Optical circuit for compensating for optical lead fluctuations in RFOG

Abstract

The present disclosure relates to a system and method for compensating for optical lead fluctuations in an RFOG, comprising: a source for generating a first optical signal and a second optical signal; an optical resonator; a first lead and a second lead connected to the source and the resonator, where the source couples the first optical signal and the second optical signal into the first lead and the second lead, and the first lead and the second lead couple the first optical signal and the second optical signal into the optical resonator for propagation in opposite directions within the optical resonator; a tapping device coupled to the optical resonator, the optical resonator coupling portions of the first optical signal and the second optical signal out of the optical resonator; a combiner that combines the first optical signal and the portion of the second optical signal to produce a combined output; and a photodetector that generates a beat signal from the combined output.

Description

[0001] Statement Regarding Federally Sponsored Research or Development

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

[0003] Gyroscopes have been used to measure the rate of rotation or change in angular velocity about an axis of rotation. A basic conventional fiber optic gyroscope (FOG) includes a light source, a beam generation device, and a fiber optic coil that is coupled to the beam generation device, where the coil surrounds a region. The beam generation device generates an optical signal for coupling into the coil, where at least two optical signals propagate in opposite directions. For example, one optical signal propagates along the core of the fiber in a clockwise (CW) direction, while the other optical signal propagates along the core of the fiber in a counterclockwise (CCW) direction. Many FOGs utilize glass-based optical fibers that guide light along a solid glass core of the fiber. When the RFOG is undergoing rotation, the two counter-propagating (e.g., CW and CCW) optical signals experience different path lengths as they propagate around the rotational closed optical path within the coil. The difference in the two path lengths is proportional to the rate of rotation perpendicular to the enclosed area.

[0004] In a typical resonant fiber optic gyroscope (RFOG), the counter-propagating light beams are highly coherent and circulate through multiple turns of the fiber optic coil. Typically, a device (such as a fiber optic coupler) redirects a portion of the light that has passed through the coil back into the coil for recirculation through the fiber optic coil. Typically, the light passes through the fiber optic coil multiple times. The beam generation device modulates and / or shifts the frequency of each counter-propagating light beam such that the resonant frequency of the resonant coil can be observed. The resonant frequency for each of the CW path and the CCW path through the coil is based on the condition of constructive interference, where light waves that have passed through the coil a different number of times are in phase or constructively add at any point within the coil. Due to this constructive interference condition, a light wave having a particular wavelength can be said to be "resonant" when the round-trip resonator optical path length is equal to an integer multiple of the wavelength. Rotation about the axis of the coil creates different optical path lengths for clockwise and counterclockwise propagation, and thus creates a shift between the corresponding resonant frequencies of the resonator. The frequency difference measured by tuning the CW beam frequency and the CCW beam frequency to match the resonant frequency shift of the closed optical path caused by rotation can be used to determine the rate of rotation. Summary of the Invention

[0005] This document describes systems and methods for an optical circuit for compensating for optical lead fluctuations in an RFOG. In certain embodiments, a resonant fiber optic gyroscope (RFOG) includes at least two optical signal sources configured to generate a first optical signal and a second optical signal; and an optical resonator. The RFOG includes a first lead connected to the light source and the optical resonator, wherein the light source couples the first optical signal into the first lead, and the first lead couples the first optical signal into the optical resonator for propagation in the optical resonator in a first direction. The RFOG also includes a second lead connected to the light source and the optical resonator, wherein the light source couples the second optical signal into the second lead, and the second lead couples the second optical signal into the optical resonator for propagation in the optical resonator in a second direction, wherein the second direction is opposite to the first direction. Additionally, the RFOG includes a tap device coupled to the optical resonator, wherein the tap device couples out portions of the first optical signal propagating in the optical resonator and portions of the second optical signal propagating in the optical resonator. Further, the RFOG includes a combiner coupled to the tap device, wherein the combiner combines the portions of the first optical signal and the portions of the second optical signal to produce a combined output. Moreover, the RFOG includes one or more photodetectors coupled to receive the combined output from the combiner, wherein the one or more photodetectors generate a beat signal from the combined output. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate only some embodiments associated with the scope of the appended claims. Thus, the described and illustrated embodiments should not be considered as limiting the scope. The drawings and the specification describe exemplary embodiments and their features with additional specificity and detail, wherein:

[0007] FIG. 1 is a block diagram illustrating an exemplary system for compensating for optical lead fluctuations in an RFOG;

[0008] FIGS. 2A and 2B are graphs illustrating the effect of sideband heterodyne modulation in an exemplary system for compensating for optical lead fluctuations in an RFOG;

[0009] Figure 3 is a block diagram illustrating a system implementing an optical circuit for compensating for optical lead fluctuations in an RFOG in accordance with aspects of the present disclosure;

[0010] Figure 4 is a schematic diagram of an RFOG implementing an optical circuit for compensating for optical lead fluctuations in accordance with aspects of the present disclosure;

[0011] Figure 5FIG. 0 is a schematic illustration of an RFOG implementing an optical circuit for compensating optical lead fluctuations in accordance with aspects of the present disclosure;

[0012] Figure 6 FIG. 1 is an illustration of components of an optical assembly of an optical circuit implementing compensation for optical lead fluctuations in an RFOG in accordance with aspects of the present disclosure; and

[0013] Figure 7 FIG. 2 is a flow diagram of a method for compensating lead fluctuations in accordance with aspects of the present disclosure.

[0014] By convention, the various features shown in the drawings are not drawn to scale, but the drawings are shown to emphasize features relevant to the exemplary embodiments. DETAILED DESCRIPTION

[0015] The following detailed description refers to the accompanying drawings that form a part of this specification. The drawings illustrate specific illustrative embodiments. However, it should be understood that other embodiments may be used and logical, mechanical, and electrical changes may be made.

[0016] Systems and methods for compensating optical lead fluctuations using an optical circuit in an RFOG are described herein. In particular, the optical circuit picks up an optical signal inside the resonator or immediately after the optical signal exits the resonator coil. For example, the optical circuit picks up the CW optical signal and the CCW optical signal of the resonator. The system may use the picked-up optical signals to measure input lead fluctuations and compensate the gyroscope output to provide accurate rotation rate information.

[0017] In some RFOGs, temperature fluctuations in the optical fiber connecting the light source to the optical resonator may cause slight changes in the optical frequency of the input laser before the input laser reaches the resonator and propagates in the clockwise and counterclockwise directions along the resonator. Since these light waves are used to detect the accurate resonance frequency of the resonator, the RFOG may incorrectly determine that the resonance frequency of the resonator has changed when the resonance has not changed. If the input lead fluctuations are not exactly the same for the CW input wave and the CCW input wave, the incorrect changes observed in the CW resonance frequency and the CCW resonance frequency will be different, which translates into an incorrect observation of the rotation rate change ("bias error"). These changes in the optical frequency of the input light occur due to changes in the refractive index and physical path length of the optical fiber, resulting in fluctuations in their optical path length or a change in the phase of the input light. However, it is difficult to match the optical fibers of the two signal paths with high precision. Thus, temperature fluctuations may cause a difference in the frequency of the signals near the light source and on the input path to the optical resonator. The RFOG can detect frequency differences as small as one-tenth of a microhertz, but even small temperature changes can cause undesirable bias errors.

[0018] Since the frequency measurements of the clockwise optical signal and the counterclockwise optical signal are made by devices close to the clockwise light source and the counterclockwise light source, additional bias errors may occur in the electrical circuit in the feedback loop in the RFOG due to time-varying electrical phenomena such as offset voltages and noises that drift over time. The time-varying electrical phenomena also cause a frequency difference between the clockwise optical signal frequency and the counterclockwise optical signal frequency close to the light source and the clockwise optical signal frequency and the counterclockwise optical signal frequency close to the optical resonator of the RFOG. Therefore, there is a need to reduce the above-mentioned bias errors.

[0019] One way to reduce the bias error is to use a low-coupling-ratio tap device in the optical resonator coil path. The tap device taps out a small amount of light in both the CW direction and the CCW direction, combines those light waves, and detects the combined light wave to form a beat signal. The beat signal is processed and used to compensate for the bias error in the gyroscope output caused by input lead fluctuations. In a typical system, a portion of the CW optical signal and a portion of the CCW optical signal are tapped out before reaching the resonator, but as close as technically possible to the resonator input to detect and compensate for input lead fluctuations. In some embodiments, the tap device can be a fiber optic coupler or a small beam deflector in the resonator path on the input optical component. However, tapping out light before the optical signal enters the resonator has significant limitations in an RFOG in which the input light is modulated to enable signal processing. For example, when this method is used in an RFOG that uses sideband heterodyne detection (SHD) modulation (described later), the specific implementation of SHD may cause severe signal fading of the resulting beat signal. The systems and methods described herein allow for reducing the errors that occur during beat compensation for lead fluctuations.

[0020] In the embodiments described herein, the tap device taps out the optical signal from the resonator after the optical signal has propagated through a so-called "transmission port" within the resonator. When the optical signal is transmitted out of the resonator, two coupling devices can be used, one on each output arm of the resonator. The coupling devices extract portions of the optical signals propagating in the CW direction and the CCW direction, combine the extracted optical signals, and form a beat signal. The propagation of light within the resonator can also filter out unwanted modulation sidebands to ensure that the beat signal is not affected by large variations caused by the modulation implemented in some RFOG embodiments.

[0021] FIG. 1 is a block diagram illustrating an exemplary system 100 for an RFOG. The system 100 includes a clockwise (CW) light source 102 and a counterclockwise (CCW) light source 104. The CW light source 102 includes circuitry that provides an optical signal for coupling into the resonator 110 for propagation in the CW direction within the resonator 110. Similarly, the CCW light source 104 includes circuitry that provides an optical signal for coupling into the resonator 110 for propagation in the CCW direction within the resonator 110. As used herein, a light source can be a laser (such as a diode laser, a vertical cavity surface emitting laser, etc.), a light emitting diode, an amplifier for amplifying light from another light source, or even a combination of the foregoing devices. Although the terms CW and CCW describe the direction of propagation within the resonator 110, these terms are relative and mean that the optical signals provided by the CW light source 102 and the CCW light source 104 propagate in opposite directions within the resonator 110. As used herein, the resonator 110 (also referred to as a resonator coil) can refer to a multi-turn or multi-loop optical fiber having a device that recirculates light within the resonator coil.

[0022] As described herein, the optical signals provided by the CW light source 102 and the CCW light source 104 are coupled into the resonator 110 through the CW lead 106 and the CCW lead 108, respectively. In particular, the CW lead 106 couples the light generated by the CW light source 102 into the resonator 110, and the CCW lead 108 couples the light generated by the CCW light source 104 into the resonator 110.

[0023] The propagation of light through the CW lead 106 and the CCW lead 108 is sensitive to temperature variations. In particular, temperature variations can cause fluctuations in the frequency of the optical signals propagating towards and through the resonator 110. For example, temperature variations can change the refractive index and the physical path length of the CW lead 106 and the CCW lead 108, and these changes in physical properties cause frequency fluctuations in the optical signals. The temperature gradient increases the difficulty of matching the lead length fluctuations in the optical signal path lengths of the CW lead 106 and the CCW lead 108 with high precision. This difficulty in matching the signal path lengths means that temperature fluctuations can cause a difference in the frequencies of the signals coupled into the resonator 110 from the CW light source 102 and the CCW light source 104. When the RFOG detects rotation based on a frequency difference as small as one tenth of a microhertz, even small temperature variations can result in an undesired bias error.

[0024] In addition, frequency measurements of the CW optical signal and the CCW optical signal are performed near the CW light source 102 and the CCW light source 104. The CW light source 102 and the CCW light source 104 use an electrical circuit to generate the corresponding optical signals, control the frequency of the optical signals, and perform a feedback loop involving time-varying electrical phenomena. The time-varying electrical phenomena also cause a difference between the frequencies of the CW optical signal and the CCW optical signal near the light source and the frequencies of the CW optical signal and the CCW optical signal near the resonator 110, which also results in an undesired bias error.

[0025] To address the bias error, when the signals provided to the resonator 110 are provided to the resonator 110 from the leads, they may interfere with each other. Then, the resulting beat signal can be used to correct the bias caused by the difference in the resonance frequencies of the CW optical signal and the CCW optical signal due to time-varying fluctuations. To extract the CW optical signal and the CCW optical signal, the system 100 may include a tap-off device 112 that taps off a portion of the light provided for propagation in both the CW direction and the CCW direction within the resonator 110 before the light propagates within the resonator 110. As described herein, the tap-off device 112 can be any device that couples the light to the resonator 110 before the light provided through the CW lead 106 and the CCW lead 108 propagates within the resonator 110. The tap-off device 112 can be an optical coupler placed between the CW lead 106 and the CCW lead 108 and the resonator 110. Alternatively, the tap-off device 112 can be implemented as an optical component on an optical assembly, such as a silicon optical bench.

[0026] In certain embodiments, the system 100 includes a combiner 118. In particular, the first output optical path 114 and the second output optical path 116 from the tap-off device 112 are coupled to the combiner 118. The combiner 118 is an optical device that receives different optical signals through different input ports and combines the received signals for output through an output port. For example, the combiner 118 can receive the CCW optical signal from the first output optical path 114 and the CW optical signal from the second output optical path 116, and combine these signals for output to the combined optical path 120.

[0027] In additional embodiments, the system 100 includes a beat detector 122 that receives the combined signal from the combined optical path 120. The beat detector 122 detects the combined signal to form a beat signal. The beat signal represents the frequency interval between the CW optical signal and the CCW optical signal. The beat signal can be processed to measure the input lead fluctuations and compensate for the bias caused by the lead fluctuations. Thus, the RFOG can provide more accurate rotation rate information.

[0028] In certain specific implementations, the RFOG can implement sideband heterodyne detection (SHD). When the RFOG implements SHD, a common frequency modulation modulates the CW optical signal and the CCW optical signal. Then, an additional sideband heterodyne detection (SHD) modulation frequency modulates the CW optical signal and the CCW optical signal, and the CW SHD modulation frequency is slightly different from the CCW sideband modulation frequency. The sideband modulation frequency can be relatively high compared to the common modulation frequency. The application of SHD frequency modulation to the CW optical signal and the CCW optical signal results in harmonic components of the optical signal centered at the respective carrier frequencies of the CW optical signal and the CCW optical signal, where the sideband modulation frequency generates separate harmonic components for the CW direction and the CCW direction. Generally, the sideband modulation frequencies can each be set to a frequency close to half of the free spectral range (FSR) of the resonator 110 or an odd multiple of half of the FSR. As used herein, the FSR of the resonator 110 is the frequency difference between adjacent resonance frequencies of the resonator 110. Therefore, every other harmonic of the sideband modulation is at or near the resonance frequency of the resonator 110.

[0029] In an RFOG that implements SHD, where the sidebands are generated at half of the FSR by modulating at that frequency or near that frequency, or at an odd multiple of half of the FSR by modulating at that frequency or near that frequency, the RFOG can adjust the carrier frequencies of the CW optical signal and the CCW optical signal to be at the resonance peak of the resonator 110 or at the middle between the resonance peaks of the resonator 110. When the carrier frequency is set to the middle frequency between the resonance peaks of the resonator 110, the odd harmonics of the CW optical signal and the CCW optical signal will be at the resonance peaks of the resonator 110. Therefore, the first-order harmonics of the CW optical signal and the CCW optical signal pass through the resonator 110 and interfere with each other. Then, the CW optical signal and the CCW optical signal are detected, and the detected signals are demodulated at a modulation frequency that is twice the sideband modulation frequency. At this time, the CW demodulated signal and the CCW demodulated signal are then demodulated at a common low-frequency modulation frequency. Therefore, the resonance frequencies in the CW direction and the CCW direction are determined because the difference between them is due to rotation (in the absence of bias error). However, the bias error is caused by input lead fluctuations.

[0030] However, systems implementing a tap device (such as system 100) are prone to errors where the tap device taps an optical signal from a lead before the signal resonates within a resonator. In particular, in systems implementing an SHD, tapping an optical signal before resonator 110 is prone to errors. When light is tapped before resonator 110 after being modulated by the SHD, the optical signal will include multiple spectral components that subsequently beat against each other. Moreover, when different components beat against each other, the differences between the different components may cause the amplitude of the combined effect of the different beats to effectively be zero. When the amplitude of the combined effect of different beats is zero, the RFOG may not be able to obtain phase information for compensating lead fluctuations. FIGS. 2A and 2B illustrate errors that may occur when attempting to compensate for lead fluctuations in a typical RFOG (such as the RFOG in system 100).

[0031] FIG. 2A is a diagram illustrating the frequency components of a carrier and a first-order sideband provided to a resonator through a lead. As shown, FIG. 2A illustrates a CW frequency component 200 and a CCW frequency component 202. The CW frequency component 200 includes a CW carrier component 201, an upper CW sideband 207, and a lower CW sideband 205. Similarly, the CCW frequency component 202 includes a CCW carrier component 203, an upper CCW sideband 211, and a lower CCW sideband 209. Although only one set of sidebands is illustrated for each direction, there are also higher-order sidebands, but their amplitudes decrease proportionally with the order of the sideband.

[0032] When tap device 112 couples an optical signal out of the lead before the optical signal propagates within the resonator, all components of the signal are affected by lead fluctuations. Since lead fluctuations can change the frequencies of the two signals, ideally (without modulation, no sidebands present), the fluctuations in the carrier and the beats of each beam represent the true frequency fluctuations observed by the resonator. In such cases, if the distance between the optical pickups of the beat signals is a negligibly small distance from the input to the resonator, there is no error. The ideal beat signal contains rotation rate information without lead length error. In non-ideal beat schemes with modulation and a large number of sidebands on the light, the frequency sidebands of the CW input and the CCW input will beat against each other at a frequency close to the beat frequency between the carriers. These beats between the sidebands combined with the beats between the carriers can cause the total signal on the beat detector to periodically become zero amplitude. This results in a noisy and practically unusable beat signal because the phase and frequency are indeterminable. For example, CW sideband 205 and CCW sideband 209 will beat against each other at a frequency close to the frequency at which CW carrier 201 and CCW carrier 203 will beat. Similarly, CW sideband 207 and CCW sideband 211 will beat against each other at a frequency close to the frequency at which CW carrier 201 and CCW carrier 203 will beat. The combination of all these beat signals produces a time-varying signal as depicted in FIG. 2B.

[0033] FIG. 2B illustrates a graph of the amplitude of a beat formed from a combination of signals provided by leads. The detected signal includes beats between various sideband pairs and carrier pairs. That is, the detected signal includes a beat between sidebands 205 and 209 (lower sideband beat) plus a beat between carriers 201 and 203 (carrier beat) plus a beat between sidebands 207 and 211 (upper sideband beat). As shown, graph 240 illustrates a first periodic portion 241 and a second periodic portion 243. The first period portion 241 corresponds to a time when the carrier beat and the sideband beat are additive in amplitude. The second period portion 243 corresponds to a time when the carrier beat and the sideband beat are less additive, thus reducing the magnitude of the signal. As described, when transitioning between 243 and 241, sometimes the total signal magnitude is zero. Thus, when the carrier beat and the sideband beat cancel each other out, the amplitude of the combined effect periodically passes through zero. When the amplitude of the combined effect is zero, the RFOG cannot obtain beat phase and frequency information without bias errors due to lead fluctuations.

[0034] Figure 3 is a block diagram illustrating a system 300 for compensating for optical lead fluctuations using an optical circuit within an RFOG, where a tap device taps light out of the resonator 310 after the light has propagated within the resonator. Similar to system 100, system 300 includes a clockwise (CW) light source 302 and a counterclockwise (CCW) light source 304, which function substantially similar to CW light source 102 and CCW light source 104. CW light source 302 includes a circuit that provides an optical signal for coupling into resonator 310 for propagation in the CW direction within resonator 310. Similarly, CCW light source 304 includes a circuit that provides an optical signal for coupling into resonator 310 for propagation in the CCW direction within resonator 310. As used herein, resonator 310 (also referred to as a resonator coil, although resonator 310 includes a recirculation device in the optical component to recirculate light within the coil and, in addition to the coil, provides means for introducing / extracting light into / out of the coil) may refer to a multi-turn or multi-loop optical fiber having a device for recirculating the light within the resonator coil.

[0035] As described herein, the optical signals provided by the CW light source 302 and the CCW light source 304 are coupled into the resonator 310 through the CW lead 306 and the CCW lead 308, respectively. In particular, the CW lead 306 couples the light generated by the CW light source 302 into the resonator 310, and the CCW lead 308 couples the light generated by the CCW light source 304 into the resonator 310. In some embodiments, the CW lead 306 and the CCW lead 308 are coupled to an optical component for coupling the CW optical signal and the CCW optical signal into the resonator 310, or are coupled to the resonator 310 using different optical components (such as optical couplers, free space optics, etc.).

[0036] As mentioned above, the propagation of light through the CW lead 306 and the CCW lead 308 is sensitive to temperature changes. In particular, temperature changes may cause fluctuations in the frequency of the optical signal propagating towards and through the resonator 310. When the RFOG detects rotation based on a frequency difference as small as one tenth of a microhertz, even small temperature changes can cause an undesired bias error. In addition, frequency measurements of the CW optical signal and the CCW optical signal are made near the CW light source 302 and the CCW light source 304. The CW light source 302 and the CCW light source 304 use electrical circuits to generate the corresponding optical signals, control the frequency of the optical signals, and perform a feedback loop involving time-varying electrical phenomena. The time-varying electrical phenomena also cause a difference between the frequencies of the CW optical signal and the CCW optical signal near the light source and the frequencies of the CW optical signal and the CCW optical signal near the resonator 310, which also results in an undesired bias error.

[0037] In certain specific implementations, where the RFOG implements SHD, a common frequency modulation modulates the CW optical signal and the CCW optical signal, and an additional SHD modulation frequency modulates the CW optical signal and the CCW optical signal, where the CW SHD modulation frequency is slightly different from the CCW sideband modulation frequency. Additionally, the RFOG can adjust the carrier frequencies of the CW optical signal and the CCW optical signal to be at the resonance peak of the resonator 310 or at an intermediate between the resonance peaks of the resonator 310. When the carrier frequency is set to an intermediate frequency between the resonance peaks of the resonator 310, the odd harmonics of the CW optical signal and the CCW optical signal will be at the resonance peaks of the resonator 310. Thus, the first-order harmonics of the CW optical signal and the CCW optical signal pass through the resonator 310 and interfere with each other. Then, the CW optical signal and the CCW optical signal are detected, and the detected signals are demodulated at a modulation frequency that is twice the sideband modulation frequency. At this time, the CW demodulated signal and the CCW demodulated signal are then demodulated at the common low-frequency modulation frequency. Thus, the resonance frequencies in the CW direction and the CCW direction are determined because the difference between them is due to rotation (in the case of no bias error). However, the bias error is caused by input lead fluctuations.

[0038] As described above, to address the bias error, the signals exiting the resonator 310 are provided to the tap device 112 and the combiner 318 to interfere with each other as they are provided to the resonator 310 from the leads and then propagate within the resonator 310. Then, the resulting beat signal can be used to correct the bias or give a signal without bias error, which is caused by the difference in the resonance frequencies of the CW optical signal and the CCW optical signal due to time-varying fluctuations. However, when the first-order harmonics are in resonance, the variation between different harmonics of the CW optical signal and the CCW optical signal may cause the combination of the upper and lower adjacent harmonics, such that the amplitude of the associated beat from the interference of the CW optical signal and the CCW optical signal may experience signal fading due to the interference signal. Since the combination of the first-order harmonics may cause the associated signal to have a zero amplitude, the presence of the first-order harmonics may cause an error in the resulting beat signal, which has a negative impact on the compensation for lead fluctuations. Moreover, if the optical signal is tapped out before propagating within the resonator 310, the presence of multiple harmonics and carrier frequencies may also cause signal fading (i.e., the signal amplitude periodically becomes smaller or zero over time) in a way that combines due to the interference of multiple components. Since the lead fluctuations bias the rotation information provided by the RFOG, the presence of the first-order harmonics has a negative impact on the ability of the RFOG to compensate for lead fluctuations and the resulting bias error.

[0039] In some embodiments, system 300 suppresses the first harmonic before interfering the CW optical signal and the CCW optical signal. When the first harmonic is suppressed, the first harmonic cannot be combined in a way that drives the associated beat signal between the CW optical signal or the CCW optical signal to a small amplitude or zero amplitude. Therefore, system 300 can use the generated beat signal to more accurately compensate for the bias caused by lead fluctuations. In particular, system 300 can suppress the first harmonic by centering the carrier frequency at the resonance peak of resonator 310 and then causing the CW optical signal and the CCW optical signal to beat against each other after propagating within resonator 310. System 300 suppresses the first harmonic because centering the carrier frequency at the resonance peak causes odd harmonics to deviate from resonance. Therefore, the odd harmonics will destructively interfere within resonator 310. When the carrier and the even harmonics resonate within resonator 310, the amplitude of the carrier will be substantially greater than the amplitude of the second harmonic, such that the combination of the second harmonic will be less than the amplitude of the component at the carrier frequency. In this case, the beat signal does not reach zero amplitude. Therefore, the beat signal between the CW optical signal and the CCW optical signal can be detected to compensate for lead fluctuations.

[0040] In some embodiments, in order to extract the CW optical signal and the CCW optical signal after propagating through resonator 310, system 300 may include a tap device 112 that taps a portion of the light propagating in both the CW direction and the CCW direction within resonator 310. As described herein, tap device 112 can be any series of devices that couple the light propagating along the resonance path in resonator 310 to an optical path external to resonator 310. In some specific implementations, tap device 112 can be an optical coupler mounted on the resonance path of resonator 310. In other specific implementations, tap device 112 can be implemented as an optical component on an optical assembly, such as a silicon optical bench. In some specific implementations, tap device 112 couples a small portion of the light propagating in the resonance path of resonator 310 into a first output optical path 314 and a second output optical path 316. For example, tap device 112 can couple 30% of the light propagating in the resonance path of resonator 310 into the first output optical path 314 and the second output optical path 316.

[0041] When the tap device 112 is an optical coupler, the optical coupler is an optical device that transfers optical signals between optical fibers. In particular, two optical paths pass through the optical coupler, where the optical coupler brings the two optical paths into a coupling region. Within the coupling region, light couples between different optical paths. The portion of the light that couples out of a particular optical path depends on the optical characteristics of the different optical paths within the coupling region. Regarding the placement of the optical coupler within the resonant path of the resonator 310, the light propagating within the resonator 310 passes through the optical path in the optical coupler that is part of the resonant optical path of the resonator. For example, the resonant optical path may have a first port and a second port on opposite sides of the optical coupler. A CW optical signal is received through the first port, and a portion of the CW optical signal is output through the second port. Similarly, a CCW optical signal is received through the second port, and a portion of the CCW optical signal is output through the first port. Additionally, based on the optical characteristics of the optical paths within the coupling region, a portion of the light in the resonant optical path is coupled into a coupling path, where the coupling path has a first coupled port and a second coupled port. For example, a portion of the CW optical signal received through the first port is coupled to be output through the second coupled port, and a portion of the CCW optical signal received through the second port is coupled to be output through the first coupled port. The first coupled port may be coupled to the first output optical path 314, and the second coupled port may be coupled to the second output optical path 316.

[0042] When the tap device 112 is implemented in an optical assembly, the optical assembly can be a platform for mounting and aligning optical components on a silicon substrate, such as a silicon optical bench. The optical components can include various waveguides, mirrors, lenses, beam splitters, polarizers, wave plates, prisms, and other optical components. The optical assembly can receive light through one or more input ports, perform various modifications on the light, and provide the light to one or more output ports. When the optical assembly incorporates the tap device 112, the optical assembly that introduces the CW optical signal and the CCW optical signal into the resonator 310 can also include optical components in the resonant path of the resonator 310 that couple a portion of the light in the resonant path out of the resonant path of the resonator 310. In particular, the optical assembly can couple the light propagating in the CCW direction to a first output port that is coupled to the first output optical path 314, and couple the light propagating in the CW direction to a second output port that is coupled to the second output optical path 316.

[0043] In some embodiments, system 300 includes a combiner 318. In particular, a first output optical path 314 and a second output optical path 316 from the tap device 112 are coupled to the combiner 318. The combiner 318 is an optical device that receives different optical signals through different input ports and combines the received signals for output through an output port. For example, the combiner 318 may receive a CCW optical signal from the first output optical path 314 and a CW optical signal from the second output optical path 316, and combine these signals for output to the combined optical path 320.

[0044] In additional embodiments, system 300 includes a beat detector 322 that receives the combined signal from the combined optical path 320. The beat detector 322 detects the combined signal to form a beat signal. The beat signal includes a dominant tone at a frequency within the frequency interval between the carriers of the CW optical signal and the CCW optical signal. The beat signal can be processed to measure input lead fluctuations and compensate for the bias caused by lead fluctuations, or can be configured to give rate information without bias due to lead length fluctuations. Thus, the RFOG can provide more accurate rotation rate information.

[0045] Figure 4 is a block diagram of a system incorporating an optical circuit for compensating for optical lead fluctuations in an RFOG 400. In particular, the optical circuit in the RFOG 400 can use a tapped optical coupler 427 in the resonant path of the resonator 423 to couple light out of the resonator for compensating for fluctuations in leads 406 and 408. The light coupled out of the resonator 423 by the tapped optical coupler 427 can be used to detect and measure input lead fluctuations for compensating for the bias in the gyroscope output caused by fluctuations in the leads. Additionally, as shown, the RFOG 400 also implements SHD when modulating and demodulating the CW optical signal and the CCW optical signal.

[0046] Figure 4The exemplary RFOG 400 illustrated in the example includes a main light source 435, a CW light source 413, a CCW light source 449, an integrated photonics circuit (IPC) 469, an optical bench (or optical bench circuit) 443, a CW optical phase-locked loop (CW OPLL or CW OPLL circuit) 411, a CCW optical phase-locked loop (CCW OPLL or CCW OPLL circuit) 447, a Pound-Drever-Hall (PDH) servo (or PDH servo circuit) 463, a fiber optic coil (resonator coil) 423, which becomes a resonator when combined with some components on the optical bench 443 (here, the element 423 is referred to as the "resonator", which technically includes the resonator coil and some elements on the optical bench 443), a CCW resonance tracking servo (or CCW resonance tracking servo circuit) 401, and a CW resonance tracking servo (or CW resonance tracking servo circuit) 455. The illustrated embodiment is described for teaching purposes, and embodiments of the present invention may be implemented using a system having a configuration different from the Figure 4 configuration illustrated in the example. The signals emitted by the CW light source 413, the CCW light source 449, and the main light source 435 may be referred to as CW optical signals, CCW optical signals, and main optical signals, respectively.

[0047] For example, the photonics circuit of the integrated photonics circuit 469 and the optical components 443 are each integrated; however, in other embodiments, the photonics circuit of the integrated photonics circuit 469 and / or the optical components 443 may be implemented as separate components. Thus, the integrated photonics circuit 469 may include discrete photonics circuits. The discrete photonics circuits may be integrated on a common substrate, where the common substrate includes semiconductors and / or insulators. In some specific implementations, the substrate may be a doped or undoped semiconductor. The integrated photonics circuit 469 integrates various components on the substrate. The various integrated components may include the main light source 435, the CW light source 413, the CCW light source 449, the CW OPLL 411, the CCW OPLL 447, modulators 437 and 439, optical couplers, a first photodetector 471, and a second photodetector 473 on the substrate. In some specific implementations, some or all of the components on 469 may be implemented as a multi-frequency laser source (MFLS). An MFLS is a device capable of emitting laser light at multiple frequencies. Alternatively, the components listed as part of the integrated photonics circuit 469 may be separate components coupled to each other by waveguides, optical fibers, wires, or other communication media.

[0048] The integrated photonic circuit 469 is configured to condition optical signals generated by a master light source 435, a CW light source 413, and a CCW light source 449, respectively, for introduction into the resonator 423. The master light source 435 generates a master optical signal that is used to stabilize the frequency of the light provided by the CW light source 413 and the CCW light source 449, and specifically provides very low frequency noise between these sources. Additionally, the light provided by the master light source 435 can be phase modulated by a common phase modulator 439 and be heterodyned with the CW optical signal and the CCW optical signal. For example, the master optical signal can be modulated by the common phase modulator 439 to provide modulation for the CW optical signal and the CCW optical signal such that the resonance frequency of the resonator 423 can be determined. The modulated master optical signal can be coupled with the CW optical signal generated by the CW light source 413 into a photodetector 471 such that the optical phase-locked loop 411 will apply the common modulation to the CW optical signal. Similarly, the modulated master optical signal can then be coupled with the CCW optical signal generated by the CCW light source 449 into a photodetector 473 such that the optical phase-locked loop 411 will apply the common modulation to the CW optical signal.

[0049] In some specific implementations, the heterodyne signals generated by the first photodetector 471 and the second photodetector 473 are respectively coupled into mixers 409 and 445, where these signals are heterodyned with the signals generated by the respective CW resonance tracking servo 455 and CCW resonance tracking servo 401. Then, the mixers 409 and 445 provide the heterodyned signals to the CW OPLL 411 and the CCW OPLL 447. When the CW resonance tracking servo 455 and the CCW resonance tracking servo 401 track the CW resonance and the CCW resonance of the resonator 423, the signals provided by the CW OPLL 411 and the CCW OPLL 447 will drive the carrier frequencies of the light generated by the CW light source 413 and the CCW light source 449 to the resonance of the resonator 423. In particular, the signals provided by the CW OPLL 411 and the CCW OPLL 447 drive the light generated by the CW light source 413 and the CCW light source 449 at the resonance frequency of the resonator 423.

[0050] In some embodiments, the CW optical signal generated by the CW light source 413 is coupled into the sideband heterodyne modulation phase modulator 415, which modulates the CW optical signal at a frequency that is essentially a multiple of half of the free spectral range of the resonator 423. Similarly, the CCW optical signal generated by the CCW light source 449 is coupled into the sideband heterodyne modulation phase modulator 451, which also modulates the CCW optical signal at a frequency that is essentially a multiple of half of the free spectral range of the resonator 423. Thus, the sideband heterodyne modulation phase modulator 415 provides a CW optical signal at the harmonic sidebands of the optical signal generated by the CW light source 413, where the harmonic sidebands are at the modulation frequency applied by the sideband heterodyne modulation phase modulator 415. Similarly, the sideband heterodyne modulation phase modulator 451 provides a CCW optical signal at the harmonic sidebands of the optical signal generated by the CCW light source 449, where the harmonic sidebands are at the modulation frequency applied by the phase modulator 451.

[0051] In some specific implementations, the CW optical signal provided by the sideband heterodyne modulation phase modulator 415 is provided to the coupler 417, which provides the CW optical signal to the circulator 419. Additionally, the main optical signal provided by the main light source 435 may also be modulated by the phase modulator 437, which modulates the main optical signal at the same frequency referred to as the PDH modulation frequency, which is used to determine the resonant frequency of the resonator for PDH loop operation. The modulated main optical signal is also provided to the coupler 417. Thus, the coupler 417 couples the CW optical signal from the sideband heterodyne modulation phase modulator 415 and the modulated main optical signal from the phase modulator 437 onto the same optical path, herein referred to as the CW optical path. The CCW optical signal provided by the sideband heterodyne modulation phase modulator 451 is coupled from the integrated photonics circuit 469 into the CCW optical path, which couples the CCW optical signal to the circulator 441.

[0052] In some embodiments, the circulator 419 receives the CW optical signal and the main optical signal and couples the CW optical signal and the main optical signal into the lead 406. The lead 406 provides the CW optical signal and the main optical signal to the optical component 443. Similarly, the circulator 441 receives the CCW optical signal and couples the CCW optical signal into the lead 408. The lead 408 provides the CCW optical signal to the optical component 443.

[0053] In some embodiments, the structure of the optical assembly 443 can include any component or device that supports the mounting and coupling of optical components. Additionally, the optical assembly 443 can include a substrate material that serves as a base onto which optical components can be placed, applied, or coupled. Such optical components can include lenses (e.g., ball lenses, concave lenses, convex lenses, compound lenses, gradient index (GRIN) lenses, etc.), beam splitters, combiners, polarizers, optical circulators, or other optical devices. However, in other exemplary embodiments, when extracting an optical signal from the resonator 423 or within the resonator path itself, the number of optical components should be reduced to prevent any unnecessary power attenuation. Additionally, the optical assembly 443 can optionally include an optical bench of a suitable size for use within the RFOG 400. In some cases, the optical assembly 443 can be an optical bench constructed from a silicon or glass substrate, also known as a silicon optical bench.

[0054] In additional embodiments, the optical assembly 443 is coupled to leads 406 and 408. The optical components of the optical assembly 443 receive a CW optical signal and a primary optical signal from lead 406 and couple the CW optical signal and the primary optical signal into the resonator 423 for propagation in the CW direction around the resonator 423. Similarly, the optical components of the optical assembly 443 receive a CCW optical signal from lead 408 and couple the CCW optical signal into the resonator 423 for propagation in the CCW direction around the resonator 423. Additionally, the optical assembly 443 can include optical components that couple an optical signal out of the resonator 423 for performing resonance tracking. For example, the optical assembly 443 can couple light propagating in the CW direction out of the resonator 423 and into lead 408 toward the circulator 441. Moreover, the optical assembly 443 can couple light propagating in the CCW direction out of the resonator 423 and into lead 406 toward the circulator 419. Additionally, the optical assembly 443 can couple light propagating in the CW direction out of the resonator 423 for detection by the detector 421 at the reflection port in the case where the resonator is used for a Pound-Drever-Hall (PDH) feedback loop to lock the CW optical frequency to the resonator. Alternatively, although not shown in Figure 4 the architecture can be configured such that the optical assembly 443 couples light from the CCW direction via the CCW reflection port to form the basis of a PDH feedback loop. The optical assembly 443 is also configured to provide maximum optical coupling of optical power from one end of the fiber coil within the resonator 423 to the other end of the coil within the resonator 423. Thus, the optical assembly 443 couples light into and out of the resonator 423 while also completing the resonance loop of the resonator 423.

[0055] In some embodiments, the circulator 419 couples the CCW optical signal into the CCW resonant tracking loop with the CCW resonant tracking servo 401. For example, the circulator 419 receives the CCW optical signal from the lead 406 and provides the CCW optical signal to the CCW detector 407. Then, the CCW detector 407 converts the optical signal into an electrical signal for demodulation. Similarly, the circulator 441 couples the CW optical signal into the CW resonant tracking loop with the CW resonant tracking servo 455. For example, the circulator 441 receives the CW optical signal from the lead 408 and provides the CW optical signal to the CW detector 461.

[0056] In some embodiments, when the RFOG 400 implements sideband heterodyne detection, the CW resonant tracking loop and the CCW resonant tracking loop perform double demodulation. For example, within the CCW resonant tracking loop, the electrical signal generated by the CCW detector 407 is demodulated by the demodulator 405 at twice the sideband heterodyne frequency, which is equal to twice the modulation frequency applied to the CCW optical signal by the sideband heterodyne modulation phase modulator 451. After demodulation by the demodulator 405, the signal is further demodulated by the demodulator 403 at the common modulation frequency, which is associated with the frequency applied by the common phase modulator 439. Then, the CCW resonant tracking servo 401 uses the doubly demodulated signal to identify the adjustment to the frequency of the CCW optical signal to track the resonance of the resonator 423 in the CCW direction. Similarly, within the CW resonant tracking loop, the electrical signal generated by the CW detector 461 is demodulated by the demodulator 459 at twice the sideband heterodyne frequency, which is equal to twice the modulation frequency applied to the CW optical signal by the sideband heterodyne modulation phase modulator 415. After demodulation by the demodulator 459, the signal is further demodulated by the demodulator 457 at the common modulation frequency, which is associated with the frequency applied by the common phase modulator 439. Then, the CW resonant tracking servo 455 uses the doubly demodulated signal to identify the adjustment to the frequency of the CW optical signal to track the resonance of the resonator 423 in the CW direction.

[0057] In additional embodiments, the electrical signal from the detector 421 is provided to the demodulator 467, which demodulates the electrical signal at the Pound-Drever-Hall (PDH) modulation frequency. Then, the demodulated signal is offset by the PDH loop offset by the summer 465. Then, the signal is provided to the PDH loop circuit 463. The PDH loop circuit 463 is configured to lock the optical signal of the master light source 435 to the same frequency as the resonant frequency of the resonator 423 or to a frequency intermediate between the resonant peaks of the resonator 423.

[0058] In some embodiments, the RFOG 400 includes a tap optical coupler 427 that is placed within the resonator 423 to couple a portion of the light propagating within the resonator 423 out of the resonator 423. The function of the tap optical coupler 427 can be similar to the tap device 112 described above with respect to Figure 3 Specifically, the resonator 423 can circulate an optical signal propagating in the CW direction into the tap optical coupler 427. The tap optical coupler 427 can couple a small portion of the optical signal circulating in the CW direction into the CW tap line 431. Also, the resonator 423 can circulate an optical signal propagating in the CCW direction into the tap optical coupler 427. The tap optical coupler 427 can couple a small portion of the optical signal circulating in the CCW direction into the CCW tap line 425. The functions of the CW tap line 431 and the CCW tap line 425 can be similar to Figure 3 the first output optical path 314 and the second output optical path 316 in

[0059] In some embodiments, the RFOG 400 can include a combiner 429. The function of the combiner 429 is similar to the combiner 318 described above with respect to Figure 3 Specifically, the combiner 429 is coupled to both the CCW tap line 425 and the CW tap line 431. The combiner 429 receives optical signals from the CCW tap line 425 and the CW tap line 431, combines the optical signals, and outputs the combined optical signal through an output port for reception by a detector 433. The function of the detector 433 can be similar to the beat detector 322 described in Figure 3 Specifically, the detector 433 detects the combined optical signal to form a beat signal. The beat signal is a frequency that represents the frequency interval between the CW optical signal and the CCW optical signal. The beat signal can be processed to measure the input lead fluctuations of the leads 406 and 408 and compensate for the bias caused by the lead fluctuations in the output of the RFOG, or to provide a signal without lead fluctuation errors to provide accurate rotation rate information.

[0060] In some embodiments, a beat signal may be provided to a processor that processes the beat signal to correct for fluctuations in leads 406 and 408 and in the leads connecting integrated photonic circuit 469 and circulators 419 and 441. The processor may be a computing device (such as a general-purpose processor, a multi-core processor, multiple processors, an application-specific circuit, etc.). The functions performed by the processor may be implemented using software, firmware, hardware, or any suitable combination thereof. The processor and other computing devices may be supplemented or incorporated by a specially designed application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA). The processor and other computing devices may also include software programs, firmware, or other computer-readable instructions or operate in conjunction with them to perform the various processing tasks, computations, and control functions used in the present methods and systems.

[0061] The present method may be implemented by computer-executable instructions (such as program modules or components) executed by a processor or other computing device. Generally, program modules include routines, programs, objects, data components, data structures, algorithms, etc. that perform specific tasks or implement specific abstract data types.

[0062] The memory may be any suitable computer-readable storage medium, which includes, for example, non-volatile memory devices, including semiconductor memory devices such as random access memory (RAM), read-only memory (ROM), electrically erasable programmable ROM (EEPROM), or flash memory devices; magnetic disks such as internal hard disks or removable disks; optical storage devices such as compact discs (CDs), digital versatile discs (DVDs), Blu-ray discs, or other media that can carry or store the desired program code as computer-executable instructions or data structures.

[0063] Figure 5 is a block diagram of a system incorporating an optical circuit for compensating for optical lead fluctuations in RFOG 500. In particular, the optical circuit in RFOG 500 may use optical components on optical assembly 543 to couple light out of the resonator for compensating for fluctuations in leads 506 and 508. The light coupled out of the transmission port of resonator 523 by optical assembly 543 may be used to detect and measure input lead fluctuations for compensating the gyroscope output based on the measured fluctuations in the leads. Moreover, components with similar numbers in RFOG 400 and 500 perform similar functions. For example, coupler 517 and coupler 417 may perform similar functions.

[0064] In some embodiments, the RFOG 500 may couple a CW optical signal into the circulator 519 for coupling into the lead 506. Moreover, the RFOG 500 may couple a CCW optical signal into the circulator 519. The leads 506 and 508 couple the CW optical signal and the CCW optical signal onto the optical component 543 for coupling into the resonator 523. The CW optical signal and the CCW optical signal propagate around the resonator 523, and when the CW optical signal and the CCW optical signal pass through the optical component 543, the transmission ports on the optical component 543 couple portions of the CW optical signal and the CCW optical signal out of the resonant path of the resonator 523. For example, the optical components on the optical component 543 may direct light out of the transmission ports of the resonator 523, and its function is similar to the tap device 112 described above in Figure 3 . The optical components on the optical component 543 may couple light back onto the leads 506 and 508.

[0065] In some embodiments, the RFOG 500 includes lead couplers 575 and 577. The lead couplers 575 and 577 are optical couplers located on the leads 506 and 508, respectively. When the lead couplers 575 and 577 receive optical signals from the circulators 519 and 541, the lead couplers 575 and 577 couple the optical signals into the optical component 543. However, when the lead coupler 575 and the lead coupler 577 receive optical signals from the transmission ports on the optical component 543, the lead couplers 575 and 577 couple the optical signals onto the tap lines 525 and 531. For example, the CW optical signal may be coupled by the optical coupler 577 onto the CW tap line 531. Additionally, the CCW optical signal may be coupled by the optical coupler 575 onto the CCW tap line 525. Both the CW tap line 531 and the CCW tap line 525 couple the CW optical signal and the CCW optical signal into the combiner 529. The function of the combiner 529 is similar to the combiners 318 described above with respect to Figure 3 and the combiner 429 in Figure 4 . The combiner 529 receives optical signals from the CCW tap line 525 and the CW tap line 531, combines the optical signals, and outputs the combined optical signal through an output port for reception by the detector 533. The function of the combiner 5 is similar to the beat detector 322 described in Figure 3 and the detector 533 in Figure 5 . In particular, the detector 533 detects the combined optical signal to form a beat signal, which contains a frequency in the frequency interval between the CW optical signal and the CCW optical signal.

[0066] Figure 6FIG. 0 is a diagram of an optical component on the optical assembly 600, where the optical component can direct the combined optical signal from the transmission port to the photodetector. For example, the optical assembly 600 can be similar to the optical assembly 543 described above. As shown, the optical assembly 600 can be coupled to the end of the resonator. For example, the optical assembly 600 can receive the light propagating in the resonator from the first end 601 and the second end 603. The optical signal from the first end 601 can pass through the first lens 617, and the optical signal from the first end 603 can pass through the second lens 625. The first lens 617 and the second lens 625 can collimate the light received from the resonator coil onto the optical component on the optical assembly 600.

[0067] In some embodiments, the first lens 617 collimates the received optical signal for passing through the first rotating half-wave plate 619, and the second lens 625 collimates the received optical signal for passing through the second rotating half-wave plate 623. The first rotating half-wave plate 619 and the second rotating half-wave plate 623 are mounted on the optical assembly 600. The first rotating half-wave plate 619 can rotate the polarization orientation of the received optical signal for passing through the first polarizer 633, and the second rotating half-wave plate 623 can rotate the polarization orientation of the received optical signal for passing through the second polarizer 641. The first polarizer 633 and the second polarizer 641 can filter the received optical signal such that the passed optical signal is in a specific polarization.

[0068] In some embodiments, the optical signal passes through the first polarizer 633 and is incident on the first beam splitter 613, and the optical signal passes through the second polarizer 641 and is incident on the second beam splitter 615. The first beam splitter 613 and the second beam splitter 615 can reflect a portion of the received optical signal and allow a portion of the received optical signal to pass through. In some specific implementations, each of the first beam splitter 613 and the second beam splitter 615 can reflect a small portion of the optical signal. For example, the first beam splitter 613 and the second beam splitter 615 can reflect 3.5% of the incident optical signal. When the first beam splitter 613 and the second beam splitter 615 reflect the incident optical signal, the first beam splitter 613 and the second beam splitter 615 direct the incident optical signal out of the resonant path of the resonator as the first tapped optical signal 627 and the second tapped optical signal 629, respectively. The light passing through the first beam splitter 613 and the second beam splitter 615 passes through and enters one of the first end 601 and the second end 603. For example, the optical signal incident on the second beam splitter 615 passes through the second beam splitter 615 and enters the first end 601. The transmitted portion of the optical signal incident on the first beam splitter 613 passes through the first beam splitter 613 and enters the second end 603.

[0069] In another embodiment, the first tapped optical signal 627 passes through the first additional beam splitter 635, and the second tapped optical signal 629 passes through the second additional beam splitter 639. The beam splitters 635 and 639 are provided to bring the optical signals from the inputs 602 and 604 to the resonator. The optical signal from the CW input 602 travels through the spherical lens 618 to the left side of the polarization element 633 to reach the beam splitter 635, then to the beam splitter 613, and is reflected into the resonator coil end 601 for propagation in the CW direction. Similarly, the optical signal from the CCW input 604 travels through the spherical lens 624 to the right side of the polarization element 641 to reach the beam splitter 639, then to the beam splitter 615, and is reflected into the resonator coil end 603 for propagation in the CCW direction in the resonator. When a portion of the CW optical signal propagating in the resonator is tapped out by reflection from the beam splitter 615, i.e., the beam splitter 615 outputs an optical signal as "transmission from the resonator". The tapped optical signal is directed to the beam splitter 639. A portion of the power transmitted through the beam splitter 639 is then directed to the reflector 657 (via transmission through the beam splitter 649), and is directed to the beam splitter 659 to form optical signal paths 661 and 663 for beat detection. Similarly, when a portion of the CCW optical signal propagating in the resonator is tapped out by reflection from the beam splitter 613, i.e., the beam splitter 613 outputs an optical signal as "transmission from the resonator". The tapped optical signal is directed to the beam splitter 635. A portion of the power transmitted through the beam splitter 635 is then directed to the reflector 653 (via transmission through the beam splitter 647), and then to the reflector 655 and then to the beam splitter 659 to form optical signal paths 661 and 663 for beat detection. The optical signal paths 661 and 663 each have a collinear output derived from portions of the optical signals traveling in the CW and CCW directions. Additionally, it should be noted that the first additional beam splitter 635 and the second additional beam splitter 639 can separately split the first tapped optical signal 627 and the second tapped optical signal 629, and direct portions of the first tapped optical signal 627 and the second tapped optical signal 629 back to where the optical signals enter the assembly 606, i.e., into the ports on the left side of the polarizer 633 and into the ports on the right side of the polarizer 641. Each of those portions of the output lights 627 and 629 reflected by the beam splitters 635 and 639 respectively is not used for the beat signal because they are respectively directed back Figure 5Circulators 519 and 541, in which they are used for the CCW output signal and the CW output signal, as previously discussed. In some specific embodiments, the first additional beam splitter 635 and the second additional beam splitter 639 may reflect most of the first tapped optical signal 627 and the second tapped optical signal 629 back to the circulator as the CCW resonance tracking signal and the CW resonance tracking signal.

[0070] In another embodiment, portions of the first tapped optical signal 627 and the second tapped optical signal 629 that pass through the first additional beam splitter 635 and the second additional beam splitter 639 then impinge on the transmission port beam splitters 647 and 649. The transmission port splitters 647 and 649 direct half (or some other portion) of the first tapped optical signal 627 and the second tapped optical signal 629 to the transmission port outputs 645 and 651. The transmission port outputs 645 and 651 may direct the optical signal out of the optical assembly 600 to components above the optical assembly 600 (such as a photodetector, such as Figure 4 detector 421 in). For example, the transmission port outputs 645 and 651 can be diffraction gratings or micromirrors that receive the optical signal and direct the optical signal out of the optical assembly 600. The transmission outputs 645 and 651 respectively indicate the power circulating in the resonator in the CCW direction and the CW direction, and can be used for input power stabilization or other purposes, but not for beat detection.

[0071] In certain embodiments, portions of the first tapped optical signal 627 and the second tapped optical signal 629 that pass through the transmission port splitters 647 and 649 are directed to the combination beam splitter 659 by a series of mirrors 653, 655, and 657. For example, a portion of the first tapped optical signal 627 that passes through the transmission port splitter 647 is directed by mirrors 653 and 655 to one side of the combination beam splitter 659. Also, a portion of the second tapped optical signal 629 that passes through the transmission port splitter 649 is directed by mirror 657 to the other side of the combination beam splitter 659. The combination beam splitter 659 can be a 50% beam splitter that allows half of the incident light to pass through the combination beam splitter 659 and reflects the other half of the incident light. When the first tapped optical signal 627 and the second tapped optical signal 629 impinge on opposite sides of the combination beam splitter 659, combined portions of the first tapped optical signal 627 and the second tapped optical signal 629 are directed from each surface of the combination beam splitter 659. For example, the first combined signal 661 (including a nominal half of the first tapped optical signal 627 and the second tapped optical signal 629 incident on the combination beam splitter 659) is directed towards the mirror 665. The second combined signal 663 (including the other nominal half of the first tapped optical signal 627 and the second tapped optical signal 629 incident on the combination beam splitter 659) is directed towards the mirror 667.

[0072] In another embodiment, mirrors 665 and 667 direct the combined optical signals toward beat note outputs 669 and 671. For example, mirror 665 reflects the first combined signal 661 toward the first beat note output 669. Also, mirror 667 reflects the second combined signal 663 toward the second beat note output 671. The first beat note optical components 669 and 671 can be components similar to the transmission port outputs 645 and 651. Thus, the first beat note output 669 can direct the first combined signal 661 to a beat note detector near the optical assembly 600. The second beat note output 671 can also direct the second combined signal 663 to a beat note detector near the optical assembly 600. As described above, the optical assembly 600 can extract portions of the propagating light within the resonator for combination and detection.

[0073] Figure 7 is a flowchart of a method 700 for compensating for optical lead fluctuations in an RFOG. As shown, the method 700 proceeds at 701, where a first optical signal is generated. Additionally, the method 700 continues at 703, where a second optical signal is generated. Also, the method 700 continues at 705, where the first optical signal is coupled into the resonator using a first lead to propagate in a first direction. Additionally, the method 700 continues at 707, where the second optical signal is coupled into the resonator using a second lead to propagate in a second direction opposite to the first direction. Further, the method 700 continues at 709, where portions of the first optical signal and the second optical signal are tapped out of the resonator. The method 700 also continues at 711, where a beat note is detected from the combination of the portions of the first optical signal and the second optical signal.

[0074] Exemplary implementation

[0075] Embodiment 1 includes a resonant fiber optic gyroscope (RFOG), the RFOG including: at least two optical signal sources configured to generate a first optical signal and a second optical signal; an optical resonator; a first lead connected to the light source and the optical resonator, wherein the light source couples the first optical signal into the first lead, and the first lead couples the first optical signal into the optical resonator for propagation in the optical resonator in a first direction; a second lead connected to the light source and the optical resonator, wherein the light source couples the second optical signal into the second lead, and the second lead couples the second optical signal into the optical resonator for propagation in the optical resonator in a second direction, wherein the second direction is opposite to the first direction; a tap device coupled to the optical resonator, wherein the tap device couples out a portion of the first optical signal propagating in the optical resonator and a portion of the second optical signal propagating in the optical resonator from the optical resonator; a combiner coupled to the tap device, wherein the combiner combines the portion of the first optical signal and the portion of the second optical signal to produce a combined output; and one or more photodetectors coupled to receive the combined output from the combiner, wherein the one or more photodetectors generate a beat signal from the combined output.

[0076] Embodiment 2 includes the RFOG according to Embodiment 1, wherein the tap device includes an optical coupler placed in the resonant path of the optical resonator, wherein the optical coupler couples the portion of the first optical signal into a first tap wire and the portion of the second optical signal into a second tap wire, wherein the first tap wire and the second tap wire are coupled to the combiner.

[0077] Embodiment 3 includes the RFOG according to any one of Embodiments 1 to 2, wherein the first lead and the second lead are coupled to an optical component that guides the first optical signal and the second optical signal into the optical resonator and guides the first optical signal and the second optical signal out of the optical resonator.

[0078] Embodiment 4 includes the RFOG according to Embodiment 3, wherein the optical component includes an optical part that guides the portion of the first optical signal into a first tap wire and the portion of the second optical signal into a second tap wire, wherein the first tap wire and the second tap wire are coupled to the combiner.

[0079] Example 5 includes the RFOG according to any one of Examples 3 to 4, wherein the optical assembly includes optical components, the optical components include the tapping device and the combiner, and wherein the optical assembly provides the combined output to the one or more photodetectors.

[0080] Example 6 includes the RFOG according to any one of Examples 1 to 5, the RFOG further including a beat processor configured to receive the beat signal from the one or more photodetectors and compensate for the measurement of fluctuations in the first lead and the second lead.

[0081] Example 7 includes the RFOG according to any one of Examples 1 to 6, wherein the first optical signal and the second optical signal are each modulated using common modulation and sideband heterodyne modulation.

[0082] Example 8 includes the RFOG according to Example 7, wherein the common modulation modulates the first optical signal and the second optical signal to be within the free spectral range of the optical resonator.

[0083] Example 9 includes a method, the method including: generating a first optical signal; generating a second optical signal; coupling the first optical signal into a resonator using a first lead to propagate in a first direction; coupling the second optical signal into the resonator using a second lead to propagate in a second direction opposite to the first direction; tapping off portions of the first optical signal and the second optical signal from the resonator; and detecting a beat from the combination of the portions of the first optical signal and the portions of the second optical signal.

[0084] Example 10 includes the method according to Example 9, wherein coupling the first optical signal into the resonator and coupling the second optical signal into the resonator includes: coupling the first optical signal into an optical assembly through the first lead; coupling the second optical signal into the optical assembly through the second lead; and introducing the first optical signal and the second optical signal from the optical assembly into the resonator.

[0085] Example 11 includes the method according to Example 10, wherein optical components on the optical assembly direct the portions of the first optical signal and the portions of the second optical signal to a combiner through a first tap wire and a second tap wire.

[0086] Embodiment 12 includes the method according to any one of Embodiments 10 to 11, wherein the tapping is performed by an optical component on the optical assembly, and the optical component combines the portion of the first optical signal and the portion of the second optical signal to output a combination of the portion of the first optical signal and the portion of the second optical signal.

[0087] Embodiment 13 includes the method according to any one of Embodiments 9 to 12, wherein the tapping is performed by an optical coupler located in the resonant path of the resonator, and the optical coupler couples the portion of the first optical signal into a first tap line and couples the portion of the second optical signal into a second tap line.

[0088] Embodiment 14 includes the method according to any one of Embodiments 9 to 13, and the method further includes any one of the following: compensating for the measurement of fluctuations in the first lead and the second lead based on the detected beat; and providing a measurement of the rotation rate without errors caused by fluctuations in the first lead and the second lead based on the detected beat.

[0089] Embodiment 15 includes the method according to any one of Embodiments 9 to 14, and the method further includes modulating the first optical signal and the second optical signal using common modulation and sideband heterodyne modulation.

[0090] Embodiment 16 includes the method according to Embodiment 15, wherein the sideband heterodyne modulation is performed at a frequency associated with the free spectral range of the resonator.

[0091] Embodiment 17 includes a system that includes: at least two optical signal sources configured to generate CW optical signals and CCW optical signals; an optical resonator coil; a first lead connected to the light source and the optical resonator coil, wherein the light source couples the CW optical signal into the first lead, and the first lead couples the CW optical signal into the optical resonator coil to propagate in the CW direction within the optical resonator coil; a second lead connected to the light source and the optical resonator coil, wherein the light source couples the CCW optical signal into the second lead, and the second lead couples the CCW optical signal into the optical resonator coil to propagate in the CCW direction within the optical resonator coil; a tap-off device coupled to the optical resonator coil, wherein the tap-off device couples out a portion of the CW optical signal propagating in the optical resonator coil and a portion of the CCW optical signal propagating in the optical resonator coil; a combiner coupled to the tap-off device, wherein the combiner combines the portion of the CW optical signal and the portion of the CCW optical signal to produce a combined output; one or more photodetectors coupled to receive the combined output from the combiner, wherein the one or more photodetectors generate a beat signal from the combined output; and a beat processor configured to receive the beat signal from the one or more photodetectors and compensate for measurements of fluctuations in the first lead and the second lead.

[0092] Embodiment 18 includes the system according to Embodiment 17, wherein the tap-off device includes an optical coupler placed in the resonant path of the optical resonator coil, wherein the optical coupler couples the portion of the CW optical signal into a first tap-off wire and couples the portion of the CCW optical signal into a second tap-off wire, and wherein the first tap-off wire and the second tap-off wire are coupled to the combiner.

[0093] Embodiment 19 includes the system according to any one of Embodiments 17 to 18, wherein the first lead and the second lead are coupled to an optical component that guides the first optical signal and the second optical signal into the optical resonator coil and guides the first optical signal and the second optical signal out of the optical resonator coil, wherein the optical component includes an optical part that guides the portion of the CW optical signal into a first tap-off wire and guides the portion of the CCW optical signal into a second tap-off wire, and wherein the first tap-off wire and the second tap-off wire are coupled to the combiner.

[0094] Embodiment 20 includes the system according to any one of Embodiments 17 to 19, wherein the first lead and the second lead are coupled to an optical component that directs the CW optical signal and the CCW optical signal into the resonator and directs the CW optical signal and the CCW optical signal out of the resonator, wherein the optical component includes optical elements that include the tap device and the combiner, and wherein the optical component provides the combined output to the one or more photodetectors.

[0095] Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art will recognize that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. It is, therefore, evident that the invention is limited only by the claims and their equivalents.

Claims

1. A resonant fiber gyroscope (RFOG), the RFOG comprising: at least two optical signal sources (302, 304), the at least two optical signal sources configured to generate a first optical signal and a second optical signal; An optical resonator (310); a first lead (306) connected to the light source (302) and the optical resonator (310), wherein the light source (302) couples the first optical signal into the first lead (306), and the first lead (306) couples the first optical signal into the optical resonator (310) for propagation in a first direction within the optical resonator (310); a second lead (308) connected to the light source (304) and the optical resonator (310), wherein the light source (304) couples the second optical signal into the second lead (308), and the second lead (308) couples the second optical signal into the optical resonator (310) for propagation in a second direction within the optical resonator (310), wherein the second direction is opposite to the first direction; a tapping device (112) coupled to the optical resonator (310), wherein the tapping device (112) couples a portion of the first optical signal propagating in the optical resonator (310) and a portion of the second optical signal propagating in the optical resonator (310) out of the optical resonator (310); a combiner (318) coupled to the tap device (112), wherein the combiner (318) combines the portion of the first optical signal and the portion of the second optical signal to produce a combined output; and One or more photodetectors (322) coupled to receive the combined output from the combiner (318), wherein the one or more photodetectors (322) generate a beat signal from the combined output.

2. The RFOG of claim 1 , wherein the tapping device ( 112 ) comprises an optical coupler ( 427 ) placed in a resonant path of the optical resonator ( 310 ), wherein the optical coupler ( 427 ) couples the portion of the first optical signal into a first tap line ( 431 ) and couples the portion of the second optical signal into a second tap line ( 425 ), wherein the first tap line ( 431 ) and the second tap line ( 425 ) are coupled to the combiner ( 429 ).

3. A method comprising: generating a first optical signal; generating a second optical signal; coupling the first optical signal into a resonator (310) using a first lead (306) to propagate in a first direction; coupling the second optical signal into the resonator (310) using a second lead (308) to propagate in a second direction opposite to the first direction; tapping a portion of the first optical signal and a portion of the second optical signal from the resonator (310); as well as A beat note is detected from a combination of the portion of the first optical signal and the portion of the second optical signal.

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