TECHNICAL FIELD 
     The present invention relates to path length control apparatus (PLC) for optical devices and in particular to a co-fired piezoelectric transducer that can be used in a PLC for a ring laser gyroscope and method of making the same. 
     BACKGROUND OF THE INVENTION 
     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 move 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. Three planar non-adjacent sides of the block form the mirror mounting surfaces for three mirrors at the comers of the optical path, which is triangular in shape. 
     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 and 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 for maximum signal. 
     One such known PLC  10  for a block  12  of a RLG  14  is illustrated in FIGS. 1-2. The PLC  10  includes a piezoelectric transducer (PZT)  16  which is secured to a mirror  18  via an epoxy-based adhesive  20 . The epoxy adhesive  20  completely covers the interface (defined by a lower surface  22  of the PZT  16  and an upper surface  24  of the mirror  18 ) between the PZT  16  and the mirror  18 . The mirror  18  is secured to a mirror mounting surface  26  of the optical block  12 . The mirror  18  communicates with laser bores  32  (only partially shown) of an optical cavity  34  (only partially shown) of the block  12 . The bores  32  partially form a portion of the closed loop optical path  38  defined by the optical cavity  34 . As seen in FIG. 1, the mirror  18  reflects counter-rotating laser light beams  40  at its respective corner of the closed loop optical path  38 . 
     Conventional PZT  16  (perhaps shown best in FIG. 2) is defined by a pair of piezoelectric elements  42  and  44 . A conductive tab  45  is sandwiched between the elements  42  and  44 , which are bonded to the conductive tab  45  by thin layers of conductive epoxy. Opposite polarity conductive tabs  41  and  43  are adhered to the outer major surfaces of elements  42  and  44 , respectively, also by thin layers of conductive epoxy. The opposite polarity leads  47  and  49  extend from the positive conductive tabs  41  and  43 , respectively. Another lead  48  extends from the negative conductive tab  45 . As shown in FIG. 1, the opposite polarity leads  47  and  49  are electrically connected to form a single lead  46 , and the leads  46  and  48  extend from the PZT  16  and are connected to terminals  50  and  52  of a wireboard element  54 . Leads  58  and  59  extend from the terminals  50  and  52 , respectively, of the wireboard element  54  and are coupled to a regulated voltage source (not shown) which is in turn coupled to a detector (not shown) which monitors the intensity of the light beams  40 . The PZT  16  takes an applied voltage delivered by the regulated voltage source, in response to a signal provided by the detector, and turns this voltage into small but precisely controlled mechanical movement. This mechanical movement of the PZT  16  affects translational movement (as represented by double-headed arrow  60 ) of the mirror  18 , and thereby controls the laser light beam path length. 
     SUMMARY OF THE INVENTION 
     The present invention is a multi-layer PZT fabricated as a multi-layer ceramic assembly. The multi-layer PZT of the present invention has contacts, which are electrically connected to other layers within the multi-layer PZT, formed directly on the top layer of the PZT, and the regulated voltage source can be coupled directly to the PZT at the top layer contacts. The present invention is a multi-layer piezoelectric transducer that can be used as a path length control apparatus of an optical device. The multi-layer piezoelectric transducer includes a plurality of ceramic layers so as to form a stack, wherein each ceramic layer has first and second opposing surfaces. The plurality of ceramic layers includes a top layer at a first end of the stack having a top conductive pattern formed on the first surface thereof. The top conductive pattern includes a negative contact and a positive contact. The plurality of ceramic layers also includes at least one poled ceramic layer having a conductive pattern formed on the first surface thereof. The plurality of ceramic layers include additional poled ceramic layers having alternating conductive patterns formed on the first surface thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a portion of a prior art path length control apparatus for a ring laser gyroscope incorporating a prior art piezoelectric transducer. 
     FIG. 2 is an isometric view of the prior art piezoelectric transducer shown in FIG.  1 . 
     FIG. 3 is an isometric view of a second embodiment of a multi-layer piezoelectric transducer according to the present invention. 
     FIG. 4 is a cross-sectional view of the multi-layer piezoelectric transducer of FIG. 3 taken along the line  8 — 8 . 
     FIG. 5 is a top, plan view of the top conductive pattern of the multi-layer piezoelectric transducer of FIG.  3 . 
     FIG. 6 is a top, plan view of the negative conductive pattern of the multi-layer piezoelectric transducer of FIG.  3 . 
     FIG. 7 is a top, plan view of the positive conductive pattern of the multi-layer piezoelectric transducer of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A multi-layer PZT  200  is shown in FIGS. 3-4 and can be used as a path length control apparatus of an optical device. PZT  200  comprises a stack  202  of circular ceramic layers that includes a top ceramic layer  204  at a first end of the stack  202  and alternating negative ceramic layers  206  and positive ceramic layers  208 . At the second end of the stack  202  opposite the first end is a bottom ceramic layer  209 , which, as described below, may be a negative ceramic layer, a positive ceramic layer, or a substantially unpoled ceramic layer. Although the PZT  200  is shown in FIGS. 3-4 as having two negative ceramic layers  206  and one positive ceramic layers  208 , it is to be understood that the PZT  200  can be fabricated with any number of negative ceramic layers  206  and positive ceramic layers  208 . The ceramic layers of the stack  202  typically have dimensions that are similar to the dimensions of the ceramic layers of PZT  100  described above. 
     The top ceramic layer  204  has a top conductive pattern  210  (perhaps shown best in FIG. 5) formed on an upper surface thereof, each negative ceramic layer  206  has a negative conductive pattern  212  (shown in FIG. 6) formed on an upper surface thereof, and each positive ceramic layer  208  has a positive conductive pattern  214  (shown in FIG. 7) formed on an upper surface thereof. As explained in detail below, the bottom ceramic layer  209  has either a negative conductive pattern  212  or a positive conductive pattern  214  formed on an upper surface thereof. 
     Negative castilation  226  that covers the side of the stack  202  is formed nearside edge  228 . A negative contact  216  (described below) that is formed in the top conductive pattern  210 , the negative conductive patterns  212 , and the negative castilation  226  are shaped and located so that the negative castilation  226  intercontacts the negative contact  216  of the top conductive pattern  210  and each of the negative conductive patterns  212 . Positive castilation  230  that connects to each layer of the stack  202  are formed on a second side edge  232 . A positive contact  218  (described below) that is formed in the top conductive pattern  210 , the positive conductive patterns  214 , and the positive castilation  230  are shaped and located so that the positive castilation  230  interconnects the positive contact  218  of the top conductive pattern  210  and each of the positive conductive patterns  214 . 
     The top conductive pattern  210  (perhaps shown best in FIG. 5) includes a negative contact  216  and a positive contact  218 . In the embodiment shown, the negative contact  216  has a generally semicircular shape with the circular periphery near the first side edge  228 . The positive contact  218  is generally cresent-shaped. The negative contact  216  and the positive contact  218  are separated and electrically isolated from each other by a channel  224  formed in the top conductive pattern  210  in which no conductive material is applied. The negative and positive contacts  216  and  218  serve as terminals to which a regulated voltage source (not shown) of an optical device such as a RLG can be coupled to the PZT  200 . 
     The negative conductive pattern  212 , shown in FIG. 6, is generally circular except for a crescent-shaped cutout portion  238  near the second side edge  232  in which no conductive material is present. The negative castilation  226  connects to the negative conductive pattern  212  so that the conductive coatings of the negative pattern castilation (shown in FIG. 4) formed on the surfaces of the stack  202  near side  228  can electrically connect the negative conductive pattern  212  to the other negative conductive patterns  212  and the negative contact  216 . The positive castilation  230  connects to the positive conductive pattern  214  so that the conductive coatings of the positive pattern castilation (shown in FIG. 4) formed on the surfaces of the stack  202  near side  232  can electrically connect the positive conductive pattern  214  to the other positive conductive patterns  214  and the positive contact  218 . The negative conductive pattern  212  does not extend to the peripheral edge of the negative ceramic layer  206  and instead a channel  240  separates and electrically isolates the rest of the negative conductive pattern  212  from the peripheral edge of the negative ceramic layer  206 . Preferably, all the negative conductive patterns  212  formed on ceramic layers of the stack  202  have substantially the same shape. 
     The positive conductive pattern  214 , shown in FIG. 7, is generally circular except for a crescent-shaped cutout portion  242  near the first side edge  228  in which no conductive material is present. The positive castilation  230  connects to the positive conductive pattern  214  so that the conductive coatings of the positive pattern castilation (shown in FIG. 4) formed on the surfaces of the stack  202  near side  232  can electrically connect the positive conductive pattern  214  to the other positive conductive patterns  214  and the positive contact  218 . The negative castilation  226  connects to the negative conductive pattern  212  so that the conductive coatings of the negative pattern castilation (shown in FIG. 4) formed on the surfaces of the stack  202  near side  228  can electrically connect the negative conductive pattern  212  to the other negative conductive patterns  212  and the negative contact  216 . The positive conductive pattern  214  does not extend to the peripheral edge of the positive ceramic layer  208  and instead a channel  244  separates and electrically isolates the rest of the positive conductive pattern  214  from the peripheral edge of the positive ceramic layer  208 . Preferably, the positive conductive patterns  214  formed on ceramic layers of the stack  202  are all substantially the same. Also, it is preferable that the positive conductive patterns  214  are mirror images of, and have substantially the same shape as, the negative conductive patterns  212  so that the bending imparted to the PZT  200  by each of the positive ceramic layers  208  is symmetrical to the bending imparted to the PZT  200  by each of the negative ceramic layers  206 . 
     If the ceramic layer immediately adjacent the bottom ceramic layer  209  is a negative ceramic layer  206  having a negative conductive pattern  212  formed thereon (as shown in FIGS.  3 - 4 ), then preferably the bottom ceramic layer  209  has a positive conductive pattern  214  formed on an upper surface thereof so that a voltage can be developed across the immediately adjacent negative ceramic layer  206  when a voltage is developed across the negative and positive contacts  216  and  218 . Likewise, if the ceramic layer immediately adjacent the bottom ceramic layer  209  is a positive ceramic layer  208  having a positive conductive pattern  214  formed thereon, then preferably the bottom ceramic layer has a negative conductive pattern  212  formed on an upper surface thereof so that a voltage can be developed across the immediately adjacent positive ceramic layer  208  when a voltage is developed across the negative and positive contacts  216  and  218 . 
     The bottom ceramic layer  209  can be formed as an unpoled ceramic layer (as shown in FIGS.  3 - 7 ). The bottom surface  211  of such an unpoled bottom ceramic layer  209  need not have a conductive pattern formed thereon. This allows a better epoxy bond to be formed between the bottom surface  211  of the PZT  200  and the optical device to which the PZT  200  is being attached. But, such an unpoled ceramic layer  209  that does not have a conductive pattern formed on its bottom surface  211  will not apply a bending force to the PZT  200  upon application of a voltage to the negative and positive contacts  216  and  218  and instead will resist the bending force provided by the negative and positive ceramic layers  206  and  208 . 
     Alternatively, the bottom ceramic layer  209  can be formed as a poled ceramic layer. If the poled bottom ceramic layer  209  in such an embodiment has a positive conductive pattern  214  formed on the upper surface thereof, preferably the bottom surface  211  of such a poled bottom ceramic layer  209  would have a negative conductive pattern  212  (connected to the other negative conductive patterns  212 ) formed thereon so that a voltage can be developed across the bottom ceramic layer  209  during the poling step. Likewise, if the poled bottom ceramic layer  209  has a negative conductive pattern  212  formed on the upper surface thereof, preferably the bottom surface  211  of such a poled bottom ceramic layer  209  would have a positive conductive pattern  214  (connected to the other positive conductive patterns  214 ) formed thereon so that a voltage can be developed across the bottom ceramic layer  209  during the poling step. In operation, a poled bottom ceramic layer  209  will apply a bending force to the PZT  200  upon application of a voltage to the negative and positive contacts  216  and  218  and will not resist the bending force provided by the negative and positive ceramic layers  206  and  208 . However, the epoxy bond that would be formed between the conductive pattern formed on the bottom surface  211  of the bottom ceramic layer  209  and the optical device would be less secure. 
     The negative and positive ceramic layers  206  and  208  (along with the bottom ceramic layer  209  if the bottom ceramic layer  209  is to be poled) can be poled at the same time by applying an appropriate voltage across the negative castilation  226  (which is in electrical contact with the negative conductive patterns  212 ) and the positive castilation  230  (which is in electrical contact with the positive conductive patterns  214 ) in the same manner that the ceramic layers of PZT  100  are poled. Also, as with PZT  100 , to improve the bending symmetry of PZT  200 , it is preferred that the amount of the top ceramic layer  204  that is poled during the poling step is reduced. 
     Negative and positive leads from external circuitry such as a regulated voltage source (not shown in FIGS. 3-7) can be connected to the negative and positive contacts  216  and  218 , respectively. 
     The PZT  200  shown in FIGS. 3-7 can be used as a PLC in an optical device such as a RLG. A regulated voltage source and/or other circuitry can be coupled to the contacts  120  and  122  of PZT  100  and the contacts  216  and  218  of PZT  200 . Thus, a wireboard element need not be attached to a PZT according to the present invention in order to provide a point at which a regulated voltage source or other circuitry can be coupled to the PZT. The regulated voltage source can be used to apply a voltage to the multi-layer PZT, which turns this voltage into small but precisely controlled mechanical movement in order to maintain a constant light path length in an optical device such as a RLG. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention. For example, the number of layers used and the shape of the final PZT can be varied to suit the particular application for which the PZT is fabricated.
A multi-layer PZT comprises a plurality of stacked ceramic layers. The stack of ceramic layers includes a top ceramic layer on which negative and positive contacts for electrically coupling the PZT to external circuitry are formed. The stack of ceramic layers also includes at least one negatively poled ceramic layer having a negative conductive pattern formed thereon and at least one positively poled ceramic layer having a positive conductive pattern formed thereon. The PZT also includes a negative pattern interconnect for electrically connecting the negative contact and the negative conductive pattern and a positive pattern interconnect for electrically connecting the positive contact and the positive conductive pattern. The multi-layer PZT can be fabricated using a ceramic co-firing process.