prevent crosstalk between the input and output electrical signals. Thus, like the optical transceiver module of the first embodiment, this optical transceiver module exhibits reduced crosstalk. 
         [0033]    While the optical transceiver module of the present embodiment has been described as including the shielding member  40 , this member may be omitted where the electromagnetic waves emitted from the lead wires  30  are of low intensity, since in such a case it is sufficient that the stem protrusions  13  block the electromagnetic waves emitted from the light-emitting device power supply pins  14 . 
       Third Embodiment 
       [0034]    A third embodiment of the present invention provides an optical transceiver module that exhibits lower optical crosstalk than the optical transceiver module of the second embodiment. 
         [0035]      FIG. 4  is a diagram showing an optical transceiver module of the present embodiment. This optical transceiver module differs from that of the second embodiment in that it additionally includes a dielectric multilayer film filter  42 . The dielectric multilayer film filter  42  has a function to transmit only a particular wavelength of light. Specifically, according to the present embodiment, the dielectric multilayer film filter  42  allows only the received beam (of wavelength λ 2 ) to pass through. This filter exhibits lower filtering performance when filtering light incident at different angles than that of the received beam (of wavelength λ 2 ). Further, it has a thin plate-like structure so that it can be mounted within the shielding member  40 . 
         [0036]    The dielectric multilayer film filter  42  is disposed within the shielding member  40  such that it is located immediately above the photodetector  16 . According to the present embodiment, the received beam passes through this filter before entering the photodetector  16 . 
         [0037]    The optical transceiver module of the present embodiment operates in the same manner as the optical transceiver module of the first embodiment. 
         [0038]    In the case of a single-package optical transceiver module containing both the light emitting device  12  and the photodetector  16 , unwanted light generated from the light emitting device  12  may reach the photodetector  16 , which may result in interference with the received beam incident to the photodetector  16 . (This interference is referred to as “optical crosstalk.”) To prevent this, the optical transceiver module of the present embodiment includes the dielectric multilayer film filter  42  disposed immediately above the photodetector  16 , as described above, to prevent unwanted light from entering the photodetector  16 . Since the dielectric multilayer film filter  42  is mounted within the shielding member  40  (as described above), only light that has passed through the pinhole  32  enters the filter. That is, only light in a limited range of incident angles reaches the dielectric multilayer film filter  42 , allowing the filter to properly function. Thus, the optical transceiver module of the present embodiment includes the dielectric multilayer film filter  42  to prevent unwanted light from entering the photodetector  16  and thereby reduce optical crosstalk. 
       Fourth Embodiment 
       [0039]    A fourth embodiment of the present invention provides an optical transceiver module in which the dielectric multilayer film filter  42  can be more easily mounted, as compared to the optical transceiver module of the third embodiment. 
         [0040]      FIG. 5  is a diagram showing an optical transceiver module of the present embodiment. This optical transceiver module differs from that of the third embodiment in that the dielectric multilayer film filter  42  is mounted at a different location. Specifically, according to the present embodiment, the dielectric multilayer film filter  42  is disposed on the top surface of the top wall of the shielding member  40 , immediately above the pinhole  32 . It is fixed to the shielding member  40  by an adhesive. This arrangement allows the received beam to pass through the dielectric multilayer film filter  42  before entering the photodetector  16 . The dielectric multilayer film filter  42  reduces optical crosstalk, as in the third embodiment. 
         [0041]    The optical transceiver module of the present embodiment operates in the same manner as the optical transceiver module of the first embodiment. 
         [0042]    In fabrication of the optical transceiver module of the third embodiment, the dielectric multilayer film filter  42  must be bonded to the shield member  40  before the shield member  40  is fixed to the stem  38 , since the dielectric multilayer film filter  42  is disposed within the shield member  40 . It should be noted that soldering is the easiest way to fix the shielding member  40  to the stem  38 . However, in the case of the optical transceiver module of the third embodiment, such soldering may cause thermal damage to the dielectric multilayer film filter  42 , resulting in degraded filtering characteristics. In the case of the optical transceiver module of the present embodiment, on the other hand, since the dielectric multilayer film filter  42  is disposed on the exterior of the shielding member  40 , it can be bonded to the shielding member  40  after the shield member  40  is soldered to the stem  38 , thus avoiding causing thermal damage to the dielectric multilayer film filter  42 . 
         [0043]    Thus, the present invention allows an optical transceiver module to exhibit reduced crosstalk between the input and output signals without increasing its component mounting space. 
         [0044]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may by practiced otherwise than as specifically described. 
         [0045]    The entire disclosure of a Japanese Patent Application No. 2007-028551, filed on Feb. 7 th  2007 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.