Abstract:
A crystal oscillator including a laser light source for emitting a laser beam to an aligned quartz crystal coupled to an oscillator circuit by an optical feedback network. The optical feedback network is responsive to variations of misalignment of the laser beam with the crystal and correction signals generated for introduction back to the crystal to bring its frequency back to a constant standard frequency output.

Description:
Priority claim based on Ser. No. 60/141,010 filed Jun. 29, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of electronic crystal oscillators, and more particularly to a frequency stability control for crystal oscillators including dielectric resonator oscillators, commonly referred to as DRO which reduces the number components, increases reliability and substantially reduces size, weight, and power consumption. 
     2. Brief Description of the Prior Art 
     In the past, it has been the usual practice to stabilize the frequency of crystal oscillators through the employment of low phase noise transistors biased at their minimum noise figure operating point and an automatic gain control loop to keep the voltage amplitude across the crystal constant. The oscillator is further stabilized by employing another loop for temperature control in order to maintain the crystal and the frequency sensitive elements at the optimum temperature of the crystal&#39;s high turnover point. For crystal oscillators with stringent long term stability requirements, an additional means is provided taking the form of a frequency control loop which is implemented with a Rubidium or Cesium cell known as the “physics package” serving as a series resonant circuit of extremely high quality (Q) factor. However, this approach to high frequency stability design is highly labor intensive, complicated and expensive due to the difficulty of circuit adjustment and number of component parts required. 
     In particular, when the frequency control loop is included in the “physics package”, the loop contains circuitry to excite the lamp, and a thermostat for temperature control of the lamp. Additional circuitry is required to excite the electric field inside the lamp as well as servo-amplifiers, sweep circuits, and a low frequency error signal generator. The loop further includes a lock detector, a phase modulator and frequency multipliers. This degree of complexity reduces the reliability of the oscillator and substantially contributes further to an increase in size, weight, and power consumption. Therefore, the present invention is intended to overcome the above recited shortcomings of conventional Rubidium and Cesium standards. 
     Also, it has been the conventional practice to frequency stabilize microwave DRO&#39;s by phase locking the oscillator to another signal typically derived from a crystal source. This approach to high frequency stability design is highly labor intensive, complicated, expensive and inefficient because it requires successive multiplication and filtering of a low frequency signal with higher stability at the expense of higher power consumption and lower reliability. Also, as part of the multiplication process, the enhanced phase noise of the crystal source appears at the reference frequency resulting in loop design trade-offs between noise band widths and lock-in ranges. 
     Therefore, a long-standing need has existed to provide a DRO phase-locking technique which extends to stability enhancement of lower frequency crystal oscillators while overcoming the above problems. 
     SUMMARY OF THE INVENTION 
     Accordingly, the above problems and difficulties are avoided by the present invention which provides a novel technique to stabilize the frequency of a crystal oscillator by passing through the crystal or reflecting from the crystal two light beams. These two beams are formed from one common laser source passing through a beam splitter. After passing through the crystal or reflected from the crystal, the beams are combined and allowed to interact. Since these beams propagate through different path lengths, the result of their interaction is a beam where energy is polarized at an angle that changes if the difference in path propagation length changes. The combined beam is split again by a splitter-polarizer lens, resulting in two beams in which the energy of one contains vertical polarization only and the energy in the other horizontal polarization only. Any shift in propagation path length difference due to crystal displacement changes, causes the magnitude of these energies to change in opposite directions. The light of these polarized beams is photo-detected and converted to electric signals, and by comparing these two signals an error signal is obtained that is used to change the excitation current that passes through the crystal. The piezoelectric effect forces the crystal displacement change back to zero thus achieving frequency stability control and phase noise reduction. 
     Also, the present invention provides a frequency stability for a DRO or a crystal oscillator by employing a laser beam and interferometer. In another form of the invention, a laser source generates a beam to a beam splitter which then introduces the split beam to a mirror that then introduces the reflection to a dielectric resonator as well as to a beam combiner followed by introduction to a beam splitter and then through photo detectors to a phase/frequency control loop circuit. The output of the circuit is then looped back to the dielectric resonator via an interferometer including a matching network. Thereby, the phasing frequency of the dielectric resonator is adjusted by the feedback loop. 
     Therefore, it is among the primary objects of the present invention to provide a frequency stability means for a DRO or crystal oscillator employing a laser beam source and an interferometer. 
     Another object of the present invention is to provide an improved means for gaining long-term frequency stability of crystal standards or free-running DRO&#39;s by at least one order of magnitude. 
     Yet another object of the present invention is to improve the phase noise ratio of crystal standards and free-running DRO&#39;s by employing laser techniques. 
     Yet another object is to provide a stabilized DRO which is lighter in weight, more reliable in performance and far more economical to produce and fabricate than can be achieved with conventional phase-locked DRO&#39;s. 
     A further object resides in providing a frequency standard which is lighter in weight, more reliable in performance and more economical to fabricate than conventional standards. 
     Also, it is among the primary objects of the present invention to provide a novel frequency stabilizing means for a crystal oscillator, which employs a laser beam and a Michelson interferometer. 
     Another object of the present invention is to provide a long-term frequency stability control of a crystal oscillator by improving the standards by at least one order of magnitude. 
     Another object of the present invention is to improve the phase noise characteristics of crystal oscillators by at least a factor or two. 
     Another object of the present invention is to provide a novel laser controlled crystal oscillator that reduces the size, weight and power consumption of conventional frequency standards. 
     Still a further object of the present invention is to provide a novel frequency control stabilization means for crystal oscillators employing a laser control feature, that is more reliable and more economical to fabricate, calibrate and maintain than otherwise possible with conventional frequency standards. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood with reference to the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a diagrammatic view illustrating the laser controller crystal oscillator incorporating the present invention; 
     FIG. 2 is block diagram showing the control sequence and arrangement of components employed in the oscillator system of FIG. 1; 
     FIGS. 3 and 4 are diagrammatic views of a metallic vibrating crystal; 
     FIG. 5 is a diagrammatic view of a laser controlled DRO in accordance with the present invention; and 
     FIG. 6 is a block diagram of the laser controlled DRO illustrated in FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, the novel laser controlled crystal oscillator is indicated in the general direction of arrow  10  and is illustrated as being provided with a crystal oscillator circuit  20  that includes automatic gain control and temperature control loops. A laser source  11  generates a beam which is directed to a beam splitter  12 , beams  15  and  16  go through or are reflected by crystal  17  and then directed to a beam combiner  13 . The combined beam is introduced to and incides upon a splitter-polarized lens  14 . One of the two emerging beams contains vertically polarized light only while the other contains horizontally polarized light only. These two beams incide a pair of photo detectors  19 . Electric currents generated by the photo detectors are introduced via lines  23  and  24  to the oscillator circuit  20 . Therefore, it can be seen that the frequency control loop is implemented by the laser source, the beam splitter, the beam combiner, the splitter-polarizer and the photo detector circuits. 
     A beam of coherent monochromatic light is emitted from a laser diode acting as the laser source  11  and is split into two beams  15  and  16  so that phase and amplitude coherence is established between the beams. The beams are transmitted through the quartz crystal or reflected from the quartz crystal  17  at two specific locations where the lateral displacement sensitivity of the crystal is maximum. The refracted or reflected beams are routed to the beam combiner  13  where they interact resulting in a beam with polarization angle changes if the path length difference between beams  15  and  16  changes. The combined beam is made to incide on the splitter-polarizer lens  14  from which two beams emerge, one vertically polarized only and the other horizontally polarized only. These two beams are directed to photo detectors  19  where their light intensity is converted to electric signals and by comparing their amplitude an error signal is formed that drives the frequency control loop. Any crystal displacement changes within a quarter wavelength of the laser light will generate an error signal that changes the phase of the excitation current that passes through the crystal. The piezoelectric effect forces the crystal&#39;s displacement back to zero to achieve frequency stability control and reduction of phase noise. Thus, the accuracy of the crystal oscillator is greatly improved, especially when employed in clock or time-keeping applications. 
     Referring now in detail to FIG. 3, the quartz crystal  17  is illustrated with an advanced metallization pattern that enables the laser beams to pass through the points of maximum displacement sensitivity and also provide the electrical contacts required for crystal excitation. The metallization pattern is accomplished so as not to impede, dampen, obstruct or prohibit the transmission of laser light through the crystal. 
     FIG. 3 depicts a preferred approach for preparing the metallization of a crystal when a light transmission technique is to be used instead of a light reflection technique. Since an electrical connection is required at diametrically opposed points, the metallization would take the form of two equal paths so as not to interrupt the symmetry or temperature effects, but will allow laser light to pass through the crystal at optimum inflection points in order to control the frequency of operation of oscillator  10 . Numeral  25  indicates the crystal while numeral  26  indicates the metallization which provides the same pattern on opposite side of the crystal. The contacts are indicated by numerals  27  and  28  on opposite sides of the crystal respectively. The inflection points are represented by Rb and Ra. The contacts  27  and  28  are connected to input electrode or terminal  30  and output electrode or terminal  31  which are then carried through a mounting block  32  and couple with the oscillator circuit. Electrode  29  is a grounding center pin to prevent noise and electrostatic build-up on the case of the oscillator or device. The center of the crystal within the circular metallization  26  is exposed and transparent to passage of the light beams. 
     FIG. 4 illustrates a similar quartz crystal  25  to that shown in FIG. 3; however, crystal  39  is a quartz crystal having metal reflecting surfaces  40  and  41  receiving beams  15  and  16  respectively. The beams are reflected back out of the crystal to beam combiner  13  preparatory for introduction to the photo detectors. 
     Referring to FIGS. 5 and 6, another embodiment of a crystal oscillator is a laser controller system for a dielectric resonant oscillator as indicated in the general direction of arrow  110 . The system for control includes a laser source  111  for generating a beam  112  which is received by a beam splitter  113  and the output beams  127  and  128  of the beam splitter are introduced to a light reflecting surface  114  on a dielectric resonator  115  via a partially reflecting mirror  118 . The reflected beams from the surface  114  are indicated by numerals  116  and  117  which are processed back through the partially reflecting mirror  118 . The two reflected beam outputs from the partially reflecting mirror represented by numerals  130  and  131  which are then introduced to a beam combiner  120  which then introduces its single beam output to a beam splitter  121 . The beam splitter  121  may also be referred to as a polarizer and its output is transmitted to a pair of photo-electric circuits or detectors  122  and  123  prior to introduction to a phase/frequency loop circuit  124  via lines  125  and  126 . 
     In more detail, a light beam  112  projected from the diode in the laser source  111  is split into the two beams  127  and  128  which provide phase and amplitude coherence between the beams. The beams are reflected back from the surface  114  of the resonator to mirrored surface  118 . The two reflected beams  130  and  131  are combined in the beam combiner  120  and allowed to interact yielding a fringe pattern. The resulting beam is split into the two beams  125  and  126  by the splitter-polarizer  121  and applied to individual photo detectors  122  and  123 . The voltages on lines  125  and  126  from photo detectors  122  and  123  are compared forming an error signal that drives the phase/frequency control loop  125 . Any resonator or crystal displacement changes within half a wavelength of the laser light will generate a signal that changes the phase of the excitation field interacting with the resonator  115  or current that passes through the crystal of the resonator. The piezoelectric effect forces the crystal&#39;s displacement change back to zero thus achieving phase/frequency stability control and reducing phase noise. In the DRO  133 , the field interaction between the resonator and the transmission lines of the external circuit force the field change back to zero in order to achieve phase/frequency control. 
     A varactor diode  134  tunes the oscillator by changing the frequency thereof. Micro strip transmission lines are indicated by numerals  132 ,  135  and  136  and numeral  137  is a FET. A bypass capacitor  137  diverts RF energy to ground. A coupling capacitor  138  joins a coaxial adapter  140  to a transmission line  141 . 
     An interferometer of the Michelson type may be employed in the circuit to produce simplicity and improve the long-term frequency stability of a dielectric resonator. The oscillator circuit interferometer is directly coupled to the phase/frequency control loop circuit  124  and forms a part thereof which includes various electronic components including a matching network. 
     The transmission line  136  couples RF energy from the drain of the FET into the dielectric resonator (frequency determining element) which in turn couples energy to the gate of the FET  137 , (via transmission line  135 ) closing the feedback path and thus establishing oscillations. The mirrored or polished surface  114  reflects the two light beams used for optical loop control. Error signal developed by comparison of the two reflected light beams acts upon the varactor  134  keeping the oscillator frequency constant. The optical loop is sensitive to displacement changes in the top surface of the resonator and these changes are translated into electric signals for frequency control. 
     In view of the foregoing, it can be seen that the frequency stability means of the present invention for a DRO or a crystal oscillator is greatly improved with respect to the phase noise of crystal standards and free-running DRO&#39;s. Also, it can be seen that the shortcomings of conventional DRO phase-locking techniques are avoided and also the present invention extends to stability enhancement of lower frequency crystal oscillators. 
     The inventive concept stabilizes the frequency of an oscillator by means of an optical feedback loop using split laser beams that may be used in controlling frequency stability in such applications as atomic clocks with less power, weight, cost and higher reliability. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.