Abstract:
A highly reliable optical add/drop device is described. The optical add/drop device has an external tube and a ring, wherein the external tube and the ring are made of metallic material. A WDM filter is fixed in the ring. The ring is inserted in the external tube at the middle portion and fixed therein by soft solder technique. A single fiber collimator and a dual fiber collimator are respectively coupled with the opposite surfaces of the filter, and aligned in the external tube. Additionally, the single and dual fiber collimators are also fixed therein by soft solder technique, as two metal tubes respectively hold the single and dual fiber collimators. In the invention, the WDM filter is rigidly fixed to avoid tilting during temperature variation. Moreover, the invention provides an optical add/drop device with low insertion loss and reflection loss.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to an optical device. More particularly, the present invention relates to a highly reliable optical add/drop device. 
     2. Description of Related Art 
     In optical fiber technology, optical add/drop (OAD) devices, sometimes referred as wavelength division multiplexed units, are used to combine or separate optical signals having different wavelengths. The OAD devices have been utilized to significantly enhance the signal capacity of optical communication systems. An OAD system simultaneously transmits multiple information signals on a single waveguide medium at different wavelengths or channels. Examples of such communication systems include telecommunication systems, cable television systems, local area networks (LANs) and wide area networks (WANs). As highly developing of optical communication, the requirements for OAD devices with high performance and reliability are increased. 
     The reliability of OAD devices generally depends on the designing and packaging technology. In conventional OAD devices, one way to assemble the OAD devices is bonding all optical parts together by applying epoxy. A typical structure of the OAD device includes a dual fiber collimator with a first gradient index (GRIN) lens, a wavelength division multiplexed (WDM) filter and a single fiber collimator with a second GRIN lens. A light beam carried on a plurality of wavelengths are multiplexed together or demultiplexed separately for transmission by the OAD devices. A light beam having different wavelengths is input into a conventional OAD device via one fiber of the dual fiber collimator. The light beam is transmitted to the first GRIN lens for collimating, and then incident on the WDM filter. The light passing through the WDM filter is transmitted to the single fiber collimator. The remainder of the composite signal is reflected back to the first GRIN lens and then transmitted to another optical fiber. 
     In one type of conventional OAD device, the WDM filter is fixed on one end surface of the first GRIN lens based on epoxy bonding. Therefore, the angle between the filter and the incident light is adjusted to achieve a lowest transmission loss, and fixed. Thereafter, the single fiber collimator and the dual fiber collimator are then packaged by applying a heat-curing epoxy to assemble the OAD device. The conventional packaging structure provides the OAD devices with good performance. However, the OAD devices according to conventional method have a risk of failure when they are operated during temperature variation. In general, the epoxy applied on the edge of the filter and the first GRIN lens non-uniformly expands or shrinks during temperature variation, and tilts the filter. Further, the light beam emitted from the WDM filter may deviate, and thus the insertion loss and reflection loss are increased. Because of the expansion and shrinkage problem of the epoxy, there is still a need in the art to provide new packaging structure by decreasing the usage of epoxy to obtain highly reliable operation. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a reliable OAD device in which a ring is used to mount a filter therein, and thereby prevent the filter from tilting. 
     It is another object of the present invention to provide a reliable OAD device, in which by soldering the ring with an external tube via the openings at the middle sidewall of the external tube, the ring with the filter therein and other optical parts of the OAD device can be packaged rigidly. Hence, the OAD device can be reliably performed during temperature variation. 
     In one aspect, the present invention provides a structure for packaging an optical add/drop (OAD) device with a filter. The structure includes a ring and an external tube. At least three corners of the filter contact the inner sidewall of the ring, and the filter tightly wedges in the ring. The external tube has an inner diameter substantially equal to the outer diameter of the ring. The external tube has at least one opening at the middle sidewall, and the ring is positioned in the external tube to block the opening. Moreover, the ring is soldered with the external tube to combine them together. 
     In another aspect, the present invention provides an optical add/drop device that includes a filter, a ring, an external tube, a single fiber collimator and a dual fiber collimator. At least three corners of the filter contact the inner sidewall of the ring, and the filter wedges in the inner hole of the ring. An external tube has an inner diameter substantially equal to the outer diameter of the ring. The external tube has a middle group of openings in the middle sidewall, and two groups of openings adjacent to the terminal ends of the external tube respectively. As the ring is positioned at the middle of the external tube to block the center openings, the sidewall of the ring is seen via the center openings. The ring is soldered with external tube via the center openings so as to fix the ring in the external tube. The single fiber collimator and the dual fiber collimator are coupled to each other in the external tube by soldering the inner metal tubes with the external tube via the another two groups of openings, respectively. 
     According to the OAD device of the present invention, the filter can be well fixed in the external tube through the ring. By soldering the ring, the inner metal tube of the dual fiber collimator and the inner metal tube of the single fiber collimator with the external tube via the openings, the optical parts can be well assembled inside the external tube. Therefore, optical path shift in conventional OAD device caused by the degradation and deformation of epoxy can be eliminated to attain high performance and reliability. 
     The feature of the invention is that the diameter of each center openings is less than the thickness of the ring. As the external tube and the ring are made of metal, the ring and the external tube can be soldered together via the middle openings without utilizing any adhesive material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, wherein: 
     FIG. 1 is a schematic perspective view of an external tube according to one preferred embodiment of the present invention; 
     FIG. 2 is a schematic perspective view of a fixing ring according to one preferred embodiment of the present invention; 
     FIG. 3 is a schematic cross-sectional view of the fixing ring of FIG. 2; and 
     FIG. 4 is a schematic cross-sectional view of an optical add/drop device according to one preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention provides an optical add/drop (OAD) device that includes a ring and an external tube to assemble all optical parts into a fixed optical path in a high thermal environment and during long-term operation without a heat absorption problem, thereby to obtain high reliability. 
     FIG. 1 is a schematic perspective view of an external tube according to one preferred embodiment of the present invention. Referring to FIG. 1, the external tube  100  is a cannular tube, of which the center portion is hollow, so the external tube  100  has a center hole  110  in the center of the tube. The inner diameter of the external tube  100  is Dit. The external tube  100  has a plurality of openings  120  on the sidewall of the external tube  100 . Among the openings  120 , a first group of the openings  124  is set circularly around the annular sidewall. The first group of the openings  124  may include four circular openings that are used to tetragonal-fix an optical part inside the external tube  100 . The second and third groups of the openings  122 ,  126  are set circularly around the annular sidewall of the external tube  100  adjacent to the terminal end of the external tube  100 , respectively. In other words, the second group of the openings  122  is set adjacent to one end of the external tube  100 , and the third group of the openings  126  is set adjacent to the other end. The second and third groups of the openings  122 ,  126  may also include four circular holes to fix optical parts, respectively. 
     FIG. 2 is a schematic perspective view of a ring according to one embodiment of the present invention. FIG. 3 is a schematic cross-section corresponding to FIG.  2 . Referring to FIGS. 2 and 3 simultaneously, the ring  200  is circular, and made of a metal, a glass, or other compatible materials. The ring  200  has an outer diameter Def. The outer diameter Def of the ring  200  is substantially equal to the inner diameter Dit of the external tube  100 . The ring  200  has an interior hole  222 , and the inner diameter of the ring  200  is Dif. In the invention, the external tube  100  and the ring  200  are preferably made of metallic material, such as stainless steel. 
     An optical filter  300  is positioned into the interior hole  222  of the ring  200 . The filter  300  includes a wavelength division multiplexed (WDM) filter, a dense WDM (DWDM) filter, a coarse WDM (CWDM), a wide-band WDM (WWDM) or a narrow-band WDM (NWDM). The filter  300  typically has a tetragonal structure as shown in FIG.  2 . The diagonal length W of the filter  300  is substantially equal to the inner diameter Dif of the ring  200 . The filter  300  is wedged in the interior hole  222  tightly such that at least three corners of the filter  300  touch the sidewall of the interior hole  222 . Additionally, an adhesive material (not shown), such as heat-curing epoxy, is applied to the corners of the filter  300  and the gap between the filter  300  and the sidewall of the ring  200  so as to ensure the filter  300  being fixed in the ring  200 . Consequently, the filter  300  doesn&#39;t directly adhere to the GRIN lens of the collimator. The filter  300  is steadily fixed in the ring  200 , even if the epoxy has a little thermal deformation, so that optical path deviation problem will not occur. 
     Referring to FIG. 3, when the filter  300  is fixed in the ring  200 , the filter  300  can be stopped by a stop portion  210 . The stop portion  210  is at one side of the interior hole  222 , at the end of the filter  300 , thereby to assist in the mounting of the filter  300  in the ring  200 . For example, as the shape of the stop portion  210  is circular, the hole  224  inside the portion  210  is smaller than the interior hole  222 . Except for the circular shape, the stop portion  210  can be at least two protrusions to stop the filter  300  at the desired position in the interior hole  222 . Of course, the stop portion  210  can be modified to other shapes without blocking light from traveling through the ring  300 . 
     FIG. 4 is a schematic cross-section of an OAD device according to one embodiment of the present invention. Referring to FIG. 4, after the filter  300  is fixed in the ring  200 , the ring  200  holding the filter  300  is placed in the external tube  200 . Next, the ring  200  is positioned in the external tube  100  corresponding to the position of the middle group of the openings  124 . Because the thickness of the ring  200  is larger than the diameter of each opening  124 , the ring  200  blocks the middle group of the openings  124 . Any of various fusible materials  134 , such as tin or lead, is positioned in the middle group of the openings  124 , and heated so as to solder the ring  200  and the external tube  100  together. In another way, the ring  200  holding the filter  300  therein can be fixed in the external tube  100  by applying adhesive material in the middle group of the openings  124 . Consequently, the temperature variation cannot tilt the filter  300 , because the filter  300  is rigidly fixed in the external tube  100 . 
     After fixing the ring  200  in the external tube  100 , a dual fiber collimator  400  and a single fiber collimator  500  are packaged into the external tube  100  to form the OAD device of the present invention. The dual fiber collimator  400  includes a first glass ferrule  410  grasping a pair of fibers  412 ,  414 , a first GRIN lens  420 , and a first glass tube  430  holding the first glass ferrule  410  and the first GRIN lens  420 . The first glass ferrule  410  grasping the fibers  412 ,  414  is aligned with the first GRIN lens  420  within the first glass tube  430 . After the dual fiber collimator  400  obtains a lowest reflection loss, the first glass ferrule  410  and the first GRIN lens  420  are fixed in the first glass tube  430  by using adhesive material. Further, a first metal tube  440  having an outer diameter equal to the inner diameter of the external tube  100  is employed to sheathe the first glass tube  430 , wherein an adhesive material (not shown) is applied between the first metal tube  440  and the first glass tube  430  to combine them. The dual fiber collimator  400  is inserted in one end of the external tube  100  adjacent to the filter  300 , and blocks the second group of the openings  122 . In this case, the gap between the dual fiber collimator  400  and the filter  300  tends to zero. Next, the fusible material  134  mentioned above is positioned in the second group of the openings  122 , and heated so as to solder the first metal tube  440  and the external tube  100  together. In another way, the dual fiber collimator  400  having the first metal tube  440  can be fixed in the external tube  100  by applying adhesive material in the second group of the openings  124 . 
     Similarly, the single fiber collimator  500  includes a second glass ferrule  510  grasping a fiber  512 , a second GRIN lens  520 , and a second glass tube  530  holding the second glass ferrule  510  and the second GRIN lens  520 . The second glass ferrule  510  grasping the fiber  512  and the second GRIN lens  520  are inserted in the second glass tube  530 , and then a beam transmitted in the fiber  512  is incident on the second GRIN lens  520 . After obtaining collimating light from the second GRIN lens  520 , the second glass ferrule  510  and the second GRIN lens  520  are fixed in the second glass tube  530  by using adhesive material. A second metal tube  540  having an outer diameter equal to the inner diameter of the external tube  100  is employed to sheathe the second glass tube  530 , wherein an adhesive material (not shown) is applied between the second metal tube  540  and the second glass tube  530  to combine them. The single fiber collimator  500  is inserted in the other end of the external tube  100  opposite to the dual fiber collimator  400 . Next, the single fiber collimator  500  is aligned with the dual fiber collimator  400  to obtain the lowest insertion loss. After obtaining the lowest insertion loss and the optimum position of the single fiber collimator  500  relative to the dual fiber collimator  400 , the fusible material  134  mentioned above is positioned in the third group of the openings  126 , and heated so as to solder the second metal tube  540  and the external tube  100  together. In another way, the single fiber collimator  500  having the second metal tube  540  can be fixed in the external tube  100  by applying adhesive material in the third group of the openings  126 . 
     According to above description, the present invention provides an OAD device, which has a ring for fixing a filter therein and an external tube with openings on the sidewall. The ring fixing a filter therein, the dual fiber collimator and the single fiber collimator are soldered with the external tube, so all of the optical parts are steadily packaged inside the external tube. The OAD device of the invention has better reliability in the long-time high-power operation. 
     As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. It is intended that various modifications and similar arrangements included within the spirit and scope of the appended claims be covered, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.