GB2181295A - Optical gyroscope - Google Patents

Abstract

An optical gyroscope with the operational attributes of a ring laser gyroscope and a conventional fibre optic gyroscope is comprised by a solid state laser diode (4), or equivalent high coherence source, whose output is tightly coupled into a coil (1) of low loss single mode optical fibre, so that the coil forms an optical resonant circuit for the laser diode and establishes a single frequency output of high coherence. On rotation of the coil about its axis, the degeneracy of clockwise and counter clockwise optical resonances is removed by the Sagnac effect, thus giving a frequency splitting of the diode output which is proportional to the rate of rotation. <IMAGE>

Description

SPECIFICATION Optical gyroscope This invention relates to an optical gyroscope and in particularto a fibre optic gyroscope.

A conventional fibre optic gyroscope comprises a multi-turn coil of optical fibre and beam splitters and combiners by means of which light fro a single laser is propagated in both directions simultaneously in the coil to provide rotation sensitive output signals at a photodetector. When the output signals are combined interference fringe patterns are developed which, in a stationary system, form a fixed pattern whose share depends on the nature of the imaging optics. If system is rotated about the coil axis fringe excursions take place and by suitable processing rotational rate information can be extracted.

Various measurement problems are encountered with the conventional fibre optic gyroscope but may be overcome such as discussed in our GB Patent Applications Nos. 8132314 and 8301654 (Serial Nos. ) (J.S.Heeks-32; M.C.Bone-3).

According to the present invention there is provided an optical gyroscope including a coil of single mode optical fibre and a high coherence optical source whose output is tightly coupled into the optical fibre, which coil forms an optical resonant circuit for the high coherence optical source.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates schematically one embodiment offibre optic gyroscope according to the present invention; and Figure2 illustrates schematically another embodiment of fibre optic gyroscope according to the present invention.

The basic architecture offibre optic gyroscope according to the present invention is shown in Figure 1.

It comprises a loop or coil of low loss single mode fibre 1 which is illustrated with two turns but may alternatively be a single turn or multi-turn as in a conventional fibre optic gyroscope. The two ends 2 and 3 ofthefibre loop are directly coupled to a respective output face of a solid state laser diode 4. A primary consideration for the laser to fibre interface is thatthe junction should transmit the light with low loss, that is the modal match between radiation from the laser output facet and the fibre end should be maximised.

There are well known procedures for achieving this objective. Thus an optical matching medium can be provided between the laser output facet and the fibre end by introducing a liquid between the surfaces or by applying a solid coating to one of the surfaces.

The refractive index and thickness of the matching medium would be chosen to provide a quarter wave transformation between the optical path in the laser and in the fibre. Alternatively, the fibre end can be rounded to provide a light collecting termination or a lens can be interposed between the laser and the fibre.

Sincethe laser diode istightly coupled to thefibre input it is ensured that its optical oscillating frequency is defined by the response of the fibre circuit. To a first approximation the optical load experienced by the laser diode will be equivalentto a single fibre of a length equal to half the length of the loop and terminated by a perfect reflecting end. For low loss fibre, with a loss of a few tenths of a dB per Km, a loop a few metres in extent will exhibit a Q comparableto that of a ring laser cavity. Hence an extremely narrow laserlinewidth (high coherence) can be obtained. In other words the fibre coil forms the optical resonant circuitforthelaserdiodeand henceasinglefrequ- ency output of high coherence.Instead of using a solid state laser diode, any equivalent high coherence optical source may be employed.

On rotation of the fibre coil about its axis the degeneracy in the resonant frequency of the clockwise and counter-clockwise optical paths is removed by the Sagnac effect and the laserwill coupleto both re- sonances, causing frequency splitting of its output.

The difference in optical frequencies is proportional to the rate of rotation.

The sensitivity (frequency resolution) and dynamic rangewill be determined bytheoptical Qofthefibre system, which is proportional to loop length and inversely proportional to optical loss, and by the separation of the optical frequency resonances, which is inversely proportional to loop length, respectively.

The output of the gyroscope can be taken by very lightly coupling to the optical path within the loop by means of a bidirectional coupling 5 (Figure 1 ) comprising a respective small loop offibre 6 and an associated photodetector diode 7. The optical beat frequency between counter propagating waves is thus "read" via the photodetector diode.

The fibre optic gyroscope of the present invention may beconsidered as a simplefibre equivalent of a ring laser gyroscope.

At low rotation ring laser gyroscopes exhibit the phenomenon of mode sticking, due to back reflection of light from one path into the other. This is removed by dithering the optical path length, usually by means of movement of one of the mirrors. By analogy, the same effect in a fibre optic gyroscope as proposed here can be removed by incorporating a phase modulator in the fibre path. A simpleform of such a modulator comprises a length ofthe fibre wound on a piezo-electricformer8 (Figure 1).

An alternative agreement for coupling the laser to thesensorfibre loop is shown in Figure 2.

In Figure2thefibre ends 2 and 3 aredisposedsubstantially in parallel and adjacent a single outputface ofthe laser diode 4, whereas in Figure 1 the fibre ends are substantially axially aligned and adjacent opposite output faces of the laser diode 4.

If the fibre of the loop is a polarisation maintaining fibre, or ifthe loop includes a polarising single mode fibre filter, optical recipriocity will be ensured.

The fibre gyroscope of the present invention, which may be termed a guided wave resonant optical gyroscope, thus combines the operational attributes of a ring laser gyroscope and the conventional fibre optic gyroscope.

Claims (11)

1. An optical gyroscope including a coil of single mode optical fibre and a high coherence optical source whose output is tightly coupled into the optical fibre, which coil forms an optical resonant circuit for the high coherence optical source.

2. An optical gyroscope as claimed in claim 1, wherein the fibre is of low loss.

3. An optical gyroscope as claimed in claim 1 or claim 2 wherein upon rotation of the coil its axis the degeneracy in the resonantfrequency of clockwise and anti-clockwise optical paths in the coil is removed bythe Sagnac effect and a frequency splitting ofthe output of the high coherence optical source is caused, which difference in optical frequency is proportional to the rate of rotation.

4. An optical gyroscope as claimed in claim 3 and including, for detecting the outputofthe high coher ence optical source, means very lightly coupled to the optical fibre coil.

5. An optical gyroscope as claimed in any one of the preceding claims, wherein the high coherence optical source is a solid state laser diode.

6. An optical gyroscope as claimed in claim 5 wherein the ends ofthe optical fibre constituting the coil are disposed in parallel and adjacent a single output face ofthe laser diode.

7. An optical gyroscope as claimed in claim5 wherein the ends of the optical fibre constituting the coil are axially aligned and disposed adjacent respective opposite output faces of the laser diode.

8. An optical gyroscope as claimed in any one of the preceding claims and including a phase modulator associated with a portion of the optical fibre coil.

9. An optical gyroscope as claimed in any one of the preceding claims wherein the fibre is a polarisa tion maintaining fibre whereby to ensure optical recipriocity.

10. An optical gyroscope as claimed in any one of claims 1 to 8 and including a polarising single mode fibre filter in the coil wherebyto ensure optical re cipriocity.

11. An optical gyroscope substantially as herein described with reference to Figure 1 or Figure 2 ofthe accompanying drawings.