Abstract:
Ring laser gyroscope that includes a gyroscope block, a radio frequency transmitting device, and a radio frequency energy source. The gyroscope block has at least one discharge bore containing a gain medium, and the radio frequency transmitting device is located within the gyroscope block in proximity to at least one discharge bore and located so as to encompass the discharge bore. The radio frequency energy source is configured to apply a pulsed radio frequency signal to the transmitting device.

Description:
PRIORITY CLAIM 
   This application is a CIP of U.S. patent application Ser. No. 11/040,469 filed Jan. 21, 2005. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to ring laser gyroscopes, and more specifically, to systems and methods for utilizing pulsed radio frequencies within ring laser gyroscopes. 
   At least some known ring laser gyroscopes (RLGs) utilize a direct current (D.C.) voltage discharge in order to start and maintain laser beams within a discharge cavity located in a block of the RLG. A discharge cavity is also sometimes referred to as a gain bore or discharge bore. In such a utilization, D.C. electrodes must be in direct contact with a gain medium of the laser that is contained within the discharge bore. In order to prevent external materials from leaking around these D.C. electrodes, an interfacial seal is used to bond the electrodes to the laser block. The integrity of such interfacial seals has historically limited the temperature range, reliability, and lifetime of RLGs which employ the interfacial seals. 
   Often the gain necessary to sustain the laser beams within an RLG require discharge currents which are powerful enough to sputter cathode material from the electrodes into the gain medium. This sputtering contaminates the gain medium which results in shortening the laser lifetime and hence gyro reliability and performance. Additionally, the cathode or cathodes, depending upon the RLG configuration, pump gases from the gain medium producing undesirable gas mix changes. 
   Other known ring laser gyroscopes employ capacitively coupled radio frequency (RF) energy which maintain the laser beams within the gyroscope through discharge of the RF energy. In such gyroscopes, electrodes transmitting RF energy are deposited onto an outer surface of the laser block. Still another known RLG employs an inductive coil wrapped around one leg of the discharge bore within the laser block. In this gyroscope embodiment, the inductive coil may be embedded within the laser block itself. As still another alternative, a capacitively coupled RF apparatus which includes two plates, is embedded within the laser block. When utilizing such an apparatus, one leg of the discharge bore is juxtaposed between two of the plates. 
   These RLGs couple continuous wave RF energy into the gain medium of a ring laser gyroscope thereby eliminating the need for electrodes within the discharge bores and the resulting problems associated with the sealing of the laser block. However, dynamic impedance characteristics of the gain medium within the discharge bore can cause problems related to controlling an amount of power delivered to the gain medium when utilizing such continuous wave (CW) RF signals. 
   SUMMARY OF THE INVENTION 
   In one aspect, a ring laser gyroscope is provided that comprises a gyroscope block having at least one discharge bore containing a gain medium, a radio frequency (RF) transmitting device, and an RF energy source. The transmitting device is within the gyroscope block in proximity to at least one discharge bore. The RF energy source is configured to apply a pulsed RF signal to the RF transmitting device, the RF transmitting device located such that the pulsed RF signal is applied to the gain medium. 
   In another aspect, a method for pumping a gain medium within a discharge bore of a ring laser gyroscope is provided. The method comprises locating an RF transmitting device in proximity to the discharge bore and providing a pulsed RF signal to the transmitting device such that the pulsed RF signal is applied to the gain medium. 
   In still another aspect, a ring laser gyroscope is provided which comprises a gain medium, a radio frequency (RF) transmitting device, and an RF energy source. The RF energy source applies a signal to the RF transmitting device. The signal initiates a discharge from the RF transmitting device within the gain medium. The signal is a pulsed RF signal having a duty cycle between zero and one. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
       FIG. 1  is a top view of one embodiment of a ring laser gyroscope which includes an inductive coil and a pulsed RF supply. 
       FIG. 2  is a graph illustrating one embodiment of a signal provided by the pulsed RF supply shown in  FIG. 1 . 
       FIG. 3  is a block diagram illustrating one embodiment of a pulsed RF supply for utilization within a ring laser gyroscope. 
       FIG. 4  is a top view of a ring laser gyroscope configured with capacitive plates coupled to a pulsed RF supply. 
       FIG. 5  is a side view of the ring laser gyroscope of  FIG. 4  further illustrating the capacitive plates. 
       FIG. 6  is a top view of a gyroscope block which incorporates multiple pairs of capacitive plates which may be utilized with a pulsed RF supply. 
       FIG. 7  is a top view of a gyroscope block which incorporates multiple inductive coils which may be utilized with a pulsed RF supply. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a top view of one embodiment of a ring laser gyroscope (RLG)  10  in which a pulsed radio frequency (RF) is applied to a gain medium. Utilization of pulsed RF reduces average RF power provided to RLG  10  as compared to RLGs which utilize a continuous wave RF signal to initiate and maintain a laser beam within RLG  10 . RLG  10  comprises a gyroscope block  12 , transducer mirrors  14 ,  16 , a readout mirror  18 , discharge bores  20 ,  22  and  24 , and an inductive coil  26 . Inductive coil  26  is one embodiment of an RF transmitting device as further described below. Gyroscope block  12 , in alternative embodiments, is fabricated from one or more of Zerodur.RTM. (Zerodur is a registered trademark of SCHOTT AG), silica, or another comparable material having stable temperature expansion characteristics. Transducer mirrors  14 ,  16  and readout mirror  18  are bonded to corners of gyroscope block  12  to form a gas tight seal. A gain medium, for example, helium neon (HeNe) gas may be employed within discharge bores  20 ,  22 , and  24 . Upon discharge of the RF signals from inductive coil  26 , counter propagating continuous laser beams  28  are induced within RLG  10 . Initiation and maintaining a continuous laser beam within RLG  10  utilizing, for example, RF energy, is sometimes referred to as gain pumping of a gain medium. 
   In the embodiment illustrated, inductive coil  26  is wound around RLG discharge bore  22 , for example, and is embedded within gyroscope block  12 . Inductive coil  26  is fabricated from any suitable conductive material and may be constructed in accordance with well known coil winding techniques. Inductive coil  26  may be embedded by depositing or printing onto gyroscope block  12 , for example, or by drilling holes through gyroscope block  12 . A first terminal  30  of inductive coil  26  is connected by a conductor  32  to a pulsed RF supply  34 . Pulsed RF supply  34  is sometimes referred to as an RF energy source. A second terminal  36  of inductance coil  26  is connected by conductor  38  to a second terminal of RF supply  34 . 
   A pulsed RF signal (shown in  FIG. 2 ) is transmitted from pulsed RF supply  34  to an RF transmitting device (e.g., inductive coil  26 ) which substantially encompasses discharge bore  22  which contains the gain medium of RLG  10 . This RF signal initiates a discharge which starts and maintains a laser beam within gyroscope block  12 . By utilizing a pulsed RF signal, as opposed to a continuous wave RF signal to pump the gain medium within discharge bores  20 ,  22 , and  24  of RLG  10 , the average RF power consumed is reduced by, for example, ten times the base-10 log of the duty cycle of the pulsed RF signal. In the described embodiments, the duty cycle is a number between zero and one. In addition, altering the duty cycle and/or power envelope of the pulsed RF also provides an additional mechanism to control the laser discharge and the power of the optical laser output beam. 
   The power reduction achieved through utilization of a pulsed RF signal is further illustrated in  FIG. 2 , which is a graph  50  illustrating a pulsed RF signal  52  and a power envelope  54  generated by pulsed RF signal  52 . Graph  50  illustrates an approximate ⅓ duty cycle, which is sometimes referred to as a 33% duty cycle. Pulsed RF signal  52  is composed of a sequential time series of pulses of frequency f 0  spaced by a time of T. The pulse width is αT where α is the duty cycle with a unitless range of 0&lt;α&lt;1. The average power is then the maximum power multiplied by α, or Pave=αPmax. The average power delivered by pulsing the RF signal is 10*log 10 (α) less than the CW case (α=1). For example, utilization of pulsed RF supply  34  configured with a duty cycle of 0.1 will result in an average power of 10 dB less than a ring laser gyroscope which utilizes a continuous wave RF supply. 
   While power envelope  54  is illustrated as being rectangular, the description should not be construed as being limited to a rectangular power envelope. Any arbitrary shaped power envelope may be incorporated. In addition, neither the pulse period T, nor the duty cycle α are limited to a constant value. In other words, a variable pulse period and/or a variable duty cycle may be incorporated into the embodiments described herein. 
     FIG. 3  is a block diagram of pulsed RF supply  34  which also illustrates a connection to RLG  10 . Pulsed RF supply  34  includes an RF oscillator  100 , an RF driver amplifier  102  configured to amplify an output of RF oscillator  100 , an impedance matching device  104  receiving an output of RF driver amplifier  102 , and an RF power amplifier  106  receiving the output of impedance matching device  104 . An output of RF power amplifier  106  is received by an output impedance matching unit  108  which is configured to output the pulsed RF signal onto conductors  32  and  38  for transmission to inductive coil  26 . 
   Pulsed RF supply  34  further includes a pulse control circuit  110  and a pulse drive circuit  112 , which in combination are configured to control a duty cycle of the RF signal output by pulsed RF supply  34 . Pulse drive circuit  112 , in one embodiment, is configured to provide signal conditioning, for example, amplification, filtering, and/or impedance matching, to a signal output by pulse drive circuit  110 . In alternative embodiments, pulse control circuit  110  is fabricated utilizing a programmable integrated circuit (PIC), a microprocessor, microcontroller or FPGA (field programmable gate array), depending on the level of pulse control desired. 
   In one embodiment, a method for producing a pulsed RF signal is to modulate the bias current on an active device (i.e., RF driver amplifier  102 , RF power amplifier  106 ) with a high frequency switch. Pulse control circuit  110  provides such a switch. As illustrated, pulse control circuit  110  is configured to control a duty cycle of the RF signal produced by RF oscillator  100  by switching off and on (e.g., modulating the bias current of) RF driver amplifier  102 . In such an embodiment, pulse control circuit  110  is configured as a high frequency switch. 
   In addition, to control an envelope of RF power output by pulsed RF supply  34 , pulse control circuit  110  is further configured to provide an enabling signal to RF power amplifier  106 . By providing controlling signals to both RF driver amplifier  102  and RF power amplifier  106  utilizing pulse control circuit  110 , which is in one embodiment user configurable, a user is able to control the output of pulsed RF supply  34 . Specifically, both the duty cycle of the generated RF signal and the shape of a power envelope output by pulsed RF supply  34  are user programmable. 
   As described above, inductive coil  26  is one embodiment of an RF transmitting device which can be utilized with the RF energy source of pulsed RF supply  34 .  FIG. 4  illustrates an alternate embodiment of RLG  150  which may be utilized with pulsed RF supply  34 . RLG  150  is, similar to RLG  10  (shown in  FIG. 1 ) except that capacitive plates  152  and  154  (capacitive plate  154  not shown in  FIG. 4 ) have been substituted for inductive coil  26 . 
     FIG. 5  schematically shows a side view of ring laser gyroscope  150 . One gain bore section (e.g., discharge bore  22 ) is juxtaposed between embedded capacitive plates  152  and  154 . Capacitive plate  152  is coupled by conductor  32  to pulsed RF supply  34 , and capacitive plate  154  is coupled by conductor  38  to pulsed RF supply  34 . Any suitable conductive material may be used to form the capacitive plates. For example, conductive adhesive strips may be utilized to form capacitive plates  152  and  154 . 
     FIG. 6  illustrates another alternative embodiment of a portion of a RLG  200  which can be utilized which pulsed RF supply  34 . RLG  200  includes a gyroscope block  202  and a plurality of pairs of capacitive plates. Specifically gyroscope block  202  includes six pairs of capacitive plates. In  FIG. 6 , capacitive plates  204 ,  206 ,  208 ,  210 ,  212  and  214  are shown. Similarly to  FIG. 4 , capacitive plates which are opposite the discharge bore corresponding to capacitive plates  204 ,  206 ,  208 ,  210 ,  212  and  214  are not shown. The embodiment of  FIG. 6  should be construed as an illustration and not a limitation. Those of skill in the art will understand that a greater or lesser number of such plates may be used in configurations employing the methods and apparatus described herein. 
     FIG. 7  illustrates still another alternative embodiment of a portion of a RLG  300  which includes a gyroscope block  302  and a plurality of conductance coils  304 ,  306 ,  308 ,  310 ,  312 , and  314  that may be employed in place of the capacitive plates  204 - 214  illustrated in  FIG. 6 . At least one advantage to using multiple capacitive plates  204 - 214  or inductive coils  304 - 314  for each discharge bore of a RLG is that it allows for the utilization of several phases of the RF drive source. Utilization of several phases of RF power promotes a smoothing of the output power and allows the use of lower frequencies than might otherwise be necessary. 
   Multiple phases may be employed with any of the embodiments described herein using multiple RF transmitting device, for example, either inductive coils or capacitive plates. For example, in an embodiment using two pairs of capacitive plates, the capacitive plate pairs may be driven with a phase difference of 90 degrees. Other configurations may be similarly driven to maintain the desired smoothing effect. Operation of single and multiple inductive coils and capacitive plates are described in more detail in U.S. Pat. No. 5,381,436 entitled “Ring Laser Gyro Employing Radio Frequency For Pumping of Gain Medium” which issued to Nelson et al., the entire subject matter of which is hereby incorporated by reference in its entirety. 
   A method of generating and delivering a pulsed RF signal to an RF energy transmitting apparatus, for example inductive coil  26 , is herein provided utilizing components which may be situated on a printed-circuit board (PCB) substrate within a ring laser gyroscope assembly. The pulsed RF device described herein utilized for power gas discharge within a ring laser gyroscope provides improved gas discharge lifetime and reduced cost for the gyroscope as compared to the above described DC discharge ring laser gyroscopes. 
   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.