U.S. patent application number 16/900419 was filed with the patent office on 2021-12-16 for systems and methods to reduce differential harmonics of resonance tracking modulation in a resonant fiber optic gyroscope.
This patent application is currently assigned to Honeywell International Inc.. The applicant listed for this patent is Honeywell International Inc.. Invention is credited to Chuck Croker, Lee K. Strandjord, Norman Gerard Tarleton.
Application Number | 20210389128 16/900419 |
Document ID | / |
Family ID | 1000004904149 |
Filed Date | 2021-12-16 |
United States Patent
Application |
20210389128 |
Kind Code |
A1 |
Strandjord; Lee K. ; et
al. |
December 16, 2021 |
SYSTEMS AND METHODS TO REDUCE DIFFERENTIAL HARMONICS OF RESONANCE
TRACKING MODULATION IN A RESONANT FIBER OPTIC GYROSCOPE
Abstract
Systems and methods are provided to reduce at least one
differential harmonics of a resonance tracking modulation in a
resonant fiber optic gyroscope (RFOG). The fundamental frequency of
the resonance tracking modulation of each of the clockwise and
counter clockwise optical signals is substantially identical;
however, the amplitude and phase of the Nth harmonic of a clockwise
(CW) resonance tracking modulation and the Nth harmonic of a
clockwise (CCW) resonance tracking modulation may differ due to
non-linearities in the RFOG. Embodiments of the invention diminish,
e.g., reduce to zero such vectoral difference. Differential
harmonics may be generated at one or more harmonics.
Inventors: |
Strandjord; Lee K.; (Tonka
Bay, MN) ; Tarleton; Norman Gerard; (Glendale,
AZ) ; Croker; Chuck; (Glendale, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Honeywell International Inc. |
Morris Plains |
NJ |
US |
|
|
Assignee: |
Honeywell International
Inc.
Morris Plains
NJ
|
Family ID: |
1000004904149 |
Appl. No.: |
16/900419 |
Filed: |
June 12, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01C 19/727 20130101;
G01C 19/721 20130101 |
International
Class: |
G01C 19/72 20060101
G01C019/72 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0001] This invention was made with Government support under
Government Contract Number FA9453-18-C-045 awarded by AFRL. The
Government has certain rights in the invention.
Claims
1. A system for diminishing differential harmonics of common
resonance tracking modulation in a resonant fiber optic gyroscope
(RFOG), comprising: beat note servo circuitry configured to receive
an electrical beat note signal and to generate at least one
differential harmonic alternating current (AC) error signal, where
the at least one differential harmonic AC error signal is generated
using the electrical beat note signal, where the electrical beat
note signal is created from a beat note of a first optical signal
generated by a first optical laser and a second optical signal
generated by a second laser, where the first optical signal and the
second optical signal are modulated with the common resonance
tracking modulation, where the first optical signal circulates in a
first direction in a resonator of the RFOG, and where the second
optical signal circulates in a second direction in the resonator of
the RFOG that is opposite the first direction; differential
harmonic correction circuitry configured to generate at least one
error cancellation signal, where each error cancellation signal is
generated using a corresponding differential harmonic AC error
signal, and where each error cancellation signal and its
corresponding differential harmonic AC error signal correspond to a
differential harmonic selected to be diminished; and adder
circuitry configured to add the at least one error cancellation
signal to an offset frequency signal to the differential harmonics,
where the offset frequency signal comprises a frequency used to
tune a carrier frequency of one of the first optical signal and the
second optical signal to a resonant frequency in respectively one
of the first direction and the second direction; wherein each
differential harmonic of the common resonance tracking modulation
is a vectoral difference between a harmonic of the common resonance
tracking modulation of the first optical signal and a same harmonic
of the common resonance tracking modulation of the second optical
signal, and where a harmonic may be a fundamental frequency.
2. The system of claim 1, wherein the beat note servo circuitry is
further configured to generate a beat note frequency.
3. The system of claim 1, wherein the beat note servo circuitry
comprises: beat note (BN) analog to digital converter (ADC)
circuitry comprising an input and an output, where the output of
the BN ADC circuitry is configured to receive and digitize the
electrical beat note signal; BN digital mixer circuitry comprising
a first input, a second input, and an output, where the first input
of the BN digital mixer circuitry is configured to receive the
electrical beat note signal, which has been digitized, from the BN
ADC circuitry, and where the output of the BN digital mixer
circuitry is configured to generate the at least one differential
harmonic AC error signal; digital loop filter circuitry comprising
an input and an output, where the input of the digital loop filter
circuitry is coupled to the output of the BN digital mixer
circuitry; and a BN numerically controlled oscillator (NCO)
comprising an input and an output, where the input of the BN NCO is
coupled to the output of the digital loop filter circuitry, and
where the output of the BN NCO is coupled to the second input of
the BN digital mixer circuitry.
4. The system of claim 3, wherein the output of the digital loop
filter circuitry is configured to be generate a beat note
frequency.
5. The system of claim 1, further comprising a transimpedance
amplifier coupled to an input of the beat note (BN) servo circuitry
and configured to convert the electrical beat note signal from a
current signal to a voltage signal.
6. The system of claim 1, wherein the differential harmonic
correction circuitry comprises at least one error reduction circuit
configured to generate an error cancellation signal using a
differential harmonic AC error signal for a differential harmonic
selected to be cancelled; and wherein each error reduction circuit
comprises: sine-cosine signal generator (SCG) circuitry comprising
a first output configured to provide a sine signal and second
output configured to provide a cosine signal at a same frequency;
first digital mixer circuitry comprising a first input, a second
input, and an output, where the first input of the first digital
mixer circuitry is configured to receive the at least one
differential harmonic AC error signal, and where the second input
of the first digital mixer circuitry is coupled to the first output
of the SCG circuitry; second digital mixer circuitry comprising a
first input, a second input, and an output, where the first input
of the second digital mixer circuitry is configured to receive the
at least one differential harmonic AC error signal, and where the
second input of the second digital mixer circuitry is coupled to
the first output of the SCG circuitry; first accumulator circuitry
comprising an input and an output, where the output of the first
digital mixer circuitry is coupled to the input of the first
accumulator circuitry; second accumulator circuitry comprising an
input and an output, where the output of the second digital mixer
circuitry is coupled to the input of the second accumulator
circuitry; first digital multiplier circuitry comprising a first
input, a second input, and an output, where the first input of the
first digital multiplier circuitry is coupled to the output of the
first accumulator circuitry, and where the second input of the
first digital multiplier circuitry is coupled the first output of
the SCG circuitry; second digital multiplier circuitry comprising a
first input, a second input, and an output, where the first input
of the second digital multiplier circuitry is coupled to the output
of the second accumulator circuitry, and where the second input of
the second digital multiplier circuitry is coupled the second
output of the SCG circuitry; and signal combiner circuitry
comprising a first input, a second input, and an output, where the
first input is coupled to the output of the first digital
multiplier circuitry, and where the second input is coupled to the
output of the second digital multiplier circuitry.
7. The system of claim 6, wherein the differential harmonic
correction circuitry further comprises adder circuitry configured
to combine the output of each error reduction circuit.
8. A method for diminishing differential harmonics of common
resonance tracking modulation in a resonant fiber optic gyroscope
(RFOG), comprising: modulating the common resonance tracking
modulation on a first optical signal and a second optical signal;
receiving an electrical beat note signal, where the electrical beat
note signal is created from a beat note of the first optical signal
generated by a first optical laser and the second optical signal
generated by a second laser; generating at least one differential
harmonic alternating current (AC) error signal; generating at least
one error cancellation signal, where each error cancellation signal
is generated using a corresponding differential harmonic AC error
signal, and where each error cancellation signal and its
corresponding differential harmonic AC error signal correspond to a
differential harmonic selected to be diminished; and adding the at
least one error cancellation signal to an offset frequency signal
to the differential harmonics, where the offset frequency signal
comprises a frequency used to tune a carrier frequency of one of
the first optical signal and the second optical signal to a
resonant frequency in respectively one of a first direction and a
second direction; wherein each differential harmonic of the common
resonance tracking modulation is a vectoral difference between a
harmonic of the common resonance tracking modulation of the first
optical signal and a same harmonic of the common resonance tracking
modulation of the second optical signal, and where a harmonic may
be a fundamental frequency.
9. The method of claim 8, further comprising digitizing the
electrical beat note signal, where the at least one differential
harmonic AC error signal is generated with the electrical beat note
signal which has been digitized.
10. The method of claim 8, further comprising converting the
electrical beat note signal from a current signal to a voltage
signal.
11. A resonant fiber optic gyroscope (RFOG) configured to diminish
differential harmonics of common resonance tracking modulation,
comprising: an optical fiber coil comprising a first port and a
second port; optical bench circuitry comprising a first input, a
second input, a first port, a second port, a first output, a second
output, and a third output, and where the first port and the second
port of the optical bench circuitry are configured to be coupled
respectively to the first port and the second port of the optical
fiber coil, and further configured to generate a first electrical
signal, a second electrical signal, and a Pound-Drever-Hall (PDH)
electrical signal, where the PDH electrical signal has an amplitude
that varies based upon a differential phase of a first optical
signal being injected into the optical fiber coil and the first
optical signal circulating in the optical fiber coil; PDH servo
circuitry comprising an input and an output, where the input is
configured to receive the PDH electrical signal; a master laser
comprising an input configured to be coupled to the output of the
PDH servo circuitry, and configured to generate a master optical
signal; a first slave laser configured to generate the first
optical signal modulated by the common resonance tracking
modulation which circulates in the optical fiber coil in a first
direction; a second slave laser configured to generate a second
optical signal modulated by the common resonance tracking
modulation which circulates in the optical fiber coil in a second
direction, where the first direction is opposite the second
direction; first optical phase lock loop (OPLL) circuitry
configured to receive the first optical signal and the master
optical signal; second OPLL circuitry configured to receive the
second optical signal and the master optical signal and to provide
a second OPLL signal to the second slave laser; an integrated
photonics circuitry coupled to the first slave laser and configured
to receive the first optical signal, coupled to the second slave
laser and configured to receive the second optical signal, coupled
to the master laser and configured to receive the master optical
signal, coupled to the first OPLL circuitry and configured to
provide the first optical signal and the master optical signal to
the first OPLL circuitry, coupled to the second OPLL circuitry and
configured to provide the second optical signal and the master
optical signal to the second OPLL circuitry, configured to provide
the first optical signal and the master optical signal to the first
input of the optical bench circuitry, and configured to provide the
second optical signal to the second input of the optical bench
circuitry; beat note servo circuitry configured to receive an
electrical beat note signal and to generate at least one
differential harmonic alternating current (AC) error signal and to
generate a beat note frequency, where the at least one differential
harmonic AC error signal is generated using the electrical beat
note signal, where the electrical beat note signal is created from
a beat note of the first optical signal generated by the first
optical laser and the second optical signal generated by the second
laser, where the first optical signal circulates in the first
direction in a resonator of the RFOG, and where the second optical
signal circulates in the second direction in the resonator of the
RFOG that is opposite the first direction, where the resonator is
formed by the optical fiber coil and a portion of the optical bench
circuitry; rate calculation circuitry coupled to the beat note
servo circuitry and configured to receive the beat note frequency
and to determine a rate of rotation of the optical fiber coil
around a center axis; differential harmonic correction circuitry
configured to generate at least one error cancellation signal,
where each error cancellation signal is generated using a
corresponding differential harmonic AC error signal, and where each
error cancellation signal and its corresponding differential
harmonic AC error signal correspond to a differential harmonic
selected to be diminished; first resonance tracking servo circuitry
configured to receive the first electrical signal generated by the
optical bench circuitry used to generate a first offset frequency
signal comprising a frequency used to tune a carrier frequency of
the first optical signal generated by the first slave laser to a
resonant frequency in the first direction; and second resonance
tracking servo circuitry configured to receive the second
electrical signal generated by the optical bench circuitry used to
generate a second offset frequency signal comprising a frequency
used to tune a carrier frequency of the second optical signal
generated by the second slave laser to a resonant frequency in the
second direction, and to receive the at least one error
cancellation signal; wherein each differential harmonic of the
common resonance tracking modulation is a vectoral difference
between a harmonic of the common resonance tracking modulation of
the first optical signal and a same harmonic of the common
resonance tracking modulation of the second optical signal, and
where a harmonic may be a fundamental frequency.
12. The RFOG of claim 11, wherein the beat note servo circuitry
comprises: beat note (BN) analog to digital converter (ADC)
circuitry comprising an input and an output, where the output of
the BN ADC circuitry is configured to receive and digitize an
electrical beat note signal; BN digital mixer circuitry comprising
a first input, a second input, and an output, where the first input
of the BN digital mixer circuitry is configured to receive the
electrical beat note signal, which has been digitized, from the BN
ADC circuitry, and where the output of the BN digital mixer
circuitry is configured to generate the at least one differential
harmonic AC error signal; digital loop filter circuitry comprising
an input and an output, where the input of the digital loop filter
circuitry is coupled to the output of the BN digital mixer
circuitry, where the output of the digital loop filter circuitry is
configured to be generate a beat note frequency; and a BN
numerically controlled oscillator (NCO) comprising an input and an
output, where the input of the BN NCO is coupled to the output of
the digital loop filter circuitry, and where the output of the BN
NCO is coupled to the second input of the BN digital mixer
circuitry.
13. The RFOG of claim 11, further comprising a transimpedance
amplifier coupled to an input of the beat (BN) servo circuitry and
configured to convert the electrical beat note signal from a
current signal to a voltage signal.
14. The RFOG of claim 11, wherein the differential harmonic
correction circuitry comprises at least one error reduction circuit
configured to generate an error cancellation signal a differential
harmonic AC error signal for a differential harmonic selected to be
cancelled; and wherein each error reduction circuit comprises:
sine-cosine signal generator (SCG) circuitry comprising a first
output configured to provide a sine signal and second output
configured to provide a cosine signal at a same frequency; first
digital mixer circuitry comprising a first input, a second input,
and an output, where the first input of the first digital mixer
circuitry is configured to receive the at least one differential
harmonic AC error signal, and where the second input of the first
digital mixer circuitry is coupled to the first output of the SCG
circuitry; second digital mixer circuitry comprising a first input,
a second input, and an output, where the first input of the second
digital mixer circuitry is configured to receive the at least one
differential harmonic AC error signal, and where the second input
of the second digital mixer circuitry is coupled to the first
output of the SCG circuitry; first accumulator circuitry comprising
an input and an output, where the output of the first digital mixer
circuitry is coupled to the input of the first accumulator
circuitry; second accumulator circuitry comprising an input and an
output, where the output of the second digital mixer circuitry is
coupled to the input of the second accumulator circuitry; first
digital multiplier circuitry comprising a first input, a second
input, and an output, where the first input of the first digital
multiplier circuitry is coupled to the output of the first
accumulator circuitry, and where the second input of the first
digital multiplier circuitry is coupled the first output of the SCG
circuitry; second digital multiplier circuitry comprising a first
input, a second input, and an output, where the first input of the
second digital multiplier circuitry is coupled to the output of the
second accumulator circuitry, and where the second input of the
second digital multiplier circuitry is coupled the second output of
the SCG circuitry; and signal combiner circuitry comprising a first
input, a second input, and an output, where the first input is
coupled to the output of the first digital multiplier circuitry,
and where the second input is coupled to the output of the second
digital multiplier circuitry.
15. The RFOG of claim 14, wherein the differential harmonic
correction circuitry further comprises adder circuitry configured
to combine the output of each error reduction circuit.
16. The RFOG of claim 11, each of the first resonance tracking
servo circuitry and the second resonance tracking servo circuitry
comprise: resonance tracking analog to digital converter (ADC)
circuitry configured to receive the first electrical signal and to
digitize the first electrical signal; first digital demodulator
circuitry comprising an output, a first input, and a second input,
and configured to receive the first electrical signal, which has
been digitized, at the first input of the first digital demodulator
circuitry, and to receive a common modulation frequency at the
second input of the first digital demodulator circuitry; gain
circuitry comprising an input and an output, where the input of the
gain circuitry is coupled to the output of the first digital
demodulator circuitry; resonance tracking (RT) accumulator
circuitry comprising an input and an output, where the input of the
RT accumulator circuitry is coupled to the output of the gain
circuitry; and direct digital synthesizer (DDS) circuitry
comprising an input and an output, where the input of the DDS
circuitry is coupled to the output of the RT accumulator
circuitry.
17. The RFOG of claim 16, wherein each of the first resonance
tracking servo circuitry and the second resonance tracking servo
circuitry further comprises: RT servo adder circuitry comprising an
output coupled to the input of the DDS circuitry, and a first
input; and a RT numerically controlled oscillator coupled to the
first input and configured to provide the common resonance tracking
modulation.
18. The RFOG of claim 17, wherein each of the first resonance
tracking servo circuitry and the second resonance tracking servo
circuitry further comprises a second digital demodulator comprising
a first input, a second input, and an output, and configured to
receive the first electrical signal, which has been digitized, at
the first input of the second digital demodulator, and to receive a
signal having a frequency of twice sideband heterodyne detection
(SHD) frequency at the second input of the second digital
demodulator; and a SHD numerically controlled oscillator; wherein
the RT servo adder circuitry further comprises a second input
coupled to the SHD numerically controlled oscillator.
19. The RFOG of claim 16, further comprising a RT servo adder
circuitry comprising an output coupled to the input of the DDS
circuitry, and a first input configured to receive the at least one
error cancellation signal from the differential harmonic correction
circuitry.
20. The RFOG of claim 16, further comprising a common resonance
tracking modulation (CRTM) generator configured to generate an
analog common resonance tracking modulation signal; and wherein the
integrated photonics circuitry further comprises a phase modulator
configured to phase modulate the master optical signal provided to
the first OPLL circuitry and the second OPLL circuitry.
Description
BACKGROUND
[0002] The resonant fiber optic gyroscope (RFOG) shows promise of
meeting challenging demands of a large number of inertial guidance
applications. To meet cost and size requirements, much of the RFOG
laser source optical functions may be employed with silicon
photonics (SiP) chip technology. Many of the optical functions of
integrated components of the SiP (such as waveguides, optical
couplers and splitters, intensity modulators, and photodiodes) can
perform just as well or even better than their discrete optical
device counterparts. However, a satisfactory optical phase or
frequency modulator(s) are difficult to implement in silicon while
meeting harmonic distortion requirements because of the silicon
frequency/phase modulators inherent nonlinearity. Such
non-linearity generates spurious signals which generate error when
determining RFOG rotation rate.
SUMMARY
[0003] The following summary is made by way of example and not by
way of limitation. It is merely provided to aid the reader in
understanding some of the aspects of the subject matter described.
A method for diminishing differential harmonics of common resonance
tracking modulation in a resonant fiber optic gyroscope is
provided. The method comprises: modulate the common resonance
tracking modulation on a first optical signal and a second optical
signal; receive a beat note electrical signal, where the beat note
electrical signal is created from a beat note of a first optical
signal generated by a first optical laser and a second optical
signal generated by a second laser; generate at least one
differential harmonic alternating current (AC) error signal;
generate at least one error cancellation signal using, where each
error cancellation signal is generated using a corresponding
differential harmonic AC signal, and where each error cancellation
signal and its corresponding differential harmonic AC signal
correspond to a differential harmonic desired to be diminished; and
add the at least one error cancellation signal to an offset
frequency signal to the differential harmonics, where the offset
frequency signal comprises a frequency used to tune a carrier
frequency of one of the first optical signal and the second optical
signal to a resonant frequency in respectively one of the first
direction and the second direction; wherein each differential
harmonic of the common resonance tracking modulation is a vectoral
difference between a harmonic of common resonance tracking
modulation of the first optical signal and a same harmonic of
common resonance tracking modulation of the second optical signal,
and where a harmonic may be a fundamental frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Understanding that the drawings depict only exemplary
embodiments and are not therefore to be considered limiting in
scope, the exemplary embodiments will be described with additional
specificity and detail through the use of the accompanying
drawings, in which:
[0005] FIG. 1 illustrates a block diagram of one embodiment of a
resonator fiber optic gyroscope configured to reduce differential
harmonics;
[0006] FIG. 2 illustrates a block diagram of one embodiment a beat
note servo;
[0007] FIG. 3 illustrates a block diagram of one embodiment of
differential harmonic correction circuitry;
[0008] FIG. 4 illustrates a block diagram of one embodiment of a
counter clockwise resonance tracking servo;
[0009] FIG. 5 illustrates a block diagram of one embodiment a
resonator fiber optic gyroscope employing common resonance tracking
modulation applied to a portion of an optical signal generated by a
master laser; and
[0010] FIG. 6 illustrates a flow diagram of an exemplary method of
reducing differential harmonics of resonance tracking modulation in
a resonant fiber optic gyroscope.
[0011] In accordance with common practice, the various described
features are not drawn to scale but are drawn to emphasize specific
features relevant to the subject matter described. Reference
characters denote like elements throughout Figures and text.
DETAILED DESCRIPTION
[0012] In the following detailed description, reference is made to
the accompanying drawings that form a part hereof, and in which is
shown by way of illustration specific illustrative embodiments.
However, it is to be understood that other embodiments may be
utilized, and that structural, mechanical, and electrical changes
may be made. Furthermore, the method presented in the drawing
figures and the specification is not to be construed as limiting
the order in which the individual steps may be performed. The
following detailed description is, therefore, not to be taken in a
limiting sense. Also, it will be understood that when a device or
signal is referred to as being "coupled" to an element, it can be
coupled directly to the element, or intervening elements may also
be present.
[0013] Embodiments provide systems and methods to reduce at least
one differential harmonics in a resonant fiber optic gyroscope.
Harmonic means a fundamental or higher order harmonic frequency.
Each differential harmonic of a resonance tracking modulation (or
differential harmonic) in an RFOG means a vectoral difference
between a harmonic of resonance tracking modulation of a clockwise
(CW) optical signal and a same harmonic of the resonance tracking
modulation of a counter clockwise (CCW) optical signal, and where a
harmonic may be a fundamental frequency. Each of the CW resonance
tracking modulation and the CCW resonance tracking modulation
comprise a signal having an amplitude, a phase, and a fundamental
frequency. The fundamental frequency of the resonance tracking
modulation of each of the CW and CCW optical signals is
substantially identical; however, the amplitude and phase of the
Nth harmonic of a clockwise (CW) resonance tracking modulation and
the Nth harmonic of a counter clockwise (CCW) resonance tracking
modulation may differ due to non-linearities described elsewhere
herein. Embodiments of the invention diminish, e.g., reduce to zero
such vectoral difference. Differential harmonics may be generated
at one or more harmonics. Differential harmonics may be generated,
for example, by pick-up, frequency dependent gain, and
non-linearities in the RFOG (e.g., due to phase modulator(s)).
Resonance tracking modulation means modulation applied to a CW
optical signal and a modulation applied to a CCW optical signal to
permit detection of a resonance in a direction that the CW optical
signal propagates in a resonator of the RFOG and detection of a
resonance in a direction that the CCW optical signal propagates in
the resonator. Common resonance tracking modulation means the
modulation frequency applied (by frequency or phase modulation) to
the CW optical signal and the modulation frequency applied (by
frequency or phase modulation) to the CCW optical signal are the
same, i.e. the same frequency. The resonance tracking modulation
used herein is common resonance tracking modulation.
[0014] FIG. 1 illustrates a block diagram of one embodiment of a
resonator fiber optic gyroscope 100 configured to reduce
differential harmonics. Reduction of differential harmonics is
facilitated by a beat note servo (or beat note servo circuitry) 180
and a differential resonance tracking modulation error reduction
circuitry (or differential harmonic correction circuitry) 182. The
differential harmonic correction circuitry 182 generates at least
one error cancellation signal (error cancellation signal(s)) 185
which diminishes, e.g., cancels, one or more differential harmonics
generated elsewhere in the RFOG 100.
[0015] The exemplary RFOG 100, illustrated in FIG. 1, includes a
master laser 102, a clockwise (CW) slave laser 104, a
counterclockwise (CCW) slave laser 105, integrated photonics
circuitry (IPC) 110, an optical bench (or optical bench circuitry)
130, a CW optical phase lock loop (CW OPLL) (or CW OPLL circuitry)
108, CCW optical phase lock loop (CCW OPLL) (or CCW OPLL circuitry)
120, a Pound-Drever-Hall (PDH) servo (or PDH servo circuitry) 106,
a beat note servo (or beat note servo circuitry) 180, the
differential harmonic correction circuitry 182, rate calculation
circuitry 184, an optical fiber coil (fiber coil) 150, a CCW
resonance tracking servo (or CCW resonance tracking servo
circuitry) 198, and CW resonance tracking servo (or CW resonance
tracking servo circuitry) 199. This illustrated embodiment is
described for pedagogical purposes, and the embodiments of the
invention can be implemented using systems with configurations
which differ from the configuration illustrated in FIG. 1. Signals
emitted by the CW slave laser 104, the CCW slave laser 105, and the
master laser 102 may be referred to herein respectively as the CW
optical signal, the CCW optical signal, and the master optical
signal.
[0016] For example, the photonics circuitry of the integrated
photonics circuitry 110 and the optical bench 130 are each
integrated; however, in other embodiments, the photonics circuitry
of the integrated photonics circuitry 110 and/or the optical bench
130 may be implemented in discrete form. Thus, the integrated
photonics circuitry 110 may be just photonics circuitry in discrete
form. Integrated means formed on a common substrate, e.g.,
comprised of a semiconductor and/or an insulator; for example, the
substrate may be a doped or undoped semiconductor.
[0017] Further, the RFOG architecture of FIG. 1 is just one
architecture which can be used with the techniques described herein
to reduce differential harmonics. For example, the architecture
illustrated in FIG. 2 of U.S. Pat. No. 9,587,945 may be used in the
alternative. For example, a beat note servo could be coupled to
outputs of the phase modulators 242, 246, and a differential
harmonic correction circuitry (coupled to the beat note servo)
could be coupled to one of the resonance tracking electronics 268,
274--of U.S. Pat. No. 9,587,945. U.S. Pat. No. 9,587,945 is
incorporated by reference herein in its entirety.
[0018] The integrated photonics circuitry 110 integrates waveguides
112, waveguide beam splitters 114, a first photodetector PD1, and a
second photodetector PD2 on a substrate 111. Optionally, the
substrate is a semiconductor or insulator, and the waveguides 112,
the waveguide beam splitters 114, the first photodetector PD1, and
a second photodetector PD2 are formed from a semiconductor.
Optionally, silicon may be used as the semiconductor material. Each
of the waveguide beam splitters 114 may function as a beam splitter
and/or as a beam combiner. Note, embodiments of the invention may
use waveguide beam splitter(s) 114 which are not part of the
integrated photonics circuitry 110. Photodetectors described herein
may be implemented by photodiodes or other suitable components.
[0019] The integrated photonics circuitry 110 is configured to
direct optical signals generated by the respectively the master
laser 102, the CW slave laser 104 and the CCW slave laser 105
through the integrated photonics circuitry 110 to the first
photodetector PD1, the second photodetector PD2, and external
circuitry (subsequently described). The CW slave laser 104, the CCW
slave laser 104, and the master laser 102 are coupled respectively
to the integrated photonics circuitry 110 through a first input
103a, a second input 103b, and a third input 103c of the integrated
photonics circuitry 110. A first output 117a and a second output
117b of the integrated photonics circuitry 110 are configured to be
coupled--through optical waveguide 112' (e.g., optical fiber) and
waveguide beam splitters 114--to respectively a first input 119a
and a second input 119b of the optical bench 130; thus, the first
output 117a and the second output 117b are configured to provide
respectively the optical signal from the CW slave laser 104 and the
optical signal from the master laser 102, and an optical signal
from the CCW slave laser 105, to respectively the first input 119a
and the second input 119b of the optical bench 130. The first
output 117a and the second output 117b of the integrated photonics
circuitry 110 are also configured to be coupled, through waveguide
beam splitter 114, to a third photo detector PD3; thus, the first
output 117a and the second output 117b are configured to provide
portions of the optical signals from the CW slave laser 104, the
master laser 102, and the CCW slave laser 105 to the photodetector
PD3. An electrical output of the third photodetector PD3 is
configured to provide an electrical signal (proportional to the
optical power incident upon the third photodetector PD3) to an
input of the beat note servo 180. Optionally, in alternative to the
embodiment illustrated in FIG. 1, the optical signal from the
master laser 102 is not combined with the optical signal of the CW
slave laser 104 in the IPC 110 so that the optical signal of the
master laser 102 is not received by third photodetector PD3, but
rather the optical signal of the master laser 102 is combined with
the optical signal of the CW slave laser 104 prior to being input
to the optical bench 130 at the first input 119a so that the
optical signal from the master laser 102 is not received by the
third photodetector PD3 and the beat note servo 180.
[0020] The first photodetector PD1 is configured to receive
(through optical waveguides 112 and waveguide beam splitters 114)
portions of the optical signal generated by the master laser 102
and the optical signal generated by the CW slave laser. The second
photodetector PD2 is configured to receive (through optical
waveguides 112 and waveguide beam splitters 114) portions of the
optical signal generated by the master laser 102 and the optical
signal generated by the CCW slave laser 105. An electrical output
of the first photodetector PD1 is configured generate an electrical
signal having an amplitude proportional to the optical power
incident upon the first photodetector PD1. The electrical signal
generated by the first photodetector PD1 is configured to be
combined at the CW OPLL 108 with a CW offset frequency signal 192
generated by the CW resonance tracking servo 199. The CW OPLL 108
controls a carrier frequency of the optical signal emitted by the
CW slave laser 104. An electrical output of the second photodiode
PD2 is configured generate an electrical signal having an amplitude
proportional to the optical power incident upon the second
photodetector PD2. The electrical signal generated by the second
photodetector PD2 is configured to be combined at the CCW OPLL 120
with a CCW offset frequency signal 190 generated by the CCW
resonance tracking servo 198. The CCW OPLL 120 controls a carrier
frequency of the optical signal emitted by the CCW slave laser
105.
[0021] The CCW offset frequency signal 192 and the CW offset
frequency signal 190 each comprise a frequency used to tune
respectively the CCW slave laser 105 and the CW slave laser 104 so
that the carrier frequencies of the optical signals of the CCW
slave laser 105 and the CW slave laser 104 are tuned to
respectively CCW and CW resonant frequencies of a resonator 148
formed by the optical fiber coil 150 and a portion of the optical
bench 130 allowing optical signals to circulate through ports of
the optical fiber coil 150 coupled to the optical bench 130. If
optional Sideband Heterodyne Detection (SHD) modulation (described
elsewhere herein) is used, then the carrier frequencies of the
optical signals of the CCW slave laser 105 and the CW slave laser
104 are tuned to either respective CCW and CW resonant frequencies
of a resonator 148 or substantially in the middle of two adjacent
resonances in each of the CCW and CW paths of the resonator
148.
[0022] The optical bench 130 couples optical signals into and out
of the resonator 148, and completes the resonator 148, e.g., with
free-space optical components. Components of the optical bench 130
may be integrated on a substrate (as discussed elsewhere herein)
and/or may be discrete implementations.
[0023] The embodiment of the optical bench illustrated in FIG. 1
comprises a first collimating lens 135, a second collimating lens
134, a first optical circulator 136, a second optical circulator
138, a first mirror 140, a second mirror 142, a third collimating
lens 147, a fourth collimating lens 145, a fourth photodetector
PD4, a fifth photodetector PD5, and a sixth photodetector. Each
collimating lens, for example, may be a ball lens. The optical
bench may be implemented in other ways.
[0024] The beat note servo 180 is configured to generate at at
least one output at least one differential harmonic alternating
current (AC) error signal (differential harmonic AC error
signal(s)) 181. Optionally, the beat note servo 180 is also
configured to generate at least one output at the beat note
frequency. The differential harmonic AC error signal(s) 181
comprise a differential harmonic AC error signal for each
differential harmonic.
[0025] The differential harmonic correction circuitry 182 is
configured to receive the generated differential harmonic AC error
signal(s) 181, and to generate at least one error cancellation
signal (error cancellation signal(s)) 185 used to suppress one or
more differential harmonics. Each error cancellation signal (of the
error cancellation signal(s) 185) is intended to diminish, e.g.,
cancel, a corresponding differential harmonic for which a
corresponding differential harmonic correction circuitry (of the
differential harmonic correction circuitry) is designed to
diminish. Each error cancellation signal comprises a tone having a
corresponding frequency, amplitude, and phase. Each error reduction
circuit (of the differential harmonic correction circuitry) is used
to generate a unique error cancellation signal used to cancel a
unique differential harmonic.
[0026] A resonance tracking servo, e.g., the CCW resonance tracking
servo 198, is configured to receive the at least one error
cancellation signal 185 and to add the at least one error
cancellation signal 185 to a corresponding offset frequency signal
(e.g., CCW offset frequency signal 190), a resonance tracking (RT)
modulation signal, and optionally a SHD modulation signal. The CW
offset frequency signal 192 and the CCW offset frequency signal
190, generated respectively by the CW resonance tracking servo 199
and the CCW resonance tracking servo 198, are used to adjust the
carrier frequency of respectively the CW slave laser 104 and the
CCW slave laser 105.
[0027] Thus, the CCW offset frequency signal 190 also comprises the
error cancellation signal(s) 185 which diminish, e.g., cancel, one
or more differential harmonics in the resonance tracking
modulation. Although FIG. 1 illustrates that the error cancellation
signal(s) 185 is included in the CCW offset frequency signal 190,
alternatively the error cancellation signal(s) 185 can be included
in the CW offset frequency signal 192 instead of the CCW offset
frequency signal 190; thus, the differential harmonic correction
circuitry 182 is coupled to the CW resonance tracking servo 199
instead of the CCW resonance tracking servo 198. The error
cancellation signal(s) 185 may be added to either a frequency of
the CCW offset frequency signal 190 or a frequency of the CW offset
frequency signal 192 with a signal combiner (or signal combiner
circuitry), e.g., an adder (or adder circuitry); the signal
combiner may optionally be located in the corresponding resonance
tracking servo.
[0028] Optical signals from the CW slave laser 104 and the master
laser 102 (coupled through optical waveguide 112') are collimated
by a first collimating lens 135 and are directed towards the first
optical circulator 136. Mirrors described herein are partially
reflective, and thus partially transmissive. Optionally, the
mirrors may reflect ninety-nine percent of incident energy and
transmit one percent of the incident energy; however, each mirror
may have a different proportion of reflected and transmitted
energy.
[0029] The electrical output of the fifth photodetector PD5 is
configured to provide an electrical signal proportional to the
optical power incident upon the fifth photodetector PD5. The
electrical signal provided by the fifth photodetector PD5 is
configured to be provided to the CCW resonance tracking servo 198
to be used to generate the CCW offset frequency signal 190 used to
maintain the carrier frequency of the CCW slave laser 105 on a CCW
resonance of the resonator 148.
[0030] Optical signals from the CCW slave laser 104 (coupled
through optical waveguide 112') are collimated by a second
collimating lens 134 and are directed towards second optical
circulator 138. The second optical circulator 138 has a first
output that directs a portion of the optical signals from the CCW
slave laser 104 to a second mirror 142 and then to a fourth
collimating lens 145 (and thus to a second port of the resonator
148), and a second output that couples a portion of the optical
signals from the CW slave laser 104 and the master laser 102
circulating in the CW direction in the resonator 148 and emitted by
the second mirror 142 of the resonator 148 to a fourth
photodetector PD4.
[0031] The electrical output of the fourth photodetector PD4 is
configured to provide an electrical signal proportional to the
optical power incident upon the fourth photodetector PD4. The
electrical signal provided by the fourth photodetector PD4 is
configured to be provided to the CW resonance tracking servo 199 to
be used to generate the CW offset frequency signal 192 used to
maintain the carrier frequency of the CW slave laser 104 on a CW
resonance of the resonator 148.
[0032] In the embodiment illustrated in FIG. 1, the resonator 148
comprises the optical fiber coil 150, the third collimating lens
147, the fourth collimating lens 145, the first mirror 140, and the
second mirror 142. A portion of the CW optical signal, circulating
in the CW direction of the resonator 148, in the resonator 148 is
coupled out of the resonator 148 by the first mirror 140 and
directed towards a sixth photodetector PD6. The electrical output
of the sixth photodetector PD6 is configured to provide an
electrical signal proportional to the optical power incident upon
the sixth photodetector PD6. The electrical signal (PDH electrical
signal) provided by the sixth photodetector PD6 at the reflection
port is configured to be provided to the PDH servo 106 that
controls the frequency of the master laser 102 to a frequency with
a fixed offset from a center of a resonance dip detected using the
sixth photodetector PD6.
[0033] Another portion of the CW optical signal, circulating in the
CW direction of the resonator 148, is coupled out of the resonator
148 by the second mirror 142 and directed towards the second
circulator 138. Such other portion of the CW optical signal emitted
by the resonator 148 is transmitted by the second circulator 138 to
the fourth photodetector PD4. Thus, the electrical signal generated
by the fourth photodetector PD4 is proportional to the optical
power of the CW optical signal incident upon the fourth
photodetector PD4.
[0034] A portion of the CCW optical signal circulating in the CCW
direction of the resonator 148 is coupled out of the resonator 140
by the first mirror 140 and directed towards the first circulator
136. Such portion CCW optical signal circulating in the CCW
direction in the resonator 148 and emitted by the resonator 148 is
transmitted by the first circulator 136 to the fifth photodetector
PD5. Thus, the electrical signal generated by the fifth
photodetector PD5 is proportional to the optical power of the CCW
optical signal incident upon the fifth photodetector PD5. Further
in this embodiment, a preponderance of optical energy, e.g., about
ninety eight to ninety nine percent, of the CW optical signal and
the CCW optical signal propagating through the first mirror 140 and
the second mirror 142 recirculates through the resonator 148, and
is not emitted by the resonator 148.
[0035] Optical signals, including the CW optical signal, emitted
from the CW slave laser 104 and the master laser 102 are received
by and emitted by the first collimating lens 135. Such optical
signals are directed by the first collimating lens 135 to the first
circulator 136. Then, such optical signals are directed from the
first circulator 136 to the first mirror 140. A first portion of
such optical signals, including the CW optical signal, are
reflected by the first mirror 140 to the third collimating lens
147, and thus such optical signals, including the CW optical
signal, are injected into the resonator 148; a second portion of
such optical signals, including the CW optical signal, are
transmitted by the first mirror 140 to be incident upon the sixth
photodetector PD6. A third portion of the optical signals,
including the CW optical signal, circulating in the resonator 148
is also reflected by the first mirror 140 to also be incident upon
the sixth photodetector. The second portion interferes at the sixth
photodetector PD6 with the third portion of the optical signal,
including the CW optical signal, reflected from the circulating
optical signals, including the CW optical signal. The differential
phase arising from interference between the second portion and the
third portion of the CW signal results in the PDH electrical signal
(generated by the sixth photodetector PD6) whose amplitude varies
based upon the amount of differential phase; the amplitude of the
signal is utilized by the PDH servo 106 to adjust the carrier
frequency of the master laser 102. This reduces relative phase
noise between carrier frequency of the master slave laser 105 and
the resonance frequencies of the resonator 148.
[0036] The CCW optical signal, emitted from the CCW slave laser 105
are received by and emitted by the second collimating lens 134. The
CCW optical signal is directed by the second collimating lens 134
to the second circulator 138. Then, the CCW optical signal is
directed from the second circulator 138 to the second mirror 142. A
portion of the CCW optical signal is reflected by the second mirror
142 to the fourth collimating lens 145. Thus, the CCW optical
signal is injected into the resonator 148.
[0037] To determine rate of rotation, .OMEGA., a center axis 196 of
the optical fiber coil 150, a difference between the CW and CCW
resonance frequencies of the fiber optic ring resonator 148 of the
RFOG 100 is determined by the rate calculation circuitry 184 using
the beat note frequency. To measure the resonance frequencies, the
CW optical signal of the CW slave laser 104 and the CCW optical
signal of the CCW slave laser 105 are used in an embodiment to
probe resonance frequencies of the resonator 148 in respectively
the CW and the CCW directions. The resulting CW and CCW optical
signals are used to generate the beat note signal used to determine
rate of rotation.
[0038] Resonance tracking modulation is applied to each of the CW
optical signal and the CCW optical signal coupled to the resonator
148, and is used to detect the resonance frequencies in each of the
CW and the CCW directions of the resonator 148. Optionally, the
phase modulation is generated with a pair of phase modulators
located between the output of the CW slave laser 104 and the first
input 119a, and between the output of the CCW slave laser 105 and
the second input 119b. For pedagogical purposes, phase modulation
and phase modulator(s) are referenced herein; however, frequency
modulation and frequency modulator(s) may be used in place of the
phase modulation and phase modulator(s).
[0039] Optionally, Sideband Heterodyne Detection (SHD) modulation,
at relatively high frequency (typically greater than 1 MHz) is also
applied to both the CW optical signal and the CCW optical signal to
reject signals due to optical backscatter. For example, the CCW
offset frequency signal 192 and the CW offset frequency signal 190
are SHD frequency or phase modulated respectively in the CCW
resonance tracking servo 198 and the CW resonance tracking servo
199; however, the SHD modulation may be performed (and the SHD
modulator(s) and/or corresponding SHD modulator drive circuitry may
be located) elsewhere, e.g., in IPC 110. The SHD modulation
produces a signal at the resonator output (e.g., at the output of
the first circulator 136 coupled to the fifth photodetector PD5 in
the illustrated embodiment) at twice the frequency of the SHD
modulation frequency when the laser carrier frequency are at some
fixed offset from resonance, and the laser first-order sidebands
generated by the SHD modulation are on resonance, or when the
carrier frequency of a corresponding slave laser and the
second-order sidebands generated by the SHD modulation are on
resonance. The SHD modulation sidebands are about the carrier
frequency of a corresponding slave laser.
[0040] When SHD modulation is employed, the resonance tracking
modulation results in an amplitude modulation (AM), on the CW
optical signal emitted by the resonator 148 and incident upon the
fourth photodetector PD4 and on the CCW optical signal emitted by
the resonator 148 and incident upon the fifth photodetector PD5,
that is at twice the respective SHD frequency. When the laser
first-order sidebands generated by the SHD modulation are on
resonance, or when the CW laser carrier frequency and the CW laser
second-order sidebands generated by the SHD modulation are on
resonance, the amplitude modulation of the CW and CCW resonator
output signals has no content at the resonance tracking modulation
frequency.
[0041] Differential harmonics of the resonance tracking modulation
may be generated by imperfections, such as nonlinearity, in the
phase (or frequency) modulator drive circuitry, the phase (or
frequency) modulator(s), and/or possibly other components (e.g.,
other circuitry). Such modulators, driver circuitry, and/or other
components are used to provide resonant tracking modulation.
Optionally, the resonant tracking modulation is applied at a
fundamental frequency of 7 kHz; thus, second and higher order
harmonics may thus occur at 14 kHz, 21 kHz, 28 kHz, etc. Difference
between the CW and CCW components can result in differential
harmonics, which can create an error in determined rotation rate of
the RFOG.
[0042] Since the CW and CCW optical signals are locked onto
adjacent resonances, the carrier frequencies of the CW and CCW
optical signals will be separated by one free spectral range (FSR)
of the gyro resonator. Therefore, the beat note signal from the
third photodetector PD3 will have a carrier at a frequency equal to
a FSR plus the frequency shift due to rotation of the optical fiber
coil 150 about its center axis 196. Differential harmonics on the
resonance tracking modulations will produce sidebands about the
beat note signal carrier frequency. The difference between the
carrier frequency and sidebands of the beat note signal will be
equal to the frequency of the differential harmonic. The beat note
servo 180 demodulates the beat note signal at the beat note carrier
frequency. Therefore, the sidebands due to differential harmonics
are frequency down converted to baseband, and show up at their
respective frequencies on the beat note servo 180 output to the
differential harmonic correction circuitry 182. The demodulated (or
baseband) sidebands are an alternating current error signals for
control loops that diminish, e.g., eliminate, the differential
harmonics of the resonance tracking modulation. Each AC error
signal corresponds to a unique differential harmonic. To diminish,
e.g., eliminate, differential harmonics, the differential harmonic
correction circuitry 182 is configured to receive at least one
differential harmonic AC error signal (differential harmonic AC
error signal(s)) 181 from the beat note servo 180. Using the
received differential harmonic AC error signal(s) 181, the
differential harmonic correction circuitry 182 generates a
differential harmonic correction signal for each of one or more of
the at least one differential harmonic desired to be corrected. The
implementation of the differential harmonic correction circuitry
182 determines which of the at least one differential harmonic the
error cancellation signal(s) 185 (generated by the differential
harmonic correction circuitry 182) is intended to correct. Prior to
discussing the differential harmonic correction circuitry 182
further, the beat note servo 180 will now be described.
[0043] FIG. 2 illustrates a block diagram of one embodiment a beat
note servo 280. However, other implementations of a beat note servo
may be used as an alternative to the design of FIG. 2.
[0044] Returning to FIG. 2, an input to the beat note servo 280 is
configured to receive a signal from the third photodetector PD3.
Because the output of the third photodetector PD3 may be a current
signal, optionally, an optional beat note transimpedance amplifier
(TIA) 220 may be inserted between the third photodetector PD3; the
beat note TIA 220 is configured to convert a current signal
generated by the third photodetector PD3 to a voltage signal which
is provided to the input of, and can be processed by, the beat note
servo 280.
[0045] The third photodetector PD3 is configured to provide an
electrical signal (or a beat note signal) proportional to the
optical power incident upon the third photodetector PD3 of a beat
note created by the interference of the CW optical signal and the
CCW optical signal at the third photodetector PD3. The beat note
servo 280 is configured to detect, or measure, the frequency of the
beat note signal. The frequency of the beat note signal is the
difference between the optical carrier frequencies of the clockwise
optical signal and the counter clockwise optical signal. The beat
note servo is optionally designed to measure and provide at one of
its outputs a beat note frequency 227 in the presence of other
frequency components. The beat note frequency 227 is the frequency
of the beat note arising from the CW and CCW optical signals
incident at the third photodetector PD3. In the illustrated
embodiment, the beat note servo 280 comprises a digital phase
locked loop which performs such detection function.
[0046] The digital phase locked loop, and thus the beat note (BN)
servo 280, is implemented with digital signal processing.
Optionally, the beat note servo 280 comprises an BN analog to
digital converter (ADC) (or BN ADC circuitry) 222, a BN digital
mixer (or BN digital mixer circuitry) 224, a digital loop filter
(or digital loop filter circuitry) 228, and a BN numerically
controlled oscillator (NCO) 229. Optionally, the digital loop
filter 228 comprises digital gain circuitry and/or a digital filter
(e.g., an integrator circuitry with a zero (integrator with zero)
228 or a low pass filter with a cut-off frequency). The digital
gain circuitry is configured to increase or diminish the digital
loop filter 228, and hence the amplitude of the signal provided at
the output of the BN digital mixer 224. The digital filter may be
implemented with an infinite or finite impulse filter. The BN
digital mixer 224 may be implemented with a digital multiplier.
[0047] The signal output from the third photodetector PD3, and
output from the beat note TIA 220, is an analog signal. The BN ADC
222 digitizes the analog signal (whether a current signal or a
voltage signal). The beat note servo 280 locks an output frequency
of the BN NCO 229 to the digitized analog signal, but with a
90-degree phase shift from the digitized analog signal so that the
average output of the BN digital mixer 224 is at or near zero. When
the digital loop filter 228 is an integrator with a zero, the
average output of the BN digital mixer 224 thus is at or near zero.
A first input of the BN digital mixer 224 is configured to receive
the digitized analog signal. A second input of the BN digital mixer
224 is configured to be coupled to an output of the BN NCO 229, and
to receive an output signal from the BN NCO 229. The output of the
digital mixer is configured to be coupled to an input of the
digital loop filter 228. An output of the digital loop filter 228
is configured to be coupled to an input of the BN NCO 229.
[0048] The output of the BN digital mixer 224 is configured to
generate an output signal comprising the at least one differential
harmonic AC error signal. If the beat note servo 280 closed loop
bandwidth is less than the frequency of each baseband differential
harmonic desired to be extracted from the beat note servo 280, then
the output of the BN digital mixer 224 is configured to be coupled
to an input of the differential harmonic correction circuitry 182.
However, if the beat note servo 280 closed loop bandwidth is not
less than the frequency of each differential harmonic AC error
signal desired to be extracted from the beat note servo 280, then
the output of the digital loop filter 228 is configured to be
coupled to the input of the differential harmonic correction
circuitry 182. In one embodiment, a bandwidth and gain of the
digital loop filter 228 may be such that the beat note servo 280
closed loop bandwidth is between one and two kilohertz; however,
the bandwidth may be outside of this range. An alternative to
selecting an output of the beat note servo 280 at the input or
output of the loop filter 228 depending upon the bandwidth of the
loop filter 228 (which is used when utilizing a difference of the
frequencies output by the BN NCO 229 and the BN ADC 222), the
output of the beat note servo 280 may be taken at the input of the
loop filter 280 (regardless of the bandwidth of the loop filter
228) when using a sum of the frequencies output by the BN NCO 229
and the BN ADC 222 and coupled to the DHCC 180. If the sum of
frequencies is used, then the loop filter 330 is a band or high
pass filter having a filter characteristic that attenuates the
difference frequency component and passes the sum frequency
component. (When the difference frequency component is used, then a
low or band pass filter is used as the loop filter 228 to filter
out the sum frequency and optionally a DC component.) In the event
that the sum frequency component is used, then, the subsequently
described sine-cosine signal generator 332x are configured to
generate output signals having a frequency that has been increased
by twice the beat note frequency generated by the output of the
loop filter 330.
[0049] The digital loop filter 228 and the output frequency of the
BN NCO 229 are designed so that the difference between initial
output frequency of the BN NCO 229 and the frequency of the
electrical beat signal falls within half the frequency of the
nearest undesired frequency components, e.g., modulation sidebands
generated by the SHD modulation which could be 100 Hz away from the
frequency of the electrical beat signal. As a result, the digital
phase lock loop locks the BN NCO 229 onto the beat note carrier
frequency and 90-degree phase of the digitized electrical beat note
signal, and not onto a frequency of the other frequency
components.
[0050] The digital loop filter 228 is configured to generate, at
the output of the digital loop filter 228, a frequency of the beat
note signal. The rate calculation circuitry 184 is configured to
receive the frequency of the beat note signal from the beat note
servo 180, e.g., from the output of the digital loop filter 228.
The output of the digital loop filter 228 is configured to be
coupled to an input of the BN NCO 229, and to provide frequency
control words to the BN NCO 229.
[0051] FIG. 3 illustrates a block diagram of one embodiment of
differential harmonic correction circuitry 382. The differential
harmonic correction circuitry 382 can be implement in other ways,
e.g., using in phase or quadrature phase circuitry alone and then
shifting a phase of a signal from a signal generator to generate
cancellation component; thus, also, no signal combiner would be
required.
[0052] The differential harmonic correction circuitry 382 may be
implemented in different ways, including using an analog
implementation using a direct digital synthesizer for the sine
cosine generator, and analog mixers in lieu of digital mixers,
e.g., digital multipliers. The differential harmonic correction
circuitry 382 comprises at least one error reduction circuit 331x.
Each error reduction circuit is configured to generate an error
cancellation signal at a frequency of a corresponding differential
harmonic, and to be coupled to one resonance tracking servo (e.g.
the CCW resonance tracking servo 198). The error cancellation
signal diminishes, e.g., cancels, a corresponding differential
harmonic signal so as to reduce RFOG bias. If more than one error
reduction circuit is utilized so that more than one error
correction signal is generated for more than one differential
harmonic, then, optionally, the outputs of each error reduction
circuit (and thus the error correction signals) may be combined,
e.g., with an optional differential harmonic correction circuitry
(DHCC) adder (or DHCC adder circuitry) 339.
[0053] Optionally, the differential harmonic correction circuitry
382 also includes a filter 330, e.g. a band pass or low pass
filter. The filter 330 is coupled between the input of the
differential harmonic correction circuitry 382 and each error
reduction circuit 331x. The filter 330 is used to diminish or
suppress signals out of band of the differential harmonic AC error
signal(s) 181.
[0054] Each error reduction circuit 331x is configured to have an
input coupled to an output of the beat note servo 180 (e.g., the
output of the BN digital mixer 224 or the output of the loop filter
228) configured to provide the output signal comprising at least
one differential harmonic AC error signal. Thus, each error
reduction circuit 331x (of the differential harmonic correction
circuitry 382) is used to generate a unique error cancellation
signal used to cancel a unique differential harmonic.
[0055] Optionally, the differential harmonic correction circuitry
382 further comprises a filter 330 (e.g., a passband filter
configured to only pass the at least one differential harmonics
demodulated to baseband) that is coupled between the input of each
error reduction circuit and the output of the beat note servo 180
(e.g., the output of the BN digital mixer 224) configured to
provide the output signal comprising the at least one differential
harmonics demodulated to baseband.
[0056] Each error reduction circuit 331x comprises a sine-cosine
signal generator (SCG or sine/cosine signal generator circuitry)
332x, a first digital mixer (or first digital mixer circuitry)
334x-1, a second digital mixer (or second digital mixer circuitry)
334x-2, a first digital multiplier (first digital multiplier
circuitry) 337x-1, a second digital multiplier (second digital
multiplier circuitry) 337x-2, a first accumulator (or first
accumulator circuitry) 336x-1, a second accumulator (or second
accumulator circuitry) 336x-2, and a signal combiner (SC) (or SC
circuitry) 338a. The digital mixers may be implemented as digital
multipliers (or digital multiplier circuits). The sine/cosine
signal generator 332x may be implemented with an NCO (or NCO
circuit).
[0057] The SCG 332x generates sine and cosine signals having a
frequency near or equal to the carrier frequency of the
differential harmonic to be reduced so that the corresponding error
reduction circuit 331x acts as a notch filter with a finite
bandwidth and diminishes any signal in the finite bandwidth. `Near`
means that the carrier frequency of the differential harmonic has
to fall within a bandwidth--having sufficient gain to suppress
differential harmonics--of a loop of the RFOG formed by the BN
servo 180, the differential harmonic correction circuitry 182, a
corresponding RT servo, a corresponding OPLL, and a corresponding
slave laser. Typically, the frequency of the SCG 332x has to be
within 1 Hz of the carrier frequency of the differential harmonic
sought to be diminished by the corresponding error reduction
circuit 331x.
[0058] The sine/cosine signal generator generates a sine wave
signal and a cosine wave signal (which are ninety degrees out of
phase). Each sine and cosine signal generated by a SCG 332x is used
as local oscillator signals for respectively corresponding first
digital mixer 334x-1 and second digital mixer 334x-2, and as
carrier signals for respectively corresponding first digital
multiplier 337x-1 and second digital multiplier 337x-2.
[0059] A sine signal (Sin.) generated by the SCG 332x is coupled to
a first input of the first digital mixer 334x-1 and a first input
of a first digital multiplier 337x-1. The cosine signal (Cos.)
generated by the SCG 332x is coupled to a first input of the second
digital mixer 334x-2 and a first input of the second digital
multiplier 337x-2.
[0060] AC error components are configured to be coupled to a second
input of each of the first digital mixer 334x-1 and the second
digital mixer 334x-1. Each of the first digital mixer 334x-1 and
the second digital mixer 334x-2 translates to baseband in phase and
quadrature phase components of the differential harmonic AC error
signal corresponding to the frequency of the sine and cosine
signal. Such baseband signals may be respectively referred to as an
in phase differential harmonic direct current (DC) error signal and
a quadrature phase differential harmonic DC error signal. The in
phase differential harmonic DC error signal and the quadrature
phase differential harmonic DC error signal are used to control the
amplitude of respectively an in phase error cancellation signal and
a quadrature phase error cancellation signal.
[0061] Each of the corresponding first accumulator 336x-1 and the
second accumulator 336x-2 integrates the baseband in phase and
quadrature phase components. Each accumulator may be referred to as
accumulator circuitry. Such integration adjusts amplitude of the
corresponding in phase error cancellation signal and quadrature
phased cancellation signal.
[0062] A first input of each of the first digital multiplier 337x-1
and the second digital multiplier 337x-2 respectively the
integrated baseband in phase component and the integrated baseband
quadrature phase component. Each digital multiplier may be referred
to as digital multiplier circuitry. A second input of each of the
first digital multiplier 337x-1 and the second digital multiplier
337x-2 respectively receives the sine signal and the cosine signal.
The first digital multiplier 337x-1 multiplies the integrated
baseband in phase component and the sine signal. The second digital
multiplier 337x-2 multiplies the quadrature phase component and the
cosine signal. Such multiplication by the first digital multiplier
337x-1 respectively generates the in phase error cancellation
signal for the corresponding differential harmonic at the output of
the first digital multiplier 337x-1. Such multiplication by the
second digital multiplier 337x-2 respectively generates the
quadrature phase error cancellation signal for the corresponding
differential harmonic at the output of the second digital
multiplier 337x-2.
[0063] Inputs of a corresponding signal combiner 338x are
respectively coupled to the outputs of the first digital multiplier
337x-1 and the second digital multiplier 337x-2. The signal
combiner may be referred to as signal combiner circuitry. The
signal combiner 338x combines the in phase error cancellation
signal and the quadrature phase error cancellation signal for the
corresponding differential harmonic, and provides (at an output of
the SC 338x) a resulting error cancellation signal for the
differential harmonic (formed by the in phase and quadrature
components) at an output of the signal combiner 338x and at the
frequency of the sine and cosine signals generated by the
corresponding SCG 332x. Each error cancellation signal at an output
of the SC 338x is an AC signal at baseband. Optionally, if more
than one error reduction circuit 331x is used, the output of each
error reduction circuit (and each error cancellation signal) may be
combined by another signal combiner (e.g., the DHCC adder 339).
Such signal combiner or adder may be respectively referred to as
signal combiner circuitry or adder circuitry.
[0064] FIG. 4 illustrates a block diagram of one embodiment of a
CCW resonance tracking servo 498. In an alternate embodiment, the
error cancellation signal(s) 185 may be added in the CW resonance
tracking servo 199. The illustrated CCW resonance tracking servo
498 comprises a direct digital synthesizer (DDS) 440, a RT servo
digital adder (RT servo adder) 442, a CCW resonance tracking (RT)
NCO 444, a RT accumulator 445, gain circuitry 446, a first digital
demodulator 447, and an RT ADC 449. Optionally, the CCW resonance
tracking servo 498 includes a CCW SHD NCO 443 and a second digital
demodulator 448.
[0065] The CCW resonance tracking servo 498, e.g., the RT ADC 449,
is configured to receive the electrical signal generated by the
fifth photodetector PD5. The RT ADC 449 digitizes the electrical
signal and provides digitized electrical signal to a first input of
the first digital demodulator 447 or optionally to a first input of
the second digital modulator 448 (if SHD modulation is used).
[0066] Optionally, if SHD modulation is used, a second input of the
second digital demodulator 448 is configured to receive a signal
having a frequency of twice the CCW SHD modulation frequency
(f.sub.CCW,SHD). The second digital demodulator 448 generates a
signal at the output of the second digital demodulator 448 that is
the digitized electrical signal demodulated at twice the CCW SHD
modulation frequency.
[0067] The output of the optional second digital demodulator 448
(or alternatively the output of the RT ADC) is coupled to a first
input of the first digital demodulator 447 which is configured to
optionally receive the digitized electrical signal demodulated at
twice the CCW SHD modulation frequency (or alternatively a
digitized electrical signal generated by the fifth photodetector
PD5). The first digital demodulator 447 is also configured to
receive a signal having a common modulation frequency (f.sub.cm) at
a second input of the first digital demodulator 447. The first
digital demodulator 447 demodulates--the digitized electrical
signal demodulated at twice the CCW SHD modulation frequency or the
digitized electrical signal generated by the fifth photodetector
PD5--at the common modulation frequency. The average output of the
first digital demodulator 447 is an error signal coupled to the CCW
OPLL 120 to facilitate a resonance tracking control loop to
maintain the CCW slave laser 105 on a CCW resonance of the
resonator 148. The output of the first digital demodulator 447 is
coupled to an input of the gain circuitry 446. The gain circuitry
446 applies a gain (or an attenuation) to the resonance tracking
error signal from demodulator 447 to ensure the resonance tracking
loop is stable and that the RFOG 100 has a desired bandwidth. An
output of the gain circuitry 446 is coupled to an input of the RT
accumulator 445. The RT accumulator 445 integrates any non-zero
input so as to drive its average input to zero by outputting a
control signal to keep the CCW slave laser on resonance. The output
of RT accumulator 445 is an offset frequency signal (e.g., CCW
offset frequency signal) summed at the RT servo adder 442 with an
output from the CCW RT NCO 444, and optionally an output from the
optional CCW SHD NCO 443. The optional CCW SHD NCO 443 outputs a
sinusoidal digital signal at the CCW SHD frequency. The CCW RT NCO
44 outputs a sinusoidal digital signal at the resonance tracking
modulation frequency. The output of the RT servo adder 442 is
coupled to an input of the DDS 440. The DDS 440 converts digital
data at its input to an analog signal which provides an offset
frequency for the CCW OPLL 120. Optionally, the CCW SHD modulation
signal frequency or phase modulates the DDS 440 output frequency at
the SHD modulation frequency. The CCW resonance tracking modulation
frequency or phase modulates the DDS 440 output frequency at the
resonance tracking modulation frequency. Thus, the DDS 440 provides
an analog signal at its output with a carrier frequency equal to an
offset frequency that centers the OPLL on a resonance peak, along
with a signal modulated by the resonance tracking modulation
frequency, and also optionally by the SHD modulation frequency. The
resonance tracking modulation frequency, and if used, the optional
SHD modulation frequency, frequency or phase modulate the CW and
CCW offset frequencies generated by a DDS in respectively each of
the CW resonance tracking servo and the CCW resonance tracking
servo.
[0068] Note, typically, if one of the resonance tracking servos,
e.g., the CCW resonance tracking servo, is implemented as described
above with respect to FIG. 4, then the other resonance tracking
servo, e.g., the CW resonance tracking servo, would be similarly
implemented except that it would not receive at least one error
cancellation signal 185. However, resonance tracking modulation by
phase or frequency modulation can be applied in alternative
ways.
[0069] The resonator fiber optic gyro (RFOG) 100 of FIG. 1 may
employ resonance tracking modulation with frequency or phase
modulating the CW and CCW offset frequencies using RT NCOs.
Alternatively, the differential modulation correction circuitry can
be employed to correct differential modulation harmonics when other
methods of common resonance tracking modulation are employed. FIG.
5 illustrates a block diagram of one embodiment of an RFOG 500
employing a common resonance tracking modulation applied to a
portion of an optical signal generated by the master laser 102 and
which is coupled to the CW OPLL 108 and the CCW OPLL 120. A common
resonance tracking modulation (CRTM) generator 115 generates, at an
output of the CRTM generator 115, an analog resonance tracking
modulation signal. The CRTM generator 115 is used in lieu of the CW
and CCW RT NCOs in the CW and CCW resonance tracking servo
described with respect to FIG. 1. The output of the CRTM generator
115, and thus the analog modulation signal, is coupled to an
optical phase modulator (PM) 118 on the IPC 110 through a fourth
input 103d of the IPC 110. Optionally, the integrated photonics
circuitry 110 integrates the optical phase modulator 118 on the
substrate 111. The optical phase modulator 118 may be formed from a
semiconductor, e.g. silicon, if the rest of the IPC 110 is formed
from semiconductor. Since the portion of the optical signal
generated by the master laser 102 used by the CW OPLL 108 and the
CCW OPLL 120 is phase modulated at the resonance tracking
modulation frequency, the CW OPLL 108 and CCW OPLL 120 transfer the
resonance tracking modulation to the CW slave laser 104 and the CCW
slave laser 105 with a high degree of commonality. However,
imperfections in the optics devices can lead to differential
harmonic modulation. For example, to optical phase modulator 118
can also generate intensity modulation at a harmonic of the
resonance tracking modulation. Differential harmonic modulation in
the CW optical signal and the CCW optical signal (respectively
generated by the CW slave laser 104 and the CCW slave laser 105)
can arise respectively from the CW OPLL 108 and the CCW OPLLs
because each has a different response to the intensity modulation.
If the implementation of FIG. 5 is employed, then RT NCOs (444)
would not be utilized in the corresponding resonance tracking
servos as illustrated in FIG. 4.
[0070] FIG. 6 illustrates a flow diagram of an exemplary method 600
of reducing differential harmonics of resonance tracking modulation
in a resonant fiber optic gyroscope. To the extent the method 600
shown in FIG. 6 is described herein as being implemented in the
systems shown in FIGS. 1-5, it is to be understood that other
embodiments can be implemented in other ways. The blocks of the
flow diagrams have been arranged in a generally sequential manner
for ease of explanation; however, it is to be understood that this
arrangement is merely exemplary, and it should be recognized that
the processing associated with the methods (and the blocks shown in
the Figures) can occur in a different order (for example, where at
least some of the processing associated with the blocks is
performed in parallel and/or in an event-driven manner).
[0071] In block 660, modulate common resonance tracking modulation
on a CW optical signal, e.g., emitted from a CW slave laser, and on
a second optical signal, e.g., emitted from a second slave laser.
In block 662, receive a beat note electrical signal of a CW optical
signal and a CCW optical signal, e.g., emitted respectively by
lasers for example the CW slave laser 104 and the CCW slave laser
105. In block 664, using the beat note signal, generate at least
one AC error signal, where each AC error signal corresponds to a
unique differential harmonic. In block 667, use the at least one AC
error signal, generate at least one error cancellation signal,
where each error cancellation signal corresponds to an AC error
signal and a differential harmonic desired to be suppressed. In
block 669, adding the at least one error cancellation signal to an
offset frequency signal to diminish, e.g., cancel, the differential
harmonics in the RFOG.
EXAMPLE EMBODIMENTS
[0072] Example 1 includes A system for diminishing differential
harmonics of common resonance tracking modulation in a resonant
fiber optic gyroscope (RFOG), comprising: beat note servo circuitry
configured to receive an electrical beat note signal and to
generate at least one differential harmonic alternating current
(AC) signal, where the at least one differential harmonic AC signal
is generated using the beat note signal, where the electrical beat
note signal is created from a beat note of a first optical signal
generated by a first optical laser and a second optical signal
generated by a second laser, where the first optical signal and the
second optical signal are modulated with the common resonance
tracking modulation, where the first optical signal circulates in a
first direction in a resonator of the RFOG, and where the second
optical signal circulates in a second direction in the resonator of
the RFOG that is opposite the first direction; differential
harmonic correction circuitry configured to generate at least one
error cancellation signal using, where each error cancellation
signal is generated using a corresponding differential harmonic AC
signal, and where each error cancellation signal and its
corresponding differential harmonic AC signal correspond to a
differential harmonic desired to be diminished; and adder circuitry
configured to add the at least one error cancellation signal to an
offset frequency signal to the differential harmonics, where the
offset frequency signal comprises a frequency used to tune a
carrier frequency of one of the first optical signal and the second
optical signal to a resonant frequency in respectively one of the
first direction and the second direction; wherein each differential
harmonic of the common resonance tracking modulation is a vectoral
difference between a harmonic of common resonance tracking
modulation of the first optical signal and a same harmonic of
common resonance tracking modulation of the second optical signal,
and where a harmonic may be a fundamental frequency.
[0073] Example 2 includes the system of Example 1, wherein the beat
note servo is further configured to generate a beat note
frequency.
[0074] Example 3 includes the system of and of Examples 1-2,
wherein the beat note servo comprises: a beat note (BN) analog to
digital converter (ADC) circuitry comprising an input and an
output, where the output of the BN ADC circuitry is configured to
receive and digitize an electrical beat note signal; a BN digital
mixer circuitry comprising a first input, a second input, and an
output, where the first input of the BN digital mixer is configured
to receive a digitized electrical beat note signal from the BN ADC
circuitry, and where the output of the BN digital mixer circuitry
is configured to generate the at least one differential AC error
signal; digital loop filter circuitry comprising an input and an
output, where the input of the digital loop filter circuitry is
coupled to the output of the BN digital mixer circuitry; and a BN
numerically controlled oscillator (NCO) comprising an input and an
output, where the input of the BN NCO is coupled to the output of
the digital loop filter circuitry, and where the output of the BN
NCO is coupled to the second input of the BN digital mixer
circuitry.
[0075] Example 4 includes the system of Example 3, wherein the
output of the digital loop filter circuitry is configured to be
generate a beat note frequency.
[0076] Example 5 includes the system of any of Examples 1-4,
further comprising a transimpedance amplifier coupled an input of
the BN servo circuitry and configure to convert the beat note
electrical signal from a current signal to a voltage signal.
[0077] Example 6 includes the system of any of Examples 1-5,
wherein the differential harmonic correction circuitry comprises at
least one error reduction circuit configured to generate an error
cancellation signal using a differential harmonic AC signal for a
differential harmonic desired to be cancelled; and wherein each
error reduction circuit comprises: sine-cosine signal generator
(SCG) circuitry comprising a first output configured to provide a
sine signal and second output configured to provide a cosine signal
at a same frequency; first digital mixer circuitry comprising a
first input, a second input, and an output, where the first input
of the first digital mixer is configured to receive at least one
differential harmonic AC error signal, and where the second input
of the first digital mixer is coupled to the first output of the
SCG circuitry; second digital mixer circuitry comprising a first
input, a second input, and an output, where the first input of the
second digital mixer is configured to receive at least one
differential harmonic AC error signal, and where the second input
of the second digital mixer is coupled to the first output of the
SCG circuitry; first accumulator circuitry comprising an input and
an output, where the output of the first digital mixer circuitry is
coupled to the input of the first accumulator; second accumulator
circuitry comprising an input and an output, where the output of
the second digital mixer circuitry is coupled to the input of the
second accumulator; first digital multiplier circuitry comprising a
first input, a second input, and an output, where the first input
of the first digital multiplier circuitry is coupled to the output
of the first accumulator circuitry, and where the second input of
the first digital multiplier circuitry is coupled the first output
of the SCG circuitry; second digital multiplier circuit comprising
a first input, a second input, and an output, where the first input
of the second digital multiplier circuitry is coupled to the output
of the second accumulator circuitry, and where the second input of
the second digital multiplier circuitry is coupled the second
output of the SCG circuitry; and signal combiner circuitry
comprising a first input, a second input, and an output, where the
first input is coupled to the output of the first digital
multiplier circuitry, and where the second input is coupled to the
output of the second digital multiplier circuitry.
[0078] Example 7 includes the system of Example 6, wherein the
differential harmonic correction circuitry further comprises adder
circuitry configured to combine the output of each error reduction
circuit.
[0079] Example 8 includes a method for diminishing differential
harmonics of common resonance tracking modulation in a resonant
fiber optic gyroscope (RFOG), comprising: modulate the common
resonance tracking modulation on a first optical signal and a
second optical signal; receive a beat note electrical signal, where
the beat note electrical signal is created from a beat note of a
first optical signal generated by a first optical laser and a
second optical signal generated by a second laser; generate at
least one differential harmonic alternating current (AC) error
signal; generate at least one error cancellation signal using,
where each error cancellation signal is generated using a
corresponding differential harmonic AC signal, and where each error
cancellation signal and its corresponding differential harmonic AC
signal correspond to a differential harmonic desired to be
diminished; and add the at least one error cancellation signal to
an offset frequency signal to the differential harmonics, where the
offset frequency signal comprises a frequency used to tune a
carrier frequency of one of the first optical signal and the second
optical signal to a resonant frequency in respectively one of the
first direction and the second direction; wherein each differential
harmonic of the common resonance tracking modulation is a vectoral
difference between a harmonic of common resonance tracking
modulation of the first optical signal and a same harmonic of
common resonance tracking modulation of the second optical signal,
and where a harmonic may be a fundamental frequency.
[0080] Example 9 includes the method of Example 8, further
comprising digitizing the beat note electrical signal, where the at
least one differential AC error signal is generated with the
digitized beat note electrical signal.
[0081] Example 10 includes the method of any of Examples 8-9,
further comprising converting the beat note electrical signal from
a current signal to a voltage signal.
[0082] Example 11 includes a resonant fiber optic gyroscope (RFOG)
configured to diminish differential harmonics of common resonance
tracking modulation, comprising: an optical fiber coil comprising a
first port and a second port; optical bench circuitry comprising a
first input, a second input, a first port, a second port, a first
output, a second output, and a third output, and where the first
port and the second port of the optical bench are configured to be
coupled respectively to the first port and the second port of the
optical fiber coil, and further configured to generate a first
electrical signal, a second electrical signal, and an PDH
electrical signal, where the PDH electrical signal has an amplitude
that varies based upon a differential phase of a first optical
signal being injected into the optical fiber coil and a first
optical signal circulating in the optical fiber coil;
Pound-Drever-Hall (PDH) servo circuitry comprising an input and an
output, where the input is configured to receive the PDH electrical
signal; a master laser comprising an input configured to be coupled
to the output of the PDH servo circuitry, and configured to
generate a master optical signal; a first slave laser configured to
generate the first optical signal modulated by a common resonance
tracking modulation which circulates in the optical fiber coil in a
first direction; a second slave laser configured to generate the
second optical signal modulated by the common resonance tracking
modulation which circulates in the optical fiber coil in a second
direction, where the first direction is opposite the second
direction; first optical phase lock loop (OPLL) circuitry
configured to receive the first optical signal and the master
optical signal; second OPLL circuitry configured to receive the
second optical signal and the master optical signal and to provide
a second OPLL signal to the second slave laser; an integrated
photonics circuitry coupled to the first slave laser and configured
to receive the first optical signal, coupled to the second slave
laser and configured to receive the second optical signal, coupled
to the master laser and configured to receive the master optical
signal, coupled to the first OPLL circuitry and configured to
provide the first optical signal and the master optical signal to
the first OPLL circuitry, coupled to the second OPLL circuitry and
configured to provide the second optical signal and the master
optical signal to the second OPLL circuitry, configured to provide
the first optical signal and the master optical signal to the first
input of the optical bench circuitry, and configured to provide the
second optical signal to the second input of the optical bench
circuitry; beat note servo circuitry configured to receive an
electrical beat note signal and to generate at least one
differential harmonic alternating current (AC) signal and to
generate a beat note frequency, where the at least one differential
harmonic AC signal is generated using the beat note signal, where
the electrical beat note signal is created from a beat note of the
first optical signal generated by the first optical laser and the
second optical signal generated by the second laser, where the
first optical signal circulates in a first direction in a resonator
of the RFOG, and where the second optical signal circulates in a
second direction in the resonator of the RFOG that is opposite the
first direction, where the resonator is formed by the optical fiber
coil and a portion of the optical bench circuitry; rate calculation
circuitry coupled to the beat note servo circuitry and configured
to receive the beat note frequency and to determine a rate of
rotation of the fiber coil around a center axis; differential
harmonic correction circuitry configured to generate at least one
error cancellation signal using, where each error cancellation
signal is generated using a corresponding differential harmonic AC
signal, and where each error cancellation signal and its
corresponding differential harmonic AC signal correspond to a
differential harmonic desired to be diminished; first resonance
tracking servo circuitry configured to receive the first electrical
signal generated by the optical bench circuitry used to generate a
first offset frequency signal comprising a frequency used to tune a
carrier frequency of the first optical signal generated by the
first slave laser to a resonant frequency in the first direction;
and second resonance tracking servo circuitry configured to receive
the second electrical signal generated by the optical bench
circuitry used to generate a second offset frequency signal
comprising a frequency used to tune a carrier frequency of the
second optical signal generated by the second slave laser to a
resonant frequency in the second direction, and to receive the at
least one error cancellation signal; wherein each differential
harmonic of the common resonance tracking modulation is a vectoral
difference between a harmonic of common resonance tracking
modulation of the first optical signal and a same harmonic of
common resonance tracking modulation of the second optical signal,
and where a harmonic may be a fundamental frequency.
[0083] Example 12 includes the RFOG of Example 11, wherein the beat
note servo comprises: a beat note (BN) analog to digital converter
(ADC) circuitry comprising an input and an output, where the output
of the BN ADC circuitry is configured to receive and digitize an
electrical beat note signal; a BN digital mixer circuitry
comprising a first input, a second input, and an output, where the
first input of the BN digital mixer is configured to receive a
digitized electrical beat note signal from the BN ADC circuitry,
and where the output of the BN digital mixer circuitry is
configured to generate the at least one differential AC error
signal; digital loop filter circuitry comprising an input and an
output, where the input of the digital loop filter circuitry is
coupled to the output of the BN digital mixer circuitry, where the
output of the digital loop filter circuitry is configured to be
generate a beat note frequency; and a BN numerically controlled
oscillator (NCO) comprising an input and an output, where the input
of the BN NCO is coupled to the output of the digital loop filter
circuitry, and where the output of the BN NCO is coupled to the
second input of the BN digital mixer circuitry.
[0084] Example 13 includes the RFOG of any of Examples 11-12,
further comprising a transimpedance amplifier coupled an input of
the BN servo circuitry and configure to convert the beat note
electrical signal from a current signal to a voltage signal.
[0085] Example 14 includes the RFOG of any of Examples 11-13,
wherein the differential harmonic correction circuitry comprises at
least one error reduction circuit configured to generate an error
cancellation signal using a differential harmonic AC signal for a
differential harmonic desired to be cancelled; and wherein each
error reduction circuit comprises: sine-cosine signal generator
(SCG) circuitry comprising a first output configured to provide a
sine signal and second output configured to provide a cosine signal
at a same frequency; first digital mixer circuitry comprising a
first input, a second input, and an output, where the first input
of the first digital mixer is configured to receive at least one
differential harmonic AC error signal, and where the second input
of the first digital mixer is coupled to the first output of the
SCG circuitry; second digital mixer circuitry comprising a first
input, a second input, and an output, where the first input of the
second digital mixer is configured to receive at least one
differential harmonic AC error signal, and where the second input
of the second digital mixer is coupled to the first output of the
SCG circuitry; first accumulator circuitry comprising an input and
an output, where the output of the first digital mixer circuitry is
coupled to the input of the first accumulator; second accumulator
circuitry comprising an input and an output, where the output of
the second digital mixer circuitry is coupled to the input of the
second accumulator; first digital multiplier circuitry comprising a
first input, a second input, and an output, where the first input
of the first digital multiplier circuitry is coupled to the output
of the first accumulator circuitry, and where the second input of
the first digital multiplier circuitry is coupled the first output
of the SCG circuitry; second digital multiplier circuit comprising
a first input, a second input, and an output, where the first input
of the second digital multiplier circuitry is coupled to the output
of the second accumulator circuitry, and where the second input of
the second digital multiplier circuitry is coupled the second
output of the SCG circuitry; and signal combiner circuitry
comprising a first input, a second input, and an output, where the
first input is coupled to the output of the first digital
multiplier circuitry, and where the second input is coupled to the
output of the second digital multiplier circuitry.
[0086] Example 15 includes the RFOG of Example 14, wherein the
differential harmonic correction circuitry further comprises adder
circuitry configured to combine the output of each error reduction
circuit.
[0087] Example 16 includes the RFOG of any of Examples 11-15, each
of the first resonance tracking servo circuitry and the second
resonance tracking servo circuitry comprise: a resonance tracking
analog to digital converter (ADC) circuitry configured to receive
the first electrical signal and to digitize the first electrical
signal; first digital demodulator circuitry comprising an input, a
first input, and a second input, and configured to receive the
digitized first electrical signal at the first input of the first
digital demodulator, and to receive a common modulation frequency
at the second input of the first digital demodulator circuitry;
gain circuitry comprising an input and an output, where the input
of the gain circuitry is coupled to the output of the first digital
demodulator circuitry; resonance tracking (RT) accumulator
circuitry comprising an input and an output, where the input of the
RT accumulator circuitry is coupled to the output of the gain
circuitry; and direct digital synthesizer (DDS) circuitry
comprising an input and an output, where the input of the DDS
circuitry is coupled to the output of the RT accumulator
circuitry.
[0088] Example 17 includes the RFOG of Example 16, wherein each of
the first resonance tracking servo circuitry and the second
resonance tracking servo circuitry further comprises: RT servo
adder circuitry comprising an output coupled to the input of the
DDS circuitry, and a first input; and RT numerically controlled
oscillator coupled to the first input and configured to provide
common resonance tracking modulation.
[0089] Example 18 includes the RFOG of Example 17, wherein each of
the first resonance tracking servo circuitry and the second
resonance tracking servo circuitry further comprises a second
digital demodulator comprising a first input, a second input, and
an output, and configured to receive the digitized first electrical
signal at the first input of the second digital demodulator, and to
receive a signal having a frequency of twice sideband heterodyne
detection (SHD) frequency; and SHD numerically controlled
oscillator; wherein the RT servo adder circuitry further comprises
a second input coupled to the SHD numerically controlled
oscillator.
[0090] Example 19 includes the RFOG of any of Examples 16-18,
further comprising a RT servo adder circuitry comprising an output
coupled to the input of the DDS circuitry, and a first input
configured to receive the at least one error cancellation signal
from the differential harmonic correction circuitry.
[0091] Example 20 includes the RFOG of any of Examples 16-19,
further comprising a common resonance tracking modulation (CRTM)
generator configured to generate an analog common resonance
tracking modulation signal; and wherein the integrated photonics
circuitry further comprises a phase modulator configured to phase
modulate the master optical signal provided to the first OPLL
circuitry and the second OPLL circuitry.
[0092] Although specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that any arrangement, which is calculated to achieve the
same purpose, may be substituted for the specific embodiment shown.
This application is intended to cover any adaptations or variations
of the present invention. Therefore, it is manifestly intended that
this invention be limited only by the claims and the equivalents
thereof.
* * * * *