Abstract:
In the beam path of an Optical Cross Connect between the front face of a fiber block and a moveable mirror array are placed a telecentric lens and multi-surface optical element. The lens is placed adjacent the front face with a front focal plane coinciding with the front face. The substantially parallel beam path axes between the front face and the telecentric lens are converted by the lens into dispersing directions towards the optical element. Discrete optical surfaces of the optical element redirect the dispersing beam paths in a fashion such that the beam paths coincide in the following with corresponding moveable mirrors of a mirror array. Pitches of arrayed fiber ends and of the optical surfaces as well as the moveable mirrors are independently selectable. The telecentric lens simultaneously focuses the signal beams with improved beam separation and reduced signal loss.

Description:
CROSS REFERENCE 
   This application is a divisional of U.S. patent application Ser. No. 10/354,887, filed on Jan. 29, 2003, and claims priority therefrom under 35 U.S.C.§ 120. The priority application is currently pending. This application also cross-references and hereby incorporates by reference U.S. patent application Ser. No. 10/354,901, entitled “ASSEMBLED MULTI-SURFACE OPTICAL COMPONENT AND METHOD FOR FABRICATING” and filed Jan. 29, 2003. 

   FIELD OF INVENTION 
   The present invention relates to Optical Cross-connect Switches (OXC). Particularly, the present invention relates to OXC with combined signal beam focusing. 
   In the field of optical telecommunication, Optical Cross-connect Switches (OXC) provide simultaneous switching of up to several thousands of signal beams. Generally, there is a continuing demand for OXC that can be more efficiently fabricated with ever increasing numbers of simultaneously switched signal beams. 
   For best understanding of the improvements provided by the present invention, a prior art OXC  100  may be initially described by referring to prior art  FIG. 1 . The prior art OXC  100  includes a bundle of incoming fibers  101  and outgoing fibers  112  that pass through a laser card  102  where a laser light is inserted and aligned with each beam path. Laser light is propagating in the same direction with the telecommunication signals along the input fibers  104 . The laser light is propagating in the opposite direction with the telecommunication signals along the output fibers  111 . 
   The fibers  104 ,  111  terminate with their fiber ends  208  (see  FIG. 2 ) at the front face  113 ,  114  of the fiber blocks  105 ,  110 , as is exemplarily depicted in  FIG. 2 . The signal beams emit/impinge at the front faces  113 ,  114  substantially collinear with the emitting laser beams. Each propagating signal beam is separately reflected together with its laser beam by a moveable mirror arrayed in the mirror arrays  106 ,  108 . The movement of the moveable mirrors is coordinated such that a switching of the signal beams is accomplished by spatially reorienting the beam paths between the mirror arrays  106 ,  108 . 
   The laser beams injected by the laser card are deflected by the moveable mirrors in the same fashion as the signal beam. A dichroic flat  109  placed in the beam paths between the mirror arrays  106 ,  108  is configured for filtering the laser light by reflecting only the signal beams. The laser light impinges a PDA detector  103  placed immediately behind the dichroic flat  109 . The PDA detector  103  recognizes the impinging coordinates of the lasers, which is part of a feedback loop utilized by the processor  107  to monitor and control the moveable mirrors. Each telecommunication beam is reflected along its path between the fiber blocks  105 ,  110  at the mirror array  106 , the dichroic flat  109  and the mirror array  108 . 
   Light emits from the fiber ends  208  (see  FIG. 2 ) with a certain dispersion angle making it necessary to focus the signal beams before directing it towards the mirror array  106 . Also, the switched signal beams need to be focused and narrowed before impinging the fiber ends  208  at the fiber block  110 . In the prior art OXC  100 , this is accomplished by separately focusing each signal beam. As is illustrated in the prior art  FIGS. 2 ,  3 , a number of lenses  204  is arrayed and positioned in alignment with their corresponding fiber ends  208 . The lenses  204  are fabricated with high precision into a lens plate  205  that is fixed with its frame  206  to the main housing  200  after a precision alignment procedure. 
   As can be seen in prior art  FIG. 3  there are certain design limitations associated with the use of a lens plate  205 . Signal beams emit/impinge the fiber ends  208  within the conical beam boundaries  301 . To provide sufficient spacing for the alignment and positioning, the lens plate  205  needs to be spaced apart the end faces  113 ,  114 . In order to capture the entire signal beam, each lens  204  has to have a diameter that at least equals the lateral extension of a conical boundary  301  where the signal beam impinges the lens  204 . Also, due to precision limitations of the optical elements in the beam path, the signal beam may have a certain scattering angle with which it propagates towards the opposing fiber block  110 . This may result in an extended width of it, which also needs to be captured by the lenses  204 . Hence, the lenses  204  have to be substantially larger in diameter than the diameter of the fiber ends  208 . 
   In addition to the required lens diameter, the lenses  204  need to be sufficiently spaced to each other for fabrication purposes. The design requirements for lens diameter and lens pitch mainly limit the minimal pitch  303 , with which the fiber ends  208  are arrayed within the fiber blocks  105 ,  110 . At the time this invention was made, an exemplary pitch  303  of a prior art OXC  100  is about 1 mm. 
   The monolithic fabrication of the small-scale lenses  204  is very cost intensive. Each lens  204  has to be fabricated with the same precision. Scaling of an OXC  100  for a larger number of simultaneously switched telecommunication signals is limited by the increasing costs associated with the fabrication of the lens plate  205 . 
   Due to the small lens sizes, the achievable focusing precision is relatively low compared to larger size lenses. As a result, the optical path between the fiber blocks  105 ,  110  needs to be kept as short as possible, which in turn defines the required tilt range of each movable mirror. Unfortunately, the efficiency of the OXC  100  is significantly influenced by the precision and speed with which the two axes tilt movement of each moveable mirror is accomplished. For that purpose it is desirable to have the maximum required tilt range of the moveable mirrors at a minimum. At the time this invention was made, an exemplary tilt angle of a moveable mirror is about 8 degrees. 
   The fabrication of the small scale lenses  204  results also in limited surface quality of each lens  204 , which in turn induces a certain loss of signal strength. At the time this invention was made, the loss of signal strength in an exemplary prior art OXC  100  is about 2 dB. It is desirable to reduce this loss. 
   The separate focusing of each signal beam with a monolithic lens array requires also substantially parallel beam propagation between the fiber blocks  105 ,  110  and their adjacent mirror arrays  106 ,  108 . Thus, dimensional scaling of the mirror array is dependent on scaling of the fiber blocks pitch and lens array. 
   Finally, the use of a laser card  102  is a cost intensive device for injecting laser light into the signal beams. Optical elements have to be additionally provided for separately injecting the laser light into each signal line. It is desirable to have an OXC with a configuration in which laser light may be injected into the signal beams without need of a separate laser card. 
   The prior art OXC design of prior art  FIGS. 1 ,  2 ,  3  has significant limitations summarized as follows:
         1) the fabrication of the lens plate  205  is highly cost intensive since for each fiber end  208  a separate lens  204  needs to be provided;   2) as a consequence of using a lens array, the signal beams propagate substantially parallel between the fiber blocks  105 ,  110 . This requires simultaneous scaling of fiber blocks  105 ,  110  and mirror arrays  106 ,  108 ;   3) miniaturization and scaling of the prior art OXC  100  is limited by the precision and cost with which the lens array  205  may be fabricated;   4) low focusing precision of the lens array  205  requires short beam paths between the fiber blocks  105 ,  110  and consequently increased tilt angles of the moveable mirrors;   5) reduced fabrication precision of the lens array results in loss of signal strength; and   6) cost intensive use of a laser card to inject laser light into the signal beams.       

   To overcome the limitations described above, an OXC design is needed in which:
         1) focusing of the signal beams is accomplished in a simplified fashion and without use of a micro lens array to reduce fabrication cost, loss of signal strength;   2) scaling and miniaturization of fiber blocks, mirror arrays, dichroic flat and detector is highly independent of beam paths and individual elements&#39; design constrains within the OXC; and   3) injecting laser into the signal beams without use of a separate laser card.       

   The OXC described in the following addresses this needs. 
   SUMMARY 
   The OXC of the present invention utilizes telecentric lenses in combination with multi-surface optical elements for a simultaneous focusing and directing of the signal beams between the fiber blocks and the moveable mirrors. A telecentric lens is placed adjacent to each of the two fiber blocks&#39; front faces such that a front focal plane of the telecentric lens coincides with the respective fiber block&#39;s front face. The telecentric lenses accomplish two tasks simultaneously. Firstly, each signal beam is converted from a dispersing condition towards the telecentric lenses into a converging condition away from the telecentric lenses. The beams propagate towards the mirror array with converging beam widths, which results in improved beam separation and minimal loss of signal strength. 
   Secondly, each telecentric lens redirects the signal beams&#39; axes from a substantially parallel direction at the front faces to dispersing directions such that the distance between adjacent beam path axes increases with the distance away from the telecentric lenses. The signal paths coincide centrally with optical surfaces of a multi-surface optical element placed at a certain distance away from the telecentric lens. The pitch of the beam axes at the multi-surface element is a multiple of the pitch with which the signal beams emit/impinge the fiber block providing for a larger scale fabrication of the optical surfaces. The multi-surface element redirects each beam path separately and in a fashion such that each beam path coincides with a single moveable mirror of a mirror array placed adjacent the optical element. The optical surfaces are positioned and oriented in a fashion that corresponds on one hand to the direction of the dispersing path axes and on the other hand to the mirror arrays&#39; pitch and distance to the optical element. Thus, by introducing a telecentric lens, the pitches of the fiber ends, the optical surfaces and the arrayed moveable mirrors may be separately selected. 
   In the preferred embodiment, the optical surfaces are planar mirrors that are easily fabricated. In that context it is referred to the cross-referenced application. Also, the use of a telecentric lens instead of arrayed micro lenses greatly reduces signal losses and provides for more flexibility in the configuration of the beam paths between the mirror arrays. As a consequence, the mirror arrays may be placed in a greater distance to each other, which in turn reduces the maximal required tilt angle of the moveable mirrors. The telecentric lens may be in a commercially available configuration. 
   The telecentric lens may be configured with a front focal length sufficiently long such that a beam splitter may be placed between the fiber blocks and the telecentric lenses. Through the beam splitter, the signal beams are accessed for laser injection and other monitor functions eliminating the need for the laser card. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  shows a schematic view of a prior art Optical Cross Connect [OXC]. 
       FIG. 2  shows a perspective exploded view of a prior art fiber block with a lens plate. 
       FIG. 3  shows a section view of a prior art fiber block and a lens plate. 
       FIG. 4  depicts a simplified OXC of the present invention. 
       FIG. 5  schematically illustrates a portion of the beam paths of an OXC in accordance with the present invention. 
       FIG. 6  shows a portion of an OXC with a multi-surface optical element in alternate configuration. 
       FIG. 7  shows a fiber block and a telecentric lens with a beam splitter assembly placed between them. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 4 , an Optical Cross Connect [OXC]  500  of the present invention includes a housing  501  is connected to an incoming fiber string  424  and an outgoing fiber string  425 . The optical fibers of the incoming string  424  are inserted in fiber block  420  and terminate at the fiber block&#39;s  420  front face  421 . The optical fibers of the outgoing string  425  are inserted in the fiber block  426  and terminate at the fiber block&#39;s  426  front face  428 . Signal beams emit at the first front face  421 , propagate along the main path  484  through the OXC  500  and impinge the second front face  428 . 
   The emitting signal beams impinge a first front side  431  of a first telecentric lens  430 . The telecentric lens  430  is configured in a well known fashion to simultaneously transfrom the distinct signal beams&#39; propagation characteristic such that the signal beams emit from telecentric lens&#39;  430  first back side  432  with dispersing beam axes  482 A and converging beam widths  486  (see  FIG. 5 ). 
   Along the main path  484  and following the telecentric lens  430  in direction of signal beam propagation is placed a first multi-surface optical element  440  that has a number of discrete optical surfaces  441 . In the preferred embodiment, the discrete optical surfaces  441  are planar mirrors. Each of the surfaces  441  has a unique position and orientation with respect to preferably one impinging signal beam such that all signal beams are independently redirected towards a number of moveable mirrors arrayed at the front  451  of a first mirror array  450 . Hence, after impinging the surfaces  441 , the signal beams propagate away from the first optical element  440  and towards the first mirror array  450  along beam axes  482 B. 
   The first optical element  440  provides for an individual redirecting of each signal beam. In the preferred embodiment, the beam axes  482 A are converted from a dispersing condition into a converging condition of the beam axes  482 B. 
   Between first mirror array  450  and second mirror array  460 , the switching of the signal beams takes place by correspondingly actuating the mirrors of both mirror arrays  450 ,  460 . The switching of signal beams takes place by spatially redirecting them while they are propagating from mirror array  450  to mirror array  460 . The spatial redirected beams remain within the boundaries  488  and  489 . The signal beams propagating between mirror array  450  and  460  impinge and are reflected by a dichroic flat  470 , which filters control laser beams from the signal beams. 
   The signal beams impinge the moveable mirrors of the mirror array  461  and are redirected again towards a second optical element  445  having discrete optical surfaces  443 . Between first and second mirror array  450 ,  460 , the signal beams propagate within the boundaries  488  along beam axes that change as a result of the induced switching operation(s) performed by moveable mirrors. At the second mirror array  460  the signal beams&#39; axes are again brought into a stable condition with their beam axes dispersing in constant directions  482 C away from the mirror array  460 . The beam axes  482 C are spatially oriented in correspondence to the position of the second optical surfaces  443  where they are redirected towards a second telecentric lens  433 . The signal beams propagate from the second multi-surface optical element  445  again with converging beam axes  482 D towards the second back side  434  of the second lens  433 . 
   The second telecentric lens  433  induces a simultaneous transformation to the signal beams in a fashion such that the signal beams emitting on the second front side  435  impinge at predetermined locations on the second front face  428 . The predetermined locations are within the boundaries of the fiber ends of the second fiber string  425  and the signal beams are injected again into the optical fibers of the second fiber string  425 . 
   The upper portion of the OXC  500  including the first fiber block  420 , the first telecentric lens  430 , the first optical element  440  and the first mirror array  450  is preferably symmetrical to the lower portion of the OXC  500  including the second fiber block  426 , the second telecentric lens  433 , the second optical element  445  and the second mirror array  460 . All signal beams propagate within the OXC  500  within the boundaries  481 ,  486 ,  487 ,  488  and  489 . The terms upper portion and lower portion pertain to the  FIG. 4  and are introduced solely for the purpose of ease of understanding. The telecentric lenses  430 ,  433  have symmetry axes that preferably coincide with main path  484 . 
   Now turning to  FIG. 5 , the geometrical configuration of the upper portion of the OXC  500  is described in detail. Due to the preferably symmetrical configuration of upper and lower portion, the teachings presented in the following for the upper portion may be applied to the lower portion with inverted propagation directions of the signal beams as can be well appreciated by anyone skilled in the art. The fiber ends are parallel arrayed in the fiber block  420  with a first pitch  423 . Consequently, the signal beams emit with substantially parallel first beam axis  480  and a certain, well-known scattering angle that results in conical beam boundaries  481 . Since the telecentric lenses  430 ,  433  provide a simultaneous beam transformation, the signal beams may overlap when they impinge/emit the front faces  431 ,  435 . This is particularly advantageous because the minimization of the first pitch  423  is no longer influenced by dimensional limitations of the involved optical components. 
   In the present invention the first pitch  423  may be provided with 0.22 mm compared to about 1 mm in the prior art. This example demonstrates the dramatic increase of signal beam density achieved by utilizing telecentric lenses  430 ,  433 . Reducing the pitch from 1 mm to 0.22 mm increases the signal beam density more than twenty fold. 
   Since in the present invention the signal beams may overlap between the fiber block fronts  421 ,  428  and the lens fronts  431 ,  435 , the distance between then may be freely selected. The lenses  433 ,  430  are accordingly configured such that a front focus of the lenses  433 ,  430  coincides with the fiber block fronts  421 ,  428 . The distance  402  may be defined sufficiently large for additional optical component(s) to be placed between the fiber block fronts  421 ,  428  and lens fronts  431 ,  435 . Such additional optical component(s) may provide an interaction with the signal beams similar to that of a well-known laser card and will be described further below together with  FIG. 6 . 
   For the purpose of ease of understanding, the main path  484  is shown in  FIG. 5  as a straight line with the optical elements  440 ,  445 , the mirror arrays  450 ,  460  and the dichroic flat  470  considered as being translucent. Also, the orientation of the optical surfaces  441 ,  443  is shown in  FIG. 5  without regards to their function and solely for ease of understanding of some general dimensional relations of the OXC  500 . 
   The beam transformation performed by the lens  430  includes a transformation from substantial parallel beam axes  480  towards the front side  431  into dispersing beam axes  482 A away from the back side  432 . The beam transformation performed by the lens  430  also includes a transformation from dispersing beam widths  481  towards the lens front  431  into converging beam widths  486  away from the lens back  432 . 
   The beam transformation performed by the lens  433  includes a transformation from dispersing beam axes  482  towards the back side  434  into substantial parallel beam axes  480  away from the front side  435 . The beam transformation performed by the lens  433  also includes a transformation from converging beam widths  486  towards the lens back  434  into dispersing beam widths  481  away from the lens front  435 . 
   The beam widths  486  approach zero towards a reference plane  483 , which is preferably perpendicular to the lenses&#39;  430 ,  433  symmetry axes. It is desirable to have each beam&#39;s widths  486  equal at the moveable mirror arrays  450 ,  460 . Hence, the distances  413 ,  408  between the mirror arrays  450 ,  460  and the dichroic flat  470  are preferably substantially equal. For equal beam widths  486  at the moveable mirrors  450 ,  460  the reference place  483  ideally coincides with the dichroic flat  470 . 
   The most peripheral beam axes  482  are at a certain distance  410  at the reference plane  483 . A first scaling rate of the beam axes  481  is defined as the proportion between the distance  410  and a distance  415  between the reference plane  483  and a common point  403  where the beam axes  482  intersect. The common point  403  is a theoretical point inside the telecentric lenses  430 ,  433  and is defined by a well known combination of individual lenses lined up inside the telecentric lenses  430 ,  433 . The scale of the optical elements  440 ,  445  is defined in correspondence to the first scaling rate and a distance  405  of the optical elements  440 ,  445  to the common point  403 . In the preferred embodiment, the telecentric lenses  430 ,  433  are preferably substantially equal with equally positioned common point  403 . As may well be appreciated by anyone skilled in the art, the widths  446  and second pitch  448  of the optical surfaces  443  are a function of the first scaling rate, the distance  405  and the number of signal beams along second pitch direction. 
   Each optical surface  441 ,  443  has a distinct angular orientation  444 , which is defined for its position relative to the main path  484  in accordance with well-known optical principles for redirecting optical beams and in conjunction with optical properties of the optical surface. 
   A second scaling rate is defined as the proportion between the distance of the most peripheral impinging locations on the optical elements  440 ,  445  and the distance  409  plus distance  408 . A third pitch  452 ,  463  with which the moveable mirrors are arrayed on the mirror arrays  450 ,  460  is a function of the second scaling rate, the distances  413 ,  408  and the number of signal beams along third pitch direction. 
   As illustrated in  FIG. 5 , the optical surfaces  441  are orientated such that the beam axes  482 B preferably coincide at the mirror array  460 . This condition may be applied for the lower portion in the way that the beam axes  482 C preferably coincide at the mirror array  450 . Consequently, a maximum tilt angle  407  of the moveable mirrors may be a trigonometric function of the second scaling rate. The maximum tilt angle  407  is thus reduced by reducing the second scaling rate. 
   Turning now to  FIG. 6 , an alternate embodiment of the OXC  500  is described. There, the multi-surface optical elements  440 ,  445  feature a reference surface  449  along which the optical surfaces  441 ,  443  are aligned. The reference surface  449  may have a continuous geometric configuration that corresponds to the directional change between beam axes  482 A and  482 B in accordance with the physical laws of optical reflection. For more details refer to the cross-referenced application. 
   Finally  FIG. 7  may be described in more detail. There an embodiment of the OXC  500  is depicted in which a beam splitter  701  is placed between the fiber blocks  420 ,  428  and the lenses  430 ,  433 . The beam splitter  701  splits signal beam portions  782  off the signal beams and directs them towards monitoring device  740 , which may be a well-known InGaAs camera for monitoring signal strength. At the same time, the beam splitter  701  injects laser beams  781  coming from a secondary beam splitter  710 . The secondary beam splitter  710  receives laser pulses  783  from a lasing device  720 , which may be for example a vertical cavity surface emitting laser array[VCSEL]. Such VCSEL are commercially available with standardized pitch  723 . Making the first pitch  423  equal to the standardized pitch  723  is a significant factor for directly inserting the laser into the signal beams. The amount of additional optical components may be kept to a minimum. 
   The secondary beam splitter  710  directs a laser light portion  784  towards a laser monitoring device such as a well-known PSD3. 
   An exemplary OXC  500  in accordance with a preferred embodiment of the present invention may have the following characteristics: 
   
     
       
             
             
           
         
             
                 
             
           
           
             
               path length 484: 
               1100 mm; 
             
             
               throughput loss: 
               1 db; 
             
             
               maximum tilt angle 407: 
               3.5 degrees; 
             
             
               first pitch 423: 
               0.22 mm; 
             
             
               number of mirrors/pitch 452 of mirror arrays 
               30 × 40/1 mm; 
             
             
               450, 460: 
             
             
               mirror tilt accuracy: 
               0.0015 degrees; 
             
             
               required angular accuracy of optical surfaces 
               0.01 degrees; 
             
             
               440, 445: 
             
             
               mainframe size of housing 501: 
               10.5 × 6.1 × 3.5 inches. 
             
             
                 
             
           
        
       
     
   
   Accordingly, the scope of the invention described in the specification above is set forth by the following claims and their legal equivalent. 
   Accordingly, the scope of the invention described in the specification above is set forth by the following claims and their legal equivalent.