Abstract:
An optical module that implements with a tri-plexer assembly is disclosed. The tri-plexer module comprises a bi-directional module for transmitting digital input and output signals and an analogue optical assembly for receiving analog optical signals. The bi-directional module installs both a light-emitting device and a light-receiving device in a signal package. The analogue optical assembly is assembled such that the optical axis thereof makes a substantially right angle with the optical axis of the bi-directional module. The signal ground of the module is common to the analogue module and to a section for receiving the digital data; while, the chassis ground or the frame ground in the module is isolated from the signal ground.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority of U.S. Provisional Patent Application Ser. No. 61/071,003 filed on Apr. 8, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical module  1  that handles three types of optical signals and is usable in the optical CATV system. 
     2. Related Prior Arts 
     The PON system (Passive Optical Network system) transmits and receives two types of optical signals, each having wavelengths of 1.31 μm and 1.55 μm through a single optical fiber  50 . A bi-directional optical module  1  applicable to this PON system is necessary to provide, in addition to a light receiving device and a light-transmitting device, a WDM (Wavelength Division Multiplexing) coupler or a WDM filer. A Japanese Patent Application published as JP-2005-099482A has disclosed such a bi-directional optical module  1 . This optical module  1  has two devices each installing the semiconductor light-receiving device or the semiconductor light-transmitting device. 
     A United States Patent, the U.S. Pat. No. 7,093,988, has disclosed another type of the bi-directional optical module  1  as one modification of the module above, in which a semiconductor device to receive signal light and another semiconductor device to transmit another signal light are housed within a single package accompanied with the WDM filter. In this modified optical module  1  shown in  FIG. 1  of the prior art, within the package comprised of the stem  114  and the cap  110  with the lens is enclosed with the light-transmitting device  113  and the light-receiving device  112 . The light emitted from the light-transmitting device  113  couples with the optical fiber  132  after it is reflected by the WDM filter  115  and focused by the lens  116 . On the other hand, the light provided from the optical fiber  132  couples with the light-receiving device  112  after it is focused by the lens  116  and passes through the WMD filter  115 . 
     Such a bi-directional module with a single package, when it is installed in an optical communication system, may leave a space in a side of this bi-directional module for another functional device to be installed therein. From the viewpoint of the system configuration, one request to transmit another optical signal in an analog form, in addition to digital optical signals each being transmitted in up-and-down directions, has become practical; in particular, the optical CATV (Cable Television) system has strongly demand this new configuration. 
     The conventional CATV system that uses a copper cable has transmit analogue video signals, whose frequency range is 100 to 700 MHz, from the center station to respective subscribers by the frequency multiplexing, while, each subscriber has upload digital signals with a frequency range below 50 MHz to the center station. Currently, the optical CATV system first converts the downward video signal into an optical signal with the same frequency spectrum of the analogue electrical signal explained above. In addition, the CATV system now strongly demands to access the internet system optically. Thus, the current CATV system is necessary to handle three types of optical signals, one is the downward video signal and the other two are the upward and the downward digital signals for the internet access. 
     To satisfy the request in the current CATV system mentioned above, a new architecture has been proposed in which three optical signals are transmitted by multiplexing different wavelengths. The downward video signal assigns 1.55 μm, the downward digital signal assigns 1.49 μm, and the upward digital signal assigns 1.31 μm. The optical module  1  applicable to such a new optical CATV system provides the bi-directional module with the single package that handles the upward and the downward digital signals, and an optical-receiving module for the analog video signal set in the side of the module where the conventional bi-directional module with two or more packages assembles the light-receiving module. This type of the optical module  1  is what is called as tri-plexer. 
     The analog video signal has a frequency range from 40 to 870 MHz with the frequency multiplexing configuration and the wavelength of 1.55 μm, while, the downward digital signal may have several standards, for instance, a standard whose transmission speed up to 620 Mbps is called as BPON, those up to 1.25 Gbps is called as EPON, and those up to 2.5 Gbps is called as GPON. The present invention relates to this type of the tri-plexer and an optical module  1  installing such a tri-plexer. 
     SUMMARY OF THE INVENTION 
     A feature of the present invention relates to an optical module applicable to the GPON system that transmits two digital signals with wavelengths of 1.3 μm and 1.48 μm and one analog signal with a wavelength of 1.55 μm. The optical module comprises a tri-plexer optical subassembly that provides an analog module, a bi-directional module, and a WDM filter. The analog module receives the optical signal with the wavelength of 1.55 μm. The bi-directional module, which has a single package that commonly installs a semiconductor light-receiving device and a semiconductor light-emitting device, receives the first digital signal with a wavelength of 1.48 μm and transmits the second digital signal with a wavelength of 1.3 μm. The WDM filter discriminates the optical signal with the wavelength of 1.55 μm from the other optical signals with wavelengths of 1.48 μm and 1.3 μm, respectively. In the present optical module, the analog module and the bi-directional module are assembled substantially in a right angle with respect to each other to isolate an analog circuit from a digital circuit electrically. 
     The optical module may further comprise a housing, a printed circuit board and a ground plate. The housing includes a base, which is made of resin, a heat sink and a metal shell to form a space within which the tri-plexer subassembly and the circuit board is installed. The metal shell provides a frame ground of the optical module, while, the signal ground is implemented on the printed circuit board. The printed circuit board installs the first circuit that is coupled with the analog module and processes the analog signal and a second circuit that is coupled with the bi-directional module and processes the digital signal. The first circuit is electrically isolated from the second circuit except for the signal ground. The heat sink, which is set within a ceiling of the metal shell and is made of relatively thicker metal, provides a heat-dissipating path from the electronic circuit to the exterior of the optical module through the metal shell. 
     The ground plate is fixed to the bi-directional module and to the printed circuit board so as to distinguish the lead pins for the light-receiving section from the lead pins for the light-transmitting section. Accordingly, the ground plate has functions of: (1) grounding the bi-directional module on the signal ground on the printed circuit board; (2) electrically shielding the light-receiving section from the light-transmitting section; (3) supporting the rear portion of the tri-plexer optical subassembly with respect to the printed circuit board; and (4) dissipating heat generated in the bi-directional module directly to the printed circuit board without coming in contact with the heat sink, which may thermally isolate the tri-plexer optical sub-assembly from the heat sink. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of the optical module according to an embodiment of the invention; 
         FIG. 2  illustrates the metal shell assembled with the heat sink in the interior thereof; 
         FIG. 3A  illustrates the base made of resin without the printed circuit board, and  FIG. 3B  illustrates the base assembled with the printed circuit board but omitting the tri-plexer optical subassembly; 
         FIG. 4  is a magnified view showing a portion where the tri-plexer optical subassembly in a front portion thereof is installed on the base; 
         FIG. 5  illustrates the interior of the optical module where the printed circuit board, the tri-plexer optical subassembly are assembled with the base but the metal shell is omitted; 
         FIG. 6  is a perspective view of the tri-plexer optical subassembly with the ground plate of the first embodiment, which is installed in the optical module; 
         FIG. 7  shows behaviors of the bit error rate of the bi-directional module in conditions of G 3 : without any crosstalk, G 5 : without the ground plate, and G 4 : with the ground plate; 
         FIG. 8  shows an effect of the existence of the ground plate measured through the noise power spectrum; 
         FIGS. 9A and 9B  show another shape of the ground plate according to the second embodiment of the invention; 
         FIGS. 10A and 10B  minutely describe the arrangement of the ground plate with the stem of the bi-directional module according to the second embodiment shown in  FIGS. 9A and 9B ; 
         FIG. 11A  schematically illustrates anther arrangement of the ground plate according to the third embodiment of the invention, and  FIGS. 11B and 11C  illustrate still another type of the ground plate according to the fourth embodiment of the invention; and 
         FIG. 12A  schematically illustrates a modified arrangement of the stem of the bi-directional module that provides a groove to set the ground plate therein,  FIG. 12B  illustrates a still modified arrangement of the stem that provides a hollow to set the ground plate therein, and  FIG. 12C  is a cross section taken along the line L-L′ in  FIG. 12A  or  FIG. 12B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, preferred embodiments according to the present invention will be described as referring to accompanying drawings. 
       FIG. 1  is an exploded view of an optical module  1  according to an embodiment of the present invention. The optical module  1  installs a printed-circuit board (hereafter denoted as PCB)  30 , a tri-plexer optical sub-assembly (hereafter denoted as OSA)  51 , and a heat sink  20  within a package comprised of a base  40  and a metal shell  10 . The metal shell  10  is made of stainless plate with a thickness of about 0.2 mm, which is hard to absorb and to dissipate heat generated in the OSA  51  and the ICs on the PCB  30 . Accordingly, the interior of the shell  10 , which corresponds to the ceiling of the module  1 , attaches a heat sink  20  made of aluminum (Al) slab with a thickness of about 3 mm to absorb heat from the OSA  51  and to conduct heat. 
       FIG. 2  illustrates the interior of the shell  10  assembled with the heat sink  20 . The shell  10  is formed by cutting and bending a single metal sheet, and provides self lock latches  14  in four corners thereof to fix the sides  10   a  and heat sink latches  13  to assemble the heat sink  20  in respective four sides  10   a . These self lock latches  14  and heat sink latches  13  are formed by cutting a portion of the sides  10   a  in the U-shape and bending inwardly an inner portion of this U-shaped cut to form a stopper. The shorting fingers  11  ground the shell  10  by making contact with the ground pattern on the PCB  30 . On the other hand, the non-shorting fingers  12  only mechanically fixes the optical module  1  to the mother board (not shown in the figures) that mounts this optical module  1  by penetrating in the via-hole on the mother board and bending a tip portion of the non-shorting finger  12  at the other side of the mother board. The non-shorting fingers  12  may fix the optical module by soldering them to the land attributed with the via-hole of the mother board. By electrically isolating the via-hole from the ground in the mother board, the shell  10  of the optical module may be also isolated from the mother ground. The base latches  15  are provided to fix the base  40  with the shell  10 . 
     The heat sink  20  shown in  FIG. 2  provides a hollow  22  and a plurality of terraces  21 . The hollow  20  receives the tri-plexer OSA  51  but not comes in directly contact with the tri-plexer OSA  51 , while, one of terraces  21  comes in directly contact with the ICs for the digital signal mounted on the PCB  30  that generate heat, and the other terrace  21  comes in contact with the shield box  32  to shield a portion of the PCB  30  where the analogue circuit is mounted. That is, the ICs and the shield box  32  comes in thermally contact with the corresponding terraces  21  via a thermal sheet, thermal grease, or the like. On the other hand, the tri-plexer OSA  51  may be thermally isolated from the heat sink  20 . Because the tri-plexer OSA  51  mounts a light-emitting device, typically a laser diode (hereafter denoted as LD), whose performances are most sensitive to temperatures, the thermal isolation of the heat dissipating path of the tri-plexer OSA  51  from the heat sink may make the operation of the tri-plexer OSA  51  in stable. 
       FIG. 3A  illustrates the base  40 . The base  40  is made of resin, plastics or the like with a thickness of about 0.5 mm, at a thinnest portion thereof, and has a box shape. Several ribs  41  are formed inside of respective sides  40   a , and the PCB  30  is mounted on these ribs  41 , that is, the top of respective ribs supports the PCB  30  to secure a space under the PCB  30 . On sides facing to each other are formed with a snap fit  42  with a hooked end  42   a . The PCB  30  is placed on the rib  41  and snapped with this hooked end  42   a . The base  40  further provides a post  43  with a stepped top  43   a  in a center portion of the base  40 . Inserting a tip with a smaller diameter of the stepped top  43   a  into a via-hole  33   a  of the PCB  30 , the post  43  not only supports but aligns the PCB  30 . Three connector via-holes  44  are formed in a corner of the base  40  where the analogue circuit for the video signal is assembled. The analog signal received by the tri-plexer OSA  51  and processed by the analogue circuit on the PCB  30  may be extracted from the optical module  1  through lead pins that pass the connector via-holes  43 . The center hole is for the lead pin for the analogue ground, while the holes in both sides are for lead pins for the analogue signals complementary to each other. 
       FIG. 3B  illustrates the base  40  assembled with the PCB  30 .  FIG. 3B  omits the tri-plexer OSA  51 , electronic components such as ICs on the PCB  30 , and the shield box  32  for the analogue circuit. The PCB  30  configures with a row of lead pins  31   a  along one side thereof opposite to the side where the tri-plexer OSA  51  is to be mounted. Another side of the PCB  30  arranges the other three lead pins for the analog signal, the center of which has a smaller diameter compared with the side pins. In the present embodiment, the center pin corresponds to the analog ground, while, the side pins transmit the analog signals as described above. 
       FIG. 4  illustrates the tri-plexer OSA  51  mounted on the base  40 . As shown in  FIG. 4 , the OSA  51  is fixed on the base  40  such that the body portion  53  of the OSA  51 , in which the WDM filter is mounted as explained later, is set on the terrace  44  of the base  40  and a portion  51   c  with a smaller diameter between two flanges  51   b  is put between the hooked latches  47  protruding from the bottom  40   b  of the base  40 . The hook  47   a  in a tip of this latch  47  protrudes toward the other latch  47 . When the OSA  51  is to be set, these hooked latches  47  are bent so as to broaden the hooked end  47   a  from the root portion thereof, and, after the OSA  51  in the portion  51   c  between the flanges  51   b  is set on the saddle  45 , these hooked ends  47   a  press the portion  51   c  against the saddle  45 . The base  40  in the bottom  40   b  thereof provides the holes  46  in the root portion of the latch  47  to facilitate the broadening of the latch  47  when the OSA  51  is to be set. 
       FIG. 5  illustrates the assembly of the PCB  30  with the electronic circuit thereon and the OSA  51  each mounted on the base  40 . Comparing  FIG. 5  with  FIG. 3B  and  FIG. 6 , the tri-plexer OSA  51  is set in a preset position on the base  40  as facing the analogue module  54  toward the center of the PCB  30 . The PCB  30  provides the T-shaped cut to set the analogue module  54  in the center bar of this T-shape continuous to a cut corresponding to the horizontal bar of the T-shape for setting the body  53  of the tri-pexer OSA  51 . The analogue module  54  extends three lead pins (SIC; Vcc and GND) to be soldered on each pattern on the PCB  30 . The GND lead pin is common to the package of the tri-plexer OSA  51  by directly connecting with the stem  54   a  of the analogue module  54 . While, the digital signals, namely, the Tx digital signal and the Rx digital signal, are extracted from the stem  55   a  of the digital module  55  such that the lead pins  55   b  for the Tx signal are connected in the back side of the PCB  30 ; while the lead pins  55   c  for the Rx signal are guided to the front side of the PCB  30 . The digital module  55 , which is a portion removing the analog module  54  and the body portion  53  from the OSA  51 , will be hereinafter called as the bi-directional module. 
     Between the Tx lead pins  55   c  and the Rx lead pins  55   b  is provided with a ground plate  56  whose both ends are soldered with the ground patterns on the PCB  30 . This ground plate  56  secures the electrical isolation between the Tx signal and the Rx signal. Moreover, the ground for the Tx signal and that for the Rx signal are also electrically isolated from each other in the present OSA  51 . The ground pin for the Rx signal is directly connected to the package of the OSA  51  and is common to the analogue ground, while, the ground for the Tx signal is isolated from the Rx ground. This is because the Tx signal has the largest amplitude among signals in the tri-plexer OSA  51 . Moreover, the Tx signal has the current mode to drive the LD. Accordingly, the Tx signal causes the strongest influence to the other signals, namely, the analogue signal and the Rx digital signal. Without the reinforced ground for the digital Tx signal, the common impedance is inherently formed in the ground pattern, which becomes a noise source to degrade not only the sensitivity of the receiver but the crosstalk between the Tx signal and the Rx signal. This crosstalk may be reduced by isolating the Tx ground from the Rx ground. 
     The analogue signal, because the amplitude thereof is far smaller than that of the digital signals and it configures the frequency multiplexed signal, is sensitive to the electromagnetic radiation. Accordingly, the shield box  32  covers the whole portion of the analogue circuit on the PCB  30  to be able to escape from the EMI radiation. The Tx digital signal, which is conducted in the back side of the PCB  30  from the OSA  51 , is guided to the IC  35   b  installed on the top surface of the PCB  30  through the via-holes. Thus, the back surface of the PCB  30  does not install any active devices. 
     Along one edge of the PCB  30  is arranged with connector pins  31  to transmit the digital signals. Because the optical module  1  shown in the figures processes the digital signal whose transmission speed is 2.5 Gbps at fastest in the GPON system, the connector pins  31  are unnecessary to provide the impedance matched arrangement. The connector pins  31  of the present configuration may transmit such digital signals with the speed of 2.5 Gbps without substantial degradation in the signal form. An arrangement, where both sides of the signal pins (Tx, Rx and /Rx, here “/” means the complimentary signal) are provided with pins with the low impedance such as the power supply Vcc and the ground GND, may transmit the signal with suppressed degradation. 
     The OSA  51  provides a sleeve  57  in the front end thereof with a pair of flanges  51   b . As already explained, the OSA  51  is fixed to the base  40  by setting the necked portion  51   c  between the flanges  51   b  by the hooked latch  47 . The OSA  51  provides, continuous to the sleeve  57 , a body  55  that installs the WDM filter to divide the light with the wavelength of 1.55 μm, which transmits the analog signal, from the light with the wavelengths shorter than 1.55 μm, which transmits the digital signals. The WDM filter of the present embodiment is configured with a multi-layered dielectric film on a substrate transparent for wavelengths shorter than 1.55 μm. The light with the wavelength of 1.55 μm heads for the analogue module  54  by being reflected by the WDM filter. The analogue module  54  only mounts the photodiode on the stem  54   a  thereof without any pre-amplifier thereon. 
     The light with the wavelengths shorter than 1.55 μm heads for the bi-directional module  55 . Strictly, the light coming from the optical fiber transmits through the WDM filter or the light output from the bi-direction module  55  is emitted from the OSA  51  to the optical fiber. The bi-directional module  55  configures with lead pins extending from the stem  55   a  thereof, in which an upper half of the stem  55   a  arranges the Rx lead pins  55   c  for the Rx signals (SIG, /SIG, Vcc, Vpd and GND), while the bottom half thereof provides the lead pins  55   b  for the Tx signals (SIG, Vcc, MON).  FIG. 6  illustrates the tri-plexer OSA  51  in upside down. As already explained, the stem  55   a  does not extrude the Tx ground pin. That is, the LD in the bi-directional module  55  is electrically connected between the lead pins, SIG and Vcc, to switch the current provided to the LD by the signal applied to the pin SIG. The photodiode to monitor the optical output from the LD is connected between the pins Vcc and MON. 
     The photodiode that receives the digital Rx signal is a type of the avalanche photodiode (hereafter denoted as APD) in the present embodiment. The APD is necessary to be biased with a several tens of voltages to show the carrier multiplication function. This bias voltage is given through the lead pin Vpd. The bi-directional module  55  installs a pre-amplifier to convert the photocurrent output from the APD into a voltage signal and to amplify this voltage signal. The pre-amplifier is powered by the voltage supplied through the lead pin Vcc and outputs two signals complementary to each other from the lead pins, SIG and /SIG. 
     The stem  55   a  of the bi-directional module  55  further provides a ground plate  56 , whose plane arrangement is like an H-shape. One of four branches of this H-shape is wound around the stem  55   a  so as to surround the lead pins for the Tx signal, while the branches opposite to and diagonal to the branch above explained are soldered with the ground pattern on the PCB  30 . Thus, the lead pins for the Tx signal are surrounded by the ground plate  56  connected to the analogue ground on the PCB  30 , which may form a quasi-shielding of the Tx signal pins and may reduce the crosstalk from the Tx signal to the Rx signal. 
     Thus, the present bi-directional module  55 , even the tri-plexer OSA  1  installs the Tx device and the Rx device in the common package, namely, on the common stem  55   a , the lead pins  55   b  for the Tx signal and those  55   c  for the Rx signal are geometrically divided into two groups, one of which is coupled in the top side of the PCB  30 , while the other is connected with the back side of the PCB  30 , with the ground plate  56  arranged so as to separate these two groups of the lead pins and surround the lead pins  55   b  for the Tx signal. Accordingly, the Rx signal may be isolated from the Tx signal; in particular, the Rx signal may be isolated from the lead pins to supply the driving current of the LD, which may improve the crosstalk. 
       FIG. 7  shows an experimental result of the reduction of the crosstalk. The experiment measured the bit error rate (BRT) with the photodiode by supplying the driving current to the LD to emit light with the power of, for instance, 1.5 dBm as varying the optical receiving power to the photodiode. The behavior G 3  corresponds to a condition where no driving current was supplied, which was equivalent to the no-crosstalk condition, the behavior G 5  corresponds to a condition where the driving current was supplied and the bi-directional module without ground plate, and the behavior G 4  denotes the result when the LD was driven and the bi-directional module with the ground plate between two groups of lead pins. 
     When the LD was turned off, that is, no driving current was provided thereto, the bit error rate became 1×10 −13  for the receiving power of −28 dBm at the point P 1 . While, the laser diode was practically driven with the substantial current, the same bit error rate of 1×10 −13  was obtained for the optical receiving power of −25.8 dBm at the point P 3 , which degraded by 2 dBm. Finally, the bi-directional module provided with the ground plate of the embodiment above described showed the same bit error rate for the receiving power of −27 dBm at the point P 2 , which was half of that obtained without the ground plate. 
       FIG. 8  compares the noise appeared in the output of the digital Rx signal depending on the implementation of the ground plate. In this experiment, the receiver circuit was powered on under no optical input signal, while, the laser diode was driven with a substantial switching current whose primary speed was 2.5 Gbps through a Bessel-Thomson filter with the cut-off frequency of 1.25 GHz. The experiment compared the magnitude and the spectrum of the noise appeared in the output of the receiver circuit. 
     As shown in  FIG. 8 , the output of the receiver circuit showed a difference of about 3 to 5 dBm for a unit frequency depending on the implementation of the ground plate. In particular, the bi-directional module showed the improvement greater than 10 dBm around 150 MHz. Total difference of the noise power accumulated in whole frequencies became about 2.5 dB, which demonstrated the effect of the ground plate. 
     Next, some modifications of the ground plate will be described as referring to  FIGS. 9 to 12 .  FIGS. 9A and 9B  illustrate a bi-direction module  55  with the ground plate  156  according to the second embodiment of the invention, where  FIG. 9A  shows the bi-directional module  55  viewed from the top, while,  FIG. 9B  shows the module viewed from the bottom. 
     This bi-directional module  55  may be also coupled with the circuit provided on the PCB  30 . The bi-directional module  55  provides a metal stem  55   a  connected with the signal ground, an LD  57 , a light-receiving device such as a photodiode (PD)  58  and a metal cap to form a cavity within which the LD  57  and the PD  58  are installed. The stem  55   a  with a primary surface  55   d  and a back surface  55   e  has a disk shape with a diameter of, for instance, 5.6 mm. The LD  57  is mounted on the primary surface  55   d  through an LD sub-mount, while, the PD  58  is also mounted on the primary surface  55   d  of the stem  55   a  though a PD sub-mount. On the primary surface  55   d  is provided with a monitor PD and a pre-amplifier to amplify the signal output from the PD  58 , which are not shown in  FIG. 9A . 
     The bi-directional module  55  further installs a WDM filter, not shown in  FIG. 9A , on the primary surface  55   d  thereof. The PD  58  is set on a center portion of the stem  55   a , while, the LD  57  is set in a position offset from the center of the stem  55   a . Between the LD  57  and the PD  58  is provided with the WDM filter; specifically, the WDM filter is arranged on an optical axis vertically extending from the PD  58  and as being inclined by 45° to the optical axis. The optical fiber is arranged above the WDM filter as the lens is interposed between the fiber and the WDM filter. The WDM filter reflects light emitted from the LD  57  toward the optical fiber, while, transmits light provided from the optical fiber toward the PD  58 ; thus, the function of the bi-direction with respect to the single fiber may be realized. 
     The bi-directional module  55  illustrated in  FIGS. 9A and 9B  provides three Tx lead pins  55   b , four Rx lead pins  55   c  and one ground pin  55   f . Three Tx lead pins correspond to the Tx power supply VccT, the Tx signal SigT and a monitored signal MON, while, four Rx lead pins correspond to a pair of complementary signals, SigR and /SigR, the Rx power supply VccR, and a bias for the PD Vpd, respectively. These lead pins, although they pass through the stem  55   a , are fixed to the stem  55   a  as they are electrically isolated from the stem  55   a  with sealing glass. 
     Tip end of respective lead pins,  55   b  and  55   c , protrude from the primary surface  55   d  of the stem  55   a ; while, the other end thereof extend from the back surface  55   e . A plurality of bonding wires with a diameter of about 25 μm electrically connect the tip end of the lead pins with the electrodes of the LD  57  and those of the PD  58 . Specifically, two of the Tx lead pins  55   b , SigT and VccT, are connected with electrodes of the LD and the rest of the Tx lead pins, MON, is connected with one of electrodes of the monitor PD. The rest electrode of the monitor PD is connected with the Tx power supply VccT. While, the Rx lead pin Vpd is connected with the PD  58 , one of rest three lead pins, VccR, is connected with the pre-amplifier to provide the power thereto, and the rest two lead pins, SigR and /SigR, are connected with the outputs of the pre-amplifier. The case pin  55   f  is directly connected with the stem  55   a  and extends from the back surface  55   e  of the stem  55   a . As illustrated in  FIG. 5 , the other ends of respective Tx lead pins  55   b , Rx lead pins  55   c  and the case pin  55   f  are soldered with the wiring patterns on the PCB  30 . 
     The stem  55   a , as illustrated in  FIGS. 10A and 10B , comprises a Tx section E 1  and an Rx section E 2 .  FIG. 10A  illustrates the arrangement of the primary surface  55   d  of the stem  55   a , while,  FIG. 10B  illustrates the arrangement of the back surface  55   e  of the stem  55   a . Between these two sections, E 1  and E 2 , is intersected with a boundary S. Three Tx lead pins  55   b  position within the Tx section E 1 , while, four Rx lead pins  55   c  and the case pin  55   f  position in the Rx section E 2 . The length of the intersection S is nearly equal to the diameter of the stem  55   a . The LD  57  is surrounded by the boundary S and three Tx lead pins  55   b , while, the PD  58  is surrounded by the boundary S, four Rx lead pins  55   c  and the case pin  55   f.    
     Referring back to  FIGS. 9A and 9B , the optical module  1  implemented with the bi-directional module  55  shown in  FIGS. 9A and 9B , discriminates the signal ground from the case ground to enhance the noise tolerance and the EMI tolerance. In the present optical module  1 , the ground for the Rx section E 2  is connected with the signal ground, while, the ground for the Tx section E 1  is grounded in the case ground. Specifically, the case pin  55   f  set in the Rx section E 2  directly extends from the stem  55   a  to be grounded in the signal ground on the PCB  30 . Moreover, the ground plate  156  shown in  FIGS. 9A and 9B , which is directly connected with the stem  55   a , is also grounded to the ground pattern on the PCB  30  through the wing portion  156   a . The ground plate  156  includes the wing portions  156   a  in both ends thereof and the center bar  156   b  connecting the wing portions  156   a . The center bar  156   b  is fixed to the back surface  55   e  of the stem  55   a  by soldering. Both ends of the center bar  156   b  extends from the edge of the stem  55   a . The wing portions  156   a  provide a plurality of holes  156   c  to fix them with the PCB  30  with screws. The center bar  156   b  is set along the boundary S between two sections, E 1  and E 2 , as shown in  FIGS. 10A and 10B . A region in the back surface  55   e  including the Tx section E 1  arranges three Tx lead pins  55   b , while, another region including the Rx section E 2  arranges four Rx lead pins  55   c  and the case lead pin  55   f . Thus, the ground plate  156  may electrically isolate the region including the Tx section E 1  from the other region including the Rx section E 2  in the back surface  55   e  of the stem  55   a , and the Tx lead pins  55   b  may be also isolated from the Rx lead pins  55   c.    
     An outer shape of the ground plate  156  is not restricted to those illustrate in  FIGS. 5 ,  6 ,  9 A and  9 B. Other shapes may be considered to show the same function and effect with those of the ground plates,  56  and  156 .  FIG. 11A  illustrates one type of the ground plate  256  showing a similar function with the ground plates,  56  and  156 , but has a different shape, and  FIGS. 11B and 11C  show still another ground plate  356  with different shape. 
     The ground plate  256  shown in  FIG. 11A  provides a shortened center bar  256   b  arranged on the boundary S between two sections, E 1  and E 2 . Both ends of the center bar  256   b  are within the back surface  55   e  of the stem  55   a , and a length of this center bar  256   b  is comparable with a size of the LD  57  or the PD  58 . This center bar  256   b  is surrounded by three Tx lead pins  55   b , four Rx lead pins  55   c  and the case pin  55   f.    
     The ground plate shown in  FIG. 11B  provides a center bar  356   b  including first to third portions,  356   c  to  356   e . The first portion  356   c  is set along the boundary S. Both ends of the first portion  356   c  are within the back surface  55   e  of the stem  55   a . The second and third portions,  356   d  and  356   e , extend from the end of the first portion  356   c  along the back surface  55   e . The second and third portions,  356   d  and  356   e , are set within the Tx section E 1  so as to surround the three Tx lead pins  55   b  and the region where the LD  57  is mounted. The second and third portions,  256   d  and  256   e , may provide a plurality of hoes to fix the bi-directional module  55  with the ground plate  256  to the PCB  30 . 
     The bi-directional module  55  shown in  FIG. 11C  provides a similar center bar with that shown in  FIG. 11B  but the second and third portions extend to the direction opposite to those in  FIG. 11B . Specifically, the second and third portions,  356   d  and  356   e , are arranged so as to put the four Rx lead pins  55   c  and the region where the PD  58  is mounted therebetween. The second and third portions,  356   d  and  356   e , surrounds the Rx section E 2  and the region where the PD  78  is mounted. Other arrangements of the ground plate  356  are the same as those shown in  FIG. 11B . 
       FIGS. 12A and 12B  explain another arrangement of the stem. The stem  155   a  in the back surface  155   e  thereof may provide a groove  155   g  or a hollow  155   h  to arrange the ground plate,  56 ,  156 ,  256  or  356 , on the boundary S.  FIG. 12C  is a cross section taken along the line L-L′ viewed from a direction intersecting the boundary S. Other arrangements of the stem,  155   a  or  255   a , are the same with those of the stem  55   a  except that the modified stem,  155   a  or  255   a , provides the groove  155   g  or the hollow  155   h.    
     The back surface  155   e  of the stem  155   a  shown in  FIG. 12A  provides the groove  155   g  along the boundary S. The center bar of the ground plate  56  or else which is set within this groove  155   g  may be further soldered with the stem  155   a . Because the position of the ground plate  56  is easily determined by this groove  155   g , the assembly of the ground plate  56  or else with the stem  155  may be facilitated. 
     The back surface  255   e  of the other stem  255   a  shown in  FIG. 12B  provides the hollow  255   g  in a position on the boundary S. The ground plate  56  or else may be set in the center bar  56   b  thereof within this hollow  255   h  and may be further soldered with the stem  255   a . In this embodiment, the center bar of the ground plate  56  or else may be easily set within this hollow  255   g , the assembly of the ground plate  56  or else with the stem  255  may be facilitated. 
     Thus, the ground plate  56  or else according to the present invention provides following three functions: (1) reinforcing the ground potential by being electrically connected with the ground patter on the PCB  30  and electrically isolating the Rx unit from the Tx unit; (2) mechanically supporting the rear end portion of the tri-plexer OSA  51 ; and (3) thermally stabilizing the bi-directional module  55  by dissipating heat generated in the bi-directional module  55  to the PCB  30 . 
     The semiconductor devices mounted on the stem  55   a  are active devices to generate heat. Especially, the laser diode shows large temperature dependence in performances thereof. Effective heat conduction from the stem  55   a  to the PCB  30  through the ground plate may improve the temperature characteristic of the bi-directional module. It is further important for the bi-directional module that the bi-directional module installs both the LD and the PD with the pre-amplifier on the same stem. Heat generated by the pre-amplifier strongly affects the performance of the LD. 
     A conventional optical module, even it is a bi-directional module, generally secures the heat-dissipating path from the sleeve in the front portion to the housing of the optical module that installs the bi-directional module. To fix the sleeve to the housing may concurrently secure the heat-dissipating path. Heat generated by the device within the bi-directional module is indirectly conducted to the housing. Specifically, the heat is once conducted to the stem, then to the cap mechanically fixed to the stem, then to the sleeve attached in the top of the cap and finally to the housing from the sleeve, which is inefficient heat conduction path. The ground plate of the present optical module may secure the effective heat conduction path from the stem directly to the PCB. 
     In addition, the optical module according to the embodiments explained above distinguishes the heat dissipating path for the ICs on the PCB  30  from the path for the bi-directional module  55 . The former path from the ICs is secured by the direct contact between the ICs with the heat sink  20  attached in the inner ceiling of the metal shell  10 , while, the latter path is independently secured from the stem  55   a  of the bi-directional module  55  to the PCB  30  through the ground plate,  56 ,  156 ,  256  or  356 , which may release the active devices in the bi-directional module  55  from the heat generated in the ICs. 
     The ground plate fixed to or solder on the PCB in the wing portions thereof may release the lead pins from the mechanical stress. The bi-directional module installs both the Tx function and the Rx function in the same package, which forces to increase the count of lead pins to be prepared. On the other hand, the optical module not restricted to those of the bi-directional module is continuously requested to make the outer diameter further smaller. One solution for these contradictory requests is to form the lead pins with a smaller diameter. Accordingly, the lead pins become delicate against the mechanical stress. The conventional bi-directional module is fixed by arranging the sleeve in the front thereof against the housing of the optical transceiver or the optical module. An embodiment of the present application, although the front portion of the tri-plexer OSA may be supported by setting the necked portion between two flanges of the sleeve on the saddle  45  and the body  53  on the terrace  44  of the base  40 , the rear end portion of the OSA  51  is left as substantially free. 
     When the bi-directional module with a slender cylindrical shape is held by the sleeve in one end thereof, the other end must be held and supported by the lead pins soldered to the PCB. Thus, the lead pins have to provide two functions, one is to transmit electrical signals and the other is to support the end portion of the module. However, as explained above, thinner lead pins with a diameter of, sometimes less than 0.5 mm, are often result in the breaking. The ground plate of the invention with the wing portions may support the rear end portion of the bi-directional module in stead of the lead pins, which may make the diameter of the lead pin further smaller. 
     While the preferred embodiments of the present invention have been described in detail above, many changes to these embodiments may be made without departing from the true scope and teachings of the present invention. The present invention, therefore, is limited only as claimed below and the equivalents thereof.