Abstract:
An optical transceiver including: a light emitting unit for emitting light having a predetermined wavelength; a light reception unit for receiving light; an optical waveguide an end surface of which is introduced into a body of the optical transceiver. The optical waveguide receives and releases light through the end surface. The optical transceiver also includes a light branching unit deposited between the light emitting unit and the end surface of the optical waveguide at a distance from each of them, and is within directional angles of the light emitted from the light emitting unit and light released from the end surface of the optical waveguide. The light branching unit allows the light emitted from the light emitting unit to enter the end surface of the optical waveguide, and the light released from the end surface of the optical waveguide to be received by the light reception unit.

Description:
OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    The present invention relates to an optical transceiver that transmits and receives optical signals via a piece of optical fiber.  
           [0003]    (2) Description of the Related Art  
           [0004]    In recent years, a fiber-to-the-user system using the WDM (Wavelength Division Multiplexing) technology for transmitting and receiving a plurality of optical signals via a piece of optical fiber has been proposed. The WDM is a communication system in which a plurality of optical signals having different wavelengths are multiplexed and transferred simultaneously.  
           [0005]    Optical transceivers are installed in home of users being subscribers to the fiber-to-the-user system and in the system operator&#39;s stations. The optical transceivers have (a) a function to convert electric signals into optical signals and output the optical signals and (b) a function to convert received optical signals into electric signals and input the electric signals.  
           [0006]    Now, an optical transceiver disclosed in Japanese Laid-Open Patent Application No. 11-202140 will be described, for example.  
           [0007]    The optical transceiver has an optical substrate, a semiconductor laser, a photodiode, and a WDM filter. An optical waveguide is formed on the optical substrate. Also, the semiconductor laser, photodiode, and WDM filter are mounted on the optical substrate.  
           [0008]    The optical transceiver can couple optical signals emitted from the semiconductor laser with the optical waveguide with a high coupling efficiency. Also, in the optical transceiver, the semiconductor laser and the photodiode are arranged with a great distance and the WDM filter in between. This construction provides a high crosstalk characteristic. It is considered that such an optical transceiver is suitable for long-distance and broadband fiber-to-the-user systems.  
           [0009]    However, for small- to medium-sized fiber-to-the-user systems having optical fiber communication networks for medium or short distances of no longer than 30 km and having a communication speed of up to approximately 250 Mbps, the above-described high coupling efficiency and crosstalk characteristic are considered to be overdesigned.  
           [0010]    In the manufacturing of the above-described optical transceiver, (1) the semiconductor laser should be aligned with the optical axis of the optical waveguide, (2) the optical waveguide should be aligned with a slit of the WDM filter which is formed by a dicing saw, and (3) the optical fiber being an external transfer path should be aligned with the optical axis of the optical waveguide. The performance of the three alignments requires a high level of accuracy, thus requiring an expensive, high-precision mounting apparatus and a high production cost.  
         SUMMARY OF THE INVENTION  
         [0011]    The object of the present invention is therefore to provide an optical transceiver that is suitable for small- to medium-sized fiber-to-the-user systems that do not require high coupling efficiency and crosstalk characteristic, and can be manufactured at a low cost.  
           [0012]    The above object is fulfilled by an optical transceiver comprising: a light emitting unit operable to emit light having a predetermined wavelength; a light reception unit operable to receive light; an optical waveguide an end surface of which is introduced into a body of the optical transceiver, the optical waveguide operable to receive and release light through the end surface; and a light branching unit which is deposited between the light emitting unit and the end surface of the optical waveguide at a distance from each thereof so as to be within (i) a directional angle of the light having the predetermined wavelength emitted from the light emitting unit and (ii) a directional angle of light released from the end surface of the optical waveguide, and operable to allow the light emitted from the light emitting unit to enter the end surface of the optical waveguide and allow the light released from the end surface of the optical waveguide to be received by the light reception unit.  
           [0013]    With the above-described construction, the mounting tolerance of the light emitting unit is increased. This eliminates the necessity of a high-precision mounting and an expensive mounting apparatus in manufacturing the optical transceiver of the present invention. That is to say, the manufacturing cost can be reduced.  
           [0014]    In the above optical transceiver, a light intercepting film may be formed in the light branching unit so as to prevent the light having the predetermined wavelength emitted from the light emitting unit from being received by the light reception unit.  
           [0015]    Alternatively, the above optical transceiver may further comprise a light intercepting unit which is deposited on an optical path from the light branching unit to the light reception unit, of the light having the predetermined wavelength emitted from the light emitting unit.  
           [0016]    With the above-described construction, it is possible to intercept the light emitted from the light emitting unit so that the light is not received by the light reception unit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    These and the other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention.  
         [0018]    In the drawings:  
         [0019]    [0019]FIG. 1 shows an arrangement of optical parts constituting the optical transceiver in Embodiment 1;  
         [0020]    [0020]FIG. 2 is a cross-sectional view of the optical transceiver taken substantially along line A-A′ of FIG. 1;  
         [0021]    [0021]FIG. 3 is a cross-sectional view of the optical transceiver taken substantially along line C-C′ of FIG. 1;  
         [0022]    [0022]FIG. 4 is a cross-sectional view of a photodiode having a light refracting film;  
         [0023]    [0023]FIG. 5 is a plot of the excess loss between the semiconductor laser and the optical fiber (or optical waveguide) vs. the amount of displacement of the semiconductor laser;  
         [0024]    [0024]FIG. 6 is a plot of the coupling efficiency between the semiconductor laser and the optical fiber (or optical waveguide) vs. the amount of displacement of the semiconductor laser;  
         [0025]    [0025]FIG. 7 is a plot of the coupling efficiency between the semiconductor laser and the optical fiber vs. the amount of displacement of the semiconductor laser in the conventional optical transceiver;  
         [0026]    [0026]FIG. 8 shows a part of the optical transceiver having the wavelength branch filter in which a light intercepting unit is formed;  
         [0027]    [0027]FIG. 9A shows an example of the shape of the light intercepting unit formed in the wavelength branch filter;  
         [0028]    [0028]FIG. 9B shows an example of the shape of the light intercepting unit formed in the wavelength branch filter;  
         [0029]    [0029]FIG. 10 is a cross-sectional view of a photodiode having a light refracting film and a 1.3 μm-wavelength-light absorbing layer;  
         [0030]    [0030]FIG. 11 shows a light-receiving photodiode when viewed from below;  
         [0031]    [0031]FIG. 12 shows an arrangement of optical parts constituting the optical transceiver in Embodiment 2;  
         [0032]    [0032]FIG. 13 shows the filter block in Embodiment 2;  
         [0033]    [0033]FIG. 14 shows the filter block in Embodiment 2 on a surface of which a light intercepting film is formed;  
         [0034]    [0034]FIG. 15 shows an arrangement of optical parts constituting the optical transceiver in Embodiment 3;  
         [0035]    [0035]FIG. 16 shows an arrangement of optical parts constituting the optical transceiver in Embodiment 4;  
         [0036]    [0036]FIG. 17 shows a metal light intercepting wall formed on the optical platform;  
         [0037]    [0037]FIG. 18 shows a metal light intercepting wall formed on the optical platform;  
         [0038]    [0038]FIG. 19 is a plot of the angle α formed by the optical axis of the semiconductor laser with the optical axis of the optical fiber vs. the coupling efficiency between the semiconductor laser and the optical fiber;  
         [0039]    [0039]FIG. 20 is a plot of the distance between the optical fiber and the light-receiving photodiode vs. the coupling efficiency;  
         [0040]    [0040]FIG. 21 shows an example of the shape of an end of the optical fiber facing the wavelength branch filter; and  
         [0041]    [0041]FIG. 22 shows an example of the shape of an end of the optical fiber facing the wavelength branch filter. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0042]    The following describes several embodiments of an optical transceiver of the present invention, with reference to the attached drawings.  
       Embodiment 1  
       [0043]    Construction  
         [0044]    The following describes the construction of an optical transceiver in Embodiment 1.  
         [0045]    [0045]FIG. 1 shows an arrangement of optical parts constituting the optical transceiver in Embodiment 1. FIG. 2 is a cross-sectional view of the optical transceiver taken substantially along line A-A′ of FIG. 1. FIG. 3 is a cross-sectional view of the optical transceiver taken substantially along line C-C′ of FIG. 1.  
         [0046]    The optical transceiver includes an optical platform  10 , a semiconductor laser  11 , a photodiode  12 , a photodiode  13 , a wavelength branch filter  14 , a member  16 , an optical fiber  17 , and a resin  19 . Now, each optical part will be described.  
         [0047]    Optical Platform  10   
         [0048]    The optical platform  10  is a silicon substrate. On a surface of the optical platform  10 , slits  18  and  15 , in which the optical fiber  17  and the wavelength branch filter  14  are respectively disposed, are formed. An angle θ which the central axis (axis A-A′) of the slit  18  forms with the central axis (axis B-B′) of the slit  15  is 120 degrees. The reason why the angle is set to 120 degrees will be described later.  
         [0049]    Optical Fiber  17   
         [0050]    The optical fiber  17  receives an optical signal at an end surface  171  where the core is exposed. The optical fiber  17  also releases an optical signal, which has been received at another end surface, from the end surface  171 . As shown in FIGS.  1  to  3 , the optical fiber  17  is disposed in the slit  18  formed on the surface of the optical platform  10 , with the member  16  deposited thereon.  
         [0051]    Semiconductor Laser  11   
         [0052]    The semiconductor laser  11  has a function to emit an optical signal being a laser beam having wavelength of 1.3 μm, from light emitting points  111  and  112  in the direction of the A-A′ axis. As shown in FIG. 1, the semiconductor laser  11  is dice-bonded with the surface of the optical platform  10  so that the optical fiber  17  is approximately aligned with the optical axis. A distance “m” between the end surface  171  of the optical fiber  17  and the light emitting points  111  of the semiconductor laser  11  is at least 100 μm. The reason why the distance m is set to no smaller than 100 μm will be described later.  
         [0053]    Photodiode  12   
         [0054]    The photodiode  12  receives an optical signal that has a wavelength of 1.3 μm and is emitted from the semiconductor laser  11 , and monitors the received optical signal. As shown in FIG. 1, the photodiode  12  is dice-bonded with the surface of the optical platform  10 .  
         [0055]    Now, the photodiode  12  in Embodiment 1 will be described with reference to FIG. 4. Though FIG. 4 is a cross-sectional view of a photodiode  13 , the figure can be applied also to the photodiode  12 .  
         [0056]    An optical signal released from the light emitting point  112  of the semiconductor laser  11  is refracted on a light refracting film  131  and enters into a light reception unit  132 . It is desirable that the distance between the semiconductor laser  11  and the photodiode  12  is as short as possible so that the light reception unit  132  can receive the light signal efficiently.  
         [0057]    Photodiode  13   
         [0058]    The photodiode  13  receives an optical signal having a wavelength of 1.55 μm released from the end surface  171  of the optical fiber  17 . As shown in FIG. 1, the photodiode  13  is dice-bonded with the surface of the optical platform  10  at a position where it can efficiently receive the optical signal with the wavelength of 1.55 μm that has been reflected by the wavelength branch filter  14 .  
         [0059]    Now, the photodiode  13  in Embodiment 1 will be described with reference to FIG. 4.  
         [0060]    An optical signal released from the end surface  171  of the optical fiber  17  is reflected by the wavelength branch filter  14 , then refracted on the light refracting film  131  of the photodiode  13  and enters into the light reception unit  132 . In the photodiodes of the optical transceiver in Embodiment 1, the light reception unit  132  has a light reception diameter of no smaller than 100 μm. This arrangement is done for the purpose of reducing the degradation of sensitivity caused by displacement in a direction perpendicular to the optical axis.  
         [0061]    It is desirable that the length of an optical signal path between the end surface  171  of the optical fiber  17  and an end surface of the photodiode  13  on the optical signal incident side is as short as possible, and the photodiode  13  is disposed at a position where this condition is met. The photodiode  13  and the optical fiber  17  should be disposed so as not to interfere the optical signal path between thereof which changes with the reflection on the wavelength branch filter  14 .  
         [0062]    Wavelength Branch Filter  14   
         [0063]    The wavelength branch filter  14  is plate-shaped, approximately 10-30 μm thick and allows optical signals having the wavelength of 1.3 μm to pass through itself, and reflects optical signals having the wavelength of 1.55 μm. The wavelength branch filter  14  is fit into the slit  15  formed on the surface of the optical platform  10 .  
         [0064]    Resin  19   
         [0065]    The resin  19  is made of a material that has a refractive index that is consistent with the refractive index of the core of the optical fiber  17 . The resin  19  prevents the optical signal from being reflected by the end surface  171  of the optical fiber  17 , providing an effect of substantially reducing the distance between optical parts. As shown in FIGS. 1 and 2, the resin  19  fills spaces between optical parts, covering the optical parts The optical signals released from the light emitting point  112  of the semiconductor laser  11  and released from the end surface  171  of the optical fiber  17  radiate at certain directional angles when they pass through the resin  19 .  
         [0066]    Comparison with Conventional Apparatus  
         [0067]    The following describes how the optical transceiver in Embodiment 1 eliminates three causes of the high production cost in manufacturing the conventional optical transceiver disclosed in Japanese Laid-Open Patent Application No. 11-202140.  
         [0068]    Cause 1  
         [0069]    One of the causes of the high production cost of the conventional optical transceiver is the smallness of the mounting tolerance. That the mounting tolerance is small indicates that an expensive mounting apparatus should be installed to achieve a high-precision mounting.  
         [0070]    For the conventional optical transceiver, the semiconductor laser and the optical waveguide are directly coupled with each other, or coupled via a light-gathering means such as a lens or a diffraction means such as a diffraction grating. The major goal of either of the coupling methods is to increase the coupling efficiency. However, these coupling methods also have a negative aspect that the excess loss and the coupling efficiency greatly change depending on an amount of relative displacement between the semiconductor laser and the optical waveguide.  
         [0071]    Now, (i) the relationships between the excess loss between the semiconductor laser and the optical fiber (or optical waveguide) and the amount of displacement and (ii) the relationships between the coupling efficiency between the semiconductor laser and the optical fiber (or optical waveguide) and the amount of displacement will be discussed with reference to FIGS. 5 and 6, for each of the conventional and present-invention optical transceivers.  
         [0072]    [0072]FIG. 5 is a plot of the excess loss between the semiconductor laser and the optical fiber (or optical waveguide) vs. the amount of displacement of the semiconductor laser. FIG. 6 is a plot of the coupling efficiency between the semiconductor laser and the optical fiber (or optical waveguide) vs. the amount of displacement of the semiconductor laser.  
         [0073]    In FIG. 5, if the mounting tolerance is regarded to be expressed by the displacement values that do not cause the excess loss between the semiconductor laser and the optical waveguide to exceed 2 dB, the mounting tolerance of the semiconductor laser in the conventional optical transceiver is ±1.6 μm. To meet this condition, a high-precision mounting apparatus is required.  
         [0074]    On the other hand, as understood from FIG. 5, the mounting tolerance of the semiconductor laser in the optical transceiver in Embodiment 1 is ±4 μm.  
         [0075]    [0075]FIG. 6 indicates that since the optical signal released from the light emitting point  111  radiates, if the distance between the semiconductor laser  11  and the optical fiber  17  is great enough as is the case with the optical transceiver in Embodiment 1, the amount of reduction of the coupling efficiency, which varies depending on the displacement of the semiconductor laser  11 , is small.  
         [0076]    As understood from the above description, to manufacture the optical transceiver in Embodiment 1, an expensive mounting apparatus is not necessary since a high mounting precision is not required.  
         [0077]    Also, for a medium-sized fiber-to-the-user system having an optical fiber communication network for approximately 30 km of medium distance in which only approximately 0.1-0.2 W of optical output is required, if the distance m is great and the coupling efficiency is low as is the case with the optical transceiver in Embodiment 1, it hardly affects the transmission or reception of optical signals.  
         [0078]    Cause 2  
         [0079]    Now, the mounting tolerance of the optical fiber for a case where it is coupled with the optical waveguide in the conventional optical transceiver will be described with reference to FIG. 7. FIG. 7 is a plot of the coupling efficiency between the semiconductor laser and the optical fiber vs. the amount of displacement of the semiconductor laser in the conventional optical transceiver.  
         [0080]    As shown in FIG. 7, for the conventional optical transceiver to obtain the coupling efficiency with the optical fiber of no lower than −2 dB, the mounting tolerance of ±2 μm is necessary. Since it is difficult for the passive alignment method to meet this condition, the active alignment method has been used. However, the active alignment method generates much expense in time and effort, and increases the mounting cost as much.  
         [0081]    On the other hand, in the optical transceiver in Embodiment 1, the optical fiber  17  also serves as the optical waveguide, and there is no need to adjust the optical coupling between the optical waveguide and the optical fiber.  
         [0082]    Cause 3  
         [0083]    Now, the precision of the slit in which the wavelength branch filter is fitted in the conventional optical transceiver will be discussed.  
         [0084]    According to the optical path design for the conventional optical transceiver, the optical signal emitted from the semiconductor laser is reflected by the WDM filter and enters the optical fiber being an external transfer path. However, when such an optical path design including a reflection system is adopted, the optical signal maybe partially lost on the optical path due to the displacement of the WDM filter in the position or the angle. For this reason, it was necessary to install an expensive dicing apparatus that can form a slit with high precision.  
         [0085]    On the other hand, the optical path for the optical transceiver in Embodiment 1 is designed so that the optical signal emitted from the semiconductor laser  11  passes through the wavelength branch filter  14 . As a result, in the optical transceiver in Embodiment 1, the displacement of the wavelength branch filter  14  in the position or the angle hardly affects the optical coupling at the end surface  171  of the optical fiber  17 . Also, the wavelength branch filter  14  is approximately 10 μm to 30 μm thick. That is, it is thin enough. As a result, when the optical signal emitted from the semiconductor laser  11  passes through the wavelength branch filter  14 , only a small amount of the optical signal is lost.  
         [0086]    The mounting precision of the wavelength branch filter  14  is determined by the machining precision of the slit  15 . When an ordinary dicing apparatus is used, the slit can be formed with the precision of approximately ±10 μm. This level of the slit formation precision is enough for the optical transceiver of the present invention, which uses a photodiode having light reception diameter of no smaller than 100 μm, to suppress the light reception sensitivity.  
         [0087]    The mounting tolerance of the photodiode  13  varies depending on the mounting conditions or shape or the like of the photodiode, but compared with the mounting tolerance of the semiconductor laser  11 , it is less exacting and does not become a large problem in mounting.  
         [0088]    Modification  
         [0089]    Now, a modification of Embodiment 1, in which the wavelength branch filter  14  has a light intercepting unit, will be described. FIG. 8 shows a part of the optical transceiver where a light intercepting unit  141  is formed in the wavelength branch filter  14 .  
         [0090]    The light intercepting unit  141  is aimed to prevent the photodiode  13  from receiving the optical signal that has been emitted from the semiconductor laser  11  and has radiated at a directional angle β. The light intercepting unit  141  may be a vapor-deposition metal film which can be formed on a filter surface of the wavelength branch filter  14  with ease. Alternatively, the light intercepting unit  141  may be a wavelength selection film that has a high reflectivity for the wavelength of the semiconductor laser.  
         [0091]    As shown in FIGS. 9A and 9B, the light intercepting unit  141  is not formed on the entire filter surface of the wavelength branch filter  14 , but has a window  142  therein. The window  142  should be large enough not to interfere the coupling between the semiconductor laser  11  and the optical fiber  17 . Considering the filter mounting precision, it is desirable that the window  142  has a diameter or width of approximately 50 μm to 150 μm, with the optical axis being the center of the diameter or width.  
         [0092]    Supplement  
         [0093]    (1) The photodiode for receiving a wavelength of 1.3 μm may have a light reception unit of a pass-band structure that has no sensitivity for the light having a wavelength of 1.55 μm.  
         [0094]    (2) FIG. 10 is a cross-sectional view of a photodiode for receiving a wavelength of 1.55 μm which may be used in the optical transceiver of the present invention. As shown in FIG. 10, the photodiode has a 1.3 μm-wavelength-light absorbing layer  133  below the light reception unit  132 , where the 1.3 μm-wavelength-light absorbing layer  133  passes through the light having the wavelength of 1.55 μm and absorbs the light having the wavelength of 1.3 μm. Alternatively, the photodiode may be a photodiode chip on a surface of which a metal light intercepting pattern is formed.  
         [0095]    (3) To prevent the light reception sensitivity from degrading due to reflections, a no-reflection coating may be applied to the light refracting film  131  of the photodiode for receiving the light.  
         [0096]    (4) A material having a refractive index close to the refractive index of the optical fiber may be used as a material of a substrate of the wavelength branch filter  14  so as to prevent excess reflection.  
         [0097]    (5) The semiconductor laser  11  is not limited to the general Fabry-Perot structure, but may be a DFB laser.  
         [0098]    (6) The structure of the optical waveguide of the semiconductor laser  11  is not limited to the ordinary parallel stripe structure, but may be the taper stripe structure that can change the light spot size.  
         [0099]    (7) FIG. 11 shows a light-receiving photodiode when viewed from below. As shown in FIG. 11, a light intercepting metal  134 , which is made of the same material as the material of the back-surface electrodes may be attached to the incident surface of the light-receiving photodiode, by the vapor deposition.  
         [0100]    (8) To enhance the coupling efficiency of the optical signal between the semiconductor laser  11  and the optical fiber  17 , an optical fiber  17 A or an optical fiber  17 B may be used, where an end of the optical fiber  17 A is cut at a bevel as shown in FIG. 21, and an end of the optical fiber  17 B is chamfered into a conical shape as shown in FIG. 22.  
         [0101]    (9) If the semiconductor laser  11  is set to emit the optical signal having the wavelength of 1.55 μm, the photodiode  13  is set to receive the optical signal having the wavelength of 1.3 μm.  
         [0102]    (10) It has been difficult to downsize an optical transceiver that uses a conventional optical waveguide because downsizing of the substrate of the optical waveguide is difficult. However, in the optical transceiver in Embodiment 1, since an optical fiber  17  installed in the optical transceiver itself is also used as an optical waveguide, the slit in which the optical fiber is fitted can be formed by the known technologies of lithography and etching. This enables the optical platform  10  to be downsized. A silicon substrate is used for the optical platform  10 . As a result, a V-shaped slit can be formed with great precision by wet etching.  
       Embodiment 2  
       [0103]    The following describes an optical transceiver in Embodiment 2 with reference to the attached drawings.  
         [0104]    Since the optical transceiver in Embodiment 2 have many characteristics in common with the optical transceiver in Embodiment 1, only characteristics unique to Embodiment 2 will be explained in the following description. The difference is that a filter block is used as the wavelength branch filter.  
         [0105]    Construction  
         [0106]    [0106]FIG. 12 shows an arrangement of optical parts constituting the optical transceiver in Embodiment 2.  
         [0107]    In the optical platform  10  in Embodiment 2, a slit  20  is formed instead of the slit  10  in Embodiment 1. Also, a filter block  21  is fitted into the slit  20 .  
         [0108]    The filter block  21  is made of glass, and is no smaller than 100 μm thick. As shown in FIG. 12, the filter block  21  is deposited between the semiconductor laser  11  and the optical fiber  17 , and has an oblique surface which forms an angle of approximately 120 degrees with the optical axis of the optical fiber  17 .  
         [0109]    [0109]FIG. 13 shows the filter block  21  formed in the optical platform  10 . A wavelength branch filter  211  is formed on the oblique surface which forms an angle of approximately 120 degrees with the optical axis of the optical fiber  17 . The wavelength branch filter  211  is a multi-layered dielectric film.  
         [0110]    In Embodiment 1, a very thin, 10-to-30 μm-thick wavelength branch filter is used. In the case of such a wavelength branch filter, it is difficult to discern between the front and back surfaces, and there is a fear that the automatic mounting efficiency is decreased due to warpage, which is apt to happen because of its thinness. In contrast, as is the case with Embodiment 2, when the filter block  21  being no smaller than 100 μm thick is used, an image recognition during aligning of the optical path is easier and the damage is less apt to be caused during a handling operation than in the case where the thin wavelength branch filter  14  is used. This prevents reduction in mounting yields.  
         [0111]    Supplement  
         [0112]    (1) As shown in FIG. 14, a light intercepting film  212  may be formed on certain portions of a surface of the filter block  21 . The light intercepting film  212  may be a vapor-deposition metal film which can be formed with ease and provide an effective light intercepting property. Alternatively, the light intercepting film  212  may be a wavelength selection film that has a high reflectivity for the wavelength of the optical signal emitted from the semiconductor laser  11 .  
         [0113]    (2) The mounting precision of the filter block  21  is not affected by the machining precision of the slit  20 , but depends on only the mounting precision of the filter block  21 . When an ordinary manufacturing tool is used, the mounting precision is approximately ±10 μm, and the sensitivity degradation is approximately 5%.  
         [0114]    (3) When it is intended to obtain a high coupling efficiency by reducing the distance between the semiconductor laser  11  and the optical fiber  17 , the thickness of the filter block  21  can be reduced for this purpose.  
         [0115]    (4) It is desirable that the filter block  21  is made of a material that have a refractive index that is close to a refractive index of the core of the optical fiber  17 . When glass is used as the material of the filter block  21 , the oblique surface can be formed by the glass press working. Also, when plastic is used as the material of the filter block  21 , the oblique surface can be formed by the metal mold working with ease. Also, when silicon is used as the material of the filter block  21 , the oblique surface can be formed by the selective etching using an off substrate. It should be noted here that when silicon is used as the material of the filter block  21 , a no-reflection coating should be applied onto the side of the filter block  21  opposite the oblique surface side. Also, since the refractive index of silicon is different from that of the core of the optical fiber  17 , the design of the optical system should be changed by considering the difference between the refractive indexes.  
         [0116]    (5) In FIG. 13, the wavelength branch filter  211  of the filter block  21  is formed on the side of the optical fiber  17 . However, the wavelength branch filter  211  may be formed on the side of the semiconductor laser  11 .  
       Embodiment 3  
       [0117]    The following describes an optical transceiver in Embodiment 3 with reference to the attached drawings. Since the optical transceiver in Embodiment 3 have many characteristics in common with the optical transceiver in Embodiment 1, only characteristics unique to Embodiment 3 will be explained in the following description. The difference is that the optical transceiver in Embodiment 3 has an intercepting filter that prevents the optical signal emitted from the semiconductor laser from entering into the light-receiving photodiode.  
         [0118]    Construction  
         [0119]    [0119]FIG. 15 shows an arrangement of optical parts constituting the optical transceiver in Embodiment 3.  
         [0120]    In the optical platform  10  in Embodiment 3, a slit  22  is formed in parallel to the slit  15 , as well as the slits  15  and  18  described in Embodiment 1. An intercepting filter  23  is fitted into the slit  22 . The slit  22  is deep enough for the intercepting filter  23  to intercept the optical signal that travels radiating after emitted from the semiconductor laser  11 .  
         [0121]    A wavelength selection film, which has a high reflectivity for the wavelength of the optical signal emitted from the semiconductor laser  11 , is formed on a filter surface of the intercepting filter  23 .  
         [0122]    The above-described construction prevents the optical signal emitted from the semiconductor laser  11  from entering into the photodiode  13 .  
         [0123]    In FIG. 15, the intercepting filter  23  sticks in the optical fiber  17 . However, the intercepting filter  23  may be arranged so as to be in the vicinities of an end surface of the optical fiber  17  in so far as the intercepting filter  23  does not interfere the coupling of the semiconductor laser  11  with the optical fiber.  
       Embodiment 4  
       [0124]    The following describes an optical transceiver in Embodiment 4 with reference to the attached drawings. Since the optical transceiver in Embodiment 4 have many characteristics in common with the optical transceiver in Embodiment 1, only characteristics unique to Embodiment 4 will be explained in the following description. The difference is that the optical axis of the semiconductor laser is slanted from the optical axis of the optical fiber to form a certain angle, with the light emitting point of the semiconductor laser being the vertex, and that a metal light intercepting wall is provided to prevent the optical signal emitted from the semiconductor laser from entering into the light-receiving photodiode.  
         [0125]    Construction  
         [0126]    [0126]FIG. 16 shows an arrangement of optical parts constituting the optical transceiver in Embodiment 4.  
         [0127]    As shown in FIG. 16, the semiconductor laser  11  and the photodiode  12  are arranged so that the optical axis of the optical fiber  17  forms an angle α with the optical axis of the semiconductor laser  11 , with the light emitting point  111  being the vertex. The arrangement is made to reduce the amount of the optical signal entering into the photodiode  13 , out of the optical signal emitted from the semiconductor laser  11 .  
         [0128]    [0128]FIG. 19 is a plot of the angle α vs. the coupling efficiency between the semiconductor laser  11  and the optical fiber  17 . As shown in FIG. 19, when the angle α is in a range from 3 to 5 degrees, the loss of the coupling efficiency is approximately 1 to 2 dB. This level of coupling efficiency loss is not regarded as a big problem.  
         [0129]    A metal light intercepting wall  24  is formed by performing the metal plating on a metal on the optical platform  10 . Here, the height of the metal light intercepting wall  24  will be discussed.  
         [0130]    [0130]FIG. 17 is a cross-sectional view of the optical transceiver taken substantially along line D-D′ of FIG. 16. As shown in FIG. 17, the optical signal emitted from the semiconductor laser  11  travels radiating at a certain directional angle. The directional angle needs to be taken into consideration when the height of the metal light intercepting wall  24  is determined. Basically, the metal light intercepting wall  24  should be higher than the light refracting film  131  of the photodiode  13 . For example, if the directional angle of the semiconductor laser  11  is 20 degrees, the metal light intercepting wall  24  is required to be 30 to 40 μm in height.  
         [0131]    With the above-described construction, it is possible to prevent the optical signal emitted from the semiconductor laser  11  from entering into the photodiode  13 .  
         [0132]    Supplement  
         [0133]    (1) To prevent the optical signal reflected by the dicing surface of the slit  15  from entering into the photodiode  13 , a slit  241  as shown in FIG. 18 may be formed in the optical platform  10 .  
         [0134]    (2) To achieve a high intercepting property, the metal light intercepting wall  24  may be deposited at a position close to the slit  18  so that the photodiode  13  is hidden when viewed from the light emitting point  111 .  
         [0135]    (3) It is possible to increase the light intercepting property by depositing the photodiode  13  at a position out of the range of the directional angle of the semiconductor laser  11 . Meanwhile, as understood from FIG. 20, the coupling efficiency decreases as distance between the photodiode  13  and the wavelength branch filter  14  increases. As a result, the location of the photodiode  13  may be determined in accordance with a desired coupling efficiency value.  
         [0136]    Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.