Abstract:
A compact PD unidirectivity solution for an optical tap monitor, which reduces the overall size of optical tap module, is provided. The solution is to use lensing to separate the light from the input and output fibres, and then add a mask or spacer in front of the monitor PD to prevent any of the light from the output fiber from entering the photodetector package.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Patent Application No. 61/251,981 filed Oct. 15, 2009, which is incorporated herein by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a compact optical tap monitor, and in particular to an optical tap monitor including a uni-directivity solution preventing superfluous light from entering the photodetector housing for increasing the accuracy of the photodetector measurement. 
       BACKGROUND OF THE INVENTION 
       [0003]    With reference to  FIG. 1 , a conventional integrated optical tap monitor  10  in accordance with the present invention includes two waveguides, an input fiber  1  and an output fiber  2 , a collimating lens combination  3 , a tap filter or tap coating  4 , an imaging lenses combination  6 , and one photodetecter (PD) package  8 , including a PD chip  9 . 
         [0004]    Light, launched from the input fiber  1 , is collimated by the lens combination  3 , and directed onto the tap filter or coating  4 . A first portion of the collimated beam is reflected by the tap filter or coating  4  to the lens combination  3 , which focuses the first portion into the output fiber  2 . A second portion of the collimated beam is focused by the imaging lens combination  6  onto the PD chip  9  to monitor the output light power of the input beam. 
         [0005]    Unfortunately, any light launched or back reflected from the output fiber  2  will also be focused onto the PD chip  9 , providing incorrect measurements of the power of the light portion coming from the input fiber  1 . 
         [0006]    An attempt at improving the conventional optical tap module is disclosed in U.S. Pat. No. 7,333,693, issued Feb. 19, 2008 to Nagata et al, in which light from the input fiber and light from the output fiber are directed in slightly different directions by an imaging lens. Unfortunately, unwanted reflected light will still enter the packaged photodetector  8 , resulting in overly high power readings by the photodetector chip  9  due to multiple reflections off the walls of the photodetector package  8 . 
         [0007]    An object of the present invention is to overcome the shortcomings of the prior art by providing a lensing arrangement, which separates the light coming from the input fiber and the output fiber, and a mask for preventing any of the light from the output fiber from entering the photodetector package. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, the present invention relates to a compact optical tap monitor device comprising: 
         [0009]    an input waveguide for launching an optical signal; 
         [0010]    a collimating lens for collimating the optical signal; 
         [0011]    a tap filter for receiving the collimated optical signal at an acute angle of incidence, for reflecting a first portion of the optical signal at an acute angle of reflection, and for passing a second portion of the optical signal; 
         [0012]    an output waveguide spatially separated from the input waveguide for outputting the first portion of the optical signal; 
         [0013]    an imaging lens for focusing the second portion of the optical signal, and for spatially separating light from the input waveguide from light from the output waveguide; 
         [0014]    a photodetector including an active area for receiving the second portion of the optical signal and for providing a measure of the optical power in the second portion; 
         [0015]    a mask covering a portion of the active area blocking light from the output waveguide from the active area of the photodetector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0017]      FIG. 1  illustrates a conventional optical tap monitor; 
           [0018]      FIG. 2  illustrates a cross-sectional view of an optical tap monitor in accordance with the present invention; 
           [0019]      FIG. 3  illustrates a photodetector package of the optical tap monitor of  FIG. 2 ; and 
           [0020]      FIG. 4  illustrates an alternative embodiment of the mask of the photodetector package of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    With reference to  FIG. 2 , an integrated optical tap monitor  11  in accordance with the present invention includes two waveguides, an input fiber  12  and an output fiber  13 , a collimating lens combination, e.g. graded index lens  14 , a tap filter or tap coating  16 , an imaging lenses combination, e.g. graded index lens  17 , a mask or spacer  18 , and one monitor photodetecter (PD) package  19 , including a photodetector (PD)  21 . Any waveguide or any light transmission medium can be used in place of the input and output fibers  1  and  2 . 
         [0022]    Light, launched from the input fiber  1 , is collimated by the lens combination  14  forming collimated light A C , and directed onto the tap filter or coating  16 . A first portion B F  of the collimated beam A C  is reflected by the tap filter or coating  16  at an acute angle of reflection in the lens combination  14 , which focuses the first portion B F  into the output fiber  13 . A second smaller (1% to 10%) portion C F  of the collimated beam A C  is focused by the imaging lens combination  17  through a clear portion of the mask  18  into the PD package  19  and onto the PD  21  to monitor the output light power of the input beam A. Ideally, the optical axes OA 1  of the collimating lens  14  and the optical axis OA 2  of the imaging lens  17  are aligned colinear with each other, with the input fiber  12  spaced the same distance therefrom as the output fiber  13 . Any light D F  launched r reflected from the output fiber  13 , which passes through the tap filter  16  is directed to a masked portion of the mask  18 , which prevents the light D F  from entering into the PD package  19  and from onto the photodetector  21 . 
         [0023]    Due to the symmetry of the input and output fibers  12  and  13  and the collimating and imaging lenses  14  and  17 , the tapped portion C F  of the input light A is directed to one side of the optical axes OA 1  and OA 2  of the lenses  14  and  17 , while the superfluous light D F  is directed to the opposite side of the optical axes OA 1  and OA 2 . Accordingly, the photodetector  21  is preferably positioned on the one side of the optical axes OA 1  and OA 2 , while the masked portion of the mask  18  is positioned on the opposite side of the optical axes OA 1  and OA 2 . Other arrangements, in which symmetry between the input and output fibers  12  and  13 , and the lenses  14  and  17  are also possible. 
         [0024]    The separated distance between the focused light C F  and the superfluous light D F  exiting the lens  17  depends on the distance between the input and output fibers  12  and  13 , and the combination of the focal lengths of the collimating lens  14  with the focusing lens  17 . Unfortunately, without the mask  18 , unwanted reflected light will still enter the packaged photodetector  19 , resulting in overly high power readings by the photodetector  21 . 
         [0025]    The typical structure of an integrated PD monitor is illustrated in  FIG. 1 ; however, the monitor PD  11  with mask or spacer  18  can be reduced in size, and a packaged PD  19  can be connected to the lens  17  directly, so the whole assembly size becomes very compact integrated unit, as illustrated in  FIG. 2 . A dual fiber pigtail  22 , containing both the input and output fibers  12  and  13 , can be connected together, e.g. fixed with adhesive, with the collimating lens  14 , and the imaging lens  17  with the tap filter  16  coated on either one of the collimating lens  14  or the imaging lens  17  or on a separate substrate therebetween. An example of the size of the packaged PD  11  with mask or spacer  18  is &lt;1.8 mm×1.8 mm×1.2 mm. 
         [0026]    The shape and design of the monitor PD package  19  is not essential, and depends on the structure of the overall assembly. The uni-directivity refers to light coming from one direction having much more power than light coming from another direction, e.g. when light is launched from the input fiber  12 , the photodetector  21  has a normal response I 1 , but when light is launched from the output fiber  13 , the photodetector  21  has a much lower response I 2 . Typical requirements call for −10*log(I 2 /I 1 )&gt;15 dB or more. 
         [0027]    Below is a chart of experimental results for the PD package  19  in accordance with the present invention indicating directivity above 19 dB in all cases. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Photocurrent of PD 11 
                   
               
             
          
           
               
                   
                 Input 
                 Incident from input 
                 Incident from output 
                 Directivity 
               
               
                 Number 
                 Power 
                 fiber 1 
                 fiber 2 
                 (dB) 
               
               
                   
               
               
                 1 
                 4 mW 
                 40 μA 
                 0.1 μA 
                 26.0 
               
               
                 2 
                 4 mW 
                 38 μA 
                 0.4 μA 
                 19.8 
               
               
                 3 
                 4 mW 
                 42 μA 
                 0.1 μA 
                 26.2 
               
               
                 4 
                 4 mW 
                 34.5 μA   
                 0.3 μA 
                 20.6 
               
               
                 5 
                 4 mW 
                 46.5 μA   
                 0.3 μA 
                 21.9 
               
               
                   
               
             
          
         
       
     
         [0028]    With reference to  FIG. 3 , a typical packaged photodetector  21  includes a photodetector chip  25  with an active area  22  mounted on a substrate  23 . Solder pads  24  are provided on the substrate  23  for electrically connecting the photodetector chip  25  with electrical leads  26  extending from opposite sides of the photodetector  21 . A transparent, e.g. clear glass, window  27  is placed over the photodetector chip  25  to protect it from elements in the environment. 
         [0029]    Using the mask or spacer  18  prevents the reflected light from the output fiber  13  from entering the photodetector package  19  and being detected by the PD chip  25 . The mask portion  28  can be rectangular in shape, thereby having a straight edge parallel to the edge of the window  27 , covering a fraction of the opening of the photodetector package  19  and the active area  22  on the side of the photodetector package  19  to where the reflected light D F  is directed by the lens  17 , as shown in  FIG. 2 . The exact shape of the mask portion  28  is not essential; however, it ideally covers ⅓˜⅔, preferably 0.4 to 0.6, of the PD active area  22  based on the imaging points spacing distance and direction of the reflected light to the PD chip  25 , as illustrated in  FIG. 4 , for intersecting the reflected light and preventing it from becoming incident on the active area  22  of the PD chip  25 . The masked portion  28  can be placed on or over the glass window  27 , as illustrated in  FIG. 3  or disposed inside the glass window  27  directly on or over the PD chip  25  covering a portion of the active area  22  where the reflected light would enter, as illustrated in  FIG. 4 , thereby preventing the PD chip  25  from measuring the reflected light. 
         [0030]    The mask portion  28  can be made out or any suitable material, e.g. metal, plastic, epoxy, glue or any shading light material. Optically absorbing or reflecting coatings can also be used.