Abstract:
A package structure of a WDM device is disclosed. The package structure disclosed here can effectively reduce the amount of the adhesive leaking during the manufacturing process and increase the yield of the manufacturing process. The package includes: a tubular fixing unit having a low thermal expansion coefficient; a first collimator partially embedded in the tubular fixing unit; a second collimator partially embedded in the tubular fixing unit and a filter located between the first and second collimators inside the tubular fixing unit. The first and second collimators are fixed to the tubular fixing unit through an adhesive, respectively. Besides, an external metal tube encompasses the tubular fixing unit. In cooperation with the two metal caps positioned at the two ends of the external metal tube, the disclosed package structure can be protected from the damage otherwise caused by heat, electromagnetic waves or vibration of the ambient environment.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the package structure of a wavelength division multiplexing [WDM] device and, more particularly, to a package structure of a WDM device which can effectively reduce the amount of adhesive leaking during the manufacturing process and increase the yield of the manufacturing process of the WDM device.  
         [0003]     2. Description of Related Art  
         [0004]     Referring to  FIG. 1 , which is a sectional view of the conventional package structure of a WDM device. The package structure of the WDM device  1  comprises: a metal shell  11 , an insulating protective layer  12 , a first GRIN lens  13 , a second GRIN lens  14  and an IR-cut lens  15  wherein the first GRIN lens  13 , the second GRIN lens  14  and the IR-cut lens  15  are fixed to the insulating protective layer  12  through UV adhesives  131 ,  141   151 , respectively. The metal shell  11  encompasses the outer surface of the insulating protective layer  12  and protects the elements of the package structure of the WDM device inside it. Besides, the terminal of the first GRIN lens  13  is connected with a fiber pigtail  132  and the terminal of the second GRIN lens  14  is connected with another fiber pigtail  142 .  
         [0005]     As clearly shown in  FIG. 1 , the relative positions of the first GRIN lens  13 , the second GRIN lens  14  and the IR-cut lens  15  are determined by the thicknesses of the UV adhesive  131 ,  141 ,  151 . As a result, during the manufacturing process of the package structure of the WDM device, these elements must be installed carefully and the thickness of all the UV adhesives must be precisely controlled. Therefore, these elements must be fixed to the insulating protective layer  12  separately and this operation is particularly time-consuming. Besides, since the outer radii of these elements (the first GRIN lens  13 , the second GRIN lens  14  and the IR-cut lens  15 ) are much smaller than the inner radius of the insulating protective layer  12 , these elements cannot be fixed to the insulating protective layer  12  by the embedding method.  
         [0006]     Hence, as shown in  FIG. 1 , since these UV adhesives  131 , 141 , 151  fixing these elements all have certain thickness, there are some disadvantages for having such thick VW adhesives, which are as follows: 
        (a) Since the UV adhesive is extremely expensive it costs a lot to form these thick UV adhesives to fix the elements;     (b) Since the UV adhesive is formed through the transformation from the original liquid-state precursor into the solid-state adhesive when it is exposed to a UV light, if the UV adhesive is too thick, it is likely that only the liquid-state precursor near the surface of a drop is transformed into solid-state adhesive. This means the liquid-state precursor near the center region of the drop still remains in its original liquid-state. Therefore, even with certain exposure to the UV light, the whole drop is likely to remain in a composition of a liquid-state precursor and solid-state adhesive and flows only gradually. As a result, the relative position relation of the elements (the first GRIN lens  13 , the second GRIN lens  14  and the IR-cut lens  15 ) cannot be efficiently maintained; and     (c) As described in (b), since the drop is still flowing slowly, the UV adhesive is likely to flow onto the optical surfaces of these elements. Therefore, the UV adhesives contaminate the optical surfaces of these elements and make the optical efficiency of these elements deteriorate.        
 
         [0010]     In summary, since the manufacturing process of the conventional package structure of the WDM device has the following disadvantages: (a) its working steps are complex; (b) it cannot precisely define the positions of the elements; (c) it involves a large amount of expensive UV adhesive during the whole process; and (d) the UV adhesive flows onto and damages the optical surfaces of the elements easily. As a result, it is desirable to provide an improved package of a WDM device to mitigate and/or obviate the aforementioned problems.  
       SUMMARY OF THE INVENTION  
       [0011]     The package structure for a wavelength division multiplexing device of the present invention comprises: a tubular fixing unit having a low thermal expansion coefficient; a first collimator partially embedded in the tubular fixing unit; a second collimator partially embedded in the tubular fixing unit; and a filter mounted inside the tubular fixing unit and located between the first collimator and the second collimator. Wherein the first collimator and the second collimator are fixed to the tubular fixing unit through an adhesive, respectively.  
         [0012]     Another package structure for a wavelength division multiplexing device of the present invention comprises: a tubular fixing unit having a low thermal expansion coefficient; a first collimator partially embedded in the tubular fixing unit, where an IR-cut coating is formed on the surface of the first collimator inside the tubular fixing unit; and a second collimator partially embedded in the tubular fixing unit. Wherein the first collimator and the second collimator are fixed to the tubular fixing unit through an adhesive, respectively.  
         [0013]     Therefore, by having the package structure of the present invention, the amount of the adhesive leaking during the manufacturing process of a WDM device can be reduced and the yield of the manufacturing process can also be increased. In addition, since the relative position of each optical element (e.g. the first GRIN lens, infrared cut lens and the second GRIN lens) are determined and maintained by the tubular fixing unit (a glass tube or a metal tube), the optical path (e.g. the collimation) is easily maintained once all the optical elements are assembled in their predetermined positions. The labor for carefully aligning all the optical elements involved can also be saved. Besides, the manufacturing process of the WDM device of the present invention can be greatly simplified and the manufacturing amount of the WDM device having the package structure of the present embodiment can be dramatically increased compared to that of the WDM device having the conventional package structure.  
         [0014]     The type of the first collimator in the package structure of the WDM device of the present invention is not limited; preferably the first collimator is a GRIN lens. The type of the second collimator in the package structure of the WDM device of the present invention is not limited; preferably the second collimator is a GRIN lens. The type of the adhesive fixing the first collimator to the tubular fixing unit in the package structure of the WDM device of the present invention is not limited, preferably the first collimator is fixed to the tubular fixing unit through a UV cure adhesive or a thermal cure adhesive. The type of the adhesive fixing the second collimator to the tubular fixing unit in the package structure of the WDM device of the present invention is not limited; preferably the second collimator is fixed to the tubular fixing unit through a UV cure adhesive or a thermal cure adhesive. The material of the tubular fixing unit of the package structure of the WDM device of the present invention is not limited; preferably the tubular fixing unit is made of glass, metal or ceramic.  
         [0015]     Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a sectional view of the conventional package structure of a WDM device.  
         [0017]      FIG. 2A  is a perspective view of the package structure of a WDM device according to the first preferred embodiment of the present invention.  
         [0018]      FIG. 2B  is a sectional view of the package structure of a WDM device according to the first preferred embodiment of the present invention.  
         [0019]      FIG. 3A  is a perspective view of the package structure of a WDM device according to the second preferred embodiment of the present invention.  
         [0020]      FIG. 3B  is a sectional view of the package structure of a WDM device according to the second preferred embodiment of the present invention.  
         [0021]      FIG. 4A  is a perspective view of the package structure of a WDM device according to the third preferred embodiment of the present invention.  
         [0022]      FIG. 4B  is a sectional view of the package structure of a WDM device according to the third preferred embodiment of the present invention.  
         [0023]      FIG. 5A  is a perspective view of the package structure of a WDM device according to the fourth preferred embodiment of the present invention.  
         [0024]      FIG. 5B  is a sectional view of the package structure of a WDM device according to the fourth preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     With reference to  FIG. 2A , there is shown the first preferred embodiment of a package structure of a wavelength division multiplexing device (WDM) of the present invention. The package structure for a WDM of the present embodiment includes a glass tube  21 , a first GRIN lens  22 , a second GRIN lens  23 , and a cube-shaped IR-cut lens  24 . As shown in  FIG. 2A , part of the first GRIN lens  22  and part of the second GRIN lens  23  are embedded in the glass tube  21 . The glass tube  21  is made of glass the thermal expansion coefficient of which is very low. In the present embodiment, the diameter of the inner wall of the glass tube  21  is about 1.8 mm, while the diameters of the outer surface of the first GRIN lens  22  and the second GRIN lens  23  are both about 1.79 mm. That is, the diameters of the outer surface of these two GRIN lens  22 , 23  are close in size to the diameter of the inner wall of the glass tube  21 . Besides, the terminal of the first GRIN lens  22  outside the glass tube  21  is connected with a single fiber pigtail  221 . Likewise, the terminal of the second GRIN lens  23  outside the glass tube  21  is connected with a dual fiber pigtail  231 .  
         [0026]     In the gap between the outer surface of the first GRIN lens  22  and the inner wall of the glass tube  21 , a UV cure adhesive  251  is injected for fixing the embedded part of the first GRIN lens  22  to the glass tube  21  securely. Similarly, in the gap between the outer surface of the second GRIN lens  23  and the inner wall of the glass tube  21 , another UV cure adhesive  252  is injected for fixing the embedded part of the second GRIN lens  23  and the glass tube  21 .  
         [0027]     The package structure of the first preferred embodiment of the present invention is manufactured through the following process.  
         [0028]     First, the glass tube  21 , the first GRIN lens  22 , the second GRIN lens  23  and the cube-shaped infrared cut lens  24  are provided. The first GRIN lens  22  is embedded into the glass tube  21  partially. As described above, since the diameter of the outer surface of the first GRIN lens  22  (about 1.79 mm) is a little smaller than the diameter of the inner wall of the glass tube  21 (about 1.8 mm), a small gap is formed between the outer surface of the first GRIN lens  22  and the inner wall of the glass tube  21 . Subsequently, after a small amount of liquid-state precursor of a UV cure adhesive is injected at the seam between the small gap and the ambient outer space, the liquid-state precursor flows into the small gap slowly and stops flowing when the small gap is filled with the liquid-state precursor as a result of the “siphon effect”. Then, the liquid-state precursor filling the small gap is exposed to a UV radiation (UV curing process). As a result, the liquid-state precursor is transformed into the solid-state adhesive (UV cure adhesive) inside the small gap. In addition, a UV gun or a UV oven can provide the UV radiation to the liquid-state precursor in the present embodiment.  
         [0029]     After the first GRIN lens is securely fixed to the glass tube  21 , the cube-shaped infrared cut lens  24  is squeezed into the glass tube  21  through the opening on the other side of the glass tube  21 . The cube-shaped infrared cut lens  24  with a surface  241  having an IR-cut coating is squeezed into the glass tube  21  until being stopped by the first GRIN lens  22 . Since the diagonal dimension (about 1.77 mm) of the cube-shaped infrared cut lens  24  of the present embodiment is a little smaller than the diameter of the inner wall of the glass tube  21  (about 1.8 mm), the cube-shaped infrared cut lens  24  can be fixed inside the glass tube  21  easily without using any adhesive (e.g. a UV cure adhesive or a thermal cure adhesive).  
         [0030]     After the cube-shaped infrared cut lens  24  is fixed inside the glass tube  21 , the second GRIN lens  23  is then squeezed into the glass tube  21  through the same opening through which the cube-shaped infrared cut lens  24  has been squeezed. The second GRIN lens  23  is squeezed into the glass tube  21  until being stopped by the cube-shaped infrared cut lens  24 .  
         [0031]     Similarly, there is also a small gap located between the outer surface of the second GRIN lens  23  and the inner wall of the glass tube  21 . A small amount of liquid-state precursor of a UV cure adhesive is injected at the seam between the small gap and the ambient outer space. Again, due to the “siphon effect”, the liquid-state precursor flows into the small gap slowly and stops flowing when the small gap is filled with the liquid-state precursor. Then the liquid-state precursor filling the small gap is exposed to a UV radiation (UV curing process). As a result, the liquid-state precursor is transformed into the solid-state adhesive (UV cure adhesive) inside the small gap.  
         [0032]     Furthermore, the terminal of the first GRIN lens  22  outside the glass tube  21  is connected with a single fiber pigtail  221  through a thermal cure adhesive or a UV cure adhesive. Similarly, the terminal of the second GRIN lens  23  outside the glass tube  21  is connected with a dual fiber pigtail  231  through a thermal cure adhesive or a UV cure adhesive. The resulting package structure of a WDM device of the present invention according to the present embodiment is as shown in  FIG. 2A .  
         [0033]     As shown in  FIG. 2B , the package structure of a WDM device is then inserted into an external metal tube  26  and the two terminals of the external metal tube  26  are respectively covered with the external metal caps  271 ,  272  for protecting the WDM device. (e.g. protecting the WDM device from heat, electromagnetic interference, or impact).  
         [0034]     Therefore, by having the package structure of the present invention illustrated above, the amount of the adhesive leaking during the manufacturing process of the WDM device can be reduced and the yield of the manufacturing process can also be increased. In addition, since the relative position of each optical element (e.g. the first GRIN lens, the IR-cut lens, and the second GRIN lens) are determined and maintained by the glass tube, the optical path (e.g. the collimation) is easily maintained once all the optical elements are assembled in their predetermined positions. Hence, by using the package structure of the WDM device of the present invention, the labor for carefully aligning all the optical elements involved can be saved, the manufacturing process of the WDM device of the present invention can be greatly simplified and the yield of the manufacturing process can also be effectively improved.  
         [0035]     With reference to  FIG. 3A , there is shown the second preferred embodiment of a package structure of a WDM device of the present invention. The package structure for a WDM of the present embodiment includes a metal tube  31 , a first GRIN lens  32 , a second GRIN lens  33 , and a cube-shaped IR-cut lens  24 . As shown in  FIG. 3A , part of the first GRIN lens  32  and part of the second GRIN lens  33  are embedded in the metal tube  31 . The metal tube  31  is made of metal the thermal expansion coefficient of which is low. In the present embodiment, the diameter of the inner wall of the metal tube  31  is about 1.8 mm, while the diameters of the outer surface of the first GRIN lens  32  and the second GRIN lens  33  are both about 1.79 mm. That is, the diameters of the outer surface of these two GRIN lens  32 , 33  are close in size to the diameter of the inner wall of the metal tube  31 . Besides, the terminal of the first GRIN lens  32  outside the metal tube  31  is connected with a single fiber pigtail  321 . Likewise, the terminal of the second GRIN lens  33  outside the metal tube  31  is connected with a dual fiber pigtail  331 .  
         [0036]     In the gap between the outer surface of the first GRIN lens  32  and the inner wall of the metal tube  31 , a thermal cure adhesive  351  is injected for fixing the embedded part of the first GRIN lens  32  to the metal tube  31  securely. Similarly, in the gap between the outer surface of the second GRIN lens  33  and the inner wall of the metal tube  31 , another thermal cure adhesive  352  is injected for fixing the embedded part of the second GRIN lens  33  and the metal tube  31 .  
         [0037]     The package structure of the second preferred embodiment of the present invention is manufactured through the following process.  
         [0038]     First, the metal tube  31 , the first GRIN lens  32 , the second GRIN lens  33  and the cube-shaped infrared cut lens  34  are provided. The first GRIN lens  32  is embedded into the metal tube  31  partially. As described above, since the diameter of the outer surface of the first GRIN lens  32  (about 1.79 mm) is a little smaller than the diameter of the inner wall of the metal tube  31 (about 1.8 mm), a small gap is formed between the outer surface of the first GRIN lens  32  and the inner wall of the metal tube  31 . Subsequently, after a small amount of liquid-state precursor of a thermal cure adhesive is injected at the seam between the small gap and the ambient outer space, the liquid-state precursor flows into the small gap slowly and then stops flowing when the small gap is filled with the liquid-state precursor as a result of the “siphon effect”. Then, the liquid-state precursor filling in the small gap is heated (thermal curing process). As a result, the liquid-state precursor inside the small gap is transformed into the solid-state thermal cure adhesive  351 . In the present embodiment, an oven can provide the heat required in the thermal curing process.  
         [0039]     After the first GRIN lens  32  is securely fixed to the metal tube  31 , the cube-shaped infrared cut lens  34  is squeezed into the metal tube  31  through the opening on the other side of the metal tube  31 . The cube-shaped infrared cut lens  34 , the surface  341  of which having an IR-cut coating, is squeezed into the metal tube  31  until being stopped by the first GRIN lens  32 . Since the diagonal dimension (about 1.77 mm) of the cube-shaped infrared cut lens  34  of the present embodiment is a little smaller than the diameter of the inner wall of the metal tube  31  (about  1 . 8  mm), the cube-shaped infrared cut lens  34  can be fixed inside the metal tube  31  easily without using any adhesive (e.g. a UV cure adhesive or a thermal cure adhesive).  
         [0040]     After the cube-shaped infrared cut lens  34  is fixed inside the metal tube  31 , the second GRIN lens  33  is then squeezed into the metal tube  31  through the same opening through which the cube-shaped infrared cut lens  34  has been squeezed. The second GRIN lens  33  is squeezed into the metal tube  31  until being stopped by the infrared cut lens  34 .  
         [0041]     Similarly, there is also a small gap located between the outer surface of the second GRIN lens  33  and the inner wall of the metal tube  31 . A small amount of liquid-state precursor of a thermal cure adhesive is injected at the seam between the small gap and the ambient outer space. Again, due to the “siphon effect”, the liquid-state precursor flows into the small gap slowly and stops flowing when the small gap is filled with the liquid-state precursor. Then the liquid-state precursor filling the small gap is heated (thermal curing process). As a result, the liquid-state precursor inside the small gap is transformed into the solid-state thermal cure adhesive  352  inside the small gap. In the present embodiment, an oven can provide the heat required in the thermal curing process.  
         [0042]     Furthermore, the terminal of the first GRIN lens  32  outside the metal tube  31  is connected with a single fiber pigtail  321  through a thermal cure adhesive or a UV cure adhesive. Similarly, the terminal of the second GRIN lens  33  outside the metal tube  31  is connected with a dual fiber pigtail  331  through a thermal cure adhesive or a UV cure adhesive. The resulting package structure of a WDM device of the present invention according to the present embodiment is as shown in  FIG. 3A .  
         [0043]     As shown in  FIG. 3B , the package structure of a WDM device is then inserted into an external metal tube  36  and the two terminals of the external metal tube  36  are respectively covered with the external metal caps  371 ,  372  for protecting the WDM device. (e.g. protecting the WDM device from heat, electromagnetic interference, or impact).  
         [0044]     Therefore, by having the package structure of the present invention illustrated above, the amount of the adhesive leaking during the manufacturing process of the WDM device can be reduced and the yield of the manufacturing process can also be increased. In addition, since the relative position of each optical element (e.g. the first GRIN lens, the IR-cut lens, and the second GRIN lens) are determined and maintained by the metal tube; the optical path (e.g. the collimation) is easily maintained once all the optical elements are assembled in their predetermined positions.  
         [0045]     Moreover, since hundreds, even thousands of the metal tubes each having the liquid-state precursor of the thermal cure adhesive in their small gaps can be heated in an oven at the same time, the manufacturing amount of the WDM device having the package structure of the present embodiment can be dramatically increased compared to that of the WDM device having the conventional package structure.  
         [0046]     Hence, by using the package structure of the WDM device of the present invention, the labor for carefully aligning all the optical elements involved can be saved, the manufacturing process of the WDM device of the present invention can be greatly simplified and the yield of the manufacturing process can also be effectively improved.  
         [0047]     With reference to  FIG. 4A , there is shown the third preferred embodiment of a package structure of a WDM device of the present invention. The package structure for a WDM of the present embodiment includes a glass tube  41 , a first GRIN lens  42  and a second GRIN lens  43  wherein an IR-cut coating is formed on the surface  432  of the second GRIN lens  43 . As shown in  FIG. 4A , part of the first GRIN lens  42  and part of the second GRIN lens  43  are embedded in the glass tube  41 . The glass tube  41  is made of glass the thermal expansion coefficient of which is very low. In the present embodiment, the diameter of the inner wall of the glass tube  41  is about 1.8 mm, while the diameters of the outer surface of the first GRIN lens  42  and the second GRIN lens  43  are both about 1.79 mm. That is, the diameters of the outer surface of these two GRIN lenses  42 , 43  are close in size to the diameter of the inner wall of the glass tube  41 . Besides, the terminal of the first GRIN lens  42  outside the glass tube  41  is connected with a single fiber pigtail  421 . Likewise, the terminal of the second GRIN lens  43  outside the glass tube  41  is connected with a dual fiber pigtail  431 .  
         [0048]     In the gap between the outer surface of the first GRIN lens  42  and the inner wall of the glass tube  41 , a UV cure adhesive  441  is injected for fixing the embedded part of the first GRIN lens  42  to the glass tube  41  securely. Similarly, in the gap between the outer surface of the second GRIN lens  43  and the inner wall of the glass tube  41 , another UV cure adhesive  442  is injected for fixing the embedded part of the second GRIN lens  43  and the glass tube  41 .  
         [0049]     The package structure of the third preferred embodiment of the present invention is manufactured through the following process.  
         [0050]     First, the glass tube  41 , the first GRIN lens  42  and the second GRIN lens  43  the surface  432  of which has an IR-cut coating are provided. The first GRIN lens  42  is embedded into the glass tube  41  partially. As described above, since the diameter of the outer surface of the first GRIN lens  42  (about 1.79 mm) is a little smaller than the diameter of the inner wall of the glass tube  41 (about 1.8 mm), a small gap is formed between the outer surface of the first GRIN lens  42  and the inner wall of the glass tube  41 . Subsequently, after a small amount of liquid-state precursor of a UV cure adhesive is injected at the seam between the small gap and the ambient outer space, the liquid-state precursor flows into the small gap slowly and stops flowing when the small gap is filled with the liquid-state precursor as a result of the “siphon effect”. Then the liquid-state precursor filling the small gap is exposed to a UV radiation (UV curing process). As a result, the liquid-state precursor is transformed into the solid-state adhesive (UV cure adhesive) inside the small gap. In addition, a UV gun or a UV oven can provide the UV radiation to the liquid-state precursor in the present embodiment.  
         [0051]     After the first GRIN lens  42  has been fixed to the glass tube  41 , the second GRIN lens  43  is then squeezed into the glass tube  41  through the opening on the other side of the glass tube  41 . Similarly, there is also a small gap located between the outer surface of the second GRIN lens  43  and the inner wall of the glass tube  41 . A small amount of liquid-state precursor of a UV cure adhesive is injected at the seam between the small gap and the ambient outer space. Again, due to the “siphon effect”, the liquid-state precursor flows into the small gap slowly and stops flowing when the small gap is filled with the liquid-state precursor. Then the liquid-state precursor filling the small gap is exposed to a UV radiation (UV curing process). As a result, the liquid-state precursor is transformed into the solid-state adhesive (UV cure adhesive) inside the small gap.  
         [0052]     Furthermore, the terminal of the first GRIN lens  42  outside the glass tube  41  is connected with a single fiber pigtail  421  through a thermal cure adhesive or a UV cure adhesive. Similarly, the terminal of the second GRIN lens  43  outside the glass tube  41  is connected with a dual fiber pigtail  431  through a thermal cure adhesive or a UV cure adhesive. The resulting package structure of a WDM device of the present invention according to the present embodiment is as shown in  FIG. 4A .  
         [0053]     As shown in  FIG. 4B , the package structure of a WDM device is then inserted into an external metal tube  45  and the two terminals of the external metal tube  45  are respectively covered with the external metal caps  461 ,  462  for protecting the WDM device. (e.g. protecting the WDM device from heat, electromagnetic interference, or impact).  
         [0054]     Therefore, by having the package structure of the present invention illustrated above, the amount of the adhesive leaking during the manufacturing process of the WDM device can be reduced and the yield of the manufacturing process can also be increased. In addition, since the relative position of each optical element (e.g. the first GRIN lens and the second GRIN lens) are determined and maintained by the glass tube, the optical path (e.g. the collimation) is easily maintained once all the optical elements are assembled in their predetermined positions. Hence, by using the package structure of the WDM device of the present invention, the labor for carefully aligning all the optical elements involved can be saved, the manufacturing process of the WDM device of the present invention can be greatly simplified and the yield of the manufacturing process can also be effectively improved.  
         [0055]     With reference to  FIG. 5A , there is shown the fourth preferred embodiment of a package structure of a WDM device of the present invention. The package structure for a WDM of the present embodiment includes a metal tube  51 , a first GRIN lens  52  and a second GRIN lens  53  wherein an IR-cut coating is formed on the surface  532  of the second GRIN lens  53 . As shown in  FIG. 5A , part of the first GRIN lens  52  and part of the second GRIN lens  53  are embedded in the metal tube  51 . The metal tube  51  is made of metal the thermal expansion coefficient of which is low. In the present embodiment, the diameter of the inner wall of the metal tube  51  is about 1.8 mm, while the diameters of the outer surface of the first GRIN lens  52  and the second GRIN lens  53  are both about 1.79 mm. That is, the diameters of the outer surface of these two GRIN lens  52 , 53  are close in size to the diameter of the inner wall of the metal tube  51 . Besides, the terminal of the first GRIN lens  52  outside the metal tube  51  is connected with a single fiber pigtail  521 . Likewise, the terminal of the second GRIN lens  53  outside the metal tube  51  is connected with a dual fiber pigtail  531 .  
         [0056]     In the gap between the outer surface of the first GRIN lens  52  and the inner wall of the metal tube  51 , a thermal cure adhesive  541  is injected for fixing the embedded part of the first GRIN lens  52  to the metal tube  51  securely. Similarly, in the gap between the outer surface of the second GRIN lens  53  and the inner wall of the metal tube  51 , another thermal cure adhesive  542  is injected for fixing the embedded part of the second GRIN lens  53  and the metal tube  51 .  
         [0057]     The package structure of the fourth preferred embodiment of the present invention is manufactured through the following process.  
         [0058]     First, the metal tube  51 , the first GRIN lens  52  and the second GRIN lens  53  the surface  532  of which has an IR-cut coating are provided. The first GRIN lens  52  is embedded into the metal tube  51  partially. As described above, since the diameter of the outer surface of the first GRIN lens  52  (about 1.79 mm) is a little smaller than the diameter of the inner wall of the metal tube  51  (about 1.8 mm), a small gap is formed between the outer surface of the first GRIN lens  52  and the inner wall of the metal tube  51 . Subsequently, after a small amount of liquid-state precursor of a thermal cure adhesive is injected at the seam between the small gap and the ambient outer space. The liquid-state precursor flows into the small gap slowly and stops flowing when the small gap is filled with the liquid-state precursor as a result of the “siphon effect”. Then the liquid-state precursor filling the small gap is heated (thermal curing process). As a result, the liquid-state precursor inside the small gap is transformed into the solid-state thermal cure adhesive  541 . In the present embodiment, an oven can provide the heat required in the thermal curing process.  
         [0059]     After the first GRIN lens  52  is securely fixed to the metal tube  51 , the second GRIN lens  53  is then squeezed into the metal tube  51  through the opening on the other side of the glass tube  51 . Similarly, there is also a small gap located between the outer surface of the second GRIN lens  53  and the inner wall of the metal tube  51 . A small amount of liquid-state precursor of a thermal cure adhesive is injected at the seam between the small gap and the ambient outer space. Again, due to the “siphon effect”, the liquid-state precursor flows into the small gap slowly and stops flowing when the small gap is filled with the liquid-state precursor. Then the liquid-state precursor filling in the small gap is heated (thermal curing process). As a result, the liquid-state precursor inside the small gap is transformed into the solid-state thermal cure adhesive  542  inside the small gap. In the present embodiment, an oven can provide the heat required in the thermal curing process.  
         [0060]     Furthermore, the terminal of the first GRIN lens  52  outside the metal tube  51  is connected with a single fiber pigtail  521  through a thermal cure adhesive or a UV cure adhesive. Similarly, the terminal of the second GRIN lens  53  outside the metal tube  51  is connected with a dual fiber pigtail  531  through a thermal cure adhesive or a UV cure adhesive. The resulting package structure of a WDM device of the present invention according to the present embodiment is as shown in FIG  5 A.  
         [0061]     As shown in  FIG. 5B , the package structure of a WDM device is then inserted into an external metal tube  55  and the two terminals of the external metal tube  55  are respectively covered with the external metal caps  561 ,  562  for protecting the WDM device. (e.g. protecting the WDM device from heat, electromagnetic interference, or impact).  
         [0062]     Therefore, by having the package structure of the present invention illustrated above, the amount of the adhesive leaking during the manufacturing process of the WDM device can be reduced and the yield of the manufacturing process can also be increased. In addition, since the relative position of each optical element (e.g. the first GRIN lens and the second GRIN lens) are determined and maintained by the metal tube, the optical path (e.g. the collimation) is easily maintained once all the optical elements are assembled in their predetermined positions.  
         [0063]     Moreover, since hundreds, even thousands of the metal tubes each having the liquid-state precursor of the thermal cure adhesive in their small gaps can be heated in an oven at the same time, the manufacturing amount of the WDM device having the package structure of the present embodiment can be dramatically increased compared to that of the WDM device having the conventional package structure.  
         [0064]     As a result, by having the package structure of the present invention, the amount of the adhesive leaking during the manufacturing process of a WDM device can be reduced and the yield of the manufacturing process can also be increased. In addition, since the relative position of each optical element is determined and maintained by the tubular fixing unit (a glass tube or a metal tube), the optical path (e.g. the collimation) is easily maintained once all the optical elements are assembled in their predetermined positions. The labor for carefully aligning all the optical elements involved can also be saved. Besides, the manufacturing process of the WDM device of the present invention can be greatly simplified and the manufacturing amount of the WDM device having the package structure of the present embodiment can be dramatically increased relative to that of the WDM device having the conventional package structure.  
         [0065]     Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.