GB2108652A

GB2108652A - Fibre-optic interferometer gyroscope

Info

Publication number: GB2108652A
Application number: GB8132314A
Authority: GB
Inventors: John Stuart Heeks
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-10-27
Filing date: 1981-10-27
Publication date: 1983-05-18
Also published as: DE3239068A1; GB2108652B

Abstract

A fibre-optic interferometer gyroscope includes means 18, 19 for applying a first phase modulation in the optical path of the loop 10 for alternating periods tau , where tau is the loop transit time, and means 16, 17 for synchronously applying a second phase modulation for alternating periods of 2 tau . In synchronism with the phase modulation the output of the light source (laser 14) is switched for alternating periods of duration tau . Feedback means controls the magnitude and sense of the phase shift of modulation means 18, 19 and 16, 17, so that the amplitude modulation at the photodetector resulting from rotation of the gyroscope is nulled; the feedback signal being a measure of the rotation rate. <IMAGE>

Description

SPECIFICATION Fibre-optic gyroscope This invention relates to fibre-optic interferometer gyroscopes utilising the Sagnac effect.

The use of a multi-turn coil of optical fibre in which, by means of beam splitters and combiners, light from a single laser is propagated in both directions simultaneously to provide rotation sensitive output signals at a photodetector is known. Such an arrangement is described in, for example, "Sensitivity analysis of the Sagnaceffect optical-fibre ring interferometer" by Shih Chun Lin and Thomas G. Giallorenzi in Applied Optics, Vol. 18, No. 6, 1 5 March, 1979. When the output signals are combined interference fringe patterns are developed which, in a stationary system, from a fixed pattern whose share depends on the nature of the imaging optics. If the system is rotated about the coil axis fringe excursions take place and by suitable processing rotational rate information can be extracted.

Depending on the physical details of the system (e.g. operating wavelength A, fibre length L etc.) and the range of rotational velocities to be monitored, systems operating within a single fringe or over many fringes can be envisaged.

Considering initially operation within one fringe measurement an examination of the form of the output signal will show that there are measurement difficulties, three of which are i) the static nature of the sensor output (d.c. for constant angular velocity), ii) the non-linearity of output current with phase deviation, and iii) the pedestal lever arising from spurious optical signals.

The basic homodyne system can be improved, as shown by Lin and Giallorenzi, by taking a second matching complementary fringe pattern and applying this to a differential amplifier. The differential phase shift of 1 800 in the second fringe can be conveniently arranged by providing an extra reflection in its transmission path. This technique eliminates common mode signals, particularly the average level of the noise (i.e. the pedestal signal). A further benefit of this arrangement is that the two outputs can give a measure of the total energy at the sensor output and a feedback control signal can be derived for maintaining constant source power.

It is difficult however to get adequate stability in such d.c, systems and in addition low frequency noise can be serious with some detectors. A translation of the measurement to an intermediate frequency eliminates these problems. Modulation of the input signal can provide an a.c. measuring system. Lin and Giallorenzi suggest some principles for effecting modulation and synchronous detection in a homodyne system.

However such modulation is performed it is clear that the counter-rotating optical signals must be separated and a differential phase perturbation applied, According to the present invention there is provided a fibre-optic interferometer gyroscope including means for applying a first phase modulation in the loop optical path for alternating periods of duration T, where T is the transit time in the optical loop, means for applying a second phase modulation in the loop for alternating periods of duration 2T, and means for switching the light source for the loop for alternating periods of duration T in synchronisation with said first phase modulation periods which in turn is synchronised with said second phase modulation periods, and feedback control means to which the output of the gyroscope is applied, said feedback control means producing phase locked control signals for the phase modulation means.

The invention also provides a method of operating a fibre-optic interferometer gyroscope asymmetrically in the time domain wherein a first phase modulation at a frequency f = 2T, where T is the optical transit time in fibre-optic loop, is applied to the optical signals in the loop synchronously with both a second phase modulation at a frequency f = t r and a switching of the light source for the gyroscope at a frequency f = 2T, the amplitude and sense of the modulation at frequency f ---- = 2tT being controlled by a feedback signal derived from the interferometer output and being a measure of the rotation rate.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates schematically a fibre-optic gyroscope according to the invention, Fig. 2 illustrates the phase deviation output characterisation from a basic fibre-optic gyroscope arrangement, Fig. 3 illustrates driving waveforms and associated phase responses for the arrangement of Fig. 1, and Fig. 4 illustrates schematically a form of integrated phase nuller and modulator for use with the arrangement of Fig. 1.

The fibre-optic gyroscope shown in Fig. 1 consists essentially of a single or multi-turn coil of optical fibre 10, which is coupled via focussing lenses 11, 12 and a balanced beam splitter 13 to a laser 14 and a photodetector 1 5. (Ignore for the moment the other components in the Figure.) Light launched from the laser 4 is split equally at the beam splitter 1 3 and coupled into each end of the fibre 10, where it is propagated round the coil in both directions simultaneously. Upon emergence the two light outputs from the fibre are each split again equally at the beam splitter and half of each output will reach the photodetector 1 5. The two half outputs reaching the photodetector will mutually interfere at the plane of the photodetector.In general the superposition of the two output waves results in an interference pattern of concentric interference rings. In a well adjusted optical system only the central fringe is present and this central area is focussed onto the photodetector. If now the gyroscope is rotated about the axis of the coil, phase differences occur in the two outputs from the fibre which give rise to a change of light intensity at the photodetector.

The photodetector response to the changing phase deviation A0 arising from the rotation will have the form shown in Fig. 2, in which the output current i is at a central peak for zero rotational velocity falling to a first null and then rising to a second peak and so on as the speed of rotation is increased.

To eliminate the inherent d.c. nature of the output when the gyroscope is rotated at a constant angular velocity, phase modulation of the optical signals can be utilised. To illustrate how this phase modulation is accomplished consider a phase shifter 1 8, of electro-optic or other type, positioned at one end of the fibre loop or coil as in Fig. 1. This phase shifter is driven by a phase modulator 19 which applies a bias signal to the shifter for alternate periods of duration T, where T is the optical transit time in the loop or coil 1 0. As a consequence of the asymmetric placement of the phase shifter 1 8 every other transit of the clockwise wave will experience an electrically derived increment of phase shift and alternate anticlockwise transits will experience an identical phase shift.This leads to a phase modulation on the Sagnac signal at frequency 2T with resulting amplitude modulation at the photodetector output of the interferometer.

With the addition of synchronous switching of the laser output, it is possible to apply the externally applied phase shift to only one of the contra-propagating waves. This forms the basis for cancelling the Sagnac signal and operating the sensor in a closed loop phase nulled circuit. This mode of operating the sensor in a closed loop phase nulled circuit. This mode of operation is the one actually illustrated schematically in Fig. 1 with the phase shifter 1 8 forming the phase nuller in conjunction with the synchronously switched laser source. Modulation is now applied in a second electro-optic phase shifter 1 6 which is also interposed between lens 11 and the fibre end and is driven by a phase nuller 17 which applies a bias signal for alternate periods of duration 2T.As a consequence of the asymmetric placement of the phase shifter 16 and the relative phase of the driving waveforms to shifters 1 6 and 18, as indicated in Fig. 3, every other transit of the clockwise optical signal will experience an electrically derived increment of phase shift and alternate anticlockwise transits will experience an identical phase shift. This leads to a phase modulation of the Sagnac signal at 4T with resulting amplitude modulation at the photodetector output of the interferometer. With no Sagnac phase displacement, and zero modulation signal applied to the phase shifter 18, there will be zero modulation component at frequency 4T at the photodetector output. This situation corresponds to the slope of the curve in Fig. 2 at the zero phase point.Conversely the photodetector output at frequency -41T will be at its maximum value when the Sagnac phase deviation has increased to 7r/2. The action of the closed control loop is, via the amplitude and sense of the drive to phase shifter 18, to drive the phase difference to zero. The amplitude and sense of the drive to phase shifter 1 8 in this phase locked condition then represents a measure of the rotation rate.

It is convenient to fabricate the two electrooptic phase shifters 1 6 and 1 8 for phase modulation and phase nulling respectively as an integrated optical device as shown in Fig. 4. Phase nuller 19 also controls switching of the light source 14. In practice this is achieved by a separate electro-optic amplitude modulator (not shown) at the laser output since this is easier to implement than direct switching of the laser itself.

Driving waveforms for the two phase shifters and the laser switching together with the associated phase modulation response signals are given in Fig. 3. Separate phase modulators are preferable in order to avoid the filtering problem which would arise if both modulating waveforms were applied to the same modulator. A separate moduiator for the bias signal also allows it to be made longer for handling the multi-fringe condition at the upper end of the dynamic range.

A clear advantage of the phase modulator architecture is that, by virtue of its relative simplicity, it forms an ideal circuit for implementation in integrated optics. In fact the whole of the optics, outside the fibre coil, laser and detector, could be combined on a single fourport optical integrated circuit. As regards the system considerations, the contra-propagating waves are at precisely the same frequency and the minimum phase (path) differences which establish the phase modulation and nulling signals are, in a monolithically integrated component, under the highest degree of control. As shown in Fig. 4 the integrated optical device comprises a body 20 of lithium niobate having an optical guiding channel 21 diffused into one surface, e.g. diffused titanium. Two sets of metal electrodes, 22, 22a for the phase nuller and 23, 23a for the phase modulator, are put down on the surface of the body adjacent the channel 21. When a set of electrodes are biased by a voltage an electric fieid is set up across the channel and this field effectively alters the refractive index of the channel, causing a phase shift to be imposed on light passing along the channel.

Claims

1. A fibre-optic interferometer gyroscope including means for applying a first phase modulation in the loop optical path for alternating periods of duration, where is the transit time in the optical loop, means for applying a second phase modulation in the loop for alternating periods of duration 2, and means for switching the light source for the loop for alternating periods of duration in synchronisation with said first phase modulation periods which in turn is synchronised with said second phase modulation periods, and feedback control means to which the output of the gyroscope is applied, said feedback control means producing phase locked control signals for the phase modulation means.

2. A gyroscope according to claim 1 wherein the means for applying the first and second phase modulation comprise first and second electrooptic phase shifters coupled to the end(s) of the fibre-optic loop.

3. A gyroscope according to claim 2 wherein said electro-optic phase shifters are realised as integrated optics devices each comprising a diffused channel optical waveguide in a body of lithium niobate with biasing electrode means adjacent the channel whereby the refractive index of the channel may be modulated.

4. A gyroscope according to claim 3 wherein the channel is formed by the diffusion of titanium.

5. A gyroscope according to claim 3 or 4 wherein both the first and second phase shifters are formed as single integrated optics device having a common channel with two sets of biasing electrodes.

6. A gyroscope according to any preceding claim wherein the light source is a laser.

7. A gyroscope according to claim 6 wherein the means for switching the light source comprises an amplitude modulation optical device at the output of the laser.

8. A fibre-optic interferometer gyroscope substantially as described with reference to the accompanying drawings.

9. A method of operating a fibre-optic interferometer gyroscope asymmetrically in the time domain wherein a first phase modulation at a frequency f = 2T, where z is the optical transit time in fibre-optic loop, is applied to the optical signals in the loop synchronously with both a second phase modulation at a frequency f = and a switching of the light source for the gyroscope at a frequency f = 2z, the amplitude and sense of the modulation at frequency f = being controlled by a feedback signal derived from the interferometer output and being a measure of the rotation rate.

New claims or amendments to claims filed on 20-4-82.

Superseded claims 1.

New or amended claims: CLAIMS

1. A fibre-optic interferometer gyroscope including means for applying a first phase modulation in the loop optical path for alternating periods of duration T, where T is the transit time in the optical loop, means for applying a second phase modulation in the loop for alternating periods of duration 2T, and means for switching the light source for the loop for alternating periods of duration T in synchronisation with said first phase modulation periods which in turn is synchronised with said second phase modulation periods, and feedback control means to which the output of the gyroscope is applied, said feedback control means producing phase locked control signals for the phase modulation means.