Closed loop operation of broadband resonator optical gyroscopes using sawtooth phase modulation

Abstract

Various examples of a closed loop optical gyroscope are disclosed. The closed loop optical gyroscope includes a broadband light source configured to generate a broadband light signal. A broadband optical signal propagates in the optical resonator and is coupled into and out of the optical resonator through the optical coupler. The phase modulator phase modulates the optical signal based on the sawtooth modulation signal. The optical signal repropagates in different directions in the optical resonator. The optical signal is then received and analyzed to determine parameters of the phase modulator. The one or more processors configure the phase modulator based on the determined parameters.

Description

Background Art

[0001] Resonator fiber optic gyroscopes (RFOGs) typically utilize a narrow linewidth laser source to generate the optical signal necessary for rotation rate measurement. RFOGs typically operate by propagating an optical signal generated by a laser through an optical resonator in a counter-propagating direction. When the RFOG is subjected to rotation about its sensing axis, the resonant frequency of the optical resonator undergoes a frequency shift due to the Sagnac effect; this frequency shift in the resonant frequency can then be used to determine the degree of rotation to which the RFOG has been subjected. Summary of the invention

[0002] The details of one or more embodiments are set forth in the following description. Features shown or described in conjunction with an exemplary embodiment may be combined with features of other embodiments. Thus, any of the various embodiments described herein may be combined to provide additional embodiments. If necessary, various aspects of the embodiments may be modified to adopt concepts from various patents, applications, and publications as identified herein to provide yet additional embodiments.

[0003] In one example, a closed-loop optical gyroscope is disclosed. The closed-loop optical gyroscope includes a broadband light source configured to generate an optical signal having a broadband frequency range. The closed-loop optical gyroscope includes a first optical coupler of an optical resonator, the first optical coupler coupled to the broadband light source and configured to couple the optical signal into the optical resonator. The optical signal propagates through the optical resonator in a first direction. The closed-loop optical gyroscope includes a second optical coupler of the optical resonator, the second optical coupler coupled to the first optical coupler. The second optical coupler is configured to couple the optical signal out of the optical resonator. The closed-loop optical gyroscope includes a phase modulator coupled to the second optical coupler. The phase modulator is configured to generate a phase-modulated optical signal from the optical signal based on a sawtooth modulation signal. The second optical coupler is configured to provide the phase-modulated optical signal back into the optical resonator. The phase-modulated optical signal propagates through the optical resonator in a second direction different from the first direction to the first optical coupler. The first optical coupler is configured to couple the phase-modulated optical signal out of the optical resonator. The closed-loop optical gyroscope includes a control servo system coupled to a first optical coupler and a phase modulator. The control servo system is configured to receive a signal corresponding to a phase-modulated optical signal from the first optical coupler and to adjust at least one parameter of the phase modulator based on the received signal.

[0004] In another example, a system is disclosed. The system includes a broadband light source configured to generate an optical signal having a broadband frequency range. The system includes a first optical coupler of an optical resonator, the first optical coupler coupled to the broadband light source and configured to couple the optical signal into the optical resonator. The optical signal propagates through the optical resonator in a first direction. The system includes a second optical coupler of the optical resonator, the second optical coupler coupled to the first optical coupler. The second optical coupler is configured to couple the optical signal out of the optical resonator. The system includes a phase modulator coupled to the second optical coupler. The phase modulator is configured to generate a phase-modulated optical signal from the optical signal based on a sawtooth modulation signal. The second optical coupler is configured to provide the phase-modulated optical signal back into the optical resonator. The phase-modulated optical signal propagates through the optical resonator in a second direction different from the first direction to the first optical coupler. The first optical coupler is configured to couple the phase-modulated optical signal out of the optical resonator. The system includes at least one processor coupled to the first optical coupler and the phase modulator. The at least one processor is configured to receive a signal corresponding to the phase modulated optical signal from the first optical coupler and is configured to adjust at least one parameter of the phase modulator based on the received signal.

[0005] In yet another example, a method for operating a closed-loop optical gyroscope is disclosed. The method includes generating an optical signal having a broadband frequency range. The method includes coupling the optical signal into an optical resonator. The optical signal propagates through the optical resonator in a first direction; coupled, the method includes coupling the optical signal out of the optical resonator. The method includes modulating the optical signal with a sawtooth modulation signal to generate a phase-modulated optical signal. The method includes coupling the phase-modulated optical signal back into the optical resonator. The phase-modulated optical signal propagates through the optical resonator in a second direction different from the first direction. The method includes coupling the phase-modulated signal out of the optical resonator. The method includes receiving a signal corresponding to the phase-modulated optical signal. The method includes adjusting at least one parameter of the phase modulator based on the received signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered to be limiting of the scope of the invention, the exemplary embodiments will be described with additional specificity and detail through use of the drawings, as briefly described below and as described in detail later in the Detailed Description.

[0007] Figure 1 A block diagram of an exemplary closed-loop optical gyroscope as described in one or more embodiments is shown.

[0008] Figures 2 to 3A graphical representation of various modulation schemes applied to optical signals in an optical gyroscope is shown, as described in one or more embodiments.

[0009] Figure 4 A graphical representation of sinusoidal and square wave phase demodulation applied to a phase modulated optical signal is shown, as described in one or more embodiments.

[0010] Figure 5 A flow chart illustrating an exemplary method for operating a closed-loop optical gyroscope, as described in one or more embodiments.

[0011] According to common practice, the various features described are not necessarily drawn to scale, instead emphasis is placed on specific features relevant to the exemplary embodiments. DETAILED DESCRIPTION

[0012] In the following detailed description, reference is made to the accompanying drawings which form a part thereof and in which specific illustrative embodiments are shown by way of illustration. However, it should be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the methods presented in the drawings and the specification should not be interpreted as limiting the order in which the various steps may be performed. Therefore, the following detailed description should not be considered to have a limiting meaning.

[0013] The signals generated by the narrow-linewidth lasers in the RFOG are generally more susceptible to the optical Kerr effect, a nonlinear optical phenomenon that modifies the propagation characteristics of an optical signal depending on its intensity. Broadband lasers (i.e., wide-linewidth sources that generate optical signals with a broadband frequency range) can reduce the optical Kerr effect present in RFOGs, but also introduce additional problems in RFOGs: the broadband nature of the laser source and the narrow resonance peak of the optical resonator can cause the output intensity of the optical signal to be significantly reduced after propagating through the optical resonator. As a result, the output signal may be too weak for the detection and processing circuits to determine the rotation rate.

[0014] One option to overcome the losses caused by implementing a broadband light source is to utilize an acousto-optic modulator (AOM) in an open-loop gyroscope configuration. While operational, this approach is not suitable for certain applications. For example, AOMs are bulky and expensive devices, making them unsuitable for photonic-based integrated gyroscopes or other small-scale RFOG designs. AOMs may also lack the performance required for more precise implementations of optical gyroscopes.

[0015] Embodiments of the present disclosure improve upon the RFOG architecture. Specifically, embodiments of the present disclosure utilize closed-loop operation of the RFOG through adaptive sawtooth phase modulation. Thus, the RFOG can compensate for the power loss of the optical resonator and maintain a substantially constant power response for determining a stable rotational speed. Such a system can operate in closed-loop operation and maintain scale factor linearity.

[0016] Figure 1 A block diagram of an exemplary optical gyroscope is shown, generally indicated at 100. Optical gyroscope 100 includes various components, including a broadband light source (BLS) 102, a circulator 104, an optical coupler 106, an optical resonator 110, an optical coupler 116, a circulator 118, a phase modulator 120, and a control servo system 126. Although not shown in FIG. Figure 1 Although not specifically labeled in the drawings, the components of the optical gyroscope 100 are coupled via suitable guided optics. For example, the components of the optical gyroscope 100 may be coupled via optical waveguides and / or free-space optics such as lenses, mirrors, and beam splitters. In some examples, the optical gyroscope 100 is implemented on an integrated photonic substrate such as a photonic chip.

[0017] The optical gyroscope 100 generally operates by propagating optical signals through different optical paths where they are ultimately processed to determine the rate of rotation experienced by the gyroscope. The optical signals within a broadband frequency range are initially generated by a broadband light source 102. In some examples, the broadband light source 102 is a broadband laser configured to generate optical signals by amplified spontaneous emission (ASE). Thus, the optical signals generated by the broadband light source 102 have a broad spectral linewidth and low coherence. In some examples, the broadband light source 102 generates signals at a selected frequency / wavelength range or intensity that reduces the effects from the optical Kerr effect to negligible or makes such effects negligible.

[0018] exist Figure 1 In the example shown, the optical signal generated by the broadband light source 102 is provided to the circulator 104. The circulator 104 is configured to provide the received signal to a different optical path according to the received signal. For the optical signal generated by the broadband light source 102, the circulator 104 provides them to the optical coupler 106. The optical coupler 106 is configured to couple at least a portion of the optical signal into the optical resonator 110. The optical waveguide is cut at an angle at the element 108 to prevent a portion of the optical signal that is not transmitted into the optical resonator 110 from being back-reflected so that these signals do not interfere with other signals received by the optical coupler 106 (including the optical signal generated by the broadband light source 102 and the phase-modulated optical signal, as further described herein).

[0019] The function of the optical resonator 110 is to pass a portion of the coupled optical signal at one or more resonant frequencies of the optical resonator. When the optical coupler 106 receives the optical signal from the circulator 104, it enables the optical signal to propagate in the optical resonator 110 in a first direction. Figure 1 In the example shown, the optical signal coupled by the optical coupler 106 propagates in a counterclockwise (CCW) direction. However, in other examples, the optical gyroscope 100 can be designed so that the optical signal initially propagates in a clockwise (CW) direction. As the optical signal propagates in the optical resonator, some frequency components of the optical signal (especially frequency components that are not close to the resonant frequency of the optical resonator) will cause destructive interference and will not resonate. Therefore, the frequency components of the optical signal corresponding to the resonant frequency of the optical resonator 110 will be transmitted to the output port, while other components will not.

[0020] To reduce rate measurement errors, the optical resonator 110 may include a polarizer configured to filter out portions of the optical signal corresponding to undesirable polarization states. Figure 1 In the example shown, the optical resonator 110 includes a polarizer 114 that is configured to filter out portions of the optical signal corresponding to an unwanted polarization mode of the optical resonator 110. Thus, only portions of the optical signal corresponding to a selected polarization mode of the optical resonator 110 are allowed to pass through the polarizer 114 and will each be transmitted back and forth through the optical resonator 110.

[0021] After propagating through the optical resonator 110, a portion of the optical signal is coupled out of the optical resonator 110 via the optical coupler 116. The optical signal is then provided to the circulator 118 and subsequently to the phase modulator 120. The phase modulator 120 is configured to modulate the phase of the optical signal based on the modulation signal applied to the optical signal. Specifically, the phase modulator 120 is configured to generate a phase modulated optical signal using a sawtooth modulation signal. The sawtooth modulation signal includes as shown in Figures 2 to 3 In some examples, the phase modulator 120 is an electro-optical phase modulator. Using such a phase modulator simplifies the architecture of the optical gyroscope 100 and reduces the size and cost of implementing the frequency shift of the optical signal.

[0022] Once modulated by the phase modulator 120, the modulated optical signal (referred to as the "phase modulated optical signal") propagates in the reverse direction. After modulation, the phase modulated optical signal is provided to the circulator 118, which then passes the phase modulated optical signal back to the optical coupler 116. The optical coupler 116 is configured to couple at least a portion of the phase modulated optical signal into the optical resonator 110. While some of the signal strength is coupled into the optical resonator, the remaining amount of the signal is angle-chopped at the element 122.

[0023] The optical coupler 116 couples the phase modulated optical signal through the optical resonator 110 in the second direction. Figure 1 In the example shown, the phase modulated optical signal propagates in the CW direction. The phase modulated optical signal having a frequency component corresponding to the resonance peak of the optical resonator 110 propagates through the optical resonator 110, while non-resonance frequency components are blocked. Figure 1 In the example shown, the optical resonator 110 also includes a polarizer 112 that is configured to filter out unwanted polarization modes of the optical resonator 110. Thus, only a portion of the phase modulated optical signal corresponding to the selected polarization mode of the optical resonator 110 passes through the polarizer 114 and will be coupled out of the optical resonator 110.

[0024] After propagating through the optical resonator 110, the optical coupler 106 is configured to couple a portion of the phase modulated optical signal out of the optical resonator 110, where the phase modulated optical signal is directed to the circulator 104. Since the phase modulated optical signal propagates in a direction toward the broadband light source 102, the circulator 104 acts as an isolator to prevent the phase modulated signal from propagating back into the broadband light source 102. Instead, the phase modulated optical signal propagates from the circulator 104 to the photodetector 124. The photodetector 124 is configured to convert the phase modulated optical signal from an optical signal to a corresponding electrical signal based on the power of the received signal. The electrical counterpart of the phase modulated optical signal is then provided to the control servo system 126.

[0025] To determine the rotation rate, the control servo system 126 includes or is coupled to one or more processors (described as a single processor for simplicity). For example, the processor can demodulate the electrical signal and determine the resonant frequency shift based on the power of the demodulated signal corresponding to the phase modulated optical signal. For example, the received signal can indicate a power difference corresponding to a change in the rotation rate. The power can be used to adjust the slope of the sawtooth modulated signal, and the slope difference between two consecutive peaks of the sawtooth modulated signal can be used to determine the frequency shift. The processor can then determine the rotation rate based on the frequency shift.

[0026] like Figure 1 As shown, the optical gyroscope 100 is a closed-loop optical gyroscope because the output from the optical resonator 110 indicating the rotation rate is also used as feedback to adjust the operation of the phase modulator 120 (where "closed loop" includes the signal ultimately propagating from the phase modulator 120 to the control servo system 126 and the control signal being input to the phase modulator 120). Specifically, the control servo system 126 includes a control circuit that is configured to adjust the operation of the phase modulator 120 based on the electrical signal from the photodetector 124. In some examples, the control servo system 126 configures at least one parameter of the phase modulator 120, such as the characteristics of the sawtooth modulation signal used by the phase modulator 120 to modulate the optical signal. For example, as previously described, the control servo system 126 is configured to adjust the slope of the sawtooth modulation signal based on the power fluctuation determined from the received electrical signal. The control servo system 126 can send a control signal that configures the phase modulator 120. In response to receiving the control signal for adjusting the slope of the sawtooth modulation signal, the phase modulator 120 generates a sawtooth modulation signal having an adjusted waveform slope corresponding to the parameter set by the control servo system 126. For example, the frequency shift of the phase modulated optical signal may correspond to the shifted resonance frequency of the optical resonator 110 due to the rotation. In this way, the optical gyroscope 100 (specifically, the control servo system 126) may compensate for the power fluctuation of the average power of the phase modulated optical signal detected by the control servo system 126.

[0027] Figures 2 to 3 A graphical representation showing various modulation schemes applied to the optical signal in an optical gyroscope. Figure 2 shows the modulation scheme applied to the optical signal corresponding to the slow rotation rate, while Figure 3 A modulation scheme applied to an optical signal corresponding to a fast spin rate is shown.

[0028] refer to Figure 2, four graphical representations are shown. Graph 200A shows the output intensity as a function of the frequency of two curves 230, 232, wherein the peak of each curve 230, 232 corresponds to the resonant frequency of the optical resonator 110. The first curve 230 of graph 200A represents the resonant power output of the optical signal in the absence of rotation. In contrast, the second curve 232 of graph 200A represents the resonant power output of the optical signal in the presence of a slow rotation rate around the sensing axis of the optical gyroscope. Both curves 230, 232 approximate a bell-shaped curve. As shown in graph 200A, the frequency of the maximum value between the two curves 230, 232 shifts, which corresponds to the shift in the resonant frequency of the optical resonator when the optical gyroscope undergoes rotation. In addition, although not explicitly shown in graph 200A, each curve 230, 232 may include multiple peaks periodically spaced apart in frequency, wherein the frequency difference between two adjacent frequency peaks of the same curve is referred to as the free spectral range. For each curve 230 , 232 in graph 200A, only one frequency peak is shown for illustrative purposes.

[0029] Graph 200B shows three different frequency modulated waveforms: waveform 202 (corresponding to no rotation, i.e., a reference waveform), waveform 204 (corresponding to slow rotation observed through an open-loop configuration), and waveform 206 (corresponding to slow rotation observed through a closed-loop configuration). For each respective waveform 202, 204, 206, the horizontal axis represents frequency relative to an origin (the origin corresponding to the resonant frequency of the optical resonator when no rotation is observed), and the vertical axis represents time associated with each frequency. It should be noted that the vertical axis of graph 200B is defined to be specific to each respective waveform 202, 204, 206, and is not relative to each other (e.g., waveform 202 corresponding to no rotation is not shown as appearing before waveform 204 corresponding to open-loop rotation). Instead, each different waveform 202, 204, 206 is shown vertically to illustrate and compare different attributes of each waveform.

[0030] As shown in graph 200B, each waveform 202, 204, 206 resembles a square wave showing a frequency modulation corresponding to the phase modulation applied by phase modulator 120. The maximum magnitude of each waveform 202, 204, 206 corresponds to a specified frequency f1 above the origin of its corresponding curve, while the minimum magnitude of each waveform 202, 204, 206 corresponds to a specified frequency f2 below the origin of its corresponding curve. Thus, the magnitude difference between the maximum frequency and the minimum frequency can be defined based on a frequency interval that characterizes the resonant peak of its corresponding curve, such as the full width at half maximum (FWHM) of the curve.

[0031] Reference is now made to graph 200C, which illustrates a phase modulation signal applied to an optical signal after exiting an optical resonator. The phase modulation signal is used to modulate the optical signal by the phase modulator 120. For example, waveform 208 is a phase modulation signal that would be applied to the optical signal in the absence of rotation, waveform 210 is a phase modulation signal that would be applied to the optical signal in an open loop configuration during slow rotation, and waveform 212 is a phase modulation signal that would be applied to the optical signal in a closed loop configuration during slow rotation. Specifically, graph 200C is similar to graph 200B in that three different waveforms 208, 210, 212 are shown vertically for teaching comparison, with the horizontal axis representing the magnitude of the phase modulation and the vertical axis representing time (which, like graph 200B, is defined to be specific to each respective waveform rather than being defined to be absolute for all waveforms).

[0032] Referring to graph 200C, each waveform 208, 210, 212 may be composed of one or more time periods t representing the periodicity of the phase modulation. a ,t b Definition. In some examples, t a represents the time associated with the first phase peak, and t b represents the time associated with the second phase peak. As shown in graph 200C, the t of waveforms 208, 210, 212 a =t b , but in general these may not be equal. When phase modulation is applied to an optical signal corresponding to no rotation, a sawtooth modulated signal is shown as waveform 208, where the phase amplitude φ varies between a maximum value and a minimum value. As shown for waveforms 208 and 210, the minimum value of the phase amplitude is equal to the negative amplitude of the maximum value, and the waveforms are symmetrical about the zero amplitude axis. The phase modulation applied to generate waveform 210 is the same as waveform 208 because, in an open loop configuration, no feedback is provided to the phase modulator to adjust the degree of phase modulation. In contrast, when the phase amplitude φ is φ, the phase amplitude φ varies between a maximum value and a minimum value. Figure 1 When the optical gyroscope is subjected to rotation in a closed-loop configuration, the phase modulation signal, after adjustment by the control servo system 126, is represented by waveform 212. For waveform 212, the maximum phase amplitude is greater than the minimum phase amplitude, and therefore, the sawtooth phase modulation waveform is not symmetrical about the zero amplitude axis. As the rotation rate changes, the characteristics of the phase modulation signal may change based on feedback received from the control servo system 126. For example, when the optical gyroscope is subjected to a large rotation rate, the feedback from the control servo system 126 may be used to adjust the slope of the phase modulation waveform 212, such as in Figure 3 is further described in the context of .

[0033] In an optical gyroscope, it is advantageous to keep the output signal power constant to limit deviations due to power fluctuations or due to power levels that may be too high or too low for the sensitivity of the detection circuit. However, when the gyroscope undergoes rotation, the power of the optical signal decreases when propagating back into the optical resonator due to the frequency shift of the resonant frequency of the optical resonator, especially in an open loop configuration. The power output for three different cases is shown in graph 200D, which shows a graph of signal amplitude as a function of time based on the curve shown in graph 200A. When there is no rotation, the power output is shown as waveform 220. To generate waveform 220, the amplitude of curve 230 is plotted starting from the origin and the frequency is shifted back and forth from its maximum frequency f1 to its minimum frequency f2 (e.g., an interval defined by the FWHM of the curve), thereby producing a rectangular output of amplitude that repeats for each iteration.

[0034] When the optical gyroscope undergoes rotation, the power waveform will represent 222 or 224, depending on the configuration. In an open loop configuration, the waveform generated is 222, which shows power fluctuations between the maximum amount of one output (corresponding to one side of curve 232) and the maximum amount of the other output (corresponding to the opposite side of curve 232). This fluctuation occurs because the frequency interval used to sample the amplitude is asymmetric in an open loop configuration that undergoes rotation. For example, as shown in graph 200A, the amplitude sampling starts from the origin of curve 232 to point 234, and the sampling is performed until point 236 of curve 232, where the frequency at point 234 is not equal to the negative value of the frequency at point 236. Therefore, since the amplitude change on the right hand side of curve 232 relative to the origin is less than the amplitude change on the left hand side of curve 232 relative to the origin, the resulting power waveform 222 will have power fluctuations.

[0035] In contrast, from Figure 1 The power waveform 224 of the closed-loop configuration of the embodiment maintains (at least substantially) the same power output. This is due to the feedback provided to the phase modulator 120 and the subsequent adjustment of the phase modulation signal used to generate the phase modulated optical signal. Since the phase modulated optical signal compensates for the resonant frequency shift of the optical resonator, the amplitude of the curve 232 is sampled according to intervals symmetrical about its peak (i.e., the maximum frequency of the interval is equal to the negative value of the minimum frequency), which maintains the constant power output shown by the waveform 224, even when the optical gyroscope undergoes rotations of different magnitudes about its sensing axis. When implementing a closed-loop configuration with dynamic feedback to the phase modulator, the advantages achieved by using a broadband light source (e.g., less optical Kerr effect) can be exploited while compensating for its weaknesses (such as power losses caused by propagation through the optical resonator).

[0036] As the gyroscope experiences different rotation rates, the resonant frequency of the optical resonator 110 will shift, and the power of the phase modulated optical signal will begin to fluctuate due to the changed resonant conditions. Therefore, the output power of the phase modulated optical signal will change according to the new rotation. In some examples, the control servo system 126 is configured to determine the power of the electrical signal corresponding to the phase modulated optical signal. The control servo system 126 or another processor can determine the frequency shift based on the power. For example, the control servo system 126 can determine the change in power from the electrical signal and use the change in power to adjust the slope of the sawtooth phase modulation signal. a With t b The slope difference between φ and φ can be used to determine the frequency difference between the CW signal and the CCW signal, and therefore determine the rotation rate experienced by the gyroscope.

[0037] In addition, the control servo system 126 controls the phase modulation signal applied by the phase modulator 120 based on the power of the received electrical signal. Referring to graph 200B, waveforms 202, 204 and 206 are generated by the time derivatives of their corresponding phase modulation waveforms 208, 210 and 212. Waveform 202 corresponds to curve 208 and represents the waveform when the optical gyroscope is not experiencing rotation. In an open-loop configuration (i.e., an open-loop configuration in which no feedback is given to the phase modulator), the frequency-modulated waveform 204 corresponds to curve 210, and therefore, waveforms 202 and 204 are identical in form. This is because the same phase modulation waveform is applied in the open-loop configuration as when there is no rotation. In contrast, waveform 206 corresponds to curve 212 and represents a closed-loop configuration. In such a closed-loop configuration (see Figure 1 ), when the optical gyroscope undergoes a rotation, such as Figure 2 The waveform 206 is shown to have different characteristics than the waveform 202. For example, the amplitude between the two waveforms 202 and 206 is offset by a value df, corresponding to the frequency difference. Therefore, by knowing the slope of the phase modulation applied by the phase modulator 120, the frequency offset can then be determined.

[0038] Reference now Figure 3 , which shows that Figure 2 300A, 300B, 300C, 300D are similar to the modulation scheme shown in FIG. 300B, but when the optical gyroscope undergoes rapid rotation. Figure 2 Their respective corresponding graphs are shown, and the effect of rapid rotation on each of these graphs will be described in turn.

[0039] As shown in graph 300A, when the gyroscope undergoes faster rotation, the two curves 330, 332 will be separated by a larger frequency shift between their respective peaks. In other words, as the gyroscope undergoes faster rotation about its sensing axis, the difference in the resonance peaks of the optical resonator in the CCW direction and the CW direction will shift more.

[0040] Referring to graphs 300B to 300C, the difference between the open loop configuration and the closed loop configuration is more easily shown during rapid rotation. Since no feedback is provided to the modulator in the open loop configuration, waveform 310 will be the same as waveform 308 when the gyroscope is not experiencing rotation. Moreover, since frequency is related to the time differential of phase, the frequency waveforms 302, 304 will be the same between a gyroscope not experiencing rotation and a gyroscope in the open loop configuration even during rapid rotation. However, during rapid rotation, the power output of the open loop configuration can be significantly reduced, as shown by waveform 322. This is due to the smaller amplitude changes in curve 332 (i.e., from point 334 of curve 332 to point 336 of curve 332) sampled at the frequency interval of the no-rotation curve 330, as would correspond to the phase waveform 308 for the open loop configuration. Therefore, waveform 322 may be too small to be reliably detected, and thus may affect the accuracy of the rotation rate measurement determined by the open loop gyroscope.

[0041] In contrast, Figure 1 The modulation scheme of the closed-loop configuration shown maintains a substantially constant power level even during rapid rotation. Referring to graph 300C, waveform 312 is generated by increasing the slope of the phase modulation applied to the optical signal to generate a phase modulated optical signal. The waveform 312 applied to the optical signal enables the generation of a phase modulated optical signal having a frequency corresponding to the resonant frequency of the optical resonator (as determined based on a signal provided to a control servo system). A characteristic of waveform 312 is that the slope of the phase amplitude increases from the slope of waveform 212 (corresponding to slow rotation). Additionally, in some examples, phase waveform 312 is not symmetrical about a zero amplitude axis, but rather each value of the phase amplitude is in the interval 0<=φ<=φ max In some examples, when the phase amplitude φ exceeds the maximum value φ max When the phase amplitude is reduced, a waveform such as waveform 340 may be generated. This waveform 340 corresponds to a reduced phase amplitude, but maintains the same slope as waveform 312. Since the slopes of waveforms 312 and 340 are the same, the frequency waveform generated from either of these phase waveforms 312 and 340 will be the same as waveform 306. Based on the offset between waveform 306 and waveform 302, the frequency difference df (corresponding to the resonant frequency without rotation) is easily determined. Referring to graph 300D, Figure 1The power waveform 324 in the closed loop configuration is the same as the power waveform 320 when the gyroscope is not experiencing rotation, thereby enabling the output power of the gyroscope to be substantially constant even when the gyroscope is experiencing rapid rotation. This is in contrast to the power waveform 322 of the open loop configuration which may be significantly reduced due to rapid rotation.

[0042] Figure 4 A graphical representation 400 of sinusoidal and square wave phase demodulation applied to a phase modulated optical signal is shown. Specifically, the graph 400 shows the normalized output as a function of the change in Sagnac frequency (in MHz). Two curves are shown in the graph 400, where curve 402 corresponds to the output of the sinusoidal demodulated signal and curve 404 corresponds to the output of the square wave demodulated signal. The sinusoidal demodulated signal is used to demodulate the phase modulated optical signal modulated from the sinusoidal modulated signal, while the square wave demodulated signal is used to demodulate the phase modulated optical signal generated in Figure 1 , that is, the phase-modulated optical signal modulated by the sawtooth modulation signal applied from the phase modulator 120.

[0043] like Figure 4 As shown, for any frequency change, the normalized output of curve 404 corresponding to square wave demodulation generally produces a higher normalized output. For positive frequency changes, the normalized output peaks at approximately 6×10 -5 (greater than the 4×10 -5 output), while for negative frequency changes, the normalized output peak reaches about -6×10 -5 (the absolute value is greater than -4×10 -5 output). Therefore, to achieve Figures 1 to 3 The sawtooth phase modulation scheme shown instead of sinusoidal phase modulation can improve the sensitivity of the gyroscope. As mentioned earlier, dynamically adjusting the characteristics of the sawtooth phase modulation in a closed loop configuration can further improve the detection and stability of the gyroscope.

[0044] Figure 5 A flow chart showing an exemplary method for operating an optical gyroscope. The method 500 may be performed via a Figures 1 to 4 The present invention is implemented by the techniques described herein, but may also be implemented via other techniques. For ease of explanation, the blocks of the flowchart are arranged in a generally sequential manner; however, it should be understood that this arrangement is merely exemplary, and it should be recognized that the processing associated with the methods described herein (and the blocks shown in the figure) may occur in a different order (e.g., where at least some of the processing associated with the blocks is performed in parallel and / or in an event-driven manner).

[0045] The method 500 includes generating an optical signal having a broadband frequency range at block 502. For example, an ASE laser or other broadband light source may generate light having a broad spectral linewidth. At block 504, the method 500 includes coupling the optical signal into an optical resonator. The functions of block 504 may be performed by an optical coupler 106 that receives the broadband optical signal and couples the signal into an optical resonator 110. Once coupled into the optical resonator, the optical signal propagates in a first direction. Figure 1 In the example shown, the optical signal coupled from the optical coupler 106 into the optical resonator 110 propagates in a CCW direction. While propagating through the optical resonator, the optical signal experiences a minimal amount of attenuation at the resonant frequency of the optical resonator, which may shift when the gyroscope experiences rotation.

[0046] The method 500 also includes coupling the optical signal out of the optical resonator at block 506. The functionality of block 506 may be performed by the optical coupler 116, which receives the optical signal from the optical resonator 110 and couples it to an external waveguide. After coupling the optical signal out of the optical resonator, the optical signal is modulated using a sawtooth modulation signal (block 508). In so doing, a phase modulated optical signal is generated from the optical signal.

[0047] Proceeding to block 510, method 500 includes coupling the phase modulated optical signal into the optical resonator. The functionality of block 510 may be performed by optical coupler 116, which receives the phase modulated optical signal from phase modulator 120. Once coupled into the optical resonator, the phase modulated optical signal propagates in a second direction that is different from the first direction. Figure 1 In the example shown, the phase modulated optical signal propagates in the CW direction.After propagating in the optical resonator, the phase modulated optical signal is coupled out of the optical resonator, for example, by the optical coupler 106 (block 512).

[0048] The method 500 also includes receiving a signal corresponding to the phase modulated optical signal at block 514. For example, Figure 1As shown, the phase modulated optical signal is provided to the photodetector 124, which converts the phase modulated optical signal into an electrical signal and sends it to the control servo system 126. After receiving the electrical signal, the method 500 also includes adjusting at least one parameter of the phase modulator based on the received signal (block 516). Block 516 is performed as part of a closed-loop configuration, where feedback is provided to control the phase modulator. In some examples, the control servo system may generate a control signal to the phase modulator. The at least one parameter may include a parameter of a waveform generated by the phase modulator, such as a slope of a phase modulation signal used by the phase modulator when modulating the optical signal. In some examples, the control servo system may control the phase modulator so that the phase modulation applied to the optical signal maintains a constant output power even during changes in the rotation rate. For example, the slope of the adjusted phase modulation signal may be determined based on a power or amplitude so that the resulting phase modulated optical signal compensates for the power loss of the optical resonator, wherein the power or amplitude is determined based on the received signal.

[0049] Optionally, the method 500 includes generating an adjusted phase modulated optical signal based on at least one parameter at block 518. This function is performed by the phase modulator after receiving a control signal from the control servo system 126. When the gyroscope experiences faster rotation, the phase modulated signal may be a waveform 312 with a greater slope of the phase amplitude to compensate for the increased rotation rate. During slower rotation, the phase modulated signal may be a waveform 212 with a smaller slope of the phase amplitude. The phase modulator then generates an adjusted phase modulated optical signal returned to the optical resonator while adjusting the phase modulated signal. The function described at block 518 is an optional function because, in at least some implementations, there may be no rotation rate change and therefore no need to adjust the phase modulation applied to the optical signal.

[0050] The techniques herein provide a combination of advantages over conventional optical gyroscopes. First, the use of a broadband light source rather than a narrow linewidth light source reduces the effects of the optical Kerr effect and optical backscattering. Second, the use of an electro-optic phase modulator reduces the size and weight requirements of the gyroscope. Third, resonant detection in a closed-loop configuration compensates for power losses due to changes in the resonant frequency in the optical resonator during rotation; in particular, feedback control of the phase modulator enables generation of a substantially constant output power response and stabilizes the signal used to measure the rotation rate.

[0051] The methods and techniques described herein may be implemented at least in part in digital electronic circuits, or with a programmable processor (e.g., a special purpose processor or a general purpose processor such as a computer) firmware, software, or various combinations of each. Apparatus embodying these techniques may include appropriate input and output devices, a programmable processor, and a storage medium tangibly embodying program instructions for execution by the programmable processor. A process embodying these techniques may be performed in the following manner: a programmable processor executes an instruction program to perform a desired function by operating on input data and generating appropriate outputs. These techniques may be advantageously implemented in one or more programs executable on a programmable system, the programmable system comprising at least one programmable processor coupled to receive data and instructions from a data storage system, at least one input device, and at least one output device, and to transmit data and instructions to the data storage system, at least one input device, and at least one output device. Generally speaking, the processor will receive instructions and data from a read-only memory and / or a random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices such as erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and flash memory devices; magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and digital video disks (DVDs). Any of the foregoing may be supplemented by or incorporated in specially designed application specific integrated circuits (ASICs).

[0052] Exemplary embodiments

[0053] Embodiment 1 includes a closed-loop optical gyroscope, the closed-loop optical gyroscope comprising: a broadband light source, the broadband light source configured to generate an optical signal having a broadband frequency range; and a first optical coupler of an optical resonator, the first optical coupler coupled to the broadband light source and configured to couple the optical signal into the optical resonator, wherein the optical signal propagates through the optical resonator in a first direction; a second optical coupler of the optical resonator, the second optical coupler coupled to the first optical coupler, wherein the second optical coupler is configured to couple the optical signal out of the optical resonator; a phase modulator, the phase modulator coupled to the second optical coupler, wherein the phase modulator is configured to modulate the signal based on a sawtooth generating a phase-modulated optical signal from the optical signal; wherein the second optical coupler is configured to provide the phase-modulated optical signal back into the optical resonator, wherein the phase-modulated optical signal propagates through the optical resonator in a second direction different from the first direction to the first optical coupler, wherein the first optical coupler is configured to couple the phase-modulated optical signal out of the optical resonator; and a control servo system coupled to the first optical coupler and the phase modulator, wherein the control servo system is configured to receive a signal corresponding to the phase-modulated optical signal from the first optical coupler, and is configured to adjust at least one parameter of the phase modulator based on the received signal.

[0054] Embodiment 2 includes the closed-loop optical gyroscope according to embodiment 1, wherein the control servo system is configured to determine a power of a signal corresponding to the phase modulated optical signal, wherein the control servo system is configured to adjust at least one parameter of the phase modulator based on the power.

[0055] Embodiment 3 includes a closed-loop optical gyroscope according to any one of embodiments 1 to 2, wherein the control servo system is configured to send at least one control signal including at least one adjusted parameter to the phase modulator, wherein the phase modulator is configured to modulate the optical signal based on the at least one control signal.

[0056] Embodiment 4 includes the closed-loop optical gyroscope of any one of embodiments 1 to 3, wherein the at least one parameter that is adjusted includes a slope adjustment of the sawtooth modulation signal.

[0057] Embodiment 5 includes the closed-loop optical gyroscope according to embodiment 4, wherein the phase modulator is configured to generate an adjusted phase modulation signal based on the slope adjustment.

[0058] Embodiment 6 includes the closed-loop optical gyroscope of any one of embodiments 1 to 5, wherein the phase modulator is configured to reduce the amplitude of the sawtooth modulation signal while maintaining the slope of the sawtooth modulation signal.

[0059] Embodiment 7 includes a closed-loop optical gyroscope according to any one of embodiments 1 to 6, which includes at least one polarizer, which is coupled to the optical path of the optical resonator, wherein the at least one polarizer acts as a filter to allow optical signals in a first polarization state to pass and to attenuate optical signals in another polarization state.

[0060] Embodiment 8 includes a system comprising: a broadband light source configured to generate an optical signal having a broadband frequency range; and a first optical coupler of an optical resonator, the first optical coupler coupled to the broadband light source and configured to couple the optical signal into the optical resonator, wherein the optical signal propagates through the optical resonator in a first direction; a second optical coupler of the optical resonator, the second optical coupler coupled to the first optical coupler, wherein the second optical coupler is configured to couple the optical signal out of the optical resonator; and a phase modulator coupled to the second optical coupler, wherein the phase modulator is configured to modulate the optical signal based on a sawtooth modulation signal by the optical signal. generating a phase modulated optical signal; wherein the second optical coupler is configured to provide the phase modulated optical signal back into the optical resonator, wherein the phase modulated optical signal propagates through the optical resonator to the first optical coupler in a second direction different from the first direction, wherein the first optical coupler is configured to couple the phase modulated optical signal out of the optical resonator; and at least one processor coupled to the first optical coupler and the phase modulator, wherein the at least one processor is configured to receive a signal from the first optical coupler corresponding to the phase modulated optical signal, and is configured to adjust at least one parameter of the phase modulator based on the received signal.

[0061] Embodiment 9 includes the system of embodiment 8, wherein the at least one processor is configured to determine a power of a signal corresponding to the phase modulated optical signal, wherein the at least one processor is configured to adjust at least one parameter of the phase modulator based on the power.

[0062] Embodiment 10 includes a system according to any one of Embodiments 8 to 9, wherein the at least one processor is configured to send at least one control signal including at least one adjusted parameter to the phase modulator, wherein the phase modulator is configured to modulate the optical signal based on the at least one control signal.

[0063] Embodiment 11 includes the system of any one of Embodiments 8 to 10, wherein the at least one parameter that is adjusted includes a slope adjustment of the sawtooth modulation signal.

[0064] Embodiment 12 includes the system of embodiment 11, wherein the phase modulator is configured to generate an adjusted phase modulated signal based on the slope adjustment.

[0065] Embodiment 13 includes the system of any one of Embodiments 8 to 12, wherein the phase modulator is configured to reduce the amplitude of the sawtooth modulation signal while maintaining the slope of the sawtooth modulation signal.

[0066] Embodiment 14 includes a system according to any one of Embodiments 8 to 13, the system comprising at least one polarizer coupled to an optical path of an optical resonator, wherein the at least one polarizer acts as a filter to pass an optical signal in a first polarization state and attenuate another polarization state of the optical resonator.

[0067] Embodiment 15 includes a method for operating a closed-loop optical gyroscope, the method comprising: generating an optical signal having a broadband frequency range; coupling the optical signal into an optical resonator via a first optical coupler, wherein the optical signal propagates through the optical resonator in a first direction; coupling the optical signal out of the optical resonator via a second optical coupler; and modulating the optical signal using a sawtooth modulation signal via a phase modulator to generate a phase-modulated optical signal; coupling the phase-modulated optical signal back into the optical resonator via the second optical coupler, wherein the phase-modulated optical signal propagates through the optical resonator in a second direction different from the first direction; coupling the phase-modulated signal out of the optical resonator via the first optical coupler; receiving a signal corresponding to the phase-modulated optical signal at a control servo system; and adjusting at least one parameter of the phase modulator based on the received signal.

[0068] Embodiment 16 includes the method of embodiment 15, comprising determining a power of a signal corresponding to the phase modulated optical signal; and adjusting at least one parameter of the phase modulator based on the power.

[0069] Embodiment 17 includes the method according to any one of embodiments 15 to 16, comprising: adjusting the slope of the sawtooth modulation signal; and generating an adjusted phase modulation signal based on the adjusted slope of the sawtooth modulation signal.

[0070] Embodiment 18 includes a method according to any one of Embodiments 15 to 17, comprising: filtering an optical signal in a first polarization state of the optical resonator via at least one polarizer coupled to an optical path of the optical resonator; and attenuating an optical signal in another polarization state of the optical resonator.

[0071] Embodiment 19 includes the method of any one of embodiments 15 to 18, comprising determining the rotation rate based on the slope of the sawtooth modulation signal.

[0072] Embodiment 20 includes a method according to embodiment 19, wherein determining the rotation rate includes: determining a change in power associated with a phase modulated optical signal; and determining a difference in the slope of a sawtooth modulated signal based on the change in power; and determining the rotation rate based on the difference in the slope.

[0073] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that the present invention be limited only by the claims and the equivalents thereof.

Claims

1. A closed-loop optical gyroscope, comprising: a broadband light source configured to generate an optical signal having a broadband frequency range; a first optical coupler coupled to the broadband light source and configured to provide the optical signal to an optical resonator, wherein the optical signal propagates through the optical resonator in a first direction; a second optical coupler coupled to the first optical coupler, wherein the second optical coupler is configured to couple the optical signal out of the optical resonator; a phase modulator coupled to the second optical coupler, wherein the phase modulator is configured to generate a phase-modulated optical signal from the optical signal based on a sawtooth modulation signal; wherein the second optical coupler is configured to provide the phase-modulated optical signal back to the optical resonator, wherein the phase-modulated optical signal propagates through the optical resonator to the first optical coupler in a second direction different from the first direction, wherein the first optical coupler is configured to couple the phase modulated optical signal out of the optical resonator, and a control servo system coupled to the first optical coupler and the phase modulator, wherein the control servo system is configured to receive a signal from the first optical coupler corresponding to the phase-modulated optical signal and to adjust at least one parameter of the phase modulator based on the received signal.

2. The closed-loop optical gyroscope of claim 1 , wherein the control servo system is configured to determine a power of the signal corresponding to the phase modulated optical signal, wherein the control servo system is configured to adjust the at least one parameter of the phase modulator based on the power.

3. A method for operating a closed-loop optical gyroscope, comprising: generating an optical signal having a broadband frequency range; coupling the optical signal into an optical resonator via a first optical coupler, wherein the optical signal propagates through the optical resonator in a first direction; coupling the optical signal out of the optical resonator via a second optical coupler; modulating the optical signal with a sawtooth modulation signal via a phase modulator to generate a phase modulated optical signal; coupling the phase modulated optical signal back into the optical resonator via the second optical coupler, wherein the phase modulated optical signal propagates through the optical resonator in a second direction different from the first direction; coupling the phase modulated signal out of the optical resonator via the first optical coupler; receiving, at a control servo system, a signal corresponding to the phase modulated optical signal; as well as At least one parameter of the phase modulator is adjusted based on the received signal.

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