GB2100855A

GB2100855A - Sideband modulating/ demodulating fibre optic gyroscope

Info

Publication number: GB2100855A
Application number: GB8118808A
Authority: GB
Inventors: John Stuart Heeks
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-06-18
Filing date: 1981-06-18
Publication date: 1983-01-06
Also published as: GB2100855B; DE3220389A1

Abstract

A fibre optic interferometer gyroscope including a coil (1) of optical fibre the two ends of which are coupled to an optical Y-junction (4) having electro-optical properties, means (7) for irradiating the coil to produce counter propagating optical signals therein, means for detecting the combined output signals from the Y-junction, means for impressing (9, 10) a first triangular waveform modulation having a predetermined amplitude in opposite senses on the two optical output signals from the coil in the Y-junction means for simultaneously impressing (10, 9) a second square waveform modulation synchronous with the first modulation likewise on the two optical output signals, means for adjusting the amplitude of the second modulation to maintain zero phase movement of the interference pattern and zero distortion of the continuous sinewave amplitude modulation of the combined optical signals resulting from the first triangular waveform modulation, and timing control means for the optical input and output signals and the modulation means. <IMAGE>

Description

SPECIFICATION Sideband modulating/demodulating fibre optic gyroscope This invention relates to fibre optic interferometer gyroscopes utilizing the Sagnac effect and in particular to a method and means for achieving sideband modulation and demodulation of the counter propagating optical signals in the fibre optic loop.

The use of a multi-turn coil of optical fibre in which, by means of beam splitters and combiners, light from a single coherent source, e.g. a laser, is propagated in both directions in the coil simultaneously to provide rotation sensitive output signals at a photodetector is known. Such an arrangement is described in, for example "Sensitivity Analysis of the Sagnac-Effect Optical Fiber Ring Interferometer" by Shih-Chun Lin and Thomas G. Giallorenzi in Applied Optics, Vol. 18, No. 6, 15 March, 1 979. When the output signals are combined interference fringe patterns are developed which, in a stationary system, are of infinite width, uniform and constant. If the system is rotated about the coil axis fringe excursions take place, and by suitable processing, rotational rate information can be extracted.

Although the fibre optic gyro has many extremely attractive operational features many technical difficulties need to be solved before the full potential of the instrument can be realised.

There are problems associated with both the performance of the components and, closely related, the signal processing architecture.

According to the present invention there is provided a method of demodulating optical signals in a fibre optic interferometer gyroscope wherein a first triangular waveform modulation having a predetermined amplitude is impressed in opposite senses on the two counter-propagating optical signals being outputted from the optical fibre coil and a second square waveform modulation synchronous with the first modulation is simultaneously likewise impressed on the two optical signals, the two double modulated optical signals being combined to form an interference pattern and the amplitude of the second square wave modulation is adjusted to maintain zero phase movement of the interference pattern and zero distortion of the continuous sinewave amplitude modulation of the combined optical output resulting from the first triangular waveform modulation.

The invention also provides a fibre optic interferometer gyroscope including a coil of optical fibre and two ends of which are coupled to an optical Y-junction the optical properties of the branches of which are variable by applied electrical fields, means for feeding the coil with an optical signal to produce counter propagating optical signals in the coil, means for detecting the combined output signals from the Junction, means for impressing a first triangular waveform modulation having a predetermined amplitude in opposite senses on the two optical output signals from the coil in the Junction, means for simultaneously impressing a second square waveform modulation synchronous with the first modulation likewise on the two optical output signals in the Junction, means for adjusting the amplitude of the second modulation to maintain zero phase movement of the interference pattern formed by the combined modulated optical signals and zero distortion of the continuous sinewave amplitude modulation of the combined optical signals resulting from the first triangular waveform modulation and timing control means for the optical input and output signals and the modulation means.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 illustrates the basic principle of the invention, Fig. 2 illustrates schematically a fibre optic gyro arrangement according to the invention, Fig. 3 illustrates schematically an integrated optics subsystem arrangement for a two-coil version of a fibre optic gyro, and Fig. 4 illustrates schematically a single coil fibre optic gyro having separate input and output means coupled to the coil by three port optical circulator means.

In the arrangement of Fig. 1 a pulse of light, which has been previously launched into both ends of a fibre optic sensor loop 1, propagates in both directions round the loop simultaneously and reaches the ends 2, 3 of the loop. The counter propagating light waves in the loop are emptied at time t = 0 through an optical Y-junction device 4.

This device is basically a pair of optical waveguides 5, 6 which converge into a single optical waveguide 7 and are formed in a substrate 8 of electro-optically responsive material. The two branches 5, 6 are coupled to respective ends 2,3 of the fibre loop. The optical waveguides may be formed by, for example, indiffusion of titanium into lithium niobate. A first electrode pattern 9 is deposited on the surface of the substrate 8 and extends close to and parallel with the outer boundaries of the branches 5, 6. A second electrode 10 is formed on the surface of the substrate 8 between the branches 5, 6. Consider first the effect of a triangular waveform electrical modulation applied between electrodes 9 and 10.

The triangular modulation impresses an amplitude modulation on the combined optical output via the rate of change of phase which the electro-optic effect induced in opposite senses in the Y-junction branches. Under the condition of zero Sagnac phase effect and an integral number of differential half wavelength phase shift impressed via the modulator, the output waves represent a continuous sinewave amplitude modulation (of 100% modulation index) on the resultant optical carrier. In effect, harmonic multiplication occurs between the modulating triangular wave of frequency 1 f= t' and the photodiode output envelope, the harmonic factor being 2n, where n is the number of halfwavelengths of impressed phase shift at the maximum modulation excursion.If now a further, synchronous square wave modulation is applied via electrodes 10 and 9 the phase of the amplitude modulated envelope will vary without the introduction of phase discontinuities at the singular points of the applied modulation envelope. The phase movement is linearly related to the amplitude of the square wave.

The amplitude of the triangular waveform is chosen to give the integral half wavelength phase condition and the square wave amplitude is then used to hold the output of the interferometer (formed by the electro-optic device) in its zero phase, zero distortion sinewave modulation condition. The amplitude of the square wave necessary to achieve this balance represents a measure of the rotation rate of the fibre optic loop about the loop axis. The reference phase of the phase locked loop is taken from that of the triangular waveform modulation and the control is fed back via the square wave amplitude.

In practice, perfect triangular and square modulating waveforms are not realisable but due to discrimination against unwanted harmonics in the tuned amplifier filter 21, the performance of the system is relatively insensitive to degradation.

in the modulating waveforms. Thus, considering the triangular waveform along, if this degenerates to the limiting condition of a sinewave, the filtered output envelope of the photodetector will still be sinusoidal if the adjacent sidebands of the fundamental waveforms are rejected by the tuned amplifier. For a given light intensity through the Mach Zehnder, however, the system will now be less efficient due to the power lost in the spurious harmonic outputs. Taking the ultimate limit of both the triangular and square modulating waveforms degenerating into sinewaves in quadrature phase, and it is readily apparent that the phase control will only be linear for a small deviation, i.e. for small amplitude of the phase control sinewave.

The complete fibre optic gyro is schematically illustrated in Fig. 2. Light from a laser 11 is applied via a lens 14, a half silvered mirror 1 5, a second lens 1 6 and the electro-optic interferometer device 4 to the fibre optic loop 1. After time T laser bias switch 12 is opened and the triangular waveform generator 1 7 is energised to apply the triangular modulation to the interferometer.

Simultaneously the square wave generator 1 8 is energised. The combined light output from the interferometer device 4 is collected via lens 16, passes through the half silvered mirror 1 5 and is focussed by a third lens 1 9 onto a photodetector 20. The output of photodetector 20 is amplified in a tuned amplifier 21 and applied to a phase comparator 22 which is enabled by the timing control circuit 1 3. The phase difference signal output of comparator 22 is amplified and applied as an amplitude control signal to the square waveform generator 1 8 to bring the output of the interferometer back to the zero phase condition as explained above.

Clearly, as described so far, the sensor can only work for half the time, as the loop needs to be filled from the light source with no applied modulation. Hence the source is switched on for a time T equal to the transit time in the loop, and then switched off for a further period of time T to allow the loop to empty via the electro-optic interferometer. This may or may not be significant according to whether or not the sampled system satisfies the Nyquist criterion for the maximum rate of change of rotation rates to be considered.

To overcome this problem it is possible to use a more complex switched loop system as shown in Fig. 3. Separate input and output fibres 30, 31 are cross-coupled via a switched directional coupler 32 which can be formed as optical waveguides in the same substrate as the interferometers. In this arrangement two side by side interferometers 33, 34 are formed on the substrate and they are coupled to two identical coaxial optical fibre loops 35, 36. The directional coupler and the interferometers are switched so that whilst one coil is being filled via fibre 30 with light from a continuously energised laser (not shown) for the period T the first coil is emptied and the second coil is filled. Thus by using two alternating coil functions a substantially continuous operation of the gyro can be obtained.

An alternative arrangement is shown in Fig. 4, in which a single coil 40 is continuously and simultaneously filled and emptied from a single light source (not shown) and itscontinuous outputs are taken to a single interferometer 41. In this case optical circulators 42, 43, are used to isolate the input from the output.

Claims

1. A method of demodulating optical signals in a fibre optic interferometer gyroscope wherein a first triangular waveform modulation having a predetermined amplitude is impressed in opposite senses on the two counter-propagating optical signals being outputted from the optical fibre coil and a second square waveform modulation synchronous with the first modulation is simultaneously likewise impressed on the two optical signals, the two doubly modulated optical signals being combined to form an interference pattern and the amplitude of the second square wave modulation is adjusted to maintain substantially zero phase movement of the interference pattern and substantially zero distortion of the continuous sinewave amplitude modulation ofthe combined optical output resulting from the first triangular waveform modulation.

2. The method according to claim 1 wherein the two ends of the coil are coupled to an optical splitter/combiner which is in turn coupled to a source of light and to a photodetector alternately for equal periods of time T where z is the transit time of optical signals in the fibre optic coil.

3. The method according to claim 2 wherein two identical coils with associated splitter/combiners are employed the input/outputs of which are respectively coupled alternately to a light source and a photodetector and vice versa.

4. The method according to claim 1 wherein the two ends of the fibre optic coil are each coupled to a respective optical circulator whereby light from a source may be continuously entered into each end of the coil and continuous light from the two ends of the coil may be applied to an optical combiner after the step of simultaneously doubly modulating the light outputs.

5. A method of demodulating optical signals in a fibre optic interferometer gyroscope substantially as hereinbefore described with reference to Fig. 1 and 2 or Fig. 3 or Fig. 4 of the accompanying drawings.

6. A fibre optic interferometer gyroscope including a coil of optical fibre the two ends of which are coupled to an optical Y-junction the optical properties of the branches of which are variable by the applied electrical fields, means for feeding the coil with an optical signal to produce counter propagating optical signals in the coil, means for detecting the combined output signals from the Junction, means for impressing a first triangular waveform modulation having a predetermined amplitude to vary the electrical fields in opposite senses on the Y-junction branches carrying the two optical output signals from the coil, means for simultaneously impressing a second square waveform modulation synchronous with the first modulation likewise on the branches of the Junction, means for adjusting the amplitude of the second modulation to maintain substantially zero phase movement of the interference pattern formed by the combined modulated optical signals and substantially zero distortion of the continuous sinewave amplitude modulation of the combined optical signals resulting from the first triangular waveform modulation, and timing control means for the optical input and output signals and the modulation means.

7. A gyroscope according to claim 6 including means for feeding both ends of the coil via the Y-junction with light signals which are switched on for periods equal to the transit time of light in the coil alternating with equal periods of time when no light is fed to the coil, and means for detecting the modulated light received from the two ends of the coil via the Junction.

8. A gyroscope according to claim 6 including two identical coaxial fibre optic coils each connected to the branches of a respective electro-optic Junction, means for feeding one of the Y-junctions with light from a source, means for receiving light from the other Junction, switchable directional coupling means for cross-coupling the feeding and receiving means and a timing control means for energising said directional coupling means periodically for periods of time equal to the light propagation time in a coil alternating with equal periods of time when the directional coupling means is not energised.

9. A gyroscope according to claim 6 wherein the two ends of the coil are coupled to the branches of the Y-junction by respective optical circulators, the gyroscope including means for feeding the two ends of the coil with light via the two optical circulators.

10. A gyroscope according to any one of claims 6 to 9 wherein the Y-junction comprises a pair of optical waveguides which converge into a single optical waveguide, said waveguides being formed within a substrate of electro-optically responsive material the surface of which is provided with electrode patterns extending close to and parallel with the waveguide boundaries whereby electrical signals may be applied to effect modulation of light propagated in the waveguides.

1 A gyroscope according to claim 8 wherein the Y-junctions and the directional coupling means comprise optical waveguides formed within a single substrate of electro-optically responsive material the surface of which is provided with electrode patterns extending close to and parallel with the waveguide boundaries whereby electrical signals applied to the electrodes effect modulation of the light in those waveguides constructed to form Y-junctions and crosscoupling of the light in those waveguides constructed to form a directional coupling means.

1 2. A gyroscope according to claim 10 or 11 wherein the waveguides are formed by indiffusion of titanium into a lithium niobate substrate.

13. A fibre optic gyroscope substantially as described with reference to Fig. 1 or Figs. 1 or 2 or Fig. 3 or Fig. 4 of the accompanying drawings.