Systems and methods for efficient relative intensity noise reduction for broadband resonator optical gyroscopes

Abstract

A method of operating a resonator optical gyroscope, the method comprising: generating an optical signal having a broadband frequency range; coupling the optical signal into an optical resonator (OR) for propagation in a first direction and coupling the optical signal out of the OR after the optical signal passes through the OR in the first direction; applying phase modulation to the optical signal coupled out of the OR to generate a phase modulated optical signal; filtering a first portion of the phase modulated optical signal to generate a filtered phase modulated optical signal; generating a first electrical signal indicative of a power level and RIN of the filtered phase modulated optical signal; coupling a second portion of the phase modulated optical signal into the OR to propagate in a second direction, and coupling the second portion out of the OR after the phase modulated optical signal passes through the OR in the second direction; generating a second electrical signal indicative of a power level of the phase modulated optical signal after passing through the OR in a second direction; and determining a rotation rate based on the first electrical signal and the second electrical signal.

Description

Background Art

[0001] Resonator fiber 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-propagation direction. When the RFOG is subjected to rotation about its sensing axis, the resonant frequency of the optical resonator shifts due to the Sagnac effect. The 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] In some aspects, a resonator optical gyroscope includes a broadband light source and an optical resonator, the broadband light source being configured to generate an optical signal having a broadband frequency range. The resonator optical gyroscope also includes a first optical coupler configured to receive an optical signal from the broadband light source and couple the optical signal into the optical resonator so that the optical signal propagates through the optical resonator in a first direction. The resonator optical gyroscope also includes a second optical coupler coupled to the optical resonator, wherein the second optical coupler is configured to couple the optical signal out of the optical resonator after the optical signal passes through the optical resonator in the first direction. The resonator optical gyroscope also includes a phase modulator configured to apply phase modulation to the optical signal that has been coupled out of the optical resonator by the second optical coupler to generate a phase modulated optical signal. The resonator optical gyroscope also includes an optical filter configured to filter a first portion of the phase modulated optical signal to generate a filtered phase modulated optical signal. The second optical coupler is configured to couple a second portion of the phase modulated optical signal into the optical resonator so that the phase modulated optical signal propagates through the optical resonator in a second direction. The first optical coupler is configured to couple the second portion of the phase modulated optical signal out of the optical resonator after the phase modulated optical signal passes through the optical resonator in the second direction. The resonator optical gyroscope also includes a relative intensity noise detector configured to convert the filtered phase modulated optical signal into a first electrical signal, the first electrical signal indicating a power level of the filtered phase modulated optical signal and a relative intensity noise. The resonator optical gyroscope also includes a second photodetector configured to convert the phase modulated optical signal into a second electrical signal, the second electrical signal indicating a power level of the phase modulated optical signal after passing through the optical resonator in the second direction. The resonator optical gyroscope also includes one or more circuits configured to determine a rotation rate based on the first electrical signal and the second electrical signal.

[0003] In some aspects, a resonator optical gyroscope includes a broadband light source and an optical resonator, the broadband light source being configured to generate an optical signal having a broadband frequency range. The resonator optical gyroscope also includes a first optical coupler configured to receive an optical signal from the broadband light source and couple the optical signal into the optical resonator so that the optical signal propagates through the optical resonator in a first direction. The resonator optical gyroscope also includes a second optical coupler coupled to the optical resonator, wherein the second optical coupler is configured to couple the optical signal out of the optical resonator after the optical signal passes through the optical resonator in the first direction. The resonator optical gyroscope also includes a phase modulator configured to apply phase modulation to the optical signal that has been coupled out of the optical resonator by the second optical coupler to generate a phase-modulated optical signal. The resonator optical gyroscope also includes an optical resonator filter configured to filter a first portion of the phase-modulated optical signal to generate a filtered phase-modulated optical signal. The second optical coupler is configured to couple a second portion of the phase modulated optical signal into the optical resonator so that the phase modulated optical signal propagates through the optical resonator in a second direction. The first optical coupler is configured to couple the phase modulated optical signal out of the optical resonator after the phase modulated optical signal passes through the optical resonator in the second direction. The resonator optical gyroscope also includes a relative intensity noise detector configured to convert the filtered phase modulated optical signal into a first electrical signal, the first electrical signal indicating a power level of the filtered phase modulated optical signal and a relative intensity noise. The resonator optical gyroscope also includes a second photodetector configured to convert the phase modulated optical signal after passing through the optical resonator in the second direction into a second electrical signal, the second electrical signal indicating a power level of the phase modulated optical signal. The resonator optical gyroscope also includes one or more circuits configured to determine a rotation rate based on the first electrical signal and the second electrical signal.

[0004] In some aspects, a method of operating a resonator optical gyroscope includes generating an optical signal having a broadband frequency range using a broadband light source. The method also includes coupling the optical signal into an optical resonator using a first optical coupler for propagation in a first direction. The method also includes coupling the optical signal out of the optical resonator using a second optical coupler after the optical signal passes through the optical resonator in the first direction. The method also includes applying phase modulation to the optical signal coupled out of the optical resonator by the second optical coupler to generate a phase-modulated optical signal. The method also includes filtering a first portion of the phase-modulated optical signal using an optical filter to generate a filtered phase-modulated optical signal. The method also includes generating a first electrical signal indicating a power level and a relative intensity noise of the filtered phase-modulated optical signal using a relative intensity noise detector. The method also includes coupling a second portion of the phase-modulated optical signal into the optical resonator using a second optical coupler for propagation in a second direction. The method also includes coupling the phase-modulated optical signal out of the optical resonator using the first optical coupler after the phase-modulated optical signal passes through the optical resonator in the second direction. The method also includes generating a second electrical signal with a second photodetector, the second electrical signal indicating a power level of the phase modulated optical signal after passing through the optical resonator in the second direction. The method also includes determining a rotation rate based on the first electrical signal and the second electrical signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Understanding that the drawings depict only some embodiments and are therefore not to be considered limiting in scope, exemplary embodiments will be described with additional specificity and detail using the accompanying drawings, in which:

[0006] Figure 1 is a block diagram of an example system;

[0007] Figure 2 is a graph of the spectrum of an optical signal at different levels in an example optical gyroscope;

[0008] Figure 3A is a graph of the transmission spectra of the resonator optical gyroscope and the optical filter;

[0009] Figure 3B is a graph of the transmission spectra of the resonator optical gyroscope and the optical filter; and

[0010] Figure 4 is a flow chart of an example method for relative intensity noise subtraction for a resonator optical gyroscope.

[0011] As is common practice, the various features described are not necessarily drawn to scale, emphasis instead being placed on specific features relevant to the example 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. In addition, 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] RFOGs using broadband light sources have recently emerged as potential devices for rotation rate sensing, with reduced fiber length, simpler configuration, and potentially competitive performance. However, RFOGs using broadband light sources have problems associated with the broadband light source spectrum being significantly filtered by the resonator, making the relative intensity noise at the rate detector much higher than that experienced by interferometer fiber optic gyroscopes (IFOGs).

[0014] One potential option for reducing relative intensity noise is to measure the relative intensity noise of the optical signal at a point in the optical path after it has passed through the optical resonator and phase modulation has been applied. The measured relative intensity noise is then subtracted from the electrical signal generated by the rate photodetector for the rate output of the optical resonator gyroscope. However, the problem with this approach is that the spectrum at the rate detector is significantly different from the spectrum at other points in the optical path of the optical resonator gyroscope due to the filtering of the optical signal by the optical resonator. The light reaching the rate detector passes through the optical resonator twice, while the light reaching the RIN subtraction detector passes through the optical resonator only once. The mismatch in the spectra makes simple relative noise intensity subtraction techniques ineffective.

[0015] The techniques described herein address the problem of relative intensity noise by utilizing an optical filter at a specific location in the optical path that modifies the spectrum of an optical signal sampled to generate an electrical signal for relative intensity noise subtraction to obtain a spectrum that more closely matches the spectrum of a phase-modulated optical signal sampled for rate detection. In some examples, a servo mechanism is used to adjust or tune the optical filter to better match the spectrum over time. The techniques described herein improve relative intensity noise subtraction and make the rate detection performance of a resonator optical gyroscope using a broadband light source more accurate.

[0016] Figure 1 A block diagram of an example resonator optical gyroscope 100 is shown in which an effective relative intensity noise subtraction technique may be implemented. Figure 1In the example shown, the resonator optical gyroscope 100 includes various components, including a broadband light source 102, a first circulator 104, a first optical coupler 106, an optical resonator 108, a second optical coupler 110, a second circulator 112, a phase modulator 114, an optical filter 116, a relative intensity noise (RIN) detector 118, a rate photodetector 120, and a RIN subtraction and rate detection circuit 122. Although in Figure 1 Although not specifically labeled, the components of the resonator optical gyroscope 100 are coupled via suitable guiding optics. For example, the components of the resonator 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 resonator optical gyroscope 100 is implemented on an integrated photonic substrate such as a photonic chip.

[0017] The resonator optical gyroscope 100 generally operates by propagating optical signals through different optical paths, where the optical signals are ultimately processed to determine the rate of rotation experienced by the resonator optical gyroscope 100 RIN subtraction and rate detection circuit 122 .

[0018] The broadband light source 102 is configured to generate an optical signal having a broadband frequency range. In some examples, the broadband light source 102 is configured to generate an optical signal via amplified spontaneous emission (ASE). In some examples, the broadband light source 102 is a rare earth doped optical fiber or other waveguide light source. For example, the broadband light source 102 can be an optical fiber doped with rare earth ions (such as erbium, ytterbium, etc.). In such examples, the broadband light source 102 is optically pumped (for example, using a 980nm or 1480nm pump laser), absorbs the pump light, and emits an optical signal. In some examples, the broadband light source 102 can be a superluminescent laser diode, a light emitting diode (LED), etc. In any case, the optical signal generated by the broadband light source 102 has a continuous broadband spectrum and low coherence, which reduces the impact from the optical Kerr effect or makes such impact negligible.

[0019] exist Figure 1 In the example shown, the optical signal generated by the broadband light source 102 is provided to the first circulator 104. Figure 1 In the example shown, the first circulator 104 is a three-port device that is configured to output a signal received at one port to another specific port. Figure 1 In the example shown, an optical signal received from a broadband light source 102 at a first port (shown as port 1 of a first circulator 104) is output to a second port (shown as port 2 of the first circulator 104) coupled to a first optical coupler 106. Figure 1In the example shown, the optical signal received at the second port from the first optical coupler 106 is output to a third port (shown as port 3 of the first circulator 104 ) and then output to the rate photodetector 120 .

[0020] The first circulator 104 is configured to provide an optical signal received from the broadband light source 102 to the first optical coupler 106, and the first optical coupler 106 is configured to couple at least a portion of the optical signal into the optical resonator 108. In some examples, the optical fiber extending through the first optical coupler 106 is beveled to prevent back reflections of a portion of the optical signal that is not transmitted into the optical resonator 108. In other examples, an optical isolator or similar device may also be used to prevent back reflections. In any case, preventing back reflections ensures that these signals do not interfere with other signals received by the rate photodetector 120 (including the optical signal generated by the broadband light source 102 and the phase modulated optical signal, as further described herein).

[0021] The optical resonator 108 is configured to pass a portion of the coupled optical signal at one or more resonant frequencies of the optical resonator 108. When the first optical coupler 106 receives the optical signal from the first circulator 104, it directs the optical signal to propagate in the optical resonator 108 in a first direction. Figure 1 In the example shown, the optical signal coupled into the optical resonator 108 by the first optical coupler 106 propagates in a counterclockwise (CCW) direction; however, in other examples, the resonator optical gyroscope 100 may be designed so that the optical signal initially propagates in a clockwise (CW) direction. As the optical signal propagates in the optical resonator 108, some frequency components of the optical signal (particularly frequency components that are not close to the resonant frequency of the optical resonator 108) will destructively interfere and not resonate. Therefore, the frequency component of the optical signal corresponding to the resonant frequency of the optical resonator 108 will be transmitted to the output port (the second optical coupler 110), while other frequency components will not be transmitted.

[0022] After propagating through the optical resonator 108, a portion of the optical signal is coupled out of the optical resonator 108 via the second optical coupler 110. Optical filtering of the optical signal in the first path through the optical resonator 108 reshapes the broadband spectrum to match the optical resonator transmission line. Figure 1 As shown, the optical signal coupled out of the optical resonator 108 has a significantly reduced power level and is channelized in the CCW direction through multiple resonances.

[0023] The optical signal coupled out of the optical resonator 108 via the second optical coupler 110 is then sent to the second circulator 112. Figure 1In the example shown, the second circulator 112 is a three-port device that is configured to output a signal received at one port to another specific port. Figure 1 In the example shown, an optical signal received at a first port (shown as port 1 of the second circulator 112) from the second optical coupler 110 is output to a second port (shown as port 2 of the second circulator 112) coupled to the phase modulator 114. Figure 1 In the example shown, the optical signal received from the phase modulator 114 at the third port (shown as port 3 of the second circulator 112 ) is output to the first port and then to the second optical coupler 110 .

[0024] exist Figure 1 In the example shown, the optical signal output from port 2 of the second circulator 112 is provided to the phase modulator 114. The phase modulator 114 is configured to modulate the phase of the optical signal based on the phase modulation signal sent to the phase modulator 114 from the RIN subtraction and rate detection circuit 122. In some examples, the phase modulator 114 is configured to generate the phase modulated optical signal using a sawtooth waveform. In some examples, the phase modulator 114 is an electro-optical phase modulator.

[0025] Once modulated by the phase modulator 114, the modulated optical signal (referred to as a "phase-modulated optical signal") is sent back to the optical resonator 108 in the opposite direction through the second circulator 112 and the second optical coupler 110. After modulation, the phase-modulated optical signal is provided to the third port of the second circulator 112. The phase-modulated optical signal received at the third port of the second circulator 112 is output to the first port of the second circulator 112, which then passes the phase-modulated optical signal back to the second optical coupler 110. The second optical coupler 110 is configured to couple at least a portion of the phase-modulated optical signal into the optical resonator 108.

[0026] When some of the signal strength is coupled into the optical resonator 108, the remaining amount of the signal propagates toward the optical fiber extending through the second optical coupler 110. In some examples, the optical fiber extending through the second optical coupler 110 is beveled to prevent back reflections of a portion of the phase-modulated optical signal that is not transmitted into the optical resonator 108. In other examples, an optical isolator or similar device may also be used to prevent back reflections. In any case, preventing back reflections ensures that these signals do not interfere with other signals received by the second optical coupler 110.

[0027] The second optical coupler 110 couples the phase-modulated optical signal in the second direction through the optical resonator 108. Figure 1In 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 108 propagates through the optical resonator 108, while non-resonant frequency components are blocked.

[0028] After the phase-modulated optical signal propagates through the optical resonator 108, the first optical coupler 106 is configured to couple a portion of the phase-modulated optical signal out of the optical resonator 108, and the phase-modulated optical signal is directed to the first circulator 104. Since the phase-modulated optical signal propagates in a direction toward the broadband light source 102 and is received at the second port of the first circulator 104, the first circulator 104 acts as an isolator to direct the phase-modulated optical signal to the rate photodetector 120 and prevent the phase-modulated optical signal from propagating back to the broadband light source 102.

[0029] The phase modulated optical signal propagates from the first circulator 104 to the rate photodetector 120. The rate photodetector 120 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 phase modulated optical signal. The electrical signal corresponding to the phase modulated optical signal is then provided to the RIN subtraction and rate detection circuit 122.

[0030] The spectrum of the optical signal generated by the broadband light source 102 changes significantly after the optical signal passes through the optical resonator 108 and phase modulation is applied. Figure 2 is a graph 200 of the spectrum of a single-resonance optical signal at various stages of the optical path of the resonator optical gyroscope 100. Figure 2 In the example shown, line 202 represents the spectrum of the optical signal after passing through the optical resonator 108 in the first direction and before phase modulation is applied using the phase modulator 114. Figure 2 As can be seen in FIG. 1 , the spectrum of the optical signal is quite well defined after passing through the optical resonator 108 , which is described above as the filtering / channelization effect of the optical resonator 108 .

[0031] exist Figure 2 In the example shown, line 204 represents the spectrum of the phase modulated optical signal before being coupled back into the optical resonator 108 by the second optical coupler 110 in the second direction. Figure 2 In the example shown, the peaking shown in line 204 representing the spectrum of the phase modulated optical signal corresponds to the sidebands of the optical wave after phase modulation has been applied using the phase modulator 114 .

[0032] exist Figure 2In the example shown, line 206 represents the spectrum of the phase modulated optical signal provided to rate photodetector 120 after passing through optical resonator 108 in the second direction. Figure 2 As can be seen in FIG. 1 , the power level of the resonance peak of the phase modulated optical signal provided to the rate photodetector 120 is reduced compared to the power level of the phase modulated optical signal before it passes through the optical resonator 108 in the second direction. In addition, the power level of the sideband of the phase modulated optical signal provided to the rate photodetector 120 is significantly reduced compared to the power level of the sideband of the phase modulated optical signal before it passes through the optical resonator 108 in the second direction.

[0033] Due to Figure 2 Given the mismatch in the spectra represented by lines 204, 206 in FIG. 1 , current techniques for relative intensity noise subtraction are insufficient to adequately reduce the relative intensity noise.

[0034] exist Figure 1 In the example shown, the resonator optical gyroscope 100 also includes an optical filter 116 optically coupled between the phase modulator 114 and port 3 of the second circulator 112 to provide more effective relative intensity noise reduction. Figure 1 In the example shown, the third optical coupler 115 is configured to couple at least a portion of the phase modulated optical signal into the optical filter 116. In some examples, the phase modulated optical signal passes through the optical filter 116 in the same direction as the first direction of the optical resonator 108. Figure 1 In the example shown, the phase modulated optical signal passes through the optical filter 116 in the CCW direction. The optical filter 116 transmits the phase modulated optical signal and generates a filtered phase modulated optical signal. The optical filtering of the optical signal in the path through the optical filter 116 reshapes the broadband spectrum to match the optical filtering effect of the optical resonator 108 in the second path through the optical resonator 108.

[0035] The optical filter 116 is configured to filter the phase modulated optical signal so that the spectrum of the phase modulated optical signal used for relative intensity noise subtraction approximately matches the spectrum of the phase modulated optical signal provided to the rate photodetector 120. Figure 1 In the example shown, the optical filter 116 is an optical resonator filter, and the following description is based on this example. However, it should be understood that other types of optical filters may also be used as long as the optical filter has similar characteristics to those described below.

[0036] After propagating through the optical filter 116, a portion of the filtered phase-modulated optical signal is coupled out of the optical filter 116 via the fourth optical coupler 117. The filtered phase-modulated optical signal coupled out of the optical filter 116 via the fourth optical coupler 117 is provided to the RIN detector 118. In some examples, the RIN detector 118 is configured to measure the intensity fluctuation of the filtered phase-modulated optical signal at the phase modulation frequency. In some examples, the RIN detector 118 is configured to measure the direct current (DC) power and noise of the filtered phase-modulated optical signal. In some examples, the RIN detector 118 is configured to convert the filtered phase-modulated optical signal from an optical signal to a corresponding electrical signal based on the power and relative intensity noise of the received phase-modulated optical signal. In some examples, the RIN detector 118 is configured to output an electrical signal indicating the power level and relative intensity noise at the modulation frequency of the filtered phase-modulated optical signal, and provide the electrical signal to the RIN subtraction and rate detection circuit 122.

[0037] In some examples, optical filter 116 is designed to have a cavity that is similar in size to the cavity of optical resonator 108. In other examples, optical filter 116 is designed to have a smaller cavity than the cavity of optical resonator 108 and to have a higher finesse than optical resonator 108. Optical filter 116 may differ from optical resonator 108 in these or other respects as long as the optical transmission spectrum of optical filter 116 for the filtered phase modulated optical signal approximately matches the optical transmission lineshape of optical resonator 108 for the phase modulated optical signal.

[0038] In some examples, the optical filter 116 is designed to have a free spectral range (FSR) that is an integer multiple (n=1, 2, 3, . . . ) of the FSR of the optical resonator 108 . Figure 3A An optical transmission spectrum 302 of the optical filter 116 and an optical transmission spectrum 304 of the optical resonator 108 are shown, where the integer ratio of the FSR of the optical filter 116 to the FSR of the optical resonator 108 is n = 10. It should be understood that this is an example, and any integer ratio of one or greater of the FSR of the optical filter 116 to the FSR of the optical resonator 108 may be achieved for the resonator optical gyroscope 100.

[0039] In some examples, optical filter 116 is designed such that the line shape of optical filter 116 is approximately equal to the line shape of optical resonator 108 . Figure 3B is for the resonant peak (e.g. Figure 3A350 of an optical transmission spectrum 352 of an optical resonator 108 and an optical transmission spectrum 354 of an optical filter 116 with respect to the central resonance peak shown in FIG. 350 . In some examples, the line shape of optical filter 116 is considered to be approximately equal to the line shape of optical resonator 108 if the line shape of optical transmission spectrum 354 of optical filter 116 overlaps with optical transmission spectrum 352 of optical resonator 108 by ninety-five percent or more within the order of the resonance peak. Figure 3B In the example shown, the line shape of the optical transmission spectrum 354 of the optical filter 116 matches the optical transmission spectrum 352 of the optical resonator 108 to better than −20 dB. FIG. 3A to FIG. 3B Deviations below -20 dB shown in do not contribute much to the relative intensity noise difference, so matching the top part of the resonant peak is more important (and effective).

[0040] RIN subtraction and rate detection circuit 122 is configured to subtract the electrical signal received from RIN detector 118 from the electrical signal received from rate photodetector 120 to remove relative intensity noise from the electrical signal received from rate photodetector 120 .

[0041] In some examples, the electrical signal from RIN detector 118 and / or the electrical signal from rate photodetector 120 is modified prior to subtraction. In some such examples, the gain of the electrical signal from RIN detector 118 and / or the gain of the electrical signal from rate photodetector 120 is modified to equalize the power levels of the electrical signals prior to subtraction. The difference or ratio between the direct current (DC) power levels between the electrical signal from RIN detector 118 and the electrical signal from rate photodetector 120 may be used to determine a gain adjustment to apply to the electrical signal from RIN detector 118 and / or the electrical signal from rate photodetector 120.

[0042] To determine the rotation rate, the RIN subtraction and rate detection circuit 122 is configured to demodulate the electrical signal from the rate photodetector 120 after subtracting the relative intensity noise using the electrical signal from the RIN detector 118, and is further configured to determine the resonant frequency shift based on the power of the demodulated signal corresponding to the phase modulated optical signal. For example, the electrical signal received from the rate photodetector 120 may indicate a power difference corresponding to a change in the rotation rate.

[0043] In some examples, to better match the spectrum of the filtered phase-modulated optical signal coupled out of the optical filter 116 and the spectrum of the phase-modulated optical signal coupled out of the optical resonator 108, the optical filter 116 is locked to the optical resonator 108 so that the optical filter 116 does not drift from the optical resonator 108 over time. In some examples, the RIN subtraction and rate detection circuit 122 or another circuit is configured to adjust at least one characteristic of the optical filter 116 to lock the optical filter 116 to the optical resonator 108. In some examples, the RIN subtraction and rate detection circuit 122 or another circuit is configured to adjust the temperature of the optical filter 116. In other examples, the RIN subtraction and rate detection circuit 122 or another circuit is configured to modify the length of the optical filter 116 (e.g., using a piezoelectric transducer). In some examples, the RIN subtraction and rate detection circuit 122 is configured to lock the optical filter 116 to the optical resonator 108 by maximizing the DC power level output by the RIN detector 118.

[0044] In some examples, the power level of the phase modulated optical signal can also be used to adjust the slope of the triangular phase modulation signal (which shifts the frequency of the optical signal), and the slope difference between two consecutive cycles of the phase modulation signal (which applies square wave frequency modulation to the optical signal) can be used to determine the frequency shift. RIN subtraction and rate detection circuit 122 can then determine the rotation rate based on the frequency shift.

[0045] In some examples, Figure 1 The resonator optical gyroscope 100 shown in FIG. 1 is configured to operate as a closed-loop optical gyroscope because the output from the rate photodetector 120 indicating the rotation rate is also used as feedback to adjust the operation of the phase modulator 114 (wherein the "closed loop" includes the propagation of the signal from the phase modulator 114 ultimately to the RIN subtraction and rate detection circuit 122 and the control signal input to the phase modulator 114). In some examples, the RIN subtraction and rate detection circuit 122 includes a control circuit configured to adjust the operation of the phase modulator 114 based on the electrical signal from the rate photodetector 120. In some examples, the RIN subtraction and rate detection circuit 122 configures at least one parameter of the phase modulator 114, such as a characteristic of the phase modulation signal used by the phase modulator 114 to modulate the optical signal. For example, as previously described, the RIN subtraction and rate detection circuit 122 is configured to adjust the slope of the sawtooth phase modulation signal based on the rotation-induced change determined from the received electrical signal. The RIN subtraction and rate detection circuit 122 may send a control signal that configures the phase modulator 114.

[0046] In response to receiving the control signal for adjusting the slope of the phase modulated signal, the phase modulator 114 generates an adjusted phase modulated signal having a slope of a waveform corresponding to the parameter set by the RIN subtraction and rate detection circuit 122. For example, the phase modulated optical signal may have a frequency shift corresponding to the shifted resonant frequency of the optical resonator 108 due to the rotation. In this way, the resonator optical gyroscope 100 (and specifically the RIN subtraction and rate detection circuit 122) can compensate for power fluctuations of the average power of the phase modulated optical signal detected by the RIN subtraction and rate detection circuit 122.

[0047] In some examples, to reduce rate measurement errors, the optical resonator 108 may include a polarizer (not shown) configured to filter out portions of the optical signal corresponding to undesired polarization states. For example, the optical resonator 108 may include a polarizer (not shown) configured to filter out portions of the optical signal corresponding to an undesired polarization mode of the optical resonator 108. Thus, only portions of the optical signal corresponding to the selected polarization mode of the optical resonator 108 are allowed to pass through the polarizer and are transmitted through the optical resonator 108 each round trip.

[0048] Figure 4 A flow chart of an example method 400 of operating a resonator optical gyroscope is shown. Figures 1 to 3B The common features discussed with respect to the example systems in can include similar characteristics to those discussed with respect to method 400, and vice versa. In some examples, blocks of method 400 are performed by resonator optical gyroscope 100 described above.

[0049] Method 400 includes generating an optical signal having a broadband frequency range (block 402). In some examples, the optical signal is generated using a broadband light source, which may be an ASE light source, a rare-earth doped fiber light source, a superluminescent laser diode, a light emitting diode, or the like.

[0050] The method 400 also includes coupling an optical signal into the optical resonator to propagate in a first direction (block 404); and coupling the optical signal out of the optical resonator after the optical signal has completely traversed the optical resonator in the first direction (block 406). In some examples, the optical signal is coupled into and out of the optical resonator using a first optical coupler and a second optical coupler, respectively.

[0051] The method 400 also includes applying phase modulation to the optical signal (block 408). In some examples, applying phase modulation to the optical signal includes using a sawtooth waveform.

[0052] The method 400 also includes filtering the first portion of the phase modulated optical signal using an optical filter (block 410). In some examples, the filtering is performed using an optical filter (e.g., an optical resonator filter). In some examples, the optical filter is designed to have a free spectral range that is an integer multiple of the free spectral range of the optical resonator and to match the line shape of the resonance peak of the optical signal output from the optical filter with the line shape of the resonance peak of the optical signal output from the optical resonator. In some examples, the optical filter is locked to the optical resonator by adjusting one or more characteristics of the optical filter.

[0053] Method 400 also includes generating an electrical signal corresponding to the filtered phase modulated optical signal (block 412). In some examples, the electrical signal is generated by a relative intensity noise detector and indicates a power level and relative intensity noise of the filtered phase modulated optical signal.

[0054] The method 400 also includes coupling a second portion of the phase modulated optical signal into the optical resonator to propagate in a second direction (block 414); and coupling the phase modulated optical signal out of the optical resonator after the optical signal has traversed the optical resonator in the second direction (block 416). In some examples, the phase modulated optical signal is coupled into and out of the optical resonator using a second optical coupler and a first optical coupler, respectively.

[0055] The method 400 also includes generating a second electrical signal corresponding to the phase modulated optical signal after passing through the resonator in the second direction (block 418). In some examples, the electrical signal is generated by a photodetector and indicates a power level of the phase modulated optical signal.

[0056] The method 400 also includes determining a rotation rate based on the first electrical signal and the second electrical signal (block 420). In some examples, determining the rotation rate based on the first electrical signal and the second electrical signal includes subtracting the first electrical signal from the second electrical signal. In some such examples, determining the rotation rate based on the first electrical signal and the second electrical signal includes adjusting a gain of the first electrical signal and / or the second electrical signal prior to the subtraction. In some examples, determining the rotation rate includes demodulating the electrical signal and determining a resonant frequency shift based on a power of the demodulated electrical signal corresponding to the phase modulated optical signal.

[0057] As described herein, by filtering a portion of a phase modulated optical signal using an optical filter, relative intensity noise subtraction for a broadband RFOG can be performed in an effective and efficient manner. In addition, by designing the optical filter as described herein and locking it to an optical resonator, the optical filter can be more compact and effective for relative intensity noise subtraction. Overall, the systems and methods described herein can significantly reduce the rate error caused by relative intensity noise of a broadband RFOG, enabling higher performance to be achieved at a low cost.

[0058] In various aspects, the system elements, method steps, or examples described in the present disclosure (such as, for example, the RIN subtraction and rate detection circuit 122 or components thereof) may be implemented on one or more computer systems including a central processing unit (CPU), a graphics processing unit (GPU), a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), and / or similar devices including hardware-executable code for implementing those elements, processes, or examples, the code being stored on a non-transitory data storage device. These devices include or operate using software programs, firmware, or other computer-readable instructions for performing various methods, process tasks, computations, and control functions.

[0059] These instructions are usually stored on any suitable computer storage medium for storing computer-readable instructions or data structures. Computer-readable media can be implemented as any available medium that can be accessed by a general or special-purpose computer or processor or any programmable logic device. Suitable processor-readable media may include storage devices or memory media, such as magnetic media or optical media. For example, storage devices or memory media may include conventional hard disks, compact disk-read-only memory (CD-ROM), volatile or non-volatile media (such as random access memory (RAM) (including but not limited to synchronous dynamic random access memory (SDRAM), double data rate (DDR) RAM, RAMBUS dynamic RAM (RDRAM), static RAM (SRAM) etc.), read-only memory (ROM), electrically erasable programmable ROM (EEPROM) and flash memory etc. Suitable processor-readable media may also include transmission media transmitted via communication media (such as networks and / or wireless links), such as electrical signals, electromagnetic signals or digital signals.

[0060] The methods and techniques described herein may be implemented 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 a combination thereof. 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. Processes embodying these techniques may be performed by a programmable processor executing a program of instructions to perform a desired function by operating on input data and producing appropriate outputs. These techniques may advantageously be implemented in one or more programs executable on a programmable system, the programmable system including at least one programmable processor coupled to receive data and instructions from and transmit data and instructions to a data storage system, at least one input device, and at least one output device. Generally, 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 EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and DVD disks. Any of the foregoing may be supplemented by, or incorporated in, specially designed application specific integrated circuits (ASICs).

[0061] Example Implementation

[0062] Embodiment 1 includes a resonator optical gyroscope, the resonator optical gyroscope comprising: a broadband light source configured to generate an optical signal having a broadband frequency range; an optical resonator; a first optical coupler configured to receive the optical signal from the broadband light source and couple the optical signal into the optical resonator so that the optical signal propagates through the optical resonator in a first direction; a second optical coupler coupled to the optical resonator, wherein the second optical coupler is configured to couple the optical signal out of the optical resonator after the optical signal passes through the optical resonator in the first direction; a phase modulator configured to apply phase modulation to the optical signal that has been coupled out of the optical resonator by the second optical coupler to generate a phase modulated optical signal; an optical filter configured to filter a first portion of the phase modulated optical signal to generate a filtered phase modulated optical signal; wherein the second optical coupler is a first optical coupler configured to couple a second portion of the phase modulated optical signal into the optical resonator so that the phase modulated optical signal propagates through the optical resonator in a second direction, wherein the first optical coupler is configured to couple the second portion of the phase modulated optical signal out of the optical resonator after the phase modulated optical signal passes through the optical resonator in the second direction; a relative intensity noise detector configured to convert the filtered phase modulated optical signal into a first electrical signal, the first electrical signal indicating a power level and a relative intensity noise of the filtered phase modulated optical signal; a second photodetector configured to convert the phase modulated optical signal into a second electrical signal, the second electrical signal indicating a power level of the phase modulated optical signal after passing through the optical resonator in the second direction; and one or more circuits configured to determine a rotation rate based on the first electrical signal and the second electrical signal.

[0063] Embodiment 2 includes the resonator optical gyroscope of Embodiment 1, wherein the one or more circuits are configured to lock the optical filter to the optical resonator by adjusting at least one characteristic of the optical filter.

[0064] Embodiment 3 includes the resonator optical gyroscope of Embodiment 2, wherein the one or more circuits are configured to utilize a piezoelectric transducer to adjust the temperature of the optical filter and / or modify the length of the optical filter.

[0065] Embodiment 4 includes the resonator optical gyroscope of any one of Embodiments 1 to 3, wherein the optical filter is configured such that the line shape of the resonance peak of the filtered phase-modulated optical signal is approximately equal to the line shape of the resonance peak of the phase-modulated optical signal.

[0066] Embodiment 5 includes the resonator optical gyroscope of any of Embodiments 1 to 4, wherein the one or more circuits are configured to determine the rotation rate based on the first electrical signal and the second electrical signal by subtracting the first electrical signal from the second electrical signal.

[0067] Embodiment 6 includes the resonator optical gyroscope of Embodiment 5, wherein the one or more circuits are configured to adjust a gain of the first electrical signal and / or the second electrical signal before subtracting the first electrical signal from the second electrical signal.

[0068] Embodiment 7 includes the resonator optical gyroscope of Embodiment 6, wherein the one or more circuits are configured to adjust the gain of the first electrical signal and / or the second electrical signal based on a difference between DC power levels of the first electrical signal and the second electrical signal.

[0069] Embodiment 8 includes the resonator optical gyroscope of any of Embodiments 1 to 7, wherein the one or more circuits are configured to lock the optical filter to the optical resonator by maximizing a DC power level output by the relative intensity noise detector.

[0070] Embodiment 9 includes a resonator optical gyroscope, the resonator optical gyroscope comprising: a broadband light source configured to generate an optical signal having a broadband frequency range; an optical resonator; a first optical coupler configured to receive the optical signal from the broadband light source and couple the optical signal into the optical resonator so that the optical signal propagates through the optical resonator in a first direction; a second optical coupler coupled to the optical resonator, wherein the second optical coupler is configured to couple the optical signal out of the optical resonator after the optical signal passes through the optical resonator in the first direction; a phase modulator configured to apply phase modulation to the optical signal that has been coupled out of the optical resonator by the second optical coupler to generate a phase modulated optical signal; an optical resonator filter configured to filter a first portion of the phase modulated optical signal to generate a filtered phase modulated optical signal; wherein the second an optical coupler configured to couple a second portion of the phase-modulated optical signal into the optical resonator so that the phase-modulated optical signal propagates through the optical resonator in a second direction, wherein the first optical coupler is configured to couple the phase-modulated optical signal out of the optical resonator after the phase-modulated optical signal passes through the optical resonator in the second direction; a relative intensity noise detector configured to convert the filtered phase-modulated optical signal into a first electrical signal, the first electrical signal indicating a power level and a relative intensity noise of the filtered phase-modulated optical signal; a second photodetector configured to convert the phase-modulated optical signal after passing through the optical resonator in the second direction into a second electrical signal, the second electrical signal indicating a power level of the phase-modulated optical signal; and one or more circuits configured to determine a rotation rate based on the first electrical signal and the second electrical signal.

[0071] Embodiment 10 includes the resonator optical gyroscope of embodiment 9, wherein the optical resonator filter has a higher finesse than the optical resonator.

[0072] Embodiment 11 includes the resonator optical gyroscope of any of Embodiments 9-10, wherein the one or more circuits are configured to lock the optical resonator filter to the optical resonator by adjusting at least one characteristic of the optical resonator filter.

[0073] Embodiment 12 includes the resonator optical gyroscope of any one of Embodiments 9 to 11, wherein the optical resonator filter is configured such that a free spectral range of the optical resonator filter is an integer multiple of a free spectral range of the optical resonator.

[0074] Embodiment 13 includes the resonator optical gyroscope of any of Embodiments 9 to 12, wherein the one or more circuits are configured to determine the rotation rate based on the first electrical signal and the second electrical signal by subtracting the first electrical signal from the second electrical signal.

[0075] Embodiment 14 includes the resonator optical gyroscope of any of Embodiments 9 to 13, wherein the one or more circuits are configured to adjust the gain of the first electrical signal and / or the second electrical signal before subtracting the first electrical signal from the second electrical signal.

[0076] Embodiment 15 includes the resonator optical gyroscope of Embodiment 14, wherein the one or more circuits are configured to adjust the gain of the first electrical signal and / or the second electrical signal before subtracting the first electrical signal from the second electrical signal.

[0077] Embodiment 16 includes a method of operating a resonator optical gyroscope, the method comprising: generating an optical signal having a broadband frequency range using a broadband light source; coupling the optical signal into an optical resonator using a first optical coupler to propagate in a first direction; coupling the optical signal out of the optical resonator using a second optical coupler after the optical signal passes through the optical resonator in the first direction; applying phase modulation to the optical signal coupled out of the optical resonator by the second optical coupler to generate a phase-modulated optical signal; filtering a first portion of the phase-modulated optical signal using an optical filter to generate a filtered phase-modulated optical signal; generating a first relative intensity noise detector using a relative intensity noise detector. an optical resonator for propagating in a second direction; after the phase-modulated optical signal passes through the optical resonator in the second direction, coupling the phase-modulated optical signal out of the optical resonator using the first optical coupler; generating a second electrical signal using a second photodetector, the second electrical signal indicating a power level of the phase-modulated optical signal after passing through the optical resonator in the second direction; and determining a rotation rate based on the first electrical signal and the second electrical signal.

[0078] Embodiment 17 includes the method of Embodiment 16, further comprising locking the optical filter to the optical resonator by adjusting at least one characteristic of the optical filter.

[0079] Embodiment 18 includes the method of any one of Embodiments 16 to 17, wherein determining the rotation rate based on the first electrical signal and the second electrical signal includes: subtracting the first electrical signal from the second electrical signal.

[0080] Embodiment 19 includes the method of any one of Embodiments 16 to 18, further comprising adjusting a gain of the first electrical signal and / or the second electrical signal before subtracting the first electrical signal from the second electrical signal.

[0081] Embodiment 20 includes the method described in any one of Embodiments 16 to 19, further comprising locking the optical filter to the optical resonator so that the line shape of the resonance peak of the spectrum of the filtered phase-modulated optical signal is approximately equal to the line shape of the resonance peak of the spectrum of the phase-modulated optical signal.

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

Claims

1. A resonator optical gyroscope (100), the resonator optical gyroscope comprising: a broadband light source (102) configured to generate an optical signal having a broadband frequency range; An optical resonator (108); a first optical coupler (106) configured to receive the optical signal from the broadband light source (102) and couple the optical signal into the optical resonator (108) such that the optical signal propagates through the optical resonator (108) in a first direction; a second optical coupler (110) coupled to the optical resonator (108), wherein the second optical coupler (110) is configured to couple the optical signal out of the optical resonator (108) after the optical signal passes through the optical resonator (108) in the first direction; a phase modulator (114) configured to apply phase modulation to the optical signal that has been coupled out of the optical resonator (108) by the second optical coupler (110) to generate a phase modulated optical signal; an optical filter (116) configured to filter a first portion of the phase modulated optical signal to generate a filtered phase modulated optical signal; wherein the second optical coupler (110) is configured to couple a second portion of the phase modulated optical signal into the optical resonator (108) such that the phase modulated optical signal propagates through the optical resonator (108) in a second direction, wherein the first optical coupler (106) is configured to couple the second portion of the phase modulated optical signal out of the optical resonator (108) after the phase modulated optical signal passes through the optical resonator (108) in the second direction; a relative intensity noise detector (118) configured to convert the filtered phase modulated optical signal into a first electrical signal indicative of a power level and a relative intensity noise of the filtered phase modulated optical signal; a second photodetector (120) configured to convert the phase-modulated optical signal into a second electrical signal, the second electrical signal indicating a power level of the phase-modulated optical signal after passing through the optical resonator (108) in the second direction; and One or more circuits (122) configured to determine a rotation rate based on the first electrical signal and the second electrical signal.

2. The resonator optical gyroscope (100) of claim 1, wherein the one or more circuits (122) are configured to lock the optical filter (116) to the optical resonator (108) by adjusting at least one characteristic of the optical filter (116).

3. The resonator optical gyroscope (100) of any one of claims 1 to 2, wherein the one or more circuits (122) are configured to determine the rotation rate based on the first electrical signal and the second electrical signal by subtracting the first electrical signal from the second electrical signal.

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