Two servo loop passive ring laser gyroscope

A passive ring resonator gyroscope comprising a single piece body having an integral first and second resonator cavity. The first resonator cavity contains a single frequency laser means to provide a sharply tuned single frequency light source to the second resonator cavity. The single frequency light source is sharply tuned and is split to form first and second sources. The second resonator cavity is a passive high Q cavity having a closed second optical path. The two light sources are fed to the second resonator and propagate as CW and CCW beams within the second resonator. A first servo tunes the frequency of the linear laser to the resonance peak of the CW beam in the second resonator. A second servo means is provided to shift the frequency of the CCW beam to its resonance peak. By converting both servo error outputs into frequency, the relative frequency difference between the CW and CCW beams are recorded as the frequency shift in response to the body rate rotation about the sensitive axis. The second resonator cavity is located and dimensioned in relation to the first resonator cavity to have similar path length changes in response to the induced body dimension changes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to optical gyroscopes and more particularly 
to passive ring resonator gyroscopes; these have bias frequency errors 
resulting from mechanically or thermally induced dimensional changes that 
influence tuning. 
2. Description of Prior Art 
This application relates to an application titled "PASSIVE RING RESONATOR 
GYROSCOPE", filed Nov. 29, 1984, having Ser. No. 676,322, and having three 
inventors, two of which are common to this application and having a common 
assignee. This previous application described a laser gyro having a single 
piece body incorporating a linear laser light source, a passive resonant 
cavity and which relies on three active servo loops for operation. 
In a passive ring resonator gyroscope, a pair of monochromatic light beams 
counterpropagate about closed-loop optical paths, where the paths form a 
resonator. The stability of the path length between reflective surfaces 
forming the closed path is critical in maintaining resonance in the 
passive ring resonator cavity since dimensional changes contribute to bias 
frequency errors. The relationship between a linear laser.sup.1 and a ring 
resonator to form a prior art passive ring resonator gyro is depicted in 
an article by S. EZEKIEL and S. R. BALSAMO titled "A Passive Ring Laser 
Gyroscope", Applied Physics Letters, Vol. 30, No. 9, May 1, 1977, pg. 
478-480. (NOTE: Usually a resonator is conceived as a linear or standing 
wave resonator in which the light completes an optical round trip by 
reflecting off a mirror and retracing its path. These forward and backward 
waves create a standing wave in the cavity. In a ring resonator, the light 
completes an optical round trip without retracing its path and hence the 
path encloses an area as shown in Ezekiel's paper.) 
FNT .sup.1 For description of lasers and resonators refer to: Yariv, A., 
QUANTUM ELECTRONICS (John Wiley & Sons, 1975) or Sargent, M., et.al., 
LASER PHYSICS (Addison-Wesley Pub., 1974). 
In the passive ring resonator, such as that described in the EZEKIEL 
reference, the two beams, traveling in opposite directions around the 
closed-loop optical path, are injected into the passive ring resonator 
from a single frequency light source. As the ring resonator gyroscope 
cavity rotates in inertial space, the two counterpropagating beams travel 
unequal path lengths. This path difference, due to rotation in inertial 
space, gives rise to a relative frequency difference (Sagnac effect.sup.2) 
between the two counterpropagating beams. (NOTE: A ring resonator, as 
opposed to a linear resonator, can exhibit the Sagnac effect and detect 
inertial rotation.) 
The relative frequency difference is detected as an electrical beat signal 
which is then electronically interpreted to indicate the direction and 
inertial rate of rotation of the passive gyro about the gyro's sensitive 
axis. The sensitive axis of the gyro is along the direction normal to the 
plane of the passive resonator. 
FNT .sup.2 E. J. Post, "Sagnac Effect", Review of Modern Physics, Vol. 39, No. 
2, Apr. 1967, p. 475-493. 
The single frequency light source for the passive resonator is typically an 
external linear laser. Spectra Physics Inc. of Sunnyvale, Calif. produces 
stabilized lasers with the required characteristics. 
It is known that bias errors in the detected signal of a ring resonator 
gyro result from dimensional changes in the laser and in the passive ring 
resonator. Bias errors also result from Fresnel Drag; these errors arise 
from the presence of gases (e.g. air) in the path of the 
counterpropagating beams in the resonator. Bias errors are typically 
characterized as a frequency difference between the two beams which is not 
related to the rotation rate. Bias errors are sometimes detected as a 
frequency difference in the absence of rotation or as post calibration 
changes in the frequency difference for a specific absolute inertial 
rotation rate. 
The Passive Ring Resonator Gyroscope of the type described in the EZEKIEL 
reference is typically constructed by placing optical elements, such as 
mirrors, beamsplitters, etc. on an optical bench. The location, spacing 
and geometrical relationships between the elements of the gyro function to 
enhance the passive ring resonator gyroscope's sensitivity and stability. 
Experimental passive ring resonator gyroscopes typically have path lengths 
of a few meters making them unsuitable for use as a navigational 
instrument. The large size of prior art passive ring resonator gyroscopes, 
such as that characterized in the EZEKIEL reference, also contributes to 
the likelihood of bias errors due to mechanical coupling and mechanical 
drift of the optical elements in response to physical and thermal forces 
acting on the laser and on the optical table or bench. 
SUMMARY OF THE INVENTION 
The objective of this invention is to provide a passive ring resonator gyro 
suitable for use as a navigational instrument having reduced bias errors 
and bias error sensitivity while having enhanced stability and 
sensitivity. This is accomplished by constructing a linear and ring 
resonator from one body in which the total resonator path length is 
substantially below a half meter. 
Another objective of this invention is to provide a Passive Ring Laser 
Gyroscope using two frequency tracking servos for peaking the intensity of 
the CW (clockwise) and CCW (counterclockwise) propagating light beams in 
the passive cavity, eliminating the extra servo used in prior art systems 
to stabilize the first linear laser cavity. 
A particular embodiment of this innovative passive ring resonator gyroscope 
has a single piece body, typically fabricated from a block of glass 
ceramic material such as ZERODUR .sup.R, (a trademark of the JENA.sup.ER 
GLASSWERK SCHOTT & GEN. of MAINZ, GERMANY), which forms a fixed reference 
frame for all required optical elements, including the integral first and 
second resonator cavities. A laser means such as a linear or "L" shaped 
laser uses the first cavity, when operated with suitable excitation, 
functions as a linear laser providing a source of single mode TEM.sub.oo, 
single frequency light for the second resonator cavity. 
In a more particular alternative embodiment, the laser means, first 
resonator cavity has a transmitting optical ports for transmitting 
stabilized single frequency light. The internal body-mounted reflective 
surfaces are coupled to and mechanically spaced by the body. A gain 
medium, such as a Helium Neon gas mixture is contained in the first 
resonator cavity. 
A means for exciting this gain medium to induce lasing in the first 
resonator cavity is provided. The single frequency light source is 
directed through at least one transmitting optical port of the first 
resonator. 
The second resonator cavity and its reflective elements form a passive high 
Q cavity having a closed second optical path tuned to resonate at 
substantially the same frequency of the first resonator cavity. A means 
for coupling the single frequency light from the first to the second 
resonator cavity and for forming CW and CCW light beams in said second 
resonator is implemented using conventional mirrors, lenses and 
beamsplitters. 
A significant feature of this passive ring resonator gyroscope is that the 
second resonator cavity is oriented and dimensioned in relation to the 
first resonator cavity to have substantially equivalent optical path 
length changes in response to any induced body dimensional changes. 
Furthermore, the fixed relationship of the linear laser and the passive 
ring resonator eliminates beam misalignment caused by the relative motion 
of the two resonators. Bias errors are diminished since the ring resonator 
is a passive device and has no internal excitation to frequency shift the 
cavity resonances. Bias errors due to axial gas flow is not a problem in 
the passive resonator. Bias errors are further diminished by the 
evacuation of the second resonator since there will be no effect due to 
Fresnel drag. Taken together, these features form a gyroscope with 
increased stability and reduced bias errors. 
A cavity servo means is provided for controlling the resonant frequency of 
the first resonator high Q cavity to track the resonant frequency of the 
second resonator CW beam. A significant feature of this invention is that 
only two servo loops are needed since the first linear cavity frequency is 
locked to the CW resonance. The third servo loop required by prior art 
systems to stabilize and fix the frequency of the linear laser is 
eliminated. 
The cavity servo means includes several elements such as a means responsive 
to the second resonator clockwise single frequency light beam for 
providing resonant clockwise beam optical excitation. The cavity servo 
means also includes a means responsive to the clockwise beam optical 
excitation for detecting the intensity of the second resonator clockwise 
beam and for providing a CW intensity control signal to characterize the 
intensity of the second resonator clockwise beam. In addition, a first 
servo amplifier means is included and is responsive to the clockwise 
intensity control signal for providing a first cavity path length control 
signal. The first servo means enables the first cavity to track the 
resonant frequency of the second resonator CW beam. 
A first cavity path length adjusting means responsive to the first cavity 
path length control signal is provided to shift the resonant frequency of 
the first cavity. The first cavity path length adjusting means shifts the 
frequency of the first single frequency light source in response to the CW 
intensity control signal to maximize the intensity of the second resonator 
CW beam. Using this servo method, the servo normally used to stabilized 
the linear first cavity to its own intensity maximum is eliminated. 
An output servo means is provided for shifting the frequency of the first 
cavity single frequency light as it enters the second resonator to form 
the CCW beam, to the intensity peak of the CCW beam in the second 
resonator in response to an input body rate about the sensitive axis of 
the gyro. 
The output servo means further comprises a means responsive to the second 
resonator counterclockwise beam for providing resonant optical excitation. 
A means responsive to the counterclockwise optical excitation for 
detecting the intensity of the second resonator counterclockwise beam and 
for providing a CCW intensity control signal to characterize the intensity 
of the CCW beam is included along with an output servo amplifier means 
responsive to the CCW intensity control signal for shifting the frequency 
of the first cavity single frequency light source to peak the intensity of 
the second resonator CCW beam. The frequency of the single frequency light 
source before entering the second resonator to form the counterclockwise 
beam is constantly adjusted to compensate for effective path length 
changes due to body rate inputs to the passive ring resonator gyroscope 
sensitive axis and to peak the intensity of the CCW beam and to compensate 
for the effect of the first servo on the first cavity's resonant 
frequency. 
In this more particular embodiment, a means for detecting the frequency 
difference between the clockwise and the counterclockwise beams provides a 
signal representing a measure of the input body rate.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, an embodiment of the invention passive ring 
resonator gyroscope is depicted having a single piece body 12 having 
integral first and second resonator cavities 14, 16. The first cavity 
contains elements forming a standing wave "L" shaped laser, referred to as 
the linear laser. The output of the linear laser is not influenced by 
inertial rate inputs to the gyroscope. 
The first resonator cavity 14 has a transmitting optical port means at 
transmissive port 20 for transmitting stabilized single frequency light as 
ray 17, and at least two internal body-mounted reflective surfaces such as 
partially transmissive mirrors 24, 26 and a mirror surface 25 on a 
piezoelectric transducer, such as PZT1 64. Each reflective surface is 
coupled to and mechanically spaced by the body 12. 
An appropriate gain medium, such as a mixture of Helium and Neon, is 
contained in the first resonator cavity. 
Block 28 represents a means for exciting the gain medium to induce lasing 
in the first resonator cavity. This element is typically a controllable 
current source capable of providing an output voltage capable of exceeding 
the ionization potential of the gas mixture and having an output current 
capability of one to ten milliamperes. 
Referring again to FIG. 1, the single frequency light, represented by 
phantom line 17, is shown directed through the first resonator 
transmitting optical port means, such as partially transmissive mirrors 24 
to beamsteering optics 58. Beamsteering optics 58 couples the single 
frequency light to the second resonator's clockwise beam entry at 
partially transmissive MIRROR 3, 78. 
Referring to FIG. 1, the second cavity 16 is a ring resonator cavity 
depicted as having a receiving optical port means, such as MIRROR 78 for 
receiving stabilized single frequency light. The second resonator has at 
least three reflective surfaces such as those designated by reference 
numbers 44, 46, 48, 49, that form a closed optical path and enclosing an 
area. The passive ring resonator gyroscope sensitive axis 54 is normal to 
the plane of the enclosed area. 
The FIG. 1 phantom block 58 represents a means for coupling the single 
frequency light source by optically deflecting the light to beamsteer the 
single frequency light as it exits the first resonator cavity transmitting 
port means 20 to the second resonator cavity receiving optical port means, 
such as MIRROR 78. 
Beam splitter 19, and MIRRORS 87, 88 represent a means for beamsplitting 
and beamsteering. The single frequency light 17 exits the first resonator 
cavity transmitting port means at partially transmissive MIRROR 2 at 20 at 
frequency Fo. Beamsplitter 19 receives the stabilized single frequency 
light from 17 and divides the light Fo into the first and second single 
frequency light sources represented by rays 22 and 23 to the second 
resonator cavity receiving optical port means, such as partially 
transmissive MIRROR 3. MIRROR 3 represents a receiving optical port means 
for coupling the first and second single frequency light sources into the 
second resonator cavity to provide clockwise and counterclockwise beams 
respectively within said second resonator closed optical path 16. 
Referring to both FIG. 6A and 6B, the elements within phantom blocks 76, 
90, 92, and PZT 1, 64 represent a cavity servo means responsive to the 
intensity of the clockwise beam for adjusting the path length of the first 
resonator cavity 14 between mirror surfaces 25 and 26 to maximize the 
intensity of light sensed by DET 1, 90 of the clockwise beam. The CW beam 
is frequency stabilized to its intensity peak. The phrase "frequency 
stabilization" is understood to mean phase sensitive detecting and is also 
meant to include the principle of servo locking the laser output to its 
intensity peak. 
Elements within phantom blocks 76, 96, 95, 98 and 93 represent an output 
servo means responsive to the amplitude of the counterclockwise beam for 
adjusting the frequency of the counterclockwise beam by adjusting the 
second single frequency light source 23 to compensate for effective path 
length changes due to body rate inputs to the passing ring resonator 
gyroscope sensitive access. The output servo means operates to maximize 
the intensity of the counterclockwise beam. 
The first and second resonator cavities induces output signal frequency 
bias error in response to body dimension changes, such as those induced by 
temperature changes. The second resonator cavity is positioned, i.e. in 
parallel alignment, and dimensioned in relation to the first resonator 
cavity to experience relatively equivalent optical path length changes in 
response to induced body dimension changes. 
Referring to FIGS. 6A and 6B, the means for exciting the gain medium to 
induce lasing in the first resonator cavity to provide a stabilized single 
frequency light source further comprises elements within the LINEAR LASER 
such as anode 80, cathode 82, and current source means 28. Current source 
28 has a current source terminal, such as 74 and a return terminal 75, for 
coupling a control current from the current source terminal 74 to the 
anode means 80, through the gain medium to induce lasing, to the cathode 
means 82 and thence to the current source return terminal 75. 
The cavity servo means 90, 92 and 64 includes partially transmissive MIRROR 
4, 76. MIRROR 4 functions as a means responsive to the second resonator 
clockwise single frequency light beam for providing clockwise beam 97 for 
optical excitation to DETECTOR 1, 90. 
DETECTOR 1, 90 represents a means responsive to the clockwise beam optical 
excitation for detecting the intensity of the second resonator clockwise 
beam 97 and for providing a CW intensity control signal on signal line 91 
to characterize the intensity of the second resonator clockwise beam 97. 
Referring to FIG. 6A, phantom block 92 represents a first servo amplifier 
means responsive to the clockwise intensity control signal on line 91 for 
providing a first cavity path length control signal on line 65 to PZT 1, 
64. 
PZT 1, 64 is a piezoelectric transducer and functions as a first cavity 
path length adjusting means responsive to the first cavity path length 
control signal on signal line 65 for adjusting the first cavity optical 
path length between MIRRORS 25, 26 to maximize the intensity of the second 
resonator CW beam by shifting the frequency of the first cavity single 
frequency light source 17, such that the shifted frequency Fo+F1 
corresponds to the frequency of maximum intensity of the second resonator 
CW beam. 
Referring again to FIGS. 6A and 6B, the output servo means also comprises 
partially transmissive MIRROR 4, 76. The embodiment of FIG. 6B also uses 
MIRROR 4 as a means responsive to the second resonator counterclockwise 
beam 94 for providing optical excitation to DET 2, 96 at frequency Fo+F2. 
DETECTOR 2, 96 represents a means responsive to the counterclockwise 
optical excitation for detecting the intensity of the second resonator 
counterclockwise beam 94 at frequency Fo+F2 and for providing a CCW 
intensity control signal on control line 83 to characterize the intensity 
of the CCW beam. 
Phantom block 95, block 98 and AO2, 93 represents an output servo amplifier 
means responsive to the CCW intensity control signal from DETECTOR 2 on 
signal line 83 for shifting the frequency of the second single frequency 
light source 23 to frequency Fo+F2 at the intensity peak of the second 
resonator CCW beam. 
AO SERVO DRIVER, 98 has a reference signal generator 79 to provide a 
reference carrier signal F1 at typically 80 MHz. A dither signal source is 
characterized in FIG. 6A by sinusoidal signal generator source 50. The 
sinusoid dither signal is supplied via signal line 71 to second resonator 
PZT 2, 86 and PZT 3, 84 to modulate the second resonator resonant 
frequency at a fixed low frequency, typically selected to be in the range 
of 100 to 1000 Hz. The Fm is provided as a reference to the FIRST SERVO 
AMPLIFIER input 51 and to the OUTPUT SERVO AMPLIFIER MEANS INPUT 42. 
The AO SERVO DRIVER 98 modulates the first single frequency light source at 
ray 22 with the reference carrier signal F1 via AO1 94, to provide a 
reference carrier modulated clockwise beam at frequency Fo+F1. The cavity 
servo means 76, 90, 92, 64 is responsive to the intensity of the clockwise 
beam for adjusting the path length of said first resonator cavity 14 to 
maximize the intensity of the reference carrier modulated clockwise beam. 
The output servo means 76, 96, 95, 98, 93 modulates the second single 
frequency light source at ray. 23 with a shifted reference carrier signal 
F2 to provide a shifted reference carrier modulated counterclockwise beam 
at frequency Fo+F2. The output servo means is responsive to the amplitude 
of the shifted reference carrier modulated counterclockwise beam for 
adjusting the frequency of the shifted reference carrier signal frequency 
to maximize the intensity of the shifted reference carrier modulated 
counterclockwise beam at frequency Fo+F2. 
Counter 66 represents a means for detecting the frequency difference 
between the reference carrier F1 and the shifted reference carrier signal 
F2 in response to rotation of the single piece body about the sensitive 
axis and to provide a frequency difference signal of F1-F2 to serve as a 
measure and direction of the rotation rate of the body about its sensitive 
axis. 
The first servo amplifier means 92 has a LOCK-IN amplifier means 54 having 
a first input coupled to the CW intensity control signal on signal from 
DETECTOR 1 on line 91, and a second input responsive to the dither signal 
at 51 from generator 50 for mixing and amplifying the CW intensity signal 
from line 91 with the dither signal from generator 50 and for providing a 
synchronously demodulated and amplified clockwise control signal on signal 
line 62. 
The first servo amplifier means 92 also has an integrator means 59 having 
an input responsive to the amplified clockwise control signal on line 62 
for providing an integrated error signal on line 30. 
High voltage amplifier means 32 has an input 29 coupled to the integrated 
clockwise control signal for providing the first cavity path length 
control signal on signal line 65. 
The first cavity path length adjusting means is represented by at least one 
electromechanical transducer such as PZTl 64 attached to reflective 
surface 25 positioned to adjust the first resonator optical path length 
between reflective surfaces 25 and 26 to maximize the second resonator 
clockwise beam intensity. PZTl has an input terminal 7 coupled to receive 
the first cavity path length control signal on signal line 65. 
The output servo amplifier means is characterized to modulate the CW beam 
with reference carrier signal at frequency F1. The output servo operates 
to continuously adjust the shifted reference carrier output frequency F2 
in response to the CCW intensity control signal from DET 2 on signal line 
83 by using synchronous detection in response to a dither signal source 
such as the output from oscillator source 50. 
In an alternative embodiment, the output servo amplifier means 95 is 
responsive to the CCW intensity control signal. It has a LOCK-IN amplifier 
means 57 having a first input coupled to the intensity control signal on 
signal line 41, and second input responsive to the dither signal one line 
42 for providing a synchronously demodulated amplified frequency control 
signal on control line 52. 
Integrator means 77 has an input responsive to the synchronously 
demodulated amplified frequency control signal for providing an integrated 
frequency control signal on signal line 72. 
AO servo driver means 98 is included and has a first input 72 coupled to 
the integrated frequency control signal. The AO servo driver has a 
reference signal generator 79 that provides a predetermined reference 
frequency signal F1 of typically 80 MHz. 
The voltage controlled oscillator 89 is also included and has a center 
frequency established by the predetermined reference frequency signal F1 
typically centered at 80 MHz. The voltage controlled oscillator (VCO) 89 
has an input 39 responsive to the integrated frequency control signal on 
line 72 for providing a shifted reference carrier frequency signal F2. The 
frequency of this signal is shifted from the reference carrier frequency 
by a difference characterized by the integrated frequency control signal. 
A first RF amplifier means is represented by triangle 81. This RF AMP is 
responsive to the reference carrier signal for providing a clockwise 
carrier drive signal on signal line 68 that has a frequency equal to the 
reference carrier frequency signal frequency. This amplifier has a signal 
input coupled to the output 67 of the SIGNAL GEN 79. 
A second RF amplifier means is represented by triangle 85. This RF AMP is 
responsive to the shifted carrier reference signal F2 for providing a 
counterclockwise carrier drive signal. 
This amplifier has a signal input coupled to the output of the VCO 89 at 
VCO output 43. The second RF amplifier 85 is responsive to the 
counterclockwise carrier frequency signal for providing a counterclockwise 
carrier drive signal at frequency F2. 
A first acousto-optic coupler AO1, 94 is responsive to the clockwise 
carrier drive signal on signal line 68 for modulating the frequency of the 
first single frequency light source 22 at a frequency equal to the 
reference carrier signal frequency F1. 
A second acousto-optic coupler AO2, 93 is responsive to the 
counterclockwise carrier drive signal on signal line 69 for modulating the 
frequency of the second single frequency light source 23 at the frequency 
of the shifted carrier reference signal frequency F2. 
Counter means 66 is responsive to the reference carrier signal F1 and the 
shifted carrier reference signal F2 for providing a gyro body rate output 
signal proportional to the difference F1-F2. The frequency difference 
signal is processed and scaled to provide a signal characterizing the 
rotational body rate about the gyro sensitive axis. 
In a preferred alternative embodiments of the passive ring resonator 
gyroscope, the Free Spectral Range of the first resonator cavity is 
adjusted to be equal to the Free Spectral Range of the second resonator 
cavity. 
The use of an L-shaped laser as the first resonator cavity above the 
passive resonator appears to offer the best promise for dimensional 
compensation. However, FIG. 7 shows a simpler embodiment using a first 
resonator cavity characterized to form a straight linear laser. The second 
resonator cavity 16 is characterized as being positioned and dimensioned 
relative to the first resonator cavity to experience relatively equivalent 
optical path length changes along one axis in response to the induced body 
dimension changes. 
Bias errors relating to temperature induced body dimension changes or to 
changes from external body forces, such as those transmitted to the body 
12 via shock mounts, are cancelled thereby providing enhanced stability. 
In another more particular alternative embodiment of the passive ring 
resonator gyroscope, the means for exciting the gain medium to induce 
lasing in the first resonator cavity to provide a stabilized single 
frequency light source further comprises at least one anode, such as anode 
80, at least one cathode, such as cathode 82, and a current source means 
block 28, such as that shown in FIG. 6B. Block 28 is powered from a 
voltage source such as V+with respect to a reference potential such as 
ground, having a current source terminal 74 and a return terminal 75 for 
coupling a controlled current from the current source terminal 74 to the 
anode means 80, through the gain medium (not shown) to induce lasing, to 
the cathode means 82 and thence to the current source return terminal 75. 
In a typical Helium-Neon laser, it is customary to reference the anode to 
a potential at or near ground and to drive the cathode from a high 
negative potential source. 
FIGS. 6A and 6B, show a block diagram of an alternative embodiment of the 
passive ring resonator gyroscope in which the first resonator tuning means 
has a means responsive to the single frequency light for providing optical 
excitation, such as partially transmissive MIRROR 2, 20 and BEAMSPLITTER 
BS1, 19 and mirror 87. The current source means 28, and the first 
resonator optical path 14 between mirrored surfaces 25 and 26, and a gain 
medium (not shown) such as a mixture of Helium and Neon gas resides within 
the sealed first resonator 14 and functions to provide the single 
frequency light source to partially transmitting MIRROR 2, 20. 
Beamsplitter BS1, 19 provides the optical excitation from the linear laser 
as ray 22 at frequency Fo to form the CW beam in the second cavity exiting 
at MIRROR 4, 76 to be incident on Detector 1, 90. 
DETECTOR 1, 90, operating with the first servo amplifier 92, represents a 
cavity servo amplifier means responsive to the clockwise beam optical 
excitation for detecting the intensity of the second resonator clockwise 
beam and for providing an optical path length control signal to 92 on 
control line 91 to maximize the intensity of the second resonator 
clockwise beam. By the phrase "detecting the intensity of the second 
resonator clockwise beam", we also mean to include the concept of 
positioning the DETECTOR and characterizing the control means to be 
responsive to a reflected intensity minimum. In the present embodiment, 
DETECTOR 1 and 2 are responsive to an intensity maximum. 
Optical detector units (such as DETECTOR 1, 2, 90 and 96) typically contain 
a reversed biased PIN diode and a low noise preamplifier. A detector such 
as the SD-00-12-12-231 manufactured by the Silicon Detector Corp. of 
Newbury Park, Calif. is suitable for use with a Helium Neon laser light 
source. 
DETECTOR 2, 96, OUTPUT SERVO AMPLIFIER 95 and AO SERVO DRIVER 98 represent 
a means responsive to the counterclockwise optical excitation for 
detecting the intensity of the second resonator counterclockwise beam and 
for providing a counterclockwise frequency control signal on control line 
69 to AO2 to adjust the frequency of the second CCW single frequency light 
source. 
The elements within OUTPUT SERVO AMPLIFIER, PHANTOM BLOCK 95 and AO SERVO 
DRIVER, PHANTOM BLOCK 98 represent an output servo amplifier means 
responsive to the counterclockwise frequency control signal for adjusting 
the frequency of the counterclockwise beam to maximize the intensity of 
the counterclockwise beam. The frequency of the counterclockwise beam is 
constantly adjusted by AO2 93 in response to the frequency control signal 
on line 69 to compensate for effective path length changes due to body 
rate inputs about the passive ring resonator gyroscope sensitive axis 54. 
In another more particular alternative embodiment of the passive ring 
resonator gyroscope, the cavity servo amplifier means responsive to the 
optical path length control signal from DETECTOR 1 has a LOCK-IN amplifier 
means represented by LOCK-IN AMPLIFIER 54 having a first input coupled to 
the clockwise intensity control signal from DETECTOR 1 on control line 91, 
a second input responsive to the dither signal at terminal 51 for mixing 
and amplifying the clockwise intensity control signal with the dither 
signal and for providing an amplified clockwise control signal. 
Integrator means represented by INTEGRATOR 59 has an input responsive to 
the amplified clockwise control signal for providing an integrated 
clockwise control signal. 
A high voltage amplifier means to drive the electromechanical transducer is 
represented by the HI VOLT DC element 33 within phantom block 32. The HI 
VOLT DC amplifier has input 29 coupled to the integrated clockwise control 
signal on signal line 30 to provide a buffered output control voltage on 
signal line 65 referred to as the first cavity pathlength control signal. 
The cavity servo amplifier means also includes at least one 
electromechanical transducer such as PZT2 86 and PZT3 84 mounted behind a 
reflective surface such as 48 and 44. The PZT's are used to modulate the 
second resonator optical path length 16 at the dither frequency Fm. Input 
terminals 8, 9 are coupled to receive the dither signals on signal line 71 
from dither signal source 50. 
Several reference frequencies are defined in FIG. 6B. By the symbol Fo, we 
mean the operating frequency of the linear laser which, when added to the 
reference carrier signal frequency, provides a light source at the CW 
intensity peak. By F1 we mean the frequency that AO1 is driven at to 
modulate the frequency of the clockwise beam at the frequency of the 
reference carrier signal. The clockwise beam has a frequency of Fo+F1 and 
is at CW peak resonance. By F2 we mean the frequency that AO2 is driven at 
to modulate the frequency of the counterclockwise beam at the frequency of 
the shifted reference carrier signal to achieve peak CCW resonance. The 
CCW beam has a frequency of Fo+F2. 
A voltage controlled oscillator (VCO) 89 is shown to be phase-locked to 
reference carrier signal F1 through input 40. The VCO has a center 
frequency at the reference frequency F1 (typically 80 MHz). The frequency 
selected for actual use will depend on the AO devices selected for use. 
The voltage controlled oscillator input 39 is responsive to the integrated 
frequency control signal and provides a counterclockwise frequency signal 
at output terminal 43 at frequency F2. The frequency difference between F1 
and F2 is characterized by the integrated frequency error signal input at 
39. 
A first RF amplifier means represented by RF AMP 81 is responsive to the 
reference carrier signal F1 for providing a clockwise carrier drive signal 
at terminal 68. A second RF amplifier means represented by RF AMP 85 is 
responsive to the shifted reference carrier signal F2 for providing a 
counterclockwise carrier drive signal at terminal 69. A first 
acousto-optic modulator AO1, 94 is shown on FIG. 6B responsive to the 
clockwise carrier drive signal for shifting the frequency of the linear 
laser Fo to adjust the Fo+F1 frequency to equal the CW resonance intensity 
peak. 
A second acousto-optic coupler AO2, 93 is responsive to the 
counterclockwise carrier drive signal on control line 69 to shift the 
frequency of the linear laser to Fo+F2. 
A counter means 66 phased locked to the reference clock through 53 is 
provided to measure F2 and compare its value with the reference frequency 
F1 to determine the inertial rotation rate. 
Each of the LOCK-IN AMPLIFIERS depicted in FIG. 6A functions to mix a 
dither signal, such as Fm, typically near 1kHz, with a DETECTOR output 
signal that contains information at Fm. Using this synchronous 
demodulation or phase sensitive detection technique, control information 
is obtained in high noise environments. Amplifiers of this type are 
sensitive to the phase relationship between the input information signal 
and the reference or dither signal to the lock-in amplifier. An in-phase 
information signal will provide an output response of one polarity and an 
out-of-phase relationship will result in an output signal of an opposite 
polarity. A typical LOCK-IN AMPLIFIER is the PRINCETON APPLIED RESEARCH 
Model 124A manufactured at Princeton, N.J. Amplifiers of this type, 
typically high Q DC amplifiers, provide a gain of several thousand at the 
reference or dither frequency and have virtually no gain at other 
frequencies above or below the reference frequency. For an information 
signal of a fixed level at the reference frequency, the amplifier 
typically provides a fixed DC level response at its output. 
The output of the LOCK-IN AMPLIFIER 57, i.e. the amplitude frequency 
control signal 52, is fed to the input of INTEGRATOR 77. The output 72 of 
INTEGRATOR 77 provides an integrated frequency control signal that is 
coupled to an input of the AO SERVO DRIVER 98. The INTEGRATOR 77 responds 
by integrating the output of the lock-in amplifier. A constant level out 
of the LOCK-IN AMPLIFIER will typically result in a constantly increasing 
or decreasing output from the integrator. The output servo control loop is 
characterized to respond by driving the A02 device as required to peak the 
CCW beam intensity at Fo+F2 and to drive the information signal at Fm from 
DETECTOR 2 to zero. 
The AO SERVO DRIVER 98 has RF AMP 81 and 85 with gains centered around F1. 
These amplifiers are characterized to typically operate at 80 MHz and to 
provide the required power level (typically 1 to 2 Watts) to AO1 and A02 
on control lines 68 and 69, respectively. SIGNAL GEN 79 provides a 
reference carrier output signal at output 67 on signal line 73 at 
frequency F1 (typically 80 MHz) to RF AMP 81. Phase-lock operation 
requires that counter 66, VCO 89, and the output of RF AMP 81 all be 
referenced to the output of frequency of SIGNAL GEN 79 at 67. Arrangements 
using a master clock at typically 10 MHz (not shown) with appropriate 
multiplication and/or divide circuits to phase-lock the VCO and counter 
are anticipated. The VCO output 43 at F2 is coupled to the input of RF AMP 
85 and to the input of COUNTER 66. The VCO is characterized in this 
configuration to operate with a center frequency of F1. The VCO output 
varies in frequency by +/-DELTA as a function of its scale factor and the 
level of the analog integrated frequency control signal 72 from INTEGRATOR 
77. DELTA is the variation in frequency of F2 from F1 and is the amount by 
which the CW frequency Fo+F1 and the CCW frequency at Fo+F2 must be 
shifted up or down in frequency to their corresponding peak resonance in 
the passive cavity. DELTA is counted by Counter 66 and is a measure of the 
body input rate about the gyro's sensitive axis. 
The cavity servo means has FIRST SERVO AMPLIFIER 92 which has LOCK-IN 
AMPLIFIER 54, INTEGRATOR 59 and HI VOLT AMP 32. LOCK-IN AMPLIFIER 54 has a 
first input coupled to the output of DETECTOR 1 via control line 91 and a 
second input coupled to receive the output of the Fm dither oscillator 
from terminal 51. The operation of this LOCK-IN is essentially equivalent 
to that of LOCK-IN AMPLIFIER 57 discussed above. The output 62 of LOCK-IN 
AMPLIFIER 54, the amplified clockwise control signal, is coupled to the 
input of INTEGRATOR 59. The output of INTEGRATOR 59, the integrated 
clockwise control signal 30, is coupled to input 29 of HI VOLT AMP 32. The 
HI VOLT AMP 32 has a HI VOLTAGE DC AMPLIFIER 33 capable of providing an 
output voltage of over 100 V needed to drive the PZT1. The signal to PZT1 
is the first cavity pathlength control signal. It is a slowly varying 
control signal as a result of INTEGRATOR 59. 
PZT2 and PZT3 86, 84 shown in FIG. 6B represent piezoelectric transducers. 
PZT2 and PZT3 each function as an electromechanical transducer, attached 
to reflective surfaces 48, 44 so as to modulate the second resonator 
optical path length at the dither frequency Fm. Each PZT has an input 
terminal 8, 9 coupled via signal line 71 to the dither signal source 50. 
In alternative embodiments of this type, shown in FIG. 1, the first 
resonator cavity is characterized to form a straight or "L-shaped" or 
"U-shaped" linear laser. The manufacturability of the invention gyroscope 
is increased by the use of a straight linear laser as shown in FIG. 7. 
FIG. 2 is a top view of a preferred embodiment single piece body 12 showing 
partially transmissive mirror 20, mirror 21 and PZT1, 64. Single frequency 
light passes through mirror 20 and is redirected by beamsteering optics to 
the second passive cavity 16 (not shown). PZT1, 64 is a piezoelectric 
transducer having a mirrored surface at the corner of the first resonator 
cavity 14 for circulating light through the gain bore within the first 
resonator 14 (of FIG. 4 and FIG. 1). 
FIG. 3 shows PZT1, 64 on the surface of the passive ring resonator 
gyroscope body 10. Section lines 4--4 and 5--5 show the section line 
locations for FIGS. 4 and 5, respectively. 
FIG. 4 shows a sectioned view of the first resonator cavity 16 viewed from 
the bottom looking up. 
FIG. 5 shows a sectional view of the second resonator cavity viewed from 
the top looking down. The L-shape of the first resonator cavity in FIG. 4 
is dimensioned and positioned in this embodiment to reside directly above 
the second resonator cavity of FIG. 5 and and to be essentially congruent 
with one half of the second resonator cavity and therefore experiences 
relatively equivalent optical path length changes in response to induced 
body dimension changes such as those resulting from mechanical forces 
applied to the body or to those resulting from thermally induced strain. 
FIG. 7 represents another alternative embodiment of a body 34 for use in 
fabricating a more produceable passive ring resonator gyroscope. The 
configuration of body 34 includes a first cavity 35 for use as the gain 
bore for a linear laser. Vent holes 37 and 38 lead to anode and cathode 
locations on the top surface of the body 34. 
The frequency of the free running linear laser 17 typically varies over a 
few MHz (megahertz). In this invention, intensity stabilization control 
means is expected to provide operation with a stability of a few hundred 
kilohertz. 
The DETECTOR 1, 90 block is selected to have high gain and low noise. A 
slow dither or modulation frequency Fm is superimposed on top of the CW 
resonance frequency Fo+F1. The optical feedback signal from DETECTOR 1, 90 
is demodulated by mixing it with the reference Fm in the LOCK-IN 
amplifier. By choosing a suitable Fm, the LOCK-IN output will be a 
discriminator curve proportional to the slope of the gain curve or 
resonance signal. This discriminator curve is integrated to provide the 
compensating feedback+or-error control voltage to PZT1. The PZT changes 
the length of the cavity to maximize the output such that the detector 
output is zero and the cavity is locked on the top of the gain curve. This 
concept of frequency stabilization using frequency modulation and phase 
sensitive detection is used in the linear laser servo and in the cavity 
servo loops. 
OPERATION 
Due to the Sagnac effect, the path lengths of the clockwise (CW) and 
counterclockwise (CCW) beams in ring resonator differ as a function of 
body rotation rates. Consequently, the resonant frequencies for CW and CCW 
light in the passive cavity have a corresponding difference dependence. 
See Reference 2 on page 3 of this document. The goal of the optics and 
electronics of the Passive Ring Resonator Gyroscope (PRRG) is to detect 
the frequency difference that arises between the CW and CCW beams due to 
inertial rotations. The intent of the single body linear laser/passive 
cavity design is to eliminate bias errors caused by the resonant 
frequencies of the linear laser and passive cavity changing with respect 
to one another. In this single block, these bias errors will also be 
independent of input rotation rates. The two cavities are arranged 
mechanically and geometrically to insure that shifts in laser frequency 
arising from body dimensional changes are experienced by both the passive 
resonator cavity and the linear laser cavity and therefore reduce the 
effect of such bias errors. 
To obtain such bias compensation, the Free Spectral Range of the first 
resonant cavity 14 is made equal to the Free Spectral Range of the second 
resonant cavity 16. The Free Spectral Range (F.S.R.)=c/p where c is the 
speed of light and p is the optical round trip pathlength of a resonator. 
Thus, the F.S.R. of a square passive ring resonator with sides of length L 
is c/4L. To meet the F.S.R. requirement, two cases arise contingent upon 
the linear resonator design. If the first resonator forming the linear 
laser is comprised of a mirror with no curvature, i.e. a flat mirror, and 
a mirror with a predetermined radius of curvature, i.e. a curved mirror, 
the required resonator length between mirrors is L. The F.S.R. of this 
flat mirror, curved mirror resonator equals c/4L. However, a resonating 
using two curved mirrors requires a resonator length between mirrors equal 
to 2L. This configuration also has a F.S.R. equaling c/4L. With the 
criteria (F.S.R.).sub.Laser =(F.S.R.).sub.Passive Resonator =c/4L, it is 
apparent that any change in the F.S.R. of the laser resonator will equal 
any change in the F.S.R. of the passive ring resonator which may arise 
from thermal or mechanical changes. 
Mirrors with piezoelectric (PZT) backings supply a modulation means to the 
passive resonator cavity; this modulation effects both the CW and CCW 
beams. This modulation or dither frequency Fm is a sinusoid typically 1 
kHz. 
Light leaving the passive cavity from the CW direction is detected and 
phase sensitive demodulated by employing a lock-in amplifier. The error 
signal drives a piezoelectric mirror in the linear laser to bring it into 
the CW beam's resonance. In the preferred embodiment, the DC error signal 
is applied to PZT1. The modulation signal Fm is applied to PZT2 and PZT3. 
The bandwidth of the gyroscope is limited by the bandwidth of PZTs used in 
this system. The unique property of this configuration is that only two 
servo loops are needed. The servo loop used in prior art control systems 
to frequency stabilize the first cavity source laser to its intensity gain 
peak is eliminated. The first cavity's frequency output is second cavity's 
CW resonance peak using one servo loop. The need for two different 
modulation frequencies, Fm and the need for summing high voltage DC 
amplifiers are eliminated. The single Fm reference signal is applied to 
the second cavity's PZTs and the DC error voltage is applied to the first 
cavity's PZT1. The first servo forces the first cavity to track the CW 
resonance while the second output servo tracks the CCW resonance at all 
times including intervals when body rate inputs are changing and thereby, 
changing the path lengths of the CW and CCW beams. 
Acousto-Optics 
Each of two single frequency light sources encounters a non-linear optical 
device which shifts the frequency of the input light. The AO devices are 
typically made up of a piezoelectric transducer attached to a crystal. The 
AO device is fabricated from a piezoelectric crystal material such as 
quartz, TeO.sub.2 (Telurium Dioxide) or PbMoO.sub.4 (Lead Molybdate). In 
the preferred embodiment, the transducer, driven at F1, establishes an 
acoustic wave F1 in the crystal. Associated with the acoustic wave is a 
varying index of refraction due to the compressions and rarefactions of 
atomic density in the crystal. Incoming light diffracts off this induced 
grating giving rise to many orders of diffracted beams. The Oth order beam 
is at the frequency of the single frequency light, Fo. The first order 
beam has the frequency Fo+Fs, where Fs is the frequency of the acoustic 
wave. The output from the AO device has the discrete frequencies Fo+nFs 
where n=0,1,2 . . . each separated in space by an angle proportioned to n. 
Thus, we see that by shifting the AO wave frequency by 1 Hz, we can add 1 
Hz to the frequency of the first order beam exiting the AO device. (See 
e.g., Optical Waves in Crystals, A. Yariv & P. Yeh (John Wiley & Sons) 
AO Servo 
The acousto-optic device is an essential element in the output servo loop. 
CCW light is detected and phase sensitive demodulated producing a dc error 
signal. A voltage controlled oscillator (VCO) receives this error signal 
and outputs a sinusoid whose frequency is related to the input rotation 
rate. Specifically, the output servo alters the frequency of the light 
supplied to the CCW beam path via an A02 bringing that light into the CCW 
resonance. The output of the VCO and the reference carrier frequency is 
compared in a separate counter to determine the rotation rate of the gyro 
about its sensitive axis. 
Common Mode 
Two acousto-optic devices are used so each beam encounters similar phase 
shifts. A02 is driven by the VCO and shifts the CCW light to Fo+F2 where 
F2 displaced from F1 by an amount related to the input rotation rate. AO1 
is in the CW beam path. It is driven by the signal generator which shifts 
the CW light to Fo+F1. By passing the CW light through an acousto-optic 
device, common mode bias errors are eliminated with respect to the CCW 
beam. 
Although the invention has been disclosed and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration as an 
example only and is not to be taken by way of limitation. The spirit and 
scope of this invention is to be limited only by the terms of the appended 
claims.