Abstract:
A three-storied structure optical communications module having a top case, a middle case and a bottom case which are piled and coupled in a vertical direction. The middle case has a silicon bench with M (channel number) lightwaveguides, a first WDM 1 , a second WDM 2 , M (channel number) laser diodes for generating transmitting signals λ 1 . The top case has a set of λ 2  receiving photodiodes PD 1 s of the channel number M. The bottom case has another set of λ 3  receiving photodiodes PD 2 s. λ 2  signal beams and λ 3  signal beams propagating in optical fibers go into the lightwaveguides on the middle case. The λ 2  signal beams are reflected midway on the lightwaveguides by the WDM 1  upward to the photodiodes PD 1 s on the top case and are converted into photocurrents by the photodiodes PDs. The λ 3  signal beams are reflected halfway on the lightwaveguides by the WDM 2  downward to the photodiodes PD 2 s and are converted into photocurrents by the photodiodes PD 2 s. The λ 1  yielded by the laser diodes make their way in the lightwaveguides, go into a multichannel fiber and propagate in the multichannel fiber. The three-storied structure allows two sets of M-channel receiving signals and a set of M-channel transmitting signals to be received or transmitted. The three-storied structure enables the module to alleviate a necessary area and reduce optical, electrical interchannel crosstalk.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an optical communications module having two sets of M (M;channel number) photodiodes and one set of M laser diodes, suitable for multichannel bidirectional optical communications systems which transmit a variety of optical signals by making use of two kinds of receiving signals λ 2  and λ 3  and one kind of a transmitting signal λ 1  via M channel optical fibers. The optical communications module includes M laser diodes (LDs) for making and transmitting M channel signals of a λ 1  band, M photodiodes (PD 1 s) for receiving M channel signals of a λ 2  band and M photodiodes (PD 2 S) for receiving M channel signals of a λ 3  band sealed in a package. The module has 3M optoelectronic device chips (M LDs+2M PDs). There is no device containing so many optoelectronic chips in a package yet.  
           [0003]    This application claims the priority of Japanese Patent Application No.2002-176524 filed on Jun. 18, 2002, which is incorporated herein by reference.  
           [0004]    A single channel bi-directional optical communications system makes use of a single optical fiber for transmitting downward and upward optical signals in two directions. An LD/PD module for the single channel bi-directional communications should have a laser diode of making transmitting signals, a photodiode of receiving signals and a signal dividing part (e.g., y-branch) for dividing propagating signals into transmitting ones and receiving ones. Requirements imposed upon the signal dividing part are small division loss, small optical crosstalk, weak electromagnetic and electric crosstalk.  
           [0005]    Optical crosstalk means noise generation in a photodiode (PD) caused by invasion of strong light beams emitted from a laser diode (LD). The optical crosstalk is a serious problem for simultaneous bi-directional communications. Transmitting signal wavelength λ 1  emitted from the laser diodes (LD) is different from receiving signal wavelength λ 2  which has been sent from another port (a subscriber or a station). Conventional photodiodes employed for communications modules have an InGaAs light receiving layer (active layer) which has sensitivity within a wavelength range between 1.0 μm and 1.6 μm.  
           [0006]    The InGaAs photodiodes sense both λ 1  and λ 2 . Sensitivity of λ 2  is a reason for causing optical crosstalk in a simultaneous bi-directional LD/PD module.  
           [0007]    Another problem is electrical crosstalk among laser diodes. Strong current for driving lasers leaks in a package and a bench. The strong noise current has an influence upon the driving state of neighboring laser diodes.  
           [0008]    Besides, there is also electromagnetic crosstalk among laser diodes and photodiodes. Noise current generates electromagnetic waves which fly in space to the photodiodes of high impedance in the same package and perturb the actions of the photodiodes.  
         DESCRIPTION OF THE RELATED ART  
         [0009]    There have been some different types of allocating a laser diode and a photodiode in a module. FIG. 8 shows a three dimension, separated type which disposes a laser diode  86  along an extension (axial line) of an optical fiber  85 , positions a Wavelength Division Multiplexer (WDM)  87  slanting at 45 degrees at a middle point between the fiber  85  and the laser diode  86 , and allocates a photodiode  88  along a line vertical to the axial line. A transmitting beam of λ 1  emitted from the laser diode  86  passes the WDM  87  and is converged into the fiber  85  by a lens. A receiving beam λ 2  propagating in the fiber  85  goes to the WDM  87 , is reflected by the WDM and goes into the photodiode  88 .  
           [0010]    The module makes use of the WDM for separating light beams of different wavelengths. The WDM is an optical element which is produced by piling two kinds of transparent dielectric films with different refractive indices in turn on a substrate. The WDM plays a role of allowing the first wavelength λ 1  to pass but reflecting the second wavelength λ 2  with nearly 100%. The WDM has a definite reflection rate and definite transparency rate for wavelengths other than λ 1  and λ 2 . People have proposed various modules having wavelength selective filters (WDMs) which reflect 45 degree incidence beams of a definite wavelength and allows 45 degree incidence beams of another definite wavelength to pass without loss.  
           [0011]    {circle over (1)} Masahiro Ogusu, Tazuko Tomioka and Shigeru Ohshima, “Receptacle Type Bidirectional WDM Module I”, Proceeding of the 1996 Electronics Society Conference of IEICE, C-208, p208 (1996).  
           [0012]    {circle over (1)} has an independently packaged photodiode, an independently packaged laser diode, an independent WDM and a free space for propagation of optical beams. Since the photodiode is packaged in a metallic case, the module has an advantage of no optical crosstalk. Separation of the photodiode from the laser diode by individual cases needs a wide volume, which raises the cost and the volume of the LD/PD module.  
           [0013]    Some people have proposed planar type modules having a Y-branched lightwaveguide for dividing optical paths for a signal transmitting portion (LD portion) and a signal receiving portion (PD portion). The module has a silicon bench, Y-branched lightwaveguides formed on the silicon bench, a laser diode laid behind an end of the first lightwaveguide, a photodiode positioned behind an end of the second lightwaveguide and a wavelength selective filter positioned at the y-branch. Another end of the third lightwaveguide is joined to an outer optical fiber. Transmitting light emanating from the laser enters the lightwaveguide, passes the wavelength selective filter, propagates in the third lightwaveguide and goes into the outer fiber to a central station.  
           [0014]    {circle over (2)} M. Kuribayasi, H. Isono, T. Kunikane, Y. Omori and T. Emori, “Optical Bi-directional Module With WDM Using Silica Waveguides”, Proceeding of the 1993 IEICE Fall Conference, C-158, p4-238 (1993).  
           [0015]    {circle over (2)} proposed a planar type Y-branched LD/PD module having a Y-branch dividing part, a photodiode for sensing 1.55 μm signals and a laser diode for generating 1.3 μm signals encapsulated in a common package. {circle over (2)} made a planar module by making Y-branched GeO 2  doped SiO 2  light waveguides on a silicon bench, installing a WDM at a Y-branch point, laying a photodiode at a final end of a branch, positioning a laser diode at a final end of another branch and joining an outer fiber to an initial point of a stem of the lightwaveguide. The planar Y-branched module has advantages of leveling the laser and photodiode on the same height and small optical crosstalk due to the Y-branch.  
           [0016]    However, the Y-branch causes difficulties. Excess curvature will increase bending loss at the Y-branch. Avoiding the excess curvature requires a large length of the lightwaveguide in the vicinity of the Y-branch, which requires a long silicon bench. Since the photodiode and the laser diode are arranged side by side on the bench, the bench should have a large width for reducing electric and electromagnetic crosstalk. The Y-branch and the parallel LD/PD mounting enlarge the module. A single channel planar Y-branch module may be still allowable. Application of the Y-branched module to multichannel devices would lead to an impractical, bulky device. The defect of the bulky module would be more serious for application to multichannel modules having a plurality of lasers and photodiodes.  
           [0017]    Someone proposed another type of y-branched modules which have a silicon bench, a Y-branched lightwaveguide made on the silicon bench, a WDM positioned at the branch, a laser diode laid at an initial end of the branch, a photodiode installed at a final end of a stem, and an outer fiber joined to an initial end of another branch. 1.55 μm signals propagating from the outer fiber go into the lightwaveguide via the initial end, pass the WDM straightforward, and enter the photodiode. 1.3 μm signals emitted from the laser diode run in the branch, arrive at the WDM, are selectively reflected by the WDM, propagate in the waveguide and go into the outer fiber.  
           [0018]    Somebody proposed a third type of LD/PD modules without Y-branch. The module has a flat bench, a linear lightwaveguide formed on the bench, a WDM slantingly laid on the lightwaveguide for dividing paths in slanting directions, a laser diode mounted at a rear end of the lightwaveguide and a photodiode mounted slantingly above the WDM. Input signals propagating from an outer fiber go into the lightwaveguide, are reflected by the WDM slantingly upward and are supplied to the photodiode.  
           [0019]    {circle over (3)} T. Uno, T. Nishikawa, M. Mitsuda, G. Tohmon and Y. Matsui, “Hybridly integrated LD/PD module with passive-alignment technology”, Proceeding of the 1997 Electronics Society Conference of IEICE, C-3-89, p198 (1997).  
           [0020]    {circle over (3)} suggested an LD/PD module having a silicon bench with a lower front part and a higher rear part with a V-groove, a glass plate with a V-groove mounted upon the lower front part of the silicon bench, an optical fiber fitted commonly on the bench V-groove and the glass V-groove, a laser diode (LD) laid at an extension of the fiber on the rear part of the silicon bench, a WDM inserted into a slanting groove formed by cutting slantingly the fiber and glass plate and a photodiode mounted upon the glass plate just before the WDM. {circle over (3)} also divided optical paths for the LD and PD in a vertical direction.  
           [0021]    The photodiode (PD) should be slightly higher than the laser diode (LD) in {circle over (3)}. The extra glass submount is glued on the silicon bench and the photodiode is mounted on the submount. The submount raises the photodiode to a point higher than the laser. The distance from the WDM to the PD is short. The PD is in contact with the fiber. Difference of heights between the LD and the PD is one or half of the diameter of the fiber. Though the paths are vertically separated by the WDM, the photodiode is nearly on the same level as the laser.  
           [0022]    {circle over (4)} Japanese Patent Laying Open No.2001-203419, “LIGHT-EMITTING DEVICE”, which was filed by the same applicant as the present invention, proposed a vertical separation path type LD/PD module. The module was assembled by making a longitudinal SiO 2  lightwaveguide on a silicon bench, making a lower step at a rear end of the silicon bench, mounting a laser diode on the lower step, building a slanting WDM filter midway on the lightwaveguide, and installing a photodiode slantingly in front of the WDM filter on the lightwaveguide. A path to the photodiode, which is raised by the WDM, is short. The photodiode is mounted just on the lightwaveguide. Difference of heights between the laser and the photodiode is nearly equal to one or half of thickness of the lightwaveguide. Though the paths are separated in a vertical direction by the WDM, the laser and the photodiode are laid nearly on the same level.  
           [0023]    {circle over (5)} Japanese Patent Laying Open No.11-218651, “OPTICAL TRANSMISSION AND RECEPTION MODULE”, which was filed by the same applicant as the present invention, proposed a module having a common ground metallize, an LD part built on a top surface of the common ground metallize, a PD part formed on a bottom surface of the metallize. FIG. 9 shows a sectional view of the module proposed by {circle over (5)}. {circle over (5)}has a first plate  95  with a vertical hole and a second plate  99  with another vertical hole. The first plate  95  and the second plate  99  sandwich a common ground metallize G. The first plate  95  is a transmitting portion having an end of a fiber  105 , a lightwaveguide  96 , a wavelength division multiplexer (WDM)  97  and a laser diode  98  aligning along ap extension of the fiber  105 . The second plate  99  is a receiving portion having a photodiode (PD)  102  below the vertical hole and a preamplifier (AMP)  103  fitted on the bottom.  
           [0024]    A λ 1  transmitting signal light beam emitted from the laser diode (LD)  98  goes into the lightwaveguide  96 , passes the WDM  97 , and enters the fiber  105 . A λ 2  receiving signal light beam running in the fiber  105  goes into the lightwaveguide  96  and shoots the WDM  97 . The λ 2  beam is reflected by the WDM  97  downward into the vertical holes and is guided to the photodiode  102 . The photodiode  102  yields photocurrent in proportion to the receiving light signals. The preamplifier AMP 1   03  amplifies the photocurrent. The middle ground metallize G is commonly connected both to the LD part and to the PD part. The middle ground metallize G prevents electromagnetic noise produced at the LD part from invading the photodiode. The module makes use of the WDM for dividing LD and PD optical paths in a vertical direction. A long path from the WDM to the photodiode (PD) enables the photodiode to separate far from the lightwaveguide  96  and the laser  98 , which reduces optical noise for the photodiode  102 . Separation of the PD part from the LD part by two plates  95  and  99  succeeds in suppressing the optical noise. The ground G decreases electromagnetic noise. The double insulating plates reduce electric noise. {circle over (5)} was a sophisticated LD/PD module contrived by the applicant.  
           [0025]    {circle over (5)} had been contrived as a single-channel transmitting/receiving module or an ONU (optical network unit). {circle over (5)} had a single laser diode (LD), a single photodiode and a single lightwaveguide. There was no need for installing a plurality of pairs of lasers and photodiodes. Furthermore, the inventors found a surprising aspect that the middle ground G between the LD part and the PD part acts to increase noise by playing a role of an antenna emitting LD signals instead of decreasing electromagnetic crosstalk. The silicon plates  95  and  99  are semi-conductive, which incurs electric crosstalk via the silicon plates.  
           [0026]    There have been little requirements for multichannel modules including a plurality of pairs of lasers and photodiodes. Almost all modules which have been proposed so far have been single channel modules with a single pair of a laser and a photodiode. Multichannel modules will be required in near future.  
           [0027]    Multichannel modules are favorable for modules equipped at a central station, for saving a space for accommodating modules. In addition to the multichannel communications, some of single channel modules require both an analog receiving portion and a digital receiving portion. The multichannel communications would require a hybrid single channel module having a laser (LD) and two photodiodes (PDs). An M-channel analog/digital communications would demand a 3M channel module including 3M optoelectronic devices in a package. A central station, which treats with many subscribers (ONUs), will demand small-sized, low cost multichannel modules.  
           [0028]    A multichannel three function module having 3M chips (M; channel number) requires being a small-sized module by curtailing a volume per chip. The multichannel module should be immune from optical, electrical and electromagnetic crosstalk.  
           [0029]    What requires multichannel optical communications modules? Why multichannel systems are demanded? The reason should be clarified before describing contrivances of the present invention. The present invention aims at an improvement of bidirectional simultaneous optical communications.  
           [0030]    At an early stage, a 1:16 communications network had been investigated. The 1:16 network joins a single optical fiber starting from a station to sixteen fibers deriving from sixteen subscribers (ONUs: optical network units) at a 1:16 coupler installed near the subscribers. N denotes the number of subscribers (ONUs). The number of fibers connecting the station to the subscribers can be reduced from N to N/16. The prototype 1:16 network had advantages of alleviating the number of necessary optical fibers and reducing the number of station modules for delivering signals to the subscribers. On the contrary, the 1:16 network turned out to have weak points of complexity of a controlling system and lack of elasticity of designing.  
           [0031]    A simpler system which uses a single fiber per subscriber (1:1 type) becomes more promising than the prototype 1:16 system. Without branching couplers, the simple system needs N fibers for connecting the station to N subscribers (ONUs). The simple system allocates one independent fiber to one subscriber. Use of independent fibers enables the system to add extra functions. However, the simple system would have a drawback of a vast increment of the number of modules equipped at the central station.  
           [0032]    In the case, it would be preferable for a station to have complex modules having four, eight, sixteen, . . . pairs of lasers and photodiodes. The number of modules equipped at the station can be reduced from N to N/4, N/8, N/16, . . . , where N is the number of subscribers. Namely, the central stations prefer multichannel modules to single-channel ones for reducing the total number of modules.  
           [0033]    In near future, digital/analog hybrid communications systems having analog channels and digital channels will be requested. For example, when telephones and facsimiles are transmitted as digital signals and television broadcasting is transmitted as analog signals, the downward signals will include analog and digital signal modes. In the case, a subscriber requires a single channel three device type module having LD(λ 1 )+PD(λ 2 )+PD(λ 3 ). A central station should equip many three device type modules having a plurality of sets of LD(λ 1 )+PD(λ 2 )+PD(λ 3 ). The present invention will give multichannel three device type modules to a central station as well as single channel three device type modules to subscribers.  
           [0034]    A purpose of the present invention is to provide a small sized multichannel LD/PD module which can include a plurality of sets of LD/2PD. Another purpose of the present invention is to provide a low-cost LD/PD module which can reduce cost per channel. A further purpose of the present invention is to provide an LD/PD module which can alleviate optical crosstalk, electrical crosstalk and electromagnetic crosstalk between LDs and PDs and between PDs and PDs.  
         SUMMARY OF THE INVENTION  
         [0035]    The present invention proposes a three-storied module having a top case including λ 2  receiving photodiodes PD 1  and PD 1  leadpins, a middle case including a connector with fibers, a lightwaveguide-carrying silicon bench, a first WDM 1 , a second WDM 2 , λ 1  transmitting laser diodes LDs and LD leadpins, a bottom case including λ 3  receiving photodiodes PD 2  and PD 2  leadpins, the top, middle and bottom cases being piled and unified in a vertical direction, a transparent resin filling the top, middle and bottom cases, and a package encapsulating the top, middle and bottom cases. λ 1  transmitting signal beams are yielded by the lasers (LD), go into the lightwaveguide on the silicon bench, pass the WDM 2  and WDM, and propagate in the fibers. λ 2  receiving signal beams run in the fibers downward, go into the lightwaveguides on the silicon bench, are reflected upward by the WDM 1 , and are guided into the photodiodes PD 1  in the top case. λ 3  receiving signal beams run in the fibers downward, go into the lightwaveguides on the silicon bench, are reflected downward by the WDM 2 , and are guided into the photodiodes PD 2  in the bottom case.  
           [0036]    Floor holes have been perforated on the top case and the middle case for allowing receiving signal beams to pass therethrough. Sets of multichannel beams coming from a ribbonfiber and propagating in the lightpaths are divided into upward beams λ 2  and into downward beams λ 3  by two WDMs (wavelength division multiplexers). The upward λ 2  beams pass the upper floor holes and enter the upper photodiodes PD, in the top case. The downward λ 3  beams pass the middle floor holes and enter the lower photodiodes PD 2  in the bottom case. The upper photodiodes PD 1  are separated from the middle laser diodes LDs by the upper (top) case bottom. The lower photodiodes PD 2  are separated from the middle laser diodes LDs by the middle case bottom. Since the cases are opaque, optical crosstalk between the PDs and the LDs is greatly suppressed.  
           [0037]    The cases which are not silicon but plastics or ceramics, are insulators. The top floor is insulated by the upper case from the middle floor. The bottom floor is electrically separated from the middle floor by the insulating middle case. Electrical crosstalk between the laser diodes and the photodiodes is depressed by the isolation of the insulating cases. Electrical circuits are also separated. The middle floor has an individual electrical circuit for the laser diodes prepared in the middle case. The top floor has another individual electrical circuit for the upper set of photodiodes PD 1  (and preamplifiers). The bottom floor has another separated electrical circuit for the bottom set of the photodiodes PD 2 . Separation of wirings is effective for suppressing electrical crosstalk. Unlike the known {circle over (5)}, the present invention has no common ground metallize. Lack of the common grounds is effective for suppressing electromagnetic crosstalk.  
           [0038]    The inventors have noticed that insertion of a common ground between the transmitting part and the receiving part suggested by {circle over (5)} increases electromagnetic crosstalk by playing a role of an antenna contrary to the common sense of electronics technology. The present invention excludes such a common ground metallize from the modules for avoiding the problem of common ground-induced electromagnetic crosstalk.  
           [0039]    The highest story and the lowest story are provided with photodiodes PD 1  and PD 2  for sensing different receiving light signals with different wavelengths λ 2  and λ 3 . The middle story is provided with laser diodes LDs for producing transmitting light signals with a wavelength λl. Two wavelength division multiplexers WDM 1  and WDM 2  enable the three stories to arrange sets of the highest PD 1 s (λ 2 ), lowest PD 2 s (λ 3 ) and middle LDs(λ 1 ). The transmitting light signals λ 1  does not fly in space. The receiving light signals λ 2  and λ 3  experience space flight. A transparent resin should be preferably filled in the cases for the sake of the space flight. The transparent resin with a refractive index akin to the lightwaveguides has a function of reducing reflection loss. If an inertia gas were filled in the package instead of resin, the reflection loss would be large at interfaces. Elastic silicone resins are suitable for reducing the loss. Filling of the elastic resin has another function of protecting the devices by alleviating external shock.  
           [0040]    The whole of the piled three cases is molded by an opaque, hard resin as a plastic package, for example, an epoxy resin. The present invention allocates a set of LDs and WDMs on the middle floor, a first set of PDs on the top (highest) floor and a second set of PDs on the lowest (bottom) floor for reflecting receiving signals by the WDMs on the middle. Three sets of LDs, PDs and PDs are disposed in a vertical direction. Vertical arrangement of the devices can curtail horizontal sizes. The storied structure enables the present invention to align a plurality of same function chips (LDs and PDs) chip by chip. The present invention is suitable for multichannel LD&amp;2PD modules.  
           [0041]    The present invention proposes the three-storey module with the upper case, the middle case, and the bottom case for allotting a PD 1  set, a PD 2  set and an LD set to different storeys at different heights. Vertical separation of the PD 1  set and PD 2  set from the LD set enables the module to decrease electric, optical and electromagnetic crosstalk between LDs and PDs. The vertical arrangement allows the module to alleviate an occupying area and to reduce the size of the module. The three-story PD 1 /LD/PD 2  modules are suitable for multichannel optical communications due to the reduction of a size and the decrease of interchannel crosstalk. The channel number is, for example, four, eight, sixteen or so. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]    [0042]FIG. 1 is an axially sectioned side view of Embodiment 1 of a multichannel PD/LD/PD module having a top case (PD 1 s), a middle case (lightwaveguides &amp; LDs) and a bottom case (PD 2 s).  
         [0043]    [0043]FIG. 2 is a laterally sectioned front view of Embodiment 1 for showing λ 3  receiving beams being reflected down by a second WDM 2  and going into the photodiodes (PD 2 ) in the bottom case.  
         [0044]    [0044]FIG. 3 is a laterally sectioned front view of Embodiment 1 for showing λ 2  receiving beams being reflected upward by a first WDM 1  and going into the photodiodes (PD 1 ) in the top case.  
         [0045]    [0045]FIG. 4 is a perspective view of Embodiment 1 having the top case, the middle case and the bottom case, leadpins, a connector and a resin package enclosing the cases.  
         [0046]    [0046]FIG. 5 is a plan view of a top case of Embodiment 2 of an eight channel PD/LD/PD module.  
         [0047]    [0047]FIG. 6 is a plan view of a middle case of Embodiment 2 of the eight channel PD/LD/PD module.  
         [0048]    [0048]FIG. 7 is a plan view of a bottom case of Embodiment 2 of the eight channel PD/LD/PD module.  
         [0049]    [0049]FIG. 8 is a plan view of a conventional LD/PD module arranging an optical fiber, a photodiode (λ 2 ) and a laser diode (λ 1 ) in three vertical directions and placing a WDM at a center for allowing a λ 1  laser beam to pass and for reflecting a λ 2  beam to the photodiode.  
         [0050]    [0050]FIG. 9 is a vertically sectioned view of a planar type LD/PD module proposed by Japanese Patent Laying Open No. 11-218651. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0051]    The present invention makes use of three cases, i.e., top, middle and bottom cases, and allots the middle case to a transmitting (LD) part, the top and bottom cases to receiving (PD) parts. The transmitting portion maintains a connector, a silicon bench with waveguides, a wavelength division multiplexers (WDM 1  and WDM 2 ), a set of laser diodes (LDs), and a set of LD leadpins. The top case contains a set of λ 2  receiving photodiodes (PD 1 s), a set of PD leadpins, optionally and a set of preamplifiers for amplifying photocurrents of the PD 1 s.  
         [0052]    The bottom case contains a set of λ 3  photodiodes (PD 2 s), a set of PD leadpins, optionally and a set of preamplifiers for amplifying photocurrents of the PD 2 s.  
         [0053]    Receiving light signals λ 2  and λ 3  propagate in ribbonfibers, a connector and waveguides to the WDM 1  and WDM 2 . λ 2  light is reflected upward by the WDM, to the PD 1 s on the top floor. λ 3  light is reflected downward by the WDM 2  to the PD 2 s on the bottom floor. The LDs produce transmitting signal light λ 1 , which is introduced into the waveguides. Propagating in the waveguides, λ 1  passes the WDM 1  and WDM 2  with little loss. Three-storied cases are supplied with a transparent resin having a refractive index similar to the waveguides for decreasing reflection/scattering loss at interfaces. A hard rigid resin encapsulates the three-storied cases.  
         [0054]    There are six allowable modes of pairing one of LDs, PD 1 s and PD 2 s with one of λ 1 , λ 2  and λ 3 . Furthermore, if the module employs a 1.3 μm band, 1.4 μm band and 1.55 μm band as λ 1 , λ 2  and λ 3 , the pairing allows six probable choices. Of course, this invention is applicable to all probable allotments.  
         [0055]    For avoiding confusion and clarifying mutual relations, λ 1  is determined to a transmitting signal wavelength emitted by the laser diodes (LDs), λ 2  is assigned to a receiving wavelength sensed by the upper photodiodes (PD 1 s) and λ 3  is allotted to another receiving wavelength detected by the bottom photodiodes (PD 2 s). What selectively reflects λ 2  is the first WDM 1 . The second WDM 2  reflects λ 3 .  
         [0056]    The wavelengths λ 1 , λ 2  and λ 3  correspond to anyone of a 1.3 μm band, 1.4 μm band and 1.55 μm band. Multichannel communications makes use of many different wavelengths for a single band for exchange signals with many subscribers. The 1.3 μm band includes a plurality of wavelengths which are close to 1.3 μm but are slightly different with each other. The number of the different wavelengths is equal to the channel number M. The 1.4 μm band also includes M different wavelengths akin to 1.4 μm. The 1.55 μm has M different wavelengths. Thus, M optoelectronic devices are required for M different wavelengths. M denotes, therefore, the channel number, the wavelength number and the device number. The WDM 1  and WDM 2  have complex dialectic multilayered structures for selecting different wavelengths. Contrivances enable two WDMs to divide the three wavelength bands.  
         [0057]    [Top floor (upper case, highest floor; PD 1 s; λ 2 )] 
         [0058]    The top floor accommodates M λ 2 -photodiodes PD 1 s for sensing λ 2  beams, metallized patterns, wires and leadpins on an upper case. The upper case has holes for guiding M-channel beams. Optionally, M preamplifiers AMP, for amplifying photocurrents of the M photodiodes PDs are installed on the upper floor. In addition, the upper floor can include capacitors for stabilizing source voltages or other electric elements. The photodiodes (PD 1 s), the preamplifiers and the electric elements are connected by wires to the metallized patterns. The M-channel photodiodes are denoted by suffixing as PD 1a , PD 1b , PD 1M .  
         [0059]    [Middle floor (middle case; λ 1 ; LDs, WDM 1 , WDM 2 )] 
         [0060]    The middle floor contains M λ 1  emitting laser diodes (LDs), M first wavelength selective filters (WDM 1 s; wavelength division multiplexers), M second wavelength selective filters (WDM 2 s), a silicon bench having M V-grooves, metallized patterns, wires and leadpins. The laser diodes (LDs) and metallized patterns are connected to the leadpins via wires. The M channel laser diodes can be individually expressed by suffixing as LD a , LD b , LD c , . . . , LD M . The first wavelength selective filters, which align in a lateral direction in a slanting groove, are denoted by suffixing as WDM 1c , WDM 1b , WDM 1c , . . . , WDM 1M . The WDM 1 s reflect λ 2  beams. The λ 2  beams include M different wavelengths in M channel communications. The WDM 1a , WDM 1b , WDM 1c , WDM 1M  are different in the layered structure in a rigorous case. However, when the wavelength differences between neighboring channels are sufficiently small in three bands, a common WDM 1  can discern λ 1  from λ 2  and λ 3 . Second wavelength selective filters WDM 2 s are similar to the first WDM 1 s.  
         [0061]    [Bottom floor (lowest case; PD 2 ; λ 3 )] 
         [0062]    The bottom floor accommodates M λ 3 -photodiodes PD 2 s for sensing λ 3  beams, metallized patterns, wires and leadpins in a lowest case. Optionally, M preamplifiers (AMP 2 s) for amplifying photocurrents of the M photodiodes PD 2 s are installed on the bottom floor. In addition, the lowest floor can include capacitors for stabilizing source voltages or other electric elements. The photodiodes (PD 2 s), the preamplifiers and the electric elements are connected by wires to the metallized patterns. M-channel photodiodes are denoted by suffixing as PD 2a , PD 2b , . . . ,PD 2M .  
         [0063]    [Transparent resin] 
         [0064]    λ 2  beams reflected by WDM 1  and λ 3  beams reflected by WDM 2  propagate in free space. Inner rooms in the upper, middle and lower cases are filled with a transparent resin without air gap for reducing reflection or scattering of the beams at interfaces between the fibers/waveguide and the free space. The conditions imposed upon the resin are transparency and a refractive index similar to the fibers (refractive index n=1.43). Candidates are transparent silicone resins or transparent acrylate resins, which have a refractive index nearly equal to silica fibers. The transparent resin, which maintains residual elasticity after hardening, has an advantage of protecting photodiodes, lasers, preamplifiers and wires from outer shock.  
         [0065]    [Bench (substrate)] 
         [0066]    The bench is a base for building lightwaveguides, WDMs and laser diodes. A good candidate for the bench is a silicon single crystal substrate. Besides, ceramic benches or polymer benches are also available.  
         [0067]    [Cases] 
         [0068]    The upper, middle and lower cases can be produced by insert-molding a leadframe with a resin in a metallic die. Insert-molding dispenses with an extra step of printing metallized patterns on cases. A candidate resin is a liquid crystal polymer, which gives lowcost cases. However, ceramic cases are more sophisticated candidates for the upper, middle and lower cases. In this case, metallizes are made by printing or evaporation on the ceramic case and leadpins are brazed to peripheral metallizes on the ceramic case. The ceramic cases are rich in airtight sealing, reliability and thermal diffusion.  
         [0069]    [Lightwaveguides] 
         [0070]    Polymer lightwaveguides excel in productivity and cost. If the bench is a silicon substrate, quartz lightwaveguides are congenial to the bench. The quartz waveguide is a set of a SiO 2  cladding layer and a GeO 2  doped SiO 2  core enclosed by the cladding. The quartz lightwaveguide has an advantage of loss lower than the polymer lightwaveguide. The present invention is applicable both to the silicon and the polymer lightwaveguides.  
         [0071]    [Number M of LD, PD 1 , PD 2 )] 
         [0072]    The module of the present invention contains one or more than one transmitting/receiving unit. The number of the units is denoted by M (M≧1). It is convenient to determine M as a multiple of four. For example, M=4, M=8, M=12, M=16, M=24 or M=36 are suitable candidates. M is equal to the number of the fibers and the number of the channels. Then, M can be called as a channel number. The present invention allots PD 1 s, LDs and PD 2 s to the upper floor, the middle floor and the bottom floor respectively. Vertically divided allocation of PD 1 s, LDs and PD 2 s saves a horizontal area of the module. Vertical division of a plurality of LD/2PD sets. Thus, the present invention is suitable for multichannel communications modules.  
         [0073]    An optical communications system includes a central station, a plurality of subscribers (ONUs) and fibers connecting the station to the subscribers. In general, a λ 1  (1.3 μm) band is assigned to upward signals from the subscribers to the central station. A λ 2  (1.48 μm) band and λ 3 (1.55 μm) band are assigned to downward signals from the central station to the subscribers. In this case, λ 1  is a transmitting light wavelength generated by a laser, λ 2  is a receiving light wavelength and λ 3  is another receiving light wavelength in the subscribers. The relation is reverse at the central station.  
         [0074]    The number of the subscribers is denoted by N. The ONU module is a single channel module (M=1) having a single set of one laser and two photodiodes. On the ONU, λ 1  is LD light (1.3 μm), λ 2  is a digital receiving light (1.48 μm) and λ 3  is an analog receiving light (1.55 μm). On the contrary, multichannel modules are more favorable than single channel ones for the central station. The number of necessary modules can be reduced from N to N/M by employing M-channel modules at the central station.  
         [0075]    [Leadframe] 
         [0076]    The upper case and the lower case are complex packages unified to a metal leadframe. Insert-molding produces plastic cases unified with metallic leadframes. Photodiodes should be loaded not upon the silicon bench but upon the leadframe. Receiving beams reflected by the WDM pass the floor holes of the upper and middle cases and go into photodiodes. The floor holes are perforated not on the rigid silicon bench but are preliminarily bored on the thin metal leadframe at a preparatory step of making leadframes. There is no step for boring the floor holes on the cases.  
         [0077]    The aforementioned known reference {circle over (5)} Japanese Patent Laying Open No.11-218651 includes a step of perforating a throughhole on a rigid silicon bench by a drill for guiding receiving light to the bottom of the bench, which requires much time and skill. Silicon is a hard material. It is not an easy task to pierce a throughhole from top to bottom by mechanical tools. Wet etching or dry etching cannot pierce such a deep hole in silicon. The present invention, which perforates holes on plastic cases at the time of molding, is far feasible than {circle over (5)}.  
         [0078]    [Optical crosstalk] 
         [0079]    The present invention enjoys an advantage of reduction of optical crosstalk, electric crosstalk and electromagnetic crosstalk in addition to the merit of reduction of a unit volume per a PD or per an LD. The three storey structure enables the middle, upper case bottoms to protect the upper photodiodes PD 1 s and bottom photodiodes PD 2 s from middle strong LD stray rays. Optical crosstalk is suppressed by the opaque case floors of the three storey package. The opaque package is effective for shielding strong LD light in the present invention. Silicon in the known {circle over (5)}, which is transparent for wavelengths from 1 μm to 1.6 μm, is impotent to shield near-infrared light of 1 μm to 1.6 μm to the contrary.  
         [0080]    [Electric crosstalk] 
         [0081]    The present invention separates the first set of photodiodes PD 1 s and the second set of photodiodes PD 2 s from the set of laser diodes LDs in vertical directions and the horizontal directions by the cases and the resin. The distance and the resin succeeds in decreasing electric crosstalk between the LDs and the PDs. The aforementioned known reference{circle over (5)} Japanese Patent Laying Open No.11-218651 intervenes an LD and a PD with a silicon bench. But, silicon is not an insulator but a semiconductor having sufficient conductivity. The isolating silicon bench itself conducts electricity and causes electric crosstalk between the LD and the PD in {circle over (5)}. On the contrary, the present invention succeeds in reducing the electric crosstalk by isolating the PDs from the LDs by the insulating plastic cases.  
         [0082]    [Electromagnetic crosstalk] 
         [0083]    The present invention separates the photodiodes PD 1 s and PD 2 s from the laser diodes LDs in horizontal and vertical directions and isolates the grounds and the power source patterns on the three floors. The distance and the isolation of the grounds and patterns succeeds in alleviating electromagnetic crosstalk between the LDs and the PD 1 s or PD 2 s. The aforecited known reference {circle over (5)} Japanese Patent Laying Open No.11-218651 sandwiches a single ground metallize with a PD-carrying silicon bench and an LD-carrying silicon bench. The silicon benches intervene between an LD and a PD. The ground is a common ground for both the LD circuit and the PD circuit. OS thought that the bisecting common ground would be effective to shield electromagnetic waves from the LDs to the PDs.  
         [0084]    But, the truth is otherwise. The thin metal ground with high resistance cannot be a true ground but acts as an antenna for causing electromagnetic noise. Strong currents or electromagnetic signals from the laser diodes fluctuate the level of the thin metal ground in {circle over (5)}. The ground level of the photodiode circuit is perturbed by the fluctuation of the common metal ground. Then, the ground of the preamplifier accompanying the photodiodes fluctuates, which induces large electromagnetic noise in the receiving signals. This is an origin of electromagnetic crosstalk between the laser diode and the photodiode.  
         [0085]    [EMBODIMENT  1  (Upper PD 1 s, Middle LDs, Lower PD 2 s ; FIGS.  1  to  4 )] 
         [0086]    [0086]FIG. 1 shows a vertically sectioned side view of Embodiment 1 allotting a first set of photodiodes PD 1 s to an upper floor, a set of laser diodes to a middle floor and a second set of photodiodes PD 2 s to a lower floor. FIG. 2 is a laterally sectioned front view for showing lightpaths of λ 3  being downward reflected by a second WDM 2  at the middle floor, passing middle floor holes and going into the photodiodes PD 2 s at the bottom floor in Embodiment 1. FIG. 3 is another laterally sectioned front view for showing lightpaths of λ 2  being upward reflected by a first WDM, at the middle floor, passing upper floor holes and going into the photodiodes PD 1 s at the top (upper) floor in Embodiment 1.  
         [0087]    A package has a three-storeyed structure consisting of a lower case  3 , a middle case  1  and an upper case  2 .  
         [0088]    This is an example of a four channel module (M= 4 ). An eight channel, sixteen channel or one channel can be easily built by the teaching of the present invention.  
         [0089]    The middle case  1  is a top-opening vessel having a bottom plate  5 , a front wall  6 , a rear wall  8  and side walls  9 . An inner space enclosed by the middle case  1  is named a middle floor B. The middle case  1  has a cavity for accommodating a silicon bench  4 . The silicon bench  4  is a rectangular silicon single crystal plate. Lightwaveguides G a , G b , G c  and G d  are formed parallel in the longitudinal direction upon the silicon bench  4 . The lightwaveguides are made from fluoric polyimide. A core/clad structure is formed by differentiating refractive indices by doping the core or clad with an impurity. The resin lightwaveguides have advantages of low cost and facile fabrication.  
         [0090]    Otherwise, silica lightwaveguides can be produced by oxidizing a surface of the silicon bench  4  into silicon dioxide (SiO 2 ). The dielectric lightwaveguides have an advantage of low propagation loss in spite of complicated fabrication and high cost. FIG. 1, which is a vertical section cut along a longitudinal line, shows a lightwaveguide G on the silicon bench  4 . Four parallel lightwaveguides G a , G b , G c  and G d  are made on the silicon bench as shown in FIG. 2. FIG. 2 and FIG. 3 exhibit an example of a four channel (M=4). Eight, sixteen, lightwaveguides are also available on the silicon bench. Four channel transmitting parts including laser diodes LD a , LD b , LD c  and LD d  behind ends of the lightwaveguides on a rear region of the silicon bench.  
         [0091]    V-grooves V a , V b , V c  and V d  are made at a front region of the lightwaveguides by anisotropic etching. The middle case  1  has a front aperture. An optical connector  7  is sustained in the front aperture of the front wall  6 .  
         [0092]    The connector (MT connector)  7  maintains terminals of element fibers FB a , FB b , FB c  and FB d  of a four channel ribbonfiber. The element fibers are cut at points behind the connector  7  into short tails.  
         [0093]    The cut short tails of the ribbonfiber are embedded onto the V-grooves V a , V b , V c  and V d  on the silicon bench  4 . The number of fibers, the number of lightwaveguides and the number of V-grooves are all the same (M). The laser diodes LD a , LD b , LD c  and LD d  are mounted on the rear region of the silicon bench  4 . The number of the LDs is also equal to the channel number M.  
         [0094]    In the example, the top and bottom cases dispense with silicon benches. The middle floor B makes use of a silicon bench for aligning optical axes of the laser diodes LD a , LD b , LD c  and LD d , the lightwaveguides G a , G b , G c  and G d  and the optical fibers FB a , FB b , FB c  and FB d  with accuracy. The middle case has a plurality of outward extending leadpins  10  which have been insert-molded with the case. Electrodes (cathodes and anodes) of the laser diodes LD a , LD b , LD c  and LD d  are joined to the leadpins  10  with wires  12 . In the example, four laser diodes LD a , LD b , LD c  and LD d  are connected with eight wires to the leadpins  10 , though one wire  12  appears in FIG. 1. In the middle floor, two wavelength selective filters WDM 1  and WDM 2  are mounted at an intermediate region of the lightwaveguides for selectively reflecting receiving signal light λ 2  and λ 3  upward and downward.  
         [0095]    The WDM 1 , which is closer to the lasers (LDs) than the WDM 2 , is slanting upward at an angle of 30 degrees. The WDM 1  reflects a set of receiving beams λ 2  propagating in the fibers FBs slantingly upward but allows transmitting beams λ 1  emanating from the lasers (LDs). The λ 2  beams are sensed by the upper photodiodes PD 1 s on the upper floor A.  
         [0096]    The WDM 2 , which is closer to the fibers (FBs) than the WDM 1 , is slanting downward at  30  degrees. The WDM 2  reflects another set of receiving beams λ 3  propagating in the fibers FBs slantingly downward but allows transmitting beams λ 1  emanating from the lasers (LDs). The λ 3  beams are detected by the lower photodiodes PD 2 s on the ground floor C. The λ 3  is infrared light. The silicon bench  4  is transparent to λ 3 . There is no hole on the silicon bench. But, the bottom plate  5  is opaque for λ 3 . Perforation of beam-guiding holes J a , J b , J c  and J d  on the bottom plate  5  is necessary for guiding λ 3  from the middle floor B to the lower floor C. In addition to the beam-guiding holes, the bottom plate  5  of the middle case I has bores  47  and  48  for allocating the lowest floor C with a transparent resin  50 .  
         [0097]    The upper case  2  is a resin-made rectangular package with a top opening. An inner space encapsulated by the upper case  2  is an upper floor A. The upper case  2  has a front wall  16 , a back wall  18 , a bottom plate  15 , side walls  19  and  19 . A plurality of metallized patterns  22  are printed on the bottom plate  15 . M channeling (M=4) photodiodes PD 1a , PD 1b , PD 1c  and PD 1d  are mounted on metallized pads on the bottom for receiving light signals λ 2  selectively reflected by the first WDM 1  laid on the middle floor. The figures denote bottom incidence type photodiodes epi-up fixed on the pads. Otherwise, top incidence type photodiodes can be epi-down mounted on the pads for sensing light signals for the WDM 1 .  
         [0098]    Holes H a , H b , H c  and H d  are perforated on the bottom plate  15  of the upper case  2  for guiding light beams reflected by the WDM 1  of the middle floor B. Preamplifiers AMP 1a , AMP 1b , AMP 1c  and AMP 1d  are installed adjacently to the photodiodes PD 1a , PD 1b , PD 1c  and PD 1d  for preamplifying photocurrents induced in the photodiodes. Electric elements  20  are furnished for shielding photodiode circuits from external noise. Lots of leadpins  23  and  24  are fixed in holes of the side walls  19  of the upper case  2 . Metallized patterns  22  formed on the bottom  15  are joined by wires  25  and  26  to the leadpins  23  and  24 . Details of wirings between the metallizes and the leadpins are omitted in the figures.  
         [0099]    Wires  29  connect the photodiodes PD 1 s to the preamplifiers AMP 1 s. Some of the electric elements  20  are joined by wires (omitted) to the PD 1 s and AMP 1 s. The leadpins are provided to the case by insert-molding a leadframe with a resin in a metallic mold. Several resin-guiding holes  27  and  28  are perforated on the bottom  15  of the upper floor. The resin-guiding holes allow a fluid resin to flow down into and fill the lower floors B and C overall.  
         [0100]    The lower case  3  is also a resin rectangular vessel with a top opening. An inner space encapsulated by the lower case  3  is a lower floor C. The lower case  3  has a front wall  36 , a back wall  38 , a bottom plate  35 , and side walls  39  and  39 . A lot of metallized patterns  42  are printed on the bottom plate  35 . M channeled (M= 4 ) photodiodes PD 2a , PD 2b , PD 2c  and PD 2d  are mounted on metallized pads on the bottom for receiving light signals selectively reflected by the second WDM 2  laid on the middle floor. The figures denote top incidence type photodiodes epi-up fixed on the pads. Otherwise, bottom incidence type photodiodes can be epi-down mounted on the pads for sensing light signals for the WDM 2 .  
         [0101]    Holes J a , J b , J c  and J d  are perforated on the bottom  5  of the middle case  1  for guiding light beams λ 3  reflected by the WDM 2  of the middle floor B. Preamplifiers AMP 2a , AMP 2b , AMP 2c  and AMP 2d  are installed in the vicinity of the photodiodes PD 2a , PD 2b , PD 2c  and PD 2d  for preamplifying photocurrents induced in the photodiodes. The lower floor C has electric elements  40  for shielding photodiode circuits from external noise. A plurality of leadpins  43  and  44  are fixed in holes of the side walls  39  and  39  of the lower case  3 .  
         [0102]    Metallized patterns  42  formed on the bottom  35  are joined by wires  45  and  46  to the leadpins  43  and  44 . Details of wirings between the metallizes and the leadpins are omitted in the figures.  
         [0103]    Wires  49  connect the photodiodes PD 2 s to the preamplifiers AMP 2 s. Some of the electric elements  40  are joined by wires (omitted) to the PD 2 s and AMP 2 s. The leadpins are provided to the case by insert-molding a leadframe with a resin in a metallic mold.  
         [0104]    The three cases are unified by piling the middle case  1  upon the lower case  3 , piling the upper case  2  on the middle case  1  and gluing the piled three with an adhesive. In the coupled three-stories, the first set of photodiodes PD 1a , PD 1b , PD 1c  and PD 1d  on the highest floor A are positioned above the lightwaveguides G a , G b , G c  and G d  on the middle floor. The second set of photodiodes PD 2a , PD 2b , PD 2c  and PD 2d  in the bottom case  3  align just beneath the lightwaveguides G a , G b , G c  and G d  on the middle floor B.  
         [0105]    The first set of photodiodes PD 1a , PD 1b , PD 1c  and PD 1d  on the top floor A is located on loci of the upward beams reflected by the WDM 1  and passing the bottom holes. The second set of photodiodes PD 2a , PD 2b , PD 2c  and PD 2d  on the bottom floor C is located on loci of the downward beams reflected by the WDM 2 .  
         [0106]    The top floor (the highest case A) is supplied with a transparent fluid resin  50  of a low refractive index, for example, an acrylate (thermosetting or ultraviolet setting) resin or silicone (thermosetting or ultraviolet setting) resin. The resin fluid passes the holes  27  and  28  and arrives at the middle floor (B). The resin fills the middle floor. Then, the fluid resin passes the holes  47  and  48  and fills the bottom floor (C). The transparent resin  50  protects the PDs, LDs, WDMs and wires.  
         [0107]    In the top floor A, the transparent resin  50  is in tight contact with the first set of photodiodes PD 1a , PD 1b , PD 1c  and PD 1d , the first set of preamplifiers AMP 1a , AMP 1b , AMP 1c  and AMP 1d , wires  29  and metallizes  22 . In the middle floor B, the transparent resin  50  is in contact with the silicon bench  4 , lightwaveguides, the set of laser diodes LD a , LD b , LD c  and LD d , the WDM 1  and WDM 2 , wires  12  and leadpins  10 . In the bottom floor C, the transparent resin  50  comes into contact with the second set of photodiodes PD 2a , PD 2b , PD 2c  and PD 2d , the second set of preamplifiers AMP 2a , AMP 2b , AMP 2c  and AMP 2d , wires  49  and metallizes  42 .  
         [0108]    The transparent resin  50  is a resin which is hardened by heat (thermosetting) or ultraviolet rays (ultraviolet setting). The hardened transparent resin  50  has a refractive index (1.4-1.5) similar to optical fibers (silica fibers). The transparent resin  50  decreases reflection loss at ends of fibers or lightwaveguides.  
         [0109]    Three storey half products have been produced by the steps mentioned hitherto. The coupled upper case, the middle case and the bottom case are packaged by transfermolding with an outer rigid resin. The outer resin is an opaque water-proof sturdy resin, for example, an epoxy resin.  
         [0110]    [0110]FIG. 4 is a perspective view of a resin packaged optical communications module. The whole is enclosed by a resin package  52 . Parallel leadpins  10  for laser diodes extend backward from the package  52 . The LD leadpins are parts of a middle LD leadframe. The top floor (upper floor A) has PD 1  leadpins  23  and  24  extending from the sides. The bottom floor (lower floor C) has PD 2  leadpins  43  and  44  extending from the sides.  
         [0111]    [0111]FIG. 4 shows a connector  7  at the front of the module. Ends of fibers FB a , FB b , FB c  and FB d  appear on the forefront of the connector  7 . The pitch of the fibers is 250 μm (0.25 mm). Four fibers stretch in a width of 250 μm×3+125 μm=875 μm. Guidepins  11  enable the module to attach to or detach from a ribbonfiber connector. In this example, this module has a 10 mm width, a 35 mm length and a 6 mm height.  
         [0112]    The functions of the three-storied communications modules of the present invention are described. The middle floor (B) contains an M channel λ 1  transmitting (LD) portion. The top floor (A) has an M channel λ 2  receiving (PD) portion. The bottom floor (C) has an M channel λ 3  receiving (PD) portion.  
         [0113]    The laser diodes LD a , LD b , LD c  and LD d  on the middle floor (B) generate transmitting light signal beams of a λ 1  band. The transmitting signal beams propagate in the lightwaveguides G a , G b , G c  and G d  on the silicon bench, go into the element fibers FB a , Fb b , FB c  and FB d  and run in the fibers to counterpart subscribers or central stations.  
         [0114]    λ 2  receiving signal beams running in the fibers FB a , FB b , FB c  and FB d  go into the lightwaveguides and are reflected by the first WDM 1  upward. The λ 2  beams pass the holes H a , H b , H c  and H d  and enter the photodiodes PD 1a , PD 1b, PD   1c  and PD 1d , which yield photocurrents in proportional to the signals. The photocurrents are amplified by the neighboring preamplifiers AMP 1a , AMP 1b , AMP 1c  and AMP 1d . Amplified electric signals are output via wires and leadpins to outer circuits.  
         [0115]    λ 3  receiving signal beams running in the fibers FB a , FB b , FB c  and FB d  go into the lightwaveguides and are reflected by the second WDM 2  downward. The λ 3  beams pass the holes J a , J b , J c  and J d  and enter the photodiodes PD 2a , PD 2b , PD 2c  and PD 2d  which yield photocurrents in proportional to the signals. The photocurrents are amplified by the neighboring preamplifiers AMP 2a , AMP 2b , AMP 2c  and AMP 2d . Amplified electric signals are output via wires and leadpins to outer circuits.  
         [0116]    [Fabrication of Embodiment 1 (four-channel PD/LD/PD module)] 
         [0117]    Production processes of Embodiment 1 are described. Straight polymer lightwaveguides G a , G b , G c  and G d  of a 250 μm pitch are produced. In practice, a 1 mm thick single crystal silicon wafer of a diameter of 20 cm to 30 cm is prepared. Lightwaveguides, V-grooves, steps and metallizes are produced on predetermined chip areas assumed on the wafer by wafer-processing. Then, the silicon wafer is scribed and divided into a plurality of silicon bench chips of 1.5 mm×7.5 mm.  
         [0118]    A transparent core/cladding resin is coated on the silicon wafer by a spin coating method. The cladding layer is formed to a 10 μm thickness on the silicon wafer. The core layer is made into a 5 μm thickness. The core layer is formed into 6.5 μm wide separated core lines by photolithography with dry etching. A cladding layer of a 10 μm thickness is further formed upon the core lines and the cladding layer. A cladding/core/cladding triplet waveguide structure is made. The core has a section of a 5 μm height and 6.5 μm width. Waveguides mean cores in the triplet structure.  
         [0119]    Two slanting grooves of a 20 μm width are cut across the lightwaveguides G a , G b , G c  and G d  on the silicon bench by dicing processing. Normals of the slanting grooves are slanting to the lightwaveguides at ±30 degrees. The first WDM 1  and the second WDM 2  are inserted and fixed in the slanting grooves. The lower steps behind the final ends of the lightwaveguides are metallized. The hitherto described steps (of making steps, lightwaveguides, metallizes and grooves) are done on the silicon wafer by wafer processing. The round wafer is scribed and cut into silicon benches of 1.5 mm×7.5 mm×1 mm.  
         [0120]    After the processed wafer has been divided into individual chips, 1.3 μm laser diodes (LDs) LD a , LD b , LD c  and LD d  are die-bonded on a rear part behind the lightwaveguides with an AuSn solder. The LDs are easily and exactly aligned to the lightwaveguides G a , G b , G c  and G d  by marks which have been preliminarily printed on a rear part of the silicon bench. Wavelength division multiplexers WDM 1  and WDM 2  are inserted and fixed in the slanting grooves. The WDMs are made by piling a plurality of sets of at least two different kinds of dielectric films in turn on a polymer substrate. The first WDM, has a function of reflecting 30 degree slantingly forward incidence 1.48 μm (λ 2 ) beams and permitting 30 degree slanting rearward incidence LD beams of 1.3 μm (λ 1 ) to pass. The second WDM 2  has a function of reflecting 30 degree slantingly forward incidence 1.55 μm (λ 3 ) beams and permitting 30 degree slanting rearward incidence LD beams of 1.3 μm (λ 1 ) to pass.  
         [0121]    The middle case  1  is made by insert-molding a liquid crystal polymer with a leadframe with wiring patterns in a mold. The middle case  1  has a 5 mm width, a 25 mm length and a 1.5 mm height. Holes for leading resins, holes for admitting light beams and a cavity for supporting the silicon bench  4  are formed on the middle case  1 . The processed silicon bench  4  is embedded on the cavity of the middle case  1 .  
         [0122]    The upper (top) case  2  is made by insert-molding a liquid crystal polymer with a leadframe in a mold. The leadframe has wiring patterns for the first set of photodiodes PD 1 s and the first set of preamplifiers AMPIs and 0.1 mm φ bottom holes H a , H b , H c  and H d  for guiding light beams. The metallic leadframe is made of a metallic thin plate by punching holes and patterns. The size of the top case is a 5 mm width, a 25 mm length and a 1.5 mm height.  
         [0123]    The first set of photodiodes PDs and preamplifiers AMP 1 s is bonded on the wiring patterns of the leadframe on the top case  2 . Optionally, capacitors, coils and resistors are bonded on the leadframe by silver(Ag) paste for the sake of noise shielding. The figures show capacitors as an example. The electric elements are not indispensable. 25 μm φ Au wires are wirebonded for connecting wiring patterns to electrode pads on the photodiodes PDs and the preamplifiers AMP 1 s.  
         [0124]    The bottom case  3  is also produced by insert-molding a liquid crystal polymer with a leadframe in a mold. The leadframe has wiring patterns for photodiodes PD 2 s and preamplifiers AMP 2 s. The bottom case  3  has a size of 5 mm×25 mm×1.5 mm.  
         [0125]    The second set of photodiodes PD 2 s and the second set of preamplifiers AMP 2 s are mounted on the wiring patterns on the leadframe of the bottom case  3 . Sometimes, capacitors, coils and resistors are fitted on the leadframe by Ag-paste for attenuating external noise. Electrode pads of the photodiodes PD 2 s and the preamplifiers AMP 2 s are joined to wiring patterns on the leadframe by 25 μm φ Au wires.  
         [0126]    Preliminarily marks have been allotted to a set of photodiode chips PD 2 s fixed upon the bottom case  3 . The marks should be inscribed on predetermined spots on the silicon bench and predetermined spots on the middle case  1 . The bottom case  3  and the middle case  1  are unified by observing the middle case marks (bench marks) and bottom marks by image-processing through a microscope, positioning the middle and bottom cases at predetermined places by the observation of the bench marks and the case marks, supplying an ultraviolet setting resin to the cases  1  and  3 , laying the middle case  1  just upon the bottom case  3 , irradiating the resin by ultraviolet rays and fitting the middle case  1  to the bottom case  3 . A similar process joins the top case  2  to the middle case  1 . Then, the three-storied structure is built up.  
         [0127]    Finally, the three-storied cases are resin-packaged by transfermolding into a pertinent shape with a hard resin, as shown in FIG. 4. The size of the packaged module is 10 mm×35 mm×6 mm.  
         [0128]    [EMBODIMENT  2  (M= 8 , PD 1 /LD/PD 2 ; FIGS.  5  to  7 )] 
         [0129]    This invention has an advantage of reducing a necessary space, which is conspicuous in multichannel communications systems. Then, an eight channel case (M=8) is now described with referring to FIG. 5, FIG. 6 and FIG. 7. The M=8 module has a fundamental structure similar to the M=4 module of Embodiment 1.  
         [0130]    [0130]FIG. 5 is a plan view of an upper (top) case  2 . In FIG. 5, the top case  2  has a bottom plate  15 , a front wall  16 , a rear wall  18 , and side walls  19  and  19 . The bottom plate  15  has metallized patterns and a leadframe with leadpins  23  and  24 . FIG. 5 omits details of the metallized patterns. Eight photodiodes PD 1a , PD 1b , PD 1c , . . . , PD 1h , for receiving a λ 2  band are mounted on the metallizes of the top case  2 . Preamplifiers AMP 1a , AMP 1b , AMP 1c , . . . , AMP 1h  are installed in the vicinity of the photodiodes. Besides, the photodiodes and the preamplifiers, electric elements  20 , for example, capacitors, coils or resistors, are installed on wiring patterns on the bottom plate  15 . The optoelectronic chips (PDs), the AMPs, the electric elements and the wiring patterns are joined by wires.  
         [0131]    [0131]FIG. 6 is a plan view of a middle case  1 . The middle case  1  has a bottom plate  5 , a front wall  6 , a rear wall  8 , and side walls  9  and  9 . The bottom plate  5  has a central cavity and resin leading holes. A rectangular silicon bench  4  is stored on the central cavity. The silicon bench  4  has been provided with V-grooves, lightwaveguides, metallized patterns and slanting grooves by evaporation, CVD, photolithography or printing at the stage of a silicon wafer in the wafer-process. The silicon wafer is scribed and cut into a plurality of benches. The silicon bench  4  has eight V-grooves V a , V b , V c , . . . , V h , eight lightwaveguides G a , G b , G c , . . . , G h , a WDM 1 , a WDM 2  and eight laser diodes LD a , LD b , LD c , . . . , LD h . An MT connector  7  having eight tails of an eight channel ribbonfiber is fixed on a front hole of the front wall  6  of the middle case  1 . The tails of the ribbonfiber are inserted and fixed in the V-grooves. The laser diodes LD a , LD b , LD c , . . . , LD h , metallized patterns, and leadframe  10  are connected by wires.  
         [0132]    [0132]FIG. 7 is a plan view of a bottom case  3 . The bottom case  3  comprises a bottom plate  35 , a front wall  36 , a rear wall  38  and side walls  39  and  39 . The bottom plate  35  has embedded leadpins and printed metallized patterns. FIG. 7 omits details of the metallized patterns. Eight photodiodes PD 2a , PD 2b , . . . , PD 2h  are die-bonded on metallized pads on the bottom plate. Preamplifiers AMP 2a , AMP 2b , AMP 2c , . . . , AMP 2h  are furnished on metallized patterns in the vicinity of the photodiodes for amplifying photocurrents of the photodiodes. Further electric elements  40  (capacitors, resistors) are upholstered on the bottom plate. The optoelectronic chips, the preamplifiers, and the electric elements are connected by wires.  
         [0133]    Embodiment 2 is completed by piling the bottom case  3 , the middle case  1  and the upper case  2  in this order, gluing the cases together, injecting a transparent resin  50  into the cases, and molding the whole with a rigid outer resin  52  into a plastic packaged device.