Abstract:
An optical lens device assembly is provided which is capable of being substantially free from a deviation in an optical axis of an optical lens caused by errors of its manufacturing, thus preventing a drop in coupling efficiency and enabling easy alignment of the optical axis of the optical lens.  
     One of end faces of two optical lens is used as a lens plane and the two optical lens are optically in series coupled. The two optical lenses are placed in a manner that their non-lens planes face each other.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical lens device assembly comprising two optical lens devices and more particularly to an ideal optical lens device assembly being made up of microlenses.  
           [0003]    2. Description of the Related Art  
           [0004]    In optical communications, in order to guide signal light from, for example, a laser diode used as a light emitting element into an optical fiber, a combination of microlenses each having a diameter of a hundred and several tensμ m is employed.  
           [0005]    As shown, for example, in “Proceeding SPIE” (Vol. 3631, p234-243) issued in April 1999 which has disclosed such the microlens, a plurality of microlenses each having a columnar shape is collectively by bundling many cylindrical optical elements each having an outer diameter being equal to that of an optical fiber to be optically coupled to the microlens and performing photolithography and etching processing, by one operation, on end faces of the optical elements to collectively form specified lens planes on each of end faces of many optical elements.  
           [0006]    Also, by forming many lens planes on an optical substrate made up of, for example, a silicon crystal at regular intervals and then coating lens portions containing each lens plane with a etching mask and by performing etching processing on the substrate region being exposed from the above mask to form many columnar microlenses portions each having a lens plane at each end on an optical substrate and then by separating each of the microlenses portions from the optical substrate, many columnar microlenses can be also formed collectively.  
           [0007]    Such the microlens is used in a form of a lens device assembly constructed by combining a first microlens adapted to convert diverging light emitted from a light emitting element to a collimated beam with a second microlens adapted to gather the collimated beam transferred through the microlens at an end of an optical fiber. To guide the diverging light emitted from the light emitting element into the optical fiber, the lens device assembly made up of the first and second microlenses is placed between the light emitting element and the optical fiber.  
           [0008]    The optical fiber is placed in a V-shaped groove formed, by photolithography and etching technologies, which are generally used in Si LSI manufacturing processs, relative to the light emitting element with high accuracy, on a substrate, thus enabling highly accurate alignment of an optical axis of the light emitting element and that of the optical fiber. Moreover, as described above, by serially placing two columnar microlenes each having an outer diameter being approximately equal to that of the optical fiber in the V-shaped groove, it is made possible to fitly place the two microlenses by a passive alignment method using no monitoring light and without the occurrence of a deviation in the optical axis.  
           [0009]    However, the conventional microlens has a problem. That is, the conventional microlens made up of a columnar optical element with a lens plane on its one end generally has a shape of a truncated cone because a diameter of the optical element is easily changed along its optical axis due to errors in manufacturing the microlens. When two microlenses having the truncated cone shape are serially placed in the groove, a great deviation occurs in the optical axis depending on a form of placement of these two microlenses.  
         SUMMARY OF THE INVENTION  
         [0010]    In view of the above, the present invention has an object that is to provide an optical lens device assembly which can decrease a deviation between optical axes of two optical lenses caused by manufacturing errors, thus can prevent a drop in coupling efficiency.  
           [0011]    According to a first aspect of the present invention, there is provided an optical lens device assembly including:  
           [0012]    two optical lens devices each having a truncated cone shape as a whole and being made up of an optical element one end face of which serves as a lens plane and another end face of which serves as a non-lens plane, and being placed on a reference plane so as to be optically coupled in series; and  
           [0013]    wherein the two optical lens devices are placed in a manner that the non-lens planes of the two optical lens devices face each other.  
           [0014]    In this optical lens device assembly, the two optical lens devices may be placed in a manner that either smaller end faces of the two optical lens devices face each other or larger end faces of the two optical lens devices face each other.  
           [0015]    Also, the two optical lens devices may be placed so as to be symmetric with respect to a virtual intermediate plane between the two optical lens devices.  
           [0016]    Also, a smaller end face of one optical lens device may face a larger end face of another optical lens device.  
           [0017]    Also, the two optical lens devices may be placed in a manner that smaller end faces of the two optical lens devices are aligned in same direction and larger end faces of the two optical lens devices are aligned in same direction.  
           [0018]    According to a second aspect of the present invention, there is provided an optical lens device assembly including:  
           [0019]    two optical lens devices each having a truncated cone shape as a whole and being made up of an optical element one end face of which serves as a lens plane and another end face of which serves as a non-lens plane, and being placed on a reference plane so as to be optically coupled in series; and  
           [0020]    wherein the two optical lens devices are placed in a manner that planes of outgoing light of the two optical lens devices serve as the lens plane.  
           [0021]    In this optical lens device assembly, the two optical lens devices may be placed in a manner that either smaller end faces of the two optical lens devices face each other or larger end faces of the two optical lens devices face each other.  
           [0022]    Also, the two optical lens devices may be placed so as to be symmetric with respect to a virtual intermediate plane between the two optical lens devices.  
           [0023]    Also, a smaller end face of one optical lens device may face a larger end face of another optical lens device.  
           [0024]    Also, the two optical lens devices may be placed in a manner that smaller end faces of the two optical lens devices are aligned in same direction and larger end faces of the two optical lens devices are aligned in same direction.  
           [0025]    Moreover, in the foregoing, the two optical lens devices may be placed in a concave groove formed on a substrate providing the reference plane in which optical fiber being optically coupled to the optical lens device is placed.  
           [0026]    Also, the two optical lens devices may be microlenses.  
           [0027]    Also, the two optical lens devices may be microlens made of a silicon crystal substrate.  
           [0028]    With the above configurations, by placing both the microlenses according to the specified placements described above respectively, the optical axes of the two optical microlenses can be adjusted easily. Therefore, even if each of the microlenses is changed to a truncated cone shape due to the manufacturing errors so that it can not show a cylindrical shape as a whole, the deviation between the optical axes can be decreased. As a result, it is possible to prevent easily a drop in the substantial coupling efficiency. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:  
         [0030]    [0030]FIG. 1 is a diagram illustrating a form of placement of two microlenses employed in an embodiment of the present invention;  
         [0031]    [0031]FIG. 2 is a diagram illustrating a form of placement of two microlenses not employed in the embodiment of the present invention;  
         [0032]    [0032]FIG. 3 is a diagram illustrating an angular difference between an optical axis of each of the microlenses of FIGS. 1 and 2 and a reference optical axis provided in the embodiment of the present invention;  
         [0033]    [0033]FIG. 4 is a graph showing a relation between the angular difference and lens coupling efficiency provided in the embodiment of the present invention as shown in FIG. 3;  
         [0034]    [0034]FIG. 5 a diagram illustrating another form of the placement of two microlenses employed in an embodiment of the present invention;  
         [0035]    [0035]FIG. 6 a diagram illustrating another form of the placement of two microlenses not employed in the embodiment of the present invention;  
         [0036]    [0036]FIG. 7 is a diagram illustrating an angular difference between an optical axis of each of the microlenses of FIGS. 5 and 6 and a reference optical axis provided in the embodiment of the present invention;  
         [0037]    [0037]FIG. 8 is a graph showing a relation between the angular difference and lens coupling efficiency provided in the embodiment of the present invention as shown in FIG. 7;  
         [0038]    [0038]FIG. 9 is a plan view of an optical apparatus in which a microlens assembly of the present invention is used; and  
         [0039]    [0039]FIG. 10 is a perspective view illustrating one example of a microlens of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    Best modes of carrying out the present invention will be described in further detail using various embodiments with reference to the accompanying drawings.  
       Embodiment  
       [0041]    [0041]FIG. 1 is a diagram illustrating a form of placement of two microlens employed in an embodiment of the present invention. Prior to detailed descriptions of the placement of the two microlenses by referring to FIG. 1, an application example is explained to which a microlens and a microlens assembly of the present invention are used.  
         [0042]    As shown in FIG. 9, the optical lens device assembly  10  (made up of microlenses  10   a  and  10   b ) of the present invention is used as an optical unit for optical communications. In FIG. 9, the micorlenses  10   a  and  10   b  make up the optical lens device assembly  10  which is used to guide a signal light emitted from a laser diode  12  serving as a light emitting element to an end of an optical fiber  13  placed on a crystal substrate  11  serving as a support substrate.  
         [0043]    As the crystal substrate  11 , for example, a silicon crystal substrate is employed. On the crystal substrate is formed, by an etching method, a V-shaped concave groove  14  used to perform positioning of the optical fiber  13 . The optical fiber  13  is fitly supported on the crystal substrate  11  by placing a part of its portion surrounding a wall of the optical fiber  13  into the V-shaped concave groove  14 . The laser diode  12  operating as the light emitting element is fixed in the vicinity of a terminating portion of the concave groove  14  on a surface  11   a  of the crystal substrate  11  through a well-known electrode  12 ′ formed on the crystal substrate  11 , so that an optical axis of a light emitting plane  12   a  of the laser diode  12  is aligned exactly with an optical axis of the optical fiber  13  positioned by the concave groove  14 .  
         [0044]    The optical fiber  13 , when having received signal light having a wavelength of, for example, 1.3 μm or 1.5 μm which is emitted from the laser diode  12  at an end portion of the optical fiber  13 , operates to guide the received signal light to its required portion. Moreover, the optical fiber  13  can be constructed of a single mode optical fiber having an outer diameter of, for example, 125 μm.  
         [0045]    The optical lens device assembly  10  (made up of the microlenses  10   a  and  10   b ) is placed between the laser diode  12  and the optical fiber  13  so as to guide the signal light emitted from the light emitting plane  12   a  to the end portion of the optical fiber  13 .  
         [0046]    The optical lens device assembly  10  is made up of two microlenses  10   a  and  10   b , as described above. Each of the microlens  10   a  and  10   b  is constructed of an optical element having an approximately cylindrical shape as a whole, which has an diameter being approximately equal to that of the optical fiber  13 . As described above, the two microlenses  10   a  and  10   b  are arranged in the concave groove  14  in a manner that they are placed between the laser diode  12  and the optical fiber  13 . As a material for each of the microlenses  10   a  and  10   b,  an optical element being suitable for a wavelength of light to be handled can be selected. For example, if light having a wavelength of 1.3 μm or 1.5 μm is handled, a low-loss material in the wavelength band of 1.3 μm or 1.5 μm such as a silica or silicon can be employed.  
         [0047]    In FIG. 9, the microlens  10   a  placed in the vicinity of the laser diode  12  has a lens plane at one end face  15   a  of the microlens  10   a  facing the laser diode  12  and the lens plane has a collimating function to convert diverging light emitted from the light emitting plane  12   a  of the laser diode  12  to a collimated beam. An other end face  15   b  of the microlens  10   a  is a flat plane implementing no lens functions.  
         [0048]    The microlens  10   b  placed in the vicinity of the optical fiber  13  has a lens plane at its one end  16   a  facing the optical fiber  13  and the lens plane has a light-gathering function to gather the collimated beam transferred from the microlens  10   a  at an end of the optical fiber  13 . An other end face  16   b  of the microlens  10   b  is a flat plane implementing no lens function. Each of the microlenses  10   a  and  10   b  can be configured so as to have not only the collimating function but also desired optical characteristics.  
         [0049]    As each of lens planes of the mircrolenses  10   a  and  10   b , either of a well-known diffractive optical element (in other words, diffractive type lens plane) using a diffraction phenomenon or a refractive optical lens element using a refraction phenomenon may be employed as necessary. For example, a computer-generated hologram (CGH) can be used as the diffractive optical element.  
         [0050]    Both the microlenses  10   a  and  10   b  are arranged serially in the concave groove  14  with the flat end faces  15   b  and  16   b  both serving as non-lens planes being faced each other so that they can guide diverging light emitted from the laser diode  12  to the optical fiber  13 .  
         [0051]    So long as the outer diameter of the cylindrical optical element constituting both the microlenses  10   a  and  10   b  is equal to that of the optical fiber  13 , surrounding side face portions extending between both ends of the microlenses  10   a  and  10   b  can be partially placed in the concave groove  14  in the crystal substrate  11  defining a reference plane so that an optical axis of the optical lens device assembly  10  made up of the microlenses  10   a  and  10   b  coincides exactly with a reference axis defined by the laser diode  12  and the optical fiber  13 .  
         [0052]    However, as shown in FIG. 10 being a magnified view of each of the microlenses  10   a  and  10   b,  a diameter of the end face  15   a  of the microlens  10   a  or end face  16   a  of the micorlens  10   b  is slightly different, due to errors in manufacturing the microlenses  10   a  and  10   b , from that of the other end face  15   b  of the micorlens  10   a  or end face  16   b  of the micorlens  10   b  both serving as non-lens planes, as a result, providing the microlenses  10   a  and  10   b  each having a truncated cone shape.  
         [0053]    In the example as shown by FIG. 10, each of the microlenses  10   a  and  10   b  has a thickness H of, for example, 100 μm, their smaller end face  15   a  or  16   a  being a lens plane has an diameter D1 of, for example, 125,μm which is equal to the outer diameter of the optical fiber  13 , and their larger end face  15   b  or  16   b  being a non-lens plane has an diameter D2 being slightly larger by, for example, 0.2 μm to 0.7 μm than the above diameter D1.  
         [0054]    Instead of above case, the smaller end face  15   a  or  16   a  may be the non-lens plane, then the larger end face  15   b  and  16   b  may be the lens plane. In this case, the diameter of the larger end face  15   b  or  16   b  being a lens plane is 125 μm, and the diameter of the smaller end face  15   a  or  16   a  being a lens plane is slightly smaller by, for example, 0.2 μm to 0.7,μm than 125 μm.  
         [0055]    Also, the thickness H of each of the microlenses  10   a  and  10   b , diameters D1 of the end faces  15   a  and  16   a , and D2 of the end faces  15   b  and  16   b  are not limited to the values shown above and various values may be used.  
         [0056]    Moreover, it is not necessary for both the microlenses  10   a  and  10   b  to have the same dimensions and/or the same truncated cone shapes and they can be configured by the microlens  10   a  and  10   b  each having a different dimension and/or a different truncated cone shape. However, in order to simplify descriptions of them, in the following description, an example is explained in which both the microlenses  10   a  and  10   b  have the same dimensions and the same truncated cone shapes.  
         [0057]    An angle “θ” formed by an optical axis “L” of the microlens  10   a  or  10   b  and a ridge line “G” defined by a surrounding side face portion  15   c  of the microlenses  10   a  or  10   b  corresponds to a deviated angle “θ” of the microlens optical axis from the reference optical axis defined by the laser diode  12  and the optical fiber  13  when each of the microlenses  10   a  and  10   b  is placed in the concave groove  14  defining a reference plane. The above angular deviation changes coupling efficiency of the optical lens device assembly  10  to be obtained when signal light emitted from the laser diode  12  has reached an end face of the optical fiber  13 . To obtain a relation between the deviated angle (θ) and the coupling efficiency, an example model is studied in which the microlenses  10   a  and  10   b  are placed in a manner so as to be symmetric with respect to a virtual intermediate plane P1 between both the mirolenses  10   a  and  10   b  as shown in FIGS. 1 and 2.  
         [0058]    When the microlenses  10   a  and  10   b  are placed in a manner so as to be symmetric with respect to the virtual intermediate plane P1, there are two forms of placement of the microlens  10   a  and  10   b , one being a form in which the microlenses  10   a  and  10   b  are placed with their smaller end faces being faced each other and another being a form in which the microlenses  10   a  and  10   b  are placed with their larger end faces being faced each other. The relation between the deviated angle (θ) and the coupling efficiency to be obtained in each of the two form of placement was calculated using the model shown in FIG. 3.  
         [0059]    In the example model shown in FIG. 3, a focal length “f1” of the first micorlens  10   a  is 80 μm, a focal length “f2” of the second microlens  10   b  is 360 μm, a radius of a beam waist “ω” of the laser diode  12  is 1.0 μm and a radius of the beam waist “ω” of the optical fiber  13  is 4.6 μm. In this model, absolute values of the deviated angle (θ) of the microlens  10   a  and of the microlens  10   b  are the same but directions of the deviation are different from each other.  
         [0060]    A graph showing simulation results in each form of the placement of the microlenses  10   a  and  10   b  illustrated in FIG. 1( a ) to FIG. 1( d ) and FIG. 2( a ) to FIG. 2( d ) is provided in FIG. 4. In FIG. 4, the deviated angle (θ) (degree) is plotted as abscissa and the coupling efficiency (dB) of the optical lens device assembly  10  at each deviated angle (θ) as ordinate.  
         [0061]    A line  16  shown in FIG. 4 represents the simulation results obtained when the microlenses  10   a  and  10   b  are placed with their smaller end faces  15   a  and  16   a  being faced each other or with their larger end faces  15   b  and  16   b  being faced each other and when the lens planes of the microlenses  10  and  10   b  are formed at end faces being opposite to end faces (non-lens planes) facing each other.  
         [0062]    In the form of the placement illustrated in FIG. 1( a ), the larger end face  15   b  of the microlens  10   a  placed on a side of the laser diode  12  serves as an incident plane on which a lens plane is formed. Moreover, the smaller end face  15   a  of the microlens  10   a  is used as a non-lens plane and serves as an exit plane. On the other hand, the smaller end face  16   a  of the microlens  10   b  placed on a side of the optical fiber  13  is used as a non-lens plane and serves as an incident plane. The larger end face  16   b  of the microlens  10   b  serves as an exit plane on which a lens plane is formed.  
         [0063]    In the form of the placement shown in FIG. 1( b ), the smaller end face  15   a  of the microlens  10   a  placed on the side of the laser diode  12  serves as an incident plane on which a lens plane is formed. The larger end face  15   b  of the microlens  10   a  is used as a non-lens plane and serves as an exit plane. On the other hand, the larger end face  16   b  of the microlens  10   b  placed on the side of the optical fiber  13  is used as a non-lens plane and serves as an incident plane. The smaller end face  16   b  of the microlens  10   b  serves as an exit plane on which a lens plane is formed.  
         [0064]    As is apparent from the line  16 , in the case of the first form of the placement shown in FIG. 1( a ) and FIG. 1( b ), so long as the deviated angle (θ) is within a range of ±5°, at any deviated angle (θ), the coupling efficiency as high as about −2 dB was obtained.  
         [0065]    A line  17  illustrated in FIG. 4 represents simulation results obtained when the micorlenses  10   a  and  10   b  are placed so as to be symmetric with respect to the virtual intermediate plane P1 as shown in FIG. 1( c ) and FIG. 1 ( d ) and planes of outgoing light of both the microlenses  10   a  and  10   b  are used as lens planes.  
         [0066]    In the form of the placement shown in FIG. 1( c ), the larger end face  15   b  of the microlens  10   a  placed on the side of the laser diode  12  serves as a incident plane and is used as a non-lens plane. The smaller end face  15   a  of the microlens  10   a  is used as a lens plane and serves as a exit plane. On the other hand, the smaller end face  16   a  of the microlens  10   b  is used as a non-lens plane and serves as a incident plane. The larger end face  16   b  of the microlens  10   b  serves as a exit plane on which a lens plane is formed.  
         [0067]    In the form of the placement shown in FIG. 1( d ), the smaller end face  15   a  of the microlens  10   a  placed on the side of the laser diode  12  is used as a non-lens plane and serves as an incident plane. The larger end face  15   b  of the microlens  10   a  serves as an exit plane on which a lens plane is formed. On the other hand, the larger end face  16   b  of the microlens  10   b  placed on the side of the optical fiber  13  is used as a non-lens plane and serves as an incident plane. The smaller end face  16   a  of the microlens  10   b  serves as an exit plane on which a lens plane is formed.  
         [0068]    As is apparent from the line  17  obtained in the case of the second form of the placement shown in FIG. 1( c ) and FIG. 1( d ), if the deviated angle (θ) is within a range of ±2°, the coupling efficiency of −3 dB, which does not present a problem from a practical view, can be obtained.  
         [0069]    On the other hand, another form of placement is shown in FIG. 2( a ) to FIG. 2( d ) in which the micorlenses  10   a  and  10   b  are placed so as to be symmetric with respect to the virtual intermediate plane P1 and in a manner that the smaller end faces  15   a  and  16   a  face each other or the larger end faces  15   b  and  16   b  face each other and in a manner that the end faces on which lens planes are formed face each other or the end faces serving as planes of incident light are used as lens planes.  
         [0070]    That is, in the form of the placement shown in FIG. 2( a ) and FIG. 2( b ), the microlenses  10   a  and  10   b  are placed with their lens planes being faced each other and, in the form of the placement shown in FIG. 2( c ) and FIG. 2( d ), the microlenses  10   a  and  10   b  are placed so that the end face serving as the incident plane is used as a lens plane, that is, when the smaller end face of either one of the microlenses  10   a  and  10   b  serve as lens plane, the larger end face of another microlens is used as the lens plane.  
         [0071]    The line  18  shown in FIG. 4 represents the characteristics as obtained when the microlenses  10   a  and  10   b  are placed in the form of the placement shown in FIG. 2( a ) and FIG. 2( b ) and the line  19  represents the characteristics as obtained when the microlenses  10   a  and  10   b  are placed in the form of the placement shown in FIG. 2( c ) and FIG. ( d ).  
         [0072]    As is apparent from both lines  18  and  19 , when the microlenses  10   a  and  10   b  are placed in the form of the placement shown in FIG. 2( a ) to FIG. 2 ( d ) and when the deviated angle (θ) is within a range not exceeding ±10°, a great drop exceeding −3 dB in the coupling efficiency, which is not negligible from a practical view, occurs.  
         [0073]    Thus, when the two microlenses  10   a  and  10   b  both having the truncated cone shapes are used in combination as the optical lens device assembly  10  and when the microlenses  10   a  and  10   b  are placed in a manner that the smaller end face  15   a  of the microlens  10   a  faces the smaller end face  16   a  of the microlens  10   b  or the larger end face  15   b  of the microlens  10   a  faces the larger face  16   b  of the microlens  10   b , as described by referring to FIG. 1( a ) to FIG. 1( d ), by employing the form of the placement in which end faces of the microlenses  10   a  and  10   b  that face each other are used as the non-lens plane or planes of outgoing light of the microlenses  10   a  and  10   b  are used as lens planes, even if the angular deviation between the reference optical axis and the optical axes of the microlenses  10   a  and  10   b  occurs due to errors in manufacturing the microlenses  10   a  and  10   b , so long as the deviated angle is within a range of ±2°, the drop in the coupling efficiency is negligible from a practical point of view and therefore it is made possible to easily perform alignment of the optical axes of both the microlenses  10   a  and  10   b  by a passive alignment method without causing the substantial drop in the coupling efficiency.  
         [0074]    Particularly, by employing the first form of the placement of the microlenses  10   a  and  10   b  as shown in FIG. 1( a ) and FIG. 1( b ) in which the smaller end faces  15   a  and  16   a  or the larger end faces  15   b  and  16   b  existing on a side being opposite to end faces being faced each other are used as lens planes, even when the deviated angle (θ) exceeds ±5°, the coupling efficiency being as high as −2 dB is achieved and, as a result, even if there are errors in manufacturing the microlenses  10   a  and  10   b , highly accurate passive alignment can be easily implemented.  
         [0075]    In the above description, when the microlenses  10   a  and  10   b  are placed so as to be symmetric with respect to a virtual intermediate plane P1, the relation between the deviated angle (θ) and the coupling efficiency is defined based on the model as shown in FIG. 3.  
         [0076]    Next, deviated angle (θ) between optical axes of the microlenses  10   a  and  10   b  and the reference optical axis is considered when smaller end faces of the microlenses  10   a  and  10   b  and larger end faces of the microlenses  10   a  and  10   b  are aligned in the same direction.  
         [0077]    In the form of the placement in which smaller end faces of the microlenses  10   a  and  10   b  and larger end faces of the microlens  10   a  and  10   b  are aligned in the same direction, as shown in FIGS. 5 and 6, one lens plane formed in one end face of either one of the microlens  10   a  or  10   b  is arranged so as to be approximately parallel to the lens plane formed in the end face of another microlens.  
         [0078]    In this case, as illustrated in FIG. 7, absolute values of the deviated angles (θ) of the microlenses  10   a  and  10   b  and their directions of the deviation are the same. Moreover, a focal length f1 of the microlens  10   a , focal length f2 of the second microlens  10   b , radius of a beam waist “ω” of the laser diode  12  and radius of the beam waist “ω” of the optical fiber  13  applied in FIG. 7 are the same as in FIG. 3.  
         [0079]    [0079]FIG. 8 shows simulation results obtained when the microlenses  10   a  and  10   b  are arranged in the form of the placement shown in FIG. 5( a ) to FIG. 5( b ) and in FIG. 6( a ) to FIG. 6( d ). In FIG. 8, as in the case in FIG. 4, the deviated angle (θ) (degree) is plotted as abscissa and the coupling efficiency (dB) of the optical lens device assembly  10  at each deviated angle (θ) as ordinate.  
         [0080]    A line  20  shown in FIG. 8 represents simulation results obtained when the smaller end face  15   a  of the microlens  10   a  and the smaller end face  16   a  of the microlens  10   b  are aligned in the same direction and the larger end face  15   b  of the microlens  10   a  and the larger end face  16   b  of the microlens  10   b  are also aligned in the same direction and, at the same time, non-lens planes of the microlenses  10   a  and  10   b  face each other as shown in FIGS.  5 ( a ) and  5 ( b ).  
         [0081]    In the form of the placement shown in FIG. 5( a ), the larger end face  15   b  of the microlens  10   a  placed on the side of the laser diode  12  serves as an incident plane on which a lens plane is formed. Moreover, the smaller end face  15   a  of the microlens  10   a  is used as a non-lens plane and serves as an exit plane. On the other hand, the larger end face  16   b  of the microlens  10   b  placed on a side of the optical fiber  13  is used as a non-lens plane and serves as an incident plane. The smaller face end  16   a  of the microlens  10   b  serves as an exit plane on which the lens plane is formed.  
         [0082]    In the form of the placement shown in FIG. 5( b ), the smaller end face  15   a  of the microlens  10   a  placed on the side of the laser diode  12  serves as an incident plane on which a lens-plane is formed. Moreover, the larger end face  15   b  of the microlens  10   a  is used as a non-lens plane and serves as an exit plane. On the other hand, the smaller end face  16   a  of the microlens  10   b  placed on the side of the optical fiber  13  is used as a non-lens plane and serves as an incident plane. Moreover, the larger end face  16   b  of the microlens  10   b  serves as an exit plane on which the lens plane is formed.  
         [0083]    As is apparent from the line  20  in FIG. 8, in the case of the third form of the placement shown in FIG. 5( a ) and FIG. 5( b ), so long as the deviated angle (θ) is within a range of ±4°, the coupling efficiency as high as about −3 dB was obtained at any deviated angle (θ).  
         [0084]    A line  21  shown in FIG. 8 represents simulation results obtained when the exit planes of the microlenses  10   a  and  10   b  are used as the lens planes in the fourth form of the placement as shown in FIGS.  5 ( c ) and ( d ).  
         [0085]    In the form of the placement shown in FIG. 5( c ), the larger end face  15   b  of the microlens  10   a  placed on the side of the laser diode  12  serves as an incident plane and is used as a non-lens plane. Moreover, the smaller end face  15   a  of the microlens  10   a  is used as a lens plane and serves as an exit plane. On the other hand, the larger end face  16   b  of the microlens  10   b  placed on the side of the optical fiber  13  is used as a non-lens plane and serves as an incident plane. Moreover, the smaller end face  16   a  of the microlens  10   b  serves as an exit plane on which a lens plane is formed.  
         [0086]    In the form of the placement shown in FIG. 5( d ), the smaller end face  15   a  of the microlens  10   a  placed on the side of the laser diode  12  is used as a non-lens plane and serves as an incident plane. Moreover, the larger end face  15   b  of the microlens  10   a  serves as an exit plane on which a lens plane is formed. On the other hand, the smaller end face  16   a  of the microlens  10   b  placed on the side of the optical fiber  13  is used as a non-lens plane and serves as an incident plane. Moreover, the larger end face  16   b  of the microlens  10   b  serves as an exit plane on which a lens plane is formed.  
         [0087]    As is apparent from the line  21  obtained in the case of the fourth form of the placement shown in FIG. 5( c ) and FIG. 5( d ), if the deviated angle (θ) is within a range of ±2°, as in the case of the second form of the placement, the coupling efficiency of −3 dB, which does not present a problem from a practical view, can be obtained.  
         [0088]    As stated above, mirolenses  10   a  and  10   b  are placed so that the smaller end faces  15   a  and  16   a  or the larger end faces  15   b  and  16   b  are aligned in a same direction respectively. On the other hand, other two forms are shown by FIG. 6( a ) to FIG. 6( d ).  
         [0089]    That is, in one form of the placement as shown in FIGS.  6 ( a ) and  6 ( b ), the lens planes of the micorlenses  10   a  and  10   b  are located face to face each other. Also in another form of the placement as shown FIGS.  6 ( c ) and  6  ( d ), the larger end faces  15   b  and  16   b  or the smaller end faces  15   a  and  16   a  of the microlenses  10   a  and  10   b  are lens planes and serve as incident planes.  
         [0090]    A line  22  shown in FIG. 8 represents simulation results obtained in the case of the form of the placement shown in FIGS.  6 ( a ) and  6  ( b ). A line  23  shown in FIG. 8 represents simulation results obtained in the case of the form of the replacement shown in FIGS.  6 ( c ) and  6  ( d ).  
         [0091]    As is apparent from both the lines  22  and  23 , when the microlenses  10   a  and  10   b  are placed in the form of the placement shown in FIG. 6( a ) to FIG. 6 ( d ) and when the deviated angle (θ) is within a range not exceeding ±1°, a great drop exceeding −3 dB in the coupling efficiency, which is not negligible from a practical view, occurs.  
         [0092]    Thus, when the two microlenses  10   a  and  10   b  both having the truncated cone shapes are used in combination as the optical lens device assembly and when the microlenses  10   a  and  10   b  are placed in a manner that their smaller end faces  15   a  and  16   a  and their larger end faces  15   b  and  16   b  are aligned in the same direction, as described by referring to FIG. 5( a ) to FIG. 5( d ), by employing the form of the placement in which end faces of the microlenses  10   a  and  10   b  that face each other are used as the non-lens plane or exit planes of the microlenses  10   a  and  10   b  are used as lens planes, even if the angular deviation between the reference optical axis and the optical axes of the microlenses  10   a  and  10   b  occurs due to errors in manufacturing the microlenses  10   a  and  10   b,  so long as the deviated angle is within a range of ±2°, the drop in the coupling efficiency is negligible from a practical point of view and therefore it is made possible to easily perform alignment of the optical axes of both the microlenses  10   a  and  10   b  by a passive alignment method without causing the substantial drop in the coupling efficiency.  
         [0093]    Particularly, by employing the third form of the placement of the microlenses  10   a  and  10   b  as shown in FIG. 5( a ) and FIG. 5( b ) in which the end faces of the microlenses  10   a  and  10   b  that face each other are used as the non-lens plane, even when the deviated angle (θ) exceeds ±4°, the coupling efficiency being as high as −3 dB can be achieved and, as a result, even if there are errors in manufacturing the microlens  10   a  and  10   b , highly accurate passive alignment can be easily implemented.  
         [0094]    Moreover, by employing the forms of the placement shown in FIGS.  1 ( a ) and  1 ( b ) or in FIGS.  5 ( a ) and  5 ( b ), it is made possible to reliably prevent reflected light at the non-lens plane of the microlens  10   a  from being fed back to the light emitting element.  
         [0095]    Generally, in order to prevent reflected light, for example, an antireflection coating may be used which is to be formed on both the incident plane and exit plane of the microlens  10   a . The antireflection coating is effective on light entering the lens from an outside of the lens. However, it is not effective on light proceeding toward an exit plane in the lens from the incident plane of the microlens  10   a  and, as a result, part of the light proceeding toward the exit plane within the microlens  10   a  is reflected by the exit plane therein.  
         [0096]    Therefore, if the microlens  10   a  is of the truncated cone shape, since a deviation of the angle (θ) relative to the reference optical axis occurs in the optical axis of the microlens  10   a , it is made possible to prevent the light internally reflected at the exit plane from being fed back to the laser diode  12 .  
         [0097]    Therefore, by preventing the internally reflected light from being fed back to the laser diode  12 , since a factor leading to instability caused by the feed-back light in oscillation of the laser diode  12  can be removed, the first form of the placement of the microlenses  10   a  and  10   b  shown in FIGS.  1 ( a ) and  1 ( b ) and the third form of the placement shown in FIGS.  5 ( a ) and  5 ( b ) are advantageous.  
         [0098]    It is apparent that the present invention is not limited to the above embodiments but may be changed and modified without departing from the scope and spirit of the invention.