GB2156070A - Ring laser gyroscopes - Google Patents

Abstract

It is known to use ring laser gyroscopes to measure rotation by measuring the rotation of the laser cavity, but more difficult to obtain the direction of rotation because it is difficult to maintain precise alignment of the optical system generating the moving interference pattern which gives the direction of rotation. The system described does not rely on such accurate alignment. Respective portions of the two oppositely travelling waves 2 and 3 are incident on a partly-transmissive partly-reflective and partly-absorptive beamsplitter 1 to form beams each comprising components of both portions and directed towards respective photosensors, the components in each beam interfering so that the intensity of the light falling on the photosensors is modulated in dependence on rotation, the modulation at one detector being out of phase with that at the other according to the absorption characteristics. These are chosen to give phase quadrature. <IMAGE>

Description

SPECIFICATION Ring laser gyroscopes This invention relates to ring laser gyroscopes.

A ring laser gyroscope comprises a laser cavity with three or more mirrors arranged so as to define a closed loop, path or ring for the laser beam. The laser beam comprises two waves of light travelling in opposite directions around the closed path defined by the laser cavity, the waves having optical frequencies which are determined by the resonance properties of the cavity. When the cavity is not rotating, the two waves have the same resonant frequencies as they traverse equal paths in equal times. However, when the cavity rotates each of the two waves experiences a change in resonant frequency producing an overall frequency difference which can be related to the rate of rotation of the cavity.

This enables the ring laser gyroscope to be used as a rotation sensing device. In order to utilise this, portions of each wave are optically superimposed to produce an interference fringe pattern which is modulated at the frequency difference between the two waves in the cavity. The modulated output optical signal obtained is a sine wave which has its frequency proportional to the rate of rotation of the cavity.

In many cases, however, it is not sufficient to determine only the rate of rotation of the cavity, the direction of rotation may also be required. To obtain the direction of rotation, the two wave portions may be misaligned by the optical system so as to create a fringe pattern consisting of a series of straight light and dark bands, with a sinusoidal intensity variation across the pattern. When the cavity now rotates, the fringe pattern moves, the direction of movement being related to the direction of rotation of the cavity. The fringe intensity is detected by each of two photodiode elements which are spaced apart by 1/4 of the fringe separation, and which hence produce two sine wave outputs in quadrature. These sine wave outputs are then converted to pulse-trains which can be processed by logic circuits to give the direction of rotation.In order to generate the fringe pattern properly, the wave superimposing optical system has to be very carefully set up and aligned. It is difficult to maintain this alignment for anything other than relatively short periods, particularly when operating under difficult environmental conditions.

According to one aspect of the invention, there is provided a ring laser gyroscope having an output sensor wherein respective portions of the two waves travelling in opposite directions within the laser cavity of the gyroscope are received and made incident on a partially-transmissive, partially-reflective and partially-absorptive beamsplitter surface so as to form two beams which each comprise components of both said portions and which are direcred onto respective ones of two lightsensitive detector elements, the components of each beam interfering with one another so that the intensity of the light falling on the two detectors is modulated in time in dependence upon rotation of the gyroscope about its sensitive axis and the intensity modulation at one detector is out-of-phase with that at the other detector, the magnitude of the phasedifference being determined by the absorption characteristics of said surface and being substantially nearer to phase quadrature than to either of in-phase and anti-phase.

According to a second aspect of the invention, there is provided a ring laser gyroscope in which two laser beam waves travel in opposite directions around the laser cavity, the gyroscope having an output sensor which is arranged for receiving respective portions of said two waves via a partially-transmissive one of the laser cavity mirrors and combining means including a partially-transmissive, partially-reflective and partially-absorptive beamsplitter surface, the combining means being operable for receiving said portions and for forming two beams each of which comprises interfering components of both said portions, and two light-sensitive detector elements positioned for receiving respective ones of said two beams and for providing output signals in response to the light incident on them, the arrangement being such that the intensity of light falling on each detector element is modulated in time in dependence upon the rotation of the gyroscope about its sensitive axis, the intensity modulation at one detector element being out-of-phase with that at the other, and the magnitude of the phase difference being determined by the absorption characteristics of said beamsplitter surface and being substantially nearer to phase quadrature than to either in-phase or anti-phase.

For a better understanding of the invention reference will now be made by way of example to the accompanying drawings in which: Figure 1 shows a beamsplitter which partially transmits, partially reflects and partially absorbs two waves of different frequencies incident on it; Figures 2 and 3 illustrate the outputs obtained from two photosensitive diodes positioned to detect the modulated fringe patterns for clockwise and counterclockwise rotation respectively of the ring laser gyroscope; Figure 4 illustrates one embodiment of the invention; and Figure 5 illustrates a further embodiment of the invention.

Referring initially to Fig. 1, a beamsplitrer 1 has two waves, 2 and 3 respectively, of different frequencies incident on it at a point 4 on its surface. Respective parts of each incident wave are transmitted, reflected and absorbed by the beamsplitter 1, and as can be seen from the figure, the reflected part of wave 2 combines with the transmitted part of wave 3 to produce a fringe pattern at 5.

Similarly, the transmitted part of wave 2 and the reflected part of wave 3 produce a fringe pattern at 6. The absorption characteristics of the beamsplitter 1 determines the phase relationship obtained between the two fringe patterns formed at 5 and 6 respectively and by choosing an appropriate absorption for the beamsplitter the two patterns can be made to be in phase quadrature. Two separate photodiodes (not shown) are arranged to receive the respective patterns such that the electrical signals from the photo-diodes are also in phase quadrature.

Fig. 2 shows the variation in output signal obtained from the two photo-diodes for clockwise rotation of the ring laser gyroscope.

It can be seen that the two signals are in phase quadrature with the upper trace lagging the lower trace. Similarly, in Fig. 3, the upper trace leads the lower trace by n/2 for counterclockwise rotation.

Referring now to Fig. 4, a gyroscope cavity mirror 10 has two waves 11 and 1 2 respectively, of different frequency incident on it.

Part of each wave is refracted through the mirror 10 and is then incident on a beamsplitter 1 3. Wave 11 is incident on the beamsplitter 1 3 at a point 1 3a. Here the wave 11 is divided into two parts 11 a and 11 b. Part 11 a is reflected at the point 1 3a onto high reflectivity reflectors 14 and 1 5 and is then refracted towards a corner cube 1 6. The other part 11 b of the wave 11 is refracted directly towards the cube 1 6. Both wave parts 11 a and 11 b are incident normally on the surface 1 6a of the cube and pass undeviated to surface 1 6b where they are totally internally reflected to surface 1 6c as shown. At surface 1 6c the two parts are again reflected and emerge undeviated from the cube 16.Wave part 11 b is reflected by another high reflectiv ity reflector 1 7 and is directed towards a pair of intensity monitors (not shown). Wave 11 a is incident at point 1 3c on the beamsplitter 1 3 where it combines with wave part 1 2a of the wave 1 2 to produce a fringed output 19, and after further reflection of the combined wave at reflector 18, a second fringed output 20. Wave part 12a is produced at point 1 3b on the beamsplitter 1 3 where the incident wave 12 is divided into parts 1 2a and 1 2b respectively. Part 1 2a is reflected by reflector 1 8 to the point 1 3c where it combines with wave part 11 a as previously mentioned.Wave part 1 2 b is refracted at point 1 3 b towards the intensity monitors. The two fringed outputs 1 9 and 20 are in phase quadrature. Using suitably placed detectors these outputs can be monitored.

In Fig. 5, a beamsplitter 30 is used in conjunction with gyroscope mirror 33 to refract and reflect the wave parts. Again waves 31 and 32 are obtained from the gyroscope cavity (not shown) via gyroscope mirror 33.

Wave 31 is divided at point 30a on the beamsplitter 30 to produce two wave parts 31 a and 31 b. Part 31 b is reflected by the mirror 33 at point 33a and is then refracted to an intensity monitor (not shown). Part 31 a is refracted towards a corner cube 34, where it is twice totally internally reflected emerging from the cube 34 to be incident at point 30c on the beamsplitter 30. Wave 32 is incident on the beamsplitter at point 30b where part of it 32b is transmitted on towards an intensity monitor (not shown). The reflected part 32a is reflected again by the mirror 33 and returned to point 30c of the beamsplitter where it combines with wave part 31 a to produce fringed outputs 35 and 36, output 36 being further reflected by mirror 33. Again the fringed outputs 35 and 36 are in phase quadrature.

The wave parts ill and 12b in Fig. 4 and 31 b and 32b in Fig. 5 may be monitored to determine the uncombined light levels in the system with special regard to 'lock-in control' (LIC) or 'path length control' (PLC) of the gyroscope system.

In both Figs. 4 and 5, the quadrature of the fringed outputs does not depend on prism alignment as with previous systems and the system is environmentally stable.

The beamsplitters stable.

The beamsplitters 1 3 and 30 may comprise a suitable single metallic layer, aluminium for example, or a combination of layers of different metals.

Claims (8)

1. A ring laser gyroscope having an output sensor wherein respective portions of the two waves travelling in opposite directions within the laser cavity of the gyroscope are received and made incident on a partiallytransmissive, partially-reflective and partiallyabsorptive beamsplitter surface so as to form two beams which each comprise components of both said portions and which are directed onto respective ones of two light-sensitive detector elements, the components of each beam interfering with one another so that the intensity of the light falling on the two detectors is modulated in time in dependence upon rotation of the gyroscope about its sensitive axis and the intensity modulation at one detector is out-of-phase with that at the other detector, the magnitude of the phase-difference being determined by the absorption characteristics of said surface and being substantially nearer to phase quadrature than to either of in-phase and anti-phase.

2. A ring laser gyroscope in which two laser beam waves travel in opposite directions around the laser cavity, the gyroscope having an output sensor which is arranged for receiving respective portions of said two waves via a partially-transmissive one of the laser cavity mirrors and combining means including a partially-transmissive, partially-reflective and partially-absorptive beamsplitter surface, the combining means being operable for receiving said portions and for forming two beams each of which comprises interfering components of both said portions, and two light-sensitive detector elements positioned for receiving respective ones of said two beams and for providing output signals in response to the light incident on them, the arrangement being such that the intensity of light falling on each detector element is modulated in time in dependence upon the rotation of the gyroscope about its sensitive axis, the intensity modulation at one detector element being out-ofphase with that at the other, and the magnitude of the phase difference being determined by the absorption characteristics of said beamsplitter surface and being substantially nearer to phase quadrature than to either in-phase or anti-phase.

3. A ring laser gyroscope according to claim 2, wherein the back surface of said one laser cavity mirror is reflective, and the sensor is positioned so as to direct said portions onto said back surface from the beamsplitter surface to produce the reuired interference.

4. A ring laser gyroscope according to claim 3 wherein said combining means further includes a prism arranged to reflect one of said components and direct it back onto said beamsplitter surface.

5. A ring laser gyroscope according to claim 2, wherein said combining means includes a plurality of reflective surfaces positioned so as to receive components of said portions from the beamsplitter surface and to direct these components to produce the required interference.

6. A ring laser gyroscope according to claim 5, wherein said combining means further includes a prism arranged to reflect components of both portions and to direct them back onto said beamsplitter surface.

7. A ring laser gyroscope according to any one of claims 2 to 6, wherein the output sensor includes respective detector means for receiving a part of each portion and forming signals for the lock-in control and/or path length control of the gyroscope.

8. A ring laser gyroscope substantially as hereinbefore described with reference to Figs.

4 and 5 of the accompanying drawings.