Abstract:
An optical module has first and second optical parts. Each of the optical parts has an optical path extending in a longitudinal direction and an end face intersecting the optical path. The end face of the first optical part faces the end face of the second optical part. Light enters the end face of the second optical part from the end face of the first optical part. An adhesive forms a joint for connecting the end faces of the parts to each other in peripheral portions about the optical paths of the parts.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical module used in an optical device such as an optical multiplexer, an optical demultiplexer, and an optical multiplexer/demultiplexer used in an optical communication system based on a wavelength division multiplexing (WDM) transmission modes such as a dense wavelength division multiplexing (DWDM) transmission mode and a coarse wavelength division multiplexing (CWDM) transmission mode.  
           [0002]    Three-port filter modules such as a filter module  20  shown in FIG. 13 have been conventionally used as optical modules. The filter module  20  has first and second optical fiber collimators  21 ,  22 . The first collimator  21  has a first gradient index rod lens  23 , and the second collimator  22  has a second gradient index rod lens  24 . A wavelength selective interference film  25  is provided between the first and second rod lenses  23 ,  24 .  
           [0003]    The first optical fiber collimator  21  includes the first rod lens  23  and a single-core capillary  27  having a single mode optical fiber  26 . The optical fiber  26  and the single-core capillary  27  form a single optical fiber chip  31 .  
           [0004]    The second optical fiber collimator  22  includes the second rod lens  24  and a dual core-capillary  30  having two optical fibers  28 ,  29 . The optical fibers  28 ,  29  and the dual-core capillary  30  form a dual optical fiber chip  32 .  
           [0005]    The first and second rod lenses  23 ,  24  of the first and second optical fiber collimators  21 ,  22  are coaxially arranged in the filter module  20  to form an integral center piece  33 . The filter module  20  is manufactured by aligning optical fiber chips  31 ,  32  relative to the center piece  33  and fixing the chips  31 ,  32  to the center piece  33 . That is, the center piece  33 , with which the first and second rod lenses  23 ,  24  are coaxially integrated, is first assembled. Then, the optical fiber chips  31 ,  32  are aligned with and fixed to the center piece  33  to form the filter module  20 .  
           [0006]    Therefore, alignment and fixing must be performed only twice for aligning and fixing each of the optical fiber chips  31 ,  32  to the center piece  33 . This shortens the time required for assembly, and thus reduces the cost. Also, since the number of parts is reduced, inexpensive and highly reliable filter modules are produced.  
           [0007]    In the above described filter module  20 , the optical fiber chips  32 ,  31  are adhered to the center piece  33 . The following is an example of adhering methods.  
           [0008]    When aligning and fixing the optical fiber chip  32  to the rod lens  24  of the center piece  33 , inclined faces of the rod lens  24  and the dual-core capillary  30  are placed parallel to face each other. Optical adhesive  35  having a predetermined refractive index is applied to penetrate the space between the rod lens  24  and the capillary  30 . The adhesive  35  is hardened to bond the rod lens  24  and the capillary  30  to each other. Then, reinforcing adhesive  36  is applied to cover the circumference of the optical adhesive  35  and is hardened. Thereafter, with the same procedure, the optical fiber chip  31  is aligned with and fixed to the rod lens  23  of the center piece  33 .  
           [0009]    Since such a bonding process is performed, the optical adhesive  35  exists in the optical paths between the optical fibers  28 ,  29  and the rod lens  24  and in the path between the optical fiber  26  and the rod lens  23 .  
           [0010]    Even if the optical adhesive  35  has a transparency, the transmittance of the adhesive is not be 100% in the entire usable range of temperatures of the filter module  20 , for example, in a range from 0 to 70° C. Also, troubles in the boding process or improper maintenance cause foreign matter and bubbles to enter the optical adhesive  35 . In such a case, the optical adhesive  35  in the optical paths is an impediment to light. No significant drawbacks exist when optical signals of low optical power, for example, signals of several milliwatts, are used. However, using light signals of high optical power, for example, signals of several hundreds of milliwatts produces heat and thus damages the optical adhesive  35 . This may decrease the performance.  
           [0011]    Therefore, in optical communications systems, conventional filter modules cannot be used in apparatuses and lines in which optical signals of a high optical power are used. That is, conventional filter modules can only be applicable to apparatuses and lines in which optical signals of a high optical power are not used. For example, there is a possibility that the conventional filter modules cannot be applied to lines of optical communications systems in which optical signals of high optical power, for example, optical signals of several watts are used, specifically, in a system where pump light for an optical amplifier is used.  
         SUMMARY OF THE INVENTION  
         [0012]    Accordingly, it is an objective of the present invention to provide an optical module that has no adhesive in optical paths and is applicable to apparatuses in a optical communications system using optical signals of a high power, and to lines for conveying amplifying pump lights having a high power.  
           [0013]    To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an optical module having first and second optical parts, and an adhesive is provided. Each of the first and second optical parts has an optical path extending in a longitudinal direction and an end face intersecting the optical path. The end face of the first optical part faces the end face of the second optical part. Light enters the end face of the second optical part from the end face of the first optical part. The adhesive forms a joint for connecting the end faces of the parts to each other in peripheral portions about the optical paths of the parts.  
           [0014]    Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0016]    [0016]FIG. 1( a ) is a schematic view showing an optical module according to a first embodiment;  
         [0017]    [0017]FIG. 1( b ) is an enlarged partial cross-sectional view shoring a joint of the optical module of FIG. 1( a );  
         [0018]    [0018]FIG. 1( c ) is an enlarged partial cross-sectional view shoring another joint of the optical module of FIG. 1( a );  
         [0019]    [0019]FIG. 2 is a schematic view showing a process step 1 for assembling an optical module;  
         [0020]    [0020]FIG. 3 is a schematic view showing a process step 2 for assembling the optical module;  
         [0021]    [0021]FIG. 4 is a schematic view showing a process step 3 for assembling the optical module;  
         [0022]    [0022]FIG. 5 is a side view showing the appearance of the optical module in the process step 3;  
         [0023]    [0023]FIG. 6 is a schematic view showing a process step 4 for assembling the optical module;  
         [0024]    [0024]FIG. 7 is a schematic view showing a process step 5 for assembling the optical module;  
         [0025]    [0025]FIG. 8 is a side view showing the appearance of the optical module in the process step 5;  
         [0026]    [0026]FIG. 9( a ) is a schematic view showing an optical module according to a second embodiment;  
         [0027]    [0027]FIG. 9( b ) is a schematic view showing the center piece of the optical module shown in FIG. 9( a );  
         [0028]    [0028]FIG. 9( c ) is a partially enlarged cross-sectional view showing the center piece of FIG. 9( b );  
         [0029]    [0029]FIG. 9( d ) is a side view showing the appearance of the center piece of FIG. 9( b );  
         [0030]    [0030]FIG. 10( a ) is a cross-sectional view showing an optical module according to a third embodiment;  
         [0031]    [0031]FIG. 10( b ) is a side view showing a lower half of the housing to which the main body of the optical module is fixed;  
         [0032]    [0032]FIG. 10( c ) is a side view showing the upper half of the housing;  
         [0033]    [0033]FIG. 11( a ) is a cross-sectional view showing a modification of the lens holder used in the first embodiment;  
         [0034]    [0034]FIG. 11( b ) is a side view showing the lens holder of FIG. 11( a );  
         [0035]    [0035]FIG. 12( a ) is a cross-sectional view showing another modification of the lens holder used in the first embodiment;  
         [0036]    [0036]FIG. 12( b ) is a side view showing the lens holder of FIG. 12( a ); and  
         [0037]    [0037]FIG. 13 is a cross-sectional view showing a prior art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    First to third embodiments of the present invention will now be described with reference to the drawings. Throughout the description, the same or like components have the same reference numbers in all embodiments.  
         [0039]    FIGS.  1 ( a ),  1 ( b ), and  1 ( c ) illustrate an optical module  40  according to a first embodiment.  
         [0040]    The optical module  40  is a three-port filter module having a center piece  44  and two optical fiber chips  45 ,  46 . The center piece  44  has two gradient index rod lenses  41 ,  42  and a filter located between the lenses  41 ,  42 . The filter is a wavelength selective interference film  43  and is coaxial with the lenses  41 ,  42 . The optical module  40  is formed by aligning the first and second optical fiber chips  45 ,  46  relative to the first and second rod lenses  41 ,  42  of the center piece  44 , and then fixing the fiber chips  45 ,  46  to the rod lenses  41 ,  42 .  
         [0041]    The first rod lens  41  and the dual optical fiber chip  45  form a dual optical fiber collimator. The second rod lens  42  and the single optical fiber chip  46  form a single optical fiber collimator.  
         [0042]    The dual optical fiber chip  45  includes two single mode optical fibers (hereinafter referred to as first and second optical fibers), and a dual-core capillary  49  for holding the first and second optical fibers  47 ,  48 . On the other hand, the single optical fiber chip  46  has a single optical fiber  50  and a single-core capillary  51  for holding the optical fiber  50 .  
         [0043]    One endface of the first rod lens  41  (right endface as viewed in FIG. 1( a )) is polished to be an endface that is perpendicular to the optical axis. The left endface of the rod lens  41  is polished to be an endface that is inclined relative to a plane perpendicular to the optical axis by a predetermined angle (for example, eight degrees). The second rod lens  42  is identical with the first rod lens  41 .  
         [0044]    The diameter of the first and second rod lenses  41 ,  42  is, for example, 1.8 mm, and the length of the lenses  41 ,  42  is, for example, 0.245 pitch. The pitch refers to the sinusoidal period of a light ray traveling in a gradient index rod lens. A wavelength selective interference film  43  is formed on the endface of the rod lens  41 . The interference film  43  is a dielectric film having a wavelength selecting property. The interference film  43  is an edge filter that, in the light wavelength region used in the conventional optical communications, transmits all the light rays in a wavelength band about 1.55 μm, and reflects all the light in the wavelength band about 1.48 μm. The light rays in the wavelength band about 1.55 μm are light rays in a wavelength region λ1 ranging from 1.53 to 1.58 μm. The light rays in the wavelength band about 1.48 μm are light rays in a wavelength region λ2 ranging from 1.45 to 1.49 μm. The wavelength selective interference film  43  may be a bandpass filter that transmits or reflects the light rays in a particular wavelength band ranging from a few nanometers to a few tens of nanometers falling in the light wavelength region used in the conventional optical communications.  
         [0045]    The radiating ends of the optical fibers  47 ,  48  and the right end of the dual-core capillary  49  are polished to be a flush inclined face that is inclined (for example, by eight degrees) relative to a plane perpendicular to the core axis of each of the optical fibers  47 ,  48 . The radiating end of the optical fiber  50  and the left end of the single-core capillary  51  are polished to be a flush inclined face that is inclined (for example, by eight degrees) relative to a plane perpendicular to the core axis of the optical fiber  50 .  
         [0046]    The dual optical fiber chip  45  is aligned with and fixed to the center piece  44  such that the inclined face of the dual-core capillary  49  and the inclined face of the rod lens  41  are face each other in parallel. Likewise, the single optical fiber chip  46  is aligned with and fixed to the center piece  44  such that the inclined face of the single-core capillary  51  and the inclined face of the rod lens  42  are face each other in parallel.  
         [0047]    An antireflection film  52  is formed on each of the inclined face of the rod lens  41 , the inclined face of the dual-core capillary  49 , the left endface of the rod lens  42 , the inclined face of the rod lens  42 , and the inclined face of the single-core capillary  51 . The antireflection film  52  has a property to reduce the reflectivity to a predetermined wavelength (1.55 μm) in a usable wavelength region to a value equal to or less than a predetermined value (for example, 0.2%).  
         [0048]    The wavelength selective interference film  43  may be formed on either one of the facing endfaces of the rod lenses  41 ,  42 . However, if the length of the rod lenses  41 ,  42  is equal to or less than 0.245 pitch, it is more reasonable to form the film  43  on the endface of the rod lens  42 .  
         [0049]    The center piece  44  is formed by fixing the two rod lenses  41 ,  42  to the inner surface of a cylindrical lens holder  53 , such that the rod lenses  41 ,  42  are coaxial and the endfaces face each other with the wavelength selective interference film  43  in between.  
         [0050]    The inner surface of the lens holder  53  is accurately machined to coaxially hold the rod lenses  41 ,  42 . Therefore, the axes of the rod lenses  41 ,  42  are aligned simply by inserting the rod lenses in the lens holder  53 . To facilitate adjusting of the positions of the rod lenses  41 ,  42  in the lens holder  53  for creating a predetermined lens distance, indications, such as markings, are preferably provided on the rod lenses  41 ,  42  and on the lens holder  53 .  
         [0051]    In the optical module  40  of this embodiment, the inclined face of the rod lens  41  and the inclined face of the dual-core capillary  49  (end face of the first optical fiber chip) are fixed by adhesive  60  at a peripheral portion that is outward of a center portion forming an optical path. Likewise, the inclined face of the rod lens  42  and the inclined face of the single-core capillary  51  (end face of the second optical fiber chip) are fixed by the adhesive  60  at a peripheral portion that is outward of a center portion forming an optical path. A layer of air exists inward of the adhesive  60 .  
         [0052]    If the diameter of the rod lenses  41 ,  42  is, for example, 1.8 mm, the adhesive  60  is preferably applied such that the adhesive does not enter a region of a radius 0.5 mm from the center of the lenses  41 ,  42 , that is, such that an inner diameter D of the adhesive  60  is more than 1.0 mm (0.5×2 mm).  
         [0053]    Further, the joint of the rod lens  41  and the dual-core capillary  49 , which are fixed with the adhesive  60 , and the joint between the rod lens  42  and the single-core capillary  51 , which are fixed with the adhesive  60 , are reinforced with adhesive  61  applied to cover the entire circumference of the adhesive  60 .  
         [0054]    As the adhesive  60 , an ultraviolet curing adhesive or a visible light curing adhesive high in viscosity is used. Accordingly, the adhesive  60  does not penetrate into the center when applied to the peripheral portion. As the reinforcing adhesive  61 , a thermosetting epoxy adhesive is used.  
         [0055]    An assembly procedure of the optical module  40  will now be described with reference to FIGS.  2  to  8 .  
         [0056]    In process step 1, the center piece  44  shown in FIG. 2 is assembled.  
         [0057]    First, the first and second rod lenses  41 ,  42  are inserted in the lens holder  53  such that the wavelength selective interference film  43  and the endface of the second rod lens  42  face each other at a predetermined distance as shown in FIG. 2. At this time, the phases of the inclined faces of the first and second rod lenses  41 ,  42  are matched. To accurately match the phases, a referential marking (not shown) is preferably provided on the circumference of each of the rod lenses  41 ,  42 .  
         [0058]    Thereafter, a small amount of the adhesive  59  is scooped with a micro spatula  62  and applied to the space between the lens holder  53  and each of the first and second rod lenses  41 ,  42 . At this time, the adhesive  59  is prevented from entering the space between the first and second rod lenses  41 ,  42  (see FIGS.  1 ( a ) and  1 ( b )). When the adhesive  59  is hardened, the first and second rod lenses  41 ,  42  are fixed to the lens holder  53 , and the center piece  44  is completed.  
         [0059]    In process step 2, the dual optical fiber chip  45  is aligned as shown in FIG. 3, on an aligning device, relative to the rod lens  41  of the center piece  44 , such that the rod lens  41  and the two optical fibers  47 ,  48  are in a predetermined arrangement relative to each other. In the predetermined arrangement, the antireflection film  52  on the inclined face of the rod lens and the antireflection film  52  on the inclined face of the dual-core capillary  49  are parallel and face each other with a predetermined space in between.  
         [0060]    In process step 3, as shown in FIGS. 4 and 5, the antireflection film  52  on the inclined face of the rod lens  41  and the antireflection film  52  on the inclined face of the dual-core capillary  49  are fixed to each other with the adhesive  60  in a peripheral portion outward of the center portion used as an optical path. At this time, as shown in FIG. 4, the inner diameter D of the adhesive  60  is controlled to exceed approximately 1.0 mm.  
         [0061]    The fixing process is performed immediately after the alignment of process step 2. That is, fixing with the adhesive  60  is executed while the rod lens  41  of the center piece  44  on the aligning device and the dual optical fiber chip  45  are kept aligned. In the fixing process, an appropriate amount of the adhesive  60  is scooped with a micro spatula. The adhesive  60  is then applied to the peripheral space between the rod lens  41  and the dual-core capillary  49  such that adhesive  60  lies over both the rod lens  41  and the dual-core capillary  49 .  
         [0062]    After applying the adhesive  60 , if the adhesive  60  is an ultraviolet curing adhesive, the adhesive  60  is exposed to ultraviolet rays (center wavelength=365 nm) as necessary to be hardened.  
         [0063]    If the adhesive  60  used in the fixing process is an ultraviolet curing adhesive or a visible light curing adhesive, the adhesive  60  must have the following characteristics.  
         [0064]    (a) The viscosity of the adhesive  60  before being hardened must be relatively high, for example, 35000 cps, so that the adhesive  60 , when applied to the peripheral portion, does not penetrate into the inner portion of the space between the rod lens  41  and the dual-core capillary  49 . If the viscosity of the adhesive  60  before being hardened is low, for example, less than 35000 cps, the adhesive  60  might enter a space of 10 to 50 μm.  
         [0065]    (b) The glass transition temperature Tg of the adhesive  60  must be higher than the usable range of temperatures of the optical module, and is preferably equal to or higher than 100° C.  
         [0066]    In process step 4, the single optical fiber chip  46  is aligned as shown in FIG. 6 on an aligning device relative to the rod lens  42  of the center piece  44 , such that the second rod lens  42  and the single optical fiber  50  are in a predetermined arrangement relative to each other.  
         [0067]    In process step 5, as shown in FIGS. 7 and 8, the antireflection film  52  on the inclined face of the rod lens  42  and the antireflection film  52  on the inclined face of the single-core capillary  51  are fixed to each other with the adhesive  60  in a peripheral portion outward of the center portion used as an optical path. This fixing process is performed in the same manner as process step 3. The adhesive  60  used in this fixing process must also have the properties (a) and (b) as that in process step 3.  
         [0068]    In process step 6, at the joint of the rod lens  41  and the dual-core capillary  49 , and at the joint between the rod lens  42  and the single-core capillary  51 , the reinforcing adhesive  61  is applied to cover the entire circumference of the hardened adhesive  60 . For example, a thermosetting epoxy adhesive is used as the reinforcing adhesive  61 . After being applied, the adhesive  61  is hardened in an air of 100° C. for six hours.  
         [0069]    The optical module  40  shown in FIG. 1( a ) is thus completed. For example, when a light signal containing mixed light components with center wavelengths within the wavelength ranges λ1, λ2 enters the optical fiber  47 , only the component of the center wavelength within the range λ1 passes the wavelength selective interference film  43 . The passed light is condensed by the rod lens  42  and coupled with the optical fiber  50 . The remainder of the light, that is, the light component of the center wavelength within the range λ2 is reflected by the interference film  43 . The reflected light is condensed by the rod lens  41  and is coupled with the optical fiber  48 . In this manner, an optical signal having the center wavelength within the wavelength ranges λ1, λ2 is split.  
         [0070]    The above configured first embodiment provides the following advantages.  
         [0071]    (1) The inclined face of the rod lens  41  and the inclined face of the dual-core capillary  49  are fixed to each other with the adhesive  60  in an peripheral portion that is outward of the center portion in which an optical path is formed. Likewise, the inclined face of the rod lens  42  and the inclined face of the single-core capillary  51  are fixed to each other with the adhesive  60  in an peripheral portion that is outward of the center portion in which an optical path is formed. Therefore, the rod lens  41  and the dual optical fiber chip  45  are connected to each other without adhesive in the optical path. Also, the rod lens  42  and the single optical fiber chip  46  are connected to each other without adhesive in the optical path. Therefore, the adhesive  60  is not damaged by optical signals of high power, and the performance of the module is not degraded. Thus, in optical communications systems, the optical module can be applied to apparatuses using optical signals of high power and lines through which pump light of high power for an optical amplifier is transmitted. Further, since no adhesive exists in the optical paths, deterioration of optical characteristics due to adhesive is suppressed for an extended period.  
         [0072]    (2) The adhesive  60  does not exist in the optical path between two optical parts, that is, between the rod lens  41  and the dual optical fiber chip  45 , and between the rod lens  42  and the single optical fiber chip  46 . Accordingly, the adhesive  60  need not be selected from adhesives having an optimal refractive index for matching the refractive indexes of two optical parts. This broadens the options of adhesive.  
         [0073]    (3) As the adhesive  60 , which is applied to the peripheral portions of optical parts, is an ultraviolet curing adhesive or a visible light curing adhesive, and the viscosity of the used adhesive is sufficiently high so that the adhesive does not penetrate inward from the peripheral portions. The time for exposing the adhesive  60  to ultraviolet or visible light to harden the adhesive  60  is no more than a few minutes. Thus, the time for fixing two optical parts with the adhesive  60  is shortened.  
         [0074]    (4) The bonding strength of two optical parts is increased by the reinforcing adhesive  61 , which increases the overall rigidity.  
         [0075]    (5) The antireflection film  52 , the reflectivity of which is less than usable wavelength by a predetermined value, is formed on each of the end faces of two optical parts. That is, the antireflection film  52  is formed on the inclined faces of the rod lens  41  and the dual-core capillary  49 , and on the inclined faces of the rod lens  42  and the single-core capillary  51 . This reduces the reflectivity at center portions of each end face of two optical parts, which center portions are used as optical paths.  
         [0076]    The second embodiment will now be described. FIGS.  9 ( a ) to  9 ( d ) illustrate an optical module  40 A according to the second embodiment.  
         [0077]    The optical module  40 A of the second embodiment is different from the optical module  40  of the first embodiment shown in FIG. 1 only in the structure of a center piece  44 A.  
         [0078]    In the optical module  40 A, endfaces of the two rod lenses  41 ,  42  face each other with the wavelength selective interference film  43  in between. The rod lenses  41 ,  42  are coaxially fixed to each other with adhesives  64 ,  65  to form a center piece  44 A.  
         [0079]    The wavelength selective interference film  43  is formed on the endface of the first rod lens  41 . The endface of the second rod lens  42  and the wavelength selective interference film  43  are fixed to each other with the adhesive  64  in the peripheral portions. Specifically, portions outward of the optical path are fixed to each other with the adhesive  64 .  
         [0080]    Further, the two rod lenses  41 ,  42  are fixed with the reinforcing adhesive  65  covering the entire circumference of the adhesive  64 . The adhesive  64  is the same as the adhesive  60 , and the adhesive  65  is the same as the adhesive  61 .  
         [0081]    In addition to the advantages (1) to (5), the second embodiment provides the following advantages.  
         [0082]    The second embodiment provides an optical module that can be used in a center piece type filter module that does not use a cylindrical lens holder. Specifically, the optical module of the second embodiment may be applied to an optical demultiplexer/multiplexer that is used in an apparatuses using optical signals of high power of an optical communications system or in lines through which pump light of high power for an optical amplifier is transmitted.  
         [0083]    Since a cylindrical lens holder is not used in the center piece  44 A, the number of parts is reduced. This lowers the cost of the optical module.  
         [0084]    Forming the wavelength selective interference film  43  on the endface of the rod lens  41  facilitates the manufacture of the center piece  44 A.  
         [0085]    Since the endface of the second rod lens  42  that is perpendicular to the optical axis and the wavelength selective interference film  43  are adhered to each other at peripheral portions outward of the center portion used as an optical path, the two rod lenses  41 ,  42  are fixed to each other without adhesive in the optical path.  
         [0086]    The bonding strength of the two rod lenses  41 ,  42  is increased by the reinforcing adhesive  65 , which increases the rigidity of the center piece  44 A.  
         [0087]    The third embodiment will now be described.  
         [0088]    FIGS.  10 ( a ),  10 ( b ),  10 ( c ) illustrate an optical module  40 B according to the third embodiment.  
         [0089]    The optical module  40 B has a housing  70  for accommodating the main body of an optical module, for example, the optical module  40  of the first embodiment.  
         [0090]    The housing  70  is a cylindrical body having lids  74 ,  75  at the left and right ends. An accommodation space  71  for the optical module  40  is defined in the housing  70 . The lids  74 ,  75  have through holes  72 ,  73  through which an optical fiber extends, respectively. The housing  70  is formed with a resin lower half  80  and a resin upper half  90 . The circumference of distal portions of the lid  74 ,  75  have a circular cross-section. Heat-shrinkable tubes  76 ,  77  are attached to the distal circumference of the lid  74 ,  75 , respectively. The heat-shrinkable tubes  76 ,  77  are engaged with notches  78 ,  79  formed in the lids  74 ,  75 , respectively, so that the heat-shrinkable tubes  76 ,  77  are prevented from being disengaged.  
         [0091]    Accommodation recesses  81 ,  91  are formed in the halves  80 ,  90 . When the housing is assembled, the recesses  81 ,  91  define the accommodation space  71 . Lid pieces  82 ,  92  are formed in left portions of the halves  80 ,  90 . When the housing is assembled, the lid pieces  82 ,  92  form the lid  74 . Lid pieces  83 ,  93  are formed in right portions of the halves  80 ,  90 . When the housing is assembled, the lid pieces  83 ,  93  form the lid  75 .  
         [0092]    An engaging piece  84  projects from a left portion of the lower half  80 . The engaging piece  84  has an engaging hole  84   a . An engaging piece  95  projects from a right portion of the upper half  90 . The projection  94  has an engaging hole  95   a . On the other hand, a projection  85  projects from a right portion of the lower half  80 . The projection  85  engages with the engaging hole  95   a . A projection  94  projects from a left portion of the upper half  90 . The projection  94  engages with the engaging hole  84   a.    
         [0093]    A procedure for assembling the housing  70  will now be described.  
         [0094]    As shown in FIG. 10( b ), the optical module  40  is placed in the accommodation recess  81  of the lower half  80 . Then, the bottom of the lens holder  53  and the inner surface of the lower half  80  are fixed to each other with adhesive  86 .  
         [0095]    Thereafter, the edges of the halves  80 ,  90  are brought into contact such that the optical fibers  47 ,  48  pass through the through hole  72  of the lid  74 , and the optical fiber  50  passes through the through hole  73  of the lid  75 . The halves  80 ,  90  are thus integrated. At this time, the engaging pieces  84 ,  95  of the halves  80 ,  90  are elastically deformed, and the projections  85 ,  94  of the halves  80 ,  90  engage with the corresponding engaging holes  95   a ,  84   a . The housing  70  is thus assembled.  
         [0096]    Then, the optical fibers  47 ,  48  and the optical fiber  50  are fixed to the through hole  72  and the through hole  73  with adhesive, respectively.  
         [0097]    Thereafter, the heat-shrinkable tubes  76 ,  77  are attached to the distal circumference of the lids  74 ,  75 , respectively. The heat-shrinkable tubes  76 ,  77  are engaged with the notches  78 ,  79 . The heat-shrinkable tube  76  prevents the optical fibers  47 ,  48  from being bent abruptly. The heat-shrinkable tube  77  prevents the optical fiber  50  from being bent abruptly. As the material for the heat-shrinkable tubes  76 ,  77 , for example, heat-shrinkable silicone rubber tubes are preferable because of the low cost and flexibility.  
         [0098]    The above configured third embodiment provides the following advantages.  
         [0099]    The optical module  40 B hermetically accommodates the main body of the optical module, that is, the optical module  40 , in the housing  70 . Therefore, the durability of the optical module is improved.  
         [0100]    The optical fibers  47 ,  48  and the optical fiber  50  are fixed to the through holes  72 ,  73  of the left and right lids  74 ,  75  of the housing  70 . The optical fibers  47 ,  48  and the optical fiber  50  are held by the flexible heat-shrinkable tubes  76 ,  77 , respectively, and extend outward. Therefore, the optical fibers  47 ,  48  and the optical fiber  50  are prevented from being bent abruptly. That is, the optical fibers  47 ,  48 ,  50  are prevented from being damaged by abrupt bending, and the performance of the optical module is maintained.  
         [0101]    The housing  70  is formed of the resin lower half  80  and the resin upper half  90 . The halves  80 ,  90  are formed by dividing a cylindrical body having the accommodation space  71  and the left and right lids  74 ,  75 . The lids  74 ,  75  have the through holes  72 ,  73  for receiving optical fibers, respectively. Therefore, the housing that is inexpensive and easy to assemble is provided.  
         [0102]    It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.  
         [0103]    In the first embodiment shown in FIG. 1 and the second embodiment shown in FIG. 9, the present invention is applied to three-port filter modules. However, the present invention may be applied to devices other than filter modules. That is, the present invention may be applied to optical modules having two optical parts the facing end faces of which are connected to each other with adhesive, and light is transmitted from one of the parts to the other. For example, the present invention may be applied to an optical module having minute optical elements, which are, for example, two lenses similar to gradient index lenses. Alternatively, the present invention may be applied to an optical module having a minute gradient index lens and an optical fiber.  
         [0104]    In the first embodiment, the lens holder  53  may be replaced by a lens holder  53 A shown in FIGS.  11 ( a ) and  11 ( b ). Three lateral holes  53   a  are formed in the circumference of the lens holder  53 A. The adhesive  59  can be applied through the lateral holes  53   a . This permits the rod lenses  41 ,  42  to be easily adhered to and fixed to the inner surface of the lens holder  53 .  
         [0105]    In the first embodiment, the lens holder  53  may be replaced by a lens holder  53 B shown in FIGS.  12 ( a ) and  12 ( b ). An expanding slot  53   b  is formed in the circumference of the lens holder  53 B along the entire axial length of the lens holder  53 B. The adhesive  59  can be applied through the slot  53   b . This permits the rod lenses  41 ,  42  to be easily adhered to and fixed to the inner surface of the lens holder  53 .  
         [0106]    In the first and second embodiments, the wavelength selective interference film  25  is used as a filter. However, the interference film may be replaced by a half mirror.  
         [0107]    Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.