JP2009047684A - Rfog modulation error correction - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a system and method for modulation error correction. <P>SOLUTION: An example 40 of this system subjects first and second laser beams 182 and 184 to common phase/frequency modulation, subjects a first modulated beam to first intensity modulation, and subjects a second modulated beam to second intensity modulation. Signals output are demodulated in accordance with the frequency of the common phase/frequency modulation. Next, among the demodulated signals, a first demodulated signal is demodulated in accordance with the frequency of the intensity modulation of the first beam, and a second demodulated signal is demodulated in accordance with the frequency of the intensity modulation of the second beam. Then, the rotation velocity is determined in accordance with the demodulated signals. The frequencies of intensity modulation are unequal, and as such, are out of accord with each other. Each of the light beams subjected to intensity modulation is coded as a unique code. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to RFOG modulation error correction.

  A resonator fiber optic gyro (RFOG) senses rotation by detecting the frequency difference between the clockwise and counterclockwise resonant frequencies of the fiber optic resonator. The resonant frequency of the fiber resonator is probed by one or more narrow band lasers. The resonator converts the optical frequency into light intensity. For example, in the case of the reflective resonator shown in FIG. 1, the light intensity detected by the photodetector will be maximized when the optical frequency moves away from the resonant frequency. However, the light intensity rapidly decreases to the minimum at the resonance frequency when the optical frequency approaches the resonance frequency. Thus, one way to detect the resonant frequency is to detect the minimum power at the detector. However, this method is not accurate enough to perform rotation sensing and therefore another more accurate method is required.

  A very accurate measurement of the resonant frequency can be made by using optical frequency or phase modulation, or by using cavity length modulation of the resonator. FIG. 1 shows a prior art RFOG using cavity length modulation. FIG. 2 shows how modulation allows an accurate measurement of the resonance frequency. Section (a) in FIG. 2 shows the resonance dip that would be observed at the photodetector when the frequency of light is swept. Section (a) also shows the resonator response to sinusoidal frequency modulation at three different nominal optical frequencies. If the nominal optical frequency is to the right of the center of the resonance dip, then the intensity signal will be at the photodetector that is at the modulation frequency and is in phase with the modulation. If the nominal optical frequency is to the left of the center of the resonance dip, the intensity signal will be present at a photodetector that is at the modulation frequency but 180 degrees out of phase due to the modulation. In the case of an ideal resonator and modulation, when the nominal optical frequency is at the center of the resonance dip, which is the resonance frequency, the intensity signal at the photodetector is only at the even harmonics of the modulation frequency, not the modulation frequency. Will have ingredients. Therefore, the resonance frequency is detected when the intensity signal is zero at the modulation frequency.

  The photodetector converts the intensity signal into a voltage signal. As the voltage signal passes through the demodulator (phase detector), the demodulator output as a function of the nominal optical frequency will look like that shown in section (b) of FIG. In the ideal case, the demodulator output passes through zero when the nominal optical frequency is exactly at the resonant frequency.

FIGS. 3-1 and 3-2 show examples of the resonance tracking electronics associated with FIG. CW signal from the photodetector is demodulated at frequency f _m. The output of the demodulator is an error signal that crosses zero when the CW light wave is on the resonant frequency of the ring resonator. This error signal is integrated by an integrator. The output of the integrator controls the laser frequency to the resonant frequency of the resonator. The CCW control loop is similar except that the integrator output goes to a voltage-frequency converter, which output is at Δf and is proportional to the rotational speed.

  FIG. 4 shows an RFOG with common phase modulation of the laser light before it is split into two beams that will back propagate through the coil. There are a number of different phase modulators that can be used. Some examples include a fiber wound around a PZT transducer, an integrated optical chip, or a bulk electro-optic modulator.

  FIG. 5 shows an RFOG with common frequency modulation of the laser light before it is split into two beams that will back propagate through the coil. Some common ways to modulate the laser frequency are to modulate the injection current of a diode laser or a diode pump laser, or to modulate the length of the laser cavity with PZT or some type of MEMS device.

  There are imperfections in the gyro that can cause errors in detecting the resonant frequency, and hence errors in rotation sensing. There are two types of modulator imperfections that can lead to rotation sensing errors. One type is modulator intensity modulation. Even if the intended modulation is cavity length, optical frequency or optical phase, a non-ideal modulator will also generate a modulation of light intensity that may have a component at the modulation frequency. Undesirable intensity modulation will be detected by the demodulator and interpreted as a signal indicative of a non-resonant condition. In that case, the resonator tracking electronics will move the laser frequency away from the resonant frequency until the standard resonator intensity signal accurately cancels the unwanted intensity signal. Deviation from the resonant frequency results in a rotation sensing error if the undesired intensity signal differs between the two backpropagating light waves. However, if the undesired intensity signal can be formed identically, no rotation sensing error will occur.

  Another modulator imperfection that can lead to rotation sensing errors is modulation distortion. Modulation distortion can occur in the modulator drive electronics or modulator. The ideal modulation is a sinusoidal modulation at a single frequency. However, distortion can lead to higher harmonic generation in the modulation. Even harmonic modulation will lead to resonance detection errors that can lead to rotation sensing errors.

  One way to reduce or eliminate rotation sensing errors due to modulator imperfections is to use a common modulation of two backpropagating light waves. This is done by using the same modulator for both backpropagating light waves. 1, 4 and 5 show various RFOG configurations that use common modulation. By using the same modulator, the resonance detection error is the same for both clockwise and counterclockwise directions. Since the rotation measurement is the difference between the detected clockwise and counterclockwise resonance frequencies, the common error is canceled by the rotation measurement (common mode blocking).

  One drawback to common modulation is that the gyro system becomes sensitive to other imperfections associated with optical back reflection or back scattering within the resonator. Backscattered light will result in two types of errors. One type of error (interference type) is associated with optical interference between the backscattered wave and the P wave reaching the photodetector. The other type of error (intensity type) is related to the intensity of the backscattered wave that is modulated by modulation across the resonant dip exactly like the P wave.

  A way to eliminate the intensity type error found in the prior art is to use independent phase modulation of the counterpropagating beam before it enters the resonator. This method is illustrated in FIG. The frequency of phase modulation of each beam is set so as to be different from each other and not in harmony with each other. Thus, the intensity signal of the backscattered light is not at the same frequency as the intensity signal of the P wave, and can be blocked to a very high degree by the synchronous demodulator used in the signal processing electronics. The disadvantage of using independent modulation of backpropagating light waves is that the modulator imperfections are no longer offset by common mode rejection.

FIG. 7 shows an example of the resonance tracking electronics associated with FIG. The CW signal from the photodetector is demodulated at frequency fm _{, 2} . The output of the demodulator is an error signal that crosses zero when the CW light wave is on the resonant frequency of the ring resonator. This error signal is integrated by an integrator. The output of the integrator controls the laser frequency to the resonant frequency of the resonator. The CCW control loop is similar except that the signal from the photodetector is demodulated at a frequency fm _{, 1} where the output of the integrator goes to the voltage-frequency converter, and the output of the converter is Δf. Yes, proportional to rotation speed.

  There is a need to have an RFOG configuration that eliminates rotation sensing errors due to modulator imperfections and backscatter.

  The present invention provides a system and method for performing modulation error correction while maintaining the prevention of intensity-type backscatter errors. An example of this system provides common phase / frequency modulation to the first and second laser beams, first intensity modulation to the first modulation beam, and second intensity modulation to the second modulation beam. . The first and second beams are then sent through the optical resonator. Next, the signal output from the optical resonator is analyzed.

  The analysis demodulates the output signal associated with the first beam based on the frequency of intensity modulation of the first beam and the output associated with the second beam based on the frequency of intensity modulation of the second beam. Demodulating the signal. The demodulated output signal is then demodulated according to the frequency of the common phase / frequency modulation, and the rotational speed is determined based on the demodulated signal.

In one aspect of the invention, the frequency of intensity modulation is unequal and not harmonized, and intensity modulation encodes each light beam with a unique code.
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.

  FIG. 8 illustrates an embodiment of the present invention that eliminates rotation sensing errors due to modulator imperfections and backscatter. RFOG 40 includes light source 50, beam splitters 52-56, photodetectors 60, 62, resonator 68, modulators 72, 74, 76 and associated modulator oscillators 82, 84, 86, frequency shifter 90, and processor 94. Including.

  The common cavity length modulator 76 performs resonance detection and eliminates errors due to modulation imperfections. Cavity length modulation can be performed by a piezoelectric tube wound with a resonator fiber, or a piezoelectric element placed on a resonator mirror. Independent intensity modulation (intensity modulators 72, 74) of the two back-propagating beams is used to encode each light wave with a unique code so that unwanted intensity signals due to backscattering are blocked by signal processing. Can be done. The intended intensity modulation frequencies are generated differently and not in a harmonic relationship. Thus, the frequency of the undesired intensity signal due to backscattering is not the same as the P-wave resonant signal and will therefore be blocked by the signal demodulator included in the processor 94. The intended intensity modulation frequencies are set differently and unharmoniously, and the common modulation is performed by the cavity length modulator 76. The intended intensity modulation (modulators 72, 74) may be a sine wave or a square wave. Examples of intensity modulators 72, 74 include a Mach-Zehnder interferometer, a MEMS-based intensity modulator, or an integrated optical intensity modulator that includes a Pockels cell followed by a polarizer. The intensity modulators 72, 74 can be implemented with any type of common modulation.

FIGS. 9-1 and 9-2 show examples of resonance tracking electronics associated with FIG. Both CW and CCW signals are demodulated at a common modulation frequency fm _{, 3} . The CW signal from the photodetector is modulated at the frequency fm _{, 2} . The output of the demodulator is an error signal that crosses zero when the CW light wave is on the resonant frequency of the ring resonator. This error signal is integrated by an integrator. The output of the integrator controls the laser frequency to the resonant frequency of the resonator. The CCW control loop is similar except that the signal from the photodetector is demodulated at a frequency fm _{, 1} where the output of the integrator goes to the voltage-frequency converter, and the output of the converter is Δf. Yes, proportional to rotation speed.

  The backscatter error interference type can be reduced or eliminated by locking the clockwise light wave on a different resonant frequency than the counterclockwise light wave. Thus, the interference between the backscattered wave and the P wave generates an error signal at a frequency equal to the difference between the two wave frequencies. By locking on different resonance frequencies corresponding to different resonance longitudinal modes, the frequency difference becomes very high, in the megahertz-gigahertz range. Since rotation measurements are typically made at frequencies below 100 Hz, high frequency errors due to interference-type backscattering can be easily prevented by low-pass filtering the gyro rotation signal. FIG. 10 is a teaching of the prior art that uses two lasers to lock onto various resonant frequencies. In this configuration, each laser has the disadvantage that it is modulated at a different frequency for resonance detection and is therefore sensitive to modulation imperfections.

FIGS. 11-1 and 11-2 show examples of resonance tracking electronics associated with FIG. The CW signal from the photodetector is demodulated at frequency fm _{, 2} . The output of the demodulator is an error signal that crosses zero when the CW light wave is on the resonant frequency of the ring resonator. This error signal is integrated by an integrator. The output of the integrator controls the laser 2 frequency to the resonance frequency of the resonator. The CCW control loop is similar except that the signal from the photodetector is demodulated at a frequency fm _{, 1} where the output of the integrator controls the laser 1 frequency to the resonant frequency of the resonator.

  FIG. 12 shows the use of two lasers (slave laser 122 phase locked to master laser 120). This method can provide two independent frequencies to operate in various resonant modes to prevent interference type errors. By phase locking the slave laser 122 to the master laser 120, the frequency modulation of the master laser 120 will also appear on the slave laser 122. This is not a true common frequency modulation, but if a high bandwidth phase locked loop is used, modulation distortion will be mainly seen between the master laser 120 and the slave laser 122. Thus, modulation distortion and backscatter error can be reduced or eliminated for more than one laser configuration when independent intensity modulation is used. One drawback to this configuration is that the undesired intensity modulation of the two lasers may also have a fairly rare component that would not be eliminated by common mode blocking.

FIGS. 13-1 and 13-2 show examples of resonance tracking electronics associated with FIG. CW signal from the photodetector is demodulated at frequency f _m. The output of the demodulator is an error signal that crosses zero when the CW light wave is on the resonant frequency of the ring resonator. This error signal is integrated by an integrator. The output of the integrator controls the laser 2 frequency to the resonance frequency of the resonator. The CCW control loop is similar except that the output of the integrator controls the laser 1 frequency to the resonant frequency of the resonator.

  FIG. 14 shows an example of the present invention used with two laser configurations 180. One disadvantage of this configuration is that the undesirable intensity modulation of the two lasers 182, 184 may also have a fairly rare component that would not be eliminated by common mode blocking. However, other ways can be used to reduce the effects of undesirable intensity modulation, such as using intensity control by using an intensity servo loop.

FIGS. 15-1 and 15-2 show examples of resonance tracking electronics for the system of FIG. Both CW and CCW signals are demodulated at a common modulation frequency fm _{, 3} . The CW signal from the photodetector is demodulated at frequency fm _{, 2} . The output of the demodulator is an error signal that crosses zero when the CW light wave is on the resonant frequency of the ring resonator. This error signal is integrated by an integrator. The output of the integrator controls the laser 2 frequency to the CW resonance frequency of the resonator. The CCW control loop is similar except that the signal from the photodetector is demodulated at a frequency fm _{, 1} where the output of the integrator controls the laser 1 frequency to the CCW resonance frequency of the resonator.

FIG. 16 shows an RFOG configuration 220 using common cavity length modulation, independent intensity modulation, and the use of two lasers 222,224.
FIG. 17 shows a similar configuration 280 but with three lasers 282-286. The advantage of these two configurations is that multiple rotation sensing errors are reduced or eliminated. Common cavity length modulation eliminates errors due to modulator imperfections, independent intensity modulation eliminates intensity type errors due to backscattering, and the use of multiple lasers eliminates interference type errors due to backscattering Provide a means. Further, in the three laser configuration 280, the independent intensity modulators 290, 292 encode each laser beam with a unique code so that the signal processing is performed with the second laser 284 and the third laser 286. It becomes possible to separate the relevant signals, and both lasers send light in the same direction in the resonator and on the same photodetector.

18-1 and 18-2 show examples of resonance tracking electronics for the system shown in FIG. The CW signal from the photodetector is split into two channels. Both channels are demodulated at a common modulation frequency fm _{, 4} . The channel used to control the laser 2 is then demodulated at fm _{, 2} . The channel used to control the laser 3 is demodulated at fm _{, 3} . The CCW signal is demodulated at fm _{, 4} and then at fm _{, 1} . The output of the integrator controls the laser 1 frequency to the resonator CCW resonance frequency.

  FIG. 19 illustrates an example process 300 performed by a processor (eg, processor 94) included in the system shown above. First, at block 304, common phase / frequency modulation is applied to the first and second laser beams. At block 308, a first intensity modulation is applied to the first modulated beam. At block 310, a second intensity modulation is applied to the second modulated beam. At block 312, first and second beams are sent through the optical resonator. In block 316, the signal output from the optical resonator is analyzed (see FIG. 20).

  FIG. 20 shows the process performed in block 316 of FIG. First, at block 324, the output signals associated with the first and second beams are demodulated according to the frequency of the common modulation. Next, at block 326, the signal associated with the first beam is demodulated based on the frequency of intensity modulation of the first beam. At block 328, the output signal associated with the second beam is demodulated based on the frequency of intensity modulation of the second beam. At block 332, the rotational speed is determined based on the demodulated signal. The demodulation steps can be performed in various orders.

The processor may be an analog and / or digital processor.
Embodiments of the invention in which an exclusive right or privilege is claimed are defined as follows.

It is a figure which shows embodiment of a prior art. It is a figure which shows embodiment of a prior art. FIG. 3A is a diagram illustrating an embodiment of the prior art. FIG. 3-2 is a diagram illustrating an embodiment of the prior art. It is a figure which shows embodiment of a prior art. It is a figure which shows embodiment of a prior art. It is a figure which shows embodiment of a prior art. FIG. 7A is a diagram illustrating an embodiment of the prior art. FIG. 7-2 is a diagram illustrating an embodiment of the prior art. It is the schematic which shows embodiment of this invention. FIG. 9A is a schematic diagram illustrating an embodiment of the present invention. FIG. 9-2 is a schematic view showing an embodiment of the present invention. It is a figure which shows embodiment of a prior art. FIG. 11A is a diagram illustrating an embodiment of the prior art. FIG. 11-2 is a diagram illustrating an embodiment of the prior art. It is a figure which shows embodiment of a prior art. FIG. 13A is a diagram illustrating an embodiment of the prior art. FIG. 13-2 is a diagram illustrating an embodiment of the prior art. It is the schematic which shows embodiment of this invention. FIG. 15A is a schematic diagram illustrating an embodiment of the present invention. FIG. 15-2 is a schematic diagram illustrating an embodiment of the present invention. It is the schematic which shows embodiment of this invention. It is the schematic which shows embodiment of this invention. FIG. 18A is a schematic diagram illustrating an embodiment of the present invention. FIG. 18-2 is a schematic view showing an embodiment of the present invention. 2 is a flow diagram illustrating an example of a process performed in accordance with the present invention. 2 is a flow diagram illustrating an example of a process performed in accordance with the present invention.

Claims (3)

At least one laser (50) configured to generate at least two light waves;
An optical fiber loop configured to allow the light wave generated by the at least one laser to move in a clockwise (cw) and counterclockwise (ccw) direction in the optical fiber loop; A resonated resonator (68);
At least two intensity modulators (72, 74) configured to perform intensity modulation of the light wave prior to introduction into the resonator;
Two photodetectors (60, 62) configured to detect a component of the light wave rotating to cw and ccw in the resonator;
A processor (94) in signal communication with the photodetector and configured to block an intensity signal due to backscatter based on the intensity modulation and to determine a rotational speed based on the detected component. A resonator optical fiber gyro (RFOG) comprising:
The resonator further comprises a cavity length modulator configured to reduce errors due to modulation imperfections;
The at least one laser comprises only one laser and a beam splitter configured to split a light wave generated by the laser into two light waves;
The at least two intensity modulators comprise two intensity modulators, one for modulating one of the split light waves and the other for modulating the other of the split light waves. ,
The frequency of the intensity modulation of the light waves is unequal and not harmonious, and the intensity modulator encodes each light wave with a unique code. The at least one laser is
A master laser configured to generate light waves;
A slave laser configured to generate a light wave;
A phase lock loop configured to lock a phase of the light wave generated by the slave laser to the light wave generated by the master laser;
The RFOG of claim 1, wherein the frequency of the intensity modulation is unequal for the light waves and is not in a harmonic relationship, and the intensity modulator encodes each light wave with a unique code.   The at least one laser comprises three lasers, and the at least two intensity modulators comprise three intensity modulators each associated with a respective one of the three lasers, of the intensity modulators Two outputs are selectively provided within the resonator in one of the cw or ccw directions, the resonator further comprising a cavity length modulator configured to reduce errors due to modulation imperfections. 2. The RFOG of claim 1, wherein the frequency of the intensity modulation is unequal with respect to the light wave and is not harmonized, and the intensity modulator encodes each light wave with a unique code.

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