Abstract:
The module has a planar platform. A straight light guide is formed on the planar platform for guiding incoming light and outgoing light. A filter is interposed slantingly in the light guide for reflecting a part of light propagating in the light guide but for allowing the rest of the light to pass through. An LD or LED is mounted at an extension of the light guide for producing sending signal light and introducing the sending light into the light guide. A photodiode is mounted on the platform at a point on a path of the reflected light for sensing the incoming (receiving signal) light. Guide pins are fixed to the platform for attaching to and detaching from an optical connector having holes and an optical fiber. In the attached state, the end of the light guide of the module faces the end of the fiber in the connector.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an LD/PD (laser diode/photodiode) module or an LED/PD (light emitting diode/photodiode) module as a sending/receiving apparatus used at base ports (broadcasting station) and subscribers in a unidirectional or bidirectional optical communication system which transmits optical signals of different wavelength in a unidirectional direction or bidirectional directions. In particular, this invention relates to an LD/PD or an LED/PD module which ensures facile attachment to and detachment from an optical connector. The word “LD/PD module” is adopted to signify both an “LD/PD module” and an “LED/PD module” hereafter for simplicity. At a subscriber port, an incoming light is called “receiving signal light” and an outgoing light is called “sending signal light”. 
     This application claims the priority of Japanese Patent Application No.10-58806(58806/1998) filed Feb. 23, 1998 which is incorporated herein by reference. 
     2. Description of Related Art 
     [EXPLANATION OF BIDIRECTIONAL OPTICAL COMMUNICATION] 
     Recent development of technology has reduced transmission loss of optical fibers and has improved the properties of semiconductor laser diodes (hereafter indicated as LDs) and semiconductor photodiodes (hereafter indicated as PDs). The improvements of fibers, LDs and PDs enable us to transmit various types of information by light signals. The transmission is called “optical communication”, since light signals carry information. The types of information for sending or receiving at stations or at subscriber ports are, for example, telephones, facsimiles or televisions. In particular, people have vigorously tried optical communication based on long wavelength light (near infrared), for example, of a 1.3 μm wavelength or of a 1.55 μm wavelength. Recently bidirectional transmission attracts attention, since only a single optical fiber can send signals both in a forward direction and in a backward direction at the same time. The system of the communication is called a “bidirectional communication system”. Fortunately the bidirectional system saves one optical fiber. 
     FIG. 1 schematically indicates a multiwavelength bidirectional optical communication system which adopts a plurality of wavelengths for sending signals simultaneously both in a forward direction and in a backward direction. 
     One station is connected to a plurality of subscribers (ONUs) by optical fibers. Although FIG. 1 shows only a single subscriber for drawing convenience, many subscriber ports are connected to the central station. The fiber from the station branches at many bisecting points into a plurality of fibers linking with individual subscribers. 
     The central station amplifies the signals of telephones or televisions as digital signals or analog signals and drives a semiconductor laser (LD 1 ) which produces λ1 light responsive to the amplified signals. The light of λ1 emitted from the LDI (P 1 ) enters an optical fiber  1  as light signals of λ1. A wavelength division multiplexer (WDM)  2  introduces the λ1 light into an intermediate optical fiber  3 . Another wavelength multiplexer (WDM)  4  allocates the λ1 light to an optical fiber  5 . A photodiode (PD 2 ) senses the λ1 signals for converting the optical signals to electric signals (P 3 ). A receiver apparatus on the subscriber side amplifies and processes the electric signals (P 3 ) for reproducing a voice or image. The signals flowing from the station to the subscribers are called “downward signals”. The direction is called a “downward direction”. 
     On the contrary, a subscriber converts electric signals of a facsimile or a telephone into λ2 light signals by a semiconductor laser diode (LD 2 ) which oscillates at a wavelength λ2 (P 4 ). Going into a fiber  6 , the λ2 light passes the WDM  4 , propagates in the intermediate optical fiber  3  to the station. The WDM  2  allocates the λ2 light into a fiber  7 . A photodiode (PD 1 ) senses the λ2 light for converting into electric signals (P 2 ). Converters or signal processing circuits on the station side regenerate telephone voice or facsimile images. The direction of the signal flow from the subscribers to the station is called an “upward direction”. The signals are called “upward signals”. 
     The above system appropriates λ1 to downward signals and λ2 to upward signals exclusively. Another system uses only one wavelength in common for both upward and downward signals. A further system doubly allocates two wavelengths λ1 and λ2 both for upward and downward signals. Separation of two wavelengths is an important problem in the optical communication system which carries different wavelength signals in an optical fiber. 
     [Explanation of wavelength division multiplexer] 
     Both the station and the subscribers require a device for discriminating wavelengths and separating one wavelength from others. A WDM is a device having such a function. In FIG. 1, the WDMs  2  and  4  play the role of distinguishing and separating different wavelengths. A WDM either joins λ1 to λ2 for introducing them into a fiber or extracts only one wavelength light from two wavelengths propagating in a fiber. WDMs play an important role in multiwavelength bidirectional optical communication systems. 
     Various types of wavelength division multiplexers have been suggested. FIG. 2 indicates a WDM constructed by optical fibers or optical waveguides. Two optical paths  8  and  9  lie in a close relation at a part  10  for allowing the exchange of power. The distance D and the length L of the close portion  10  determine the modes of coupling. In the example, when λ1 enters the path  8  (P 1 ), the same wave appears in a path  11  (P 3 ). Going into a path  12  (P 4 ), λ2 appears in a path  9  instead of the path  8  (P 2 ). The coupling portion  10  gives wavelength selectivity to the device. 
     FIG. 3 shows another WDM which uses a multilayered mirror. The WDM consists of two rectangular isosceles triangle columns  13  and  14 , and a dielectric multilayer mirror  15  formed on the slanting plate of the columns. The two columns are glued at the dielectric mirror  15  for making a square column. Selection of the refractive index and the thickness gives the dielectric multilayer the wavelength selectivity of allowing one wavelength λ1 shooting at 45 degrees to the multilayer to pass through and of reflecting another wavelength λ2 at a right angle. This dielectric layer type WDM can be the WDMs  2  and  4  in the optical communication system of FIG.  1 . The WDM is sometimes called a wave-division-integration device. Fiber-type WDMs and glass block type WDMs are already on sale. 
     An example of an LD/PD module on a subscriber side is explained by referring to FIG. 16. A single mode optical fiber  16  spreading from the central station is connected by an optical connector  17  to an optical fiber  18  of a subscriber (ONU) module. The ONU module has a fiber-type WDM  21  which couples the fiber  18  to a fiber  19  with wavelength selectivity. A contact portion  20  exchanges light power. An optical connector  22  couples the fiber  18  to an LD module  25  in the ONU. Another optical connector  23  joins the fiber  19  to a PD module  27 . 
     The LD module  25  and the fiber  24  are part of an upward system. 1.3 μm light carries signals from the subscriber to the station through the upward system. The fiber  26  and the PD module  27  are part of a downward system. The station sends 1.55 μm light carrying signals to subscribers through the downward system. The PD module  27  converts the optical signals into electric signals. The LD module  25 , a signal sending device, includes an electric circuit for amplifying and modulating the signals of telephones and facsimiles, and a laser diode (LD) for converting electric signals into optical ones. The PD module  27 , a receiving device, contains a photodiode for converting optical signals from the station into electric signals, an amplification circuit for amplifying the optical signals and a demodulation circuit for restoring the television signals or telephone signals. The WDM  21  has a function for separating 1.55 μm light from 1.3 μm light. This example allots 1.3 μm light to the upward system and 1.55 μm light to the downward system. 
     This invention provides an improved LD/PD module for bidirectional multiwavelength optical communication. The LD/PD module signifies a device having an LD for producing optical signals, a PD for receiving optical signals and peripheral electric circuits for amplifying, modulating or demodulating signals. Related art is explained with regard to the parts. 
     [EXPLANATION OF KNOWN SEMICONDUCTOR LASER DIODE] 
     FIG. 4 shows a known laser diode device  28 . The device  28  includes a semiconductor laser diode (LD) chip  29  and a monitoring photodiode (PD) chip  30 . The laser diode chip  29  is vertically mounted on a side surface of a pole  31  on a header  32 . The laser diode  29  emits light in a vertical direction. Photodiode chip  30  is mounted upon the top of the header  32  at a spot at which the laser shoots a backward light. A plurality of lead pins  33  implanted on the bottom of the header  32 . A cap  34  covers the top surface of the header  32 . 
     The cap  34  has a window  35  at the center. The light beams are emitted in both directions vertically from the laser diode  29 . A lens  37  is fixed just above the window  35  by a lens holder  36 . A conical housing  38  covers the top of the lens holder  36 . A ferrule  39  pierces through a hole of the housing  38 . The ferrule  39  holds an end of an optical fiber  40 . The ends of the ferrule  39  and the fiber  40  are polished in a slanting plane of about eight degrees for impeding reflected light from going back into the laser. The holder  36  is aligned at an optimum spot to the header  32  by sliding the holder  36  and measuring the light power at the other end of the fiber  40 . The housing  38  is also aligned to the lens holder  36  by the same manner. Wires connect the pads of the laser diode chip  29  and the photodiode chip  30  to the lead pins  33 . 
     The lens  37  converges the light beams emitted from the laser  29  on the end of the fiber  40 . The light beams go into the fiber  40 . Since the laser is modulated by a driver circuit with an electric signal, the light carries the signal. The output of the laser diode is monitored by the monitoring photodiode  30 . The material of the laser determines a wavelength between 1.3 μm and 1.55 μm of the light produced by the laser. 
     [EXPLANATION OF KNOWN PHOTODIODE DEVICE] 
     FIG. 5 shows a known photodiode device. The photodiode device has a header  42  as a package base. A photodiode chip  41  is die-bonded upon the header  42 . Lead pins  43  project from the bottom of the header  42 . A cap  44  protects the top surface of the header  42 . An opening  45  is perforated at the center of the cap  44 . A cylindrical lens holder  46  encircles the cap  44  upon the header  42 . The lens holder  46  holds a lens  47  just above the photodiode chip  41 . 
     The lens holder  46  has a conical housing  48  on the top. A ferrule  49  grips an end of an optical fiber  50 . The ferrule  49  pierces a top hole of the housing  48  for fixing the fiber end to the housing  48 . The ends of the ferrule  49  and the fiber  50  are slantingly polished for suppressing reflected light from returning back to the laser diode. 
     The holder  46  and the housing  48  are aligned at optimum spots by introducing light into the fiber from the farther end, measuring the light power by the PD  41  and maximizing the power input in the PD  41 . The material of the light receiving layer of the PD  41  determines the range of wavelengths of the light detectable to the PD device. Silicon photodiodes are available for sensing visible light. However, silicon photodiodes are irrelevant for the present invention which aims at building an LD/PD module relying upon infrared light. Sensing of infrared light requires compound semiconductor photodiodes having InP as a substrate which has a relevant band gap and an InGaAs or an InGaAsP light receiving layer which has a narrow band gap for absorbing near infrared light. 
     Prior proposed LD/PD modules are not yet. Subscribers would be mostly households. Thus, the optical communication network should have a market as wide as telephones. However, consumers would not buy such an LD/PD module for optical communication unless the price of the module fell to a level as low as a telephone. For optical communication to become prevalent the subscriber sending-/receiving device must be inexpensive. The prior LD/PD apparatus proposed in FIG. 16 cannot be made as inexpensive as the price of a telephone, since the apparatus is only an assembly of a PD module, an LD module and a WDM module. The price is the sum of the modules. 
     A high price impedes the sending-/receiving devices from becoming prevalent among subscribers. Efforts have been to made to reduce the number of parts or diminishing the size of such devices for lowering their cost. Attempts for lowering the cost are now explained. 
     [A. Spatial separating beam type module (a receptacle containing a WDM, a PD and an LD)] 
     FIG. 6 shows a candidate proposed by Masahiro Ogusu, Tazuko Tomioka and Sigeru Ohshima, “Receptacle Type Bi-directional WDM Module I”, Electronics Society Conference of Japanese Electronics, Information and Communication, C-208, p208(1996). The module is built in a rectangular housing  60 . A WDM filter  61  is centrally positioned in a 45 degree slanting direction to sides of the housing  60 . Three drum lenses  62 ,  63  and  64  lie in radial directions at three sides of the housing  60 . A photodiode  66  is mounted on one side in front of the drum lens  62 . A laser diode  68  is fixed on another side in front of the lens  63 . The lens  64  lies at the end of the fiber  69 . 
     In practice, the module consists of two separable parts. One part having the fiber end can be detached from and attached to the other part (housing  60 ) containing the filter  61 , the PD  66  and the LD  68 . In the coupled state, an external fiber  69  is connected via the WDM  61  to the PD  66  and the LD  68 . The lenses  64 ,  62  and  63  prevent the light emanating from the fiber  69  from diverging spatially in the receptacle. The LD  68  emits 1.3 μm light which penetrates the WDM filter  61  slantingly and goes into the fiber  69  for sending optical signals. 
     The incoming light propagating in the fiber  69  is 1.55 μm light which is reflected by the WDM filter  61 . The reflected light is transmitted via the lens  62  into the PD  66 . The module of FIG. 6 is smaller than the previously explained module of FIG.  16 . However, the receptacle type module still contains two independent optoelectronic devices of the LD and the PD and the indispensable WDM filter in addition to three lenses for preventing light from dispersing spatially. The alignment of parts is still as difficult as the module of FIG.  16 . The cost is similar to the cost of FIG.  16 . 
     [B. Y-bisecting wave guide type module (FIG.  7 )] 
     FIG. 7 shows another LD/PD module proposed by Naoto Uchida, Yasufumi Yamada, Yoshinori Hibino, Yasuhiro Suzuki &amp; Noboru Ishihara, “Low-Cost Hybrid WDM Module Consisting of a Spot-Size Converter Integrated Laser Diode and a Waveguide Photodiode on a PLC Platform for Access Network Systems”, IEICE TRANS. ELECTRON., VOL.E80-C, NO.1, P88, January 1997. An insulating silicon substrate  70  is adopted as a base. A transparent quartz optical waveguide  71  is produced on the insulating silicon substrate  70 . A corner of the waveguide  71  is cut into a step  72 . Doping of impurity makes narrow Y-bisecting paths  73 ,  74 ,  76 ,  77  and  78  on the waveguide  71 . 
     This example has two Y-branches. The first Y-branch has a WDM  75  at the cross point. The WDM  75  has wavelength selectivity that reflects 1.55 μm light but allows 1.3 μm light to pass through. Electrode patterns  79 ,  80 ,  81  and  82  are evaporated upon the step  72  of the waveguide. A light source  83  which has a bottom electrode is bonded on the electrode patterns  79  and  80 . The light source  83  is either an LED or an LD for emitting 1.3 μm light from a point  85  on an end surface. 
     A PD chip  84  is bonded on the farther electrodes  81  and  82  on the step  72  for sensing 1.3 μm light. The light spreading in the optical fiber  88  contains both 1.3 μm light and 1.55 μm light. The light goes into the path  74  and attains to the WDM  75  which reflects 1.55 μm light to another fiber  87 . 1.3 μm light continues along its way to the Y-branched paths  77  and  78 . The half light reaching the LED  83  is of no use. The other half power shoots the PD  84  at a side point  86  for detecting the sent signals. The LED (or LD) makes sending signal light of 1.3 μm which propagates in the path  78 , the WDM  75  and the path  74  and enters the fiber  88 . 
     This example uses the WDM only for excluding 1.55 μm light. The biggest drawback of the proposed module is the difficulty of producing the planar Y-branched waveguide. Fabrication of curved Y-branch on a waveguide layer is far more difficult than the production of a straight path on a waveguide. In addition, the junction of fibers to the ends of the paths  73  and  74  is also a difficult task. Thus, the proposal gives us no means for solving the problem of making inexpensive LD/PD modules yet. 
     [C. Upward reflection WDM type LD/PD module (FIG.  8 )] 
     FIG. 8 shows another LD/PD module proposed by Tomoaki Uno, Tohru Nishikawa, Masahiro Mitsuda, Genji Tohmon, Yasushi Matsui, “Hybridly integrated LD/PD module with passive-alignment technology”, 1997 Conference of Electron, Information, Communication Society, C-3-89p198(1997). Mounting an LD and a PD on a common substrate allows the module to reduce cost and minimize size. In FIG. 8, a silicon substrate  90  provides a base to the module. Straight scribing makes a longitudinal V-groove  91  in the middle on the Si substrate  90 . An end of an optical fiber  92  is inserted and fixed in the V-groove  91 . Slanting cutting makes a deep slanting notch  93  midway across the fiber  92  and the V-groove  91  into the Si substrate  90 . A fiber end  94  is cut and separated from the fiber  92  by the notch  93 . The slanting notch  93  holds a WDM filter  95  in it. A PD  96  is mounted before the slanting WDM filter  95  above the V-groove  91  on the top of the Si substrate  90 . A step part  97  is made by cutting the back end of the Si substrate  90 . An LD chip  98  lies upon the step part  97 . The LD  98  emits 1.3 μm light  99  for sending signals. The sending light  99  propagates in the fiber  92  and the WDM  95  to the station (not shown). The receiving light of 1.55 μm  100  running in the fiber  92  is reflected by the WDM  95  to be light  101  and sensed by the PD  96 . The light path turns upward at the branch. 
     The module of FIG. 8 seems to have a simple structure. However, it is difficult to bury a fiber into a V-groove, and to align an LD and a PD to the fiber by sliding the LD and the PD and monitoring the light power sensed by the PD. A single-mode fiber has a 10 μm core diameter and a 125 μm cladding diameter. Insertion of the WDM filter  95  requires the notch to cut the thick cladding. Thus, two fibers are equivalently separated by (125 μm+WDM) at the notch  93 . The wide separation increases the reflection loss between the fibers  94  and  92 . The LD light leaks at the gap. 
     SUMMARY OF THE INVENTION 
     The LD/PD module of the present invention comprises a platform, a light guide straight formed at a center of the platform for guiding light, a plurality of guide pins fixed on the platform for connecting the platform to an optical connector (not shown in FIG.  9 ), a filter laid on the light guide for reflecting a portion of the light and for allowing another portion of the light to pass through, a photodiode mounted upon the platform for detecting the reflected light the by the filter, a light source (LD or LED) mounted upon the platform at the back of the light guide for generating sending light carrying signals. The end of the light guide faces an optical fiber in a good alignment in an optical connector by inserting the guide pins into sockets of the optical connector. Received light having passed the fiber in the connector enters the light guide, reflects on the filter and makes its way to the PD which converts light signals to electric signals. Sending light from the light source (LD or LED) penetrates the filter, is transmitted through the light guide and enters the fiber in the optical connector. 
     The light guide is a straight waveguide produced by enhancing partially refractive index by doping impurity to a linear part in a transparent material of the platform. The light guide is not a fiber glued on the platform but a waveguide made in the platform by impurity doping. The platform is transparent at least at the top. The waveguide on the platform can be made by transparent inorganic glass material or transparent resins. Quartz is the best material for the waveguide. 
     This invention is also applicable to a simpler LD/PD module which uses a single wavelength λ both for sending signals and for receiving signals. In the mono-wavelength system, the filter reflects a part of λ light and allows the other part of λ light to pass at a definite rate. In addition to the mono-wavelength system, this invention is also applicable to a two-wavelength system which sends λ1 light and receives λ2 light. In such a system, the filter has two roles of reflecting all λ1 and allowing all λ2 to pass. In this case, the filter is a WDM filter. 
     The filter can be produced by piling a plurality of dielectric films of different refractive indexes in turn for providing a desired ratio of reflection to penetration for a definite range of wavelengths. For example, a filter with wavelength selectivity or a partial reflection rate can be made by piling a plurality of dielectric films of different thicknesses and different refractive indexes on a glass substrate in turn or by piling a plurality of dielectric films in layers on a transparent plastic substrate. 
     The LD/PD module has, for example, an InGaAs or an InGaAsP photodiode having an InP substrate and an InGaAs or InGaAsP light receiving layer. Signal light is near infrared light, e.g., of 1.3 μm or 1.55 μm. The module has an LD of, e.g., an InGaAsP active layer. 
     Otherwise, a Si—PD can be used as a photodiode. In the case of the Si—PD, signal light should be visible light of a wavelength between 0.7 μm and 0.8 μm. InGaAsP lasers would be replaced by GaAIAs lasers as light sources. Both top surface incidence type PDs and bottom surface incidence type PDs can be used as photodetectors. Additional mounting of a pre-amplifier which amplifies a photocurrent of the PD raises sensitivity, enabling the PD to detect weak signals and suppressing external noise. 
     The sizes of the guide pin, diameter, length, or spacing, should be determined for enabling the pins to fit with the optical connector, which is, for example, a MT connector or a mini connector. 
     This invention is based upon a different concept from prior LD/PD modules. All the conventional modules employ a common concept of a module having a fiber in itself. Prior modules have communicated with other optical devices thorough the fiber. In contrast to prior fiber-housing modules, this invention advocates a fiberless LD/PD module which can attach to or detach from an optical connector directly. This invention joins the light guide to the fiber in a connector by coupling the module to the connector with the guide pins and the holes. Then, this module need not include a fiber in itself. Since there is no fiber, this module requires no means for fixing a fiber or for adjusting a fiber to other optical parts. 
     Advantages of the present invention will now be explained. 
     (1) The first advantage of the present invention is facile attachment or detachment to/from an optical connector. In particular, the embodiment of FIG. 9 has guide pins directly on the Si platform for attaching to or detaching from an MT connector or a mini-MT connector which makes it useful. For example, the module can be formed by digging V-grooves on the platform, inserting guide pins into the V-grooves and fixing the guide pins in the V-grooves. Thus, a first feature of the invention is its ability to attach and detach from ready-made optical connectors, e.g., MT connectors or mini-MT connectors. 
     (2) A second feature of the invention is a straight light guide produced on the Si platform and a WDM filter interposed on the light guide. It is easy to make a simple, straight light guide. This invention dispenses with a WDM coupler (FIG. 2 or FIG. 7) or a filter by surface wave guides. Optical circuits based upon a surface wave guide are called PLCs (Planar Lightwave Circuits). The WDM filter in FIG.  2  and the LED/PD module in FIG. 7 are produced by PLCs including complex components, e.g., Y-branches, confluences (coupling parts) and curves. The function of separating or integrating light waves and the function of selecting waves by wavelengths requires such complex components. But it is difficult to produce light waveguides including curves, Y-branches or confluences. This invention makes a light guide by raising the refractive index of the platform locally. But the light guide is a straight line. This invention includes neither branch, curve nor confluence of waveguides. The light guide this invention uses is not a waveguide but a simple light guide. The light guide is a single-mode guide which can easily be fabricated with a high yield. 
     The PLC (Planar Lightwave Circuit) technology aims at high-performance and sophistication by allotting WDMs, branches or confluences to the PLCs in general. This invention does not take such a complicated way but adopts a simple, straight light guide on a platform, which alleviates the difficulty of fabrication. If an optical fiber were to be glued to an end of a light guide like FIG. 7, it would require a difficult adjustment and would invite an error of the position of the fiber. This invention which does not glue an optical fiber to the light guide is immune from the difficult adjustment. 
     FIG. 10 shows a section of a light guide of the module. The longitudinal direction of the light guide is assigned to x-direction. This is a yz-section of the Si platform intersecting the light guide at right angles. The platform is a Si substrate  110 . A SiO 2  buffer layer  111  is made upon the Si substrate  110  by a flame deposition method or a sputtering method. A germanium-doped high refractive index SiO 2  layer  112  is produced overall upon the SiO 2  buffer layer  111 . Sides of the Ge-doped high refractive index SiO 2  layer are removed by photo etching with a mask having a pattern of central stripe. A central linear stripe part  112  of the Ge-dopes SiO 2  survives. A low-refractive index SiO 2  layer  113  is formed upon the stripe  112  and the buffer layer  111  by flame deposition or sputtering. Thus a light guide is produced. The Ge-doped high refractive index SiO 2  stripe  112  is assigned to a core. The peripheral low-refractive index SiO 2  layer  113  is a cladding layer. It is feasible to make such a simple light guide, since the light guide includes neither curve, confluence nor branch. 
     (3) A third feature of the invention is its low coupling loss. The module of the present invention can be coupled to an optical connector with high accuracy, since a single Si platform contains the light guide and the V-grooves for the guide pins. The precise assemblage ensures a low coupling loss. Matured mask alignment technology and the sophisticated lithography enable the light guide to interpose correctly at the middle of the guide pins. Since the partner optical connector has been made to have an end of the fiber at a middle point between the pin-holes, the end of the fiber of the connector faces exactly the end of the light guide of the module in the coupled state. 
     (4) A fourth feature of the invention is the smallness of the module. The WDM filter is made from an assembly of thin film dielectric layers. Piling dielectric films on a thin glass or a thin plastic substrate makes a small WDM filter unlike the bulky glass block WDM of FIG.  3 . Unlike the WDM of FIG. 2 which selects wavelengths along a coupled path and divides different wavelengths in a plane, the WDM of the invention divides the wavelengths slanting to the plane. The embodiment of FIG. 9 bends the receiving light at 90 degrees by the WDM. The incidence direction and the refraction direction are not in parallel to the WDM. The bent receiving light advances in the vertical path  117 , shoots the slanting plane at 45 degrees, turns upward and goes into the bottom of the PD  137 . 
     (5) The fifth feature is the pre-amplifier mounted on the same platform. The existence of the amplifier in the same package raises the sensitivity as the receiving set. If the amplifier is positioned out of the package, external noise is liable to invade in the module via wires or lead pins. This invention gives the module high sensitivity and strong resistance against external noise by amplifying the photocurrent of the PD by the pre-amplifier within the module. 
     (6) The signal sending port can be built only with an LD. Preferably, an additional monitoring PD is included in the sending port. The monitoring PD is mounted behind the LD for stabilizing the output power of the LD in the example. In FIG. 12, the light guide  115  is coupled in butting joint to the LD  132 , which dispenses with a lens. The monitoring PD  133  is a bottom-incidence type PD in FIG.  12 . The slanting plane  154  allows the beams from the light guide to emerge from the groove up to the PD. The set of the V-groove and the bottom-incidence type PD simplifies the light paths on the platform. 
     (7) A further feature of the invention is its simple wiring. This module produces Au-metallized patterns on the Si platform for facilitating the connections of the LD, the PDs and the amplifier via lead pins to external circuits. FIG. 13 shows an example of metallized patterns on the platform. Another set of patterns is also available other than the examples of FIG.  9  and FIG.  13 . 
     (8) Another feature of the invention is a compact appearance without a tangling tail of a fiber. FIG. 14 denotes the appearance of the module. The module has no fiber tail but has only the guide pins and the lead pins. FIG. 15 shows the coupled state of the module and the MT connector having a single-mode fiber communicating to the central station. Since an optical connector can be irreversibly coupled to the module, the module itself need not contain a fiber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of multiwavelength bidirectional optical communication system. 
     FIG. 2 is a schematic view of a WDM filter or an optical coupler making use of optical fibers or waveguides. 
     FIG. 3 is a schematic view of a WDM filter or an optical coupler in a rectangular column made by piling dielectric multilayers on a diagonal surface of a rectangular isosceles triangle column, gluing the triangle column to a partner column into a square column. 
     FIG. 4 is a vertically-sectioned view of a conventional LD module. 
     FIG. 5 is a vertically-sectioned view of a conventional PD module. 
     FIG. 6 is a horizontally-sectioned view of a prior-art receptacle-type multiwavelength LD/PD module. 
     FIG. 7 is an oblique view of a prior-art Y-branch type multiwavelength LD/PD module. 
     FIG. 8 is a sectional view of a prior-art LD/PD module including an upward slanting WDM which reflects incoming light upward. 
     FIG. 9 is an oblique view of an embodiment LD/PD module of the present invention. 
     FIG. 10 is a sectional view of the platform having a light guide on the surface. 
     FIG. 11 is a zy-sectioned view of the embodiment having a bottom-incidence type PD for receiving signal light emitted from the light guide and reflected by the slanting plane in a groove. 
     FIG. 12 is a zx-sectioned view of the embodiment having a PD monitoring the light power of the LD. 
     FIG. 13 is a plan view of another embodiment platform having metallized patterns and V-grooves. 
     FIG.  14 ( a ) is a plan view of the embodiment LD/PD module sealed in a package. 
     FIG.  14 ( b ) is a front view of the embodiment LD/PD module sealed in a package. 
     FIG. 15 is a plan view of a print circuit board having the LD/PD module to which an optical connector is attached. 
     FIG. 16 is a schematic view of a prior-art LD/PD module. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 9 shows an embodiment of the present invention. A Si substrate having a SiO 2  top layer is used as a platform  110 , because photolithography and other technology are matured for Si wafers and Si devices. As alternatives, a ceramic plate, a plastic plate or a metal plate could be used for platform  110 . The silicon platform  110  has light guides  114  and  115  (in x-direction) on the surface in the middle. The light guides  114  and  115  are produced by enhancing the refractive index of the SiO 2  in x-direction partially. 
     FIG. 10 shows a section of the light guide on the silicon platform  110 . Oxidization or sputtering can be used to provide a SiO 2  buffer layer  111  on the silicon platform  110 . Sputtering or CVD produces further a Ge-doped higher refractive index SiO 2  layer  112  overall on the SiO 2  buffer layer  111 . Photoetching with a mask removes side parts of the Ge-doped SiO 2    112  except a middle stripe. A Ge-doped SiO 2  middle stripe remains on the buffer layer  111 . Lower-refractive index SiO 2  cladding layer  113  is deposited on the Ge-doped higher refractive index layer  112  and the SiO 2  buffer layer  111  by sputtering or CVD. The higher refractive index stripe  112  is the light guides  114  and  115 . The light guides guide light because of their higher refractive index than the neighboring SiO 2  cladding layer  113 . 
     Vertical paths  116  and  117  are made in y-direction in the vicinity of the middle point ofthe light guides  114  and  115 . The vertical paths  116  and  117  also conduct light. The vertical paths  116  and  117  can be either vacant grooves or Ge-dope light guides. The groove vertical paths allow light to disperse slightly, since the light propagates in space. The light guide vertical paths suppress light from dispersing. 
     Photoetching makes two longitudinal V-grooves  118  and  119  in x-direction on the platform  110 . The V-grooves  118  and  119  sustain guide pins  120  and  121 . The guide pins  120  and  121  are metal bars, plastic bars or ceramic bars. An adhesive fixes the guide pins  120  and  121  on the V-grooves  118  and  119 . The number, the length, the diameter or the spacing of the guide pins should be determined for fitting the holes of a partner optical connector. When the partner optical connector has three or four holes, the platform should have three or four guide pins in accordance with the holes. 
     Relative spacings between the optical guide  114  and the guide pins  120  and  121  are an important factor. When the holes of the optical connector receive the guide pins  120  and  121 , the optical fiber of the connector should face just on the optical guide  114  in the longitudinal direction. Preferably, the guide pins are parallel with the optical guides. Non-parallel guide pins, however, are also allowable, if the fiber coincides with the optical guide  114  in the joined state. 
     In the example, the V-grooves  118  and  119  have slanting ends  122  and  123 . The platform  110  has a rear plateau  124 , The plateau  124  has a plurality of printed electrode patterns  125 , . . . ,  131  extending in the y-direction. These electrodes connect a laser diode (LD)  132  emitting light and a photodiode (PD)  133  to external circuits. The LD  132  is mounted on the rear plateau  124  at a point of an extension of the light guide  115 . The LD  132  is settled at the position for guiding the light emitted from the LD  132  to the light guide  115 . A bottom electrode of the LD  132  is bonded upon the electrode pattern  125 . A top electrode of the LD  132  is connected to another electrode pattern  126  by a wire. The electrode patterns  125  and  126  carry an electric current for driving the LD  132 . 
     The PD  133  is bonded on the electrode pattern  128  at the back of the LD  132 . The PD  133  is not a signal receiving PD but a monitoring PD of sensing the output of the LD  132 . A top electrode of the PD  133  is joined to the electrode pattern  127  by a wire. If the heights of the LD  132  and the PD  133  are different, a structure of FIG. 12 is contrived for guiding the rear emitted beams into the PD  133 . 
     In FIG. 12, the LD  132  is mounted upside down on the platform  110 . Namely, the luminous part touches the platform  110 . Front beams  146  emitted from the front end of the LD  132  go into an end of the light guide  115 . Rear beams  147  emitted from the rear end of the LD  132  shoot a side of the PD  133 . The rear illuminated type PD  133  is insensitive to the side beams. A cavity  153  is perforated between the LD  132  and the PD  133 . A slanting plane  154  of the cavity  153  reflects beams  148  emitted from the LD  132 . Reflected beams  149  goes up to the bottom of the PD  133 . The PD  133  senses the light beams going upward from the bottom for stabilizing the power of the laser  132 . 
     An oblique groove  134  crosses the light guides  114  and  115  and the vertical paths  116  and  117  at a bisecting angle at the crossing point Q in the SiO 2  cladding layer  113 . Φ is the cross angle between the light guides and the vertical paths. The oblique groove  134  inclines at an angle Φ/2 both to the vertical paths and the light guides. Any angle other than 90 degrees is also allowable. An example of Φ=90 degrees is explained hereafter. In the example, the oblique groove  134  meets with the light guides  114  and  115  at 45 degrees. However, it is possible to position the filter at the middle point in the light guide at 30 degrees to the light guide. 
     A filter  136  is inserted into the oblique groove  134  at the cross point Q. The filter  136  has a transparent substrate of glass or plastics and dielectric multilayers piled on the substrate. The filter  136  obtains the wavelength selectivity of reflection or penetration by piling two kinds of dielectric layers of different thicknesses and refractive indexes. 
     When the optical communication system employs only one common wavelength λ for both the sending signal and the receiving signal, time is shared for sending signals and for receiving signals. This time-sharing system is called a ping-pong transmission. For example, the sending signal rides on 1.3 μm light and the receiving signal rides also on 1.3 μm light. In the ping-pong transmission, the filter  136  should have the role of allowing a part of the wavelength light to pass and of reflecting the other part of the same wavelength light at a definite rate. For example, the filter  136  should have a function of reflecting and leading the common wavelength light of a incidence angle of 45 degrees with a ratio of penetration:reflection=1:1. 
     Allotment of different wavelengths of sending signals and receiving signals gives simultaneous bidirectional transmission to the communication system. λ2 is the wavelength of sending signals. λ1 is the wavelength of receiving signals. In this case, the filter  136  should have 100% transparence to the 45 degree incidence sending light λ2 and should have 100% reflection to the 45 degree incidence receiving light λ1. The simultaneous bidirectional transmission requires such a wavelength-selective filter. Namely, the filter  136  should be a WDM filter. However, when the light is reflected at 45 degrees, the amount of light entering the PD becomes a little smaller due to the difference of reflectivity according to the direction of polarization. This is a defect for the WDM filter positioned inclining at 45 degrees to the light guide. In such case, it is suitable to let the light reflect at 30 degrees by arranging a WDM filter at 30 degrees to the light guide. A PD  137  is mounted on an end of the vertical path  117 . This is a PD for receiving signals, which has a role different from the monitoring PD  133 . The PD  137  is connected to an amplifier  138  by a wire  139 . 
     An inlet and outlet of signal light is the front end  135  of the light guide  114 . The signal light from the central station goes into the light guide  114  at the inlet/outlet point  135 . The incoming light is reflected by the filter  136  to the vertical path  117  and goes into the PD  137  thorough a bottom opening. In this case, the beam height in the vertical path  117  is different from the PD  137 . The difference of height requires a contrivance as shown in FIG.  11 . FIG. 11 is a section taken along yz-plane at the WDM  136 . Incoming light  145  reflected by the WDM  136  gets ahead in the vertical path  117 , shoots a slanting plane  143 , and enters the bottom surface incidence type PD  137 . The PD  137  has a light receiving region  141  near the top surface and a p-electrode  142  on the top. A ring n-electrode  140  is formed on the bottom. The PD  137  is bonded on a metallized pattern  144  of the platform  110 . 
     The function of the LD/PD module is clarified. Inserting the guide pins  120  and  121  into the holes of a partner optical connector joins the module to the optical connector. The optical fiber in the connector just faces the light guide  114  of the module. Propagating in an optical fiber from the central station, the signal light transfers from the connector to the light guide  114 , turns to the right at the WDM  136 , makes its way along the vertical path  117  to the PD  137  and produces a photocurrent in the PD  137 . On the contrary, the sending signal light is produced by the LD  132 , goes into the light guide  115 , passes the WDM  136 , progresses in the light guide  114  and is infused into the fiber of the connector. This LD/PD module has no pig tail of a fiber. The LD/PD module can be easily connected to an optical connector by inserting the guide pins into the holes. 
     FIG. 14 shows a appearance of an assembled LD/PD module  170  which has a case  171  and the platform sealed in the case  171 . The guide pins  120  and  121  project from the case  171 . The bottom end of the case  171  has a plurality of downward lead pins  175 . The lead pins  175  are connected to inner electrode patterns for giving electric power to the LD, the PD, the monitoring PD, sending signals to the LD, and extracting receiving signal from the PD and power signal from the monitoring PD. 
     FIG. 15 shows the LD/PD module installed on a printed circuit board  189  for information processing and coupled with a optical connector  180 . The printed circuit board  189  includes electric circuits for converting signals to be sent into a form congruous to sending, for amplifying received signals and for reproducing signals. Besides the circuits, the printed circuit board  189  has the LD/PD module  170  soldered on it. The optical connector  180  is, e.g., an MT (miniature) connector or a mini-MT(mini-miniature) connector. A connector  180  has a pair of holes  183  and  184  which correspond to the guide pins  120  and  121  of the module  170 . The end of the optical fiber  181  is exactly interposed at the middle point between the holes  183  and  184  in the connector  180 . Insertion of the guide pins  120  and  121  into the holes  183  and  184  brings the fiber  181  into contact with the light guide  114  of the module  170 . The downward signal light of 1.55 μm from the central station transfers to the light guide  114 , turns to the right at the WDM filter  136 , enters the PD  137  and makes a photocurrent in proportion to the light power. 
     The sending signal light emitted from the LD  132  makes its way in the light guides  115  and  114 , transfers into the fiber  181  in the connector  180 , propagates in the fiber and arrives at the central station. Although this LD/PD module has no optical fiber in itself, it is facile to attach or detach the LD/PD module to or from the optical connector. 
     FIG. 13 is a plan view of a platform having another disposition of metallized patterns. Dotted line denotes the light guides  114  and  115 . A metallized pattern  156  is a base pad for mounting a PD for signal reception. A bottom n-electrode of the PD is bonded upon the pattern  156 . A metallized pattern  157  in the vertical path (a V-groove in the example) enhances the reflection of the slanting plane for reinforcing the power of signal light entering the PD. A wide metallized pattern  155  is a base for soldering a bottom of a pre-amplifier. A top p-electrode of the PD is connected to an input pad of the pre-amplifier. An LD will be die-bonded upon an end of a metallized pattern  160 . A top stripe electrode of the LD is connected to a metallized pattern  161  by a wire. A monitoring PD is mounted upon a pattern  158 . A metallized pattern  159  heightens the reflection of the slanting plan  154 . A top p-electrode of the monitoring PD is connected to a pattern  162  a wire.