Patent Description:
Resonator fiber optic gyroscopes are a promising next generation gyroscope technology that meets various needs in many navigation and inertial stabilization markets. The resonator fiber optic gyroscope (RFOG) is a navigation gyroscope that typically uses two optical signals, where one optical signal propagates around a resonator in a clockwise (CW) direction and the other optical signal propagates in a counter-clockwise (CCW) direction. Two different lasers are employed to eliminate the bias error due to back reflection. The RFOG is susceptible to errors, which can vary with a temperature of the core of an optical fiber of the resonator. One such error occurs in the bias, which represents an offset error in measured rotation rate.

The optical Kerr effect is a fundamental bias error source in an RFOG and often creates bias noise higher than navigational grade requirements. The Kerr effect is caused by the different phase shifts between the self-phase modulation (SPM) and cross-phase modulation (XPM) phenomenon. The SPM due to the copropagating CW beam generates less phase shift than the XPM due to the counter-propagating CCW beam in the RFOG. Therefore, if the CW and CCW beams encounter differential power fluctuations, the RFOG will have a non-reciprocal phase shift and cause bias error.

Prior approaches for Kerr effect reduction focuses on differential power stabilization. A common method is to reduce the Kerr bias error by stabilizing the CW and CCW beam power. But the asymmetry Kerr bias error is sensitive to even common intensity fluctuation. Moreover, the intensity stabilization loop does not reduce the Kerr bias error due to common loss change in the cavity.

<CIT> relates to methods and systems for an intensity stabilized resonator fiber optic gyroscope.

Modulation methods and systems for non-reciprocal Kerr reduction in a resonator fiber optic gyroscope (RFOG) according to the independent claims <NUM>, <NUM>, and <NUM> are disclosed.

In the following detailed description, embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense.

Modulation methods and systems for reducing or eliminating Kerr effect bias instability in resonator fiber optic gyroscopes are described herein.

In developing the present methods and systems, the discovery was made that common loss variations change the beam intensity distributions non-reciprocally and generate Kerr bias error in a resonator fiber optic gyroscope (RFOG). In the present methods, the clockwise (CW) and counterclockwise (CCW) input power is balanced to make the intensity distributions the same. This eliminates or reduces the Kerr bias due to asymmetrical loss distribution. The present methods can reduce the Kerr bias error below navigation grade level.

The common loss and power change in a resonator cavity of the RFOG can also cause non-reciprocal phase shifts. The present approach provides a method to minimize the Kerr effect due to common loss and power variations.

In one approach, a method for reducing or eliminating Kerr effect bias sensitivity in a resonator fiber optic gyroscope (RFOG) uses a common intensity modulation scheme. In this approach, a common intensity modulation is applied to the CW and CCW beams. If the power distribution between the CW and CCW beams is not balanced, the intensity modulation will create a bias modulation signal. This bias modulation signal can be used as an indicator for non-reciprocal intensity distribution. The Kerr bias due to asymmetrical loss distribution is minimized by balancing the CW and CCW input power.

In another approach, a method for reducing or eliminating Kerr effect bias sensitivity uses a common loss modulation scheme. In this approach, a polarization state in the optical resonator of the RFOG is adjusted to modify a cavity loss in the optical resonator and create a loss modulation. In one technique, a piezoelectric transducer (PZT) coupled to the optical resonator is used to apply pressure to a fiber of the optical resonator to adjust the loss due to the polarization change. The polarization state of the resonator changes and modifies the cavity loss.

In another technique using a common loss modulation scheme, two polarizers in an RFOG are used to force the resonator to only allow one polarization in the cavity. The fiber resonance typically allows two polarizations for propagation in the cavity. By mis-aligning one of the polarizers away from its eigen polarization state, additional cavity loss is created. By modulating the misalignment, a loss modulation is generated. For example, an angle of one polarizer can be adjusted with respect to an angle of the other polarizer to modulate a cavity loss in the resonator.

Further details related to the present methods and systems are described as follows and with reference to the drawings.

<FIG> is flow diagram of a method <NUM> for reducing or eliminating a Kerr effect bias sensitivity in an RFOG by using common intensity modulation, according to one exemplary implementation. The method <NUM> is applicable to an RFOG that generally comprises an optical resonator in optical communication with a first light source and a second light source, such as laser devices. The first light source emits a first optical signal at a first power level in a clockwise (CW) direction, and the second light source emits a second optical signal at a second power level in a counterclockwise (CCW) direction.

The method <NUM> comprises adding a common intensity modulation to the first optical signal emitted from the first light source at the first power level, to adjust the first power level and produce an intensity modulated CW signal (block <NUM>); and adding the common intensity modulation to the second optical signal emitted from the second light source at the second power level, to adjust the second power level and produce an intensity modulated CCW signal (block <NUM>). The method <NUM> further includes introducing the intensity modulated CW signal into the optical resonator such that the intensity modulated CW signal propagates in a CW direction in the optical resonator (block <NUM>); introducing the intensity modulated CCW signal into the optical resonator such that the intensity modulated CCW signal propagates in a CCW direction in the optical resonator (block <NUM>); and detecting rotation rate signals that are output from the optical resonator at a common intensity modulation frequency (block <NUM>).

As shown in <FIG>, method <NUM> adjusts a power level ratio between the first and second power levels of the first and second optical signals based on the common intensity modulation frequency, to determine a modified power level ratio value where a Kerr effect bias instability due to an asymmetrical loss distribution is reduced or eliminated (block <NUM>). The method <NUM> maintains the modified power level ratio value in the RFOG such that the Kerr effect bias instability is reduced or eliminated (block <NUM>).

<FIG> is a schematic illustration of a RFOG <NUM> according to one embodiment, which is configured to reduce or eliminate the Kerr effect bias sensitivity, such as by using common intensity modulation as shown in the method of <FIG>. The RFOG <NUM> includes an optical resonator <NUM>, such as fiber optic ring resonator, which is in optical communication with a first laser device <NUM>, and a second laser device <NUM>, through an optical path. In one embodiment, optical resonator <NUM> can be a fiber optic coil wound around a core and about an axis around which rotation is sensed. In another embodiment, optical resonator <NUM> can be formed on an optical chip using waveguides.

A first intensity modulator <NUM> is optically coupled between an output of first laser device <NUM> and a first optical circulator <NUM>, which is optically coupled to optical resonator <NUM> through a first bus waveguide <NUM>. A second intensity modulator <NUM> is optically coupled between an output of second laser device <NUM> and a second optical circulator <NUM>, which is optically coupled to optical resonator <NUM> through a second bus waveguide <NUM>. The first bus waveguide <NUM> is coupled to optical resonator <NUM> at a first coupling region <NUM> on a first side of optical resonator <NUM>. The second bus waveguide <NUM> is coupled to optical resonator <NUM> at a second coupling region <NUM> on a second side of optical resonator <NUM>.

As shown in <FIG>, a modulation unit <NUM> is operatively coupled to first and second intensity modulators <NUM>, <NUM>. The modulation unit <NUM> is configured to send a common intensity modulation signal to first and second intensity modulators <NUM>, <NUM>. Some of the optical power from the two beams from laser devices <NUM> and <NUM> is tapped from intensity modulators <NUM> and <NUM> and sent to a pair of respective detectors <NUM> and <NUM>, which measure the beam modulation amplitudes separately (modulation amplitude = modulated power/DC power). Then, the modulation amplitudes are compared between the two beams (block <NUM>). A feedback loop <NUM> is used to ensure that the two beams have the same modulation amplitude. The foregoing structure provides an example of a means to equalize the modulation amplitude between the two beams.

A first optical detector <NUM> is in optical communication with optical resonator <NUM> through first optical circulator <NUM>. A second optical detector <NUM> is in optical communication with optical resonator <NUM> through second optical circulator <NUM>. A rate calculation unit <NUM> is configured to receive electrical signals output from first and second optical detectors <NUM>, <NUM>, and calculate a rotation rate signal for RFOG <NUM>. A demodulation unit <NUM> is configured to receive the rate signal output from rate calculation unit <NUM>, and the common intensity modulation signal from modulation unit <NUM>.

A control loop <NUM> is operatively coupled between an output of demodulation unit <NUM> and an input of second intensity modulator <NUM>. In an alternative embodiment, control loop <NUM> can be operatively coupled between an output of demodulation unit <NUM> and an input of first intensity modulator <NUM>.

During operation of RFOG <NUM>, first laser device <NUM> emits a first optical signal that propagates in a clockwise (CW) direction to an input of first intensity modulator <NUM>. The first intensity modulator <NUM> varies the optical power of the first optical signal, based on the common intensity modulation signal from modulation unit <NUM>, to produce an intensity modulated CW signal. The intensity modulated CW signal is sent to optical resonator <NUM> through first optical circulator <NUM> and first bus waveguide <NUM>. The intensity modulated CW signal is coupled into optical resonator <NUM> at coupling region <NUM> and propagates in a CW direction in optical resonator <NUM>.

The second laser device <NUM> emits a second optical signal that propagates in a counter-clockwise (CCW) direction to an input of second intensity modulator <NUM>. The second intensity modulator <NUM> varies the optical power of the second optical signal, based on the common intensity modulation signal from modulation unit <NUM>, to produce an intensity modulated CCW signal. The intensity modulated CCW signal is sent to optical resonator <NUM> through second optical circulator <NUM> and second bus waveguide <NUM>. The intensity modulated CCW signal is coupled into optical resonator <NUM> at coupling region <NUM> and propagates in a CCW direction in optical resonator <NUM>.

The first optical detector <NUM> receives the intensity modulated CCW signal, which is coupled out of optical resonator <NUM> at coupling region <NUM> into bus waveguide <NUM> and through first optical circulator <NUM>. The second optical detector <NUM> receives the intensity modulated CW signal, which is coupled out of optical resonator <NUM> at coupling region <NUM> into bus waveguide <NUM> and through second optical circulator <NUM>. The first and second optical detectors <NUM>, <NUM> convert the received signals to electrical signals, which are sent to rate calculation unit <NUM> for further processing to determine the rotation rate. The demodulation unit <NUM> receives a rate signal output from rate calculation unit <NUM>, and the common intensity modulation signal from modulation unit <NUM>. The rate signal has a sine wave at the common intensity modulation frequency when the CW and CCW power is not balanced for unevenly distributed Kerr effects. The demodulation unit <NUM> uses the received rate signal to compute the amplitude of the sine wave at the common intensity modulation frequency.

The control loop <NUM> receives an output signal from demodulation unit <NUM>, which is used to adjust the power level ratio to balance the CW and CCW input power, to reduce or eliminate the sine wave in the rate signal. In one embodiment, control loop <NUM> balances the CW and CCW input power by adjusting second intensity modulator <NUM> to control a direct current (DC) laser power in the CCW direction. In an alternative embodiment, control loop <NUM> balances the CW and CCW input power by adjusting first intensity modulator <NUM> to control the DC laser power in the CW direction. The control loop <NUM> determines a modified power level ratio value where a Kerr effect bias instability due to an asymmetrical loss distribution in RFOG <NUM> is reduced or eliminated, and maintains the modified power level ratio value in RFOG <NUM>. The control loop <NUM> operates as a feedback loop to determine the modified power level ratio value where the bias instability is reduced or eliminated such as going to zero. This modified power level ratio value can then be locked in by control loop <NUM>.

<FIG> is flow diagram of a method <NUM> for reducing or eliminating a Kerr effect bias sensitivity in an RFOG by employing common loss modulation, according to an exemplary implementation. The method <NUM> is applicable to an RFOG that generally comprises an optical resonator in optical communication with a first light source and a second light source, such as laser devices. The first light source emits a first optical signal at a first power level in a CW direction, and the second light source emits a second optical signal at a second power level in a CCW direction.

The method <NUM> comprises adding a first intensity modulation to the first optical signal emitted from the first light source, to adjust the first power level (block <NUM>); and adding a second intensity modulation to the second optical signal emitted from the second light source, to adjust the second power level (block <NUM>). For example, a first intensity modulator or variable optical attenuator (VOA) can be used to adjust the first power level of the first optical signal and produce a CW signal; and a second intensity modulator or VOA can be used to adjust the second power level of the second optical signal and produce a CCW signal.

The method <NUM> further includes introducing the CW signal into the optical resonator such that the CW signal propagates in a CW direction in the optical resonator (block <NUM>); and introducing the CCW signal into the optical resonator such that the CCW signal propagates in a CCW direction in the optical resonator (block <NUM>). The method <NUM> generates a common loss modulation in the optical resonator by adjusting a polarization state in the optical resonator to modify a cavity loss in the optical resonator (block <NUM>); and detects modulated rate signals that are output from the optical resonator and a common loss modulation frequency (block <NUM>).

As shown in <FIG>, method <NUM> adjusts a power level ratio between the first and second power levels of the first and second optical signals based on the common loss modulation frequency, to determine a modified power level ratio value where a Kerr effect bias instability due to an asymmetrical loss distribution is reduced or eliminated (block <NUM>). The method <NUM> maintains the modified power level ratio value in the RFOG such that the Kerr effect bias instability is reduced or eliminated. (block <NUM>).

<FIG> is a schematic illustration of a RFOG <NUM> according to another embodiment, which is configured to reduce or eliminate the Kerr effect bias sensitivity, such as by using common loss modulation as shown in the method of <FIG>. The RFOG <NUM> includes an optical resonator <NUM>, such as a fiber optic ring resonator, which is in optical communication with a first laser device <NUM>, and a second laser device <NUM>. The optical resonator <NUM> also includes a piezoelectric transducer (PZT) <NUM>, which is configured to modulate the cavity loss of optical resonator <NUM> via polarization variations. In one embodiment, PZT <NUM> is attached to a piece of optical fiber of optical resonator <NUM>, such that PZT <NUM> is configured to sinusoidally press the optical fiber at a modulation frequency. Such a modulation varies the polarization state of the cavity and modulates the cavity loss.

A first intensity modulator <NUM> (or VOA) is optically coupled between an output of first laser device <NUM> and a first optical circulator <NUM>. A second intensity modulator <NUM> (or VOA) is optically coupled between an output of second laser device <NUM> and a second optical circulator <NUM>. The first optical circulator <NUM> is optically coupled to optical resonator <NUM> through a first bus waveguide <NUM>. The second optical circulator <NUM> is optically coupled to optical resonator <NUM> through a second bus waveguide <NUM>. The first bus waveguide <NUM> is optically coupled to optical resonator <NUM> at a first coupling region <NUM> on a first side of optical resonator <NUM>. The second bus waveguide <NUM> is optically coupled to optical resonator <NUM> at a second coupling region <NUM> on a second side of optical resonator <NUM>.

As illustrated in <FIG>, a modulation unit <NUM> is operatively coupled to PZT <NUM>. The modulation unit <NUM> is configured to send a common cavity loss modulation signal such as a sine wave signal to PZT <NUM>. A first optical detector <NUM> is in optical communication with optical resonator <NUM> through first optical circulator <NUM>. A second optical detector <NUM> is in optical communication with optical resonator <NUM> through second optical circulator <NUM>. A rate calculation unit <NUM> is configured to receive electrical signals output from first and second optical detectors <NUM>, <NUM>, and calculate a rotation rate signal for RFOG <NUM>. A demodulation unit <NUM> is configured to receive the rate signal output from rate calculation unit <NUM>, and the sine wave signal from modulation unit <NUM>.

During operation of RFOG <NUM>, first laser device <NUM> emits a first optical signal that propagates in a CW direction to first intensity modulator <NUM>, which outputs a CW signal that is sent to optical resonator <NUM> through first optical circulator <NUM> and first bus waveguide <NUM>. The CW signal is coupled into optical resonator <NUM> at coupling region <NUM> and propagates in a CW direction in optical resonator <NUM>.

The second laser device <NUM> emits a second optical signal that propagates in a CCW direction to second intensity modulator <NUM>, which outputs a CCW signal that is sent to optical resonator <NUM> through second optical circulator <NUM> and second bus waveguide <NUM>. The CCW signal is coupled into optical resonator <NUM> at coupling region <NUM> and propagates in a CCW direction in optical resonator <NUM>.

The modulation unit <NUM> sends the sine wave signal to PZT <NUM>, which applies pressure to the optical fiber of optical resonator <NUM> to adjust a cavity loss due to a polarization change. The cavity eigen polarization is defined by a polarizer in the gyroscope. Any polarization modes that do not align with the eigen mode experience high loss in the cavity. The pressure applied to the optical fiber via PZT <NUM> changes the fiber birefringence and polarization state of optical resonator <NUM>. The change in the polarization state modifies the cavity loss in optical resonator <NUM> to produce a common loss modulation, which generates a bias modulation at the same modulation frequency as the common loss modulation. The bias modulation can be minimized or eliminated by balancing the CW and CCW input power.

For example, first optical detector <NUM> receives the modulated CCW signal with the common loss modulation, which is coupled out of optical resonator <NUM> at coupling region <NUM> into bus waveguide <NUM> and through first optical circulator <NUM>. The second optical detector <NUM> receives the modulated CW signal with the common loss modulation, which is coupled out of optical resonator <NUM> at coupling region <NUM> into bus waveguide <NUM> and through second optical circulator <NUM>. The first and second optical detectors <NUM>, <NUM> convert the received signals to electrical signals, which are sent to rate calculation unit <NUM> for further processing to determine the rotation rate. The demodulation unit <NUM> receives a rate signal output from rate calculation unit <NUM>, and the common cavity loss modulation signal from modulation unit <NUM>. The rate signal has a sine wave at the common loss modulation frequency. The demodulation unit <NUM> uses the received signals to compute the sine wave at the common loss modulation frequency.

The control loop <NUM> receives an output signal from demodulation unit <NUM>, which is used to adjust the power level ratio to balance the CW and CCW input power, to reduce or eliminate the sine wave in the rate signal. In one embodiment, control loop <NUM> balances the CW and CCW input power by adjusting second intensity modulator <NUM> to control the DC laser power in the CCW direction. In an alternative embodiment, control loop <NUM> balances the CW and CCW input power by adjusting first intensity modulator <NUM> to control the DC laser power in the CW direction. The control loop <NUM> determines a modified power level ratio value where a Kerr effect bias instability due to an asymmetrical loss distribution in RFOG <NUM> is reduced or eliminated, and maintains the modified power level ratio value in RFOG <NUM>.

<FIG> is a schematic illustration of a RFOG <NUM> according to an alternative embodiment, which is configured to reduce or eliminate the Kerr effect bias sensitivity, such as by using common loss modulation. The RFOG <NUM> includes an optical resonator <NUM>, such as a fiber optic ring resonator, which is in optical communication with a first laser device <NUM>, and a second laser device <NUM>, through an optical path. The optical resonator <NUM> is coupled to a bus waveguide <NUM> such as an optical fiber. The first laser device <NUM> is configured to emit a CW beam at a first power level, and the second laser device <NUM> is configured to emit a CCW beam at a second power level.

Various discrete optical components can be employed to direct the CW and CCW beams to optical resonator <NUM>. For example, a first optical coupler <NUM> can direct the CW beam to a first set of mirror reflectors <NUM>, <NUM>, which reflect the CW beam to a first polarizer <NUM>. The first polarizer <NUM> can be placed in the cavity close to a CW input configured to receive the CW beam. The first polarizer <NUM> passes a first polarized beam to a second optical coupler <NUM>, which couples the first polarized beam into bus waveguide <NUM> for propagation in optical resonator <NUM>. A third optical coupler <NUM> directs the CCW beam to a second set of mirror reflectors <NUM>, <NUM>, which reflect the CCW beam to a second polarizer <NUM>. The second polarizer <NUM> can be placed in the cavity close to a CCW input configured to receive the CCW beam. The second polarizer <NUM> passes a second polarized beam to a fourth optical coupler <NUM>, which couples the second polarized beam into bus waveguide <NUM> for propagation in optical resonator <NUM>.

A common loss modulation can be generated in optical resonator <NUM> by adjusting a polarization state in optical resonator <NUM> to modify a cavity loss in optical resonator <NUM>. The polarization state in optical resonator <NUM> is adjusted by producing a misalignment of one of the first or second polarizers <NUM>, <NUM> away from its eigen polarization state to modify the cavity loss in optical resonator <NUM>. The misalignment is modulated to generate a common loss modulation frequency. The misalignment can be produced by using micro-electromechanical systems (MEMS) actuated polarizers.

For example, an angle of first polarizer <NUM> can be adjusted with respect to a resonant polarization mode, or an angle of second polarizer <NUM> can be adjusted with respect to a resonant polarization mode. A power level ratio between the first and second power levels of the CW and CCW beams can be adjusted based on the common loss modulation frequency, to determine a modified power level ratio value where a Kerr effect bias instability in RFOG <NUM> is reduced or eliminated.

Additional details and examples related to the present approach are described as follows.

Simulations have shown that the CW and CCW beams in a RFOG have different intensity distributions when the RFOG has asymmetrical loss distribution. Such non-reciprocal intensity distribution makes the RFOG sensitive to both common power and common loss variations via the Kerr effect. This effect is illustrated in <FIG>, which is a diagram representing the asymmetric loss distribution in an optical fiber. For a CW beam <NUM> with a CW intensity (Icw), a loss <NUM> in the resonator is represented as Lw - Iloss. For a CCW beam <NUM> with a CCW intensity (Iccw), a loss <NUM> in the resonator is represented as Iccw- Iloss. The asymmetric loss p ηloss(%) in the resonator between losses L<NUM> and L<NUM> is indicated at <NUM>.

<FIG> indicates that the asymmetric loss distribution in the resonator can lead to an asymmetric Kerr effect, which in turn leads to bias even with common intensity fluctuation and loss fluctuation. A nonreciprocal phase change can be represented by the expression: <MAT> with Δφ = n<NUM>L<NUM> × [Icw - Iccw(<NUM> - ηloss)] + n<NUM>L<NUM> × [Icw(<NUM> - ηloss) - Iccw] = n<NUM>(L<NUM> + L<NUM>) × (Icw - Iccw) + ηloss × n<NUM>(L<NUM>Iccw - L<NUM>Icw).

The above expression: n<NUM>(L<NUM> + L<NUM>) × (Icw - Iccw) represents the primary Kerr effect, which is due to differential power fluctuations. The above expression: ηloss × n<NUM> (L<NUM>Iccw - L<NUM>Icw) represents the asymmetric Kerr effect, which is due to common loss/power fluctuations.

Even with both the CW and CCW beams under the same intensity modulation, this can still lead to a bias in the RFOG. In one example, a CW input has zero dB coupling loss, while a CCW input has a <NUM> dB coupling loss in an RFOG. Assuming both the CW and CCW ports have a <NUM> mW input power with <NUM> % intensity modulation at frequency f<NUM>, the CW loop sees <NUM>µW power variation while the CCW loop only experiences <NUM>µW power variation. This power difference will generate a bias modulated at f<NUM> due to the Kerr effect. The power ratio can be adjusted to reduce or eliminate the bias. In this example, the CCW power can be doubled to eliminate the modulated bias signal.

<FIG> is an example graph of the bias with respect to time for CW and CCW beams in a resonator of an RFOG, prior to application of the present approach of adjusting the power ratio to reduce bias sensitivity to common loss change and common power fluctuations. A plot <NUM> in the graph of <FIG> has a sinusoidal shape for the bias with respect to time. In this example, the CW and CCW beams are under the same intensity modulation, the CW power is equal to the CCW power, and the common modulation leads to a bias modulation.

<FIG> is an example graph of the bias with respect to time for CW and CCW beams in the resonator of the RFOG, after application of the present approach of adjusting the power ratio to reduce bias sensitivity to common loss change and common power fluctuations. A plot <NUM> in the graph of <FIG> shows a substantial reduction in sinusoidal shape for the bias with respect to time. In this case, the CW and CCW beams are under same intensity modulation, but the power ratio is adjusted such that CW power is equal to <NUM> × CCW power. This results in the bias modulation being substantially reduced (in this case, a <NUM>,<NUM> time reduction). Therefore, by adjusting the ratio between the CW and CCW power, the present method can reduce the bias due to common intensity fluctuation and common loss variation.

<FIG> is an example graph of the bias peak to peak (PP) modulation with respect to a power ratio at a <NUM> common modulation amplitude, with the power ratio being adjusted to reduce bias sensitivity. As indicated by a plot <NUM> in the graph of <FIG>, by adjusting the power ratio between the CW and CCW power, the bias modulation peak to peak value is reduced to almost zero. In this case, the total ratio stability is about <NUM> ppm.

The processing units and/or other computational devices used in the method and system described herein may be implemented using software, firmware, hardware, or appropriate combinations thereof. The processing unit and/or other computational devices may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In some implementations, the processing unit and/or other computational devices may communicate through an additional transceiver with other computing devices outside of the system, such as those associated with a management system or computing devices associated with other subsystems controlled by the management system. The processing unit and/or other computational devices can also include or function with software programs, firmware, or other computer readable instructions for carrying out various process tasks, calculations, and control functions used in the methods and systems described herein.

The methods described herein may be implemented by computer executable instructions, such as program modules or components, which are executed by at least one processor or processing unit. Generally, program modules include routines, programs, objects, data components, data structures, algorithms, and the like, which perform particular tasks or implement particular abstract data types.

Instructions for carrying out the various process tasks, calculations, and generation of other data used in the operation of the methods described herein can be implemented in software, firmware, or other computer readable instructions. These instructions are typically stored on appropriate computer program products that include computer readable media used for storage of computer readable instructions or data structures. Such a computer readable medium may be available media that can be accessed by a general purpose or special purpose computer or processor, or any programmable logic device.

Claim 1:
A method comprising:
providing a resonator fiber optic gyroscope (<NUM>), RFOG, that comprises an optical resonator (<NUM>) in optical communication with a first light source that emits a first optical signal at a first power level in a clockwise, CW, direction, and a second light source that emits a second optical signal at a second power level in a counterclockwise, CCW, direction;
adding a common intensity modulation to the first optical signal emitted from the first light source, to adjust the first power level and produce an intensity modulated CW signal;
adding the same common intensity modulation to the second optical signal emitted from the second light source, to adjust the second power level and produce an intensity modulated CCW signal;
providing means to equalize a modulation amplitude between the first and second optical signals;
introducing the intensity modulated CW signal into the optical resonator (<NUM>), such that the intensity modulated CW signal propagates in a CW direction in the optical resonator;
introducing the intensity modulated CCW signal into the optical resonator (<NUM>), such that the intensity modulated CCW signal propagates in a CCW direction in the optical resonator;
detecting the intensity modulated CW and CCW signals that are output from the optical resonator;
outputting a first electrical signal based on the detected intensity modulated CCW signal, and outputting a second electrical signal based on the detected intensity modulated CW signal;
determining a rotation rate signal based on the first and second electrical signals, wherein the rotation rate signal comprises a sine wave at a common intensity modulation frequency;
adjusting a power level ratio between the first and second power levels of the first and second optical signals based on the sine wave at the common intensity modulation frequency, to determine a modified power level ratio value where the sine wave in the rotation rate signal is reduced or eliminated, such that a Kerr effect bias instability due to an asymmetrical loss distribution in the RFOG is reduced or eliminated; and
maintaining the modified power level ratio value in the RFOG such that the Kerr effect bias instability is reduced or eliminated.