Abstract:
An apparatus including a passive wavelength division multiplexing (WDM) demultiplexer (DeMUX) or a passive WDM multiplexer (MUX), an active photo diode (PD) array or an active laser diode (LD) array, and a compressing device disposed between the passive WDM DeMUX or the passive WDM MUX and the active PD array or the active LD array. The compressing device changes the optical spot pitch of the passive WDM DeMUX or the passive WDM MUX o match the pitch of the active PD array or the active LD array. A compression ratio can be adjusted by changing the incident angle of the incident beam to the compressing device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/999,431, filed Feb. 25, 2014, which claims the benefit of U.S. Provisional Application Ser. No. 61/853,787, filed Mar. 10, 2013. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to an optical spot array pitch compressor that compresses the optical spot array pitch of a passive wavelength division multiplexing (WDM) multiplexer (MUX) or demultiplexer (DeMUX) to match the pitch of an active laser diode (LD) array in a transmitter optical sub-assembly (TOSA) or photo diode (PD) array in a receiver optical sub-assembly (ROSA) and more specifically to an optical spot array pitch compressor that can compresses the optical spot array pitch with varying compression ratio. 
       BACKGROUND 
       [0003]    Wavelength division multiplexing (WDM) is used to increase the communication bandwidth or the number of communication channels in optical communications. A number of optical signals carried by light having different wavelengths are input and propagating in a single optical fiber. A WDM multiplexer (MUX) is used to combine a number of optical signals carried by light having different wavelengths into a fiber. To detect each signal, the combined light exiting from the fiber is decomposed into its components having different wavelengths using a WDM demultiplexer (DeMUX). Each component corresponds to an optical signal. Typically, the optical signals decomposed by the DeMUX form an optical spot array on a cross-section plane. The pitch of the array is determined by the DeMUX. The optical signals are detected by an array of photo diodes (PD) in a receiver optical sub-assembly (ROSA). 
         [0004]    Similarly, an array of laser diodes (LD) in a transmitter optical sub-assembly (TOSA) emits optical signals. Typically, the optical signals emitted by the LD array in the TOSA form an optical spot array on a cross-section plane. The optical signals are combined by a WDM MUX into an optical fiber. Accordingly, the optical spot array pitch of the passive WDM MUX or DeMUX must match the optical spot array pitch of the active LD or PD array. 
         [0005]    The pitch of an active LD or PD array may be 3.05 mm, which is the diameter of the transistor outline (TO) can for packaging LD or PD. On the other hand, the pitch of an active LD or PD array may be 0.25 mm for an integrated LD or PD. The pitch of an active LD and PD array may be any number, which is determined by the manufacture of the device. Similarly, the pitch of a passive WDM MUX and DeMUX may be any number as well, which is determined by its manufacturer. Accordingly, an optical spot array pitch compressor to match the pitch of a passive WDM MUX or DeMUX with the pitch of an active LD or PD array is required. Furthermore, the optical spot array pitch compressor must be able to provide a varying compressor ratio. Especially, when the passive WDM MUX and DeMUX and the active LD and PD array are not made based on the same specification, or are made by different manufacturers. It is appreciated that it is almost impossible to compress the optical spot pitch of a passive WDM MUX or DeMUX to as small as 0.25 mm pitch using traditional free space optics. 
         [0006]    U.S. Pat. No. 7,023,620 to Sandberg et al. discloses a device  100  to provide beam pitch compression using a group of mirror as shown in  FIG. 1 . Four beams  102 ,  104 ,  106 , and  108  having wavelengths λ 1 , λ 2 , λ 3 , and λ 4 , respectively, are separated into a first group including beams  102  and  104 , and a second group including beams  106  and  108 . The first group including beams  102  and  104  is reflected 90° by a first mirror  110 . The second group including beams  106  and  108  is reflected 90° by a second mirror  112 . Second mirror  112  has a hole or window to allow beam  104  passing through. If there are more than four beams, second mirror  112  must have a periodical structure mirror-window-mirror-window to reflect beams of the second group and to transmit beams of the first group. Device  100  will change the order of beams  102 ,  104 ,  106 , and  108  to a new order of beams  102 ,  106 ,  104 , and  108 . The special structure of second mirror  112  will increase the cost. Device  100  will have a fix compression ratio instead of a varying compression ratio. 
         [0007]    U.S. Pat. No. 4,627,690 to Fantone discloses an anamorphic prism  200  for beam compression as shown in  FIG. 2 . An incident beam  202  enters anamorphic prism  200  normally from a right angle surface  204 . After having two total internal reflections (TIR) at an inclined surfaces  206  and a flat surface  208 , incident beam  202  is refracted from inclined surface  206  to the air becoming an output beam  210 . Incident beam  202  originally has a beam diameter Win. Output beam  210  has a compressed beam diameter Wout. The compression ratio is Win/Wout. Anamorphic prism  200  requires the following conditions be satisfied. 
         [0000]    
       
         
           
             
               
                 
                   
                     n 
                     = 
                     
                       
                         cos 
                          
                         
                             
                         
                          
                         α 
                       
                       
                         cos 
                          
                         
                             
                         
                          
                         3 
                          
                         α 
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
             
               
                 
                   
                     η 
                     = 
                     
                       
                         
                           W 
                           in 
                         
                         
                           W 
                           out 
                         
                       
                       = 
                       
                         2 
                         + 
                         
                           1 
                           n 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    where n is the refractive index of anamorphic prism  200 , a is an apex angle  212  of anamorphic prism  200 , and η is the compression ratio. 
         [0008]    Apex angle  212 , a, is determined in a range of 17° to 19° by the refractive index n. Compression ratio η, which is in a range of 2 to 3, is also determined by the refractive index n. Due to the small apex angle (17°˜19°), the prism must have a long length L  214  to fully transmit the beam through the prism. Furthermore, inclined surface  206  includes an area of TIR  216 , which is not coated, and an area of refraction  218 , which is anti-reflection (AR) coated. To separate two areas  216  and  218  in an AR coating process, the prism may not be small. Anamorphic prism  200  has a fix compression ratio instead of a varying compression ratio. 
         [0009]    Accordingly, an optical spot array pitch compressor to match the pitch of a WDM MUX or DeMUX with the pitch of a LD or PD array, which is simple, small, low cost, and capable of providing a varying compression ratio, is desired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Non-limiting and non-exhaustive embodiments of the present application are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
           [0011]      FIG. 1  shows a beam pitch compression using a group of mirror. 
           [0012]      FIG. 2  shows a beam compression using an anamorphic prism. 
           [0013]      FIG. 3  shows a typical 4-channel ROSA including a zigzag WDM DeMUX. 
           [0014]      FIG. 4  shows an embodiment of a ROSA including an optical wedge between passive and active components. 
           [0015]      FIG. 5  shows an optical wedge having refractive index n, apex angle α, and output angle θ. 
           [0016]      FIG. 6  shows plots of compression ratio as function of apex angle α for refractive index n=1.5 (broken line) and n=1.75 (solid line). 
           [0017]      FIG. 7  shows light transmitting an optical wedge having incident angle β and output angle θ. 
           [0018]      FIG. 8  shows a plot of output angle θ as function of incident angle β for refractive index n=1.748 and apex angle a=31.1°. 
           [0019]      FIG. 9  shows a plot of compression ratio η as function of incident angle β. 
           [0020]      FIG. 10  shows an embodiment of a ROSA including two optical wedges between passive and active components. 
           [0021]      FIG. 11  shows an embodiment of a ROSA including a grating between passive and active components. 
       
    
    
       [0022]    Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present application. 
       DETAILED DESCRIPTION 
       [0023]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present application. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present application. 
         [0024]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present application. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or sub-combinations in one or more embodiments. 
         [0025]    A free space optical sub-assembly (OSA) integrates a passive component such as a WDM MUX or DeMUX and an active component such as a LD or PD array. A typical WDM MUX or DeMUX comprising thin film filters (TFF) is based on a zigzag optical path design. U.S. Pat. No. 6,201,908 to Grann and U.S. Pat. No. 6,769,816 to Capewell et al. show examples of a zigzag WDM DeMUX.  FIG. 3  shows a typical 4-channel ROSA  300  including a zigzag WDM DeMUX. 
         [0026]    ROSA  300  includes a passive WDM DeMUX  302  and an active PD array  304 . Passive WDM DeMUX  302  includes four TFFs,  310 ,  312 ,  314 , and  316 , and three mirrors,  320 ,  322 , and  324 . Active PD array  304  includes four lenses  350 ,  352 ,  354 , and  356 , and four PDs,  360 ,  362 ,  364 , and  366 . A wavelength multiplexed beam  306  having λ 1 , λ 2 , λ 3 , and λ 4  wavelengths is output from an optical fiber and enters into ROSA  300  through a common port  308 . Beam  306  is directed to TFF  310 . Light having wavelength λ 1  is transmitted through TFF  310 , focused by lens  350  and detected by PD  360 . The rest of beam  306  is reflected by TFF  310  toward mirror  320 . Beam  306  is directed to TFF  312  by mirror  320 . Light having wavelength λ 2  is transmitted through TFF  312 , focused by lens  352  and detected by PD  362 . The rest of beam  306  is reflected by TFF  312  toward mirror  322 . Beam  306  is directed to TFF  314  by mirror  322 . Light having wavelength λ 3  is transmitted through TFF  314 , focused by lens  354  and detected by PD  364 . The rest of beam  306  is reflected by TFF  314  toward mirror  324 . Beam  306  is directed to TFF  316  by minor  324 . Light having wavelength λ 4  is transmitted through TFF  316 , focused by lens  356  and detected by PD  366 . 
         [0027]    It is appreciated that ROSA and TOSA, in principle, have the same structure, but the optical path is reversed. Accordingly,  FIG. 3  can be seen as a TOSA, in which the PD array is replaced with a LD array, and the optical path is reversed. Instead of a PD detecting light beam, a LD is emitting a light beam. Minor modification may further be made. For example, in a TOSA, TFF  316  or the last TFF may be removed. An optical isolator may be disposed at common port  308 . In the disclosure, a ROSA is described in general, one skilled in the art would understand that the same principle applies to a TOSA as well, by reversing the optical path and replacing the PD array with a LD array. 
         [0028]    Recently, the pitch of the active LD or PD array decreases while the pitch of optical spot array generated by a traditional WDM MUX or DeMUX comprising TFFs based on a zigzag optical path design does not significantly decrease because it is difficult to reduce the size of the traditional passive WDM MUX or DeMUX. The typical numbers for pitch of the active LD or PD array currently include 0.25, 0.5, 0.75, 1.5 and 3.05 mm. The TO can provide a pitch of 3.05 mm. However the integrated technique provides a pitch of 0.25 mm. Accordingly, a solution is sought to solve how to couple the optical spot array from a passive WDM MUX or DeMUX to an active LD or PD array. 
         [0029]      FIG. 4  is an exemplary embodiment of an ROSA  400  including an optical wedge  402  between passive and active components, according to the present application.  FIG. 4  is essentially the same as  FIG. 3 . The difference between  FIG. 4  and  FIG. 3  is optical wedge  402  disposed between passive WDM DeMUX  302  and active PD array  404 . A light beam  410  having wavelength λ 1  is bent by optical wedge  402  becoming a light beam  420 . A light beam  412  having wavelength λ 2  is bent by optical wedge  402  becoming a light beam  422 . A light beam  414  having wavelength λ 3  is bent by optical wedge  402  becoming a light beam  424 . A light beam  416  having wavelength λ 4  is bent by optical wedge  402  becoming a light beam  426 . The pitch of optical spot array of passive WDM DeMUX  302 , which is the separation between beams  410  and  412 , beams  412  and  414 , and beams  414  and  416 , is compressed by optical wedge  402 . In other words, the separation between beams  420  and  422 , beams  422  and  424 , and beams  424  and  426  is smaller than the separation beams  410  and  412 , beams  412  and  414 , and beams  414  and  416 . Thus, PD array  404  having smaller pitch can be used. 
         [0030]    It is appreciated that the number of light beams is not limited to four. Any number is possible. Accordingly, the number of mirrors and TFFs may be any number as well. 
         [0031]    A compression ratio η is a function of refractive index n, apex angle a, and output angle θ, of optical wedge  402 .  FIG. 5  shows optical wedge  402  having refractive index n, apex angle a, and output angle θ, according to the present application. Optical wedge  402  comprises a right angle surface  502 , an inclined surface  504 , and a flat surface  506 . Incident beams  508  and  510  are incident normally on right angle surface  502 , and transmitted through optical wedge  402 , before they are bent becoming output beams  512  and  514 , respectively, having output angle θ. 
         [0032]    Since the incident beams are incident normally, compression ratio η, which is Win/Wout, is a function of refractive index n and apex angle a of optical wedge  402 , as shown in  FIG. 5 . Win is a separation of two adjacent incident beams, and Wout is a separation of two adjacent output beams. Plots of compression ratio as function of apex angle a for refractive index n=1.5 (broken line) and n=1.75 (solid line) are shown in  FIG. 6 , according to the present application. The compression ratio η as function of apex angle a and refractive index n is given in Equation 3. 
         [0000]    
       
         
           
             
               
                 
                   
                     η 
                     = 
                     
                       
                         cos 
                          
                         
                             
                         
                          
                         α 
                       
                       
                         cos 
                          
                         
                           [ 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                              
                             
                               ( 
                               
                                 
                                   n 
                                    
                                   sin 
                                 
                                  
                                 
                                     
                                 
                                  
                                 α 
                               
                               ) 
                             
                           
                           ] 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
         [0033]    Increase in an incident angle  702 , β, will change an output angle  704 , θ, as shown in  FIG. 7 , according to the present application. A plot showing the relationship of output angle θ and incident angle β for refractive index n=1.748 and apex angle a=31.1° is given in  FIG. 8 , according to the present application.  FIG. 8  shows that increase in incident angle β will result in decrease in output angle θ. The output angle θ as function of input angle β, apex angle α and refractive index n is given in Equation 4. 
         [0000]    
       
         
           
             
               
                 
                   
                     θ 
                     = 
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                        
                       
                         { 
                         
                           n 
                            
                           
                               
                           
                            
                           
                             sin 
                              
                             
                               [ 
                               
                                 α 
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                                     sin 
                                     
                                       - 
                                       1 
                                     
                                   
                                    
                                   
                                     ( 
                                     
                                       
                                         sin 
                                          
                                         
                                             
                                         
                                          
                                         β 
                                       
                                       n 
                                     
                                     ) 
                                   
                                 
                               
                               ] 
                             
                           
                         
                         } 
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
         [0034]      FIG. 9  shows a plot of compression ratio η as function of incident angle β, according to the present application. The plot in  FIG. 9  is calculated using data of  FIG. 8 .  FIG. 9  shows that for the incident angle range of β=−2° to β=1.8°, compression ratio is decreasing in the range η=2.4 to η=1.8. The compression ratio η as function of input angle β, apex angle α and refractive index n is given in Equation 5. 
         [0000]    
       
         
           
             
               
                 
                   
                     η 
                     = 
                     
                       
                         
                           cos 
                            
                           
                             [ 
                             
                               α 
                               - 
                               
                                 
                                   sin 
                                   
                                     - 
                                     1 
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       sin 
                                        
                                       
                                           
                                       
                                        
                                       β 
                                     
                                     n 
                                   
                                   ) 
                                 
                               
                             
                             ] 
                           
                         
                         
                           cos 
                            
                           
                             [ 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
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                                 ( 
                                 
                                   
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                                      
                                     
                                         
                                     
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                                     β 
                                   
                                   n 
                                 
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                             ] 
                           
                         
                       
                       × 
                       
                         
                           cos 
                            
                           
                               
                           
                            
                           β 
                         
                         
                           cos 
                            
                           
                             { 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                                
                               
                                 [ 
                                 
                                   n 
                                    
                                   
                                       
                                   
                                    
                                   
                                     sin 
                                      
                                     
                                       ( 
                                       
                                         α 
                                         - 
                                         
                                           
                                             sin 
                                             
                                               - 
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                                             ( 
                                             
                                               
                                                 sin 
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                                                  
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                                               n 
                                             
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                                 ] 
                               
                             
                             } 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
         [0035]    Accordingly, after a single optical wedge is made and disposed between the passive WDM DeMUX and the active PD array, the compression ratio can be adjusted by changing the incident angle as shown in Equation (5). A beam incident to the right angle surface of an optical wedge is refracted into the optical wedge. The beam is transmitted in the optical wedge and arriving at the inclined surface of the optical wedge, and is refracted to the air. Thus, no TIR occurs in the optical wedge. As mentioned previously, it is appreciated that the embodiment and the calculation may be applied to a TOSA comprising a passive WDM MUX and an active LD array by reversing the light path. 
         [0036]      FIG. 10  shows that a second optical wedge  1002  may be included in an exemplary embodiment  1000 , according to the present application.  FIG. 10  is essentially the same as  FIG. 4 . The difference between  FIG. 10  and  FIG. 4  includes that second optical wedge  1002  is added between a first optical wedge  402  and PD array  404 . Thus, embodiment  1000  of  FIG. 10  provides a two level compression. The compression ratio of the two level compression of  FIG. 10  is higher than the compression ratio of  FIG. 4 . Second optical wedge  1002  also compensates for some second order effects generated by first optical wedge  402 , because the directions of optical wedge  1002  and optical wedge  402  are opposite. The compression ratio can be adjusted by changing the incident angle as described previously. A plurality of optical wedges may be included in an embodiment of the present application to further increase the compression ratio. 
         [0037]      FIG. 11  shows an exemplary embodiment  1100  replacing optical wedge  402  with a grating  1102 , according to the present application. In  FIG. 4 , incident beams  410 - 416  are refracted by optical wedge  402  becoming bent output beams  420 - 426 . Incident beams  410 - 416  may be diffracted by grating  1102  becoming bent output beams  420 - 426 , as shown in  FIG. 11 . The compression ratio can be adjusted by changing the incident angle as well. 
         [0038]    An apparatus is disclosed that comprises a passive WDM DeMUX or a passive WDM MUX, an active PD array or an active LD array, and a compressing device disposed between the passive WDM DeMUX or the passive WDM MUX and the active PD array or the active LD array. The compressing device changes the optical spot pitch of the passive WDM DeMUX or the passive WDM MUX to match the pitch of the active PD array or the active LD array. The compressing device may be a single optical wedge, a first and a second wedges, plurality of optical wedges, or a grating. A compression ratio can be adjusted by changing the incident angle of the incident beam to the compressing device. 
         [0039]    While the present application has been described herein with respect to the exemplary embodiments and the best mode for practicing the application, it will be apparent to one of ordinary skill in the art that many modifications, improvements and sub-combinations of the various embodiments, adaptations and variations can be made to the application without departing from the spirit and scope thereof. For the disclosed methods, the steps need not necessarily be performed sequentially. 
         [0040]    The terms used in the following claims should not be construed to limit the application to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.