Abstract:
An optical diplexer having a birefringent crystal for providing substantially a whole number of wavelength retardation to first optical energy having said first polarization type, and having a first wavelength, fed to a first end of said crystal, and exiting a second end of said crystal with said first polarization type and said first wavelength and for providing substantially an odd integer number of half wavelength retardation to second optical energy having said first polarization type, having a second wavelength, fed to said second end of said crystal and exiting said first end of said crystal with said second polarization type and said second wavelength. The system includes an electronically actuated polarization aligner for adjusting phase retardation of the second energy fed to the second end of the crystal prior to such second energy entering the second end of the birefringent crystal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 11/132,532 filed on May 19, 2005, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates generally to laser optical fiber communication systems and more particularly to optical diplexers used in such systems. 
       BACKGROUND 
       [0003]    As is known in the art, a laser communication system includes a plurality of transceivers, each one being adapted to transmit optical frequency signals for example a laser transmitter to one or more other transceivers in the system and to receive optical frequency signals transmitted to such one of the transceivers by such one or more other ones of the transceivers. 
         [0004]    As is also known in the art, the optical frequencies have been standardized by the International Telecommunications Union (ITU) to be a comb of frequencies beginning with a frequency of 193 TeraHertz (THz) and separated by 100 GHz. Thus, the frequency of any signal, f k =193 THz+k (0.1 THz) where k is an integer. The value of k is sometimes referred to as the channel designation. Thus, each transceiver has odd channels, i.e., where k is an odd integer) and even channels (where k is an even integer). Further, for any one of the transceivers, odd channels are used for transmitting signals and the signals received by such one of the transceivers are in even channels, or vice versa (i.e., even channels are used for transmitting signals and the signals received by such one of the transceivers are in odd channels). It should also be noted that the transmitted energy and the received energy have the same type of polarization, e.g., vertical. Thus, it follows that if, for example, two transceivers are to communicate with each one, to provide effective isolation between the receive and transmits channels therein, one of the transceivers uses transmits signals with say vertical polarization in the odd channels and receives signals with vertical polarization in the even channels while the other one of the pair of transceivers transmits signals with vertical polarization in the even channels and receives signals with vertical polarization in the odd channels 
         [0005]    As is also known in the art, a diplexer is sometimes used in the transceiver to separate the transmitted and received signals having the same polarization type into separate paths; the transmitted signal emanating from a laser transmitter in the transceivers passing along one path (i.e., a transmit path) and the received signals being directed along a different path to a laser energy receivers in the transceiver (i.e., a receive path). 
         [0006]    One such diplexer includes: (1) a birefringent crystal (e.g., retardation wave plate) for providing an integral number of wavelength phase retardation for signals in the, for example, odd channels, e.g., the transmitted signals, and an odd number of half wavelength phase retardation for signals in the received signals; and (2) a polarization beam splitter between the crystal and: (a) the transmitter disposed in the transmit path; and (2) the receiver disposed in the receive path. Thus, during transmit, energy from the transmitter having a frequency in, for example, one of the odd channels, and having, for example, vertical polarization, passes along the transmit path though the polarization beam splitter and then through the crystal as vertically polarized light of the same frequency for external propagation to another one of the transceivers in the system. During receive, energy from the other one of the transceivers in the system, which transmits signal of vertical polarization but, in this example, with a frequency in an even channel, passes though the birefringent crystal; however, here the birefringent crystal changes the polarization in the received signal from vertical polarization to horizontal polarization. The horizontally polarized signal is directed by the polarization bean splitter to the receiver along the receive path. 
         [0007]    As is also known in the art, it is important that the length of the optical path through the birefringent crystal be very accurately controlled, which is particularly difficult for high order retardation plates having a thickness of several mm. Furthermore, once an element is fabricated, the optical path length through the element is typically fixed, and is not adjustable. Therefore, if the element is made to the wrong length, the element has to be scrapped and a new one fabricated. One adjustable retardation plate is discussed in U.S. Pat. No. 6,704,143, entitled “Method and apparatus for adjusting an optical element to achieve a precise length”, inventors Han, et al. issued Mar. 9, 2004. 
       SUMMARY 
       [0008]    In accordance with the present invention, an optical diplexer having a polarizing beam splitter for transmitting a first polarization type and for deflecting a second polarization type. The diplexer includes a birefringent crystal for providing substantially a whole number of wavelength retardation to first optical energy having said first polarization type, and having a first wavelength, fed to a first end of said crystal, and exiting a second end of said crystal with said first polarization type and said first wavelength and for providing substantially an odd integer number of half wavelength retardation to second optical energy having said first polarization type, having a second wavelength, fed to said second end of said crystal and exiting said first end of said crystal with said second polarization type and said second wavelength. The system includes an electronically actuated polarization aligner for adjusting phase retardation of the second energy fed to the second end of the crystal prior to such second energy entering the second end of the birefringent crystal. 
         [0009]    In one embodiment, the polarization aligner is an electronically controllable waveplate. 
         [0010]    In one embodiment, the polarization aligner includes a pair of the polarization aligner includes a pair of electronically controllable waveplates which, in combination, allows more flexible and precise control of the birefringent retardation. 
         [0011]    In one embodiment, the optical diplexer includes a polarizing device disposed: (a) between a source of the second optical energy fed to the second end of the birefringent crystal; and/or (b) between a source of the first optical energy and the first end of the polarizing beam splitter. 
         [0012]    In accordance with another feature of the invention, a phase aligner is provided comprising a pair of electronically controllable waveplates having fast axis oriented in different directions. 
         [0013]    In one embodiment, the phase aligner includes a fixed birefringent element. 
         [0014]    In one embodiment the fixed birefringent element is a waveplate. 
         [0015]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a diagram of an laser optical communication system according to the invention; 
           [0017]      FIG. 2  is a block diagram of an exemplary one of a pair of transceivers used in the system of  FIG. 1  according to the invention; and 
           [0018]      FIG. 3  is a block diagram of an exemplary one of a pair of transceivers used in the system of  FIG. 1  according to another embodiment of the invention. 
       
    
    
       [0019]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0020]    Referring now to  FIG. 1 , a laser communication system  10  is shown here having a pair of transceivers (XCVRs)  12   a,    12   b.  Transceiver  12   a  transmits optical communication signals to transceiver  12   b  using odd channels while transceiver  12   b  transmits optical communication signals to transceiver  12   a  using even channels. More particularly, here transceiver  12   a  transmits optical signals having frequencies f k =193 THz+kS, where S is the channel spacing, (e.g., here 0.1 THz) and where k is an odd integer (i.e., odd channels) and transceiver  12   b  transmits optical signals having frequencies f k =193 THz+k (0.1 THz) where k is an even integer (i.e., even channels). It follows then that transceiver  12   a  includes a receiver, adapted to receive optical signals having frequencies f k =193 THz+k (0.1 THz) where k is an even integer and that transceiver  12   b  includes a receiver adapted to receive optical signals having frequencies f k =193 THz+k (0.1 THz) where k is an odd integer. 
         [0021]    Referring now to  FIG. 2  an exemplary one of the identically constructed transceivers  12   a,    12   b,  here transceiver  12   a,  is shown to include a laser transmitter  14  and a laser receiver  16 . As is well known in the art, additional passive components (not shown) such as lenses and mirrors may lie between the two transceivers for shaping the optical beams, passing them through optical fiber, or sending them through free space. The optical communication signal produced by the laser transmitter  14 , indicated by dotted arrow  18 , is vertically polarized, indicated by arrows  19 , and has the odd channels described above, i.e., k is an odd integer. The light from the transmitter  14  is first passed through a collimator  20 . The output of the collimator  20  is passed though a polarization beam splitter  24  so that any here horizontally polarized component, indicated by arrows  21 , in the optical energy is directed to an optical energy absorber  22  while only here the vertically polarized energy passes through a polarization beam splitter (PBS)  24  to an electronically controllable birefringent interleaving diplexer  26 . 
         [0022]    The electronically controllable birefringent interleaving diplexer  26  includes a polarization beam splitter (PBS)  28 , a birefringent crystal  30 , and a polarization aligner  31 . The polarization aligner  31  includes a first electronically controllable waveplate  32 , here for example, a liquid crystal waveplate (LCWP) having a 45 degree fast axis orientation relative to the vertical axis, i.e., the fast axis is 45 degrees with respect to the direction as arrow  19 , a second electronically controllable waveplate  34 , here for example, a liquid crystal waveplate having a vertical fast axis orientation, i.e., the fast axis is along the same direction as the direction as arrow  19 , and a fixed birefringent element, here a quarter wave plate QWP  36  having a 45 degree fast axis orientation aligned with the fast axis of the first electronically controllable waveplate  32 , i.e., the fast axis is 45 degrees with respect to the direction as arrow  19 , all serially arranged as shown along a common optical path indicated by arrows, P. 
         [0023]    Thus, the fast axis orientation of the liquid crystal waveplate  32  is different from the fast axis orientation of the liquid crystal waveplate  34 . 
         [0024]    A polarization beam splitter  42  is disposed between an entrance/exit aperture  40  of the transceiver  12   a  and the electronically controllable birefringent interleaving diplexer  26 . The polarization beam splitter  42  passes vertically polarized light indicated by arrow  19  to entrance/exit aperture  40  and directs horizontally polarized light, indicated by arrow  21  pointing out of the plane of  FIG. 2 , to an optical energy absorber  33 . 
         [0025]    The electronically controllable birefringent interleaving diplexer  26  includes a controller  44  for producing electrical signals to the liquid crystal quarter wave plates  32 ,  34  in a manner to be described. 
         [0026]    In order to understand the effect of the crystal  30 , let the center frequency of the k th  channel be F k  and its wavelength be λ k . These are related by 
         [0000]        F   k   =c/λ   k   =F   0   +kS    (1) 
         [0000]    where throughout this discussion when numerical values are given they are understood to be examples for the particular choices of F 0 , here 193.0 THz, and S, the channel spacing, here 100.0 GHz, i.e. 0.1 THz; k is an integer. The diplexer  26  is desired to function as a half-wave plate for channels with odd k and as a zero-wave plate for the channels with even k. As is well known in the art, by this is meant that the birefringent retardation L divided by the wavelength be an integer for the even-numbered channels and one-half plus an integer for the odd-numbered channels, i.e., 
         [0000]    
       
         
           
             
               
                 
                   L 
                   = 
                   
                     
                       
                         ( 
                         
                           
                             k 
                             0 
                           
                           + 
                           k 
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             λ 
                             k 
                           
                           / 
                           2 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         ( 
                         
                           
                             k 
                             0 
                           
                           + 
                           k 
                         
                         ) 
                       
                        
                       
                         
                           c 
                           
                             2 
                              
                             
                               F 
                               k 
                             
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where k 0  is an even integer and the second form follows from Eq. 1. This equation gives a relationship which must hold for all k but a fixed value of L, which we may express differently in terms of F 0  and S by solving it for F k  and using Eq. 1: 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                     k 
                   
                   = 
                   
                     
                       
                         F 
                         0 
                       
                       + 
                       kS 
                     
                     = 
                     
                       
                         
                           c 
                           
                             2 
                              
                             L 
                           
                         
                          
                         
                           k 
                           0 
                         
                       
                       + 
                       
                         
                           c 
                           
                             2 
                              
                             L 
                           
                         
                          
                         
                           k 
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    This is satisfied by the following choice of the birefringent retardation and the integer k 0 : 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       L 
                       = 
                       
                         
                           L 
                           0 
                         
                         ≡ 
                         
                           c 
                           
                             2 
                              
                             S 
                           
                         
                         ≈ 
                         
                           1.5 
                            
                           
                               
                           
                            
                           mm 
                         
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         k 
                         0 
                       
                       = 
                       
                         
                           
                             F 
                             0 
                           
                           S 
                         
                         = 
                         1930 
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       note 
                        
                       
                           
                       
                        
                       
                         
                           L 
                           0 
                         
                         
                           k 
                           0 
                         
                       
                     
                     = 
                     
                       
                         
                           λ 
                           0 
                         
                         2 
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0027]    Thus, with L=L 0 ≡c/2S=1.499 millimeter (mm) here, a retardation is provided which varies in a prescribed way with wavelength, i.e., changes by one-half a wavelength as the optical frequency changes by 100 GHz. 
         [0028]    As will now be shown, the roles of even-k and odd-k channels may be interchanged by appropriate setting of the variable waveplates  32 ,  34 . As will be discussed, such setting results in a change in the total birefringence and thus acts a small change of L away from its nominal value L 0 , viz., L=L 0 +d. The new frequency of the kth channel will be given by solving Eq. 2 for the values F k ., denoted (for L shifted by d) F′ k . Performing this algebra one finds that the shift in the kth channel from its nominal (i.e., for d=0) value is given by 
         [0000]        F   k   −F′   k ={( k   0   +k ) d/ ( L   0   +d )} S.    (5) 
         [0000]    Noting that k 0  is large compared with unity, we may choose d to be a certain (small) value such that the term in curly braces equals unity for k=0. The necessary value of d is (cf. Eq. 4) approximately λ 0 /2, where λ 0  is the wavelength of the zeroth channel. The shift of the zero-th channel (i.e. the one with k=0) is exactly S, i.e. this even channel has shifted to an odd-channel frequency. The shift will be approximately S as long as k is small compared with k 0 . For example, over a span of 40 channels covering the conventional laser-communication band, the shift will be within 40/1930=2% of S. Such small frequency errors are acceptable for an interleaver to be used with many laser communication systems. 
         [0029]    The LCWP  32  adjacent to the fixed crystal  30  is preferably oriented so that its birefringent fast axis is parallel with that of the crystal  30 . Thus, electrically varying its retardation, as is well known in the art of liquid crystal variable retarders, directly changes the total birefringence of the two elements, crystal  48  plus LCWP  32 . For cases where the LCWP  32  has a highly stable fast axis as a function of voltage, or where the system requirements are not too stringent, these two elements (LCWP  32  plus crystal  48 ) are all that are required for proper diplexer operation. The control voltage for this LCWP  32  is set so that the combination of the LCWP  32  and the crystal  48  results in the desired total retardation. However, provision of a second LCWP  34  allows compensation for imperfections in the whole system as will now be described. In this case, the control voltage for this LCWP  32  is set so that the combination of the LCWP  32  and the crystal  48  results in an excess retardation of a quarter wave, resulting in circular polarization of the light which enters the crystal from the right as linear (vertical) polarization. Since real LCWP  32  may have a slightly variable fast axis, the second LCWP  34  is oriented with its fast axis at approximately 45° to that of LCWP  32 . This permits effectively adjusting the birefringemnt fast axis of the combination of LCWPs  32  and  34  to be exactly aligned at 45 degrees. Small changes in the control voltage of the two LCWP&#39;s  32 ,  34  allows achieving very pure circular polarization of light exiting LCWP  34 . The remaining element, i.e., the fixed quarter-wave plate  36 , is oriented with its fast axis along the slow axis of the LCWP  32 , thereby converts the light to the desired linear polarization. Thus, the combination of three elements serially arranged LCWP  32 , LCWP  34 , and fixed quarter-wave plate  36  disposed along the common path indicated by the arrows, P, act like a very pure electrically variable retardation which may be set from zero to some positive value, effectively controlling the total birefringent retardation of the polarization aligner in its entirety. The control signals may be developed in a manner as shown in  FIG. 2  and/or in a closed-loop manner by providing the absorber  33  with a photodetector  33 ′ as shown in  FIG. 3  and using familiar hill-climbing servo techniques to minimize the power incident thereunto. 
         [0030]    Thus, referring to the arrangement shown in  FIG. 2 , very small errors from temperature variations in the crystal  30  retardation are corrected. That is, the retardation L changes with temperature. This temperature error correction signal may be generated by including a thermocouple or other temperature-sensing device  52  ( FIG. 2 ). The signal from the temperature sensing device  52  is compared with a reference signal on line  53  representative of a nominal temperature condition at which the crystal provides the ideal phase retardation L. Variations from the reference signal thus represents an error signal related to the variation of the actual retardation of the crystal, ΔL, from the ideal retardation, L, which generates the required voltages for the liquid crystals  32 ,  34  to thereby compensate for the temperature effects on the crystal retardation. The error should be negligible over the ±20 nm (i.e., ±2.5 THz) band used for laser communications. 
         [0031]    It is noted that the crystal  30  is a high order waveplate having a length in combination with aligner  31  selected to provide a phase retardation L such that optical signals having wavelengths in the odd channels experience a phase retardation of a whole number of wavelengths and thus pass therethrough without a change in polarization whereas optical signal having wavelengths in the even channels experience a phase retardation of an odd number of half wavelengths to thereby rotate the polarizations of such optical signals by 90 degrees, i.e., vertically polarized optical signals having wavelengths in the even channels are converted into horizontally polarized optical signals. 
         [0032]    In operation, optical communication signals from transmitter  14  have frequencies f k =193 THz+k (0.1 THz) where k is an odd integer (i.e., the odd channels) and pass, after collimation by collimator  20 , through polarization beam splitter  24 . The vertically polarized light, having frequencies f k =193 THz+k (0.1 THz) where k is an odd integer, pass, as vertically polarized light, though the electronically controllable birefringent interleaver  26  and then through the polarization beam splitter  42  to the through the entrance/exit aperture  40  for transceiver  12   b.  It is noted in the signals provided by the controller  44  are used to compensate for any temperature effects on the retardation L of the crystal  28  as well as to perform the interchange of the roles of even and odd channels as desired. 
         [0033]    During receive, optical communication signals from transceiver  12   b  having frequencies f k =193 THz+k (0.1 THz) where k is an even integer (i.e., an even channels), pass through polarization beam splitter  42  as vertically polarized light, any horizontal components being directed to an absorber  32 . The vertically polarized light, having frequencies f k =193 THz+k (0.1 THz) where k is an even integer, pass, as vertically polarized light, though the electronically controllable birefringent interleaving diplexer  26 ; here, however, because the received optical signals are in the even channels, the crystal  30  in combination with aligner  31  provides an odd number of half wavelengths to such received optical signals and thereby converts such received optical signals to horizontally polarized light. The horizontally polarized light in the received signals is then directed by the polarization beam splitter  28  to the receiver  16 . 
         [0034]    From Eq. 3, above, it was shown that choosing L=c/2S, i.e. L=1.499 millimeter (mm), will simultaneously satisfy the equations for all k whether k is odd or k is even. It should be noted that if L is “detuned” by a half-wave, the even and odd channels will be interchanged. This “detuning” is performed by providing proper voltages to the liquid crystal waveplates. Thus, the same transceiver may be used for transceiver  12   a  and  12   b;  one, here transceiver  12   b,  having a voltage applied to the liquid crystal waveplates  32 ,  34  to establish the “detuned” while transceiver  12   a  does not have the “detuned” retardation. 
         [0035]    Thus, the use of the polarization aligner  31  enables both transceivers  12   a,    12   b,    FIG. 1 , to be constructed identically (i.e., with the same crystal  30  retardation, L). More particularly, referring to  FIG. 1 , and assuming that neither transceiver  12   a,    12   b  includes the polarization aligner  31  ( FIG. 2 ), the transceiver  12   a  must have a crystal  30  with a different retardation L than the corresponding retardation in transceiver  12   b  in order to communicate with transceiver  12   b.  Thus, two types of transceivers would be required, i.e., transceivers with different crystals  30 . When the transceivers incorporate polarization aligners  31 , both transceivers may have the same nominal crystal retardation, L≃L 0 , and the voltages applied to the polarization aligner  31  of one of the transceivers would be different from the other one of the transceivers to provide the requisite additional odd number of half wavelength retardation to one of the two transceivers. Provision of the polarization aligners  31  also allows simultaneously providing any necessary compensation for temperature effects and also for small errors in fabrication of the two crystals as described above. 
         [0036]    As is known, the state of polarization of a light wave is defined by two parameters (e.g., orientation and aspect ratio or “ellipticity” of the polarization ellipse). To change a polarization from an arbitrary given state A to a given state B therefore requires, in general, two degrees of freedom (“2DOF”). In accordance with the invention, those are the settings (voltages) of two LCWP&#39;s  32 ,  34  of the polarization aligner  31 . The input states to the polarization aligner  31  result from the operation by the crystal  30 , having known and fixed fast axis but somewhat variable retardation, on the lightwaves just to its right, which are vertically or horizontally polarized. The resulting states to the left of crystal  30  have known orientation, namely, vertical/horizontal, and variable ellipticity. The 2DOF polarization controller comprising LCWP&#39;s  32  and  34  can transform any such state into perfect circular polarization, or in fact to any state near perfect circular, but not in general to perfect vertical polarization. Thus a fixed birefringent element, here QWP  36 , is used to transform the set of states near circular to a set of states near vertical. Even in the presence of small errors in the orientation and retardation of the fixed birefringent element QWP  36 , there will be a state near circular which is transformed into perfect vertical polarization by that element. That state is within the available output space of the 2DOF polarization aligner comprising the two LCWP&#39;s  32  and  34 . Thus, here in the diplexer  26 , the polarization aligner having two LCWP&#39;s  32 ,  34  with a fixed QWP  38  to their left provides the desired control. 
         [0037]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.