GB2190188A - A fibre optic gyroscope - Google Patents

Abstract

The application describes a fibre optic gyroscope comprising a short coherence length optical source (2), a fibre optic loop (1), means to inject the light into the loop in opposite directions such that it recirculates around the loop (1), and means (5) to detect the interference of light which has travelled several times around the loop but in opposite directions. The gyroscope may comprise a reflector (8) outside the loop to reflect light from the source back to the loop. Alternatively, the said loop (10) is within another loop, whereby light from the source (2) is split before the first mentioned loop and is injected into that loop from both directions. <IMAGE>

Description

SPECIFICATION Fibre optic gyroscopes The present invention relates to fibre optic gyroscopes using short coherence length sources and recirculating (i.e. resonant) loops.

The area of optical fibre gyroscopes has been an exciting research area for the last decade and has now reached the industrial arena with the first fibre gyroscopes arriving on the market last year. These gyroscopes are of the Sagnac loop configuration where injected light makes one pass in each of two directions around a loop of fibre and then enters to a signal processing station. The sources used are very short coherence length optical emitters (eg. superluminescent diodes).

A short coherence length source has been found to be necessary in order to reduce the effects of backscatter and Kerr nonlinearities.

Research has been carried out at several laboratories to improve on the situation described above by using a resonant configuration for the fibre-optic gyroscope. Such a topology would allow the injected light to pass through the gyroscope sensing loop several times, picking up rotation information on each pass, before reaching the signal processing area. Such an approach would be more sensitive than an standard gyroscope, if the same length of fibre were used, or alternatively it could have the same sensitivity while using a reduced length of fibre. Previous research in this area of resonant gyroscopes has not achieved satisfactory results primarily due to the need for a long coherence length optical source. Such a source inevitably led to problems with backscatter and nonlinearities.

The invention aims to provide resonant gyroscope configurations which use a short coherence length optical emitter. Such a configuration should show the high sensitivity expected of a resonant architecture without sufferring from excessive backscatter or nonlinear problems.

According to the invention there is provided a fibre optic gyroscope comprising a short coherence length optical source, a fibre optic loop, means to inject the light into the loop in opposite directions, and means to detect the interference of light which has travelled at least once around the loop but in opposite directions.

Thus, the invention is concerned with resonant gyro architectures where due to the reciprocal nature of the topology a short coherence length source may be used. By reciprocal nature we mean that the resonant loop will be traversed twice, once in each direction.

Thus a short coherence length source may be used, reducing the effects of backscatter and non-linearities present in other resonant gyro configurations.

Embodiments of the invention are described in more detail below, by example only, and with reference to the accompanying drawings, wherein: Figs. 1A to 1F illustrate one fibre optic gyroscope according to the invention; and Fig. 2 illustrates an alternative embodiment.

Figure 1A shows a gyroscope which utilizes both a resonant (or recirculating) loop 1 and a short coherence length optical source 2. There are two fibre-to-fibre couplers 3,4 shown (though these could be replaced by bulk optic beamsplitters). One 3 is used to channel part of the light returning from the gyroscope to a detector 5 and the other 4 is used to close the recirculating loop. Depending upon the signal processing scheme used and the source characteristics, various components 6,7 may need to be added in the locations shown.

These components may be phase modulators, frequency shifters, polarizers, or polarization controllers, for example. The fibre used is assumed to be either conventional single mode fibre or polarization preserving fibre at the source optical waveguide. The source 2 is a light emitter with a coherence length much less than any optical paths which lead to interferometric mixing.

Figures 1B to 1F show the operating principle behind this gyroscope configuration. Assume that short coherence length light is launched into gyroscope. In order to characterize what the output from the system will look like one needs to consider all of the possible optical paths through the gyroscope from the source 2 to the detector 5.

The first (i.e. shortest) path is shown in 1B.

In this case the light never enters the sensing loop, it simply bypasses the loop, reflects off of the reflector 8, and then bypasses the loop again on its way to the detector 5. This path is unique in that there are no other optical paths of this length.

Figures 1C and 1D show the next two paths through the system. In each case the light circles the loop once, in the clockwise direction for figure 1C and in the counterclockwise direction for figure 1 D, so that the two light signals which reach the detector have travelled the same distance (neglecting non-reciprocal effects). If the loop is rotating then these two paths will be of slightly different lengths. Since the two optical signals are coherent, having travelled approximately the same distances, this small mismatch will be converted into an intensity variation on the detector which is proportional to the rotation rate of the loop. Thus the system acts as a gyroscope.

A conventional Sagnac gyroscope operates by mixing light which has travelled once in the clockwise direction around a loop with light which has travelled once in the counterclockwise direction. The present configuration improves on this by allowing the light to travel several times around the loop and still inter fere at the detector.

Figures 1E and 1 F, for example, show the next two paths which the light can take. In Figure 1E the light travels around the loop twice in the counterclockwise direction before reaching the detector, while in Figure 1F two light travels around the loop twice in the clockwise direction. As with the other matched paths described above, these two optical signals are coherent and carry information on the rotation rate of the loop. Since they have recirculated the loop twice they have twice the mismatch of the light described above and thus increase the sensitivity of the gyroscope.

Such a configuration could have many recirculations of light providing a large increase in performance over the standard configuration.

Problems with backscatter and non-linearities have been reduced by the use of the short coherence length source. The above configuration does have a liability, however, in that there are more paths through the system than are necessary. For example, the first (i.e. shortest) path is incoherent with all the other paths and results in a large fraction of the optical power simply going to a nonsensitive offset on the detector. Also, there are two other paths of the same length as those shown in Figures 1E and 1F, namely one recirculation of the loop in the clockwise direction and then one recirculation in the counterclockwise direction (and cw followed by ccw).

The light which travels these paths is relatively small but has an effect on the system performance.

An alternative configuration which still allows a short coherence length source to be used in a resonant (or recirculating) topology but removes the unwanted paths mentioned in the last paragraph is shown in Figure 2. In this case, rather than use a reflector to send the light back through the loop, the light is split before the loop and injected into the loop from both directions. This configuration is identical to the standard Sagnac gyroscope configuration, except that a recirculating loop 10 has been added within the sensing loop.

Both of the systems described with reference to Figs. 1 and 2 can be extended further to a three axis system and also to use a multimode fibre loop, as described below.

Figs. 3a and 3b show a system in which three fibre rings 11, 21, 31 are cascaded in series to form the three axis device. There are many different paths that the light can take through this system. Using a source 2 of very low coherence ensures that only light beams having similar propagation delays will mix coherently giving an output signal. The sensitivity to rotation of a particular loop is dependent upon the length of the fibre forming the loop and the radius. Adjustment of these two parameters allows all three loops to maintain the same sensitivity whilst having different time delays. Incorporating a modulator 12 in each loop operating at different frequencies ensures that the rotation rate affecting each loop can be individually accessed, by processing at the appropriate frequency.

One of the limitations to the sensitivity of the gyroscope described in the earlier embodiments is the reciprocal action of the single mode coupler. In an ideal situation the sensitivity can be greatly enhanced by maximising the power input to the loop whilst minimising the power out of the loop. Reciprocal action of the coupler does not allow both conditions to be met. However, as shown in Fig. 4 a single mode fibre input 13 coupled to a multimode fibre loop 41 via a single to multimode coupler 15 gives effectively non-reciprocal operation allowing most of the power to be coupled in and very little extracted per. transit.

The light consequently undergoes many transits of the loop acquiring additional rotation induced phase shift each transit. This arrangement will also behave as a delay line filter and a power storage loop. The single to multimode loop can directly replace the single mode loops in all of the previous configurations.

It should be stressed that in practice all of the above embodiments will require various fibre optic components to be used to enhance the signal processing. Also, these configurations are not restricted to fibre optic systems but could be constructed from bulk optic components.

Claims (8)

1. A fibre optic gyroscope comprising a short coherence length optical source, a fibre optic loop, means to inject the light into the loop in opposite directions such that it recirculates around the loop, and means to detect the interference of light which has travelled several times around the loop but in opposite directions.

2. A gyroscope according to claim 1, comprising a reflector outside the loop to reflect light from the source back into the loop.

3. A gyroscope according to claim 1, wherein the said loop is within another loop, whereby light from the source is split before the first mentioned loop and is injected into that loop from both directions.

4. A gyroscope according to any preceding claim, comprising a cascade of three of said first-mentioned loops in series, whereby a three axis device is formed.

5. A gyroscope according to claim 4, wherein the length and radius of each loop is adjustable to adjust the sensititivy to rotation.

6. A gyroscope according to claim 5, wherein each loop incorporates a modulator.

7. A gyroscope according to any preceding claim, wherein a single mode fibre from said source is coupled to the first-mentioned loops by a single to multimode coupler, said loops being of multimode fibre.

8. A fibre optic gyroscope substantially as herein described and as illustrated in the accompanying drawings.