Abstract:
In an angular rate sensor including a plurality of thin film waveguides and a directional coupler, a plurality of interdigitated transducers arranged to propagate energy into the waveguides and detect the resulting counterpropagating waves.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the field of optical angular rate sensors of the passive ring resonator type typified in Lawrence U.S. Pat. No. 4,326,803. In such an instrument a passive waveguide provides a closed loop propagation path for optical signals about a measurement axis. Coherent optical signals are coupled into the path for propagation in opposite directions therearound: as pointed out by Sagnac, the effective length of this path varies with the direction and rate of the rotation of the instrument about the measurement axis. The frequencies of the oppositely propagating signals are adjusted to make them resonant in the propagation path, and output coupling means samples the two frequencies, maintains them at resonant values, and determines their difference as a measure of the rate of rotation of the instrument. 
     SUMMARY OF THE INVENTION 
     The present invention makes use of acousto-optic deflector/modulator means having a novel structure to supply and sample the counterpropagating signals in such a resonator, for maintaining them resonant and determining their frequency difference. 
     Various advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and objects attained by its use, reference should be had to the drawing which forms a further part hereof, and to the accompanying descriptive matter, in which there are illustrated and described certain preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing, in which like reference numerals identify corresponding parts throughout the several views, 
     FIG. 1 illustrates a first embodiment of the invention, 
     FIG. 2 gives details of frequency shifting apparatus used in the invention, and 
     FIG. 3 shows a complete system usable in practice of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to FIG. 1, there is shown a first embodiment of the invention using thin film passive optical waveguides. A first single mode channel thin film waveguide 10 formed on a planar substrate 11 of Z n  O/Si acts as a ring resonator and provides a closed loop propagation path about an axis 12 normal to the substrate: the substrate is mounted for rotation about that axis by suitable means not shown. A second similar waveguide 13 has a mid-portion 14 arranged to coact with waveguide 10 as a directional coupler 15. Optical signals are supplied to the ends 16 and 17 of waveguide 13 from waveguides 20 and 21 respectively of a pair of similar crossed-channel acousto-optic devices 22 and 23. A controllable frequency diode laser suggested at 24 acts through a power divider 25 to supply first and second optical signals at a frequency f 0 , on waveguides 26 and 27, to devices 22 and 23. An electro-optical modulator 28 acts on waveguide 10 to continuously vary the effective path length therein, and hence the resonant frequency thereof, sinusoidally through a small range. 
     Waveguide 20 extends beyond shifter 22 at 30 to a detector 31, and waveguide 21 extends beyond shifter 23 at 32 to detector 33. Waveguides 26 and 27 also extend beyond shifters 22 and 23 to terminate at traps 34 and 35, all respectively. 
     Devices 22 and 23 are of the same nature, and FIG. 2 shows device 23 schematically in more detail. It includes a crossed-channel waveguide area 40, the channels crossing at an angle which for parameters appropriate to this device might be about 3.6 degrees. An interdigitated transducer or generator 41, the center frequency f 1  of which is such that the corresponding Bragg angle is equal to one half the intersection angle of the waveguides, launches a surface acoustic wave 42 across area 40, so that a portion of the laser energy from power divider 25 entering at 27 does not proceed to 35, but is deflected to emerge at 21. The shifted laser beam supplied to end 17 of waveguide 13 is of frequency (f 0  +f 1 ). A first portion of this beam is coupled into ring resonator 10 in the clockwise direction as seen in FIG. 1, and a second portion is reflected at coupler 15 and continues through waveguides 13 and 20, shifter 22, and waveguide 30 to detector 31. Likewise at coupler 15 a portion of the clockwise beam in waveguide 10 is coupled back to waveguide 13 and hence through waveguide 20, shifter 22, and waveguide 30 to detector 31. 
     In the same way, frequency shifter 22 has a generator 43, and a portion of the signal supplied at 26 from power divider 25 does not proceed to 34 but is displaced to emerge at 20. Generator 43 is excited at a frequency f 2 , and the shifted laser beam supplied to end 16 of waveguide 13 is of frequency (f 0  +f 2 ). A portion of this beam is coupled into ring resonator 10 in the counterclockwise direction, as seen in FIG. 1, and a portion is reflected at coupler 15 and continues through waveguides 17 and 21, shifter 23, and waveguide 32 to detector 33. Likewise at coupler 15 a portion of the counterclockwise beam in waveguide 10 is coupled back to waveguide 13 and hence through waveguide 21, shifter 23, and waveguide 32 to detector 33. 
     Attention is now directed to FIG. 3, where it is shown that laser diode 24 is energized from a power amplifier 50: varying the power supplied at 50 varies the frequency f 0  of the laser slightly by changing its temperature. 
     As shown in FIG. 3, generator 43 is excited at frequency f 2  by a constant frequency oscillator 51. Modulator 28 is energized from a constant frequency oscillator 52. Oscillator 52 also presents a reference signal to a pair of demodulators 53 and 54. The output of detector 33 is supplied through a preamplifier 55, an amplifier 56, demodulator 53 and a servo 57 to power amplifier 50. The output of detector 31 is supplied through a preamplifier 60, an amplifier 61, demodulator 54 and a servo 62 to a voltage controlled oscillator 63, the output of oscillator 63 being combined in a single side band mixer 64 with the output of constant frequency oscillator 51, to supply generator 41 of frequency shifter 23. 
     The angular rate signal from the device is taken as the output 66 of oscillator 63. 
     In one proposed embodiment of the invention, laser 24 operated at a wavelength of 0.83 microns or a frequency of 3.6×10 8  megahertz, oscillator 51 was at 600 megahertz frequency, and oscillator 52 was at 25 kilohertz frequency. 
     OPERATION 
     The operation of this angular rate sensor is as follows. Consider first the state of the device when it is not rotating about axis 12. An optical signal of frequency f 0  +f 2  is being supplied by frequency shifter 22 through waveguide 13 and coupled counterclockwise into ring 10 at coupler 15, but a portion of the signal passes coupler 15 and goes through frequency shifter 23 to detector 33. It is a characteristic of coupler 15 that the amount of energy transferred between waveguides 13 and 10 is greatest at the resonant frequency of the ring. This means that for resonance the output of detector 33 is minimum, and f 0  is to be adjusted to minimize the intensity of f 0  +f 2  as sensed at detector 33. 
     The signal in loop 10 has been modulated at 28, and components 55, 56, 53, 57, and 50 operate to change the frequency of laser 24 to minimize the intensity of the signal reaching the detector, and hence to cause the counterclockwise optical signal to achieve a resonance in the ring 10 by varying the frequency f 0  : frequency f 2  does not change. 
     At the same time an optical signal of frequency f 0  +f 1  is being supplied by frequency shifter 23 and coupled clockwise into ring 10. A portion of this signal passes through shifter 22 to detector 31. This signal is also modulated at 28. 
     Components 60, 61, 54, and 62 function to control the frequency of voltage controlled oscillator 63 to modify the output of fixed oscillator 51 so that the sum of these outputs, namely fl, has such a value that when combined with f 0  set as above-described, the sum f 0  +f 1  is equal to f 0  +f 2 , the resonance frequency of the ring 10. 
     Now suppose the unit rotates about axis 12 at a rate Ω, in a clockwise direction. The effective length of the resonator in the counter-clockwise direction decreases, so that its resonant frequency increases, and the signal to detector 31 decreases, and f 0  is adjusted as described above until f 0  +f 2  is equal to the new resonant frequency. At the same time the effective length of the resonator in the clockwise direction decreases, so that its resonant frequency increases, and f 1  is adjusted as described above until f 0  +f 1  is equal to the new resonant frequency. The frequency of oscillator 63 required to do this is a measure of the rate of rotation Ω, and can be taken as the output of VCO 63 at 66. 
     Numerous characteristics and advantages of the invention have been set forth in the foregoing description, together with details of the structure and function of the invention, and the novel features thereof are pointed out in the appended claims. The disclosure, however, is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts, within the principle of the invention, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.