Abstract:
An optical rotator includes a pair of waveplates which receive a polarized beam having a first state and outputs a polarized beam having a second state rotated 90° with respect to the first state. In the illustrative embodiment, the first and second waveplates are physically coupled to one another.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical devices. More specifically, the present invention relates to optical rotators. 
     2. Description of the Related Art 
     For optical systems, there is often a need to rotate the polarization state of an optical beam. For this purpose, optical rotators are made of quartz are often used. A half waveplate made of birefringent material can also be employed to transform a given linear polarization state into a different linear polarization state. A specific orientation of the half waveplate is necessary to achieve this result, while the optical rotator can have any orientation. These devices are generally passive and reciprocal. Electro-optical crystals are active waveplates, where phase is proportional to applied voltage. Non-reciprocal applications require Faraday rotators. 
     An optically active rotator rotates the plane of polarization of light passing through it. A typical optically active rotator is generally a fairly long device, requiring a large block of optical grade material such as quartz. One of the advantages of an optically active medium used in fabricating a rotator is that the rotator can be fabricated that rotates the transmitted beam by any desired angle, not just 90°. One of the disadvantages to employing an optically active medium is that high quality material is not available at all wavelengths. 
     Other inventions by this Applicant also assigned to Raytheon Company include: 
     “Reeder Rod” Ser. No. 09/482,230, filed Jan. 13, 2000; “Reeder Compensator” Ser. No. 09/482,376, filed Jan. 13, 2000; and “Waveplate Polarization Rotator” Ser. No. 09/482,378, filed Jan. 13, 2000; These applications are incorporated herein by reference. 
     Thus, there is a need in the art for a rotator which is much thinner than an equivalent 90° quartz rotator using much less optical grade material. There is a Other need for a rotator would be made of any uniformly birefringent medium. This would extend the possible wavelength range of operation as optically active rotators are not currently available at all wavelengths. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the optical rotator of the present invention. The inventive optical rotator includes a pair of waveplates which receive a polarized beam having a first state and outputs a polarized beam having a second state rotated 90° with respect to the first state. In the illustrative embodiment, the rotator includes first and second waveplates formed as a monolithic element. In another illustrative embodiment, the waveplates are coupled to one another by an optical spacer. 
     A rotator constructed in accordance with the present teachings should be much thinner than the equivalent 90° quartz rotator and, thus, use much less optical grade material. Moreover, the inventive rotator is compact and can be made of any uniformly birefringent medium. The uniformly birefringent material extends the possible wavelength range of operation. In addition, in accordance with the present teachings, rotators of larger size are possible because of the reduced amount of material required for a given diameter rotator. 
     A rotator constructed in accordance with the present teachings may be used for laser applications that require a strongly pumped laser rod that exhibits more than a quarter-wave of thermal birefringence. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrative of an optical rotator constructed in accordance with the teachings of the present invention. 
     FIG. 2 is an illustration of an alternate configuration of the optical rotator according to the present invention. 
     FIGS. 3A-3C illustrate alternative embodiments of the optical rotator according to the present invention. 
     FIG. 4 illustrates a thermal birefringence compensated laser rod employing the optical rotator depicted in FIG.  1 . 
    
    
     DESCRIPTION OF THE INVENTION 
     Illustrative embodiments and exemplary applications will now be described with preference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 1 is a diagram illustrative of an optical rotator constructed in accordance with the teachings of the present invention. In the illustrative embodiment, the Reeder Rotator  10  is designed to rotate any input polarization state by 90°. Hence, the inventive rotator (hereinafter ‘Reeder Rotator’) is the waveplate equivalent of a 90° optically active rotator (such as quartz). As illustrated in FIG. 1, the Rotator  10  is formed from two half waveplates  20  and  30  oriented 45° apart. It should be noted that the optical rotator according to the present invention can be formed from a single, monolithic structure without departing from the scope of the present teachings. Alternatively, the Reeder Rotator  10  can be formed from two half waveplates  20 ,  30  oriented 45° apart and separated by an optically passive material, i.e., an optical spacer  40 , where optical power through the Reeder Rotator  10  is a consideration, from a cooling standpoint. See FIG.  2 . Implementation of a rotator with two half waveplates is a particularly novel aspect of the present invention. 
     Before discussing the preferred embodiments of the present invention in further detail, some background discussion on polarization “tracing” with Jones matrices will first be given to aid in the understanding of the invention. Jones matrices are 2×2 matrix operators that are used to trace polarization states, where an electric field is written as a two-element column vector. The field state after the optic described by the Jones matrix J is                  E   _     =     J   _                     or           [   1   ]                 (           E   x   ′               E   y   ′           )     =       J        (           E   x               E   y           )       .             [   2   ]                                
     Jones matrices are very simple in the coordinate system of the optic described herein, but these principle axes are not always lined up with the lab reference frame, i.e., horizontal (x) and vertical (y). Rotation matrices allow for conversion between these different frames of reference. Thus, a general Jones matrix is of the form 
     
       
           J=R   —   J   principle   R   + ,  [3] 
       
     
     with rotations to and from the optic&#39;s principle axes. A rotation matrix is                R   =     (           cos                 θ           sin                 θ                 -   sin                   θ           cos                 θ           )       ,           [   4   ]                                
     where the previous plus and minus subscripts referred to whether the rotation angle was +θ or −θ. A waveplate with a phase difference of φ at an angle of θ has a Jones matrix of              W   =       (           cos                 θ             -   sin                   θ               sin                 θ           cos                 θ           )          (                        ϕ   2             0           0                      ϕ   2               )          (           cos                 θ           sin                 θ                 -   sin                   θ           cos                 θ           )               [   5   ]                                
     where absolute phase is not of concern. Note that it is implied that R=R(θ) and W=W(φ,θ). 
     The Jones matrix for a 90° optically active rotator is                Rot     90      °       =       R        (       -   90        °     )       =       (         0         -   1             1       0         )     .               [   6   ]                                
     which swaps the field components and puts a 180° phase shift on one of them, i.e.,                  Rot     90      °            (           E   x               E   y           )       =         (         0         -   1             1       0         )          (           E   x               E   y           )       =     (           -     E   y                 E   x           )               [   7   ]                                
     The sign difference between a rotator and a rotation matrix is due to the fact that a rotator rotates a field while a rotation matrix rotates the coordinate system. 
     The Reeder Rotator of the present invention is the waveplate equivalent to the optical rotator discussed immediately above. As mentioned above, the inventive rotator consists of two waveplates at an angle of 45° with respect to each other. The Jones matrix for this configuration is:                          W     λ   /   2            (     0      °     )              W     λ   /   2            (     45      °     )         =       (         i       0           0         -   i           )          [       1   2          (         1         -   1             1       1         )          (         i       0           0         -   i           )          (         1       1           1       1         )       ]                     =       (         0         -   1             1       0         )     =     Rot     90      °           ,                 [   8   ]                                
     where the half waveplate at 45° swaps the components and the one 0° puts on the 180° phase difference. Of course, this interpretation depends on absolute orientation, even though the behavior of the Reeder Rotator does not. The only requirement is that the two half waveplates be 45° apart in angle (or −45°, or ±135°, some angles giving a −90° rotation). Since this waveplate rotator rotates every linear input state by 90°, it must work that way independent of absolute orientation, i.e., 
     
       
           W   {fraction (λ/2)} (θ) W   {fraction (λ/2)} (45°+θ)=Rot 90°  for all θ.  [9] 
       
     
     It should be mentioned that the Reeder Rotator can be used anywhere that an optically active 90° rotator is used. One of the advantages of the Reeder Rotator is that it is much thinner than an equivalent optically active rotator, using much less material and taking up much less space. Thus, the Reeder Rotator can be made out of any uniform birefringent medium, which extends the possible wavelength range of operation. 
     In addition, it is often possible to separate two parallel beams with the same linear polarization state but with different wavelengths by making the waveplates employed in the Reeder Rotator multi-order, e.g., a net half waveplate for the first wavelength and a net full wave for the second. Thus, the first wavelength is rotated 90°, while the second is not rotated at all. It should be noted that the same could be done with a single waveplate if the polarization state of the beam is known and fixed. 
     FIG. 2 is an illustration of an alternate configuration of the optical rotator according to the present invention. As shown in FIG. 2, the Reeder Rotator  10 ′ is formed from two half waveplates  20 ,  30  oriented 45° apart and separated from one another by an optical spacer  40 , e.g., a section of white YAG. It should be noted that the Reeder Rotator  10 ′ is preferable to the Reeder Rotator  10  when optical power through the rotator is a consideration, from a cooling standpoint. It should also be noted that both Reeder Rotator  10 ′ and Reeder Rotator  10  rotate any input polarization state by 90°. It will be appreciated that other variations and alternative configurations are possible, as shown in FIGS. 3A-3C (see the teachings of which are incorporated herein by reference), which collectively illustrate a waveplate rotator that consists of either three waveplates or several groups of three waveplates. In an exemplary case, the outer two waveplates, which are generally denoted  20   n  and  30   n , are quarter waveplates, while the inner waveplate, generally denoted  50   n , has a phase difference that equals twice the desired rotation angle, i.e., 90°. Although an infinite number of variations are possible, it will also be appreciated that most of these variations require too many elements to make these variations practical. 
     One application of the Reeder Rotator is thermal birefringence compensation, which is shown here as for the exemplary thermal birefringence compensated laser rod  100  illustrated in FIG. 4 (see application Ser. No. 09/482,230 the teachings of which are incorporated herein by reference). 
     FIG. 4 illustrates a thermal birefringence compensated laser rod employing the optical rotator depicted in FIG.  1 . The laser rod  100  depicted in FIG. 4 makes use of the Scott-Dewit compensation scheme for correcting the thermal birefringence. However, instead of using a thick 90° quartz rotator, which is typically about 1.5 cm long, it uses a Reeder Rotator  10  made of sapphire, which, in the illustrative embodiment, is approximately 132 μm long, disposed between two sections of optical gain material  110 . The monolithic rod structure  100  can be made via diffusion bonding or any other suitable technique. 
     High transmission may be observed when the Reeder Rotator  10  is placed between crossed polarizers, independent of rotator orientation. Moreover, low transmission may be observed when the Reeder Rotator  10  is placed between parallel polarizers, independent of rotator orientation. This should indicate that the Reeder Rotator  10  works according to the mathematical analysis presented above. Although the Reeder Rotator  10  employed in the laser rod illustrated in FIG. 4 can be fabricated from optical quality sapphire material, the present invention is not so limited. 
     Advantageously, the Reeder Rotator  10  is much thinner than the equivalent 90° quartz rotator and, thus, uses much less optical grade material. Moreover, the Reeder Rotator  10  can be used anywhere that smaller size is desired. Furthermore, the inventive Rotator can be made of any uniformly birefringent medium, which extends the possible wavelength range of operation, since good optically active rotators are not available at all wavelengths. In addition, Reeder Rotator  10  of a larger size are possible because of the reduced amount of material required for a given diameter rotator. 
     It should be noted that the Reeder Rotator  10  according to the present invention advantageously can be used for laser applications that require a strongly pumped laser rod that exhibits more than a quarter wave of thermal birefringence. 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications applications and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,