Abstract:
A method and structure for packaging fiber optics devices hermetically are provided. The packaging structure comprises a fiber optics sub-assembly that has one or more fibers extending out, a housing cap, and a sleeve. Sealants are permeated into narrow gaps between the fiber optics sub-assembly and other components through a capillary effect to achieve their tight bonding and air-tightness. The packaging method is different from and superior to conventional methods using a soldering process.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and structure for packaging a fiber optics device of fiber communications, and more particularly to a method and structure utilizing a sealant permeated into narrow gaps between components of the fiber optics device through a capillary effect to achieve a hermetical package.  
       BACKGROUND OF THE INVENTION  
       [0002]     Currently a typical fiber optics device for fiber communications is assembled by first joining optical parts and mechanical parts into sub-assemblies by using sealants. Then a soldering process is conducted to package the sub-assemblies together as a whole into an airtight device.  FIG. 1  is a cross-sectional view of a conventional optical add/drop filter packaged by a soldering process. As shown in  FIG. 1 , various parts are composed together using a sealant into a common port  30  and a transmission port  31  respectively. Then the common port  30  and the transmission port  31  are further packaged together by a soldering process. Within the common port  30 , a dual-fiber pigtail  2 , a pair of fibers  3   a  and  3   b , a GRIN lens  4 , a first glass tube  8   a , and a filter  5  are joined together to form a dual-fiber collimator. On the other hand, within the transmission port  31 , a GRIN lens  6 , a single-fiber pigtail  7 , a fiber  3   c , and a second glass tube  8   b  are joined together to form a single-fiber collimator. In a typical operation of the optical add/drop filter, a multi-wavelength light beam shoots into the common port  30  via the fiber  3   b . A light with a particular wavelength transmits through the filter  5  and is then focused by the second GRIN lens  6  on an end of the single fiber pigtail  7  adjacent to the GRIN lens  6 . The light then emanates out from the fiber  3   c . In addition, the filter  5  reflects lights with other wavelengths and they are focused by the first GRIN lens  4  on an end of the dual fiber pigtail  2  adjacent to the GRIN lens  4 . The lights then emanate out from the fiber  3   a.    
         [0003]     The functionality and long-term stability of a fiber optics device such as the optical add/drop filter are highly sensitive to the air-tightness of the device. As shown in  FIG. 1 , narrow gaps (about 0.005˜0.3 mm) exist between the first glass tube  8   a  and the dual-fiber pigtail  2 , and between the first glass tube  8   a  and the first GRIN lens  4 . The gaps are filled with a sealant through a capillary effect to achieve bonding and air-tightness. The first glass tube  8   a  is then inserted into a metallic tube  9   a  and a narrow gap (about 0.005˜0.3 mm) therebetween is also filled with a sealant to achieve tight bonding and air-tightness. Similarly, a sealant is filled into narrow gaps between the second glass tube  8   b  and the single-fiber pigtail  7 , and between the second glass tube  8   b  and the second GRIN lens  6 . The second glass tube  8   b  is then inserted into a metallic tube  9   b  and a narrow gap therebetween is also filled with a sealant to achieve hermetical packaging. Then the common port  30  and transmission port  31  are further packaged in a housing tube  11 . Before fixing the relative positions of the two ports in the housing tube  11 , the two ports are usually shifted and tilted so that the light with the particular wavelength emanating out of the common port  30  and entering into the transmission port  31  should have the maximum intensity (i.e., minimum insertion loss). In other words, the two ports may not be aligned on a same axis. Larger gaps are therefore reserved for position adjustment between the housing tube  11  and the first metallic tube  9   a , and between the housing tube  11  and the second metallic tube  9   b . A typical procedure is to adjust the two metallic tubes  9   a  and  9   b  dynamically until their coupling can achieve the maximum light intensity inside the housing tube  11 . Then a solder  12  is used to join and seal the housing tube  11  and the first metallic tube  9   a , and the housing tube  11  and the second metallic tube  9   b , respectively.  
         [0004]     Based on the foregoing description, the package of a typical fiber optics device such as an optical add/drop filter according to a prior art is first to form tightly bonding sub-assemblies by permeating sealants into narrow gaps between various components of the sub-assemblies. Then a soldering process is used to join these sub-assemblies together as a whole into an airtight device. However the soldering process has a number disadvantages. First, during the manufacturing process the heat generated by the soldering process would affect the device components and the light coupling to adjust the relative positions of sub-assemblies becomes difficult, which is not an easy task. The soldering process will also introduce extra stresses into the device, which will be released gradually afterwards and the functionality and long-term stability of the device will therefore be affected. In addition, two additional metallic tubes and two additional glass tubes are required. Moreover, the metallic tubes and the housing tube have to be plated with gold for alloying with the solder tin. These not only increase the dimension of the device, but also increase its material cost.  
       SUMMARY OF THE INVENTION  
       [0005]     This present invention is directed to obviate the disadvantages of using a soldering process in the package of conventional fiber optics devices. These disadvantages include: 
    (a) The heat generated by the soldering process during the manufacturing process would affect the device components and the light coupling to adjust the relative positions of sub-assemblies becomes difficult.     (b) The soldering process will introduce extra stresses into the device, which will be released gradually afterwards and the functionality and long-term stability of the device will therefore be affected.     (c) Two additional metallic tubes and two additional glass tubes are required. Moreover, the metallic tubes and the housing tube have to be plated with gold for alloying with the solder tin. These not only increase the dimension of the device, but also increase its material cost.    
 
         [0009]     To obviate the foregoing disadvantages, a packaging method according to the present invention mainly comprises the following steps: 
    (a) Prepare a fiber optics sub-assembly with a specific function that has one or more fibers extending from its both ends.     (b) Insert a first end of the sub-assembly into a housing cap and fill the narrow gap between the housing cap and the sub-assembly with a sealant to achieve their tight bonding and air-tightness.     (c) Reserve a section (whose length is d1) of the fibers outside a second end of the sub-assembly.     (d) Strip the protective coating of a section of the fibers, starting from a position that has a distance d1 from the second end of the sub-assembly, up to a length d2.     (e) Insert the second end of the sub-assembly into a hole of a sleeve whose aperture only allows the fibers to pass through so that the stripped sections of the fibers are surrounded entirely by the sleeve, and fill the narrow gap between the stripped fibers and the sleeve hole with a sealant to achieve their tight bonding and air-tightness.     (f) Surround the housing cap and the sleeve with a metal housing tube and fill the narrow gaps between the metal housing tube and the housing cap, and between the metal housing tube and the sleeve with a sealant to achieve their tight bonding and air-tightness.    
 
         [0016]     Compared to the prior arts, the present invention basically permeates sealants into the narrow gaps between various device components so that the device can be achieved hermetical packaging. As a soldering process is avoided during light aligning, a fiber optics device with better optical performance, long-term stability, and lower cost can be obtained.  
         [0017]     The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a cross-sectional view of a conventional optical add/drop filter packaged by a soldering process.  
         [0019]      FIG. 2  is a schematic diagram showing various components of a miniature optical add/drop filter.  
         [0020]      FIG. 3  is a cross-sectional view of a miniature optical add/drop filter packaged according to a first embodiment of the present invention.  
         [0021]      FIG. 4  is a schematic diagram showing, in a miniature optical add/drop filter packaged according to a first embodiment of the present invention, a section of a fiber is reserved behind a fiber optics sub-assembly to buffer the stress resulted from temperature variations.  
         [0022]      FIG. 5  shows the relationship of reserved length d1 of the fiber  272  versus thermal expansion coefficient of the metal housing tube  243  under the conditions that the inner section  320  has a length 20 mm, the thermal expansion coefficient of the fiber optics sub-assembly is 7×10 −6 /° C., and the thermal expansion coefficient of the fiber is 0.5×10 −6 /° C.  
         [0023]      FIG. 6  is a cross-sectional view of a multi-port fiber optics device packaged according to a first embodiment of the present invention.  
         [0024]      FIG. 7  is a cross-sectional view of a fiber optics assembly with sleeves at its both ends according to a second embodiment of the present invention.  
         [0025]      FIG. 8  is a cross-sectional view of a fiber optics device packaged according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]      FIG. 2  is a schematic diagram showing various components of a miniature optical add/drop filter  310  comprising a dual-fiber pigtail  210 , a first GRIN lens  200 , a wavelength-division multiplexing (WDM) filter  230 , a second GRIN lens  201 , a single-fiber pigtail  220 , and fibers  270 ,  271  and  272 . An adhesive  250  is applied at the interfaces between various components for intensification of the interfacings.  FIG. 3  is a cross-sectional view of a miniature optical add/drop filter packaged according to the present invention. As shown in  FIG. 3 , the packaging is conducted as follows. The dual-fiber pigtail  210  is inserted into a housing cap  241  whose length is d3. The housing cap  241  should be made of a material such as metal, glass, or ceramic, that is completely moisture-proof. The housing cap  241  has an appropriate thermal expansion coefficient and is not easy to rust. A narrow gap  291  (about 0.005˜0.3 mm) could exist between the housing cap  241  and the dual-fiber pigtail  210 . A sealant such as epoxy resin is then used to permeate into the gap  291  through a capillary effect to achieve tight bonding and air-tightness. The output fiber  272  extends from an output end of the single-fiber pigtail  220  for an appropriate distance. After a distance d1 away from the pigtail  220 , a protective coating outside a section of fiber  272   a  is stripped for a length d2. The protective coating is usually made of acrylic for protecting the fiber inside. However the protective coating is usually too soft to have a strong bonding with the sealant and therefore has to be stripped. Then the fiber  272  is slipped into a hole  245  of a sleeve  242  whose aperture only allows the fiber  272  to pass through. The sleeve  242  is made of a same material as the housing cap  241  and has a length slightly greater than d2 so that the section  272   a  can be surrounded entirely. A narrow gap  294  (about 0.005˜0.3 mm) would exist between the sleeve  242  and the fiber  272   a . A sealant is then used to permeate into the gap  294  to achieve tight bonding, air-tightness, and protection of the exposed fiber  272   a . In the end, the housing cap  241  and the sleeve  242  are surrounded with a housing tube  243 . A narrow gap  292  (about 0.005˜0.3 mm) would exist between the housing tube  243  and the housing cap  241 , and between the housing tube and the sleeve  242 . A sealant is then used to permeate into the gap  292  to achieve tight bonding and air-tightness for the whole device. The housing tube  243 , besides being completely moisture-proof, not easy to rust, and with appropriate strength, should have a compatible thermal expansion coefficient with those of other components.  
         [0027]     As shown in  FIG. 3 , the miniature optical add/drop filter  310  is confined inside an inner section  320  of the package by the housing cap  241 , the sleeve  242 , and the housing tube  243 . The materials for the housing cap  241 , the sleeve  242 , and the housing tube  243  should be chosen to have their thermal expansion coefficients compatible with that of the fiber optics sub-assembly  310  so that, under temperature variations, stresses between them can be reduced. The thermal expansion coefficient of the fiber optics sub-assembly  310  is about 5×10 −6 ˜9×10 −6 /° C., derived from a weighted computation including individual thermal expansion coefficient of every sub-assembly component. A material for the housing tube  243  therefore is better to have its thermal expansion coefficient within the range 5×10 −6 ˜9×10 −6 /° C. In general, the difference in terms of thermal expansion coefficients among the housing tube and the fiber optics sub-assembly is better under 30×10 −3 /° C. and the smaller the better (as shown in  FIG. 5 ).  
         [0028]     In addition, the section of the fiber  272  whose length is d1 is reserved to buffer the stress resulted from temperature variations. Due to a flexibility of the fiber  272 , this section of the fiber  272  will be bended as the fiber  272  is under compression resulted from a temperature dropping from a high temperature to a low temperature and the housing tube  243  contracting more than the fiber optics sub-assembly  310  does. As shown in  FIG. 4 , the fiber  272  is bended into  272   c . If the curvature of  272   c  has a diameter larger than 40 mm, such a bending will not cause any damage or functional degradation to the fiber optics device.  FIG. 5  shows the relationship of the reserved length d1 of the fiber  272  versus the thermal expansion coefficient of the housing tube  243  under the conditions that the inner section  320  has a length 20 mm, the thermal expansion coefficient of the fiber optics sub-assembly  310  is 7×10 −6 /° C., and the thermal expansion coefficient of the fiber is 0.5×10 −6 /° C. As shown in  FIG. 5 , the reserved length d1 of the fiber  272  has to be longer as the materials used for the metal housing tube  243  has a thermal expansion coefficient more greater than that of the fiber optics sub-assembly  310 .  
         [0029]     The packaging structure according to the present invention can be applied to the packaging of other fiber optics devices besides the miniature 3-port optical add/drop filter described above. Examples of these fiber optics devices include, but are not limited to, multi-port optical add/drop filters, optical couplers, optical isolators, polarization beam splitters, or other fiber optics sub-assemblies composed of hybrid components.  FIG. 6  is a sectional view of a multi-port fiber optics device packaged according to a first embodiment of the present invention.  FIG. 6  has a structure almost identical to that of  FIG. 3 . The differences lie in that a fiber optics sub-assembly  330  has two fibers  272  and  273  extending out of a second end of the sub-assembly  330 . Protective coatings of the fiber  272  and  273  are stripped for a length d2 starting from an appropriate distance d1 after the second end of the sub-assembly  330  and therefore expose fiber sections  272   a  and  273   a . A hole  245  at the center of the sleeve  242  has an aperture only big enough to allow fibers  272  and  273  to pass through. The sub-assembly  330  is a fiber optics assembly with a specific function and it can be one of the various product types mentioned above. Based on its product type, the sub-assembly  330  can have one or more fibers extending out of its both ends.  
         [0030]      FIG. 7  is a cross-sectional view of a fiber optics sub-assembly  330  packaged with sleeves at its both ends according to a second embodiment of the present invention. The sub-assembly components are joined together as what is shown in  FIG. 3 . As this structure is more susceptible to external impacts, the package is filled with a softer buffer material  400  such as silicon or rubber.  
         [0031]      FIG. 8  shows a fiber optics device packaged according to a third embodiment of the present invention. As shown in  FIG. 8 , a fiber optics sub-assembly  352  comprising VCSEL, receiver, or MEMS is first positioned and fixed to a fiber optics collimator  300  to achieve an optimal light coupling effect. Then the collimator  300  is fixed to a TO-Can  351  and they are slipped into a housing tube  243  together. A narrow gap  295  (about 0.005˜0.3 mm) between the metal housing tube  243  and the TO-Can  351  is then filled with a sealant to achieve tight bonding and air-tightness. The other end of the collimator  300  is packaged in a same way as what is shown in  FIG. 6 .  
         [0032]     Referring to  FIGS. 3, 6 ,  7 , and  8 , if the lengths of the housing cap  241 , the sleeve  242 , and the TO-Can  351  (i.e., d3, d2, and d4, respectively) are extended longer, then their contact surfaces with the fiber optics sub-assembly  310 ,  330 , and the metal housing tube  243  will become larger, and an even better tight bonding and air-tightness can be achieved.  
         [0033]     Using sealants in the aforementioned assembly methods will contribute to a lower cost. However, if cost is not an issue, some variations can be applied to the assembly methods based on a same packaging structure described above. In  FIGS. 3, 6 ,  7 , and  8 , tight bonding and air-tightness between the housing tube  243  and the housing cap  241 , and between the housing tube  243  and the sleeve  242  can also be achieved using tin soldering or laser welding. The difference between the tin soldering or laser welding here and those used in prior arts lies in that no light coupling is required in the packaging structures according to the present invention as the light coupling is already done between the components of the fiber optics sub-assemblies  310  and  330 . Attention therefore only has to be focused on not to bend the fibers severely. In this way, fiber optics devices can be packaged quickly without sacrificing their optical performance. Similarly, tin soldering or glass soldering can also be used between the sleeve  241  and the fibers  272   a  and  273   a  for fast packaging.  
         [0034]     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.