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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical gyroscopes and more particularly
to passive ring resonator gyroscopes which have bias and frequency errors
resulting from mechanically or thermally induced dimensional changes.
2. Description of Prior Art
This application relates to three previously submitted applications, Ser.
No. 676,322, "PASSIVE RING RESONATOR GYROSCOPE", filed 11/29/84, inventor
Sanders et al; Ser. No. 701,891, "TWO SERVO LOOP PASSIVE RING LASER
GYROSCOPE", filed 2/13/85, inventors SooHoo and Valle; and Ser. No.,
839,292, "TWO SOURCE LASER PASSIVE RING LASER GYROSCOPE", filed 3/13/86,
inverter SooHoo, and all having common assignee.
The first 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.
The second previous application described a passive laser gyroscope having
a single piece body and having a single linear laser light source and a
passive resonant cavity. This gyro uses first and second active servo
loops for operation.
The third application describes a single body passive ring laser gyroscope
having a first laser source to produce a clockwise beam and a second laser
source to produce a counterclockwise beam, both beams circulating in a
sealed, evacuated passive cavity within the same body.
In a passive ring resonator gyroscope, a pair of monochromatic light beams
counterpropagate about a closed-loop optical path, which forms a high Q
resonant optical circuit. 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 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.) For
a 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)
between the two counterpropagating beams. (NOTE: A ring resonator, as
opposed to a linear resonator, can exhibit the Sagnac effect and detect
inertial rotation.) E. J. Post, "Sagnac Effect", Review of Modern Physics,
Vol. 39, No. 2, April 1967, p. 475-493.
The relative frequency difference is detected as a changing interference
fringe pattern 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.
The single frequency light source for the passive resonator is typically an
external linear laser. Spectra Physics Inc. of Sunnyvale, CA. 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 phase locked passive ring
resonator gyro suitable for use as a navigational instrument having
reduced bias errors and bias error sensitivity while having substantially
enhanced stability and sensitivity. This is accomplished by constructing a
gyro having a single source laser and a ring resonator within a single
housing or one piece body in which the total resonator path length is
substantially below a half meter.
Another objective of this invention is to provide a single source passive
ring laser gyroscope using two phase locked tracking servos for peaking
the intensity of the CW (clockwise) and CCW (counterclockwise) propagating
light beams in the passive cavity.
Another objective of the invention is to change the passive cavity's path
length with a control signal to achieve resonance locking of the cavity to
the center frequency of the linear laser without the need for periodic AC
modulation of the passive cavity's path length.
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.RTM., (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 first and second
resonator cavities. A laser means is composed of a linear or "L" shaped
laser that uses the first cavity. This laser, when operated with suitable
excitation, functions as a linear laser providing a source of single mode
TEM.sub.oo, single frequency light for the third resonator cavity.
In a more particular alternative embodiment, the laser means first
resonator cavity has a transmitting optical port for transmitting
stabilized single frequency light to the second resonator cavity. The
internal body-mounted reflective surfaces are coupled to and mechanically
spaced by the single piece body. A gain medium, such as a Helium Neon gas
mixture is contained in the first resonator cavity but the second cavity
is evacuated.
A means for exciting the gain medium, including a power source, to induce
lasing in the first resonator cavity is provided. The output of the single
frequency light sources is directed through at least one transmitting
optical port of each of the first and second resonators.
The second resonator cavity and its reflective elements form a passive high
Q cavity having a closed optical path tuned to resonate at substantially
the same frequency as the first resonator cavity. Means are provided for
modulating the frequency of the light source and means are provided for
splitting this source into first and second rays and coupling the first
and second rays into the second passive resonator cavity, thereby forming
CW and CCW light beams in the second resonator. These means are
implemented using conventional beamsplitters, mirrors, and lenses.
The second resonator cavity is oriented and dimensioned in relation to the
first and second resonator cavities to have substantially equivalent
optical path length changes in response to any induced body dimensional
changes.
Bias errors are diminished since the passive ring resonator is a passive
device and has no internal excitation to frequency shift the cavity
resonances. Bias errors due to axial gas flow or Fresnel drag is
eliminated since the second resonator is evacuated. Taken together, these
features form a gyroscope with increased stability and reduced bias
errors.
A cavity servo means controls the resonant frequency of the laser light
source. The cavity servo means has a servo loop that locks the first
laser's single frequency light source relative to the FCW resonance. The
cavity servo loop is synchronized with a first oscillator that frequency
modulates light leaving the laser via an electro-optic modulator at a
frequency Fm (typically 10 MHz). The first oscillator also provides a
reference signal to a first and second phase-sensitive detector to obtain
a demodulated error signal for integration. The integrated error signal
from an integrator provides a second cavity path length control signal for
the resonator. The first servo means enables the first cavity to track the
resonant frequency of the second resonator's FCW beam.
The first cavity linear laser has a first cavity path length adjusting
means such as a piezoelectric transducer responsive to the first cavity
path length control signal for shifting the resonant frequency of the
first cavity. The first cavity path length adjusting means shifts the
frequency of the single frequency light source in response to the
integrated phase locked error signal applied to the PZT to maximize the
intensity of the second resonator CW beam. The frequency of the CW beam is
upshifted by an acousto-optic modulator (AO1) driven by a reference
oscillator before it is injected into the second resonator.
A second servo is provided for frequency shifting the FCCW beam in the
second cavity as it enters the second resonator to form the CCW beam. The
second servo means is also referenced to the first oscillator. A (Fm)
frequency modulated light source is up shifted in frequency by an
acousto-optic coupler (AO2) driven by a voltage controlled oscillator
(VCO). The VCO adjusts its output frequency in response to a second servo
control signal and adds enough of a frequency increase to the Fm modulated
laser light source to shift the center frequency of the FCCW beam entering
the second cavity to the second cavity (passive resonator) line width
resonant point. A portion of the CCW beam is extracted from the resonator
and a photodetector responds to the beat signal at the frequency of the Fm
signal. The Fm signal thus extracted is coupled to a second phase
sensitive detector, also referenced to the first oscillator. If the
upshifted frequency modulated light source, FCCW, is above or below the
line center of the CCW cavity resonance, the second phase sensitive
detector develops an error signal having a magnitude related to the CCW
frequency error and a polarity related to the position of the error above
or below the cavity's CCW peak resonance.
In this embodiment, a means for detecting the frequency difference between
a clockwise upshift oscillator and a counterclockwise VCO upshift
oscillator provides a signal representing a measure of the input body
rate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a prospective view of the phase locked passive ring resonator
gyroscope.
FIG. 2 is a top planar view of the two source passive ring resonator
gyroscope 10 of FIG. 1 showing the first and second resonator cavities in
phantom. Left and right partially transmissive mirrors 43, 41 represent
first and second partially transmissive and receiving ports.
FIG. 3 is a front view of the phase locked passive ring resonator gyroscope
body.
FIG. 4 is a rear view of the phase locked passive ring resonator gyroscope
body.
FIG. 5 is a cross sectional view of the phase locked passive ring resonator
gyroscope body taken along line 5--5 of FIG. 3.
FIG. 6 is a cross sectional view of the phase locked passive ring resonator
gyroscope body taken along sectional line 6--6 of FIG. 3 and viewed from
the bottom.
FIG. 7 is a combination schematic and block diagram of the associated
electronics and optical elements of the phase locked passive ring
resonator gyroscope using two servo control loops.
FIG. 8 is a schematic of a phase sensitive detector.
FIG. 9 shows a waveform depicting the response characteristic and center
frequency of a passive resonator and depicting the center frequency of a
HeNe laser.
FIG. 10a shows the relative position of two detector diodes in relation to
a laser source on the left and a passive cavity on the right.
FIG. 10b shows the response characteristic of the rightmost detector.
FIG. 10c shows the response characteristic of the lower detector.
FIG. 11a shows a wave form that represents the spectral amplitude
characteristic or intensity of a laser source that is frequency-modulated
over a range of from (Fo-Fm) to (Of+Fm), where Fo is the laser center
frequency.
FIG. 11b shows a wave form represents the output voltage response from a
phase sensitive detector as the laser source is tuned over a range
extending from (Fo-Fm) to (Fo+Fm) while the passive resonator is tuned to
resonate at a relatively fixed frequency Fo.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, an embodiment of the invention phase detector
passive ring resonator gyroscope 10 is depicted having a single piece body
12 having integral first, and second resonator cavities 15, 16. The term
"integral" is meant to convey the idea that the body 12 is formed from one
homogeneous piece of material such as ZERODUR. The top layer has a first
cavity that serves as a linear laser. The output of the linear laser is
not influenced by inertial rate inputs to the gyroscope.
The first laser means (cavity 15) is shown sourcing single frequency light
at frequency Fo through the first resonator partially transmissive mirror
20 to E0 (electro-optic modulator) 17. The beam is frequency modulated by
E0 17 and at frequency +/-Fm and passes to beamsplitter 21 to form first
and second frequency modulated rays 22, 14, respectively.
A first oscillator, OSC1, 33 provides a sinusoidal reference signal at a
frequency Fm (typically 50 MHz) to E0 17 and as a reference signal to PSD1
27 and PSD2 61 (Phase Sensitive Detector).
The E0 device is typically a phase modulator device obtained from a company
such as LASERMETRICS, Electro-Optics Div. at 196 Coolidge Ave., Englewood,
NJ 07631, or one such as Model 10-P-22-3-2-2.5 obtained from QUANTUM
TECHNOLOGY, INC. of 2620 Iroquois Ave., Sanford, Fla. 32771.
The E0 17 frequency modulates light from laser 15 to add frequency side
bands at (Fo-Fm) and (Fo+Fm) where Fo is the laser center frequency and
where Fm is the frequency output of OSC1 33.
Beamsteering optics, such as first prism 18 couples the first frequency
modulated ray as beam 22 through AO1 (acousto-optic modulator) 29. AO1 is
a frequency shifter driven by OSC2, 45, a sinusoidal oscillator typically
operating at a second reference frequency in the range of from 50 MHz to
80 MHz. The AO1 device is structured to upshift the frequency of the first
frequency modulated ray 22 to a frequency F1 higher than that of the first
frequency modulated ray.
FIG. 8 is a schematic of a PSD (phase sensitive detector) such as PSD1 27
and PSD2 61. FIG. 8 shows the structure of a PSD to be a passive device.
The circuit of FIG. 8 is a type referred to as a double balanced mixer.
The PSD1 and PSD2 mixers 27, 61 of FIG. 7, respectively, operate as
phase-sensitive detectors in applications where the frequency of signal
applied to the L0 terminal 810 is the same as the frequency of the signal
applied to the RF terminal 820. The PSD1 and PSD2 mixers 27, 61 are
typically commercial components such as the OAD-6 from the MINI-CIRCUITS
COMPANY at P.O. Box 166, Brooklyn, NY 11235. A further discussion of the
operation of balanced mixers is found in an article entitled "Mixers As
Phase Detectors", RF SIGNAL PROCESSING COMPONENTS, by the Watkins-Johnson
Company, 1983/84, p. 605-631.
The L0 (local oscillator) terminal 810 is driven by a reference signal such
as Fm from OSC1, 33 as shown in FIG. 7. The RF (radio frequency) terminal
820 typically receives a signal for detection such as the output signals
from DET 1 (detector 1) 31 on signal path 34 or from DET2 63 via signal
output 64. The output signal from a phase sensitive detector is fed from
the IF terminal 830 to load R1. The signal at IF terminal 830 is filtered
and typically has a zero volt value where the frequency of the signal at
the L0 terminal 810 is the same as the frequency of the signal at the RF
terminal 820 and where the phase difference is zero degrees. With a fixed
frequency signal, such as Fm, present at terminal 810 and with a fixed
frequency input signal of the same frequency present at the RF input
terminal 820 the IF port will output a DC signal related to the phase of
the L0 and RF signals. A DC output signal develops at terminal 830 of one
polarity for an input RF signal having the same phase relationship as that
of the L0 signal; an opposite polarity signal will result at the IF
terminal 830 for an input RF signal having an opposite phase relationship
with that of the L0 signal.
The waveform in FIG. 11a represents the spectral content of light 32 and 40
incident on DET1 31 and DET2 63 with the detectors being located as shown
in FIG. 7. The upper waveform of FIG. 11 is obtainable by monitoring the
input beams 32 and 40 with a spectrum analyzer. The waveform of FIG. 11b
depicts the signal response of PSD1 and PSD2 at their respective IF
terminals 830 in FIG. 8 or such as 28, 56 in FIG. 7, as the center
frequency of the laser is gradually tuned over a frequency range extending
from below (Fo-Fm) to a high frequency of (Fo+Fm) and while leaving the
spectral feature, in this case, the resonator cavity 16 having a fixed
center frequency at F0.
Referring again to FIG. 1 and to FIG. 7, BS1 21 splits the frequency
modulated light source into first and second frequency modulated rays 22,
14 respectively. The first frequency modulated ray 22 passes to AO1, 29.
AO1 aperture 68 acts as a propagating beam source to provide a propagating
beam 67. Beam 22 is upshifted in frequency by AO1, 29 to form the
propagating beam 67. Propagating beam 67 is directed to partially
transmissive mirror MIR1, 43. The portion of the propagating beam 43 that
passes through MIR1 into the second resonator cavity forms the propagating
light beam (FWC) within the second resonator cavity.
The second frequency modulated ray 14 is first reflected by MIR5, 47 to
A02, 49. Ray 14 is upshifted in frequency by A02. Aperture 70 of A02
serves as a counterpropagating beam source to provide a counterpropagating
beam 69. FCCW within the second resonator cavity counterpropagating light
beam. directed to partially transmissive mirror MIR4, 41. The portion of
the counter propagating beam 69 that passes through MIR4 forms the
counterpropagating light beam FCCW within the second resonator cavity.
Counterpropagating beam 69 is a source for the counterclockwise light
beam, FCCW as it passes through MIR4, a partially transmissive mirror 41.
The. A02 device 49 upshifts the frequency of ray 14 by F2 Hertz to form
the couterpropagating beam 69 that enters cavity 16 via MIR4, at 41.
Acousto-optic device A02 49 and VCO 51 in combination represent a means
responsive to a second frequency modulated ray for frequency shifting the
second frequency modulated ray in response to a second control signal from
the output of AMP3, 53.
The second resonator cavity 16 is a passive high Q evacuated cavity having
a closed optical path with first, second, third, and fourth segments, 44,
46, 48, 50 tuned in combination to resonate at a frequency derived from
the first resonator cavity. The sensitive axis, characterized by Vector
54, and shown in FIG. 3 and FIG. 4, is essentially normal to the plane of
the closed second optical path established by the plane of segments 44,
46, 48, 50.
Part of the propagating beam is reflected off the cavity 16 at MIR1 43 and
is focused on output detector 31 at DET1 receiving aperture 32. Detector
DET1 31 and DET2 63 are typically silicon photodiode amplifier assemblies.
First detector 31 provides a detected FCW signal on signal line 34 to the
RF input of PSD1 27. PSD1 provides a positive or negative phase error
signal on signal line 28 to INTEG1 23 for integration. INTEG1 provides a
first phase control signal on signal line 38 to the inputs of AMP1 19 and
AMP2 25. AMP1 conditions the phase control signal and provides drive
signal to PZT1 13 to shift the frequency of linear laser 15 in a direction
that will drive the phase error signal on signal line 28 to zero volts.
Second detector 63 provides a detected FCCW signal on signal line 64 to the
RF input of PSD2 61. PSD2 provides a positive or negative phase error
signal on signal line 56 to INTEG2 65 for integration. INTEG2 65 provides
a second phase control signal on signal line 59 to the inputs of AMP3 53.
AMP3 53 conditions the phase control signal and provides drive signal to
VCO 51 which changes the frequency that the A02, 49 shifts the FCCW beam
in a direction that will drive the phase error signal on signal line 56 to
zero volts.
These elements represent in combination, a cavity servo means responsive to
the first phase error signal. They provide control signals to the first
laser transducer PZT1 to control the resonant frequency of the first
single frequency light sources F0. The first single frequency light source
is tuned to control and maintain the clockwise propagating (FCW) light
beam in the second resonator at peak resonance.
The second single frequency light source is tuned to control and maintain
the counterpropagating light beam (FCCW) at peak resonance.
OSC2 45, represents a second reference oscillator that operates in
combination with AO1, 29 as shown within phantom block 60 to form a means
for shifting the center frequency of the first frequency modulated ray 22
by a fixed offset frequency F1. VCO 51 is the oscillator that operates in
combination with A02, 49, as shown in phantom block 75, to shift a ray 69
into peak resonance of the CCW modulated beam.
Output counter means 71 is provided for measuring and outputting the
frequency difference between said fixed offset frequency and said variable
offset frequency. The measured frequency difference representing the
difference in frequency due to an input gyro body rate about said
sensitive axis increased by the fixed frequency of the fixed offset
frequency. Output counter means 71 is typically a counter such as a HP3335
by Hewlett Packard for use in a laboratory, but in alternative product
designs, the counter would be fabricated from conventional high speed
logic circuit elements such as MECL or ECL logic by MOTOROLA suitable for
use at frequencies at and above F1, the frequency of the reference signal
generator (to the AO).
The first partially transmissive and receiving port 43 is characterized to
receive and pass a portion of the propagating beam 67 into the second
resonator cavity 16 to form the propagating light beam (FCW).
The second partially transmissive and receiving optical port 41 is
characterized to receive and pass the counterclockwise beam 69 into the
second resonator cavity 16 to form a counterpropagating light beam (FCCW).
PZT2 and PZT3 35, 57 shown in FIG. 7 represent piezoelectric transducers.
PZT2 and PZT3 each function as an electromechanical transducer, attached
to reflective surfaces 37, 39 so as to modulate the second resonator's
optical path length to maintain the propagating light FCW at peak
resonance within cavity 16. PZT2, 35 is a piezo-electric transducer
attached to a mirrored surface, that represents a dynamic path length
adjusting means for adjusting the optical path length of the second
resonator cavity and has an input terminal coupled via a signal line 26 to
the output of AMP2. PZT2 is adjusted, by the first control signal from
first integrator INTEG1, 23 via the output 26 of AMP2, 25 for adjusting
the optical path length of the second resonator cavity 16 to maintain the
propagating beam FWC at peak resonance
PZT3, 57 is a static optical path length adjusting means that provides an
initial coarse adjustment for the cavity 16. PZT3, 57 is adjusted by
adjusting the output voltage of DC TUNING SOURCE 58.
The first detector 31 receives a large portion of the FCW light beam
reflected off the first partially transmissive and receiving port MIR1,
43.
The second cavity detector 63 receives a large portion of the
counterpropagating light beam FCCW reflected off the second partially
transmissive and receiving port MIR4, 41.
Mirrors 43 and 41 have reflective surfaces positioned to direct the
reflected incident beams to their respective detectors 31 and 63. The
intensity of the light striking detector 31 or 63 has a high background
level that dips as the resonant cavity 16 achieves resonance in response
to a shift in frequency of the respective laser source. FIG. 10a
characterizes a laser source directing a beam at a representative port
with a portion of the beam being reflected to a detector such as detector
63. FIG. 10b depicts a peak response to incident light from detector 1009.
FIG. 10c shows a dip in the background of the light intensity 1013
striking the detector such as 1005 as the laser source at 1001 is tuned to
the resonance point of the passive cavity 1007.
Conversely, detector 63 senses light sourced from resonator cavity 16 via
second partially transmissive port MIR4, mirror 41, to detector receiving
aperture 40. The background light including signal information incident at
aperture 40 dips to a minimum as the cavity is tuned to resonance.
FIG. 9 depicts the pass band of a typical passive cavity having a resonance
peak at 1002. The approximate frequency spread between reference 1004 and
1006 represents a typical frequency range between the half-power points
and is included in FIG. 9 along with the indicated laser center frequency,
to provide the reader with a visual appreciation of the "Q" of the second
resonator. 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 the intensity peak of the passive
cavity.
A HeNe laser typically has an instantaneous line width of less than one Hz
but the operating frequency is subject to considerable jitter.
The dip represented by FIG. 10c would also typically have half-power points
separated by 100 kHz. Typically, a HeNe input laser would have its output
at 4.74.times.10.sup.14 Hz injected along path 1003 into the passive
cavity 1007.
FIG. 6 shows the second resonator cav | | |
