Ring resonator gyroscope

Abstract: A ring resonator gyroscope (10) in which the clockwise and counter-clockwise light beams are frequency shifted by an equal but opposite amount and then have their frequencies varied in antiphase relative to eachother about a mean frequency (f o + f₁ or f o - f₁) so any backscatter signal is at the difference frequency and hence locking at zero rate is avoided.

Description: The present invention relates to a ring resonator gyroscope and relates particularly not exclusively, to a ring resonator gyroscope having a means for reducing the disruptive effect of any back scatter signal. Presently known ring resonator gyroscopes comprise a laser for producing laser light at a first frequency, light splitting means in the form, for example, of integrated optics for splitting said light into first and second light components and a ring resonator in the form of a fibre optic bundle for receiving said first light component in a first direction therearound and the second light component in a second direction therearound. Phase modulators act to apply a positive frequency shift to the first light component and a negative frequency shift of equal magnitude to the second light component in order to maintain both light components on resonance at the same time. Photodetectors act to detect the first and second light components after passing around the resonator and act to detect changes in the interference patterns which are then electronically interpreted to indicate the direction and inertial rate of rotation of the gyroscope about the gyro's sensitive axis. One of the problems associated with the above mentioned arrangement is its susceptibility to backscatter effects which result in inaccurate rate readings. EP-A-0405821 also discloses a ring resonator gyroscope basically similar to the above described known ring resonator gyroscope with the exception that sine wave modulation to the two phase modulators is used at different frequencies to give frequency shifting using sideband locking. It is an object of the present invention to provide a ring resonator gyroscope which reduces and possibly eliminates the problems of backscatter effects on the accuracy of rate readings and avoids lockon at zero rate. Accordingly, the present invention provides a ring resonator gyroscope including a laser for producing laser light at a first frequency, light splitting means for splitting said light into first and second light components, a ring resonator for receiving said first light component in a first direction therearound and for receiving said second light component in a second direction therearound, frequency shifting means for applying a positive frequency shift to said first light component and a negative frequency shift of equal magnitude to said second light component such that both light components are on resonance at the same time, resonance locking means for maintaining said first light component on resonance, and beam detecting means for detecting said first and second light components after passing around the resonator, including frequency varying means for varying the frequency of the shifted first and second light components at a frequency (W m ) in antiphase to each other about a mean reference frequency (f₀ + f₁ or f₀ - f₁) to produce a dither, with the frequency shifting means utilising serrodyne phase modulation to shift the frequency. It will be appreciated that there is a varying difference frequency between the light in the two directions around the resonator. This means that any backscatter signal is at this difference frequency which is large compared to the 10KHz used for demodulation. The gyro servos will therefore not be disrupted by the effects of backscatter so there will be no locking at zero rate and the scale factor of the gyro will be linear. Preferably, the frequency varying means is operable to vary the frequency of the shifted first and second light components sinuisoidally about said mean reference frequency. Advantageously, the same magnitude of frequency shift is applied to the first and second light components thereby to benefit from common mode rejection between the path length loop and the frequency loop. Advantageously, the gyroscope is provided with error signal generating means for monitoring the frequency variation of said first or second light components at said photodetector and for generating an error signal for driving servos to lock both light components to line centre. Conveniently, the gyroscope includes path length changing means for changing the path length of the ring resonator so as to facilitate locking onto the resonance. Alternatively, the gyroscope may include laser frequency varying means for varying the frequency of the laser light so as to facilitate locking onto resonance. Preferably, the frequency of the laser-light as produced by the laser is substantially 1MHz, thereby to ensure any spurious sidebands from an imperfect Serrodyne are well clear of the resonance line shape. Advantageously, the frequency of the shifted first and second light components is varied at a frequency of substantially 10KHz. In one embodiment the gyroscope includes phase sensitive detector means for detecting the phase of the light at one or other of said beam detector means and for producing an output for adjusting a voltage on a light intensity varying means so as to vary the relative intensity of light in said first and second directions. The present invention will now be more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic of a ring resonator gyroscope according to the present invention, Figure 2 is a graph of the intensity response at a photodetector as either the frequency of the light or the length of the fibre is changed, Figure 3 illustrates a perfect Serrodyne signal as used for frequency shifting, Figure 4 illustrates the resonance peaks for frequency shifted clockwise and counter-clockwise beams together with associated Serrodyne signals. Figure 5 illustrates an imperfect Serrodyne signal, Figure 6 illustrates the response of the resonator with sidebands, Figure 7 illustrates the output of the photodetector at line centre, Figure 8 illustrates the output of the photodetector with positive offset from line centre, Figure 9 illustrates the output of the photodetector with negative offset from line centre, and Figure 10 is a schematic diagram of the electronics required to drive the optics illustrated in Figure 1. Referring now to the drawings in general but particularly to Figure 1, a ring resonator gyroscope of the invention comprises a narrow linewidth laser 12 from which laser light 14 is launched into fibre optics 16 and then enters an integrated optics circuit 18. In the integrated optics circuit 18 the light is split into two by splitting means 20 so as to produce first and second light components or beams 22, 24 for directing clockwise and anti-clockwise around a ring resonator 26. Phase modulators 28 and 30 frequency shift the first and second light components respectively. Photodetectors 32, 34 act to detect the clockwise and anti-clockwise light components 22, 24 after they have been around the resonator 26. The signal from photodetector 32 is used to drive a path length control loop 36. The function of this is to keep the light locked onto resonance and is achieved by adjusting the length of the fibre of the resonator 26 by means of a piezo 38. Alternatively, wavelength adjusting means shown schematically at 40 may be employed for adjusting the wavelength of the laser. Figure 2 illustrates the intensity response at the photodetector 32 as either the frequency of the light or the length of the fibre is changed. On resonance there is a sharp resonance dip 42 which has the same turning position in the two directions when the gyroscope is at rest. When there is rotation the resonances in the clockwise and the counter-clockwise directions are split by the Sagnac effect. This frequency splitting is proportional to the rotation rate. A frequency shift is introduced by the two phase modulators 28, 30 so that both directions are on resonance at the same time. The frequency shift is produced by a Serrodyne phase modulation, a perfect form of which is shown in Figure 3. The phase varies from 0 to 2π in a time t and is then reset to zero instantaneously. The frequency shift induced is then given by 1/t Hz. Means (not shown) are provided for varying the duration of the ramp in order to vary the frequency shift. Figure 4 illustrates the case where a frequency shift of 900KHz is applied to the clockwise light component and 1100KHz is applied to the counter-clockwise light component. At zero rate this shows that in the clockwise direction the left hand side of the resonance is scanned, while in the counter-clockwise direction the right hand side of the resonance is scanned. Also shown in Figure 4 are the two corresponding Serrodyne waveforms 46, 48. The frequency of the two Serrodyne waveforms is given by: fcw = f₀ + f₁ Sin W m t fccw = f₀- f₁ Sin W m t Where fcw and fccw are the frequencies of the clockwise and counter-clockwise Serrodyne beams. The value of f₁ is set to about 100KHz so that the full linewidth of the resonator is scanned at an angular frequency W m . A typical value for W m is 10KHz and this is chosen for convenience of demodulation. In the example shown in Figure 4, the value of f₀ is 1MHz. This value is chosen so that any spurious sidebands 50 from an imperfect Serrodyne will be well clear of the resonance line shape as shown in Figure 7 and not affect the servos. Figure 5 illustrates an imperfect Serrodyne waveform 52 when the reset takes a finite time and there is a maximum phase not equal to 2π radians. The effect of this is to generate the sidebands 50 as illustrated in Figure 6, with the sideband spacing being equal to 1MHz. Figure 6 shows that these sidebands are many linewidths away from line centre, as the full half width (54) of the resonance is about 100KHz, and hence the sidebands with f₀ set at 1MHz are many half widths away from the main resonance and hence spurious signals are avoided. Referring now to Figure 4, at a later time the clockwise beam will have a frequency shift of 1100KHz, so the right hand side 56 of the resonance will be sampled and the counter-clockwise beam will have a frequency shift of 900 KHz so the left hand side of the resonance will be sampled. Figure 7 illustrates the output of the photodetector as a function of time as the Serrodyne frequency f CW goes up and down. On line centre the photodetector output 58 is the frequency doubled version of the input Serrodyne frequency. Off line centre with a positive offset the photodetector output is shown in Figure 8, where is a signal of W m on the photodetector out of phase with the frequency variation, while Figure 9 illustrates the case with a negative offset when the photodetector has an in-phase signal W m . The signal at W m on the photodetector can thus act as an error signal to drive a servo to lock to line centre as is well known. For the resonator gyro two servos are employed. One servo 60 locks the one direction of light onto resonance by changing the fibre length, or the laser frequency as discussed above and this is called the path length servo. The second servo 62 adjusts the relative frequency of the two Serrodyne modulations to bring both directions into resonance at the same time. This is shown in Figure 10 which illustrates schematically the electronics required to drive the optics illustrated in Figure 1. Referring now particularly to Figure 10, the signal from the two photodetectors 32, 34 is amplified at 64 and then passed through a low pass filter 66 to remove any residual signal at the frequency shifting of 1MHz. A differential amplifier 68 is used in the path length loop. Demodulation at 20KHz on the output 70 is used to control a common mode rejection loop. This ensures that the signals at 20KHz from the two photodetectors are matched and equalises the gains in the two signal channels, so that the difference in signal at 10KHz is only dependent on the difference in tuning positions between the directions, which is the correct signal to drive the frequency servo. The output of the phase sensitive detector 70 after low pass filtering is used to adjust a voltage on a coupler on the integrated optics 18 which alters the relative intensity of light in the two directions. The output of the differential amplifier is demodulated also at 10KHz, after low pass filtering (shown by the loop order control 76) the signal drives a high voltage amplifier (HVA) 78 and this in turn drives the piezo 38 to control the path length so that the mean path is on resonance. For the frequency loop a sum amplifier 80 is used, and a notch filter 82 operating at 20KHz, but clocked at 10KHz, removes the residual 20KHz signal. The signal at 10KHz is then demodulated by the phase sensitive detector 84 and after low pass filtering at 86 the signal feeds to the two voltage controlled oscillators 88, 90 controlling phase modulators 28, 30 respectively. The 10KHz signal is summed into the error signal and this composite signal drives the two voltage controlled oscillators in antiphase. Therefore if the error signal is zero the two voltage controlled oscillators will be running at the same frequency, with an antiphase frequency deviation at 10KHz. A sum frequency lock is also applied to the outputs of the two voltage controlled oscillators by substracting the output of the voltage controlled oscillator 88 from 2MHz and a phase locked loop 92 compares this signal with the output of voltage controlled oscillator 90. This signal 94 is filtered at 96 and fed to the two oscillators acting in phase, so that this error signal alters both frequencies up and down together. It will be appreciated from the above that there is a sinusoidally varying difference frequency between the light in the two directions going around the resonator 26. This means that any backscatter signal is at this difference frequency which is large compared to the 10KHz used for demodulation. The servos will therefore not be disrupted by the effects of backscatter so there will be no locking at zero rate and the scale factor of the gyro will be linear. The same frequency is applied to both phase modulators in the two directions which means that the benefits of common mode rejection can be achieved between the path length loop and the frequency loop. The gyro quality is dependent upon the operation of the frequency loop, so it is very important to insulate this loop from the effects of path length offsets. This scheme should allow a significant path length desensitisation for the frequency loop. Other schemes were a difference frequency is applied for modulation and demodulation in the two directions, have no common mode rejection so that the frequency loop is exposed to any path length errors. The sum amplifier and common mode rejection circuit in Figure 10 should allow a factor of 100 in common mode rejection. If the frequency deviation in the two directions is in phase, then the light would travel around the resonator with the same frequency and thereby be very susceptible to backscatter effects.

Claims: A ring resonator gyroscope including a laser (12) for producing laser light (14) at a first frequency, light splitting means (20) for splitting said light into first and second light components (22, 24), a ring resonator (26) for receiving said first light component in a first direction therearound and for receiving said second light component in a second direction therearound, frequency shifting means (28, 30) for applying a positive frequency shift to said first light component (22) and a negative frequency shift of equal magnitude to said second light component (24) such that both light components are on resonance at the same time, resonance locking means (36) for maintaining said light components on resonance, and beam detecting means (32, 34) for detecting said first and second light components after passing around the resonator, characterised by including frequency varying means for varying the frequency of the shifted first and second light components at a frequency (W m ) in antiphase to each other about a mean reference frequency (f₀ + f₁ or f₀ -f₁) to produce a dither, with the frequency shifting means (28, 30) utilising serrodyne phase modulation to shift the frequency. A ring resonator gyroscope as claimed in claim 1, in which the frequency varying means is operable to vary the frequency of the shifted first and second light components sinusoidally about said mean reference frequency. A ring resonator gyroscope as claimed in claim 1 or claim 2 in which the same magnitude of frequency shift is applied to the first and second light components thereby to facilitate common mode rejection. A ring resonator gyroscope as claimed in any one of claims 1 to 3, including error signal generating means for monitoring the frequency variation of said first or second light components at said photodetector and for generating an error signal for driving servos to lock both light components to line centre. A ring resonator gyroscope as claimed in any one of claims 1 to 4, including path length changing means (38) for changing the path length of the ring resonator (26) so as to facilitate locking onto the resonance. A ring resonator gyroscope as claimed in any one of the claims 1 to 4, including laser frequency varying means for varying the frequency of the laser light so as to facilitate locking onto resonance. A ring resonator gyroscope as claimed in any one of claims 1 to 6, in which the frequency of the laser light (14) as produced by the laser (12) is substantially 1 MH z . A ring resonator gyroscope as claimed in any one of claims 1 to 7, in which the frequency of the shifted first and second light components is varied at a frequency of substantially 10kHz. A ring resonator gyroscope as claimed in any one of claims 1 to 8, including phase sensitive detector means (70, 84) for detecting the phase of the light at one or other of said beam detector means (32, 34) and for producing an output for adjusting a voltage on a light intensity varying means so as to vary the relative intensity of light in said first and second directions.

Inventors: MALVERN ALAN RICHARD BRITISH A, MALVERN, ALAN RICHARD, BRITISH AEROSPACE SYSTEMS
Applicants: BRITISH AEROSPACE, BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1992
Main classification: G01C19
IPC codes: G01C19