Abstract:
A piezoelectric transducer is described that is configured for use within a path length control apparatus of an optical device. The transducer comprises at least one void formed within a central region of the piezoelectric transducer, the one void or alternatively, the multiple voids, utilized at least in part to limit a curvature induced into a mirror during operation of the piezoelectric transducer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to a path length control apparatus (PLC) for optical devices, and more specifically, to a PLC apparatus that includes an improved surface for reflecting laser beams within a ring laser gyroscope (RLG).  
         [0002]     A ring laser gyroscope (RLG) is commonly used to measure the angular rotation of an object, such as an aircraft. Such a gyroscope has two counter-rotating laser light beams that propagate within a closed loop optical path or “ring” with the aid of successive reflections from multiple mirrors. The closed path is defined by an optical cavity that is interior to a gyroscope frame or “block.” In one type of RLG, the block includes planar top and bottom surfaces that are bordered by six planar sides that form a hexagon-shaped perimeter. The block is sometimes referred to as a laser block assembly. Three planar non-adjacent sides of the block form the mirror mounting surfaces for three mirrors at the corners of the optical path, which is triangular in shape.  
         [0003]     Operationally, upon rotation of the RLG about its input axis (which is perpendicular to and at the center of the planar top and bottom surfaces of the block), the effective path length of each counter-rotating laser light beam changes. A frequency differential is produced between the beams that is nominally proportional to angular rotation. This differential is then optically detected and measured by signal processing electronics to determine the angular rotation of the vehicle. To maximize the signal out of the RLG, the path length of the counter-rotating laser light beams within the cavity must be adjusted. Thus, RLGs typically include a path length control apparatus (PLC), the purpose of which is to control the path length for the counter-rotating laser light beams to maximize the output signal.  
         [0004]     Such PLCs typically include a piezoelectric transducer (PZT) secured to a mirror that is in turn secured to a mirror mounting surface of the laser block assembly (LBA). The mirror is in communication with bores in an optical cavity of the LBA. The bores form a portion of the closed loop optical path or ring defined by the optical cavity. The mirror reflects the counter-rotating laser light beams at its respective corner of the closed loop optical path. As such, a flatness of the mirror can affect the reflection of the counter-rotating laser light beams and thus operation of the RLG.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     In one aspect, a piezoelectric transducer configured for use within a path length control apparatus of an optical device is provided. The transducer comprises at least one void formed within a central region of the piezoelectric transducer. The single void, or alternatively the multiple voids, are utilized at least in part, to limit a curvature induced into a mirror during operation of the piezoelectric transducer.  
         [0006]     In another aspect, a method for limiting an amount of curvature induced into a mirror during operation of a piezoelectric device attached to the mirror is provided. The piezoelectric device includes one or more piezoelectric layers adjacently stacked and the process comprises forming a void through a central region of at least the piezoelectric layer adjacent the mirror and attaching a non-piezoelectric stiffening block to the mirror within the void.  
         [0007]     In still another aspect, a method for limiting an amount of curvature induced into a mirror during operation of a piezoelectric device attached to the mirror is provided. The piezoelectric device includes one or more piezoelectric layers and the method comprises coating a surface of at least one of the piezoelectric layers with an electrode material, the electrode material having a void formed therein adjacent a central region of the respective piezoelectric layer, and forming a stack of piezoelectric layers, the one or more coated surfaces substantially parallel to a reflective surface of the mirror.  
         [0008]     In yet another aspect, a ring laser gyroscope is provided that comprises a laser block assembly, a mirror, and a piezoelectric transducer. The laser block assembly comprises an optical path bored therein and the mirror comprises a reflective surface and a non-reflective surface. The reflective surface is attached to the laser block assembly and in optical communication with the optical path. The piezoelectric transducer is attached to the non-reflective surface of the mirror, and comprises at least one void located in a central region of the piezoelectric transducer. The void is configured to limit a curvature induced into the mirror during operation of the piezoelectric transducer.  
         [0009]     In another aspect, a path length control apparatus for a ring laser gyroscope is provided that comprises a mirror comprising a reflective surface and a non-reflective surface and a piezoelectric transducer. The reflective surface is configured for attachment to a laser block assembly of the ring laser gyroscope and the piezoelectric transducer is attached to the non reflective surface of the mirror. The piezoelectric transducer comprises at least one void located in a central region of the piezoelectric transducer and the at least one void is configured to limit a curvature induced into the mirror during operation of the path length control apparatus. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a cross-sectional view of a path length control apparatus, including a piezoelectric transducer, that is attached to a laser block assembly.  
         [0011]      FIG. 2  is a cross-sectional view of one embodiment of piezoelectric transducer.  
         [0012]      FIG. 3  is a cross-sectional view of a piezoelectric transducer which includes voids formed therein.  
         [0013]      FIG. 4  is a top view of the piezoelectric transducer of  FIG. 3 .  
         [0014]      FIG. 5  is a side view illustrating the operation of a non-piezoelectric block placed within the voids of the piezoelectric transducer of  FIG. 3 , with respect to a mirror of the piezoelectric transducer.  
         [0015]      FIG. 6  is a cross-sectional view of another embodiment of piezoelectric transducer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]      FIG. 1  illustrates a path length control (PLC) apparatus  10  attached to a laser block assembly (LBA)  12  of a ring laser gyroscope (RLG). PLC apparatus  10  includes a piezoelectric transducer (PZT)  16  which is secured to a mirror  18  via an epoxy-based adhesive  20 . Epoxy adhesive  20  covers the interface (defined by a lower surface  22  of PZT  16  and an upper surface  24  of mirror  18 ) between PZT  16  and mirror  18 . Mirror  18  is secured to a mirror mounting surface  26  of LBA  12 . Mirror  18  is configured for communication with laser bores  32  within an optical cavity  34  of LBA  12 . Bores  32  form a portion of a closed loop optical path  38  defined by the optical cavity  34 .  
         [0017]     As illustrated by  FIG. 1 , mirror  18  reflects counter-rotating laser light beams  40  at a respective corner of the closed loop optical path  38 . PZT  16  includes at least a pair of piezoelectric elements, or layers,  42  and  44 . A plurality of piezoelectric layers, for example piezoelectric elements  42  and  44 , are sometimes collectively referred to as a piezoelectric device. PZT  16  utilizes a voltage applied to piezoelectric elements  42  and  44  and delivered by a regulated voltage source (not shown) which is attached to contacts  46 . Contacts  46  are electrically connected to piezoelectric elements  42  and  44 . Regulation of the applied voltage is in response to a signal provided by a detector (not shown) that monitors the intensity of the light beams  40 . Application of the applied voltage results in small, but precisely controlled, mechanical movements of piezoelectric elements  42  and  44  in a direction perpendicular to a top surface  48  of PZT  16 . This mechanical movement of piezoelectric elements  42  and  44  of PZT  16  affects translational movement of mirror  18 , and thereby controls the path length of the laser light beam  40  (e.g., a length of closed loop optical path  38 ).  
         [0018]      FIG. 2  is a cross-sectional view of a multi-layered PZT  60 , which includes a stack  61  of alternating negative and positive co-fired ceramic piezoelectric layers. Co-fired ceramic piezoelectric layers are layers that are “fired” together when they are fabricated, as opposed to being fabricated separately and then later bonded together in a multi-layered stack. Multi-layered PZT  60  may include, for example, a top layer  62 , a bottom layer  68 , and alternating negative  64  and positive  66  layers therebetween. Multi-layer PZT  60  also includes contacts  70 , which are electrically connected to one or more of the above described layers within multi-layer PZT  60 . Such contacts are typically formed directly on top layer  62  of PZT  60 . A regulated voltage source can be coupled directly to PZT  60  utilizing contacts  70  on top layer  62 . Multi-layer PZT  60  therefore includes a plurality of ceramic layers including top layer  62 , negative layers  64 , positive layers  66 , and bottom layer  68  so as to form a stack  61  in which each ceramic layer has first and second opposing surfaces.  
         [0019]     In one embodiment, top layer  62  includes a top conductive pattern formed on its first surface  72 . The top conductive pattern includes a negative contact  74  and a positive contact  76 . Bottom layer  68  also includes a conductive pattern formed on its first surface. Layers  64  and  66  each include alternating conductive patterns formed on the first surface thereof. In such a multi-layer configuration, the co-fired ceramic layers  62 ,  64 ,  66 ,  68  are more tightly coupled to mirror  18  since they lack an epoxy layer between each ceramic layer. Therefore, almost all of the distortion in the ceramic stack  61  is directly imparted into mirror  18 .  
         [0020]     Sometimes, with conventional PZTs, for example, PZT  16  and multi-layered PZT  60 , in which the PLC driver is bonded directly to the transducer mirror, curvature in the mirror due to stresses or other factors may cause multi-moding of the laser beam that is directed towards (and reflected from) mirror  18 . In multi-layered PZT  60 , this multi-moding occurs more often, for example, in approximately 30-50% of the laser block assemblies which utilize a PZT similar to PZT  60 . This is particularly true, for example, because only thin layers  20 , for example, from about 0.0005″ to about 0.001″ of epoxy are typically used to attach the mirror  18  to the driver. This multi-moding interferes with the laser mode that the LBA  12  uses to get accurate count data (and therefore navigation data).  
         [0021]      FIG. 3  is a cross-sectional view multi-layered PZT  100 , which includes a stack  102  of alternating negative and positive co-fired ceramic piezoelectric layers,  104  and  106  respectively, attached to mirror  108  with a layer of epoxy  110 . Contacts  112  attached to top layer  114  operate similarly to contacts  70  (shown in  FIG. 2 ) as described above. Although not shown in  FIG. 3 , piezoelectric layers  104  and  106  include having alternating conductive patterns formed on their top surfaces providing a mechanism for electrical contact with contacts  112 .  
         [0022]     While generally similar to PZT  60  (shown in  FIG. 2  and described above), ceramic layers  104  and  106  of PZT  100  each include a void  120  or hole formed therethrough. Void  120  is roughly centered at a position generally co-linear to a perpendicular of the mirror and within an area  122  where a laser beam strikes mirror  108 . In one embodiment, void  120  is circular in shape and therefore, ceramic layers  104  and  106  have a shape similar to a washer. In other embodiments (not shown), the void is configured in other geometric shapes, including, but not limited to, a square, a rectangle, and an oval. Area  122  is sometimes referred to as a critical region. Within void  120  is a block  124  of non-piezoelectric material that is also bonded to mirror  108  with epoxy  110 . By bonding block  124  to mirror  108 , area  122  of mirror  108  is constrained to retain a surface with an improved flatness as compared to mirrors within known PZTs, for example PZTs  16  and  60 .  
         [0023]     The bonded non-piezoelectric material (e.g., block  124 ) acts through the bond of epoxy  110  to distribute stresses placed on mirror  108  by changes in temperature and voltage, for example, from mirror to block  124 . A surface area of block  124 , in one embodiment, is greater than area  122  since stress concentrations are greatest at a perimeter of block  124 . Therefore, moving area  122  (the critical region of mirror  108 ) farther from the perimeter of block  124  (closer to a center of block  124 ) reduces the effects of the stress concentrations.  FIG. 4  is a top view of PZT  100  further illustrating void  120  through top layer  114 , non-piezoelectric block  124  and area  122  of mirror  108 .  
         [0024]      FIG. 5  is a side view of mirror  108 , epoxy  110 , and block  124  which illustrates the stress distribution of block  124  and the improved flatness of mirror  108  within area  122 . The actual dimensions of block  124  and void  120  are dependent on the amount of voltage available and the flexibility (ease of driving the mirror) of mirror  108 . However, it is important is that void  120  is of a large enough diameter so that stresses are minimized in area  122  of mirror  108  at which the laser beam  40  reflects and thus minimizes any curvature.  
         [0025]      FIG. 6  is a side view of another embodiment of PZT  150  which is also configured to minimize a flexibility of mirror  152  within an area  154  from which a laser beam reflects. More specifically, PZT  150  includes a stack  156  of alternating negative and positive co-fired ceramic layers,  158  and  160  respectively, attached to mirror  152  with a layer of epoxy  162 . On a surface of a number of layers  158  and  160 , an electrode material  166  is screen printed thereon. Electrode material  166  is screen printed, in one embodiment, to include a void  170  therein which is larger than a critical region  172  of mirror  152 . Voids  170  render unscreened portions  174  of layers  158  and  160  inactive. The inactive portions  174  of layers  158  and  160  do not react to electrical signals applied to electrode material  166  from contacts  176  and therefore act to constrain a flatness of mirror  152  within critical region  172 .  
         [0026]     As above, the actual dimensions of voids  170  within electrode material  166  are dependent on the amount of voltage available and the flexibility (ease of driving the mirror) of mirror  152 . However, and similarly to voids  120  and non-piezoelectric block  124  in PZT  100 , it is important is that voids  170  are of a large enough diameter so that stresses are minimized in critical region  172  of mirror  152  at which the laser beam  40  reflects to minimize any curvature.  
         [0027]     The above described embodiments make the path length control (PLC) mirrors for laser devices such as ring laser gyroscopes less susceptible to beam area curvature within the mirrors due to thermal, voltage, and other displacement effects. In one embodiment, a stiffening block  124  is provided behind a critical area  122  of mirror  108  which limits the curvature induced into the mirror by the PLC driver. In this embodiment, a separate material (e.g., stiffening block  124 ) is located within voids  120  formed in the piezoelectric material,  104  and  106 , attached to a back side of mirror  108 . In another embodiment, stiffening is provided through a lack of screen printed electrode material  166  (e.g., voids  170 ) applied to piezoelectric material,  158  and  160 . Voids  170  within screen printed material  166  is sometimes referred to as an inactive area within the PLC driver. The inactive area  154  in the ceramic of the piezoelectric driver causes a decoupling of any bending motion within an active area of piezoelectric layers  158  and  160  (e.g., the area that is coated electrode material  166 ) from the inactive area  154  behind mirror  152 .  
         [0028]     While PZT  100  and PZT  150  are described as being formed from co-fired ceramic layers, it is to be understood that PZTs which are formed from individual ceramic layers that are bonded together after fabrication, for example, similar to PZT  16  (shown in  FIG. 1 ) may benefit from incorporation of the embodiments described herein. For example, voids may be formed in the ceramic layers and the epoxy utilized to bond the layers together to facilitate insertion of a non-piezoelectric stiffening block within the void and attached to the mirror. Similarly, voids may be formed in the electrode material that is applied to the surface of such ceramic layers, to provide a similar effect on the flexibility of a mirror to which such piezoelectric devices are attached.  
         [0029]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.