RING RESONATOR GYRO

Abstract: In a ring resonator gyro the CW and CCW beams are modulated to provide each with a spectrum including a carrier and an upper and a lower side band. The upper side band of one beam is kept at a resonance by a path length control loop 24 and the upper side band of the other beam is kept at a resonance by a rate control loop 40.

Description: This invention relates to a ring resonator gyroscope. In typical examples of such gyroscopes, clockwise (CW) and counterclockwise (CCW) beams are passed in opposite directions around a resonator loop and each direction is kept at resonance by modifying the effective optical length around the loop either by directly changing the length of the resonator loop, e.g. by a piezo-electric transducer, or by modifying the frequency of the associated beam or by varying the source frequency. When both directions are at resonance, the rate of rotation applied to the gyro may be determined by detecting the frequency difference between the resonant CW and CCW beams and dividing the difference by a scale factor. In existing proposals for ring resonator gyroscopes the frequencies of the CW and CCW beams have been shifted by acoustic optic deflectors or serrodyne phase modulation so that the CW and CCW beams are at different frequencies. U.S. Patent US-A-4815851 discloses a passive ring resonator gyroscope in which two Bragg cells are used for frequency shifting in a technique which employs locking of the carrier or line centre of the carrier signal. According to this invention, there is provided a ring resonator gyro, comprising:- a resonator loop means defining a clockwise (CW) path and a counterclockwise (CCW) path, each path having respective CW and CCW frequency responses with resonances, means for injecting into said loop means clockwise (CW) and counterclockwise (CCW) beams, means for modulating said CW and CCW beams to provide each with a frequency spectrum comprising a carrier and two side bands, wherein said gyro further comprises control means for controlling the gyro to cause a selected one of said CW side bands and a selected one of said CCW side bands to be kept substantially at respective CW and CCW resonances. The invention may be performed in various ways and an embodiment thereof will now be described by way of example, reference being made to the accompanying drawings, in which:- Figure 1 is a schematic diagram of the optical arrangement of a ring resonator gyro in accordance with the invention; Figure 2 is a graph illustrating the filter functions of the clockwise direction and the counter clockwise direction when rotation is applied to the ring resonator gyro of Figure 1, and Figure 3 is a schematic diagram of one of the control systems for controlling the path length and setting the frequency in the ring resonator gyro of Figure 1. Referring to Figure 1, the optical components of the example of ring resonator gyro comprise a laser source 10, an integrated optics chip 12, a coupler 14, a resonator ring 16 of fibre wound on a piezo-electric ring 18 and photodetectors 20 and 22. Light from the laser source 10 is fed by a fibre to the integrated optics chip 12 where the following functions are performed: beam splitting and recombining; beam polarisation and phase modulation of the CW and CCW beams by phase modulators PM1 and PM2 respectively. Light entering the integrated optics chip 12 is polarised and then split at 24 into two equal parts. The parts then pass through the phase modulators PM1 and PM2 and are then injected into the resonator ring 16 through the coupler 14. The filter functions of the resonator ring 16 in the CW and CCW directions, when rotation is applied, are shown in Figure 2. It will be seen that the CW and CCW "combs" are shifted with respect to one another by an amount proportional to rotation rate. In operation, the control system shown in Figure 3 locks both the CW and CCW directions onto resonance and reads out the difference in frequency between the two resonance combs. On leaving the resonator ring 16, the light is split at 26 on the return path through the integrated optics chip 12 and the CW and CCW portions are directed to the two photodetectors 20 and 22 respectively. The photodetectors in this example have a broad bandwidth of up to 20MHz with a reasonably low noise figure. The light intensity on the detector may be typically between 2 and 10 µW. The resonator coil is typically 10 metres long and so the free spectral range or mode spacing is about 20MHz. Referring now to Figures 2 and 3, a frequency of 5MHz (i.e. about one quarter of the free spectral range) is applied to the phase modulator PM1 with a low amplitude so that phase (or frequency) modulation occurs, giving rise to a spectrum of a carrier (at the operating frequency of the laser source) with two sidebands, the upper sideband (USB) and the lower sideband (LSB), which are 180° out of phase. In this example, a path length control loop 24 is used to alter the fibre length using a voltage onto the piezo-electric ring 18 to lock the USB of the CCW beam to a resonance. In the path length control loop, a 5MHz signal from a source 28 is frequency modulated at 30 with a deviation of +/- 100KHz at a frequency 10KHz and then applied to the phase modulator PM1 to apply a frequency modulation to the CCW beam so that it has a spectrum of a carrier with a USB and a LSB. The 10KHz frequency modulation has the effect of scanning the laser light (i.e. the USB) across the resonance.When the CCW beam leaves the resonator ring 16 it passes to the photodetector 22 which provides a signal which is fed to a mixer 32 which is supplied with the fixed 5MHz and which incorporates a low pass filter to obtain the information at 10KHz. The signal is amplitude modulated at 10KHz on the 5MHz signal. After leaving the mixer 32 the signal is split with one part passing to a phase sensitive detector 34 which demodulates the signal. The demodulated signal is integrated at 36 and amplified at 38 and then supplied to the piezo-electric ring 18. A rate control loop 40 operates to lock the USB of the CW beam to a resonance. In the rate control loop, a (5+δ)MHz signal from a source 42 is frequency modulated at 44 with a deviation of +/- 100KHz at a frequency of 10KHz and then applied to the phase modulator PM2 to apply a frequency modulation for the CW beam to provide a carrier with USB and LSB. When the CW beam leaves the resonator ring 16 it passes to the photodetector 20 which supplies a signal to a mixer 44 also supplied with the (5+δ) MHz signal and having a low pass filter to obtain the 10KHz information. The signal from the mixer 44 is supplied to a differential amplifier 46 with the signal from the other mixer 32 and the difference in light intensity between the two directions is then demodulated by a phase sensitive detector 48 and the demodulated signal integrated at 50 and applied to a voltage controlled oscillator 52 to set the value of δ. The output (i.e. rate) signal may be obtained by mixing the 5MHz and (5 + δ) MHz signals, or by using a difference counter and in this way a digital output proportional to rate is obtained, i.e. δ is the output frequency proportional to rate. For both the path length control loop 24 and the rate control loop 40, when the carrier, the USB and LSB are away from a resonance there is a large signal at 5MHz. When the carrier is on resonance, there is no signal at 10 MHz because the carrier is notched out by the filter function of the resonator. There will also be a phase reversal around the resonance which would, if required, enable locking to be achieved on the carrier by phase-sensitive detection techniques at 5MHz. In the described embodiment, however, locking is achieved on one side band (USB) where there is a phase reversal of the 5MHz signal. Referring specifically to Figure 2, the frequency of 5MHz is a fixed frequency and ensures that, when the USB is at a resonance, the LSB is midway between two resonances. In this way there is no signal variation from the LSB as this is on a flat part of the curve. In order to lock onto a sideband, the 5MHz CCW (and 5+δ MHz CW) are modulated with a deviation of 100KHz and a frequency of 10KHz, as described above. Both directions have the same frequency modulation. The USB and the LSB will sweep in opposite directions due to the 10KHz modulations and this is used to ensure that the USB is locked, rather than the LSB, by looking at the phase of the servo. In the system illustrated in Figures 1 and 3, the differencing of the outputs from the two detectors gives some common mode rejection since a path length error would otherwise appear on the rate loop as a rate error. Further improvement may be obtained using the scheme described and illustrated in our published European Application EP-A-363027 to which reference is directed in which, when both paths are at resonance, the relative strengths of the signals incident on the photodetectors at twice the 10KHz modulation frequency are compared and used to adjust an automatic gain control loop to null the gain differences between the photodetectors and associated preamplifiers. Whilst in the above embodiment the USBs of the CW and CCW beams have been locked on to resonance it will of course be understood that for either or both of the CW and CCW beams the LSB may be locked on to resonance instead.

Claims: A ring resonator gyro, comprising:- a resonator loop means (16) defining a clockwise (CW) path and a counterclockwise (CCW) path, each path having respective CW and CCW frequency responses with resonances, means (12) for injecting into said loop means (16) clockwise (CW) and counterclockwise (CCW) beams, means (28,30, PM1; 42,44, PM2) for modulating said CW and CCW beams to provide each with a frequency spectrum comprising a carrier and two side bands (USB or LSB), characterised in that said gyro further comprises control means (24;40) for controlling the gyro to cause a selected one of said CW side bands (eg. USB) and a selected one of said CCW side bands (eg. USB) to be kept substantially at respective CW and CCW resonances. A ring resonator gyro according to Claim 1, wherein said control means (24;40) is operable to control one of said CW and said CCW beams to shift the frequency of the associated side band (USB) relative to the carrier to maintain said side band (USB) substantially at resonance, and further to cause a shift in the frequency response of the other of said CW and CCW paths to maintain the side band associated with the other of said CW and CCW beams at resonance. A ring resonator gyro according to Claim 2, wherein said control means includes means (18) for modifying the optical path length of said resonator loop means (16) thereby to cause said shift in the frequency response of the other of said CW and CCW beams. A ring resonator gyro according to any preceding claim, wherein said control means (24;40) applies a fixed frequency modulating signal to one of said CW and CCW beams and a variable frequency modulating signal to the other of said CW and CCW beams. A ring resonator according to any preceding claim, wherein said side band is the upper side band. A ring resonator gyro according to any preceding claim, wherein the frequency spacing of the side bands (USB, LSB) from the respective carrier is approximately one quarter the free spectral range of the resonator loop means (16). A ring resonator gyro according to any preceding claim, wherein said means for modulating (28,30, PM1; 42,44,PM2) includes a servo loop including first detector means (20) and first amplifier means for receiving a portion of the CW beam and providing a CW signal, a second servo loop having second detector means (22) and second amplifier means for receiving a portion of the CCW beam and providing a CCW signal, and gain control means responsive to the intensities of the CW and CCW beams detected at resonance for controlling said first and second amplifier means substantially to null the gain differences between the first and second servo loops. A ring resonator gyro according to Claim 1, wherein said control means includes a first control loop (24) responsive to changes in optical path length of the resonator and a second control loop (40) responsive to changes in applied rate, each control loop maintaining a respective one of the CW and CCW side bands at said respective resonances. A ring resonator gyro according to Claim 9, including means (28) for modulating a common carrier beam with a fixed frequency to provide one of said CW and CCW beams and means (42) for modulating said carrier beam with an adjustable frequency to provide the other of said CW and CCW beams.

Inventors: MALVERN, ALAN RICHARD,  PB 514, MALVERN, ALAN RICHARD, PB 514
Applicants: BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: G01C19
IPC codes: G01C19, H01S3