Apparatus and method for enhanced path sound detection

Abstract

An apparatus and method for enhancing beat detection. The present invention discloses a lower noise and complexity technique for compensating for time-varying phase variations of an optical fiber in a resonant fiber optic gyroscope (RFOG). A quadrature component derived from the beat electrical signal and a difference between a clockwise resonant frequency and a counterclockwise resonant frequency of an optical resonator coil of the RFOG is determined. The quadrature product is converted to a phase, which is differentiated with respect to time to obtain a beat correction signal.

Description

[0001] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under FA9453-20-C-0013 awarded by the Air Force Research Laboratory. The Government has certain rights in this invention. Background Art

[0003] For a given diameter, a resonant fiber gyroscope (RFOG) has better signal-to-noise sensitivity ("rotation sensitivity") in rotation sensing than laser ring gyroscopes and interferometer fiber gyroscopes. Therefore, an RFOG is desirable because a given level of rotation sensitivity can be achieved using a smaller RFOG, thereby reducing the size of an inertial navigation system incorporating the RFOG. The rotation of the RFOG is determined by the difference between the clockwise resonant frequency and the counterclockwise resonant frequency of the fiber ring resonator. The resonant frequency is measured using optical signals from light sources (clockwise light source and counterclockwise light source, respectively) that generate clockwise and counterclockwise optical signals, which are aligned, referenced, or locked to the resonant frequency of the resonator. Therefore, the frequency difference between the clockwise and counterclockwise optical signals at the input of the resonator is a measure of the rotation. Typically, frequency measurements of such clockwise and counterclockwise optical signals are performed at locations close to the clockwise and counterclockwise light sources.

[0004] The time-varying optical path length of the optical fiber coupling the light source to the optical resonator may change the frequency of the clockwise and counterclockwise optical signals propagating through the optical fiber. This may be caused by temperature fluctuations or mechanical vibrations that result in time-varying indices of the refractive index and physical path length of the optical fiber, resulting in a time-varying optical phase of the optical signal propagating through the optical fiber. The optical fibers of the clockwise and counterclockwise optical signal paths can only be matched with limited accuracy. Therefore, when path length fluctuations occur, the frequency difference between the clockwise and counterclockwise optical signal frequencies will not be the same at locations close to the light source and close to the optical resonator of the RFOG. If the pickup point used to measure the optical frequency is not close enough to the resonator input, this may result in rotation rate errors (also known as offset errors), which can be challenging. RFOGs can be designed to measure frequency differences as low as tens of microhertz. Therefore, even very small temperature fluctuations may produce undesirable offset errors in the RFOG.

[0005] Since the frequency measurement of the clockwise and counterclockwise optical signals is performed at a location close to the clockwise and counterclockwise light sources, additional offset errors due to time-varying electrical phenomena (such as offset voltages that drift over time and 1 / f noise) may occur in the circuits in the feedback loop in the RFOG. Such time-varying electrical phenomena also cause frequency differences between the clockwise and counterclockwise optical signal frequencies close to the light source and the clockwise and counterclockwise optical signal frequencies close to the optical resonator of the RFOG. Therefore, it is necessary to reduce such offset errors.

[0006] U.S. Patent No. 10,837,779 (hereinafter "the '779 patent") describes a technique for achieving this using a phase locked loop; the '779 patent is hereby incorporated by reference herein. However, this technique is more complex, difficult to optimize, and is susceptible to gain interactions, resulting in higher noise in the beat note measurement. Summary of the invention

[0007] In some aspects, the technology described herein relates to a method for generating a beat note correction signal configured to reduce an offset error in a resonant fiber gyroscope, the resonant fiber gyroscope comprising a light source optically coupled to an optical resonator coil, wherein the light source is configured to generate a clockwise optical signal configured to propagate clockwise around the optical resonator coil and a counterclockwise optical signal configured to propagate counterclockwise around the optical resonator coil, the method comprising: receiving a resonant frequency of the optical resonator coil in each of a clockwise direction and a counterclockwise direction; receiving a beat note electrical signal, the beat note electrical signal derived from a portion of the clockwise optical signal and a portion of the counterclockwise optical signal; determining a difference frequency, the difference frequency being a difference between the resonant frequency of the optical resonator coil in the clockwise direction and the counterclockwise direction. A method for generating a beat note correction signal by mixing the sine signal with the digitized beat note electrical signal and removing a first set of mixing products other than a first mixing product closest to zero Hz; a method for generating a beat note correction signal by mixing the cosine signal with the digitized beat note electrical signal and removing a second set of mixing products other than a second mixing product closest to zero Hz; a method for generating a beat note correction signal by using a rectangular coordinate to polar coordinate converter using the first low pass filtered signal and the second low pass filtered signal; and a method for generating a beat note correction signal by differentiating the phase signal with respect to time.

[0008] In some aspects, the technology described herein relates to a non-transitory machine-readable medium storing data, which, when loaded into a programmable circuit, causes the programmable circuit to instantiate a circuit configured to generate a beat note correction signal, the beat note correction signal being configured to reduce an offset error in a resonant fiber gyroscope, the resonant fiber gyroscope comprising a light source optically coupled to an optical resonator coil, wherein the light source is configured to generate a clockwise optical signal configured to propagate clockwise around the optical resonator coil and a counterclockwise optical signal configured to propagate counterclockwise around the optical resonator coil, wherein the programmable circuit is further configured to: receive a resonant frequency of the optical resonator coil in each of a clockwise direction and a counterclockwise direction; receive a beat note electrical signal derived from a portion of the clockwise optical signal and a portion of the counterclockwise optical signal; determine determining a difference frequency that is a function of the resonant frequency of the optical resonator coil in the clockwise direction and the resonant frequency of the optical resonator coil in the counterclockwise direction; generating (a) a sine signal as a function of the difference frequency and (b) a cosine signal as a function of the difference frequency; digitizing the beat note electrical signal; generating a first low pass filtered signal by mixing the sine signal with the digitized beat note electrical signal and removing a first set of mixing products other than a first mixing product that is closest to zero Hz; generating a second low pass filtered signal by mixing the cosine signal with the digitized beat note electrical signal and removing a second set of mixing products other than a second mixing product that is closest to zero Hz; generating a phase signal using a rectangular to polar converter using the first low pass filtered signal and the second low pass filtered signal; and generating a beat note correction signal by differentiating the phase signal with respect to time.

[0009] In some aspects, the technology described herein relates to a resonant fiber gyroscope (RFOG), comprising: a light source comprising at least one laser and configured to generate a clockwise (CW) optical signal and a counterclockwise (CCW) optical signal; a beat note detector configured to receive a first portion of the CW optical signal and a first portion of the CCW optical signal, and to generate a beat note electrical signal using the first portion of the CW optical signal and the first portion of the CCW optical signal; an optical resonator coil; and a resonator closure optical device configured to receive a second portion of the CW optical signal. and a second portion of the CCW optical signal into the optical resonator coil, and transmitting a third portion of the CW optical signal and a third portion of the CCW optical signal from the optical resonator coil; a CW rate detector configured to convert a portion of the third portion of the CW optical signal into a CW electrical signal; a CCW rate detector configured to convert a portion of the third portion of the CCW optical signal into a CW electrical signal; a CW resonant tracking loop circuit configured to determine a resonant frequency of the optical resonator coil in a CW direction; and a CW resonant tracking loop circuit configured to determine a resonant frequency of the optical resonator coil in a CW direction. a CCW resonant tracking loop circuit configured to determine the resonant frequency of the optical resonator coil in the CCW direction; and a beat note corrector circuit configured to: receive the resonant frequency of the optical resonator coil in each of a clockwise direction and a counterclockwise direction; receive the beat note electrical signal; determine a difference frequency that is a function of the resonant frequency of the optical resonator coil in the clockwise direction and the resonant frequency of the optical resonator coil in the counterclockwise direction; generate (a) a sine signal as a function of the difference frequency and (b) a cosine signal as a function of the difference frequency; signal; digitize the beat note electrical signal; generate a first low pass filter signal by mixing the sine signal with the digitized beat note electrical signal and removing a first group of mixing products except a first mixing product that is closest to zero Hz; generate a second low pass filter signal by mixing the cosine signal with the digitized beat note electrical signal and removing a second group of mixing products except a second mixing product that is closest to zero Hz; generate a phase signal using the first low pass filter signal and the second low pass filter signal using a rectangular coordinate to polar coordinate converter; and generate a beat note correction signal by differentiating the phase signal with respect to time. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting of the scope of the invention, the exemplary embodiments will be described with additional specificity and detail through use of the accompanying drawings.

[0011] in:

[0012] Figure 1 A block diagram of one embodiment of an RFOG with enhanced beat detection is shown;

[0013] Figure 2 A block diagram showing one embodiment of a resonator closure optical device;

[0014] Figure 3 A block diagram of one embodiment of a beat note corrector is shown; and

[0015] Figure 4 A flow chart of an exemplary method of determining a beat note correction signal is shown.

[0016] According to common practice, the various features described are not necessarily drawn to scale, but rather are used to emphasize specific features relevant to the exemplary embodiments. Reference characters denote similar elements throughout the figures and text. DETAILED DESCRIPTION

[0017] In the following detailed description, reference is made to the accompanying drawings which form a part thereof and in which specific illustrative embodiments are shown by way of illustration. However, it should be understood that other embodiments may be utilized and that structural, mechanical and electrical changes may be made. Furthermore, the methods presented in the drawings and the specification should not be interpreted as limiting the order in which the various steps may be performed. Therefore, the following detailed description should not be considered to be limiting.

[0018] In order to solve the above problems by reducing the complexity, optimization difficulty and noise in the beat note measurement, a technique for generating a beat note correction signal using the difference between the clockwise resonant frequency and the counterclockwise resonant frequency of the optical resonator coil of the RFOG as a reference signal is disclosed. Such a technique is a technical improvement of the RFOG, which reduces the offset error in the RFOG caused by the time-varying index of the refractive index of the optical fiber and the physical path length.

[0019] A beat note electrical signal is received and digitized. The frequency of the clockwise optical signal and the frequency of the counterclockwise optical signal are received, and the difference between the frequency of the clockwise optical signal and the frequency of the counterclockwise optical signal is generated; the difference between the frequency of the clockwise optical signal and the frequency of the counterclockwise optical signal is called a reference frequency. A sine signal and a cosine signal are generated as a function of the reference frequency. The sine signal and the cosine signal are orthogonal to each other. Each of the sine signal and the cosine signal is mixed with the digitized beat note electrical signal to generate two sets of mixing products. Each set of mixing products is filtered to remove all mixing products except the mixing product closest to zero Hz in frequency. The two filtered mixing products are converted into a phase difference relative to the reference frequency. A beat note correction signal is determined by differentiating the phase difference relative to time. The clockwise optical signal and the counterclockwise optical signal are locked to the clockwise resonant frequency and the counterclockwise resonant frequency, respectively. In order to more accurately determine the rotation rate of the RFOG or its resonator, the difference between the frequency of the clockwise light signal and the frequency of the counterclockwise light signal is adjusted (or offset) by the beat note correction signal.

[0020] The resonance tracking loop determines the resonant frequencies of the clockwise and counterclockwise paths in the optical resonator. The beat note detector generates an electrical signal that is a beat note of the clockwise and counterclockwise signals emitted from the resonator.

[0021] Figure 1 A block diagram of one embodiment of an RFOG with enhanced beat detection 100 is shown. Figure 1 The implementation of is only one RFOG architecture that can implement the present invention and is therefore described for teaching purposes only.

[0022] In the illustrated embodiment, the RFOG with enhanced beat note detection (RFOG) 100 includes an optical resonator coil (or resonator) 165, a resonator closure optical device 118, a first coupling optical device 113, a second coupling optical device 116, a CW rate detector 102, a CCW rate detector 101, a beat note detector 114, a beat note corrector (or a beat note corrector circuit) 115, a CW splitter (or CW splitter) 112, a CCW splitter (or CCW splitter) 117, a CW resonant tracking loop (or a CW resonant tracking loop circuit) 104, a CCW resonant tracking loop (or a CCW resonant tracking loop circuit) 103, and a light source 110.

[0023] Optionally, the light source 110 includes at least one laser. Optionally, when a single laser is used, an acousto-optic modulator or other frequency shifting device may be used to generate another optical signal having a carrier frequency different from the carrier frequency of the optical signal generated by the laser. Optionally, each detector is implemented using a photodiode.

[0024] The RFOG 100 optionally includes a processing system (or processing circuit) 179; however, the optional processing system 179 may be located external to the RFOG 100. Optionally, the beat note corrector 115 may be included in the optional processing system 179. The optional processing system 179 is configured to determine a rotation rate (Ω) 107 of the RFOG 100 or its optical resonator coil 165 about a winding axis 108 of the optical resonator coil 165 (e.g., the RFOG 100). The optical resonator coil 165 is composed of turns or windings of optical fiber around the winding axis 108.

[0025] The light source 110 is configured to provide a clockwise (CW) light signal (CW, CW light signal, or CW signal) 161 having a carrier frequency f1 at a first output, and a counterclockwise (CCW) light signal (CCW, CCW light signal, or CCW signal) 162 having a carrier frequency f2 at a second output. Optionally, the carrier frequencies f1 and f2 may be separated by one or more free spectral ranges. The CW optical splitter 112 is configured to be optically coupled to the first output of the light source 110 (e.g., via a first optical fiber) and to receive the CW light signal 161. The CCW optical splitter 117 is configured to be optically coupled to the second output of the light source 110 (e.g., via a second optical fiber) and to receive the CCW light signal 162. The CW optical splitter 112 is optically coupled to the beat note detector 114 and to the first coupling optical device 113 (e.g., through a third optical fiber), and is configured to transmit a first portion of the CW optical signal 161 to the beat note detector 114 and a second portion of the CW optical signal 161 to the first coupling optical device 113. The CCW optical splitter 117 is optically coupled to the beat note detector 114 and to the second coupling optical device 116 (e.g., through a fourth optical fiber), and is configured to transmit a first portion of the CCW optical signal 162 to the beat note detector 114 and a second portion of the CCW optical signal 162 to the second coupling optical device 116. Optionally, each optical splitter 112, 117 can be an optical coupler, but can alternatively be another type of optical device. Optionally, each coupling optical device 113, 116 can be an optical circulator, but can alternatively be another type of optical device, such as an optical coupler. The beat note detector 114 is configured to generate a beat note electrical signal (BNES) based on incident power or intensity of the first portion of the CW optical signal 161 and the CCW optical signal 162 and a phase difference therebetween.

[0026] For clarity, a portion of the CW optical signal 161 and a portion of the CCW optical signal 162 optically coupled to the beat note detector 114 may be obtained from the optical resonator coil 165, the resonator closure optical device 118, or (eg, Figure 1161 and the CCW optical signal 162 are optically coupled to the beat note detector 114 at or as close to the optical resonator coil 165 as possible.

[0027] Figure 2 A block diagram of one embodiment of a resonator closure optical device 218 is shown. The resonator closure optical device 218 includes a first optical reflector (or first reflector) 221 and a second optical reflector (or second reflector) 222. The first optical reflector 221 is configured to receive a second portion 223 of the CW optical signal 161, and optically couple (e.g., reflect) a portion thereof, i.e., a third portion (CW') 263, to the optical resonator coil 165, e.g., the first resonator port 175. The first optical reflector 221 is further configured to optically couple a portion of the second portion (CCW') 264 that circulates counterclockwise around the optical resonator coil 165 (i.e., a fourth portion (CCW") 273) to the first coupling optical device 113.

[0028] The second optical mirror 222 is configured to receive the second portion 224 of the CCW optical signal 162 and optically couple (e.g., reflect) a portion thereof, i.e., a third portion (CCW′) 264, to the optical resonator coil 165, for example, through the second resonator port 176. The second optical mirror 222 is further configured to optically couple a portion of the third portion (CW′) 263 that circulates clockwise around the optical resonator coil 165, i.e., a fourth portion (CW″) 272, to the second coupling optical device 116.

[0029] Return to Figure 1 , the first coupling optics 113 is configured to optically couple a second portion 223 of the CW optical signal 161 to, for example, a third port of the resonator closure optics 118. The second coupling optics 116 is configured to optically couple a second portion 224 of the CCW optical signal 162 to, for example, a fourth port of the resonator closure optics 118. The optical resonator coil 165, for example, a first resonator port and a second resonator port thereof, is configured to be optically coupled to the resonator closure optics 118, for example, to the first port and the second port of the resonator closure optics 118, respectively.

[0030] A portion of the second portion 223 of the CW optical signal 161 (or a third portion (CW′) 163 of the CW optical signal 161) is optically coupled from the resonator closure optics 118 to the optical resonator coil 165, for example, through the first resonator port 175. The third portion (CW′) 163 travels clockwise around the optical resonator coil 165 and the winding axis 108.

[0031] A portion of the second portion 224 of the CCW optical signal 162 (or a third portion (CCW′) 164 of the CCW optical signal 162) is optically coupled from the resonator closure optics 118 to the optical resonator coil 165, for example, through the second resonator port 176. The third portion (CCW′) 164 travels counterclockwise around the optical resonator coil 165 and the winding axis 108.

[0032] The CW rate detector 102 is optically coupled to the second coupling optics 116. A portion of the fourth portion (CW") 172 is optically coupled from the resonator closure optics 118 to the second coupling optics 116. All or a portion of the fourth portion (CW") 172 is optically coupled from the second coupling optics 116 to the CW rate detector 102. For didactic purposes, the entirety of the fourth portion (CW") 172 is shown as coupled to the CW rate detector 102. The CW rate detector 102 is configured to generate an electrical CW optical signal (ECCWS) 126 based on an incident power or intensity of a portion of the CW" 172, and may be used to detect an optional common modulation frequency that dithers the frequency f1 of the CW optical signal 161 (modulates the CW optical signal 161). The optional common modulation frequency may be used to find the resonant frequency of the optical resonator coil 165 in the CW direction.

[0033] The CCW rate detector 101 is optically coupled to the first coupling optics 113. A portion of the fourth portion (CCW") 173 is optically coupled from the resonator closure optics 118 to the first coupling optics 113. All or a portion of the fourth portion (CCW") 173 is optically coupled from the first coupling optics 113 to the CCW rate detector 101. For teaching purposes, the entirety of the fourth portion (CCW") 173 is shown as coupled to the CCW rate detector 101. The CCW rate detector 101 is configured to generate an electrical CCW optical signal (ECCWS) 127 based on the incident power or intensity of a portion of the CCW" 173, and can be used to detect an optional common modulation frequency that dithers the frequency f2 of the CCW optical signal 162 (modulates the CCW optical signal 162). The optional common modulation frequency can be used to find the resonant frequency of the optical resonator coil 165 in the CCW direction.

[0034] The CW rate detector 102 is electrically coupled to a CW resonant tracking loop (or CW resonant tracking loop circuit) 104 and is configured to generate an electrical CW optical signal (ECWS) 126 that is provided to the CW resonant tracking loop 104. Using the ECWS 126, the CW resonant tracking loop 104 determines the resonant frequency (f1′) of the optical resonator coil 165 in the CW direction. The CW resonant tracking loop 104 is electrically coupled to the light source 110 and the beat note corrector 115 and is configured to provide an electrical signal representing the resonant frequency (f1′) in the CW direction to the light source 110 and the beat note corrector 115. The light source 110 is configured to use the electrical signal representing the resonant frequency (f1′) in the CW direction to adjust the frequency f1 of the CW optical signal 161 to be equal to the resonant frequency f1′ in the CW direction.

[0035] The CW resonant tracking loop 103 is further configured to be electrically coupled to the processing system 179 and to provide an electrical signal representative of the resonant frequency ( f1 ′) in the CW direction to the processing system 179 .

[0036] The CCW rate detector 101 is electrically coupled to a CCW resonant tracking loop (or a CCW resonant tracking loop circuit) 103, and is configured to generate an electrical CCW optical signal (ECCWS) 127 that is provided to the CCW resonant tracking loop 103. Using the ECCWS 127, the CCW resonant tracking loop 103 determines the resonant frequency (f2′) of the optical resonator coil 165 in the CCW direction. The CCW resonant tracking loop 103 is electrically coupled to the light source 110 and the beat note corrector (or a beat note corrector circuit) 115, and is configured to provide an electrical signal representing the resonant frequency (f2′) in the CCW direction to the light source 110 and the beat note corrector 115. The light source 110 is configured to use the electrical signal representing the resonant frequency (f2′) in the CCW direction to adjust the frequency f2 of the CCW optical signal 162 to be equal to the resonant frequency f2′ in the CCW direction. The CCW resonant tracking loop 103 is further configured to be electrically coupled to the optical processing system 179 and to provide an electrical signal representative of the resonant frequency (f2′) in the CCW direction to the optical processing system 179 .

[0037] The beat note corrector 115 is configured to generate a beat note correction signal δf and optionally a beat note radius monitoring signal. The beat note corrector 115 utilizes an open loop implementation that is less complex to implement than the closed loop implementation of the '779 patent.

[0038] Figure 3A block diagram of one embodiment of a beat note corrector 315 is shown. The beat note corrector 315 includes a subtractor 335, a numerically controlled oscillator (NCO) 334, a first multiplier 332, a second multiplier 333, a digital converter (ADC) 331, a first low pass filter (LPF1) 336, a second low pass filter (LPF2) 353, a Cartesian coordinate to polar coordinate converter 338, and a differentiator 339. Optionally, the beat note corrector 115 includes at least one of the following: a high pass filter (HPF) 351, a first block averager 337, a second block averager 354, a third low pass filter (LPF3) 351, and a fourth low pass filter (LPF4) 355.

[0039] The digitizer 331 is configured to receive the beat note electrical signal BNES from the beat note detector 114, and to digitize the beat note electrical signal BNES. The beat note electrical signal BNES is an information transmission signal, and its data is extracted by the beat note corrector 115 using a reference signal. The subtractor 335 is configured to receive the resonance frequency (f1') in the CW direction of the optical resonator coil 165 (from the CW resonance tracking loop 104), receive the resonance frequency (f2') in the CCW direction of the optical resonator coil 165 (from the CCW resonance tracking loop 103), and subtract the latter from the former, i.e., f1'-f2'. The difference frequency f1'-f2' is a reference frequency. In other embodiments, the difference frequency, i.e., the reference frequency, is f2'-f1'; for teaching purposes, the difference frequency and the reference frequency f1'-f2' will be described herein.

[0040] The NCO 334 is configured to receive a reference frequency and a sine function (sin(f1'-f2')) and a cosine function (cos(f1'-f2')) that generate a difference frequency thereof. The first multiplier 332 is configured to receive the sine function of the difference frequency and the digitized beat note electrical signal, and multiply the sine function with the digitized beat note electrical signal, and generate a first mixed signal. Optionally, the beat note corrector 315 includes an HPF 351, which is configured to remove any DC component from the digitized beat note electrical signal. When the HPF 351 is used, the first multiplier 332 receives the high-pass filtered digitized beat note electrical signal. The second multiplier 333 is configured to receive the digitized beat note electrical signal and the cosine function, and is also configured to multiply the cosine function with the digitized beat note electrical signal, and generate a second mixed signal. Optionally, the beat note corrector 315 includes a HPF 351 configured to remove any DC component from the digitized beat note electrical signal. When the HPF 351 is used, the second multiplier 333 receives the high pass filtered digitized beat note electrical signal.

[0041] The first low pass filter 336 is configured to receive the first mixed signal and is further configured to generate a first low pass filtered signal, wherein the power of all frequency components of the first mixed signal except the lowest frequency component is reduced compared to the lowest frequency component of the first mixed signal. Optionally, the beat note corrector 315 includes a first block averager 337, which is configured to perform down sampling of the first low pass signal (i.e., reduce the bit rate). When the first block averager 337 is used, the Cartesian coordinate to polar coordinate converter 338 receives the first block averaged low pass filtered signal. The second low pass filter 353 is configured to receive the second mixed signal and is further configured to generate a second low pass filtered signal, wherein the power of all frequency components of the second mixed signal except the lowest frequency component is reduced compared to the lowest frequency component of the second mixed signal. Optionally, the beat note corrector 315 includes a second block averager 354, which is configured to perform down sampling of the second low pass signal (i.e., reduce the bit rate). When the second block averager 354 is used, the Cartesian to Polar converter 338 receives the second block averaged low pass filtered signal

[0042] The first low-pass filtered signal and the second low-pass filtered signal correspond to Cartesian coordinate values, for example, x-axis values ​​and y-axis values, respectively. The Cartesian coordinate to polar coordinate converter 338 is configured to receive the first low-pass filtered signal and the second low-pass filtered signal, and use such first low-pass filtered signal and the second low-pass filtered signal to generate a phase signal θ. Optionally, the Cartesian coordinate to polar coordinate converter 338 is implemented using a coordinate rotation digital calculation (CORDIC) algorithm; however, any Cartesian coordinate to polar coordinate converter algorithm can be used. If there is a frequency difference between the difference frequency and the beat note electrical signal, the phase signal θ changes with time. Optionally, the Cartesian coordinate to polar coordinate converter 338 is further configured to generate an optional beat note radius signal (R) 356, which is a marker of the amplitude of the beat note electrical signal (BNES), for example, which can be used to monitor the level of the beat note electrical signal (BNES). The differentiator 339 is configured to receive the phase signal θ and differentiate the phase signal (θ) with respect to time to generate a frequency also referred to as the beat note correction frequency δf. Optionally, the beat note corrector comprises a third low pass filter 351 electrically coupled to the output of the differentiator 339 , configured to reduce noise in the beat note correction signal δf and to provide a low pass filtered beat note correction signal δf′ at the output of the third low pass filter 351 .

[0043] Return to Figure 1The optional processing system 179 is coupled to the beat note corrector 115 and is configured to receive the beat note correction frequency δf from the beat note corrector 115. The optional processing system 179 is also coupled to the CW resonant tracking loop 104 and the CCW resonant tracking loop 103. The processing system is configured to receive the resonant frequency (f1′) in the CW direction of the optical resonator coil 165 and the resonant frequency (f2′) in the CCW direction of the optical resonator coil 165 from the CW resonant tracking loop 104 and the CCW resonant tracking loop 103, respectively.

[0044] Using the beat note correction frequency δf, the resonant frequency (f1') of the optical resonator coil 165 in the CW direction, and the resonant frequency (f2') of the optical resonator coil 165 in the CCW direction, the optional processing system 179 is configured to determine the rotational angular velocity (Ω) 107 about the winding axis 108 of the optical resonator coil 165. The optional processing system 179 (e.g., the memory circuits therein) may include program instructions executed by the optional processing system 179 (e.g., the processor circuits therein) to determine the rotational angular velocity (Ω) 107 of the optical resonator coil 165 about the winding axis 108 of the optical resonator coil 165. The rotational angular velocity (Ω) 107 is a function of (a) the difference frequency (f1'-f2' or f2'-f1', as described elsewhere herein) and (b) offset by the beat note correction frequency δf (Δf). For clarity, the offset may be achieved by subtracting or adding the beat note correction frequency δf from the difference frequency. For teaching purposes, the function Δf may be implemented as follows: f1′-f2′-δf; however, the function Δf may be implemented in alternative ways. Optionally, the rotational angular velocity Ω107 around the winding axis 108 is equal to:

[0045] (λ*Δf*P) / (4*A),

[0046] Where λ is substantially equal to the average wavelength of the clockwise optical signal 161 and the counterclockwise optical signal 162 , A is the area enclosed by the optical resonator coil 165 , and P is the circumference of the optical resonator path of the optical resonator coil 165 and the resonator closure optical device 118 .

[0047] Figure 4 4 shows a flow chart of an exemplary method 440 for determining a beat note correction signal δf in an RFOG. Figure 4 The method 440 shown herein is described as using Figures 1 to 3The method 440 is implemented by one or more of the devices shown, and it should be understood that other embodiments may be implemented in other ways. For ease of explanation, the blocks of the flowchart are arranged in a generally sequential manner; however, it should be understood that this arrangement is merely exemplary, and it should be recognized that the processing associated with these methods (and the blocks shown in the figure) may occur in a different order (for example, where at least some of the processing associated with these blocks is performed in parallel and / or in an event-driven manner). Optionally, the method 440 utilizes Figures 1 to 3 The method 440 is implemented by one or more of the devices shown, for example, at least partially in the beat note corrector (or beat note corrector circuit) 115. The method 440 is used to correct differential errors relative to the carrier frequencies f1 and f2.

[0048] In block 441, a resonant frequency (f1') in the CW direction of the optical resonator coil 165 and a resonant frequency (f2') in the CCW direction of the optical resonator coil 165 are received. In block 442, a beat note electrical signal (BNES) is received. The beat note electrical signal is derived from a portion of the clockwise optical signal and a portion of the counterclockwise optical signal, for example, as further described elsewhere herein.

[0049] In block 443, the difference frequency is determined as a function of f1' and f2'. In block 444, a sine signal and a cosine signal are generated as a function of the difference frequency.

[0050] The beat note electrical signal is digitized in block 445. Optionally, the digitized beat note electrical signal is high pass filtered.

[0051] In block 446, a first low pass filtered signal is generated by mixing the sinusoidal signal with the digitized beat note electrical signal and removing the mixing products except for the mixing products closest to zero Hz (i.e., DC). Optionally, the first low pass filtered signal is block averaged to reduce the sampling rate of the first low pass filtered signal.

[0052] In block 447, a second low pass filtered signal is generated by mixing the cosine signal with the digitized beat note electrical signal and removing mixing products other than the mixing products closest to zero Hz (i.e., DC). Optionally, the second low pass filtered signal is block averaged to reduce the sampling rate of the second low pass filtered signal. In block 448, the first low pass filtered signal and the second low pass filtered signal are used to generate a phase signal θ, for example, as described elsewhere herein.

[0053] In block 449, a beat note correction signal δf is generated by differentiating the phase signal θ with respect to time. Optionally, the beat note correction signal δf is low pass filtered. Optionally, a beat note radius signal (R) 356 is generated, which is a marker of the amplitude of the beat note electrical signal BNES, e.g., as described elsewhere herein. Thus, the beat note radius signal 356 can be used to monitor the level of the beat note electrical signal BNES. Optionally, the beat note radius signal (R) 356 is low pass filtered. In optional block 460, the rotation rate Ω107 of the winding axis 108 around the optical resonator coil 165 is determined using the beat note correction signal and the difference frequency, e.g., as described elsewhere herein.

[0054] Optionally, blocks 441-460 are performed in the optional processing system 179. Alternatively and optionally, blocks 441-449 are performed in the beat note corrector 115, and optional block 460 is performed in the optional processing system 179. However, the blocks of method 440 may be implemented differently in the beat note corrector 115 and the optional processing system 179, or may be performed in other devices.

[0055] The processor circuits described herein may include one or more microprocessors, microcontrollers, digital signal processing (DSP) elements, application specific integrated circuits (ASICs), and / or field programmable gate arrays (FPGAs). In this exemplary embodiment, the processor circuits include or function with software programs, firmware, or other computer-readable instructions for performing the various process tasks, calculations, and control functions of the methods described herein. These instructions are typically tangibly embodied on any storage medium (or computer-readable medium) for storing computer-readable instructions or data structures.

[0056] The memory circuits described herein can be implemented using any available storage media (or computer-readable media) that can be accessed by a general or special computer or processor or any programmable logic device. Suitable computer-readable media may include storage or memory media, such as semiconductors, magnetic media, and / or optical media. For example, computer-readable media may include conventional hard disks, compact disks-read-only memories (CD-ROMs), DVDs, volatile or non-volatile media such as random access memories (RAMs) (including but not limited to dynamic random access memories (DRAMs)), read-only memories (ROMs), electrically erasable programmable ROMs (EEPROMs), and / or flash memory. The above combinations are also included within the scope of computer-readable media.

[0057] The method of the present invention can be implemented as computer-readable instructions (such as program modules or applications) that can be stored in a computer-readable medium and executed by a processor circuit. Typically, a program module or application includes routines, programs, objects, data components, data structures, algorithms, etc. that perform specific tasks or implement specific abstract data types.

[0058] The techniques described herein may be implemented using programmable circuits (eg, FPGAs) that, when loaded with data stored in a non-transitory machine-readable medium, cause a programmable circuit array to instantiate circuits configured to implement such techniques.

[0059] Although the present teachings have been described with respect to one or more embodiments, the illustrated embodiments may be changed and / or modified without departing from the scope of the appended claims. In addition, although specific features of the present disclosure may be described with respect to only one of several embodiments, such features may be combined with one or more other features of other embodiments as may be desired or advantageous for any given or specific function. In addition, to the extent that the terms "include," "comprise," "have," "have," "have," "with," or variations thereof are used in the specific embodiments and claims, such terms are intended to be included in a manner similar to the term "include." The term "at least one of..." is used to indicate that one or more of the listed items may be selected. As used herein, the term "one or more of..." with respect to a series of items such as, for example, A and B or A and / or B refers to A alone, B alone, or A and B. The term "at least one of" is used to indicate that one or more of the listed items may be selected.

[0060] The term "about" or "substantially" means that the specified value or parameter may vary slightly as long as the variation does not cause the process or structure to be inconsistent with the illustrated embodiment. Finally, "exemplary" means that the description is used as an example and does not imply that it is an ideal condition. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. Therefore, it is apparent that the present invention is limited only by the claims and their equivalents.

[0061] Exemplary embodiments

[0062] Embodiment 1 includes a method for generating a beat note correction signal configured to reduce an offset error in a resonant fiber gyroscope, the resonant fiber gyroscope comprising a light source optically coupled to an optical resonator coil, wherein the light source is configured to generate a clockwise optical signal configured to propagate clockwise around the optical resonator coil and a counterclockwise optical signal configured to propagate counterclockwise around the optical resonator coil, the method comprising: receiving a resonant frequency of the optical resonator coil in each of a clockwise direction and a counterclockwise direction; receiving a beat note electrical signal derived from a portion of the clockwise optical signal and a portion of the counterclockwise optical signal; determining a difference frequency, the difference frequency being a difference between the resonant frequency of the optical resonator coil in the clockwise direction and the counterclockwise optical signal; The invention relates to a method for producing a beat note correction signal by mixing the sine signal with the digitized beat note electrical signal and removing a first set of mixing products other than a first mixing product closest to zero Hz; a second low pass filtered signal is generated by mixing the cosine signal with the digitized beat note electrical signal and removing a second set of mixing products other than a second mixing product closest to zero Hz; a phase signal is generated using a rectangular to polar converter using the first low pass filtered signal and the second low pass filtered signal; and a beat note correction signal is generated by differentiating the phase signal with respect to time.

[0063] Embodiment 2 includes the method of embodiment 1, further comprising determining a rotation rate about a winding axis of the optical resonator coil using the difference frequency and the beat note correction signal.

[0064] Embodiment 3 includes the method according to any one of embodiments 1 to 2, further comprising low pass filtering the beat note correction signal.

[0065] Embodiment 4 includes a method according to any one of Embodiments 1 to 3, the method further comprising high-pass filtering the digitized beat note electrical signal to remove any DC component; wherein the first low-pass filtered signal and the second low-pass filtered signal are generated using the digitized beat note electrical signal after high-pass filtering.

[0066] Embodiment 5 includes a method according to any one of embodiments 1 to 4, the method also comprising reducing the sampling rate of each of the first low-pass filtered signal and the second low-pass filtered signal; wherein the phase signal is generated using the first low-pass filtered signal and the second low-pass filtered signal with reduced sampling rate.

[0067] Embodiment 6 includes the method according to any one of embodiments 1 to 5, further comprising generating a beat radius signal, the beat radius signal being an indicia of an amplitude of the beat note electrical signal.

[0068] Embodiment 7 includes a non-transitory machine-readable medium storing data, which, when loaded into a programmable circuit, causes the programmable circuit to instantiate an instantiated circuit configured to generate a beat note correction signal, the beat note correction signal being configured to reduce an offset error in a resonant fiber gyroscope, the resonant fiber gyroscope comprising a light source optically coupled to an optical resonator coil, wherein the light source is configured to generate a clockwise optical signal configured to propagate clockwise around the optical resonator coil and a counterclockwise optical signal configured to propagate counterclockwise around the optical resonator coil, wherein the instantiated circuit is configured to: receive a resonant frequency of the optical resonator coil in each of a clockwise direction and a counterclockwise direction; receive a beat note electrical signal, the beat note electrical signal being derived from a portion of the clockwise optical signal and a portion of the counterclockwise optical signal; determine a difference frequency, the difference frequency The invention relates to a method for generating a beat note correction signal by mixing the sine signal with the digitized beat note electrical signal and removing a first set of mixing products other than a first mixing product closest to zero Hz; generating a second low pass filtered signal by mixing the cosine signal with the digitized beat note electrical signal and removing a second set of mixing products other than a second mixing product closest to zero Hz; generating a phase signal using a rectangular coordinate to polar coordinate converter using the first low pass filtered signal and the second low pass filtered signal; and generating a beat note correction signal by differentiating the phase signal with respect to time.

[0069] Embodiment 8 includes a non-transitory machine-readable medium storing data according to embodiment 7, wherein the instantiated circuit is further configured to determine a rotation rate about a winding axis of the optical resonator coil using the beat note correction signal and the difference frequency.

[0070] Embodiment 9 includes the non-transitory machine-readable medium storing data according to any one of embodiments 7-8, wherein the instantiated circuit is further configured to low pass filter the beat note correction signal.

[0071] Embodiment 10 includes a non-transitory machine-readable medium storing data according to any one of Embodiments 7 to 9, wherein the instantiated circuit is further configured to high-pass filter the digitized beat note electrical signal to remove any DC component; wherein the first low-pass filtered signal and the second low-pass filtered signal are generated using the digitized beat note electrical signal after high-pass filtering.

[0072] Embodiment 11 includes a non-transitory machine-readable medium storing data according to any one of Embodiments 7 to 10, wherein the instantiated circuit is further configured to reduce the sampling rate of each of the first low-pass filtered signal and the second low-pass filtered signal; wherein the phase signal is generated using the first low-pass filtered signal and the second low-pass filtered signal with reduced sampling rates.

[0073] Embodiment 12 includes a non-transitory machine-readable medium storing data according to any one of Embodiments 7 to 11, wherein the instantiated circuit is further configured to generate a beat radius signal, the beat radius signal being an indicia of an amplitude of the beat note electrical signal.

[0074] Embodiment 13 includes a resonant fiber gyroscope (RFOG), the RFOG comprising: a light source comprising at least one laser and configured to generate a clockwise (CW) optical signal and a counterclockwise (CCW) optical signal; a beat note detector configured to receive a first portion of the CW optical signal and a first portion of the CCW optical signal, and to generate a beat note electrical signal using the first portion of the CW optical signal and the first portion of the CCW optical signal; an optical resonator coil; a resonator closure optical device configured to receive a second portion of the CW optical signal and a second portion of the CCW optical signal; a second portion of the CCW optical signal coupled into the optical resonator coil, and a third portion of the CW optical signal and a third portion of the CCW optical signal transmitted from the optical resonator coil; a CW rate detector configured to convert a portion of the third portion of the CW optical signal into a CW electrical signal; a CCW rate detector configured to convert a portion of the third portion of the CCW optical signal into a CW electrical signal; and a CW resonant tracking loop circuit configured to determine a resonant frequency of the optical resonator coil in a CW direction. a CCW resonant tracking loop circuit configured to determine the resonant frequency of the optical resonator coil in the CCW direction; and a first circuit configured to: receive the resonant frequency of the optical resonator coil in each of a clockwise direction and a counterclockwise direction; receive the beat note electrical signal; determine a difference frequency that is a function of the resonant frequency of the optical resonator coil in the clockwise direction and the resonant frequency of the optical resonator coil in the counterclockwise direction; generate (a) a sine signal as a function of the difference frequency and (b) a cosine signal as a function of the difference frequency ; digitize the beat note electrical signal; generate a first low pass filter signal by mixing the sine signal with the digitized beat note electrical signal and removing a first group of mixing products except a first mixing product closest to zero Hz; generate a second low pass filter signal by mixing the cosine signal with the digitized beat note electrical signal and removing a second group of mixing products except a second mixing product closest to zero Hz; generate a phase signal using a rectangular coordinate to polar coordinate converter using the first low pass filter signal and the second low pass filter signal; and generate a beat note correction signal by differentiating the phase signal with respect to time.

[0075] Embodiment 14 includes the RFOG of Embodiment 13, wherein the first circuit is further configured to determine a rotation rate about a winding axis of the optical resonator coil using the beat note correction signal and the difference frequency.

[0076] Embodiment 15 includes the RFOG of any one of Embodiments 13-14, wherein the first circuit is further configured to low pass filter the beat note correction signal.

[0077] Embodiment 16 includes an RFOG according to any one of Embodiments 13 to 15, wherein the first circuit is further configured to high-pass filter the digitized beat note electrical signal to remove any DC component; wherein the first low-pass filtered signal and the second low-pass filtered signal are generated using the digitized beat note electrical signal after high-pass filtering.

[0078] Embodiment 17 includes an RFOG according to any one of Embodiments 13 to 16, wherein the circuit is further configured to reduce the sampling rate of each of the first low-pass filtered signal and the second low-pass filtered signal; wherein the phase signal is generated using the first low-pass filtered signal and the second low-pass filtered signal with reduced sampling rate.

[0079] Embodiment 18 includes the RFOG of any one of Embodiments 13 to 17, wherein the first circuit is further configured to generate a beat note radius signal, the beat note radius signal being an indicia of an amplitude of the beat note electrical signal.

[0080] Embodiment 19 includes the RFOG of any one of Embodiments 13 to 18, wherein the light source includes two lasers.

[0081] Embodiment 20 includes the RFOG of any one of Embodiments 13 to 19, wherein the first circuit includes a beat note corrector and a processing system.

[0082] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that the present invention be limited only by the claims and the equivalents thereof.

Claims

1. A method for generating a beat note correction signal configured to reduce an offset error in a resonant fiber gyroscope, the resonant fiber gyroscope comprising a light source optically coupled to an optical resonator coil, wherein the light source is configured to generate a clockwise light signal configured to propagate clockwise around the optical resonator coil and a counterclockwise light signal configured to propagate counterclockwise around the optical resonator coil, the method comprising: receiving a resonant frequency of the optical resonator coil in each of a clockwise direction and a counterclockwise direction; receiving a beat note electrical signal derived from a portion of the clockwise light signal and a portion of the counterclockwise light signal; determining a difference frequency, the difference frequency being a function of a resonant frequency of the optical resonator coil in the clockwise direction and a resonant frequency of the optical resonator coil in the counterclockwise direction; generating (a) a sine signal as a function of the difference frequency and (b) a cosine signal as a function of the difference frequency; digitizing the beat note electrical signal; generating a first low pass filtered signal by mixing the sinusoidal signal with a digitized beat note electrical signal and removing a first set of mixing products except a first mixing product closest to zero Hertz; generating a second low pass filtered signal by mixing the cosine signal with the digitized beat note electrical signal and removing a second set of mixing products except for a second mixing product closest to zero Hertz; generating a phase signal using a rectangular to polar converter using the first low pass filtered signal and the second low pass filtered signal; generating the beat note correction signal by differentiating the phase signal with respect to time; as well as Using the difference frequency and the beat note correction signal, a rotation rate about a winding axis of the optical resonator coil is determined.

2. A resonant fiber gyroscope (RFOG), comprising: a light source comprising at least one laser and configured to generate a clockwise (CW) light signal and a counterclockwise (CCW) light signal; a beat note detector configured to receive a first portion of the CW optical signal and a first portion of the CCW optical signal, and to generate a beat note electrical signal using the first portion of the CW optical signal and the first portion of the CCW optical signal; Optical resonator coil; a resonator closure optical device configured to couple a second portion of the CW optical signal and a second portion of the CCW optical signal into the optical resonator coil and to transmit a third portion of the CW optical signal and a third portion of the CCW optical signal from the optical resonator coil; a CW rate detector configured to convert a portion of the third portion of the CW optical signal into a CW electrical signal; a CCW rate detector configured to convert a portion of the third portion of the CCW optical signal into a CCW electrical signal; a CW resonant tracking loop circuit configured to determine a resonant frequency of the optical resonator coil in a CW direction; a CCW resonant tracking loop circuit configured to determine a resonant frequency of the optical resonator coil in a CCW direction; and A first circuit, wherein the first circuit is configured to: receiving the resonant frequency of the optical resonator coil in each of a clockwise direction and a counterclockwise direction; receiving the beat note electrical signal; determining a difference frequency, the difference frequency being a function of the resonant frequency of the optical resonator coil in the clockwise direction and the resonant frequency of the optical resonator coil in the counterclockwise direction; generating (a) a sine signal as a function of the difference frequency and (b) a cosine signal as a function of the difference frequency; digitizing the beat note electrical signal; generating a first low pass filtered signal by mixing the sinusoidal signal with a digitized beat note electrical signal and removing a first set of mixing products except a first mixing product closest to zero Hertz; generating a second low pass filtered signal by mixing the cosine signal with a digitized beat note electrical signal and removing a second set of mixing products except for a second mixing product closest to zero Hertz; generating a phase signal using a rectangular to polar converter using the first low pass filtered signal and the second low pass filtered signal; as well as A beat note correction signal is generated by differentiating the phase signal with respect to time.

3. The RFOG of claim 2, wherein the first circuit is further configured to determine a rotation rate about a winding axis of the optical resonator coil using the beat note correction signal and the difference frequency.

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