Abstract:
An improved wavelength division multiplexing (WDM) coupler includes a dual fiber collimator assembly ( 27 ), a filter device ( 29 ), and a single fiber collimator assembly ( 31 ). The filter device comprises a holder ( 26 ) and a filter ( 24 ) received therein. The holder has a first opening ( 21 ) and a second opening ( 23 ) communicating therewith. The holder firmly secures both the dual fiber collimator assembly and the single fiber collimator assembly therein. A relative position and orientation of a first GRIN lens ( 22 ) and the filter can be adjusted during assembly by sliding the first GRIN lens in or out of and by microtilting the GRIN lens in the second opening while monitoring reflection insertion loss. A DWM coupler which is more easily adjusted during assembly and which has a more stable structure is thus obtained.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to a wavelength division multiplexing (WDM) coupler, and more particularly to a WDM coupler which is adjustable during assembly and which has a stable structure after assembly. 
   2. Related Art 
   In optical fiber technology, wavelength division multiplexing (WDM) couplers are used to combine or separate optical signals having different wavelengths. As WDM couplers are being more broadly applied in the telecommunications, data communications and cable television (CATV) industries, WDM couplers having higher levels of performance and reliability are required. 
   A WDM coupler comprises two collimator assemblies and a filter member. To minimize transmission losses of light signals, distances between and relative orientations of the two collimator assemblies and the filter member have to be fixed precisely. 
   Referring to  FIG. 4 , a conventional WDM coupler disclosed in U.S. Pat. No. 6,343,166 comprises two collimator assemblies (not labeled), a filter  624 , and a filter holder  626 . The filter  624  is partially received in the filter holder  626  to form a filter subassembly (not labeled). An aperture  627  within the filter holder  626  extends between the filter  624  and lens  622 . The filter subassembly is aligned with the two collimator assemblies to transmit light signals having a preselected wavelength and to reflect all other. However, the filter holder  626  has a predetermined structure and the aperture  627  has a predetermined length, which limits readjustments of the distance between the filter  624  and the lens  622 . 
   Another conventional WDM coupler is disclosed in U.S. Pat. No. 6,282,339. Although the disclosed coupler also has two collimator assemblies and a filter, the filter is directly attached to a face of a collimating lens using adhesive. The adhesive can act as a contaminant between the lens and filter, which can cause problems. 
   An improved WDM coupler which is efficiently adjustable during assembly and which has a stable structure after assembly is desired to overcome the above-mentioned problems. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide an improved WDM coupler which is efficiently adjustable during assembly and which has a stable structure after assembly. 
   To achieve the above-mentioned objects, a wavelength division multiplexing (WDM) coupler comprises a dual fiber collimator assembly, a filter device, and a single fiber collimator assembly. The dual fiber collimator assembly comprises a dual fiber ferrule and a first GRIN lens. The filter device comprises a holder and a filter received therein. The holder has a first opening at one end and a second opening at another end which communicates with the first opening. The single fiber collimator assembly comprises a single fiber ferrule and a second GRIN lens. A metal ring surrounding an end of the first GRIN lens is used to engage with the second opening of the holder, thereby helping to fix alignment of the first GRIN lens with the filter. A relative position between the first GRIN lens and the filter is adjusted during assembly to obtain optimal optical performance, and then the first GRIN lens is secured in the holder, fixing that relative position. Next, an end of the second GRIN lens is secured in the first opening of the holder, while signals transmitted from the first GRIN lens to the second GRIN lens are monitored to assure a minimum insertion loss of no greater than about 0.3 dB. A DWM coupler having a more stable structure is thus obtained. The DWM coupler is also more easily adjusted during assembly than those of the prior art. 
   Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of a preferred embodiment of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of an incompletely assembled wavelength division multiplexing (WDM) coupler according to a preferred embodiment of the invention; 
       FIG. 2  is a cross-sectional view of the assembled WDM coupler of  FIG. 1 ; 
       FIG. 3  is a perspective view of a filter holder used in the WDM coupler of  FIG. 1 ; and 
       FIG. 4  is a cross-sectional view of a three-port filter assembly of the prior art. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to FIG.  1  and  FIG. 2 , there is shown a wavelength division multiplexing (WDM) coupler  10 . The WDM coupler  10  comprises a dual fiber collimator assembly  27 , a single fiber collimator assembly  31  and a filter device  29 . The filter device  29  is arranged between the dual fiber collimator assembly  27  and the single fiber collimator assembly  31 . A metal tube  30  surrounds the dual fiber collimator assembly  27 , the filter device  29  and the single fiber collimator assembly  31  to fix them therein. 
   The dual fiber collimator assembly  27  comprises a dual glass ferrule  16 , a first graded index (GRIN) lens  22 , a glass tube  14 , and a metal housing  12 . The dual glass ferrule  16  has a left end (not labeled) and a right end (not labeled) and defines a passageway  161  therebetween. The passageway  161  receives an input optical fiber  18  and a reflective optical fiber  20  therein. The left end of the ferrule  16  together with ends of the input optical fiber  18  and the reflective optical fiber  20  held within the ferrule  16  is polished at a 7 to 8 degree angle with respect to a plane constructed perpendicular to a longitudinal axis of the dual fiber collimator assembly  27 . A right end of the first GRIN lens  22  is ground at an angle identical to that of the left end of the ferrule  16  and is spaced a predetermined distance from the left end of the ferrule  16 . The dual glass ferrule  16  and the first GRIN lens  22  are aligned and fixed in the glass tube  14  and the glass tube  14  is surrounded by the metal housing  12  for further protection. A left end portion of the first GRIN lens  22  is exposed outside the metal housing  12  and a metal ring  25  surrounds the first GRIN lens  22  at the left end thereof. 
   The filter device  29  comprises a holder  26  and a filter  24 . The holder  26  receives the filter  24  at one end and is engaged with the metal ring  25  at the other end. Referring to  FIG. 3 , the holder  26  is cylindrically-shaped and has a tubular sidewall (not labeled), an open right end (not labeled), and an annular-shaped left end (not labeled) which defines a first opening  21  through its center. The open right end and an inside of the holder  26  will be designated as a second opening  23 . The first opening  21  is in communication with the second opening  23 . A diameter of the first opening  21  is smaller than a diameter of the second opening  23 . The filter  24  is attached to an inside wall (not labeled) of the annular-shaped left end, adjacent to and covering the first opening  21 . An outer diameter of the filter  24  is smaller than the diameter of the second opening  23  and bigger than the diameter of the first opening  21 , such that light from the first GRIN lens  22  passes through the filter  24  and the first opening  21  to the single fiber collimator assembly  31 . The second opening  23  receives and is engaged with the metal ring  25  surrounding the first GRIN lens  22 . An outer diameter of the metal ring  25  is smaller than the diameter of the second opening  23  so that the dual fiber collimator assembly  27  can be adjusted up and down, back and forth, and can be tilted in a certain range. A plurality of slots  231  is defined inside the open right end of the holder  26 , which is advantageous to dispose adhesive to secure the metal ring  25 . 
   The single fiber collimator assembly  31  comprises a single glass ferrule  32 , a second GRIN lens  34 , a glass tube  36  and a metal housing  37 . The single glass ferrule  32  has a left end (not labeled) and a right end (not labeled) and defines a passageway  321  therebetween. The passageway  321  receives an output optical fiber  38 . The right end of the ferrule  32  together with the end of the output optical fiber  38  held within the ferrule  32  is polished at a 7 to 8 degree angle with respect to a plane constructed perpendicular to a longitudinal axis of the single fiber collimator assembly  31 . The left end of the second GRIN lens  34  is ground at an angle identical to that of the right end of the ferrule  32  and is spaced a predetermined distance from the right end of the ferrule  32 . The single glass ferrule  32  and the second GRIN lens  34  are aligned and fixed in the glass tube  36  and the glass tube  36  is surrounded by the metal housing  37  for further protection. A right end portion of the GRIN lens  34  is exposed outside the glass tube  36  and the metal housing  37 . 
   In assembly, the filter  24  is attached to the inside wall of the annular-shaped left end of the holder  26 , adjacent to and covering the first opening  21  using silicon adhesive. The dual fiber collimator assembly  27  and single fiber collimator assembly  31  are also assembled using adhesives. The first GRIN lens  22  surrounded by the metal ring  25  is then inserted into the second opening  23  of the holder  26  after the dual fiber collimator assembly  27  is properly assembled and secured by silicon adhesive. A relative position and annular orientation between the first GRIN lens  22  and the filter  24  is adjusted while signals transmitted from the input optical fiber  18  to the reflective optical fiber  20  are being monitored. When a position and orientation is achieved at which monitored reflection insertion losses are at a minimum, the holder  26  is fixed to the metal ring  25  using silicon adhesive injected into the slots  231 . The dual fiber collimator assembly  27  with the attached filter device  29  is then,positioned in the metal tube  30 . Then the single fiber collimator assembly  31  is inserted into the tube  30  and is aligned with and inserted into the first opening  21 . The orientation and position of the single fiber collimator assembly  31  is adjusted relative to the filter  24  while the signals transmitted through the input optical fiber  18  to the output optical fiber  38  are being monitored. When indicated insertion losses are minimized at a value no greater than about 0.3 dB, the dual fiber collimator assembly  27  with the filter device  29  and the single fiber collimator assembly  31  are secured within the metal tube  30 . 
   In operation, light signals are transmitted from the input optical fiber  18  through the first GRIN lens  22 , collimates the light signals. Light signals of a wavelength determined by the properties of the filter  24  passes through the filter  24 . All other wavelengths are reflected back through the GRIN lens  22  and are coupled into the reflective optical fiber  20 . The light signals passes by the filter  24  are coupled into the output optical fiber  38 . 
   Compared with a WDM coupler of the prior art, the WDM coupler  10  is easier to adjust and to align during assembly. The distance between the first GRIN lens  22  and the filter  24  can be adjusted to be shorter than that indicated by the U.S. Pat. No. 6,343,166, and this should result in a lower value of insertion losses and in greater thermal stability. Furthermore, unlike the U.S. Pat. No. 6,282,339, contamination of the first GRIN lens  22  is avoided since the filter  24  is not attached to the first GRIN lens  22 . Moreover, the first GRIN lens  22  can be micro-adjusted to align with the filter  24  at different angular orientations, thereby further lowering return losses. The right end of the second GRIN lens  34  snuggly fits into the first opening  21  of the holder  26 , so that the second GRIN lens  34  is firmly secured by the holder  26  at one end. A DWM coupler having a more stable structure is thus obtained, and this DWM coupler is easy to adjust during assembly. Furthermore, the holder  26  and the metal ring  25  are both made of metal having excellent conductivity, which is advantageous for the thermal stability of the WDM coupler  10 . 
   It is believed that the present invention and its advantages will be understood from the foregoing description and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.